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

正如Matsuda等人，（1997）的结果表明，提供冷却年龄分别约为300℃、240℃和110℃的钛铁矿、锆石和磷灰石裂变径迹年龄均小于希达山花岗岩的形成年龄。希达花岗质岩石的磷灰石裂变径迹年龄范围较窄，为40 ~ 50 Ma，而锆石裂变径迹年龄范围较宽，为99 ~ 172 Ma。他们指出，最新的隆升和冷却年龄，假定等于磷灰石裂变径迹年龄，闭合温度为110℃，在希达山脉，40-50 Ma。此外，他们还得出结论，该年龄与希达山脉中隆起平原的形成年龄相对应。本工作是他们在确定日本列岛隆起平原形成时代方面工作的一个扩展。本文测定了花岗质岩石和白垩纪火山岩的磷灰石裂变径迹年龄。磷灰石裂变径迹年龄分别为：北上山82-100 Ma，阿武熊山46-78 Ma（大部分为55-70 Ma），冈山46-78 Ma（大部分为55-70 Ma）。新泻县的后藤为6.08 Ma，希达山为40-50 Ma（其径迹长度的模式为缓慢冷却），中部地区的花岗质和酸性火山岩为45-55 Ma，中国地区为16-30 Ma，淡路岛和四国岛为28-46 Ma。每个地区的年龄都很小。日本各岛屿地区的降温幅度约为110℃。这表明，由磷灰石裂变径迹测年推断出的隆起平原的形成时代在白垩纪以后各地存在差异。

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.