Abstract
K-Ar ages and fission-track ages of granitic rocks in the Hatagawa Fault Zone (HFZ), NE Japan were measured to examine the cooling history of the HFZ. The HFZ is an NNW-SSE trending fault zone in the Cretaceous granitic rocks, and consists of a conspicuous cataclasite and two types of mylonite with a sinistral sense of shear. The cataclasite zone is NNW-SSE trending and continuous over at least 40 km with a maximum thickness of 100 m. One type of mylonite is low-T mylonite, which is mainly developed for a length of 6 km along the HFZ. The other is high-T mylonite, which is widely distributed in the HFZ. Most of K-Ar ages of hornblende and biotite from granitic rocks are about 110 Ma and show no obvious differences along the strike of the HFZ or among different granite bodies. This implies that the granitic rocks in the HFZ have a similar cooling history and cooled rapidly from closure temperature of hornblende to that of biotite. Zircon fission-track analysis shows little possibility of reheating of the granitic rocks. This supports the formation of cataclasite and mylonite during the cooling of the granitic bodies. Fission-track ages of zircon and apatite from the samples in and near the areas where the low-T mylonite is developed are older than those for other areas. Infiltration of near-surface derived water into the low-T mylonite after plastic deformation may account for the accelerate cooling of granitic bodies.
