Geochemistry of Felsic Volcanics from Gurapratap Singh and Diri, Pali Dist., Rajasthan (Part - II, Trace Elements)

Felsic volcanics of the Gurapratap Singh and Diri, belong to the Malani volcanic suite of Precambrian age. Major element chemistry of these volcanics shows systematic variations with progress of differentiation. The distribution and behaviour of trace elements in these rocks is in accord with their major element data and the distribution patterns are normal with no anomalous concentrations of the elements studied. With the progress of fractionation there is a smooth decrease in Sr and increase in Rb which along with Ba are the main trace contents of these rocks. Unlike Sr and Rb which have a linear distribution pattern, Ba shows a curvilinear trend, i.e., an initial increase from andesite to dacite and rhyodacite followed by a decrease amongst rhyolites. The behaviour of ratios like Ni/Co, Fe/Zn, Mg/Li and K/Rb agrees with a model of fractional crystallisation. Furthermore, the trace element abundances of the volcanics point to their crustal source. The work also shows that in felsic magmas Ba and Sr are sensitive indicators for tracing possible differentiation and that rhyolitic rocks depleted in Ba are enriched in Rb and that in last stages of differentiation Ba/Rb ratio shows rapid decrease.