Abstract
The NNW-trending Onikobe-Yuzawa Mylonite Zone extends for 45 km from Onikobe Caldera, Miyagi Prefecture to Yuzawa, Akita Prefecture, northeast Honshu, Japan. The mylonite was derived from Cretaceous tonalite and granodiorite. The field relationship between the mylonite and the foliated tonalite is gradational. The mylonite is intruded by the Paleogene massive tonalite. The K-Ar ages of the mylonite range from 75.6 Ma to 88.5Ma. These facts indicate that the mylonitization proceeded during Late Cretaceous. Dynamically recrystallized quartz aggregates in mylonite are distinguished into P-and S-types. They were partly changed into blastomylonite through the intrusion of the younger tonalite. The micro-structure of the mylonites shows left-lateral shear deformation. The pre-Tertiary metamorphic and granitic rocks on the western side of the mylonite zone are comparable to those in the Abukuma Belt. The pre-Tertiary basement rocks on the east of the mylonite zone are composed of highly to weekly metamorphosed volcanic and sedimantary rocks probably of Paleozoic ages, and the Cretaceous granitic rocks, which are similar to those of Kitakami Belt. Hence, the Onikobe-Yuzawa Mylonite Zone is probably a northern extension of the Hatagawa Fracture Zone, which is a boundarv between the Abukuma and Kitakami Belts.
