Strain localization in brittle–ductile shear zones: fluid-abundant vs. fluid-limited conditions (an example from Wyangala area, Australia)

Abstract

Abstract. This study focuses on physiochemical processes occurring in a brittle-ductile shear zone at both fluid-present and fluid-limited conditions. In the studied shear zone (Wyangala, SE Australia), a coarse-grained two-feldsparquartz-biotite granite is transformed into a medium-grained orthogneiss at the shear zone margins and a fine-grained quartz-muscovite phyllonite in the central parts.The orthogneiss displays cataclasis of feldspar and crystalplastic deformation of quartz. Quartz accommodates most of the deformation and is extensively recrystallized, showing distinct crystallographic preferred orientation (CPO). Feldspar-to-muscovite, biotite-to-muscovite and albitization reactions occur locally at porphyroclasts' fracture surfaces and margins. However, the bulk rock composition shows very little change in respect to the wall rock composition. In contrast, in the shear zone centre quartz occurs as large, weakly deformed porphyroclasts in sizes similar to that in the wall rock, suggesting that it has undergone little deformation. Feldspars and biotite are almost completely reacted to muscovite, which is arranged in a fine-grained interconnected matrix. Muscovite-rich layers contain significant amounts of fine-grained intermixed quartz with random CPO. These domains are interpreted to have accommodated most of the strain. Bulk rock chemistry data show a significant increase in SiO 2 and depletion in NaO content compared to the wall rock composition.We suggest that the high-and low-strain microstructures in the shear zone represent markedly different scenarios and cannot be interpreted as a simple sequential development with respect to strain. Instead, we propose that the microstructural and mineralogical changes in the shear zone centre arise from a local metasomatic alteration around a brittle precursor. When the weaker fine-grained microstructure is established, the further flow is controlled by transient porosity created at (i) grain boundaries in fine-grained areas deforming by grain boundary sliding (GBS) and (ii) transient dilatancy sites at porphyroclast-matrix boundaries. Here a growth of secondary quartz occurs from incoming fluid, resulting in significant changes in bulk composition and eventually rheological hardening due to the precipitation-related increase in the mode and grain size of quartz. In contrast, within the shear zone margins the amount of fluid influx and associated reactions is limited; here deformation mainly proceeds by dynamic recrystallization of the igneous quartz grains.The studied shear zone exemplifies the role of syndeformational fluids and fluid-induced reactions on the dominance of deformation processes and subsequent contrasting rheological behaviour at micron to metre scale.

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

“…The occurrence of isolated quartz grains dispersed in the feldspathic matrix is consistent with precipitation in cavities generated during viscous grain boundary sliding (Fusseis et al, 2009;Menegon et al, 2015;Spruzeniece and Piazolo, 2015). Most of the feldspar grain boundaries appear pitted in the BSE images (Figs.…”

“…We attempted a more quantitative estimate of the P , T conditions using the two-feldspar thermometry (Stormer, 1975;Whitney and Stormer, 1977). However, given the ultrafine grain size of the recrystallized feldspar and the analytical limitations outlined in Sects.…”

Mid-crustal shear zone development under retrograde conditions: pressure–temperature–fluid constraints from the Kuckaus Mylonite Zone, Namibia

“…The three samples fall on a broad compositional trend, manifested by increases in Si and Na coupled to decreases in Fe, Mg and Ca, indicating a degree of open-system behaviour during their KMZ-related metamorphic history (e.g. Fyfe et al, 1978;Spruzeniece and Piazolo, 2015). Sample KMZ28 is largely unaffected by shearing and is also the least altered, whereas KMZ30 is pervasively recrystallized, mylonitised and also the most strongly metasomatized.…”

“…Therefore it seems equally or more likely that a new shear zone within a dry granulite terrane should initiate either along an existing well-oriented fabric (Worley and Wilson, 1996;Montesi, 2013;Rennie et al, 2013), or around a stress riser such as a pretectonic granitic intrusion (Goodwin and Tikoff, 2002;Rennie et al, 2013). Spruzeniece and Piazolo (2015) describe a similar example of deformation localization into a metre-scale fine-grained ultramylonite with increased metasomatic Si content and suggest that fluids were required to initiate metasomatism and localized deformation, but were introduced through brittle precursor fracture systems. We do not see such definite evidence for a brittle precursor, but also prefer a conclusion where deformation localization allowed localized fluid flow, either through a brittle precursor or activation of existing fabrics, locally or further afield.…”

Multiphysics Modeling of a Brittle‐Ductile Lithosphere: 1. Explicit Visco‐Elasto‐Plastic Formulation and Its Numerical Implementation

“…An additional limitation of a purely viscous approach to the formation of shear bands relates to the viscosity contrast between the highly deformed band and the surrounding matrix. Such a gradient will induce overpressure within the band, which will favor fluid to escape into the matrix contrasting the presence of channelized fluid as commonly observed within mylonitic shear zones (Bauer et al, 2000;Spruzeniece & Piazolo, 2015;White et al, 1980).…”