Late Neoproterozoic metamorphic evolution of Larsemann Hills, East Antarctica: Insights from pseudosection modelling and monazite–zircon geochronology of Easther Island porphyroblastic gneiss

The Larsemann Hills sector of East Antarctica preserves complexly deformed Mesoproterozoic to Neoproterozoic-aged composite gneisses (Sostrene orthogneiss) and interleaved sequence of metasedimentary rocks (Brattstrand paragneiss). The Easther Island porphyroblastic gneiss, a unit of erstwhile ‘Brattstrand paragneiss’, is investigated in this work. Easther Island porphyroblastic gneiss comprises garnet + biotite + quartz + plagioclase + K-feldspar + ilmenite + sillimanite and preserves thick plagioclase corona over coarse-grained garnet. Conventional thermometry and phase equilibria modelling indicate temperatures of 830°C defining peak attained during metamorphism followed by decompression at ~4 kbar pressure. Zircon U–Pb isotopic data yield Concordia ages of 725 ± 43 and 566 ± 15 Ma, respectively. Whereas chemical geochronological data of in-situ monazites provide a unimodal CHIME age of 510 ± 27 Ma with no early Neoproterozoic signature. The new data is discussed in light of the much-debated relict Upper Tonian to Lower Cryogenian (800–700 Ma) event registered by the Larsemann Hills sector, followed by an early Paleozoic orogenic event related to the final amalgamation of Gondwana.