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

Abstract

The Precambrian basement of the Schirmacher Hills, Queen Maud Land, East Antarctica has evolved through multiple episodes of deformation and metamorphism. The rocks have suffered at least five phases of deformation. The imprint of the early deformation, D₁, is preserved in some mafic isolated enclaves. The second and the third deformations (D₂ and D₃) are the dominant deformations of this area and produced isoclinal folds with transposition of earlier cleavages. The later deformations, D₄ and D₅, produced two sets of open, upright folds. Detailed mineralogical, textural, mineral chemical studies and geothermobarometry on khondalite, leptynite as well as different varieties of enderbite and mafic granulites have revealed that the rocks suffered two phases of metamorphism under granulite facies conditions followed by an amphibolite facies overprint. M₁ is broadly coeval with D₁ only in mafic granulite enclaves within enderbitic gneiss, and took place at ca. 10 Kbar, 900°C. The mafic magma, parental to the enclaves, probably crystallized at 11.2 Kbar. Following post-peak near isobaric cooling, the mafic granulites were transported to shallower levels by the enderbitic magma. M₂, recorded in all the lithologies, occurred at ca. 8 Kbar, 800-850°C and synchronous with D₂. Post peak M₂ evolution of the rocks was characterized by a pressure — temperature drop of 2 Kbar and 200°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 out. M₃, synchronous with D₃, took place at 6 Kbar, 600-650°C and evolved hydrous fluid flux. Correlation with available structural and geochronological data shows that both M₁ and M₂ could be of Grenvillian event. M₃ could well be Pan-African age.