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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.

The Borborema and Benin-Nigeria provinces of NE Brazil and NW Africa, respectively, are key areas in the amalgamation of West Gondwana by continental collision during the Brasiliano/Pan-African orogenies. Both are underlain by complex basement: Nigeria has c. 3.05 Ga Archaean crust but no known Palaeoproterozoic rocks .2.0 Ga; in NE Brazil, 2.6-3.5 Ga Archaean rocks form small cores within Palaeoproterozoic gneiss terrains affected by plutonism at c. 2.17 Ga. Both regions exhibit Late Palaeoproterozoic (c. 1.8 Ga) rift-related magmatism and metasedimentary sequences overlying the basement. The Seridó Group of NE Brazil (,0.65 Ga) is similar to the Igarra Sequence in SW Nigeria. The Ceará Group, which may date back to c. 0.85 Ga, is a passive margin deposit on crust thinned during initiation of an oceanic domain. In both provinces, basement and sedimentary cover were involved in tangential tectonics that resulted in crust-thickening by nappe-stacking associated with closure of this ocean. Frontal collision between c. 0.66 and 0.60 Ga later evolved to an oblique collision, generating northsouth continental strike-slip shear zones at c. 0.59 Ga. In NE Brazil, the main Pan-African suture is probably buried beneath the Parnaíba Basin. The Transbrasiliano Lineament, interpreted as the prolongation of the Kandi-4850 Lineament in Hoggar, may represent a cryptic suture. Opening of the Atlantic Ocean in the Mesozoic led to the break-up of Pangaea (and West Gondwana) and the consequent individualization of the South America and Africa continents, each one containing part of the Brasiliano/Pan-African fold belt. The Borborema Province (Almeida et al. 1981) in northeastern Brazil (Fig. 1) was built during the Brasiliano/Pan-African orogeny, as the result of convergence and final collision of the São LuisWest Africa and São Francisco -Congo cratons, in the context of West Gondwana amalgamation.The present structural framework of the province (Fig. 2) dates from the end of the Brasiliano/ Pan-African orogeny, forming a mosaic of independent domains juxtaposed along large crustal-scale shear zones (Vauchez et al. 1995) in a continental-scale collage (Van Schmus et al. 1998).Since the first attempts to reconstruct West Gondwana, the similarity of geological features between NE Brazil and NW Africa were used to argue for the juxtaposition of the two continents. However, there is still much uncertainty, and correlation between the two domains remains imprecise . The northern part of the Borborema Province, north of the Patos Lineament, and the Benin -Nigeria province are the key for correlation between the continents (Fig. 2). Integration of recent geological and geochronological data allows re-evaluation of the geological framework of the two provinces and provides new insights for the correlation between the two provinces.

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