GEOLOGIC EVOLUTION AND INITIAL CONSTRAINTS ON THE PETROGENESIS OF THE MCDERMITT VOLCANIC CENTER, NORTHERN NV AND SOUTHERN OR

The mid-Miocene McDermitt caldera on the northern NV - southern OR border is commonly considered to mark initial silicic volcanism associated with the Yellowstone hotspot track, first proposed by Pierce and Morgan (1992). It was also interpreted to consist of 5, partly overlapping calderas. Our new work substantially revises this picture to show that silicic magmatism began at ~16.5 Ma, after emplacement of Steens basalts likely erupted from vents to the North, but consisted of a mix of metaluminous and peralkaline rhyolite lavas, and that a single, 30 x 40 km caldera formed during eruption of a zoned rhyolite-dacite ignimbrite at ~16.2 Ma. Effusive eruptions of peralkaline and metaluminous rhyolites and icelandites continued in and around what was to become the caldera until immediately before eruption of the caldera-forming tuff. Intracaldera tuff (ICT) is spectacularly exposed along the west side of the caldera where Basin and Range faulting has revealed pre-caldera lavas overlain by a complete section of ICT and megabreccia, locally strongly rheomorphic, to post-collapse icelandite lava. ICT varies from aphyric, high-SiO₂, peralkaline rhyolite to abundantly porphyritic, metaluminous dacite. Feldspar changes from sanidine to anorthoclase along this range. Numerous other silicic volcanic centers about as old as McDermitt, including several older calderas, are present in NW NV and SE OR. These include the Santa-Rosa Calico volcanic center (Brueseke et al., 2008), the Soldier Meadow and Virgin Valley calderas, and various silicic units in the High Lava Plains of central Oregon (Jordan et al., 2004).

Despite the importance attached to McDermitt in the literature, there is a noticeable lack of petrologic studies when compared to the amount of published work on the other silicic eruptive centers in the Yellowstone hotspot track. The magma chambers associated with McDermitt and the other mid-Miocene silicic centers in the region were formed in crust composed of accreted terranes, as opposed to the cratonal crust in which the Yellowstone plateau and Snake River Plain volcanic systems were developed (Nash et al., 2006). Initial ⁸⁷Sr / ⁸⁶Sr values of rhyolites in McDermitt range from 0.704473 to 0.706395, indicating that the petrogenesis of the McDermitt magmas must have had an upper crustal component.