Felsic pyroclastic rock originated from subaqueous eruption in the Espírito Santo sedimentary basin: an association with the tectonic-sedimentary model.

Abstract

Felsic pyroclastic rock originated from subaqueous eruption in the Espírito Santo sedimentary basin: an association with the tectonic-sedimentary model. This paper presents the first outcrop description of rhyolitic pyroclastic rocks that constitute small extrusive rock bodies in the Espírito Santo Sedimentary Basin. The observed outcrops show horizontal tabular rock bodies, about 5m thick, with horizontal extension less than 200 m. The rhyolitic bodies are intercalated in Paleogene fluvial and lacustrine sandstone layers of the Rio Doce Formation. The microscopic observations exhibit a-phase quartz phenocrysts and small quartz fragments, suggesting explosive volcanic activity. The matrix is intensely devitrified by sericitisation and silicification due to hydrothermal reaction between hot glass shard and lacustrine water. This rhyolitic magmatism is a heat source, which has possible relationship to the NNW-SSE regional fracture system, with variation to NW-SE, the so-called Colatina Lineament, which seems to be important for petroleum deposits formation. This is the first information of Cenozoic felsic magmatism of the onshore region of Espírito Santo, and the first report of subaqueous rhyolitic deposit originated from pyroclastic eruption in continental Brazil. Keywords: Rhyolite, subaqueous eruption, hydrothermalism, Rio Doce Formation, Espírito Santo basin.