Geochemical Characteristics of Paleoproterozoic Granites in the Rizhao Area, Shandong Province: Implications for the Breakup of the Columbia Supercontinent

The Rizhao area in Shandong Province lies on the northeastern margin of the Yangtze block, south of the Tanlu fault zone. It is a key region for investigating the breakup processes of the Columbia supercontinent. Due to poor exposure of Paleoproterozoic granitic rocks in this area, the timing of their formation, source characteristics, and tectonic setting have not been adequately addressed. To elucidate the emplacement ages, petrogenesis, and geological background of these Paleoproterozoic granites, representative units, including meta-alkali feldspar granite, meta-monzogranite, meta-granodiorite, and meta-tonalite, are investigated in this study through petrographic, geochemical, zircon U-Pb geochronological, and Lu-Hf isotopic analyses. Geochemical results indicate that all granites in the study area belong to the subalkaline, high-K series and can be classified as A-, S-, and Ⅰ-type granites. LA-ICP-MS zircon U-Pb dating yields a weighted mean ²⁰⁷Pb/²⁰⁶Pb age of 1980 ± 18 Ma for the meta-alkali feldspar granite, indicating its emplacement during the late stage of the Columbia supercontinent breakup. Zircon Lu-Hf isotopic compositions show εHf(t) values of –10.5 to –7.3, T_DM ages of 2747–2613 Ma, and T_\mathrmDM^\mathrmC ages of 3753–3474 Ma, suggesting that the magma was produced by partial melting of the early Archean primitive continental crust. Combined tectonic discrimination diagrams and trace-element signatures reveal that the Paleoproterozoic granites in the region were formed in a post-collisional extensional setting, implying that the Rizhao area may represent one of the earliest sites where the breakup of the Columbia supercontinent was initiated. This study provides essential geochronological and geochemical constraints for reconstructing the position of the Yangtze block within the Columbia supercontinent and for understanding its rifting processes.