Abstract

The middle Miocene McDermitt volcanic field of southeastern Oregon and northern Nevada is a caldera complex that is temporally and spatially associated with the earliest flood lavas of the Columbia River Basalt Group, the Steens Basalt. The topographically prominent caldera west of McDermitt, Nevada, has commonly been considered the starting point for the time-transgressive Yellowstone hotspot trend. In the original work defining the field, seven weakly to moderately peralkaline rhyolitic ignimbrites were identified to have erupted from seven calderas over an interval of ∼1 m.y. following emplacement of Steens Basalt flood lavas. Aided by 47 new high-precision 40Ar/39Ar ages and extensive trace-element geochemistry, we refine the volcanic stratigraphy to four major ignimbrites: 16.468 ± 0.006 Ma (2σ) Tuff of Oregon Canyon, 16.415 ± 0.007 Ma Tuff of Trout Creek Mountains, 16.328 ± 0.013 Ma Tuff of Long Ridge, and 15.556 ± 0.014 Ma Tuff of Whitehorse Creek. New geologic mapping has identified the sources of the two oldest ignimbrites at two newly delineated, overlapping calderas in the northern McDermitt volcanic field: the ∼20 × 24 km Fish Creek caldera, formed on eruption of the Tuff of Oregon Canyon, and the ∼20 × 26 km Pole Canyon caldera, formed ∼50 k.y. later on eruption of the compositionally similar Tuff of Trout Creek Mountains. Ring-fracture lavas of these two calderas lie outboard of those related to the youngest caldera in the field, the ∼13 × 12 km Whitehorse caldera, which is entirely nested within the Pole Canyon caldera.

The new mapping and chronology of the northern McDermitt volcanic field make clear that there is a linear ∼N20°W trend of mafic, intermediate, and rhyolitic volcanism that extends southwest from the northern McDermitt volcanic field, through McDermitt caldera and the Santa Rosa–Calico center, to the northern Nevada Rift. A similar linear trend is observed ∼75 km to the west, where the Hawks Valley–Lone Mountain center and the calderas of the High Rock caldera complex define an ∼N20°E trend radiating south-southwest from Steens Mountain. The temporal, spatial, and compositional patterns of rhyolitic magmatism along both trends are consistent with rapid southward propagation of flood basalt dike swarms associated with emplacement of the Yellowstone plume head.

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