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 consists of granulites and layered mafic and granite bodies. Diagnostic ultra-high-temperature mineral assemblages are recognized among the granulites, spinel + quartz being the most common. Ultra-high-temperature conditions are recovered from several rock samples using conventional thermobarometry, based on the orthopyroxene and garnet pair, and on the Zr-in-rutile thermometer, with values above 900°C. In some outcrops, an intense retrometamorphism affected the rocks, and the granulite facies mineral assemblage was replaced by typical greenschist facies assemblage such as kyanite + chloritoid + muscovite + chlorite. In this sample, the high-grade aspect in outcrop is preserved, as attested to by the presence of leucosome and residue. Thermodynamic modelling indicates that intense water influx occurred, and temperatures around 500°C were necessary to produce the observed mineral assemblage, at pressures below 5 kbar. With the present composition, thermodynamic modelling points to the possible generation of spinel + quartz, should the rock be submitted to ultra-high-temperature conditions and some Fe 3+ was present.