Age and petrogenesis of the Roaring Mountain rhyolites, Yellowstone Volcanic Field, Wyoming

Extracaldera rhyolites in the Norris-Mammoth Corridor of the Yellowstone Volcanic Field (YVF) appear to be unrelated to intracaldera volcanism, resulting instead from a new crustal magma source derived from northeastward propagation of the Yellowstone "melting anomaly." The youngest extracaldera rhyolite unit, the Roaring Mountain Member (RM), is chemically distinct from the previous extracaldera lavas, reverting to more primitive compositions.;This study suggests that the majority of the RM rhyolites erupted from the same large-scale silicic magma system. Based on geochemistry and 40Ar/39Ar geochronology, the Crystal Spring mingled rhyolite and Obsidian Cliff rhyolite erupted concurrently at 59.1 +/- 2.0 ka. The mafic enclaves within the Crystal Spring mingled rhyolite are compositionally similar to the Swan Lake Flat basalts. Their presence confirms mafic magmatism persisted after the latest basalt eruptions at 209 ka. The residual Obsidian Cliff magma subsequently underwent fractional crystallization and erupted as the main flow of the Gibbon River rhyolites in 52.0 +/- 8.5 ka. Two other flows within the Gibbon River rhyolites may have evolved as small, independent melts that erupted concurrently with the main Gibbon River flow. 238U/230Th zircon geochronology indicates magma residence times of up to 160 ka.