Phase relations in high-pressure metapelites in the system KFMASH (K2O-FeO-MgO-Al2O3-SiO2-H2O) with application to natural rocks

Using a previously published, internally consistent thermodynamic dataset and updated models of activity-composition relations for solid solutions, petrogenetic grids in the model system KFMASH (K2O-FeO-MgO-Al2O3-SiO2-H2O) and the subsystems KMASH and KFASH have been calculated with the software THERMOCALC 3.1 in the P-T range 536 kbar and 400-810 degreesC, involving garnet, chloritoid, biotite, carpholite, talc, chlorite, staurolite and kyanite/sillimanite with phengite, quartz/coesite and H2O in excess. These grids, together with calculated AFM compatibility diagrams and pseudosections, are shown to be powerful tools for delineating the phase equilibria and P-T conditions of pelitic high-P assemblages for a variety of bulk compositions. The calculated equilibria and mineral compositions are in good agreement with petrological observation. The calculation indicates that the typical whiteschist assemblage kyanite-talc is restricted to the rocks with extremely high X-Mg values, decreasing X-Mg in a bulk composition favoring the stability of chloritoid and garnet. Also, the chloritoid-talc paragenesis is stable over 19-20 kbar in a temperature range of ca. 520-620 degreesC, being more petrologically important than the previously highlighted assemblage talc-phengite. Moreover, contours of the calculated Si isopleths in phengite in P T and P-X pseudosections for different bulk compositions extend the experimentally derived phengite geobarometers to various KFMASH assemblages.