Deformation behaviour of feldspar in greenschist facies granitoide shear zones

• Applicant: Professorin Dr. Claudia Trepmann
• Subject Area: Palaeontology ; Mineralogy, Petrology and Geochemistry
• Term: from 2015 to 2019
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 277689670

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.

• DFG Programme: Research Grants
• International Connection: France
• Cooperation Partner: Emilie Janots, Ph.D.