Glacial Isostatic Adjustment–Driven Hydrothermal Activity Recorded by Sheet-Crack Cements in Marinoan Cap Dolostones, South China: Fluid-Inclusion Constraints

Glacial isostatic adjustment (GIA) profoundly influences postglacial tectonics, sea-level change, and geothermal systems; however, its role during the deglaciation of the Neoproterozoic Snowball Earth remains poorly constrained. Sheet-crack is one of the diagnostic structures in Marinoan (635 Ma) cap carbonates that overlie glacial tillites. Available geochemical evidence indicates that sheet-crack cements precipitated immediately following crack formation, potentially preserving GIA-driven fluid signatures. This study presents fluid inclusion microthermometric data from sheet-crack quartz and barite cements along shelf-to-slope transects on the Yangtze Block, South China. Fluid inclusion homogenization temperatures (Th) reveal the occurrence of a ubiquitous low-temperature hydrothermal event, with most Th values clustering at ∼145 °C–190 °C and ∼120 °C–180 °C in platform facies and slope facies, respectively. Fluid salinities in platform facies exhibit unimodal distributions and low to moderate (∼2–8 wt % NaCl eq.), while those in slope facies show broadly continuous distributions, with most values concentrated at intermediate-to-high salinities (∼6–15 wt % NaCl eq.) and a subordinate low-salinity tail. These patterns are consistent with varying contributions from recirculated, geothermally heated seawater and basinal brines. We propose a refined model in which GIA-related geothermal anomalies promoted a widespread low-temperature hydrothermal circulation event, responsible for sheet-crack cementation shortly after the Marinoan deglaciation, South China.