[SGL34-P07] Formation ages of the Konosuyama Formation and the Shinobu Diorite and the provenance of the Douge Formation in the Noto Peninsula, central Japan
Keywords:Miocene, Neogene, U–Pb dating, zircon, SW Japan
The Oligocene or younger volcanic and sedimentary rocks widely occur in the Noto Peninsula. This study aims to explore the formation ages of the Konosuyama Formation and the Shinobu Diorite and the provenance of the Douge Formation in the Noto Peninsula, central Japan. We performed the U-Pb dating of igneous and detrital zircons from these and related geological units.
In the north of the Noto Peninsula, the Oligocene Konosuyama Formation, consisting mainly of andesite, is widely distributed, and the Shinobu Diorite intrudes into it (Kano, 2018). In the northwest of the Noto Peninsula, on the other hand, the Besshodake Andesite (the former Anamizu Formation) and the Nawamata and Douge formations (e.g. Ozaki, 2010) cover the constituent rocks of the Hida Belt. The Douge Formation unconformably covers the Nawamata Formation and consists mainly of conglomerate, with minor sandstones, mudstones, and dacite tuffs. The conglomerate contains round pebbles to cobbles of mainly andesite, rhyolite, tuff, and granite.
We collected lava of the Konosuyama Formation, the Shinobu Diorite, and a granite cobble and sandstone of the Douge Formation. Moreover, we collected the Shirakawa Granite cutting the Nohi Rhyolite in the Shirakawa Village of Gifu Prefecture. We extracted zircons from these rock samples and conducted U-Pb dating with the LA-ICPMS equipped in the Graduate School of Environmental Studies of Nagoya University. We adopted concordant data with the error ellipse (2σ) on the concordia diagram (a 206Pb/238U–207Pb/235U plot) intersecting the concordia curve.
Results of the zircon U–Pb dating
The dating results are shown in the attached table.
The weighted mean age of the youngest age cluster of the Konosuyama Formation and the Shinobu Diorite was 32.8 ± 2.3 Ma and 28.09 ± 0.67 Ma, respectively. Hence, the andesite of the Konosuyama Formation likely formed at about 33 Ma, and the Shinobu Diorite intruded into it at about 29 Ma.
Four zircon age clusters of 23 Ma, 59–74 Ma, 173–201 Ma, and 252–267 Ma were obtained from the sandstone sample of the Douge Formation, and the granite cobble from the same outcrop contained 181–206 Ma zircons. The age of this granite cobble is similar to that of the Hida Younger Granites (e.g., Takahashi et al., 2010).In addition, the formation age of the Hida Older Granites was about 266–229 Ma (Horie et al., 2010; Takahashi et al., 2018). Hence, it is likely that 173–201 Ma and 252–267 Ma zircons in the Douge sandstone are from the Hida Granites. The source of 23 Ma zircons in the Douge sandstone is probably the 22–23 Ma Moonstone Rhyolite (Ota et al., 2019; Yamada, 2019) to the south of the study area. The source of the 59–74 Ma zircons from the Douge sandstone is most likely the Nohi and Futomiyama rhyolites of 66–72 Ma (Hoshi et al., 2016; Kaneko et al., 2019). Moreover, the 60–66 Ma Shirakawa Granite, which cuts the Nohi Rhyolite 100 km to the south of the study area, is a probable source of zircons in the Douge sandstone. In this way, the provenance of Douge sandstone may have covered the distribution areas of the Nohi Rhyolite, Fotomiyama Group, Shirakawa Granite, and Moonstone Rhyolite distributed in the Toyama and Gifu prefectures to the south.
On the other hand, we have not confirmed the zircon supply from the Bessyodake Andesite (K–Ar age of 15–17 Ma: Shibata et al., 1981) covered by the Douge Formation, the Konosuyama Formation, and the Shinobu Diorite, which are widely distributed to the east of the Douge Formation. This result is concordant with the northward paleocurrent direction in the Douge formation (Yoshioka et al.,1984).
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