Fragments of the early Paleozoic orogenic belt from Tokyo Metropolis, Japan Zircon U-Pb ages of high P/T-type metagabbro and granitoids of the Kurosegawa belt in eastern Kanto Mountains

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 d...

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Bibliographic Details
Published inChishitsugaku zasshi Vol. 126; no. 10; pp. 551 - 561
Main Authors Sawada, Hikaru, Isozaki, Yukio, Sakata, Shuhei
Format Journal Article
LanguageJapanese
Published Tokyo The Geological Society of Japan 15.10.2020
Japan Science and Technology Agency
Subjects
Cambrian
Crop yield
Geochronometry
granitoid
Islands
Isotopes
Kurosegawa belt
metagabbro
Metamorphic rocks
Mountains
Ophiolites
Ordovician
Paleozoic
Serpentinite
Silurian
Subduction
Yields
Zircon
Japan
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Summary: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.
ISSN:0016-7630
1349-9963
DOI:10.5575/geosoc.2020.0026