Petrogenesis and geochemical characteristics of pyroxenite dykes in and around Salem Mafic–Ultramafic complex, southern India: an arc-related origin of Alaskan-type

Abstract

Arc-related origin of pyroxenites in association with Alaskan-type tectonics has been described in many mafic–ultramafic complexes across the globe. The Salem Mafic–Ultramafic Complex (SMUC) is one such Neo-p roterozoic Alaskan-type complex exposed at the north-er n margin of the Cauvery Suture Zone (CSZ), Southern Granulite Terrane, south India. The Complex consists of mafic and ultramafic sequences along with several occurrences of pyroxenite intrusions of varied thick-ness in the form of dykes. Similar pyroxenite dykes were also observed in and around the Complex at several locations within the basement hornblende gneiss, trending in the NE–SW and E–W directions. Petrography of these dykes indicated websterite variety with cumulate textures and reveals the dominance of clinopyroxene along with orthopyroxene, primary amphi bole, minor plagioclase and oxide minerals like magnetit e, ilmenite and spinels. The whole-rock chemistry of 10 representative samples showed enrichment of LIL elements (Sr, K, Rb, Th) and depletion of HFSE (Hf, Ti, Y, Yb) with normalized primitive mantle and N-MORB . The clinopyroxene mineral chemistry represented tholeiitic signatures with high Mg# values (Mg/(Mg + Fe)) up to 0.91, and the two-pyroxene thermobarometry of these pyroxenites yielded re-equilibrium crystallization temperatures of 820–932  C with moderate pressures at 11–12 kbar. Various tectonic discrimination plots of clinopyroxene mineral chemistry together with whole-rock chemistry favoured their origin under arc settings with the interactions of fluid-related subduction zone m etasomatism relevant to Neoproterozoic Alaskan-typ e tectonics.