Fragments of the early Paleozoic orogenic belt from Tokyo Metropolis, Japan:

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  • 東京都産古生代前期造山帯の断片：
  • 東京都産古生代前期造山帯の断片 : 関東山地東部,黒瀬川帯高圧型変斑れい岩および花崗岩類のジルコンU-Pb年代
  • トウキョウトサン コセイダイ ゼンキ ゾウザンタイ ノ ダンペン : カントウ サンチ トウブ,クロセガワタイ コウアツガタ ヘン ハンレイ ガン オヨビ カコウガンルイ ノ ジルコン U-Pb ネンダイ
  • Zircon U-Pb ages of high P/T-type metagabbro and granitoids of the Kurosegawa belt in eastern Kanto Mountains
  • 関東山地東部，黒瀬川帯高圧型変斑れい岩および花崗岩類のジルコンU-Pb年代

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Abstract

<p>The Japanese Islands have evolved as a subduction-related orogenic belt since the Cambrian, as recorded by lower Paleozoic granitoids and blueschists. These older rocks occur as disrupted tectonic blocks within serpentinite mélange zones (e.g., the Kurosegawa belt in southwest Japan), which form a discontinuous belt less than a few kilometers wide. Using LA-ICP-MS, this study reports in-situ zircon U-Pb ages for high-pressure metagabbro and granitoids from the Sakamoto area in Tokyo. Zircon grains from the metagabbro yield an age of ca. 490 Ma, which is coeval with similar rocks from the Kurosegawa belt reported in the islands of Kyushu and Shikoku. Zircon grains from the granitoids yield an igneous age of ca. 440 Ma, whereas xenocrystic or inherited zircon grains range in age from 2400 to 500 Ma. Similar zircon ages have been reported from granitoids of the Kurosegawa belt in Kyushu, Shikoku, and the Kii Peninsula. Results from the present study confirm that the dated igneous and metamorphic rocks in the study area represent some of the oldest components of the Kurosegawa belt, and that the belt extended discontinuously to the eastern Kanto Mountains. The ca. 490 Ma upper Cambrian (Furongian)-Lower Ordovician metagabbro (ophiolites) and the ca. 440 Ma Upper Ordovician-lower Silurian (Llandovery) granitoids near Tokyo metropolis thus indicate development of a >800 km arc-trench system across the Japanese area from the early Paleozoic.</p>

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