Formation age and metasomatism of the sub-continental lithospheric mantle beneath southeast China: Sr-Nd-Hf-Os isotopes of Mingxi mantle xenoliths

This study presents comprehensive geochemical data of spinel- and garnet-facies mantle peridotites entrained in the Cenozoic basalts from Mingxi, southeastern China. Whole rock major elements suggest that most Mingxi peridotites have been subjected to polybaric melting, i.e., <15% for the lherzolites and ∼15-25% for the harzburgites. Presence of amphibole/phlogopite in a few samples indicates the local occurrence of modal metasomatism, whereas mineral chemistry suggests that most Mingxi peridotites have experienced cryptic metasomatism by various agents, including carbonatitic melts, silicate melts and Fe-Ti-rich melts. Clinopyroxenes in the harzburgites have 87Sr/86Sr and 143Nd/144Nd ratios of 0.703121-0.704557 and 0.512793-0.513106 (εNd of +3 ∼ +9), which are more enriched than those in the lherzolites (87Sr/86Sr of 0.702238-0.703898, 143Nd/144Nd of 0.512918-0.514763, and εNd of +5 ∼ +42). The 176Hf/177Hf ratios of clinopyroxenes in harzburgites are comparable to the lherzolites, i.e., 0.282888-0.285485 (εHf of +4 ∼ +96) and 0.283259-0.285791 (εHf of +15 ∼ +106), respectively. Neither Sm-Nd nor Lu-Hf isotopes yielded meaningful isochron ages owing to the metasomatic overprinting. Clinopyroxenes in two samples are characterized by extremely radiogenic 176Hf/177Hf ratios in association with less extreme 143Nd/144Nd ratios, resulting in a strong Nd-Hf decoupling relative to the mantle array. Whole rock Os isotopes of the Mingxi peridotites range from 0.11685 to 0.13035, yielding Re depletion (TRD) ages no older than 1.79 Ga, and do not show a correlation with Re/Os ratios. In contrast, they display a roughly positive correlation with bulk rock Al2O3 contents, which can be explained by mixing of ancient mantle with fertile convecting upper mantle. Therefore, we suggest that the SCLM beneath Mingxi is juvenile and recently accreted from the convective mantle. The Mingxi peridotites with old Os ages reflect the ancient mantle in the convecting upper mantle, and have no relationship with the overlying old crust with ages of the Late Paleoproterozoic. The newly formed SCLM beneath Mingxi has been metasomatized by fluids/melts released from the subducted paleo-Pacific Ocean, which was initiated since the Mid-Permian.