Geochemical, isotopic and geochronological characterization of tholeiitic dykes of the Southeastern Coast of Brazil: geodynamical implications

Abstract

The geodynamical processes related to the genesis of large igneous provinces are scarcely known, and they are essential to the understanding of ocean spreading tectonics, including South Atlantic opening. In this context, the southern area of the Brazilian territory has a great relevance, since it holds one of the biggest magmatic events of the globe: the Paraná Magmatic Province (PMP). It consists of more than 600.000 km³ of lava flows and an extensive intrusive igneous activity, which includes sills and three main dyke swarms: Ponta Grossa, Florianópolis, and Serra do Mar. The tholeiitic dykes of Serra do Mar Swarm (SMS) have some peculiar features that need to be better investigated, since previous studies have shown that magmatism may have lasted from the Jurassic up to the Paleogene, possibly exhibiting a significant compositional variability. Most of the SMS dykes are basic in composition, presenting high contents of titanium (HTi; TiO2 > 2%). Their geochemical signatures are commonly similar to the HTi lava flows of the PMP, even though dykes with very particular signatures, not encountered in the lava flows, were found. 40Ar-39Ar dating show that these bodies were emplaced mainly between 133 and 130 Ma, with some occurrences between 125 and 120 Ma. However, K-Ar geochronology suggested some older ages (193-144 Ma), which are in agreement with paleomagnetic data. The SMS also has dykes with low titanium contents (LTi; TiO2 < 2%) in the highland regions of Rio de Janeiro, and also along the coast between Cabo Frio and Búzios. There is not a consensus about the ages of those coastal dykes, because the Ar-Ar age spectra are disturbed. For this reason, there are age reports in the literature of 133-138 Ma, as well as of 55 Ma. The latter are corroborated by new paleomagnetic data, which also suggest the existence of a group of ages around 100 Ma. The purpose of this project is the geochemical, isotopic and geochronological (Ar-Ar) investigation of representative samples of the SMS tholeiitic dykes, in order to distinguish different groups by their ages and geochemical/isotopic signatures. The sample selection will be based on paleomagnetic information to identify the intrusion episodes. The study aims to investigate if: a) the magmatism started during the Jurassic; b) the emplacement of the LTi dykes of the Cabo Frio-Búzios coast occurred in three different magmatic pulses (133, 100 and 55 Ma); c) these episodes are related to distinct geochemical and isotopic (Sr-Nd-Pb) signatures. The 40Ar/39Ar dating will be carried out by the student herself (by way of a BEP-FAPESP financial help request), in the Argon Geochronology Laboratory of Earth Sciences of the University of Queensland, Australia (UG-AGES), in collaboration with Professor Paulo Vasconcelos. (AU)