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

Granulites in the Bolingen Islands comprise two structural-lithologica l 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 domi nated 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 i n the southern Bolingen Islands are associated with normal movements a nd may represent time correlatives of composite D-3 events in the nort hern Bolingen Islands where no pseudotachylite was found. Later pegmat ite-parallel shears (D-7) formed throughout the Bolingen Islands. The granulite-grade D-2, D-3 and D-5 events are composite in nature, meani ng that these events comprise several truncating foliations and series of overprinting folds. Each deformation is associated with a unique l ineation direction and sense of shear, which forms a good basis for st ructural correlations. The granulites preserve textures that record 30 0-500 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 de compression textures can be explained in a model involving extensional collapse and exhumation (D-3-6) Of thickened crust (0(2)). However, b ased on regional correlations it appears that D-2 compression took pla ce at 1000 Ma, while D-3-7 extension occurred 500 million years later. If correct, these correlations imply a more complicated thermal and e xhumation history. Brittle events alternating with kinematically relat ed, pervasive, granulite-grade ductile events, hint at temperature flu ctuations during high-grade metamorphism in the southern Bolingen Isla nds. This is consistent with the observations that pseudotachylite geo metries are independent of lithology, pre-existing layering or mylonit es. Rocks that do not contain recrystallized pseudotachylite preserve little evidence of thermal fluctuations during the interval D-3-D-7. T hus, polymetamorphic histories in relatively anhydrous rocks may be di fficult to recognize in the absence of detailed structural evidence.