Tectonothermal evolution of the ultrahigh temperature metapelites in the Rauer Group, east Antarctica

Abstract

The Mg–Al-rich Mather Paragneiss in the Rauer Group experienced three distinct stages of tectonothermal evolution: (1) An initial pre-peak stage at 10–11 kbar and ∼850 °C followed by prograde heating that converted kyanite to sillimanite, and partial melting that produced coarse-grained garnet–orthopyroxene-bearing leucosomes, at ∼12 kbar and 960–970 °C. (2) Initial near-isothermal decompression to ∼10 kbar and ∼1000 °C, resulting in the development of corona textures of orthopyroxene + sapphirine on garnet grains and sapphirine/orthopyroxene-K-feldspar-dominated leucosome pockets or sapphirine–orthopyroxene–cordierite-bearing leucosome stringers. (3) Subsequent decompression–cooling to ∼7 kbar and ∼800 °C involving the development of symplectic sapphirine + cordierite, orthopyroxene + cordierite and biotite + cordierite. The proposed P– T path suggests that the Mather Paragneiss had a shared P– T history with the Filla Paragneiss, with prograde P– T stage evolving in the kyanite stability field. The recognization of kyanite in stage (1), together with the peak ultrahigh temperature metamorphic event, is probably related to late Mesoproterozoic collision and arc accretion (∼1100–1000 Ma) that was overprinted by decompression–cooling and retrograde metamorphism during the early Palaeozoic (∼500 Ma) tectonic event.