Late Cretaceous and Paleogene tholeiitic intrusions from Região dos Lagos (RJ): 40Ar/39Ar thermochronological data inversion and nature of mantle sources

Abstract

The magmatism related to the tectonic processes responsible for the Gondwana supercontinent breakup and the Paraná Magmatic Province (PMP) genesis still poses several unanswered questions. One of them refers to the duration of the PMP main extrusive phase; another one concerns the number and duration of previous and subsequent magmatic events. In addition, the relationship between magmatism, coastal uplift, development of the Sudeste Continental Rift and erosion processes still lacks more detailed quantitative studies.Even though the PMP volcanism occurred in high extrusion rates during a relatively short period (1-3 Ma), the tholeiitic intrusions related to the Serra do Mar Dyke Swarm (SMDS) present ages extending from the Jurassic to the Paleogene periods. Such age values were obtained by both 40Ar/39Ar and paleomagnetism methods and imply that the Brazilian Southeast region was affected by a long period of magmatism before and after the supercontinent breakup. However, the 40Ar/39Ar ages are mostly incomplete, some results are doubtful, and proper quantitative models were never applied to evaluate possible thermal effects in the geochronological data. In this context, the Região dos Lagos (RJ) is important for preserving evidence of successive tholeiitic dyke intrusions in ~132, ~100-90 and ~60-50 Ma. The first of them is either contemporary or immediately posterior to the main volcanic PMP phase, though its mantle sources have distinct geochemical characteristics. The second one also does not display similar sources to those of the PMP and is contemporary to important margin uplift events in the Brazilian Southeast region, to an increase in the South Atlantic expansion rate, and to the dyke intrusions in the Namibia coast. The third one, much more recent, is related to coastal dykes whose thermal effect upon the previous groups was never characterized. This project proposes a new approach in the 40Ar/39Ar geochronology and thermochronology study of these events in order to determine their duration, thermal effects, and influences upon the morphotectonic evolution of the South America Southeast region.For this purpose, detailed investigations of the ages and thermal histories of the different intrusive magmatic groups, xenoliths inside the intrusions and contact regions with the country rocks will be performed. Considering that the Ar diffusivity in K-bearing minerals can be described by the Multidomain Diffusion model (MDD), each domain will be characterized, allowing the intrusions and country rock thermal histories definition through the inversion of age spectra curves. Such methodology was never explored in this region before, and is based on a thermochronological method, which displays higher closure temperatures than all the ones applied in previous studies (e.g., AFT, (U-Th)/He, etc.). The resultant curves will enable the reconstruction of the thermal events that affected the basement rocks and the detection of the influences caused in the 40Ar/39Ar obtained for the intrusive Brazilian Southeast rocks. Finally, trying to achieve a better understanding about the mantle role in the margin relief evolution, which until now is poorly known, an elemental and isotopic characterization of the recent tholeiitic magmatism will be carried out. Such investigation will provide information about the role of asthenosphere (N-MORB) and/or lithosphere mantle sources in the dyke genesis.