Geology and evolution of the McDermitt caldera, northern Nevada and southeastern Oregon, western USA
AuthorHenry, Christopher D.
Castor, Stephen B.
Starkel, William A.
Ellis, Ben S.
Wolff, John A.
Laravie, Joseph A.
McIntosh, William C.
Heizler, Matthew T.
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The McDermitt caldera (western USA) is commonly considered the point of origin of the Yellowstone hotspot, yet until now no geologic map existed of the caldera and its geology and development were incompletely documented. We developed a comprehensive geologic framework through detailed and reconnaissance geologic mapping, extensive petrographic and chemical analysis, and high-precision Ar-40/Ar-39 dating. The caldera formed during eruption of the 16.39 +/- 0.02 Ma (n = 3) McDermitt Tuff (named here), which is strongly zoned from peralkaline, aphyric, high-Si rhyolite (comendite) to metaluminous, abundantly anorthoclase-phyric, trachydacite, or Fe-rich andesite (icelandite). The McDermitt caldera formed in an area that had undergone two episodes of Eocene intermediate volcanism at 47 and 39 Ma and major middle Miocene volcanism that led continuously to caldera formation. Eruption of the Steens Basalt, the oldest Miocene activity, began before 16.69 Ma. Exclusively mafic magmas erupted initially. Rhyolite lavas erupted as early as 16.69 Ma, contemporaneous with upper parts of the Steens Basalt, and the proportion of rhyolite increased steadily until eruption of the McDermitt Tuff. Precaldera silicic activity was diverse and almost entirely effusive, including metaluminous to mildly peralkaline, sparsely anorthoclase-phyric rhyolite to peralkaline, aphyric rhyolite to quartz-sanidine-sodic amphibole porphyritic rhyolite at 16.41 Ma. Metaluminous to peraluminous biotite rhyolite lavas and domes were emplaced around what is now the caldera wall in 4 areas at 16.62, 16.49, and 16.38 Ma. Eruption of the McDermitt Tuff generated the irregularly keyhole-shaped, 40 x 30-22 km McDermitt caldera. Collapse occurred mostly along a narrow ring-fault zone of discrete faults with variable downwarp into the caldera between faults. Minor parallel faults locally widen the zone to as much as 6 km. We find no evidence that the McDermitt caldera consists of multiple nested calderas, as previously postulated. Total collapse was no more than similar to 1 km, and total erupted volume was similar to 1000 km(3), of which 50%-85% is intracaldera tuff. Uncertainties in these estimates arise because an intracaldera tuff section is exposed only along the western edge of the caldera, and outflow tuff is not completely mapped. Megabreccia and mesobreccia are common in intracaldera tuff in the complete section. Intracaldera tuff is strongly rheomorphic, scrambling the compositional zoning, which is locally better preserved in outflow sections. However, outflow tuff is also strongly rheomorphic where it was deposited over steep topography. The caldera underwent post-collapse resurgent uplift driven by intrusion of icelandite magma, which also erupted from two major vents and as widespread lavas. Major igneous activity around the McDermitt caldera lasted from before 16.7 to 16.1 Ma, although minor high-alumina olivine tholeiite lavas were emplaced ca. 14.9 Ma in the youngest recognized igneous activity. Tuffaceous sediments as much as 210 m thick filled the caldera, although whether deposition preceded, followed, or spanned resurgence is unresolved. Although formed during regional extension, the caldera is cut only at its western and eastern margins by much younger, high-angle normal faults that resulted in gentle (similar to 10 degrees) eastward tilting of the caldera. Numerous hydrothermal systems probably related to caldera magmatism and focused along caldera structures produced Hg, Zr-rich U (some along the western caldera ring fracture dated as 16.3 Ma), Ga, and minor Au mineralization. Lithium deposits formed throughout the intracaldera tuffaceous sediments, probably ca. 14.9 Ma. Silicic volcanism around the McDermitt caldera is some of the oldest of the Yellowstone hotspot, but the caldera is younger than two known calderas in northwestern Nevada. Published data show that silicic activity as old as at McDermitt is also found from the Silver City area of southwestern Idaho to the Santa Rosa-Calico volcanic field in Nevada east of McDermitt. As recognized by others, an area of similar to 40,000 km(2) mostly in northwestern Nevada and southeastern Oregon underwent major silicic activity before ca. 16.0 Ma. The McDermitt caldera is similar to calderas of the middle Cenozoic ignimbrite flareup of the Great Basin, especially in strong compositional zoning and large volume of erupted tuff, collapse along a distinct ring-fracture system, abundance of megabreccia in intracaldera tuff, and resurgence. The greatest differences are that McDermitt is larger in area than all except a few flareup calderas and underwent far less collapse (similar to 1 km versus 3-4 km to as much as 6 km). The McDermitt caldera may be an analog for the buried calderas of the central Snake River Plain, which also erupted voluminous rhyolitic tuffs. However, the Snake River Plain ignimbrites are metaluminous and homogeneous in bulk chemistry, with plagioclase, sanidine, and minor quartz as common phenocrysts, and rarely contain pumice or lithics.
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