Basaltic ring structures of the Serra Geral Formation at the southern Triangulo Mineiro, Agua Vermelha region, Brazil

Author(s)

Rodrigues Crincoli Pacheco, Fernando Estevao ; Caxito, Fabricio de Andrade ; de Moraes, Lucia Castanheira ; Marangoni, Yara Regina ; Zanon dos Santos, Roberto Paulo ; Pedrosa-Soares, Antonio Carlos

• Document type: Journal article
• Source: JOURNAL OF VOLCANOLOGY AND GEOTHERMAL RESEARCH; v. 355, p. 13-pg., 2018-04-15.

Abstract

The Serra Geral Formation constitutes a continental magmatic province on the southern part of South America within the Parana basin. Basaltic magmatism of the Serra Geral Formation occurred as extrusions at around 134.5 to 131.5 My ago. The formation is part of the Parana-Etendeka large igneous province, spanning South America and southwestern Africa. The main extrusion mechanism was probably through fissures related to extensional regime during the breakup of Gondwana in the Cretaceous. Basaltic ring structures (BRS) with tens of meters of diameter, cropping out downstream of Grande river at Agua Vermelha hydroelectric dam in southern Triangulo Mineiro region, enable the study of the mechanism of extrusion. The origin of the BRS has been subject to differing interpretations in the past, either collapsed lava flows or central conduits. Detailed geological mapping at 1:1000 scale, stratigraphic, petrographic and gravimetric analysis of the most well preserved of the BRS, with a 200 m diameter, has enabled the description of thirteen different basalt lava flows, along with single a central lava lake and a ring dyke structure. The central flow, interpreted as a preserved lava lake, comprises vesicle- and amygdale-rich basalt, spatter, ropy and degassing structures. The most basal of the thirteen lava flows has massive basalt containing geodes filled with quartz. Above, the lava flows show massive basalt with vertical columnar jointing where is possible to identify the top and bottom of each individual flow, with gentle dips towards the perimeter of the structure. A prominent ring dyke dipping towards the lava lake presents horizontal columnar jointing and cuts the basal and central flows. The gravimetric analysis shows a weak negative Bouguer anomaly on the center of the BRS. The proposed model describes the volcanism of the region in three main steps: (1) fissure flow occurs with lava input; (2) this lava cools and crystallizes cementing most of the fissures, promoting the formation of localized central conduits; and (3) the presence of dissolved gas in lava produces ring and radial fractures around the solidified lava lake. The magma uses some of the ring fissures to ascend and the following lava flows assume the ring shape of the dyke vent. Thus, the BRS in Agua Vermelha region can be interpreted as remnants of central conduits representing the late stage magmatism of the Serra Geral Formation. (C) 2017 Elsevier B.V. All rights reserved. (AU)

• FAPESP's process: 12/06082-6 - The Paraná-Etendeka magmatic province in Brazil: temporal and petrologic relationships between the tholeiitic and alkaline magmatism and geodynamic implications
• Grantee: Excelso Ruberti
• Support Opportunities: Research Projects - Thematic Grants