JOURNAL OF PETROLOGY
VOLUME 38
NUMBER 8
PAGES 1021–1046
1997
Petrogenesis of Tertiary Andesite Lava Flows
Interlayered with Large-Volume Felsic
Ash-Flow Tuffs of the Western USA
DANIEL R. ASKREN1∗, MICHAEL F. RODEN2 AND JAMES A. WHITNEY2
1
DEPARTMENT OF GEOLOGY AND PHYSICS, GEORGIA SOUTHWESTERN STATE UNIVERSITY, AMERICUS, GA 31709, USA
2
DEPARTMENT OF GEOLOGY, UNIVERSITY OF GEORGIA, ATHENS, GA 30602, USA
RECEIVED OCTOBER 11, 1995 REVISED TYPESCRIPT ACCEPTED MARCH 18, 1997
The San Juan volcanic field (Colorado), the Indian Peak
volcanic field (Utah–Nevada) and the central Nevada volcanic
field formed during Oligocene and Miocene times. Each field is
characterized by >3000 km3 of rhyolitic and dacitic ash-flow
tuff sheets and smaller volumes (<300 km3) of interlayered
andesitic lavas. In each field, andesite lavas erupted from vents
within and peripheral to calderas formed by approximately
contemporaneous felsic ash-flow eruptions. Olivine andesites occur
only peripheral to calderas, whereas hornblende andesites occur
only within calderas. Pyroxene andesites occur at both locations.
The parental magmas of the andesites formed by partial melting
of mantle material, as shown by the presence of olivine or Crspinel in some units. Compositional evolution of andesites appears
to have been controlled in part by crystal fractionation at 0·2–0·8
GPa, based on compositional and mineralogic comparisons with
liquid lines of multiple saturation from experiments involving
data for andesitic compositions. Modeling of bulk compositional
variations suggests crystal fractionation may have been accompanied
by mixing with dacitic or rhyolitic magmas. At locations beneath
present-day calderas, upward-migrating basaltic to andesitic
magmas encountered dacitic or rhyolitic magmas. The more felsic
magmas gravitationally trapped the rising mafic magmas. These
trapped magmas evolved from basaltic or olivine andesitic
compositions to hornblende andesitic compositions by crystal
fractionation combined with mixing with dacitic or rhyolitic
magmas. Subsequent eruption or crystallization of dacitic and
rhyolitic magmas removed the density contrasts and allowed
hornblende andesites to erupt within calderas. In contrast, parental
mafic magmas at locations peripheral to present-day calderas
ascended without encountering dacitic or rhyolitic magmas, and
these erupted as olivine andesites. Thus andesites initially evolved
∗Corresponding author. Telephone: (912) 931-2329. Fax: (912) 9312770. e-mail: [email protected]
separately from the region’s voluminous dacite and rhyolite
magmas, but later mixing with the felsic magmas influenced
compositions of many andesites.
KEY WORDS:
andesite; ash-flow tuff; fractional crystallization; magma
mixing
INTRODUCTION
The San Juan (Colorado), Indian Peak (Utah–Nevada)
and the central Nevada volcanic fields each developed
a large, nested caldera complex in response to episodic
eruptions of large-volume ash-flow tuffs (Fig. 1) during
the Oligocene and Miocene as volcanic activity moved
along a southward-sweeping front across the western
USA (between latitudes 39° and 37°N; (Best et al.,
1989b). These fields are characterized by individual
tuffs as large as 3000 km3 and relatively small volumes
of interlayered andesite lavas (<300 km3). In each
volcanic field, these andesites were the most mafic
magma erupted contemporaneously with the felsic ash
flows (Askren, 1992). Precaldera volcanic activity (35–30
Ma) in the San Juan field was dominated by extrusion
of andesitic lavas before the voluminous ash-flow
eruptions; in contrast, precaldera andesitic activity was
minimal in the other fields.
 Oxford University Press 1997
JOURNAL OF PETROLOGY
VOLUME 38
NUMBER 8
AUGUST 1997
Fig. 1. Regional map of western USA showing selected Tertiary volcanic fields (Ekren et al., 1974; Ratté et al., 1984; Steven et al., 1984;
Lipman & Sawyer, 1988; Best et al., 1989a, 1993) and plate tectonic situation at time of volcanism. Approximate location of convergent
and oblique transform plate boundaries at 30–20 Ma from Lipman (1992).
Proposed models for development of many large
felsic magma systems require close genetic relationships
with more mafic magmas. For example, Lipman et al.
(1978, p. 75) suggested that silicic ash-flow tuffs in the
central San Juan volcanic field are ‘genetically related
differentiates’ of more mafic magmas. In contrast,
Whitney (1988) proposed that hydrous basalt and
andesite magmas may provide heat and volatiles
necessary for anatexis within the crust. Best et al.
(1989b) suggested that the production of voluminous
ash-flow tuffs of the Indian Peak volcanic field was
initiated by mafic magma production in the mantle.
Huppert & Sparks (1988) modeled formation of silicic
magmas by emplacement of basaltic sills into the crust.
The presence of andesite lava flows between ash-flow
tuff sheets in all three fields provides an opportunity
to examine relationships between silicic magmas and
contemporaneous, more mafic ones. Our approach was
to use field relations, mineral compositions and bulk
rock compositions to constrain andesite petrogenesis
and the role of these andesites in the development of
the larger felsic magma systems. We also compare
compositions of inferred parental magmas with those
of modern arc magmas and discuss the effects of
distance to the Tertiary paleosubduction zone on
parental magma composition.
FIELD RELATIONSHIPS
In all three volcanic fields, andesites have been named
as stratigraphic units bracketed by felsic tuffs (e.g.
Lipman, 1975; Best et al., 1989b). Therefore all
stratigraphically correlative andesites belong to the
same unit. Most such units outcrop within very limited
areas, consistent with eruption from single or closely
spaced vents. However, four units include outcrops
that are widely scattered, small and not necessarily
related; for these, we restricted our study to large
exposures from single locations. In each field, andesitic
lavas were erupted from vents within and peripheral
to calderas formed by approximately contemporaneous
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ASKREN et al.
ANDESITES OF WESTERN USA
Fig. 2. Outcrop locations of andesite lavas interlayered with dacite
and rhyolite ash-flow tuffs in the San Juan volcanic field. Black,
hatched and stippled areas represent approximate outcrop locations
of olivine, pyroxene and hornblende andesites, respectively. Calderas
related to contemporaneous ash-flow tuffs are shown, and associated
tuffs are listed in the stratigraphic column; LGS is southern extension
of La Garita caldera. Abbreviations of ash-flow tuffs are: NM,
Nelson Mtn Tuff; RC, Rat Creek Tuff; SS, Snowshoe Mtn Tuff;
WP, Wason Park Tuff; CR, Carpenter Ridge Tuff; FC, Fish Canyon
Tuff; MP, Masonic Park Tuff. Abbreviations for andesite units are:
BH, andesite of Bristol Head; TM, volcanics of Table Mtn; HU,
Huerto Andesite; SM, Sheep Mtn Andesite (see text for references).
felsic ash-flow eruptions (Figs 2, 3 and 4). Olivine
andesites occur only peripheral to calderas, whereas
hornblende andesites occur only within calderas. Pyroxene andesites occur in both locations.
San Juan Volcanic Field
Volcanism in the San Juan field began at ~35 Ma
with eruption of the dominantly andesitic lava flows
of the Conejos Formation (Lipman, 1975; Zielinski &
Lipman, 1976; Lanphere, 1988; Lipman & Sawyer,
1988; Colucci et al., 1991). At ~30 Ma, activity changed
to voluminous eruptions of dacitic and rhyolitic ashflow tuffs (e.g. Fish Canyon Tuff; Whitney & Stormer,
1985). Minor amounts of andesitic magmas erupted
between eruptions of felsic magmas. Following this
explosive ash-flow ‘stage’, volcanism changed to eruptions of bimodal basalt and rhyolite lavas of the
Hinsdale Formation at ~26 Ma (Lipman et al., 1970).
Andesitic units associated with the dacitic and rhyolitic
ash flows in the central San Juan field are defined by
their ascending stratigraphic position as follows (Fig.
2; Lipman, 1975): Sheep Mountain Andesite, Huerto
Andesite, volcanics of Table Mountain and the andesite
of Bristol Head. Generally, these units are two-pyroxene
andesites, although some samples of Huerto Andesite
contain olivine instead of orthopyroxene, and the
volcanics of Table Mountain as well as the andesite
of Bristol Head commonly contain hornblende and
biotite in addition to orthopyroxene (Lipman, 1975;
Williams, 1991; Askren et al., 1991; Askren, 1992).
The Sheep Mountain Andesite outcrops along the
slopes of Saddle and Sheep Mountains as 200–300 m
thick, nearly continuous exposures of phenocryst-rich
lava flows and interlayered breccia (Lipman, 1975).
These outcrops may form a portion of an exogenous
dome. Exposures on the two mountains extend for ~5
km each and are ~4 km apart. Saddle and Sheep
Mountains are separated by an erosional valley cut by
the west fork of the San Juan River. We infer that
the Sheep Mountain Andesite was originally continuous
across this valley and covered an area of at least 35
km2 with a volume of at least 10 km3. The Sheep
Mountain Andesite is underlain by the 28·6 Ma
Masonic Park Tuff and overlain by the 27·8 Ma
dacitic Fish Canyon Tuff (Lanphere, 1988). Locally,
the Sheep Mountain Andesite is overlain by the
Chiquito Peak Tuff (28·4 Ma; Lipman et al., 1996), a
newly recognized dacite with a source in the southeastern San Juan volcanic field.
The Huerto Andesite includes all andesites stratigraphically bracketed by the Fish Canyon and Carpenter Ridge Tuffs (27·6 Ma; Lipman, 1975; Lanphere,
1988). Many outcrops of this andesite are small (<1
km2) and widely scattered. We have therefore restricted
our study to the two largest Huerto Andesite exposures
(combined volume of ~200 km3); these two separate
areas are composed of numerous lava flows and minor
amounts of interlayered breccia (Askren et al., 1991).
Although samples from both areas include two-pyroxene
andesites, some from the area west of La Garita
caldera (Fig. 2) contain olivine instead of orthopyroxene.
Most contacts between the andesite and the Fish Canyon
Tuff are conformable; however, local disconformities are
present where andesite lavas fill small (5 m wide)
channels within the Fish Canyon Tuff. The two
exposures of Huerto Andesite discussed here and by
Askren et al. (1991) are separated by >20 km, so
magmas probably erupted from two separate chambers.
However, magmas and preeruptive equilibration conditions that produced andesites at both areas were
similar (Askren et al., 1991).
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AUGUST 1997
Fig. 3. Outcrop locations of andesite lavas interlayered with dacite and rhyolite ash-flow tuffs in the Indian Peak volcanic field (shading
patterns have same significance as in Fig. 2). Abbreviations of ash-flow tuffs are: IM, Isom Fm; RG, Ripgut Fm; LD, Lund Fm; RS,
Ryan Spring Fm; WS, Wah Wah Springs Fm; CW, Cottonwood Wash Fm; ED, Escalante Desert Fm. Abbreviations for andesite units
of these formations are: IA, Isom Fm; LA, Lund Fm; RA, Ryan Springs Fm; WA, Wah Wah Springs Fm; EA, Escalante Desert Fm (see
text for references).
The volcanics of Table Mountain are a series of
intermediate composition lava flows, domes, breccias
and tuffs (Steven & Lipman, 1973; Williams, 1991)
that are stratigraphically bound by the Carpenter Ridge
and Wason Park (27·2 Ma; Lanphere, 1988) Tuffs.
The lava flows and associated pyroclastics cover an
area of ~70 km2 and have a volume of ~15 km3. We
believe the volcanics of Table Mountain are the product
of a single magmatic system because inferred eruptive
vents occur within a 10 km2 area (Williams, 1991), no
other lithologies are interlayered with these rocks, and
all associated eruptions occurred within 0·4 Ma (as
indicated by ages of felsic ash-flow tuffs; Lanphere,
1988).
The andesite of Bristol Head (Steven, 1967; Steven
et al., 1974) outcrops along the steep sides and gently
inclined summit of Bristol Head. This unit consists of
a few thin lava flows with a total thickness of <300
m and an exposed area of ~10 km2. Continuous
exposures along the south, vertical face of Bristol Head
affirm that this unit is composed of a closely related
sequence of lava flows. The andesite of Bristol Head
is underlain by the Wason Park Tuff. No rocks overlie
the andesite. It is probably older than the Nelson
Mountain Tuff (26·1 Ma; Lanphere, 1988), and it
could be older than the Snowshoe Mountain Tuff (Fig.
2; Steven et al., 1974).
Indian Peak volcanic field
The Indian Peak field (Best et al., 1989a, 1989b)
includes five nested calderas and inferred calderas in
the eastern Basin and Range Province along the
1024
ASKREN et al.
ANDESITES OF WESTERN USA
Fig. 4. Outcrop locations of andesite lavas interlayered with dacite
and rhyolite ash-flow tuffs in the central Nevada volcanic field (black
area represents approximate outcrop locations of Cr-spinel-bearing
andesite of Pritchards Station; other shading patterns have same
significance as in Fig. 2). Abbreviations of ash-flow tuffs are: FT,
Fraction Tuff; PF, Pahranagat Fm; BT, tuff of Big Ten Peak; GK,
tuff of Goblin Knobs; LC, tuff of Lunar Cuesta; SP, Shingle Pass
Tuff; OL, tuff of Orange Lichen; MT, Monotony Tuff; HC, Tuff
of Hot Creek Canyon; WB, Windous Butte Fm; PT, Pancake
Summit Tuff; SC, Stone Cabin Fm. Abbreviations for andesite units
are: CM, volcanics of Citadel Mtn; RV, andesite of Reveille; PS,
andesite of Pritchards Station (see text for references).
Utah–Nevada border (Fig. 3). Volcanic activity began
at ~35–34 Ma with eruptions of small volumes of
rhyolitic tuffs and lavas onto Paleozoic and minor
Mesozoic sedimentary rocks (Best et al., 1989a). Voluminous ash flows (some >1000 km3) erupted from
32 to 27 Ma in compositional cycles (Best et al., 1989a):
generally, each cycle commenced with a rhyolitic ash
flow, followed by a more voluminous dacitic ash flow.
The eruptive cycles terminated with a trachydacitic
ash flow. Intercalated between these felsic tuffs are
small volumes of andesitic lava flows.
As for the San Juan field, the andesite units are
named on the basis of their stratigraphic relationships
with intercalated ash-flow tuffs (Best et al., 1989a,
1989b). From oldest to youngest, the units are (Fig.
3): andesites of the Escalante Desert, Wah Wah Springs,
Ryan Springs, Lund and Isom Formations. Generally,
these andesites contain phenocrysts of clino- and
orthopyroxene, plagioclase and magnetite. Additionally,
hornblende is common in andesites of the Escalante
Desert and Isom Formations and rare in the andesite
of the Ryan Springs Formation. Olivine and Cr-spinel
occur in the andesite of the Wah Wah Springs
Formation and in one sample from the andesite of
the Escalante Desert Formation. Cr-spinel occurs as
inclusions in olivine of both units. It also occurs as
discrete, small crystals (<0·25 mm) in close proximity
to olivine in the former unit, where its subhedral
morphology and lack of reaction rims suggest equilibrium.
The andesite of the Escalante Desert Formation
includes all intermediate lavas stratigraphically bracketed by the felsic Marsden and Lamerdorf Tuff
members of the Escalante Desert Formation (~32 Ma;
Best & Grant, 1987; Best et al., 1989a). Outcrops of
this andesite are small (<1 km2) and widely scattered.
We restricted our study to two specific areas. One
area is along the northern edge of the Indian Peak
caldera complex, and the other area is ~15 km
northeast of these calderas. Outcrops at both areas
cover ~10 km2 each and are composed of a series of
discontinuously exposed lava flows. These lavas are
chiefly two-pyroxene andesites; however, hornblende
and biotite are additionally present in some samples
from the area along the northern edge of the complex.
The andesite of the Wah Wah Springs Formation
is defined as intermediate lavas stratigraphically bracketed by the felsic tuffs of the Cottonwood Wash (30·6
Ma) and Wah Wah Springs Formations (30 Ma; Best
& Grant, 1987; Best et al., 1989a). Several small
outcrops (<5 km2) of andesite lava of this age are
widely distributed throughout the field. We investigated
the largest of these outcrops (~4 km2). This exposure
is ~40 km northeast of the calderas associated with
felsic ash-flow eruptions. Contacts between this andesite
and the felsic tuffs are conformable. The exposure is
a 100 m thick sequence of dense to vesicular olivine
andesite lava flows.
The andesite of the Ryan Springs Formation is
defined as intermediate lavas stratigraphically bracketed
by two felsic ash-flow tuff members of this formation
(the older Greens Canyon and the younger Mackleprang
Members; ~28 Ma). The andesitic lavas occur as a
series of dense, slightly oxidized, small (<1 km2) outcrops
within a 25 km2 area located inside the White Rock
caldera. Locally, this unit is conformably bound by
the felsic tuff members; however, some outcrops are
underlain by felsic tuffs of the Wah Wah Springs
Formation or overlain by Quaternary alluvium. Our
samples are from outcrops stratigraphically overlain or
underlain by felsic tuff members of the Ryan Springs
Formation. Although outcrops are small and discontinuous, we believe they represent a closely related
sequence of lavas: outcrops are distributed over a small
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JOURNAL OF PETROLOGY
VOLUME 38
area, no other lithologies are interlayered between
these lavas and felsic members of the Ryan Spring
Formation, and lithologic characteristics of the andesitic
lavas at each outcrop are similar.
The andesite of the Lund Formation overlies the
dacitic tuff of the Lund Formation (27·9 Ma; Best &
Grant, 1987; Best et al., 1989a) and occurs along the
outside edge of the White Rock caldera (source of the
dacitic tuff ). This andesite outcrops as a single, 200
m thick sequence of poorly exposed lavas which covers
~10 km2.
The andesite of the Isom Formation occurs as
intermediate lavas stratigraphically bracketed by the
older Ripgut Formation and younger felsic tuffs of the
Isom Formation (~27 Ma; Best et al., 1989a). We
investigated one 150 m thick sequence of vesicular
lava flows that outcrops over 1 km2 along the sides
and top of White Rock Peak. This location is in the
center of the felsic caldera complex, and lithologies
include pyroxene and hornblende andesites.
Central Nevada volcanic field
Volcanic rocks that make up the central Nevada field
overlie chiefly Paleozoic sedimentary rocks. Volcanic
activity commenced at ~35 Ma (Best et al., 1993), and
voluminous rhyolitic and dacitic ash-flow tuffs erupted
from this time until ~18 Ma (Best et al., 1993).
Volumetrically minor andesitic lavas are interlayered
between these tuffs (Fig. 4; each <5 km3). Miocene to
Pliocene Basin and Range faulting deformed the caldera
complex, and Quaternary basalts covered portions of
the complex (Ekren et al., 1974; Feuerbach et al., 1993).
Andesitic rocks in this field have been mapped in
three separate areas (Fig. 4) as unnamed units that
are stratigraphically between felsic ash-flow tuffs. From
oldest to youngest, these will be referred to informally
here as the andesite of Pritchards Station (Dixon et
al., 1972), the andesite of Reveille (Ekren et al., 1973)
and the volcanics of Citadel Mountain (Scott & Trask,
1971; Snyder et al., 1972). The andesites of Pritchards
Station and Reveille contain phenocrysts of clino- and
orthopyroxene, plagioclase, minor magnetite and minor
ilmenite. In addition, the former andesite contains rare
microphenocrysts and inclusions of Cr-spinel (<0·25
mm) in clinopyroxene phenocrysts. The latter andesite
contains hornblende phenocrysts. The volcanics of
Citadel Mountain contain phenocrysts of clino- and
orthopyroxene, plagioclase, hornblende, Fe–Ti oxides
and minor amounts of biotite.
The andesite of Pritchards Station outcrops ~10–20
km north of the central Nevada caldera complex (Fig.
4) as a 100–200 m thick series of discontinuous lava
flows. These lavas outcrop around the base of Park
NUMBER 8
AUGUST 1997
Range, a 100 km2, 600 m high horst bound on its
east and west sides by grabens. This horst is capped
by 300–400 m of dacitic Windous Butte Tuff (31·3
Ma; Best et al., 1993), which is underlain locally by
the felsic tuff of Cottonwood Canyon and the andesite
of Pritchards Station. Locally, the Stone Cabin Formation (35·3 Ma; Best et al., 1993), a felsic ash-flow
tuff, is present beneath the andesite (Dixon et al.,
1972). Because this andesite outcrops around the base
of this horst on the north, east and west sides, we
infer that andesite is continuous beneath the Windous
Butte Formation from one end of the horst to the
other, and that the andesite erupted from a single
vent or closely related vents.
The andesite of Reveille underlies the Monotony
Tuff (27·3 Ma; Best et al., 1993) in a single area within
the south central portion of the central Nevada caldera
complex (Fig. 4). This 100 m thick andesite covers
~10 km2, and it is composed of a poorly exposed
series of lava flows.
The volcanics of Citadel Mountain are a sequence
of intermediate lavas and tuffs underlain by the tuffs
of Buckskin Point (undated) and Lunar Cuesta (25·4
Ma; Best et al., 1993) near Citadel Mountain (Snyder
et al., 1972). This 350 m thick sequence covers ~20
km2. The unit was previously described as an unnamed
andesite by Ekren et al. (1974), but our data indicate
that these volcanics are predominantly dacite with
subordinate andesite.
RESULTS
Phenocryst compositions
Compositions of phenocrysts were determined by wavelength dispersive X-ray spectrometry using a Cameca
SX-50 electron microprobe at the University of South
Carolina. Generally, 15 kV accelerating voltage, a 15
nA beam current and a minimum spot size were
utilized, although a relatively large spot size was used
in the case of feldspar to minimize Na migration during
analyses. Data reduction incorporated procedures that
correct for the influence of matrix atomic number, Xray absorption and secondary fluorescence. Repeated
analyses of US National Museum standards indicate
analytical uncertainties of ~1% for all major elements.
Comprehensive data sets and sampling locations have
been given by Askren (1992) or can be obtained by
writing to the first author; representative modes and
compositions can be found in Table 1.
Plagioclase is a ubiquitous phenocryst and ranges in
modal abundance from <1% to 34%. Samples with
the lowest modal abundance typically have relatively
calcic plagioclase; the most calcic of these are An89 in
the andesite of the Isom Formation and An88 in the
1026
1027
99·33
—
0·29
19·9
13·7
0·45
11·3
<0·04
2·01
0·58
51·1
2·3
Cpx
CM12
99·16
—
<0·04
1·43
20·7
0·81
23·2
<0·04
0·82
0·20
52·0
0·8
Opx
CM12
0·35
—
3·6
46·1
—
34·0
—
0·46
—
0·06
17·5
1·18
0·04
99·34
Modal %:
SiO2
TiO2
Al2O3
Cr2O3
FeO
MnO
MgO
CaO
Na2O
K 2O
Total
98·50
20·5
14·7
0·21
8·39
0·20
4·12
0·53
49·5
3·3
Cpx
Plag
Mineral:
IA1
IA1
Sample:
Isom Formation
100·12
—
0·03
1·33
27·8
0·29
14·41
0·03
2·34
0·19
53·7
0·1
Opx
IA1
Indian Peak volcanic field
0·51
99·27
K 2O
4·69
Na2O
Total
11·0
CaO
—
Cr2O3
0·05
28·7
Al2O3
MgO
—
0·62
53·7
SiO2
TiO2
—
14·8
Modal %:
MnO
Plag
Mineral:
FeO
CM12
Sample:
Volcanics of Citadel Mtn
Central Nevada volcanic field
97·10
0·92
2·4
11·5
14·5
0·12
11·1
—
12·2
2·66
41·7
2·0
Amph
IA1
97·08
0·91
1·28
11·4
12·8
0·43
14·7
—
8·34
1·62
45·6
0·1
Amph
CM12
98·77
—
0·28
19·8
14·6
0·34
10·1
<0·04
2·15
0·60
50·9
3·4
Cpx
RV3
99·92
0·39
4·77
10·9
0·04
—
0·82
—
28·2
—
54·8
13·5
Plag
LA8
98·88
—
0·29
19·0
14·2
0·34
11·41
<0·04
2·10
0·64
50·9
6·2
Cpx
LA8
Lund Formation
100·20
0·19
2·66
15·2
0·08
—
0·67
—
31·9
—
49·5
21·6
Plag
RV3
Andesite of Reveille
98·74
—
<0·04
1·81
24·5
0·40
17·1
0·10
2·06
0·27
52·5
1·5
Opx
LA8
99·06
—
<0·04
1·60
23·7
0·46
19·4
<0·04
1·24
0·26
52·4
3·8
Opx
RV3
99·56
0·07
1·30
18·0
0·10
—
0·59
—
34·3
—
45·2
12·3
Plag
PS2
99·93
0·35
3·53
13·2
0·07
—
0·58
—
30·0
—
52·2
3·1
Plag
PS13
98·88
0·48
5·00
9·77
0·02
—
0·41
—
27·4
—
55·8
5·3
Plag
RA5
99·39
—
0·39
18·6
15·5
0·30
10·1
<0·04
2·08
0·32
52·1
2·1
Cpx
RA5
100·11
—
<0·04
1·85
23·8
0·44
19·0
0·69
1·10
0·33
52·9
3·2
Opx
RA1
Ryan Spring Formation
96·93
0·90
1·89
11·3
13·6
0·11
13·1
—
12·4
3·13
41·1
<0·1
Amph
RV10
96·58
0·93
1·81
11·2
15·4
0·14
11·1
—
11·0
1·70
43·3
0·1
Amph
RA5
98·54
—
0·19
19·7
16·0
0·16
8·1
0·33
3·19
0·57
50·3
2·5
Cpx
PS2
Andesite of Pritchards Station
Table 1: Representative compositions and modal abundances of phenocrysts from andesitic volcanic rocks
99·19
—
<0·04
1·87
26·0
0·32
16·2
0·16
1·62
0·32
52·7
1·2
Opx
PS2
99·04
—
0·04
1·64
22·1
0·44
21·4
0·06
1·87
0·35
51·4
1·7
Opx
PS13
99·65
0·45
3·4
13·2
0·15
—
0·75
—
29·9
—
51·8
0·3
Plag
WA4
98·98
—
0·24
21·3
16·0
0·10
6·16
0·69
2·97
0·72
50·8
0·4
Cpx
WA4
99·23
—
—
0·13
44·2
0·20
16·0
<0·04
—
<0·04
38·7
8·9
Oliv
WA4
Wah Wah Springs Formation
98·68
—
0·25
19·8
14·3
0·24
10·2
0·28
2·42
0·69
50·5
18·9
Cpx
PS13
97·80
—
—
0·06
7·42
0·35
30·0
47·5
11·4
1·07
<0·04
0·1
Spnl
WA4
96·26
—
—
0·09
5·59
0·32
50·3
25·0
9·64
5·32
<0·02
<0·1
Spnl
PS2
ASKREN et al.
ANDESITES OF WESTERN USA
1028
0·65
—
0·05
13·2
3·14
0·45
98·99
MgO
CaO
Na2O
K 2O
Total
30·4
Al2O3
MnO
—
FeO
51·1
12·7
Modal %:
SiO2
Plag
TiO2
EA1
Mineral:
EA14
98·83
—
0·22
21·4
13·7
99·27
—
<0·04
0·97
19·2
1·35
25·3
1·35
0·10
51·0
0·7
Opx
EA10
97·64
1·23
1·82
12·5
16·1
—
9·65
11·5
2·44
42·4
<0·1
Amph
EA15
94·82
9·32
0·45
—
13·0
0·14
16·6
13·7
5·31
36·3
<0·1
Bio
EA1
99·24
—
—
0·14
36·9
0·40
24·0
—
—
37·8
0·9
Oliv
BH21
100·73
0·57
4·66
10·4
<0·05
—
0·40
28·3
—
56·4
17·3
Plag
BH21
99·55
—
0·37
21·2
14·9
0·85
8·51
1·01
0·21
52·5
2·2
Cpx
BH21
99·81
—
<0·04
1·06
24·8
0·98
18·9
0·63
0·14
53·3
<0·1
Opx
97·79
0·93
2·42
11·2
13·4
0·21
12·5
13·1
3·23
40·8
0·8
Amph
BH27
96·51
9·37
0·51
—
14·7
0·20
14·4
13·6
6·33
37·4
<0·1
Bio
BH21
99·07
0·82
4·80
9·99
0·06
—
0·50
27·4
—
55·5
24·2
Plag
SM3
99·32
—
0·39
17·8
14·0
0·34
12·1
3·26
0·73
50·7
7·9
Cpx
SM3
99·72
—
0·06
1·88
22·5
0·61
20·7
2·23
0·54
51·2
2·8
Opx
SM3
NUMBER 8
98·29
0·61
4·79
10·1
<0·05
0·49
9·81
1·09
0·22
51·9
6·7
Cpx
EA14
Sheep Mountain Andesite
VOLUME 38
—
0·29
27·9
—
54·6
17·0
Plag
EA14
Andesite of Bristol Head
Escalante Desert Formation
Sample:
San Juan volcanic field
Indian Peak volcanic field
Table 1: continued
JOURNAL OF PETROLOGY
AUGUST 1997
ASKREN et al.
ANDESITES OF WESTERN USA
Fig. 5. Compositions and modal abundances of plagioclase phenocrysts. Crosses indicate mean composition for each unit. Solid boxes
indicate one standard deviation from the mean. Stippled boxes mark
total observed range of compositions. Values to the right of each
box are ranges of modal abundances of plagioclase phenocrysts
observed for each unit.
andesite of Pritchards Station (Fig. 5). Mean values
within each andesite unit range from An44 to An78
(Fig. 5). Normal and reverse zoning with changes in
compositions of 10–30% An and oscillatory zoning
with changes in compositions of 1–10% An are
common. Such compositional variations together with
complex zoning are typical of many orogenic andesites
(Gill, 1981).
Clinopyroxene is present as phenocrysts in all lavas,
whereas orthopyroxene is generally restricted to olivinefree lavas. Both orthopyroxene, ranging in composition
from En60 to En79 [classified as enstatite, following
Morimoto et al. (1988)] as well as clinopyroxene,
Wo33–44, Fs8–20, En40–52 [classified as augite, following
Morimoto et al. (1988)] are dominated by quadrilateral
components and are similar to pyroxenes of typical
orogenic andesites (Gill, 1981). Some isolated grains
in samples IA3, WA4 and EA1 contain significant
contents of components outside the pyroxene quadrilateral (up to 6·5% Al2O3, 0·5% Na2O and 1%
TiO2). These components may reflect crystallization at
elevated pressures (Wass, 1979; Bédard et al., 1988),
although these grains appear to be in textural equilibrium with other phases.
Amphibole is common (modal abundances up to
6%) in the more SiO2-rich andesites and occurs with
olivine in only one sample (EA1). In most cases,
amphiboles are associated with two pyroxenes and
appear texturally to be in equilibrium with the groundmass phases. Most amphiboles belong to the calcic
amphibole group of Leake (1978), although those in
the andesites of the Isom Formation and Bristol Head
are transitional from calcic amphibole to sodic–calcic
amphibole. Biotite is much rarer than amphibole and
is always associated with amphibole. Biotites are
generally Fe rich and have ubiquitous coronas of iron
oxide granules.
Olivine is a relatively rare phenocryst which is
restricted to lavas with <60% SiO2, as has been
observed for andesitic lavas elsewhere (e.g. Luhr,
1990). Olivine compositions range from Fo57 to Fo87.
Texturally, the olivines appear to be in equilibrium
except in sample EA1, which also contains hornblende
and biotite. In this sample olivine is rimmed by crystals
of orthopyroxene, suggesting a reaction relationship
between olivine and melt.
Chromium-spinel (containing up to 48% Cr2O3)
occurs as inclusions in olivine in one sample (EA1) of
the andesite of the Escalante Desert Formation and
as inclusions as well as discrete grains in the andesite
of the Wah Wah Springs Formation. In the andesite
of Pritchards Station, this mineral occurs as inclusions
in clinopyroxene and as discrete grains. Cr-spinel
inclusions are sub- to euhedral, and grain boundaries
between the inclusions and host phenocrysts show no
indication of disequilibrium. The olivine and clinopyroxene host phenocrysts in the andesites of the Wah
Wah Springs Formation and Pritchards Station appear
to be in equilibrium with matrix phases, whereas the
olivine grains hosting Cr-spinel in the andesite of the
Escalante Desert Formation are rimmed by orthopyroxene. The occurrence of spinel without olivine in
the andesite of Pritchards Station may indicate earlyformed olivine reacted with melt. Such a process is
consistent with the crystallization history of many
basalts in which olivine and Cr-spinel coprecipitate
early (Haggerty, 1976; Danyushevsky & Sobolev, 1996).
Magnetite, with or without ilmenite, is present as
phenocrysts and in the groundmass in all units; pervasive
secondary oxidation affected these phases in most cases.
Whole-rock compositions
Approximately one hundred samples were analyzed for
major element abundances by X-ray fluorescence
spectrometry (Askren, 1992). Many of these were
also analyzed for selected trace elements by X-ray
fluorescence spectrometry, and samples of each unit
1029
JOURNAL OF PETROLOGY
VOLUME 38
with minimum and maximum SiO2 contents were
analyzed for trace elements by neutron activation
(Table 2). Most lavas have 55–65% SiO2 and 1–5%
MgO, all are quartz-normative, and most have relatively
low total alkali contents and thus are basaltic andesites
to dacites (Fig. 6). Three of the four units from the
San Juan field include trachyandesites. In the Great
Basin, andesites predominate except for the relatively
MgO-rich (7–8%) basaltic andesites of the Wah Wah
Springs Formation and the dacites of Citadel Mountain.
There are some intriguing inter-field and intra-field
distinctions between andesite units. The lavas from the
central Nevada and San Juan fields show nearly
identical covariation of MgO with SiO2 contents (Fig.
7), whereas the lavas from the Indian Peak field have
slightly higher MgO contents relative to SiO2. In the
Indian Peak field, the andesite of the Wah Wah Springs
Formation is richer in MgO than all other units; in
the San Juan field, the Sheep Mountain Andesite is
distinctly richer in MgO than other San Juan andesites.
Phosphorus content [and K2O in Askren (1992)] at a
fixed SiO2 content (58%) tends to increase from west
to east through all three fields and is especially high
in the lavas of the San Juan field. Here, P2O5 decreases
with increasing SiO2, whereas it is only poorly correlated
with SiO2 in the other two fields. One unit in the
central Nevada field, the andesite of Reveille, has
unusually low P2O5 contents (<0·2%), perhaps because
of fractionation of apatite (an accessory phase in all
units) from parental magmas. Titania shows similar
behavior to P2O5. Titania is inversely correlated with
SiO2 in the lavas of the two eastern fields, perhaps
because of fractionation of Fe–Ti oxides. In central
Nevada, however, TiO2 content is nearly constant,
with the exception of the andesite of Pritchards
Station—which has the highest TiO2 content (up to
1·3%) of any unit studied here. In this andesite,
phenocrysts of Fe–Ti oxides are small and rare,
and Cr-spinel inclusions are present in clinopyroxene.
Therefore these lavas may have relatively high TiO2
contents because Cr-spinel, rather than Fe–Ti oxides,
was an early-fractionating phase. Lower TiO2 contents
in more SiO2-rich lavas from this unit may have
resulted from subsequent fractionation of Fe–Ti oxides.
Generally, the andesitic lavas from the three volcanic
fields have similar abundances of incompatible trace
elements (Ba, Sr, Zr, Rb, La, Th and Ta; Fig. 8).
However, the relatively alkaline lavas of the San Juan
field (Fig. 6) tend to have lower abundances of Rb,
La and Th than andesitic lavas from the other two
fields. Differences also exist between units from the
same field. For example, the andesite of Reveille is
characterized by higher Ba and Sr and lower Zr, La,
Th and Ta than the andesite of Pritchards Station
(Fig. 8). Moreover, contrasting trends of trace element
NUMBER 8
AUGUST 1997
abundances with increasing SiO2 occur: Ba increases
with increasing SiO2 content in the andesite of Pritchards Station and decreases with SiO2 content in the
andesite of Reveille. As SiO2 increases within a single
unit, Rb generally increases, whereas Zr most commonly
remains constant, Sr decreases or remains constant
and Ba may increase or decrease (Fig. 8).
Compositionally, most of the lavas are similar to the
average orogenic andesites of Gill (1981) and have low
total FeO/MgO ratios typical of calc-alkaline rocks
(e.g. Miyashiro, 1974; Gill, 1981). Some of the relatively
alkaline lavas from the San Juan field, however, have
relatively high FeO/MgO ratios. All of the lavas show
important trace element similarities with subductionrelated lavas: on a chondrite-normalized abundance
diagram (Fig. 9) the lavas have negative slopes with
large relative depletions of Nb, Ta and Ti compared
with elements of similar compatibility. All of the lavas
have high Ba/Ti ratios (>450), and most have La/Nb
(>2), Ba/La (>15) and La/Th (<7) ratios typical of
subduction-related magmas (Gill, 1981). The Nb and
Ta depletions may result from retention of these
elements by a residual mineral in refractory mantle
material (McDonough, 1991) or by reaction between
primary melt and mantle peridotite (Kelemen et al.,
1993). Ti depletions may be enhanced by subsequent
crystal fractionation of Fe–Ti oxides. Likewise, P
and Eu are depleted relative to elements of similar
compatibility, and these depletions probably result from
crystal fractionation of apatite and plagioclase.
DISCUSSION
Inferred crystallization temperatures
The presence of two pyroxenes in many of the andesites
allowed us to estimate magmatic temperatures following
Frost & Lindsley (1992) and Lindsley & Frost (1992).
The secondary oxidation of Fe–Ti oxides and scarcity
of ilmenite precluded use of oxide minerals to estimate
magmatic temperatures. Calculated pyroxene equilibration temperatures of the andesites range from 830
to 1180°C (Fig. 10; Table 3). These temperatures
are similar to estimated magmatic temperatures from
orogenic andesites (Gill, 1981) and are higher than the
calculated magmatic temperatures of the associated
felsic ash-flow tuffs, except for the tuffs of the Isom
Formation (Fig. 11). Silica content of the andesites
and pyroxene equilibration temperatures are inversely
correlated for many units (Fig. 11). Moreover, calculated
pyroxene equilibration temperatures correlate with phenocryst populations: most hornblende andesites have
pyroxene equilibration temperatures less than or equal
to those of pyroxene andesites, and pyroxene andesites
have calculated temperatures less than or equal to
1030
1031
8·5
6·4
Sc
Co
10
0·84
Lu
Cr
2·63
Yb
33
1·03
Tb
Y
6·62
Hf
312
1·74
Zr
8·3
612
Sr
Eu
108
Ce
Sm
63
La
1·04
19
Nb
Ta
9
98·47
Total
Th
2·28
LOI
129
0·30
P2O5
1137
3·96
K 2O
Rb
2·32
Na2O
Ba
1·64
3·78
CaO
0·10
MgO
5·20
MnO
15·1
0·79
Fe2O3
Al2O3
TiO2
63·0
dact
Rock type:
SiO2
CM12
Sample:
–
–
–
–
–
26
–
–
327
–
–
660
–
–
–
24
–
124
1152
100·69
1·59
0·26
3·92
2·52
3·98
1·30
0·12
5·32
15·8
0·78
65·1
dact
CM24
Central Nevada volcanic field
25
17·1
17·5
0·46
1·80
23
0·66
4·56
238
1·27
5·3
878
64
37
0·37
22
4
61
971
100·73
0·33
0·19
2·27
2·70
6·96
3·68
0·09
7·58
17·2
0·83
58·9
ands
RV3
19
9·6
10·1
0·22
1·50
15
0·73
4·42
217
1·13
5·3
771
68·9
41
0·49
14
5
90
1403
98·99
0·48
0·17
3·13
2·49
4·89
1·88
0·06
5·07
17·2
0·62
63·0
dact
RV10
137
19·9
17·8
0·62
2·64
32
0·89
6·32
307
1·56
7·2
548
84
49
0·85
39
6
75
744
99·82
0·62
0·26
2·21
1·84
7·34
3·99
0·09
8·49
15·7
1·18
58·1
ands
PS2
42
17·5
16·3
0·44
2·89
30
1·08
6·78
309
1·86
9·0
588
103
57·5
0·78
19
8
75
899
100·74
0·59
0·31
2·46
1·89
6·74
2·18
0·09
8·02
16·1
1·26
61·1
ands
PS5
49
13·4
11·1
0·40
2·72
38
0·98
7·25
325
1·75
8·7
704
109
65·3
0·72
24
10
101
1409
99·93
1·26
0·26
2·83
2·29
5·76
1·59
0·06
4·83
16·6
0·95
63·5
dact
PS13
Table 2: Representative major and trace element analyses of andesitic volcanic rocks
20
8·1
8·6
0·38
2·04
26
0·83
5·72
262
1·84
7·72
778
101·7
51·1
0·83
12
11
97
1170
99·38
0·54
0·31
3·56
3·07
4·41
1·06
0·11
5·07
17·3
0·65
63·3
dact
BH21
18
14·2
12·1
0·51
2·61
31
0·85
4·58
275
1·90
7·54
807
76·8
36·6
0·69
16
6
103
1200
100·66
0·48
0·46
3·07
3·79
5·69
2·44
0·11
7·41
17·1
0·91
59·2
trchand
BH27
San Juan volcanic field
83
28·0
21·2
0·41
2·48
28
1·10
4·86
205
1·70
7·43
530
42·4
38·5
0·67
14
5
66
891
99·31
0·38
0·38
2·57
3·96
6·15
3·98
0·11
9·62
15·3
1·06
55·8
trchand
SM3
65
24·3
18·7
0·46
2·82
32
0·99
4·87
214
1·73
7·7
567
78·4
44·9
0·62
8
6
73
890
100·69
0·67
0·39
3·08
2·99
6·11
3·54
0·17
8·86
15·8
0·98
58·1
trchand
SM4B
ASKREN et al.
ANDESITES OF WESTERN USA
ands
61·2
0·77
15·4
7·12
0·07
2·53
5·87
2·34
3·21
0·21
1·35
100·07
716
108
15
23
0·90
46
33
735
6·70
1·38
196
4·31
0·69
20
1·66
0·31
16·2
18·2
29
Rock type:
SiO2
TiO2
Al2O3
Fe2O3
MnO
MgO
CaO
Na2O
K 2O
P 2O5
LOI
Total
Ba
Rb
Th
Nb
Ta
La
Ce
Sr
Sm
Eu
Zr
Hf
Tb
Y
Yb
Lu
Sc
Co
Cr
678
101
12
9
0·64
44
76
714
6·26
1·38
187
3·92
0·75
20
1·91
0·31
19·6
20·0
31
59·0
0·83
15·6
7·88
0·10
3·03
6·33
2·35
3·00
0·22
1·08
99·42
ands
IA3
940
98
11
9
0·59
62
108
718
7·56
1·61
249
5·27
0·76
29
2·05
0·40
19·7
25·7
98
56·6
0·90
15·7
8·10
0·11
4·42
6·45
2·94
3·05
0·27
0·93
99·47
trchand
LA8
999
78
10
13
0·64
67
132
1034
9·80
2·19
293
5·90
1·09
32
2·34
0·39
15·4
16·8
33
58·8
0·83
16·1
7·72
0·12
2·99
6·37
2·68
2·73
0·41
0·83
99·58
ands
RA5
1016
128
17
18
0·65
74
123
509
8·25
1·61
286
6·79
0·84
30
2·51
0·35
12·7
13·8
55
63·4
0·75
15·6
5·74
0·07
2·24
4·52
2·71
3·37
0·23
1·32
99·95
dact
RA6
691
70
4
20
0·63
47
92
609
8·30
1·98
249
5·51
0·94
24
1·43
0·23
17·8
33·4
790
56·8
1·01
14·4
8·04
0·12
7·75
6·27
2·73
2·55
0·38
0·79
100·84
bsltand
WA4
EA1
1239
126
16
12
0·81
79
145
838
12·30
2·59
290
6·43
0·75
34
2·74
0·46
15·2
22·9
164
57·1
1·04
15·5
7·52
0·08
4·29
6·68
2·95
3·84
0·48
0·74
100·22
trchand
EA4
679
59
6
8
0·33
40
70
503
5·44
1·19
174
3·70
0·62
25
1·91
0·35
27·0
32·0
339
57·3
0·78
14·2
7·43
0·13
5·74
8·48
2·17
2·13
0·20
2·28
100·84
ands
EA11B
685
131
14
9
0·96
60
102
362
7·05
1·48
230
5·70
0·74
22
2·07
0·34
14·3
12·7
27
64·2
0·94
14·4
6·39
0·04
1·49
4·59
2·44
3·25
0·22
0·89
98·85
dact
EA14
677
101
15
13
1·10
50
97
435
7·40
1·44
225
5·92
0·86
30
2·18
0·38
16·1
16·0
28
62·9
0·97
15·3
7·01
0·10
2·42
5·65
2·35
2·78
0·20
0·37
100·05
dact
419
64
1·2
14
1·3
16·3
39·7
284
6·3
2·11
248
4·6
0·90
10
2·22
0·32
32
37
13
59·0
1·02
17·4
6·73
0·10
1·51
6·73
4·36
2·80
0·47
Mean
Ref. stds∗
(13)
(2)
(0·1)
(5)
(0·2)
(0·3)
(0·8)
(4)
(0·4)
(0·01)
(6)
(0·1)
(0·13)
(2)
(0·21)
(0·02)
(1)
(1)
(5)
(0·51)
(0·04)
(0·22)
(0·11)
(0·02)
(0·08)
(0·11)
(0·50)
(0·05)
(0·02)
(1r)
VOLUME 38
1032
NUMBER 8
Major elements were analyzed with a Philips PW 1410 X-ray fluorescence spectrometer (University of Georgia) using fused disks (after Norrish & Hutton, 1967).
Barium, Rb, Sr, Y, Zr, and Nb were also analyzed by X-ray fluorescence spectrometry using pressed powder pellets (after Potts, 1987). Thorium, Hf, Ta, REE, Sc,
Co and Cr were analyzed by instrumental neutron activation as described by Askren et al. (1991). Rock types are from classification of LeBas et al. (1986), after
renormalization to 100% dry; ands, andesite; dact, dacite; bsltand, basaltic andesite; trchand, trachyandesite.
∗Mean and standard deviation (1r) of replicate analyses of US Geological Survey standards analyzed as unknowns. Reference standards used are: AGV-1 (major
elements, Rb, Zr), G-2 (Nb, Y), SY-3 (Ba, Sr), BHVO-1 ( Th, Ta, La, Ce, Sm, Eu, Hf, Tb, Yb, Lu), and BCR-1 (Sc, Co, Cr).
IA1
Sample:
Indian Peak volcanic field
Table 2: continued
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ANDESITES OF WESTERN USA
whereas clino- and orthopyroxene phenocrysts in the
same sample yield temperatures of 860°C (Fig. 11). In
rare cases, chemical disequilibrium between zones or
populations of pyroxene crystals cannot be the result
of cooling in a closed system. For example, clinopyroxene in sample IA1 yields a temperature of 1150°C,
but orthopyroxene yields a temperature of 990–1050°C
(Fig. 10).
Magmatic pressure estimates
Fig. 6. IUGS classification (LeBas et al., 1986) of intermediate
volcanic rocks from the central Nevada, Indian Peak and central
San Juan volcanic fields. All samples from each volcanic field fall
within labeled patterned areas except three samples from the Indian
Peak field (square symbols, from the Escalante Desert Formation)
and one sample from the San Juan field (cross symbol, from the
volcanics of Table Mountain).
those of olivine- or Cr-spinel-bearing andesites [temperatures of olivine andesites estimated as minimum
temperatures from clinopyroxene using the geothermometer of Lindsley & Frost (1992)].
Although there is overall consistency between calculated pyroxene equilibration temperatures, bulk SiO2
content and phenocryst populations, in some cases our
calculations suggest that the two pyroxenes were not
completely in equilibrium. For example, the absolute
ranges of calculated temperature (>100°C) within the
andesites of Reveille and Pritchards Station are larger
than predicted from the observed ranges of bulk SiO2
content: the 5% variation in bulk SiO2 is consistent
with about a 50°C temperature variation [at constant
P, f (O2); e.g. Grove et al., 1982; Baker & Eggler, 1983;
Luhr, 1990].
Similar large temperature ranges are estimated for
andesites of the Isom and Escalante Desert Formations.
These large temperature ranges may suggest disequilibrium, or such ranges may record a temperature
interval of crystallization. For example, sample EA11B
(andesite of the Escalante Desert Formation) contains
very large (>1 cm diameter) clinopyroxene phenocrysts,
and temperatures estimated from analyses of single
crystals vary by >100°C (Fig. 10). Such large ranges
of calculated temperatures may document magma
cooling and compositional evolution. This inference is
particularly likely for sample EA15 (andesite of Escalante
Desert Formation); clino- and orthopyroxene inclusions
in biotite yield equilibration temperatures of 1120°C,
Magmatic pressures were estimated using phenocryst
assemblages and the position of whole-rock or matrix
compositions relative to liquid lines of multiple saturation in the plagioclase- and magnetite-saturated
pseudoternary system olivine–diopside–silica+orthoclase (Ol–Di–SiOr) of Baker & Eggler (1983, 1987; Fig.
12; Table 3). This particular system is appropriate
because of the ubiquitous occurrence of plagioclase
and magnetite phenocrysts in the lavas under consideration. The system has been calibrated for dry and
2 wt % water contents; positions of the liquid lines
and the olivine–orthopyroxene peritectic in this projection are more sensitive to total pressure than to
other parameters such as water content and f (O2).
Thus two criteria can be used to infer magmatic
pressures: (1) the position of whole-rock or matrix
compositions, i.e. estimates of melt compositions, relative
to the experimental liquid lines; (2) the position of the
peritectic as inferred from the presence or absence of
olivine phenocrysts relative to the experimental peritectics. Melt compositions are not equivalent to bulk
compositions, so we also investigated calculated matrix
compositions (the net phenocryst composition of each
sample was calculated from modal abundances, typical
densities and average compositions of phenocryst phases;
the matrix composition was calculated by subtracting
the net phenocryst composition from the measured
bulk composition). Typically, calculated matrix compositions are slightly higher in SiOr and lower in Ol
than bulk compositions of the same samples (as
illustrated by bulk and matrix compositions of Pritchards
Station samples in Fig. 12). Thus pressures estimated
from bulk and calculated matrix compositions are
similar. However, calculated matrix compositions of
the andesite of the Wah Wah Springs Formation are
significantly higher in SiOr and lower in Ol than are
bulk compositions (Fig. 12). This unit contains up to
9% modal olivine, and bulk compositions may reflect
crystal accumulation; calculated matrix compositions
of this unit are a better approximation of melt
compositions.
If the andesitic magmas have assimilated or mixed
with other materials, then estimated pressures may not
1033
Fig. 7. Major element abundances vs silica contents. Filled symbols represent olivine andesites (and the Cr-spinel-bearing pyroxene andesite of Pritchards Station). Partially filled symbols
represent pyroxene andesites. Open symbols represent hornblende andesites. Symbols for andesites of the central Nevada field are: triangles, volcanics of Citadel Mtn; circles, andesite of
Reveille; squares, andesite of Pritchards Station. Symbols for andesites of the Indian Peak field are: triangles, andesite of the Isom Fm; diamonds, andesite of Lund Fm; crosses, andesite of
Ryan Spring Fm; circles, andesite of Wah Wah Springs Fm; squares, andesite of Escalante Desert Fm. Symbols for andesites of the San Juan field are: triangles, andesite of Bristol Head;
crosses, volcanics of Table Mtn; circles, Huerto Andesite; squares, Sheep Mtn Andesite.
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Fig. 8. Abundances of selected trace elements (p.p.m.) vs SiO2 for samples from the three areas. Symbols same as in Fig. 7. Circled fields are ranges of compositions of felsic ash-flow tuffs;
stars are abundances of individual samples of ash-flow tuffs (Lipman et al., 1982; Phillips, 1989; Best et al., 1989a). Abbreviations used to label ash-flow tuff data are the same as used in
Fig. 2, Fig. 3 and Fig. 4. Arrows are calculated paths of Rayleigh crystal fractionation using mean mineral–melt distribution coefficients; double arrows for Sr are paths using minimum
and maximum mineral–melt distribution coefficients (see text for references). f=0·5 at arrow heads. Data for Huerto Andesite (circled field labeled HU) are from Askren et al. (1991).
ASKREN et al.
ANDESITES OF WESTERN USA
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Fig. 9. Chondrite-normalized trace element diagram for andesites
from the central Nevada (dark-shaded field), Indian Peak (stippled
field) and San Juan (light-shaded field) volcanic fields. Normalizing
values (except Eu and Ba) and element ordering (most incompatible
to the left) are from Thompson et al. (1984). Eu normalizing value
calculated assuming Eu/Sm=0·377 (Boynton, 1984). Ba normalizing
value calculated assuming Ba/La=9·96 (Anders & Grevesse, 1989).
Upper- and lower-crustal compositions are from Taylor & McLennan
(1985). Windous Butte Tuff compositions are from Phillips (1989).
Carpenter Ridge Tuff compositions are from Dorais (1987).
be reliable. However, if the compositions of these other
materials lie along liquid lines of multiple saturation,
then minor mixing would not change estimated pressures (Baker, 1987). Given the vast volumes of associated
felsic volcanics, material that andesitic magmas may
have mixed with or assimilated includes more felsic
compositions near the SiOr apex (i.e. dacite or rhyolite).
Mixing or assimilating such compositions would not
greatly offset plotted compositions from liquid lines of
multiple saturation, so pressure estimates would not be
greatly affected.
Askren et al. (1991) previously used this experimental
system to estimate equilibration pressures of the Huerto
Andesite to be between 0·2 and 0·5 GPa. Unlike the
Huerto Andesite, other San Juan andesites lack olivine
phenocrysts and contain orthopyroxene, although they
are as Ol-rich in the projection (Fig. 12) as the Huerto
NUMBER 8
AUGUST 1997
Andesite. This observation suggests that they evolved
beyond the olivine–orthopyroxene peritectic at higher
pressures than the Huerto lavas because the peritectic
shifts towards more Ol-rich compositions as pressure
increases. The limited experimental calibration does
not allow us to specify an upper pressure limit.
For the lavas from the other two fields it is difficult
to precisely estimate pressures of equilibration because
of the lack of olivine. An important exception is the
andesite of the Wah Wah Springs Formation—this
andesite contains olivine phenocrysts, and calculated
matrix compositions plot along the 0·1 MPa and 0·2
GPa liquid lines (Fig. 12). Thus if the matrix compositions are indicative of melt compositions, then the
presence of olivine phenocrysts suggests relatively low
pressure (<0·2 GPa) equilibration given the migration
of the peritectic towards SiOr-rich compositions with
decreasing pressure in this system. Other lavas from
the same field apparently equilibrated at somewhat
higher pressures because they lack olivine even though
their bulk compositions are relatively Ol-rich in the
projection. The lavas from central Nevada all lack
olivine and all plot at SiOr-rich compositions where all
four liquid lines intersect (Fig. 12). Such phase relationships do not constrain the pressure. However,
Cr-spinel in the andesite of Pritchards Station suggests
that olivine was stable in parental, slightly more mafic
magmas (e.g. Fig. 12); such magmas may have been
stable at low pressures (<0·2 GPa).
Effects of crystal fractionation
The inverse correlation between SiO2 and MgO, CaO,
TiO2 and phenocryst equilibrium temperatures in the
andesitic lavas (Fig. 7 and Fig. 11) is compatible with
chemical variation that resulted from cooling and
crystal–melt separation within a closed system. Thus
as a first step to explain the chemical variation, we
modeled fractional crystallization for each andesitic
unit utilizing the least-squares-based XLFRAC model
of Stormer & Nicholls (1978). In this model (Table
4), phenocryst compositions are combined and subtracted from a parental magma composition (in this
case the most mafic andesitic lava in each unit) in an
attempt to match the bulk compositions of the most
evolved lava in the unit. The quality of the fit can be
evaluated by examining the sum of squares of residual
oxides; a value of <2 indicates a satisfactory fit (Stormer
& Nicholls, 1978). For the andesites modeled here, the
sums of squares of residual oxides are <1·4, and
this indicates that crystal fractionation is a plausible
explanation of the major element variation. In addition,
the modeled abundances of fractionating phenocrysts
are generally similar to the observed abundances of
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ASKREN et al.
ANDESITES OF WESTERN USA
Table 3: Summary of estimated magmatic conditions
Andesite unit
Sample
Bulk SiO2
Olivine
Opx
Temperature
Pressure
range (°C)
range (GPa)
Central Nevada
Citadel Mountain
Reveille
Pritchards Station
CM12
63·0
+
944–988
CM23
65·7
+
969–1052
RV3
58·9
+
1044–1053
RV10
63·0
+
921–953
PS2
58·1
+
1142–1176
PS5
61·1
+
1093–1096
PS13
63·5
+
1019–1035
IA3
59·0
+
1142–1179
IA5
59·8
+
971–1088
>0·2?
>0·2
>0·2
Indian Peak
Isom
>0·2?
IA1
61·2
+
1152–1155
Lund
LA8
56·6
+
1046–1078
>0·2
Ryan Spring
RA1
57·0
+
1024–1085
>0·2
RA5
58·8
+
1028–1102
+
RA6
63·4
Wah Wah Springs
WA4
56·8
+
Escalante Desert
EA1
57·1
+
EA4
EA15
987–1006
1050–1160
<0·2
+
1070–1108
>0·2
57·3
+
1087–1131
61·1
+
827–1132
EA2
61·8
+
983–1075
EA12
62·6
+
934–1052
EA14
62·9
+
818–837
EA11B
64·2
+
989–1105
BH26
58·3
+
971–1010
BH23
61·1
+
941–959
BH21
63·3
+
823–935
TM10A
61·3
+
939–993
TM11
63·4
+
959–971
HU47
54·8
+
1064–1069
HU42
56·6
+
1058–1060
HU40
58·6
+
1034–1076
HU15
58·2
+
1046–1076
SM3
55·8
+
1102–1128
SM4B
58·1
+
1065–1102
San Juan
Bristol Head
Table Mountain
Huerto
Sheep Mountain
+
>0·5?
>0·5
0·2–0·5
>0·5
+ indicates presence of phenocrysts of olivine or orthopyroxene (Opx). Temperature ranges given for variation observed
from clinopyroxene compositions.
modal phenocrysts. For example, plagioclase is the
dominant fractionating phase in most of the XLFRAC
models (from 5 to 23%) and is the dominant phenocryst
in most of the lavas (from 3 to 32%, Table 1; Appendix
I of Askren, 1992).
If crystallization within a closed system is the
explanation for the observed chemical variation, then
trace element abundance variations in the andesitic
lavas should also be duplicated by a closed-system
crystallization model. We utilized simple Rayleigh
crystal fractionation (Cox et al., 1979) to model trace
element abundances (Fig. 8 and Fig. 13). Bulk distribution coefficients were calculated using phenocryst
proportions generated from the major-element
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Fig. 10. Clinopyroxene temperatures vs orthopyroxene temperatures [using geothermometer of Frost & Lindsley (1992) and Lindsley &
Frost, 1992)]. Symbols are the same as used for Fig. 7. Samples EA11B and IA1 (bold symbols) are discussed in text. Range of estimated
temperatures for each sample is indicated by error bars. Samples with estimated temperature range <50°C are symbolized without error
bars.
XLFRAC models (Table 4) along with published
mineral–melt distribution coefficients (Gill, 1981; Watson & Green, 1981; Green & Pearson, 1987). The
XLFRAC model is based on equilibrium crystallization
(i.e. homogeneous crystals), whereas the Rayleigh fractionation model is based on ideal fractional crystallization (i.e. normally zoned crystals). Thus the
combination of these two models will produce crystals
which are chemically homogeneous with respect to
major elements but strongly zoned with respect to
trace elements. Such crystals are probably not common
in nature; rather, major and trace element zoning are
commonly found in the same crystal (e.g. Shimizu,
1981). Therefore, the trace element model cannot be
used to quantitatively test the closed system fractionation
model, but it can be used as a qualitative test:
incompatible trace elements should increase and compatible elements should decrease with evolution of
andesitic magmas. Of the elements modeled, most (Ba,
Rb, Th, Nb, Ta, Zr, Hf, Y and REE excluding
Eu) should increase in concentration as crystallization
proceeds; Sr and Eu, and in some cases the middle
REE (if apatite fractionates), should decrease with
crystallization.
The variations of most trace elements are qualitatively
consistent with closed system fractional crystallization,
but two elements exhibit trends suggestive of open
system behavior. For example, in most units Ba
concentrations remain nearly constant or decrease as
SiO2 contents increase, although all of the fractionating
phases (plagioclase, clinopyroxene, orthopyroxene, amphibole and magnetite) have relatively small (<0·3)
partition coefficients for Ba. Similarly, Zr concentrations
remain nearly constant as SiO2 increases, although Zr
also is excluded from the crystallizing phases. Variations
in ratios of incompatible trace elements (e.g. K/Rb,
Ba/Rb, Zr/Rb) show even poorer agreement with
models of closed system fractionation. These ratios
should remain constant in a system evolving by crystal
fractionation (Fig. 13), but they show a three-fold
change within some units (e.g. andesite of Reveille,
andesite of the Escalante Desert Formation). Fractionation of zircon would invalidate generalizations
concerning Zr, but the andesitic magmas did not have
the compositional and thermal parameters consistent
with zircon saturation (e.g. Watson & Harrison, 1983).
Evidence for magma mixing
The above results indicate that the andesitic magmas
behaved as open systems during compositional evolution. Possible processes include crustal assimilation
and magma mixing; these processes were evaluated
using the mathematical expressions of DePaolo (1981;
Fig. 13). Assimilants [average compositions of upper
and lower crust from Weaver & Tarney (1984)] and
plotting parameters (K/Rb and Ba/Rb vs Rb) are those
used by Colucci et al. (1991) for assimilation–fractional
crystallization modeling of precaldera andesitic lavas
of the southeast San Juan field. In addition, Zr/Rb has
been evaluated because Rayleigh crystal fractionation
models failed to predict the variation of Zr abundances
in the andesitic units discussed here.
Modeled crystal fractionation combined with crustal
contamination by upper- or lower-crustal material does
not produce the large decreases in K/Rb, Ba/Rb or
Zr/Rb in samples from the studied areas (Fig. 13).
Increasing the ratio of assimilated material:fractionated
crystals (r) to values >0·5 slightly improves the agreement
1038
ASKREN et al.
ANDESITES OF WESTERN USA
Fig. 11. Estimated magmatic temperatures of andesite lavas and
felsic ash-flow tuffs vs bulk silica contents. Symbols for andesites are
the same as used in Fig. 7. Each symbol represents mean value of
estimated temperatures for each sample. Outlined areas show total
range of calculated temperatures and measured silica contents for
units. Bold symbols for sample EA15 are temperature estimates for
phenocrysts and inclusions (discussed in text). Patterned fields are
ranges of temperature estimates for ash-flow tuffs, and labels for
these fields are the same as used in Figs 2–4 (see text for references).
between the model and observed trace element abundances, but this is not geologically reasonable (e.g.
insufficient heats of solution; Bowen, 1928). Similar
results led Colucci et al. (1991) to suggest that the
precaldera lavas of the southeast San Juan field were
contaminated by material with relatively low K/Rb
and Ba/Rb ratios and Rb abundances such as many
A-type granites (Whalen et al., 1987). Here, we have
modeled crystal fractionation combined with contamination by felsic ash-flow tuffs (Fig. 13); these
models match observed data better than models that
use upper and lower crust. The close temporal and
spatial relationships of the andesite lavas and the felsic
ash-flow tuffs are consistent with mixing between
andesitic and more felsic magmas. Moreover, mixing
is shown in isolated andesite samples by highly variable
plagioclase and clinopyroxene compositions in some
samples, and mixing is documented in various tuffs by
reports of very calcic plagioclase (Whitney & Stormer,
1985; Phillips, 1989) as well as by the presence of
mafic inclusions (Dorais, 1987; Whitney et al., 1988).
Isotopic compositions of the andesites are consistent
with this idea: published 87Sr/86Sr and 18O ratios of
felsic tuffs (Lipman et al., 1978; Larson & Taylor,
1986; Riciputi & Johnson, 1990; Riciputi et al., 1995)
are typically higher than those of andesites (Askren &
Roden, 1992), but the lowest isotopic ratios of the
tuffs overlap the highest observed ratios in the andesites
in each area.
A combination of fractional crystallization and magma
mixing with rhyolitic or dacitic magmas can explain
the compositional variations of andesitic units studied
here (Fig. 13). The assimilation–fractional crystallization
model of DePaolo (1981) can be used to evaluate
combined fractional crystallization and magma mixing
because the physical state of the contaminant (solid or
liquid) does not affect the mathematical treatment.
Thus assimilation of solid material and mixing with
magma can be treated identically. For all andesitic
units in each area, K/Rb, Ba/Rb and Zr/Rb values
are better explained by mixing with magmas of
compositions similar to selected interlayered ash-flow
tuffs than by assimilation of average crustal compositions. For example, in the central Nevada field,
Zr/Rb values of samples from the andesite of Pritchards
Station and from the volcanics of Citadel Mountain
can be explained by a combination of mixing (with
magmas of composition similar to the rhyolitic portions
of the Windous Butte Tuff; Phillips, 1989) and fractional
crystallization. In the Indian Peak field, combined
fractional crystallization and mixing (with magmas of
composition similar to the dacitic Wah Wah Springs
Tuff; Best et al., 1989a) can reproduce observed K/
Rb and Ba/Rb ratios for most samples from andesites
of the Escalante Desert, Lund and Ryan Springs
Formations. In the San Juan field, combined fractional
crystallization and mixing (with magmas of composition
similar to the rhyolitic Carpenter Ridge Tuff; Whitney
et al., 1988) can reproduce observed Ba/Rb and Zr/
Rb ratios for the andesite of Bristol Head; this model
can also reproduce observed Zr/Rb ratios of the Sheep
Mountain and the Huerto Andesites. Mixing of mafic
magmas with magma that erupted to form the Carpenter Ridge Tuff was documented by Whitney et al.
(1988), although that mafic magma was compositionally
distinct from the andesitic lavas discussed here.
Petrogenesis of parental magmas
The parental magma of the andesitic lavas was probably
basaltic magma formed by partial melting in the
1039
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Fig. 12. Projection of bulk compositions of samples with anhydrous modal mineralogy onto magnetite- and plagioclase-saturated, Ol–Di–SiOr
pseudoternary diagram of Baker (1987). Symbols are the same as used in Fig. 7. The 0·1 MPa and 0·8 GPa cotectics (dashed lines) are
dry; 0·2 and 0·5 GPa cotectics (continuous lines) are for compositions with 2 wt % H2O. Stars are olivine–opx–liquid reaction points for
0·1 MPa, 0·2 GPa, 0·5 GPa and 0·8 GPa cotectics. Filled field of central Nevada compositions is possible composition of olivine andesite
parental to Cr-spinel-bearing andesite of Pritchards Station. Circled field of central Nevada compositions is range of calculated matrix
compositions of andesite of Pritchards Station (bulk compositions shown as squares). Circled field of Indian Peak compositions is range of
matrix compositions of andesite of Wah Wah Springs Fm (bulk compositions shown as filled circles). Circled field of San Juan compositions
is range of Huerto Andesite compositions (0·2–0·5 GPa; Askren et al., 1991).
mantle. In each area, some andesitic units contain
olivine or Cr-spinel. These relatively mafic andesites
probably evolved from their basaltic parents chiefly by
fractional crystallization, as indicated by equilibrium
textures of the most mafic andesites and by relatively
low abundances of MgO, CaO (Fig. 7) and Cr (Table
2) and high abundances of SiO2 compared with typical
basalts. One exception, the andesite of the Wah Wah
Springs Formation, has high abundances of MgO,
CaO and Cr. We found no textural evidence for
magma mixing in the more mafic andesites, and the
presence of olivine and Cr-spinel makes it unlikely
that they formed by melting in the crust. Most olivine
phenocrysts are Fe rich (<Fo85) compared with olivine
in mantle peridotite (e.g. Ringwood, 1975), and thus
the lavas are not primary, mantle-derived magmas.
The andesites of the Wah Wah Springs Formation are
exceptional; these lavas contain Mg-rich olivine (Fo83–87)
and are probably compositionally close to their parental
magmas.
The basaltic parent magmas probably had the same
arc-type trace element signature that the andesitic lavas
1040
SM3
1041
PS2
Pritchards Station
CM16
PS12
RV10
14·2
17·7
7·3
5·1
10·9
14·0
22·5
12·7
5·5
14·7
14·3
13·5
17·5
16·8
8·1
6·2
0·6
8·0
17·9
22·9
14·0
0·8
1·3
3·2
4·6
2·9
3·2
6·2
7·1
7·4
2·8
2·1
11·4
2·5
4·5
6·3
3·7
4·6
Opx
3·1
1·8
0·1
0·1
3·0
2·9
3·2
0·4
1·3
2·1
2·2
2·5
5·3
3·5
Mt
0·1
0·8
6·9
Amph
32·5
33·1
10·8
13·3
31·8
40·6
41·8
25·3
10·6
24·5
27·4
25·8
29·7
31·1
Total
Sum of
6·4
5·3
1·6
1·8
5·1
6·8
6·7
2·7
2·0
1·7
3·8
6·2
6·0
5·2
0·06
0·25
0·58
0·11
0·49
0·71
0·45
1·38
0·07
0·98
0·20
0·28
1·30
0·62
SiO2 variation squares
Observed
0·13
0·25
0·38
0·17
0·31
0·42
0·34
0·59
0·13
0·50
0·20
0·26
0·57
0·39
error
Standard
Mineral abbreviations: Plag, plagioclase; Cpx, clinopyroxene; Opx, orthopyroxene; Mt, magnetite; Amph, amphibole. Sum of squares is the
summation of the squares of differences between calculated and observed oxide concentrations. Table Mountain fractionation model modified after
Williams (1991). Huerto model from Askren et al. (1991).
CM12
RV3
Reveille
EA11B
EA15B
Citadel Mountain
Central Nevada
EA11B
Escalante Desert
RA7
EA15B
RA1
Ryan Springs
LA3
IA1
EA7
LA8
Lund
SM4B
HU44
EA7
IA3
Isom
Indian Peak
HU47
Sheep Mountain
TM19
TM10A
Huerto
TM10A
TM44
Table Mountain
BH21
BH26
Bristol Head
San Juan
Cpx
Plag
Initial
Final
Subtracted phenocrysts
Compositions
Table 4: Summary of crystal fractionation models using XLFRAC (Stormer & Nicholls, 1978)
ASKREN et al.
ANDESITES OF WESTERN USA
Fig. 13. K/Rb, Ba/Rb and Zr/Rb plotted vs Rb (p.p.m.) for andesite units. FC curves are modeled paths for fractional crystallization. AFC Lower Crust curves are modeled paths for
combined fractional crystallization and assimilation of lower crust. AFC Upper Crust curves are modeled paths for combined fractional crystallization and assimilation of upper crust. MFC
Tuff curves are modeled paths for combined fractional crystallization and mixing with rhyolitic or dacitic magmas. Tick marks on curves indicate relative mass of magma remaining (f ) in
increments of 0·1. References for compositions of crust and felsic magmas are given in text. Assimilation rate/crystallization rate (r)=0·5 for assimilation and mixing curves.
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have because fractionation of crystals typically found
in basalts is not capable of depleting the magmas in
Nb and Ta or producing elevated La/Nb and Ba/La
ratios. Ilmenite is the only phase observed in these
samples with Nb and Ta partition coefficients greater
than unity, and these coefficients are not sufficiently
high to explain the large observed depletions (Green
& Pearson, 1987). The lavas in the San Juan field are
higher in alkali elements, but otherwise the lavas from
the three fields are compositionally similar. In all, these
data support the contention of Coney & Reynolds
(1977) that the Benioff zone at this time had a very
low angle of dip, and consequently subduction-related
magmas erupted at great distances (>1000 km) from
the trench. Likewise, our contention that olivine
andesites are products of mantle-derived basalts is
consistent with the model of Zandt et al. (1995), in
which upwelling of hot asthenosphere resulted from
detachment of a 100 km deep, flat subducted slab.
Relative positions of magma chambers
In all three volcanic fields, magmatic pressure estimates
are similar (0·2–0·8 GPa) and require equilibration at
mid- to shallow crustal levels. These depths are similar
to estimated depths for silicic ash-flow tuff magma
chambers in the San Juan (0·2–0·8 GPa: Matty &
Stormer, 1985; Whitney & Stormer, 1985; Krause &
Stormer, 1986; Whitney et al., 1988; Johnson &
Rutherford, 1989), Indian Peak (0·2–0·3 GPa: Christiansen et al., 1988; Christiansen & Best, 1989; Best et
al., 1989a) and central Nevada fields (0·2–0·7 GPa:
Phillips, 1989; Christiansen & Best, 1989). This would
suggest that the andesites or their parental magmas
paused at similar levels in the crust (~6–25 km)
and then erupted rapidly (i.e. without significant
reequilibration) regardless of the distance they initially
ascended from the crust–mantle boundary.
Olivine andesites occur exclusively outside of ashflow tuff calderas, and hornblende andesites occur
exclusively inside calderas (Figs 2–4). One unit outside
the central Nevada caldera complex contains Cr-spinel
but no olivine. The olivine andesites are typically more
mafic (e.g. lower SiO2 and higher MgO abundances)
than most pyroxene andesites. However, some pyroxene
andesites are as mafic as the olivine andesites, and
they also occur outside caldera margins. Thus, the
most mafic lavas are located relatively far (10–40 km)
from ash-flow caldera rims. Given the shift of the
olivine–orthopyroxene peritectic with pressure, the relatively mafic pyroxene andesites occurring outside the
calderas (Sheep Mountain Andesite; some samples from
the Escalante Desert Formation) may have equilibrated
at slightly higher pressures than olivine andesites of
similar bulk composition (Fig. 12). Conversely, more
felsic hornblende andesites and pyroxene andesites occur
within calderas. The relatively consistent distribution of
hornblende andesites and olivine andesites relative to
the caldera walls may be a tool to assist in the location
of ash-flow tuff calderas in volcanic fields.
This correlation between mineralogy and location
relative to felsic ash-flow tuff calderas indicates that
the felsic magma chambers may have impeded the
ascent of andesitic magmas because the density of
felsic magma (2·45 g/cm3 for dacitic magma; Whitney
& Stormer, 1985) is less than that of typical olivine
andesite magma (2·66 g/cm3 at 54·6% SiO2; Gill,
1981). We propose that gravitationally entrapped andesite magmas (e.g. Whitney, 1988) continued evolving
by fractional crystallization and mixing with overlying
felsic magmas to more felsic, i.e. hornblende andesite,
compositions (2·56 g/cm3 at 59·4% SiO2; Gill, 1981).
Subsequent eruption of felsic magmas then provided
unobstructed conduits for the eruption of hornblende
andesite magmas, as felsic magma chambers emptied
or remaining magma crystallized. If emptied, then
felsic magma no longer remained to trap andesitic
magmas. If crystallized, then the increased density of
crystallized dacitic magma (to 2·65 g/cm3 for a typical
granodiorite) allowed hornblende andesite magma to
rise and erupt.
CONCLUSIONS
Andesitic lavas in the San Juan, Indian Peak and
central Nevada volcanic fields had parental magmas
with arc-type trace element signatures. Parental magmas
to these andesites were derived by partial melting of
mantle material, as evidenced by the presence of olivine
or Cr-spinel in lavas from each area. Compositional
evolution appears to be controlled chiefly by crystal
fractionation at 0·2–0·8 GPa because bulk and matrix
compositions follow liquid lines of multiple saturation
when plotted on appropriate experimental phase diagrams, and phenocryst modes are consistent with
experimental phase relations. Large dacitic or rhyolitic
magma chambers associated with felsic ash-flow tuffs
may have blocked the ascent of some andesitic magmas
until density contrasts were removed by eruption or
crystallization of felsic magmas. While trapped, such
andesitic magmas crystallized and mixed with the more
felsic overlying magmas. This combined fractionation
and mixing allowed these andesitic magmas to evolve
to hornblende andesite compositions. Thus, olivinebearing andesites occur only peripheral to ash-flow
calderas and hornblende andesites occur within and
along caldera walls.
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In each field, the andesitic magmas equilibrated at
similar depths and evolved by combined fractional
crystallization and mixing with felsic magmas at middleto upper-crustal levels. Mixing of andesitic and felsic
magmas was probably limited and did not produce
tremendous volumes of hybrid dacitic magma. Instead,
the intrusion of basaltic magmas, parental to the
andesites, may have provided the heat responsible for
the crustal anatexis which produced the felsic ash-flow
tuffs.
ACKNOWLEDGEMENTS
The authors thank Peter Lipman, Myron Best and
Eric Christiansen for their invaluable insights on field
and petrologic relationships, and Myron Best and an
anonymous reviewer for detailed comments on the
manuscript. We thank Tom Williams for his observations on the volcanics of Table Mountain. Microprobe assistance was provided by Scott Vetter and Jim
Wittke, and analyses were supported by a National
Science Foundation grant. XRF assistance was provided
by Chris Fleisher. Field work was supported by GSA
Grant 3714-87 and Sigma Xi Grant in Aid of Research
(to D.R.A.).
REFERENCES
Anders, E. & Grevesse, N., 1989. Abundances of the elements:
meteoric and solar. Geochimica et Cosmochimica Acta 53, 197–214.
Askren, D. R., 1992. Origin of andesites interlayered with largevolume ash-flow tuffs in the western United States. Ph.D.
Dissertation, University of Georgia, Athens, 281 pp.
Askren, D. R. & Roden, M. F., 1992. Regional isotopic trends of
andesites interlayered with large-volume ash-flow tuffs in the
western United States. EOS Transactions of the American Geophysical
Union 74, 349.
Askren, D. R., Whitney, J. A. & Roden, M. F., 1991. Petrology
and geochemistry of the Huerto Andesite, San Juan volcanic
field, Colorado. Contributions to Mineralogy and Petrology 107, 373–386.
Baker, D. R., 1987. Depths and water content of magma chambers
in the Aleutian and Marianas island arcs. Geology 15, 496–499.
Baker, D. R. & Eggler, D. H., 1983. Fractionation paths of Atka
(Aleutian) high-alumina basalts: constraints from phase relations.
Journal of Volcanology and Geothermal Research 18, 387–404.
Baker, D. R. & Eggler, D. H., 1987. Compositions of anhydrous
and hydrous melts coexisting with plagioclase, augite, and olivine
or low-Ca pyroxene from 1 atm to 8 kbar—application to the
Aleutian volcanic center of Atka. American Mineralogist 72, 12–28.
Bédard, J. H. J., Francis, D. M. & Ludden, J., 1988. Petrology and
pyroxene chemistry of Monteregian dykes: the origin of concentric
zoning and green cores in clinopyroxenes from alkali basalts and
lamprophyres. Canadian Journal of Earth Science 25, 2041–2058.
Best, M. G. & Grant, S.K., 1987. Stratigraphy of the volcanic
Oligocene Needles Range Group in southwestern Utah. US
Geological Survey Professional Paper 1433A.
NUMBER 8
AUGUST 1997
Best, M. G., Christiansen, E. H. & Blank, H.R., 1989a. Oligocene
caldera complex and calc-alkaline tuffs and lavas of the Indian
Peak volcanic field, Nevada and Utah. Geological Society of America
Bulletin 101, 1076–1090.
Best, M. G., Christiansen, E. H., Deino, A. L., Grommé, C. S.,
McKee, E. H. & Noble, D. C., 1989b. Eocene through Miocene
volcanism in the Great Basin of the western United States. New
Mexico Bureau of Mines and Mineral Resources Memoir 47, 91–133.
Best, M. G., Scott, R. B., Rowley, P. D., Swadley, W. C., Anderson,
R. E., Grommé, C. S., Harding, A. E., Deino, A. L., Christiansen,
E. H., Tingey, D. G. & Sullivan, K. R., 1993. Oligocene–Miocene
caldera complexes, ash-flow sheets, and tectonism in the central
and southeastern Great Basin. In: Lahren, M. M., Trexler, J. H.
& Spinosa, C. (eds) Crustal Evolution of the Great Basin and the Sierra
Nevada, Field Trip Guidebook for Cordilleran/Rocky Mountain Sections of
the Geological Society of America. Reno: University of Nevada, pp.
285–312.
Bowen, N. L., 1928. The Evolution of Igneous Rocks. Princeton, NJ:
Princeton University Press, 334 pp.
Boynton, W. W., 1984. Cosmochemistry of the rare earth elements.
In: Henderson, P. (ed.) Rare Earth Element Geochemistry. Amsterdam:
Elsevier, pp. 63–114.
Christiansen, E. H. & Best, M. G., 1989. Compositional contrasts
among middle Cenozoic ash-flow tuffs of the Great Basin, western
United States. New Mexico Bureau of Mines and Mineral Resources
131, 51.
Christiansen, E. H., Best, M. G. & Hoover, J. D., 1988. Petrology
of a trachytic tuff from a calc-alkaline volcanic center: the
Oligocene Isom Tuff, Indian Peak volcanic field, Utah and
Nevada. EOS Transactions of the American Geophysical Union 69,
1494–1495.
Colucci, M. T., Dungan, M. A., Ferguson, K. M., Lipman, P. W.
& Moorbath, S., 1991. Precaldera lavas of the southeast San
Juan volcanic field: parent magmas and crustal interactions. Journal
of Geophysical Research 96(B8), 13413–13434.
Coney, P. J. & Reynolds, S. J., 1977. Cordilleran Benioff zones.
Nature 270, 403–406.
Cox, K. G., Bell, J. D. & Pankhurst, R. J., 1979. The Interpretation
of Igneous Rocks. London: George Allen & Unwin, 450 pp.
Danyushevsky, L. V. & Sobolev, A. V., 1996. Ferric–ferrous ratio
and oxygen fugacity calculations for primitive mantle-derived
melts—calibration of an empirical technique. Mineralogy and Petrology
57, 229–241.
DePaolo, D. J., 1981. Trace element and isotopic effects of combined
wallrock assimilation and fractional crystallization. Earth and
Planetary Science Letters 53, 189–202.
Dixon, G. L., Hedlund, D. C. & Ekren, E. B., 1972. Geologic map
of the Pritchards Station Quadrangle, Nye County, Nevada. US
Geological Survey Miscellaneous Investigations Map I-728.
Dorais, M. J., 1987. The mafic enclaves of the Dinkey Creek
Granodiorite and Carpenter Ridge Tuff: a mineralogical, textural
and geochemical study of their origins with implications for the
generation of silicic batholiths. Ph.D. Dissertation, University of
Georgia, Athens, 185 pp.
Ekren, E. B., Rogers, C. L. & Dixon, G. L., 1973. Geologic and
Bouguer gravity map of the Reveille Quadrangle, Nye County,
Nevada. US Geological Survey Miscellaneous Investigations Map I-806.
Ekren, E. B., Quinlivan, W. D., Snyder, R. P. & Kleinhampl, F.
J., 1974. Stratigraphy, structure, and geologic history of the Lunar
Lake caldera of northern Nye County, Nevada. US Geological
Survey Journal of Research 2, 599–608.
Feuerbach, D. L., Smith, E. I., Walker, J. D. & Tangeman, J. A.,
1993. The role of the mantle during crustal extension: constraints
1044
ASKREN et al.
ANDESITES OF WESTERN USA
from geochemistry of volcanic rocks in the Lake Mead area,
Nevada and Arizona. Geological Society of America Bulletin 105,
1561–1575.
Frost, B. R. & Lindsley, D. H., 1992. Equilibria among Fe–Ti
oxides, pyroxenes, olivine, and quartz: Part II, Application. American
Mineralogist 77, 1004–1020.
Gill, J. B., 1981. Orogenic Andesites and Plate Tectonics. Berlin: SpringerVerlag, 390 pp.
Green, T. H. & Pearson, N. J., 1987. An experimental study of
Nb and Ta partitioning between Ti-rich minerals and silicate
liquids at high pressures and temperatures. Geochimica et Cosmochimica
Acta 51, 55–62.
Grove, T. L., Gerlach, D. C. & Sando, T. W., 1982. Origin of
calc-alkaline series lava at Medicine Lake volcano by fractionation,
assimilation and mixing. Contributions to Mineralogy and Petrology 80,
160–182.
Haggerty, S. E., 1976. Opaque mineral oxides in terrestrial igneous
rocks. In: Rumble, D. (ed.) Reviews in Mineralogy 3, Hg101–Hg175.
Huppert, H. E. & Sparks, R. S. J., 1988. The generation of granitic
magmas by intrusion of basalt into continental crust. Journal of
Petrology 29, 599–624.
Johnson, M. C. & Rutherford, M. J., 1989. Experimentally determined
conditions in the Fish Canyon Tuff, Colorado, magma chamber.
Journal of Petrology 30, 711–737.
Kelemen, P. B., Shimizu, N. & Dunn, T., 1993. Relative depletion
of niobium in some arc magmas and the continental crust:
partitioning of K, Nb, La and Ce during melt/rock reaction in
the upper mantle. Earth and Planetary Science Letters 120, 111–134.
Krause, K. W. & Stormer, J. C., 1986. Mineralogy, geochemistry
and magmatic conditions in the Wason Park Tuff, central San
Juan volcanic field, Colorado. EOS Transactions of the American
Geophysical Union 67, 1269.
Lanphere, M. A., 1988. High-resolution 40Ar/39Ar chronology of
Oligocene volcanic rocks, San Juan Mountains, Colorado. Geochimica et Cosmochimica Acta 52, 1425–1434.
Larson, P. B. & Taylor, H. P., 1986. 18O/16O ratios in ash-flow
tuffs and lavas erupted from the central Nevada caldera complex
and the central San Juan caldera complex, Colorado. Contributions
to Mineralogy and Petrology 92, 146–156.
LeBas, M. J., LeMaitre, R. W., Streckeisen, A. & Zanettin, B.,
1986. A chemical classification of volcanic rocks based on the
total alkali–silica diagram. Journal of Petrology 27, 745–750.
Leake, B. E., 1978. Nomenclature of amphiboles. American Mineralogist
63, 1023–1052.
Lindsley, D. H. & Frost, B. R, 1992. Equilibria among Fe–Ti
oxides, pyroxenes, olivine and quartz: Part I, Theory. American
Mineralogist 77, 987–1003.
Lipman, P. W., 1975. Evolution of the Platoro caldera complex
and related volcanic rocks, southeastern San Juan Mountains,
Colorado. US Geological Survey Professional Paper 852.
Lipman, P. W., 1992. Magmatism in the Cordilleran United States;
progress and problems. In: Burchfield, B. C., Lipman, P. W. &
Zoback, M. L. (eds) The Cordilleran Orogen: Conterminous United States.
Geology of North America G3. Boulder, CO: Geological Society of
America, pp. 481–514.
Lipman, P. W. & Sawyer, D. A., 1988. Preliminary geology of the
San Luis Peak quadrangle and adjacent areas, San Juan volcanic
field, southwestern Colorado. US Geological Survey Open-file Report
88.
Lipman, P. W., Steven, T. A. & Mehnert, H. H., 1970. Volcanic
history of the San Juan Mountains, Colorado, as indicated by
potassium–argon dating. Geological Society of America Bulletin 81,
2327–2352.
Lipman, P. W., Doe, B. R., Hedge, C. E. & Steven, T. A., 1978.
Petrologic evolution of the San Juan volcanic field, southwestern
Colorado: Pb and Sr isotope evidence. Geological Society of America
Bulletin 89, 59–82.
Lipman, P. W., Bowman, H. R., Knight, R., Millard, H. T.,
Pallister, J. S., Street, K., Wollenberg, H. & Zielinski, R. A.,
1982. Instrumental neutron activation analyses of Cenozoic volcanic
rocks, phenocrysts, and associated intrusions from the southern
Rocky Mountains and adjacent areas. US Geological Survey Openfile Report 82.
Lipman, P. W., Dungan, M. A., Brown, L. L. & Deino, A., 1996.
Recurrent eruption and subsidence at the Platoro-caldera complex,
southeastern San Juan volcanic field, Colorado—new tales from
old tuffs. Geological Society of America Bulletin 108, 1039–1055.
Luhr, J. F., 1990. Experimental phase relations of water- and sulfursaturated arc magmas and the 1982 eruptions of El Chichón
volcano. Journal of Petrology 31, 1071–1114.
Matty, D. J. & Stormer, J. C., 1985. Magmatic conditions of the
Snowshoe Mountain Tuff, central San Juan volcanic field,
Colorado. EOS Transactions of the American Geophysical Union 66,
396.
McDonough, W. F., 1991. Partial melting of subducted oceanic
crust and isolation of its residual eclogitic lithology. Philosophical
Transactions of the Royal Society of London 335, 407–418.
Miyashiro, A., 1974. Volcanic rock series in island arcs and active
continental margins. American Journal of Science 274, 321–355.
Morimoto, N., Fabries, J., Ferguson, A. K., Ginzburg, I. V., Ross,
M., Seifert, F. A., Zussman, J., Aoki, K. & Gottardi, G., 1988.
Nomenclature of pyroxenes. American Mineralogist 73, 1123–1133.
Norrish, K. & Hutton, J. T., 1969. An accurate X-ray spectrographic
method for the analysis of a wide range of geological samples.
Geochimica et Cosmochimica Acta 33, 431–453.
Phillips, L. V., 1989. Petrology and magmatic evolution of the largevolume ash-flow tuffs of the central Nevada caldera complex,
Nye County, Nevada. Ph.D. Dissertation, University of Georgia,
Athens, 285 pp.
Potts, P. J., 1987. A Handbook of Silicate Rock Analysis. New York:
Chapman and Hall, pp. 226–285.
Ratté, J. C., Marvin, R. F. & Naeser, C. W., 1984. Calderas and
ash-flow tuffs of the Mogollon Mountains, southwest New Mexico.
Journal of Geophysical Research 89, 8713–8732.
Riciputi, L. R. & Johnson, C. M., 1990. Nd- and Pb-isotope
variations in the multicyclic central caldera cluster of the San Juan
volcanic field, Colorado, and implications for crustal hybridization.
Geology 18, 975–978.
Riciputi, L. R., Johnson, C. M., Sawyer, D. A. & Lipman, P. W.,
1995. Crustal and magmatic evolution in a large multicyclic
caldera complex—isotopic evidence from the central San Juan
volcanic field. Journal of Volcanology and Geothermal Research 67, 1–28.
Ringwood, A. E., 1975. Composition and Petrology of the Earth’s Mantle.
New York: McGraw–Hill, 618 pp.
Scott, D. H. & Trask, N. J., 1971. Geology of the Lunar Crater
volcanic field, Nye County, Nevada. US Geological Survey Professional
Paper 599I.
Shimizu, N., 1981. Trace element incorporation into growing augite
phenocrysts. Nature 289, 575–577.
Snyder, R. P., Ekren, E. B. & Dixon, D. L., 1972. Geologic map
of the Lunar Crater Quadrangle, Nye County, Nevada. US
Geological Survey Miscellaneous Investigations Map I-700.
Steven, T. A., 1967. Geologic map of the Bristol Head Quadrangle,
Colorado. US Geological Survey Geologic Quadrangle Map GQ-631.
Steven, T. A. & Lipman, P.W., 1973. Geologic map of the Spar
City Quadrangle, Colorado. US Geological Survey Geologic Quadrangle
Map GQ-1052.
1045
JOURNAL OF PETROLOGY
VOLUME 38
Steven, T. A., Lipman, P.W., Hail, W. J., Barker, F. & Luedke, R.
G., 1974. Geologic map of the Durango Quadrangle, southwestern
Colorado. US Geological Survey Miscellaneous Investigations Map I-764.
Steven, T. A., Rowley, P. D. & Cunningham, C. G., 1984. Calderas
of the Marysvale volcanic field, west central Utah. Journal of
Geophysical Research 89, 8751–8764.
Stormer, J. C. & Nicholls, I. A., 1978. XLFRAC—a program for
the interactive testing of magmatic differentiation models. Computers
in Geosciences 4, 143–159.
Taylor, S. R. & McLennan, S. M., 1985. The Continental Crust: its
Composition and Evolution. Oxford: Blackwell Scientific, 312 pp.
Thompson, R. N., Morrison, M. A., Hendry, G. L. & Parry, S. J.,
1984. An assessment of the relative roles of crust and mantle in
magma genesis: an elemental approach. Philosophical Transactions of
the Royal Society of London, Series A 310, 549–590.
Wass, S. Y., 1979. Multiple origins of clinopyroxenes in alkali
basaltic rocks. Lithos 12, 115–132.
Watson, E. B. & Green, T. H., 1981. Apatite/liquid partition
coefficients for the rare earth elements and Sr. Earth and Planetary
Science Letters 56, 405–421.
Watson, E. B. & Harrison, T. M., 1983. Zircon saturation revisited:
temperature and composition effects in a variety of crustal magma
types. Earth and Planetary Science Letters 64, 295–304.
Weaver, B. L. & Tarney, J. T., 1984. Empirical approach to
estimating the composition of the continental crust. Nature 310,
575–577.
NUMBER 8
AUGUST 1997
Whalen, J. B., Currie, K. L. & Chappell, B. W., 1987. Atype granites: geochemical characteristics, discrimination and
petrogenesis. Contributions to Mineralogy and Petrology 95, 407–419.
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 100, 1886–1897.
Whitney, J. A. & Stormer, J. C., 1985. Mineralogy, petrology, and
magmatic conditions from the Fish Canyon Tuff, central San
Juan volcanic field, Colorado. Journal of Petrology 26, 726–760.
Whitney, J. A., Dorais, M. J., Stormer, J. C., Kline, S. W. & Matty,
D. J., 1988. Magmatic conditions and development of chemical
zonation in the Carpenter Ridge Tuff, central San Juan volcanic
field, Colorado. American Journal of Science 288A, 16–44.
Williams, T. J., 1991. Petrology and geochemistry of the volcanics
of Table Mountain, San Juan volcanic field, south central
Colorado. M.S. Thesis, University of Georgia, Athens, 150 pp.
Zandt, G., Myers, S. C. & Wallace, T. C., 1995. Crust and mantle
structure across the Basin and Range–Colorado Plateau boundary
at 37 degrees N latitude and implications for Cenozoic
extensional mechanism. Journal of Geophysical Research 100(B6),
10529–10548.
Zielinski, R. A. & Lipman, P. W., 1976. Trace element variations
at Summer Coon volcano, San Juan Mountains, Colorado, and
the origin of continental-interior andesite. Geological Society of America
Bulletin 87, 1477–1485.
1046