Ductile deformation of K-feldspar in dry eclogite facies shear zones in the Bergen Arcs, Norway

Abstract

The deformation microstructures of K-feldspar of dry eclogite facies shear zones in syenitic granulites are studied in detail. The eclogite facies overprint occurred syntectonically. The pyroxenes, feldspars, quartz and apatite in the shear zones recrystallized dynamically during shearing, partially changing their chemical composition. The metamorphic paragenesis is represented by omphacitic clinopyroxene, K-feldspar, plagioclase, rutile, ilmenite and quartz. During dry eclogite facies shearing, K-feldspar deformation was predominantly plastic. The dominant microstructures are undulatory extinction, kink bands //(010), deformation bands and micro shear zones //(100) and //(010), although rare intra- and intercrystalline microfractures are observed. Stress-induced flame perthites and ‘replacement’ bands are present but uncommon. Grain-size reduction via dynamic recrystallization occurred by two different mechanisms. The recrystallization mechanism switches from subgrain rotation in the wall rocks to low-temperature grain-boundary migration in the shear zones, due to an increase in strain rate. Similarities of the lattices of K-feldspar and plagioclase lead to the suggestion that both minerals deform in a similar style under ‘dry’ eclogite facies conditions. Therefore, we assume that the proven style of deformation is a dominant feature of high-pressure deformation in continental root zones where fluid activity is low.