祁连与伊豆-小笠原玻安岩的地球化学特征和成因模型对比

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。

Comparison in geochemical characteristics and genesis models of different boninites between Qilian Orogen and Izu-Bonin arc system

Boninites are characterized by high-Si (＞52 wt.%), high-Mg (＞8 wt.%), and low-Ti (＜0.5 wt.%). Boninite is thought to be originated from the partial melting of the refractory mantle induced by fluids released from the subducting seafloor during the subduction initiation. Therefore, study on petrogenesis of boninite is of great significance to further understand the geodynamic mechanism of subduction initiation. Although contribution of subducted materials for the boninitic magma source is significant as people commonly believed, the varying enriched extents of incompatible elements among different boninites reflect the complex physicochemical properties of subducting slab-derived fluids for the formation of boninites. In this study, we compared boninite from the Izu-Bonin-Mariana (IBM) arc system and the North Qilian orogenic belt, and found many geochemical differences between them. Compared to Izu-Bonin boninites, boninite from the North Qilian orogeny does not show U-shaped rare earth element (REE) pattern or enriched light REEs and Zr-Hf, while both of them have highly varied ratios of fluid-soluble element to incompatible element (e.g. Ba/La) and high (87Sr/86Sr)i values. These characteristics reflect the contribution of slab-derived fluids / melts to the magma source for Qilian/Izu-Bonin boninite, respectively. Different from the Izu-Bonin boninite, the Qilian boninite is likely to be produced in a mature subduction system with back-arc spreading centers. Combined with previous studies, we proposed two potential ways for the formation of the Qilian boninite unrelated to the seafloor subduction initiation: (1) the back-arc lithosphere extension and the hot mantle upwelling provided a suitable temperature and pressure for the formation of boninite magmas, with the contribution of the hydrated/serpentinized mantle; (2) the corner flow carried the residual peridotite of the back-arc mantle into the sub-arc/fore-arc mantle, which can be melted and possibly induce dehydration of the subducting slab again to produce boninite magmas.