Crystallographic control and texture inheritance during mylonitization of coarse grained quartz veins
Abstract Quartz veins within Rieserferner pluton underwent deformation during post-magmatic cooling at temperature around 450°C. Different crystallographic orientations of cm-sized quartz vein crystals conditioned the evolution of microstructures and crystallographic preferred orientations (CPO) during vein-parallel simple shear up to high shear strains (γ≈10). For γ glide in ribbons with c-axis sub-parallel to the shear zone vorticity Y-axis. Ribbons with c-axis orthogonal to Y (XZ-type ribbons) were stronger and hardened more quickly: they show lower aspect ratios and fine (grain size ~10–20μm) recrystallization along sets of microshear zones (μSZs) exploiting crystallographic planes. Distortion of XZ-type ribbons and recrystallization preferentially exploited the slip systems with misorientation axis close to Y. New grains of μSZs initiated by subgrain rotation recrystallization (SGR) and thereupon achieved high angle misorientations by a concurrent process of heterogeneous rigid grain rotation around Y associated with the confined shear within the μSZ. DauphinE twinning occurred pervasively, but did not play a dominant role on μSZ nucleation. Recrystallization became widespread at γ>2 and pervasive at γ≈10. Ultramylonitic quartz veins are fine grained (~10μm, similar to new grains of μSZ) and show a CPO banding resulting in a bulk c-axis CPO with a Y-maximum, as part of a single girdle about orthogonal to the foliation, and orientations at the pole figure periphery at moderate to high angle to the foliation. This bulk CPO derives from steady-state SGR associated with preferential activity, in the different CPO bands, of slip systems generating subgrain boundaries with misorientation axes close to Y. The CPO of individual recrystallized bands is largely inherited from the original crystallographic orientation of the ribbons (and therefore vein crystals) from which they derived. High strain and pervasive recrystallization were not enough to reset the initial crystallographic heterogeneity and this CPO memory is explained by the dominance of SGR. This contrast with experimental observation of a rapid erasure of a pristine CPO by cannibalism from grains with the most favourably oriented slip system under dominant grain boundary migration recrystallization. Highlights Variably oriented quartz vein crystals show different strength during shear at 450°C. Prism was the most efficient slip system. The grain size of new grains is independent on bulk strain and microstructure. High strain and complete recrystallization did not erase the initial vein CPO. The dominant misorientation axis of new grains coincides with the vorticity axis.
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