Zircon U–Pb–Hf Isotopic and Trace Element Analyses for Oceanic Mafic Crustal Rock of the Neoproterozoic–Early Paleozoic Oeyama Ophiolite Unit and Implication for Subduction Initiation of Proto-Japan Arc

The Oeyama ophiolite unit is one of the oldest components of the subduction orogenic belts in the Japanese Islands and is important for understanding the initiation of plate subduction in the proto-Japan region. This study performed U–Pb and Lu–Hf isotopic and trace element analyses of zircon in gabbroic rocks from the Oeyama mafic–ultramafic complex, the type locality of the Oeyama ophiolite unit. This study obtained a weighted average zircon²⁰⁶Pb/²³⁸U age of 544 ± 4 Ma, which indicates an igneous age of the gabbroic rock. Trace element analysis of the zircons revealed that the host rock was derived from the mantle depleted of incompatible elements such as the depleted mid-oceanic ridge mantle. The present igneous age and geochemical features of the zircons are consistent with previous work for other parts of the Oeyama ophiolitic unit. The isotopic signature of Hf also indicated that the gabbroic rock was derived from the depleted mantle domain. The Hf isotopic signature is more depleted than those represented by zircons in the jadeitite associated with the Oeyama ophiolite unit. These results demonstrated that the older crustal material was involved in the initial oceanic plate subduction along the proto-Japan arc.