Timing and metamorphism of the root of a magmatic arc on the West Gondwana margin: a case study of the Socorro Nappe and São Roque Domain in south-eastern Brazil

The assembly of West Gondwana resulted in a high-grade nappe system of south-eastern Brazil that comprises a magmatic arc remnant (Socorro-Guaxupé Nappe). The UHT metamorphism is product of the collision between the Paranapanema block (active margin) and the São Francisco craton (passive margin) and decompression, but also related to a pre-collisional stage (Rocha et al., 2017; Tedeschi et al., 2018). The U-Pb ID-TIMS and U-Th-PbT EPMA monazite peak metamorphic ages systematically decrease towards the lower nappes describing an orogenic front migration from ca. 630-570 Ma (Campos Neto et al., 2011; Westin et al., 2021). The southernmost segment (Socorro Nappe) comprises gneisses with varied degrees of migmatization that unveil UHT-HT conditions, e.g., spinel + quartz and melt (quartz + K-feldspar) inclusions in garnet, neosomes with garnet + orthopyroxene and garnet + cordierite that are associated with amphibole ± pyroxene-bearing mesocratic gneisses. The paragneisses host a peak assemblage of garnet + sillimanite (needle trail inclusions) whereas biotite + sillimanite in the foliation are potentially retrograde. SEM-MLA and BSE imaging of monazite reveal complex microstructures. Monazite from the paragneisses displays homogeneous, moderate zonation and/or cloudy internal structures. A spinel-cordierite-garnet leucosome hosts peritectic garnet in which monazite inclusions are cloudy and weakly zoned. EPMA core-to-rim profiles of garnet show Fe-Mg zonation in a paragneiss whereas none in the neosomes. The overall composition is almandine and pyrope (XAlm 55-70%; XPrp 25-37%) and low Ca. We combine geothermobarometry and petrochronology of monazite to understand the metamorphic transition from lower to upper crustal rocks in the Neoproterozoic orogen.