Carbonated Big Mantle Wedge Extending to the NE Edge of the Stagnant Pacific Slab: Constraints from Late Mesozoic-Cenozoic Basalts from Far Eastern Russia

It has been suggested that the carbonated mantle reflected by Mg-Zn isotopic anomalies of Cenozoic intraplate basalts from East Asia coincides with the stagnant West Pacific slab in the mantle transition zone. However, the northern boundary of such carbonated domain beneath East Asia is uncertain. Late Mesozoic-Cenozoic intraplate basalts are widespread in far eastern Russia and thus provide an opportunity to examine this issue. Here we report major-trace element contents and Sr-Nd--Mg-Zn isotopic compositions for 9 Late Mesozoic-Cenozoic basaltic samples from the Khanka Block and Sikhote-Alin accretionary complex. They are characterized by large variations in SiO2 contents (41 wt.% to 50 wt.%) and CaO/Al2O3 (0.50 to 0.97), enrichments of large-ion lithophile elements (LILE), positive Nb-Ta anomalies and strongly negative K, Pb, Zr, Hf, Ti, Y anomalies in primitive mantle-normalized trace element spider diagram. Furthermore, the rocks show good correlations of Ti/Ti* with Hf/Hf*, La/Yb, Fe/Mn and trace element contents (e. g., Nb). In addition, they have lighter Mg and heavier Zn isotope compositions than the BSE estimates, coupled with depleted Sr-Nd isotope compositions. These elemental and isotopic characteristics cannot be explained by alteration, magma differentiation or diffusion, but are consistent with the partial melting of carbonated peridotite. By and large, the Late Mesozoic-Cenozoic basalts from far eastern Russia bear very similar geochemical characteristics as those Na-series Cenozoic basalts from eastern China. The extended region of Mg-Zn isotopic anomalies is roughly coincident with the stagnant West Pacific slab beneath East Asia, and all of these alkali basalts can be generated from mantle sources hybridized by recycled Mg-carbonates from the Pacific slab stagnant in the mantle transition zone. We infer that (1) the carbonated big mantle wedge extends to the NE edge of the West Pacific slab and may have also appeared in the Late Mesozoic due to the effect of the Paleo-Pacific slab beneath this region, and (2) decarbonation of stagnant slabs in the mantle transition zone is a key mechanism for carbon outgassing from deep mantle to surface via intraplate alkali melts.