Petrochemical Criteria of Staurolite Stability in Metapelites at Medium-Temperature Low- and Medium-Pressure Metamorphism

Staurolite stability in rocks of zonal metamorphic complexes and the subsequent decomposition of this mineral in the rocks at increasing temperature is a criterion for distinguishing the staurolite metamorphic zone. Along with other metamorphic zones, this zone reflects the temperature distribution configuration when the metamorphic zoning was formed. Staurolite-bearing mineral assemblages are formed in metapelites of appropriate chemical composition at metamorphic temperatures of ~500–650°C. The fact that staurolite is stable within a relatively narrow temperature range makes it possible to predict the presence or absence of this mineral in rocks if representative data on the composition of their protoliths are available. We propose a method for identifying potentially staurolite-bearing rocks using computational petrochemical modules, whose values are calibrated on natural rocks and modeled rock compositions that can occur in nature. Four petrochemical modules are proposed in the form of specified proportions of major chemical components in metamorphic rocks: Al 2 O 3 /SiO 2 , CaO/FM (where FM = FeO t + MgO), K 2 O/FM, and Na 2 O/FM. They can be used as criteria for estimating the possibility of staurolite crystallization in rocks when a suitable temperature (and pressure) is reached. These modules make it possible to select and more reliably specify the boundaries of the staurolite zone when, for example, maps of metamorphism are produced.