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

Hikaru Sawada,Mitsuhiro Nagata,Takafumi Hirata,Sota Niki

doi:10.3390/min12010107

Abstract

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 zircon 206Pb/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.

Highlights

Ophiolites are fragments of oceanic lithosphere, including ultramafic rocks of the upper mantle and oceanic crustal mafic rocks preserved in the subduction continental margins or continental collision zones [1,2]
This result confirms that the mafic rocks of the Oeyama ophiolite unit (i.e., Unit II according to Kurokawa (1985) [39]) was formed and accreted almost simultaneously along the plate subduction zone of the proto-Japan arc
Trace element analysis of the zircons revealed that the host rock was derived from the mantle depleted of incompatible elements such as mid-oceanic ridge basalt (MORB)

Summary

Introduction

Ophiolites are fragments of oceanic lithosphere, including ultramafic rocks of the upper mantle and oceanic crustal mafic rocks preserved in the subduction continental margins or continental collision zones [1,2]. There are several types of ophiolite formation mechanisms, intra-oceanic forearcs with mid-oceanic ridge (MOR)-like magmatism are especially important tectonic activity for initiation of subduction and subsequent orogenic crustal evolution [3]. 540 Ma Oeyama ophiolite unit, which has a MOR-like geochemical signature [8]. The jadeitites have been reported as blocks in serpentinite mélange structurally under the Oeyama ophiolite unit. The jadeitite is monomineralic high P/T-type metasomatic rock composed of jadeite, which occurred in the subduction channel and the mantle wedge during serpentinization [15]. The Oeyama ophiolite unit is key to understanding the initiation of plate subduction in the proto-Japan region

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Results

Discussion

Conclusion