Internship in the argon Geochronology Laboratory of the School of Earth Sciences in the University of Queensland (UQ-AGES - Australia): 40Ar/39Ar dating of Cabo Frio-Búzios tholeiitic dykes

Abstract

Recent geochronological and paleomagnetic data of the Serra do Mar Dyke Swarm (SMDS, Southeastern Brazil) indicate that the tholeiitic magmatism might have started during the Jurassic period (~156 Ma) and lasted until the Paleogene period (~55 Ma). These data imply that these intrusions have occurred during a much wider time interval than the volcanism of the Paraná Magmatic Province (133-134 Ma).The Paleogene ages concentrate in the Northeastern portion of the Rio de Janeiro State, especially in the coast (Cabo Frio - Búzios region). The youngest ages were obtained by 40Ar/39Ar measurements with no well-defined plateaus that indicate maximum ages of 55 Ma for these dykes. These results have been confirmed by recent paleomagnetic data, which have displayed two intrusion phases (55 and 102 Ma). Some researchers, however, have related these younger ages to regional re-heating caused by the younger widespread alkaline magmatism, which affected the same region.The SMDS dykes have usually basic composition and present high contents of titanium (HTi; TiO2>2%) and incompatible trace elements. In the Cabo Frio-Búzios region, however, the tholeiitic dykes have low titanium contents (LTi, TiO2<2%). Furthermore, they can be divided in two different igneous suites based on geochemical data. They are also depleted in incompatible trace elements and have lower Sr isotope ratios in comparison to the other dykes of the swarm. The 40Ar/39Ar geochronology is one of the major objectives of the student's Master project (Process No. 2014/22948-9), and the internship in the Argon Geochronology Laboratory of the School of Earth Sciences in the University of Queensland (UQ-AGES - Australia) is previewed in the initial project description. These data may determine if the emplacement of the LTi dykes is indeed younger than the HTi ones, as well as if they occurred in two different magmatic pulses, as indicated by the previous studies. The 40Ar/39Ar ages will provide ways to investigate whether there is any relationship between the geochemical and isotopic signatures of the LTi dykes with different intrusion pulses and their genetic links with the opening of South Atlantic Ocean. We have selected 4 samples for 40Ar/39Ar dating of plagioclase grains based on petrography, preliminary geochemical results, and paleomagnetic data. The samples have displayed paleomagnetic poles close to those of ~55 Ma and ~102 Ma igneous rocks outcrop in the southern region of South America. The petrographic studies have shown that most of the plagioclase phenocrysts have been altered by sericite in their cores, whereas the borders remain fresh; the plagioclases present in the groundmass are generally fresh. In order to obtain reliable 40Ar/39Ar ages and avoid the same problems encountered by previous dating attempts, the analyses will take place both in the plagioclases of the groundmass and in the borders of the phenocrysts. The age spectra of each grain will be carefully interpreted based on the petrographic descriptions and Ar diffusion modelling. In addition, it may be useful to obtain the Ar spectra of the sericite cores to try to determine the ages of the plagioclase alteration.Because of the complexity of the problem, these analyses must take place in a totally equipped laboratory. The UQ-AGES fits our purposes. The study will also be under supervision of Professor Paulo Vasconcelos, who has a vast experience in Ar analyses in altered samples and is very interested in our project. The fresh plagioclases and sericite-altered grains have already been sent to UQ-AGES and from there they were sent to neutron irradiation in a nuclear reactor at Oregon State University, USA. We calculate that a nearly three-month period in Queensland will be sufficient to proceed with the following steps of Ar analysis and data interpretation, starting January 2016, consisting of Scanning Electron Microscope analysis, 40Ar/39Ar analysis, data processing and interpretation. (AU)