GEOCHEMISTRY OF ARCHEAN BIMODAL VOLCANIC-ROCKS OF THE SANDUR SUPRACRUSTAL BELT, DHARWAR CRATON, SOUTHERN INDIA

In the late Archaean Sandur supracrustal belt of the Dharwar craton, S outhern India, bimodal (mafic-felsic) volcanic rocks are encountered i n its eastern region (Copper Mountain region). The mafic volcanic rock s are represented by Al-depleted picritic basalt and tholeliitic basal t. Picritic basalt has a HREE - depleted pattern. Tholeliitic basalt e xhibits two types of REE patterns; (a) slightly depleted - to flat - L REE and unfractionated HREE patterns, and (b) LREE - enriched and HREE -depleted patterns. The felsic volcanic racks are Na-Rhyolites, which show calc-alkaline affinity and LREE - enriched and HREE-depleted chon drite-normalised patterns. Geochemical signatures show that the picrit ic basalt owes its origin to melting of a mantle diapir at depths arou nd 100 km. The LREE - depleted tholeiites are not genetically related to the picritic basalt. They were derived through high degree partial melting of a depleted shallow mantle source. The internal variations o bserved in the LREE - depleted tholeiitic suite are due to derivation of magma through different degrees of melting of the same mantle sourc e. REE-based petrogenetic modelling shows that the LREE - depleted and LREE - enriched tholeiites are not related to a common parent magma. The LREE - enriched tholeiites were generated from a mantle source, wh ich has been metasomatized by subduction related melt/fluid phase. REE chemistry shows that the mafic and felsic volcanic rocks, though spat ially and temporally associated, are not genetically related. The HREE depleted rhyolites were derived by partial melting of a garnet - amph ibole bearing source, possibly a subducting oceanic crust. Feild geoch emical and petrogenetic aspects suggest that the magmatic rocks of the study area evolved in an active plate margin environment.