A calculated petrogenetic grid for the system K₂O‐FeO‐MgO‐Al₂O₃‐SiO₂‐H₂O, with particular reference to contact‐metamorphosed pelites

Abstract

ABSTRACT A petrogenetic grid is presented for the system KFMASH (K 2 O‐FeO‐MgO‐Al 2 O 3 ‐SiO 2 ‐H 2 O), including biotite, muscovite, K‐feldspar, chlorite, chloritoid, staurolite, cordierite, garnet, orthoamphibole, orthopyroxene, spinel, andalusite, sillimanite, kyanite, quartz and corundum with H 2 O in excess, which was calculated using the computer program THERMOCALC and the Powell and Holland internally consistent thermodynamic dataset. By removing the normal constraint of having quartz in excess, both quartz‐bearing and quartz‐absent equilibria are shown. Quartz‐absent equilibria are particularly relevant at high‐ T and low‐ P conditions, because of their common occurrence at these conditions. The calculated mineral assemblage and mineral compositional variations in terms of FeMg‐1 and (Fe, Mg)SiAl ‐2 exchange vectors are broadly compatible with observations on natural rocks, particularly when non‐KFMASH components are taken into account.