T72 Strain in the Footwalls of Detachment Faults: New Insights on the 40th Anniversary of the Seminal GSA Memoir “Cordilleran Metamorphic Core Complexes”

Tuesday, 27 October 2020
1:30 PM - 5:30 PM
Meeting Rooms

RELATING TI CONCENTRATIONS TO QUARTZ MICROSTRUCTURE IN THE MYLONITIC FOOTWALL OF A DETACHMENT, PIONEER METAMORPHIC CORE COMPLEX, IDAHO

Paper number: T72-5
Program: Technical Sessions
Time: 2:20 PM – 2:35 PM

Authors

Abstract

Quartz is capable of accommodating large strains through ductile deformation and can play a key role in the evolution of detachment shear zones. Microstructural analyses and geochemical techniques, such as Ti-in-quartz thermobarometry, can be used to extract a record of deformation and pressure-temperature conditions from ductilely deformed quartz, providing an exhumation history for the shear zone. In order to investigate the interplay between quartz microstructure and geochemistry during ductile deformation in nature, we sampled mylonitic Kinnikinik quartzite from the footwall of the Wildhorse detachment, the bounding structure of the Pioneer metamorphic core complex. The quartzite contains a foliation and macroscopic stretching lineation parallel to the detachment fault plane and extension direction. The quartz fabric is dominated by three microstructures: minimally deformed relic quartz grains, highly elongate quartz ribbons with evidence for subgrain rotation, and fully recrystallized quartz. Ti concentrations were measured across these microstructures using EPMA and compared to cathodoluminescence (CL) images. A strong correlation between Ti concentration and CL intensity was found and used to create semi-quantitative maps of Ti concentration that clearly reflect quartz microstructure. The highest Ti concentrations (up to 64 ppm) occur in the relic quartz, indicating equilibration at temperatures approaching 650 °C. Similarly high Ti concentrations are also found in quartz ribbons, while recrystallized quartz is characterized by lower Ti concentrations (7-42 ppm) corresponding to equilibration between 380-580 °C. These correlations suggest Ti concentrations are re-equilibrated during recrystallization but not during the development of quartz ribbons, which preserve high-temperature Ti concentrations. We further investigate the relationship between Ti concentration and microstructure through detailed EBSD mapping. Understanding this relationship will allow for a more detailed analysis of the exhumation history preserved in ductilely deformed quartzite as well as insight into how microstructure development and geochemistry interact at the micro scale.

Geological Society of America Abstracts with Programs. Vol. 52, No. 6

doi: 10.1130/abs/2020AM-358835

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