Geochemistry of the Archaean Metasedimentary Rocks from the Ramagiri Schist Belt, Eastern Dharwar Craton, India:Implications to Crustal Evolution

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.