Effectiveness for Determination of Depositional Age by Detrital Zircon U–Pb Age in the Cretaceous Shimanto Accretionary Complex of Japan

Abstract

Detrital zircon U–Pb ages indicate the crystallization age. Therefore, it is necessary to evaluate the effectiveness of determining the age of deposition using zircon age data. We carried out U–Pb dating of detrital zircons from sandstone at eight sites in the Cretaceous Shimanto accretionary complex on Kii Peninsula, Japan, with the aim of evaluating the accuracy of U–Pb zircon ages as indicators of the depositional age of sedimentary rocks by comparing zircon ages with radiolarian ages. Our results reveal zircons of late Cretaceous age, and the youngest peak ages are in good agreement with depositional ages inferred from radiolarian fossils. In addition, the youngest peak ages become younger as tectono-structurally downwards, and this tendency is clearer for the zircon ages than for the radiolarian ages. These results indicate that newly crystalized zircons were continuously supplied to the sediment by constant igneous activity during the late Cretaceous and that zircon ages provide remarkably useful information for determining the age of deposition in the Cretaceous Shimanto accretionary complex.