Quartz microstructures and crystallographic preferred orientation: Which shear sense do they indicate?

Abstract

Crystallographic preferred orientation (CPO) and shape fabrics of dynamically recrystallized quartz are currently used as shear sense indicators. We show that the CPO and shape fabric in a polyphase rock do not necessarily indicate the shear sense at the global scale of the shear zone but rather at the local scale of the deforming and recrystallizing quartz aggregates. In lower amphibolite facies shear zones in the Gran Paradiso metagranodiorite, magmatic quartz grains have recrystallized dynamically by subgrain rotation and grain boundary migration and deform inside a very fine-grained feldspar–mica matrix. The quartz CPO has a peripheral [c]-axis maximum inclined synthetically with the local shear sense. The orientation of the surface fabric is related to the orientation of the [c]-axis maximum and the microscopic shear sense in the quartz aggregate. The geometry of the surface fabric ranges from monoclinic to symmetric depending on the relative contribution of grain boundary migration. It is inferred that flow partitioning between the quartz aggregates and the matrix controls the local kinematics. CPOs are only reliable shear sense indicators in polyphase rocks if the most highly strained parts are analyzed where spin of the aggregates with respect to the shear zone boundary has ceased.