Improved exhumation history estimates for the Pliocene Tanigawa-dake granites using thermochronometry, Al-in-Hbl geobarometry and 1D heat numerical modeling
熱年代学、角閃石アルミ地質圧力計、一次元熱モデルに基づいた中新世後期から鮮新世の谷川岳花崗岩類における改善された削剥史
南 沙樹*; 末岡 茂
; 福田 将眞
; Malatesta, L.*; Kohn, B. P.*; 河上 哲生*; 東野 文子*; 梶田 侑弥*; 田上 高広*
Minami, Saki*; Sueoka, Shigeru; Fukuda, Shoma; Malatesta, L.*; Kohn, B. P.*; Kawakami, Tetsuo*; Higashino, Fumiko*; Kajita, Yuya*; Tagami, Takahiro*
We attempt to constrain a reliable exhumation history for the youngest plutons, central Tanigawa-dake area, using two methods: (1) Al-in-Hbl geobarometry to estimate emplacement depths and (2) 1D numerical simulation to explore the best exhumation/cooling histories that fit the reported zircon U-Pb, zircon and apatite (U-Th)/He (ZHe and AHe) dates. As a result of Al-in-Hbl geobarometry, solidification pressures of 0.9-2.6 kbar were obtained. Emplacement depths of 3.4-9.5 km were estimated from the pressures and a crustal density of 2.7 g/cm
. Exhumation rates were calculated to be 1.0-2.9 mm/yr for the youngest pluton, assuming an intrusive age of ca. 3.3 Ma. As a result of the 1D numerical simulation, the best exhumation rate is estimated to be ca. 1.2 mm/yr and the best emplacement depth is ca. 4.0 km. Comparing with the AHe-derived exhumation rate (0.8-1.7 mm/yr) in the same pluton, the geobarometry-derived exhumation rate (1.0-2.9 mm/yr) are comparable or higher. The modeled exhumation rate (1.2 mm/yr) is within the range of the exhumation rates estimated by the AHe age. This indicates that the AHe date of the ca. 3.3 Ma pluton does not reflect the initial cooling but exhumation. Consequently, the exhumation rate calculated from the AHe dates and current geothermal gradient were consistent with those obtained from the combination of geobarometry, zircon U-Pb dating and numerical thermal modeling.