The ⁴⁰Ar/³⁹Ar ages and tectonic setting of the middle Eocene northeast Nevada volcanic field

Abstract

Widespread middle to late Eocene calc‐alkalic volcanism, which formed the Northeast Nevada Volcanic Field, marks the earliest Tertiary volcanism in the northern Basin and Range. The central part of this major field in northeast Nevada and adjacent Utah is herein defined by 23 40Ar/39Ar ages that range from 42.6 to 39.0 Ma, rock chemistry from 12 localities, stratigraphic position of the volcanic rocks above a regional middle Eocene unconformity, volcanic setting, and lithology. The type area is at Nanny Creek, in the northern Pequop Mountains, Nevada, where rhyolite ash flow tuffs are overlain by a thick section of intercalated andesitic to dacitic flows and flow breccias and rhyolite ash flow tuffs. The intermediate composition rocks are locally derived throughout the volcanic field, whereas the sources for rhyolite ash flow tuffs are unknown. The uniform and widespread occurrence of the andesitic and dacitic flows and flow breccias strongly suggests that the upper crust was perforated by intermediate composition magma across the entire region. In the central part of the field the middle Eocene volcanic rocks rest with depositional angular discordance on deformed middle Paleozoic to Triassic strata; ostracode‐bearing limestone, probably of early Eocene age, is locally present below the volcanic rocks. In the western and southeastern parts of the field these middle Eocene volcanic rocks rest with depositional angular discordance on lower Eocene lacustrine strata of the Elko and White Sage Formations, respectively. This angular discordance documents a middle Eocene deformational event previously unrecognized in the region.

Ash-flow tuffs and paleovalleys in northeastern Nevada: Implications for Eocene paleogeography and extension in the Sevier hinterland, northern Great Basin

“…4) (McIntosh et al, 1990;John et al, 2008). All published ages cited in Table 1 are by step-heating of bulk samples of sanidine or biotite (Hofstra, 1994;Brooks et al, 1995aBrooks et al, , 1995bMueller et al, 1999). Most of these ages agree with the results obtained in this study, although bulk samples can contain xenocrysts.…”

“…45 Ma in northeastern Nevada and was part of a southward-migrating belt of magmatism that swept from Washington and Idaho, through northeastern Nevada and northwestern Utah, and into central Nevada in the Oligocene (Stewart, 1980;Christiansen and Yeats, 1992;Brooks et al, 1995aBrooks et al, , 1995bHumphreys, 1995;Henry and Ressel, 2000a;this study). Eocene magmatism in northeastern Nevada was dominated by andesitic to dacitic lavas and compositionally similar intrusions in numerous centers.…”

“…An episode of extension that tilted the Elko Formation near Elko ~10° to 15° to the southeast before ca. 38 Ma is well established (Brooks et al 1995a;Henry and Faulds, 1999;Henry et al, 2001;Haynes, 2003;Hickey et al, 2005). Most cooling and presumed uplift from extension of the Ruby Mountains is interpreted to have occurred between 36 and 20 Ma based on K-Ar and 40 Ar/ 39 Ar ages (Kistler et al, 1981;Dokka et al, 1986;McGrew and Snee, 1994) and at ca.…”

“…Most published K-Ar ages are insuffi ciently precise to help with correlation and are not considered. Thirty-fi ve whole-rock, ash-fl ow tuff samples were analyzed for major and trace elements by XRF (X-ray fl uorescence) at Washington State University (Table 2) and were combined with published analyses in Mueller (1992), Brooks et al (1995aBrooks et al ( , 1995b, and Wallace (2003a). Comparison of the new data with published analyses of the same units and, in a few cases, the same outcrops, indicates some interlaboratory bias, which complicates correlation.…”

Middle Miocene to Holocene tectonics, basin evolution, and paleogeography along the southern margin of the Snake River Plain in the Knoll Mountain–Ruby–East Humboldt Range region, northeastern Nevada and south-central Idaho

“…From oldest to youngest, they are the basement sequence, the Clover Creek sequence, and the middle Miocene to Pliocene sequence. The basement sequence includes Precambrian to Triassic continental margin clastic, carbonate, and igneous rocks that are overlain in places by 46-29 Ma Eocene to Oligocene volcanic and sedimentary rocks (Brooks et al, 1995;Mueller et al, 1999;Haynes, 2003;Henry, 2008;Lund Snee, 2013;McGrew and Snoke, 2015;Lund Snee et al, 2016). In most places, the basement sequence is overlain by the middle Miocene to Pliocene sequence, which is composed largely of the ca.…”