Document Type
Thesis
Abstract
Kyanite quartzite is a minor but distinctive rock type that occurs throughout the Piedmont Province of central Virginia. The origin of these quartzites has been debated, with suggested protoliths ranging from aluminous sandstone to hydrothermally altered volcanic rock. I present new observations and geochemical data bearing on this problem from all significant occurrences of kyanite quartzite in Virginia. The whole-rock dataset consists of 23 samples from 9 separate localities. All quartzites are mineralogically similar, and contain variable amounts of kyanite, white mica, and rutile. Pyrite occurs sporadically, but is locally abundant. Topaz is a notable accessory in some samples, and lazulite occurs in one sample. Major element compositions are dominated by SiO, and A203, and show low, but variable TiO2, Fe2O3-T, and K2O. All other major element oxides occur in negligible amounts. Concentrations of many trace elements (Ni, Co, Zn, Ga, Nb, Y, Rb) are near or below detection limits in most samples. Samples from Madisonville contain extremely high Cr (~1200 ppm), a factor of 2 higher than Cr-rich samples from nearby Baker Mt. All other samples contain negligible Cr. High-Cr rocks contain lower Zr/TiO2 than low-Cr rocks, consistent with contrasting (mafic vs. felsic) igneous protoliths for each type. Rare earth element patterns show a curious “scoop”, with relative depletion of some HREE. The unusually low Ga/AI values displayed by most rocks suggests some mechanism for the preferential removal of Ga relative to Al. Thermodynamic evaluation of numerous potential complexing agents in hydrothermal fluids indicates that S probably plays a key role in Ga removal. A good analogue for the formation of the kyanite quartzite protoliths is represented by epithermal, high-sulfidation alteration systems. Such systems are locally characterized by severe leaching of all soluble elements in hot acidic (pH <2) fluids, which can result in kaolinite-quartz residues (which could be later metamorphosed to kyanite-quartz rocks). In addition, the high activity of sulfate in such systems may enable the removal of Ga relative to Al (and may also cause the scooped REE pattern). Finally, the kyanite quartzites all reside within volcanic arc terranes, the setting for high-sulfidation systems in modern analogues.
Date Awarded
2002
Department
Geology
Advisor 1
Brent E. Owens
Recommended Citation
Pasek, Mathhew Adam, "Regional Geochemical Constraints on the Petrogenesis of Kyanite Quartzites in the Piedmont Province of Virginia and an Evaluation of Anomalous Ga/Al Values" (2002). Geology Senior Theses. William & Mary. Paper 103.
https://scholarworks.wm.edu/geologyseniors/103
