INVESTIGATION OF A SMALL IGNEOUS COMPLEX ON THE NORTH END OF BURRO MESA, BIG BEND NATIONAL PARK, TEXAS

We investigated a small volcanic center located on the north end of Burro Mesa in Big Bend National Park, Texas. Three major stratigraphic units and several minor ones are centered around an intrusive core. The units include lava flows, agglomerates, breccias, air-fall tuffs, irregularly-shaped plutons, and a pair of apparently unrelated pyroclastic flow deposits. The total aggregate thickness of the units in the vicinity of this vent is about 120 m but their lateral extent is limited to an area of a few square kilometers. The age of the extrusive units, based on stratigraphic relationships, is at least in part younger than the upper part of the Chisos Formation. The younger of the two pyroclastic flow deposits in the map area overlies the Tule Mountain Trachyandesite (Ttmt), which is the uppermost formal member of the Chisos Formation, along the northeast side of Burro Mesa near the main park road. The older pyroclastic flow deposit occurs below Ttmt and it is correlated with the Mule Ear Spring Tuff Member of the Chisos Formation. The remaining extrusive units are not seen in contact with other previously mapped units.

The erupted materials associated with the vent are mainly rhyolitic with one basaltic unit. The rocks are aphanitic and commonly have phenocrysts of plagioclase and/or alkali feldspar with rarer mafic minerals. Quartz is common in the groundmass but not as a phenocryst phase. Chemically, the rocks show quite a bit of variability in terms of silica and other oxides. Similarly, alumina saturation ranges from peralkaline to peraluminous. The intrusive core or spine has a faint blue tint similar in a broad sense to some outcrops of the Burro Mesa Rhyolite Member of the South Rim Formation. Regardless of its relationship to other units, this vent is unlike others previously described in the park. It is not a caldera or lava dome but has characterisitics perhaps most closely related to a spatter cone. It was down-faulted and preserved within a previously unmapped graben. Although the units mapped around this vent are relatively localized, their finer-grained equivalents may have been preserved as undifferentiated volcaniclastic deposits. Perhaps many of the small plutons in the park are more deeply-eroded examples of this vent and at one time may have been small eruptive centers.