Origin of the Felsic and Basaltic Dikes and Flows in the Rajula-Palitana-Sihor Area of the Deccan Traps, Saurashtra, India: A Geochemical and Geochronological Study

Abstract

Rhyolite, trachyte, pitchstone, and granophyre dikes are associated with mafic dolerite dikes and basaltic flows of the northwestern part of the Deccan flood basalt province in the Saurashtra Peninsula, India. Felsic dikes, exposed in the Rajula area of Saurashtra, are similar in age to the basaltic flows of neighboring Palitana. The ages of both the felsic and mafic rocks straddle the ~65 Ma Cretaceous-Tertiary boundary and correspond to the main Deccan flood basalt episode. Palitana is centered on an elongated gravity high whose major axis is NE-SW, and Rajula is located on its southwestern flank. Unlike the younger Bombay felsic rocks from the western coast of India, which have been explained as partial melts of gabbros in deep crustal sills or previously erupted basalts, the incompatible-element characteristics of the Rajula rocks indicate that the Rajula rhyolites, trachytes, and dacites may have been generated by an almost complete melting of upper crustal rocks at the southwestern flank of the Rajula-Palitana-Sihor magmatic body. High potential temperatures of the Deccan plume, quick migration of the hot basaltic parent magma through lithospheric weak trends, and collection and residence of magma in upper-crustal magma chambers before eruption may have produced the right conditions to melt the upper crust in the vicinity of the Rajula-Palitana-Sihor magma chamber. On the other hand, the andesite located northeast of the magmatic body possibly evolved by assimilation of upper-crustal wall rocks accompanied by 5-10% crystallization of a Rajula-type basalt near the wall of the magma chamber. The Sihor rhyolites may also have been derived from the Sihor basalts through fractional crystallization accompanied by crustal assimilation. The Rajula granophyres, however, do not show any involvement of the upper crust in their genesis. These may have a history similar to that of the Bombay rocks and may have erupted in response to rifting along the Cambay rift.