Petrogenesis of acid rocks of the deccan traps

The acid rocks of the Deccan Traps including microgranites, felsites, rhyolites and related rocks are contined to tectonically weak zones in Western India. The significance of their distribution is discussed and the opinions expressed on the genesis of the rocks are critically reviewed. In their formation, fractional crystallization of tholeiitie basalt magma, which is supposed to be the parental one, was aided in certain localities by the melting of the sialic crust and assimilation or partial melting of the pre-Deccan Trap rocks. Evidence for the latter is found in the gradational contacts of the acid rocks with the earlier rocks, their association with cruptive centres or faulted zones and absence of any definite trend of variation in petrographical and chemical characters. Some of the rhyolitic rocks are also formed due to hydrothermal alteration of sedimentaries along the Narmada Valley.