Geochemical and geochronological constraints on the origin and emplacement of the Shimo‐ondori diorites in Shikoku, Southwest Japan

Abstract

AbstractIn this study, the LA‐ICP‐MS zircon U–Pb dating of the Shimo‐ondori diorites in the Shimanto accretionary complex of SW Japan provides ~130 Ma, representing the timing of their crystallization ages. Combined with the geological occurrence, that age clearly indicates that the diorites occur as blocks, not as intrusive rocks as suggested by previous studies. Moreover, the ages of the Shimo‐ondori diorites are suggesstive that they could be influential for the estimate of the early‐Cretaceous tectonic evolution for the eastern Asian margin. Their whole‐rock chemical compositions show high MgO, Ni and Cr contents, and low total FeO/MgO ratios, indicating that they were crystallized from high magnesian andesite (HMA) magmas. Moreover, their TiO2 and REE compositions suggest that they were formed by the same processes as the sanukites. And, the zircon Hf isotopic ratios (εHf [~130 Ma] = +9.9 − +17.5), which is close to or slightly lower than that of the ~130 Ma depleted mantle, suggest that the wedge‐mantle materials were predominantly involved in the formation of the dioritic magmas. Their geochronological and geochemical similarities of the Shimo‐ondori diorites with the early Cretaceous adakites and HMAs in the eastern Asian margin suggest that they might have been formed possibly by the same slab rollback of the Izanagi plate at the early Cretaceous. After the crystallization of the Shimo‐ondori diorites, they were delivered and deposited as blocks in a trench site with the surrounding sedimentary rocks of the Shimanto accretionary complex.