SHRIMP U/Pb Zircon Dating of Neoproterozoic Granitic Magmatism and Collision in the Pelotas Batholith,Southernmost Brazil

The introduction of robust geochronological methods for age determinations of the southernmost segment of the Neoproterozoic terranes of Brazil, namely the Dom Feliciano Belt, provides important clues for unraveling the complex evolution of the Brasíliano/Pan-African orogeny in this southwestern portion of the Gondwana supercontinent. Except for associated small schist belts and post-orogenic foreland basins, the belt is represented in this region of southeastern South America by the Pelotas Batholith. Precise SHRIMP U/Pb zircon geochronological techniques based on the study of 95 individual spots on 74 zircon crystals (three samples) and on Nd-isotopic determinations (three samples) are used to assess the late Neoproterozoic history of the belt, especially the orthogneisses interleaved with the batholithic plutons. Three petrotectonic associations were selected for detailed isotopic investigations—the Pinheiro Machado syncollisional monzogranites, the widespread Piratini gneiss tonalitic xenoliths, and the Arroio dos Ratos (now Encantadas) gneiss. The results allow the establishment of the timing and ages of the metamorphic peak and early magmatism. We demonstrate that evolution of Neoproterozoic magmatism within the Pelotas Batholith occurred through a long-lived crustal recycling process from the Paleoproterozoic Rio de la Plata craton. Three major events are recognized, two corresponding to crustal granite generation by partial melting of Paleoproterozoic protoliths at ～780 and ～610 Ma, and one related to the high-grade syncollisional metamorphic peak at ～630 Ma. The data also yield precise criteria to distinguish between thrust-related granitoids of the Dom Feliciano belt and older orthogneisses, both previously interpreted as a unique, pre-collisional, Brasíliano Cycle magmatic-arc association.