The results of recent investigations on the stability limits of staurolite have been combined together with those of the present study to develop a semi-quantitative model of the P-T-fo2-X relations of staurolite±quartz±magnetite. The problem with respect to the hydroxyl content of staurolite has been analysed; it is concluded that no evidence has yet been mustered to discount the idealised stoichiometry proposed by Naray-Szabó & Sasvari (1958), at least as a limiting composition. The stability limits of staurolite±magnetite have been calculated from the experimental data for the equilibria involving quartz. Also the conditions over which the assemblage cordierite+magnetite+quartz could be stable, as well as a quantitative model for the fo2-P stability of almandine ± quartz have been deduced theoretically.An analysis is presented of the paragenetic relations of staurolite in common pelitic schists. It is suggested that the formation of staurolite at the expense of either chloritoid or chlorite, rather than the unqualified first appearance of staurolite as proposed by Winkler (1970), should define a 'staurolite-in' isograd in the range of 500-575 °C. In regional metamorphism, chloritoid, staurolite, and aluminum silicates should, under equilibrium conditions, be unstable relative to almandine in graphitic pelitic schists involving magnetite (chloritoid/staurolite/Al2SiO6+magnetite+quartz↑almandine+O2+H2O). The limits of P-T conditions over which staurolite and cordierite may coexist in natural assemblages have been deduced; it is restricted, almost entirely within the field of andalusite, between 500-700 °C, and 2-6 kbars, thus defining the range of P-T conditions for the 'low-pressure intermediate'- or 'Buchan'-type amphibolite facies discussed by Miyashiro (1961). In assemblages involving staurolite and andalusite, cordierite rather than almandine should usually be stable; the reverse holds for assemblages involving staurolite and sillimanite.
ASJC Scopus subject areas
- Geochemistry and Petrology