17:15 〜 19:15
[SMP28-P16] 接触変成帯の温度構造と貫入熱モデリングを用いた地殻の古地温勾配推定：関東山地金峰山花崗岩の例
キーワード：甲府花崗岩体、接触変成作用、花崗岩、熱モデリング、炭質物ラマン温度計
The geothermal gradient in the crust is one of the key parameters determining important crustal processes, such as the distribution of seismogenic zones and magma generation. The geothermal gradient is also strongly influenced by crustal deformation, as well as the existence of fluids and magma, and varies greatly with time and location. Therefore, knowledge of the paleo-crustal geotherm can provide insights into the tectonics and magmatic activity of that period. Evidence on the paleo-crustal geotherm has mainly been obtained from petrological studies of limitedly exposed regional metamorphic rocks. Alternatively, relatively unexplored approaches that combine contact metamorphic rocks and thermal modeling can provide additional insights into the crustal geotherm, particularly in areas with significant spatiotemporal formation around subduction zones (e.g., Nobe et al., 2021; Yamaoka et al., 2023).
In this study, we investigate the Miocene Kinpusan pluton in the Kanto Mountain Range, which intruded into the Shimanto Cretaceous accretionary complex, to constrain the crustal geotherm. Recent radiometric dating studies revealed that the formation of the pluton occurred during the earliest phase of the Izu–Bonin–Mariana Arc collision with the Honshu arc, around 15 Ma (Saito et al., 2007; Sawaki et al., 2020). The thermal structure of the contact metamorphic aureole surrounding the Kinpusan pluton was determined using Raman spectroscopy of carbonaceous material, with temperatures ranging from approximately 600 °C near the intrusive contact to ~250 °C at 2000 m from the contact (e.g., Beyssac et al., 2002; Kouketsu et al., 2014; Aoya et al., 2010). The emplacement depth of the pluton is estimated to be ~6 km (~170 MPa) based on the application of the Al-in-hornblende geobarometer (Mutch et al., 2016) and MagMaTab (Weber & Blundy, 2024). A one-dimensional analytical approach was employed for thermal modeling, assuming an instantaneous intrusion of a spherical magma body (Carslaw & Jaeger, 1959). While this assumption aligns with the inferred geological distribution of the pluton, the incremental emplacement history of the Kinpusan pluton (Takahashi et al., 2021; Murakami et al., 2024) suggests that the calculated geothermal gradient represents a lower limit. Fitting of the observed thermal structure of the contact metamorphic aureole to the thermal model requires an initial geothermal gradient of 25–35°C/km and a significantly higher initial magma temperature, which can be inferred under saturated condition of H2O estimated by rhyolite-MELTS (Gualda et al., 2012) modeling. This result indicates that the water content of the Kinpusan magma should be further verified through petrological estimations of the magma temperature. If this condition is not met, it is highly likely that significant hydrothermal convection occurred around the intrusive contact, influencing the local thermal structure. The estimated geothermal gradient is lower than that of similar periods in the forearc region in SW Japan (Sakaguchi, 1996), and may correspond to a transient cooling period immediately after the collision of an immature island arc.
References
Aoya et al. (2010), doi:10.1111/iar.12057; Beyssac et al. (2002), doi:10.1046/j.1525-1314.2002.00408.x; Carslaw & Jaeger (1959), ISBN: 0198533039; Gualda et al. (2012), doi:10.1093/petrology/egr080; Kouketsu et al. (2014), doi:10.1111/iar.12057; Much et al. (2016), doi:10.1007/s00410-016-1298-9; Murakami et al. (2024), JpGU Abstract, SCG45-P04; Nobe et al. (2021), doi:10.14863/geosocabst.2021.0_221; Sakaguchi et al. (1996), doi:10.1130/0091-7613(1996)024%3C0795:HPGWRS%3E2.3.CO;2; Saito et al. (2007), doi:10.1093/petrology/egm037; Sawaki et al. (2020) doi:10.1111/iar.12361; Takahashi et al. (2021), doi:10.15006/chs20201056005; Yamaoka et al. (2023), doi:10.1130/g51563.1; Weber et al. (2024), doi:10.1093/petrology/egae020
In this study, we investigate the Miocene Kinpusan pluton in the Kanto Mountain Range, which intruded into the Shimanto Cretaceous accretionary complex, to constrain the crustal geotherm. Recent radiometric dating studies revealed that the formation of the pluton occurred during the earliest phase of the Izu–Bonin–Mariana Arc collision with the Honshu arc, around 15 Ma (Saito et al., 2007; Sawaki et al., 2020). The thermal structure of the contact metamorphic aureole surrounding the Kinpusan pluton was determined using Raman spectroscopy of carbonaceous material, with temperatures ranging from approximately 600 °C near the intrusive contact to ~250 °C at 2000 m from the contact (e.g., Beyssac et al., 2002; Kouketsu et al., 2014; Aoya et al., 2010). The emplacement depth of the pluton is estimated to be ~6 km (~170 MPa) based on the application of the Al-in-hornblende geobarometer (Mutch et al., 2016) and MagMaTab (Weber & Blundy, 2024). A one-dimensional analytical approach was employed for thermal modeling, assuming an instantaneous intrusion of a spherical magma body (Carslaw & Jaeger, 1959). While this assumption aligns with the inferred geological distribution of the pluton, the incremental emplacement history of the Kinpusan pluton (Takahashi et al., 2021; Murakami et al., 2024) suggests that the calculated geothermal gradient represents a lower limit. Fitting of the observed thermal structure of the contact metamorphic aureole to the thermal model requires an initial geothermal gradient of 25–35°C/km and a significantly higher initial magma temperature, which can be inferred under saturated condition of H2O estimated by rhyolite-MELTS (Gualda et al., 2012) modeling. This result indicates that the water content of the Kinpusan magma should be further verified through petrological estimations of the magma temperature. If this condition is not met, it is highly likely that significant hydrothermal convection occurred around the intrusive contact, influencing the local thermal structure. The estimated geothermal gradient is lower than that of similar periods in the forearc region in SW Japan (Sakaguchi, 1996), and may correspond to a transient cooling period immediately after the collision of an immature island arc.
References
Aoya et al. (2010), doi:10.1111/iar.12057; Beyssac et al. (2002), doi:10.1046/j.1525-1314.2002.00408.x; Carslaw & Jaeger (1959), ISBN: 0198533039; Gualda et al. (2012), doi:10.1093/petrology/egr080; Kouketsu et al. (2014), doi:10.1111/iar.12057; Much et al. (2016), doi:10.1007/s00410-016-1298-9; Murakami et al. (2024), JpGU Abstract, SCG45-P04; Nobe et al. (2021), doi:10.14863/geosocabst.2021.0_221; Sakaguchi et al. (1996), doi:10.1130/0091-7613(1996)024%3C0795:HPGWRS%3E2.3.CO;2; Saito et al. (2007), doi:10.1093/petrology/egm037; Sawaki et al. (2020) doi:10.1111/iar.12361; Takahashi et al. (2021), doi:10.15006/chs20201056005; Yamaoka et al. (2023), doi:10.1130/g51563.1; Weber et al. (2024), doi:10.1093/petrology/egae020