Recovering P–T–t paths from ultra-high temperature (UHT) felsic orthogneiss: An example from the Southern Brasília Orogen, Brazil

Abstract

Mineral assemblages that are diagnostic of ultrahigh-temperature (>900 °C) metamorphism are generally restricted to Mg- and Al-rich rocks. However, in many metamorphic terrains such compositions do not occur, and the rocks are dominated by orthogneisses that typically contain high-variance mineral assemblages that are not diagnostic of particular pressure–temperature (P–T) conditions. Here we use whole-rock and mineral chemistry, thermodynamic phase equilibrium modelling, and zircon U–Pb–Hf isotopic data to constrain the P–T–time history of granulites from the Socorro-Guaxupé Nappe of the Southern Brasília Orogen. These rocks occur as part of a stack of Neoproterozoic nappes that were assembled during the formation of West Gondwana. Based on phase equilibrium modelling, the peak assemblage in a garnet-bearing felsic granulite (ortho- and clinopyroxene, garnet, K-feldspar, plagioclase, quartz, ilmenite, and silicate melt) records pressures of around 11.5 kbar and temperatures of ~1000 °C, consistent with the high-Al content of orthopyroxene. The retrograde path records a small decrease in pressure to 9 kbar marked by orthopyroxene replacing garnet and the exsolution of magnetite from ilmenite followed by near-isobaric cooling that led to the development of coronas of hornblende around pyroxene due to crystallization of the last vestiges of melt. Zircon cores yielding U–Pb ages of ca. 625 Ma are interpreted as dating close-to-peak conditions during high-temperature metamorphism. In one sample, zircon rims and ‘soccer ball’ zircon neoblasts both have high U contents and yield ages of ca. 615 Ma, interpreted to mark the onset of melt crystallization. In two other samples, low-U rims and ‘soccer ball’ zircons grains yield ages of ca. 600–595 Ma, interpreted to date the end of melt crystallization. These data reveal a protracted regional metamorphic event in which the lower crust remained at granulite-facies conditions for at least 30 Myr during collision and assembly of West Gondwana.