Heterogeneous Exhumation Mechanisms in Neoproterozoic Collisional Metamorphism: P–T Constraints From Pelotas and Punta del Este Terranes, Southern Brazil and Uruguay

ABSTRACT

Pre‐orogenic orthogneisses with Tonian‐Cryogenian crystallization ages from the Eastern and Southern Dom Feliciano Belt are preserved as orthogneiss xenoliths and roof pendants intruded by voluminous post‐collisional magmatism during the amalgamation of Western Gondwana. These rock fragments occur in different localities in Southern Brazil and Uruguay, encompassing distinct units that include the gneisses from Chácara das Pedras and Piratini, and the Cerro Bori Orthogneisses and Chafalote Metamorphic Suite (CMS) granulites from the Cerro Olivo Complex. These rocks may be related to one another, as they share similar crystallization ages of approximately 800 Ma, subduction‐related geochemical affinities, and record evidence of a collisional metamorphic event at approximately 650 Ma. However, their common metamorphic history is poorly characterized, because most felsic‐intermediate protoliths lack suitable mineral paragenesis for conventional geothermobarometric and P–T modelling techniques. This work presents a study of the metamorphic evolution of these high‐grade sequences, based on petrography, thermobarometry, and pseudosection modelling. Peak metamorphic conditions vary significantly across the studied units: from 3.2–7.5 kbar and 520°C–670°C in the Chácara das Pedras Gneiss, to 4.5–6.2 kbar and 690°C–800°C in the Piratini Gneiss, 7.1–7.5 kbar and 765°C–795°C in the CMS granulites, and 4.3–5.5 kbar and 625°C–740°C in the Cerro Bori Orthogneiss. In conjunction with available literature data, the Punta del Este Terrane records higher average pressures (ca. 6–8 kbar) than the Pelotas Terrane (ca. 5 kbar). Variable preservation of near‐isothermal decompression features reflects different exhumation mechanisms: channel flow in CMS granulites and rapid erosion in Piratini Gneiss, versus slower exhumation in Cerro Bori and Chácara das Pedras units. Metamorphic field gradients (27°C/km–39°C/km) indicate moderate thermal regimes comparable to Himalayan‐type collisional settings, supporting the correlation of these orthogneisses as metamorphic remnants of the Piratini Orogeny within the Brasiliano/Pan‐African orogenic cycle.