VOLCANOLOGY AND PETROLOGY OF BURRO MESA RHYOLITE, SOUTHWESTERN BIG BEND PARK, TEXAS

Burro Mesa Rhyolite sensu stricto crops out discontinuously within a 16 by 8 km area in fault blocks alongside Ross Maxwell Drive. Domes and thick flows of comendite were erupted from eight known centers; others are suspected. Centers began with surge and ignimbrite eruptions forming tuff rings, which culminated with the issue of lava flows up to 150 m thick that travelled up to several km from their vents. Surge and ignimbrite deposits are normally nonwelded, except where heat was available from overlying lava. One center, Cerro Castellan, has an exposed feeder dike that cut upwards through pyroclastic deposits and expanded to produce a volcanic dome. The nearby Horseshoe Canyon center erupted the spectacular pyroclastic deposits of Tuff Canyon, a well-known exhibit of the park. Within Horseshoe Canyon, thick rhyolite lava is exposed in canyon walls, revealing basal breccia, recumbent folds, steeply-dipping ramp structures, and upper breccia.

Burro Mesa Rhyolite occurs in two major varieties: widespread sparsely-porphyritic and less widespread, abundantly-porphyritic, peralkalic rhyolite, the latter only known from Burro Mesa proper. Trace element modeling suggests that fractionation of 15-27 weight percent of observed phenocrysts (anorthoclase, quartz, fayalite) could generate the more evolved abundantly-porphyritic rhyolite from parental sparsely-porphyritic rhyolite. Small enclaves of mafic trachyte occur within several flows, but their interaction with enclosing silicic magma has rendered petrologic modeling of deeper parts of the magma systems difficult.

Detailed mapping of the Cerro Castllan quadrangle and surrounding areas should provide added insight into the volcanic history of the park, enriching visitors and geologists alike.