Garnet-forming reactions and recrystallization in high-grade mylonite zones, MacRobertson Land, east Antarctica |
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| Authors: | R W WHITE G L CLARKE |
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| Institution: | Department of Geology and Geophysics, University of Sydney, NSW 2006, Australia |
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| Abstract: | Proterozoic granulite facies gneisses in MacRobertson Land, east Antarctica, are cut by numerous D5 mylonite-ultramylonite zones of probable Cambrian age. In garnet-absent mafic two-pyroxene gneisses and garnet-bearing charnockitic orthogneisses, the mylonite-ultramylonite zones are characterized by the growth of garnet at the expense of ilmenite, pyroxene and plagioclase. Textures within each mylonite zone can vary from protomylonitic to ultramylonitic. A range of mineral textures involving M5 garnet is developed corresponding to variations in deformation intensity. In protomylonites, garnet occurs as coronas on orthopyroxene-plagioclase and ilmenite-plagioclase boundaries, and as overgrowths on earlier garnet. In ultramylonites, fine-grained orthopyroxene-plagioclase-garnet ± quartz ± clinopyroxene intergrowths and poikilitic garnet are common. Garnet growth in all shear zones is accompanied by shifts in the compositions of neoblastic minerals occurring with garnet, consistent with local chemical equilibrium having been attained during recrystallization. Mylonitization is inferred to have occurred at P ～ 6.5 kbar. Temperature estimates for M5 vary between 550 and 797 C, which may reflect variations and uncertainties associated with the calibrations used and/or partial re-equilibration during cooling. The presence of post-tectonic, coronate garnet in some mylonite zones indicates that garnet continued to form exclusively in the mylonite zones after movement had ceased and is interpreted to reflect the effects of localized strain heating. |
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| Keywords: | Colbeck Archipelago east Antarctica garnet Proterozoic recrystallization ultramylonite |
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