AGE DISTRIBUTION AND COMPOSITION OF DETRITAL ZIRCONS IN THE PERMIAN BRUSHY CANYON FORMATION, DELAWARE BASIN, WEST TEXAS

While the Delaware Basin is well known for the Permian reef complex, it also represents a significant sink for Late Paleozoic clastic sediment. The oldest clastic unit in the basin, the Brushy Canyon Formation (BCF), represents a delta complex that prograded to the southeast during the early to middle Permian. Both the progradation direction of the delta complex and paleoflow indicators in the BCF led previous workers to suggest a source area to the north in the nearby Ancestral Rocky Mountains (ARM). One of the southernmost Late Paleozoic ARM uplift, the Ute Pass Uplift, exposed portions of the ca. 1.1 Ga Pikes Peak Batholith (PPB), a granitoid complex distinctive for both its age and its alkaline composition. Duncan et al. (this volume) show that zircon crystals derived from the PPB have a distinctive trace element composition. The goal of this study is to evaluate the provenance of the BCF and to search for 1.1 Ga zircons derived from the Ute Pass uplift. We performed LA-ICPMS U-Pb geochronology and trace element analysis on more than 200 CL-imaged, well rounded detrital zircon grains from two fine-grained quartz arenite samples collected from one channel complex in the lower BCF. Spot analyses of a single, inclusion-free, growth domain in each zircon crystal from these two samples yielded identical age distributions. Measured ages range from about 380 Ma to 3740 Ma with 40% of the ages clustered tightly about 520 Ma. Broader peaks, each representing about 10% of the measured ages, occur from 1000 Ma to 1150 Ma, from 1600 Ma to 1800 Ma, and from 2600 Ma to 2800 Ma. Minor peaks, each representing < 5% of the measured ages, occur at about 380 Ma, 430 Ma, 1260 Ma and 1440 Ma. The predominance of ca. 520 Ma zircon ages, the paucity of ca. 1.4 Ga zircon ages, and the abundance of pre-2.5 Ga zircon ages in the BCF are not consistent with a provenance in the southern ARM. Moreover, the ca. 1.1 Ga zircon crystals from the BCF have trace element compositions (Th, U, Nb, Ta, Y, Hf, REE patterns) distinct from ca. 1.1 Ga zircon crystals from the Ute Pass uplift. Possible sources for the BCF include other ARM ranges (including those in the Wyoming Province), the southern Oklahoma aulacogen, and the Ouachita-Marathon range, which formed the southern boundary of the Delaware Basin.