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A new quantitative approach to constraining mineral equilibria in sapphirine-bearing ultrahightemperature (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 Ana´polis-Itauc¸u Complex, central Brazil, where modelling of mineral equilibria indicates peak metamorphic conditions of about 9 kbar and 1000°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-Al 2 O 3 -SiO 2 (FMAS) univariant reaction orthopyroxene + sillimanite ¼ 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 H 2 O . 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 H 2 O sections in FMAS-H 2 O (FMASH), for which the corresponding a H 2 O is commonly higher than that experienced by UHT granulites. A corollary of this dependence of mineral equilibria on a H 2 O is that local variations in a H 2 O may explain the formation of cordierite without significant changes in P-T conditions, particularly without marked decompression.

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The Barro Alto Complex and Juscelândia volcanosedimentary sequence are exposed in the central part of the Neoproterozoic Brasília belt of central Brazil. The former is a large (approximately 150 km long), boomerang-shaped, mafic-ultramafic, layered complex formed by two different intrusions metamorphosed under granulite facies. These rocks are tectonically overlain by rocks of the Juscelândia volcanosedimentary sequence, represented mainly by biotite-gneiss and amphibolite, or amphibolite facies metamorphic equivalents of rhyolite and basalt, respectively. New SIMS U-Pb zircon data and Sm-Nd isochron data presented herein help clarify the igneous and metamorphic evolution of the Juscelândia volcanosedimentary sequence, as well as its relationship with the Barro Alto Complex. Zircon grains from two biotite gneisses were analyzed by SIMS (SHRIMP) and indicate Mesoproterozoic dates, approximately 1.28 Ga, interpreted as the time of bimodal volcanism in a tectonic setting transitional between a continental rift and an ocean basin. Metamorphism is constrained by Sm-Nd garnet-whole-rock isochrons for garnet amphibolite and pelitic schists of the Juscelândia sequence, as well as for clinopyroxene-garnet amphibolite and garnet granulite of the Barro Alto Complex, which give ages between 0.74 and 0.76 Ga, in agreement with SIMS dates for metamorphic zircon rims. These new data are significant, because they establish that a single metamorphic event affected both the Barro Alto Complex and the Juscelândia sequence. Based on these new data, we present a modified tectonic model for the Brasília belt. q

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