Redox processes in subducting oceanic crust recorded by sulfide-bearing high-pressure rocks and veins (SW Tianshan, China)
Redox processes in subducting oceanic crust recorded by sulfide-bearing high-pressure rocks and veins (SW Tianshan, China)
复制标题
含硫化物高压岩石和矿脉记录的俯冲洋壳氧化还原过程（中国西南天山）
DOI：
10.1007/s00410-016-1284-2
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发表时间：
2016-09-01
影响因子：
3.5
通讯作者：
Wang, Xin-Shui
中科院分区：
文献类型：
--
作者：
Li, Ji-Lei;Gao, Jun;Wang, Xin-Shui
弧下地幔和弧岩浆的氧化性质通常被解释为俯冲相关的氧化流体或熔体从下降的板块向地幔楔体迁移的结果。这一点尤其重要，因为次弧岩浆的氧化状态似乎在弧相关矿床的形成中起着关键作用。然而，对俯冲洋壳氧化还原状态的直接约束很少。本文对中国西北部阿克亚孜高压（HP）地块中含硫化物和氧化物榴辉岩、蓝片岩、云母片岩、榴辉岩相和逆行脉进行了详细的岩石学研究，以期对俯冲洋板中记录的氧化还原过程有更深入的了解。岩石中硫化物以黄铁矿为主，少量为磁黄铁矿、黄铜矿、斑铜矿、辉钼矿、闪锌矿和辉铜矿，并有斑铜矿与黄铜矿共生的溶出结构。磁铁矿、钛铁矿和黄铁矿以包裹体形式出现在石榴石中，而硫化物在基体中占主导地位。基质中较大的黄铁矿颗粒含有石榴石、辉长石等HP指数矿物的包裹体。然而，在逆行样品中，通常观察到磁铁矿取代黄铁矿结构。榴辉岩相和逆行脉表现出两个流体事件，分别表现为早期含硫化物和晚期含磁铁矿矿物组合。结构和岩石学证据表明，硫化物主要形成于HP变质期。矿物组合转变表明，随着深度的增加，俯冲洋壳的相对氧逸度略有下降。然而，根据氧质量平衡计算，基于氧摩尔量（nO(2)），在HP变质过程中氧化还原条件保持不变。在浅层(
The oxidized nature of the sub-arc mantle and hence arc magmas is generally interpreted as a result of the migration of subduction-related oxidizing fluids or melts from the descending slab into the mantle wedge. This is of particular importance seeing that the oxidization state of sub-arc magmas seems to play a key role in the formations of arc-related ore deposits. However, direct constraints on the redox state of subducted oceanic crust are sparse. Here, we provide a detailed petrological investigation on sulfide-and oxide-bearing eclogites, blueschists, micaschists, eclogite-facies and retrograde veins from the Akeyazi high-pressure (HP) terrane (NW China) in order to gain insight into the redox processes recorded in a subducting oceanic slab. Sulfides in these rocks are mainly pyrite and minor pyrrhotite, chalcopyrite, bornite, molybdenite, sphalerite and chalcocite, including exsolution textures of bornite-chalcopyrite intergrowth. Magnetite, ilmenite and pyrite occur as inclusions in garnet, whereas sulfides are dominant in the matrix. Large pyrite grains in the matrix contain inclusions of garnet, omphacite and other HP index minerals. However, magnetite replacing pyrite textures are commonly observed in the retrograded samples. The eclogite-facies and retrograde veins display two fluid events, which are characterized by an early sulfide-bearing and a later magnetite-bearing mineral assemblage, respectively. Textural and petrological evidences show that the sulfides were mainly formed during HP metamorphism. Mineral assemblage transitions reveal that the relative oxygen fugacity of subducted oceanic crust decreases slightly with increasing depths. However, according to oxygen mass balance calculations, based on the oxygen molar quantities (nO(2)), the redox conditions remain constant during HP metamorphism. At shallow levels (