Geobarometry and geothermometry of plagioclase-biotite-garnet-muscovite assemblages

The assemblage, plagioclase-biotite-garnet-muscovite is widespread in garnet to lower sillimanite zone metasedimentary rocks. The equilibria, (1) pyr+gr+mu=3an+phl and (2) alm+gr+mu=3an+ann, involve a change in Al coordination from 6 to 4 and in Mg-Fe coordination from 8 to 6 and should be strongly pressure dependent. Using an ionic solution model we can define solid activity products for (1) and (2). Using a linear least squares analysis of activity products and estimated P-T from natural assemblages we can derive equilibrium constant equations for (1) and (2). Combining either of these equations with that for Mg-Fe exchange for garnet-biotite from Ferry and Spear (1978) allows estimation of P and T of metamorphism. Pressures estimated from these equilibria compare favorably to pressures estimated from garnet-plagioclase-Al2SiO5-quartz equilibria.

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