Platinum-group elements in igneous rocks of the Kutch rift basin, NW India : Implications for relationships with the Deccan volcanic province

Abstract Platinum-group elements (PGE) have been determined in mafic igneous rocks from the Kutch region, Gujarat, north–northwest of the Deccan Volcanic Province (DVP) of west central India. The PGE data are supported by determination of Cu, Ni, Cr and S together with Nd and Sr isotopic compositions. PGE in tholeiitic flows of the southern Kutch were compared with Deccan basalts of the Western Ghats region of the DVP and with a variety of alkaline mafic rocks from Kutch. Data are presented as mantle normalized Ni-PGE-Cu plots, PGE-base metal ratio plots and Nd–Sr initial ratio diagrams. Tholeiitic flows from the southern coast of Kutch, thought to represent the earliest eruptive phase of the DVP, are characterized by Ni-PGE-Cu mantle normalized profiles similar to those of the Western Ghats. Both suites have very high proportions (99%) of Pt, Pd and Rh relative to Ir and Ru and represent highly evolved compositions relative to parental mantle melts. Mafic alkaline intrusions occur in uplifts in southern Kutch along with the tholeiites, and in uplifts in northern Kutch where tholeiites are absent. Alkaline intrusions in the south are generally characterized by much lower PGE abundances than the tholeiites and much higher proportions of Ir plus Ru relative to total PGE. They also have supermantle Cu/Pd ratios which together with overall low PGE contents suggest parental melts which were equilibrated with an immiscible sulfide liquid. Some of these alkaline intrusions are thought to carry a significant ocean island basalt (OIB) component. Comparison of four alkaline intrusions from the southern Kutch with Hawai'i OIB in Ni–Cu–Pd–Ir metal ratio plots along with comparison in a Nd–Sr initial ratio diagram is generally supportive of this contention. Alkaline intrusions from the northern Kutch include a spectrum of emplacement modes represented by sills, small diapirs and a large differentiated igneous complex. Very low PGE contents prevail generally, but there are exceptions. The large Nir Wandh igneous complex is mainly a gabbro with low PGE content but includes a basal pyroxenite with Pt–Pd–Cu levels similar to the tholeiitic flows. Sadara sill, an intrusion of some 17 km length, and the pyroxenite member of the Nir Wandh complex have mantle normalized profiles similar to the Kutch tholeiitic flows with respect to Pt–Pd–Cu but differ with higher Ir and Ru values than the tholeiites. Thus, the alkaline and tholeiitic volcanism of Kutch is well differentiated by PGE.