40AR/39AR AGES, COMPOSITION, AND LIKELY SOURCE OF FALLOUT TUFFS IN THE DUCHESNE RIVER FORMATION, NORTHEASTERN UTAH

Thin fallout tuffs in the fluvial deposits of the Duchesne River Formation (DRF) in the Uinta Basin, Utah, are evidence that volcanism was active in northern Nevada and Utah in the late Eocene. Most of the tuffs have rhyolitic mineral assemblages including biotite, sanidine, plagioclase, and probably quartz. Some beds also contain allanite, zircon, apatite, titanite, and glass shards. Apatite is fluorapatite and has around 1-0.5 wt% REEs. Allanite from 4 samples contains about 0.65 apfu REEs which is about 0.3 apfu less than the highly evolved Bishop Tuff. Allanite from DRF tuffs also contains less Al than in the Bishop Tuff (0.9 vs. 1.3 apfu). Glass from one sample is rhyolitic with about 75 wt% SiO₂. Titanite in one tuff has a significantly lower Fe+Al (0.078 apfu) and a higher ratio of REEs/Y (9.6) than other Neogene Great Basin tuffs (Fe+Al=0.11 apfu and REEs/Y=3.6) but total REEs are only slightly lower (0.024 vs. 0.031 apfu). Based on this anomalous composition titanite may be detrital. The same tuff also contains many detrital high Or_>90 alkali feldspar gains. After exclusion of detrital grains, clusters of sanidine compositions at Or₇₃ and Or₈₀ suggests at least two volcanic episodes. Only one tuff retained plagioclase which ranged from An₂₅ to An₄₅. Clusters of biotite compositions suggests 3 or 4 distinct episodes of volcanism. Eruption temperatures calculated from biotite range from 650-730°C.

⁴⁰Ar/³⁹Ar dates constrain the timing of volcanism. One plagioclase and one sanidine separate from two different samples yielded ages of 39.5 and 39.36 Ma respectively. These dates, along with the compositional data on the ash beds probably limits the eruptive sources for these fallout tuffs to the Northeast Nevada or Tuscarora volcanic fields which have similar phenocrysts assemblages, rhyolitic compositions, and both were active during the late Eocene.