Long-lived high- to ultrahigh-temperature metamorphism constrained by reconstructing P-T-(t) paths of metasedimentary rocks: Example from the Neoproterozoic Guaxupé Nappe, Southernmost Brasília Orogen, SE Brazil

Worldwide occurrences of metamorphic rocks that underwent ultrahigh-temperature metamorphism ( T > 900 °C) demonstrate that the continental crust has experienced heat anomalies that drastically disrupted its thermal state, mostly during mountain-building events throughout space and time. In few cases, it is debated whether this type of metamorphism is related to local heat anomalies or to progressive regional metamorphism. An example of this occurs in the Guaxupé Nappe, southernmost Brasília Orogen (SE Brazil), formed in an arc to continental collision setting during the Neoproterozoic amalgamation of Western Gondwana. In this contribution, we reconstruct the metamorphic P – T –(t) paths of two metasedimentary migmatites from different crustal depths of the Guaxupé Nappe to constrain the conditions, extent and timing of high-grade metamorphism, evaluate the presence of a metamorphic gradient and explore the potential heat sources responsible for the extreme metamorphic conditions in the area. The garnet diatexite (#OS14b), collected from intermediate crustal levels, exhibits peak P – T conditions of ~925–1040 °C and 8.5–11 kbar (depths of ~28–37 km and T / P ratio of ~1088–945 °C/GPa), and retrograde conditions of ~680–755 °C and 5.5–6.5 kbar (depths of ~18–22 km). The garnet-cordierite metatexite (#RSJ09), collected from upper crustal levels, yields peak P – T conditions of ~835–910 °C and 6–7 kbar (depths of ~20–23 km and T / P ratio of ~1392–1300 °C/GPa), and retrograde conditions of ~665–815 °C and < 5.3 kbar (depths above 18 km). These results indicate that both rocks experienced a clockwise P–T path characterized by high- to ultrahigh-temperature peak conditions, followed by decompression and cooling down to amphibolite facies conditions, which likely reflect the exhumation of the nappe. These findings, combined with others from the literature, also suggest the existence of a metamorphic gradient within the nappe, marked by decreasing peak P–T conditions toward the structural top. The scattered monazite dates from the garnet diatexite indicate that either the high-temperature conditions persisted for at least ~45 Myr, reaching its peak at ca. 633 ± 10 Ma, the geochronological data were modified, possibly by processes similar to those occurring in zircon such as diffusion-induced resetting, leading to a misleading interpretation of prolonged metamorphism. Older ages, up to 701 ± 33 Ma, likely represent the prograde metamorphic path while younger ages, down to 542 ± 48 Ma, may reflect retrograde stages. Based on our results, we suggest that high-grade metamorphism may have been initiated in an arc-magmatic setting, but the most plausible driver of the high- to ultrahigh-temperature metamorphism in the Guaxupé Nappe appears to be the upwelling of hot asthenosphere beneath the stack during continental collision. • Reconstructed P – T paths of metasedimentary migmatites confirms extreme metamorphism. • Metamorphic gradient marked by decreasing P – T conditions toward the structural top. • Extreme metamorphism likely persisted for ~45 Myr, reaching its peak at ca. 633 ± 10 Ma. • Asthenospheric upwelling during continental collision is a plausible heat source.