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Significance of greentones for the formation of accretionaly complex of Japanese Islands during Jurassic to Paleogene

侏罗纪至古近纪绿带对日本列岛增生杂岩形成的意义

基本信息

批准号：
07304039
负责人：
MIYASHITA Sumio
金额：
$ 7.3万
依托单位：
Niigata University
依托单位国家：
日本
项目类别：
Grant-in-Aid for Scientific Research (A)
财政年份：
1995
资助国家：
日本
起止时间：
1995 至 1997
项目状态：
已结题

来源：
https://kaken.nii.ac.jp/grant/KAKENHI-PROJECT-07304039/
关键词：
greenstone accretionary complex ophiolite Japanese Island Jurassic-Paleogene

项目摘要

The results obtained from this research project are summarized as follows.Petrological features and mode of occurrences of greenstones especially exposed in the Sorachi-Yezo belt were reexamined. Geochemical characteristics in respect to minor elemetns, rare earth elements, isotopic compositions of the greenstone, which are previously regarded to have N-MORB signatures, show that they are rather resemble to those of oceanic plateau basalts. In particular, Nb contents are slightly higher than N-MORB whereas HFS elements such as Y and Zr are depleted than N-MORB.Furthermore, intrusive relations of the greenstones against chert were found from several localities, indicating that the igneous activity initiated after the deposition of pelagic sediments. Therefore, these intrusive rocks may correspond to the initial stage of magmatism such as hot-spot type or oceanic plateau type magmatism, Geochemical and age data for the greenstones in the Mikabu belt in Southwest Japan have been carried out also.In situ greenstones with MORB features which were prpbably generated by the subduction of ocean ridge, are extensively described in the shimanto belt and Hidaka belt from Okinawa Island to Hokkaido along thorough Japanese Island. Age determination, mode of occurrence, petrographical and petrological features were studied for them. Systematic younging toward north of radiolarilan fossil ages in the sediments associating with the insitu greenstones bcomes to clear. Besides, interaction between basaltic rocks and intruded sediments was clarified in respect to chemical exchange between them. Some LIL elements such as K and Rb are significantly removed from the sediments whereas Si is added into the sediments near the boundary. Thus, such chmical characteristics of the silicifie sediments can be used as an indicator showing basaltic intrusions into unsolidifed sediments.

本研究项目取得的成果总结如下：重新研究了特别是索拉奇-耶佐带出露的绿岩的岩石学特征和产状模式。过去认为具有N-MORB特征的绿岩，其微量元素、稀土元素、同位素组成等地球化学特征与大洋高原玄武岩十分相似。特别是，Nb含量略高于N-MORB，而HFS元素，如Y和Zr比N-MORB亏损。此外，侵入关系的绿岩对燧石被发现从几个地方，表明岩浆活动开始后，远洋沉积物的沉积。因此，这些侵入岩可能对应于岩浆活动的初始阶段，如热点型或洋坪型岩浆活动。本文还对日本西南部三部带的绿岩进行了地球化学和年龄测定，发现具有MORB特征的原地绿岩，可能是洋中脊俯冲作用的产物。广泛分布于冲绳本岛至北海道的四万多带和日高带。研究了它们的时代、产状、岩相学特征。与原地绿岩伴生的沉积物中放射虫化石时代向北的系统划分逐渐清晰。此外，还阐明了玄武质岩石与侵入沉积物之间的化学交换作用。一些LIL元素，如K和Rb被显着地从沉积物中去除，而Si被添加到靠近边界的沉积物中。因此，硅化沉积物的这些化学特征可作为指示玄武质侵入未固化沉积物的标志。