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Abstract

The Precambrian granulites of Enderby Land, Antarctica, were
metamorphosed at temperatures as high as 900 °C and then underwent slow
isobaric cooling at about 10 kb pressure. The mafic and pelitic
granulites are characterised by the presence of retrograde reaction
coronas, a variety of exsolution assemblages in clinopyroxenes and
compositional zoning in minerals. The recognition of staggered closure
temperatures of cation exchange for different mineral systems permits
the use of geothermometers and geobarometers to evaluate the P-T
cooling paths of these granulites.
The pelitic rocks contain coexisting spinel-quartz, sapphirine-quartz,
hypersthene-sillimanite-quartz and osumilite on a regional
scale. Osumilite is present in a variety of mineral assemblages, most
of which have not previously been reported. These assemblages have
been used to construct theoretical P-T diagrams for the stability of
osumilite in the K2O-Mg0-Al2O3 -SiO2 and K2O-FeO-Mg0-Al2O3 -SiO2 chemical
systems.
Secondary cordierite has developed by a variety of mineral reactions
on cooling of these higher temperature-pressure equivalents of coexisting
garnet and cordierite. Available Fe-Mg partition data for garnet-cordierite
would imply equilibrium at temperatures of 500-600 °C, which is
inconsistent with both mineral compatibilities in these rocks and
temperature estimates based on other geothermometers. It is suggested
that the garnet-cordierite Fe-Mg exchange reaction may be dependent upon
PH2O. An experimental study to test this possibility has proved
inconclusive, but theoretical considerations indicate that this exchange
reaction cannot be considered applicable as a quantitative geothermometer
because of our current lack of understanding.
Study of the mafic granulites from Enderby Land demonstrates the
control of differing bulk rock compositions upon the variety of reactions
involved in the development of garnet with cooling through the pyroxene
granulite-eclogite transition. Data obtained from the staggered
closure temperatures of cation exchange in different mineral systems
supports the proposal for a curvature to the pyroxene granulite-eclogite
transition at low temperatures. The formation of garnet in metamorphosed
quartz tholeiite dykes is consistent with previous descriptions of the
continuous reaction of plagioclase feldspar with orthopyroxene and opaque
oxide to produce garnet and clinopyroxene or quartz. The disappearance
of plagioclase feldspar in the more mafic, quartz-free rocks involves
a different series of reactions in which garnet and an aluminous clinopyroxene
are formed. At still lower temperatures a second generation of
plagioclase feldspar may form through exsolution from aluminous clinopyroxene.
This feldspar would eventually react at still lower temperatures
to form a Ca-rich garnet and kyanite.
A marked compositional dependence to the garnet-clinopyroxene Fe-Mg
exchange reaction is evident from this study of the Enderby Land
granulites, and an experimental investigation of this effect has been
carried out. The results of this study reconcile previous inconsistencies
in the temperature and pressure dependence of the Fe-Mg distribution
coefficient for coexisting garnet-clinopyroxene determined in previous
experimental studies of simple systems, complex basalt, garnet peridotite
and grospydite compositions. The derived geothermometer yields similar
estimates for garnet-clinopyroxene equilibration for neighbouring metamorphic
rocks of different composition and different KD values. In
addition, temperature estimates using this geothermometer are more
consistent with independent temperature estimates based on other geothermometers
than previous estimates which did not correct for the
Ca-effect.
Application of this geothermometer to the temperature of equilibration
of garnet-peridotite and eclogite xenoliths in kimberlites has
revealed inconsistencies in our current concepts of strongly silica-undersaturated
basalt magma petrogenesis.

Item Type:	Thesis - PhD
Authors/Creators:	Ellis, David John
Keywords:	Granulite
Copyright Holders:	The Author
Copyright Information:	Copyright 1979 the Author - The University is continuing to endeavour to trace the copyright owner(s) and in the meantime this item has been reproduced here in good faith. We would be pleased to hear from the copyright owner(s).
Additional Information:	Library has additional copy on microfiche. Thesis (Ph.D.)--University of Tasmania, 1979. Bibliography: l. 266-287
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