Fluid–rock interaction and evolution of a high-pressure/low-temperature vein system in eclogite from New Caledonia: insights into intraslab fluid flow processes
Fluid–rock interaction and evolution of a high-pressure/low-temperature vein system in eclogite from New Caledonia: insights into intraslab fluid flow processes
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DOI：
10.1007/s00410-016-1295-z
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发表时间：
2016-10
影响因子：
3.5
通讯作者：
S. Taetz;T. John;M. Bröcker;C. Spandler
中科院分区：
文献类型：
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作者：
S. Taetz;T. John;M. Bröcker;C. Spandler
一个复杂的高压/低温脉系统，横切Pouebo Ecloetry Melange（北方新喀里多尼亚）的榴辉岩寄主岩石，记录了蓝片岩到榴辉岩的过渡和石榴石-石英-多硅白云母脉的相关形成。地质温压法（Grt-Cpx-Ph，金红石中的Zr）和假截面计算表明，约为峰值变质条件。540 °C和1.9- 2.2GPa。岩石学和地球化学观测以及假断面建模表明，主脉网络是由脱水过程形成的，该过程收集了内部衍生的流体，这些流体与潜进变质作用期间含水相（角闪石、闪石、绿帘石）的分解有关。新形成相的较低固体体积和孔隙流体压力的相关增加导致脉的形成，从而允许这些流体的积聚和通道化排空。由于流体-岩石系统一直处于化学平衡状态，因此这些矿脉不显示交代蚀变边缘。第二个静脉类型（运输静脉）记录的叠加流入的外部流体略有不同的组成，最有可能是有关类似的脱水反应，在其他部分的俯冲板。由于源岩的成分差异，这些流体与渗透的岩石体积不平衡，并通过溶解-沉淀过程诱导形成不同的交代边缘。质量平衡计算表明，Ca，Na和Li通过外部流体添加到织边。LILE和HREE在较小程度上也被移动并从织边中移除。轻稀土元素主要被新形成的矿物（如绿帘石）所缓冲。岩石学证据表明，所研究的矿脉系统形成时，样品仍然是一个连贯的俯冲板片的一部分。Rb-Sr年代学研究表明，该事件发生在38.2 ± 0.3Ma。这个年龄是ca。600万年前比迄今推测的变质年龄高峰，这表明一个更复杂和持久的变质历史的PEM。
A complex high-pressure/low-temperature vein system that cross-cuts eclogitic host rocks of the Pouébo Eclogite Melange (northern New Caledonia) records the prograde blueschist-to-eclogite transition and associated formation of garnet–quartz–phengite veins. Geothermobarometry (Grt–Cpx–Ph, Zr-in-rutile) and pseudosection calculations indicate peak metamorphic conditions of ca. 540 °C and 1.9–2.2 GPa. Petrological and geochemical observations as well as pseudosection modelling suggest that the main vein network is formed by dehydration processes that collected internally derived fluids related to the breakdown of hydrous phases (amphibole, chlorite, epidote) during prograde metamorphism. The lower solid volume of the newly formed phases and the associated increase in pore fluid pressure lead to the formation of veins that allowed for accumulation and channelized evacuation of these fluids. Such veins do not show metasomatic alteration selvages because the fluid–rock system had been in chemical equilibrium. A second vein type (transport veins) records the superimposed influx of external fluids with slightly different composition that most likely are related to similar dehydration reactions in other parts of the subducting slab. Due to the source-rock-imposed compositional differences, these fluids are not in equilibrium with the infiltrated rock volume and induce the formation of distinct metasomatic selvages by dissolution–precipitation processes. Mass-balance calculations show that Ca, Na and Li are added to the selvage by the external fluid. LILE and to a lesser extend also HREE are mobilized and removed from the selvage. The LREE are predominantly buffered by newly formed minerals (e.g. epidote). Petrological evidence implies that the studied vein system formed while the sample was still part of a coherent subducting slab. Rb–Sr geochronology indicates that this occurred at 38.2 ± 0.3 Ma. This age is ca. 6 myr younger than the hitherto presumed peak metamorphic age, suggesting a more complex and protracted metamorphic history for the PEM.