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

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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.