Linked microstructural and geochemical evolution of mylonitic quartzite during exhumation of a core complex

Abstract

A novel method for combining microstructural analysis and Ti-in-quartz measurements is used to characterize interactions between quartz deformation and Ti equilibration during progressive deformation and cooling in a mylonitic shear zone. Mylonitic quartzite from the Wildhorse detachment, the major bounding structure of the Pioneer metamorphic core complex (Idaho, USA), contains a continuum of microstructures: minimally deformed relict grains, quartz ribbons, and a matrix dominated by subgrain rotation recrystallization. EBSD and semi-quantitative Ti mapping via EPMA and cathodoluminescence imaging are used to correlate quartz microstructure and Ti concentration [Ti]. The Ti-in-quartz thermobarometer is used to estimate P-T conditions during and prior to mylonitization. Relict and ribbon quartz preserve high [Ti] (∼60 ppm) indicative of temperatures >650 °C, whereas recrystallized quartz contains low [Ti] (<5 ppm), reflecting equilibration to temperatures <450 °C. The high-T record may reflect either relict conditions prior to detachment deformation or conditions that prevailed in the early stages of strain localization in the detachment, whereas the low-T record likely reflects the final stages of ductile deformation. The preservation of multiple microstructures with distinct [Ti] facilitates construction of a multi-phase history of linked ductile deformation and Ti equilibration during cooling.