Back reaction between restite and melt; implications for geothermobarometry and pressure-temperature paths

Partial melting is an important process in high-grade metamorphism and is responsible for the production of large volumes of melt in the Earth's crust. Unless melt extraction is complete, in-situ crystallizing melt back-reacts with the restite upon cooling. In the Northampton Block, Western Australia, spinel formed by high-temperature ( approximately 850 degrees C) partial melting of metapelites is separated from leucosome by garnet + or - sillimanite or cordierite + or - sillimanite coronas. Formation of the coronas by back reaction with a silica-undersaturated melt is inferred because: (1) textural evidence indicates that quartz was exhausted as a result of progressive dehydration melting before formation of spinel+melt; (2) the coronitic phases are never in direct contact with quartz of the leucosome; (3) barometry on cordierite- and garnet-bearing equilibria with spinel gives conflicting pressures (8.0+ or -0.5 and 3.3+ or -0.2 kbar, respectively, at 850 degrees C) if reactions with quartz are assumed, but pressure estimates converge on 6.5+ or -0.5 kbar at lower silica activity, a (sub SiO2) = 0.85+ or -0.02. These results indicate that the use of corona textures for the derivation of pressure-temperature (P-T) conditions and pressure-temperature vectors in migmatites should be reconsidered. Such coronas may result from a heating-cooling cycle without substantial change in pressure. Back reaction with residual melt is likely to be an important process in migmatites generally, affecting both the solid residuum and the leucosome composition.