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[SVC29-02] The Sanzugawa Caldera: a caldera that experienced a large caldera-forming eruption in the Quaternary
Keywords:caldera, U-Pb dating, zircon, Quaternary, Tohoku region
The Sanzugawa Caldera is the largest caldera system out of more than 80 calderas formed during the Miocene–Pliocene time in the NE Japan arc or the Tohoku region (Ito et al., 1989; Yoshida et al., 2014). The caldera is filled with pumice flow deposits (the Torageyama Formation) that were produced during the caldera formation. The formation is mainly composed of crystal-rich dacite lapilli tuff, tuff breccia and tuff with >1500 m thick (Oki, 2016). The caldera formation period had been assumed to be 6–5 Ma and 4–3 Ma based on K-Ar dating method (Takeno, 1988), while Ito (2000) assumed that the volcanic activity that formed the Sanzugawa Caldera continued as late as 1 Ma based on zircon fission-track (FT) dating method. Oki (2016) also assumed the formation age to be 1.2 Ma based on K-Ar. Meanwhile, in the "Volcanoes of Japan" database by Geological Survey of Japan (https://gbank.gsj.jp/volcano/Quat_Vol/volcano_data/D42.html), there is a comment that "The Sanzugawa Caldera was formed at 6 Ma, and it is unacceptable that it was formed at 1.2 Ma asserted by Oki (2016)".
It is important to clarify the formation age of the Sanzugawa Caldera because it is closely related to a volcano development model which asserts that calderas were formed in the Pliocene and stratovolcanoes were formed in the Quaternary in the NE Japan arc (Yoshida et al., 2014). Here, I applied the zircon U-Pb method to date the Torageyama Formation. The results were as follows: The sample dated as 6.0 ± 1.6 Ma (2σ) by K-Ar (Takeno, 1988) that was collected in the central uplift zone of the caldera yielded a U-Pb age of 4.38 ± 0.12 Ma. The sample dated as ~1 Ma by FT (Ito, 2000) yielded single crystal U-Pb ages of 6–1 Ma. The sample dated as 1.2 ± 0.4 Ma by FT (Ito, 1996) yielded a U-Pb age of 1.70 ± 0.14 Ma. Those FT dated samples were collected in the western basin of the caldera. Therefore, it seems reasonable to assume that the caldera–forming volcanic activity initiated at ~6 Ma and it continued as late as 2–1 Ma or ~1.5 Ma in the western part of the Sanzugawa Caldera. A caldera-forming volcanic activity at 3–1.4 Ma was also reported for the Akakura Caldera, to the south of the Sanzugawa Caldera (Otake, 2000). These indicate that a series of caldera-forming eruptions occurred at ~2 Ma in the southern part of Akita Prefecture and in the northern part of Yamagata Prefecture. This may contradict the assumption that a volcanic lull exists during 3.5–1.5 Ma in the NE Japan arc (Kimura et al., 2015, Mahony et al., 2016) and call the need to date 'Pliocene' calderas in the NE Japan arc by zircon U-Pb.
References:
Ito, 1996, Ph. D. thesis of Kyoto Univ. Fac. Sci., 122p; Ito, 2000, J. Geotherm. Res. Soc. Japan 22, 9–21; Ito et al., 1989, Mem. Geol. Soc. Japan., 32, 409–429; Kimura et al., 2015, Geochem. Geophys. Geosyst., 16, doi:10.1002/2015GC005854; Mahony et al., 2016, Geochem. Geophys. Geosyst., 17, doi:10.1002/2016GC006362; Oki, 2016, JpGU, SVC48-P06; Otake, M., 2000, Jour. Geol. Soc. Japan, 106, 205–222; Takeno, 1988, Rept. Geol. Surv. Japan, 268, 191–210; Yoshida et al., 2014, Geol. Soc. Spec. Publ., 385, 335–387.
It is important to clarify the formation age of the Sanzugawa Caldera because it is closely related to a volcano development model which asserts that calderas were formed in the Pliocene and stratovolcanoes were formed in the Quaternary in the NE Japan arc (Yoshida et al., 2014). Here, I applied the zircon U-Pb method to date the Torageyama Formation. The results were as follows: The sample dated as 6.0 ± 1.6 Ma (2σ) by K-Ar (Takeno, 1988) that was collected in the central uplift zone of the caldera yielded a U-Pb age of 4.38 ± 0.12 Ma. The sample dated as ~1 Ma by FT (Ito, 2000) yielded single crystal U-Pb ages of 6–1 Ma. The sample dated as 1.2 ± 0.4 Ma by FT (Ito, 1996) yielded a U-Pb age of 1.70 ± 0.14 Ma. Those FT dated samples were collected in the western basin of the caldera. Therefore, it seems reasonable to assume that the caldera–forming volcanic activity initiated at ~6 Ma and it continued as late as 2–1 Ma or ~1.5 Ma in the western part of the Sanzugawa Caldera. A caldera-forming volcanic activity at 3–1.4 Ma was also reported for the Akakura Caldera, to the south of the Sanzugawa Caldera (Otake, 2000). These indicate that a series of caldera-forming eruptions occurred at ~2 Ma in the southern part of Akita Prefecture and in the northern part of Yamagata Prefecture. This may contradict the assumption that a volcanic lull exists during 3.5–1.5 Ma in the NE Japan arc (Kimura et al., 2015, Mahony et al., 2016) and call the need to date 'Pliocene' calderas in the NE Japan arc by zircon U-Pb.
References:
Ito, 1996, Ph. D. thesis of Kyoto Univ. Fac. Sci., 122p; Ito, 2000, J. Geotherm. Res. Soc. Japan 22, 9–21; Ito et al., 1989, Mem. Geol. Soc. Japan., 32, 409–429; Kimura et al., 2015, Geochem. Geophys. Geosyst., 16, doi:10.1002/2015GC005854; Mahony et al., 2016, Geochem. Geophys. Geosyst., 17, doi:10.1002/2016GC006362; Oki, 2016, JpGU, SVC48-P06; Otake, M., 2000, Jour. Geol. Soc. Japan, 106, 205–222; Takeno, 1988, Rept. Geol. Surv. Japan, 268, 191–210; Yoshida et al., 2014, Geol. Soc. Spec. Publ., 385, 335–387.