New magnetostratigraphy constrains in the southern Paraná magmatic province (Herveiras and Gramado Xavier areas), Rio Grande do Sul state, South Brazil: Implications for the timing between volcanic sources

Abstract

This paper undertakes a new paleomagnetic study of 51 lava flows that crop out in Southern Paraná Magmatic Province. The volcanic rocks are basalt of low-Ti content type (18 flows) and Palmas silicic type (33 flows) corresponding to the Caxias do Sul, Barros Cassal, and Santa Maria sub-types covering all lithostratigraphic units of the region. This study is part of a broad magnetic study in the Brazilian part of the Paraná-Etendeka LIP to understand the temporal relationship between the volcanic units of different compositions allowing the investigation of the contemporary nature of the multiple sources. The characteristic remanent magnetization (ChRM) for the lavas was obtained by alternating field (a.f.) and thermal demagnetization methods, and is interpreted as a thermo-remanent magnetization carried by stable pseudo-single domain magnetite grains, being acquired during the cooling of the lava flows. The high-quality of the ChRM results from both areas allows us to interpret that the emplacement of all volcanic rocks was geologically very fast, as verified by the symmetrical direction of both polarities (normal and reverse i.e. positive reversal test) of the geomagnetic field associated with the stratigraphy of all lava flows. The passage from pahoehoe to rubbly basalts was marked by a change of polarity of the geomagnetic field. Our results suggest that all volcanic rocks were emplaced by at least two volcanic events as shown by the polarity changes, and the sources of basaltic and silicic rocks of different compositions were active at the same time.