Sapphirine — Orthopyroxene — Garnet Granulite from Schirmacher Hills, Central Dronning Maud Land

Abstract

A third locality of sapphirine granulite was discovered in the Schirmacher Hills, central Dronning Maud Land, East Antarctica; one unique for the association with orthopyroxene and garnet. Sapphirine occurs as inclusions together with orthopyroxene within garnet and as discrete grains together with spinel, secondary orthopyroxene in cordierite coronas. Orthopyroxene porphyroblasts have the highest Al₂O₃ contents (∼10 wt.-%) so far reported from Dronning Maud Land. The earlier of two metamorphic stages inferred for the sapphirine-bearing granulite is characterized by breakdown of sapphirine and orthopyroxene to form garnet at peak conditions approaching 950–1050°C and 9–10 kbar. The later stage is decompression when spinel-cordierite-orthopyroxene formed at about 950–980°C and 8 kbar. The age of the ultrahigh-temperature event could be ca. 1.15 Ga, somewhat older than the late Mesoproterozoic granulite-facies event elsewhere in central Dronning Maud Land. Metamorphic temperatures estimated to exceed 900°C imply that the tectonothermal regime in the Schirmacher Hills was distinct and could be critical in determining relationships between central Dronning Maud Land and metamorphic belts in southern Africa and in understanding the evolution of Rodinia and Gondwana.