TEXTURAL FEATURES OF METAMORPHIC QUARTZ AS REVEALED BY SCANNED CATHODOLUMINESCENCE (SEM-CL): POTENTIAL USE IN PROVENANCE STUDIES

Monotonous under the petrographic microscope, quartz grains show a range of textural features when examined by SEM-CL. Preliminary studies have shown that quartz grains from different sources (volcanic, plutonic, metamorphic) differ in appearance when examined by SEM-CL. In this study, metamorphic source rocks, soils, and stream sediments from the Llano Uplift in Texas were surveyed for SEM-CL features, with the objective to ultimately use the results for provenance studies. The textures observed in the metamorphic rocks under study (schist and gneiss) include: (1) low to moderate intensity mottled/patchy CL with smooth gradients between areas of different intensity; (2) semi parallel bands/lineations of differing CL intensity; (3) cataclasic texture reflecting fragmentation and subsequent cementation/healing, and (4) low intensity (dark) lineations. Texture (1) is interpreted as a result of metamorphic annealing that obliterated pre-existing CL features; texture (2) is considered a reflection of slip/gliding during deformation at high pressures and temperatures; and deformation at somewhat lower P/T conditions deformation may have produced the fracturing seen in texture (3). Field relationships suggest that the dark lineations of texture (4) originated as fractures related to stress caused by granitic intrusions, and were healed with quartz deposited by hydrothermal fluids. These features survive without alteration into soils and stream sediments, but their recognition is grain-size dependent. Sand size or larger quartz grains typically allow identification of the metamorphic source, whereas at coarse silt size approximately 50% of the quartz grains can still be identified as metamorphic. At finer grain sizes the proportion of identifiable grains drops sharply. Because quartz is chemically and mechanically very resistant, as well as ubiquitous in most sediments, understanding SEM-CL features from different source rock types has the potential to greatly aid provenance determination.