Neoproterozoic high-pressure metamorphism and tectonic constraint from the nappe system south of the São Francisco Craton, southeast Brazil

Abstract

The Neoproterozoic giant nappe system exposed south of the São Francisco craton underwent a minimum of 150 km of near-horizontal northeastward displacement. This nappe system comprises an uppermost unit derived from a plutonic magmatic arc terrane with high-pressure, high-temperature intermediate to mafic granulites at its base (Socorro–Guaxupé nappe), underlain in sequence by a high-pressure kyanite granulite nappe, an amphibolite facies metapelite nappe, and finally by a quartzite nappe which forms the lowest part of the pile and rests on a parautochthonous unit related to the passive margin of the craton. The kyanite granulites crystallized ca 13 kbar, 750°C and have only at their top sillimanite related to T-increase overprinting ( T=890°C), a consequence of downward heat advection from the overlying Socorro–Guaxupé granulites equilibrated at 900°C. Pressures of 13–14 kbar are documented in the underlying metapelite nappe with temperatures of 640–670°C, in which lenses of eclogite indicate P max of ca 17.5 kbar. Phyllites of the parautochthonous unit are directly overlain by HP granulites in the north and record P=7 kbar and T=500°C. The distribution of metamorphic facies is reminiscent of inverted metamorphism with eclogites from subduction-related metamorphism and kyanite granulites from early extrusion of subducted continental crust. The metamorphic conditions of the parautochthon may represent a stage achieved during late metamorphic thinning of the nappe pile. The overall picture suggests that high-pressure units recrystallized in a subduction zone located to the WSW. The frontal thrusts of the nappe system show a late, north northeastward transported thin-skinned pattern.