Modelling of mineral equilibria in ultrahigh-temperature metamorphic rocks from the Anapolis-Itaucu Complex, central Brazil

A new quantitative approach to constraining mineral equilibria in sapphirine-bearing ultrahigh-temperature (UHT) granulites through the use of pseudosections and compatibility diagrams is presented, using a recently published thermodynamic model for sapphirine. The approach is illustrated with an example from an UHT locality in the Anapolis-Itaucu Complex, central Brazil, where modelling of mineral equilibria indicates peak metamorphic conditions of about 9 kbar and 1000 degrees C. The early formed, coarse-grained assemblage is garnet-orthopyroxene-sillimanite-quartz, which was subsequently modified following peak conditions. The retrograde pressure-temperature (P-T) path of this locality involves decompression across the FeO-MgO-Al2O3-SiO2 (FMAS) univariant reaction orthopyroxene + sillimanite = garnet + sapphirine + quartz, resulting in the growth of sapphirine-quartz, followed by cooling and recrossing of this reaction. The resulting microstructures are modelled using compatibility diagrams, and pseudosections calculated for specific grain boundaries considered as chemical domains. The sequence of microstructures preserved in the rocks constrains a two-stage isothermal decompression-isobaric cooling path. The stability of cordierite along the retrograde path is examined using a domainal approach and pseudosections for orthopyroxene-quartz and garnet-quartz grain boundaries. This analysis indicates that the presence or absence of cordierite may be explained by local variation in a(H2O). This study has important implications for thermobarometric studies of UHT granulites, mainly through showing that traditional FMAS petrogenetic grids based on experiments alone may overestimate P-T conditions. Such grids are effectively constant a(H2O) sections in FMAS-H2O (FMASH), for which the corresponding a(H2O) is commonly higher than that experienced by UHT granulites. A corollary of this dependence of mineral equilibria on a(H2O) is that local variations in a(H2O) may explain the formation of cordierite without significant changes in P-T conditions, particularly without marked decompression.