Calculated phase equilibria in K2O‐FeO‐MgO‐Al2O3‐SiO2‐H2O for silica‐undersaturated sapphirine‐bearing mineral assemblages

Abstract

AbstractSilica‐undersaturated, sapphirine‐bearing granulites occur in a large number of localities worldwide. Such rocks have historically been under‐utilized for estimating P–T evolution histories because of limited experimental work, and a consequent poor understanding of the topology and P–T location of silica‐undersaturated mineral equilibria. Here, a calculated P–T projection for sapphirine‐bearing, silica‐undersaturated metapelitic rock compositions is constructed using THERMOCALC for the FeO‐MgO‐Al2O3‐SiO2 (FMAS) and KFMASH (+K2O + H2O) chemical systems, allowing quantitative analysis of silica‐undersaturated mineral assemblages. This study builds on that for KFMASH sapphirine + quartz equilibria [Kelsey et al. (2004) Journal of Metamorphic Geology, vol. 22, pp. 559–578]. FMAS equilibria are significantly displaced in P–T space from silicate melt‐bearing KFMASH equilibria. The large number of univariant silica‐undersaturated KFMASH equilibria result in a P–T projection that is topologically more complex than could be established on the basis of experiments and/or natural assemblages. Coexisting sapphirine and silicate melt (with or without corundum) occur down to c. 900 °C in KFMASH, some 100 °C lower than in silica‐saturated compositions, and from pressures of c.≤1 to ≥12 kbar. Mineral compositions and composition ranges for the calculated phases are consistent with natural examples. Bulk silica has a significant effect on the stability of sapphirine‐bearing assemblages at a given P–T, resulting in a wide variety of possible granulite facies assemblages in silica‐undersaturated metapelites. Calculated pseudosections are able to reproduce many naturally occurring silica‐undersaturated assemblages, either within a single assemblage field or as the product of a P–T trajectory crossing several fields. With an understanding of the importance of bulk composition on sapphirine stability and textural development, silica‐undersaturated assemblages may be utilized in a quantitative manner in the detailed metamorphic investigation of high‐grade terranes.