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Deformation behaviour of feldspar in greenschist facies granitoide shear zones

绿片岩相花岗岩剪切带中长石的变形行为

基本信息

批准号：
277689670
负责人：
Professorin Dr. Claudia Trepmann
金额：
--
依托单位：
Institut des Sciences de la Terre (ISTerre)
依托单位国家：
德国
项目类别：
Research Grants
财政年份：
2015
资助国家：
德国
起止时间：
2014-12-31 至 2018-12-31
项目状态：
已结题

来源：
https://gepris.dfg.de/gepris/projekt/277689670?language=en
关键词：
Deformation behaviour feldspar greenschist facies

项目摘要

Objective of the proposed study is to elucidate the feldspar deformation behaviour at greenschist facies conditions relevant for the rheological properties of continental crust. Uncertainties in models for the rheological properties are partly due to a poor knowledge of the deformation mechanisms that are actually taking place in granitoid rocks at inaccessible depth. The deformation behaviour of feldspar, the most abundant mineral in the continental crust, is characterized by an interaction of brittle, dissolution-precipitation and crystal-plastic processes, which is difficult to evaluate in experiments given the problematic extrapolation of experimental conditions to reasonable natural conditions. However, microfabrics of metamorphic granitoid rocks record the grain-scale deformation mechanisms and involved chemical reactions proceeding during their geological history. This usually includes deformation and modification through several stages in space (depth, i.e., P, T conditions) and/or time. For deciphering the rock record this implies both, challenge and chance to resolve these different stages. Here, we propose to use the deformation record of mylonitic pegmatites from the Austroalpine basement south to the western Tauern window. The pegmatites represent a relatively simple Ca-poor granitoid system mineralogically dominated by albite-rich plagioclase, K-feldspar and quartz. The structural, crystallographic and chemical characteristics of the feldspar microfabrics will be determined via micro-analytical techniques to identify the deformation mechanisms. To constraint deformation conditions (stress, temperature) and to infer the relative sequence of processes, the feldspar microfabrics will be correlated to the microfabrics of quartz and REE-minerals that grew contemporaneously with or after deformation. In-situ U-Th-Pb dating of the REE-minerals will be performed in cooperation with Dr. Emilie Janots to identify different deformation stages with respect to time. The inferred age of REE-minerals allows a correlation to the Alpine deformation history, i.e., to the tectonometamorphic eoalpine event (Cretaceous) or to the activity of the DAV shear zone (Paleogene). The findings will be used to address the following key questions: Which role do brittle mechanisms, dissolution-precipitation creep and crystal-plastic processes play on the feldspar deformation behaviour? Are different deformation stages in space (depth, i.e. P, T conditions) and/or time recorded? What are the rheological implications (e.g., stress history, episodic deformation at transient high stresses, (re)activations of shear zones at different metamorphic conditions)? The findings contribute to elucidate the rheological properties of continental crust.

研究目的是阐明绿片岩相条件下长石的变形行为与陆壳流变学性质的关系。流变学性质的模型的不确定性部分是由于对花岗岩类岩石在不可到达的深度实际发生的变形机制的认识不足。长石是大陆地壳中最丰富的矿物，其变形行为的特点是脆性、溶解-沉淀和结晶-塑性过程的相互作用，由于将实验条件外推到合理的自然条件存在问题，因此难以在实验中进行评估。然而，变质花岗岩类岩石的显微组构记录了其地质历史中的颗粒尺度变形机制和所涉及的化学反应。这通常包括通过空间（深度，即，P、T条件）和/或时间。对于破译岩石记录，这意味着解决这些不同阶段的挑战和机会。在这里，我们建议使用变形记录的糜棱岩伟晶岩从南阿尔卑斯山基底西部陶恩窗口。伟晶岩代表了一个相对简单的贫钙花岗岩类系统，矿物学上以富含钠长石的斜长石、钾长石和石英为主。将通过显微分析技术确定长石微组构的结构、晶体学和化学特征，以确定变形机制。约束变形条件（应力，温度），并推断过程的相对顺序，长石微组构将相关的石英和稀土矿物的微组构，生长的同时或变形后。将与Emilie Janots博士合作，对稀土矿物进行现场U-Th-Pb测年，以确定不同的变形阶段。稀土矿物的推断年龄允许与阿尔卑斯山变形历史相关，即，与构造变质古阿尔卑斯事件（白垩纪）或与DAV剪切带活动（古近纪）有关。研究结果将用于解决以下关键问题：脆性机制，溶解-沉淀蠕变和晶体塑性过程中发挥的作用长石变形行为？是否记录了空间（深度，即P、T条件）和/或时间上的不同变形阶段？流变学的含义是什么（例如，应力历史、瞬时高应力下的幕式变形、不同变质条件下剪切带的（再）活化）？这一发现有助于阐明大陆地壳的流变学性质。