U-Pb zircon ages of granitic rocks from Kagasawa, Hida Mountains

Abstract

Laser-ablation ICP-MS U−Pb dating was carried out on rock samples from granitic dikes of the Hida Metamorphic Belt at Kagasawa, Toyama City, where Precambrian Rb−Sr whole-rock ages disagree with Triassic (240-230 Ma) CHIME zircon ages. Two medium-grained granodiorite samples yielded zircon ²³⁸U-²⁰⁶Pb weighted mean ages of young age clusters of 241.3±3.0 Ma and 229.3±1.9 Ma, and a coarse-grained granite yielded a zircon ²³⁸U-²⁰⁶Pb weighted mean age of 241.4±2.0 Ma. These ages are similar to the CHIME zircon ages. The medium-grained granodiorites contain several Precambrian zircons with core ages of ca. 2500 Ma, 2000 Ma, and 1600 Ma. The coarse-grained granite, however, contains no older zircon.