EXTRACTION OF EBSD QUARTZ DEFORMATION FABRIC DOMAINS IN POLYMINERALIC ROCKS USING THE MTEX TOOLBOX FOR MATLAB

Quartz deformation patterns can be interpreted to reveal information about deformation temperatures, strain rates, and fluid content using optical petrographic microscopy and/or electron backscatter diffraction (EBSD). However, many of the basic assumptions about quartz deformation regimes, and the different quartz c- and a-axis pole figure patterns, are based on a homogenous monomineralic rock. In practice, many rocks are quite heterogeneous, and consequently the way quartz deforms is dramatically affected by the presence of weaker phases which may accommodate much of the strain the rock experienced. Moreover, micas and feldspar may interrupt the interconnectedness of quartz throughout the sample, as well as pin quartz grains and prevent them from moving.

In order to investigate the effects of spatial distribution of quartz within mineralogically heterogeneous samples, we determined a method of extracting different quartz domains. Using the MTEX toolbox in MATLAB, we were able to separate quartz ribbon domains from quartz grains which are pinned or trapped and are consequently unable to migrate. We analyzed over 40 thin sections of metagraywackes, schists, and gneisses from across the Southern Appalachians, which optically provide evidence for grain boundary migration in quartz, and temperatures exceeding 550 °C. These heterogeneous rocks have pockets of quartz which are commonly pinned by micas and are separated from other quartz domains by networks of feldspar. Using our method, we determined that in many cases the quartz ribbons have a strong lattice preferred orientation and an improved signal of deformation patterns, whereas the non-ribbon grains have a diffuse shotgun pattern and contribute little to a meaningful deformation pattern. The pole figures for the entire rock are a composite of these two distinct groups of quartz.