Miocene transition from an active continental margin to a trench–arc–basin system in Northeast Asia: Constraints from compositional variations of basaltic rocks in the Russian Far East

The timing and mantle processes of the Cenozoic transition from an active continental margin to a trench–arc–basin system in northeast Asia remain controversial. Here we present major and trace element, and Sr–Nd–Pb–Hf isotope data for Cenozoic basaltic rocks from the Russian Far East, which provide new constraints on the deep mantle processes during the transition. The basaltic rocks formed mainly in four stages. The results show that middle Eocene basaltic rocks have arc-like geochemical features, such as depletions in high-field-strength elements, indicating an origin by partial melting of a mantle wedge that had been modified by slab-derived fluids and recycled sediments. Early and middle Miocene basaltic rocks also have arc-like geochemical features, and have relatively low contents of fluid-mobile elements and high contents of fluid-immobile elements, as compared with the middle Eocene basaltic rocks. This suggests that these magmas were derived by the melting of previously melted lithospheric mantle that had been modified by slab-derived fluids. In contrast, late Miocene basalts have ocean island basalt-like geochemical features, such as enrichments in high-field-strength elements, indicating their derivation by partial melting of the asthenospheric mantle. The temporal variations in the mantle sources of the Cenozoic basaltic rocks in the Russian Far East, along with an abrupt change in SrNd isotopic compositions at ca. 20 Ma, demonstrate that the transition from an active continental margin to a trench–arc–basin system in northeast Asia occurred at ca. 20 Ma.