EXPLORATION OF THE TERNEROS CREEK RHYOLITE DOME ON THE TASCOTAL MESA FAULT, PRESIDIO COUNTY, TEXAS

The Terneros Creek Rhyolite (Pgtc) is a series of lava domes flanking the Solitario laccocaldera in the Big Bend Region of Texas. One dome is located on the property of Midwestern State University’s Dalquest Desert Research Station, 5 km north of Solitario; it covers an area of ~1 km² and is bisected and offset by the east-west trending Tascotal Mesa Fault. This study characterizes the chemical composition and mineral content of this exposure of the Terneros Creek Rhyolite to better constrain its relationship with the strata it intruded. Samples were collected from across the dome and evaluated through XRF, XRD, and optical microscopy.

Based on findings around the northern edge of the dome, the Pgtc was emplaced into a unit composed of a black chert-bearing conglomerate with interbedded lacustrine limestone. Though the fault movement is largely strike-slip, downward movement along the fault has upturned the limestone beds bordering the dome and formed an escarpment. Toward the center of the dome, a wedge of younger strata (the conglomerate-limestone and the overlying Rawls Fm.) is dropped-in where the fault passes through the center of the dome. Samples collected from across the dome vary from grayish-blue, brownish-red, yellow-pink, and white or orange with fracture-controlled, Liesegang-like banding.

Preliminary findings show that composition to be similar across the dome, exhibiting slight variations with proximity to the dome’s margins and to the fault. For example, the XRD only resolved quartz and sanidine within the two marginal samples, whereas two samples located in the interior of the dome contained not only quartz and potassium feldspar, but sodium amphibole. XRF data indicate a range in silica from 72.3 to 73.6 wt.%. Iron (as Fe₂O₃) varied between 2.6 and 3.4 wt.%, with MgO up to 0.14 wt.%. The greatest variation was in interior sample 08, which had the highest MnO at 0.5 wt.% and Na₂O at 4.4 wt.%, and the lowest K₂O at 5.6 wt.%, leading us to believe that the sample is relatively unaltered. Fault-adjacent sample 03 exhibited elevated CaO at 0.46 wt.%. Optical microscopy revealed a flow-fabric within the samples, indicated by an alignment of larger grains. Relatively large (0.5 mm) fluorite grains were a notable and consistent feature of the Pgtc samples.