Molybdenum isotopic evidence for the initiation of a big mantle wedge beneath eastern Asia

The subduction of the Paleo-Pacific plate and the formation of a big mantle wedge have influenced the tectonic-magmatic evolution of eastern Asia. However, the timing and mechanism of big mantle wedge formation remain obscure. Here we report molybdenum (Mo) isotope compositions of Mesozoic-Cenozoic mafic rocks in eastern China, which provide constraints on the geodynamics of Paleo-Pacific subduction. The >125 Ma rocks have high δ⁹⁸Mo values of −0.27‰ to 0.17‰ and arc-like features, whereas the <125 Ma rocks have low δ⁹⁸Mo values of −0.67‰ to −0.04‰ and ocean-island basalt (OIB)-like features. The mantle sources of these two types of rocks contain subducted Paleo-Pacific slab components at sub-arc and mantle transition zone depths, respectively. Therefore, the Paleo-Pacific subduction involves shallow subduction yielding a small mantle wedge in the early stage and deep subduction yielding a big mantle wedge in the late stage. The dramatic change in Mo isotopes reveals that the big mantle wedge beneath eastern Asia was initiated at ∼125 Ma.