Geology and Evolution of the Mcdermitt Caldera, Northern Nevada and Southeastern Oregon, Western USA GEOSPHERE; V

Research Paper THEMED ISSUE: Cenozoic Tectonics, Magmatism, and Stratigraphy of the Snake River Plain–Yellowstone Region and Adjacent Areas

GEOSPHERE Geology and evolution of the McDermitt caldera, northern Nevada and southeastern Oregon, western USA GEOSPHERE; v. 13, no. 4 Christopher D. Henry1, Stephen B. Castor1, William A. Starkel2, Ben S. Ellis3, John A. Wolff2, Joseph A. Laravie4, William C. McIntosh5, 5 doi:10.1130/GES01454.1 and Matthew T. Heizler 1Nevada Bureau of Mines and Geology, University of Nevada, 1664 N. Virginia Street, Reno, Nevada 89557, USA 2School of the Environment, Washington State University, PO Box 642812, Pullman, Washington 99164, USA 18 figures; 7 tables; 1 plate; 2 supplemental files 3Institute of Geochemistry and Petrology, ETH Zurich, NW Clausiusstrasse 25, 8092 Zurich, Switzerland 4Great Basin GIS, 803 Clover Drive, Spring Creek, Nevada 89815, USA CORRESPONDENCE: chenry@unr.edu 5New Mexico Bureau of Geology and Mineral Resources, New Mexico Institute of Mining and Technology, 801 Leroy Place, Socorro, New Mexico 87801, USA

CITATION: Henry, C.D., Castor, S.B., Starkel, W.A., Ellis, B.S., Wolff, J.A., Laravie, J.A., McIntosh, W.C., and Heizler, M.T., 2017, Geology and evolution of the ABSTRACT dera tuff in the complete section. Intracaldera tuff is strongly rheomorphic, McDermitt caldera, northern Nevada and southeast‑ scrambling the compositional zoning, which is locally better preserved in out ern Oregon, western USA: Geosphere, v. 13, no. 4, The McDermitt caldera (western USA) is commonly considered the point flow sections. However, outflow tuff is also strongly rheomorphic where it p. 1066–1112, doi:10.1130/GES01454.1. of origin of the Yellowstone hotspot, yet until now no geologic map existed of was deposited over steep topography. the caldera and its geology and development were incompletely documented. The caldera underwent post-collapse resurgent uplift driven by intrusion Received 7 November 2016 Revision received 3 March 2017 We developed a comprehensive geologic framework through detailed and re of icelandite magma, which also erupted from two major vents and as wide Accepted 2 May 2017 connaissance geologic mapping, extensive petrographic and chemical analy spread lavas. Major igneous activity around the McDermitt caldera lasted from Published online 17 July 2017 sis, and high-precision 40Ar/39Ar dating. The caldera formed during eruption before 16.7 to 16.1 Ma, although minor high-alumina olivine tholeiite lavas of the 16.39 ± 0.02 Ma (n = 3) McDermitt Tuff (named here), which is strongly were emplaced ca. 14.9 Ma in the youngest recognized igneous activity. zoned from peralkaline, aphyric, high-Si rhyolite (comendite) to metaluminous, Tuffaceous sediments as much as 210 m thick filled the caldera, although abundantly anorthoclase-phyric, trachydacite, or Fe-rich andesite (icelandite). whether deposition preceded, followed, or spanned resurgence is unresolved. The McDermitt caldera formed in an area that had undergone two episodes Although formed during regional extension, the caldera is cut only at its west of Eocene intermediate volcanism at 47 and 39 Ma and major middle Miocene ern and eastern margins by much younger, high-angle normal faults that re volcanism that led continuously to caldera formation. Eruption of the Steens sulted in gentle (~10°) eastward tilting of the caldera. Basalt, the oldest Miocene activity, began before 16.69 Ma. Exclusively mafic Numerous hydrothermal systems probably related to caldera magmatism magmas erupted initially. Rhyolite lavas erupted as early as 16.69 Ma, contem and focused along caldera structures produced Hg, Zr-rich U (some along the poraneous with upper parts of the Steens Basalt, and the proportion of rhyolite western caldera ring fracture dated as 16.3 Ma), Ga, and minor Au mineral increased steadily until eruption of the McDermitt Tuff. Precaldera silicic activ ization. Lithium deposits formed throughout the intracaldera tuffaceous sedi ity was diverse and almost entirely effusive, including metaluminous to mildly ments, probably ca. 14.9 Ma. peralkaline, sparsely anorthoclase-phyric rhyolite to peralkaline, aphyric rhyo Silicic volcanism around the McDermitt caldera is some of the oldest of lite to quartz-sanidine-sodic amphibole porphyritic rhyolite at 16.41 Ma. Meta the Yellowstone hotspot, but the caldera is younger than two known calderas OLD G luminous to peraluminous biotite rhyolite lavas and domes were emplaced in northwestern Nevada. Published data show that silicic activity as old as at around what is now the caldera wall in 4 areas at 16.62, 16.49, and 16.38 Ma. McDermitt is also found from the Silver City area of southwestern Idaho to the Eruption of the McDermitt Tuff generated the irregularly keyhole-shaped, Santa Rosa–Calico volcanic field in Nevada east of McDermitt. As recognized 40 × 30–22 km McDermitt caldera. Collapse occurred mostly along a narrow by others, an area of ~40,000 km2 mostly in northwestern Nevada and south OPEN ACCESS ring-fault zone of discrete faults with variable downwarp into the caldera be eastern Oregon underwent major silicic activity before ca. 16.0 Ma. tween faults. Minor parallel faults locally widen the zone to as much as 6 km. The McDermitt caldera is similar to calderas of the middle Cenozoic ig We find no evidence that the McDermitt caldera consists of multiple nested nimbrite flareup of the Great Basin, especially in strong compositional zoning calderas, as previously postulated. Total collapse was no more than ~1 km, and large volume of erupted tuff, collapse along a distinct ring-fracture system, and total erupted volume was ~1000 km3, of which 50%–85% is intracaldera abundance of megabreccia in intracaldera tuff, and resurgence. The greatest tuff. Uncertainties in these estimates arise because an intracaldera tuff section differences are that McDermitt is larger in area than all except a few flareup This paper is published under the terms of the is exposed only along the western edge of the caldera, and outflow tuff is not calderas and underwent far less collapse (~1 km versus 3–4 km to as much as CC‑BY license. completely mapped. Megabreccia and mesobreccia are common in intracal 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. How partly because McDermitt was thought to be the oldest silicic caldera of the ever, the Snake River Plain ignimbrites are metaluminous and homogeneous track (Pierce and Morgan, 1992, 2009; Parsons et al., 1994; Zoback et al., 1994; in bulk chemistry, with plagioclase, sanidine, and minor quartz as common Camp and Ross, 2004). Recent work shows that several calderas in northwest- phenocrysts, and rarely contain pumice or lithics. ern Nevada are slightly older, and silicic volcanism ca. 16 Ma or older that may record initiation of the hotspot is widespread through Nevada, southeastern INTRODUCTION Oregon, and southwestern Idaho (Fig. 2; Noble et al., 1970; Rytuba and McKee, 1984; Castor and Henry, 2000; Cummings et al., 2000; Christiansen et al., 2002; The McDermitt caldera is a well-known and well-exposed but poorly docu Bonnichsen et al., 2008; Brueseke et al., 2008; Shervais and Hanan, 2008; mented part of the Yellowstone hotspot track in northern Nevada and south- Wypych et al., 2011; Coble and Mahood, 2012, 2016; Streck et al., 2015; Ben- eastern Oregon, USA (Figs. 1 and 2) that is important for many reasons. son and Mahood, 2015, 2016). This wide footprint may indicate the geographic 1. The caldera has long been cited as the point of initiation of the hotspot extent of the initial Yellowstone plume head (Pierce and Morgan, 2009; Coble track (Pierce and Morgan, 1992). The interrelated Columbia River Basalt Group and Mahood, 2012). The oldest rhyolites erupted contemporaneously with late and Yellowstone hotspot constitute the most prominent intraplate Cenozoic parts of Steens Basalt lavas and dikes, the oldest part of Columbia River Basalt magmatic province in the United States (Pierce and Morgan, 1992, 2009; Wolff activity, and are in and south of the southern part of Steens Basalt distribution. et al., 2008; Reidel et al., 2013). Although the origin of the Yellowstone hotspot 2. The McDermitt caldera is a large collapse feature that formed during is debated, in part because of the opposite Newberry trend (Fig. 1; Humphreys eruption of magma that was strongly zoned from peralkaline aphyric rhyo- et al., 2000; Christiansen et al., 2002; Jordan et al., 2004; Foulger et al., 2015), lite to metaluminous porphyritic trachydacite (Rytuba and Conrad, 1981; Con- it is commonly considered to have initiated at or near the McDermitt caldera, rad, 1984; Rytuba and McKee, 1984; Starkel et al., 2012, 2014; Starkel, 2014).

120° ID 01115° 00 200300 km WA Silicic centers of Ye llowstone hotspot track Columbia River McDermitt and other 16 Ma calderas. Dotted are Basalt other silicic volcanic Province centers (see Figure 2) Figure 1. McDermitt caldera in relation Accreted ~Sr 0.706 line, Craton to Yellowstone hotspot track and Co 45° Terrane margin lumbia River Basalt Province. Lines with arrows show generalized trends of Yel lowstone and Newberry tracks. S—Santa Rosa–Calico volcanic field (Brueseke and Hart, 2008). Silicic volcanic centers of the Craton 45° Yellowstone track: O—Owyhee-Humboldt; MT B—Bruneau-Jarbidge; T—Twin Falls, P— Ye llowstone Picabo; H—Heise. From Pierce and Morgan Newberry (1992, 2009); Christiansen et al. (2002); Ellis et al. (2012); Camp et al. (2013); Reidel et al. Steens H (2013); Streck et al. (2015). K+ is location lavas of Kimberly drillhole (Knott et al., 2016). CA—California; ID—Idaho; MT—Montana; P NV—Nevada; OR—Oregon; UT—Utah; T WA—Washington; WY—Wyoming. OR K + B CA NV O McDermittS caldera Accreted UT Craton WY 40° 120° Terrane 115°

120° 119° Source of ~75 km 118° 117° 116° Later Yellowstone hotspot volcanic centers Dinner Creek Tuff ~16 Ma Early Yellowstone hotspot calderas Early Yellowstone non-caldera or uncertain Rooster Comb caldera volcanic centers tuff of Leslie Gulch 15.9 Ma 43° Magnetic anomalies of Glen and Ponce (2002) + Silver City Range 43° Areas of Eocene igneous activity Rhyolite lavas 16.7, 16.3 Ma 0 50 100 Km Steens Mtn

Bruneau-Jarbidge Rhyolite tuffs Whitehorse and older? calderas 12.5–10.5 Ma tuff of Whitehorse Creek 15.67 Ma ?tuff of Trout Creek Mts 16.49 Ma Juniper Mtn Rhyolite lavas, tuffs Hawks Valley-Lone 14.5–13.7 Ma Mtn volcanic field Oregon Santa Rosa-Calico Rhyolite lavas Trout Canyon Mts volcanic field 16.6 Ma Rhyolite lavas 42° OR Creek Mts ID 16.7 Ma 42° CA NV Virgin Valley caldera Idaho Canyon Tuff Jarbidge Rhyolite 16.56 Ma Rhyolite lavas 16.3 Ma 39 Ma Mts 36 Ma

Bilk Creek High Rock caldera McDermitt caldera Summit Lake Tuff orest Range McDermitt Tuff Nor 41 Ma 16.43 Ma 16.39 Ma Rhyolite lavas 16.7 Ma ther

Pine F

36 Ma n Ne Warner Range Tuscarora Cottonwood Creek k Rock Range v 39–40 Ma ada Rift

Blac

41° 41°

120° 119° 118° 117° 116°

Figure 2. Shaded relief digital elevation model of the McDermitt caldera region showing known and proposed early Yellowstone hotspot calderas and other silicic volcanic centers (Noble et al., 1970; Rytuba and McKee, 1984; Castor and Henry, 2000; Manley and McIntosh, 2002; Bonnichsen et al., 2008; Brueseke and Hart, 2008; Brueseke et al., 2008, 2014; Wypych et al., 2011; Hausback et al., 2012; Henry et al., 2012a; Coble and Mahood, 2012, 2016; Streck et al., 2015; Benson and Mahood, 2015, 2016; Benson et al., 2017; Mahood and Benson, 2017), the northern Nevada rift and related magnetic anomalies (John et al., 2000; Glen and Ponce, 2002), and areas of Eocene magmatism (Axelrod, 1966; Castor et al., 2003; Colgan et al., 2006a, 2011; Lerch et al., 2008). Our interpretation of calderas in the High Rock–Virgin Valley area shown here (Castor and Henry, 2000; Hausback et al., 2012) differs from that of Coble and Mahood (2016). CA—California; ID—Idaho; NV—Nevada; OR—Oregon.

Sample Numb rock location map unit Age (Ma) UncertaintyLat27 Long27 Analyst SiO2 TiO2 Al2O3 FeO* MnO MgO CaO Na2O K2O P2O5 LOI MD-8Btuff of Whitehorse CreWhitehorseTtw 15.67 0.05 42.20254 -118.18473 WSU XRF74.59 0.16 12.592.330.060.110.243.716.200.02 Despite many rocks being peralkaline, the caldera area underwent at least three mitt caldera by X-ray fluorescence and inductively coupled plasma–mass MD-8C tuff of Whitehorse CreWhitehorseTtw 42.20254 -118.18473 WSU XRF74.56 0.16 12.572.220.020.140.644.944.720.04 MD-9 tuff of Whitehorse CreWhitehorseTtw 42.20810 -118.18257 WSU XRF75.19 0.16 12.661.720.020.120.494.924.690.02 H12-258 tuff of Trout Creek MoNW calderaTtt 42.03798 -118.11394 ALS ICP-OES, ICP-MS 78.280.3410.77 1.74 0.02 0.05 0.36 3.85 4.54 0.04 1.51 H04-46 tuff of Trout Creek MoOregon Canyon Ttt 16.49 0.05 42.23743 -117.98673 WSU XRF75.21 0.290 10.56 3.62 0.097 0.00 0.66 4.62 4.73 0.074 C04-09 tuff of Trout Creek MoBretz Mine Ttt 16.45 0.04 42.04800 -117.87167 WSU XRF74.43 0.367 10.83 4.31 0.104 0.05 0.40 4.53 4.81 0.032 C04-08 tuff of Trout Creek MoBretz Mine Ttt 16.41 0.03 42.04800 -117.87200 WSU XRF75.68 0.295 9.89 4.52 0.093 0.05 0.27 4.47 4.51 0.036 episodes of biotite-bearing, metaluminous rhyolite eruptions that raise inter- spectrometry (ICP-MS) at Washington State University (http://cahnrs .wsu H04-44 tuff of Oregon Canyon Oregon Canyon Tto 16.57 0.05 42.20846 -117.97573 WSU XRF76.61 0.201 11.30 2.56 0.022 0.00 0.07 4.22 4.86 0.020 H07-174 tuff of Oregon Canyon Angel Canyon Tto 42.13892 -117.88774 WSU XRF75.94 0.226 11.28 2.76 0.059 0.05 0.28 3.89 5.50 0.018 H07-175 tuff of Oregon Canyon Angel Canyon Tto 42.13892 -117.88774 WSU XRF, lCP/MS 76.30 0.224 11.34 2.76 0.038 0.09 0.17 4.36 4.69 0.025 H07-176 tuff of Oregon Canyon Angel Canyon Tto 42.13895 -117.88791 WSU XRF, lCP/MS 76.37 0.19611.26 2.66 0.038 0.18 0.37 4.18 4.72 0.027 H07-177 tuff of Oregon Canyon Angel Canyon Tto 42.13913 -117.88840 WSU XRF, lCP/MS 76.46 0.19211.25 2.70 0.0530.12 0.22 4.28 4.71 0.017 H07-186 tuff of Oregon Canyon North rimTto 42.08374 -118.06801 WSU XRF, lCP/MS 73.13 0.357 12.08 4.32 0.126 0.15 0.46 4.50 4.83 0.049 esting questions about the petrogenesis of all silicic magmatism at McDermitt. .edu/soe /facilities /geolab/) or by ICP–optical emission spectrometry (major H07-187 tuff of Oregon Canyon North rimTto 42.08374 -118.06801 WSU XRF, lCP/MS 67.60 0.690 13.47 6.93 0.2430.38 0.62 4.83 5.05 0.187 H11-198 Aurora icelandite north intracaldera Tia 42.02674 -117.92682 ALS ICP-OES, ICP-MS 63.371.0413.80 8.75 0.41 0.59 3.44 4.07 3.73 0.34 3.96 H11-199 Aurora icelandite north intracaldera Tia 42.02541 -117.92870 ALS ICP-OES, ICP-MS 63.541.0613.76 8.50 0.31 0.59 3.10 4.03 4.36 0.34 1.98 H07-198 TivRoundMt Tiv 41.81757 -118.05543 WSU XRF60.87 1.62 13.899.000.211.584.693.793.590.75 MES369 TivRound MtnTiv 41.825 -118.045 DOE61.83 1.76 14.078.210.181.924.703.003.710.62 BM-V icelandite Black Mtn. Tiv 41.898 -118.031 Wallace et al, 1980 60 1.89 14 9.30 1.73 4.43 3.57 3.60.7 BM-F icelandite Black Mtn. Tiv 41.898 -118.031 Wallace et al, 1980621.3213.39.151.583.613.583.690.7 3. The caldera is well exposed, has been negligibly affected by later exten- oxides) and ICP-MS (trace elements) at ALS Chemex (http://www .alsglobal A-1Aurora lava Cottonwood Cr Til 42.021 -117.933 Wallace et al, 198061.71.0313.69.230.673.383.784.130.33 A-SAurora lava Cottonwood Cr Til42.021 -117.933 Wallace et al, 1980 61 1.21 14.7 9.36 0.38 2.01 4.18 4.66 0.41 H12-251 rhyolite of McDermitt southeast intracalderaTrm 41.75209 -117.94892 ALS ICP-OES, ICP-MS 72.200.5013.12 3.70 0.03 0.29 0.90 3.84 5.12 0.11 1.25 H12-252 rhyolite of McDermitt southeast intracalderaTrm 41.75719 -117.94512 ALS ICP-OES, ICP-MS 71.180.6314.20 3.38 0.04 0.13 0.71 4.39 5.08 0.13 1.32 10McD225 rhyolite of McDermitt Washburn Creek in TrmTrm 42.00636 -117.86027 WSU XRF72.87 0.507 13.40 2.76 0.021 0.21 0.95 3.98 5.20 0.114 09-Mcd-33 rhyolite of McConnell eastern caldera Trm 16.49 0.08 41.88367 -117.87946 WSU XRF, lCP/MS 73.96 0.33912.81 2.60 0.0600.14 0.82 3.62 5.61 0.047 sion, and nicely illustrates many aspects of caldera development, including .com/services-and-products/geochemistry/geochemistry-testing-and-analysis; H07-189 McDermitt Tuff lithic upper ash-flow tuff, basTmtu 41.55556 -118.05456 WSU XRF74.96 0.329 11.47 3.41 0.0550.28 0.45 4.13 4.88 0.053 H07-190 McDermitt Tuff lithic upper ash-flow tuff, topTmtu 41.55556 -118.05456 WSU XRF74.08 0.294 11.15 3.26 0.0720.10 2.02 4.22 4.77 0.041 H12-259 feeder dike, McDermitNW caldera wall Tmv 41.93443 -118.17275 ALS ICP-OES, ICP-MS 72.580.3712.66 3.75 0.11 0.09 0.99 3.69 5.66 0.04 4.02 H95-103 feeder dike, McDermitvi tro dike, porphyriticTmv 16.301 0.02 41.78934 -118.14460 WSU XRF72.77 0.39 12.554.030.130.141.144.714.090.05 1 H95-83 Tmt icelandite vitmegarheomorphic foldsTmt 41.72478 -118.11678 ALS ICP-OES, ICP-MS 64.710.9113.76 8.00 0.29 0.69 3.12 4.30 3.91 0.30 3.84 09-McD-56 Tmt icelandite vitmegarheomorphic foldsTmt NAD83 41.72303 -118.11707 WSU XRF, ICP-MS 67.640.6613.64 6.35 0.25 0.32 2.29 4.47 4.23 0.15 abundant precaldera magmatism, voluminous caldera-forming eruption, a method ME-MS81d) (Supplemental Table S1 ). Seven samples were analyzed 09-McD-131 Tmt icelandite vitsouth of Moonlight TmtNAD83 41.77999 -118.14456 WSU XRF67.28 0.66 13.526.660.260.322.384.404.360.15 10-McD-220 Tmt icelandite vit camping canyon S of Aurora TmtNAD83 42.03483 -117.95391 WSU XRF67.20 0.74 13.877.010.300.312.173.794.430.18 H95-82 McDermitt Tuff Moonlight Tmt 41.7250 -118.1283WSU XRF76.16 0.27 11.402.880.060.000.983.105.130.02 H04-41 McDermitt Tuff IntracalderaTmt 16.3480.01 42.01573 -117.94867 WSU XRF75.69 0.27 11.382.830.030.060.273.935.420.03 MD-3AMcDermitt Tuff Moonlight Tmt 41.78939 -118.14008 WSU XRF73.36 0.45 12.204.110.120.301.634.193.550.09 complex ring-fracture fault system, and post-collapse magmatism and resur- at both laboratories; analyses agree well for the same constituents. We also

1Supplemental Table 1. Chemical analyses. Please gence. Both intracaldera and outflow McDermitt Tuff, the caldera-forming tuff, used six published analyses, which were of major oxides only, from Wallace visit http://doi .org /10 .1130/GES01454 .S1 or the full- are intensely rheomorphic to the degree that their pyroclastic origin is sub- et al. (1980) and Castor et al. (1982). text article on www.gsapubs.org to view Supplemen- stantially obscured in many outcrops. However, the occurrence of abundant, Determining the timing of igneous activity in and around the McDermitt tal Table 1. voluminous rhyolite lavas aids in the discrimination of pyroclastic and effusive caldera was a critical aspect of this study. Published K-Ar dates demonstrate deposits. The McDermitt Tuff has both similarities and differences with volu- that the McDermitt caldera is middle Miocene (Greene, 1976; Rytuba and minous rhyolitic ignimbrites of the central Snake River Plain, where source McKee, 1984) but have large uncertainties, and some are demonstrably inaccu- calderas are entirely buried, but may have an analog in the McDermitt caldera. rate, so are of limited use. Therefore, we obtained single crystal 40Ar/39Ar dates 4. Although mercury is the best-known element of economic significance on sanidine and anorthoclase from 26 samples, which provide the most pre- related to the caldera and the only one that has had significant mining, the cal- cise reproducible ages, and step-heating dates on 5 plagioclase, 1 matrix, and dera also contains known deposits or concentrations of uranium, lithium, gal- 3 potassium feldspars formed during mineralization (Table 2). Table 2 also lists lium, gold, and zirconium. Most of these deposits are, or probably are, related the summed age data of three or four individual sanidine dates on each of the to caldera igneous activity and structures. The caldera contains significantly regional tuffs of the Trout Creek Mountains and Oregon Canyon, respectively. more mineral potential than has been exploited. All ages in this report are calculated relative to the Fish Canyon Tuff sanidine Despite this significance, the basic geology of the caldera and its spa- standard at 28.201 Ma (Kuiper et al., 2008) and a total 40K decay constant of tial, temporal, and geochemical evolution are not well known. Our efforts to 5.463e-10/a (Min et al., 2000). Table 1. 40Ar/39Ar analytical data. understand McDermitt geology include detailed and reconnaissance geologic Initial dating was done with a single collector MAP215–50, whereas dating 40 39 37 39 36 39 39 40 ID Ar/ Ar Ar/ Ar Ar/ Ar ArK K/Ca Ar* Age ±1σ (x 10-3)(x 10-15 mol) (%) (Ma) (Ma) mapping of the caldera and construction of a 1:70,000-scale map (Plate 1), since 2012 was done with a multicollector Argus VI instrument, which is sig- 2016 H16-12, Sanidine, J=0.0029858±0.01%, IC=0.99978±0.0006153, NM-285B, Lab#=65207 40 39 35 3.0880.0280 0.1886 1.469 18.298.316.469 0.043 Ar/ Ar dating of the sequence of igneous rocks and several of the mineral nificantly more sensitive and precise (Heintz et al., 2014; Heizler, 2011; Heizler 28 3.0950.0164 0.2006 0.750 31.298.116.481 0.082 05 3.0980.0227 0.1630 3.067 22.598.516.562 0.012 01 3.1300.0238 0.2620 2.561 21.497.616.578 0.014 deposits, and geochemical analysis of the igneous rocks (this study; Starkel, et al., 2014; Hereford et al., 2016). With these advances in mass spectrometry, 31 3.0720.0210 0.0625 2.362 24.399.516.582 0.026 33 3.1150.0185 0.1824 1.786 27.598.316.621 0.035 32 3.1040.0133 0.1406 4.545 38.398.716.628 0.014 2014; Starkel et al., 2012, 2014). the greatest limiting factors in precise dating are the quality of dated material 26 3.1330.0270 0.2402 1.556 18.997.816.629 0.040 06 3.1580.0133 0.3165 3.809 38.497.116.639 0.011 and reduction of reactor fluence gradients. ARGUS VI data yield lower stan- 04 3.0830.0112 0.0586 4.940 45.699.516.642 0.007 34 3.1470.0170 0.2635 2.794 30.197.616.663 0.023 27 3.0820.0187 0.0346 3.086 27.399.716.680 0.020 dard deviation, but higher MSWD (mean square of weighted deviates) values 29 3.4560.0220 1.2732.335 23.289.116.7220.030 STUDY METHODS 30 3.3040.0215 0.7314 4.980 23.893.516.7650.015 2 13 3.2570.0261 1.1650.279 19.589.5 15.81 0.22 compared to MAP-215–50 data (Supplementary Table S2 ). That is, the mean is 12 3.0780.0227 0.1192 2.833 22.598.916.513 0.022 09 3.0760.0130 0.0910 2.849 39.199.216.542 0.022 01 3.0650.0196 0.0433 3.177 26.099.616.564 0.020 The major objectives of this study included (1) deciphering the igneous more tightly constrained (accurate) with the ultrahigh precision analysis of the 02 3.0900.0181 0.1198 1.968 28.298.916.574 0.032 05 3.1080.0244 0.1760 8.773 20.998.416.587 0.008 03 3.0860.0232 0.1004 2.879 22.099.116.588 0.022 and structural evolution of the McDermitt caldera, (2) assessing published ARGUS VI, but the data are overdispersed relative to the cited precision. Much 15 3.1420.0295 0.2865 2.706 17.397.416.595 0.024 11 3.0820.0162 0.0765 3.848 31.699.316.601 0.017 multicaldera interpretations and proposed ash-flow tuff sources (Rytuba and of the overdispersion can be accounted for when considering both vertical and 04 3.0900.0164 0.1001 3.844 31.199.116.607 0.016 10 3.1090.0190 0.1658 3.714 26.898.516.607 0.017 14 3.1000.0153 0.1345 4.520 33.498.816.607 0.014 McKee, 1984), (3) estimating the area and amount of collapse relevant to gen- circumferal fluence gradients in the irradiation trays. Despite tightly packaging 06 3.0800.0209 0.0677 4.699 24.499.416.609 0.014 08 3.0070.0066 -1.80880.760 77.6 117.8 19.208 0.084 07 3.1820.0157 -2.12280.365 32.6 119.8 20.650.20 eral caldera evolution models, (4) providing a comprehensive geologic frame- crystals in sub–2 mm irradiation pits that are not deeper than 4 mm, the grain 03 3.1990.0201 0.1152 3.609 25.399.017.185 0.010 Mean age ± 2s n=24 MSWD=5.50 27.8 ±14.6 16.607 0.015 work for thorough petrogenetic studies (Starkel, 2014), and (5) analyzing the to grain difference in fluence is measurable. This variation causes the high association of numerous mineral deposits and occurrences in and around the MSWD values for the ARGUS VI data. There is probably geological variation 1Supplemental Table 2. Comprehensive 40Ar/39Ar data. Please visit http://doi.org/10.1130/GES01454 caldera with caldera igneous activity and structure. To accomplish these ob- at the sub–per mil level that is now detectable, but until fluence gradients can .S2 or the full-text article on www.gsapubs.org to jectives, we mapped the geology of the caldera at an overall scale of 1:70,000 be further reduced, assessing geological scatter is somewhat ambiguous. If view Supplemental Table 2. (Plate 1) through a combination of detailed 1:24,000-scale mapping in selected we assume that all scatter is reactor related, and if all grains from all holes areas and less detailed mapping throughout. We also conducted petrographic, (monitors and unknowns) are analyzed, the weighted mean values can be geochemical, and geochronologic analyses of samples that span the Cenozoic used to calculate age and J-factor and the uncertainty would be proportional evolution of the caldera area. to the sum of the inverse variance of each analysis. However, we take a more Phenocryst contents and abundances were determined by point counts conservative approach to determining uncertainty by increasing the calculated and visual estimates of more than 100 thin sections of representative sam- weighted mean error by the square root of the MSWD value. This is conser- ples of all rock types (Table 1). Chemical analyses were obtained of 102 rocks vative because a significant contribution to the high MSWD values is grain to spanning all the intermediate to silicic igneous activity around the McDer- grain variation of neutron flux.

NEVADA BUREAU OF MINES AND GEOLOGY Prepared with support from the Nevada Division of Minerals and the US Department of Energy. PLATE 1

420 117°52'30"W

Ts i Tra2 Qay Tbrm 73 Tto Tmt Tra2 118°0'0"W Ts i 68 30 H05-28

Qfo Qay Qay Qfy Ts i Ts i GEOLOGIC MAP OF THE MCDERMITT CALDERA, HUMBOLDT Tt46o60 4660 Ts i Tvu Ts 14 Tbrm Qfy 4 HD7-186 Ts i Tmt 10 27 430 118°7'30"W 60 Ts Tto Qay COUNTY, NEVADA AND HARNEY AND MALHEUR COUNTIES, OREGON Ta rv Qfy Tmt Ttt Ts i 17 Qay B''' Ttt Qay Tra Ttt Tmt Tmt Qfo 8 35 Ttt 1 2 3 4 Tmt 7 Tmt 1 Ttt 9 1 Christopher D. Henry , Stephen B. Castor , William A. Starkel , Ben S. Ellis , 10 40 Tmt 12 Tis 12 Ta ro Qay Tmt Ts i Qay 2 4 5 5 18 Qay 15 H15-67 Tmt Tmt Ts i Qfy John A. Wolff , Joseph A. Laravie , William C. McIntosh , and Matthew T. Heizler Ttt Tmt Tmt 19 Tmt Tis 6 9 Tvu H05-23P H16-12 Qay 33 H04-39 Qay Ttt Tmt Tvu 65 1 85 Tmt Tra Tra2 Qfo 80 60 46 Tmt Nevada Bureau of Mines and Geology, University of Nevada, Reno Qfo Tis Tis9 Qay Tvu Ttt 50 50 2 Tbl Tis Tra Ts i 12 Tia Qfy Ttt Ttt 5 Qfo 90 Tra 30 School of the Environment, Washington State University, Pullman 20 Tra Trq 2028 Tmt Qfo Qay TttC04-07 3 Tmt Tis 22 Tvu C04-06 Ta rv Qay Tmt Tia Qfo Institute of Geochemistry and Petrology, ETH Zurich, Switzerland Tmt Qfo Qfo Trq 5 4 Ttt Qay Qfo Great Basin GIS, Spring Creek, NV 12 Tis 25 51 Tis Ta ro 5 80 Qfo Qfo Qfo Tmt 70 New Mexico Bureau of Geology and Mineral Resources, New Mexico Tech 4 10 Tis Ta r Ttt 5 Tis Tia 10 Tis Qfy Tmt Tia Ttt? Tis Qay Qay Qfy H11-199 2017 Tmt 18 Qfo Qfo 4 00 10 Tia Tis Qfo Qfy Ts Qfo QaTiy a 75 Tia Tia Tis H04-40 10 Tis Tis Qfo Ta r 541523 9 118°15'0"W Tmt

25 10-McD-225 Qfp Ta r Trm 42°0'0"N Qay Qfy Tis B'' Tmt Tis Qm OREGON Tis Tis Qfy Qm - Mine dumps Ts i 42°046 '0"N NEVADA Ts Tmt Qay Trm 50 Qay - Younger alluvium

4 ry Qfp Qay Tis Trm Qfp - Flood plain deposits na Qfy Qfy - Younger alluvial fan deposits 4 at er Qpl Tmt Qfy Qpl - Pluvial lake bottom deposits Tis Qfy Qu Qay Qps Qls Qct Qps - Pluvial lake shoreline deposits Ts i Trm Qls - Landslide deposits Qfo Tmt Qfo Tis Qfy Qct - Colluvium and Talus Qfo - Older alluvial fan deposits Qfo Ts b Tis Kg

Kg 14.9 Tbl Late basaltic lavas (high-alumina olivine tholeiite) H04-55 Ts b Qfy Intracaldera sedimentary deposits Ts Trm Tis (moat sediments) Known or probable area of lithium mineralzation Kg Kg Late icelandite vent complex and lavas Qfo Tmt? Tiv - Vent complex 16.12, 16.40 Tiv Til Tia Qfy Til - Lavas Qfy Tia - Aurora lavas Tis Tis 10 Trm Trm - Rhyolite of McDermitt Creek (possibly part of McDermitt Tuff) Ts i Qfo Qfo 16.41 Tlm Tlm - Late mesobreccia Qay Qfy Qfy Qfy Tmtu Tmt 10 Tmt - McDermitt Tuff 45 Tis Tmtu - Late intracaldera tuff TmTlvm 16.36 Tmt Tmb TsH12-259 43 Tmb - Megabreccia blocks and mesobreccia Tmv? Trau? Tmv Tmv - Vitrophyric tuff dikes 10 Tis Qay Tis Qfo Qm Ts b Tmt Trau? Tis Trm Ts i

Tv1 Tis Tis Qm WESTERN CALDERA WALL NORTHERN EASTERN CALDERA WALL SOUTHERN Tis (Thacker Pass to south CALDERA WALL CALDERA WALL 75 MD-6 Tmt Tmt Qay Mcd Qm of Disaster Peak) TiTssi 16.36 Tbrh3 Tv1 Trm Qm Qfo Qfy Biotite rhyolite lavas Qfy Trau Tbrh2 Ts b Ts of Hoppin Peaks Til Tis Tbrh1 Qay Ts i 16.38 4640 Tta Til Trm 4640 Qfo C05-280 Trau Sparsely anorthoclase- 25 Tis Qls Trau 16.40 Kg Qfo Tis Qfo phyric rhyolite lavas Kg Qls Qfo

Qay Tiv Tis H04-49 Ta r Aphyric rhyolite lavas Tta Qfo Tis Tpr Peralkaline rhyolite lava Tram 16.40 Tpr Qfp Ts b Tta Til 16.42 Tpr Peralkaline rhyolite lava 16.40 Peralkaline rhyolite lava Trau Biotite rhyolite Qay e lava domes of Tv2 Til

Qfy ocen Mendi Suri Tv2 Tv Qfo 2 90 83 Qfp Mi Ts i Sparsely anorthoclase- Aphyric rhyolite lavas 15 20 16.46 Tvu Tra2 Ta r 25 Qay 09-McD-33 Trau phyric rhyolite lava 80 Tis Qay 10 Ts i 45 Ttt Tuff of Trout Creek Mountains Tbrm 16.49 Qfy Qfy Trm Tram 16.49 41°52'30"N Qay Qay 11-McD-311 Sparsely anorthoclase- Qfy Trau 16.50 Tram 21 3 Tpi phyric rhyolite lava Tis Tv2 16.53 Precaldera intermediate lavas Tv2 Trm H04-51 41°52'30"N M Tv Qps Tvu Volcanic rocks, undivded 2 Volcanic rocks, Sparsely anorthoclase- Qfy Trm Tra O undivided phyric rhyolite lavas 12 80 Tmt Trm N Ts i 12 Qpl Kg NE AT Qfy Trm Trm Tpi Precaldera intermediate lavas Qct Tbrh2 AN ZO 45 Tpi Precaldera intermediate lavas Qay Qay Ts50 b 16.56 Tto Tuff of Oregon Canyon Tmb Qct Qfy TmTs bb M Qct Til ? Ts Qfo T Ts Traw Tbrh2 O Ts b 55 Ta rv Taro - Older aphyric rhyolite lavas Qfy Qay Ts b Ta ro U Ts i H04-53 A''' Tarv - Older aphyric rhyolite vent Qfo Ts i 44 UL Tbrh3 Biotite rhyolite lavas and TN Qct Tbr Trq 25 3 A 16.62 16.61 Quart-sandine rhyolite lava dome Tvs4433 A'' domes of the Moonlight Mine Qfo F Qay A Ts NI Qay Ts b Qay Qay Ts 26 Tmb Qfy Qay TsTmi b 26 17 S Qfo Qfo K Qct Sparsely anorthoclase- Kg Ts 7 Traw Ts A Qfy AF Qfo Ts b Ts i Tpr phyric rhyolite lava Tbrh2 Tvu Tvs Qfy U Qfo Qfo Tpr Tvs P E Ts, Tsb, Steens Basalt, TL Qay Qfo 16.65 Tta Anorthoclase-phyric tuff including coarsely 23 Tpr Qps Qay Tvs B' Kg Tbrh2 plagioclase-phyric Qfy Qa OZ y Qfy 14 Tiv 21 Tbrh1 I N Sparsely anorthoclase- type in Tsb Qfo Qct Tral EN Tmb H07-198 16.69 phyric rhyolite lava 46 QfQfy y Tvs Tis Qps 46 30 Tpr Tis Tis PP 30 (north of map area) 400000m.E Qfy Kg Qfo Qfo O Tis Tsb Qfy 9 Ts b Tmt Qfo H 20 Ts i Kg Qps Qfy Qps Tmb Tram32 12 Qps Qfo Ts i Ta r Kg Tis Qpl Qps QctQfo 64 Qay Ts i Tbrh1 Qps Ts i Tmb 85 Qfy 90 Tmv Tvs Volcaniclastic sedimentary deposits Qpl Qfo 20 Tmv 10 Qc48t Qfo Qps 81 Qfy Tbr Tpr Tmv Qps A' Tis? Tbr Tmb Tbrh1 Ts i 15 Tmv80 Qfo Qps C95-105 Tmvb Tbr 72 21 Tv2 39 Ma andesite A Kg C95-107 H95-103 Qfy Qfo C95-103A Qay Tmtu Qfo Qfy Tv1 Qps Qct Trm Qfy Eocene 47 Ma dacite and andesite Qct QpQps s Qfo Qfo Ta r Qay Qfy Qps Qfy Qfoy Tpr Qps Qfy eous Qct 17 Qps Qfo Kg Granodiorite and related intrusive rocks

Qfo ac Qfy Qct Qct

Qfo et ^ Qfy Qps Qay Qfo Qps

Qfo Cr Qay Qfy Qct Tis Qps GN MN Qps Qfy Qps Qps Qay Qfo Tpr Qps Trm Qps Qay Tis Qps Qps 1º 00' 18º 00' Qfo Qfy Qfy Qfy Qfy 430 Qps Qct 50 × 10 × Trm Qfo 50 UTM GRID AND Tis Til? Qps 1983 MAGNETIC NORTH 41°45'0"N Tbr 55 Qps Qfo DECLINATION AT CENTER OF SHEET 41°45'0"N ? Til Tbr 50 Qps Contact Solid where certain and location accurate, long-dashed Qpl Qfy Tbr Qfo where approximate, short-dashed where inferred, dotted where concealed; Qps Qay Tis 4 Qfo queried if identity or existence uncertain. Qay Tis 09-McD-53 Tis 80 Qfo 60 Tbl Til Qfo Qps QfQpy s Qct 30 Internal contact Solid where certain and location accurate. Qfy 8 117°52'30"W Tmb 22 Qps Qfy Tbr 80 Qps Gradational contact Dashed where approximately located. Indicates Qfy Qct 5 85 17 21 Qfy Qct seperation of detailed mapping from areas of reconnaisance mapping. Tis Qfo Qfo Qfy Tbr Qct 3 Qfo 46 Tm48b 46 20 Qpl 27 Tmt Qps 20 Qfy 90 Tmb 23 Qps ? Qay 90 90 Fault Solid where certain and location accurate, dashed where Tmb 66 Tpr QpsQaQfyo Qct Tbl Qps approximate, dotted where concealed; queried if identity or Qps QfQfo y 70 H12-250 existence uncertain. Ts i15 Qfo 19 Tbl Tis? 21 Tlm QctTmt : ? Qct 21Qay 45 10 Tis Normal fault Solid where certain and location accurate, dashed where Tbl Tlm 45 Tbl Qay Qps approximate, dotted where concealed; queried if identity or Qfo WLC67-13.5 Qay Qfo 38 existence uncertain. Ball on downthrown side. Qfy 20 WLC43-180.2 25 Qfo Tis? Qps Qfy Qay Qfo ? Qfy Tbr 52 Tmt Tbl Caldera margin Solid where certain and location accurate, dashed Tbl Qps Qfo Qfy where approximate, dotted where concealed; queried if identity Qps WLC-58C Qfy or existence uncertain. Ticks on downthrown side. 30 Tbl Qay Tbl Qfy 42 Qfy Tmt Tis Tbl Qps Strike and dip of bedding Qps Qfy Qfo 41 Tis 10 o Qfo Ta r Inclined Tis Adjoining 30' x 60' Qps See accompanying text for unit descriptions and Strike and dip of compaction foliation in ash-flow tuff quadrangle names Qfy Qps geology and evolution of the McDermitt caldera. ³47 Qps Inclined 123 4 Qfy Qfy Qps Qps Strike and dip of compaction foliation in ash-flow tuff that underwent secondary flow 5867 Qps ´56 118°7'30"W Inclined 9 10 11 12 Qps Ta r Tmt Strike and dip of flow banding or flow foliation in volcanic rocks 13 14 15 16 Tmt? Qps Tmt? 33 Qfy Qay Tmt ¦ Inclined 1 Bluejoint Lake Tis? Qfy 2 Steens Mountain Qps Tmt? Tpr Ta r Sample locality 3 Jordan Valley Qps H04-40 4 Triangle Qps Tmt? Qfo 5 Adel 6 Alvord Lake Qay Line of cross section Qps 7 Louse Canyon Qps Qfy Qps QpsQps AA' 8 Riddle 16 18 9 Vya Qps Tpr Tmt 10 Denio Tvu 420 11 Quinn River Valley Map and cross sections are at 12 Bull Run Mountains Qps Tmt? 1:70,000 scale Qfy 13 High Rock Canyon Qps 14 Jackson Mountains

N Qps Qfy 15 Osgood Mountains Qfy 4610 000m. Qfo 16 Tuscorora 11-McD-290 Qay B 10 Tmt?

46 Qfy Qps Qay Qfy Ta r Qay Qps Qfy Scale 1:70,000 Qps 41°37'30"N 41°37'30"N Qfo 01234kilometers Qps Qfy Tvu Qps A A''' Qps feet meters 410 Qfo Qay Moonlight 010.5m23iles Tmt? 9000 Mine A' Montana Section A'' Qps Round Western Mountains B–B' Jordan Hoppin Tmtu Tis Mountain Escarpment Tmb Til Meadow Peaks 05,000 10,000 15,000 feet Tmv Quinn River 2000 Qps Kings River Tis Flat Tbrh3 Tbrh2 6000 Tpr Tiv Qfy Qfy Tmt Tmt Tbrh1 Valley Valley Ts i Tmt Tmt Tmt Ta r Trau Tpi Tis Tis Ta r CONTOUR INTERVAL 50 METERS

? ? Trm

$ ? Trm Tpi

& $ Tvu $

$ $ Tmt $ $ Tbr Tmt Tpi Tpi & QaQfyy 3000 Tvu Tvu Tvu Tvu Tpi 1000 Projection: Universal Transverse Mercator, Zone 11, Tmt Q & ? Q 12 Qps ? Kg North American Datum 1983 (m) Qfo Tvu Tvu Qfy Base map: U.S. Geological Survey Denio 0 0 Some Quaternary units generalized as unit Q on cross section. Some thin surficial Quaternary units have been omitted. 30'x60' quadrangle (1979) 118°0'0"W W E U.S. Geological Survey Alvord Lake 30'x60' quadrangle (1982) U.S. Geological Survey Quinn River Valley 30'x60' quadrangle (1985) U.S. Geological Survey Louse Canyon 30'x60' quadrangle (1984)

B B’ feet meters B'' Double H Nevada Oregon Bretz Flattop 9000 Section Field work completed in 2015 Mountains Montana A–A' Humboldt Co. Malheur Co. Mine Round Black Supported by the Nevada Division of Minerals and the U.S. Department Mountains Tmt of Energy, Yucca Mountain Project Office, Las Vegas, NV, Tmt Mountain B'' Mountain Tra Thacker Til Tra 2 2000 contract 1950125-03. Tpi 6000 Tmt McDermitt Pass Qfy Tiv Tiv Tia Tis Tpi Compilation by Christopher D. Henry

Qfy Tmt Creek Tis Ta ro Qfy

& Tpi Ts ? ? Cartography and map production in ESRI ArcGIS v10.3.1

Tmt $

Ta r Ta r Tis Tmt Tmt Tmt $ ? $ (ArcGeology v1.3) by Irene M. Seelye

? Tpr? ? Ts b? ? $

Tmt $ Symbology (per FGDC-STD-013-2006) Ta r ? Tpr? Ta r $ 1000 3000 & Tvu ? Tpi ? Second Edition, April 2017 Tvu Ts b & ? Tpr? ? Ts b Ta r Tvu Ta r Nevada Bureau of Mines and Geology 0 0 2175 Raggio Pkwy. Some thin surficial Quaternary units have been omitted. Reno, Nevada 89512 S N ph. (775) 682-8766 www.nbmg.unr.edu; [email protected] Plate 1. Geologic map of the McDermitt caldera by Henry et al. (2016). To view Plate 1 at full size, please visit http://doi.org/10.1130/GES01454.S3.

TABLE 1. COMPOSITIONAL AND PETROGRAPHIC CHARACTERISTICS OF IGNEOUS ROCKS OF THE McDERMITT CALDERA Composition Phenocrysts**

Map Age Rock Na2O + † § Rock type unit (Ma) name* SiO2 FeOAI K2OTotal Plagioclase SanidineAnorthoclase Quartz Amphibole ClinopyroxeneBiotiteOlivineOther Postcaldera late basaltic lavas Tbl14.9 HAOT 50 9.90.242.7 0–200–20, 1 x 40–2, ≤1 0–1, ≤1 Fe-rich andesite (icelandite) domes and lavas southwestern caldera Til16.1FA56110.6 6.254, to 15 ≤1, to 8 Aurora lavas Tia16.41 FA 60–649.4–8.50.82–0.78 7.8–8.82–3 2–3, 22–4, 2≤1, ≤1 fa, ≤1, ≤1 Black Mountain Tiv, TilFA60–62 9.4–9.30.70–0.74 7.2–7.35–7 5, 2 Round Mountain Tiv16.3FA61–62 9.4–8.80.64–0.73 7.4–6.7 5–154–15, to 2 rhyolite of McDermitt Creek (McDermitt Tuff?) Trm16.41 R70–73 4.5–3.7–2.8 0.85–0.919.5–9.0 1–2, 1–2 15–25, 1–6<1<1, ≤1 fa, traceap McDermitt Tuff 16.39PR74–77 3.8–2.81.00–1.12 8.2–9.40–5 trace0–5, to 2naa?, trace, 0.2 TmtTD–R68–74 6.57–3.8 0.91–1.018.7–9.8 5–305–30, to 2.5trace–1, to 1fa, ≤1, to 1 FA–TD64–67 8.03–7.070.80–0.92 8.47–8.72301 30, to 2.5≤1, to 0.8 fa, ≤1, to 1 Precaldera peralkalline rhyolite lava Tpr16.40 PR 76 2.5–3.31.0–1.168.6–9.7 12–17 5–12, 2–4, 2–5, 1–2 naa?, trace, <1, 0.4–2aen, mon, some 0.2 naa (vapor chatoyant phase) biotite rhyolites biotite rhyolite lava of Hoppin Peaks Tbrh 16.36 MR 75 1.7–1.90.868.6–8.8 3–6trace–2, 1–2, 1–2 1–3, 1–1.5 trace, 1trace–1, fa, trace, 16.38 1–2 ≤1.5 0.5 biotite rhyolite of Mendi Suri Tbrm 16.49MR760.9 0.93 961.5, 22, 2–31, 1.5hbl, trace, 0.3<1, 0.5 quartz-sanidine rhyolite Trq16.61 MR 76–771.6–1.90.88–0.90 8.6–8.4 15–20 2, 0.2–2 5–7, 0.2–27–10, 0.5–2trace, 0.5 biotite rhyolite of Moonlight Mine Tbr16.62 MR 71–752.6–1.90.84–0.98 10.4–9.4 10–16 6–10, 1.51–3, 2.5trace, <1 1–3, 1.5ap, zr distal ash-flow tuffs tuff of Trout Creek Mountains Ttt16.49 PR 74–763.6–4.51.07–1.24 9.0–9.4 14–20 6–7, 25–12, 2–3naa?, <1, 0.8trace –0.3, ≤1 aen tuff of Oregon Canyon Tto16.56 PR–TD77–67 2.7–5.71.09–0.96 9.8–8.7 6–154–8, 2–4 1–4, 1–2.5 0–1, 1aen anorthoclase-phyric ash-flow tuff Tta16.65 R742.1 0.93 9.5108, 1–3trace, 0.5<1, 1 aphyric and sparsely porphyritic rhyolite lavas aphyric rhyolite Tar, 16.35 AR 75–763.1 1.12–1.17 9.0–9.50 Taro, 16.46 Tarv anorthoclase rhyolite Traw, 16.35 R75–76 3.2 1.07–1.16 9.0–9.31–3 1–2, ≤2 <1?, 0.5 Tra, 16.46 Tram, Trau anorthoclase rhyolite Tral 16.69R 72 1.80.9310.34–5 4, ≤1.5 trace, ≤0.4 Precaldera intermediate lavas basaltic andesite to dacitic lava Tpi16.56 BA 55–6211.4–6.50.60–0.65 5.8–7.80–5 0–5, 1–15trace –1, ≤1 fa 16.40 FA 62–679.4–4.80.57–0.87 6.2–8.5 5–154–10, 1–30–2, 1–2 2–3, ≤0.8 ap Steens Basalt Steens Basalt Ts ≥16.56 B49–54 12.0–10.30.37–0.57 3.7–5.30–5 ≤4, to 2≤1≤1 Tsb≥16.69PB46–51 13.7–11.60.29–0.42 2.9–4.5~40 ≤40, to 60 olivine Eocene younger Eocene andesite Tv2 39.2 A60–62 6.6–6.30.62–0.65 8.1–7.73025, 5<1, 11, 13,1 ap older Eocene dacite and andesite Tv1 46.7 A–D 62–645.3–4.00.52–0.57 6.5–6.93523, 3–4 hbl, 5, 1.34–5, 4zr *R—rhyolite, PR—peralkaline rhyolite, AR—aphyric rhyolite, MR—metaluminous to peraluminous rhyolite, BA—basaltic andesite, FA—Fe–rich andesite (icelandite), B—basalt, PB—porphyritic basalt, TD—trachydacite, D—porphyritic dacite, A—andesite, HAOT—high–alumina olivine tholeiite. †Weight percent, major elements normalized to 100% volatile free. §AI—agpaitic or peralkalinity index (molar Na + K/Al). **Total—mean or range in volume percent followed by size in mm. Abbreviations: tr—trace, hbl—hornblende, fa—fayalite, all—allanite, ap—apatite, zr—zircon, aen—aenigmatite, mon—monazite, naa—sodic amphibole.

TABLE 2. 40Ar/39Ar DATES OF IGNEOUS ROCKS AND MINERALIZATION, McDERMITT CALDERA Single crystal analyses LatLong Age (Ma) Lab Mass Area* Sample UnitMineral wtd mean ±2σ K/Ca ±2σ n† MSWD No.** NAD27 spectrometer Postcaldera rocks NW MD-8Tuff of Whitehorse Creek sanidine 15.67 0.05 49.8 126 18/200.9455600 42.20254–118.18473 MAP215-50 NH11-199Tia, Aurora lava, icelandite anorthoclase 16.412 0.0164.1 3.113/14 3.61 62964 42.02541–117.92870 Argus VI NE H04-55 Tr m, rhyolite of McDermitt Creek (Greene, 1976) anorthoclase 16.417 0.0181.3 1.015/15 3.09 61038 41.96699–117.83488 Argus VI NE 10-McD-225 Tr m, rhyolite of McDermitt Creek (Greene, 1976)plag/ 16.39 0.10 0.80.9 9/15 0.82 59958 42.00636–117.86027 MAP215-50 anorthoclase McDermitt tuff NW H15-67 Tmt, northwestern outflow anorthoclase 16.403 0.00910.63.2 23/252.0064234 42.06208–118.15160 Argus VI NW H12-259 Tmv, northwestern tuff dike anorthoclase 16.374 0.0126.8 5.312/18 2.60 63543 41.93443–118.17275 Argus VI N H05-23P Tmt, northern outflow anorthoclase 16.386 0.0086.0 3.312/13 2.97 61039 42.06038–117.86914 Argus VI N H04-40 Tmt, northern intracaldera, highly rheomorphic anorthoclase 16.348 0.0146.4 1.914/18 4.00 61034 42.01573–117.94867 Argus VI W H95-103 Tmv, western tuff dike anorthoclase 16.301 0.0244.3 3.415/16 6.19 61030 41.78934–118.14460 Argus VI S H07-191 Tmt, southern outflow anorthoclase 16.346 0.0107.4 5.18/8 4.05 61042 41.55587–118.05610 Argus VI Precaldera rocks N H04-39 Tra, anorthoclase rhyolite lava anorthoclase 16.35 0.04 15/220.7555609 42.05767–117.91479 MAP215-50 E H04-53 Tbrh3, biotite rhyolite lava of Hoppin Peak, highest sanidine 16.362 0.02424.63.7 14/186.7561037 41.85060–117.81474 Argus VI E H04-51 Tbrh1, biotite rhyolite lava of Hoppin Peak, lowest sanidine 16.384 0.06924.413.315/15 8.46 61036 41.87344–117.83797 Argus VI E H04-49 Trau, anorthoclase rhyolite lava anorthoclase 16.397 0.0617.4 4.414/15 8.16 61035 41.89883–117.81177 Argus VI W C95-107 Tpr, peralkaline rhyolite lava, Moonlight Mine sanidine 16.399 0.016 133 15914/14 16.3961032 41.78833–118.15333 Argus VI SE H12-250 Tpr, peralkaline rhyolite lava, southeast caldera wall sanidine 16.427 0.015 137 26 15/1513.64 64997 41.72092–117.94967 Argus VI SW 11-McD-290 Tpr, peralkaline rhyolite lava, south of caldera sanidine 16.404 0.018 256 7437/7 9.98 61044 41.63468–118.10511 Argus VI NE H04-47 Tra, anorthoclase rhyolite lava, Oregon Canyon anorthoclase 16.40 0.04 10 17.2 19/192.1955598 42.23777–117.98662 MAP215-50 N C04-07 Tra2, anorthoclase rhyolite lava anorthoclase 16.462 0.0186.3 1.412/13 8.94 61033 42.04817–117.87184 Argus VI NE H05-28 Tbrm, biotite rhyolite dome of Mendi Suri sanidine 16.487 0.01583.247.73/3 7.79 61041 42.09532–117.87168 Argus VI E 09-McD-33 Tram, anorthoclase rhyolite lava (McConnell Canyon) anorthoclase 16.496 0.0792.5 2.413/14 17.9361043 41.88367–117.87946 Argus VI N mean of 3Ttt, tuff of Trout Creek Mountains sanidine 16.49 0.03 ~500 35/35 MAP215-50 N mean of 4Tto, tuff of Oregon Canyon sanidine 16.57 0.02 ~200 49/49 MAP215-50 N H04-44 Tto, tuff of Oregon Canyon sanidine 16.517 0.013 203 24718/19 7.49 64232 42.20846–117.97573 Argus VI NE H16-12Trq, quartz-sanidine rhyolite lava dome sanidine 16.607 0.01527.814.624/26 5.50 65207 42.06029–117.84299 Argus VI W C95-103A Tbr, biotite rhyolite dome, Moonlight Mine sanidine 16.619 0.02026.713.413/14 55.3461031 41.78833–118.15417 Argus VI W C05-280 Tta, anorthoclase-phyric tuff anorthoclase 16.651 0.0088.8 4.312/16 15.8861853 41.90204–118.16437 Argus VI NE H05-26 Tral, anorthoclase rhyolite lava anorthoclase 16.691 0.0215.9 1.911/13 29.7861040 42.18131–117.91709 Argus VI (continued)

To improve sample quality, steps in addition to typical magnetic and den- (Table 2). Dating of anorthoclase probably is ~2–3 three times less precise. sity mineral separation are taken. All feldspars were leached with 5% HF for Argus VI dates are reported to three decimals, whereas MAP215–50 and step ~1 h to remove adhering matrix, alteration products, or other non-feldspar heating data are reported to two decimals in Table 2 and throughout the text. material. However, this leaching does not remove all such material, espe- Complete analytical data and plots are provided in Supplemental Table S2 cially encapsulated melt, mineral, or fluid inclusions (Ramos et al., 2016) (see footnote 2). or alteration products along cleavage planes. Any grains containing such material that could be identified with binocular and petrographic micro- scope examination were discarded, but not all non-feldspar material can PREVIOUS WORK be identified this way. Anorthoclase, which has lower K content and is generally more fractured than sanidine, thus allowing for greater alteration, is Greene (1972, 1976) first recognized the caldera from geologic mapping particularly susceptible to these issues. These points are illustrated herein. of the northeastern part (Jordan Meadow 15′ quadrangle; Greene, 1972) and The results of our dating suggest that the resolution of sanidine is in the low reconnaissance in surrounding areas; he recognized many of the essential rock tens of thousands of years at the ca. 16 Ma age of the McDermitt caldera units, much of the northern caldera fault and flexure boundary, and that intra-

TABLE 2. 40Ar/39Ar DATES OF IGNEOUS ROCKS AND MINERALIZATION, McDERMITT CALDERA (continued) Single crystal analyses LatLong Age (Ma) Lab Mass Area* Sample UnitMineral wtd mean ±2σ K/Ca ±2σ n† MSWD No.** NAD27 spectrometer Mineralization S WLC43-180.2K-feldspar in Li deposits K-feldspar 14.87 0.05 54.4 7/12 15.240.26173 78 14.920.0361056 41.70949–118.06058 Argus VI W C95-105Adularia, Moonlight Mine U mineralization adularia 16.32 0.10 94.9 9/13 16.380.25599041.7880 –118.1597 MAP215-50 NE McD Adularia, McDermitt Mine Hg mineralization adularia no plateau 16.670.14295 1.216.56 0.12 64958 41.9195 –117.8139 Argus VI Postcaldera rocks SE WLC67-13.5 Tbl, late basalt (high-alumina olivine tholeiite) matrix 14.90 0.74 92.5 10/1215.89 1.10 292615.181.1661065 41.71017–118.07190 Argus VI SC H07-198 Tiv, Fe-rich andesite (icelandite) vent plagioclase 16.22 0.13 89.8 11/13 16.32 0.23 2832616.11 0.19 64197 41.81757–118.05543 Argus VI SW 09-McD-53 Til, Fe-rich andesite (icelandite) lava plagioclase 16.08 0.10 93.8 9/10 16.12 0.03 2731315.76 0.10 62989 41.74001–118.10789 Argus VI Miocene precaldera rocks N C04-06 Tpi, precaldera intermediate lava, icelandite plagioclase 16.53 0.16 80.6 8/10 16.54 0.19 2951216.57 0.15 55652 42.04800–117.87212 MAP215-50 Eocene volcanic rocks W11-McD-311 Tv2, andesite lava plagioclase 39.18 0.12 94.2 7/8 39.09 0.22 32087 39.02 0.17 61052 41.87447–118.15990 Argus VI W MD-6 Tv1, dacite lava plagioclase 46.74 0.41 80.4 8/10 46.91 0.27 27715 46.30 0.28 55651 41.91674–118.18456 MAP215-50 Note: Ages in bold are best estimates of eruption or deposition age. Weighted mean (wtd mn) 40Ar/39Ar ages of feldspars (plag—plagioclase) calculated with inverse varience as weighting factor. Uncertainty is after Taylor (1982) (times square root mean square of weighted deviates, MSWD, when MSWD > 1). Error also includes uncertainty of J-factor and reactor interfering correction factors. All 40Ar/39Ar analyses were done at the New Mexico Geochronological Research Laboratory (NMGRL; methodology in McIntosh et al., 2003). Neutron flux monitor Fish Canyon Tuff sanidine (FC-1) with assigned age of 28.201 Ma (Kuiper et al., 2008). Minerals were separated from crushed, sieved samples by standard magnetic and density techniques, feldspars were leached with dilute HF to remove matrix, and all were hand-picked. Decay constants are after Min et al. (2000); λtotal = 5.463 x 10–10 yr–1. Isotopic abundances are after Steiger and Jäger (1977); 40K/K = 1. 167 x 10–4. *Area of caldera or caldera wall; N—north, S—south, E—east, W—west, NW—northwest, NE—northeast, SE—southeast, SW—southwest, SC—south central. Italics indicate outside map area. NAD27—North American Datum of 1927. †Number of single grains used in age calculation/total number of grains analyzed. §%39Ar—percentage of 39Ar used to define plateau age; steps—number of steps in plateau/total number of steps. **Laboratory number (NMGRL analysis number, tied to Supplemental Table S2 [see text footnote 2]).

caldera tuff had undergone secondary laminar flow and folding. Greene (1976) GEOLOGY OF THE McDERMITT CALDERA reported petrographic and major oxide chemical data and K-Ar dates that were largely repeated and supplemented in Rytuba and McKee (1984). Pre-Cenozoic Rocks In the best-known publication about McDermitt caldera geology, Rytuba and McKee (1984) interpreted five overlapping calderas, as well as two to the The oldest exposed rocks in the map area are Cretaceous granodiorite northwest, and correlated specific units to each caldera (Fig. 3); however, they (Figs. 4A, 4B) and abundant crosscutting felsic to mafic dikes. Large irregular did not provide a geologic map. Rytuba and colleagues (Rytuba, 1976; Rytuba bodies of diorite cut the granodiorite, and aplite dikes cut both. The grano and Glanzman, 1978, 1979; Glanzman and Rytuba, 1979) showed generalized, diorite is undated but almost certainly Cretaceous and possibly the same age page-sized geologic maps that do not separate the calderas. Conrad (1984) as plutons in the Santa Rosa Range 20 km to the east, where U-Pb zircon dates thoroughly described the compositionally zoned ash-flow tuff but provided no reveal 2 pulses ca. 101 Ma and 94 Ma (Brown and Hart, 2013). Triassic–Jurassic sample locations, so what he studied relative to our current understanding metasedimentary rocks that are the host rocks for the plutons in adjacent of caldera geology is uncertain. Hargrove and Sheridan (1984) documented ranges (Compton, 1960; Wyld et al., 2003; Purcell and Hart, 2012) are not ex- megarheomorphic folds in intracaldera tuff. posed in the McDermitt map area but probably occur in the subsurface. A considerable amount of published work focused on mineral deposits, particularly U (e.g., Rytuba, 1976; Rytuba and Glanzman, 1979; Rytuba et al., 1979; Wallace and Roper, 1981; Roper and Wallace, 1981; Castor et al., 1982; Eocene Volcanic Rocks (Tv1, Tv2) Garside, 1982; Dayvault et al., 1985; Peterson et al., 1988; Castor and Henry, 2000) and Hg (Yates, 1942; Fisk, 1968; Hetherington and Cheney, 1985; McCor- Two sequences of Eocene andesite to dacite lavas that crop out in the mack, 1986; Noble et al., 1988). Less has been published about Ga (Rytuba northwestern caldera wall are the oldest Cenozoic rocks in the McDermitt et al., 2003) and Li (Glanzman et al., 1978; Rytuba and Glanzman, 1979; Western area. Both sequences directly overlie an irregular surface cut into Cretaceous Lithium NI43–101, 2014; Stillings and Morissette, 2012). Vikre et al. (2016) sum- granitic rock and are overlain by middle Miocene Steens Basalt (Fig. 4A). The marized publicly available data on all deposit types. Each of these contributed older group (Tv1; see Plate 1 for map unit symbols), with a plagioclase 40Ar/39Ar to understanding overall caldera geology. date of 46.74 ± 0.41 Ma (MD-6, Table 2), crops out in a 3 × 2 km area ~3 km

GEOSPHERE | Volume 13 | Number 4 Henry et al. | Geology and evolution of the McDermitt caldera Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/13/4/1066/3995069/1066.pdf 1073 by guest on 26 September 2021 Research Paper 1983; Rytuba et al., 1983a, 1983b; Minor, 1986; Minor et al., 1989a, 1989b; and this study), and proposed calderas of Rytuba and McKee (1984) and Mahood Benson (2017). Figure 3. Google Earth image of McDermitt caldera showing caldera and faults interpreted in this study, approximate limit of outflow McDermitt Tuff (Tmt; from Rytuba and Curtis, Range Range Pine F Pine F 42 ° Mountains Mountains Pueb Pueb McDe rm Appro Benson (2017) with names in bl Calderas of (1984) with names in y Calderas and f Caldera and caldera-related f orest orest lo lo Pueb itt ximate limit of outflo Mines and prospects with name in white lo Whitehorse area of Mahood an d aults of Rytuba and McK Creek Mts Creek Mts tuff of tuff of T T rout rout ello Mountains Bilk Creek w McDer w type aults of this study ue type Pe Catlo Pe Catlo mitt ak w w Whitehorse T ee + uff

Mountains Mountains T T outflo Appro

rout Creek rout Creek

r r r ve ve Ri Ri Kings Kings Vi ew Or ev w McDer ximate limit,

Pe Disaster Pe Disaster

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Va Kings vera M yon o McDer

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Creek Creek

Mountains Mountains Doub Doub

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mitt Creek Creek mitt mitt Bretz a yo yo n n Au Sur Mendi

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Va mitt lley rn mitt T uff Santa R eak s

osa Rang 42 ° e

A Steens Disaster Cretaceous Basalt (Tsb) B Peak granodiorite (Kg) Ts Orevada View Eocene dacite (Tv1, 46.7 Ma) Tsb Steens Basalt

Figure 4. (A) Eocene dacite (Tv1, 46.7 Ma) filling irregular topography developed on Cretaceous granodiorite, northwestern Cretaceous granodiorite caldera wall. Steens Basalt overlies both dacite and granodiorite in background along flatter surface. Dacite cliff is 10 m high. See Plate 1, Table 1, and text for map unit symbols and descriptions in this and subsequent figures. (B) Steens Basalt overlying Cretaceous granodiorite along planar contact at Disaster Peak and Orevada View. Unit Tsb is ~300 m thick C D at Disaster Peak. (C) Steens Basalt with plagioclase phenocrysts as much as 5 cm long in unit Tsb. Observable part of pen P is 5 cm long. (D) Photomicrograph, pre Steens caldera intermediate lava (Tpi, C04–06, Basalt (Tsb) northern caldera wall). Plagioclase (P) and clinopyroxene (C) phenocrysts in matrix of Precaldera intermediate plagioclase, clinopyroxene, Fe-Ti oxides, C lava (Tpi, C04-06, 16.53 ± 0.16 Ma) and finely devitrified glass. Cross-polar ized light. Horizontal field is 3.2 mm.

P P

south of Disaster Peak (Fig. 3; Plate 1). The unit consists of porphyritic andesite CaO, Sr, Cr, and Th than Miocene rocks but are similar in other major oxides

to dacite lava flows and minor dikes (~62%–64% SiO2) with phenocrysts of and trace elements. plagioclase, biotite, and hornblende, and interbedded volcaniclastic rocks con- The Eocene rocks are part of the Eocene–Oligocene southwestward sweep taining angular to rounded blocks of volcanic rocks and sparse granitic rocks to of magmatism across the Great Basin, probably resulting from rollback of the 40 cm in diameter. The total sequence is as much as 200 m thick. shallow Laramide slab (Humphreys, 1995; Henry and John, 2013; Best et al., The younger group (Tv2), with a plagioclase 40Ar/39Ar date of 39.18 ± 2016). Late Eocene intermediate volcanic rocks crop out throughout northern 0.12 Ma (11-McD-311, Table 2), consists of several, poorly exposed flows that Nevada (Fig. 2), as far west as the Warner Range in California (Colgan et al., total ~125 m in thickness and crop out in a narrow 4 × 0.4 km belt just south of 2011), and at least as far east as the Tuscarora volcanic field (Castor et al.,

Tv1. A single analyzed sample has 60% SiO2 and phenocrysts of plagioclase, 2003). The nearest similar rocks are 39 Ma, 60%–63% SiO2, plagioclase-horn- biotite, and clinopyroxene. blende-biotite trachyandesite intrusions in the Pine Forest Range, 50 km to The Eocene rocks are calc-alkaline, metaluminous, and I-type, which dis- the west-southwest (Colgan et al., 2006a). Andesite to dacite lavas erupted tinguishes them from all the Miocene rocks, which are A-type (Fig. 5). Lower in several episodes in the 40–39 Ma Tuscarora volcanic field, ~150 km to the

peralkalinity mostly reflects much higher Al2O3, because the Eocene and Mio- east-southeast (Castor et al., 2003). Other late Eocene rocks are 36–35 Ma ba- cene rocks have similar total alkalies (Fig. 6). The Eocene rocks also have lower saltic lava flows in the Black Rock and Santa Rosa Ranges (Lerch et al., 2008;

FeO, MnO, TiO2, Y, and heavy rare earth elements (HREEs), and variably higher Brueseke and Hart, 2008).

12 A

H05-26 10 Trachydacite

0 (wt% ) Trachyandesite 2 ++ 8 O + K 2

Na Rhyolite 6 Dacite Andesite

4 55 60 65 70 75 80 SiO2 (wt%) 1.4 B Peralkaline Icelandite vents and lavas 1.2 rhyolite of McDermitt Creek 3 O

2 McDermitt Tuff Peralkaline rhyolites

O)/Al 1.0 2 C04-10A Quartz-sanidine rhyolite + Figure 5. (A) Total alkali (Na2O + K2O)-SiO2 + Biotite rhyolites plot of igneous rocks of the McDermitt O + K + caldera. (B) Peralkalinity versus alumina

2 Regional ash-flow tuffs 0.8 Metaluminous Aphyric rhyolites index of McDermitt caldera rocks. (C) Zr Peraluminous Sparsely anorthoclase- versus 104 * Ga/Al plot.

mol (N a phyric rhyolites 0.6 Precaldera intermediate lavas Eocene andesites and dacites

0.4 0.6 0.7 0.8 0.9 1.01.1

mol Al2O3/(Na2O + K2O + CaO) C 700

500 Tbr Moonlight Mine Increasing SiO2 in McDermitt Tuff 300

Tbrh A-type + + Hoppin Peaks granite

Zr (ppm ) C04-10A I- and S-type granite Tbrm, H05-28 100 Mendi Suri

2 104 Ga/Al 4 5 6

3 Icelandite vents and lavas (Tiv, Til, Tia) A Rhyolite of McDermitt Creek (Trm) TiO2 (wt%) McDermitt Tuff (Tmt) Peralkaline rhyolite (Tpr) 2 + Quartz-sanidine rhyolite (Tqr) Biotite rhyolites (Tbr, Tbrm, Tbrh) Regional ash-flow tuffs (Tta, Tto, Ttt) Aphyric rhyolites (Tar) 1 Sparsely anorthoclase- phyric rhyolites (Tra, Traw, Tram, Tral, Trau) Precaldera intermediate lavas (Tpi) H05-28 Eocene andesites and dacites (Tv1, Tv2) C04-10A Tbrm B C 11 Al2O3 (wt%) 10 FeO* (wt%) 16 9 H05-26 8 C05-280 7 14 C04-10A H05-28 6 Tbrm 5 12 4 3 2 H05-26 + 10 1 C05-280 +H05-28 C04-10A Tbrm Figure 6. Harker variation diagrams of Mc 0.5 D 60 65 70 75 Dermitt igneous rocks. (A) TiO2. (B) Al2O3. 1.2 E SiO2 (wt %) (C) FeO*. (D) Mol (Na2O + K2O)/Al2O3. mol (Na O + K O)/Al O (E) MnO. (F) Zr (ppm). (G) La/Yb. 2 2 2 3 0.4 MnO (wt%)

1.0 0.3 ++ 0.8 0.2

0.1 0.6 C05-280 ++ 600 40 H05-26 60 65 70 75 F G SiO2 (wt %) C05-280 500 Zr (ppm) La/Yb H05-26 30 + 400 Tbr Eastern Moonlight Mine + caldera wall 300 20 Tbrh 200 Hoppin Peaks ++ C05-280 C04-10A 10 100 Tbrm, H05-28 Mendi Suri H05-28 C04-10A Tbrm 55 60 65 70 75 80 55 60 65 70 75 80 SiO2 (wt%) SiO2 (wt%)

Middle Miocene Precaldera Volcanism Steens Basalt (Tsb, Ts) around the McDermitt caldera. Mafic lavas around the McDermitt caldera underlie the tuffs of Oregon Canyon and Trout Creek The following discussion is mostly arranged by age, oldest to youngest. Mountains, fit the formal definition of Camp et al. (2013), and are here termed However, distinct categories of rocks, (1) Steens Basalt and precaldera inter- the Steens Basalt. We divide the Steens Basalt into two units based on stra- mediate lavas, (2) sparsely porphyritic to aphyric rhyolite lavas, (3) distal ash- tigraphy and the presence of coarsely plagioclase-phyric lavas common to the flow tuffs, and (4) biotite-bearing rhyolites, are grouped because of their petro- Steens Basalt (Fig. 4C). The lower unit Tsb consists of numerous thin (≥2 m) graphic, compositional, and possible genetic relationships. basalt to basaltic andesite lavas that include both aphyric and coarsely plagioclase-phyric lavas. The porphyritic rocks have plagioclase phenocrysts as Volcaniclastic Sedimentary Deposits (Tvs) much as 6 cm long as well as 2–3-mm-diameter olivine and clinopyroxene. Unit Tsb is ~300 m thick at Disaster Peak, the best-exposed and most complete A probable middle Miocene, volcaniclastic sedimentary sequence (Tvs) section; Tsb is equivalent to unit Ts (Steens Basalt) of Minor et al. (1989b) at that consists of conglomerate, sandstone, mudstone, and siltstone crops out Catlow Peak along the northwest continuation of the Disaster Peak–Orevada in the caldera wall east of Horse Creek. This unit filled a topographic low at View ridge (Fig. 3). An overlying unit Ts ~75 m thick is predominantly aphyric least 100 m deep cut into Cretaceous granitic rock and underlies the Steens basalt flows and is stratigraphically equivalent to unit Taf (andesitic flows) of Basalt (Tsb). Conglomerate layers contain small (≤10 cm, most ≤1 cm) clasts Minor et al. (1989b), which is basalt according to the analyses of Camp et al. of granitic rock, probable Eocene volcanic rocks, and possible Miocene vol (2013). Detailed petrographic and geochemical studies demonstrate that both canic rocks. The presence of accretionary lapilli-bearing, shard-bearing, and Tsb and Ts of this study (Starkel, 2014; Starkel et al., 2014) and Ts and Taf biotite-bearing ash beds in the upper part of the unit and fining-upward bed of Minor et al. (1989b) at Catlow Peak (Camp et al., 2013) are Steens Basalt. sets indicate deposition during active volcanism, which was probably the ear- Minor et al. (1989b) mapped tuff of Oregon Canyon capping their unit Taf at liest part of middle Miocene activity. Catlow Peak. Unit Tsb crops out only around the northwestern wall of the caldera as far Steens Basalt (Tsb, Tb) and Precaldera Intermediate Lavas (Tpi) south as Horse Creek. Unit Ts occurs in the same area as well as across most of the northern caldera wall. Rocks of both units may underlie younger rocks Middle Miocene volcanism began in the McDermitt caldera area before farther south. 16.691 ± 0.021 Ma, the oldest dated rock of this study and ca. 300 ka. before Precaldera intermediate lavas (Tpi). Effusive volcanism continued after eruption of the McDermitt Tuff and caldera collapse (Table 2). Precaldera rocks emplacement of the tuff of Oregon Canyon but became more silicic than

are predominantly Steens Basalt (Tsb, Tb) and intermediate composition lavas the Steens Basalt. Unit Tpi consists of intermediate (~56%–66% SiO2; Figs. (Tpi), which are extensively exposed around the northern half of the caldera 5 and 6), aphyric to sparsely plagioclase-phyric lavas that overlie the tuff of and regionally (Fig. 4B; Greene, 1972, 1976; Brueseke et al., 2007; Bondre and Oregon Canyon or tuff of Trout Creek Mountains where the former is absent. Hart, 2008; Brueseke and Hart, 2008; Jarboe et al., 2010; Camp et al., 2013), The flows may be as much as 425 m thick in the northern caldera wall, al- along with numerous interbedded local to distal silicic lavas and tuffs. Erupted though the sequence may be repeated by unrecognized faults resulting from rocks generally became more silicic through time, although rhyolite lavas are caldera collapse, and the bounding tuffs crop out discontinuously. Unit Tpi is locally interbedded with Steens Basalt low in the stratigraphic section. These equivalent to unit Tif (intermediate flows) of Minor et al. (1989b) northwest of rhyolitic units are particularly important because they are dated more precisely the caldera. than the more mafic rocks, and so provide tight age constraints on Steens Unit Tpi and other Miocene intermediate rocks of the McDermitt caldera

magmatism (Henry et al., 2006; Mahood and Benson, 2016, 2017). generally match the characteristics of icelandite, i.e., a high FeO*, TiO2, MnO,

Definition of Steens Basalt. Camp et al. (2013) reviewed the controversy low Al2O3, and simple phenocryst assemblage, compared to calc-alkaline ande over what constitutes the Steens Basalt, stratigraphic unit versus geochemical sites (Fig. 6; Carmichael, 1964), and the term commonly has been applied to rock type (e.g., Brueseke et al., 2007), and formally proposed stratigraphic lim- the McDermitt rocks (Wallace et al., 1980; Noble et al., 1988). McDermitt rocks

its to the unit. Based on 2 measured sections 13 km northwest and 15 km north are significantly more potassic and span a wider SiO2 range than the original

of the northern margin of the McDermitt caldera, Camp et al. (2013) restricted icelandites (59%–65% SiO2; Carmichael, 1964). However, others have applied

the Steens Basalt to rocks below the tuffs of Oregon Canyon or Trout Creek the term to a wider SiO2 range and moderately potassic rocks (e.g., 54%–63%;

Mountains, distinct marker units that crop out along the northern margin. Av- van der Zander et al., 2010). Many McDermitt rocks fit either SiO2 range, so erage single-collector MAP215–50 sanidine 40Ar/39Ar dates for these tuffs are we use the term here despite its imperfect match with the original icelandites. 16.57 ± 0.02 Ma and 16.49 ± 0.03 Ma, respectively (n = 4 and 3, respectively, Unit Tpi consists mostly of icelandites or trachyandesites with 8%–10% Table 2; Fig. 7), and a new multicollector Argus VI date on the tuff of Oregon FeO* (Wallace et al., 1980; Figs. 5 and 6). As with all Miocene rocks, they plot Canyon is 16.517 ± 0.013 Ma (H04-44; Table 2). in the A-type field on a Ga/Al vs Zr plot; they are along linear trends with other

McDermitt Volcanic Field: Igneous Rocks and Mineral Deposits 15.5 16.0 Age (Ma) 16.5 17.0

14.87 ± 0.05 K feldspar, Li, WLC43-180.2 Mineral Deposits U, Moonlight Mine, C95-105 M Hg, McDermitt Mine ? McD 14.90 ± 0.74 Tbl, WLC67-13.5 Postcaldera rocks tuff of Whitehorse Creek, MD-8 M Til, icelandite lava, 09-McD-53 Tiv, icelandite vent, H07-198 Tia, Aurora lava, icelandite, H11-199 Tr m, rhyolite, McDermitt Creek, H04-55 Tr m, rhyolite, McDermitt Creek, 10-McD-225 Figure 7. Timing of middle Miocene igne M ous activity in and around the McDermitt McDermitt Tuff Tmt, mean of 3 best dates; see text caldera. Activity began with eruptions Tmt, NW outflow, H15-67 of Steens Basalt between ca. 16.85 and Tmt, N outflow, H05-23P Tmt, NW tuff dike, H12-259 16.74 Ma (Camp et al., 2013; Mahood and Tmt, N intracaldera, H04-40 Benson, 2016) and progressed continu Tmt, W tuff dike, H95-103 ously to eruption of the McDermitt Tuff ca. Tmt, S outflow, H07-191 16.39 ± 0.02 Ma, then was followed prob Precaldera rocks ably more sporadically by postcaldera ice biotite rhyolite of Tbrh3, H04-53 Tbrh1, H04-51 landites to ca. 16.1 Ma. The tuff of White Hoppin Peaks horse Creek erupted ca. 15.7 Ma, possibly peralkaline rhyolites Tprw, western wall, C95-107 from a caldera northwest of the McDer Tpre, eastern wall, H12-250 mitt caldera (Fig. 3). The youngest igneous Tprs, southern wall, 11-McD-290 activity in the caldera area was eruption anorthoclase rhyolites Trau, H04-49 of high-alumina olivine tholeiite lavas (Tbl) Tra2, C04-07 ca. 14.9 Ma. Most mineralization was de Tra, H04-39 monstrably or probably contemporaneous M Tra, H04-47 M Tram, 09-McD-33 with the major pulse of igneous activity, but formation of lithium deposits appears Tpi, icelandite, C04-06 precaldera intermediate lavas M to have occurred much later, ca. 14.9 Ma. biotite rhyolite of Mendi Suri Tbrm, H05-28 Ttt, tuff of Trout Creek Mts, n=3 distal tuffs M Tto, tuff of Oregon Canyon, H04-44 Tto, tuff of Oregon Canyon, n=4 M Adularia quartz-sanidine rhyolite lava dome Trq, H16-12 Matrix With 2 σ error bars biotite rhyolite of the Moonlight Mine Tbr, C95-103A Plagioclase Older MAP data anorthoclase-phyric tuff Tta, C05-280 with “M” Anorthoclase anorthoclase rhyolite Tral, H05-26 Sanidine Steens Basalt (Mahood and Benson, 2016, 2017) (Ts, Tsb) (Camp et al., 2013)

15.5 16.0 Age (Ma) 16.5 17.0

Miocene rocks in most major oxide and trace element plots, e.g., FeO* and Zr. Two small outcrops of mafic icelandite (~56%–57% SiO2) that appear be- However, they have distinctly lower MnO and Ba and higher Sr contents at a neath rhyolite lavas in the southeastern caldera wall are included in unit Tpi.

given SiO2 than do the least silicic rocks of the McDermitt Tuff (Fig. 6). These rocks are demonstrably older than 16.41 Ma, and the tuffs of Oregon Plagioclase from an icelandite lava that underlies the tuff of Trout Creek Canyon and Trout Creek Mountains are not exposed in this area. Their pres- Mountains at Flattop in the northeastern caldera wall yielded a 40Ar/39Ar date ence indicates that precaldera intermediate lavas (Tpi) occur at least beneath of 16.53 ± 0.16 Ma (C04-06, Table 2; Fig. 4D). The precaldera intermediate lavas the southern part of the caldera. Younger rhyolites cover all rocks farther south. can be no younger than ca. 16.41 Ma, the average age of the peralkaline rhyo- However, aeromagnetic highs that coincide with the western faulted edge of lite lava (Tpr; Table 2), which overlies intermediate lavas in the eastern caldera the Double H Mountains (U.S. Geological Survey, 1972) suggest that interme- wall and at the Moonlight Mine (Plate 1). diate or mafic rocks probably underlie the rhyolites.

Sources for Steens Basalt and precaldera intermediate lavas. No dikes or Stevenson and Wilson, 1997), or through fusion of tephra due to heating and other unequivocal sources for the Steens Basalt or the precaldera intermediate compaction by overlying lavas. The fall deposits are not indicators that the lavas have been found in the McDermitt map area. However, Glen and Ponce flows are lava-like ignimbrites, because the flows invariably have basal brec- (2002) identified three prominent, curvilinear, approximately north-trending cias that overlie the fall deposits with a sharp contact (Bonnichsen and Kauff- magnetic bands, the central of which extends through the McDermitt cal- man, 1987; Henry and Wolff, 1992). dera and suggests an underlying dike swarm (Fig. 2). The nearest known dike The only identified source for any of these lavas is a circular vent of geo- swarm, which extends south from Steens Mountain to the Pueblo Mountains, chemically distinct aphyric rhyolite (Tarv vent for unit Taro) ~900 m in diame- coincides with the western magnetic lineation. Minor (1986) and Minor and ter in the northeastern caldera wall. The vent has radial, inward-dipping flow Wager (1989) found several 1–6-m-wide, porphyritic basalt dikes in the north- bands and abundant, locally glassy, carapace breccia. The other lavas proba- ern Bilk Creek Mountains that they interpreted to be feeders to the Steens Ba- bly also have nearby sources, as suggested by the coarseness of interbedded salt, but no magnetic band is present there. pyroclastic-fall deposits (Fig. 8A). The anorthoclase-phyric and aphyric rhyolites mostly plot as two coher- Sparsely Porphyritic to Aphyric Rhyolite Lavas (Units of Tra and Tar) ent and contiguous groups, suggesting that the aphyric rhyolites are more evolved versions of the porphyritic rocks (Figs. 5, 6, and 9). With the exception Sparsely anorthoclase-phyric (Tra, Tral, Tra2, Traw, Tram, Trau) to aphyric of obsidian nodules from unit Taro, the aphyric rhyolites are highly enriched rhyolite (Tar, Taro, Tarv) lavas that are interbedded with middle and upper in incompatible elements, e.g., Zr and REEs, have large Eu anomalies, and are parts of the Steens Basalt or precaldera intermediate lavas (Tpi) crop out in depleted in compatible elements such as Ba. Obsidian from Taro (C04-10A) the northern, eastern, and southern caldera walls. The different units desig- has low Zr and REEs, which suggest affinity to nearby biotite rhyolite despite nate lavas at different locations or stratigraphic positions. The anorthoclase being aphyric. rhyolites have 2%–8% phenocrysts, mostly of anorthoclase and lesser clinopy- The aphyric to sparsely porphyritic rhyolite lavas of this study are shown roxene. The aphyric rhyolites commonly have nonhydrated glass, especially as ash-flow tuff, units 1 and 2, in the southern and southeastern caldera wall, in upper breccias. The lavas are 10 m to as much as 200 m thick; they erupted and as ash-flow tuff, units 4-5 undivided, in the northern caldera wall (Rytuba, 1 throughout the precaldera activity, and dates range from 16.691 ± 0.021 to 976; Rytuba and Glanzman, 1978, 1979; Glanzman and Rytuba, 1979). However, 16.35 ± 0.04 Ma (Fig. 7; Table 2), all on anorthoclase rhyolites. In keeping with the rocks are lavas.

the general upward increase in SiO2, the typically lower SiO2 (72%–75%) an-

orthoclase rhyolites mostly underlie the higher SiO2 (75%–76%) aphyric rhyo- Distal Ash-Flow Tuffs lites where they occur together. The anorthoclase rhyolites are metaluminous or peraluminous to slightly peralkaline, whereas the aphyric rhyolites, with Anorthoclase-phyric ash-flow tuff (Tta). Three rhyolitic ash-flow tuffs from one exception, are peralkaline (Figs. 5 and 6). uncertain sources are also interbedded with Steens-type rocks. The oldest tuff

The oldest anorthoclase rhyolite is a 16.691 ± 0.021 Ma, 72% SiO2 lava (sam- is an anorthoclase-phyric rhyolite (16.651 ± 0.008 Ma, Fig. 7; C05-280, Table 2) ple H05-26, Table 2) from the lower rhyolite unit of Rytuba and Curtis (1983), the that crops out only in a small area ~7 km south of Disaster Peak in the western lowest exposed unit along the range front ~13 km north of the caldera (Fig. 3). caldera wall, where it overlies a thin sequence of units Tv2 and Ts that overlies The rhyolite is directly overlain by mafic lavas, including some with plagioclase Cretaceous granodiorite. This is also the oldest middle Miocene ash-flow tuff in laths as much as 1 cm long, and is ~120 m stratigraphically below the tuff of the region, not identified in any previous studies (Castor and Henry, 2000; Coble Oregon Canyon. No similar rhyolite lava occurs in the stratigraphically equiv- and Mahood, 2012, 2016). The source is unknown. Its anorthoclase phenocrysts alent Catlow Peak section (Camp et al., 2013). The 16.651 ± 0.008 Ma anortho- and composition suggest that it may be related to the anorthoclase-phyric rhyo

clase-phyric ash-flow tuff (Tta; see following discussion of Distal Ash-Flow lite lavas, but the tuff has higher Al2O3 and lower FeO* and is more similar to Tuffs) overlies basalts of unit Ts and may be a pyroclastic deposit related to the sample H05-26 than to other anorthoclase-phyric rhyolites (Fig. 6). anorthoclase-phyric lavas. All other anorthoclase or aphyric rhyolites overlie Tuff of Oregon Canyon (Tto). The 16.57 ± 0.02 Ma (n = 4 samples by tuff of Oregon Canyon and have ages ranging from 16.49 ± 0.08 or 16.462 ± MAP215–50) and 16.517 ± 0.013 Ma (Argus VI, H04-44, Table 2) tuff of Ore-

0.018 to 16.35 ± 0.04 Ma (Table 2). The only anorthoclase rhyolite lava in the gon Canyon is a mildly peralkaline, mostly high-SiO2 rhyolite distinguished western caldera wall is an undated flow (Traw) in unit Tpi along Horse Creek. from the anorthoclase-phyric tuff by age, composition, and the presence of Pyroclastic-fall deposits consisting of coarse, poorly sorted pumice, lo- sanidine, quartz, and aenigmatite (a Na-Fe-Ti silicate found in peralkaline cally with obsidian blocks, are nearly ubiquitous between flows (Fig. 8A). The rocks) phenocrysts (Figs. 5 and 6; Table 1). Rytuba and Curtis (1983) named pyroclastic eruptions were probably precursors to lava extrusion. Many fall the tuff of Oregon Canyon for its occurrence in Oregon Canyon ~15 km north deposits are strongly welded, either from agglutination or welding during of the caldera (Fig. 3). The tuff crops out locally in the northern caldera wall, emplacement, both indicating near-vent deposition (Sparks and Wright, 1979; and Minor et al. (1989b) and Camp et al. (2013) showed it capping the Steens

Outflow McDermitt Tuff (Tmt) A B Pyroclastic-fall deposit Sparsely anorthoclase-phyric between Aphyric rhyolites (Tar) rhyolite lava (Tra2, C04-07, 16.462 ± 0.018 Ma)

Precaldera intermediate lavas (Tpi)

Figure 8. (A) Pumice-obsidian fall deposit (Tar), Double H Mountains south of cal dera. Nonwelded to welded fall deposits Biotite rhyolite of Mendi Suri consisting of poorly sorted pumice (to (Tbrm, H05-28, 16.487 ± 0.015 Ma) 12 cm) and obsidian blocks (to 15 cm) are commonly interbedded with flows. (B) Google Earth view southwest across 1.1 km diameter lava dome of biotite rhyolite of Mendi Suri (white mountain in Basque, Tbrm) 5 km northeast of the Mc Dermitt caldera. Ridge at top is Flattop, which is capped by outflow McDermitt Biotite rhyolite lava of Hoppin Peaks Peralkaline rhyolite lava Tuff (Tmt). (C) Basal breccia of rhyolite lava C (Tbrh1, H04-51,16.384 ± 0.069 Ma) D (Tpr; H12-250, 16.427 ± 0.015 Ma) of Hoppin Peaks (Tbrh1). View is 1 m high. (D) Peralkaline rhyolite lava (Tpr, south Stony rhyolite eastern caldera wall) consisting of basal vitrophyric breccia and main stony rhyo Basal breccia lite. Rhyolite overlies welded fall deposit. Hammer in this and subsequent figures is Basal vitrophyric 42 cm long. breccia

Welded f all

Basalt at Catlow Peak northwest of the caldera. The tuff of Oregon Canyon cation for a local source for the tuff (see discussion of Washburn caldera). is highly enriched in incompatible elements and depleted in compatible ele Reported sanidine 40Ar/39Ar dates of the tuff of Oregon Canyon are 16.654 ± ments (Starkel, 2014). Zoning to less evolved compositions is indicated by 0.025 Ma (weighted mean of eight aliquots of one sample from Catlow Peak; an upper trachydacite that is only locally preserved and pumice in the rhyo Jarboe et al., 2010) and 16.573 ± 0.014 Ma (weighted mean of eight samples;

lite that contains ~67% SiO2 (Fig. 6). Rytuba and McKee (1984) proposed a Mahood and Benson, 2016, 2017), all calculated with respect to 28.201 Ma caldera source partly overprinted by the northeastern part of the McDermitt for Fish Canyon sanidine. Our data, coupled with these published results, re- caldera. However, we do not find evidence for a caldera there, or an indi- veal an age range of 0.137 Ma (0.8%, 16.517 ± 0.013 Ma to 16.654 ± 0.025).

AB All samples Peralkaline Rhyolites (Tpr) 100 100 ite

10 10 /chrondr Ro ck

1 1 La Ce Pr NdPmSmEu Gd Tb Dy Ho Er TmYb Lu La Ce Pr NdPmSmEu Gd Tb Dy Ho Er TmYb Lu CD Rhyolite of McDermitt Creek (Trm) McDermitt Tuff (Tmt) H95-83 64.7% 100 09-McD-56 67.6% 100

10 10

Figure 9. Chondrite-normalized rare-earth element plots of McDermitt igneous rocks; normalization is from Sun and Mc 1 1 Donough (1989). (A) All samples. (B) Per La Ce Pr NdPmSmEu Gd Tb Dy Ho Er TmYb Lu La Ce Pr NdPmSmEu Gd Tb Dy Ho Er TmYb Lu alkaline rhyolites. (C) McDermitt Tuff; the EFQuartz-Sanidine two least silicic samples are labeled with Rhyolite (Trq) Biotite Rhyolites (Tbr) %SiO2. (D) Rhyolite of McDermitt Creek. Aphyric Rhyolites (Tar) (E) Aphyric rhyolites. (F) Biotite rhyolites. 100 100 (G) Postcaldera icelandites. (H) Precaldera C04-10A intermediate lavas.

H05-28 Tbrm 10 10

1 1 La Ce Pr NdPmSmEu Gd Tb Dy Ho Er TmYb Lu La Ce Pr NdPmSmEu Gd Tb Dy Ho Er TmYb Lu GH Postcaldera icelandites Precaldera intermediate lavas (Tpi) (Tia, Tiv) 100 100

10 10

1 1 La Ce Pr NdPmSmEu Gd Tb Dy Ho Er TmYb Lu La Ce Pr NdPmSmEu Gd Tb Dy Ho Er TmYb Lu

The New Mexico Geochronology Research Laboratory (NMGRL) ARGUS VI against Quaternary in the adjacent basin and in a smaller western window and MAP215–50 data vary by ca. 50 ka and are nearly equal at 2s, and this beneath overlying rocks. The dome is at least 5 km across and 250 m thick, and small difference is likely not significant considering there is only a single the base is not exposed. The dome underlies older aphyric rhyolite (Taro) and ARGUS VI analysis (i.e., error not adjusted for Student’s t-test). The overall precaldera intermediate lavas (Tpi, Plate 1). Vitrophyre that crops out at low data demonstrate the ongoing problem of high-precision comparison of data elevation in its southernmost extent, adjacent to Quaternary cover, may mark between laboratories. The relatively old age of 16.654 ± 0.0025 Ma from Jarboe the base of the dome. Vitrophyre that is exposed beneath precaldera interme- et al. (2010) is consistent with offsets between the NMGRL and the Berkeley diate lavas in the small western window probably marks the former top of the Geochronology Center (BGC), that may be related to similar inaccuracies of dome. The rock is strongly flow banded; layers are planar and subhorizontal in BGC data reported by Niespolo et al. (2016). much of the dome and turn steeply upward and are folded in its upper part. A Tuff of Trout Creek Mountains (Ttt). The 16.49 ± 0.03 Ma (n = 3) tuff of Trout few major ledges give the impression of separate flow lobes, but no breccia or Creek Mountains (Ttt) is the youngest regional tuff in the McDermitt map area, vitrophyre that could mark separate flows have been found. underlies the McDermitt Tuff around much of the northwestern caldera wall, Phenocrysts consist of smoky quartz, sanidine, plagioclase, and a trace of and thins to ~1 m in the northeastern wall. The tuff of Trout Creek Mountains is clinopyroxene (Table 1). Although it does not contain biotite, the quartz-sani- one of the most strongly peralkaline rocks in the region, with particularly low dine rhyolite is petrographically and compositionally similar (plagioclase, low

Al2O3 and high FeO*, Zr, Y, and REEs (Fig. 6), and contains sanidine, quartz, FeO*, Zr, and AI, and high Al2O3; Figs. 5 and 6) to the biotite rhyolites in most sodic amphibole, aegirine-augite, and aenigmatite phenocrysts (Table 1). respects, so we include it with them. The quartz-sanidine rhyolite is indistin- Rytuba and McKee (1984) proposed a source almost entirely buried beneath guishable from the biotite rhyolite of the Moonlight Mine (Tbr) in sanidine Pueblo Valley, ~40 km northwest of the McDermitt caldera (Fig. 3), but we sug- 40Ar/39Ar age and K/Ca (16.607 ± 0.015 Ma, 27.8 ± 14.8, H16-12, Table 2). gest a possible closer source (see discussion of Whitehorse caldera). Mahood Biotite rhyolite of Mendi Suri (Tbrm). Biotite rhyolite crops out as two light and Benson (2017) reported a weighted mean age of 16.521 ± 0.015 Ma from colored ~1.5-km-wide and ≥140 m exposed thickness bodies that are either shal- seven samples of the tuff of Trout Creek Mountains. low intrusions into, or lava domes underlying, unit Tpi (precaldera intermediate lavas) at Mendi Suri (Figs. 3 and 8B), ~5 km northeast of the northeastern Biotite Rhyolites margin of the caldera. The rock is massive to strongly and steeply flow banded. Several north-northeast–striking dikes of biotite rhyolite cut the northern body. Sparsely porphyritic biotite rhyolites were emplaced at four separate areas The southern body has an upper vitrophyre breccia, and clasts of similar rhyo and times (Fig. 7) along or near the western, northeastern, and eastern mar- lite occur in nearby conglomerate. Obsidian nodules (C04-10A, Supplemental gins of the caldera. The presence of biotite, hornblende (at Mendi Suri), and Table S1 [see footnote 1]) compositionally similar to the distinctive main body

plagioclase in high-SiO2 rhyolite, and trace zircon and metaluminous to peralu- (H05-28) erode out of the top of an aphyric rhyolite (Taro) ~4 km south of the minous composition distinguish these rocks from the alkaline and commonly domes; this suggests that the rocks are related despite the difference in pheno peralkaline rocks of the McDermitt caldera (Table 1; Figs. 5, 10A, and 10B). crysts. The Mendi Suri domes contain 6%–10% phenocrysts of sanidine, plagio Biotite rhyolite of the Moonlight Mine (Tbr). Biotite rhyolite lava crops out clase, quartz, biotite, and a trace of hornblende (Fig. 10B). Both the main body

discontinuously along the southwestern caldera margin from the Moonlight and the obsidian nodules have ~76% SiO2, distinctive flat REE patterns, large Eu Mine south to Thacker Pass (Fig. 11A). It is at least 150 m thick at the mine, anomalies, high concentrations of most incompatible elements (Rb, Y, Nb, Ga,

and the base is nowhere exposed. Similar rock occurs as lithics in McDermitt Pb, and Th), and the lowest Zr and TiO2 contents of any rhyolites (Figs. 5, 6, and Tuff and as megabreccia (notably in the megarheomorphic folds of Hargrove 9). A sanidine 40Ar/39Ar date is 16.487 ± 0.015 Ma (H05-28, Table 2). and Sheridan, 1984) through the same area and as far north as Horse Creek. Biotite rhyolite of Hoppin Peaks (Tbrh). Three lava flows totaling ~230 m With a sanidine 40Ar/39Ar date of 16.619 ± 0.020 Ma (C95-103A, Table 2), this is thick cap Hoppin Peaks along the eastern margin of the caldera. The flows, the second oldest, demonstrably locally derived silicic rock in the caldera area. which were interpreted as strongly rheomorphic intracaldera tuff by Rytuba and The Moonlight rhyolite has 10%–16% phenocrysts, the most among the biotite McKee (1984), consist mostly of strongly flow banded and folded stony rhyolite rhyolites, consisting of plagioclase, sanidine, biotite, and minor quartz (Fig. with basal breccia characteristic of lavas (Fig. 8C). The stratigraphically lowest

10A; Table 1). It is rhyolite with 70%–75% SiO2 (Figs. 5 and 6). The rock is al- and highest flows have basal vitrophyres as much as 10 m thick in the lowest tered through much of its distribution, particularly at the Moonlight Mine, with flow. All flows contain 3%–6% phenocrysts of quartz, sanidine, plagioclase, and coarse hydrothermal quartz, adularia as disseminations and replacing feldspar biotite, and the lowest flow contains a trace of pyroxene. All flows are compo- 40 39 phenocrysts, pyrite, arsenopyrite, fluorite, and various uranium minerals, es- sitionally similar with 75% SiO2 (Figs. 5 and 6). Sanidine Ar/ Ar dates on the pecially uranium-rich zircon (Castor and Henry, 2000). lowest and highest flows are 16.384 ± 0.069 (H04-51) and 16.362 ± 0.024 Ma

Quartz-sanidine rhyolite lava dome (Trq). A porphyritic, high-SiO2 rhyolite (H04-53), respectively (Table 2). These dates indicate that the rhyolite of Hoppin lava dome crops out northeast of the caldera in one large contiguous area Peaks erupted closely preceding eruption of the McDermitt Tuff.

Biotite rhyolite of B Biotite rhyolite of Q B A the Moonlight Mine Mendi Suri (Tbr, C95-103A, (Tbrm, H05-29) 16.619 ± 0.020 Ma)

Figure 10. Photomicrographs (cross-polar P ized light, except D). (A) Biotite rhyolite of Moonlight Mine (Tbr, C95–103A). Highly S resorbed quartz (Q), sanidine (S), and Q B P biotite (B) phenocrysts in devitrified and moderately silicified groundmass. Hori B zontal field is 3.2 mm. (B) Biotite rhyolite of Mendi Suri (Tbrm, H05-29). Sanidine, P plagioclase (P), and biotite phenocrysts in S finely devitrified groundmass. Horizontal field is 3.2 mm. (C) Peralkaline rhyolite lava S (Tpr, H12–250, southeastern caldera wall). Vitrophyre with phenocrysts of clear, C Peralkaline rhyolite D euhedral sanidine, resorbed quartz, and (Tpr, H12-250, clinopyroxene (C). Bright spot in lower left 16.427 ± 0.15 Ma) Peralkaline rhyolite is a spherulite. Horizontal field is 2.4 mm. (Tpr, H07-194) (D) Peralkaline rhyolite lava (Tpr, H07-194, Double H Mountains south of caldera). Q Partly spherulitically devitrified vitrophyre S with phenocrysts of clear euhedral sani dine and quartz. Circle with FI is a fluid S inclusion. Similar inclusions in sanidine would be difficult to detect and could FI A measurably affect high-precision 40Ar/39Ar dates. Horizontal field is 3.2 mm. Q

Geochemistry. The biotite rhyolites are distinct from other Miocene rocks in Peralkaline Rhyolite Lava (Tpr)

being metaluminous to peraluminous, having higher Al2O3, lower FeO*, MnO,

and HREEs, and mostly lower TiO2 and Zr (Figs. 5, 6, and 9). The four suites are What is probably a single lava flow of peralkaline rhyolite crops out in

distinct from each other. The Moonlight Mine suite is mostly low-SiO2 rhyo three areas around the southern margin of the caldera. The most prominent

lite, whereas the other three have 75%–76% SiO2. Moonlight rocks also have outcrops are as cliffs along the escarpment into Kings River Valley centered

lower Rb and Y and higher TiO2 and Zr, more like the alkaline Miocene rocks. on the Moonlight Mine, where a single flow is as much as 180 m thick (Fig. The single sample from Mendi Suri (H05-28) and the compositionally similar 11A), continuous along strike for ~8 km, and has both basal and upper hy-

but aphyric obsidian nodules (C04-10A) have very low TiO2, FeO*, P2O5, Sr, Zr, drated vitrophyre and breccia. A similar thick flow with an upper vitrophyric Ba, and REEs, high Rb and Nb, and the deepest Eu anomalies. The four suites breccia forms cliffs for 3.5 km along the escarpment south of Thacker Pass. also plot as distinct groups on a plot of Ga/Al versus Zr. The quartz-sanidine The flow in the southeastern caldera wall crops out for 1.5 km, is covered by rhyolite (Trq) is distinct from all other rhyolites in having the highest light REE aphyric rhyolites to the north and south, is as much as 55 m thick, and has a concentrations and steepest REE patterns (Figs. 6 and 9). well-exposed basal breccia of hydrated vitrophyre that overlies welded fall

B A Tis Intracaldera sediments (Tis) Tmtu

Tmb vera Canyon Vitrophyre Figure 11 (on this and following page). Caladikes (Tmv) McDermitt Tuff (A) Complete section of precaldera lavas (Tmt; 16.39 ± 0.02 Ma) [biotite rhyolite of the Moonlight Mine (Tbr) and peralkaline rhyolite lava (Tpr)], Peralkaline Rhyolite Biotite Rhyolite (Tpr; C95-107, intracaldera McDermitt Tuff (Tmt), and (Tbr; C95-103A, 16.399 ± 0.016 Ma) post-collapse intracaldera sedimentary Caldera16.619 ± 0.020 Ma) deposits (Tis) above the Moonlight Mine, fault western edge of caldera. Also shown are Moonlight northeast-striking vitrophyre pyroclastic Mine dikes (Tmv), mesobreccia layer (Tmb), and p Quaternary scar upper part of McDermitt Tuff (Tmtu). Conti nuity of rocks across Calavera Canyon pre cludes a caldera boundary through there. C Face is ~650 m high. (B) Vitrophyre layer D with glassy aphyric pumice and a few lithic fragments in megarheomorphic fold sec Non-strained tion (Hargrove and Sheridan, 1984), south western part of caldera (Fig. 11D). (C) Strain partitioning in rheomorphic intracaldera McDermitt Tuff showing highly strained zone below and nonstrained zone above. (D) View southeast to megarheomorphic folds (Hargrove and Sheridan, 1984), south western part of caldera. Face is ~450 m high. Highly strained

deposits (Fig. 8D). In all areas, the main part of the flow is light tan, mas- phenocrysts, rather than anorthoclase, distinguishes this unit from the other sive to flow banded, and crystalline. The rock everywhere contains 12%–17% precaldera peralkaline rhyolites. phenocrysts of euhedral and inclusion-free sanidine, commonly smoky These rhyolites are moderately peralkaline, although two samples proba-

quartz, clinopyroxene, aenigmatite, monazite, magnetite, ilmenite, and trace bly have lost Na2O and gained K2O (Figs. 5, 6, and 9). Despite the great differ- apatite (Figs. 10C, 10D). Probable vapor-phase sodic amphibole is common. ence in phenocrysts, the peralkaline and aphyric rhyolites are compositionally

All analyzed samples are indistinguishable with 76% SiO2 and low Al2O3 and similar, with overlapping concentrations of incompatible and compatible ele- high FeO characteristic of peralkaline rhyolites (Figs. 5, 6, and 9). Sanidine ments and large Eu anomalies. yielded indistinguishable 40Ar/39Ar dates in the western (16.399 ± 0.016 Ma, C95–107), southwestern (16.404 ± 0.018 Ma, 11-McD-290), and southeastern McDermitt Tuff and Related Rocks (Tmt, Tmb, Tmtu, Tmv) areas (16.427 ± 0.015 Ma, Fig. 7; H12-250, Table 2); mean = 16.410 ± 0.015 Ma. The source of the unit is unknown. Similarity in rock type, phenocryst as- Collapse of the McDermitt caldera occurred during eruption of what we semblage, composition, and age suggests that the three outcrops are part of herein name the McDermitt Tuff, which is zoned from peralkaline, aphyric, 2 a single flow across an ~450 km area. The presence of quartz and sanidine high-SiO2 rhyolite to metaluminous, abundantly anorthoclase-phyric, high-

E F

Figure 11 (continued). (E) Moderately rheo morphic tuff with steeply dipping primary compaction foliation and some early vapor phase cavities parallel to that foliation, overprinted by late vapor phase release that generated subhorizontal cavities. View is ~60 cm wide. (F) Highly sheared bands in megarheomorphic folds (Har grove and Sheridan, 1984), southwestern part of caldera (Fig. 11D). Most deforma tion probably was taken up by shearing in H these bands. (G) Strongly rheomorphic tuff G with flow fold and highly stretched pum ice(?), Moonlight Mine section. (H) Linea tions (toward 115° in foliation 72°, 21°) in McDermitt Tuff, Moonlight Mine section. Lineation trend is approximately con stant and approximately perpendicular to caldera wall in this area, which suggests that some rheomorphism may have been slumping down growing caldera wall.

SiO2 trachydacite and icelandite (Figs. 5, 6, 11, and 12; Table 1; Conrad, 1984; southern wall in the Double H Mountains, where the vertical zonation is lo- Starkel et al., 2012; Starkel, 2014). The tuff ponded to a thickness of at least cally well preserved. 610 m in the caldera, where it is commonly strongly rheomorphic (see also Introduction of the name McDermitt Tuff is necessitated by the confusing Hargrove and Sheridan, 1984). This rheomorphism generally folded together application of previous informal names. The McDermitt Tuff includes the alkali the different petrographic and compositional zones (Fig. 12A); a probable rhyolite of Jordan Meadow of Greene (1976) and parts of the tuff of Long Ridge original vertical zonation is at best poorly preserved in intracaldera tuff (Tmt). of Rytuba and McKee (1984), who provided only a 1:1,000,000-scale map that Megablocks and mesobreccia lenses of precaldera rocks (Tmb) are common shows all volcanic rocks in the region as a single unit. The general distribution in intracaldera tuff and are especially well exposed along the western es- of the tuff of Long Ridge, members 1, 2, 3, and 4-5 undivided, are depicted carpment of the Montana Mountains. A sparsely porphyritic tuff (Tmtu) that on page-sized maps in Rytuba (1976), Rytuba and Glanzman (1978, 1979), and overlies mesobreccia at the top of the Moonlight Mine section is probably a Glanzman and Rytuba (1979) that encompass approximately the same area late variant of the McDermitt Tuff. Vitrophyric pyroclastic dikes (Tmv), prob- as our Plate 1. Members 1 and 2 appear to be entirely sparsely porphyritic or ably feeders to late tuff eruptions, are present along the western side of the aphyric rhyolite lavas (our units Tra and Tar). Member 3, which is shown only caldera. Outflow tuff that is variably rheomorphic crops out in the northern in the Double H Mountains, partly coincides with our McDermitt Tuff. Member caldera wall in the Oregon Canyon and Trout Creek Mountains and in the 4-5 undivided, which is shown only north of the caldera, partly coincides with

H04-32B B MD-3A A

C C A A

Figure 12. Photomicrographs, McDermitt Tuff (Tmt, A-D) and Vitrophyric tuff dikes (Tmv, E, F). A is anorthoclase, C is clino pyroxene. (A) McDermitt Tuff (H04-32B, outflow in southern caldera wall). Inter banded aphyric (left) and abundantly porphyritic (right) phases. Cross-polarized C D light. Horizontal field is 30 mm. (B) MD-3A, MD-21 intracaldera vitrophyre, Moonlight Mine H05-23A section. Anorthoclase and clinopyroxene C phenocrysts in matrix containing variably colored, aphyric pumice that are probably compositionally varied. Horizontal field N is 3.2 mm. (C) MD-21, outflow in south ern caldera wall. Euhedral anorthoclase and clinopyroxene phenocrysts in matrix A A of devitrified shards and pumice. Cross- C polarized light. Horizontal field is 3.2 mm. (D) H05-23A, outflow in northern caldera wall. Anorthoclase, clinopyroxene, and probable sodic amphibole (N) phenocrysts in devitrified matrix. Cross-polarized light. Horizontal field is 0.9 mm. (E) Vitrophyre A dike (H12-259, northwestern caldera wall). Anorthoclase and clinopyroxene phenocrysts in densely welded, shard- and fine pumice-rich matrix. Horizontal Vitrophyre dike field is 3.2 mm. (F) Vitrophyre dike (H95- E Vitrophyre dike F (Tmv, H95-103 103, Moonlight Mine section). Anhedral (Tmv, H12-259 16.301 ± 0.024 Ma) anorthoclase phenocryst with abundant 16.374 ± 0.012 Ma) clinopyroxene inclusions. Cross-polarized light. Horizontal field is 3.2 mm.

C C

A A

our McDermitt Tuff where it caps Flattop, and the phenocryst assemblage of with 68% SiO2 is the most common low-SiO2 end member, but icelandite with

member 5 (anorthoclase and minor ferrohedenbergite and fayalite; Rytuba as little as 64% SiO2 is present at the top of the section in the southwestern rim and McKee, 1984) most closely matches the McDermitt Tuff. However, mem- (H95-83; Supplemental Table S1 [see footnote 1]). Incompatible elements Rb,

ber 4-5 also includes sparsely porphyritic or aphyric rhyolite lavas that underlie Y, Zr, Nb, Hf, Ta, Pb, Th, and U increase with increasing SiO2, whereas compati precaldera intermediate lavas (Tpi) that underlie McDermitt Tuff. Rytuba and ble elements Sr and Ba decrease. All REEs are incompatible, except for Eu, McKee (1984) identified member 4 as the tuff of Trout Creek Mountains, but which is incorporated in anorthoclase (Fig. 9; Starkel et al., 2012). The least described it as having 26% anorthoclase and 1% quartz phenocrysts, unlike silicic rocks have positive Eu anomalies and are similar to precaldera interme- the sanidine-quartz–rich tuff of Trout Creek Mountains. The page-sized maps of diate rocks (Tpi) except for distinctly different MnO trends. The McDermitt Tuff Rytuba (1976), Rytuba and Glanzman (1978, 1979), and Glanzman and Rytuba makes a linear trend starting at the edge of the I- and S-type granite field on

(1979) also showed most of what we map as intracaldera McDermitt Tuff as a Ga/Al versus Zr plot because of increasing Zr and Ga and decreasing Al2O3

various units of amalgamated “rhyolite intrusive rocks, flows, and tuffs” (fig- with increasing SiO2. ure 2 of Rytuba and Glanzman, 1978); no tuff of Long Ridge is shown inside the Most tuff is devitrified, but vitrophyre zones are common around mega- McDermitt caldera. Based on our comprehensive new work and the confusion breccia and mesobreccia (Fig. 11B). Pumice is generally present in these zones about what constitutes all of the tuffs of Rytuba and McKee (1984), the older and constitutes 10 vol% to as much as 50 vol%. Mixed pumice populations, names should be abandoned. with aphyric pumice in porphyritic rock, and vice versa, are common (Fig. 12B). Lithics vary from sparse to highly abundant and from millimeter size to blocks as large as 10 m in diameter. Clasts represent a nearly complete selection of Intracaldera Tuff (Tmt) precaldera rocks, including Cretaceous granitic rocks, probable Eocene biotite dacite, coarsely plagioclase-phyric to aphyric Steens Basalt, particularly abun- Because of the intense rheomorphic intrafolding of the different petro- dant biotite rhyolite, and peralkaline rhyolite. The largest blocks are mapped as graphic-compositional types, we map most intracaldera tuff as a single unit megabreccia. Lenses of mesobreccia with clasts as large as ~1 m in diameter (Tmt). More detailed 1:24,000-scale mapping of the Calavera Canyon 7.5′ quad- in a poorly exposed matrix are especially prominent in and at the top of the rangle, approximately centered on Calavera Canyon and Horse Creek, where Moonlight Mine section (Figs. 11A, 13A, and 13C). intracaldera tuff is well exposed along the western escarpment, allows greater The McDermitt Tuff is moderately to intensely rheomorphic throughout subdivision of the petrographic variants (Starkel, 2014). The western escarp- the caldera (Figs. 11C–11H). Sparse, weakly rheomorphic rocks are nearly un- ment exposes a complete section of intracaldera tuff as well as precaldera deformed or have pumice stretched only in one direction. Strain partitioning, units: Steens Basalt (Tsb, Ts) and precaldera intermediate rocks (Tpi), biotite where one layer is significantly stretched and an adjacent, apparently indis- rhyolite of the Moonlight Mine (Tbr), and peralkaline rhyolite lava (Tpr), to tinguishable layer is not strained, is developed in what were probably origi- overlying intracaldera sedimentary deposits (Tis) (Fig. 11A). It is notable that nally stratigraphically high parts of the tuff (Fig. 11C). The megarheomorphic no other tuff underlies the McDermitt Tuff where the base is exposed. The tuff folds of Hargrove and Sheridan (1984) represent moderately to highly strained is thickest, 610 m, at Garden Creek immediately south of Calavera Canyon, rock where large-scale folds have developed, and aphyric and abundantly and generally >500 m thick along the escarpment (Fig. 3; Plate 1). Because the porphyritic rocks are interbanded (Fig. 11D). Most of the folded rock is easily precaldera rocks dip variably into the caldera along the western margin (see recognizable as ash-flow tuff with stretched but recognizable pumice ± lithics discussion of Western Margin herein), the tuff may thicken eastward (Plate (Fig. 11E). Lenses of more highly sheared rock in the folds probably took up 1, section A-A′). A drillhole (WNC-58C; D.P. Bryan, Western Lithium Corpora- most of the strain and indicate an extreme example of strain partitioning (Fig. tion, written commun., 2013) in the southern part of the caldera encountered 11F). Hargrove and Sheridan (1984, p. 8632) interpreted that biotite rhyolite was 544 m of intracaldera tuff and bottomed in it. Elsewhere, only the uppermost lava that was interbedded and cofolded with McDermitt Tuff in a “plastic man- parts of intracaldera tuff are exposed. Nevertheless, the tuff is at least 290 m ner”; however, we find that biotite rhyolite existed entirely as megablocks and (Rock Creek) and 220 m thick (McDermitt Creek) in canyons in the southern and mesobreccia and that compaction around the blocks probably made a minor northern parts of the caldera, respectively. contribution to apparent folding. In the most uniformly strained rocks, foliation

The greatest volume of McDermitt Tuff is aphyric, high-SiO2 rhyolite (77%; appears more like that of lava (Fig. 11G). Pumice either was not present or is so Figs. 5 and 6). Phenocryst content, mostly anorthoclase with minor clinopyrox- elongate that its original discontinuity is difficult to recognize. Most intracaldera

ene and fayalite, increases continuously to ~30 vol% with decreasing SiO2 con- tuff appears to have deformed to this degree, because rock exposed in canyons

tent (Fig. 12; Table 1). Anorthoclase in higher-SiO2 rhyolites is generally more of Pole, Washburn, and McDermitt Creeks (south to north through the caldera)

euhedral and less inclusion rich than those in lower-SiO2 rocks (cf. Figs. 12B, all have this appearance. Flow folds do not appear to have regular orientations, 12C with 12E, 12F). The most silicic rocks also contain sparse sodic amphibole a conclusion also reached by Hargrove and Sheridan (1984). Lineations along

phenocrysts consistent with peralkalinity (Fig. 12D). High-SiO2 trachydacite foliation surfaces in the Moonlight Mine section (Fig. 11H) trend regularly ~110°,

Mesobreccia B Vitrophyre A dike (Tmv) Vitrophyre dike (Tmv) McDermitt Tuff (Tmt)

Subhorizontal columnar joints

Vitrophyre layer in intracaldera tuff (Tmt)

Peralkaline C Rhyolite

Figure 13. (A) Vitrophyre pyroclastic dike (Tmv, Moonlight Mine section) cuts steeply through rheomorphic intracaldera McDermitt Tuff and through left side of mesobreccia knob, dark area just below box, which shows location of C. Mesobreccia knob is ~6 m high. (B) Vitrophyre pyroclastic dike with subhorizontal columnar joints. Same as dike in A. Height Steens from columnar joints to top of outcrop is ~ 45 m high.(C) Mesobreccia (location shown Basalt in A) containing angular to subrounded clasts of peralkaline rhyolite, Steens Basalt, and other pre-McDermitt Tuff rock types.

nearly perpendicular to the caldera wall there, which suggests that some rheo deformed rocks. The folds demonstrably predate nearly flat-lying mesobreccia morphism may have been slumping off the growing wall. The vitrophyre pyro- lenses that overlie the megarheomorphic folds, similar folds at the Moonlight clastic dikes (Tmv) cut vertically through highly rheomorphic rock, indicating Mine section, and the upper tuff (Tmtu) at the Moonlight Mine section. that rheomorphism occurred during emplacement, therefore, during caldera Strong rheomorphism of McDermitt Tuff throughout the caldera suggests collapse. This interpretation slightly contrasts from Hargrove and Sheridan that slumping off a growing caldera wall was not the only operative process. (1984, p. 8629), who concluded that “…large-scale folds in the deformed se- Deposition and slumping over irregular precaldera topography or irregular col- quence developed shortly after caldera collapse” because the folds underlie un- lapse of the subsiding caldera block may also be significant.

Because of the intense rheomorphism, some intracaldera McDermitt Tuff significantly younger (years to thousands of years?) than the underlying tuff; is not immediately recognizable as ash-flow tuff. Much of the intracaldera however, field relations only provide a relative age, and dating is precluded by tuff is massive to steeply foliated aphyric rhyolite. Bands of variably anortho- alteration. Petrographically similar, probably correlative, late outflow tuff also clase-phyric rhyolite in sharp contact with aphyric rhyolite constitute as much is present in the Double H Mountains south of the map area (H07-189, H07-190; as 50% of the tuff in some locations (Fig. 12A) but are essentially absent through Supplemental Table S1 [see footnote 1]). large areas elsewhere. Greene (1972, 1976, p. 36) recognized this characteristic, stating “The alkali rhyolite of Jordan Meadow consists of interlayered aphyric, sparsely porphyritic, and abundantly porphyritic alkali rhyolites....” Outflow Tuff

Northern caldera wall. McDermitt Tuff crops out discontinuously along Megablocks and Mesobreccia Lenses (Tmb) the northern caldera wall but may have been more continuous before erosion. Excellent cliff exposures at Flattop have thin (~1 m) layers of abundantly Large megablocks and mesobreccia lenses of finer blocks are common in porphyritic trachydacite within thick aphyric to sparsely porphyritic rhyolite McDermitt Tuff, and mesobreccia is between McDermitt Tuff (Tmt) and upper that are folded into mostly low-amplitude, long-wavelength folds (≤5 × 100– tuff (Tmtu) along the western escarpment. Mesobreccia (Fig. 13C) contains an- 300 m) with limb dips no more than 10° (Fig. 14A). The tuff locally steepens gular to subrounded clasts of peralkaline rhyolite (Tpr), Steens Basalt (Ts, Tsb), into ramps with near vertical dips. Lower contacts of the more porphyritic rock and other pre-McDermitt Tuff rock types. are gradational over 1–2 m, whereas upper contacts are sharp. Sparse, abundantly porphyritic pumice as much as 20 cm long and common aphyric, mafic to intermediate lithic fragments as much as 5 cm in diameter occur in the Vitrophyre Pyroclastic Dikes (Tmv) lower parts of the aphyric to sparsely porphyritic rock (Fig. 14B). The sparsely porphyritic phase contains the probable sodic amphibole phenocrysts (Fig. Vitrophyre pyroclastic dikes are present in two areas. Numerous dikes cut 12D). The tuff at Flattop overlies a sparsely anorthoclase-phyric rhyolite lava intracaldera McDermitt Tuff just south of Calavera Canyon in the Moonlight (Tra2) with a similar phenocryst assemblage as part of the tuff. However, this Mine section. The dikes mostly form northeast-striking, steeply northwest-dip- underlying rock is not part of the McDermitt Tuff because it is distinctly older ping planar bodies, 1–10 m thick, commonly with columnar joints approxi- (16.462 ± 0.018 Ma; C04-07; Table 2; Fig. 7) and stratigraphically distinct. The mately perpendicular to walls (Figs. 11A, 13A, and 13B). The dikes are locally rhyolite lava has an upper vitrophyric breccia overlain by thin tuffaceous sedi slightly nonplanar, particularly where they bend around or through meso- mentary deposits that record a significant time and depositional break from breccia. Common pumice, lithics, and densely welded, eutaxitic textures in the overlying tuff. the dikes demonstrate their pyroclastic origin (Fig. 12E). The dikes cut sharply South of caldera in Double H Mountains. Outflow McDermitt Tuff crops through highly rheomorphic tuff, and therefore postdate rheomorphism. Some out as much as 13 km south of the caldera margin in the Double H Mountains bodies identified as dikes may be vertically dipping tuff. A single northern dike (Fig. 3; Plate 1). The tuff overlies aphyric rhyolite lava (Tar) throughout this area. ~3 km southeast of Disaster Peak cuts the Steens Basalt (Ts) in the caldera Outflow McDermitt Tuff shows an abrupt transition from thin (≤10 m) wall also strikes northeast, but dips more gently northwest, ~40°–45°, and is as welded, normal ash-flow tuff to thick (≥70 m) strongly rheomorphic tuff where much as 30 m thick. All dikes are glassy, moderately to abundantly porphyritic it was deposited over the steep margin of an aphyric rhyolite lava (Figs. 14C,

with 68%–72% SiO2 (H12-259, H95-103; Figs. 12E, 12F; Supplemental Table S1 14D). The lava has a thick, upper vitrophyric breccia, so is distinctly an older [see footnote 1]). All these features suggest that the dikes fed late eruptions of unit, not a lower phase of the tuff. The thin outflow tuff has an aphyric base McDermitt Tuff but prior to eruption of the upper tuff (Tmtu), which is much overlain by a sparsely porphyritic layer 1–2 m thick, in turn overlain by more less porphyritic. abundantly porphyritic tuff. Where the tuff thickens, flow bands and folds de- velop and progress to isoclinal, recumbent, and sheath folds, all indicating transport toward 140°. Aphyric and more porphyritic bands are intimately Upper Tuff (Tmtu) folded, similar to rheomorphic tuff in the caldera. Sparse lithics to 5 cm are the only preserved indicator of a pyroclastic origin. Still farther south, beyond the A sparsely anorthoclase-phyric, strongly eutaxitic, lithic-bearing, densely map area, bands of abundantly porphyritic rock as much as 3 m thick are com- welded, and moderately rheomorphic ash-flow tuff as much as 35 m thick over- plexly interfolded with aphyric to sparsely porphyritic rock. This rheomorphic lies an equally thick mesobreccia layer (Tmb) at the top of the Moonlight Mine unit is overlain by two thin cooling units of nonrheomorphic, sparsely anortho- section (Fig. 11A). The rock is distinctive because of pervasive alteration of clase-phyric tuff that may correlate with the late, post-mesobreccia tuff (Tmtu) feldspars and variable silicification. This variant is a late eruption that may be at the Moonlight section.

Outflow McDermitt Tuff at Flattop A Outflow McDermitt Tuff B aphyric, high SiO2 rhyolite Flattop, northeastern caldera wall trachydacite pumice

Abundantly Figure 14. (A) Outflow McDermitt Tuff porphyritic showing low-amplitude folds at Flattop, trachydacite northeastern caldera wall. Cliff is ~10 m high. (B) Outflow McDermitt Tuff at Flat

top showing aphyric, high SiO2 phase with sparse lithic fragments and abun dantly porphyritic trachydacite pumice over abundantly porphyritic trachydacite phase. (C) Thickening of outflow McDer Thickening of outflow McDermitt Tuff mitt Tuff over the end of an aphyric rhyo C over the end of an aphyric rhyolite lava (Tar) D lite lava (Tar), Double H Mountains south Double H Mts south of caldera of caldera. Box shows area of D. Cliff in thick part of McDermitt Tuff is ~60 m high. (D) Recumbent folds in outflow McDer mitt Tuff where it thickened abruptly in C. Pencil is 16 cm long.

Upper vitrophyre breccia at top of aphyric rhyolite lava (Tar)

Recumbent folds in outflow McDermitt Tuff

Massive aphyric rhyolite lava (Tar)

Overall distribution of outflow McDermitt Tuff. Delineating the original dis- caldera, based on descriptions of the probably correlative tuff of Long Ridge tribution of outflow tuff is hampered by uncertainty in correlation with mapped given by Rytuba and Curtis (1983), Rytuba et al. (1983a, 1983b), Minor (1986), units in areas of published geologic maps, by lack of mapping in other areas, Minor and Wager (1989), and Minor et al. (1989a, 1989b). No detailed mapping and by uncertainty in the amount of erosion of distal tuff (Fig. 3). The McDer- is available for the southern Bilk Creek Mountains southwest of the caldera. mitt Tuff is probably present in much of the southern Oregon Canyon and Trout McDermitt Tuff is not present in the eastern caldera wall or in the Santa Rosa Creek Mountains and northern Bilk Creek Mountains north and west of the Range farther east (Brueseke and Hart, 2008; our observations).

Age John, 2013). Method 1 uses a caldera area of ~930 km2, average intracaldera tuff thickness between 500 and 800 m, the distribution of outflow tuff as dis- The 40Ar/39Ar dates were determined on anorthoclase from six samples, cussed here, and average outflow thickness between 50 and 100 m. The lower one intracaldera tuff, three outflow tuffs, and two pyroclastic dikes (Fig. 7; estimate of 600 km3, based on only 500 m of intracaldera tuff, is probably too Table 2). The 6 dates average 16.36 ± 0.04 Ma, but individual dates do not low. A significant uncertainty is the thickness of intracaldera tuff, because a overlap within their 2s sigma uncertainties. All dates were done on the Argus complete section is only exposed along the western margin. This approach VI multicollector mass spectrometer. However, dates generally correlate posi also does not consider dispersed ash.

tively with K/Ca of analyzed anorthoclase and with SiO2 of host samples. Method 2, using the same caldera area and estimated collapse between Some anorthoclase in H95-103, which has the youngest apparent age and 1000 and 1200 m (the greatest estimated collapse from along the southern lowest K/Ca, is particularly inclusion rich (Table 2; Fig. 12F). Whether the dif- margin), yields 930–1116 km3. Uncertainty in thickness of intracaldera tuff, and ferences in K/Ca result from differences in anorthoclase composition or the so the amount of collapse, is significant, but this approach accounts for all presence of inclusions is unknown. Magnetic separation and hand-picking erupted magma including dispersed ash. Considering all the data and their should remove grains with abundant clinopyroxene inclusions, but might not significant uncertainties, a total volume of ~1000 km3 seems reasonable. eliminate grains with few, small inclusions. In addition, analyses of H15-67, H12-259, and H05-23P were done most recently using concurrent HF leaching and sonification to remove impurities; this probably generated cleaner Postcaldera Magmatism anorthoclase samples. These three samples yielded the oldest dates and average 16.388 ± 0.015 Ma (Fig. 7), which is probably the best estimate of Rhyolite of McDermitt Creek (Trm) eruption age, although analytically indistinguishable from the mean of all six. The slightly older age of these three samples is more consistent with dates The rhyolite of McDermitt Creek (Greene, 1972, 1976) is a group of low-

on post–McDermitt Tuff units. The scatter in anorthoclase dates contrasts with SiO2 (71%–73%) porphyritic rhyolite lavas that crop out in the northeastern the tighter clustering of sanidine dates from the probably single lava flow of part of the caldera. Much of the rock resembles highly rheomorphic McDer- peralkaline rhyolite (Tpr) and illustrates the greater precision and accuracy of mitt Tuff, and both units are indistinguishable in age. However, the rhyolite of sanidine dates. Mahood and Benson (2017) and Benson et al. (2017) reported McDermitt Creek overlies McDermitt Tuff, locally with a basal flow breccia and a weighted mean age of 16.433 ± 0.022 Ma from 4 samples of the tuff of Long vesicular rock (Greene, 1976), and contains no interlayered aphyric rock. The Ridge, the probable correlative of the McDermitt Tuff, from north and north- lavas are more coarsely porphyritic than McDermitt Tuff, with anorthoclase west of the McDermitt caldera map area. phenocrysts as much as 8 mm long, and contain plagioclase, which is absent in the tuff. Two single crystal 40Ar/39Ar dates are 16.417 ± 0.018 Ma (H04-55) on anorthoclase and 16.39 ± 0.10 Ma (10-McD-225) on a mix of plagioclase Volume and anorthoclase (Fig. 7; Table 2). These dates indicate that the rhyolite of McDermitt Creek was emplaced immediately after eruption of McDermitt Tuff We estimate a total volume of McDermitt Tuff of between 600 and 1100 and caldera collapse. km3 dense rock equivalent, ~50% to 85% of which is intracaldera (Table 3). Chemical analyses also support the rhyolite of McDermitt Creek not being These estimates use two approaches: (1) using known and estimated distri- part of the McDermitt Tuff (Figs. 5, 6, and 9). The rhyolite has lower HREEs at

butions and thicknesses of intracaldera and outflow tuff, and (2) using the the same SiO2 content and lower MnO, similar to MnO in the biotite rhyolites area and estimated total collapse of the caldera (John et al., 2008; Henry and and along the lower precaldera icelandite trend.

TABLE 3. AREA, THICKNESS, AND VOLUME OF McDERMITT TUFF Area Tuff thickness Volume Intracaldera (km2) (m) (km3) Minimum Maximum MinimumMaximum MinimumMaximum (%) (%) Caldera 930 500 800 465 744 63 85 Outflow 2720 50 100 136 272 Total 3650 601 1016 Amount of collapse 1000 m 1200 m 930 1116 50 67

Icelandite Lavas and Vents (Tiv, Til, Tia) Mountain and Round Mountain are similar, with 60%–62% SiO2 and sparse phenocrysts of plagioclase and clinopyroxene. Jordan Meadow Mountain is Post-collapse volcanic vents (Tiv) or lavas (Til, Tia) of icelandite that range a slightly northeast-elongated area of icelandite lavas. They are petrographi-

from ~56% to 64% SiO2 crop out in 6 areas in the caldera. Vents at Round cally similar to the Round Mountain rocks, but we have no chemical analyses. Mountain and Black Mountain retain subcircular, resistant rims, ~700 m in Plagioclase from the Round Mountain vent yielded an 40Ar/39Ar date of 16.22 ± diameter, of dense, locally glassy, icelandite and an interior basin, both sur- 0.13 Ma (H07-198 plateau; 16.32 ± 0.23 Ma isochron; Table 2). rounded by flows and agglutinated scoria (Fig. 15A). Round Mountain and The Aurora lavas (Tia), the host for U mineralization at the Aurora de- Black Mountain also correlate well with regional (1:250,000 scale) aeromag- posit (Wallace et al., 1980; Wallace and Roper, 1981; Roper and Wallace, 1981), netic highs (Fig. 16), as do the lavas mapped near Jordan Meadow Mountain, are a sequence of icelandite lavas, flow breccias, and volcaniclastic depos- which suggests a possible vent there. Lavas as much as 120 m thick extend its just inside the northeastern caldera wall. The lavas overlie McDermitt Tuff ~1.5 km outward from Black Mountain. Greene (1976) suggested that the and underlie tuffaceous lacustrine sediments (unit Tis). Aurora lavas include

area mapped as lavas 2 km northeast of Black Mountain is another vent, but the most silicic postcaldera rocks (60%–64% SiO2) and contain plagioclase this area does not correlate with an aeromagnetic high. The rocks at Black (anorthoclase in the most silicic rocks; Fig. 15B), clinopyroxene, and fayalite.

A B Aurora lava (Tia, H11-199, 16.412 ± 0.016 Ma)

Round Mountain (Tiv, ca. 16.3 Ma) Figure 15. (A) Icelandite vent complex at A Round Mountain; top, flat part is ~1.3 km across. Looking west at resistant rim of dense icelandite surrounding core of less resistant scoria breccia. (B) Photomicro graph, Aurora lava (Tia, H11–199). Anortho A clase (A) phenocrysts with resorbed rims A in matrix of anorthoclase laths, clino pyroxene, Fe-Ti oxides, and finely devitri fied glass. Leaching with dilute HF and hand-picking removed rims for single crystal 40Ar/39Ar dating. Crossed-polarized light. Horizontal field is 2.3 mm. (C) Late basaltic lava (Tbl) banked against aphyric or very sparsely porphyritic rhyolite lava C D Intracaldera sedimentary deposits (Tar) in southeastern caldera wall at Pole Sentinel Rock (Tis) Creek north of Sentinel Rock (upper left), (Tar) which is a sparsely porphyritic rhyolite Pyroclastic-fall deposits lava. Variably welded pyroclastic-fall de posits composed of pumice and obsidian Late basaltic lava are well exposed between the two rhyolite (Tbl, ca. 14.9 Ma) flows. Small cliff at left edge of photo is ~20 m high. (D) Intracaldera sedimentary deposits (Tis) composed of generally well-bedded, variably tuffaceous sand Aphyric rhyolite lava stone, siltstone, and mudstone exposed in (Tar, ca. 16.4 Ma) a hectorite pit near Thacker Pass, southern part of the caldera. Face is ~3 m high.

3000 2800 31 0 2 2400 0 3200 2400 5 23 260 00 3000 2900 00 + + 0

2700 0 2600 250 _

2600

2700 _ _ 2300 2400 2300 2500 2500 2400 500 2 0 200 2500260 118°15′ 117°45′ 42°+ _ + 42° _ 2200 2500 2000 2600 2600 _ 2300 2500 2700 00

0 _ 27 0

240 2800 280 2700 2900

Black 2900 2500 2600 Mountain + 2700 2800 Figure 16. Aeromagnetic contours (U.S. + +3000 Geological Survey, 1972) overlain on geol ogy of the McDermitt caldera (simplified 2600 from Plate 1 with transparent colors).

2500 The resurgent uplift, which exposed intra 2700 _ + caldera McDermitt Tuff in the middle of the

_2600 caldera, coincides with a magnetic high that, in turn, coincides with vents at Black Round Mountain and Round Mountains and probable sub surface distribution of late icelandite (Tiv). 2400 + 2600 290 _ + 0 2600 0 _ 2800 250

+ 2500 118°15 +41°45′ ′ 2500 600 2 2400

Quaternary range-front faults 2300 2400 2500 Other faults

Caldera ring-fracture faults _ All: dashed where approximately 2600 _ located; dotted where concealed 200 2300 0 2 Aeromagnetic contours 270 in nanoteslas Plus sign: aeromagnetic high; minus sign: aeromagnetic low. 2800 290 + 0 _

0 0 2300 0 0510 km + 0 25 240

70 2 _

A single crystal anorthoclase date is 16.410 ± 0.020 Ma (H11-199, Fig. 7; Table 2). The best-exposed deposits are coarse, commonly silicified conglomerate This is indistinguishable from the age of the McDermitt Tuff and indicates the and breccia at the base of the unit near the western margin of the caldera. Aurora lavas erupted immediately after ash-flow eruption. These basal deposits are poorly sorted with angular to subangular clasts as

Distinctly less silicic lavas (56% SiO2) with vitreous plagioclase laths as much as 1 m diameter of McDermitt Tuff and precaldera volcanic rocks, partic- much as 15 mm long overlie poorly exposed intracaldera sediments (Tis) in ularly biotite rhyolite, and lesser granite. These coarse deposits are as much as the southwestern part of the caldera. The plagioclase yielded a 40Ar/39Ar date 15 m thick near the western margin and probably resulted from erosion of the of 16.08 ± 0.10 Ma (16.12 ± 0.03 Ma isochron; 09-McD-53, Table 2). The source nearby caldera wall soon after collapse, similar to mesobreccia but postdating of these lavas is unknown. all tuff eruption.

The icelandite lavas overlap in SiO2 with the precaldera icelandites, but Well-bedded, planar to cross-laminated tuffaceous sandstone, siltstone, most have slightly higher alkalies and peralkalinity index than the precaldera mudstone, tephra, and minor conglomerate and fossiliferous limestone over- icelandites (Figs. 5, 6, and 9). Aurora lavas plot along the McDermitt Tuff trend lie the coarse deposits. These finer deposits are sparsely exposed in a few in MnO and Ba, and have positive Eu anomalies like the least silicic tuff sam- canyons and in mine and exploration workings in the northern part of the cal- ples. In contrast, the young southern sample (09-McD-53) plots along the pre- dera, where they are locally silicified by hydrothermal alteration, but poorly caldera icelandite trend and does not have a positive Eu anomaly. exposed to commonly covered by Quaternary fan deposits elsewhere, especially around Thacker Pass in the south. The tuffaceous sediments have been Late Basalt (Tbl; High-Alumina Olivine Tholeiite) altered to smectite, zeolites, and potassium feldspar (Glanzman and Rytuba, 1979) and are locally silicified to opal or chalcedony (Yates, 1942; Wallace and Flows and a small plug of aphyric to moderately porphyritic basalt are the Roper, 1981; Hetherington and Cheney, 1985; McCormack, 1986). youngest igneous rocks in the caldera area. The flows are poorly exposed in Where exposed, the rocks occur mostly as float and commonly form light the southern part of the caldera, where they overlie intracaldera sediments and dark bands on aerial photographs that correspond with varying densities (Tis) and were deposited against aphyric rhyolite in the southeastern caldera of sagebrush and other vegetation. The more easily observed northern de- wall (Fig. 15C). The ~300-m-diameter plug intrudes intracaldera sediments in posits consist of glass shards, fine pumice, and mineral and rock fragments. the northern part of the caldera. The only chemical analysis, of a southern Petrified wood, diatoms, fish, and leaves occur in tuffaceous sediments and aphyric flow from drill core, shows high-alumina olivine tholeiite (Hart et al., unidentified shells in limestone (Yates, 1942; Greene, 1976; Glanzman and Ry- 1984; Brueseke and Hart, 2008) with distinctive low Ti, K, P, Rb, Nb, and REEs tuba, 1979; Castor et al., 1982). (WLC67-13.5, Supplemental Table S1 [see footnote 1]) markedly unlike Steens The sediments are thickest in the northern caldera, where drilling encoun- Basalt. An imprecise matrix 40Ar/39Ar date on the same sample is 14.90 ± tered as much as 210 m at the Aurora U deposit (Wallace and Roper, 1981), and 0.74 Ma (Table 2). in the southern caldera at Thacker Pass, where drilling by Western Lithium Cor- poration encountered as much as 190 m. No more than ~120 m are preserved Intracaldera Sedimentary Deposits (Tis) in the western rim, where the deposits are substantially eroded. A common interpretation is that the sediments were deposited post- A heterogeneous sequence of mostly poorly exposed, poorly lithified, resurgence in a moat between the caldera wall and the resurgent uplift in the predominantly tuffaceous sedimentary deposits (Fig. 15D) accumulated in a middle of the caldera (Rytuba and Glanzman, 1979; Glanzman and Rytuba, lacustrine basin in the caldera following collapse. They are preserved, and 1979; Castor et al., 1982; Garside, 1982). If true, the sediments probably never possibly were deposited, in an irregular doughnut-shaped rim or moat around covered much of the resurgent uplift and certainly have been removed now. the margin of the caldera. Stratigraphic relations with dated rocks indicate the Interbedding with late basalt (Tbl) is consistent with deposition over a long sediments were deposited over a relatively long time. Through most of the cal- period, some of which would have been post-resurgence. However, the in- dera, the sediments overlie intracaldera McDermitt Tuff. In a small area in the ward dip of sediments near the caldera wall and the gentle dip off the uplift northeast, they overlie rhyolite of McDermitt Creek (Trm) or Aurora lavas (Tia), suggest that some sediments may have been deposited before resurgence. both dated as 16.4 Ma. They underlie 16.1 Ma icelandite in the southwestern The intracaldera sedimentary deposits are similar to the Creede Formation of part of the caldera and are interbedded with ca. 14.9 Ma late basalt (Tbl) near intracaldera sediments in the Creede caldera of Colorado. The Creede Forma- Thacker Pass. Along most of the northern and northwestern structural margins tion has a thin sequence of coarse breccia overlain by much thicker lacustrine of the caldera, the sedimentary rocks overlie precaldera rocks in the caldera sediments, dominantly post-resurgence deposition, but possibly some prere- wall. The sediments close to the structural margin around most of the caldera surgence or synresurgence deposition (Larsen and Crossey, 1996; Larsen and dip ~10° inward and locally to ~25°. Our few measurements also indicate that Lipman, 2016). More structural, stratigraphic, and facies data on the sediments they dip gently off the central resurgent dome-topographic high in the middle are needed to evaluate the full timing of deposition relative to resurgence at of the caldera. McDermitt.

CALDERA STRUCTURE the Aurora U deposit, which varies from ~1400 to 1560 m (Table 4; Wallace and Roper, 1981; J. Schloderer, Oregon Energy, written commun., 1985). If intracal- Caldera Collapse–Ring-Fracture System dera McDermitt Tuff is ~600 m thick, the fault zone has >1 km displacement, possibly significantly more. We recognize a single, keyhole-shaped McDermitt caldera that is ~45 km An anastomosing zone of faults, with tens of meters to ~100 m displace- north-south by 30–22 km east-west (Fig. 3; Plate 1). Although some previous ment mostly down toward the caldera, parallels this main fault away from the studies also recognized a single McDermitt caldera (e.g., Wallace and Roper, caldera in the northern wall. The zone is ~1 km wide near the Bretz Mine and 1981), the best-known study interpreted five overlapping nearly circular cal- as much as 6 km wide in the northwestern caldera wall. Individual faults form deras in the same area, although it showed them only in a generalized, zones as wide as 70 m, e.g., at sample location H07-186 (tuff of Oregon Can- 1:1,000,000-scale, page-sized figure (Rytuba and McKee, 1984). Because of yon). Rocks in the overall fault zone dip gently, ~10° or less, into the caldera these different interpretations, we describe the geometry of the caldera, its (Fig. 17A). Dip decreases across faults away from the caldera, and rocks in the structural margin, and field relationships around the caldera in detail. Our wall more than a few kilometers outboard dip gently northward. Local tilting in interpretation of a single caldera is based on continuity of a single intracal- some fault zones generated significantly steeper dips; e.g., the tuff of Oregon dera tuff (Tmt), of a single outflow tuff correlative with the intracaldera tuff, Canyon dips 60° southward in the fault zone at H07-186 (Plate 1). of the ring fault system, and of intracaldera sedimentary deposits (Tis), as well as evidence discounting the other proposed calderas (see Discussion). Western Margin Older middle Miocene tuffs are absent where the base of the McDermitt Tuff is exposed in the caldera in the western escarpment, although the base is not A similar major fault zone with subsidiary parallel faults continues around exposed throughout the rest of the caldera. Only the tuffs of Oregon Canyon the northwestern caldera margin to Horse Creek, where erosion along the and Trout Creek Mountains underlie McDermitt Tuff in adjacent caldera walls. major west-dipping Montana Mountains fault zone exposes much deeper The continuity of intracaldera McDermitt Tuff and sedimentary deposits were levels (Fig. 17B). The caldera structural margin in and south of the canyon com- discussed herein; here we describe the ring fault system. bines wide areas of steep downwarp into the caldera and discrete but variably Minor extension of the McDermitt caldera preserved its original shape. A exposed faults. In the northern part of Horse Creek, a major fault along Cherry single major caldera fault is well exposed or reasonably interpreted beneath Creek juxtaposes various Miocene volcanic rocks down against Cretaceous post-collapse intracaldera sedimentary deposits (Tis) around the entire caldera granodiorite (Kg). An inner fault separates intracaldera McDermitt Tuff (Tmt) (Fig. 3; Plate 1). Smaller displacement concentric faults, which commonly en- and sedimentary deposits (Tis) from older Miocene and Eocene rocks and is compass zones where precaldera rocks are warped down into the caldera, are partly overlapped by sedimentary deposits. Rocks between the two faults dip also present outside the major fault, especially along the northern and western into the caldera, locally as steeply as 50°, probably as a result of slump during margin. Moderate to high-amplitude aeromagnetic highs and lows that mark collapse (e.g., Steens Basalt, Tsb, Ts; Fig. 17B). The outer fault appears to die various precaldera rocks outline the caldera around the northern, western, and out southward, but the steep inward dip continues (e.g., the Tvs dip is as steep southern margins (Fig. 16). as 44°). The inner fault continues but is poorly exposed; it separates the Steens Basalt on the west from precaldera intermediate lavas (Tpi), peralkaline rhyo- Northern Margin lite (Tpr), and McDermitt Tuff (Tmt) on the east from north to south. A major fault, probably the continuation of the inner fault from the north, The major ring-fracture fault is marked by the juxtaposition of intracaldera is well exposed in mine workings at the Moonlight Mine, dips ~50° E, and sep- sedimentary deposits (Tis) against precaldera rocks in the caldera wall. This arates precaldera biotite rhyolite of the Moonlight Mine (Tbrm) in the hanging relationship is especially clear in the Bretz Mine, where coarse sediments and wall from Cretaceous granodiorite (Kg) (Fig. 17C). We interpret this as a caldera minor megabreccia are against rhyolite in what is partly a buttress unconfor- collapse fault rather than an antithetic Basin and Range fault, because adu- mity (Wallace and Roper, 1981; Fig. 17A). Blocks in megabreccia and smaller laria in U-mineralized breccia in the fault zone yields a 40Ar/39Ar date of 16.32 ± clasts in sedimentary deposits are flow banded and folded spherulitic sparsely 0.10 Ma (C95-105, Table 2), which provides a minimum age for the fault. porphyritic rhyolite, which crops out in the adjacent wall. This major fault is Total collapse along the western margin is difficult to estimate because cor- exposed for ~4.5 km length along the northeastern margin and is covered be- relative rocks are not exposed on opposite sides of faults and much of the dis- neath intracaldera sedimentary deposits farther west and along much of the placement was accommodated by steep downwarping. However, Cretaceous western caldera margin. granodiorite (Kg) crops out at ~1945 m west of Horse Creek and is not exposed Minimum displacement across this major fault zone of 420–580 m is esti below precaldera rocks at the Moonlight Mine at ~1400 m. This indicates at mated from the ~1980 m elevation of the base of outflow McDermitt Tuff at least 545 m downdrop into the caldera, consistent with the ~450–610 m total Flattop (Fig. 3) and the elevation of the top of intracaldera tuff in drillholes at thicknesses of intracaldera McDermitt Tuff along this margin (Table 4).

West wall A North wall B Sparsely anorthoclase- Oregon Canyon Mts k phyric rhyolite (Tra) Cretaceous e re granodiorite (Kg) C ry Cher McDermitt ? Tuff (Tmt)

Steens Basalt Tsb Bretz Mine

Horse Creek

C Moonlight Mine D Altered Biotite South wall rhyolite (Tbr) Outflow McDermitt Tuff (Tmt) Double H Mts Aphyric rhyolite (Tar)

Caldera fault Intracaldera sedimentary deposits (Tis) mostly covered by thin veneer of Qf

Altered Cretaceous granodiorite (Kg)

E East wall Figure 17. (A) North wall of caldera looking west from the Bretz Mine (Fig. 3). Intracaldera McDermitt Tuff (Tmt) Rocks in the Oregon Canyon Mountains on skyline are flat-lying to gently north ward dipping. Dip changes to gently southward dipping, into the caldera, across Aphyric rhyolite a series of small-displacement faults. (B) West wall of caldera looking north Intracaldera sediments (Tis) lava (Tar) mostly covered by Qf across Horse Creek. Main caldera fault here has Cretaceous granodiorite in the wall and McDermitt Tuff over slumped rocks including steeply inward dipping Steens Basalt in the caldera. The skyline is ~500 m above Horse Creek. (C) Cal dera fault at the Moonlight Mine dips ~50° into the caldera, has dip-slip striae, Late basalt (Tbl) and separates Cretaceous granodiorite from pre-collapse biotite rhyolite of the Moonlight Mine, both highly altered with U mineralization. Exposed fault face is approximately 2 m high. (D) South wall of caldera (looking southwest) with outflow McDermitt Tuff overlying aphyric rhyolite in the wall and intracaldera sedimentary deposits in foreground mostly covered by Quaternary fan deposits. (E) East wall of caldera looking north from near Pole Creek; scarp is ~200 m high. Aphyric rhyolite in the wall dips gently away from the caldera. Intracaldera Mc Dermitt Tuff in the background is overlain by intracaldera sedimentary deposits, mostly covered by Quaternary fan deposits, and by late basalt.

TABLE 4. AMOUNT OF COLLAPSE, McDERMITT CALDERA board segments truncate at the east-northeast segment. This east-northeast Minimum Maximum segment may have reactivated older (pre-Cenozoic?) structures during col- Part of caldera (m) (m) lapse. It parallels the vitrophyre pyroclastic dikes (Tmv), and both are strongly North 580 >1000 discordant to other collapse structures. The eastern end of the east-northeast West 545 >610 segment disappears beneath Quaternary deposits. However, the caldera wall South 1100–1200 ≥1100–1200 presumably continues beneath Quaternary deposits to connect to the eastern Southeast 250 >>250 end of the northern caldera margin (Fig. 3; Plate 1). The difference in elevation Northeast segment 475 >475 between rhyolite of Hoppin Peaks (Trbh) and intracaldera sedimentary rocks (Tis) in the McDermitt Mine (≤1400 m) requires at least 475 m of collapse. Add- ing rhyolite of McDermitt Creek and intracaldera tuff would greatly increase Southern Margin this estimate. Fisk (1968) reported a rhyolite dike along the northeast segment at the Cordero Mine; however, he also said that the dike bottomed at ~210 m The Montana Mountains fault zone displaces the structural margin beneath depth, which complicates interpretation of the body as intrusive or extrusive. Quaternary deposits for 9 km southward from the Moonlight Mine to Thacker Pass, where it reappears separating McDermitt Tuff from biotite rhyolite of the Resurgence Moonlight Mine (Tbrm) and undivided precaldera volcanic rocks (Tvu). The southern structural margin is mostly marked by a steep, concave north-facing Resurgence of the McDermitt caldera created a structural and topographic topographic scarp as much as 300 m high and ~15 km long (Fig. 17D). Intracal- high where intracaldera McDermitt Tuff crops out at its highest elevations in dera McDermitt Tuff is locally exposed at the base of the topographic scarp and the center of the caldera (Fig. 18; Plate 1, section B-B′). Intracaldera tuff crops is separated from aphyric rhyolite (Tar) in the caldera wall by a nonexposed out at slightly more than 2000 m elevation immediately north and south of fault. Intracaldera sedimentary rocks (Tis) that overlie caldera faults are ex- Round Mountain, ~700 m higher than the 1300 m top of tuff in drillhole WLC- posed nearby but are mostly covered by Quaternary deposits. 58C in Thacker Pass in the southern part of the caldera (Fig. 3). This structural- Total collapse along the southern margin is at least 1100–1200 m, based on topographic expression is most apparent in a 3× exaggerated Google Earth outcrop of outflow tuff in the caldera wall at 1800–1940 m (Table 4). Drillhole (https://www .google.com /earth/) image (Fig. 18). Without this clear geologic WLC-58C (Plate 1) bottomed in intracaldera tuff at 738 m elevation. and topographic expression, doming would be difficult to confirm because of the lack of good structural markers. Foliation in intracaldera tuff, which is the primary structural marker in most calderas (e.g., John et al., 2008), is un- Eastern Margin suitable at McDermitt because the tuff is so strongly rheomorphic that paleo- horizontal is rarely preserved. Obtaining attitudes in the poorly exposed and The southeastern caldera margin mostly consists of a single, north-north- commonly slumped intracaldera sedimentary deposits (Tis) is difficult, so they east–striking fault that extends ~23 km from the southern margin. The fault is are of limited use. Whether doming preceded, followed, or overlapped with expressed as a low topographic scarp (100 to as much as 270 m) with mostly sediment deposition remains uncertain. aphyric to sparsely porphyritic rhyolite (Tar) and locally underlying peralka- Doming was accomplished partly by upwarping of intracaldera tuff and line rhyolite (Tpr) and icelandite lava (Tpi) in the caldera wall (Fig. 17E). Poorly probably partly by faults approximately tangential to the uplift. Upwarp is ap- exposed intracaldera sedimentary deposits (Tis), mostly covered by Quater- parent around most of the dome where the surface slopes steeply outward nary fans, occur at the south and north ends of this segment; rhyolite of Mc- (Fig. 18). In addition, east-striking, down-to-the-south faults separate tuff on Dermitt Creek (Trm) is in the middle. Late basaltic lava (Tbl) banked against the north from sediments on the south in Thacker Pass. Because of their ori- aphyric rhyolite (Tar) in the caldera wall at the south end. Two faults parallel entation, perpendicular to north-striking extensional faults, these faults were this north-northeast segment at its northern end ~2 and 4 km to the east. most likely active during resurgence. If so, resurgence postdated some sedi- Total collapse along this margin must be >>250 m (Table 4), the difference ment deposition. between the highest elevation of precaldera rhyolite of Hoppin Peaks (Trbh, The topographic high drops north of Long Ridge into the McDermitt Creek ~1875 m) to the east and the surface elevation in the caldera (~1624 m). Adding drainage, then rises slightly north of the creek in the northern part of the cal- the thickness of rhyolite of McDermitt Creek (Trm) and intracaldera tuff (Tmt) dera. It is unknown whether this topographic pattern results from variation in would greatly increase this estimate, probably to at least 1000 m. original doming or from erosion along McDermitt Creek, which is the largest The caldera margin turns abruptly east-northeast for ~11 km to the Cordero drainage in the caldera and the only one that crosses the entire resurgent uplift. and McDermitt Mines at the northern end of the north-northeast segment The resurgent uplift coincides well with an aeromagnetic high and with the (Fig. 3; Plate 1). The main north-northeast segment and the two parallel out- highest anomalies at the late icelandite vents at Black and Round Mountains

LR B ? ? Figure 18. Looking north across resurgent R Intracaldera sedimentary ? dome (dotted line is approximate bound deposits (Tis) ary) marked by uplifted intracaldera Mc Dermitt Tuff (Google Earth image with 3× vertical exaggeration). Caldera is approxi mately 40 km north to south. Intracaldera McDermitt Tuff (Tmt) sedimentary deposits (Tis) underlie most of low area between dome and caldera wall (red line). R (Round Mountain) and B (Black Mountain) are vent areas (Tiv) for postcaldera icelandites, whose underlying intrusions may have driven resurgence (see Fig. 16). LR is Long Ridge.

03 km

(Fig. 16; U.S. Geological Survey, 1972). This pattern suggests that post-collapse value Li-smectite used for specialty drilling fluids, yielded small production intrusion or rise of icelandite magma caused resurgence, and some magma and are currently being developed on a larger scale (Odom, 1992; Western actually erupted. Based on dates on icelandites, resurgence could have oc- Lithium Corporation, 2014; now LithiumAmericas; Fig. 3). The lithium is be- curred almost immediately after ash-flow eruption and caldera collapse or as ing evaluated for use in Li-ion electric batteries (http://lithiumamericas.com much as ca. 0.1 Ma or more later (Table 2; Fig. 7). /companies/lithium -nevada -corp/). All non-lithium deposits and prospects are along or just inside the ring-fracture system of the caldera (Fig. 3), and most reports emphasize the importance MINERALIZATION of caldera faults as conduits for hydrothermal fluids. The Moonlight Mine ex- emplifies this relationship. Uranium occurs as breccia fill along the western The McDermitt caldera has a wide range of associated mineral depos- ring-fracture fault at the mine, and the age of related adularia (16.32 ± 0.10 its and elemental enrichment, far more mineralization than in other known Ma; C95-105; Table 2) indicates that mineralization occurred soon after caldera Yellowstone hotspot calderas (Table 5; Figs. 3 and 7). This discussion focuses collapse. Intrusions are interpreted to be the heat source for alteration at the on how mineralization is related to caldera activity and structure; references Moonlight Mine. For example, Rytuba (1976, 1994) and Rytuba et al. (1979) cited in Table 5 provide more complete descriptions of many deposits, and variably cited rhyolite domes or a peralkaline rhyolite dike as intrusions along Vikre et al. (2016) summarized publicly available data on most. The Cordero, the western ring fracture and as host for the Moonlight and Horse Creek ura- McDermitt, Bretz, and Opalite Mines were the largest mercury producers in the nium deposits. However, we find these rocks to be precaldera biotite (Tbr) and U.S. between 1933 and 1989. Uranium deposits were evaluated extensively but peralkaline rhyolite (Tpr) lavas. Although ring-fracture intrusions are common yielded only minor production in the 1950s from the Moonlight Mine. Gallium in many calderas and are logical sources of heat and possibly metals, ring-frac- and gold anomalies have also been investigated. Deposits of hectorite, a high- ture intrusions are notably scarce at McDermitt.

TABLE 5. MINERAL DEPOSITS AND PROSPECTS ASSOCIATED WITH THE McDERMITT CALDERA Associated elements Major Mine or Estimated T* component prospect Major Minor (°C)Host rock§ Age and relative age Age source NotesReferences†

Uranium Moonlight Zr, As, Sb, F, Cu, Mo, Ag,Te, Tl Bi, Hg, Au 330?, 340 ± 7Tbr, Kg 16.32 ± 0.10 Ma Adularia Much U in hydrothermal1, 2, 3, 4, 5 Horse Creek Zr, As, Sb, F, Ag, Au, Bi, Mo, Tl Cu, Hg, Te, REE Tb, Tpi, Tpr, Tmt Same as Moonlight?Host rock U-rich zircon1, 2, 3, 4, 5 Big Bend Spring Zr, As, Sb, Hg Mo Tmt ≤TmtHost rock U-rich zircon present 1, 2, 3 Old Man Spring Th, Sb, As, Mo, Hg, YZr, FTmt, basal Tis? ≤Basal TisHost rock U-rich zircon present 1, 2, 3 Aurora As, Tl, Sb, Mo, F, Hg, Li Tia, Tis, Tmt ~Tis Host rock 2, 3, 4, 5, 6, 7, 8 Mercury, uranium Bretz As, Sb, Mo, FZr, Au, Ag 205 ± 5Tis <16.4 Ma, ~Tis Host rock 1, 3, 5, 6, 7, 9 OpaliteSbZr, As, Mo 195 ± 5Tis ~Tis Host rock 1, 3, 5, 6, 7, 9 Mercury CorderoAu, Ag, Ga Cu, Mo, Zn, Pb Trau?, Tar? ≤16.4 Ma Host rock Ag,Au possibly deeper 10, 11, 12 part of Hg system McDermittAs, Sb, Se, Cu, Mo, Tl, F, Te 200 ± 5Tis 16.67 ± 0.14; 15.7 ± Adularia and K-Ar 4, 10, 11, 14, 15, 16 0.4 Ma 12.6 ± 0.7 Ma Alunite K-Ar, 13 supergene? Lithium Kings Valley IF, Rb Mo lowTis 14.87 ± 0.05 Ma On spatially Mineral assemblage is 4, 5, 9, 17, 18 associated typical of closed hydrologic K-feldspar system diagenesis Hectorite Thacker Pass Tis≤TisHost rock In upper part of lithium deposit 19 Bull Basin Tis≤TisHost rock Gallium McDermittAs, Sb, Zn Tis~TisHost rock 20 low Gold Albisu Ag,As, Hg Mo, Sb, Co Tmt, Tpi ≤TmtHost rock 21, 22 *T—temperature estimates from Rytuba (1976) and Rytuba and Glanzman (1979) except for hectorite and lithium, which is based on young age and association with diagenetic minerals. ? indicates uncertainty (see text for details). †References: 1—Castor et al. (1982), 2—Dayvault et al. (1985), 3—Castor and Henry (2000), 4—Rytuba (1976), 5—Rytuba and Glanzman (1979), 6—Wallace and Roper (1981), 7—Roper and Wallace (1981), 8—Myers (2005), 9—Glanzman et al. (1978), 10—Yates (192), 11—Fisk (1968), 12—Childs (2007), 13—Rytuba and McKee (1984), 14—Hetherington and Cheney (1985), 15—McCormack (1986), 16—Noble et al. (1988), 17—Western Lithium Corporation (2014), 18—Stillings and Morissette (2012), 19—Odom (1992), 20—Rytuba et al. (2003), 21—Minor et al. (1988), 22—Western Lithium Corporation, D.P. Bryan (2015, written commun.). §For host rock symbols, see text Figures 6 and 7.

The timing of other uranium mineralization is defined only as being and caldera collapse and early during deposition of the intracaldera sedi- younger than host rocks (Table 5). However, similar element assemblages, mentary deposits (Tis). especially enrichment in zirconium at Moonlight, Horse Creek, Big Bend The Aurora uranium deposit, although enriched in some of the same ele- Spring, and Old Man Spring, and their spatial association in the western ments as at the Moonlight deposit, is not enriched in zirconium (Castor and part of the caldera, suggest genetic and possibly temporal ties. Zirconium Henry, 2000). Aurora probably is unrelated to the Moonlight hydrothermal sys- enrichment appears to decrease upward, with the greatest enrichment at tem, consistent with its location in the northeastern part of the caldera more the Moonlight Mine and the least at Old Man Spring, where mineralization than 20 km from the other deposits. probably extends into basal intracaldera sedimentary deposits (Castor et al., The timing of mercury mineralization at the McDermitt Mine, which is 1982). All of these occurrences have U-rich, hydrothermal zircon, which hosts hosted by intracaldera sedimentary deposits (Tis), is poorly constrained. most of the uranium at the Moonlight Mine (Castor and Henry, 2000). All U-Zr Step-heating of fine-grained (≤50 µm) adularia intergrown with opal did not mineralization may have formed at the same time, shortly after tuff eruption yield a plateau (McD; Table 2, Supplemental Table S2 [see footnote 2]), but

indicates that mineralization occurred approximately coincident with caldera bly contemporaneous with eruption of late basalt lavas that are interbedded formation. Individual steps yielded ages ranging from 16.15 ± 0.10 to 16.97 ± with the intracaldera sedimentary deposits in the southern part of the cal- 0.10 Ma; the largest single step, with 25% of released 39Ar, is 16.25 ± 0.09 Ma. dera. The implications of this date for the timing of lithium deposition are All steps but one were 32%–38% radiogenic, probably reflecting the presence less certain. If hectorite and Li-rich smectite formed during diagenesis, then of some residual opal with abundant atmospheric Ar in the mineral concen- at least some lithium concentration resulted from diagenesis. The date seems trate. An isochron of all steps is 16.67 ± 0.14 Ma (McD; Table 2), but this age consistent with the sedimentary deposits taking a considerable time to accu- is older than those of ash-flow eruption, caldera collapse, and deposition of mulate and become altered by low-temperature groundwater diagenesis. A intracaldera sedimentary deposits. A K-Ar date on alunite of 12.6 ± 0.7 Ma was low-temperature origin is consistent with proposed mechanisms of formation interpreted to indicate a second, younger hydrothermal episode (Rytuba and of Li-rich brines in closed basins (Davis et al., 1986) and extreme enrichment McKee, 1984; Noble et al., 1988). However, the date more likely indicates super and posteruptive depletion of lithium in some rhyolites of the western United gene formation of alunite in a low-sulfidation epithermal system in which adu States (Hofstra et al., 2013). The presence of illite is the primary evidence for laria is the primary K alteration phase (J.K. McCormack, 2016, personal com- higher temperatures, but its significance is debated. Problems with an igne- mun.; McCormack, 1986). ous-driven hydrothermal system include distribution of lithium enrichment Stratiform lenses containing Li-enriched illite or smectite are found in throughout the caldera, lack of association with igneous rocks except locally intracaldera sedimentary deposits (Tis) throughout the caldera (Glanzman with the small-volume and areally restricted basalts, and association with and Rytuba, 1979; Odom, 1992; Western Lithium Corporation, 2014; Fig. 3). probable low-temperature diagenesis. The relative contributions of low-temperature diagenetic versus higher Regardless of the exact origin and conditions of lithium mineralization, temperature hydrothermal fluids in concentrating lithium are uncertain. the different types of deposits and elemental concentrations and multiple Glanzman and Rytuba (1979) and Rytuba and Glanzman (1979) proposed that episodes of igneous activity at the McDermitt caldera suggest considerable the lithium mineralization resulted from leaching of lithium from nonwelded potential for overprinting by multiple hydrothermal systems and diagenesis. glass in the sedimentary deposits and concentration in potassium feldspar– Further determination of deposit types, mineralogic sites of concentrated ele- rich alteration zones; they reported that alteration mineralogy progresses ments, age, formation temperature, and potential source rocks (e.g., modern from almost fresh glass, to a mix of zeolites, to analcime, and to potassium determination of lithium concentrations in unaltered rocks) is needed to eval- feldspar, and interbedded mudstones are altered to dioctahedral smectite uate the origin of deposits. and trioctahedral smectite including hectorite. Our recent work additionally found interlayered illite/smectite and illite. Except for illite, this mineral zonation is typical of that that forms during closed hydrologic system diagenesis, EXTENSION where groundwater evolves through rock-water interaction and evaporation in a closed basin such as in the McDermitt caldera to become highly alkaline The McDermitt caldera is within the Basin and Range Province, but the and saline (Surdam, 1977; Langella et al., 2001). Most closed-system dia minor tilting of the caldera demonstrates that it has undergone only minor genesis occurs at near surface temperature, ≤50 °C (Langella et al., 2001), extension, which probably began ca. 12 Ma based on thermochronology although diagenesis in a volcanically active area could be at higher tempera- of the Santa Rosa Range to the east and the Pine Forest Range to the west tures. Glanzman and Rytuba (1979) did not report a temperature of diagen- (Figs. 2 and 3; Colgan et al., 2004, 2006a, 2006b). Quaternary faults bound esis, but interpreted that hydrothermal alteration overprinted the diagenetic both the western and eastern sides of the Montana Mountains (Plate 1). The sequence at five locations in the caldera: at the McDermitt, Bretz, and Opalite west-dipping Montana Mountains fault zone forms the western edge of the Mines, a location 2 km south of the Bretz Mine, and a general area along the Montana Mountains and the Double H Mountains to the south, and dies out western part of the caldera. Based on studies of the diagenesis of Paleogene– northward into the Trout Creek Mountains (Plate 1; U.S. Geological Survey, Neogene deposits of the Gulf Coast, the presence of illite could indicate tem- 2016). The east-dipping Hoppin Peaks fault zone forms the eastern edge of peratures of 100 °C or more (Boles and Franks, 1979; Bourdelle et al., 2013). the Montana Mountains and is an antithetic set to the larger displacement, However, Turner and Fishman (1991) found that pore-water chemistry can be west-dipping Santa Rosa Range fault system that forms the western front of more important than temperature in illite formation and that illite can form the Santa Rosa Range. at near-surface temperatures in saline alkaline lakes similar to the McDermitt The caldera is gently east tilted, based on the overall higher elevations of caldera setting. the west side of the caldera block, relative elevations of poorly stratified or Potassium feldspar from the Western Lithium Corporation Kings Val- exposed rocks across the caldera, and especially on obvious gentle, ~10°–12° ley Stage I lens at Thacker Pass yielded a 40Ar/39Ar date of 14.87 ± 0.05 Ma east dips of better-layered volcanic rocks to the south and north, e.g., in the (WLC43-180.2; Table 2). This date indicates that potassium feldspar forma- Double H Mountains (our observations) and in the Trout Creek Mountains of tion occurred long after most caldera-related magmatism, although possi- Orevada View (Plate 1) and farther north (Minor et al., 1989b).

DISCUSSION position (Starkel et al., 2012), and precise 40Ar/39Ar age, to be continuously exposed from north of McDermitt Creek to near Thacker Pass, through all How Many Calderas? three proposed calderas. In the most clearly described relationships, Rytuba and McKee (1984) pro- Our interpretation of a single caldera contrasts with the conclusions of posed that the Calavera caldera collapsed during eruption of the tuff of Double Rytuba and McKee (1984), who interpreted three partly overlapping calderas H (formerly member 1 of the tuff of Long Ridge; Rytuba and Glanzman, 1978, composing a McDermitt caldera complex (Fig. 3; Table 6) plus two adjacent 1979), an aphyric comendite that they showed in the southern and southeast- and partly overlapping calderas, one younger and one older than the caldera ern caldera wall where we mapped aphyric rhyolite lava (Tar). Rytuba and complex. Our interpretation of a single caldera is based on continuity of a sin- McKee (1984) placed the northern boundary of the caldera through Calavera gle intracaldera tuff (Tmt), the ring fault system, and intracaldera sedimentary Canyon; that boundary is shown overlapped by the southwestern margin of deposits (Tis), and absence of evidence for the other proposed calderas. Here the younger Jordan Meadow caldera slightly to the northeast, and the Jor- we discuss significant aspects of the geology that we interpret either simi- dan Meadow boundary is shown as buried or projected through continuous larly or differently in comparison to Rytuba and McKee (1984). A limitation intracaldera McDermitt Tuff (Fig. 3). As initially discussed by Castor and Henry is that published depictions of the previous interpretation consist only of a (2000), precaldera rocks [biotite rhyolite of the Moonlight Mine (Tbr) and per- 1:1,000,000-scale page-sized map (Rytuba and McKee, 1984), and generalized, alkaline rhyolite (Tpr)], intracaldera McDermitt Tuff (Tmt), and post-collapse page-sized geologic maps do not show the individual calderas (Rytuba, 1976; intracaldera sedimentary deposits (Tis) are not displaced and do not change Rytuba and Glanzman, 1978, 1979; Glanzman and Rytuba, 1979). thickness across Calavera Canyon or anywhere within several kilometers to the north or south; this precludes a caldera boundary fault there (Fig. 11A). Proposed McDermitt Caldera Complex Hargrove and Sheridan (1984) came to a similar conclusion.

Rytuba and McKee (1984) divided the McDermitt caldera as identified here Proposed Adjacent Overlapping Calderas into three separate calderas: from north to south, the Long Ridge, Jordan Meadow, and Calavera calderas (Fig. 3; Table 6). An immediate problem is Washburn caldera: Proposed source of the tuff of Oregon Canyon. Rytuba that their only caldera depiction, Figure 1 of Rytuba and McKee (1984), shows and McKee (1984) interpreted a Washburn caldera, the source of the tuff of the Long Ridge caldera blending continuously into the Jordan Meadow cal- Oregon Canyon, partly overprinted by and partly exposed northeast of the dera with no boundary, and their text identifies no map boundary between northeastern part of the McDermitt caldera (Fig. 3). However, we find no evi intracaldera tuffs, so it is impossible to know where one interpreted caldera dence for any caldera in this area (see following). Rytuba and McKee (1984, ends and the other starts. In contrast, we find intracaldera McDermitt Tuff, p. 8618) interpreted the northwestern Washburn caldera structural margin to indistinguishable in outcrop characteristics, phenocryst assemblages, com- be an arcuate, 6-km-wide zone of 5 down-to-the-southeast faults that “dis-

TABLE 6. CALDERAS OF THE McDERMITT AREA Rytuba and McKee (1984) This study Proposed calderaProposed caldera-forming tuff Caldera? Key points WhitehorseTuff of Whitehorse Creek Ye s, but possibly for tuff of Trout Tuff of Whitehorse Creek 47 m thick in caldera, suggests deposited in existing caldera. Creek Mountains, not for tuff of Whitehorse Creek Hoppin Peaks Biotite rhyolite of Hoppin Peaks Not a calderaProposed intracaldera tuff consists of 3 rhyolite lavas totaling 210 m thick, no structural margin, and proposed margin appears to go through middle of lava flows. McDermitt Long Ridge Tuff of Long Ridge member 5 Part of single McDermitt calderaIntracaldera McDermitt Tuff, indistinguishable in outcrop characteristics, phenocryst assemblage, caldera Jordan Tuff of Long Ridge members Part of single McDermitt caldera composition, and 40Ar39Ar age, crops out continuously through entire McDermitt caldera. complex Meadow 2, 3 Structural margin is continuous around entire caldera. Proposed Calavera margin goes Calavera Tuff of Double H (tuff of Long Part of single McDermitt caldera uninterrupted through precaldera, intracaldera, and postcaldera rocks. Tuff of Long Ridge Ridge member 1) members 1, 2, and 3 are rhyolite lavas. WashburnTuff of Oregon CanyonNot a calderaNo structural margin and no intracaldera tuff or postcaldera fill despite exposure of rocks older than tuff of Oregon Canyon in proposed caldera. Tuff of Oregon Canyon farther from proposed caldera appears to be closer to source, which may be in the Whitehorse caldera area (Benson and Mahood, 2015).

place the tuff of Oregon Canyon downward 250 m.” Figure 5a of Rytuba and Benson et al. (2017) interpreted a source caldera that is overprinted by the McKee (1984), a generalized geologic cross section, shows the following. Whitehorse Creek caldera (see following; Figs. 2 and 3). A less likely alter- (1) The northwestern 4 faults have a few tens to ~100 m displacement of native is that the tuff of Oregon Canyon correlates with the petrographically the tuff of Oregon Canyon, which maintains constant thickness across the and compositionally similar Idaho Canyon Tuff, which erupted from the Virgin faults. (2) The tuff was displaced ~600 m down across the southeasternmost Valley caldera 65 km to the west (Noble et al., 1970; Castor and Henry, 2000). fault, which coincides with the caldera boundary shown in Figure 3 but is However, Coble and Mahood (2016) argued that the two tuffs are not the same in an area for which no published geologic map is available. Rytuba and based on small compositional differences. McKee (1984) seemed to indicate that the tuff is nowhere exposed in the Whitehorse caldera: Proposed source of the tuff of Whitehorse Creek. The proposed source caldera. (3) Intracaldera tuff is interpreted to be covered ~15–20-km-diameter Whitehorse caldera is ~20 km northwest of the McDer- beneath ~600 m of caldera-filling rhyolite lavas (the upper rhyolite, Turf, mitt caldera and is interpreted to have formed during eruption of the tuff of of Rytuba and Curtis, 1983), which are ~150 m thick northwest of the fault. Whitehorse Creek (Figs. 2 and 3; Rytuba et al., 1981; Rytuba and McKee, 1984). The abrupt change in thickness of these post-tuff lavas is cited as evidence The caldera interpretation is reasonable; a shallow topographic basin con- that “define the northwest margin of the caldera.” However, these lavas are tains tuffaceous sediments including diatomite with five rhyolite domes along not exposed south of the southeasternmost fault in the proposed caldera. the probable ring fracture (Rytuba et al., 1981; Benson et al., 2017). However, (4) A “400-m-thick sequence of iron-rich andesite flows, ponded along the the caldera being the source of the tuff of Whitehorse Creek is less certain. northwestern ring zone, indicates continued subsidence of the caldera”; This tuff, for which we obtained a sanidine 40Ar/39Ar date of 15.67 ± 0.04 Ma these are displaced ~200 m across the southeastern fault but do not change (MD-8, Table 2), crops out in the eastern part of the caldera where it laps thickness across it: in addition, “These lavas are restricted to areas adjacent onto the caldera wall (Rytuba et al., 1981, 1983b). The tuff of Whitehorse to the Washburn caldera.” (Rytuba and McKee, 1984, p. 8618). Rytuba and Creek is nearly flat lying in the caldera but dips moderately, to 28°, into the McKee (1984) also interpreted the northeast-striking part of the McDermitt caldera where it is compacted against the southeastern caldera wall (Rytuba caldera margin north of Hoppin Peaks to be a reactivated part of the Wash- et al., 1983b; our observations); this indicates that the caldera wall existed burn caldera. when the tuff was deposited. Moreover, Rytuba et al. (1981) reported that In contrast to point 2, Peterson et al. (1988) did not show the southeastern- a drillhole to a depth of 226 m in the southern part of the caldera encoun- most fault, but showed an ~1.5-km-long, thin (≤100 m?) lens of tuff of Oregon tered 136.5 m of caldera-fill sedimentary deposits, 47 m of intracaldera tuff Canyon ~2 km south of Mendi Suri, which would be close to the middle of of Whitehorse Creek, and 43 m of rhyolite lava to the bottom of the hole. the proposed Washburn caldera. If present, the tuff would indicate little dis- Although a caldera-forming tuff could compact against its own caldera placement across the southeasternmost fault and remarkably thin intracaldera wall, the thinness of the tuff of Whitehorse Creek inside the caldera sug- tuff. However, we found that area to consist entirely of precaldera intermediate gests that the tuff was deposited in an existing caldera. The 47 m thickness lavas (unit Tpi). In contrast to point 3, we found no great thickness of rhyolite of intracaldera tuff is very small compared to the thickness of intracaldera lavas that could correlate with the upper rhyolite. In contrast to point 4, Rytuba McDermitt Tuff and seems insufficient to generate caldera collapse. The et al. (1983a, 1983b) showed the same iron-rich andesite map units cropping Whitehorse caldera may have formed during eruption of an older tuff, the out at least 15 km to the northwest. We find similar icelandite lavas around most likely being the 16.49 Ma tuff of Trout Creek Mountains, based on that most of the McDermitt caldera. unit’s distribution. From new geologic mapping and geochronology, Benson The most telling evidence against a Washburn caldera is that the quartz- et al. (2017) came to a similar conclusion and even interpreted calderas for sanidine rhyolite lava dome (Trq), which predates tuff of Oregon Canyon both the tuffs of Trout Creek Mountains and Oregon Canyon to be nested (Fig. 7; Table 2), occupies the proposed center of the caldera. The presence of beneath the Whitehorse caldera (Fig. 2). this older rock and absence of tuff of Oregon Canyon in the section preclude Hoppin Peaks caldera: Proposed source of the tuff of Hoppin Peaks. Rytuba this area being a caldera for the tuff. and McKee (1984) interpreted a Hoppin Peaks caldera, the source of their tuff Sparse evidence suggests that the source for the tuff of Oregon Canyon of Hoppin Peaks, east of the McDermitt caldera and largely buried beneath could be significantly north or west of the McDermitt caldera. The tuff at the Quinn River Valley (Fig. 3). Much of the cited evidence consists of a closed proposed northwestern caldera wall just northwest of the southeasternmost 20 mg gravity low in the valley. However, the rhyolite of Hoppin Peaks consists fault of point 1 is ~35 m thick with an ~8-m-thick poorly welded zone at the of three separate rhyolite lavas that lack any evidence of being pyroclastic, top, has few lithics ≤2 cm in diameter, and shows no evidence of rheomor- e.g., there are no nonwelded facies, no pumice, fiamme, or lithics in any rock, phism. The tuff of Oregon Canyon in Oregon Canyon 10 km to the northwest and they are cumulatively only 210 m thick. The proposed structural margin of appears to be much more proximal, ~60 m thick, more coarsely (to 15 cm di- Rytuba and McKee (1984, Fig. 1 therein) appears to go through the middle of ameter) and abundantly lithic, and strongly rheomorphic with flow folds and the lava outcrop. The gravity anomaly is better interpreted to indicate basin fill ramps to vertical. These characteristics suggest a source farther north, and in an extensional basin (Figs. 2 and 3).

Timing of Steens Basalt and Precaldera Intermediate Magmatism tion and caldera collapse, although a small (~2.5 × 3.5 km) caldera erupted a and Initiation of the Yellowstone Hotspot local tuff ca. 15.5 Ma, and high-alumina olivine tholeiite magmatism occurred only early in the field. The Steens Basalt has been difficult to date precisely, but new40 Ar/39Ar Silicic volcanism as old as ca. 16 Ma is even more widespread, from the dating indicates initiation between ca. 16.85 and 16.74 Ma. Camp et al. (2013; High Rock caldera complex to the west, north to the Lake Owyhee volcanic see also Brueseke et al., 2007; Jarboe et al., 2010; Barry et al., 2013) reported field, and approximately located Dinner Creek center (Oregon) and Silver City 7 new 40Ar/39Ar dates on plagioclase separates and whole rocks and 41 pub- Range (Idaho), and east to the Jarbidge Rhyolite (Fig. 2; Noble et al., 1970; lished Steens Basalt dates from many sources, and 14 dates on silicic rocks Cummings et al., 2000; Bonnichsen et al., 2008; Wypych et al., 2011; Hausback that overlie the Steens Basalt (including some of our data), all recalculated to et al., 2012; Brueseke et al., 2014; Streck et al., 2015; Benson and Mahood, the 28.201 Ma age of the Fish Canyon Tuff sanidine monitor (Kuiper et al., 2008) 2016; Coble and Mahood, 2016). The Jarbidge Rhyolite is distinctly old rela- used in this study. Based on these data, Camp et al. (2013) concluded that eruptive to its longitude along the Yellowstone track, petrologically distinct, and tion of their formally proposed Steens Basalt may have begun ca. 16.85 Ma may be more related to regional extension (Brueseke et al., 2014). Neverthe- and lasted ca. 0.3 Ma (Fig. 7). Based on 40Ar/39Ar dates on silicic tuffs inter- less, early Yellowstone silicic activity is widespread, extending over an area bedded with Steens Basalt and assumed basalt accumulation rates, Mahood of ~40,000 km2, but is concentrated in discrete centers with abundant silicic and Benson (2016, 2017) estimated a slightly younger time of initiation, ca. activity. Silicic rocks are absent in major sections and in known or probable 16.74 Ma. source areas of the Steens Basalt away from the silicic centers, e.g., at Steens Our data, with four dates between 16.691 ± 0.021 and 16.607 ± 0.015 Ma on Mountain and in the Catlow Peak section in the Trout Creek Mountains just three lavas and one tuff probably all in the middle or upper part of the Steens northwest of the McDermitt caldera (Fig. 3; Camp et al., 2013). Although ba- Basalt, and the 16.56 ± 0.02 Ma (n = 4) or 16.517 ± 0.013 Ma dates on the cap- saltic activity began in the northern Nevada rift at about the same time as ping tuff of Oregon Canyon, are consistent with this Steens Basalt eruption regional Steens magmatism, silicic volcanism did not begin until much later, timing (Fig. 7). Except for the tuff of Oregon Canyon, none of these units are in ca. 15.7 Ma (John et al., 2000). the two sections examined by Camp et al. (2013) near the McDermitt caldera, Hart et al. (1984) identified a distinctive high-alumina olivine tholeiite so precise correlation into those sections is not possible. Precaldera interme- magma type that is widely distributed through northeastern California, south- diate lavas (Tpi) overlie the tuff of Oregon Canyon and continued at least until eastern Oregon, southwestern Idaho, and northern Nevada; it began erupt- the 16.53 ± 0.16 Ma date on icelandite in the north wall. Icelandite continued to ing ca. 10. 5 Ma, and continued to later than 0.1 Ma throughout most of this erupt inside the caldera until 16.08 ± 0.10 Ma, although whether this rock rep- area. Recognition of high-alumina olivine tholeiite lavas and dikes as old as ca. resents continued magmatism related to the precaldera lavas or late eruption 16.8 Ma in the Santa Rosa field (Brueseke and Hart, 2008) and 14.90 ± 0.74 Ma from the McDermitt magma chamber is unknown. at McDermitt (Table 2) confirms the geographic distribution, but significantly Although not the oldest caldera (Fig. 2), the McDermitt volcanic field has increases its time of initiation. These results further support high-alumina some of the oldest silicic volcanism in the region, ca. 16.7 Ma. The Virgin olivine tholeiite being a small volume but integral component of regional mag- Valley caldera, source of the 16.56 Ma Idaho Canyon Tuff, is the oldest cur- matism (Hart et al., 1984). rently identified caldera (Castor and Henry, 2000; Hausback et al., 2012;Coble and Mahood, 2016). A possible caldera for the tuff of Oregon Canyon in the Continuity of Magmatism and Peralkaline versus Whitehorse area northwest of McDermitt could be slightly older (Benson and Metaluminous‑Peraluminous Activity Mahood, 2015; Benson et al., 2017). Rhyolite lavas as old as ca. 16.7 Ma are also abundant in the Santa Rosa–Calico volcanic field immediately to the east Field relations and high-precision 40Ar/39Ar dating indicate that magmatism (Fig. 2; Brueseke and Hart, 2008), to the west in the Hawks Valley–Lone Moun- around the McDermitt caldera was essentially continuous from the beginning tain volcanic field (Wypych et al., 2011), and far to the northeast in the Silver of Steens Basalt eruptions between ca. 16.85 and 16.74 Ma (Brueseke et al., City, Idaho, area (Bonnichsen et al., 2008). 2007; Jarboe et al., 2010; Camp et al., 2013; Barry et al., 2013; Mahood and The timing and, aside from a major ash-flow eruption, style of magmatism Benson, 2016, 2017; this study) through eruption of the McDermitt Tuff and at McDermitt are similar to that in the Santa Rosa–Calico volcanic field east caldera collapse ca. 16.39 Ma, to post-collapse volcanism. Erupted rocks gen- of McDermitt (Fig. 2; Brueseke and Hart, 2008; Brueseke et al., 2008). Santa erally became more silicic through time. The oldest eruptions were ~47–49

Rosa–Calico magmatism started with eruption of the Steens Basalt, was fol- wt% SiO2 lavas of the Steens Basalt (Camp et al., 2013, and references therein; lowed by a wide range of mafic through silicic lavas, and mostly ended ca. Starkel, 2014; Starkel et al., 2014). Minor rhyolite erupted as early as 16.691 ± 16.2 Ma, with some minor, mostly mafic magmatism much later, ca. 14 Ma 0.021 Ma, and rhyolite predominated by ca. 16.5 Ma. The precaldera interme- (Brueseke and Hart, 2008). Significant differences are that the voluminous diate lavas (Tpi) are between the Steens Basalt and the rhyolite lavas in com-

Santa Rosa–Calico magmatism did not culminate with major ash-flow erup- position (56%–66% SiO2; Supplemental Table S1 [see footnote 1]) and strati-

graphic position; they overlie the tuff of Oregon Canyon (16.56 ± 0.04 Ma, n = 4, is based heavily on study of the ignimbrite flareup, particularly in the United MAP215–50; 16.517 ± 0.013 Ma, Argus VI) and partly underlie the tuff of Trout States (Table 7; Lipman, 1984, 2007; Best et al., 2013a, 2013b, 2016; Henry and Creek Mountains (16.49 ± 0.03 Ma, n = 3, MAP215–50). John, 2013; Lipman and Bachmann, 2015). Peralkaline and biotite-bearing (metaluminous-peraluminous) rhyolites Important characteristics of the McDermitt caldera are (1) generation by overlapped closely in space and time around the McDermitt caldera, but are eruption of a strongly zoned, probably high-temperature peralkaline rhyolite somewhat antithetic in detail. The variably peralkaline, sparsely anortho- to metaluminous icelandite, (2) large area (~930 km2) and volume (~1000 km3) clase-phyric to aphyric rhyolite lavas (various Tra, Tar) that erupted semicontin- but relatively shallow collapse (~1 km), (3) major resurgence, (4) extensional uously from 16.69 to eruption of the McDermitt Tuff at 16.39 Ma are nearly ab- setting that was not undergoing active extension (Colgan et al., 2004, 2006a, sent in the southwestern wall, which consists mostly of the 16.619 ± 0.020 Ma 2006b; Colgan and Henry, 2009), and possibly (5) location in accreted island arc biotite rhyolite of the Moonlight Mine (Tbr). The closest overlap between or transitional terrane crust (Fig. 1). peralkaline and metaluminous activity is in the eastern caldera wall, where The McDermitt caldera is most like calderas of the ignimbrite flareup, 16.38 ± 0.07 Ma biotite rhyolite of Hoppin Peaks was deposited closely after the especially in producing pyroclastic deposits with strong compositional zon- underlying 16.397 ± 0.061 Ma anorthoclase and aphyric rhyolite. The 16.410 ± ing, large volume, and resurgence, as well as most subsidiary characteristics 0.015 Ma (n = 3) sanidine-quartz-phyric peralkaline rhyolite (Tpr) appears to (Table 7). McDermitt and many ignimbrite flareup calderas also have similar have been a distinct event unlike any of the other rhyolites. We have not found crustal settings and tectonic environments. The McDermitt caldera is in thin, any evidence that the different magma systems interacted during eruption. accreted island arc crust, as are the westernmost of the ignimbrite flareup cal- However, their close spatial and temporal proximity would seem to allow for deras (Henry and John, 2013; John et al., 2015). The tectonic setting of McDer- the possibility (e.g., Allan et al., 2012). mitt may have been more extensional than that of the ignimbrite flareup, but Emplacement of icelandite lavas after caldera collapse suggests (1) that the both occurred during a neutral environment that lacked any active deforma- caldera magma chamber retained some magma after tuff eruption, or (2) that tion (Best and Christiansen, 1991; John et al., 2008; Colgan and Henry, 2009; regional, less silicic magmatism was reestablished after interruption by the Henry and John, 2013). The most notable differences are that the McDermitt caldera system density filter, which prevented the rise of higher density, more caldera is larger in area than all except a few ignimbrite flareup calderas, but mafic magmas through lower density, more silicic magmas of the caldera. The has undergone far less collapse, ~1 km versus typically 3–4 km. The propor- similarity between postcaldera and precaldera icelandites hampers choosing tion of intracaldera versus outflow tuff is similar for McDermitt and ignimbrite between these alternatives. That the immediately postcaldera Aurora lava ice- flareup calderas and appears to be independent of other variables, includ- landites have MnO (Fig. 6E) and Ba contents that plot on the McDermitt Tuff ing amount of collapse. The paucity of ring-fracture volcanism at McDermitt trend supports alternative 1. That the much younger southern icelandite lavas is also anomalous, especially given that ring-fracture rhyolites are abundant plot along the precaldera icelandite trend may support alternative 2 once suffi- around the other early hotspot calderas, Virgin Valley and High Rock (Castor cient time elapsed. Determining the regional distribution and timing of similar and Henry, 2000; Hausback et al., 2012; Coble and Mahood, 2016). The simi- magmatism could help distinguish between the alternatives. larities and differences with ignimbrite flareup calderas apply even to the rare peralkaline ignimbrite flareup caldera. The 25.4 Ma Questa caldera of northern New Mexico is ~15 km in diameter, underwent more than 2 km of collapse Comparison of the McDermitt Caldera with Other Silicic Calderas during eruption of 500 to possibly 1000 km3 of slightly zoned peralkaline rhyo lite, and has resurgent and ring-fracture intrusions of peralkaline rhyolite in- Here we compare the McDermitt caldera to other calderas associated with distinguishable in age and composition to the erupted tuff (Lipman et al., 1986; voluminous, silicic magmatism but spanning the range from strongly peralka- Lipman, 1988; Zimmerer and McIntosh, 2012). line to calc-alkaline (metaluminous), and (2) evaluate to what extent the McDer- The McDermitt caldera is less similar to those associated with strongly per- mitt caldera is an analog for the volcanic centers associated with voluminous alkaline magmas (Mahood, 1984; Table 7). The McDermitt Tuff shares strong rhyolites but buried beneath younger rocks of the central Snake River Plain compositional zoning and is at the low end of the alkalinity range, but the (Table 7). Comparisons are made with strongly peralkaline calderas, for which caldera is much larger in area, erupted volume, and amount of collapse, and erupted tuffs have peralkalinity indices >1.1 to 2.0 (Mahood, 1984), and with underwent resurgence, which is rare among strongly peralkaline calderas. calderas of the ignimbrite flareup of western North America. The ignimbrite These comparisons suggest that magma system volume and derivative flareup formed a semicontinuous belt of ignimbrites and source calderas from eruptible volume may be the most important factors in determining most cal- the Great Basin of the United States to the southern end of the Sierra Madre dera characteristics. Intrinsic variables such as peralkalinity and inferred high Occidental in Mexico (Swanson and McDowell, 1984; Ferrari et al., 2007; Mc- temperature strongly encouraged the intense rheomorphism shown by the Dowell and McIntosh, 2012; Henry et al., 2012b; Best et al., 2013a, 2016). Overall McDermitt Tuff and most strongly peralkaline tuffs, but seem to have had little understanding of the geometry, evolution, and geotectonic setting of calderas influence on caldera character.

TABLE 7. CHARACTERISTICS OF THE McDERMITT CALDERA AND OTHER SILICIC CALDERAS McDermitt CalderaBuried Yellowstone hotspot calderas Strongly peralkaline calderas Ignimbrite flareup, western North America Precaldera volcanismMajor, diverse; Steens Basalt to ?Shield built from abundant low viscosity Mostly major (to minor and possibly none) intermediate to rhyolite lavas lavas Ash-flow tuff character Strongly zoned, peralkaline high-Si High-temperature, generally Commonly strongly zoned from Generally zoned, calc-alkaline, rhyolite (comendite) to metaluminous homogeneous, low- or high-Si pantellerite to crystal-rich trachyte; high metaluminous to mildly peraluminous trachydacite (icelandite) rhyolites temperature high-Si rhyolite to dacite andesite; low- temperature alkalic systems are rare Phenocrysts Anorthoclase, pyroxene, fayalite Plagioclase, sanidine, augite, pigeonite, Alkali feldspar, scarce quartz Quartz, sanidine, plagioclase, biotite, rare quartz and hypersthene, ilmenite, hornblende, clinopyroxene, apatite, zircon magnetite, zircon, apatite Peralkalinity index 1. 12–0.80 ~0.7–0.9 >1.1 to 2.00 ~0.7–0.9

(molar Na2O + K2O/Al2O3) Structural collapse area (km2) 930; larger than all except a few Calderas are inferred but not exposed Mostly ~10–100, rarely to 300 ~100–600, rarely 1600 (Windous Butte, Ignimbrite Flareup Great Basin), 2500? (La Garita, southern Rocky Mountains) Topographic wall No more than 1 km outside structural ?Mostly very little outside structural Varies from many kilometers outside margin margin structural margin to almost none Vertical collapse and/or subsidence ~1 km ?Hundreds of meters, rarely to 1 km Hundreds of meters to 6 km Tuff volume (km3)~1000 To at least 1000 Mostly 2–10, rarely to 100s ≤100 to ~6000; maximum >5000 = Fish Canyon Tuff (southern Rocky Mountains), 5900 Wah Wah Springs (Great Basin) Megabreccia and mesobreccia Common ?Intracaldera tuff rarely exposed Abundant, probably universally present Outflow distance <20 km ?Few kilometers to tens of kilometers, Tens of kilometers to as much as 250 km commonly limited by island or rift where flowed in paleovalleys setting Intracaldera tuff (%) 50–85 ?Poorly known because rarely exposed Variable, 80 to ~50 RheomorphismCommon, intenseCommon, intenseCommon, intenseRare, and only with most alkalic Postcaldera igneous activityVolumetrically minor icelandite as central Common, voluminous trachyte lavas Major to none, ring-fracture intrusions and vents and lavas, few if any ring-fracture from central vent lava domes very common bodies Resurgent upliftYes, through center of caldera ? Rare and with little structural uplift: Mostly yes, in some cases strong doming Pantelleria and Mount Suswa only Resurgent Intrusion Ye s, probably icelandite that also erupted ?Probably trachyteCommon; some compositionally similar to as late vents and lavas less evolved part of erupted tuff Ring-fracture Intrusions Absent or sparse? Variable, common along outer concentric Abundant to common to minor fractures Part of Caldera ComplexNo? Commonly part of nested complexBoth complexes and individual calderas are common Related hydrothermal activity and Abundant, diverse (U, Hg, Au, Li) Minor to none? Probably none Highly variable: none, to major mineralization hydrothermal systems without deposits, to a few major deposits (Au, Cu) Tectonic setting In extensional setting, but negligible total Extensional, at least partly during active Commonly associated with active but Neutral tectonic environment, with extension until ~4 Ma after caldera extension low-magnitude extension; e.g., East no measureable extension or any activity Africa Rift contemporaneous deformation Crustal setting Accreted island arc terrane North American craton Variable North American craton, to transitional-rifted craton, to accreted island arc terrane Implications for Magma Chamber Large area but small subsidence may ?ShallowVariable area with small to very large imply thin (few kilometers) tabular subsidence implies generally thick, magma chamber tabular magma chambers Note: From this study, Mahood (1984), Swanson and McDowell (1984), McIntosh et al. (1992), Christiansen (2001), Christiansen (2005), Lipman (2007), Christiansen and McCurry (2008), Lipman and McIntosh (2008), Bonnichsen et al. (2008), John et al. (2008), McDowell and McIntosh (2012), Henry and John (2013), Ellis et al. (2013), Best et al. (2013a, 2013b, 2016), and Lipman and Bachmann (2015). ?—no data; buried.

The characteristics of the McDermitt caldera have several possible impli- dant in the Santa Rosa–Calico field (Brueseke and Hart, 2008, 2009) but are not cations for the underlying magma chamber. Given the general interpretation reported from the younger volcanism of the SRP. Furthermore, the variety in that caldera area approximates magma chamber area (Lipman, 1997, 2007; compositions of rhyolites in terms of peraluminosity and mineral assemblage Christiansen, 2005), the McDermitt magma chamber probably was large and (the common occurrence of biotite-bearing rhyolite) is at odds with the rhyo- underlay the entire caldera. The small amount of interpreted collapse suggests lites of the Snake River Plain–Yellowstone system, where hydrous phases are that the erupted part was only a thin cap to the chamber, overlying a sub- rarely found (Christiansen, 2001; Drew et al., 2013; Troch et al., 2017). At Mc- stantial volume of more intermediate composition or noneruptible, crystal-rich Dermitt the intermediate compositions are effusive, and so such compositions mush. Subsequent resurgence implies that (1) sufficient magma remained in could have erupted in the younger systems but remain obscured within the the chamber to generate resurgence, or (2) a deeper magma source remained poorly exposed calderas. However, two main lines of evidence argue against sufficiently active to quickly provide significant magma volumes that were in- this: (1) no such intermediates are observed at Yellowstone where intracaldera distinguishable from the least evolved tuff. lavas are abundant (Christiansen, 2001; Stelten et al., 2015), and (2) samples from drillcores in the CSRP exhibit the same basalt-rhyolite compositional bi- McDermitt: An Analog for the Central Snake River Plain? modality as the surficial record (Anders et al., 2014; Knott et al., 2016). These petrologic differences may reflect different crustal environments, i.e., accreted The new mapping and geochemical and petrological constraints described terrane for the McDermitt caldera, and craton for the CSRP (Fig. 1). here make the McDermitt volcanic field one of the best studied of the entire The McDermitt Tuff shows some similarities but also some important differ- Yellowstone system. With this information we can evaluate to what extent Mc- ences in comparison with most CSRP ignimbrites. The anhydrous mineralogy Dermitt represents a potential analog for the central Snake River Plain (CSRP) and relatively high inferred magmatic temperature of the McDermitt Tuff are supereruption-producing centers, which are concealed beneath later basalt somewhat similar to those of CSRP ignimbrites, which show a general decrease (Fig. 1; Table 7; Bonnichsen et al., 2008). in magmatic temperatures toward Yellowstone (Nash et al., 2006; Branney et al., Similarity in erupted volumes suggests that McDermitt is a good analog 2008). The moderate to dense welding of the McDermitt Tuff is more similar for the CSRP calderas in terms of caldera style and geometry. The Kimberly to the ignimbrites from the Yellowstone volcanic field than to the lava-like ig- drillcore in the CSRP east of McDermitt encountered more than 1.3 km of nimbrites of the CSRP. The McDermitt Tuff also contains abundant pumice (typi Castleford Crossing ignimbrite (Fig. 1; Knott et al., 2016), by far the thickest cally expressed as fiamme) and abundant lithic clasts of a wide variety of rock ignimbrite recorded from the province, and consistent with an intracaldera types (Figs. 11B and 12B), both features that are atypical of the CSRP ignimbrites. deposit. Other drillholes have mostly encountered outflow deposits. For ex- While the CSRP ignimbrites do contain lithic clasts, typically these occur as small ample, among numerous units encountered in core in the eastern SRP, the centimeter-scale clasts at low proportions (<5%–10%) and are almost uniquely thickest was a 200-m-thick lava and the thickest ignimbrite was ~30 m (Anders composed of antecedent CSRP rhyolites with glassy lithologies seemingly over- et al., 2014). This core bottomed in rhyolite, allowing the potential for greater represented with respect to surface lithologies (Ellis and Branney, 2010). The thickness beneath, but the recovered drillcore produced 9 different units over a relative paucity of lithics in the CSRP ignimbrites has been attributed to the in- thickness of ~360 m, consistent with an extra-caldera sequence. ferred distance to a proposed caldera; however, the general scarcity of pumice The relatively thin occurrence of intracaldera McDermitt Tuff (with the asso (in both ignimbrite and fallout deposits) cannot readily be explained in such a ciated caveat of only having reliable observations from the western caldera way. In terms of deposit architecture, the large compositional zonation exhibited margin) at first appears similar to the intracaldera thickness of the Yellowstone by the McDermitt Tuff is unknown in the CSRP, where deposits show extremely ignimbrites. While the 400 m maximum thickness contour of Member A, Lava limited heterogeneity in bulk compositions (Ellis et al., 2013; Wolff et al., 2015). Creek Tuff, could perhaps be considered similar to the 600 m of measured Branney et al. (2008) suggested that the extent to which the unusual char- McDermitt Tuff, we note that the isopachs of deposit thickness illustrated in acteristics of Snake River Plain–type volcanism are displayed is at a maximum Christiansen (2001) for the major Yellowstone ignimbrites are still interpola- around the Bruneau-Jarbidge region and decreases to the east and west. Our tions, because much of the current exposure is composed of postcaldera lavas detailed study of the McDermitt volcanic system confirms this idea, illustrating (e.g., Stelten et al., 2015), and locations where both the base and top of an both similarities and differences with the rhyolites of the Snake River Plain ignimbrite sheet can be seen are scarce, even with the density of drillcores at across a range of petrology, geochemistry, and physical appearance. Yellowstone. Petrologically, the CSRP rhyolites are significantly unlike the McDermitt CONCLUSIONS volcanic system, which shows more similarities with the other early hotspot centers (High Rock and Virgin Valley; Castor and Henry, 2000; Hausback et al., The McDermitt caldera formed ca. 16.39 Ma in an area that had undergone 2012; Coble and Mahood, 2016). The McDermitt system produced voluminous abundant prior igneous activity, the most important being major middle Mio- intermediate compositions (e.g., icelandites, andesites), which are also abun- cene volcanism that led continuously to caldera formation, as well as two pre-

ceding episodes of Eocene intermediate volcanism at 47 and 39 Ma. Diverse provided funding in 1995. Starkel received funding from the Edmap component of the U.S. Geo- middle Miocene activity began with eruption of voluminous basalt lavas of the logical Survey National Cooperative Geologic Mapping Program, the Hugh E. McKinstry Fund of the Society of Economic Geologists Foundation, Inc., the American Philosophical Society, and Steens Basalt, probably between ca. 16.85 and 16.74 Ma (Camp et al., 2013; Washington State University (Pullman). Mahood and Benson, 2016, 2017). The oldest rhyolites erupted by 16.69 Ma, and subsequent activity became progressively more dominated by rhyolite. Precaldera rhyolites were particularly diverse, ranging from voluminous per- REFERENCES CITED alkaline (aphyric to sparsely anorthoclase phyric to moderately quartz-sani- Allan, S.R.A., Wilson, C.J.N., Millet, M.-A., and Wysoczanski, R.J., 2012, The invisible hand: Tec- dine ± sodic amphibole phyric) to common but less voluminous metalumi- tonic triggering and modulation of a rhyolitic supereruption: Geology, v. 40, p. 563–566, doi: 10.1130/G32969.1. nous (moderately quartz-sanidine-plagioclase-biotite phyric). Although not the Anders, M.H., Rodgers, D.W., Hemming, S.R., Saltzman, J., DiVenere, V.J., Hagstrum, J.T., oldest caldera of the Yellowstone hotspot, the McDermitt area has the oldest Embree, G.F., and Walter, R.C., 2014, A fixed sublithospheric source for the late Neogene documented rhyolitic activity. track of the Yellowstone hotspot: Implications of the Heise and Picabo volcanic fields: Jour- The McDermitt caldera formed during eruption of magma zoned from nal of Geophysical Research, v. 119, p. 2871–2906, doi:10.1002/2013JB010483. Axelrod, D.I., 1966, Potassium-argon ages of some western Tertiary floras: American Journal of aphyric, high-Si, peralkaline rhyolite to abundantly anorthoclase-phyric, Science, v. 264, p. 497–506, doi:10.2475/ajs.264.7.497.

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