Granulite-grade garnet pyroxenite from the Kolli-massif, southern India: implications for Archean crustal evolution

Abstract

Pyroxenite is a relatively rare but important rock type of mafic-ultramafic series, and provides a useful tool to understand deep crustal and crust mantle processes. Here we present petrology, phase equilibrium modelling and P-T estimates, zircon U–Pb geochronology and Hf isotopes on a recent find of garnet websterite from the Jambumalai hillock, Kolli-massif, southern India. The rock types in this location include metamorphosed orthopyroxenite, clinopyroxenite, garnet websterite and garnetite among which, garnet websterite is the most abundant variety. Petrological studies show that the peak metamorphic mineral assemblage of the garnet websterite is Grt + Cpx + Opx ± Ilm. The garnet is Fe rich (XMg = 0.39–0.41) whereas the clinopyroxene composition varies from augite to sub-augite (XMg = 0.72–0.79). The orthopyroxene shows a compositional range between ferrosilite and hypersthene (XMg = 0.61–64). Clinopyroxene in some of the garnet websterite samples exhibits exsolution microstructure with blebs of orthopyroxene. The orthopyroxene is Ca poor and Fe rich and displays the features of ‘inverted pigeonite’. Phase-equilibria modelling in the NCFMASH system yielded a P-T estimate of c.22 Kbar and c.980 °C. Zircon U–Pb analysis using SIMS yielded a weighted mean age of 2526 ± 38 Ma, which is regarded as the age of metamorphism. Zircon Lu–Hf isotope data shows initial εHf values between +2 and −7.7, indicating a dominantly reworked crustal source for the protolith. Our results suggest high pressure and high temperature metamorphism at the base of a thickened Archean continental crust.