A subduction-related metasomatically enriched mantle origin for the Luoboling and Zhongliao Cretaceous granitoids from South China: implications for magma evolution and Cu–Mo mineralization

<div><p>The Luoboling granodiorite porphyry and Zhongliao porphyritic biotite-granodiorite occur within the Zijinshan ore field in Fujian Province, southeast China. LA–ICP–MS zircon U–Pb dating yields ages of 103.1 ± 1.1 Ma (2σ, MSWD = 3.1) and 95.9 ± 0.6 Ma (2σ, MSWD = 1.2) for the Luoboling and Zhongliao intrusions, respectively. All rocks show high SiO₂, K₂O, and light rare earth element levels, variable CaO and Fe₂O₃^T, but low heavy rare earth element and high field strength element (Nb, Ta, Ti) concentrations. They also exhibit uniform initial ⁸⁷Sr/⁸⁶Sr ratios (0.7064–0.7068) and ε_Nd(t) values (−4.0 to −2.6), falling within the compositional field of Cretaceous basalts and mafic dikes in the Cathaysia Block. Together with their relatively high Hf isotopic ratios (ε_Hf(t) = −5.8 to +0.7), these data suggest that the Luoboling and Zhongliao intrusions were derived from a subduction-enriched mantle source. We infer fractional crystallization processes involving early fractionation of clinopyroxene and olivine, and subsequent fractionation of garnet forming the Luoboling granodiorite porphyry, eventually followed by amphibole- and biotite-dominated crystallization, with minor accessory mineral contribution, producing the (younger) Zhongliao porphyritic biotite-granodiorite. Asthenospheric mantle sources may have also contributed to generating the melts as indicated by initial Pb isotopic compositions and Hf isotopic compositions. The generation of these intrusions was associated with the Pacific subduction in an extensional setting during the Cretaceous.</p></div

Accurate Determination of Silver Isotopic Composition in Silicate Rocks with Low Silver Abundance

Owing to the extremely low abundance of silver in the Earth’s crust and mantle, it remains a major challenge to eliminate matrix impurities to obtain accurate silver isotopic compositions in silicate rocks. To determine precise and accurate silver isotopic data in silicates, we have modified the traditional pretreatment procedures, assessed isobaric interference and matrix effects, and analyzed silver isotopic compositions in silicates. By modification of the silicate digestion and ion-exchange procedures, efficient elution of silver was achieved. The doping experimental results indicated that the matrix effect induced by Ti and Cr could be satisfactorily corrected using the internal standard Pd isotope pair of 108Pd–106Pd. The modified chemical chromatographic method effectively separates Ni from Ag in silicate samples, thereby minimizing the significant isobaric interference from Ni cations. As a result, the shifts in the δ109Ag value caused by cations can be corrected to less than 0.02‰. There are considerable shifts down to −0.82‰ in δ109Ag from the accepted δ109Ag value when soluble metasilicate is present in the solution, which might explain the discrepancies in measured δ109Ag values for silicate materials. To accurately analyze silver isotopic compositions, especially of silicates with extremely low silver abundance, a silver standard doping method with an optimum doping proportion (sample-to-standard material ratio of 2:8) has been shown to produce an acceptable measurement uncertainty from 0.04 to 0.06‰ (2SD). The high-precision δ109Ag value determined in this study for ultramafic rocks from Balmuccia and the basalt reference material, BHVO-2, of −0.044 ± 0.062‰ is consistent with that of −0.16 ± 0.07‰ reported by previous studies. Our study paves the way for the more extensive use of silver isotopes in studies of terrestrial/extraterrestrial rocks, something that will be of help in constraining the sources of precious metals in polymetallic ore deposits as well as core formation and volatile-element depletion in the early solar system