Potential Mesozoic reference zircon from the Unazuki plutonic complex: geochronological and geochemical characterization



OT4 zircon collected from Oto-dani quartz diorite in Unazuki, in the northeastern part of the Hida Belt, Southwest Japan, was evaluated using microbeam analysis for its qualification as a Mesozoic reference zircon based on U–Pb dating. The characterization of OT4 zircon was carried out using a sensitive high-resolution ion microprobe (SHRIMP II). Chemical zoning was revealed in some of the grains with a relatively high concentration of U and Th (up to 988 and 1054 ppm, respectively). The weighted mean 206Pb/238U age corrected by 207Pb was 191.1 ± 0.3 Ma (95% confidence limit) and was found to be almost constant for all the grains. The high crystallinity, the constant REE abundance patterns, and the homogeneous Hf concentrations (8651 ± 466 ppm) indicate that the grains were not affected by any geological events after crystallization. Furthermore, the homogeneous Hf concentrations and consistency of the Ti-in-zircon temperature (733 ± 34°C) and the Zr saturation temperature (728°C) suggest that the OT4 zircon crystallized over a short time. The OT4 zircon contains enough radiogenic 206Pb (>2 ppm) and 207Pb (>0.1 ppm) to be detected by microbeam analysis, which indicates its potential of being an effective Mesozoic reference used in microbeam U–Pb dating.