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Title: The metamorphic histories of some proterozoic granulites from East Antarctica
Author: Fitzsimons, Ian Christopher William
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 1991
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The Proterozoic Complex of East Antarctica is an extensive metamorphic terrain, which comprises various outcrop areas preserving granulite-facies mineral assemblages ascribed to a 1000 Ma metamorphic event. This study has focused on two areas of this terrain: the Brattstrand Bluffs coastline of Prydz Bay, Princess Elizabeth Land, and the Nemesis Glacier region in the northern Prince Charles Mountains of Mac. Robertson Land. Both areas are composed of granulite-facies gneisses and migmatites of igneous and sedimentary origin, with a complex history of deformation, anatexis and intrusion at high grade. The structure of both areas is dominated by a flat-laying, layer-parallel, foliation. This foliation is locally overprinted by folds and shear zones which are flat lying in the Brattstrand Bluffs coastline, but upright in the Nemesis Glacier region. Pressure-temperature (P-T) estimates were derived through application of suitable barometers and thermometers to selected specimens of garnet-orthopyroxene-plagioclase-quartz gneiss from both areas. A spread of temperature data from garnet-orthopyroxene thermometers, and pressure data from a barometer based on the solubility of alumina in coexisting garnet and orthopyroxene, are attributed to a variable extent of retrograde Fe-Mg exchange. P-T estimates were corrected for this exchange by using the Fe-Mg distributon coefficient required to bring the pressures derived from the exchange-sensitive barometer into agreement with pressures derived from garnet-orthopyroxene-plagioclase-quartz barometers, which are relatively insensitive to Fe-Mg exchange. Correct peak P-T estimates are c. 6.0 kbar and 860oC for the Brattstrand Bluffs coastline and c. 6.5 kbar and 800oC for the Nemesis Glacier region. Zonation trends in garnet, orthopyroxene and plagioclase from the two areas are consistent with a retrograde P-T path gradient of c. 17 bar/oC in the Brattstrand Bluffs coastline, and c. 6 bar/oC in the Nemesis Glacier region. Well-layered and migmatitic metapelites in the Brattstrand Bluffs coastline exhibit abundant field evidence for the generation and extraction of leucocratic melts at the metamorphic peak, and metapelite mineral assemblages imply low values of αHSO (less than 0.2). Textural studies indicate that H2O was partitioned in the silicate melt phase as melting progressed, and that the final stages of melting proceeded through incongruent reactions such as biotite + sillimanite + quartz = garnet + cordierite + K-feldspar + melt, which require fluid absence. Mineral assemblage development subsequent to the melting, for example spinel-cordierite symplectites after garnet and sillimanite, indicates decompression from pressures over 5 kbar to c. 3 kbar. Integration of microtextures with the structural history indicates that some of this decompression was synchronous with localized development of flat-lying folds and shear fabrics. This combination of decompression and sub-horizontal shearing is attributed to progressive extension after the metamorphic peak. Hydrous fluids were released by the melts as they crystallized, and were transported along the shear zones, thus preserving anhydrous assemblages in most of the rocks.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available