A Transgressive Depositional Setting for the Paleogene Shahejie Formation in the Qikou Depression, Eastern China: Inferences from the REE Geochemistry of Carbonates

Rare earth element (REE) and Y concentrations, and ⁸⁷Sr/⁸⁶Sr ratios were analyzed in 33 carbonate samples from the Paleogene Shahejie Formation in the Qikou depression, eastern China, with the goal of determining depositional environments and post-depositional conditions of carbonates in the region. The REE and Y concentrations were normalized to the post-Archean Australian shale (PAAS) standard. The La*_PAAS/Yb*_PAAS ratios of 0.35–1.52, where *_PAAS indicates values for the PAAS standard, show light REE enrichment and heavy REE depletion in most samples. Values of La*_PAAS (0.775–1.284) and Ce*_PAAS (0.822–1.224), coupled with a relatively flat REE distribution, indicate that the Shahejie carbonates were deposited in lacustrine environments. Values of Y anomalies (1.009–1.527) and Y/Ho ratios (28.43–45.00) in the Shahejie Formation are greater than those of lacustrine carbonates and closer to those of marine carbonates, indicating that diagenetic fluids were probably influenced by seawater. In the carbonates from Well Kou-42, Eu anomalies (1.171–1.604), ⁸⁷Sr/⁸⁶Sr ratios (0.708 001–0.710 893), and high homogenization temperatures (104–151 ºC) suggest that the carbonates were affected by hydrothermal fluids. The REEs and Sr isotope ratios show that the carbonates from the Shahejie Formation in the Qikou depression were deposited in lacustrine environments, and were influenced by seawater and hydrothermal fluids.