Tracing the origin of Permian volcanic ash fall layers in the Paraná Basin using zircon U-Pb ages, Lu-Hf isotopes, and trace element contents

Abstract

This study investigates the origin of Permian volcanic ash layers preserved in the sedimentary units of the Paraná Basin (PB), Brazil, and Uruguay, through the integration of U-Pb geochronology, Lu-Hf isotopes, and trace element data in zircon. The aim is to evaluate a possible genetic link between these ash layers and volcanic rocks of the Choiyoi Igneous Province (CIP) in western Argentina. A total of 183 zircon spots were analyzed - 129 from PB formations (Rio Bonito, Irati, Mangrullo, Estrada Nova, Teresina, and Rio do Rasto) and 54 from CIP units (El Imperial, Cerro Carrizalito, Agua de los Burros) - using previously dated grains by SHRIMP. Zircons from the main Permian PB units, e.g. Irati, Estrada Nova and Rio do Rasto Formation, show crystallization ages between 274 and 269 Ma, εHf_(t) values ranging from −6 to +4, and crustal model ages (T_DM^c) between 1400 and 1100 Ma. These values are compatible with those from intermediate and upper Choiyoi units, which show zircon ages between 264 and 251 Ma and similar isotopic signatures within uncertainties. Zircons from the Rio Bonito Formation mostly yielded older Carboniferous ages (304-302 Ma), suggesting a different volcanic source, with a minor Permian population (∼298-297 Ma). The El Imperial Formation includes most Ordovician zircons (483-450 Ma) and one younger Permian grain (∼297 Ma), possibly reflecting recycling of older crustal sources, such as the Famatinian or Ocloyic belts. Geochemical proxies (Th/U, Ce/Ce∗, Eu/Eu∗, Zr/Hf) and textures support a volcanic origin for the zircon crystals, which exhibit calc-alkaline compositional signatures and crystallization under oxidizing conditions, and lack evidence for abrasion owing to sedimentary transportation. The overlap in their crystallization age, isotopic composition, and trace elements between PB and CIP zircons points to a shared magmatic history. The data suggests that the CIP was likely a primary source of ash for the PB during the Permian, while older ash layers may be linked to earlier, unrecognized magmatic episodes.