世界一若い露出プルトンの冷却史 : 北アルプス, 滝谷花崗閃緑岩の年代と冷却モデル  [in Japanese] Cooling history of the youngest exposed pluton in the World : The Plio-Pleistocene Takidani Granodiorite (Japan Alps, central Japan)  [in Japanese]

最も若い露出プルトンの冷却史解明のため, 滝谷花崗閃緑岩 (北アルプス) の熱年代学的な研究を行った。垂直方向に累帯する岩相から4試料について鉱物年代を測定した結果, 黒雲母・カリ長石の鉱物年代 (K-Ar法, Rb-Sr法) は岩相によらず1.2-1.1 Maに集中することが判明した。ホルンブレンドのK-Ar年代は1.93-1.20 Ma であり, 深部相ほど若い傾向がある。これは深部ほど固結開始時期が遅れたためであろう。測定鉱物の閉鎖温度から冷却曲線を求めた結果, 平均冷却率は岩体浅所で350℃/Ma, 深部では550℃/Ma以上を示した。三次元単純熱伝導モデル計算による冷却曲線は指数減衰を示し, 岩体浅所での直線状の熱年代学的冷却曲線と大きく異なるため, 冷却前半 (2.2-1.2 Ma) では深部からの熱流入が, 後半では隆起活動による急速冷却が推定された。深部ほど冷却率が大きいのは, 冷却開始が深部では遅いため熱流入の効果は短期間で弱く, 単純熱伝導冷却に近づいたためと解釈できる。

Thermochronological investigations have been carried out on samples from various levels in the Plio-Pleistocene Takidani Granodiorite in order to elucidate the cooling history of the youngest exposed pluton on Earth. The Takidani Granodiorite is a pluton (13×4 km) situated along a major axis of the Japan Alps (Hida Mountain Range) that was intruded into late Pliocene age (2.4 Ma) volcanic rocks (Hotaka Andesite), and a vertical variation in texture and compositionis observable from the top to a level 1800 m below the roof. The pluton is a vertically zoned pluton, grading from equigranular biotite-hornblende granodiorite (TGL) in the deepest through hornblende-biotite granodiorite (TGM) to hornblende-bearing porphyritic biotite granite (TGU) at the shallowest level. A zircon fission track age for TGM is 0.80±0.02 Ma. K-Ar ages on biotites and K-feldspars for four specimens covering the vertical variations clusterina range from 1.2 to 1.1 Ma, as well as Rb-Sr biotite age (1.12±0.06 Ma). Three hornblende measurements for TGM (R49859) and TGL (KH479, R49857) yielded discordant K-Ar ages of 1.93±0.16 Ma, 1.46±0.10 and 1.20±0.34 Ma, respectively. These age discrepancy among the three specimens can be explained by the slower start in cooling at deeper levels of the pluton. On the basis of the calculated closure temperatures of the dated coexisting minerals, cooling Temperature-time (T-t) paths of the Takidani Granodiorite are derived. The average cooling rates are 350℃/m.y. for TGU and TGM, and >550℃/m.y. for TGL at temperature below 650℃. The cooling T-t path generated using five coexisting mineral ages from TGM shows an early linear, not exponential, decline. A simple conductive cooling lead exponential decline of temperature, and for the TGM, the simulated cooling curve is lower than the thermochronological linear T-t path in the early stage of the cooling, and higher in the late stage. The temperature discrepancy between the two T-t path indicates high thermal input (e.g. periodic replenishment of the magma chamber) in the early stage and rapid cooling caused by regional uplift in the late stage. For the TGL, the cooling rates increased due to slower start in cooling and decreasing thermal input from deeper levels.