APPLICATION OF SCANNED COLOR CATHODOLUMINESCENCE OF QUARTZ TO PROVENANCE INVESTIGATIONS OF SHALES AND MUDSTONES

A wide range of primary source rocks, igneous, metamorphic, and volcanic, was examined with a scanned color cathodoluminescence detector (GATAN Chroma CL) in order to define characteristic textures as well as colors for provenance studies of fine grained clastics. We find that although textural details, such as healed fractures, mottling, oscillatory zoning, and cataclasic texture, may be too small to be reliably identified in quartz grains in shales (often only a few ten microns in size), CL color is nonetheless a good guide to provenance. For example, low grade metamorphic quartz typically luminesces in the red-orange region of the spectrum, whereas grains derived from high grade metamorphic rocks (gneiss) luminesce bright bluish-green. Quartz grains that are derived from intermediate metamorphic grades (schists) are characterized by dull colors ranging from dull reddish-brown to dull bluish-purple. Both volcanic and plutonic (granite) derived quartz grains show intense blue CL colors, but distinguishing textures are common enough in the silt range to allow differentiation. Plutonic quartz silt tends to have small healed fractures of low CL intensity, whereas volcanic quartz lacks this feature. Textural features as well as CL color characteristics survive the cycle of weathering, transport and reburial. Because we observed within quartz grains core regions of different color and texture, it appears that additionally, CL based provenance fingerprints can survive into the realm of low grade metamorphism. Because quartz grains are ubiquitous in shales and mudstones, and because scanned color CL surveys can readily be conducted on polished thin sections, it represents a very rapid method for provenance assessment of shales and mudstones.