Petrogenesis of Mesozoic Adakitic Granites in AB3 ‐2 Buried Hill, Eastern Bohai Sea

The Bohai Sea, characterised by the most intense craton destruction and the thinnest crust within the North China Craton, is a key region for investigating such destruction processes. Nevertheless, Mesozoic magmatism in the Bohai Sea, though extensive, remains poorly constrained, hindering our understanding of deep material properties and the spatiotemporal heterogeneity of craton destruction. Here we present a systematic study of newly discovered Mesozoic granites in AB3‐2 Buried Hill, southern Bozhong Sag, using zircon U–Pb dating and whole‐rock major and trace element geochemistry. These granites yield zircon U–Pb isotopic ages of 125–123 Ma, representing the first reported Cretaceous intrusive body in the Bohai Sea. Integrated with the observation of amphibole and magnetite, they show low P 2 O 5 (0.11–0.12 wt.%) and Zr (64.4–75.8 ppm) and exhibit positive correlations of Y, Th with Rb, indicating an affinity with I‐type granite. Their differentiation indices (90.07–91.81) are significantly lower than those of highly fractionated granites. High Si contents, elevated Zr/Hf and K/Rb ratios, together with low Al, Fe, Ca and K/Ba ratios, suggest a low degree of fractional crystallisation. Meanwhile, they are enriched in large‐ion lithophile elements (e.g., Ba) and show negative anomalies in high‐field‐strength elements (e.g., Nb, Ta), resembling the characteristics of mature continental crust. Additionally, high Sr contents, elevated Sr/Y, La/Yb ratios, coupled with low Y, Yb, jointly point to an adakitic affinity. And low Mg (0.47–0.58 wt.%), Mg # (25–38) and Ni (6.16–7.17 ppm) contents agree well with experimental melts derived from eclogite (1–4 GPa), supporting an origin from thickened lower crust. Notably, these Cretaceous granites share similar adakitic geochemical compositions with Jurassic CD9‐1 granites (ca. 165 Ma) in the same sag, implying an analogous thickened lower crust source. In contrast to contemporaneous magmatism in Liaodong and Jiaodong Peninsula, it is suggested that significant lithospheric thinning beneath the Bohai Sea had not significantly occurred before the Early Cretaceous (ca. 125 Ma).