1. science
2. geology
3. volcanology
4. volcanic eruptions

PETROGENESIS OF THE OLIGOCENE
EAST TINTIC VOLCANIC FIELD, UTAH
by
Daniel K.
Jeffrey D.
Eric H. Christiansen2, Choon-Sik Kim3,
2
David G. Tingey , Stephen T. Nelson2, and Douglas S. Flamm2
Moore1,
Keith2,
ABSTRACT
The early Oligocene East Tintic volcanic field of central Utah, located near the eastern margin of the
Basin and Range Province, consists of mafic to silicic volcanic (mostly intermediate-composition lava
flows) and shallow intrusive rocks associated with the formation of small, nested calderas. Radiometric
ages establish a minimum age for initiation (34.94 ± 0.10 Ma) and cessation (32.70 ± 0.28 Ma) of East
Tintic magmatism. The igneous rocks of the field are calc-alkalic, potassic, silica-oversaturated, and metaluminous, and can be categorized into the following three compositional groups: the shoshonite-trachyte
series, the trachyandesite series, and the rhyolite series. Based on composition and phenocryst assemblage,
the shoshonite-trachyte series is divided into two groups: a clinopyroxene group and a two-pyroxene
group. The rhyolite series consists of three field units: the Packard Quartz Latite, the Fernow Quartz
Latite, and the rhyolite of Keystone Springs. The trachyandesite series is by far the most voluminous. This
series is also subdivided into a clinopyroxene group and a two-pyroxene group. Temperature and oxygen
fugacity estimates indicate that shoshonite-trachyte series magmas were the hottest and least oxidizing and
that two-pyroxene trachyandesite series magmas were the coolest and most oxidizing.
Clinopyroxene shoshonite-trachyte series magma evolved mainly by fractional crystallization. The
high K2O, Rb, and Al2O3/CaO ratios and modest SiO2 enrichment of these rocks appear to result from
extensive, high-pressure fractional crystallization of clinopyroxene (without plagioclase). Two-pyroxene
shoshonite-trachyte series magma was likely produced by mixing between mafic and silicic clinopyroxene
shoshonite-trachyte series magmas at low pressure. Assimilation of crustal material appears not to have
been important for shoshonite-trachyte series magmas. We believe that parental clinopyroxene shoshonite-trachyte series magma originated in the mantle wedge above a Cenozoic subduction zone and then
interacted with older subduction-metasomatized lithospheric mantle. Rhyolite series magma was likely the
differentiate of a lower crustal partial melt. Trachyandesite series magma likely evolved by magma mixing and subsequent fractional crystallization. Trace-element compositions indicate that the mixing that
produced trachyandesite series magmas was between mafic clinopyroxene shoshonite-trachyte series
magma and Fernow Quartz Latite magma, at low pressure for two-pyroxene trachyandesite series magma,
and at high pressure for clinopyroxene trachyandesite series magma.
INTRODUCTION
The East Tintic Mountains of central Utah are a
Basin and Range-style horst that exposes Paleozoic sedimentary rocks and the volcanic, sedimentary, and shallow intrusive rocks of the early Oligocene East Tintic
volcanic field (figure 1). In the East Tintic mining district, sulfide-rich alteration and mineralization (Lindgren
and Laughlin, 1919; Lovering, 1949; Morris and
Lovering, 1979) are the products of magmatism (Ames,
1962; Morris and Lovering, 1979; Keith and others,
1989; Hannah and others, 1991; Moore, 1993). The East
Tintic volcanic field represents an ideal locale for studying the processes responsible for the genesis of subduction-related, potassic, silica-saturated magmas related to
ore genesis. This report explores the petrogenesis of the
igneous rocks of the East Tintic volcanic field.
GEOLOGIC SETTING
To establish stratigraphic and temporal relationships,
the bedrock geology of the central East Tintic Mountains
1
2
3
(figure 1) was mapped at a scale of 1:24,000 using conventional methods (Morris, 1975; Hannah and Macbeth,
1990; Kim, 1992; Moore, 1993; Keith and others, in
preparation). Fresh samples were collected from most
igneous rock units. Standard petrographic techniques
were used to determine rock texture and modal mineralogy. New 40Ar/39Ar age determinations were done on a
VG1200S automated mass spectrometer using standard
techniques (like those of Harrison and Fitzgerald, 1986)
at the University of California at Los Angeles by S.T.
Nelson, and are reported in table 1.
The East Tintic Mountains are characterized by volcanic and sedimentary rocks of early Oligocene age (following the scale of Hansen, 1991) that lie unconformably
upon folded and faulted Paleozoic sedimentary rocks, all
of which are cut by shallow intrusions associated with
the volcanic rocks (figure 1). The west side of the range
is more deeply eroded and exposes older rocks. Morris
and Anderson (1962) studied the Paleozoic-Tertiary
unconformity and concluded that there was substantial topographic relief at the onset of volcanism. An
40Ar/39Ar sanidine age of 34.94 ± 0.10 Ma from the
Department of Geology, Brigham Young University-Idaho, Rexburg, ID 83460 [email protected]
Department of Geological Sciences, Brigham Young University, Provo, UT 84602 [email protected]
Department of Geology, Pusan National University, Pusan, South Korea
Central Utah — Diverse Geology of a Dynamic Landscape
164
112˚07’ 30’’ W
112˚00’ W
40˚00’ N
N
0
2
km
Goshen
Valley
Tintic
Valley
39˚45’ N
Figure 1. Index map and generalized geologic map of
the East Tintic volcanic field. Geologic contacts are
from Keith and others (in preparation).
Ore-related
latite flows
Ore-related
monzonite intrusions
Other volcanic and
intrusive rocks undivided
Salt Lake
City
Tintic
District
Utah
Paleozoic sedimentary
rocks undivided
39
1
40Ar / 40
39Ar
Table
Summary
of new
Ar radiometric
Table 1.1.Summary
of new
Ar /radiometric
ages.1 ages.
Sample2
Mineral
3
Latitude (N)
Longitude(W)
Jx10-6
Fusions
40/36
Ar
40/39
Ar
MWSD4
Age (Ma)5
1 6
1
SP292
SP192
TJ77
TJ197
TJ108
ET134
biotite
sanidine
sanidine
biotite
biotite
biotite
40.174296°
40.204185°
ET134
ET121
hornblende hornblende
39°48’23” 39°53’35” 39°49’21” 39°59’37” 39°59’37” 39°57’59”
111.955592° 111.977750° 112°2’54”
112°3’2”
112°4’7”
112°2’38” 112°2’38” 112°3’51”
7865
8
-
7862
10
-
7856
10
-
7860
5
-
7858
5
-
7870
7
-
34.71
0.19
34.18
0.24
34.03
0.18
33.87
0.13
33.72
0.08
33.34
0.15
7869
-
7876
-
294±7.60
301±4.90
2.37±0.01
2.33±0.02
10.75
1.96
33.29
0.09
32.70
0.28
The details of the analytical methods used for age determinations are described in Moore (1993).
Samples are in stratigraphic order: the youngest are on right.
Location data based on NAD27.
4 MWSD = mean weighted standard deviation.
5 The age reported for biotite and sanidine analyses are mean ages; hornblende analyses are isotope correlation ages.
6 1σ = one-sigma standard deviation.
2
3
2007 UGA Publication 36 — Willis, G.C., Hylland, M.D., Clark, D.L., and Chidsey, T.C., Jr., editors
Fernow Quartz Latite, one of the oldest volcanic units,
establishes a minimum age for initiation of magmatism
in the East Tintic region (Utah Geological Survey and
New Mexico Geochronology Research Laboratory,
2007; Keith and others, in preparation). Clark (2003)
obtained a similar argon age (34.83 ± 0.15 Ma) for the
Fernow to the south in Sage Valley. East Tintic magmatism continued until at least 32.70 ± 0.28 Ma (table 1).
The two “SP” samples in table 1 are from locations just
north of the East Tintic Mountains and are from units that
overlie possibly the oldest East Tintic volcanic field
unit—a welded ash-flow tuff, the Packard Quartz Latite.
Christiansen and others (2007, this volume) describe
these “SP” units and associated volcanics.
Outcrops of Paleozoic sedimentary rocks are most
abundant in the northern and southern parts of the range
(figure 1). These Paleozoic rocks experienced significant
folding and faulting during the Mesozoic orogenies of
western North America. Farmer and DePaolo (1983)
suggested that basement rocks beneath this portion of the
Basin and Range Province are Proterozoic. Nelson and
others (2002) show that the nearby Santaquin Complex
was accreted to the Archean craton no earlier than ca.
1700 Ma and underwent metamorphism prior to ca. 1670
Ma.
Moore (1993) reported on the volcanic stratigraphy
and age relations of the field (see also the stratigraphic
summary of Hintze, 1988, p. 149). This work substantially updates and revises that of Morris and others
(Morris and Anderson, 1962; Morris, 1975; Morris and
Lovering, 1979), and Hannah and Macbeth (1990).
Further clarification of the volcanic stratigraphy and age
relations is ongoing by Keith and others (in preparation).
At least 100 km3 of magma was erupted during the
roughly 2-million-year life span of the volcanic field
(Moore, 1993). The magmatic history of the field consists of the formation of nested calderas that produced
small deposits of tuff and numerous lava flows. By volume, roughly twice as much lava as ash was erupted.
The volcanic field is one of the easternmost members of
the Tintic–Deep Creek magmatic belt, an east-west elongate zone of Cenozoic magmatism that extends from the
Colorado Plateau westward into the Deep Creek Range
of west-central Utah (Stewart and others, 1977). Activity
in nearby magmatic centers, including those in the West
Tintic Mountains (Stein and others, 1990), Salt Creek
area (Keith and others, 1991; New Mexico Geochronology Research Laboratory and Utah Geological Survey,
2005), and Bingham mining district (Moore, 1973), was
roughly contemporaneous with that in the early Oligocene East Tintic volcanic field. Severinghaus and
Atwater (1990) reconstructed the time-integrated geometry and thermal history of the Farallon and Vancouver
plates, which were subducted beneath western North
America during the Cenozoic. Best and others (1989),
Best and Christiansen (1991), and Christiansen and others (2007, this volume) suggest that, beginning at around
45 Ma in northern Utah and Nevada, volcanism spread
southward across what is now the Basin and Range
165
Province. These observations are compatible with production of subduction-related magmas beneath the East
Tintic volcanic field during the early Oligocene, although
East Tintic magmatism would have been behind the
active volcanic front, which was several hundred kilometers farther south.
COMPOSITION
Whole-rock major- and trace-element analyses were
obtained by wavelength dispersive X-ray fluorescence
spectrometry using a Seimens SRS 303 at Brigham
Young University-Provo. Representative analyses are
given in table 2. A description of techniques and the
complete geochemical data set (as an Excel file) are
available at http://www.geology.byu.edu/faculty/ehc
under the heading Resources. Phenocryst compositions
were determined using a JEOL JXA-8600 Superprobe by
Choon Sun Kim at the University of Georgia with an
accelerating potential of 15 keV, a 15 nA beam, and
counting times of 40s for standards and 20s for unknowns.
Igneous rocks of the East Tintic volcanic field are
potassic, silica-oversaturated, metaluminous to slightly
peraluminous, and range from shoshonite to rhyolite—
53 to 78 weight percent (wt. %) SiO2; (table 2; figure 2).
Compositions plot mostly in the alkali-calcic field on the
modified alkali-lime versus silica diagram of Frost and
others (2001). On the silica versus FeOtotal/(FeOtotal +
MgO) diagram of Frost and others (2001), using the
dividing line of Miyashiro (1974), East Tintic samples
straddle the ferroan/magnesian boundary. Compositional
trends for major and trace elements are shown on SiO2
variation diagrams (figures 3 and 4). Representative
mineral modes are shown in table 3. We group the compositions of field units of the East Tintic volcanic field
into the following three compositional series: the
shoshonite-trachyte series, the trachyandesite series, and
the rhyolite series. These series are based on compositional groupings and trends that we determined were petrogenetically important. We describe these series in this
section and interpret them in the next.
Shoshonite-trachyte series rocks are petrogenetically
important (see below), but volumetrically small. This
series includes the most mafic samples of the field and
ranges from 53 to 57 and 63 to 68 wt. % SiO2. Compared
with trachyandesite series rocks at equal SiO2, shoshonite-trachyte series rocks have higher K2O, Rb, Zr, Ba,
and Al2O3/CaO and lower Ni, Cr, Fe2O3, and MgO (table
3; figures 3 and 4). Based on composition and phenocryst assemblage, the shoshonite-trachyte series is divided into two groups: a clinopyroxene (cpx) group, and a
two-pyroxene (2-px) group. Compositionally, the cpx
group follows a tight mineral-control line, whereas the 2px group is more scattered (e.g., figures 3e and 4f). The
cpx group consists of the shoshonite of Buckhorn
Mountain and the Latite Ridge Latite units, and the 2-px
group consists of the Dry Herd Canyon Latite and the
Big Canyon Latite units (Moore, 1993). Patterns on
Series
Shoshonite-Trachyte
Group
Clinopyroxene
Sample TD55 TD39 TD40 TJ198B ET153
Latitude (39°N) 48’20”
Long. (112°W) 4’45”
SiO2
TiO2
Al2 O3
Fe2 O3
MnO
MgO
CaO
Na2 O
K2 O
P2O5
Anal. Total
LOI
1
51’13”
5’38”
53.90 55.66
1.40
1.16
15.54 16.81
9.10
8.61
0.14
0.15
5.16
3.16
8.54
6.85
2.95
2.42
2.74
4.47
0.54
0.70
100.90 99.60
1.65
1.28
217
315
127
48
105
21
91
782
24
204
11
1115
45
95
47
11
8
3
213
5
14
82
87
21
124
1144
30
260
11
1417
61
110
59
25
31
12
51’10”
5’38”
55’4”
1’33”
55’30”
2’3”
Rhyolite
TJ74
Two-pyroxene Fernow
Two-Pyroxene
TD47 TD1.5 TD45 TD63 TD65 TD46E TD46C TD66 TD56
49’14”
1’3”
46’15”
5’56”
55’17”
0’52”
55.59 64.00 66.3
1.15
0.88
0.7
16.89 17.38 16.72
8.53
4.66
4.16
0.14
0.11
0.05
3.24
1.18
0.78
6.40
2.88
1.51
2.64
3.34
2.57
4.71
6.42
6.96
0.71
0.32
0.24
99.40 101.17 99.9
1.47
0.96
0.88
67.26 56.75 63.11
0.56
1.23 0.88
15.14 16.74 18.46
3.36
7.50 4.11
0.06
0.22 0.10
0.38
2.31 1.16
1.32
7.75 4.39
5.02
3.24 2.71
6.69
3.67 4.74
0.21
0.59 0.34
99.40 100.60 101.10
1.72
1.45 1.17
219
5
13
84
90
21
144
1176
31
255
11
1441
53
111
58
32
31
11
27
5
1
8
66
19
272
271
44
485
19
1313
87
138
70
36
60
11
57
4
3
15
87
21
254
482
42
445
17
1495
88
143
70
33
64
11
45
6
2
12
85
20
265
343
42
496
18
1369
79
143
68
37
68
11
Trachyandesite
202
8
10
42
101
23
136
672
33
282
11
1127
77
133
61
18
31
6
70
1
1
12
76
20
168
728
35
325
13
1371
82
129
65
24
37
9
45’37”
6’49”
77.59
0.25
12.24
1.27
0.02
0.30
1.26
2.65
4.36
0.05
98.80
0.90
17
2
9
3
18
12
152
200
13
115
16
577
23
46
20
30
24
8
49’13”
4’8”
49’10”
4’19”
47’26”
6’28”
58.28 59.52 59.74
1.06
1.04
1.12
15.56 15.61 17.25
8.34
7.89
6.64
0.13
0.12
0.13
3.55
2.90
2.35
5.88
5.58
5.90
2.96
2.93
2.26
3.74
3.97
4.02
0.48
0.45
0.59
101.10 100.20 99.90
0.05
1.20
0.03
161
18
15
45
93
23
139
731
29
239
12
1175
67
110
52
22
16
4
172
49
20
36
83
20
134
694
29
240
12
1224
55
105
48
25
22
7
132
1
3
14
89
19
133
822
36
272
12
1210
66
124
60
18
22
6
46’25”
6’33”
49’10”
4’24”
54’21”
5’20”
51’42”
6’0”
60.01 60.32 60.32 61.30 63.15 65.25 58.57
1.15
0.97
1.06
0.98 0.90 0.70 1.03
16.75 15.91 16.87 16.45 16.34 15.39 16.14
6.47
6.34
6.33
6.96 5.14 4.86 7.92
0.13
0.11
0.10
0.12 0.12 0.09 0.13
2.65
2.33
2.00
2.19 1.61 1.75 3.24
5.85
5.10
5.50
5.19 4.07 2.40 5.31
2.21
4.01
2.74
2.33 4.02 4.35 2.60
4.21
4.43
4.59
4.07 4.36 4.88 4.53
0.57
0.49
0.47
0.41 0.29 0.34 0.54
99.40 101.20 101.80 99.60 98.80 99.50 98.90
0.08
0.07
2.71
1.33 0.89 0.56 1.04
60.27
0.87
15.76
7.04
0.12
2.50
4.17
3.95
4.86
0.44
99.80
1.27
164
5
6
22
81
19
132
777
36
275
12
1240
61
117
60
22
23
9
139
59
21
57
76
21
182
661
27
286
14
1337
51
110
50
29
37
8
162
21
10
39
85
22
162
739
29
233
12
1347
59
109
50
25
27
8
47’19”
5’19”
Clinopyroxene
TD6 TD71B TJ146 ET188 TJ126
129
8
7
22
74
20
171
747
34
287
11
1360
62
111
60
19
22
7
47’44”
3’17”
116
3
5
18
91
21
140
605
35
283
11
1148
67
128
61
22
27
6
49’48”
5’55”
85
3
6
12
61
20
143
652
32
318
12
1288
62
118
58
18
21
6
51’25”
3’22”
97
8
4
24
66
20
168
426
25
213
12
1159
52
93
43
24
28
5
Major elements are reported as weight percent oxide and are normalized to 100% on a volatile-free basis. LOI is loss on ignition at 1000°C for 4 hours. Trace elements are reported as parts per million.
161
39
22
77
87
22
155
783
29
280
12
1190
78
131
47
28
26
7
Central Utah — Diverse Geology of a Dynamic Landscape
V
Cr
Ni
Cu
Zn
Ga
Rb
Sr
Y
Zr
Nb
Ba
La
Ce
Nd
Pb
Th
U
166
Table 2. Representative whole-rock compositions1.
2007 UGA Publication 36 — Willis, G.C., Hylland, M.D., Clark, D.L., and Chidsey, T.C., Jr., editors
16
Clinopyroxene Shoshonite-Trachyte Series
Two-pyroxene Shoshonite-Trachyte Series
14
Na2O + K2O (wt. %)
167
Two-pyroxene Trachyandesite Series
Clinopyroxene Trachyandesite Series
12
Rhyolite Series
Trachyte
Keystone
10
Latite
Packard
8
Shoshonite
6
Fernow
Potassic
Trachybasalt
4
Rhyolite
Dacite
Basaltic
Andesite
Basalt
Andesite
2
0
35
40
45
50
55
60
65
70
75
SiO2 (wt. %)
Figure 2. International Union of Geological Sciences (IUGS) classification diagram (Le Bas and others, 1986) for East Tintic rocks.
Table 3. Representative phenocryst modes.
Series
Group
Shoshonite-Trachyte
Rhyolite
Clinopyroxene
Two-pyroxene Fernow
Sample1 TD18 TD39 TD55 ET153 TJ198B TD47 TJ181
Latitude (39°N) 50’10” 51’13” 48’20” 55’30”
Clinopyroxene
TD45 TD46C TD46E TD56 TD63 TD68 TD6 TJ126
TJ51
51’48”
55’4”
46’15”
55’42”
45’37”
46’25”
1’33”
5’56”
3’23”
6’49”
6’33”
Phenocryst % 19.8 34.2 8.8
4.5
Lithic % 5.0
Points Counted 2000 2000 2000 1959
11.1
6.1
2264
15.0
2000
31
2261
47.9
1.1
2000
32.9
2000
14.1 23.9 32.1 35.0 22.3 37.0
3.8
2000 2000 2000 2000 1000 2000
31.0
2000
Plagioclase
Sanidine
Quartz
Clinopyroxene
Orthopyroxene
Biotite
Hornblende
Olivine
Fe-Ti Oxides
Apatite
3.6
15.2
8.0
trace
58.5
17.7
9.4
14.4
trace
62.4
25.4
5.1
7.1
trace
33.5
26.7
31.7
6.7
2.9
trace
64.4
12.5
9.4
4.0
9.7
trace
68.7 58.8
9.2
14.2 12.0
8.5
9.7
8.5
8.8
trace trace
55.3
1.5
13.4
19.8
9.5
trace
Longitude (112°W) 4’43”
1
Trachyandesite
Two-pyroxene
60.1
30.3
9.6
trace
5’38”
57.3
31.9
10.8
trace
4’45”
31.8
68.2
trace
trace
2’3”
63.0
trace
26.1
trace
10.9
trace
47’26” 47’19” 49’13” 49’20” 47’44” 51’42”
6’28”
5’19” 4’8”
52.0
24.5
11.4
trace
11.4
trace
4’29” 3’17”
6’0”
63.1 65 63.2
10.9 5.8 15.4
4.4 10.3
9.2
14.3
6.3
12.4 12.6 7.0
trace trace trace
3’30”
Samples were chosen to represent the variations in unit mineral mode. Sample – field unit correlations, and a complete list of all measured modal analyses are reported in
Moore (1993).
Central Utah — Diverse Geology of a Dynamic Landscape
168
7
0.8
A
6.5
6
0.6
5.5
P2O5 (wt. %)
K2O (wt. %)
E
0.7
Shoshonitic
5
4.5
4
High-K
3.5
0.5
0.4
0.3
0.2
0.1
3
2.5
0
Medium-K
52
54
56
58
60
62
64
66
68
70
72
74
52
76
54
56
58
60
62
6
8
4.5
7
Fe2O3 (wt. %)
Na2O (wt. %)
68
70
72
74
76
4
3.5
3
2.5
F
9
5
2
6
5
4
3
2
1.5
1
1
52
54
56
58
60
62
64
66
68
70
72
74
52
76
54
56
58
60
SiO2 (wt. %)
1.6
62
64
66
SiO2 (wt. %)
68
70
72
74
5.5
C
1.5
76
G
5
1.4
4.5
1.3
1.2
MgO (wt. %)
TiO2 (wt. %)
66
10
B
5.5
64
SiO2 (wt. %)
SiO2 (wt. %)
1.1
1
0.9
0.8
0.7
4
3.5
3
2.5
2
0.6
1.5
0.5
1
0.4
0.5
0.3
0
0.2
52
54
56
58
60
62
64
66
68
70
72
74
52
76
54
56
58
60
62
64
66
68
70
72
74
9
19
D
18
76
SiO2 (wt. %)
SiO2 (wt. %)
H
8
CaO (wt. %)
7
Al2O3 (wt. %)
17
16
15
14
6
5
4
3
2
1
13
0
12
52
54
56
58
60
62
64
66
SiO2 (wt. %)
68
70
72
74
76
52
54
56
58
60
62
64
66
SiO2 (wt. %)
68
70
72
74
76
Figure 3. Major-element variation diagrams for East Tintic rocks. The classification lines in figure 3A are after Ewart (1982). Symbols
as for figure 2.
2007 UGA Publication 36 — Willis, G.C., Hylland, M.D., Clark, D.L., and Chidsey, T.C., Jr., editors
140
3000
A
120
169
E
2700
2400
Ba (ppm)
Ni (ppm)
100
80
60
2100
1800
1500
1200
40
900
20
600
0
300
52
54
56
58
60
62
64
66
SiO2 (wt. %)
68
70
72
74
52
76
350
56
58
60
62
64
66
68
70
72
74
76
SiO2 (wt. %)
190
B
300
54
F
175
160
145
Ce (ppm)
Cr (ppm)
250
200
150
100
115
100
85
70
50
55
0
40
52
54
56
58
60
62
64
66
SiO2 (wt. %)
68
70
72
74
76
52
54
56
58
60
C
240
64
66
68
70
72
74
76
G
280
220
260
200
240
180
Rb (ppm)
160
140
120
100
80
220
200
180
160
140
60
120
40
20
100
0
80
52
54
56
58
60
62
64
66
68
70
72
74
52
76
54
56
58
60
SiO2 (wt. %)
62
64
66
68
70
72
74
76
SiO2 (wt. %)
1100
1000
400
900
Sr (ppm)
450
350
300
H
1200
D
500
Zr (ppm)
62
SiO2 (wt. %)
260
V (ppm)
130
800
700
600
500
250
400
200
300
150
200
100
100
52
54
56
58
60
62
64
66
SiO2 (wt. %)
68
70
72
74
76
52
54
56
Figure 4. Trace-element variation diagrams for East Tintic rocks. Symbols as for figure 2.
58
60
62
64
66
SiO2 (wt. %)
68
70
72
74
76
170
chondrite-normalized trace-element diagrams of mafic
shoshonite-trachyte series samples (especially TD55) are
smooth curves—suggesting they are relatively undifferentiated rocks—that peak at Th, and have negative Nb
and Ti anomalies (figure 5). Negative Sr, P, and Ti anomalies become larger as SiO2 increases. We attribute this
to fractionation of the observed minerals (table 2). In
evolved shoshonite-trachyte series samples positive light
rare earth element (LREE) and Zr anomalies increase
with evolution, suggesting the absence of LREE- and Zrsequestering phases like monazite, xenotime, or allanite
and zircon, respectively. Zircon saturation calculations
show that shoshonite-trachyte series magma was, for its
composition, too hot and too Na-, K-, and Ca-rich to stabilize zircon (Watson and Harrison, 1983).
The rhyolite series is distinguished by high silica
content (70 to 78 wt. %), and consists of three field units:
the Fernow Quartz Latite, the rhyolite of Keystone
Springs, and the Packard Quartz Latite (Moore, 1993).
Because rhyolite series compositions do not overlap in
silica content with the other series, comparisons with
those series are not meaningful. The units which comprise this series have relatively uniform mineral modes
and chemical compositions, except that the Packard
Quartz Latite has extremely high concentrations of Ba,
~2700 parts per million (ppm), and relatively high concentrations of Sr, ~425 ppm. Contamination by crustal
rocks could explain the anomalous composition of
Packard rhyolite series magma. The Jurassic Arapien
Shale, which contains evaporite deposits, is exposed near
the volcanic field (Hintze, 1980). It is possible that this
unit, or some other unit containing evaporites, underlies
the volcanic field at depths where rhyolite series magma
could have been contaminated by them. If this did occur,
the magma preferentially assimilated Sr and Ba, because
the concentrations of other elements that would be abundant in evaporite deposits (e.g., Rb) appear not to have
been affected. Some of the silicic rocks in the Soldiers
Pass area to the north of the volcanic field also display
elevated Ba concentrations (Christiansen and others, 2007,
this volume).
The trachyandesite series is by far the most voluminous. Rocks of this series range from 56 to 68 wt. %
SiO2 (i.e., between the mafic and silicic rocks of the volcanic field) and are characterized by the lack of strong
mineral control (e.g., figures 3g, 4b, 4d, and 4g). Based
on differences in mineral mode, this series is subdivided
into two groups: a cpx group, and a 2-px group. There
are no significant compositional differences (for the elements we measured) between these two groups; however, intrusions of 2-px group magma are associated with
mineralization while those of the cpx-group are not
(Keith and others, 1993, 1997; Stavast and others, 2006).
The 2-px group, shown in figure 1 as the ore-related latite
flows and monzonite instrusions, consists of the Latite of
Rock Canyon, the Silver City Monzonite, the Copperopolis Latite Tuff, and the Andesite of Rock Canyon
units (Moore, 1993; Stavast and others, 2006). The cpxgroup consists of the Latite of Sunrise Peak and the
Central Utah — Diverse Geology of a Dynamic Landscape
North Standard Latite, both non-mineralizing units
(Moore, 1993). Trachyandesite sample patterns on chondrite-normalized trace-element diagrams show negative
Sr, P, and Ti anomalies that become larger as SiO2
increases (figure 5). We attribute this to fractional crystallization of the observed phenocryst assemblages.
The temperatures and oxygen fugacities of shoshonite-trachyte series and trachyandesite series rocks were
estimated by Kim (1992) using the compositions of
pyroxenes (Davidson and Lindsley, 1985) and Fe-Ti
oxides (Andersen and Lindsley, 1988). For cpx shoshonite-trachyte series magma, three determinations from a
silicic unit (Latite Ridge Latite) yielded Fe-Ti oxide temperatures between 950 and 960°C and oxygen fugacity
values between 0.88 and 0.94 log units above the fayalite-magnetite-quartz oxygen buffer (FMQ) (Kim,
1992). Additional temperature information for cpx
shoshonite-trachyte series rocks can be gleaned from a
model for apatite saturation derived from experiments
(Watson, 1980). Assuming apatite saturation occurred in
the mafic samples of this series as predicted by peak concentrations of P2O5 at 55 wt % SiO2 (figure 3e), the saturation model predicts a magmatic temperature of
~1000°C. These independent estimates of cpx shoshonite-trachyte series magmatic temperatures are consistent—as expected, the less evolved magma is hotter. For
2-px shoshonite-trachyte series magma, determinations
from two lava flows (Latite of Dry Herd Canyon and
Latite of Rock Canyon) yielded 2-px temperatures
between 1063 and 1162°C, an Fe-Ti oxide temperature of
1028°C, and oxygen fugacity values between 0.6 and 1.7
above FMQ. For 2-px trachyandesite series magma, four
determinations from an extrusive unit (Latite of Rock
Canyon) and four from an intrusive unit (Silver City
Monzonite) yielded 2-px temperatures between 904 and
990°C, Fe-Ti oxide temperatures between 888 and
958°C, and oxygen fugacity values between 1.72 and
2.45 log units above FMQ. No temperature or oxygen
fugacity estimates were made for cpx trachyandesite
series or rhyolite series samples. The mineral assemblages of rhyolite series units indicate that rhyolite series
magma was significantly cooler than shoshonite-trachyte
series or trachyandesite series magma. These temperature and oxygen fugacity estimates support the series
groupings we created based on compositional characteristics.
PETROGENESIS
Major- and trace-element trends on SiO2 variation
diagrams (figures 3 and 4) indicate strong mineral control in the evolution of magmas of the East Tintic volcanic field, but are too incoherent to be explained by evolution along a single fractional crystallization line. These
trends and major- and trace-element models suggest that
magma mixing was also important in generating the
observed compositions (explained below). Major-element modeling of fractional crystallization was done
2007 UGA Publication 36 — Willis, G.C., Hylland, M.D., Clark, D.L., and Chidsey, T.C., Jr., editors
171
1000
Rock/Primitive Mantle
Shoshonite-Trachyte Series
100
10
1
0.1
Rb
Ba
U
K
Nb
La
Ce
Sr
Nd
P
Zr
Ti
Y
Ti
Y
1000
Rhyolite Series
Rock/Primitive Mantle
100
10
1
0.1
Rb
Ba
U
K
Nb
La
Ce
Sr
Nd
P
Zr
1000
Rock/Primitive Mantle
Trachyandesite Series
100
10
1
0.1
Rb
Ba
U
K
Nb
La
Ce
Sr
Nd
P
Zr
Ti
Y
Figure 5. Trace-element patterns for each of the compositional series using the normalizing values of McDonough and Sun (1995).
Symbols as for figure 2.
172
using mass balance calculations (Stormer and Nicholls,
1978) with the phenocryst compositions of Kim (1992)
and the whole-rock chemical analyses collected for this
study (table 2). Trace-element modeling of fractional
crystallization was done using multi-sequence fractional
crystallization calculations (Allégre and Minster, 1978).
The parameters for the major- and trace-element fractional crystallization models are found in tables 4 and 5.
The trace-element fractional crystallization and mixing
models were applied to Rb, Sr, Ba, Ni, Cr, Sc, V, Zr, U,
Th, La, and Ce compositions with equal success. The fits
of the models to Rb, Sr, and Ba (and Zr and Cr) data are
shown in figures 6, 7, and 8.
Shoshonite-Trachyte Series
Major- and trace-element models suggest that fractional crystallization dominated the evolution of cpx
shoshonite-trachyte series magmas, which range in composition from ~53 to 68 wt. % SiO2. The composition of
the most mafic sample, TD55, was used as the parent
magma in modeling the petrogenesis of this series. The
whole-rock and olivine compositions of this sample have
too little MgO and Ni to be a simple partial melt of mantle peridotite, indicating that some amount of evolution
took place in the primitive magma generated by partial
melting to produce the composition of this sample.
Based on changes in the phenocryst (= fractionating)
assemblage, the evolution of this series was divided into
three segments: (1) olivine + clinopyroxene, (2) plagioclase + clinopyroxene + magnetite + apatite, and (3) plagioclase + pyroxene + biotite + hornblende + magnetite
+ apatite. The first segment models changes from samples TD55 to TD39, the second from samples TD39 to
TJ198B, and the third from samples TJ198B to ET153.
The inception of plagioclase and biotite fractionation for
trace-element models was estimated from inflection
points on, respectively, SiO2 vs. Sr (and Ba) and SiO2 vs.
Cl (and F) variation diagrams.
Central Utah — Diverse Geology of a Dynamic Landscape
The first segment of the major-element fractional
crystallization model (table 4) predicts ~13% fractionated material and has a very large sum of the squared residuals, 3.66. Adding plagioclase and/or magnetite does not
improve the model. The second segment of the majorelement model predicts subtraction of ~37% additional
material, and has a small sum of the squared residuals,
0.46. A major-element model that combines segments
one and two has a low sum of the squared residuals, 0.07,
and predicts ~73% subtracted material. The third segment of the major-element model predicts subtraction of
~14% additional material, and has a sum of the squared
residuals equal to 0.79.
Table 5 shows the mineral modes and partition coefficients that make up the trace-element fractional crystallization models. These models are illustrated in figure 6.
The first segment models evolution from the parent
magma (TD55) to TD39 by removing 30% of the magma
as olivine and clinopyroxene. The second segment models evolution from TD39 to TJ198B by removing an
additional 33% of the magma as plagioclase, clinopyroxene, magnetite, and apatite. The third segment models
evolution from TJ198B to ET153 by removing an additional 17% of the magma as plagioclase, pyroxene, biotite, hornblende, and apatite. A likely cause for the scatter of the data about the trace-element models is that
sample compositions evolved from a parent magma more
mafic than TD55, but followed slightly different fractional crystallization paths. As noted above, while only
the fit of the model to Rb, Sr, and Ba data are shown in
figure 6, the model was applied with equal success to Ni,
Cr, Sc, V, Zr, U, Th, La, and Ce compositional data.
Segments one and three of the major-element fractional crystallization models are impaired because SiO2
changes so little over these segments. Small SiO2
changes do not allow the necessary natural averaging
effects needed in mass balance calculations. The majorelement models are also impaired by the mineral compositions upon which they are based. The models would be
Table4.4.Major-element
Major-element
fractional
crystallization
Table
fractional
crystallization
models1models
for clinopyroxene
shoshonite-trachyte
series magma.series magma.
for clinopyroxene
shoshonite-trachyte
1
Segment
1
Parent
Daughter
TD 55
TD39
2
1+2
3
TD39
TD55 TJ198B
TJ198B TJ198B ET153
Plagioclase
Clinopyroxene
Biotite
Olivine
Magnetite
83.0
17.0
-
43.0
33.7
11.7
11.6
51.3
33.3
7.1
2.0
6.4
68.0
23.3
3.2
5.5
Sum % Subtracted
13.3
37.6
73.2
14.4
3.66
0.46
0.07
0.79
Sum R
1
2
Phenocryst modes represent weight percent crystals subtracted (as % of all
mineral phases) from the parent magma.
2
Sum of the squares of the residuals of the nine elements used.
2007 UGA Publication 36 — Willis, G.C., Hylland, M.D., Clark, D.L., and Chidsey, T.C., Jr., editors
173
Table5.5.Trace-element
Trace-element
fractional
crystallization
model parameters.
Table
fractional
crystallization
model parameters.
Mineral Mode1 & Partition Coefficient2 (D)
Series
Pl
San
Cpx
Opx
Bt
Hb
Ol
Ap
Dbulk
-
-
0.31
0.01
0.14
0.03
-
-
-
0.68
0.01
0.00
0.01
0.11
0.39
0.05
0.010
0.044
0.016
Clinopyroxene mode 0.60
DRb 0.02
ShoshoniteTrachyte
DSr 3.00
F= 0.70–0.37 3
DBa 0.48
-
0.30
0.01
0.08
0.03
-
-
-
-
trace4
0.11
0.39
0.05
0.016
1.826
0.297
Clinopyroxene mode 0.67
DRb 0.02
ShoshoniteTrachyte
DSr 3.00
F= 0.37–0.20 3
DBa 0.48
-
0.04
0.01
0.08
0.03
-
0.20
4.00
0.27
10.0
-
-
trace4
0.11
0.39
0.05
0.814
2.069
2.323
Clinopyroxene
ShoshoniteTrachyte
F=1.0–0.70 3
1
mode
DRb
DSr
DBa
Two-pyroxene
ShoshoniteTrachyte 5
mode
DRb
DSr
DBa
0.60
0.02
3.00
0.48
-
0.22
0.01
0.08
0.03
0.07
0.02
0.02
0.02
-
-
-
trace4
0.11
0.39
0.05
0.382
0.707
1.830
Two-pyroxene
Trachyandesite 5
mode
DRb
DSr
DBa
0.60
0.02
3.00
0.48
-
0.14
0.01
0.08
0.03
0.09
0.02
0.02
0.02
0.03
4.00
0.27
10.0
0.02
0.05
0.23
0.35
-
trace4
0.11
0.39
0.05
0.140
1.835
0.606
Clinopyroxene
Trachyandesite 5
mode
DRb
DSr
DBa
0.59
0.02
3.00
0.48
0.01
0.38
9.40
6.60
0.15
0.01
0.08
0.03
-
0.17
4.00
0.27
10.0
-
-
trace4
0.11
0.39
0.05
0.140
1.835
0.606
Pl = plagioclase; San = sanidine; Cpx = clinopyroxene; Opx = orthopyroxene; Bt = biotite; Hb = hornblende; Ol = olivine;
Ap = apatite.
2
Partition Coefficients are from Arth (1976) and Henderson (1990). Because the partition coefficients for Rb, Sr, and Ba in
Fe-Ti oxides are all 0, their modes were not included in the table, though magnetite was included in the models.
3
F = the fraction of liquid remaining.
4
The apatite modes used were: 0.001% for segment 1 (F = 1-0.7) and 0.005% for all other segments and models.
5
For this series, bulk D was calculated using an average of the sample mineral modes (table 3).
174
Central Utah — Diverse Geology of a Dynamic Landscape
Figure 6. The cpx shoshonite-trachyte series trace-element fractional crystallization model, which uses the data of table 5. The heavy
line is the model. The model was applied to Rb, Sr, Ba, Ni, Cr, Sc, V, Zr, U, Th, La, and Ce compositional data with equal success.
The fit of the model to Rb, Sr, and Ba data is shown here—Rb vs. Sr in figure A, Rb vs. Ba in figure B, and Sr vs. Ba in figure C. Fvalues indicate fraction of melt remaining (F = 1.0 indicates all melt; F = 0 indicates all solid). The cpx shoshonite-trachyte series fractional crystallization model (heavy line) is also the 2-px shoshonite-trachyte series mixing envelope. The arrow indicates the direction
of compositional change caused by fractional crystallization (after mixing) for 2-px shoshonite-trachyte series magma. Symbols as
for figure 2.
2007 UGA Publication 36 — Willis, G.C., Hylland, M.D., Clark, D.L., and Chidsey, T.C., Jr., editors
more accurate if we had mineral composition data for
samples from each segment. Unfortunately, the data of
Kim (1992) are from a single cpx shoshonite-trachyte
series sample. In spite of these problems, the major-element models do predict roughly the same amount of fractionated material as the trace-element models (tables 4
and 5). We attribute the discrepancies between the
major- and trace-element models to the problems with
the major-element model just described.
Taken as a whole, the major- and trace-element models predict that ~80% of an initial magma with the composition of TD55 was subtracted as phenocrysts during
the evolution from shoshonite to trachyte (from TD55 to
ET153). The percent of magma subtracted as minerals to
produce trachyte from the actual parent magma would be
greater than 80% since TD55 is too Ni- and MgO-poor to
be a direct partial melt of peridotite. Our calculations
suggest that fractional crystallization dominated the evolution of cpx shoshonite-trachyte series magma.
Two-pyroxene shoshonite-trachyte series samples
have compositions that lie between the mafic and the silicic cpx shoshonite-trachyte series samples (figures 4 and
6). These compositional variations, supported by magmatic temperatures and oxygen fugacity values, are consistent with production of 2-px shoshonite-trachyte series
magmas by mixing between mafic and silicic cpx
shoshonite-trachyte series magmas. Low Zr concentrations of silicic 2-px shoshonite-trachyte series samples
(figure 4d) are, however, not consistent with this genetic
model. Since silicic 2-px shoshonite-trachyte series
magma at ~1000°C would be zircon-undersaturated by
roughly 400 ppm Zr, the low Zr concentrations could not
have been produced by zircon removal (Watson and
Harrison, 1983). Mixing in a small amount of Fernow
rhyolite series magma would explain the Zr-concentration discrepancy. We propose that 2-px shoshonite-trachyte series magma was generated by mixing between
mafic and silicic cpx shoshonite-trachyte series magma,
with perhaps some involvement of Fernow rhyolite series
magma in generating silicic 2-px shoshonite-trachyte
series magma.
The compositional characteristics of shoshonite-trachyte series magma (e.g., negative Nb and Ti anomalies),
the temporal and spatial association with a subducting
oceanic plate (Severinghaus and Atwater, 1990), and the
association with subduction-related mineralization (e.g.,
porphyry Cu-Mo and Ag-Au vein deposits; Lindgren and
Laughlin, 1919; Morris and Lovering, 1979; Keith and
others, 1989) suggest that cpx shoshonite-trachyte series
magma originated above a Cenozoic subduction zone.
Many workers have proposed a fundamentally basaltic
view for arc-related magmatism (e.g., Hildreth, 1981;
Fyfe, 1982; Grove and Kinzler, 1986). These models
predict injection and ponding of basaltic magma at or
near the base of the crust. As noted before, shoshonitetrachyte series samples are characterized by high
Al2O3/CaO, Zr, Rb, and K and modest SiO2 enrichment.
Clinopyroxene and/or plagioclase fractionation affects
the Al2O3/CaO ratios in cogenetic fractionating magmas.
175
The high Al2O3/CaO ratio of shoshonite-trachyte series
magma is likely due to clinopyroxene fractionation
(without plagioclase). Meen (1987) proposed that highpressure fractional crystallization of orthopyroxene from
basaltic magma at or near the base of continental crust
could cause the K2O/SiO2 ratio of the magma to increase
dramatically. There is no orthopyroxene in cpx shoshonite-trachyte series samples, but the SiO2 content and
K2O/SiO2 ratio in clinopyroxene is essentially the same
as in orthopyroxene. Further, Draper and Johnston
(1992) show that in Mg-rich arc basalts, clinopyroxene is
stable at high pressure whereas both clinopyroxene and
orthopyroxene are stable at low pressure. We propose
that the elevated K2O, Rb, and Al2O3/CaO ratio and modest SiO2 enrichment of cpx shoshonite-trachyte series
magma result from extensive high-pressure fractionation
of clinopyroxene (with plagioclase absent) from a primitive magma. As fractionation progressed, K2O and H2O
concentrations would have increased to levels sufficient
to stabilize biotite at the expense of clinopyroxene.
Rhyolite Series
There is little direct evidence for the source of rhyolite series magma. The fundamentally basaltic models
for arc magmatism mentioned above predict that continental crustal melts would be produced by heat released
from ponded, crystallizing basaltic magma. As these
continental partial melts would be intimately associated
with the basaltic magmas, extensive mixing between the
two would be expected. We believe that Fernow,
Keystone, and Packard rhyolite series magmas are likely
the differentiates of lower crustal partial melts, and that
these partial melts were produced as a result of the heat
produced by the crystallization (including clinopyroxene) of ponded primitive cpx shoshonite-trachyte series
magma in the lower crust.
Trachyandesite Series
Trachyandesite series samples have compositions
that lie between the mafic and silicic magmas of the volcanic field (figures 3 and 4). The compositional variations of this series indicate mineral control, but are too
varied to have been produced by fractional crystallization
alone along a single trend (e.g., figures 3a, 3d, 4d, 4f, and
4g). These compositional characteristics, supported by
magmatic temperature and oxygen fugacity estimates,
suggest that magma mixing and subsequent fractional
crystallization governed the generation and evolution of
trachyandesite series magma. Compositional variations
suggest that the mafic end member for mixing was mafic
cpx shoshonite-trachyte series magma. Zirconium concentrations (figure 4d) as well as estimated magmatic
temperatures and oxygen fugacity values indicate that
silicic shoshonite-trachyte series magma can be ruled out
as the silicic end member for the trachyandesite series.
Compositional variations are, however, consistent with
rhyolite series magma as the silicic end member of mix-
176
ing. The compositional trends illustrated in figure 7 indicate that neither Packard nor Keystone rhyolite series
magmas are the silicic end members of mixing—since
Packard rhyolite series magma has too much Ba and too
little Zr, and Keystone rhyolite series magma has too little Cr and Rb/Zr ratios that are too low. We propose that
trachyandesite series magmas were generated by mixing
between mafic cpx shoshonite-trachyte series and
Fernow rhyolite series magmas. Figure 8 illustrates the
compositions that could be produced by the combined
effects of magma mixing and fractional crystallization.
The fractional crystallization arrows in figure 8 were calculated using the parameters in table 5 and indicate the
direction in which primitive (recently mixed) trachyandesite series magmas would evolve by fractionation (of
the observed minerals). We are not able to determine the
relative importance of magma mixing and fractional
crystallization in generating trachyandesite series magmas. For example, widening the mixing envelope to
Central Utah — Diverse Geology of a Dynamic Landscape
include silicic cpx shoshonite-trachyte series magmas is
a valid alternative, but nearly eliminates the need for
fractional crystallization. Our model allows an important
role for both magma mixing and subsequent fractional
crystallization and is meant to be illustrative, not definitive. We suggest that primitive trachyandesite series
magmas were produced by magma mixing (within a mixing envelope similar to the one we have proposed) and
then evolved (often to compositions outside the mixing
envelope) by fractional crystallization.
If the magmas of both trachyandesite series groups
were produced by this mechanism, why does one group
contain only clinopyroxene while the other contains both
clinopyroxene and orthopyroxene? In addition, if 2-px
shoshonite-trachyte magma was produced by mixing
between two orthopyroxene-absent magmas, why does it
contain orthopyroxene? As mentioned above, Draper
and Johnston (1992) show that in high-Mg arc basalts,
only clinopyroxene is stable at high pressure whereas
Figure 7. Cr vs. Ba (A) and Zr vs. Rb/Zr (B) variation diagrams, showing (1) the rhyolite series magma responsible for mixing with
mafic cpx shoshonite-trachyte series magma to produce trachyandesite series magmas was Fernow rhyolite series magma rather
than Keystone or Packard rhyolite series magmas, and (2) assimilation of crustal material was not important in generating the compositions of shoshonite-trachyte series magma. Symbols as for figure 2.
2007 UGA Publication 36 — Willis, G.C., Hylland, M.D., Clark, D.L., and Chidsey, T.C., Jr., editors
177
Figure 8. The trace-element magma mixing and fractional crystallization models for trachyandesite series magmas. The models were
applied to Rb, Sr, Ba, Ni, Cr, Sc, V, Zr, U, Th, La, and Ce compositional data with equal success. The fit of the model to Rb, Sr, and
Ba data is shown here—Rb vs. Sr in figure A, Rb vs. Ba in figure B, and Sr vs. Ba in figure C. The mafic end member of mixing is
mafic cpx shoshonite-trachyte series magma (represented by the mafic portion of the fractional crystallization model of figure 6). The
silicic end member of mixing is Fernow rhyolite series magma. Arrows show the directions that magmatic compositions would be displaced, after mixing, as a result of fractional crystallization. The directions were calculated using the parameters of table 5. Symbols
as for figure 2.
178
both clinopyroxene and orthopyroxene are stable at low
pressure. Assuming that their interpretations regarding
pyroxene stability are applicable to the mafic and intermediate magmas of the East Tintic volcanic field, then
the observed differences in mineral assemblages (specifically pyroxene) could indicate that phenocryst assemblages in 2-px trachyandesite series and 2-px shoshonitetrachyte series magmas last equilibrated at low pressure,
and that the phenocryst assemblages of cpx trachyandesite series and cpx shoshonite-trachyte series magmas
last equilibrated at high pressure. This may indicate that
mixing occurred in these groups at those pressures.
Assimilation
The role of assimilation of crustal material can be
estimated most effectively with isotopic data; however,
the variability of incompatible trace-element ratios is
also a reasonably good indicator of open-system processes. Figure 7 illustrates the variability of two incompatible trace elements, Rb and Zr. The Rb/Zr ratio is relatively constant for shoshonite-trachyte series samples,
which is consistent with our interpretations that magmatic differentiation for cpx shoshonite-trachyte series
magma was dominated by fractional crystallization, that
2-px shoshonite-trachyte series magma may have been
generated by mixing of mafic and silicic cpx shoshonitetrachyte series magma (because mixing would not
change the ratio), and that assimilation was not significant for shoshonite-trachyte series magma. If we have
interpreted the origin of Fernow rhyolite series magma
correctly, then the mixing of Fernow rhyolite series and
shoshonite-trachyte series magmas could be considered
crustal assimilation.
CONCLUSIONS
We propose that fractional crystallization and magma
mixing controlled the evolution of magmas in the East
Central Utah — Diverse Geology of a Dynamic Landscape
Tintic volcanic field. The cpx shoshonite-trachyte series
magma evolved mainly by fractional crystallization. The
high K2O, Rb, and Al2O3/CaO ratios and modest SiO2
enrichment of this series appear to result from extensive,
high-pressure fractional crystallization of clinopyroxene
(without plagioclase). The 2-px shoshonite-trachyte
series magma was likely produced by magma mixing
between mafic and silicic shoshonite-trachyte series
magma at low pressure. Assimilation appears not to have
been important for the shoshonite-trachyte series.
Fernow rhyolite series magma was likely the differentiate of a lower crustal partial melt. Trachyandesite series
magma evolved by magma mixing—between mafic
shoshonite-trachyte series and Fernow rhyolite series
magmas—and subsequent fractional crystallization, at
low pressure for 2-px trachyandesite series magma and at
high pressure for cpx trachyandesite series magma. We
believe parental cpx shoshonite-trachyte series magma
originated in the mantle wedge above a Cenozoic subduction zone and then interacted with older subductionmetasomatized lithospheric mantle. Figure 9 is a schematic diagram that illustrates our petrogenetic interpretations—namely, how cpx shoshonite-trachyte series and
Fernow rhyolite series magmas evolved and interacted to
produce the magmas of the East Tintic volcanic field.
Our petrogenetic model has important implications for
the genesis of ore bodies in the Tintic mining district and
related areas (see Keith and others, 1993, 1997; Stavast
and others, 2006).
ACKNOWLEDGMENTS
This study was funded by grants from the Brigham
Young University Department of Geology and National
Science Foundation (# EAR-9114980). We thank Dave
Wark (Rensselaer Polytechnic Institute), Tobi Kosanke
(Shell Oil Company), and Don Clark, Tom Chidsey, and
Grant Willis (Utah Geological Survey) for helpful reviews of this manuscript.
Figure 9. Schematic diagram showing the proposed petrogenesis of East Tintic volcanic field magmas. Subduction-related cpx
shoshonite-trachyte series magma ponded at or near the base of the crust. Heat from the crystallizing magma induced melting in
continental material producing rhyolite series melts. Two-pyroxene shoshonite-trachyte magma was produced by mixing between
primitive and evolved shoshonite-trachyte series magma at low pressure. Trachyandesite series magma was generated by mixing
between Fernow rhyolite series and mafic cpx shoshonite-trachyte series magmas, at high pressure for cpx trachyandesite magma
and at low pressure for 2-px trachyandesite magma.
2007 UGA Publication 36 — Willis, G.C., Hylland, M.D., Clark, D.L., and Chidsey, T.C., Jr., editors
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