Ascertaining the emplacement mechanism of oceanic basaltic lavas is important in understanding how ocean floor topography is produced and oceanic plates evolve, particularly during the early stages of crustal development of a supra‐subduction zone. A detailed study of the volcanic stratigraphy at International Ocean Discovery Program (IODP) Site U1438 in the Amami Sankaku Basin, west of the Kyushu–Palau Ridge, has revealed the development of lava accretion and ridge topography on the Philippine Sea plate at about 49 Ma. Igneous basement rocks penetrated at Site U1438 are the uppermost 150 m of ~6 km‐thick oceanic crust, and comprise, in a downhole direction, sheet flows (12.6 m), lobate sheet flows (61.3 m), pillow lavas (50.7 m), and thin sheet flows (25.3 m). The lowermost sheet flows are intercalated with layers of limestone and epiclastic tuff. Lithofacies analysis reveals that the lowermost sheet flows, limestone, and tuff formed on an axial rise, the pillow lavas were emplaced on a ridge slope, and the lobate sheet flows formed off ridge on an abyssal plain. The lithofacies of the basement basalt corresponds to the upper portions of fast‐spreading oceanic crust, suggesting that subduction initiation was associated with intermediate to fast rates of seafloor spreading. The surface sheet flows are olivine–clinopyroxene‐phyric basalt and differ from the lower basalt flows that contain phenocrysts of olivine and plagioclase, with or without clinopyroxene. The depleted chrome‐spinel composition and olivine–clinopyroxene phenocryst assemblage in the surface sheet flows suggests a slight contribution of water for magma generation not present for the lower basalt flows. Considering the lithological difference between the backarc and forearc oceanic crust in the Izu–Bonin–Mariana arc, with sheet flow dominant in the former, seafloor spreading occurred faster in the later stage of subduction initiation.

中文翻译：

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伊祖-波宁-马里亚纳弧系中原弧形玄武岩的侵位过程

确定海底玄武岩熔岩的进位机制对于理解海床地形是如何产生的以及海床板块如何演化至关重要，特别是在超俯冲带地壳发育的早期阶段。在九州-帕劳山脊以西的Am美三国盆地国际海洋发现计划（IODP）站点U1438上的火山地层的详细研究表明，菲律宾海板上约49 Ma的熔岩吸积和山脊地形发育。在U1438处穿透的火成岩基底岩石是约6 km厚的大洋地壳的最上层150 m，在井下方向上包括表层流（12.6 m），叶状表层流（61.3 m），枕形熔岩（50.7 m）。和薄板流（25.3 m）。最低层的表层流夹层有石灰岩和碎屑凝灰岩层。岩相分析显示，最低层的片流，石灰岩和凝灰岩在轴向上升时形成，枕形熔岩被置于山脊斜坡上，而叶状片流在深海平原的山脊上形成。地下玄武岩的岩相与快速扩散的洋壳的上部相对应，这表明俯冲作用的开始与海底扩散的中速到快度有关。地表层流是橄榄石－斜向辉石－玄武岩，与下部玄武岩流不同，下部玄武岩流含有橄榄石和斜长石的表晶，有或没有斜向辉石。表层流中贫化的铬-尖晶石成分和橄榄石-斜向辉石的隐晶石组合表明，水对岩浆生成的贡献很小，而较低的玄武岩流则不存在。