Using two-pyroxene mafic granulites to reconstruct ultra-high temperatures and the retrograde <i>P</i>-<i>T</i> path of metamorphism in the Socorro-Guaxupe Nappe (Southern Brasilia Orogen, SE Brazil)

Abstract

In the southernmost part of the Southern Brasilia Orogen ( SE Brazil), regional ultra-high temperature metamorphism has been reported mainly in garnet-bearing granulites and (garnet)- orthopyroxene-bearing leucosomes at the base of the Socorro-Guaxupe Nappe, but detailed petrological studies focusing on garnet-absent mafic granulites at the upper crustal levels of this terrain are still scarce and the tectonic setting in which these extreme thermal conditions were attained are still under debate. In this study, we focus on reconstructing the metamorphic P-T paths of (garnet-absent) mafic granulites at intermediate crustal levels of the Socorro-Guaxupe Nappe using petrography, compositional maps, phase diagram modelling and Ti-in-quartz geothermometry. Our results indicate that the mafic granulites record peak P- T conditions of similar to 970 degrees C and 9.0-9.5 kbar (thermobaric ratio of similar to 1078 degrees C GPa(-1)) and their retrograde paths are characterized by both decompression and cooling, down to similar to 840-850 degrees C and 5.5-6.0 kbar. Our peak temperature results are consistent with other estimates for granulites and orthopyroxene-bearing leucosomes throughout the nappe, but the (peak) pressure results indicate points to its decrease towards the structural top. The similarity of retrograde paths throughout the Guaxupe Nappe suggests similar exhumation dynamics at different crustal depths. Furthermore, a prominent decompression vector along with the thermal peak, indicated by intergrowth microtextures and chemical zoning, coupled with compositional isopleths, suggests regional metamorphism during the continental collision between the Paranapanema and the Sao Francisco paleoplates. [Graphics] .