Direct age determination for an Upper Permian accretionary complex (Kirinai Formation), Kitakami Mountains, Northeast Japan

Abstract

This study presents the first direct age determination for the Kirinai Formation in the Kitakami Mountains, Northeast Japan, and a new model that correlates the Kirinai Formation with Upper Permian accretionary complexes of the Ultra-Tamba belt, Southwest Japan. The correlation is based on similarities in lithology, age, and structural relationships. The Kitakami Mountains are geotectonically divided into several belts, with subduction-related complexes occurring in the North Kitakami and Nedamo belts, whereas Ordovician ophiolites and stratigraphically overlying Paleozoic–Mesozoic sedimentary sequences are respectively exposed in the so-called “Hayachine tectonic belt” and South Kitakami belt. The Kirinai Formation, previously thought to belong to the North Kitakami belt, is tectonically underlain by the Jurassic Kamaishi and Tassobeguchi formations of the North Kitakami belt, and is overlain by the Lower Carboniferous Nedamo Complex of the Nedamo belt. The Kirinai Formation is lithostratigraphically subdivided into three members: a lower member dominated by black phyllitic mudstone with minor amount of bedded siliceous mudstone, a middle member consisting mainly of greenish gray massive sandstone and phyllitic mudstone, and an upper member composed of phyllitic mudstone locally intercalated with felsic tuff. The radiolarian fauna obtained from the lower member includes age-diagnostic species such as Follicucullus sp. cf. F. dilatatus Rudenko and Albaillella sp. cf. A. protolevis Kuwahara, corresponding to the interval from the Neoalbaillella ornithoformis Assemblage Zone to the lower part of the Neoalbaillella optima Assemblage Zone of Kuwahara et al. (1998). These findings indicate a Changhsingian (Upper Permian) assignment for the lower member of the Kirinai Formation. This newly obtained age and the obtained correlations provide the key to understanding the original relationships among the various subduction-related complexes in Japan.