Rochas dioríticas do platô da ramada, rio grande do sul, e sua relação com o vulcanismo alcalino da formação acampamento velho, neoproterozoico do escudo sul-rio-grandense

Abstract

Studies carried out in the Ramada Plateau (Western of the Rio Grande do Sul State, Brazil), have allowed the characterization of a dioritic hipabissal body, which is intrusive in pyroclastic rocks of the Acampamento Velho Formation. This unit consists of volcanic rocks of predominantly silicic composition in association with basic rocks, and it corresponds to the extrusive portion of the sodic alkaline magmatism of the Neoproterozoic age (570-550 Ma), related to postcollisional events of the Brasiliano-Pan Africano cycle of Sul-Rio-Grandense Shield. The intrusive body ranges from diorite to quartz-monzonite and its sodic alkaline affinity is attested by the large-ion lithophile elements (LILE), high-field strength elements (HFSE) and rare earth elements (REE) contents, whose characteristics indicate its correlation with the high Ti magmatism of Acampamento Velho Formation, described in the Ramada Plateau. The alkaline magmatism evolution of Acampamento Velho Formation in this region can be explained mainly by fractional crystallization processes, involving three main stages, according to petrogenetic models presented in this work. However, given the huge volume of high-silica rhyolitic rocks and the complexity involved in the generation and evolution of these magmas, probably crustal assimilation has occurred together with fractional crystallization processes. The identification and characterization of intermediate rocks related to Acampamento Velho Formation in Ramada Plateau thus indicate that the magmatism must have evolved from basic to acidic compositions. But, the premise of bimodal volcanism still remains due to the absence of andesitic lavas, which can be attributed to a 'density barrier' created by the differentiation of basic to intermediate liquids enriched in FeO. Therefore, such liquids were probably placed in shallow crustal levels.