Compositional variations of chromiferous spinel in Mg-rich rocks of the Deccan Traps, India

Abstract

Composition of chromiferous spinel included in olivines of Mg-rich basalts and gabbros of the Deccan Traps (Gujarat and Western Ghats) are reported here. They vary from Al-rich compo- sitions (Al2O3 = 53 wt.%; Cr#, 100Cr/(Cr + Al) = 12) to Cr-rich compositions (Cr2O3 =5 1 wt.%; Cr# = 84), and from Cr-Al rich compositions towards Cr-rich Ti-magnetite (TiO2 up to 23 wt.%, ulvospinel up to 67 mol.%). The Mg# (100Mg/(Mg + Fe 2+ )) of spinel decreases from 81 to nearly zero. The highest Cr# has been found in the Bushe Fm., Thakurvadi Fm., and some high-Ti basalts of the Pavagadh section, whereas some of the low-Ti basalts of Saurashtra have Al-rich composi- tions typical of spinels found in mid-ocean ridge basalts. The chemical composition of the Deccan Trap spinels is completely different compared to that observed in mantle spinel suites, with very few exceptions. The decreasing Al and increasing Fe and Ti of spinel seems to be mainly the result of decrease of Mg in the locally coexisting melts and favourable cationic substitutions in the lat- tice. There is barely any evidence of general relationships between the composition of the Deccan spinels and inferred mantle sources of the host magmas. Pyroxene inclusions in spinels may wit- ness a high-pressure stage of crystallization, but the possibility of non-equilibrium crystallization, or even magma mixing, cannot be ruled out. Overall, the compositional ranges of chromiferous spinel in the Deccan Traps closely match those observed in the other Large Igneous Provinces having mafic/ultramafic intrusions and mafic magma compositions (e.g., Siberian Traps, Karoo, Emeishan).