I 

I 

I 

The Seaman Volcanic CenterA Rare Middle Tertiary 

Stratovolcano in Southern Nevada 

U.S. GEOLOGICAL SURVEY BULLETIN 2052 
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Ave. 
The Seaman Volcanic CenterA Rare Middle Tertiary 

Stratovolcano in Southern Nevada 

By EDWARD A. DU BRAY 

U.S. GEOLOGICAL SURVEY BULLETIN 2052 

Geology and geochemical evolution of a rare, 

well-preserved middle Tertiary stratovolcano 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1993 
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary 

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

For sale by 

USGS Map Distribution 

Box 25286, Building 810 

Denver Federal Center 

Denver, CO 80225 

Any use of trade, product, or finn names in this publication is for descriptive purposes only and 

does not imply endorsement by the U.S. Government. 

Library of Congress Cataloging-in-Publication Data 

Du Bray, E. A. 

The Seaman volcanic center ; a rare middle Tertiary stratovolcano in southern Nevada I by Edward A. du Bray. 

p. em. - (U.S. Geological Survey bulletin; 2052) 

Includes bibliographical references. 

Supt. of Docs. no.: 1.19.3:B2052 

1. Volcanoes-Nevada-Lincoln County. 2. Geology, Stratigraphic-Tertiary. 3. Geochemistry-Nevada-Lincoln 

County. I. U.S. Geological Survey. II. Title. III. Series. 

QE75.B9 no. 2052 

[QE524] 

557.3 s-dc20 93-2837 

[551.2'1'0979314] CIP 
CONTENTS 

Abstract................................................................................................................................. 1 

Introduction........................................................................................................................... 1 

Geology of the Serunan Volcanic Center.............................................................................. 3 

Petrography........................................................................................................................... 3 

Geochemistry of the Seaman Volcanic Center..................................................................... 4 

Whole-Rock Chemistry.............................................................................................. 4 

Mineral Chemistry...................................................................................................... 10 

Major Element Least-Squares Fractionation-Mixing Modeling........................................... 11 

Evolution of the Seaman Volcanic Center............................................................................ 12 

Summary and Conclusions................................................................................................... 17 

References Cited................................................................................................................... 18 

FIGURES 

1. Generalized geologic map of the Seaman volcanic center, Lincoln County, Nevada................................................... 2 

2. Photomicrographs showing petrographic features of components of the Seaman volcanic center ... ........................... 5 

3. Alkali-silica classification diagram showing analyses of samples from the Seaman volcanic center........................... 6 

4. Ternary AFM diagram showing compositions of samples from the Seaman volcanic center...................................... 7 

5. Diagrrun showing the variation of rubidium and strontium abundances in samples from the Seaman volcanic 

center.............................................................................................................................................................................. 8 

6. Chondrite-normalized diagram showing compositions of samples from the Seaman volcanic center ......................... 8 

7. Chondrite-normalized extended trace element diagram showing compositions of samples from the Seaman 

volcanic center ... .. ...... ................ .. . . . . . ............. ..... .. ..... .... . . .. . . ... .. ... ................. ... ... ............. .............................................. 9 

8. Enrichment-depletion diagram showing major- and trace-element abundances of the Serunan volcanic center dike 

normalized to those of the lava flow unit....................................................................................................................... 14 

9. Tectonic setting-trace element discriminant variation diagrams for components of the Seaman volcanic center........ 17 

TABLES 

1. Major-oxide and normative mineral composition for samples of the Seaman volcanic center, Lincoln County, 

Nevada........................................................................................................................................................................... 6 

2. Trace element data for samples of the Serunan volcanic center, Lincoln County, Nevada........................................... 7 

3. Instrumental neutron activation data for samples of the Seaman volcanic center, Lincoln County Nevada................. 7 

4. Average major-oxide, normative, and trace-element compositions of components of the Seaman volcanic center, 

Lincoln County, Nevada................................................................................................................................................ 8 

5. Representative mineral compositions for phenocryst phases in samples of the Seaman volcanic center, Lincoln 

County, Nevada.............................................................................................................................................................. 10 

6. Results of major element least-squares crystal fractionation-mixing modeling for samples of the Seaman volcanic 

center, Lincoln County, Nevada.................................................................................................................................... 12 

01 

THE SEAMAN VOLCANIC CENTER-A RARE 

MIDDLE TERTIARY STRATOVOLCANO IN 

SOUTHERN NEVADA 

By EDWARD A. DUBRAY 

ABSTRACT 

The Seaman volcanic center is a composite, middle 

Tertiary stratovolcano in southern Nevada; it is about 10 

km (6 mi) in diameter and has a reconstructed volume of 

about 20 km3 (5 mi3). The center is principally composed 

of interbedded dacite lava and pyroclastic flows whose 

eruption built a stratovolcano that was intruded by rhyolite 

of a hypabyssal core plug and its associated radial dike. 

Almost continuous chemical variation, from the oldest to youngest components of the center, is consistent 

with periodic tapping of a magma chamber whose contents became progressively more evolved by fractionation, 

through sidewall crystallization, of observed phenocryst 

phases. Major- and trace-element abundance variation and 

covariation groupings, mixing-model calculations, and rare 

earth element patterns indicate derivation of magma represented by the core plug and radial dike by fractionation 

and accumulation of hypersthene, calcic plagioclase, hornblende, biotite, apatite, and magnetite from magma represented by the lava flows. The presence of phenocrysts and 

glomerocrysts composed of various phenocryst combinations indicates that the phases noted above were on the 

liquidus and accumulating in crystal-rich zones within the 

chamber. 

The development of a stratovolcano in the southern 

Great Basin, an area in which middle Tertiary volcanism 

was dominated by ash-flow eruption from caldera sources, 

may result from a number of factors. As indicated by the 

scarcity of hydrous silicate phases, particularly in the flow 

rocks, stratovolcano construction may indicate development of a small chamber that contained relatively volatile 

poor magma. Older rocks that host the volcanic center 

contain a relatively low density of intersecting faults. The 

absence of structural features conducive to ash-flow 

caldera formation may also have favored stratovolcano 

formation. Finally, the volume of the Seaman volcanic 

center is an order of magnitude less than that characteristic 

of ash-flow caldera-forming systems. 

INTRODUCTION 

The Seaman volcanic center is clearly denoted as a 

small, circular volcanic feature on the geologic map of 

Nevada (Stewart and Carlson, 1978). The west-central part 

of the Seaman Range is dominated by the Seaman volcanic center (fig. 1 ), a feature first noted and described by 

Tschanz and Pampeyan (1961, 1970) and believed by D.C. 

Noble (University of Nevada-Reno, written common., 

1967, 1970, and 1971, to Ekren and others, 1977) to be a 

partially eroded composite stratovolcano. Information presented here confirms that the Seaman volcanic center is a 

middle Tertiary stratovolcano whose gross morphology is 

remarkably well preserved. 

The Seaman volcanic center, about 200 km (125 mi) 

north of Las Vegas in Lincoln County, Nevada, has a midpoint near lat 37°55' N., long 115°09' W. Ekren and others 

(1977) suggested that the Seaman volcanic center was 

active in late Oligocene and early Miocene time. Its age is 

bracketed between the ages of the underlying Lund Formation (27.9 Ma; Best and Grant, 1987) and overlying 

rhyolite tuff of the Seaman Range (26.0-27.4 Ma; Ekren 

and others, 1977, and Taylor and others, 1989). Rocks of 

the Seaman volcanic center are overlain by the oldest (unit 

1 of du Bray and Hurtubise, in press) of the rhyolite tuffs 

of the Seaman Range. Taylor and others (1989, fig. 8) 

portrayed the tuff of Hamilton Spring, in the adjacent (to 

the east) North Pahroc Range, in a stratigraphic position 

similar to that of unit 1 of the rhyolite tuffs of the Seaman 

Range; Taylor and Bartley ( 1988) presented an age of 27.4 

Ma for the tuff of Hamilton Spring. Assuming that the tuff 

of Hamilton Spring is coeval with unit 1 of the rhyolite 

tuffs of the Seaman Range, development of the Seaman 

volcanic center is restricted to the period between 27.9 and 

27.4 Ma. 

The extent to which the Seaman volcanic center was 

inundated and buried, perhaps completely covered, by 

younger, onlapping volcanic rocks is unknown; however, 

the remarkable morphologic preservation of this middle 
115"12' 115 ·os · 

37"56' 

37"54' 

CONTOUR INTERVAL 50 METERS 0 ~ 1 2 4 KILOMETERS 

0 2 MILES 

a:: 

UJ 

EXPLANATION + SAMPLE LOCATiON AND NUMBER 

l 45 STRIKE AND DIP OF BEDDING 

f AND COMPACTION FOLIATION 

EJ SURFICIAL J DEPOSITS 

G ANDESITE LAVA l 

6 RHYOLITE TUFF OF 

THE SEAMAN RANGE 

~~a RADIAL DIKE !lB CORE PLUG 

~B LAVA FLOWS 

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FORMATION 

~ JOANA LiME STONE J 

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.E~KO I IRENO I 

lo CARSON ELy II 

~CITY • 

' I '"ZEAWAN VOLCANIC CENTER 1 

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""" ' ' VEGAS LAS ~ I ""-,, . r-

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Figure 1. Generalized geologic map of the Seaman volcanic center, Lincoln County, Nevada. Six-digit numbers beginning with 201 indicate sampling sites for 

samples with geochemical data (tables 1-4). 

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GEOLOGY OF THE SEAMAN VOLCANIC CENTER 3 

Tertiary stratovolcano implies that it may have been completely covered and protected from dissection by slightly 

younger volcanic rocks. Miocene to Pliocene Basin and 

Range block faulting and subsequent erosion have stripped 

onlapping rocks from the flanks of the stratovolcano and 

exhumed it. The core of the volcanic center has been 

deeply dissected, and the core plug is exposed. 

The development of an isolated stratovolcano in this 

part of the Great Basin, during a period dominated by ashflow caldera volcanism (Best, Christiansen, and others, 

1989, figs. 3, 7), is unusual. Best, Christiansen, and others (1989, p. 93) noted that "Cenozoic lavas are widely 

scattered throughout the Great Basin* * *lavas occur as 

isolated domes and flows within a sequence of ash-flow 

deposits" and that "In contrast to the more or less contemporaneous San Juan volcanic field* * *large constructional 

volcanoes and associated volcanic debris flows (lahars) are 

absent in most Great Basin eruptive centers." 

Ekren and others ( 1977) suggested that rhyolitic ashflow tuff at Hancock Summit, 58 km (36 mi) to the southwest, and tuffaceous rocks to the north and northwest in 

the Golden Gate Range and to the north in the southern 

Egan Range are derived from the Seaman volcanic center. 

His correlations are implausible, however, for a number of 

reasons. Deposits presumed to be derived from the Seaman volcanic center could have travelled to these relatively distant sites only as ash flows of the type derived 

from caldera-forming eruptions. Evidence of caldera-forming eruptions associated with the Seaman volcanic center 

is absent, and far-travelled ash flows are unlikely to have 

been produced by a stratovolcano. Ash-flow deposits presumed to be derived from the Seaman volcanic center 

should be preserved, at least locally, even if in very thin 

layers in the Seaman Range; however, all of the ash flows 

preserved in the Seaman Range either are from known 

sources unrelated to the volcanic center or are compositionally (E.A. du Bray, unpub. data, 1992) and petrographically (Hurtubise and du Bray, in press, table 3) dissimilar 

to components of the volcanic center. It is unlikely that 

deposits presumed to be derived from the Seaman volcanic center are present in surrounding ranges because 

deposits related to the center are not known in the Seaman 

Range, exclusive of the center itself. For these reasons, the 

correlation offered by Ekren and others (1977) is not 

valid. 

Acknowledgments.-The work reported on here is an 

outgrowth of the mineral resource assessment of the U.S. 

Bureau of Land Management Weepah Spring wilderness 

study area. The author is particularly indebted to J.S. Pallister and G.B. Sidder, whose incisive reviews dramatically improved this report. In addition, G.P. Meeker is 

thanked for assistance rendered in obtaining electron 

microprobe mineral chemistry data and M. W. Bultman is 

thanked for determining densities of samples. 

GEOLOGY OF THE SEAMAN 

VOLCANIC CENTER 

The Seaman volcanic center is about 10 km (6 mi) in 

diameter. Its components include an accumulation, as 

much as 250 m. (820 ft) thick, of lava flows and associated 

lahars and interbedded pumiceous pyroclastic flows, a 

hypabyssal core plug approximately 2 km (1.2 mi) in 

diameter, and a radial dike 50 m (165ft) wide and approximately 4 km (2.5 mi) long. 

The volume of material erupted from the Seaman 

volcanic center is probably between 15 and 20 kffi3 

(3.6-4.8 mi3). These estimates are the result of computing 

the volume of a cone having dimensions similar to those 

of the Seaman volcanic center. A volume of 15 kffi3 was 

computed for a cone whose top is truncated (to represent 

the present-day height of the volcano), whereas a volume 

of 20 km3 was computed for a cone whose top was reconstructed by upward extrapolation of the stratovolcano 

flanks. By analogy to the high-level apophyses inferred by 

Sillitoe (1973) to intrude the bases of stratovolcanoes that 

develop above porphyry copper systems, the unexposed 

part of the solidified core plug in the Seaman volcanic 

center is considered to have a volume of at most several 

cubic kilometers. 

The igneous products of the Seaman volcanic center 

range in composition from andesite (minor) to dacite 

(major) to rhyolite (minor). Andesite to dacite lava flows 

dip radially away from the central part of the volcanic 

center and form prominent cliffs. Associated lahars are 

composed of many varicolored, poorly sorted, bouldery, 

and pumiceous debris flows. Lahar deposits are especially 

voluminous on the western flank of the volcanic center in 

cliffs north of the westward-flowing drainage heading at 

Oreana Spring (fig. 1). The hypabyssal core plug is 

weakly hydrothermally altered in some places and weathers to bouldery outcrops. The dike, also affected by weak 

deuteric alteration in some places, forms prominent castellated outcrops along its 4 km (2.5 mi) length. The dike 

and core plug intrude, and therefore are younger than, the 

lava flows; because the contact between the dike and plug 

is not exposed, age relations between the dike and plug 

are unknown. 

The volcanic center is in one of the least deformed 

parts of the Seaman Range. Evidence that might indicate 

localization of the volcanic center by major structural features is absent, although rocks of the Seaman volcanic 

center probably conceal some older faults (Hurtubise and 

du Bray, 1992). 

PETROGRAPHY 

Lavas erupted from the Seaman volcanic center are 

light-brownish-gray to dark-gray dacite that forms 
4 THE SEAMAN VOLCANIC CENTER-A RARE MIDDLE TERTIARY STRATOVOLCANO 

prominent cliffs composed of radially outward dipping 

flows. The dacite contains about 10 percent crystals. Phenocrysts (fig. 2A), in decreasing order of abundance, are 

subhedral, strongly (oscillatory) zoned plagioclase (0.2-4 

mm), subhedral hypersthene (0.2-3 mm), trace amounts of 

anhedral to subhedral magnetite (0.1 mm), and anhedral 

quartz (2 mm), and accessory allanite, apatite (abundant as 

minute inclusions in hypersthene, fig. 2B), and rare oxyhornblende in a pilotaxitic, devitrified-glass groundmass 

that contains plagioclase microlites. Glomerocrysts composed of plagioclase, hypersthene, and magnetite are a 

characteristic feature of the dacite lava flows (fig. 2C); 

trace amounts of brown interstitial glass are present in 

some glomerocrysts. The lava flow unit (fig. 1) also contains massively bedded, varicolored, poorly sorted, bouldery lahars and (or) block and ash pyroclastic flow 

deposits; groundmass is ash and contains abundant pumice 

blocks 1-5 em in diameter. 

The core plug consists of light-greenish-gray to 

light-brownish-gray, fine-grained, porphyritic, hypabyssal 

rhyolite and dacite that weathers to rounded boulders and 

outcrops. The plug contains about 8 percent crystals. 

Phenocrysts, in decreasing order of abundance, are subhedral zoned plagioclase (1.5-3 mm), subhedral hornblende laths altered to biotite (0.5-2 mm), and trace 

amounts of anhedral, resorbed and embayed quartz (2-4 

mm), subhedral biotite (1-1.5 mm), subhedral hypersthene (0.5-1 mm), and anhedral magnetite (0.1 mm) in 

a fine-grained groundmass of quartz and feldspar (fig. 

2D). Plagioclase glomerocrysts are present in some samples of the core plug. 

The radial dike is grayish-black to light-yellowishgray and light-gray, porphyritic rhyolite to dacite. The 

dike is inhomogeneous with respect to petrographic and 

compositional characteristics along its length and across 

its width, and it is enclosed in a distinctive black-glass 

chilled margin several meters thick. The dike contains 

about 10 percent crystals. Phenocrysts, in decreasing order 

of abundance, are subhedral zoned plagioclase (0.5-4 

mm), anhedral sanidine (1-8 mm), anhedral, deeply 

embayed quartz (1-3 mm), subhedral hornblende (0.5 

mm), subhedral hypersthene (0.2-0.4 mm), and trace 

amounts of subhedral biotite (0.1-1 mm) and anhedral 

magnetite (0.1 mm) in a devitrified-glass groundmass that 

contains feldspar microlites (fig. 2E). 

Phenocrysts in some Seaman volcanic center samples have textures that imply phenocryst-melt disequilibrium; others show the effects of deuteric alteration. The 

rims of most pyroxene crystals (as well as cores in some 

cases) in samples of the lava flows are altered to opaque 

masses of fine-grained iron-titanium oxide minerals. 

Hornblende laths in many samples of the core plug have 

been replaced by biotite. Crystals that form some glomerocrysts show disequilibrium textures; a second generation 

of plagioclase rims primary grains whose cores are 

altered to sericite, and pyroxene crystals are almost uniformly replaced by felted, opaque intergrowths of irontitanium oxide minerals. 

GEOCHEMISTRY OF THE SEAMAN 

VOLCANIC CENTER 

WHOLE-ROCK CHEMISTRY 

All of the geochemical data presented here were 

determined in analytical laboratories of the U.S. Geological Survey in Denver, Colorado. Major-oxide analyses 

(table 1) were performed (analysts, J.E. Taggart, A.J. Bartel, and K. Stewart) using X-ray fluorescence techniques 

(Taggart and others, 1987). FeO:FeO* (total iron as FeO) 

molar ratios were adjusted to 0.85 and major-oxide abundances recalculated to 100 percent, on an anhydrous basis. 

Trace element abundances in table 2 were determined (by 

du Bray) by energy-dispersive X-ray fluorescence spectroscopy (Elsass and du Bray, 1982) using 109Cd and 

241 Am radioisotope excitation sources; the accuracy of 

this type of data is discussed by Sawyer and Sargent 

(1989). Trace element data in table 3 were determined 

(analyst, J.R. Budahn) by instrumental neutron activation 

analysis (INAA) (Baedecker and McKown, 1987). 

Trace element data obtained by X-ray fluorescence 

spectroscopy were used to identify the range of compositional variation for seventeen samples of Seaman volcanic 

center components. A representative subset of nine samples was selected in order to depict the range of major 

element variation of components of the volcanic center. 

Similarly, the least evolved lava sample, the most evolved 

dike sample, and a representative sample of the core plug 

were selected for instrumental neutron activation analysis. 

The system of the International Union of Geological 

Sciences (Le Bas and others, 1986) was used to classify 

the rocks of the Seaman volcanic center (table 1, fig. 3). 

The lava flows range in composition from andesite to dacite although most of the flows are dacite. The core plug 

and the radial dike are transitional from dacite to rhyolite. 

The core plug and radial dike contain 0-0.19 (average 

0.05, table 4) and 0.30-1.57 (average 0.99, table 4) percent normative corundum (table 1), respectively. 

All components of the Seaman volcanic center are 

subalkaline (fig. 3) and contain roughly equal amounts of 

Na20 and K20. Average Na20+K20 values (table 4) are 

5.90, 6.98, and 7.00 at Si02 values of 65.4, 70.6, and 

70.5, respectively, for the lava flows, central plug, and 

radial dike (all in weight percent, normalized to 100 percent, anhydrous). Compositions of components of the Seaman volcanic center plot near the Cascade calc-alkaline 

trend on an AFM (Na20+K20-FeO-MgO) diagram (fig. 4) 

and are within the calc-alkaline field as defined by Irvine 

and Baragar (1971). 
GEOCHEMISTRY OF THE SEAMAN VOLCANIC CENTER 5 

Abundances for most trace elements in components 

of the Seaman volcanic center (tables 2, 3) are within 

ranges characteristic of calc-alkaline, middle Tertiary volcanic rocks of the Basin and Range province (Best, Christiansen, and Blank, 1989). Average barium abundances in 

Figure 2. Photomicrographs showing petrographic features of 

components of the Seaman volcanic center. A, Phenocrysts of plagioclase (PI) and hypersthene (Hy) in devitrified glass groundmass. 

Lava flow. B, Very fine grained apatite (Ap) needles in hypersthene 

(Hy) phenocryst. Lava flow. C, Plagioclase (PI)-hypersthene 

(Hy)-iron-titanium oxide (It) glomerocryst in devitrified glass 

groundmass. Lava flow. D, Plagioclase (PI) glomerocryst in devitrified glass groundmass containing phenocrysts of plagioclase and 

altered mafic silicate (Ms) minerals. Core plug. E, Phenocrysts of 

plagioclase (PI), hypersthene (Hy), and hornblende (Hb) in devitrified glass groundmass. Radial dike. 

Seaman volcanic center components (824, 1,315, and 974 

ppm in the radial dike, core plug, and lava flows, respectively) are elevated relative to barium abundances characteristic of granite or the crust, 600 and 425 ppm, 

respectively (Krauskopf, 1979). Rb/Sr ratios range from a 

low value of 0.21 in the lava flows to a more evolved 

value of 0.56 in the radial dike (table 4). Rubidium and 

strontium abundances vary systematically from the oldest 

(elevated strontium and depleted rubidium) to the youngest (depleted strontium and elevated rubidium) components of the Seaman volcanic center (fig. 5). Note, 
6 THE SEAMAN VOLCANIC CENTER-A RARE MIDDLE TERTIARY STRATOVOLCANO 

Table 1. Major-oxide and CIPW normative mineral composition for samples of the Seaman volcanic center, Lincoln Cmmty, Nevada. 

[Major oxide data normalized to 100 weight percent, anhydrous; LOI, loss on ignition; Fe2+:Fe3+ set to 0.8; Sum0

, original analytical total; FeT03

, analytical 

value for total iron as Fe2

0 3_. Leaders(--), not present] 

Unit Radial dike CoreQlug Lava flows 

SamEleNo. 201411 201412 201414 201407 201410 201417 201409 201415 201423 

Major-oxide composition 

Si02 68.84 69.84 72.84 70.62 71.13 70.04 68.81 64.87 62.55 

Al20 3 15.81 15.96 14.36 14.99 14.84 14.93 15.22 16.88 17.32 

Fe20 3 0.54 0.49 0.37 0.46 0.46 0.55 0.62 0.88 1.03 

FeO 2.74 2.52 1.90 2.35 2.33 2.83 3.16 4.48 5.23 

MgO 1.03 .54 .58 .83 .61 .95 1.04 1.14 1.68 

CaO 3.67 3.06 2.04 3.19 2.91 3.52 3.65 4.89 5.94 

N~O 3.55 3.09 2.87 3.01 3.07 3.13 3.11 3.45 2.94 

~0 3.08 3.78 4.64 4.05 4.19 3.48 3.74 2.54 2.22 

Ti02 .55 .56 .27 .37 .34 .42 .48 .58 0.76 

P205 .14 .15 .08 .10 .09 .10 .11 .18 .21 

MnO .06 .01 .04 .03 .04 .04 .06 .10 .11 

LOI 1.83 2.62 2.14 1.75 1.53 2.17 1.05 1.66 1.53 

slllllo 100.19 99.38 99.83 99.43 99.49 99.57 99.93 99.88 100.27 

FeT03 3.51 3.18 2.43 2.99 2.97 3.59 4.07 5.73 6.72 

CIPW normative mineral composition 

Q 

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25.69 

0.30 

18.19 

30.03 

17.28 

6.35 

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.34 

28.95 

1.57 

22.35 

26.13 

14.22 

4.66 

0.72 

1.06 

.34 

32.09 

1.10 

27.40 

24.30 

9.59 

4.28 

0.54 

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PERCENT 

Figure 3. Alkali-silica classification diagram (Le Bas and others, 

1986) showing analyses (table 1) of samples from the Seaman volcanic center. Alkaline-subalkaline dividing line from Irvine and 

Baragar (1971). 

28.81 28.05 25.44 20.39 18.92 

0.19 0.01 

24.73 20.57 22.13 15.00 13.12 

25.98 26.49 26.28 29.18 24.91 

13.82 16.41 16.54 23.06 27.48 

4.95 6.31 6.83 9.56 11.65 

.33 0.72 0.49 

.66 0.80 .90 1.27 1.49 

.64 .80 .91 1.11 1.45 

.22 .24 .26 .44 .51 

however, the continuous variation of rubidium and strontium abundances between components of the Seaman volcanic center and the overlap between abundances of these 

two elements between at least one of the lava flow samples and some core plug and radial dike samples. 

Chondrite-normalized rare earth element patterns for 

samples of the Seaman volcanic center have a gentle negative slope and small to very small negative europium 

anomalies (fig. 6). Patterns for lava flow and radial dike 

samples are almost parallel, whereas a core plug sample 

has a crossing pattern; it is light rare earth element 

enriched and heavy rare earth element depleted relative to 

the lava and dike samples. The magnitude of the europium 

anomaly increases from the lava flows and core plug to 

the radial dike. Light- and heavy-rare earth element abundances are about 100 and 10 times chondri tic values, 

respectively; (La/Lu)N values are 6.7 and 15.5 in lava flow 

and core plug samples, respectively. Total rare earth element contents range from 127 (radial dike) to 157 (core 

plug and lava flow) ppm. The relatively less evolved (with 

respect to Si02 abundances and Rb-Sr ratios, for instance) 

lava flows have rare earth element contents that are 
GEOCHEMISTRY OF TIIE SEAMAN VOLCANIC CENTER 7 

Table 2. Trace element data for samples of the Seaman volcanic center, Lincoln County, Nevada. 

[In parts per million] 

Unit Radial dike Core(!lug Lava flows 

Sample No. 201411 201412 201414 201407 201410 201416 201417 201386 201408 

Rb 127 137 177 128 128 118 108 80 78 

Sr 326 262 200 380 326 372 366 434 414 

y 22 25 23 20 19 18 21 39 28 

Zr 163 162 135 153 148 148 175 233 208 

Nb 13 14 15 11 12 13 11 15 15 

Ba 876 796 800 1,327 1,366 1,242 1,326 930 793 

La 34 31 35 47 41 42 54 34 40 

Ce 58 58 65 73 82 64 90 69 68 

Nd 31 32 34 42 36 30 34 36 37 

Unit Lava flows 

Sample No. 201409 201413 201415 201418 201419 201423 201426 201429 

Rb 108 91 58 84 93 66 73 122 

Sr 387 457 446 388 385 437 401 344 

y 20 27 27 32 27 35 28 19 

Zr 154 187 190 195 191 214 205 147 

Nb 11 13 14 13 15 12 12 10 

Ba 1,201 996 1,008 905 927 880 833 1,268 

La 40 36 34 43 39 32 38 45 

Ce 65 67 68 74 71 67 70 69 

Nd 35 30 30 34 29 34 34 34 

Table 3. Instrumental neutron activation data for samples of the F 

Seaman volcanic center, Lincoln County, Nevada. 

[In parts per million. bdl, below detection limit] EXPLANATION 

+RADIAL DIKE 

Unit Radial dike Core(!lug Lava flows .&CORE PLUG 

Sample No. 201414 201407 201423 +LAVA FLOWS 

Ba 804 1,360 886 

Co 4.39 3.98 10.9 

Ni 8.2 6.2 bdl 

Cr 10.3 8.82 18.3 

Cs 6.23 3.21 1.79 

Hf 4.20 4.25 5.27 

Rb 181 124 57.3 '\ 

Sb 0.310 0.150 0.237 CALCTa 2.71 0.985 0.820 ALKALINE TREND 

Th 15.1 12.8 7.65 

u 5.80 4.11 2.03 CALC-ALKALINE 

Zn 60.3 57.3 104 

Zr 117 147 180 

Sc 5.37 7.61 18.7 M 

La 31.2 38.8 33.6 

Ce 58.1 74.3 70.6 Figure 4. Ternary AFM (Na20+K20-Feo* -MgO) diagram 

Nd 24.5 30.6 32.7 showing compositions of samples from the Seaman volcanic cenSm 5.55 5.49 6.77 ter. Cascade calc-alkaline trend (solid line) and tholeiitic-calcEu .792 1.18 1.56 alkaline (dashed line) dividing lines from Irvine and Baragar 

Gd 4.32 4.42 6.42 (1971). Tb 0.607 0.580 0.931 

Tm bdl 0.293 0.534 

Yb 1.82 1.78 3.43 

Lu 0.270 0.260 0.519 
8 THE SEAMAN VOLCANIC CENTER-A RARE MIDDLE TERTIARY STRATOVOLCANO 

Table 4. Average major-oxide, CIPW normative, and traceelement compositions of components of the Seaman volcanic 

center, Lincoln COWlty, Nevada. 

[n, number of analyses from which mean and standard deviation were 

calculated. Major-oxide and CIPW normative data in weight percent, 

normalized to I 00 percent, anhydrous; LOI, loss on ignition; trace element 

data in parts per million; leaders ( --), not detected; .±, calculated standard 

deviation] 

Radial dike Core plug Lava flows 

n=3 n=3 n=3 

Major-oxide composition 

Si02 70.50±2.08 70.59±0.55 65.41±3.17 

Al203 15.38±0.88 14.92±0.07 16.47±1.11 

Fe20 3 0.47±0.08 0.49±0.06 0.84±0.21 

FeO 2.39±0.43 2.50±0.28 4.29±1.05 

MgO 0.72±0.27 0.80±0.17 1.28±0.35 

CaO 2.92±0.82 3.21±0.31 4.83±1.14 

N~O 3.17±0.35 3.07±0.06 3.17±0.26 

JSO 3.83±0.78 3.91±0.37 2.83±0.80 

Ti02 0.46±0.17 0.38±0.04 0.61±0.15 

P205 0.12±0.04 0.10±0.01 0.17±0.05 

MnO 0.04±0.03 0.03±0.04 0.09±0.03 

LOI 2.20±0.40 1.82±0.33 1.41±0.32 

CIPW normative mineral composition 

Q 28.91 28.36 21.58 

c 0.99 0.05 

or 22.65 23.09 16.75 

ab 26.82 25.97 26.79 

an 13.70 15.26 22.37 

hy 5.10 5.61 9.35 

di 0.40 

mt 0.68 0.71 1.22 

il 0.87 0.72 1.15 

aE 0.29 0.24 0.40 

Trace element composition 

n=3 n=4 n=10 

Rb 147±27 121±10 85±19 

Sr 263±63 361±24 409±31 

y 23±2 20±1 28±6 

Zr 153±16 156±13 190±31 

Nb 14±1 12±1 13±2 

Ba 824±45 1,315±52 974±153 

La 33±2 46±6 38±4 

Ce 60±4 77±11 69±3 

Nd 32±2 36±5 33+3 

systematically greater than those of the more evolved 

radial dike. 

Spidergrams (Thompson and others, 1983) (fig. 7) 

facilitate comparison of incompatible trace element abundances in components of the Seaman volcanic center. Patterns are relatively smooth (with superimposed negative 

Ba, Nb, Ta, Sr, P, and Ti anomalies) and have a gentle 

negative slope. Normalized abundances are from about 600 

to 9 times chondritic values. Chondrite-normalized patterns 

for the radial dike and lava flow are the most and least, 

respectively, negatively sloping. The radial dike is characterized by the most well developed negative anomalies. 

200-

• 7 

2-; 100-

::::>0::: 

;;w 

_Q_ 

CD 

::::> 

0::: 

+RADIAL DIKE 

.&CORE PLUG 

+LAVA FLOWS OL-------~'---------~'------~~~--~ 

100 200 300 400 

STRONTIUM, IN PARTS PER MILL I ON 

Figure 5. Diagram showing the variation of rubidium and strontium abundances in samples from the Seaman volcanic center. 

Lines connect sample points whose major oxide compositions were 

evaluated in mixing model calculations (table 5). Numbers next to 

points depict the sequence, starting with 1 and ending with 6, in 

which compositions were derived in mixing calculations; point 0 

depicts the sam pie whose composition was designated as that of the 

most primitive liquid in this system. Numbers followed by the letter A indicate alternate liquid evolution paths, as described in text. 

1 , 00 

w 

1--

0::: 

0 

~ 10 

I 

u 

""-.. 

w 

_j 

o._ 

2 

<( 

(/) 

~-:r~-+--------+---+-

EXPLANATION 

tRADIAL DIKE, SAMPLE 201414 

•CORE PLUG, SAMPLE 201407 

+LAVA FLOWS, SAMPLE 201423 

0. 1~~----._--~--~--~~------~--~~ 

La Ce Nd Sm Eu Tb Dy Tm Yb 

Figure 6. Chondrite (Anders and Ebihara, 1982)-normalized 

diagram showing compositions (table 3) of samples from the 

Seaman volcanic center. 

Compositional data for each of the components of the 

Seaman volcanic center are characterized by groups of 

oxides and elements whose abundances display mutual 

covariation. Abundances of total iron, Ti02, CaO, Al203, 

MgO, P205, MnO, Sr, Zr, and Y and of Si02, K20, Rb, 

and Ba vary systematically. These covariance relations 

suggest that compositional variation within the Seaman 

components is controlled by observed phenocryst phases. 

In samples of each of the three Seaman components, 

total iron, Ti02, and CaO abundances are strongly correlated; correlation coefficients (r) exceed 0.9. Abundances 

of MgO are also correlated with this group of elements. In 
GEOCHEMISTRY OF THE SEAMAN VOLCANIC CENTER 9 

1 J 000 

w 

1-100 

0::: 

0 

z 

0 

I 

0 10 

"" w 

_j 

Q_ 

2 

<{ EXPLANATION 

(/) 1 .RADIAL DIKE, SAMPLE 201414 

•CORE PLUG, SAMPLE 201407 

+LAVA FLOWS, SAMPLE 201423 

0.1~--------------~----~----~----------------~----._~ 

RbBaTh K NbTaLaCeSrNd P SmZrHfTi TbY TmYb 

Figure 7. Chondrite (Thompson and others, 1983)-normalized extended trace element diagram showing compositions (tables 1-3) of 

samples from the Seaman volcanic center. Trace elements are arranged in order of increasing compatibility to the right. 

samples of the core pluton and lava flows, r values for 

MgO and these elements exceed 0.9 (except total iron in 

core pluton samples, 0.79). In samples of the radial dike, r 

values for MgO and these elements exceed 0.6 (except 

Ti02, r is 0.4). Abundances of Al203 and P205 in samples 

of the radial dike and of the lava flows are also strongly 

correlated (r exceeds 0.95) with total iron, Ti02, and CaO 

abundances. In samples of the lava flows, MnO abundances display strong covariance (r exceeds 0.88) with all 

of the oxides (except MgO, r is 0.75) mentioned above. 

Abundances of strontium, yttrium, and zirconium vary 

systematically with abundances of most of the oxides 

described above (r exceeds 0.8 in most cases), though 

abundances of yttrium in radial dike samples do not 

exhibit this behavior. 

In samples of each of the three components of the 

Seaman volcanic center, abundances of K20, Si02, and 

rubidium have r values in excess of 0.95; barium joins this 

group in samples of lava flows. In samples of the radial 

dike, abundances of K20, Si02, and rubidium co-vary as 

do those of Na20, CaO, barium, and strontium (r exceeds 

0.85, except CaO and barium is 0.76). 

Major-oxide and trace-element covariance relations 

discussed above may reflect control by fractionation and 

(or) accumulation of hypersthene, calcic plagioclase, hornblende, biotite, apatite, and magnetite. The compositions 

of hypersthene ([Mg,Fe ]Si03) and hornblende ([Na,K]-

Ca2[Mg,Fe,Al]s[Si6Al2][0H,Fh) are appropriate to control total iron, MgO, zirconium, and yttrium and some 

CaO and MnO abundance variations in components of the 

Seaman volcanic center. Hypersthene in Seaman rocks 

contains distinctive, accessory amounts of apatite, which 

controls P205, strontium, and some of the CaO abundance 

variations. Zirconium and yttrium are inferred to occur as 

trace constituents, substituting for other cations, in hypersthene and hornblende; likewise strontium is inferred to 

substitute for calcium in apatite. The fact that yttrium and 

other oxides of the total iron-Ti02-CaO-Al203-MgOP205-MnO group do not display systematic covariation in 

dike samples suggests that yttrium was partitioned into a 

phase (an iron-titanium oxide mineral or unidentified trace 

mineral) other than hypersthene or hornblende during evolution of the magma represented by the radial dike. MgO 

covariations in dike samples suggest that the abundance of 
10 THE SEAMAN VOLCANIC CENTER-A RARE MIDDLE TERTIARY STRATOVOLCANO 

Table 5. Representative mineral compositions for phenocryst phases in samples of the Seaman volcanic center, Lincoln County, 

Nevada. 

[n, number of analyses from which mean and standard deviation were calculated; leaders(-), not detected; H20 ill hornblende and biotite determined by 

difference] 

Plagioclase Hvoersthene Hornblende Biotite 

Sample no ............. 201386 201413 201386 201413 201413 201410 

n=6 n=8 n=9 n=6 n=6 n=4 

Mineral composition 

Si02 45.94±0.50 48.80±0.60 50.21±0.34 50.32±0.25 41.63±0.34 36.34±1.44 

AI203 33.94±0.27 32.06±0.51 0.45±0.06 0.76±0.20 10.90±0.28 13.32±1.13 

FeO 0.18±0.04 0.23±0.03 30.85±0.98 29.84±0.92 19.51±0.34 17.24±1.82 

MgO 14.79±0.77 16.00±0.82 9.05±0.15 13.26±1.62 

CaO 17.45±0.47 15.25±0.51 1.66±0.09 1.46±0.07 10.69±0.07 

N~O 1.38±0.19 2.75±0.30 2.29±0.49 0.20±0.10 

K20 0.04±0.01 .08±0.02 0.76±0.04 8.91±0.26 

Ti02 0.17±0.07 0.11±0.06 2.55±0.08 3.81±0.51 

MnO 0.68±0.06 0.66±0.06 0.19±0.03 0.15±0.03 

~0 2.43±0.28 6.78±0.27 

Sum 98.93 99.16 98.80 99.15 100.00 100.00 

Structural formula 

No. oxygens 32 6 24 24 

Si 8.542 8.999 1.986 1.972 6.299 5.624 

AI 7.439 6.969 0.014 0.028 "1.701 2.376 

AI 0.007 0.007 0.243 0.056 

Ti 0.005 0.003 0.290 0.443 

Fe+2 0.029 0.035 1.021 0.978 2.470 2.231 

Mn 0.023 0.022 0.025 0.019 

Mg 0.872 0.935 2.041 3.058 

Ca 3.476 3.013 0.070 0.061 1.733 

Na 0.498 0.983 0.672 0.059 

K 0.009 0.019 0.147 1.760 

OH 2.460 6.596 

Site occupancy 

z 15.981 15.968 2.000 2.000 8.000 8.000 

y 1.928 1.945 2.552 5.807 

X 4.012 4.050 0.070 0.061 5.069 1.819 

End-member mineral proportions 

Ab 12.5 24.5 Wo 3.6 

An 87.3 75.0 En 44.4 

Or 0.2 0.5 Fs 52.0 

MgO may have been controlled by a phase, possibly 

biotite, other than hypersthene or hornblende. In lava 

flow samples, strong abundance covariation between MnO 

and other oxides discussed above suggests that MnO was 

principally sited in hypersthene or hornblende in the lava 

flows but in some other phase (again perhaps biotite) in 

the other two Seaman volcanic center components. The 

K20, Si02, rubidium, and barium abundance covariance 

relations discussed above are considered to reflect fractionation and (or) accumulation of biotite, whereas calcic 

plagioclase controls Na20, CaO, barium, and strontium 

abundance variations. Rubidium, barium, and strontium 

are inferred to be present as trace constituents, substituting 

for potassium and sodium in plagioclase and biotite, as 

appropriate. 

3.1 

47.4 

49.5 

MINERAL CHEMISTRY 

Mineral compositions (table 5) were determined 

using an ARL SEM Q electron microprobe equipped with 

six wavelength dispersive spectrometers. A 20-micron 

beam was used to analyze phenocrysts. Sample current 

was 15 nA (on brass), and accelerating voltage was 15 kV. 

Natural and synthetic materials were used as standards. 

Compositions were determined for least three points distributed across the width of at least two grains per mineral 

species during 20-second analyses. 

The average compositions of plagioclase in samples 

201386 and 201413 (table 5) are statistically distinct and 

are calcium-rich and calcium-poor bytownite, respectively. 

The plagioclase within each of these samples is 
MAJOR ELEMENT LEAST-SQUARES FRACTIONATION-MIXING MODELING 11 

compositionally zoned; grain cores are calcium rich and 

sodium poor relative to grain rims. Compositional zonation spans 5-10 mol percent in the anorthite component. 

The compositions of orthopyroxene in samples 

201386 and 201413 (table 5) are very similar. The compositions of these phenocrysts plot in the ferrohypersthene field of Deer and others (1966). Core to rim 

zonation is much less pronounced in pyroxenes of the 

Seaman volcanic center than in plagioclase; zonation of 

several percent in the enstatite and ferrosalaite end members was identified. 

The composition of hornblende in sample 201413 

. (table 5) is distinct in that its Ti02 and Na20+K20 contents are elevated relative to those of common hornblende. These features and the fact that this amphibole 

contains subequal cationic amounts of iron and magnesium are consistent with its having an affinity to kaersutitic amphibole. The low standard deviations associated 

with the mean oxide abundances calculated from six 

analyses (two analysis points for each of three grains) 

indicates that these phenocrysts are relatively compositionally homogeneous. 

The composition of mica in sample 201410 (table 5) 

is that of a slightly magnesium enriched biotite (Foster, 

1960). Similar to the hornblende in sample 201413, the 

biotite in sample 201410 is somewhat titanium enriched. 

Moderate standard deviations associated with the mean 

oxide abundances calculated from four analyses (two analysis points for each of two grains) indicate that these phenocrysts are somewhat compositionally inhomogeneous. 

Slightly low (relative to mica analyses; Deer and others, 

1966), analytical totals and somewhat elevated calculated 

(by difference) H20 contents for these micas imply that 

they have been affected by weak deuteric or hydrothermal 

alteration, in accord with petrographic observations. 

MAJOR ELEMENT LEAST -SQUARES 

FRACTIONATION-MIXING 

MODELING 

In order to more rigorously evaluate the role of phenocryst fractionation or accumulation in the compositional 

evolution of the Seaman volcanic center, major element 

least-squares crystal fractionation-mixing models were calculated using PETMIX IV (Kevin W. Laurent, U.S. Geological Survey, unpub. software, version 2.1b). The 

sequential derivation of progressively more evolved 

major-oxide compositions (table 1) by fractionation of 

phenocrysts having compositions like those determined in 

samples of Seaman components (table 5) is evaluated in 

table 6. 

Numerical input to the modeling process was modified and selected somewhat from that presented in tables 1 

and 5. All iron in whole-rock and mineral compositions 

was recalculated to the ferrous state. The compositions of 

plagioclase and hypersthene from two lava flow samples 

(201386 and 201413) were utilized in the modeling process. Hypersthene and plagioclase compositions in sample 

201386 are considerably more primitive (higher FeO and 

CaO, respectively) than those in sample 201413 and were 

used as fractionation-mixing phases only in the first modeling increment. Plagioclase and pyroxene were used 

throughout the modeling process, which is in accord with 

their occurrence as phenocrysts in all samples of Seaman 

volcanic center components. Hornblende and biotite compositions (table 5) determined for phenocrysts in samples 

201413 and 201410, respectively, were used in modeling 

steps 3-7, which is in accord with the absence of these 

phases in samples represented by the first two modeling 

steps. Magnetite and apatite, present in all samples of Seaman components, were used throughout the modeling process; their compositions were selected from Colucci and 

others (1991, analysis HMM 130 OX8) and Deer and others (1966, apatite analysis no. 1), respectively, because the 

compositions of these phases are relatively constant. 

The numerical modeling process allows minerals to 

be subtracted or added during least-squares calculations. 

Numerical output from the modeling process indicating 

that a mineral was subtracted to drive computed liquid 

compositions toward the model endpoint identifies a phase 

that crystallized and was mechanically separated (fractionated) from the residual liquid. Numerical output indicating that a mineral was added indicates a mineral that was 

mechanically added to the residual liquid. 

The physical process of crystal fractionation is logical 

and intuitive, whereas the process for crystal addition is 

not. Components of the Seaman volcanic center are characterized by the presence of glomerocrysts, which indicate 

that crystallization was occurring in the chamber. The 

physical process of crystal addition, suggested by output 

from the modeling process (table 6, numerical entries preceded by a + ), involves recombination (by convective stirring) of crystals nucleated in one part of the chamber with 

residual magma from another part of the chamber. 

The results of major element least-squares crystal 

fractionation-mixing modeling are considered meaningful 

when the quantity :Er2 (table 6), the sum of the squares of 

the residuals for each of the evaluated major element components, is less than the value ascribed to analytical uncertainty (approximately 0.5-1.0 weight percent) for the 

analyses being modeled. Values of :Er2 greater than 1.0 

indicate that the proposed model has not addressed petrologic processes or components operative in the genesis of 

the compositions being evaluated. 

The value of :Er2 is less than 0.35 in six of nine 

models for the Seaman volcanic center and is less than 

0.15 in four of these. Consequently, models calculated for 

sequential derivation of the Seaman components by 
12 THE SEAMAN VOLCANIC CENTER-A RARE MIDDLE TERTIARY STRATOVOLCANO 

Table 6. Results of major element least-squares crystal fractionation-mixing modeling for samples of the Seaman volcanic center, 

Lincoln County, Nevada. 

[Entries in "Step" column refer to incremental episodes modeled along the inferred fractionation path, compare with figure 5. Numbers under mineral headings 

are mass percentages of each phase removed (or, if preceded by a plus sign ( + ), added) in each fractionation step; F, mass percent of derived liquid composition 

relative to parent liquid] 

Plag!oclase Hvoersthene Hornblende Biotite 

Step Parent Derived 201386 201413 201386 

1 201423 201415 7.69 4.82 

2 201415 201409 12.62 

3A 201409 201407 3.64 

3 201407 201417 +1.69 

or 

3 201409 201417 0.60 

4 201417 201411 +4.45 

5 201411 201410 7.54 

6 201410 201412 +7.01 

7 201412 201414 9.31 

fractionation-mixing are valid. The three highest l:r2 values are 0.7592, 0.6480, and 0.4683 for derivation of 

compositions represented by samples 201410, 201409, 

and 201411, respectively. In all of these cases the major 

contributions to elevated l:r2 values involve large residuals, of opposite sign, for Na20 and K20. These components are notoriously mobile in glassy volcanic rocks and 

are easily redistributed by circulating hydrothermal fluids 

or during devitrification and hydration (Lipman, 1965). 

All three of these samples show signs of weak alteration 

or glassy groundmass devitrification. The high residuals 

for these three models probably reflect an alkali remobilization component not addressed by the calculated model. 

Consequently, the results of calculations involving these 

three samples are more meaningful than the l:r2 values 

indicate. 

The sequence in which compositions were incrementally modeled was primarily determined by evaluation of 

rubidium versus strontium abundances (fig. 5). Variations 

in the relative abundances of these two elements are a reliable indicator of degree of geochemical behavior because 

rubidium is incompatible in most intermediate-composition igneous systems and strontium is compatible (Hanson, 

1980). Therefore, of the nine samples of the Seaman volcanic center for which major element data are available 

the two samples having the highest strontium and lowest 

rubidium abundances were initially selected as potential 

starting points for the fractionation-mixing modeling. In 

tum, the sample having the next highest rubidium and 

next lowest strontium abundances was identified as the 

starting point for the next composition increment to be 

modeled. This pattern was continued until the sample 

having the second highest rubidium and second lowest 

strontium abundances was used to model derivation of the 

most evolved sample by fractionation-mixing of observed 

phenocryst phases. 

The rubidium and strontium abundances of the two 

potential starting point samples (201415 and 201423, table 

201413 Magnetite Apatite 201413 201410 F u2 

0.06 0.31 87.12 0.1053 

1.79 1.76 0.10 83.92 0.6480 

1.57 0.77 0.17 93.84 0.0568 

+1.92 +0.09 0.18 +3.40 3.38 103.53 0.0670 

+2.28 0.55 0.11 0.63 2.93 97.45 0.0160 

0.98 0.08 0.29 +1.01 +1.02 105.27 0.4683 

2.18 +0.32 +0.45 4.74 +2.24 88.55 0.7592 

+1.12 +0.89 0.20 4.55 +1.14 105.42 0.2251 

+0.34 1.07 +0.24 1.57 +0.71 89.34 0.3123 

2) are indistinguishable within analytical precision 

(Sawyer and Sargent, 1989), but the abundances (table 1) 

of the more refractory major element components (including total iron, MgO, CaO, and Ti02) are distinctly higher 

in sample 201423. Consequently, the composition of sample 201423 was selected as that from which to begin fractionation-mixing modeling. 

Similarly, although the rubidium and strontium abundances of samples 201410 and 201411 are indistinguishable, the more refractory major element components are 

distinctly higher in sample 201411; accordingly, the modeling sequence was constructed such that the composition 

of sample 201411 was derived from that of sample 

201417 and was used as the starting point for derivation of 

the composition of sample 201410. 

Two paths were modeled between major element compositions for samples 201409, 201407, and 201417 (paths 3 

and 3A, fig. 5, table 6). The rubidium and strontium abundances for sample 201407 do not plot near the trend exhibited by the compositions for the remainder of the modeled 

sample compositions. Consequently, an alternate path from 

the composition of sample 201409 to that of sample 

201417, skipping the composition of sample 201407 was 

modeled. Both paths are characterized by very low values 

of l:r2 and neither path is obviously more meaningful. 

EVOLUTION OF THE SEAMAN 

VOLCANIC CENTER 

Field relations indicate that the Seaman volcanic center is a composite stratovolcano that developed by episodic lava flow eruption and by emplacement of 

associated pyroclastic flows and lahars; final magmatic 

activity at the Seaman volcanic center is denoted by the 

hypabyssal core plug and associated radial dike, both of 

which intruded the earlier erupted products. The Seaman 
EVOLUTION OF THE SEAMAN VOLCANIC CENTER 13 

volcanic center was constructed between about 27.9 and 

27.4 Ma, between emplacement of the underlying Lund 

Formation and the oldest of the overlying ash-flow tuffs of 

the Seaman Range. Significantly larger stratovolcanoes 

have developed in considerably shorter time periods. 

Mount Shasta in northern California, for instance, has a 

volume 20 times greater than that of the Seaman volcanic 

center and developed in 0.35 m.y. (Christiansen and 

Miller, 1989). The 0.5 m.y. available for development of 

the Seaman volcanic center is therefore more than ample. 

The thick section of lahars and pyroclastic flows in 

the northwestern part of the Seaman volcanic center fills a 

large topographic depression in the lava flow sequence. 

These deposits indicate that lava interacted with water 

during at least the waning stages of lava flow eruption. 

The large volume of lahars, together with their coarseness 

and lack of sorting, indicates rapid deposition and incorporation of a large volume of water, possibly in the form of 

snowpack or glacial ice. The bedded character of the 

lahars and the lack of voluminous debris flow deposits in 

the area immediately surrounding the Seaman volcanic 

center argue against a sector collapse origin for these 

deposits. The three-dimensional shape and large volume 

of the lahar deposits indicate that the northwestern flank 

of the stratovolcano was breached during or after emplacement of the lava flows; the breaching process was probably directly linked to the processes responsible for lahar 

generation. 

The radial dike probably represents extraction of the 

final, most evolved fraction of Seaman magma and 

emplacement into a local, northeast-trending fracture. The 

orientation of the fracture that controlled localization of 

the radial dike may have been influenced by the prevailing 

regional stress field; fracture development probably began 

during solidification of the core plug. Delaney and others 

(1986) indicated that in the general case the orientation of 

dike-hosting fractures is "perpendicular to the direction of 

least compressive regional stress acting at the time of dike 

emplacement." Taylor and Bartley (1988) and du Bray and 

Hurtubise (in press) showed that normal faults active in 

nearby areas immediately prior to development of the Seaman volcanic center are oriented northeast to north-northeast, a direction that implies the least principal stress 

direction was oriented northwest to west-northwest. In 

accord with theoretical predictions, the northeast-trending 

radial dike of the Seaman volcanic center was emplaced in 

a fracture that is perpendicular to the direction of the least 

compressive regional stress. 

Volatile phase exsolution results in a net volume 

expansion (Burnham and Ohmoto, 1980), which imposes 

stress on overlying rocks. Crystallization of magma represented by the core plug may have resulted in volatile saturation, fluid phase exsolution, and consequent volume 

expansion. Hence, lava flows of the Seaman volcanic 

center were fractured in the direction perpendicular to the 

minimum regional stress direction by volume expansion 

in the underlying, solidifying reservoir. The resulting 

fracture served as a conduit for magma represented by 

the radial dike. 

Major-oxide and trace-element compositions of the 

components of the Seaman volcanic center indicate that 

progressively more evolved magmas were erupted during 

evolution of the stratovolcano (fig. 8). Most notably, abundances of total iron, MgO, CaO, Ti02, P205, MnO, and 

compatible trace elements (including Sr, Co, Cr, Zn, and 

Sc) are highest in samples of the lava flows, whereas 

abundances of Si02, total alkalis, and incompatible trace 

elements (including Rb, Cs, Ta, Th, and U) are highest in 

samples of the central plug and radial dike (tables 3, 4). 

Major-oxide abundances vary smoothly and continuously 

(fig. 5) between the components of the volcanic center, 

and compositions of some lava flow samples (samples 

from the highest stratigraphic positions) overlap those of 

the core plug and radial dike (fig. 3). The youngest lavas 

have compositions similar to those of the core plug and 

radial dike; the last fraction of liquid that was erupted was 

compositionally similar to magma represented by the core 

plug and radial dike. 

It is noteworthy that the last phase of magmatism 

(represented by the core plug and radial dike) associated 

with the Seaman volcanic center is the most geochemically evolved. This trend is opposite to that predicted by 

top-down tapping of a chemically zoned chamber 

(Hildreth, 1981). Several possibilities for eruption of 

geochemically primitive magma followed by emplacement 

of somewhat more evolved magma are considered below. 

The reversed chemical zonation may result from a 

two-stage process. The lava flows could have been 

erupted so soon after emplacement of magma into an 

upper crustal magma chamber that development of compositional zonation in the chamber was precluded. After 

eruption of the lava flows, the residual magma may have 

differentiated and become compositionally zoned; the 

more evolved, upper part of the chamber is then represented by the youngest lava flows and by the core plug 

and the radial dike. This two-stage process would probably result in a compositional discontinuity between the 

lava flows and subsequent igneous products that is not 

present in compositional data for the Seaman volcanic 

center (figs. 3-5). In addition, this two-stage process 

probably could not generate the volume of lava flows 

derived from the Seaman volcanic center. The presence of 

interbedded lava flows, pyroclastic flows, and lahars indicates complex and episodic eruptions over a considerable 

time span that probably are incompatible with a simple 

two-stage model. 

Alternatively, reversed chemical zonation may result 

from eruption dynamics prevailing during evacuation of a 

normally zoned chamber. Very high eruption rates could 

cause deeper, less evolved parts of the chamber to be 
14 THE SEAMAN VOLCANIC CENTER-A RARE MIDDLE TERTIARY STRATOVOLCANO 

3 1-

w 

~ 

0 2 - ....J 

<( 

0 

<( 

0::: 

~ 0 

....J 

...... m L.a.. 

<( 

> 

u <( 

....J 

0 I I I I I I I I I I I I I I 

Na AI p Ca Ti Mn Co 

Mg Si K Sc Cr Fe Ni 

rum 

I I I I I I I I 

Zn Sr Zr Sb 

Rb Y Nb Cs 

mmrn m 

I I I I I I I I 

Ba Ce Sm Gd 

La Nd Eu Tb 

m 

I I I I 

Tm Lu Ta 

Yb H f Th 

-

-

I 

u 

Figure 8. Enrichment-depletion diagram showing major- and trace-element abundances of the Seaman volcanic center dike normalized 

to those of the lava flow unit. 

extracted while more evolved magma in the upper part of 

the chamber is effectively necked off and prevented from 

entering eruption conduits. Similar, discontinuously (or 

reversely) zoned ash-flow tuff deposits derived from Plinian eruptions were considered by Boden ( 1989) to reflect 

rapid eruption from a discontinuously (stepwise) zoned 

chamber. Boden indicated that the subsequent eruption 

(emplacement) of more evolved material from the same 

chamber may "reflect eruption of residual silicic layers of 

magma that restratified below the vent during pauses in 

the eruptive cycle." However, the eruption rates and 

dynamics that prevail during Plinian eruptions and during 

multiple eruptions of lava from a stratovolcano over an 

extended period of time are different to the extent that the 

necking-off process is an unlikely explanation for the 

compositional variation of rocks of the Seaman volcanic 

center. 

The preferred model for eruption and emplacement of 

progressively more evolved magmas from the Seaman volcanic center involves a variation on the process of sidewall crystallization described by Sawka and others (1990) 

for zoned plutons of the Sierra Nevada batholith. This 

solidification model is invoked for magma chambers in 

which cooling is inferred to proceed from the margin 

inward by conductive heat loss to host rock forming the 

chamber walls. As cooling proceeds, mafic silicate minerals, calcic plagioclase, and trace phases crystallize along 

the chamber walls. Sawka and others (1990) described 

processes of interstitial liquid migration toward the top of 

the chamber that may not be applicable to evolution of the 

Seaman volcanic center. Instead, the melt in the Seaman 

chamber is inferred to become more evolved by removal 

of components necessary for sidewall crystallization of 

these refractory phases. Continuous sidewall crystallization, convective mixing between refractory-componentdepleted sidewall melt and undepleted melt inboard from 

the crystallization front, and periodic tapping of the compositionally evolving, convectively mixed central part of 

the chamber results in an inward-crystallizing magma 

chamber characterized by more evolved compositions 

from the walls inward and eruption of progressively more 

evolved lavas. 

A series of high-level reservoirs above the main 

chamber may have controlled lava flow eruption. The last 
EVOLUTION OF THE SEAMAN VOLCANIC CENTER 15 

of these reservoirs may be represented by the core plug 

and radial dike. The progressively more evolved composition of lavas erupted from the Seaman volcanic center and 

the most evolved compositions of its core plug and radial 

dike are logical consequences of sidewall crystallization, 

residual liquid differentiation, and magma emplacement in 

small, high-level, eruption-producing reservoirs. 

Thus, the hypabyssal core plug of the Seaman volcanic center probably represents the top of the final in a 

series of small volume reservoirs and not a large, solidified, high-level magma chamber that directly fed lava flow 

eruptions. Initially, these reservoirs probably developed in 

older rocks (Paleozoic carbonate rocks and Oligocene ashflow tuff) that host the volcanic center and, ultimately, as 

the stratovolcano grew in height, in the base of the stratovolcano itself. Sillitoe (1973) depicted bodies such as the 

core plug of the Seaman volcanic center as apophyses 

above much larger plutons whose tops are approximately 

4 km beneath the stratovolcano base. Similarly, Pallister 

and others (1992) indicated that juvenile material for the 

1980 eruption of Mount St. Helens was supplied from a 

small reservoir whose top was about 7 km below the 1980 

crater floor. By analogy, the core plug of the Seaman volcanic center may represent the solidified equivalent of the 

last, high-level reservoir from which lava flows were 

erupted. Eruption from this reservoir probably ceased following eruption of the youngest lava flows when magma 

supply from the underlying chamber diminished. With an 

end to effusive activity the volcanic center became inactive and the final reservoir solidified to the core plug. 

Sparse gravity data (du Bray and others, 1987, fig. 3) 

indicate a broad, low-amplitude high associated with the 

area of the Seaman volcanic center. Dacite lava flows and 

the core plug of the center are more dense (2.51 and 2.61 

g/cm3, respectively, E.A. du Bray, unpub. data, 1992) than 

the thin accumulation of dacite to rhyolite ash-flow tuff 

(1.4-2.5 g/cm3, average about 2.1 g/cm3; Cook, 1965, and 

Williams, 1967) exposed at the surface in this area but are 

significantly less dense than the thick section of carbonate 

sedimentary rocks (2.6-2.8 g/cm3; Dobrin, 1976, p. 

455-456) in the shallow subsurface throughout the Seaman Range (du Bray and others, 1987, p. E7). Taken 

together, these features suggest that a pluton, the solidified 

magma chamber that periodically supplied the series of 

small, high-level reservoirs beneath the evolving stratovolcano and is more dense than its host Paleozoic carbonate 

rocks, is present beneath the Seaman volcanic center. 

The density (greater than 2.8 g/cm3) required for the 

inferred pluton to have an associated gravity anomaly high 

relative to the enclosing Paleozoic carbonate rocks indicates a quartz diorite to gabbro composition (Dobrin, 

1976). Such a pluton represents the refractory, early crystallized, relatively more dense part of a solidified magma 

chamber that was periodically tapped to supply the series 

of high-level reservoirs. The composition of magma in 

these reservoirs, and thus the composition of the erupted 

products, would have become progressively more evolved 

with time as a result of crystallization and fractionation in 

the underlying source chamber. 

With the exception of the core plug and radial dike, 

hypabyssal to exogenous volcanic features such as those 

that intruded lava flows on the flanks of stratovolcanoes of 

the modem Cascades volcanic arc, including Mount 

Mazama (Bacon, 1983) and Mount St. Helens (Crandell, 

1987), are not exposed in the deeply incised flanks of the 

Seaman volcanic center. The Seaman volcanic center thus 

probably evolved entirely by central vent eruption. By 

analogy to construction of stratovolcanoes of the modem 

Cascade arc, numerous eruptive episodes were probably 

required to complete construction of the Seaman volcanic 

center. Because the core plug and dike are all that remains 

of the magmatic plumbing system of the Seaman volcanic 

center, it is likely that successive eruptive episodes obliterated previously developed, central exogenous domes in a 

manner similar to that which characterized early (MayOctober 1980) central dacite dome development at Mount 

St. Helens following its May 1980 eruption (Swanson and 

others, 1987). The solidified, unerupted contents of highlevel reservoirs inferred to have preceded that represented 

by the central plug may form stocks in the shallow subsurface beneath the Seaman volcanic center. 

Rare earth element abundances of the radial dike and 

core plug, in particular heavy rare earth element abundances, which are depleted relative to those characteristic 

of the lava flows, are consistent with hypersthene and 

hornblende fractionation. Mineral-matrix distribution coefficients for hypersthene range from 0.1 to 1.14 (Hanson, 

1980), whereas those for hornblende range from 1 to 14. 

Mineral-matrix rare earth element patterns for hypersthene 

and hornblende have gentle positive slopes and small negative europium anomalies (Hanson, 1980); heavy rare 

earth element enrichment in hornblende is maximized at 

dysprosium or erbium and flattens or tails off in ytterbium 

and lutetium. Although hypersthene fractionation will 

cause heavy rare earth elements to be depleted relative to 

light rare earth elements in the residual melt, its fractionation will not appreciably reduce the rare earth element 

content of the residual liquid because hypersthene-melt 

distribution coefficients are less than one in most cases. 

Consequently, heavy rare earth element abundance reduction between the lava flows and the core plug and radial 

dike was principally controlled by hornblende fractionation. The cause of light rare earth element depletion of 

the radial dike relative to the lava flows and core plug is 

unknown but may reflect late-stage fractionation of a light 

rare earth element-enriched accessory phase such as allanite or monazite (Miller and Mittlefehldt, 1982). 

Abundance variations involving rubidium and strontium indicate that feldspar fractionation was important in 

compositional evolution of the Seaman volcanic center. 
16 THE SEAMAN VOLCANIC CENTER-A RARE MIDDLE TERTIARY STRATOVOLCANO 

Feldspar-melt distribution coefficients for rubidium and 

strontium in intermediate composition rocks (Hanson, 

1980) indicate that feldspar fractionation preferentially 

removes strontium relative to rubidium from the melt 

phase. Feldspar crystallization causes residual liquids 

(such as those represented by the youngest parts of the 

Seaman volcanic center, namely the core plug and radial 

dike) to become progressively rubidium enriched relative 

to strontium. Europium (fig. 6) and strontium and barium 

(fig. 7) abundances are most depleted in the most evolved 

component of the volcanic center (radial dike), which is 

also consistent with a derivation of the radial dike that 

involved feldspar fractionation. Similarly, the negative 

phosphorous and titanium anomalies (fig. 7) are probably 

a consequence of apatite and iron-titanium oxide fractionation, respectively. Co-nucleation of apatite and hypersthene is indicated by the fact that apatite is included as 

very fine grained needles within hypersthene phenocrysts. 

Consequently, apatite and hypersthene fractionation are 

linked. As hypersthene phenocrysts grew, incorporated 

minute apatite crystals, and separated from the melt phase, 

they removed apatite and the components contained 

therein. 

The presence of plagioclase-hypersthene glomerocrysts in most samples of the Seaman volcanic center provides additional, tangible evidence that these phases 

played an essential role in the chemical evolution of the 

components of the center. Fractionation-mixing calculations support the inference that varying amounts of plagioclase, hypersthene, hornblende, biotite, apatite, and 

magnetite fractionation and convective remixing with 

glomerocrysts controlled compositional evolution of components of the volcanic center. These phases probably 

crystallized along the walls of the cooling magma chamber, and residual silicate liquid continued convective overtum, mixing, and evolution in the central part of the 

chamber. Eruption dynamics may have caused some of the 

sidewall crystal accumulations to be disrupted and mixed 

back into residual silicate liquids such that they are almost 

ubiquitously present in components of the volcanic center. 

Southward-sweeping, middle Tertiary volcanism in 

the Basin and Range province is considered to be a consequence of decreasing convergence rate between the Pacific 

and North American plates, stagnation of shallow subduction along the North American plate margin, subsequent 

foundering of the subducted Pacific plate, and pronounced 

asthenospheric backflow beneath the continental lithosphere of western North America (Lipman, 1980; Best and 

Christiansen, 1991). The Seaman volcanic center is a 

minor manifestation of this volcanic arc magmatism. 

Several trace element characteristics of components 

of the Seaman volcanic center confirm the inference that 

the center evolved in a volcanic arc setting. In particular, 

negative Nb-Ta anomalies (fig. 7) are considered to be a 

hallmark of magmas generated in a volcanic arc setting 

(Wood and others, 1979; Gill, 1981). Baffa ratios greater 

than 450 and La/Nb ratios of 2-7 are also considered to 

indicate magma genesis in a volcanic arc setting. Baffa 

ratios are 297, 1 ,380, and 1,080 in the radial dike, core 

plug, and lava flows of the Seaman volcanic center, 

respectively, whereas La/Nb ratios are 2.1, 3.5, and 2.8, 

respectively. 

Pearce and others (1984) recognized that granitic 

rocks from different settings are characterized by distinctive geochemical signatures, and they developed trace element discriminant diagrams to infer tectonic setting. Trace 

element abundance variations in coeval volcanic and plutonic rocks from any given terrane should be similar. Consequently, the tectonic setting-trace element discriminant 

diagrams developed by Pearce and others (1984) are used 

herein to evaluate the tectonic setting that prevailed during 

the development of the Seaman volcanic center. Samples 

from the volcanic center plot in the volcanic arc field on 

these diagrams (fig. 9), consistent with development of the 

center in a volcanic arc setting and with the regionally 

based inference that the Basin and Range province was a 

volcanic arc terrane during middle Tertiary time. 

The existence of the isolated Seaman stratovolcano in 

a region dominated by caldera-style volcanism (Best, 

Christiansen and others, 1989) is unusual. Several factors, 

including the volume of the associated magma chamber, 

the volatile content of contained magma, and the structural 

setting, may have caused the Seaman volcanic center to 

develop as a stratovolcano rather than as an ash-flow 

caldera. 

The small volume of the Seaman volcanic center is 

outside the range of values characteristic of caldera-forming ash-flow eruptions. Smith (1979) indicated that large, 

epicontinental caldera-forming eruptions involve magma 

volumes of 100-1,000 km3, whereas eruptions with volumes of 1-100 km3 form stratovolcanoes and small epicontinental ring structures. The total volume (exposed 

plus inferred volume of unexposed core plug) of the Seaman volcanic center is only about 20 km3, at the low end 

of the stratovolcano-small ring structure range. The volume of magma emplaced into the Seaman volcanic center 

chamber was probably insufficient to initiate the processes 

that drive and sustain ash-flow caldera-style eruptions. 

Magmatic volatile saturation and the rapid vesiculation that occurs when a magma is depressurized are essential to initiation of ash-flow eruptions (Smith, 1979). 

Relatively low inferred magma volatile contents may have 

contributed to the style of eruption at the Seaman volcanic 

center. Primary hydrous silicate minerals (hornblende and 

biotite), whose nucleation indicates relatively volatile rich 

magma, are almost absent from lava flow samples of the 

Seaman volcanic center and are not abundant in samples 

of the core plug. Samples of the lava flows and core plug 

contain rare phenocrysts of hornblende, whereas some 

samples of the core plug contain biotite phenocrysts; most 
SUMMARY AND CONCLUSIONS 17 

1,000~------------------------------~ 

z 

0 

_I 

_I 

WITHIN PLATE 

~ VOLCANIC ARC ioo n::: 

w 

o._ 

(f) 

1---

n::: 

<( 

o._ 

z 10 

2 

:J 

co 

0 

z 

10,000 

z 

0 

_I 

_I 

21,000 

n::: 

w 

o._ 

(f) 

I--

n::: 100 <( 

o._ 

z 

2 

:J 

0 

co 

:J 

n::: 1 

1 

•RADIAL DIKE 

.& CORE PLUG 

+LAVA FLOWS 

OROGENIC 

10 100 1,000 

YTTRIUM, IN PARTS PER MILL I ON 

WITHIN PLATE 

.RADIAL DIKE 

.&CORE PLUG 

+LAVA FLOWS 

VOLCANIC ARC OROGENIC 

10 100 1 '000 

YTTRIUM+ NIOBIUM, 

IN PARTS PER MILLION 

10,000 

Figure 9. Tectonic setting-trace element discriminant variation diagrams for components of the Seaman volcanic center; abundances 

expressed in parts per million. Tectonic setting-composition 

boundaries from Pearce and others (1984). A, Rubidium versus 

yttrium plus niobium. B, Niobium versus yttrium. 

contain phenocrysts of pyroxene and lesser amounts of 

hornblende. 

Volatile contents (LOI, tables 1, 4) of components of 

the Seaman volcanic center are low and may corroborate 

the inference, from the scarcity of hydrous phenocryst 

phases, that magmas of the Seaman volcanic center were 

relatively volatile poor. It is important to note, however, 

that measured volatile abundances in fresh volcanic rocks 

are typically less than magmatic abundances because volatile components are lost through exsolution and outgassing 

accompanying and following eruption. Analyzed volatile 

contents may, however, be greater than magmatic values 

due to hydration that accompanies glassy groundmass 

devitrification. The best estimates of pre-eruption volatile 

contents are derived from analysis of glass inclusions; no 

glass inclusions have been identified in samples of the 

Seaman volcanic center. The lava flows, core plug, and 

radial dike contain 1.41, 1.82 and 2.20 weight percent volatile constituents, respectively (LOI, table 4). 

Estimated magmatic volatile contents, which provide 

an indication of volatile contents necessary for initiation 

of ash-flow eruptions, are summarized by Whitney (1988). 

These estimates range from 2.2 (Bishop Tuff) to 4.0 (Fish 

Canyon Tuff) weight percent H20. Relatively elevated 

volatile abundances probably did not develop in the chamber of the Seaman volcanic center until late in its evolution. The volatile abundances in magmas of the center 

were probably too low to initiate an ash-flow calderaforming eruption. 

The structural setting in the rocks that host the Seaman volcanic center also may have inhibited the rapid 

vesiculation necessary for the onset of a caldera-forming 

eruption. A densely faulted and fractured terrane is more 

likely to experience catastrophic failure, leading to spontaneous depressurization and vesiculation in an underlying 

magma chamber, than a geologically coherent terrane. The 

density of intersecting faults in older rocks exposed 

peripheral to the Seaman volcanic center is low (du Bray 

and Hurtubise, in press), with the result that the host rocks 

of the center confined the center to a relatively coherent 

eruption style. 

SUMMARY AND CONCLUSIONS 

The Seaman volcanic center in southern Nevada is a 

composite, middle Tertiary stratovolcano principally composed of dacite lava flows and interbedded pyroclastic 

flows and lahars, a hypabyssal core plug, and a radial 

dike. The volcano developed through initial eruption of 

lava flows, forming a stratovolcano, and subsequent intrusion and solidification of a core plug and a radial dike. 

The Seaman volcanic center probably evolved from 

magma emplaced into a chamber that cooled and solidified from its walls inward. Sidewall crystallization 

resulted in a refractory-component-depleted residual silicate liquid, which by mixing with undepleted magma 

caused the composition of undepleted magma in the central part of the chamber to become progressively evolved. 

Chemical and field relations suggest that the volcanic center erupted progressively more evolved lava flows from a 

series of small, high-level reservoirs. Geochemical zonation in the chamber of the volcanic center probably 

resulted primarily from fractionation of plagioclase, 

hypersthene, and hornblende. The chamber of the Seaman 
18 THE SEAMAN VOLCANIC CENTER-A RARE MIDDLE TERTIARY STRATOVOLCANO 

volcanic center may have formed a stratovolcano instead 

of a caldera because· of its small size, relatively low volatile content, and a structural setting not conducive to 

development of an ash-flow caldera. 

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complex, southwestern Nevada volcanic field: Journal of 

Geophysical Research, v. 94, p. 6021-6040. 

Sillitoe, R.H., 1973, The tops and bottoms of porphyry copper 

deposits: Economic Geology, v. 68, p. 799-815. 

Smith, R.L., 1979, Ash-flow magmatism: Geological Society of 

America Special Paper 180, p. 5-27. 

Stewart, J.H., and Carlson, J.E., 1978, Geologic map of Nevada: 

U.S. Geological Survey, scale 1:500,000. 

Swanson, D.A., Dzurisin, D., Holcomb, R.T., Iwatsubo, E.Y., 

Chadwick, W.W., Casadevall, T.J., Ewert, J.W., and Heliker, 

C.C., 1987, Growth of the lava dome at Mount St. Helens, 

Washington (USA) 1981-1983: Geological Society of 

America Special Paper 212, p. 1-16. 

Taggart, J.E., Lindsay, J.R., Scott, B.A., Vivit, D.V., Bartel, A.J., 

and Stewart, K.C., 1987, Analysis of geologic materials by Xray fluorescence spectrometry, in Baedecker, P.A., ed., 

Methods for geochemical analysis: U.S. Geological Survey 

Bulletin 1770, p. El-El9. 

Published in the Central Region, Denver, Colorado 

Manuscript approved for publication December 18, 1992 

Edited by Judith Stoeser 

Type composed by Shelly A. Fields 

Graphics prepared by Edward du Bray 

Taylor, W.J., and Bartley, J.M., 1988, Stratigraphy and structure in 

the North Pahroc and eastern Seaman Ranges, in Weide, D.L., 

and Faber, M.W., eds., Cordilleran Section fieldtrip 

guidebook: Geological Society of America, p. 5-6. 

Taylor, W.J., Bartley, J.M., Lux, D.R., and Axen, G.J., 1989, 

Timing of Tertiary extension in the Railroad Valley-Pioche 

transect, Nevada-Constraints from 40 Arf39 Ar ages of volcanic 

rocks: Journal of Geophysical Research, v. 94, p. 7757-7774. 

Thompson, R.N., Morrison, M.A., Dickin, A.P., Hendry, G.L., 

1983, Continental flood basalts ... Arachnids rule OK?, in 

Continental flood basalts and xenoliths: Cheshire, England, 

Shiva Publishing, p. 158-185. 

Tschanz, C.M., and Pampeyan, E.H., 1961, Preliminary geologic 

map of Lincoln County, Nevada: U.S. Geological Survey 

Mineral Investigations Field Studies Map MF-206, scale 

1:200,000. 

---1970, Geology and mineral deposits of Lincoln County, 

Nevada: Nevada Bureau of Mines and Geology Bulletin 73, 

188 p. 

Whitney, J.A., 1988, The origin of granite-The role and source of 

water in the evolution of granitic magmas: Geological Society 

of America Bulletin, v. 100, p. 1886--1897. 

Williams, P.L., 1967, Stratigraphy and petrography of the 

Quichapa Group, southwestern Utah and southeastern Nevada: 

Seattle, University of Washington, Ph.D. thesis, 139 p. 

Wood, D.A., Joron, J.-L., and Treuil, M., 1979, A re-appraisal of 

the use of trace elements to classify and discriminate between 

magma series erupted in different tectonic settings: Earth and 

Planetary Science Letters, v. 45, p. 326--336. 

*U.S. GOVERNMENT PRINTING OFFICE: 1993- n4-049/66047 

SELECTED SERIES OF U.S. GEOLOGICAL SURVEY PUBLICATIONS 

Periodicals 

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Technical Books and Reports 

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of wide and lasting interest and importance to professional scientists and 

engineers. Included are reports on the results of resource studies and of 

topographic, hydrologic, and geologic investigations. They also include 

collections of related J?apers addressing different aspects of a single scientific topic. 

Bulletins contain significant data and interpretations that are of 

lasting scientific interest but are generally more limited in scope or geographic coverage than Professional Papers. They include the results of 

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as collections of short papers related to a specific topic. 

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