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首页> 外文期刊>Contributions to Mineralogy and Petrology >K-feldspar-quartz and K-feldspar-plagiclase phase boundary interaction in garnet-orthopyroxene gneiss's from the Val Strona di Omegna, Ivrea-Verbano Zone, northern Italy
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K-feldspar-quartz and K-feldspar-plagiclase phase boundary interaction in garnet-orthopyroxene gneiss's from the Val Strona di Omegna, Ivrea-Verbano Zone, northern Italy

机译：意大利北部Ivrea-Verbano区Val Strona di Omegna的石榴石-邻苯二酚片麻岩中的K-长石-石英和K-长石-plagiclase相边界相互作用

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A detailed study based on textural observations combined with microanalysis (back scattered electron imaging (BSE) and electron microprobe analysis (EMPA)) and microstructural data transmission electron microscopy (TEM) has been made of K-feldspar micro-veins along quartz-plagioclase phase and plaaio-clase-plagioclase grain boundaries in granulite facies. orthopyroxene-garnet-hearing gneiss's (700-825 deg C. 6-8 kbar) from the Val Strona di Omegna. Ivrea-Verbano Zone, northern Italy. The K-feldspar micro-veins are commonly associated with quartz and plagioclase and are not found in quartz absent regions of the thin section. This association appears to represent a localised reaction texture resulting from a common high grade dehydration reaction, namely: amphibole + quartz =orthopyroxene+ clinopyroxene + plagioclase + K-feldspar + H_2O, which occurred during the granulite facies metamorphism of these rocks. There are a number of lines of evidence for this. These include abundant Ti-rich biotite, which was apparently stable during granulite facies metamorphism, and total lack of amphibole, which apparently was not. Disorder between Al and Si in the K-feldspar indicates crystallisation at temperatures > 500 deg C. Myrmekite and albitic rim intergrowths in the K-feldspar along the K-feldspar-plagioclase interface could only have formed at temperatures > 500-600 deg C. Symplectic intergrowths of albite and Ca-rich plagioclase between these albitic rim intergrowths and plagioclase suggest a high temperature grain boundary reaction, which most likely occurred at the start of decompression in conjunction with a fluid phase. Relatively high dislocation densities (>2 X 10~9 to 3 X 10~9/cm~2) in the K-feldspar suggest plastic deformation at temperatures > 500 0C. We propose that this plastic deformation is linked with the extensional tectonic environment present during the mafic underplating event responsible for the granulite facies metamorphism in these rocks. Lastly, apparently active garnet grain rims associated with side inclusions of K-feldspar and quartz and an exterior K-feldspar micro-vein indicate equilibrium temperatures within 20-30 deg C of the peak metamorphic temperatures estimated for the sample (770 deg C). Contact between these K-feldspar micro-veins and Fe-Mg silicate minerals, such as garnet, orthopyroxene clinopyroxene or biotite along the interface, is observed to be very clean with no signs of melt textures or alteration to sheet silicates. This lends support to the idea that these micro-veins did not originate from a melt and, if fluid induced, that the water activity of these fluids must have been relatively low. All of these lines of evidence point to a high grade origin for the K-feldspar micro-veins and support the hypothesis that they formed during the granulite facies metamorphism of the metabasite layers in an extensional tectonic environment as the consequence of localised dehydration reactions involving the breakdown of amphibole in the presence of quartz to orthopyroxene, clinopyroxene, plagioclase, K-feldspar and H_2O. It is proposed that the dehydration of the metabasite layers to an orthopyroxene-garnet-bearing gneiss over a 4-km traverse in the upper Val Strona during granulite facies metamorphism was a metasomatic event initiated by the presence of a high-grade, low H_2O activity fluid (most likely a NaCl-KCl supercritical brine), related to the magmatic underplating event responsible for the Mafic Formation; and that this dehydration event did not involve partial melting.
机译：基于质构观察结合微观分析（背散射电子成像（BSE）和电子微探针分析（EMPA））以及显微结构数据传输电子显微镜（TEM），对沿石英斜长石相的钾长石微静脉进行了详细研究粒相中的Plaio-clase-Plagioclase晶界。来自Val Strona di Omegna的邻苯二甲rox-石榴石听觉片麻岩（700-825摄氏度6-8 kbar）。意大利北部Ivrea-Verbano区。钾长石微静脉通常与石英和斜长石有关，而在薄断面的石英不存在的区域中找不到。这种联系似乎代表了由一种常见的高级脱水反应引起的局部反应织构，即：闪石+石英=邻苯二甲酚+斜py石+斜长石+ K长石+ H_2O，发生在这些岩石的粒岩相变质过程中。有很多证据可以证明这一点。这些包括丰富的富钛黑云母，在粒岩相变质过程中显然是稳定的，以及完全缺乏闪石，显然不是。钾长石中的Al和Si之间的无序现象表明在> 500摄氏度的温度下会发生结晶。只有在> 500-600摄氏度的温度下，才能在钾长石中沿钾长石-斜长石界面形成白云母和任意缘共生体。钠长石和斜长石之间的钠长石和富含钙的斜长石的辛共生表明存在高温晶界反应，这很可能发生在减压开始时与液相结合。钾长石中较高的位错密度（> 2 X 10〜9至3 X 10〜9 / cm〜2）表明在> 500 0C的温度下塑性变形。我们建议这种塑性变形与负责这些岩石中的花岗石相变质的镁铁质基底作用期间存在的伸展构造环境有关。最后，与钾长石和石英的侧面夹杂物以及外部钾长石微静脉相关的明显活跃的石榴石晶粒边缘表明平衡温度在样品估计的最高变质温度（770摄氏度）的20-30摄氏度之内。观察到这些钾长石微脉与Fe-Mg硅酸盐矿物（例如石榴石，邻苯二茂铁斜发石或黑云母）之间沿界面的接触非常干净，没有熔融结构或片状硅酸盐蚀变的迹象。这支持以下观点：这些微静脉不是源自熔体，并且，如果诱发了流体，则这些流体的水活度必须相对较低。所有这些证据都表明钾长石微脉具有较高的成因，并支持了这样的假说，即它们是在伸展构造环境中变位辉石层的粒状相变质过程中形成的，这是由于局部脱水反应引起的。石英存在下邻苯二酚分解为邻位邻苯二酚，斜向茂，斜长石，钾长石和H_2O。有人认为，在粒岩相变质过程中，在Val Strona上部4公里的横断面上，变质岩层脱水成含邻二甲苯-石榴石的片麻岩是一种高，低H_2O活性引发的变质事件。流体（最可能是NaCl-KCl超临界盐水），与负责镁铁质组的岩浆下覆事件有关;并且该脱水事件不涉及部分熔化。

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