Origin of Late Cenozoic Basaltic Magmatism in Inner Mongolia, NE China: Constraints from Sr–Nd–Hf–Pb–Mo–He Isotopes

This paper presents a study of the late Cenozoic Chifeng basalts (CBs) of NE China, including their olivine He isotopic compositions, whole-rock major- and trace-element contents, and whole-rock Sr-Nd-Hf-Pb-Mo isotopic compositions, with the aim of constraining their mantle source. Results show that the basalts have high MgO, low CaO contents, and high FeOT/MnO values, which indicate that their mantle lithology was most likely pyroxenite. The CBs also exhibit ocean-island-basalt-like trace-element patterns (e.g., enrichment in light rare earth elements and high-field-strength elements) and have depleted Sr-Nd-Hf and relatively radiogenic Pb isotopic compositions, requiring both depleted and enriched components in their mantle. The low olivine He (3He/4He = 0.8-5.5 Ra) and whole-rock Mo (delta 98/95Mo = -0.71 parts per thousand to -0.18 parts per thousand) isotopic values of the CBs, together with geophysical evidence, indicate that the rocks were derived from a depleted MORB mantle (DMM) enriched by recycled oceanic crust that was sourced from the mantle transition zone (MTZ). During the late Cenozoic, ascending wet mantle plumes triggered by dehydration of a stagnant Pacific oceanic slab are inferred to have transported preexisting recycled Pacific oceanic crust from the MTZ into the overlying asthenosphere mantle. The upwelling Pacific oceanic crust reacted with asthenospheric mantle peridotite (i.e., DMM) to produce mantle pyroxenite, whose partial melting at shallow depths generated the CBs. Considering the low delta 98/95Mo values of both the CBs and coeval potassic basalts from NE China, we speculate that there may be a low delta 98/95Mo reservoir in the MTZ beneath NE China. The Chifeng basalts (CBs) exhibit relatively low 3He/4He ratios ranging from 0.8 to 5.5 RaThe mantle source end-members of CBs were the depleted mid-ocean-ridge basalt mantle and recycled oceanic crustA low delta 98/95Mo reservoir is inferred in the mantle transition zone beneath NE China