P-T-X (Fe-Mg) Relations of Cummingtonite-sillimanite-cordierite-quartz-H 2 O Equilibrium: Implications on Evolution of Metamorphic Rocks

The P-T-X (Fe-Mg) phase relations of the cummingtonite-sillimanite-cordierite-quartz-fluid assemblage have been constructed considering the model system FeO-MgO-Al 2 O 3 -SiO 2 -H 2 O (FMASH) and using available experimental and thermodynamic data. The assemblage represents a divariant reaction: cum + sil + qtz= crd + H 2 O, the Fe- and Mg-end member reactions of which have positive slope in P-T space. Isobaric T-X (Fe-Mg) loop and isothermal P-X (Fe-Mg) loop show that, at constant pressure, both cummingtonite and cordierite become Fe-richer with increasing temperature, whereas, at constant temperature, this assemblage is displaced towards Mg-richer compositions with increasing pressure. The study of the P-T-X relationships of cummingtonite-sillimanite-cordierite-quartz-H 2 O equilibrium suggests that the cummingtonite—cordierite bearing assemblages are stable at the low-pressure metamorphic environment.