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The Granulite Facies Terrains of Southern Peninsular India

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The Granulite Facies Terrains of Southern Peninsular India 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
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