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

近年来提出秩父带是由三个地体组成的复合地体。从北到南分别是，北秩父，黑川和南秩父。其中，含有前志留纪基底组合的黑川地体被认为是阐明日本西南构造的关键地体。这是因为黑川地体的动物成分和古地磁资料有力地支持了它可能曾经构成冈瓦纳的一部分的假设。该项目的目的是为了解黑家川地体是否起源于以结晶基底为特征的复盖层序层序地层提供基础资料。为此目的，计划了下列办法：(1)利用放射虫化石重新鉴定了以花岗质砾岩为特征的“乌苏金型砾岩”的年龄；(2)对黑泽川构造带的花岗质岩石与“乌苏金型砾岩”中采集的花岗质砾岩进行了岩石学研究，确定了两者之间的物源联系；(3)利用锆石U-Pb对构造带的花岗质岩石与该砾岩中采集的花岗质砾岩进行了定年。这些研究的结果表明了以下几点：(1)黑家川地体中的“usuginu型砾岩”是陆相碎屑覆盖地层的一部分，年龄在中二叠世至中侏罗统之间；(2)黑家川构造带花岗质岩石和砾岩中采集的花岗质砾岩的全岩分析显示，黑家川构造带花岗质岩石具有典型的钙碱性岩系亲缘关系，但两者的起源可能并不密切相关；(3)初步定年结果表明，该砾岩花岗质巨石中分离的锆石U-Pb年龄比预期年轻250Ma。最后一个结果尤其重要，可能是当时黑川地体俯冲的直接证据。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