Petrologia e correlação geoquímica da Formação Vale do Sol, Grupo Serra Geral : província magmática Paraná-Etendeka

Abstract

The Paraná-Etendeka Magmatic Province is a Cretaceous event (131-135 Ma) associated with the breakup of Western Gondwana and the opening of the South Atlantic Ocean. In Brazil, the rocks fall within the Serra Geral Group, subdivided into formations: Torres, Vale do Sol, Palmas, Barros Cassal, Esmeralda to the south, and Pitanga and Paranapanema to the north. In this context, the rocks of the Vale do Sol Formation, the subject of this study, represent the main phase of magmatism in the southern part of the province. This unit consists of a stack of lavas with a total thickness of over 500 m, composed of basaltic andesites, and characterized by thick tabular flows (> 20 m). The relationship between the main phases of magmatism and the generation of intermediate compositions in large igneous provinces, as well as the petrological processes involved in magma differentiation, are still poorly understood, and petrological models related to the generation of these are scarce. Detailed stratigraphic profiles were constructed in the northeastern part of Rio Grande do Sul (RS), around São Marcos and in the Serra da Rocinha, to investigate the internal stratigraphy of the Vale do Sol Formation. Along these profiles, flows range from 9 to 90 meters in thickness, and are characterized by a poorly vesicular/amygdaloidal base, a massive core, and a fragmented top. Detailed sampling allowed for petrological characterization of the unit. The flows are aphyric with fine to very fine grain texture and are composed of plagioclase, augite, and oxides, ±pigeonite ± olivine. Chemically, they are basaltic andesites of the Gramado and Esmeralda types, with some transitional terms. These can be grouped into three categories based on TiO2, P2O5, MgO, and Zr: Group 1 (G1) with TiO2 < 1.3%, Group 2 (G2) with TiO2 between 1.3 and 1.5%, and Group 3 (G3) with TiO2 > 1.5%. Compositions vary within the stratigraphy, with the base being formed by primitive G1 flows, followed by differentiated G3 flows. This transition occurs abruptly. The sequence is overlain by G2 flows and, finally, a package of acid rocks. The RA profile also contains an additional package of intermediate G2 flows and an upper acid unit (Anita Garibaldi). Stratigraphic correlation provides important information about the magnitude and lateral continuity of volcanic events. Future investigations with an emphasis on modelling petrological processes will provide insights into the understanding of the dynamics of large igneous provinces and how crustal petrological processes are associated with eruptive dynamics, and the mechanisms responsible for the transition from the main phase to the final stages of magmatism in LIPs.