Early Cretaceous diabases, lamprophyres and andesites-dacites in western Shandong, North China Craton: Implications for local delamination and Paleo-Pacific slab rollback

Early Cretaceous tectono-magmatic activity in the eastern North China Craton has long been linked to cratonic destruction, but no systematic investigations about the geodynamic processes have been performed. Early Cretaceous diabases, lamprophyres, andesites and dacites are distributed in the western Shandong Peninsula, and these rocks have a close relationship as they formed synchronously. Zircon U-Pb dating results show that the diabases and lamprophyres formed dykes or sills at 133-127 Ma and that the andesites erupted at similar to 125 Ma. The diabases could be divided into high- and low-Mg# value groups. The high-Mg# (> 60) diabases have lower SiO2 (48.44-54.68 wt%) and higher MgO (7.41-19.30 wt%) contents than the low-Mg# (< 60) ones (SiO2 : 52.90-56.41 wt%; MgO: 2.70-4.37 wt%). The lamprophyres (Mg# values of 58-70) exhibit low SiO2 (45.04-49.99 wt%) and moderate MgO (4.77-9.87 wt%) contents. The diabases and lamprophyres are enriched in large ion lithophile elements (e.g., Ba, Th and U) and light rare earth elements, are depleted in Nb and Ta, and feature negligible Eu anomalies. They have lower zircon epsilon(Hf)(t) values (- 12 to -19) and whole-rock epsilon(Nd)(t) values (2 to -12), with (Sr-87/Sr-86)(i) ratios of 0.7066-0.7097. The andesites and dacites have intermediate SiO2 contents (60.10-64.95 wt%), high Mg# values (56-65), high Sr (1005-1300 ppm), and low Y (11.3-12.9 ppm) contents, with high Sr/Y ratios (78-115), showing adakitic affinity. They have lower zircon e(Hf)(t) (-12 to - 26) and whole-rock epsilon(Nd)(t) (- 9 to -13) values and high (Sr-87/Sr-86)(i) ratios (0.7109-0.7111). We suggest that the diabases and lamprophyres were derived from the partial melting of enriched lithospheric mantle, and the parental magmas of some of them had more contribution from the asthenosphere. The andesites and dacites were derived from the partial melting of delaminated thickened lower crust with addition of mantle material. Since the late Mesozoic, local delamination may have occurred beneath the Shandong Peninsula. During the Early Cretaceous, the rollback process of the Paleo-Pacific slab beneath the Shandong Peninsula could have facilitated delamination of the lithospheric mantle, and enhances the activity of trans-lithospheric faults.