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.