Whiteschists from the Monte Rosa Nappe were examined in the field as well as with petrographic, geochemical, and isotopic methods to constrain the controversial origin of these rocks in their Alpine metamorphic context. Whiteschists occur as ellipsoidal-shaped, decametric-sized bodies, within a Permian metagranite, and consist mainly of chloritoid, talc, phengite, and quartz. The transition from whiteschist to metagranite is marked by multiple sharp mineralogical boundaries defining concentric zones unrelated to Alpine deformation. The development of reaction zones, as well as the geometry of the whiteschist suggest a pervasive fluid infiltration, facilitated and canalized by reaction fingering. Whole-rock compositions of whiteschists and metagranites indicate an enrichment in MgO and H 2 O and depletion of Na 2 O, CaO, Ba, Sr, Pb, and Zn in the whiteschist relative to the metagranite. Trace- and rare-earth elements, together with all other major elements, notably K 2 O and SiO 2 , were within uncertainty not mobile. Such a K and Si saturated, Na undersaturated fluid is not compatible with previous interpretations of fluids derived from ultramafic rocks, evaporites, or Mg-enriched seawater due to mantle interactions. Together with the large variations in δD and δ 18 O values, this indicates large fluid fluxes during metasomatism. Calculated δD and δ 18 O values of fluids in equilibrium with the whiteschist support a magmatic–hydrothermal fluid source, as does the chemical alteration pattern. Bulk rock 87 Sr/ 86 Sr ratios in whiteschists confirm a pre-Alpine age of fluid infiltration. The 87 Sr/ 86 Sr ratios in whiteschists were subsequently partially homogenized in a closed system during Alpine metamorphism. In conclusion, the granite was locally affected by late magmatic–hydrothermal alteration, which resulted in sericite–chlorite alteration zones in the granite. The entire nappe underwent high-pressure metamorphism during the Alpine orogeny and the mineralogy of the whiteschist was produced during dehydration of the metasomatic assemblage under otherwise closed-system metamorphism. While each whiteschist locality needs to be studied in detail, this in-depth study suggests that many whiteschists found in granitic bodies in the Alps might be of similar origin.

中文翻译：

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在 Monte Rosa 推覆（西阿尔卑斯山）中通过交代作用和变质作用的白片岩成因

来自 Monte Rosa Nappe 的白片岩在野外进行了研究，并使用岩相学、地球化学和同位素方法来限制这些岩石在其高山变质背景下有争议的起源。白片岩在二叠纪变花岗岩中以椭圆形、十米大小的体形式出现，主要由绿泥石、滑石、菱镁矿和石英组成。从白片岩到变花岗岩的过渡以多个尖锐的矿物边界为标志，这些边界定义了与高山变形无关的同心带。反应区的发展，以及白片岩的几何形状表明普遍的流体渗透，通过反应指法促进和疏通。白片岩和变花岗岩的全岩成分表明 MgO 和 H 2 O 富集，Na 2 O、CaO、Ba、Sr、Pb、和 Zn 在白片岩中相对于变花岗岩。痕量和稀土元素，连同所有其他主要元素，特别是 K 2 O 和 SiO 2 ，​​都在不确定范围内，不可移动。由于地幔相互作用，这种 K 和 Si 饱和、Na 不饱和流体与先前对源自超基性岩、蒸发岩或富含镁的海水的流体的解释不兼容。连同 δD 和 δ 18 O 值的巨大变化，这表明交代过程中存在较大的流体通量。与白片岩平衡的流体的计算 δD 和 δ 18 O 值支持岩浆-热液流体源，化学蚀变模式也是如此。白片岩中的块状岩石 87 Sr/86 Sr 比率证实了流体渗透的前高山时代。白片岩中的 87 Sr/86 Sr 比率随后在高山变质作用期间在封闭系统中部分均质化。综上所述，花岗岩局部受到晚期岩浆-热液蚀变的影响，导致花岗岩中出现绢云母-绿泥石蚀变带。整个推覆在阿尔卑斯造山运动期间经历了高压变质作用，白片岩的矿物学是在封闭系统变质作用下交代组合脱水过程中产生的。虽然需要详细研究每个白片岩地区，但这项深入研究表明，在阿尔卑斯山的花岗岩体中发现的许多白片岩可能具有相似的起源。这导致花岗岩中的绢云母-绿泥石蚀变带。整个推覆在阿尔卑斯造山运动期间经历了高压变质作用，白片岩的矿物学是在封闭系统变质作用下交代组合脱水过程中产生的。虽然需要详细研究每个白片岩地区，但这项深入研究表明，在阿尔卑斯山的花岗岩体中发现的许多白片岩可能具有相似的起源。这导致花岗岩中的绢云母-绿泥石蚀变带。整个推覆在阿尔卑斯造山运动期间经历了高压变质作用，白片岩的矿物学是在封闭系统变质作用下交代组合脱水过程中产生的。虽然需要详细研究每个白片岩地区，但这项深入研究表明，在阿尔卑斯山花岗岩体中发现的许多白片岩可能具有相似的起源。