A metamorphic perspective on the Pan African overprint in the Amery area of Mac. Robertson Land, East Antarctica

Abstract

The Amery area of Mac. Robertson Land lies between the early Palaeozoic granulite terrain of Prydz Bay and Meso-Neoproterozoic granulites in northern Prince Charles Mountains (nPCM). In contrast to the nPCM which shows an apparently simple near-isobaric history, granulites exposed in the Amery area contain reaction textures suggesting a more complex evolution. Peak-M₁ Mesoproterozoic assemblages formed at c. 700 MPa and 800°C and initially underwent a near-isobaric cooling. A subsequent increase in temperature (M₂) resulted in the formation of cordierite-spinel assemblages at ~450 MPa and 700°C in metapelite. The timing of M₂ is not firmly established, however existing data strongly suggest it is an early Palaeozoic event coeval with tectonism in Prydz Bay to the north-east. Thus the metamorphic evolution of granulites in the Amery area reflects a terrain-scale thermal interference pattern between two unrelated orogenic events. In rocks not recording post-M₁ isobaric cooling, the superposition of M₂ on M₁ assemblages resulted in the formation of M₂ cordierite-spinel symplectites at the expense of peak M₁ garnet and sillimanite. This texture, commonly interpreted to reflect near-isothermal decompression, has no relevance in terms of a single tectonothermal event in the Amery area.