Separation of exhumation and post-intrusion cooling with thermochronology, Al-in-Hbl geobarometry, and numerical thermal modeling: an example from Central Japan

Granites are generally emplaced several kilometers deep. Therefore, areas where granites younger than ~5 Ma are exposed must have been uplifted and exhumed rapidly. Young granites are distributed along convergent plate boundaries [1]. The Japanese islands, consisting of active island arcs, have some young granites, such as the world’s youngest Kurobegawa granite of ~0.8 Ma [2] in Hida mountain range, central Japan. The Tanigawa-dake area, in the southern end of the Northeast Japan arc, hosts such granites of late Miocene to Pliocene ages ranging from ~6.0–5.5 Ma, ~4.0 Ma to 3.3–3.2 Ma (zircon U-Pb) [3,4]. Previous studies [4] also reported zircon (U-Th)/He dates (ZHe) of 3.3–1.4 Ma and apatite (U-Th-Sm)/He (AHe) dates of 2.8–1.0 Ma for these young granites and the Cretaceous granites. Exhumation rates of 0.3–1.7 mm/yr were estimated by AHe dates and assumption of constant geothermal gradients of 40–60 °C/km [6]. However, the AHe dates might reflect initial cooling phase of the young plutons as well as cooling derived from exhumation, potentially leading to an overestimation of exhumation rates in the Tanigawa-dake area.

This study aims to constrain a more reliable exhumation history. We applied two methods for the youngest pluton (~3.3 Ma): (1) Al-in-Hbl geobarometry [5] to estimate the emplacement depth and (2) 1D numerical simulation of geothermal structure based on heat advection-diffusion-production equation [7] to explore the best cooling/exhumation histories consistent with the reported zircon U-Pb age, ZHe and AHe dates. As a result of Al-in-Hbl geobarometry, solidification pressures of 0.9–2.6 kbar were estimated. Emplacement depths derived from these pressures are 3.4–9.5 km by assuming the granites density of 2.7 g/cm³. Exhumation rates were calculated to be 1.0–2.9 mm/yr for the youngest pluton, assuming an intrusive age of ~3.3 Ma [3]. In the 1D heat advection-diffusion-generation model, the best exhumation rates are ~1.2 mm/yr and the best emplacement depth is ~4.0 km. Comparing with the exhumation rate estimated from the AHe age of ~1.0 Ma [4] in the same pluton (0.8–1.7 mm/yr), the geobarometry method yielded similar or higher exhumation rates (1.0–2.9 mm/yr). Similarly, the modeled rate (1.2 mm/yr) fits with the exhumation rate estimated using AHe age. This indicates that the initial cooling was finished by the time of the AHe date for ~3 Ma pluton, i.e., the previous geothermal structure in this area had relaxed to the current one. Consequently, the exhumation rates calculated from AHe date and current geothermal gradient were consistent with those obtained from the combination of geobarometry, zircon U-Pb, ZHe and AHe datings and numerical thermal modeling.

 

References [1] Harayama (1992) Geology, 20, 657–660, [2] Ito et al. (2013) Sci. Rep., 3:1306, [3] Minami et al. (2021) EPS., 73:231, [4] Minami et al. (2023) Thermo2023 abstract, p.129, [5] Mutch et al. (2016) Contr. Mineral. and Petrol., 171:85, [6] Tanaka et al. (2010) EPS., 56, 1191–1194, [7] Murray et al. (2018) G-Cubed, 19, 3739–3763.