摘要：Deformation microstructures of albitic plagioclase and K-feldspar were investigated inmylonitic pegmatites from the Austroalpine basement south of the westernTauern Window by polarized light microscopy, electron microscopy and electronbackscatter diffraction to evaluate feldspar deformation mechanisms atgreenschist facies conditions. The main mylonitic characteristics arealternating almost monophase quartz and albite layers, surroundingporphyroclasts of deformed feldspar and tourmaline. The dominant deformationmicrostructures of K-feldspar porphyroclasts are intragranular fractures at ahigh angle to the stretching lineation. The fractures are healed or sealed bypolyphase aggregates of albite, K-feldspar, quartz and mica, which also occuralong intragranular fractures of tourmaline and strain shadows around otherporphyroclasts. These polyphase aggregates indicate dissolution–precipitationcreep. K-feldspar porphyroclasts are partly replaced by albite characterizedby a cuspate interface. This replacement is interpreted to take place byinterface-coupled dissolution–precipitation driven by a solubility differencebetween K-feldspar and albite. Albite porphyroclasts are replaced atboundaries parallel to the foliation by fine-grained monophase albiteaggregates of small strain-free new grains mixed with deformed fragments.Dislocation glide is indicated by bent and twinned albite porphyroclasts withinternal misorientation. An indication of effective dislocation climb withdynamic recovery, for example, by the presence of subgrains, is systematicallymissing. We interpret the grain size reduction of albite to be the result ofcoupled dislocation glide and fracturing (low-temperature plasticity).Subsequent growth is by a combination of strain-induced grain boundarymigration and formation of growth rims, resulting in an aspect ratio of albitewith the long axis within the foliation. This strain-induced replacement bynucleation (associated dislocation glide and microfracturing) and subsequentgrowth is suggested to result in the observed monophase albite layers,probably together with granular flow. The associated quartz layers showcharacteristics of dislocation creep by the presence of subgrains, undulatoryextinction and sutured grain boundaries. We identified two endmember matrixmicrostructures: (i) alternating layers of a few hundred micrometres' width,with isometric, fine-grained feldspar (on average 15µm in diameter)and coarse-grained quartz (a few hundred micrometres in diameter),representing lower strain compared to (ii) alternating thin layers of sometens of micrometres' width composed of fine-grained quartz (<20µm in diameter)and coarse elongated albite grains (long axis of a few tens ofmicrometres) defining the foliation, respectively. Our observations indicatethat grain size reduction by strain-induced replacement of albite (associateddislocation glide and microfracturing) followed by growth and granular flowsimultaneous with dislocation creep of quartz are playing the dominating rolein formation of the mylonitic microstructure.
