THE PETROGRAPHY AND GEOCHEMISTRY OF THE SOUTH RIM FORMATION, BIG BEND NATIONAL PARK, TEXAS

The South Rim Formation consists of metaluminous to peralkalic quartz trachyte, rhyolite, and high-silica rhyolite lavas, lava domes, ash-flow tuffs, and related intrusive rocks. These are the products of numerous volcanic vents that erupted about 33-32 Ma and 29-28 Ma throughout Big Bend National Park, Texas.

The Boot Rock and Pine Canyon Rhyolite Members range in SiO₂ from 62 to 72%. Variation diagrams demonstrate distinct decreases in the major-element oxides, Sr, Ba, and Eu with increasing SiO₂. These trends are consistent with fractional crystallization of an assemblage dominated by alkali feldspar; additional support for this model is supported by the abundance of euhedral alkali feldspars in all samples and by trace-element and major-element mass balance modeling.

The Burro Mesa Rhyolite and Emory Peak Rhyolite Members all have greater than 72% SiO2. For a given concentration of SiO₂, the Burro Mesa Rhyolite is more strongly enriched in FeO, K₂O, Rb, Y, Zr, Nb, and Th than the Emory Peak Rhyolite. Despite varying chemical trends, both of these rhyolites are petrographically similar, consisting of aphyric to alkali feldspar and quartz phyric comendite.

Based on the trace-element geochemistry, it is very unlikely that these high silica rhyolites evolved from the same quartz trachyte-rhyolite suite via fractional crystallization. These data in conjunction with the younger age for the Burro Mesa Rhyolite (28-29 Ma) suggest that this unit—although petrographically very similar—is not related to any other rocks in, and might be better excluded from, the South Rim Formation.