THE ROLE OF BASALT IN ERUPTION OF QUATERNARY RHYOLITES NORTH OF THE YELLOWSTONE CALDERA

The Yellowstone Volcanic Field has produced 3 large-scale caldera-forming eruptions at 2.1 Ma, 1.3 Ma and 640 ka. The Yellowstone Caldera, produced at 640 ka, has been infilled with intracaldera rhyolite lavas. They are petrographically similar high-silica rhyolites and do not have direct evidence for mafic magma interactions, nor are they closely associated spatially with young basalts.

In contrast, extracaldera rhyolites erupted north of the Yellowstone Caldera are spatially associated with young basalt lavas, and several contain evidence for magma mingling. These define a geochemically and isotopically coherent group that is spatially restricted to the Norris-Mammoth corridor, and erupted adjacent to, or through, lava flows of the Swan Lake Flat basalt. ⁴⁰Ar/³⁹Ar dates of 350 ka and 209 ka indicate a minimum interval of ~140 ka for basalt eruptions at the surface. Four rhyolite lavas with mafic enclaves, having ⁴⁰Ar/³⁹Ar dates of 316 ka, 301 ka, 263 ka and 59 ka, record basaltic injection at depth over an ~260 ka interval. Combined, these data record mafic magmatism in the Norris-Mammoth corridor over a ~290 ka interval.

Beginning with eruption of the Willow Park rhyolite at 326 ka, an ~270 ka interval of eruptions records chemical evolution consistent with fractional crystallization punctuated by recharge and mixing events associated with the mingled lavas, and ending with the most recent eruption of the Gibbon River rhyolite at 56 ka. The youngest Roaring Mountain member rhyolites (118-56 ka) are petrographically distinct, being nearly aphyric, in contrast to the porphyritic textures of the previous Obsidian Creek member rhyolites (326-134 ka). The Roaring Mountain member rhyolites are fayalite bearing and record Ti-in-Quartz and 2-feldspar temperatures as high as 975 ^oC. This reflects a recent up temperature excursion of the magma system due to renewed mafic input.

These observations support the establishment and evolution of a young rhyolitic magma system north of Yellowstone Caldera, supported by basaltic input which occurred over a nearly 300 ka interval. The 59 ka age for the youngest mingled lava, along with evidence for increased temperatures of rhyolite magmas, suggests the possibility of future eruptions in an area which may represent the leading edge of the advancing Yellowstone melting anomaly.