EXTRACALDERA RHYOLITES NORTH OF THE YELLOWSTONE PLATEAU VOLCANIC FIELD CALDERA COMPLEX: AN EVOLVING SILICIC MAGMA SYSTEM AND SITE OF FUTURE LARGE VOLUME ERUPTIONS?

The Obsidian Creek (OC) and Roaring Mountain (RM) member extracaldera rhyolites erupted north of the YPVF caldera complex at ~ 526 - 106 ka (⁴⁰Ar/³⁹Ar) to 80 ka (K/Ar) as porphyritic and aphyric high silica rhyolite domes/flows. These eruptions may mark the most recent migration of the YPVF melting anomaly, yet their relation to the magma system that produced caldera-forming eruptions at 2.1, 1.3 and 0.64 Ma is unclear. Previous work has suggested that these small volume rhyolites are the product of several independent magma batches unrelated to the main subcaldera system.

Our preliminary results indicate that the OC and RM rhyolites are related temporally, spatially and geochemically, and may be derived from a single magma system that evolved over an ~ 200 ky (⁴⁰Ar/³⁹Ar) time period. Most rhyolites cluster within restricted groups for εNd (-12 to -13), ⁸⁷Sr/⁸⁶Sr (0.710 - 0.714), and Pb (²⁰⁶Pb/²⁰⁴Pb=17.1 - 17.5, ²⁰⁷Pb/²⁰⁴Pb=15.5 - 15.6, ²⁰⁸Pb/²⁰⁴Pb=38.1 - 38.4). Isotopic compositions show that these are hybrid magmas isotopically distinct from the main subcaldera system. The two oldest flows (358 ka and 526 ka) are spatial, temporal, and isotopic outliers, and are therefore unrelated to the younger units.

The presence of four mingled rhyolite-basalt lavas emphasizes the role of basalt in generating and modifying these silicic magmas. Three mingled lavas from the OC rhyolites suggest an influx of mafic melts early in the lifespan of the magma system (326 - 263 ka). Subsequent OC eruptions (226 - 134 ka) exhibit an evolution consistent with 5% - 40% FXL as indicated by trace element modeling using modal phenocryst abundances. The younger (118 - 80 ka) RM rhyolites record a change to less evolved compositions suggesting that they may represent a separate magma batch, or that the OC magma system was rejuvenated by influx of mafic magma that triggered subsequent eruptions of an aphyric, less evolved magma. RM rhyolites record a second apparent FXL trend. Observed magma mingling in one of the aphyric lavas supports a recharge event.

Data from the extracaldera rhyolites therefore suggest evolution of a magma system of substantial longevity (~200 ka) possibly sustained by mafic recharge, which introduces the possibility for a build up phase to future caldera forming eruptions north of the Yellowstone caldera complex.