Melt‐rock interaction between ascending melt and peridotite results in mantle metasomatism and also leads to compositional modification of the primary melt. While this process is known to occur, it is less well understood how the reactions and the composition of the resulting magma temporally evolve. Here whole‐rock major and trace element, Sr‐Nd‐Pb‐Hf isotopes, and olivine major element composition of Quaternary Nuominhe basalts in the Greater Khingan Range of northeast China are presented to unravel how melt‐rock interaction modified the composition of the high‐MgO potassic basalts as time progressed. The Nuominhe basalts are predominantly basanite with high MgO (8.1–16.8 wt %) and high total alkali content (K2O + Na2O = 6.0–9.2 wt %). They have high K2O/Na2O ratios (K2O/Na2O = 0.77–1.24) and low SiO2 and Al2O3 content (SiO2 = 44.4–48.7 wt %, Al2O3 = 10.5–13.2 wt %). They are characterized by enrichment in strongly incompatible elements, positive Ba, K, and Sr and negative Th, U, Zr, Hf, and Ti anomalies, similar to the composition of enriched mantle (EM1)‐type oceanic island basalts (OIBs). Their isotopic composition also compares to that of EM1‐type OIBs (i.e., with 87Sr/86Sr = 0.70467–0.70483, εNd = −4.1 to −1.5, εHf = −0.3 to 2.3, 206Pb/204Pb = 17.03–17.36). These elemental and isotopic characteristics are consistent with the interpretation that the potassium‐rich melts were derived from recycled crustal materials with EM1 signature. Phlogopite‐bearing mantle xenoliths and zoned olivine xenocrysts with high Fo89–92 and low CaO (<0.1 wt %) core and low Fo75–86 and high CaO (>0.1 wt %) rim composition record interaction between the ascending melt and mantle peridotite. Basalts erupted during late stages (Late Pleistocene and Holocene) of activity at the Nuominhe volcanic field show notably higher SiO2 content, Rb/Nb, Ba/Nb, K/La, and Ba/La, and lower MgO content than early‐stage basalts (Early and Middle Pleistocene), which we infer to reflect a temporally decreasing extent of melt‐rock interaction. During early stages of melt ascent, a reaction zone between melt channels and unreacted peridotite formed; at later stages this reaction zone effectively sealed the ascending melt from further reaction, resulting in increasing Rb/Nb, Ba/Nb, K/La, and Ba/La signatures of the erupted lavas.

In addition to magma‐rock and rock‐rock reaction, magma‐magma interaction at mantle depth has recently been proposed as an alternative mechanism to produce the compositional diversity of intraplate basalts. However, up to now no compelling geochemical evidence supports this novel hypothesis. Here we present geochemistry for the Longhai basalts from Fujian Province, southeastern China, which demonstrates the interaction between two types of magma at mantle depth. At Longhai, the basalts form two groups, low‐Ti basalts (TiO2/MgO < 0.25) and high‐Ti basalts (TiO2/MgO > 0.25). Calculated primary compositions of the low‐Ti basalts have compositions close to L + Opx + Cpx + Grt cotectic, and they also have low CaO contents (7.1–8.1 wt %), suggesting a mainly pyroxenite source. Correlations of Ti/Gd and Zr/Hf with the Sm/Yb ratios, however, record binary mixing between the pyroxenite‐derived melt and a second, subordinate source‐derived melt. Melts from this second source component have low Ti/Gd and high Zr/Hf and Ca/Al ratios, thus likely representing a carbonated component. The Sr, Nd, Hf, and Pb isotopic compositions of the high‐Ti basalts are close to the low‐Ti basalts. The Sm/Yb ratio of the high‐Ti basalts, however, is markedly elevated and characterized by crossing rare earth element patterns at Ho, suggesting that they have source components comparable to the low‐Ti basalts, but that they have experienced garnet and clinopyroxene fractionation. We posit that mingling of SiO2‐saturated tholeiitic magma with SiO2‐undersaturated alkaline magma might trigger such fractionation. Therefore, the model of magma‐magma interaction and associated deep evolution of magma in the mantle is proposed to explain the formation of Longhai basalts. It may, moreover, serve as a conceptual model for the formation of tholeiitic to alkaline intraplate basalts worldwide.

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Eastern China has experienced widespread and voluminous, basaltic to andesitic intraplate magmatism in the Cenozoic. Seismic tomography and kinematic reconstructions show that magmas have erupted above the stagnant Pacific plate, which currently extends within the mantle transition zone for >1500 km beyond the active arc. Seismic studies also show in intriguing detail that low‐velocity wave anomalies reach from the mantle transition zone or below to major volcanic centers, suggesting an intimate relation between the magmatism and components derived from the mantle‐transition zone and possibly from deeper levels. Here, we provide new petrological, mineral chemical, and whole‐rock geochemical data showing that Cenozoic basaltic andesitic and andesitic magmas that have erupted above the present‐day edge of the subducted Pacific plate are largely unfractionated, ≥1130‐1160±15‐50 °C hot, mantle‐derived melts that were volatile‐poor (undegassed melt CO2 and H2O of ~2400‐3100±500 ppm and ~0.5‐0.6±0.4‐1.1 wt%), and relatively oxidized (~FMQ to ~FMQ+1.3±1.0). We further infer that the magmas were derived by partial melting of oxidized, H2O‐ and CO2‐poor eclogite‐rich sources (~FMQ‐1.0 to ~FMQ). We use this to suggest that the low‐velocity seismic anomalies below Eastern China largely reflect the presence of oxidized and enriched, hot and not necessarily particularly hydrous mantle domains as common interpretation has suggested. This is crucial information for quantifying material flow above and around the subducted Pacific plate and for constraining residence times of subducted components in Eastern Asia and globally.

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