Terrane accretion and upward extrusion of high-pressure granulites in the Neoproterozoic nappes of southeast Brazil: Petrologic and structural constraints

The high-grade crystalline nappes exposed southeast of the Sao Francisco craton comprise two distinct units of mainly granulite facies rocks that represent a composite section of Neoproterozoic deep continental crust: the Socorro-Guaxupe nappe above, derived from an are terrane, and the Tres Pontas-Varginha nappe below. Metamorphism in the Tres Pontas-Varginha nappe is characterized by the exceptional preservation of kyanite granulites (700-750 degrees C, 15 kbar), and followed by limited retrogression. Maximum temperatures around 900-950 degrees C were reached toward the base of the overlying Socorro-Guaxupe nappe during the intrusion of charnockitic-mangeritic magmas. Lower-pressure metamorphism, accompanied by anatexis, prevailed at shallower crustal levels. Our petrological results document an inverted thermal structure with isobaric heating of the top of the high-pressure granulite nappe. Both granulite nappes were transported more than 200 km eastward above lower nappes involving reworked basement and passive margin units, both metamorphosed to high-pressure but lower-temperature conditions. Significant thinning and cooling of the two granulite nappes may have occurred before their emplacement onto the lower nappes. The proposed geodynamic scenario considers that continental subduction took place westward underneath Neoproterozoic oceanic lithosphere. The two granulite units crystallised at similar to 45 km depths under distinct paleogeotherms within this subduction zone around 630 Ma. The kyanite granulites were rapidely exhumed through the mechanism of low-angle forced extrusion, whereas syncollisional collapse affected the soft, anatectic middle crust of the overlying are terrane. The final emplacement of the thinned nappe pile onto the cold Sao Francisco craton and its platform cover, with at most, anchizonal to greenschist-facies metamorphism, occurred around 600 Ma.