Situated along the Yanbu Suture Zone, the Al-Wask ophiolite is one of the largest and best-preserved ophiolite sequences in the Proterozoic Arabian shield. A mantle section of serpentinized ultramafics is structurally overlain by a crustal section of gabbros and pillow lavas. The whole ophiolite sequence is capped by pelagic sedimentary cover, and tectonically emplaced over a metamorphosed island-arc volcano-sedimentary succession. Situated along the Yanbu Suture Zone, the Al-Wask ophiolite is one of the largest and best-preserved ophiolite sequences in the Proterozoic Arabian shield. A mantle section of serpentinized ultramafics is structurally overlain by a crustal section of gabbros and pillow lavas. The whole ophiolite sequence is capped by pelagic sedimentary cover, and tectonically emplaced over a metamorphosed island-arc volcano-sedimentary succession. The Al-Wask ultramafic rocks are strongly deformed, metamorphosed, and altered by carbonatization and silicification. Samples dominated by antigorite indicate upper greenschist to lower amphibolite facies peak metamorphic grade, whereas samples dominated by lizardite and magnesite preserve lower grade conditions that we interpret as a cooling path buffered to low CO2 activity by the increasing stability of magnesite with decreasing temperature. Nearly all the primary silicate minerals have been replaced by serpentine minerals, leaving only relics of primary olivine and chromian spinel. Petrographic observation of relict olivine and spinel and of mesh and bastite textures in the serpentines suggest that the peridotite protoliths were mainly harzburgite with minor dunite. Whole-rock compositions of serpentinites show low CaO (<0.1 wt.%), Al2O3 (<1.5 wt.%), and Y (<0.4 ppm) combined with high Mg# (0.90–0.92), Ni, Co, and Cr contents; all these indicate a highly refractory mantle protolith. The mineral chemistry of relict primary spinel and olivine provides additional petrogenetic and geodynamic indicators. The high Cr# (> 60) and low TiO2 (≤0.2 wt. %) of spinel and high forsterite contents (90–92) of associated olivine indicate residual mantle that underwent extensive partial melt extraction. The whole-rock and mineral chemistry of the serpentinized ultramafic rocks are both consistent with extracted melt fractions from ∼32 to 38 percent. This extent of melting is typical of fore-arc supra-subduction zone settings, which is the most likely tectonic environment for formation and preservation of the Al-Wask ophiolite. Two types of magnesite deposits can be distinguished in the Al-Wask mantle section: an early generation of massive magnesite and a later generation of magnesite veins. Hence the Al-Wask ophiolite underwent multiple stages of carbonatization, likely involving different sources of CO2-bearing fluids. The massive magnesite likely formed at relatively high temperature during cooling from peak metamorphic condition from CO2-bearing fluid probably derived from decomposition of subducted carbonates. Using thermodynamic calculations in the simple MgO-SiO2-H2O-CO2 system, we constrain the path of the reaction boundary where lizardite and magnesite can coexist at equilibrium. On the other hand, the cryptocrystalline magnesite veins fill tectonic fractures and likely formed at low temperature and shallow levels, after serpentinization and ophiolite obduction.

中文翻译：

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沙特阿拉伯阿拉伯地盾 Al-Wask 蛇绿岩中蛇纹石化超镁铁质和伴生菱镁矿矿床的成因和地球动力学演化

Al-Wask蛇绿岩位于延布缝合带沿线，是元古代阿拉伯地盾中最大、保存最完好的蛇绿岩层序之一。蛇纹石化超镁铁质的地幔部分在结构上被辉长岩和枕状熔岩的地壳部分覆盖。整个蛇绿岩层序被远洋沉积盖层覆盖，并在构造上位于变质的岛弧火山沉积层序之上。Al-Wask蛇绿岩位于延布缝合带沿线，是元古代阿拉伯地盾中最大、保存最完好的蛇绿岩层序之一。蛇纹石化超镁铁质的地幔部分在结构上被辉长岩和枕状熔岩的地壳部分覆盖。整个蛇绿岩层序被远洋沉积盖覆盖，并在构造上位于一个变质的岛弧火山沉积序列上。Al-Wask 超基性岩通过碳化和硅化发生强烈变形、变质和蚀变。以叶蛇纹石为主的样品表明上部绿片岩到下部角闪岩相峰值变质品位，而以蜥蜴岩和菱镁矿为主的样品保留了较低品位条件，我们将其解释为通过菱镁矿随着温度降低而增加稳定性而缓冲到低 CO2 活动的冷却路径。几乎所有的原生硅酸盐矿物都被蛇纹石矿物取代，只留下原生橄榄石和铬尖晶石的遗迹。对残余橄榄石和尖晶石以及蛇纹岩中的网状和氟碳岩结构的岩相学观察表明橄榄岩原岩主要是菱镁矿和少量纯英岩。蛇纹岩的全岩成分显示出低 CaO (<0.1 wt.%)、Al2O3 (<1.5 wt.%) 和 Y (<0.4 ppm) 以及高 Mg# (0.90–0.92)、Ni、Co 和 Cr内容; 所有这些都表明地幔原岩具有高度的耐火性。残存的原生尖晶石和橄榄石的矿物化学提供了额外的岩石成因和地球动力学指标。尖晶石的高 Cr# (> 60) 和低 TiO2 (≤0.2 wt. %) 以及伴生橄榄石的高镁橄榄石含量 (90-92) 表明残余地幔经历了广泛的部分熔融提取。蛇纹石化超基性岩的全岩和矿物化学与提取的熔体分数从 32% 到 38% 一致。这种熔融程度是前弧超俯冲带环境的典型特征，这是最有可能形成和保存 Al-Wask 蛇绿岩的构造环境。在 Al-Wask 地幔剖面可以区分两种类型的菱镁矿矿床：早期的块状菱镁矿和后期的菱镁矿脉。因此，Al-Wask 蛇绿岩经历了多个碳酸化阶段，可能涉及不同来源的含 CO2 流体。块状菱镁矿很可能是在相对较高的温度下从含 CO2 流体的峰值变质状态冷却过程中形成的，这可能来自俯冲碳酸盐的分解。在简单的 MgO-SiO2-H2O-CO2 系统中使用热力学计算，我们限制了蜥蜴矿和菱镁矿可以在平衡状态下共存的反应边界路径。另一方面，隐晶菱镁矿脉充填构造裂缝，可能形成于低温浅层，经过蛇纹石化和蛇绿岩外展。