LATE CARBONIFEROUS AGE CONFIRMED FOR THE OCEANIC PLATE OF PANTHALASSA PRESERVED IN THE KADOMA UNIT OF THE JURASSIC ACCRETIONARY COMPLEX IN NORTHEAST JAPAN

Abstract

Accretionary complexes in the Japanese Islands preserve material from parts of the oceanic plates of Panthalassa that have subducted and are now lost. The age of the oceanic plate that is incorporated into the Jurassic accretionary complex of Japan has been previously estimated from scant data on Carboniferous bedded chert. In this study, we investigated a Carboniferous to Permian basalt-chert sequence in the Jurassic accretionary complex of the North Kitakami-Oshima belt in Northeast Japan. The sequence is composed of basaltic rocks, red bedded chert and grey bedded chert in ascending order. The basaltic rocks at the base contain red cherty nodules that yielded conodonts indicating Bashkiran to early Moscovian, the latter age being more likely (late Carboniferous). The red bedded chert yielded middle Sakmarian (early Permian) radiolarians. The grey bedded chert yielded conodonts indicating latest Kungurian to earliest Roadian. Thus, our study section demonstrates a formation of the oceanic plate in the late Carboniferous and a following deposition of pelagic siliceous sediments. This is the first time that basaltic rocks within the Jurassic accretionary complex of Japan are directly dated. While previous data need careful reviewing, our results demonstrate that the oldest part of the oceanic plate within the Jurassic accretionary complex is Serpukhovian or older and the youngest part may be as young as the Sakmarian.