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 r esponsible for the production of large volumes of melt in the Earth's crust . Unless melt extraction is complete, in-situ crystallizing melt back-react s with the restite upon cooling. In the Northampton Block, Western Australi a, spinel formed by high-temperature (similar to 850 degrees C) partial mel ting of metapelites is separated from leucosome by garnet +/- sillimanite o r cordierite +/- sillimanite coronas, Formation of the coronas by back reac tion with a silica-undersaturated melt is inferred because: (1) textural ev idence indicates that quartz was exhausted as a result of progressive dehyd ration 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 pre ssures (8.0 +/- 0.5 and 3.3 +/- 0.2 kbar, respectively, at 850 degrees C) i f reactions with quartz are assumed, but pressure estimates converge on 6.5 + 0.5 kbar at lower silica activity, a(SiO2) = 0.85 +/- 0.02. These result s indicate that the use of corona textures for the derivation of pressure-t emperature (P-T) conditions and pressure-temperature vectors in migmatites should be reconsidered, Such coronas may result from a heating-cooling cycl e without substantial change in pressure. Back reaction with residual melt is likely to be an important process in migmatites generally, affecting bot h the solid residuum and the leucosome composition.