Clarifying temperature-pressure paths via structures in granulite from the Bolingen Islands, Antarctica

Abstract

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 (D2). 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 (D3). In the southern Bolingen Islands, D3 effects overprint D2 to locally form a composite gneissic layering, which is overprinted by pseudotachylite zones (Dipt) that are reactivated as granulite-grade ultramylonites (D4Um) and folded in a later high-grade gneissic layering (D5), once more truncated by recrystallized pseudotachylite (D6). D3-6 events in the southern Bolingen Islands are associated with normal movements and may represent time correlatives of composite D3 events in the northern Bolingen Islands where no pseudotachylite was found. Later pegmatite-parallel shears (D7) formed throughout the Bolingen Islands. The granulite-grade D2, D3 and D5 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° C). Most textures resulted from the overprint of D2 by lower pressure D3 assemblages, but decompression continued during and after D3. The decompression textures can be explained in a model involving extensional collapse and exhumation (D3-6) of thickened crust (D2). However, based on regional correlations it appears that D2 compression took place at 1000 Ma, whileD3-7 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 D3-D7. Thus, polymetamorphic histories in relatively anhydrous rocks may be difficult to recognize in the absence of detailed structural evidence. © 1995, Taylor and Francis Group, LLC.