10:00 〜 10:15
[SCG44-03] Exhumation histories of the Pliocene granitoid in the Tanigawa-dake area based on thermochronology and 1D heat modeling
キーワード：鮮新世花崗岩、削剥、熱年代学、1次元熱数値モデリング、Al-in-Hbl地質圧力計
Granitoids are generally emplaced at crustal depths deeper than several kilometers. Therefore, areas where young granitoids are exposed must have been uplifted and exhumed rapidly. Granitoids younger than ~5 Ma are distributed along convergent plate boundaries in the world, which are characterized by active seismic and magmatic events [1]. However, exhumation histories and mechanism of such young granitoids remain poorly understood. Thermochronology is often used to quantitatively estimate the exhumation rates by translating from cooling rates of rock samples with current geothermal gradients. However, in the case of young plutons, the exhumation rates based on thermochronology might be overestimated (e.g., [2]). This is because thermochronological dates of young granitoids might reflect both post-intrusive cooling and exhumation-derived cooling. Therefore, the post-intrusive cooling should be distinguished to clarify more reliable exhumation rates.
The Tanigawa-dake area in Japanese islands located in a northeastern part of the junction zone of the Honshu arc and the Izu-Bonin arc. Late Miocene to Pliocene granitoids (~6.0–3.2 Ma) are exposed in the Tanigawa-dake area revealed using zircon U-Pb dating [3] [4]. Previous research reported zircon and (U-Th)/He (ZHe) dates of 3.3–2.6 Ma and apatite (U-Th-Sm)/He (AHe) dates of 3.3–1.0 Ma for each locality, which represents cooling histories below ~200 ºC [4]. Then, AHe-derived exhumation rates (EAHe) of 0.3–1.8 mm/yr were estimated from these AHe dates and current geothermal gradient of 40–60 ºC/km [5]. In this study, 1D numerical modeling based on heat advection-diffusion-production equations were applied for three localities at two ~4.0 Ma plutons and a ~3.3 Ma pluton to obtain geological meaningful exhumation rates. This method is expected to estimate more accurate exhumation rates and emplacement depths by exploring modeled cooling histories which align with the reported ZU-Pb, ZHe and AHe dates.
As a result, the modeling method suggests that either (1) exhumation at a constant rate after intrusion or (2) 2 steps exhumation history, characterized by an extremely rapid exhumation phase immediately after intrusion for two localities at ~3.3 Ma and ~4.0 Ma plutons. The modeled exhumation rates for the samples are estimated to be 0.9–1.5 mm/yr after ~2.3 Ma, which are consistent with the EAHe of 0.5–1.8 mm/yr, suggesting that the AHe date of the samples do not reflect the post-intrusive cooling but rather its exhumation. Meanwhile, for the other ~4.0 Ma pluton, the method yielded modeled cooling paths that are consistent with the measured dates only when 2 steps exhumation histories were applied. Note that the 2 steps exhumation histories are debatable because amount of exhumation (3–6 km) for 0.3–1.0 Myr is too large. It implies that the sample could have been reheated by the ~3 Ma intrusion.
For the ~3.3 Ma pluton, the emplacement depth of 6.1–6.5 km [4] estimated using Al-in-Hbl geobarometry supports the modeled 2 steps exhumation history because of the consistency with the modeled emplacement depth of 5-7 km obtained by the 2 steps exhumation test. Together with the modeled rate of the ~4.0 Ma pluton, it is revealed that the central part of the Tanigawa-dake area was rapidly exhumed at a rate of 0.9–1.4 mm/yr after ~2.3 Ma. Numerical modeling method enhances the interpretation of the cooling dates of young plutons, providing reliable exhumation history constrained by multiple thermochronological dates. A more robust exhumation history after intrusion was estimated by comparing it with the depth estimated by Al-in-Hbl geobarometry.
References [1] Harayama (1992) Geology, 20, 657-660, [2] Murray et al. (2018) G-Cubed, 19, 3739–3763, [3] Minami et al. (2021) EPS. 73:231, [4] Minami et al. (2023) Thermo2023 abstract, p.129, [5] Geothermal potential map in Japan (2009) AIST webpage.
Acknowledgements
This study was supported by JST SPRING, Grant Number JPMJSP2110.
The Tanigawa-dake area in Japanese islands located in a northeastern part of the junction zone of the Honshu arc and the Izu-Bonin arc. Late Miocene to Pliocene granitoids (~6.0–3.2 Ma) are exposed in the Tanigawa-dake area revealed using zircon U-Pb dating [3] [4]. Previous research reported zircon and (U-Th)/He (ZHe) dates of 3.3–2.6 Ma and apatite (U-Th-Sm)/He (AHe) dates of 3.3–1.0 Ma for each locality, which represents cooling histories below ~200 ºC [4]. Then, AHe-derived exhumation rates (EAHe) of 0.3–1.8 mm/yr were estimated from these AHe dates and current geothermal gradient of 40–60 ºC/km [5]. In this study, 1D numerical modeling based on heat advection-diffusion-production equations were applied for three localities at two ~4.0 Ma plutons and a ~3.3 Ma pluton to obtain geological meaningful exhumation rates. This method is expected to estimate more accurate exhumation rates and emplacement depths by exploring modeled cooling histories which align with the reported ZU-Pb, ZHe and AHe dates.
As a result, the modeling method suggests that either (1) exhumation at a constant rate after intrusion or (2) 2 steps exhumation history, characterized by an extremely rapid exhumation phase immediately after intrusion for two localities at ~3.3 Ma and ~4.0 Ma plutons. The modeled exhumation rates for the samples are estimated to be 0.9–1.5 mm/yr after ~2.3 Ma, which are consistent with the EAHe of 0.5–1.8 mm/yr, suggesting that the AHe date of the samples do not reflect the post-intrusive cooling but rather its exhumation. Meanwhile, for the other ~4.0 Ma pluton, the method yielded modeled cooling paths that are consistent with the measured dates only when 2 steps exhumation histories were applied. Note that the 2 steps exhumation histories are debatable because amount of exhumation (3–6 km) for 0.3–1.0 Myr is too large. It implies that the sample could have been reheated by the ~3 Ma intrusion.
For the ~3.3 Ma pluton, the emplacement depth of 6.1–6.5 km [4] estimated using Al-in-Hbl geobarometry supports the modeled 2 steps exhumation history because of the consistency with the modeled emplacement depth of 5-7 km obtained by the 2 steps exhumation test. Together with the modeled rate of the ~4.0 Ma pluton, it is revealed that the central part of the Tanigawa-dake area was rapidly exhumed at a rate of 0.9–1.4 mm/yr after ~2.3 Ma. Numerical modeling method enhances the interpretation of the cooling dates of young plutons, providing reliable exhumation history constrained by multiple thermochronological dates. A more robust exhumation history after intrusion was estimated by comparing it with the depth estimated by Al-in-Hbl geobarometry.
References [1] Harayama (1992) Geology, 20, 657-660, [2] Murray et al. (2018) G-Cubed, 19, 3739–3763, [3] Minami et al. (2021) EPS. 73:231, [4] Minami et al. (2023) Thermo2023 abstract, p.129, [5] Geothermal potential map in Japan (2009) AIST webpage.
Acknowledgements
This study was supported by JST SPRING, Grant Number JPMJSP2110.