Analysis of Equilibrium in Garnet-Biotite-Sillimanite Gneisses from Quebec

Abstract

An analysis is presented of equilibrium in six specimens of garnet—biotite—sillimanite—plagioclase—potash feldspar—quartz … gneiss from a metamorphic terrain in south-western Quebec. A nearly uniform Ti content of biotite may be accounted for by an equilibrium (a) involving biotite, sillimanite, quartz, garnet, potash feldspar, and H2O. The nature of the distribution of Fe and Mg between garnet and biotite may be accounted for by another equilibrium (b) involving the same mineral suite, or by a simple exchange equilibrium (c) involving only garnet and biotite. The distribution of Mn between garnet and biotite is accounted for by an exchange equilibrium (d). A nearly uniform value of the ratio Ca content of plagioclase/Ca content of garnet may be accounted for by an equilibrium (e) involving plagioclase, garnet, sillimanite, and quartz. A proposed equilibrium (f) involving biotite, quartz, ilmenite, potash feldspar, sillimanite, and H2O conflicts with equilibrium (a) and was evidently not established in the gneisses. The factors governing the Ca content of biotite remain largely unknown. Some of these equilibria form potential indicators of relative geologic temperature, pressure, and chemical potential of H2O.