GEOLOGICAL RELATIONSHIPS IN HIGH-GRADE BASEMENT GNEISS OF THE NORTHERN PRINCE CHARLES MOUNTAINS, EAST ANTARCTICA

High-grade gneiss in the northern Prince Charles Mountains, East Antarctica, has a complex intrusive and deformational history. Outcrop is dominated by homogeneous felsic orthogneiss, which encloses boudinaged mafic and ultramafic units. These boudins preserve structures (S1, F2) not seen in the host gneiss, and are interpreted as transposed and boudinaged dykes. A sedimentary protolith is inferred for less homogeneous felsic gneiss interlayered with semi-pelite, calc-silicate and rare pelite. These basement lithologies were deformed into a series of flat-lying structures (S3, F4) consistent with progressive horizontal shear, and then into a series of upright structures (F5) culminating in the development of regional synforms, antiforms and monoclines (F6) separated by zones of intense upright fabric (S6). The D3 to D6 time interval was associated with several episodes of partial melting which produced discordant leucogneiss bodies, and with the emplacement of mafic dykes and charnockite plutons correlated with 950 to 1000 Ma charnockite elsewhere in East Antarctica. The stability of granulite assemblages throughout the D3 to D6 interval is attributed to a widespread 1000 Ma metamorphic event. Thermobarometry of garnet-orthopyroxene-plagioclase-quartz gneiss and pelite yield peak conditions of 700-800-degrees-C and 0.6-0.7 GPa for this Proterozoic metamorphism. Petrogenetic grid constraints on calc-silicate assemblages indicate peak temperatures of 830-degrees-C, suggesting that the lower temperatures derived by thermometry have been reset. Mineral assemblages in interlayered felsic and calc-silicate units imply H2O-rich conditions during prograde metamorphism, but indicate that peak metamorphism was fluid absent, or associated with volatile fluid buffering on a local scale. Calc-silicate reaction textures reflect a retrograde evolution dominated by cooling, which is supported by mineral zonation trends in the garnet-orthopyroxene-plagioclase-quartz gneiss. Post-D6 intrusive and deformational events reflect a decrease in grade to greenschist facies and a transition from ductile to brittle deformation between 950 and 500 Ma.