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

• Published in: Solid earth (Göttingen) 2019-01, Vol.10 (1), p.95-116
• Main Authors: Hentschel, Felix, Trepmann, Claudia A, Janots, Emilie
• Format: Article
• Language: English
• Subjects: Aggregates; Analysis; Aspect ratio; Boundaries; Chemical precipitation; Deformation; Deformation mechanisms; Dislocation; Dislocation mobility; Dissolution; Dissolving; Earth science; Electron backscatter diffraction; Electron microscopy; Feldspars; Fractures; Grain boundaries; Grain boundary migration; Grain size; Greenschist facies; Growth; Isometric; Light diffraction; Light microscopy; Low temperature; Mica; Microscopy; Microstructure; Misalignment; Mountains; Nucleation; Particle size; Pegmatite; Plagioclase; Polarized light; Properties; Quartz; Size reduction; Solifluction; Thin films; Tourmaline; Width
• ISSN: 1869-9529; 1869-9510; 1869-9529
• DOI: 10.5194/se-10-95-2019

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 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 at a high angle to the stretching lineation. 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. These polyphase aggregates indicate dissolution–precipitation creep. K-feldspar porphyroclasts are partly replaced by albite characterized by a cuspate interface. This replacement is interpreted to take place by interface-coupled dissolution–precipitation driven by a solubility difference between K-feldspar and albite. Albite porphyroclasts are replaced at boundaries parallel to the foliation by fine-grained monophase albite aggregates of small strain-free new grains mixed with deformed fragments. Dislocation glide is indicated by bent and twinned albite porphyroclasts with internal misorientation. An indication of effective dislocation climb with dynamic recovery, for example, by the presence of subgrains, is systematically missing. We interpret the grain size reduction of albite to be the result of coupled dislocation glide and fracturing (low-temperature plasticity). Subsequent growth is by a combination of strain-induced grain boundary migration and formation of growth rims, resulting in an aspect ratio of albite with the long axis within the foliation. This strain-induced replacement by nucleation (associated dislocation glide and microfracturing) and subsequent growth is suggested to result in the observed monophase albite layers, probably together with granular flow. The associated quartz layers show characteristics of dislocation creep by the presence of subgrains, undulatory extinction and sutured grain boundaries. We identified two endmember matrix microstructures: (i) alternating layers of a few hundred micrometres' width, with isometric, fine-grained feldspar (on average 15 µm in diameter) and coarse-grained qua