Garnet zoning and orthopyroxene + plagioclase coronitisation |
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| Authors: | Jacques Kornprobst |
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| Institution: | (1) Department of Geology, 5, rue Kessler, F-63000 Clermont Ferrand, France |
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| Abstract: | A very uncommon garnet-zoning—Ca poor centre and Ca rich margin—has been observed, in a granulite facies rock. Calculation of equilibrium at each stage of the garnet growth led to apply in this particular case, as well as for the most garnet-zoning instances, the Rayleigh fractionation model, the fractionation factor  Ca being less than 1 at the beginning of the crystallization. Subsequent coronitisation, at lower P and/or higher T, involved the development of orthopyroxene and plagioclase in equilibrium with a thin (15  thick) Ca-Mg poor and Fe rich outer cuticle of the garnet: thus the garnet lattice appears to be not so refractory as previously admitted, and in most coronitisation processes involving garnet such an outer cuticle must be considered as a component of the new mineral assemblage. |
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