Tectonic evolution of the Neoproterozoic São Gabriel block, southern Brazil: Constraints on Brasiliano orogenic evolution of the Rio de la Plata cratonic margin

Abstract

Precambrian rocks of southernmost Brazil display a complex tectonometamorphic evolution since the Archean. Neoproterozoic juvenile rocks in the São Gabriel block offer a unique opportunity to study unequivocal Brasiliano fabric and tectonics. The stratigraphic succession comprises the Palma Group, which consists of metasedimentary rocks and juxtaposed (ultra-)mafic to intermediate volcanic and volcanoclastic rocks formed mainly in a subduction zone environment, as well as the juvenile metadiorites, -tonalites and -trondhjemites of the Cambaí Complex, which represent relics of magmatic arcs. Four deformational events, the first three characterized by ductile deformation, can be distinguished in the São Gabriel block. The first deformation, D1, led to the formation of a first foliation and metamorphic layering, possibly related to a first folding event. The D2 event reached amphibolite facies metamorphic peak conditions and corresponds to southeast-directed thrusting. D1 and D2 can be observed only in the lower Palma Group rocks. D3 led to the tectonic juxtaposition of different lithologic units along major SW–NE-striking shear zones, which defines the final overall geometry of the area represented by a dextral transpressive, southeast-vergent ductile thrust stack. Ductile SW–NE-oriented strike-slip shearing occurred predominantly in syntectonic (syn-D3) plutonic rocks of the Cambaí Complex due to the lower viscosity and strength of the magmatic rocks, whereas deformation in the wall rocks (Palma Group) was accommodated mainly by southeast-vergent folding. The main part of the D3 deformation took place under upper greenschist to lower amphibolite facies metamorphic conditions. The last deformation, D4, is characterized by semi-brittle, southeast-directed thrusting. A plate tectonic model for the São Gabriel block comprises (1) eastward subduction of oceanic crust and approximately 880 Ma accretion of the intraoceanic Passinho arc to the eastern border of the Rio de la Plata Craton; (2) west-dipping subduction beneath the newly formed active continental margin, development of a 750–700 Ma magmatic arc, and related sedimentation and their imbrication in the forearc and backarc regions; and (3) collision with another continental block (Encantadas block) at approximately 700–670 Ma. A change from southeast-directed thrusting (D2) to localized strike-slip tectonics and oblique folding during D3 occurred during progressive structural evolution due to oblique subduction. Oblique convergence and collision was accommodated by orogen-parallel movements, facilitated by syntectonic magmatic activity. The São Gabriel block represents a major Brasiliano collision zone. The oceanic basin facing the eastern margin of the Rio de la Plata Craton might be traced to the north and could be linked to Neoproterozoic juvenile oceanic crust in the western Brasília belt.