Conditions of ultra-high temperature metamorphism and its retrogression in the Neoproterozoic Anápolis-Itauçu Complex of the Brasília Fold Belt

The Anápolis-Itauçu Complex is part of the Neoproterozoic Brasília Fold Belt and constitutes its metamorphic core, or internal zone. It is constituted of granulites and layered mafic and granite bodies. Diagnostic ultra-high temperature mineral assemblages are recognized among the granulites, being spinel + quartz the most common. Ultra-high temperature conditions are recovered from several rocks using conventional thermobarometry, based on the orthopyroxene and garnet pair, and on the Zr-in-rutile thermometer, with values above 900 o C. In some outcrops, an intense retrometamorphism affected the rocks, and the granulite facies mineral assemblage was replaced by typical greenschist facies ones such as kyanite + chloritoid + muscovite + chlorite. In this sample, the high-grade aspect in outcrop is preserved, as attested by the presence of leucosome and residue. Thermodynamic modeling indicates that intense water influx occurred, and temperatures around 500 o C were necessary to produce the observed mineral assemblage, at pressures lower than 5 kbar. With the present composition, thermodynamic modeling points to a possible generation of spinel + quartz, should the rock be submitted to ultra-high temperature conditions and some Fe 3+ were present.