Geology and detrital zircon U-Pb age of the Cretaceous Mugitani Formation in the Shimanto Belt, central Kii Peninsula, Southwest Japan

Abstract

We have investigated the Mugitani Formation located in the structurally upper part of the Cretaceous accretionary complex in the Shimanto Belt, central Kii Peninsula. The Mugitani Formation is newly divided into two units: the Izuo Unit and the Sebato Unit. The Izuo Unit is composed mainly of mudstone, broken beds of sandstone and mudstone, and mixed rock with minor amounts of sandstone and chert. The Sebato Unit is composed mainly of mudstone with minor amounts of broken beds of sandstone and mudstone, basalt, and mixed rock. The bedding planes generally strike WNW-ESE and dip moderately northward. In addition, we also carried out UPb dating of detrital zircons separated from three sandstones to clarify the depositional age of the Mugitani Formation. The youngest zircon grain ages and/or the youngest cluster ages indicate ca.Ma, presenting latest Early Cretaceous (Albian). Furthermore, we found no Late Cretaceous zircon grains that are known to have been produced by the intensive igneous activity in Southwest Japan. Based on the youngest zircon grain ages, the youngest cluster ages, and age compositions, the depositional age of the Mugitani Formation corresponds to that of the Yukawa Formation, ranging from Albian to Cenomanian.

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

“…In the Cretaceous Shimanto AC, YC1σ ages of (106.6 ±4.9) Ma (2σ, MSWD = 0.5), (108.6 ±3.6) Ma (2σ, MSWD = 0.2), and (105.5 ±3.5) Ma (2σ, MSWD = 1.0) (samples 15111701, 15100202, and 15083005, respectively) have been reported previously for the Mg1 Unit of the Mugitani Complex by Shimura et al () (the YC1σ ages were revised in the erratum to Shimura et al () cited in doi:10.5575/geosoc.erratum_2018.02.) (Figure ).…”

“…Distributions of YSG and YC1σ ages for the detrital zircon U–Pb dating results of Shimura et al () and this study. The YC1σ ages (samples 15111701, 15100202, and 15083005) of Shimura et al () were revised in the erratum to Shimura et al () cited in doi:10.5575/geosoc.erratum_2018.02. Age uncertainties are given at the 2σ level.…”

“…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.…”

“…The reproducibility of NIST SRM 610 (%2SD) was 1.0 %–4.8 %, 0.8 %–4.6 %, and 0.9 %–3.8 % for the 206 Pb/ 238 U, 208 Pb/ 232 Th, and 207 Pb/ 206 Pb ratios, respectively, and these analytical values were reflected as the external errors for the obtained ages (values of %2SD that are < 5 % are considered reliable). In addition, we refer to the youngest single grain ages and the youngest cluster ages reported by Shimura et al () in our discussions. The reproducibility of NIST SRM 610 in analyzing these youngest ages was 1.1 %–4.1 %, 1.7 %–4.8 %, and 1.3 %–3.0 % for the 206 Pb/ 238 U, 208 Pb/ 232 Th, and 207 Pb/ 206 Pb ratios, respectively, falling within 5 %.…”

“…For our study, we focused on the Mugitani Complex in the Kouyasan Belt. Shimura, Tokiwa, Takeuchi, and Yamamoto () carried out U–Pb dating of detrital zircons from this complex, and they reported the youngest single grain (YSG) ages and the youngest cluster (YC1σ) ages (Dickinson & Gehrels, ) to be ca . 110–100 Ma (the youngest cluster ages were revised in the erratum to Shimura et al () cited in doi:10.5575/geosoc.erratum_2018.02.…”