The influence of grain boundary fluids on the microstructure of quartz-feldspar mylonites Neil S. Mancktelow a, * , Giorgio Pennacchioni b a Geologisches Institut, ETH-Zentrum, CH-8092 Zu ¨rich, Switzerland b Dipartimento di Geologia, Paleontologia e Geofisica, Universita ` di Padova and CNR—Istituto di Geoscienze e Georisorse (Sezione di Padova), Italy Received 15 August 2002; received in revised form 17 April 2003; accepted 24 April 2003 Abstract Quartz-rich domains in pre-Alpine, water-deficient, amphibolite facies (510 – 580 8C, 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 (An 33-38 ) 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. q 2003 Elsevier Ltd. All rights reserved. Keywords: Quartz; Feldspar; Deformation; Microstructure; Mylonites; Crystallographic preferred orientation (CPO); Fluid-rock interaction 1. Introduction Quartz and feldspar are the most common minerals in the crust and overwhelmingly dominate in acidic magmatic rocks. On subsequent deformation under elevated tempera- tures and pressures, these rocks may develop localized shear zones now marked by quartz-feldspar mylonites (e.g. Burg and Laurent, 1978; Lister and Price, 1978; Berthe ´ et al., 1979; Ramsay and Allison, 1979). The deformational microstructure of these two minerals depends primarily on temperature, strain rate and water activity (Jaoul et al., 1984; Kronenberg and Tullis, 1984; Ord and Hobbs, 1986; Tullis and Yund, 1987, 1989; Hirth and Tullis, 1992; Gleason and Tullis, 1995; Kohlstedt et al., 1995; Dunlap et al., 1997; Stipp et al., 2002a,b; and many others). Because the differential stress dependence on temperature is exponential, whereas the dependence on strain rate is linear (Newtonian viscous) or less than linear (power-law creep), temperature is generally considered to have the most important effect. The onset of crystal plastic behaviour in quartz is usually taken to occur around 280–300 8C (e.g. Sibson et al., 1979; Van Daalen et al., 1999, Sto ¨ ckhert et al., 1999) and the transition in dominant dynamic recrystalliza- tion mechanism from subgrain rotation to grain boundary migration generally at around 400 8C (e.g. Urai et al., 1986; Mancktelow, 1990). A considerably higher temperature for this transition at around 550 8C was reported by Stipp et al. (2002a,b), who considered the process in terms of variation in strain rate. The initiation of crystal plastic deformation 0191-8141/03/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0191-8141(03)00081-6 Journal of Structural Geology 26 (2004) 47–69 www.elsevier.com/locate/jsg * Corresponding author. Tel.: þ 41-1-632-3671; fax: þ41-1-632-1030. E-mail address: neil@erdw.ethz.ch (N.S. Mancktelow).