U-Pb and Sm-Nd geochronology of the neoproterozoic granitic-gneissic Dom Feliciano belt, Southern Brazil

The Brasiliano Cycle in southern Brazil and Uruguay is represented by three major NE-SW trending geotectonic units: the Vila Nova belt, Tijucas belt and Dom Feliciano belt. The Vila Nova belt is located in western part of Rio Grande do Sul State; its evolution took place between 900 and 700 Ma and it corresponds to one of the few areas with juvenile accretion during the Neoproterozoic in Brazil. The Tijucas belt, situated between the Vila Nova and Dom Feliciano belts, consists of a rift-related Mesoproterozoic (?) volcano-sedimentary sequence which was strongly deformed during the Brasiliano cycle. The Dom Feliciano belt is located along the eastern coast of southern Brazil and Uruguay and is a typical granite-gneiss-migmatite terrane. This belt is a key area for understanding West Gondwana assembly during Neoproterozoic and Early Paleozoic times, because of its direct connection to the Gariep and Damara belts in southern Africa. esent study defines the main tectonic phases of the ca. 600 Ma Dom Feliciano event in the São Feliciano belt. U-Pb zircon data for flat-lying gneisses yield ages between 610 ± 5 Ma and 616 ± 2 Ma, which we believe correspond to the approximate age of thrusting. The strike-slip deformation (main transcurrent phase) is well dated by U-Pb zircon ages for the syn-transcurrent granites (Arroio Moinho Granite, 595 ± 1 Ma; Encruzilhada do Sul Granite, 594 ± 5 Ma). These results indicate a relatively rapid evolution, from about 620 Ma (upper limit for the age of the gneiss) to 594 Ma (syn-trancurrent granites), for the known thrust related and strike-slip related tectonic phases of the Dom Feliciano belt. results can be considered in three major groups. The first group (I) includes Brasiliano gneisses, granitoids, and one anorthosite with TDM ages of ca. 2.0 Ga and very negative εeNd(600) values. They may represent either direct melting of Transamazonian (Paleoproterozoic) basement or extensive contamination with older material of Paleoproterozoic to Archean age. The second group (II) includes granitoids and gneisses with TDM model ages from 1.31 to 1.41 Ga. The third group (III) comprises samples with TDM ages between 1.58 to 1.75 Ga. For groups II and III it is clear these rocks or their protoliths represent pre-Brasiliano continental crust. Unlike Group I rocks, groups II and III granites and gneisses may also contain a small fraction of a juvenile Brasiliano material. However, we have not yet found any sample from the Dom Feliciano belt with a Neoproterozoic TDM age and positive εNd value at 600 Ma that could be considered largely juvenile. on results from the Vila Nova belt, in which the main orogenic process developed between 753 and 704 Ma, we conclude that the Vila Nova belt was stable for over 100 Ma before the Dom Feliciano event reached its peak. It is probable that the collage of terranes in the Dom Feliciano belt and the region comprised by the Tijucas and Vila Nova belts were assembled during the Dom Feliciano event (ca. 600 Ma).