Geochemical constrains on pyroxenites from Aniyapuram Mafic–Ultramafic Complex, Cauvery Suture Zone, southern India: Suprasubduction zone origin

Abstract

The Aniyapuram Mafic and Ultramafic Complex (AMUC) is a dismembered ophiolite of Neoarchean to Paleoproterozoic age, exposed in south-central part of the Cauvery Suture Zone (CSZ), Southern Granulite Terrane (SGT), India. The complex comprises ultramafic rocks like serpentinized peridotites of harzburgite variety and pyroxenites of websterite variety along with metagabbros, amphibolites, plagiogranites and metacherts. The pyroxenites in the complex occur in the form of dykes, veins and lenses in association with serpentinized peridotites and metagabbros. The chemical compositions of olivine from the peridotites show a moderate forsterite content (Fo88–87) and spinels are enriched in aluminum (Al2O3 > 60 wt%) with depletion of Cr contents (Cr# < 0.5). The pyroxenites are composed of orthopyroxenes with dominance of clinopyroxene and sulphide occurrences (up to 62 wt% S) along the grain boundaries of pyroxenes. The whole rock geochemistry of these pyoxenites shows enrichment of LILE and depletion of HFSE with negative Nb anomalies on N-MORB and primitive normalization. The mineral chemistry of clinopyroxenes from the pyroxenites shows tholeiitic in nature with high Mg# ratios [Mg/(Mg + Fe) = 0.70–0.88]. These mineral chemistry results together with whole rock chemistry reveal their origin in Island arc setting of supra-subduction zone (SSZ) tectonics possibly evolved by the interaction of subduction derived fluids of host serpentinized peridotites. The estimated two-pyroxene thermobarometry of these pyroxenites represents varied re-equilibrium temperatures of 820–980°C and medium to slightly high pressures of 10–12 kbar. With the available age relationship from the complex, the study supports as a strong evidence for the occurrence of Neoarchean to Paleoproterozoic suprasubduction tectonics and associated lithologies of formation in the terrane.