Continental subduction and exhumation of the lower crust in a hot orogen: Insights from high-pressure (ultra) high-temperature granulite in the Pouso Alto Nappe, Southeast Brazil

The Pouso Alto Nappe (PAN) is part of the Southern Brasília Orogen (SBO) hinterland and primarily comprises Kfeldspar + garnet + kyanite + rutile-bearing paragneiss, interpreted as a residual high-pressure (HP) granulite. The PAN granulite was studied by combining detailed microstructural and petrographic descriptions with monazite petrochronology, thermodynamic modeling, and trace element thermometry. The P–T–t trajectory of the PAN granulite reveals that it experienced HP, and possibly ultrahigh temperature (UHT), metamorphic conditions during the SBO evolution. The prograde subsolidus metamorphic conditions occurred between ca. 670 and 620 Ma. Subsequently, the granulite underwent water-undersaturated partial melting during heating and burial, reaching peak conditions of ≥1000 ◦C and 18–19 kbar at a maximum age of ca. 620 Ma. The proposed prograde path indicates that the granulite was involved in the continental subduction channel, reaching depths of ca. 70 km, corresponding to the interface between the lower crust and lithospheric mantle in a double-thickened crust. The granulite facies metamorphic conditions would have been attained primarily through the accumulation of heat production elements and mantle heat flow. Decompression and cooling of the melt-weakened rocks commenced after the delamination of denser material of lithosphere. Subsequently, the previously meltweakened granulite initiates the in-sequence ductile flow, driven by gravitational force exerted by the weight of the orogen. The PAN granulite likely records the transition from a wedge-shaped orogenic belt to an orogenic plateau during continental collision development.