As rochas vulcânicas mesozóicas ácidas da Bacia do Paraná: litoestratigrafia e considerações geoquímico-estratigráficas

Abstract

The mesozoic acidic volcanics of Paraná Basin covers an area of 64,000 km² or 2.5% of the total volume of Serra Geral Formation. Two main types are recognized and are named Palmas (aphyric rocks) and Chapecó (porphyritic rocks). The Palmas type (2% of total volume) dominate over the Chapecó (0.5% vol.) one. In the Paraná State, it is possible to found the Chapecó overlaying the Palmas volcanic rocks. The geochemical data shows that Chapecó volcanics have distinctly higher Ba, Nb, La, Ce, Zr, P. Nd, Y, Yb, Lu e K and lower concentrations of Rb, Th and U relative to Palmas rocks. The Nb concentration in Chapecó rocks is 2.58 times higher than Palmas leading to the suggestion that the magmatic sources of Palmas and Chapecó rocks are different. The Rb/Ba of Chapecó (0,09-0,11) and Palmas rocks (0,27-0,31) indicates that crustal assimilation process is very important to explain the origin of these rocks, mainly the Palmas type. The geochemical data allow us to recognize five different types of Palmas volcanic rocks named Santa Maria, Caxias do Sul, Anita Garibaldi (Peate el al., 1997), Clevelândia and Jacuí, and three different types of Chapecó volcanics, named Ourinhos, Tamarana (Peate el al., 1997) e Guarapuava, which are strongly correlated with their regional distribution in Paraná Basin, and appear to have evolved from different magma-types. The regional and temporal relationships of the different acidic volcanic rocks point to diacronous stratigraphic units.