The Serra da Bolívia complex: The record of a new Neoproterozoic arc-related unit at Ribeira belt

Abstract

The Neoproterozoic arc related rocks of the central segment of the Ribeira belt crop out at the Oriental Terrane that is subdivided into three thrust sheets verging northwestern ward, each of them with subduction related rocks. The recently discovered Serra da Bolívia complex crops out at the Cambuci domain that represents the lowermost thrust sheet of the Oriental terrane. The unit comprises orthogneisses of varied compositions, ranging from basic to acid rocks. Hornblende bearing coarse grained orthogneisses, with granodiorite to quartz-diorite compositions, is the most common association in the complex; followed by quartz poor rocks such as monzo-gabbros, monzodiorites, monzonites and syenites. Coarse grained gabbros (norites) and leucogabbros occur as lenses inside the orthogneisses.New geological, geochemical and geochronological data for recently described arc-related orthogneisses of the Serra da Bolívia complex is presented. The connection with other arc-related rocks of the Araçuaí and southern Ribeira belt in the context of West Gondwana reconstructions is also explored.Chemical data indicates at least three groups of sub-alkaline rocks and one group of alkaline to transitional character (SH). Among the sub-alkaline groups three calc-alkaline groups (CA1, CA2 and CA3) with progressive enrichment in Alkalis are discriminated. Tectonic discriminate diagrams suggest that the calc-alkaline groups display compositions compatible with arc settings, while the shoshonite group, with more evolved compositions indicates progressive maturity during arc development. Few basic rocks indicate CAB or IAT filiations.Obtained U–Pb ages for tonalites and gabbros yielded values between ca. 596 and 591Ma, but an older TIMS age pointed to ca. 620Ma. The youngest monzonite orthogneiss yielded ca. 574Ma, similar to the metamorphic ages previously described for the Ribeira belt. Sm-Nd data resulted on Tdm model ages of ca. 1.72Ga and 2.04Ga, except for three samples, which display Mesoproterozoic model ages. The initial ɛNd, calculated for 605Ma, which is the average age for the complex are between −8 and −12.Regional correlations suggest that the Serra da Bolivia Complex represents the inner cordilleran magmatic arc of the Ribeira belt that continues northward to the Galiléia arc (G1 association of the Araçuaí belt), and southwards to the Socorro arc at the interference zone of the Brasília belt. Combined U–Pb data indicate the development of a cordilleran arc between ca. 650 and 590Ma. Youngest syenitic to monzonitic bodies are coeval with the metamorphic episode and intrude the older rocks of the arc. The other two previously described arc related complexes, Rio Negro and Serra da Prata arcs are located easternmost at the belt and display contrasting more juvenile signatures and older ages reaching ca. 840M.Looking to the West Gondwana scenario, during Tonian to Cryogenian time, subduction all around older and stables plates resulted on the generation of large volumes of arc-related rocks. In most of the belts, only the plutonic zones of the magmatic arcs are exposed. Based on time span of rock crystallization of all the arc related rocks of the Ribeira belt, the subduction period lasted at least 240 million years. Even though subduction was breaking down into three individual zones, the tectonic scenario suggests oceanic plate consumption in a protracted evolution. The initial phases of most magmatic arcs are represented by more primitive and juvenile rocks, but at the end of the subduction period, at the Ediacaran, a more cordilleran setting was achieved. By the end of the Ediacaran period and the beginning of Cambrian, all the pieces were attached together in several diachronic collisions leading to the final amalgamation of West Gondwana.