ABSTRACT

The Cretaceous to early Paleogene magmatism occurred in the Honshu Arc actively and the formation age for this period shifted regionally in the Southwestern and Northeastern Japan Arcs, respectively. The Tadamigawa older-stage granites are located to the west of the Tanakura Tectonic Line (TTL), which is the boundary of the Northeastern Japan Arcs and the Ashio belt, and the formation age and source of the Tadamigawa older-stage granites were unclear. In this study, we determined the formation age of the Tadamigwa older-stage granites by zircon U-Pb and whole-rock and mineral Rb-Sr datings and estimated the origin of the granites by the initial Sr isotopic ratio. The Tadamigawa older-stage granites have two intrusion ages, 107–93 Ma and ca. 62 Ma. In addition, the older granites show +4.6ε_Sr – +20.4ε_Sr, and the younger granites show +64.0ε_Sr. The Tadamigawa older-stage granites to the west of the TTL of northeastern Japan comprise two types of plutons with different ages and origins.