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

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;Thermal modelling experiments produced a model for FMC thermal evolution involving rapid cooling from peak conditions, known from ;FMC structural-metamorphic features resemble those predicted beneath the 'mylonite front,' within metamorphic core complexes; few other examples exist where structural-magmatic relationships are so clear. The Fosdick Mountains therefore serve as an important example of a mid-crustal deformation zone and provide new insights into the processes envisioned for maintaining constant crustal thickness and a flat moho beneath extended terranes such as the Basin and Range Province of the United States.