P‐T grids for silica‐undersaturated granulites in the systems MAS (n+ 4) and FMAS (n+ 3)‐tools for the derivation of P‐T paths of metamorphism

Abstract

Abstract Considering the minerals cordierite (Cd), sapphirine (Sa), hypersthene (Hy), garnet (Ga), spinel (Sp), sillimanite (Si) and corundum (Co) in the system FeO‐MgO‐Al2O3‐SiO2 (FMAS), the stable invariant points are [Co], [Ga], [Cd] and [Sa]. Constraints imposed by experimental data for the system MAS indicate that under low PH2o conditions the invariant points occur at high temperature (> 900° C) and intermediate pressure (7‐10 kbar). This temperature is higher than that commonly advocated for granulite facies metamorphism. In granulites Fe‐Mg exchange geothermometers may yield temperatures of 100–150° C below peak metamorphic conditions and evidence for peak temperatures is best preserved by relict high‐temperature assemblages and by Al‐rich cores in orthopyroxene. Application of the FMAS grid to some well‐documented granulite occurrences introduces important constraints on their P‐T histories. Rocks of different bulk compositions, occurring in close proximity in the field, may record distinct segments of their P‐T paths. This applies particularly to rocks with evidence for reaction in the form of coronas, symplectites and zoned minerals. Consideration of curved reaction boundaries and XMs isopleths may explain apparently contradictory results for the stability of cordierite obtained from low‐temperature experiments and thermochemical calculations on the one hand and hightemperature experimental data on the other.