geochronological analysis of the evolutional history of basement rocks in the Japanese Islands

利用CHIME定年和火成岩的x射线荧光光谱（XRF）和EPMA分析，研究了日本列岛基底岩的演化历史；变质岩和沉积岩。主要结果如下：(1)日本中部Nohi流纹岩最低火山层序Agigawa焊接凝灰岩片CHIME锆石年龄为85*5 Ma， allme年龄为86*7 Ma。Agigawa焊接簇状片在年代上与日良变质带构造后的新四郎闪长岩和三桥花岗闪长岩相同。酸性岩浆活动在100 Ma左右高T/P Ryoke带剥蚀变质后，在85 Ma左右同时广泛开始。(2)新鲜岩石中的黑云母具有明显的10 a反射，但其na修饰具有明显的10和12.5 a反射。这表明，即使来自新鲜岩石的黑云母也含有贫钾黑云母和蛭石微域。富钾黑云母的Fe2+/Fe3+比值高于贫钾黑云母和蛭石。这说明八面体薄片中铁的氧化导致k损失以维持局部静电平衡，并解释了K-Ar黑云母年龄与CHIME独居石年龄的偏差。(3)里奥克片麻岩CHIME独居石年龄集中在102 ~ 98 Ma之间，可解释为下角闪岩相条件下独居石的形成时间。该变质峰与地质上最古老的约95 Ma的东、西两部分相隔约550 km的岩体同时存在。东部从95 Ma持续到68 Ma，西部在85 Ma停止。这些新的年龄数据不支持目前的观点，即里奥克带的深部成矿作用向东变得更年轻。CHIME年龄结合现有地质资料表明，在91 ~ 85 Ma的时间跨度内，西部的侵蚀速度（1.5mm/y）快于东部（0.8mm/y）。

The evolutional history of basement rocks in the Japanese Islands was investigated on the basis of the CHIME dating and the XRF and EPMA analyses of igneous ; metamorphic and sedimentary rocks. The main results are as follows.(1)The CHIME zircon age is 85*5 Ma and the CHIME allanite age is 86*7 Ma for the Agigawa welded tuff sheet in the lowest volcanic sequence of the Nohi Rhyolite in central Japan. The Agigawa welded tuft sheet is chronologically identical with the post-tectonic Shinshiro Tonalite and Mitsuhashi Granodiorite in the Ryoke metamorphic belt. Acidic magmatism started simultaneously and widely at about 85 Ma after the denudation of the high T/P Ryoke belt metamorphosed at about 100 Ma.(2)Biotite from fresh rock shows a sharp 10 A basal reflection, but its Na-modification gives distinct 10 and 12.5 A reflections. This shows that biotite even from fresh rocks contains microdomains of K-poor biotite and vermiculite. The Fe2+/Fe3+ ratio is higher in K-rich biotite than in K-poor biotite and vermiculite. This suggests oxidation of iron in octahedral sheet causes the K-loss to maintain local electrostatic balance, and accounts for the deviation of K-Ar biotite ages from CHIME monazite ages.(3)The CHIME monazite ages for the Ryoke gneisses are concentrated between 102 and 98 Ma, and are interpreted as the time of monazite formation under lower amphibolite facies conditions. The peak metamorphism is contemporaneous with the geologically oldest plutons of at about 95 Ma in both the eastern and western parts separated by about 550 km. Plutonism continued from 95 Ma to 68 Ma in the eastern part, whereas in the western part it ceased at 85 Ma. These new age data do not support a current view that plutonism in the Ryoke belt becomes younger towards the east. The CHIME ages coupled with available geological data show that the western part was eroded rapidly (1.5mm/y) than the eastern part (0.8mm/y) over the time span from 91 Ma to 85 Ma.