抄録
Radiometric ages of zircons in psammitic rocks from the Nishiki and Tsuno groups were determined from 238U/206Pb ratio and isotopic compositions of Pb which are analyzed by Sensitive High Resolution Ion Microprobe (SHRIMP II). Detrital zircons in the Nishiki sandstones show a continuous distribution from 320 Ma to 240 Ma, which is similar to those from the Tsuno psammitic schists; about 380-220 Ma. The youngest zircons from the Nishiki and Tsuno groups are 238.7±5.9 Ma and 218.6±11.4, respectively. It was estimated as upper limits of the depositional ages.
Detrital zircon age in the psammitic rocks shows continuous variation from 380 Ma to 220 Ma, whereas K-Ar age of white mica corresponding to metamorphic age is around 220 Ma in both the groups. Hence, it is realistic to conclude that the youngest age is close to the depositional age and clastic rocks in the Tsuno Group is Triassic in origin. Probability distributions for zircon ages in psammitic rocks from both groups, calculated by a multi-peak Gauss fitting, indicate that provenances for the clastic materials in both the groups overlap widely and are similar to those in the Mino and Hida belts.
Acidic tuff in the Nishiki group occurs in a hemi-pelagic unit different from the sandstone of trench-fill sediment. Zircons in the tuffs show apparently two distinctive U-Pb ages; about 2700-2400 Ma and 320-260 Ma. The former is considered to be accidental materials derived from Archean basement which will be a part of the Sino-Korean craton with 2700-2500 Ma. Least squares fitting for the latter younger cluster yields a concordia intercept age of 270.3±8.5 Ma (2σ) which probably represents a depositional age for the tuff.
