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

根据岩浆岩、变质岩和沉积岩的CHIME定年、XRF和EPMA分析，研究了日本群岛基岩的演化历史。主要研究结果如下：(1)日本中部Nohi流纹岩最低火山层序中Agigawa焊接凝灰岩岩片的脉状锆石年龄为85×5 Ma，脉状尿灰石年龄为86×7 Ma。Agigawa焊接簇状岩片在年代学上与Ryoke变质带中的后构造信四郎英云闪长岩和三桥花岗闪长岩一致。高T/P Ryoke带剥蚀后，酸性岩浆作用开始于约85 Ma，变质时间约为100 Ma。(2)新鲜岩石中的黑云母具有明显的10A基面反射，但其钠变质作用具有明显的10A和12.5A反射。这表明，即使是来自新鲜岩石的黑云母也含有贫钾黑云母和蛭石的微域。富钾黑云母的Fe2+/Fe3+比值高于贫钾黑云母和蛭石。这说明八面体片状岩石中的铁的氧化使K的损失维持了局部的静电平衡，并解释了K-Ar黑云母年龄与独居石年龄的偏差。(3)Ryoke片麻岩的独居石年龄集中在102~98 Ma之间，并被解释为下角闪岩相条件下独居石的形成时间。峰期变质作用与东西相距约550公里的地质年代最久远的95 Ma左右的深成岩体同时发生。东部从95 Ma持续到68 Ma，西部在85 Ma停止。这些新时代的数据不支持目前的观点，即Ryoke带的政治主义向东变得更年轻。结合已有的地质资料，西段的侵蚀速度(1.5 mm/年)明显快于东部(0.8 mm/年)，时间跨度为91~85 Ma。

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.