Alkali Granites with Pan-African Affinities from Kerala, S. India

Abstract

In the Kerala region of the south Indian shield alkali-granite plutons ranging in age from 550 to 740 Ma intrude Precambrian charnockites and migmatites. All of them are spatially related to regional fault-lineaments. Their geochemistry is similar to Pan-African post-kinematic plutons from the Afro-Arabian shield. They are of granitic to adamellitic composition, with high $SiO_{2}, K_{2}O$ (up to 12%), and $Na_{2}O$, and low CaO, FeO, and MgO. Their major element composition, together with a low $^{87}Sr/^{86}Sr_{I}$ (0.703) from one pluton, denote A-type affinities. However, local charnockites have such low Rb and radiogenic $^{87}Sr$ that the granites could have formed by crustal melting. Their overall trace element characteristics are typical of alkaline granites in general, but the details show that most resemble more recent volcanic-arc granites, and one pluton is typical of a within-plate setting. The presence of $CO_{2}$-rich fluid inclusions supports the notion that the enrichment of rare-earth and high field strength elements is related to high fluxes of carbonic fluids during the magma evolution. The situation of the plutons along rift-related faults suggests that the magma-tism manifests an extensional phase associated with the pre-rift tectonics of the Indian continent and the supercontinent of which it was a part during the Pan-African.