"Usugimu-type conglomerate" and Processes of Collision and Accretion of the Kurosegawa Landmass

近年来，有人提出秩父带是由三个带组成的复合带。从北到南分别是北方秩父、黑濑川和南秩父。其中，含有前志留纪基底组合的黑濑川组被认为是阐明日本西南部构造的关键组。这是因为黑濑川古陆的动物群和古地磁数据为它可能曾经是冈瓦纳大陆的一部分的假设提供了有力的支持。本项目的目的是提供基础数据，以了解黑濑川古陆是否起源于一个地层古陆，其特征是存在结晶基底，覆盖层为连贯的层序。为此，计划采取以下方法：（1）用放射虫化石重新鉴定以花岗质巨砾为特征的“Usuginutype砾岩”的时代;（2）用Pem ...更多信息 对黑濑川构造带的花岗质岩石和从“Usuginu型砾岩”中采集的花岗质巨砾进行了物源学研究，确定了它们之间的物源联系;（3）对构造带的花岗质岩石和从砾岩中采集的花岗质巨砾进行了锆石U-Pb定年研究。（1）黑濑川组中的“Usuginu型砾岩”是一个大陆碎片的覆盖层的一部分，其时代为中二叠世至中侏罗世，（2）对黑濑川构造带的花岗质岩石和从砾岩中采集的花岗质漂砾的全岩分析表明，（3）测年初步结果表明，该砾岩花岗质巨砾中分离的锆石U-Pb年龄比预期的要年轻250 Ma。最后一个结果特别重要，可能是当时沿着黑濑川江俯冲的直接证据。少

In recent years, it has been suggested that the Chichibu Belt is a composite terrane consisting of three terranes. Those are, from north to south, Northern Chichibu, Kurosegawa and Southern Chichibu terranes. Among them, the Kurosegawa terrane, which contains a pre-Silurian basement assemblage, is considered to be the key terrane in elucidating the tectonics of Southwest Japan. This is because the faunal content and paleomagnetic data of the Kurosegawa terrane provide compelling support for the postulate that it may once have constituted part of Gondwana.The purpose of this project is to provide basic data to understand whether or not the Kurosegawa terrane originated as a stratigraphic terrane characterized by the presence of crystalline basement with covering strata of coherent sequences. For this purpose, following approaches were planned; (1) Reappraisal of the age of the "Usuginutype conglomerate", which characteristically contains granitic boulders, by radiolarian fossils, (2) Pe … More trological study of granitic rocks of the Kurosegawa Tectonic Zone and granitic boulders collected from the "Usuginu-type conglomerate" to determine the provenancial linkage between them, and (3) Dating of granitic rocks of the tectonic zone and granitic boulders collected from the conglomerate by U-Pb on zircons.As a result of these studies the following points were made clear: (1) The "Usuginu-type conglomerate" in the Kurosegawa terrane, which is part of the covering strata to a continental fragment, ranges in age from mid Permian to Middle Jurassic, (2) Whole rock analysis of both granitic rocks of the Kurosegawa Tectonic Zone and granitic boulders collected from the conglomerate shows typical calc-alkaline rock series affinities, but indicates that both may not have been closely related in origin, and (3) A preliminary result of dating indicates that U-Pb ages of zircons separated from granitic boulders of the conglomerate are 250Ma surprisingly younger than expected. The last result is particularly important and might be a direct evidence for a subduction around that time along the Kurosegawa terrane. Less