Zircon Crystallization Timings of Granitoids in the Aoyama Area, Ryoke Belt, Southwest Japan

We performed U–Pb zircon dating using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) for gneissose and massive granitoids from the Aoyama area, Ryoke belt, Southwest Japan. This is the first comprehensive report of U–Pb zircon ages from the Aoyama area, a key region to discuss an along-arc variation of the lower crustal magma generation in the Ryoke belt. Solidification timings of the Kabuto Granodiorite, Ao Granite, Misugi Tonalite, Joryu Tonalite, and Kimigano Granodiorite are considered to be ca. 80, ca. 72, ca. 74, ca. 88, and ca. 112–99 Ma, respectively. These timings are consistent with published intrusion relationships. Kimigano Granodiorite characteristically gives old magmatic ages, showing the magmatic activity predating the regional high-temperature metamorphism in the Aoyama area. The scattered U–Pb dates indicate either a long duration of zircon crystallization or rejuvenation of U–Pb dates of preexisting zircon grains. Magmatic activities are continuously observed for ~15 Myr, following the ~10 Myr “lull” after the solidification of the Kimigano Granodiorite at ca. 112–99 Ma. There is an ~8 Myr age gap between the youngest gneissose granitoid of the Joryu Tonalite and the oldest massive granitoid of the Kabuto Granodiorite. Zircon grains of the Kimigano Granodiorite and the Joryu Tonalite could be, respectively, affected by regional Ryoke metamorphism and by contact metamorphism due to the intrusion of the adjacent Ao Granite.