NEW CONSTRAINTS ON THE TIMING OF VOLCANISM AND ASSOCIATED INTRUSIVE ROCKS IN BIG BEND NATIONAL PARK USING 40AR/39AR AND U-PB SHRIMP GEOCHRONOLOGY

This study presents 19 new 40Ar/39Ar ages for volcanic rocks associated with the Chisos and South Rim Formations and four new U-Pb SHRIMP zircon ages for three fayalite-bearing intrusions and one gabbro sill from Big Bend National Park (BBNP).

Recent 40Ar/39Ar ages have redefined the volcanic evolution for BBNP. Chisos Formation (CF) rocks range in age from 46.6 to 28.8 Ma. The oldest lava flow dated at 46.62±0.07 Ma comes from the Alamo Creek lavas followed by thick sections of air-fall tuffs. An indurated tuff from this same section yielded an age of 40.62±0.11 Ma. A series of thick basaltic lava flows, known as the Bee Mountain basalts (BMB), were erupted at 33.74±0.09 Ma. Stratigraphically above the BMB is the only regional ash-flow tuff that occurs in the Park, known as the Mule Ear Spring Tuff (MEST). The source area for this tuff is still unknown, but its deposition occurred at 33.50±0.21 Ma. The MEST is overlain by the Tule Mountain Trachyandesite (TMT). Its thickness varies considerably, with the thickest flows occurring in the southwest part of the Park. A sample from the TMT yielded ages of 30.29±0.05 Ma and 30.29±0.11 Ma. The eruption of the Wasp Spring Ignimbrite (WSI) signaled a new eruptive episode of felsic volcanism in the Park. Two samples of the WSI yielded ages of 29.48±0.19 Ma and 29.22±0.12 Ma. We believe the WSI is related to the eruption of the Burro Mesa Rhyolite (BMR) at 29.33±0.15 Ma based on an average of five ages for the BMR. A rhyolite dike, dated at 28.81±0.10 Ma, cross cuts the CF and represents the youngest igneous unit identified. Five rhyolite samples from the South Rim Formation yield an average age of 32.09±0.14 Ma, similar to previous ages obtained in the area, and are coeval with the eruption of the Pine Canyon Caldera, the only know caldera in the Park dated at 32 Ma.

Three fayalite-bearing intrusions; two related to the Rosillos Mountains, yielded zircon 206Pb/238U SHRIMP ages of 32.3±0.2 Ma and 32.1±0.2 Ma; the third from Grapevine Hills yielded an age of 32.9±0.2 Ma. These ages are contemporaneous with the eruption of the Pine Canyon Caldera. A fourth sample, a gabbro sill from Mariscal Mountain, yielded a zircon 206Pb/238U SHRIMP age of 46.5±0.3 Ma, similar to the Alamo Creek Lavas of the CF. Continued geologic mapping, age determinations, and geochemistry will help unravel the complex eruptive and plutonic history for Big Bend National Park.