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

日本西南部四万十带变质岩的 K-Ar 年代学

基本信息

批准号：
04452079
负责人：
ITAYA Tetsumaru
金额：
$ 4.48万
依托单位：
Okayama University of Science
依托单位国家：
日本
项目类别：
Grant-in-Aid for General Scientific Research (B)
财政年份：
1992
资助国家：
日本
起止时间：
1992 至 1993
项目状态：
已结题

来源：
https://kaken.nii.ac.jp/en/grant/KAKENHI-PROJECT-04452079/
关键词：
Subduction zone Accretionary complex Shimanto belt Metamorphic rocks Low grade Fine-grained mica K-Ar age Laser probe KーAr年代

项目摘要

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.

本文研究的物质为极低品位沉积岩中的极细粒变质白云母。因此，我们必须建立一种材料的分离和浓缩技术。利用所建立的方法，我们从石漫滩沉积岩石中分离了物质，并用K-Ar法测定了它们的年龄。几个大小的分数被确定了日期。结果表明，小于2微米的组分是变质的，较大的组分有时比化石年龄更老，表明混合了一些碎屑云母。我们还在日本西南部侏罗纪增生杂岩中发现了碎屑云母，并研究了这种碎屑云母对K-Ar年龄的影响以及使这种影响显着减弱的方法。并与低级变质岩中的云母进行了对比。来自断裂带的云母在结晶度上已经扩散，但仍可用于K-Ar年代学。我们还开发了用于极少量白色云母或细粒单晶的激光探针式Ar-Ar定年系统。将材料的温度控制在+-1ﾟC的相对精度为500~1500ﾟC是可能的。通过求解俯冲减弱后的二维热传导方程，对造山过程进行了新的约束，对大洋板块在弧下主动俯冲时形成的弧沟系统的反向热结构恢复进行了数值实验。结果表明，高压变质岩经历了海脊俯冲或大陆碰撞的幕式折返。