Oligocene melting of subducted mélange and its mantle dynamics in northeast Asia

Melting of subducted mélange can potentially transport mass from the slab-mantle interface to the mantle wedge in subduction zones. The mélange diapir model was primarily proposed from the results of laboratory experiments and thermodynamic modeling. However, the melting mechanisms of mélange diapirs in subduction zones remain unclear. To further constrain the mantle dynamics of a mélange diapir, we studied Oligocene alkaline intermediate rocks on the northeast Asian continental margin. We report whole-rock geochemical and Sr-Nd-Pb-Mg-Zn isotope data and show that these rocks formed by partial melting of mélange. We conclude that a diapir was the mechanism for Oligocene melting of the mélange. We also identified younger rocks formed by melting of mélange in the eastern part of northeast Asia, implying an eastward shift in such magmatism since the Oligocene. Our results and the tectonic setting indicate that melting of mélange diapirs occurred preferentially during tectonic transitions, such as the formation of a back-arc basin triggered by trench-perpendicular mantle flow. The low-viscosity mantle with an incompressible stress field triggered melting of the mélange diapirs. Interactions occurred between the mélange diapirs and carbonated peridotites, constraining the depth of mélange-mantle interactions to the asthenosphere, which is deeper than the depth inferred in previous studies.