Mineralogy and petrology of some xenolith-bearing alkaline dykes associated with deccan magmatism, south of Bombay, India

Abstract

A regional, N-S trending dyke-swarm, parallel to the west coast of India, immediately postdates the main Deccan tholeiitic volcanism at around 60 Ma. Around Murud-Janjira, 70 km south of Bombay, the swarm inc1udes strongly alkaline minor intrusions: lamprophyre (camptonite-damkjemite and monchiquite) dykes, plugs of nephelinite-basanite, and more evolved tephriphonolite dykes. Xenoliths in the lamprophyres inc1ude sodic granulites and deformed diopsidic c1inopyroxenites, of presumed crustal and mantle origin respectively. The mafic intrusions show high enrichments in LIL elements relative to MORB, and mostly 'primary' mg (molecular Mg/[Mg+Fe2+]), Cr and Ni values. They also carry magnesian olivines (FO sub(82-86)) with aluminous chromite inc1usions. They must therefore have derived from enriched mantle sources, and show little evidence of crustal contamination. However, extremely complex zoning occurs in their c1inopyroxenes and micas : despite a limited overall En sub(33-48)Fs sub(4-20)Wo sub(44-50) range, different clinopyroxene grains even within single dykes may show Na increasing or decreasing with increasing, decreasing or static mg from core to rim (i.e. normal, inverse or reverse zoning). This implies that the magmas have undergone varying degrees of polybaric crystallization during their ascent through the crust, plus mixing of more primitive and slightly more fractionated pulses at crustal levels, further complicated by disaggregation of incorporated mantle materials. The Murud mafic dykes represent the most primitive and deepest derived of all Deccan magmas, and thus have considerable implications as regards the trigger and source of the Deccan magmatism itself. A connection with a contemporaneous tripie junction in the Bombay area is possible.