Metamorphic P–T paths of metapelitic granulites from the Larsemann Hills, East Antarctica

Abstract

Through detailed textural observations, a peak M1 assemblage garnet+orthopyroxene+cordierite+K-feldspar has been identified in a structurally early Al-rich metapelitic granulite lens from the Larsemann Hills, East Antarctica. The M1 assemblage has been overprinted by M2 cordierite corona and M3 orthopyroxene+cordierite symplectite on garnet grains. Quantitative modeling for the peak M1 assemblage via the THERMOCALC program in the KFMASH system suggests that it was formed by the crossing of the univariant reaction garnet+biotite=cordierite+orthopyroxene+K-feldspar+melt under P–T conditions of 6–8kbar and 840–880°C, followed by post-peak near isobaric cooling. However, the average P–T calculations for the boron-bearing pelitic granulite indicate that peak M1 conditions reached ~9.0kbar and ~900°C, and the overprinting M2 assemblage formed under P–T conditions of ~7.0kbar and 800–850°C, reflecting a post-peak near isothermal decompression. P–T estimates show that M3 conditions reached 4–5kbar and 700–750°C. These imply that the M1 metamorphic evolution of the region displays contrasting P–T paths, while M2 to M3 evolution indicates a decompression-cooling process. The available chronological data support that the M1 metamorphic evolution occurred during the late Proterozoic (1000–900Ma) Grenvillian high-grade compression tectonic event (D1), and was accompanied by strong magmatism, showing a close affinity to the northern Prince Charles Mountains and the Rayner complex. However, the overprinted M2 to M3 metamorphic evolution formed during the early Palaeozoic (~530Ma) Pan-African high-grade tectonic events (D2–D3), and was associated with an important intracontinental reworking. This study presents an example for interpreting a complex polymetamorphic history.