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 (SHRIMPII). Chemical zoning was revealed in some of the grains with a relatively high concentration of U and Th (up to 988 and 1054ppm, respectively). The weighted mean Pb-206/U-238 age corrected by Pb-207 was 191.1 +/- 0.3Ma (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 +/- 466ppm) 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 degrees C) and the Zr saturation temperature (728 degrees C) suggest that the OT4 zircon crystallized over a short time. The OT4 zircon contains enough radiogenic Pb-206 (>2ppm) and Pb-207 (>0.1ppm) to be detected by microbeam analysis, which indicates its potential of being an effective Mesozoic reference used in microbeam U-Pb dating.