Multiple Subduction–Exhumation Cycles and Episodic Hydration–Metasomatism of an Oceanic Eclogite Block: Implications for Thermal Structure Evolution and Mass Transfer at the Subduction Interface

Abstract

The rarity of natural rocks that specifically record the physicochemical conditions of the subduction interface strongly hampers pertinent studies on metamorphic trajectories of high‐pressure (HP) rocks and related fluid–rock interactions. This study examines an oceanic eclogite block from the Huwan shear zone in the Hong'an orogen. Eclogites from different domains exhibit considerable differences in texture, mineral assemblage, and major‐element zoning patterns. Pseudosection modeling and geothermobarometry on representative eclogites yield distinct pressure–temperature ( P – T ) paths with divergent evolutionary vectors. Coupled Lu–Hf and Sm–Nd dating on an eclogite with three garnet generations yields a reverse age relationship, with the Lu–Hf date (273.7 ± 0.6 Ma) approximately constraining the growth age of the garnet rim given its higher weight fraction of Lu. Multiple age clusters of metamorphic zircons (∼353–237 Ma) are interpreted as reflecting the time of disparate HP metamorphic events. Combined with previous data, a long‐term yo‐yo‐like path is delineated in response to multiple subduction–exhumation cycles within the subduction interface. We attribute the discrepant P – T estimates and isotopic dates of eclogites to the formation of reaction rinds through episodic hydration–metasomatism, facilitated by externally derived aqueous, carbon‐bearing, and sulfur‐bearing fluids. The block records a higher geothermal gradient than other eclogites, attributed to the combined effects of subduction‐interface geotherms, exothermic hydration, and potential shear heating. Within the interface, reduced sulfur species (e.g., HS ⁻ ) in up‐dip migrating fluids can be effectively filtered by reprecipitating as pyrite through redox reactions with detached HP rocks, sometimes accompanied by carbonate dissolution and silica metasomatism.