Buffering of metamorphic reaction temperature by metamorphic fluids in mayuan group, northern fujian province

Abstract

Metamorphic rocks in Mayuan Group in northern Fujian Province are outcropped in the form of Barrow belts from chlorite, biotite, garnet, staurolite, kyanite to sillimanite zones. The calculations - from geothermometer indicate that the metamorphic temperature of kyanite zone (470°C- 500°C) is obviously lower than that of garnet zone and staurolite zone. It means that kyanite zone was formed before garnet zone and staurolite zone. This is evidently inconsistent with the traditional theory of metamorphic belts. Chemical equilibrium calculations of fluid composition and thermodynamical analysis of metamorphic reactions show that the lower metamorphic temperature of kyanite zone is resulted not from the effect of metamorphic pressure and retrogressive metamorphism but from the buffering of temperature of dehydration metamorphic reaction forming kyanite by fluids with lower XH2o. The buffered temperature range is up to 100°C-200°C. The paper discusses the buffering mechanism and buffering intensity from thermodynamics and emphasizes the important role of metamorphic fluids in the studies of metamorphic facies and metamorphic belts.