Insights into mantle metasomatism during oceanic subduction from late Paleozoic (∼260 Ma) high-Mg diorites in the northeastern Tibetan Plateau

Abstract

Subduction recycled components can be incorporated into the mantle through aqueous solution/melt metasomatism; however, tracing the source and nature of metasomatic agents, as well as reconstructing the melting reactions of metasomatic mantles, has consistently posed significant challenges. In this work, we performed an extensive study involving petrology, geochronology, mineral chemistry, geochemistry including Sr-Nd-Pb-Hf isotopes, and numerical modeling on the newly discovered high-Mg diorites in the northeastern Tibetan Plateau to unravel their source reactions between subducting components and the mantle wedge. Zircon UPb data reveals that three diorites crystallized at ∼260 Ma, synchronous with the subduction of the Paleo-Zongwulong oceanic slab. These rocks show magnesian characteristics of amphibole and biotite, and most of plagioclase cores produce intermediate anorthite contents (An_36–49). The variable ⁸⁷Sr/⁸⁶Sr_(i) ratios ranging from 0.70656 to 0.7094 and most ε_Hf(t) values between −4.1 and + 4.7, but homogeneous ε_Nd(t) values from −5.4 to −2.3 indicate the diorites are derived from an enriched mantle source. The observed enrichment in large ion lithophile elements and depletion in high field strength elements further indicate the characteristics typical of arc igneous rocks. High Th/Nb ratios (0.54–1.37) and Th/Yb ratios (2.42–4.7), alongside low Sr/Th ratios (24.51–59.83) and narrow Pb isotopic variations (²⁰⁶Pb/²⁰⁴Pb_(i) of 18.09–18.34, ²⁰⁷Pb/²⁰⁴Pb_(i) of 15.59–15.63 and ²⁰⁸Pb/²⁰⁴Pb_(i) of 38.14–38.37), imply the presence of sediment components in the mantle source. Pyroxenites have been identified as their potential source, as indicated by whole-rock chemistry (e.g., high 10000*Zn/Fe ratios). Numerical modeling suggests that these diorites formed through the addition of ∼1 % aqueous solutions and 6 %–20 % hydrous melts into the mantle wedge. Therefore, the high-Mg diorites discovered in the northeastern Tibetan Plateau provide valuable insights into the interactions between the crust and mantle, which are associated with slab subduction during the late Paleozoic era.