Evolution of microstructure and melt topology in partially molten granitic mylonite: Implications for rheology of felsic middle crust

摘要

The deformation study of midcrustal porphyritic granite reveals exceptionally high strain intensities of feldspar aggregates compared to stronger quartz. Three types of microstructures corresponding to evolutionary stages of deformed granite were recognized: (1) the metagranite marked by viscous flow of plagioclase around strong alkali feldspar and quartz, (2) quartz augen orthogneiss characterized by development of banded mylonitic structure of recrystallized plagioclase and K-feldspar surrounding augens of quartz, and (3) banded mylonite characterized by alternation of quartz ribbons and mixed aggregates of feldspars and quartz. The original weakening of alkali feldspar is achieved by decomposition into albite chains and K-feldspar resulting from a heterogeneous nucleation process. The subsequent collapse of alkaline feldspar and development of monomineralic layering is attributed to the onset of syn-deformational dehydration melting of Mu-Bi layers associated with production of ～2% melt. The final deformation stage is marked by mixing of feldspars which is explained by higher melt production due to introduction of external water. An already small amount of melt is responsible for extreme weakening of the feldspar because of Melt Connectivity Threshold effect triggering grain boundary sliding deformation mechanisms. The grain boundary sliding controls diffusion creep at small melt fraction and evolves to particulate flow at high melt fractions. Strong quartz shows a dislocation creep deformation mechanism throughout the whole deformation history marked by variations in the activity of the slip systems, which are attributed to variations in stress and strain rate partitioning with regard to changing rheological properties of the deforming feldspars.