Application of neutron activation analysis to the study of low-titanium tholeiitic dykes of the Serra do Mar Swarm (RJ)

Abstract

The tholeiitic dykes of the Serra do Mar Swarm (SMS) outcrop along the coast of the São Paulo and Rio de Janeiro states (Southeastern Brazil), spreading over the Rio Paraiba Valley. Previous studies carried out in such dykes showed that they are related to the tectonic and magmatic processes that took place during the South Atlantic Ocean opening and gave rise to the Paraná Magmatic Province (PMP). The dykes are vertical to subvertical and preferentially NE-SW trending, with thicknesses ranging from centimeters to tens of meters. Most of the SMS intrusive rocks have relatively high concentrations of TiO2 (>3wt%), except for occurrences in the Região dos Lagos (Costa Azul Suite), as well as around Nova Friburgo Town (Serrana Suite), where they are characterized by basic rocks with low TiO2 (<2wt%) contents, whose genesis are still not completely understood. Therefore, a detailed study is being carried out in rocks of the Suite Serrana, with the determination of the concentrations of the rare earth (REE: La, Ce, Nd, Sm, Eu, Tb, Yb, and Lu) and other trace (Ta, Th, U, Hf, Cs, Sc, and Co) elements by instrumental neutron (thermal and epithermal) activation analysis, which has accuracy and precision required for petrogenetic studies. The data obtained so far show a substantial enrichment of highly incompatible trace elements (light REE, Th and U) and a remarkable geochemical similarity with the low-TiO2 basaltic flows of Gramado type found in southern PMP, suggesting important processes of crustal contamination during the genesis of the investigated dykes.