Petrogenesis of Low Potassic Quartz Normative Deccan Tholeiites

Among the ten-flow basaltic sequence in the NE of the Deccan province, flow 5 is considered as the most 'primitive' (K₂O, 0.16%; Rb, 2 ppm; Pb, 2 ppm and ⁸⁷Sr/⁸⁶Sr, 0.7039). 190 metres of the ten-flow lava pile is singularly quartz normative and the absence of olivine and olivine tholeiites is conspicuous. The unique 'Deccan character', i.e., enrichment of iron and titania is observed but silica (48.26%), K₂O (0.2%) and coherent trace elements (Rb, 6 ppm, Pb : 6 ppm) are strikingly low. Occurrence of similar low potassic rocks are observed from elsewhere in the Deccan region. Such rocks will extend the diversity of Deccan basalts in the low potassium end of the spectrum and are shown to have a definite resemblance to the oceanic tholeiites in spite of difference in the 'environment'.

Petrogenesis of these low potassic quartz normative tholeiites, taking flow 5 as the unmodified melt is discussed in the light of the generally accepted models. Its derivation as partial melts of Upper Mantle composition analogous to garnet peridotite is not favoured. These tholeiites seem to have been derived as almost 'total melts' of possible eclogite lenses and patches in the Upper Mantle.