ANALYSIS OF PHENOCRYST TEXTURES AND SIZE DISTRIBUTION, CARLTON RHYOLITE, WICHITA MOUNTAINS, SOUTHWESTERN OKLAHOMA

This study reports on the textural and mineralogical character of Carlton Rhyolite cropping out on Pratt Hill and near the base of Mount Scott in the Wichita Mountains, Oklahoma. The brick red, highly fractured rhyolite is distinctly porphyritic (~25-35% phenocrysts). Phenocrysts of Qtz (50-65%) and Ksp (35-50%) are set in a fine-grained granular matrix of Qtz and Ksp. Minor opaque minerals (<3%) are typically associated with Ksp phenocrysts. Phenocrysts vary from euhedral, planar crystals to embayed, anhedral crystals, with less than 1% of crystals forming clusters. The abundance of Qtz phenocrysts decreases from ~19% to ~16% and Ksp phenocrysts increases from ~9% to ~17% moving up the stratigraphic section. In lower sections of the formation, small Qtz phenocrysts (~0.75mm) are rare and larger crystals (up to ~2.2mm) are extensively embayed. Throughout the section, resorbed Qtz phenocrysts are ubiquitous, where embayed Ksp phenocrysts are less common. Rare flow-banding is present low in the section and has yet to be observed higher in the section. Qtz phenocrysts in the upper section of the formation exhibit extensive brittle fracturing, whereas within Ksp phenocrysts fractures may diffuse to a braided pattern and reconnect with fractures in the matrix on the other side of the crystal, rather than forming a single transgranular fracture. Preliminary Crystal Size Distribution analysis of the Qtz and Ksp phenocrysts across the stratigraphic section yielded roughly concave up graphs, although reliability is suspect due to small sample size. Ksp had relatively sparse nucleation at depth and slower crystallization; whereas Qtz had denser nucleation and crystallized more rapidly, leading to less euhedral crystals. The anhedral, embayed texture of the phenocrysts suggests resorption during magma ascent, with Qtz undergoing more resorption than Ksp. The variation in modal abundance also seems to suggest a zoned magma chamber, with the top being rich in Qtz, grading into a Ksp-rich bottom. The fractured nature of the phenocrysts is consistent with deformation during a later tectonic event, rather than during emplacement and suggests deposition as a flow, rather than as an explosive pyroclastic airfall.