SHRIMP U-Pb age of SoriZ93 zircon from the Sori Granodiorite, Northeast Japan: a potential reference zircon of Late Cretaceous age

SoriZ93 zircon was separated from residual mineral fraction after the preparation of the SORI93 biotite standard from the Sori Granodiorite in the Ashio Mountains, Northeast Japan, and analyzed for its U–Pb age using a sensitive high resolution ion microprobe (SHRIMP). The zircon grains of SoriZ93 are prismatic with pyramidal ends or broken prismatic fragments. Most zircons are 100–250 μm long and 50–150 μm wide. The zircons are clear crystals and colorless to pale yellow, although some grains are brown with optically low transparency. Cathodoluminescence (CL) imaging of the SoriZ93 zircons showed a fine oscillatory zoning, which is a typical characteristic of zircons in granitic rocks. A xenocrystic core was not present in the zircons. Although some mineral inclusions were present in the zircons, it is possible to select a typical analytical area with a dimension of 30 μm necessary for the microbeam technique. The analytical results of the colorless zircons provided a weighted mean 207Pb corrected 206Pb/238U age of 93.9 ± 0.6 Ma (95% confidence, MSWD = 0.97). This 206Pb/238U age is 1.3 m.y. older than the K–Ar age of the SORI93 biotite, indicating that the granodiorite cooled to a closure temperature of the K–Ar biotite system within a short time interval. Although some grains of the SoriZ93 zircons show high U concentration, a selection of colorless zircons provided the precise age to be used for the calibration and reference for zircons of the Late Cretaceous.