Geochronology and geochemistry of Early Cretaceous bimodal volcanic rocks from Erguna Massif, NE China: evidence for the back-arc extension of the Mongol-Okhotsk orogenic belt

The Manzhouli-Xin Barag Right Banner area, located in the southwestern Erguna Massif, is characterized by the distribution of a large volume of Mesozoic volcanic rocks and many polymetallic deposits. However, the tectonic setting of this area during the Late Mesozoic remains controversial. In this study, zircon U-Pb ages range from 142.8 +/- 1.8 to 140.4 +/- 1.6 Ma for the alkali basalt from the Meiletu Formation and 140 +/- 0.9 Ma for alkaline rhyolites from the Baiyingaolao Formation, forming a bimodal volcanic rock assemblage. The initial Sr-87/Sr-86 ratios range from 0.705365 to 0.705674, and the epsilon(Nd)(t) values vary from + 0.55 to + 1.47. The alkaline rhyolites show epsilon(Hf)(t) values ranging from + 5.3 to + 8.6, corresponding to T-DM1 and T-DM2 ages in the range of 0.48-0.58 Ga and 0.64-0.86 Ga, respectively. Combined with the major and trace element compositions, the Meiletu alkali basalts are characterized by Nb enrichment (averaging 23.6 ppm) and were derived from partial melting of metasomatized subcontinental lithospheric mantle and underwent moderate fractional crystallization of minerals (e.g. olivine, clinopyroxene, apatite, and plagioclase). The alkaline rhyolites show A1-type granite affinity and originated from partial melting of the mafic lower crust accompanied by fractional crystallization. This suite of bimodal volcanic rocks was mainly dominated by the Mongol-Okhotsk tectonic regime, and the Erguna Massif was in a back-arc extensional setting triggered by slab rollback during the Early Cretaceous.