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首页> 外文期刊>Contributions to Mineralogy and Petrology >Modelling phase-assemblage diagrams for magnesian metapelites in the system K_2O-FeO-MgO-Al_2O_3-SiO_2-H_2O:geodynamic consequences for the Monte Rosa nappe, Western Alps
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Modelling phase-assemblage diagrams for magnesian metapelites in the system K_2O-FeO-MgO-Al_2O_3-SiO_2-H_2O:geodynamic consequences for the Monte Rosa nappe, Western Alps

机译：系统K_2O-FeO-MgO-Al_2O_3-SiO_2-H_2O系统中镁质变石的相组装图建模：对西方阿尔卑斯山蒙特罗莎推覆的地球动力学后果

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摘要

Magnesian metamorphic rocks with metapelitic mineral assemblage and composition are of great interest in metamorphic petrology for their ability to constrain P-T conditions in terranes where metamorphism is not easily visible. Phase-assemblage diagrams for natural and model magnesian metapelites in the system KFMASH are presented to document how phase relationships respond to water activity, bulk composition, pressure and temperature. The phase assemblages displayed on these phase diagrams are consistent with natural mineral assemblages occurring in magnesian metapelites. It is shown that the equilibrium assemblages at high pressure conditions are very sensitive to a(H_2O). Specifically, the appearance of the characteristic HP assemblage chloritoid-talc-phengite-quartz (with excess H2O) in the magnesian metapelites of the Monte Rosa nappe (Western Alps) is due to the reduction of a(H_2O). Furthermore, the mineral assemblages are determined by the whole-rock FeO/(FeO + MgO) ratio and effective Al content X_A as well as P and T. The predicted mineral associations for the low- and high-Z_A model bulk compositions of magnesian metapelites at high pressure are not dependent on the X_A variations as they show a similar sequence of mineral assemblages. Above 20 kbar, the prograde sequence of assemblages associated with phengite (with excess SiO_2 and H_2O) for low- and high-l^ bulk compositions of magnesian metapelites is: carpholite-chlorite - > chlorite-chloritoid - > chloritoid-talc - > chloritoid-talc-kyanite - > talc-garnet-kyanite - > garnet-kyanite ± biotite. At low to medium P-T conditions, a low- X_A stabilises the phengite-bearing assemblages associated with chlorite, chlorite + K-feldspar and chlorite + biotite while a high-J^A results in the chlorite-phengite bearing assemblages associated with pyrophyllite, andalusite, kyanite and carpholite. A high-XA magnesian metapelite with nearly iron-free content stabilises the talc-kyanite-phengite assemblage at moderate to high P-T conditions. Taking into account the effective bulk composition and a(H_2O) involved in the metamorphic history, the phase-assemblage diagrams presented here may be applied to all magnesian metapelites that have compositions within the system KFMASH and therefore may contribute to gaining insights into the metamorphic evolution of terranes. As an example, the magnesian metapelites of the Monte Rosa nappe have been investigated, and an exhumation path with P-T conditions for the western roof of the Monte Rosa nappe has been derived for the first time. The exhumation shows first a near-isothermal decompression from the Alpine eclogite peak conditions around 24 kbar and 505 deg C down to approximately 8 kbar and 475 deg C followed by a second decompression with concomitant cooling.
机译：具有变质矿物组合和组成的镁质变质岩对变质岩石学具有极大的兴趣，因为它们能够约束不易见到变质作用的地层中的P-T条件。给出了系统KFMASH中天然和模型镁质变石的相组装图，以记录相关系如何响应水活度，体积组成，压力和温度。这些相图上显示的相组合与发生在镁质变质岩中的天然矿物组合一致。结果表明，高压条件下的平衡组装对a（H_2O）非常敏感。具体而言，在蒙特罗莎尿布（西阿尔卑斯山）的镁质变石中，特征性的HP组合类胡萝卜素-滑石-辉石-石英（具有过量的H2O）的出现是由于a（H_2O）的减少。此外，矿物组合是由整个岩石中的FeO /（FeO + MgO）比和有效Al含量X_A以及P和T决定的。低Z-A和高Z_A模型镁质变质岩的预测矿物组成高压下的X_A变化不依赖于X_A变化，因为它们显示出相似的矿物组合序列。高于20 kbar时，镁质偏闪石的低和高l ^本体组成的与亚硫酸盐（含过量的SiO_2和H_2O）相关的组合的前移顺序为：陨石-亚氯酸盐->亚氯酸盐-类固醇->亚氯酸盐-滑石->亚氯酸盐-滑石蓝晶石->滑石石榴石-蓝晶石->石榴石-蓝晶石±黑云母。在中低PT条件下，低X_A可使与亚氯酸盐，亚氯酸盐+ K长石和亚氯酸盐+黑云母有关的含亚硫酸盐的组合稳定，而高J ^ A则导致与叶蜡石，红柱石有关的含亚氯石-辉石的组合，蓝晶石和Carpholite。高XA镁含量的亚辉石具有几乎不含铁的成分，可在中等至高P-T条件下稳定滑石-蓝晶石-辉石组合。考虑到有效的体积组成和变质史中涉及的a（H_2O），此处给出的相组装图可应用于系统KFMASH中具有组成的所有镁质变质岩，因此可能有助于深入了解变质演化。的地形。例如，已经研究了蒙特罗莎棉布的镁质变质岩，并且首次推导了具有P-T条件的蒙特罗莎棉布西屋顶的掘尸路径。掘出过程显示，首先从大约24 kbar和505℃的高山榴辉岩峰条件降到大约8 kbar和475℃的近等温减压，然后进行第二次减压并伴随冷却。

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