Rare earth element and Sr–Nd isotope geochemistry of phosphatic rocks in Neoproterozoic Ediacaran Doushantuo Formation in Zhangcunping section from western Hubei Province, South China

Abstract

Phosphatic rocks are widely distributed in Neoproterozoic Ediacaran Doushantuo Formation in Yangtze Platform, South China. In this study, we examined samples from the Zhangcunping section in western Hubei Province with major element, rare earth element (REE) and Sr–Nd isotopic analyses on the phosphorites and the host dolostone rocks. We employed an acid-leaching method to separate the carbonate (acetic acid leachable) and phosphate (hydrochloric acid leachable) fractions in all the samples. The geochemical differences between the different fractions can be recognized based on the P2O5, REE contents and Sr–Nd isotopic characteristics. The carbonate fractions in the rocks display low total REE concentrations with seawater-like REE patterns, super-chondritic Y/Ho ratios, moderate negative Ce anomalies and positive Eu anomalies, indicating these carbonates have retained the signatures of paleo-seawater. On the contrary, the phosphate fractions display high total REE concentrations with hat-shaped REE patterns, generally lower Y/Ho ratios, weak negative Ce anomalies and positive Eu anomalies, which may have recorded the features of diagenetic fluids. Negative Ce anomalies and positive Eu anomalies displayed in the carbonate fractions of the studied samples may also indicate the anoxic environments of ocean basins in South China during the deposition of the Neoproterozoic Ediacaran Doushantuo Formation. A geochemical comparison between Lower Phosphorite Layer and Upper Phosphorite Layer indicates a greater degree of diagenesis occurred in the Lower Phosphorite Layer than the Upper Phosphorite Layer. The negative εNd(t) values of the samples suggest continental crustal sources for the REEs, and the lower values in the phosphorite samples (−6.9 to −6.0) than the dolostone samples (−5.5 to −3.8) may suggest different processes for them, and it is likely that the REEs in the phosphorites have been modified by diagenetic fluids with REE contributions of pore fluids derived from the clastic silicates in the strata which generally have lower εNd(t) values, whereas the dolostone took the REEs from the ambient seawater mainly.