Crystallographic preferred orientations (CPOs) are widely used to infer deformation conditions in the ductile crust and mantle. However, the effects of secondary phases on CPO evolution are not fully understood, despite the compositional diversity of Earth’s lithosphere. Here we examine the role of plagioclase on the evolution of quartz CPOs through general shear experiments at 900 °C and 1.1 GPa. Pistons featuring an “asperity” on the shear plane were used, providing a novel way of visualizing CPO evolution under increasing shear strains and with material flow path variations. With the addition of albite, grain boundary pinning inhibits a transition from basal to prism slip, which is observed in single-phase quartz aggregates deformed under the same conditions. Simultaneously, crystallographic axes are modified by rotations around the kinematic vorticity axis, resulting in pronounced weakening of [c]-axis CPOs in the two-phase experiments, where quartz [c]-axes are oriented perpendicular to the vorticity axis. After only modest amounts of deformation (shear strain, γ < 5), CPOs in the single- and two-phase materials become markedly different. These findings highlight complications in using CPOs to infer deformation conditions in well-mixed polymineralic mylonites typical of most lithospheric shear zones.

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