11:00 〜 13:00
[SCG42-P03] Thermal/denudation histories in the Tanigawa-dake region based on multi-thermochronometries
キーワード：谷川岳、ジルコン ウラン-鉛年代、低温熱年代学、熱史/削剥史、後期中新世-鮮新世花崗岩、水上
Granites are generally formed at a crustal depth of a few to dozen kilometers. Regions where young granites are exposed could have been extremely rapidly uplifted and denudated. In fact, granites younger than ~5 Ma have been identified along plate convergent boundaries in the world [1]. Such young granites were also reported in Japanese islands located along subduction zones, whose intrusive ages were determined based on zircon U-Pb dating (closure temperature (Tc) > ~900 ℃), e.g., for the Kurobegawa granite in the Hida mountain range [2] and the Tanzawa Tonalite complex in the South Fossa Magna [3].
The Tanigawa-dake area is located on the back arc side of Northeast Japan arc and is included in Niigata-Kobe tectonic zone [4]. The lithology of this area is mainly composed of late Cretaceous to Paleogene granites (e.g., Minakami quartzdiorite) and Pliocene Tanigawa-dake granites (Akayu, Tanigawa and Makihata bodies) [5]. For the Tanigawa and Makihata bodies, the intrusive ages were estimated to be ~4.0–3.2 Ma from the zircon U-Pb ages [5], whereas cooling ages around ~350–400 ℃ and ~280 ℃ were obtained by biotite K-Ar ages (~3.9–3.1 Ma [6,7]) and zircon fission track ages (~3.3–2.9 Ma [8]), respectively. In this study, we performed (1) zircon U-Pb dating for unmeasured localities of the Tanigawa-dake Pliocene granites in [5], including the Akayu body, to determine the precise intrusive ages and (2) zircon and apatite (U-Th)/He dating (the Tc are 160–200 ℃ and 55–80 ℃, respectively) to estimate the thermal/denudation histories at the shallow crust. As a result, we obtained zircon U-Pb ages of ~6.0–3.2 Ma together with [5], zircon (U-Th)/He (ZHe) ages of ~3.1–1.3 Ma, and apatite (U-Th)/He (AHe) ages of ~3.3–1.2 Ma for the Tanigawa-dake granites and ~2.4 Ma for the Minakami quartzdiorite. Considering difference in the zircon U-Pb ages, the three bodies of the Tanigawa-dake granites intruded at different timings, ~6.0–5.5 Ma, ~4.0 Ma and ~3.3–3.2 Ma. The cooling paths were plotted based on the zircon U-Pb, ZHe and AHe ages, and the cooling rates between Tc of AHe and surface temperature (10 ℃) were converted into the denudation rates assuming that the cooling reflects denudation rather than initial cooling of the granitic magma. The caluculated rates range ~0.5–0.6 mm/yr for the Minakami quartzdiorite and the Makihata body in the eastern side of the main ridge, and ~1.1 mm/yr for the Tanigawa body in the western side. All AHe ages in the eastern side at ~2.8–2.0 Ma, in spite of the zircon U-Pb ages of ~109.4 Ma [5] and ~4.0 Ma, indicate that the area was denudated uniformly at this timing. In addition, denudation rates caluculated from AHe ages in the Tanigawa-dake area are comparable to those of the Tanzawa mountain [9] and the Ou Backbone range [10], and are faster than those of the back-arc side of the Northeastern Japan arc [10]. Thus, the denudation rates in the Tanigawa-dake area are equivalent to the rates of active crustal movement areas, such as an arc-arc collision zone (Tanzawa mountain) and strain concentrated zone (Ou Backbone range).
Acknowledgements
This study was funded by the Ministry of Economy, Trade and Industry (METI), Japan as part of its R&D supporting program titled "Establishment of Advanced Technology for Evaluating the Long-term Geosphere Stability on Geological Disposal Project of Radioactive Waste (Fiscal Years 2021), Grunt Number JPJ007597". This study was also supported by MEXT KAKENHI Grant Number 21K03730.
References
[1] Harayama (1992) Geology, 20, 657–660, [2] Ito et al. (2013) Sci. Rep. 3:1306, [3] Tani et al. (2010) Geology, 38, 215–218, [4] Sagiya et al. (2000) Pure and Applied Geophys, 157, 2303–2322, [5] Minami et al. (2021) EPS. 73:231, [6] Ohira and Honda (1999) FTNL, 12, 61–63, [7] Kawano et al. (1992) Jour. Geol. Soc. Japan, 87, 221–225, [8] Sato (2016) Bull. Gunma. Mus. Nat. His., 20, 85–104, [9] Yamada & Tagami (2008) Jour. Geophys. Res., 113, B03402, [10] Sueoka et al. (2017) EPS. 69:79.
The Tanigawa-dake area is located on the back arc side of Northeast Japan arc and is included in Niigata-Kobe tectonic zone [4]. The lithology of this area is mainly composed of late Cretaceous to Paleogene granites (e.g., Minakami quartzdiorite) and Pliocene Tanigawa-dake granites (Akayu, Tanigawa and Makihata bodies) [5]. For the Tanigawa and Makihata bodies, the intrusive ages were estimated to be ~4.0–3.2 Ma from the zircon U-Pb ages [5], whereas cooling ages around ~350–400 ℃ and ~280 ℃ were obtained by biotite K-Ar ages (~3.9–3.1 Ma [6,7]) and zircon fission track ages (~3.3–2.9 Ma [8]), respectively. In this study, we performed (1) zircon U-Pb dating for unmeasured localities of the Tanigawa-dake Pliocene granites in [5], including the Akayu body, to determine the precise intrusive ages and (2) zircon and apatite (U-Th)/He dating (the Tc are 160–200 ℃ and 55–80 ℃, respectively) to estimate the thermal/denudation histories at the shallow crust. As a result, we obtained zircon U-Pb ages of ~6.0–3.2 Ma together with [5], zircon (U-Th)/He (ZHe) ages of ~3.1–1.3 Ma, and apatite (U-Th)/He (AHe) ages of ~3.3–1.2 Ma for the Tanigawa-dake granites and ~2.4 Ma for the Minakami quartzdiorite. Considering difference in the zircon U-Pb ages, the three bodies of the Tanigawa-dake granites intruded at different timings, ~6.0–5.5 Ma, ~4.0 Ma and ~3.3–3.2 Ma. The cooling paths were plotted based on the zircon U-Pb, ZHe and AHe ages, and the cooling rates between Tc of AHe and surface temperature (10 ℃) were converted into the denudation rates assuming that the cooling reflects denudation rather than initial cooling of the granitic magma. The caluculated rates range ~0.5–0.6 mm/yr for the Minakami quartzdiorite and the Makihata body in the eastern side of the main ridge, and ~1.1 mm/yr for the Tanigawa body in the western side. All AHe ages in the eastern side at ~2.8–2.0 Ma, in spite of the zircon U-Pb ages of ~109.4 Ma [5] and ~4.0 Ma, indicate that the area was denudated uniformly at this timing. In addition, denudation rates caluculated from AHe ages in the Tanigawa-dake area are comparable to those of the Tanzawa mountain [9] and the Ou Backbone range [10], and are faster than those of the back-arc side of the Northeastern Japan arc [10]. Thus, the denudation rates in the Tanigawa-dake area are equivalent to the rates of active crustal movement areas, such as an arc-arc collision zone (Tanzawa mountain) and strain concentrated zone (Ou Backbone range).
Acknowledgements
This study was funded by the Ministry of Economy, Trade and Industry (METI), Japan as part of its R&D supporting program titled "Establishment of Advanced Technology for Evaluating the Long-term Geosphere Stability on Geological Disposal Project of Radioactive Waste (Fiscal Years 2021), Grunt Number JPJ007597". This study was also supported by MEXT KAKENHI Grant Number 21K03730.
References
[1] Harayama (1992) Geology, 20, 657–660, [2] Ito et al. (2013) Sci. Rep. 3:1306, [3] Tani et al. (2010) Geology, 38, 215–218, [4] Sagiya et al. (2000) Pure and Applied Geophys, 157, 2303–2322, [5] Minami et al. (2021) EPS. 73:231, [6] Ohira and Honda (1999) FTNL, 12, 61–63, [7] Kawano et al. (1992) Jour. Geol. Soc. Japan, 87, 221–225, [8] Sato (2016) Bull. Gunma. Mus. Nat. His., 20, 85–104, [9] Yamada & Tagami (2008) Jour. Geophys. Res., 113, B03402, [10] Sueoka et al. (2017) EPS. 69:79.