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

Sawada, Hikaru*; Niki, Sota*; Nagata, Mitsuhiro   ; Hirata, Takafumi*

The Oeyama ophiolite is one of the oldest components in Japanese Islands and important to reveal the initiation of plate subduction along proto-Japan. This study performed U-Pb-Hf isotopic and trace element analyses of zircon in gabbroic rocks. The weight mean of Pb/U dates from zircons of the Oeyama ophiolite is 544 4 Ma (2 sigma). Trace element analysis of the zircons exhibit that the host rock was derived from the mantle depleted of incompatible elements like mid-oceanic ridge basalt. The present igneous age and geochemical feature of the zircons is consistent with previous work for other part of the Oeyama ophiolite. Zircon Lu-Hf isotopic analysis also indicate that the gabbroic rock was derived from the depleted mantle domain. The Hf isotopic signature is more depleted than those of the zircons in jadeitite associated with the Oeyama ophiolite. This result implies that the older crustal material was involved into the initial oceanic plate subduction along the proto-Japan arc.