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

Abstract

The high‐grade crystalline nappes exposed southeast of the São Francisco craton comprise two distinct units of mainly granulite facies rocks that represent a composite section of Neoproterozoic deep continental crust: the Socorro‐Guaxupé nappe above, derived from an arc terrane, and the Três Pontas‐Varginha nappe below. Metamorphism in the Três Pontas‐Varginha nappe is characterized by the exceptional preservation of kyanite granulites (700–750°C, 15 kbar), and followed by limited retrogression. Maximum temperatures around 900–950°C were reached toward the base of the overlying Socorro‐Guaxupé 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 ∼ 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 arc terrane. The final emplacement of the thinned nappe pile onto the cold São Francisco craton and its platform cover, with at most, anchizonal to greenschist‐facies metamorphism, occurred around 600 Ma.