Geology of the Osayama serpentinite melange in the central Chugoku Mountains, southwestern Japan:320 Ma blueschist-bearing serpentinite melange beneath the Oeyama ophiolite

Abstract

A blueschist-bearing serpentinite melange is developed beneath the Osayama peridotite body of the Oeyama ophiolite in central Chugoku Mountains. The Osayama serpentinite melange is a typical tectonic melange with serpentinite matrix. Blueschist-facies schists (ca. 320 Ma) of the Sangun-Renge metamorphic belt, fragments of the Oeyama ophiolite (serpentinized peridotite, gabbro, dolerite) and related metasomatic rocks (albitite, jadeitite, omphacitite, tremolite schist, etc.) are enclosed as tectonic blocks of various sizes (10 cm to 1.5 km in length) in the serpentinite matrix, which consists of schistose, friable, fine-grained serpentinite with pebble to boulder-size fragments of serpentinized peridotite. Petrology of the peridotite blocks suggests that the melange matrix has been derived from widely varying residual peridotites of the Oeyama ophiolite in central Chugoku Mountains, which is mainly composed of homogeneous, massive, clinopyroxene-bearing harzburgite. The blueschist blocks are characterized by high Al (sub 2) O (sub 3) content (up to 12.9 wt.%) in glaucophane and presence of glaucophane + lawsonite (or pumpellyite) and high-Al glaucophane + epidote assemblages. These characteristics suggest very high-P/T metamorphism as in the case of jadeite-glaucophane type, which is quite different from the Sangun-Chizu metamorphism affecting the underlying tectonic unit. In the Osayama serpentinite melange, gabbroic rocks derived from the Oeyama ophiolite occur only as blocks and have suffered blueschist-facies metamorphism. Various metasomatic rocks of metabasite-origin as well as tremolite schist of peridotite-origin may also have suffered the same metamorphism. These facts suggest that a part of the Oeyama ophiolite (representing supra-subduction zone wedge mantle) has been tectonically eroded by subducting oceanic lithosphere, and has experienced a high-P/T metamorphism in a deep part of the subduction zone with the Sangun-Renge metamorphic rocks (subducted oceanic sediments). The Osayama serpentinite melange was probably tectonically formed along a thrust fault between the Ordovician Oeyama ophiolite and Carboniferous Sangun-Renge blueschist nappe during the emplacement of the Oeyama ophiolite.