Significance of the Archaean Bimodal Volcanics from the Ramagiri-Schist Belt in the Formation of Eastern Dharwar Craton

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.