Abstract Supra-subduction zone (SSZ) ophiolites formed in forearc settings and the Izu–Bonin–Mariana (IBM) in-situ forearc show conversions from normal mid-ocean ridge basalt (MORB) to boninitic affinities, which are widely interpreted as rules for the evolving chemical geodynamics of subduction initiation magmatism (SIM). Nevertheless, forearc sequence generated by subduction initiation need not always include boninitic rocks. Here we reconstruct a forearc chemostratigraphy without boninitic affinity based on the Central Palawan and Amnay (CPA) ophiolites of the west Philippines to show diversity in melt evolution of SIM. Zircon U–Pb dating on the MORB-type Amnay gabbros reveal a 10 m.y. of seafloor spreading from 33 to 23 Ma. These ages together with the progressively enriched zircon in-situ oxygen isotopes ( δ 18 O= 1.62–10.1‰) and whole rock elemental and Nd–Hf isotopic signatures ([La/Yb]N= 0.19–3.45; Nb/Yb= 0.07–5.24; eNd= 2.1–10.2; eHf= 11.2–24.1) indicate that the depleted MORB mantle (DMM)-type source of the Amnay ophiolite was getting enriched by fluxing of aqueous slab fluid in the early stage followed by addition of adakitic and Nb-enriched slab melts in the later stage. This distinct mantle evolution together with higher eHf at a given eNd of the Amnay mafic samples than the contemporaneous South China Sea (SCS) oceanic samples do not support previous interpretation that the Amnay ophiolite was a fragment of the SCS. However, the 33–23 Ma Amnay ophiolite can be the younger part of the 34 Ma forearc-type Central Palawan ophiolite, and both (i.e. the CPA ophiolites) represent on-land fragments of the NW Sulu Sea. We propose that the 34–23 Ma CPA ophiolites and the NW Sulu Sea constitute forearc basin of the late Oligocene–middle Miocene island arc–backarc basin duo of the Cagayan Ridge and SE Sulu Sea, associated with subduction of the Cretaceous to Eocene Proto-SCS since 34 Ma. The CPA ophiolites lack boninite but show geochemical progression from normal to enriched MORB affinities, whose melt evolution differs from that of the IBM SIM. We attribute the differences in SIM and variations in forearc chemostratigraphy to the subduction of a younger slab with thick terrigenous sediment cover of the Proto-SCS versus an older slab with thin pelagic sediment cover of the Pacific Plate.

中文翻译：

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俯冲起始岩浆作用和弧前化学地层变化过程中板片控制的元素同位素富集

摘要 前弧环境中形成的超俯冲带 (SSZ) 蛇绿岩和伊豆-博宁-马里亚纳 (IBM) 原位前弧显示从正常洋中脊玄武岩 (MORB) 到博宁岩亲和性的转变，这被广泛解释为俯冲起始岩浆作用（SIM）的演化化学地球动力学。然而，由俯冲开始产生的弧前序列不一定总是包括博尼质岩。在这里，我们基于菲律宾西部的 Central Palawan 和 Amnay (CPA) 蛇绿岩重建了一个没有 boninitic 亲和力的前弧化学地层学，以显示 SIM 熔体演化的多样性。MORB 型 Amnay 辉长岩上的锆石 U-Pb 测年揭示了从 33 到 23 Ma 延伸 10 米的海底。这些年龄连同逐渐富集的锆石原位氧同位素 (δ 18 O= 1.62–10)。1‰) 和全岩元素和 Nd–Hf 同位素特征（[La/Yb]N= 0.19–3.45；Nb/Yb= 0.07–5.24；eNd= 2.1–10.2；eHf= 11.2–24.1）表明耗尽的 MORB Amnay蛇绿岩的地幔（DMM）型源在早期通过含水板坯流体的流动而变得富集，随后在后期添加埃达克质和富含Nb的板坯熔体。这种独特的地幔演化以及在 Amnay 基性岩样品的给定终点比同期南海 (SCS) 海洋样品更高的 eHf 不支持先前的解释，即 Amnay 蛇绿岩是 SCS 的碎片。然而，33-23 Ma Amnay 蛇绿岩可能是 34 Ma 前弧型 Central Palawan 蛇绿岩的年轻部分，两者（即 CPA 蛇绿岩）都代表 NW 苏禄海的陆上碎片。我们提出 34-23 Ma CPA 蛇绿岩和 NW 苏禄海构成了晚渐新世-中中新世岛弧-卡加延海脊和苏禄海东南部弧后盆地的弧前盆地，与白垩纪向始新世原始的俯冲有关。自 34 Ma 以来的 SCS。CPA蛇绿岩缺乏boninite，但显示出从正常到富MORB亲和性的地球化学进展，其熔体演化与IBM SIM不同。我们将 SIM 的差异和弧前化学地层学的变化归因于原始 SCS 具有厚陆源沉积物覆盖的年轻板块与太平洋板块具有薄远洋沉积物覆盖的旧板块的俯冲。与自 34 Ma 以来白垩纪向始新世 Proto-SCS 的俯冲有关。CPA蛇绿岩缺乏boninite，但显示出从正常到富MORB亲和性的地球化学进展，其熔体演化与IBM SIM不同。我们将 SIM 的差异和弧前化学地层学的变化归因于原始 SCS 具有厚陆源沉积物覆盖的较年轻板块与太平洋板块具有薄远洋沉积物覆盖的较老板块的俯冲作用。与自 34 Ma 以来白垩纪向始新世 Proto-SCS 的俯冲有关。CPA蛇绿岩缺乏boninite，但显示出从正常到富MORB亲和性的地球化学进展，其熔体演化与IBM SIM不同。我们将 SIM 的差异和弧前化学地层学的变化归因于原始 SCS 具有厚陆源沉积物覆盖的年轻板块与太平洋板块具有薄远洋沉积物覆盖的旧板块的俯冲。