Deccan Trap volcanism

The Deccan Traps, now occupying an area of 200,000 sq. miles, must originally have been more wide-spread. Their thickness in Western India reaches 6000 ft. They have been encountered at depths of over 1500 ft. in Kathiawar and Sind (Pakistan), and have been faulted down to a depth of over 6000 ft. in the Cambay area. They are composed of numerous flows whose thickness varies from a few ft. to 200 ft. The flows are often compact in the lower portions and vesicular in the upper portions. Over most of the area (east of the Western Ghats) the rock is a tholeiitic basalt (50 to 51.5 % silica) whose pyroxene is intermediate in composition between pigeonite and diopside, and whose CIPW norm generally shows the presence of some quartz. In the Bombay Kathiawar region there are numerous eruptive Centres associated with a large range of differentiated types comprising both very basic and acid types. The study of the analyses of the various types indicates the presence of both the alkali-olivine basalt as well as the Calc-alkali lines of differentiation, which has brought up the question of the nature of the primary magma. It is noted that while the greater part of the area shows tholeiitic rock, olivine basalts and connected types appear in the more western areas, perhaps as a result of the local tectonic conditions. Recent geophysical data point to the presence of an « oceanic basalt » layer all around the earth both in oceanic and continental crust, while a less dense (presumably tholeiitic) layer overlies it (below the sial) in the continental segments. The « oceanic basalt » should therefore be taken as representing the primary magma, and tholeiite as a major type derived from it in the continental crust.