SIERRA QUEMADA—A FAILED CALDERA, BIG BEND NATIONAL PARK

Sierra Quemada (SQ) is a mid-Oligocene domal structure about 4 km in diameter ringed by rhyolitic dikes in the southern Chisos Mountains, Texas. Regional volcanic strata of the Chisos Formation dip ~ 20-40º away from the structure, with no significant discontinuity across the ring dikes except for local irregularities in dip within the dome interior. Ring dikes dip ~ 80º inward, suggesting that initial vertical ring dikes were tilted 10º during continued post-emplacement doming. No evidence of wall breccia, caldera collapse, or of caldera-filling deposits is present. Small vents (< 1 Km² area) consisting of extremely lithic-rich rhyolitic explosion breccia (Teb) are scattered within the perimeter of ring dikes. Lithic fragments within Teb (mm-size to 200 m long) consist of vertically oriented, highly elongate, Tertiary, Cretaceous, and Paleozoic strata, the larger of which have relic bedding. No outflow deposits of Teb were found. Remnants of thin carapace-like rhyolitic flows, fed by large rhyolitic dikes, lie within the area of the structural dome. A 31-Ma (preliminary age) basaltic flow outside the NW side of the structure was deformed by doming. Altered Teb matrix sanidines yielded a ⁴⁰Ar/³⁹Ar age of 29.5 Ma. Additionally, a 29-Ma (preliminary age) rhyolitic dike indicates that the domal feature and igneous activity significantly postdates the early-Oligocene 33.64-Ma Mule Ear Spring Tuff (Tmet), which had previously been attributed to SQ caldera eruption. Local epidote veins suggest post-emplacement low-grade thermal alteration, possibly related to contemporaneous gabbroic dikes within SQ. The tight grouping of REEs and Hf/Ta vs La/Yb from Tmet differ significantly from those of possible rhyolitic sources within SQ confirming SQ was not the source of Tmet. Therefore, we conclude that SQ is a relatively young, magmatically driven, domed area that produced a non-eruptive ring dike complex followed by minor explosion breccia from small, localized vents. Similar domal and ring-dike features of questionable eruptive history also occur elsewhere in the southern Chisos Mountains and Big Bend National Park area.