GEOLOGICAL RELATIONSHIPS IN HIGH-GRADE GNEISS OF THE BRATTSTRAND BLUFFS COASTLINE, PRYDZ BAY, EAST ANTARCTICA

The Brattstrand Bluffs coastline of Prydz Bay, east Antarctica consists of Proterozoic granulite and subordinate late intrusions. Two gneissic associations have been identified, one comprising interlayered felsic and mafic orthogneiss, and the other comprising metapelite-dominated paragneiss. The orthogneiss exhibits fabrics (S1) and migmatitic textures developed in metamorphic events not experienced by the paragneiss, which is interpreted as a cover sequence deposited onto the orthogneiss after these early events. Cover and basement were tectonically interleaved (D2), and then metamorphosed during a Proterozoic granulite event (approximately 1000 Ma). Geothermobarometry of garnet-orthopyroxene-plagioclase-quartz semi-pelite yields peak pressure-temperature estimates of 0.6 GPa and 850-degrees-C for this event. Peak metamorphism was associated with development of a penetrative foliation (S3), and generation and extraction of leucocratic melts from metapelite. Textural studies of metapelitic migmatite indicate that partial melting proceeded through incongruent biotite melting reactions such as: biotite + sillimanite + quartz = garnet + cordierite + K-feldspar + melt, which proceed in the absence of a volatile fluid phase. Retrograde mineral textures, such as spinel-cordierite symplectites after garnet and sillimanite, indicate decompression from peak pressures to 0.3-0.4 GPa, which was partly synchronous with localized development of flat-lying folds and shear fabrics (D4). This combination of decompression and subhorizontal shear is attributed to extensional collapse of overthickened crust. Further retrograde evolution was characterized by cooling, emplacement of a granite pluton and dykes, and D5 shortening to produce a regional antiform under amphibolite to granulite conditions. A pegmatite suite was intruded at approximately 770 Ma, and mylonite development, further granite intrusion and a greenschist overprint occurred at 500 Ma.