JpGU-AGU Joint Meeting 2020

Presentation information

[E] Poster

S (Solid Earth Sciences ) » S-GL Geology

[S-GL33] Geologic structure and tectonic history of East Asia and Japanese Islands

convener:Jun Hosoi(Geological Survey of Japan, Institute of Geology and Geoinformation, AIST), Makoto Otsubo(Geological Survey of Japan, Research Institute of Earthquake and Volcano Geology)

[SGL33-P06] Provenance of the Yezo forearc basin in Northeast Japan from detrital zircon U–Pb age spectra

*Koh Kubomi1, Shin-ichi Sano2, Minami Murakami3, Hiroto Ota1, Mitsuhiro Nagata1, Koshi Yamamoto4, Shigeru Otoh2 (1.Graduate School of Science and Engineering, University of Toyama, 2.Academic Assembly, Faculty of Sustainable Design, University of Toyama, 3.OYO Corporation, 4.Graduate School of Environmental Studies, Nagoya University)

Keywords:Cretaceous, Detrital Zircon, Yezo forearc basin, Yezo Group, Futaba Group, Nakaminato Group

In Hokkaido, the Cretaceous to Paleogene Yezo Group narrowly extends in the north direction, and the Upper Cretaceous Futaba and Nakaminato groups sporadically occur in the Pacific side of Northeast Japan1). They are thought to have been deposited in the forearc basin along the East Asian active continental margin, called the “Yezo forearc basin”1). The modal composition of sandstones2),3) of the Yezo Group, and the detrital zircon age spectra4) suggested that its provenance is the East Asian continental margin. However, we have not been able to identify the specific provenance of each formation of the Yezo Group and its equivalents. In this presentation, we discuss the provenance of the Yezo forearc basin from the detrital zircon age spectra of sandstone samples and the database of the age and distribution of igneous rocks in East Asia.

Geological Setting
The Yezo Group consists mainly of black mudstone and sandstone. It is subdivided into the Shuparogawa (Srs), Hikagenosawa (Hgs), Mikasa (Mks), Haborogawa (Hbs), and Hakobuchi formations (Hks1, Hks2), in ascending order5). The Futaba Group, consisting mainly of sandstone and mudstone, unconformably covers the granites of the Abukuma Belt. The group comprises the Ashizawa (Aos), Kasamatsu, and Tamayama formations (Tys), in ascending order6). The Nakaminato Group consists mainly of turbidite and is subdivided into the Hiraiso and Isoai formations (Is8), in ascending order7). The codes for the nine sandstone samples analyzed in this study are in the parentheses.

Analytical Method
We separated detrital zircons from each sandstone sample, measured their U/Pb isotopic ratios with LA-ICPMS, and calculated the U–Pb ages. YC is the weighted mean of the youngest zircon 206Pb/238U age and the other ages with the error bar (±2σ) overlapping with the youngest age.

Figure 1 summarizes the result of our measurements of the Yezo, Futaba, and Nakaminato groups.

Samples Srs, Hgs, and Mks have zircons crystallized during the magmatic hiatus in Korea8) (158–138 Ma), 230–210 Ma, and 2400–1800 Ma zircons. The database indicates that igneous rocks of these ages are widely exposed in South China9). In addition, paleomagnetic studies10) suggested that the Hikagenosawa and Takinosawa formations of the Yezo Group was deposited at 10°N and moved to the north during the Cretaceous. Thus, the provenance of the Yezo Group was probably South China during the Aptian to Cenomanian.
The detrital zircons from sample Aos are mostly 120–100 Ma in age and likely originated from the 120–100 Ma granites in the Abukuma Belt11). 96–83 Ma zircons occupy more than 70% of the detrital zircons from sample Tys and likely originated from the ca. 90–80 Ma igneous rock bodies in the Inner Zone of Southwest Japan12).
About 95% of the detrital zircons from sample Hbs are 100–80 Ma. Thus, coeval, 100–80 Ma igneous rocks of an active igneous belt must have occupied the hinterland of sample Hbs.
From the Hakobuchi Formation of the Yezo Group, sample Hks1 has 250–203 Ma, 310 Ma, and 490 Ma detrital zircons, and about 18% of the detrital zircons from sample Hks2 are Paleozoic (excluding Permian) ones (Fig. 1). Sample Is8 has 210 Ma and 1800 Ma zircons. 500–400 Ma and 300 Ma igneous rock bodies widely crop out in Far East Russia13). The provenance of the Yezo and Nakaminato groups in the latest Cretaceous was probably Far East Russia. These lines of evidence strongly suggest that the provenance of the Yezo forearc basin continuously changed from south to north.

Reference 1) Ando, 2003: J. Asian Earth Sci., 21, 921–; 2) Kiminami, 1986: J. Sediment. Soc. Japan, 4, 1–; 3) Takashima et al., 2001: J. Geol. Soc. Japan, 107, 359–; 4) Murakami et al.,2016: 2016JpGU: SGL37-P01; 5) Takashima et al., 2004: Cretaceous Res., 25, 365–; 6) Ando et al., 1995: J. Geol., 104, 284–; 7) Masukawa and Ando, 2018: Cretaceous Res, 91, 362–; 8) Lee et al., 2010: Isl. Arc, 19, 647–; 9) Wang et al., 2013: Gondwana. Res., 23, 1273–; 10) Tamaki and Itoh, 2008: Isl. Arc, 17, 270–; 11) Tsuchiya et al.,2013: J. Geol. Soc. Japan., 119, 154–; 12) Iida et al., 2015: Isl. Arc, 24, 205–; 13) Bi et al., 2014: J. Asian Earth Sci., 96, 301–.