THE PINE CANYON RHYOLITE: A NEWLY RECOGNIZED OCCURRENCE OF PANTELLERITE IN BIG BEND NATIONAL PARK, TEXAS

The Pine Canyon Rhyolite (PCR) represents the caldera-forming ignimbrite of the Pine Canyon caldera in the eastern Chisos Mountains in Big Bend National Park, Texas. It was first described by Maxwell et al. (1967), who named it the “Brown Rhyolite” and defined it as the lowest member in the South Rim Formation. We and other previous workers have described the Pine Canyon caldera as a downsag ash-flow tuff caldera approximately 6-7 km in diameter that experienced considerable post-collapse trachytic to comenditic volcanism. Post-collapse volcanism occurs both as maar-type surge deposits, block-and-ash flow tuff deposits, and lava domes that erupted within and along the margin of the caldera and as ash-flow tuffs, lava flows, and lava domes that erupted outside of the caldera. The caldera did not experience resurgent doming. The Pine Canyon rhyolite has an estimated volume of >9.5 km3. These characteristics are typical of calderas associated with strongly peralkalic magmatism.

The PCR pantellerite has an agpaitic index (mol Na+K/Al) of 1.21, which is less than values reported for samples from Pantelleria (1.34-1.99) but is similar to the Gomez Tuff (1.18-1.38), the only other recognized pantellerite in Trans-Pecos Texas. The trace element geochemistry of the PCR is very similar to type pantellerite. PCR is characterized by an extreme LREE/HREE enrichment (normalized Ce/Yb=8.57), which is slightly higher than type pantellerite (n-Ce/Yb=6.75 – 7.91). PCR has a more pronounced negative Eu anomaly (Eu/Eu*=0.16) than does type pantellerite (Eu/Eu*=0.33 – 0.53); this is most likely because type pantellerite is more strongly alkalic than PCR pantellerite. Incompatible trace element concentrations for Zr (1850 ppm), Nb (319 ppm), and Th (50 ppm) are similar to type pantellerite (822 – 2168 ppm Zr, 253-520 ppm Nb, 17 – 50 ppm Th), as are Rb/Sr ratios (54 for PCR, 22 – 61 for Pantelleria).