Unusual exsolution phenomena in omphacite and partial replacement of phengite by phlogopite + kyanite in an eclogite from the Erzgebirge
Unusual exsolution phenomena in omphacite and partial replacement of phengite by phlogopite + kyanite in an eclogite from the Erzgebirge
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DOI：
10.1007/s004100000161
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发表时间：
2000
影响因子：
3.5
通讯作者：
E. Schmädicke;W. Müller
中科院分区：
文献类型：
--
作者：
E. Schmädicke;W. Müller
在Erzgebirge晶体复合体中，榴辉岩出现在三个不同的高压（HP）单元（1，2和3）中，记录了对比的压力（P）-温度（T）条件。高压1号机组的榴辉岩在柯石英稳定性场中经历了约33 kbar/850 °C的峰值变质条件。通常，这些榴辉岩从HP-单元1都非常相似，与榴辉岩峰组合绿辉石-石榴石-柯石英-钾长石，很少伴有蓝晶石，绿辉石系统偏离化学计量组成。相比之下，最近在Blumenau附近发现的榴辉岩在矿物学和地球化学上与HP单元1的典型榴辉岩不同。这种不寻常的榴辉岩揭示了榴辉岩平衡组合绿辉石-石榴石-柯石英-多硅白云母-金云母-蓝晶石，并产生870 °C和>29 kbar的变质峰条件。在多硅白云母和石榴石的部分消耗下，金云母和蓝晶石的形成有明确的结构证据。绿辉石具有化学计量比，并含有丰富的出溶纹层，其中最厚的经电子探针鉴定为石英。用透射电子显微镜（TEM）研究了更细的层。Oligoclase被确定为出溶相。其他的薄层被证明是由K-白云母组成，也被解释为出溶。出溶之前，绿辉石组合物必须是阳离子缺乏，作为其他常见的HP-1单元榴辉岩。这些非化学计量的组合物被归因于部分取代的Ca-Eskola辉石组分，计算为平均8摩尔%的绿辉石在HP-单元1榴辉岩。根据实验，这种替代在P > 30 kbar时变得显著。K-白云母的出溶可能表明原始绿辉石中的羟基缺陷，也有利于高压。富钙-埃斯科拉单斜辉石中的斜长石和钾白云母出溶体未见报道。绿辉石具有无序的C2/c结构;在一种情况下，存在由C2/c到P2/n转变产生的非常小（几十纳米）的反相域。这些特征可能指示了一个短暂的热历史和快速的构造过程。
In the Erzgebirge Crystalline Complex, eclogites occur in three different high pressure (HP) units (1, 2 and 3) recording contrasting pressure (P)–temperature (T) conditions. Eclogites from HP-unit 1 experienced peak metamorphic conditions in the coesite stability field at about 33 kbar/850 °C. Commonly, these eclogites from HP-unit 1 are all very similar, with an eclogitic peak assemblage of omphacite–garnet–coesite–K-feldspar, rarely accompanied by kyanite, and omphacites systematically deviating from a stoichiometric composition. In contrast, an eclogite recently found near Blumenau, is mineralogically and geochemically different from the typical eclogites of HP-unit 1. This unusual eclogite reveals the eclogitic equilibrium assemblage omphacite–garnet–coesite–phengite–phlogopite–kyanite, and yields metamorphic peak conditions of 870 °C and >29 kbar. There is clear textural evidence of the formation of phlogopite and kyanite under partial consumption of phengite and garnet. Moreover, the omphacite is stoichiometric and contains abundant exsolution lamellae, the thickest of which were identified as quartz by the electron microprobe. The finer lamellae were studied by transmission electron microscopy (TEM). Oligoclase was identified as an exsolution phase. Other lamellae proved to consist of K-white mica, also interpreted as exsolution. Prior to exsolution, the omphacite composition must have been cation-deficient, as that of the other, common HP-unit 1 eclogites. These non-stoichiometric compositions are ascribed to partial substitution by the Ca-Eskola pyroxene component, which calculates to an average of 8 mol% for omphacite in HP-unit 1 eclogites. According to experiments, this substitution becomes significant at P > 30 kbar. Exsolution of K-white mica may indicate hydroxyl defects in the original omphacite, also favoured by high pressure. Oligoclase and K-white mica exsolution from Ca-Eskola-rich clinopyroxene has not previously been reported. The omphacite has a disordered C2/c structure; and in just one case very small (a few tens of nanometres) antiphase domains, resulting from the C2/c to P2/n transformation, are present. These features may indicate a brief thermal history and rapid tectonic processes.