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[SCG47-P03] Uplift/denudation histories around the Tanigawa-dake region based on (U-Th)/He thermochronometries
Granites younger than a few Ma are supposed to be exposed to Earth’s surfaces associated with extremely rapid uplift and denudation because granites are generally formed at a depth of a few to dozen kilometers. Such young granites are mainly identified along plate convergence boundaries (Harayama, 1992). In Japan, granites yielding a few Ma of zircon U-Pb ages have been reported in the Hida range, Northern Japanese Alps (Ito et al., 2013) and in Tanzawa mountains, South Fossa Magna region (Tani et al., 2010). Moreover, application of lower-temperature thermochronology based on fission track and (U-Th)/He dating suggested possibilities of extremely rapid uplift and denudation in these regions (Ito and Tanaka, 1999; Yamada and Harayama, 1999; Yamada, 1999; Yamada and Tagami, 2008).
Such young granodiorites are also exposed in the Tanigawa-dake region (the Tanigawa-dake body; Sato, 2016), which were formed in the Miocene to Pliocene based on biotite K-Ar dating (Sato,2016) and whole rock Rb-Sr dating (Ohira et al., 1999). However, the formation ages were not determined accurately because 1) the closure temperature of biotite K-Ar system is lower than the granitic solidus by ~300°C and because 2) the Rb-Sr age has a large error and was obtained only at 1 locality. Recently, Minami et al (2021) reported the zircon U-Pb ages at 4 localities in the Tanigawa-dake region. Zircon U-Pb ages were estimated to be 3.19 ± 0.15 Ma and 3.32 ± 0.15 Ma in the western body of the Tanigawa-dake granodiorite, 3.95 ± 0.14 Ma in the eastern body, and 109.3 ± 3.2 Ma in the late Cretaceous Minakami granodiorite. These dating results provided the accurate formation ages of granites in the Tanigawa-dake region. On the other hands, uplift and denudation histories and its mechanism in the Tanigawa-dake region were poorly understood. Although Ohira et al (1999) reported the ZFT ages of 3.3-2.9 Ma and the AFT age of 2.4 Ma in the Tanigawa-dake granodiorite, the cooling history was discussed only at 1 locality.
This study aims to clarify uplift and denudation histories in the Tanigawa-dake region based on apatite and zircon (U-Th)/He dating (hereinafter, AHe and ZHe dating). The closure temperatures of AHe and ZHe systems are ~60°C and ~180°C, being one of the lowest in the general thermochronometers (e.g. Ault, 2019). The closure temperatures are equivalent to crustal depths of 2 and 6 km assuming a general geothermal gradient. Thus, we can estimate the uplift and denudation histories in shallower crust than 2 and 6 km with these methods.
We newly obtained the AHe and ZHe ages at 6 localities; the ages are hereinafter shown as weighted mean ages ± 95% confidence intervals. The ZHe and AHe ages were calculated at 2.6-1.4 Ma and 2.5-1.3 Ma in the western body, 3.1-2.7 Ma and 3.3-2.0 Ma in the eastern body, and 2.4 ± 0.2 Ma and 30.4 ± 0.9 (1σ) Ma in the Minakami granite - the ZHe age of Minakami granite is used as reference due to dispersed single grain ages. Together with the zircon U-Pb ages, the AHe and ZHe ages at each locality are consistent each other considering the relations of the closure temperatures. The AHe and ZHe ages are less likely to reflect the initial cooling of granitic intrusion because these ages differ from the corresponding zircon U-Pb ages over the ranges of 2σ. The Cretaceous Minakami granite were cooled slowly above ~180°C based on the reference ZHe age, and then rapidly exhumed together with the Tanigawa-dake granodiorite in 3-2 Ma considering the AHe ages. The time-temperature paths obtained from the cooling ages and the closure temperatures suggest that 1 locality of the western body was cooled more slowly than the other localities of the Tanigawa-dake granodiorite and that the mean denudation rate of 2 localities changed in ~3 Ma.
Such young granodiorites are also exposed in the Tanigawa-dake region (the Tanigawa-dake body; Sato, 2016), which were formed in the Miocene to Pliocene based on biotite K-Ar dating (Sato,2016) and whole rock Rb-Sr dating (Ohira et al., 1999). However, the formation ages were not determined accurately because 1) the closure temperature of biotite K-Ar system is lower than the granitic solidus by ~300°C and because 2) the Rb-Sr age has a large error and was obtained only at 1 locality. Recently, Minami et al (2021) reported the zircon U-Pb ages at 4 localities in the Tanigawa-dake region. Zircon U-Pb ages were estimated to be 3.19 ± 0.15 Ma and 3.32 ± 0.15 Ma in the western body of the Tanigawa-dake granodiorite, 3.95 ± 0.14 Ma in the eastern body, and 109.3 ± 3.2 Ma in the late Cretaceous Minakami granodiorite. These dating results provided the accurate formation ages of granites in the Tanigawa-dake region. On the other hands, uplift and denudation histories and its mechanism in the Tanigawa-dake region were poorly understood. Although Ohira et al (1999) reported the ZFT ages of 3.3-2.9 Ma and the AFT age of 2.4 Ma in the Tanigawa-dake granodiorite, the cooling history was discussed only at 1 locality.
This study aims to clarify uplift and denudation histories in the Tanigawa-dake region based on apatite and zircon (U-Th)/He dating (hereinafter, AHe and ZHe dating). The closure temperatures of AHe and ZHe systems are ~60°C and ~180°C, being one of the lowest in the general thermochronometers (e.g. Ault, 2019). The closure temperatures are equivalent to crustal depths of 2 and 6 km assuming a general geothermal gradient. Thus, we can estimate the uplift and denudation histories in shallower crust than 2 and 6 km with these methods.
We newly obtained the AHe and ZHe ages at 6 localities; the ages are hereinafter shown as weighted mean ages ± 95% confidence intervals. The ZHe and AHe ages were calculated at 2.6-1.4 Ma and 2.5-1.3 Ma in the western body, 3.1-2.7 Ma and 3.3-2.0 Ma in the eastern body, and 2.4 ± 0.2 Ma and 30.4 ± 0.9 (1σ) Ma in the Minakami granite - the ZHe age of Minakami granite is used as reference due to dispersed single grain ages. Together with the zircon U-Pb ages, the AHe and ZHe ages at each locality are consistent each other considering the relations of the closure temperatures. The AHe and ZHe ages are less likely to reflect the initial cooling of granitic intrusion because these ages differ from the corresponding zircon U-Pb ages over the ranges of 2σ. The Cretaceous Minakami granite were cooled slowly above ~180°C based on the reference ZHe age, and then rapidly exhumed together with the Tanigawa-dake granodiorite in 3-2 Ma considering the AHe ages. The time-temperature paths obtained from the cooling ages and the closure temperatures suggest that 1 locality of the western body was cooled more slowly than the other localities of the Tanigawa-dake granodiorite and that the mean denudation rate of 2 localities changed in ~3 Ma.