Geologic study of oceanic plate components in the northern Chichibu belt

关东秩父带北方主要由侏罗纪陆源碎屑岩和三叠纪层状硅质岩、晚古生代灰岩和绿岩等规模不等的洋板块成因外来岩块组成。不同地质时代地块的产出方式和共生性表明秩父带是与大洋板块俯冲有关的增生杂岩。认为俯冲作用发生在沿着东亚大陆边缘。杂岩体中独居石碎屑颗粒的U-Th-Pb年龄（200 Ma和1900 Ma）支持了这一观点。将三坝川变质带与白垩纪变质带并列的侏罗纪增生杂岩划分为三个构造地层单位：风早下、上吉田和柏木。Kazahaya-toge单元的放射虫生物地层时代为砂层交替， ...更多信息 一套为泥岩，时代为早侏罗世晚期至中侏罗世早期，被认为是弧前盆地沉积。上吉田单元由砂岩和泥岩组成，包括层状燧石、石灰岩和镁铁质火山岩和/或火山碎屑岩的外来洋板块，其时代为早中侏罗世晚期至中侏罗世中期。该单元内夹有丰富的磷灰石和金云母等特殊的弹性体层，可能来源于碱性镁铁质火成岩。金云母的K-Ar年龄约为250 Ma。柏木单元由浅绿灰色板岩、千枚状黑色板岩、深灰色硅质板岩和灰色至浅绿色丛岩交替组成，并伴有少量层状燧石和石灰岩块。该单元主要存在于半远洋沉积物中。该单元的片理结构表现出与晚侏罗世三卡布绿色岩和三坝川变质岩的变质递变。增生杂岩中的镁铁质岩块主要占据了晚古生代至早中生代洋底的一部分。岩石学特征和地质时代表明，它们可能是伊那木板块洋坪的一部分。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