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Orógenos colisionais funcionam como cápsulas do tempo que podem registrar grandes eventos geológicos que ocorreram durante a evolução da Terra. A reconstituição dos diferentes estágios de tais eventos depende, entretanto, de uma série de fatores, como a composição favorável do protólito e as condições de preservação. A reconstrução de trajetórias P-T-t de rochas metamórficas encontradas em tais ambientes, através da combinação de múltiplas metodologias, é essencial para desvendar as condições físicas e a duração dos estágios pré-, sin- e pós-colisão. Nesta contribuição, investigamos as trajetórias P-T-t progressivas e retrogressivas de metapelitos grafitosos ricos em Al e Fe do Grupo Carrancas na Nappe Carrancas, localizados no extremo sul do Orógeno Brasília. A combinação de mapas composicionais quantitativos, modelagem de diagramas de fases em equilíbrio (Theriak-Domino), termometria de espectroscopia Raman em materiais carbonosos (RSCM) e datação química de monazita via microssonda eletrônica permitiu a reconstrução da trajetória progressiva e dos estágios metamórficos pós-pico. As amostras analisadas registram um aumento de temperatura de ~60 ºC, atingindo 600 – 620 ºC e 12 – 14 kbar (estaurolita-cianita-granada-muscovita xisto) e 595 – 630 ºC e 9,5 – 11,5 kbar (estaurolitagranada-muscovita xisto) em ca. 600–570 Ma e 580–560 Ma, respectivamente. Os gradientes geotérmicos correspondentes combinados com as idades in situ permitem concluir que essas condições metamórficas foram alcançadas durante a transição entre os estágios de subducção e colisão. A rápida extrusão para aproximadamente 28 km ocasionada pela colisão ocorreu sob 7 – 9 kbar e 560 – 580 ºC. Os novos dados apresentados neste trabalho indicam um metamorfismo dt/dp intermediário, com a trajetória P-T-t progressiva de de alta pressão resultante de um único evento metamórfico relacionado à subducção-colisão., 4° Encontro dos Pesquisadores de Pós-Doutorado do IGc/USP, Instituto de Geociências da Universidade de São Paulo, São Paulo, SP, Brasil, 29 e 30 de novembro de 2022.

The reactivation of shear zones under brittle conditions during exhumation due to extensional collapse of the thickened continental crust is a common feature of post-collisional settings. New illite K-Ar analyses and (U-Th)/He data help establish the timing of exhumation and residual deformation during the final stages of orogenic activity in the Kaoko Belt, NW Namibia. Dating of clay-sized white mica from fractionated samples can be used to estimate late-stage deformation in shear zones and constrain the transition from the ductile to the brittle regime. XRD characterization of the dated material indicates crystallization under epizonal conditions (>300°C), which, together with the lack of correlations with age in fractionated samples are suggestive of protracted mineral growth. This supports the interpretation that K-Ar ages at 510-480 Ma in the analyzed samples set a maximum age for the end of ductile activity in the Kaoko Belt, after which brittle faulting is interpreted to have been the predominant style of deformation. This is in accordance with zircon thermochronological data, which predominantly record fast cooling towards temperatures below 200°C shortly thereafter. When compared with equivalent data in South America, the new results reveal that, while the peak orogenic stage of the Dom Feliciano Belt (650-570) took place many tens of Myr earlier than in the Kaoko Belt (580-550 Ma), the transition from ductile to brittle deformation (535-465 Ma and 510-480 Ma, respectively) and rapid exhumation into upper crust conditions (550-400 Ma and 500-350 Ma, respectively) has much more significant overlaps. This observation suggests that, with the progression of the main deformation front from present- day South America to Africa at the time of the collisional process in the Kaoko Belt, the late-stage deformation and eventual post-orogenic exhumation of the Dom Feliciano Belt may have been controlled by the far-field tectonic influence of the continuing orogenic activity to the West. The late stages of the Kaoko Belt, on its turn, are also influenced by a continuation of the progressive migration of the orogenic front towards the Damara Belt, thus highlighting the interconnected character of deformation and exhumation within the entire orogenic system., 4° Encontro dos Pesquisadores de Pós-Doutorado do IGc/USP, Instituto de Geociências da Universidade de São Paulo, São Paulo, SP, Brasil, 29 e 30 de novembro de 2022.

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.