EVALUATING SEM-CL TEXTURES IN QUARTZ GRAINS FOR PROVENANCE STUDIES OF MUDSTONES AND SHALES

In SEM-CL, quartz grains from different sources differ distinctly in appearance. Igneous, metamorphic, and volcanic source rocks, soils, and stream sediments were examined for SEM-CL textural elements that can be used in provenance determination.

Plutonic quartz is characterized by overall bright CL, interspersed diffuse streaks of lower intensity CL, and cross-cutting healed fractures filled with low luminescent (black) quartz. Metamorphic quartz mainly shows low to moderate intensity mottled/patchy CL with smooth gradients between areas of different intensity, and cataclasic texture reflecting fragmentation and subsequent cementation/healing. These textures can be overprinted (but not obscured) by low luminescent (black) fracture fill quartz. Oscillatory zoning characterizes volcanic quartz (tends to follow grain outlines unless breakage occurred). Some volcanic quartz simply showed uniform low intensity CL, possibly a result of low temperature formation during post-eruption alteration. Oscillatory banding is also common in quartz from subvolcanic intrusions (has cross-cutting healed black fracture fills in contrast to eruptive volcanic quartz).

Although these textural features of quartz grains survive the cycle of weathering, transport and reburial, source identification is nonetheless grain size dependent. At 250 microns or larger, grains are large enough to show textural features in a clear enough context for reliable source rock identification. At 100 microns diameter, spacing of diagnostic features (oscillatory zoning, fracture density, cataclast size etc.) becomes important. Example: when cataclasis produces large syntectonic fragments, only 50% of quartz grains may still be attributable to a metamorphic source (no clear identification can be made for the remaining grains), whereas with fine fragmentation the success rate may reach 75%. At coarse silt size (63 microns), closely spaced diagnostic elements may permit identification of up to 60% of grains, but wide spacing may lower the success rate to 15% or less. Although mudstones and shales contain an abundance of quartz grains that are too small for reliable provenance identification by SEM-CL, they also contain appreciable quantities of coarse silt size and larger quartz grains that allow for direct prima facie evaluation of provenance.