Magma batches in the Timber Mountain magmatic system, Southwestern Nevada Volcanic Field, Nevada, USA

Joumalofvolcanologv

and geothermal meaxh

ELSEVIER

Journal of Volcanology

and Geothermal

Research 78 (1997) 185-208

Magma batches in the Timber Mountain magmatic system, Southwestern Nevada Volcanic Field, Nevada, USA James G. Mills Jr. ap*,Benjamin W. Saltoun b, Thomas A. Vogel b a Geology and Geography Department, DePauw Unic!er.sity,Greencastle, IN 46135, USA b Department of Geological Sciences, Michigan State Uniuersity, East Lansing, MI 48823, USA Received 30 July 1996; accepted 30 March 1997

Abstract The common occurrence of compositionally and mineralogically zoned ash flow sheets, such as those of the Timber Mountain Group, provides evidence that the source magma bodies were chemically and thermally zoned. The Rainier Mesa and Ammonia Tanks tuffs of the Timber Mountain Group are both large volume (1200 and 900 km3, respectively) chemically zoned (57-78 wt.% SiO,) ash flow sheets. Evidence of distinct magma batches in the Timber Mountain system are based on: (1) major- and trace-element variations of whole pumice fragments; (2) major-element variations in phenocrysts; (3) major-element variations in glass matrix; and (4) emplacement temperatures calculated from Fe-Ti oxides and feldspars. There are three distinct groups of pumice fragments in the Rainier Mesa Tuff: a low-silica group and two high-silica groups (a low-Th and a high-Th group). These groups cannot be related by crystal fractionation. The low-silica portion of the Rainier Mesa Tuff is distinct from the low-silica portion of the overlying Ammonia Tanks Tuff, even though the age difference is less than 200,000 years. Three distinct groups occur in the Ammonia Tanks Tuff: a low-silica, intermediate-silica and a high-silica group. Part of the high-silica group may be due to mixing of the two high-silica Rainier Mesa groups. The intermediate-silica group may be due to mixing of the low- and high-silica Ammonia Tanks groups. Three distinct emplacement temperatures occur in the Rainier Mesa Tuff (869, 804, 723°C) that correspond to the low-silica, high-Th and low-Th magma batches, respectively. These temperature differences could not have been maintained for any length of time in the magma chamber (cf. Turner, J.S., Campbell, I.H., 1986. Convection and mixing in magma chambers. Earth-Sci. Rev. 23, 255-352; Martin, D., Griffiths, R.W., Campbell, I.H., 1987. Compositional and thermal convection in magma chambers. Contrib. Mineral. Petrol. 96, 465-475) and therefore eruption must have occurred soon after emplacement of the magma batches into the chamber. Emplacement temperatures of the pumice fragments from the Ammonia Tanks Tuff show a continuous gradient of temperatures with composition. This continuous temperature gradient is consistent with the model of storage of magma batches in the Ammonia Tanks group that have undergone both thermal and chemical diffusion. 0 1997 Elsevier Science B.V. Keywords: magma zonation; ash-flow

* Corresponding

tuff; geochemistry;

petrology

author.

0377-0273/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PZZ SO377-0273(97)00015-Z

186

J.G. Mills Jr. et al./ Journal of Volcanology and Geothermal Research 78 (1997) 185-208

1. Introduction 1.1. Purpose

The Timber Mountain Group of the Southwestern Nevada Volcanic Field (SWNVF) contains two large volume ash flow tuffs that erupted at approximately 11.6 Ma (Byers et al., 1976a; Sawyer et al., 1994). Based on chemical and mineralogical analyses of glassy pumice fragments, the Rainier Mesa Tuff (11.60 Ma, 1200 km3) and the Ammonia Tanks Tuff (11.45 Ma, 900 km3) exhibit a range of magmatic compositions from trachyandesite to high-silica rhyolite (Rose, 1988; Mills, 1991). De Silva and Wolff (1995) have recently suggested that the shape of the magma chamber controls the degree of zonation within the magma body, and that the degree of zonation is a function of the efficiency of sidewall convective fractionation. Convective fractionation is very inefficient in magma chambers with large aspect ratios (De Silva and Wolff, 1995) and large volume magma chambers are constrained to have large aspect ratios if they are not to occupy unrealistic proportions of the crust. Such constraints do not occur in small volume magma chambers as these systems may have small aspect ratios. Convective fractionation is, however, very efficient at producing strong zonation in these types of systems. Our work on the large volume tuffs of the Timber Mountain Group supports the hypothesis that zoning of this large volume system is not due to convective fractionation. We conclude that the significant geochemical, mineralogic and geothermometric variations represented in these ash flow sheets are due to emplacement of new distinct magma batches into the Rainier Mesa and Ammonia Tanks magma systems, and not in situ differentiation (crystal fractionation). 1.2. Background The common occurrence of compositionally and mineralogically zoned ash flow sheets, such as those of the Timber Mountain Group, provides strong evidence that the source magma bodies were chemically and thermally zoned. Among the first studies to establish this premise was the study of the Topopah Spring Tuff of the Paintbrush Group from the SWNVF by Lipman et al. (1966). Subsequent studies

of zoned ash flow sheets supported the concept that large chemical, mineralogical and thermal gradients are common features of magma bodies (for a review see Smith, 1979; Hildreth, 1981; Mahood, 1981; Bacon et al., 1981; Crecraft et al., 1981; Baker and McBimey, 1985; De Silva and Wolff, 1995; Cambray et al., 1995). Much of the effort of recent studies has been to evaluate the origin and evolution of zoned magma bodies by studying the ash flow sheets. In almost all studies of zoned ash flow sheets, the inferred trend towards the top of the parental magma bodies is increasing silica, decreasing temperature and decreasing phenocryst content. Because of the effects of magma withdrawal processes (Spera et al., 1986) and topography (Valentine et al., 1992) on compositional zonation in ash flow sheets, it is not clear if the zoned ash flow sheets reflect the presence of continuous chemical gradients or discrete layers in the magma body. Regardless of the exact configuration of the zoning in the magma body, it is generally accepted that zoning (or layering) is common in high-level silicic magma bodies (cf. Fisher and Schminke, 1984; De Silva and Wolff, 1995). However, conclusions based on the presence of chemical gradients are, to a great extent, model dependent. Many previous studies have assumed that the dominant volume of the parental magma body evolved to a large extent, by differentiation of an originally homogeneous magma (cf. Broxton et al., 1989; Warren et al., 1989). If silicic magma batches were derived by melting and extractive processes rather than by differentiation of a homogeneous body, then the chemical zoning in the magma body could be complex. Several models for the origin of chemically and thermally zoned magma bodies have been presented in previous studies. Most models that involve large scale differentiation of a homogeneous magma are generally based on sidewall crystallization (cf. Mittlefehltd and Miller, 1983; McBimey et al., 1985; Baker and McBimey, 1985; De Silva and Wolff, 1995). In this model, the magma evolves by density differences due to crystallization and cooling on the margins of the magma chamber, although some interaction with the wall rock (assimilation) may take place (Shaw, 1965; Hildreth, 1981; Spera et al., 1984; Lowell, 1985; McBimey et al., 1985; Nilson et al., 1985; McBimey and Nilson, 1986; Trial and

J.G. Mills Jr. et al./Joumal

of Volcanology and Geothennal Research 78 (1997) 185-208

Spera, 1990; Farmer et al., 1991). Zoning can also result from interactions (assimilation or melting) with the roof of the magma chamber. Hildreth’s innovative thermogravitational diffusion model (soret diffusion) (Hildreth, 1981) has been disproved by measurements of thermal diffusion coefficients that indicated diffusion was inefficient in generating large chemical gradients in magma bodies (e.g., Lesher and Walker, 1991). At the opposite end of the conceptual spectrum are models that involve controls by partial melting and melt extraction, which were first proposed by Marsh ( 1984) and expanded upon by Bergantz ( 1989) and Sawyer (1994). There is recent petrologic evidence (e.g., Sampson and Cameron, 1987; Vogel et al., 1987, 1989a,b; Schuraytz et al., 1989, 1991; Civetta et al., 1991; Lu, 1991; Hervig and Dunbar, 1992; Orsi et al., 1995) that many magma bodies may have had distinct compositional groups that were not related by fractionation processes. Chemically distinct magma batches that are unrelated by fractionation processes can result from melting and extraction processes (Sawyer, 1994). These magma batches can be subsequently modified by crystal fractionation, assimilation and magma mixing processes, and if enough time is available, the magma

To Reno I

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I

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117O f

187

batches can become stratified in the magma chamber according to their densities. An extreme case of the combining of magma batches is the underplating of a silicic magma body by an independently generated mafic magma (Wiebe, 1994; Coleman et al., 1995; Metcalf et al., 1995). The extremely large compositional range from trachyandesite to high-silica rhyolite (57-78 wt.% SiO,) in the two large ash flow sheets of the Timber Mountain Group provides an ideal setting to evaluate models for the origin of chemically zoned magma bodies. Although the Ammonia Tanks Tuff is separated from the Rainier Mesa Tuff by less than 200,000 yr (Sawyer et al., 1994) the Ammonia Tanks Tuff represents a distinct influx of new magma based on data presented in this paper.

2. Regional setting The Timber Mountain-Oasis Valley caldera complex, located in the southern Great Basin, was active from approximately 13 to 9.5 Ma (Fig. 1; Byers et al., 1976a; Christiansen et al., 1977). Volcanic activity at the Timber Mountain-Oasis Valley caldera complex was part of a larger igneous event that

116 O30 I I

116' I I

Silent Canyon Caldera \

I

To Las Vegas

’

0 SAMPLING SITES Fig. 1. Location of the southwestern Nevada Volcanic Field and associated are indicated with solid circles and corresponding sample group numbers.

calderas (modified

from Byers et al., 1976a). Sample localities

188

J.G. Mills Jr. et al. /Journal

of Volcanology and Geothermal Research 78 (1997) 185-208

occurred throughout the Basin and Range Province from the Early Eocene to the Early Pliocene (37-5 Ma) (Eaton, 1984). This episode of igneous activity may have been related to subduction of the Farallon plate and later regional crustal extension (Lipman et al., 1972; Snyder et al., 1976; Cross and Pilger, 1978; Eaton, 1979, 1984; Zoback et al., 1981; Axen et al., 1990). Quaternary volcanic activity, predominantly basaltic in nature, occurs sporadically throughout the Basin and Range Province and represents extension-related igneous activity (Eaton, 1979, 1984; Feurerbach et al., 1993). The SWNVF consists of six major calderas formed between > 15 and 7.5 Ma. Eruptions of the Paintbrush Group and Timber Mountain Group were responsible for the development of the Oasis Valley and Timber Mountain calderas, respectively (Fig. 1; Byers et al., 1976a; Christiansen et al., 1977). The Paintbrush Group has a minimum volume of 2200 km3 and was erupted periodically over 100,000 yr (12.8-12.7 Ma) (Broxton et al., 1989; Sawyer et al., 1994). Detailed petrologic, geochemical and mineralogical studies of each member of the Paintbrush Group were performed by Lipman et al. (1966), Schuraytz et al. (1986, 19891, Broxton et al. (19891, Warren et al. (19891, Flood (1987) and Flood et al. (1989a,b). Results of these studies have shown that: a sharp interface existed between two magmas in the parental magma body of the Topopah Spring Tuff; the overlying members of the Paintbrush Group (Pah Canyon, Yucca Mountain and Tiva Canyon tuffs) were generated by fractional crystallization of an originally homogeneous magma body that was periodically erupted and replenished; and finally, that the Pah Canyon Tuff was produced by mixing of the two Topopah Spring Tuff magmas. Approximately at 11.6 and 11.45 Ma, eruption of the Rainier Mesa and Ammonia Tanks tuffs, respectively, resulted in formation of the Timber Mountain caldera (Byers et al., 1976a; Christiansen et al., 1977; Broxton et al., 1989). Re-calculation of older K/Ar dates and additional “OAr/ 39Ar dating by Sawyer et al. (1994) indicates that the time interval between eruptions of the Rainier Mesa and Ammonia Tanks tuffs is approximately 150,000 yr. Resurgence of the Timber Mountain caldera occurred after the eruption of the Ammonia Tanks Tuff. Volcanism waned following resurgence and

was limited to two small-volume ash flow tuffs (Tuff of Fleur-de-lis Ranch and Tuff of Cutoff Road), a series of rhyolite flows along the caldera rim, and minor basalt flows (Byers et al., 1976a,b; Christiansen et al., 1977).

3. Geochemistry 3.1. Sample selection The use of glassy pumice fragments from ash flow tuffs rather than whole-rock ash flow tuff samples to evaluate magma evolution has been discussed by many workers (Lipman, 1967, 1971; Walker, 1972; Wolff, 1985; Hildreth and Mahood, 1985; Schuraytz et al., 1989). Geochemical and mineralogical reconstruction of the original magmatic compositions based upon whole-rock tuff samples is very difficult based upon detailed, individual glass shard analyses by an electron microprobe. Individual shard analyses in a whole-rock sample will provide melt compositions, but magma compositions must include phenocryst phases present within that particular system or magma batch. Chemical and mineralogic analyses of whole-rock tuff samples represents a variable mixture of matrix glass, phenocrysts, and xenolithic material that will in most cases not be representative of the true magmatic composition(s). Glassy pumice fragments, however, most nearly represent quenched packets of distinct magma compositions. Glassy pumice fragments of a commingled nature exist within both members of the Timber Mountain Group. Commingling is often recognized by distinct color and textural banding in the pumice fragments. All attempts were made in this study to reject pumice fragments that exhibited evidence of commingling based on color banding, geochemical banding based on electron microprobe traverses and disequilibrium phenocryst assemblages. As shown by Wolff (1985), consideration of sample size is critical to minimizing the effects of magma fragmentation and phenocryst control of bulk pumice fragment chemistry due to high crystal/glass ratios. Thus, collection of pumice fragments was limited to samples greater than 4 cm in diameter. Sampling of

J.G. Mills Jr. et al./Joumal

of Volcanology and Geothermal Research 78 (1997) 185-208

glassy pumice fragments from only the basal unwelded zone, vitrophyric zone and unwelded top of the ash flow sheet could be construed as a potential source of sample bias. However, the upper poorly to non-welded zone of the ash flow sheet contains the full range of pumice compositions that were erupted from the magma body as conclusively shown by Rose (19881, Schuraytz et al. (1989) and Vogel et al. (1989a,b). Valentine et al. (1992) used theoretical modeling of explosive eruptions to suggest that compositional breaks, such as those observed in both members of the Timber Mountain Group and in the outflow facies of other ignimbrites, may be an artifact of the presence of a topographic barrier (caldera rim) during eruption and emplacement of the ash flow sheet, This effect is not significant in the Timber Mountain

12,

I

I

I

I

11

UJ 10

Q)

‘-

B

9

3

a 7

:5:

60

65

70 so

SiO

75

60

*

2

Fig. 2. Alkalis (Na,O+ K,O) and Ba versus SiO, (all data normalized to 100% in all of the figures) for the Rainier Mesa and Ammonia Tanks tuffs.

189

Group because in every section of the ash flow sheets, the same compositional breaks occur. If compositional breaks were due to topographic barriers, the breaks would not occur in all sections. Therefore, in order to identify and evaluate distinct magma compositions in the Timber Mountain Group, this study concentrates on the glassy pumice fragments and fiamme found in the unwelded to densely welded glassy zones of the individual tuffs. These pumice fragments and fiamme most nearly represent the original pre-eruptive magmatic geochemical and mineralogical compositions as each fragment represents a relatively unaltered quenched packet of magma (cf. Wolff, 1985). Although secondary hydration of the glassy pumice has occurred, and some alkali exchange may have taken place (Lipman et al., 1969; Jezek and Noble, 19781, the geochemical data do not indicate significant alkali exchange (Fig. 2). 3.2.

Pumice

fragment

distribution:

Rainier Mesa Tufs

Pumice fragment compositions become more diverse with increasing stratigraphic height. The unwelded to partially welded base of the Rainier Mesa Tuff is composed of high-silica rhyolite pumice fragments containing quartz, feldspar and biotite. Based on Th, Nb, Rb and other trace elements there are two high-silica pumice fragment populations. The range of pumice fragment compositions becomes greater with increasing stratigraphic height. Amphibole, pyroxene and feldspar-rich mafic pumice fragments become volumetrically more significant upsection. All pumice fragment compositions that occur throughout the sheet are represented within the upper few meters of the top of the Rainier Mesa Tuff. This feature has been noted in several other ash flow deposits: the Mount Mazama pyroclastic flows (McBimey. 1968; Noble et al., 1969; Druitt and Bacon, 1986); ash flow sheets from the Aso caldera, Japan (Lipman, 19671; the Topopah Spring Tuff of the Paintbrush Group, Nevada (Lipman et al., 1966; Schuraytz et al., 1989); the Tiva Canyon Tuff of the Paintbrush Group (Flood et al., 1989a,b); pyroclastic deposits derived from the Las Canadas caldera on Tenerife Island (Canary Islands) (Wolff and Storey, 1984); and in the 1912 ash flow of the Valley of Ten Thousand Smokes, Alaska (Hildreth, 1983).

J.G. Mills Jr. et al./Joumal

190

3.3. Pumice fragment

distribution:

of Volcanology and Geothermal Research 78 (1997) 18.5-208

Ammonia

Tanks

T&l-

Early studies based on whole-rock tuff geochemical analyses led Byers et al. (1968, 1976a) to infer that the lower unit of Ammonia Tanks Tuff was reversely zoned with respect to silica. They reported whole-rock tuff silica concentrations ranging from 68 wt.% SiO, at the base to 78 wt.% SiO, at the top. Whole-rock analyses of tuffs from the upper portion of the upper cooling unit were not reported, but this unit was also inferred to be reversely zoned based on the presence of a quartz latitic base (66 wt.% SiO,) (Byers et al., 1976a).

Table 1 (a) Representative Sample No.:

major- and trace-element R8-1

R8-7

analyses

R8-35

High-silica rhyolite pumice fragments from the compound cooling unit of the Ammonia Tanks Tuff form a significant but not dominant population of the pumice fragments at the base of the lower unit, and decrease rapidly (usually within one meter of the base) in abundance towards the top of the upper unit. The base of the upper unit, however, does not always contain high-silica rhyolite pumice fragments. All pumice fragment compositions found in the lower portion of the sheet can be found at the very top of the compound cooling unit. It is questionable whether the Ammonia Tanks Tuff can be labeled as either a normal or reversely zoned tuff for two reasons: (1) the high-silica rhyolite pumice-rich zone at the base

for the Rainier Mesa Tuff R8-42

RI l-20

R8-20

R8m22

Rll-9

Rll-5

R21-13

Major-element oxides (wt.%) 76.30 0.11 12.00 0.58 0.06 0.00 0.48 2.52 5.70 0.01

75.62 0.20 11.72 0.87 0.04 0.03 0.51 2.18 6.10 0.02

74.19 0.27 12.59 1.02 0.05 0.15 0.75 2.86 5.67 0.03

72.16 0.26 13.40 1.18 0.05 0.16 0.78 2.39 7.13 0.05

66.46 0.43 16.09 1.89 0.08 0.7 1 1.61 4.34 5.38 0.11

63.61 0.60 16.99 2.88 0.13 0.95 2.40 4.44 4.77 0.22

60.01 0.75 17.86 4.03 0.11

E,%

76.08 0.10 11.95 0.40 0.07 0.00 0.40 2.94 5.29 0.01

Total

97.24

97.76

97.29

97.58

97.56

97.10

96.99

96.89

SiO, TiO, AlA Fe0 MnO MS0 CaO Na,O K2O

Trace elements (X-ray$uorescence) Rb 265 271 Sr 2 ND Y 30 26 zr 68 80 Nb 29 23 La 35 28 Ba 132 169 Trace elements (INAA) (ppm, SC 3.8 Hf 3.5 Th 25.4 La 21.36 Ce 51.0 Sm 5.5 EU 0.12 Tb 0.66 Yb 4.3 LU 0.42

3.3 2.3 11.6 30.32 36.3 5.22 0.19 0.52 1.67 0.24

(ppm) 141 34 21 153 15 79 157

1.9 4.7 33.4 68.72 110.4 6.04 0.59 0.68 1.66 0.16

121 68 21 193 16 89 433

165 97 16 210 16 85 408

1.9 5.4 37.5 86.5 1 138.1 6.43 0.69 0.69 1.41 0.10

2.0 4.5 31.2 82.03 135.5 6.03 0.60 0.06 1.18 0.09

125 335 19 407 12 129 1319

3.7 9.4 33.8 132.40 212.8 6.70 1.49 0.67 1.92 0.20

165 559 24 547 15 127 1939

3.3 12.8 25.0 114.45 179.9 7.14 2.03 0.62 2.04 0.23

1.54 3.79 4.30 4.10 0.40

118 917 20 515 14 82 2067

6.3 9.9 18.2 95.82 171.3 7.87 2.43 1.09 2.70 0.40

59.14 0.90 17.46 4.83 0.12 I .79 4.2 I 3.91 4.17 0.54

54.98 I .54 16.69 7.02 0.14 3.36 5.67 3.54 3.13 0.72

97.07

96.79

123 858 22 381 13 105 1826

9.4 8.3 18.8 96.04 190.5 9.41 2.57 1.48 3.23 0.48

67 1099 31 281 12 104 1365

13.1 6.1 12.2 86.02 153.4 10.72 2.87 0.68 2.28 0.42

191

J.G. Mills Jr. et al./ Journal of Volcanology and Geothermal Research 78 (1997) 185-208 Table 1 (continued) (b) Representative Sample No.:

major- and trace-element A4-12

Major-element oxides (wt.%) SiO 76.15 TiO: 0.14 AU’, 11.90 Fe0 0.73 MnO 0.09 MgO 0.21 CaO 0.41 Na,O 3.27 K2O 5.15 p2°5

Total

A4-53

analyses for the Ammonia Tanks Tuff

A5-68

A4-70

A4-55

A4-16

A5-72

A5-63

A5-75

0.01

74.77 0.17 12.23 0.72 0.07 0.00 0.41 2.95 6.08 0.01

72.31 0.23 13.83 1.02 0.08 0.11 0.72 3.72 5.93 0.03

70.42 0.32 14.35 1.35 0.09 0.19 0.91 3.74 6.02 0.05

67.90 0.36 15.98 1.47 0.09 0.32 1.15 4.53 6.12 0.06

65.02 0.54 16.61 1.84 0.10 0.52 1.31 4.28 6.77 0.11

63.35 0.68 17.24 2.81 0.16 0.84 2.14 4.64 5.26 0.16

61.86 0.69 17.58 3.01 0.11 1.11 2.52 4.27 5.28 0.23

60.23 0.87 17.47 3.72 0.12 1.51 3.15 4.63 4.93 0.34

57.68 1.11 17.69 4.90 0.12 2.17 4.08 4.60 4.20 0.47

98.06

97.41

97.98

97.44

97.98

97.10

97.28

96.66

96.97

97.02

Trace elements IX-ray fluorescence) (ppm) Rb 232 188 152 Sr 4 2 70 Y 28 39 32 Zr 123 165 223 Nb 31 32 22 La 46 65 37 373 Ba 197 169

146 131 30 296 27 78 620

123 180 28 326 23 116 852

Trace elements (INAA) Cppm) 1.6 I .2 Hf 3.6 5.7 Th 24.5 27.7 La 34.3 I 48.07 Ce 51.1 85.7 Sm 5.39 8.12 Eu 0.14 0.39 Tb 0.38 0.65 Yb I .42 3.73 Lu 0.24 0.36

2.7 5.7 18.2 74.21 103.4 7.83 0.80 0.58 1.43 0.25

3.4 8.7 20.9 103.28 1171.5 8.62 1.32 0.61 3.17 0.33

SC

A5-71

1.2 6.9 23.0 52.72 102.7 6.81 0.50 0.75 2.66 0.46

is not always present; and (2) depending on the type and number of pumice fragments present within the whole-rock tuff at any given location, the overall composition of the whole-rock tuff sample will be highly variable. Two of the three stratigraphic sections chosen for sampling contained pumice fragment populations that were compositionally heterogeneous from the base to the top of the ash flow sheet. The silica content of the pumice fragments in the basal portions of the sections located along the southern margin of Pahute Mesa range from 69.6 to 78 wt.% SiO, whereas at the top of the sections, the range increases from 59.4

85 155 28 542 14 242 1168

6.3 12.2 19.5 232.94 393.0 13.12 2.60 0.72 2.37 0.49

122 372 28 720 20 170 2478

6.9 15.6 16.8 200.17 295.3 10.61 3.34 0.57 2.45 0.40

76 520 30 819 17 131 3560

6.2 17.7 13.3 134.56 238.7 9.61 2.99 0.73 2.54 0.44

78 613 31 865 20 118 3990

6.8 18.8 12.0 137.60 228.7 10.35 3.50 0.72 2.37 0.40

66 819 29 982 11 107 4703

8.0 18.6 11.4 128.25 217.2 8.93 3.30 0.72 2.40 0.31

to 77.7 wt.% SiO, (samples from the analytical groups ACXX and AS-XX). A third section along the northern cauldron margin a few kilometers to the southwest of the other two sections, has a distinct 1.5 m thick base consisting entirely of quartz, feldspar and biotite-bearing rhyolitic pumice fragments. Chemical analyses of these pumice fragments indicate that they are identical in composition to other high-silica rhyolite pumice fragments in the ash flow sheet. Above this rhyolitic base, the pumice fragments rapidly become heterogeneous in composition over a 5 to 10 cm interval in a fashion similar to the two sections to the northeast. Compositionally banded

J.G. Mills Jr. et al. / Journal of Volcanology and Geothermal Research 78 (19971 185-208

192

pumice are common in all observed sections of the Ammonia Tanks Tuff. Commingling of rhyolitic and trachydacitic to trachyandesitic magmas appears to have been common throughout the eruption of the Ammonia Tanks Tuff.

analysis. Once the pumice fragments were returned to the laboratory a final suite of samples was selected for analysis based upon color, texture, mineral assemblages, and electron microprobe traverses. Locations of the sampled sections are limited to outflow sheets north and northeast of the caldera due to several factors: poor exposure and incomplete stratigraphic sections to the south and southeast, pervasive hydrothermal alteration to the west and southwest, complete burial of the Rainier Mesa Tuff within the caldera, and incomplete stratigraphic sections and complete devitrification of the Ammonia Tanks Tuff on the resurgent dome (Fig. 1).

3.4. Sample site selection Potential collecting localities were identified from USGS geologic maps (see Byers et al., 1976a, for a map index). The selection of outcrops to be sampled was based on stratigraphic control of the underlying and overlying units, degree of exposure, absence of hydrothermal alteration, and the presence of an unwelded base and/or top at the exposed section. Partially exposed or eroded units (where only unwelded bases or tops were present) were collected only when nearby exposures provided the missing top or bottom of the section. Sampling locations are shown in Fig. 1. Exact locations of the sampling sites are available from the first author. At the sampling sites, pumice fragments were collected over an area of 10 m* (1 m high by 10 m wide) approximately every 1.5 m up section in the unwelded to partially welded zones. Glassy pumice fragments were selected based on differences in color, texture, phenocryst assemblage, and size. In all, over 1400 pumice fragments were collected for potential

Table 2 Accuracy

and precision

Maior-element

data for the major-element

oxides: X-rav fluorescence:

Std. dev. Std. error mean

oxides and trace elements

USGS standard G-2: N = 5

AI,G,

Fe0

MgG

CaO

Na,O

K,O

TiO,

P20s

MnO

0.56 0.25

0.30 0.13

0.16 0.07

0.019 0.0008

0.02 0.009

0.04 0.018

0.01 0.004

0.01 0.004

0.004 0.002

0.001 0.001

USGS standard G-2, N = 10

Cr

Ni

Cu

Zn

Rb

Sr

Y

Zr

Nb

Ba

8.8 2.8

4.2 1.3

5.1 1.8

4 1

2

3 1

2.4 0.8

2 1

3.8 1.2

140 44

Trace elements: induced neutron activation SY -2; Japanese Std Jg-1 a

Avg. std. error est. Max. std. error est.

All major-element and seven trace-element concentrations (Rb, Sr, Y, Zr, Nb, La, Ba) were analyzed for on a Rigaku S-Max X-ray fluorescence spectrophotometer using fused glass disks at Michigan State University. The remainder of the trace elements (SC, Hf, Th, La, Ce, Sm, Eu, Tb, Yb, Lu) were analyzed for by Instrumental Neutron Activation Analysis at Michigan State University (Mills, 1991). All major-element data have been normalized to 100% anhydrous conditions to compensate for secondary hydration and to facilitate ease of comparison between samples. Representative analyses for

SiO 2

Trace elements: X-ray fluorescence;

Std. dev. Std. error mean

3.5. Analytical technique

I

analysis. USGS Stds BIR- 1, G-l, G-2, QLO-1, RGM-1, STM-1, W-2; Canadian

Stds MRG-

SC

Hf

Th

La

Ce

Sm

Eu

Tb

Yb

Lu

0.612 1.23

0.386 0.593

1.053 2.219

3.781 8.217

6.109 11.712

0.421 0.802

0.102 0.234

0.185 0.234

0.288 0.48 1

0.037 0.059

1,

J.G. Mills Jr. et al. /Journal

each tuff are presented in Table la and precision data are presented complete major- and trace-element as replicate analyses, are available spreadsheet tile from the authors.

of Volcanology and Geothermal Research 78 (1997) 185-208

and b. Accuracy in Table 2. The data set, as well as an ASCII or

tion diagrams for the Rainier Mesa Tuff reveal a clustering of data into two main groups, a mafic group (trachyandesite to rhyolite) with a range from 57 to 70 wt.% SiO, and a felsic group (rhyolite to high-silica rhyolite) from 74 to 78 wt.% SiO, (Fig. 3). The clustering of data can be observed on majorelement plots, but is more pronounced on trace-element versus SiO, plots (Fig. 5). The 4 wt.% silica gap is consistent with other large volume ash flow tuffs that may exhibit compositional gaps of up to 7 wt.% SiO, (Fig. 1; Hildreth, 1981). The total range in silica concentration (57-78 wt.% SiO,) for the Rainier Mesa Tuff is the largest thus far documented for an ash flow tuff (Mills and Rose, 1986) and

3.6. Data Major-element analyses were performed on 124 pumice fragments of the Rainier Mesa Tuff and on 71 pumice fragments of the Ammonia Tanks Tuff. Major-element variation diagrams are presented in Figs. 3 and 4 for the Rainier Mesa Tuff and Ammonia Tanks Tuff, respectively. Major-element varia-

Rainier Mesa Tuff

60

65

70

75

8055

60

Si02 Fig. 3. Major-element

193

65

70

75

80

Si02 oxide variation diagrams

for the Rainier Mesa Tuff.

194

J.G. Mills Jr. et al. /Journal

of Volcanology and Geothermal Research 78 (1997) 185-208

Ammonia Tanks Tuff

0.4 P,Os

0.2

0.0

6 5

60

65

70

sii Fig. 4. Major-element

75

60 55

60

65

70

75

v

60

SiO 2

2

oxide variation diagrams for the Ammonia Tanks Tuff.

greatly expands the whole-rock compositional range reported by Byers et al. (1976a,b). Major-element data for the Ammonia Tanks Tuff range from 59 to 78 wt.% SiO, with the data clustering into two major groups: a mafic group (59-74 wt.% SiO,) and a felsic group (75-78 wt.% SiO,). Using the Total Alkali-Silica classification system of Le Bas et al. (1986), pumice fragments from each member of the Timber Mountain Group range from trachyandesite to rhyolite (approximately 57 to 78 wt.% SiO,). In this study, all pumice fragment compositions that

have anhydrous SiO, concentrations greater than 74 wt.% SiO, will be classified as high-silica rhyolite. Chemical discrimination between the Rainier Mesa Tuff and Ammonia Tanks Tuff can be done on the basis of alkalis vs. SiO, for those pumice fragments with a bulk SiO, concentration less than 75 wt.%, and with selected trace elements (Fig. 2). Mills (1991) and Cambray et al. (1995) showed also that CaO, and Sr and Ba can also be used to discriminate between the Rainier Mesa and Ammonia Tanks tuffs.

J.G. Mills Jr. et al./ Journal of Volcanology and Geothermal Research 78 (1997) 185-208

195

Ammonia Tanks Tuf’ff

Rainier Mesa Tuff

--,

30 I

Th

Th 20

10 Hf mu 0 ” c,....,. -55

5

-L.I_J” 60

65

70

75

55

60

SiO 2

Fig. 5. Yb, La, Th, Hf variation diagrams diagrams for each tuff.

70

75

"

Sii 2

for the Rainier Mesa and Ammonia

For the Rainier Mesa Tuff, trace-element concentration ranges are also large (Fig. 5). On several trace-element plots (Rb, SC, Y, Nb, Th and La), the high-silica rhyolite data can divided into two populations (Fig. 5; Cambray et al., 1995). Plots of La against Th (Fig. 6) are especially useful for distinguishing the different populations of magmas in the Rainier Mesa Tuff. For this paper, the designation low-Th, high-silica rhyolite and high-Th, high-silica rhyolite will be used to identify the respective highsilica rhyolite components. Th was chosen because the two high-silica rhyolite types form discrete popu-

65

Tanks tuffs. Note the three compositional

groups in all of the

lations using this element (Figs. 5 and 6). Altematively, Nb and Rb could also be used to discriminate between the two high-silica rhyolite components. For the Ammonia Tanks Tuff, the trace elements Ba, Zr, Yb, Th, La and Hf have different trends in concentration than do the same elements in the Rainier Mesa Tuff (Figs. 5 and 6). In addition, the absolute abundance of Zr, Hf, Ba, Nb, La and Ce is significantly higher in the Ammonia Tanks Tuff than in the Rainier Mesa Tuff. In the Ammonia Tanks Tuff, the trace-element data can be used to further divide the mafic pumice fragment population into

J.G. Mills Jr. et al. /Journal

196

of Volcanology and Geothermal Research 78 (1997) 185-208

Table 3 (a) Modal petrography

of the Rainier Mesa Tuff

Sample

Pumice (wt.% SiO,)

PLG

R23-21 R23-11 R23-24 Rll-9 h

51 59 60 61

R8-45 R8-23 ’

SAN

QTZ

BIO

HRN

10.3 18.4 15.6

3.0 2.2 1.7

66 67

15.9 C

0.8

R8-25 R8-39 R8-24- 1 R8-26 h

67 61 68 68

28.0 ’ 13.9 c 20.7 ’

Rl l-2 R25-21-l R8-44 R23-4 ’

68 68 69 69

11.5 7.8 7.1

13.1 1.9 0.8

R23-13 Rll-3’

70 75

1.4

5.8

1.0

2.0

trace

Rl8-2 Rl l-7 R23-22 ’

15 76 76

2.4 0.4

0.1 5.5

2.3 3.7

0.1 trace

trace

R21-44 Rl8-3 Rl8-8 b

76 77 77

2.6 4.8

2.8 4.8

1.3 1.7

0.4 0.2

Rl8-16 R21-40 Rl8-1 ’

71 78 78

0.6 1.8 ’

6.8

6.2 trace

0.3 trace

QTZ

BIO

(b) Modal petrography

CPX

MT/IL

DRE a

Point count

6.4 6.1 5.0

0.1

1.7 2.4 0.9

21.4 29.7 23.3

1000 1000 1000

trace

0.4

0.8

17.9

1000

2.8 2.1 3.0

0.7 0.1 0.1

0.8 0.6 0.5

32.3 16.7 24.6

1000 1000 1000

1.5 1.7

0.3 0.4 trace

0.3 0.8 0.1

26.1 12.6 9.6

1000 1000 1000

0.5

0.7

17.5

1000

trace 0.3

4.9 9.9

1000 1000

0.3 trace

7.4 11.5

1000 1000

0.2 0.2

14.1 2.0

1000 1000

CPX

MT/IL

DRE a

Point count

1.6

trace

of the Ammonia Tanks Tuff

Sample

Pumice (wt.% SiO,)

PLG

SAN

A5-75 A5-69 A5-63 A5-31 ’ A5-72 A5-30 A5-7 I A21-1 ’ A5-58 A5-9 A4-70 ’ A5-35 A5-68 A21-5 b A5-29 A5-79 ’ A5-3 ’ A21-18’ A5-56 b

59 61 62 63 64 65 67 69 69 70 70 73 74 75 77 77 78 78 78

21.5 22.3 ’ 19.3 c

21.5

2.1 2.5 2.5

1.2 0.2 0.7

1.2 0.2 1.1

26.6 24.8 23.6

1000 1000 514

11.3 14.8 12.5

0.3 4.7 10.5

2.3 2.3 0.6

0.3

0.3 trace 0.9

15.2 22.1 24.4

500 502 503

6.3 7.6

7.0 1.7

0.3 0.3

trace trace

13.8 9.6

501 500

4.8 1.5

3.8 7.0

0.3 trace

trace trace

8.9 8.5

500 458

2.8

1.4

trace

5.3

500

a DRB = Dense Rock Equivalent. ’ Samples analyzed by electron microprobe ’ All feldspar grains counted as plagioclase.

only.

1.1

HRN

0.3

J.G. Mills Jr. et al. /Journal

of Volcanology and Geothermal Research 78 (1997) 185-208

~Ammonia Tanks Tuff_ i

I

2ot

,dY’

La

.rnlfl

IntermediateSilica

Ll

101

r1

:

3’

_~

1

40

30

20

10

Th

Rainier Mesa Tuff 1

i La

Lower-SilicaGroup

c

1

40

Fig. 6. La versus Th for the Rainier Mesa and Ammonia Tanks tuffs. Note the three distinct groups for each tuff. This data and those in Fig. 5 can be used to reject fractional crystallization of the low-silica magma to produce the high-silica magmas in the Rainier Mesa Tuff. Neither fractional crystallization or magma mixing can be rejected as methods for producing the intermediate to higher silica magmas in the Ammonia Tanks Tuff from this data. The low-silica group in the Ammonia Tanks Tuff has a trend at a right angle to the trend in the intermediate- and high-silica groups, and this may indicate mixing between the low- and high-silica magma batches (see text).

mafic and intermediate groups. Thus, based on the major- and trace-element data, the Ammonia Tanks Tuff can be divided into low-, intermediateand high-silica groups (Figs. 5 and 6). 4. Mineralogy Phenocryst assemblages in the Rainier Mesa Tuff and Ammonia Tanks Tuff are similar. Plagioclase,

197

sanidine, quartz, biotite, clinopyroxene, orthopyroxene and Fe-Ti oxides make up the bulk of the phenocryst phases present in each tuff. The trachyandesites of both tuffs are sanidine and quartz free whereas the high-silica rhyolites of each tuff are clinopyroxene/orthopyroxene free. In both tuffs, orthopyroxene occurs as irregularly shaped cores in many of the clinopyroxene grains. Hornblende is present in significant quantities in the Rainier Mesa Tuff trachyandesite and in trace quantities in the high-silica rhyolite of both the Rainier Mesa and Ammonia Tanks tuffs (Mills, 1991; Saltoun, 1995). Olivine does not occur as forsteritic phenocrysts within the Timber Mountain Group with the exception of one grain in the Ammonia Tanks Tuff. Manganese-rich fayalites, however, are present in the Ammonia Tanks Tuff and are unusual in that they have not been documented in volcanic rocks prior to this occurrence. Manganese-rich fayalites are more commonly associated with Mn-rich metasomatized sedimentary rocks such as those at the Bluebell Mine, British Columbia (Mossman and Pawson, 1976; Mills and Rose, 1991). Trace phenocryst phases are also similar between tuffs. Apatite, chevkinite, perrierite and zircon are common to both tuffs while monazite occurs only in the Rainier Mesa Tuff and titanite occurs only in the Ammonia Tanks Tuff. The modal petrography of each tuff is presented in Table 3a and b along with the Dense Rock Equivalent modal percentages for each pumice fragment sample. Table 4 contains information on the range of composition of each phenocryst phase as analyzed by electron microprobe and the pattern of zoning within particular phenocryst species. Detailed petrographic analyses and compositional data are presented and discussed by Mills (1991). A comprehensive and detailed study of the mineralogy, mineral chemistry and glass chemistry for whole-rock samples of the Timber Mountain Group has also been performed by Warren et al. (1989). Composition plots of plagioclase and sanidine are presented in Fig. 7. These plots show that the plagioclase and alkali feldspar phenocrysts have a similar compositional range in the two tuffs. However, detailed analyses of feldspars from the high- and lowTh, high-silica Rainier Mesa Tuff (Saltoun, 1995) show that there are differences in the plagioclase

198

J.G. Mills Jr. et al. /Journal

of Volcanology and Geothetnzat Research 78 (1997) 185-208

Table 4 Phenocryst

abundances

and compositions

for the Timber Mountain

Phenocryst species

Range of occurrence (wt.% SiO,)

Rainier Mesa Tuff Plagioclase

51-17

Sanidine

68-77

a

Group

Compositional range b (mafic to felsic) ’

Abundance range, DRE d (mafic to felsic)

Zoning pattern

Edge An sz -An, 3 Core An,,-An,, Edge Or,,Cna *-

28.0-I .8%

normal

13.1-0.1%

none

3.0%-trace 1.O-6.2% O-l-0.5%. trace

normal

none

Qr47Cn2.z Core Or,,Cn, Biotite Quartz Clinopyroxene

57-77 10-71 60-70.76

Qr5sCn3.4 Mg# 0.70-0.60 N.A. Wo,En,,Fs,Wo37En47Fs,6 Mg# 0.77-0.72

s-

64-74

Wo,En,,,Fs,,-

trace

Amphibole

57-66,70,17

WozEnssFs,4 Mg# 0.71-0.66 Mg# 0.71-0.63

6.4%-trace,

Ammonia Tanks Tuff Plagioclase

59-71

Sanidine

6477

Biotite Quartz Clinopyroxene

Orthopyroxene

Orthopyroxene

Amphibole Olivine

normal

trace, trace

none

21.54-2.19%

normal

0.3- 1.40% (10.47% max.) 2.66%-trace 1.12% 1.20-0.26%

none

59-74 17 59-69

An,oAb&‘zs Edge Or,, -Or,, Core Or,-Or,, Mg# 0.67-0.80 N.A. Wo,En,,Fs,-

normal

64-69

Wo,,En.t,Fsr, Mg# 0.82-0.72 Wo,En,,Fs,,-

trace

none

trace trace

none none

59 74,71

Edge At+-An,, Core An,,-An,, Grdmass

WozEn,,Fs,, Mg#0.74-0.70 Mg# 0.70 ‘%7

normal

Tel*-Te,, N.A. = not analyzed. a Range of occurrence units are based upon whole pumice fragment silica concentrations (anhydrous). b Compositional Range units are based upon electron microprobe analyses. ’ Matic to felsic refers to the whole pumice fragment composition (i.e., trachyandesite to rhyolite). d Abundance range is based upon Dense Rock Equivalent (DRE) calculations.

from each of these groups. The average plagioclase from the low-Th group is An,,, whereas those from the high-Th group is An,,. About 5% of the plagioclase from the high-silica Rainier Mesa Tuff are compositionally zoned from Ca-rich cores to Na-rich rims. Fig. 7 shows the zoned plagioclase grains from the high- and low-Th groups fall on different trends. The sanidine grains from the high- and low-Th groups are identical.

The clinopyroxene and orthopyroxene phenocrysts in both the Rainier Mesa Tuff and Ammonia Tanks Tuff are compositionally similar. The clinopyroxene grains range in composition from to Wo,,En,, Fs ,6. Clinopyroxene Wo&n,,Fs, grains are usually zoned with a Wo mole fraction range of up to kO.5. Orthopyroxene cores in the clinopyroxene grains range in composition from Wo,En,,Fs,, to Wo,En,5Fs,,.

J.G. Mills Jr. et al./ Journal of Volcanology and Geothermal Research 78 (1997) 185-208

199

Or

Ab

Rainier Mesa Tuff

An Or

Or

An

Ab

Rainier Mesa Tuff

An

Pb

Ammonia Tanks Tuff

Fig. 7. Feldspar compositions from the Rainier Mesa and Ammonia Tanks tuffs. The upper left diagram is all of the feldspars from the Rainier Mesa Tuff (Mills, 1991). The lower left diagram is the zoned feldspars from the low- and high-Th, high-silica Rainier Mesa Tuff. Note how each of these fall on separate evolutionary trends. The lower right diagram is all of the feldspars from the Ammonia Tanks Tuff.

Biotites in the Rainier Mesa Tuff have a lower Mg# [Mg/(MgO + FeO) wt.%] value when compared to biotites from similar pumice fragment compositions in the Ammonia Tanks Tuff, and occur on entirely different TiO,-Mg# trends (Fig. 8). Biotites from the high-silica Rainier Mesa Tuff occur in two compositional groups reflecting the high-Th and low-Th magma groups (Fig. 8). 4.1. Pumice fragment

matrix glass

Compositional data for the matrix glass of the pumice fragments in the Timber Mountain Group reveal distinct differences between samples of the matrix glasses from the Rainier Mesa Tuff and Ammonia Tanks Tuff (Fig. 9). The matrix glass for both members becomes more silicic from the trachyandesite to high-silica rhyolite. However, for wholepumice compositions of less than 70 wt.% silica, the matrix glass compositions are distinctly different for each tuff; the glass compositions from the Rainier Mesa Tuff are nearly constant, averaging about 71

wt.% SiO,, whereas the glass compositions from the Ammonia Tanks Tuff become more siliceous (65-70 wt.% SiO,). For the most mafic pumice fragments, the Ammonia Tanks has a higher concentration of CaO, Fe0 and TiO, than the Rainier Mesa Tuff, and for these oxides the matrix compositions as a whole, become more evolved as whole-pumice silica compositions increase. In the Ammonia Tanks Tuff for the high-silica pumice fragments, the range in silica content of the matrix glasses is relatively constant (76 wt.% SiO,).

5. Geothermometry

and geobarometry

In addition to trace-element evidence, Fe-Ti oxide temperature and oxygen fugacity data for magmas of the Timber Mountain Tuff provide evidence for the existence of separate magma batches in the system over time. New data from this study and re-calculation of data from Lipman (197 1) are presented below.

J.G. Mills Jr. et al./Journal

200

of Volcanology ana’ Geothermal Research 78 (19971 185-208

High-Th Magma

0” I=

LJ 0.45

0.55 0.50 MgO/(MgChFeO)in Biotite

0.60

Fig. 8. Biotite compositions of the Rainier Mesa and Ammonia Tanks tuffs. The upper diagram is MgO/(MgO+ FeO) versus TiO, in biotites from pumice fragments. Note that the biotites from the Rainier Mesa Tuff have a lower MgO/(MgO +FeO) than the biotites from the Ammonia Tanks Tuff (data from Mills, 1991). The lower diagram is TiO, versus MgO/(MgO+FeO) in biotites from the high-silica portion of the Rainier Mesa Tuff. Note how the high- and low-Th groups have distinct biotite compositions

(data from Saltoun,

1995).

Ulvospinel and ilmenite compositions of Lipman (197 1) for the Rainier Mesa Tuff were calculated using QUILF4 (Table 5; Anderson et al., 1993). The calculated emplacement temperatures and oxygen fugacities for Lipman’s data range from 742°C - 13.68 log fo, for rhyolitic samples (77 wt.% SiO,) to 843°C - 11.21 log fo, for samples of trachydacite (68 wt.% SiO,). The calculated temperatures using Lipman (197 1) Ammonia Tanks Tuff data range from 740°C - 13.60 log fo, in the high-silica rhyolite to 864°C - 11.33 log f,, in the trachydacite. New temperature and oxygen fugacity calculations from the present study using the QUILF4 program (Anderson et al., 19931, exhibit a significantly larger range in values than those of Lipman (1971)

(Table 5). The Rainier Mesa Tuff varies from 708°C - 14.62 log fo, for the high-silica rhyolite to 878°C - 10.84 log fez for the trachyandesite. The Ammonia Tanks Tuff exhibits a similar range in temperature and oxygen fugacity (Table 5). Minimum temperature and oxygen fugacity data in the Ammonia Tanks Tuff occur in the high-silica rhyolite at 728°C - 13.92 log f,, while the maximum temperature and oxygen fugacity occur in the trachyandesite at 883°C - 11.35 log fo,. Samples from this study for the Rainier Mesa Tuff, plot as three distinct populations on a temperature versus SiO, and Th diagram (Fig. 10). Fig. 10 shows the temperatures that were calculated from Fe-Ti oxide and two-feldspar feldspar geothermometry (see below). For the feldspar geothermometry, a pressure of 4.2 kbar was assumed based on quartzamphibole geobarometry (see below). The low-silica, high-temperature samples have a nearly constant temperature (Fe-Ti oxide temperature of 869”C), whereas the high-silica samples occur as two distinct populations. In addition, three samples from the Rainier Mesa Tuff had temperatures calculated from pyroxene geothermometry using QUILF4 (at 4.2 kbar) and were compared to the Fe-Ti oxide and feldspar temperatures. The Fe-Ti oxide, feldspar and pyroxene temperatures, respectively, for the three samples are: R8-44, 856, 852 and 892°C; R23-13, 862, 846 and 871°C; and R8-45, 861, 846 and 893°C. The Ammonia Tanks Tuff has a continuous variation of calculated temperatures with SiO,. This is very different than the distinct temperature-compositional groups in the Rainier Mesa Tuff. Fig. 10 shows both the Fe-Ti oxide and feldspar (calculated at 4.2 kbar) temperatures versus SiO, concentration of the pumice fragments for the Ammonia Tanks Tuff. The coexistence of sanidine and plagioclase in these samples allowed for equilibrium calculations between the orthoclase, albite and anorthite components of these feldspars. These data are presented in Table 6. Using the feldspar thermometry model of Fuhrman and Lindsley (1988) in the SOLVCALC program by Wen and Nekvasil (1994), ternary feldspar temperatures for the high-silica samples were calculated. As the ternary-feldspar model is pressure dependent, the necessary geobarometric data were

J.G. Mills Jr. et al./Joumal

0

1 CaO

of Volcanology and Geothermal Research 78 (1997) 185-208

2

Wt. % in Glass

3

00

0.4

0.2

~-a+

Wt. %

201

0.6

in Glass

Fig. 9. Composition of the glass matrix versus SiO, compositions of pumice fragments for the Rainier Mesa and Ammonia Tanks tuffs. Note that for the lower silica Rainier Mesa Tuff the glass matrix compositions are nearly constant. Also note that for the pumice compositions below 70% SiO,, the matrix glass from the Ammonia Tanks is less evolved than the matrix glass from Rainier Mesa Tuff. This precludes the evolution of the Ammonia Tanks liquid by fractional crystallization of the Rainier Mesa liquid.

obtained using coexisting amphibole-quartz phenocrysts from R23-13, a high-silica pumice fragment. A pressure estimate of 4.2 kbar obtained from this technique was used in the ternary-feldspar data reduction (see below). However, using a lower pressure reduced the lower temperature by about 30°C and had minimal effect on the higher temperature. For example, sample Rl l-7 has a calculated temperature at 4 kbar of 735°C; at 1 kbar it is 706°C. Sample R8-44 has a calculated temperature at 4 kbar of 844°C; at 1 kbar it is 837°C. Data from the ternary-feldspar calculations are concordant with the Fe-Ti oxide temperature data and provide further support for discrete magma batches in the high-silica portion of the magmatic system. Oxygen fugacity values as compared to the QFM buffer, do not indicate a change in fo, with respect to the different magma batches in either tuff (Lange, pers. commun., 1996). AQFM values for all of the magma types in the Timber Mountain Group, with the exception of the trachyandesite in the Ammonia Tanks Tuff, are relatively uniform and only range from 1.9 to 2.2 AQFM. The trachyandesite of the

Ammonia Tanks Tuff has a AQFM value of 1.5. These data do not support or reject the hypothesis of discrete magma batches in the Timber Mountain Group. 5.1. Amphibole-quartz

geobarometry

Using the method of Hammarstrom and Zen (1986), six homblendes in equilibrium with quartz, from sample R23-13 (70 wt.% SiO,), were used to estimate the pressure of hornblende crystallization. One of the six grains was analyzed along the edge and within the core. The calculated pressures of formation from these grains range from 4.8 to 3.9 kbar. The average value is 4.2 kbar with a standard deviation of 0.28 kbar. Pressure estimates calculated from the analyzed grain both near the edge and in the core vary by 0.2 kbar (edge = 4.0 kbar, core = 4.2 kbar). 5.2. Melt inclusions Vogel and Aines (1996) measured H,O, CO, and major-element concentrations from melt inclusions

202

J.G. Mills Jr. et al./ Journal

Table S Fe-Ti oxide temperature Sample No. a

and oxygen fugacity

SiO, b

Temp

(wt.%)

(“a

of Volcanology

and Geothermal

data for the Timber Mountain f 02

Research

78 (1997) 185-208

Group using QUILF4 (Anderson

Sample No.

SiO, b (wt.%)

Temp

et al., 1993)

fox

(“0

Rainier Mesa Tuff LIP17 LIP14 LIP15 LIP16 R18-16 Rll-7 RlS-2 R21-5 R23-13 LIP1 8 R8-44 R21-1 LIP19 R8-25 R8-45 R23-26 Rll-5 R23-24 R21-13

78 77 77 77 71 76 75 75 70 69 69 69 68 67 66 64 61 60 57

Ammonia Tanks Tuff A5-3 78 A5-37 78 LIP20 77 LIP2 1 77 LIP22 17 LIP23 71 A5-29 77 A5-68 74 A5-35 73 A4-70 72 A5-9 70 A5-58 69 LIP24 68 LIP25 66 A5-30 66 A5-72 64

760 746 742 750 799 742 761 809 862 842 856 869 843 798 861 879 871 878 878

- 13.09 - 13.46 - 13.68 - 13.31 - 12.43 - 13.60 - 13.11 - 11 .I7 - 11.22 - 11.26 - 11.20 -11.02 - 11.21 - 12.24 -11.02 -11.01 - 10.83 - 11.04 - 10.84

710 728 748 747 745 740 723 739 779 776 793 797 864 855 871 883

- 14.33 - 13.92 - 13.48 - 13.35 - 13.49 - 13.60 - 14.22 - 14.32 - 12.70 - 13.06 - 12.53 - 12.82 - 11.33 - 11.08 - 11.18 -11.35

High-Th high-silica R18-16 R26-14 R18-2 R23-7

Low-Th high-silica R18-6 Rll-7 R18-15

rhyolite 75.40 74.61 72.70 72.48

801 818 787 810

-

12.53 11.75 13.49 Il.81

708 735 723

- 14.62 - 13.70 - 14.26

rhyolite 75.00 74.07 73.13

a Data from Mills (1991), Saltoun (1995) b Whole pumice silica concentration.

in phenocrysts of the Topopah Spring Member of the Paintbrush Group and Rainier Mesa and Ammonia Tanks tuffs of the Timber Mountain Group. The goal of their study was to determine if volatile gradients were present in the pre-eruptive magmatic system for each ash flow sheet. Using melt inclusions within feldspar and quartz phenocrysts, they determined that

the inclusions from the Rainier Mesa Tuff contained a maximum of 659 ppm CO, and 5.9% H,O while the inclusions from the Ammonia Tanks Tuff contained a maximum of 180 ppm CO, and 4.67% H,O. The majority of the inclusions contained little or no CO, and low concentrations of H,O indicating decompression of the melts and loss of volatiles. The

J.G. Mills Jr. et al. /Journal

presence of CO, in many of the melt inclusions does indicate, however, that the magmas most likely evolved a gas phase prior to eruption. In addition, Vogel and Aines (1996) determined that the composition of melt inclusion glasses from the mafic pumice phenocrysts represented a range of compositions whereas melt inclusion glasses from the most felsic pumice phenocrysts had very limited compositions. Vogel and Aines (1996) interpreted this to represent mixing of the mafic magma with a Ammonia Tanks Tuff SD --.--- ~~~~~ .-. ~~--~

. 7

E’ 0.

l

0 l

0 . 0 l

b

203

of Volcanology and Geothemal Research 78 (1997) 185-208

Low-Si, Oxide Low-SF&d Lav-Th, hMi. Oxide Lmv-Th, hi-Sl. Feld Low-Th, hiSi. Oxide Hi-Th, hiSi, Feld

SiOi

Rainier Mesa Tuff Y---7

Table 6 Mean compositions of coexisting and Ammonia Tanks tuffs

feldspar

Sample a

An

Ab

Or

Cn

R23-7 ks R23-7 pl R23-8 ks R23-8 pl R23-4 ks R23-4 pl R18-14 ks RlS-14 pl R26- 19 ks R26-19 pl R18-9 ks R18-9 pl R18-15 ks R18-15 pl Rll-7 ks R-l 1-7 pl R18-2 ks R18-2 pl R18-6 ks R18-6 pl R8-44 ks R8-44 pl R23-13 ks R23-13 pl R8-45 R8-45 pl R21-1 R21-lpl Rll-5 ks RI l-5 pl R23-6 ks R23-26 pl R21-5 ks R21-5 pl

0.014 0.189 0.014 0.206 0.012 0.310 0.011 0.148 0.016 0.212 0.008 0.136 0.007 0.129 0.011 0.134 0.019 0.177 0.020 0.190 0.021 0.259 0.024 0.224 0.03 1 0.248 0.015 0.478 0.03 1 0.354 0.026 0.267 0.016 0.185

0.368 0.716 0.377 0.714 0.383 0.630 0.350 0.780 0.369 0.715 0.366 0.787 0.360 0.794 0.388 0.792 0.350 0.755 0.344 0.734 0.382 0.661 0.374 0.674 0.375 0.667 0.347 0.498 0.367 0.599 0.369 0.649 0.373 0.722

0.617 0.095 0.609 0.080 0.597 0.059 0.639 0.072 0.614 0.073 0.623 0.078 0.633 0.076 0.599 0.074 0.636 0.067 0.642 0.075 0.586 0.078 0.581 0.101 0.565 0.081 0.633 0.023 0.569 0.045 0.581 0.08 1 0.609 0.100

0.003 0.000 0.000 O.C@O 0.000 0.001 0.000 0.001 0.002 0.000 0.003 0.000 0.000 0.001 0.000 0.002 0.002 0.000 0.002 0.001 0.019 0.003 0.021 0.001 0.029 0.003 0.005 0.001 0.033 0.002 0.024 0.003 0.003 0.000

a ks = alkali feldspar; pl = plagioclase

Th Fig. 10. The upper diagram is of emplacement temperatures calculated from Fe-Ti oxide thermometry and coexisting feldspar thermometry (at 4.2 kbar) for the Ammonia Tanks Tuff. Note the continuous distribution of temperature and composition. Contrast this distribution with those in the middle and lower diagrams for the Rainier Mesa Tuff (all data from Mills, 1991). The middle and lower diagrams are for emplacement temperatures calculated from Fe-Ti oxide thermometry and coexisting feldspar thermometry (at 4.2 kbar) for the Rainier Mesa Tuff plotted against SiO, and Th. Note how the three magma batches occur at three distinct temperatures (see text for discussion; all data from Mills, 1991; Saltoun, 1995).

from the Rainier Mesa

feldspar.

more silicic magma. In both tuffs some melt inclusions are significantly more evolved than the matrix glasses. This is consistent with mixing of a more evolved magma, and is difficult to produce by crystal fractionation.

6. Discussion 6.1. Major- and trace-element data of pumice fragments The major- and trace-element data for the Rainier Mesa Tuff and the Ammonia Tanks Tuff document

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the wide range of magmatic compositions present in the Timber Mountain system (Figs. 3 and 4). Within the Rainier Mesa Tuff at least three distinct chemical groups are present (Cambray et al., 19951, a lower silica group and two high-silica groups (Fig. 6). The Ammonia Tanks Tuff (Fig. 4) can be broken into low-, intermediate- and high-silica groups based on Yb, Th, La and Hf versus SiO, plots (Figs. 5 and 6). However, in contrast to the data from the Rainier Mesa Tuff, these data do not permit rejection of crystal fractionation within the Ammonia Tanks magma for the origin of the chemical variation. On all of these plots, magmas with less than 65 wt.% SiO,, and greater than 75 wt.% SiO, are distinct and on the La versus Th or SiO, plots, the low-silica samples occur on distinctly different trends than the intermediate- and high-silica samples. These are best explained as distinct magma batches with mixing of the low-silica and high-silica magma batches to produce the intermediate magma compositions in the Ammonia Tanks Tuff. For samples more mafic than rhyohte, the Ammonia Tanks ash flow sheet has a lower CaO and a higher alkali content than the Rainier Mesa ash flow sheet (Fig. 2). Except for the highest silica samples there are major differences in trace elements between the two ash flow sheets (Figs. 5 and 6). High concentrations of Ba, Hf and La (Figs. 2 and 5) in the lower silica portion of the Ammonia Tanks Tuff compared to similar composition magmas in the Rainier Mesa Tuff clearly demonstrates that these magmas could not have been derived by fractional crystallization of the Rainier Mesa magma. The younger Ammonia Tanks magma represents the influx of a new magma batch.

6.2. Chemical ments

data of matrix glass of pumice frag-

For pumice fragments of lower than 70 wt.% SiO,, the Ammonia Tanks Tuff is distinctly more mafic than the Rainier Mesa Tuff (Fig. 91. This clearly demonstrates that the younger, more mafic composition magmas from the Ammonia Tanks Tuff could not be derived by crystal fractionation or magma mixing of the older magmas from the Rainier Mesa Tuff.

6.3. Mineralogic

data

In general, the mineral assemblages of the two tuffs are similar. The major differences are the occurrence of hornblende and monazite in the Rainier Mesa Tuff and the occurrence of titanite in the Ammonia Tanks Tuff. There are major differences in the chemistry of the biotites in the two tuffs. Biotites from the Ammonia Tanks Tuff have higher Mg numbers than pumice fragments of the same general silica content in the Rainier Mesa Tuff (Fig. 8). This is inconsistent with a crystal fractionation model for the origin of the Ammonia Tanks magmas from Rainier Mesa magmas. The distinct plagioclase and biotite compositions for the two high-silica magma groups of the Rainier Mesa Tuff (Figs. 7 and 8) demonstrate that they evolved from separate magma batches. 6.4. Emplacement

temperatures

Thermal data derived from the analysis of Fe-Ti oxides and coexisting pyroxenes and ternary-feldspar thermometry demonstrate the presence of magma batches in the Rainier Mesa Tuff. These data show that the three magma types defined by the trace-element chemistry occur in three distinct temperature groups (Fig. 10). Using the Fe-Ti oxide temperatures, the lower silica group has a mean temperature of 869°C (T = 9.5, the low-Th, high-silica group has a mean temperature of 723°C (T = 15, and the highTh, high-silica group has a mean temperature of 804°C (T= 13.3. Fig. 10 shows the temperatures for the three magma batches and these data clearly document three discrete temperatures for the magma batches. The Fe-Ti oxide emplacement temperatures for the Ammonia Tanks Tuff have a linear temperature variation with composition (Fig. 10) ranging from 880°C for the low-silica samples to 710°C for the high-silica samples. In contrast to the Rainier Mesa Tuff, the Ammonia Tanks Tuff contains no discrete grouping of temperature with respect to composition.

7. Conclusion The Timber Mountain Group is an example of a highly zoned (57-78 wt.% SiO,), voluminous mag-

J.G. Mills Jr. et al./ Journal of Volcanology and Geothennal Research 78 (1997) 185-208

matic system (> 2200 km3). The high degree of zonation reflected in tuffs of the Timber Mountain Group is consistent with the model of De Silva and Wolff (1995) that in situ differentiation is not the dominant process in large volume magmatic systems that exhibit large ranges of compositional zonation. De Silva and Wolff (1995) argue that small aspect ratio magmatic systems, such as the sill-shaped Timber Mountain system (Cambray et al., 1995) may be highly zoned, but that the origin of the zonation must be due to other factors such as compositionally distinct magma batches with independent origins. Within the Timber Mountain Group, the Rainier Mesa Tuff formed from three chemically, mineralogically and thermally distinct magma batches. The temperature differences between the magma batches could not have been maintained for any length of time in the magma chamber (Turner and Campbell, 1986; Martin et al., 1987), indicating that eruption of the magma occurred almost immediately after emplacement of the magma batches in the chamber. In contrast the Ammonia Tanks Tuff formed from magmas with a linear distribution of temperatures with respect to composition. This would be consistent with chemical and thermal diffusion immediately after emplacement of the different magma batches (a low- and high-silica magma) into the chamber. Some of the high-silica Ammonia Tanks magma may be due to mixing of the low-Th and high-Th, high-silica Rainier Mesa magma (Cambray et al., 1995), however, Huysken et al. (1994) and Huysken (1996) have shown that some of the high-silica tephra-fall samples that immediately preceded the eruption of Ammonia Tanks Tuff are unique. These data are still consistent, however, with limited magma mixing between the low- and high-silica Ammonia Tanks magma to produce the intermediate compositions. The lower silica portion of the Ammonia Tanks Tuff could not have been derived from Rainier Mesa magmas and represents a new magma batch. Farmer et al. (1991) concluded that the Ammonia Tanks Tuff was derived from an isotopically different source than the Rainier Mesa Tuff based on the difference in the lNd and “Sr/ 86Sr values (cf. Tegtmeyer and Farmer, 1990; Farmer and Tegtmeyer, 1991). Although Farmer et al. (1991), interpreted differences in oxygen isotopes of the tuffs to indicate assimilation of different wall rocks, these data are also

205

consistent with different magma batches for each of the two tuffs.

Acknowledgements

Rebecca Lange is sincerely thanked for her careful and constructive review of the manuscript. A second anonymous reviewer is thanked for their helpful comments. The Earth Sciences Department of Lawrence Livermore National Laboratory is gratefully acknowledged for their support of research on the origin and evolution of high-level magmatic systems in southwest Nevada. Part of this research has been supported by NSF EAR890361 to TAV.

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