Geologia e petrologia da porção nordeste do platô da Ramada - escudo sul-rio-grandense

Abstract

The study area this work is in the northeast portion of the Plateau of Ramada, region located in the midwest of Rio Grande do Sul, about 20 km south of the city Vila Nova do Sul. The Ramada plateau consists of the vulcan-sedimentars units of Camaquã Basin, which in this area essentially involves rocks of the Acampamento Velho Formation and secondarily at the base, rocks of the Maricá Group and Hilary Formation. In the region studied, which is an area of 60 km², is in the western portion of the Acampamento Velho Formation, with the presence of rhyolites and pyroclastic rocks ranging from tuft to breach. To the east of the study area, there is the western portion of the Granito Ramada Complex with syenogranites. With smaller representation in the area, there are rocks of the Maricá Group to the northwest and southeast. In the first stage of the work objective was to a detailed study, through literature, to seek knowledge then existing on the study area and the rock types that compose it, as well as review the status of the related art the knowledge of volcanic-sedimentares sequence of Camaquã Basin and its related plutonic rocks, react to the rocks of the area. Sequentially, was performed detailed geological map existing of the study area in scale 1: 50000, through aerial photographs and satellite images, correlated with petrographic data (macroscopic and microscopic) resulting from this work. Highly silicified sandstones of the Maricá Group form the basis of volcanic sequence of the study area setting land more recessed than the other rock types. Immediately superimposed on these occur the andesite of the Hilário Formation, also setting more recessed and flat topographies and are succeeded by ignimbrites, reoignimbrites and rhyolites of the Acampamento Velho Formation forming a high plateau, called Ramada Plateau. The syenogranite lithofacies of the Granito Ramada Complex intrude rocks of the Maricá Group and Hilário Formation being synchronous or subsequente to volcanic rocks of the Acampamento Velho Formation and sustain reliefs shaped coxilhas, setting a gentle swell ground. The rocks of the Acampamento Velho Formation in the study area showed field relationships, composition and color very similar to the Ramada Granitic Complex syenogranites. This fact supports the hypothesis that these two rock units at the Ramada Plateau together can be part of a plutono-volcanic event, with episodes of caldera collapse. A record in the area of study that can be associated with the episode of caldera collapse is the presence of rhyolite dikes of the Acampamento Velho Formation. In this case, this expression of dikes is the ease of magma rise generated by failures caused by collapse of the caldera.