是否为阿尔卑斯山:Chenaillet蛇绿岩(西阿尔卑斯)变质作用的新岩石学约束

IF 3.5 2区 地球科学 Q1 GEOLOGY
Alberto Corno, Chiara Groppo, Alessandro Borghi, Pietro Mosca, Marco Gattiglio
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引用次数: 3

摘要

Chenaillet蛇绿岩代表了西阿尔卑斯利古里亚-皮埃蒙特海洋中保存非常完好的部分。它由海洋岩石圈序列形成,包括折返地幔、各种镁铁质侵入岩(即广义辉长岩)和世界著名的枕状玄武岩序列。除了与海洋岩石圈密切相关的稀少角砾岩外,没有沉积覆盖层暴露出来。从历史上看,Chenaillet蛇绿岩以其非常低温-低压的阿尔卑斯变质作用而闻名,这归因于造山过程。然而,实际上缺乏旨在限制阿尔卑斯变质作用的峰值压力-温度(P–T)条件的研究,到目前为止,普遍关注的是其高温变质作用和地球化学特征。本文研究了辉长岩和钠长石/碱性正长岩两种岩石,它们的岩石学特征揭示了Chenaillet蛇绿岩的复杂变质史。通过对矿物组合、爆破/变形关系和矿物化学数据的详细分析,可以区分两个变质事件:一个是早期的高温事件(已在文献中报道),另一个是后期的低温高压事件,这是本文首次认识到的。辉长岩中的洛氏体残余物和钠长岩中的间隙绿辉石的出现,有力地证明了低温高压事件。对辉长岩样品进行的热力学建模(即通过等化学相图)表明,该单元的最低温度为9kbar和300°C,最高温度为15 kbar和450°C。将这些P–T条件与根据观察到的矿物组合推断的钠长石条件重叠,可以将阿尔卑斯峰变质作用限制在10–11 kbar和340–360°C。这些P–T条件表明上覆推覆岩堆的厚度约为35–40 km,这与造山或埋藏过程不相容,而是与阿尔卑斯造山运动相关的俯冲过程一致。我们认为,与Chenaillet蛇绿岩逃脱阿尔卑斯变质作用的普遍看法相反,我们的新数据有力地支持了它经历了低温蓝片岩相变质作用的观点,其证据仍然可以在那些更好地记录和保存它的(少数)岩石中追踪。这一发现引发了关于西阿尔卑斯山等复杂造山带俯冲和折返过程的新的挑战性问题。
本文章由计算机程序翻译,如有差异,请以英文原文为准。

To be or not to be Alpine: New petrological constraints on the metamorphism of the Chenaillet Ophiolite (Western Alps)

To be or not to be Alpine: New petrological constraints on the metamorphism of the Chenaillet Ophiolite (Western Alps)

The Chenaillet Ophiolite represents a very well-preserved portion of Ligurian-Piedmont ocean in the Western Alps. It is formed from an oceanic lithospheric succession comprising exhumed mantle, various mafic intrusives (i.e., gabbro sensu lato), and a world-renowned sequence of pillow basalts. Apart from scarce breccias closely related to oceanic lithosphere, no sedimentary cover is exposed. Historically, the Chenaillet Ophiolite has been known for its very low temperature–low pressure Alpine metamorphism, ascribed to obduction processes. However, studies aimed at constraining the peak pressure–temperature (P–T) conditions of Alpine metamorphism are virtually lacking, the general focus having been so far on its high temperature metamorphism and geochemical features. In this paper, we investigate two kinds of rocks: gabbro and albitite/alkali syenite, whose petrographic features shed light on the complex metamorphic history of the Chenaillet Ophiolite. Detailed analyses of mineral assemblages, blastesis/deformation relationships, and mineral chemical data allow two metamorphic events to be distinguished: an earlier, high temperature event (already reported in the literature) and a second, later low temperature, high pressure event, recognized here for the first time. The low temperature, high pressure event is strikingly testified by the occurrence of lawsonite relicts in the gabbro and of interstitial omphacite in the albitite. Thermodynamic modelling (i.e., via isochemical phase diagrams) performed on a gabbro sample suggests for this unit a minimum of 9 kbar and 300°C and a maximum of 15 kbar and 450°C. Overlapping these P–T conditions with those inferred for the albitite based on the observed mineral assemblage allows the Alpine peak metamorphism to be constrained to 10–11 kbar and 340–360°C. These P–T conditions suggest a thickness of the overlying nappe stack of about 35–40 km, which is incompatible with obduction or burial processes, and instead consistent with subduction processes related to the Alpine orogeny. We argue that, opposite to the common belief that the Chenaillet Ophiolite escaped Alpine metamorphism, our new data strongly support the idea that it experienced low temperature-blueschist-facies metamorphism, whose evidence can still be tracked in those (few) rocks that better recorded and preserved it. This finding generates new challenging questions regarding both subduction and exhumation processes in complex orogens such as the Western Alps.

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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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