40AR/39AR AGE AND PETROGRAPHY OF THE TUFF OF LYLE SPRING, YELLOWSTONE VOLCANIC FIELD, IDAHO

Investigation of small-volume rhyolites that erupt between caldera-forming eruptions can aid in understanding the timescales of magmatic rejuvenation. The Tuff of Lyle Spring within the Yellowstone Volcanic Field is one such rhyolite that occurred after the eruption of the Huckleberry Ridge Tuff and preceded the Mesa Falls Tuff. Unlike the other small-volume rhyolite domes extruded between these two caldera-forming eruptions, the Tuff of Lyle Spring was the only explosive eruption. We have identified two units within this tuff. The lower tuff (sample 15TLS-1) is distinguished by a medium gray color with 1-5 mm elongated, compressed, parallel lithic clasts. Phenocrysts of sanidine, plagioclase, biotite, and quartz are apparent in hand samples and thin sections. Samples from the upper unit (15TLS-2) have a lighter gray to pink color, the same phenocryst assemblage, and moderately to lightly compressed lithic clasts up to 60 mm in their longest dimension. We observe phenocrysts of sanidine and quartz, as well as spherulitic texture of the groundmass within the larger lithic clasts of sample 15TLS-2. ⁴⁰Ar/³⁹Ar single crystal fusions of 1-3 mm sanidine grains from sample 15TLS-1 were performed to evaluate the accuracy of the previously reported average K/Ar date of 1.19 ± 0.01 Ma. Although originally interpreted to be mineralogically similar and thus closely related to the adjacent effusive Bishop Mountain Flow and the Mesa Falls Tuff, our new results indicate this small-volume explosive eruption is temporally correlative with the Bishop Mountain Flow, but significantly older than the Mesa Falls Tuff.