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.