Thermometry of quartz mylonites: Importance of dynamic recrystallization on Ti-in-quartz reequilibration

Quartz mylonites from the Tonale Fault Zone in the Alps (northern Italy) have been investigated by the Ti-in-quartz geothermometer (TitaniQ) in order to test its applicability to measure deformation temperatures. The eastern part of the Tonale Fault Zone was contact metamorphosed by the synkinematic intrusion of the Adamello pluton, forming an similar to 800 m wide mylonitic shear zone, with a synkinematic temperature gradient from similar to 280 degrees C at the frictional-viscous transition to similar to 700 degrees C at the pluton contact as derived from metamorphic mineral assemblages. Deformation microstructures from quartz mylonite samples, systematically collected across the mylonitic shear zone, display the entire range of dynamic recrystallization in quartz, which comprise bulging recrystallization (BLG), subgrain rotation recrystallization (SGR), and grain boundary migration recrystallization (GBM). TitaniQ geothermometry yields the near-peak deformation temperature for quartz mylonites deformed at metamorphic temperatures above similar to 540 degrees C in the zone of GBM. However, for mylonites formed under lower temperatures in the zones of SGR and BLG, the preexisting Ti concentrations were not reset. It is suggested that this is due to the sluggish Ti volume diffusion rates below 500 degrees C and the short duration of contact metamorphism and deformation. Even in the higher temperature samples the reequilibration of Ti-in-quartz content was achieved by grain boundary migration rather than by volume diffusion. Hence, our results show that GBM is crucial for the reequilibration of Ti-in-quartz, while quartz mylonites deformed by either BLG or SGR, which predominate in natural shear zones at greenschist facies metamorphic conditions, most likely yield inherited temperatures.