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.