祁连玻安岩地球化学特征数据集

玻安岩为一类具有特殊地球化学性质的岩石，具有高 SiO2（＞52%）、高 MgO（＞8%）和低 TiO2（＜0.5%）等特征。前人认为其形成主要是在俯冲起始阶段大洋板块所释放的流体导致亏损程度较高的难熔地幔楔发生熔融，因此其成因的研究对深入理解板块俯冲起始等地球动力学问题具有重要意义。虽然普遍认为俯冲物质对玻安岩岩浆源区具有重要贡献，但玻安岩中元素的不同富集程度反映了复杂的俯冲板片流体物理化学性质和对玻安岩形成的不同影响。通过对比分析伊豆–小笠原（Izu-Bonin）和北祁连造山带大岔大坂地区玻安岩样品，发现二者具有明显的地球化学差异：与伊豆–小笠原玻安岩相比，大岔大坂玻安岩中没有呈现“U”型稀土配分模式，不富集轻稀土元素或 Zr、Hf 等元素；而二者流体活动性/不相容元素比值（如Ba/La）变化较大，并具有较高的 (87Sr/86Sr)i。这些特征反映了俯冲板片释放的流体和熔体分别对大岔大坂和伊豆–小笠原玻安岩岩浆地幔源区的贡献，从而表明大岔大坂玻安岩形成过程与伊豆–小笠原玻安岩所代表的俯冲初始形成模型不同，更可能形成于存在弧后扩张作用的成熟岛弧阶段。结合区域地质背景和前人研究，本文针对大岔大坂玻安岩成因提出了两种与俯冲初始阶段无关的可能形成机制：① 玻安岩产出于弧后扩张中心，弧后岩石圈的拉张环境和较热的地幔上隆区为玻安质岩浆的 形成提供了温压条件，充分交代的水化地幔楔和蛇纹岩化地幔也参与了玻安质岩浆的形成；② 虽与弧后扩张中心相关，但玻安岩的产出位于前弧或弧。由于弧后地幔对弧下深度地幔楔进行侧向加热，导致地幔楔内部对流重新启动，弧后地区已经熔融出弧后玄武岩的残余橄榄岩进入前弧–弧下地幔楔，地幔楔底部和俯冲板片表面被重新加热而发生变质脱水，富水流体交代上部地幔楔使其部分熔融形成玻安质岩浆。Boninite is a type of rock with distinctive geochemical traits, featuring high SiO₂ (>52 wt%), high MgO (>8 wt%), and low TiO₂ (<0.5 wt%). Prior researchers proposed that its formation primarily stems from melting of highly depleted refractory mantle wedge triggered by fluids released from the oceanic plate during the initial subduction stage; therefore, studies on boninite genesis are of great significance for in-depth comprehension of geodynamic issues such as the initiation of plate subduction. Although subducted materials are widely recognized to make substantial contributions to the magma source regions of boninites, the variable enrichment degrees of elements in boninites reflect the complex physico-chemical properties of subducted slab fluids and their distinct influences on boninite formation. Through comparative analysis of boninite samples from the Izu-Bonin and Dachadaban areas of the North Qilian Orogenic Belt, notable geochemical discrepancies were observed: compared with Izu-Bonin boninites, Dachadaban boninites do not display a U-shaped rare earth element (REE) partition pattern, and are not enriched in light rare earth elements (LREEs) or elements such as Zr and Hf. Conversely, their fluid-mobile/incompatible element ratios (e.g., Ba/La) vary significantly, and exhibit higher initial (87Sr/86Sr) ratios. These features reflect the respective contributions of fluids and melts released from the subducted slab to the magmatic mantle source regions of Dachadaban and Izu-Bonin boninites, indicating that the formation process of Dachadaban boninites differs from the initial subduction-related formation model represented by Izu-Bonin boninites, and they are more likely to have formed during the mature island arc stage with back-arc spreading. Combined with regional geological settings and previous research, this paper puts forward two possible formation mechanisms for Dachadaban boninites that are unrelated to the initial subduction stage: 1. Boninites form at back-arc spreading centers. The extensional environment of the back-arc lithosphere and the relatively hot mantle upwelling zone provide the necessary temperature and pressure conditions for the generation of boninitic magmas, and fully metasomatized and hydrated mantle wedges as well as serpentinized mantles also participate in the formation of boninitic magmas. 2. Although associated with back-arc spreading centers, boninites occur in the forearc or arc. Owing to lateral heating of the subarc mantle wedge by the back-arc mantle, convection within the mantle wedge restarts, and residual peridotite remaining after back-arc basalt formation via partial melting in the back-arc region enters the forearc-subarc mantle wedge. The bottom of the mantle wedge and the surface of the subducted slab are reheated and undergo metamorphic dehydration, and water-rich fluids metasomatize the upper mantle wedge, triggering partial melting to form boninitic magmas.