Occurrence of Tertiary bimodal volcanism in the Mucuri Basin

Abstract

Since the last century two distinct ages of magmatic activity have been recognized in the Espírito Santo and Mucuri basins. The first, interpreted as Cretaceous, is related to the South Atlantic opening and is registered by its tholeiitic basalts of the Barremian aged Cabiúnas Formation. The second and most expressive volcanic activity in the basin was formed during Tertiary times and generated basalts described as of an alkaline nature, from the Abrolhos Formation. Recent studies of core samples obtained from two wells made by Petrobras in the offshore part of the Mucuri Basin identified volcanic rocks of intermediate to acid nature on the top of the Abrolhos Formation. The presence of rhyolites and tra-chytes on the top of basalts characterized for the first time a bimodal volcanism in the submersed part of this basin. In both these lithologies, the substitution of the matrix minerals by carbonate and zeolites, is probably due to the interaction of the rocks with seawater. The two sampled wells are something like 120km apart, which suggests that due to small lateral continuity, characteristic of acid to intermediate magmas, these so distant occurrences should reflect proximity with their volcanic conduits. The presence of volcanic rock fragments immediately overlaying the calcarenites, base of the Caravelas Formation, documented in the core samples, supports the registration of the extrusion of  intermediate to acid lithotypes in the final stages of the Abrolhos volcanism. Furthermore, other occurrences of rhyolitic and ignimbrite rocks are reported over the São Mateus crystalline basement regions, Espírito Santo State and in the northeastern part of Minas Gerais State. Also they appear as intercalated levels in the sandstones of the Rio Doce Formation in the most southern part of Bahia State and could have a cogenetic character and be related to the ending of the Abrolhos Tertiary magmatism the dimension of which has been underestimated until the present. Geochemical, geo-chronological and isotopic studies of these rocks are of extraordinary importance to define the extension of basic to acid volcanism and, consequently, to contribute to a better understanding of the tectonic evolution of the Mucuri and Espírito Santo basins.