LATE ARCHAEAN GRANULITE FACIES METAMORPHISM IN THE VESTFOLD HILLS, EAST ANTARCTICA

Abstract

Granulites of the Vestfold Hills record a pulsed end-Archaean to early Palaeoproterozoic M1–M2 evolution that is distinct from other Archaean areas in East Antarctica and cratonic domains placed adjacent to East Antarctica in Gondwana reconstructions. Pressure and temperature conditions of the end-Archaean to earliest Palaeoproterozoic (2501–2496 Ma) M1 granulite facies metamorphism in the Vestfold Hills have been constrained from mineral assemblages and thermobarometry of Fe-rich paragneisses. Reintegrated compositions of exsolved subcalcic clinopyroxenes and pigeonites in a metaironstone yield temperatures of 895 ± 35 °C, whilst reintegrated compositions of perthitic feldspars in semipelitic paragneisses give minimum estimates of 860 ± 30 °C. These results rule out the extreme ultrahigh temperature (UHT) conditions previously proposed for M1 in the Vestfold Hills. Pressures of metamorphism during M1 are estimated as 8.1 ± 0.9 kb at 850 ± 40 °C from hercynite + sillimanite + almandine + corundum and retrieved Fe–Mg–Al relations in orthopyroxene coexisting with garnet. A second metamorphic event, M2, occurred at 600–660 °C and 6–8 kb based on thermometry of recrystallised pyroxene neoblasts and thermobarometry applied to M2 garnet–quartz symplectites formed on orthopyroxene and garnet. The intervening emplacement of the magmatic Crooked Lake Gneiss Group precursors occurred at similar or shallower pressures prior to D2–M2, an event that caused tectonic interleaving and reactivation of the Vestfold Hills basement at mid-crustal depths in the earliest Palaeoproterozoic, prior to its unroofing to shallower levels (3–5 kb) by 2470 Ma. The lack of correlative Archaean histories in areas that were formerly adjacent in Gondwanan reconstructions is consistent with the Vestfold Hills region either being exotic to the East Antarctic Shield until the final (Neoproterozoic to Cambrian) amalgamation of Gondwana, or being accreted to part of East Antarctica in a Proterozoic event distinct from the Rayner–Eastern Ghats tectonism that united much of India with Antarctica at 1000–900 Ma.