Influence of the subduction of the Pacific plate on the mantle characteristics of South China: Constraints from the temporal geochemical evolution of the Mesozoic basalts in the Jitai Basin

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The Pacific plate since the Mesozoic is subducting into the mantle beneath the eastern Asia, and the involved recycled crustal material has greatly affected the mantle geochemistry. However, the initial timing of the subduction remains controversial, partly due to the insufficient understanding of the geochemical evolution of the Mesozoic mantle. The present study provides new constraints on this aspect using whole-rock and olivine phenocryst compositions of Mesozoic basalts from the Jitai Basin of South China, which is part of the eastern Asian volcanic belt. Multiple OIB-type basalts outcrop in the Jitai Basin ranging in age from 168 Ma to 90 Ma. In comparison with normal mantle, these basalts exhibit higher (87Sr/86Sr)i, Ba/Th, Nb/La and lower εNd(t). Their olivine phenocrysts display high Ni contents and low Mn and Ca contents. Together, these indicators suggest that components of recycled oceanic crust were involved in the mantle source of the basalts. Moreover, combined with the observations from the Hebu basalts, from 168 Ma to 63 Ma, it shows that the component of the recycled oceanic crust was continuously supplied to the mantle source of the Jitai Basin basalts. Considering that the Jitai Basin basalts have whole-rock and olivine phenocryst compositions similar to those of the Cenozoic basalts from eastern China, which were originated from a stagnant Pacific plate in the mantle transition zone, we propose that the same origin may be the best candidate for the components of recycled oceanic crust in the mantle source of the Jitai Basin basalts. Finally, it is possible to estimate that the westward subduction of the Pacific plate probably started in an earlier stage.