Análise estrutural e petrográfica de milonitos a SW do Domo de Santana, Santana da Boa Vista, RS

Abstract

The Santana da Boa Vista municipality is a region with a unique relief. The mixture of topographic elements ─ such as narrow mountains and depressions that form valleys and streams that flow into the Camaquã River ─ give meaning to the term "Boa Vista" in the name of the municipality. All these elements form a geological context that is the subject of many controversies regarding the formation and deformation of these structures. In this context, the present work aims to collaborate with new data and structural and petrographic interpretations of milonites which occur in the Santana Dome, in the Santana da Boa Vista region, associated with rocks of the Paleoproterozoic basement (Encantadas Complex) and with neoproterozoic supracrustals (Porongos Metamorphic Complex). The study was carried out based on the detailed geological and structural study in the southwestern region of the Santana Dome, close to the flank line of this regional antiform, where the occurrence of quartz-milonites, phylonnites and milonitic granitoids. From the analysis of stereograms elaborated with field data, it was observed that the attitude of the milonitic foliation of the region plunges into SW and SE and the stretch lineation has a plunge for SW. In petrographic and microstructural analysis, kinematic indicators were obtained that indicate top movement to NE, typical of thrusting. Microstructures of low to medium temperature (280-500°C) were also analyzed as quartz recrystallization mechanisms and brittle mechanisms in feldspar porfiroclasts (<500°C). Thus, some evidences obtained in this work, such as the presence of small-scale intercalated phylonnites, with quartz-milonites, quartz ribbons and myrmequites in felsic granitoids reinforce the hypothesis of fluid percolation and silica remobilization to form quartz rich rocks. These processes are probably related to the segregation of quartz e feldspar from the quartz-feldspathic matrix of milonitized granitoids, generating quartz and white mica, marking regions of strain softening.