THE EFFECT OF DEFORMATION ON ACCESSORY PHASE DISTRIBUTION IN A QUARTZITE MYLONITE

In extensional shear zones bounding metamorphic core complexes, strong crystal fabric anisotropy and pervasive hydrothermal circulation creates a dynamic environment for element transport. In this study, we focus on the distribution of accessory phases because their presence is necessary for application of trace element thermobarometric techniques that rely on estimating trace element activity. To understand how accessory phases relate to trace element abundance in quartz, we document changes in distribution, modal percent, and geometry of accessory phases (rutile, zircon, monazite, ilmenite, apatite, others) in a suite of quartzite mylonite representing different structural levels beneath a detachment.

Samples analyzed are 4 quartzite mylonites and 1 ‘protolith’ quartzite collected along a ~1000 m section of the ductilely-deformed footwall of the Shuswap metamorphic core complex, SE British Columbia. Samples range from coarse-grained quartzite in the deepest levels of the traverse to finely recrystallized quartzite mylonite directly below the detachment fault. Previous work determined quartz trace element composition, quartz microstructures, and O and H stable isotope values on co-existing quartz-white mica pairs (thermometry). To investigate phase geometry in quartzite mylonite, we applied X-ray Computed Tomography (XRCT). Images were processed to extract quantitative data for mode, and geometry orientation of accessory phases. Phase distribution was analyzed using electron microprobe EDS mapping via the distribution of major and trace elements (Si, Al, Fe, Ca, Ti, Zr, Ce, P). Finally, EBSD textural analysis was performed to measure the crystallographic orientation of rutile.

Results show that with increasing deformation and fluid-rock interaction, accessory phases decrease in abundance and size. Multiple techniques document development of shape preferred orientation. From protolith to most deformed, the mean distance between accessory phases nearly doubled. This has direct implications for the rock buffering capacity and for determination of trace element activity for the application of trace element thermobarometry. This study demonstrates that deformation and fluid interaction affect accessory phase constituents in deformed rocks.