Relações entre terrenos Embu e Ocidental na porção central da Faixa Ribeira, no leste do estado de São Paulo

Abstract

The geological and geophysical characterization of a major contact zone between two lithospheric blocks (Embu and Occidental) is a fundamental step for the advancement of knowledge of orogenic roots and for the formulation of tectonic models in continental collision environments. These collisions have imposed Ribeira Belt a structural trend and have developed a system of dextral transpressive shear zones, many of them marking the borderlines between the tectonic terrains. This dissertation has sought to characterize Embu and Occidental and the contact between them in the eastern region of the state of São Paulo (Brazil) by means of lithogeochemical and geochronological investigations, investigations at surface as well as through subsurface geophysical survey. Cubatão Shear Zone (CSZ), with apparent thickness between 500m and 3km in the region between the cities of Cunha and São Luis do Paraitinga (São Paulo), displays mylonites structurally and mineralogically similar to phyllites, with hydrothermal contribution and dextral kinematic indicators. CSZ separates metasedimentary rocks of Embu terrain (called Embu Complex) of metasediments and ortoderivated rocks of Occidental terrain (respectively, Andrelândia Megasequence and Natividade da Serra Batholith). Different behavior of deformation is observed on regions adjacent to ZCC. The mylonitic deformation extends to gneiss/schist of Embu Complex, but without hydrothermal contribution to the deformation process. Andrelândia Megasequence has mylonitic foliation, but it is difficult to notice the relationship with preterit deformational characteristics. The geophysical research has enabled us to compare the geometric aspects of each tectonic domain based on resistivity (Magnetotelluric Method). Embu Complex displays resistivity variations throughout its domain, with values that are lower when near ZCC and higher in more distal portions. Andrelândia Megasequence is more conductive than paraderivated rocks of Embu Complex. The lithogeochemistry comparison of Embu Complex rocks with the Andrelândia Megasequence shows divergence related to trace-elements and similarity in rare earth elements. In CSZ, values of trace-elements are similar to those observed in both terrains, which may indicate that the shear zone has affected both domains. The analysis of ortoderivated rocks also has demonstrated different behavior. In Embu terrain, Lagoinha Batholith has sodium peraluminous feature, suggesting metasedimentary protolith (type S granitoid). The amphibolite has from basic to intermediate feature, with tectonic ambience of mid-oceanic ridge type, island arc and high aluminum tholeiitic. These different tectonic ambiences may represent different generations of basalts. In Occidental terrain, Natividade da Serra Batholith displays a peraluminous from calc-alkaline to tholeiitic pattern enriched in potassium. The amphibolite is classified as subalkaline basalt and tectonic ambience of intraplate type or oceanic island. The patterns of rare earth and of trace-elements display a differentiation among orthogneiss, with higher values in Natividade da Serra Batholith. Geochronological analyses of Embu Complex indicate older sources, dating up to 3891 Ma, and may represent a cratonic source (São Franscisco Craton?). The more recent ages (817-938 Ma) indicate Brasiliano metamorphic event. In Andrelândia Megasequence, Archean ages have been found. The more recent ones vary from 639 and 788 Ma, associated with partial loss of PB due to Brasiliano metamorphic event. Their sources of sediment have more recent ages that are present in Embu Complex, exposing differences between the terrains and suggesting that both were not close throughout the Proterozoic evolution