Geologic study of oceanic plate components in the northern Chichibu belt

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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