CATHODOLUMINESCENCE IMAGING OF HIGH-GRADE MICROSTRUCTURES IN QUARTZ, CENTRAL GNEISS BELT, ONTARIO, CANADA

Cathodoluminescence (CL) has been used to image rocks from the Parry Sound domain (PSD) in the Central Gneiss Belt of the Grenville Orogen. The domain reached granulite facies conditions; amphibolite facies shear zones along the domain margins resulted from hydration and retrogression. The samples analyzed in this study are (a) protolith-shear zone pairs from meter-scale structures and (b) from the Parry Sound and Twelve Mile Bay shear zones that bound the PSD. The microstructural features revealed by CL in the PSD are dark within otherwise brightly luminescing quartz grains; the most ubiquitous features include the following. (1) Wide (~25-100μm), diffuse-edged mantles within grain boundaries; commonly these will extend into grains as fingers or wedges. (2) Straight or gently curved lines that vary in thickness from ~2μm to diffuse-edged lines ~20μm that either (a) extend across grains or (b) start at the grain boundary and terminate mid-grain. Some of these lines correspond to fluid inclusion trails. (3) Finer (~2-5μm) branched lines that are curved and/or sinuous; these typically wind across entire grains and commonly form closed loops. (4) Large portions of grains, or entire grains, with low luminescence; samples from km-wide shear zones have pervasively low luminescing quartz with very few distinguishable features. CL is paired with optical observations, analysis of water species content by Fourier transform infrared spectroscopy (FTIR) and analysis of titanium content by electron microprobe. In this study, we compare features in quartz from high-grade rocks in different deformational regimes in order to constrain the microstructural history as it relates to rheological evolution of the PSD.