Metamorphic Reactions in the Kyanite and Sillimanite Zones of the Barrovian Type Area

Metamorphic conditions in the staurolite, kyanite, and sillimanite zones of the Barrovian type area have been calculated using a variety of equilibria. Temperatures ranged from 550°C (transition zone) to 650°C (sillimanite zone) with P ˜ 6000 bars and XH2O ˜ 0·6. Metamorphism was progressive, and the exact nature of the continuous reactions responsible for the formation of index minerals was strongly controlled by bulk rock Mg/(Mg + Fe). Thus, in magnesian rocks staurolite broke down to kyanite + biotite, whereas in iron-rich rocks it broke down to garnet + muscovite and kyanite was not produced. Sillimanite formed both from kyanite and by dehydration reactions of staurolite and white mica.