Possible lateral flow in the Big Mantle Wedge of Northeast China revealed by the geochemistry of Cenozoic basalts

In eastern Asia, the subducting Pacific slab is stagnant in the mantle transition zone (MTZ), and the upper mantle above the slab is termed the Big Mantle Wedge (BMW). However, how the BMW runs and controls the distribution of the above volcanism remains poorly understood. To test possible convective patterns of the BMW, we compare the geochemical compositions of Cenozoic basalts from different localities in eastern Northeast China. These basalts exhibit broad ⁸⁷Sr/⁸⁶Sr (0.7037–0.7054) and ¹⁴³Nd/¹⁴⁴Nd (0.51250–0.51288) ratios and compose binary mixing trends on the plots of ⁸⁷Sr/⁸⁶Sr versus ¹⁴³Nd/¹⁴⁴Nd and selected elemental ratios (e.g., Rb/Nb, Nb/Th and MgO/SiO₂). Statistically, the compositions of basalts strongly correlate with their spatial distributions. The closer to the Songliao basin with thinner lithosphere, the more depleted; the closer to the Changbaishan Volcano with thicker lithosphere, the more enriched. Furthermore, we simulate the decompressional melting process of a lithologically heterogeneous mantle, by using a previously-established software REEBOX PRO (Brown and Lesher, 2016). The results demonstrate that melting of the vertical-upwelling mantle within a limited depth range beneath eastern Northeast China cannot adequately account for the observed broad chemical variations unless the deep-sourced enriched melts have been partially removed prior to the upwelling mantle reaching a thinner lithosphere. Therefore, we conclude that vertical mantle upwelling beneath Changbaishan Volcano might change into lateral flows at the lithosphere-asthenosphere boundary, which could be responsible for the scattered Cenozoic volcanism. This attempt links geochemical variations in continental basalts to geodynamics of the BMW and provides an example to verify possible mantle convective patterns quantitatively for other areas in eastern Asia.