Deformation of feldspar at greenschist facies conditions – the record of mylonitic pegmatites from the Pfunderer Mountains, Eastern Alps
Authors: Hentschel Felix , Trepmann Claudia A. , Janots Emilie 
Published: January 2019

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 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 quartz (a few hundred micrometres in diameter),
representing lower strain compared to (ii) alternating thin layers of some
tens of micrometres' width composed of fine-grained quartz (<20 µm in diameter)
and coarse elongated albite grains (long axis of a few tens of
micrometres) defining the foliation, respectively. Our observations indicate
that grain size reduction by strain-induced replacement of albite (associated
dislocation glide and microfracturing) followed by growth and granular flow
simultaneous with dislocation creep of quartz are playing the dominating role
in formation of the mylonitic microstructure.