Orthopyroxene–Corundum in Mg–Al-rich Granulites from the Oygarden Islands, East Antarctica

Abstract High-Mg–Al, silica-undersaturated metapelites from the Oygarden Group of islands, East Antarctica, preserve clear evidence for the stable coexistence of the assemblage orthopyroxene + corundum in natural rocks. The quartz-absent metapelite occurs as pods and isolated layers within a high-strain zone related to deformation during the c. 0·93 Ga Rayner Structural Episode. Assemblages that include orthopyroxene, corundum, sapphirine, sillimanite, cordierite, garnet and kornerupine are developed across a pre-existing compositional zoning, leading to contrasting mineral Fe–Mg ratios. The assemblage orthopyroxene–corundum is shown to exist in only a very restricted range of bulk compositions and P–T histories. Simplified qualitative FMAS grids have been constructed for kornerupine-absent and -present systems, illustrating MAS terminations and divariant equilibria that help to describe the mineral assemblage and reaction history. Reaction textures that include coronas of sapphirine and sillimanite separating orthopyroxene and corundum, and symplectites of orthopyroxene + sapphirine ± cordierite/plagioclase, orthopyroxene + sillimanite ± cordierite/plagioclase and orthopyroxene + sapphirine + sillimanite embaying garnet, imply a clockwise P–T–t evolution. Conditions of P > 9–10 kbar and T ≈ 800–850°C were attained prior to an initial phase of decompression that was accompanied by heating of up to ≈100°C. Peak temperatures of T ≈ 850–900°C were achieved at P ≈ 9 kbar followed by near-isothermal decompression to pressures of P ≈ 5 kbar. This clockwise isothermal decompression path contrasts markedly with anticlockwise isobaric cooling paths recorded elsewhere in the Rayner Complex, and reflects a second phase of orogenesis within the Rayner Structural Episode.