Large, Persistent Rhyolitic Magma Reservoirs Above Columbia River Basalt Storage Sites: the Dinner Creek Tuff Eruptive Center, Eastern Oregon

Cenozoic Tectonics, Magmatism, and Stratigraphy of the Snake River Plain–Yellowstone Region and Adjacent Areas themedStreck issue et al. Large, persistent rhyolitic magma reservoirs above Columbia River Basalt storage sites: The Dinner Creek Tuff Eruptive Center, eastern Oregon

Martin J. Streck1,*, Mark L. Ferns2, and William McIntosh3 1Department of Geology, Portland State University, P.O. Box 751, Portland, Oregon 97207, USA 2College of Arts and Sciences, Eastern Oregon University, One University Boulevard, La Grande, Oregon 97850-3672, USA 3Earth and Environmental Science Department, New Mexico Tech, 801 Leroy Place, Socorro, New Mexico 87801, USA

ABSTRACT basaltic andesite (~56 wt% SiO2) in compo- ern Oregon–northern Nevada have been a center- sition are found in two of the cooling units. piece in the model to connect fl ood basalts to Our understanding of the Yellowstone Major and trace element compositions of the present location of the Yellowstone hotspot hotspot and its connection to fl ood basalts of the more mafi c components match the com- (e.g., Pierce and Morgan, 1992, 2009). However, the Columbia River Basalt province (west- positions of nearby Grande Ronde Basalt rhyolites have not been associated much with ern and northwestern USA) has grown tre- flows and dikes. Compositional similari- the fl ood basalt stage until recently (Coble and mendously over the past decades since the ties between cognate mafi c components and Mahood, 2012; Streck and Ferns, 2012). model was fi rst proposed in 1972. Despite Grande Ronde Basalt fl ows are direct evi- We report our fi rst results on one of the main strong support for a plume origin of the dence for coeval mafi c and silicic magmatism silicic systems, the Dinner Creek Tuff eruptive entire Yellowstone–Columbia River Basalt linking DITEC and Grande Ronde Basalt center (DITEC), which was active near the cen- magmatic province, new non-plume mod- eruptions. Furthermore, finding Grande ter of fl ood basalt eruptive sites, where new age els have emerged to explain early fl ood Ronde Basalt magmas as coeruptive com- dates suggest that activity overlaps with main- basalt volcanism. Unresolved issues of the ponent in Dinner Creek Tuff suggests that stage CRBG activity, and where petrological early fl ood basalt stage include the location Grande Ronde Basalt magmas were stored data suggest tapping and interaction of CRBG of crustal magma reservoirs feeding these beneath Dinner Creek Tuff rhyolites, thereby magmas during explosive silicic eruptions. voluminous eruptions and to what extent providing the fi rst direct evidence for the The Dinner Creek Welded Tuff was originally these were associated with contemporaneous location of a storage site of Columbia River defi ned as a single ignimbrite that erupted from silicic reservoirs. Basalt magmas. Shallow crustal rhyolitic the Castle Rock caldera (Wood, 1976; Rytuba This study focuses on the newly defi ned reservoirs active during ca. 16–15 Ma that and Vander Meulen, 1991) and was found to ca. 16–15 Ma Dinner Creek Tuff Eruptive yielded tuffs of the DITEC and other sur- extend over an area of ~2000 km2 centered along Center that overlaps in time and space with rounding contemporaneous and widespread the Malheur River (Haddock, 1967). Our work fl ood basalt volcanism of the Columbia River rhyolites of the area likely imposed control shows that the original Dinner Creek Welded Basalt Group. New work on distribution, on timing and place of eruption of Columbia Tuff and other correlative ignimbrites mapped lithologic variations, geochemical composi- River Basalt Group lava fl ows. elsewhere in the same stratigraphic position tions, and eruption ages indicate that the consist of a minimum of four discrete ignimbrite extensive Dinner Creek Welded Tuff (herein INTRODUCTION sheets that we herein name the Dinner Creek Dinner Creek Tuff) and associated mapped Tuff, extending over an area >25,000 km2. We and unmapped ignimbrites include a mini- Our understanding of the Columbia River further propose that several well-known fallout mum of 4 discrete cooling units that spread Basalt province (western USA) and its likely tuff deposits of Oregon, Nevada, Idaho, and out over an area of ~25,000 km2. Widespread connection to the Yellowstone hotspot has grown Washington erupted from the same center, fallout deposits in northeast Oregon and the tremendously since the Yellowstone volcanic active from ca. 16 to ca. 15 Ma. Fallout tuffs neighboring states of Nevada, Idaho, and fi eld was fi rst proposed as the present location of either correlate with ignimbrites (cf. Nash and Washington have now been compositionally a continental hotspot (e.g., Morgan, 1972). There Perkins, 2012) or fall between ignimbrite erup- correlated with the redefi ned Dinner Creek is now strong support for a plume origin for tions. Minor amounts of mafi c magmas found Tuff. Compositional coherence between the the entire Yellowstone hotspot track and fl ood in the Dinner Creek Tuff match CRBG magmas ignimbrite sheets and fallout deposits indi- basalts of the Columbia River Basalt Group and thus provide direct evidence for CRBG cate a common source, herein referred to (CRBG) (Pierce and Morgan, 2009). However, fl ood basalt reservoirs beneath this large and as the Dinner Creek Tuff eruptive center the decades-long controversy as to whether this long-lived rhyolitic eruptive center. (DITEC). large igneous province (LIP) is due to the arrival Cognate mafi c components (glass shards, of a deep mantle plume continues, and new non- METHODS pumice shards, and mafi c globules) that plume models for the origin of the CRBG have

range from dacite (~68 wt% SiO2) to Fe-rich been proposed (e.g., Liu and Stegman, 2012). Major and trace element compositions of bulk Age-progressive rhyolites of the Snake River tuff and pumices were determined by X-ray *Corresponding author email: [email protected]. Plain starting at the oldest centers in southeast- ﬂ uores cence and by inductively coupled plasma–

Geosphere; April 2015; v. 11; no. 2; p. 226–235; doi:10.1130/GES01086.1; 7 ﬁ gures; 3 tables; 7 supplemental ﬁ les. Received 3 June 2014 ♦ Revision received 12 November 2014 ♦ Accepted 15 January 2015 ♦ Published online 17 February 2015

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mass spectrometry (ICP-MS) at the Washington tance furnace. After each heating step, followed that the ash-fl ow tuff extended over an area of State University GeoAnalytical Laboratory. by gas cleanup, the isotopic composition of Ar ~2000 km2. Haddock (1967) and Wood (1976) Major element composition of glasses and was analyzed using a MAP 215–50 mass spec- proposed that the Dinner Creek Tuff erupted feldspars were determined with the Oregon trometer. ArArCALC software (Koppers, 2002) from a vent in the vicinity of Castle Rock (Fig. State University (OSU) CAMECA SX100 elec- was used to reduce the isotopic data and make 1A) in the area proposed to be part of the Castle tron microprobe, which was operated remotely age calculations. Further details of ana lyti- Rock caldera (Rytuba and Vander Meulen, from Portland State University. For analysis of cal procedures were described in Duncan and 1991). Thick sequences (~70 m) of rheomorphic the glass, we employed an accelerating volt- Keller (2004; see also the OSU laboratory web- tuff crop out north of Ironside Mountain and at age of 15 kV, a beam current of 8 nA, and a site, http:// www .coas .oregonstate .edu /research Castle Rock, along what we consider to be the defocused beam (10 µm diameter). Peak and /mgg /chronology .html). northern and southern margins of the DITEC. background counting was done as follows (in Single-crystal analyses were performed at the The DITEC has not been studied in any detail, seconds): 10/5 for Na, Al, Si, K; 20/10 for Ca, New Mexico Geochronology Research Labora- in part due to concealment by a thick sequence Fe, Ti, Mn, P; and 30/15 for S, Cl, and Mg. First tory at New Mexico Tech (Socorro). HF-cleaned of younger mafi c lava fl ows, which are probably and short counting of Na and K was targeted to alkali feldspar separates and interspersed related to the Strawberry Volcanics (cf. Steiner minimize loss under the electron beam. Natural Fish Canyon Tuff (FCT) sanidine monitors in and Streck, 2013). mineral standards were used for calibration. We machined Al discs were enclosed in evacuated monitored our calibration during each session quartz tubes and irradiated at the Denver U.S. DINNER CREEK TUFF—THIS STUDY with natural rhyolitic and basaltic glass stan- Geological Survey TRIGA reactor. Individual

dards. Analysis conditions were similar for feld- grains were fused by CO2 laser and analyzed Our data on lithology, chemistry, and petrog- spar with the exception of a 15 nA beam current, using a Thermo Argus VI mass spectrometer. raphy combined with new age dates allow us a more focused beam, and analysis of a smaller Pychron software (Ross, 2014) was used to con- to update the distribution of the Dinner Creek range of elements. trol analysis and reduce data. Tuff, to determine chronostratigaphic eruptive Laser ablation ICP-MS analyses of trace ele- For all age calculations, we used a FCT age of units, to correlate the tuff with regional fallout ment concentrations in glasses were done in 28.201 Ma (Kuiper et al., 2008). tuffs, and to establish petrogenetic connections the W.M. Keck Collaboratory for Plasma Mass to mafi c magmas of the CRBG. Spectrometry at OSU using a Thermo XSeries DINNER CREEK TUFF—ORIGINAL II ICP-MS instrument coupled with a Photon WORK AND LIKELY SOURCE AREA Distribution Machines G2 ArF Excimer laser ablation system. Analyses using a spot size of mostly 40 µm The recognized type section of the Dinner We can correlate widely distributed tuff out- and pulse rate of 10 Hz were conducted in an He Creek Ash Flow Tuff, which we refer to as the crops (with local names or unnamed) with Din- atmosphere and an He fl ow rate of ~0.8 l/min was Dinner Creek Tuff, is along the Malheur River ner Creek Tuff and distinguish Dinner Creek used to transfer ablated material to the plasma. gorge between the small towns of Juntura and Tuff units from other widespread middle to late Analyses and data reduction followed the gen- Harper in eastern Oregon (Fig. 1). Here the Din- Miocene ignimbrites in eastern Oregon based eral procedures outlined in Loewen and Kent ner Creek Tuff forms an important and easily on major and trace element geochemistry, litho- (2012). Calibration of unknown glasses was per- recognizable stratigraphic marker near the top logical characteristics, and stratigraphic position formed via analysis of basaltic glass GSE-1G, of a thick section of mafi c lava fl ows (Kittle- (Fig. 2A). Previously, workers did not attempt to using concentrations from the GeoReM compi- man et al., 1965, 1967). All mafi c lava fl ows correlate the ash-fl ow tuff outcrops. lation (georem.mpch-mainz.gwdg.de) and 29Si below the tuff are referred to as the basalt of Our data show numerous outcrops of generic as an internal standard. Reported precision for Malheur Gorge and lava fl ows above the tuff Miocene welded tuff that had been mapped individual analyses includes the reproducibility as Hunter Creek Basalt. The basalt of Malheur across northeast Oregon (e.g., Pardee, 1941; of standard measurements, and the uncertainty gorge at the type section grades upward from a Prostka, 1962, 1967; Brooks et al., 1976) to be in the accepted composition of the calibration basal series of mafi c fl ows that are correlative correlative with one or more outfl ow units of standard and in the concentration of the internal to the Steens Basalt, through an intermedi- the Dinner Creek Tuff. The Mascall ignimbrite standard element for each unknown. Accuracy ate sequence of mafi c fl ows correlative to the of Davenport (1971) is also determined to be was monitored via analysis of BCR-2G glass Imnaha Basalt, into an upper series of iron-rich Dinner Creek Tuff. This correlation shows that and for most elements measured concentra- basaltic andesite and andesite lava fl ows known Dinner Creek Tuff extends into central Oregon tions were within ~5%–10% of the reported as the Birch Creek basalt that are correlative to and includes extensive unmapped areas of ash- concentrations in this glass. Exceptions were the Grand Ronde Basalt, as are iron-rich ande- fl ow tuff near and south of John Day (Fig. 1). Cr, P, and Zn, which were within ~20%–25% of site lava fl ows of the overlying Hunter Creek Outcrops near John Day were included in the accepted values. Basalt (Hooper et al., 2002; Camp et al., 2003; southern facies of the Columbia River Basalt The Ar-Ar incremental heating experiments Barry et al., 2013; Ferns and McClaughry, 2013; by Brown and Thayer (1966) (Fig. 1). The Din- were performed at OSU. Crystallization ages Reidel and Tolan, 2013). ner Creek Tuff was originally considered to be on acid-rinsed fresh feldspars were determined Pardee (1941) was the fi rst to map a rhyolite a single ignimbrite, confi ned to the Malheur by the 40Ar/39Ar incremental heating method at tuff that separated older mafi c from younger gorge region. Our work indicates that the Dinner the Noble Gas Mass Spectrometry laboratory at mafi c lava fl ows in eastern Oregon. Others, Creek Tuff includes four separate cooling units OSU. All samples were loaded into quartz vials such as Gilully (1937) and Brooks et al. (1976), at stratigraphic positions similar to the type sec- containing small quantities of mineral monitor mapped discontinuous exposures of ash-fl ow tion in the Malheur gorge. Evidence for these FCT-3 biotite and irradiated at the OSU TRIGA tuff as far east as Richland, Oregon. The earliest cooling units is given in the following. research reactor. Samples were heated in a systematic work in the Dinner Creek type section Outcrops are discontinuous and severely bro- double-vacuum, thermocouple-controlled resis- was done by Haddock (1967), who determined ken up by Neogene faults, some of which have

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UM B A

M Baker City Pleasant Valley T. John Day BU PG PB SC Mascall IS Ignimbrite PB DITEC CR Vale WA Bully Creek T C H A MG WB original J CRBG extent ID Burns OR VV 0 30 60 km NV C BC

Figure 1. Regional overview, distribution of the Dinner Creek Tuff, and locations of correlative fallout units. (A) Orange dots—investigated Dinner Creek Tuff locations (this study); red—database of Oregon Department of Geology and Mineral Industries showing original Dinner Creek Tuff and unnamed tuffs correlated with Dinner Creek Tuff; thick orange line—envelope around most distal outcrops; dashed orange oval—likely source area of Dinner Creek Tuff eruptive center (DITEC); yellow—original distribution of Dinner Creek Welded Tuff and previous generalized distributions of local ignimbrites here correlated with Dinner Creek Tuff (see text); gray dashed line—hypothesized location of crustal magma reservoirs of Columbia River Basalt Group (CRBG) magmas (after Wolff et al., 2008). Abbreviations: MG— Malheur Gorge, CR—Castle Rock, H—town of Harper, IS—Ironside Mountains, J—town of Juntura, PG—Picture Gorge, PB—Paulina Basin, T—tuff. (B) Yellow frame is areal coverage of A. Stars indicate fallout localities: M—Mascall Formation, PB—Paulina Basin, BU— Bully Creek Formation, SC—Succor Creek (after Nash and Perkins, 2012), UM—Umatilla (after Ferns, personal data). (C) Stars indicate regional fallout localities (after Nash and Perkins, 2012): NV—Nevada, BC—Buffalo Canyon, C—Carlin, VV—Virgin Valley , ID— Idaho, WB—White Bird, WA—Washington, A—Asotin. Dark and light blue—CRBG (after Camp and Ross, 2004); red—distribution of 17–12 Ma rhyolites (after Christiansen et al., 2002).

>1000 m of cumulative vertical displacement ther than indicated by preserved outcrops. It is Lithology, Mineralogy, and Composition (Ferns et al., 2010). Much of the outcrop area diffi cult to estimate how much landscape of the is obscured by younger volcanic and sedimen- area was covered by tuff. The wide distribution Units of the Dinner Creek Tuff range from tary cover. The best preserved sections occur in and comparatively thin deposits suggest that welded, marked by a basal vitrophyre typically canyons or along fault ridges where they were Dinner Creek Tuff units are low-aspect-ratio overlain by primarily devitrifi ed zones, to only capped by younger lava fl ows. Even within the ignimbrites emplaced at high energy, represent- incipiently welded tuff sections throughout. All canyons, outcrops are often obscured by land- ing landscape-mantling deposits (Freundt et al., cooling units are crystal poor with phenocryst slide and talus deposits. 1999). This suggests that most of the area was contents of <1%–5%. Outcrop thickness is typically 3–8 m; thicker covered by Dinner Creek Tuff. Using our cur- Outcrops of Dinner Creek Tuff range from exposures (~20 m) are found in areas more rent distribution area with conservative thick- pumice poor (~5%) to pumice rich (~30%); proximal to the presumed source, where the nesses yields a volume of the erupted magma pumice is typically ~2–4 cm in the longest tuff can be ~70 m (Haddock, 1967; this study). of ~300 km3 dense-rock equivalent (DRE) of dimension, and rarely exceeds 15 cm. Colors Thicker tuff can also occur more distally and the combined cooling units. The known extent range from white to tan to dark gray, but in suggests that topography-controlled thicken- of associated fallout tuffs preserved probably single outcrops only one or two pumice colors ing and thinning occurs locally. Current Din- represents a magmatic volume of an additional are evident. ner Creek Tuff outcrops enclose an area of 300 km3 DRE, and therefore doubles the vol- The mineralogy of single cooling units is ~25,000 km2. Ash-fl ows probably traveled far- ume erupted from the DITEC. dominated by a single feldspar type (anortho-

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Figure 2. (A) Nb versus Zr of A Or Unit 3 & 4 all four Dinner Creek Tuff cool- n=3 N=13 ing units relative to 15–7 Ma 20 regional rhyolitic ash-ﬂ ow tuffs. Unit 1 Data of 43 Dinner Creek Tuff n=6 N=37 (DIT) bulk analyses are plot- 10 ted. Data sources for other tuffs: Unit 2 n=5 Wildcat Creek Welded Tuff N=58 (WCT)—Hooper et al. (2002); Rattlesnake Tuff (RST)— Streck and Grunder (1997); Ab 10 mol % 20 An Devine Canyon Ash-Flow Tuff (DCT)—Wacaster and Streck Figure 3. Dinner Creek Tuff feldspar compo- (personal data); Prater Creek sitions: n—number of samples; N—number Ash-Flow Tuff (PCT), Spring of crystals analyzed; Or—orthoclase; An— Creek Tuff (SCT), Leslie Gulch anorthite; Ab—albite. Three analyses were Tuff Member (LGT)—our data. obtained on each crystal (with a few excep-

Numbers are SiO2 wt% values tions). Unit number refers to Dinner Creek and lines approximately con- Tuff units (see text). tour variations within Dinner B Creek Tuff samples (all units); ≥ green arrow means that DCT glass with 75wt% SiO2. Outcrops of these are compositions extend beyond seldom welded, but are typically pumiceous and shown range. Sample MS-11-27 in the middle of the age spectrum. Tuff samples is a sample from a distal location with dacitic bulk compositions are complex, that yielded the highest Nb and consisting of high-silica rhyolite to dacitic glass Zr concentrations. (B) Zr versus shards, pumice shards of dacitic composition,

SiO2 for bulk analyses of Din- and andesitic glassy globules (microscoria) ner Creek Tuff of all units (open (Fig. 5). Outcrops of dacitic bulk composition black circles); color indicates are distinctly darker than either low- or high- samples currently identiﬁ ed as silica rhyolite outcrops. Tuff belonging to this one of the four ignimbrite units eruptive unit yielded the youngest age. The based on a combination of bulk rhyo litic component of each individual cool- and glass composition, age, feld- ing unit seems compositionally unzoned to spar composition, and lithology little zoned. There are only small compositional (see Supplemental Table 1 [see variations between low-silica rhyolite and high- footnote 1]). silica rhyolite, and some incompatible trace element variations are near analytical uncertainties (Fig. 2; Supplemental Table 1 [see footnote 1]).

The most mafi c components (~56 wt% SiO2) are typically recorded as variously vesicular clase or Na-rich plagioclase) with subordinate and 5; Supplemental Table 22). Lithic fragments glassy globules (Fig. 5) (cf. Sumner and Wolff, amounts of titanomagnetite (Fig. 3; Supple- in the tuffs are mostly reworked tuff fragments 2003). Petrographic features (glassy, vesicular, mental Table 11). Pyroxenes are exceedingly with compositions matching the host tuff. Bulk friable textures), compositional continuity with rare. Precise accessory phase mineralogy is cur- tuffs with high-silica rhyolitic composition have more silicic compositions, and direct contact of rently uncertain due to the crystal-poor nature of exclusively high-silica rhyolite glass shards and andesitic with more silicic glasses all argue for the tuff. are generally pumice poor and welded. High- the globules recording a liquid component of the Bulk tuff samples of the Dinner Creek Tuff silica rhyolite units are the oldest (see follow- Dinner Creek Tuff magmatic systems. Dacitic range from high-silica to low-silica rhyolite to ing). Bulk compositions of low-silica rhyolite or glass shards and dacitic pumices are in linear dacitic, and composition varies with cooling those straddling the low-silica–high-silica rhyo- compositional continuum between the ande- unit (Fig. 2; Supplemental Table 1 [see foot- lite boundary typically contain pumice of low- sitic component and the rhyolites on element- note 1]). Glass shards and other glassy compo- silica rhyolite, a subordinate amount of pumice element variation diagrams (Fig. 5). nents observed in the tuff have a wider range, with dacitic composition, and mostly contain from high-silica rhyolite to andesite (Figs. 4 Age Data of Ignimbrite Cooling Units and 2Supplemental Table 2. Glass compositions of Correlation with Regional Fallout Tuffs 1Supplemental Table 1. Bulk chemical compo- shards, pumice shards, scoria shards (mafi c glob- sition of Dinner Creek Tuff ignimbrite units 1–4. ules), and pumices. If you are viewing the PDF of Our current age information on the lithologi- If you are viewing the PDF of this paper or read- this paper or reading it offl ine, please visit http:// ing it offl ine, please visit http:// dx .doi .org /10 .1130 dx .doi .org /10 .1130 /GES01086 .S2 or the full-text cal units in combination with glass and feldspar /GES01086 .S1 or the full-text article on www article on www .gsapubs .org to view Supplemental data suggest a minimum of four distinct cooling .gsapubs .org to view Supplemental Table 1. Table 2. units. We have obtained the following ages on

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diatomite-bearing sedimentary unit with several intercalated thin fallout ash beds distributed east to northeast of Vale (Brooks and O’Brien, 1992; Ferns et al., 1993; Nash and Perkins, 2012). Rhyolite with a glass and feldspar composition close to that of Dinner Creek Tuff unit 3 erupted again ca. 15 Ma (14.88 ± 0.15 Ma; Table 1), along with substantial amounts of dacitic and more mafi c magmas to generate Dinner Creek Tuff unit 4 (Figs. 3 and 4). Dinner Creek Tuff unit 1 can be correlated with fallout tuffs in central Nevada (ash buf94– 623 of Buffalo Canyon; Nash et al., 2006), the Carlin Basin in northern Nevada, and the Paulina Basin in eastern Oregon (Figs. 1 and 4; Table 2) (Nash and Perkins, 2012). Cooling unit 2 of the Dinner Creek Tuff has no known coeruptive fallout ash deposits. How- ever, a regionally widespread fallout unit with glass compositions overlapping those of Dinner Creek ignimbrite unit 2 closely precedes eruption of cooling unit 2 at 15.97 ± 0.04 Ma (pre- Figure 4. Rhyolite (>74.5 wt% SiO2) glass compositions of Dinner Creek ash-fl ow tuff units and regional fallout tuffs. All data for fallout tuff glass and glass of Dinner Creek Tuff unit viously 15.77 ± 0.04; Swisher, 1992) (Table 2; 3 are from Nash and Perkins (2012); abbreviations of fallout localities are as in Figure 1. Fig. 4). This 15.97 Ma fallout is the prominent Each data point of this study is the average of several analyses per shard (typically 3 analy- Mascall ash of the John Day Fossil Beds near ses) (Supplemental Table 2 [see footnote 2]). Dinner Creek Tuff samples with glass data are the Picture Gorge locality in eastern Oregon following: Unit 1—MS-MC02–09, MS-11–27, MS-11–54. Unit 2—MS-PCIT1, MS-11–30, and several other correlative fallout exposures, MS-11–32, MS-10–1. Unit 3—Bully Creek Tuff (Nash and Perkins, 2012) equivalent to one in Oregon (Bully Creek Formation) below MS-12–38. Unit 4—MS-11–11, MS-11–20. the tuff of Bully Creek, three in Nevada (Buf- falo Canyon, Carlin, Virgin Valley), one in Idaho (White Bird), and one in Washington (Asotin) the Dinner Creek Tuff: 16.16 ± 0.02, 15.95 ± should not be confused with the Mascall ash, (Fig. 1; Table 2) (Nash and Perkins, 2012). 0.09, 15.53 ± 0.15, 15.48 ± 0.13, 15.46 ± 0.02, which is a prominent fallout tuff within the Mas- Glass compositions of these fallout deposits 15.45 ± 0.15, and 14.88 ± 0.15 Ma (Table 1; call Formation at its type locality near Picture match that of Dinner Creek Tuff unit 2 (Fig. 4). Supplemental Figs. 13, 24, and 35). Our new ages Gorge (Fig. 1) (Davenport, 1971). Older pub- Based on this, we can say that the Mascall ash is cluster at three intervals. The fi rst interval is ca. lished ages on the Dinner Creek Tuff (Supple- older than the ca. 15.5 Ma ash-fl ow tuff, but the 16 Ma, the second is at 15.5 Ma, and the last ca. mental Table 3 [see footnote 6]), or what was fallout unit and Dinner Creek Tuff unit 2 have 15 Ma. One age date of 16.2 Ma was reported suspected as such, range from 16 to 14 Ma but overlapping glass compositions (Fig. 4; Table 2; for the Mascall ignimbrite that we correlate here cluster around 15.3 Ma, the preferred age of the Supplemental Table 2 [see footnote 2]). with the Dinner Creek Tuff (Davenport, 1971; tuff (Cummings et al., 2000; Hooper et al., 2002; Cooling unit 3 may be correlated with the Supplemental Table 36). The Mascall ignimbite Camp et al., 2003) (Supplemental Table 3 [see 15.66 ± 0.07 Ma (recalculated from the original crops out in and around the Paulina Basin and footnote 6]). In contrast, Nash et al. (2006) and 15.46 ± 0.07 Ma; Downing and Swisher, 1993) Nash and Perkins (2012) argued that the Dinner basal fallout tuff (Lough ash) at Succor Creek 3 Supplemental Figure 1. Ar-Ar age plateau plots. Creek Tuff should be assigned an age of 15.9 Ma near the Oregon-Idaho state border (Fig. 1). The If you are viewing the PDF of this paper or reading it offl ine, please visit http:// dx .doi .org /10 .1130 (recalculated to 16.0 Ma). It is now clear that Lough ash was previously correlated with the /GES01086 .S3 or the full-text article on www one Dinner Creek Tuff cooling unit has an age tuff of Bully Creek based on glass compositions .gsapubs .org to view Supplemental Figure 1. of ca. 16 Ma and it is the high-silica rhyolitic (Nash and Perkins, 2012). Our obtained age of 4 Supplemental Figure 2. Single crystal age dating ash-fl ow tuff that we designate as Dinner Creek 15.46 ± 0.02 Ma for the previously undated tuff results. If you are viewing the PDF of this paper or reading it offl ine, please visit http:// dx .doi .org /10 Tuff unit 1. Samples of Dinner Creek Tuff unit 2 of Bully Creek based on single-crystal Ar-Ar .1130/GES01086 .S4 or the full-text article on www produced ages from 15.53 to 15.45 Ma, are low- dating questions this correlation, if the age for .gsapubs .org to view Supplemental Figure 2. silica rhyolite in bulk composition, and contain the Lough ash is correct (Table 1). 5Supplemental Figure 3. Single crystal age dat- An20 plagioclase (Fig. 3). Dinner Creek Tuff unit The 15.02 ± 0.08 Ma (originally 14.93 Ma) ing results. If you are viewing the PDF of this paper 3 produced an age of 15.46 Ma and is low-silica Obliterator fallout tuff, that occurs in Succor or reading it offl ine, please visit http:// dx .doi .org /10 .1130/GES01086 .S5 or the full-text article on www rhyolite, albeit with some geochemical varia- Creek, in western and northwestern Idaho, and .gsapubs .org to view Supplemental Figure 3. tions. Unit 3 can be distinguished from unit 2 as possibly in Nevada (Nash and Perkins, 2012), 6Supplemental Table 3. Age compilation of radio- it contains anortho clase and different glass com- has the correct age to be correlated with the metric ages of Dinner Creek Tuff including correla- position (Figs. 3 and 4). Dinner Creek Tuff unit 14.88 Ma ignimbrite unit 4. However, an ε iso- tive units. If you are viewing the PDF of this paper Nd or reading it offl ine, please visit http:// dx .doi .org /10 3 includes the tuff of Bully Creek (dated here at topic ratio of –9.1 is much too low to be derived .1130 /GES01086 .S6 or the full-text article on www 15.46 ± 0.02 Ma; Table 1) that is near the middle from an eruptive site that gave rise to the Dinner .gsapubs .org to view Supplemental Table 3. of the Bully Creek Formation, a tuffaceous, Creek Tuff (cf. Nash et al., 2006).

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Figure 5. (A) Photomicrograph shows glassy mafi c globule with a SiO2 content of 56 wt%; longest dimension is ~7 mm. (B, C) Glass composition of mafi c and intermediate components of Dinner Creek Tuff (DIT) relative to regional mafi c lavas. Dashed line encircles data for Grande Ronde Basalt (B). All regional mafi c lavas are recognized as correlative with upper Grande Ronde Basalt lava fl ows. Data sources for regional lavas: Grande Ronde Basalt—Wolff et al. (2008); Hunter Creek Basalt—Hooper et al. (2001), Ferns and McClaughry (2013),

this study. Icelandites include Fiddlers Hell lavas, 60 wt% SiO2 Hunter Creek Basalt unit, and icelandites immediately north and south of Ironside Mountain—Ferns McClaughry (2013), this study. Dikes: Brooks (2006). (D) Normalization plot is based on primitive (prim.) mantle composition by Sun and McDonough (1989). Composition of maﬁ c globules for normalization plot is average of several glass analyses by laser ablation–inductively coupled plasma–mass spectrometry (all trace elements and Ti; n = 5) and by electron microprobe (K, P; n = 8) (Supplemental Table 4 [see footnote 7]). Compositions of Hunter Creek Basalt and Birch Creek basalt are from this study and of Grande (Grd.) Ronde Basalt are samples GR50 and GR51 of Wolff et al. (2008; see text).

In summary, ages, geochemical data, and CRBG Units Coeruptive with end at geomagnetic chron C5Cn.1n, which cor- mineral compositions indicate that at least 4 Dinner Creek Tuff responds to an age of 15.95 Ma (Fig. 6). There- discrete ignimbrites were emplaced between fore, most Dinner Creek Tuff eruptions (the ﬁ rst ca. 16 and ca. 15 Ma (Table 3). The earliest Radiometric ages of ignimbrites and fall- 3 ignimbrites, units 1–3, and several fallout units, and possibly largest eruptions generated Dinner out tuffs originating from the DITEC range e.g., Mascall ash and Lough ash) are in the same Creek Tuff unit 1 and extensive fallout deposits. from 16.1 to 15 Ma. Main phase CRBG mag- eruptive time window as the most voluminous Dinner Creek Tuff units 2 and 3 were emplaced mas, consisting of the Steens, Imnaha, and of all CRBG units, the Grande Ronde Basalt nearly simultaneously at 15.5 Ma. Units 2 and Grand Ronde Basalts and accounting for 92% (Fig. 6) (Barry et al., 2013). Alternatively, only 3 were preceded by compositionally similar of all CRBG volume (Camp and Ross, 2004), Dinner Creek Tuff unit 1 and associated fallout fallout tuffs ca. 15.9 and ca. 15.7 Ma, based erupted in <1–1.5 m.y. starting ca. 16.9 Ma (Jar- tuffs would coincide with Grande Ronde Basalt on existing age information (Table 2). Dinner boe et al., 2010; Barry et al., 2013). Based on activity (Jarboe et al., 2010). Our geological evi- Creek Tuff unit 2 is almost as voluminous as the Barry et al. (2013) chronology, the Grande dence unequivocally indicates that equivalents to unit 1. Dinner Creek Tuff unit 4 was erupted Ronde Basalt eruptions took place from ca. 16 to Grande Ronde Basalt lavas are 16 Ma or younger near 15 Ma and appears to be the least volumi- 15.6 Ma, while the Jarboe et al. (2010) chronol- (see following). The last silicic eruptions leading nous of the four units. ogy indicates a beginning at 16.54 Ma and an to Dinner Creek Tuff unit 4 in either chronology

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TABLE 1. INCREMENTAL HEATING AGE DETERMINATIONS (OREGON STATE UNIVERSITY) AND SINGLE CRYSTAL AGES (NEW MEXICO TECH), DINNER CREEK TUFF Plateau Weighted Isochron Sample Material Age* Steps plateau 39Ar Plateau† Age Isochron identiﬁ cation Unit dated (Ma) ±2σ (n) (%) MSWD (Ma) ±2σ MSWD CR-U31c 1 plagioclase 15.95 0.09 9 92.38 0.58 15.88 0.17 0.61 CR-U10b 2 plagioclase 15.53 0.15 11 99.4 0.31 15.52 0.23 0.31 MS-PCIT1 2 plagioclase 15.48 0.13 12 100 0.49 15.49 0.15 0.46 CR-U2a 2 plagioclase 15.45 0.15 10 96.3 1.61 15.38 0.45 1.64 MS-11-20 4 anorthoclase 14.88 0.15 5 96.95 1.05 14.91 0.17 1.20

Number of Single crystal crystals MS-12-29 1 plagioclase 16.16 0.02 9 1.1 MS-12-38 3 anorthoclase 15.46 0.02 13 0.4 Note: All ages calculated relative to Fish Canyon Tuff sanidine age at 28.201 Ma (cf. Kuiper et al., 2008). MSWD—mean square of weighted deviate. Additional information in Supplemental Figures 1–3 (see text footnotes 3, 4, and 5, respectively). *Preferred weighted mean ages. †See Supplemental Figure 1 (text footnote 3).

TABLE 2. REGIONAL FALLOUT TUFFS ORIGINATING FROM DINNER CREEK TUFF ERUPTIVE CENTER Name of dated Reported age Source Recalculated age Correlated fallout tuff localities fallout tuff and locality (Ma) of age (Ma) (after Nash and Perkins, 2012) FCT, 27. 84* FCT, 28.201* Lough Ash–Succor Creek, southeast OR 15.46 ± 0.07 1 15.66 ± 0.07 Mascall ash–Mascall Formation at Picture Gorge, OR 15.77 ± 0.07 2 15.97 ± 0.07 Bully Creek Formation, OR; Virgin Valley, NV; White Bird, ID; Asotin, WA Buffalo Canyon, NV 15.9 ± 0.06 3 16.0 Paulina Basin, OR; Carlin Basin, NV FCT, 28.02* Carlin Basin, NV 16.3 ± 0.25 4 16.4 ± 0.25 Paulina Basin, OR (16.02)† (16.12)† Buffalo Canyon, NV Note: OR—Oregon; NV—Nevada; ID—Idaho; WA—Washington. Sources: 1—Downing and Swisher (1993); 2—Swisher (1992); 3—Nash and Perkins (2012; reﬂ ects stratigraphically interpolated age); 4—Wallace et al. (2008). *Age of Fish Canyon Tuff (FCT) used to calculate reported ages. †Correlation age, Wallace et al. (2008).

TABLE 3. DINNER CREEK TUFF ERUPTIVE CENTER Dinner Creek Tuff Fallout tuff Ma Unit Bulk Glass Feldspar Ma Localities Glass

16.15–16.0 1 hi-Si loCa-loFe An10 16.0 PB, BC, C loCa-loFe

15.95 M, BU, WB, A, BC, VV, UM? hiCa-miFe

15.66 SC loCa-hiFe

15.54–15.47 2 lo-/hi-Si hiCa-miFe An20 ê ?

15.46 3 lo-Si loCa-hiFe Anorth

14.99 4 dacite loCa-mi-hiFe Anorth

Oregon: BU (Bully Creek F.) M (Mascall), PB (Paulina Basin), SC (Succor Creek) hi-Si: high-silica rhyolite (>75% SiO2) UM (Umatilla; Ferns, unpubl.) lo-Si: low-silica rhyolite (< 75% SiO2) Nevada: BC (Buffalo Canyon), C (Carlin), VV (Virgin Valley) dacite (<69% SiO2) Idaho: WB (White Bird) loCa: low CaO (~0.5 wt.%) Washington: A (Asotin) hiCa: high CaO (~0.9 to >1 wt.%)

An10, An20: An content of plagioclase loFe: low FeO* (~2.0 wt.%) Anorth: Anorthoclase miFe: middle FeO* (~2.4 wt.%) hiFe: high FeO* (~2.8 wt.%)

are in the eruptive phase of the Wanapum 2002; Ferns and McClaughry, 2013). Field evi- Creek basalt and Hunter Creek Basalt, respec- Basalt (15.5–15 Ma, Barry et al., 2013; or 15.3– dence includes mafi c lavas underlying and over- tively (Cummings, 2000; Hooper et al., 2002; 14.6 Ma, Wolff and Ramos, 2013). lying Dinner Creek Tuff in the Malheur gorge Camp et al., 2003). The overlying Hunter Creek Eruption sites of mafi c units correlative with and the occurrence of dikes and other vent prox- Basalt has two distinct compositions, one basal- Grande Ronde Basalt are located peripherally imal deposits. Underlying and overlying mafi c tic andesitic and the other icelanditic, reaching

to the inferred sources of Dinner Creek Tuff lavas are fractionated and incompatible trace 60 wt% SiO2 (Ferns and McClaughry, 2013). magmas (Cummings et al., 2000; Hooper et al., element–rich basaltic andesites of the Birch The Birch Creek basalt and Hunter Creek Basalt

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suggested that these two magmas may have Dinner Creek Tuff: Prolonged Silicic erupted contemporaneously. We investigate Reservoir above CRBG Magma this further and use our compositional glass Storage Sites data obtained on shards, pumice shards, mafi c globules, and pumices that we found in one of The inference that mafi c components of the the two 15.5 Ma cooling units (unit 2) and in Dinner Creek Tuff are correlative with upper units the 14.9 Ma ignimbrite (unit 4). Comparing of the Grande Ronde Basalt allows us to identify these intermediate to mafi c components of the the DITEC as one location where mafi c magmas Dinner Creek Tuff to regionally occurring lavas of the CRBG resided in crustal reservoirs. The reveals the following. The more mafi c the com- intimate relationships between the mafi c com- Figure 6. Age relationships of ignimbrites ponents of the Dinner Creek Tuff are, the more ponents and rhyolites of the Dinner Creek Tuff (Table 1) and fallout tuffs (Table 2) origi- strongly they resemble regional CRBG magmas indicate that upper Grande Ronde Basalt com- nating from the Dinner Creek Tuff erup- (Fig. 5). The basaltic andesitic mafi c globules ponents were stored below a long-lived (1 m.y.) tive center (DITEC) in relation to eruption of the Dinner Creek Tuff have major and trace mid-Miocene rhyolite magmatic system. period of Grande Ronde Basalt lavas based element glass compositions within the range The details on the existence of the silicic on the Barry et al. (2013) and Jarboe et al. of basaltic andesitic composition of the Hunter reser voir (or reservoirs), such as whether it (2010) chronology. Creek and Birch Creek lavas, mafi c dikes of waxed and waned, and whether it existed con- Mormon Basin, basaltic andesites of Fiddlers tinuously or periodically, are not known. What- Hell, icelandites immediately north and south of ever the status and distribution of these rhyolites, are correlative with R1 and R2 lavas of the upper Ironside Mountain, and members of the Grande multiple eruptions led to crystal-poor rhyolites. Grande Ronde Basalt, respectively (Reidel and Ronde Basalt (Fig. 5). The icelanditic (~60–63 Some of the erupted rhyolite batches have some

Tolan, 2013; Barry et al., 2013). These occur on wt% SiO2) components of the Dinner Creek subtle, yet unique, chemical and mineralogical the southern side of the DITEC. In this regard, if Tuff are also similar to the regionally exposed, fi ngerprints. Such differences between Dinner Hunter Creek Basalt is correlative to Wapshilla more evolved Hunter Creek and Fiddler Hell Creek Tuff units 2 and 3, the ages of which are Ridge member of the Grande Ronde Basalt lavas, although there are some differences; for unresolvable by our new age data, suggest either (Reidel and Tolan, 2013), this would require that example, FeO* of ~9 wt% in Dinner Creek Tuff two compositionally silicic systems operating this R2 unit is ca. 16 Ma or younger, because glass compositions is lower than the ~11 wt% of nearby ca. 15.7–15.5 Ma, or that both evolved it overlies Dinner Creek Tuff unit 1 in Malheur regional lava fl ows (Fig. 5). In general, observed in short succession at the same locus. Wide- Gorge, which in turn would confl ict with an age differences are consistent with Dinner Creek spread fallout tuffs were either associated with of 16.3 based on the magnetostratigraphy of R2 Tuff icelanditic compositions having evolved ash-fl ow eruptions or preceded ignimbrites. (C5Cn.1r chron) (cf. Jarboe et al., 2010). from basaltic andesitic Hunter Creek magmas All rhyolites are compositionally very similar, On the northern side, Fiddlers Hell icelanditic slightly differently than icelanditic Hunter suggesting cogenetic or cospatial relationships lavas (15.51 ± 0.1 Ma; Ferns and McClaughry, Creek magmas. The difference of evolutionary (Fig. 2; Supplemental Table 1 [see footnote 1]). 2013), with compositions close to icelandites paths is likely due to mixing of Dinner Creek Therefore, petrogenetic stages leading to each of the Hunter Creek Basalt, erupted at several Tuff rhyolites with Hunter Creek magmas in rhyolite must have been nearly identical, gen- localities (near the towns of Baker City and La addition to fractional crystallization, similar to erating individual batches of chemically practi- Grande, Oregon) and constitute the top of the what is observed in the 7.1 Ma Rattlesnake Tuff cally unzoned rhyolites. Grande Ronde Basalt (Ferns and McClaughry, (Streck and Grunder, 1999). Evidence for that is In summary, persistent silicic reservoirs gen- 2013). We have also found icelanditic lavas with seen in icelanditic and dacitic Dinner Creek Tuff erating the erupted Dinner Creek Tuff rhyolites comparable compositions immediately north compositions plotting on mixing lines between developed above a major storage site of Grande and south of Ironside Mountain (Fig. 1), which rhyolites and basaltic andesitic compositions Ronde Basalt magmas that likely persisted for is the likely northern terminus of the DITEC. of Dinner Creek Tuff mafi c globules as well as several hundred thousand years (Table 3). The Dikes of basaltic andesitic compositions like Hunter Creek Basalt magmas (Fig. 5; Supple- storage site of Grande Ronde Basalt magma Hunter Creek Basalt were mapped 20 km north- mental Table 47). identifi ed here (i.e., DITEC) is within the area east of the proposed venting site for the Dinner Based on the compositional overlap com- of the previously hypothesized crustal magma Creek Tuff (Brooks, 2006). bined with concurrent ages of Hunter Creek and reservoir area of the CRBG (Wolff et al., 2008; other equivalents of Grande Ronde Basalt with Wolff and Ramos, 2013). DISCUSSION Dinner Creek Tuff unit 1, we make the inference that basaltic andesitic to icelanditic components Eruption versus Storage Sites of CRBG Mafi c Dinner Creek Tuff: Records of of the Dinner Creek Tuff are in fact components Grande Ronde Basalt Magmas of upper Grande Ronde Basalt magmas, even Numerous rhyolitic centers with ages rang- though they postdate the main eruption phase of ing from 16.5 to 15 Ma surround the DITEC in Close relationships in terms of time and Grande Ronde Basalt. eastern Oregon (Figs. 1 and 7) (Cummings et al., space of lavas of Hunter Creek Basalt (i.e., 2000; Ferns and McClaughry, 2013; Steiner upper Grand Ronde Basalt) with the Dinner 7Supplemental Table 4. LA-ICPMS trace elemen- and Streck, 2013; our data). Independent of the Creek Tuff have been found ~20 km southeast tal concentrations of mafi c to intermediate composi- model that leads to the production of rhyolites, of the proposed source area from where Evans tion glass in Dinner Creek Tuff. If you are viewing mafi c magma is ultimately involved in their pro- the PDF of this paper or reading it offl ine, please (1990) and Evans and Binger (1998) reported visit http:// dx .doi .org /10 .1130 /GES01086 .S7 or the duction either as heat source to induce melting, a mafi c vitrophyric lense with Hunter Creek full-text article on www .gsapubs .org to view Supple- as material source for fractional crystallization Basalt composition in Dinner Creek Tuff; they mental Table 4. processes, or generally to maintain rhyolites at

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logically with individual cooling units but also precede ignimbrite eruptions. Baker City The fi rst ash-fl ow tuff eruptions (unit 1, 16.1– 16 Ma) were the most silicic, producing high- Dooley Mtn. silica rhyolites. Later eruptions (units 2 and 3, Unity CRBG crustal 15.5 Ma) were high- to low-silica rhyolite, some John Day 16.5 15.5 storage site (Wolff et al., 2008) with ubiquitous dark pumices of dacitic composition. Bulk compositions of tuff deposited Strawberry DITEC Cottonwood during the last eruptions (unit 4, 14.9 Ma) are 16.2 Vale dacite, but the lower SiO in this unit is due to 16.1 2 substantial commingling of dacitic and ande sitic 16.3 Littlefield components with high-silica rhyolite. Major Buchanan OIG and trace element compositions of rhyolite of all four cooling units are surprisingly simi- Burns 16.1 LOVF15.9 lar and differ markedly from all other regional 16.0 Mahogany/ Miocene ash-fl ow tuffs in eastern Oregon. This Three Fingers 50 km evidence and current fi eld data on thickness and distribution suggest a common source area for all ignimbrites and fallout tuffs. This common Figure 7. Major 16–15 Ma regional rhyolite occurrences (lavas source we defi ne as the DITEC. domes and tuffs without Dinner Creek Tuff) surrounding Dinner Coeruptive basaltic andesitic magmatic com- Creek Tuff eruptive center (DITEC). Rhyolites in red based on ponents of the Dinner Creek Tuff are composi- Oregon Department of Geology and Mineral Industries database tionally indistinguishable from regional mafi c without Dinner Creek Tuff, rhyolites in pink were more or less lava fl ows belonging to the upper Grande Ronde unknown (Strawberry, Unity) or were thought to be younger. Solid Basalt of the CRBG, and this implies that Grande dashed lines show Lake Owyhee Volcanic Field (LOVF) and Oregon Ronde Basalt magmas were ponded below Din- Idaho graben (OIG) (Cummings et al., 2000); orange dashed oval ner Creek Tuff rhyolites. This effectively pins shows inferred DITEC; dashed gray oval shows inferred location down one location where CRBG magmas were of Columbia River Basalt Group (CRBG) crustal storage sites (by stored in the crust. Therefore, Dinner Creek Tuff Wolff et al., 2008). Numbers are ages (in Ma) and are near onset of rhyolites and other widespread 16–15 Ma rhyo- activity of select centers based on our data. lites of the area possibly acted as rheological and density barriers, allowing CRBG magmas stored underneath to erupt after rhyolites or in or near liquidus temperatures in the shallow crust other large-scale geologic features (Glen and peripheral areas after traveling in dikes for many (cf. Johnson, 1991). If the Dinner Creek Tuff Ponce, 2002; Figs. 1 and 4). The presence of kilometers, as proposed by Wolff et al. (2008). rhyolites are the result of a thermal pulse induced cognate Grande Ronde Basalt glassy globules in Our study highlights the close spatial and age by mafi c lavas of the CRBG, then other contem- the Dinner Creek Tuff supports the geographic relationship of mafi c magmas with rhyolites at poraneous silicic centers are likely also under- position of the centralized Grande Ronde stor- the youngest continental fl ood basalt province plated by CRBG magmas. Direct evidence for age area by Wolff et al. (2008). We therefore we know. Understanding rhyolites may provide storage of mafi c magmas beneath rhyolites, like hypothesize that the timing and location of erup- important complementary data on the existence the one documented here for the Dinner Creek tion sites for Grande Ronde Basalt and younger and location of storage areas of mafi c magmas. Tuff, is rare. Reports of coeruptive intermediate CRBG magmas were in part infl uenced by the to mafi c magmas during rhyolite eruptions are buffering effect of trapped rhyolitic crustal melts. ACKNOWLEDGMENTS more typically from ignimbrite eruptions rather This work was supported by a Portland State Uni- than lava fl ows (e.g., Streck and Grunder, 1999). CONCLUSIONS versity (PSU) Faculty Enhancement Grant and par- Rhyolite magma chambers in the upper crust tially supported by National Science Foundation grant can act as rheological and possible density bar- Our study focuses on the newly defi ned EAR-1220676 to Streck. We thank Barbara Nash and riers to prevent eruption of underlying mafi c DITEC, a long-lived silicic center in eastern John Wolff for detailed formal reviews and Shan de Silva and Matthew Brueseke for their editorial work, magmas (Valentine, 1993). Ponded mafi c mag- Oregon that is part of other numerous silicic all of which helped to improve this manuscript. We mas below rhyolites would either be redirected centers overlapping in time and space with fl ood thank the following PSU students: Arron Steiner, to peripheral areas or erupt after rhyolites. Wolff basalt volcanism of the CRBG and thus demon- who provided data on outcrops and compositions of et al. (2008) suggested that the main CRBG strating abundant rhyolite activity during fl ood the Dinner Creek Tuff within the Strawberry volcanic magmas (Steens, Imnaha, and Grande Ronde basalt stage. fi eld; Erik Shafer and Tracy Handrich, who prepared undergraduate theses on glass and feldspar composi- Basalts) were staged in a common area in the The Dinner Creek Tuff, formerly considered tions; and Chris Ricker, who collected and analyzed crust before migrating laterally along a series of a single ignimbrite deposit, is determined to Dinner Creek Tuff samples in the area around the town radial fractures to form the well-known Colum- consist of a minimum of 4 cooling units erupt- of Unity. The paper was written while Streck was on a bia River Basalt dike swarms. The DITEC and ing over a time span of 1 m.y from ca. 16 to sabbatical stay at the University of Hannover and the 2 University of Stuttgart, Germany; he thanks his respec- known contemporaneous rhyolite centers are at 15 Ma and covering ~25,000 km . Well-known tive hosts Francois Holtz and Hans-Joachim Massone the central, uplifted core area at the intersection fallout tuff deposits in Oregon and elsewhere for their support, and Gerlinde Ehlers and Wolfgang of aeromagnetic lineaments, dike swarms, and can be matched both chemically and chrono- Ehlers for their hospitality at Neustadt, Hannover.

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