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首页> 外文期刊>Contributions to Mineralogy and Petrology >The composition of nanogranitoids in migmatites overlying the Ronda peridotites (Betic Cordillera, S Spain): the anatectic history of a polymetamorphic basement
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The composition of nanogranitoids in migmatites overlying the Ronda peridotites (Betic Cordillera, S Spain): the anatectic history of a polymetamorphic basement

机译：隆达橄榄岩上的蒙脱石中的纳米粒类固醇的组成（西班牙贝蒂科迪勒拉）：多变质地下室的麻醉史

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The study of the composition of primary melts during anatexis of high-pressure granulitic migmatites is relevant to understand the generation and differentiation of continental crust. Peritectic minerals in migmatites can trap droplets of melt that forms via incongruent melting reactions during crustal anatexis. These melt inclusions commonly crystallize and form nanogranitoids upon slow cooling of the anatectic terrane. To obtain the primary compositions of crustal melts recorded in these nanogranitoids, including volatile concentrations and information on fluid regimes, they must be remelted and rehomogenized before analysis. A new occurrence of nanogranitoids was recently reported in garnets of mylonitic metapelitic gneisses (former high pressure granulitic migmatites) at the bottom of the prograde metamorphic sequence of Jubrique, located on top of the Ronda peridotite slab (Betic Cordillera, S Spain). Nanogranitoids within separated chips of cores and rims of large garnets from these migmatites were remelted at 15 kbar and 850, 825 or 800 degrees C and dry (without added H2O), during 24 h, using a piston cylinder apparatus. Although all experiments show glass (former melt) within melt inclusions, the extent of rehomogenization depends on the experimental temperature. Experiments at 850-825 degrees C show abundant disequilibrium microstructures, whereas those at 800 degrees C show a relatively high proportion of rehomogenized nanogranitoids, indicating that anatexis and entrapment of melt inclusions in these rocks likely occurred at pressures <= 1.5 GPa and temperatures close to 800 degrees C. Electron microprobe and NanoSIMS analyses show that experimental glasses are leucogranitoid and peraluminous, though define two distinct compositional groups. Type I melt inclusions correspond to K-rich, Ca- and H2O-poor leucogranitic melts, whereas type II melt inclusions represent K-poor, Ca- and H2O-rich granodioritic to tonalitic melts. Type I and II melt inclusions are found in most cases at the cores and rims of large garnets porphyroclasts, respectively. We tentatively interpret these two distinct melt compositions as suggesting that these former migmatites underwent two melting events under contrasting fluid regimes, possibly during two different orogenic periods. This study demonstrates the strong potential of melt inclusions studies in migmatites and granulites in order to unravel their anatectic history, particularly in strongly deformed rocks where most of the classical anatectic microstructures and macrostructures have been erased during deformation.
机译：研究高压花岗辉石辉腾岩初生熔体的组成对了解大陆地壳的形成和分化具有重要意义。蒙脱石中的包晶矿物可以捕集在地壳麻醉过程中通过不一致的熔化反应形成的熔滴。这些熔融夹杂物通常在缓慢冷却地层时会结晶并形成纳米颗粒。为了获得记录在这些纳米颗粒物中的地壳熔体的主要成分，包括挥发物浓度和有关流体状态的信息，必须在分析之前将其重熔并重新均化。最近有报道称，在隆布橄榄岩板块顶部（贝蒂科迪勒拉，西班牙），朱布利格渐进变质层序的底部，出现了一种新的纳米类固醇类物质，其分布在长石质变质层序的底部的长石质变质片麻岩的石榴石中（以前的高压粒状米格氏岩）。在24小时内，使用活塞式汽缸装置，在15 kbar和850、825或800摄氏度的温度下，将这些石榴石的大石榴石芯和轮缘的分离碎片中的纳米类固醇重熔，并干燥（不添加H2O）。尽管所有实验均显示玻璃（前熔体）在熔体夹杂物中，但均质化程度取决于实验温度。在850-825摄氏度的温度下进行的实验显示出丰富的不平衡微观结构，而在800摄氏度的温度下进行的实验则显示出相对较高比例的均质纳米粒状质，这表明这些岩石中的熔融物夹杂物和夹杂物的发生可能是在压力<= 1.5 GPa和温度接近800摄氏度。电子探针和NanoSIMS分析表明，实验玻璃虽然定义了两个不同的组成组，但它们都是类镜质的和全铝质的。 I型熔体夹杂物对应于富钾，Ca和H2O贫乏的白云母熔体，而II型熔体夹杂物代表贫钾，富含Ca和H2O的粒二向熔体对tonalitic熔体。在大多数情况下，分别在大型石榴石卟啉石的核心和边缘发现I型和II型熔融夹杂物。我们试探性地将这两种不同的熔体成分解释为，暗示这些前辉石在不同的流体状态下经历了两次熔融事件，可能在两个不同的造山期。这项研究表明，在蒙脱石和花岗石中进行熔融包裹体研究具有很强的潜力，以揭示其滑石的历史，特别是在强烈变形的岩石中，在变形过程中大部分经典的滑石微结构和宏观结构都被消除了。

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