No abstract

We measured the uranium-lead (U-Pb) and fission-track (FT) ages of single zircon grains from three samples (TKB , TKB , and TKB ) of tephras that are intercalated with Neogene to Quaternary deposits in the Horonobe area, Hokkaido, Japan, to understand their eruption ages. The weighted mean U-Pb ages and pooled FT ages of the youngest zircon grains are .. Ma ( σ) and .. Ma ( σ) for TKB , and .. Ma ( σ) and .. Ma ( σ) for TKB , respectively. Their U-Pb and FT ages overlap within their respective σ uncertainties, which suggests that their ages correspond to the eruption ages. Specifically, the TKB tephra age is consistent with the diatom fossil ages from the overlying and underlying silt deposits. The FT ages for most of the zircons in TKB are reset to their U-Pb ages or younger. The pooled FT age for all of the analyzed zircons ( .. Ma; σ) and weighted mean U-Pb age for zircon grains within the youngest age range ( .. Ma; σ) are coincident within their respective σ uncertainties. Therefore, their ages could also show the eruption age. TKB and TKB are from the Quaternary Sarabetsu Formation, and their U-Pb and FT ages correspond to the stratigraphy. Although TKB lies at the boundary between the Yuchi and Koetoi formations, its U-Pb and FT ages are significantly older than the reported depositional age of the Yuchi Formation. The Neogene to Quaternary depositional age of this horizon tends to be older from west to east in the Horonobe area. The older age of TKB , which is located in the easternmost part of the study area, is consistent with this stratigraphic trend.

No abstract

The Harachiyama Formation comprises Lower Cretaceous volcanic rocks, which are distributed in the eastern margin of the Kitakami Mountains in northeastern Japan. Whole-rock chemical analysis, UPb zircon dating and Hf isotope analysis of the Harachiyama Formation were performed to constrain the formation age and the magmatic origin was discussed. The lava and tuff samples of the Harachiyama Formation from the Omoto and Tsukue areas supported island-arc rhyolitic rocks (SiO 2 content > 70％) , and yielded the weighted mean UPb ages of 127.8 ± 3.4 Ma and 129.2 ± 2.6 Ma (2σ) , respectively. Eighteen zircon grains from two samples, dated between 141.6 Ma and 123.9 Ma, yielded positive ε Hf (t) values between +5.0 and +8.7. The UPb ages and ε Hf (t) values are consistent with those of the Kitakami Granitoid reported in previous studies. Therefore, it is suggested that the Early Cretaceous volcanic rocks in northeastern Japan, such as the Harachiyama Formation, have the same magmatic origin as the Kitakami Granitoid.

We performed zircon U–Pb dating on the Pliocene Tanigawa-dake granites (Makihata and Tanigawa bodies) and the Cretaceous Minakami quartzdiorite, Northeast Japan Arc. Concordia ages were estimated to be 3.95 ± 0.11 Ma (± 2 sigma) for the Makihata body, 3.18 ± 0.13 Ma and 3.32 ± 0.15 Ma for the Tanigawa body, and 109.4 ± 2.2 Ma for the Minakami quartzdiorite. The Minakami quartzdiorite is possibly correlated to the bedrock in the Ashio belt because the age of the Minakami quartzdiorite is consistent with the zircon U–Pb ages of the earliest Tadamigawa granites (107–62 Ma) which are distributed to the northeast of the Tanigawa-dake region and belong to the Ashio belt. All the zircon U–Pb ages of the Tanigawa-dake granites are older than the previously reported cooling ages, i.e., K–Ar ages and zircon fission-track ages, being consistent with their difference in closure temperature. On the basis of these results, we concluded that the intrusive ages of the Tanigawa-dake granites are ~ 4–3 Ma, which are among the youngest exposed plutons on Earth. The U–Pb ages of the Makihata body and the Tanigawa body are different significantly in the 2 sigma error range. Thus, the Tanigawa body intruded later than the Makihata body by ~ 0.7 Myr. Graphical Abstract