Li and O isotopes of mantle xenoliths from deep fault-related Cenozoic basalts in eastern China: The role of subducted components in the generation of the heterogeneous lithospheric mantle

Abstract

It has been known that the lithospheric mantle underneath eastern China is geochemically heterogeneous, with different isotopic compositions across the South China and North China blocks. The distribution of Cenozoic basalts (<23 Ma) with abundant mantle xenoliths in eastern China is mainly controlled by lithospheric-scale faults. In this paper, we report new Li and O isotopic data for these mantle xenoliths and demonstrate that the mantle was variably modified via the Pacific subduction. Most olivine and orthopyroxene grains from the peridotite xenoliths in the South China block display lower δ18O values (4.43 to 5.77‰ and 4.77 to 5.55‰; respectively) than those of normal mantle, possibly reflecting impregnation of the slab melt of altered gabbroic precursors in the mantle transition zone (MTZ). The positive correlation between Li (1.40–6.54 ppm) and δ7Li (−28.41 to 20.83‰) of olivine grains suggests that the SCLM was metasomatized by hydrated slab-derived, isotopically heavier Li melts penetrating the frontal mantle wedge. The decoupling of Li and O isotopic systems implies that both hydrated mantle wedge and MTZ control the mass transportation and melt/fluid flow circulation beneath South China via the Pacific subduction. Comparatively, the higher δ18O values (4.90–6.41‰) and negative correlations between Li (1.39–6.02 ppm) and δ7Li (−23.55 to 22.54‰) in olivine grains from the North China block suggest a major contribution from the eclogitic slab that was metasomatized by the carbonatitic agent. Our findings corroborate the wide storage of carbonated eclogites in the MTZ beneath North China. We highlight that the stagnant slab in the MTZ might have controlled the refertilization of the lithospheric mantle beneath North China.