Brittle grain-size reduction of feldspar, phase mixing and strain localization in granitoids at mid-crustal conditions (Pernambuco shear zone, NE Brazil)

Abstract

Abstract. The Pernambuco shear zone (northeastern Brazil)is a large-scale strike-slip fault that, in its eastern segment, deforms granitoids at mid-crustal conditions. Initially coarsegrained (> 50 µm) feldspar porphyroclasts are intensively fractured and reduced to an ultrafine-grained mixture consisting of plagioclase and K-feldspar grains (< 15 µm) localized in C' shear bands. Detailed microstructural observations and electron backscatter diffraction (EBSD) analysis do not show evidence of intracrystalline plasticity in feldspar porphyroclasts and/or fluid-assisted replacement reactions. Quartz occurs either as thick (∼ 1-2 mm) monomineralic veins transposed along the shear zone foliation or as thin ribbons (≤ 25 µm width) dispersed in the feldspathic mixture. The microstructure and c axis crystallographicpreferred orientation are similar in the thick monomineralic veins and in the thin ribbons, and they suggest dominant subgrain rotation recrystallization and activity of prism < a > and rhomb < a> slip systems. However, the grain size in monophase recrystallized domains decreases when moving from the quartz monomineralic veins to the thin ribbons embedded in the feldspathic C' bands (14 µm vs. 5 µm respectively). The fine-grained feldspar mixture has a weak crystallographic-preferred orientation interpreted as the result of shear zone parallel-oriented growth during diffusion creep, as well as the same composition as the fractured porphyroclasts, suggesting that it generated by mechanical fragmentation of rigid porphyroclasts with a negligible role of chemical disequilibrium. Once C' shear bands were generated and underwent viscous deformation at constant stress conditions, the polyphase feldspathic aggregate would have deformed at a strain rate 1 order of magnitude faster than the monophase quartz monomineralic veins, as evidenced by applying experimentally and theoretically calibrated flow laws for dislocation creep in quartz and diffusion creep in feldspar. Overall, our data set indicates that feldspar underwent a brittle-viscous transition while quartz was deforming via crystal plasticity. The resulting rock microstructure consists of a two-phase rheological mixture (fine-grained feldspars and recrystallized quartz) in which the polyphase feldspathic material localized much of the strain. Extensive grain-size reduction and weakening of feldspars is attained in the East Pernambuco mylonites mainly via fracturing which would trigger a switch to diffusion creep and strain localization without a prominent role of metamorphic reactions.

Hierarchical creep cavity formation in an ultramylonite and implications for phase mixing

“…This implies that either the synkinematic viscosity contrast between the polyphase and the quartz domains was small (Smith, 1975;Viegas et al, 2016), or that the quartz layer was not able to achieve localisation because the local temperature fluxes were efficiently dissipated (Peters et al, 2015). Boudinage by either of these processes is considered a ductile instability through which irrecoverable change occurs and grows over time (cf.…”

“…9). This is typically interpreted as indicating a transition from GSI creep to GSS creep accommodated by viscous grain-boundary sliding (Mitra, 1978;Etheridge and Wilkie, 1979;Kerrich et al, 1980;Behrmann and Mainprice, 1987;Závada et al, 2007;Kilian et al, 2011;Herwegh et al, 2014;Menegon et al, 2015;Viegas et al, 2016).…”

Syn-kinematic hydration reactions, grain size reduction, and dissolution–precipitation creep in experimentally deformed plagioclase–pyroxene mixtures

“…The large Burgers vectors in plagioclase are unfavorable for intracrystalline deformation, especially at the comparatively low experimental temperatures of 800 • C. Dislocation glide and climb have been suggested to be active in plagioclase under both natural and experimental conditions (e.g., Tullis and Yund, 1985;Shaocheng and Mainprice, 1987;Rybacki and Dresen, 2000;Shigmeatsu and Tanaka, 2000;Kruse et al, 2001;Lapworth et al, 2002;Stünitz et al, 2003;Ji et al, 2004;Barreiro et al, 2007;Mehl and Hirth, 2008) but usually are not considered to accommodate large amounts of strain. Recrystallization takes place by different mechanisms including neo-crystallization (e.g., Fitz Gerald and Stünitz, 1993;Rosenberg and Stünitz, 2003;Brander et al, 2012;Fukuda and Okudaira, 2013;Mukai et al, 2014) or by growth of fragments formed by fracturing (e.g., Stünitz et al, 2003;Viegas et al, 2016). In fine-grained aggregates, diffusion creep (in the broadest sense), often dissolutionprecipitation creep (DPC), is the main strain-accommodating process described for polycrystalline plagioclase aggregates (e.g., Fitz Gerald and Stünitz, 1993;Jiang et al, 2000;Lapworth et al, 2002;Rosenberg and Stünitz, 2003;Brander et al, 2012;Fukuda and Okudaira, 2013;Mukai et al, 2014;Viegas et al, 2016).…”

“…Recrystallization takes place by different mechanisms including neo-crystallization (e.g., Fitz Gerald and Stünitz, 1993;Rosenberg and Stünitz, 2003;Brander et al, 2012;Fukuda and Okudaira, 2013;Mukai et al, 2014) or by growth of fragments formed by fracturing (e.g., Stünitz et al, 2003;Viegas et al, 2016). In fine-grained aggregates, diffusion creep (in the broadest sense), often dissolutionprecipitation creep (DPC), is the main strain-accommodating process described for polycrystalline plagioclase aggregates (e.g., Fitz Gerald and Stünitz, 1993;Jiang et al, 2000;Lapworth et al, 2002;Rosenberg and Stünitz, 2003;Brander et al, 2012;Fukuda and Okudaira, 2013;Mukai et al, 2014;Viegas et al, 2016).…”

Weak and slow, strong and fast: How shear zones evolve in a dry continental crust (Musgrave Ranges, Central Australia)

“…K-feldspar shows fractures decorated with subgrains (Appendix A2). The clasts are mantled by recrystallized grains with outwardly decreasing grain size down to smaller than 1 micron and limited host control on new grain orientation (Appendix A2; compare Viegas et al, 2016). Quartz recrystallized in ribbons of about 50 micron width by subgrain rotation and subordinate grain boundary migration.…”