Metamorphism of Ultramafic and Mafic Enclaves Within Granulites, Schirmacher Oasis, East Antarctica

Abstract Mafic and ultramafic enclaves from Schirmacher Oasis preserve evidences for the earliest metamorphic event. In this study, this event has been constrained through reaction textures, mineral chemistry and thermobarometry by application of conventional and multireaction methods. Mafic enclaves comprise melanocratic pargasite-bearing gabbro, gabbroic anorthosite and ultramafic enclaves are spinel- and garnet-bearing websterite, ortho- and clinopyroxenite which occur as deformed enclaves within foliated norite. Deformed metanorite dykes and metagabbro pods occur within metasedimentary granulite. Most of the enclaves are massive, without chromite layering, while few preserve deformed modal layering at centimeter-scale. It is likely that the enclaves represent mantle restites during a partial melting event. Reaction textures within enclaves indicate a decompressive sense of the P-T path inferred from garnet-bearing websterite, deformed metanorite dyke and melanocratic pargasite-bearing gabbro, recorded by breakdown of blastic garnet to symplectites of orthopyroxene and highly calcic plagioclase and by the decomposition of pargasite in metagabbro to symplectites of pyroxenes and plagioclase. Thermobarometric calculations confirm that these enclaves were metamorphosed to granulite-facies conditions at ~ 8 kbar and ~ 800 °C, and underwent high-temperature decompression to a mid-crustal level.