Volcanic succession and feeder systems of acidic lava-domes of Serra Geral Formation in São Marcos-Antônio Prado region, South Brazil

Abstract

In the São Marcos (RS) and Antonio Prado (RS), the Serra Geral Formation exposes at the base basalts of pahoehoe type, coveredby basalts of the ´a´ā type. The first succession was generated by a low rate of eruption in a closed flow system allowed the flow toreach distances > 100 km of the source.T he ´a´ā lava flow types were generated by higher rates of eruption andt ransported in openchannels where rapid cooling prevented long distances from the source to be reached. The two types of basalts are low-TiO2 tholeiiticand the morphology of flows is not related to variations in SiO2 and MgO contents. Above these rock types outcrop acidic volcanicrocks geochemically of Caxias Group (Palmas Subgroup). Dimension stones extraction exposed the inner portions of the acidicfeeder dikes with vertical magmatic foliations. The lava domes have exogenous characteristics and horizontal foliations. We proposea model for the generation of domes involving the diapirically rise of acids magmas that become vesicular and more viscous, thatstop near the surface. New magmatic pulses extracted “pieces” of the vesicular fraction generating autobreccias in the conduit andvertical structures that extend laterally toward the surface organizing the lava domes with vitrophyres in the base and in the top, witha thin massive phaneritic core. Magmatic textures of the domes are typical of effusive units and the identification of the feeder dykesin the area allows the understanding of the emplacement process of acidic flows in the Serra Geral Formation.

Keywords

• Pahoehoe
• ´a´ā
• Large Igneous Provinces
• Lava domes
• Paraná-Etendeka Province.