Genesis of the post-caldera eastern Upper Basin ... - Springer Link

Contrib Mineral Petrol (2012) 164:205–228 DOI 10.1007/s00410-012-0733-9

ORIGINAL PAPER

Genesis of the post-caldera eastern Upper Basin Member rhyolites, Yellowstone, WY: from volcanic stratigraphy, geochemistry, and radiogenic isotope modeling Chad J. Pritchard • Peter B. Larson

Received: 2 November 2010 / Accepted: 24 February 2012 / Published online: 16 March 2012 Ó Springer-Verlag 2012

Abstract An array of samples from the eastern Upper Basin Member of the Plateau Rhyolite (EUBM) in the Yellowstone Plateau, Wyoming, were collected and analyzed to evaluate styles of deposition, geochemical variation, and plausible sources for low d18O rhyolites. Similar depositional styles and geochemistry suggest that the Tuff of Sulphur Creek and Tuff of Uncle Tom’s Trail were both deposited from pyroclastic density currents and are most likely part of the same unit. The middle unit of the EUBM, the Canyon flow, may be composed of multiple flows based on a wide range of Pb isotopic ratios (e.g., 206Pb/204Pb ranges from 17.54 to 17.86). The youngest EUBM, the Dunraven Road flow, appears to be a ring fracture dome and contains isotopic ratios and sparse phenocrysts that are similar to extra-caldera rhyolites of the younger Roaring Mountain Member. Petrologic textures, more radiogenic 87 Sr/86Sr in plagioclase phenocrysts (0.7134–0.7185) than groundmass and whole-rock ratios (0.7099–0.7161), and d18O depletions on the order of 5% found in the Tuff of Sulphur Creek and Canyon flow indicate at least a twostage petrogenesis involving an initial source rock formed by assimilation and fractional crystallization processes,

Communicated by M. Schmidt.

Electronic supplementary material The online version of this article (doi:10.1007/s00410-012-0733-9) contains supplementary material, which is available to authorized users. C. J. Pritchard (&) Geology Department, Eastern Washington University, Cheney, WA 99004-2439, USA e-mail: [email protected] P. B. Larson School of Earth and Environmental Sciences, Washington State University, Pullman, WA 99164-2812, USA

which cooled and was hydrothermally altered. The source rock was then lowered to melting depth by caldera collapse and remelted and erupted. The presence of a low d18O extra-caldera rhyolite indicates that country rock may have been hydrothermally altered at depth and then assimilated to form the Dunraven Road flow. Keywords Yellowstone caldera Assimilation Post-caldera Ring fracture dome Hydrothermally altered source rock Low d18O rhyolite Caldera subsidence Laser strontium

Introduction The eastern Upper Basin Member of the Plateau Rhyolite (EUBM) consist of the Tuff of Uncle Tom’s Trail, the Tuff of Sulphur Creek, the Canyon flow, and the Dunraven Road flow, from oldest to youngest (Christiansen 2001). Of particular interest is the low d18O nature of the EUBM magmas and increased 87Sr/86Sr, 206Pb/204Pb, and 207 Pb/204Pb radiogenic isotope ratios when compared to other Yellowstone rhyolites (Hildreth et al. 1991). These units were first documented as low d18O rhyolites by Friedman et al. (1974). The connection between low d18O and increased 87Sr/86Sr, 206Pb/204Pb, and 207Pb/204Pb isotope ratios was initially described by Hildreth et al. 1991. Variations in radiogenic isotope ratios following a caldera collapse may be expected due to either assimilation of wall rock or magma mixing; however, determining the processes that lead to d18O ratio depletions has lead to multiple and often contradictory hypotheses. For example, Friedman et al. (1974) in their original description of the d18O values proposed that magmas absorbed ambient low d18O meteoric groundwater to produce the rhyolitic low O

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ratio signatures, although Taylor (1986) has demonstrated the physical problems with this model. More recent workers favor the involvement of low d18O hydrothermally altered source rocks as a significant component of the magmas, either by incorporation of altered roof rocks (Hildreth et al. 1984, 1991) or by subsidence of hydrothermally altered volcanic rocks to melting depths (Bindeman and Valley 2001). This study of the EUBM was initiated to measure geochemical variations within each of the rhyolites and to compare them with other rhyolites from Yellowstone. Variations within individual rhyolite eruptions can be used to better define characteristics of the magmatic feeding system that produced the Yellowstone Plateau Rhyolite. Here, evidence is provided that suggests that the Tuff of Uncle Tom’s Trail and the Tuff of Sulphur Creek are portions of the same unit, the Canyon flow may be comprised of multiple rhyolites, and that the Dunraven Road flow is more geochemically akin to an extra-caldera rhyolite. This paper also proposes that Energy-Constrained Recharge, Assimilation, and Fractional Crystallization models (EC-RAxFC) established by Bohrson and Spera (2007) may best explain radiogenic isotope variation and the high-silica values found in the EUBM. Current models for rhyolite genesis at the Yellowstone caldera require significant volumes of basaltic intrusions to partially melt the crust, on the order of equal proportions of basalt to the melted rhyolite (Bindeman et al. 2008). Modeling rhyolite genesis at Yellowstone using partial melting would predict higher uplift rates than may be required to develop significant quantities of high-silica rhyolites by EC-RAxFC processes. Pre-Yellowstone caldera geologic history The Yellowstone caldera is located in northwestern Wyoming, at the easternmost extension of the Snake RiverYellowstone volcanic track (Christiansen 2001). Basement rocks that are exposed around the caldera include Archean granitoids and biotite schists, Paleozoic and younger shallow marine sedimentary rocks, and Eocene Absaroka igneous rocks (Christiansen 2001). These lithologies are potential contributors to the younger rhyolitic magmas. The southern Beartooth Mountains, located just north of Yellowstone Park, are comprised of Archean granodiorite, granite to granodiorite gneiss, andesitic amphibolite, and granite biotite schist. U–Pb measurements of zircon grains and whole-rock Pb ratios have been used to estimate the age of the Archean Beartooth Mountains at approximately 2.8 Ga. This Archean block, which here is considered to represent Archean rocks at depth beneath the caldera, was uplifted during the Laramide orogeny (Wooden and Mueller 1988). Granitic gneiss, garnet-biotite schist, norite,

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and migmatite are exposed in the northern portion of the park (Christiansen 2001). The Mississippian Madison Limestone outcrops extensively in the northeastern corner of the park and to the south of the caldera, although other sedimentary units have been mapped, for example the Mississippian Lodgepole Limestone, Devonian Three Forks and Jefferson clastic Formations, Ordovician Bighorn Dolomite and older Cambrian sedimentary units (Love and Keefer 1975; Prostka et al. 1975; Christiansen 2001). The presence of deeper sedimentary bedrock outside of the caldera ring fracture zone is suggested by hydrocarbons in the Washburn Hot Springs (Fournier 1989) and by CO2 emissions (Werner and Brantley 2003). Eocene Absaroka volcanics are present around the northeastern corner of the Yellowstone caldera. These Eocene magmas intruded the deep continental crust and then mixed in shallow reservoirs (Feeley et al. 2002). Exposed stocks of the Eocene Mt. Washburn volcano, just north of the EUBM outcrops, indicate that the southern portion of the edifice foundered into the caldera during collapse (Feeley et al. 2002). Lithic fragments of the Mt. Washburn volcanics are present in the Tuff of Uncle Tom’s Trail (Christiansen 2001). Finally, Basin and Range tectonics extends through the park and is present along much of the Snake River-Yellowstone Track, though volcanic rocks associated with the track are not generally faulted (Leeman et al. 2008). Yellowstone volcanic background Volcanism from the Yellowstone Plateau produced at least three periods of large-volume caldera-forming eruptions with the first approximately 2.059 ± 0.004 Ma and the latest at approximately 640 ± 0.2 ka based on 40Ar-39Ar measurements (Lanphere et al. 2002). The three calderaforming eruptions deposited, from oldest to youngest: Huckleberry Ridge Tuff (HRT), Mesa Falls Tuff (MFT), and the Lava Creek Tuff (LCT). HRT was deposited by at least three eruptions that were the largest known eruptions in the Yellowstone area (Christiansen 2001). Of the three HRT members, the youngest, Member C (HRT-C) has a substantially smaller estimated initial erupted volume (Christiansen 2001) and also has a unique radiogenic isotopic signature (Doe et al. 1982; Hildreth et al. 1991). The complex history of pre-caldera and post-caldera collapse volcanism involves the eruption of basalt and rhyolite on the Yellowstone Plateau (Fig. 1). Following the deposition of the youngest caldera-forming eruption of the LCT numerous basalts and rhyolites erupted outside of the caldera (extra-caldera), and numerous rhyolites erupted from within the caldera (intracaldera). Post-LCT rhyolites are grouped into the Plateau Rhyolite as defined by


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Christiansen and Blank (1972). 40Ar-39Ar measurements of extra-caldera basalts and rhyolites have yielded ages from 590 to 80 ka (Bennett 2002; Nastanski 2002; Christiansen et al. 2007). Ages of the younger intracaldera Central Plateau Member of the Plateau Rhyolite were measured using 238U-230Th and range between 166 and 70 ka (Vazquez and Reid 2002; Vazquez et al. 2009) and 40Ar/39Ar ages estimates between 159 and o 60 ka (Dallegge 2008; Simon et al. 2008). The oldest of the intracaldera rhyolites, the Upper Basin Member (UBM), are approximately 516–255 ka, based on 40Ar–39Ar measurements (Gansecki et al. 1996; Bindeman et al. 2008). The UBM has been subdivided into the eastern Upper Basin rhyolites (EUBM), including the Tuff of Uncle Tom’s Trail, Tuff of Sulphur Creek (479 ± 10 ka), Canyon flow (484 ± 15 ka), the Dunraven Road flow (486 ± 42 ka), and the western Upper Basin Rhyolites (WUBM) including the Biscuit Basin flows and Scaup Lake flow (Gansecki et al. 1996). The Biscuit Basin flows have been subdivided into the north, south (255 ± 11 ka), middle (516 ± 7 ka), and east flows (Gansecki et al. 1996; Bindeman and Valley 2001; Bindeman et al. 2008; Girard and Stix 2009). The Scaup Lake flow is the youngest WUBM rhyolite, 257 ± 13 ka, and is located southeast of the Biscuit Basin Flows (Christiansen et al. 2007). The older UBM rhyolites and Blue Creek flow (post –HRT) are the first known intracaldera rhyolites that followed caldera collapse and have up to 5% depletion in d18O values with slightly elevated radiogenic Sr and Pb signatures (Hildreth

et al. 1984; Bindeman and Valley 2001). Other rhyolites from Yellowstone also have d18O ratio depletions. The Central Plateau Member has depletions of approximately 2–3%, and the MFT and LCT have depletions of 1–2% when compared to ‘‘normal Yellowstone rhyolites’’ (Hildreth et al. 1984; Bindeman et al. 2008). The UBM exhibits an age progression in d 18O values where the older units such as Middle Biscuit Basin flow and Tuff of Sulphur Creek/Canyon flow show larger d18O depletions and younger units such as south Biscuit Basin flow and Scaup Lake show less of d18O depletion (Gansecki et al. 1996; Bindeman et al. 2008). One exception to this trend is the less d18O depleted Dunraven Road flow, although as noted in this paper, this flow is geochemically distinct from the other UBM units. The EUBM are located within, and straddling, the northeastern caldera margin and are north of the Sour Creek resurgent Dome (Fig. 1). Generally, these flows contain up to 35% phenocrysts (Christiansen 2001). However, the youngest of the EUBM, the Dunraven Road flow, contains fewer phenocrysts, 8% or less, which differs from other UBM rhyolites (Girard and Stix 2009). Previous studies describe the Tuff of Uncle Tom’s Trail as deposited from pyroclastic density currents, the Tuff of Sulphur Creek as an agglutinated ash-fall tuff, and the younger units as rhyolitic lava flows (Christiansen and Blank 1972). Geochemical studies of these units have concluded that many of the flows are from isolated magma bodies (Girard and Stix 2009), whereas the next series of

Fig. 1 a Map of Yellowstone volcanic area, showing approximate locations of the three calderas, extent of Plateau Rhyolite: Upper Basin Member, Central Plateau Member, and northern extra-caldera rhyolites. Upper Basin member has been broken into a western and

eastern group. Map modified from Gansecki et al. (1996) and Christiansen (2001). b Close-up of the eastern Upper Basin Member rhyolites. Circles are sample locations (location for each sample is given for samples in the electronic appendix)

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flows, the Central Plateau Member of the Plateau Rhyolite, are thought to have erupted from a single large magma body located within the caldera (Vazquez et al. 2009). These studies generally agree with a model of silicic magma genesis presented by Huppert and Sparks (1988), where basaltic intrusions melt, or assimilate, crustal material in small batches which coalesce into a single large magma chamber over time. This study presents new radiogenic isotope values for whole rock and separated glass for a majority of the UBM rhyolites, including the North, South, and East Biscuit Basin flows and the Dunraven Road flow, and presents a model for the petrogenesis of the EUBM.

Sampling and methods A sampling permit was issued by the National Park for the three summers of sampling. Each sample was located using a hand held Garmin GPS unit, and sample locations are listed in the Online Resource (whole-rock data). Care was taken to collect the freshest and least-altered samples possible. Glass separates were picked using a specimen microscope from chipped and sieved (500 micron) vitric hand samples to ensure the purest possible sample. However, microphenocrysts may be present in samples even though microphenocrysts were not commonly observed in thin section. Analytical data were collected in the Washington State University GeoAnalytical Laboratory. Major element concentrations were measured from ground fused beads using a ThermoARL XRF (Tables 1 and 2, Online Resource). Trace elements were measured using a HP4500 ICP-MS (Tables 1 and 2, Online Resource). Glass and mineral analyses for major elements were collected using a JEOL 8500F field emission electron microprobe with a spot size of 10 microns, 20 keV accelerating voltage, and 16 nA beam current (Table 2, Online Resource). Glass and mineral trace element data were collected using a New Wave UP-213 laser ablation system and analyzed with a Finnigan Element 2 HR-inductively coupled plasma mass spectrometer (LA-ICP-MS), and results are presented in Table 3 and the Online Resource. Glass and pyroxene analyses were conducted using a 16-micron beam with 72% 12–14 J/cm2 at a scan speed of 8 microns/s. Feldspar analyses were conducted using a 30-micron beam, approximately 12 J/cm2 at a scan speed of 15 microns/s. Background levels in the vacuum sample holder were collected for 10 s and used for corrections, and standards NBS612 and BCR were routinely run for calibration purposes. Specific isotopes analyzed during LAICP-MS included Rb85, Sr88, Y89, Cs133, Ba138, La139, Ce140, Pr141, Nd146, Sm147, Eu151, Gd157, Gd157, Tb159, and Pb208.

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Sr and Nd isotopic ratio analyses were conducted on whole-rock powders and glass separates (Table 2). Samples were dissolved in H3BO3 and twice with 6.0 M HCl, in clean Teflon crucibles. Samples were then dissolved in 6 M HCl and H2O for column chemistry. Initial column separation was done using AG50 W-X8 resin, with a series of washes using 2.5 M HCl and 6.0 M HCl. First-stage Sr elution was conducted using 2.5 M HCl, and REEs (Nd) were separated using 6.0 M HCl. Samples were dried down and treated with 14 M twice distilled HNO3 to remove organic residue from resin. Second-stage column chemistry for Sr separation was conducted using Sr-spec resin. We used 3 M HNO3 to load sample and to rinse resin and then 0.5 M HNO3 to rinse resin and elute Sr. Nd was separated from other REE using HDEHP resin and 0.14 M HCl for loading sample, rinsing resin, and elution. Samples were dried down and treated with 14 M twice distilled HNO3 to remove organic residue from resin. Pb isotope analyses were conducted on whole-rock powders and glass separates (Table 2). Sample dissolution involved fluxing with HF and HNO3 (10:1), drying down, and fluxing with 8 M HBr in Savillex crucibles. Samples were then dissolved in 0.5 M HBr to load into columns, with AG1-X8 resin. Columns were repeatedly rinsed with H2O, 0.5 HBr, 1.0 M HCl, and 6.0 M HCl. Pb was eluted using four additions of 6.0 M HCl. All samples for radiogenic isotopic analyses were put into solution with 2% HNO3, and measured ratios were collected using a Thermo-Finnigan Neptune MC-ICP-MS. Ratios for each sample were collected 75 times and averaged and then corrected for mass bias and interferences. Standards used were NBS 987 for Sr, Ames Nd for Nd, and NBS 981 for Pb. 87 Sr/86Sr ratios of separated plagioclase phenocrysts were measured using a New Wave UP-213 laser ablation system and analyzed using a Thermo-Finnigan Neptune multi-collector ICP-MS (LA-MC-ICP-MS) in the GeoAnalytical Laboratory at Washington State University (data presented in Online Resource). Ratios for each sample were collected 100 times, averaged, and then corrected for mass bias and interferences. An in house plagioclase standard, IBS was used for corrections. See Ramos et al. (2004) for further discussion of the method and http://www.wsu.edu/*geolab for additional instrument information. Oxygen isotopic ratios were collected on mineral separates using a laser fluorination vacuum extraction line (Table 4). Gas analyses were measured using a Finnigan Delta S MAT IRMS. Analyses of samples were bracketed by analyses of standards, UWG-2 (Valley et al. 1995) and an in house MM-1 quartz standard. A more detailed description of O ratio measurement techniques is presented in Takeuchi and Larson (2005). Oxygen isotope ratios are reported in per mil (%) and are relative to VSMOW.


Results Physical description of the EUBM rhyolites The EUBM is approximately 481 ± 8 ka (Gansecki et al. 1996) and has been described as including four units: Tuff of Uncle Tom’s Trail, Tuff of Sulphur Creek, Canyon flow, and the Dunraven Road flow, from oldest to youngest. The Tuff of Uncle Tom’s Trail is located between Upper and Lower Falls in the Canyon area of Yellowstone. Extensive hydrothermal alteration and poor exposures mask the thickness of the unit, but assuming it makes up the walls of the Canyon, it is at least 35 m thick over an approximately 2 km2 area (Christiansen 1999, 2001). Outcrops are generally well bedded, poorly sorted, and contain pumice-rich layers and lithics, ranging from Eocene Absaroka volcanics to obsidian fragments. Christiansen (2001) describes this unit as a pyroclastic flow deposit overlain by a diamicton. The Tuff of Sulphur Creek is approximately 230 m thick and spans an elongated area of 119 km2, making up the wall of the Canyon and extending many kilometers to the east. The Tuff in the walls of the Canyon is densely welded and massive with faint columnar jointing and very faint lenses of increased phenocryst concentrations. The type location is north of Sulphur Creek where the unit grades, vertically and laterally, into a bedded deposit just outside of the caldera (Christiansen and Blank 1972). Approximately 230 m to the east along the same cliff where Christiansen and Blank (1972) described this unit, there is a complete basal section of the Tuff of Sulphur Creek. At this outcrop, there are two layers (each approximately 2 m thick) of poorly sorted ash, pumice, and blocks overlying Eocene Absaroka volcanics. Welding increases from the base to the top of each layer. However, both layers are still moderately welded and do not allow for sieving of undisturbed samples (Fig. 2). Many of the pyroclastic features, such as fiamme, rounded and fragmented pumices (imbricated at the upper portion of the bottom layer), poorly sorted glass fragments, and lithics are preserved. Separating these two layers is a thin, approximately 1 cm, layer of sand to fine gravel sized clast-supported black obsidian fragments and welded pumices. These black obsidian fragments have a distinct chemical composition of a more evolved high-silica rhyolite and can be found in trace abundances throughout the basal layers. Similar black obsidian fragments are found in the Tuff of Uncle Tom’s Trail. Overlying the basal layers is a vitrophyre, which displays increased welding toward the top of the 2- to 3-mthick layer. Pyroclastic features are present in thin sections from the base, but are absent in samples from the top because welding has obscured most features. Overlying the basal vitrophyre is a thick section of devitrified tuff containing fiamme ranging from 1 mm to 10 cm long. Outside

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of the caldera, the Tuff of Sulphur Creek is comprised of thinly bedded devitrified layers with variable vesicle and lithic content. Some of the thin (approx 0.25 m thick) layers contain angular glass fragments up to 5 cm in length within an ashy groundmass that show similar chemical composition to the rest of the unit. Field observations and thin section analyses indicate that the Tuff of Sulphur Creek is most likely a pyroclastic flow deposit. The type location, initially described as an agglutinated ash-fall tuff by Christiansen and Blank (1972), may be better described as a veneer deposit where the pyroclastic flow encountered a topographic high and the upper, more dilute portions of the current overtopped topography created by ring fracture blocks. Such lateral variability lithofacies are common features of pyroclastic density current deposits (Branney and Kokelaar 2002). If the Tuff of Sulphur Creek was emplaced as a veneer deposit, this could explain why large blocks of Absaroka volcanic are not found at the base, as found in the Tuff of Uncle Tom’s Trail. However, similar coarse sand sized obsidian fragments are found in the base of both units. Both units were most likely pyroclastic density currents, share similar appearance in hand sample, and spatially appear to be the east and western extents of one unit; therefore, we believe that the Tuff of Sulphur Creek most likely correlates with the Tuff of Uncle Tom’s Trail. As discussed later in this paper, correlation between the units is not possible due to extensive hydrothermal alteration of the Tuff of Uncle Tom’s Trail. The next highest unit in the EUBM is the Canyon flow. This unit covers the Tuff of Sulphur Creek over an area of 118 m2 and is approximately 20 m thick. The Canyon flow is only discernable from the Tuff of Sulphur Creek where flow banding is present. Outcrops are generally found in depressions within the Tuff of Sulphur Creek and appear to fill topographic lows. Outcrops consist of a basal vitrophyre that can be up to 4 m thick, but is commonly on the order of 1 m thick. Thick basal vitrophyre can contain spherulites up to 5 cm in diameter, but spherulites are generally minor where the basal vitrophyre is thinner. The stony interior is generally a light gray devitrified rock with light red to pink bands and can be up to 20 m thick in some areas. The glassy upper portion of the Canyon flow is 2 m thick and in multiple locations is a highly vesiculated breccia. At the eastern end of the Canyon flow, the upper portions also contain a thin layer of volcanic sediments. Distinguishing between possible multiple flows is difficult due to sparse exposures. The Dunraven Road flow overlies the Canyon flow in the northeastern portion of the caldera. The Dunraven Road flow has been described by Christiansen (2001) as having a vitric basal breccia and extending to the Canyon rim, covering an area of 4 km2. Further mapping and chemical

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Fig. 2 Generalized crosssection of the eastern Upper Basin Member rhyolites

analysis have found that the Dunraven Road flow is only approximately a quarter of the originally mapped size (1.2 km2) and resembles a rhyolitic dome centered on the northeastern portion of the ring fracture (Fig. 3). The previously mapped basal breccia contains up to 20% phenocrysts and is chemically similar to the upper breccia of the Canyon flow. Mapping this unit has also revealed that the Dunraven Road flow sample analyzed by Bindeman and Valley (2001) was most likely taken from the Canyon flow, which explains discrepancies between stable isotope ratios measured by Bindeman and Valley (2001) and Hildreth et al. 1984. The central portion of the Dunraven Road flow is exposed within a valley mapped as the ring fracture/ caldera boundary by Prostka et al. (1975). The core of this high-silica rhyolite is approximately 20 m thick. The core is devitrified and heavily flow banded, almost swirly, in parts. Surrounding the core is a layer of vitrophyre that can be up to approximately 10 m thick and at the extremities of the unit is heavily spherulitic, such as is found in multiple road-cuts along Dunraven Pass Road. Closer to the core of the unit, however, the glass lacks spherulites and can contain black and red banded obsidian. Whole-rock geochemistry EUBM whole-rock geochemistry and glass geochemistry are presented in Hildreth et al. (1991) and Girard and Stix (2009). However, for this study, samples were collected

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Fig. 3 Map of the Dunraven Road flow, original and larger Dunraven Road flow map unit from Prostka et al. (1975) and Christiansen (1999). Smaller and darker Road flow map unit from this study

throughout and across the units and describe greater heterogeneity than described in previous work. A summary of whole-rock and glass data are presented in Tables 1 and 2. Data are presented in the Online Resource. Analyses of the Tuff of Uncle Tom’s Trail were completed using microprobe and LA-ICP-MS on pumices as

Contrib Mineral Petrol (2012) 164:205–228 Table 1 Average whole-rock analyses of EUBM

Unit

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Tuff of Sulphur Creek

Canyon flow

n = 23

n = 29

2r

Dunraven Road flow 2r

n = 12

2r

XRF (wt%) SiO2

76.45

1.25

76.10

1.31

76.45

1.25

TiO2

0.34

0.04

0.32

0.12

0.34

0.04

Al2O3

12.56

0.38

12.61

0.61

FeO*

1.54

0.96

1.80

0.58

1.54

0.96

MnO

0.03

0.02

0.04

0.04

0.03

0.02

MgO

0.06

0.06

0.09

0.12

0.06

0.06

CaO

0.66

0.21

0.75

0.42

0.66

0.21

Na2O

3.34

0.25

3.27

0.81

3.34

0.25

K2O

4.99

0.25

4.98

0.54

4.99

0.25

0.03 100.00

0.01

0.03 100.00

0.02

0.03 100.00

0.01

P2O5 Total

12.6

0.38

ICP-MS (ppm) La Ce Pr

17.3

Nd

62.8

Sm

12.9

Eu Gd Tb Dy Ho

1.92 11.6 1.97 11.9

17.9 41.2 4.45 16.1 3.28 0.37 3.16 0.53 3.06

76.5 146 16.9 61.4 12.7 1.94 11.7 1.99 12.1

10.75 24.0 2.80 12.2

58.9 117 12.8 45.8

16.9 45.3 4.00 15.0

3.28

9.60

3.08

1.15

0.72

0.09

5.22

8.59

2.76

0.93

1.49

0.43

5.96

9.13

2.48

2.35

0.62

2.43

1.32

1.80

0.47

Er

6.34

1.58

6.59

3.60

4.88

1.20

Tm

0.92

0.23

0.95

0.48

0.71

0.16

Yb Lu

5.63 0.86

1.40 0.22

5.87 0.89

2.68 0.41

4.43 0.66

0.94 0.15

Ba

1019

132

1016

205

673

Th

24.1

1.56

23.1

1.86

20.9

Nb

49.3

1.88

46.6

6.81

33.6

Y

57.9

61.5

42.89

46.0

Hf

10.8

Ta U Pb Rb Cs Sr Sc Data for individual samples are attached in electronic appendix

78.1 149

Zr

3.45 5.95 31.9 173 3.41 73.3 5.80 369

well as XRF and ICP-MS on large angular glass inclusions. Many of the samples have been hydrated and hydrothermally altered, although attempts were made to isolate the most pristine samples. Therefore, correlation between the Tuff of Uncle Tom’s Trail and the Tuff of Sulphur Creek based on geochemistry alone is not definitive, but due to both tuffs having mapped correlations in the field and

17.6 0.68 0.16 0.62 12.67 12.3 1.36 21.81 1.72 40.6

10.4

35 1.57 1.69 13.7

2.18

7.05

0.40

3.28

0.39

2.50

0.12

5.69

0.69

4.53

0.38

30.1 170 3.72 76.4 5.78 354

3.96 21.9 1.95 42.12 2.12 108.0

31.1 170 2.98 20.1 4.00 194

6.20 4.3 1.19 5.19 0.89 14.1

generally similar chemistries, we propose that the two are indeed the same unit, with Tuff of Uncle Tom’s Trail representing the westernmost extent of a larger ignimbrite. The Tuff of Sulphur Creek and Canyon flow are highsilica rhyolites and show relatively high concentrations of FeO, TiO2, Sr, Ba, and Sc when compared to other Yellowstone rhyolites, which may indicate less fractional

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Table 2 Upper Basin Member and Archean Gneiss—Major elements, trace elements, and radiogenic isotope ratios Member Unit Northing Easting (12) Lithology Age (Ma) Age error Sample # XRF (wt%) SiO2 TiO2 Al2O3

Upper Basin TSC 4956461 547171 Vitric 0.479 0.01 08CP12

Upper Basin TSC 4956400 547245 Picked glass 0.479 0.01 0835Ga

Upper Basin CAN 4951810 542014 Vitric 0.484 0.015 08CP26

Upper Basin CAN 4952622 543804 Tuff 0.484 0.015 0822

Upper Basin CAN 4949551 555907 Picked glass 0.484 0.015 0934Ga

Upper Basin DRF 4955387 540434 Picked glass 0.486 0.042 07CP15

Upper Basin DRF 4955840 542283 Obsidian 0.486 0.042 08CP01

76.24

77.76

76.55

75.99

77.15

77.59

77.34

0.38

0.24

0.34

0.31

0.25

0.12

0.13

13.1

12.0

12.6

12.5

12.0

12.2

12.1

Upper Basin DRF 4955642 542334 Tuff 0.486 0.042 0921

77.57 0.12 12.1

FeO*

0.83

1.10

1.35

2.08

1.41

0.88

1.18

1.22

MnO

0.02

0.03

0.03

0.06

0.04

0.02

0.03

0.01

MgO

0.18

0.06

0.07

0.14

0.09

0.04

0.07

0.01

CaO

0.98

0.38

0.83

0.72

0.49

0.47

0.53

0.37

Na2O

3.55

2.68

2.50

3.42

3.18

3.48

3.49

3.44

K2O

4.67

5.78

5.65

4.80

5.37

5.20

5.13

5.15

P2O5

0.02

0.02

0.02

0.03

0.02

0.02

0.01

0.01

Total

100.00

100.00

100.00

100.00

100.00

100.00

100.00

100.00

99.33

97.30

96.17

99.44

91.57

96.73

98.88

97.55

62.52

37.26

Raw sum ICP-MS (ppm) La

72.7

Ce

136

74.24 142

78.43 150

71.87 138

76.22 148

Pr Nd

16.0 57.9

15.79 55.88

17.00 61.35

15.63 56.17

16.74 60.19

Sm

12.0

11.16

12.65

11.50

Eu

2.3

0.83

1.91

1.73

Gd

10.4

60.39 112

119

73

12.80 45.68

13.39 48.38

7.66 26.93

12.37

9.49

10.15

5.72

1.12

0.69

0.72

0.69

10.52

11.63

10.57

11.14

8.78

9.32

5.11

Tb

1.74

1.79

1.96

1.81

1.95

1.50

1.58

0.93

Dy

10.67

11.07

12.12

11.20

11.81

9.19

9.71

5.70

Ho

2.12

2.23

2.44

2.28

2.36

1.82

1.95

1.16

Er

5.68

6.10

6.86

6.24

6.52

4.92

5.33

3.21

Tm

0.84

0.89

0.99

0.91

0.93

0.72

0.76

0.49

Yb

5.15

5.53

6.17

5.72

5.85

4.35

4.77

3.11

Lu

0.79

0.87

0.95

0.87

0.89

0.66

0.72

0.46

Ba

1163

677

1034

1016

698

671

664

656

Th

22.36

25.14

23.56

21.77

24.62

23.07

20.14

20.39

Nb

46.05

48.59

48.45

42.97

48.12

34.24

33.45

33.75

Y Hf

53.10 11.60

58.12 8.67

63.83 10.75

58.82 9.53

62.45 8.68

47.02 7.06

51.20 7.06

27.03 6.79

Ta

3.24

3.53

3.41

3.05

3.42

2.54

2.50

2.50

U

5.68

6.06

5.85

5.51

6.01

4.67

4.53

4.35

Pb

25.37

32.02

30.86

31.90

30.10

32.10

31.44

28.49

Rb

152.0

Cs

3.73

Sr

110.49

Sc

6.46

Zr

404.2 Pb/204Pb

235.0 5.05 26 4.8 285.5

186.6

162.7

5.94

4.11

83.05

78.47

5.74

5.39

372.3

318.3

174.0 4.04 27 5.1 282.9

172.4

172.2

167.3

3.27

3.38

1.84

19.28

20.14

20.31

3.44

4.00

191.5

190.7

3.91 195.1

206

17.78

17.84

17.81

17.54

17.86

16.68

16.67

16.68

207

15.64

15.65

15.65

15.59

15.65

15.45

15.44

15.45

Pb/204Pb

123


213

Table 2 continued Member Unit Northing Easting (12) Lithology Age (Ma) Age error Sample # 208

Pb/204Pb

Sr/86Srm

Upper Basin TSC 4956461 547171 Vitric 0.479 0.01 08CP12 38.36

Upper Basin TSC 4956400 547245 Picked glass 0.479 0.01 0835Ga 38.4

Upper Basin CAN 4951810 542014 Vitric 0.484 0.015 08CP26

Upper Basin CAN 4952622 543804 Tuff 0.484 0.015 0822

38.37

38.23

Upper Basin CAN 4949551 555907 Picked glass 0.484 0.015 0934Ga

Upper Basin DRF 4955387 540434 Picked glass 0.486 0.042 07CP15

38.40

37.79

Upper Basin DRF 4955840 542283 Obsidian 0.486 0.042 08CP01 37.77

Upper Basin DRF 4955642 542334 Tuff 0.486 0.042 0921 37.81

87

0.71569

0.71575

0.71597

0.71605

0.71318

0.71334

2r error

0.00005

0.00007

0.00004

0.00007

0.00004

0.00007

143

Nd/144Nd

0.512152

0.512181

0.512163

0.512224

0.512177

0.511965

0.511976

0.511982

2r error

0.000029

0.000023

0.000012

0.000023

0.000023

0.000029

0.000012

0.000023

Epsilon Nd Member Unit Northing Easting (12) Lithology Age (Ma) Age error Sample #

-9.5

-8.9 Upper Basin SBBF 4925596 510161 Picked glass 0.255 0.011 YC96-2Gb

-9.3

-8.1

-9.0

-13.1

Upper Basin NBBF 4931015 486037 Picked glass 0.2?

Upper Basin EBBF 4924479 487852 Picked glass 0.5?

08YS-14BGb

08YS-15BGb

-12.9

-12.8 Precambrian Bedrock 4973680 556305 Granitic-gniess 2.8 0.05 08CP061

XRF (wt%) SiO2 TiO2

77.41 0.19

75.42 0.34

65.66 0.86

Al2O3

12.15

12.36

15.84

FeO*

1.09

2.29

5.05

MnO

0.03

0.05

0.07

MgO

0.08

0.19

1.52

CaO

0.41

0.84

3.98

Na2O

3.18

3.34

3.98

K2O

5.43

5.11

2.76

P2O5

0.01

0.05

0.28

Total

100.00

100.00

100.00

96.01

91.81

98.35

72.77

39.04

Raw sum ICP-MS (ppm) La Ce

76.22 138

Pr Nd

15.61 53.78

Sm Eu

38.62 66.8

140

86.7

7.57 25.78

15.69 55.68

10.53

5.17

11.49

6.99

0.87

0.30

1.31

2.02

Gd

9.07

4.26

10.31

5.55

Tb

1.55

0.78

1.82

0.70

Cy

9.50

4.84

10.75

3.78

Ho

1.92

0.96

2.22

0.70

Er

5.20

2.58

6.00

1.79

Tm

0.76

0.39

0.87

0.25

Yb

4.75

2.41

5.40

1.54

Lu

0.73

0.36

0.83

Ba Th

430 26.44

75 13.45

873 24.67

10.25 38.32

0.25 1907 10.55

123

214


Table 2 continued Member Unit Northing Easting (12) Lithology Age (Ma) Age error Sample #

Upper Basin SBBF 4925596 510161 Picked glass 0.255 0.011 YC96-2Gb

Upper Basin NBBF 4931015 486037 Picked glass 0.2?

Upper Basin EBBF 4924479 487852 Picked glass 0.5?

08YS-14BGb

08YS-15BGb

Precambrian Bedrock 4973680 556305 Granitic-gniess 2.8 0.05 08CP061

Nb

43.99

24.56

43.17

9.15

Y

47.19

20.50

56.26

17.71

Hf

6.35

3.90

9.86

7.11

Ta

3.31

1.76

3.15

0.73

U

6.20

3.26

5.79

3.42

Pb

27.01

13.89

28.02

15.12

Rb

168.7

Cs

4.09

Sr

22

108.2

169.2

1.88

4.01

5

52

Sc

2.9

1.1

5.5

Zr

176.0

110.8

356.9

Pb/204Pb

99.1 2.21 438.71 13.88 282.4

206

17.69

17.62

17.87

207

15.61

15.60

15.66

16.51

208

38.37 0.70993

38.34 0.71019

38.52 0.71954

40.52 0.72873

0.00007

0.00007

0.00007

0.00004

Pb/204Pb

Pb/204Pb 87 Sr/86Srm 2r Error 143

Nd/

144

Nd

2r Error Epsilon Nd

21.91

0.512253

0.512253

0.512151

0.510803

0.000023

0.000023

0.000023

0.000012

-7.5

-7.5

-9.5

-35.8

TSC tuff of sulphur creek, CAN Canyon flow, DRF dunraven road flow, BBF biscuit basin flow (south, north, and east) a

Picked glass was analyzed using 8 mm beads, approximately 1 g of powdered sample

b

Picked glass from samples supplied by Kathryn Watts and Dr. Iilya Bindeman, sample YC96-2G from Bindeman and Valley (2001)

crystallization and/or a more fertile source. Melt inclusion data presented by Gansecki (1998) were approximately 1.2% water and very similar glass compositions for the Canyon flow. These values are low when correlated to the calculated temperatures for these flow, possibly indicating a dehydration phase of the source rock or melt. Differences between the major elements are within error, although the melt inclusions generally have lower SiO2 and higher Al2O3. Dunraven Road flow has higher SiO2 concentrations, but generally plots with other rhyolites from Yellowstone for most elements and does not show increased FeO, TiO2, Sr, Ba, and Sc as found in the Tuff of Sulphur Creek and Canyon flow. Dunraven Road flow has depleted trace element values compared to Tuff of Sulphur Creek and Canyon flow (Fig. 4). Petrography and mineral chemistry The mineral assemblages of the Tuff of Uncle Tom’s Trail, Tuff of Sulphur Creek, and Canyon flow are very similar.

123

Tuff of Uncle Tom’s Trail contains small (5 mm) obsidian fragments with depleted chemistry, auto-brecciated glass, and porphyric sections of groundmass appear to be very similar to the basal sections of Tuff of Sulphur Creek. As found in this study and by Girard and Stix (2009), the mineralogy of the Dunraven Road flow is very different from the other EUBM. Tuff of Sulphur Creek contains phenocrysts in variable concentrations, from approximately 10 to 20% in the basal sections with 20–35% in the stony interior. Plagioclase phenocrysts make up approximately 5–15% of samples and range from An18 to An37, with an average of An25 (Table 3). Cores of larger, sometimes sieved, plagioclases are generally more calcic, An24 to 30, similar to observations by Girard and Stix (2009). Magma compositional changes are also indicated by a small proportion of the larger plagioclase laths having coronas with compositions of alkali feldspar. Antirapakivi texture is also observed in samples from Canyon flow and HRT-C, which may indicate that either cooler temperatures or mixing with silicic magma occurred just


215

Table 3 Average results from mineral analysis Unit


Canyon flow

Dunraven Road flow


Canyon flow

Mineral

Plagioclase

Plagioclase

Plagioclase

Alkali feldspar

Alkali feldspar

Microprobe (wt%)

n = 57

2r

n = 29

2r

n = 23

2r

n = 113

2r

n=8

2r

SiO2

61.94

2.43

61.95

3.00

61.28

1.29

64.73

1.15

64.80

0.78

Na2O

7.49

0.65

7.56

0.79

8.69

0.38

5.16

0.48

5.29

0.21

Al2O3

23.53

1.74

24.00

2.21

23.15

0.67

19.73

0.54

19.95

0.26

K2O

1.57

0.81

1.39

0.68

1.15

0.38

7.97

0.84

7.80

0.37

FeO

0.32

0.06

0.31

0.04

0.32

0.05

0.20

0.05

0.20

0.05

BaO

0.22

0.14

0.22

0.13

0.18

0.11

1.60

0.69

1.72

0.22

CaO Total

5.02 100.10

1.88 0.99

5.28 100.70

2.28 0.62

4.88 99.66

0.62 0.88

0.76 100.15

0.30 0.95

0.82 100.58

0.15 0.69

An

24.55

9.12

25.60

10.85

22.22

2.90

3.87

1.52

4.16

0.72

Ab

66.33

5.38

66.39

7.14

71.55

2.80

47.68

3.78

48.63

1.78

Or

9.12

4.71

8.01

3.93

6.23

2.03

48.45

5.13

47.21

2.20

LA-ICP-MS (ppm)

n=5

Rb

5.36

5.41

3.35

3.83

3.49

2.38

123.53

101.35

51.57

0.16

Sr

433.78

78.13

355.35

304.99

190.63

24.89

250.84

183.16

407.82

74.31

Y Cs

0.98 0.03

2.21 0.07

0.80 0.00

0.68 0.00

0.34 0.01

0.41 0.06

2.58 0.18

4.41 0.17

5.90 0.10

4.80 0.18

Ba

1374

806

1331

1345

976

254

7841

5394

10341

63

La

20.45

12.20

18.60

17.07

14.50

2.86

12.16

8.26

14.51

6.76

Ce

30.40

14.83

25.61

22.11

21.67

3.45

14.07

14.26

14.08

5.40

Pr

2.08

0.99

1.79

1.51

1.37

0.27

1.03

1.20

0.93

1.08

Nd

6.37

4.79

5.05

4.20

3.62

0.77

3.34

4.43

3.00

4.67

Sm

0.70

0.64

0.49

0.44

0.35

0.08

0.58

1.17

0.47

0.86

Eu

6.40

1.43

5.26

4.65

2.33

0.23

4.72

2.46

7.43

0.16

Gd

0.52

0.52

0.76

0.70

0.24

0.08

0.64

0.90

5.41

1.35

Tb

0.04

0.07

0.03

0.04

0.02

0.02

0.07

0.14

0.06

0.12

Pb

20.04

5.35

17.80

15.75

19.30

2.28

37.99

17.97

29.66

0.58

n=6

n=7

n=9

n=2

Single-point data attached in electronic appendix

prior to eruption (Fig. 5). Alkali feldspar phenocrysts make up approximately 5–20% of the samples and range from Or40 to Or55, with an average of Or48. Alkali feldspars from Tuff of Sulphur Creek, Canyon flow, and HRT member C contain high amounts of Ba, generally over 1 weight percent, which is a greater concentration than found for other units at Yellowstone (Gansecki 1998). Quartz phenocrysts make up 5–15% and are commonly fragmented and occasionally embayed. Oxides, mostly ilmenite and titano-magnetite, are also present in variable abundances, about 2–5%. Oxides tend to have rather high Mn concentrations (approximately 1 weight percent). Trace pyroxene, zircon, and amphibole were also observed in samples from the Tuff of Sulphur Creek. Fine grained lithic fragments are also present up to approximately 5% in the basal ash, vitrophyre, and some of the upper banded portions of the veneer deposit. Thin sections made from the basal poorly sorted ash, and pumice

contain a large variety in grain sizes and variably sized bubble wall shards, though these features may also be found in airfall deposits close to the vent or some rhyolitic lava flows (Fig. 6a, b). Canyon flow phenocrysts are variable in concentration, but not to the extent as in the Tuff of Sulphur Creek. Phenocrysts generally make up 15–35%, and feldspars are variable in composition. Plagioclase phenocrysts make up approximately 5–15% of samples and range from An19 to An40 with an average of An25 (Table 3). Similar to Tuff of Sulphur Creek, cores of larger plagioclase tend to have a slightly higher anorthite component at An24 to An40, when compared to rims of An19 to An27. Plagioclase textures are similar to those in the Tuff of Sulphur Creek, including sieved cores, resorbed rims, and occasional anti-rapakivi (Fig. 5). Alkali feldspar phenocrysts make up approximately 5–25% of the samples and range from Or40 to Or54,

123

216

Fig. 4 Rare Earth Element plot for whole-rock samples from the EUBM. Values are normalized to Bulk Silicate Earth (McDonough and Sun 1995)

Fig. 5 a Backscatter image of anti-rapakivi texture from the Canyon flow, sample 08CP21B. b Distance from rim versus percent anorthite (An), albite (Ab), and orthoclase/sanidine (Or) from results from electron microprobe transect from the rim to core-region. At the core of the crystal is a titanomagnetite, nine other points from the alkali feldspar rim and plagioclase core yielded similar results as shown in the graph. The plagioclase phenocrysts appear to have undergone some diffusion and resorption prior to the crystallization of the alkali feldspar rim, with the contact at 205, measurement along the contact yielded approximately 49% SiO2 and 8% FeO (wt%). Extent of crystal is outlined with a thin black line

123


with an average of Or48. Quartz phenocrysts make up 5–15% of the samples and are commonly embayed. Pyroxenes are trace components in the stony interior of the Canyon flow. Pyroxenes can make up to 6% of the basal vitrophyre and are commonly associated with plagioclase and oxides in aggregates. In basal samples, approximately 4% are augite, Wo39–40En31–35Fs25–29, (similar to data presented by Hildreth et al. 1991; Girard and Stix 2009) and 2% are ferrosilite, Wo3–4En41–43Fs53–55. Ferrosilite was found exclusively in aggregates, whereas the augite was found in aggregates and as isolated crystals. Pyroxenes contain more Mg than others measured from the caldera (Hildreth et al. 1984). Other phenocrysts included oxides and trace zircon. Quartz rich lithic fragments were prevalent in the basal vitrophyre, but were not found in the stony interior. The Dunraven Road flow is very distinct from the other EUBM units in that it is aphanitic with up to 8% plagioclase phenocrysts. Phenocryst compositions are very different to those in the Tuff of Sulphur Creek, ranging from An19 to An24, with an average of An22 and no discernable difference from core to rim. Plagioclase phenocrysts also contain much less Sr, Ti, and Rb and are generally lower in trace element concentrations when compared to the other EUBM rhyolites (Table 3). Oxides can range up to about 5% and are generally titanomagnetite, with much less Mn than the other EUBM rhyolites. Quartz and sanidine may be found as trace minerals, but are often very small. Black and red obsidian samples from the Dunraven Road flow commonly contain features that resemble small red ribbons throughout thin sections (Fig. 6c). We have interpreted these features as microfractures due to the presence of occasional conchoidal fracturing. Backscatter images of the ribbons (Fig. 6e) show that the edges of the ribbons can have slightly higher FeO concentrations. The slight increase in FeO is interpreted as precipitated and oxidized iron. Oxidation may have occurred during cooling or as part of the weathering process. Crystallites are also present as small cross-shaped splinters on the order of 100 nm and tend to accumulate in bands and blobs. The presence of crystallites or microscopic crystal nucleations as described in Best (2003) may indicate that the magma was too viscous to allow crystal growth (Fig. 6d). Temperature and viscosity Magmatic temperatures of the EUBM were estimated with applicable geothermometers. Using plagioclase-melt (Putirka 2008), the Tuff of Sulphur Creek and Canyon flow were approximately 890 and 887°C, respectively. Calculations used variable concentrations of H2O (1.8–3%). Temperatures ranged from 857 to 825°C for the Dunraven Road flow. The temperature of the Dunraven Road flow is


Fig. 6 a, b 50 times magnification photomicrographs from the base of Tuff of Sulphur Creek, sample 08CP29, showing poorly sorted bubble wall shards and ash. c 50 times magnification image of Dunraven Road flow, slide 08CP01, showing ribbon like features may be oxidized microfractures. Crystallites are also present as isolated

217

features and clumped into masses, as depicted in the lower left hand corner. d 200 times magnification image of crystallites from the same slide as (c). e A backscatter image of microfractures, showing lighter colors around the edges of fractures

123

218


Fig. 7 87Sr/86Sr (480 ka) and 147Nd/144Nd plot of rhyolitic groups from Yellowstone. Eocene volcanics (Feeley et al. 2002), and Beartooth Mountains (Wooden and Mueller 1988) plotted for comparison. Basalts from within the Yellowstone Plateau area, HRT—Huckleberry Ridge Tuff (members A, B, & C), LCT—Lava Creek Tuff (members A & B), BBF—Biscuit Basin flows (denoted by N-north, S-south, M-middle, and E-east), bc—Blue Creek flow, mj— Mt. Jackson Rhyolites. Dunraven Road flow plots within the extracaldera zone. Data for Yellowstone Samples from Doe et al. 1982; Hildreth et al. 1991; Bennet, MS thesis—UNLV, 2002;

Nastanski, MS thesis—UNLV, 2002; Vazquez et al. 2009; and this study. The Scaup Lake flow is plotted with an unpublished 87Sr/86Sr value of 0.709838 ± 11, measured by TIMS (Vazquez, personal communication). Models A thru D are EC-RAxFC (Bohrson and Spera 2007) curves, discussed in text, and raw data are supplied in the Online Resource. Teq/Mao values for each model are Model A 894.03°C/2.091, Model B 910.05°C/1.848, Model C Junction Butte basalt—933.37°C/1.496 (Swan Lake basalt: 915.88°C/1.759), and Model D 1025.17°C/0.970

most likely lower than other EUBM rhyolites by approximately 33–65°C. Ternary feldspar thermometry (Putirka 2008) was used on rims of plagioclase and alkali feldspars from the Tuff of Sulphur Creek and Canyon flow and yielded temperatures of 868 and 866°C, respectively. Using pyroxene-melt geothermometers (Putirka 2008), the Canyon flow temperature was calculated to be approximately 860°C. However, previous studies have measured temperatures using oxide–oxide geothermometry for the Tuff of Sulphur Creek and Canyon flow from 850 to over 900°C (Hildreth et al. 1984; Bindeman and Valley 2001). The temperature for the Tuff of Sulphur Creek is, therefore, at least 850°C, and the Dunraven Road flow is estimated to be approximately 816°C. Based on these temperatures and average whole-rock chemistry, the viscosity for each flow has been estimated at 8 9 106 Pas for Tuff of Sulphur Creek, 4 9 106 Pas for Canyon flow, and 2 9 107 Pa s for Dunraven Road flow (Giordano et al. 2008). Calculated

viscosities are for atmospheric pressure and do not include effect of crystals and bubbles; however, the values are presented to illustrate the relative melt viscosities.

123

Radiogenic isotope ratios Radiogenic isotope data are presented in Table 2 as well as Figs. 7 and 8. Most of the UBM rhyolites have more radiogenic 87Sr/86Sr, 206Pb/204Pb, and 207Pb/204Pb ratios than other Yellowstone rhyolites (Doe et al. 1982; Hildreth et al. 1991). HRT, member C (HRT-C), contains more radiogenic 87Sr/86Sr, 206Pb/204Pb, 207Pb/204Pb, and lower 144 Nd/143Nd ratios when compared to other Yellowstone volcanics, including the EUBM. The Blue Creek flow and Mt. Jackson Rhyolites, which are post-HRT-collapse rhyolites, show similar trends as the UBM rhyolites, with depleted d18O values and somewhat enriched 87Sr/86Sr values (Fig. 7) (Hildreth et al. 1991).


Pb and Nd isotope ratios measured on a sample from just west of Silver Cord Falls suggest that there may be more than one flow in this unit. Sample 0822 from Silver Cord Falls contained 206Pb/204Pb = 17.54, 207Pb/204Pb = 15.59, and 208Pb/204Pb = 38.23, which are lower than other whole-rock Pb isotope measurements from the Canyon flow, which are 17.81, 15.65, and 38.37, respectively. This may indicate that the area near Silver Falls contains a younger flow as the sample appeared to be taken from the brecciated upper part of the unit, along the top of the Canyon rim. Similar sequences also appear in the WBUM, where older units contain more radiogenic isotopic ratios than younger flows. The name Silver Cord Falls flow is here referred to this flow found stratigraphically on top of the Canyon Flow in the Ribbon Lake/Silver Cord Falls area. The Dunraven Road flow contains very similar ratios of 87 Sr/86Sr, 206Pb/204Pb, 207Pb/204Pb, and 144Nd/143Nd when compared to extra-caldera rhyolites, most notably the Roaring Mountain Member. The Dunraven Road flow also contains low 208Pb/204Pb ratios when compared to other Yellowstone rhyolites (Fig. 8), indicating that the source rock may have had very low Th. Three Dunraven Road flow samples were run at different times to validate the results, and all the data for the Dunraven Road flow (except d18O ratios) are relatively homogeneous. 87 Sr/86Sr was measured on picked plagioclase phenocrysts from the Tuff of Sulphur Creek, Canyon flow, and Dunraven Road flow (Fig. 9). Some grains are in equilibrium with the melt, whereas most have more radiogenic values. There does not seem to be any correlation between core, rim, or size of the phenocryst, although samples were fractured (chipped) prior to picking. Thus, small plagioclase phenocrysts may have been fractured portions of larger grains. Data may be interpreted as evidence that the rhyolites are rejuvenated crystal mush (Girard and Stix 2009), but that the pre-existing magma had more radiogenic values than the replenishing magma, which may be the case if assimilation of Archean crust influenced rhyolite genesis. Figure 9 also illustrates that even though plagioclase phenocryst compositions vary, the glass versus whole-rock samples did not greatly vary, suggesting that radiogenic Sr is still useful for correlation of units. Stable isotope ratios The EUBM and WUBM rhyolites have been shown to be low d18O rhyolites by multiple studies (Friedman et al. 1974; Hildreth et al. 1984; Bindeman and Valley 2001; and Bindeman et al. 2008). Stable isotope data are presented in Table 4. Our measurements of stable isotope ratios for the Tuff of Sulphur Creek and the Canyon flow generally agree with previously measured quartz and feldspar ratios,

219

although they are occasionally higher than those of Hildreth et al. (1984), Bindeman and Valley (2001), and Bindeman et al. (2008). Quartz from the base of the Tuff of Sulphur Creek (picked from pumices) and from the vitric base of the Canyon flow consistently have higher d18O values (2.4%, n = 4) than the rest of the unit, which averaged 1.8% (n = 10). Quartz picked from the base of Canyon flow also exhibited higher d18O values (3.0%, n = 2) when compared to the rest of the unit, which averaged 2.5% (n = 4). Not including the basal portions of each flow and assuming a magma temperature of 850°C, the Tuff of Sulphur Creek magma was approximately 1.2% and the Canyon flow magma was approximately 1.8% (Zhao and Zheng 2003). d18O values for plagioclase separates from the Dunraven Road flow ranged from 2.6 to 4.8% (n = 9). These values are very comparable with those found by Hildreth et al. (1984). Sample YL96-4 from Bindeman and Valley (2001) was most likely from the Canyon flow, based on the location and description of the sample, though published as a sample from the Dunraven Road flow. We also analyzed quartz grains from the devitrified core of the Dunraven Road flow. Quartz values from the Dunraven Road flow were significantly lower than the plagioclase feldspar (1.5%); however, quartz was only observed in elongated vesicles and not in the vitric portions of the flow. Quartz present in the devitrified section of Dunraven Road flow is most likely vapor phase and thus crystallized in atmospheric conditions, thus inheriting lower d18O values. The average plagioclase phenocryst d18O value for the Dunraven Road flow was 4.0%. Assuming a magmatic temperature of 800°C, the magmatic value of the Dunraven Road flow is approximately 5%, with an error of approximately 1% due to high variability in the plagioclase phenocrysts (Zhao and Zheng 2003). This value agrees well with d18O values measured from obsidian (Hildreth et al. 1984). Studies of quartz and zircon d18O values by Bindeman and Valley (2001) and Bindeman et al. (2008) found that minerals from rhyolites that immediately followed caldera collapse have normal d18O cores with low d18O rims, whereas the younger Central Plateau Member and Scaup Lake flow contain low d18O cores with normal d18O rims (Bindeman et al. 2008). Such observations suggest that older units had a source rock that was initially not depleted in d18O, whereas younger units had a source that was depleted in d18O ratios and shifted to more normal values (Bindeman et al. 2008). Bindeman et al. 2008 also found that HRT members B and C, LCT, and MFT also exhibit some degree of depleted d18O values, whereas HRT member A contains somewhat higher d18O values. We analyzed grains from a sample of the Archean basement and found that quartz, plagioclase, alkali feldspar, and biotite are 8.7, 7.0, 7.0, and 3.8%, respectively. If values

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220 Fig. 8 Measured 208Pb/204Pb and 207Pb/204Pb versus 206 Pb/204Pb plot of rhyolitic groups from Yellowstone. Basalts from within the Yellowstone Plateau area, HRT—Huckleberry Ridge Tuff (members A, B, & C), LCT— Lava Creek Tuff (members A & B), BBF—Biscuit Basin flows (denoted by N-north, S-south, M-middle, and E-east). Data for Yellowstone Samples from Doe et al. 1982; Hildreth et al. 1991; Bennet, MS thesis—UNLV, 2002; Nastanski, MS thesis— UNLV, 2002; Vazquez et al. 2009; and this study. Beartooth Mountain samples are highly variable and span the entire plot area, and more

Fig. 9 Results from laser 87 Sr/86Sr for plagioclase picked from the Tuff of Sulphur Creek, Dunraven Road flow, and Canyon flow. Maximum and minimum 87Sr/86Sr values of glass and whole rock for each unit are depicted for comparison

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Fig. 10 Generalized cartoon of the development of UBM rhyolites, not to scale. Letters correspond to EC-RAxFC models. A—example of a possible source material of Tuff of Sulphur Creek and Canyon flow with high amount of assimilation and fractional crystallization (low Teq), B—example of a possible source material of ‘‘normal Yellowstone rhyolites’’ (e.g., Lava Creek Tuff, Central Plateau) from

221

a homogenized magma chamber, C—example of a possible source material of the Dunraven Road flow, a small melt from the hydrothermally altered ring fracture (basalt dikes not presented on picture), most likely at depths closer to other magmas, but illustrated at shallower to press the concept of a different magma chamber. D—not pictured, as it represents an extra-caldera setting

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222


Discussion

Table 4 Results of single-grain stable isotope analysis Sample

Type

18

d O

VSMOW

07-CP-12.4

Plagioclase

0.3 ± 0.1

Rhyolite genesis is generally thought to involve partial melting of the crust (Doe et al. 1982; Hildreth et al. 1991; and Christiansen 2001). However, multiple processes for rhyolite genesis have been presented in the literature, where the only similar factor is that the rhyolites have generally undergone fractional crystallization prior to eruption. We would like to address the plausible processes that can lead to the genesis to the EUBM and to lesser extent rhyolites from the Yellowstone volcanic area in general. As illustrated by Hildreth et al. (1991) and emphasized in this paper, deciphering the genesis of these rhyolites is complex and given the diversity of the crust, the concept of making simple mixing curves is fairly futile. However, integrating data from physical volcanology, petrology, geochemistry, and geophysics allow a conceptual model for understanding genesis of the Yellowstone rhyolites. Any model requires the consideration of the following observations:

07-CP-01

Plagioclase

0.4 ± 0.2

1.

Tuff of Sulphur Creek 07-CP-01.1

Quartz

1.5 ± 0.2

07-CP-01.2

Quartz

2.4 ± 0.2

07-CP-03

Quartz

1.7 ± 0.2

07-CP-04

Quartz

1.9 ± 0.2

07-CP-09.1

Quartz

1.9 ± 0.1

07-CP-09.2

Quartz

1.8 ± 0.1

07-CP-10.1

Quartz

2.1 ± 0.1

07-CP-10.2

Quartz

1.9 ± 0.1

07-CP-12.1

Quartz

1.8 ± 0.1

07-CP-12.2 08CP28.1

Quartz Quartz

1.9 ± 0.1 2.4 ± 0.2

08CP28.2

Quartz

2.3 ± 0.2

07-CP-03

Plagioclase

0.3 ± 0.2

07-CP-04

Plagioclase

0.3 ± 0.2

07-CP-12.3

Plagioclase

0.3 ± 0.1

Canyon Flow 08CP26.2

Quartz

2.9 ± 0.2

08CP26.1

Quartz

2.3 ± 0.2

0920.1

Quartz

2.4 ± 0.3

0920.2

Quartz

2.5 ± 0.3

0934.1

Quartz

3.9 ± 0.3

0934.2

Quartz

4.6 ± 0.3

08CP26.2

Plagioclase

0.9 ± 0.2

0920.4 0934

Plagioclase Plagioclase

1.0 ± 0.3 0.8 ± 0.3

08CP26.1

Alkali feldspar

1.8 ± 0.2

0920.3

Alkali feldspar

1.7 ± 0.3

0934

Alkali feldspar

2.7 ± 0.3

0934

Clinopyroxene

0.7 ± 0.3

08CP01.1

Plagioclase

3.6 ± 0.3

08CP01.2

Plagioclase

5.7 ± 0.3

3.

4.

Dunraven Road Flow

08CP01.3

Plagioclase

3.5 ± 0.3

08CP01.4

Plagioclase

3.5 ± 0.3

08CP03.1

Plagioclase

3.6 ± 0.3

08CP07.1

Plagioclase

4.1 ± 0.3

08CP02.2

Plagioclase

4.8 ± 0.3

08CP02.3

Plagioclase

3.8 ± 0.3

08CP02.4 08CP02.1

Plagioclase Vapor phase quartz

2.6 ± 0.3 1.9 ± 0.3

08CP07.2

Vapor phase quartz

1.2 ± 0.3

from the Archean basement reflect the source values, then depletions found in the EUBM may be as high as 5.8%, assuming an original magmatic d18O value of 7.0% as reflected by the value of Archean alkali feldspar.

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2.

5.

Isotopic shifts are found following the large scale caldera collapses at 2.1 Ma and 640 Ka (Hildreth et al. 1991), slight d18O depletions have also been found in HRT members B and C, LCT, MFT, and the rhyolites of the Central Plateau member (Hildreth et al. 1984; Bindeman et al. 2008). Low-velocity zones from seismic tomography depict the top of crystallizing magma body at depths of 8 km (Husen et al. 2004) and possibly basaltic sills at a depth of 15 km (Wicks et al. 2006). Plagioclase phenocrysts have oligoclase to andesine cores with oligoclase rims (Girard and Stix 2009), and most grains (independent of core or rim location) contain more radiogenic 87Sr/86Sr ratios than the host magma. Zircon ages do not generally exceed 2.5 Ma, and no Archean age zircons have been found in Yellowstone rhyolites. Zircons from caldera-forming eruptions indicate that crystallization occurred over the span of 10s to 100s of thousands of years (Bindeman et al. 2008). Quartz and zircons from Canyon flow contain cores with higher d18O values than the rims (Bindeman et al. 2008). The base of the Tuff of Sulphur Creek and Canyon flow contains phenocrysts with higher d18O values than the rest of the flow. The base of the Tuff of Sulphur Creek also contains up to 20% fewer phenocrysts than that of the stony interior.

It has been suggested that Yellowstone rhyolites may be formed by (1) partial melting of Earth’s crust (Doe et al. 1982; Hildreth et al. 1991; Christiansen 2001), (2) melting of previous volcanic deposits from the Yellowstone


volcanic area (Bindeman and Valley 2001), (3) partial melting of hydrothermally altered un-erupted magma and volcanic deposits with a rejuvenation of crystal mushes (Girard and Stix 2009), and (4) in this paper, we model the source rocks of the EUBM rhyolites using the EnergyConstrained Recharge, Assimilation, and Fractional Crystallization model (EC-RAxFC), as developed by Spera and Bohrson (2002) and Bohrson and Spera (2007). Our model suggests that, similar to Girard and Stix (2009), the source rock for the EUBM was un-erupted material from above the magma chamber. The following is a brief description of these plausible mechanisms for rhyolite genesis. Partial melting of the crust is the generally accepted method to form voluminous rhyolites and explains the absence of intermediate volcanic rocks at Yellowstone. We have used batch melting equations to produce a model where an approximately 30% partial melt can produce the general rare-earth element values found in Yellowstone rhyolites, including the Tuff of Sulphur Creek and Canyon flow. If the magma chamber started with a general Yellowstone radiogenic signature (for example, LCT, member A), the more radiogenic signature of the EUBM can be achieved by assimilation of 15–25% of more radiogenic crustal material (Hildreth et al. 1991). To form low d18O rhyolites requires the crustal assimilant to be approximately -12%. On the scale of a crystal, more calcic cores from plagioclase phenocrysts for the Tuff of Sulphur Creek and Canyon flow have been described ‘‘as retained residual feldspars from partial melting of the crust’’ (Doe et al. 1982). To test this hypothesis, plagioclase crystals from Archean crust at Lamar Canyon were measured. Analyzed Archean plagioclase compositions were An19–30, Ab69–81, and Or1. If plagioclase phenocrysts were inherited from Archean gneiss, this would require significant exsolution of the grains or recrystallization to increase Or percent, and decrease the Ab component prior to eruption of the EUBM. Although occasional perthitic textures are observed, extensive exsolution lamellae are not preserved in most phenocrysts from the EUBM (Hildreth et al. 1984). Laser 87Sr/86Sr measurements of plagioclase phenocrysts are generally more radiogenic than the surrounding glass, but require that 87Sr/86Sr values of the crustal material were approximately 0.717–0.718. This is significantly less than our measured value of Archean granitic gneiss from Lamar Canyon (0.72873 ± 4) and significantly less than estimated values of Hildreth et al. (1991), but within the measurements of the Beartooth Mountains (Fig. 7) (Meen 1987). However, models using 87Sr/86Sr of 0.718, or less, fail to produce the whole-rock and glass radiogenic values of Tuff of Sulphur Creek, Canyon flow, or Middle and East Biscuit Basin flows. Melting of hydrothermally altered volcanic rocks from previous eruptions of Yellowstone rhyolites has been suggested as the source for UBM rhyolites and other volcanic

223

centers along the eastern Snake River—Yellowstone Hotspot Track (Bindeman and Valley 2001; Bindeman et al. 2007, 2008). UBM radiogenic values are then explained in this model by inheritance from previously erupted Yellowstone rhyolites (Bindeman et al. 2008). The only other Yellowstone volcanic rock to have such an anomalous radiogenic isotope composition is HRT-C. HRT-C also contains similarities to the UBM rhyolites, such as high Ba concentrations in alkali feldspar, and anti-rapakivi texture samples. However, HRT-C only has an estimated volume of 250 km3 (Christiansen 2001). The approximate volume of Tuff of Sulphur Creek and Canyon flow is a little over 100 km3. Based on volumes of HRT-C and EUBM, over half of the deposition of HRT-C would have been deposited 60 km to the northeast, where no HRT-C has been mapped outside, or within the caldera. Furthermore, more radiogenic values preserved in phenocrysts suggests that the source rock contained higher radiogenic values than older volcanic events at Yellowstone, except for HRT member C. We have applied the EC-RAxFC model for initial magma generation at Yellowstone (Bohrson and Spera 2007) as presented in Fig. 6. We prefer EC-RAFC because 1.

2.

3.

4.

5.

6.

87

Sr/86Sr and 143Nd/144Nd for EUBM rhyolites and HRT-C can be modeled with measured data, whereas simple mixing of crust with mantle-derived material requires unrealistically low Nd and/or high Sr concentrations in end members. The model also illustrates relatively small changes in 143Nd/144Nd when compared to larger shifts in 87Sr/86Sr. Plagioclase phenocrysts contain higher and lower 87 Sr/86Sr ratios than glass and whole rock, suggesting a mixture of grains that are ‘‘older (un-erupted magma with higher amounts of assimilation)’’ and ‘‘younger (recharging magma)’’. There is an ample heat source, basalt injections, to allow for recharge of magma chambers and allow for assimilation with less crystallization than described by Glazner (2007). Assimilation of crust by multiple magma chambers allows for large amounts of rhyolite to be formed without exorbitant uplift rates, and emplacement rates on the order of 100s of thousands of years can be achieved by ‘‘incremental assembly’’ (Coleman et al. 2004). Units with increased radiogenic isotope ratios generally plot on curves with lower equilibration temperatures, higher amounts of assimilation, and further along the EC-RAxFC. These units may have been near solidus magma bodies above the magma chamber. Inherited Archean zircon ages may not be anticipated due to the initial magmatic temperatures exceeding 850°C (Miller et al. 2003).

123

224

7.


Similar problems with creating 87Sr/86Sr and Nd/144Nd values for rhyolites in the Eastern Snake River Plain have been explained by using EC-AFC (McCurry et al. 2008), although intermediate rocks are present in that system. 143

EC-RAxFC models can be made for all of the Yellowstone rhyolites using measured data and variable equilibration temperatures. UBM protoliths are formed with relatively high amounts of assimilation and fractional crystallization, resulting in lower equilibration temperatures. Four models are shown in Fig. 7. Models A and B, were based on a mixture of Junction Butte Basalt (Hildreth et al. 1984) and Archean Basement (Sample 08CP61, this study), with recharging magma that is similar to Junction Butte Basalt. We used the Lamar Canyon as a proxy for the Archean basement, as it is the closest to our field area and was representative in appearance to much of the granitic to gneissic basement in the area. Models C and D replaced Junction Butte Basalt with Swan Lake Basalt (Bennett 2002) values, as the rocks being modeled were extra-caldera rhyolites, although models using either basalt yielded similar results. Variables used in the models include initial basaltic temperature of 1,184°C, calculated using MELTS (Ghiorso and Sack 1995), initial wall rock temperature of 450°C (using lower wall rock temperatures for Model A, did not substantially change the model), resulting magma solidus of 750°C (estimated from MELTS and assumed to be lower than the magmatic temperatures for EUBM). The initial magma composition used Model curves are presented in Fig. 7, where Models A and B, and C and D use the initial variables as described above with the exception of equilibration temperatures (Teq)/mass of country rock involved in the RAFC event (Mao). Teq/Mao (K/kg) values for each model are Model A 894.03°C/2.091, Model B 910.05°C/1.848, Model C Junction Butte Basalt— 933.37°C/1.496 (Swan Lake Basalt: 915.88°C/1.759), and Model D 1025.17°C/0.970. Variables and results of the models are presented in the Online Resource. Variations in Teq are reasonable as the magmatic system below Yellowstone has been reestablished after two large caldera collapses (2.1 Ma and 640 ka), and the wall rock temperature has most likely increased as the magma system has grown over time. Models A to D have increasing values of Teq to represent the influence of a slightly warmer wall rock with time. We propose that Model A represents initial intrusions into cool wall rock. Model B is a homogenized magma chamber that feeds a bulk of Yellowstone rhyolitic volcanism. Models C and D (erupted units north of the caldera) appear to reach equilibrium at higher temperatures. Magma ascent seems to follow extensional structures north of the caldera and may require less interactions with the wall rock to rise through the upper crust and is

123

generally considered the product of small and isolated magma chambers (Hildreth et al. 1991). EC-RAxFC models generally fit the measured radiogenic isotope, and trace element data for the pre-hydrothermally altered source rock for the EUBM. Variables for the model were based on measured data, whereas basic mixing requires unrealistic trace element concentrations for more radiogenic UBM and HRT member C. This model also implies that magma bodies in the Yellowstone caldera are initially made of smaller intrusions that have accumulated, or focused, in one area to produce a large rhyolitic magma chamber, as described for the Central Plateau Member rhyolites (Vazquez and Reid 2002; Vazquez et al. 2009). Therefore, once a rhyolitic system is established by EC-RAxFC, the system contains a more radiogenic value than the intruding basalts, which establishes two distinct radiogenic sources as described by Christiansen (2001). As the amount of assimilation and fractional crystallization continue, the magma plots further away from the initial magma values using the EC-RAxFC model. As can be seen in Fig. 7, post-collapse rhyolites and HRT-C contain increased radiogenic ratios due to increased assimilation of crust and thus plot further away from the Yellowstone Basalt. This indicated that these magmas has proceeded along the EC-RAxFC curve to the point that they could no longer erupt until they were lowered to melting temperatures following successive caldera collapses. HRT-C is distinct from members A and B. It has been argued by Hildreth et al. (1991) that HRT-C is in fact only a contaminated version of members A and B due to similar phenocryst assemblage and locality within the caldera of member A. Observations of anti-rapakivi texture may not be isolated to member C of the HRT, although the presence of anti-rapakivi texture implies that a secondary recharge, or mixing event took place prior to eruption. HRT-C may have initially been at similar temperatures and pressures as of HRT members A and B; however, even if the intrusive emplacement of HRT was similar for all members, HRT-C may have erupted under different circumstances. For example, HRT-C magma may have had higher levels of assimilation that were already too crystal rich to erupt, but was recharged or subsided during caldera collapse during the eruption of HRT member A and B. Bindeman et al. (2008) describe variable d18O values in zircons from HRT members B and C, indicating that some digestion of hydrothermally altered material occurred prior to the eruption of the two younger HRT members, possibly indicating that assimilation of hydrothermally altered rock occurred just prior to eruption. Post-HRT rhyolites, such as Blue Creek flow, also show slightly increased radiogenic Sr ratios than common at Yellowstone, with an associated depletion in d18O values (Hildreth et al. 1991). Timing between the members of HRT is not known, but evidence


suggests that HRT-C and Blue Creek may have formed similarly to EUBM rhyolites and that the HRT-C is more characteristic of a post-caldera rhyolite, although it is considered a caldera-forming eruption. The lack of intermediate rocks within the Yellowstone Plateau is problematic. The models show that magmas initially stayed basaltic as fractionation begins, during which time the wall rock is warmed with no, or very little, assimilation occurring. Once the equilibration temperature is achieved, magma can change from mafic to felsic over small temperature increments. This observation was noted by Annen (2011) and during our modeling. Therefore, initially the magma is fractionating basalt until the thermal threshold for assimilation is reached (equilibration temperature) followed by rapid differentiation of the magma. Once multiple small melts coalesce to a larger magma, the differentiated silicic magma deflects any further basaltic injections. Any stalled mafic intrusions would most likely act as a heat source and possibly replenish the basal section of the felsic magma chamber (Bachmann and Bergantz 2006). This seems reasonable for Yellowstone, as the first known volcanic unit is the Junction Butte Basalt, followed by a rhyolite dominated system, while basalts are restricted to the extra-caldera setting. The crust under Yellowstone is also thicker than the rest of the Snake River Plain (Christiansen 2001), requiring basaltic magma to rise over greater distances than other Snake River Plain systems and providing an opportunity for more extensive mid-crustal magma pooling. This could increase the likelihood of a system dominated by high-silica rhyolites with peripheral basalts. Some EC-RAxFC factors that require consideration are that source rocks can vary greatly in trace element concentrations. Also, the magmas do not model well for radiogenic Pb ratios, based on similar ratios for the initial mafic input and recharging magma. Christiansen (2001) described radiogenic lead coming from contamination by mixing with small amounts of basalt. However, in the case of radiogenic Pb ratios, both basaltic and wall rock values are extremely variable. For example, 206Pb/204Pb can vary from 15.02 to 36.36 in the Beartooth Mountains (Wooden and Mueller 1988; Meen 1987) and basalt 206Pb/204Pb ratios vary from 16.10 to 17.89. However, the more radiogenic isotope signature of the UBM requires a dominance of assimilated older bedrock. Formation of low d18O rhyolites can be achieved by melting/assimilation of hydrothermally altered rocks (Taylor 1986; Hildreth et al. 1991). To achieve the approximately -5.0 to -5.8% shifts found in post-caldera rhyolites, it has been proposed that meteoric water was the dominant fluid in hydrothermal alteration of source rocks and also suggested very shallow melting (Bindeman and

225

Valley 2001; Bindeman et al. 2008). However, such shifts may also be influenced by time-integrated fluid flux during contact metamorphism over distances of approximately 10 km and at depth greater than a few kilometers (Dipple and Ferry 1992). We propose that equilibration of d18O during contact metamorphism may be a small portion of the hydrothermal alteration with meteoric water extending to depths along structural features. Meteoric water is the main fluid in hydrothermal systems at Yellowstone (Fournier 1989; Larson et al. 2009). Modeling of caldera formation suggests that high angle normal faults are produced along the ring fracture with lower angle normal faults toward the center of the caldera, the approximate location of the resurgent domes, and may act as conduits for the establishment of deep hydrothermal circulation (Hardy 2008). Therefore, meteoric water circulation and fluid-flux equilibration may be focused in areas such as the ring fracture and resurgent dome areas (Larson and Taylor 1986), which is where the UBM rhyolites are located.

Similar sources for the Tuff of Sulphur Creek and Canyon flow Similar whole-rock chemistry, isotopic composition, phenocryst assemblage and composition, as well as juxtaposed position of the flows indicate that the Tuff of Sulphur Creek and Canyon flow are genetically similar. Pb isotopic ratios measured from a sample of Canyon flow taken from the Silver Cord Falls area indicate that there may be more than one flow mapped in the Canyon flow unit and that this flow may show isotopic ratios comparable to the younger Central Plateau Member, North and South BBFs, and Scaup Lake. However, these samples are not generally distinguishable by major and trace element compositions. We prefer EC-RAxFC to explain the protolith for Tuff of Sulphur Creek and Canyon flow as un-erupted magma, or a rind/upper portion of the initial magma chamber that intruded into cooler crust and fractionally crystallized to the point that it was unable to erupt (e.g., low Teq). The intrusion may be as old as the HRT, 2.1 Ma, as this period of volcanism appears to have the highest amount of magma generated in the Yellowstone volcanic area, and zircon grains have been dated to this time period (Bindeman et al. 2008). These intrusions were more radiogenic than common Yellowstone rhyolites because of the high degree of assimilation. Cooled intrusions overlying the magma chamber were hydrothermally altered during a long history of underlying magmatic activity. The protolith was then dropped to melting depths during the last caldera collapse. We envision that the protolith was assimilated by any remaining magma or slowly melted by latent heat from fractionated crystals. Phenocrysts from this protolith were

123

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then rejuvenated by the intrusion of rhyolite and erupted along the ring fracture/resurgent dome normal faults. Heterogeneity in these flows indicates that either the source rock was heterogeneous or that mixing occurred prior to eruption as suggested by Girard and Stix (2009). Figure 10 presents a generalized cartoon depicting the events leading to eruption of the EUBM members based upon EC-RAxFC models A and C. The close proximity of the Mt. Washburn volcanics suggests that Absaroka volcanics are a likely candidate as a source rock. Indeed, the Absaroka volcanics are described as forming by cryptic hybrid magmas, which involves the stalling of magma within the crust (Feeley et al. 2002). However, the distinct radiogenic signature of the Absarokas is very different than Yellowstone volcanics, with lower 87Sr/86Sr and much more radiogenic 144Nd/143Nd (Fig. 7) and most likely did not contribute significantly to the intra-caldera rhyolites.

Extra-caldera source for Dunraven Road flow Petrology, geochemistry, and radiogenic isotope ratios suggest that the genesis of the Dunraven Road flow is different from the rest of the EUBM. Small isolated volume, stratigraphic makeup, petrologic features such as microfractures and crystallites, and highly fractionated geochemistry indicate that this unit had a high viscosity and is most likely a ring fracture dome. Furthermore, radiogenic isotope ratios suggest that it formed as an extracaldera rhyolitic dome, most comparable to the Roaring Mountain Member. From this, we have determined that the source of the Dunraven Road flow is most likely from the extra-caldera side of the ring fracture. Figure 3 is a modified map of the Dunraven Road flow, based upon our field work. Of particular interest is that low d18O feldspars and glass measurements by Hildreth et al. (1984) suggest the source rock had been hydrothermally altered. Modeling the petrogenesis of the Dunraven Road flow seems to match well with other extra-caldera rhyolites and suggests that these rhyolites were formed by assimilation of Archean crust and extensive fractional crystallization. The same model also can be used to model other extracaldera rhyolites, excluding Cougar Creek and Riverside flows, which would require a higher equilibration temperature to achieve the radiogenic values (Hildreth et al. 1991; Nastanski 2002). Using a one-stage EC-RAxFC model requires the source rock to be approximately 0% to explain the d18O value measured for Dunraven Road flow, 5%. Thus, the hydrothermally altered protolith in this case was most likely Archean rocks at upper crustal depths, indicating that fracture porosity and resulting hydrothermal alteration may extend to melting depths and transmit

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enough meteoric water to sufficiently lower d18O values of the wall rock. During assimilation, it is assumed that the margins of the intrusion digested the most country rock and that plagioclase phenocrysts crystallizing in this region were possibly entrained in the eruptible magma, which also contains plagioclase phenocryst in equilibrium with the glass values for 87 Sr/86Sr, or the initial magma had slightly more radiogenic value, which was lowered by subsequent recharge. This seems more plausible for Dunraven Road flow as the measured plagioclase are generally only 0.0015 87Sr/86Sr higher than the glass, where as Tuff of Sulphur Creek and Canyon flow contain phenocrysts that are up to 0.0025 87Sr/86Sr higher than the glass values. However, it is also plausible that some mixing did occur prior to eruption.

Conclusions The most significant conclusions of this work are summarized below. 1.

2.

3.

4.

5.

The oldest members of the EUBM, the Tuff of Uncle Tom’s Trail and Tuff of Sulphur Creek, are most likely the same unit and were deposited as a pyroclastic density current based on the field and petrologic observations. The Canyon flow is most likely composed of multiple flows, with at least one younger flow exposed near the Silver Cord Falls area. The youngest member of the EUBM is most likely an extra-caldera unit similar to the Roaring Mountain Member and is the only known low d18O rhyolite erupted from outside the caldera boundaries. The formation of the Dunraven Road flow can be best explained by a single EC-RAxFC event assuming that hydrothermal alteration extended to depths along the ring fracture zone. Results from this study did not support the formation of Tuff of Sulphur Creek and Canyon flow by onestage partial melting of Archean basement material. Also, based upon location and geochemistry of volcanic rocks from Yellowstone, our results did not support the hypothesis that the formation of EUBM was by remelting of hydrothermally altered rocks that had been erupted on the surface and then subsided to melting depths. Data and modeling suggest that the formation of the Tuff of Sulphur Creek and Canyon flow involved the hydrothermal alteration of the cooled margins of the magmatic system feeding the Yellowstone volcanic area. These un-erupted source rocks were modeled using EC-RAxFC, based on the initial magma similar


being similar to the Junction Butte basalt. The assimilate used in the modeling was Archean Gneiss from Lamar Canyon, which is the closest Archean bedrock to the location of the EUBM. Following the collapse of the Yellowstone Caldera, the un-erupted source rock was melted and erupted to deposit the Tuff of Sulphur Creek and Canyon flow. These findings generally agree with the concept of rejuvenating an un-erupted crystal mush, as presented by Girard and Stix (2009). Acknowledgments Greatly appreciated field help was given by Allen Anderson, William Starkel, and Kevin Tarbert. Sample analyses were conducted at Washington State University GeoAnalytical Laboratories with help from Dr. Rick Conrey, Charles Knaack, Dr. Garret Hart, and Dr. Jeffery Vervoort. Sampling permits were issued by Yellowstone National Park, Research Permit Office, with help from Christie L. Hendrix. We greatly appreciate discussions, 87Sr/86Sr values for the Scaup Lake flow, and manuscript review from Dr. Jorge Vazquez. Samples from the Biscuit Basin flows and valued discussions by Dr. Ilya Bindeman, and Kathryn Watts were also appreciated. Discussions and manuscript reviews were also greatly appreciated by Meghan Lunney, Dr. Benjamin Ellis, Dr. Richard Conrey, Dr. John Wolff, Dr. Wendy Bohrson, Dr. Guillaume Girard, and an anonymous editor. This study was funded by National Science Foundation grant #EAR-0609475, awarded to Dr. Peter B. Larson and by the School of Earth and Environmental Sciences, Washington State University.

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