TITANIUM IN QUARTZ GRAINS WITHIN A RHYOLITE DIKE FROM THE SOUTHERN OKLAHOMA AULACOGEN, MEDICINE PARK, OKLAHOMA

The Eocambrian magmatism associated with the Southern Oklahoma Aulacogen (SOA) is exposed in the Wichita Mountains, the rugged terrain extending from Lawton to near Altus, Oklahoma. The waning stages of igneous activity include shallow dikes of porphyritic rhyolite, exampled by the well-characterized exposure (see O’Donnell, MS Thesis, Missouri Science & Tech) that cuts into SOA granite and meta-rhyolite at Medicine Park.

The rhyolite is characterized by phenocryrsts and monomineralic glomerocrysts of turbid pink alkali feldspar and clear quartz set in a fine-grained matrix of alkali feldspar and quartz. It contains small, but abundant iron oxides (<20 µm)of varying Ti content. It has few other minerals, including altered hornblende and biotite, minor Fe-rich allanite grains, and minor zircon.

Electron microscope cathodoluminescence (CL) of the quartz phenocrysts and glomerocrysts reveals fine oscillatory zoning in blue CL intensity. Grains display subtle higher intensities in cores and near the rims. Grains are variably resorbed, but facets and embayments both exhibit a thin (~20 µm) homogenous rim. Fractures and Fe-rich inclusions are marked with bright red CL. Matrix quartz grains also exhibit fine oscillatory zoning and variable CL intensities. Matrix grains typically show mantles and zones that are marked by bright red CL. The blue intensity changes track with titanium concentrations in the quartz crystals. Electron microanalysis of phenocrysts exhibit average Ti concentrations of 80 ± 5 ppm; with core levels near 100 ppm, dipping to less than 80 ppm for much of the grain, and returning to core-like concentrations near the rim. These are mantled by variable-Ti rims (65 – 134 ppm). A survey of 21 matrix grains yielded Ti as low as 51 ± 6 ppm to concentrations as high as 212 ± 7 ppm.

Applying the Thomas et al. (2010, CMP, 160) Ti-in-quartz thermobarometer (TitaniQ) produces an average temperature for the phenocrysts of 623 ± 20ºC, assuming a P=0.2 GPa and an TiO₂ activity of 0.51. The thin post-resorption rim temperatures range from 550-625 ºC, at 0.1 GPa and the same αTiO₂. The groundmass Ti variation produces comparably inconsistent TitaniQ estimates, from 529 to 676 ºC (mean = 601 ± 20 ºC). Despite the variability, the results suggest similar crystallization temperatures for phenocrysts and matrix quartz.