NEW HIGH PRECISION 40Ar/39Ar MEASUREMENTS OF THE HUCKLEBERRY RIDGE TUFF: MEMBER C - A CALDERA-FORMING, POST-CALDERA RHYOLITE, YELLOWSTONE, WY, USA

New high precision ⁴⁰Ar/³⁹Ar measurements of sanidine crystals from the Huckleberry Ridge Tuff were used to calculate ages of Member A and B as approximatly 2.13 Ma and Member C approximatly 2.11 Ma. From these new ages we present Huckleberry Ridge Tuff Member C as a separate unit that shares many similarities to younger post-caldera rhyolites in the Yellowstone Plateau. Similarities include increased compatible trace elements and ⁸⁷Sr/⁸⁶Sr, ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, as well as lower ¹⁴⁴Nd/¹⁴³Nd ratios when compared to other Yellowstone rhyolites. However, Huckleberry Ridge Tuff member C erupted approximately 20,000 years after the larger caldera collapse, whereas, the Upper Basin Member Rhyolites erupted at least 120,000 following the Lava Creek Tuff. EC-RAxFC modelling of rhyolites following the two major eruptions suggests that the source rocks were emplaced during the initial volcanism at Yellowstone, subsided during caldera collapse, and erupted with distinct isotopic values. Lower oxygen isotope ratios are found in younger post-collapse rhyolites, but are not found in Huckleberry Ridge Tuff Member C as the sourced rocks had not been subject to long-lived hydrothermal systems. New ages differentiate Huckleberry Ridge Tuff Member C as a separate eruption following Members A and B and helps better determine the architecture of the large silicic magma system below Yellowstone.