Deccan Traps and Associated Rocks of the Bassein Area

The area of Bassein lies 30 miles north of Bombay. Within a compass of about 150 square miles the rocks encountered here are basalt flows and dykes, dolerite dykes, ·diorites, thin veins of aplite, a single dyke of camptonite and pyroclasts.

An explanation is offered for the origin of felsic rocks of differing composition. It is suggested that the initial rhyolitic magma is not a product of differentiation of Deccan basalt but originated as a result of partial fusion of the sialic crust during long continued ascent and outpouring of vast volume of basalt lavas.

Rocks of dioritic composition are described, and it has been shown that the diorite and its associates like meladiorite. leucodiorite, granophyre, andesite. etc., are the result of contamination of rhyolite magma with the country rock dolerite. In support of this, chemical, microscopic and field evidences are cited. Thin stringers of aplite are considered to be the residual liquid product of the contaminated rhyolite magma.

Detailed optical investigations on the pyroxenes and feldspars of basalts and dolerites are carried out. While pyroxene is mainly augite, pigeonite and/or orthopyroxene are conspicuous by their absence. Important minerals of various other rock types are also described.