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High-precision U-Pb zircon ages of 125.755 ± 0.061 Ma and 124.122 ± 0.048 Ma, respectively, represent the onset and termination of Yixian Formation in the Jin-Yang basin, bracketing its duration to 1.633 ± 0.078 Myr.

Our current knowledge regarding the distribution patterns of ancient and recent recycled materials, as well as the origin of intraplate basalts in the eastern Asia Big Mantle Wedge (BMW), is limited. To address this, we conducted a detailed geochemical analysis of olivine melt inclusions (OLMI) in nephelinite samples. The normal OLMIs detected in our investigation exhibit geochemical features that closely resemble those of the hosting nephelinite, indicating a consistent association with a carbonated mantle source. Additionally, we identified a distinct group of anomalous OLMIs that displayed different geochemical characteristics from the hosting nephelinite but showed similarities to regional alkali basalts sourced from pyroxenite. The observed geochemical diversity in the nephelinite OLMIs suggests an interaction between deeply derived carbonated melts originating from the flattened Pacific slab in the mantle transition zone and pyroxenite in the lithospheric mantle. Carbonated fluid‐fluxed melting is key to basalt formation in the BMW. Plain Language Summary: The Big Mantle Wedge (BMW) refers to the mantle regions situated between the flattened subducted slab in the mantle transition zone (MTZ) and the overlying plates. In order to better understand the distribution patterns of recycled materials and the origin of intraplate basalts within the Eastern Asia BMW, we have focused on studying the Cenozoic basalts from the eastern North China region. Initially, it was believed that these basalts were formed through decompression melting of the mantle caused by back‐arc extension associated with the subduction of the Pacific plate. However, recent studies have revealed unique characteristics of these basalts, such as high water content and isotopic values that differ from those of the depleted mantle. This suggests a possible connection between the basalts and decarbonation and dehydration processes occurring within the stagnant subducted slab in the MTZ. According to the proposed model, carbonated melts/fluids released from the stagnant slab interacted with the lithospheric mantle, resulting in the transformation of the melts and the involvement of pyroxenite in the lithospheric mantle. The pyroxenite forms through ancient melt metasomatism events. This interaction ultimately leads to the generation of basalts with a wide range of SiO2 content. Key Points: The olivine melt inclusions (OLMI) originate from either a carbonated mantle source or ancient pyroxenite in the lithosphereThe geochemical diversity in the OLMI reflects the interaction between deeply derived carbonated melts and pyroxeniteCarbonated fluid‐fluxed melting stands as a crucial mechanism in the formation of basalt within the Big Mantle Wedge [ABSTRACT FROM AUTHOR]

How eclogite/pyroxenite‐derived melts evolve through the refractory lithosphere above a plume remains poorly understood. Here we conducted layered experiments of reaction between tholeiitic melts and harzburgite at 2–3 GPa, 1,400–1,500°C, with a run duration ranging from 2 to 24 hr. The resulting residual melts exhibit lower SiO2, TiO2, Al2O3, FeO, CaO, and total alkali contents, higher Ni, MgO, and Mg#, and almost constant CaO/Al2O3 compared to the initial tholeiitic melts. The compositions of the residual melts are influenced by factors such as the melt/harzburgite mass ratio, temperature, and run duration. Decreasing the melt/rock ratio or increasing temperature and run duration leads to a greater extent of assimilation. Under disequilibrated conditions (2 hr), the residual melts have higher SiO2, FeO, and MgO, and lower CaO, Al2O3, and total alkali contents compared to those under equilibrated conditions. The results suggest that interface reactions involving olivine dissolution and orthopyroxene precipitation, and chemical diffusion occur simultaneously during the interaction process. The compositions of the residual melts are largely controlled by interface reactions within 2 hr, followed by dominant chemical diffusion between the melts and refertilized harzburgite from 2 to 24 hr. Based on the experimental results, we propose a two‐stage model for the origin of Hawaiian shield stage parental magmas. Eclogite/pyroxenite‐derived tholeiitic melts first react with harzburgite, with varying melt/rock ratios, to produce residual melts in the deep lithosphere. These residual melts subsequently mix with plume peridotite‐derived melts at shallow depths, contributing to the geochemical diversity observed in Hawaiian shield stage lavas. Plain Language Summary: We conducted experiments to understand how melts derived from subducted oceanic crust interact with surrounding rocks during their journey through the Earth's lithosphere. By simulating melt‐rock reaction along wall‐rock boundary under high‐pressure and high‐temperature conditions, we found that the interaction mechanism involves olivine dissolution and orthopyroxene precipitation. The extent of interaction between the melts and rocks depends on factors such as the amount of melt present and the duration of the interaction. We also observed that certain chemical reactions took place at the interface between the melts and rocks, which affect the composition of the resulting melts. Our findings suggest that the formation of magmas with specific chemical compositions, similar to those observed in Hawaiian shield stage lavas, can be elucidated by the reaction between tholeiitic melts derived from eclogite/pyroxenite and harzburgite. The harzburgite was generated through prior high‐degree melting of peridotite influenced by the high temperatures in the plume setting. We propose a two‐stage model involving initial reactions in the deep lithosphere followed by mixing of the melts in shallower depths. This model provides insights into the processes that contribute to the diversity of magmas observed in Hawaiian volcanic islands. Key Points: Eclogite melts reacting with harzburgite produce high SiO2, MgO, and Ni tholeiitic meltsResidual melt composition is influenced by melt size and reaction time, involving interface reactions and diffusionEclogite melt interaction with harzburgite, followed by mixing with peridotite melts, accounts for Hawaiian shield stage lava generation [ABSTRACT FROM AUTHOR]