The relative stability of garnet-cordierite and orthopyroxene-sillimanite-quartz assemblages in metapelitic granulites: experimental data

Abstract The equilibrium between garnet-cordierite and orthopyroxene-sillimanite-quartz assemblages is of fundamental importance in constructing the phase relations of metapelitic granulites. Previous studies have inferred widely differing forms, pressure-temperature positions and dP/dT slopes for the reaction. The experiments presented here show that, for the KFMASH system, the reaction is grt + crd + kfs + qz = opx + sil + L, running from left to right with increasing pressure. The reaction extends from 900 (± 10)°C, 8.9 (± 0.3) kbar to 950 (± 10)°C, 7.8 (± 0.4) kbar, i.e. it has a negative dP/dT. Comparison with previous data shows the vital importance of accounting for the volatiles contained in cordierite and shows that the high-pressure stability limit of volatile-undersaturated cordierite is between that for volatile-saturated cordierite and volatile-absent cordierite. Natural assemblages are modelled most closely by experimental studies involving volatile-undersaturated cordierite, with the experimental data of this paper being most applicable to mineral assemblages low in carbon dioxide and calcium. The data of Hensen & Green (1970, 1971, 1972, 1973) are more suitable for natural assemblages high in carbon dioxide and calcium. Both sets of data constrain the reaction to lie between 8.0 and 11.0 kbar at 900°C and therefore, the occurrence of the orthopyroxene-sillimanite-quartz assemblage should not be taken as a necessary indicator of metamorphic pressures in excess of 11 kbar (Bertrand et al., 1991).