Origin of the Tonaru body in the Sanbagawa metamorphic belt, SW Japan

• Published in: The island arc 2020-01, Vol.29 (1), p.n/a
• Main Authors: Aoki, Shogo, Aoki, Kazumasa, Chiba, Kentaro, Sakata, Shuhei, Tsuchiya, Yuta, Kato, Daichi
• Format: Article
• Language: English
• Subjects: Belts; Eclogite; Geochronology; Isotopes; Magma; oceanic arc; Radiometric dating; Sanbagawa metamorphic belt; Subduction (geology); Tonaru; trace element; Trace elements; Zircon; zircon U–Pb dating
• ISSN: 1038-4871; 1440-1738
• DOI: 10.1111/iar.12332

Zircon U–Pb dating of the Tonaru metagabbro body in the Sanbagawa metamorphic belt, southwest Japan, suggests that igneous events at ca 200–180 Ma were involved in the protolith formation. The trace element compositions of the Tonaru zircons are enriched in U (a fluid‐mobile element) and Sc (an amphibole‐buffered element), and depleted in Nb (a fluid‐immobile element), suggesting that the parental magmas related to the Tonaru metagabbros formed in an arc setting. Integration of our results with previous studies of the metasedimentary rocks in the Tonaru body clearly indicates that the protoliths of the Tonaru body were produced by oceanic‐arc magmatism. With the previous geochronological and geological studies, the tectono‐magmatic–metamorphic history of the Tonaru and other mafic bodies in the Sanbagawa metamorphic belt may be summarized as follows: (i) the protolith formation by the oceanic‐arc magmatic event had occurred at 200–180 Ma; (ii) the protoliths were accreted in the trench at ca 130–120 Ma; and (iii) they were completely subducted into the depth of the eclogite‐facies condition after 120 Ma. Illustration of the tectono‐magmatic and metamorphic histories of the Tonaru and Iratsu bodies in the Sanbagawa metamorphic belt. (a) The oceanic‐arc magmatism forming the mafic rocks (protoliths of the Tonaru and Iratsu bodies) occurred at ca 200–180 Ma. (b) The Tonaru and Iratsu bodies were accreted and subducted in the trench by ca 130–120 Ma, and the eclogite metamorphism occurred after ca 120 Ma. 西南日本三波川変成帯・別子地域に産するエクロジャイト–角閃岩岩体の1つである東平変はんれい岩体からジルコンを抽出し、そのU–Pb年代と微量元素濃度から原岩形成年代とその形成場の制約を試みた。火成起源ジルコンのU−Pb年代測定値は、200 Maから180Maの値を示した。また、その微量元素組成は、島弧マグマ起源ジルコンに特徴的に見られる高U/Yb、高Sc/Yb、および低Nb/Ybを示した。これらの結果と、隣接するその他の岩体からこれまで報告されている地質学・岩石学的特徴を考慮すると、東平はんれい岩体を含む別子地域周辺に産するエクロジャイト–角閃岩岩体の原岩の多くは200 Maから180 Maの火成活動の間に形成された海洋性島弧起源であり、それら原岩は130–120 Ma頃に当時の海溝に達した後、沈み込みに伴う変成作用を被ったと考えられる。