Implications of crystallographic preferred orientations in quartz mylonites

Quartz crystallographic preferred orientations (CPO), often reduced to solely inspecting the (0001) polefigure, are frequently used to deduce finite strain type, shear sense and deformation temperature of deformed rocks. While the direct dependency of CPO and deformation temperature can actually be rejected (Kilian & Heilbronner, 2017), a so-called opening-angle thermometer is still commonly applied. Using various datasets of natural quartz mylonites formed under different metamorphic conditions, we strongly suggest that a finite, steady-state quartz CPO is the result of mostly two independent, texture-forming deformation processes, dislocation glide (strain-dependent) and dynamic recrystallization by oriented nucleation and growth (stress-dependent). Those processes occur simultaneously and their relative contributions depend on factors such as strain rate, deformation temperature and grain boundary mobility, amongst others. In combination with flow kinematics, the contributions of both texture-forming processes determine the finite CPO. The quartz CPO features are at most only indirectly related to temperature and should not be applied as a thermometer. However, a useful application of quartz CPO includes usage as a simple proxy of rock rheology (i.e. flow at low or high stress conditions), mostly robust against post-kinematic annealing.

Kilian, R., Heilbronner, R., 2017, https://doi.org/10.5194/se-8-1095-2017