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 Antarctica. Rhyolites with occasional granophyres from the two nunataks are compositionally similar except for high Ba in Littlewood. Basalts, of tholeiitic to calc-alkaline nature are mainly from the Bertrab nunatak and show compositional variation (basalt to basaltic-trachy-andesite) both 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 elemental abundances and their distribution patterns in the rhyolites from the two nunataks are broadly similar with comparable total REE (Bertrab 80.33; Littlewood 86.82), moderate LREE/HREE fractionation (Ce_N/Yb_N= 4.36 in Bertrab and 4.33 in Littlewood) and negative europium anomdies.

Overall major and trace elemental chemistry of the acid volcanics coupled with the systematic compositional variation from the basic 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 have evolved in a compressional tectonic regime and has intruded (extruded) in an extensional rift tectonic setting.