Abstract Olivine (Ol) and orthopyroxene (Opx) are the primary liquidus phases of boninite in modern subduction settings and in many ophiolite complexes. It is thus straightforward to expect the formation of harzburgite cumulate resulting from boninite magma evolution. However, such magmatic harzburgite has been rarely studied. Here, we report the results of our study on such harzburgite from the Troodos ophiolite complex. The Troodos cumulate harzburgite (locally lherzolite) is characteristically interlayered with dunite, showing varying thickness on millimeter to decimeter scales, as the result of volumetrically varying multiple pulses of melt injection into the evolving magma chamber. We illustrate the development of the interlayered cumulate by phase equilibrium analysis. The parental melt of each pulse begins to crystallize olivine to form a dunite layer before reaching the Ol-Opx cotectic, along which Ol and Opx coprecipitate to form a harzburgite layer. Periodical replenishment will result in dunite-harzburgite interlayered cumulate. In cases when replenishment may be delayed, the melt along the Ol-Opx cotectic can evolve to the Ol-Opx-clinopyroxene (Cpx) eutectic to form harzburgite with some Cpx or lherzolite. The calculated melts in equilibrium with spinels in the cumulate are characteristic of boninite in major element compositional spaces. The calculated melts in equilibrium with Cpx and Opx in the cumulate share the same as, or identical to, trace element patterns of the Troodos boninite (both glasses and bulk-rock compositions). Petrological modeling of the boninite magma evolution shows a crystallization order of Ol, Opx, Cpx, plagioclase. Our study also emphasizes the importance in considering dunite-harzburgite/lherzolite cumulate when interpreting seismic structure of the crust in subduction settings, especially in rock sequences associated with subduction initiation thought to be indicated by boninite magmatism.

中文翻译：

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作为鲕岩岩浆演化结果的岩浆斜方辉石的起源——使用来自 Troodos 蛇绿岩复合体的层状斜方辉石-纯辉石堆积的说明

摘要 橄榄石 (Ol) 和斜方辉石 (Opx) 是现代俯冲环境和许多蛇绿岩复合体中钐岩的主要液相相。因此，可以直接预期由钆岩岩浆演化导致的菱镁矿堆积物的形成。然而，这种岩浆菱镁矿很少被研究。在这里，我们报告了我们对来自 Troodos 蛇绿岩复合体的这种方辉石的研究结果。Troodos cumulate harzburgite (locally lherzolite) 特征性地与纯长岩夹层，在毫米到分米尺度上显示不同的厚度，这是由于熔体注入演化中的岩浆房的多次脉冲体积变化的结果。我们通过相平衡分析说明了层间累积的发展。每个脉冲的母体熔体在到达 Ol-Opx 共晶之前开始使橄榄石结晶形成纯晶岩层，Ol 和 Opx 沿该晶界共沉淀形成菱镁矿层。定期补充将导致纯石英-菱镁矿夹层堆积。在补给可能延迟的情况下，沿着 Ol-Opx 共晶的熔体可以演变为 Ol-Opx-单斜辉石 (Cpx) 共晶，形成带有一些 Cpx 或锂辉石的方辉石。计算出的与尖晶石相平衡的熔体是主要元素组成空间中钐岩的特征。计算出的与累积中 Cpx 和 Opx 平衡的熔体与 Troodos boninite（玻璃和块状岩石成分）的微量元素模式相同或相同。Boninite 岩浆演化的岩石学模型显示了 Ol、Opx、Cpx、斜长石的结晶顺序。我们的研究还强调了在解释俯冲环境中的地壳地震结构时考虑单晶-方辉石/锂辉石堆积的重要性，特别是在与被认为是由博宁岩岩浆作用指示的俯冲起始相关的岩石序列中。