Tectonic Evolution of Two Contrasting Schist Belts in Southernmost Brazil: A Plate Tectonic Model for the Brasiliano Orogeny

Abstract

Two Brasiliano schist belts (Porongos belt and São Gabriel block) of contrasting geotectonic setting exhibiting a distinct structural evolution are exposed in the southern Brazilian shield east of the Rio de la Plata craton. Trace-element and isotope data (Sm-Nd, Rb-Sr) demonstrate that the São Gabriel block and Porongos belt represent two distinct tectonostratigraphic blocks, with the former consisting of Neoproterozoic juvenile rocks, and the latter being characterized by intense reworking of old continental crust. The São Gabriel block in the west consists of Neoproterozoic relics of two Brasiliano magmatic arcs, an intra-oceanic arc, and a younger continental arc or active continental margin. The Porongos belt located on the Encantadas block, in contrast, formed in a passive-margin setting on thinned continental crust in an extensional or transtensional regime. Basin formation was accompanied by volcanism due to partial melting of the stretched continental crust. Comparison of the structural evolution and age data provide constraints on the timing of tectonic juxtaposition of the two terranes. Plate tectonic evolution started with opening of an oceanic basin to the east of the Rio de la Plata craton since at least 0.9 Ga, possibly earlier. First subduction occurred at about 0.87 Ga, leading to accretion of an intra-oceanic island arc to the passive margin of the Rio de la Plata craton. An active continental margin developed above a west-directed subduction zone beneath the block consisting of the Rio de la Plata craton and the attached island arc. At the same time, the Porongos basin formed on stretched continental crust of the Encantadas passive margin. The final stage of the São Gabriel event represents the collision of the Rio de la Plata craton with the Encantadas block between about 700 to 660 Ma. Progressive shortening resulted in SE-directed brittle thrust faulting in the São Gabriel block, and left-lateral ductile shear at the Dorsal de Canguçu Shear Zone (DCSZ) led to DB4 NW-vergent folding and thrusting in the Porongos belt. Activity at the DCSZ ceased at about 620 Ma, whereas deformation became localized in transcurrent shear zones accompanied by synkinematic granite intrusions in the Dom Feliciano belt farther to the east. Post-tectonic granites of 600-580 Ma are widespread in both western and eastern areas. However, shearing continued in localized fault zones lasting at least until 540 Ma as recorded by fault-related granites.