Phase equilibria for metapelites in the system K2O-FeO-MgO-Al2O3-SiO2-H2O (KFMASH)

Using the internally-consistent thermodynamic dataset of Holland and updated models of activity-composition relation 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 0.05 similar to 1.2 GPa and 450 similar to 900degreesC, involving biotite, muscovite, K-feldspar, chlorite, chloritoid, staurolite, cordierite, garnet, orthopyroxene, spinel, andalusite, sillimanite, kyanite, quartz (in excess), melt and H2O (in excess for subsolidus part, not excess for supersolidus part). 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 pelitic low-medium pressure metamorphism. The calculated equilibria and mineral assemblages predicted by the grid is consistent with successive prograde development in fields. Especially, adding melts makes it possible to interpret quantitatively the wide migmatization in fields.