Reaction rind formation in the Catalina Schist: Deciphering a history of mechanical mixing and metasomatic alteration
Reaction rind formation in the Catalina Schist: Deciphering a history of mechanical mixing and metasomatic alteration
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卡塔利娜片岩中的反应壳形成：破译机械混合和交代蚀变的历史
DOI：
10.1016/j.chemgeo.2014.06.024
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发表时间：
2014
期刊：
影响因子：
3.9
通讯作者：
R. J. Walker
中科院分区：
文献类型：
--
作者：
S. Penniston‐Dorland;J. Gorman;G. Bebout;P. Piccoli;R. J. Walker
在与俯冲相关的卡特琳娜片岩中，反应区或“外皮”在变质镁铁质块体与化学上不同的混杂岩基质的接触处发育。这些外皮具有复杂的化学成分，无法用简单的单一过程解释。高品位和低品位反应果皮的比较，包括现场观察、大块岩石地球化学、薄片结构观察、矿物成分分析和热力学模型，为果皮形成过程的性质和时间提供了约束。块体岩石中高亲铁元素（HSE，包括 Os、Ir、Ru、Pt、Pd 和 Re）以及 MgO、Cr 和 Ni 的变化提供了对富含这些元素的混杂岩基质的超镁铁质材料与来自块体的镁铁质材料混合的物理过程的深入了解。在低品位和高品位反应层中都发现了这些元素浓度的富集，这表明富集这些元素并产生反应层的过程发生在俯冲带的各种 P-T 条件下。与相关反应外皮中的石榴石相比，角闪岩级变铁岩块中石榴石的化学成分存在差异，这表明至少有一些混合发生在石榴石块中的生长之前，可能是在与产生石榴石的温度条件不同的情况下进行的。在所有反应层中都观察到通常被认为是流体流动元素（例如 K2O、Ba 和 Rb）的块体岩石变化。在高品位中，K2O、Ba 和 Rb 的富集与多硅白云石和绿泥石取代石榴石和角闪石有关。由于多硅白云石是 K、Ba 和 Rb 的主要宿主，因此通过流体添加这些元素似乎是石榴石后生长现象。这项研究强调了混杂岩带交代作用和机械混合过程对影响板片-地幔界面地球化学演化的重要性。从俯冲板片发出并进入地幔楔和弧熔岩源区的任何“流体”（含水或硅酸盐熔体）必然反映这些过程产生的混合成分。
In the subduction-related Catalina Schist, reaction zones or ‘rinds’ developed at contacts of metamorphosed mafic blocks with chemically distinct mélange matrix. These rinds exhibit complex chemical compositions that defy simple, single-process explanations. A comparison of high-grade and low-grade reaction rinds, including field observations, bulk-rock geochemistry, thin section textural observations, mineral compositional analyses, and thermodynamic modeling provides constraints on the nature and timing of the rind-forming processes. Bulk-rock variations in the highly siderophile elements (HSE, including Os, Ir, Ru, Pt, Pd and Re) and MgO, Cr, and Ni provide insight into the physical processes of mixing of ultramafic material of the mélange matrix, rich in these elements, with mafic material derived from the blocks. Enrichments in the concentrations of these elements are found in both low-grade and high-grade reaction rinds suggesting that the processes that enrich these elements and create the reaction rinds occur over a wide range ofP–Tconditions in subduction zones. Differences in the chemical composition of garnets in an amphibolite-grade metamafic block, compared with garnets in the associated reaction rind, suggest that at least some of the mixing occurred before the growth of garnet in the blocks, perhaps atP–Tconditions different from those producing the garnet. Bulk-rock variations in elements commonly considered fluid-mobile, such as K2O, Ba and Rb, are observed in all reaction rinds. At high-grade, enrichments of K2O, Ba and Rb are associated with replacement of garnet and amphibole by phengite and chlorite. Because phengite is the major host for K, Ba and Rb, the addition of these elements by fluids appears to have been a post-garnet growth phenomenon. This study highlights the significance of processes of metasomatism and mechanical mixing in mélange zones for influencing the geochemical evolution of the slab–mantle interface. Any “fluids” (hydrous or silicate melts) emanating from subducting slabs and entering the mantle wedge and arc lava source regions would necessarily reflect the hybrid compositions created by these processes.