THE RELATIVE STABILITY OF GARNET-CORDIERITE AND ORTHO-PYROXENE-SILLIMANITE-QUARTZ ASSEMBLAGES IN METAPELITIC GRANULITES - EXPERIMENTAL-DATA

The equilibrium between garnet-cordierite and orthopyroxene-sillimanit e-quartz assemblages is of fundamental importance in constructing the phase relations of metapelitic granulites. Previous studies have infer red widely differing forms, pressure-temperature positions and dP/dT s lopes 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 e xtends from 900 (+/- 10)degrees C, 8.9 (+/- 0.3) kbar to 950 (+/- 10)d egrees 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 v olatiles contained in cordierite and shows that the high-pressure stab ility 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 involvin g volatile-undersaturated cordierite, with the experimental data of th is paper being most applicable to mineral assemblages low in carbon di oxide and calcium. The data of Hensen and Green (1970, 1971, 1972, 197 3) are more suitable for natural assemblages high in carbon dioxide an d calcium. Both sets of data constrain the reaction to lie between 8.0 and 11.0 kbar at 900 degrees C and therefore, the occurrence of the o rthopyroxene-sillimanite-quartz assemblage should not be taken as a ne cessary indicator of metamorphic pressures in excess of 11 kbar (Bertr and et al., 1991).