Cathodoluminescence of quartz as an evaluation indicator for mylonitization

Cathodoluminescence (CL) microscopy and spectroscopy provide useful information on existence and distribution of impurity elements related to crystal chemistry and lattice defects produced by destruction or distortion of atomic linkages with a high-spatial resolution, which should be more informative to characterize the crystal-chemical features of minerals. In this study, we have conducted to characterize CL features of the quartz in mylonitic rocks distributed in the Ryoke metamorphic belt along the Median Tectonic Line for the evaluation of mylonitization. Mylonite, protomylonite and their source rocks (Hiji tonalite) were collected from the outcrops around an eastern part of Iida City, Nagano Prefecture. CL imaging was carried out using the Luminoscope with a cooled CCD camera. CL spectra were obtained by an SEM-CL system consisting of SEM with a grating monochromator. CL Spectral data were corrected for total instrumental response. Color CL imaging of the mylonitic rocks shows yellow to khaki for plagioclase, blue for K-feldsper, purple to crimson for quartz and occasionally cream to yellow for minute zircon and apatite. In CL spectroscopy, the quartz gives a broad band at 640-750 nm in a red region and a broad emission band at around 390 nm in a blue region. In the quartz, the CL emission in a blue region around 390 nm can be assigned to structural defects related to Al3+-M+ (M+ is typically Li+, Na+, K+ or H+). Its intensity decreases with the progress of the mylonitization in host rocks. However, the emission intensity in a red region due to structural defects of NBOHC with O3≡Si-O-H precursor (635 nm) and substitutional Fe3+ (750 nm) shows an increase in accordance with the progress of the mylonitization. The intensities of red and blue emissions in the quartz CL show a converse relation depending on the degree of the mylonitization in host rocks. It suggests that the stress-stain due to the mylonitization might cause the dissociation of Al3+-M+ bond, and subsequent the formation of NBOHC with O3≡Si-O-H (H as M+) precursor and the defect related to Fe3+ after substitution of Al3+. Therefore, CL characterization of the quartz could be used for evaluating the mylonitization process.