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Title: Oxygen isotope study of migmatites and associated rocks from Nanga Parbat-Harmosh Massif, Pakistan Himalaya.
Author: Leslie, Karen Ann.
Awarding Body: Royal Holloway, University of London
Current Institution: Royal Holloway, University of London
Date of Award: 1998
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Collision of the Indian and Eurasian plates (-65Ma) caused the formation of the Himalayan chain. The Nanga Parbat-Haramosh Massif within the western Himalaya, Pakistan, is experiencing the fastest uplift anywhere in the world (-10mmyr-1). The massif contains amphibolite facies pelitic and semipelitic migmatites with subordinate calc-silicates and amphibolites. Intruded into these migmatites are very young tourmaline leucogranites (2- l OMa). Only in the central portion of the massif is the granulite facies sillimanite-K-feldspar-cordierite assemblage found. The migmatites themselves contain lit-par-lit quartzo-feldspathic leucosomes (cros) and cross-cutting cordierite-bearing seams (cros). Tourmaline leucogranites have been derived from muscovite vapour-absent melting of this basement, while cordierite seams have been derived from biotite-dehydration melting of these migmatites. The rapid uplift experienced by the massif reduced the effects of post migmatisation mineral-mineral oxygen isotope diffusional exchange. Thus, primary fractionation values are more likely to be preserved. To study the effects of migmatisation on the fractionation of oxygen isotopes, mineral separates from mesosomes, leucosomes and seams were analysed by laser-fluorination mass spectrometry. The use of a Nd: YAG laser required the establishment of a method to analyse quartz and alumino-silicates. Proxy heating with a standard ferromagnesian mineral is followed by a mass balance calculation to obtain 818Oquartz/feldspafr om the mixture. Precision for non-ferromagnesian minerals is ±0.4%o (2sd) for a standard quartz, NBS-28, using this method. Results indicate that Mgtrmelt-mesosomaen d Ofeldmelt-mesosomaer e positive. It is considered unlikely that there was substantial fluid infiltration during migmatisation because quartz shows large infra- and inter-sample variability, indicating that 8180 values have not been homogenised by fluids. Any fluids that were present are likely to have been metamorphic rather than meteoric in origin, because the latter have particularly low 8180 values (-15%o to -16%o). Reequilibration between mesosome and leucosome produced diffusion profiles within quartz. Within quartz, an array of fluid inclusion types (H2O and CO2), salinities and homogenisation temperatures, suggest that fluids have been present within the massif over much of its history. Mineral-mineral fractionations suggest fast cooling in a fluid undersaturated environment.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Geology Geology Mineralogy Sedimentology Geochemistry