University of Tasmania
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Garnet-orthopyroxene assemblages as pressure-temperature indicators

posted on 2023-05-26, 05:33 authored by Harley, SL
Granulites from the Napier province\ Enderby Land Antarctica were metamorphosed at 900-950 °c and 7-10 kilobars durinq and subsequent to a major tectonothermal episode at 3000 million years before the present. Rocks presently exposed at the surface in southwestern Enderby Land indicate a regional increase in pressures of metamorphism towards the Scott Mountains - Casey Bay region where crustal thicknesses of at least 30 kilometres were attained at that time. Subsequent to the peak granulite facies metamorphic event the Napier province granulites underwent a prolonged phase of near-isobaric cooling at depth to P-T conditions of 650-700°C and 5-8 kilobars by 2500 million years before the present. A study of the pyroxene granulites from the Napier province demonstrates the near-isobaric cooling path in the development of a variety of retrograde corona textures involving the formation of garnet by reactions between pyroxenes plagioclase and opaque phases. Garnet-orthopyroxene bearing felsic and pelitic granulites display a variety of recrystallisation and corona textures which usually involve the formation of secondary garnet and occasionally orthopyroxene. Chemical zoning in these minerals and the compositions of distinct generations of the garnets and pyroxenes are used to constrain the P-T metamorphic evolution of these granulites and hence the evolution of the Napier province. The P-T conditions of metamorphism and cooling history of the qranulites of the Napier province have in part been determined by the application of several previously calibrated qeothermometers and geobarometers to mineral assemblages common throughout the province. Most information on the physical conditions of metamorphism and later cooling suggested by zoning relationships in coexisting phases has been obtained however using a new experimental calibration of the P-T-compositional dependence of the solubility of A1 203 in orthopyroxene coexisting with garnet and an experimental study of the distribution of Fe and Mg between garnet and orthopyroxene. The P-T-compositional dependence of the solubility of A1 203 in orthopyroxene coexisting with garnet has been experimenta11y determined in the P-T range 5-30 kilobars and 800-1200°C in the system FeO-MgO-Al z03-SiOZ CFMAS) and extended 1nto the system CaO-FeO-MgOA1 Z03-SiOZ (CFMAS) in a further set of experiments designed to quantify the effects of the calcium content of garnet on the A1 Z03 content of coexistinq orthopyroxene. The geobarometer developed from these experimental studies and app1ied to granulites from the Napier province compares favourably with recent experimental data obtained by other workers and yields reasonable and consistent results for a variety of well-documented natural occurrences previously reported in the literature. The P-T-compositional dependence of the distribution of Fe and Mg between garnet and orthopyroxene has been experimerltally calibrated in FMAS and CFMAS enabling the construction of an approximate geothermometer for use in granulite facies assemblages and in garnet peridotite xenoliths. Non-ideal model-dependent interaction parameters for garnet derived from Fe-Mg distribution experimental data obtained in CFMAS are in good agreement with previous estimates by other experimental investigators."


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