This paper explores the evolutional process of back-arc basin (BAB) magma system at final spreading stage of extinct BAB, Shikoku Basin (Philippine Sea) and assesses its tectonic evolution using a newly discovered oceanic core complex, the Mado Megamullion. Bulk and in-situ chemical compositions together with in-situ Pb isotope composition of dolerite, oxide gabbro, gabbro, olivine gabbro, dunite, and peridotite are presented. Compositional ranges and trends of the igneous and peridotitic rocks from the Mado Megamullion are similar to those from the slow- to ultraslow-spreading mid-ocean ridges (MOR). Since the timing of the Mado Megamullion exhumation corresponds to the very end of the Shikoku Basin opening, the magma supply was subdued and highly episodic, leading to extreme magma differentiation to form ferrobasaltic, hydrous magmas. In-situ Pb isotope composition of magmatic brown amphibole in the oxide gabbro is identical to that of depleted source mantle for mid-ocean ridge basalt (MORB). In the context of hydrous BAB magma genesis, the magmatic water was derived solely from the MORB source mantle. The distance from the back-arc spreading center to the arc front increased away through maturing of the Shikoku Basin to cause MORB-like magmatism. After the exhumation of Mado Megamullion along detachment faults, dolerite dikes intruded as a post-spreading magmatism. The final magmatism along with post-spreading Kinan Seamount Chain volcanism were introduced around the extinct back-arc spreading center after the opening of Shikoku Basin by residual mantle upwelling.

本文探讨了弧后盆地 (BAB) 岩浆系统在已灭绝的四国盆地（菲律宾海）BAB 最后扩张阶段的演化过程，并利用新发现的洋核复合体 Mado Megamullion 评估其构造演化。提供了大量和原位化学成分以及辉绿岩、氧化物辉长岩、辉长岩、橄榄石辉长岩、纯长岩和橄榄岩的原位 Pb 同位素组成。Mado Megamullion 火成岩和橄榄岩的成分范围和趋势与慢到超慢扩张的洋中脊（MOR）相似。由于 Mado Megamullion 挖掘的时间对应于四国盆地开放的末期，岩浆供应受到抑制且高度间歇，导致岩浆极端分异形成铁玄武岩、含水岩浆。氧化辉长岩中岩浆棕色角闪石的原位Pb同位素组成与洋中脊玄武岩(MORB)的贫化源地幔相同。在含水 BAB 岩浆成因的背景下，岩浆水仅来自 MORB 源地幔。随着四国盆地的成熟, 弧后扩张中心到弧锋的距离增加, 引起类似 MORB 的岩浆作用。Mado Megamullion 沿拆离断层出土后，辉绿岩岩脉作为扩张后的岩浆侵入。在四国盆地开放后，残余地幔上升流在已灭绝的弧后扩张中心周围引入了最终的岩浆作用以及扩张后的基南海山链火山活动。