MAFIC AND INTERMEDIATE MAGMATISM AT MCDERMITT CALDERA, NEVADA–OREGON: ICELANDITE TRIGGER FOR CALDERA FORMATION

The McDermitt volcanic center is a roughly 2,000 km² topographic feature in northern Nevada and southern Oregon, USA. The center is dominated by the ~45 x ~25 km McDermitt caldera, formed during eruption of the ~1,000 km³ anorthoclase-phyric McDermitt Tuff at 16.39 ± 0.02 Ma (Henry et al., 2017). Local effusive eruptions of basalts, intermediate lavas, and rhyolitic domes and flows both preceeded and followed caldera formation. The occurrence of a significant volume of intermediate lavas, albeit subordinate to rhyolite and regional Steens Basalts, contrasts with the sharply bimodal character of most of the Yellowstone hotspot province. Precaldera basalts older than 16.5 Ma consist of aphyric to coarsely plagioclase phyric Steens Basalt, with close affinities to nearby contemporaneous lavas at Catlow Peak and the type locality of Steens Mountain, Oregon (Camp et al., 2013). They are overlain at McDermitt by intermediate lavas, erupted between 16.5 Ma and caldera formation at 16.39 Ma. Intermediate lavas are aphyric to sparsely porphyritic Fe-rich icelandites with phenocryst assemblages of plagioclase, augite, Ca-poor pyroxene, Fe-rich olivine and Fe-Ti oxides. Similar lavas overlie the McDermitt Tuff within the caldera, and evolved icelandite (63–65% SiO₂, 8–9% FeO*) occurs as a component within the McDermitt Tuff. The latest volcanism at McDermitt (~15 Ma) is represented by scattered high-alumina olivine tholeiite lavas at a few locations close to the caldera margin; these lavas are not petrogenetically related to the earlier rocks of the center. Icelandites can be modeled as derived from Steens Basalt via coupled assimilation and fractional crystallization at mid-crustal depths. A few pre-caldera, several post-caldera, and all McDermitt Tuff icelandites are anomalously enriched in Ba and have positive Eu anomalies, indicating assimilation of alkali feldspar. Potassium, however, is not strongly enriched in these samples. This is evidence for interaction of icelandite magma with anorthoclase-dominated cumulates, such as crystal mush in the lower parts of the McDermitt Tuff magma body, and indicates that recharge by icelandite played a major role in mobilising the McDermitt Tuff rhyolite towards eruption.

References: Camp et al. (2013) GSA Spec Paper 497, 87–116; Henry et al. (2017) Geosphere 13/4, 47p.