Petrogenesis of rodingites along the northern margin of the North China Craton: Constraints from zirconology and whole-rock geochemistry

Rodingite is a metasomatic product formed from various rock types by aqueous fluids, providing a direct record of lithosphere-hydrosphere interactions and serving as a key carrier of surface water into Earth's deep interior（water content >1.5wt.%）. Determining the properties of metasomatic fluids is crucial for understanding the petrogenesis of rodingites. In this study,we conducted a detailed study of mineral inclusions, U-Pb ages, trace elements, and Hf-O isotopes of zircon, as well as wholerock geochemical and Sr-Nd-Hf isotopic data on rodingites and associated felsic alkaline veins/dikes along the northern margin of the North China Craton（NMNCC）, with aims to elucidate their formation ages, petrogenesis, and geodynamic settings. The results show that zircons from syenite and albite syenite veins/dikes exhibit clear oscillatory zoning, a wide range of Th/U ratios（0.0001–4.073）, low concentrations of rare-earth elements（REEs）, and low crystallization temperatures（<600°C）. These characteristics suggest that the zircons crystallized from fluids separated from late-stage melts during magma evolution. Consequently, the zircon U-Pb ages obtained through Secondary Ion Mass Spectrometry（SIMS） and Laser Ablation Inductively Coupled Plasma Mass Spectrometry（LA-ICPMS） analyses（389.1±3.5 to 390.2±1.6 Ma） record the timing of hydrothermal fluid activity. Zircons within the rodingites contain grandite, hematite, and aqueous fluid inclusions, indicating their formation during rodingitization, with an oxidizing, water-rich fluid acting as the metasomatic agent. SIMS and LA-ICPMS analyses of these zircons yielded U-Pb ages of 388.1±3.9 and 388.6±4.6 Ma, which register the formation ages of the rodingites. Both the syenite and albite syenite samples display high total alkali contents（9.9–13.9 wt.%）, consistent initial Sr isotopic ratios（0.7047–0.7051）,negative εNd（t）(-10.1 to-8.5) and εHf（t）(-13.9 to-13.7) values, and slightly elevated zircon δ18O values（6.20‰±0.18‰）compared to mantle zircon. These characteristics align with those of the contemporaneous alkaline complexes in the NMNCC,suggesting that the syenite and albite syenite veins/dikes originated from late-stage melts produced by regional alkaline magmatism. The rodingites are characterized by high Ca and low Si contents, as well as enrichment in total alkalis（Na2O+K2O） and large ion lithophile elements（Ba, K, and Sr）. They display homogeneous initial Sr isotopic ratios（0.7046–0.7047）, negative εNd（t）(-6.8 to-6.1) and εHf（t）(-9.8 to-8.3) values, and slightly elevated zircon δ18O values（5.79‰±0.10‰ to 5.91‰±0.18‰）. These geochemical features are intermediate between those of the protoliths and the metasomatic fluids, as represented by pyroxenitehornblendite and syenite-albite syenite, respectively. Considering the regional geological evidence, we proposed that the rodingites in the NMNCC formed through the interaction of oxidizing, aqueous fluids derived from regional alkaline magmas with ultramafic rocks in an extensional setting. This formation process differs from the commonly observed rodingites in ophiolite suites, where reducing fluids produced by serpentinization or carbonate fluids generated by the dehydration of subducted sediments are the predominant metasomatic agents. Therefore, the rodingites in the NMNCC may represent a previously unidentified type of rodingite.