The influence of grain boundary fluids on the microstructure of quartz-feldspar mylonites

Quartz-rich domains in pre-Alpine, water-deficient, amphibolite facies (510–580 °C, 250–450 MPa), pegmatite mylonites from Mont Mary (MM), western Alps, preserve a fine dynamically recrystallized grain size, without significant annealing, despite the high synkinematic temperatures and subsequent static greenschist facies Alpine overprint. The microstructure is dramatically different from more typical water-rich amphibolite facies mylonites, such as from the Simplon Fault Zone in the central Alps, where the recrystallized grain size is on the millimetre-scale. The difference reflects the dominant strain-induced recrystallization mechanism: (1) progressive subgrain rotation and grain boundary bulging for the dry MM examples; and (2) fast grain boundary migration for the wet Simplon examples. The grain boundary microstructure imaged with SEM is also very different, with most grain boundaries in the dry MM samples lacking porosity, whereas grain boundaries in the wet samples are decorated by a multitude of pores. Quartz grain boundaries from both wet and dry samples are locally coated by thin (100’s of nanometres), possibly amorphous, silica films. Despite the differences in microstructure, the crystallographic preferred orientations (CPOs) of quartz-rich domains from both areas are very similar. Water-deficient conditions hinder grain boundary mobility and thereby modify the dominant recrystallization mechanism(s) but apparently have little influence on the intracrystalline slip systems, as reflected in the CPOs (strong c-axis Y maxima). In MM mylonites, both K-feldspar and plagioclase (An33-38) dynamically recrystallize, consistent with the inferred metamorphic conditions. Under water-deficient conditions, mid- to lower-crustal rocks can deform heterogeneously under transitional ductile-brittle conditions at high differential stress (for MM ca. 300–500 MPa, as estimated for dry Mohr–Coulomb failure) and preserve this high-stress microstructure, because of the low mobility of dry grain boundaries.