Bulletin of Volcanology

, Volume 70, Issue 3, pp 315–342

Miocene silicic volcanism in southwestern Idaho: geochronology, geochemistry, and evolution of the central Snake River Plain

  • Bill Bonnichsen
  • William P. Leeman
  • Norio Honjo
  • William C. McIntosh
  • Martha M. Godchaux
Research Article

DOI: 10.1007/s00445-007-0141-6

Cite this article as:
Bonnichsen, B., Leeman, W.P., Honjo, N. et al. Bull Volcanol (2008) 70: 315. doi:10.1007/s00445-007-0141-6

Abstract

New 40Ar-39Ar geochronology, bulk rock geochemical data, and physical characteristics for representative stratigraphic sections of rhyolite ignimbrites and lavas from the west-central Snake River Plain (SRP) are combined to develop a coherent stratigraphic framework for Miocene silicic magmatism in this part of the Yellowstone ‘hotspot track’. The magmatic record differs from that in areas to the west and east with regard to its unusually large extrusive volume, broad lateral scale, and extended duration. We infer that the magmatic systems developed in response to large-scale and repeated injections of basaltic magma into the crust, resulting in significant reconstitution of large volumes of the crust, wide distribution of crustal melt zones, and complex feeder systems for individual eruptive events. Some eruptive episodes or ‘events’ appear to be contemporaneous with major normal faulting, and perhaps catastrophic crustal foundering, that may have triggered concurrent evacuations of separate silicic magma reservoirs. This behavior and cumulative time-composition relations are difficult to relate to simple caldera-style single-source feeder systems and imply complex temporal-spatial development of the silicic magma systems. Inferred volumes and timing of mafic magma inputs, as the driving energy source, require a significant component of lithospheric extension on NNW-trending Basin and Range style faults (i.e., roughly parallel to the SW–NE orientation of the eastern SRP). This is needed to accommodate basaltic inputs at crustal levels, and is likely to play a role in generation of those magmas. Anomalously high magma production in the SRP compared to that in adjacent areas (e.g., northern Basin and Range Province) may require additional sub-lithospheric processes.

Keywords

Caldera Crustal foundering Ignimbrite flare-up Lava flow Rhyolite Rifting Snake River Plain 40Ar-39Ar dating 

Introduction

The SRP volcanic province is an elongate, topographically depressed feature that extends some 600 km from the Yellowstone Plateau across southern Idaho and into northern Nevada (Fig. 1, inset). Initial magmatism along this trend coincided with early phases of Columbia Plateau flood basalt volcanism, and the province is characterized by a diachronous early phase of silicic volcanism that generally migrated northeastwardly to Yellowstone with time. Together, these observations have led to the prevailing view that magmatism is related to migration of the North American plate over the putative Yellowstone hotspot or mantle plume (Armstrong et al. 1975; Leeman 1982a, 1989; Pierce and Morgan 1992). We assess this notion primarily from the viewpoint of the silicic volcanism, with specific emphasis on the central SRP (CSRP).
Fig. 1

Map of southwestern Idaho showing important volcanic features and key locations discussed in the text: western Snake River Plain graben (WSRP), central Snake River Plain (CSRP) magmatic province, Black Rock escarpment (BRE), Browns Bench escarpment (BB), Rogerson-Twin Falls area (RTF), Owyhee Front area (OF), Grasmere escarpment (GE), Jacks Creek area (JC), West Bennett Mountain area (WBM), Magic Reservoir eruptive center (MREC), Picabo/Lake Hills area (PH), and Trapper Creek area (TC). Streams shown include the Snake (SR), Bruneau (BR), and Jarbidge (JR) rivers and Salmon Falls Creek (SFC). Population centers are shown in shaded circles. Inset shows map area with respect to the entire Snake River Plain—Yellowstone volcanic province, including eastern Snake River Plain (ESRP); loci of other areas discussed in text include: McDermitt (MD), Juniper Mountain/Owyhee Humboldt (JM), and Yellowstone Plateau (YP) eruptive centers and source area for Arbon Valley Tuff (AVT). Filled black squares show locations for photographs in Figs. 2 and 3. Idaho-Nevada state border is located at 42°N

This paper presents a synthesis of the regional geology and our current understanding of the volcanic stratigraphy, followed by a description of magmatic evolution in the CSRP. We then consider the petrogenetic and tectonic processes underlying silicic magmatism and their implications for development of the SRP-Yellowstone province as a whole. Our intent is to present what we know or can infer from work to date, and to focus attention on some of the remaining outstanding problems. A major take-home message is that the scale of magmatism associated with this province is enormous. We propose that silicic magmatism was driven primarily by infusion of great volumes of mantle-derived basalt into the crust, and conclude that much of the underlying crust was affected by partial melting and by deformation required to accommodate the volumes of magma involved. Origin(s) of the basaltic magmas and their link to sublithospheric processes are fundamental questions that will be addressed elsewhere.

Regional setting

The earliest manifestation of silicic magmatism that is considered to be related to the SRP province was eruption of peralkalic silicic volcanics associated with the ca. 16–14 Ma McDermitt caldera complex (Rytuba and McKee 1984; Conrad 1984) and vents in nearby ranges (Brueseke et al. 2007) in north-central Nevada. This volcanism is closely associated in time and space with early phases of Columbia Plateau flood basalt activity (cf. Camp and Ross 2004), leading to the view that it is hotspot-related. Rhyolitic activity directly associated with the western Snake River Plain (WSRP) first occurred in southwestern Idaho (Juniper Mountain/Owyhee-Humboldt eruptive center; JM) around 15–13 Ma (Ekren et al. 1984; Manley and McIntosh 2002).

Between roughly 12 and 7 Ma, NE–SW crustal extension in the WSRP produced a conspicuous NW-trending fault-bounded graben or rift that diverges from the main hotspot track (Fig. 1). This WSRP rift zone, the eastern SRP (ESRP), and the Owyhee Plateau (including the JM center) converge in the CSRP where the exceptionally prolific Bruneau-Jarbidge (BJ; Bonnichsen 1982a) and Twin Falls (TF; Wright et al. 2002) eruptive centers produced numerous regionally extensive ignimbrites and large-volume rhyolite lavas in late Miocene time. Overall, the cumulative volume of silicic magma produced in the CSRP may dwarf that produced at Yellowstone by several-fold (Perkins and Nash 2002). CSRP volcanism is the main focus of this paper, and is described in more detail in the next section.

A notable flare-up between 11.7–10.2 Ma produced numerous distinctive rhyolites from widely distributed vents in the BJ center, along the margins of the WSRP graben (e.g., Owyhee Front, OF; Bonnichsen et al. 2004), the Mount Bennett Hills to the north (MBH; Leeman 1982b, c; Honjo 1990), and as far east as the Pocatello area (e.g., Arbon Valley tuff, or AVT; cf. Kellogg et al. 1994). Eruptions of these units conspicuously diverge from the widely accepted age-space pattern for early SRP volcanism. The AVT is also anomalous with regard to its composition and mineralogy—particularly the presence of abundant biotite phenocrysts, which are notably absent in other early SRP rhyolites.

With exception of the AVT and related rhyolites, silicic volcanism in the ESRP was concentrated between 7 to 4 Ma and produced several regionally extensive ignimbrites and associated airfall tuffs that are attributed to the Heise volcanic center (Morgan et al. 1984; Morgan and McIntosh 2005). This sequence was onlapped to the east by ash flow tuffs produced from the well-known Yellowstone eruptive center beginning around 2 Ma (Christiansen 2001).

Most rhyolites of the SRP-Yellowstone region are metaluminous in composition, have high temperature anhydrous mineral assemblages (cf. Perkins et al. 1995), and have distinctive Sr and Nd isotopic compositions compared to those from north-central Nevada and eastern Oregon (Leeman et al. 1992a; Streck and Grunder 2007; Nash et al. 2006; Brueseke et al. 2007). This distinction coincides closely with the inferred locus of the western Idaho suture zone between cratonic North America to the east and accreted oceanic terranes to the west. Thus, the nature of the underlying lithosphere appears to have influenced compositions of the rhyolites—with a crustal signature more prevalent to the east.

Regarding the hypothesis that SRP magmatism is related to the Yellowstone hotspot, we also note that patterns of SRP volcanic activity differ in several important respects from typical oceanic hotspot magmatism. In any given area within the SRP, initial silicic volcanism persisted for several million years prior to onset of basaltic volcanism but, once initiated, the latter continued sporadically (up to Quaternary time locally). Delayed appearance of basaltic volcanism in the SRP may be explained by a crustal ‘density filter’ effect, but its persistence long after the North American plate migrated past the Yellowstone hotspot is seemingly inconsistent with basalt derivation from a narrowly focused source (e.g., a plume tail). Also, whereas silicic volcanism is a minor component of oceanic volcanism, the cumulative volume of rhyolitic magma (on the order of tens of thousands of km3; Perkins and Nash 2002) places the SRP among the most productive silicic igneous provinces known (Mason et al. 2004).

To understand the origin of the silicic magmatism, as well as the influence of the underlying basement, we have developed a comprehensive stratigraphic and petrologic framework for CSRP Miocene volcanism that spanned some 6–7 m.y. We describe the most important eruptive units, present new stratigraphic and age information, and document evolutionary trends of eruptive style, volume, geochemistry, and petrology.

Overview of central Snake River Plain geology

The main focus area for this study covers some 140 km SE to NW and 170 km SW to NE in south-central Idaho, exceeding an area of 20,000 km2. Representative stratigraphic sequences are described from the following areas (see Fig. 1): (1) Bruneau-Jarbidge (BJ) eruptive center [with partly equivalent units from the Grasmere escarpment (GE) and Jacks Creek (JC) areas to the west]; (2) the Rogerson-Twin Falls region (RTF)—which includes units from the Twin Falls (TF) eruptive center and the Rogerson graben-Browns Bench (BB) area near the Idaho-Nevada boundary (Andrews et al. 2007); and (3) the Mount Bennett Hills (MBH) along the northern margin of the SRP (Bonnichsen et al. 1989; Honjo 1990; Wood and Clemens 2002). Most silicic volcanic rocks in this broad region are inferred to have erupted from vents within the CSRP that are now largely buried beneath a veneer of younger basalts and fluvial-lacustrine sediments.

Voluminous explosive and effusive eruptions of rhyolite in the BJ region produced a large (95 × 55 km) elliptical zone of structural collapse (Fig. 1); this zone is elongated NW to SE and encloses about 4,100 km2 (Bonnichsen 1982a). The principal silicic volcanic products of the BJ center include at least nine high-temperature ignimbrites (Fig. 2), collectively known as the Cougar Point Tuff (CPT), and a number of mostly younger large-volume rhyolite lava flows that fill the interior of the BJ center (Bonnichsen 1982a, b; Bonnichsen and Citron 1982). Along the southern and western margins of the BJ center, there is evidence of major structural collapse in the form of faults and downwarping of volcanic strata into the basin. Owing to younger cover, loci of the northern and eastern margins are inferred on the basis of geophysical evidence, distributions of silicic lavas outside of the basin, and the assumption that such units erupt from caldera-like vents. In reality, it is likely that BJ eruptions originated from a plexus of nested calderas and perhaps fissure-like vents. Loci of buried ring faults may be inferred from the distribution of post-rhyolite basalt shield volcanoes; these define a series of semi-circular arcs, each convex toward the east and varying from 12 to 45 km in diameter across (Bonnichsen and Godchaux 2002). To the west and northwest (GE and JC areas), CPT units are onlapped by rhyolitic lava flows of local origin.
Fig. 2

Photograph of Black Rock Escarpment, along east side of the Bruneau River. Section exposes ignimbrite cooling units of the Cougar Point Tuff (CPT) as described in Appendix 6; these are designated by Roman numerals as follows: III (12.66 ± 0.02 Ma), V (12.07 ± 0.04 Ma), VII (11.81 ± 0.03 Ma), IX (11.56 ± 0.07 Ma), X (∼11.3 Ma), XI (11.22 ± 0.07 Ma), XII (∼11.1 Ma), XIII (10.79 ± 0.04 Ma), and XV (∼10.5 Ma). There are no units corresponding to Roman numerals I, II, IV, VI, VIII, XIV, or XVI and higher. Total section is approximately 450 m thick at this locality. Note that CPT unit X and intercalated rhyolite lava flows are missing from the section at this locality

The TF eruptive center to the east is the presumed source area for additional silicic ignimbrites and rhyolite lava flows that are well exposed in the Cassia Mtns. and adjacent highlands (Hughes and McCurry 2002; Wright et al. 2002). Some of these units originated from northerly sources buried in the CSRP, and onlap distal exposures of CPT units. The N–S trending Rogerson graben developed during the time of silicic volcanism in this part of the SRP. This structure is bounded by the prominent Browns Bench (BB; Fig. 3) escarpment on its western side and by a series of smaller escarpments to the east. A number of ignimbrites that are partly correlative with those of the BJ and TF units are well exposed along BB, and sporadically elsewhere in the eastern CSRP.
Fig. 3

Photograph of Browns Bench escarpment, along west side of the Rogerson graben near Corral Creek (sec. 15, T. 15 S., R 14 E. and vicinity), which we propose as the reference section. A sequence of at least twelve ignimbrite cooling units (outflow facies) is exposed in this area. Numbers correspond to the BBU-Number units as described in Appendix 6; The lowermost unit (BBU-1) is not exposed at this site and the uppermost units (BBU-9 to BBU-12) are present but not visible in this view. The portion of the section shown is stratigraphically or temporally equivalent to CPT units V to XV shown in Fig. 2. Total section is approximately 400 m thick at this locality

Along the north margin of the CSRP, in the MBH, Miocene silicic volcanic rocks similar to those to the south unconformably overlie Eocene volcanic rocks (Challis Volcanics) and granitic rocks of the Idaho batholith. The western MBH rhyolites comprise two sequences of multiple tuffs and/or lavas erupted from southerly sources, also buried within the CSRP. Many of the individual units are equivalent at least in age to rhyolites of the BJ area. Structural relations between these packages suggest that early development of the WSRP bounding faults was in part syn-volcanic. Later, we show that rhyolites from the MBH and the BJ area define similar temporal trends in composition—suggesting that all are related to a common (or similarly) evolving magmatic system(s). Volcanism in the eastern MBH differs from that in the southern highlands in having a unique rejuvenation of silicic magmatism between 6–3 Ma, represented by the Magic Reservoir eruptive center (MREC) (Leeman 1982c; Honjo et al. 1986). The earliest MREC products include ferrolatites that clearly formed by mixing between coeval basalt and rhyolite magmas (Honjo and Leeman 1987). Because MREC rhyolites differ from older CSRP rhyolites with regard to mineralogy and composition and appear to be derived from an unrelated magmatic system (Leeman 2004), they are not considered further in this paper.

For brevity, considerable auxiliary supporting information is presented in electronic Appendices as follows: A1, locations of dated samples; A2, summary descriptions of stratigraphic units; A3, all available major/trace element data; A4, averaged analyses for each stratigraphic unit; A5, compiled temperature estimates for CSRP rhyolites; and A6, discussion of stratigraphic relations.

Geochronology of rhyolite units

Critical to understanding the systematics of CSRP magmatism, and that of the province as a whole, is establishment of a regional geochronological framework. Early reconnaissance studies (Armstrong et al. 1975, 1980) provided the basis for time-transgressive magmatism that is often cited in support of the hotspot hypothesis for origin of the SRP. More recent work substantiates the impression that silicic magmatism generally decreases in age northeasterly toward Yellowstone (e.g., Pierce and Morgan 1992; Manley and McIntosh 2002; Morgan and McIntosh 2005) but, as previously noted, also confirms that there are notable exceptions to a simple age progression. Except for Yellowstone (Doe et al. 1982; Hildreth et al. 1991; Christiansen 2001; Bindeman and Valley 2001a, b; Vazquez and Reid 2002), evolutionary details remain sparse for these older eruptive centers, although recent tephrochronology studies of SRP rhyolite tuffs and ashfall deposits (Perkins et al. 1995, 1998; Perkins and Nash 2002) provide improved chronology for the regional stratigraphic framework.

To further elucidate time-stratigraphic relations, we report new high-precision single-crystal sanidine 40Ar-39Ar laser fusion dates for 24 samples of middle and late Miocene rhyolites from southwestern Idaho and adjacent areas in Nevada (Table 1; Appendix 1). Nine dated samples are from units in the BJ region, three are from units in the BB-Rogerson graben area, nine are from rhyolite units along the southwestern flank of the WSRP, and the three remaining samples are from older rhyolite units in the Owyhee Range or adjacent to the WSRP graben. For two units, multiple samples were dated with close agreement in age. Chemical analyses for many of these dated samples are included in Appendix 3 or presented in Bonnichsen et al. (2004).
Table 1

New 40Ar-39Ar dates for west and south-central SRP rhyolite units

ID number

Sample

Unit

Na

K/Ca ± 2σ

Age ± 2σ (Ma)

Location

W. Long.

N. Lat.

Bruneau-Jarbidge Region

    

1

I-452

CPT III

11

27.0 ± 6.0

12.67 ± 0.08

Bruneau Canyon

115.65

42.05

2

I-569

CPT III

15

26.9 ± 6.2

12.64 ± 0.06

Bruneau Canyon

115.64

42.01

3

I-1084

CPT IX

17

13.2 ± 2.2

11.56 ± 0.07

Bruneau Canyon

115.63

42.02

4

X-174

CPT XI

21

15.5 ± 3.6

11.22 ± 0.07

Jarbidge Canyon

115.41

42.00

5

I-551

CPT XIII

13

14.6 ± 4.6

10.82 ± 0.06

Near House Ck.

115.02

42.01

6

I-541

CPT XIII

15

14.3 ± 2.7

10.80 ± 0.06

Ross Pasture Ck.

115.12

41.99

7

I-809

CPT XIII

15

12.2 ± 9.0

10.75 ± 0.07

Bruneau Canyon

115.63

42.01

8

I-1119

Cedar Tree rhy.

14

17.2 ± 6.3

10.16 ± 0.09

Bruneau Canyon

115.68

42.30

9

S-487

Bruneau Jasper rhy.

14

15.8 ± 3.1

9.50 ± 0.06

Bruneau Canyon

115.64

42.34

Rogerson-Twin Falls Region

10

I-3918

BBU-7

19

13.7 ± 3.1

10.91 ± 0.07

Browns Bench Escarp.

114.79

42.15

11

I-3929

BBU-9

17

19.8 ± 16.5

10.22 ± 0.09

Browns Bench Escarp.

114.81

42.13

12

I-3503

Rabbit Springs ignim.

11

19.1 ± 3.7

10.37 ± 0.13

Backwaters area

114.76

42.06

Western SRP Region

13

I-3673

Jump Ck., Shares Snout mbr.

15

21.6 ± 3.0

11.69 ± 0.06

Shares Snout

116.82

43.38

14

I-3478

Jump Ck., Buck Mountain mbr.

15

0.3 ± 0.1

11.56 ± 0.25

Buck Mountain

116.88

43.38

15

I-3846

Reynolds Creek rhy.

20

21.3 ± 8.3

11.48 ± 0.09

Reynolds Ck. Can.

116.71

43.28

16

I-3803

Wilson Creek ignim.

16

30.8 ± 10.6

11.42 ± 0.08

near Wilson Ck.

116.74

43.36

17

I-3253

Wilson Creek ignim.(?)

16

40.2 ± 11.7

11.41 ± 0.05

Hill 2597, Owyhee Front

116.77

43.42

18

I-3664

Wilson Creek ignim.

14

21.3 ± 2.2

11.34 ± 0.11

Wilson Bluff

116.77

43.32

19

I-3467

Upper Browns Ck. ignim.

15

30.6 ± 4.5

11.20 ± 0.02

SW of Oreana

116.44

43.01

20

I-3246

Cerro Otono, Hill 2471

15

61.3 ± 14.5

11.14 ± 0.03

Hill 2471, Owyhee Front

116.72

43.40

21

I-3672

Cerro Otono, Hlll 3036

15

56.3 ± 13.4

11.03 ± 0.07

Hill 3036, Owyhee Front

116.74

43.40

Older Rhyolite Units, Owyhee Mtns.

22

I-1706

Swisher rhy., Poison Ck.

12

14.2 ± 8.0

14.21 ± 0.11

SW of Grand View

116.30

42.73

23

I-3063

Silver City Rhyolite

14

14.6 ± 4.2

16.66 ± 0.08

Silver City Range

116.61

43.07

24

I-3065

Silver City Rhyolite

13

13.2 ± 3.8

16.33 ± 0.12

Silver City Range

116.63

43.07

See Appendix 1 for more detailed sample descriptions.

aNumber of mineral grains included in averages

Where direct comparison can be made with previously available age information, the new ages generally agree within the stated analytical uncertainties (typically less than 0.1 Ma). For the earliest CPT ignimbrite (CPT III), two samples from the BJ area give ages (12.64, 12.67 Ma) that are indistinguishable from that reported (12.67 Ma; Perkins et al. 1995) for a correlative sample from Bruneau Canyon. The three determinations average 12.66 ± 0.02 Ma.

Perkins et al. (1995) infer ages of other CPT units from analyses of distal ash deposits from the Trapper Creek (TC, Fig. 1) section east of the Cassia Mtns.—more than 100 km east of our sample localities. For example, they report an age (10.94 Ma) for tephra they correlate with the tuff of Big Bluff (Cassia Mtns.) as well as, possibly, the penultimate CPT ignimbrite (CPT XIII). We report a similar age for one of the BB ignimbrites (BBU-7; 10.91 Ma). However, our three samples of CPT XIII from the BJ area give slightly lower ages (10.75, 10.80, 10.82; avg. = 10.79 ± 0.04 Ma), raising the possibility that multiple ignimbrites erupted over a short time span. On the other hand, these dates are indistinguishable within their respective analytical uncertainties. The new ages confirm the earlier chronology of Perkins et al. (1995, 1998). Other dates are discussed in a stratigraphic context in Appendix 6.

Along the Owyhee Front (OF; Fig. 1) and northern flank of the WSRP graben, several rhyolite units erupted, mainly as lavas, between 11.7–11.0 Ma (cf. Bonnichsen et al. 2004; Wood and Clemens 2002; Table 1). They clearly post-date activity in the nearby Juniper Mountain center, and are difficult to relate to a simple time-transgressive model for onset of SRP silicic magmatism. Although these rhyolites are compositionally distinct from the CPT ignimbrites, activity in these two contiguous areas clearly overlapped in time. Remarkably, WSRP eruptive activity falls within the time span of the three largest eruptive and collapse episodes in the BJ center (ca. 11.6, 11.2, and 10.9 Ma), suggesting the intriguing possibility that eruptions from widely separated and compositionally distinct reservoirs could have been triggered by related factors (e.g., episodes of faulting). Moreover, the unusual biotite-bearing Arbon Valley tuff (AVT) also erupted from vents in the ESRP (ca. 10.2 Ma; Kellogg et al. 1994) contemporaneous with waning stages of explosive volcanism in the BJ area—in this case, significantly earlier than postulated migration of its source region (Pocatello area) over the Yellowstone hotspot. Although different in terms of composition and eruptive volume, these examples demonstrate that silicic magmatism was widely distributed (over 400 km!) across southern Idaho between 10–12 Ma.

Published age determinations (adjusted to currently accepted decay constants), albeit often of lesser precision, further constrain timing and correlations of the units described in this paper (cf. Armstrong et al. 1975, 1980; Hart and Aronson 1982; Honjo et al. 1986; Clemens and Wood 1993). For example, in the MBH, available dates (11.0 ± 0.5, 11.0 ± 0.6 Ma) for rhyolites of the Mount Bennett group are consistent with the reversed magnetic polarity of most units. With reference to the geomagnetic polarity time-scale of Cande and Kent (1995), these units likely erupted during chron 5r (11.1–12 Ma) and so are slightly older than the indicated ages (albeit within the analytical errors). Rocks of the Danskin group and most eastern MBH rhyolites have normal polarity consistent with assignment to chron 5n (9.8–11.1 Ma); equivalent units are overlain in the eastern MBH by the City of Rocks rhyolite (9.15 ± 0.13 Ma; chron 4An). Thus, the entire MBH rhyolite section likely was emplaced between ca. 11.5–9.0 Ma.

Stratigraphic relations throughout the CSRP have been established in varying detail via local geologic investigations combined with geochronologic, paleomagnetic, petrographic, and geochemical information (Appendices 24). Comprehensive chemical and isotopic analyses for representative samples from the CSRP region are given in Table 2. Appendix 6 details our current understanding of the field and stratigraphic relations in widely separated localities upon which we develop a regional stratigraphic model. Although definitive correlations remain to be established in detail, this information provides a self-consistent regional framework for understanding magmatic evolution in the CSRP.
Table 2

Compositions of representative Cougar Point Tuff, Bruneau-Jarbidge, and Mount Bennett Hills rhyolites

Sample

I-569

I-841

I-463

X-37

I-459

I-407

I-445

I-411

I-1208

I-529

I-1001

I-693

L80-78

L80-85

L80-64

Unit

CP III

CP VII

CP XI

CP XIII

CP XV

Mary’s Ck

Ind. Batt

Brun. Jasp.

Sheep Ck

Dorsey Ck

Dorsey Ck

Horse Bu

Ticr

Tigsu

Tigsl

Age (Ma)

12.66 ± 0.02

11.81 ± 0.03

11.22 ± 0.07

10.82 ± 0.06

10.50

11

9.80

9.50 ± 0.06

9.30

8.10

8.10

8

9.15 ± 0.13

9.8

9.8

SiO2

76.70

73.29

75.25

75.46

73.85

72.52

70.13

74.14

70.58

72.45

73.88

69.89

70.26

72.51

72.81

TiO2

0.15

0.49

0.31

0.28

0.42

0.56

0.66

0.37

0.67

0.52

0.45

0.77

0.79

0.61

0.59

Al2O2

12.64

13.01

12.33

12.33

12.87

13.04

14.14

13.01

13.38

12.82

12.97

13.54

13.48

12.46

13.16

FeO*

1.30

2.99

2.15

2.17

2.76

3.61

4.14

2.50

4.88

3.83

2.92

4.68

4.19

4.21

3.44

MnO

0.02

0.05

0.02

0.04

0.05

0.05

0.06

0.04

0.08

0.07

0.03

0.07

0.07

0.07

0.05

MgO

0.36

0.42

0.19

0.23

0.50

0.35

0.67

0.27

0.44

0.39

0.34

0.63

0.84

0.34

0.04

CaO

0.64

1.35

0.83

0.81

1.40

1.28

2.44

1.05

2.18

1.65

1.31

2.44

2.35

1.64

1.83

Na2O

2.56

2.52

2.95

2.53

2.90

3.25

2.71

2.73

3.04

2.78

3.16

3.22

3.04

2.77

2.76

K2O

5.62

5.78

5.94

6.12

5.18

5.22

4.90

5.83

4.62

5.39

4.88

4.58

4.79

5.30

5.20

P2O5

0.02

0.09

0.03

0.04

0.06

0.11

0.14

0.05

0.12

0.09

0.06

0.17

0.17

0.08

0.12

XRF trace elements (ppm)

Ba

95

989

858

674

1155

1125

1192

1141

1138

1180

1137

1061

1130

1201

1235

Rb

313

220

239

220

188

194

172

201

151

173

165

151

153

166

160

Sr

16

87

40

33

81

123

138

68

133

107

99

129

134

103

120

Zr

218

483

430

456

465

499

589

431

579

561

501

555

501

647

507

Y

86

62

72

89

67

76

67

75

75

75

66

66

59

79

69

Nb

60

41

43

48

39

35

42

47

45

46

46

42

37

50

43

Pb

38

31

33

33

30

28

27

30

27

28

28

28

23

29

26

INAA trace elements (ppm)

Sc

1.7

5.7

3.4

3.2

4.4

6.8

7.6

4.3

9.2

6.6

6.5

9.1

8.1

6.9

7.4

Co

0.6

2.7

1.1

0.6

2.2

3.3

4.5

1.7

3.8

2.4

2.1

5.6

6.5

2.9

3.8

Cs

7.20

4.92

5.00

4.40

4.01

3.26

3.31

3.86

3.09

3.05

2.51

2.82

2.73

3.00

2.82

La

82.0

78.4

88.0

99.3

84.6

85.2

84.9

89.1

82.6

84.8

87.2

77.6

71.7

81.1

94.5

Ce

156

153

167

191

158

155

149

179

166

159

166

153

142

162.5

155

Nd

65.0

62.4

65.5

80.3

65.5

66.0

65.8

72.0

71.3

71.0

72.2

65.5

60.7

71.7

80.8

Sm

12.8

11.7

12.3

15.1

11.9

12.9

12.3

13.8

14.1

13.6

13.9

12.5

11.4

14.1

15.5

Eu

0.22

1.49

1.02

1.24

1.48

2.00

2.02

1.82

2.75

2.35

2.24

2.34

2.12

2.44

2.42

Tb

1.97

1.63

1.71

2.26

1.74

2.08

1.98

2.21

2.29

2.03

2.16

1.98

1.64

2.1

2.13

Tm

1.15

0.87

0.92

1.21

0.91

1.07

0.92

1.02

1.04

1.05

0.99

0.95

0.84

1.12

0.98

Yb

7.80

5.65

5.94

7.88

6.06

6.63

5.90

6.80

7.06

6.73

6.71

6.17

5.47

7.06

6.34

Lu

1.17

0.88

0.96

1.23

0.95

1.05

0.94

1.07

1.13

1.06

1.07

0.96

0.83

1.11

0.99

Hf

8.1

12.9

11.8

13.1

12.4

13.6

14.8

13.2

15.6

14.5

14.3

15.0

13.4

16.2

13.4

Ta

5.10

3.13

3.12

3.47

3.07

2.81

2.96

3.50

3.33

3.44

3.52

3.12

2.91

3.49

3.12

Th

44.5

32.2

35.0

29.4

32.3

29.4

28.7

32.6

25.9

26.4

28.9

24.9

25.1

26.3

25.6

U

12.3

8.5

9.1

9.2

8.1

6.2

6.9

8.4

6.7

7.2

7.0

6.4

6.5

6.88

6.6

B

15.0

9.6

10.0

12.0

11.5

5.3

7.5

12.0

9.4

8.0

6.3

6.7

7.8

8.0

87Sr/86Sr-I

0.70865

0.71031

0.70935

0.70938

0.70976

0.71044

0.71062

0.71150

0.71220

0.71194

0.71209

0.71215

0.71210

0.71293

143Nd/144Nd

0.512213

0.512209

0.512259

0.512264

0.512234

0.512266

0.512252

0.512307

0.512278

0.512316

0.512314

0.512314

0.512286

0.512317

0.512282

143Nd/144Nd-I

0.512193

0.512192

0.512245

0.512248

0.512219

0.512249

0.512238

0.512293

0.512264

0.512304

0.512302

0.512302

0.512273

0.512302

0.512267

eps Nd-I

−8.4

−8.4

−7.4

−7.3

−7.9

−7.3

−7.6

−6.5

−7.3

−6.3

−6.4

−6.4

−6.9

−6.3

−7.0

δ18O

3.8

0.2

2.6

1.1

2.1

3.4

2.2

1.5

1.5

3.8

Major element analyses by XRF (Univ. of Edinburgh), normalized to 100% anhydrous with all Fe as FeO*. Trace elements by XRF (Univ. of Edinburgh) or INAA (Open Univ.). Sr and Nd isotopic analyses by TIMS (Open Univ.) are reported relative to 87Sr/86Sr = 0.71025 for NBS-987 and 143Nd/144Nd = 0.51184 for Johnson-Matthey standards. Initial ratios (denoted by −I) are computed for the ages cited in the table, and epsilon Nd is computed for the initial Nd isotopic ratio and the CHUR value at time of eruption. O isotopic values are from Boroughs et al. (2005), except for sample L80-78 (measured at SMU). δ18O is reported in per mil relative to 9.6‰ for the NBS-968 standard; all data are analyses of feldspar. Nd isotopic analysis for I-463 is from equivalent unit in Cathey and Nash (2004). Sr and Nd isotopic data for I-445 are from M. McCurry on an equivalent sample, I-1143 (personal communication, 2004). Ages in italics are estimated from stratigraphy and dated units. Units are described in Appendix 2.

Endash indicates not measured.

Composition and time (CAT) groups

Provisional temporal relations have been established for most well characterized rhyolite units from the BJ, RTF, and MBH subareas of the CSRP for which there is both detailed stratigraphic control and fairly extensive analytical data for most units. To examine temporal trends in composition, we rely on unit averages for the major rhyolite units (Appendix 4) based on nearly 460 individual major and over 280 trace element analyses (Appendix 3).

Our first order observations are that (1) bulk compositions of rhyolite units erupted within relatively short time intervals define a narrow range despite sampling over a broad region, and (2) compositions vary more strongly as well as systematically with time in each subarea. Accordingly we define ‘composition and time’ (CAT) groups (Table 3) that include more than 95% of the recognized CSRP rhyolite units; most groups include both ignimbrite and lava flow units although the proportion of lavas increased with time. Individual CAT groups typically range from 0.2 to 0.5 Ma in duration, although the youngest intervals are longer and defined with less precision. Temporal variations in composition are readily illustrated by differences in TiO2, FeO, and SiO2 contents. Similar patterns are apparent for other major oxides and trace elements; these are discussed later in a petrologic context.
Table 3

Composition and time (CAT) groups, CSRP rhyolite units

CAT Group

Age Range (Ma)

%TiO2 range

Magnetic polarity

Grasmere Escarpment and Jacks Ck areas

Bruneau-Jarbidge and area to SW

Rogerson-Twin Falls and Cassia Mts

North margin of SRP (Mt. Bennett Hills, etc.)

Stage F

13

5.5–7.5

0.64–0.85

N

  

Shoshone Falls lf

Magic Reservoir lf, ign

Sand Springs ign

 

Stage E

12B

7.5–9.0

0.41–0.60

N

 

Dorsey Creek lf

Greys Landing ign

Lake Hills 4 ign

Juniper-Clover Area lf

McMullan Ck 2–4 ign

 

Three Creek ign

  

12A

7.5–9.0

0.68–0.85

N,R

  

Castleford Crossing ign

City of Rocks rhy

Balanced Rock lf

 

Horse Butte Area rhy

 

11

9.0–9.5

0.56–0.68

N

Horse Basin lf

Sheep Creek lf

Up Crows Nest lf

Lake Hills 1–3 ign

 

Poison Creek lf

  
 

Basalt flows

Basalt flows

Basalt flows

Stage D

10B

9.5–10.0

0.33–0.42

R

Perjue Canyon lf

Bruneau Jasper lf

Dry Gulch ign

King Hill Creek III ign

Tigert Springs lf

   

10A

9.5–10.0

0.56–0.83

N,R

 

Indian Batt lf

BBU 10–12 ign

Dempsey Meadows rhy

Triguero Homestead lf

Browns View ign

Gwin Springs ign

Long Draw lf

Wooden Shoe Bu ign

King Hill Creek II ign

  

Cold Springs VI rhy

9

10.0–10.4

0.36–0.59

N

 

Cedar Tree lf

House Creek ign

Thorn Creek rhy

 

BBU 9 ign

King Hill Creek I rhy

 

Rabbit Springs ign

Cold Springs rhy

 

Low Crows Nest ign

 

Stage C

8B

10.4–10.7

0.30–0.48

N

 

CPT XV ign

  

8A

10.4–10.7

0.39–0.67

N

  

BBU 8 ign

Rattlesnake/High Spr rhy

Jackpot Mem 7 ign

Knob ign

Steer Basin ign

 

7

10.7–11.0

0.22–0.38

N

 

CPT XIII ign

BBU 7 ign

Frenchman Spr/Dive Ck rhy

 

Jackpot Mems 1–6 ign

Henley rhy

 

Big Bluff ign

 

Stage B

6

11.0–11.2

0.46–0.57

N,R

Marys Creek lf

CPT XII ign

 

Bennett Mtn ign

 

Black Rock Escp lf

 

Fir Grove ign

5

11.2–11.5

0.26–0.43

R

Grasmere Escarp. ign

CPT XI ign

BBU 5–6 ign

Windy Gap rhy

   

Deer Springs ign

 

CPT X

Magpie Basin ign

Unnamed Rev Pol rhy

4

11.5–11.7

0.31–0.41

N

 

CPT IX ign

BBU 4 ign

Unnamed Nor Pol rhy

Stage A

3

11.7–11.9

0.38–0.59

R

Buckhorn ign

CPT VII ign

BBU 3 ign

Willow Creek rhy

Crab Creek ign

   

Halfway Gulch lf

   

O X Prong lf

   

Rattlesnake Ck rhy

   

2

11.9–12.4

0.23–0.32

N,R

Badlands-Poison Ck rhy

CPT V ign

BBU 2 ign

 

1

12.4–13.0

0.12–0.28

R

 

CPT III ign

BBU 1 ign

 

Rattlesnake Dr ign

  

Whiskey Draw ign

  

See text and Appendices 2A–C and 6 for unit descriptions and details of regional stratigraphy. Italicized units have been dated by K–Ar or Ar–Ar methods. Stages A–F correspond to areal distribution maps in Fig. 4.

ign, ignimbrite; lf, lava flow; rhy, rhyolite (flow or rheomorphic tuff) unit

Coherency of the CAT groups is most robust in the first phase of CSRP volcanic activity, prior to onset of regional rifting (ca. 9.5 Ma) that is marked by the earliest eruptions of basaltic lavas. During this early period the character of individual CAT groups oscillated between more or less felsic compositions, whereas later, both felsic and relatively mafic rhyolites are represented in certain time intervals. In such cases, these are designated as ‘A’ or ‘B’ CAT groups, respectively (Table 3), and the respective rocks generally occur in geographically distinct regions—likely signifying development of more diverse reservoirs or melt lenses within the crust. Paleomagnetic polarity is usually consistent for all units in a particular CAT group, although exceptions occur (e.g., CAT group 6).

Table 3 shows units assigned to each CAT group within the main geographic regions discussed in Appendix 6. Most CAT groups include units that are widely distributed across the CSRP; in some cases (e.g., CAT group 5) units of similar composition are found in all subareas. This is unsurprising for large ignimbrite outflow sheets, but is notable when both ignimbrite and lava flow units are included (e.g., CAT group 6). In the latter case we surmise that eruptive style or depositional facies varied laterally within a restricted time interval, but that the magma system being tapped was relatively homogeneous over wide distances or, if different magma reservoirs were tapped, they somehow evolved in a consistent fashion.

We emphasize that our definition of CAT groups is strictly empirical and does not imply that all units within any particular group are exactly correlative or precisely cogenetic. It is likely that some ignimbrites are the distal equivalents of others in the same CAT group, or that some successions of ignimbrites or lavas in the same CAT group may have been sequentially erupted from the same (or similar) magma batch(es). For example, CPT XI and the Grasmere escarpment ignimbrite (CAT group 5) occur in adjacent areas and probably represent deposits of the same eruptive event. On the other hand, the superposed CPT XII ignimbrite and Black Rock Escarpment lava flow (CAT group 6) have similar geochemical attributes and probably represent sequential eruptions from a common (or related) magma batch(es).

Because of incomplete exposure or preservation, or due to burial by younger rocks, the full extent of the large ignimbrites has not been established. Nor has detailed unit-by-unit stratigraphic correlation across the region yet been achieved. In the following discussion, we use CAT groups as stratigraphic proxies to overcome these limitations and to estimate broad-scale CSRP rhyolite volumes and temporal variations of magma generation and eruption rates. Using this approach we have developed a model for the evolution of CSRP volcanism and accompanying tectonism (detailed below) that allows us to compare the pattern of CSRP volcanism with that elsewhere in the SRP-Yellowstone hotspot track. This model also serves to guide future stratigraphic investigations that will hopefully lead to refinements in our understanding of SRP volcanic evolution.

Evolution of rhyolitic volcanism in the central Snake River Plain

Areal distribution of rhyolitic volcanism in the CSRP is portrayed as a function of time in Fig. 4. For simplicity, we divide the volcanic record into six temporal stages (denoted as A to F from oldest to youngest). Stages A to C constitute the CSRP ignimbrite flare-up and comprise the dominant volume of CSRP rhyolite output. Stage D spans a waning of the caldera-forming phase and transition to the rifting phase, and Stages E to F represent eastward migration and eventual waning of silicic magmatism in the area. The overall pattern shows that rhyolitic volcanism started in the western CSRP, expanded across the entire region, and then died toward the east. While the notion of simple time-transgressive migration of silicic magmatism for the SRP seems valid on a large scale, in detail magmato-tectonic activity in the CSRP was certainly more complex.
Fig. 4

Distribution of CSRP rhyolites over time. Stages a to f correspond to time intervals (in Ma) discussed in the text; Table 3 shows CAT groups assigned to each interval. Shaded areas indicate known extent of welded ignimbrites and rhyolite lavas erupted during each stage; distal airfall deposits are not included. Dashed lines denote probable source or vent areas for the respective time intervals based on distributions of rhyolite lavas and geologic inference. Geographic areas are indicated for reference as follows: Bruneau-Jarbidge (BJ), Jacks Creek (JC), Cassia Mountains (CM), Mount Bennett Hills (MBH), and Twin Falls (TF). Light gray shaded areas indicate highlands bounding the CSRP

During stage A (13.0–11.7 Ma, CAT groups 1–3, Fig. 4a), volcanism was concentrated in the western part of the region, and culminated with large eruptions that formed the voluminous CPT VII ignimbrite unit and its probable correlatives to the east (BBU-3), west (Buckhorn ignimbrite) and north (Willow Creek rhyolite). The combined volume of these units suggests that significant crustal subsidence occurred, probably resulting in formation of now-buried calderas.

During stage B (11.7–11.0 Ma, CAT groups 4–6, Fig. 4b) widespread rhyolite volcanism in the western and middle parts of the CSRP formed the voluminous CPT XI ignimbrite unit in the BJ region and its equivalents (the Grasmere escarpment ignimbrite and Windy Gap rhyolite) in the west and north. Stage B ended with the relatively mafic units of CAT group 6 and includes extrusion of the first documented rhyolite lavas in the BJ region (the Black Rock Escarpment and Marys Creek flows). The voluminous ignimbrite eruptions of CAT group 5 undoubtedly resulted in significant subsidence, and possibly caldera formation, in the western and middle part of the CSRP. The relatively mafic CAT group 6 rhyolites erupted at the end of stage B could represent continued evolution of the earlier magma system, following new recharge of the system by basaltic magmas. Stage B is coeval with eruption of genetically unrelated rhyolites along the Owyhee Front and southwestern margin of the WSRP graben (Bonnichsen et al. 2004).

During stage C (11.0–10.4 Ma, CAT groups 7, 8A, 8B, Fig. 4c) the zone of active volcanism spread eastward, producing enormous eruptions that formed the tuff of Big Bluff in the Cassia Mtns., and Jackpot ignimbrite members 1–6 (BBU-7 unit) in the Rogerson graben. The source region for these units likely is centered in the vicinity of Hollister and Rogerson, just west of the Cassia Mtns.—an area that is now buried under basalt flows. The CPT XIII ignimbrite may be slightly younger; although this unit was distributed farther west, its composition is similar to other magmas of CAT group 7, and we consider them to be related genetically. The compositionally similar Henley and Frenchman Springs rhyolites (WBM) also belong to CAT group 7. Because they were erupted respectively before and after faulting that separates the older Bennett Mountain and younger Danskin Mountain groups, their eruption brackets a time of caldera development and/or major subsidence in the northwestern part of the CSRP. Stage C closed with eruption of numerous ignimbrites from CAT groups 8A and 8B. All of these except CPT XV (CAT group 8B) are more mafic than the voluminous ignimbrites of CAT group 7, and they may represent a more mafic and hotter continuation of that magmatic phase. The timing of major ignimbrite eruptions during stage C suggests that the source calderas formed in a piecemeal fashion and ultimately resulted in major crustal subsidence.

During stage D (10.4–9.5 Ma, CAT groups 9, 10A, 10B, Fig. 4d) rhyolitic volcanism was broadly distributed across the CSRP. In the BJ region, volcanism switched from an explosive to an effusive mode, whereas a number of ignimbrites were erupted in the eastern part of the CSRP. Stage D coincides with a dramatic decrease in the overall eruption rate for silicic magmas. The earliest basalts intercalated within the rhyolite succession also appear at the end of stage D in the BJ (cf. Appendix 6); basalts of similar age also occur in the eastern MBH region (McHan basalt, Leeman 1982c), and as far east as the Malta Range (southeast of Twin Falls; Armstrong et al. 1975).

Stage E (9.5–7.5 Ma, CAT groups 11, 12A, 12B, Fig. 4e) is represented by rhyolitic volcanism in many areas of the CSRP—but mostly concentrated in the east. Both ignimbrites and rhyolite lava flows formed during this interval. Particularly notable are the very large and comparatively mafic rhyolite lavas erupted in the BJ region, even though the total volume of rhyolite erupted and the eruption rate were diminished considerably compared to earlier stages.

During the final phase of CSRP rhyolitic volcanism, stage F (7.5 to 5.5 Ma, CAT group 13; Fig. 4f), the focus of volcanic activity shifted entirely to the east, where comparatively small ignimbrite and lava flow units were erupted. Following stage F, rejuvenated silicic magmatism of the MREC is conspicuously anomalous with regard to the generally eastward-younging trend of the SRP-Yellowstone province silicic magmatism.

Huge rhyolite volume and catastrophic crustal foundering

The subsided interior of the CSRP region occupies about 14,000 km2. We believe it contains a fairly continuous rhyolite accumulation that is partly obscured by younger basalts and sediments. By analogy with Yellowstone, the subsided material likely includes thick deposits of intracaldera ignimbrites and lavas filling depressions that formed above partially emptied magma chambers. Nearly all of the rhyolite eruptions appear to have originated from sources in the subsided interior of the CSRP, whereas extracaldera ignimbrite outflow sheets and airfall tuffs dominate exposures in the flanking areas. The BRE, BB, WBM, and Cassia Mtns. sections expose ignimbrite successions on the order of at least 500 m in thickness. Thus, to estimate the minimum (i.e., ignoring intracaldera fill) rhyolite volume erupted from the 14,000 km2 subsided zone we have assumed an average thickness of 500 m. The cumulative volume of all CSRP units is estimated conservatively to be at least 7000 km3, most of which erupted in a little more than two million years (12.7–10.5 Ma) with recurrence intervals of roughly 200–300 ka. Inclusion of the volumes of dispersed airfall tuff deposits (that could constitute more than 30% of the mass of explosive eruptions; Mason et al. 2004), regionally correlative ignimbrites, and intracaldera fill, if known, would significantly increase estimates of the cumulative volume of material erupted from the BJ region, perhaps by 50 to 100%. From studies of tephras correlative with the CPT, Perkins and Nash (2002) estimate a cumulative eruptive volume on the order of 104 km3.

Considering the thicknesses and lateral continuity of the individual units in the various CAT groups, we have estimated the relative and the cumulative volume of rhyolite erupted through time (Fig. 5). Despite the incomplete exposure and continuity of units, this diagram provides a first order picture of CSRP rhyolite productivity. The more prolific early caldera-forming stage (12.7 to 10.5 Ma) was dominated by three major eruptive episodes (corresponding to CAT groups 3, 5, and 7) that comprise nearly three-fourths of the rhyolite erupted in the CSRP. Subsequently, eruptive volumes and rates diminished and recurrence intervals appear to have increased. These patterns reflect important changes in the energetics and processes underlying CSRP magmatism. We revisit this topic later, after reviewing compositional evolution of these rocks.
Fig. 5

Temporal variation in a relative and b cumulative volume of rhyolitic magma extruded in the CSRP; percentages are given because relative volumes are better constrained than absolute volumes. Intracaldera fill and distal airfall deposits (for which volumes are sizable, but unknown) are not included. Perkins and Nash (2002) suggest that total volume for the CPT and equivalent units may have exceeded 10,000 km3. Points plotted correspond to individual CAT groups; letters (A–F) correspond to developmental stages discussed in text. Eruption rates and volumes were clearly highest between 12–10 Ma, with notable spikes corresponding to the three most voluminous ignimbrite events (CPT units VII, XI, and XIII)

Finally, another important facet of CRSP magmatism concerns evidence for magmato-tectonic subsidence and crustal deformation. It is clear that most of the CSRP underwent significant subsidence in concert with, and possibly punctuated by, a series of episodic ignimbrite eruptions, as evidenced by major normal faulting cited earlier (e.g., at WBM, BB, and Rogerson graben areas). Of particular note are (1) the outbreak of compositionally distinct rhyolites in the WSRP graben essentially coeval with the period (11.7 to 11.0 Ma) of most intense activity of the CSRP magmatic system, and (2) eruption of the 10.2 Ma Arbon Valley tuff and related rhyolites in the ESRP (Kellogg et al. 1994) coinciding with major eruptions at the end of the CSRP ignimbrite flare-up (CAT group 9). Considering that CSRP activity postdates the initial manifestation of regional magmatism by several million years (and thus is unlikely to be associated with a plume head), the contemporary residence of multiple silicic magma bodies within the crust over horizontal distances of hundreds of kilometers is enigmatic in the context of the hotspot model. Assuming that crustal melting is driven by intrusion of basaltic magma, then regionally extensive basalt production is implied rather than localized, time-transgressive production. Also, the coincidence of regional rhyolite eruptions with major ignimbrite events in the CSRP suggests that all may be triggered by a common factor—perhaps, activation of a system of normal faults or triggered by major eruptive events. If so, this, too, seems inconsistent with a simple eastward progression of Basin and Range style extension at rates comparable to passage of North America over the Yellowstone hotspot, as proposed by Rodgers et al. (1990). Prevailing N- to NW-trending orientations of the principal syn-volcanic normal faults in this region suggest that the crust was extended primarily in a SW–NE direction—essentially parallel to the axis of the hotspot track. We propose that this deformation style reflects a combination of regional extension and crustal weakening due to injections of enormous volumes of mantle-derived basaltic magma and large-scale partial melting of crustal rocks. Widespread rhyolite outbreaks could have been triggered by catastrophic foundering of the weakened crust (cf. Elkins-Tanton and Hager 2000). Further support for this type of scenario is provided by petrologic considerations.

Compositional evolution of CSRP silicic volcanism

Here we review temporal variations in compositional and petrologic evolution of CSRP silicic magmatism in the context of our inferred regional stratigraphy. Discussion is focused mainly on the BJ and RTF areas, where stratigraphic relations and timing are best understood, and on the MBH where partly equivalent units have been studied in some detail. Ages are based on available radiometric dates combined with conservative interpolations for intervening units as constrained by stratigraphic correlations between sections. Although sequences of eruptive units from these areas are considered to be representative, we recognize that stratigraphic sections may be incomplete due to limited exposure or restricted areal distribution of some units. Also, particularly for long-traveled ignimbrites, local stratigraphic sections could include units derived from multiple sources having different petrogenetic histories. Despite these limitations, such sections provide useful insights into large-scale magma reservoir processes as well as constraints on petrologic scenarios for magma production.

Data description

Compositional data used in this paper are based mostly on XRF (some 355 major element and 175 selected trace element) analyses of multiple samples from the described BJ and RTF units; all but the lowermost RTF units and one BJ unit are represented by at least three major element analyses. Data for MBH rhyolite units are based on combined XRF (n = 39) and inductively coupled plasma optical emission spectroscopy (ICP-OES) analyses (n = 65). Precision and accuracy of these data are comparable based on replicate analyses of standards and multiple samples from common units (Appendices 3, 4). Analyzed samples are mostly whole-rock vitrophyres representing rapidly quenched magmatic material; such samples were obtained from basal parts of ignimbrites and from basal and upper parts of lavas. Where vitrophyres were not preserved, devitrified interior samples were analyzed; in general, compositions of the two types of samples are similar for any given unit. Averages and standard deviations for all well-studied stratigraphic units are given in Appendix 4, with major elements normalized to 100% on an anhydrous basis and all iron reported as FeO*; with exception of a few probably altered samples, all available data are included in these averages. These averages are used in compositional diagrams unless otherwise noted. Statistics presented in Appendix 4A demonstrate that overall compositional variability (RSD, or coefficient of variation) within individual lava flow units is generally comparable to analytical precision; data for ignimbrites tend to have RSDs about twice analytical precision on average for most reported elements. Compared to literature data for many ignimbrites (e.g., Hildreth 1979, 1981), CSRP units are relatively homogeneous (see below). However, welded basal airfall ash occasionally is distinctive from the overlying bulk ignimbrite (e.g., CPT III, CPT V, CPT XV); such analyses are excluded from the unit averages and treated separately. For a representative subset of BJ samples, the XRF data are complemented by more comprehensive trace element analyses using combined instrumental neutron activation and ICP-OES methods (Table 2). Sr and Nd isotopic analyses were obtained on these samples using thermal ionization mass spectrometry; these data are corrected for isotopic fractionation and reported relative to 87Sr/86Sr = 0.71025 for NBS-987 and 143Nd/144Nd = 0.51184 for Johnson–Matthey Nd and 0.51264 for BCR-1 standards. Published Nd isotopic data for CPT tuffs (Cathey and Nash 2004) were normalized to the same standard values to enable direct comparison with our results. Initial ratios are reported based on measured or inferred ages of each sample. Available δ18O data for feldspars or quartz in BJ area rhyolites show them to be variably depleted in 18O compared to most rhyolites worldwide (Boroughs et al. 2005); representative δ18O values from their work and one new datum for a MBH rhyolite are listed in Table 2.

Electron microprobe analyses of phenocryst minerals of representative CSRP rhyolites have been used to calculate two-feldspar, two-pyroxene, and Fe–Ti oxide mineral temperatures based on analyses of contiguous coexisting phases that are most likely to have attained equilibrium (Appendix 5; data references given therein). The two-pyroxene thermometers tend to produce the highest and most consistent temperature estimates (cf. Honjo et al. 1992); these were used to ‘re-tune’ the Zr-saturation thermometry of Watson and Harrison (1983), which in turn was applied to infer magmatic temperatures using average compositions for all sampled eruptive units. These values correlate well (not shown) with total FeO* in the rhyolites. Thus, FeO* is a useful and simple proxy for both relative magmatic temperature and degree of evolution.

Overview of compositional variations

Reliance on whole-rock samples to represent magmatic compositions for silicic ignimbrites can be problematic for several well-documented reasons (cf. Hildreth 1979, 1981; Streck and Grunder 1997, 2007) including: presence of initial compositional variations (e.g., tapping of compositionally zoned magma systems), heterogeneities related to eruption and deposition (e.g., crystal winnowing or concentration, incorporation of lithic clast impurities, etc.), or post-eruptive alteration (e.g., alkali exchange with meteoric fluids). Ideally, vitric pumice clasts are considered to be most representative of magmatic samples, but such clasts are rarely found in SRP ignimbrites–presumably due to high eruptive temperatures and extensive welding in most units. Alternatively, glass matrix from flows or shards from airfall deposits can be used to estimate melt composition. Such data suggest that several CSRP ignimbrites exhibit weak compositional zoning from base to top (though not in any consistent sense), and that glasses typically are slightly more evolved (e.g., higher SiO2, lower FeO*) than equivalent bulk-rock samples (Honjo 1990; Cathey and Nash 2004). In detail, some units carry multiple compositionally distinct populations (or ‘modes’) of glass shards–signifying (syn-eruptive?) extraction and incomplete mingling of magmas from multiple distinct or zoned reservoirs at depth (Cathey and Nash 2004; Andrews et al. 2007). The complexity of CSRP ignimbrites also is recorded by the presence of multiple ‘modes’ of phenocryst mineral populations, occurrence of xenoliths and/or xenocrysts derived from magmas as mafic as basalt, diverse glomerocrysts presumably derived from silicic magmas at differing stages of evolution, and rare lithic clasts of diverse crustal origins (Bonnichsen and Citron 1982; Honjo et al. 1992; Wright et al. 2002; Cathey and Nash 2004). Although such complexities must ultimately be factored into interpretation of the petrogenesis of CSRP rhyolites, this task largely awaits more detailed studies.

In the interim, we have evaluated the gross compositional features of these rhyolites based on their bulk rock chemistry. Our most fundamental observations concern (1) the relatively small compositional diversity of most CSRP rhyolites, (2) their persistent metaluminous and crystal-poor (typically <10%) character, and (3) their similar mineralogy (and particularly the absence of hydrous phenocryst phases in virtually all units). Rarely, variations may result from stratigraphic uncertainties. For example, Cathey and Nash (2004) attribute significant compositional differences between samples nominally of CPT XV collected from two localities (Bruneau vs Jarbidge River drainages) to sampling of distinct ignimbrite flow units. As illustrated by our unit averages (often representing widely separated samples, and in some cases both lower and upper vitrophyres), variation for most major and many trace (e.g., Rb, Sr, Ba, Zr, Y, Nb) elements is comparable to analytical precision for the rhyolite lavas, and only slightly greater than analytical precision for most CPT ignimbrites (notable exceptions being CPT XV and CPT III). Regardless of cause, internal variations within most individual emplacement units (including correlative samples from widely separated localities) are relatively small despite the aforementioned evidence for compositional heterogeneities in their source regions. For example, average SiO2 for all CPT ignimbrite units ranges from 73–77%, but the maximum standard deviation for any given unit is ±0.8% (1σ, absolute); comparable values for BJ rhyolite lavas are 70–74% and ±0.7%. The largest intra-unit major element variations are observed for FeO* as follows: 1.1–3.1% (±0.36%; 1σ, absolute) for tuffs and 2.5–4.6 (±0.34%) for lavas (Appendix 4). Direct comparison with electron microprobe data for CPT glasses (Honjo 1990; Cathey and Nash 2004) shows the latter to have lower FeO* than our averages for most CPT ignimbrites by as much as 1% absolute (Fig. 6). Although this probably reflects the more evolved nature of the crystal-free glass, analytical or sampling biases or alteration may also contribute to the apparent differences. Because our data average a greater number of samples, and include both glass as well as entrained crystals, we consider them to be representative of significant volumes of CPT magmas. More importantly, because we observe limited compositional diversity within most multiply-sampled units compared to observed diversity between units, the averages provide useful snapshots of the overall magma system at various times.
Fig. 6

Comparison of bulk rock (shaded field; this study, AVG ± SD from Appendix 4) and glass (Cathey and Nash 2004) compositions for CPT ignimbrites. The latter represent average shard analyses from basal airfall ash (X), and basal (filled circles) and upper (open circles) vitrophyres. Cathey and Nash (2004) note that some scatter in glass compositions reflects sampling of different flow units (e.g., XVb vs. XVj) and, in some cases, effects of alteration (e.g., low FeO* points in parentheses). In general, bulk rock data show less variation than glass data. Analytical error is typically within symbol size

Temporal compositional trends

We next take a closer look at compositional variations recorded in our stratigraphic sections, relying on those elements that are least sensitive to post-eruptive alteration. Temporal trends in SiO2 and TiO2 for BJ, MBH, and RTF area rhyolites are compared in Fig. 7. Coherency between these trends for widely separated sections (BJ, BB, MBH) strongly supports our proposed stratigraphic correlations for the major units. When compared in detail, occasional anomalies observed may be explained by non-uniform distribution or preservation of individual outflow sheets and (especially for MBH) uncertainties in age control. Small offsets in compositions of correlated or time-equivalent ignimbrite units also could reflect heterogeneities or zoning within individual or related outflow sheets. The large-scale temporal trends suggest that at specific times eruptive products were tapping either a common large and evolving magma reservoir, or multiple smaller magma sources that were following similar evolutionary paths. The later rhyolite lava flows are particularly important in this regard because approximately coeval lavas from widely separated (up to 50–100 km) sources are similar in composition. Because most lavas likely traveled no more than tens of kilometers from their sources, it is unlikely that widely separated lavas were fed from common reservoirs.
Fig. 7

Major element temporal variations in a BJ and MBH, and b RTF rhyolite units. Data plotted are unit averages (Appendix 4). Note that few analyses are available for the lowermost RTF units, and at least two units may be affected by post-eruptive silicification (indicated by question mark). Error bars represent standard deviation on unit averages (omitted on MBH symbols for clarity) and conservative age uncertainties. Data from BJ and RTF sections are connected by lines to highlight temporal variations; tuffs (solid and half-filled symbols, solid line) and lavas (open symbols, dashed line) are distinguished. Lower unit of the Tuff of McMullen Creek (Tmc), from the Twin Falls eruptive center, falls off the BJ-RTF trends. To facilitate comparison, shaded trends are shown in ‘A’ panels corresponding to variations for RTF samples

A second key point is that, between 12.7 and 9 Ma, the magma system as a whole tended to become more mafic (or less evolved) over time, with compositions gradually varying from relatively high- to low-SiO2 rhyolite. This evolutionary trend is clearly defined by increasing TiO2 with time. TiO2 is positively correlated with FeO*, MgO, CaO, Zr, Sr, and Ba, and inversely correlated with SiO2, K2O, and Rb. Superimposed small-scale fluctuations within the general progression suggest variations in magma chamber or magma supply dynamics. For example, following the large CPT VII and XI eruptions and again near 10.5 Ma, the system followed small excursions back toward higher-SiO2 rhyolite. Between 9 and 8 Ma (not precisely dated), we note a marked shift to relatively evolved rhyolite; rhyolites younger than 8 Ma (and mainly restricted to the RTF area) resume a trend toward less evolved compositions similar to that seen earlier. This part of the record could correspond to a waning phase in the CSRP system or, alternatively, the younger RTF units may sample a different magma system having a distinct evolutionary history.

Because the locally derived (i.e., near source) BJ rhyolite lavas define continuations of the CPT trends, we are reasonably confident that this record corresponds to evolution of the CSRP magma system (sensu latu), and not incursions of distal units from unrelated eruptive centers. One such exception may be the Tuff of McMullen Creek (Tmc). Wright et al. (2002) propose that Tmc rhyolites were derived from a distinct ‘Twin Falls’ eruptive center, and are unrelated to those from the BJ center. Major element, Rb, Sr, and Zr, and various element and isotopic ratios for the lower member of this unit are indeed distinct from the BJ/RTF trends (Figs. 7, 8 and 9)—thus supporting their interpretation. On the other hand, compositions of the upper Tmc units are not so distinctive that fundamentally different sources are required. If the Tmc is sourced from a distinct Twin Falls eruptive center (Fig. 1) as proposed then, based on similar compositions of upper Tmc and essentially coeval late BJ rhyolite lavas, it appears that coherent evolution in the rhyolite sources may extend for lateral length scales on the order of at least 100 km.
Fig. 8

Temporal variation of trace element compositions in BJ, RTF, and MBH rhyolites, as defined by unit averages (Appendix 4). Data are superimposed to facilitate comparison; broad shaded lines approximate the ‘main trend’ variations referred to in text. Despite small offsets in some cases, trends and major inflections are similar for all three widely separated areas. Error bars show typical ranges for each unit. Composition of lower Tmc unit clearly diverges from patterns defined by rhyolites from the main trend

Fig. 9

Temporal variation of selected element ratios in rhyolites of the CSRP (again, note anomalous lower Tmc unit). Significant fluctuations in composition with respect to the main trend (broad shaded curves) signify short-term variation in the magmatic system due to competing magmatic processes. Vectors show predicted effects of fractional crystallization (FC), fractional melting (FM), and recharge by less evolved magmas (MR)

Considering the heterogeneity in mineralogy, shard chemistry, etc. observed at the hand sample scale (Cathey and Nash 2004), it seems unlikely that these magmas tapped an evolving well-mixed single reservoir. An alternative scenario is that the magma system consisted of a plexus of melt lenses distributed over a wide region in the crust, each of which evolved in parallel over time—producing an array of similar though not identical compositions at any given time. Eruptions apparently triggered extraction and intimate mingling of magmas from multiple lenses to simultaneously produce broadly similar eruptive products characterized by small-scale heterogeneities. The physical processes by which this occurred are unclear and deserve further scrutiny.

The similarity of long-term compositional changes in eruptive products over a regional length-scale appears to record significant evolutionary patterns in the CSRP ‘magma system’ proper. Such trends are reasonably attributed to competing effects of recharge by less evolved magma vs. differentiation within the magma system. For example, coherent upsection increases in TiO2 and decreases in SiO2 (e.g. between 12.7–9.0 Ma) are inconsistent with progressive fractional crystallization (FC) of a simple evolving magma reservoir, but could reflect either recharge of less evolved magmas into the system or fractional melting of progressively more refractory source volumes over nearly 4 m.y. Such trends are commonly observed within zoned ignimbrites (e.g., Bishop Tuff) that, over the geologically instantaneous time scale of eruptions, presumably tap deeper, less evolved magma from compositionally stratified reservoirs (e.g., Hildreth 1979, 2004). However, this explanation seems unlikely for the long-term CSRP trends because (1) the time scales involved are much greater than expected residence times (perhaps on the order of 105 years) for most silicic magma bodies (cf. Vazquez and Reid 2002), and (2) the overall variations are significantly larger than those within individual CSRP eruptive units. Notable reversals in the long-term trend (e.g., following the large CPT VII or CPT XI eruptions, or recorded by rhyolite lavas erupted between 9–8 Ma) are consistent with crystallization of magma reservoirs over long repose intervals (ca. 250 ka) between ignimbrite pulses. Such reversals could record episodes of cooling owing to waning rates of basalt influx into the crust.

The major element temporal patterns are mimicked by trace element variations (Fig. 8). Ba, Sr, Zr, and Rb contents define coherent fluctuations with time for the three areas. Notably, MBH samples appear to be slightly enriched in Ba (as reflected in both ICP and XRF analyses), yet concentrations of other elements are comparable to those in BJ/RTF rhyolites of similar age; we tentatively suggest that this could reflect small lateral variations in either source compositions or reservoir processes. With decreasing age, BJ area rhyolites become progressively depleted in Rb (280 to 150 ppm), and enriched in FeO* (1.1 to 4.7%), Ba (<250 to 1250 ppm), Sr (<20 to 140 ppm), Zr (300 to 610 ppm); 87Sr/86Sr and 143Nd/144Nd ratios also increase (see below). On a short time-scale all these parameters fluctuate noticeably—particularly during the early history of the system—and it is not always possible to correlate the variations in detail across the entire region. However, significant trend reversals are common to all sections within the precision of our stratigraphic control. Between ca. 12.7–9 Ma, Ba/Y (<10 to 24), Ba/Nb (<10 to 30), Zr/Nb (<8 to 13.5), Ba/Rb (<1 to 7.5), and K/Rb (<200 to 290) ratios exhibit general upsection increases, whereas K/Ba (ca. 250 to 30), Nb/Y (1.0 to 0.6), K/Sr (ca 3000 to 250)*, Rb/Sr (ca. 18 to 1)*, Pb/Nb (0.69 to 0.47)*, Rb/Y (4.5 to 2.2)*, and Th/Nb (0.75 to 0.45)* generally decrease, with significant trend reversals or oscillations at particular time horizons (Fig. 9 shows representative examples of these trends). In general, all of these patterns are consistent with the existence of a dynamic magma system that became progressively less evolved with time. Trends for element ratios marked above by (an asterisk) show small reversals consistent with a transient increase in the proportion of evolved magma between 9 and 7 Ma; such excursions are consistent with evolution of stored magma during lulls in the recharge sequence (although other explanations may be viable). Overall, these compositional trends suggest that the magma system (sensu latu) was open to short-term communication between differently evolving silicic magma reservoirs (more vs less evolved), and/or recharge by mantle-derived basaltic magma (Hildreth et al. 1991).

Calculated initial Sr and Nd isotopic ratios for BJ rhyolites (Fig. 10) display sizable ranges (87Sr/86Sr = 0.7086–0.7124; 143Nd/144Nd = 0.51220–0.51234), and both generally increase with time albeit with small fluctuations. These data record progressively greater incorporation of both radiogenic Sr and Nd as the rhyolites become more mafic. Note that Tmc rhyolites are distinct with regard to 87Sr/86Sr, although they have similar Nd compositions (Wright et al. 2002). Notably, rhyolite 143Nd/144Nd evolves toward values seen in SRP basaltic lavas (Menzies et al. 1984; Lum et al. 1989; Leeman et al. 1992a), whereas 87Sr/86Sr diverges from the basaltic range with time. Additional reservoirs that could be involved in evolution of SRP rhyolites include Archean basement, as exposed adjacent to the NE SRP (e.g., Wooden and Mueller 1988) or inferred from xenoliths to underlie much of the province (Leeman et al. 1985), Proterozoic basement that may project beneath parts of the NW SRP (Fleck and Wooden 1997), and plutonic (granodiorite–granite) rocks of the Cretaceous Idaho batholith and related Eocene intrusions (Clarke et al. 1989; Fleck and Criss 1985; Criss and Fleck 1987). Most of the ancient crustal rocks are characterized by low 143Nd/144Nd (<0.511) and varied 87Sr/86Sr (as low as 0.702 to greater than 0.95), whereas the Cretaceous/Eocene granitoids are more similar to SRP basalts in having less extreme Sr (0.707–0.715) and Nd (0.5117–0.5124) ratios. Collectively, these data suggest relatively little contribution from Archean crustal rocks to CSRP rhyolite magmas—particularly considering that the rhyolites evolve away from such material with time. It is more likely that they represent melts of Cretaceous/Eocene basement rocks, although contributions from SRP basaltic magmas or melts of their intrusive equivalents are possible.
Fig. 10

Temporal variation of initial Sr and Nd isotopic ratios for BJ rhyolites, calculated for time of eruption. Data are from this study (Table 2), Wright et al. (2002), Cathey and Nash (2004), and Nash et al. (2006). Arrows emphasize general trends. For comparison, isotopic ranges are shown for Miocene-Recent basalts from the north CSRP (Leeman, unpublished data)

Petrologic implications

Possible origins of voluminous early rhyolites

Two end member processes have been considered for origins of SRP rhyolites: crustal anatexis vs differentiation of basaltic magmas (Leeman 1982b; Christiansen and McCurry 2007). A first order observation bearing on this matter concerns the strong bimodality of SRP magmatism. As seen in Fig. 11, essentially all of the silicic units studied in this paper, as well as most other published SRP and Yellowstone analyses (Leeman 1982b; Morgan et al. 1984; Hildreth et al. 1984, 1991; Hughes and McCurry 2002), plot in the BJ rhyolite field, whereas most SRP basalts cluster near the lower SiO2 end of the labeled basalt field. These rocks collectively account for the majority of erupted magmas in the province. Intermediate composition lavas occur in a few places, such as Craters of the Moon lava field (Leeman et al. 1976), but in nearly all cases these are volumetrically minor and represent extreme differentiates of basaltic magmas. Only near Cedar Butte (ESRP) is there a compositional continuity to rhyolitic compositions. McCurry et al. (2007) consider the latter occurrence as a possible example of derivation of rhyolite from basaltic parental magma. However, intermediate-composition rocks could also form by mixing between rhyolite and evolved basalt, or by partial fusion of basaltic intrusions. It is also worth noting that a number of SRP olivine tholeiites contain high SiO2 interstitial glass (e.g., Leeman and Vitaliano 1976; Leeman unpub. data); however, such glasses typically are richer in Fe, Mg, Ti, and Mn than any SRP rhyolite, and represent less than 1% of the rock. In our view, such differences and the sparcity of intermediate composition rocks make it improbable that the voluminous early SRP rhyolites form primarily by differentiation of basaltic liquids. Rather, we consider it more plausible that influx of basaltic magma causes widespread crustal anatexis to produce the rhyolitic magmas (Huppert and Sparks 1988; Annen and Sparks 2002). Below, we consider rhyolite-forming processes in the context of data for the BJ eruptive center.
Fig. 11

Rock classification diagram showing bimodal character of dominant basalt-rhyolite volcanism in the SRP. Data shown are averages for the BJ rhyolite units (lavas and ignimbrites) and representative olivine tholeiitic basalts from the north CSRP. Intermediate composition lavas [e.g., Craters of the Moon (COM); Leeman et al. 1976] are volumetrically minor; they represent extreme differentiates and are unlikely to lead to significant production of rhyolite from primitive basaltic magmas. Compositions of Idaho batholith plutonic rocks and crustal xenoliths from SRP lavas are indicated for comparison; these compositions overlap extensively so are not subdivided (Leeman, unpublished compilation)

Implied sources of rhyolite magmas

Based on detailed analyses of representative BJ rhyolites (Table 3), it is clear that their overall compositions are very similar to estimates of average crust (Taylor and McLennan 1985). Crust-normalized element profiles are nearly horizontal with exception of elements (e.g., Ba, Sr, Eu, transition metals) that are easily fractionated by the common phenocryst phases (Fig. 12), or B for which source abundances may be lower than in the average crust estimate used (cf. Leeman et al. 1992b). Essentially, partial melting of typical crustal rocks appears to be a viable process for rhyolite formation. Sr and Nd isotopic data preclude significant involvement of Archean basement, but could be reconciled with calcalkalic basement rocks similar to the Idaho batholith, and such a source has been proposed to account for low δ18O in the BJ rhyolites (Boroughs et al. 2005). Furthermore, experimental studies indicate that metaluminous rhyolite liquids can be produced by melting of calcalkalic plutonic rocks at upper crustal depths (Creaser et al. 1991; Patino-Douce 1997; cf. Cathey and Nash 2004). Contributions of melt from partial fusion of local basaltic rocks (Streck and Grunder 1999; Annen and Sparks 2002) appear limited to a subordinate role by the Sr and O isotopic data. However, at Yellowstone low δ18O in certain late cycle rhyolites has been attributed to extensive remelting of earlier cycle rhyolites that had undergone 18O-depletion as a result of hydrothermal alteration following catastrophic caldera collapse (Hildreth et al. 1984, 1991; Bindeman and Valley 2001a,b). If this scenario applies to the CSRP silicic centers (e.g., given that all BJ units are 18O-depleted as far as we know), alteration on a much larger (i.e., ∼crustal) scale seems required.
Fig. 12

Compositions of CPT tuffs and BJ rhyolite lavas normalized to average crust composition of Taylor and McLennan (1985). Maximum melt/crust enrichment factors (for U and Th; level 1) range between 7–10 for most CPT samples. For simple batch melting of ‘average crust’ these values correspond to minimum melt fractions between 10–15% (see text). Most other elements shown have enrichment factors between 4–5 (level 2), consistent with either higher degrees of melting (20–25%) or systematically higher bulk distribution coefficients. Because CPT III sample (I-569) appears to have been more strongly affected by crystal fractionation (note low Ba, Zr, and larger enrichments of incompatible elements), estimates of melt fraction (∼5–10%) based on this sample are unrealistically low. See Table 2 for data and unit assignments

First-order constraints on degree of melting

To model the physical process of CSRP silicic melt production, an estimate of degree of melting is required. Assuming ‘average crust’ (Taylor and McLennan 1985) to be a valid source, the extent of melting can be constrained from the maximum rhyolite/crust enrichment factors. These are highest for U and Th (∼9–10 for most CPT samples and 7–8 for most BJ lavas; Fig. 12). If bulk mineral/melt partition coefficients approach zero for these elements (cf. Bea 1996), simple batch melting can produce the observed enrichments with 10–15% melting. The fact that enrichment factors for the younger lavas are lower than those for the earlier ignimbrites suggests that degree of melting could have increased with time. Also, these are likely minimal estimates because (1) enrichment factors could be enhanced by FC processes, and (2) distribution coefficients could be enhanced by the presence of accessory minerals (Bea 1996) or, for elements like Sr, Eu, and Ba, affinities for feldspar minerals. For example, enrichment factors (ca. 4–5) for the other incompatible elements can be explained by the same model if bulk distribution coefficients for those elements are slightly higher (∼0.10–0.15). For any given model, assuming a less fertile source (e.g., ‘average lower crust’) will require lower melt fraction, and vice versa. Further consideration of magmatic differentiation effects and more complex melting models will be presented elsewhere.

Temperature and composition relations

The absence of hydrous phenocrysts in most SRP rhyolites and available mineral thermometry (Appendix 5) indicate that SRP rhyolites are unusually hot and water-undersaturated magmas. Minimum temperatures exceeding 950°C are indicated by pyroxene thermometry in many CSRP samples (Honjo et al. 1992), and Zr-saturation thermometry suggests that most CSRP rhyolites had magmatic temperatures above 850–900°C. The general enrichment of FeO* with time fundamentally signifies that progressively more mafic and hotter rhyolites were produced by fractional melting of the crust. This scenario would be favored by repeated injection of basaltic magmas into zones of crustal melting, in which case successive melts would be more depleted in incompatible elements and enriched in compatible ones—consistent with the overall temporal variations defined by CSRP rhyolites. The observed small oscillations on these trends signify that, in reality, melt production is more complex and may involve other factors, including migration of melt generation zones within the crust, mingling of magmas produced over large crustal domains, heterogeneities in the crust, etc.

It is also instructive to compare CSRP rhyolites with those from other centers associated with the Yellowstone hotspot (see references in regional overview). Here we compare each center in terms of temporal variation in FeO* (Fig. 13) that, as noted earlier, correlates with inferred magmatic temperature and is an inverse indicator of magmatic evolution. In this diagram, rhyolites of the CSRP are unique in showing progressively increasing FeO* over time. All other centers show generally decreasing FeO* with time, as expected for systems that evolve via FC processes. In other words, the behavior observed at most centers appears consistent with dominant control by progressive cooling of discrete magma bodies, or, in the case of the intensely studied Yellowstone system, repeatedly emplaced discrete magma batches (Hildreth et al. 1991; Bindeman and Valley 2001a, b). Long-term compositional variations of CSRP rhyolites (and particularly those of the BJ center proper) are inconsistent with such scenarios, and control by additional processes—such as fractional melting of the crust–seems required (cf. Creaser et al. 1991).
Fig. 13

Temporal variation in chemistry of CSRP rhyolites (14–3 Ma). Included are data for CPT (our averages), Owyhee front (OF), Mount Bennett Hills (MBH), Rogerson/Twin Falls (RTF; Tmc = McMullen Ck.) areas, and Magic Reservoir eruptive center (MREC: Tmr/yt, Tyd). Comparative data are shown for younger rhyolites from Yellowstone (YP) and older rhyolites from the Juniper Mtn. (JM) and McDermitt (MD) eruptive centers. Regression lines through data from all eruptive centers, except the BJ/TF/MBH group, have negative slopes for FeO* and positive slopes for SiO2 vs age—both consistent with magmas becoming more evolved with time. See text for data references

A physical model for SRP silicic magmatism

Scale of melt production and tectonic implications

Assuming progressive crustal melting to be an appropriate general model, and that rhyolite melt production is proportional to available heat input into the crust, we next consider the volume of basalt input into the crust that is needed to produce the inferred amount of rhyolite produced. Table 4 presents three bracketing scenarios as follows: (1) lower limit case based on documented effusive volumes of rhyolite from the Yellowstone eruptive center (Hildreth et al. 1991; Christiansen 2001), (2) CSRP case using the volume estimate of Perkins and Nash (2002), and (3) an upper limit (but likely) case assuming a ratio of 1:2 between extruded and intruded magma (cf. Annen and Sparks 2002). Because melt volumes are uncertain, and the model simple, the calculations discussed here are intended only to provide ‘order of magnitude’ estimates for the scale of melt production. We consider the simplest of scenarios in which the heat to warm crustal rocks to their ‘dry’ solidus (i.e., from ∼700° to ∼1000°C) and to produce a unit mass of rhyolite magma (Cp•ΔT and ΔHfus of crust) is provided by latent heat of crystallization of intruded basaltic magma (ΔHxtlzn). Using appropriate enthalpies and heat capacity (cf. Bohrson and Spera 2003), the mass of basalt required is roughly 2.5 times that of the rhyolite produced (or a volumetric ratio near 2). Note that this is a minimal estimate; with different assumptions, the basalt:rhyolite mass ratio could be several times higher.
Table 4

Scenarios for estimating magma supply in the Y-SRP province

Silicic magma production

YP-extrusive rhyolite (scenario 1)

CSRP-minimum (scenario 2)

CSRP-likely (I:E=2) (scenario  3)

Duration of eruptive center (Ma)

2

2

4

Estimated volume of rhyolite magma (km3)

6,000

10,000

30,000

Average rhyolite supply rate (km3/year)

0

0.01

0.01

Source volume assuming 15% partial melting (km3)

40,000

66,667

200,000

Thickness (km) of melt zone (radius = 50 km)

5.1

8.5

25.5

Basaltic power source

Volume of basalt (km3) needed to heat crust & melt rhyolite source (assuming minimal basalt/rhyolite volume ratio = 2)a

12,000

20,000

60,000

Average (minimal) basalt supply rate (km3/year)b

0.01

0.01

0.02

Thickness (km) intruded (radius = 50 km)

1.53

2.55

7.64

Rate of layer thickening (mm/year)

0.76

1.27

1.91

YP, Yellowstone Plateau; CSRP, central Snake River Plain; Ext, extruded magma; I:E, intrusive/extrusive ratio

aΔH required = Cp*ΔT (heating wall rocks) + ΔH (fusion); thermodynamic values are from Bohrson and Spera (2003)

bcomparative rates are ∼0.2 km3/year for Kilauea and 0.017 km3/year for the entire Hawaiian Ridge (Crisp 1984; Robinson and Eakins 2006)

If total rhyolite production in the CSRP is taken as 104 km3 (over 4 m.y.), the corresponding volume of basalt required is at least twice that amount. Taking the Yellowstone Plateau as an analog for horizontal scale, the underlying magmatic system could have a diameter up to about 100 km, which may be reasonable for the BJ center. This scale is also consistent with lateral extent of an inferred mafic sill complex beneath the ESRP (Rodgers et al. 2002). For basalt intrusion into a cylindrical domain of such diameter, and given the parameters in scenario [2], an equivalent thickness of about 2.6 km of basalt must be injected into the crust. The time-integrated magma influx rate is at least 0.01 km3/year—i.e., similar to the integrated rate (0.017 km3/year) estimated for Hawaiian magmatism (Crisp 1984; Robinson and Eakins 2006).

These estimates will increase to the extent that a significant fraction of the rhyolitic magma is intruded or that basaltic intrusion is confined to a smaller areal footprint. For example, assuming a conservative intrusive:extrusive ratio of 2:1 (cf. Annen and Sparks 2002; our scenario [3]), the total rhyolite volume would be about 3*104 km3, and the corresponding lens of basalt would be about 7.6 km thick. This calculation is likely to be an underestimate considering that the inferred mafic lens beneath the ESRP is several kilometers thicker (cf. Peng and Humphreys 1998; Rodgers et al. 2002). Annen and Sparks (2002) present a more detailed analysis in which it is demonstrated that, after an incubation time of up to 106 years, frequent small additions of basalt will maximize efficiency of secondary melt generation in the crust, and that crystalline differentiates of the basalt are likely to remelt along with country rocks to produce silicic magmas having a hybrid ‘crust-mantle’ composition. The petrologic implications of such models for the SRP will be explored elsewhere.

It is also instructive to consider effects of magmatism in terms of crustal modification within a zone of partial melting and melt migration. Assuming a melt fraction of 15% (minimal) and the cylindrical geometry used above, melting scenarios considered (Table 4) imply partial melt zone thicknesses ranging from roughly 4 to 25 km, the latter being a conservative figure for the CSRP. Even if melt fraction approaches 25%, scenario [3] predicts a vertical dimension for the zone of melting (15 km) approaching half the crustal thickness. Solidification of injected basalt and extraction of lower density rhyolitic magma will thus contribute to an overall densification of a significant volume of crust (Leeman 1982a) and may contribute to observed positive magnetic intensity and Bouguer gravity anomalies coincident with parts of the SRP (Mabey 1982). Below we elaborate on the physical significance of these calculations.

In passing, we note that similar volume considerations also effectively preclude FC of associated basaltic magmas as the origin of most SRP rhyolite. Such models require about 95% (or greater) FC to produce suitable daughter liquids (e.g., production of rhyolite with 150 ppm Rb from basalt with 5 ppm Rb requires a closed system F-value of 0.033, or ∼97% solidification). Using the same geometric assumptions as above, the corresponding volume ratio of basalt:rhyolite (approaching 20–30) implies a thickness (22–33 km) of basaltic cumulates exceeding half that of the present-day crust! This volume of basaltic magma far exceeds that required in the anatectic model.

Even if silicic magmas are produced solely by crustal anatexis, basaltic intrusion likely has resulted in significant reconstitution of the original crust. Notably, given that the Moho (base of crust) beneath the SRP is nearly flat at ∼40 km (Hill and Pakiser 1967; Sparlin et al. 1982; Peng and Humphreys 1998), intrusion of the requisite basalt volume creates a serious ‘room problem’. Second, seismic refraction studies (cf. Pakiser 1989; Smith and Braile 1994) indicate that the lower crustal layer (Vp = 6.7 km/s) is thicker beneath the WSRP (∼30 km) than under either the ESRP or Yellowstone (both ∼20 km); effectively, this translates to the presence of a higher velocity (i.e., denser) lower crust toward the west. A plausible explanation for this observation is that the density of the WSRP lower and middle crust has been selectively increased owing to greater inputs of basaltic magma (Leeman 1982a, 1989). The room problem may be alleviated by Basin and Range style extension (i.e., parallel to the ESRP axis or, equivalently, to the vector of North American plate motion), effectively elongating the SRP (cf. Leeman 1982a, 1989; Parsons et al. 1998). There is little evidence supporting significant NW-SE extension across the SRP (Peng and Humphreys 1998; Rodgers et al. 2002). Horizontal flow of warm and weak lower crust is also possible, but in itself will not accommodate the room problem.

A simple 2-D model was used to evaluate the possible role of extension in explaining all of these observations. Figure 14 portrays the extent of crustal attenuation as a function of time for a constant rate of pure shear extension, and the corresponding volume of new material (i.e., basaltic ‘new crust’) that must be added to maintain a constant crustal thickness. Based on geologic reconstructions, Rodgers et al. (2002) estimate the magnitude of extension along the SE and NW margins of the ESRP to be at least 21 and 15%, respectively, since roughly 10 Ma—i.e., a few percent per million years Rodgers et al. (1990) also demonstrate that extension in tectonic basins flanking the SRP migrated from west to east since mid-Miocene time, in which case cumulative extensional strain is likely to increase westward along the SRP. Although quantitative estimates for the western region are lacking, we have noted (cf. Appendix 6) the presence of several major N- to NNW-trending normal fault zones that experienced significant displacement contemporaneous with silicic volcanism in the CSRP, and we note that the NW-trending western SRP graben is a zone of significant SW-NE extension (cf. Wood and Clemens 2002).
Fig. 14

SRP extensional model for progressive crustal modification either as (1) temporal variation at a given location, or (2) a cross-sectional snapshot from east (YP) to west (WSRP). Model assumes original crustal thickness of 40 km and extension at a constant rate of 2%/m.y. Assuming conservation of volume (or area in two dimensions), and negligible extension out of the plane of cross-section, the original crust thins with time as indicated by shallowing Moho (M0) and Conrad (C0) discontinuities. To maintain crustal thickness near 40 km (M1; as suggested by seismic data) requires addition of new crust (i.e., basalt) equivalent in thickness to the shaded area between curves M0 and M1 (though not restricted to the geometry shown). New crust added below the Bruneau-Jarbidge (BJ) eruptive center is equivalent to a vertical accumulation of at least 8 km of basalt. Final mass distribution is such that the lower 60–70% of present-day crust below WSRP has an average “lower crustal” P wave velocity (∼6.7 km/s) UC and LC represent upper and lower crust. See text for details

In Fig. 14 we assume a constant strain rate (2% per m.y.) equivalent to that documented for the ESRP. Since 15 Ma, this corresponds to 30% (or about 150 km) cumulative extension, with significantly greater strain concentrated in the west-central SRP. To maintain a constant apparent crustal thickness, the model predicts that addition of at least 7.7 km of new crust must be added beneath the area (west-central SRP) that began to extend at ∼12 Ma; this agrees well with the above estimate (scenario [3]) of basalt addition required to produce CSRP silicic magmas. Also, according to this model, material with lower crustal seismic properties would appear to make up about 60% of the present-day crust, whereas the seismic estimate is closer to 75% (Hill and Pakiser 1967). This discrepancy also suggests that we have underestimated the actual basalt input into the crust. All of these perspectives can be brought into closer agreement if the input is about 1.5 times that calculated for scenario [3]. In this case the room problem is more severe and requires greater extension—either at a higher constant rate (∼3%/Ma) or, if extension is non-uniform, selectively concentrated across the western and central SRP. Uncertainties in magma volumes, intrusive geometries, and other factors, allow considerable latitude in the model results, so we do not claim that they are unique—nevertheless, internal consistency between the various constraints considered point to the need for astounding modification of the crust. Clearly, the diachronous nature of lithospheric deformation and its relation to magmatic evolution of the SRP warrant further detailed investigation—particularly with regard to the feedback effects of magmatism on crustal strength and strain history.

In summary, this model can (1) accommodate significant input of basaltic magma leading to densification of the crust as well as extensive partial melting to produce SRP rhyolitic magmas, and (2) account for protracted inputs (and eventually throughputs) of basaltic magma over the duration of SRP magmatism – including an incubation period to generate the earliest rhyolites and extending to recent time over much of the province. Production of quartz-bearing silicic magmas having light oxygen isotopic compositions and other observed compositional features is consistent with melting at mid- to upper crust levels. The inferred heat source is contemporaneous injection of mantle-derived basaltic magmas that, upon cooling, form a mafic sill complex—i.e., imaged by receiver function studies at depths between about 10–20 km beneath the ESRP (Peng and Humphreys 1998), and implied to exist beneath much of the SRP on the basis of observed Bouguer gravity and magnetic intensity highs (Mabey 1982) and elevated heat flow (Blackwell and Steele 1992).

Discussion

The dominant CSRP silicic magma system persisted for more than 4 Ma producing a volume of rhyolitic magma on the order of 104 km3. Broad distribution of multiple ignimbrite units and compositionally similar rhyolite lavas implies the existence of large reservoir systems that at times could deliver more than 1,000 km3 of magma. Whereas these magmas were relatively homogeneous at specific event horizons, they exhibit large and coherent compositional variation over the life of the magmatic system. In detail they typically comprise complex, sometimes multi-modal mineral assemblages that seemingly require contributions from multiple discrete magma lenses in the crust. The magma systems perhaps resembled a melt-filled ‘crustal sponge’ rather than large ‘convecting vats’. Parallel generation and storage in a crustal plexus (cf. Dufek and Bergantz 2005) could possibly overcome the problem of accumulating sufficient magma to feed the largest ignimbrite events (Jellinek and DePaolo 2003) but leaves open the question of how such magmas are efficiently extracted. In any case the extracted magmas appear to have mixed efficiently enough to produce relatively homogeneous eruptions with only small compositional zoning. High magmatic temperatures would lower melt viscosity and enhance mixing and homogenization. The models of Annen and Sparks (2002) imply that persistently high mineral equilibration temperatures (∼900–1000°C) for CSRP rhyolites require repeated injections (trickle feed?) of basaltic magma over protracted time intervals. Moreover a precursory incubation period (on the order of 105–106 years) may be required prior to onset of the earliest rhyolitic eruptive activity—in which case the CSRP crust could have received significant basaltic inputs as early as ∼13.5 Ma.

The eruption of distinct rhyolites over long horizontal distances (e.g., from western to eastern SRP at 11.7–10.2 Ma) implies the near-contemporaneous existence of discrete crustal melt and accumulation zones over distances of some 400 km. This implies widely distributed injection of basaltic magmas into the crust beneath much of the SRP well before 11 Ma. Also, intermittent continuation of basaltic magmatism to Pleistocene or younger time over much of the SRP is difficult to reconcile with a Hawaiian-style (‘point source’ or ‘plume tail’) hotspot model. We propose that both features are more consistent with tectonic control by lithospheric extension – as demonstrated for the Great Basin to the south (Harry and Leeman 1995).

Both the general eastward younging age progression of SRP silicic magmatism (and inferred mafic plutonism), and the persistence of this activity once initiated, could be related to Basin and Range style extension that, essentially, has lengthened the apparent hot-spot track. The apparent age progression is consistent with diachronous eastward migration of the onset of extension, coupled with sustained stretching once deformation began. A simple extensional model involving at least 2% stretching per m.y. (ca. 10 mm/y) can accommodate the volume of mafic magma needed to sustain silicic magmatism, while balancing crustal attenuation (i.e., maintaining near-constant crustal thickness), and simultaneously increasing lower crustal density and seismic P wave velocity to match inferred crustal structure.

What processes drive SRP magmatism? In our view, this question fundamentally concerns origin of the associated basaltic magmas. Considering the basaltic inputs inferred to sustain voluminous SRP silicic magmatism, the integrated melt production rate for the SRP clearly exceeds that associated with Basin and Range extension to the south or north. While this observation provides a compelling incentive for invoking a mantle hotspot model, the causes for excess melt productivity associated with the SRP are far from clear. For example, origin of the basalts can be viewed in terms of two competing generic processes, both of which are potentially important: [1] decompression melting of lithospheric mantle due to tectonic stretching (Harry and Leeman 1995), and [2] decompression melting of hot ascending asthenospheric mantle (McKenzie and Bickle, 1988). Of these, the first could possibly account for latitudinal differences in melt production – for example, if the lithospheric mantle is compositionally more fertile beneath the SRP. It could also better account for the timing and spatial pattern of mafic magmatism. The second process could result in excess melt productivity by promoting a local positive thermal anomaly in underlying mantle. As noted earlier, the style of SRP magmatism does not conform well to patterns expected for this process, but it remains to be seen what influence the continental lithosphere imposes on expressions of deeper magma production and ascent. These issues will be addressed in greater detail elsewhere (cf. Leeman 2005).

Conclusions

Our principal conclusions are summarized as follows:
  1. 1.

    Although the silicic magmatism associated with the SRP broadly tends to decrease in age from SW to NE along the trace of the Yellowstone hotspot track, in detail there are important exceptions at several scales. In the CSRP, rhyolitic volcanism occurred sporadically throughout the entire area, and apparently was related to piecemeal foundering of the crust in a series of major, perhaps catastrophic, caldera-forming events that did not follow a simple diachronous pattern in time and space.

     
  2. 2.

    CSRP rhyolitic volcanism consisted of an early (12.7 to 9.5 Ma), very voluminous, caldera-forming stage and a later (9.5 to 5.5 Ma), less voluminous, rifting stage. Ignimbrite eruptions dominated the caldera-forming stage, whereas rhyolite lava flow eruptions dominated the rifting stage; in fact, both eruptive styles occurred during each stage. Basaltic eruptions were coeval with the second stage of this activity.

     
  3. 3.

    A regional stratigraphic framework is proposed based on physical stratigraphy, geochronology, and compositional variations. These data allow us to define composition and time (CAT) groups, in which ignimbrites and/or lavas erupted within a single time interval, over distances as great as about 102 km, have similar bulk compositions and mineralogy. Implicitly, rhyolites of a single CAT group provide a time-slice sample of the evolving regional-scale reservoir system. In particular, the 12.7 to 9.0 Ma CAT groups define a coherent general temporal variation from more to less evolved compositions and toward higher temperatures over time, albeit with small short-term reversals along the way.

     
  4. 4.

    These coherent patterns over great distances suggest that the underlying magma systems are of regional extent, and are consistent with either increasingly refractory crustal sources, increasing inputs of basaltic magma, or both, with time. These trends, and the volume scale of CSRP silicic magmatism appear to be unique among eruptive centers of the SRP-Yellowstone province.

     
  5. 5.

    The overall volume of the rhyolite erupted over about 7 million years was immense. It exceeds 7,000 km3, and possibly is well within the 10,000–20,000 km3 range considering that volumes of intracaldera fill and distal airfall deposits have not been included. An unknown, but perhaps larger volume of magma may have been retained as intrusions within the crust.

     
  6. 6.

    The rhyolitic melts were produced dominantly by partial melting of pre-existing crustal materials, as indicated by the persistent presence of restitic glomerocrystic aggregates, low-δ18O signature and other geochemical characteristics of CSRP rhyolites. Incompatible trace element data are consistent with at least 10–15% melting, assuming an ‘average crust’ source. Sr and Nd isotopic data resemble compositions of Idaho batholith granitoids, and seemingly preclude significant involvement of either Archean crust or dominant contributions from the coeval basaltic magmas or melts of their intrusive counterparts.

     
  7. 7.

    Intrusion of huge volumes of basaltic magma into the crust provided the energy to drive CSRP silicic magmatism. Significant crustal deformation was required to accommodate the volume of basalt needed for energy balance. We present a model that conceptually integrates basalt intrusion and compositional reconstitution of the crust with Basin and Range style extension. Lithosphere-scale extension could also contribute to production of the basaltic magmas.

     
  8. 8.

    Eruptions between 11.7–10.2 Ma of voluminous mineralogically and compositionally distinct rhyolites across much of the SRP, during the height of the CSRP ignimbrite flare-up, indicate that (1) significant basaltic injections into the crust were widespread prior to that time, and (2) magmatism did not follow a simple hotspot-like time-transgressive pattern as previously believed. Rather, basaltic magmatism likely was significant across the province from mid-Miocene to Quaternary time. Also, the respective 11.7–10.2 Ma rhyolites demonstrate that large silicic magma systems were at least partly coeval over a distance of some 400 km. We suggest that magmato-tectonic events, such as proposed for the CSRP, may have triggered activity in adjacent areas if silicic magma accumulations were primed to erupt.

     
  9. 9.

    We envision a magmatic system that at any given time involves a wide swath of crust that is occupied by a swarm of coeval but distinct melt lenses, and that these reservoirs mimic each other not only in composition but also in the pace and degree of their evolution. Large eruptions presumably extract magma from multiple discrete lenses rather than from a single zoned magma chamber. Syneruptive mixing may partly homogenize these melt ‘parcels’, but is inefficient to the extent that mineralogical and even glass shard heterogeneities are preserved on a small (cm) length scale. This model implies that magma bulk composition may partly depend on the length-scale of melt extraction (hence the total volume of an eruption), on the rate at which melt parcels can be extracted and commingled, on the heterogeneity and melting history of the crustal source rocks, and on thermal gradients across the melt zone that influence the degree of melting for any given lens.

     

Modeling of such a dynamic melting regime is a fertile research goal for the future. We also leave as an open question the details of how crustal deformation and magmatism are linked. To constrain this relationship, and to test the concepts presented here, will require additional information regarding (1) the distribution and magnitude of tectonic strain across the entire SRP and its flanks, and (2) the density distribution and seismic structure of the underlying crust. Also, investigations are needed to quantify how magmatism at the scale we envisage will modify the thermal structure and strength of the crust; such information may provide insights into processes of magma accumulation and eruption. Finally, although we propose that tectonic processes have a significant influence on SRP magmatism, we are reluctant to rule out entirely some linkage between sublithospheric processes and generation of the associated basaltic magmatism (cf. Humphreys et al. 2000; Yuan and Dueker 2005; Waite et al. 2006).

Acknowledgements

Our efforts leading to this paper benefited from the assistance of many people over some 30 years. The Idaho Geological Survey, Rice University, New Mexico Bureau of Mines and Mineral Resources, and Mount Holyoke College supported aspects of this research, and the U.S. Geological Survey and the Idaho Department of Water Resources contributed financial and logistical support. Leeman acknowledges grants EAR-8018580 and EAR-8320358 from the National Science Foundation and continued support by the Division of Earth Sciences since joining NSF. We thank M. Rhodes and P. Dawson (University of Massachusetts), D. Cornelius (Washington State University), and G. Fitton (University of Edinburgh) for help with XRF analyses. E. Pestana and M. Dehn (Rice University) assisted with ICP analyses and N. Rogers (Open University) provided neutron activation analyses. Help in the field and many discussions were provided by the ‘Friends of Rhyolite’: J. Bernt, D. Kauffman, M. Jenks, J. Wolff, S. Boroughs, P. Larson, G. Gillerman, M. Branney, T. Barry, G. Andrews, B. Ellis, J. Sumner, M. McCurry, S. Hughes, M. Perkins, D. Clemens, B. Hirt, C. Manley, G. Citron, T. Gregg, A. Semple, C. White, S. Wood, C. Henry, M. Ferns, A. Grunder, M. Cummings, M. Norman, J. Aranda-Gomez, G. Aguirre-Diaz, G. Labarthe, J. McPhie, N. Riggs, R. Smith, R. Christiansen, L. Morgan, and W. Hackett. Finally, we are indebted to Henny Cathey, Martin Streck, Derek Schutt, Barbara Nash, and Lina Patino whose reviews and comments helped us improve this paper. Of course, we take responsibility for any errors in interpretation and hope that, as research continues on SRP rhyolites, our ideas will be modified for the better.

Supplementary material

Copyright information

© Springer-Verlag 2007

Authors and Affiliations

  • Bill Bonnichsen
    • 1
  • William P. Leeman
    • 2
  • Norio Honjo
    • 3
  • William C. McIntosh
    • 4
  • Martha M. Godchaux
    • 5
  1. 1.Idaho Geological SurveyMoscowUSA
  2. 2.National Science FoundationArlingtonUSA
  3. 3.Rice UniversityHoustonUSA
  4. 4.New Mexico Inst. of Mining and TechnologySocorroUSA
  5. 5.Mount Holyoke CollegeMoscowUSA

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