Geology and 40 Ar/ 39 Ar geochronology of the mid-miocene McDermitt volcanic field

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1 Geology and Ar/ 39 Ar geochronology of the mid-miocene McDermitt volcanic field Geology and Ar/ 39 Ar geochronology of the middle Miocene McDermitt volcanic field, Oregon and Nevada: Silicic volcanism associated with propagating flood basalt dikes at initiation of the Yellowstone hotspot Thomas R. Benson, Gail A. Mahood, and Marty Grove Department of Geological Sciences, Stanford University, 450 Serra Mall, Building 320, Stanford, California 94305, USA 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 Ar/ 39 Ar ages and extensive trace-element geochemistry, we refine the volcanic stratigraphy to four major ignimbrites: ± Ma (2σ) Tuff of Oregon Canyon, ± Ma Tuff of Trout Creek Mountains, ± Ma Tuff of Long Ridge, and ± 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 trb@ stanford.edu 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. INTRODUCTION The Yellowstone Snake River Plain province is an ~600-km-long, time-transgressive locus of explosive and effusive volcanism that was initiated during the middle Miocene in eastern Oregon and northern Nevada coeval with the eruption of the Columbia River Basalt Group lavas. Magmatism has since steadily propagated northeastward toward Wyoming, where activity has most recently occurred during the Quaternary in Yellowstone National Park (Fig. 1). Various geodynamic models have been proposed to explain the space-time progression of volcanism, including the migration of the North American plate over the tail of a deep mantle plume (e.g., Pierce and Morgan, 1992, 2009; Camp and Ross, 2004; Smith et al., 2009; Xue and Allen, 2010; Darold and Humphreys, 2013; Camp et al., 2015), or melting and extension in response to convection within the upper mantle (e.g., Anderson, 1994; Christiansen et al., 2002; James et al., 2011; Fouch, 2012; Foulger et al., 2015). Characterization of the source locations and timing of major rhyolite eruptions along a linear trend has been critical to the development of the hypotheses for the origin of Snake River Plain volcanism. Notably, this includes detailed work on ca Ma rhyolites of the province where many calderas are exposed or inferred (e.g., Christiansen, 2001; Morgan and McIntosh, 2005; Ellis et al., 2012; Anders et al., 2014) and within the central and western Snake River Plain, where voluminous, hot, dry rhyolite eruptions occurred from ca. 15 to 10 Ma (e.g., Mc- Curry et al., 1997; Boroughs et al., 2005; Bonnichsen et al., 2004, 2008; Branney et al., 2008). Perhaps most important to the origin of the Yellowstone Snake River Plain system, however, was the ca Ma rhyolite volcanism in Oregon, northern Nevada, and southwestern Idaho that occurred contemporaneous with the voluminous eruption of the Columbia River and Steens flood basalts (Fig. 1). This relationship has prompted researchers to suggest that initial silicic volcanism was associated with a mantle plume head centered roughly at Steens Mountain (e.g., Camp et al., 2003; Shervais and Hanan, 2008; Coble and Mahood, 2012; Fig. 1). New field investigations coupled with high-precision Ar/ 39 Ar geochronology has enabled researchers to more precisely define the sequence of the largest silicic eruptions in this area, notably at High Rock caldera complex (Coble and Mahood, 2016), Lake Owyhee volcanic field (Nash and Perkins, 2012; Streck et al., 2015; Benson and Mahood, 2016), and McDermitt volcanic field (Henry et al., 2016; this study). The location and timing of the oldest rhyolite eruptions in McDermitt volcanic field are of critical importance in developing a physical model for the onset of Yellowstone Snake River Plain volcanism because of the hypothesis that formation of the McDermitt volcanic field was the initial manifestation of Snake River Plain volcanism (e.g., Pierce and Morgan, 1992, 2009; Branney et al., 2008; Leeman et al., 2008; Shervais and Hanan, 2008; Ellis et al., 2012). In this paper, we refine stratigraphic relationships of regional ignimbrites and their relationships to the McDermitt volcanic field through field GSA Bulletin; September/October 2017; v. 129; no. 9/10; p ; doi.org / /B ; 13 figures; 3 tables; Data Repository item ; published online 23 June Geological For permission Society to of copy, America contact editing@geosociety.org Bulletin, v. 129, no. 9/ Geological Society of America

2 Benson et al. 124 W 120 W 116 W JdF Pacific plate map extent Cascade Volcanoes Basin and Range Y OREGON Monument High Lava Plains Trend Lake Owyhee Volcanic Field CR Chief Joseph 120 km IDAHO Columbia River Basalt Group lavas and dikes Younger CRB Steens Basalt Mid-Miocene rhyolitic centers Lava centers s BB D TM HH LJ High Rock Complex JB Steens Mtn CC B H HV V? SW? Steens M S R SC??McDermitt Volcanic Field Fig N J SR NNR I Snake River Plain/ Yellowstone Trend NEVADA Figure 1. Regional map showing the distribution of mid-miocene (ca Ma) volcanism in the Pacific Northwest (after Benson and Mahood, 2016). Extents of Columbia River Flood Basalt (CRB) Group members are split into Steens Basalt and younger members (Imnaha, Grande Ronde, Picture Gorge, Wanapum, Saddle Mountain) based on Reidel et al. (2013a), and generalized locations of feeder dikes are after Tolan et al. (1989), Camp et al. (2013), and Reidel et al. (2013b). locations are from this study, Rytuba and McKee (1984), Rytuba and Vander Meulen (1991), Benson and Mahood (2016), Coble and Mahood (2016), and Henry et al. (2016), and are labeled with the following symbols: V Virgin Valley caldera, B Badger Mountain caldera, H Hanging Rock caldera, CC Cottonwood Creek caldera, M McDermitt caldera, R Rooster Comb caldera, CR proposed caldera at Castle Rock. Contemporaneous lava centers are shown as yellow dots and are labeled with the following symbols: SC Silver City, LJ Little Juniper Mountain, HH Horsehead Mountain, JB Jackass Butte, SW Swamp Creek Rhyolite, TM Twenty Mile Creek Rhyolite, BB Bald Butte, D Drum Hill, HV Hawks Valley Lone Mountain, S Sleeper Rhyolite, SR Santa Rosa Calico, I Ivanhoe, J Jarbidge. Other symbols: Y Yellowstone caldera, JdF Juan de Fuca plate, NNR northern Nevada Rift. Isopleths of and Sr/ 86 Sr i are after Benson and Mahood (2016, and references therein). mapping, geochemistry, and high-precision Ar/ 39 Ar geochronology. These new data allow us to delineate three overlapping calderas in the northern McDermitt volcanic field. The distribution of volcanic activity in this nested caldera complex and in the southern McDermitt volcanic field and in the surrounding region leads us to new insights into the petrogenetic processes involved during impingement of a mantle plume on continental lithosphere. METHODS Ten months of field work involving geological mapping and sample collection were performed between 2012 and 2015 in the northern Mc- Dermitt volcanic field and in adjacent areas of northern Nevada and southern Oregon. The majority of mapping was performed at the 1:24,000 scale in northern McDermitt volcanic field, field-checking and remapping contacts previously mapped in published 7.5 quadrangles (Rytuba et al., 1982a 1982g; Rytuba and Curtis, 1983; Rytuba et al., 1983a, 1983b; Peterson and Tegtmeyer, 1987; Minor and Wager, 1989; Minor et al., 1989a 1989c). Standard methods used for the geochemical, electron microscopy, and volume estimate calculations are described in Appendix A of the GSA Data Repository file. 1 Important new details of the Ar/ 39 Ar methods, which bear on the precision and accuracy of the reported ages, are summarized next. 1 GSA Data Repository item , field and laboratory methodology; supplemental geochronology figures and data; mineral chemistry; whole-rock geochemistry, is available at /datarepository /2017 or by request to editing@ geosociety.org. Ar/ 39 Ar Geochronology Of ~300 samples collected in McDermitt volcanic field and the surrounding region for this study, 47 whole-rock rhyolite lava and ignimbrite samples were selected for Ar/ 39 Ar analysis. All ages reported here are based on analyses of samples included within a single irradiation, in order to maximize the ability to analytically resolve ages of closely spaced eruptive units. Details of the sample preparation and irradiation procedures are provided in Appendix A (see footnote 1). The Ar/ 39 Ar measurements were single-crystal, CO 2 laser fusion analyses performed with a Nu Instruments Noblesse multicollecting mass spectrometer fitted with a high-mass Faraday cup and two low-mass ion-counting detectors (Coble et al., 2011). The Noblesse was interfaced with a 1028 Geological Society of America Bulletin, v. 129, no. 9/10

3 Geology and Ar/ 39 Ar geochronology of the mid-miocene McDermitt volcanic field LabVIEW-automated, all-metal extraction line system to acquire Ar/ 39 Ar analyses by one of two procedures. Most gas aliquots were sufficiently large (e.g., > mol 39 Ar) to use the multicollecting procedure that involves a peak hop between the mass station used to measure Ar 38 Ar- 36 Ar and that used to measure 39 Ar and 37 Ar (method 3 in Coble et al., 2011). Smaller gas aliquots were dynamically selected to be measured in monocollection mode using the axial ion counter. Either procedure requires 300 s per Ar/ 39 Ar analysis. The overall approach for standardizing the Ar/ 39 Ar measurements has been described in Coble et al. (2011). The measurement procedures employed in this study were standardized using a newly synthesized reference gas prepared in collaboration with Andrew Calvert at the U.S. Geological Survey (Menlo Park, California) that was put into service in June This gas was generated by mixing previously described aliquots of argon ( Ar/ 36 Ar = from reservoir U041 and 38 Ar from reservoir U039; Miiller, 2006) with 39 Ar extracted by fusing neutron-irradiated kalsilite glass. When standardized against atmospheric argon ( Ar/ 36 Ar True = ± 0.31; Lee et al., 2006; Ar/ 36 Ar Measured = ± 0.), the mass discrimination corrected Ar/ 39 Ar, 38 Ar/ 39 Ar, and 36 Ar/ 39 Ar values measured on the Faraday cup of the Stanford Noblesse were ± 0.008, ± , and ± , respectively (2s standard errors). The 39 Ar contributed per aliquot, determined via measurements of Ar sensitivity performed with weighed aliquots of GA1550 biotite (McDougall and Wellman, 2011), was mol. Correction factors for argon isotopic ratios measured daily over the course of measurements performed for this study for each of the two procedures are shown in Appendix A (see footnote 1). As indicated, Ar isotopic ratios measured by multicollection measurements were reproducible to 0.12 (2s standard error) for a 39 Ar abundance of mol based upon 275 measurements performed over a 60 s interval (Appendix A, A C [see footnote 1]). Monocollection measurements from mol 39 Ar gas fractions were reproducible to 0.15 for Ar/ 39 Ar and 0.25 for Ar/ 39 Ar for 119 measurements performed over the same interval (Appendix A, D F [see footnote 1]). In calculating Ar/ 39 Ar ages, we used a decay constant of l = yr 1 (Steiger and Jäger, 1977), which remains consistent with current best estimates (Renne et al., 2011). There is currently no consensus regarding the true Ar/ K of ca. 28 Ma Fish Canyon sanidine (FCs) or any other flux monitor in use for Ar/ 39 Ar analysis. The range of K-Ar ages ascribed to FCs on the basis of different approaches defines a spread of more than 1% (e.g., Phillips and Matchan, 2013) and thus requires that the accuracy of Ar/ 39 Ar results must also be percent level at best. In this study, we assigned FCs a K-Ar age of Ma (e.g., Renne et al., 1998) and quote only analytical uncertainties to maintain consistency with previous studies in the area that adopted such a value (e.g., Brueseke et al., 2007; Coble and Mahood, 2016). J-factor data, calculated as outlined in McDougall and Harrison (1999), appear in Appendix B (see footnote 1), with values used for each sample listed in Appendix C (see footnote 1). Inverse isochron model ages were calculated as described in Mahon (1996). We prefer inverse isochron ages to weighted mean of total gas ages because most of our samples indicate trapped Ar/ 36 Ar ratios in the range (Table 1), which are distinctly lower than the atmospheric value of (Lee et al., 2006). While the difference between inverse isochron ages and weighted mean ages that assume atmospheric trapped argon tends to be negligible for highly radiogenic feldspars, many samples analyzed in this study had radiogenic Ar ( Ar) contents <90% (Table 1; Appendix C [see footnote 1]). A plot comparing inverse isochron and weighted mean ages from representative samples of all four McDermitt volcanic field ignimbrites is shown in Figure 2. Inverse isochron ages for all 47 samples are summarized in Table 1, and all inverse isochron plots are provided in Appendix D (see footnote 1). Most samples analyzed in this study have mean square of weighted deviates (MSWD) values near unity, indicating that the single-crystal age distributions represent homogeneous populations. For those units in which we analyzed several samples, our pooled ages were calculated from the error-weighted mean of inverse isochron ages from individual samples (results in bold font in Table 1). The errors associated with all model ages reported in this manuscript are 2s standard deviations for inverse isochron ages and 2s standard errors for weighted mean ages, indicated by ±± following the guidelines of Renne et al. (2009). MSWD values reflect propagation of all analytical errors (see individual data tables in Appendix C [see footnote 1] for details). The uncertainties associated with model ages include additional systematic errors associated with constants that apply to the entire irradiation ( Ar/ 39 Ar K, 38 Ar/ 39 Ar, 36 Ar/ 37 Ar Ca, and 39 Ar/ 37 Ar Ca correction factors and positional error in the J-factor). Inclusion of these systematic errors (1s model error in Table 1) facilitates comparison of our results with those reported in other studies (e.g., Jarboe et al., 2008, 2010). We accomplished this comparison with studies that used different standard or decay constant values by recalculating previously reported Ar/ 39 Ar ages using a FCs K-Ar age of Ma and a decay constant of yr 1. We were unfortunately unable to apply this approach to results reported in Henry et al. (2016) because the J-factors and Ar/ 39 Ar ratios necessary for this recalculation were not provided. Instead, we arbitrarily lowered Henry et al. s (2016) results 100 k.y. to account for their use of a Ma K-Ar age for FCs and a decay constant of yr 1. Finally, we note that estimation of absolute errors in the Ar/ 39 Ar model ages requires propagation of additional systematic errors associated with the K decay constant and a >1% error in the age of FCs. We did not apply this step, since all samples in this study were co-irradiated to allow precise calculation of relative age differences. GEOLOGIC BACKGROUND The McDermitt volcanic field is a mid-miocene volcanic center along the Oregon-Nevada border consisting of basaltic to rhyolitic lavas, calderas, and outflow sheets of associated rhyolitic ignimbrites. The McDermitt volcanic field straddles the Sr/ 86 Sr i isopleth (Fig. 1), which delineates the boundary between ~30-km-thick crust primarily composed of compositionally primitive accreted mafic terranes in the west and transitional cratonal crust to the east (e.g., Armstrong et al., 1977). Further east, Precambrian cratonal crust is present with a thickness that exceeds 35 km (e.g., Eagar et al., 2011; Stanciu et al., 2016). The Paleozoic to Jurassic Black Rock terrane basement that underlies the McDermitt volcanic field is composed of Paleozoic sedimentary rocks, early Mesozoic arc-related mafic volcanic rocks, and Late Triassic to Early Jurassic deep-marine back-arc basin sediments (Wyld and Wright, 2001, and references therein). These rocks are intruded by middle Cretaceous granitic rocks (N. van Buer, 2012, personal commun.), upon which most of the Miocene volcanic strata were deposited. Basin and Range extension gradually increases from <1% north of the High Lava Plains (Trench et al., 2012; Ford et al., 2013) to ~50% south of the study area in central Nevada (Colgan et al., 2006b; Fig. 1). In the study area, faulting began at ca. 12 Ma and produced a system of high-angle normal faults that collectively accommodate a modest amount (<20%) of crustal extension (Colgan et al., 2006a; Lerch et al., 2008; Fig. 1). This faulting resulted in the exposure of as much as 1 km of Miocene stratigraphy in the footwalls of normal faults, enabling the detailed joint stratigraphic and Ar/ 39 Ar geochronologic analysis that is a significant component of this study. Geological Society of America Bulletin, v. 129, no. 9/

4 Benson et al. TABLE 1. SUMMARY OF NEW Ar/ 39 Ar AGES FOR THE NORTHERN MCDERMITT VOLCANIC FIELD Latitude ( N) Longitude ( W) Rock type Fsp type Inv. isochron age (Ma) An. err An. err (1σ) (2σ) MSWD Model err (1σ) No. grains Ar/ 36 Ar Ar/ 36 Ar Ar trapped err 2σ (%) Unit Sample no. Irr. ID Whitehorse caldera Buckskin Mtn hb rhyoilte lava TB D DL S / S. camp rhyolite lava TB D SL S / W. Willow Ck rhyolite lava TB D SL S / Red Mtn rhyolite lava TB D DL S / Willow Butte rhyolite lava TB D DL S / Flagstaff Butte rhyolite lava TB D DL S / Buckskin Mtn rhyolite lava TB D DL, SL S / S. Flagstaff Ranch rhyolite lava TB D DL S / N. Flagstaff Ranch rhyolite lava TB D DL S / Red Lookout Butte rhyolite lava TB D DL, SL S / Tuff of Whitehorse Creek TB D NWI S / TB D DWI S / TB-522C 12-33D MWI S / TB D DWI S / Bearclaw rhyolite lava TB-362A 12-16D DL S / TB D DL S / NE campground rhyolite lava TB D SL S / N. Whitehorse Butte rhyolite lava TB D DL, SL S / Whitehorse Butte rhyolite lava TB D DL S / Camp turnoff rhyolite lava TB D SL S / Tule Rims Tule Rims rhyolite lava TB D VL S / McDermitt caldera Tuff of Long Ridge TB C DWI A / TB C RI A / TB C RI A / EW C DWI A / Pole Canyon caldera Tuff of Trout Creek Mountains TB-225A 12-10C DWI S / TB-265B 12-11C DWI S / TB C DWI S / TB C DWI S / TB C DWI S / TB C DWI S / EW C DWI S / Fish Creek rhyolite lava TB D DL S / Fish Creek caldera Tuff of Oregon Canyon TB C DWI S / MC7B 12-03C DWI S / ML C DWI S / TB C MWI S / TB C MWI S / TB C DWI S / TB C DWI S / TB D RI S / Antelope Creek rhyolite lava TB D DL S / N. Red Mountain rhyolite lava TB D SL S / TB D DL S / Whitehorse Cyn rhyolite lava TB D DL S / TB D SL S / TB D DL S / Note: Irr. irradiation; Fsp. feldspar; Inv. inverse; An. analytical; MSWD mean square of weighted deviates; err error; Mtn Mountain; hb hornblende; Ck Creek; Cyn Canyon; DL devitrified lava; SL silicified lava; VL vitric lava; NWI nonwelded ignimbrite; MWI moderately welded ignimbrite; DWI densely welded ignimbrite; RI rheomorphic ignimbrite; S sanidine; A anorthoclase. Ages in bold are weighted mean averages of inverse isochron ages obtained on all samples from that unit. Initial Eruption of the Columbia River Basalts and Relationship to Silicic Volcanism The Steens Basalt is the earliest member of the Columbia River Basalt Group, with a total estimated volume of ~31,800 km 3 (e.g., Mankinen et al., 1987; Camp et al., 2003, 2013; Brueseke et al., 2007; Hooper et al., 2007; Jarboe et al., 2008, 2010; Fig. 1). The thickest section (~1 km) of Steens Basalt occurs at Steens Mountain (Fig. 1). There, the lower part of the section is dominated by tholeiitic basalt, whereas the upper part is more chemically varied and includes alkali basalt, trachybasalt, and trachybasaltic andesite (Johnson et al., 1998; Camp et al., 2003). In the most recent definitive work on the chemistry of the Steens Basalt, Camp et al. (2013) subsumed lavas at the top of the type section, which were originally mapped separately as andesite lavas by Johnson et al Geological Society of America Bulletin, v. 129, no. 9/10

5 Geology and Ar/ 39 Ar geochronology of the mid-miocene McDermitt volcanic field Tuff of Oregon Canyon (TB-433) Ar/ 36 Ar trapped ±± 2.8 Weighted Mean Age ±± Ma Tuff of Trout Creek Mountains (TB-456) Ar/ 36 Ar trapped ±± 5.1 Weighted Mean Age 16.2 ±± Ma Ar/ Ar Age (Ma) 36 Ar/ Ar Age (Ma) Inverse Isochron Age ±± Ma MSWD = Ar/ Ar Inverse Isochron Age ±± Ma MSWD = Ar/ Ar Ar/ 36 Ar trapped ±± 3.6 Tuff of Long Ridge (TB-455) Tuff of Whitehorse Creek (TB-322) Ar/ 36 Ar trapped ±± Weighted Mean Age ±± Ma Weighted Mean Age ±± Ma 36 Ar/ Ar Inverse Isochron Age ±± Ma MSWD = Age (Ma) Ar/ Ar 36 Ar/ Ar Inverse Isochron Age ±± Ma MSWD = Age (Ma) Ar/ Ar Figure 2. Inverse isochron plots of representative data from the four major McDermitt volcanic field ignimbrites, with insets showing weighted mean ages. Gray bands indicate 95% confidence intervals, and gray data points indicate grains not included in the age calculation. We prefer inverse isochron ages since weighted mean ages assume trapped Ar/ 36 Ar = and result in significant scatter for many samples, as reflected in these representative insets. Full data appear in Appendices C and D (see text footnote 1). MSWD mean square of weighted deviates. (1998), within their basaltic trachyandesite and basaltic andesite part of the upper Steens (Fig. 3). Under this definition, all flows of upper and lower Steens Basalt are chemically constrained to have concentrations of Ba <900 ppm and Cr >1 ppm. In the McDermitt volcanic field, Steens Basalt units exposed in the Trout Creek and Oregon Canyon Mountains (unit Tbs in Fig. 3) are similar in composition to the basalt, trachybasalt, trachybasaltic andesite, and basaltic andesite of upper and lower Steens Basalt at the type locality at Steens Mountain (Johnson et al., 1998), with Ba <900 ppm and Cr >1 ppm (Fig. 3). In both ranges, Steens Basalt lavas are capped by trachyandesite to trachyte lavas (unit Tt in Figs. 3 and 4) that are more evolved than Steens Basalt, with Ba >900 ppm (Fig. 3). Previous workers classified these intermediate rocks variously as andesite basalt (Carlton, 1969), potassic icelandite (Wallace et al., 1980), iron-rich andesite and quartz latite ( Rytuba et al., 1982a, 1982e; Rytuba and Curtis, 1983; Rytuba et al., 1983b), basaltic ande site (Minor, 1986), andesite (Mankinen et al., 1987), and basaltic trachyandesite (Camp et al., 2013). Geological Society of America Bulletin, v. 129, no. 9/

6 Benson et al. Figure 3. Compositions of Steens Basalt (Tbs) and trachyte and trachyandesite lavas of McDermitt volcanic field (Tt). Data are compared to all members of Steens Basalt at Steens Mountain, including capping basaltic andesite (Johnson et al., 1998; Camp et al., 2013). Steens Basalt is characterized by Ba <900 ppm and Cr >1 ppm. In the Trout Creek Mountains, the section of Steens Basalt lavas is as much as ~450 m thick (Minor, 1986). The overlying trachyandesite and trachyte lavas (Tt) form a 300-mhigh stratocone centered east of Pole Canyon Ba (ppm) McDermitt Volcanic Field Steens Basalt (Tbs) Intermediate lavas (Tt) Steens Basalt Cr (ppm) ~7 km south of the map border in Figure 4A. Steens Basalt and the overlying intermediate lavas ceased erupting in the Trout Creek Mountains prior to eruption of the first ignimbrite of McDermitt volcanic field, the ca Ma Tuff of Oregon Canyon, which conformably overlies them. In contrast, in the Oregon Canyon Mountains (Aspen Spring section in Fig. 5), ~90 m of Steens Basalt and ~80 m of trachyandesite and trachyte lavas lie above the Tuff of Oregon Canyon and are in turn capped by the ca Ma Tuff of Trout Creek Mountains. The vent for these intercalated mafic and intermediate lavas appears to be near Twelvemile Summit, where they reach a maximum combined thickness of ~300 m. The trachyte and trachyandesite lavas flowed further north and west than the underlying Steens Basalt, as they occur directly on top of Tuff of Oregon Canyon with no intercalated Steens Basalt in Whitehorse Canyon and Antelope Creek (Tt in Figs. 4 and 5). Flows of Steens Basalt that Tioc Tule Rims Pole Canyon Qc Tbc Titc Trout Ck Qc Tioc Titc Tttr Tbc dikes A Fig. 7 section location Tttr Tiwc Ts Red Lookout Butte Buckskin Mtn Qc Trtr Tlwy Tbc Tlwy Flagstaff Butte Ts Qc Qc Tlwh Tbc Tttr Tbs Ts Tlty Tbc Tiwc Ts N. Red Mtn Tlwy Pole Cn Red Mtn W Qc 5 km Whitehorse x Ts Tlwo Titc Qc Tloo Tbc Ts Willow Butte Tt camp Ts Qc Titc Trout Ck Mtns Whitehorse Ranch Tt Tioc Tiwc Tlwo Willow Ck Tilr Tiwc Whitehorse Ck Qc Whitehorse Butte Tiwc Tloo Tsb Tilr Tlto Ar/ 39 Ar & geochemistry Geochemistry W Qc Antelope Ck Titc Fish Ck A Tt Titc Fish Creek Tt Tt Tilr Tilr Oregon Cn Mtns Topographic margin (dotted where covered) Structural margin Normal fault (dashed where inferred) Lava vent Whitehorse Road N N Figure 4 (on this and following page). Geologic map of northern McDermitt volcanic field, schematic cross section, and correlation of map units. Geologic map, field-checked and modified from Rytuba et al. (1982a 1982e); Rytuba et al. (1983a, 1983b); and Peterson and Tegtmeyer (1987). X marks the location of well MC-7 (see Rytuba et al., 1981), and camp marks the location of the campground at Willow Creek Hot Springs. Schematic cross section of the northern McDermitt volcanic field calderas. Correlation of map units. Ck Creek, Cn Canyon, Mtn Mountain, Hbl hornblende Geological Society of America Bulletin, v. 129, no. 9/10

7 Geology and Ar/ 39 Ar geochronology of the mid-miocene McDermitt volcanic field occur between Tuff of Oregon Canyon and the intermediate lavas are restricted to the eastern Oregon Canyon Mountains and Blue Mountain region (Fig. 5). The presence of feeder dikes and spatter deposits in both the Trout Creek Mountains and Oregon Canyon Mountains (Minor, 1986; Mankinen et al., 1987; Bondre and Hart, 2008; Camp et al., 2013; this study) is consistent with local sources for the Steens Basalt and intermediate lavas (in contrast to far-traveled, more voluminous flows observed farther north in the Grande Ronde Member of the Columbia River Basalt Group; e.g., Swanson et al., 1975; Tolan et al., 1989; Reidel et al., 2013a). This resulted in the development of significant paleo topog raphy that controlled the distribution and thickness of ignimbrites within McDermitt volcanic field. Estimates for the age of Steens Basalt are ca Ma, based on relatively lowprecision analyses of plagioclase feldspar and K 2 O-poor basaltic matrices (Mankinen et al., 1987; Camp et al., 2003; Brueseke et al., 2007; Jarboe et al., 2008, 2010; Barry et al., 2013). Recently, high-precision Ar/ 39 Ar ages on feldspar separates from ignimbrites and other tuffs interbedded with Steens Basalt lavas, including those mentioned here, have refined the estimate for the duration of this earliest flood basalt volcanism to ca Ma in the vicinity of the study area (Mahood and Benson, 2017). IGNIMBRITE STRATIGRAPHY OF THE MCDERMITT VOLCANIC FIELD Previous Stratigraphy In Rytuba and McKee s (1984) initial description of the McDermitt volcanic field, they identified seven regionally extensive tuffs sourced to seven different calderas. Based upon field data and K-Ar ages, they recognized the 16.1 Ma Tuff of Oregon Canyon, 15.8 Ma Tuff of Trout Creek Mountains, 15.7 Ma Tuff of Double H, 15.6 Ma Tuff of Long Ridge members 2 and 3, 15.6 Ma Tuff of Long Ridge member 5, 15.5 Ma Tuff of Hoppin Peaks, and 15.0 Ma Tuff of Whitehorse Creek (Table 2). Castor and Henry (2000), Starkel (2014), and Henry et al. (2016) subsequently reinterpreted the tuff of Hoppin Peaks as a package of metaluminous rhyolitic lavas and consolidated the Tuff of Double H and Tuff of Long Ridge members 2, 3, and 5 into a single tuff they called the McDermitt Tuff, for which they reported a preferred Ar/ 39 Ar age of ±± 0.03 Ma (Table 2). Refined Stratigraphy Based upon new Ar/ 39 Ar ages, the stratigraphic relations of ignimbrites, and geochemical correlations of outflow sheets, we refine the ignimbrite stratigraphy of McDermitt volcanic field to four major ignimbrites. Listed in stratigraphic order with the new preferred ages, these are: (1) ±± Ma Tuff of Oregon Canyon, zoned from high-silica alkali rhyolite to trachyte; (2) ±± Ma Tuff of Trout Creek Mountains, zoned from high-silica alkali rhyolite to low-silica alkali rhyolite; (3) ±± Ma Tuff of Long Ridge, zoned from high-silica alkali rhyolite to low-silica alkali rhyolite; and (4) ±± Ma Tuff of Whitehorse Creek, a low-silica alkali rhyolite (Fig. 6; Tables 1 and 3). The ages, compositions, field characteristics, and distributions of these ignimbrites are described in detail next Ma Tuff of Oregon Canyon The first major ignimbrite known to erupt from McDermitt volcanic field is the peralkaline Tuff of Oregon Canyon, which resulted in collapse of an ~20 24 km caldera we name the Fish Creek caldera (Fig. 4A). The tuff is typified in the northern McDermitt volcanic field by a complete section exposed in Trout Creek lake sediments A Whitehorse A Titc 1600 Flagstaff Butte Qc Qc Tsw Tlwy Qc 10 Tlwy OCT Steens Tbs PCC Tsp sed PCC Tsp sed 1200 Steens Tbs TCT Titcic TCT Titcic elev (m) Tbc Tbc Tsw WHC cls sed Tsw Tsw TiwcWHT ic Tiwc Tiwc Pole Canyon 5000 m 100 m Titc Whitehorse Butte Tiwc fish Tlto Tt ck rhy Tlwy Tiwc Tlwo FCC Tsf sed? PCC sed PCC Tsp sed FCC Tsfsed? Steens Tsp Tbs TCT ic TCT ic OCT ic? OCT ic? Titc? Titc? Tioc? Tioc? 3x vertical exaggeration Fish Creek SEDIMENTARY ROCKS WHITEHORSE CALDERA POLE CANYON CALDERA FISH CREEK CALDERA OTHER VOLCANIC ROCKS Qc Ts Tsw Tsp Tsf Quaternary cover Whitehorse Pole Canyon Fish Creek Basinal sediments Tsb Postcaldera rhyolite lava Hbl-bearing rhyolite lava Tlwh Tlwy Tiwc Tlwo Precaldera rhyolite lava Tuff of Whitehorse Creek CORRELATION OF MAP UNITS Pole Canyon Volcanics Tlty Titc Tlto Fish Creek rhyolite lava Tuff of Trout Creek Mtns Figure 4 (continued). Tuff of Oregon Canyon Tioc Tloo Precaldera rhyolite lava Postcaldera basalts Tule Rims trachybasaltic Tule Rims andesite Tbc rhyolite lava Tttr Trtr Tilr Tuff of Long Ridge Tt Tbs Intermediate lavas Steens Basalt Geological Society of America Bulletin, v. 129, no. 9/

8 Benson et al. 119 W 118 W Trout Creek 16. Steens Mtn Mickey Hot Springs 30 km Aspen Spring Titc Titc Tioc Tbs Big Sand Gap Blue Mtn Tt x 0.5 Tbs x 0.5 Tioc N Pueblo Mtns Tilr Titc Tioc MBT Tbs Pueblo Mtns Trout Ck Mtns Oregon Cyn Mtns Long Ridge Whitehorse Canyon Tilr Titc Tbs 42 N Idaho Cyn TBM Tilr ICT Bilk Ck Mtns Tt x 0.5 Tioc Tloo Santa Rosa- Calico Tbs Windy Pt Double H Mtns 119 W 118 W Figure 5. Map showing the calderas of McDermitt volcanic field, extent of associated outflow ignimbrites, and stratigraphic sections with dated ignimbrites. Topographic margins of the calderas are shown in thick black lines. Extents of ignimbrites are shown by thin lines: orange Tuff of Oregon Canyon (Tioc), green Tuff of Trout Creek Mountains (Titc), purple Tuff of Long Ridge (Tilr), light blue Tuff of Whitehorse Creek (Tiwc). These extents are constrained by outcrops with chemical analyses (outlined circles) and without chemical analyses (no outline). Tuff of Long Ridge indicated in the Santa Rosa Calico center is a correlation with unit Tp1 of Brueseke and Hart (2008); all other chemical data are from this study. Insets: Sections of outflow ignimbrite sheets with dated samples: ignimbrites of McDermitt volcanic field are shown in same colors as above, yellow other ignimbrites (MBT Tuff of Monument Basin, ICT Idaho Canyon Tuff, TBM Tuff of Big Mountain), dark gray Steens Basalt (Tbs), light gray trachyte and trachyandesite lavas (Tt). Ck Creek, Cyn Canyon, Mtn Mountain, Pt Point. In some sections, Tbs and Tt are shrunk by 50%. Italicized dates are from Coble and Mahood (2016); all other dates are from this study (see Table 1). (Fig. 4A). It contains three main parts: (1) an initial black, lithic-rich, densely welded base overlain by (2) the main eruptive phase of the ignimbrite, which is a light-blue ignimbrite with microphenocrysts of sodic amphibole, grading to (3) densely welded, maroon-colored ignimbrite rich in fiamme and in lithics of basalt to rhyolite lava, all of which are as large as 10 cm in maximum dimension. All three parts of the ignimbrite are weakly porphyritic, with ~10 vol% phenocrysts of sodic sanidine (8%; Appendix E [see footnote 1]), quartz (2%), and minor sodic amphibole, clinopyroxene, fayalite, Fe-Ti oxides, and apatite. In this section, the ignimbrite is zoned from a high-silica alkali rhyolite to trachyte (Appendix F [see footnote 1]). The early portion of the ignimbrite (black to the middle of the blue portion of the tuff) is slightly reversely chemically zoned (Fig. 7), with compati ble element TiO 2 (wt%) decreasing from 0.36 to 0.15 and FeO (wt%) decreasing from 2.88 to 2.52 up section, and incompatible elements slightly increasing over the same interval (Rb: ppm; Zr: ppm). The mid-blue to late maroon portion of the ignimbrite is normally zoned ( wt% TiO 2 ; wt% FeO; ppm Rb; ppm Zr; Fig. 7; Appendix G [see footnote 1]). We correlated outcrops of outflow ignimbrite as Tuff of Oregon Canyon based on observed phenocryst assemblages and the abundances of trace and minor elements measured on ~45 samples throughout the region (Figs. 5 and 8; 1034 Geological Society of America Bulletin, v. 129, no. 9/10

9 Geology and Ar/ 39 Ar geochronology of the mid-miocene McDermitt volcanic field TABLE 2. CORRESPONDENCE OF UNITS WITH OTHER STUDIES OF MCDERMITT VOLCANIC FIELD Rytuba and McKee (1984) Henry et al. (2016) This study -forming unit -forming unit or lava -forming unit Tuff of Whitehorse Creek Whitehorse Tuff of Whitehorse Creek Whitehorse Tuff of Long Ridge Member 5 Long Ridge McDermitt Tuff McDermitt Tuff of Long Ridge McDermitt Tuff of Long Ridge Members 2 and 3 Jordan Meadow Tuff of Double H Calavera Tuff of Hoppin Peaks Hoppin Peaks Biotite rhyolite lava of Hoppin Peaks n/a Tuff of Trout Creek Mountains Pueblo Tuff of Trout Creek Mountains Pole Canyon Tuff of Oregon Canyon Washburn Tuff of Oregon Canyon Fish Creek Appendix G [see footnote 1]). The early black, lithic-rich portion of the ignimbrite is present only in Trout Creek and locally in Tule Rims. The main phase is the most widespread, with outcrops extending west of the Pueblo Mountains and east toward Blue Mountain (Fig. 5). Outcrops of the late maroon, lithic- and pumice- rich portion of the tuff occur throughout northern McDermitt volcanic field and extend north of Mickey Hot Springs (Fig. 5), where we, like Sherrod et al. (1989), correlated the Mickey ignimbrite of Hook (1981) as Tuff of Oregon Canyon. We mapped the unit in Tule Rims mapped as rhyolite and breccia by Rytuba et al. (1982b) as the upper portion of Tuff of Oregon Canyon based on its stratigraphic position below the Tuff of Trout Creek Mountains, a matching phenocryst assemblage, and compositions that fall along the mafic end of the trend for Tuff of Oregon Canyon (Fig. 8). A unit mapped as a lahar by Rytuba et al. (1982e, their unit Tl), which is locally preserved beneath the Tuff of Trout Creek Mountains immediately south of the Fish Creek caldera in the northern Trout Creek Mountains, we interpreted as a proximal fall deposit related to the Tuff of Oregon Canyon. This poorly sorted and weakly stratified deposit is incipiently to moderately welded. It consists of subangular pumice lapilli that are chemically similar to the trachytic end of the Tuff of Oregon Canyon (Fig. 9A; TB 601B in Appendix G [see footnote 1]), and it contains lithics that range in size from <1 cm to >20 cm, one of which is chemically and petrographically similar to the early-erupted portion of the Tuff of Oregon Canyon (TB-601L1). The poor sorting, abundant lithics, and rapid vertical variations in degree of welding within this unit (e.g., Wright, 1980; Mahood, 1984) suggest that these outcrops are high-temperature, near-vent fall deposits equivalent to the maroon-colored ignimbrite west of the caldera. We obtained eight new Ar/ 39 Ar inverse isochron ages on outflow of Tuff of Oregon Canyon from throughout the McDermitt volcanic field (Fig. 6; Table 1). At our type section in Trout Creek (starting coordinates: N, W), ages on the early black vitrophyric portion of the tuff (TB-429; Whitehorse Creek MVF ignimbrite ages (others) MVF ignimbrite samples (this study) Northern MVF rhyolite lava samples (this study) Age (Ma) ±± Ma), the devitrified, blue main eruptive portion (TB-433; ±± Ma) and the late maroon portion (TB-435; ±± Ma) agree within analytical uncertainty (Table 1; Figs. 6 and 7), and they are older Long Ridge Trout Creek Oregon Canyon Figure 6. Age systematics within northern McDermitt volcanic field (MVF), with vertical bars indicating preferred ages and 2σ errors for the four McDermitt volcanic field ignimbrites. The top panel shows agreement of our preferred Ar/ 39 Ar ages for McDermitt volcanic field ignimbrites compared to previous studies (1 Henry et al., 2016; 2 Jarboe et al., 2008). Ages on individual samples obtained in this study appear in the middle (ignimbrite) and bottom (lava) panels. TABLE 3. SUMMARY OF ERUPTIVE UNITS IN THE MCDERMITT VOLCANIC FIELD Age (Ma) Volume (km 3 ) Unit name Map unit Whitehorse caldera (13 12 km) Postcaldera lavas Tlwy, Tlwh ca Tuff of Whitehorse Creek Tiwc ± Precaldera lavas Tlwo ca McDermitt caldera (30 km) -related rhyolite lavas n/a ca Tuff of Long Ridge Tilr ± Pole Canyon caldera (20 26 km) Pole Canyon volcanics Tlto ca Tuff of Trout Creek Mountains Titc ± Fish Creek rhyolite lava Tlty ca Fish Creek caldera (20 24 km) Tuff of Oregon Canyon Tioc ± Precaldera lavas Tloo ca Mafic and intermediate lavas Trachybasaltic andesite of Tule Rims Tta ca Ma 10 Intermediate lavas of Oregon Canyon Mountains Tt ca Ma 200 Intermediate lavas of Trout Creek Mountains Tt ca Ma 200 Steens Basalt Tbs ca Ma 31,800 Age data for pre- and postcaldera lavas from Henry et al. (2016). Volume data from Camp et al. (2013); age data from this study, Mahood and Benson (2017). than the ages of 16.1 ±± Ma and 16.4 ±± Ma obtained at the same locality on the stratigraphically higher Tuff of Trout Creek Mountains (Table 1). West of the Pueblo Mountains, outflow of the Tuff of Oregon Canyon is Geological Society of America Bulletin, v. 129, no. 9/

10 Benson et al. height in section (m) TiO 2 (wt %) Rb (ppm) Zr (ppm) exposed below the Tuffs of Long Ridge and Trout Creek Mountains and above a nonwelded, vapor-phase altered, metaluminous, biotitebearing ignimbrite we informally named Tuff of Monument Basin, which we suggest originated from the nearby Hawks Valley Lone Mountain center based on its mineralogic and compositional similarity to lavas analyzed by Wypych et al. (2011). At this locality, we obtained ages on Tuff of Oregon Canyon of ±± Ma (TB-224) and ±± Ma (MC7B), which are analytically indistinguishable, and which are younger, within analytical uncertainty, than the underlying ±± Ma Tuff of Monument Basin (Mahood and Benson, 2017) and older than the ages for the Tuff of Trout Creek Mountains and overlying Tuff of Long Ridge also sampled in that section (Table 1). Additional ages were obtained on outflow of the Tuff of Oregon Canyon at Catlow Peak ( ±± Ma; ML-304), Aspen Spring ( ±± Ma; TB-444), and west of Whitehorse Ranch ( ±± Ma; TB-264). Together, these eight samples of Tuff of Oregon Canyon give a weighted mean age of ±± Ma (0.014 Ma model error). This preferred age is in excellent agreement with an age of ±± 0.02 Ma (mean of four samples) obtained by Henry et al. (2016), but it differs slightly from an age of ±± Ma reported by Jarboe et al. (2010) based on one sample of outflow ignimbrite collected at Catlow Peak. This slight disagreement may simply be a function of the small sample size (8 grains) of the Jarboe et al Nb (ppm) Ba (ppm) La (ppm) 16.4 ±± Ma Tuff of Trout Creek Mtns 16.1 ±± Ma Tuff of Oregon Cyn ±± Ma ±± Ma ±± Ma Figure 7. Illustration of compositional variation of the Tuff of Oregon Canyon and Tuff of Trout Creek Mountains along a detailed traverse in Trout Creek (starting coordinates: N, W). Three samples of the Tuff of Oregon Canyon and two samples of Tuff of Trout Creek Mountain at this locality were dated via Ar/ 39 Ar geochronology, with each ignimbrite yielding ages that agree within analytical uncertainty. Cyn Canyon, Mtns Mountains. (2010) analysis. In this study, we found that it was necessary to analyze at least 20 single crystals to obtain a clear indication of sample homogeneity and consistency with stratigraphic relationships Ma Tuff of Trout Creek Mountains The Tuff of Trout Creek Mountains erupted during collapse of the newly defined ~20 24 km Pole Canyon caldera. Tuff of Trout Creek Mountains conformably overlies Tuff of Oregon Canyon in the Pueblo and Trout Creek Mountains, whereas in the Oregon Canyon Mountains, a thick stack of upper Steens and intermediate lavas erupted between the two ignimbrites. The outflow ignimbrite is thickest (~100 m) where it banked in against the southern and eastern margins of the older Fish Creek caldera. The Tuff of Trout Creek Mountains is composed of two members, both of which are present in Trout Creek above exposures of Tuff of Oregon Canyon. First-erupted Member A is peralkaline high-silica rhyolite (A.I. [agpaitic index] = 1.24; Appendix F [see footnote 1]) characterized by ~15 vol% phenocrysts of sodic sanidine (Appendix E [see footnote 1]), ~3% quartz, ~2% sodic amphibole, and trace amounts clinopyroxene and Fe-Ti oxide in a dark-green matrix. Xenotime occurs in cavities of the tuff as a vapor-phase mineral. After a short hiatus sufficient to form a crystal-rich black to blue-black basal vitrophyre the relatively more voluminous and more strongly porphyritic Member B erupted. It is a low-silica alkali rhyolite (A.I. = 1.04; Appendix F [see footnote 1]) distinguished by 25 vol% phenocrysts of ~4 mm blocky sodic sanidine (Appendix E [see footnote 1]) and 5% smoky quartz, along with trace clinopyroxene, amphibole, and aenigmatite in a teal-green matrix with abundant (up to 5 vol%) lithic fragments of mafic to intermediate lava. All analyses of both members of the Tuff of Trout Creek Mountains, from ~80 samples collected throughout the region (Fig. 5), fall along a compositional trend similar to that defined by the Tuff of Oregon Canyon (but chemically distinct from the younger Tuffs of Long Ridge and Whitehorse Creek; Fig. 8A). Detailed sampling in Trout Creek demonstrates that Members A and B constitute a normally zoned section with no significant compositional change at the contact between the two members (Fig. 7). Compatible elements increase up section ( wt% TiO 2 ; ppm Ba), and incompatible elements decrease (e.g., ppm Rb; ppm Zr; Fig. 7; Appendix G [see footnote 1]). We identified outcrops of Members A and B using compositional data and phenocryst assemblages. Member A flowed dominantly to the west of the caldera, as most outcrops occur in the Trout Creek Mountains, Tule Rims, and the Pueblo Mountains (Figs. 4A and 5). East of the caldera, Member A is absent, but the moreevolved Member A composition is represented by an ~30 cm fall deposit (TB-359 in Appendix G [see footnote 1]) that is preserved under ~1 m of fall and surge deposits underneath Member B (TB-360) in Whitehorse Creek. Along the southeastern wall of the Fish Creek caldera, we correlate a thin (~1 m) nonwelded ignimbrite containing reversely graded lithic fragments of mafic lava as much as 4 cm in diameter to Member A. Overlying this ignimbrite, there are finegrained fall deposits and the basal vitrophyre of Member B. Member B is more widespread, as it is preserved in all directions: from west of the Pueblo Mountains, to east of Blue Mountain (Fig. 5), and from the McDermitt in the south to Big Sand Gap in the north (Fig. 5). We obtained seven new Ar/ 39 Ar ages from different samples of the Tuff of Trout Creek Mountains. At the section in Trout Creek, Members A and B yielded indistinguishable ages of 16.1 ±± Ma (TB-439 in Table 1) and 16.4 ±± Ma (TB-443), respectively. In the dip-slope section on the west side of the Pueblo Mountains (Fig. 5), the Tuff of Trout Creek Mountains yielded an age of ±± Ma (TB-225A), which is younger than the ages obtained on the underlying Tuffs of Monument Basin and Oregon Canyon, and older than the age obtained for the overlying Tuff of Long Ridge in that locality. An age of ±± Ma (TB-265B) obtained on 1036 Geological Society of America Bulletin, v. 129, no. 9/10

11 Geology and Ar/39Ar geochronology of the mid-miocene McDermitt volcanic field Zr (ppm) A 900 Ignimbrites 700 Whitehorse Ck Long Ridge Trout Ck Mtns Oregon Cyn 500 an outcrop of Tuff of Trout Creek Mountains at North Red Mountain is consistent with its stratigraphic position above the ± Ma North Red Mountain rhyolite lava (Table 1). In the Oregon Canyon Mountains, three ages were obtained on samples of outflow stratigraphically below the ca Ma Tuff of Long Ridge: ±± Ma (TB-451 in Table 1), ±± Ma (TB-456), and ±± Ma (EW-218). The seven inverse isochron ages on samples from seven localities yielded a weighted mean age of ±± Ma (0.015 Ma model error; Table 1). This preferred age is in agreement with a weighted mean age of ±± 0.03 Ma obtained by Henry et al. (2016) on four samples of the ignimbrite. 300 Nb (ppm) B Zr (ppm) TiO2 (wt %) Lavas hb-bearing post-wht pre-wht Tule Rims pre-tct pre-oct Rb (ppm) Tuff of Trout Creek Mtns 300 Nb (ppm) 45 Tuff of Whitehorse Ck Tuff of Oregon Cyn TiO2 (wt %) Tuff of Long Ridge Rb (ppm) Figure 8. Compositions of rhyolite ignimbrites and lavas of McDermitt volcanic field. (A) Plots of TiO2 and Rb vs. Zr and Nb showing samples of McDermitt volcanic field ignimbrites analyzed by energy-dispersive X ray fluorescence. (B) Plots of TiO2 and Rb vs. Zr and Nb showing trends of McDermitt volcanic field ignimbrites (best-fit polynomial lines using data in A) and lavas of northern McDermitt volcanic field: hb hornblende; TCT Tuff of Trout Creek Mountain; WHT Tuff of Whitehorse Creek; OCT Tuff of Oregon Canyon. Ck Creek, Cyn Canyon, Mtns Mountains Ma Tuff of Long Ridge Throughout the study region, the Tuff of Long Ridge overlies Tuff of Trout Creek Mountains. The Tuff of Long Ridge is widely distributed because it is the most voluminous of the ignimbrites of the McDermitt volcanic field, and because it flowed across a landscape previously paved by two older ignimbrites. In their study of the McDermitt volcanic field, Rytuba and McKee (1984) and Conrad (1984) characterized ignimbrites that they proposed were sourced from a topographically prominent, composite collapse structure (including their Calavera, Long Ridge, and Jordan Meadow calderas; Fig. 10) km centered ~20 km west of the town of McDermitt, Nevada. These ignimbrites include the Tuff of Double H, which is exposed largely south of the caldera, and the composite Tuff of Long Ridge, which occurs throughout the volcanic field. They divided the Tuff of Long Ridge into several members. Pheno cryst abundances are highly variable within and between members, from aphyric to moderately porphyritic (3 15 vol% alkali feldspar, minor clinopyroxene and fayalite, and trace ilmenite, sodic amphibole, and quartz). Compositions of all members of the Tuff of Long Ridge fall along a single compositional trend from high-silica alkali rhyolite to trachyte (Fig. 8A), leading Rytuba and McKee (1984) and Conrad (1984) to suggest that they erupted from a single compositionally zoned magma chamber. In a reinterpretation of the stratigraphy of the southern McDermitt volcanic field, Castor and Henry (2000) and Henry et al. (2016) combined the tuffs of Double H Mountains and Members 2, 3, and 5 of the Tuff of Long Ridge of Rytuba and McKee (1984) into a single unit they termed the McDermitt Tuff (Table 2). They reported this tuff as being zoned from aphyric, per alka line high-silica rhyolite to non-peralkaline, high-silica trachydacite with abundant pheno- Geological Society of America Bulletin, v. 129, no. 9/

12 Benson et al. B A densely welded upper nonwelded to densely welded member crystal clot lithic nonwelded lower nonwelded member C Tuff of Trout Creek Mountains ponds thickly against older Fish Creek wall and flows over Fish Creek rhyolite lava Tuff of Long Ridge Trachyte lava Tuff of Trout Ck Mtns Whitehorse Canyon rhyolite lava Tuff of Trout Ck Mtns D Flagstaff Butte Flagstaff Butte rhyolite lava Pole Cyn volcanics Whitehorse lake sediments Tuff of Oregon Cyn Steens Basalt Pole Canyon lake sediments Quaternary alluvium Figure 9. Field photographs from northern McDermitt volcanic field. (A) Densely welded fall deposit of Tuff of Oregon Canyon along the north end of the Trout Creek Mountains. (B) Welding variations in the Tuff of Whitehorse Creek along Willow Creek. (C) Picture looking north from the southern rim of Whitehorse Canyon along the eastern topographic margin of the Fish Creek caldera, showing how Tuff of Trout Creek Mountains ponded at the base of the topographic wall and flowed over a wall developed in a precaldera lava (Tloo). (D) Picture looking east at the sediment unconformity southwest of Flagstaff Butte. Flat-lying Whitehorse caldera lake sediments were deposited atop slightly northeast-dipping sediments of the older Pole Canyon caldera. Red lines indicate normal faults. Cyn Canyon Geological Society of America Bulletin, v. 129, no. 9/10

13 Geology and Ar/ 39 Ar geochronology of the mid-miocene McDermitt volcanic field 42 N Alvord Desert Pueblo Pueblo Mountains W Tule Rims WHC Pole Canyon Trout Creek Mountains McDermitt Long Ridge W Fish Creek Oregon Canyon Mountains Blue Mountain 10 km Washburn OREGON NEVADA Hoppin Peaks Figure 10. Map comparing locations of calderas of the McDermitt volcanic field as originally proposed by Rytuba and McKee (1984; thin lines with italicized names) with updated and corrected locations based on this study and Henry et al. (2016; thick lines). WHC Whitehorse caldera. Pine Forest Range Bilk Creek Mountains Calavera Santa Rosa Mountains crysts of anorthoclase and minor amounts of clinopyroxene and fayalite. They characterized the caldera near McDermitt as resulting from a single collapse event that formed a single caldera they called the McDermitt caldera, rather than being a nested, composite feature formed by several eruptions of closely related magma. Our samples from ~ outcrops of ignimbrite, most of them north of the McDermitt caldera, fall along the compositional trend (Fig. 8A) defined by the Tuff of Double H and Members 2, 3, and 5 of the Tuff of Long Ridge of Rytuba and McKee (1984). Such outcrops in the Oregon Canyon and Trout Creek Mountains commonly contain several flow units that range in phenocryst content from aphyric to ~15 vol% feldspar, with minor mafic phases and no quartz. A section of tuff detailed ~4 km north of Blue Mountain (Fig. 5) consists of a thin vitrophyre with <10 vol% phenocrysts of predominantly 2 3 mm alkali feldspar overlain by densely welded, dark-purple low-silica alkali rhyolite with 5 7 vol% phenocrysts of 1 2 mm feldspar and scarce 2 7 mm lithic fragments (EW-210; Appendix F [see footnote 1]). This is overlain by a perlitic vitrophyre with 5 vol% phenocrysts of 1 2 mm feldspar, which locally contains lithic fragments 3 mm to 10 cm in diameter. Above this vitrophyre, there is an orange to brick-red nonwelded flow unit containing sparse lithics of black obsidian and devitrified rhyolite lava <0.5 cm in diameter. This is overlain by the main flow unit: thick, densely welded, light- to darkpurple, rheomorphic ignimbrite transitioning into a vapor-phase altered, moderately welded, high-silica alkali rhyolite ignimbrite characterized by a light-blue matrix, ~3 vol% phenocrysts of mm euhedral alkali feldspar, and lithics of 1 3 cm intermediate and rhyolite lava (EW-213 in Appendix F [see footnote 1]). The main flow unit, which we correlate to Member 5 of the Tuff of Long Ridge of Rytuba and McKee (1984) and Conrad (1984), is widespread throughout the Trout Creek and Oregon Canyon Mountains, whereas the under lying flow units, which we interpret as corresponding to Members 2 and 3, are present only locally. We interpret the distribution and thicknesses of these flow units as indicating that the main eruption from the southern McDermitt volcanic field was Tuff of Long Ridge Member 5, and the preceding tuffs of Double H Mountains and Long Ridge Members 2 and 3 were relatively minor eruptions that were locally preserved or, in the case of Tuff of Double H Mountains, flowed mostly south of the caldera. The eruption of multiple cooling units, as reflected in the presence of multiple vitrophyres, probably occurred in rapid succession (within tens of years), given that there is no evidence for erosion between the cooling units, and all samples of outflow ignimbrite fall along the same compositional trend (Fig. 8A). In this paper, we use the term Tuff of Long Ridge to refer to the composite section of ignimbrite sourced from the southern McDermitt volcanic field. We prefer the name Tuff of Long Ridge to McDermitt Tuff (Henry et al., 2016) because it has precedence, and because the geographic feature of Long Ridge is made up of the ignimbrite, whereas it is not present near the town of McDermitt. Moreover, four major ignimbrites erupted at McDermitt volcanic field, so calling this unit McDermitt Tuff may generate unnecessary confusion. We retain the name McDermitt caldera from Castor and Henry (2000) because this is how the caldera has been identified informally in most published literature. Our regional reconnaissance work expands the distribution of the Tuff of Long Ridge to the west and south of areas identified in previous studies (Fig. 5). To the west, it occurs locally at Oregon End Table and widely west of the southern Pueblo Mountains, where partially welded Tuff of Long Ridge lies conformably above the Tuffs of Monument Basin, Oregon Canyon, and Trout Creek Mountains (Fig. 5; TB-226 in Appendix G [see footnote 1]). At Idaho Canyon to the southwest, the Tuff of Long Ridge occurs Geological Society of America Bulletin, v. 129, no. 9/