In this contribution we present new insights on the evolution of the Dom Feliciano Belt, southernmost Man-tiqueira Province, integrating new whole-rock Sm–Nd isotopic data for the Arroio Grande Ophiolite (Punta del Este Terrane, Brazil/Uruguay border) with previously published bulk-rock and isotope geochemistry of the South Adamastor paleo-ocean metamafic rocks located in Uruguay (Paso del Dragón Complex, Punta del Este Terrane) and Namibia (Chameis Subterrane, Marmora Terrane, Gariep Belt). For the regional geology, the new data corroborate previous hypothesis and demonstrate the depleted mantle features of the amphibolites and meta-gabbros of the studied ophiolite. The Arroio Grande Ophiolite rocks are compared with its Uruguayan and Namibian counterparts, demonstrating their isotopic and geochemical similarities and differences, and the back-arc affinity of the South Adamastor paleo-ocean. The MORB-affinity amphibolites from the Arroio Grande Ophiolite-Paso del Dragón Complex are, so far, the most juvenile rocks in the eastern sector of the Dom Feliciano Belt, yieldingεNd(640 Ma) between+7.3 and+9, and high 147Sm/144Nd (>0.169) and 143Nd/144Nd(640 Ma) ratios (0.51219–0.51229). The South Adamastor is interpreted in this paper as an internal back-arc ocean, with limited lateral extension, opened at around 750–650 Ma as the result of the closure of the older Charrua-Goianide paleo-ocean during the Brasiliano/Pan-African orogenic cycle and final configuration of the West Gondwana paleocontinent.