CLARIFYING TEMPERATURE-PRESSURE PATHS VIA STRUCTURES IN GRANULITE FROM THE BOLINGEN ISLANDS, ANTARCTICA

Granulites in the Bolingen Islands comprise two structural-lithological domains. The southern Bolingen Islands are dominated by felsic-mafic gneiss and a foliation with an east-plunging lineation that formed in response to compression (D-2). The northern Bolingen Islands are dominated by sediments and a series of related foliations with a southwest-plunging lineation that formed in response to extension (D-3). In the southern Bolingen Islands, D-3 effects overprint D-2 to locally form a composite gneissic layering, which is overprinted by pseudotachylite zones (D-4pt) that are reactivated as granulite-grade ultramylonites (D-4um) and folded in a later high-grade gneissic layering (D-5), once more truncated by recrystallized pseudotachylite (D-6) D-3-6 events in the southern Bolingen Islands are associated with normal movements and may represent time correlatives of composite D-3 events in the northern Bolingen Islands where no pseudotachylite was found. Later pegmatite-parallel shears (D-7) formed throughout the Bolingen Islands. The granulite-grade D-2, D-3 and D-5 events are composite in nature, meaning that these events comprise several truncating foliations and series of overprinting folds. Each deformation is associated with a unique lineation direction and sense of shear, which forms a good basis for structural correlations. The granulites preserve textures that record 300-500 MPa of decompression at elevated temperatures (> 800 degrees C). Most textures resulted from the overprint of D-2 by lower pressure Dg assemblages, but decompression continued during and after D-3. The decompression textures can be explained in a model involving extensional collapse and exhumation (D-3-6) Of thickened crust (0(2)). However, based on regional correlations it appears that D-2 compression took place at 1000 Ma, while D-3-7 extension occurred 500 million years later. If correct, these correlations imply a more complicated thermal and exhumation history. Brittle events alternating with kinematically related, pervasive, granulite-grade ductile events, hint at temperature fluctuations during high-grade metamorphism in the southern Bolingen Islands. This is consistent with the observations that pseudotachylite geometries are independent of lithology, pre-existing layering or mylonites. Rocks that do not contain recrystallized pseudotachylite preserve little evidence of thermal fluctuations during the interval D-3-D-7. Thus, polymetamorphic histories in relatively anhydrous rocks may be difficult to recognize in the absence of detailed structural evidence.