K-Ar chronology of metamorphic rocks in Shimanto belt, SW Japan

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The materials to be studied in this work were very fine grained metamorphic white micas from very low grade matasedimentary rocks. Therefore, we had to establish a separation and concentration technique of the materials. Using the established technique, we have separated the materials from the Shimanto matasedimentary rocks and dated them with K-Ar method. Several size fractions were dated. The results revealed that the fractions less than 2 microns were metamorphic and the larger fractions were sometimes older than fossil ages, suggesting mixture of some detrital micas. We also found the detrital micas from the Jurassic accretionary complexes in SW Japan and examined the influences to K-Ar ages and the methods to make the effects significantly decreased.This type of fine grained white micas occur in fault shear zones and geothermally altered volcanic rocks. The micas from them were also examined to compare with those from the low grade metamorphic rocks. The micas from the fault zones have been diffused in crystallinity but are still useful for K-Ar chronology.We have also developed laser probe argon extraction system for very small amount of white micas or fine grained single crystal to proceed Ar-Ar dating. It is possible to control temperatures of the materials with a relative accurracy of +-1ﾟC between 500 and 1500ﾟC.Numerical experiments on recovery of reversed thermal structure in arc-trench system which have been formed during active subduction of oceanic plate under the arc, were carried out by solving a 2-D heat conduction equation after the decease of subduction to make new constraints on orogenic process. The results suggest that the high pressure metamorphic rocks have exhumed episodically with a trigger by ridge subduction or continental collision.