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

在这项工作中要研究的材料是非常细粒的变质白色云母从非常低级的杂岩沉积岩。因此，我们必须建立一种材料的分离和浓缩技术。利用已建立的技术，我们从四万托火山碎屑沉积岩中分离出物质，并用K-Ar法测定了它们的年龄。对几个粒级进行了年代测定。结果表明，小于2 μ m的云母组分为变质云母，较大的云母组分年龄大于化石年龄，可能是碎屑云母的混合物。我们还在日本西南部的侏罗纪增生杂岩中发现了碎屑云母，并研究了其对K-Ar年龄的影响以及使这种影响显著降低的方法。并与浅变质岩中的云母进行了对比。断裂带云母结晶度已扩散，但仍可用于K-Ar年代学，我们还研制了激光探针氩提取系统，对极少量的白色云母或细粒单晶进行Ar-Ar定年。在500 ~ 1500 ℃范围内，可将材料温度控制在± 1 ℃的相对精度。通过求解俯冲结束后的二维热传导方程，对大洋板块在弧下活动俯冲过程中形成的弧-沟系统的反转热结构进行了恢复的数值试验，对造山过程进行了新的约束。研究结果表明，高压变质岩的幕式折返是在洋脊俯冲或大陆碰撞的触发下发生的。

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.