5:15 PM - 6:45 PM
[SVC30-P05] Zircon U-Pb dating of large-scale pyroclastic flow deposits from Naruko and Onikobe calderas, northwestern Miyagi Prefecture, Japan
Keywords:caldera, U-Pb dating, Pyroclastic flow deposits, zircon
The Quaternary Naruko and Onikobe calderas are clustered in northwestern Miyagi Prefecture, Japan. In this caldera cluster, Neogene calderas such as Sanzugawa and Akakura, are adjacent to the west of Naruko and Onikobe calderas (Otake, 2000). Large-scale pyroclastic flow deposits from Naruko and Onikobe calderas are located east - southeast of the calderas, and consist of four units: Ikezuki pyroclastic flow deposit (pfl) (250±80 ka: FT age) (hereafter, age errors are shown as 2σ) and Shimoyamasato pfl (210±90 ka: FT age) originated from Onikobe caldera, meanwhile Nisaka pfl (88-112 ka: from tephrochronology) and Yanagisawa pfl (40-50 ka: 14C age, etc.) originated from Naruko caldera (e.g. Tsuchiya et al., 1997; Nagahashi et al., 2007; Tomiya and Miyagi, 2020; Nakai, 1988). To determine the absolute ages with higher accuracy and precision, we performed zircon U-Pb dating of these pyroclastic flow deposits. Zircon grains in white pumice of Ikezuki, Shimoyamasato, and Nisaka pfls were extracted and U-Pb dating was carried out based on the method of Ito (2023). Yanagisawa pfl is too young for the application of U-Pb dating (approx.>100 ka, Ito, 2014) so 14C dating was carried out on charcoal using accelerator mass spectrometry (AMS) at Yamagata University.
The U-Pb ages for 155 zircon grains commonly show three ranges of age peaks as <1 Ma, 6-9 Ma, and 90-100 Ma, for all the three pfls (Fig. 1). We focused on the youngest peak of <1 Ma because zircon crystallized at the last phase of the magmatic system should be adopted to the estimation of eruption age. As a result, a U-Pb age of 233 ± 56 ka (n = 10) was obtained for Ikezuki pfl; while, 420 ± 210 ka (n = 1) and 201 ± 98 ka (n = 5) were obtained for Shimoyamasato and Nisaka pfls, respectively. Charcoal samples in the Yanagisawa pfl yielded a 14C calendar age of about 32-33 cal ka.
For the Ikezuki pfl, our U-Pb age has a smaller error (± 56 ka) than the FT age in a previous study (± 80 ka). Adopting the newly obtained U-Pb results for Ikezuki pfl, we speculate that the time separating the eruption of Ikezuki and Shimoyamasato pyroclastic flows, from Onikobe caldera, is shorter than previously thought. The U-Pb dates of Shimoyamasato and Nisaka pfls in this study were not adoptable due to insufficient zircon, resulting in larger errors.
The age of Yanagisawa plf is more than 8 thousand years younger than previously reported (Nakai, 1988). As multiple flow units were recognized in Yanagisawa pfls (Tsuchiya et al., 1997), we suggest that the difference between our 14C date and previous dates could come from dating different flow units. In future work, we will do systematic sampling and dating of multiple flow units of Yanagisawa pfl.
The second age peak range of 6-9 Ma can be correlated with the U-Pb ages of ejecta from adjacent Sanzugawa caldera reported by Ito (2023). This suggests that magmatic activity in Naruko and Onikobe calderas occurred at 6-9 Ma, the same as in Sanzugawa caldera. The oldest age peak range of 90-100 Ma matches with the age of Cretaceous granitoids, which are basement rocks in this area, suggesting the zircons are either xenocrysts mechanically incorporated or were captured at the partial melting of the crust during the generation of silicic magmas under Naruko and Onikobe calderas. More work is required to clarify this suggestion.
Reference
Ito, H. (2014) JVGR. 289, 210-223.
Ito, H. (2023) Front. Earth Sci. 10:964773.
Ludwig, K.R. (2012) Berkeley Geochronol. Center Spec. Publ, 5, 75.
Nagahashi, Y. et al., (2007) Quat Res. 464, 305-325.
Nakai, N. (1988) Historical Archives Collection of Tohoku region. 23, Historical Museum in Tohoku region and Stone Age Culture Discussion Group, 52.
Otake, M. (2000) JGSJ. 106, 205-222.
Tomiya, A and Miyagi, I. (2020) Bull Volcanol Soc Jpn. 65, 13-18.
Tuchiya, N. et al., (1997) Explanatory Text of the Geological map of Japan, Scale 1:50,000 “Iwagasaki”
The U-Pb ages for 155 zircon grains commonly show three ranges of age peaks as <1 Ma, 6-9 Ma, and 90-100 Ma, for all the three pfls (Fig. 1). We focused on the youngest peak of <1 Ma because zircon crystallized at the last phase of the magmatic system should be adopted to the estimation of eruption age. As a result, a U-Pb age of 233 ± 56 ka (n = 10) was obtained for Ikezuki pfl; while, 420 ± 210 ka (n = 1) and 201 ± 98 ka (n = 5) were obtained for Shimoyamasato and Nisaka pfls, respectively. Charcoal samples in the Yanagisawa pfl yielded a 14C calendar age of about 32-33 cal ka.
For the Ikezuki pfl, our U-Pb age has a smaller error (± 56 ka) than the FT age in a previous study (± 80 ka). Adopting the newly obtained U-Pb results for Ikezuki pfl, we speculate that the time separating the eruption of Ikezuki and Shimoyamasato pyroclastic flows, from Onikobe caldera, is shorter than previously thought. The U-Pb dates of Shimoyamasato and Nisaka pfls in this study were not adoptable due to insufficient zircon, resulting in larger errors.
The age of Yanagisawa plf is more than 8 thousand years younger than previously reported (Nakai, 1988). As multiple flow units were recognized in Yanagisawa pfls (Tsuchiya et al., 1997), we suggest that the difference between our 14C date and previous dates could come from dating different flow units. In future work, we will do systematic sampling and dating of multiple flow units of Yanagisawa pfl.
The second age peak range of 6-9 Ma can be correlated with the U-Pb ages of ejecta from adjacent Sanzugawa caldera reported by Ito (2023). This suggests that magmatic activity in Naruko and Onikobe calderas occurred at 6-9 Ma, the same as in Sanzugawa caldera. The oldest age peak range of 90-100 Ma matches with the age of Cretaceous granitoids, which are basement rocks in this area, suggesting the zircons are either xenocrysts mechanically incorporated or were captured at the partial melting of the crust during the generation of silicic magmas under Naruko and Onikobe calderas. More work is required to clarify this suggestion.
Reference
Ito, H. (2014) JVGR. 289, 210-223.
Ito, H. (2023) Front. Earth Sci. 10:964773.
Ludwig, K.R. (2012) Berkeley Geochronol. Center Spec. Publ, 5, 75.
Nagahashi, Y. et al., (2007) Quat Res. 464, 305-325.
Nakai, N. (1988) Historical Archives Collection of Tohoku region. 23, Historical Museum in Tohoku region and Stone Age Culture Discussion Group, 52.
Otake, M. (2000) JGSJ. 106, 205-222.
Tomiya, A and Miyagi, I. (2020) Bull Volcanol Soc Jpn. 65, 13-18.
Tuchiya, N. et al., (1997) Explanatory Text of the Geological map of Japan, Scale 1:50,000 “Iwagasaki”