NEW 40AR/39AR AGES FROM BIG BEND NATIONAL PARK

Previous geochronological studies of Big Bend National Park suggest that volcanic activity started around 47 Ma. However, with limited K/Ar and 40Ar/39Ar ages obtained for the Park, it is difficult to address the overall timing of volcanism, as well as the emplacement of fayalite granites and other igneous intrusions in the region.

This study presents 7 new 40Ar/39Ar ages for various rhyolites and ash-flow tuffs. Four sanidine ages from 3 rhyolites and 1 trachyte dike from the South Rim Formation in the Chisos Mountains yield ages ranging from 32.17 to 31.93 Ma. The other three sanidine ages are from ash-flow tuffs: Welded tuff from Cerro Castellan (CC) at 28.96±0.14 Ma, Burro Mesa Rhyolite (BMR) collected due east of CC at 29.23±0.14 Ma, and BMR from Burro Mesa Pouroff at 29.40 ±0.17 Ma. Of particular importance to volcanic activity in the Park is the eruption of the BMR, because there are multiple lava flows and eruptions sources that provide a precise stratigraphic marker for unraveling the volcanic stratigraphy. However, BMR type samples from Emory Peak, Cerro Castellan, and Upper Burro Mesa yield ages of 31.93±0.13 Ma, 28.3±0.1 Ma, and 29.40±0.17, respectively, indicating that the BMR erupted over a much broader time period than previously thought.

Continuing geologic mapping and geochronology studies on the BMR, ash-flow tuffs, dikes, sills, basalts, lava flows, and intrusive bodies will help resolve the volcanic and intrusive evolution for Big Bend National Park and its genetic relationship to the Trans-Pecos volcanic field.