U-Pb ages of detrital zircon from the tuffaceous sandstone of the Shimanto Belt in the Kii Peninsula

Abstract

We carried out the U-Pb dating on detrital zircons obtained from tuffaceous sandstone in the Nyunokawa Formation of the Shimanto Belt in the Kii Peninsula to clarify its depositional age. The youngest peak age from the Nyunokawa Formation indicates 63.0±1.8 Ma (2σ). On the other hand, the age of detrital zircons from the Otonashigawa Group indicates 50.8±1.0 Ma (2σ) corresponding to radiolarian ages obtained from this group. Therefore, there is a major gap between the depositional ages of the Nyunokawa Formation and the Otonashigawa Group. In addition, the age from the Ryujin Formation distributed along the thrust bounded between the Nyunokawa Formation and the Otonashigawa Group shows 55.8±2.5 Ma (2σ). Basically, the depositional ages become younger as tectono-stratigraphically downward (north to south), and the tendency corresponds to the characteristics yielding an accretionary complex.