Zircon U-PbSHRIMP dating of a Neoproterozoic overprint in Paleoproterozoicgranitic-gneissic terranes, southern Brazil

The Neoproterozoic Florianopolis batholith provides the major clue for unra veling the pre-Brasiliano Cycle evolution of Santa Catarina State, southern Brazil, because it contains septa of basement units such as the Camboriu a nd Aguas Mornas complexes reactivated during the younger Brasiliano orogeny at about 600 Ma. We establish precise U-Pb zircon SHRIMP ages and Nd isoto pic evolution of four key samples from the septa. The zircon population displays complex internal structures in three of the four dated samples. The complex crystals have well-preserved magmatic cores and altered rim domains. The core popu lations were dated at about 2180 Ma (Aguas Mornas complex), about 2160 Ma (Camboriu complex amphibolite xenoli th), and 2000 Ma (Camboriu complex orthogneiss), whereas the ages of the me asured rims and altered domains are about 590 Ma in all three samples. On t he other hand, the Presidente Nereu tonalite has a simpler zircon populatio n without core/rim separation, and yields magmatic ages about 2200 Ma. The Nd isotopic analyses of the three samples emphasize the polycyclic evol ution of the batholith, which is much more complex than previously realized . The evolution is marked by two successive Paleoproterozoic accretionary o rogenic events at about 2200-2175 Ma and by a crustal melting event at abou t 2000 Ma, which had an Archaean source (Sm-Nd depleted mantle model age, T -DM = 2860 Ma). These orogenic events were followed 1.4 billion years later by a Neoproterozoic hydrothermal/metamorphic overprint at about 599 Ma. Th is time gap permits us to identify a long intracratonic period-the Atlantic a supercontinent. The unraveling of the Proterozoic evolution and the preci se characterization of the basement remnants show that the Paleoproterozoic was a crust-building, orogenic interval and was the major source of the mu ch later Neoproterozoic granites. The integrated use of back-scattered elec tron and cathodoluminescence imaging with SHRIMP spot dating and Nd isotope s provide a clear understanding of the timing of crustal generation and def ormation in southern Brazil, including strong evidence for a long-lasting s upercontinent cycle.