Geologia e petrologia das unidades vulcanossedimentares ediacaranas da Bacia do Camaquã na região da Serrinha, Vila Nova do Sul, RS

Abstract

In the Serrinha region there is an important record of Ediacaran age rocks of the Sul-rio-grandense Shield, represented by a volcanosedimentary sequence of the Camaquã Basin, developed in the post-collision period of the Brasiliano Pan-African orogeny. This region is located about 20 km south of the city of Vila Nova do Sul, and has an area of 60 km². The area studied is situated on the São Gabriel Terrain and the basement of the region is composed of granite-gneiss rocks of the Cambaí Complex. Geological, petrographic, mineral and whole rock geochemical studies allowed the characterization of the volcanic and sedimentary rocks of Serrinha. The oldest unit that occurs in Serrinha comprises sedimentary rocks of the Maricá Group, especially argillites, sandstones and conglomerates, deposited in unconformity on the basement and which, in general, present a high degree of silicification. They are rocks generated in continental, coastal and marine environments, with some significant volcanic contribution. Portions with concentrations of heavy minerals suggest separation of these minerals by coastal marine currents. Overlapping the rocks of the Maricá Group, and appearing in a restricted portion of the study area, occur andesitic flows associated with Hilário volcanism. They are porphyritic and alkaline potassic rocks, and are associated to this unit by the geological, petrographic and geochemical similarities of them with the andesites of other regions of the Sul-rio-grandense Shield. The Acampamento Velho Formation is the top unit of the sequence in the study area and consists of a predominantly acidic volcanic unit predominantly composed of pyroclastic rocks in the lower portion (ignimbrites and reoignimbrites) and effusive in the upper portion (rhyolites). There is also occurrence of subvolcanic rocks in the form of dikes with acid and basic composition that intruded the other units. All the rocks of the Acampamento Velho Formation studied have alkaline sodic affinity and intra-plate characteristics, corroborating studies of rocks of this unit in other regions of the Sul-rio-grandense Shield.