Metamorphic reactions are commonly driven to completion within shear zones thanks to fluid circulation, making the re-equilibration of the mineral assemblage one of the dominant processes. Despite the important role of H₂O in such processes, forward thermodynamic modelling calculations commonly assume either H₂O-saturated conditions or only fluid loss during prograde evolution to peak conditions. These assumptions influence the understanding of shear zones during the retrograde evolution. Here, we investigate the P–T–MH₂O retrograde evolution of the Mt. Bracco Shear Zone (MBSZ), an Alpine ductile tectonic contact which marks the boundary between two HP units in the Dora-Maira Massif (Western Alps, Italy). After the eclogite-facies peak (at 500–520°C and 1.8–2.2 GPa), the subsequent mylonitic event is constrained at amphibolite-facies conditions, continuing its evolution at decreasing pressure and temperature during rock exhumation, from ~590°C, 1.0 GPa down to ~520°C, 0.7 GPa. The P/T–MH₂O forward modelling highlights different behaviour for the two analysed samples. After reaching a minimum H₂O content at the transition from eclogite- to amphibolite-facies conditions, a significant fluid gain is modelled for only one of the two analysed samples just before the mylonitic event. The MBSZ then evolves towards H₂O-undersaturated conditions. This work thus underlines the necessity of investigating the H₂O evolution within shear zones, as the H₂O content is susceptible to change through the P–T path, due to dehydration reactions or fluid infiltration events. Furthermore, lithological heterogeneities influence possible different fluid circulation regimes in shear zones, resulting in externally or internally derived fluid gain.

中文翻译：

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评估 H2O 含量在剪切带构造变质演化中的重要性：以 Dora-Maira 地块（西阿尔卑斯山）为例

由于流体循环，变质反应通常在剪切带内完成，这使得矿物组合的重新平衡成为主要过程之一。尽管H ₂ O 在此类过程中发挥着重要作用，正向热力学建模计算通常假设H ₂ O 饱和条件或仅在逐步演化至峰值条件期间发生流体损失。这些假设影响了对逆行演化过程中剪切带的理解。在这里，我们研究了布拉科山剪切带（MBSZ）的P-T-M H _{2 O 逆行演化，这是一个高山}韧性构造接触面，标志着多拉-迈拉地块（西阿尔卑斯山，意大利）。在榴辉岩相峰值（500-520°C和1.8-2.2 GPa）之后，随后的糜棱岩事件被限制在角闪岩相条件下，在岩石折返过程中压力和温度降低的情况下继续其演化，从~590°C， 1.0 GPa 降至 ~520°C，0.7 GPa。 P / T – M H ₂ O 正演模型突出显示了两个分析样本的不同行为。在从榴辉岩相过渡到角闪岩相条件时达到最低 H ₂ O 含量后，仅对糜棱岩事件之前的两个分析样品之一进行了显着的流体增加建模。然后MBSZ 向H ₂ O 不饱和条件演化。因此，这项工作强调了研究剪切区内H ₂ O 演变的必要性，因为由于脱水反应或流体渗透事件， H ₂ O 含量很容易通过P-T路径发生变化。此外，岩性非均质性影响剪切带中可能不同的流体循环状况，导致外部或内部衍生的流体增益。