The influence of pressure,temperature and bulk composition on the appearance of garnet in orthogneisses — an example from South Harris,Scotland |
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| Authors: | Bernard J Wood |
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| Institution: | Department of Geology, University of Manchester, Manchester Great Britain |
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| Abstract: | Granulite facies metamorphism of the igneous complex of South Harris has produced garnet-clinopyroxene-plagioclase assemblages from olivine-normative rocks and 2 pyroxene-plagioclase-quartz assemblages from quartz-normative rocks. The appearance of garnet can be considered in terms of two complex reactions:Olivine + plagioclase1 → (Ca, Mg, Fe) garnet + plagioclase2(olivine-normative) (A)Orthopyroxene + plagioclase1 → (Ca, Mg, Fe) garnet + plagioclase2 + quartz (quartz-normative) (B)For bulk compositions of the South Harris rocks the equilibrium pressure for reaction (A) has been exceeded whereas that for reaction (B) was not reached. Estimated physical conditions of metamorphism bracketed by these and other reactions are: 800–860°C and 10–13 kbar. These estimates, based on experimental data on simple systems combined with thermodynamic models of the solid solutions involved are in good agreement with extrapolated pressures for the experimentally determined appearance of garnet in basaltic compositions (Green and Ringwood, 1967). The latter give 9–12 kbar in the temperature range of interest. The calculations are also consistent with the occurrence of kyanite in associated metapelites and with the stability of spinel-lherzolite during the granulite metamorphism. |
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