Hua-Wen Cao

The Yidun Terrane, located in the eastern Sanjiang Tethys domain of southwestern China, records extensive Late Cretaceous magmatism, yet its geodynamic setting remains controversial. Zircon U–Pb dating yields a crystallization age of 103.37 ± 0.51 Ma. The rocks are characterized by high SiO₂ contents (73.9–78.9 wt.%), ferroan compositions, predominantly high-K calc-alkaline and weakly peraluminous affinities, elevated 10,000 × Ga/Al values (2.81–3.29), and low P₂O₅ contents (0.03–0.04 wt.%). Integrated mineralogical and whole-rock geochemical evidence indicates an A-type affinity, while incompatible-element geochemical characteristics further suggest an A2 affinity. Their highly evolved compositions and depletion in Eu, Sr, Ba, and P are consistent with substantial fractional crystallization, although source composition and melting conditions may also have contributed to these features. Zircon εHf(t) values range from −1.1 to +0.5, with two-stage Hf model ages of 1.14–1.24 Ga, whereas whole-rock εNd(t) values range from −5.2 to −4.9, with two-stage Nd model ages of 1.30–1.32 Ga, indicating that the magma was derived predominantly from Mesoproterozoic crust with only limited juvenile input. Regional comparisons reveal substantial spatial differences among the Late Cretaceous plutons, but the available dataset is insufficient to establish a continuously sampled north–south evolutionary trend. Overall, the Queershan pluton most likely formed in a post-collisional extensional setting following the Lhasa–Qiangtang collision, and this process may have been further enhanced by slab break-off of the Bangong–Nujiang Ocean. In contrast, the younger magmatism in the southern Yidun Terrane is more readily reconciled with rollback-related back-arc extension associated with continued northward subduction of the Yarlung Zangbo Neo-Tethys.

The occurrence of CO ₂ fluid inclusions in fluorite from the Dadaogoumen deposit (North China Craton) provides critical insights into volatile dynamics in cratonic hydrothermal systems. Although carbonic inclusions are rare in most fluorite deposits, their enrichment indicates a major contribution from CO ₂ ‐bearing crustal fluids; however, the underlying processes remain poorly constrained. This study integrates fluid inclusion microthermometry with H‐O and Sm‐Nd isotope analyses to clarify the origin, evolution, and mineralization mechanisms of the hydrothermal fluids. Three distinct fluid inclusion types are identified: aqueous liquid‐rich, aqueous vapour‐rich, and H ₂ O‐CO ₂ ‐NaCl three‐phase inclusions, which define a two‐stage mineralization sequence. The early stage is characterized by high‐temperature (204°C–360°C) and moderate‐salinity (3.0–8.1 wt.% NaCl eqv.) fluids, whereas the late stage records lower temperatures (105°C–210°C) and salinities (0.3–4.7 wt.% NaCl eqv.). CO ₂ ‐bearing primary fluid inclusions likely played a key role in triggering early fluorite precipitation. The occurrence of CO ₂ ‐bearing inclusions in this vein‐type fluorite system underscores a complex interplay among multiple crustal volatile sources, potential magmatic inputs, hydrothermal circulation, and structural controls. H‐O isotopic compositions (δD = −121.4 to −98.2‰; δ ¹⁸ O = −9.62 to −4.19‰) plot entirely within the meteoric‐water field, indicating the dominance of meteoric fluids. Sm‐Nd isochron data, together with geological evidence, constrain the timing of fluorite mineralization to the Early Cretaceous, and εNd(t) values (−12.3 to −10.8) indicate derivation from ancient, evolved continental crust. The CO ₂ ‐bearing inclusions are interpreted to have originated from deeply circulating meteoric waters that interacted with carbonate‐bearing and organic‐rich crustal lithologies, releasing CO ₂ into the hydrothermal system and facilitating fluorite precipitation.