赤石山地中央部の小渋川地域に分布する陸源砕屑岩から得られた前期白亜紀最末期の砕屑性ジルコン

抄録

We conducted U-Pb dating of detrital zircons from three clastic rock samples, which have been classified into the Chichibu Belt in the Koshibu-gawa area, central Akaishi Mountains. U-Pb ages of the samples fall into two main age groups at 280-100 Ma and 2600-1700 Ma. The youngest single grain ages are 105.5±1.3 Ma, 109.6±1.2 Ma, and 111.1±1.9 Ma, and the youngest cluster ages are 115.9±1.0 Ma, 115.1±0.4 Ma, and 113.1±1.3 Ma. These Aptian to Albian (latest Early Cretaceous) ages are considerably younger than previously reported youngest ages from the Jurassic accretionary complex of the Chichibu Belt in the southern part of the Koshibu-gawa area (Okuchausu-yama area). Our results indicate that the geological bodies, including clastic rocks studied in the Koshibu-gawa area, should be assigned to the Cretaceous accretionary complex of the Shimanto Belt, and not to the Jurassic accretionary complex of the Chichibu Belt. Reexamination of the structural evolution and geological architecture in the Koshibu-gawa area is thus warranted.