CONTRIBUIÇÃO AO ESTUDO DAS ROCHAS VULCÂNICAS DA BACIA DO PARANÁ NA REGIÃO DE SANTA MARIA, RS

Abstract

The volcanic rocks which occur in the southern part of the Paraná basin, in Santa Maria region, Rio Grande do Sul, Brazil, are piled up in four lava flows with intertrapp sandstones and some related sills. Their radiometric age varies between 118 - 123 m.y. Descriptions of the structural aspects of the flows are made, likewise a careful study of the mineralogy and petrography in several samples collected in different positions and localities where occur each flow. The lava flows are characterized by differences in composition which helped the identification of the stratigraphic position of each one of them. Modal composition indicated a basalt to andesite composition for the three lower lava flows. Normative compositions, howewer, indicated more differentiated facies - latites and trachybasalts. Differences are due to unidentified mineralogical phases present in the matrix of the rocks. The fourth upper flow, which shows the largest thickness and horizontal distribution, presents two different textural facies: a vitrophiric one and the other a granophiric. Normative composition indicated a dellenitic nature for the former one and a riolitic one for the latter. The rocks follow the general trend known for the basaltic rocks of the Paraná basin but the upper flow is established as the most differentiated already found in the magmatic province.