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Garnet-orthopyroxene assemblages as pressure-temperature indicators

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Harley, SL (1981) Garnet-orthopyroxene assemblages as pressure-temperature indicators. PhD thesis, University of Tasmania.

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Abstract

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.

Item Type: Thesis (PhD)
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Date Deposited: 20 Jul 2009 05:37
Last Modified: 11 Mar 2016 05:56
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