Characterization and P-T evolution of melt-bearing ultrahigh-temperature granulites:: An example from the Anapolis-Itaucu Complex of the Brasilia Fold Belt, Brazil

Sapphirine-quartz occurs in orthopyroxene-garnet granulites in two areas similar to22 km apart within 'common' granulites of the Anapolis-Itaucu Complex, recording extreme P-T conditions: a minimum of 1030-1050degreesC at similar to10 kbar. In one area, the post-peak evolution is constrained to begin at P > 10 kbar by down-temperature stability of garnet-orthopyroxene-sillimanite-quartz and the absence of cordierite. In a second area, the post-peak evolution is constrained by a succession of melt-present reactions that occur at P < 10 kbar, inferred from microstructural relations between garnet, orthopyroxene and sillimanite, and coronae or symplectites involving corundum, sapphirine, spinel, cordierite, plagioclase, sillimanite, orthopyroxene, biotite and ilmenite. The retrograde segment of the P-T path was further constrained by biotite-producing reactions. A composite P-T path was constructed from samples that exhibit different amounts of retrograde reaction, reflecting different amounts of melt retention. Using back-calculated compositions for garnet and orthopyroxene, thermobarometry yields 1012-960degreesC and 9.7-8.1 kbar; these P-T results underestimate 'peak' conditions, in part as a result of modification of garnet compositions in rocks in which some melt was retained. In samples from both areas, orthopyroxene porphyroblasts have high Al2O3 (12.9-9.7 wt %) in cores, which suggests maximum T > 1150degreesC in both areas. After decompression at the metamorphic peak, the P-T path for both areas followed a near-isobaric cooling stage to <900degreesC, but these two paths are separated by similar to3 kbar. This ultrahigh-temperature metamorphism occurred in a collisional tectonic setting, in which the extreme thermal perturbation probably was a result of asthenosphere replacement of mantle lithosphere.