GEOLOGY OF A PART OF SOUTHERN TRAVANCORE WITH SPECIAL REFERENCE TO THE PETROLOGY OF THE SYENITE AT PUTTETTI

30 Apr GEOLOGY OF A PART OF SOUTHERN TRAVANCORE WITH SPECIAL REFERENCE TO THE PETROLOGY OF THE SYENITE AT PUTTETTI

GEOLOGY OF A PART OF SOUTHERN TRAVANCORE WITH SPECIAL REFERENCE TO THE PETROLOGY OF THE SYENITE AT PUTTETTI

 

 

 

 

 

 

 

 

 

Thesis submitted to

The University of Travancore

for the Degree of

Master of Science

 

 

 

 

 

 

 

 

 

 

 

 

 

 

By

K.V. Krishnan Nair, B.Sc.

Research Assistant

University of Travancore Trivandrum

1954

CERTIFICATE

 

 

This is to certify that this Thesis is an authentic record of the work carried out by the author under my supervision and guidance and that no part thereof has been presented before for any other degree.

 

 

 

(Supervising Teacher)

Head of the Division of

Mineral Survey & Research

University of Travancore

Trivandrum

August 26, 1954.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

PREFACE

 

Generally speaking, the geology of Travancore is largely unknown, for, ever since the outstanding work of the pioneer geologists Dr. William King and Mr. R. Bruce Foote during the early eighties of the last century, no progress worth the name has been achieved in the exposition of the geological history of the State. Therefore, the contributions of Dr. King and Mr. Bruce Foote continue to be esteemed as landmarks in the field of geological research in this State.

 

In 1947, the University of Travancore organized the Division of Mineral Survey and Research under the directorship of Mr. T.R.M. Lawrie, B.Sc., F.G.S., the main object envisaged being to conduct a systematic geological survey of the State. During the three years of his tenure of office, Mr. Lawrie was chiefly concerned with the field training of the departmental personnel, and towards the close of 1949 regular survey work was commenced, and for this purpose, areas in Southern Travancore were selected to be mapped first.

 

Subsequent to his training, the writer was assigned the area surrounding Puttetti for detailed mapping. This thesis largely embodies the results of his investigations made during field work.

 

For convenience in presentation, this thesis has been divided into three parts. The first part is devoted for a brief general report on the geology of the area; part two, which is also the main body of the thesis, deals with the problem that was taken up for special study; and part three contains two papers based on topics having direct bearing on the main work.

 

The author wishes to place on record his deep sense of gratitude to Sr. K. Kunjunni Menon, B.Sc.(Madras)., M.S.(Yale)., M.A.I.M.E., for his inspiring guidance, and constructive criticisms throughout the course of this work. He is thankful to Sri. K.V. Nayar of the University Chemistry Department for valuable advice regarding chemical analyses. His thanks are also due to Sri. C.V. Paulose, Research Assistant, for the interest he evinced in this work. He is indebted to the University of Travancore for deputing him to Madras for a short term, to undergo special training the use of the Universal Stage.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

PART ONE

 

 

 

 

 

 

 

 

 

GEOLOGY OF THE AREA

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

CHAPTER – I

 

1. Introduction and previous work

 

Geologically speaking, Travancore is largely ‘unknown’ in that, ever since the general reconnaissance works of the pioneer geologists Messrs William King and R. Bruce Foote in the early eighties of the nineteenth century, the attention of the later workers was confined more to location and prospecting of industrial minerals rather than the systematic geologic survey. The contributions of King (1882) and Foote (1883) are reckoned at present as the most valuable sources of information concerning the geology of Travancore, and we are indebted in particular to Bruce Foote for his enlightening account of the geology of Southern Travancore.

 

The region, that was geologically mapped, (Map No.1) is situated on the coastal tract of Southern Travancore, and corresponds to the area represented in one inch topographic sheet 58 H/4. It covers an area of over thirty square miles and lies within parallels of latitude 8º 10’ and 8º 15’ and longitudes 77º 10’ and 77º 15’.

 

This part is triangular-shaped, the port of Kolachel, the fishing village of Taingapatam, and the high-hills near Kappiyara marking the south-east, north-west, and north-east corners respectively.

 

A net-work of motorable roads, cart-tracks, and footpaths facilitates easy access to most of the outcrops in the region. Good sections of the sedimentary rocks can conveniently be examined especially near the coast, where these are exposed in deep gullies and other water channels which are dry during most part of the year.

 

2. Physiography

 

Physiographically, four distinct zones can be recognized in Southern Travancore as a whole, trending lengthwise, and almost parallel to the coast. These zones are:

1. The mountains represented by the Western Ghats
2. The vast stretch of Alluvium
3. The plateau formed by the Tertiary formation
4. The coastal dunes made up of Recent sands

 

Being situated on the costal tract, the region under study is composed mainly of the last two physiographic zones viz., the Plateau and the Dunes.

 

In climate, Southern Travancore is neither as humid as the rest of the State, nor as arid as Tinnevelly and other places lying beyond the Western Ghats. Rainfall is low, and occurs during the South-West Monsoon. Hence the natural vegetation is of a semi-arid type. Palmyra Palm (Borassus flabelliformis), Acacia planifrons – a short stumpy tree with umbrella-like foliage, and others adapted for a desert habit are common in the flora. This part is industrially important, and the manufacture of palmyra fibre and coir, poultry-farming and fish-curing, form some of the major items of industry.

 

The region has a gentle slope towards the south-west, and the drainage also follows this direction. As typical of a semi-arid country, rainfall, in the form of occasional cloud-bursts, has caused numerous gullies to be formed in the region and drainage is effected along these. The only major stream of any importance, a part of which at least is contained in the area, is the Kuzhithurai River. This river, which originates far away in the Western Ghats, flows into the backwaters at Taingapatam, just before reaching the sea. Its outlet into the sea is controlled by a sand bar, which permits communication with the sea only during monsoon when the river maintains a considerable discharge.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

CHAPTER – II

 

1. Stratigraphy

 

Peninsular India, of which Travancore forms the southernmost part, when compared with rest of India, has a most imperfectly developed geological record. It is believed (Wadia, 1944, p.48) that the region has probably never been submerged beneath the sea, except temporarily and locally, and for this reason, no considerable thickness of marine sediments was ever deposited in the interior of the land mass during post Cambrian period.

 

Therefore in Travancore, the geological column is not represented by rocks of the Paleozoic and Mesozoic era. The oldest sedimentary rocks are of Tertiary age. The stratigraphical sequence in the area under study is as follows:

 

Recent		Blown Sands formed by the coastal dunes, and the red ‘Teris’; alluvium
Sub-recent (?)		Laterite
Tertiary		Warkilli Beds
Archaean		Garnetiferous Gneiss, Charnockite, Pegmatites and Veins

 

 

 

 

 

2. Petrology

 

• Archaean Rocks

 

The archaean group of crystalline rocks is considered to be the oldest known rocks, and forms the basement on which later sediments were deposited. This group of rocks is represented chiefly by garnetiferous gneiss (leptynite), and charnockite.

 

• Garnetiferous Gneiss (Leptynite)

 

This is a garnet-rich, light-coloured rock which is also referred to as leptynite. Fresh outcrops are exposed in some of the quarries at Kappiyara and Kolachel. Generally the garnetiferous gneiss or leptynite has a better expressed foliation than the charnockites. Usually, the rock has a white colour and the garnet appear as bright spots rendering the foliation conspicuous. Often the white colour of the leptynite is quite a remarkable feature of the rock, and Chacko (1921, p. 17) attributes the white colour to the alteration undergone by the feldspars. The foliation strikes N.W. – S.E. and dips 50º – 60º to the N.E. locally there seems to be a change in the direction of dip. However, the exact reason is not known.

 

The rock is medium-grained, and composed essentially of quartz, feldspar, pink garnet and mica with smaller amounts of graphite and iron ores. The greater bulk of the rock is made of the two minerals quartz and feldspar. The feldspar is represented by orthoclase, plagioclase and perthite. The micas are largely biotite and phlogopite, and tend to be concentrated along foliation. The average specific gravity of the rock, as determined on six specimens, was found to be 2.65.

 

The leptynite is comparatively less resistant to weathering than the charnockites. Examination of a number of outcrops shows that the process of weathering is most conspicuous along foliation planes and joints. The progress of rock decay along planes of foliation leads to large slabs of the rock to be dislocated, causing the formation of dip-slopes along flanks. The same feature accounts for the typical serrated appearance of the summit. In outcrops, where the surface is generally even, the effects of differential weathering are marked. The easy weatherability of the feldspars results in feldspar portions to be fluted in contrast to quartz which stands out. Weathered surfaces are usually stained by solution carrying limonite, evidently formed by the decomposition of the iron-bearing constituents of the rock.

 

The ultimate product of weathering is laterite. It may often be noted that the laterite formed ‘in situ’ from a garnetiferous gneiss is lighter in colour than that formed from a charnockite. This is probably due to the relative sparseness of the ferromagnesian minerals in some of the garnetiferous gneiss. In places, where soil profiles exist, the various stages in the transformation of the rock into laterite can be observed.

 

• Charnockite

 

Charnockite is more abundant than the garnetiferous gneiss in the area, and is represented practically by the intermediate variety. The acid variety is comparatively rare. The rock is generally grayish-blue in colour, holocrystalline, and medium to coarse-grained. The characteristic grayish-blue colour helps in the easy identification of the rock in the field. The average specific gravity was found to be 2.61 for the acid variety, and 2.70 for the intermediate variety. The somewhat higher specific gravity of the intermediate charnockite is evidently due to the presence of a good amount of ferromagnesian minerals.

 

Megascopically, feldspar, quartz, hypersthenes, micas, garnet, iron ores, and pyrite can be identified. The feldspar and quartz are usually greenish or bluish-gray in colour, and together they make the greater bulk of the rock. Feldspar is present in greater quantity than quartz. While hypersthene is constantly present in all the specimens examined, the remaining minerals occur in variable quantities, all of which are not always represented in a single hand specimen. Garnet is of the almandine variety, and, though it is an important constituent of the garnetiferous gneiss, it is no less conspicuous in the charnockite. The rock is tough, massive, and does not easily yield to the hammer. Fracture surfaces appear subconchoidal. Foliation, though seen in many outcrops, is less marked and sometimes very indistinct in the charnockites of the area.

 

In a few places, outcrops of the acid and the intermediate varieties are seen close by, but do not show plausible evidences to indicate their mutual relations. Some of the quarries in the area especially those near Karingal Camp-shed, expose sections where the charnockite and the garnetiferous gneiss occur as local patches in it.  The rock at the junction is so badly weathered and stained that the nature of the association could not be clearly observed. At Kolachel, a quarry face has exposed charnockite at the base and garnetiferous gneiss at the top. Both rocks are texturally alike, and garnet is present almost equally in both. The junction between the two rocks shows gradual merging of the one into the other.

 

• Syenite

 

In addition to the garnetiferous gneiss and the charnockites, the crystalline rocks of the area include yet another rock type having features quite dissimilar to those characterizing the other two rock types. This is a syenite occurring as a composite, low ridge, exposures of which are conspicuously seen at a number of places in and around the place known as Puttetti. The texture ranges from medium to coarse, the coarse type being the most prevalent. The syenite is considered to be genetically related to the charnockites. Since the petrology of this syenite is discussed in Part Two of this Thesis, further details are not given here.

 

• Pegmatites, Dykes and Veins

 

Pegmatites are common in both the garnetiferous gneiss and the charnockite. These generally strike in all directions, and vary in width from fraction of an inch to about 8 inches. The pegmatites are characteristically coarse, possess more or less the same colour as the respective host rock, and mostly carry feldspar, quartz, and mica. Rarely garnet, tourmaline, and hornblende occur in the pegmatites.

 

A dolerite dyke is found to outcrop at Appiyodu. This has a variable width of about six inches to a foot, and is exposed to a length of about thirty feet, in a brook. It dips almost vertically and has a N.W. – S.E. strike.

 

A very conspicuous mass of vein quartz outcrops on the eastern side of the syentite ridge. This has an average width of about 250 feet and apparently trends in a W.N.W. – E.S.E. direction. The quartz is very much weathered and is clearly exposed for a distance of over hundred feet. Similar occurrences of masses of quartz are not uncommon in other parts of the State. Chacko (1919, p.4) refers to similar occurrences in Mundakayam District and considers them as “differentiation products of or segregations in the charnockites of the district”.

 

Chalcedony occurs as pockets and insignificant veins, and is exposed in some of the water channels near about the syenite occurrence. The chalcedony shows different colours in the various outcrops, as milky white, brown and pale blue. Sometimes mottling is also observed.

 

An olive green mineral occurs as a small pocket in a brook at Appiyodu. This mineral has been identified to be apatite.

 

 

 

 

 

• Tertiary

 

• Warkilli beds

 

The tertiary beds in Travancore are represented by fresh-water deposits formed under shallow water conditions, and occurring as extensive outliers practically all along the coast from Quilon to Cape Comorin. A typical stratigraphic section is exposed in the cliff at Warkilli, the type area, and hence the formation is known as the Warkilli formation. At Warkilli, the section has a thickness of about 138 feet (King, W., 1938, p.98) and consists of beds of laterites, sands, and sandy clays alum clays, lignite beds, and sands. The formation is fossiliferous at the type area. However, the formation in the rest of the State is not known to have yielded fossils. A lithologically similar formation of Tertiary age, known as the Cuddallore Series, extends along the Coromandel Coast, and has been provisionally laid down by King (1882, p.92), and Foote (1883, p.25) to be equivalent of the Warkilli formation.

 

In Southern Travancore, sections of the Warkilli formation generally consists of a fewer number of beds than the case in the north, and are as a rule patchy and inconspicuous. Beds generally consist of sandstones, grits, and clays. Lignite is not present.

 

In the area under reference, outcrops of sandstone and grits are exposed at Taingapatam, Amanad, Kizhkulam, Midalam, and Kolachel along the coast. At Taingapatam, soft mottled grits are found in a well section. The grit beds near Midalam show a distinctly conglomeratic character. Deep gullies expose fair thicknesses of coarse conglomeratic mottled grits, capped by red loam. Clay galls are frequently found enclosed in the grit beds. The outcrops, being very small and scattered, are not separately represented in the map.

 

• Sub-Recent

 

• Laterite

 

Laterite is an important rock unit seen abundantly in the stratigraphic column of the area. Two varieties are distinguishable, primary or residual and secondary or detrital laterite. The primary laterite is generally restricted in distribution to higher altitudes, and as much, it is found to cap the archaean crystalline rocks where it is formed by the alteration ‘in situ’ of the rocks concerned. The detrital variety is the top member of the Warkilli beds and is commonly found in the low lying areas, particularly the coast. It may be noted that the two varieties cannot be separated one from the other.

 

In some parts of the area, sub-aerial denudation of the ‘teris’ has exposed large outcrops of primary laterite at the base. These outcrops usually retain the original structures of the gneissic rocks, the alteration of which ‘in situ’ has resulted in the formation of the laterite. Sometimes, remnants of pegmatites and veins are observed in the laterite, and these clearly indicate the primary mode of origin of the laterite. This is further supported by the presence of angular quartz in the material.

 

Two good outcrops of laterite have been noted, one (K 53) on the eastern side of the road near Karingal Camp-shed, and the other (K 48) about 1½ miles north-west of Karingal Market. At K 53, the gradual passage from the massive fresh rock to the soft and finally hard vermicular, ferruginous laterite is remarkably seen. Exfoliation appears to be the initial stage in the disintegration of the rock, which is closely followed by laterisation. The original structures such as foliation, veins and pegmatites are traceable even after the transformation. At K 48, a quartz vein about 4 inches wide is seen in the laterite. The section exposed here is nearly 30 feet thick. On the top, there is about 10 feet of laterite which merges into 15 feet of reddish-yellow and white mottled quartzose clay. Further down, about 5 feet of white quartzose-feldspathic-lithomarge is seen. The quartz grains are strikingly angular, and the interstices between them are filled by the partly kaolinised feldspar. The lithomargic zone should naturally lead imperfectly to the fresh country rock below. Other exposures are seen near Kizhkulam.

 

Outcrops of laterite in the northern half of the area are almost invariably primary in nature as seen from the characteristics already mentioned. However, towards the coast the predominant variety is detrital laterite, and the two varieties usually merge in such a way as to obliterate all possibilities of distinction. Such a situation arises at Amanad near the coast, where excellent sections of laterite are exposed in deep gullies.  In this place, on the seaward side, the laterite overlies the Warkilli beds and hence detrital in nature, and further inland the laterite is primary as shown by its typical structures. Again, the detrital nature of the laterite on the seaward side is supported by the presence of numerous well-rounded pebbles of quartz in it. The zone in-between the two, which is also of laterite, does not show any clue as to its origin. The passage from the one to the other is quite insensible. Though topographic expressions may aid in distinguishing primary laterite in the highlands, lower down, the two types of laterite have more or less flat tops, and hence topographic considerations are not of much help. This apparent confusion may probably due to a general subaqueous erosion in post-tertiary period, when the topographical irregularities in areas of the two types of laterites were removed thereby rendering the general surface even and uniform. In the words of King (1882, p.92) “Whatever form of denudation may have produced the now much worn terrace of the gneissic portion of the country, the same also determined the general surface of the Warkilli beds. Indeed, it gradually dawned on me while surveying this country, having the remembrance of what I had seen of the plateaus and terraced low-land in Malabar in previous years, that here, clearly, on this western side of India is an old marine terrace, which must of later date than the Warkilli beds”.

 

• Recent

 

• Alluvium

 

Though alluvium is found deposited in most of the depressed parts in the area, its occurrence in the form of two, extensive stretches, one in the east-west direction, from the flanks of the Kappiyara Hills up to Taingapatam; and another a smaller one, occupying the eastern part of the area, are notable. The source of the alluvium is largely the weathered products of the crystalline rocks, laterite, and red loam. It is mainly composed of sands, sandy clay and clay with an admixture of various hydrated oxides of iron imparting to it a rather pale reddish-brown colour. When intimately mixed with organic matter the alluvium has an almost black colour.

 

• ‘Teri’ (Red Loam)

 

‘Teri’ is the local term for a peculiar type of red sand hills considered to be Pleistocene to Recent in age. This formation is found to cover extensive areas all over the coastal tract of Peninsular India.

 

In Travancore, the ‘teris’ are confined to the south, and extend as a regular belt from Trivandrum up to Cape Comorin, a distance of about 50 miles. It has a variable width of 2 to 6 miles, the maximum being attained around Nagercoil. The ‘teris’ are found to overlie the older rocks such as laterite, tertiary, and archaean rocks.

 

In the area under reference, three conspicuous patches are found to occur at Amanad, Midalam, and Kolachel respectively. Based on their topographic expression, the ‘teris’ in Travancore have been provisionally classified (Menon, 1950, p.6) into ‘Dome or Ridge teris’, ‘Plateau teris’, and ‘Loose teris’. According to this classification, the ‘teris’ at Amanad which is a compact one, belongs to the class ‘Dome or Ridge teris’ and the ‘teri’ at Kolachel and Midalam being flat, comes under ‘Plateau teri’. Generally, the ‘teris’ are traversed by networks of deep gullies with precipitous walls. At Amanad, where the ‘teri’ is of the ‘Dome or Ridge type’, the pattern of the gullies is remarkably radial. Some of the deep gullies are 35 to 40 feet in depth corresponding to the local thickness of the red loam in that place.

 

In composition, the ‘teris’ mostly consist of sub-angular to rounded quartz grains and magnetic iron ores such as magnetite and ilmenite. Some of the other heavy minerals identifiable under the microscope are zircon, sillimanite, rutile, monazite, kyanite, andalusite, and a few others. The binding material is ferruginous clay. It is remarkable that the ‘teris’, which are mostly deposited in an area of garnetiferous gneisses, do not practically contain garnets. This attracted the attention of Bruce Foote (1883, p.33), who, in his report on the geology of South Travancore remarked thus:

 

“Common as garnet sand is on the beaches of South Travancore, I never yet found a grain of it in the ‘teri’ sand where the latter was pure and had not been mixed with beach sand”.

 

This absence or rarity is largely due to the fact that under tropical weathering, garnet is highly unstable.

 

A large number of the constituent quartz grains appear frosted, evidently due to wind action. Generally, bedding is absent, although this feature is locally seen. This would indicate local activity of fluvial agents in the deposition of the ‘teri’ sediments.

 

The question of the origin of the ‘teris’ is a much disputed one. According to the observations of the writer, the ‘teris’ seem to have been derived ultimately from laterites and other products of weathering of the archaean and tertiary rocks. However, before final deposition the sediments appear to have been reworked. The features of deposition suggest formation under warm, moist conditions, associated in pluvial periods.

 

• Blown Sands

 

The blown sands occupy a narrow strip fringing the coast. Usually quartz grains make the greater bulk of the beach sands. Small amounts of ilmenite, rutile, monazite, garnet, sillimanite and zircon are also found. At Kolachel, there are conspicuous sand dunes which are almost composed of black sands. Further, along the shore at Kolachel, concentration of ilmenite, garnet, and monazite are found. The sources of the sands are, at least in large parts, the archaean crystalline rocks, the tertiary beds and the ‘teris’.

 

3. Economic aspects

 

From the economic point of view, mineral sands, mica and graphite deserve consideration. Regarding the mineral sands, Kolachel is one among the richest places in Travancore. Though the deposit is not being worked at present, it holds considerable potentialities. The sporadic occurrence of mica of the phlogopite variety has been reported from different parts of the area. Prospecting for this mineral was being carried out during the war years, when prospecting centres in the area supplied hundreds of pounds of the mineral. At three places near about Karingal, prospecting operations showed occurrence of graphite. These occurrences are not regular and do not seem to have any continuity. The deposits are not being worked at present.

 

There is an almost inexhaustible reserve of building stones, and road material obtainable from rocks such as the charnockites and the leptynites. These are being actively quarried in a number of places.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

PART TWO

 

 

 

 

 

 

 

 

 

 

 

 

 

PETROLOGY OF THE SYENITE AT PUTTETTI

 

 

 

 

 

 

 

 

 

 

 

 

CHAPTER – III

 

1. Introduction and previous work

 

Although the most general and abundant crystalline rock types of Southern Travancore, including of course, those of the area mapped, are represented by the charnockites and the leptynites, there occurs over a limited area in and around the place called Puttetti in the Eraniel Taluk, a rock type which is apparently distinct from the rest of the crystalline series of the surrounding area.

 

The rock which generally resembles the charnockites, particularly the intermediate variety, on closer examination can be seen to be entirely made up of feldspar and mafic minerals to the complete exclusion of quartz, which feature is perhaps the most outstanding one that distinguishes it from the usual charnockites of the surrounding area.

 

Some parts of this rock massif contain unusually large and beautiful crystals of zircon, and this has been attracting from time to time, the attention of geologists from other parts of India, as well as abroad.

 

No detailed work on the zircon bearing rocks appears to have been attempted by earlier geologists till now, and obviously therefore, previous literature on the subject is very scanty, and what little is available, seems to concern more about the zircon crystals than the associated rock.

 

According to the available literature, the occurrence of this rock was first noticed by Masillamani, a former State Geologist, more than fourty years ago. In his report (1080, M.E.=1911, p.2), he stated “Two new rocks hitherto not known in South Travancore came under my notice in the Eraniel Taluk. One is syenite and the other has not been definitely identified yet. The former occurs in the western border of Eraniel and consists essentially of feldspars, pyroxenes, and limonite, evidently an intermediate facies of the charnockite series. This rock is remarkable in zircon being present in great abundance”.

 

“Closely associated to the syenite is the other rock which consists of feldspar, and a green ferromagnesian mineral which has been identified as pale green augite. The specific gravity of the rock is 3.21 and will indicate it to belong to the basic series”.

 

In the course of the systematic geological mapping of the area dealt with in the foregoing pages, (Part one of this thesis), the writer became interested in making a special study of the zircon-bearing syenite, about which as previously mentioned, only very little information is available. Therefore, during the period1952-1953, the writer camped off and on at Puttetti, surveying the syenite and the associated rocks, examining every accessible outcrop in the area, and collecting suitable samples for subsequent petrographic and chemical analyses. In all, not less than one hundred hand specimens were collected, and an equal number of microslides were prepared by the writer himself, for the purpose of the present investigation.

 

The following pages contain an account of the observations made in the field as well as in the laboratory, based on which an attempt is made to ascertain the genesis of the syenite rock and its possible relation to the charnockite, with which it is seen to be closely associated at Puttetti.

 

2. Field Characters

 

The crystalline rocks in and around Puttetti, when carefully examined in the field, may be seen to be constituted of a few varieties, and may be arbitrarily grouped as:

1. Diopside Granulite
2. Diopside Syenite
3. Diopside Syenite (zircon-bearing)
4. Intermediate Charnockite, and
5. Acid Charnockite

 

On a broader basis, these five arbitrary varieties could conveniently be brought under two main divisions, the Syenite suite, comprising of the first three varieties, and the Charnockite suite, consisting of the intermediate and acid varieties of charnockite. Examination of the outcrops whatever they occur side by side, indicate a merging of one variety with the other. Hence the five varieties are considered to be local facies having many features in common, and believed to have originated from a common magma.

 

In the accompanying map (No.2), tow shades of the same colour are given for a single rock suite. The darker shade represents localities where actual outcrops are present, while the lighter shade corresponds to places where extension of the same rock suite is inferred upon field evidences, though outcrops are absent.

 

A glance at the map would give a general idea about the distribution of the two main rock suites, in the area. The Syenite suite of rocks, consisting of three varieties already mentioned, are seen as three ridges, the one on the north being the largest. This and the smaller ridge on the west, strike N.N.W., while the southern ridge strikes almost N.W. in general, the ridges may be said to lie in a position, coinciding with the regional strike of the crystalline rocks of Southern Travancore. The largest ridge has a length of about three-fourth of a mile and a maximum width of about 800 feet attained near the northern end. The largest individual outcrop in the ridge is about 750 feet long and 375 feet broad.

 

Surrounding the ridges composed of the syenite suite of rocks, and also occupying practically the rest of the area surveyed, is the charnockite suite, represented mostly by the intermediate variety and to a very small extent by the acid variety. Outcrops of charnockite are seen only at a few places. These are invariably small in size, and appear to crowd towards the western side of the main syenite ridge.

 

It is noteworthy that nowhere in the area, any distinct contact between the syenite suite and the charnockite suite of rocks could be observed. On the contrary, in a few places, especially bordering the northern half of the main ridge, a gradual merging of the syenite with the charnockite, could be noticed. In the vicinity of Appiyodu, and in two other places, (K8 & K9; K6 & K21) a traverse from the syenite ridge towards the nearest outcrop of charnockite would reveal traces of quartz gradually appearing in the peripheral part of the syenite body. On account of paucity of outcrops between the syenite and the charnockite, it was not possible to observe the nature of the interlying rock.

 

The five varieties of rocks, which are grouped under syenite and charnockite suites, as already mentioned have certain common features, but at the same time display some features characterizing each variety and these are summarized below.

 

Syenite Suite		1. Diopside Granulite 2. Diopside Syenite 3. Diopside Syenite (zircon-bearing)
Charnockite Suite		1. Intermediate Charnockite 2. Acid Charnockite

 

• Syenite Suite

 

Although diopside granulite is grouped as a separate variety, it is not seen to occur as a separate rock body. In the area under reference, this variety is found outcropping as bands and irregular patches at four places in the diopside syenite of the main ridge, as marked in the map. These are without any clear-cut contact with the host rock. The rock is typically medium-grained, greenish-gray coloured, and composed of feldspar and ferromagnesian minerals. The distribution of the constituent minerals is such as to impart a granulitic appearance to the rock. Frequently, the ferromagnesian mineral grains show a tendency to coalesce and form conspicuous clots, dark basic patches, and segregations.

 

The second variety, diopside syenite is more or less identical with the previously mentioned diopside granulite in the matter of mineralogy. However, in structure and texture it is notably different. It is typically a greenish-gray coloured, coarse-grained, generally foliated variety, and while foliation is clearly developed (Plate I) in most of the outcrops, in some the foliation is less marked. The foliation is evidently the result of the parallel alignment of the ferromagnesian minerals. This variety constitutes the greater part of the syenite suite of rocks and occurs in all the three ridges. In the field, the rock is found associated with diopside granulite in certain outcrops, and with the zircon-bearing diopside syenite in certain others. In all these cases, the rocks concerned appear to merge with each other.

 

The next variety is the zircon-bearing diopside syenite. It is a coarse-grained, greenish-gray rock, essentially composed of feldspar and a ferromagnesian mineral. It contains large, well-developed crystals of zircon, and this feature distinguishes the rock from the previously described diopside syenite. Some flakes of mica are also noted. Foliation is generally absent. Though the constituent minerals do not appear conforming to any particular arrangement in hand specimens, in the field it is possible to notice a crude type of foliation marked by the ferromagnesian mineral. The zircon crystals are distributed haphazardly, and while these occur in large numbers at some places, they are not apparently seen at other spots in the same outcrop. Towards the periphery of the man ridge, a very careful examination has revealed traces of quartz to be present.

 

• Charnockite Suite

 

Intermediate charnockite is well represented in the area, but outcrops are comparatively few and small in size. The largest outcrop occurs between the two syenite ridges on the eastern half of the map. Since most of the charnockite exposures in the area are being actively quarried, it was possible to examine fresh quarry faces in a number of instances. The rock is compact, tough and has a grayish-blue colour. It is essentially composed of feldspar, quartz, and hypersthenes, with subordinate amounts of garnet and mica. In texture, the rock is medium to coarse-grained. Abrupt changes in texture are a remarkable feature, and can be seen in hand specimens. Such coarse-grained portions of the rock show a striking similarity in appearance to the typical syenite. Sometimes, foliation is markedly displayed and is brought about by the linear arrangement of the dark constituents. Since all the charnockite outcrops are widely separated from the syenite ridges, it was not possible to observe their mutual relations in the field.

 

Compared to the intermediate charnockite, the acid variety is light coloured and is represented only to a very small extent in the area. Though, mineralogically, the two varieties are almost similar, the acid variety is characterized by a lesser amount of the ferromagnesian mineral, with practically no garnet. It is a compact and tough rock with a medium texture. Outcrops are very few, located far apart, and do not enable to observe their relation with the other rock types.

 

In addition to the rocks described above, other rocks occurring in the area include a dolerite dyke (?) and a massive quartz vein. The dolerite dyke has a variable width of six inches to a foot and is exposed in a brooklet for a length of about ten feet. Beyond this, it covered by a thick layer of alluvium. The dyke strikes in a N.W.-S.E. direction. The quartz vein is marked on the eastern side of the map (No.2). It is exposed on the side of a low hill for a distance of about 100 feet and has a width of about 250 feet. Since it gradually passes into lateritised material, the boundaries are not clearly seen. The quartz is impure and is in an advanced stage of disintegration.

 

3. Weathering

 

The effects of weathering on the surface of the rocks of the syenite and charnockite suites are typical of each group, and this feature considerably helps to distinguish rocks of any one suite from the other in the field, even when fresh exposures are absent.

 

When compared with the charnockite, the syenite is in an advanced state of weathering and this is easily explained in virtue of the high feldspar content and coarse texture of the former. Being thus easily prone to weathering, the effects have apparently penetrated to considerable depths in the syenite ridge, so that the deciphering of the mutual relations among the members of the syenite suite is very much hindered. The weathered surface of the syenite has a grayish-white colour, and is full of small pits and irregular depressions, obviously caused by the removal of the weathered products. Frequently, cavernous hollows (Plate II & III) are formed as a result of differential weathering and solution. Exfoliation, and weathering along joints which causes blocks of the syenite to be isolated from the rest of the rock, are common. The isolated blocks disintegrate and form talus at the base of the ridge (Plate IV & V).

 

Exfoliation and spheroidal weathering are characteristic features of weathering in the charnockites. The freshly exposed surface after the removal of the overlying exfoliated layer is comparatively even and smooth (Plate VI). Spheroidal weathering is clearly seen in the loose boulders.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

CHAPTER – IV

 

1. Petrography & Mineralogy

 

The arbitrary division of the crystalline rocks of Puttetti into five varieties or types was already mentioned under Field Characters. This division is evidently found to facilitate in great measure their petrographic descriptions also. The scheme containing the two major suites and their sub-divisions is repeated below for ready reference.

 

Syenite Suite		1. Diopside Granulite 2. Diopside Syenite 3. Diopside Syenite (zircon-bearing)
Charnockite Suite		1. Intermediate Charnockite 2. Acid Charnockite

 

• Syenite Suite of Rocks

 

• Diopside Granulite

 

1. Megascopic Characters

 

In hand specimens, the rock is seen to be composed essentially of a greenish-gray pyroxene, and a greenish-white feldspar. These two constituents are present in almost equal amounts, and are uniformly distributed. The rock is medium-grained, and the disposition of the constituents is such as to give it a typically granulitic aspect. However, in a few specimens, the pyroxene as well as the feldspar grains show a tendency to occur in clusters thereby imparting a crude gneissic appearance to the rock. No other mineral is identifiable with the naked eye. But a few specks of a brownish mineral resembling sphene, are seen under a magnifying glass. The rock has an average specific gravity of 2.98.

 

1. Microscopic Characters (Typical Slide Nos. K16, K19, K24A, K24B, K24C, K24D, K24E, K24F, K41A, K41B, and K41C)

 

In thin sections, the rock has typically a granular texture, and the grains are more or less of the same size. It is seen to consist of feldspar and diopside in almost equal proportions. The other ferromagnesian minerals found are hornblende, sphene and mica. The minor accessories include zircon, iron ores, apatite, calcite, and a little secondary quartz. Original quartz is absent.

 

Feldspar is mostly microperthite, and the characteristic spindles can easily be distinguished under a moderate magnification. Often a single patch of microperthite is observed to contain a few relicts of the clear non-perthitic host feldspar (K19 & K41C). Small amounts of orthoclase also occur. Generally, the feldspars show undulose extinction, which may be due to the effect of strain. Calcite of a secondary origin is found developed along some of the cracks in the feldspar grains. The optical constants of the microperthite are as follows:

 

Extinction relative to the basal cleavage is 10º

2 V = 80º ± 2.

Optically negative

Diopside is light green in colour, non-pleochroic, and does not show any idiomorphic outline. Some of the grains have diallagic cleavages, and certain others contain numerous cracks. The diopside often completely encloses patches of feldspar, a feature which would indicate a secondary origin for the mineral. Many grains contain strings of bubbly inclusions with a parallel alignment. The mineral has the following optical properties:

 

Colour – pale green to green

Pleochroism – nil

Extinction – Z Λ C = 35º to 40º

2 V = 58º ± 2.

Optically positive

 

Hornblende is of a pale yellowish-brown colour, and is strongly pleochroic in shades of brown. Grains do not exhibit idiomorphism, and they appear to be secondary in origin after the diopside (Hallows, 1923, p.258) and mica. Irregular patches of clear feldspar, mica, and occasional granules of apatite, are often found enclosed in the mineral. Optical properties of the mineral are as follows:

 

Colour – pale yellowish-brown

Pleochroism pronounced with the following colour schemes:

X = pale yellow

Y = yellowish-brown

Z = dark brown

Absorption – X < Y < Z

Extinction – Z Λ C = 35º ± 2.

2 V = 62º

Optically negative

Sphene is honey brown in colour and is strikingly pleochroic. It is anhedral, and usually occurs either in the plates of feldspar or pyroxene, or at their contact. The grains show high relief, and are usually full of cracks which contain opaque inclusions of iron ores. The optical properties are as follows:

 

Colour – honey-brown

Pleochroism:

X = pale yellow to colourless

Y = pale greenish-brown

Z = honey-brown

Biaxial and optically positive

2 V could not be determined due to the small size of the grains.

 

Mica is generally colourless to pale yellow, and is strongly pleochroic. It is mostly seen to occur in close association with diopside and hornblende, and shows corroded and irregular margins, (K19 & K41C) (Plate VII, Fig. 3). The mica appears to be primary in origin, and has the following optical properties:

 

Colour – colourless to pale yellow

Pleochroism:

X = colourless to pale yellow

Y = yellow

Z = yellowish-brown

Extinction – straight

 

Zircon, iron ores, apatite, calcite and a little secondary quartz go to form the assemblage of minor accessories. Grains of zircon, with their corners more or less rounded, are seen in some of the sections (K41A & K41B). Iron ores include ilmenite having the characteristic pitted appearance, visible under reflected light. Platy inclusions of colourless apatite are found in the feldspar and also in the ferromagnesian minerals. Calcite and blebs of quartz are found along cracks in the feldspars.

 

• Diopside Syenite

 

1. Megascopic Characters

 

The diopside syenite is a medium to coarse-grained, greasy-gray, massive rock consisting essentially of feldspar and a mafic mineral (diopside), the former being predominant. The mafic mineral occurs either as bands of variable width having a rough parallelism with the general regional strike (N.W.–S.E.), or as clusters and segregations showing all sorts of odd shapes. The feldspar, where it is in contact with the mafic mineral, is sometimes stained brown, apparently caused by the alteration of the latter. The mafic mineral is greenish-black in colour, and individual anhedral grains vary in size from pin-points to those three to four centimeters in diameter. A few grains of pyrrhotite are also occasionally present. The average specific gravity of the rock is 2.8.

 

1. Microscopic Characters (Typical Slide Nos. K8A, K8B, K8C, K8D, K32A, K32B, K32C, K32D, & K32E)

 

Thin sections of the rock exhibit a coarse granular texture (Hypautomorphic-haphazard-granular, Johannsen, 1949, Vol. III, p.54), the individual grains of the minerals being mostly anhedral. Feldspar constitutes about 50 to 60 percent of the bulk of the rock, the rest being composed mainly of diospide. The minor accessories are mica, pyrrhotite, apatite, calcite, and secondary quartz. Original quartz is absent.

 

As in the case of the diopside granulite, the feldspar forms the most important constituent, and is largely represented by microperthite. The perthitic intergrowth is so fine, that it can be distinguished only under moderate magnification and optimum conditions of illumination. A feldspar which is non-perthitic is also present, and the values of its optical constants indicate it to be a plagioclase of the oligoclase variety. The absence of twinning is an interesting feature of the plagioclase. There are strings of minute globular inclusions noticeable in the body of the feldspar grains. The non-perthitic feldspar is in part found to be orthoclase. The optical characters of the feldspars are as follows:

 

Microperthite

Extinction relative to the basal cleavage is 10º to 13º.

2 V = 78º ± 2.

Optically negative.

 

Oligoclase

2 V = 86º

Optically negative.

 

Orthoclase

2 V = 70º ± 2.

Optically negative.

 

Diopside is pale green in colour, non-pleochroic, and does not show crystal outlines. Diallagic cleavages are seen in some of the grains. The body of individual grains of the mineral in many instances contains numerous cracks. Inside the cracks, a somewhat brownish, fibrous, pleochroic mineral is seen. The optical characters of the mineral could not be discerned. It has a high birefringence, and may probably be a fibrous variety of hornblende. Frequently, grains of diopside are found to enclose irregular plates of feldspar, and this feature suggests a secondary origin for the diopside after feldspar (K8A, K8C, & K8D). These feldspar remnants show wavy extinction effects which recall to mind a crude resemblance to the pattern of spherulitic structure. Secondary calcite is often found filling the cracks in the diopside. Optical characters of diopside are as follows:

 

Colour – pale green

Pleochroism – nil

Extinction – C Λ Z = 39º

2 V = 60º ± 2.

Optically positive.

 

Minor accessories

 

Mica is of the biotite variety, and appears to be gradually transforming into diopside (K8A, K8C, & K8D), as is inferred from the corroded and embayed nature of the margins of mica. It is pale yellow in colour and has a pronounced pleochroism. Pyrrhotite is another accessory, and is found only in some of the slides. It usually occupies the cracks of other minerals and has the characteristic bronze-yellow colour under reflected light. Apatite is colourless and is found as inclusions in the feldspar and diopside. Calcite (K8B, K8C, & K8D), in most cases is associated with blebs of released quartz.

 

• Diopside Syenite (Zircon-bearing)

 

1. Megascopic Characters

 

The rock has a greenish-gray colour. Most of the specimens examined are distinctly coarse-grained, although a few tend to be medium in texture. The latter specimens (K14 & K21) are strikingly similar to some of the medium-grained charnockites in appearance. Mineralogically, the rock is composed of feldspar, and a greenish-black ferromagnesian mineral (diopside). The feldspar constitutes more than seventy percent of the entire bulk of the rock. Minor accessories that can be distinguished in hand specimens are zircon, mica, sulphide and iron ores, the latter two being magnetic. Large and well-developed crystals of zircon are seen in some of the hand specimens.

 

The feldspar has a greenish-gray colour, which largely contributes to the general colouration of the rock also. Hand specimens of the rock are frequently seen to be composed entirely of feldspar alone, or in association with a subordinate amount of diopside. The diopside is non-uniformly distributed, and when present, is found either as individual grains or as ill-defined aggregates which widely vary from 0.5 to 5.0 centimeters along the longest direction. The sulphide ores have very much altered resulting in the formation of a dirty looking limonite stain on the surface of the rocks. On being treated with diluted hydrochloric acid, some specimens produced effervescence along cracks. This evidently indicates the presence of calcite as has been subsequently confirmed by microscopic examination of thin sections. The average specific gravity of the rock, determined on twelve specimens, is 2.75.

 

1. Microscopic Characters (Typical Slide Nos. K1A to K1N, K2A to K2L, K3A to K3L, K6A to K6E, K7, K14, K20, K22, K29, K30A, K30B, K34 and R.S. 55)

 

On account of the coarse texture of the rock, and the sparse distribution of the minerals present, it was not possible to observe all the constituents together in any single slide under the microscope. Therefore, more than fifty slides of the rock were examined to determine the important properties of the various mineral constituents. As seen from the numerous slides, the appears to have a typical texture of a syenite, referred hypautomorphic-haphazard-granular (Johannsen, 1949, Vol. III, p.54). Microscopic study revealed that the rock is essentially composed of feldspar and a ferromagnesian mineral which is pale green to green in colour.  Other ferromagnesian minerals, found to occur in smaller quantities, are hornblende, sphene and mica. The minor accessories include zircon, sulphide ores, iron ores, apatite, calcite and secondary quartz. Original quartz is absent.

 

It may also be mentioned, that the rock appears to have undergone metamorphism subsequent to its emplacement, as may be inferred from the occurrence of secondary minerals, and other features to be referred to in the following pages.

 

Feldspar is the most important constituent of the rock, making up nearly seventy five to eighty percent of the total bulk. The feldspar includes microperthite, plagioclase and a little orthoclase. The plagioclase seems to belong to more than one generation, the chemical composition correspondingly ranging from albite to andesine. However, oligoclase is the most common.

 

Microperthite is the most conspicuous mineral present in every one of the slides examined. The fine, sharp and highly regular microperthitic structure is visible under a moderate magnification of about seventy diameters. The perthitic structure is best seen on (010) cleavage flakes. Certain optimum conditions of illumination are necessary for the structure to be visible in ordinary light.

 

A fairly bright source of light is essential, and this should be ‘stopped down’ with the substage diaphragm to enable maximum visibility of the fine intergrowth. Even the relatively coarse microperthite become invisible or indistinct, if the illumination is not suitably adjusted. Further, the perthitic intergrowth appears more distinct under crossed nicols, with the mineral section near its position of extinction. Plate VII, Fig. 1 shows the mineral as it appears under ordinary light. The faint line which runs obliquely across the figure is the trace of the basal cleavage. It is observed that the fine spindles of the intergrowth make an angle of 73º with the trace of the basal cleavage. The individual spindles are spear-shaped, and have comparatively higher index of refraction. Consequently, they stained out in relief, giving the whole patch a furrowed appearance.

 

Some of the feldspar individuals appear to have the microperthitic structure developed only in patches (K6C & K6D), leaving relicts of the clear host feldspar all around. The patchy nature of the microperthite may probably be due to differential or selective abstraction of the calcium necessary for the formation of other calcium-bearing minerals. It might as well have taken place during a period when the rock got the present metamorphic impress. Further, there seems to be some sort of a gradation in the development of the microperthitic structure. In some of the slides, it is visible only very faintly, even under the optimum conditions of illumination. Such intergrowths may probably approximate to the submicroscopic variety of perthite in the feldspars of the Norwegian Syenites (Brogger, 1890, pp.524-551).

 

Most of the microperthite show undulatory extinction, suggesting possibly the effect of stress. The mean extinction angle with reference to the trace of the basal cleavage ranges from 10º to 12º. The value obviously represents the average extinction of the two sets of spindles (soda & potash) that have intergrown to form the microperthite. The two sets of spindles appear to extinguish almost simultaneously. The relatively higher index of refraction of one set of lamellae, suggests that it may be a member of the plagioclase series which has a higher index of refraction.

 

The optic axial angle of the microperthite was determined on the Leitz Universal Stage. The value obtained, ranges from 75º to 83º. The sign is negative.

 

Following the work of Bogglid (cited by Spencer, 1930, p.305), who endeavoured to deduce a progressive relationship between the extinction angle and chemical composition of schillerised potash-soda feldspars collected from different localities, Spencer (1030, p.305) has observed and described a similar relationship in perthitic feldspars. Figure 1 (reproduction of Fig. No. 4, Spencer, 1930, p.342) refers to this relationship. According to this graph (Fig. 1) for an angle of extinction of 10º to 12º as obtained for the microperthite from the rocks under study, the percentage of soda appears to range from 38 to 51 percent.

 

Spencer (1030, p.344) have given another graph regarding the relationship existing between optic axial angle and composition. The optic axial angle determined for the microperthite under study ranges from 75º to 83º and according to the above graph (Fig. 2) (reproduction of Fig. No. 6, Spencer, 1930, p.343), the percentage of soda is found to vary from 40 to 50 percent.

 

The plagioclases are very rarely twinned. Twins, wherever developed, are so fine as to escape notice. In other cases, both albite and pericline twins are present. Bent twin lamellae are also seen, suggesting the effect of stress (Plate VIII, Fig. 1 & 2). The effect of stress has also caused the bending of the cleavage traces, which should normally b parallel and straight (Plate VIII, Fig. 3 & 4).

 

The values of the optical constants, suggest that the various plagioclase feldspars belong to different generations, and vary in composition from albite to andesine, oligoclase being the most common variety. Zoning of a faint nature was noticed in two of the thin sections.

2 V = 86º ± 2.

Optically negative.

 

Orthoclase is the common potash feldspar, and is found in close association with the microperthite and plagioclase. The optical constants are as follows:

 

2 V = 68º

Optically negative.

 

The feldspars usually have numerous opaque acicular inclusions. They are mostly without any definite orientation. But in some of the sections, they are arranged remarkably parallel to one another, and may be easily mistaken for cleavage traces. Some of these inclusions have a rhombic outline (K1B), and a brown colour, and may be haematite (Rosenbusch, 1903, p.165). Needles of another mineral, presumably rutile, also occur as inclusions.

 

The feldspars are altered to varying degrees, rendering their thin sections turbid when viewed under ordinary light. Alteration seems to have started along cracks and cleavages. Minute scales of sericite (?) (Krishnan, 1926, p.395) and white mica (?) are also noticed on the altered plagioclases in a few instances (K1B, K1C, K1D & K20). They exhibit a “spangled effect” (King, 1947, p.47) on rotation of the microscope stage under crossed nicols. Sometimes, small patches of plagioclase, which are more acid than the usual plagioclase of the rock, are observed. These are almost always fresh, and found associated with calcite and secondary quartz. The above mentioned patches of plagioclase evidently seem to be secondary in origin.

 

The specific gravity of the feldspar, as determined on a sample that was crushed and carefully handpicked under a binocular microscope, was found to be 2.61. The same sample was subjected chemical analysis, the result of which is given in Table VI. From the chemical analysis, it is obvious that the amounts of soda and potash are practically the same in the feldspar. The source of the phosphate in the analysis is evidently the apatite found in the feldspar.

 

Diopside is the most important ferromagnesian mineral in the rock. In thin sections, it is anhedral, pale green to green in colour and non-pleochroic. Some of the grains show diallagic cleavages; and others have numerous cracks. Frequently, the diopside is seen to enclose feldspar with irregular margins, and this relation tends to suggest a secondary origin for the diopside after the feldspar. The microperthite and plagioclase could possibly have contributed to the formation of the diopside. The disposition of some of the diopside grains, with ill-defined and irregular margins, may indicate that the adjacent feldspar has contributed some material such as calcium, necessary for their formation. The calcium having been thus abstracted, the plagioclase seems to have become more acid. This has been supported by the values of the optic axial angle determined on such patches.  Blebs and patches of plagioclase thus formed can be seen associated with released quartz near the frayed margins of the diopside (K1F). Another feature noted is that patches of microperthite adjoining diopside grains are devoid of perthitic intergrowth towards their contact. This may possibly be explained as being due to the depletion of calcium from the microperthite.

 

Alteration of the diopside has resulted in the formation of hornblende (K1D, K1L, K1M, K2B, K2C, K2E, K2L, K3B & K3E). The change appears to have originated along the cracks and cleavage traces of the diopside. The green colour of the pyroxene changes to greenish brown. The hornblende thus formed is pleochroic. These transformations in the diopside are often accompanied by the release of quartz and calcite. Sphene is also frequently seen in close association with the diopside (K1F & K3E). The diposide has the following optical constants:

 

Colour – pale green to green.

Pleochroism – nil

Extinction – C Λ Z = 38º

2 V = 60º ± 2.

Optically negative.

 

A piece of the fresh rock was crushed and about 5 grammes of diopside, almost free from inclusions, were carefully hand-picked under the binocular microscope. The specific gravity of the material was determined to be 3.48. The same material was chemically analysed. The result of the analysis is given in Table VI.

 

Hornblende is invariably of secondary origin (K1D, K1L, K1M, K2E, K2L, K3H & K3I), after diopside, mica and feldspar (Ghosh, 1941, p.47). It is dirty pale green in colour and is pleochroic. The following are the optical properties:

 

Colour – pale green.

Pleochroism:

X = yellowish brown

Y = pale greenish brown

Z = dirty greenish brown

Extinction – C Λ Z = 18º to 27º

Birefringence = 0.022 (Berek Compensator)

2 V could not be determined due to the high absorption.

Absorption – X < Y < Z.

 

Sphene is irregular in distribution and is honey-brown in colour. This mineral is often found associated with the ferromagnesian constituents, and sometimes is enclosed by them (K1F, K3H, K6D & K6F). Slides examined under the microscope do not reveal any indication as to the exact nature of its origin. However, grains of the mineral do not show crystal outlines, and are frequently seen along with calcite and diopside. Some of the grains are full of cracks, containing dark opaque particles of iron ores. Under the microscope, the mineral is recognized by its high relief, strong birefringence and marked pleochroism. In some of the sections, it is also found altering to leucoxene (K1F & K6D). further, diopside is seen to be developed at the expense of sphene, which may probably be the source of the calcium necessary for the formation of diopside. This is often accompanied by the release of iron ores (K1F, K6D & K6E).

 

Optical properties:

 

Colour – honey-brown.

Pleochroism marked with the following colour scheme:

X = pale yellow to colourless

Y = pale greenish-brown

Z = honey-brown

Biaxial and optically positive. The optic axial angle could not be measure due to the small size of the grains.

 

Mica is represented by biotite, and appears to be original. In some of the sections, the mineral is absent. It is usually pale greenish-brown in colour, with pronounced pleochroism. The occurrence of flakes of the mineral as cores and fringes of other minerals, particularly the ferromagnesian minerals, suggest that it is an original constituent.

 

Zircon is seen a well-developed crystals (Plate X, Fig. 1), of a brown colour. Under the microscope, it has a pale brown colour, and is easily distinguished by its high relief and strong birefringence. Some of the sections give good interference figures, which are uniaxial positive. A detailed account of zircon is given in Appendix I.

 

Sulphide and iron ores include pyrrhotite, ilm,enite and magnetite. Pyrrhotite shows the characteristic bronze-yellow colour, in thin sections under reflected light.in some of the sections studied, the mineral is found filling the cracks in the feldspars and diopside (K3B, K3D, K3F, K3J & K6E) (Plate VII, Fig. 2).

 

Ilmenite, in thin sections, shows the typical silvery-gray colour and pitted appearance under reflected light. In specimen K14, the mineral shows an abundant and almost even distributed throughout. Some of the grains show idiomorphic outlines. The usual alteration product is leucoxene.

 

Apatite is a minor accessory, and is found as platy inclusions in the feldspars, and also in the ferromagnesian minerals. The grains are usually anhedral, and are colourless. Distinguished in thin sections by the characteristic pitted appearance (K1B, K1D, K2C, K6A, K6B, K6C, K6D & K6E), high relief and low polarization colours (Plate X, Fig. 3). Basal sections are isotropic between crossed nicols, and some of them give a negative uniaxial figure. Grains of the mineral picked out from the crushed specimens, were treated with hydrochloric acid (Dana, 1949, p.704). On examination with a hand lens, oily bubbles of hydrofluoric acid were found floating, and the grains completely dissolved in the hydrochloric acid. This indicates that the mineral is the common flourapatite.

 

Calcite and quartz, both secondary, are often found together. Calcite has resulted from the plagioclase and also possibly from sphene (?). It is found as minute grains, scattered all over the body of the altered plagioclases, and also seen as irregular patches filling the cracks of the same. Formation of the diopside at the expense of the feldspars, seems to have resulted in the production of calcite, which is often associated with blebs of secondary quartz.

 

• Charnockite Suite of Rocks

 

• Intermediate Charnockite

 

1. Megascopic Characters

 

The rock is greenish to bluish-gray in colour and has a greasy lusture, and consists of feldspar, quartz, hypersthene, garnet and mica. It is a compact, tough, and medium to coarse-grained rock. Both foliated as well as faintly foliated types are commonly met with. Whenever foliation is present, it is usually characterized by a somewhat parallel arrangement of the garnet and mica.

 

The feldspar dominates over the rest of the minerals, and has a bluish-gray colour. Quartz occurs in subordinate amounts, and is coloured almost like the feldspars. Hypersthene is represented by a few grains, the typical bronze-brown colour of which enables easy identification in a hand specimen. The garnet is pink coloured, and is almost found only in the foliated variety of the rock. Mica has a brownish colour and is identified as biotite. The average specific gravity of the rock is 2.74.

 

 

1. Microscopic Characters (Typical Slide Nos. K45A, K45B, K45C & K49A)

 

Under the microscope, the thin sections of the rock generally exhibit a xenomorphic granular texture (Plate IX, Fig. 1). The constituent minerals are equigranular. Besides feldspars, quartz, hypersthene, mica and garnet, it is also possible to see iron ores, apatite and zircon in the slides. None of the above minerals, except the minor accessory zircon, is idiomorphic.

 

The quartz is seen to occur either as interstitial grains, or as blebs enclosed in the feldspars and the ferromagnesian minerals. Some of the quartz grains display undulose extinction suggesting strain effects.

 

Feldspars constitute more than fifty percent of the bulk of the rock. Plagioclase is the predominant type and is in part twinned according to the albite and pericline laws. The majority of the plagioclase grains are untwined. A characteristic feature in some of the twinned plagioclase is the tendency of individual lamellae to tapper towards one end. Microperthite, and a little orthoclase are also represented in the feldspar assemblage. Rarely antiperthite is also found. Myrmekite is seen occasionally developed (K49A). The plagioclase ranges in composition from albite to labrodorite. The optical characters of the feldspars are mentioned below:

 

Albite:

 

2 V = 70º ± 2.

Optically positive.

Oligoclase:

 

2 V = 86º

Optically negative.

 

Andesine:

 

2 V = 86º ± 2.

Optically positive.

 

Labrodorite:

 

2 V = 77º ± 2.

Optically negative.

 

Orthoclase:

 

2 V = 68º ± 2.

Optically negative.

 

While hypersthene is seen to be the important ferromagnesian mineral in charnockite of the non-garnetiferous variety (K45), it is only sparingly present in the garnetiferous variety. It is pale yellowish-green in colour, and is pleochroic. In some of the microsections, particularly that of the garnetiferous variety, a fibrous, brownish mineral can be seen occurring along cracks. It appears to be secondary hornblende. Optical properties of the mineral could not be discerned on account of the fibrous habit of the mineral. The optical properties for the hypersthene are as follows:

 

Colour – yellowish-green.

Pleochroism:

X = pink

Y = pale yellowish-green

Z = greenish-blue

Extinction – straight

Birefringence – 0.014 (Berek Compensator)

2 V = 64º

Optically negative.

 

Hypersthene is found altering to chlorirte (K45B), having a dirty green colour. A few grains of hypersthene are found to enclose patches of mica and feldspar, which feature may probably imply a secondary origin for hypersthene. However, conclusive proof as to the exact origin is wanting, in the slides examined.

 

Garnet is a conspicuous constituent in the garnetiferous variety (K49A) of the charnockite. In this rock, it would seem that the rarity of hypersthene is made good by the relative abundance of garnet. The garnet is pale pink in colour, anhedral and isotropic. It frequently encloses quartz, feldspar, apatite and zircon. The presence of pools of quartz and feldspar inside the garnet seems to indicate a secondary origin for the garnet.

 

Mica is present in both garnetiferous and non-garnetiferous varieties of the intermediate charnockite and has a brownish-black colour. Some of the flakes contain patches of feldspar with corroded borders, thereby showing that the mica is secondary in origin after the feldspar, the necessary iron being probably derived from the adjacent grains of iron ores. A single perfect cleavage parallel to the base (001) is clearly visible. The grains are highly pleochroic in shades of brown. The optical properties are given below:

 

Colour – Brownish-black

Pleochroism – pronounced with the under-mentioned colour scheme:

X = yellow

Y = reddish-brown

Z = dark opaque brown

Absorption – Z > Y > X

Extinction – straight

Optically negative.

 

The minor accessories are represented by iron ores, apatite and zircon. Iron ores are found as irregular grains. Apatite occurs as large, irregular plates abutting against the hypersthene, and is sometimes enclosed in the plates of feldspar. It clearly shows the characteristic pitted appearance, and has a high relief. Zircon occurs mostly as sub-rounded crystals, and is highly birefringent. It is very common in most of the slides examined.

 

• Acid Charnockite

 

1. Megascopic Characters

 

Similar to the intermediate charnockite, the colour of this rock is greenish to bluish-gray, but of a lighter shade. It is very hard, tough, and has a waxy lusture. When hammered, it makes a clinking sound. Fractured surfaces are subconchoidal. Mineralogically, it consists of almost equal parts of feldspar and quartz, with subordinate amounts of ferromagnesian minerals such as hypersthene and mica. The ferromagnesium minerals are scattered wide apart, and do not assume as much prominence as they do in the intermediate charnockites. The average specific gravity of the rock is 2.64.

 

1. Microscopic Characters (Typical Slide Nos. K9, K10A, K10B, K18A, K18B & K18C)

 

In addition to the constituent discernible megascopically, the rock, in thin sections, is seen to consist of accessory minerals such as iron ores and zircon. The texture is xenomorphic-granular (Johannsen, 1949, Vol. III, p.54). The feldspars formed about half the bulk. An equal amount of quartz is also present. None of the minerals possesses crystal outlines. As seen under microscope, the rock appears to be a typical granulitic type (Plate IX, Fig. 2) of acid charnockite.

 

The feldspars consist of microperthite, and a lesser amount of orthoclase. The miccroperthite appears remarkably similar to that observed in the case of the syenite described previously. Optical determinations tend to show, that most of the non-perthite feldspar in the slides are orthoclase. An effect of strain, to which the rock has been subjected, is evidenced by the undulatory extinction shown by the feldspars in general. The optical properties are as follows:

 

Microperthite:

 

2 V = 75º to 82º.

Optically negative.

 

Orthoclase:

2 V = 70º ± 2.

Optically negative.

 

The quartz grains occur as large plates of fairly uniform size. It is a primary constituent, and occupies the space left over by the feldspar and ferromagnesian minerals. Frequently blebs of quartz are enclosed in the feldspar grains. Most of the quartz grains exhibit strain effects causing undulatory extinction. Minute, opaque, dust-like inclusions are often seen in the quartz plates.

 

Hypersthene is the same plae yellowish-green and pleochroic variety, occurring in the intermediate charnockite. Unlike as in the intermediate charnockite, the number of grains in the acid variety is fewer. Here also, a brownish, fibrous mineral considered to be secondary hornblende is found to occupy cracks. Optical properties of the hypersthene are as follows:

 

Colour – pale yellowish-green.

Pleochroism, marked as in the intermediate charnockite.

X = pale pink

Y = yellowish-green

Z = greenish-blue

Extinction – straight

Birefringence – could not be determined due to the altered nature of the grains.

2 V = 62º ± 2.

Optically negative.

 

Mica is of the usual variety – biotite. A few flakes could be discerned in slides (K9, K10), and it is possible to see the trace of the basal cleavage (001). The mica is pleochroic, and shows straight extinction.

 

There are mainly iron ores and zircon (as minor accessories), the former being more abundant. Some of the grains of the iron ores are frequently seen altered to leucoxene – an opaque, dirty, brownish-white substance. This obviously indicates the presence of ilmenite in the iron ores.

 

Zircon grains are comparatively few, and are sub-rounded. These exhibit high order interference colours, and stand out in high relief in contrast to the enclosing feldspar and quartz grains.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

CHAPTER – V

 

1. Chemical and Modal Analyses and their Interpretation

 

Chemical analyses are deemed to be of immense value in comparing groups of igneous rocks. For this purpose, the usual practice is to compute from the chemical analyses and the Niggli symbols, normative minerals and other relevant values, as required for the problem in hand.

 

The data of the chemical analyses may be directly used to construct variation diagrams. Such a diagram will illustrate graphically many characters of a group of rocks that is investigated. The values for the different oxides are plotted as ordinates against those for silica as abscissae.  In the place of the weight percentages, it is the practice among petrologists of the Niggli school to plot the Niggli symbols (al, fm, c, alk) as ordinates against the Si values.

 

With a view to examine the possible chemical relationship among the five interesting rock types from Puttetti, chemical analyses of fresh samples representing each of these five types, viz: diopside granulite, diopside syenite, diopside syenite (zircon bearing), intermediate charnockite and acid charnockite, were performed. Since there are apparent differences in the structure and texture of the various rock types, care was taken to ensure that the samples selected were sufficiently representative of the rock types concerned.

 

In the case of the coarse-grained rock, and those in which the distribution of the constituents is not uniform, larger bulks of the respective samples were taken, and after crushing and powdering, the quantity was suitably reduced by standard methods (Hillebrand et al., 1953, p. 809), as required for the analyses. The results of the chemical analyses, together with the C.I.P.W norms and Niggli values, are entered in Tables I to V. In the case of the intermediate charnockite, two specimens were analysed (IV. A and IV. B in the Tables). The two analyses show some variation in silica content. On account of the difference in the values of silica content, both the analyses have been considered in the construction of the variation diagrams.

 

For the purpose of estimating the modal composition of the five rock types brought under the syenite and charnockite suites, a selected number of microsections of the rock type concerned were used. While the modal composition of the medium-grained rocks could be dependably estimated from a small number of thin sections, in the case of the coarse-grained rocks it was necessary to examine a large number of thin sections because individual slides hardly represented all the constituents of the particular rock type. Thus the modal composition given in the Tables I to V corresponds to averages of a number of determinations.

 

In addition to the various rock types, some of the important constituent minerals such as feldspar, diopside and zircon were also chemically analysed. In this case, samples obtained by crushing the rock were sieved, and the fraction retained on the hundred mesh sieve was treated in bromoform, and the heavy crop, after being properly washed and dried, was further concentrated in the Franz Isodynamic Separator. The fractions thus obtained were spread under a binocular microscope, and grains of the required mineral were carefully hand-picked, taking particular care to exclude all those that contained inclusions. All the three mineral samples used for the chemical analyses were obtained from the zircon-bearing diopside syenite. The results are entered in Table VI.

 

Examination of the chemical analyses of the five Puttetti rock types, given in Tables I to V, shows that the silica ranges between 56 and 73 percent. In the three rock types comprised in the syenite suite, it may be seen that an insignificant amount of quartz appears in the norm, although not seen in the mode. In the case of the intermediate and acid charnockites, the figures for quartz in the mode as well as in the norm compare satisfactorily.

 

The quantity of alumina present in all the rock types is almost not more than what is necessary to satisfy the combined alkalies and lime. This view is identically the same as that of Washington (1916, p.334). However, the normative corundum in the two charnockites may be explained by the presence of mica in the rocks (Groves, 1935, p.174).

 

For purposes of the present investigation, variation diagrams (Fig. 3 & Fig. 4) have been constructed. In Fig. 3, the values for the oxides are plotted as ordinates against the silica value as abscissae. Fig. 4 is constructed by plotting the Niggli values al, fm, c and alk, against Si.

From Fig. 3, the following observations may be made.

 

1. There is a steady and progressive increase in the silica content of the five rock types in the order diopside granulite, diopside syenite, diopside syenite (zircon-bearing), intermediate charnockite, and acid charnockite. The silica content is the minimum for the diopside granulite and the maximum in the acid charnockite.
2. The curves representing the oxides of iron show a more or less steady decline towards the acid charnockite.
3. The curves for caO and MgO start high at the diopside granulite end, and after showing a somewhat steep fall towards the diopside syenite (zircon-bearing), continues as a gentle slope with increase of silica. However, the curves are typically concave upward.
4. The soda curve is convex upward, the highest position being attained near about diopside syenite. The potash curve starts low at the diopside granulite end, and shows a gradual rise with silica increase.

 

It may be seen from the plottings of the Niggli symbols (Fig. 4) that the alkali and alumina curves show a steady increase, while those of lime, and total iron and magnesium, show a steady decrease with increasing silica.

 

In Fig. 5 are given two variation diagrams (Rajagopalan, 1947, p.244); one ‘A’ is the differentiation diagram of calc-alkali series after Niggli, and the other ‘B’ being the differentiation diagram of the charnockite series. These are reproduced for the purpose of comparison with the Niggli variation diagram obtained for the Puttetti rocks (fig. 4). It may be seen there from that in all three, there is a general similarity in the tendencies of the curves.

 

Commenting on the two diagrams ‘A’ and ‘B’ (Fig. 5), Rajagopalan (1947, p.245) has observed thus: “On a closer study of the two diagrams it will be seen that there is not nearly a general similarity in the tendencies of the curve, but that the intersection point of al and fm curves is very nearly at the same Si value. This point, known as the Isofalic point, gives an Si value which is characteristic for each petrographic province. Thus, the isofalic point for Circum Pacific or calc-alakaline rocks falls at Si = 180 or more, while the same in the alkaline suites – both Atlantic and Mediterranean – rarely exceeds Si = 160.”

 

Based on the above, Rajagopalan has observed that a strong affinity to the calc-alkaline suite is indicated by the charnockite series, which has an Si value of 216 for its isofalic point (Fig. 5, ‘B’).

 

When considered on similar lines, the Niggli variation diagram of the Puttetti rocks (Fig. 4) shows that the intersection point corresponds to an Si value of about 180, which evidently refers the Puttetti suite of rocks to the calc-alkaline series.

 

Usually variation diagrams, in whatever ways they are constructed, indicate the regular changes from one rock to another in their order of age, at the same time reflecting the changes overtaking the parental magma that undergoes differentiation.

 

With a view to examine whether the crystalline rocks of Puttetti are igneous or not, the Al, c, Alk and Al, S, F, triangular diagrams of Osann’s system (Johannsen, 1949, Vol. I, p.79) have been constructed (Fig. 6 and Fig. 7). It was found that all the above rocks clearly fall in the igneous field.

 

The values S, Al, F and Al, C, Alk for the rocks of Puttetti together with the nearest classes into which the Puttetti rock types fall, are given in Table VII. The nearest rock classes were found out by comparing the values obtained for the Puttetti rocks with the corresponding typical values given by Osann (Johannsen, 1949, Vol. I, Table XVIII, p.81).

 

Table VIII gives a comparative statement of chemical analyses of intermediate charnockites and syenites. Herein, two analyses relate to the intermediate charnockite and diopside syenite (zircon-bearing), from the area under investigation. A perusal of the table will enable to visualize the chemical affinities that exist between the charnockites and syenites.

 

 

 

 

 

 

 

 

 

 

 

Table I

Diopside Granulite

 

Chemical analysis		Modal composition		C.I.P.W. Norm		Niggli values
Constituents	Weight percent
SiO2	56.61	Quartz	—	Q	2.04	si	137.40
TiO2	Trace	Microperthite	32.3	Or	11.67	qz	-2.60
Al2O3	11.42	Non-perthitic feldspar	26.5	Al	24.10	al	16.00
Fe2O3	2.16	Diopside	39.8	An	12.51	fm	43.00
FeO	4.72	Sphene	1.4	C	—	c	31.00
MnO	0.21			Di	37.47	alk	10.00
MgO	7.79			Hy	8.78	k	0.31
CaO	12.10			Il	—	mg	0.61
Na2O	2.87			Mt	3.25
K2O	1.95			Ht	—
P2O5	0.07			Ap	0.34
ZrO2	—			Py	—
SO3	—			Zr	—
H2O	0.45
H2O	0.07
Total	100.43
Sp. Gr.	2.98
Locality: Puttetti, Southern Travancore
Classification: II. 5. 4(5). 4

Analyst: K.V. Krishnan Nair

 

 

Table II

Diopside Syenite

 

Chemical analysis		Modal composition		C.I.P.W. Norm		Niggli values
Constituents	Weight percent
SiO2	59.67	Quartz	—	Q	0.42	si	184.00
TiO2	0.25	Microperthite	29.3	Or	25.02	qz	-4.80
Al2O3	14.98	Non-perthitic feldspar	40.3	Al	39.30	al	27.20
Fe2O3	1.61	Diopside	28.9	An	7.51	fm	26.10
FeO	3.64	Mica	1.1	C	—	c	25.50
MnO	0.06	Apatite	0.4	Di	23.47	alk	22.20
MgO	2.74			Hy	0.79	k	0.37
CaO	7.42			Il	0.46	mg	0.48
Na2O	4.63			Mt	2.18
K2O	4.25			Ht	—
P2O5	0.16			Ap	0.34
ZrO2	—			Py	—
SO3	—			Zr	—
H2O	0.67
H2O	0.08
Total	100.21
Sp. Gr.	2.81
Locality: Puttetti, Southern Travancore
Classification: II. 5. 3. 4

Analyst: C.V. Paulose

 

 

Table III

Diopside Syenite (zircon-bearing)

 

Chemical analysis		Modal composition		C.I.P.W. Norm		Niggli values
Constituents	Weight percent
SiO2	61.34	Quartz	—	Q	2.10	si	218.40
TiO2	0.34	Microperthite	45.64	Or	35.58	qz	6.40
Al2O3	14.87	Non-perthitic feldspar	35.65	Al	35.63	al	31.00
Fe2O3	0.80	Diopside	12.55	An	3.89	fm	22.00
FeO	3.82	Hornblende	3.18	C	—	c	19.00
MnO	0.10	Pyrrhotite	2.08	Di	17.25	alk	28.00
MgO	1.52	Ilmenite	0.03	Hy	1.06	k	0.50
CaO	4.99	Magnetite		Il	0.61	mg	0.40
Na2O	4.21	Zircon	0.27	Ht	—
K2O	5.98	Sphene	0.25	Ap	0.11
P2O5	0.04	Mica	0.35
ZrO2	1.20	Apatite	—
SO3	0.05
H2O	0.74
H2O
Total	100.00
Sp. Gr.	3.11
Locality: Puttetti, Southern Travancore
Classification: II. 5. 2. 3

Analyst: N. Jayaraman

 

Table IV. A.

Intermediate Charnockite

 

Chemical analysis		Modal composition		C.I.P.W. Norm		Niggli values
Constituents	Weight percent
SiO2	65.27	Quartz	—	Q	16.56	si	265.00
TiO2	Trace	Microperthite	14.6	Or	21.13	qz	57.00
Al2O3	16.12	Non-perthitic feldspar	51.9	Ab	37.73	al	38.00
Fe2O3	0.86	Hypersthene	3.4	An	12.51	fm	19.00
FeO	1.94	Ilmenite	1.7	C	0.31	c	16.00
MnO	0.08	Magnetite		Di	—	alk	27.00
MgO	1.57	Zircon	0.1	Il	—	k	0.35
CaO	3.64	Mica	2.3	Mt	1.16	mg	0.51
Na2O	4.49	Garnet	4.9	Ht	—
K2O	3.54	Apatite	2.2	Ap	2.12
P2O5	0.85			Py	—
ZrO2	—			Zr	—
SO3	—
H2O	0.92
H2O	0.35
Total	99.63
Sp. Gr.	2.72
Locality: Puttetti, Southern Travancore
Classification: I. 5. 2. 2

Analyst: K.V. Krishnan Nair

Table IV. B.

Intermediate Charnockite

 

Chemical analysis		Modal composition		C.I.P.W. Norm		Niggli values
Constituents	Weight percent
SiO2	70.28	Not determined		Q	22.08	si	332.00
TiO2	Trace			Or	33.80	qz	100.00
Al2O3	14.40			Ab	29.34	al	40.00
Fe2O3	0.73			An	7.23	fm	17.00
FeO	1.14			C	—	c	10.00
MnO	Trace			Di	2.04	alk	33.00
MgO	1.45			Hy	4.72	k	0.51
CaO	1.96			Il	—	mg	0.56
Na2O	3.48			Mt	—
K2O	5.51
P2O5	0.01
ZrO2	—
SO3	—
H2O	0.84
H2O	0.24
Total	100.04
Sp. Gr.	2.68
Locality: Puttetti, Southern Travancore
Classification: I. 4. 1. 3

Analyst: K.V. Krishnan Nair

 

 

 

 

 

Table V

Acid Charnockite

 

Chemical analysis		Modal composition		C.I.P.W. Norm		Niggli values
Constituents	Weight percent
SiO2	73.24	Quartz	26.5	Q	31.26	si	405.60
TiO2	Trace	Microperthite	35.1	Or	35.58	qz	167.60
Al2O3	13.83	Non-perthitic feldspar	29.6	Ab	20.96	al	45.20
Fe2O3	0.51	Hypersthene	6.4	An	6.67	fm	11.30
FeO	0.83	Ilmenite	1.5	C	0.81	c	9.00
MnO	Trace	Magnetite		Di	—	alk	34.50
MgO	0.58	Mica	0.9	Hy	3.22	k	0.61
CaO	1.51			Il	—	mg	0.44
Na2O	2.49			Mt	—
K2O	5.97			Ht	—
P2O5	0.16			Ap	0.34
ZrO2	—
SO3	—
H2O	0.76
H2O	0.18
Total	100.06
Sp. Gr.	2.61
Locality: Puttetti, Southern Travancore
Classification: I. 4. 2. 2

Analyst: K.V. Krishnan Nair

 

Table VI

Chemical Analyses of the Three Principal Minerals of Diopside Syenite (zircon-bearing)

 

Constituents	Feldspar	Diopside	Zircon
SiO2	64.39	46.99	30.51
TiO2	—	Trace	0.01
Al2O3	18.42	9.10	3.13
Fe2O3	0.09	18.24	0.23
FeO	0.05	4.00	0.15
MnO	—	0.43	Trace
MgO	0.65	2.45	0.03
CaO	1.93	16.03	0.15
Na2O	6.12	1.11	0.13
K2O	5.74	1.08	0.10
P2O5	1.44	0.97	0.03
ZrO2	—	—	65.27
SO3	—-	—	0.02
H2O	0.98	0.24	0.23
H2O	0.50	0.03	0.04
Total	100.39	100.63	100.03
Analyst:	K.V. Krishnan Nair	K.V. Krishnan Nair	N. Jayaraman

 

 

 

 

 

 

 

 

Table VII

Osann’s System

 

Name & Hand Specimen No.	S	Al	F	Al	C	Alk	Rock type of Osann
Acid charnockite (K18)	17.3	1.9	0.8	10.2	2.0	7.8	Siliceous alkali-lime granite
Intermediate charnockite (K28)	16.6	2.0	1.4	9.7	2.4	7.9	Granite
Intermediate charnockite (K49)	15.7	2.3	2.0	9.5	3.9	6.6	Granite
Diopside syenite (zircon-bearing) (K1)	15.1	2.1	2.8	8.0	4.7	7.3	Alkali-lime-rich syenite
Diopside syenite (K8)	14.1	2.1	3.8	7.4	6.7	5.9	Alkali-lime-rich syenite
Pyroxene granulite (K24)	12.5	1.5	6.0	5.7	10.9	3.4	Essexite shonkinite

 

 

 

 

 

 

 

 

Table VIII

Comparative Statement of Chemical Analyses of Intermediate Charnockites and Syenites

 

Constituents	1	2	3	4	5	6	7
SiO2	65.7	60.52	63.85	61.34	62.50	61.60	60.51
TiO2	Trace	0.62	0.83	0.34	—	—	1.52
Al2O3	16.12	15.92	14.87	14.87	16.50	15.10	14.16
Fe2O3	0.86	3.00	2.32	0.80	2.40	2.00	1.60
FeO	1.94	5.66	5.07	3.82	2.00	2.20	8.02
MnO	0.08	0.17	0.07	0.10	—	—	0.17
MgO	1.57	1.19	3.29	1.52	1.90	3.70	0.17
CaO	3.64	5.47	4.48	4.99	4.20	4.60	4.14
Na2O	4.49	4.38	3.72	4.21	4.40	4.30	3.47
K2O	3.54	2.36	1.09	5.98	4.60	4.50	4.38
P2O5	0.85	0.34	0.08	0.04	—	—	0.58
ZrO2	—	—	—	1.20	—	—	—
SO3	—	—	—	0.05	—	—	—
S	—	—	0.15	—	—	—	—
H2O	0.92	0.48	0.11	0.74	0.60	0.70	0.10
H2O	0.35						0.90
Other oxides	—	—	—	—	1.30	1.00	—
Total	99.63	100.11	99.89	100.00	100.40	99.70	99.91
Sp. Gr.	2.72	2.79	2.70	3.11	2.73	—	—

Continued………

 

 

 

 

 

Table VIII (contd.)

 

C.I.P.W. Norm	1	2	3	4	5	6	7
Q	16.56	12.06	20.95	2.10	10.26	7.92	11.07
Or	21.13	13.90	6.67	35.58	27.24	26.69	26.13
Ab	37.73	37.20	31.44	35.63	37.20	36.16	28.82
An	12.51	16.68	20.57	3.89	11.68	6.39	10.15
C	0.31	—	—	—	—	—	—
Di	—	6.63	1.36	17.26	1.35	7.97	6.12
Hy	6.94	6.75	13.14	1.06	5.88	7.83	10.96
Il	—	1.22	1.52	0.61	—	—	2.89
Mt	1.16	4.31	3.25	1.16	3.48	3.02	2.32
Ht	—	—	—	—	—	—	—
Ap	2.12	1.01	—	0.11	3.02	2.35	1.34
Py	—	—	—	0.12	—	—	—
Zr	—	—	—	1.83	—	—	—
Niggli values
si	265.00	202.70	224.00	218.50	225.50	210.45	Not given
qz	57.00	25.90	20.00	6.40	22.16	14.57
al	38.00	31.40	31.00	31.00	35.00	26.68
fm	19.00	29.80	38.00	22.00	23.00	30.53
c	16.00	19.60	16.00	19.00	16.00	16.80
alk	27.00	19.20	15.00	28.00	26.00	23.97
k	0.35	0.26	0.16	0.50	0.41	0.51
mg	0.51	0.20	0.42	0.40	0.45	0.62

Continued………

 

 

 

 

 

Index to Table VIII

 

1. Intermediate Charnockite, Puttetti, Southern Travancore.

Analyst: K.V. Krishnan Nair, 1954.

 

2. Intermediate Charnockite, Myladi, Southern Travancore.

Analyst: C.V. Paulose, M.Sc. Thesis, 1953.

 

3. Intermediate Charnockite, Yercaud, Salem District, Madras.

Analyst: H.S. Washington (1916, p.328).

 

4. Diopside Syenite (zircon-bearing), Puttetti, Southern Travancore.

Analyst: N. Jayaraman, 1953.

 

5. Syenite, Plauen, near Dresden, Germany (cited in Table 2, p.181, by Pirsson and Knopf).

 

6. Syenite, little Belt Mountains, Montana (cited in Table 2, p.181, by Pirsson and Knopf).

 

7. Hornblende-pyroxene syenite gneiss, Vermontville, Saranac.

Analysts: G. Kahan and R.B. Ellestad (Buddington, 1952, p.66).

 

 

 

 

 

 

CHAPTER – VI

 

1. Petrogenetic Considerations

 

• General remarks on the rocks of the syenite family

 

It was Rosenbusch who first clearly defined “Syenite” as quartz-free orthoclase rocks regardless of the presence or absence of hornblende. In 1823, before the discovery of plagioclase as a separate variety of feldspar, von Leonhard noticed that two kinds of feldspars, such as ‘feldspath’ (orthoclase) and more rarely ‘feldstein’ (plagioclase) were essential in a syenite, and this usage has been followed by later workers.

 

Johannsen (1949, Vol. III, p.52) has described “syenite” as a “plutonic rock of hypautomorphic-granular texture and consisting of the mineral combination orthoclase, less acid plagioclase and usually some dark mineral. Quartz is absent or is present only as an accessory”. According to him, “the quartz must form less than 5 percent of the total leucocrates”.

 

The syenites are plutonic rocks of intermediate composition. The essential constituents are alkali feldspars and a mafic mineral, the former being dominant. Sometimes, plagioclase within the range oligoclase-andesine is present in subordinate amounts. The mafic minerals are commonly represented by the pyroxenes, amphiboles or micas. Very often unsaturated minerals (Shand, 1947, p.118) such as the feldspathoids, may substitute the feldspars in varying degree to form feldspathoidal syenites. However, the two groups of syenites are syngenetic in occurrence. Syenites are closely allied to granites on one side and nepheline-syenites on the other. They also grade through monzonite to diorite, and the augite syenites may grade to gabbro.

 

The syenites embrace a group of rocks of considerable diversity. In regard to their relative abundance, Shand (1947, p.272) has observed: “the whole amount of syenite in the lithosphere is insignificant in comparison with granite or gabbro”. Daly’s (1910, p.90) profound study on the alkaline rocks of the world which also includes syenites and monzonites, led him to infer, that “the visible alkaline rocks of the world probably constitute less than one percent of the total visible igneous rocks”. In the same paper, he listed in geographical order the more important of the recorded occurrences of the alkaline rocks in the world, numbering 143 in all. The list shows the wide distribution of the alkaline rocks in all the continents, and probably in every latitude from Greenland to the Antarctic. The majority of the syenites occurs as marginal modifications of granites, and as satellitic stocks related to larger granite batholiths. Some however form independent dykes, laccoliths and masses of irregular shape.

 

Reference to geological literature pertaining to syenites in India shows that the non-feldspathoidal syenites are not usually found to occur as extensive bodies at all, but are practically seen to occur as transitionary stages of the feldspathoidal as well as non-feldspathoidal rocks. In the feldspathoidal syenites, augite and hornblende syenites commonly mark transitionary stages.

 

The well-known occurrences of syenites in India, which have been studied in some detail, are essentially feldspathoid-bearing, though locally these show transition to non-feldspathoid syenites. These include the syenites at Sivamalai, Coimbatore District (Holland, 1901); Mount Girnar, Kathiawar (Evans, 1901; Krishnan, 1926); Sarnu, Jodhpur (Holland, 1902); Vizagpatam (Walker, 1907); and Kishengarh, Rajputana (Heron, 1924).

 

In general, syenites associated with non-feldspathoidal rocks are very few in number. Nevertheless, some instances, where syenites are found to occur in close association with charnockites have been noted in India, as well as other parts of the world. In almost every such instance, it has been possible to postulate a genetic relationship between the two kinds of rocks.

 

• Charnockite-Syenite relationship at Puttetti

 

In the area surrounding Puttetti, the dominant rock is charnockite, particularly of the intermediate type. The syenite extends over a smaller area and occurs in close association with the charnockite. Frequently, outcrops of intermediate charnockite resemble those of syenite so strikingly, that often distinction between the two is possible only on careful examination of the mineralogy. While it was not possible to trace any contact between the two rocks anywhere in the area, field indications are such as to suggest a gradual passage of one into the other. Further, microscopical and chemical data strongly suggest a genetic relationship between the charnockite and the syenite. In other words, in the matter of origin, there seems to be much in common between the two kinds of rocks. Therefore, it is only proper to review briefly the salient features and genetic implications of the charnockite series of rocks before passing on to the petrogenetic considerations of the syenite.

 

• Charnockite series of rocks

 

As originally proposed by Holland (1900), the charnockite series of rocks grouped together in one petrographical province, but varying in composition from acid charnockite through intermediate and basic varieties to ultrabasic pyroxenites. He regarded the several types of the series as differentiated phases of crystallization of a normal plutonic magma, intrusive into the associated granitoid gneiss. Mineralogically, the rocks are distinctive in almost constant presence of the highly pleochroic rhombic pyroxene. Further, the rocks of this group have a widespread granulitic texture. Each of the above-mentioned four divisions has characteristic mineralogical, physical and chemical features which aid in their proper identification.

 

The charnockites are widely distributed in Peninsular India, where they largely constitute some of the mountains such as Nilgiris, Palnis, Shevroys and Annamalais. The rocks are well represented in the Ghats section in Travancore and can be traced to extend up to Cape Comorin. As a rule, these charnockite masses are irregular in shape, and sometime show roughly lenticular form. It is not uncommon to see small hills constituted by the basic varieties. While the masses of the rocks have uniform general characters over large areas, there are structural and compositional variations suggesting local differentiation. The rocks are usually banded, but very often bands could not be traced for any considerable distance. This apparent banding or foliation is explained by Holland (1900, p.176) as “probably induced during the process of consolidation, although of course it may have been accentuated in many perhaps, in most, exposures by continued exertion of the forces which determined the main physical conformation of S. India in very early geological times”.

 

• Views on the origin of the charnockite series of rocks

 

In regard to the mode of origin of the charnockites, it may be seen from the preceding section that Holland considers an igneous origin. In support of this view, Holland (1900, p.243) has mentioned the peculiar form and structure of many of the massifs. The presence of schlieren often brecciated and invaded by the residual magma, as observed by him at Ootacamond (1900, p.219) and the presence of appophyses and dykes of charnockite protruding into the country rock as at Salem (1900, p.219), and Coorg (1900, p.228), have been cited by him as favouring an igneous origin.

 

This view of Holland has been upheld by many others including Walker (1902, p.7); Washington (1916); Masillamani (1916, p.6); Chacko (1921, p.8); Crookshank (1938; p.424); Rajagopalan (1947); Pascoe (1950, p.117); Narasingha (1950) and Krishnan (1951, p.318 and 319).

 

On the other hand, there is a school which considers the charnockites to be platonically metamorphosed igneous rocks – this school includes Stilwell (1918); Vredenburg (1918); Groves (1935) and Prider (1945, p.171).

 

Tyrrell (1948, p.317) considers that the charnockite series “are either of primary igneous crystallization under conditions of high temperature and great uniform pressure or they represent plutonic igneous rocks of the usual characters, which have undergone slow recrystallization in the solid state on being subjected to conditions of plutonic metamorphism”.

 

Ghosh (1941) is of opinion that the basic and ultrabasic portions of the series are formed by heat and pressure metamorphism, but the intermediate and acid rocks owe their origin to a process of assimilation.

 

Rama Rao (1945), in his work on the charnockite rocks of Mysore, has observed: “……., I would regard the charnockite not as belonging to a petrographic province as originally interpreted – namely the differentiated phases of crystallization of a normal plutonic intrusive magma – but as of a metamorphic province wherein the combined effects of a repeated series of alterations under different periods of metamorphism of a composite series of rock formation of different ages, have given rise to a series of hypersthene granulites of very variable composition”.

 

Quensel’s (1950) observations about the origin of charnockites, to quote his own words are that “the basic charnockites are of primary igneous origin, derived from the basic igneous rocks belonging to the Archaean gneiss complex”, and further, “the intermediate charnockite presumed to be hybrid rocks formed by the complete assimilation of rock components of different chemical composition can hardly be assigned a definite position in the sequence of plutonic metamorphism”.

 

• Review of occurrences of inter-related charnockite and syenite

 

Some occurrences of syenites associated with charnockite in a more or less similar manner as at Puttetti in Southern Travancore, have been recorded from elsewhere in India, as well as from other parts of the world. Most of these occurrences have been described in some detail, while a few, which have not evidently been studied, are mentioned in geological literature. Before passing into the genetic aspects of the syenite under investigation, it is worthwhile to make a brief survey of the various similar occurrences.

 

The earliest report, in which striking similarity between syenites and charnockites in India, appears to have been made by Captain J. Allardyce (1836). When studying the granite formation and direction of the primary Mountain Chain of South India, he evidently recognized a similarity between what he called as the “Primitive trap allied to Sienitic granite” of Pallavaram and the principal rock masses of the Nilgiris, Shevroys, Western Ghats and Ceylon.

 

In the same year, Benza described the rocks of Nilgiris thus: “The lowest visible rocks of the Nilgiris is the primitive unstratified class including true granite, pegmatite, Sienitic granite and hornblende rocks; Sienitic gneiss and hornblende slate are occasionally seen but they belong to the outskirts of the hills”.

 

Captain Outchterlony (1848) distinguished between the “granite”, “Sienite” and “hornblende rock” of the Nilgiris on the one hand and the “beds of gneiss” met with in the plains on the others, and referred to the mass or nucleus of the mountain as “granite frequently passing into sienite”.

 

In the Kalahandi State, and the Vizagpatam District, Walker (1902) observed that the members of the charnockite suite instead of maintaining their characteristic individuality over extensive areas tend to assume notable modification. One such modified form is stated to be a massive variety of bluish-black hypersthene-syenite. From the same are in Vizagpatam, Walker has mentioned having collected specimens closely similar to the intermediate form of charnockite and other related to the Canadian Anorthosites.

 

In a subsequent paper (1907) the same author has mentioned about Elaelolite syenites near Koraput in Vizagpatam occurring associated with the charnockite.

 

In the course of an elaborate study of the ‘Igneous Complex of the Blue Ridge Region, Virginia’, Watson and Cline (1916) found that “In the middle and northern parts of the Blue Ridge and the adjacent portions of the Piedmont Plateau in Virginia, one of the dominant igneous rocks of granitoid type is a quartz-bearing pyroxene-syenite. The igneous complex of which pyroxene syenite is the chief type, may represent a Precambrian batholithic intrusion, exposed at intervals for a distance of 150 miles belt up to 20 miles or more in width. Differentiation of the syenite magma has given rise to a variety of related rocks, some of which are of particular interest”.

 

Studies of the igneous complex forming the central core of the Blue Ridge in middle and northern Virginia, are sufficiently advanced to indicate, that the rock types exhibit certain kinships which mark them as differentiates from a common magma, ad that this igneous complex designated by the writer as the Blue Ridge petrographic province, shows certain important differences in mineralogy and chemistry from the igneous rocks which enter into the composition of the Piedmont Plateau to the east.

 

Subsequently, in the same paper (1916, p.219), the authors have compared the syenite with charnockite. The relevant part from the above paper may be quoted here.

 

‘There appears to be remarkably close correspondence in mineral composition of the most abundant variety of charnockite of intermediate composition and the Blue Ridge Virginia Syenite”.

 

In his preliminary report on the Geology of Eraniel, Kalkulam and Vilavancode Taluks in Southern Travancore, Masillamani (1911) has stated that the syenite occurring in Eraniel Taluk is “evidently an intermediate facies of the charnockite series”.

 

In North Arcot, there appears to be a gradual passage from intermediate and basic forms of charnockite to augite syenite and hornblende gneiss. Frequently, the augites of the syenite is found changing to hornblende. According to Fermer (1932), on account of the absence of hypersthene in some of these rocks, the term charnockite is almost inaaplicable. Further it was suggested thatthese augite syenites are a late phase of the charnockite intrusion.

 

The chief rock type of Cochin is charnockite, and its co-existence with augite syenite was recorded by Sen Gupta and Chatterjee (1936). The syenite is holocrystalline – sometimes very coarsely crystalline rock – almost wholly made up of grayish or flesh-coloured feldspar (generally microperthitic) with spots of green augite. Quartz is sometimes present in small quantity. Monzonitic varieties are also known. Sen Gupta and Chatterjee believe that the augite syenite and monzonite are but variants of the same magma.

 

In the well known work on the charnockite rocks of Mysore, Rama Rao (1945, p.48) has mentioned the occurrence of rock types corresponding to acid charnockite and syenitic type as local variations at the marginal fringes of an ultrabasic charnockite exposure near Dodkanya.

 

Pascoe (1950, p.129) has stated that towards the north of Travancore, overlying the charnockite and having the appearance of passing down into it, there is a granular hornblende granite or syenite, with well developed gneissose structure. However, the exact relationship of this rock with the charnockite is said to be obscure.

 

In the Adirondacks of North America, pyroxene syenitic and quartz syenitic rocks are found to occur extensively. These comprise two different series which occur as separate sheet –like masses, namely the Diana Stark Complex and the Tupper-Saranac Complex respectively. The members of the Tupper-Saranac Complex according to Buddington (1952) have features that characterize the charnockite suite of rocks, “and this term seems appropriately applicable to at least part of the members of the Tupper-Saranac Complex”.

 

• Features common to the syenite and charnockite suites of rocks at Puttetti and their probable genetic import

 

From the numerous instances cited in the preceding section, it is evident, that the close association of syenites and charnockites in the field is more or less a widely recognized fact, and that the association between the two kinds of rocks may be considered as an expression of their mutual genetic relationship.

 

Results obtained from the field, petrographical, mineralogical, and chemical investigations of the crystalline rocks of Puttetti, denote certain kinships that seem to exist in the syenite and charnockite type of rocks in the area. Moreover, these kinships lead to conclude that the syenite and charnockite are most probably derived from a common magma.

 

In order to ascertain the validity of the above tentative conclusion, it is desirable to consider the salient characteristics of the rocks concerning thereto, and examine the merits of their genetic implications.

 

1. Significant field characteristics

 

Some of the field characteristics, displayed by the rock types of the syenite suite such as diopside granulite, diopside syenite and diopside syenite (zircon-bearing) among themselves and with the associated charnockites, are highly significant in deciphering the genetic relationship of the syenite and the charnockite suites of rocks of Puttetti area. These characteristics have been already described in detail, and therefore the same require only brief mention here.

 

1. The close association of the syenite and charnockite suite of rocks as seen from their occurrences side by side without any visible trace of contact.
2. The insensible merging of the two suites of rocks indicated by the gradual appearance of free quartz at the periphery of the syenite body in the zone which separates the syenite type from the charnockite type. In otjer words, it is possible to collect specimens, along a traverse from the syenite ridge to the nearest charnockite outcrop that will range in the amount of quartz from a minimum in the periphery of the syenite to a maximum in the typical charnockite.
3. The coincidence in the direction of strike of foliation of the syenite with that of the country rocks, essentially represented by the charnockites. It may also be mentioned that the direction of elongation of the syenite ridge corresponds to the general strike direction.
4. The similarity in colour and texture shown by fresh outcrops of diopside syenite and intermediate charnockite. The similarity is so remarkably perfect that charnockite and syenite could be distinguished from each other only on a close observation of mineralogy in hand specimens.

 

1. Significant petrographical and mineralogical characteristics

 

Comparison of petrographical and mineralogical details clearly shows a number of characteristics to be common to both the syenites and the charnockite suites of rocks. Some of the significant points which strongly imply a genetic relationship of the two kinds of rocks are noted below.

 

1. Texturally, the diopside syenite and the intermediate charnockite are alike, in that both are typically medium to coarse-grained.
2. Uneven distribution of ferromagnesian minerals is a feature observed in rocks of the two suites.
3. The mineral assemblage which constitutes the charnockites is seen to characterize the syenites too, with the exception of quartz and hypersthene. However, diopside and sphene which are absent in the charnockites of the area are found in the syenite.
4. Feldspar, which is an essential constituent of the charnockite, and at the same time, the predominant constituent of the syenite, is made up of microperthite and non-perthitic varieties in both the rocks. The plagioclase is the same in both the rock suites, and usually ranges in composition from albite to andesine, and rarely to labrodorite. Twinning is generally absent in both.
5. Post-consolidation metamorphic evidences, such as pressure phenomena and formation of secondary minerals, are displayed by the two rock suites practically to the same degree.

 

The absence of hypersthene in the rocks of the syenite suite is somewhat conspicuous. Even so is the absence of diopside in the rocks of the charnockite suite in Puttetti. Sphene is also not found in the charnockite, but its absence is nothing unexpected since hypersthene and sphene are believed to be almost mutually exclusive in the charnockites (Rama Rao, 1945, p.25).

 

The examination of numerous microsections of the rocks of the syenite and charnockite suites has not been fruitful, either in tracing any possible relationship between the two minerals hypersthene and diopside, or in satisfactorily explaining the apparent anomaly concerning the nature of distribution of these two minerals in the various rock types of the two suites. However, related literature mentions about the relationship between hypersthene and diopside, and some of the prevalent views may profitably be referred to in this connection.

 

The alteration of monoclinic pyroxene into hypersthene has been noted in the charnockites by Groves (1935, p.146), Rama Rao (1945, p.19), Quensel (1950, p.277) and many others. In Varberg Charnockite Series, hypersthene is the most characteristic mineral, and occurs in different types in varying amounts. It is nearly always accompanied by a monoclinic pyroxene, but the proportions between the two pyroxenes are subject to considerable variation. This can, as Quensel 91950, p.238) pointed out, in certain cases go so far, that only the one or the other of the pyroxenes is present.

 

Ghosh views the relationship between hypersthene and diopside from a different angle. He (Ghosh, 1941, p.8) considers, that diopside, which is usually a primary mineral, may as well as be found secondary after hornblende, and probably also after hypersthene under the effect of regional metamorphism, as a product of reversible reaction.

 

The absence of hypersthene in some of the charnockites of Cochin in the Travancore-Cochin State, was noted by Sen Gupta and Chatterjee (1936, p.6). Though it is the characteristic mineral of the charnockites, the authors have considered their absence as not significant, in as much as hypersthene is a secondary mineral after augite. Moreover, the same authors have adduced the occurrence of secondary hypersthene as an evidence of subsequent metamorphism of the charnockite, formed out of a primary igneous crystallization.

 

In a recent work, regarding the petrogenesis of the charnockite of the Cape Comorin area in Southern Travancore, Paulose (1953, p.32) noted the complete absence of clinopyroxene in the intermediate charnockite, which is hypersthene-bearing. Further, among the four types of charnockites of the basic division, of which all the four bear hypersthene, clinopyroxene was found only in three types, while in the remaining one type the same was not seen. He considers this rock as a transitionary one between the basic charnockite and the other members of the charnockite suite.

 

Therefore, hypersthene may have a secondary origin from diopside; also diopside could probably be formed from hypersthene, as believed by Ghosh. Though at this stage, no specific reason could be assigned for the apparently partial distribution of diopside and hypersthene in the different crystalline rock types of Puttetti, which are considered as descending from a common magma as concluded from the present study, yet, the statement made by Quensel (1950, p.233) tend to show, that the partial distribution of diopside and hypersthene in genetically related rocks does not matter much as far as the present work is concerned.

 

• Significant characteristics of chemical data

 

The chemical analyses of syenites and intermediate charnockites from Puttetti and other places (Table VIII) are highly interesting in their import on the genesis of the syenite at Puttetti. These significant points are mentioned below, and they strongly imply that the crystalline rock types of Puttetti have a common origin, and represent variants of the same magma, evolved by differentiation.

 

1. Regarding chemical composition, the syenite from Puttetti, compares favourably with other syenites from different parts of the world. More interesting than this, is the general similarity in chemical composition, notably in respect of the major constituents, between the syenites and the intermediate charnockites.
2. The curves in the variation diagrams (Fig. 3) relating to the crystalline rock types of Puttetti, trend in such a manner as to indicate that the different rock types concerned are but variants of a common magma.
3. The Niggli variation diagram (Fig. 4) referring to the crystalline rock types of Puttetti, on comparison with his (Niggli’s) original diagram (reproduced in Fig. 5 ‘B’) illustrating the differentiation of the calc-alkaline series, shows a close similarity.
4. It is evident from Fig. 4 that the rock types of Puttetti have a strong affinity to the calc-alkaline series. It may be recalled that calc-alkaline affinities of the charnockite series in St. Thomas Mount and Kondapalle Hill Ranges have been shown by Rajagopalan (1947, p.246) and Srirama Rao (1947, p.154) respectively.

 

• Conclusions

 

In the course of a discussion on the rocks of the syenite clan, Daly (1933, p.481) observed that, “A single mode of origin for the rock species of the syenite clan is largely probable, and a complete list of the actual modes may long elude formation. Still more, obscure is the quantitative importance of the various mechanisms”. He concluded the same discussion with the statement that, “Only a few varieties of rock belonging to the syenite clan are considered to be true hybrids unaffected by differentiation”.

 

Obviously the consideration of such characteristics as are common to both the charnockite and syenite suites of rocks, leads to the recognition of a kinship among the rock types of the two suites. The absence of intrusive contacts and effects in the field, excludes the possibility of intrusion from consideration. Further, the gradual passage of the syenite suite of rocks among themselves, and to the charnockite suite, as seen in the field forms a substantial line of evidence to reckon a common origin for the crystalline rock types of Puttetti.

 

In more or less similar situation, Watson and Cline (1916, p.223-24), while investigating the granites and syenites of the Blue Ridge Region, Virginia, noted the absence of definite contacts between the two types and the gradual passage of one into the other characterized by the progressive increase in quartz content regularly from the syenite to the granite. This led to the conclusion, that the two types represented differentiation phases of the same magma, and that the granite is an acid extreme of the syenite.

 

The comparative statement of the chemical analyses of syenites and intermediate charnockites given in Table VIII clearly brings out a close correspondence between the syenites and the charnockites regarding chemical affinity.  Osann’s diagrams (Figs. 6, 7) in respect of the crystalline rock types of Puttetti, denote an igneous mode of origin, and also emphasize a gradual variation from the acid charnockite at one end to diopside granulite at the other.

 

The variation diagrams (Figs. 3 & 4) show, that the rock types of the charnockite and the syenite suites have resulted from the differentiation of a common magma. From the Niggli varistion diagram, which also shows magmatic differentiation, a calc-alkali affinity in regard to the crystalline rock types of Puttetti is eseen. It is interesting to recall the calc-alkali affinity of the charnockites of the St. Thomas Mount and Kondapalle Hill Ranges as shown by Rajagopalan (1947, p.246) and Srirama Rao (1947, p.154) respectively. Evidently the calc-alkali affinity may be considered to be a characteristic of some charnockites, and at the same time displayed by the crystalline rock types of Puttetti.

 

From the foregoing discussion, it is clear that genetically the charnockite and syenite suites of rocks are closely connected. In other words, the events that marked the genesis of the charnockite suite, have largely contributed to the formation of the syenite suite also.

 

The granite family in the Northwest Adirondacks is represented by rocks of many varieties including some prominent occurrences of syenites. Buddington 91947, p.22-23) considers 85 percent of the granitic rocks of the region as produced by consolidation of magma, in part modified by incorporation of country rock.

 

The quartz syenitic series of the Adirondacks igneous Complexes (p.24-25) show two facies: (1) the Diana-Stark complexes with such predominant varieties such as augite syenite, augite and hornblende quartz syenite, and hornblende granite, (2) Tupper-Saranac complex (related to charnockite series) wherein predominant varieties are augite, hypersthene syenite and augite-hypersthene-hornblende quartz syenite.

 

Some geologists interpret pyroxene–bearing granitic rocks and granitic members of the charnockite series in other regions as products of migmatisation, metasomatism, and mobilization or rheomorphism by emanations. However, based on evidences gathered from the Adirondacks region, Buddington (1947, p.24-25) concluded that the quartz syenitic series in the region are fundamentally due to consolidation of magma, which was intruded from depth and changed in composition by differentiation and in part, by incorporation of country rock.

 

In the earlier portions of this chapter, a number of instances in Travancore-Cochin, India, and other parts of the world, of inter-related charnockite – syenite occurrences with their respective modes of origin wherever known, were briefly outlined. It is interesting to note, that practically all these instances have more or less a similar mode of origin, namely magmatic differentiation. For purposes of the present discussion, the occurrences of syenites in Travancore-Cochin may be recalled with greater emphasis.

 

Masillamani (1911) in describing the geology of Eraniel, Kalkulam and Vilavancode Taluks in Southern Travancore, referred the syenite occurring in Eraniel Taluk as “an intermediate facies of the charnockite series”. Evidently, this occurrence refers to the syenite of the crystalline rocks of Puttetti. Sen Gupta and Chatterjee (1936, p.3) considered the augite syenite and monzonite of Cochin as variants of one and the same magma. Pascoe 91950, p.129) has mentioned about a hornblende syenite associated with charnockite towards the north of Travancore, but the mode of origin is stated to be obscure.

 

From a recent study of the charnockites of Cape Comorin area in Southern Travancore, Paulose (1953, p.106) concluded that, the intermediate and the acid charnockites of the area were formed primarily by a process of differentiation of a palingenetic rock magma, and were subsequently subjected to metamorphism.

 

Though a detailed investigation of the mode of genesis of the charnockites of the area does not, strictly speaking, come within the purview of the present work, obviously, according to the trend of reasoning, adopted in deciphering the mode of origin of the syenite suite of rocks, the mode of origin of the charnockites does have a direct bearing on the problem in so far as the two rock suites descent from the same parent magma as evidenced by field and laboratory findings.

 

Therefore, before concluding the genetic consideration of the syenite suite of rocks, a brief reference dealing on the relevant genetic aspects of the charnockites may also be made.

 

One among the outstanding hypotheses on the origin of the charnockite series of rocks is that propounded by Holland (1900), according to which the members of the charnockite series of rocks have resulted from the differentiation of an igneous magma. Previous works on the charnockites in different parts of Travancore-Cochin by Masillamani (1911), Chacko (1919 & 1921) and Sen Gupta and Chatterjee (1936) have invariably indicated magmatic differentiation as the chief factor in the genesis of the charnockites. Paulose (1953) have also favoured differentiation of an intrusive magma in regard to the Cape Comorin charnockites, but has considered the magma as having been modified before consolidation by incorporation of country rock. He further noted that the charnockite also bear evidences of metamorphic impress.

 

Furthermore, Osann’s diagrams (Figs. 6, 7), which have shown an igneous mode of origin for the syenite suite of rocks, have also indicated at the same time an igneous mode of origin for the charnockite rocks of the area. Other kines of evidences, which have already been described, only go to support this view.

 

Thus the sum total of all the evidences, gathered both from the field and the laboratory as well, taken together, would lead to the conclusion, that the charnockite suite of rocks is fundamentally igneous in origin, and that the same magma which gave rise to the charnockites on the one side by differentiation, further resulted in producing the diopside granulite, diopside syenite and diopside syenite (zircon-bearing), which three types are collectively designated as the syenite suite of rocks. Incidently, this conclusion is consistent with Daly’s view (1933, p.481) that “Only a few varieties of rock belonging to the syenite clan are considered to be true hybrids, unaffected by differentiation”.

 

Certain evidences in the field, particularly observed in the outcrops of the diopside granulite, tend to suggest, that prior to consolidation of magma incorporated some pre-existing basic rock. This is inferred from the presence of irregularly distributed clots, patches and segregations of a dark-coloured basic mineral seen to occur locally in the diopside granulite. Such occurrences as the above were not noticed in the associated charnockite suite of rocks and the remaining two types of the syenite suite of rocks. It is quite probable, that the initial parent magma attacked a pre-existing basic rock of the area, but the assimilation was practically complete in the differentiated fraction which gave rise to the charnockite suite of rocks, whereas the portion of the magma which produced the syenite suite of rocks could not complete the assimilation, thereby resulting in the local occurrence of partly-digested relicts of basic rock. These patches, on close examination in the field are seen to pass gradually into the surrounding host rock.

 

The rock types of the syenite and charnockite suites bear effects of metamorphism wherein heat and pressure conditions seem largely to have controlled. The metamorphic impresses are shown particularly by the presence of, (1) gneissic foliation developed in varying degrees, (2) cataclastic phenomena such as undulose extinction, bent cleavages and distorted twin lamellae, (3) secondary minerals, and (4) perthites. The metamorphic effects are seen more or less alike in the two suites of rocks, and therefore it may be inferred, that the two rock suites were metamorphosed almost simultaneously subsequent to consolidation.

 

Usually the age relationship between two rock types are established on the evidence furnished by dykes, apophyses and similar structures of any one rock found in the other. However, the complete absence of such features as mentioned above in the rock types at Puttetti has made it difficult to determine the age relationship of the rock of the area.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

SUMMARY

 

1. The crystalline rocks of Puttetti are formed by a Syenite suite of rocks consisting of diopside granulite, diopside syenite, diopside syenite (zircon-bearing); and a Charnockite suite of rocks consisting of intermediate and acid charnockites.
2. The Syenite suite occurs as a composite low ridge, trending in a direction coinciding with the regional strike of foliation of the rocks of the area, and surrounded by charnockites, particularly of the intermediate type.
3. The five rock types, which go to make the two rock suites of the area, do not show any intrusive relation, either among themselves, or between the two suites. On the contrary, there is a transition from the syenite ridge towards the surrounding intermediate charnockite, marked by the gradual appearance of quartz.
4. The essential minerals of the syenite suite are feldspars including microperthites and diopside. Other mafic constituents are hornblende, sphene, and mica. Minor accessories include zircon, iron ores, apatite, calcite, pyrrhotite, and quartz. In the charnockites, feldspars including microprthite, quartz, hypersthene, garnet and mica are essentially found, while iron ores, apatite and zircon occur as accessories.
5. Chemical analyses of typical samples of the five rock types were made and the results were plotted in different ways. From these plotting, it was observed, that the Puttetti rocks are igneous in origin and formed by the differentiation of one and the same magma.
6. Based on field and laboratory findings, it is concluded that the rocks of the syenite suite and charnockite suite were derived from the same parent magma by a process of differentiation.
7. Further, certain field evidences, particularly observed in the outcrops of the diopside granulite, suggest, that the parent magma was somewhat modified by the incorporation of some basic rock before final consolidation.
8. Metamorphic effects of the same kind are noted in all the rock types, and it is inferred that the two rock suites were metamorphosed almost simultaneously subsequent to consolidation.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

PART THREE

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

APPENDICES I & II

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Appendix I: A note on the large crystals of zircon in Puttetti

 

Considerable interest has been evinced by geologists and others in the somewhat widespread occurrence of large, well-formed crystals of zircon, disseminated in the gravel and soil over an area of about six acres in the neighbourhood of Puttetti and Kizhkulam in Southern Travancore. The parent rock is not far to seek; a diligent search would reveal it to be none other than the diopside syenite (described in Part II) of the syenite ridge, by the age-long weathering of which the stable zircons came to be released, and lie where they are found at present.

 

Zircon, an orthosilicate of zirconium is a common accessory mineral in many granites, syenites and gneisses. Therefore, the sediments that are formed from the above rocks usually contain concentrates of the mineral. Thus, in the beach sands of Travancore, zircon forms an important constituent. Generally, it occurs in the rocks in microscopic crystals in quantities not sufficient to enable large-scale extraction.

 

The occurrence of zircon at Appiyodu (Maps 1 & 2) in Southern Travancore was recorded by Masillamani as early as the year 1911. He found zircon occurring as a surface deposit, and also “in situ” in a pegmatite in which feldspar predominates with very small amounts of mica.

 

The presence of zircon crystals near Appiyodu was noted by the writer, during his survey of the systematic geology of the area, described in Part I of this thesis. Well-developed, large crystals, evidently weathered out of the parent rock, were found scattered in the bed of a small stream in the neighbouring valley. On being traced further up, it was found that the zircons progressively concentrated, and later, large crystals were actually found embedded in the hard laterite forming the elevated ground adjoining the syenite ridge.

 

At Kizhkulam, behind the Primary School, ziron is found in the gravels in greater abundance than at Appiyodu. This area being under cultivation, ploughing the ground year after year results in zircon crystals which remained at depth being brought to the surface. Davidson (1946) has referred to this occurrence as “a very rich alluvial deposit”. A casual search for a few minutes would enable one to collect not less than a pound of the crystals. The edges and corners of the individual crystals are found to be somewhat worn out as a result of constant abrasion.

 

At Puttetti, about a mile east of Kizhkulam, zircon is found to occur ‘in situ’ in the diopside syenite. In regard to this occurrence, Davidson (1946) remarked, “a zircon-rich pegmatite which is considerably richer than any occurrence that I know of (possibly there are some equally rich occurrences in Madagascar)”. The crystals are unusually large, and form a conspicuous accessory of the syenite. Since fresh surfaces of the rock are not seen in the area, fresh exposures were made by blasting with the use of explosives. It was observed, that far from having any uniformity in the matter of distribution, the crystals occurred either as clusters or separated far apart in the syenite body.

 

At Puttetti, single crystals of zircon are found to measure from a fraction of an inch up to an inch or more in length. Rarely some crystals are even 2 inches long. The crystals are usually brown in colour, bit some of the smaller ones are honey-yellow. The lusture is usually adamantine. Crystals are mostly translucent, but some of the smaller ones are nearly transparent. The average specific gravity is 4.66.

 

Lacroix (1922, Vol. I., p.236), has recorded a similar occurrence of zircon discovered by Mr. Gauge, in the Mount Ampanobe. There, in the red earth were found abundant crystals of zircon with normal optical properties and density. Some of them have been found to be more than 10 centimeters along the vertical axis and weigh many kilograms.

 

Crystal habit

 

As a rule, crystals of zircon from the area show the prismatic and pyramidal faces to be well-developed. The typical features of the tetragonal form is well-displayed by some of the smaller crystals. In some crystals, it is observed, that the prismatic face terminates through two pyramids, in succession. The terminal pyramid makes an angle of 44º to 54º with the projected prismatic face, and the pyramid between the terminal pyramid and prismatic face makes an angle of 18º to 22º with the prismatic face. Parallel intergrowths of two zircon crystals are frequently seen. In this case, the faces and edges of one of the two individual crystals are found parallel to the corresponding faces and edges of the other. Rarely outgrowths of smaller crystals of zircon are found to emerge from the prismatic faces of larger crystals.

 

The most common variety of zircon occurring in the area is that which is elongated along the vertical axis, the horizontal axes being equal to each other. Occasionally, certain crystals are found to be distorted in different ways. One such instance is where one-pair of opposite lying prismatic faces is more flattened than the other pair, so much so the two horizontal axes are unequal, and the plane containing the two horizontal axes is rectangular in shape. Unequal development of the pyramidal faces is also caused by distortion. These distortional features may possibly be attributed as due either to restricted conditions under which crystallization took place, or to non-uniform conditions of pressure. No twinned crystal was noticed in the area.

 

To study the effects of heat on the physical properties, such as colour and specific gravity, a few crystals ‘hammered out’ of the host rock, after noting the initial colour and specific gravity, were heated to about 800ºC for nearly an hour. On cooling, it was noticed that the colour generally turned to white except locally where the colour remained unchanged. Richter (1932, cited by Brogger, 1890, p.102) has mentioned that hyacinth from Ceylon, when heated to red hot, changed its colour. Rivot (taken from the review of G. Spezia’s paper: Atti della Reale Accademia della Scienze di torino, Vol. xii, 1876. Mineralogical Magazine, 1877, No. 7, p.138), attributed colour change in zircons when heated, to the destruction of organic matter present in the zircon. In the same context, Chandler’s view is mentioned, according to which “changes of colour are caused by the different degrees of heat to which the mineral has been exposed”. Spezia explained the phenomenon as “alteration due to a difference in the state of oxidation of a colouring metallic oxide”. Kennard & Howell (1936, p.721) have mentioned, that “the cause of the colour in zircons generally has been attributed to the presence of impurities, particularly, iron, copper, titanium, chromium, vanadium, zinc, uranium, thorium, hafnium, and magnesium”.

 

The zircon crystals from Puttetti area are found to contain about 0.23 percent of ferric oxide and the reduction of this oxide to the ferrous state on heating, is probably the reason for the colour change.  One peculiarity is, that on the application of heat, the smaller crystals tend to lose colour easily, whereas in the case of larger ones the change in colour is not easily perceived. In a few cases, however, under the same conditions, no change in colour has been noticed at all.

 

As a result of heating, it was noticed that the specific gravity of the zircons from Puttetti has been increased by 0.10 to 0.18. Brogger (1890, p.102) has cited Svanberg to have observed a similar change in the specific gravity of zircon when heated red hot. Church (1875, p.323) has pointed out that all zircons do not show increase in specific gravity on being heated.

 

The effects of heat on some of the loose crystals disseminated in the gravelat Kizhkulam were also observed. It was found, that there was no change in colour and specific gravity in this aces. The crystals are deep-brown in colour, and are richer in the content of ferric oxide. According to Spezia (taken from the review of G. Spezia’s paper: Atti della Reale Accademia della Scienze di torino, Vol. xii, 1876. Mineralogical Magazine, 1877, No. 7, p.138), zircons rich in iron “do not become quite colourless, but assume a greenish tint”.

 

Chudoba and Stackelburg (1937, p.196) believed, that there is a basic connection between colour and internal structure or density of zircon. According to them, the density of zircon of lowest specific gravity can be considerably increased by heating to about 1450º. As particular examples, they have mentioned about the zircon of density 4.15 which could be raised to a density of 4.63 by heating. According to the same authors, zircons of low density are composed of amorphous SiO₂ and amorphous ZrO₂, while those of high density consist of crystallized ZrSiO₄. This amorphous nature of the components ZrO₂ and SiO₂, they believe, can have a leading influence in the density of the mineral.

 

Zircon from Puttetti area was found to be radioactive. Detection and determination of the radioactivity was facilitated by the use of a sensitive X-ray Electroscope (Mathai, 1943), originally devised by C.T.R.Wilson (1901) for the investigation of the natural ionization of gases. The instrument was subsequently modified to suit measurement of radioactivity of some of the feebly active mineral sands of Travancore. It consists of a gold leaf system fixed within a thick lead chamber, below which the specimens are mounted. The discharge of the gold leaf, due to the X-ray ionization is measured through glass windows with a low power microscope fitted with a micrometer eye piece. The insulation of the instrument is secured through a bed of sulphur. The instrument retains a charge for forty-eight hours, and it is sensitive enough to respond to the presence a few grains of monazite sand.

 

Zircon was powdered in an agate mortar, and a uniform layer of the powdered mineral was taken in a glass disc. The activity of the mineral was compared to that of a sample of known uranium content, and the uranium equivalent of the mineral was found to be 0.35. In this connection, it may be mentioned that ‘Malacon’ or brown zircon of Madagascar was reported to be invariably radiocactive, due to the presence of thorium in it (Lacroix, 1922, p.242).

 

The radioactivity of the zircon from the Puttetti area was also determined by a somewhat sensitive gamma counter (Viswanathan Nair and Krishnan Nair, 1953). The experiment was done at the Phycics Department of the University of Lucknow. Thorium in Beta active, but its fifth disintegration product is sufficiently strong in gamma activity to be measured by the Counter. The instrument was first standardized for the purpose of determining the background of cosmic shower which continue throughout the experiment. The counts due to the cosmic radiation were never constant, but varied between 30 and 60 per minute. The average of fifteen counts, each of one minute duration and made in-between the countings with the sample, was 47.5 counts per minute.

 

Next, the counts were taken with pure Thoria (ThO₂). Ten gms. of pure thoria were spread uniformly on a gummed cellophane of an area just enough to cover the counter tube. The average of fifteen readings was 115.7 counts per minute.

 

The same process was then repeated using 10 gms. of the powdered sample of zircon . The average of fifteen counts was in this 48.25 counts per minute.

 

Assuming that the gamma radiation in the crystal was due to thorium, the percentage of thoria in the sample was calculated to be 1.1.

 

The same experiment was repeated, this time on a sample of powdered zircon sands obtained from the beach concentrates along the coast of Travancore. It was found that the gamma activity of this sample was very low, compared o that of the fresh crystals ‘hammered out’ of the parent rock at Puttetti.

 

Chemical analysis of a sample of the fresh zircon crystal, obtained by crushing the host rock, was made. The results are entered in the Table VI. In this analysis, thorium has not been estimated. Quantitative estimation, subsequently done in the laboratory, gave the amount of thorium to be 0.83 percent. The radioactivity is therefore attributed to the thorium present in the mineral.

 

 

 

 

 

 

Appendix II: Five axis universal stage

 

The optical constants of minerals were determined by the Leitz Five-Axis Universal Stage. The instrument is provided with the following axes and is diagrammatically represented in Fig. 8.

 

1. Inner Vertical Axis (I.V.)
2. Inner East-West Axis (I.E.W.)
3. North-South Axis (N.S.)
4. Outer Vertical Axis (O.V.)
5. Outer East-West Axis (O.E.W.)

 

In the Four Axis Universal Stage, ‘2 V’ can be determined only in a section containing ‘Y’ in the horizontal plane, often with the aid of a stereographic projection, whereas in the Five Axis Universal Stage, 2 V can be measured in a section of random orientation. If ‘Y’ is in the horizontal plane, direct measurement of the 2 V is possible, but in case the ‘XZ’ plane is horizontal, 2 V can be determined using the Berek method.

 

A fragment of a mineral mounted on the universal stage, can be so oriented that one of its critical vibration directions is rendered parallel to the axis of the microscope and the others perpendicular to it; one north-south and the other east-west.

 

Adjustment of the Universal Stage

 

A strong source of illumination is necessary for universal stage work, and the source of light should be perfectly centered, to ensure accuracy.

The cross hairs are perfectly adjusted with reference to the vibration planes of the nicols. This can be effected as follows:

 

Take (010) section of gypsum, and determine the extinction angle from the cleavage – say 35º. Suppose the stage was turned to the right to get the above value.

 

Next, put the section, upside down, and get the extinction value, which will now be to the left. Let us suppose that it is 41º. The eye piece must be turned anticlockwise by 3º which means the cross hairs are turned now.

 

Thus the extinction value to the right must be equal to the value on the left, on reversing the section; the measurement being made with reference to the same cleavage.

 

The cross hairs having been adjusted, the objective is centered accurately.

 

Next, the universal stage is fixed on to the microscope stage by two thumb screws and has to be centered accurately so that it’s vertical axis may coincide with the microscope axis. This is done as follows:

 

1. Clamp the microscope stage and rotate the I.V., observing the centre of rotation choosing a dust particle on the glass plate.
2. If the centre of rotation does not coincide with the cross hairs, loosen gently the thumb screws holding the universal stage and bring this centre of rotation to the cross hairs. Clamp the thumb screws and test by rotating on the inner vertical axis. By successive adjustments, the centre of rotation of the universal stage can be very nearly brought to coincide with the cross hairs. Now, a rotation on the outer vertical axis should also be centered reasonable well. The thumb screws are clamped well in this position, and the slide mounted on the stage with liquid contacts. The next adjustment is such that the axis of the section coincides with the E-W axis.

 

The elevation of the mount is adjusted as follows:

 

For this, rotate on the outer east-west axis and note the movement of a particle on the slide. If the particle moves in the same direction as the rotation, the mount is too low; if in the opposite direction, too high. The elevation of the mount is adjusted by turning the collar which supports the inner stage plate in the desired direction. Perfect adjustment would ensure an object in the centre of the cross hairs remain almost stationary, on rotation of the O.E.W. now, on a rotation of the N.S. and E.W. axes, the particle should very nearly remain in position, as it was made to do for the rotation on the O.E.W. axis.

 

Next, the horizontal axes are aligned with respect to the vibration direction of the nicols which are considered set – polarizer N.S., and the analyzer E.W. then the tube of the microscope is raised and focused on some dust particle on the upper surface of the upper hemisphere. The O.E.W. axis is rotated and the movement of the dust particle is observed. If it does not follow the north-south cross hairs, sufficient rotation on the microscope stage is given to make it do so, and the microscope stage is fixed in this position. The vernier reading is recorded, as it is the zero position of the microscope stage for the setting, which should be carefully maintained for all the future readings.

 

Similarly, the north-south axis is rotated and the dust particle is made to follow the east-west cross hair, by suitable adjustment on the outer vertical axis.

 

Orientation procedure for the determination of 2 V and sign

 

In the orientation of a mineral mounted on the universal stage, the three vibration directions are set in such a way that one is parallel to the microscope axis and the others parallel to the polarizer and analyze respectively. Extinction is the optical feature employed in the orientation of the mineral. Extinction reveals vibration differences which are related to the planes of optic symmetry. The planes of optic symmetry are adjusted to the cross hairs by rotating the crystal to and from extinction. The following procedure is adopted in critically orienting a mineral, for the determination of 2 V and sign.

 

1. The inner vertical axis of the universal stage is turned and the mineral brought to extinction. The traces of the two vibration directions in the horizontal plane are now parallel to the polarizer and analyzer.
2. Extinction is tested by rotating on the outer east-west axis and north-south axis, noting on which axis the crystal departs least from extinction. It is advantageous to have this one as the north-south axis, as otherwise the outer east-west axis which has a greater freedom of movement cannot be used to the maximum advantage. If therefore, the rotation on the outer east-west horizontal axis gave less departure from extinction, the inner vertical axis should be turned through 90º. Now, one symmetry plane strikes roughly east-west and dips north or south.
3. To bring this symmetry plane parallel to the plane of analyzer, the inner east-west is inclined to a few degrees north or south. Now the mineral departs from the position of extinction. Extinction is restored by suitable rotation on the inner vertical axis. As before, the position of extinction is tested on the north-south axis, noting whether it departs more or less from extinction, than before. If more, then the direction of inclination on the inner east-west axis is reversed and extinction again restored by rotation on the inner vertical axis. North-south axis is again rotated on either side and extinction tested. By successive increments of inclination on the inner east-west axis, and testing on the north-south axis, a position is found for the inner east-west and inner vertical axes, at which, the crystal will remain at extinction when tested by wide rotations on the north-south axis. This step is done carefully, as final accuracy depends mainly on it. The desired setting for one symmetry plane being vertical and parallel to the plane of vibration of the analyzer, i.e., east-west, is obtained. But one optic symmetry plane mat dip east or west and the other will be normal to this. Suppose on optic symmetry plane dips east by 70º, the other optic plane will be dipping 20º west. The first one must be rotated clockwise by 20º to render it vertical. In other words, the north-south axis must be tilted clockwise. However, since the mineral is already at extinction, a rotation on the north-south axis under the present setting will only keep the mineral dark. Hence, the outer east-west axis is inclined so that the mineral departs from extinction; i.e., some light is observed. Then and then only, the north-south axis should be tilted to get darkness. The outer east-west axis is now rotated back to its zero position. Now, the mineral is set on the stage in such a way that one optic symmetry plane is vertical, another parallel to the polarizer and the third parallel to the analyzer.

 

Although the mineral is thus oriented with its cardinal directions parallel to those of the microscope, the recognition of the vibration directions X, Y, and Z, still remain to be made. For this, optic plane is identified and the relative velocities of the transmitted rays are also determined. This is done as follows:

 

1. Turn the microscope stage to 45º, say anti-clockwise. This makes one vibration direction northeast-southwest, and another northwest-southeast.
2. Rotate both ways on the outer east-west axis and observe if there is a fall of interference colour leading to darkness. If the optic axial plane is vertical and the optic angle not very wide, positions of extinction will be reached on either side and the values will be equal also. In such a position of extinction, the optic axis is obviously parallel to the microscope axis. Hence the accuracy can be checked by rotating the stage of the microscope when extinction should persist. The angle between the two optic axes is measured and this gives the value of the optic angle (2 V).
3. The presence of optic axes indicates the north-east direction as ‘Y’. by a suitable compensator, it is found out whether ‘Y’ is faster or slower. In case, ‘Y’ is found to be slower, the other vibration direction in the horizontal plane is ‘X’; that coinciding with the microscope axis is ‘Z’; ‘Z’ is the acute bisectrix; the mineral is therefore positive. If on the other hand, ‘Y’ proves to be fast, that in the horizontal plane will be ‘Z’ and that coinciding with the microscopic axis is ‘X’. The mineral is then negative.

 

If in step 2, the optic axes are not obtained, it may be that the optic axial plane is in the plane containing the microscope axis and the axis of rotation. A rotation of 90º on the outer vertical axis will render the optic axial plane normal to the axis of rotation. The optic axial angle (2 V), and sign are then determined as before.

 

In case the optic axes are not obtained in either procedures, then ‘Y’ can be presumed to be vertical, in which case 2 V cannot be determined in the usual way. In similar instances, the Berek method may be employed.

 

 

 

 

Berek’s Procedure

 

This procedure can be employed for the determination of the optic axial angle (2 V), for the three possible orientations. But since direct reading of the optic angle is possible when ‘Y’ is horizontal and the optic axial angle is not very large, this method may be conveniently used as a check.

 

The method is as follows:

1. The mineral is set in the critical position.
2. The outer vertical axis is turned through 45º, say clockwise.
3. Now the outer east-west axis is turned through 54.7º in a direction opposite to the general inclination of the mineral mount, to ensure a wide latitude of movement.
4. The microscope stage is rotated anti-clockwise still the mineral reaches the position of extinction, and the angle is noted.
5. The next step is to read the value of the optic axial angle (2 V), corresponding to the extinction angle obtained, by referring to the Berek’s graph (Emmons, 1943, p.31).

 

This method is only rarely adopted as slight changes in the extinction angle will affect the value of the optic axial angle (2 V) to a considerable extent. The error is much greater in the case of minerals having a small optic axial angle (2 V). Determination of the exact extinction position of coloured minerals like the amphiboles and pyroxenes which have greater absorption is often very difficult, even with the use of a sensitive tint. This may be avoided to a certain extent by taking the average of the two readings when the mineral reaches and departs from the position of extinction.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

BIBLIOGRAPHY

 

Allardyce, Captain	1836	“On the Granitic formation and direction of the Primary Mountain Chains of South India” Madras Jour. of Lit. and Sci., Vol. IV, pp.327-335. (Cited by Holland, T.H., 1900, p.122)
Barth, Tom, F.W.	1952	“Theoretical Petrology”. John Wiley and Sons.
Brogger, W.C.	1890	“Die Mineralien de Syenitpegmatitgange der Sudnorwegischen Augit – und Nephelinsyenite. Specieller Theil, Beshreibung der auf den  Syenitpegmatitgangen der Sudnorwegischen Augit – und Nephelinsyenite Vorkomenden Mineralien. pp.101-106, and 525-551.
Buddington, A.F.	1948	“Origin of Granitic Rocks of the Northwest Adirondacks”. Geol. Soc. Amer. Mem., No.28, pp.21-43.
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	1921	“A Sketch of the Geology of Travancore”. Rec. Dept. Geol. Trav., Vol. I, pp.1-43.
Chaudoba, K. & Stackelburg, M. von.	1937	“Optical properties, density and structure of zircon”. The Gemmologist, Vol. VIII, No.75, pp.193-196.
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Daly, A.	1910	“Origin of the Alkaline Rocks”. Bull. Geol. Soc. Amer., Vol. 21, pp.87-118.
	1933	“Igneous Rocks and the Depths of the Earth”. McGraw Hill Book Company.
Dana, E.S.	1949	“A Text Book of Mineralogy”. John Wiley and Sons, Inc.
Davidson, C.F.	1946	Letter dated 15-1-’46 from Dr. Davidson, Cliff House, Trivandrum to the Dewan of Travancore.
Emmons, R.C.	1943	“The Universal Stage”. Mem. 8, Geol. Soc. Amer.
Evans, J.W.	1901	“Monchiquite from Mount Girnar (Junagadh)”. Quart. Jour. Geol. Soc., Vol. LVII, pp.38-54.
Fermor, L.L.	1932	General Report of the Geological Survey of India for the year 1931. Rec. Geol. Surv. Ind., Vol. LXVI, Pt.1, pp.1-150.
Foote, R.B.	1883	“On the Geology of South Travancore”. Rec. Geol. Surv. Ind., Vol. XV, Pt.1, pp.20-35.
Ghosh, P.K.	1941	“The Charnpckite Series of Bastar State and Western Jeypore”. Rec. Geol. Surv. Ind., Vol. LXXV, pp.1-55.
Grout, F.F.	1932	“Petrography and Petrology”. McGraw Hill Book Company, Inc.
Groves, A.W.	1932	“Petrology of the Western Rift of Central Africa”. Geol. Mag., Vol., LXXIX, No.821, pp.497-510.
	1935	“The Charnpckite Series of Uganda, British East Africa”. Quart. Jour. Geol. Soc., Vol. XCI, for 1935, pp.150-207.
Harker, A.	1902	“Petrology for Students”. Cambridge University Press.
	1909	“The Natural History of Igneous Rocks”. Methuen and Company Ltd.
Heron, A.M.	1924	“Soda-bearing suite of the north-east end of the Aravallis”. Rec. Geol. Surv. Ind., Vol. LXVI, Pt.2, pp.177-197.
Hillibrand, Lundell, Bright, Hoffman	1953	“Applied Inorganic Analysis”. John Wiley and Sons, Inc.
Holland, T.H.	1900	“The Charnockite Series, a group of Archaean Hypersthene Rocks in Peninsular India”. Mem. Geol. Surv. Ind., Vol.XXVIII, Pt.2, pp.1-249.
	1901	“The Sivamalai Series of Elaeolite-Syenites and Corrundum-Syenites in the Coimbatore District, Madras Presidency”. Mem. Geol. Ind., Vol.XXX, Pt.3, pp.169-224.
	1902	Mem. Geol. Surv. Ind., Vol.XXXV, p.92 (cited by Heron, 1924).
Holmes, A.	1930	“Petrographic Methods and Calculations”. Thomas Murby and Company.
	1931	“The Association of Acid and Basic Rocks in Central Complexes”. Geol. Mag. Pol., Vol. LXVIII, whole series No.804, pp.241-255.
	1932	“The Origin of Igneous Rocks”. Geol. Mag., Vol. LXIX, No.822, pp.543-558.
Johannesen, A.	1948	“A Descriptive Petrography of the Igneous Rocks”. Vol. I, The University of Chicago Press.
	1949	“A Descriptive Petrography of the Igneous Rocks”. Vol. II to IV, The University of Chicago Press.
Kennard, T.G. & Howell, H.D.	1936	“Spectrographic Examination of Siamese Zircon”. Amer. Miner., Vol.21, p.271.
King, W.	1882	“General Geology of South Travancore”. Rec. Geol. Surv. Ind., Vol.XV, Pt.2, pp.87-102.
King, B.C.	1947	“The Textural features of the Granites and invaded rocks of the Singo batholith of Uganda and their petrogenetic significance”. Quart. Jour. Geol. Soc., Vol.CIII, Pt.1, No.409, pp.37-64.
Knight, Hallows, K.A.	1923	“Basic and Ultrabasic members of the Charnockite Series in the Central Province”. Rec. Geol. Surv. Ind., Vol. LV, Pt.3, pp.254-259.
Krishnan, M.S.	1926	“The Petrography of rocks from the Girnar and Osham Hills, Kathiavar, India”. Rec. Geol. Surv. Ind., Vol. LVIII, Pt.4, pp.380-424.
	1951	“Beswada Gneiss, Khondalites and Leptynites”. Rec. Geol. Surv. Ind., Vol. 81, Pt.2, pp.315-320.
Kunjunni Menon, K.	1950	“General features of the Teris of South Travancore”. Ind. Geogr. Jour., Vol.XXV, pp.1-9.
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	1922	“Mineralogie de Madagascar”. Tome, I, p.236.
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	1907	“Petrological Study of some Rocks from the Hill Tracts, Vizagpatam District, Madras Presidency”. Rec. Geol. Surv. Ind., Vol.XXXVI, Pt.1, pp.1-18.
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GEOLOGY OF A PART OF SOUTHERN TRAVANCORE WITH SPECIAL REFERENCE TO THE PETROLOGY OF THE SYENITE AT PUTTETTI

 

 

 

 

 

 

 

 

 

Thesis submitted to

The University of Travancore

for the Degree of

Master of Science

 

 

 

 

 

 

 

 

 

 

 

 

 

 

By

K.V. Krishnan Nair, B.Sc.

Research Assistant

University of Travancore Trivandrum

1954

CERTIFICATE

 

 

This is to certify that this Thesis is an authentic record of the work carried out by the author under my supervision and guidance and that no part thereof has been presented before for any other degree.

 

 

 

(Supervising Teacher)

Head of the Division of

Mineral Survey & Research

University of Travancore

Trivandrum

August 26, 1954.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

PREFACE

 

Generally speaking, the geology of Travancore is largely unknown, for, ever since the outstanding work of the pioneer geologists Dr. William King and Mr. R. Bruce Foote during the early eighties of the last century, no progress worth the name has been achieved in the exposition of the geological history of the State. Therefore, the contributions of Dr. King and Mr. Bruce Foote continue to be esteemed as landmarks in the field of geological research in this State.

 

In 1947, the University of Travancore organized the Division of Mineral Survey and Research under the directorship of Mr. T.R.M. Lawrie, B.Sc., F.G.S., the main object envisaged being to conduct a systematic geological survey of the State. During the three years of his tenure of office, Mr. Lawrie was chiefly concerned with the field training of the departmental personnel, and towards the close of 1949 regular survey work was commenced, and for this purpose, areas in Southern Travancore were selected to be mapped first.

 

Subsequent to his training, the writer was assigned the area surrounding Puttetti for detailed mapping. This thesis largely embodies the results of his investigations made during field work.

 

For convenience in presentation, this thesis has been divided into three parts. The first part is devoted for a brief general report on the geology of the area; part two, which is also the main body of the thesis, deals with the problem that was taken up for special study; and part three contains two papers based on topics having direct bearing on the main work.

 

The author wishes to place on record his deep sense of gratitude to Sr. K. Kunjunni Menon, B.Sc.(Madras)., M.S.(Yale)., M.A.I.M.E., for his inspiring guidance, and constructive criticisms throughout the course of this work. He is thankful to Sri. K.V. Nayar of the University Chemistry Department for valuable advice regarding chemical analyses. His thanks are also due to Sri. C.V. Paulose, Research Assistant, for the interest he evinced in this work. He is indebted to the University of Travancore for deputing him to Madras for a short term, to undergo special training the use of the Universal Stage.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

PART ONE

 

 

 

 

 

 

 

 

 

GEOLOGY OF THE AREA

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

CHAPTER – I

 

1. Introduction and previous work

 

Geologically speaking, Travancore is largely ‘unknown’ in that, ever since the general reconnaissance works of the pioneer geologists Messrs William King and R. Bruce Foote in the early eighties of the nineteenth century, the attention of the later workers was confined more to location and prospecting of industrial minerals rather than the systematic geologic survey. The contributions of King (1882) and Foote (1883) are reckoned at present as the most valuable sources of information concerning the geology of Travancore, and we are indebted in particular to Bruce Foote for his enlightening account of the geology of Southern Travancore.

 

The region, that was geologically mapped, (Map No.1) is situated on the coastal tract of Southern Travancore, and corresponds to the area represented in one inch topographic sheet 58 H/4. It covers an area of over thirty square miles and lies within parallels of latitude 8º 10’ and 8º 15’ and longitudes 77º 10’ and 77º 15’.

 

This part is triangular-shaped, the port of Kolachel, the fishing village of Taingapatam, and the high-hills near Kappiyara marking the south-east, north-west, and north-east corners respectively.

 

A net-work of motorable roads, cart-tracks, and footpaths facilitates easy access to most of the outcrops in the region. Good sections of the sedimentary rocks can conveniently be examined especially near the coast, where these are exposed in deep gullies and other water channels which are dry during most part of the year.

 

2. Physiography

 

Physiographically, four distinct zones can be recognized in Southern Travancore as a whole, trending lengthwise, and almost parallel to the coast. These zones are:

1. The mountains represented by the Western Ghats
2. The vast stretch of Alluvium
3. The plateau formed by the Tertiary formation
4. The coastal dunes made up of Recent sands

 

Being situated on the costal tract, the region under study is composed mainly of the last two physiographic zones viz., the Plateau and the Dunes.

 

In climate, Southern Travancore is neither as humid as the rest of the State, nor as arid as Tinnevelly and other places lying beyond the Western Ghats. Rainfall is low, and occurs during the South-West Monsoon. Hence the natural vegetation is of a semi-arid type. Palmyra Palm (Borassus flabelliformis), Acacia planifrons – a short stumpy tree with umbrella-like foliage, and others adapted for a desert habit are common in the flora. This part is industrially important, and the manufacture of palmyra fibre and coir, poultry-farming and fish-curing, form some of the major items of industry.

 

The region has a gentle slope towards the south-west, and the drainage also follows this direction. As typical of a semi-arid country, rainfall, in the form of occasional cloud-bursts, has caused numerous gullies to be formed in the region and drainage is effected along these. The only major stream of any importance, a part of which at least is contained in the area, is the Kuzhithurai River. This river, which originates far away in the Western Ghats, flows into the backwaters at Taingapatam, just before reaching the sea. Its outlet into the sea is controlled by a sand bar, which permits communication with the sea only during monsoon when the river maintains a considerable discharge.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

CHAPTER – II

 

1. Stratigraphy

 

Peninsular India, of which Travancore forms the southernmost part, when compared with rest of India, has a most imperfectly developed geological record. It is believed (Wadia, 1944, p.48) that the region has probably never been submerged beneath the sea, except temporarily and locally, and for this reason, no considerable thickness of marine sediments was ever deposited in the interior of the land mass during post Cambrian period.

 

Therefore in Travancore, the geological column is not represented by rocks of the Paleozoic and Mesozoic era. The oldest sedimentary rocks are of Tertiary age. The stratigraphical sequence in the area under study is as follows:

 

Recent		Blown Sands formed by the coastal dunes, and the red ‘Teris’; alluvium
Sub-recent (?)		Laterite
Tertiary		Warkilli Beds
Archaean		Garnetiferous Gneiss, Charnockite, Pegmatites and Veins

 

 

 

 

 

2. Petrology

 

• Archaean Rocks

 

The archaean group of crystalline rocks is considered to be the oldest known rocks, and forms the basement on which later sediments were deposited. This group of rocks is represented chiefly by garnetiferous gneiss (leptynite), and charnockite.

 

• Garnetiferous Gneiss (Leptynite)

 

This is a garnet-rich, light-coloured rock which is also referred to as leptynite. Fresh outcrops are exposed in some of the quarries at Kappiyara and Kolachel. Generally the garnetiferous gneiss or leptynite has a better expressed foliation than the charnockites. Usually, the rock has a white colour and the garnet appear as bright spots rendering the foliation conspicuous. Often the white colour of the leptynite is quite a remarkable feature of the rock, and Chacko (1921, p. 17) attributes the white colour to the alteration undergone by the feldspars. The foliation strikes N.W. – S.E. and dips 50º – 60º to the N.E. locally there seems to be a change in the direction of dip. However, the exact reason is not known.

 

The rock is medium-grained, and composed essentially of quartz, feldspar, pink garnet and mica with smaller amounts of graphite and iron ores. The greater bulk of the rock is made of the two minerals quartz and feldspar. The feldspar is represented by orthoclase, plagioclase and perthite. The micas are largely biotite and phlogopite, and tend to be concentrated along foliation. The average specific gravity of the rock, as determined on six specimens, was found to be 2.65.

 

The leptynite is comparatively less resistant to weathering than the charnockites. Examination of a number of outcrops shows that the process of weathering is most conspicuous along foliation planes and joints. The progress of rock decay along planes of foliation leads to large slabs of the rock to be dislocated, causing the formation of dip-slopes along flanks. The same feature accounts for the typical serrated appearance of the summit. In outcrops, where the surface is generally even, the effects of differential weathering are marked. The easy weatherability of the feldspars results in feldspar portions to be fluted in contrast to quartz which stands out. Weathered surfaces are usually stained by solution carrying limonite, evidently formed by the decomposition of the iron-bearing constituents of the rock.

 

The ultimate product of weathering is laterite. It may often be noted that the laterite formed ‘in situ’ from a garnetiferous gneiss is lighter in colour than that formed from a charnockite. This is probably due to the relative sparseness of the ferromagnesian minerals in some of the garnetiferous gneiss. In places, where soil profiles exist, the various stages in the transformation of the rock into laterite can be observed.

 

• Charnockite

 

Charnockite is more abundant than the garnetiferous gneiss in the area, and is represented practically by the intermediate variety. The acid variety is comparatively rare. The rock is generally grayish-blue in colour, holocrystalline, and medium to coarse-grained. The characteristic grayish-blue colour helps in the easy identification of the rock in the field. The average specific gravity was found to be 2.61 for the acid variety, and 2.70 for the intermediate variety. The somewhat higher specific gravity of the intermediate charnockite is evidently due to the presence of a good amount of ferromagnesian minerals.

 

Megascopically, feldspar, quartz, hypersthenes, micas, garnet, iron ores, and pyrite can be identified. The feldspar and quartz are usually greenish or bluish-gray in colour, and together they make the greater bulk of the rock. Feldspar is present in greater quantity than quartz. While hypersthene is constantly present in all the specimens examined, the remaining minerals occur in variable quantities, all of which are not always represented in a single hand specimen. Garnet is of the almandine variety, and, though it is an important constituent of the garnetiferous gneiss, it is no less conspicuous in the charnockite. The rock is tough, massive, and does not easily yield to the hammer. Fracture surfaces appear subconchoidal. Foliation, though seen in many outcrops, is less marked and sometimes very indistinct in the charnockites of the area.

 

In a few places, outcrops of the acid and the intermediate varieties are seen close by, but do not show plausible evidences to indicate their mutual relations. Some of the quarries in the area especially those near Karingal Camp-shed, expose sections where the charnockite and the garnetiferous gneiss occur as local patches in it.  The rock at the junction is so badly weathered and stained that the nature of the association could not be clearly observed. At Kolachel, a quarry face has exposed charnockite at the base and garnetiferous gneiss at the top. Both rocks are texturally alike, and garnet is present almost equally in both. The junction between the two rocks shows gradual merging of the one into the other.

 

• Syenite

 

In addition to the garnetiferous gneiss and the charnockites, the crystalline rocks of the area include yet another rock type having features quite dissimilar to those characterizing the other two rock types. This is a syenite occurring as a composite, low ridge, exposures of which are conspicuously seen at a number of places in and around the place known as Puttetti. The texture ranges from medium to coarse, the coarse type being the most prevalent. The syenite is considered to be genetically related to the charnockites. Since the petrology of this syenite is discussed in Part Two of this Thesis, further details are not given here.

 

• Pegmatites, Dykes and Veins

 

Pegmatites are common in both the garnetiferous gneiss and the charnockite. These generally strike in all directions, and vary in width from fraction of an inch to about 8 inches. The pegmatites are characteristically coarse, possess more or less the same colour as the respective host rock, and mostly carry feldspar, quartz, and mica. Rarely garnet, tourmaline, and hornblende occur in the pegmatites.

 

A dolerite dyke is found to outcrop at Appiyodu. This has a variable width of about six inches to a foot, and is exposed to a length of about thirty feet, in a brook. It dips almost vertically and has a N.W. – S.E. strike.

 

A very conspicuous mass of vein quartz outcrops on the eastern side of the syentite ridge. This has an average width of about 250 feet and apparently trends in a W.N.W. – E.S.E. direction. The quartz is very much weathered and is clearly exposed for a distance of over hundred feet. Similar occurrences of masses of quartz are not uncommon in other parts of the State. Chacko (1919, p.4) refers to similar occurrences in Mundakayam District and considers them as “differentiation products of or segregations in the charnockites of the district”.

 

Chalcedony occurs as pockets and insignificant veins, and is exposed in some of the water channels near about the syenite occurrence. The chalcedony shows different colours in the various outcrops, as milky white, brown and pale blue. Sometimes mottling is also observed.

 

An olive green mineral occurs as a small pocket in a brook at Appiyodu. This mineral has been identified to be apatite.

 

 

 

 

 

• Tertiary

 

• Warkilli beds

 

The tertiary beds in Travancore are represented by fresh-water deposits formed under shallow water conditions, and occurring as extensive outliers practically all along the coast from Quilon to Cape Comorin. A typical stratigraphic section is exposed in the cliff at Warkilli, the type area, and hence the formation is known as the Warkilli formation. At Warkilli, the section has a thickness of about 138 feet (King, W., 1938, p.98) and consists of beds of laterites, sands, and sandy clays alum clays, lignite beds, and sands. The formation is fossiliferous at the type area. However, the formation in the rest of the State is not known to have yielded fossils. A lithologically similar formation of Tertiary age, known as the Cuddallore Series, extends along the Coromandel Coast, and has been provisionally laid down by King (1882, p.92), and Foote (1883, p.25) to be equivalent of the Warkilli formation.

 

In Southern Travancore, sections of the Warkilli formation generally consists of a fewer number of beds than the case in the north, and are as a rule patchy and inconspicuous. Beds generally consist of sandstones, grits, and clays. Lignite is not present.

 

In the area under reference, outcrops of sandstone and grits are exposed at Taingapatam, Amanad, Kizhkulam, Midalam, and Kolachel along the coast. At Taingapatam, soft mottled grits are found in a well section. The grit beds near Midalam show a distinctly conglomeratic character. Deep gullies expose fair thicknesses of coarse conglomeratic mottled grits, capped by red loam. Clay galls are frequently found enclosed in the grit beds. The outcrops, being very small and scattered, are not separately represented in the map.

 

• Sub-Recent

 

• Laterite

 

Laterite is an important rock unit seen abundantly in the stratigraphic column of the area. Two varieties are distinguishable, primary or residual and secondary or detrital laterite. The primary laterite is generally restricted in distribution to higher altitudes, and as much, it is found to cap the archaean crystalline rocks where it is formed by the alteration ‘in situ’ of the rocks concerned. The detrital variety is the top member of the Warkilli beds and is commonly found in the low lying areas, particularly the coast. It may be noted that the two varieties cannot be separated one from the other.

 

In some parts of the area, sub-aerial denudation of the ‘teris’ has exposed large outcrops of primary laterite at the base. These outcrops usually retain the original structures of the gneissic rocks, the alteration of which ‘in situ’ has resulted in the formation of the laterite. Sometimes, remnants of pegmatites and veins are observed in the laterite, and these clearly indicate the primary mode of origin of the laterite. This is further supported by the presence of angular quartz in the material.

 

Two good outcrops of laterite have been noted, one (K 53) on the eastern side of the road near Karingal Camp-shed, and the other (K 48) about 1½ miles north-west of Karingal Market. At K 53, the gradual passage from the massive fresh rock to the soft and finally hard vermicular, ferruginous laterite is remarkably seen. Exfoliation appears to be the initial stage in the disintegration of the rock, which is closely followed by laterisation. The original structures such as foliation, veins and pegmatites are traceable even after the transformation. At K 48, a quartz vein about 4 inches wide is seen in the laterite. The section exposed here is nearly 30 feet thick. On the top, there is about 10 feet of laterite which merges into 15 feet of reddish-yellow and white mottled quartzose clay. Further down, about 5 feet of white quartzose-feldspathic-lithomarge is seen. The quartz grains are strikingly angular, and the interstices between them are filled by the partly kaolinised feldspar. The lithomargic zone should naturally lead imperfectly to the fresh country rock below. Other exposures are seen near Kizhkulam.

 

Outcrops of laterite in the northern half of the area are almost invariably primary in nature as seen from the characteristics already mentioned. However, towards the coast the predominant variety is detrital laterite, and the two varieties usually merge in such a way as to obliterate all possibilities of distinction. Such a situation arises at Amanad near the coast, where excellent sections of laterite are exposed in deep gullies.  In this place, on the seaward side, the laterite overlies the Warkilli beds and hence detrital in nature, and further inland the laterite is primary as shown by its typical structures. Again, the detrital nature of the laterite on the seaward side is supported by the presence of numerous well-rounded pebbles of quartz in it. The zone in-between the two, which is also of laterite, does not show any clue as to its origin. The passage from the one to the other is quite insensible. Though topographic expressions may aid in distinguishing primary laterite in the highlands, lower down, the two types of laterite have more or less flat tops, and hence topographic considerations are not of much help. This apparent confusion may probably due to a general subaqueous erosion in post-tertiary period, when the topographical irregularities in areas of the two types of laterites were removed thereby rendering the general surface even and uniform. In the words of King (1882, p.92) “Whatever form of denudation may have produced the now much worn terrace of the gneissic portion of the country, the same also determined the general surface of the Warkilli beds. Indeed, it gradually dawned on me while surveying this country, having the remembrance of what I had seen of the plateaus and terraced low-land in Malabar in previous years, that here, clearly, on this western side of India is an old marine terrace, which must of later date than the Warkilli beds”.

 

• Recent

 

• Alluvium

 

Though alluvium is found deposited in most of the depressed parts in the area, its occurrence in the form of two, extensive stretches, one in the east-west direction, from the flanks of the Kappiyara Hills up to Taingapatam; and another a smaller one, occupying the eastern part of the area, are notable. The source of the alluvium is largely the weathered products of the crystalline rocks, laterite, and red loam. It is mainly composed of sands, sandy clay and clay with an admixture of various hydrated oxides of iron imparting to it a rather pale reddish-brown colour. When intimately mixed with organic matter the alluvium has an almost black colour.

 

• ‘Teri’ (Red Loam)

 

‘Teri’ is the local term for a peculiar type of red sand hills considered to be Pleistocene to Recent in age. This formation is found to cover extensive areas all over the coastal tract of Peninsular India.

 

In Travancore, the ‘teris’ are confined to the south, and extend as a regular belt from Trivandrum up to Cape Comorin, a distance of about 50 miles. It has a variable width of 2 to 6 miles, the maximum being attained around Nagercoil. The ‘teris’ are found to overlie the older rocks such as laterite, tertiary, and archaean rocks.

 

In the area under reference, three conspicuous patches are found to occur at Amanad, Midalam, and Kolachel respectively. Based on their topographic expression, the ‘teris’ in Travancore have been provisionally classified (Menon, 1950, p.6) into ‘Dome or Ridge teris’, ‘Plateau teris’, and ‘Loose teris’. According to this classification, the ‘teris’ at Amanad which is a compact one, belongs to the class ‘Dome or Ridge teris’ and the ‘teri’ at Kolachel and Midalam being flat, comes under ‘Plateau teri’. Generally, the ‘teris’ are traversed by networks of deep gullies with precipitous walls. At Amanad, where the ‘teri’ is of the ‘Dome or Ridge type’, the pattern of the gullies is remarkably radial. Some of the deep gullies are 35 to 40 feet in depth corresponding to the local thickness of the red loam in that place.

 

In composition, the ‘teris’ mostly consist of sub-angular to rounded quartz grains and magnetic iron ores such as magnetite and ilmenite. Some of the other heavy minerals identifiable under the microscope are zircon, sillimanite, rutile, monazite, kyanite, andalusite, and a few others. The binding material is ferruginous clay. It is remarkable that the ‘teris’, which are mostly deposited in an area of garnetiferous gneisses, do not practically contain garnets. This attracted the attention of Bruce Foote (1883, p.33), who, in his report on the geology of South Travancore remarked thus:

 

“Common as garnet sand is on the beaches of South Travancore, I never yet found a grain of it in the ‘teri’ sand where the latter was pure and had not been mixed with beach sand”.

 

This absence or rarity is largely due to the fact that under tropical weathering, garnet is highly unstable.

 

A large number of the constituent quartz grains appear frosted, evidently due to wind action. Generally, bedding is absent, although this feature is locally seen. This would indicate local activity of fluvial agents in the deposition of the ‘teri’ sediments.

 

The question of the origin of the ‘teris’ is a much disputed one. According to the observations of the writer, the ‘teris’ seem to have been derived ultimately from laterites and other products of weathering of the archaean and tertiary rocks. However, before final deposition the sediments appear to have been reworked. The features of deposition suggest formation under warm, moist conditions, associated in pluvial periods.

 

• Blown Sands

 

The blown sands occupy a narrow strip fringing the coast. Usually quartz grains make the greater bulk of the beach sands. Small amounts of ilmenite, rutile, monazite, garnet, sillimanite and zircon are also found. At Kolachel, there are conspicuous sand dunes which are almost composed of black sands. Further, along the shore at Kolachel, concentration of ilmenite, garnet, and monazite are found. The sources of the sands are, at least in large parts, the archaean crystalline rocks, the tertiary beds and the ‘teris’.

 

3. Economic aspects

 

From the economic point of view, mineral sands, mica and graphite deserve consideration. Regarding the mineral sands, Kolachel is one among the richest places in Travancore. Though the deposit is not being worked at present, it holds considerable potentialities. The sporadic occurrence of mica of the phlogopite variety has been reported from different parts of the area. Prospecting for this mineral was being carried out during the war years, when prospecting centres in the area supplied hundreds of pounds of the mineral. At three places near about Karingal, prospecting operations showed occurrence of graphite. These occurrences are not regular and do not seem to have any continuity. The deposits are not being worked at present.

 

There is an almost inexhaustible reserve of building stones, and road material obtainable from rocks such as the charnockites and the leptynites. These are being actively quarried in a number of places.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

PART TWO

 

 

 

 

 

 

 

 

 

 

 

 

 

PETROLOGY OF THE SYENITE AT PUTTETTI

 

 

 

 

 

 

 

 

 

 

 

 

CHAPTER – III

 

1. Introduction and previous work

 

Although the most general and abundant crystalline rock types of Southern Travancore, including of course, those of the area mapped, are represented by the charnockites and the leptynites, there occurs over a limited area in and around the place called Puttetti in the Eraniel Taluk, a rock type which is apparently distinct from the rest of the crystalline series of the surrounding area.

 

The rock which generally resembles the charnockites, particularly the intermediate variety, on closer examination can be seen to be entirely made up of feldspar and mafic minerals to the complete exclusion of quartz, which feature is perhaps the most outstanding one that distinguishes it from the usual charnockites of the surrounding area.

 

Some parts of this rock massif contain unusually large and beautiful crystals of zircon, and this has been attracting from time to time, the attention of geologists from other parts of India, as well as abroad.

 

No detailed work on the zircon bearing rocks appears to have been attempted by earlier geologists till now, and obviously therefore, previous literature on the subject is very scanty, and what little is available, seems to concern more about the zircon crystals than the associated rock.

 

According to the available literature, the occurrence of this rock was first noticed by Masillamani, a former State Geologist, more than fourty years ago. In his report (1080, M.E.=1911, p.2), he stated “Two new rocks hitherto not known in South Travancore came under my notice in the Eraniel Taluk. One is syenite and the other has not been definitely identified yet. The former occurs in the western border of Eraniel and consists essentially of feldspars, pyroxenes, and limonite, evidently an intermediate facies of the charnockite series. This rock is remarkable in zircon being present in great abundance”.

 

“Closely associated to the syenite is the other rock which consists of feldspar, and a green ferromagnesian mineral which has been identified as pale green augite. The specific gravity of the rock is 3.21 and will indicate it to belong to the basic series”.

 

In the course of the systematic geological mapping of the area dealt with in the foregoing pages, (Part one of this thesis), the writer became interested in making a special study of the zircon-bearing syenite, about which as previously mentioned, only very little information is available. Therefore, during the period1952-1953, the writer camped off and on at Puttetti, surveying the syenite and the associated rocks, examining every accessible outcrop in the area, and collecting suitable samples for subsequent petrographic and chemical analyses. In all, not less than one hundred hand specimens were collected, and an equal number of microslides were prepared by the writer himself, for the purpose of the present investigation.

 

The following pages contain an account of the observations made in the field as well as in the laboratory, based on which an attempt is made to ascertain the genesis of the syenite rock and its possible relation to the charnockite, with which it is seen to be closely associated at Puttetti.

 

2. Field Characters

 

The crystalline rocks in and around Puttetti, when carefully examined in the field, may be seen to be constituted of a few varieties, and may be arbitrarily grouped as:

1. Diopside Granulite
2. Diopside Syenite
3. Diopside Syenite (zircon-bearing)
4. Intermediate Charnockite, and
5. Acid Charnockite

 

On a broader basis, these five arbitrary varieties could conveniently be brought under two main divisions, the Syenite suite, comprising of the first three varieties, and the Charnockite suite, consisting of the intermediate and acid varieties of charnockite. Examination of the outcrops whatever they occur side by side, indicate a merging of one variety with the other. Hence the five varieties are considered to be local facies having many features in common, and believed to have originated from a common magma.

 

In the accompanying map (No.2), tow shades of the same colour are given for a single rock suite. The darker shade represents localities where actual outcrops are present, while the lighter shade corresponds to places where extension of the same rock suite is inferred upon field evidences, though outcrops are absent.

 

A glance at the map would give a general idea about the distribution of the two main rock suites, in the area. The Syenite suite of rocks, consisting of three varieties already mentioned, are seen as three ridges, the one on the north being the largest. This and the smaller ridge on the west, strike N.N.W., while the southern ridge strikes almost N.W. in general, the ridges may be said to lie in a position, coinciding with the regional strike of the crystalline rocks of Southern Travancore. The largest ridge has a length of about three-fourth of a mile and a maximum width of about 800 feet attained near the northern end. The largest individual outcrop in the ridge is about 750 feet long and 375 feet broad.

 

Surrounding the ridges composed of the syenite suite of rocks, and also occupying practically the rest of the area surveyed, is the charnockite suite, represented mostly by the intermediate variety and to a very small extent by the acid variety. Outcrops of charnockite are seen only at a few places. These are invariably small in size, and appear to crowd towards the western side of the main syenite ridge.

 

It is noteworthy that nowhere in the area, any distinct contact between the syenite suite and the charnockite suite of rocks could be observed. On the contrary, in a few places, especially bordering the northern half of the main ridge, a gradual merging of the syenite with the charnockite, could be noticed. In the vicinity of Appiyodu, and in two other places, (K8 & K9; K6 & K21) a traverse from the syenite ridge towards the nearest outcrop of charnockite would reveal traces of quartz gradually appearing in the peripheral part of the syenite body. On account of paucity of outcrops between the syenite and the charnockite, it was not possible to observe the nature of the interlying rock.

 

The five varieties of rocks, which are grouped under syenite and charnockite suites, as already mentioned have certain common features, but at the same time display some features characterizing each variety and these are summarized below.

 

Syenite Suite		1. Diopside Granulite 2. Diopside Syenite 3. Diopside Syenite (zircon-bearing)
Charnockite Suite		1. Intermediate Charnockite 2. Acid Charnockite

 

• Syenite Suite

 

Although diopside granulite is grouped as a separate variety, it is not seen to occur as a separate rock body. In the area under reference, this variety is found outcropping as bands and irregular patches at four places in the diopside syenite of the main ridge, as marked in the map. These are without any clear-cut contact with the host rock. The rock is typically medium-grained, greenish-gray coloured, and composed of feldspar and ferromagnesian minerals. The distribution of the constituent minerals is such as to impart a granulitic appearance to the rock. Frequently, the ferromagnesian mineral grains show a tendency to coalesce and form conspicuous clots, dark basic patches, and segregations.

 

The second variety, diopside syenite is more or less identical with the previously mentioned diopside granulite in the matter of mineralogy. However, in structure and texture it is notably different. It is typically a greenish-gray coloured, coarse-grained, generally foliated variety, and while foliation is clearly developed (Plate I) in most of the outcrops, in some the foliation is less marked. The foliation is evidently the result of the parallel alignment of the ferromagnesian minerals. This variety constitutes the greater part of the syenite suite of rocks and occurs in all the three ridges. In the field, the rock is found associated with diopside granulite in certain outcrops, and with the zircon-bearing diopside syenite in certain others. In all these cases, the rocks concerned appear to merge with each other.

 

The next variety is the zircon-bearing diopside syenite. It is a coarse-grained, greenish-gray rock, essentially composed of feldspar and a ferromagnesian mineral. It contains large, well-developed crystals of zircon, and this feature distinguishes the rock from the previously described diopside syenite. Some flakes of mica are also noted. Foliation is generally absent. Though the constituent minerals do not appear conforming to any particular arrangement in hand specimens, in the field it is possible to notice a crude type of foliation marked by the ferromagnesian mineral. The zircon crystals are distributed haphazardly, and while these occur in large numbers at some places, they are not apparently seen at other spots in the same outcrop. Towards the periphery of the man ridge, a very careful examination has revealed traces of quartz to be present.

 

• Charnockite Suite

 

Intermediate charnockite is well represented in the area, but outcrops are comparatively few and small in size. The largest outcrop occurs between the two syenite ridges on the eastern half of the map. Since most of the charnockite exposures in the area are being actively quarried, it was possible to examine fresh quarry faces in a number of instances. The rock is compact, tough and has a grayish-blue colour. It is essentially composed of feldspar, quartz, and hypersthenes, with subordinate amounts of garnet and mica. In texture, the rock is medium to coarse-grained. Abrupt changes in texture are a remarkable feature, and can be seen in hand specimens. Such coarse-grained portions of the rock show a striking similarity in appearance to the typical syenite. Sometimes, foliation is markedly displayed and is brought about by the linear arrangement of the dark constituents. Since all the charnockite outcrops are widely separated from the syenite ridges, it was not possible to observe their mutual relations in the field.

 

Compared to the intermediate charnockite, the acid variety is light coloured and is represented only to a very small extent in the area. Though, mineralogically, the two varieties are almost similar, the acid variety is characterized by a lesser amount of the ferromagnesian mineral, with practically no garnet. It is a compact and tough rock with a medium texture. Outcrops are very few, located far apart, and do not enable to observe their relation with the other rock types.

 

In addition to the rocks described above, other rocks occurring in the area include a dolerite dyke (?) and a massive quartz vein. The dolerite dyke has a variable width of six inches to a foot and is exposed in a brooklet for a length of about ten feet. Beyond this, it covered by a thick layer of alluvium. The dyke strikes in a N.W.-S.E. direction. The quartz vein is marked on the eastern side of the map (No.2). It is exposed on the side of a low hill for a distance of about 100 feet and has a width of about 250 feet. Since it gradually passes into lateritised material, the boundaries are not clearly seen. The quartz is impure and is in an advanced stage of disintegration.

 

3. Weathering

 

The effects of weathering on the surface of the rocks of the syenite and charnockite suites are typical of each group, and this feature considerably helps to distinguish rocks of any one suite from the other in the field, even when fresh exposures are absent.

 

When compared with the charnockite, the syenite is in an advanced state of weathering and this is easily explained in virtue of the high feldspar content and coarse texture of the former. Being thus easily prone to weathering, the effects have apparently penetrated to considerable depths in the syenite ridge, so that the deciphering of the mutual relations among the members of the syenite suite is very much hindered. The weathered surface of the syenite has a grayish-white colour, and is full of small pits and irregular depressions, obviously caused by the removal of the weathered products. Frequently, cavernous hollows (Plate II & III) are formed as a result of differential weathering and solution. Exfoliation, and weathering along joints which causes blocks of the syenite to be isolated from the rest of the rock, are common. The isolated blocks disintegrate and form talus at the base of the ridge (Plate IV & V).

 

Exfoliation and spheroidal weathering are characteristic features of weathering in the charnockites. The freshly exposed surface after the removal of the overlying exfoliated layer is comparatively even and smooth (Plate VI). Spheroidal weathering is clearly seen in the loose boulders.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

CHAPTER – IV

 

1. Petrography & Mineralogy

 

The arbitrary division of the crystalline rocks of Puttetti into five varieties or types was already mentioned under Field Characters. This division is evidently found to facilitate in great measure their petrographic descriptions also. The scheme containing the two major suites and their sub-divisions is repeated below for ready reference.

 

Syenite Suite		1. Diopside Granulite 2. Diopside Syenite 3. Diopside Syenite (zircon-bearing)
Charnockite Suite		1. Intermediate Charnockite 2. Acid Charnockite

 

• Syenite Suite of Rocks

 

• Diopside Granulite

 

1. Megascopic Characters

 

In hand specimens, the rock is seen to be composed essentially of a greenish-gray pyroxene, and a greenish-white feldspar. These two constituents are present in almost equal amounts, and are uniformly distributed. The rock is medium-grained, and the disposition of the constituents is such as to give it a typically granulitic aspect. However, in a few specimens, the pyroxene as well as the feldspar grains show a tendency to occur in clusters thereby imparting a crude gneissic appearance to the rock. No other mineral is identifiable with the naked eye. But a few specks of a brownish mineral resembling sphene, are seen under a magnifying glass. The rock has an average specific gravity of 2.98.

 

1. Microscopic Characters (Typical Slide Nos. K16, K19, K24A, K24B, K24C, K24D, K24E, K24F, K41A, K41B, and K41C)

 

In thin sections, the rock has typically a granular texture, and the grains are more or less of the same size. It is seen to consist of feldspar and diopside in almost equal proportions. The other ferromagnesian minerals found are hornblende, sphene and mica. The minor accessories include zircon, iron ores, apatite, calcite, and a little secondary quartz. Original quartz is absent.

 

Feldspar is mostly microperthite, and the characteristic spindles can easily be distinguished under a moderate magnification. Often a single patch of microperthite is observed to contain a few relicts of the clear non-perthitic host feldspar (K19 & K41C). Small amounts of orthoclase also occur. Generally, the feldspars show undulose extinction, which may be due to the effect of strain. Calcite of a secondary origin is found developed along some of the cracks in the feldspar grains. The optical constants of the microperthite are as follows:

 

Extinction relative to the basal cleavage is 10º

2 V = 80º ± 2.

Optically negative

Diopside is light green in colour, non-pleochroic, and does not show any idiomorphic outline. Some of the grains have diallagic cleavages, and certain others contain numerous cracks. The diopside often completely encloses patches of feldspar, a feature which would indicate a secondary origin for the mineral. Many grains contain strings of bubbly inclusions with a parallel alignment. The mineral has the following optical properties:

 

Colour – pale green to green

Pleochroism – nil

Extinction – Z Λ C = 35º to 40º

2 V = 58º ± 2.

Optically positive

 

Hornblende is of a pale yellowish-brown colour, and is strongly pleochroic in shades of brown. Grains do not exhibit idiomorphism, and they appear to be secondary in origin after the diopside (Hallows, 1923, p.258) and mica. Irregular patches of clear feldspar, mica, and occasional granules of apatite, are often found enclosed in the mineral. Optical properties of the mineral are as follows:

 

Colour – pale yellowish-brown

Pleochroism pronounced with the following colour schemes:

X = pale yellow

Y = yellowish-brown

Z = dark brown

Absorption – X < Y < Z

Extinction – Z Λ C = 35º ± 2.

2 V = 62º

Optically negative

Sphene is honey brown in colour and is strikingly pleochroic. It is anhedral, and usually occurs either in the plates of feldspar or pyroxene, or at their contact. The grains show high relief, and are usually full of cracks which contain opaque inclusions of iron ores. The optical properties are as follows:

 

Colour – honey-brown

Pleochroism:

X = pale yellow to colourless

Y = pale greenish-brown

Z = honey-brown

Biaxial and optically positive

2 V could not be determined due to the small size of the grains.

 

Mica is generally colourless to pale yellow, and is strongly pleochroic. It is mostly seen to occur in close association with diopside and hornblende, and shows corroded and irregular margins, (K19 & K41C) (Plate VII, Fig. 3). The mica appears to be primary in origin, and has the following optical properties:

 

Colour – colourless to pale yellow

Pleochroism:

X = colourless to pale yellow

Y = yellow

Z = yellowish-brown

Extinction – straight

 

Zircon, iron ores, apatite, calcite and a little secondary quartz go to form the assemblage of minor accessories. Grains of zircon, with their corners more or less rounded, are seen in some of the sections (K41A & K41B). Iron ores include ilmenite having the characteristic pitted appearance, visible under reflected light. Platy inclusions of colourless apatite are found in the feldspar and also in the ferromagnesian minerals. Calcite and blebs of quartz are found along cracks in the feldspars.

 

• Diopside Syenite

 

1. Megascopic Characters

 

The diopside syenite is a medium to coarse-grained, greasy-gray, massive rock consisting essentially of feldspar and a mafic mineral (diopside), the former being predominant. The mafic mineral occurs either as bands of variable width having a rough parallelism with the general regional strike (N.W.–S.E.), or as clusters and segregations showing all sorts of odd shapes. The feldspar, where it is in contact with the mafic mineral, is sometimes stained brown, apparently caused by the alteration of the latter. The mafic mineral is greenish-black in colour, and individual anhedral grains vary in size from pin-points to those three to four centimeters in diameter. A few grains of pyrrhotite are also occasionally present. The average specific gravity of the rock is 2.8.

 

1. Microscopic Characters (Typical Slide Nos. K8A, K8B, K8C, K8D, K32A, K32B, K32C, K32D, & K32E)

 

Thin sections of the rock exhibit a coarse granular texture (Hypautomorphic-haphazard-granular, Johannsen, 1949, Vol. III, p.54), the individual grains of the minerals being mostly anhedral. Feldspar constitutes about 50 to 60 percent of the bulk of the rock, the rest being composed mainly of diospide. The minor accessories are mica, pyrrhotite, apatite, calcite, and secondary quartz. Original quartz is absent.

 

As in the case of the diopside granulite, the feldspar forms the most important constituent, and is largely represented by microperthite. The perthitic intergrowth is so fine, that it can be distinguished only under moderate magnification and optimum conditions of illumination. A feldspar which is non-perthitic is also present, and the values of its optical constants indicate it to be a plagioclase of the oligoclase variety. The absence of twinning is an interesting feature of the plagioclase. There are strings of minute globular inclusions noticeable in the body of the feldspar grains. The non-perthitic feldspar is in part found to be orthoclase. The optical characters of the feldspars are as follows:

 

Microperthite

Extinction relative to the basal cleavage is 10º to 13º.

2 V = 78º ± 2.

Optically negative.

 

Oligoclase

2 V = 86º

Optically negative.

 

Orthoclase

2 V = 70º ± 2.

Optically negative.

 

Diopside is pale green in colour, non-pleochroic, and does not show crystal outlines. Diallagic cleavages are seen in some of the grains. The body of individual grains of the mineral in many instances contains numerous cracks. Inside the cracks, a somewhat brownish, fibrous, pleochroic mineral is seen. The optical characters of the mineral could not be discerned. It has a high birefringence, and may probably be a fibrous variety of hornblende. Frequently, grains of diopside are found to enclose irregular plates of feldspar, and this feature suggests a secondary origin for the diopside after feldspar (K8A, K8C, & K8D). These feldspar remnants show wavy extinction effects which recall to mind a crude resemblance to the pattern of spherulitic structure. Secondary calcite is often found filling the cracks in the diopside. Optical characters of diopside are as follows:

 

Colour – pale green

Pleochroism – nil

Extinction – C Λ Z = 39º

2 V = 60º ± 2.

Optically positive.

 

Minor accessories

 

Mica is of the biotite variety, and appears to be gradually transforming into diopside (K8A, K8C, & K8D), as is inferred from the corroded and embayed nature of the margins of mica. It is pale yellow in colour and has a pronounced pleochroism. Pyrrhotite is another accessory, and is found only in some of the slides. It usually occupies the cracks of other minerals and has the characteristic bronze-yellow colour under reflected light. Apatite is colourless and is found as inclusions in the feldspar and diopside. Calcite (K8B, K8C, & K8D), in most cases is associated with blebs of released quartz.

 

• Diopside Syenite (Zircon-bearing)

 

1. Megascopic Characters

 

The rock has a greenish-gray colour. Most of the specimens examined are distinctly coarse-grained, although a few tend to be medium in texture. The latter specimens (K14 & K21) are strikingly similar to some of the medium-grained charnockites in appearance. Mineralogically, the rock is composed of feldspar, and a greenish-black ferromagnesian mineral (diopside). The feldspar constitutes more than seventy percent of the entire bulk of the rock. Minor accessories that can be distinguished in hand specimens are zircon, mica, sulphide and iron ores, the latter two being magnetic. Large and well-developed crystals of zircon are seen in some of the hand specimens.

 

The feldspar has a greenish-gray colour, which largely contributes to the general colouration of the rock also. Hand specimens of the rock are frequently seen to be composed entirely of feldspar alone, or in association with a subordinate amount of diopside. The diopside is non-uniformly distributed, and when present, is found either as individual grains or as ill-defined aggregates which widely vary from 0.5 to 5.0 centimeters along the longest direction. The sulphide ores have very much altered resulting in the formation of a dirty looking limonite stain on the surface of the rocks. On being treated with diluted hydrochloric acid, some specimens produced effervescence along cracks. This evidently indicates the presence of calcite as has been subsequently confirmed by microscopic examination of thin sections. The average specific gravity of the rock, determined on twelve specimens, is 2.75.

 

1. Microscopic Characters (Typical Slide Nos. K1A to K1N, K2A to K2L, K3A to K3L, K6A to K6E, K7, K14, K20, K22, K29, K30A, K30B, K34 and R.S. 55)

 

On account of the coarse texture of the rock, and the sparse distribution of the minerals present, it was not possible to observe all the constituents together in any single slide under the microscope. Therefore, more than fifty slides of the rock were examined to determine the important properties of the various mineral constituents. As seen from the numerous slides, the appears to have a typical texture of a syenite, referred hypautomorphic-haphazard-granular (Johannsen, 1949, Vol. III, p.54). Microscopic study revealed that the rock is essentially composed of feldspar and a ferromagnesian mineral which is pale green to green in colour.  Other ferromagnesian minerals, found to occur in smaller quantities, are hornblende, sphene and mica. The minor accessories include zircon, sulphide ores, iron ores, apatite, calcite and secondary quartz. Original quartz is absent.

 

It may also be mentioned, that the rock appears to have undergone metamorphism subsequent to its emplacement, as may be inferred from the occurrence of secondary minerals, and other features to be referred to in the following pages.

 

Feldspar is the most important constituent of the rock, making up nearly seventy five to eighty percent of the total bulk. The feldspar includes microperthite, plagioclase and a little orthoclase. The plagioclase seems to belong to more than one generation, the chemical composition correspondingly ranging from albite to andesine. However, oligoclase is the most common.

 

Microperthite is the most conspicuous mineral present in every one of the slides examined. The fine, sharp and highly regular microperthitic structure is visible under a moderate magnification of about seventy diameters. The perthitic structure is best seen on (010) cleavage flakes. Certain optimum conditions of illumination are necessary for the structure to be visible in ordinary light.

 

A fairly bright source of light is essential, and this should be ‘stopped down’ with the substage diaphragm to enable maximum visibility of the fine intergrowth. Even the relatively coarse microperthite become invisible or indistinct, if the illumination is not suitably adjusted. Further, the perthitic intergrowth appears more distinct under crossed nicols, with the mineral section near its position of extinction. Plate VII, Fig. 1 shows the mineral as it appears under ordinary light. The faint line which runs obliquely across the figure is the trace of the basal cleavage. It is observed that the fine spindles of the intergrowth make an angle of 73º with the trace of the basal cleavage. The individual spindles are spear-shaped, and have comparatively higher index of refraction. Consequently, they stained out in relief, giving the whole patch a furrowed appearance.

 

Some of the feldspar individuals appear to have the microperthitic structure developed only in patches (K6C & K6D), leaving relicts of the clear host feldspar all around. The patchy nature of the microperthite may probably be due to differential or selective abstraction of the calcium necessary for the formation of other calcium-bearing minerals. It might as well have taken place during a period when the rock got the present metamorphic impress. Further, there seems to be some sort of a gradation in the development of the microperthitic structure. In some of the slides, it is visible only very faintly, even under the optimum conditions of illumination. Such intergrowths may probably approximate to the submicroscopic variety of perthite in the feldspars of the Norwegian Syenites (Brogger, 1890, pp.524-551).

 

Most of the microperthite show undulatory extinction, suggesting possibly the effect of stress. The mean extinction angle with reference to the trace of the basal cleavage ranges from 10º to 12º. The value obviously represents the average extinction of the two sets of spindles (soda & potash) that have intergrown to form the microperthite. The two sets of spindles appear to extinguish almost simultaneously. The relatively higher index of refraction of one set of lamellae, suggests that it may be a member of the plagioclase series which has a higher index of refraction.

 

The optic axial angle of the microperthite was determined on the Leitz Universal Stage. The value obtained, ranges from 75º to 83º. The sign is negative.

 

Following the work of Bogglid (cited by Spencer, 1930, p.305), who endeavoured to deduce a progressive relationship between the extinction angle and chemical composition of schillerised potash-soda feldspars collected from different localities, Spencer (1030, p.305) has observed and described a similar relationship in perthitic feldspars. Figure 1 (reproduction of Fig. No. 4, Spencer, 1930, p.342) refers to this relationship. According to this graph (Fig. 1) for an angle of extinction of 10º to 12º as obtained for the microperthite from the rocks under study, the percentage of soda appears to range from 38 to 51 percent.

 

Spencer (1030, p.344) have given another graph regarding the relationship existing between optic axial angle and composition. The optic axial angle determined for the microperthite under study ranges from 75º to 83º and according to the above graph (Fig. 2) (reproduction of Fig. No. 6, Spencer, 1930, p.343), the percentage of soda is found to vary from 40 to 50 percent.

 

The plagioclases are very rarely twinned. Twins, wherever developed, are so fine as to escape notice. In other cases, both albite and pericline twins are present. Bent twin lamellae are also seen, suggesting the effect of stress (Plate VIII, Fig. 1 & 2). The effect of stress has also caused the bending of the cleavage traces, which should normally b parallel and straight (Plate VIII, Fig. 3 & 4).

 

The values of the optical constants, suggest that the various plagioclase feldspars belong to different generations, and vary in composition from albite to andesine, oligoclase being the most common variety. Zoning of a faint nature was noticed in two of the thin sections.

2 V = 86º ± 2.

Optically negative.

 

Orthoclase is the common potash feldspar, and is found in close association with the microperthite and plagioclase. The optical constants are as follows:

 

2 V = 68º

Optically negative.

 

The feldspars usually have numerous opaque acicular inclusions. They are mostly without any definite orientation. But in some of the sections, they are arranged remarkably parallel to one another, and may be easily mistaken for cleavage traces. Some of these inclusions have a rhombic outline (K1B), and a brown colour, and may be haematite (Rosenbusch, 1903, p.165). Needles of another mineral, presumably rutile, also occur as inclusions.

 

The feldspars are altered to varying degrees, rendering their thin sections turbid when viewed under ordinary light. Alteration seems to have started along cracks and cleavages. Minute scales of sericite (?) (Krishnan, 1926, p.395) and white mica (?) are also noticed on the altered plagioclases in a few instances (K1B, K1C, K1D & K20). They exhibit a “spangled effect” (King, 1947, p.47) on rotation of the microscope stage under crossed nicols. Sometimes, small patches of plagioclase, which are more acid than the usual plagioclase of the rock, are observed. These are almost always fresh, and found associated with calcite and secondary quartz. The above mentioned patches of plagioclase evidently seem to be secondary in origin.

 

The specific gravity of the feldspar, as determined on a sample that was crushed and carefully handpicked under a binocular microscope, was found to be 2.61. The same sample was subjected chemical analysis, the result of which is given in Table VI. From the chemical analysis, it is obvious that the amounts of soda and potash are practically the same in the feldspar. The source of the phosphate in the analysis is evidently the apatite found in the feldspar.

 

Diopside is the most important ferromagnesian mineral in the rock. In thin sections, it is anhedral, pale green to green in colour and non-pleochroic. Some of the grains show diallagic cleavages; and others have numerous cracks. Frequently, the diopside is seen to enclose feldspar with irregular margins, and this relation tends to suggest a secondary origin for the diopside after the feldspar. The microperthite and plagioclase could possibly have contributed to the formation of the diopside. The disposition of some of the diopside grains, with ill-defined and irregular margins, may indicate that the adjacent feldspar has contributed some material such as calcium, necessary for their formation. The calcium having been thus abstracted, the plagioclase seems to have become more acid. This has been supported by the values of the optic axial angle determined on such patches.  Blebs and patches of plagioclase thus formed can be seen associated with released quartz near the frayed margins of the diopside (K1F). Another feature noted is that patches of microperthite adjoining diopside grains are devoid of perthitic intergrowth towards their contact. This may possibly be explained as being due to the depletion of calcium from the microperthite.

 

Alteration of the diopside has resulted in the formation of hornblende (K1D, K1L, K1M, K2B, K2C, K2E, K2L, K3B & K3E). The change appears to have originated along the cracks and cleavage traces of the diopside. The green colour of the pyroxene changes to greenish brown. The hornblende thus formed is pleochroic. These transformations in the diopside are often accompanied by the release of quartz and calcite. Sphene is also frequently seen in close association with the diopside (K1F & K3E). The diposide has the following optical constants:

 

Colour – pale green to green.

Pleochroism – nil

Extinction – C Λ Z = 38º

2 V = 60º ± 2.

Optically negative.

 

A piece of the fresh rock was crushed and about 5 grammes of diopside, almost free from inclusions, were carefully hand-picked under the binocular microscope. The specific gravity of the material was determined to be 3.48. The same material was chemically analysed. The result of the analysis is given in Table VI.

 

Hornblende is invariably of secondary origin (K1D, K1L, K1M, K2E, K2L, K3H & K3I), after diopside, mica and feldspar (Ghosh, 1941, p.47). It is dirty pale green in colour and is pleochroic. The following are the optical properties:

 

Colour – pale green.

Pleochroism:

X = yellowish brown

Y = pale greenish brown

Z = dirty greenish brown

Extinction – C Λ Z = 18º to 27º

Birefringence = 0.022 (Berek Compensator)

2 V could not be determined due to the high absorption.

Absorption – X < Y < Z.

 

Sphene is irregular in distribution and is honey-brown in colour. This mineral is often found associated with the ferromagnesian constituents, and sometimes is enclosed by them (K1F, K3H, K6D & K6F). Slides examined under the microscope do not reveal any indication as to the exact nature of its origin. However, grains of the mineral do not show crystal outlines, and are frequently seen along with calcite and diopside. Some of the grains are full of cracks, containing dark opaque particles of iron ores. Under the microscope, the mineral is recognized by its high relief, strong birefringence and marked pleochroism. In some of the sections, it is also found altering to leucoxene (K1F & K6D). further, diopside is seen to be developed at the expense of sphene, which may probably be the source of the calcium necessary for the formation of diopside. This is often accompanied by the release of iron ores (K1F, K6D & K6E).

 

Optical properties:

 

Colour – honey-brown.

Pleochroism marked with the following colour scheme:

X = pale yellow to colourless

Y = pale greenish-brown

Z = honey-brown

Biaxial and optically positive. The optic axial angle could not be measure due to the small size of the grains.

 

Mica is represented by biotite, and appears to be original. In some of the sections, the mineral is absent. It is usually pale greenish-brown in colour, with pronounced pleochroism. The occurrence of flakes of the mineral as cores and fringes of other minerals, particularly the ferromagnesian minerals, suggest that it is an original constituent.

 

Zircon is seen a well-developed crystals (Plate X, Fig. 1), of a brown colour. Under the microscope, it has a pale brown colour, and is easily distinguished by its high relief and strong birefringence. Some of the sections give good interference figures, which are uniaxial positive. A detailed account of zircon is given in Appendix I.

 

Sulphide and iron ores include pyrrhotite, ilm,enite and magnetite. Pyrrhotite shows the characteristic bronze-yellow colour, in thin sections under reflected light.in some of the sections studied, the mineral is found filling the cracks in the feldspars and diopside (K3B, K3D, K3F, K3J & K6E) (Plate VII, Fig. 2).

 

Ilmenite, in thin sections, shows the typical silvery-gray colour and pitted appearance under reflected light. In specimen K14, the mineral shows an abundant and almost even distributed throughout. Some of the grains show idiomorphic outlines. The usual alteration product is leucoxene.

 

Apatite is a minor accessory, and is found as platy inclusions in the feldspars, and also in the ferromagnesian minerals. The grains are usually anhedral, and are colourless. Distinguished in thin sections by the characteristic pitted appearance (K1B, K1D, K2C, K6A, K6B, K6C, K6D & K6E), high relief and low polarization colours (Plate X, Fig. 3). Basal sections are isotropic between crossed nicols, and some of them give a negative uniaxial figure. Grains of the mineral picked out from the crushed specimens, were treated with hydrochloric acid (Dana, 1949, p.704). On examination with a hand lens, oily bubbles of hydrofluoric acid were found floating, and the grains completely dissolved in the hydrochloric acid. This indicates that the mineral is the common flourapatite.

 

Calcite and quartz, both secondary, are often found together. Calcite has resulted from the plagioclase and also possibly from sphene (?). It is found as minute grains, scattered all over the body of the altered plagioclases, and also seen as irregular patches filling the cracks of the same. Formation of the diopside at the expense of the feldspars, seems to have resulted in the production of calcite, which is often associated with blebs of secondary quartz.

 

• Charnockite Suite of Rocks

 

• Intermediate Charnockite

 

1. Megascopic Characters

 

The rock is greenish to bluish-gray in colour and has a greasy lusture, and consists of feldspar, quartz, hypersthene, garnet and mica. It is a compact, tough, and medium to coarse-grained rock. Both foliated as well as faintly foliated types are commonly met with. Whenever foliation is present, it is usually characterized by a somewhat parallel arrangement of the garnet and mica.

 

The feldspar dominates over the rest of the minerals, and has a bluish-gray colour. Quartz occurs in subordinate amounts, and is coloured almost like the feldspars. Hypersthene is represented by a few grains, the typical bronze-brown colour of which enables easy identification in a hand specimen. The garnet is pink coloured, and is almost found only in the foliated variety of the rock. Mica has a brownish colour and is identified as biotite. The average specific gravity of the rock is 2.74.

 

 

1. Microscopic Characters (Typical Slide Nos. K45A, K45B, K45C & K49A)

 

Under the microscope, the thin sections of the rock generally exhibit a xenomorphic granular texture (Plate IX, Fig. 1). The constituent minerals are equigranular. Besides feldspars, quartz, hypersthene, mica and garnet, it is also possible to see iron ores, apatite and zircon in the slides. None of the above minerals, except the minor accessory zircon, is idiomorphic.

 

The quartz is seen to occur either as interstitial grains, or as blebs enclosed in the feldspars and the ferromagnesian minerals. Some of the quartz grains display undulose extinction suggesting strain effects.

 

Feldspars constitute more than fifty percent of the bulk of the rock. Plagioclase is the predominant type and is in part twinned according to the albite and pericline laws. The majority of the plagioclase grains are untwined. A characteristic feature in some of the twinned plagioclase is the tendency of individual lamellae to tapper towards one end. Microperthite, and a little orthoclase are also represented in the feldspar assemblage. Rarely antiperthite is also found. Myrmekite is seen occasionally developed (K49A). The plagioclase ranges in composition from albite to labrodorite. The optical characters of the feldspars are mentioned below:

 

Albite:

 

2 V = 70º ± 2.

Optically positive.

Oligoclase:

 

2 V = 86º

Optically negative.

 

Andesine:

 

2 V = 86º ± 2.

Optically positive.

 

Labrodorite:

 

2 V = 77º ± 2.

Optically negative.

 

Orthoclase:

 

2 V = 68º ± 2.

Optically negative.

 

While hypersthene is seen to be the important ferromagnesian mineral in charnockite of the non-garnetiferous variety (K45), it is only sparingly present in the garnetiferous variety. It is pale yellowish-green in colour, and is pleochroic. In some of the microsections, particularly that of the garnetiferous variety, a fibrous, brownish mineral can be seen occurring along cracks. It appears to be secondary hornblende. Optical properties of the mineral could not be discerned on account of the fibrous habit of the mineral. The optical properties for the hypersthene are as follows:

 

Colour – yellowish-green.

Pleochroism:

X = pink

Y = pale yellowish-green

Z = greenish-blue

Extinction – straight

Birefringence – 0.014 (Berek Compensator)

2 V = 64º

Optically negative.

 

Hypersthene is found altering to chlorirte (K45B), having a dirty green colour. A few grains of hypersthene are found to enclose patches of mica and feldspar, which feature may probably imply a secondary origin for hypersthene. However, conclusive proof as to the exact origin is wanting, in the slides examined.

 

Garnet is a conspicuous constituent in the garnetiferous variety (K49A) of the charnockite. In this rock, it would seem that the rarity of hypersthene is made good by the relative abundance of garnet. The garnet is pale pink in colour, anhedral and isotropic. It frequently encloses quartz, feldspar, apatite and zircon. The presence of pools of quartz and feldspar inside the garnet seems to indicate a secondary origin for the garnet.

 

Mica is present in both garnetiferous and non-garnetiferous varieties of the intermediate charnockite and has a brownish-black colour. Some of the flakes contain patches of feldspar with corroded borders, thereby showing that the mica is secondary in origin after the feldspar, the necessary iron being probably derived from the adjacent grains of iron ores. A single perfect cleavage parallel to the base (001) is clearly visible. The grains are highly pleochroic in shades of brown. The optical properties are given below:

 

Colour – Brownish-black

Pleochroism – pronounced with the under-mentioned colour scheme:

X = yellow

Y = reddish-brown

Z = dark opaque brown

Absorption – Z > Y > X

Extinction – straight

Optically negative.

 

The minor accessories are represented by iron ores, apatite and zircon. Iron ores are found as irregular grains. Apatite occurs as large, irregular plates abutting against the hypersthene, and is sometimes enclosed in the plates of feldspar. It clearly shows the characteristic pitted appearance, and has a high relief. Zircon occurs mostly as sub-rounded crystals, and is highly birefringent. It is very common in most of the slides examined.

 

• Acid Charnockite

 

1. Megascopic Characters

 

Similar to the intermediate charnockite, the colour of this rock is greenish to bluish-gray, but of a lighter shade. It is very hard, tough, and has a waxy lusture. When hammered, it makes a clinking sound. Fractured surfaces are subconchoidal. Mineralogically, it consists of almost equal parts of feldspar and quartz, with subordinate amounts of ferromagnesian minerals such as hypersthene and mica. The ferromagnesium minerals are scattered wide apart, and do not assume as much prominence as they do in the intermediate charnockites. The average specific gravity of the rock is 2.64.

 

1. Microscopic Characters (Typical Slide Nos. K9, K10A, K10B, K18A, K18B & K18C)

 

In addition to the constituent discernible megascopically, the rock, in thin sections, is seen to consist of accessory minerals such as iron ores and zircon. The texture is xenomorphic-granular (Johannsen, 1949, Vol. III, p.54). The feldspars formed about half the bulk. An equal amount of quartz is also present. None of the minerals possesses crystal outlines. As seen under microscope, the rock appears to be a typical granulitic type (Plate IX, Fig. 2) of acid charnockite.

 

The feldspars consist of microperthite, and a lesser amount of orthoclase. The miccroperthite appears remarkably similar to that observed in the case of the syenite described previously. Optical determinations tend to show, that most of the non-perthite feldspar in the slides are orthoclase. An effect of strain, to which the rock has been subjected, is evidenced by the undulatory extinction shown by the feldspars in general. The optical properties are as follows:

 

Microperthite:

 

2 V = 75º to 82º.

Optically negative.

 

Orthoclase:

2 V = 70º ± 2.

Optically negative.

 

The quartz grains occur as large plates of fairly uniform size. It is a primary constituent, and occupies the space left over by the feldspar and ferromagnesian minerals. Frequently blebs of quartz are enclosed in the feldspar grains. Most of the quartz grains exhibit strain effects causing undulatory extinction. Minute, opaque, dust-like inclusions are often seen in the quartz plates.

 

Hypersthene is the same plae yellowish-green and pleochroic variety, occurring in the intermediate charnockite. Unlike as in the intermediate charnockite, the number of grains in the acid variety is fewer. Here also, a brownish, fibrous mineral considered to be secondary hornblende is found to occupy cracks. Optical properties of the hypersthene are as follows:

 

Colour – pale yellowish-green.

Pleochroism, marked as in the intermediate charnockite.

X = pale pink

Y = yellowish-green

Z = greenish-blue

Extinction – straight

Birefringence – could not be determined due to the altered nature of the grains.

2 V = 62º ± 2.

Optically negative.

 

Mica is of the usual variety – biotite. A few flakes could be discerned in slides (K9, K10), and it is possible to see the trace of the basal cleavage (001). The mica is pleochroic, and shows straight extinction.

 

There are mainly iron ores and zircon (as minor accessories), the former being more abundant. Some of the grains of the iron ores are frequently seen altered to leucoxene – an opaque, dirty, brownish-white substance. This obviously indicates the presence of ilmenite in the iron ores.

 

Zircon grains are comparatively few, and are sub-rounded. These exhibit high order interference colours, and stand out in high relief in contrast to the enclosing feldspar and quartz grains.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

CHAPTER – V

 

1. Chemical and Modal Analyses and their Interpretation

 

Chemical analyses are deemed to be of immense value in comparing groups of igneous rocks. For this purpose, the usual practice is to compute from the chemical analyses and the Niggli symbols, normative minerals and other relevant values, as required for the problem in hand.

 

The data of the chemical analyses may be directly used to construct variation diagrams. Such a diagram will illustrate graphically many characters of a group of rocks that is investigated. The values for the different oxides are plotted as ordinates against those for silica as abscissae.  In the place of the weight percentages, it is the practice among petrologists of the Niggli school to plot the Niggli symbols (al, fm, c, alk) as ordinates against the Si values.

 

With a view to examine the possible chemical relationship among the five interesting rock types from Puttetti, chemical analyses of fresh samples representing each of these five types, viz: diopside granulite, diopside syenite, diopside syenite (zircon bearing), intermediate charnockite and acid charnockite, were performed. Since there are apparent differences in the structure and texture of the various rock types, care was taken to ensure that the samples selected were sufficiently representative of the rock types concerned.

 

In the case of the coarse-grained rock, and those in which the distribution of the constituents is not uniform, larger bulks of the respective samples were taken, and after crushing and powdering, the quantity was suitably reduced by standard methods (Hillebrand et al., 1953, p. 809), as required for the analyses. The results of the chemical analyses, together with the C.I.P.W norms and Niggli values, are entered in Tables I to V. In the case of the intermediate charnockite, two specimens were analysed (IV. A and IV. B in the Tables). The two analyses show some variation in silica content. On account of the difference in the values of silica content, both the analyses have been considered in the construction of the variation diagrams.

 

For the purpose of estimating the modal composition of the five rock types brought under the syenite and charnockite suites, a selected number of microsections of the rock type concerned were used. While the modal composition of the medium-grained rocks could be dependably estimated from a small number of thin sections, in the case of the coarse-grained rocks it was necessary to examine a large number of thin sections because individual slides hardly represented all the constituents of the particular rock type. Thus the modal composition given in the Tables I to V corresponds to averages of a number of determinations.

 

In addition to the various rock types, some of the important constituent minerals such as feldspar, diopside and zircon were also chemically analysed. In this case, samples obtained by crushing the rock were sieved, and the fraction retained on the hundred mesh sieve was treated in bromoform, and the heavy crop, after being properly washed and dried, was further concentrated in the Franz Isodynamic Separator. The fractions thus obtained were spread under a binocular microscope, and grains of the required mineral were carefully hand-picked, taking particular care to exclude all those that contained inclusions. All the three mineral samples used for the chemical analyses were obtained from the zircon-bearing diopside syenite. The results are entered in Table VI.

 

Examination of the chemical analyses of the five Puttetti rock types, given in Tables I to V, shows that the silica ranges between 56 and 73 percent. In the three rock types comprised in the syenite suite, it may be seen that an insignificant amount of quartz appears in the norm, although not seen in the mode. In the case of the intermediate and acid charnockites, the figures for quartz in the mode as well as in the norm compare satisfactorily.

 

The quantity of alumina present in all the rock types is almost not more than what is necessary to satisfy the combined alkalies and lime. This view is identically the same as that of Washington (1916, p.334). However, the normative corundum in the two charnockites may be explained by the presence of mica in the rocks (Groves, 1935, p.174).

 

For purposes of the present investigation, variation diagrams (Fig. 3 & Fig. 4) have been constructed. In Fig. 3, the values for the oxides are plotted as ordinates against the silica value as abscissae. Fig. 4 is constructed by plotting the Niggli values al, fm, c and alk, against Si.

From Fig. 3, the following observations may be made.

 

1. There is a steady and progressive increase in the silica content of the five rock types in the order diopside granulite, diopside syenite, diopside syenite (zircon-bearing), intermediate charnockite, and acid charnockite. The silica content is the minimum for the diopside granulite and the maximum in the acid charnockite.
2. The curves representing the oxides of iron show a more or less steady decline towards the acid charnockite.
3. The curves for caO and MgO start high at the diopside granulite end, and after showing a somewhat steep fall towards the diopside syenite (zircon-bearing), continues as a gentle slope with increase of silica. However, the curves are typically concave upward.
4. The soda curve is convex upward, the highest position being attained near about diopside syenite. The potash curve starts low at the diopside granulite end, and shows a gradual rise with silica increase.

 

It may be seen from the plottings of the Niggli symbols (Fig. 4) that the alkali and alumina curves show a steady increase, while those of lime, and total iron and magnesium, show a steady decrease with increasing silica.

 

In Fig. 5 are given two variation diagrams (Rajagopalan, 1947, p.244); one ‘A’ is the differentiation diagram of calc-alkali series after Niggli, and the other ‘B’ being the differentiation diagram of the charnockite series. These are reproduced for the purpose of comparison with the Niggli variation diagram obtained for the Puttetti rocks (fig. 4). It may be seen there from that in all three, there is a general similarity in the tendencies of the curves.

 

Commenting on the two diagrams ‘A’ and ‘B’ (Fig. 5), Rajagopalan (1947, p.245) has observed thus: “On a closer study of the two diagrams it will be seen that there is not nearly a general similarity in the tendencies of the curve, but that the intersection point of al and fm curves is very nearly at the same Si value. This point, known as the Isofalic point, gives an Si value which is characteristic for each petrographic province. Thus, the isofalic point for Circum Pacific or calc-alakaline rocks falls at Si = 180 or more, while the same in the alkaline suites – both Atlantic and Mediterranean – rarely exceeds Si = 160.”

 

Based on the above, Rajagopalan has observed that a strong affinity to the calc-alkaline suite is indicated by the charnockite series, which has an Si value of 216 for its isofalic point (Fig. 5, ‘B’).

 

When considered on similar lines, the Niggli variation diagram of the Puttetti rocks (Fig. 4) shows that the intersection point corresponds to an Si value of about 180, which evidently refers the Puttetti suite of rocks to the calc-alkaline series.

 

Usually variation diagrams, in whatever ways they are constructed, indicate the regular changes from one rock to another in their order of age, at the same time reflecting the changes overtaking the parental magma that undergoes differentiation.

 

With a view to examine whether the crystalline rocks of Puttetti are igneous or not, the Al, c, Alk and Al, S, F, triangular diagrams of Osann’s system (Johannsen, 1949, Vol. I, p.79) have been constructed (Fig. 6 and Fig. 7). It was found that all the above rocks clearly fall in the igneous field.

 

The values S, Al, F and Al, C, Alk for the rocks of Puttetti together with the nearest classes into which the Puttetti rock types fall, are given in Table VII. The nearest rock classes were found out by comparing the values obtained for the Puttetti rocks with the corresponding typical values given by Osann (Johannsen, 1949, Vol. I, Table XVIII, p.81).

 

Table VIII gives a comparative statement of chemical analyses of intermediate charnockites and syenites. Herein, two analyses relate to the intermediate charnockite and diopside syenite (zircon-bearing), from the area under investigation. A perusal of the table will enable to visualize the chemical affinities that exist between the charnockites and syenites.

 

 

 

 

 

 

 

 

 

 

 

Table I

Diopside Granulite

 

Chemical analysis		Modal composition		C.I.P.W. Norm		Niggli values
Constituents	Weight percent
SiO2	56.61	Quartz	—	Q	2.04	si	137.40
TiO2	Trace	Microperthite	32.3	Or	11.67	qz	-2.60
Al2O3	11.42	Non-perthitic feldspar	26.5	Al	24.10	al	16.00
Fe2O3	2.16	Diopside	39.8	An	12.51	fm	43.00
FeO	4.72	Sphene	1.4	C	—	c	31.00
MnO	0.21			Di	37.47	alk	10.00
MgO	7.79			Hy	8.78	k	0.31
CaO	12.10			Il	—	mg	0.61
Na2O	2.87			Mt	3.25
K2O	1.95			Ht	—
P2O5	0.07			Ap	0.34
ZrO2	—			Py	—
SO3	—			Zr	—
H2O	0.45
H2O	0.07
Total	100.43
Sp. Gr.	2.98
Locality: Puttetti, Southern Travancore
Classification: II. 5. 4(5). 4

Analyst: K.V. Krishnan Nair

 

 

Table II

Diopside Syenite

 

Chemical analysis		Modal composition		C.I.P.W. Norm		Niggli values
Constituents	Weight percent
SiO2	59.67	Quartz	—	Q	0.42	si	184.00
TiO2	0.25	Microperthite	29.3	Or	25.02	qz	-4.80
Al2O3	14.98	Non-perthitic feldspar	40.3	Al	39.30	al	27.20
Fe2O3	1.61	Diopside	28.9	An	7.51	fm	26.10
FeO	3.64	Mica	1.1	C	—	c	25.50
MnO	0.06	Apatite	0.4	Di	23.47	alk	22.20
MgO	2.74			Hy	0.79	k	0.37
CaO	7.42			Il	0.46	mg	0.48
Na2O	4.63			Mt	2.18
K2O	4.25			Ht	—
P2O5	0.16			Ap	0.34
ZrO2	—			Py	—
SO3	—			Zr	—
H2O	0.67
H2O	0.08
Total	100.21
Sp. Gr.	2.81
Locality: Puttetti, Southern Travancore
Classification: II. 5. 3. 4

Analyst: C.V. Paulose

 

 

Table III

Diopside Syenite (zircon-bearing)

 

Chemical analysis		Modal composition		C.I.P.W. Norm		Niggli values
Constituents	Weight percent
SiO2	61.34	Quartz	—	Q	2.10	si	218.40
TiO2	0.34	Microperthite	45.64	Or	35.58	qz	6.40
Al2O3	14.87	Non-perthitic feldspar	35.65	Al	35.63	al	31.00
Fe2O3	0.80	Diopside	12.55	An	3.89	fm	22.00
FeO	3.82	Hornblende	3.18	C	—	c	19.00
MnO	0.10	Pyrrhotite	2.08	Di	17.25	alk	28.00
MgO	1.52	Ilmenite	0.03	Hy	1.06	k	0.50
CaO	4.99	Magnetite		Il	0.61	mg	0.40
Na2O	4.21	Zircon	0.27	Ht	—
K2O	5.98	Sphene	0.25	Ap	0.11
P2O5	0.04	Mica	0.35
ZrO2	1.20	Apatite	—
SO3	0.05
H2O	0.74
H2O
Total	100.00
Sp. Gr.	3.11
Locality: Puttetti, Southern Travancore
Classification: II. 5. 2. 3

Analyst: N. Jayaraman

 

Table IV. A.

Intermediate Charnockite

 

Chemical analysis		Modal composition		C.I.P.W. Norm		Niggli values
Constituents	Weight percent
SiO2	65.27	Quartz	—	Q	16.56	si	265.00
TiO2	Trace	Microperthite	14.6	Or	21.13	qz	57.00
Al2O3	16.12	Non-perthitic feldspar	51.9	Ab	37.73	al	38.00
Fe2O3	0.86	Hypersthene	3.4	An	12.51	fm	19.00
FeO	1.94	Ilmenite	1.7	C	0.31	c	16.00
MnO	0.08	Magnetite		Di	—	alk	27.00
MgO	1.57	Zircon	0.1	Il	—	k	0.35
CaO	3.64	Mica	2.3	Mt	1.16	mg	0.51
Na2O	4.49	Garnet	4.9	Ht	—
K2O	3.54	Apatite	2.2	Ap	2.12
P2O5	0.85			Py	—
ZrO2	—			Zr	—
SO3	—
H2O	0.92
H2O	0.35
Total	99.63
Sp. Gr.	2.72
Locality: Puttetti, Southern Travancore
Classification: I. 5. 2. 2

Analyst: K.V. Krishnan Nair

Table IV. B.

Intermediate Charnockite

 

Chemical analysis		Modal composition		C.I.P.W. Norm		Niggli values
Constituents	Weight percent
SiO2	70.28	Not determined		Q	22.08	si	332.00
TiO2	Trace			Or	33.80	qz	100.00
Al2O3	14.40			Ab	29.34	al	40.00
Fe2O3	0.73			An	7.23	fm	17.00
FeO	1.14			C	—	c	10.00
MnO	Trace			Di	2.04	alk	33.00
MgO	1.45			Hy	4.72	k	0.51
CaO	1.96			Il	—	mg	0.56
Na2O	3.48			Mt	—
K2O	5.51
P2O5	0.01
ZrO2	—
SO3	—
H2O	0.84
H2O	0.24
Total	100.04
Sp. Gr.	2.68
Locality: Puttetti, Southern Travancore
Classification: I. 4. 1. 3

Analyst: K.V. Krishnan Nair

 

 

 

 

 

Table V

Acid Charnockite

 

Chemical analysis		Modal composition		C.I.P.W. Norm		Niggli values
Constituents	Weight percent
SiO2	73.24	Quartz	26.5	Q	31.26	si	405.60
TiO2	Trace	Microperthite	35.1	Or	35.58	qz	167.60
Al2O3	13.83	Non-perthitic feldspar	29.6	Ab	20.96	al	45.20
Fe2O3	0.51	Hypersthene	6.4	An	6.67	fm	11.30
FeO	0.83	Ilmenite	1.5	C	0.81	c	9.00
MnO	Trace	Magnetite		Di	—	alk	34.50
MgO	0.58	Mica	0.9	Hy	3.22	k	0.61
CaO	1.51			Il	—	mg	0.44
Na2O	2.49			Mt	—
K2O	5.97			Ht	—
P2O5	0.16			Ap	0.34
ZrO2	—
SO3	—
H2O	0.76
H2O	0.18
Total	100.06
Sp. Gr.	2.61
Locality: Puttetti, Southern Travancore
Classification: I. 4. 2. 2

Analyst: K.V. Krishnan Nair

 

Table VI

Chemical Analyses of the Three Principal Minerals of Diopside Syenite (zircon-bearing)

 

Constituents	Feldspar	Diopside	Zircon
SiO2	64.39	46.99	30.51
TiO2	—	Trace	0.01
Al2O3	18.42	9.10	3.13
Fe2O3	0.09	18.24	0.23
FeO	0.05	4.00	0.15
MnO	—	0.43	Trace
MgO	0.65	2.45	0.03
CaO	1.93	16.03	0.15
Na2O	6.12	1.11	0.13
K2O	5.74	1.08	0.10
P2O5	1.44	0.97	0.03
ZrO2	—	—	65.27
SO3	—-	—	0.02
H2O	0.98	0.24	0.23
H2O	0.50	0.03	0.04
Total	100.39	100.63	100.03
Analyst:	K.V. Krishnan Nair	K.V. Krishnan Nair	N. Jayaraman

 

 

 

 

 

 

 

 

Table VII

Osann’s System

 

Name & Hand Specimen No.	S	Al	F	Al	C	Alk	Rock type of Osann
Acid charnockite (K18)	17.3	1.9	0.8	10.2	2.0	7.8	Siliceous alkali-lime granite
Intermediate charnockite (K28)	16.6	2.0	1.4	9.7	2.4	7.9	Granite
Intermediate charnockite (K49)	15.7	2.3	2.0	9.5	3.9	6.6	Granite
Diopside syenite (zircon-bearing) (K1)	15.1	2.1	2.8	8.0	4.7	7.3	Alkali-lime-rich syenite
Diopside syenite (K8)	14.1	2.1	3.8	7.4	6.7	5.9	Alkali-lime-rich syenite
Pyroxene granulite (K24)	12.5	1.5	6.0	5.7	10.9	3.4	Essexite shonkinite

 

 

 

 

 

 

 

 

Table VIII

Comparative Statement of Chemical Analyses of Intermediate Charnockites and Syenites

 

Constituents	1	2	3	4	5	6	7
SiO2	65.7	60.52	63.85	61.34	62.50	61.60	60.51
TiO2	Trace	0.62	0.83	0.34	—	—	1.52
Al2O3	16.12	15.92	14.87	14.87	16.50	15.10	14.16
Fe2O3	0.86	3.00	2.32	0.80	2.40	2.00	1.60
FeO	1.94	5.66	5.07	3.82	2.00	2.20	8.02
MnO	0.08	0.17	0.07	0.10	—	—	0.17
MgO	1.57	1.19	3.29	1.52	1.90	3.70	0.17
CaO	3.64	5.47	4.48	4.99	4.20	4.60	4.14
Na2O	4.49	4.38	3.72	4.21	4.40	4.30	3.47
K2O	3.54	2.36	1.09	5.98	4.60	4.50	4.38
P2O5	0.85	0.34	0.08	0.04	—	—	0.58
ZrO2	—	—	—	1.20	—	—	—
SO3	—	—	—	0.05	—	—	—
S	—	—	0.15	—	—	—	—
H2O	0.92	0.48	0.11	0.74	0.60	0.70	0.10
H2O	0.35						0.90
Other oxides	—	—	—	—	1.30	1.00	—
Total	99.63	100.11	99.89	100.00	100.40	99.70	99.91
Sp. Gr.	2.72	2.79	2.70	3.11	2.73	—	—

Continued………

 

 

 

 

 

Table VIII (contd.)

 

C.I.P.W. Norm	1	2	3	4	5	6	7
Q	16.56	12.06	20.95	2.10	10.26	7.92	11.07
Or	21.13	13.90	6.67	35.58	27.24	26.69	26.13
Ab	37.73	37.20	31.44	35.63	37.20	36.16	28.82
An	12.51	16.68	20.57	3.89	11.68	6.39	10.15
C	0.31	—	—	—	—	—	—
Di	—	6.63	1.36	17.26	1.35	7.97	6.12
Hy	6.94	6.75	13.14	1.06	5.88	7.83	10.96
Il	—	1.22	1.52	0.61	—	—	2.89
Mt	1.16	4.31	3.25	1.16	3.48	3.02	2.32
Ht	—	—	—	—	—	—	—
Ap	2.12	1.01	—	0.11	3.02	2.35	1.34
Py	—	—	—	0.12	—	—	—
Zr	—	—	—	1.83	—	—	—
Niggli values
si	265.00	202.70	224.00	218.50	225.50	210.45	Not given
qz	57.00	25.90	20.00	6.40	22.16	14.57
al	38.00	31.40	31.00	31.00	35.00	26.68
fm	19.00	29.80	38.00	22.00	23.00	30.53
c	16.00	19.60	16.00	19.00	16.00	16.80
alk	27.00	19.20	15.00	28.00	26.00	23.97
k	0.35	0.26	0.16	0.50	0.41	0.51
mg	0.51	0.20	0.42	0.40	0.45	0.62

Continued………

 

 

 

 

 

Index to Table VIII

 

1. Intermediate Charnockite, Puttetti, Southern Travancore.

Analyst: K.V. Krishnan Nair, 1954.

 

2. Intermediate Charnockite, Myladi, Southern Travancore.

Analyst: C.V. Paulose, M.Sc. Thesis, 1953.

 

3. Intermediate Charnockite, Yercaud, Salem District, Madras.

Analyst: H.S. Washington (1916, p.328).

 

4. Diopside Syenite (zircon-bearing), Puttetti, Southern Travancore.

Analyst: N. Jayaraman, 1953.

 

5. Syenite, Plauen, near Dresden, Germany (cited in Table 2, p.181, by Pirsson and Knopf).

 

6. Syenite, little Belt Mountains, Montana (cited in Table 2, p.181, by Pirsson and Knopf).

 

7. Hornblende-pyroxene syenite gneiss, Vermontville, Saranac.

Analysts: G. Kahan and R.B. Ellestad (Buddington, 1952, p.66).

 

 

 

 

 

 

CHAPTER – VI

 

1. Petrogenetic Considerations

 

• General remarks on the rocks of the syenite family

 

It was Rosenbusch who first clearly defined “Syenite” as quartz-free orthoclase rocks regardless of the presence or absence of hornblende. In 1823, before the discovery of plagioclase as a separate variety of feldspar, von Leonhard noticed that two kinds of feldspars, such as ‘feldspath’ (orthoclase) and more rarely ‘feldstein’ (plagioclase) were essential in a syenite, and this usage has been followed by later workers.

 

Johannsen (1949, Vol. III, p.52) has described “syenite” as a “plutonic rock of hypautomorphic-granular texture and consisting of the mineral combination orthoclase, less acid plagioclase and usually some dark mineral. Quartz is absent or is present only as an accessory”. According to him, “the quartz must form less than 5 percent of the total leucocrates”.

 

The syenites are plutonic rocks of intermediate composition. The essential constituents are alkali feldspars and a mafic mineral, the former being dominant. Sometimes, plagioclase within the range oligoclase-andesine is present in subordinate amounts. The mafic minerals are commonly represented by the pyroxenes, amphiboles or micas. Very often unsaturated minerals (Shand, 1947, p.118) such as the feldspathoids, may substitute the feldspars in varying degree to form feldspathoidal syenites. However, the two groups of syenites are syngenetic in occurrence. Syenites are closely allied to granites on one side and nepheline-syenites on the other. They also grade through monzonite to diorite, and the augite syenites may grade to gabbro.

 

The syenites embrace a group of rocks of considerable diversity. In regard to their relative abundance, Shand (1947, p.272) has observed: “the whole amount of syenite in the lithosphere is insignificant in comparison with granite or gabbro”. Daly’s (1910, p.90) profound study on the alkaline rocks of the world which also includes syenites and monzonites, led him to infer, that “the visible alkaline rocks of the world probably constitute less than one percent of the total visible igneous rocks”. In the same paper, he listed in geographical order the more important of the recorded occurrences of the alkaline rocks in the world, numbering 143 in all. The list shows the wide distribution of the alkaline rocks in all the continents, and probably in every latitude from Greenland to the Antarctic. The majority of the syenites occurs as marginal modifications of granites, and as satellitic stocks related to larger granite batholiths. Some however form independent dykes, laccoliths and masses of irregular shape.

 

Reference to geological literature pertaining to syenites in India shows that the non-feldspathoidal syenites are not usually found to occur as extensive bodies at all, but are practically seen to occur as transitionary stages of the feldspathoidal as well as non-feldspathoidal rocks. In the feldspathoidal syenites, augite and hornblende syenites commonly mark transitionary stages.

 

The well-known occurrences of syenites in India, which have been studied in some detail, are essentially feldspathoid-bearing, though locally these show transition to non-feldspathoid syenites. These include the syenites at Sivamalai, Coimbatore District (Holland, 1901); Mount Girnar, Kathiawar (Evans, 1901; Krishnan, 1926); Sarnu, Jodhpur (Holland, 1902); Vizagpatam (Walker, 1907); and Kishengarh, Rajputana (Heron, 1924).

 

In general, syenites associated with non-feldspathoidal rocks are very few in number. Nevertheless, some instances, where syenites are found to occur in close association with charnockites have been noted in India, as well as other parts of the world. In almost every such instance, it has been possible to postulate a genetic relationship between the two kinds of rocks.

 

• Charnockite-Syenite relationship at Puttetti

 

In the area surrounding Puttetti, the dominant rock is charnockite, particularly of the intermediate type. The syenite extends over a smaller area and occurs in close association with the charnockite. Frequently, outcrops of intermediate charnockite resemble those of syenite so strikingly, that often distinction between the two is possible only on careful examination of the mineralogy. While it was not possible to trace any contact between the two rocks anywhere in the area, field indications are such as to suggest a gradual passage of one into the other. Further, microscopical and chemical data strongly suggest a genetic relationship between the charnockite and the syenite. In other words, in the matter of origin, there seems to be much in common between the two kinds of rocks. Therefore, it is only proper to review briefly the salient features and genetic implications of the charnockite series of rocks before passing on to the petrogenetic considerations of the syenite.

 

• Charnockite series of rocks

 

As originally proposed by Holland (1900), the charnockite series of rocks grouped together in one petrographical province, but varying in composition from acid charnockite through intermediate and basic varieties to ultrabasic pyroxenites. He regarded the several types of the series as differentiated phases of crystallization of a normal plutonic magma, intrusive into the associated granitoid gneiss. Mineralogically, the rocks are distinctive in almost constant presence of the highly pleochroic rhombic pyroxene. Further, the rocks of this group have a widespread granulitic texture. Each of the above-mentioned four divisions has characteristic mineralogical, physical and chemical features which aid in their proper identification.

 

The charnockites are widely distributed in Peninsular India, where they largely constitute some of the mountains such as Nilgiris, Palnis, Shevroys and Annamalais. The rocks are well represented in the Ghats section in Travancore and can be traced to extend up to Cape Comorin. As a rule, these charnockite masses are irregular in shape, and sometime show roughly lenticular form. It is not uncommon to see small hills constituted by the basic varieties. While the masses of the rocks have uniform general characters over large areas, there are structural and compositional variations suggesting local differentiation. The rocks are usually banded, but very often bands could not be traced for any considerable distance. This apparent banding or foliation is explained by Holland (1900, p.176) as “probably induced during the process of consolidation, although of course it may have been accentuated in many perhaps, in most, exposures by continued exertion of the forces which determined the main physical conformation of S. India in very early geological times”.

 

• Views on the origin of the charnockite series of rocks

 

In regard to the mode of origin of the charnockites, it may be seen from the preceding section that Holland considers an igneous origin. In support of this view, Holland (1900, p.243) has mentioned the peculiar form and structure of many of the massifs. The presence of schlieren often brecciated and invaded by the residual magma, as observed by him at Ootacamond (1900, p.219) and the presence of appophyses and dykes of charnockite protruding into the country rock as at Salem (1900, p.219), and Coorg (1900, p.228), have been cited by him as favouring an igneous origin.

 

This view of Holland has been upheld by many others including Walker (1902, p.7); Washington (1916); Masillamani (1916, p.6); Chacko (1921, p.8); Crookshank (1938; p.424); Rajagopalan (1947); Pascoe (1950, p.117); Narasingha (1950) and Krishnan (1951, p.318 and 319).

 

On the other hand, there is a school which considers the charnockites to be platonically metamorphosed igneous rocks – this school includes Stilwell (1918); Vredenburg (1918); Groves (1935) and Prider (1945, p.171).

 

Tyrrell (1948, p.317) considers that the charnockite series “are either of primary igneous crystallization under conditions of high temperature and great uniform pressure or they represent plutonic igneous rocks of the usual characters, which have undergone slow recrystallization in the solid state on being subjected to conditions of plutonic metamorphism”.

 

Ghosh (1941) is of opinion that the basic and ultrabasic portions of the series are formed by heat and pressure metamorphism, but the intermediate and acid rocks owe their origin to a process of assimilation.

 

Rama Rao (1945), in his work on the charnockite rocks of Mysore, has observed: “……., I would regard the charnockite not as belonging to a petrographic province as originally interpreted – namely the differentiated phases of crystallization of a normal plutonic intrusive magma – but as of a metamorphic province wherein the combined effects of a repeated series of alterations under different periods of metamorphism of a composite series of rock formation of different ages, have given rise to a series of hypersthene granulites of very variable composition”.

 

Quensel’s (1950) observations about the origin of charnockites, to quote his own words are that “the basic charnockites are of primary igneous origin, derived from the basic igneous rocks belonging to the Archaean gneiss complex”, and further, “the intermediate charnockite presumed to be hybrid rocks formed by the complete assimilation of rock components of different chemical composition can hardly be assigned a definite position in the sequence of plutonic metamorphism”.

 

• Review of occurrences of inter-related charnockite and syenite

 

Some occurrences of syenites associated with charnockite in a more or less similar manner as at Puttetti in Southern Travancore, have been recorded from elsewhere in India, as well as from other parts of the world. Most of these occurrences have been described in some detail, while a few, which have not evidently been studied, are mentioned in geological literature. Before passing into the genetic aspects of the syenite under investigation, it is worthwhile to make a brief survey of the various similar occurrences.

 

The earliest report, in which striking similarity between syenites and charnockites in India, appears to have been made by Captain J. Allardyce (1836). When studying the granite formation and direction of the primary Mountain Chain of South India, he evidently recognized a similarity between what he called as the “Primitive trap allied to Sienitic granite” of Pallavaram and the principal rock masses of the Nilgiris, Shevroys, Western Ghats and Ceylon.

 

In the same year, Benza described the rocks of Nilgiris thus: “The lowest visible rocks of the Nilgiris is the primitive unstratified class including true granite, pegmatite, Sienitic granite and hornblende rocks; Sienitic gneiss and hornblende slate are occasionally seen but they belong to the outskirts of the hills”.

 

Captain Outchterlony (1848) distinguished between the “granite”, “Sienite” and “hornblende rock” of the Nilgiris on the one hand and the “beds of gneiss” met with in the plains on the others, and referred to the mass or nucleus of the mountain as “granite frequently passing into sienite”.

 

In the Kalahandi State, and the Vizagpatam District, Walker (1902) observed that the members of the charnockite suite instead of maintaining their characteristic individuality over extensive areas tend to assume notable modification. One such modified form is stated to be a massive variety of bluish-black hypersthene-syenite. From the same are in Vizagpatam, Walker has mentioned having collected specimens closely similar to the intermediate form of charnockite and other related to the Canadian Anorthosites.

 

In a subsequent paper (1907) the same author has mentioned about Elaelolite syenites near Koraput in Vizagpatam occurring associated with the charnockite.

 

In the course of an elaborate study of the ‘Igneous Complex of the Blue Ridge Region, Virginia’, Watson and Cline (1916) found that “In the middle and northern parts of the Blue Ridge and the adjacent portions of the Piedmont Plateau in Virginia, one of the dominant igneous rocks of granitoid type is a quartz-bearing pyroxene-syenite. The igneous complex of which pyroxene syenite is the chief type, may represent a Precambrian batholithic intrusion, exposed at intervals for a distance of 150 miles belt up to 20 miles or more in width. Differentiation of the syenite magma has given rise to a variety of related rocks, some of which are of particular interest”.

 

Studies of the igneous complex forming the central core of the Blue Ridge in middle and northern Virginia, are sufficiently advanced to indicate, that the rock types exhibit certain kinships which mark them as differentiates from a common magma, ad that this igneous complex designated by the writer as the Blue Ridge petrographic province, shows certain important differences in mineralogy and chemistry from the igneous rocks which enter into the composition of the Piedmont Plateau to the east.

 

Subsequently, in the same paper (1916, p.219), the authors have compared the syenite with charnockite. The relevant part from the above paper may be quoted here.

 

‘There appears to be remarkably close correspondence in mineral composition of the most abundant variety of charnockite of intermediate composition and the Blue Ridge Virginia Syenite”.

 

In his preliminary report on the Geology of Eraniel, Kalkulam and Vilavancode Taluks in Southern Travancore, Masillamani (1911) has stated that the syenite occurring in Eraniel Taluk is “evidently an intermediate facies of the charnockite series”.

 

In North Arcot, there appears to be a gradual passage from intermediate and basic forms of charnockite to augite syenite and hornblende gneiss. Frequently, the augites of the syenite is found changing to hornblende. According to Fermer (1932), on account of the absence of hypersthene in some of these rocks, the term charnockite is almost inaaplicable. Further it was suggested thatthese augite syenites are a late phase of the charnockite intrusion.

 

The chief rock type of Cochin is charnockite, and its co-existence with augite syenite was recorded by Sen Gupta and Chatterjee (1936). The syenite is holocrystalline – sometimes very coarsely crystalline rock – almost wholly made up of grayish or flesh-coloured feldspar (generally microperthitic) with spots of green augite. Quartz is sometimes present in small quantity. Monzonitic varieties are also known. Sen Gupta and Chatterjee believe that the augite syenite and monzonite are but variants of the same magma.

 

In the well known work on the charnockite rocks of Mysore, Rama Rao (1945, p.48) has mentioned the occurrence of rock types corresponding to acid charnockite and syenitic type as local variations at the marginal fringes of an ultrabasic charnockite exposure near Dodkanya.

 

Pascoe (1950, p.129) has stated that towards the north of Travancore, overlying the charnockite and having the appearance of passing down into it, there is a granular hornblende granite or syenite, with well developed gneissose structure. However, the exact relationship of this rock with the charnockite is said to be obscure.

 

In the Adirondacks of North America, pyroxene syenitic and quartz syenitic rocks are found to occur extensively. These comprise two different series which occur as separate sheet –like masses, namely the Diana Stark Complex and the Tupper-Saranac Complex respectively. The members of the Tupper-Saranac Complex according to Buddington (1952) have features that characterize the charnockite suite of rocks, “and this term seems appropriately applicable to at least part of the members of the Tupper-Saranac Complex”.

 

• Features common to the syenite and charnockite suites of rocks at Puttetti and their probable genetic import

 

From the numerous instances cited in the preceding section, it is evident, that the close association of syenites and charnockites in the field is more or less a widely recognized fact, and that the association between the two kinds of rocks may be considered as an expression of their mutual genetic relationship.

 

Results obtained from the field, petrographical, mineralogical, and chemical investigations of the crystalline rocks of Puttetti, denote certain kinships that seem to exist in the syenite and charnockite type of rocks in the area. Moreover, these kinships lead to conclude that the syenite and charnockite are most probably derived from a common magma.

 

In order to ascertain the validity of the above tentative conclusion, it is desirable to consider the salient characteristics of the rocks concerning thereto, and examine the merits of their genetic implications.

 

1. Significant field characteristics

 

Some of the field characteristics, displayed by the rock types of the syenite suite such as diopside granulite, diopside syenite and diopside syenite (zircon-bearing) among themselves and with the associated charnockites, are highly significant in deciphering the genetic relationship of the syenite and the charnockite suites of rocks of Puttetti area. These characteristics have been already described in detail, and therefore the same require only brief mention here.

 

1. The close association of the syenite and charnockite suite of rocks as seen from their occurrences side by side without any visible trace of contact.
2. The insensible merging of the two suites of rocks indicated by the gradual appearance of free quartz at the periphery of the syenite body in the zone which separates the syenite type from the charnockite type. In otjer words, it is possible to collect specimens, along a traverse from the syenite ridge to the nearest charnockite outcrop that will range in the amount of quartz from a minimum in the periphery of the syenite to a maximum in the typical charnockite.
3. The coincidence in the direction of strike of foliation of the syenite with that of the country rocks, essentially represented by the charnockites. It may also be mentioned that the direction of elongation of the syenite ridge corresponds to the general strike direction.
4. The similarity in colour and texture shown by fresh outcrops of diopside syenite and intermediate charnockite. The similarity is so remarkably perfect that charnockite and syenite could be distinguished from each other only on a close observation of mineralogy in hand specimens.

 

1. Significant petrographical and mineralogical characteristics

 

Comparison of petrographical and mineralogical details clearly shows a number of characteristics to be common to both the syenites and the charnockite suites of rocks. Some of the significant points which strongly imply a genetic relationship of the two kinds of rocks are noted below.

 

1. Texturally, the diopside syenite and the intermediate charnockite are alike, in that both are typically medium to coarse-grained.
2. Uneven distribution of ferromagnesian minerals is a feature observed in rocks of the two suites.
3. The mineral assemblage which constitutes the charnockites is seen to characterize the syenites too, with the exception of quartz and hypersthene. However, diopside and sphene which are absent in the charnockites of the area are found in the syenite.
4. Feldspar, which is an essential constituent of the charnockite, and at the same time, the predominant constituent of the syenite, is made up of microperthite and non-perthitic varieties in both the rocks. The plagioclase is the same in both the rock suites, and usually ranges in composition from albite to andesine, and rarely to labrodorite. Twinning is generally absent in both.
5. Post-consolidation metamorphic evidences, such as pressure phenomena and formation of secondary minerals, are displayed by the two rock suites practically to the same degree.

 

The absence of hypersthene in the rocks of the syenite suite is somewhat conspicuous. Even so is the absence of diopside in the rocks of the charnockite suite in Puttetti. Sphene is also not found in the charnockite, but its absence is nothing unexpected since hypersthene and sphene are believed to be almost mutually exclusive in the charnockites (Rama Rao, 1945, p.25).

 

The examination of numerous microsections of the rocks of the syenite and charnockite suites has not been fruitful, either in tracing any possible relationship between the two minerals hypersthene and diopside, or in satisfactorily explaining the apparent anomaly concerning the nature of distribution of these two minerals in the various rock types of the two suites. However, related literature mentions about the relationship between hypersthene and diopside, and some of the prevalent views may profitably be referred to in this connection.

 

The alteration of monoclinic pyroxene into hypersthene has been noted in the charnockites by Groves (1935, p.146), Rama Rao (1945, p.19), Quensel (1950, p.277) and many others. In Varberg Charnockite Series, hypersthene is the most characteristic mineral, and occurs in different types in varying amounts. It is nearly always accompanied by a monoclinic pyroxene, but the proportions between the two pyroxenes are subject to considerable variation. This can, as Quensel 91950, p.238) pointed out, in certain cases go so far, that only the one or the other of the pyroxenes is present.

 

Ghosh views the relationship between hypersthene and diopside from a different angle. He (Ghosh, 1941, p.8) considers, that diopside, which is usually a primary mineral, may as well as be found secondary after hornblende, and probably also after hypersthene under the effect of regional metamorphism, as a product of reversible reaction.

 

The absence of hypersthene in some of the charnockites of Cochin in the Travancore-Cochin State, was noted by Sen Gupta and Chatterjee (1936, p.6). Though it is the characteristic mineral of the charnockites, the authors have considered their absence as not significant, in as much as hypersthene is a secondary mineral after augite. Moreover, the same authors have adduced the occurrence of secondary hypersthene as an evidence of subsequent metamorphism of the charnockite, formed out of a primary igneous crystallization.

 

In a recent work, regarding the petrogenesis of the charnockite of the Cape Comorin area in Southern Travancore, Paulose (1953, p.32) noted the complete absence of clinopyroxene in the intermediate charnockite, which is hypersthene-bearing. Further, among the four types of charnockites of the basic division, of which all the four bear hypersthene, clinopyroxene was found only in three types, while in the remaining one type the same was not seen. He considers this rock as a transitionary one between the basic charnockite and the other members of the charnockite suite.

 

Therefore, hypersthene may have a secondary origin from diopside; also diopside could probably be formed from hypersthene, as believed by Ghosh. Though at this stage, no specific reason could be assigned for the apparently partial distribution of diopside and hypersthene in the different crystalline rock types of Puttetti, which are considered as descending from a common magma as concluded from the present study, yet, the statement made by Quensel (1950, p.233) tend to show, that the partial distribution of diopside and hypersthene in genetically related rocks does not matter much as far as the present work is concerned.

 

• Significant characteristics of chemical data

 

The chemical analyses of syenites and intermediate charnockites from Puttetti and other places (Table VIII) are highly interesting in their import on the genesis of the syenite at Puttetti. These significant points are mentioned below, and they strongly imply that the crystalline rock types of Puttetti have a common origin, and represent variants of the same magma, evolved by differentiation.

 

1. Regarding chemical composition, the syenite from Puttetti, compares favourably with other syenites from different parts of the world. More interesting than this, is the general similarity in chemical composition, notably in respect of the major constituents, between the syenites and the intermediate charnockites.
2. The curves in the variation diagrams (Fig. 3) relating to the crystalline rock types of Puttetti, trend in such a manner as to indicate that the different rock types concerned are but variants of a common magma.
3. The Niggli variation diagram (Fig. 4) referring to the crystalline rock types of Puttetti, on comparison with his (Niggli’s) original diagram (reproduced in Fig. 5 ‘B’) illustrating the differentiation of the calc-alkaline series, shows a close similarity.
4. It is evident from Fig. 4 that the rock types of Puttetti have a strong affinity to the calc-alkaline series. It may be recalled that calc-alkaline affinities of the charnockite series in St. Thomas Mount and Kondapalle Hill Ranges have been shown by Rajagopalan (1947, p.246) and Srirama Rao (1947, p.154) respectively.

 

• Conclusions

 

In the course of a discussion on the rocks of the syenite clan, Daly (1933, p.481) observed that, “A single mode of origin for the rock species of the syenite clan is largely probable, and a complete list of the actual modes may long elude formation. Still more, obscure is the quantitative importance of the various mechanisms”. He concluded the same discussion with the statement that, “Only a few varieties of rock belonging to the syenite clan are considered to be true hybrids unaffected by differentiation”.

 

Obviously the consideration of such characteristics as are common to both the charnockite and syenite suites of rocks, leads to the recognition of a kinship among the rock types of the two suites. The absence of intrusive contacts and effects in the field, excludes the possibility of intrusion from consideration. Further, the gradual passage of the syenite suite of rocks among themselves, and to the charnockite suite, as seen in the field forms a substantial line of evidence to reckon a common origin for the crystalline rock types of Puttetti.

 

In more or less similar situation, Watson and Cline (1916, p.223-24), while investigating the granites and syenites of the Blue Ridge Region, Virginia, noted the absence of definite contacts between the two types and the gradual passage of one into the other characterized by the progressive increase in quartz content regularly from the syenite to the granite. This led to the conclusion, that the two types represented differentiation phases of the same magma, and that the granite is an acid extreme of the syenite.

 

The comparative statement of the chemical analyses of syenites and intermediate charnockites given in Table VIII clearly brings out a close correspondence between the syenites and the charnockites regarding chemical affinity.  Osann’s diagrams (Figs. 6, 7) in respect of the crystalline rock types of Puttetti, denote an igneous mode of origin, and also emphasize a gradual variation from the acid charnockite at one end to diopside granulite at the other.

 

The variation diagrams (Figs. 3 & 4) show, that the rock types of the charnockite and the syenite suites have resulted from the differentiation of a common magma. From the Niggli varistion diagram, which also shows magmatic differentiation, a calc-alkali affinity in regard to the crystalline rock types of Puttetti is eseen. It is interesting to recall the calc-alkali affinity of the charnockites of the St. Thomas Mount and Kondapalle Hill Ranges as shown by Rajagopalan (1947, p.246) and Srirama Rao (1947, p.154) respectively. Evidently the calc-alkali affinity may be considered to be a characteristic of some charnockites, and at the same time displayed by the crystalline rock types of Puttetti.

 

From the foregoing discussion, it is clear that genetically the charnockite and syenite suites of rocks are closely connected. In other words, the events that marked the genesis of the charnockite suite, have largely contributed to the formation of the syenite suite also.

 

The granite family in the Northwest Adirondacks is represented by rocks of many varieties including some prominent occurrences of syenites. Buddington 91947, p.22-23) considers 85 percent of the granitic rocks of the region as produced by consolidation of magma, in part modified by incorporation of country rock.

 

The quartz syenitic series of the Adirondacks igneous Complexes (p.24-25) show two facies: (1) the Diana-Stark complexes with such predominant varieties such as augite syenite, augite and hornblende quartz syenite, and hornblende granite, (2) Tupper-Saranac complex (related to charnockite series) wherein predominant varieties are augite, hypersthene syenite and augite-hypersthene-hornblende quartz syenite.

 

Some geologists interpret pyroxene–bearing granitic rocks and granitic members of the charnockite series in other regions as products of migmatisation, metasomatism, and mobilization or rheomorphism by emanations. However, based on evidences gathered from the Adirondacks region, Buddington (1947, p.24-25) concluded that the quartz syenitic series in the region are fundamentally due to consolidation of magma, which was intruded from depth and changed in composition by differentiation and in part, by incorporation of country rock.

 

In the earlier portions of this chapter, a number of instances in Travancore-Cochin, India, and other parts of the world, of inter-related charnockite – syenite occurrences with their respective modes of origin wherever known, were briefly outlined. It is interesting to note, that practically all these instances have more or less a similar mode of origin, namely magmatic differentiation. For purposes of the present discussion, the occurrences of syenites in Travancore-Cochin may be recalled with greater emphasis.

 

Masillamani (1911) in describing the geology of Eraniel, Kalkulam and Vilavancode Taluks in Southern Travancore, referred the syenite occurring in Eraniel Taluk as “an intermediate facies of the charnockite series”. Evidently, this occurrence refers to the syenite of the crystalline rocks of Puttetti. Sen Gupta and Chatterjee (1936, p.3) considered the augite syenite and monzonite of Cochin as variants of one and the same magma. Pascoe 91950, p.129) has mentioned about a hornblende syenite associated with charnockite towards the north of Travancore, but the mode of origin is stated to be obscure.

 

From a recent study of the charnockites of Cape Comorin area in Southern Travancore, Paulose (1953, p.106) concluded that, the intermediate and the acid charnockites of the area were formed primarily by a process of differentiation of a palingenetic rock magma, and were subsequently subjected to metamorphism.

 

Though a detailed investigation of the mode of genesis of the charnockites of the area does not, strictly speaking, come within the purview of the present work, obviously, according to the trend of reasoning, adopted in deciphering the mode of origin of the syenite suite of rocks, the mode of origin of the charnockites does have a direct bearing on the problem in so far as the two rock suites descent from the same parent magma as evidenced by field and laboratory findings.

 

Therefore, before concluding the genetic consideration of the syenite suite of rocks, a brief reference dealing on the relevant genetic aspects of the charnockites may also be made.

 

One among the outstanding hypotheses on the origin of the charnockite series of rocks is that propounded by Holland (1900), according to which the members of the charnockite series of rocks have resulted from the differentiation of an igneous magma. Previous works on the charnockites in different parts of Travancore-Cochin by Masillamani (1911), Chacko (1919 & 1921) and Sen Gupta and Chatterjee (1936) have invariably indicated magmatic differentiation as the chief factor in the genesis of the charnockites. Paulose (1953) have also favoured differentiation of an intrusive magma in regard to the Cape Comorin charnockites, but has considered the magma as having been modified before consolidation by incorporation of country rock. He further noted that the charnockite also bear evidences of metamorphic impress.

 

Furthermore, Osann’s diagrams (Figs. 6, 7), which have shown an igneous mode of origin for the syenite suite of rocks, have also indicated at the same time an igneous mode of origin for the charnockite rocks of the area. Other kines of evidences, which have already been described, only go to support this view.

 

Thus the sum total of all the evidences, gathered both from the field and the laboratory as well, taken together, would lead to the conclusion, that the charnockite suite of rocks is fundamentally igneous in origin, and that the same magma which gave rise to the charnockites on the one side by differentiation, further resulted in producing the diopside granulite, diopside syenite and diopside syenite (zircon-bearing), which three types are collectively designated as the syenite suite of rocks. Incidently, this conclusion is consistent with Daly’s view (1933, p.481) that “Only a few varieties of rock belonging to the syenite clan are considered to be true hybrids, unaffected by differentiation”.

 

Certain evidences in the field, particularly observed in the outcrops of the diopside granulite, tend to suggest, that prior to consolidation of magma incorporated some pre-existing basic rock. This is inferred from the presence of irregularly distributed clots, patches and segregations of a dark-coloured basic mineral seen to occur locally in the diopside granulite. Such occurrences as the above were not noticed in the associated charnockite suite of rocks and the remaining two types of the syenite suite of rocks. It is quite probable, that the initial parent magma attacked a pre-existing basic rock of the area, but the assimilation was practically complete in the differentiated fraction which gave rise to the charnockite suite of rocks, whereas the portion of the magma which produced the syenite suite of rocks could not complete the assimilation, thereby resulting in the local occurrence of partly-digested relicts of basic rock. These patches, on close examination in the field are seen to pass gradually into the surrounding host rock.

 

The rock types of the syenite and charnockite suites bear effects of metamorphism wherein heat and pressure conditions seem largely to have controlled. The metamorphic impresses are shown particularly by the presence of, (1) gneissic foliation developed in varying degrees, (2) cataclastic phenomena such as undulose extinction, bent cleavages and distorted twin lamellae, (3) secondary minerals, and (4) perthites. The metamorphic effects are seen more or less alike in the two suites of rocks, and therefore it may be inferred, that the two rock suites were metamorphosed almost simultaneously subsequent to consolidation.

 

Usually the age relationship between two rock types are established on the evidence furnished by dykes, apophyses and similar structures of any one rock found in the other. However, the complete absence of such features as mentioned above in the rock types at Puttetti has made it difficult to determine the age relationship of the rock of the area.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

SUMMARY

 

1. The crystalline rocks of Puttetti are formed by a Syenite suite of rocks consisting of diopside granulite, diopside syenite, diopside syenite (zircon-bearing); and a Charnockite suite of rocks consisting of intermediate and acid charnockites.
2. The Syenite suite occurs as a composite low ridge, trending in a direction coinciding with the regional strike of foliation of the rocks of the area, and surrounded by charnockites, particularly of the intermediate type.
3. The five rock types, which go to make the two rock suites of the area, do not show any intrusive relation, either among themselves, or between the two suites. On the contrary, there is a transition from the syenite ridge towards the surrounding intermediate charnockite, marked by the gradual appearance of quartz.
4. The essential minerals of the syenite suite are feldspars including microperthites and diopside. Other mafic constituents are hornblende, sphene, and mica. Minor accessories include zircon, iron ores, apatite, calcite, pyrrhotite, and quartz. In the charnockites, feldspars including microprthite, quartz, hypersthene, garnet and mica are essentially found, while iron ores, apatite and zircon occur as accessories.
5. Chemical analyses of typical samples of the five rock types were made and the results were plotted in different ways. From these plotting, it was observed, that the Puttetti rocks are igneous in origin and formed by the differentiation of one and the same magma.
6. Based on field and laboratory findings, it is concluded that the rocks of the syenite suite and charnockite suite were derived from the same parent magma by a process of differentiation.
7. Further, certain field evidences, particularly observed in the outcrops of the diopside granulite, suggest, that the parent magma was somewhat modified by the incorporation of some basic rock before final consolidation.
8. Metamorphic effects of the same kind are noted in all the rock types, and it is inferred that the two rock suites were metamorphosed almost simultaneously subsequent to consolidation.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

PART THREE

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

APPENDICES I & II

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Appendix I: A note on the large crystals of zircon in Puttetti

 

Considerable interest has been evinced by geologists and others in the somewhat widespread occurrence of large, well-formed crystals of zircon, disseminated in the gravel and soil over an area of about six acres in the neighbourhood of Puttetti and Kizhkulam in Southern Travancore. The parent rock is not far to seek; a diligent search would reveal it to be none other than the diopside syenite (described in Part II) of the syenite ridge, by the age-long weathering of which the stable zircons came to be released, and lie where they are found at present.

 

Zircon, an orthosilicate of zirconium is a common accessory mineral in many granites, syenites and gneisses. Therefore, the sediments that are formed from the above rocks usually contain concentrates of the mineral. Thus, in the beach sands of Travancore, zircon forms an important constituent. Generally, it occurs in the rocks in microscopic crystals in quantities not sufficient to enable large-scale extraction.

 

The occurrence of zircon at Appiyodu (Maps 1 & 2) in Southern Travancore was recorded by Masillamani as early as the year 1911. He found zircon occurring as a surface deposit, and also “in situ” in a pegmatite in which feldspar predominates with very small amounts of mica.

 

The presence of zircon crystals near Appiyodu was noted by the writer, during his survey of the systematic geology of the area, described in Part I of this thesis. Well-developed, large crystals, evidently weathered out of the parent rock, were found scattered in the bed of a small stream in the neighbouring valley. On being traced further up, it was found that the zircons progressively concentrated, and later, large crystals were actually found embedded in the hard laterite forming the elevated ground adjoining the syenite ridge.

 

At Kizhkulam, behind the Primary School, ziron is found in the gravels in greater abundance than at Appiyodu. This area being under cultivation, ploughing the ground year after year results in zircon crystals which remained at depth being brought to the surface. Davidson (1946) has referred to this occurrence as “a very rich alluvial deposit”. A casual search for a few minutes would enable one to collect not less than a pound of the crystals. The edges and corners of the individual crystals are found to be somewhat worn out as a result of constant abrasion.

 

At Puttetti, about a mile east of Kizhkulam, zircon is found to occur ‘in situ’ in the diopside syenite. In regard to this occurrence, Davidson (1946) remarked, “a zircon-rich pegmatite which is considerably richer than any occurrence that I know of (possibly there are some equally rich occurrences in Madagascar)”. The crystals are unusually large, and form a conspicuous accessory of the syenite. Since fresh surfaces of the rock are not seen in the area, fresh exposures were made by blasting with the use of explosives. It was observed, that far from having any uniformity in the matter of distribution, the crystals occurred either as clusters or separated far apart in the syenite body.

 

At Puttetti, single crystals of zircon are found to measure from a fraction of an inch up to an inch or more in length. Rarely some crystals are even 2 inches long. The crystals are usually brown in colour, bit some of the smaller ones are honey-yellow. The lusture is usually adamantine. Crystals are mostly translucent, but some of the smaller ones are nearly transparent. The average specific gravity is 4.66.

 

Lacroix (1922, Vol. I., p.236), has recorded a similar occurrence of zircon discovered by Mr. Gauge, in the Mount Ampanobe. There, in the red earth were found abundant crystals of zircon with normal optical properties and density. Some of them have been found to be more than 10 centimeters along the vertical axis and weigh many kilograms.

 

Crystal habit

 

As a rule, crystals of zircon from the area show the prismatic and pyramidal faces to be well-developed. The typical features of the tetragonal form is well-displayed by some of the smaller crystals. In some crystals, it is observed, that the prismatic face terminates through two pyramids, in succession. The terminal pyramid makes an angle of 44º to 54º with the projected prismatic face, and the pyramid between the terminal pyramid and prismatic face makes an angle of 18º to 22º with the prismatic face. Parallel intergrowths of two zircon crystals are frequently seen. In this case, the faces and edges of one of the two individual crystals are found parallel to the corresponding faces and edges of the other. Rarely outgrowths of smaller crystals of zircon are found to emerge from the prismatic faces of larger crystals.

 

The most common variety of zircon occurring in the area is that which is elongated along the vertical axis, the horizontal axes being equal to each other. Occasionally, certain crystals are found to be distorted in different ways. One such instance is where one-pair of opposite lying prismatic faces is more flattened than the other pair, so much so the two horizontal axes are unequal, and the plane containing the two horizontal axes is rectangular in shape. Unequal development of the pyramidal faces is also caused by distortion. These distortional features may possibly be attributed as due either to restricted conditions under which crystallization took place, or to non-uniform conditions of pressure. No twinned crystal was noticed in the area.

 

To study the effects of heat on the physical properties, such as colour and specific gravity, a few crystals ‘hammered out’ of the host rock, after noting the initial colour and specific gravity, were heated to about 800ºC for nearly an hour. On cooling, it was noticed that the colour generally turned to white except locally where the colour remained unchanged. Richter (1932, cited by Brogger, 1890, p.102) has mentioned that hyacinth from Ceylon, when heated to red hot, changed its colour. Rivot (taken from the review of G. Spezia’s paper: Atti della Reale Accademia della Scienze di torino, Vol. xii, 1876. Mineralogical Magazine, 1877, No. 7, p.138), attributed colour change in zircons when heated, to the destruction of organic matter present in the zircon. In the same context, Chandler’s view is mentioned, according to which “changes of colour are caused by the different degrees of heat to which the mineral has been exposed”. Spezia explained the phenomenon as “alteration due to a difference in the state of oxidation of a colouring metallic oxide”. Kennard & Howell (1936, p.721) have mentioned, that “the cause of the colour in zircons generally has been attributed to the presence of impurities, particularly, iron, copper, titanium, chromium, vanadium, zinc, uranium, thorium, hafnium, and magnesium”.

 

The zircon crystals from Puttetti area are found to contain about 0.23 percent of ferric oxide and the reduction of this oxide to the ferrous state on heating, is probably the reason for the colour change.  One peculiarity is, that on the application of heat, the smaller crystals tend to lose colour easily, whereas in the case of larger ones the change in colour is not easily perceived. In a few cases, however, under the same conditions, no change in colour has been noticed at all.

 

As a result of heating, it was noticed that the specific gravity of the zircons from Puttetti has been increased by 0.10 to 0.18. Brogger (1890, p.102) has cited Svanberg to have observed a similar change in the specific gravity of zircon when heated red hot. Church (1875, p.323) has pointed out that all zircons do not show increase in specific gravity on being heated.

 

The effects of heat on some of the loose crystals disseminated in the gravelat Kizhkulam were also observed. It was found, that there was no change in colour and specific gravity in this aces. The crystals are deep-brown in colour, and are richer in the content of ferric oxide. According to Spezia (taken from the review of G. Spezia’s paper: Atti della Reale Accademia della Scienze di torino, Vol. xii, 1876. Mineralogical Magazine, 1877, No. 7, p.138), zircons rich in iron “do not become quite colourless, but assume a greenish tint”.

 

Chudoba and Stackelburg (1937, p.196) believed, that there is a basic connection between colour and internal structure or density of zircon. According to them, the density of zircon of lowest specific gravity can be considerably increased by heating to about 1450º. As particular examples, they have mentioned about the zircon of density 4.15 which could be raised to a density of 4.63 by heating. According to the same authors, zircons of low density are composed of amorphous SiO₂ and amorphous ZrO₂, while those of high density consist of crystallized ZrSiO₄. This amorphous nature of the components ZrO₂ and SiO₂, they believe, can have a leading influence in the density of the mineral.

 

Zircon from Puttetti area was found to be radioactive. Detection and determination of the radioactivity was facilitated by the use of a sensitive X-ray Electroscope (Mathai, 1943), originally devised by C.T.R.Wilson (1901) for the investigation of the natural ionization of gases. The instrument was subsequently modified to suit measurement of radioactivity of some of the feebly active mineral sands of Travancore. It consists of a gold leaf system fixed within a thick lead chamber, below which the specimens are mounted. The discharge of the gold leaf, due to the X-ray ionization is measured through glass windows with a low power microscope fitted with a micrometer eye piece. The insulation of the instrument is secured through a bed of sulphur. The instrument retains a charge for forty-eight hours, and it is sensitive enough to respond to the presence a few grains of monazite sand.

 

Zircon was powdered in an agate mortar, and a uniform layer of the powdered mineral was taken in a glass disc. The activity of the mineral was compared to that of a sample of known uranium content, and the uranium equivalent of the mineral was found to be 0.35. In this connection, it may be mentioned that ‘Malacon’ or brown zircon of Madagascar was reported to be invariably radiocactive, due to the presence of thorium in it (Lacroix, 1922, p.242).

 

The radioactivity of the zircon from the Puttetti area was also determined by a somewhat sensitive gamma counter (Viswanathan Nair and Krishnan Nair, 1953). The experiment was done at the Phycics Department of the University of Lucknow. Thorium in Beta active, but its fifth disintegration product is sufficiently strong in gamma activity to be measured by the Counter. The instrument was first standardized for the purpose of determining the background of cosmic shower which continue throughout the experiment. The counts due to the cosmic radiation were never constant, but varied between 30 and 60 per minute. The average of fifteen counts, each of one minute duration and made in-between the countings with the sample, was 47.5 counts per minute.

 

Next, the counts were taken with pure Thoria (ThO₂). Ten gms. of pure thoria were spread uniformly on a gummed cellophane of an area just enough to cover the counter tube. The average of fifteen readings was 115.7 counts per minute.

 

The same process was then repeated using 10 gms. of the powdered sample of zircon . The average of fifteen counts was in this 48.25 counts per minute.

 

Assuming that the gamma radiation in the crystal was due to thorium, the percentage of thoria in the sample was calculated to be 1.1.

 

The same experiment was repeated, this time on a sample of powdered zircon sands obtained from the beach concentrates along the coast of Travancore. It was found that the gamma activity of this sample was very low, compared o that of the fresh crystals ‘hammered out’ of the parent rock at Puttetti.

 

Chemical analysis of a sample of the fresh zircon crystal, obtained by crushing the host rock, was made. The results are entered in the Table VI. In this analysis, thorium has not been estimated. Quantitative estimation, subsequently done in the laboratory, gave the amount of thorium to be 0.83 percent. The radioactivity is therefore attributed to the thorium present in the mineral.

 

 

 

 

 

 

Appendix II: Five axis universal stage

 

The optical constants of minerals were determined by the Leitz Five-Axis Universal Stage. The instrument is provided with the following axes and is diagrammatically represented in Fig. 8.

 

1. Inner Vertical Axis (I.V.)
2. Inner East-West Axis (I.E.W.)
3. North-South Axis (N.S.)
4. Outer Vertical Axis (O.V.)
5. Outer East-West Axis (O.E.W.)

 

In the Four Axis Universal Stage, ‘2 V’ can be determined only in a section containing ‘Y’ in the horizontal plane, often with the aid of a stereographic projection, whereas in the Five Axis Universal Stage, 2 V can be measured in a section of random orientation. If ‘Y’ is in the horizontal plane, direct measurement of the 2 V is possible, but in case the ‘XZ’ plane is horizontal, 2 V can be determined using the Berek method.

 

A fragment of a mineral mounted on the universal stage, can be so oriented that one of its critical vibration directions is rendered parallel to the axis of the microscope and the others perpendicular to it; one north-south and the other east-west.

 

Adjustment of the Universal Stage

 

A strong source of illumination is necessary for universal stage work, and the source of light should be perfectly centered, to ensure accuracy.

The cross hairs are perfectly adjusted with reference to the vibration planes of the nicols. This can be effected as follows:

 

Take (010) section of gypsum, and determine the extinction angle from the cleavage – say 35º. Suppose the stage was turned to the right to get the above value.

 

Next, put the section, upside down, and get the extinction value, which will now be to the left. Let us suppose that it is 41º. The eye piece must be turned anticlockwise by 3º which means the cross hairs are turned now.

 

Thus the extinction value to the right must be equal to the value on the left, on reversing the section; the measurement being made with reference to the same cleavage.

 

The cross hairs having been adjusted, the objective is centered accurately.

 

Next, the universal stage is fixed on to the microscope stage by two thumb screws and has to be centered accurately so that it’s vertical axis may coincide with the microscope axis. This is done as follows:

 

1. Clamp the microscope stage and rotate the I.V., observing the centre of rotation choosing a dust particle on the glass plate.
2. If the centre of rotation does not coincide with the cross hairs, loosen gently the thumb screws holding the universal stage and bring this centre of rotation to the cross hairs. Clamp the thumb screws and test by rotating on the inner vertical axis. By successive adjustments, the centre of rotation of the universal stage can be very nearly brought to coincide with the cross hairs. Now, a rotation on the outer vertical axis should also be centered reasonable well. The thumb screws are clamped well in this position, and the slide mounted on the stage with liquid contacts. The next adjustment is such that the axis of the section coincides with the E-W axis.

 

The elevation of the mount is adjusted as follows:

 

For this, rotate on the outer east-west axis and note the movement of a particle on the slide. If the particle moves in the same direction as the rotation, the mount is too low; if in the opposite direction, too high. The elevation of the mount is adjusted by turning the collar which supports the inner stage plate in the desired direction. Perfect adjustment would ensure an object in the centre of the cross hairs remain almost stationary, on rotation of the O.E.W. now, on a rotation of the N.S. and E.W. axes, the particle should very nearly remain in position, as it was made to do for the rotation on the O.E.W. axis.

 

Next, the horizontal axes are aligned with respect to the vibration direction of the nicols which are considered set – polarizer N.S., and the analyzer E.W. then the tube of the microscope is raised and focused on some dust particle on the upper surface of the upper hemisphere. The O.E.W. axis is rotated and the movement of the dust particle is observed. If it does not follow the north-south cross hairs, sufficient rotation on the microscope stage is given to make it do so, and the microscope stage is fixed in this position. The vernier reading is recorded, as it is the zero position of the microscope stage for the setting, which should be carefully maintained for all the future readings.

 

Similarly, the north-south axis is rotated and the dust particle is made to follow the east-west cross hair, by suitable adjustment on the outer vertical axis.

 

Orientation procedure for the determination of 2 V and sign

 

In the orientation of a mineral mounted on the universal stage, the three vibration directions are set in such a way that one is parallel to the microscope axis and the others parallel to the polarizer and analyze respectively. Extinction is the optical feature employed in the orientation of the mineral. Extinction reveals vibration differences which are related to the planes of optic symmetry. The planes of optic symmetry are adjusted to the cross hairs by rotating the crystal to and from extinction. The following procedure is adopted in critically orienting a mineral, for the determination of 2 V and sign.

 

1. The inner vertical axis of the universal stage is turned and the mineral brought to extinction. The traces of the two vibration directions in the horizontal plane are now parallel to the polarizer and analyzer.
2. Extinction is tested by rotating on the outer east-west axis and north-south axis, noting on which axis the crystal departs least from extinction. It is advantageous to have this one as the north-south axis, as otherwise the outer east-west axis which has a greater freedom of movement cannot be used to the maximum advantage. If therefore, the rotation on the outer east-west horizontal axis gave less departure from extinction, the inner vertical axis should be turned through 90º. Now, one symmetry plane strikes roughly east-west and dips north or south.
3. To bring this symmetry plane parallel to the plane of analyzer, the inner east-west is inclined to a few degrees north or south. Now the mineral departs from the position of extinction. Extinction is restored by suitable rotation on the inner vertical axis. As before, the position of extinction is tested on the north-south axis, noting whether it departs more or less from extinction, than before. If more, then the direction of inclination on the inner east-west axis is reversed and extinction again restored by rotation on the inner vertical axis. North-south axis is again rotated on either side and extinction tested. By successive increments of inclination on the inner east-west axis, and testing on the north-south axis, a position is found for the inner east-west and inner vertical axes, at which, the crystal will remain at extinction when tested by wide rotations on the north-south axis. This step is done carefully, as final accuracy depends mainly on it. The desired setting for one symmetry plane being vertical and parallel to the plane of vibration of the analyzer, i.e., east-west, is obtained. But one optic symmetry plane mat dip east or west and the other will be normal to this. Suppose on optic symmetry plane dips east by 70º, the other optic plane will be dipping 20º west. The first one must be rotated clockwise by 20º to render it vertical. In other words, the north-south axis must be tilted clockwise. However, since the mineral is already at extinction, a rotation on the north-south axis under the present setting will only keep the mineral dark. Hence, the outer east-west axis is inclined so that the mineral departs from extinction; i.e., some light is observed. Then and then only, the north-south axis should be tilted to get darkness. The outer east-west axis is now rotated back to its zero position. Now, the mineral is set on the stage in such a way that one optic symmetry plane is vertical, another parallel to the polarizer and the third parallel to the analyzer.

 

Although the mineral is thus oriented with its cardinal directions parallel to those of the microscope, the recognition of the vibration directions X, Y, and Z, still remain to be made. For this, optic plane is identified and the relative velocities of the transmitted rays are also determined. This is done as follows:

 

1. Turn the microscope stage to 45º, say anti-clockwise. This makes one vibration direction northeast-southwest, and another northwest-southeast.
2. Rotate both ways on the outer east-west axis and observe if there is a fall of interference colour leading to darkness. If the optic axial plane is vertical and the optic angle not very wide, positions of extinction will be reached on either side and the values will be equal also. In such a position of extinction, the optic axis is obviously parallel to the microscope axis. Hence the accuracy can be checked by rotating the stage of the microscope when extinction should persist. The angle between the two optic axes is measured and this gives the value of the optic angle (2 V).
3. The presence of optic axes indicates the north-east direction as ‘Y’. by a suitable compensator, it is found out whether ‘Y’ is faster or slower. In case, ‘Y’ is found to be slower, the other vibration direction in the horizontal plane is ‘X’; that coinciding with the microscope axis is ‘Z’; ‘Z’ is the acute bisectrix; the mineral is therefore positive. If on the other hand, ‘Y’ proves to be fast, that in the horizontal plane will be ‘Z’ and that coinciding with the microscopic axis is ‘X’. The mineral is then negative.

 

If in step 2, the optic axes are not obtained, it may be that the optic axial plane is in the plane containing the microscope axis and the axis of rotation. A rotation of 90º on the outer vertical axis will render the optic axial plane normal to the axis of rotation. The optic axial angle (2 V), and sign are then determined as before.

 

In case the optic axes are not obtained in either procedures, then ‘Y’ can be presumed to be vertical, in which case 2 V cannot be determined in the usual way. In similar instances, the Berek method may be employed.

 

 

 

 

Berek’s Procedure

 

This procedure can be employed for the determination of the optic axial angle (2 V), for the three possible orientations. But since direct reading of the optic angle is possible when ‘Y’ is horizontal and the optic axial angle is not very large, this method may be conveniently used as a check.

 

The method is as follows:

1. The mineral is set in the critical position.
2. The outer vertical axis is turned through 45º, say clockwise.
3. Now the outer east-west axis is turned through 54.7º in a direction opposite to the general inclination of the mineral mount, to ensure a wide latitude of movement.
4. The microscope stage is rotated anti-clockwise still the mineral reaches the position of extinction, and the angle is noted.
5. The next step is to read the value of the optic axial angle (2 V), corresponding to the extinction angle obtained, by referring to the Berek’s graph (Emmons, 1943, p.31).

 

This method is only rarely adopted as slight changes in the extinction angle will affect the value of the optic axial angle (2 V) to a considerable extent. The error is much greater in the case of minerals having a small optic axial angle (2 V). Determination of the exact extinction position of coloured minerals like the amphiboles and pyroxenes which have greater absorption is often very difficult, even with the use of a sensitive tint. This may be avoided to a certain extent by taking the average of the two readings when the mineral reaches and departs from the position of extinction.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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