PRECAMBRIAN BIMODAL VOLCANISM IN WEDDELL SEA, WEST ANTARCTICA

Rhyolites and basalts constitute bulk of the bimodal volcanism in the Bertrab and Littlewood nunataks of Weddell Sea area, western Antarctic a. Rhyolites with occasional granophyres from the two nunataks are com positionally similar except for high Ba in Littlewood. Basalts, of tho leiitic to calc-alkaline nature are mainly from the Bertrab nunatak an d show compositional variation (basalt to basaltic-trachy-andesite) bo th in major (SiO2, 45.82 to 53.81%, MgO, 4.67% to 11.20% Na2O, 1.58 to 4.26%) and trace elements (V, Cr, Rb, Sr and Ba). The rare earth elem ental abundances and their distribution patterns in the rhyolites from the two nunataks are broadly similar with comparable total REE (Bertr ab 80.33; Littlewood 86.82), moderate LREE/HREE fractionation (Ce(N)/Y b(N) = 4.36 in Bertrab and 4.33 in Littlewood) and negative europium a nomalies. Overall major and trace elemental chemistry of the acid volc anics coupled with the systematic compositional variation from the bas ic to the acid end suggests a common source of origin for this bimodal volcanism. The source magma, a melt of the upper mantle appears to ha ve evolved in a compressional tectonic regime and has intruded (extrud ed) in an extensional rift tectonic setting.