Differentiation of the Deccan Trap Basalt Flow in Bombay Island

The basalt flow in the Malabar, Cumbala, Worli and Bandra Hills is the youngest lava flow in Bombay Island. It has been riddled by a network of thin veinlets which were in the past thought to be of secondary origin. A detailed examination has revealed that these veins are microaplitic and of an early hydrothermal phase being composed of alkali felspar, quartz and some hydrothermal crystallisation of quartz, zeolite, adularia and fluorite. The formation of these veins has been attributed to late stage differentiation of basaltic lava. This is perhaps a unique example of differentiation of basaltic magma after extrusion.