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.

Lithological, structural, and chronological relationships between the Sanbagawa Metamorphic Complex and the Cretaceous Shimanto Accretionary Complex on the central Kii Peninsula, SW Japan

“…The detrital zircon U–Pb ages of the Mg1 Unit mainly consist of Jurassic to Permian and Precambrian ages with a minor amount of Cretaceous ages (Shimura et al, ). These age composition patterns of the Mg1 Unit are in good agreement with those of the Albian to Cenomanian Yukawa Complex (Tokiwa et al, ), which is the Cretaceous Shimanto AC on the western Kii Peninsula. In addition, the YSG and YC1σ ages of the Yukawa Complex mainly overlap with the depositional age inferred from radiolarian assemblages (KSRG, ; Tokiwa et al, ), and these youngest ages are similar to those of the Mg1 Unit (Figure ).…”

“…These age composition patterns of the Mg1 Unit are in good agreement with those of the Albian to Cenomanian Yukawa Complex (Tokiwa et al, ), which is the Cretaceous Shimanto AC on the western Kii Peninsula. In addition, the YSG and YC1σ ages of the Yukawa Complex mainly overlap with the depositional age inferred from radiolarian assemblages (KSRG, ; Tokiwa et al, ), and these youngest ages are similar to those of the Mg1 Unit (Figure ). Thus, it is possible that the depositional ages of the sandstones in the Mg1 Unit of the Mugitani Complex can be constrained to the Albian to Cenomanian.…”

“…Summary of the distributions of YSG and YC1σ ages (obtained by U–Pb dating of detrital zircons) and depositional ages (obtained by radiolarian fossils) in the Sanbagawa MC and the Cretaceous Shimanto AC on the Kii Peninsula. YSG and YC1σ ages obtained by detrital zircon U–Pb dating are from Ota et al (), Shimura et al (), Tokiwa et al (), and this study. Depositional ages obtained by radiolarian fossils are from KSRG (, , , ), Yamamoto and Suzuki (), and YORG (, ).…”

“…The Cretaceous Shimanto AC on this area can be divided into the Kouyasan and Hidakagawa belts. Both of these belts, on which have previously been conducted the determination/constraint of the depositional ages in a clastic rocks by using radiolarian assemblages (KSRG, , , ; Yamamoto & Suzuki, ; YORG, , ) or detrital zircon U–Pb ages (Ota et al, ; Shimura et al, ; Tokiwa et al, ; this study), are composed mainly of Aptian to Maastrichtian complexes, and the age composition of the Kouyasan Belt are similar to that of the Hidakagawa Belt (Figure ). The boundary between the two belts is the out‐of‐sequence Ui Thrust, which has a sinistral sense of shear with a reverse dip‐slip component (Tokiwa et al, ).…”

“…Hirata & Nesbitt, ; Košler & Sylvester, ), and this method has been used to constrain the ages of deposition of the Sanbagawa MC and the Shimanto AC (e.g. Aoki, Iizuka, Hirata, Maruyama, & Terabayashi, ; Endo, Miyazaki, Danhara, Iwano, & Hirata, ; Hara et al, ; Knittel et al, ; Otoh et al, ; Tokiwa et al, ). As a result of these studies, many detrital zircons of Late Cretaceous age have been identified in the Sanbagawa MC (e.g.…”