The Miocene Shuangyashan basalts in northeast China: Implications for the origin of Cenozoic basalts in northeast Asia

Cenozoic basalts are widespread in northeast Asia, and their origins have been accepted as the products of different mantle end-members. A combined study of elemental and isotopic geochemistry was carried out on Miocene basalt from the Shuangyashan Basin (Northeast China) to investigate the contributions of the various mantle end-members. The Shuangyashan basalts are alkali trachybasalt (SiO2 = 49.8-50.8 wt%, Na2O + K2O = 5.45-6.09 wt%) in composition, with micro-porphyritic texture. The phenocryst is euhedral olivine, and the matrix is composed of plagioclase, clinopyroxene, olivine, and ilmenite. The basalt shows slightly high MgO (6.80-7.22 wt%), Mg# (Mg2+ x 100/(Mg2+ + Fe2+), 63.0-64.8), and Ti/Y ratios (515-597) as well as extremely low Rb/Ba (0.08-0.10), Rb/Sr (0.04-0.06), and La/Nb (0.57-0.62) ratios. The elemental and isotopic geochemistry indicates that the basalt has experienced no or negligible crustal contamination. The small variations in basalt composition are controlled by fractional crystallization of olivine and clinopyroxene. The Sr (initial Sr-87/Sr-86 = 0.70507-0.70516), Nd (epsilon(Nd)(t) = 4.13-4.33), and Hf (epsilon(Hf)(t) = 9.24-12.1) isotopes show that the basalt has been affected by the EM2 (enriched mantle 2) in spinel peridotite lithospheric mantle. Meanwhile, the basalt shows relatively high REE fractionation (La/Yb = 14.9-16.6), suggesting it may be generated from the garnet peridotite. Therefore, the basalt could be originated from garnet peridotite in the lowest lithospheric mantle or asthenosphere and overprinted by the lithospheric EM2. Based on the compiled data from the literature, Sr-Nd isotope and trace element composition of the basalts from western northeast China, eastern northeast China, and Japanese islands are different from each other. Basalts from western northeast China are characterized by low Nd-143/Nd-144, high Sr-87/Sr-86 ratios, and extremely high La/Yb ratio (mean is about 71), indicating that their origin is an EM1 (enriched mantle 1) in garnet-bearing mantle. Basalts from eastern northeast China and Japanese islands show large variation of La/Yb ratios (1.2-34), showing that both have contributions from spinel and garnet peridotites. In eastern northeast China, basalts exhibit a roughly negative correlation between Sr-87/Sr-86 and La/Yb ratios. It is speculated that the asthenospheric mantle there is dominated by DM while the lithospheric mantle contains not only DM but also EM1 and EM2. In Japanese islands, (Sr-87/Sr-86)(i) ratios of basalts vary more greatly at low La/Yb (<10) than at high La/Yb ratios (>20), and high (Sr-87/Sr-86)(i) ratios (>0.7045) can only be observed at high La/Yb ratios (>20). Both the spinel and garnet peridotite mantle beneath Japanese islands have been significantly metasomatized by the subduction-related fluid/melt.