In shear zones of the continental crust, phyllosilicates are often linked to mechanical weakening, yet their influence on rock strength and weakening mechanisms remains poorly constrained. This study investigates deformation mechanisms in biotite/phlogopite-quartz assemblages with varying mica content, deformed at 800°C, 1.0 GPa in simple shear using a modified Griggs apparatus.

Increasing mica content reduces the strength of the assemblage at quasi-steady-state conditions, but not linearly. A major weakening occurs at ~30 % vol. phlogopite, reducing viscosity by a factor of ~6, marking the transition from a quartz-supported framework to a mica-interconnected structure.

All samples exhibit grain size reduction of one to two orders of magnitude with recrystallized quartz, and phlogopite forming grains as small as 100–200 nm. Cathodoluminescence imaging reveals that increasing mica content reduces recrystallized quartz fraction and aspect ratio while broadening the quartz grain size distribution. In quartz-rich samples, dynamic recrystallization dominates, yet weak crystallographic preferred orientation and randomly oriented grains suggest solution-transfer processes are active even at 10 % vol. mica.

Phlogopite exhibits limited intracrystalline plasticity, with deformation accommodated primarily by kinking and dissolution-precipitation, as indicated by nanoscale mica grains in quartz-quartz grain boundaries. Additionally, mica composition may influence strength: fluorine-rich phlogopite samples are systematically stronger than biotite or non-F-phlogopite-bearing assemblages.

These findings highlight the dual role of phlogopite in shear zone rheology: (1) facilitating strain partitioning into weak phlogopite layers, reducing quartz deformation and recrystallization, and (2) promoting solution-transfer processes over crystal plasticity. Understanding the interplay of all these mechanisms is critical for refining models of polymineralic rock rheology.