Intraplate alkali basalts related to end-Cretaceous Deccan magmatism: implications for tectonomagmatic processes

Abstract

This study aims to investigate the earliest imprint of Deccan rift magmatism as preserved in alkali basalts from the northwestern Indian shield. The alkali basalts are petrographically classified as nephelinites and basanites. They are silica undersaturated and their high Mg numbers, CaO/Al 2 O 3 , Cr and Ni indicate their primitive character. Geochemically, they are similar to global ocean island basalts; their bulk rock trace elements and Sr–Nd–Hf isotopic signatures suggest their derivation from a garnet-bearing peridotite field. However, their elevated values of Sr/Sm, Sm/Hf, Zr/Hf, Nb/Ta; positive Ba, Sr and negative Zr, Hf spikes suggest that the magma source represents a mixture of garnet peridotites and carbonated melts. Estimated primary melt compositions closely follow the trajectory defined by the high-pressure experimental partial melting trend of a low-carbonated peridotite source. The melting environment approximates to a high mantle potential temperature. A low 87 Sr/ 86 Sr ratio and a negative correlation between 176 Hf/ 177 Hf and 143 Nd/ 144 Nd of the alkali basalts suggest that the mantle source is a mixture of a depleted Indian MORB-type mantle and an enriched mantle type 2. We correlate this event with the melting of the leading edge of the Réunion plume head during Gondwana break-up in a relatively short span of the Cretaceous/Paleogene boundary.