1 Distributed volcanic fields of the southwestern U.S.

This session invites submissions from various disciplines (e.g., petrology, tectonics, geophysics, geochronology) that explore distributed volcanism in the southwestern U.S. Topics of interest include spatio-temporal trends, magma compositions, and mantle and/or crustal controls on volcanism.

Monday, 18 May 2026
8:00 AM - 12:00 PM
Alvarado A

Burro Mesa Rhyolite: A distributed peralkaline silicic volcanic field in Big Bend National Park, Texas

Paper number: 1-6
Program: Technical Sessions
Time: 9:40 AM – 10:00 AM

Authors

Abstract

Burro Mesa Rhyolite (BMR) is exposed over ~100 km2 in western Big Bend National Park. It is composed of lava flows, pyroclastic cones, local ignimbrites and shallow intrusions emplaced during late-stage, residual Laramide compression. Rhyolite occurs in four exposure zones created by Miocene normal faulting and subsequent erosion: Zones 1 and 3 contain the largest exposures of lava, tuff cones and ignimbrites; Zones 2 and 4 are relatively uplifted, more deeply eroded and feature shallow intrusions and lava crater fillings. Estimated total volume of BMR is ~10 km3.

Nine major centers feature maar explosion craters, surrounding tuff rings, and lava. Some comenditic lava, fed from narrow dikes, travelled several kilometers from vents, but many lavas did not escape craters. The largest exposures occur at Burro Mesa proper and Goat Mountain in Zone 1, and the Horseshoe Dome – San Cristobal Dome complex in Zone 3. A local densely-welded ignimbrite up to 40 m thick in Zone 3 overlies Horseshoe Dome lava and may have been erupted from the San Cristobal dome. More deeply eroded centers are Kit Mountain and Trap Mountain in Zone 2 and Cerro Castelan in Zone 4.

BMR occurs in two varieties. Sparsely porphyritic rhyolite (LBM) with phenocrysts of alkali feldspar, fayalite and ilmenite occurs over the outcrop. Abundantly porphyritic rhyolite (UBM), occurring in two chemical varieties, is only found on Burro Mesa proper. Average age for seven LBM samples from previous work is 29.35 + 0.5 Ma. UBM lava has a younger age of 29.06 Ma, consistent with field relations. Trachyte enclaves are common in LBM, suggesting a genetic relationship. Least squares modelling indicates that UBM rhyolite may be derived by extensive fractional crystallization of LBM rhyolite. The second variety of UBM (Ross Maxwell Drive locality) is not peralkaline and distinct from other UBM rhyolite.

Geological Society of America Abstracts with Programs. Vol. 58, No. 4

doi: 10.1130/abs/2026RM-410565

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