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首页> 外文期刊>Contributions to Mineralogy and Petrology >Intracrystalline microstructures in alkali feldspars from fluid-deficient felsic granulites: a mineral chemical and TEM study
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Intracrystalline microstructures in alkali feldspars from fluid-deficient felsic granulites: a mineral chemical and TEM study

机译：缺乏流体的长英质花岗岩中长石的晶内微观结构：矿物化学和TEM研究

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摘要

Samples of essentially "dry" high-pressure felsic granulites from the Bohemian Massif (Variscan belt of Central Europe) contain up to 2-mm-large perthitic alkali feldspars with several generations of plagioclase precipitates in an orthoclase-rich host. The first generation takes the form of lenses homogeneous in size, whereas the size of a second generation of very thin albite-rich precipitates is more variable with comparatively high aspect ratios. In the vicinity of large kyanite, garnet or quartz inclusions, the first generation of plagioclase precipitates is significantly less abundant, the microstructure is coarser than in the remainder of the perthitic grain and the host is a tweed orthoclase. The first generation of precipitates formed at around 850 °C during the high-pressure stage (16-18 kbar) of metamorphism. Primary exsolution was followed by primary coarsening of the plagioclase precipitates, which still took place at high temperatures (850-700 °C). The coarsening was pronounced due to the access of fluids in the outer portions of the perthitic alkali feldspar and in more internal regions around large inclusions. The second generation of albite-rich precipitates was formed at around 570 °C. TEM investigations revealed that the interfaces between the second-generation plagioclase lamellae and the orthoclase-rich host are coherent or semi-coherent. During late evolutionary stages of the perthite, albite linings were formed at phase boundaries, and the perthitic microstructure was partially replaced by irregularly shaped precipitates of pure albite with incoherent interfaces. The albitization occurred below 400 °C and was linked to fluid infiltration in the course of deuteric alteration. Based on size-distribution analysis, it is inferred that the precipitates of the first generation were most probably formed by spinodal decomposition, whereas the precipitates of the second generation rather were formed by nucleation and growth.
机译：来自波希米亚地块（中欧的瓦里斯卡纳地带）的基本“干燥”的高压长英质花岗石样品，在富含正长石酶的宿主中含有多达2毫米大的蠕虫状碱长石，并有数代斜长石沉淀。第一代采用大小均一的透镜形式，而第二代非常薄的富含钠长石的沉淀物的尺寸在纵横比较高的情况下变化更大。在大型蓝晶石，石榴石或石英夹杂物附近，第一代斜长石沉淀物含量明显不足，微结构比其他蠕虫状颗粒更粗，宿主为花呢正长石酶。在变质的高压阶段（16-18 kbar）期间，第一代沉淀物在850°C左右形成。初次溶解后，斜长石沉淀物初次粗化，仍然在高温（850-700°C）下发生。粗大化是由于蠕虫状碱长石的外部和大包裹体周围的更多内部区域中的流体进入所致。在570°C左右形成了第二代富含钠长石的沉淀物。 TEM研究表明，第二代斜长石薄片和富含直长石的宿主之间的界面是相干或半相干的。在白云母的晚期演化阶段，钠长石衬层形成在相界处，并且蠕虫的微观结构被具有不连贯界面的不规则形状的纯钠长石沉淀物取代。仲裁过程发生在低于400°C的温度下，并且在氘代变化过程中与液体渗透有关。根据尺寸分布分析，可以推断出第一代沉淀物很可能是通过旋节线分解形成的，而第二代沉淀物则是通过成核和生长而形成的。

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