Skip to main content

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.


Mikaella Balis1, Bernhard Schulz1, Mario da Costa Campos Neto2
1TU Bergakademie Freiberg, Germany; 2University of São Paulo, Brazil
GeoMinKöln 2022
Freiberg, Sachsen