Abstract Cenozoic intraplate basalts are widespread above the Big Mantle Wedge (BMW) and its front in East Asia. While the mantle source lithology and redox-hydration state have been demonstrated to be crucial in the generation of basalts above the BMW, their nature and role in the basalts above the front of the BMW is poorly constrained. To address this, we report olivine compositions of the Quaternary Datong basalts. Datong basalts exhibit ocean island basalt-like trace-element compositions and depleted Sr–Nd isotopes with slightly enriched signatures (enriched mantle I, EMI) for tholeiitic basalts. Olivines of the Datong basalts show high Ni and Fe/Mn, and low Ca, Mn and Mn/Zn values, pointing to a pyroxenite source. Applying V and Ca partition coefficients between olivine and whole rock, respectively, the Datong basalts lie −0.44 to 0.64 log units above the fayalite–magnetite–quartz buffer for f O 2 , and contain 2.1–3.4 wt% H 2 O but highly variable H 2 O/Ce values (265–1498). Both f O 2 and H 2 O/Ce in the basalts vary with whole-rock and olivine compositions, indicating that the source was the main control. Thus, there is a heterogeneous redox–hydration state in the source, the EMI component being relatively reduced but extremely wet and recycled oceanic crust being relatively oxidized but dry. The extremely wet EMI component was probably derived from the mantle transition zone. In the light of our findings, we propose a model in which mantle upwelling carried the recycled oceanic crust and EMI component from the mantle transitional zone to shallow mantle, owing to the Pacific slab stagnating in the mantle transitional zone, to form pyroxenite, which subsequently melted to generate Datong basalts.

Geophysical, geological, and petrologic data in southwestern China have been integrated in order to characterize magmatic underplating associated with the Late Permian Emeishan large igneous province (LIP; ca. 260 Ma). Seismic reflection and refraction reveals a heterogeneous crustal structure with high-velocity layers or bodies in the upper crust (6.0–6.6 km/s), lower crust (7.1–7.8 km/s), and upper mantle (8.3–8.6 km/s). These seismically anomalous bodies are all confined in the inner zone of the prevolcanic domal structure, but are generally absent in the intermediate and outer zones. There is a decreasing trend in crustal thickness from the inner zone (>60 km, with a ∼20-km-thick high-velocity lower crust, or HVLC) via the intermediate zone (∼45 km) to the outer zone (<40 km). Because the domal uplift immediately preceding eruption of the Emeishan basalts was unambiguously related to a mantle plume, such a configuration highlights a genetic relationship between the formation of the high-velocity crust and the mantle plume that led to the eruption of the Emeishan basalts. It is proposed that the HVLC may have resulted from magmatic underplating associated with the Emeishan volcanism, whereby the fast mantle represents the residues left after extensive melt extraction from the plume head. Magmatic underplating can also account for the prolonged crustal uplift that formed the Chuandian “old land” in southwestern China. Petrologic modeling further suggests that the HVLC may represent fractionated cumulates from picritic melts and that the Emeishan basalts represent residual melts after polybaric fractionations. This relationship allows a reestimation of the volume of Emeishan magmas, which is as much as 3.8 × 10 6 km 3 , typical of plume-generated LIPs in the world.