Geochemistry of Felsic Volcanics from Gurapratap Singh and Diri, Pali District, Rajasthan (Part-I, Major Elements)

The felsic volcanics of Gurapratap Singh and Diri are mostly welded tuffs and belong to andesite-dacite-rhyolite association; they are a part of the Malani volcanic suite of Precambrian age. The volcanics are cut by rhyolite porphyry (comagmatic with volcanics) and a younger dolerite dyke. A few flows of ultrapotassic rhyolites are also found which do not conform to the main trend and presumably mark a different phase of magmatic activity.

The chemical data on 91 samples of different rock types and Harker diagrams reveal a systematic variation in all major elements with the progress of differentiation, suggesting that the entire sequence except the ultrapotassic rhyolite· is cogenetic and that the changes have been controlled by some sort of fractional crystallisation mechanism. The principal component analysis of the data reveals that in the earlier stages, the fractionating phases were soda-lime plagioclase and a mafic phase and in the later stages K-feldspar became a dominant phase. This is supported by phenocryst assemblages in these rocks. The suite also has very high inter-elemental correlation indicating that the rocks are cogenetic and a single process was responsible for their evolution. The mixing line calculation also confirms the operativeness of a single process.