Polymetamorphism in the Schirmacher Hills granulites, East Antarctica: Implications for tectonothermal reworking of an isobarically cooled deep continental crust

The Precambrian basement of the Schirmacher Hills, Queen Maud Land, East An tarctica has evolved through multiple episodes of deformation and metamorph ism. The rocks have suffered at least five phases of deformation. The impri nt of the early deformation, D-1, is preserved in some mafic isolated encla ves. The second and the third deformations (D-2 and D-3) are the dominant d eformations of this area and produced isoclinal folds with transposition of earlier cleavages. The later deformations, D-4 and D-5, produced two sets of open, upright folds. Detailed mineralogical, textural, mineral chemical studies and geothermobarometry on khondalite, leptynite as well as differen t varieties of enderbite and mafic granulites have revealed that the rocks suffered two phases of metamorphism under granulite facies conditions follo wed by an amphibolite facies overprint. M-1 is broadly coeval with D-1 only in mafic granulite enclaves within enderbitic gneiss, and took place at ca . 10 Kbar, 900 degrees C. The mafic magma, parental to the enclaves, probab ly crystallized at 11.2 Kbar. Following post-peak near isobaric cooling, th e mafic granulites were transported to shallower levels by the enderbitic m agma. M-2, recorded in all the lithologies, occurred at ca. 8 Kbar, 800 - 8 50 degrees C and synchronous with D-2. Post peak M-2 evolution of the rocks was characterized by a pressure - temperature drop of 2 Kbar and 200 degre es C respectively and textures indicative of both cooling and decompression are preserved in different rocks. The relative timing of the two, however, cannot be worked our. M-3, synchronous with D-3, took place at 6 Kbar, 600 - 650 degrees C and evolved hydrous fluid flux. Correlation with available structural and geochronological data shows that both M-1 and M-2 could be of Grenvillian event. M-3 could well be Pan-African age.