阿尔卑斯峰顶压力和蒙特罗萨推覆的构造变质史:来自意大利上阿亚斯山谷vsamuraz cirque的证据。

IF 1.8 2区 地球科学 Q3 GEOSCIENCES, MULTIDISCIPLINARY
Swiss Journal of Geosciences Pub Date : 2021-01-01 Epub Date: 2021-10-29 DOI:10.1186/s00015-021-00397-3
Joshua D Vaughan-Hammon, Cindy Luisier, Lukas P Baumgartner, Stefan M Schmalholz
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引用次数: 2

摘要

蒙特罗萨推覆体由广泛的岩性组成,记录了与高山峰变质作用相关的条件。虽然从以前的研究中推断出的峰值温度条件在很大程度上是一致的,但对阿尔卑斯高压变质事件的峰值压力的估计是可变的。小体积的绿泥石+辉白云石+滑石+石英组合的白片岩岩性的峰值压力最高可达0.6 GPa,而其伴生的变质长岩和变质花岗质岩性的峰值压力约为1.6 GPa。造成这种压力差的原因是有争议的,提出的解释包括来自不同埋藏深度的岩石的构造混合(msamuange)或由不同力学性质的岩石之间的非均匀应力条件引起的压力与静岩值的局部偏差。我们提出了详细的野外测绘结果,结构分析和Monte Rosa推覆体的一部分暴露在cirque du v raz田野地区(意大利阿亚斯山谷的头部)的新地质图。地质填图和构造分析结果表明,在Monte Rosa推覆体的西部具有构造一致性。这种结构上的一致性否定了构造差异是峰值压力变化原因的假设。构造分析表明,高寒地区主要有两大变形事件:(1)北向推覆侵位,(2)南向背褶皱。分析了新发现的白片岩体,该白片岩体位于Monte Rosa变质岩与周围变质岩的侵入接触处。这个位置与以前的白片岩不同,以前的白片岩完全嵌入在变质花岗岩中。根据变质组合图的热力学计算,高山变质峰的温度为2.1±0.2 GPa,温度为560±20°C。这些结果与先前研究的白片岩露头埋在变质岩中的变质条件相一致。因此,新的结果证实了白片岩与变长岩和变长岩之间的峰值压力差。为了更好地约束白片岩的递进压力-温度历史,我们比较了仿绿石中测量到的Mg分带与通过分数结晶伪截面模型预测的几种假设压力-温度路径的Mg分带。与变长岩和变长岩相比,白片岩的峰值压力高出约0.6 GPa,表明白片岩可能偏离了变长岩和变长岩岩性记录的渐进式埋藏路径。然而,白片岩压力偏离的确切条件仍然存在争议,因为白片岩组合中Mg分配的温度依赖性很强。我们的伪剖面结果至少表明,在白片岩中没有记录到明显的等温压力升高。
本文章由计算机程序翻译,如有差异,请以英文原文为准。

Alpine peak pressure and tectono-metamorphic history of the Monte Rosa nappe: evidence from the cirque du Véraz, upper Ayas valley, Italy.

Alpine peak pressure and tectono-metamorphic history of the Monte Rosa nappe: evidence from the cirque du Véraz, upper Ayas valley, Italy.

Alpine peak pressure and tectono-metamorphic history of the Monte Rosa nappe: evidence from the cirque du Véraz, upper Ayas valley, Italy.

Alpine peak pressure and tectono-metamorphic history of the Monte Rosa nappe: evidence from the cirque du Véraz, upper Ayas valley, Italy.

The Monte Rosa nappe consists of a wide range of lithologies that record conditions associated with peak Alpine metamorphism. While peak temperature conditions inferred from previous studies largely agree, variable peak pressures have been estimated for the Alpine high-pressure metamorphic event. Small volumes of whiteschist lithologies with the assemblage chloritoid + phengite + talc + quartz record peak pressures up to 0.6 GPa higher compared to associated metapelitic and metagranitic lithologies, which yield a peak pressure of ca. 1.6 GPa. The reason for this pressure difference is disputed, and proposed explanations include tectonic mixing of rocks from different burial depths (mélange) or local deviations of the pressure from the lithostatic value caused by heterogeneous stress conditions between rocks of contrasting mechanical properties. We present results of detailed field mapping, structural analysis and a new geological map for a part of the Monte Rosa nappe exposed at the cirque du Véraz field area (head of the Ayas valley, Italy). Results of the geological mapping and structural analysis shows the structural coherency within the western portions of the Monte Rosa nappe. This structural coherency falsifies the hypothesis of a tectonic mélange as reason for peak pressure variations. Structural analysis indicates two major Alpine deformation events, in agreement with earlier studies: (1) north-directed nappe emplacement, and (2) south-directed backfolding. We also analyze a newly discovered whiteschist body, which is located at the intrusive contact between Monte Rosa metagranite and surrounding metapelites. This location is different to previous whiteschist occurrences, which were entirely embedded within metagranite. Thermodynamic calculations using metamorphic assemblage diagrams resulted in 2.1 ± 0.2 GPa and 560 ± 20 °C for peak Alpine metamorphic conditions. These results agree with metamorphic conditions inferred for previously investigated nearby whiteschist outcrops embedded in metagranite. The new results, hence, confirm the peak pressure differences between whiteschists and the metagranite and metapelite. To better constrain the prograde pressure-temperature history of the whiteschist, we compare measured Mg zoning in chloritoid with Mg zoning predicted by fractional crystallization pseudo-section modelling for several hypothetical pressure-temperature paths. In order to reach a ca. 0.6 GPa higher peak pressure compared to the metapelite and metagranite, our results suggest that the whiteschist likely deviated from the prograde burial path recorded in metapelite and metagranite lithologies. However, the exact conditions at which the whiteschist pressure deviated are still contentious due to the strong temperature dependency of Mg partitioning in whiteschist assemblages. Our pseudo-section results suggest at least that there was no dramatic isothermal pressure increase recorded in the whiteschist.

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来源期刊
Swiss Journal of Geosciences
Swiss Journal of Geosciences 地学-地质学
CiteScore
4.50
自引率
12.90%
发文量
21
审稿时长
>12 weeks
期刊介绍: The Swiss Journal of Geosciences publishes original research and review articles, with a particular focus on the evolution of the Tethys realm and the Alpine/Himalayan orogen. By consolidating the former Eclogae Geologicae Helvetiae and Swiss Bulletin of Mineralogy and Petrology, this international journal covers all disciplines of the solid Earth Sciences, including their practical applications. The journal gives preference to articles that are of wide interest to the international research community, while at the same time recognising the importance of documenting high-quality geoscientific data in a regional context, including the occasional publication of maps.
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