Extreme ductility of feldspar aggregates—Melt‐enhanced grain boundary sliding and creep failure: Rheological implications for felsic lower crust

High‐grade orthogneisses from granulite‐bearing lower crustal unit show extreme finite strains of both K‐feldspar and plagioclase with respect to weakly deformed quartz aggregates. K‐feldspar aggregate in the most intensely deformed sample shows interstitial grains of quartz and albite, which also mark some intragranular fractures within K‐feldspar grains. Both interstitial grains and fractures are oriented mostly perpendicular to the sample stretching lineation. Quartz and albite grains within K‐feldspar bands are interpreted as crystallized from interstitial melt and the petrology study shows that the melt was produced by a metamorphic reaction in plagioclase‐mica bands. Thermodynamic Perple_X modeling shows that melt volume increase was negligible and melt amount was too small to generate considerable melt overpressure for calculated PT conditions. It is therefore suggested that dilation of K‐feldspar aggregates and fracturing of its grains represent a final creep failure state, which resulted from the cavitation process accompanying grain boundary sliding controlled diffusion creep. The consequence of cavitation‐driven dilation of K‐feldspar aggregates is the local underpressure resulting in infiltration of melt from plagioclase bands. Analogy with metallurgy experiments shows that the cavitation process, exclusively developed in cryptoperthitic K‐feldspar, can be attributed to its lower purity compared to more pure plagioclase. Contrasting rheological behavior of feldspars with respect to quartz prior to fracturing is attributed to different deformation mechanisms. Feldspars appear weaker due to grain boundary sliding accommodated by coupled melt‐enhanced diffusion creep along grain boundaries and dislocation creep within grains, in contrast to quartz deforming via grain boundary migration accommodated dislocation creep.