Geochemistry and Geochronology of the Jim Sage

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Cenozoic Tectonics, Magmatism, and Stratigraphy of the Snake River Plain– Yellowstone Region and Adjacent Areas themed issue

Geochemistry and geochronology of the Jim Sage volcanic suite, southern Idaho: Implications for Snake River Plain magmatism and its role in the history of Basin and Range extension

Alexandros Konstantinou1,*, John Valley2, Ariel Strickland2, Elizabeth L. Miller1, Chris Fisher3, Jeffrey Ver- voort3, and Joseph Wooden1,4 1Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305, USA 2Department of Geoscience, University of Wisconsin, 1215 W. Dayton Street, Madison, Wisconsin 53706, USA 3School of the Environment, Washington State University, Pullman, Washington 99164-2812, USA 4U.S. Geological Survey, Menlo Park, California 94025, USA

ABSTRACT (without the large calderas inferred for the Bryan, 2007), yet their origin is one of the SRP rhyolites) implies that there might be fundamental debates in igneous petrology and The Jim Sage volcanic suite (JSVS) an alternative mechanism for the formation geodynamics. This debate is long standing due exposed in the Jim Sage and Cotterel Moun- of the low-δ18O signature other than the pro- to the complex nature of how large silicic mag- tains of southern Idaho (USA) consists of two posed assimilation of hydrothermally altered matic systems are generated and magma stored volcanic members composed of ~240 km3 caldera blocks. One suggestion is that the for large periods of time in heterogeneous con- of Miocene rhyolite lavas separated by an north to south propagation of SRP-type low- tinental crust. Understanding the origin and interval of lacustrine sediments. It is capped δ18O rhyolitic melt along the Albion fault led longevity of large silicic complexes is of great by rheomorphic ignimbrite and as much as to off-axis magmatism. Another possibility is importance because this magmatic activity 100 m of basaltic lava fl ows probably derived that there was prior and widespread (across is often associated with mineral deposits and from the central Snake River Plain (SRP) a region wider than the SRP) hydrothermal because silicic caldera-forming supereruptions province to the north. The occurrence of vol- alteration of the crust related to its earlier pose major safety and economic risks to society. canic vents in the JSVS links the lava fl ows magmatic and faulting history. In the western United States, the Snake River to their local eruptive centers, while the adja- The eruption of SRP-type lavas in the Plain–Yellowstone (SRP-Yellowstone) vol- cent Albion–Raft River–Grouse Creek meta- hanging wall of an evolving metamorphic canic province is a prominent feature, defi ned morphic core complex exposes ~3000 km2 of core complex helps us outline the role of the by low-relief topography extending ~750 km once deep-seated rocks that offer constraints SRP magmatic province in the extensional from northwestern Nevada to Yellowstone on the composition of the potential crustal evolution of the northeastern Basin and National Park (Wyoming, Montana, and Idaho; sources of these rhyolites. U-Pb zircon ages Range. The lavas of the JSVS imply the addi- Fig. 1 inset; e.g., Pierce and Morgan, 1992, from the rhyolite lavas of the JSVS range tion of basalt, related to the SRP hotspot, to 2009; Coble and Mahood, 2012). It is one of from 9.5 to 8.2 Ma. The Miocene basalt of the crust beneath the Raft River Basin that the most productive young silicic igneous prov- 87 86 the Cotterel Mountains has an Sr/ Sri provided a heat source for crustal melting inces on Earth, active from ca. 16.5 Ma to the

composition of 0.7066–0.7075 and εNd(i) = and weakening of the deep crust; this led present day, generating large volumes (tens –3.7, and the rhyolite lavas of the JSVS have to a vertical component of crustal ﬂ ow and of thousands of cubic kilometers) of mostly 87 86 Sr/ Sri = 0.7114–0.7135 and εNd(i) values doming during extension, after the erup- basaltic and rhyolitic magmas (e.g., Christian- that range from –6.7 to –7.1. Zircon from the tion of the 9.5–8.2 Ma JSVS rhyolites. This sen, 2001). Petrologic investigations of silicic, 18 rhyolites of the JSVS range in δ Ozr (Vienna younger than 8.2 Ma component of vertical intensely welded outflow ignimbrites (e.g., standard mean ocean water, VSMOW) from motion during faulting of the Miocene strati- Branney et al., 2008) surrounding the SRP have

–0.5‰ to 5.7‰ and have εHf(i) values ranging fi ed sequence of the Raft River Basin and demonstrated that the SRP-Yellowstone prov- from –0.8 to –6.8. Based on geochronology, the rotation of the Albion fault to shallower ince is capable of producing large volumes of whole-rock major elements, trace elements, angles collectively resulted in the subhorizon- rhyolitic ignimbrite (>1000 km3) that can travel isotopes (Sr and Nd), and in situ zircon O tal detachment structure imaged seismically over great distances (e.g., Andrews and Bran- and Hf isotopic compositions, we infer that beneath the Raft River Basin. ney, 2011; Ellis et al., 2011). Individual silicic the JSVS is genetically related to the central eruptive centers in the central portion of the SRP SRP province. The eruption of the low-δ18O INTRODUCTION are largely inferred because they are mostly bur- rhyolites of the JSVS, outside of the main ied beneath younger basalt fl ows, making them topographic extent of the SRP province Large silicic igneous complexes are com- inaccessible for direct study. Ongoing borehole monly associated with continental fl ood basalt drilling studies within the SRP are trying to provinces (e.g., Paraná-Etendeka and North address some of these problems by sampling the *Email: [email protected]. Atlantic igneous provinces; Bryan et al., 2002; volcanic section directly (e.g., Shervais et al.,

Geosphere; December 2013; v. 9; no. 6; p. 1681–1703; doi:10.1130/GES00948.1; 14 ﬁ gures; 2 tables; 6 supplemental tables. Received 15 May 2013 ♦ Revision received 6 September 2013 ♦ Accepted 4 October 2013 ♦ Published online 13 November 2013

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Columbia River Basalt Paleocene-Eocene volcanic rocks 20 Km Mesozoic plutons Yellowstone Met. Core complex 02010 Idaho 45°N Batholith 16.6– 15.6 Ma 6.6– N 4.4 Ma Cotterel OR ID 12.7– 10.5– 10.2– Mts 10.5 Ma 8.5 Ma 8.0 Ma

NV Snake River Plain ID ARG

Ruby NV UT Albion Mts n Basin and Range Snake Range 40°N n 0100 200 Jim Sage Km Mts n n

Middle Mtn. gneiss Figure 2

Middle Mtn. n

Raft River extent of 32–27 Ma Basin 20° Black plutonic complex Almo Pine Mts pluton

n IDAHO 42°

UTAH n n n n n

n n n Vipoint n n n pluton n n n n 10° n Raft River n

UTAH Mts n n n n n n n

n Quaternary alluvium and basalt n Red Butte <8.2 Ma Basalt of Cotterel pluton Grouse Cenozoic Basin Miocene ignimbrite Creek Mts 8.2 Ma Upper Jim Sage lava n Middle unit (sedimentary rocks) JSVS n 9.5 Ma Lower Jim Sage lava Detachment fault, n teeth on the upper Cenozoic sedimentary rocks n plate Fault or unconformity

Emigrant High-angle normal 32–25 Ma Cenozoic plutons Pass plutonic fault, ball on the 42–31 Ma Cenozoic plutons complex down thrown side Phanerozoic metasedimentary units Middle Mountain Fault shear zone Metamorphic lower plate rocks Raft River Unconformity 113°45′ Detachment Archean Crystalline basement

Figure 1. Generalized geologic map of the Albion–Raft River–Grouse Creek Mountains. Box (dashed lines) indicates the location of Figure 2. Inset indicates the location of the Albion–Raft River–Grouse Creek metamorphic core complex (ARG) in the western U.S. and relative to the inferred eruptive centers in the Snake River Plain. Modiﬁ ed from Konstantinou et al. (2012) and references therein.

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2006; Project HOTSPOT, http://www .usu .edu Basin and Range province, few studies have assimilation (e.g., Taylor, 1986) or remelting /geo /shervais /Shervais -USU -Geology /Project tried to explain how Basin and Range exten- (e.g., Bindeman and Valley, 2001) of low-δ18O _Hotspot .html). sion is accommodated in and across the SRP- (2‰ to <–3‰) hydrothermally altered rock. Fewer studies have focused on the smaller Yellowstone province (e.g., Rodgers et al., The occurrence of low-δ18O rhyolite in Yellow- (<300 km3) eruptive centers adjacent to the 1990, 2002; Anders and Sleep, 1992; Parsons stone has been explained by the remelting of SRP province that are not associated with cal- et al., 1998; Fig. 1). In addition, there have older collapsed hydrothermally altered caldera-forming eruptions but are more accessible been very few studies focused on the relation dera blocks (e.g., Taylor, 1986, Bindeman and to study. One of the best examples of such an of magmatic addition from the SRP-Yellow- Valley, 2001). This model was then adopted for eruptive center is the Jim Sage volcanic suite stone province to the deep and shallow part the central SRP, where large-scale remelting of (JSVS; Fig. 1), developed within the evolving of the crust in the region of the Basin and buried caldera blocks resulted in the formation Miocene Raft River Basin (Konstantinou et al., Range province and/or its effects on the exten- of low-δ18O rhyolites (e.g., Bindeman et al., 2012). Volcanic vents in the JSVS link the lava sional evolution of the Basin and Range (e.g., 2007; Watts et al., 2011). In the central SRP, fl ows to their eruptive centers, and the adjacent Rodgers and McCurry, 2009). The ~240 km3 the model implies that the hydrothermal altera- Albion–Raft River–Grouse Creek metamorphic rhyolitic deposits of the JSVS are intercalated tion of the upper crust was synchronous with core complex exposes ~3000 km2 of once deep- with deposits of the Raft River Basin, which SRP magmatism and was localized above the seated rocks that offer constraints on the com- formed due to normal faulting associated with large magma chambers within the topographic position of the basement upon which the central the Miocene exhumation of the Albion–Raft expression of the province. It also predicts that SRP was built. This study focuses on the petro- River–Grouse Creek metamorphic core com- the ignimbrites and early postcaldera lavas asso- genesis of the volcanic rocks of the JSVS using plex (Konstantinou et al., 2012). Thus, the ciated with the fi rst collapse should show no zircon U-Pb geochronology, whole-rock major JSVS provides both timing constraints and effect, and there may be a decrease in the δ18O element, trace element, and isotopic composi- insight as to the possible genetic relationships signature of the ignimbrites following multiple tions (Sr and Nd), and in situ zircon O and Hf between magmatism and extensional faulting cycles of caldera collapse; as reported from the isotopic compositions. in the Basin and Range province. Heise eruptive center of the central SRP (Watts et al., 2011). Later studies from the Picabo REGIONAL GEOLOGIC SETTING Magmatism in the Snake River Plain eruptive center suggested that that hydrothermal preconditioning during Basin and Range Cenozoic Extensional and Magmatic The SRP-Yellowstone is a bimodal province extension, followed by caldera collapse, may History of the Basin and Range Province characterized by tholeiitic basalts and hot and have resulted in the generation of overabundant dry rhyolitic magmas (e.g., Branney et al., 2008; low-δ18O rhyolites (e.g., Drew et al., 2013). Following the end of regional folding and Christiansen and McCurry, 2008; Fig. 1 inset) An alternative view suggests that the low-δ18O thrust faulting during the Sevier and Laramide that have isotopic compositions that require rhyolites in the central SRP-Yellowstone rhyo- orogenies in the Cretaceous through early a signifi cant mantle component (e.g., Hil- lites (Owyhee-Humboldt, Bruneau-Jarbidge, Cenozoic (e.g., Burchfi el et al., 1992; DeCelles dreth et al., 1991; Nash et al., 2006; McCurry Twin Falls, and Picabo eruptive centers; Pierce et al., 1995), Paleocene–Oligocene volcanic and Rodgers, 2009). The province exhibits an and Morgan, 1992) were produced by melting rocks erupted across the western U.S., forming early phase of silicic magmatism that marks a of crustal rocks that underwent hydrothermal a regional pattern of magmatism that becomes northeast time-transgressive trend from north- alteration before SRP-Yellowstone magma- younger in a southward direction across the west Nevada to Yellowstone, inferred to refl ect tism (e.g., Boroughs et al., 2005; Leeman et al., northern Basin and Range (e.g., Christiansen the passage of the North American plate over 2008), such as altered Idaho batholith rocks and Lipman, 1972; Stewart et al., 1977; Best a stationary hotspot (see discussions in Mor- (Criss and Taylor 1983; Criss et al., 1984). This and Christiansen, 1991; Christiansen and Yeats, gan et al., 1984; Hooper et al., 2007; Graham view implies that the low-δ18O magmas refl ect 1992; Fig. 1). Miocene extension in the Basin et al., 2009). Initial silicic magmatism in the a signature of the crustal source that underwent and Range province has been documented by well-exposed High Rock caldera complex, the hydrothermal alteration before the Miocene, crosscutting relations, the stratigraphy and age McDermitt complex, and other dispersed erup- and thus the alteration may not be limited to of synextensional basin fi ll deposits, and low- tive centers coincided with the early phases of just the SRP caldera complexes. temperature thermochronology of fault blocks. Columbia fl ood basalt volcanism ca. 16.5 Ma Our study of the JSVS (Figs. 2 and 3) offers These data indicate that faulting began after (Coble and Mahood, 2012). insights to the origin of the low-δ18O rhyolites ca. 21 Ma and accelerated at 17–16 Ma in the One of the hallmarks of the SRP-Yellowstone as it represents an eruptive center outside the central part of the northern Basin and Range province is the abundance of low-δ18O rhyolite topographic expression of the SRP province (Miller et al., 1999; Stockli, 1999; Colgan et al., magmas, fi rst noted in Yellowstone by Fried- and one that is directly linked to its volcanic 2006a, 2006b, 2010; Colgan and Henry, 2009). man et al. (1974). An estimated >10,000 km3 vents. In particular, determining the O iso- Extensional faulting is younger at the edges of low-δ18O rhyolites erupted from the SRP- tope composition of the rhyolite lavas of the of the province; rapid slip on faults began ca. Yellowstone in the past 15 m.y. (e.g., Hil- JSVS helps to constrain the possible age and 15–10 Ma in northwestern Utah and southern- dreth et al., 1984; Bindeman and Valley, 2001; origin of upper crustal hydrothermal altera- most Idaho (Wells et al., 2000; Egger et al., Boroughs et al., 2005; Bindeman et al., 2007; tion. In addition, the adjacent Albion–Raft 2003; Konstantinou et al., 2012) and in north- Cathey et al., 2007, 2011). The most widely River–Grouse Creek metamorphic core com- western Nevada (Colgan et al., 2006b, 2008; supported explanation for low-δ18O rhyolites plex exposes ~3000 km2 of once deep-seated δ18 Egger et al., 2010). [ Owr (whole rock) ~1‰–4‰] requires a rocks that may have had similarities in terms While magmatism in the Miocene SRP- signifi cant component of oxygen to be ulti- of age and composition to the premagmatic Yellow stone province is largely synchronous mately derived from negative δ18O meteoric crust that underlies the SRP province (Fig. 1). with Miocene to present-day extension in the water (–10‰ to –18‰), and therefore requires The O-Hf isotope composition of zircon and

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113°30′0″W ′ ″ 113°20 0 W Quaternary cover Quaternary Basalt Upper lacustrine sediments N ″

0 Basalt of Cotterel ′ Welded ignimbrite 42°30 Upper rhyolite lava flow

Middle sedimentary unit JSVS Lower rhyolite lava flow Sub-volcanic intrusion Rhyolite dome Lower lacustrine sediments Metasedimentary rocks (undif.) Cotterel Mts Town of Albion Quartzite of Clarks Basin Type locality for Upper Schist of Stevens Spring member of JSVS Albion fault Schist of Upper Narrows PrecambrianElba Miocene Quartzite Archean Green Creek Complex Normal fault (ball on upper plate) C-RC C-1B Accommodation fault N ″ 0 ′ Sample location and sample name

42°20 Extent of volcanic Figure 5 rocks C-1

Jim Sage Mts

Metamorphic JS-11B lower plate rocks

21 N

″ sub-volcanic 0 ′ intrusions 42°10 Type locality for Lower domes member of JSVS 22 Inferred sub-volcanic intrusion

JS-1 0510Km 113°30′0″W 113°20′0″W

Figure 2. Simplifi ed geologic map of the Jim Sage volcanic suite (JSVS) and its location with respect to the Albion normal fault. Box (dashed lines) indicates the location of Figure 5. Blue dashed lines indicate the type localities for upper and lower members of the JSVS. On right is a simplifi ed depiction with the locations of subvolcanic intrusions and domes within the Raft River Basin. Modifi ed from existing maps by Williams et al. (1974), Smith (1982), and Pierce et al. (1983) and new geologic mapping.

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Lacustrine sediment and volcanic sandstone SRP. Volcanic rocks of the JSVS and associated with rhyolite clasts sedimentary rocks were mapped and described 1050 as part of the Salt Lake Formation by Compton Massive, aphanitic, basalt flow (1972, 1975), Williams et al. (1982), Covington 1000 Vesicular basalt flow (1983), Pierce et al. (1983), Wells (2009), and Massive, aphanitic, basalt flow us (Konstantinou et al., 2012). Borehole, well 950 log, and geophysical (seismic reﬂ ection, grav- Vesicular welded ignimbrite ity, and magnetic) data initially collected for 900 Rheomorphic ignimbrite the Raft River geothermal project during the 850 early to mid-1970s (Mabey and Wilson, 1973; 8.19±0.14 Ma; C-1B Williams et al., 1974, 1982; Covington, 1983) 800 Glassy rhyolite lava flow are available for the greater Raft River Basin with flow layering area, and were compiled for the purpose of this 750 Massive, glassy rhyolite study (Konstantinou et al., 2012; Konstantinou, lava flow with sub-vertical joints 2013). The Miocene Salt Lake Formation (and 700 the JSVS) accumulated in a topographic depres- Massive, glassy rhyolite lava flow sion or major half-graben system that formed 650 as a response to slip along the Albion normal Devitrified rhyolitic lava flow fault system, which bounds the basin to the west 600 with columnar joints Figure 3. Composite stratigra- 8.43±0.12 Ma; C-RC (Figs. 1 and 2) (Konstantinou et al., 2012). The phy of the Jim Sage volcanic Thinly laminated, onset of extensional fault slip is recorded by the 550 devitrified rhyolitic lava flow suite (JSVS) showing the strati- age of ash-fall tuffs deposited near the base of graphic locations of geochro- 500 Massive, glassy rhyolite lava flow the Miocene section ca. 14 Ma prior to the erup- nology samples. Brecciated basal contact tions of the JSVS rhyolites (Konstantinou et al., 450 Lacustrine sediments Sedimentary breccia with 2012; Konstantinou, 2013; Fig. 2; part of lower rhyolite clasts lacustrine sediments). 400 9.19±0.14 Ma; C1 Altered rhyolite sub-volcanic Geologic mapping and stratigraphic inves- intrusion Middle unit tigations of the Jim Sage and Cotterel Moun- Massive, glassy rhyolite lava flow 350 tains were carried out between 2009 and 2012, 9.34±0.12 Ma; JS-1 Glassy rhyolite lava flow with attention to the basal contacts of the rhyo-

Approximate composite stratigraphic height (m) 300 with columnar joints lite ﬂ ows (Figs. 2–5). These data were used to Massive, glassy rhyolite lava flow modify existing maps by Williams et al. (1974) 250 and Pierce et al. (1983) to produce the map in Glassy rhyolite lava flow Figure 2 and to better deﬁ ne the stratigraphy of 200 with flow layering the JSVS (Fig. 3). 150 Devitrified rhyolitic lava flow with columnar joints In Situ Zircon Sensitive High-Resolution 100 9.53±0.10 Ma; JS-11B Ion-Microprobe–Reverse Geometry Massive, glassy rhyolite lava flow

Lower member of the JSVSU-Pb Geochronology Upper member of the JSVS Basalt of Cotterel 50 Glassy rhyolite lava flow with flow layering and basal breccia In situ zircon U-Pb analyses (n = 137) from 0 Densely welded ignimbrite fi ve samples from the JSVS were carried out Lacustrine sediments using the sensitive high-resolution ion-microprobe–reverse geometry (SHRIMP-RG) at the Stanford-U.S. Geological Survey Micro Sr-Nd whole-rock isotopic compositions from METHODS Analysis Center (SUMAC). Two samples were proximal igneous and metamorphic base- collected from the lower member of the JSVS ment rocks exposed in the metamorphic core Geologic Mapping and Data Compilation (samples JS11-B, JS1), two samples from the complex have been well studied (Fig. 1; upper member (samples C-RC and C1B), and Strickland et al., 2011b; Konstantinou et al., The JSVS crops out in the Jim Sage and Cot- one from a subvolcanic intrusion (sample C1; 2013). Recent hydrogen isotope studies of terel Mountains on the eastern side of the Albion Table 1; Figs. 2 and 3; Supplemental Table 11). ductilely deformed rocks in the Raft River Mountains, which form the northern extent of the Zircon was separated by crushing and various Mountains show evidence for rock-meteoric Albion–Raft River–Grouse Creek metamorphic separation methods (Gemini table, heavy liquids, fl uid interaction near and along what was core complex (Figs. 1 and 2). The JSVS overlies once the brittle-ductile transition zone in the lacustrine sedimentary rocks and is composed 1Supplemental Table 1. Data used to calculate crust. These have been interpreted to indicate of two members of rhyolitic lavas separated by a zircon U-Pb ages of Miocene volcanic rocks. Geo- circulation of meteoric water synchronous sedimentary interval (Figs. 2 and 3). It is capped chrono logic analyses performed using the SHRIMP- RG. If you are viewing the PDF of this paper or read- with extensional deformation and coeval with by ignimbrites and basaltic fl ows (basalt of Cot- ing it offl ine, please visit http://dx.doi.org /10.1130 development of the mylonites within the meta- terel Mountains [Williams et al., 1982]; Fig. 2) /GES00948.S1 or the full-text article on www .gsapubs morphic core complex (Gottardi et al., 2011). that likely represent outfl ows from the central .org to view Supplemental Table 1.

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Cpx Cpx Cpx

Qtz Pl Pl Cpx Pl A Ox B Ox Cpx

Pumice lapilli

Pl Pl Cpx

Pl Cpx Cpx C D Ox Pl Cpx Pl Cpx

Upper rhyolite

Lower rhyolite clast E F G

Figure 4. Photographs and photomicrographs documenting the fi eld relationships and thin section appearance of the rhyolite lava fl ows of the Jim Sage volcanic suite (JSVS) and the capping ignimbrite. Width of view of photomicrographs is 5 mm. Cpx—clinopyroxene; Ox—oxide; Pl—plagioclase; Qtz—quartz. (A) Autobrecciated base of the lower member of the JSVS. (B) Autobrecciated base of upper member of the JSVS. (C) Rheomorphic ignimbrite of the upper member of the JSVS. Note the transposed and deformed shards in the photomicrograph. (D) Distal facies of the welded ignimbrite capping the upper member of the JSVS, highlighting collapsed pumice lapilli and the deformed glass shards in the photomicrograph. (E) Basal lava fl ow of the upper member of the JSVS with an entrained clast of the lower member. (F) Basal autobreccia of the lower member of the JSVS. (G) Autobreccia at the top of the lower member (base of the middle unit) of the JSVS.

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113°28'0"W Tjsu 113°26'0"W Tjsu Tjsu Tjsu Tjsu A Tjsm Tjsu Tjsu Tjsm

Tjsl Qal Tjsl Limit of sub-volcanic Tjsu 48 intrusion? Tjsm 20 Tjsu 20 51 15 16 14 48 Tjsu 42°18'0"N Tjsu Ttm 11 Tjsu 16 23 Tjsi

23 Ttm Ttm Tjsu 27 38 13 Tjsu 26 22 Ttm Ttm 17 26 8 B 16 12 Ttu Tjsi Ttu 25 Tjsu 14 15 23

17 Qal 42°17'0"N Qal Tjsi 18 16 18

Qal Km 0120.5

A B 1750 Tjsu Tjsm Tjsm Tjsi Ttu 1500 Tjsu

meters Tjsl Tjsl 1250 ? ?

Figure 5. Detailed map and cross section of the southern part of the Cotterel Mountains, showing the structural relationships of units around a subvolcanic intrusion. Note the radially dipping strata around the intrusive complex. Key is the same as in Figure 2. Abbrevia- tions: Tjsl—lower member of the Jim Sage volcanic suite (JSVS); Ttm—middle sedimentary unit separating the members of the JSVS; Tjsu—upper member of the JSVS; Tjsi—subvolcanic intrusion; Ttu—Miocene tuff younger than the JSVS; Qal—Quaternary alluvium.

Frantz magnetic separator). Zircon was hand- 156 through 270) to six to improve counting grams were constructed using Isoplot (Ludwig, selected for fi nal purity, mounted in a 25-mm- statistics for 206Pb and 207Pb, required to obtain 2003). Calculated ages for zircon are standard- diameter epoxy disc, ground and polished to a Miocene ages with acceptable analytical preci- ized relative to R33 (419 Ma; Black et al., 2004). 1 µm fi nish, and imaged in cathodoluminescence sion (~1%–2% for individual analyses, 1σ). The common Pb composition was based on the (Fig. 6) using a JEOL 5600 scanning electron The majority (n = 121) of zircons analyzed value reported by Stacey and Kramers (1975). microscope. U, Th, and Pb isotopes were mea- have concordant U-Pb ages, had low common The calculated concordia intercept ages (Fig. 7) sured on the SHRIMP-RG (using the procedure Pb, and were interpreted as magmatic zircon and weighted average 207Pb-corrected 206Pb/238U described in Strickland et al., 2011b), adjusting that crystallized just prior to eruption of the model ages are reported with 2σ errors (Table 1; the count times for each isotope and the number volcanic rocks. The data were reduced using Supplemental Table 1 [see footnote 1]). The of fi ve scans (peak-hopping cycles from mass SQUID 1 (Ludwig, 2003) and concordia dia- weighted average ages of the fi ve samples are

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TABLE 1. SUMMARY INFORMATION OF SAMPLES FROM THIS STUDY Coordinates (deg. dec. deg.) Type of analysis Sample Elevation number Unit Description NW(m) M and T U-Pb Sr Nd δ18OHf JS-10-1 lower member rhyolite lava flow 42.05053 113.45403 1629 X X X JS-10 lower member rhyolite lava fl ow 42.17989 113.42556 X X X JS-3 lower member rhyolite lava flow 42.15872 113.48694 2048 X JS-4 lower member rhyolite lava flow 42.15975 113.49156 2041 X X JS-5 lower member rhyolite lava fl ow 42.15606 113.49378 2195 X X X JS-6 lower member rhyolite lava flow 42.15564 113.49314 2194 X JS-7 lower member rhyolite lava flow 42.15478 113.49261 2176 X JS-8 lower member rhyolite lava flow 42.15336 113.49000 2145 X JS-9 lower member rhyolite lava flow 42.15292 113.48814 2104 X C-9A lower member rhyolite lava flow 42.41295 113.58193 1443 X C-11 lower member rhyolite lava flow 42.33683 113.50008 1733 X C-11B lower member rhyolite lava flow 42.33683 113.50008 1733 X JS-09-4 lower member rhyolite lava flow 42.11627 113.51707 1939 X JS-1 lower member rhyolite lava flow 42.08495 113.49483 1846 X X X JS-11B lower member rhyolite lava fl ow 42.20290 113.42440 1591 X X X X X X JS-11A upper member rhyolite lava flow 42.20290 113.42440 1591 X C-RA upper member rhyolite lava flow 42.35344 113.46592 2000 X X X C-RB upper member rhyolite lava flow 42.35119 113.47703 1950 X C-RC upper member rhyolite lava fl ow 42.35119 113.47703 1950 X X X X C-RD upper member rhyolite lava fl ow 42.34325 113.48639 1916 X X X C-RE upper member rhyolite lava fl ow 42.33794 113.50256 1713 X X X C-1A upper member rhyolite lava flow 42.36008 113.45605 2159 X C-1B upper member rhyolite lava fl ow 42.36010 113.45607 2159 X X X X X C-1C upper member rhyolite lava flow 42.36012 113.45608 2159 X C-8 upper member rhyolite lava flow 42.51690 113.47520 1690 X X C-9C upper member rhyolite lava flow 42.41295 113.58193 1443 X C-BA upper member basalt lava flow 42.51242 113.49450 1430 X X C-BB upper member basalt lava flow 42.51675 113.48958 1489 X X C-BC upper member basalt lava fl ow 42.51706 113.48894 1493 X X X C-7 upper member basalt lava flow 42.51690 113.47520 1690 X X C-10 outflow welded ignimbrite 42.33713 113.49208 1858 X JS-09-8B outflow welded Ignimbrite 42.02977 113.59873 1639 X JS-09-8C outfl ow rhyolite lava flow? 42.02977 113.59873 1639 X JS-09-2A outfl ow rhyolite lava flow? 42.08718 113.48193 1813 X JS-09-3A outfl ow rhyolite lava flow? 42.07835 113.48597 1729 X JS-09-7 outfl ow rhyolite lava flow? X C-4 subvolcanic intrusion silicified rhyolite 42.29728 113.43872 1529 X C-1 subvolcanic intrusion silicifi ed rhyolite42.29203 113.44069 1529 X X Note: Datum for sample coordinates is NAD (North American Datum) 83. M and T—whole-rock major and trace element geochemistry; U-Pb—zircon U-Pb geochronology; Sr—whole-rock Sr isotope analyses; Nd—whole-rock Nd isotope analyses; δ18O—zircon oxygen isotope analyses; Hf—zircon Hf isotope analyses; deg. dec. deg.—degree decimal degree.

preferred as the inferred crystallization ages and to the JSVS rhyolite fl ows are not certain due to chemistry assemblages (Fig. 10; Supplemental are all within error of the concordia intercept lack of continuous exposure (outfl ows). In addi- Table 33) using the JEOL JXA-733A electron ages. Zircon concentration data for U and Th tion, four samples of the basalt of Cotterel were probe at Stanford University (Stanford, Califor- were standardized against the well-character- analyzed. The samples were analyzed using nia). Pyroxene and feldspar phenocrysts were ized, homogeneous zircon standard MAD-green X-ray fl uorescence at Washington State Univer- selected and analyzed for the abundance of (Barth and Wooden, 2010). sity (Pullman, Washington, USA; 13 samples) major element oxide compositions. The oxide and Macalester College (St. Paul, Minnesota, abundances were standardized using well char- Whole-Rock Major and Trace Element USA; 24 samples); 13 samples were dissolved acterized in-house mineral standards and the Geochemistry and analyzed for trace and rare earth ele- results were normalized to six oxygen atoms for ments using inductively coupled plasma–mass pyroxene and eight for feldspar (Supplemental We selected 37 samples for whole-rock major spectrometry (ICP-MS) at Washington State Table 3 [see footnote 3]). and trace element analyses (Table 1; Fig. 8; University (Fig. 9). Analyses of 12 additional Supplemental Table 22), 15 samples from the samples of Precambrian basement rocks are Isotope Geochemistry lower member of the JSVS, 11 from the upper also included in Figure 8 (compiled from Egger member, and 6 samples of ignimbrite and lava, et al., 2003; Strickland et al., 2011b; Konstanti- We selected 15 samples from the JSVS for the stratigraphic positions of which with respect nou et al., 2013). whole-rock Sr and a subset of 7 samples for Nd analyses performed at the Stanford ICP- 2Supplemental Table 2. Whole-rock major and Electron-Microprobe Phenocryst trace element data of Miocene volcanic rocks of the Geochemistry 3Supplemental Table 3. Electron-microprobe major Jim Sage volcanic suite determined by X-ray fl uores- element compositions of feldspar and pyroxene cence and dissolution inductively coupled plasma– Three samples from the lower (JS-3, JS-7, phenocryst assemblages from the Jim Sage volcanic mass spectrometry. If you are viewing the PDF of this suite. If you are viewing the PDF of this paper or read- paper or reading it offl ine, please visit http://dx.doi JS-10) and three from the upper member ing it offl ine, please visit http://dx.doi .org /10.1130 .org /10.1130 /GES00948.S2 or the full-text article on (C-RA, C-RD, C-RE) of the JSVS were selected /GES00948.S3 or the full-text article on www .gsapubs www .gsapubs .org to view Supplemental Table 2. for electron-microprobe analyses of phenocryst .org to view Supplemental Table 3.

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9.3±0.2 Ma 2.2±0.2 ‰ 9.1±0.3 Ma B 2.4±0.2 ‰ 9.6±0.2 Ma –0.5±0.2 ‰ –5.1±0.8 –7.4±1.3 3.4±0.2 ‰ –4.7±0.1 9.4±0.4 Ma 4.0±0.2 ‰

2.7±0.2 ‰ –3.2±0.9 2.8±0.2 ‰ 11.1.±0.5 Ma 3.3±0.2 ‰ 1.8±0.2 ‰ 4.1±0.2 ‰ –2.6±0.8 1.9±0.2 ‰ 2.2±0.2 ‰ 4.2±0.2 ‰ 9.3±0.3 Ma 9.7±0.2 Ma 3.9±0.2 ‰ 3.5±0.2 ‰ A –6.8±1.2 8.1±0.3 Ma

–5.2±1.1

11.1±0.1 Ma –4.3±1.1 9.5±0.2 Ma 10.9±0.2 Ma 1.7±0.3 ‰ –6.9±0.6

9.5±0.1 Ma 10.0±0.1 Ma 8.6±0.1 Ma D –5.4±0.8

E 8.1±0.3 Ma 8.3±0.3 Ma –5.4±1.6 8.6±0.3 Ma 2.2±0.3 ‰ 8.7±0.3 Ma –5.1±1.6 C 7.9±0.4 Ma 8.3±0.3 Ma SHRIMP-RG spot with 1.9±0.3 ‰ U-Pb age; error in 1σ 2.1±0.3 ‰ 2.1±0.3‰ F 18 6.1±0.2 ‰ SIMS spot with δ O value; 5.5±0.3 ‰ error in 2σ Discordant –6.9±0.6 ca. 2550 Ma LA-ICP-MS spot with εHfi value; error in 2σ Scale bar 100 μm Samples: Discordant A=JS11-B; B=JS-1; C=C1; Discordant ca. 1745 Ma D=C-RC; E=C1-B; F= inherited ca. 1745 Ma 6.4±0.2 ‰ cores from multiple samples

Figure 6. Diagram showing representative cathodoluminescence images of zircons from the igneous rocks dated in this study showing the spot analyses for U-Pb geochronology, oxygen isotope analyses, and Hf isotope analyses. Red circles indicate the spots where 207Pb-corrected 206Pb/238U ages obtained from SHRIMP-RG (sensitive high-resolution ion-microprobe–reverse geometry) analyses, black circles indicate the spots analyzed for oxygen isotope compositions by secondary ion mass spectrometer (SIMS), and blue circles indicate spot locations from Hf isotope analyses by LA-ICP-MS (laser ablation–inductively coupled plasma–mass spectrometry). Black scale bar is 100 μm for all the samples.

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data-point error ellipses are 2σ data-point error ellipses are 2σ 0.28 0.12 Mean=9.34± 0.12 (1.3%) 14.0 Mean=9.53± 0.09 (1.0%) JS-1 14.0 Intercept at JS-11B 2 of 25 rej. MSWD=2.3 Intercept at 0 of 14 rej. MSWD=0.9 2σ error bars 9.34±0.13 Ma 2σ error bars 9.46±0.09 Ma 12.0 12.0 0.24 MSWD = 1.5 MSWD = 0.94 10.0 10.0 Common Pb 0.10 Common Pb 8.0 8.0 0.20

6.0 6.0 Pb Pb 206 206 0.16 0.08 To 2550 Ma Pb/ Pb/ inheritance 207 207 0.12

0.06 Antecrysts

0.08 Antecrysts A B 12 11 12 11 10 9 0.04 0.04 9 500 540 580 62010 660 700 740 780 520 560 600 640 680 720 760 800 238U/206Pb 238U/206Pb

data-point error ellipses are 2σ data-point error ellipses are 2σ 0.6 Mean=9.19± 0.14 (1.5%) Mean=8.43± 0.12 (1.4%) Intercept 14.0 14.0 C-1 1 of 27 rej. MSWD=2.7 Intercept at C-RC 0 of 26 rej. MSWD=1.5 8.89±0.17 Ma 2σ error bars 0.4 2σ error bars 12.0 8.25±0.17 Ma 12.0 0.5 MSWD = 2.4 MSWD = 1.4 10.0 10.0

8.0 8.0 0.4 0.3 6.0 6.0 Pb Pb Common Pb 206 206 0.3 Common Pb Pb/ Pb/ 0.2 207 207

0.2

0.1 Antecrysts 0.1 C D 16 14 12 10 8 20 18 16 14 12 10 8 0.0 0.0 300 400 500 600 700 800 900 400 500 600 700 800 900 238U/206Pb 238U/206Pb data-point error ellipses are 2σ data-point error ellipses are 2σ Mean=8.19± 0.14 (1.7%) C1-B 14.0 Conc. Age: 0 of 14 rej. MSWD=1.4 Zircon cores from five Upper Intercept: 0.5 2600 8.28 ±0.13 Ma 2σ error bars Miocene samples (n=9) ca. 2550 Ma 0.10 12.0 MSWD = 8.4 10.0 2200 0.4 8.0 Upper Intercept: 6.0 ca. 1745 Ma 1800 U

Pb 0.08 0.3 238 206

Pb/ 1400 Pb/ 206 207 0.2 1000 0.06

0.1 600

E Lower Intercepts: 10 8 ~ 80-105 Ma F 0.0 0.04 024681012 600 700 800 900 1000 207 235 238U/206Pb Pb/ U Figure 7. (A–E) Concordia diagrams and weighted averages for the samples dated in this study. The intercept or concordia ages are indicated for each sample, as well as the weighted average age of the 207Pb-corrected 206Pb/238U ages for each sample (MSWD—mean square of weighted deviates). Dashed ellipses are interpreted as antecrystic or inherited zircon and were not included in the weighted mean calculation. (F) Concordia diagram showing the distribution of inherited zircon grains from the Jim Sage volcanic suite (JSVS). The inherited grains fall into two general age groups (ca. 1754 Ma, shown in blue and 2550 Ma, shown in green) and along their discordia with Mesozoic lower intercepts (ca. 105 and 80 Ma).

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Figure 8. Whole-rock major element compositions of samples from the Jim Sage volcanic suite (JSVS), basalt of Cotterel Mountains (Williams et al., 1982), of out- Precambrian Basement A Ph flow samples with poorly constrained FLower member stratigraphic position, and Precambrian Upper memberTPh igneous rocks from the Albion–Raft River– PT P Grouse Creek metamorphic core complex.

Basalt of Cotterel Jim Sage

(A) Total alkali versus silica (G. Mahood, volcanic suite C 2013, personal commun.) diagram indicat- OutflowPhT Alkali L-Si ing the range of compositions of the JSVS TA Rhyolite

and igneous rocks of the Precambrian base- Alkali H-Si Rhyolite O

ment (used as models for crustal sources). 2 T Abbreviations: BTA—Basaltic trachyande- K BTA Rhyo- L-Si H-Si Dacite dacite site; C—Comendite; H-Si—high silica; + Alkaline Rhyo. Rhyo.

L-Si—low silica; PT—Phono-tephrite; P— O TB Phonolite; Rhyo.—rhyolite; T—Trachyte; 2 Comendite a Andesite TA—Trachyandesite; TB—Trachybasalt. N

(B) FeOt/MgO versus SiO2 diagrams of the Alkali L-Si O 91011 same suite of rocks. Fields for Snake River 2 Rhyolite

Basaltic Na Plain (SRP) basalt and rhyolite are from Andesite + High-Silica O 2 8 Christiansen and McCurry (2008). K Rhyolite Basalt Sub-Alkaline/ Tholeiitic 7 Rhyodacite Low-Silica Rhyolite 6 MS-TIMS (thermal ionization mass spectrom- 68 70 72 74 76 78 SiO2

etry) facility (Table 1; Fig. 11; Supplemental 0 2 4 6 8 10 12 14 Table 44). For more details about the procedures 45 50 55 60 65 70 75 80 used for sample dissolution and mass spectrometry, see Appendix 1. SiO2 The O isotope compositions of individual zircon domains, dated by SHRIMP-RG (n = Precambrian Basement B 70), from three samples (JS1, JS11-B, C1B) Lower member were determined in situ using an IMS-1280 at the University of Wisconsin WiscSIMS (Wis- Upper member consin Secondary Ion Mass Spectrometer) Miocene SRP

Basalt of Cotterel Jim Sage laboratory following the procedures described silicic magmas in Kita et al. (2009) and Valley and Kita (2009), Outflow volcanic suite and δ18O (Vienna standard mean ocean water, VSMOW) values were calculated with precision reported at 2 standard deviation (SD) level (see Appendix 2 for details of the method; Fig. 5 133 + 12A; Supplemental Table 5 ). A Cs beam MgO

was focused to ~10 μm spots and secondary t 18 16 ions of O and O were counted with dual O Faraday cup detectors. Groups of ~10 sample Tholeiite series Fe

Miocene SRP 4 Supplemental Table 4. Whole-rock Sr and Nd mafic magmas isotope results from Miocene volcanic rocks of the Jim Sage volcanic suite. If you are viewing the PDF of this paper or reading it ofﬂ ine, please visit http://

dx.doi.org /10.1130 /GES00948.S4 or the full-text 2 4 6 8 10 12 14 article on www .gsapubs .org to view Supplemental Table 4. Calc-alkaline series 5Supplemental Table 5. Results of zircon oxygen

isotope analyses, performed at the WiscSIMS (Uni- 0 versity of Wisconsin, Wisconsin Secondary Ion Mass 45 50 55 60 65 70 75 80 Spectrometer) facility. If you are viewing the PDF of this paper or reading it ofﬂ ine, please visit http:// dx.doi.org /10.1130 /GES00948.S5 or the full-text SiO2 article on www .gsapubs .org to view Supplemental Table 5.

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Or Wollastonite Ca Si O Basalt of Cotterel Lower member 226 Upper member Upper member Lower member CaMgSi O CaFeSi O SRP Rhyolites 26 26 Diopside Hedenbergite 1000 °C 900 °C 800 °C

Sanidine Augite

SRP rhyolites Pigeonite 800 °C Sample/Chondrite 1000 °C 900 °C Enstatite Ferrosilite

Mg226 Si O Fe226 Si O Isothermal contours at 1 atm. Anorthite Anorthoclase

10 100Dy 1000 La Ce Pr Nd Pm Sm Eu Gd Tb Ho Er Tm Yb Lu Albite OligoclaseAndesine LabradoriteL Bytownite AbAn Figure 9. Whole-rock chondrite-normalized rare earth element spider diagram of the Figure 10. Ternary diagrams for feldspar (anorthoclase-albite-ortho- rhyolite lavas of the Jim Sage volcanic suite clase) and pyroxene (Fe- enstatite-wollastonite), showing the composi- and the fl ows of the basalt of Cotterel Moun- tions of phenocryst assemblages from the lower and upper members tains (Williams et al., 1982). Chondrite nor- of the Jim Sage volcanic suite. Snake River Plain (SRP) rhyolite data malization after Boyton (1985). Snake River are from Ellis et al. (2013). Plain (SRP) rhyolite data are from Ellis et al. (2013). RESULTS sedimentary rocks locally preserve evidence of soft-sediment deformation and high-tempera- Stratigraphy of the Jim Sage Volcanic Suite ture fusion at the contact with the lower rhyolite analyses were bracketed by a total of at least lava fl ow (Fig. 3). The two members have nearly 8 analyses of KIM-5 zircon standard (Valley Distinguishing between high-temperature constant thickness (370–440 m for each mem- 2003), which are used to correct for instrument rheomorphic ignimbrites and rhyolite lavas that ber) along their strike length, except at their bias. The average value of bias was –0.31‰ and erupted from the SRP is challenging because the northern and southern exposures, where they ranged from –0.10‰ to –0.54‰. The reproduc- intense welding of these ignimbrites gives them taper rapidly and have ~30–50-m-thick terminal ibility of these groups of standard data, which a lava-like appearance (Branney et al., 2008). lobes. The two members are exposed over areas varied from 0.19 to 0.52, permil ‰ is consid- Differentiating between individual ignimbrite of ~260 km2 and have aspect ratios between ered the best indication of analytical precision, units is equally diffi cult due to their similar 1:40–1:50, and calculated volumes, based on and is reported at 2 SD. For the purpose of this appearance and geochemistry (e.g., Hildreth cross sections and map patterns, ranging from δ18 3 study, we have used the Ozr (zr—zircon) and Mahood, 1985). Many workers have used 110 to 130 km (Fig. 2). δ18 values to calculate Owr using the equation high-precision geochronology, trace element δ18 ≈ rearranged from Lackey et al. (2008): Owr geochemistry, and phenocryst geochemistry to δ18 Ozr + 0.0612 (wt% SiO2) – 2.5. correlate individual ignimbrite units and esti- 0 Hf isotopic compositions were determined mate the volumes of individual eruptions (e.g., Basalt in situ on the same zircons that were dated and Hildreth and Mahood, 1985; Bonnichsen et al., Upper Rhyolite Lower Rhyolite analyzed for O isotope compositions, from fi ve 2008; Ellis et al., 2012). In this study, we used samples of the JSVS (n = 56). The analyses the physical characteristics of the volcanic –5

were performed at Washington State Uni- deposits (thickness, aspect ratios, terminations (i)

versity’s LA-ICP-MS facility (LA—laser abla- of the ﬂ ows and basal contacts; outlined by εNd tion), following the procedure of C.M. Fisher Branney et al., 2008) to differentiate between SRP basalt SRP-Yellowstone (2013, personal commun.); for detailed meth- lavas and ignimbrites in the JSVS. –10 rhyolite ods, see Appendix 3. Individual zircon crystals The rhyolite ﬂ ows of the JSVS are exposed from two lower member samples (samples along two north-south elongate antiformal struc-

JS11-B, JS-1), two upper member samples tures that make up the rugged Jim Sage and Cot- Typical error cross at 2σ level (C1B, C-RC), and a subvolcanic intrusion terel Mountains (Fig. 2). The JSVS is composed –15 sample (sample C1) were analyzed (Fig. 12B; of two members separated by a thin sequence of 0.706 0.710 0.714 6 87 86 Supplemental Table 6 ). sediments (Fig. 3). A dark weathering, moder- Sr/ Sri ately to densely welded ignimbrite is exposed in 6Supplemental Table 6. Results of zircon Hf iso- the Raft River Basin, ~10–11 km southwest of Figure 11. Results of whole-rock Sr and Nd tope analyses, performed at the Washington State the Jim Sage Mountains, but these exposures are isotope analyses from selected samples of University laser ablation–inductively coupled plas- separated by alluvial cover from the lava fl ows the Jim Sage volcanic suite and the basalt of ma–mass spectrometry facility. If you are viewing exposed at the Jim Sage Mountains (Fig. 3). The Cotterel Mountains (Williams et al., 1982). the PDF of this paper or reading it offl ine, please The range of isotopic values from the Snake visit http:// dx.doi .org /10.1130 /GES00948.S6 or the base of the lower member overlies fi ne-grained full-text article on www .gsapubs .org to view Supple- lacustrine deposits of the Salt Lake Formation River Plain (SRP) and Yellowstone province mental Table 6. (Konstantinou et al., 2012; Fig. 3). Underlying is from Christiansen and McCurry (2008).

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5 10 A Upper Rhyolite (n=15) Upper Rhyolite (n=17) B Sample C1B Rhyolite flow 8 4 Range of zircon oxygen isotope compositions in Range of published Hf equilibrium with published isotopic compositions from rhyolites of the central 3 Central SRP values 6 (Watts et al., 2010) SRP (Nash et al., 2006)

2 4

1 2

0 0 Lower Rhyolite (n=43) Lower Rhyolite (n=25) Number 14 Sample JS1 Number Intrusion 8 12 Sample JS11-B Rhyolite flow 10 6 Range of zircon Hf isotope 8 compositions 4 interpreted as 6 antecrystic 4 2 2 0 0 –1 0 1 2 3 4 5 6 –12–10–8 –6 –4 –2 0 2 18 δ Ozr ‰ (VSMOW) εHf (i) (from individual zircon)

18 Figure 12. (A) Summary of δ Ozr (zr—zircon) compositions from the lower and upper member of the Jim Sage volcanic suite (JSVS) obtained from analyses of individual zircon domains. Also shown is the range for the central Snake River Plain (SRP) as estimated from whole-rock values reported in Watts et al. (2010). VSMOW—Vienna

standard mean ocean water. (B) Summary of εHf(i) compositions from zircon of the upper and lower member of the JSVS obtained from analyses of individual zircon domains. SRP values are from glass shards of different units reported in Nash et al. (2006).

The lower member is best exposed in the tuff, with thin beds of volcanic sandstone com- butes of silicic lava fl ows of the SRP (Branney canyons along the eastern side of the Jim Sage posed of glass shards (Fig. 3). et al., 2008; Fig. 2). In addition, based on seismic Mountains (Fig. 2). The base of the member The upper member is best exposed in the refl ection data and borehole data, it was demon- consists of a 5–30-m-thick rhyolitic auto- Cotterel Mountains, and has a locally auto- strated (Konstantinou, 2013) that these rhyolites breccia, with vitreous clasts and a matrix of brecciated base with occasional peperites and are not widely present in the Raft River Basin, strongly altered glass (Figs. 3, 4A, and 4G). clasts of the older rhyolite lavas (Figs. 3 and 4E). buried beneath younger sediments. Instead, the The rest of the lower member is made up of The peperites provide evidence that the lavas eastern limit of the rhyolite fl ows is their approx- a thick (350–400 m) sequence of massive or fl owed in water or wet sediment. The majority imate present-day exposure in the Jim Sage and layered, glassy rhyolite lava with occasional of the upper member is made up of ~400 m of Cotterel Mountains. Had these rhyolite fl ows columnar jointing and devitrifi ed interiors intercalated massive and banded lava with thick been ignimbrites, one would expect they would (Fig. 3). These rocks have aphanitic or glassy sequences characterized by columnar joints (Fig. have spread out more thinly to fi ll the east-west matrix and 20%–25% phenocrysts of euhedral 3). These lavas are similar in appearance to the extent of the fl oor of the Raft River Basin, but the plagioclase (andesine) > anhedral augite and lower member unit, with aphanitic or glassy rhyolites were not sampled in the deep boreholes pigeonite > Fe-Ti oxides > quartz > sanidine >> matrix and 20%–25% phenocrysts composed of the Raft River Basin that penetrated the lower zircon, and are similar to the phenocryst com- of euhedral plagioclase (andesine) > anhedral plate of the metamorphic core complex (Wil- positions of rhyolite erupted from the SRP augite and pigeonite > Fe-Ti oxides > quartz > liams et al., 1974, 1982; Covington, 1983). province (e.g., Bonnichsen and Citron, 1982; sanidine >> zircon (Figs. 4B, 4E, 4F). Thus, the Small subvolcanic intrusions (vents) and lava Honjo et al., 1992; Andrews et al., 2008; Ellis basal contacts of both the lower and upper mem- domes are also present. These are identifi ed by and Wolff, 2012; Fig. 4A). ber are characterized by autobreccias, typical of their silicifi ed interiors, and the fact that they The lower and upper members are separated silicic lavas, rather than a basal surge deposit dome the strata they were intruded into (Figs. by a relatively thin (0–100 m) sedimentary brec- characteristic of a welded ignimbrite (Branney 2 and 5). Based on a positive magnetic anomaly cia containing clasts derived from the underlying et al., 2008; Figs. 4A, 4B, 4E–4G). The thick- that covers the area of exposed rhyolite domes lower member (Fig. 3). The breccia is topped by nesses (lack of exposures with thickness of (Fig. 2) and on seismic refl ection profi les that a ~50–100-m-thick lacustrine sedimentary unit <30 m), aspect ratios, and abrupt lobate termi- image what appear to be intrusions (Mabey and composed of thickly bedded calc-arenite and nations are all consistent with the physical attri- Wilson, 1973; Williams et al., 1974, 1982; Cov-

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ington, 1983; Konstantinou, 2013), an elliptical yielded a weighted mean age of 8.43 ± 0.12 Ma two members of the JSVS have distinctly dif- (~4 by 2 km) cryptodome probably underlies (2σ; MSWD = 1.5; n = 26; Fig. 7D). Four zir- ferent pyroxene pair compositions. The lower the eastern portion of the Jim Sage Mountains con grains resulted in slightly older ages (ca. 9.5 member is characterized by an augite-pigeonite (Fig. 2). We infer the exposed domes and the and 11 Ma) and are interpreted to be antecrystic, pyroxene pair, and the upper member of the cryptodome to be part of the vent system for similar to observations from the Yellowstone JSVS has an augite-pigeonite pair that contains the lavas of the JSVS. caldera (Vazquez and Reid, 2002). A second, more MgO than the lower member (Fig. 10). Above the upper rhyolite member of the very similar sample collected from a higher The pyroxene pairs in the rhyolite lavas are con- JSVS is a thin (<100 m), densely welded stratigraphic position (C1B) yielded a weighted sistent with being in equilibrium at high tem- ignimbrite with an interior zone that displays mean age of 8.19 ± 0.14 Ma (2σ; MSWD = 1.4; peratures (~850–900 °C; Lindsley, 1983) and intense rheomorphic fl ow and transposed and/or n = 14; Fig. 7E). low pressures (<3 kbar), and are similar to typi- deformed shards (Fig. 4C). A less welded dis- A small number of zircon cores (n = 9) from cal central SRP rhyolites (e.g., Bonnichsen and tal phase of this ignimbrite preserves small all fi ve Miocene samples were identifi ed and Citron, 1982; Honjo et al., 1992; Andrews et al., collapsed pumice lapilli (Figs. 4C, 4D). This dated; these ages are plotted on the same concor- 2008; Ellis et al., 2010). In addition, a single ignimbrite covers most of the northern Cot- dia diagram (Fig. 7F). U-Pb isotope ratios from population of augite and pyroxene pair in both terel Mountains, forming a relatively smooth individual zircon defi ne two discordia lines with fl ow units is consistent with similar observa- dip slope on its northwestern side (Fig. 2). The poorly constrained upper intercepts ca. 2550 and tions of central SRP lavas, whereas ignimbrites ignimbrite is also exposed in the hills near the 1745 Ma (±140–150 m.y.) and Mesozoic lower from the province often have multiple popula- town of Albion (Fig. 2). Above this ignimbrite is intercepts (ca. 75 and 95 Ma ± 270 m.y.; Fig. tions of pyroxene pairs (Cathey and Nash, 2009; a capping sequence of basaltic fl ows exposed in 7F). These data points are too few for any con- Ellis and Wolff, 2012). the northern Cotterel Mountains (basalt of Cot- clusive interpretation, but there are widespread terel Mountains of Williams et al., 1982), which exposures of Archean basement in the region Whole-Rock Multicollector-ICP-MS Sr and is as much as ~100 m thick, and was dated as dated as ca. 2550 Ma, and most Eocene–Oligo- Nd Isotope Analyses 9.2 ± 1.5 Ma (1σ; K-Ar; Armstrong et al., 1975). cene plutons of the Albion–Raft River–Grouse Both the ignimbrite and the basalt of Cotterel Creek metamorphic core complex that have been The whole-rock Sr and Nd isotope results 87 86 are inferred to be erupted from the central SRP. studied have similar inherited ages (Strickland were used to calculate age-corrected Sr/ Sri ε Lacustrine sediments conformably over- et al., 2011b; Konstantinou et al., 2011, 2013). and Nd(i) values (Fig. 11; Supplemental Table 4 lie the upper rhyolite fl ow, are poorly exposed [see footnote 4]; Bouvier et al., 2008). The lower 87 86 ε along the fl anks of the Cotterel Mountains, Results from Major and Trace Element member has Sr/ Sri = 0.7114–0.7121 and Nd(i) and are known from borehole localities in the Geochemistry values that range from –6.7 to –7.1; the upper 87 86 ε eastern part of the Raft River Basin (Fig. 4). member has Sr/ Sri = 0.7130–0.7135 and Nd(i) These deposits consist mostly of thick-bedded The lower member of the JSVS has a limited values of –7.1 (Fig. 11). The basalt of Cotterel 87 86 reworked ash-fall tuffs and marls and contain range in composition, varying from low-silica Mountains has Sr/ Sri composition of 0.7066– ε clasts of rhyolite from the JSVS. alkali rhyolite to low-silica rhyolite (SiO2 = 0.7075 and Nd(i) = –3.7 (Fig. 11; Supplemental 72%–74%; Fig. 8). The upper member of the Table 4 [see footnote 4]). The isotopic composi- SHRIMP-RG U-Pb Geochronology JSVS is a low-silica alkali rhyolite with slightly tions of the rhyolite lavas of the JSVS and the

of Zircon less silica content (SiO2 = 70%–71.5%; Fig. 8) basalt of Cotterel Mountains are very similar to than the lower member. The four maﬁ c samples published data from rhyolites and basalts of the Sample JS11-B, from the lower member of are basaltic in composition and most of the out- central SRP (Fig. 11; Nash et al., 2006; Chris- the JSVS, is a two-pyroxene rhyolite lava con- ﬂ ow ignimbrite and lava samples (away from tiansen and McCurry, 2008). taining euhedral zircon (Fig. 6A). The weighted the Jim Sage and Cotterel Mountains) have mean of 23 207Pb-corrected 206Pb/238U ages compositions similar to that the lower member In Situ Zircon Secondary Ion Mass yields an age of 9.53 ± 0.10 Ma (2σ) with a of the JSVS (Fig. 8). All of the JSVS samples Spectrometer O Isotope Analyses

mean square of weighted deviates (MSWD) of are ferroan (high FeOt/MgO ratios), and dis- 2.3 (Fig. 7A). Zircon analyses (n = 14) from a tinctly different from the evolved calc-alkalic The O isotope compositions of individual ana- second sample, collected from a higher strati- composition of the underlying Archean base- lytical spots from Miocene JSVS zircons from δ18 graphic position of the lower member of the ment in the region (Fig. 8B). The rare earth the lower member range in Ozr (VSMOW) JSVS (sample JS1), yielded a weighted mean element (REE) patterns of the rhyolite samples from –0.5‰ to 4.6‰, and from the upper mem- age of 9.34 ± 0.12 Ma (2σ; MSWD = 0.9; Fig. from the lower and upper members of the JSVS ber range from 0.5‰ to 4.9‰ (Fig. 12A; Sup- 7B). Three zircon grains resulted in slightly are very similar, and show strong enrichment in plemental Table 5 [see footnote 5]). However, older U-Pb ages (ca. 10–11 Ma) and are inter- light (L) REEs relative to heavy (H) REEs, and most of the zircon analyses cluster around values δ18 preted as antecrystic (e.g., Bindeman et al., large negative Eu anomalies (Fig. 9). The basalt of Ozr (VSMOW) ranging from 0‰ to 2.5‰ 2001). Sample C1, collected from the subvol- of Cotterel Mountains has a small enrichment in (43 of 58 analyses), with only one analysis hav- δ18 canic intrusion in the southern Cotterel Moun- LREEs relative to HREEs and a small positive ing a Ozr value >5‰. The whole-rock oxy- δ18 tains, yields a weighted mean age of 9.19 ± Eu anomaly (Fig. 9). gen isotope compositions calculated from Ozr σ δ18 ≈ 0.14 Ma (2 ; MSWD = 2.7; n = 26). The high range from Owr 6.7‰ to 1.4‰ (but mostly MSWD may reﬂ ect two zircon populations with Geochemistry of Phenocryst Assemblages cluster around 2‰–4.5‰) and are similar to an irresolvable age difference (Fig. 7C). values reported from the 12–6 Ma rhyolites of Two samples from the upper member of the The lower and upper rhyolite members of the the central SRP (see compilation in Watts et al., JSVS were also collected and dated. Sample JSVS have andesine feldspar phenocrysts that 2010; Fig. 12A). A few large (~200 μm in diam- C-RC is a two-pyroxene rhyolite lava that are homogeneous in composition (Fig. 10). The eter) zircon grains were analyzed with multiple

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secondary ion mass spectrometer spots, and the Raft River–Grouse Creek metamorphic core River–Grouse Creek metamorphic core com- results show isotopic zoning within individual complex. It is unlikely that this upper part of the plex (e.g., Egger et al., 2003; Strickland et al., grains (Figs. 6A, 6B). In this limited number crust would be involved as a major source of 2011b; Konstantinou et al., 2013). We infer that δ18 of examples, we observed a decrease in Ozr contamination for Miocene magmas, because it similar rocks may underlie the whole region δ18 δ18 from core ( Ozr ~ 4‰) to rim ( Ozr ~ 2.2‰), is not thermodynamically favorable to melt and of southern Idaho, and we collectively refer to similar to observations from Yellowstone zircon assimilate carbonate rocks in igneous systems. these potential sources and/or contaminants as reported in Bindeman et al. (2008). A thick sequence (~5 km) of Neoproterozoic upper crust (Fig. 13; Table 2). Specifi cally, we quartzites and schists is beneath this carbonate used the composition of Oligocene plutons to In Situ Zircon LA-ICP-MS Hf sequence and constitutes the base of the passive represent the composition of the upper crust res- Isotope Analyses margin succession of western North America. ervoir. The bulk Sr-Nd isotopic composition of 87 86 Even though these rocks have been tectonically the upper crust was estimated to be Sr/ Sr10 = ε ε The Hf isotope results and the calculated Hf(i) attenuated (to ~1 km) in the Albion–Raft River– 0.716, and Nd(10) = –28 based on measured val- values from the upper and the lower members Grouse Creek metamorphic core complex ues from the Oligocene plutons (Table 2; Strick- of the JSVS are summarized in Figure 12B (Fig. 1), they are probably thicker to the north land et al., 2011b; Konstantinou et al., 2013; ε (Supplemental Table 6 [see footnote 6]). The (e.g., Compton et al., 1977; Compton 1972, Fig. 13A). The Hf(10) of the potential upper ε ε Hf isotope analyses yielded zircon Hf(i) values 1975), and may have underlain extensive areas crust reservoir was estimated to be Hf(10) = –32, ranging from –0.8 to –6.8, but most of the analy- of southern Idaho. The schist units have com- based on values obtained in zircon from Oligo- ses have a small range from ~–4 to –6.5 and the positions favorable for having undergone partial cene magmas in the Albion–Raft River–Grouse values obtained from each individual sample melting during Miocene magmatism. The struc- Creek metamorphic core complex (Konstanti- are homogeneous (Fig. 12B; Supplemental turally deepest and oldest rocks exposed today nou et al., 2013; Table 2). The values selected ε ε Table 6 [see footnote 6]). The lower member of in the metamorphic core complex are Archean for Hf(10) and Nd(10) do not follow the terrestrial ε the JSVS has zircon Hf(i) values that range from crystalline rocks of the Green Creek Com- array relationship established by Vervoort et al. –0.8 to –6.8, and the upper member has a range plex, which constitutes depositional basement (2011), but are based on the measured isotopic of values from –1.6 to –6.6 (Fig. 12B). The for the Neoproterozoic and Phanerozoic cover ratios of Oligocene plutons. Zircon δ18O values total range of values within each unit is larger sequence. The Green Creek Complex is com- analyzed from the Oligocene plutons and the than the 2σ error on individual zircon analysis posed primarily of augen orthogneiss with lesser Archean basement were used to estimate whole- ε ε δ18 (±1.5 Hf units). Zircons with Hf(t) values from amphibolite, diorite, tonalite, and metasedimen- rock magmatic values. These calculated Owr –0.8 to –2.1 are suggested to be antecrystic tary rocks (e.g., Compton, 1972, 1975), all of for the Archean and Oligocene rocks have a very because they have Hf isotope compositions that which have favorable compositions to undergo narrow range of 6.5‰–7.5‰, with an average δ18 are more radiogenic than the majority of zircon melting during Miocene magmatism. Rocks of value of Owr = 7.1‰ (Konstantinou et al., ε δ18 analyses, which cluster around –4 and –6.5 Hf(t) similar Archean age and isotopic composition 2013). However the Owr of the upper crustal values (Fig. 12B). have been incorporated as xenoliths in basal- potential crustal reservoir was estimated to be tic magma of the SRP (Leeman et al., 1985). either –1.5‰, similar to values estimated for the Possible Crustal and Mantle Sources for Finally the Albion–Raft River–Grouse Creek SRP (e.g., Watts et al., 2011), or ~2.5‰, similar Geochemical Modeling metamorphic core complex exposes a wide vari- to values measured from hydrothermally altered ety of Eocene–Oligocene calc-alkaline plutons rocks of the Idaho batholith (Criss and Taylor, δ18 Based on stratigraphic considerations alone, that also have compositions favorable to undergo 1983; Criss et al., 1984). This low- Owr value the Albion–Raft River–Grouse Creek metamor- melting during Miocene magmatism. Similar refl ects postmagmatic hydrothermal alteration phic core complex represents a relative vertical Eocene plutons are exposed in the Challis vol- by meteoric waters in the region of southern uplift of a minimum of 10 km, from the top of the canic fi eld (e.g., Criss and Taylor, 1983; Criss Idaho. The cause of this alteration discussed in Paleozoic section to the Archean (Konstantinou et al., 1984), and the extensive magmatic roots the following. et al., 2012). Thus the metamorphic core com- of these Cenozoic plutons may underlie large Eocene plutons in the Albion–Raft River– plex exposes extensive areas of once deep-seated areas of southern Idaho, including the region of Grouse Creek metamorphic core complex are rocks, similar to those that may underlie the cen- the SRP province. thought to have extensive deep roots, which are tral SRP and the JSVS. The Sr-Nd-Hf-O isotope For the purpose of modeling the petrogenesis not exposed, based on the time span of Eocene compositions of several igneous and metamor- of the Miocene magmas (see following discus- magmatism and the compositions of these plu- phic rocks of different ages now exposed in the sions), we made some simplifying assumptions tons (Konstantinou et al., 2013). In addition, the core complex are well characterized (Strickland about possible crustal reservoirs, assuming aver- Archean basement is inferred to include rocks et al., 2011b; Konstantinou et al., 2013). Here age isotopic compositions of the lower crust, of more mafi c compositions at depth, based on we summarize the stratigraphy and the crustal upper crust, and a mantle source for the mag- the isotopic composition of the Cenozoic igne- structure of rock units currently exposed in the matic system (following the reasoning in Kon- ous rocks (Strickland et al., 2011b; see detailed Albion–Raft River–Grouse Creek metamorphic stantinou et al., 2013) and summarized in the discussion in Konstantinou et al., 2013). A core complex. Because of their regional nature, following. However, the structure and composi- potential lower crust reservoir could be repre- these same rock units might be expected to be tion of the deep crust are likely more variable sented by the roots of Eocene plutons as well at depth beneath the SRP and southern Idaho. and complex than assumed here and shown in as by intermediate-composition Archean crys- The highest stratigraphic and/or structural levels Table 2 and Figure 13. talline rocks. Based on the interpretation (made compose a thick (6–8 km) sequence of Paleo- Silicic rocks like the Proterozoic schist units, in Konstantinou et al., 2013) that the Eocene zoic carbonates now partly exposed in the Black the Archean orthogneiss and the Eocene–Oligo- plutons in the Albion–Raft River–Grouse Creek Pine Mountains (Fig. 1; Compton et al., 1977) cene plutons that intrude them, are the domi- metamorphic core complex represent reworking and in mountain ranges surrounding the Albion– nant rock type exposed in the Albion–Raft of the deep crust, we assume that the isotopic

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TABLE 2. PARAMETERS USED IN ASSIMILATION–FRACTIONAL CRYSTALLIZATION MODELING OF THE JIM SAGE VOLCANIC SUITE, INCLUDING THE COMPOSITIONS OF THE DIFFERENT CRUSTAL AND MANTLE RESERVOIRS Sr Nd Hf ε ε δ18 Δ Reservoirs Sri (ppm) Bulk D Sr Nd (ppm) Bulk D Nd Hf (ppm) Bulk D Hf OWR Stage I: Assimilation of basalt and maﬁ c lower crust (r = 0.96) Average mantle 0.7053 135 0.1 2.0 11.5 0.1 5.0 2.5 0.1 5.6 –0.2 (pristine magma I) Lower crust 0.7108 450 0.1 –18.0 30.0 0.1 –22.0 3.5 0.1 7.0 –0.2 Stage II: Assimilation of mixed magma and intermediate lower crust (r = 0.36) Mixed magma (F = 0.992) 0.7071 392 1.5 –3.5 21.1 0.1 –0.8 4.0 0.1 5.8 –0.1 Upper crust 0.7160 150 1.5 –28.0 7.5 0.1 –32.0 2.5 0.1 –1.5 –0.1 Note: Δ represents the difference in δ18O between fractionating crystals and magma, i.e., Δ = δ18O (crystals) – δ18O magma. WR—whole rock; Bulk D—bulk partition coefﬁ cient.

composition of the Eocene plutons now exposed zircon from the Eocene plutons (Table 2; Fig. the Cenozoic is poorly constrained, primar- to the surface may refl ect the average composi- 13B; Konstantinou et al., 2013). The normal ily because of the extensive contamination of δ18 tion of the potential lower crust reservoir. The Owr of the lower crustal reservoir refl ects the mantle-derived basalts by lower crustal melts. Sr, Nd, O, and Hf isotopic composition of this inability of meteoric fl uids to infi ltrate beneath However, He isotope compositions from Mio- potential lower crust reservoir was estimated the brittle-ductile transition zone (depths of cene and Pleistocene basalts within the SRP 87 86 ε δ18 to be Sr/ Sr10 = 0.7108, Nd(10) = –18, Owr = ~10 km; Person et al., 2007) and shift the oxy- indicate that these magmas may be mixtures ε 7.1‰, and Hf(10) = –22 based on values deter- gen isotope composition of the deep crust to between a mantle plume and a lithospheric man- mined from the Eocene Emigrant Pass pluton lower and/or negative values. tle reservoir (Graham et al., 2009). For this study, (Table 2; Fig. 13; Konstantinou et al., 2013). The geochemical compositions of possible we estimated that the unmodifi ed isotopic com- δ18 ≈ This deep crustal reservoir has normal Owr asthenospheric or lithospheric mantle reservoirs position of the Miocene mantle-derived basalts 7.1‰, similar to values calculated by analyzing that contributed melts to the lower crust during represented the mean composition of three end-

Key for Figures 13A and B Isotopic Data and Modeling Compositions Basalt Upper Rhyolite 10 Lower Rhyolite 10 PREMA Model crustal Reservoir PREMA Model mantle Reservoir Mantle Array 5 central SRP 5 AFC Modeling results (crosses every F=0.2): Mantle and lower crust rhyolites BSE Mixed magma and upper crust 0 EM-II 0.12 BSE 0.27 0.21 0 Number in italics indicates total fraction of crustal 0.29 component in magma –5 0.12 SRP rhyolites 0.35 0.21 0.41 0.29 –5 0.35 –10 0.44 EM-I 0.41 (i) –15 (i) –10

SRP basalt εHf εNd 0.83 0.91 –20 Typical analytical 2σ –15 0.83 error of indivitual zircon 0.91 Hydrothermally Lower crust –25 Lower crust altered upper crust Range of –20 –30 Range of values for values for Range of lower crust values for hydrothermally Typical analytical 2σ lower crust altered upper crust Range of values for –35 –25 error of indivitual hydrothermally sample Upper crust A B altered upper crust –40 0.7020.706 0.710 0.714 0.718 0.722 –2.0 08.02.0 4.0 6.0 87 86 18 Sr/ Sri δ O wr ‰ (VSMOW)

87 86 Figure 13. (A) Sr/ Sri versus εNd(i) compositions of samples from Jim Sage volcanic suite (JSVS), the basalt of Cotterel Mountains (Williams et al., 1982) and ﬁ eld of isotopic values from the central Snake River Plain (SRP) province. PREMA—prevalent mantle; EM-I—enriched 18 18 mantle I; EM-II—enriched mantle II; BSE—bulk silicate earth. (B) δ Owr (wr—whole rock; calculated from zircon δ O) versus εHf(i) compositions determined from individual zircon grains from samples of the JSVS, and ﬁ eld of isotopic values from the central SRP province. Results from AFC (assimilation–fractional crystallization) isotopic modeling show the relative crust versus mantle components represented by the Miocene magmas (see discussion in text and Table 2). Range of SRP isotopic values is from Christiansen and McCurry (2008), Watts et al. (2010), and Nash et al. (2006). VSMOW—Vienna standard mean ocean water.

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member mantle reservoirs, using the range of and mantle sources for geochemical modeling), low A:FC rates (r = 0.34) used in modeling the values summarized in Rollinson (1993). The and the equations of simultaneous assimilation interaction of contaminated basalt with upper three mantle reservoirs used were mantle plume and fractional crystallization (AFC) derived by crustal rocks indicates high crystal fractionation and/or prevalent mantle, ancient enriched litho- DePaolo (1981). We also used the equation for rates, which were probably enhanced by the ε ε spheric mantle (lower Nd, Hf than plume value), estimating the ratio of mass of assimilated crust residence of rhyolitic magma chambers at shal- and a fl uid-enriched (higher 87Sr/86Sr than plume versus initial mass of magma from Aitcheson low crustal levels (6–10 km). Based on regional value) lithospheric mantle (Fig. 13; Table 2; ref- and Forrest (1994). We simultaneously modeled stratigraphic studies in southern Idaho, the crust erences in Rollinson, 1993). the isotopic data (Sr, Nd, O, and Hf) together at 6–10 km may have been dominated by the with the Sr, Nd, and Hf elemental composi- presence of carbonate rocks (see discussion DISCUSSION tions and tried a range of geologically reason- of possible crustal and mantle sources). How- able values for the independent variables, using ever, depending on the extensional histories Petrogenesis of the Jim Sage Volcanic Suite A:FC rates (r value in DePaolo, 1981) and bulk of the crust in this region, structurally deeper partition coeffi cients (the fi nal values used are crystalline basement and overlying Neo protero- ε The similarity of zircon Hf(i) values measured reported in Table 2). The fi nal Sr-Nd-O-Hf iso- zoic quartzites and schists may have been at from an exposed subvolcanic intrusion and the topic compositions of hybrid magmas predicted shallower levels or 5–10 km deep (they are at lava samples of the JSVS indicates that this (and from our AFC model are compared to measured the surface in the Albion–Raft River–Grouse 87 86 ε possibly other) subvolcanic intrusion is part of whole-rock values ( Sr/ Sri- Nd(i); Fig. 13A) Creek metamorphic core complex). Residence δ18 ε the volcanic vent system for the eruption of the and to the calculated Owr and Hf(i) based on of rhyolitic magma chambers at shallow, rela- lavas of the JSVS (Fig. 12B). Thus, the occur- measured isotopic values from Miocene zircon tively cool crustal levels (~5–10 km) has been δ18 ε rence of lava fl ows, lava domes, and subvolcanic (calculated Owr and Hf(i); Fig. 13B). documented today by the geophysical imaging intrusions within the Raft River Basin implies We used a two-stage assimilation model of low-velocity zones interpreted to be shallow that the JSVS erupted within the basin, and the for magma genesis (e.g., Gans et al., 1989; crystal-rich mushes beneath Yellowstone (e.g., rhyolite lavas did not fl ow very far from their Grunder, 1993; Watts et al., 2010) that involves Chu et al., 2010). vents (<30 km). The northern part of the JSVS the high-temperature interaction of mantle- The low A:FC rates used to model the rhyo- was capped by ignimbrite and basalt fl ows derived basalts with partial melts from the mod- lites of the SRP and the JSVS indicate that these derived from the central SRP. eled normal δ18O (≈7.0‰) lower crust reservoir low-silica rhyolite may have evolved by ~80% The age range of the rocks in the JSVS, (see discussion of possible crustal and mantle crystal fractionation (modeled at a partial melt together with their whole-rock major, trace ele- sources; Table 2) and high (r = 0.96) A:FC rates, fraction F = 0.2), coupled with assimilation ment, and isotopic compositions and their zircon approaching zone-refi ning values, shown by the (Fig. 8). Crystal fractionation helps to decrease oxygen and Hf isotope compositions, are very black lines in Figures 13A and 13B. The high the volume of the melt in the magmatic system similar to geochemical data sets collected on A:FC rates (r = 0.96) were chosen to refl ect the undergoing AFC processes, thus producing the high-temperature rhyolites from the central high fl ux of basaltic melts that intruded the lower low-δ18O values with smaller amounts of crustal SRP (e.g., Bonnichsen and Citron, 1982; Honjo crust during the Miocene. Based on this model- component than those predicted by simple mix- et al., 1992; Boroughs et al., 2005; Cathey et al., ing, we suggest that the basalt of the Cotterel ing models. To illustrate this, we were able to δ18 ≈ 2007, 2008; Branney et al., 2008; Christiansen Mountains and other basalts in the SRP may successfully model rhyolite magma of Owr and McCurry, 2008; Figs. 8, 11, and 12), sug- be contaminated with small amounts (~10%) 2.0‰ using the AFC model and assimilating δ18 gesting that the JSVS is clearly a part of the SRP of lower crust melts (Fig. 13), although the iso- ~30% crust with Owr = –1.5‰ by ~70% of province. The high temperature of the rhyolite topic values for the basalt may also refl ect con- already contaminated (by 10% of crustal melts) δ18 lavas (estimated from pyroxene compositions; tamination from a lithospheric mantle source. basalt with Owr = 5.8‰ (Fig. 13B). If a simple Fig. 10) is consistent with a large fl ux of astheno- This contaminated magma was further mixing model is used with the same crust-basalt δ18 sphere-derived basalt (hotspot) that in turn led to hybridized by variable amounts of partial end members ( Owr = –1.5‰ and 5.8‰ for widespread crustal melting, as suggested for the melts from the modeled upper crust reservoir crust and contaminated basalt, respectively), magmas of the SRP province (e.g., Nash et al., (red lines in Figs. 13A, 13B; isotopic values in and crust to contaminated basalt mixing ratios of δ18 2006; McCurry and Rodgers, 2009). Table 2), but at lower A:FC rates (r = 0.34). The 30%:70%, the resulting Owr would be ~3.6‰, δ18 ≈ upper crust assimilant has a negative Owr and not ~2.0‰ predicted by the AFC model. Isotopic Modeling of the Miocene Magmas –1.5‰, similar to values used in modeling of of the JSVS and Central SRP the SRP province (e.g., Watts et al., 2010, 2011). Low-δ18O Rhyolites of the JSVS Our modeling suggests that the rhyolites of the Our extensive isotopic data set from once JSVS and the SRP can be explained by the high- The low-δ18O values of the JSVS rhyolite deep-seated rocks of the Albion–Raft River– temperature interaction of crust-contaminated lavas that were erupted within the Raft River Grouse Creek metamorphic core complex allows basalt (similar to the Cotterel basalt) followed Basin, ~50 km south of the SRP province, us to use these rocks as proxies for the composi- by large amounts of assimilation of upper crust require the assimilation of hydrothermally tion of the upper and lower crust in modeling rocks. More specifi cally, our AFC modeling altered crust at shallow levels as proposed for measured compositions in Miocene volcanic indicates that the Miocene rhyolites of the SRP the SRP rhyolites. However, there are no known rocks. To model and evaluate the relative roles may represent hybrid magmas composed of long-lived caldera systems within the Raft River and components of mantle versus crustal mate- ~60% mantle-derived basalt, and ~40% crustal Basin, thus the low-δ18O signature of the JSVS rial in the Miocene magmas of the JSVS and the melts derived from two modeled crustal reser- cannot be attributed to repeated caldera collapse central SRP province, we used the isotopic con- voirs; a normal δ18O (≈7.1‰) lower crust, and a events that drop hydrothermally altered rocks straints provided by the upper and lower crust negative δ18O (≈–1.5‰) hydrothermally altered into the magmatic system (e.g., Bindeman and reservoirs (see discussion of possible crustal upper crust reservoir (Fig. 13). The relatively Valley, 2001; Bindeman et al., 2007; Watts et al.,

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δ18 ≈ 2011; Drew et al., 2013). We suggest two ways pal horizontal stress, due to the concentration of province ( Owr –1.5‰; see the discussion of to explain the low-δ18O signature of the JSVS stress around the borehole. isotopic modeling). lavas that erupted outside the SRP province. The major pitfall of this model of explaining Because there are no known caldera com- Each of these suggestions (model A and model the JSVS is achieving the physical propaga- plexes in the Raft River Basin to explain the B) (Fig. 14) has its strengths and limitations. tion of rhyolite melt within the crust, over large hydrothermal alteration of the crust, we pro- distances (~50 km; Fig. 14; model A). This is pose alternative mechanisms for the formation Model A: Low-δ18O Silicic Melt Propagation problematic due to the high viscosity of silicic of ~500 km3 of low-δ18O crust beneath the Raft Along a Preexisting Normal Fault melt that would cause them to quench during River Basin. Measured zircon δ18O values for The eruption of low-δ18O rhyolites outside injection over large distances. Few studies have deep Archean–Oligocene crustal rocks in the the main SRP province may be explained by focused on silicic melt propagation along struc- Albion–Raft River–Grouse Creek metamor- the shallow-level (<10 km) propagation of hun- tures in the upper crust, but an example may be phic core complex indicate that they have nor- δ18 δ18 δ18 ≈ dreds of cubic kilometers of low- O rhyolite the 222–196 Ma, 18 km linear trend of a silicic mal magmatic Owr values ( Owr 7.1‰; magma from the central SRP magma cham- dike swarm in the Candelaria mining district in see discussion of possible crustal and mantle bers to a magma chamber beneath the JSVS western Nevada (Thomson et al., 1995). Lat- sources; Strickland et al., 2011b; Konstantinou (Fig. 14; model A). The low-δ18O signature of eral migration of silicic magma along faults et al., 2013). These relationships imply that the the rhyolite melt is produced within the SRP- in the Aso Caldera, southwestern Japan has alteration was synchronous, or younger than Yellowstone province via one of the processes also been demonstrated (Miyoshi et al., 2013). Oligocene, and a plausible mechanism for the described herein (see discussion of Cenozoic Another problematic aspect of this model is hydrothermal alteration of the upper crust may extensional and magmatic history of the Basin that large amounts of hydrothermally altered be related to extensional deformation during the and Range province; e.g., cf. Bindeman et al., rocks with negative δ18O are required to produce formation and localized rise of metamorphic 2007; Boroughs et al., 2005, 2012; Fig. 14; the large-volume magmas of the SRP (and the core complexes. Oligocene granite-cored gneiss model A), and via AFC processes (as described JSVS; see the discussion of isotopic modeling). domes fi rst rose diapirically to the base of the in the discussion of isotopic modeling). This These extremely low δ18O rocks are rare in the upper crust over a protracted period of time (32– model is successful at explaining the similar geologic record of the region, but we envision 25 Ma; Fig. 14; Strickland et al., 2011a, 2011b; compositions of all the isotopic systems (Sr-Nd- that some of these rocks are faulted and buried Konstantinou et al., 2012). The regions around O-Hf) between the SRP and the JSVS, because beneath younger volcanic rocks within the SRP, and between the granite-core gneiss domes may the JSVS is essentially connected with the and thus inaccessible for study. have been intensely faulted and fractured (see magmatic system of the central SRP at shallow discussion in Konstantinou et al., 2012, and depths (see the discussion of isotopic modeling). Model B: Large-Scale Crustal Melting numerical modeling by Rey et al., 2011). These The propagation of the rhyolite beneath the Raft Beneath the Raft River Basin fractures would have brecciated large volumes River Basin occurs along north-south conduits An alternative explanation for the occurrence of the Archean basement and the Proterozoic perhaps controlled by the geometry of the major of the low-δ18O rhyolite lavas of the JSVS (Fig. schist units above and away from the gneiss north-south–trending normal fault (the Albion 14; model B) might entail melting of hydrother- domes. A large number of closely spaced faults fault) that bounds the Albion Mountains on its mally altered crust outside the SRP-Yellowstone may have potentially provided multiple con- eastern side. The Albion fault was an important province and directly beneath the Raft River duits for meteoric fl uids to percolate at depths Miocene crust-penetrating structure and possi- Basin. We envision that a normal δ18O (≈5.6‰) of 6–10 km. The emplacement of the Oligocene bly served as a structural weakness in the upper basaltic plumbing system of the JSVS may have plutons and the rise of the hot crust of the gneiss crust (Konstantinou et al., 2012; Konstantinou, been connected to that of the SRP-Yellowstone domes may have provided the necessary heat to 2013). Based on the extensional history of the province at deeper levels, either within the lower exchange oxygen between low-δ18O meteoric Albion–Raft River–Grouse Creek metamorphic crust or at the base of the crust (Fig. 14; model fl uids and the Archean basement, and Protero- core complex (Konstantinou et al., 2012; Kon- B). In model B, the low-δ18O rhyolite magmas zoic metapelites thus may have resulted in the stantinou, 2013), the elastic crust in the region of the JSVS may have formed by the inter action formation of a low-δ18O crustal reservoir in the of southern Idaho underwent high extensional of a large fl ux of basalt with hydrothermally brecciated and metamorphosed region between strain rates from ca. 14 until after 8 Ma, and dur- altered crust at depths of 6–8 km beneath the and around the gneiss domes. ing that time major SRP magma chambers may Raft River Basin, resulting in large-scale crustal Oxygen isotope studies of rocks within and have been just north of the Albion–Raft River– melting but not signifi cant mixing and/or assimi- around other metamorphic core complexes in Grouse Creek metamorphic core complex. lation of these crustal melts with the basalt (Fig. the western U.S. indicate that low-δ18O zones This magma chambers may have acted as large 14). Melting of the crust on an extensive scale are common in the brecciated and chloritized hetero geneous bodies in the elastic crust having (rather than AFC processes; see the discussion regions around the complexes. For example, completely different physical properties (e.g., of isotopic modeling) would require a smaller in the Valhalla gneiss dome, 1–2-km-thick 3 δ18 δ18 viscosity, stiffness) relative to the extending volume (~500 km ) of low- Owr upper crust, upper plate chloritic breccias have Owr and δ18 δ18 elastic crust. This less stiff inclusion may have with Owr of ~2.5‰ to produce the rhyolites Ofeld (feld—feldspar) that range from ~5‰ δ18 focused the extensional strain in the elastic crust of the JSVS. to –5‰, even though the mylonite zones Owr δ18 δ18 along the northern and southern tips around Because rocks with Owr of 2‰–3‰ are and Ofeld values that range from 11‰ to 2‰ the magma chamber and may have assisted in more common in the region (e.g., Criss and (Holk and Taylor, 2007). Another example is the the propagation of rhyolitic melt along north- Taylor, 1983; Criss et al., 1984; <200 km north Bitterroot metamorphic core complex, where δ18 ≈ south–trending avenues in the shallow crust. of the Raft River Basin), both the Owr chloritic breccias above the mylonite zones δ18 δ18 This process may be analogous (at a different 2.5‰ and the volume of the hydrothermally have Owr and Ofeld that range from ~6‰ to δ18 scale) to borehole wall breakouts, which form altered crust are not as problematic as the values –4‰, and the mylonite zones have Owr and δ18 symmetrically at the orientation of least princi- required for modeling the rhyolites of the SRP Ofeld values that range from 12‰ to –1‰

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SRP) look- BDTZ melt ponding Sub-continental Jim Sage volcanic suite shares a deep source with Snake River Plain magmas. Extensive shallow crustal melting of hydrothermally altered crust results in low δ O magmas of the Jim Sage volcanic suite. Mts Albion Circulation of meteoric fluids Jim Sage volcanic suite Basin Albion fault Raft River Mtns

Black Pine

18 Miocene A Model chamber (14–9.5 Ma) magmas rhyolitic magma Shallow Jim Sage Normal δ O mafic Mtns Albion 25 Ma 14 caldera SRP rhyolitic SRP magma chamber Zone of alteration SRP rhyolitic SRP Miocene hydrothermal Mtns (After 8 Ma) Basin Black Pine Raft River domes and lower crustal flow fabric granite-cored gneiss Oligocene (32–25 Ma) melt ponding

Sub-continental BDTZ Circulation of meteoric fluids N

ile Albion Mts Jim Sage volcanic suite

Brittle-Duct(BDTZ) 18 transition zone Miocene Mtns (9.5–8 Ma) Oligocene (32–25 Ma) Black Pine Intense fracturing of brittle crust Propagation of Snake River Plain-type low δ O melt along N-S trending melt conduits, gives rise to Jim Sage volcanic suite. SRP SRP Black Range Sublett magma rhyolitic Pine Mts chamber diagram View of 3D View Black Pine Mts T >8.0 Ma T centers T = 9.5 Ma T T = 13.5 Ma T SRP rhyolitic SRP SRP erruptive SRP Off-axis rhyolite Off-axis eruptive centers ~20° magma chamber 35–40° Mts ing south. U—upthrown; D—downthrown; Mts—Mountains; BDTZ—Brittle-ductile transition zone. D—downthrown; ing south. U—upthrown; Figure 14. Summaries of the two models proposed to explain the emplacement of Jim Sage volcanic suite (JSVS) and its relati 14. Summaries of the two models proposed Figure the Snake River Plain province (SRP) and motion along the Albion fault. Note that the view for the diagrams is from the north ( the diagrams is from Albion fault. Note that the view for (SRP) and motion along the Plain province the Snake River altered zone 45–50° Hydrothermally Jim Sage Albion fault Raft Future southern River Basin boundary of SRP U D U D U D Mts Mts Mts centers Cotterel Raft River Raft River SRP erruptive SRP Mts Mts Albion Albion 20 km N 20 km N 20 km N

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(Kerrich and Hyndman, 1986). In the Albion– Grouse Creek metamorphic core complex. The and trace element and isotopic compositions of Raft River–Grouse Creek metamorphic core data (and preceding discussion) establish a the rhyolite lavas are consistent with having an complex, retrogression of garnet and staurolite direct link between magmatism in the JSVS and origin similar to that of the magmas in the cen- to white mica and chlorite away from the Oli- the central SRP-Yellowstone province based tral SRP-Yellowstone province. This observa- gocene gneiss domes is widespread in the Pre- on similar timing, geochemistry, and isotopic tion, coupled with the fact that the rhyolites have cambrian schist units. This retrogression affects compositions. The data might also imply that a a low-δ18O signature, suggests that the in situ much larger volumes of crust than those that are silicic Miocene pluton with possible mafi c roots assimilation of hydrothermally altered caldera metamorphosed to sillimanite-grade rocks in the could directly underlie the Raft River Basin blocks proposed to explain the origin of low- carapaces of the actual gneiss domes. However, (Fig. 14; model B). This inferred high-level δ18O rhyolites in the SRP may not have been there has been no systematic study of the δ18O Miocene silicic magmatic system would, as for the case here. We offer two models that might values of these rocks, but such a study might the SRP, be driven by a high fl ux of basalt and explain the low-δ18O signature of the lavas in the δ18 offer insight into the Owr composition of the thermal input to the middle and lower crust of JSVS, each with their own strengths and limi- crust in this region. southern Idaho and may have led to extensive tations. The fi rst model suggests that SRP-type Another way of possibly forming a low-δ18O crustal melting at shallow levels of the crust rhyolitic melt propagated southward along the crustal reservoir is via the percolation of mete- beneath the region of the Raft River Basin and Albion fault to erupt in the center of the Raft oric fl uids deep into the crust along the Miocene the Albion–Raft River–Grouse Creek metamor- River Basin. The second model proposes that Albion fault system (Konstantinou et al., 2012), phic core complex. The structural history of the the hydrothermal alteration of the crust in the when large volumes of meteoric fl uids may have Raft River Basin (as detailed by Konstantinou region of southern Idaho may be more wide- percolated to ~8 km and exchanged oxygen with et al., 2012; Konstantinou, 2013) indicates that spread than previously thought and was related the silicic rocks of the Archean basement, the the rotation of the Albion fault to shallow angles to the earlier igneous and gneiss dome history Proterozoic schist units, or Cenozoic granites, and all of the normal faulting and rotation of or the normal faulting history of the region, thus forming a localized low-δ18O reservoir. Miocene basin fi ll strata occurred after the erup- and that the eruption of the JSVS was localized Providing support for fault-related hydrother- tion of the JSVS (after 8.2 Ma; Konstantinou above a magma chamber off axis of the SRP. It mal circulation systems are recent studies of et al., 2012). This young faulting and extension is possible that a low-δ18O silicic pluton beneath ductilely deformed rocks in the Raft River is most easily explained by coeval doming with the Raft River Basin could have provided a heat detachment that, based on their hydrogen iso- a component of vertical uplift during horizontal source for crustal melting and weakening that topes, show evidence of percolation of meteoric stretching of the crust beneath the Raft River led to extension accompanied by vertical dom- fl uids at deep crustal levels that are now exposed Basin. Horizontal extension and vertical uplift ing and rise of the Raft River Basin after the to the surface (Gottardi et al., 2011). These stud- resulted in the present-day subhorizontal geom- eruption of the JSVS rhyolites. ies show that normal fault systems provide the etry of the basal detachment imaged seismically ACKNOWLEDGMENTS necessary conduits for meteoric fl uids to circu- beneath the Raft River Basin (Covington, 1983; late to depths of 10 km in the crust, to the brittle- Konstantinou et al., 2012; Konstantinou 2013). We thank Stanford University for the Stanford ductile transition zone. The possible existence at depth of a silicic Graduate Fellowship that supported this research for Model B portrayed in Figure 14 has a major magmatic system implies (as it does for the three years, ExxonMobil for a science grant, and the limitation. If the JSVS represents large-scale SRP-Yellowstone) addition of basaltic material Leventis Foundation and the Stanford McGee Funds for providing fi nancial support for the analytical part crustal melting of rocks similar to the Archean at greater depth in the crust and concomitant of this project. Support was also provided by the basement, the Proterozoic schist units and the heating of the middle and lower crust leading National Science Foundation (NSF) Tectonics Divi- Oligocene granites exposed today in the adja- to possibly extensive crustal melting beneath the sion (grants EAR-0809226 and EAR-0948679 to cent metamorphic core complex, then the Sr- region of the Albion–Raft River–Grouse Creek Miller). Gail Mahood, Matt Coble, and Eric Gottlieb provided helpful discussions and insights about the Nd-Hf isotope compositions of the JSVS should metamorphic core complex during the Miocene. scientifi c work presented herein. Mike McCurry, Ilya be nearly identical to the compositions of rocks This crustal melting event may have resulted Bindeman, and Ben Ellis provided valuable reviews exposed in the complex. However, the rhyolites in increased mobility of the deep crust during that greatly improved the paper. We thank Noriko Kita of the JSVS have much less evolved Sr-Nd-Hf and after the eruption of the rhyolite lavas. We and Kouki Kitajima for assistance with secondary ion compositions than the compositions represented therefore interpret the extensional deformation mass spectrometer (SIMS) analysis of oxygen isotope ratios. The University of Wisconsin WiscSIMS by the silicic rocks within the Albion–Raft and vertical component of uplift after ca.8.2 Ma is partially supported by NSF grants EAR-0319230, River–Grouse Creek metamorphic core com- to be a direct consequence of greater mobility of EAR-0744079, and EAR-1053466. The Washington plex (e.g., Strickland et al., 2011b; Konstanti- the immediately underlying crust. State University Radiogenic Isotope and Geochronol- nou et al., 2013; Fig. 13). Thus model B may ogy Laboratory is partially supported by NSF grants EAR-0844149, EAR-1019877, and EAR-1119237. not effectively predict all the measured isotopic CONCLUSIONS We thank Karrie Weaver and Caroline Harris for their compositions of the magmas at the JSVS. help with the whole-rock Sr and Nd isotope analyses. The JSVS is composed of ~240 km3 of Implications for the Evolution of the rhyolite lavas, capped by central SRP–derived APPENDIX 1. METHODS FOR WHOLE- Raft River Detachment and Basin and rheomorphic ignimbrites and a thin sequence ROCK TRACER ISOTOPE ANALYSES Range Faulting (<100 m) of Miocene basalt fl ows. The rhyolite Samples were selected and processed at the Stan- lavas of the JSVS erupted from small centers ford University ICP-MS-TIMS (inductively coupled The eruption of ~240 km3 of high-tempera- within the Raft River Basin based on their phys- plasma–mass spectrometry–thermal ionization mass ture rhyolite in the synextensional Raft River ical attributes (aspect ratios and basal contact) spectrometry) facility. The samples were chipped, and rock chips were hand-selected and pulverized to a very Basin has important implications for the evo- and the occurrence of subvolcanic intrusions fi ne powder using an agate shutter box bowl. A small lution of Basin and Range faulting and the and domes. The ignimbrite and basalts are prob- aliquot of each sample (40–60 mg) was dissolved in exhumation of the adjacent Albion–Raft River– ably outfl ows from the central SRP. The major Savillex Tefl on vials using a combination of 29 N HF,

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followed by a mixture of 3:1 ratio of 10.5 N HCl and with the KIM-5 O isotope standard (Valley, 2003; Balcaen, L., De Schrijver, I., Moens, L., and Vanhaecke, F., δ18 2005, Determination of the 87Sr/86Sr isotope ratio in 14 N HNO3. An aliquot of the solution was dried and O = 5.09‰ Vienna standard mean ocean water, the salt was redissolved in HCl to prepare for column VSMOW), and polished prior to O isotope analy- USGS silicate reference materials by multi-collector chromatography. Strontium and the rare earth elements ses. Extra care was taken to achieve a smooth, fl at, ICP–mass spectrometry: International Journal of Mass Spectrometry, v. 242, p. 251–255, doi:10.1016 /j.ijms (REE) were separated from major elements and other low-relief polish. An ~2 nA primary beam of 133Cs+ μ .2004 .10.025. trace elements using an HCl elution on 200–400 mesh was focused to 10- m-diameter spots on the sample. Barth, A.P., and Wooden, J.L., 2010, Coupled elemental AG50-X8 cation exchange resin. The Sr was further A normal-incidence electron gun and carbon coat and isotopic analyses of polygenetic zircons from gra- isolated from Rb using Eichrom 20–50 μm Sr-spec were used for charge compensation. The secondary nitic rocks by ion microprobe, with implications for resin in Tefl on microcolumns. Elemental Nd was ion acceleration voltage was set at 10 kV and the O melt evolution and the sources of granitic magmas: purifi ed from the REE aliquot of the cation exchange isotopes were collected in 2 Faraday cups simultane- Chemical Geology, v. 277, p. 149–159, doi: 10.1016 column by column chromatography using Eichrom ously in multicollection mode. Four consecutive mea- /j.chemgeo .2010 .07.017. 50–100 μm LN-spec resin. The chem istry was moni- surements on KIM-5 zircon standard (δ18O = 5.09‰ Best, M.G., and Christiansen, E.H., 1991, Limited extension during peak tertiary volcanism, Great Basin of Nevada tored to ensure adequate separation of Nd from Ce and VSMOW; Valley, 2003) were analyzed before and and Utah: Journal of Geophysical Research, v. 96, Sm, the isobaric interferences of which may compro- after groups of 10–20 sample spots. The 2 SD preci- no. B8, p. 13509–13528, doi: 10.1029 /91JB00244. mise the quality of the isotopic analyses. The resulting sion for bracketing groups of eight standard analyses Bindeman, I.N., and Valley, J.W., 2001, Low-delta 18O rhyo- purifi ed Sr and Nd salts were prepared for mass spec- is reported as external precision for sample data. The lites from Yellowstone: Magmatic evolution based on

trometry analysis by redissolving them in 2% HNO3. average values of the standard analyses that bracket analyses of zircons and individual phenocrysts: Jour- Isotope analyses were performed using the Nu-Instru- each set of unknowns were used to correct for instru- nal of Petrology, v. 42, p. 1491–1517, doi: 10.1093 ments Plasma HR multicollector ICP-MS. mental bias. The average precision (reproducibility) /petrology /42 .8.1491. Each Sr isotope analysis consisted of 50 cycles of of the bracketing standards for this study ranged from Bindeman, I.N., Valley, J.W., Wooden, J.L., and Persing, H.M., 2001, Post-caldera volcanism: In situ measure- 10 s integration time, split into 2 blocks, during which ±0.13 to ±0.35 and averaged ±0.26‰ (2 SD). After ment of U-Pb age and oxygen isotope ratio in Pleisto- 83 84 6 isotopes were measured simultaneously ( Sr, Kr, O isotope analysis, ion microprobe pits were imaged cene zircons from Yellowstone caldera: Earth and 84Sr, 85Rb, 86Sr, 87Sr, 88Sr) on Faraday cups. Instrumen- by secondary electron microscopy and pits that were Planetary Science Letters, v. 189, p. 197–206, doi: tal mass fractionation and 86Kr interference on 86Sr located on obvious cracks or inclusions were excluded 10.1016 /S0012 -821X (01)00358-2. were corrected for by subtracting the 84Kr to obtain the from the data set. Bindeman, I.N., Watts, K.E., Schmitt, A.K., Morgan, L.A., natural 84Sr/88Sr ratio of 0.00675476 while iteratively and Shanks, P.W.C., 2007, Voluminous low delta 18O solving for the exponential fractionation factor and Kr APPENDIX 3. METHODS FOR HAFNIUM magmas in the late Miocene Heise volcanic ﬁ eld, correction on 86Sr, an approach similar to that of Jack- ISOTOPE ANALYSES Idaho: Implications for the fate of Yellowstone hotspot calderas: Geology, v. 35, p. 1019–1022, doi: 10.1130 son and Hart (2006). Baselines were measured at 0.25 /G24141A.1. 40 amu from peak center to account for scattering of Ar2 Hf isotopic compositions were determined on Bindeman, I.N., Fu, B., Kita, N.T., and Valley, J.W., 2008, that creates a nonlinear baseline across the Sr masses. zircons from ﬁ ve Miocene samples at Washington Origin and evolution of silicic magmatism at Yellow- Interference on 87Sr from 87Rb was small and corrected State University’s LA-MC-ICP-MS (laser ablation– stone based on ion microprobe analysis of isotopically using 85Rb and the instrumental mass fractionation multicollector–inductively coupled plasma–mass zoned zircons: Journal of Petrology, v. 49, p. 163–193, factor for Sr; 43 analyses of SRM 987 resulted in an spectrometry) facility, using the procedure from C.M doi: 10.1093 /petrology /egm075. 206 238 average 87Sr/86Sr value of 0.710144 ± 52 (2σ). Sample Fisher (2013, personal commun.). The resulting iso- Black, L.P., and 10 others, 2004, Improved Pb/ U micro- ratios were multiplied by a normalization factor deter- topic ratios were evaluated based on the quality and probe geochronology by the monitoring of a trace- element-related matrix effect: SHRIMP, ID-TIMS, mined by correcting SRM 987 analyses to a value of duration of the Hf analysis; 40 zircon Hf isotope anal- ELA-ICP-MS, and oxygen isotope documentation for 87 86 σ 176 177 Sr/ Sr = 0.71025 ± 1 (2 ; Balcaen et al., 2005) and yses yielded Hf/ Hf ratios that were corrected for a series of zircon standards: Chemical Geology, v. 205, the external error based on the 31 SRM 987 analyses interlaboratory bias using the R33 standards (correc- p. 115–140, doi: 10.1016 /j.chemgeo .2004 .01.003. was added to the analytical error for each sample. tion factor = 1.000159), and the corrected ratios, along Bonnichsen, B., and Citron, G.P., 1982, The Cougar Point The total mass of sample analyzed ranged from 20 to with measured 176Lu/177Hf and sensitive high-resolu- Tuff, southwestern Idaho and vicinity, in Bonnichsen, 50 ng, and a blank analysis yielded an estimated mass tion ion microprobe–reverse geometry ages, were B., and Breckenridge, R.M., eds., Cenozoic geology of ε ε Idaho: Idaho Bureau of Mines Geology Bulletin 26, of Sr < 0.01 ng. A secondary standard of BHVO-1 used to calculate Hf(i) and Hf(t) values. The results are was analyzed 7 times and yielded an average value of reported in Supplemental Table 6 (see footnote 6). p. 255–281. Bonnichsen, B., Leeman, W.P., Honjo, N., McIntosh, W.C., 0.703483 ± 14 (2σ) [accepted value is 0.703475 ± 17 Isotopic ratios with values ranging from εHf –25 to σ (t) Godchaux, M.M., McCurry, M., and Christiansen, (2 ); Weiss et al., 2005] after correction using the nor- –44 were excluded from further consideration (n = 8) E.H., 2008, Miocene silicic volcanism in southwestern malization factor obtained from SRM 987. because they could represent mixed isotopic composi- Idaho: Geochronology, geochemistry, and evolution of Each Nd isotope analysis consisted of 40 cycles of tions resulting from ablation and analysis of Miocene the central Snake River plain: Bulletin of Volcanology, 10 s integration time, split into 2 blocks, during which magmatic zircon and inherited zircon cores. v. 70, p. 315–342, doi: 10.1007 /s00445 -007 -0141-6. 5 isotopes were measured simultaneously (143Nd, Boroughs, S., Wolff, J., Bonnichsen, B., Godchaux, M.M., δ18 144Nd, 145Nd, 146Nd, and 144Sm), monitored for Ce and REFERENCES CITED and Larson, P., 2005, Large-volume, low- O rhyolites Sm interferences. Mass bias was exponentially cor- of the central Snake River plain, Idaho, USA: Geology, 146 144 Aitcheson, S.J., and Forrest, A.H., 1994, Quantification v. 33, p. 821–824, doi: 10.1130 /G21723.1. rected based on a Nd/ Nd ratio of 0.7219. Sam- of crustal contamination in open magmatic systems: Boroughs, S., Wolff, J.A., Ellis, B.S., Bonnichsen, B., and ple ratios were multiplied by a normalization factor Journal of Petrology, v. 35, p. 461–488, doi:10.1093 Larson, P.B., 2012, Evaluation of models for the ori- determined from 26 analyses of JNdi-1 resulted in /petrology /35 .2.461. gin of Miocene low-δ18O rhyolites of the Yellowstone/ an average 143Nd/144Nd value of 0.512087 ± 22 (2σ). 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This compares to the litic ignimbrite: Grey’s landing, southern Idaho: Geo- cations for the bulk composition of terrestrial planets: 143 144 value for BHVO-1 of Nd/ Nd = 0.512986 ± 18 logical Society of America Bulletin, v. 123, p. 725–743, Earth and Planetary Science Letters, v. 273, p. 48–57, (2σ) from Weiss et al. (2005). doi: 10.1130 /B30167.1. doi: 10.1016 /j.epsl .2008 .06.010. Andrews, G.D.M., Branney, M.J., Bonnichsen, B., and Boynton, W.V., 1985, Chapter 3: Cosmochemistry of the rare APPENDIX 2. METHODS FOR OXYGEN McCurry, M., 2008, Rhyolitic ignimbrites in the Roger- earth elements: Meteorite studies, in Henderson, P., ed., ISOTOPE ANALYSES son Graben, southern Snake River Plain volcanic prov- Rare Earth Element Geochemistry (Developments in ince: Volcanic stratigraphy, eruption history and basin Geochemistry 2): Amsterdam, Elsevier, p. 115–152. 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