DO THE YOUNG EXTRACALDERA RHYOLITES NORTH OF YELLOWSTONE CALDERA MARK THE BEGINNINGS OF A 4TH VOLCANIC CYCLE IN THE YELLOWSTONE PLATEAU VOLCANIC FIELD?

The Yellowstone Plateau Volcanic Field has produced voluminous, caldera-forming eruptions of high-silica rhyolite at ~ 2.0, 1.3, and 0.64 Ma. Volcanism since 0.64 Ma produced rhyolite domes/flows (Plateau Rhyolite). The Obsidian Creek (OC) and Roaring Mountain (RM) members erupted at 399 - 80 ka (K/Ar dates) north of the caldera complex, marking the most recent evolution of the magmatic system at the leading edge of the Yellowstone hotspot. The OC and RM members have been characterized as porhyritic and aphyric, respectively, however trace element and REE plots show no clear distinction between the two. More detailed field/petrographic evidence indicates substantial overlap in phenocryst abundance between the members. Additionally, two new mingled mafic-silicic lavas have been identified (2 were previously identified in the OC member), showing that both members record evidence of mafic recharge. Mafic enclaves from the mingled lavas are basaltic-andesite to andesite and represent physically mixed magma. Liquid state mixing is supported by enclaves with crenulated margins and exchanged phenocrysts of quartz and rhyolite glass within the mafic material, as well as reaction-rimmed clinopyroxene and mafic glass within the rhyolite. Previous work suggests extracaldera rhyolites are the product of small, independent magma batches, and are not related to the sub-caldera magma system. However, new geochemical and isotopic data show that they may be related to a single comagmatic system. Incompatible/incompatible element ratios (eg. Th/Nb and Y/Nb) show invariant trends with increasing Nb, suggesting derivation from a single source undergoing FXL. Forthcoming ⁴⁰Ar/³⁹Ar dating will test the hypothesis that these rhyolites exhibit a progressive chemical evolution with decreasing age. Extracaldera rhyolites cluster within a restricted epsilon Nd range of –12 to –13, significantly lower than the voluminous caldera-related rhyolites. These data suggest the extracaldera rhyolites were erupted from a single evolving source, independent of the main subcaldera system. Their location may indicate a migration of the current sub-caldera magma system, or development of a new magma system of substantial size and longevity north of Yellowstone Caldera.