Lateral transport of mafic sediment for the formation of analcime-bearing dolostones in a middle Eocene saline lake, Bohai Bay Basin, eastern China

Over the past decade, analcime-bearing dolostones and similar deposits have been increasingly documented and often interpreted as being related to mantle-derived hydrothermal activity. However, the universality of this association remains uncertain. In this study, the composition of sediment source areas and the direction of sediment transport in middle Eocene saline lake deposits of the Bohai Bay Basin, eastern China, were investigated to better understand the formation mechanism of such dolostones. Geochemical evidence from Th/Sc–La/Sc, Th/Sc–Cr/Th, and La/Th–Hf diagrams, low Th/Co ratios (0.26–1.07), and depletion of element Ta, together with lower 87Sr/86Sr ratios in both bulk rock (0.709408–0.709795) and carbonate fractions (0.708638–0.709006), collectively support the input of substantial mafic materials in the rock formation. Conversely, other element-based proxies, such as Al2O3/TiO2 ratios, K/Rb diagram, and REE characteristics, lack the sensitivity to detect the mafic signal. The relatively high contents of hematite and magnetite (20.2–33.1 wt.%) in the overlying sandstones, together with well-sorted, fine-grained siliciclastic grains exhibiting different roundness and distribution within the dolostone lithofacies suggest lateral transport of mafic materials. The MREE-enriched pattern and positive δ13C ratios (3.2–8.7‰) of the carbonate fraction and the absence of vertical anomalies of 87Sr/86Sr ratios, argue against direct vertical input from mantle-derived fluids. These findings suggest that lateral delivery of mafic materials was a favorable condition for the formation of analcime-bearing dolostones, whereas mantle-derived hydrothermal activity played a negligible role.