Mantle heterogeneity beneath the South China Sea: Chemical and isotopic evidence for contamination of ambient asthenospheric mantle

Abstract

Large quantities of continental lithosphere are known to enter the mantle during continental rifting, but their possible contribution in the generation of melts under a nascent ocean remains enigmatic. Here we report new in situ chemical and SrPb isotopic data for volcanic glass and feldspar in volcanic breccia in the South China Sea (SCS) formed after cessation of SCS spreading. Similar to volcanic breccia (whole rocks), the volcanic glasses and feldspars in breccia can also be grouped into early (11–8 Ma, e.g. 25R-17R) and late (<8 Ma, e.g. 10R-7R) stages. Early stage glasses and minerals in breccia display limited chemical and isotopic variability, whereas late stage glasses and feldspars show a significantly wide range in chemical and SrPb isotopic compositions. Geochemical and radiogenic isotope variations in glasses from the SCS breccia have delineated two distinct mantle sources: a depleted mantle component (DM), and an enriched mantle component (EM-2). The influence of EM-2 component is less diluted in late-stage magmatism compared to early stage magmatism. The temporal variation in compositions of volcanic glasses from the SCS are consistent with dynamic melting of a heterogeneous mantle. Due to progressive thickening of the lithosphere, low-degree melting would preferentially sample the more fertile EM-2 component during the late stage of volcanism. The new results, in combination with literature data of post-spreading lavas, provide evidence for an origin of the EM-2 (DUPAL) signature from recent introduction of continental lithosphere via delamination. We conclude that shallow recycling of continental lithosphere introduced into the upper mantle during continental rifting can be an important mechanism contributing to mantle heterogeneity.