Ordovician backarc-basin metadolerite and metabasalt of the South Kitakami Terrane, Northeast Japan

Abstract

Basement rocks that occur along the northern margin of the South Kitakami Terrane in Japan consist of Ordovician ultramafic rocks (Hayachine ultramafic complex), gneissose amphibolite (Kuromoriyama amphibolite), and mafic rocks (Kagura igneous rocks, KIR). The KIR are composed of metagabbro, metadolerite, metabasalt, and minor felsic–intermediate dikes. Although the KIR contain green hornblende due to metamorphism of greenschist to epidote–amphibolite facies, they rarely retain primary brown hornblende. Approximately 30% of the metabasalt shows porphyritic textures, with phenocrysts of saussuritized plagioclase and/or altered mafic minerals. The geochemistry of the common metadolerite and metabasalt of the KIR shows a tholeiite trend, a low TiO2 content, and high Th/Nb and Ti/V ratios. The KIR are therefore indicative of a supra-subduction zone tectonic setting, which implies a backarc origin (as also indicated by discrimination diagrams). Trace element patterns of the KIR resemble those of the backarc-basin basalt of the Japan and Yamato basins in the Japan Sea. We propose that the KIR formed during backarc spreading from the Ordovician to Early Silurian. This view is supported by the geochemical data, the tectonic setting of the Hayachine ultramafic rocks, and the provenance of clastics within Silurian sedimentary rocks. 日本語要旨 南部北上帯の北縁域に基盤岩の一つとして分布する神楽火成岩類は変斑れい岩,変ドレライト,変玄武岩からなり,少量の珪長質~中性岩岩脈を伴う.神楽火成岩類は,緑色片岩~緑れん石角閃岩相の変成作用により生じた緑色普通角閃石に加えて,まれに初生的な褐色普通角閃石を含む.変ドレライトと変玄武岩の全岩化学組成はソレアイト系列を示し,低TiO比,高Th/Nb比,高Ti/V比でも特徴付けられる.すなわち,神楽火成岩類はスープラサブダクション帯起源で,特に背弧が示唆される.微量元素組成のパターンは日本海に位置する日本海盆や大和海盆のものと類似する.全岩化学組成および先行研究で示された早池峰超苦鉄質岩コンプレックスの形成場やシルル系中の正珪岩礫やジルコン粒子の供給源などの周辺地質体情報も参考にしたうえで,神楽火成岩類がオルドビス紀~前期シルル紀にかけて背弧拡大で形成されたことを説明するモデルを示した.