Magnesium isotope constraints on contributions of recycled oceanic crust and lithospheric mantle to generation of intraplate basalts in a big mantle wedge

Abstract

Despite numerous studies, the origin of intraplate basalts from eastern Asia is still elusive. This includes the relative importance of subducted oceanic crust and metasomatized lithospheric mantle in generation of these lavas. To address this important issue, we have carried out an integrated Mg-Sr-Nd isotope study of Jiaohe garnet pyroxenite xenoliths and Cenozoic basalts from the Changbaishan-baoqing volcanic belt (CVB), NE China. The Jiaohe garnet pyroxenite xenoliths exhibit low Hf/Hf* ((HfN/(SmN × NdN)0.5, 0.7–0.9), depleted SrNd isotopic compositions (87Sr/86Sri = 0.7035–0.7041; εNd = +2.9 − +8.6) and low δ26Mg (−0.61 to −0.39‰). These features reflect both protolith heterogeneity and the effect of isotopic exchange with carbonates during subduction. The CVB basalts display variable Mg isotopic compositions. Low-Si basalts are characterized by low δ26Mg (−0.27 to −0.44‰), variable Hf/Hf* (0.7–1.4) and depleted SrNd isotopic compositions (87Sr/86Sri = 0.7039–0.7050; εNd = +1.0 − +4.7), which were attributed to partial melting of upwelling asthenospheric mantle containing recycled oceanic crustal components. In contrast, high-Si basalts display low Hf/Hf* (0.6–0.9), EM1-like SrNd isotopic compositions (87Sr/86Sri = 0.7047–0.7054; εNd = −2.4 - -0.2) and normal mantle-like δ26Mg values (−0.18 to −0.33‰). Such features can be explained by participation of metasomatized lithospheric mantle in their source. The transition from low-Si melts with low-δ26Mg to high-Si melts with normal mantle-like δ26Mg for the CVB basalts are genetically related to continental rifting, asthenospheric mantle upwelling and mechanical-chemical erosion of basal metasomatized lithosphere induced by subduction of the Pacific oceanic slab. Recycled oceanic crust and basal lithospheric mantle both played a fundamental role in generation of the intraplate basalts in eastern Asia.