Staurolite in Metabasites: <i>P-T-X</i> Conditions and the Ratios of Petrogenic Components as a Criterion of the Appearance of Staurolite

Abstract

In addition to the widespread Fe-Mg staurolite, typical for medium-temperature high-alumina metapelites, there are a number of finds of magnesian staurolite in metamorphosed mafic rocks – metabasites. Based on thermodynamic modeling and analysis of the mineral formation patterns, the most significant factors of the staurolite formation in metabasites have been revealed. For the formation of staurolite in metabasites, in contrast to staurolite in low- and medium-pressure metapelites, medium- and high-pressure conditions of metamorphism are necessary. An increase in the proportion of carbon dioxide in the composition of the water-carbon dioxide fluid has practically no effect on staurolite-forming mineral reactions, but leads to their shift to lower temperatures and higher pressures. Al, Fe, Mg, Ca are critical petrogenic rock components for the formation of magnesian staurolite, the contents and ratios of which primarily determine the stability of staurolite in metabasites. To understand the regularities of mineral formation, it seems appropriate to divide metabasites into subgroups of predominantly magnesian, iron-magnesian, and ferruginous protoliths. Based on this division, three petrochemical modules are proposed in the form of the ratio of rock-forming components: MgO/CaO, CaO/FM, Al2O3/FM, based on which it is possible to predict the appearance of staurolite in the basic rock when the corresponding P-T conditions of metamorphism are reached.