CARNET-FORMING REACTIONS AND RECRYSTALLIZATION IN HIGH-GRADE MYLONITEZONES, MACROBERTSON LAND, EAST ANTARCTICA

Proterozoic granulite facies gneisses in MacRobertson Land, east Antar ctica, 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 inv olving M5 garnet is developed corresponding to variations in deformati on intensity. In protomylonites, garnet occurs as coronas on orthopyro xene-plagioclase and ilmenite-plagioclase boundaries, and as overgrowt hs on earlier garnet. In ultramylonites, fine-grained orthopyroxene-pl agioclase-garnet +/- quartz +/- clinopyroxene intergrowths and poikili tic garnet are common. Garnet growth in all shear zones is accompanied by shifts in the compositions of neoblastic minerals occurring with g arnet, consistent with local chemical equilibrium having been attained during recrystallization. Mylonitization is inferred to have occurred at P approximate to 6.5 kbar. Temperature estimates for M5 vary betwe en 550 and 797 degrees C, which may reflect variations and uncertainti es associated with the calibrations used and/or partial re-equilibrati on during cooling. The presence of post-tectonic, coronate garnet in s ome 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.