Geochronological study of the Atokura Formation and contact igneous rocks in Shimonita-machi, Gunma Prefecture, central Japan

Bando, Koki*; Nagata, Mitsuhiro   ; Asahara, Yoshihiro*; Ishizaki, Yasuo*; Oto, Shigeru*

We studied the geochronology and geochemistry of the Cretaceous Atokura Formation called Atokura nappe and intrusive rocks in Shimonita, Gunma Prefecture. The depositional age of the Atokura Formation has not been determined whether it was the Early Cretaceous (Barremian) or Late Cretaceous (Cenomanian-Campanian). The youngest cluster dates obtained from 4 sandstone samples of the Atokura Formation are 182.8 Ma, 129.8 Ma, 123.7 Ma, and 115.1 Ma. Zircons from igneous rocks in the Atokura Formation yielded U-Pb ages of 123.2 Ma, and their zircons show Hf(t)=3.0 to 3.8. The depositional age of the Atokura Formation generally supported the Barremian, but younger zircon U-Pb date was obtained from one sample. The intrusive rocks are similar to the U-Pb dates and Hf isotopes of zircons obtained from Kitakami or Abukuma granitoids in NE Japan.