2019 Volume 125 Issue 5 Pages 329-347
Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) was used to determine U and Pb isotopic compositions of detrital zircons from sandstones of the Takaharagawa, Akataki, and Makio complexes, which are parts of the Cretaceous accretionary complex of the Shimanto Belt, central Kii Peninsula, Southwest Japan. The Takaharagawa Complex is divided into three thrust-stacked units: the Upper Sandstone, Mixed-rock, and Lower Sandstone units. There are few index fossils in the rocks, so it is controversial whether the Takaharagawa Complex is part of the late Early to Late Cretaceous accretionary complex of the Sihimanto Belt, or the Jurassic to earliest Cretaceous accretionary complex of the Chichibu Belt. The youngest detrital zircon U-Pb ages of both sandstone units of the Takaharagawa Complex are ca 115 Ma (Aptian). These ages, in conjunction with lithological characteristics, confirm that the Takaharagawa Complex is part of the Shimanto Belt. This Complex has been correlated with the Yukawa Complex in the western Kii Peninsula, based on similarities in the sandstone-dominant lithologies, but the youngest U-Pb ages of the two complexes are different. The lithology and depositional age of the Akataki Complex are similar to those of parts of the Hanazono and Miyama complexes in the Shimanto Belt, but the detrital-zircon age spectra are different. The new ages suggest that the depositional age of the Makio Complex is Campanian, which is younger than previously reported.
