P otential M esozoic reference zircon from the U nazuki plutonic complex: geochronological and geochemical characterization

OT 4 zircon collected from O to‐dani quartz diorite in U nazuki, in the northeastern part of the H ida B elt, S outhwest J apan, was evaluated using microbeam analysis for its qualification as a M esozoic reference zircon based on U – P b dating. The characterization of OT 4 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 T h (up to 988 and 1054 ppm, respectively). The weighted mean 206 P b/ 238 U age corrected by 207 P b was 191.1 ± 0.3  M a (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 H f concentrations (8651 ± 466 ppm) indicate that the grains were not affected by any geological events after crystallization. Furthermore, the homogeneous H f concentrations and consistency of the Ti ‐in‐zircon temperature (733 ± 34° C ) and the Z r saturation temperature (728° C ) suggest that the OT 4 zircon crystallized over a short time. The OT 4 zircon contains enough radiogenic 206 P b (>2 ppm) and 207 P b (>0.1 ppm) to be detected by microbeam analysis, which indicates its potential of being an effective M esozoic reference used in microbeam U – P b dating.