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Journal of Petrology | Volume 43 | Number 9 | Pages 1673-1705 | 2002
© Oxford University Press 2002
Characterization and P–T Evolution of Melt-bearing Ultrahigh-temperature Granulites: an Example from the Anápolis–Itauçu Complex of the Brasília Fold Belt, Brazil
1LABORATORY FOR CRUSTAL PETROLOGY, DEPARTMENT OF GEOLOGY, UNIVERSITY OF MARYLAND, COLLEGE PARK, MD 20742, USA
2INSTITUTO DE GEOCIÊNCIAS, UNIVERSIDADE DE BRASÍLIA, BRASÍLIA, DF, 709110-900, BRAZIL
3COMPANHIA DE PESQUISA DE RECURSOS MINERAIS (CPRM), RUA S-02, 463 APTO 101, SETOR BELA VISTA GOIÂNIA, GO 74823-430, BRAZIL
4COMPANHIA DE PESQUISA DE RECURSOS MINERAIS (CPRM) SGAN 603 CONJ. ‘J’, PARTE ‘A’, 1°, ANDAR BRASÍLIA, DF 70830 030, BRAZIL
Sapphirine–quartz occurs in orthopyroxene–garnet granulites in two areas 
22 km apart within ‘common’ granulites of the Anápolis–Itauçu Complex, recording extreme P–T conditions: a minimum of 1030–1050°C at 10 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 1012–960°C 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 > 1150°C in both areas. After decompression at the metamorphic peak, the P–T path for both areas followed a near-isobaric cooling stage to <900°C, but these two paths are separated by 3 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.
KEY WORDS: Brasília Fold Belt; granulite; melting; P–Tevolution; sapphirine–quartz; ultrahigh-temperature metamorphism
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