Thermochronology and the three-dimensional cooling pattern of a granitic pluton; An Example from the Toki granite, Central Japan

年代測定法から推定する花崗岩体の冷却過程; 土岐花崗岩体を例として

湯口 貴史; 天野 健治  ; 鶴田 忠彦  ; 檀原 徹*; 西山 忠男*

Yuguchi, Takashi; Amano, Kenji; Tsuruta, Tadahiko; Danhara, Toru*; Nishiyama, Tadao*

The cooling process of a magmatic body in the crust is a key to understanding the thermal evolution of the crust. The three dimensional spatial variations in the cooling pattern of the Toki granitic body, a zoned pluton in Central Japan, have been evaluated quantitatively by thermochronology using age dating based on the different closure temperatures of target mineral species. The Toki granite has hornblende K-Ar ages of about 74.33.7 Ma (closure temperature of 51025C; n = 2), biotite K-Ar ages of 78.53.9 to 59.71.5 Ma (30050C; n = 33), zircon fission-track ages of 75.63.3 to 51.92.6 Ma (24050C; n = 44), and K-feldspar K-Ar ages of 70.83.5 to 57.71.3 Ma (15020C; n = 21). Samples collected from 11 boreholes and seven outcrop sites in the Toki granite display spatial variations in the biotite K-Ar ages and the zircon fission-track ages, both of which have the same pattern, indicating that cooling was effectively from the roof and also from the western margin. The cooling process is affected by the extent of assimilation of the surrounding sedimentary rocks.