Fluid Migration above a Subducted Slab-Constraints on Amount, Pathways and Major Element Mobility from Partially Overprinted Eclogite-facies Rocks (Sesia Zone, Western Alps)
Fluid Migration above a Subducted Slab-Constraints on Amount, Pathways and Major Element Mobility from Partially Overprinted Eclogite-facies Rocks (Sesia Zone, Western Alps)
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DOI：
10.1093/petrology/egq087
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发表时间：
2011-03-01
影响因子：
3.9
通讯作者：
Zack, Thomas
中科院分区：
文献类型：
--
作者：
Konrad-Schmolke, Matthias;O'Brien, Patrick J.;Zack, Thomas
西阿尔卑斯塞西亚-兰佐带（SLZ）是从俯冲洋板上盘地幔深处挖出的榴辉岩相陆壳。榴辉岩相长英质岩和基性岩在SLZ内部取样，显示出不同程度的与流体流入相关的退变质叠加。弱变形的样品保存残余榴辉岩相矿物组合，显示部分流体诱导成分重新平衡沿着晶界，脆性断裂和其他流体通道。多个流体流入阶段的指示，由多硅白云母，钠长石斜长石和绿帘石，以及部分再平衡和/或过度生长的多硅白云母和钠质角闪石的替代原生绿辉石，产生特征的阶梯状的成分分带模式。所观察到的纹理，连同地图规模的样品分布，建议开放系统，普遍和反应性流体通量在大岩石体积以上的俯冲板。热力学建模表明，与围岩相互作用的流体的最小量为0中心点1-0中心点5wt%。相关系和反应织构表明K，Ca，Fe和Mg的流动性，而Al在这些中温高压流体中相对不动。此外，热力学模型表明，回收以前分馏的材料，如在核心的石榴石斑晶，在很大程度上控制了成分的再平衡的折返岩体。
The Western Alpine Sesia-Lanzo Zone (SLZ) is a sliver of eclogite-facies continental crust exhumed from mantle depths in the hanging wall of a subducted oceanic slab. Eclogite-facies felsic and basic rocks sampled across the internal SLZ show different degrees of retrograde metamorphic overprint associated with fluid influx. The weakly deformed samples preserve relict eclogite-facies mineral assemblages that show partial fluid-induced compositional re-equilibration along grain boundaries, brittle fractures and other fluid pathways. Multiple fluid influx stages are indicated by replacement of primary omphacite by phengite, albitic plagioclase and epidote as well as partial re-equilibration and/or overgrowths in phengite and sodic amphibole, producing characteristic step-like compositional zoning patterns. The observed textures, together with the map-scale distribution of the samples, suggest open-system, pervasive and reactive fluid flux across large rock volumes above the subducted slab. Thermodynamic modelling indicates a minimum amount of fluid of 0 center dot 1-0 center dot 5 wt % interacting with the wall-rocks. Phase relations and reaction textures indicate mobility of K, Ca, Fe and Mg, whereas Al is relatively immobile in these medium-temperature-high-pressure fluids. Furthermore, the thermodynamic models show that recycling of previously fractionated material, such as in the cores of garnet porphyroblasts, largely controls the compositional re-equilibration of the exhumed rock body.