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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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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