Timescale of material circulation in subduction zone: U–Pb zircon and K–Ar phengite double‐dating of the Sanbagawa metamorphic complex in the Ikeda district, central Shikoku, southwest Japan

• Published in: The island arc 2019-07, Vol.28 (4), p.n/a
• Main Authors: Nagata, Mitsuhiro, Miyazaki, Kazuhiro, Iwano, Hideki, Danhara, Tohru, Obayashi, Hideyuki, Hirata, Takafumi, Yagi, Koshi, Kouchi, Yoshikazu, Yamamoto, Koshi, Otoh, Shigeru
• Format: Article
• Language: English
• Subjects: Age; Chlorite; Chronology; Cretaceous; Dating; Deformation; Garnet; high‐pressure metamorphic rocks; K–Ar age; Lead; Major constituents; Metamorphism; Metamorphism (geology); Oceanic trenches; phengite; Plates (tectonics); Radiometric dating; Sanbagawa metamorphic complex; Sediment; Sediments; Subduction; Subduction (geology); Subduction zones; SW Japan; Time; U–Pb age; Zircon
• ISSN: 1038-4871; 1440-1738
• DOI: 10.1111/iar.12306

We have estimated the timescale of material circulation in the Sanbagawa subduction zone based on U–Pb zircon and K–Ar phengite dating in the Ikeda district, central Shikoku. The Minawa and Koboke units are major constituents of the high‐P Sanbagawa metamorphic complex in Shikoku, southwest Japan. For the Minawa unit, ages of 92–81 Ma for the trench‐fill sediments, are indicated, whereas the age of ductile deformation and metamorphism of garnet and chlorite zones are 74–72 Ma and 65 Ma, respectively. Our results and occurrence of c. 150 Ma Besshi‐type deposits formed at mid‐ocean ridge suggest that the 60‐Myr‐old Izanagi Plate was subducted beneath the Eurasian Plate at c. 90 Ma, and this observation is consistent with recent plate reconstructions. For the Koboke unit, the depositional ages of the trench‐fill sediments and the dates for the termination of ductile deformation and metamorphism are estimated at c. 76–74 and 64–62 Ma, respectively. In the Ikeda district, the depositional ages generally become younger towards lower structural levels in the Sanbagawa metamorphic complex. Our results of U–Pb and K–Ar dating show that the circulation of material from the deposition of the Minawa and Koboke units at the trench through an active high‐P metamorphic domain to the final exhumation from the domain occurred continuously throughout c. 30 Myr (from c. 90 to 60 Ma). In the Ikeda district, the depositional ages generally become younger towards lower structural levels in the Sanbagawa metamorphic complex. Our results of U–Pb and K–Ar dating show that the circulation of material from the deposition of the Minawa and Koboke units at the trench through an active high‐P metamorphic domain to the final exhumation from the domain occurred continuously throughout c. 30 Myr (from c. 90 to 60 Ma). 沈みこみ帯における物質循環の時間スケールを検討するため，池田地域の四国三波川変成コンプレックスのジルコンU–Pb年代及びフェンジャイトK–Ar年代測定を行った.  当地域に分布する三縄ユニットと小歩危ユニットは四国三波川コンプレックスの主要構成ユニットである.  構造的上位の三縄ユニットでは，約150 Maの海洋地殻に92–81 Maの海溝充填堆積物が堆積し，74–65 Maまで変成・変形作用が進行したと推定された.  この推定は，90 Ma 頃にイザナギプレートがユーラシアプレートへ沈み込んでいたことを支持する.  一方，下位の小歩危ユニットでは， 76–74 Ma の堆積年代と64–62 Maまでの変成・変形作用の進行が見積もられた.  当地域の三波川変成コンプレックス全体では，約3000万年の時間スケールで，海溝での堆積，沈み込み帯深部での変成・変形，及び地殻浅部への上昇が連続して起こっていたと推定される.