Evolution of serpentinite from seafloor hydration to subduction zone metamorphism: Petrology and geochemistry of serpentinite from the ultrahigh pressure North Qaidam orogen in northern Tibet
Evolution of serpentinite from seafloor hydration to subduction zone metamorphism: Petrology and geochemistry of serpentinite from the ultrahigh pressure North Qaidam orogen in northern Tibet
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蛇纹岩从海底水化到俯冲带变质作用的演化：藏北柴北造山带超高压蛇纹岩的岩石学和地球化学
DOI：
10.1016/j.lithos.2019.105158
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发表时间：
2019-11
期刊：
影响因子：
3.5
通讯作者：
Chen Ren-Xu
中科院分区：
文献类型：
--
作者：
Zhang Long;Sun Wei-dong;Chen Ren-Xu
通过对藏北柴达木北超高压变质带蛇纹岩的研究，揭示了蛇纹岩从海底水化到俯冲带变质的岩石地球化学演化过程。柴达木北部蛇纹岩可分为俯冲过程中未发生严重叠加的未变形蛇纹岩和俯冲过程中重结晶较好的变形安山岩蛇纹岩。岩石学和地球化学分析表明，蛇纹岩原为熔融渗流难熔深海方辉橄榄岩。结合当地的地球动力学背景，推测蛇纹岩可能起源于大陆俯冲前俯冲的大洋岩石圈。蛇纹岩相对均匀的δ18O(4.0‰-4.5‰)表明原岩受海水热液蚀变。相比之下，蜥蜴蛇纹岩的δ18O(0.6‰-2.7‰)低得多的原因是与地球表面的大气降水交换。残留的蛇纹岩结构在蛇纹岩中保存良好，橄榄石与蜥蜴、磁铁矿发生反应，辉石与滑石、透闪石、利兹石发生反应，尖晶石与铬铁矿、绿泥石发生反应。在俯冲过程中，蛇纹岩对蛇纹岩的交代作用主要是沿晶界和相互连通的细脉方向开始的，暗示了蛇纹岩在流体作用下向蛇纹岩的转化。在蛇纹岩中，部分分解生成镁质次生橄榄石，而在蛇纹岩中直接分解亚稳橄榄石，生成铁质次生橄榄石。蛇纹岩富含可流动元素，铀主要在海底蚀变过程中积累，碱性元素主要由弯曲断裂或增生楔形沉积流体引入。与沉积物平衡的流体的渗入也受到蛇纹岩高抬升的87Sr/86Sr的支持。此外，蛇纹岩的87Sr/86Sr(0.710649-0.713996)高于蛇纹岩(0.707184-0.708502)，表明前者与沉积流体的相互作用比后者更强烈。而蛇纹岩的碱度高于蛇纹岩，而铀的含量低于蛇纹岩，说明蛇纹岩在部分脱水过程中大量的碱质流失，而铀没有释放。
Serpentinite from the North Qaidam ultrahigh pressure metamorphic belt in northern Tibet is studied to provide insight into petro-geochemical evolution of serpentinite from seafloor hydration to subduction zone metamorphism. The North Qaidam serpentinite can be divided into undeformed lizardite serpentinite that was not severely overprinted during subduction and deformed antigorite serpentinite that was well recrystallized during subduction. Petrological and geochemical analyses demonstrate that the serpentinite was originally melt-percolated refractory abyssal harzburgite. Considering the local geodynamic setting, it is inferred that the serpentinite probably originated from the oceanic lithosphere that subducted before continental subduction. Relatively uniform low δ18O (4.0‰–4.5‰) of the antigorite serpentinite indicates high temperature hydrothermal alteration of protolith harzburgite by seawater. In contrast, much lower δ18O (0.6‰–2.7‰) of the lizardite serpentinite is ascribed to exchange with meteoric water at the Earth’s surface. Relict serpentinization textures are well preserved in the lizardite serpentinite, with reactions of olivine to lizardite and magnetite, pyroxenes to talc, tremolite, and lizardite, and spinel to chromite and chlorite. The replacement of lizardite by antigorite during subduction mostly initiates along grain boundaries and interconnecting veinlets, implying fluid-assisted transformation of lizardite into antigorite. Partial decomposition of antigorite produces magnesian secondary olivine in the antigorite serpentinite, while direct breakdown of metastable lizardite generates ferroan secondary olivine in the lizardite serpentinite. The serpentinite is enriched in fluid-mobile elements, with U primarily accumulated during seafloor alteration and alkalis notably introduced by sedimentary fluids at bending faults or in accretionary wedge. Infiltration by fluids equilibrated with sediments is also supported by highly elevated87Sr/86Sr of the serpentinite. Moreover,87Sr/86Sr of the antigorite serpentinite (0.710649–0.713996) is higher than that of the lizardite serpentinite (0.707184–0.708502), which implies more intense interaction of sedimentary fluids with the former than the latter. However, the lizardite serpentinite contains more alkalis and less U than the antigorite serpentinite, which indicates that large proportions of alkalis were lost during partial dehydration of serpentinite, while U was not released.
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