Bulk rock and olivine chemistry and isotopes of 106-58 Ma basalts from Liaodong Peninsula and its adjacent areas: Implications for secular evolution of the big mantle wedge in eastern China

The subducting Pacific slab stagnates in the mantle transition zone and creates a big mantle wedge (BMW) system in East Asia. A similar BMW structure may have already existed since the Early Cretaceous (>120 Ma), but how such a structure evolved from Early Cretaceous to the present day remains unclear. We address this issue by comparing compositions and source heterogeneity of the 106-58 Ma basalts from Liaodong Peninsula and its adjacent areas (LPAA) in eastern China, with those formed in the modern BMW setting. The LPAA basalts display oceanic island basalts-like trace element patterns. Elemental and isotopic compositions of these basalts and their olivine phenocrysts point to peridotite and two recycled components in their source. One recycled component is altered lower oceanic crust given the low δ18Oolivine (2.8-5.2‰) of the ~99 Ma Liaoyuan alkali basalts. The second component consists of altered upper oceanic crust and pelagic sediments indicated by high δ18Oolivine (>6.0‰), represented by the ~58 Ma Luanshishanzi alkali basalts. The depleted mantle-like isotopes of these two components suggests derivation from a young HIMU source with characteristics of the Izanagi plate (e.g., Indian Ocean-type Sr-Nd-Pb-Hf isotopes), which may have resided in the mantle transition zone at that time. Our results reveal strong similarities between chemical and source characteristics of the mantle sampled by the 106-58 Ma LPAA basalts and those derived from the modern BMW. This implies that the BMW structure has been present since the Early Cretaceous, probably having lasted more than 120 Myr, and modulating the chemical properties of the upper mantle and influencing a variety of geological processes.