U-TH DATING OF ZIRCON AND ALLANITE RIMS FROM SUGARLOAF MOUNTAIN, A HIGH SILICA RHYOLITIC DOME WITHIN THE SAN FRANCISCO VOLCANIC FIELD, NORTHERN ARIZONA

Sugarloaf Mountain is a high-silica (75.6 wt.%) rhyolite dome and has associated pumice deposits located within the San Francisco Peaks of northern Arizona. Sugarloaf rhyolite is significant because a sanidine ⁴⁰Ar/3⁹Ar plateau date of 91±2 ka and an inverse isochron date of 93±16 ka (MSWD:19) (Morgan et al., 2010) make it the youngest silicic eruption within the San Francisco Volcanic Field. The rhyolite is also noteworthy for its accessory mineral assemblage of zircon, allanite, and chevkinite. Results for allanites and/or chevkinites can be paired with those for zircons to obtain precise ²³⁸U-²³⁰Th isochron ages. The Stanford-USGS SHRIMP-RG ion microprobe was used for ²³⁸U-²³⁰Th dating of surfaces representing the final crystal growth on individual zircon and allanite crystals from five Sugarloaf samples. The zircon rims have unusually high uranium concentrations of up to 1.5 wt.%, and the allanites have thorium concentrations of 2-3 wt%. Additionally, 90% of the zircon rims yield a restricted range of model ages, assuming growth from a melt like the whole rock at ²³⁸U-²³⁰Th secular equilibrium, and a weighted mean age of 74.7+1.1/-1.0 ka (MSWD=1.6; n=31). Zircons and allanites together yield a combined ²³⁸U-²³⁰Th isochron date of 75.5± 0.8 ka, which is ~15 kyr younger than the reported ⁴⁰Ar/³⁹Ar dates. This result suggests that the Sugarloaf eruption occurred at a younger age than previously recognized.