New high-precision U-Pb zircon age of the Irati Formation (Paraná Basin) and implications for the timing of the Kungurian anoxic events recorded in southern Gondwana

Abstract

The early Permian Irati Formation is well known for its black shale oil and gas reserves and its widespread lateral correlation throughout the entire Paraná Basin. This unit records an effective water circulation restriction between the Whitehill-Irati Sea and the Panthalassa Ocean, which along with other factors, controlled the organic-rich shale deposition in several southern basins of Gondwana. The Permian was also a period of intense magmatism in this region due to the Terra Australis Orogen (Gondwanides Orogeny). Several volcanoclastic horizons are preserved within the sedimentary record, and in this study, a high-precision chemical abrasion isotope dilution U-Pb zircon age of 275.75 ± 0.29 Ma for a volcanic tuff in the Irati Formation of the southern Paraná Basin is presented. The Kungurian age agrees with previous published radioisitopic and relative ages for the Assistência Member (upper Irati Formation) and Mangrullo Formation, a stratigraphical correlated unit in Uruguay. In addition, zircon trace elements and whole-rock geochemistry allow the constraining of the magmatic characteristics of the volcanic source and its similarity with the lower Choiyoi magmatism (Argentina). Therefore, this study contributes to the understanding of the timing of black shale-bearing rocks of the Irati Formation and the anoxic events recorded in the southern Paraná Basin.