Geologic study of oceanic plate components in the northern Chichibu belt

关东山脉赤父带北部主要发育侏罗系陆相碎屑岩和三叠纪层状燧石、晚古生代灰岩、绿岩等大小不等的洋板源外来块体。不同地质时代块体的产状和存在方式表明，赤父带是一个与大洋板块俯冲有关的增生杂岩。认为俯冲发生在东亚大陆边缘。杂岩单独居石碎屑的U-Th-Pb年龄（包括200 Ma和1900 Ma）支持了这一观点。三巴川变质岩与白垩纪并置的侏罗系增生杂岩被划分为3个构造地层单元；kazahaya toge，神吉田和柏木。它们与其他单元之间的边界是断层接触。kazahaha -toge单元放射虫生物地层时代为早侏罗世晚期至中侏罗世早期，主要为砂、泥岩相间，沉积于弧前盆地。神吉田单元的时代为早侏罗世晚期至中侏罗世中期，由砂岩和泥岩组成，包括层状燧石、灰岩和基性火山和（或）火山碎屑岩组成的外来大洋板块。该单元间穿插着特殊的弹性层，包括丰富的磷灰石和绿云母，可能来源于碱性基性火成岩。绿云母的K-Ar年龄约为250 Ma。柏木单元由浅绿色灰色板岩、千层状黑色板岩、深灰色硅质板岩和灰色至浅绿色簇岩交替组成，并伴有少量的层状燧石和石灰岩块体。该单元以半深海沉积物为主。该单元的片岩结构显示出晚侏罗世米卡武绿岩和三巴川变质岩的变质级配。增生杂岩中的基性岩块体主要占据了晚古生代至早中生代的一部分海底。它们的岩石学特征和地质年代表明它们可能是伊扎那吉板块形成的海洋高原的一部分。少

The northern part of the Chichibu belt in the Kanto Mountains consists mainly of Jurassic terrigeneous clastic rocks and various-sized exotic blocks of oceanic plate origin, such as Triassic bedded chert, late Paleozoic limestone and greenstone. Mode of occurrence and cc-existence of the blocks with different geologic ages indicate that the Chichibu belt was formed as an accretion complex in relation to oceanic plate subduction. It is considered that the subduction have occurred along the East Asia continental margin. This is supported by the U-Th-Pb ages of detrital monazite grains of the complex, including 200 Ma and 1900 Ma. The Jurassic accretion complex juxtaposed between the Sanbagawa metamorphics and the Cretaceous zone is divided into three tectonostratigraphic units ; the Kazahaya-toge, Kamiyoshida and Kashiwagi. The boundaries among them and the other units are fault in contact.The radiolarian biostratigraphic age of the Kazahaya-toge unit, consisting of alternation of sandst … More one and mudstone, is of late Early Jurassic to early Middle Jurassic, and is considered to have deposited in the forearc basin. The Kamiyoshida unit, consisting of sandstone and mudstone including exotic oceanic plate blocks of bedded chert, limestone and mafic volcanic and/or volpaniclastic rocks, is late Early to middle Middle Jurassic in age. This unit intercadates peculiar elastics layers including abundant apatite and phlogopite, probably derived from alkaline mafic igneous rocks. The K-Ar age of the phlogopite is approximately 250 Ma. The Kashiwagi unit is composed of alternation of pale-greenish grey slate, phyllitic black slate, dark grey siliceous slate, and grey to pale-green tuft, with subordinate amounts of bedded chert and limestone blocks. This unit is dominated in hemipelagic sediments. Schistose texture in the unit shows a metamorphic gradation to the late Jurassic Mikabu green rocks and also to the Sanbagawa metamorphic rocks. The blocks of the mafic rocks in the accretion complex have occupied primarily a part of late Paleozoic to early Mesozoic ocean floor. Their petrologic character and geologic age suggest that they were probably a part of an oceanic plateau formed in Izanagi plate. Less