Sapphirine + quartz association in garnet: Implication for ultrahigh-temperature metamorphism at Rundvågshetta, Lützow-Holm Complex, East Antarctica

We report the occurrence of sapphirine + quartz association within garnet porphyroblast in the garnet-orthopyroxene-sillimanite granulite (Grt-Opx-Sill granulite) from Rundvågshetta in the Lützow-Holm Complex, East Antarctica. The granulites in the study area show a characteristic mineral assemblage consisting of orthopyroxene + sillimanite + quartz. The presence of sapphirine and quartz inclusions within garnet in the sapphirine-bearing Grt-Opx-Sill granulite suggests that metamorphic conditions changed from the stability field of orthopyroxene + sillimanite + quartz to that of sapphirine + quartz during the garnet growth. Peak metamorphic temperature conditions of about 1000-1100 °C are obtained by ternary feldspar thermometry for these granulites. Similar temperatures were also estimated from the Al-in-orthopyroxene geothermometer. The granulites are also characterized by coarse-grained garnet, being partly surrounded by a fine-grained symplectite composed of orthopyroxene and cordierite, whereas sapphirine + cordierite symplectitic intergrowth occurs in the matrix. These textures imply that the area underwent isothermal decompression subsequent to ultrahightemperature metamorphism. The sapphirine-bearing Grt-Opx-Sill granulite is likely to be the restitic product of partial melting and shows signs of segregation and movement of melt.