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CHIME monazite ages of the Hikami Granite and the Ono Formation in the South Kitakami Terrane
The Kitakami Massif of the Tohoku district, NE Japan, consists mainly of the South Kitakami Belt (Silurian–Cretaceous forearc shallow‐marine sediments overlaying granitoids and forearc ophiolite) and the North Kitakami Belt (a Jurassic accretionary complex). The Nedamo Belt (a Carboniferous accretionary complex) occurs as a small unit between those two belts. An accretionary unit in the Nedamo Belt is lithologically divided into the early Carboniferous Tsunatori Unit and the age‐unknown Takinosawa Unit. In order to constrain the accretionary age of the Takinosawa Unit, detrital zircon U–Pb dating was conducted. New data revealed that the youngest cluster ages from sandstone and tuffaceous rock are 257–248 and 288–281 Ma, respectively. An Early Triassic depositional age of the sandstone might be coeval with an intense magmatic activity in the eastern margin of the paleo‐Asian continent. A 30–40 Myr interval between the youngest cluster ages of the sandstone and the tuffaceous rock can be explained by the absence of syn‐sedimentary zircon in the tuffaceous rock. New detrital zircon data suggest that the Takinosawa Unit can be distinguished as an Early Triassic accretionary complex from the early Carboniferous Tsunatori Unit. This recognition establishes a long‐duration northeastward younging polarity of accretionary units, from the Carboniferous to Early Cretaceous, in the northern Kitakami Massif. The lithological features and detrital zircon spectrum suggest that the Early Triassic Takinosawa Unit in the Nedamo Belt can be comparable with the Hisone/Shingai units in the Kurosegawa Belt in Shikoku. The presence of the Early Triassic accretionary complex strongly supports a pre‐Jurassic geotectonic correlation and similarity among SW and NE Japan.
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