2017 年 123 巻 11 号 p. 977-982
A Middle Jurassic-earliest Cretaceous accretionary complex crops out in the North Kitakami Belt, located in the Kitakami Massif, Northeast Japan. The Kadoma Complex, which is distributed along the southwestern margin of the belt, contains felsic tuff with radiolarian fossils (Triassocampe? spp.) of possible Middle-Late Triassic age according to the previous work. Therefore, there is a possibility that the Kadoma Complex includes a Triassic accretionary complex.
In this study, the U-Pb ages of zircon grains from tuffaceous mudstone near the outcrop of the radiolarian-fossil-bearing felsic tuff were obtained to establish the depositional age of the Kadoma Complex. The youngest cluster of zircon grains yielded a peak age (weighted mean) of 209.4±3.7 Ma. Therefore, the tuffaceous mudstone is considered to have been deposited in the Late Triassic, given the age of radiolarian fossils from the nearby felsic tuff.
If the tuffaceous mudstone is a trench-fill deposit, then at least part of the Kadoma Complex represents a Late Triassic accretionary complex. This view provides insights into the Triassic accretionary complex, which is rare in the Japanese Islands, and into the tectonics of the early Mesozoic arc-trench system at the eastern Asian margin.
