Li, HeArculus, RichardIshizuka, OsamuHickey-Vargas, RosemaryYogodzinski, Gene MMcCarthy, AndersKusano, YukiBrandl, PhilippSavov, Ivan PTepley III, Frank JSun, Weidong2022-10-262022-10-262041-1723http://hdl.handle.net/1885/276178The magmatic character of early subduction zone and arc development is unlike mature systems. Low-Ti-K tholeiitic basalts and boninites dominate the early Izu-Bonin-Mariana (IBM) system. Basalts recovered from the Amami Sankaku Basin (ASB), underlying and located west of the IBM’s oldest remnant arc, erupted at ~49 Ma. This was 3 million years after subduction inception (51-52 Ma) represented by forearc basalt (FAB), at the tipping point between FAB-boninite and typical arc magmatism. We show ASB basalts are low-Ti-K, aluminous spinel-bearing tholeiites, distinct compared to mid-ocean ridge (MOR), backarc basin, island arc or ocean island basalts. Their upper mantle source was hot, reduced, refractory peridotite, indicating prior melt extraction. ASB basalts transferred rapidly from pressures (~0.7-2 GPa) at the plagioclase-spinel peridotite facies boundary to the surface. Vestiges of a polybaric-polythermal mineralogy are preserved in this basalt, and were not obliterated during persistent recharge-mix-tap-fractionate regimes typical of MOR or mature arcs.H.L. and W.D.S. acknowledge funding from the National Natural Science Foundation of China (91958110), the Strategic Priority Research Program (B) of the Chinese Academy of Sciences (XDB42020203) and NSFC 41473029. Funding was provided by the Australian Research Council via ANZIC through a grant to R.J.A.application/pdfen-AU© 2021 The authorshttp://creativecommons.org/licenses/by/4.0/202110.1038/s41467-021-21980-02021-11-28Creative Commons Attribution licence