Sr–Nd isotopic constraints on carbonates, conodonts, and brachiopods of Early-Middle Pennsylvanian Itaituba and Nova Olinda formations, Amazonas Basin, Brazil

Abstract

Isotopic signatures presented in carbonates and fossils herein are one of the first attempts to obtain Sr and Nd data for the Early – Middle Pennsylvanian of the Itaituba epeiric sea and to evaluate the paleoseawater and paleotectonic of the Amazonas Basin. 87Sr/86Sr data range from 0.708330 ± 0.000018 to 0.708850 ± 0.000046, with ΔSW varying from −60 to −87 reflecting the marine sedimentation, while ΔSW varying from −33 to −51 suggest a post-depositional alteration. These values are in general agreement with the 87Sr/86Sr global seawater curve evolution for this time. The increasing influx of Sr from landmasses in a restricted marine basin, the Itaituba Epicontinental Sea, reflecting enhanced continental weathering during low stand sea level is a potential explanation for the Amazonas Basin during this time. Plotted in the εNd(t = 310 Ma) isotopic global seawater curve evolution data for the Itaituba and the Nova Olinda formations shift to lower values of εNd(t = 310 Ma), when compared to the Panthalassa Ocean. Nd isotopic signature suggests an incoming of seawater masses also from other oceans (e.g. Paleo-Tethys), instead of a unique Panthalassa Ocean provenance of water during the Pennsylvanian in the Amazonas Basin. 143Nd/144Nd isotopic values showed scattered 0.511608 ± 0.000077 to 0.512270 ± 0.000012. Nd-TDM around 1.5 Ga and εNd present-day values around −15 support an old continental Nd provenance to the ancient seawater, similar to those present in the rocks from Amazonian Craton.