Petrogenesis of alkaline rocks from Murud-Janjira, in the Deccan Traps, Western India

Abstract

A late-stage rift-related tholeiite-alkalic suite of igneous intrusions cut the Deccan Traps lavas at the western Indian continental margin. The suite comprises intrusives that can be grouped into ten lithotypes on the basis of their mutual relationships. Tholeiitic types predate the alkaline rocks and greatly predominate, however, the alkaline members exhibit more diversity in mineralogy and chemistry, and are amongst the rare magmatic rocks from the Deccan that host both mantle and lower crustal xenoliths. The mineralogy of most rock types is dominated by clinopyroxene. The diversity of the alkaline rocks could be mainly accounted for by fractional crystallization and mixing between evolved and primitive melts under varying P-T conditions. Sodic and potassic lamprophyres are amongst the most primitive samples with high Mg #, FeO/MgO < 1, high Cr and also with relatively high Ba, Sr, Zr and Nb. They are the most deeply derived magmas within the Deccan Traps as is evident from the mantle and lower crustal xenoliths entrained by them. They possibly represent low degree melts of incompatible element-enriched mantle source rocks. The nephelinites are strongly porphyritic and despite their high Mg #s can be regarded as evolved magmas that have been responsible for the formation of the tephriphonolite daughter. The nephelinites have undergone contamination by lower crustal granulites. The composite intrusions of microdiorites with their complexly zoned mineralogy dominated by plagioclase and amphiboles/micas represent hybrid rocks that have resulted from mixing between tholeiitic and trachytic melts partly at depth and partly at shallow crustal levels.