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

Abstract

The mesozoic acidic volcanic rocks of Parana Basin: lithostratigraphy and geochemical stratigraphy aspects. The mesozoic acidic volcanics of Parana Basin covers an area of 64,000 km 2 or 2.5% of the total volume of Serra Geral Formation. Two main types are recognized and are named Palmas (aphyric rocks) and Chapeco (porphyritic rocks). The Palmas type (2% of total volume) dominate over the Chapeco (0.5% vol.) one. In the Parana State, it is possible to found the Chapeco overlaying the Palmas volcanic rocks. The geochemical data shows that Chapeco 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 Chapeco rocks is 2.58 times higher than Palmas leading to the suggestion that the magmatic sources of Palmas and Chapeco rocks are different. The Rb/Ba of Chapeco (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, Cax- ias do Sul, Anita Garibaldi (Peate el al., 1997), Clevelândia and Jacui, and three different types of Chapeco volcanics, named Ourinhos, Tamarana (Peate el al., 1997) e Guarapuava, which are strongly correlated with their regional distribution in Parana Basin, and appear to have evolved from different magma-types. The re- gional and temporal relationships of the different acidic volcanic rocks point to diacronous stratigraphic units.