ALTERATION OF THE CARLTON RHYOLITE GROUP BY THE WICHITA GRANITE GROUP, EASTERN WICHITA MOUNTAINS, SOUTHERN OKLAHOMA

The Wichita Mountains in southwestern Oklahoma expose Cambrian intrusions of the Wichita Granite Group into the base of the Carlton Rhyolite Group. We examined in-place rhyolite from the Fort Sill flow package to compare to 1.) granite-hosted rhyolite xenoliths, 2.) granite-contacted, largely unaltered rhyolite, and 3.) granite-contacted hornfels (strongly altered felsites mapped as Davidson Metarhyolite). These were characterized through optical and electron microscopy, the latter coupled with EDS microanalysis. As part of optical work, we digitized 19mm² areas of thin sections on a flatbed scanner at 6400 dpi in plane- and cross-polarized light. The plane-polarized images were analyzed in ImageJ and WEKA Trainable AI software to determine the concentrations of quartz, feldspar, and mafic silicates and oxides.

Preliminary results suggest that mineral content (perthite and rounded quartz phenocrysts in a matrix of quartz and perthite, with fine hematite) is similar in all materials. The quartz:feldspar ratio is highest in samples of in-place rhyolite and samples of largely unaltered contact rhyolite. Xenoliths and unaltered contact rhyolite are each indistinguishable in the field from in-place rhyolite, but produce lower quartz modes. Despite obvious alteration-induced differences, Davidson Metarhyolite quartz modes are similar those in the xenoliths. Microstructure varies strongly. Xenolith matrix is coarser than in-place rhyolite. SEM-CL imagery reveals that xenolith quartz phenocrysts are permeated by healed fractures, whereas in place-rhyolite exhibits continuous oscillatory zoning. Metarhyolites are marked by a granoblastic matrix and disaggregated phenocrysts.