Staurolite in Metabasites: P–T–X Parameters and the Ratios of Major Components as Criteria of Staurolite Stability

Abstract

Fe–Mg staurolite is a typical and widespread mineral of medium-temperature high-alumina metapelites, whereas magnesian staurolite is only relatively rarely found in metamorphosed mafic rocks (metabasites). The most significant factors controlling staurolite stability in metabasites were identified by thermodynamic modeling and analysis of the common features of the mineral-forming processes. In contrast to staurolite in low- and medium-pressure metapelites, staurolite in metabasites is stable at medium- and high-pressure metamorphism. An increase in the proportion of carbon dioxide in the water–carbon dioxide fluid shifts the staurolite-forming mineral reactions to lower temperatures and higher pressures. Al, Fe, Mg, and Ca are the major components of rocks that are critically important for the formation of magnesian staurolite in these rocks, and the contents and ratios of these components are of crucial importance for the stability of staurolite in metabasites. To understand the processes forming the mineral in metabasites, it is instrumental to subdivide metabasites into subgroups of predominantly magnesian, ferruginous–magnesian, and ferruginous protoliths. With regard to this subdivision, three petrochemical modules are proposed in the form of ratios of major components: MgO/CaO, CaO/FM, and Al₂O₃/FM, based on which it is possible to predict the stability of staurolite in mafic rocks at appropriate P–T parameters of metamorphism.