Cordierite gneisses and high temperature metamorphisms in the Fosdick Mountains, west Antarctica, with implications for breakup processes in the Pacific sector of the Mesozoic Gondwana margin [microform]

The Fosdick Metamorphic Complex (FMC), West Antarctica, is a potential example of the mid-crustal regime in which lateral flow compensates for extension in the upper crust. High grade metamorphism of sedimentary and plutonic protoliths caused extensive migmatization and anatexis, and dynamic processes assisted separation of melts. Sufficient melt buildup led to failure by vertical flattening and lateral spreading; structural and petrological investigation in the field and laboratory showed that cycles of melt accumulation occurred. Leucogranitic material in ductile shear planes and in fold hinges attests to contemporaneity of anatexis and deformation, but gneisses are nearly devoid of lineation and kinematic indicators, due to sustained high temperature metamorphism after deformation had ceased.

Petrography and thermobarometry on garnet-cordierite-sillimanite-Kfeldspar gneisses and garnet-amphibolites of FMC migmatites were used to determine peak temperatures of T = 612-700$\sp\circ$C and pressures of P = 0.50-0.55 GPa, corresponding to 17-18.5 km crustal depths. Mineral analyses were from electron microprobe analysis. Bulk rock geochemistry from Xray fluorescence was used to address questions of melt separation from source rocks. Water activity during metamorphism were low, as shown by CHN analysis of volatiles trapped in the mineral cordierite. Thermal modelling experiments produced a model for FMC thermal evolution involving rapid cooling from peak conditions, known from $\sp{40}$Ar-$\sp{39}$Ar investigation by a co-worker. The model calls upon magmatic underplating followed by translation to shallower crustal levels, consistent with thermal evolution in an extensional setting.

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