Early Cretaceous adakitic magmatism and tectonics in the Kitakami Mountains, Japan

Early Cretaceous igneous rocks in the Kitakami Mountains consist of volcanic rocks, dike rocks, and plutonic rocks, from older to younger. Plutonic rocks are composed mainly of adakitic granites in central part of zoned plutonic bodies surrounded by adakitic to non- adakitic granites in marginal part. These adakitic plutons is divided into E andWzones based on the ages and geochemistry. Zircon U-Pb ages were determined with laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) for 22 samples from 13 rock bodies including the Early Cretaceous adakitic granites in the KitakamiMountains. Zircons from the adakitic granites of E zone give older ages (127-117 Ma) compared with those of W zone (119-113 Ma). Zircon ages of the calc-alkaline to shoshonitic rocks and dike rocks range from 128-124 Ma, which are similar to the oldest rocks of the E zone (127-125 Ma). Zircon ages become younger from the northern Hashikami pluton and marginal facies of the Tanohata pluton (127-125 Ma) to southern Takase granites (118-117 Ma), in the E zone adakitic granites. Petrochemical diŠerences between the E zone and W zone rocks indicate that the adakitic melt of E zone rocks are considered to be derived from vapor-absent melting condition, while those of W zone rocks are from higher pressure and vapor-present condition. Taking all these data into consideration, the Early Cretaceous magmatisms in Kitakami can be explained by the diŠerential subduction model of the Farallon-Izanagi plates or slab rollback model accompanied with asthenospheric upwelling.