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Discussion papers | Copyright
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 21 Aug 2018

Research article | 21 Aug 2018

Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Solid Earth (SE).

Deformation of feldspar at greenschist facies conditions – the record of mylonitic pegmatites from the Pfunderer Mountains, Eastern Alps

Felix Hentschel1, Claudia A. Trepmann1, and Emilie Janots2,1 Felix Hentschel et al.
  • 1Department for Earth and Environmental Sciences, Ludwig-Maximilians-Universität München, Germany
  • 2Univ. Grenoble 1, ISTerre, 38041 Grenoble, France

Abstract. Deformation microstructures of albitic plagioclase and K-feldspar were investigated in mylonitic pegmatites from the Austroalpine basement south of the western Tauern Window by polarized light microscopy, electron microscopy and electron backscatter diffraction to evaluate the rheologically dominant feldspar deformation mechanisms at greenschist facies conditions. The main mylonitic characteristics are alternating almost monophase quartz and albite layers, surrounding porphyroclasts of deformed feldspar and tourmaline. The dominant deformation microstructures of K-feldspar porphyroclasts are intragranular fractures parallel to the main shortening direction indicated by the foliation. The fractures are healed or sealed by polyphase aggregates of albite, K-feldspar, quartz and mica, which also occur along intragranular fractures of tourmaline and strain shadows around other porphyroclasts. Polyphase aggregates at sites of dilation indicate dissolution-precipitation creep. K-feldspar porphyroclasts are partly replaced by albite characterized by a sawtooth-shaped interface. This replacement is interpreted to be by interface-coupled dissolution-precipitation driven by a solubility difference between K-feldspar and albite and is not controlled by strain. In contrast, albite porphyroclasts are replaced at sites of shortening by fine-grained monophase albite aggregates of small strain-free new grains mixed with deformed fragments. Dislocation glide is indicated by bent, kinked and twinned albite. No indication of effective dislocation climb with dynamic recovery, for example by the presence of subgrains, a crystallographic preferred orientation or sutured grain boundaries was observed. We interpret the grain size reduction of albite at sites of shortening to be the result of coupled fracturing, dislocation glide and strain-induced grain boundary migration. This strain-induced replacement by nucleation and growth leads, together with granular flow, to the monophase albite layers. The associated quartz layers in contrast, show characteristics of dislocation creep by the presence of subgrains, undulatory extinction and sutured grain boundaries. We identified two endmember matrix microstructures that correlate with strain. Samples with lower strain are characterized by layers of a few hundreds of µm width, with coarse-grained quartz and layers with isometric, fine-grained feldspar. Higher strained samples are characterized by narrow alternating layers of some tens of µm width composed of fine-grained quartz and coarse albite grains elongated parallel to the stretching lineation, respectively. These observations indicate that grain size reduction by strain-induced replacement of albite, granular flow assisted by fracturing and dissolution-precipitation together with dislocation creep of quartz are rheologically dominant.

Felix Hentschel et al.
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Felix Hentschel et al.
Felix Hentschel et al.
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Short summary
We used microscopy and electron backscatter diffraction to analyse the deformation behaviour of feldspar at greenschist facies conditions in mylonitic pegmatites of the Austroalpine basement. There are strong uncertainties about feldspar deformation, mainly because of the varying contributions of different deformation processes. We observed that deformation is mainly the result of coupled fracturing, dislocation glide, strain-induced grain boundary migration and subsequent granular flow.
We used microscopy and electron backscatter diffraction to analyse the deformation behaviour of...