The Nature and Role of Metamorphic Fluids in Formation of Metamorphic Rocks in Dehno (Mashhad)

Dehno (Mashhad) is an area of metamorphic and granitic intrusions. In some layers of rocks in schists, staurolite is present while chloritoid is absent. Study of fabric elements show that chloritoid and staurolite formed during the same deformation phase (S2) with similar pressure and temperature conditions. Chemical analyses of these rocks and investigation on geochemical diagrams show that the rocks chemically similar; suggesting that replacement of chloritoid with staurolite in sequence of layers might be controlled by metamorphic fluids. Low oxygen fugacity (low XCO2) condition in some layers which can be confirmed by the presence of graphite in the rocks; increases the stability of chloritoid and prevents the formation of staurolite. Staurolite formed during the pick metamorphism in the area; suggests that in the highest metamorphic condition, CO2 bearing fluids played significant role on metamorphic reactions and formation of different metamorphic assemblages. However, fluid flow was limited to individual layers and different fluids were not able to move to mix and make a homogeneouse fluid in the area.