Formation ages of elevated peneplains of the Japanese Islands-An approach from the fission-track dating.

Matsuda et al.,（1997）指出，提供冷却年龄分别为300℃、240℃和110℃左右的钛矿、锆石和磷灰石裂变径迹年龄比飞驒山花岗质岩石的形成年龄要年轻。飞达花岗岩的磷灰石裂变径迹年龄在40 ~ 50 Ma范围内较窄，而锆石裂变径迹年龄在99 ~ 172 Ma范围内较宽。他们指出，最新的隆升和冷却年龄与关闭温度为110℃的磷灰石裂变径迹年龄相似，40-50 Ma。此外，他们还得出结论，该年龄与飞驒山脉高架准平原的形成年龄相对应。目前的工作是他们确定日本列岛高架准平原形成年龄工作的延伸。本研究确定了花岗质岩石和白垩系火山岩的磷灰石裂变径迹年龄。磷灰石裂变径迹年龄：北上山（82-100 Ma）；阿布库马山脉，46-78 Ma（大部分为55-70 Ma）；新泻县后藤山，6.08 Ma；飞驒山脉，40-50 Ma（其轨迹长度模式为缓慢冷却）；中部地区45 ~ 55 Ma花岗岩和酸性火山岩；中央区，16-30马；浅滩岛和四国岛，28-46 Ma。每个地区的年龄都很窄。日本各岛屿的降温幅度在110℃左右。这说明白垩纪以后，由磷灰石裂变径迹测年推断出的高架准平原的形成年代在不同地区存在差异。

As Matsuda et al., (1997) made clear, the titanite, zircon and apatite fission track ages whose ages provide the cooling ages down to about 300℃, 240℃ and 110℃ respectively were younger than the formation ages of the granitic rocks of the Hida mountains. For the Hida granitic rocks, the apatite fission track ages are rather narrow in range from 40 to 50 Ma, although the zircon fission track ages are wide in range from 99 to 172 Ma. They pointed out that the latest uplift and cooling age that is assumed to be equal to the apatite fission track age with closure temperature of 110℃ was similar through the Hida mountains, 40-50 Ma. Further, they concluded that the age corresponded to the formation age of the elevated peneplain in the Hida mountains. The present work is an extension of their work for detarmination of the formation age of the elevated peneplains of the Japanese Islands. For the present work, we determined the apatite fission track ages of granitic rocks and Cretaceous volcanic rocks. The apatite fission track ages are as follows ; the Kitakami mountains, 82-100 Ma ; the Abukuma mountains, 46-78 Ma (mostly 55-70 Ma) ; Mt. Goto in Niigata prefecture, 6.08 Ma ; the Hida mountains, 40-50 Ma (the pattern of their track length is of slow cooling) ; granitic and acidic volcanic rocks in Chubu district, 45-55 Ma ; Chugoku district, 16-30 Ma ; Awaji and Shikoku islands, 28-46 Ma. The age is narrow in each area. The cooling by about 110℃ differs between areas of the Japanese islands. This suggests that the formation ages of the elevated peneplain deduced from the apatite fission track dating are different from place to place after the Cretaceous time.