Metamorphic evolution of the Bunger Hills, East Antarctica: evidence for substantial post‐metamorphic peak compression with minimal cooling in a Proterozoic orogenic event

The Bunger Hills, East Antarctica, experienced a low‐pressure granulite facies orogenic event during the Proterozoic. The stable coexistence of the S1 foliation‐parallel M1 assemblages, garnet‐cordierite‐spinel‐ilmenite and garnet‐sillimanite‐spinel‐ilmenite‐rutile, in quartz‐bearing pelitic gneisses is evidence for metamorphic peak pressures of around 4 kbar during M1, at temperatures of about 800°C. The growth of massive reaction coronas of garnet and cordierite around hercynitic spinel and iron‐titanium oxides during M2 is evidence for the destabilization of the M1 assemblages during compression. Thermodynamic calculations on the M2 assemblages indicate formation pressures of 6–7 kbar at temperatures of about 750°C. Thus, the gneisses from the Bunger Hills indicate about 2 kbar or more of compression during minimal cooling. Such a P‐T path is different from that of many other Proterozoic terranes which are characterized by isobaric cooling or decompression. A large charnockite body, which is undeformed, was intruded at ∼950°C, towards the end of compression. The low pressures during M1 can be best explained by metamorphism at mid‐crustal levels in thin continental crust in thin lithosphere above a thermal perturbation in the underlying asthenosphere. We suggest that the compression during cooling was a result of gravitational backflow in which the action of body forces between adjacent normal thickness crust and the thin crust of the Bunger Hills is 'switched on’by the thermal perturbation. Within such a model, the timing of intrusion of the charnockite exposed in the Bunger Hills is consistent with its generation by partial melting during the metamorphic maximum of the lowermost crust.