Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

Meenal Mishra ¹, V. Rajamani ²

Affiliations
1 Wadia Institute of Himalayan Geology, 33, Gen Mahadeo Singh Road, Dehra Dun - 248 006, IN
2 School of Environmental Sciences Jawaharlal Nehru University, New Delhi - 110 067, IN

Abstract

The central arm of the trident shaped Ramagiri schist belt includes a central block which consists predominantly of bimodal (mafic-felsic) volcanics, apart from minor metasedimentary units. The geological set up of the various rock types suggests that the central block could be a tectonic melange. The metatholeiites and the felsic volcanics of the central block show similar LREE - enriched and fractionated REE patterns; the metatholeiites however. show a much larger variation in abundances. It has been suggested that the protoliths to the Ramagiri central mafic suite of bimodal volcanics were formed by different extents of melting of the metasomatically enriched mantle sources and were emplaced in an island arc setting. We suggest here that partial melting of similar metatholeiites, constituting an arc crust, at - 8 kbar pressures and at -1000 °C temperatures, produced magmas to the associated felsic volcanics. The geochemical characteristics of the mafic and felsic volcanic rocks in the Ramagiri belt, as well as the adjoining Sandur belt, have many similarities to those of the modern day island arc volcanic suites. The idea that the Phanerozoic style magmatic and accretionary tectonic processes operated during the Late Archaean suggested from the studies of granitic rocks in the Eastern Dharwar craton is corroborated by the metavolcanics of the Ramagiri schist belt.

Keywords

Geochemistry, Petrogenesis, Bimodal Volcanics, Dharwar Craton, Ramagiri Schist Belt, Andhra Pradesh.

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T. S. Giritharan ¹, V. Rajamani ¹

Affiliations
1 School of Environmental Sciences, Jawaharlal Nehru University, New Delhi -110 067, IN

Abstract

The Hutti-Maski schist belt in the eastern Dharwar craton consists predominantly of mafic metavolcanics metamorphosed to amphibolite facies. The amphibolites are tholeiitic in composition with higher abundances of iron and with flat to LREE enriched rare earth patterns. Modelling of major and trace elements suggests that the tholeiitic protoliths were formed from melt enriched mantle sources by different extents of partial melting at pressures -25 kbars. The most common mafic metavolcanics in the three auriferous schist belts of eastern Dharwar craton such as the Kolar. the Ramngiri and the Hutti belts are high-iron tholeiites with LREE enriched rare earth patterns. The gold mineralization in the form of quartz-carbonate veins in the three belts is commonly hosted in high irontholeiitic protoliths. The process of formation of Fe-rich tholeiites. their metamorphism to amphibolites and their subsequent emplacement along terrane boundaries could have provided a favourable geochemical environment for gold mineralization.

Keywords

Geochemistry, Metabasalts, Petrogenesis, Gold, Hutti-Maski Belt, Karnataka.

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V. Rajamani ¹, E. J. Krogstad ², G. N. Hanson ², S. Balakrishnan ¹

Affiliations
1 School of Environmental Sciences, Jawaharlal Nehru University, New Delhi 110067, IN
2 Department of Earth and Space Sciences, Slate University of New York, Stony Brook, New York 11794, US

Abstract

Structural and geochemical evidence including major and trace elements, and Pb-isotopic data on K-feldspars, froin the granitoid rocks adjoining the Kolar Schist Belt are presented to show (1) that the Patna Granite an the NW of the belt is dissimilar to any granitoid rock E of the belt and (2) that the main granitoid gneiss immediately E of the belt, the Karnbha Gneiss, is nearly identical to the Bisanattam Granite of Narayanaswamy er al (1960).

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V. Rajamani ¹

Affiliations
1 School of Environmental Sciences, Jawaharlal Nehru University, New Delhi - 110067, IN

Abstract

The supracrustal belts of the Dharwar Supergroup in the Dharwar Craton include variably metamorphosed komatiitic and tholeiitic volcanics. Komatiites in general occur only in minor amounts. Available geochemical data of these rocks from the Holenarasipur ? Kudremukh, Bababudan and the Kolar schist belts were quantitatively modelled to infer that (1) Komatiitic magmas originated from deeper mantle sources at depths > 100 km., (2) tholeiitic magmas formed at depths < 50 km from sources different from those of komatiites and with much higher Fe/Mg ratios compared to normal mantle, (3) the two magma types are not genetically related either to one common parental magma or to one common source, (4) Dharwar magmatism could be related to rift related volcanism perhaps associated with one or more mantle plumes. High quality geochemical, including isotopic, data on a large number of samples is required to test this model of Dharwar magmatism.

Keywords

Dharwar Schist Belts, Metabasites, Mantle Plumes.

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John K. Zachariah ¹, Manoj K. Mohanta ¹, V. Rajamani ²

Affiliations
1 School of Environmental Sciences, Jawaharlal Nehru University, New Delhi - 110 067, IN
2 Earth and Space Sciences, State University of New York, Stony Brook, NY 11794-2100, US

Abstract

The gold mineralized Ramagiri Schist Belt is a volcanic dominated, late Archaean belt in the eastern Dharwar Craton. Based on the lithological association, mode of occurrence of rocks, geochemical characteristics of mantle derived rocks and the metamorphic grade of rocks, the belt is divided into three blocks that are tectonically interleaved with and surrounded by granitic rocks of distinct histories. The eastern block has amphibolite facies rocks, dominantly basic metavolcanics having light rare earth element (LREE) depleted patterns with minor banded ferruginous quartzite (BFQ). The central block includes mafic and felsic volcanics, pyroclastics, gabbroic and felsic dykes, argillites and BFQ. The volcanic rocks as well as the intrusives have LREE enriched patterns. The central block has dominantly greenschist facies rocks. The western block is made up of fine grained metabasalts with well preserved pillow structures at places, and retrogressed chlorite-actinolite and chlorite-carbonate schists in shear zones, serpentinite and BFQ. These rocks have flat to moderately LREE depleted patterns. The associated serpentinite is of residual origin, probably representing obducted pieces of Archaean oceanic lithosphere. Available age information and required tectonic settings of the volcanics necessitate subduction related magmatism and corroborate the idea of crustal genesis by accretionary processes in the eastern Dharwar Craton during late Archaean.

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J. Sahoo ¹, T. S. Giritharan ², S. Balakrishnan ², V. Rajamani ³

Affiliations
1 Department of Earth Sciences, IIT Roorkee, Roorkee - 247 667, IN
2 Department of Earth Sciences, Pondicherry University, Pondicherry- 605 014, IN
3 School of Environmental Sciences, Jawaharlal Nehru University, New Delhi - 110067, IN

Abstract

The Banded Gneissic Complex (BGC) occurring around Masuda consists of granitoids, migmatized mica schist, calc-gneiss and amphibolites. The amphibolites (few cm to tens of metre wide) are co-folded with the granitoid and calc- gneisses. The ortho-amphibolites are classified into high-Mg and tholeiitic amphibolites and they have distinct chondrite normalized Rare Earth Element (REE) patterns and Ce/Nd ratios. Modeling of their geochemical data suggests that magmas representing the high-Mg amphibolites were derived by low extents of partial melting of mantle sources more Fe-enriched than lherzolite whereas magmas representing the tholeiitic amphibolites were derived by low-extents of partial melting of sources formed by mixing of high-Mg magmas with lherzolite. The composition of this source is similar to the komatiites. The tholeiitic amphibolites have geochemical and petrogenetic characteristics of the basalts formed in island arcs. They are different from that of amphibolites found in the basal part of Aravalli Supergroup and mafic enclaves found within the BGC occurring east of Nathdwara and Udaipur.

Keywords

Geochemistry, Petrogenesis, Amphibolites, Banded Gneissic Complex, Aravalli Craton, Rajasthan.

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Jayant K. Tripathi ¹, V. Rajamani ¹

Affiliations
1 School of Environmental Sciences, Jawaharlal Nehru University, New Delhi -110 067, IN

Abstract

Mineralogy and geochemistry, including REE, of the Proterozoic Delhi quartzites have been studied to understand the processes and provenance involved in the origin of supermature quartzites. Post Archaean Australian Shale (PAAS) like chondrite normalised REE patterns, including negative Eu anomalies but with low abundance, show that the source for the quartzites was dominanted by evolved granitic rocks. High weathering indices (CIA) and strong negative Eu anomaly suggest that the source area had gone through intensive chemical weathering. We infer that the PAAS like pattern for the quartzites was inherited from the granitic source rocks through quantitative transfer of REE bearing heavy minerals. These heavy minerals (-0.7%) supplied nearly 60% of the total REE. The quantitative transfer of heavy minerals and conservation of REE patterns with respect to the source are likely due to the lack of any selective sorting of heavy minerals indicating a very low degree of recycling. One possible explanation for this type of transfer is the formation of mature weathering profiles. Mature weathering profiles on the stable craton could give rise to mature upper silica rich horizons and lower horizons rich in fine-grained clays due to illuviation and downward transport. The upper horizons could have acted as source for the quartzose sands and resistant REE bearing heavy minerals for the quartzites. The lower horizons could become a likely source for the argillaceous material overlying the quartzites during subsequent sedimentation.

Keywords

Weathering, Provenance, Quartzite, Geochemistry, Heavy Minerals, REE, Delhi Supergroup.

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Meenal Mishra ¹, V. Rajamani ¹

Affiliations
1 School of Environmental Sciences, Jawaharlal Nehru University, New Delhi - 110 067, IN

Abstract

The central block of the central arm of the trident- shaped Ramagiri Schist Belt includes volcaniclastic metasedimentary rocks comprising turbiditic metagreywacke/argillite, phyllite and siliceous shale, apart from bimodal volcanics. The greywackes exhibit primary sedimentary structures and textural immaturity and are rhythmically interbedded with the argillite. They have an intermediate amounts of quartz, low CIA, enriched and highly fractionated REE patterns ([Ce/Yb]_N=I 5-49), all characteristic features of late Archaean greywackes. These features point to a rapid denudation and sedimentation of the felsic volcanics, by turbidity flow current in an arc related basin. The intercalated argillites with the greywackes seem to have been deposited during the period of quiescence of the volcanic activity. The geochemical data of the phyllite points towards a bimodal (arc-volcanic) provenance, for these fine grained sediments. The siliceous shales are likely to have been derived from the rhyolites associated with the bimodal volcanics, after substantial chemical weathering. The clastic metasediments from the schist belts of eastern Dharwar craton (Sandur and Rarnagiri) commonly lack the negative Eu anomaly, as observed in those of many late Archaean greenstone belts elsewhere in the world as well as in the western Dharwar craton. Thus, whereas the clastic sediments of the eastern Dharwar craton were probably derived from a contemporaneous arc-crust, those of the western Dharwar craton were derived from a much older, upper continental crust, formed predominantly by intracrustal melting in the time interval of 3.4 to 3.0 Ga. We suggest from the geochemistry of the clastic metasediments of the schist belts of the Dharwar craton that (a) the western and eastern blocks of the Dharwar craton differ in a major way in the process of their formation, as proposed by Balakrishnan et al. (1999) and (b) the Dharwar craton supports the crustal growthlmodel of Gao and Wedephol (1995) that suggests intracrustal granite formation right from early Archaean i.e., 3.8 Ga onwards.

Keywords

Metasedimentary Rocks, Geochemistry, Island Arc, Europium Anomaly, Crustal Evolution, Ramagiri Schist Belt, Dharwar Craton.

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V. Rajamani ¹, K. Shivkumar ¹, G. N. Hanson ², J. W. Granath ²

Affiliations
1 School of Environmental Sciences, Jawaharlal Nehru University, New Delhi 110067, IN
2 Department of Earth and Space Sciences, State University of New York at Stony Brook, New York 11794, US

Abstract

The amphibolites in the Kolar schist belt include four textural types, schistoic, granular, massive and fibrous and occur in abundance in that order. The fibrous variety consists of two coexisting calcic amphiboles (A1 rich hornblende and A1 poor actinolite). The other three varieties contain hornblende and plagioclase (An₄₀ to An₈₅). The texture, mineralogy and mineral chemistry suggest metamorphism in the middle to upper amphibolite facies.

Major and Rare earth elements (REE) data indicate that (a) the fibrous amphibolite is somewhat similar to high-Mg basalts of Archean greenstone belts but with a strongly, heavy REE depleted pattern, (b) the other three varieties are similar to low-K oceanic tholeiites with slightly light REE depleted patterns and with strong positive Eu anomalies, and (c) the two groups do not represent primary melt compositions generated by the partial melting of a pyrolite mantle. It is concluded that the Kolar amphibolites must have been evolved from at least two separate parent magmas generated from different temperature and pressure (depth) conditions.

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