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

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-M-1 Mesoproterozoic assemblages formed at c. 700 MPa and 800 degrees C and initially underwent a near-isobaric cooling. A subsequent increase in temperature (M-2) resulted in the formation of cordierite-spinel assemblages at similar to 450 MPa and 700 degrees C in metapelite. The timing of M-2 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-M-1, isobaric cooling, the superposition of M-2 on M-1 assemblages resulted in the formation of M-2 cordierite-spinel symplectites at the expense of peak M-1 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.