Voluminous and Compositionally Diverse, Middle Miocene Strawberry Volcanics of NE Oregon: Magmatism Cogenetic with ﬂ Ood Basalts of the Columbia River Basalt Group

OLD G

The Geological Society of America Special Paper 538 OPEN ACCESS

Voluminous and compositionally diverse, middle Miocene Strawberry Volcanics of NE Oregon: Magmatism cogenetic with ﬂ ood basalts of the Columbia River Basalt Group

Arron Steiner Department of Geology, Portland State University, P.O. Box 751, Portland, Oregon 97207-0751, USA, and Department of Geology, Washington State University, P.O. Box 642812, Pullman, Washington 99164-2812, USA

Martin J. Streck Department of Geology, Portland State University, P.O. Box 751, Portland, Oregon 97207-0751, USA

ABSTRACT

The mid-Miocene Strawberry volcanic fi eld of northeastern Oregon is an example of intracontinental fl ood volcanism that produced lavas of both tholeiitic and calc- alkaline compositions derived by open-system processes. Until now, these dominantly calc-alkaline lavas have not been considered to have a petrogenetic origin similar to that of the fl ood basalts of the Pacifi c Northwest because of their calc-alkaline composition. These lavas are situated in between and co-erupted with the domi- nant volcanic fi eld of the Columbia River Basalt Group (CRBG). Due to the timing, location, and diversity of these erupted units, the Strawberry Volcanics may hold valuable information about the role of crustal modifi cation during large magmatic events such as hotspot volcanism. The earliest eruptions of the Strawberry Volca- nics began at 16.2 Ma and appear continuous to 15.3 Ma, characterized by low-silica rhyolite. High-silica, A-type rhyolite eruptions followed at 15.3 Ma. The silicic eruptions continued until 14.6 Ma, with an estimated total volume up to ~100 km3. The fi rst eruptions of the intermediate lava fl ows occurred at 15.6 Ma and continued with both tholeiitic and calc-alkaline, and transitional, lavas until 12.5 Ma. Vol- ume estimates of the intermediate lavas are ~1100 km3. The mafi c lavas are sparse (~2% of total volume) and are distributed throughout the upper sequences, and they appear to be near last to arrive at the surface. Herein, we show that the Strawberry Volcanics are not only related in time and space to the Columbia River Basalt, but they also share some chemical traits, specifi cally to the Steens Basalt. Evidence of this similarity includes: overlapping normalized incompatible trace-element patterns, selected trace-element ratios, and radiogenic isotopes. Furthermore, we compared the Strawberry rhyolites to the other mid-Miocene rhyolites of eastern

Steiner, A., and Streck, M.J., 2018, Voluminous and compositionally diverse, middle Miocene Strawberry Volcanics of NE Oregon: Magmatism cogenetic with ﬂ ood basalts of the Columbia River Basalt Group, in Poland, M.P., Garcia, M.O., Camp, V.E., and Grunder, A., eds., Field Volcanology: A Tribute to the Distin- guished Career of Don Swanson: Geological Society of America Special Paper 538, p. 41–62, https://doi.org/10.1130/2018.2538(03).

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Oregon associated with the inception of the Yellowstone–Snake River Plain hotspot and found overlapping eruption ages, trace and rare earth element compositions, and “A-type” rhyolite characteristics. This research concludes that the Strawberry Volca- nics were part of the regional basalt to rhyolite magmatism of the Yellowstone–Snake River Plain hotspot.

INTRODUCTION spatial extent and geologic relationships to regionally occurring, stratigraphically younger and older units, and delineating their The name “Strawberry Volcanics” was given to a diverse general geochemical and petrographic characteristics. These group of volcanic rocks that crop out along the southeastern works established the overall calc-alkaline character and preva- margin of the John Day valley of NE Oregon and that are mid- lence of andesitic lavas of this volcanic ﬁ eld (Fig. 2). It was also Miocene in age (Fig. 1; Thayer, 1957). Thayer (1957), Brown noted that these voluminous andesites are located in an intracon- and Thayer (1966a, 1966b), and Robyn (1977) published the tinental setting and were not generated due to subduction pro- ﬁ rst works on the Strawberry Volcanics, providing data on their cesses (Robyn, 1979). The only subsequent work of note on the

Figure 1. Regional map and simplifi ed geologic map. (A) Location of the Strawberry Volcanics and regional lavas of the Columbia River Basalt Group: MD—Monument dikes, SD—Steens dikes, CJ—Chief Joseph dikes, SRP—Snake River Plain–Yellowstone hotspot, SV—Strawberry Volcanics, MT—Montana, NV—Nevada, OR—Oregon, UT—Utah, CA—California, ID—Idaho, WA—Washington. Dashed line outlines the Snake River Plain, and circles inside the dashed line are inferred volcanic centers. Figure of the Columbia River Basalt Group lavas is molded after Camp and Ross (2004). Solid red line outlines the area shown in B, and the dashed rectangle shows area covered in Figure 3B. (B) Simplifi ed geologic map showing Strawberry Volcanics and bordering area. Field observation–identifi ed volcanic centers (asterisks) and dikes (dashed lines) are included on map: SM—Strawberry Mountain, BRM—Bull Run Moun- tain, BV—Bear Valley, CM—Canyon Mountain, HM—High Mountain, ISM—Ironside Mountain, and LV—Logan Val- ley. Stars represent towns of John Day (JD), Seneca (Sen), and Unity (UN). The geological units surrounding the Straw- berry Volcanics are based on Thayer (1956), Crowley (1960), Brown and Thayer (1966a, 1966b, 1977), Thayer et al. (1967), Greene et al. (1972), Robyn (1977), Brooks and Ferns (1979), Ferns et al. (1983), and Mullen (1983).

Figure 2. (A) Total alkali–silica diagram (Le Bas et al., 1986) with data of this study showing the compositional diversity of the volcanic suite of the Strawberry Volcanics. (B) Miyashiro (1974) tholeiitic and calc-alkaline diagram showing that maﬁ c to intermediate compositions of the Strawberry Volcanics display both tholeiitic and calc-alkaline afﬁ nities.

Strawberry Volcanics after the 1950s to 1970s was that by Goles et al. (1989), who presented isotopic data for basaltic andesites of the Slide Creek member (Thayer, 1957), located in the NNE por- tion of the fi eld. No other recent data exist on this enigmatic mid- Miocene calc-alkaline volcanic fi eld, which is surrounded by the tholeiitic lavas of the Columbia River Basalt Group. However, these earlier studies have brought to light aspects that make the Strawberry Volcanics an important fi eld to understand, including: (1) the petrogenetic evolution of a voluminous volcanic fi eld dominated by intracontinental, calc-alkaline andesites; (2) the relationship of mafi c magmas to the surrounding contemporaneous tholeiitic fl ood basalts of the Columbia River Basalt Group; and (3) the context of the largely unknown rhyolites of the Straw- berry Volcanics compared to other mid-Miocene silicic centers associated with the inception of the Yellowstone–Snake River Plain hotspot (Fig. 3). Lavas of the Columbia River Basalt Group have been heavily studied and have received considerable attention, but the volcanic fi elds spatially and temporally associated with the Figure 3. Regional distribution of major 16–15 Ma rhyolite centers (af- Columbia River basalts that erupted calc-alkaline lavas have ter Streck et al., 2015, and references therein) in relation to rhyolites of remained relatively understudied (Hooper et al., 2002; Camp the Strawberry Volcanics (in pink). DITEC—Dinner Creek Tuff erup- et al., 2003; Ferns and McClaughry, 2013). Calc-alkaline lavas tive center, depicting postulated source for four cooling units of the associated with the Columbia River Basalt Group are generally Dinner Creek Tuff (Streck et al., 2015); LOVF—Lake Owyhee volcanic fi eld; OIG—Oregon-Idaho graben, shown by solid dashed lines thought to have erupted around 13.5 Ma and to mark the transi- (Cummings et al., 2000). Numbers are ages in Ma and are near onset tion between the tholeiitic fl ood basalt stage and the Basin and of activity of select centers, except for rhyolites of the Strawberry Vol- Range extension calc-alkaline phase (Hooper et al., 2002; Camp canics, for which activity period is indicated.

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et al., 2003; Ferns and McClaughry, 2013). However, the studies lava fl ows, and domes to intermediate lava fl ows, all ranging in from Brueseke and Hart (2009) showed that calc-alkaline lavas age from 39 to 22 Ma (Robinson et al., 1984; McClaughry et of intermediate composition from the Santa Rosa–Calico volca- al., 2009; Ferns and McClaughry, 2013). Within the surround- nic fi eld were derived from tholeiitic magmas of Steens Basalt ing outcrop area of the Strawberry Volcanics, exposures of John interacting with the crust during the main fl ood basalt phase of Day Formation were found to be minimal (Brown and Thayer, the Columbia River Basalt Group. 1966a, 1966b). The goals of this paper are the following: (1) to identify The main outcrop area of the Strawberry Volcanics is bor- lavas of the Strawberry Volcanics based on physical charac- dered by units from the Columbia River Basalt Group: lavas teristics, mineralogy, geochemistry, and stratigraphy; (2) to of Picture Gorge Basalt to the west and northwest, lavas of the reevaluate the timing and stratigraphy of the Strawberry Vol- Grande Ronde Basalt to the north, lavas of the Imnaha Basalt to canics with 40Ar/39Ar age dates, placing particular emphasis the east to southeast, and Steens Basalt lavas to the south (Fig. 1). on time and space relationships to Columbia River Basalt The main pulse of the Columbia River Basalt Group erupted tho- Group magmas as previous ages include 19.8 ± 0.38 Ma to leiitic mafi c lavas during the middle Miocene (ca. 16.7–15.6 Ma), 12.4 ± 0.41 Ma suggesting onset of magmatism of the Straw- and it is the most widespread and voluminous Cenozoic volca- berry Volcanics occurred prior to the Columbia River Basalt nic unit within the Pacifi c Northwest (230,000 km3; Camp and Group; (3) to investigate the petrogenetic relationships of mafi c Hooper, 1981; Baksi, 1989; Camp and Ross, 2004; Barry et magmas of the Strawberry Volcanics to surrounding Columbia al., 2013; Camp et al., 2013, 2017; Reidel et al., 2013). Specifi - River Basalt Group units; and (4) to describe the rhyolites of cally, the ages of the Columbia River Basalt Group eruptions and the Strawberry Volcanics and explore their association with duration are: Steens Basalt 16.7–16.4 Ma; Imnaha Basalt 16.7– other mid-Miocene rhyolites associated with the Yellowstone– 16.0 Ma; Grande Ronde Basalt 16.0–15.6 Ma; Wanapum Basalt Snake River Plain hotspot. 16–15.4 Ma; and Saddle Mountains Basalt 14.0–5.5 Ma (Barry et al., 2013; Camp et al., 2013, 2017). Details of Columbia River GEOLOGIC SETTING Basalt Group chronology are still a topic of controversy and of ongoing research. These magmas erupted from mafi c shield Geologic History volcanoes and fi ssures that are now preserved as dike swarms (Wilcox and Fisher, 1966; Swanson et al., 1975; Brueseke et al., The lavas of the Strawberry Volcanics are underlain by the 2007; Bondre and Hart, 2008; Brown et al., 2014). Locations of pre-Tertiary, Paleozoic, and Mesozoic accreted terrane rocks of the dikes that produced the lavas of the Columbia River Basalt the Blue Mountains Province and Mesozoic sediments of the Group surround the Strawberry Volcanics (Fig. 1). Izee terrane (Robyn, 1977; Schwartz et al., 2010; LaMaskin et Several regional felsic ignimbrites, including the Dinner al., 2011). A Paleozoic section of the Baker terrane that crops Creek Tuff, ca. 16.2–14.9 Ma (Streck et al., 2015), Devine Can- out immediately adjacent west of Strawberry Mountain, Oregon, yon Tuff, ca. 9.7 Ma (Jordan et al., 2004), and the Rattlesnake locally known as Canyon Mountain, is a presumed ophiolite and Tuff, ca. 7.1 Ma (Streck and Grunder, 1995), reach into the area includes mafi c-ultramafi c rocks, serpentinite, and chert-argillite of the Strawberry Volcanics. Units of the Dinner Creek Tuff are mélange (Himmelberg and Loney, 1980; LaMaskin et al., 2011; intercalated among intermediate and silicic units of the Straw- Gerlach et al., 1981). Fine-grained mudstone/siltstone, argillite, berry Volcanics and younger tuffs overlie. and limestone of the Izee terrane crop out in areas southwest of Strawberry Mountain near the town of Seneca, Oregon (Fig. 1; METHODS Brown and Thayer, 1966a, 1966b). Southwest of Strawberry Mountain, these sedimentary rocks form discontinuous units. Whole-Rock Geochemistry East of Strawberry Mountain, Jurassic argillite and limestone and Cretaceous intermediate to silicic dikes and sills are exposed Major and Trace Elements near Bull Run Mountain and Ironside Mountain (Fig. 1; Thayer Fresh samples were selected for whole-rock major- and et al., 1967). trace-element analysis. Sample preparation followed the analyti- Rocks of the Clarno and John Day Formation are exposed cal procedures of the Washington State University Peter Hooper throughout the vicinity of the Strawberry Volcanics (Fig. 1). GeoAnalytical laboratory following Johnson et al. (1999) for The Clarno Formation is composed of nonmarine volcanic and X-ray fl uorescence (XRF) and inductively coupled plasma– volcaniclastic units ranging from 54 to 39 Ma in age (Swanson mass spectrometry (ICP-MS) analysis1. Samples were analyzed and Robinson, 1968). These volcanic products are thought to be derived from subduction zone magmatism and are strongly calc- alkaline with hydrous minerals phases (amphibole and biotite), 1GSA Data Repository Item 2018233, Appendix DR1—X-ray fl uorescence yet some transitional tholeiitic compositions do occur (Rogers and inductively coupled plasma–mass spectrometry data and Appendix DR2—Isotopes, is available at www. geosociety.org/datarepository/2018/, or and Novitsky-Evans, 1977). The John Day Formation is com- on request from [email protected] or Documents Secretary, GSA, P.O. posed of volcanic units ranging from silicic to intermediate tuffs, Box 9140, Boulder, CO 80301-9140, USA.

using a ThermoARL Advant’XP+ sequential XRF spectrometer and an Agilent 7700 ICP-MS. The data provided were normal- MSWD

ized to anhydrous values, and the total Fe content is reported as Isochron wt% FeO*. σ

Radiogenic Isotopes ±2 Sr, Nd, and Pb isotopic compositions were measured on 13 samples representing maﬁ c, intermediate, and rhyolitic composi- Ar

tions. Sample dissolution and chemistry were performed at the 39

radiogenic isotope clean laboratory and analyzed using thermal Ar/ 40 intercept ionization mass spectrometry (TIMS) at New Mexico State Uni- versity, Las Cruces, New Mexico. Samples were powdered and

weighed (<1 g) and digested in acid. Once completely dissolved, σ es. MSWD—mean square of weighted es. the solution was then run through a quartz column ﬁ lled with ±2 chromatography resin. Sr isotopic ratios were fractionation- corrected using 86Sr/88Sr = 0.1194, and the NBS 987 Sr standard

resulted in an average value of 0.710304 (n = 3). Nd isotopic Age (Ma) ratios were normalized to 143Nd/146Nd = 0.7219, and results for the La Jolla and JNdi-1 standards were 0.511854 (n = 1) and 0.512106 (n = 1), respectively. Pb isotopes were doped with NBS 997 Ti and normalized to 205Ti/203Ti = 2.387075. Results for NBS 981 were 206Pb/204Pb = 16.932, 207Pb/204Pb = 15.486, and MSWD 208Pb/204Pb = 36.680 (n = 6). Age corrections for Sr and Nd iso-

topic ratios were applied using the ratio of Rb and Sr whole-rock Ar Plateau element concentrations. 39 %

40Ar/39Ar Geochronology The 40Ar/39Ar geochronology was conducted at the Argon

Geochronology Research Laboratory, Oregon State University steps Plateau (OSU), Corvallis, Oregon, on a total of 15 samples: 10 groundmass concentrates, three plagioclase separates, one biotite, and Plateau Isochron one glass sample. All samples were loaded in quartz vials with σ ±2 small quantities of mineral monitor FCT-3 (28.030 ± 0.003 Ma; Renne et al., 1998) and irradiated at the OSU TRIGA research reactor. Obtained ages were later recalculated using a Fish Can- Age (Ma)

yon Tuff (FCT) age of 28.201 Ma (Kuiper et al., 2008). All DETERMINATIONS AGE HEATING INCREMENTAL 1. TABLE

samples were analyzed by the furnace incremental heating age # spectrum method using a Mass Analyzer Products (MAP) Model 215-50 mass spectrometer and analyzing between eight and 12 heating steps (Table 1). Further details of the analytical proce-

dures are described in Duncan and Keller (2004). All new age Material dated data cited in the text are quoted at 2σ (Table 1). § A GM 15.57 0.16 8 94.37 0.95 15.75 0.30 5.00 0.09 0.79 AA GM GM 14.59 14.21 0.26 0.26 8 – 99.20 88.91 0.61 4.34 14.97 14.03 0.40 0.29 5.21 5.23 0.14 0.10 20.96 2.76 A GM 14.87 0.13 6 87.43 0.63 14.83 0.25 5.61 0.09 0.76 A GM 12.52 0.12 9 98.84 0.19 12.48 0.13 4.79 0.03 0.26 B GM 12.61 0.08 13 47.27 2.56 12.65 0.24 4.46 0.08 2.75 R GM 16.16 0.17 7 84.00 8.38 16.27 0.37 5.21 0.12 32.08 RRR Plag GM GM 15.34 14.62 0.52 15.30 0.06 0.1 8 11 7 98.38 14.41 89.20 0.36 1.48 1.85 15.50 14.61 0.92 15.27 0.08 0.16 4.86 5.11 5.23 0.29 0.02 0.31 0.05 1.63 2.17 RR Glass Biotite 14.79 14.70 0.12 0.13 10 8 100.00 89.22 0.62 2.16 14.98 14.78 0.19 0.18 4.93 4.81 0.06 0.04 16.05 1.89 BABA GM GM 15.59 – 0.36 – 4 72.07 – 4.56 – 16.18 0.98 – 5.46 15.29 0.33 0.06 4.00 5.43 0.005 – BA GM 13.76 0.16 9 85.41 1.04 13.92 0.35 5.13 0.12 1.03 BA GM 13.53 0.24 5 70.78 2.25 14.10 0.48 5.50 0.18 0.99 RADIOMETRIC AGES Rock type Rock † Previous Ages of Strawberry Volcanics – – – – – – TH TH TH TH CA CA CA CA CA Robyn (1977) determined a 7-m.y.-long activity period for CA the Strawberry Volcanics, from 19.8 ± 0.38 Ma (2σ) to 12.4 ± 0.41 Ma (2σ), based on six K/Ar age dates. Of the ages reported by Robyn (1977), a single basalt lava at 14.9 ± 0.85 Ma (2σ) and

a basaltic andesite at 12.4 ± 0.41 (2σ) Ma conform best with our in this paper are the plateau ag ages referred Preferred standard R98 (4E36-14) = 28.201 ± 0.023 Ma. (FCT-NM) Tuff Fish Canyon geochronological study of the Strawberry Volcanics. On the other CA—calc-alkaline, TH—tholeiitic. CA—calc-alkaline, R—rhyolite. BA—basaltic andesite; A—andesite; Plag—plagioclase. GM—groundmass; † § # *Repeat. Note: AS-SV-151 AS-SV-291 *AS-SV-291 AS-SV-159c Sample Composition *AS-SV-144 AS-SV-179 AS-SV-156 AS-SV-144 AS-SV-173 AS-SV-188 AS-SV-82 AS-SV-14 AS-SV-230 AS-SV-190 AS-SV-109 deviates. deviates. hand, the oldest ages of 19.8 ± 0.38 Ma, 19.1 ± 0.65 Ma, and AS-SV-192

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σ 18.1 ± 0.50 Ma (2 ) that Robyn (1977) reported for lava ﬂ ows Strawberry rhyolites range from ~70 to 78 wt% SiO2 and are signiﬁ cantly older than any of our ages. Field and lithological are peraluminous (which is likely a feature of postemplacement characteristics make these units very unlike any other units of alkali mobility) to mildly peralkaline (Fig. 5). Major elements the Strawberry Volcanics. They are dacitic in composition and follow typical trends with silica enrichment, such as decreasing

contain large phenocrysts of plagioclase (~1–3 cm) and amphi- CaO, FeO*, and Al2O3, and trace-element concentrations vary bole, which matches up with units from the John Day Formation most in Sr (9–250 ppm), Zr (66–450 ppm), Ti (300–3500 ppm), described earlier herein much better. and Ba (350–1630 ppm; Table 2). We normalized our data to upper-crust values and the rhyolites plot around 1, indicating a Summary of New 40Ar/39Ar Results similar composition but with pronounced depletions in Sr, P, and Ti (Fig. 6). New 40Ar/39Ar ages from this investigation indicate that basaltic andesite to rhyolite lavas of the Strawberry Volca- Basaltic to Intermediate Lava Flows nics erupted from 16.16 ± 0.17 Ma to 12.52 ± 0.12 Ma (Fig. 4; Table 1) (cf. Steiner and Streck, 2013). Rhyolites yielded the The Strawberry Volcanics primarily consist of lava fl ows oldest ages, ranging from 16.16 ± 0.17 Ma to 14.70 ± 0.13 Ma, of basaltic andesite and andesite composition (Fig. 2) that cover which is consistent with stratigraphic fi eld observations described 3600 km2 of the Malheur National Forest and Strawberry Moun- next (Fig. 4; Table 1). Among intermediate lavas, those of calc- tain Wilderness (Fig. 1). The best exposures of these lava fl ows alkaline affi nity erupted over the longest duration, as indicated by are found in the vicinity of Strawberry Mountain proper, a gla- ages ranging from 15.59 ± 0.36 Ma to 12.52 ± 0.12 Ma (Fig. 4; cially carved mountain with a maximum elevation of ~3000 m, Table 1). The tholeiitic lavas produced a slightly narrower range for which the Strawberry Volcanics were named (Fig. 1). Within of ages of 15.57 ± 0.16 Ma to 13.53 ± 0.24 Ma. This may not be this mountain range, the total thickness of numerous basaltic signifi cant, but what is clear is that tholeiitic and calc-alkaline andesite and andesite lava fl ows is uniformly ~1000 m. Across volcanism were coeval (Fig. 4; Table 1). One basalt sample this range, individual lava fl ows show constant thicknesses of yielded an age of 12.61 ± 0.08 Ma (Fig. 4; Table 1). Other basalts ~5–10 m, with little columnar jointing, but instead display a thin are intercalated near the middle of a stratigraphic sequence of (tens of centimeters), platy appearance. Volume estimates can be lava fl ows at Strawberry Mountain and thus have ages within the conservatively calculated by using the volume of a cone, assum- range of intermediate lavas. ing that the lava thicknesses decrease from Strawberry Mountain toward the outer edges of the volcanic fi eld. If we use a radius of STRATIGRAPHY AND ERUPTIVE UNITS 32 km and a thickness of 1 km, the result is ~1100 km3, which is likely an underestimation given our approximation. Rhyolite Volcanism Calc-Alkaline Intermediate Volcanism (15.60–12.5 Ma) Rhyolites of the Strawberry Volcanics, hereafter named The oldest (15.59 ± 0.36 Ma) intermediate lava of the Strawberry rhyolites, erupted as lava domes and coulees, and Strawberry Volcanics is a calc-alkaline basaltic andesite, and fallout and ash-fl ow tuffs. Rhyolites are generally located in the youngest (12.52 ± 0.12 Ma) is an andesite vent plug located the lower part of the stratigraphy, sitting on top of Izee ter- at High Mountain (Table 1; Fig. 2). We base the calc-alkaline rane sediments or between both calc-alkaline and tholeiitic defi nition on discrimination schemes by Miyashiro (1974) and

intermediate lavas (Fig. 4). Where the rhyolites are in direct Arculus (2003). These lavas range in SiO2 (wt%) from 52.7% to contact with the intermediate lavas, there is no evidence of a 64.5% and generally form linear or collinear trends with typical signiﬁ cant time gap (e.g., paleosol or unconformities). Dikes increases or decreases as silica increases from basaltic andesite

of maﬁ c to intermediate composition crosscut these rhyolite to dacite in major elements (Fig. 2). Al2O3 ranges from 16.0 to lavas and tuffs. Phenocrysts are mostly plagioclase and quartz 17.9 wt%, and MgO and FeO* decrease with increased silica, and occasionally biotite and amphibole. Microphenocrysts are ranging from 6.2 to 2.0 wt% and 9.2 to 5.2 wt%, respectively rare (<1%) but are seen on occasion and include plagioclase, (Fig. 7). These lavas display little to no increase in FeO*/MgO

quartz, and minor pyroxene, and accessory minerals include ratios toward higher SiO2 contents (Fig. 2). Incompatible ele- zircon and apatite. These lavas are often devitrifi ed or include ments behave variably within the calc-alkaline suite of the Straw- obsidian bands between devitrifi ed sections. Ash-fl ow tuffs and berry Volcanics from basaltic andesite to dacite (Fig. 7; Table 3). fallout deposits, associated with vents proximal to this location, Trace elements that increase include Rb, Ba, Pb, and U, but all are dispersed throughout the area and intercalated among rhyo- other trace elements, including rare earth elements (REEs) are

litic lavas or are stratigraphically younger. The volume is dif- nearly constant or slightly decrease with increasing SiO2 content fi cult to estimate because of outcrop exposure, erosion, and the (Fig. 7; Table 3). expected variability in thicknesses of domes and rhyolite fl ows Calc-alkaline lava fl ows vary from aphyric to phenocryst- at emplacement. Nevertheless, we estimated a total erupted poor to containing 30% phenocrysts. The typical calc-alkaline lava rhyolite volume of between 40 and 100 km3. type is aphyric or contains <5% phenocrysts. The aphyric-type

Downloaded from http://pubs.geoscienceworld.org/books/book/chapter-pdf/4815635/spe538-03e.pdf by guest on 29 September 2021 Voluminous and compositionally diverse, middle Miocene Strawberry Volcanics of NE Oregon 47 c units are labeled to the right of . Ages of speciﬁ . 15 and unpublished) and previously dated Strawberry units dated Strawberry 15 and unpublished) previously

Figure 4. Fence diagrams for the Strawberry Volcanics. Sections are labeled A–H and can be correlated to locations on inset map Sections are labeled Volcanics. Figure 4. Fence diagrams for the Strawberry diagram. All ages in black are from this study. Ages in red are for rhyolitic units of the Dinner Creek Tuff (Streck et al., 20 Tuff Ages in red are for rhyolitic units of the Dinner Creek All ages in black are from this study. diagram. Basalt Group. in meters. CRBG—Columbia River (Robyn, 1977). Numbers on left of diagram are elevations

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Figure 5. Major-element and selected trace-element diagrams of the Strawberry Mountain rhyolites, including

(A) A/CNK-A/NK alkalinity diagram, where A/CNK = Al2O3/(CaO + Na2O + K2O), and A/NK = Al2O3/(Na2O + K2O); (B) SiO2 (wt%) vs CaO (wt%); (C) SiO2 (wt%) vs FeO*(wt%), where FeO* is the sum of both FeO and Fe2O3 (wt%); and (D–F) A-type rhyolite discrimination diagrams. Solid line in F represents the dividing line for A-type rhyolites from Whalen et al. (1987).

fl ows are best exposed along the lower section below 2000 m in at Baldy Mountain, located in the north-central area of the Straw- the cirque walls of Strawberry Mountain near Strawberry Lake. berry Volcanics (Fig. 1), despite that depictions of the Monument The calc-alkaline lavas that show greater amounts of phenocrysts Dike Swarm typically include those. Some vent site lavas have have glomerocrysts consisting of small (1–2 mm) plagioclase, microplutonic textures (e.g., the micro-norite dikes and plugs of pyroxene, and minor olivine and oxides. The more phenocrystic- Strawberry Mountain and High Mountain) and were previously rich calc-alkaline lavas are dispersed throughout the Strawberry thought to be a feature related to a long-lived, stratovolcanic edi- Mountain vicinity. fi ce that supplied the lavas for the Strawberry Volcanics (Robyn, Numerous dikes are exposed throughout the Strawberry 1977). There is no evidence for large single composite volcanoes, volcanic fi eld and appear to be the main conduit system for such as thickening of lava fl ows away from the vent locations magma extrusion at the surface through fi ssures or small-scale throughout the Strawberry Volcanics. Rather, we see lava fl ows central vent volcanoes (Fig. 1; Steiner and Streck, 2013). These extend in uniform thicknesses for tens of kilometers away from dikes are often represented by topographic highs and crosscut the vent typical of shield volcanoes or lava fl ow fi elds. lava fl ows of the Strawberry Volcanics and associated terranes (Fig. 1). Most dikes strike NNW-SSE, similar to dikes of the Tholeiitic Basalt and Intermediate Volcanism nearby (~40 km) Monument dike swarm, which gave rise to the (15.60–13.50 Ma) Picture Gorge Basalt unit of the Columbia River Basalt Group The tholeiitic lavas of the Strawberry Volcanics are geo- (Fruchter and Baldwin, 1975; Thayer, 1957). Dikes within the chemically similar to the calc-alkaline lavas and comprise com-

Strawberry volcanic ﬁ eld are clearly part of the Strawberry Vol- positions ranging from basalt to andesite with SiO2 ranging from canics and not the Monument dike swarm, based on geochemical 47.5 to 60.4 wt% (Fig. 2). These lavas are relatively high in

and petrographic similarities to the lavas of the Strawberry Vol- Al2O3 (15.4–18.2 wt%), and they vary in MgO (2.6–7.2 wt%; canics. Several dikes form a radial pattern around a central vent Fig. 7; Table 3). Incompatible element ranges are comparable

TABLE 2. STRAWBERRY MOUNTAIN RHYOLITES AS-SV- 66 144 151 173 263 175 63 133 190 98 60 179 37 Major oxides measured by X-ray ﬂ uorescence (wt% normalized)

SiO2 69.85 72.99 73.19 73.33 74.62 75.44 76.80 76.88 76.93 76.97 77.71 78.45 78.72

TiO2 0.59 0.35 0.35 0.27 0.26 0.21 0.06 0.05 0.19 0.09 0.10 0.15 0.14

Al2O3 15.17 15.00 14.57 14.24 12.98 12.96 13.20 13.35 12.66 13.16 12.40 12.36 11.64 FeO* 3.46 2.29 2.23 2.02 1.97 1.89 0.80 0.78 1.19 0.93 0.85 0.30 0.84 MnO 0.04 0.04 0.05 0.05 0.05 0.02 0.04 0.04 0.04 0.04 0.03 0.00 0.00 MgO 0.53 0.25 0.23 0.57 0.72 0.00 0.03 0.02 0.13 0.08 0.08 0.00 0.04 CaO 2.70 2.03 1.99 2.21 1.77 0.06 0.53 0.52 0.81 0.69 0.68 0.45 0.06

Na2O 4.45 3.53 3.86 3.28 3.27 5.20 3.93 3.52 4.12 3.33 3.76 3.99 4.28

K2O 3.07 3.47 3.42 3.96 4.30 4.20 4.59 4.83 3.91 4.69 4.37 4.28 4.25

P2O5 0.15 0.05 0.10 0.08 0.05 0.03 0.01 0.01 0.02 0.02 0.02 0.02 0.03 Pre- normalized 98.37 95.01 98.09 95.79 97.05 98.66 99.51 95.33 98.60 95.33 99.42 99.01 98.84 total Trace elements (ppm) Ni 3.4 3 2.5 4.5 0.40 3.4 1.9 1.5 3 2.3 0.8 3.4 1.3 Cr 4.7 4.6 5.4 6.7 9.18 3.5 2.9 3.3 3.2 3.4 3.6 3.4 3.6 V 43.1 25 29.8 27.1 35.91 4.2 1.2 2.7 5.2 2.1 1.9 2.7 3.6 Ga 17.4 14.6 15.7 15.5 14.86 24.7 17.9 17 14.4 16.7 13.7 16 21.2 Cu 5.3 2.8 7.2 6.4 9.18 1.8 0.4 0 2.7 0.4 1.1 0.5 0.0 Zn 63.4 43.4 48.5 42.1 35.11 132.7 34.0 38.7 35.2 34.2 25.4 21.2 41.3

AS-SV- 66 144 151 173 263 175 63 133 190 98 60 179 37 Trace elements (ppm), measured by inductively coupled plasma–mass spectrometry Sc 10.1 6.6 5.8 6.2 7.3 1.7 4.4 5.3 4.7 4.4 3.6 5.3 2.8 Ba 1283 1450 1485 1192 1342 1626 969 349 1255 1288 1445 1195 493 Rb 65.4 84.2 86.7 91.7 98.3 78.2 111.9 119.6 90.5 105.9 117.1 100.4 89.1 Sr 252 258 231 198 101 32 33 18 77 56 58 45 9 Zr 236 172 171 139 109 448 81 71 176 97 102 220 451 Y 28.75 19.14 17.93 24.79 23.31 61.17 36.42 45.95 35.96 33.14 22.89 43.88 52.60 Nb 11.89 9.16 8.78 8.28 8.32 25.23 19.27 23.39 12.29 15.57 8.88 13.62 23.99 Pb 11.75 13.68 13.91 13.30 13.62 14.02 15.50 15.42 13.10 14.49 16.11 14.31 7.11 U 2.43 3.42 3.67 4.70 3.72 3.49 4.42 4.79 3.42 4.43 4.58 4.04 1.85 Th 5.91 7.35 7.70 8.74 8.61 7.82 10.80 10.44 8.46 10.88 10.50 8.89 9.41 Cs 1.59 4.32 4.20 4.42 4.50 1.54 3.96 4.82 3.32 4.30 5.53 4.27 1.34 Hf 5.93 4.59 4.59 3.99 3.57 10.87 3.57 3.60 5.38 3.77 3.66 6.37 11.14 Ta 0.82 0.74 0.74 0.83 0.78 1.57 1.79 2.15 1.01 1.46 0.86 1.02 1.68 La 29.40 24.86 25.41 22.27 25.11 49.89 27.72 17.84 29.51 37.87 28.82 36.91 42.57 Ce 55.22 43.51 46.23 43.55 47.22 87.43 55.33 38.55 59.07 69.36 54.48 70.96 79.27 Pr 6.88 5.14 5.36 5.16 5.43 12.49 6.81 5.02 7.04 8.73 6.04 8.95 11.51 Nd 26.16 18.81 19.62 19.21 19.38 48.20 25.38 19.95 26.15 31.35 21.18 33.40 43.85 Sm 5.39 3.69 3.82 4.32 4.03 10.60 5.62 5.62 5.67 6.25 4.10 7.12 9.78 Eu 1.30 0.77 0.88 0.77 0.61 1.82 0.66 0.42 0.80 0.89 0.46 0.66 1.12 Gd 5.11 3.24 3.35 3.98 3.72 10.01 5.60 6.14 5.36 5.56 3.64 6.82 8.96 Tb 0.84 0.54 0.54 0.68 0.65 1.76 1.02 1.20 0.97 0.97 0.64 1.20 1.63 Dy 5.23 3.33 3.26 4.28 4.02 11.07 6.47 7.94 6.22 5.92 3.98 7.73 10.57 Ho 1.11 0.68 0.67 0.89 0.85 2.33 1.35 1.66 1.31 1.23 0.84 1.62 2.17 Er 3.08 1.95 1.88 2.55 2.42 6.52 3.84 4.74 3.88 3.49 2.42 4.57 6.03 Tm 0.46 0.30 0.29 0.39 0.38 0.97 0.58 0.73 0.60 0.53 0.37 0.69 0.91 Yb 3.03 2.02 1.93 2.61 2.49 6.07 3.70 4.74 3.96 3.53 2.49 4.34 5.85 Lu 0.49 0.33 0.32 0.42 0.39 0.91 0.60 0.74 0.63 0.56 0.41 0.69 0.87

to calc-alkaline lavas, although high ﬁ eld strength elements Tholeiitic lavas of both basaltic and andesitic composition (HFSEs) and REEs tend to be higher in more-evolved tholeiitic are dispersed throughout the stratigraphy of the Strawberry Vol- compositions (e.g., Figs. 7 and 8; Table 3). Normalized incom- canics (Fig. 4). Lavas with tholeiitic afﬁ nity are minor in volume patible trace-element patterns indicate distinct spikes at Ba and relative to calc-alkaline lavas. Tholeiitic intermediate lavas are Pb, and occasionally Ta and Nb have small troughs relative to phenocryst poor (<10%), similar to the phenocryst-poor calc- neighboring elements (Fig. 8). alkaline lavas in textures and modal mineral abundances, making

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Figure 6. (A) Upper continental crust normalized trace-element diagram. (B) Chondrite-normalized rare earth element plot of rhyolites of the Straw- berry Volcanics. Normalization values for upper continental crust are from Taylor and McLennan (1995), and chondrite from McDonough and Sun (1995).

them essentially indistinguishable in the fi eld. The vents for tho- (≤2% modal abundance). Groundmass textures can vary and leiitic lavas are assumed to have been similar to those for calc- can include ophitic texture, much like the basalts, but minerals alkaline lavas. Tholeiitic basalt lavas tend to be more massive are smaller in size (<1 mm). When phenocyrsts do occur they and cohesive in texture and lack the platy texture of the calc- include plagioclase, clinopyroxene, or olivine, or a combina- alkaline lava fl ows. These tholeiitic basalts have vesicles and tion of all three, never exceeding >10% by volume. Plagioclase commonly display diktytaxitic texture. Several tholeiitic basalts phenocrysts can be ~2 mm in length and are often anhedral to are located in the Strawberry Mountain cirque walls and can be subhedral, zoned, and with sieved and resorbed textures. When distinguished from the calc-alkaline and tholeiitic intermediate present, clinopyroxene and olivine phenocrysts range in size lavas by an ophitic texture (Table 3). Tholeiitic basalt to andesitic from 1 to 2 mm. lavas are distributed throughout the Strawberry Volcanics and are associated with nearby vent structures (e.g., dikes), which were DISCUSSION identifi ed at Strawberry Mountain proper and south to southwest of this range and east, near the town of Unity, Oregon (Fig. 1). Synopsis of Emplacement History of Tholeiitic basalt lava fl ows are characterized by the presence the Strawberry Volcanics of plagioclase, clinopyroxene, olivine, and Fe-Ti oxide. The most prevalent petrographic texture of the basalt lavas is the ophitic The Strawberry Volcanics, spanning the compositional range texture. Plagioclase is the main mineral phase, with a modal from basalt to rhyolite, have an eruption history that lasted 4 m.y. abundance of ~60%, and it ranges in size from <1 mm to 2 mm The fi rst eruption of the Strawberry Volcanics, located in the south, and is often tabular and euhedral. Oikocrysts of clinopyroxene occurred at 16.2 Ma with rhyolite lavas and domes. Succeeding (augite) are the sole pyroxene phase present, and they have a eruptions of calc-alkaline and tholeiitic intermediate lavas started modal abundance of ~30%, ranging in size from 1 mm to 3 mm. at ca. 15.6 Ma. Activity continued with eruptions of dominantly Small olivine phenocrysts (~1 mm in size) account for up to 7%. intermediate calc-alkaline lavas, accompanied by tholeiitic basalt Acicular Fe-Ti oxides are ~.05 mm in length and account for and intermediate tholeiitic lavas. Eruptions of rhyolite near the ~3% of the total visible crystals. Minor amounts of glass occupy northwestern margin of Strawberry Mountain began at 15.3 Ma the interstitial zones between crystal laths. Neither obvious dis- and continued up to 14.8 Ma. These eruptions produced aphyric equilibria of phenocrystic phases are evident nor do any xenoliths lavas forming domes and minor pyroclastic events. During this or glomerocrysts occur in thin sections of tholeiitic samples. time, eruptions at the early southernmost rhyolite fi eld contin- Intermediate lavas of tholeiitic composition are largely ued and appear to cease at 14.7 Ma. Low-volume, mafi c tholei- phenocryst-poor lavas. Groundmass crystals consist of plagio- itic eruptions appear sporadically throughout the volcanic fi eld. clase (65% modal abundance) + clinopyroxene (30% modal The youngest intermediate lava fl ow with tholeiitic affi nity is abundance) + oxides (≤3% modal abundance) and ± olivine 13.5 Ma, and we take this as the time when tholeiitic volcanism

Figure 7. Diagrams of select major and trace elements versus SiO2 for basaltic to intermediate composition lavas. Major elements are MgO, FeO, and Al2O3 (A–C), and trace elements include Rb, Ba, La, Nb, and Zr (D–H).

ceased. Tholeiitic volcanism was likely superseded by eruptions tions of the Columbia River Basalt Group (Table 1; Jarboe et al., of calc-alkaline lavas, which possibly erupted last at 13.8 Ma, as 2008; Barry et al., 2013; Camp et al., 2013, 2017). Field evidence suggested by our current age data. However, an intrusive calc- has shown that the eruption style of the mafi c to intermediate alkaline micro-norite dike at Strawberry Mountain was dated at lavas of the Strawberry Volcanics is akin to the eruptions of the 12.5 Ma, but it is unclear if any lava fl ows were produced at the Columbia River Basalt Group. Despite our noted compositional surface (Table 1). differences (i.e., calc-alkaline) compared to Columbia River Basalt Group magmas, eruptions of Strawberry Volcanics were Relationships of Mafi c Strawberry Volcanics to Flood fed from dikes leading to fi ssure eruptions at the surface. In fact, Basalt Magmatism of the Columbia River Basalt Group a number of authors (e.g., Camp and Ross, 2004; Brueseke and Hart., 2009) have shown on maps that the Monument dike swarm The close temporal and spatial relationships between the of the Columbia River Basalt Group reaches into the area of the Strawberry Volcanics and units of the Columbia River Basalt Strawberry Volcanics (Fig. 1). It is our conclusion, based on Group invite the question as to the extent to which there is a pet- geochemical data and geographical location, that these mapped rogenetic relationship, despite the predominantly calc-alkaline dikes are in fact dikes of the Strawberry Volcanics. Next, we will ≤ nature of the Strawberry Volcanics (Fig. 1). New ages presented discuss the geochemistry of mafi c lavas ( 56 SiO2 wt%) of the herein demonstrate that silicic and mafi c to intermediate magma- Strawberry Volcanics with regard to differences with and simi- tism of the Strawberry Volcanics was ongoing during the erup- larities to the various units of the Columbia River Basalt Group.

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TABLE 3. GEOCHEMICAL DATA FOR BASALT TO ANDESITE FROM STRAWBERRY VOLCANICS AS-SV- 287 109 11 217 15 16 17 285b 39b 233 32 283 156 181 Major oxides measured by X-ray fluorescence (wt% normalized)

SiO2 47.48 49.43 50.47 52.06 52.71 52.78 55.14 55.31 55.40 55.77 55.80 55.88 58.08 58.15

TiO2 2.87 2.12 1.86 1.60 1.64 1.57 1.48 1.36 1.45 1.38 1.54 1.24 1.68 1.08

Al2O3 15.36 17.16 16.17 17.09 16.26 16.45 16.78 17.64 17.71 17.83 16.98 17.62 16.31 16.89 FeO* 13.74 11.78 10.65 9.61 9.44 9.18 8.18 8.31 9.95 8.60 8.33 7.83 8.76 7.32 MnO 0.21 0.19 0.19 0.17 0.16 0.16 0.15 0.17 0.22 0.17 0.15 0.13 0.12 0.13 MgO 7.22 5.93 6.69 5.82 6.15 6.24 4.87 3.89 2.66 2.96 3.91 4.30 2.67 3.59 CaO 9.77 9.28 9.52 8.41 8.24 8.38 7.75 7.06 7.20 6.92 7.21 7.71 6.47 7.14

Na2O 2.39 3.09 3.09 3.27 3.55 3.33 3.44 4.03 4.01 4.08 3.96 3.63 3.51 3.68

K2O 0.34 0.45 0.76 1.39 1.24 1.31 1.66 1.45 0.75 1.55 1.48 1.17 1.75 1.53

P2O5 0.62 0.57 0.59 0.58 0.60 0.59 0.55 0.77 0.65 0.72 0.64 0.49 0.64 0.47 Pre- normalized 95.58 98.17 99.62 99.55 99.10 99.37 97.59 99.59 97.72 98.36 98.89 99.74 97.05 98.93 total Trace elements (ppm) Ni 79 73 69 91 96 99 64 30 45 29 45 50 14 14 Cr 211 179 167 102 193 204 106 51 55 48 67 78 29 54 V 285 278 265 233 213 204 183 151 120 139 178 177 209 159 Ga 18 21 18 19 18 18 18 19 19 19 19 19 19 18 Cu 38 46 50 72 63 55 58 48 23 52 55 49 22 24 Zn 134 126 102 102 100 99 92 107 89 101 101 91 102 90 AS-SV- 287 109 11 217 15 16 17 285b 39b 233 32 283 156 181 Trace elements (ppm), measured by inductively coupled plasma–mass spectrometry Sc 31.8 30.0 29.1 23.3 24.6 23.5 21.2 19.5 25.5 19.6 20.5 20.4 21.8 20.2 Ba 415 485 431 577 565 571 612 717 760 707 685 676 875 806 Rb 2.5 4.1 8.7 12.8 16.3 15.5 18.2 14.3 4.2 17.5 19.7 14.1 31.2 20.0 Sr 343 523 527 564 525 544 572 660 510 638 583 546 483 469 Zr 166 164 119 143 156 154 170 181 152 182 202 160 165 170 Y 38.59 34.97 25.92 24.58 27.03 26.31 25.13 29.44 40.87 28.99 34.59 23.72 30.29 28.56 Nb 14.34 13.56 11.79 10.64 16.47 16.23 15.89 20.56 10.43 20.64 19.61 12.48 11.03 11.28 Pb 3.87 4.98 3.50 5.15 5.35 4.77 5.51 6.02 4.00 6.21 5.91 5.95 7.00 7.06 U 0.23 0.33 0.41 0.43 0.52 0.55 0.62 0.65 0.49 0.67 0.74 0.65 1.01 0.90 Th 0.89 1.37 1.23 1.22 1.57 1.61 1.80 1.96 2.06 1.96 2.02 1.91 2.84 2.36 Cs 0.12 0.13 0.18 0.19 0.18 0.29 0.37 0.38 0.11 0.33 0.34 0.36 1.68 0.48 Hf 4.17 4.02 2.85 3.41 3.74 3.70 4.10 4.35 3.67 4.35 4.49 3.78 4.18 4.05 Ta 0.88 0.80 0.68 0.60 0.95 0.94 0.92 1.16 0.61 1.17 1.13 0.75 0.70 0.65

La 22.53 26.97 19.63 22.22 23.47 23.48 24.37 29.92 25.95 29.46 30.11 23.69 23.67 25.85 Ce 53.82 56.13 42.55 45.59 50.17 49.67 51.55 63.32 47.90 64.03 60.48 48.75 48.09 49.17 Pr 7.40 7.78 5.67 5.94 6.57 6.54 6.66 8.18 7.03 8.16 7.88 6.26 6.69 6.69 Nd 33.29 32.99 24.50 24.90 27.52 27.29 27.23 33.08 29.60 33.41 31.77 25.43 28.10 27.06 Sm 8.16 7.62 5.61 5.46 6.14 6.01 5.99 6.89 6.67 7.04 6.86 5.49 6.47 5.91 Eu 2.66 2.47 1.90 1.75 1.99 1.91 1.89 2.09 2.09 2.08 2.12 1.71 1.90 1.73 Gd 8.21 7.50 5.46 5.08 5.74 5.71 5.51 6.19 6.67 6.26 6.39 5.05 6.23 5.66 Tb 1.32 1.18 0.84 0.82 0.89 0.88 0.86 0.97 1.07 0.96 1.01 0.79 0.99 0.90 Dy 7.96 7.05 5.06 4.69 5.28 5.19 4.99 5.67 6.72 5.62 6.05 4.68 5.89 5.36 Ho 1.57 1.42 1.01 0.95 1.05 1.01 0.99 1.13 1.39 1.12 1.24 0.91 1.17 1.11 Er 4.13 3.72 2.68 2.53 2.76 2.67 2.57 3.08 3.86 3.00 3.35 2.45 3.09 2.96 Tm 0.57 0.51 0.38 0.36 0.38 0.38 0.36 0.44 0.56 0.43 0.47 0.35 0.44 0.43 Yb 3.46 3.15 2.29 2.22 2.36 2.36 2.28 2.74 3.44 2.72 2.93 2.14 2.72 2.67 Lu 0.54 0.49 0.35 0.35 0.39 0.37 0.37 0.45 0.57 0.45 0.49 0.35 0.42 0.42

≤ Maﬁ c ( 56 SiO2 wt%) lavas have compositions closest to Basalt Group lavas, speciﬁ cally with the Steens and Imnaha

the mantle input that gave rise to the more evolved intermedi- Basalts (Fig. 9). The higher SiO2 (52–56 wt%) lavas of the Straw- ate Strawberry magmas. We used the data of Wolff et al. (2008) berry Volcanics lie within the general trend of the Columbia River as representative of the Columbia River Basalt Group magmas. Basalt Group lavas in some elements, but they also trend away Comparison of major-element compositions shows overlap of toward increased silica in other elements. They overlap with the

the basalt lavas of the Strawberry Volcanics with Columbia River Grande Ronde Basalt in MgO, CaO, and TiO2 contents but diverge

Figure 8. (A) Primitive mantle normalized trace element. (B) Chondrite diagram for the calc-alkaline and tholeiitic suite of lavas analyzed. Normalization values are from Sun and McDonough (1989) and McDonough and Sun (1995), respectively.

≤ Figure 9. Major-element geochemistry of lavas with 56 wt% SiO2 from the Strawberry Volcanics in comparison to Columbia River Basalt Group lavas. Basalts of the Strawberry Volcanics overlap mostly with the Steens Basalt lavas and with Picture Gorge Basalt lavas while basaltic andesites and andesites mostly deﬁ ne their own space. Geochemi- cal data from the Columbia River Basalt Group are from Wolff et al. (2008).

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in Al2O3, FeO*, and P2O5 contents, with notably higher Al and P tal ranges occurs with the Strawberry Volcanics and Imnaha and and lower Fe concentrations (Fig. 9). This suggests that crustal Grande Ronde lavas, yet some elements also fall out of the range evolution does have a signifi cant impact on magmas within the compared to Steens Basalt (Fig. 10). For example, the Strawberry basaltic andesites, although incompatible trace elements may be Volcanics are enriched in Ba, Nb, Sr, and P, and enriched in the less affected than major elements and compatible trace elements. light (L) REEs (La through Nd) relative to the Imnaha Basalt On normalized incompatible trace-element diagrams and (Fig. 10). Compared to the Grande Ronde Basalt, the Strawberry REE diagrams, the greatest overlap in terms of overall pattern Volcanics are enriched in Nb, Sr, and P, but are signifi cantly with the lavas of the Strawberry Volcanics is observed with lower in Rb, Th, and U (Fig. 10). Samples of the Strawberry Vol- samples of Steens and Imnaha Basalts, as all samples of the canics are distinctly different from Picture Gorge Basalt in most Strawberry Volcanics are within the same range (Fig. 10). How- elements from Rb to Eu, but Picture Gorge Basalt interestingly ever, when compared with the average Steens Basalt pattern (or shows the same Ti troughs as samples of the Strawberry Volca- Imnaha Basalt), the Strawberry Volcanics have distinct Zr-Hf and nics (Fig. 10). Ti depletions, whereas average Steens Basalt has only a minor Diagrams using incompatible trace element ratios (e.g., depletion in Zr-Hf, and none in Ti, and Imnaha Basalt does not Ba/Zr, La/Y, U/Nb) show that the overlap of Strawberry Volca- have any of these depletions (Fig. 10). Another difference is that nics samples with Steens Basalt is again greater than with other some samples of the Strawberry Volcanics are enriched in P, groups (Fig. 11), and that magmas of the Strawberry Volcanics which could be due to select enrichment during crustal assimila- are clearly offset from magmas of the Grande Ronde and Picture tion (Streck and Grunder, 2012). Signifi cant overlap of elemen- Gorge Basalts in plots of U/Nb or La/Y, respectively (Fig. 11).

Figure 10. Primitive mantle normalized trace-element and chondrite normalized REE (rare earth element) diagrams (insets) compar- ≤ ing the maﬁ c ( 54 SiO2 wt%) tholeiitic and calc-alkaline lavas of the Strawberry Volcanics (black lines) with lavas of the Columbia River Basalt Group. For the Columbia River Basalt Group units, the transparent color indicates the unit range and the solid colored line is the unit average. The Strawberry Volcanics mostly overlap with the Steens and Imnaha samples. Normalization values are the same as in Figure 8.

≤ Figure 11. (A–E) A comparison of selected trace element ratios of the Strawberry Volcanic lavas ( 54 SiO2 wt%) with lavas of the Columbia River Basalt Group. A signiﬁ cant overlap of Strawberry basalts to andesites is visible with Steens lavas.

Whole-rock radiogenic isotopic data for samples of the greater than differences to the Strawberry Volcanics. The domi- Strawberry Volcanics show that the Strawberry Volcanics over- nant volume of the Strawberry Volcanics is at least contempo- lap with the Columbia River Basalt Group but also defi ne their raneous to eruptions of Wanapum Basalt units, but may even own isotopic space between these groups (Fig. 12; Table 4). Ini- overlap in age with upper Grande Ronde Basalt lavas—this is tial Sr isotopic data of the Strawberry Volcanics show a strong a matter of ongoing controversy and is currently unresolved. similarity to Imnaha Basalt and some minor overlap with the The Strawberry Volcanics have the greatest similarity to Steens Steens Basalt and Picture Gorge Basalt groups. Initial Nd iso- Basalt lavas with respect to trace elements but also indicate some topic values of Strawberry Volcanics are also similar to Imnaha unique geochemical features, e.g., isotopic ratios (Figs. 10, 11, and Grande Ronde Basalts, while 206Pb/204Pb value data are par- and 12) that set them apart. Some of these differences may have ticularly distinct, with much lower values than Imnaha Basalt been controlled by open-system processes as silica increased and some overlap with the other Columbia River Basalt Group and may in turn have been partly due to the mechanisms that (Fig. 12). On the other hand, Strawberry Volcanics overlap with ultimately caused most Strawberry Volcanics to develop calc- the Grande Ronde lavas in Pb isotopic ratios (Fig. 12) and plot alkaline affi nity, such as melting of local basement lithologies between the three mantle components proposed to be involved and differentiation during assimilation and fractional crystalli- in the Columbia River Basalt Group plume (Carlson, 1984). The zation processes (Brueseke and Hart, 2009; Steiner and Streck, trend of intermediate samples away from the basalt may refl ect 2013). It is surprising, however, that the Strawberry Volcanics a more complicated process of assimilating country rock. This, have more trace-element commonalities with Steens Basalt however, will be the focus of another paper. than with the immediately neighboring (~40 km) Picture Gorge To summarize, geochemical differences that exist among Basalt (Figs. 10 and 11). This may be because the Strawberry formal members for the Columbia River Basalt Group are Volcanics are slightly more evolved than the Picture Gorge

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Figure 12. Initial radiogenic isotopic ratios of the Strawberry Volcanics and ratio ﬁ elds for the Columbia River Basalt Group (Wolff et al., 2008). Plotted in D are the C1, C2, and C3 components of the Columbia River Basalt Group from Carlson (1984) and the hypothetical mantle source H after Hart (1985). Open symbols for Strawberry Volcanics indicate data from Goles et al. (1989).

Basalts. Isotopic data argue for a similar melting source to the Strawberry Rhyolites within the Context of the Mid- one for Steens Basalt and Imnaha Basalt in some respect and Miocene Rhyolite Flare-Up for Picture Gorge Basalt in others, but the Strawberry Volca- nics also defi ne their own isotopic space among the Columbia McDermitt caldera and other rhyolitic centers, such as the River Basalt Group lavas (Fig. 12). Based on the information Santa Rosa–Calico volcanic fi eld and the High Rock caldera provided here, we propose that the mantle upwelling giving rise complex located near the Oregon-Nevada border, have been to Columbia River Basalt Group is also causing magmatism of viewed as the fi rst centers of rhyolitic volcanism associated the Strawberry Volcanics and that observed geochemical differ- with the Columbia River Basalt Group fl ood basalts of the ences lie in melting slightly different mantle domains as well as Pacifi c Northwest starting at 16.5 Ma (Pierce and Morgan, locally driven open system processes. 1992; Brueseke et al., 2008, 2014; Coble and Mahood, 2012;

Pb Benson et al., 2017). Recent age determinations of long-known

204 mid-Miocene rhyolitic centers throughout eastern Oregon have

Pb/ yielded ages that are about as old as those along the Oregon- 206 Nevada state boundary and that range from 16.5 to 15.9 Ma

Pb (Streck et al., 2015; Coble and Mahood, 2016; Benson and 204 Mahood, 2016; Henry et al., 2017). These ages are from rhyo- Pb/

207 lites of the Lake Owyhee volcanic ﬁ eld and periphery, from the Dinner Creek Tuff, and also from as far north as near Baker Pb

204 City (Streck et al., 2015). Strawberry rhyolites were largely

Pb/ unknown until recently and thus were neither included as an 208 area of signiﬁ cant rhyolite volcanism nor as one of the “earli-

Nd est” mid-Miocene rhyolite centers related to Columbia River ε Basalt volcanism, despite the K-Ar age of 17.3 ± 0.36 Ma

Nd reported by Robyn (1977). Our currently oldest age on rhyo-

144 lites of the Strawberry Volcanics is 16.2 Ma (Table 1), which

Nd/ indicates that rhyolite activity at Strawberry Mountain indeed 143 started at a time when Columbia River Basalt Group volcanism

Nd was ongoing (Jarboe et al., 2010; Barry et al., 2013; Camp et 144 al., 2013, 2017). For this reason, Strawberry rhyolites also Nd/ need to be included among the earliest mid-Miocene rhyolite 143 centers that could be viewed as “plume-head”–related rhyo- Nd

144 lites. Thus, the emerging picture is that mid-Miocene rhyolitic activity in eastern Oregon (east of ~119°W) started up between Sm/

147 16.5 and 16 Ma across a wide area. The area is bounded by the following towns: Baker City in the north, Ontario in the east, McDermitt in the south, Buchanan in the west, and John Day in the northwest (Fig. 3). Strawberry rhyolites range from I-type to A-type rhyolites, = 0.1964, where CHUR—chondritic reservoir. uniform similar to other mid-Miocene eastern Oregon rhyolites (Fig. 5; CHUR

Sr Sm Nd Streck, 2014). Figure 13 is a comparison of trace elements 86 Nd) between other mid-Miocene rhyolites and Strawberry rhyolites. 144 Sr/ 87 At low Zr (ppm) concentrations (<300 ppm), the Strawberry Sm/

147 rhyolites have similar TiO2 and FeO contents (wt%) and Nb and Ba (ppm) contents as those from the High Rock caldera complex (Coble and Mahood, 2016) and the McDermitt caldera (Henry et Sr Age

86 al., 2017; see Fig. 13). The Strawberry rhyolites that have high Sr/ = 0.512638; ( = 0.512638; Zr (ppm) concentrations (>300) also are similar to samples of 87 the Dinner Creek Tuff (Streck, 2014), Lake Owyhee volcanic CHUR Sr

86 ﬁ eld (Benson and Mahood, 2016), High Rock caldera complex Nd) 144

Rb/ (Coble and Mahood, 2016), and the McDermitt caldera (Henry 87 Nd/ et al., 2017) in TiO2 (wt%), FeO (wt%), Nb (ppm), and Ba (ppm) 143 contents (Fig. 13). Models for rhyolites associated with the Columbia River Basalt Group are highly debated but, as elsewhere, are domi- TABLE 4. ISOTOPIC ANALYSIS OF MAFIC AND INTERMEDIATE LAVAS OF THE STRAWBERRY VOLCANICS (SV) VOLCANICS THE STRAWBERRY OF LAVAS OF MAFIC AND INTERMEDIATE ANALYSIS ISOTOPIC 4. TABLE nated by two different scenarios. In one, rhyolites were gener- Rb Sr = 6.54E–12; ( = 6.54E–12;

λ ated by partial melting of the crust (Clemens and Wall, 1984;

Nd Munksgaard, 1984; Pichavant et al., 1988a, 1988b; Gunnarsson 2 et al., 1998; Smith et al., 2003); the second is an origin through fractional crystallization from maﬁ c magmas (Michael, 1984; Bacon and Druitt, 1988; Mahood and Halliday, 1988; Hildreth, = 1.42E–11;

λ 1981; DePaolo et al., 1992; Hildreth and Fierstein, 2000; Lind- Sr say et al., 2001; Clemens, 2003). Variants of the partial melting model include degrees of differentiation and/or mixing follow- Note: *Tholeiitic lavas. Maﬁ c to intermediate lavas of the Strawberry volcanics c to intermediate lavas Maﬁ AS-SV-287*AS-SV-11* 47.48AS-SV-203b*AS-SV-289 50.47 53.30 2.49AS-SV-39b* 343.46AS-SV-56 14.09 53.52 8.67 0.0210 55.40 575.28AS-SV-159c* 527.15 0.70527 0.0709AS-SV-231 0.0476 9.54 57.59 56.68 14.5 0.70409 4.23AS-SV-188 413.84 0.70452 509.56 of the Strawberry volcanics lavas 14.5Rhyolite 0.0667 25.39 0.70526 61.70 22.02 14.5 0.0240 435.22AS-SV-151 0.70376 510.95 62.64 0.70408 8.16 0.70451 32.21 0.70435 0.1688 0.1248AS-SV-173 14.5 33.29 7.04 435.63 5.61 37.47 14.5 0.70392AS-SV-190 0.70441 73.19 0.70375 31.78 0.2140 410.48 24.50 15.48AS-SV-179 0.70435 14.5 0.1483 73.33 4.28 0.2642 0.70397 0.70388 86.65 AS-SV-179 0.512494 6.67 0.1338 76.93 0.70439 18.35 0.1386 0.70404 14.5 231.02extra 91.66 6.58 0.512480 0.512787 29.60 78.45 0.512773 5.44 14.5 1.0857 197.66 90.49 0.70393 28.51 0.512774 100.42 –2.4 0.1410 25.03 0.512760 1.3422 0.70429 0.70399 3.90 0.1361 76.82 0.512838 0.70444 16.06 45.10 38.889 0.1395 3.3 0.512822 3.4096 4.59 18.18 3.0 0.1313 0.512825 0.70405 6.4450 14.61 0.512875 20.86 0.70499 0.512809 78.45 38.549 0.512826 15.598 0.70416 38.544 0.70576 3.82 0.512861 0.1296 4.3 14.7 100.42 0.512814 15.21 4.32 19.62 4.0 0.512839 0.1331 15.494 15.593 0.70428 18.549 38.512 5.0 0.70437 45.10 19.21 0.512796 0.512827 4.0 38.519 6.4450 5.67 0.1176 7.12 38.466 18.844 0.512783 15.604 18.826 38.474 0.513046 26.15 0.1360 0.70583 4.3 33.40 15.608 0.512819 15.21 3.4 0.513034 15.600 38.475 18.873 15.593 0.1311 0.70444 0.1290 0.512806 18.888 38.501 8.4 0.512892 0.512833 18.856 7.12 15.602 18.855 3.9 0.512879 0.512820 33.40 38.484 15.606 38.801 18.846 5.3 4.2 15.596 0.1290 18.875 15.715 38.506 38.558 18.868 15.595 19.046 15.620 18.900 18.994 Sample SiO (wt%) (ppm) (ppm) present (Ma) initial (ppm) (ppm) ing present melting initial initialof the crust. In contrast, variants of the fractional

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Figure 13. Trace-element diagrams comparing Strawberry rhyolites (SR) to other mid-Miocene rhyolites associated with

the onset of the Yellowstone–Snake River Plain hotspot: (A) TiO2 vs. Zr, (B) FeO vs. Zr, (C) Nb vs. Zr, and (D) Ba vs. Zr. Other rhyolite ﬁ elds include: Mahogany Mountain caldera (MMC; Benson and Mahood, 2016), Dinner Creek Tuff (DIT; Streck et al., 2015), High Rock caldera complex (HRCC; Coble and Mahood, 2016), and McDermitt caldera (MD; Henry et al., 2017).

crystallization model involve variations in the initial composi- (e.g., Rollinson, 1993), but titanite has not been observed either tion of the parental magma that underwent differentiation (i.e., in andesites or in rhyolites. Furthermore, titanite is only stable in

how mafi c and enriched it was) to yield rhyolites. In the case of high-fo2 rhyolites and would produce high Nb/Ta values, given the Strawberry rhyolites, low HFSE concentrations are key fac- that partition coeffi cients are slightly higher in Ta, which is not tors to narrow down possible generation scenarios. All rhyolites seen in our data (Rollinson, 1993). The same constraint could that have lower or equal low concentrations of Nb and Ta than be formulated for the trace elements Zr and Hf, but since zircon intermediate or basaltic Strawberry magmas could not be gener- is a stable phase in more silicic magmas, it could have lowered ated via fractional crystallization (Fig. 14). Nb and Ta behave Zr and Hf contents during a very late stage. Although a plau- incompatibly throughout the compositional range from basalt to sible scenario, it is not very likely because other trace-elemental rhyolite. Therefore, differentiation-dominated scenarios would parameters that are typically affected by differentiation processes inevitably raise Nb and Ta contents and would lead to concentra- within the rhyolitic fi eld do not carry a strong late fractionation tion levels higher than values observed in the low HFSE rhyo- signal. These parameters include Ba contents and Eu/Eu*; both lites. The only mineral that could fractionate Nb and Ta is titanite are still high in most rhyolites, in other words ≥1200 ppm Ba and

Figure 14. (A) Rb versus Nb and (B) Ta versus Nb diagrams including a fractional crystallization model showing the melt evolution path starting with a basalt of the Strawberry Volcanics to 63% crystallization in 7% increments. Frac- tionation models are based on partition coefﬁ cients for Rb of 0.071, 0.031, and 0.02; for Ta of 0.04, 0.022, and 0.0025; and for Nb of 0.01, 0.0067, and 0.0013 for plagioclase, clinopyroxene and orthopyroxene, respectively (Rollinson, 1995 and Green et al., 2000). The modal proportions used are based on observed mineral assemblages and are as follows; 68% plagioclase, 17% clinopyroxene and 15% orthopyroxene.

≥0.4 Eu/Eu* (Table 2). This makes a partial melting model to mafi c magmas of the Strawberry Volcanics share petrochemical generate many of the Strawberry rhyolites compelling. Isotopic signatures with lavas of the Columbia River Basalt Group, there- compositions of the rhyolites vary and appear to have multiple fore strongly suggesting Columbia River Basalt Group lavas and sources, likely including pelitic rocks of the Baker terrane and the Strawberry Volcanics had a similar petrotectonic origin. Specifi - Cretaceous granodiorites (Table 4). cally, mafi c lavas of the Strawberry Volcanics are most similar in trace-element chemistry to the Steens Basalt and indicated some CONCLUSION overlap in radiogenic isotope ratios with the Picture Gorge Basalt, but they also defi ne their own isotopic characteristics. Toward In this paper, we have described the various units of the higher silica contents, intermediate magmas of the Strawberry Strawberry Volcanics and established the timing and duration of Volcanics appear to be increasingly a product of open-system eruptions. The following is a summary of volcanic events: differentiation (assimilation and fractional crystallization and (1) The southernmost silicic centers start erupting at 16.16 ± magma mixing) processes that causes their compositional trends. 0.17 Ma, producing plagioclase-rich, mostly low-silica rhyolite Our new ages on previously little known Strawberry rhyolites lava fl ows and lava domes, and ash-fl ow tuffs. High- and low- indicate they erupted during the height of the rhyolite fl are-up silica rhyolitic volcanism continued in this area until 14.62 ± coincident with Columbia River Basalt Group volcanism. These 0.06 Ma and was co-eruptive with other phases of the Straw- rhyolites are not fractionates of the mafi c or intermediate lavas of berry Volcanics. At 15.30 ± 0.10 Ma, the northern rhyolites the Strawberry Volcanics or Columbia River Basalt Group mag- erupted aphyric, peralkaline, A-type high-silica rhyolites, co- mas, but instead appear to have been generated by partial melting eruptive with the southern rhyolites. Activity continued here of the crust. Comparison with other mid-Miocene rhyolites of until 14.79 ± 0.12 Ma. Oregon that are associated with fl ood basalt volcanism indicates (2) Intermediate lavas of calc-alkaline composition began many rhyolites of the Strawberry Volcanics are less enriched in erupting through dikes starting at 15.59 ± 0.36 Ma. This certain typically incompatibly behaving trace elements, yet also activity continued to the end of the Strawberry Volcanics at show signifi cant compositional overlap particularly those that 12.52 ± 0.12 Ma. have A-type characteristics, which typify rhyolitic products of (3) Intermediate and mafi c tholeiitic eruptions started at the Yellowstone–Snake River Plain hotspot. 15.57 ± 0.16 Ma. Tholeiitic intermediate compositions erupted fi rst, co-erupting with other intermediate and silicic eruptions, ACKNOWLEDGMENTS and this was followed by eruptions of more mafi c composition. These eruptions continued in the same style as the calc-alkaline We would like to thank the U.S. National Science Foundation eruptions until 12.61 ± 0.06 Ma. (NSF) for the support through NSF grant EAR-1220676 to We show that the intermediate and mafi c eruptions of the Streck and the Geological Society of America for 2011 research Strawberry Volcanics co-erupted in part with the main phase of grant 9579-11 to Steiner. We would also like to thank Richard M. the Columbia River Basalt Group from an area surrounded by Conrey, Charles M. Knaack, and Scott Boroughs at Washington dike swarms producing Columbia River Basalt Group lavas. The State University for their assistance in whole-rock geochemical

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X-ray fl uorescence and inductively coupled plasma–mass thermal Research, v. 161, no. 3, p. 187–214, https://doi.org/10.1016/j spectrometry analyses and Frank Ramos for facilitating isoto- . jvolgeores.2006.12.004. Brueseke, M.E., Hart, W.K., and Heizler, M.T., 2008, Diverse mid-Miocene pic analyses. The fi rst author thanks Ashley Steiner for being silicic volcanism associated with the Yellowstone–Newberry thermal an excellent fi eld assistant and providing superior editing and anomaly: Bulletin of Volcanology, v. 70, no. 3, p. 343–360, https://doi comments along the way. Last, we thank John Wolff and Mat- .org/10.1007/s00445-007-0142-5. 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