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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. For permission to copy, contact [email protected] Geological Society of America Bulletin, 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 Pacic 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.