Granulites from Enderby Land, Antarctica : the application of experimentally determined cation partition data to estimation of pressures and temperatures of metamorphism.
Creators: name: Ellis, David John
Publisher: name: University Of Tasmania
Year: 2023

description: The Precambrian granulites of Enderby Land, Antarctica, were metamorphosed at temperatures as high as 900 ¬¨‚àûC 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 ¬¨‚àûC, 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.