Neoproterozoic magmatic arc assembly in the southern Brazilian Shield - constraints for a plate tectonic model for the Brasiliano orogeny
Saalmann, K.; Remus, M.V.D.; Hartmann, L.A.
Juvenile calc-alkaline gneisses in the São Gabriel block in southernmost Brazil show Neoproterozoic ages. Age proposals for associated metasedimentary and (ultra-)mafic metavolcanic rocks, however, range from Archean to Neoproterozoic. Sm-Nd TDM model ages and initial εNd(t) values support the existence of Neoproterozoic juvenile oceanic crust and island arcs during the Brasiliano orogenic cycle. Geochemical data indicate a subduction zone setting for the metavolcanic and metasedimentary rocks. The data also show the existence of two subsequent oceanic rock assemblages comprising a ca. 879 Ma intra-oceanic arc and a 750-700 Ma active continental margin. Four deformational events can be distinguished in the São Gabriel block. D2 at amphibolite facies metamorphic peak conditions corresponds to SE-directed thrusting. During D3, right-lateral ductile transpressive strain due to oblique convergence is partitioned in contraction-dominated zones and SW-NE oriented major transcurrent shear zones. These shear zones preferentially occur in syn-D3 plutonic rocks. D3 led to tectonic juxtaposition of the distinct lithologic units forming the final overall geometry which is represented by a SE-vergent ductile transpressive thrust stack. D4 is characterized by semi-brittle SE-directed thrusting, in parts reactivating the pre-existing shear zones. A plate tectonic model for the São Gabriel block comprises (1) subduction to the E and 0.9-0.8 Ga accretion of the Passinho intra-oceanic arc to the eastern border of the Rio de la Plata craton; (2) W-dipping subduction beneath the newly formed active continental margin, formation of a 750-700 Ma magmatic arc and related sedimentation and subsequent imbrication of the metavolcanic and metasedimentary rocks in the accretionary wedge; (3) collision with another continental block or microcontinent (Encantadas block) possibly derived from an African craton. Oblique convergence in southern Brazil is indicated by a change from SE-directed thrusting with steep dipping stretching lineations (L2) to localized wrench tectonics and oblique folding during D3 during progressive structural evolution. The juvenile rocks of São Gabriel block represent relics of a major Brasiliano oceanic basin between the Rio de la Plata craton and the Encantadas block. Subduction and arc accretion began roughly coeval with the initial stages of the break-up of Rodinia (0.9 Ga) and indicates a peripheric or isolated position of the Rio de la Plata craton in relation to the Rodinia supercontinent. The evolution from a passive margin to an active margin marks the begin of the Neoproterozoic Brasiliano cycle. The oceanic basin to the E of the Rio de la Plata craton might be traced to the N and linked with Neoproterozoic juvenile island arcs in the Brasilia belt.