The oldest known, intact sedimentary record of a nascent intraoceanic arc was recovered in a ∼100-m-thick unit (IV) above ca. 49 Ma basaltic basement at International Ocean Discovery Program Site U1438 in the Amami Sankaku Basin. During deposition of Unit IV the site was located ∼250 km from the plate edge, where Izu-Bonin-Mariana subduction initiated at 52 Ma. Basement basalts are overlain by a mudstone-dominated subunit (IVC) with a thin basal layer of dark brown metalliferous mudstone followed by mudstone with sparse, graded laminae of amphibole- and biotite-bearing tuffaceous sandstone and siltstone. Amphibole and zircon ages from these laminae suggest that the intermediate subduction-related magmatism that sourced them initiated at ca. 47 Ma soon after basement formation. Overlying volcaniclastic, sandy, gravity-flow deposits (subunit IVB) have a different provenance; shallow water fauna and tachylitic glass fragments indicate a source volcanic edifice that rose above the carbonate compensation depth and may have been emergent. Basaltic andesite intervals in upper subunit IVB have textures suggesting emplacement as intrusions into unconsolidated sediment on a volcanic center with geochemical and petrological characteristics of mafic, differentiated island arc magmatism. Distinctive Hf-Nd isotope characteristics similar to the least-radiogenic Izu-Bonin-Mariana boninites support a relatively old age for the basaltic andesites similar to detrital amphibole dated at 47 Ma. The absence of boninites at that time may have resulted from the position of Site U1438 at a greater distance from the plate edge. The upper interval of mudstone with tuffaceous beds (subunit IVA) progresses upsection into Unit III, part of a wedge of sediment fed by growing arc-axis volcanoes to the east. At Site U1438, in what was to become a reararc position, the succession of early extensional basaltic magmatism associated with spontaneous subduction initiation is followed by a rapid transition into potentially widespread subduction-related magmatism and sedimentation prior to the onset of focused magmatism and major arc building.

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来自 IODP 站点 U1438 的新生伊豆-博宁-马里亚纳弧的沉积和火山记录

在大约 100 米厚的单元 (IV) 中发现了最古老的、完整的新生洋内弧沉积记录。位于奄美三阁盆地的国际海洋发现计划站点 U1438 的 49 Ma 玄武岩基底。在单元 IV 沉积期间，该地点位于距板块边缘约 250 公里处，伊豆-博宁-马里亚纳俯冲在 52 Ma 开始。基底玄武岩上覆以泥岩为主的亚基 (IVC)，基底为深棕色含金属泥岩的薄基底层，其次是泥岩，其后为含有角闪石和黑云母的凝灰质砂岩和粉砂岩的稀疏分级层。来自这些薄层的角闪石和锆石年龄表明，产生它们的中间俯冲相关岩浆活动始于约 47 Ma 地下室形成后不久。上覆火山碎屑、沙质，重力流沉积物（亚基IVB）有不同的来源；浅水动物群和 tachylitic 玻璃碎片表明一个源火山结构上升到碳酸盐补偿深度以上并且可能已经出现。上亚单元 IVB 中玄武质安山岩层段的纹理表明，作为侵入火山中心的松散沉积物的侵位，具有基性、分化的岛弧岩浆作用的地球化学和岩石学特征。与放射性最低的伊豆-博宁-马里亚纳博宁岩相似的独特 Hf-Nd 同位素特征支持玄武质安山岩的相对较老的年龄，类似于碎屑角闪石的年代为 47 Ma。当时没有博尼特可能是由于 U1438 站点的位置离板块边缘更远。带有凝灰质层的上层泥岩（IVA 亚单元）向上进入单元 III，这是由东部增长的弧轴火山供给的楔形沉积物的一部分。在 U1438 站点，在后来成为弧后位置的地方，与自发俯冲开始相关的早期伸展玄武质岩浆作用的连续发生之后，在集中岩浆作用和主弧开始之前迅速转变为潜在广泛的与俯冲相关的岩浆作用和沉积作用建造。