GROUP DISCUSSION

ON THE GRANULITE FACIES TERRAINS OF SOUTHERN PENINSULAR INDIA

ABSTRACTS

14th & 15th February 1976 MADRAS

Sponsored by THE GEOLOGICAL SURVEY OF INDIA THE GEOLOGICAL SOCIETY OF INDIA 1976 General

SOME ASPECTS OF THE EVOLUTION OF THE INDIAN PRECAMBRIAN SHIELD

T. M. MAHADEvAN Atomic Minerals Division, Hyderabad

The granulite facies rocks of the Indian shield have a complex polymetamorphic history and constitute a tectonic unit whose space-time relationship to the' Dharwars' is not well understood. Based on observations in many parts of the shield it is suggested that the granu­ lite facies metamorphic belt represents aplate evolved in the deep dry zones of the Early Precambrian crust and its geological identity has been partially destroyed by' post-Middle Precambrian metamorphic and tectonic events. The plate has been dissected by fractures and faults leading to the mantle, along which degassing has led to influx of water and consequent diaphthoresis, development of granitic magmas at places emplaced into weak structural zones and sulphide mineralisation. Islands of the granulite facies rocks in younger gneissic complexes are relicts of the segments of the plate involved in younger orogenic regimes. The remarkably contrasting trends in evolution of the granulite facies and the 'Dharwar' formation with regard to sedimentation, tectonic styles, magmatic history, geochemistry and mineralisation are due to their independent evolution in different levels of the crust, possibly, within parallel or overlapping time ranges. The juxta position of the two regions is due to later movements.

THE TWO PYROXENE FACIES ROCKS OF SOUTHERN INDIAN PENINSULA

M. ZIAUDDIN Geological Survey ofIndia, Bangalore

The two pyroxene facies rocks mostly comprise the khondalite suite consisting of the khondalite (garnet-sillimanite-graphite schists) leptynite (garnet-granulite) representing the pelitic sediments together with quartzite, garnetiferous quartzite, garnet-sillimanite-quartzite, representing the psammitic part; and calc-gneiss, calc­ granulite and crystalline limestone representing the marly parts of the meta­ sedimentary pile. These rocks perhaps represent the oldest metasedimentary group into which the coarse augite-hypersthene granulite (meta-gabbro) and medium grained hypersthene granulite (meta-norite) perhaps intruded. These are referred to as basic charnockite. Occasionally bands of hypersthenite also occur which perhaps re­ present the ultramafic intrusives and are referred to as ultrabasic charnockite. The above basic and ultrabasic charnockites occur mostly as bands, lenses, rafts etc., within the khondalite suite. GROUP DISCUSSION ON GRANULITE FACIES 143 Intimately associated with the above suite of rocks, large bodies of hypersthene granite and granodiorite occur as roots of the mountain chains representing the batholith and stocks showing granulitic texture. Apparently, they appear to be intrusive not only into the khondalite suite of metasedimentary sequence but also into the associated ultrabasic and basic charnockites. Thus, it would appear that the ultrabasic and basic types are of mantle derivation whereas the acid intermediate type are of deep crustal origin, probably derived from secondary magma. The term 'charnockite' is usually applied to any rock which contains hyper­ sthene. Since the ultrabasic and basic types although possess consanguity of character, yet having been derived from different sources are of different age than the acid and intermediate type. Hence they should not be grouped under 'charnockite series'. These rock types should always be referred to with the qualification •ultrabasic' and' basic' and not just charnockite to distinguish them from the acid to intermediate types. The term' charnockite' should be applied only to the acid and intermediate types. The two pyroxene facies rocks regionally occupy a distinct area clearly separated from that of the migmatite (Peninsular gneiss) and present distinct metamorphic .environments, The two pyroxene facies, therefore, be equated with the Eastern Ghat facies and considered as a separate entity and not clubbed with Peninsular gneisses.

THE CHARNOCKITIC TERRAINS AND METAMORPHIC FACIES

M. S. MURTY Geology Department, S. V. University, Tirupati

The charnockites, pyroxene granulites, khondalites, calc-granulites and quartzites are in intimate association in the Eastern Ghats, and similar associations are known from several other regions in India and elsewhere. The mineral assemblages of these associations are listed below to assign them to specific mineral facies.

1. Charnockites: Quartz, orthoclase, plagioclase, hypersthene ±garnet, brown biotite. 2. Pyroxene granulites: Plagioclase, salite, hypersthene, ± brown hornblende, brown biotite, garnet, iron ore. 3. Khondalites: Quartz, K-felspar, plagioclase, garnet, sillimanite ±biotite, cordierite, hypersthene. 4. Calc granulites: Diopside, wollastonite, scapolite ± grossularite, plagioclase, quartz. 5. Quartzites: Quartz j perthite, plagioclase, garnet, sillimanite, hypersthene.

The anhydrous assemblages represent pyroxene granulite subfacies; the hydrous assemblages hornblende granulite subfacies; the wollastonite-bearing and cordierite­ bearing assemblages, cordierite granulite subfacies of granulite facies metamorphism. 144 GROUP DISCUSSION ON GRANULITE FACIES

The hydrous assemblages may have resulted from local variations of the chemical' potential of H~O during metamorphism; the wollastonite-bearing and cordierite-­ bearing assemblages could develop regionally under the same physical conditions in rocks of appropriate bulk composition. The charnockitic rocks were formed !/ot great depths under granulite facies of­ metamorphism; the charnockites were the re~ult of the crystallisation from palingenetic magmas formed at such great depths and they occupy their present position due to­ repeated uplift.

OXYGEN ISOTOPE STUDIES TO ELUCIDATE THE PETROLOGIC E"VOLUnON OF ARCHAEAN CHAR~OCXnlC C'OMPL"'itX'i~S 'OIl l"Nl)lA_

S. VISWAJ'lATHAN Atomic Minerals Division. Hyderabad

Modern mass-spectrometric and chemical-extraction techiniques make it possible to apply the principles of oxygen-isotope geochemistry to elucidate the petrologic evolution of Archaean charnockitic complexes of India (' ACCI'). Most probably, these complexes evolved from cyclic episodes of ultramafic to mafic to felsic volcanism, and cycles of penecontemporaneous sedimentation, accompanied by granitic pluton­ ism, deformation, metamorphism, and partial melting of a volcanic and volcanogenic sedimentary pile of very Early Precambrian age. It is quite likely, therefore, that some of the rocks of' ACCI ' were derived ffom more than one parent rock: ultrabasic charnockites from peridotitic komatiite and pyroxenite; basic charnockites from basaltic kornatiite, tholeiite, norite, and gabbro; intermediate charnockites from andesite and diorite; acid charnockites from dacite, rhyodacite, rhyolite, tonalite, trondhjemite, adamellite, and granite; leptynites from rhyolite and feldspathic quart­ zite; quartz-magnetite granulites from chert, ironstone, and jaspilite; calc-granulites­ from calcareous quartzites; and khondalites from porphyries, tuffs, greywacke, and shale. 18 As quartz (a mineral enriched in the heavier oxygen isotope, 0 ) and magnetite 18 (depleted in 0 ) occur in practically all the rock types of' ACCI', the paper suggests that, as a first step, <5018 quartz (10 to 16 permil in silicate igneous and metamorphic rocks) and <5018 magnetite (1 to 2 permil) be determined on as many samples as possible. The data could then be used (1) to distinguish the igneous components from the metasedimentary assemblages; (Z) to delineate within the granulite facies terrains of the Southern Peninsular Shield of India differing thermal regimes based 18 16 on the 0 - 0 fractionation betweem quartz and magnetite (in areas where they are in isotopic equilibrium); and (3) to evaluate the relative roles of progressive and retrogressive metamorphism in the evolution of the hornblende-bearing and pyroxene­ bearing granulites. GROUP DISCUSSION ON GRANULITE FACIES 145

A MODEL ENVISAGING ARCHAEAN CHARNOCKITIC TERRAINS OF INDIA AS GRANULITE-FACIES METAMORPHOSED GREENSTONE BELTS: IMPLICATIONS FOR DISCOVERIES OF NEW GOLD DEPOSITS

S. VISWANATHAN Atomic Minerals Division, Hyderabad

The recent suggestion by Viswanathan (Geol. Mag., v. 112, 1975, p. 63-69) that the pre-metamorphic evolution of Archaean charnockitic terrains followed the same trends that characterized the development of Archaean greenstone belts has important implications for exploration of new gold deposits in charnockitic terrains of the Southern Peninsular Shield of India. These implications arise because of several considerations: Gold is one of the most characteristic of the ore deposits of Archaean continental nuclei. According to current thinking, the gold in these deposits, most of which are located in greenstone belts, is believed to have originated from the ultramafic-mafic volcanic rocks including the pristine peridotitic and basaltic komati­ ites. The extraction of the gold dispersed in these rocks, and its concentration into workable ore deposits, are attributed to the wall-rock preparation effected by the post-komatiitic early tonalitic plutons, and to have hydrothermalizing influence of the later potassium-rich granitic plutons. Ifindeed, as has been suggested by Viswanathan (ibid), the Archaean charnockitic terrains of Southern India represent greenstone belts that have been metamorphosed under conditions of the granulite facies, with (a) the ultrabasic and basic charnockites being the derivatives of peridotitic and basaltic komatiites, and basalt-andesite; (b) the enderbites being the metamorphic equivalents of early Precambrian tonalites; and (c) the acid charnockites being granulite-facies metamorphosed potassium-rich granites, the chances of discovering new gold deposits in these terrains are good. The most favourable targets for exploration are those wherein the pyroxene-granul. te facies rocks have been retrograded to the hornblende-granulite and almandine­ .amphibolite facies.

METAMORPHIC FACIES OF THE SO-CALLED GRANULITE BELT

T. V. VISWANATHAN Geological Survey of India

The natural mineral assemblages and the phase characteristics like Py content of garnets, AI.Oa (Wt%) of both ortho-and clinopyroxenes and the proportions of Tschermak's molecule (as expressed by 6 ~') values) in both the ortho- and c1ino­ pyroxenes in the rocks of the so-called granulite belt provide a basis for a meta­ morphic facies classification in accordance with the principles ofthe working scheme adopted by the working group (Zwart et al, 1967) for the cartography of metamor­ phic belts of the world. The proposed scheme of classification provides a better 146 GROUP DISCUSSION ON GRANULITE FACIES understanding of the P-T conditions of equilibration of the different mineral assem­ blages and can be used as a guide for preparation of metamorphic facies map of the shield. The assemblages are grouped in the following order corresponding to increasing pressures: ASSEMBLAGES

Quartzo-!efdspathic Basic-ultrabasic Facies Pelitic to mafic Low pressure two- i) quartz+ Kvfeldspar-l- quartz + K-feldspar plagioclase+ pyroxene facies garnet (low pyrope < 22%) -l-sillimanite-l- orthopyroxene+ garnet clinopyroxene hornblende ii) quartz+K-feldspar+ I I plagioclase +orthopyroxene I (low alumina) I I I iii) cordierite (Mg)+ J I orthopyroxene (low I alumina)+K-feldspar ± I I I Intermediate i) quartz-l-Kvfeldspar-l- I plagioclase+ pressure two plagioclase+ orthopyroxene I orthopyroxene pyroxene facies (high alumina) {- (high alumina) Higher T ii) quartz-l- K-feldspar+ plagioclase+ plagioclase (antiperthite)+ orthopyroxene+ orthopyroxene+ Clino- clinopyroxene+ pyroxene (high alumina) garnet (med. pyrope 22-47%) iii) quartz-l- K-feldspar+ plagioclase (antiperthitej-l- orthopyroxene-l-Clino- pyroxene-l-garnet (med. pyrope)

Hi-ih pressure I quartz+K-feldspar+ plagioclase+ I i) t'll o-pyroxene I plagioclase-l-garnet clinopyroxene-l- facies I (high pyrope) garnet (high I I pyrope) I I I ii) quartz+K-feJdspar+ clinopyroxene I garnet (high pyrope (highalumina)+ I I more than 47% Py) garnet (high I pyrope) ±Ortho- I pyroxene (high 1 Increasing P I alumina) I I I iii) sapphirine+ blue spinel+ I orthopyroxene± sillima- I nite± cordierite ± quartz I J I iv) kyanite-l-sapphirine-l-blue t spinel± quartz±cordierite In vast areas, there are evidences to indicate that the quartzo-feldspathic assem­ blages of the two-pyroxene facies represent orthogneissic dry intercalations in amphi­ bolite facies terrains with paragneisses where metamorphism has largely been iso­ chemical. It is suggested that the term two-pyroxene facies be adopted instead of granulite facies for a rational understanding and classification of the mineral assemblages. GROUP DISCUSSION ON GRANULITE FACIES 147

A STUDY OF KHONDALITES FROM THE TYPE CHARNOCKITE AREA

SOBHEN RAY Department ofMining & Geology, Bengal Engineering College, Howrah

Khondalites, practically the universal associate of charnockites, have not received due attention hitherto, at least in the type charnockite area. In terms of mineral assemblages, three different types can be distinguished. The most common variety is (i) quartz-garnet-sillimanite-kyanite-(spinel-opaque-biotite), (ii) quartz­ garnet-sillimanite-kyanite-alkali feldspar-Ispinel-opaque-biotite) and (iii) quartz­ 'garnet-sillimanite-kyanite-biotite-(alkali feldspar-spinel-opaque-muscovite) contain­ inglittle potassium-bearing phase(s). Kyanite, though minor, is present in all these types. Besides, there are two thin bands in the area with high amounts of both biotite and alkali feldspar, which may be designated as 'impure' varieties. Hypersthene­ free cordierite granulites occur locally within the khondalite patches. Presence of folded sillimanite trails within garnet in a helicitic fashion in khondalites indicates evolution of assemblages through a reaction: biotite + silliman­ ite + quartz-s-garnet + K-feldspar + water. On the other hand, the conspicuously secondary nature of biotite (as evident from textural features), and the presence of sillimanite needles and granules intergrown with biotite and quartz in 'impure' varieties clearly indicate that both forward and backward movement in terms of this reaction have taken place, probably separated in time. The progressive step might have been followed closely by anatectic melting while the reverse reaction was initiat­ -ed by introduction of alkalies derived from the anatectic melt. Estimation of a minimum temperature in the neighbourhood of 600°-650°C in associated rocks and a load pressure of around 6.5 kilobars for the univariant kyanite-sillimanite curve cor­ responding to these temperatures corroborates the likelihood of anatexis. The absence of similar reversion in the major leptynite bands is not well understood; it may either be due to attainment of higher ,uH,o in spots and/or susceptibility of only particular types of compositions to the biotite-forming reaction. Andhra Pradesh

GRANULITES OF ANDHRA PRADESH

M. R. SUBRAMANYAM AND KULDEEP SINGH Geological Survey of India

High grade granulitic rocks in Andhra Pradesh constitute the Eastern Ghats belt which extends from south of Krishna river to as far as Srikakulam district. They consist of meta-sediments and meta-igneous rocks belonging to the khondalite and charnockite groups. The meta-sediments show a definite facies variation. Both the­ litho-units have been extensively migmatised, as indicated by the lit-par-lit injections in khondalite and the derivation of intermediate and acid charnockites due to the granitisation of earlier pyroxene granulite. Migmatisation has also resulted in the formation of garnet-biotite gneisses and other hybrid rocks from charnockites and khondalites. The granulite belt has been intruded by syntectonic and post-tectonic intrusions. An attempt has been made to elucidate the structural history and the evolution of the belt based on the available data. Karnataka

CHARNOCKITE-GNEISS COMPLEX RELATIONSHIP IN SOUTHERN KARNATAKA AND ITS BEARING ON CRUSTAL DEVELOPMENT

A. S. RAMIENGAR, M. RAMAKRISHNAN AND M. N. VISWANATHA Geological Survey ofIndia, Bangalore

The charnockites of southern Karnataka occur at the fringes of the greenstone­ migmatite terrain. The interrelationship of charnockites and migmatitic gneiss complex has been studied by the authors in the Biligirirangan and Coorg massifs with their disjointed extensions and in the lenticular masses near Kushalnagar and Karya, The contacts between the charnockites and migmatites are invariably diffuse, leading to the 'chicken-egg syndrome' of their relationship. Replication of all types of migmatitic structures by conformable charnockite bodies, discordant' tree structures' of charnockites obliterating migmatitic structures at their periphery, and prograde mineralogy of biotite in the gneisses to hypersthene in charnockites strongly suggest that the gneiss complex has been transformed in situ into charnockites without much attendant rheiditic deformation. The charnockite buds probably represent nucleii of dehydration leading at greater depths to charnockite massifs as residues of partial melting of sialic crust. Recently a major' pantectogenic' event of 2900-3000 m.y, involving the high grade schists (Sargur Schist Complex) has been identified in the gneiss complex. These schist enclaves are interleaved with the charnockites at the cratonal margins. Further, the schists themselves show progressive metamorphism from upper amphi­ bolite facies in the cratonic interior to lower granulite facies at its periphery. This suggests that the post-Sargur pantectogenic event has culminated in the granulite facies metamorphism and production of charnockites. The apparent progressive metamorphism from greenschists to granulites in the craton is the result of polymeta­ morphism and regional synclinorial structure plunging NNW. The broad conformability of the lithotectonic elements in conformity with the V-shaped configuration of the Peninsula, together with apparent progressive meta­ morphism have largely obscured craton-mobile belt relations in the region. The charnockite region lacks pronounced linearity, bounding transcurrent dislocations, and proterozoic sediments, but shows high grade metamorphism and complex defor­ mational patterns of a mobile belt. While it is closely associated with high-grade schists, it is far away from greenstone belts and has possibly formed the provenance for the latter. The mobile belt has been repeatedly reactivated, as evidenced for example by emplacement of pregranite dykes during 2500-2600 m.y, (granulite isochron). Charnockite belts have been variously interpreted as geosynclinal pile, upthrust block, paired metamorphic belt, remnant of continent-continent collision and as transform zone. The charnockite mobile belt wraps around the protocontinental mosaic of Indian Shield forming lofty mountains at the shield margin, and may re­ present a major Archaean continent-continent collision phenomenon like the present day Himalayas. The greenstone belts probably formed in rift-like fractures as a result of this collision. 10 150 GROUP DISCUSSION ON GRANULITE FACIES

GEOCHEMICAL TRENDS OF HIGH GRADE METAMORPHISM AS REFLECTED IN THE CHARNOCKITES OF KARNATAKA STATE

V. S. VENKATASUBRAMANIAN AND S. JAYARAM Department of Physics, Indian Institute of Science, Bangalore

Granulite facies rocks-charnockites and associated gneisses ofSouth Karnataka have been dated by the Rb-Sr method and their trace element constitution investigated with a view to elucidating geochemical trends at deep crustal levels. The whole rock isochron age (2670±60) m.y, is not inconsistent with the hypothesis of a high grade metamorphism of am older basement into both amphibolite and granulite facies formations. The granulite facies rocks show significant Rb-depletion and appreciable Sr and Ba enrichment resulting in enhancement of K/Rb ratios (575 ±40), and decrease of Sr/Rb (0.13 ±0.03) and K/Ba (17 ±8) ratios. Similarly, there is considerable U and Th depletion and a decrease in Th/U ratios. The ferromagnesian trace elements show a magmatic trend: Cr (81 ± 15); Ni (62±12) V (156±40) and Co (22±6), in contrast with the amphibolite facies gneisses. The mechanisms that could explain the metamorphic trend for the granitophile elements and the magmatic trends for ferro­ magnesian trace elements are discussed, with special reference to element fractionation during anatexis.

CHARNOCKITES OF BILIGIRIRANGAN AND NILGIRI HILLS

M. N. VISWANATHIAH AND J. A. K. TAREEN Mineralogical Institute, Manasa Gangotri, Mysore

The petrographic study of the hypersthene bearing granolites of Biligirirangana Hill range and Nilgiri hills show the following mineral paragenesis. The felsic members are represented by hypersthene + biotite + garnet + plagioclase + perthites + quartz granolites, which are rich in quartz but enderbitic in composition. The mafic members contain, in addition to plagioclase and minor amount quartz, the following assemblage: diopside + hypersthene, diopside + hypersthene + hornblende, diopside + hypersthene + hornblende + garnet. The mafic members are complexly intercalated with the felsic members, as bands, lensoidal bodies, streaks and stringers. The complex mineral assemblage, and compo­ site nature of the rocks is attributed to (1) the local compositional variation and (2) to the variation of Pc with reference to PI during metamorphism. Such variations are attributed to the anatectic mixing of pelitic and semipelitic sediments and the associated basic intrusives, during high grade granolite metamorphism. GROUP DISCUSSION ON GRANULITE FACIES 151

GRANULITES OF SOUTHERN KARNATAKA

T. C. DEVARAJU AND M. S. SADASHIVAIAH Department ofGeology, Karnataka University, Dharwar

The granulite facies rocks of Southern Karnataka are represented by charnockites, metasediments and metabasic minor intrusions. The associated rocks are amphibo­ lite facies gneisses (Peninsular), granites (Closepet) and basic to alkaline minor intrusions. Charnockites are predominantly in the subacid-basic range; contain two pyroxenes and plagioclase of An28 to An". Compared to the type area charnockites, these are light coloured, moderate to strongly banded and usually contain hornblende and/or biotite as chief mineral phases. The metasediments are preponderantly ferru­ ginous (with iron-rich pyroxenes), siliceous and aluminous (with sillimanite, cordie­ rite, almandine, biotite). Calcareous (with scapolite, grossularite, ferrosalite) type is of restricted occurrence. Granulites of the minor intrusion character are all basic, comparable to hornblende gabbro-norite/pyroxene diorite and they are enriched in chromium. These are unrelated to charnockites and metasediments and are much younger. Charnockites and metasediments bear conformable field relation, but there is little evidence of mineralogical and chemical gradation between the two. The granulites represent a set of geosynclinal deposits that have been metamorphosed about 3 b.y. ago under PT conditions of intermediate-low pressure granulite facies. Mineralogical/chemical variations, banding and lithological boundaries of the differ­ ent members ofthese granulites are chiefly the relic characteristics ofthe original geo­ synclinal deposits; no important migration of the component elements across the banding/lithological boundaries has taken place. Patches of charnockites occur strewn all over the gneiss areas and show all stages of gradation to the enclosing rock. There is convincing evidence of formation of the bulk of the gneisses by the retrograde metamorphism and granitization of particularly the charnockites about 2.5 to 2 b.y. ago.

GRANOLITES OF SIVASAMUDRAM AREA, KARNATAKA

B. MAHABALEswAR AND C. NAGANNA Central College, Bangalore University, Bangalore

Sivasamudram area represents a deeply eroded part of the Archaean terrain, where two-pyroxene facies rocks are exposed. The two-pyroxene facies rocks are represented by charnockitic granolites which are formed under regional hypersthene zone metamorphism. Based on the quartz, alkali felspar and plagioclase proportions, the charnockitic granolites are divided into hypersthene-perthite granolite, hornblende bearing hypersthene-pyroclase granolite, hornblende-bearing charno-enderbitic granolite and charnockitic granolite. 10* 152 GROUP DISCUSSION ON GRANULITE FACIES From the field evidences like pronounced gneissosity, rhythmic and non­ rhythmic interlayering of the different varieties of granolites with sharp contacts and interbanding of metasediments, petrographic evidences like absence of diffusion grain boundaries, presence of exsolution perthites, rounded and zoned zircons and absence of metasomatic reworking, and petrochemical evidences like simple chemical com­ position comparable to sediments, it is proposed that the source material may be of sedimentary origin. The petrochemical similarity between the sediments and charno­ ckitic granolites may be due to the fact that some part of the charnockite meta­ morphism was isochemical. It is also concluded from the petrography and mineralogy that the charnockitic granolites, especially the types which lack hornblende are formed under very high temperature, very low PH•O and very high Ps, at deeper parts of the crust. The horn­ blende-bearing charnockitic granolite (two pyroxene granolite) on the other hand could be taken as being formed under P-T conditions favourable for the coexistance of both hornblende and hypersthene i.e. under conditions of T, P, and PHzO adequate to produce the above assemblage, and probably are not formed as a consequence of diapthoresis of hypersthene granolite. The above situation can be explained by envisioning the availability of water from the environment and the behaviour of the parent rocks as partially open systems. The relation of the charnockitic granolites with respect to the associated gneisses is established b'y detailed field study and it is concluded that the gneisses are products of breaking down of the charnockitic granolites.

CHARNOCKITES FROM HULLAHALLI CHANNEL SECTIONS SOUTH OF MYSORE CITY

A. S. JANARDHANAN AND H. M. RAMACHANDRA Department ofGeology, Manasa Gangotri, Mysore

Channel sections south of HulJahaJli expose charnockite as patches and bands within the gneisses. The predominant rocks of the area are leucogneisses with bands of amphibolite (±garnet), numerous ultramafics and late pegmatite veins. Patches of typical paragneisses are seen grading on to the leucogneisses, indicating migmatitic origin. The development of gedritic anthophyllite from hornblende in the amphibo­ lites and in the paragneisses indicate high temperature of formation. Numerous ultramafic lenses ranging in composition from highly serpentinised dunites, peridotites, orthopyroxenites, norites and gabbros occur. The ultramafics have been affected by the regional metamorphism and subsequent migmatisation, as indicated by (1) the development of sagvanditic assemblages, (2) the breakdown of big lamellar orthopyroxene plates to more granular hypersthene and (3) the common occurrence in the field of boudins of ultramafics in the gneisses. The two pyroxene granulites (meta-norites) often exhibiting pseudotachylitic veins, occur as restite dykes GROUP DISCUSSION ON GRANULITE FACIES 153 and patches in the gneisses. In vertical channel sections, the granulites can be seen cutting the amphibolite bands of the gneisses. The granulite facies metamorphism has affected the paragneisses also. Ortho­ pyroxene is developed in clusters, in the paragneisses. The orthopyroxene (2Vac 70°- 75°) have anthophyllite inclusions. In the field, the paragneiss grades on to an acid charnockite with a typical greasy appearance, but still exhibiting traces of antho­ phyllite. The basic to intermediate charnockites (enderbitic) of the area can be traced from the metanorites, metagabbros and pyroxenites. The above account clearly shows that the granulites or rocks of the granulitic facies cannot form the basement rocks in the present area. The charnockites of Hullahalli area were formed by the migmatisation of meta-norites and other basics rocks, forming part of an ultramafic sequence.

GNEISS GRANULITE RELATIONS IN KABBAL, KARNATAKA

R. SRINIVASAN, B. L. SREENIVAS Geomysore Services, Bangalore AND V. N. VASUDEV Chitradurga Copper Co., Bangalore

In the Kabbal quarry, two generations of charnockites are distinguished. The earlier charnockites through diaphthoresis and migmatization have given rise to gneisses. The later charnockites have developed mainly along shear fold axes and shear fractures. The former represent a pre-Dharwar Charnockite episode and the latter may be related to the Eastern Ghat episode. Kerala

CERTAIN FACTS OF THE HIGH GRADE METAMORPHIC ROCKS OF KERALA AND THEIR SIGNIFICANCE

Y. G. K. MURTY, P. K. THAMPI, U. S. REDDY AND M. M. NAIR Geological Survey of India, Trivandrum

Charnockites and Khondalite group of rocks constitute the granulite facies terrain in Kerala. Charnockites are represented by hypersthene gneisses and granu­ lites, largely of inter~ediate composition. Pyroxene granulites, commonly called as 'basic charnockites' occur as bands. Khondalites consist of garnet-sillimanite gneiss, garnet-biotite gneiss and leucocratic quartzo-feldspathic granulites. Khonda­ lites are well developed in southern Kerala and charnockites in central Kerala. The two are separated by Achanokovil shear. Charnockites and khondalites are often intercalated. Metasedimentary quartzites, calc-granulites and magnetite quartzites, occur intercalated with members of both charnockite and khondalite groups. The charnockites although termed as pyroxene gneisses, are really layered granulites. Thin layered hypersthene granites and thick layered massive charnockites could be genetically different. The former may have resulted from sedimentary or volcano­ clastic material. The thin layered hypersthene gneisses by reaction with rising magmatic or migmatic fluids have given rise to biotite, garnet and in some cases cordierite assemblages, as seen in the quarry at Pathirikkal near Edathora. Pyroxene granulites have behaved as resisters. Development of hypersthene in charnockite group rocks and garnet-sillimanite in khondalite group under similar metamorphic conditions is attributed to the original chemical composition. Apart from charnockites and khondalites, in northern Kerala, Wynad plateau, and between Tellichery and Quilandy, biotite and biotite-hornblende gneisses have developed. The Wynad gneisses join with the Peninsular gneisses of Mysore pleteau, Another set of gneisses of granitic composition is widely developed in the Mannar area. These are intercalated with charnockites of Idikki region and overlie them. The Munnar gneisses consist of a lower grey variety containing orthoclase perthite and an upper pink variety containing predominantly microcline perthite. Plagioclase is more in grey variety. These granites and biotite gneisses of amphibo­ lite facies are distinctly different from the underlying hypersthene gneisses and granu­ lites of the two pyroxene facies. Degranitization in the granulite layer is suggested to have provided the granitizing fluids for the formation of granites and gneisses in the upper layer, thus suggesting the development of granites, gneisses and granulites during a single regional mobilization episode. Orissa

GRANULITE BELT OF ORISSA

J. SWAMI NATH AND K. SRINIVASACHARI Geological Survey ofIndia. Bhubaneshwar

The granulite belt of Orissa comprises of charnockite and khondalite groups and other associated rock types of the Eastern Ghats Super Group and extends from Andhra Pradesh into the Southern districts of Orissa. The rocks of the granulite belt have a regional NE-SW trend of foliation with a major cross fold on NW axis. They are faulted in the north along E-W Brahmani lineament. Biotite gneiss migma­ tites appear to form the basement for these granulite rocks. Intrusive into these are syenite nepheline syenite, anorthosite, pink porphyritic granite, ultramafic and mafic rocks. There are evidences of retrogression of the granulite rocks similar to those of southern Karnataka and Tamil Nadu, The granulite rocks of Orissa formed the craton for Iron-ore geosyncline, and are faulted along the boundary which brought in mantle derived ultramafic rocks. Recent work in several parts of the granulite belt indicates that rocks of the Sukma group folded with the granulite charnockite appears to be older than khonda­ lite within the granulite terrains. However some of the charnockite outcrops appear to be intrusive into the khondalite. These need detailed structural mapping. The granulite belt hosts important economic mineral deposits of the state like graphite, limestone and bauxite. The authors feel that Orissa poses problems related to strati­ graphy, tectonics, mineralogenesis and fundamental and historical geology of granu­ lite belt of Peninsular India and hence suggest detailed structural work and geo­ chronological work in selected parts of the State.

TECTONOMAGMATIC EVOLUTION OF THE CHILKA LAKE IGNEOUS COMPLEX, ORISSA

A. SARKAR, L. BHANUMATHI AND M. N. BALASUBRAHMANYAN Geological Survey ofIndia, Calcutta

The Chilka Lake segment ofthe Eastern Ghats Precambrian orogenic province is characterised by apparently isolated occurrences of anorthosite and related rocks over a wide belt amidst a catazonal environment of high grade gneisses and schists, the latter showing effects of superposed folding. Review of earlier works reveal that the anorthosites and related rocks of this belt are traditionally believed to belong to three discrete and distinct occurrences viz. (I) Balugaon-including Banpur, Sunakhala (2) Kallikota and (3) Rambha. ~ecent field and laboratory studies by the present authors and a critical evaluation of the available data, however, reveal that contrary to earlier belief, these apparently isolated occurrences of anorthositic rocks, in fact, belong to a single syntectonic composite plutonic complex. To this complex which 156 GROUP DISCUSSION ON GRANULITE FACIES evolved through a prolonged plutonic-orogenic episode of this sector of the Eastern Ghats-the authors propose the name' Chilka Lake Igneous Complex'. In spite of assembling a cogenetic suite of rocks of anorthosite kindred, the complex shows development of intrusive units with contrasting. attributes-such contrasts being ascribed to (i) nature and trend ofcrystallisation differentiation of the dioritic parent magma in a syntectonic environment and (ii) magma tectonics prior to immediate emplacement at its present level and during ascent. Causes for the generation of the dioritic parent magma is briefly discussed.

ON THE STRUCTURE AND LITHOLOGY OF THE EASTERN GHATS GROUP IN PARTS OF ORISSA

B. DASH Department ofGeology, Utkal University

Detailed studies in three critical areas of the Eastern Ghats terrain point to certain interesting structural and lithological features. The structural complexities in these areas vary in a manner suggesting the presence of long and rather straight limbs converging at closure zones with shorter limbs. This pattern is due to poly­ phase folding and imparts sinuosity, in detail, to the Eastern Ghats trend. Structural domains of this type are typically orogenic, but there are also some different struc­ tural domain-types within the terrain. These match with lithological heterogeneity and may indicate different deformational and metamorphic milieu. The broad litho­ logical mosaic of the terrain is attributed to migmatization, Charnockites, leptynites and quartzo-feldspathic gneisses (at least some of these) seem to represent products of migmatization of the pre-existing rocks. Basic granulites of the investigated areas are interpreted to represent dykes, sills (may even be flows in parts) associated with the metasediments but it is concluded that there are basic granulites of different parentage and age when the entire Eastern Ghats terrain is considered. GROUP DISCUSSION ON GRANULITE FAClES 157

GRANULITE FACIES ROCKS OF EASTERN GHATS GROUP IN ORISSA

S. NARAYANASWAMI Madras

The author proposed the 'Nilgiri/Eastern Ghats Group' for the high-grade .granulite facies rocks of'Charnockite Series' of Holland and' Khondalite Series' of Walker occurring in Tamil Nadu-Kerala-South Karnataka and in Andhra-Orissa regions in the southern and eastern parts of Peninsular India. These rocks constitute the oldest Archaean Group in the Peninsular Shield and show unequivocal evidences for the presence of a distinct geosynclinal stratigraphic assemblage of a lower meta­ volcanic' Charnockite Suite' and an upper meta-sedimentary 'Khondalite Suite'. Within the lower Charnockite Suite, there are no evidences for clear-cut intrusive relationship of a magmatically differentiated 'Charnockite Series' as originally proposed by Holland, but only a hypo-metamorphically (poly-metamorphic), palin­ genetic, migmatitic assemblage of 'Charnockite Suite' consisting of hypersthene­ bearing pyroxenite (ultrabasic charnockite), pyroxene granulite (basic charnockite), hypersthene-diopside-hornblende gneiss migmatite (intermediate charnockite) and hypersthene-bearing alaskitic granite (acid charnockite) and other genetically related rocks like birkremite, enderbite and hypersthene-quartz syenite, which Subramaniam (1959) distinguished as 'Charnockite sensu stricto' in the type area near Madras. The stratigraphically overlying 'Khondalite Suite' consists of quartzite, pyroxene­ bearing magnetite-quartzite, crystalline limestone and calc-granulite, garnet quart­ zite/garnet-sillimanite quartzite/garnet-silIimanite-graphite-felspar gneiss (khondalite proper), garnetiferous biotite gneiss migmatite, garnetiferous quartzo-felspathic granulite and gneiss (leptynite) and non-garnetiferous quartzo-felspathic (alaskitic) granulite (leptite). A structural analysis of the rocks in these regions reveals at least three distinct major periods of orogeny (folds, faults, fractures, shears, thrusts and later uplifts) in addition to the impress of much later (Purana) orogenies-the earliest having a NE-SW axial trend, followed by N-S and NNW-SSE axial trends coinciding with the later Dharwar fold axes, and a last phase of NE-SW to NNE-SSW axial trend marked by en-echelon folds and shear-fracture zones intruded by post-tectonic granitic, syenitic, carbonatite, mafic and ultramafic complexes of different periods. Subsequent studies by the author of the granulite facies rocks of Cbarnockite­ Khondalite Suites of the Eastern Ghats Group in Orissa-which was incomplete at the time the earlier papers were presented-have confirmed the above postulate of a complexly folded, poly-metamorphically differentiated group of rocks of eugeo­ synclinal assemblage. Tamil Nadu

SIGNIFICANCE OF SAPPHIRINE AND KORNERUPINE-BEARING CORDIERITE ROCKS FROM AROUND KIRANUR, TIRUCHIRAPALLI DISTRICT, TAMIL NADU

M. N. BALASUBRAHMANYAN Geological Survey ofIndia, Calcutta

The assemblages in the area afford yet another example in which "mixed " assemblages of sillimanite-almandine subfacies and hornblende granulite subfacies. occur in the polymetamorphic granulite terrains of Archaean age. Cordierite rocks, sillimanite quartzite, amphibolite and meta-limestone occur as interstratified layers. in the migmatitic biotite gneiss of the area. The paragenesis of cordierite-bearing assemblages indicate them to have been formed from pelitic sediments with an initially high MgO : FeO ratio under two cycles of metamorphism. Orthopyroxene-spinel­ sapphirine rocks with minor cordierite were initially formed under hornblende· granulite subfacies. Subsequently due to regional migmatisation Mg-rich solutions. at elevated temperatures migrated along structurally favourable fold axial zones and reacted with earlier formed rocks producing the present assemblage with cordierite· and anthophyllite under conditions of sillimanite-almandine subfacies. The study of calc-silicate assemblages also reveals conditions of sillimanite-almandine subfacies of almandine-amphibolite facies, Since sapphirine and kornerupine are mutually exclusive and with tourmaline associated with the latter, action of boron bearing mineralisers is thought to be responsible for their origin. However, the boron might also indicate an original volcanic milieu with which the rocks were associated. Interpretations of' new geology' based on plate tectonics would considerably be influenced by similar Mg-AI rich assemblages found in granulite terrains.

TECTONIC DOMAINS IN GRANULITIC ROCKS OF TAMIL NADU

V. GOPAL Tamil Nadu Mineral Development Project, Madras

Photogeological studies for the identification of regional geological structures. have been carried out over an area of about 13,000 sq. km, The good quality of the photographs and the supporting geophysical data has. made it possible to recognise several interesting tectonic features in the project area, forming parts of Salem, Dharmapuri, North and South Arcot districts. Recent studies in ERTS images of Tamil Nadu on a scale of I : 250,000 bring out the tectonic lineaments, and structures of regional scale and present a synoptic view of the entire granulitic facies rocks. An attempt is being made to correlate the available geological and geophysical information for a part of the area with the lineaments in the ERTS imagery to interpret tbe regional tectonics. From the studies carried out GROUP DISCUSSION ON GRANULITE FACIES 159 'so far, at least three tectonic domains could be recognised. The oldest appears to be the East-West domain which could be identified in several outcrops ofmigmatites and charnockites. The effect of a later, strong NE-SW domain on the EW rocks is evidenced in the form of folded folds, curvatures in trends and in 'areas of stretch­ ing'. The prominent NE-SW domain is reflected in the form of foliation strikes, fold axes, shear zones, fractures and faults. The third tectonic domain ranging in direc­ tion from north-south to NNW-SSE is also seen at several places particularly to the north of the river Cauvery.

MULTIPHASE BASIC AND ULTRAMAFIC ACTIVITY rx -GRANULITE TERRAIN OF NORTH ARCOT DISTRICT, TAMIL NADU

E. B. SUGAVANAM, V. VENKATA RAO, J. SIMHACHALAM, S. C. NAGAL AND A. K. SIMHA Geological Survey ofIndia, Madras

North Arcot district in Tamil Nadu represerits a complex high grade granulitic terrain with multistructural and polymetamorphic impress. A variety of basic and ultramafic bodies have been located in this terrain. In addition to recognition of several pulsation of migmatisation it has been possible to recognise a multiphase basic and ultramafic activity in this region, using structural criteria. (i) Predominantly concordant sills of norite (d.) represent the earliest basic magmatic activity while (ii) the composite bodies of dunite-pyroxenite-websterite­ enstatolite-hypersthenite-anorthosite represent the oldest ultramafics. Both have been emplaced in the basement complex of charnockite-metapelite. All were involved in the granulite metamorphism and' F I ' deformation. With the impress of multi­ phase deformation they are seen presently as several detached lenses and boudins with one or more members of the ultramafic bodies lacking at places. (iii) Dolerite dykes (d2) cross cutting the charnockitic group and Gingee granite show the impress of' F2' deformation. (iv) Thermally metamorphosed dark looking dolerite dykes (d.) cut across charnockitic group and Gingee granite but are affected by N-S axial plane shears of ' Fa' deformation and they are extensively epidotised. (v) Swarms of lighter dolerite dykes (d.) cut across migmatitic gneiss, north of Palar but are not affected by N-S shears of 'Fa' deformation. (vi) Dunite-pyroxenite bodies lying within the syenite-carbonatite complex represent youngest ultramafic bodies. (vii) Thin dykes of lighter coloured shonkinite cut across alkali syenite, pyroxenite and the fenitised gneisses and represent acid alkaline dykes. A few of the finer grained dykes across the charnockitic terrain have been identified as possible phonolites and tinguaites and are possibly related to the carbonatite complex. (viii) Dolerite dykes (d.) cutting across alkali syenite and shonkinite represent the youngest basic magmatic activity in this area. 160 GROUP DISCUSSION ON GRANULITE FACIES

GEOLOGY AND WHOLE ROCK CHEMISTRY OF GRANULITE AND ASSOCIATED ROCKS OF NORTH ARCOT DISTRICT, TAMIL NADU

E. B. SUGAVANAM AND V. VENKATA RAO Geological Survey of Tndia, Madras AND G. V. ANANTHA hER AND T. R. NARAYANAN KUrTY Indian Institute ofScience, Bangalore

A comprehensive study, involving the geological and whole rock chemistry of the granulite and the associated rocks of North Arcot district, Tamil Nadu has been undertaken and the available data are presented. The area comprises rocks of diverse origin involved in multiple episodes of deformation, metamorphism, each separated by acid and basic magmatic activity, different pulsations of migmatisation and metal­ logeny. Charnockites constitute the preponderant rock type in which rocks ofmeta­ sedimentary, acid, basic and ultramafic affinities occur as closely interbanded sequence. Five periods of deformation could be deciphered together with the style of individual folds as well as their interference patterns. Based on the structural sequ­ ence, a tentative stratigraphic succession of various lithologic units and different events has been worked out.

------7 Pyrite-galena bearing quartz barytes veins

6 Basic dyke activity

5 Migmatitic gneisses of different pulsations and stable under varying metamorphic conditions

4 Concordant basic sills of possible volcanic origin represented by pyroxene-granulites

3 Alkali rich pink granite (arkosic derivation)

2 Metapelites (garnetiferous cordie rite-sillimanite­ graphite-biotite schists, quartzite, magnetite quartzite)

Charnockites-acid and intermediate varieties having an igneous parentage

The whole rock chemistry of the different lithological units supports the tentative­ succession of stratigraphy, metamorphism and igneous activity. GROUP DISCUSSION ON GRANULITE FACIES 161

METALLOGENY IN THE GRANULITIC TERRAINS OF TAMIL NADU

E. B. SUGAVANAM, V. VENKATA RAO, J. SIMHACHALAM AND N. G. K. MURTHY Geological Survey ofIndia, Madras

Structural studies in parts of North Arcot district have indicated at least three periods of metallogeny in the high grade granulite terrains. These are (i) syngenetic sulphide mineralisation associated with pyroxene-granulite, (ii) sparse disseminations of sulphides in dolerite dykes and (iii) sulphides associated with quartz-barytes veins as shear fillings. Using these as keys, the better known occurrences of sulphides in the granulite terrains of other parts of Tamil Nadu are reviewed and the possibility of locating richer sulphide deposits are discussed.

MIGMATISATION IN HIGH GRADE GRANULITIC TERRAIN OF NORTH ARCOT DISTRICT, TAMIL NADU

E. B. SUGAVANAM, V. VENKATA RAO, J. SIMHACHALAM, S. C. NAGAL AND A. K. SINHA Geological Survey of India, Madras

Migmatitic gneisses of various types have been identified in and around the high .grade granulitic terrain of parts of North Arcot district, Tamil Nadu. In this area -of complex deformational and tectonic history several episodes of migmatisation -could be recognissd. Based on the structural sequence of five periods of deformation -established in this area, four different types of migmati tic gneisses have been recognised in a chronological order. The paper deals with the classification of these gneisses and their regional distribution in and around North Arcot district.

STRUCTURAL HISTORY OF TYPE AREA AROUND PALLAVARAM NEAR MADRAS

E. B. SUGAVANAM AND V. VENKATA RAo Geological Survey ofIndia, Madras

Recent studies of the type area around Pallavaram near Madras has brought out Iithological distribution and structural patterns similar to the adjacent granulite terrains of North Arcot district, Tamil Nadu. 162 GROUP DISCUSSION ON GRANULITE FACIES

MINERAL CHEMISTRY INVESTIGATIONS OF CHARNOCKITES AND ASSOCIATED ROCKS OF NORTH ARCOT DISTRICT, TAMIL NADD, SOUTH INDIA

G. V. ANANTHA IYER, T. R. NARAYANAN KUTTY, A. R. VASUDEVA MURTHY, Indian Institute ofScience, Bangalore . E. B. SUGAVANAM AND V. VENKATA RAO Geological Survey ofIndia, Tamil Nadu Circle

The changing pattern of chemical composition of minerals occurring in the charnockites, metabasites, pink granites, metapelites, Gingee granites and migmatites. of the high grade rocks of North Arcot district, Tamil Nadu, South India have been studied. Orthopyroxenes from charnockites, metapelites and quartz magnetite granu­ lites range in their composition from En» to Enro• The low Ca, AI, Cr and Ni content in them indicate that they are metamorphic pyroxenes. The dark-brown biotites with high Ti-content fall in the middle range of castonite-siderophyllite end member series, characteristic of high grade biotites. Garnets do not occur in charnockites, pyroxene' granulites, Gingee granites and migmatites. The pyrope content of garnet occurring in metapelites and pink granites range from 20- 40 mol per-cent with nearly negligible grandite component. Meta-anorthosites contain garnets of very high grandite compo­ nent. Most of the garnets plot in the granulite field closer to the almandine-pyrope base line in the ternary diagram of Coleman et al. Calciferous amphiboles are not common in the charnockites. The analysed ones are mostly from metabasites, pyroxene granulites and Gingee granite migmatites. These brown and olive green amphiboles plot within the tschermakite-pargasite fields. They are singularly charac­ terised by the absence of AlB and by the presence of FeB+. Ti and alkali contents are high. Distinct compositional variations are observed in the feldspars of charnockites, Gingee granite migmatites, pink granites and the migmatites of the Ponnaiyar river basin. In the charnockites they range from oligoclase to andesine. The Gingee granite migmatites are richer in orthoclase. The pink granites contain mesoperthites (Or., Abs) with very little anorthite. The feldspars of Ponnaiyar river basin migma­ tites are sodic and are mostly albiclase. Those of meta-anorthosites are calcic with anorthite content approaching 90. . The distribution of Fe and Mg in the co-existing biotite-orthopyroxenes of charnockites and garnet-biotite of metapelites have been studied. The KmJ~) = 0.90 and Kga(~;)o=0.30 suggest that the minerals were formed under equilibrium condi­ tions. In the metapelites of the area sill-cord-bio-gar and sill-cord-bio-hyp-garnet paragenesis observed correspond to the changing over of granoblastites to granulites. This takes place under lower granulite facies condition. Under nearly identical con­ ditions the equilibrium reaction: Biotite + quartz--orthopyroxene + orthoclase + water takes place in the preponderant charnockite rocks of this area. The absence of garnet co-existing with clinopyroxene and orthopyroxene in the rocks suggests that during high grade metamorphism the pressure did not exceed 8 Kbrs and a tempera­ ture of 800°C prevailed. Migmatites of varying types in the area were generated by anatexis under the PoT range of lower granulite facies. The temperature and the extent of melting varied depending upon the composition of the feldspars in the rocks and available water' vapour pressure at the time of their formation. GROUP DISCUSSION ON GRANULITE FACIES ]63

THE ARCHAEAN LIMESTONES OF TAMIL NADU-THEIR STRUCTURAL AND CHEMICAL CHARACTERISTICS

R. SRINIVASAN AND VINAYAK Tamil Nadu State Geology Branch, Madras

Regional studies show that in the Archaean terrain of Tamil Nadu, there is a limestone province and an iron ore province to the south and north of River Cauvery respectively. The crystalline limestones occur as long linear bands extending for 2 or 3 km, with varying widths. They are associated with gneisses, charnockites, granites etc., conforming to the regional trend of the associated calc gneisses, granitic gneisses, and occur as steeply dipping bodies and constitute a marker horizon. They are highly metamorphosed and intricately folded. The impure types are fine grained and consist of diopside, apatite, coccolite and chondrodite. They range in composition from almost calcitic type to dolomitic type. Iron-alumino silicates .and magnesian silicates form the impurities. The magnesian type is subordinate. Pure dolomites are scarce and occur only in a few localities, eg: Kayathar, Kadalai and Vallinayakapuram in Tirunelveli district, Nattam and Eliyarpatti in Madurai district etc. The predominance of calcitic limestones is in sharp contrast with the widespread dolomitization in the carbonate rocks of subsequent geologic ages in India.

CHARNOCKITIC ROCKS OF PALANI-DINDIGUL AREA, MADURAI DISTRICT, TAMIL NADU

P. NARASIMHA RAO Department ofGeology, Annamalai University, Annamalainagar

The Charnockitic rocks ofPalani-Dindigul area occurr (i) as isolated pockets and lenses of acid nature within migmatite terrain, being structurally conformable with the enclosing rocks and (ii) as large continuous horizons of acid to intermediate com­ position with local enderbitic variants and intervening bands and lenses of basic granulites. The former type are considered to be products of a low temperature process of transformation of gneisses rather than a process of high grade meta­ morphism. It is also considered that the granitization of the older schists and amphibolites might have played a role in their development by contributing the necessary chemical components for the formation of pyroxenes. The latter type are considered to be magmatic bodies intrusive into the older migmatites and meta­ sediments. The field and petrographic evidences as well as their temperatures of formation determined by felspar geologic thermometers lend support to this concept. Charnockitic rocks are found in association with the anorthosite suite of rocks round about Oddanchatram. These Charnockites are shown to be older than anortho­ sites and the two are genetically unrelated. 164 GROUP DISCUSSION ON GRANULITE FACIES

REGIONAL GEOCHEMICAL SOIL SURVEY IN PARTS OF GRANULITIC TERRAIN OF TAMIL NADU

V. SUBRAMANIAN, C. B. LAKSHMIKANTHAM AND K. P. RAMACHANDRAN Tamil Nadu Mineral Development Project, Madras

An area of roughly 10,000 sq. km forming part of Coimbatore, Salem,. Dharmapuri and North Arcot districts has been covered by geochemical soil sampling. Samples were collected at 250 metres interval and sample lines were spaced at 1 km apart. About 18,000 soil samples were collected and analysed for Cu, Zn, Co, Ni, Fe, Mn, Pb, As and Mo content. The last two elements were analysed by' colorimetry method and the remaining elements by atomic absorption method. The results of the survey indicates that geochemical survey based on soil sampling can be utilised as an effective tool for exploration purposes in granulitic terrain in spite of the fact that the soils are disturbed in most of the areas by cultiva­ tion. The geochemical samples do reflect the rock units underneath or mineralisa­ tion present. An attempt is being made to interpret the results of soil geochemistry to the geology of the area sampled taking into consideration the distribution pattern of the following elements-Cu, Zn, Co, Ni, Pb, Fe and Mn. In Samalpatti area of Dharmapuri district where detailed geochemical follow up· work is now being carried out, distribution of Cu, Zn, Co, Ni, Fe and Mn clearly define different rock units viz. gneisses, dunites, pyroxenites and syenites.

MIGMATISATION AND RETROGRESSION OF GRANULITIC ROCKS IN PARTS OF COIMBATORE AND NILGIRI HILLS DISTRICT, TAMIL NADU

K. GOPALAKRISHNA Geological Survey ofIndia

Migmatisation of granulite rocks of the Charnockite-Khondalite Supergroup in parts of Coimbatore and Nilgiri hills districts, Tamil Nadu can be classified into three groups-(i) regional migmatisation as seen in southern parts of Coimbatore district, particularly in the Anaimalai hills, (ii) migmatisation along linear belts controlled by lineaments as seen on the lower slopes and foot hills of Nilgiri hills, southern extensions of Biligirirangan hills, along the slopes of the hill tracts to the north of" Sathyamangalam, Andhiyur etc, and (iii) local injection migmatisation due to younger intrusive granites as seen in the Coimbatore plains. These three types, show migmatisation ofvarying intensities due to the difference in the geological settings and. consequently result in different grades of retrogression.

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