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[SCG41-P04] Improved exhumation history estimates for the Pliocene Tanigawa-dake granites using thermochronometry, Al-in-Hbl geobarometry and 1D heat numerical modeling
Keywords:The Tanigawa-dake area, Young granites , Exhumation history, Al-in-Hbl geobaromety, Numerical modeling, Thermochronology
Granites are generally emplaced at crustal depths of >~ several kilometers. Areas where granites <~5 Ma are exposed must have been uplifted and exhumed rapidly. Such granites are distributed along convergent plate boundaries [1]. The Japanese islands, consisting of active island arcs, expose young granites, such as the world’s youngest Kurobegawa granite of ~0.8 Ma [2] in the Hida mountain range, and Tanzawa plutonic body of ~4.0 Ma [3] in the Izu collision zone, central Japan.
Late Miocene to Pliocene granites (~6, ~4 and ~3 Ma) dated by zircon (Z) U-Pb [4,5] are exposed in the Tanigawa-dake area, located in the back-arc side of the northeastern Japan arc. Previous studies in this area reported exhumation rates of 0.3–1.7 mm/yr calculated from apatite (U-Th-Sm)/He (AHe) dates [5] and geothermal gradients of 40–60 °C/km [6]. However, due to their young intrusive age, the AHe dates might reflect both initial post-crystallisation and exhumation cooling, implying that exhumation rates might have been overestimated. In this study, we attempt to constrain a more reliable exhumation history for the youngest plutons (~3.3–3.2 Ma [4]), in the central Tanigawa-dake area, using two methods: (1) Al-in-Hbl geobarometry to estimate emplacement depths [7] and (2) 1D numerical simulation [8] to explore the best exhumation/cooling histories that fit the reported ZU-Pb, zircon (U-Th)/He (ZHe) and AHe dates. The cooling paths were simulated by accounting for both the geothermal structure of the intrusive environment (depth/thickness) and exhumation rate. The simulation is based on the heat advection-diffusion-production equation. We also report new ZHe and AHe dates for the Tanigawa-dake granites.
AHe dates of 1.7–1.0 Ma and ZHe dates of 3.1–1.5 Ma were obtained for small intrusive bodies in southern part of the study area. Samples from the northeastern part of the study area also yielded AHe and ZHe dates of 2.7 Ma and 3.3 Ma, respectively. Exhumation rates of 0.3–1.8 mm/yr were calculated using AHe dates and the geothermal gradient together with data reported in [5]. East-west transects of exhumation rates imply that the central Tanigawa-dake area may have been exhumed at a higher rate.
In Al-in-Hbl geobarometry, solidification pressures of 0.9–2.6 kbar and emplacement depths of 3.4–9.5 km were estimated using a crustal density of 2.7 g/cm3. Assuming an intrusive age of ~3.3 Ma [4], exhumation rates were calculated to be 1.0–2.9 mm/yr for the youngest pluton.
As a result of the 1D numerical simulation, the best emplacement depth is ~4.0 km, with a best exhumation rate of ~1.2 mm/yr. Compared the AHe-derived exhumation rates (0.8–1.7 mm/yr), the geobarometry-derived rates in the same pluton (1.0–2.9 mm/yr) are similar or higher. The modeled rate (1.2 mm/yr) is within the range of the exhumation rates estimated by the AHe age, indicating that the AHe date of the ~3.3 Ma pluton does not reflect the initial cooling but rather its exhumation. We conclude that the exhumation rates calculated from the AHe dates and current geothermal gradient are consistent with those obtained from using a combination of geobarometry, zircon U-Pb dating and numerical thermal modeling.
Acknowledgements
This study was funded by the Ministry of Economy, Trade and Industry, 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), Grant Number JPJ007597". This study was also supported by MEXT KAKENHI Grant Number 21K03730 and JST SPRING, Grant Number JPMJSP2110. The University of Melbourne thermochronology laboratory receives infrastructure support from the AuScope program (www.auscope.org.au) of the National Collaborative Research Infrastructure Strategy (NCRIS).
References [1] Harayama (1992) Geology, 20, 657–660, [2] Ito et al. (2013) Sci. Rep., 3:1306, [3] Tani et al. (2010) Geol. 38, 215–218, [4] Minami et al. (2021) EPS., 73:231, [5] Minami et al. (2023) Thermo2023 abstract, p.129, [6] Tanaka et al. (2010) EPS., 56, 1191–1194, [7] Mutch et al. (2016) Contr. Mineral. and Petrol., 171:85, [8] Murray et al. (2018) G-Cubed, 19, 3739–3763.
Late Miocene to Pliocene granites (~6, ~4 and ~3 Ma) dated by zircon (Z) U-Pb [4,5] are exposed in the Tanigawa-dake area, located in the back-arc side of the northeastern Japan arc. Previous studies in this area reported exhumation rates of 0.3–1.7 mm/yr calculated from apatite (U-Th-Sm)/He (AHe) dates [5] and geothermal gradients of 40–60 °C/km [6]. However, due to their young intrusive age, the AHe dates might reflect both initial post-crystallisation and exhumation cooling, implying that exhumation rates might have been overestimated. In this study, we attempt to constrain a more reliable exhumation history for the youngest plutons (~3.3–3.2 Ma [4]), in the central Tanigawa-dake area, using two methods: (1) Al-in-Hbl geobarometry to estimate emplacement depths [7] and (2) 1D numerical simulation [8] to explore the best exhumation/cooling histories that fit the reported ZU-Pb, zircon (U-Th)/He (ZHe) and AHe dates. The cooling paths were simulated by accounting for both the geothermal structure of the intrusive environment (depth/thickness) and exhumation rate. The simulation is based on the heat advection-diffusion-production equation. We also report new ZHe and AHe dates for the Tanigawa-dake granites.
AHe dates of 1.7–1.0 Ma and ZHe dates of 3.1–1.5 Ma were obtained for small intrusive bodies in southern part of the study area. Samples from the northeastern part of the study area also yielded AHe and ZHe dates of 2.7 Ma and 3.3 Ma, respectively. Exhumation rates of 0.3–1.8 mm/yr were calculated using AHe dates and the geothermal gradient together with data reported in [5]. East-west transects of exhumation rates imply that the central Tanigawa-dake area may have been exhumed at a higher rate.
In Al-in-Hbl geobarometry, solidification pressures of 0.9–2.6 kbar and emplacement depths of 3.4–9.5 km were estimated using a crustal density of 2.7 g/cm3. Assuming an intrusive age of ~3.3 Ma [4], exhumation rates were calculated to be 1.0–2.9 mm/yr for the youngest pluton.
As a result of the 1D numerical simulation, the best emplacement depth is ~4.0 km, with a best exhumation rate of ~1.2 mm/yr. Compared the AHe-derived exhumation rates (0.8–1.7 mm/yr), the geobarometry-derived rates in the same pluton (1.0–2.9 mm/yr) are similar or higher. The modeled rate (1.2 mm/yr) is within the range of the exhumation rates estimated by the AHe age, indicating that the AHe date of the ~3.3 Ma pluton does not reflect the initial cooling but rather its exhumation. We conclude that the exhumation rates calculated from the AHe dates and current geothermal gradient are consistent with those obtained from using a combination of geobarometry, zircon U-Pb dating and numerical thermal modeling.
Acknowledgements
This study was funded by the Ministry of Economy, Trade and Industry, 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), Grant Number JPJ007597". This study was also supported by MEXT KAKENHI Grant Number 21K03730 and JST SPRING, Grant Number JPMJSP2110. The University of Melbourne thermochronology laboratory receives infrastructure support from the AuScope program (www.auscope.org.au) of the National Collaborative Research Infrastructure Strategy (NCRIS).
References [1] Harayama (1992) Geology, 20, 657–660, [2] Ito et al. (2013) Sci. Rep., 3:1306, [3] Tani et al. (2010) Geol. 38, 215–218, [4] Minami et al. (2021) EPS., 73:231, [5] Minami et al. (2023) Thermo2023 abstract, p.129, [6] Tanaka et al. (2010) EPS., 56, 1191–1194, [7] Mutch et al. (2016) Contr. Mineral. and Petrol., 171:85, [8] Murray et al. (2018) G-Cubed, 19, 3739–3763.