Formation of analcime in Eocene fine-grained lacustrine sedimentary rocks from the Huanghua Depression, Bohai Bay Basin, eastern China

Sedimentary analcime holds dual significance as a proxy for paleoenvironmental reconstruction and a tracer for material transport processes; however, its genesis within fine-grained sedimentary successions remains a subject of considerable debate. The Eocene lower Shahejie Formation in the Huanghua Depression, Bohai Bay Basin of East China, serves as an ideal natural laboratory to address this issue, where analcime occurs in three modes: (1) disseminated matrix constituents; (2) ellipsoidal or patchy aggregates; and (3) pore-filling crystals within microfractures and vugs. The ubiquitous matrix analcime, characterized by a spherical morphology and low Si/Al ratios (mean: 2.17), formed through the interaction of weathering-derived mafic colloids with highly alkaline and saline lake waters resulting from solar evaporation under an arid paleoclimate. Ellipsoidal and patchy aggregates likely reflect the in-situ alteration of basaltic glass shards; however, the absence of thick tuffs and mercury anomalies, coupled with the consistent presence of fine-grained (~2–3 μm) matrix analcime, suggests that volcanic inputs were limited to short-lived, distal events. Pore-filling analcime, distinguished by high homogenization temperatures (140–266 °C), large euhedral crystals (>16 μm), and low 87Sr/86Sr ratios (0.709030–0.710234), probably originated from the circulation of hydrothermal fluids driven by local mafic intrusions. This hydrothermal circulation and alteration was spatially highly restricted, which is supported by the similarity in pyrolysis Tmax values between rocks adjacent to pore-filling analcime (mean: 430 °C) and whole rocks (mean: 434 °C). Additionally, the higher Si/Al ratios of pore-filling analcime (mean: 2.28) relative to matrix analcime (mean: 2.17) and the preservation of the subhedral to anhedral morphology of carbonate minerals confirm the localized extent of this hydrothermal overprint. This study demonstrates the complexity of analcime origin in sedimentary settings and provides an alternative framework for understanding the accumulation of widely distributed analcime in such deposits.