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, Tiju cas belt and Dom Feliciano belt. The Vila Nova belt is located in west ern 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 juvenil e accretion during the Neoproterozoic in Brazil. The Tijucas belt, sit uated between the Vila Nova and Dom Feliciano belts, consists of a rif t-related Mesoproterozoic (?) volcano-sedimentary sequence which was s trongly deformed during the Brasiliano cycle. The Dom Feliciano belt i s located along the eastern coast of southern Brazil and Uruguay and i s a typical granite-gneiss-migmatite terrane. This belt is a key area for understanding West Gondwana assembly during Neoproterozoic and Ear ly Paleozoic times, because of its direct connection to the Gariep and Damara belts in southern Africa. The present study defines the main t ectonic phases of the ca. 600 Ma Dom Feliciano event in the Sao Felici ano 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 appr oximate age of thrusting. The strike-slip deformation (main transcurre nt phase) is well dated by U-Pb zircon ages for the syn-transcurrent g ranites (Arroio Moinho Granite, 595 +/- 1 Ma; Encruzilhada do Sul Gran ite, 594 +/- 5 Ma). These results indicate a relatively rapid evolutio n, 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-s lip related tectonic phases of the Dom Feliciano belt. Sm-Nd results c an be considered in three major groups. The first group (I) includes B rasiliano gneisses, granitoids, and one anorthosite with T-DM ages of ca. 2.0 Ga and very negative epsilon(Nd)(600) values. They may represe nt 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 w ith T-DM model ages from 1.31 to 1.41 Ga. The third group (III) compri ses samples with T-DM ages between 1.58 to 1.75 Ga. For groups II and III it is clear these rocks or their protoliths represent pre-Brasilia no continental crust. Unlike Group I rocks, groups II and III granites and gneisses may also contain a small fraction of a juvenile Brasilia no material. However, we have not yet found any sample from the Dom Fe liciano belt with a Neoproterozoic T-DM age and positive epsilon(Nd) v alue at 600 Ma that could be considered largely juvenile. Based on res ults from the Vila Nova belt, in which the main orogenic process devel oped between 753 and 704 Ma, we conclude that the Vila Nova belt was s table 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 asse mbled during the Dom Feliciano event (ca. 600 Ma). (C) 1997 Elsevier S cience Ltd.