Calculated Phase Relations in High-Pressure Metapelites in the System NKFMASH (Na2O-K2O-FeO-MgO-Al2O3-SiO2-H2O)

Abstract

Using an internally consistent thermodynamic dataset and updated models of activity-composition relation for solid solutions, petrogenetic grids in the system NKFMASH (Na2O-K2O-FeO-MgO-Al2O3-SiO2-H2O) and the subsystems NKMASH and NKFASH have been calculated with the software THERMOCALC 3.1 in the P-T range 5-36 kbar and 400-810°C, involving garnet, chloritoid, biotite, carpholite, talc, chlorite, kyanite/sillimanite, staurolite, phengite, paragonite, albite, glaucophane, jadeite, with quartz/coesite and H2O in excess. These grids, together with calculated AFM compatibility diagrams and P-T pseudosections, are shown to be powerful tools for delineating the phase equilibria and P-T conditions of Na-bearing pelitic assemblages for a variety of bulk compositions from high-P terranes around the world. These calculated equilibria are in good agreement with petrological studies. Moreover, contours of the calculated phengite Si isopleths in P-T pseudosections for different bulk compositions confirm that phengite barometry is highly dependent on mineral assemblage.