Geology and 40Ar/39Ar geochronology of the mid-Miocene McDermitt volcanic field Geology and 40Ar/39Ar geochronology of the middle Miocene McDermitt volcanic field, Oregon and Nevada: Silicic volcanism associated with propagating flood basalt dikes at initiation of the Yellowstone hotspot

Thomas R. Benson†, Gail A. Mahood, and Marty Grove Department of Geological Sciences, Stanford University, 450 Serra Mall, Building 320, Stanford, California 94305, USA

ABSTRACT that extends southwest from the northern volcanism. Notably, this includes detailed work McDermitt volcanic field, through McDer- on ca. 0–10 Ma rhyolites of the province where The middle Miocene McDermitt volcanic mitt caldera and the Santa Rosa–Calico cen- many calderas are exposed or inferred (e.g., field of southeastern Oregon and northern ter, to the northern Nevada Rift. A similar Christiansen, 2001; Morgan and McIntosh, Nevada is a caldera complex that is tempo- linear trend is observed ~75 km to the west, 2005; Ellis et al., 2012; Anders et al., 2014) rally and spatially associated with the earli- where the Hawks Valley–Lone Mountain and within the central and western Snake River est flood lavas of the Columbia River Basalt center and the calderas of the High Rock cal- Plain, where voluminous, hot, dry rhyolite erup- Group, the Steens Basalt. The topographi- dera complex define an ~N20°E trend radiat- tions occurred from ca. 15 to 10 Ma (e.g., Mc- cally prominent caldera west of McDermitt, ing south-southwest from Steens Mountain. Curry et al., 1997; Boroughs et al., 2005; Bon- Nevada, has commonly been considered the The temporal, spatial, and compositional nichsen et al., 2004, 2008; Branney et al., 2008). starting point for the time-transgressive Yel- patterns of rhyolitic magmatism along both Perhaps most important to the origin of the lowstone hotspot trend. In the original work trends are consistent with rapid southward Yellowstone–Snake River Plain system, how- defining the field, seven weakly to moderately propagation of flood basalt dike swarms as- ever, was the ca. 16.5–15 Ma rhyolite volcanism peralkaline rhyolitic ignimbrites were identi- sociated with emplacement of the Yellow- in Oregon, northern Nevada, and southwestern fied to have erupted from seven calderas over stone plume head. Idaho that occurred contemporaneous with the an interval of ~1 m.y. following emplacement voluminous eruption of the Columbia River of Steens Basalt flood lavas. Aided by 47 new INTRODUCTION and Steens flood basalts (Fig. 1). This relation- high-precision 40Ar/39Ar ages and extensive ship has prompted researchers to suggest that trace-element geochemistry, we refine the vol- The Yellowstone–Snake River Plain province initial silicic volcanism was associated with a canic stratigraphy to four major ignimbrites: is an ~600-km-long, time-transgressive locus mantle plume head centered roughly at Steens 16.468 ± 0.006 Ma (2σ) Tuff of Oregon Can- of explosive and effusive volcanism that was Mountain (e.g., Camp et al., 2003; Shervais and yon, 16.415 ± 0.007 Ma Tuff of Trout Creek initiated during the middle Miocene in eastern Hanan, 2008; Coble and Mahood, 2012; Fig. 1). Mountains, 16.328 ± 0.013 Ma Tuff of Long Oregon and northern Nevada coeval with the New field investigations coupled with high-pre- Ridge, and 15.556 ± 0.014 Ma Tuff of White- eruption of the Columbia River Basalt Group cision 40Ar/39Ar geochronology has enabled re- horse Creek. New geologic mapping has lavas. Magmatism has since steadily propagated searchers to more precisely define the sequence identified the sources of the two oldest ignim- northeastward toward Wyoming, where activ- of the largest silicic eruptions in this area, nota- brites at two newly delineated, overlapping ity has most recently occurred during the Qua- bly at High Rock caldera complex (Coble and calderas in the northern McDermitt volcanic ternary in Yellowstone National Park (Fig. 1). Mahood, 2016), Lake Owyhee volcanic field field: the ~20 × 24 km Fish Creek caldera, Various geodynamic models have been pro- (Nash and Perkins, 2012; Streck et al., 2015; formed on eruption of the Tuff of Oregon posed to explain the space-time progression of Benson and Mahood, 2016), and McDermitt Canyon, and the ~20 × 26 km Pole Canyon volcanism, including the migration of the North volcanic field (Henry et al., 2016; this study). caldera, formed ~50 k.y. later on eruption American plate over the tail of a deep mantle The location and timing of the oldest rhyo- of the compositionally similar Tuff of Trout plume (e.g., Pierce and Morgan, 1992, 2009; lite eruptions in McDermitt volcanic field are Creek Mountains. Ring-fracture lavas of Camp and Ross, 2004; Smith et al., 2009; Xue of critical importance in developing a physical these two calderas lie outboard of those re- and Allen, 2010; Darold and Humphreys, 2013; model for the onset of Yellowstone–Snake River lated to the youngest caldera in the field, the Camp et al., 2015), or melting and extension in Plain volcanism because of the hypothesis that ~13 × 12 km Whitehorse caldera, which is en- response to convection within the upper man- formation of the McDermitt volcanic field was tirely nested within the Pole Canyon caldera. tle (e.g., Anderson, 1994; Christiansen et al., the initial manifestation of Snake River Plain The new mapping and chronology of the 2002; James et al., 2011; Fouch, 2012; Foulger volcanism (e.g., Pierce and Morgan, 1992, northern McDermitt volcanic field make et al., 2015). 2009; Branney et al., 2008; Leeman et al., 2008; clear that there is a linear ~N20°W trend of Characterization of the source locations and Shervais and Hanan, 2008; Ellis et al., 2012). In mafic, intermediate, and rhyolitic volcanism timing of major rhyolite eruptions along a linear this paper, we refine stratigraphic relationships trend has been critical to the development of the of regional ignimbrites and their relationships †trb@​stanford​.edu hypotheses for the origin of Snake River Plain to the McDermitt volcanic field through field

GSA Bulletin; September/October 2017; v. 129; no. 9/10; p. 1027–1051; https://​doi​.org​/10​.1130​/B31642​.1; 13 figures; 3 tables; Data Repository item 2017151; published online 23 June 2017.

GeologicalFor permission Society to of copy, America contact [email protected], v. 129, no. 9/10 1027 © 2017 Geological Society of America Benson et al.

124°W 120°W 116°W

Y OREGON 120 km JdF map extent Lake Owyhee Monument Volcanic Field

olcanoes

V Basin

and Chief Joseph Paci c Range CR IDAHO plate High La

Cascade va Plains Trend Columbia River Basalt HH R Group lavas and dikes JB SW / LJ SC end Younger CRB Steens ? e River PlainTr Steens Mtn McDermitt Snak BB ? Yellowstone Steens Basalt Volcanic Field D HV Mid-Miocene TM Fig. 4 42°N rhyolitic centers M J High Rock V Lava centers B SR Caldera H ? NN Complex 4 S Calderas CC NEVADA ? R I 0.70 0.706 Figure 1. Regional map showing the distribution of mid-Miocene (ca. 17–15 Ma) volcanism in the Pacific Northwest (after Benson and Mahood, 2016). Extents of Columbia River Flood Basalt (CRB) Group members are split into Steens Basalt and younger members (Imnaha, Grande Ronde, Picture Gorge, Wanapum, Saddle Mountain) based on Reidel et al. (2013a), and generalized locations of feeder dikes are after Tolan et al. (1989), Camp et al. (2013), and Reidel et al. (2013b). Caldera locations are from this study, Rytuba and McKee (1984), Rytuba and Vander Meulen (1991), Benson and Mahood (2016), Coble and Mahood (2016), and Henry et al. (2016), and are labeled with the following symbols: V—Virgin Val- ley caldera, B—Badger Mountain caldera, H—Hanging Rock caldera, CC—Cottonwood Creek caldera, M—McDermitt caldera, R—Rooster Comb caldera, CR—proposed caldera at Castle Rock. Contem- poraneous lava centers are shown as yellow dots and are labeled with the following symbols: SC—Sil- ver City, LJ—Little Juniper Mountain, HH—Horsehead Mountain, JB—Jackass Butte, SW—Swamp Creek Rhyolite, TM—Twenty Mile Creek Rhyolite, BB—Bald Butte, D—Drum Hill, HV—Hawks Val- ley–Lone Mountain, S—Sleeper Rhyolite, SR—Santa Rosa–Calico, I—Ivanhoe, J—Jarbidge. Other symbols: Y—Yellowstone caldera, JdF—Juan de Fuca plate, NNR—northern Nevada Rift. Isopleths of 87 86 0.704 and 0.706 Sr/ Sri are after Benson and Mahood (2016, and references therein). mapping, geochemistry, and high-precision scale in northern McDermitt volcanic field, 40Ar/39Ar Geochronology 40Ar/39Ar geochronology. These new data allow field-checking and remapping contacts previ- us to delineate three overlapping calderas in the ously mapped in published 7.5′ quadrangles Of ~300 samples collected in McDermitt northern McDermitt volcanic field. The distri- (Rytuba et al., 1982a–1982g; Rytuba and ­Curtis, volcanic field and the surrounding region for bution of volcanic activity in this nested caldera 1983; Rytuba et al., 1983a, 1983b; Peterson this study, 47 whole-rock rhyolite lava and ig- complex and in the southern McDermitt vol­ and Tegtmeyer, 1987; Minor and Wager, 1989; nimbrite samples were selected for 40Ar/39Ar canic field and in the surrounding region leads Minor et al., 1989a–1989c). Standard methods analysis. All ages reported here are based on us to new insights into the petrogenetic pro- used for the geochemical, electron microscopy, analyses of samples included within a single cesses involved during impingement of a mantle and volume estimate calculations are described irradiation, in order to maximize the ability plume on continental lithosphere. in Appendix A of the GSA Data Repository file.1 to analytically resolve ages of closely spaced Important new details of the 40Ar/39Ar methods, eruptive units. Details of the sample prepara- METHODS which bear on the precision and accuracy of the tion and irradiation procedures are provided reported ages, are summarized next. in Appendix A (see footnote 1). The 40Ar/39Ar

Ten months of field work involving geological measurements were single-crystal, CO2 laser fu- mapping and sample collection were performed 1GSA Data Repository item 2017151, field and sion analyses performed with a Nu Instruments between 2012 and 2015 in the northern Mc- laboratory methodology; supplemental geochronol- Noblesse multicollecting mass spectrometer ogy figures and data; mineral chemistry; whole-rock Dermitt volcanic field and in adjacent areas of geochemistry, is available at http://www​ ​.geosociety​ fitted with a high-mass Faraday cup and two northern Nevada and southern Oregon. The ma- .org​/datarepository​/2017 or by request to editing@​ low-mass ion-counting detectors (Coble et al., jority of mapping was performed at the 1:24,000 geosociety​.org. 2011). The Noblesse was interfaced with a

1028 Geological Society of America Bulletin, v. 129, no. 9/10 Geology and 40Ar/39Ar geochronology of the mid-Miocene McDermitt volcanic field

LabVIEW-automated, all-metal extraction line basis of different approaches defines a spread of dard or decay constant values by recalculating system to acquire 40Ar/39Ar analyses by one of more than 1% (e.g., Phillips and Matchan, 2013) previously reported 40Ar/39Ar ages using a FCs two procedures. Most gas aliquots were suffi- and thus requires that the accuracy of 40Ar/39Ar K-Ar age of 28.02 Ma and a decay constant of ciently large (e.g., >1 × 10–15 mol 39Ar) to use results must also be percent level at best. In this 5.543–10 yr–1. We were unfortunately unable to the multicollecting procedure that involves a study, we assigned FCs a K-Ar age of 28.02 Ma apply this approach to results reported in Henry peak hop between the mass station used to mea- (e.g., Renne et al., 1998) and quote only ana- et al. (2016) because the J-factors and 40Ar/39Ar sure 40Ar‑38Ar-36Ar and that used to measure lytical uncertainties to maintain consistency ratios necessary for this recalculation were not 39Ar and 37Ar (method 3 in Coble et al., 2011). with previous studies in the area that adopted provided. Instead, we arbitrarily lowered Henry Smaller gas aliquots were dynamically selected such a value (e.g., Brueseke­ et al., 2007; Coble et al.’s (2016) results 100 k.y. to account for to be measured in monocollection mode using and Mahood, 2016). J-factor data, calculated as their use of a 28.201 Ma K-Ar age for FCs and the axial ion counter. Either procedure requires outlined in McDougall and Harrison (1999), ap- a decay constant of 5.463 × 10–10 yr–1. Finally, 300 s per 40Ar/39Ar analysis. pear in Appendix B (see footnote 1), with values we note that estimation of absolute errors in The overall approach for standardizing the used for each sample listed in Appendix C (see the 40Ar/39Ar model ages requires propagation 40Ar/39Ar measurements has been described in footnote 1). of additional systematic errors associated with Coble et al. (2011). The measurement proce- Inverse isochron model ages were calculated the 40K decay constant and a >1% error in the dures employed in this study were standard- as described in Mahon (1996). We prefer inverse age of FCs. We did not apply this step, since all ized using a newly synthesized reference gas isochron ages to weighted mean of total gas ages samples in this study were co-irradiated to allow prepared in collaboration with Andrew Calvert because most of our samples indicate “trapped” precise calculation of relative age differences. at the U.S. Geological Survey (Menlo Park, 40Ar/36Ar ratios in the 288–294 range (Table 1), California) that was put into service in June which are distinctly lower than the atmospheric GEOLOGIC BACKGROUND 2015. This gas was generated by mixing previ- value of 298.6 (Lee et al., 2006). While the ously described aliquots of argon (40Ar/36Ar = difference between inverse isochron ages and The McDermitt volcanic field is a mid-Mio- 29.69 from reservoir U041 and 38Ar from res- weighted mean ages that assume atmospheric cene volcanic center along the Oregon-Nevada ervoir U039; Miiller, 2006) with 39Ar extracted trapped argon tends to be negligible for highly border consisting of basaltic to rhyolitic lavas,­ by fusing neutron-irradiated kalsilite glass. radiogenic feldspars, many samples analyzed in calderas, and outflow sheets of associated When standardized against atmospheric argon this study had radiogenic 40Ar (40Ar*) contents rhyolitic ignimbrites. The McDermitt vol­ 40 36 87 86 ( Ar/ ArTrue = 298.56 ± 0.31; Lee et al., 2006; <90% (Table 1; Appendix C [see footnote 1]). A canic field straddles the 0.704 Sr/ Sri isopleth 40 36 Ar/ ArMeasured = 299.31 ± 0.40), the mass dis- plot comparing inverse isochron and weighted (Fig. 1), which delineates the boundary between crimination–corrected 40Ar/39Ar, 38Ar/39Ar, and mean ages from representative samples of all ~30-km-thick crust primarily composed of com- 36Ar/39Ar values measured on the Faraday cup four McDermitt volcanic field ignimbrites is positionally primitive accreted mafic terranes in of the Stanford Noblesse were 10.543 ± 0.008, shown in Figure 2. Inverse isochron ages for all the west and transitional cratonal crust to the 0.3810 ± 0.0010, and 0.3821 ± 0.0008, respec- 47 samples are summarized in Table 1, and all east (e.g., Armstrong et al., 1977). Further east, tively (2s standard errors). The 39Ar contributed inverse isochron plots are provided in Appendix Precambrian cratonal crust is present with a per aliquot, determined via measurements of D (see footnote 1). Most samples analyzed in thickness that exceeds 35 km (e.g., Eagar et al., Ar sensitivity performed with weighed aliquots this study have mean square of weighted devi- 2011; Stanciu et al., 2016). The Paleozoic to Ju- of GA1550 biotite (McDougall and Wellman, ates (MSWD) values near unity, indicating that rassic Black Rock terrane basement that under- 2011), was 3.71 × 10–15 mol. Correction factors the single-crystal age distributions represent lies the McDermitt volcanic field is composed for argon isotopic ratios measured daily over homogeneous populations. For those units in of Paleozoic sedimentary rocks, early Mesozoic the course of measurements performed for this which we analyzed several samples, our pooled arc-related mafic volcanic rocks, and Late Tri- study for each of the two procedures are shown ages were calculated from the error-weighted assic to Early Jurassic deep-marine back-arc in Appendix A (see footnote 1). As indicated, mean of inverse isochron ages from individual basin sediments (Wyld and Wright, 2001, and Ar isotopic ratios measured by multicollection samples (results in bold font in Table 1). references therein). These rocks are intruded by measurements were reproducible to 0.12‰ (2s The errors associated with all model ages re- middle Cretaceous granitic rocks (N. van Buer, standard error) for a 39Ar abundance of 3 × 10–15 ported in this manuscript are 2s standard devia- 2012, personal commun.), upon which most of mol based upon 275 measurements performed tions for inverse isochron ages and 2s standard the Miocene volcanic strata were deposited. over a 60 s interval (Appendix A, A–C [see errors for weighted mean ages, indicated by “±±” Basin and Range extension gradually in- footnote 1]). Monocollection measurements following the guidelines of Renne et al. (2009). creases from <1% north of the High Lava Plains from 1.5 × 10–15 mol 39Ar gas fractions were re- MSWD values reflect propagation of all analyti- (Trench et al., 2012; Ford et al., 2013) to ~50% producible to 0.15‰ for 40Ar/39Ar and 0.25‰ cal errors (see individual data tables in Appendix south of the study area in central Nevada (Col- for 40Ar/39Ar for 119 measurements performed C [see footnote 1] for details). The uncertainties gan et al., 2006b; Fig. 1). In the study area, fault- over the same interval (Appendix A, D–F [see associated with model ages include additional ing began at ca. 12 Ma and produced a system footnote 1]). systematic errors associated with constants of high-angle normal faults that collectively ac- 40 39 40 39 In calculating Ar/ Ar ages, we used a decay that apply to the entire irradiation ( Ar/ ArK, commodate a modest amount (<20%) of crustal –10 –1 38 39 36 37 39 37 constant of l = 5.543 × 10 yr (Steiger and Ar/ Ar, Ar/ ArCa, and Ar/ ArCa correc- extension (Colgan et al., 2006a; Lerch et al., Jäger, 1977), which remains consistent with cur- tion factors and positional error in the J-factor). 2008; Fig. 1). This faulting resulted in the expo- rent best estimates (Renne et al., 2011). There is Inclusion of these systematic errors (1s model sure of as much as 1 km of Miocene stratigraphy currently no consensus regarding the true 40Ar/40K error in Table 1) facilitates comparison of our in the footwalls of normal faults, enabling the of ca. 28 Ma Fish Canyon sanidine (FCs) or any results with those reported in other studies (e.g., detailed joint stratigraphic and 40Ar/39Ar geo- other flux monitor in use for 40Ar/39Ar analysis. Jarboe et al., 2008, 2010). We accomplished this chronologic analysis that is a significant compo- The range of K-Ar ages ascribed to FCs on the comparison with studies that used different stan- nent of this study.

Geological Society of America Bulletin, v. 129, no. 9/10 1029 Benson et al.

TABLE 1. SUMMARY OF NEW 40Ar/39Ar AGES FOR THE NORTHERN MCDERMITT VOLCANIC FIELD Inv. isochron Model Latitude Longitude Rock Fsp age An. err An. err err No. 40Ar/36Ar 40Ar/36Ar 40Ar* Unit Sample no. Irr. ID (°N) (°W) type type (Ma) (1σ) (2σ) MSWD (1σ) grains trapped err 2σ (%) Whitehorse caldera Buckskin Mtn hb rhyoilte lava TB-317 12-21D 42.26712 118.38269 DL S15.369 0.0080.015 0.61 0.01921/22 293.93.6 90.0 S. camp rhyolite lava TB-321 12-27D 42.26661 118.25176 SL S15.435 0.0220.043 3.04 0.02711/22 295.71.6 53.1 W. Willow Ck rhyolite lava TB-590 12-28D 42.24478 118.24622 SL S15.446 0.0150.029 0.75 0.02222/22 293.712.591.5 Red Mtn rhyolite lava TB-573 12-14D 42.28692 118.31409 DL S15.446 0.0100.019 1.88 0.02022/22 288.77.5 92.4 Willow Butte rhyolite lava TB-593 12-25D 42.20687 118.26079 DL S15.464 0.0140.027 0.53 0.02222/22 290.54.9 88.7 Flagstaff Butte rhyolite lava TB-318 12-12D 42.19081 118.38354 DL S15.510 0.0120.023 1.24 0.02120/22 294.72.3 80.0 Buckskin Mtn rhyolite lava TB-558 12-20D 42.25534 118.41754 DL, SL S15.526 0.0160.032 0.38 0.02322/22 293.42.2 81.2 S. Flagstaff Ranch rhyolite lava TB-468 12-18D 42.18450 118.35268 DL S15.544 0.0150.030 0.75 0.02318/22 291.61.6 66.7 N. Flagstaff Ranch rhyolite lava TB-331 12-23D 42.19218 118.35779 DL S15.570 0.0120.023 1.71 0.02121/22 298.02.4 81.6 Red Lookout Butte rhyolite lava TB-563 12-02D 42.31757 118.40485 DL, SL S15.589 0.0130.026 1.86 0.02121/21 291.71.5 76.4 Tuff of Whitehorse Creek TB-322 12-31D 42.22749 118.23219 NWI S15.559 0.0220.044 0.24 0.02822/22 296.245.193.6 TB-555 12-32D 42.24480 118.42426 DWI S15.578 0.0100.020 0.48 0.02043/44 281.35.2 89.8 TB-522C 12-33D 42.31314 118.48327 MWI S15.537 0.0160.032 1.36 0.02321/22 294.110.089.4 TB-566 12-34D 42.31015 118.43772 DWI S15.532 0.0130.026 1.73 0.02220/22 291.03.3 82.5 15.556 0.0070.014 3.22 0.011 Bearclaw rhyolite lava TB-362A 12-16D 42.18745 118.32625 DL S15.540 0.0110.022 1.50 0.02022/22 295.23.9 87.7 TB-469 12-17D 42.17758 118.34011DLS 15.555 0.0110.021 1.00 0.02022/22 294.46.6 91.1 15.548 0.0080.015 0.97 0.014 NE campground rhyolite lava TB-609 12-30D 42.28671 118.24941 SL S15.553 0.0160.032 1.23 0.02314/22 294.21.7 62.0 N. Whitehorse Butte rhyolite lava TB-320 12-24D 42.26797 118.24235 DL, SL S15.575 0.0120.023 1.11 0.02120/22 293.12.1 77.7 Whitehorse Butte rhyolite lava TB-342 12-13D 42.24840 118.21720 DL S15.581 0.0110.021 0.91 0.02021/21 293.02.5 83.3 Camp turnoff rhyolite lava TB-307 12-26D 42.29710 118.26900 SL S15.592 0.0140.028 0.32 0.02219/22 291.72.6 77.4 Tule Rims Tule Rims rhyolite lava TB-567 12-07D 42.34089 118.40108 VL S16.122 0.0100.020 1.72 0.02017/22 294.93.5 90.4 McDermitt caldera Tuff of Long Ridge TB-226 12-18C 41.96416 118.75058 DWI A16.353 0.0160.031 0.88 0.02314/22 292.82.3 62.0 TB-452 12-19C 42.25548 118.01513 RI A16.343 0.0130.026 1.13 0.02219/21 293.52.3 78.0 TB-455 12-20C 42.24698 118.12354 RI A16.328 0.0160.031 1.01 0.02318/20 297.43.6 83.5 EW-210 12-21C 42.34286 117.88136 DWI A16.305 0.0110.022 1.57 0.02022/22 289.55.9 92.2 16.328 0.0070.013 2.77 0.011 Pole Canyon caldera Tuff of Trout Creek Mountains TB-225A 12-10C 41.96330 118.74812 DWI S16.423 0.0110.021 1.72 0.02021/21 291.73.9 87.7 TB-265B 12-11C 42.30632 118.32468 DWI S16.412 0.0090.017 0.91 0.01922/22 294.22.8 87.8 TB-439 12-12C 42.15867 118.45271 DWI S16.401 0.0100.020 0.92 0.02020/22 294.41.9 81.0 TB-443 12-13C 42.15890 118.45338 DWI S16.404 0.0080.015 1.35 0.01920/22 289.94.1 91.8 TB-451 12-14C 42.25564 118.01474 DWI S16.431 0.0080.015 1.29 0.01920/22 293.75.0 88.5 TB-456 12-16C 42.23906 118.11455 DWI S16.415 0.0100.020 1.55 0.02022/22 294.55.1 90.7 EW-218 12-17C 42.36816 117.96071 DWI S16.420 0.0150.030 0.57 0.02310/10 296.41.9 81.5 16.415 0.0040.007 1.58 0.008 Fish Creek rhyolite lava TB-354 12-06D 42.28041 118.18412 DL S16.445 0.0110.022 0.93 0.02021/22 295.34.4 88.6 Fish Creek caldera Tuff of Oregon Canyon TB-224 12-02C 41.96299 118.74632 DWI S16.467 0.0170.033 0.81 0.02422/22 307.415.792.6 MC407B 12-03C 41.96244 118.74628 DWI S16.484 0.0110.022 1.41 0.02022/22 291.03.6 87.6 ML-304 12-04C 42.03048 118.31897 DWI S16.494 0.0090.017 2.07 0.01922/22 297.32.7 89.2 TB-444 12-05C 42.26167 118.00729 MWI S16.461 0.0080.015 2.48 0.01922/22 290.51.5 87.4 TB-429 12-06C 42.15851 118.45157 MWI S16.450 0.0080.015 1.62 0.01919/22 294.11.8 81.7 TB-433 12-07C 42.15866 118.45172 DWI S16.468 0.0080.015 0.85 0.01920/21 293.82.8 88.3 TB-435 12-09C 42.15867 118.45187 DWI S16.457 0.0090.017 1.69 0.01922/22 296.03.2 88.6 TB-264 12-19D 42.32369 118.25404 RI S16.497 0.0180.036 0.38 0.02521/22 293.55.2 85.5 16.468 0.0030.006 3.20 0.007 Antelope Creek rhyolite lava TB-517 12-05D 42.34693 118.15824 DL S16.450 0.0140.028 1.08 0.02219/20 297.06.2 90.0 N. Red Mountain rhyolite lava TB-266 12-03D 42.31038 118.32269 SL S16.526 0.0110.022 0.61 0.02020/21 294.32.1 83.0 TB-571 12-04D 42.32951 118.32822 DL S16.513 0.0110.022 1.32 0.02017/22 291.73.1 80.9 16.520 0.0080.016 0.70 0.014 Whitehorse Cyn rhyolite lava TB-488 12-09D 42.23518 118.14332 DL S16.5110.021 0.0420.360.027 20/22292.32.5 75.0 TB-586 12-10D 42.23179 118.14403 SL S16.510 0.0110.022 0.76 0.02017/22 292.42.2 80.2 TB-355 12-11D 42.23749 118.15452 DL S16.539 0.0200.039 1.41 0.02618/22 291.92.6 75.9 16.516 0.0090.017 0.87 0.014 Note: Irr.—irradiation; Fsp.—feldspar; Inv.—inverse; An.—analytical; MSWD—mean square of weighted deviates; err—error; Mtn—Mountain; hb—hornblende; Ck— Creek; Cyn—Canyon; DL—devitrified lava; SL—silicified lava; VL—vitric lava; NWI—nonwelded ignimbrite; MWI—moderately welded ignimbrite; DWI—densely welded ignimbrite; RI—rheomorphic ignimbrite; S—sanidine; A—anorthoclase. Ages in bold are weighted mean averages of inverse isochron ages obtained on all samples from that unit.

Initial Eruption of the Columbia Mankinen et al., 1987; Camp et al., 2003, 2013; ied and includes alkali basalt, trachybasalt, and River Basalts and Relationship to Brueseke et al., 2007; Hooper et al., 2007; trachybasaltic andesite (Johnson et al., 1998; Silicic Volcanism Jarboe et al., 2008, 2010; Fig. 1). The thick- Camp et al., 2003). In the most recent defini- est section (~1 km) of Steens Basalt occurs at tive work on the chemistry of the Steens Basalt, The Steens Basalt is the earliest member Steens Mountain (Fig. 1). There, the lower part Camp et al. (2013) subsumed lavas at the top of of the Columbia River Basalt Group, with a of the section is dominated by tholeiitic basalt, the type section, which were originally mapped total estimated volume of ~31,800 km3 (e.g., whereas the upper part is more chemically var- separately as “andesite” lavas by Johnson et al.

1030 Geological Society of America Bulletin, v. 129, no. 9/10 Geology and 40Ar/39Ar geochronology of the mid-Miocene McDermitt volcanic field

Tu of Oregon Canyon (TB-433) Tu of Trout Creek Mountains (TB-456) 0.004 0.004 40Ar/36Ar trapped 40Ar/36Ar trapped 293.8 ±± 2.8 Weighted Mean Age 294.5 ±± 5.1 Weighted Mean Age 16.458 ±± 0.014 Ma 16.402 ±± 0.013 Ma 0.003 0.003 Ar Ar 40 40

/ 0.002 / 0.002

Ar 16.0 16.2 16.4 16.616.8 Ar 16.0 16.2 16.4 16.6 16.8 36 Age (Ma) 36 Age (Ma)

0.001 Inverse Isochron Age 0.001 Inverse Isochron Age 16.468 ±± 0.015 Ma 16.415 ±± 0.020 Ma MSWD = 0.85 MSWD = 1.55 0.000 0.000 0.00 0.05 0.10 0.15 0.20 0.25 0.00 0.05 0.10 0.15 0.20 0.25 39Ar/40Ar 39Ar/40Ar

Tu of Long Ridge (TB-455) Tu of Whitehorse Creek (TB-322) 0.004 0.004 40 36 40Ar/36Ar trapped Ar/ Ar trapped 297.4 ±± 3.6 296.2 ±± 45.1

Weighted Mean Age Weighted Mean Age 0.003 16.321 ±± 0.020 Ma 0.003 15.557 ±± 0.029 Ma Ar Ar 40 40

/ 0.002 / 0.002 Ar Ar 16.0 16.216.416.616.8 15.2 15.415.615.8 36 36 Age (Ma) Age (Ma) 0.001 Inverse Isochron Age 0.001 Inverse Isochron Age 16.328 ±± 0.031 Ma 15.559 ±± 0.044 Ma MSWD = 1.01 MSWD = 0.24 0.000 0.000 0.00 0.05 0.10 0.15 0.20 0.25 0.00 0.05 0.10 0.15 0.20 0.25 39Ar/40Ar 39Ar/40Ar Figure 2. Inverse isochron plots of representative data from the four major McDermitt volcanic field ignimbrites, with insets showing weighted mean ages. Gray bands indicate 95% confidence intervals, and gray data points indicate grains not included in the age calcula- tion. We prefer inverse isochron ages since weighted mean ages assume trapped 40Ar/36Ar = 298.5 and result in significant scatter for many samples, as reflected in these representative insets. Full data appear in Appendices C and D (see text footnote 1). MSWD—mean square of weighted deviates.

(1998), within their basaltic trachyandesite similar in composition to the basalt, trachy- (Fig. 3). Previous workers classified these in- and basaltic andesite part of the upper Steens basalt, trachybasaltic andesite, and basaltic termediate rocks variously as andesite basalt (Fig. 3). Under this definition, all flows of up- andesite of upper and lower Steens Basalt at (Carlton, 1969), potassic icelandite (Wallace per and lower Steens Basalt are chemically con- the type locality at Steens Mountain (Johnson et al., 1980), iron-rich andesite and quartz strained to have concentrations of Ba <900 ppm et al., 1998), with Ba <900 ppm and Cr >1 ppm latite (­Rytuba et al., 1982a, 1982e; Rytuba and Cr >1 ppm. (Fig. 3). In both ranges, Steens Basalt ­lavas and Curtis,­ 1983; Rytuba et al., 1983b), ba- In the McDermitt volcanic field, Steens Ba- are capped by trachyandesite to trachyte saltic ande­site (Minor, 1986), andesite (Man­ salt units exposed in the Trout Creek and Ore­ ­lavas (unit Tt in Figs. 3 and 4) that are more kinen et al., 1987), and basaltic trachyandesite gon Canyon Mountains (unit Tbs in Fig. 3) are evolved than Steens Basalt, with Ba >900 ppm (Camp et al., 2013).

Geological Society of America Bulletin, v. 129, no. 9/10 1031 Benson et al.

ca. 16.47 Ma Tuff of Oregon Canyon, which 2500 McDermitt Volcanic Field conformably overlies them. Figure 3. Compositions of Steens Basalt Steens Basalt (Tbs) In contrast, in the Oregon Canyon Moun- (Tbs) and trachyte and trachyandesite lavas 2000 Intermediate lavas (Tt) tains (Aspen Spring section in Fig. 5), ~90 m of McDermitt volcanic field (Tt). Data are 1500 of Steens Basalt and ~80 m of trachyandesite compared to all members of Steens Basalt at and trachyte lavas lie above the Tuff of Ore­ Steens Mountain, including capping basaltic Ba (ppm) 1000 gon Canyon and are in turn capped by the andesite (Johnson et al., 1998; Camp et al., ca. 16.42 Ma Tuff of Trout Creek Mountains. 500 2013). Steens Basalt is characterized by Ba The vent for these intercalated mafic and inter- <900 ppm and Cr >1 ppm. Steens Basalt mediate lavas appears to be near Twelvemile 100 200 300 400 500 600 Summit, where they reach a maximum com- Cr (ppm) bined thickness of ~300 m. The trachyte and trachyandesite lavas flowed further north and In the Trout Creek Mountains, the section ~7 km south of the map border in Figure 4A. west than the underlying Steens Basalt, as of Steens Basalt lavas is as much as ~450 m Steens Basalt and the overlying intermedi- they occur directly on top of Tuff of Oregon thick (Minor, 1986). The overlying trachyan- ate lavas ceased erupting in the Trout Creek Canyon with no intercalated Steens Basalt in desite and trachyte lavas (Tt) form a 300-m- Mountains prior to eruption of the first ig- Whitehorse Canyon and Antelope Creek (Tt high stratocone centered east of Pole Canyon nimbrite of McDermitt volcanic field, the in Figs. 4 and 5). Flows of Steens Basalt that

Tioc Titc Tttr Tbc 5 km Qc Whitehorse Tt Tule Rims Qc Ranch Tiwc Tttr Trtr Qc Tloo 42°20′N Tloo * Ant Tbc Tioc Tilr * Tsb elope Ck dikes N. Red Mtn Titc *Red Tiwc * Tttr Tbc Titc Lookout Tiwc Tt Butte Tiwc F Tlwy ish Ck A′ Qc * Tbc Ts Tlto Tlwy* * Red Whitehorse Ck Fish Mtn Ts * Qc camp Buckskin Tlwh Tlwo Creek Titc * Whitehorse Qc *Mtn * Caldera Tlwy Caldera Qc Ts Tilr Ts * Ts Tbc * Whitehorse Tt Qc Tbc * * Willow Butte Pole Butte * Tiwc Flagsta Willow Ck Ts * Oregon Cn Mtns Canyon Butt* e x Caldera * Qc Topographic margin A (dotted where covered) Tlty Titc Tioc Tlwo Tt Tilr * Tilr Structural margin Qc * * 42°10′N Ts Titc Normal fault (dashed where inferred)

40 39 Trout Ck Pole Cn Trout Ck Mtns Ar/ Ar & Lava vent geochemistry * Tbs Tt Whitehorse Road Fig. 7 section location Geochemistry

118°20′W 118°10′W Figure 4 (on this and following page). Geologic map of northern McDermitt volcanic field, schematic cross section, and correlation of map units. Geologic map, field-checked and modified from Rytuba et al. (1982a–1982e); Rytuba et al. (1983a, 1983b); and Peterson and Tegtmeyer (1987). X marks the location of well MC-7 (see Rytuba et al., 1981), and “camp” marks the location of the campground at Willow Creek Hot Springs. Schematic cross section of the northern McDermitt volcanic field calderas. Correlation of map units. Ck—Creek, Cn—Canyon, Mtn—Moun- tain, Hbl—hornblende.

1032 Geological Society of America Bulletin, v. 129, no. 9/10 Geology and 40Ar/39Ar geochronology of the mid-Miocene McDermitt volcanic field occur between Tuff of Oregon Canyon and the the estimate for the duration of this earliest Refined Stratigraphy intermediate lavas are restricted to the eastern flood basalt volcanism to ca. 16.64–16.43 Ma Oregon Canyon Mountains and Blue Mountain in the vicinity of the study area (Mahood and Based upon new 40Ar/39Ar ages, the strati- region (Fig. 5). ­Benson, 2017). graphic relations of ignimbrites, and geochemi- The presence of feeder dikes and spatter cal correlations of outflow sheets, we refine the deposits in both the Trout Creek Mountains IGNIMBRITE STRATIGRAPHY OF THE ignimbrite stratigraphy of McDermitt volcanic and Oregon Canyon Mountains (Minor, 1986; MCDERMITT VOLCANIC FIELD field to four major ignimbrites. Listed in strati- Mankinen et al., 1987; Bondre and Hart, 2008; graphic order with the new preferred ages, these Camp et al., 2013; this study) is consistent Previous Stratigraphy are: (1) 16.468 ±± 0.006 Ma Tuff of Oregon with local sources for the Steens Basalt and Canyon, zoned from high-silica alkali rhyolite to intermediate lavas (in contrast to far-traveled, In Rytuba and McKee’s (1984) initial descrip- trachyte; (2) 16.415 ±± 0.007 Ma Tuff of Trout more voluminous flows observed farther north tion of the McDermitt volcanic field, they iden- Creek Mountains, zoned from high-silica alkali in the Grande Ronde Member of the Colum- tified seven regionally extensive tuffs sourced to rhyolite to low-silica alkali rhyolite; (3) 16.328 bia River Basalt Group; e.g., Swanson et al., seven different calderas. Based upon field data ±± 0.013 Ma Tuff of Long Ridge, zoned from 1975; Tolan et al., 1989; Reidel et al., 2013a). and K-Ar ages, they recognized the 16.1 Ma high-silica alkali rhyolite to low-silica alkali This resulted in the development of significant Tuff of Oregon Canyon, 15.8 Ma Tuff of Trout rhyolite; and (4) 15.556 ±± 0.014 Ma Tuff of paleotopog­ raphy­ that controlled the distribution Creek Mountains, 15.7 Ma Tuff of Double H, Whitehorse Creek, a low-silica alkali rhyolite and thickness of ignimbrites within McDermitt 15.6 Ma Tuff of Long Ridge members 2 and 3, (Fig. 6; Tables 1 and 3). The ages, compositions, volcanic­ field. 15.6 Ma Tuff of Long Ridge member 5, 15.5 Ma field characteristics, and distributions of these Estimates for the age of Steens Basalt are Tuff of Hoppin Peaks, and 15.0 Ma Tuff of ignimbrites are described in detail next. ca. 16.7–16.5 Ma, based on relatively low- Whitehorse Creek (Table 2). Castor and Henry precision analyses of plagioclase feldspar and (2000), Starkel (2014), and Henry et al. (2016) 16.47 Ma Tuff of Oregon Canyon

K2O-poor basaltic matrices (Mankinen et al., subsequently reinterpreted the tuff of Hoppin The first major ignimbrite known to erupt 1987; Camp et al., 2003; Brueseke et al., 2007; Peaks as a package of metaluminous rhyolitic from McDermitt volcanic field is the peralka- Jarboe et al., 2008, 2010; Barry et al., 2013). lavas and consolidated the Tuff of Double H and line Tuff of Oregon Canyon, which resulted in Recently, high-precision 40Ar/39Ar ages on Tuff of Long Ridge members 2, 3, and 5 into a collapse of an ~20 × 24 km caldera we name the feldspar separates from ignimbrites and other single tuff they called the McDermitt Tuff, for Fish Creek caldera (Fig. 4A). The tuff is typi- tuffs interbedded with Steens Basalt lavas, which they reported a preferred 40Ar/39Ar age of fied in the northern McDermitt volcanic field including those mentioned here, have refined 16.25 ±± 0.03 Ma (Table 2). by a complete section exposed in Trout Creek

A Whitehorse Caldera 5000 m A′ Titc 100 m Titc Flagsta Butte Tt 1600 Whitehorse Butte Tiwc shTlto ck rhy Tsw Qc Tiwc Qc Qc Tlwy Tbc Tbc Tlwy Tlwo 1400 Tlwy FCCTs sed?f SteensTbs OCT PCC sed PCC sed PCCTs sedpT FCCsf sed? Steens Tsp Tsp TsWHCwT cls sed Tsw sw PCTsCp sed Tbs 1200 SteensTbs OCTiocT ic?? TCTiTtc ic TCTiTtc ic TiwcWHT ic Tiwc Tiwc TCTitcT ic?TTCitc?T ic OCTiocT ic?? elev 3x vertical exaggeration (m) Pole Canyon Caldera Fish Creek Caldera SEDIMENTARY ROCKS WHITEHORSE CALDERA POLE CANYON CALDERA FISH CREEK CALDERA OTHER VOLCANIC ROCKS

Qc Quaternary cover Hbl-bearing Ts Postcaldera rhyolite lava basalts TlwhTTlwhw CORRELATION OF MAP UNITS Tule Rims Whitehorse Tsw Postcaldera Caldera Basinal Tlwy Tu of trachybasaltic rhyolite lava andesite sediments Tiwc Whitehorse Pole Canyon Tule Rims Tbc Tttr Pole Tlwo Creek Volcanics rhyolite lava TsT b Tsp Canyon Precaldera Tlty Trtr Caldera Tu of Trout Tu of rhyolite lava Titc Tilr Caldera lake sediments Fish Creek Creek Mtns Oregon Tsf Tu of Caldera Tlto Canyon Long Ridge Tt Fish Creek Tioc rhyolite Tbs Tloo Intermediate lava Precaldera lavas rhyolite lava Steens Basalt Figure 4 (continued).

Geological Society of America Bulletin, v. 129, no. 9/10 1033 Benson et al.

119° W 118° W

Mickey Hot Springs 30 Aspen Trout Creek Steens km 16.40 Spring Mtn 16.34 Titc 16.43

Titc Tt 16.40 x 0.5 Big Sand Gap 16.46 Blue Tioc 16.47 Tbs Mtn x 0.5 16.45 Tbs Tioc 16.46 Pueblo Mtns Oregon Tbs 16.35 Tilr Cyn Mtns 16.42 Titc Trout Ck Tioc 16.47 Mtns Whitehorse MBT 16.49 Pueblo Canyon 42° N Mtns 42° N Tbs Long Tilr 16.33 Ridge Titc 16.42

Tt Idaho Cyn x 0.5 Santa Rosa- TBM 16.18 Bilk Ck Tioc Tilr Mtns Calico Tloo 16.52 ICT 16.38

Double H Tbs Mtns Windy Pt

119° W 118° W Figure 5. Map showing the calderas of McDermitt volcanic field, extent of associated outflow ignimbrites, and stratigraphic sections with dated ignimbrites. Topographic margins of the calderas are shown in thick black lines. Extents of ignimbrites are shown by thin lines: ­orange—Tuff of Oregon Canyon (Tioc), green—Tuff of Trout Creek Mountains (Titc), ­purple—Tuff of Long Ridge (Tilr), light blue—Tuff of Whitehorse Creek (Tiwc). These extents are constrained by outcrops with chemical analyses (outlined circles) and without chemical analy- ses (no outline). Tuff of Long Ridge indicated in the Santa Rosa–Calico center is a correlation with unit Tp1 of Brueseke and Hart (2008); all other chemical data are from this study. Insets: Sections of outflow ignimbrite sheets with dated samples: ignimbrites of McDermitt volcanic field are shown in same colors as above, yellow— other ignimbrites (MBT—Tuff of Monument Basin, ICT—Idaho Canyon Tuff, TBM—Tuff of Big Mountain), dark gray—Steens Basalt (Tbs), light gray—trachyte and trachyandesite lavas (Tt). Ck—Creek, Cyn—Canyon, Mtn—Mountain, Pt—Point. In some sections, Tbs and Tt are shrunk by 50%. Italicized dates are from Coble and Mahood (2016); all other dates are from this study (see Table 1).

(Fig. 4A). It contains three main parts: (1) an [see footnote 1]), quartz (2%), and minor sodic slightly increasing over the same interval (Rb: initial black, lithic-rich, densely welded base amphibole, clinopyroxene, fayalite, Fe-Ti ox- 185–197 ppm; Zr: 653–714 ppm). The mid-blue overlain by (2) the main eruptive phase of the ides, and apatite. In this section, the ignimbrite to late maroon portion of the ignimbrite is nor- ignimbrite, which is a light-blue ignimbrite with is zoned from a high-silica alkali rhyolite to tra- mally zoned (0.20–0.27 wt% TiO2; 2.50–2.88 microphenocrysts of sodic amphibole, grading chyte (Appendix F [see footnote 1]). The early wt% FeO*; 194–180 ppm Rb; 610–645 ppm Zr; to (3) densely welded, maroon-colored ignim- portion of the ignimbrite (black to the middle of Fig. 7; Appendix G [see footnote 1]). brite rich in fiamme and in lithics of basalt to the blue portion of the tuff) is slightly reversely We correlated outcrops of outflow ignimbrite rhyolite lava, all of which are as large as 10 cm chemically zoned (Fig. 7), with compatible­ as Tuff of Oregon Canyon based on observed in maximum dimension. All three parts of the ig- element TiO2 (wt%) decreasing from 0.36 to phenocryst assemblages and the abundances nimbrite are weakly porphyritic, with ~10 vol% 0.15 and FeO* (wt%) decreasing from 2.88 of trace and minor elements measured on ~45 phenocrysts of sodic sanidine (8%; Appendix E to 2.52 up section, and incompatible elements samples throughout the region (Figs. 5 and 8;

1034 Geological Society of America Bulletin, v. 129, no. 9/10 Geology and 40Ar/39Ar geochronology of the mid-Miocene McDermitt volcanic field

TABLE 2. CORRESPONDENCE OF UNITS WITH OTHER STUDIES OF MCDERMITT VOLCANIC FIELD Rytuba and McKee (1984) Henry et al. (2016) This study Caldera-forming unit Caldera Caldera-forming unit or lava CalderaCaldera-forming unitCaldera Tuff of Whitehorse CreekWhitehorse Tuff of Whitehorse CreekWhitehorse Tuff of Long Ridge Member 5 Long Ridge McDermitt Tuff McDermittTuff of Long Ridge McDermitt Tuff of Long Ridge Members 2 and 3 Jordan Meadow Tuff of Double HCalavera Tuff of Hoppin Peaks Hoppin Peaks Biotite rhyolite lava of Hoppin Peaksn/a Tuff of Trout Creek Mountains Pueblo Tuff of Trout Creek MountainsPole Canyon Tuff of Oregon Canyon WashburnTuff of Oregon Canyon Fish Creek

Trout Creek Appendix G [see footnote 1]). The early black, 1 1112 lithic-rich portion of the ignimbrite is present MVF ignimbrite ages (others) only in Trout Creek and locally in Tule Rims. The main phase is the most widespread, with MVF ignimbrite samples Long Whitehorse Ridge Oregon outcrops extending west of the Pueblo Moun- Creek (this study) Canyon tains and east toward Blue Mountain (Fig. 5). Outcrops of the late maroon, lithic- and pumice-rich­ portion of the tuff occur through- out northern McDermitt volcanic field and Northern MVF rhyolite extend north of Mickey Hot Springs (Fig. 5), lava samples (this study) where we, like Sherrod et al. (1989), correlated the Mickey ignimbrite of Hook (1981) as Tuff of Oregon Canyon. We mapped the unit in Tule Rims mapped as 15.3 15.4 15.5 15.6 15.7 15.8 15.9 16.0 16.1 16.2 16.3 16.4 16.5 16.6 16.7 “rhyolite and breccia” by Rytuba et al. (1982b) Age (Ma) as the upper portion of Tuff of Oregon Canyon Figure 6. Age systematics within northern McDermitt volcanic field (MVF), with vertical based on its stratigraphic position below the bars indicating preferred ages and 2σ errors for the four McDermitt volcanic field ignim- Tuff of Trout Creek Mountains, a matching brites. The top panel shows agreement of our preferred 40Ar/39Ar ages for McDermitt vol­ phenocryst assemblage, and compositions that canic field ignimbrites compared to previous studies (1—Henry et al., 2016; 2—Jarboe et al., fall along the mafic end of the trend for Tuff of 2008). Ages on individual samples obtained in this study appear in the middle (ignimbrite) Oregon Canyon (Fig. 8). and bottom (lava) panels. A unit mapped as a lahar by Rytuba et al. (1982e, their unit Tl), which is locally preserved beneath the Tuff of Trout Creek Mountains im- TABLE 3. SUMMARY OF ERUPTIVE UNITS IN THE MCDERMITT VOLCANIC FIELD mediately south of the Fish Creek caldera in the Age Volume Unit nameMap unit (Ma) (km3) northern Trout Creek Mountains, we interpreted Whitehorse caldera (13 × 12 km) as a proximal fall deposit related to the Tuff of Postcaldera lavasTlwy, Tlwh ca. 15.56–15.37 10 Oregon Canyon. This poorly sorted and weakly Tuff of Whitehorse Creek Tiwc 15.556 ± 0.014110 stratified deposit is incipiently to moderately Precaldera lavasTlwo ca. 15.59–15.56 12 welded. It consists of subangular pumice la- McDermitt caldera (30 × 40 km) Caldera-related rhyolite lavas* n/a ca. 16.5–16.2 80 pilli that are chemically similar to the trachytic Tuff of Long Ridge Tilr 16.328 ± 0.0131080 end of the Tuff of Oregon Canyon (Fig. 9A; Pole Canyon caldera (20 × 26 km) TB‑601B in Appendix G [see footnote 1]), and it Pole Canyon volcanics Tlto ca. 16.41 contains lithics that range in size from <1 cm to Tuff of Trout Creek Mountains Titc 16.415 ± 0.007660 >20 cm, one of which is chemically and petro- Fish Creek rhyolite lava Tlty ca. 16.45 3 graphically similar to the early-erupted portion Fish Creek caldera (20 × 24 km) Tuff of Oregon Canyon Tioc 16.468 ± 0.006510 of the Tuff of Oregon Canyon (TB-601L1). The Precaldera lavas Tloo ca. 16.52–16.47 6 poor sorting, abundant lithics, and rapid vertical Mafic and intermediate lavas variations in degree of welding within this unit Trachybasaltic andesite of Tule Rims Tta ca. 15.6 Ma 10 (e.g., Wright, 1980; Mahood, 1984) suggest that Intermediate lavas of Oregon Canyon MountainsTt ca. 16.43 Ma 200 these outcrops are high-temperature, near-vent Intermediate lavas of Trout Creek MountainsTt ca. 16.48 Ma 200 Steens Basalt† Tbs ca. 16.64–16.43 Ma 31,800 fall deposits equivalent to the maroon-colored *Age data for pre- and postcaldera lavas from Henry et al. (2016). ignimbrite west of the caldera. †Volume data from Camp et al. (2013); age data from this study, Mahood and Benson (2017). We obtained eight new 40Ar/39Ar inverse iso- chron ages on outflow of Tuff of Oregon Can- yon from throughout the McDermitt volcanic ±± 0.015 Ma), the devitrified, blue main erup- than the ages of 16.401 ±± 0.020 Ma and 16.404 field (Fig. 6; Table 1). At our type section in tive portion (TB-433; 16.468 ±± 0.015 Ma) ±± 0.015 Ma obtained at the same locality on Trout Creek (starting coordinates: 45.15851°N, and the late maroon portion (TB-435; 16.457 the stratigraphically higher Tuff of Trout Creek 118.45157°W), ages on the early black vit- ±± 0.017 Ma) agree within analytical uncer- Mountains (Table 1). West of the Pueblo Moun- rophyric portion of the tuff (TB-429; 16.450 tainty (Table 1; Figs. 6 and 7), and they are older tains, outflow of the Tuff of Oregon Canyon is

Geological Society of America Bulletin, v. 129, no. 9/10 1035 Benson et al.

distinguished by 25 vol% phenocrysts of ~4 mm 100 16.404 ±± 0.015 Ma blocky sodic sanidine (Appendix E [see foot- note 1]) and 5% smoky quartz, along with trace 90 Tu of Trout clinopyroxene, amphibole, and aenigmatite in a Creek Mtns teal-green matrix with abundant (up to 5 vol%) 70 16.401 ±± 0.020 Ma lithic fragments of mafic to intermediate lava. All analyses of both members of the Tuff of

50 Trout Creek Mountains, from ~80 samples col- lected throughout the region (Fig. 5), fall along Tu of a compositional trend similar to that defined by

height in section (m) 30 Oregon Cyn the Tuff of Oregon Canyon (but chemically dis- 16.457 ±± 0.017 Ma tinct from the younger Tuffs of Long Ridge and 10 16.468 ±± 0.015 Ma Whitehorse Creek; Fig. 8A). Detailed sampling 16.450 ±± 0.015 Ma in Trout Creek demonstrates that Members A and B constitute a normally zoned section with 0.3 0.5 130 170 500 700 25 35 300600 45 75 no significant compositional change at the con- TiO (wt %) Rb (ppm) Zr (ppm) Nb (ppm) Ba (ppm) La (ppm) 2 tact between the two members (Fig. 7). Com- patible elements increase up section (0.29–0.37 Figure 7. Illustration of compositional variation of the Tuff of Oregon Canyon and Tuff wt% TiO ; 117–208 ppm Ba), and incompat- of Trout Creek Mountains along a detailed traverse in Trout Creek (starting coordinates: 2 ible elements decrease (e.g., 189–99 ppm Rb; 42.15851°N, 118.45157°W). Three samples of the Tuff of Oregon Canyon and two samples 872–339 ppm Zr; Fig. 7; Appendix G [see of Tuff of Trout Creek Mountain at this locality were dated via 40Ar/39Ar geochronology, footnote 1]). with each ignimbrite yielding ages that agree within analytical uncertainty. Cyn—Canyon, We identified outcrops of Members A and Mtns—Mountains. B using compositional data and phenocryst as- semblages. Member A flowed dominantly to the west of the caldera, as most outcrops occur exposed below the Tuffs of Long Ridge and (2010) analysis. In this study, we found that it was in the Trout Creek Mountains, Tule Rims, and Trout Creek Mountains and above a nonwelded, necessary to analyze at least 20 single crystals to the Pueblo Mountains (Figs. 4A and 5). East of vapor-phase–altered, metaluminous, biotite- obtain a clear indication of sample homogeneity the caldera, Member A is absent, but the more- bearing ignimbrite we informally named Tuff of and consistency with stratigraphic relationships. evolved Member A composition is represented Monument Basin, which we suggest originated by an ~30 cm fall deposit (TB-359 in Appendix from the nearby Hawks Valley–Lone Mountain 16.42 Ma Tuff of Trout Creek Mountains G [see footnote 1]) that is preserved under ~1 m center based on its mineralogic and composi- The Tuff of Trout Creek Mountains erupted of fall and surge deposits underneath Member tional similarity to lavas analyzed by Wypych during collapse of the newly defined ~20 × B (TB-360) in Whitehorse Creek. Along the et al. (2011). At this locality, we obtained ages on 24 km Pole Canyon caldera. Tuff of Trout Creek southeastern wall of the Fish Creek caldera, we Tuff of Oregon Canyon of 16.467 ±± 0.033 Ma Mountains conformably overlies Tuff of Oregon correlate a thin (~1 m) nonwelded ignimbrite (TB-224) and 16.484 ±± 0.022 Ma (MC407B), Canyon in the Pueblo and Trout Creek Moun- containing reversely graded lithic fragments of which are analytically indistinguishable, and tains, whereas in the Oregon Canyon Mountains, mafic lava as much as 4 cm in diameter to Mem- which are younger, within analytical uncertainty, a thick stack of upper Steens and intermediate ber A. Overlying this ignimbrite, there are fine- than the underlying 16.479 ±± 0.026 Ma Tuff of lavas erupted between the two ignimbrites. The grained fall deposits and the basal vitrophyre Monument Basin (Mahood and Benson, 2017) outflow ignimbrite is thickest (~100 m) where it of Member B. Member B is more widespread, and older than the ages for the Tuff of Trout banked in against the southern and eastern mar- as it is preserved in all directions: from west of Creek Mountains and overlying Tuff of Long gins of the older Fish Creek caldera. the Pueblo Mountains, to east of Blue Mountain Ridge also sampled in that section (Table 1). The Tuff of Trout Creek Mountains is com- (Fig. 5), and from the McDermitt Caldera in the Additional ages were obtained on outflow of the posed of two members, both of which are pres- south to Big Sand Gap in the north (Fig. 5). Tuff of Oregon Canyon at Catlow Peak (16.494 ent in Trout Creek above exposures of Tuff of We obtained seven new 40Ar/39Ar ages from ±± 0.017 Ma; ML-304), Aspen Spring (16.461 Oregon Canyon. First-erupted Member A is different samples of the Tuff of Trout Creek ±± 0.015 Ma; TB-444), and west of Whitehorse peralkaline high-silica rhyolite (A.I. [agpaitic Mountains. At the section in Trout Creek, Mem- Ranch (16.497 ±± 0.036 Ma; TB-264). index] = 1.24; Appendix F [see footnote 1]) bers A and B yielded indistinguishable ages of Together, these eight samples of Tuff of Ore­ characterized by ~15 vol% phenocrysts of so- 16.401 ±± 0.020 Ma (TB-439 in Table 1) and gon Canyon give a weighted mean age of 16.468 dic sanidine (Appendix E [see footnote 1]), 16.404 ±± 0.015 Ma (TB-443), respectively. ±± 0.006 Ma (0.014 Ma model error). This pre- ~3% quartz, ~2% sodic amphibole, and trace In the dip-slope section on the west side of the ferred age is in excellent agreement with an age amounts clinopyroxene and Fe-Ti oxide in a Pueblo Mountains (Fig. 5), the Tuff of Trout of 16.46 ±± 0.02 Ma (mean of four samples) ob- dark-green matrix. Xenotime occurs in cavities Creek Mountains yielded an age of 16.423 tained by Henry et al. (2016), but it differs slightly of the tuff as a vapor-phase mineral. After a short ±± 0.021 Ma (TB-225A), which is younger from an age of 16.548 ±± 0.050 Ma reported by hiatus—sufficient to form a crystal-rich black than the ages obtained on the underlying Tuffs Jarboe et al. (2010) based on one sample of out- to blue-black basal vitrophyre—the relatively of Monument Basin and Oregon Canyon, and flow ignimbrite collected at Catlow Peak. This more voluminous and more strongly porphyritic older than the age obtained for the overlying slight disagreement may simply be a function of Member B erupted. It is a low-silica alkali rhyo- Tuff of Long Ridge in that locality. An age of the small sample size (8 grains) of the Jarboe et al. lite (A.I. = 1.04; Appendix F [see footnote 1]) 16.412 ±± 0.017 Ma (TB-265B) obtained on

1036 Geological Society of America Bulletin, v. 129, no. 9/10 Geology and 40Ar/39Ar geochronology of the mid-Miocene McDermitt volcanic field

an outcrop of Tuff of Trout Creek Mountains at A North Red Mountain is consistent with its strati- 900 Ignimbrites graphic position above the 16.520 ± 0.016 Ma North Red Mountain rhyolite lava (Table 1). Whitehorse Ck In the Oregon Canyon Mountains, three ages 700 Long Ridge were obtained on samples of outflow strati- Trout Ck Mtns graphically below the ca. 16.33 Ma Tuff of Oregon Cyn 500 Long Ridge: 16.431 ±± 0.015 Ma (TB-451 in

Zr (ppm) Table 1), 16.415 ±± 0.020 Ma (TB-456), and 16.420 ±± 0.030 Ma (EW-218). The seven in- 300 verse isochron ages on samples from seven lo- calities yielded a weighted mean age of 16.415 ±± 0.007 Ma (0.015 Ma model error; Table 1). This preferred age is in agreement with a 45 weighted mean age of 16.39 ±± 0.03 Ma ob- tained by Henry et al. (2016) on four samples 40 of the ignimbrite. 35 16.33 Ma Tuff of Long Ridge 30 Throughout the study region, the Tuff of

Nb (ppm) Long Ridge overlies Tuff of Trout Creek Moun- 25 tains. The Tuff of Long Ridge is widely distrib- 20 uted because it is the most voluminous of the ignimbrites of the McDermitt volcanic field, and because it flowed across a landscape previously 0.25 0.50 0.75 1.00 100 150 200 250 300 paved by two older ignimbrites. TiO (wt %) Rb (ppm) B 2 In their study of the McDermitt volcanic Lavas field, Rytuba and McKee (1984) and Conrad 900 Tu of (1984) characterized ignimbrites that they pro- hb-bearing Trout posed were sourced from a topographically post-WHT Creek prominent, composite collapse structure (in- 700 pre-WHT cluding their Calavera, Long Ridge, and Jordan Tule Rims Mtns Meadow calderas; Fig. 10) 45 × 30 km centered 500 pre-TCT ~20 km west of the town of McDermitt, Nevada. Zr (ppm) pre-OCT These ignimbrites include the Tuff of Double H, 300 which is exposed largely south of the caldera, and the composite Tuff of Long Ridge, which occurs throughout the volcanic field. They di- vided the Tuff of Long Ridge into several mem- Tu of bers. Phenocryst­ abundances are highly variable 45 Whitehorse Ck within and between members, from aphyric to 40 moderately porphyritic (3–15 vol% alkali feld- spar, minor clinopyroxene and fayalite, and trace 35 ilmenite, sodic amphibole, and quartz). Compo- 30 Tu of sitions of all members of the Tuff of Long Ridge Nb (ppm) Oregon Cyn Tu of fall along a single compositional trend from 25 high-silica alkali rhyolite to trachyte (Fig. 8A), Long 20 leading Rytuba and McKee (1984) and Conrad Ridge (1984) to suggest that they erupted from a single compositionally zoned magma chamber. 0.25 0.50 0.75 1.00 100 150 200 250 300 In a reinterpretation of the stratigraphy of the

TiO2 (wt %) Rb (ppm) southern McDermitt volcanic field,­Castor and Henry (2000) and Henry et al. (2016) combined Figure 8. Compositions of rhyolite ignimbrites and lavas of McDermitt volcanic field. the tuffs of Double H Mountains and Members

(A) Plots of TiO2 and Rb vs. Zr and Nb showing samples of McDermitt volcanic field ignim- 2, 3, and 5 of the Tuff of Long Ridge of ­Rytuba

brites analyzed by energy-dispersive X‑ray fluorescence. (B) Plots of TiO2 and Rb vs. Zr and and McKee (1984) into a single unit they termed Nb showing trends of McDermitt volcanic field ignimbrites (best-fit polynomial lines using the McDermitt Tuff (Table 2). They reported data in A) and lavas of northern McDermitt volcanic field: hb—hornblende; TCT—Tuff of this tuff as being zoned from aphyric, per­alka­ Trout Creek Mountain; WHT—Tuff of Whitehorse Creek; OCT—Tuff of Oregon Canyon. line high-silica rhyolite to non-peralkaline, Ck—Creek, Cyn—Canyon, Mtns—Mountains. high-silica trachydacite with abundant pheno-

Geological Society of America Bulletin, v. 129, no. 9/10 1037 Benson et al.

A B

densely welded upper nonwelded to densely welded member

crystal clot lithic nonwelded lower nonwelded member

C Tu of Trout Creek Mountains ponds thickly against older Fish Creek Caldera wall and ows over Fish Creek rhyolite lava Tu of Long Ridge

Tu of Trout Ck Mtns Trachyte lava

Whitehorse Canyon rhyolite lava Tu of Trout Ck Mtns

D

Flagsta Butte

Pole Cyn volcanics Tu of Oregon Cyn Flagsta Butte Steens Basalt rhyolite lava

Whitehorse Caldera lake sediments Pole Canyon Caldera lake sediments

Quaternary alluvium

Figure 9. Field photographs from northern McDermitt volcanic field. (A) Densely welded fall deposit of Tuff of Oregon Canyon along the north end of the Trout Creek Mountains. (B) Welding variations in the Tuff of Whitehorse Creek along Willow Creek. (C) Picture look- ing north from the southern rim of Whitehorse Canyon along the eastern topographic margin of the Fish Creek caldera, showing how Tuff of Trout Creek Mountains ponded at the base of the topographic wall and flowed over a wall developed in a precaldera lava (Tloo). (D) Picture looking east at the sediment unconformity southwest of Flagstaff Butte. Flat-lying Whitehorse caldera lake sediments were deposited atop slightly northeast-dipping sediments of the older Pole Canyon caldera. Red lines indicate normal faults. Cyn—Canyon.

1038 Geological Society of America Bulletin, v. 129, no. 9/10 Geology and 40Ar/39Ar geochronology of the mid-Miocene McDermitt volcanic field

Alvord Tule °W Blue 10 km

Desert Rims Mountain

-118.5°W -118.0 Fish Creek Pueblo WHC Caldera Caldera Oregon Canyon Washburn Mountains Caldera Pole Canyon Figure 10. Map comparing loca- Caldera tions of calderas of the McDer- mitt volcanic field as originally Pueblo Trout Creek Mountains proposed by Rytuba and McKee Mountains OREGON NEVADA (1984; thin lines with italicized 42°N names) with updated and cor- Hoppin rected locations based on this McDermitt Peaks study and Henry et al. (2016; Caldera Long Ridge Caldera thick lines). WHC—Whitehorse Caldera caldera.

Pine Forest Santa Range Bilk Creek Calavera Caldera Rosa Mountains Mountains

crysts of anorthoclase and minor amounts of of black obsidian and devitrified rhyolite lava cooling units, and all samples of outflow ignim- clinopyroxene and fayalite. They characterized <0.5 cm in diameter. This is overlain by the main brite fall along the same compositional trend the caldera near McDermitt as resulting from a flow unit: thick, densely welded, light- to dark- (Fig. 8A). single collapse event that formed a single cal- purple, rheomorphic ignimbrite transitioning In this paper, we use the term Tuff of Long dera they called the McDermitt caldera, rather into a vapor-phase–altered, moderately welded, Ridge to refer to the composite section of ig- than being a nested, composite feature formed high-silica alkali rhyolite ignimbrite character- nimbrite sourced from the southern McDermitt by several eruptions of closely related magma. ized by a light-blue matrix, ~3 vol% pheno- volcanic field. We prefer the name “Tuff of Our samples from ~40 outcrops of ignim- crysts of 0.5–1 mm euhedral alkali feldspar, Long Ridge” to “McDermitt Tuff” (Henry et al., brite, most of them north of the McDermitt and lithics of 1–3 cm intermediate and rhyo­ 2016) because it has precedence, and because caldera, fall along the compositional trend lite lava (EW-213 in Appendix F [see footnote the geographic feature of Long Ridge is made (Fig. 8A) defined by the Tuff of Double H and 1]). The main flow unit, which we correlate to up of the ignimbrite, whereas it is not present Members 2, 3, and 5 of the Tuff of Long Ridge Member 5 of the Tuff of Long Ridge of Rytuba near the town of McDermitt. Moreover, four of Rytuba and McKee (1984). Such outcrops in and McKee (1984) and ­Conrad (1984), is wide- major ignimbrites erupted at McDermitt vol- the Oregon Canyon and Trout Creek Mountains spread throughout the Trout Creek and Oregon canic field, so calling this unit McDermitt Tuff commonly contain several flow units that range Canyon Mountains, whereas the under­lying may generate unnecessary confusion. We retain in phenocryst content from aphyric to ~15 vol% flow units, which we interpret as corresponding the name McDermitt caldera from Castor and feldspar, with minor mafic phases and no quartz. to Members 2 and 3, are present only locally. Henry (2000) because this is how the caldera A section of tuff detailed ~4 km north of Blue We interpret the distribution and thicknesses of has been identified informally in most published Mountain (Fig. 5) consists of a thin vitrophyre these flow units as indicating that the main erup- literature. with <10 vol% phenocrysts of predominantly tion from the southern McDermitt volcanic field Our regional reconnaissance work expands 2–3 mm alkali feldspar overlain by densely was Tuff of Long Ridge Member 5, and the pre- the distribution of the Tuff of Long Ridge to the welded, dark-purple low-silica alkali rhyolite ceding tuffs of Double H Mountains and Long west and south of areas identified in previous with 5–7 vol% phenocrysts of 1–2 mm feldspar Ridge Members 2 and 3 were relatively minor studies (Fig. 5). To the west, it occurs locally at and scarce 2–7 mm lithic fragments (EW-210; eruptions that were locally preserved or, in the Oregon End Table and widely west of the south- Appendix F [see footnote 1]). This is overlain by case of Tuff of Double H Mountains, flowed ern Pueblo Mountains, where partially welded a perlitic vitrophyre with 5 vol% phenocrysts of mostly south of the caldera. The eruption of Tuff of Long Ridge lies conformably above the 1–2 mm feldspar, which locally contains lithic multiple cooling units, as reflected in the pres- Tuffs of Monument Basin, Oregon Canyon, and fragments 3 mm to 10 cm in diameter. Above ence of multiple vitrophyres, probably occurred Trout Creek Mountains (Fig. 5; TB-226 in Ap- this vitrophyre, there is an orange to brick-red in rapid succession (within tens of years), given pendix G [see footnote 1]). At Idaho Canyon to nonwelded flow unit containing sparse lithics that there is no evidence for erosion between the the southwest, the Tuff of Long Ridge occurs

Geological Society of America Bulletin, v. 129, no. 9/10 1039 Benson et al. as ~5-m-thick nonwelded, vitric ignimbrite commonly densely welded with fiamme up to ±± 0.044 Ma on a nonwelded sample collected (TB-232) sandwiched between two ignimbrites 4 cm in diameter. Lithic fragments of rhyolite at the base of the upper member at the type sec- of the High Rock caldera complex (Coble and lava and welded tuff from 0.1 through 2 cm in tion along Willow Creek (TB-322 in Table 1); Mahood, 2016), the 16.38 Ma Idaho Canyon diameter occur throughout this member but are 15.578 ±± 0.020 Ma on a densely welded sample Tuff and 16.23 Ma Tuff of Big Mountain. To locally concentrated in layers no greater than of the uppermost portion of the upper member the south, at the southern end of the Pine Forest 1 m thick. In exposures along Willow Creek, west of Buckskin Mountain (TB-555), where it Range, densely welded Tuff of Long Ridge is the two members are separated by a 2-m-thick overlies fluvial to marginal lacustrine sediments exposed north of Windy Point (Fig. 5; TB‑134A, section of thin (<1–20 cm) fall, flow, and surge we associate with the Pole Canyon caldera; and TB-134B). deposits (Fig. 9B). 15.537 ±± 0.032 Ma (TB-522C) and 15.532 We obtained four new 40Ar/39Ar ages on The relatively small volume of the Tuff of ±± 0.026 Ma (TB-566) on two moderately to outflow samples of Tuff of Long Ridge that Whitehorse Creek resulted in the preserved out- densely welded samples from Tule Rims west agree within analytical uncertainty: 16.353 flow sheet being limited to exposures in the im- of Red Lookout Butte, where the tuff overlies ±± 0.031 Ma on partially welded ignim- mediate vicinity of the caldera (Figs. 4A and 5). trachybasaltic andesite lavas of Tule Rims. The brite on the western dip slope of the southern The tuff is thickest where it banks in against the weighted mean age for these four samples of Pueblo Mountains (TB-226 in Table 1), where south and southeast wall of the older encircling 15.556 ±± 0.014 Ma (0.022 Ma model error; it is analytically distinguishable from a 16.423 Pole Canyon caldera, reaching a maximum of Table 1) is in agreement with an age of 15.57 ±± 0.021 Ma age on the underlying Tuff of 100 m thick. In the north, where the relief on the ±± 0.05 Ma reported by Henry et al. (2016). Trout Creek Mountains; 16.343 ±± 0.026 Ma on older Pole Canyon caldera wall was not great, rheomorphic ignimbrite atop a section of Tuff and the ignimbrite flowed over the relatively DELINEATION OF CALDERAS IN of Oregon Canyon, Steens Basalt, intermedi- flat topography created by older McDermitt THE NORTHERN PART OF THE ate lava, and Tuff of Trout Creek Mountains at volcanic field ignimbrites and, locally, caldera MCDERMITT VOLCANIC FIELD ­Aspen Spring (TB-452); 16.328 ±± 0.031 Ma lake sediments of the Pole Canyon caldera, the on rheomorphic ignimbrite exposed atop Tuff of ­upper member of the ignimbrite is found as In this section, we discuss the evidence for the Trout Creek Mountain in the Whitehorse Can- much as 15 km from the Whitehorse caldera; location of the topographic walls and structural yon section (TB-455); and 16.305 ±± 0.022 Ma the lower lithic-rich member did not flow as far margins of three calderas in the northern part of at the base of Tuff of Long Ridge exposed in to the north. the McDermitt volcanic field2 that formed dur- a section north of Blue Mountain (EW-210; The minimum thickness for intracaldera ig- ing eruption of three ignimbrites. The White- Table 1; Fig. 5). These four new inverse iso- nimbrite is given by the ~50 m of Tuff of White- horse caldera was delineated and named by chron ages yield a weighted mean age of 16.328 horse Creek intersected in a drill hole (core from Rytuba et al. (1981) and Rytuba and McKee ±± 0.013 Ma (0.022 Ma model error) for Tuff of well MC-7 provided by James Rytuba) located (1984) and related to eruption of the Tuff of Long Ridge. This age is slightly older than the between the structural and topographic margins Whitehorse Creek, but the two older calderas preferred age of 16.25 ±± 0.03 Ma (weighted of the Whitehorse caldera (location indicated that we identify as the sources of the Tuffs of mean of five samples) reported by Henry by X in Fig. 4A). It exhibits the same cooling Oregon Canyon (Fish Creek caldera) and Trout et al. (2016). zonation observed in the outflow sheets. Pre- Creek Mountains (Pole Canyon caldera) have caldera rhyolite lava at the base of the hole is not been previously recognized. 15.56 Ma Tuff of Whitehorse Creek overlain by ~10 m of nonwelded ignimbrite that Our general guidelines for locating the struc- The Tuff of Whitehorse Creek, initially de- contains lithics of flow-foliated rhyolite lava, tural margins of calderas are: (1) caldera ring scribed in detail by Rytuba et al. (1981) and perlitic rhyolite­ lava, mafic lava, and densely fractures can serve as conduits for postcaldera Barrow (1983), is associated with collapse of welded tuff up to 15 cm in diameter with oxi- lavas; hence, the vents for postcaldera lavas the ~10 km Whitehorse caldera (Rytuba and dized reaction rims. This in turn is overlain by may lie along ring fractures and the structural McKee, 1984), which is entirely nested within ~40 m of relatively lithic-poor ignimbrite that margin; and (2) precaldera lavas and tuffs lie the older Pole Canyon caldera (Fig. 4A). The grades from nonwelded gray tuff into dark-gray outboard of the structural margin but will gen- Tuff of Whitehorse Creek is composed of two nonwelded tuff, and, finally, an uppermost light- erally be exposed between the structural and main cooling units, both of which contain 1–3 colored nonwelded tuff. An ~140 m section of topographic margin only where caldera fill has vol% phenocrysts of sodic sanidine (Appendix caldera fill, mostly fine-grained caldera lake been removed by erosion. Due to the relatively E [see footnote 1]) and trace quartz. The lower sediments, lies above the ignimbrite. young age and lack of erosion in the calderas, member is white to gray, nonwelded, low-silica Despite significant vertical variations in thick sections of intracaldera ignimbrite are alkali rhyolite with as much as 15 vol% lithic physical appearance, the tuff is not significantly not exposed. fragments of mafic lava, flow-foliated rhyolite chemically zoned; only Nb and Cu show a sig- We drew the topographic margins of the cal- lava, and densely welded ignimbrite. The more nificant range among samples (Fig. 8A; Ap- deras along topographic highs or topographic voluminous and widespread upper member is pendix G [see footnote 1]). Pumice lapilli from breaks. These commonly truncated precaldera also low-silica alkali rhyolite in composition the gray (TB-553P in Appendix G [see footnote lavas in an arcuate manner. The topographic (Appendix F [see footnote 1]). It grades upward 1]) and black (TB-554P) portions of the tuff are walls served to contain caldera lake sediments; from a nonwelded gray base with gray pumice chemically indistinguishable from each other hence, the distribution of caldera lake sediments lapilli up to 5 cm in diameter through dark-gray and whole-rock samples of the tuff (Fig. 8A). could be used to delineate the topographic mar- ignimbrite with dark-gray pumice lapilli up to Samples of intracaldera tuff intersected by well gin. Thin ignimbrite sheets are generally inter- 10 cm in diameter. These pumice lapilli occur MC-7 match the composition of the outflow 2 throughout the tuff but are locally concentrated sheet (Fig. 8A; Appendix G [see footnote 1]). For discussion of the margins of the caldera(s) in the southern McDermitt volcanic field related to the in bands 5–20 cm thick. The uppermost portion We obtained four new ages on samples of Tuff of Long Ridge, the reader is referred to Rytuba of this member is tan to salmon in color and outflow Tuff of Whitehorse Creek: 15.559 and McKee (1984) and Henry et al. (2016).

1040 Geological Society of America Bulletin, v. 129, no. 9/10 Geology and 40Ar/39Ar geochronology of the mid-Miocene McDermitt volcanic field preted as being outflow, and, therefore, outboard unit, based on its stratigraphic position beneath the fault footwall. We conclude that the Pueblo of the topographic margin. However, in the case nonwelded Tuff of Oregon Canyon in White- caldera does not exist; instead, we locate the of nested calderas, younger ignimbrites can horse Canyon (Figs. 4A and 5). In addition, the source caldera, which we term the Pole Canyon pond to their greatest thickness against older weighted mean age for three different samples caldera, to the east, overlapping the older Fish caldera walls. Finally, because postcaldera lavas of 16.516 ±± 0.017 Ma is resolvably older than Creek caldera. This location is more consistent that erupt along ring fractures can bank against the Tuff of Oregon Canyon (Table 1). The rhyo- with the distribution of the ignimbrite (Fig. 5), topographic walls, their outcrop pattern com- lite lava dome that makes up North Red Moun- the chemical similarity of the Tuff of Trout monly defines the location of the wall, even if tain (Fig. 4A) yielded a weighted mean age of Creek Mountains to the Tuff of Oregon Canyon the wall is no longer present due to it having 16.520 ±± 0.016 Ma on two separate samples, and rhyolitic lavas in the vicinity of Whitehorse been formed by an easily erodible material such i.e., resolvably older than the Tuff of Oregon Ranch, and the presence of rocks that could be a as basinal fill. Canyon. The Antelope Creek rhyolite lava is source for the lithics of mafic and intermediate precaldera despite an age indistinguishable from lava with subordinate rhyolite lava in the Tuff of Fish Creek Caldera Tuff of Oregon Canyon (16.450 ±± 0.028 Ma; Trout Creek Mountains. Table 1) because the Tuff of Oregon Canyon We place the structural margin of the Pole Rytuba and McKee (1984) proposed that the overlies the lava (Fig. 4). Canyon caldera inboard of outflow of Tuff of Tuff of Oregon Canyon was associated with col- In the northeastern quadrant, the topographic Trout Creek Mountains outcrops south of Wil- lapse of Washburn caldera (Fig. 10), a structure wall of the caldera is drawn through the high low Butte and North Red Mountain and west largely obscured by collapse events associated points of these truncated precaldera rhyolite of Flagstaff Butte. We also place it along in- with formation of the younger McDermitt cal- lava domes. The Tuff of Oregon Canyon thick- ferred vents for basaltic lava underneath Tuff dera. We believe that a more westerly source ens and is strongly rheomorphosed against the of Whitehorse Creek north of Willow Creek for the ignimbrite is indicated by that fact that truncated precaldera Antelope Creek rhyolite (Tbc in Fig. 4) and minor-volume basalt to nearly all exposures of the outflow sheet occur lavas. Elsewhere, the thickness of the younger low-silica rhyolite lavas we map together as west of this location. We favor abandonment of Tuff of Trout Creek Mountains is particularly the Pole Canyon lavas (Tlty in Fig. 4; samples the Washburn caldera, and we identify the Fish useful in delineating the topographic mar- TB-319 and TB-464 in Appendix G [see foot- Creek caldera as the source for the Tuff of Ore­ gin because it ponded within the caldera and note 1] and samples TB-333, TB-463B, and gon Canyon based on the radial distribution of flowed over the caldera wall. This occurs at the TB-465 in Appendix H [see footnote 1]). We the ignimbrite from this newly defined struc- northeastern edge of Trout Creek Mountains, classify the Pole Canyon lavas (previously in- ture, the presence of near-vent rheomorphic where Tuff of Trout Creek Mountains banked terpreted as andesite by Rytuba et al., 1982e) ignimbrite and associated welded fall deposits in against and surmounted a topographic wall as postcaldera based on their stratigraphic po- along its northern and southern margins, and the developed in Steens Basalt, ponding to >20 m, sition below the younger Tuff of Whitehorse occurrence of rhyolitic lavas truncated by col- compared to its average thickness of ~10 m Creek and an associated precaldera rhyolite lapse along the eastern edge of the caldera that where outflow is preserved at the highest points lava to the east of Pole Canyon, from which fall along the compositional array of the Tuff of in the Trout Creek Mountains. In Whitehorse we obtained a weighted mean age of 15.548 Oregon Canyon. Canyon, the Tuff of Trout Creek Mountains ±± 0.030 Ma on two separate inverse isochron The Fish Creek caldera was mostly obscured is thick and densely welded against truncated ages (TB-362A and TB-469 in Table 1; Fig. 4). by collapse of the Pole Canyon caldera ~50 k.y. Whitehorse Canyon rhyolite lava (Fig. 9C), in We further constrained the structural margin later; as a result, only the northeast quadrant places forming steep (~75°) contacts. South of of the Pole Canyon caldera using the vent for of the caldera is exposed. South of Whitehorse Whitehorse Canyon, the 16.33 Ma Tuff of Long this rhyolite lava, as well as vents for two other Ranch (Fig. 4A), we place the structural margin Ridge also flows over the Fish Creek caldera rhyolite lavas temporally associated with the at the vent for a rhyolite lava dome at Fish Creek margin and is preserved above the Tuff of Trout much younger Tuff of Whitehorse Creek: the (Tlto in Fig. 4) that falls along the compositional Creek Mountains. The Tuff of Long Ridge is 15.581 ±± 0.021 Ma Whitehorse Butte rhyo- array of the Tuff of Oregon Canyon and that especially thick and densely welded where lite lava (TB-342 in Table 1), and the 15.526 yielded an age of 16.445 ±± 0.022 Ma (TB-354 it ponded above the ring fracture of the Fish ±± 0.032 Ma Buckskin Mountain rhyolite lava in Table 1), i.e., analytically indistinguishable Creek caldera. (TB-558). This was done on the premise that from the age of the ignimbrite. We interpret it these much younger magmas took advantage of as a postcaldera ring-fracture lava because it is Pole Canyon Caldera zones of weakness provided by the Pole Can- more crystal-rich than the tuff, occurs inboard yon caldera ring-fracture zone. of truncated rhyolite lavas resolvably older than Rytuba and McKee (1984) proposed that The topographic wall and ring-fracture faults the Tuff of Oregon Canyon, and is directly over- the source of Tuff of Trout Creek Mountains of the Pole Canyon caldera are clearly defined at lain by Tuff of Trout Creek Mountains with no was a caldera in the central Pueblo Mountains the northern terminus of the Trout Creek Moun- exposure of Tuff of Oregon Canyon interven- (Fig. 10) where they mapped megabreccia and tains in the vicinity of Pole Canyon (Fig. 4A). ing. Southwest of Whitehorse Ranch, we place heterolithologic caldera fill. We interpret these At this location, the thick stack of Steens ­Basalt the structural margin just inboard of outflow of deposits as fanglomerates related to Basin and flows that form the bulk of the Trout Creek Tuff of Oregon Canyon (TB-264 in Appendix Range faulting based on the lack of a tuffaceous Mountains was truncated during collapse of G [see footnote 1]) and three compositionally matrix in the deposits, the presence of deci­ the caldera by a series of northwest-southeast– similar (Fig. 8B) rhyolite lava domes that we meter- to meter-scale clasts of densely welded trending normal faults that drop these lavas interpret to be precaldera in age: Whitehorse Tuff of Trout Creek Mountains within the “brec- down to the north; the biggest offset (minimum Canyon rhyolite lava, North Red Mountain cia,” and the predominance of Mesozoic base- 100 m) is just south of the east-west bend in rhyolite lava, and Antelope Creek rhyolite lava. ment clasts in the deposits, which are absent as Pole Canyon (Fig. 4A), where postcaldera Pole The Whitehorse Canyon rhyolite is a precaldera lithics in the tuff itself but match the lithology of Canyon lavas (Tlty in Fig. 4A) bank in against

Geological Society of America Bulletin, v. 129, no. 9/10 1041 Benson et al. the caldera scarp to form a vertical contact with source of the Tuff of Whitehorse Creek and de- (TB-561), show strong chemical affinity to the the truncated Steens Basalt lavas. termined that it was the youngest and smallest Tuff of Whitehorse Creek and to postcaldera la- West of Pole Canyon, a late Miocene Basin- of McDermitt volcanic field calderas. Our work vas at Red Lookout Butte, so we interpret them and-Range normal fault intersects the ring- provides a more detailed map of the structural all as small-volume rhyolite lavas that erupted fracture system of the Pole Canyon caldera, and topographic margins of the caldera. within the ring-fracture zone of the White- dropping Steens Basalt, intermediate lavas, We draw the structural margin of the White- horse caldera. overlying ignimbrites, and, locally, caldera horse caldera through vents of metaluminous The occurrence of the Willow Creek Hot lake sediments down to the west. We interpret rhyolitic lavas and domes on which we obtained Spring (“camp” in Fig. 4A) along the circular fluvial and marginal lacustrine sediments in 40Ar/39Ar ages of sufficient precision to clas- pattern of postcaldera rhyolite lavas that mark this locality as having been deposited between sify them as postcaldera, i.e., younger than the the structural margin of the Whitehorse caldera the topographic and structural margins of the 15.556 ±± 0.014 Ma age of the Tuff of White- in an area with no major Basin and Range faults Pole Canyon caldera. In this region and north horse Creek: Red Mountain rhyolite lava dome suggests that caldera-related faults are responsi- toward Tule Rims, we draw the topographic (15.446 ±± 0.019 Ma, TB-573 in Table ­ 1), ble for bringing warm fluids from depth toward margin of the caldera outboard of the sediments Willow Butte rhyolite lava dome (15.464 the surface. along the ~200-m-high arcuate scarp composed ±± 0.027 Ma, TB-593), Flagstaff Butte rhyolite We also used the distribution of outflow Tuff of Steens Basalt, Tuff of Oregon Canyon, and lava dome (15.510 ±± 0.023 Ma, TB-318), and of Whitehorse Creek to constrain the structural Tuff of Trout Creek Mountains. In Tule Rims, small lava domes along Willow Creek (15.446 margin of the caldera; we draw the margin in- we attribute the northeast-southwest–trend- ±± 0.029, TB-590; 15.435 ±± 0.043 Ma, board of tuff outcrops east of Flagstaff Butte, ing normal faults that are nearly orthogonal to TB‑321). These lavas typically have ~1–5 vol% west of Willow Creek, and north of the ~50 m of typical northwest-southeast–trending Basin- (maximum 15 vol%) of alkali feldspar ± minor nonwelded tuff intersected in well MC-7. and-Range faults as representing Pole Canyon quartz, and they are metaluminous high-silica We place the topographic margin of the caldera ring fractures. The throws on late Mio- rhyolites with lower Zr/Rb than Tuff of White- Whitehorse caldera inboard of rhyolitic lava cene Basin-and Range faults decrease to zero horse Creek (Fig. 8B). Nonwelded ignimbrite domes and flows we interpret as having erupted as they approach what we interpret to be the exposed south of Willow Butte erupted in as- east of present-day Willow Creek prior to erup- structural margin of the caldera, both in Tule sociation with emplacement of the rhyolite lava tion Tuff of Whitehorse Creek (Tlwo). We des- Rims and on the faults along the southeastern that comprises the butte; it contains large gray ignate the lavas as precaldera based on the pres- margin of the caldera (Fig. 4A), presumably due pumice lapilli with a low Zr/Rb similar to post- ence of Tuff of Whitehorse Creek to the west to the relative strength of the pluton that formed caldera lavas, and it has abundant large lithics with high Nb contents (>45 ppm) not observed upon solidification of the associated underlying of quartz-rich rhyolite lava, basaltic lava, and in postcaldera rhyolite lavas (Fig. 8B), and ages magma chamber. sandstone. that are for the most part older than, though We delineate the northwest topographic We also draw the structural margin of the cal- not indistinguishable from, the Tuff of White- margin of the caldera at North Red Mountain, dera through the vent for a somewhat younger horse Creek. These lavas have 1–5 vol% pheno- where Tuff of Trout Creek Mountains (sample (15.369 ±± 0.015 Ma, TB-317) metaluminous crysts of alkali feldspar and include: the 15.581 TB-265B; Appendix G [see footnote 1]) was high-silica rhyolite lava east of Buckskin Moun- ±± 0.021 Ma rhyolite lava dome of Whitehorse deposited atop the lava between the structural tain (Tlwh), which has a phenocryst assemblage Butte (Fig. 4; TB-342 in Table 1); the 15.575 margin and the peak of North Red Mountain. different from the other postcaldera lavas: ~14 ±± 0.023 Ma lava flow to its immediate north-

The eastern and southeastern topographic mar- vol% quartz, ~9 vol% Or60 sanidine (Appendix west (TB-320), which is truncated to the west by gins of the Pole Canyon caldera are defined E [see footnote 1]), ~4 vol% magnesioferric an arcuate ring fracture; a volumetrically minor by the circular outcrop pattern of the younger hornblende, and trace apatite and zircon. The 15.553 ±± 0.032 Ma rhyolite lava that erupted Tuff of Whitehorse Creek (Fig. 4), which was presence of horizontal columnar joints along the through Pole Canyon caldera lake sediments partially confined within it. This circular out- southern margin of the main outcrop of this lava northeast of the Willow Creek campground crop pattern was identified by Rytuba et al. suggests that it intruded caldera lake sediments (TB-609); and the 15.592 ±± 0.028 Ma rhyolite (1981) and attributed to a structural depres- that have since eroded away. These lavas have dome exposed at the campground turnoff from sion. In Whitehorse Creek, the Tuff of White- lower Zr (<200 ppm) than all other northern Mc- the Whitehorse Road (TB-263, TB-307). horse Creek overlies caldera lake sediments and Dermitt volcanic field magmas (Fig. 8B), prob- We also map a thick (~600 m) blue, strongly thickens where it ramps up against the topo­ ably reflecting stabilization of zircon, but they flow-foliated rhyolite lava with ~5% pheno- graphic wall of the Pole Canyon caldera. To retain high Nb (~37 ppm) that is similar to other crysts of feldspar and trace quartz east of Pole the north, the Tuff of Whitehorse Creek flowed northern McDermitt volcanic field magmas. A Canyon (previously mapped as undifferentiated significantly farther (~20 km) from its source, small outcrop of this hornblende-bearing rhyo- tuff by Rytuba et al., 1982e) as a precaldera lava suggesting that relief on the Pole Canyon cal- lite lava also occurs atop Buckskin Mountain, dome truncated to the north by collapse of the dera topographic wall in the north was not as and, like the rhyolite of Buckskin Mountain, Whitehorse caldera. The vent for this lava dome significant as in the east and south, due to either it erupted along ring-fracture faults associated is exposed at its western margin, where flow fo- weak precaldera wall rocks or hinged collapse with the older Pole Canyon caldera (Fig. 4A). liations are dominantly vertical; away from the of the cauldron block. Weakly porphyritic rhyolite lavas north vent and eastward toward what was the basin of of Flagstaff Ranch (15.570 ±± 0.023 Ma, the Pole Canyon caldera when it erupted, flow Whitehorse Caldera TB‑331) and south of Flagstaff Ranch (15.544 foliations generally trend northeast. Two inverse ±± 0.030 Ma, TB-468) yielded ages indistin- isochron ages from this unit yield a weighted Rytuba and U.S. Geological Survey col- guishable from Tuff of Whitehorse Creek and, mean age of 15.548 ±± 0.015 Ma (TB-362A leagues (Rytuba et al., 1981; Rytuba and McKee, along with aphyric lavas west of Willow Butte and TB-469 in Table 1), which is indistinguish- 1984) identified the Whitehorse caldera as the (TB-502, TB-503) and west of Red Mountain able from the age of the Tuff of Whitehorse

1042 Geological Society of America Bulletin, v. 129, no. 9/10 Geology and 40Ar/39Ar geochronology of the mid-Miocene McDermitt volcanic field

Creek. However, the presence of ignimbrite we ments that occur exclusively in the “western ments atop older Pole Canyon caldera marginal correlate with Tuff of Whitehorse Creek above basin” being deposited on the slope between lacustrine sediments is consistent with upper this lava dome along its eastern edge (sample the topographic margin of the Pole Canyon cal- diatomaceous lacustrine sediments at this loca- TB-478 in Appendix G [see footnote 1]), its dera and a lake that formed in its center. Most tion being dominated by shallow-water diatoms petrographic and chemical similarity to a blue of the Pole Canyon caldera lake sediments were Cymbella cistula, Tetracyculus celatum, Tetra­ flow-foliated precaldera lava encountered be- either engulfed during collapse of the White- cyculus javanicus, Fragiliaria brevistraiata, neath Tuff of Whitehorse Creek in well MC-7 horse caldera or were covered by outflow Tuff and Fragilaria construens (samples 134, 177 (samples W7-729, W7-655, and W7-590 in Ap- of Whitehorse Creek and younger lavas and in Barrow, 1983), whereas the upper exposed pendix G [see footnote 1]), and its truncation to sediments. Limited exposures of diatomaceous sediments within Whitehorse caldera proper the north all point to a precaldera designation. lacustrine sediments of the Pole Canyon caldera are dominated by open-water species Melosira We therefore draw the topographic margin of occur southeast of Flagstaff Butte, where they distans, Melosira arenaria, Melosira granu­ the Whitehorse Caldera along the truncated scarp are overlain and locally fused by Tuff of White- lata, and Melosira preicelandica (samples 51, of the lava dome. horse Creek, and west of Red Mountain, where 62, 79, 88). fine-grained sediments are interbedded with Nash and Perkins (2012) correlated seven Caldera Lake Sedimentation post­caldera trachyandesite lavas and phreato­ ash layers preserved within the diatomaceous magmatic deposits. Whitehorse caldera lake sediments to re- Smith (1927) first studied lake sediments The vast majority of exposed diatomaceous gional ash falls that range in age from 15.55 to in the northern McDermitt volcanic field and lacustrine sediments—the upper interval of the 14.3 Ma. These ages are consistent with our pre- named them the Trout Creek Formation; Mac- Trout Creek Formation of Barrow (1983)—were ferred age of 15.556 ±± 0.014 Ma for the Tuff of Gintie (1933) was the first to recognize the dia- deposited after eruption of the Tuff of Whitehorse Whitehorse Creek and formation of the basin in tomaceous parts of the formation and its middle Creek in a caldera lake that filled a composite which the sediments were deposited upon cal- Miocene age; and Rytuba et al. (1979) and Bar- basin consisting of the Pole Creek caldera and dera collapse. row (1983) attributed these to deposition in the the Whitehorse caldera nested within it. Most Whitehorse caldera. Our mapping and reinter- of these diatomaceous sediments exposed at the POSTCALDERA MAFIC LAVAS pretation of the detailed M.S. thesis work by surface are white, fine-grained, and laminated, Barrow (1983) allow us to distinguish sediments with the local occurrence of cross-bedding, but Minor volumes of basalt erupted north and that were deposited in the Pole Creek caldera near the topographic margin of the Whitehorse northeast of and within both the Pole Canyon and the nested younger Whitehorse caldera. caldera, the sediments are darker colored with and Whitehorse calderas following each col- Evidence is lacking concerning the existence of a larger proportion of detrital material, includ- lapse; evidence for postcaldera mafic eruptions a caldera lake associated with the oldest caldera, ing sparse 1–2 cm rhyolitic clasts. Light-gray at the Fish Creek caldera is lacking, but the the Fish Creek caldera, because the western half layers of rhyolitic ash 1–5 cm in thickness are rele­vant stratigraphic interval is rarely exposed. was engulfed upon collapse of the Pole Creek preserved between beds of diatomaceous­ sedi- ­After collapse of the Pole Canyon caldera, mi- caldera, and there has been insufficient incision ment where preserved by capping basalts and nor volumes of basaltic lava erupted north of through Tuff of Trout Creek Mountains that in a road cut along Whitehorse Ranch Road. Pole Canyon along the structural margin, where flowed over the eastern half to expose caldera- These diatomaceous sediments and intercalated the lavas overlie low-silica rhyolite lavas of Pole fill deposits below. ash deposits were intersected in the uppermost Canyon (mapped together as Tlty in Fig. 4), and Barrow (1983) divided the Trout Creek For- ~40 m of the 226 m well MC-7 drilled near the south of Whitehorse Butte (Tbc), where they mation into two intervals at the type section margin of the Whitehorse caldera and logged by are covered by outflow of Tuff of Whitehorse ~2.5 km SW of Flagstaff Butte: (1) a lower Rytuba et al. (1981). Recovered core from the Creek. Basaltic eruptions in the Whitehorse cal- fluvial to marginal lacustrine interval character- lower ~100 m levels of the ~140 m caldera-fill dera (Tbc) are represented by lavas overlying ized by tuffs, paleosols, and alluvial and fluvial sequence is dominated by tuffaceous sandstone, the structural margin west of Whitehorse Butte, deposits with cross-bedded channel and gravel siltstone, and shale with broadleaf fossils and as pahoehoe and a‘a flows interbedded with lag deposits; and (2) a thick upper lacustrine intercalated ash deposits. This suggests that the caldera lake sediment southwest of well MC-7 portion that consists of diatomite with interbed- caldera lake that formed within the Whitehorse (mapped as “Ttcb” by Barrow, 1983), and as ded decimeter-thick vitric tuffs, characterized caldera transitioned to a lower-energy environ- phreatomagmatic deposits and mafic lavas that by a paucity of detrital components. Barrow ment with time. erupted through the center of the caldera lake (1983) sited two different basins that contained At the type section of Trout Creek Forma- and along the eastern ring fracture. A minor Trout Creek Formation: a younger, roughly cir- tion, Barrow (1983) described the older fluvial volume of basalt erupted north and northeast cular basin that he attributed to the caldera lake to marginal lacustrine sediments we attribute to of the caldera, where thin lavas overlie the Tuff of the Whitehorse caldera defined by Rytuba the Pole Canyon caldera as dipping ~15°N, with of Whitehorse Creek. Those at Tule Rims were et al. (1981), and an older, roughly northwest- an angular unconformity between them and the sourced from nearby northwest-trending dikes southeast–trending graben west of the White- flat-lying diatomaceous sediments associated (Fig. 4A). horse caldera he termed the “western basin.” with Whitehorse caldera (Fig. 9D). This occurs The most voluminous nonrhyolitic lavas to The lower fluvial to marginal lacustrine inter- southwest of the topographic wall of the White- erupt in the northern McDermitt volcanic field val of the Trout Creek Formation was mapped horse caldera, suggesting that collapse of the are the trachybasaltic andesite lavas of Tule only in the “western basin,” whereas sediments Whitehorse caldera caused sediments that were Rims (Tttr). These lavas vented from the ring- of the upper diatomaceous interval occur in deposited within the Pole Canyon caldera to fracture zone of the Pole Canyon caldera west both basins. downsag toward the Whitehorse caldera during and south of Red Lookout Butte (Fig. 4A). Barrow’s observations are consistent with collapse. Expansion of the caldera lake beyond Minor eruptions occurred in the Pole Canyon the older fluvial to marginal lacustrine sedi- the Whitehorse caldera to deposit lake sedi- caldera lake, as evidenced by phreatomagmatic

Geological Society of America Bulletin, v. 129, no. 9/10 1043 Benson et al. trachybasaltic andesite deposits interfingered DISCUSSION and Henry, 2000; Rytuba et al., 2003; Henry with brick-red tuffaceous caldera lake sediments et al., 2016). containing sparse clasts of trachybasaltic ande­ Evolution of the McDermitt Volcanic Field Following postcaldera magmatism at McDer­ site lava. The dominant volume of these trachy- mitt caldera, the McDermitt volcanic field ex- basaltic andesite lavas flowed north, where they The earliest mid-Miocene rhyolitic vol­ perienced ~700 k.y. of quiescence, interrupted are preserved as lavas with ~2 vol% phenocrysts­ canism in the McDermitt volcanic field in- only by eruption of a small-volume rhyolite­ of feldspar up to 1 cm in length. The flows over- cludes rhyolite lavas and minor tuffs, con- lava at Tule Rims at 16.12 Ma. Renewal of lie thin alluvium deposited on outflow of Tuff temporaneous with Steens Basalt, dated at volcanism is marked by eruption of rhyolite of Trout Creek Mountains and, at Tule Rims, ca. 16.59–16.47 Ma (Henry et al., 2016; this lavas at ca. 15.6 Ma along the ring fracture of a rhyolite lava that is characterized by ~5% study). The first regionally significant ignim- the Pole Canyon caldera. The 15.56 Ma erup- phenocrysts of alkali feldspar, ubiquitous flow brite, the 510 km3 Tuff of Oregon Canyon, tion of the metaluminous Tuff of Whitehorse foliations and perlitization, and local bands of erupted at 16.47 Ma from the northern part of Creek resulted in collapse of the 13 × 12 km pervasive spherulites. A maximum age for the the McDermitt volcanic field and resulted in Whitehorse caldera, completely nested within trachybasaltic andesite lavas is given by an collapse of the ~20 × 24 km Fish Creek cal- the older Pole Canyon caldera. Postcaldera age of 16.122 ±± 0.020 Ma on the underlying dera. Prior to and for a short period after this rhyolite lavas erupted along the ring-fracture rhyolite lava (Table 1); a minimum age is given first caldera-forming eruption, Steens Basalt systems of both calderas until ca. 15.44 Ma. by the overlying ca. 15.56 Ma Tuff of White- lavas erupted from vents in the Trout Creek and The youngest rhyolites are 15.37 Ma meta­ horse Creek. Oregon Canyon Mountains. Extrusion of mafic luminous, zircon- and hornblende-bearing Tule Rims trachybasaltic andesite and post- and intermediate-composition Steens Basalt la- ­lavas that erupted along the northwestern ring caldera basalts associated with the northern vas ended before eruption of the 660 km3 Tuff fracture of the Whitehorse caldera at Buckskin McDermitt volcanic field calderas are simi- of Trout Creek Mountains and resulting col- Mountain. lar in composition to postcaldera mafic lavas lapse of the ~20 × 26 km Pole Canyon caldera of the High Rock caldera complex in having at 16.42 Ma, as no Steens Basalt lavas are found Rhyolitic Volcanism Fueled by high ratios­ of large ion lithophile to high field above this ignimbrite. Intrusion of Flood Basalt strength elements compared to Steens Basalt The Tuff of Trout Creek Mountains was (Fig. 11; Johnson et al., 1998; Camp et al., sourced from essentially the same magma New information from the 40Ar/39Ar ages 2013; Coble and Mahood, 2016). This suggests chamber as the Tuff of Oregon Canyon, given of the Tuffs of Oregon Canyon, Trout Creek that these postcaldera lavas were derived from that the calderas overlap in space (Fig. 4), and Mountains, and Long Ridge, and the locations a source with a larger lithospheric component the interval between eruptions is only ~50 k.y. of their calderas, is consistent with the peralka- than the Steens Basalt. (and could be as short as 40 k.y. when taking line rhyolite magma chambers of the McDermitt analytical errors into account; Table 1). In ad- volcanic field forming as a result of injection of dition, both ignimbrites are characterized by Steens Basalt into the upper crust (e.g., Camp phenocrysts of sodic sanidine, quartz, and alkali et al., 2003; Brueseke et al., 2008; Coble and Tule Rims lavas amphibole and have overlapping trace-element Mahood, 2012). Ignimbrite stratigraphy in sev- 1250 High Rock Postcaldera postcaldera basalts differentiation trends (Fig. 8). The greater eral faulted sections in the vicinity of McDer- ma c lavas 1000 phenocryst­ abundance of the Tuff of Trout mitt volcanic field confirms findings of previ- Creek Mountains and Fish Creek rhyolite lava ous workers that eruption of the Tuff of Oregon 750 that erupted between the two ignimbrites is con- Canyon was contemporaneous with emplace-

Ba (ppm) sistent with continued crystallization of magma ment of Steens Basalt (Fig. 5; e.g., Rytuba and 500 Steens Basalt that was not tapped during initial evacuation of Curtis, 1983; Rytuba and McKee, 1984; Jarboe 250 the magma chamber during eruption of the Tuff et al., 2008, 2010; Camp et al., 2013; Mahood of Oregon Canyon. and Benson, 2017). Lavas of Steens Basalt oc- During the interval in which the Tuffs of Ore­ cur above the 16.47 Ma Tuff of Oregon Canyon, 100 150 200 250 Zr (ppm) gon Canyon and Trout Creek Mountains were but no Steens Basalt or intermediate lavas are erupting in the northern part of McDermitt vol- found overlying the Tuff of Trout Creek Moun- Figure 11. Chemistry of northern McDermitt canic field, in the southern part of the field, met- tains (Figs. 4 and 5), indicating that Steens Ba- volcanic field mafic lavas. Tule Rims trachy- aluminous and peralkaline lavas preserved in the salt eruptions ended in the northern McDermitt basaltic andesite lavas (Tttr) and basaltic Double H, Hoppin Peaks, and Oregon Canyon volcanic field by ca. 16.42 Ma (Mahood and and trachybasaltic andesite lavas associated Mountains erupted ca. 16.4–16.3 Ma (Henry Benson, 2017). with northern McDermitt volcanic field cal- et al., 2016). Eruption at 16.33 Ma of the Tuff Intrusion of mafic magma presumably con- deras (Tbc) are similar in composition to of Long Ridge, zoned from peralkaline rhyolite tinued underneath the northern McDermitt postcaldera lavas of the High Rock caldera to metaluminous trachyte, led to collapse of the volcanic field after 16.42 Ma, given that Steens- complex (Coble and Mahood, 2016). Basalts McDermitt caldera. Postcaldera metaluminous related volcanism continued in the southern from both volcanic centers have higher large low-silica rhyolite to high-silica alkali rhyolite McDermitt volcanic field for several hundred ion lithophile to high field strength element lavas that erupted in the McDermitt caldera thousand years (Table 3; Henry et al., 2016), and ratios compared to Steens Basalt. Steens Ba- ­until ca. 16.3 Ma are associated with hydrother- other major members of the Columbia River Ba- salt field follows definition of Steens Basalt mal mineralization of Hg, U, Ga, and Au along salt Group erupted until ca. 15.9 Ma, until ces- by Camp et al. (2013), encompassing lower the caldera ring fracture and Li mineralization sation of the Wanapum Member (Baksi, 2013; Steens, upper Steens, and capping basaltic in caldera lake sediments (Rytuba­ and Glanz­ Barry et al., 2013). The dearth of Steens Basalt andesite of Johnson et al. (1998). man, 1978; Rytuba and McKee, 1984; Castor above ignimbrites in the northern McDermitt

1044 Geological Society of America Bulletin, v. 129, no. 9/10 Geology and 40Ar/39Ar geochronology of the mid-Miocene McDermitt volcanic field volcanic field during this time is presumably the diagrams distinct from those for the main-stage the dikes (e.g., Camp et al., 2013). The Hawks result of a shadow zone formed by the northern peralkaline rhyolites (Fig. 8), due in part to the Valley–Lone Mountain center (Wypych et al., McDermitt volcanic field silicic magma cham- early stabilization of zircon and plagioclase in 2011), the presumed source of the Tuff of Mon- bers through which the relatively more dense the crystallizing metaluminous magmas. We ument Basin (Mahood and Benson, 2017), the basaltic magma could not ascend. hypothesize that metaluminous rhyolitic lavas four calderas of High Rock caldera complex, We speculate that late-stage magmatism in that erupted at High Rock caldera complex at and the basaltic to intermediate volcanism in the northern McDermitt volcanic field that re- ca. 15.9–14.7 Ma after a hiatus of 0.3–0.9 m.y. the Buffalo Hills (Cronquist and Camp, 2010; sulted in eruption of the caldera-forming Tuff following the peralkaline rhyolite caldera- Camp et al., 2013) are arrayed along a simi- of Whitehorse Creek and associated mafic and forming eruptions (Coble and Mahood, 2016), lar trend (Fig. 12). Steens dikes (Minor, 1986) rhyolitic lavas at ca. 15.6 Ma occurred once formed in a similar manner. and newly identified trachyte stratocones in the magma reservoirs associated with the Tuffs Trout Creek and Oregon Canyon Mountains of Oregon Canyon and Trout Creek solidified Distribution of Calderas Marks (this study) are aligned north-northwest and following a decline in the flux of Steens Ba- Two Dike Swarms are on strike with north-northwest mafic dikes salt. Once the upper crust could sustain brittle mapped in the Santa Rosa Range (Brueseke and fracture, dikes associated with the smaller flux Calderas in the McDermitt volcanic field, Hart, 2008) and the northern Nevada Rift (John of basaltic magma having a larger lithospheric High Rock caldera complex (Coble and et al., 2000), once palinspastic correction is signature caused a second round of crustal melt- ­Mahood, 2016), and the Santa Rosa–Calico made for post–15 Ma Basin-and-Range exten- ing. The complete nesting of the Whitehorse (Brueseke and Hart, 2008) and Hawks Valley– sion (Colgan et al., 2006b; Lerch et al., 2008; caldera within the older calderas suggests that Lone Mountain (Mahood and Benson, 2017) Curry et al., 2016). All calderas of McDermitt partial melting of heated and hybridized upper centers fall along two distinct trends that align volcanic field are on strike with this trend. There crust in and around the plutons associated with with the distribution and orientation of Steens are a few dikes and lavas chemically correlated the older calderas gave rise to the metaluminous Basalt dikes. Steens Basalt dikes and vents in to Steens Basalt in the Owyhee Mountains Tuff of Whitehorse Creek and associated rhyo- the Steens Mountain (Camp et al., 2013), north (Camp et al., 2013) and Pine Forest Range (Col- litic lavas.­ The metaluminous character may of Mickey Hot Springs (Brueseke et al., 2007), gan et al., 2006a) that do not fall along either reflect the more lithospheric chemical signature and in the Pueblo Mountains (Harrold, 1973; trend, demonstrating that the whole region was of the associated basalts and the greater involve- Mosher, 1989; Hart et al., 1989) trend domi- being invaded by Steens Basalt dikes, but these ment of partial melts of the upper crust than nantly ~N20°E, parallel to the range-bounding are very small in volume (<1%) compared to was the case for the more voluminous peralka- fault and to the edge of transitional crust as in- the bulk of Steens-related volcanism, which was 87 86 line rhyolites. These late-stage metaluminous dicated by the 0.704 Sr/ Sri isopleth (Fig. 1), concentrated along the High Rock and McDer- rhyolites show trends on trace-element variation which may have controlled the orientation of mitt swarms.

Figure 12. Schematic map showing two ~16 Ma Steens swarms of mid-Miocene volcanism along the Mountain IDAHO Nevada-Oregon border associated with flood OREGON basalt. Approximate location of state borders at 16 Ma are from Dickinson (2013). Loca- < 15.2 Ma High Rock 16.5 Ma tions of calderas mapped in the High Rock swarm Snake River caldera complex (Coble and Mahood, 2016) 16.4 Ma 16.5 Ma Plain volcanism and McDermitt volcanic field (this study; H * * 16.3 Ma Henry et al., 2016) and inferred sources 16.4 Ma at Hawks Valley–Lone Mountain (H) and the Santa Rosa–Calico center (S) fall along 16.3 Ma S 15.7 Ma the trend of Steens Basalt dikes and vents 16.0 Ma McDermitt in two distinct swarms. Ages of ignimbrites swarm (shown in bold from this study; Brueseke 15.7 Ma and Hart, 2008; Coble and Mahood, 2016) decrease south in both trends. Approximate NEVADA locations of mafic and intermediate rocks of Buffalo Hills/Smoke Creek and the northern Bu alo Hills Nevada Rift are represented by dashed lines. N. Nevada Orientations of dikes correlated to Steens Rift Basalt are schematically represented by thin lines after Minor (1986), Brueseke and Hart (2008), Camp et al. (2013), and Reidel et al. CALIFORNIA (2013b). Locations of stratocones discussed in the text are indicated by stars. Approximate Steens dikes locations of northern Nevada aeromagnetic anomalies are from Glen and Ponce (2002). * Stratocones 100 km

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New Ages on Ignimbrites Provide an 3 30 late renewed volcanism Volume (km ) Estimate for the Rate of Propagation Trout Oregon Creek of Basaltic Dike Swarms Canyon Mtns Southward 60 Whitehorse A propagation of 1000 McDermitt and Creek Monument Given that basalts are intrinsically difficult Long High Rock 90 Basin to date, the relatively precise 40Ar/39Ar ages B Ridge dike swarms. Idaho obtained on rhyolites of the McDermitt vol­ 500 120 Canyon canic field and High Rock caldera complex Santa Rosa lavas N Cold offer an opportunity to assess the propagation Springs 150 Summit 100 rate of the two swarms. We do this using the Lake S ages of the major­ caldera-forming ignimbrites 50 180 Soldier Yellow Meadow because these relatively large-volume units are Rock 10 15 cm/yr Canyon interpreted to have formed during the ­period Radial distance from Steens Mtn (km) of most intense intrusion of basalt into the 16.5 16.3 16.1 15.9 15.7 15.5 crust. Smaller volumes of lavas (<<1% of total­ Age (Ma) volume) precede caldera-forming eruptions south of the front of volcanism in both trends Figure 13. Plot showing southward progression of rhyolitic volcanism in both the High Rock (Fig. 12). This includes the 16.59 ±± 0.15 Ma (blue) and McDermitt (green) swarms, with data scaled to eruption volume. Greater than Nut Mountain biotite-bearing­ rhyolite lava in 99% of the total volume of rhyolitic volcanism that erupted in the region ca. 16.5–15 Ma oc- the High Rock caldera complex (N in Fig. 12; curred within these two southward-propagating swarms. Total volume of ca. 16.5–16.2 Ma Coble and ­Mahood, 2016), 16.59 ±± 0.02 Ma Santa Rosa–Calico rhyolite lavas represented by the green bar is less than 60 km3 (Brueseke anorthoclase-bearing lava in the Oregon Canyon and Hart, 2008). The volume of the Cold Springs Tuff is crudely estimated to be ~25 km3 Mountains (A in Fig. 13; Henry et al., 2016), using methods outlined in Appendix A (see footnote 1). Labeled data points discussed in 16.52 ±± 0.02 Ma biotite-bearing lava along the text are as follows: A—anorthoclase-bearing rhyolite lava in the Oregon Canyon Mountains western wall of the McDermitt caldera (B in (Henry et al., 2016), B—biotite-bearing rhyolite lava along the western McDermitt caldera Fig. 13; Henry et al., 2016), 16.3 ±± 2.3 Ma wall (Henry et al., 2016), N—Nut Mountain rhyolite lava of the High Rock caldera com- Sleeper rhyolite­ south of McDermitt caldera plex (Coble and Mahood, 2016), S—Sleeper rhyolite lava south of McDermitt caldera (Nash (S in Fig. 13; Nash et al., 1995), and ca. 16.5– et al., 1995). Error bars are shown for both the Nut Mountain and Sleeper rhyolite lavas; 16.2 Ma rhyolite lavas at the Santa Rosa–Calico all other data points have analytical errors less than 0.1 Ma at 2σ. Data are from this study, center (green bar in Fig. 13; Brueseke and Hart, Nash et al. (1995), Brueseke and Hart (2008), Coble and Mahood (2016), and Henry et al. 2008). We interpret these smaller eruptions as (2016). Mtns—Mountains. reflecting local crustal melting to produce meta- luminous compositions ahead of the main intru- sive phase of the two dike swarms. 2008; Ladderud et al., 2015), with a weighted fractures and faults within the cauldron blocks In the High Rock trend, the Tuff of Monu- mean of three reported ages in Brueseke and as the main phase of peralkaline silicic vol­ ment Basin erupted at 16.48 Ma from the Hawks Hart (2008) of 15.47 ±± 0.07 Ma (Figs. 12 and canism propagated south (Coble and Mahood, Valley–Lone­ Mountain center (personal obser- 13). The distance between the center of the Fish 2016). Similarly, in McDermitt volcanic field, vation, 2016), followed sequentially to the south- Creek caldera and the Santa Rosa–Calico center the Fish Creek rhyolite lava, Tuff of Trout Creek west by the 16.38 Ma Idaho Canyon Tuff from is ~90 km after restoring 20% Basin-and-Range Mountains, and Pole Canyon lavas erupted from the Virgin Valley caldera, 16.34 Ma Summit extension (Colgan et al., 2006b; Lerch et al., the same magma reservoir as the Tuff of Oregon Lake Tuff from the Badger Mountain caldera, 2008). This corresponds to a southeastward Canyon while the front of the swarm propagated 16.00 Ma Soldier Meadow Tuff from the Hang- propagation rate for the McDermitt swarm of southeast. ing Rock caldera, and 15.70 Ma Tuff of Yel- ~12 cm/yr (Figs. 12 and 13). Glen and Ponce Within the more northerly Chief Joseph low Rock Canyon from the Cottonwood Creek (2002) noted a progression from reverse polar- swarm, the largest dike swarm and source for caldera (Figs. 12 and 13; Coble and Mahood,­ ity dikes within the northern part of the northern the vast majority of all Columbia River Basalt 2016). The distance between the centers of the Nevada Rift to transitional and normal polarity Group lavas, dikes progressively migrated north Hawks Valley–Lone Mountain center and the dikes further south, which is consistent with the with time, as indicated by the preponderance of Cottonwood Creek caldera is 110 km, which southern propagation of the McDermitt swarm. Imnaha Basalt (ca. 16.6–16.2 Ma) dikes exposed corresponds to a propagation rate of ~14 cm/yr These simple calculations demonstrate that in the southern part of the swarm, followed by for the High Rock swarm (Figs. 12 and 13). the main front of intrusion along both swarms the progression of younger Grande Ronde Ba- In the McDermitt swarm, the 16.47 Ma Tuff occurred over the same interval (ca. 16.5– salt (ca. 16.3–15.9 Ma) dikes to the north, and of Oregon Canyon and 16.42 Ma Tuff of Trout 15.7 Ma) and at the same rate (~13 cm/yr; the restriction of feeder dikes for the youngest Creek Mountains erupted from a single magma Fig. 13). In both trends, rhyolitic volcanism members, the Wanapum and Saddle Mountain chamber that formed in the northern part of Mc- continued in a given caldera for a few hundred Basalt (<15.9 Ma), to the northern part of the Dermitt volcanic field (Figs. 4 and 12), followed thousand years, indicating that, once magma swarm (Tolan et al., 1989; Camp, 1995; Camp by the 16.33 Ma Tuff of Long Ridge from the chambers formed, volcanism continued to be and Ross, 2004; Hooper et al., 2007; Camp McDermitt caldera to the southeast (this study; fueled by intrusion while the front of the dike et al., 2013; Reidel et al., 2013b; stated ages Henry et al., 2016). Further south, the Cold swarm propagated south. In the High Rock cal- based on Jarboe­ et al., 2008, 2010; Barry et al., Springs Tuff of the Santa Rosa–Calico cen- dera complex, postcaldera rhyolite lavas and 2013; Baksi, 2013; Benson and Mahood, 2016). ter erupted 15.7–15.4 Ma (Brueseke and Hart, small-volume ignimbrites erupted along ring Ignimbrites­ that erupted within the Lake Owyhee

1046 Geological Society of America Bulletin, v. 129, no. 9/10 Geology and 40Ar/39Ar geochronology of the mid-Miocene McDermitt volcanic field volcanic field associated with the Chief Joseph a centralized focus along distinct subswarms of ca. 15.2 Ma, based on the change in the isotopic­ swarm include the 15.81 Ma Tuff of ­Leslie parallel dikes. Globally, such dike swarms have compositions of rhyolites with high eNd and eHf Gulch from the Rooster Comb caldera (Benson been linked to the impingement of mantle plume compositions similar to those of the accreted and Mahood, 2016) and the ca. 16.0 Ma Dinner heads at the base of the lithosphere, based on the terranes (0 to +6 eNd) to plume tail rhyolites of Creek and ca. 15.8 Ma Mascall tuffs from as-yet- radiating patterns of dike swarms, presence of the Snake River Plain with values consistent unmapped sources at the north end of the field coeval volcanic and plutonic rocks, lateral flow with the incorporation of more radiogenic felsic

(Swisher, 1992; Nash and Perkins, 2012; Streck away from a focal point, and rapid emplace- crust (–11 to –16 eNd). The change in the type et al., 2015). Identification of sources for these ment of swarms (e.g., Central Atlantic, Jut- of incorporated crust is also recorded in boron ignimbrites and detailed geochronology are re- land, Yakutsk, Franklin-Natkusiak, Willouran, abundance and isotopic patterns. Rhyolites west 87 86 quired before the pattern of silicic volcanism Abitibi, Keweenawan, Kola-Onega, Mackenzie, of the 0.706 Sr/ Sri isopleth retain high B/Rb can be used to estimate propagation rates of the Fort Frances, Matachewan, Mistassini events; and 11B/10B ratios of accreted terranes, with an Chief Joseph dike swarm. Baragar et al., 1996; Ernst and Buchan, 1997). appreciable amount of pelitic material relative The four radiating swarms associated with the to Snake River Plain rhyolites, which incorpo- Implications for the Yellowstone Hotspot Columbia River Basalt Group fit this model rated continental crust with relatively low B/Rb and add to the preponderance of evidence that and 11B/10B compositions (Savov et al., 2009; Radiating Pattern of Vents for Mid-Miocene a plume head was responsible for volcanism at Benson, personal observation, 2016). Volcanism Consistent with Plume Head initiation of the Yellowstone hotspot (e.g., Camp The patterns of silicic eruptions within the Impingement and Ross, 2004). This includes the high 3He/4He McDermitt volcanic field contrast with those of With the identification of the McDermitt and ratios, short duration, and high eruptive rates of the Snake River Plain province and thus serve High Rock dike swarms, it becomes clear there Columbia River Basalt Group lavas (e.g., Dod- to illustrate the different processes involved in are four exposed dike swarms associated with son et al., 1997; Hooper et al., 2007), chemical plume head and tail volcanism. In McDermitt the Columbia River Basalt Group that radiate and isotopic data indicating a common astheno- volcanic field and the adjacent High Rock cal- from a central point in the vicinity of Steens spheric mantle component for all flood basalt dera complex, caldera-forming eruptions propa- Mountain (Figs. 1 and 12): ~N20°W McDermitt magmas (Wolff et al., 2008; Wolff and Ramos, gated at ~13 cm/yr within two subswarms of swarm, ~N20°E High Rock swarm, ~N10°W 2013), and a roughly circular pattern of 17– a regionally extensive radiating dike swarm. Chief Joseph swarm, and ~N35°W Monument 15 Ma silicic volcanism centered near Steens Silicic volcanism within the Snake River Plain swarm (Camp and Ross, 2004; Hooper et al., Mountain (Coble and Mahood, 2012). follows a singular linear track no more than 2007; Camp et al., 2013; Reidel et al., 2013b; 200 km wide that youngs toward the northeast this study).3 The focal point of these dike swarms Contrasts to Plume Tail Volcanism of at a rate of 2–5 cm/yr (Christiansen and Lipman, northeast of Steens Mountain roughly coincides the Snake River Plain 1972; Pierce and Morgan, 1992; Nash et al., with the convergence of three linear aeromag- Emanating northeast from the area of plume 2006), a rate similar to, and in the opposite di- netic anomalies identified by Glen and Ponce head impingement near Steens Mountain is the rection of, the 2–4 cm/yr motion of the North (2002) and interpreted as representing mid- track of the Yellowstone hotspot (Fig. 1), char- American plate (e.g., Gripp and Gordon, 1990; crustal (12–15-km-deep) keel dikes (Ponce and acterized by the northeast-trending Owyhee- Pierce and Morgan, 1992; Puskas et al., 2007). Glen, 2008; Camp et al., 2013, 2015). The east- Humboldt, Bruneau-Jarbidge, Twin Falls, The relatively slow migration of plume tail vol- ernmost of these keel dikes fed the 16.5–15 Ma Picabo, Heise, and Yellowstone silicic centers canism through thick continental crust within mafic, intermediate, and rhyolite volcanism of (e.g., Pierce and Morgan, 1992, 2009). The spa- the Snake River Plain contributed to the produc- the northern Nevada Rift (John et al., 2000) tial relationship of flood basalts to emanating tion of large silicic magma chambers that vented within the McDermitt swarm. The presence of hotspots is recognized within the Deccan, North rhyolitic ignimbrites commonly in excess of midcrustal keel dikes and the small volume Atlantic, Parana, and Karoo flood basalt prov- 1000 km3 (e.g., Christiansen, 2001; Cathey and of ca. 16.5–15 Ma intermediate and rhyolitic inces (e.g., Morgan, 1972; Richards et al., 1989; Nash, 2004; Branney et al., 2008; Watts et al., ­lavas (~3% of total erupted volume; Coble and Ernst and Buchan, 2003), and it is interpreted as 2011; Anders et al., 2014), whereas the rapid ex- ­Mahood, 2012) that occur outside of the four the result of initial impingement of the plume pansion of the plume head beneath lithosphere main swarms of volcanism (Fig. 1) demonstrate head causing widespread flood basalts, followed containing less fertile crust led to formation of that the whole area was subject to mid-Miocene by a track of linear volcanism as the lithosphere relatively small silicic magma chambers, from heating and melting (e.g., Brueseke et al., 2008; moves over a relatively stable plume tail (e.g., which ignimbrites typically less than 1000 km3 Coble and Mahood, 2012; Streck et al., 2016). Richards et al., 1989; Griffiths and Campbell, erupted in McDermitt volcanic field and High Still, the dominant volume of melting (~97%) 1991). The transition from plume head to plume Rock caldera complex. occurred along the four dike swarms, where a tail volcanism in the Yellowstone plume occurs 87 86 large quantity of ascending basalt interacted near the 0.706 Sr/ Sri isopleth (Fig. 1), because SUMMARY with fertile crust (Camp et al., 2013; this study). the plume head impinged and spread along the The fanning pattern of the four dike swarms transition zone between thin accreted terranes New mapping and geochronology at the Mc- and diachronous nature of volcanism along and thick cratonic lithosphere and was detached Dermitt volcanic field delineate three region- them are consistent with a type II giant radiating by the thicker crust of the North American cra- ally extensive peralkaline ignimbrites and one dike swarm as defined by Ernst et al. (1995) and ton as the plate migrated southwest (e.g., Camp, smaller metaluminous ignimbrite associated Ernst and Buchan (1997), which is characterized 1995; Camp and Ross, 2004). with flood basalt volcanism. The ~510 km3 per- by volcanism that rapidly radiates outward from Nash et al. (2006) pinpointed the transi- alkaline alkali rhyolite Tuff of Oregon Canyon 3Dikes of the Prineville Basalt, which accounts for tion from widespread volcanism resulting erupted at 16.468 ±± 0.006 Ma from an ~20 × 0.3% of the total Columbia River Basalt Group vol- from a radiating plume head to a narrow zone, 24 km caldera in the northern part of the vol- ume, are not exposed (Reidel et al., 2013b). ~70 km wide, associated with a plume tail at canic field. Compositionally similar pre- and

Geological Society of America Bulletin, v. 129, no. 9/10 1047 Benson et al.

postcollapse rhyolite lavas of the Fish Canyon well MC-7. Matthew Coble and Elizabeth Miller (both Idaho, Eastern Oregon, and Northern Nevada: U.S. Geological Survey Open-File Report 2004-1222, caldera are preserved along the eastern caldera at Stanford University) provided useful feedback on the geology of the region. We thank I. Hagmann, p. 156–181. margin and along the ring fracture of the cal- R. Hampton, M. Luckett, and E. Williams for their Bonnichsen, B., Leeman, W.P., Honjo, N., McIntosh, W.C., dera. About 50 k.y. later, at 16.415 ±± 0.007 Ma, and Godchaux, M.M., 2008, Miocene silicic volcanism efforts as field assistants, and Leslie Hayden (U.S. in southwestern Idaho: Geochronology, geochemistry, 3 the ~650 km peralkaline alkali rhyolite Tuff of Geological Survey) for her assistance with electron and evolution of the central Snake River Plain: Bulletin Trout Creek Mountains erupted from the same microscopy. Matt Brueseke and W. Kirk Schleiffarth­ of Volcanology, v. 70, p. 315–342, doi:​10​.1007​/s00445​ provided substantive reviews that considerably im- -007​-0141​-6​. magma reservoir, resulting in collapse of the proved the manuscript. Benson was supported in part Boroughs, S., Wolff, J., Bonnichsen, B., Godchaux, M.M., overlapping, ~20 × 26 km Pole Canyon caldera. by a Department of Defense National Defense Sci- and Larson, P., 2005, Large-volume, low-d18O rhyolites Peralkaline volcanism continued in the south- ence and Engineering Graduate (NDSEG) Fellow- of the central Snake River Plain, Idaho, USA: Geology, ship. Research funds were provided by Penrose and v. 33, no. 10, p. 821–824, doi:​10​.1130​/G21723​.1​. east part of the field upon eruption of the 16.328 Branney, M.J., Bonnichsen, B., Andrews, G.D.M., Ellis, Mineralogy-Geochemistry-Petrology-Volcanology ±± 0.013 Ma Tuff of Long Ridge (~1000 km3), B., Barry, T.L., and McCurry, M., 2008, “Snake River (MGPV) Student Research Grants from the Geo- resulting in collapse of the McDermitt caldera. (SR)–type” volcanism at the Yellowstone hotspot track: logical Society of America and a McGee Fund Grant Distinctive products from unusual, high-temperature After an ~770 k.y. hiatus, silicic volcanism re- from Stanford University to Benson, and by the U.S. silicic super-eruptions: Bulletin of Volcanology, v. 70, newed with eruption of the ~110 km3 metalu- Geological Survey National Cooperative Geologic no. 3, p. 293–314, doi:​10​.1007​/s00445​-007​-0140​-7​. minous Tuff of Whitehorse Creek and collapse Mapping Program under award no. G15AC00435 to Brueseke, M.E., and Hart, W.K., 2008, Geology and Petrol- Mahood. ogy of the Mid-Miocene Santa Rosa–Calico Volcanic of the ~13 × 12 km Whitehorse caldera nested Field, Northern Nevada: Nevada Bureau of Mines and within the Pole Canyon caldera. Metaluminous REFERENCES CITED Geology Bulletin 113, 44 p. Brueseke, M.E., Heizler, M.T., Hart, W.K., and Mertzman, lavas associated with the Whitehorse magma Anders, M.H., Rodgers, D.W., Hemming, S.R., Saltzman, J., S.A., 2007, Distribution and geochronology of Oregon chamber erupted from 15.59 to 15.44 Ma along DiVenere, V.J., Hagstrum, J.T., Embree, G.F., and Wal- Plateau (U.S.A.) flood basalt volcanism: The Steens ring fractures of the Whitehorse caldera and the ter, R.C., 2014, A fixed sublithospheric source for the Basalt revisited: Journal of Volcanology and Geother- late Neogene track of the Yellowstone hotspot: Impli- mal Research, v. 161, no. 3, p. 187–214, doi:​10​.1016​/j​ older Pole Canyon caldera. 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