[S-CG51] ハイブリッド年代学 -多次元年代データ時代の到来-

[SCG51-02] Late dry magmatism in back-arc basin recorded as dolerite dike intrusion in the Mado Megamullion, Shikoku Basin

*秋澤 紀克¹、伊藤 健吾²、仁木 創太²、平田 岳史²、沖野 郷子¹、小原 泰彦^3,4,5 (1.東京大学　大気海洋研究所、2.東京大学　地殻化学実験施設、3.海上保安庁 海洋情報部、4.海洋研究開発機構、5.名古屋大学)

キーワード：イルメナイト U–Pb dating、背弧海盆、海洋コアコンプレックス、マントル

Back-arc basins (BAB) form an integral but often neglected part in the framework of subduction zone system. Many studies on the BAB have focused on BAB basalts because they are more accessible than to the lower crust and underlying mantle. The Shikoku Basin and Parece Vela Basin formed behind arc systems in the Philippine Sea host numerous oceanic core complexes (OCC), dome-like bathymetric highs composed of lower crust and mantle materials. One of these OCC named Mado Megamullion in the Shikoku Basin is our study target, where dolerite dike intrusions are recognized as a late magma activity. Ilmenite U–Pb dating of a dolerite sample indicates that the dolerite formed at 9.7 ± 6.3 Ma (2 sigma error, n = 24), at which the Shikoku Basin ceased opening. Geochemical tracers of Ba/Yb and Ba/Nb were used to evaluate the extent of subduction input to dolerite-forming magmas. The results demonstrated that the subduction components did not reach the back-arc spreading center and hence the dolerite-forming magma was “dry” because it was well apart from fluid-supplying slab when the Mado Megamullion was formed. The Mado Megamullion has ample potential to decode mature back-arc spreading system, whereas the OCC located in initial spreading modes of the Shikoku Basin can decipher immature back-arc spreading system. Accurate understanding of BAB evolution will come from further investigations of OCC in the Shikoku and Parece Vela back-arc basins.