The remnant rear-arc segment of the early Izu-Bonin arc, known as the Kyushu-Palau Ridge (KPR), is a key location where magmatic outputs can be constrained during the lifetime of an island arc. We present new geochemical data for coarse-grained basaltic to andesitic volcaniclastic sandstones derived from the KPR and deposited in the Amami Sankaku Basin (IODP Site U1438, Unit III rocks) in the time period 40–30 Ma. Bulk disaggregated and cleaned volcaniclastic sandstones of Unit III at Site U1438 retain primary magmatic signatures and can be used to infer the evolution of magmatic sources of the juvenile Izu-Bonin island arc through time. A sharp increase of slab-derived components to the source of KPR magmatism developed at about 35 Ma, indicated by increasing Th/La and decreasing Sm/La, Yb/Hf and Nb/Nd. Systematic variations in trace element ratios and increasing trace element abundances in younger samples through the 40-30 Ma time window are decoupled from Hf-Nd isotope ratios, which are measurably more depleted (ε_Hf =16.5–15, ε_Nd =9.6–8.2) than boninites produced during the preceding magmatic phase and sampled in the modern IBM forearc. Hafnium isotopic compositions in the Unit III sandstones remain little-changed and similar to the subducting Pacific Plate after 40 Ma and do not revert to highly radiogenic compositions of the Indian-type MORB mantle wedge which is reflected in highly-depleted basalts produced at Site U1438 and in the forearc (commonly ε_Hf≥18.0). The overall pattern recorded in Unit III sandstones indicates that the Pacific-type MORB slab-melt component, which was present in the preceding boninite phase of magmatism, persisted after 40 Ma, while the subducted sediment component in the boninite source was lost or significantly reduced. Variations in trace element ratios (at constant ε_Nd and near-constant and radiogenic ε_Hf) and in high field strength element abundances of the early Izu Bonin arc are controlled by the addition of a subducted Pacific MORB melt or supercritical fluid to the mantle wedge. A subducted MORB (slab melt) component is thus sampled throughout the early life of the Izu-Bonin arc and in the currently active Izu-Bonin arc-backarc system.

伊豆-波宁弧早期的残余后弧段，即九州-帕劳岭（KPR），是在岛弧寿命期间可限制岩浆输出的关键位置。我们提供了40〜30 Ma时期从KPR派生并沉积在Amami Sankaku盆地（IODP站点U1438，III号岩石）的粗粒玄武岩至安山岩火山碎屑岩的新地球化学数据。U1438站点的III号单元的散乱解体和清洁的火山碎屑砂岩保留了主要岩浆特征，可用于推断伊豆-波宁岛幼弧的岩浆来源随时间的演变。在大约35 Ma时，平板状成分对KPR岩浆作用源的急剧增加，表现为Th / La升高而Sm / La，Yb / Hf和Nb / Nd降低。_HF = 16.5-15，ε_的Nd = 9.6-8.2）比在前述岩浆相位产生并在现代IBM弧前采样boninites。III号单元砂岩中的compositions同位素组成在40 Ma后仍保持不变，与俯冲太平洋板块相似，并且未还原为印度型MORB地幔楔的高放射成因成分，这反映在站点U1438上高度贫化的玄武岩中并在弧前（通常ε_铪≥18.0）。第三单元砂岩中记录的总体模式表明，在前岩浆质贝氏体相中存在的太平洋型MORB板状融体成分在40 Ma之后仍然存在，而贝氏体源中俯冲的沉积物成分丢失或显着减少。 。痕量元素的比率变化（在恒定的ε_的Nd和近常数和放射性ε_铪），并在高场早期伊豆小笠原弧的强度元素丰度通过添加的俯冲太平洋MORB熔化或超临界流体的地幔楔控制。因此，在伊豆-波宁弧的早期和当前活跃的伊豆-波宁弧-反弧系统中，对俯冲的MORB（板状熔体）成分进行了采样。