Assessing the importance of H2O content in the tectono-metamorphic evolution of shear zones: A case study from the Dora-Maira Massif (Western Alps)

IF 3.5 2区 地球科学 Q1 GEOLOGY
Sara Nerone, Alessandro Petroccia, Fabiola Caso, Davide Dana, Andrea Maffeis
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Abstract

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/TMH2O 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.

Abstract Image

水含量在剪切带构造变质演化中的重要性评价——以西阿尔卑斯多拉-迈拉地块为例
由于流体循环,变质反应通常在剪切带内完成,使矿物组合的重新平衡成为主要过程之一。尽管H2O在这些过程中发挥着重要作用,但正演热力学建模计算通常假设H2O饱和状态或仅假设在向峰值状态演进过程中的流体损失。这些假设影响了对逆行演化过程中剪切带的认识。本文研究了位于意大利西阿尔卑斯地区Dora-Maira地块的阿尔卑斯韧性构造接触带——Mt. Bracco剪切带(MBSZ)的P-T-MH2O逆行演化。榴辉岩相峰后(500 ~520℃,1.8 ~ 2.2 GPa),随后糜棱岩事件被限制在角闪岩相条件下,在岩石挖掘过程中持续降温降压演化,从~590℃,1.0 GPa降至~520℃,0.7 GPa。P/ T-MH2O正演模拟强调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的条件下,达到最小的H2O含量后,在糜棱岩事件发生之前,仅对两个分析样品中的一个进行了显著的流体增加建模。然后MBSZ向水不饱和状态发展。因此,这项工作强调了研究剪切带内H2O演化的必要性,因为由于脱水反应或流体渗透事件,H2O含量容易通过P-T路径发生变化。此外,岩性非均质性影响剪切带中可能存在的不同流体循环机制,从而导致外部或内部流体增益。
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来源期刊
CiteScore
6.60
自引率
11.80%
发文量
57
审稿时长
6-12 weeks
期刊介绍: The journal, which is published nine times a year, encompasses the entire range of metamorphic studies, from the scale of the individual crystal to that of lithospheric plates, including regional studies of metamorphic terranes, modelling of metamorphic processes, microstructural and deformation studies in relation to metamorphism, geochronology and geochemistry in metamorphic systems, the experimental study of metamorphic reactions, properties of metamorphic minerals and rocks and the economic aspects of metamorphic terranes.
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