地学雑誌
Online ISSN : 1884-0884
Print ISSN : 0022-135X
論説
古生代日本の前弧地殻の変遷
─高知県中西部黒瀬川帯の古生代中期花崗岩類・砂岩のジルコンU–Pb年代─
長谷川 遼安井 敏夫堤 之恭磯﨑 行雄
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2017 年 126 巻 5 号 p. 617-640

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 To reconstruct the already lost tectonic framework of the Early-Middle Paleozoic arc-trench system, U–Pb ages of zircons are analyzed using LA-ICPMS from mid-Paleozoic arc-related granitoids and sandstones in the Kurosegawa belt, SW Japan, particularly from western-central Kochi prefecture in Shikoku where the belt attains the greatest width with the greatest variations of rock types. Dated samples include Yokokurayama granite, Ohira quartz diorite, basal sandstone of Lower Silurian Gomi Formation above the Yokokurayama granite, and basal conglomerate/overlying sandstone of Upper Devonian Ochi Formation from the Yokokurayama lenticular body in western Kochi prefecture, as well as Konomori quartz diorite from the Konomori lenticular body in Kochi city. The samples of Yokokurayama granite yield many zircons of ca. 451-439 Ma (latest Ordovician to earliest Silurian) U–Pb ages, whereas Konomori quartz diorite have only one grain, plotted on Concordia curve, of ca. 382 Ma (Late Devonian). The former ages confirm Ordovician–Silurian arc magmatism in Japan, whereas the latter identify late Devonian magmatism for the first time. The unconformably-covering sandstone of the Gomi Formation naturally contains abundant detrital zircons of almost identical ages to the underlying granite. The basal conglomerate of the Ochi Formation contains detrital zircons from Ordovician–Silurian granitoids and also some Mesoproterozoic (1400-1200 Ma) ones, suggesting an intimate link between Paleozoic Japan and the South China block. These new zircon ages from the Kuroseagawa belt suggest that a unique magmatism occurred in the forearc of Paleozoic Japan to emplace I-type granitoids in shallower crust, possibly by the subduction of a young oceanic plate. This further suggests that the Silurian mid-arc domain changed into a forearc setting with significant tectonic erosion/removal of preexisting fore-arc crust.

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© 2017 公益社団法人 東京地学協会
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