Insights into the tectonomagmatic evolution of NW Mexico:
Geochronology and geochemistry of the Miocene volcanic rocks from the
Pinacate area, Sonora
Jesús Roberto Vidal-Solano*
Departamento de Geología, Universidad de Sonora, Apdo. Postal 847, 83000 Hermosillo, Sonora, México and
Pétrologie Magmatique, Université Paul Cézanne (Aix-Marseille 3), Case Courier 441, 13397 Marseille Cedex 20, France
Alain Demant
Pétrologie Magmatique, Université Paul Cézanne (Aix-Marseille 3), Case Courier 441, 13397 Marseille Cedex 20, France
Francisco A. Paz Moreno
Departamento de Geología, Universidad de Sonora, Apdo. Postal 847, 83000 Hermosillo, Sonora, México
Henriette Lapierre†
Laboratoire de Géologie des Chaînes Alpines, UMR 5025, BP 53, 38041 Grenoble Cedex, France
María Amabel Ortega-Rivera
Estación Regional del Noroeste, Instituto de Geología, Universidad Nacional Autónoma de México, Apdo. Postal 1039, 83000 Hermosillo,
Sonora, México
James K.W. Lee
Department of Geological Sciences and Geological Engineering, Queen’s University, Kingston, Ontario K7L 3N6, Canada
ABSTRACT
Miocene volcanic rocks in the Pinacate
area, Sonora, record a progressive change in
the source of magmatism induced by asthenospheric upwelling and lithospheric thinning. 40Ar/ 39Ar age data, mineral chemistry,
and major- and trace-element contents allow
the identification of two volcanic sequences:
an oldest basaltic episode (ca. 20 Ma), and a
middle Miocene (12–15.5 Ma) sequence that
consists of mesa basalts with transitional
alkali character, calc-alkaline dacites, and
high-silica rhyolites evolving toward peralkaline liquids. Sr, Nd, and Pb isotope ratios
reveal different sources for the Miocene
basalts. The easternmost basalts have signatures indicating a Precambrian lithospheric
mantle source, while the westernmost tholeiitic to transitional basalts are related to
mixing of lithospheric and asthenospheric
mantle. Rhyolites are the result of fractional
crystallization of transitional basalt magmas
with slight contamination by Precambrian
crust. Chemical modeling shows that peral*E-mail: [email protected]
†
Deceased
kaline rhyolites are related to slightly higher
assimilation during their residence in the
upper crust but also to a change in the mantle
source of the parent basalt. The evolution of
the isotopic signatures in space and time indicates that: (1) the volcanic activity is located
over a major lithospheric boundary, i.e., the
western limit of the North American Craton;
(2) the lithosphere was progressively thinned
so that huge volumes of alkalic basalts could
access the surface during the Quaternary,
building the Pinacate Volcanic Field. Correlation between geochemical signatures and
the tectonic evolution of the western margin
of the North American Craton shows that a
progressive change in the source of magmatism can be related to the development of a
slab window during the Miocene.
Keywords: Volcanism, Mexico, geochronology, petrology, geochemistry, isotopes.
INTRODUCTION
The Pacific coast of northwestern Mexico has
been a convergent plate boundary since at least
the mid-Cretaceous. In Sonora, subductionrelated magmatism is represented by batholitic
granitoids between 90 and 40 Ma toward the west
(Damon et al., 1983; Richard et al., 1989; McDowell et al., 1997, 2001; Valencia-Moreno et al.,
2001). Meanwhile, toward the east, the subduction-related magmatism is revealed by the Late
Eocene–early Miocene large ignimbritic plateau
of the Sierra Madre Occidental (McDowell and
Keizer, 1977; McDowell and Clabaugh, 1979;
Montigny et al., 1987; Magonthier, 1988; Demant et al., 1989; Cochemé and Demant, 1991).
From Miocene to present, as the Farallon plate
fragmented and subduction under North America ended, the tectonic regime changed from a
convergent margin type, to a transtensional plate
margin style (Lonsdale, 1989; Stock and Lee,
1994). Since the mid-Cenozoic, tectonic extension has disrupted the Sierra Madre Occidental volcanic plateau, in Sonora and Chihuahua,
toward the west and toward the east, respectively.
In western Sonora, crustal extension gives rise to
the typical NNW–SSE basin and range morphology (Gans, 1997; McDowell et al., 1997; Gans et
al., 2003). This extensional regime has migrated
progressively toward the west (Gans et al., 2003,
2006; MacMillan et al., 2003, 2006), leading to
the establishment of a new frontier between the
Pacific–North America plate and the rift system
of the Gulf of California (Atwater, 1989; Stock
GSA Bulletin; May/June 2008; v. 120; no. 5/6; p. 691–708; doi: 10.1130/B26053.1; 13 figures; 6 tables; Data Repository item 2008046.
For permission to copy, contact [email protected]
© 2008 Geological Society of America
691
Vidal-Solano et al.
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91-30
HERMOSILLO
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8
VP
PV
1 2 4
3
5
3530000
8
6
7
JR98-23
6
7
3520000
Río
So
n o y ta
P03-22
2
1
P02-15
and
P02-20
31° 45’
31° 45’
JR98-20
5 Km
4
JR97-23
P02-8
JR99-83
8
to
Son
o y ta
P03-27
3
5
Altar desert
to Pto. Peñasco
113° 15’
Quaternary Pinacate Volcanic Field
Road
Trail
1 outcrop
Rhyolite
Dacite and andesite
Basalt
Figure 1. Geological sketch map of the pre-Pinacate area. 1—Sierra Suvuk; 2—Cerro Ladrilleros;
3—Sierra Batamote; 4—Cerro San Pedro; 5—Cerro El Picú; 6—Cerro Tres Mosqueteros; 7—Vidrios Viejos; 8—Lomas del Norte; PV—Pinacate Volcano (also called Santa Clara volcano).
692
Geological Society of America Bulletin, May/June 2008
Tectonomagmatic evolution of NW Mexico
Na 2 O + K 2 O
10
8
6
4
2
A
5
B
K2O
9
8
7
6
5
4
3
2
1
es
honit
Shos
4
M
3
H
high
K
R
2
B
BA
A
medium
K
1
D
Low K
40
45
50
55
60
65
70
75
50
55
60
SiO 2
65
70
75
SiO 2
Figure 2. (A) Total alkalis-silica diagram for the pre-Pinacate volcanic sequences; fields are from Le Bas et al. (1986). 1—sample 91–30,
tilted basaltic mesa NW of the Quaternary Pinacate Volcanic Field; 2—eastern basaltic outcrops lying directly on the crystalline basement;
3—basalts from Cerro San Pedro; 4—basalts from Sierra Batamote; 5—basalts on top of Sierra Suvuk; 6—andesitic and dacitic lavas from
Sierra Suvuk and Cerro Ladrilleros; 7—rhyodacites (P02–15 and P02–20); 8—ca. 12 Ma rhyolites; 9—ca. 14 Ma rhyolites. (B) K2O versus
SiO2 diagram (Peccerillo and Taylor, 1976) for the pre-Pinacate volcanic sequences.
and Hodges, 1989; Stock, 2000; Oskin et al.,
2001; Oskin and Stock, 2003).
The Pinacate Volcanic Field lies at the northern end of the Gulf of California in an arid
region that is part of the Altar desert. The volcanic field is composed of: (1) a Quaternary
volcanic shield (Santa Clara volcano of Lynch,
1981); (2) hundreds of scoria or spatter cones
covering the flanks of the shield; (3) well-preserved maars located on the lowermost slopes of
the volcano; and (4) the Miocene volcanic rocks
that crop out as scattered low-hill exposures east
of Sierra Pinacate (defined as the pre-Pinacate
volcanic sequences by Lynch [1981]).
The Miocene volcanic sequences have been
largely ignored since the reconnaissance work
reported by Lynch (1981), whereas for the Quaternary Pinacate Volcanic Field, several studies
have been done (Gutmann, 1976, 1979, 2002;
Gutmann et al., 2000; Paz-Moreno and Demant
2002, 2004). With the aim to characterize the
Miocene sequences and their chronology, discuss their petrogenesis, elucidate the correlation
between geochemical signatures and tectonic evolution of the western margin of the North American Craton, and show that a progressive change
in the source of magmatism can be related to the
development of a slab window during the Miocene, we present this study. New 40Ar/39Ar ages,
a summary of the mineral chemistry of the different rock types, their major- and trace-element
content, as well as Sr, Nd, and Pb isotope ratios
are reported here. We show that the basalts have
isotopic signatures indicating different sources,
that silicic rocks have features characteristic of
peralkaline rocks, and that they present evidence
of mixing with calc-alkaline dacitic magmas.
PRE-PINACATE VOLCANIC
SEQUENCES
Geological investigations conducted between
1998 and 2004 established the volcanic stratigraphy of the Miocene volcanic rocks (VidalSolano, 2001, 2005). According to their morphology and petrologic affinity, three main
rock types have been distinguished. (1) Sierra
Batamote and Cerro San Pedro are volcanic
mesas composed of basalts and basaltic andesites; Cerro Picú and Cerro Tres Mosqueteros
located eastward are smaller and isolated mafic
outcrops lying directly over crystalline basement. (2) Sierra Suvuk and Cerro Ladrilleros
correspond to andesitic and dacitic domes and
lava flows with more rugged shape. (3) Silicic
volcanic rocks (rhyolitic domes and pyroclastic
flow deposits) with frequent obsidian facies,
form smooth, hilly outcrops in the Lomas del
Norte and Vidrios Viejos areas (Fig. 1). Detailed
descriptions of the geological features are given
in two previous studies (Vidal-Solano, 2005;
Vidal-Solano et al., 2005).
A significant obstacle to establishing the stratigraphy in the region arises from the fact that
the Miocene outcrops are dispersed and not
directly in contact due to Late Tertiary extensional tectonics and recent covering by alluvial
fan deposits and Quaternary sand dunes.
RESULTS
All the analytical methods for Mineral chemistry, 40Ar/ 39Ar geochronology, and geochemistry are described in the GSA Data Repository
Appendix A section.1
Petrography and Mineral Chemistry
The three groups of lavas recognized in the
field are clearly delimited in the total alkalissilica diagram (Le Bas et al., 1986; Le Maitre,
1989). The mafic lavas (group 1) have compositions ranging from 48% to 57% silica (Fig. 2).
Most of these lavas are olivine to quartz normative basalts or basaltic andesites. Samples from
Cerro Picú and Cerro Tres Mosqueteros have
higher alkalis (mostly K2O) and fall therefore
in the field of alkaline lavas. However, only
one sample (JR97-24 from Cerro San Pedro)
presents normative nepheline. Based on major
elements, and other chemical criteria that will
be detailed in later sections, five types of mafic
lavas are distinguished on Figure 2A. The mafic
lavas are generally aphyric to slightly porphyritic
(<5% phenocrysts) with olivine and plagioclase
as the major phases. Clinopyroxene is relatively
uncommon as a phenocryst but abundant in the
groundmass together with olivine microcrysts
and plagioclase laths. Intersertal to intergranular
textures are the most common, but subophitic
textures are also observed.
The second group consists of lavas from
Sierra Suvuk and Cerro Ladrilleros. They plot in
the medium-K andesite and dacite fields on the
K2O versus SiO2 diagram (Fig. 2B). These lavas
1
GSA Data Repository Item 2008046, geochronological data and analytical methods description
for mineral chemistry, 40Ar/39Ar geochronology and
geochemistry of the pre-Pinacate Miocene volcanic
sequences, is available at www.geosociety.org/pubs/
ft2008.htm. Requests may also be sent to editing@
geosociety.org.
Geological Society of America Bulletin, May/June 2008
693
694
514
322
27
29
137
57
161
63
138
20
14
(continued)
499
268
28
31
107
48
152
61
136
20
15
485
346
27
29
100
40
153
60
145
21
15
512
310
28
29
101
39
158
63
142
20
14
18
426
424
35
30
116
58
236
80
200
29
34
18
401
375
40
40
121
54
247
78
194
31
34
14
486
398
41
12
40
17
258
96
279
40
29
490
665
35
33
85
31
231
85
286
41
28
18
407
307
40
40
129
59
239
75
179
29
34
9
448
266
52
39
87
56
325
90
217
34
29
56.94
58.12
51.55
50.66
58.77
51.91
55.05
22
433
690
37
26
58
30
231
120
228
34
18
25
425
675
49
21
54
30
223
119
240
37
18
58.64
9
195
121
46
103
244
162
196
98
Mg#
Rb (ppm)
Sr
Ba
Co
Cu
Cr
Ni
V
Zn
Zr
Y
Nb
32
884
1229
30
36
208
115
154
86
288
31
16
830
1008
27
42
121
63
164
69
173
22
8
791
1340
24
33
134
105
123
77
439
32
20
9
367
155
48
47
110
64
218
81
129
27
11
47.76
58.73
14.66
8.18
Qz
Ne
Hyp
Ol
65.05
60.99
59.86
48.92
57.81
50
320
476
35
52
157
65
157
85
264
38
17
58.78
59.12
15.74
13.10
17.80
16.13
18.13
16.50
0.45
13.31
3.83
2.94
11.62
13.61
15.12
18.73
16.26
2.07
13.75
17.18
17.99
9.38
7.64
2.24
28
335
615
38
51
118
40
165
77
248
38
16
62.07
58.65
19.04
13.06
4.25
99.35
99.80
0.08
1.60
99.70
99.18
0.58
1.67
99.27
98.99
0.47
0.30
99.16
99.06
99.05
99.11
3.21
99.83
100.25
1.91
3.79
100.37
100.01
98.36
100.26
Total
1.66
99.35
0.36
99.05
0.00
50.09
1.73
15.45
1.84
9.71
0.17
7.61
9.05
3.23
0.38
0.16
0.82
0.02
99.09
JR98-19
S
53.51
1.26
17.34
3.83
3.36
0.12
4.56
8.71
4.40
0.81
0.34
1.28
0.28
JR98-18
S
52.95
1.32
17.51
6.24
1.72
0.12
4.94
8.86
3.82
0.69
0.34
1.05
0.14
JR98-15
S
52.50
1.23
17.60
3.32
3.66
0.12
4.54
9.16
4.00
0.78
0.33
1.72
0.22
JR99-85
B
52.87
2.04
16.55
2.33
6.20
0.14
5.18
7.68
3.82
1.15
0.53
0.76
0.02
JR99-89
B
51.97
2.04
16.35
2.95
5.51
0.15
5.35
8.70
3.72
1.02
0.51
0.70
0.02
P02-06
B
51.17
2.28
15.71
3.78
6.51
0.19
4.98
8.26
3.58
1.29
0.83
0.27
0.31
JR98-2
B
50.48
2.22
15.61
4.72
5.86
0.19
4.90
9.09
3.50
1.10
0.93
0.38
0.08
JR99-88
B
50.41
2.07
16.49
4.96
3.93
0.15
5.65
8.07
3.80
0.96
0.42
1.90
0.24
P02-17
B
47.40
3.05
15.43
5.74
6.29
0.2
5.84
9.52
3.32
0.88
0.55
0.26
0.63
JR97-27
SP
55.97
1.34
15.47
3.13
4.74
0.13
4.37
7.16
4.09
1.87
0.26
1.15
0.15
JR97-28
SP
55.34
1.40
15.96
2.12
5.71
0.14
4.93
7.55
4.06
1.64
0.28
1.06
0.06
PI97-33
SP
53.76
2.20
15.71
2.03
8.76
0.17
4.57
7.48
3.52
1.25
0.46
0.40
0.06
JR97-23
SP
51.53
2.27
15.25
2.60
8.69
0.17
4.99
7.68
3.81
1.28
0.47
1.14
0.13
JR97-24
SP
47.83
1.62
17.02
4.99
4.81
0.16
6.55
10.59
3.45
0.68
0.27
0.07
0.32
JR98-29
TM
55.02
1.20
16.26
3.00
4.22
0.12
4.65
6.11
3.74
2.94
0.71
1.22
0.16
JR98-21
C#2
52.85
0.90
17.25
3.03
4.60
0.12
5.41
8.53
3.50
1.40
0.41
0.84
0.21
PI97-24
P
52.05
1.16
16.65
4.07
3.63
0.14
6.41
8.13
3.48
2.11
0.63
0.48
0.15
91-30
Sample no.
Locality
SiO2 (wt %)
TiO2
Al2O3
Fe2O3
FeO
MnO
MgO
CaO
Na2O
K2O
P2O5
H2O+
H2O-
Basalts and Basaltic Andesites
Olivine in the basalts and basaltic andesites
displays relatively large compositional variations. Fresh olivines (Fo82 to Fo75) have been
analyzed in plagioclase + olivine clots in the
basaltic andesite (JR97–28), whereas in most
of the basaltic samples olivine phenocrysts are
partly altered to iddingsite. Small chromiumbearing spinels are enclosed in olivine phenocrysts in the basalts from Cerro Tres Mosqueteros (JR98–30). Microcrysts (<200 µm)
from the matrix range in composition from
Fo65 to Fo44. Plagioclase, the dominant phase,
has compositions in the range An63–38. Ca-rich
phenocrysts (An75–70) in sample JR97–28 correspond to feldspars forming glomerophyric
clusters with olivine. Large (3–4 mm) euhedral
crystals in basalt P02–17 from Sierra Batamote
are, despite their size, unzoned (An63–60). Clinopyroxene displays distinct evolutionary paths
on the En-Wo-Fs classification diagram (Morimoto et al., 1988). The first type corresponds to
basalt JR97–28 from Cerro San Pedro (Fig. 3).
The tiny pyroxene crystals (<100 µm) from this
intersertal textured lava show an evolutionary
trend characterized by a decrease of the wollastonite component (Wo) without a change in
the Mg/(Mg + Fe) ratio. The pyroxene trend for
the other Cerro San Pedro samples exhibits a
decrease in both Ca and Mg and a scattered distribution of the analyses due to complex sequential growth and sector zoning, a common feature
in the subophitic textures (Hall et al., 1986). In
TABLE 1. MAJOR- (WT%) AND TRACE-ELEMENT (PPM), INDUCTIVELY COUPLED
PLASMA–ATOMIC EMISSION SPECTROSCOPY ANALYSES OF THE PRE-PINACATE MIOCENE VOLCANIC SEQUENCES
contain plagioclase and orthopyroxene phenocrysts set in a glassy groundmass including
minute plagioclase microlites and oxide grains.
In addition, amphibole and/or clinopyroxene
phenocrysts are observed in some samples. The
common glomeroporphyritic aspect of these
lavas comes from the presence of plagioclase
+ amphibole ± orthopyroxene aggregates. Two
samples located east of Cerro Ladrilleros (P02–
15 and P02–20, Fig. 1) have the same mineral
association as the dacites but higher silica contents (Table 1). They plot therefore in an intermediate position between the dacites and the
rhyolites (Fig. 2A).
The rhyolites (group 3) have high sodium
and potassium contents and relatively low alumina. As a result, some of them show a peralkaline signature [with (Na + K)/Al >1] also
indicated by the presence of normative acmite.
All the high-silica rocks (>72% SiO2) classify
as comendites (Macdonald, 1974) on the Al2O3
versus FeOt diagram (not shown). The rhyolites
have a mineral association that is composed of
K-feldspar, fayalite, and green clinopyroxene.
Late crystallizing sodic amphibole is present in
some of the peralkaline rocks.
JR98-7
S
53.53
1.29
17.46
4.30
3.39
0.12
5.61
8.49
3.30
0.67
0.33
0.75
0.11
Vidal-Solano et al.
Geological Society of America Bulletin, May/June 2008
51.20
45.75
17.54
11.97
40.91
14.77
28.29
11.72
31.67
16.35
38.88
42.75
99.34
20.30
11.71
39.69
17.15
99.80
41.44
24.14
13.77
34.30
17.46
99.41
47.30
21.95
14.08
39.91
13.08
99.65
47.82
32.39
11.34
30.32
15.52
99.50
20.70
24.54
18.68
41.96
8.38
98.84
21.53
24.77
20.05
39.76
8.68
98.87
4.46
0.76
43.01
19.70
29.11
3.90
99.11
9.92
0.74
40.80
22.69
28.55
2.71
99.12
12.50
0.99
30.78
30.01
34.00
0.51
98.83
19.32
1.00
30.33
27.26
37.08
0.17
99.47
6.89
0.95
29.31
27.70
32.45
32.17
28.41
34.50
1.75
1.43
2.58
100.59
100.08
37.23
21.77
32.30
1.96
99.23
126
14
80
3
3
4
<1
5
100
671
77
74
50.85
55.48
24.42
8.36
31.40
22.46
100.64
23
27
Rb (ppm)
32
39
37
43
52
47
47
79
74
164
258
137
Sr
638
469
487
563
520
690
640
485
475
441
449
170
180
51
12
6
7
12
12
Ba
808
811
854
867
530
567
823
787
752
819
807
832
849
55
132
15
130
39
136
21
19
Co
14
12
12
11
8
9
9
9
7
4
4
2
1
1
2
2
2
Cu
23
18
7
32
10
7
6
4
6
7
4
12
12
2
2
1
3
7
2
Cr
87
23
18
3
23
18
15
6
8
24
14
27
18
3
4
4
1
11
2
10
1
Ni
3
14
1
3
2
<1
11
12
1
14
5
<1
1
2
3
3
2
V
48
171
119
86
67
70
49
46
51
44
42
14
16
21
67
3
2
4
32
Zn
59
51
63
66
51
78
54
67
53
62
41
54
50
104
63
71
58
46
120
Zr
185
218
145
126
181
144
172
212
215
230
327
334
288
185
257
206
218
140
708
Y
17
21
21
16
17
15
22
23
21
14
13
29
30
67
17
78
65
84
63
Nb
19
16
10
11
7
7
13
13
18
8
9
20
20
50
16
35
27
47
26
Note: Abbreviations: P—Cerro El Picú; C#2—Road no. 2; TM—Cerro Tres Mosqueteros; SP—Cerro San Pedro; B—Sierra Batamote; S—Sierra Suvuk; L—Cerro Ladrilleros; LN—Lomas del Norte;
VV—Vidrios Viejos. Samples dated (P03–27 and P03–22) correspond to JR99–74 and JR97–19, respectively. For sample localities, see Vidal-Solano (2005).
24.23
9.38
31.50
22.52
16.08
9.15
30.83
27.17
99.04
4.84
0.91
48.59
57.89
Mg#
AI
99.14
100.16
0.64
1.22
13.85
7.40
34.13
25.78
10.13
5.23
32.11
29.74
Qz
Or
Ab
An
Ac
Ns
99.71
99.24
6
43
222
30
17
116
64
734
3
4
12.74
0.91
30.79
26.83
35.21
3.23
99.70
119
70
750
3
4
21
7
18
30
237
30
19
14.27
0.87
30.69
26.08
34.00
4.73
99.21
JR99-78 JR99-80 PI97-16 JR98-48 JR99-81 JR97-30 JR98-20 JR99-79 JR98-14 JR97-1 JR99-83 P02-20 P02-15 JR99-74 JR98-68 JR98-26 JR98-23 JR98-25 JR97-19 JR99-82 P02-8 P02-11
S
S
S
S
S
S
S
S
L
L
L
L
P
LN
LN
LN
VV
S
P
VV
B
B
57.49
59.66
64.15
65.58
66.91
67.09
67.79
61.18
64.54
65.24
68.86
69.11 69.96
74.33
76.03
71.97
76.07
74.25
75.89 74.79 73.49
74.83
0.98
1.12
0.66
0.67
0.60
0.43
0.80
0.62
0.58
0.60
0.37
0.08
0.10
0.10
0.10
0.46
0.19
0.15
0.30
0.31
0.03
0.20
17.79
17.12
17.71
16.75
16.39
15.35
15.80
15.91
15.64
14.92
15.36
14.23 14.31
11.55
13.02
11.69
12.53
11.41
11.23 12.88 12.90
12.12
1.75
2.77
1.55
1.01
0.85
1.89
2.95
1.47
1.21
2.31
0.83
0.89
1.48
1.15
0.50
1.55
0.84
1.13
1.40
1.55
0.99
2.11
4.31
3.31
2.35
2.44
1.89
0.27
3.52
2.76
2.78
1.55
1.98
0.56
0.15
0.38
0.98
1.91
1.13
0.51
1.58
1.46
0.07
1.43
0.11
0.10
0.09
0.07
0.07
0.07
0.07
0.07
0.02
0.07
0.05
0.07
0.06
0.02
0.02
0.02
0.06
0.03
0.03
0.02
0.04
0.07
3.82
2.59
1.90
1.46
1.25
1.20
1.24
2.85
1.68
1.09
1.18
0.03
0.10
0.16
0.05
0.01
0.13
0.12
0.35
0.37
0.05
0.08
7.11
5.92
4.65
4.69
4.12
3.71
3.75
5.72
5.10
3.46
3.20
0.78
0.80
1.04
0.58
0.42
0.74
1.56
1.73
1.98
0.93
0.45
3.73
3.96
3.60
3.61
3.62
4.77
4.59
3.66
4.00
4.65
3.49
4.80
4.61
3.82
4.33
4.07
5.35
4.14
3.95
3.26
3.33
3.78
0.87
1.23
1.38
2.00
1.53
1.54
1.94
1.94
2.30
2.35
1.87
3.16
4.83
4.56
4.80
4.70
4.52
4.34
3.06
3.33
3.79
3.65
0.26
0.31
0.21
0.23
0.26
0.20
0.21
0.20
0.20
0.13
0.13
0.03
0.03
0.04
0.04
0.02
0.03
0.04
0.08
0.09
0.33
0.05
0.93
1.50
1.34
1.37
1.98
1.84
1.53
0.59
0.84
2.13
0.16
0.17
3.61
0.60
0.28
0.44
0.27
0.16
1.29
4.18
0.42
0.27
0.09
0.12
0.19
0.14
0.92
0.08
0.19
0.11
0.23
0.31
0.05
0.24
0.23
0.14
0.06
0.01
0.01
0.83
1.97
0.67
0.09
0.02
Total
Sample no.
Locality
SiO2 (wt %)
TiO2
Al2O3
Fe2O3
FeO
MnO
MgO
CaO
Na2O
K2O
P2O5
H2O+
H2O-
TABLE 1. MAJOR- (WT%) AND TRACE-ELEMENT (PPM), INDUCTIVELY COUPLED
PLASMA–ATOMIC EMISSION SPECTROSCOPY ANALYSES OF THE PRE-PINACATE MIOCENE VOLCANIC SEQUENCES (continued)
Tectonomagmatic evolution of NW Mexico
P02–17 basalt from Sierra Batamote, pyroxene
has a trend more typical of alkaline lavas with a
slight increase in the Wo component when the
Mg/(Mg + Fe) ratio decreases (Paz-Moreno et
al., 2003; Legendre et al., 2005). These crystals
are also more titanium-rich. Ilmenite is the most
common iron-titanium oxide, but titanomagnetite also crystallizes in JR97–9 and P02–17
samples. Such minerals are typically late crystallizing phases in the basalts.
Andesitic and Dacitic Lavas
The differentiated rocks are slightly more
porphyritic than the mafic lavas (5%–10% phenocrysts). Plagioclase is by far the most abundant mineral either as phenocrysts or as microlites in the matrix. Surprisingly, plagioclase in
the dacites is more calcic than feldspars in the
basaltic lavas (Fig. 3). In Sierra Suvuk samples,
plagioclase ranges in composition from An78 to
An33, and it is even more calcic (up to An82) in
Cerro Ladrilleros dacites. However, these highCa phenocrysts are partly resorbed. Likewise,
sieve textures in Na-rich crystals are also evidence for disequilibrium. Feldspar in equilibrium with the host dacitic magma hence has a
limited compositional range of An65 to An40.
Orthopyroxene is the most common ferromagnesian phase in the dacitic lavas. Mg-rich
orthopyroxene (En75–58) is observed in the less
evolved dacite (JR98-48) with high Mg-number
[100 × Mg/(Mg + Fe)]. In the other dacites, orthopyroxene has homogeneous compositions in the
range En61–50 (Fig. 3). In some samples, augite
(Wo44–40En40–32Fs18–25) is also present. Orthopyroxene-clinopyroxene pairs in equilibrium give
crystallization temperatures in the range 1000°
to 940 °C (Wells, 1977; Lindsley, 1983). In the
other samples, amphibole, which classifies as
pargasite hornblende (Leake, 1978), accompanies orthopyroxene. Fe-Ti oxides are generally
titanomagnetite. The glassy matrix has a rhyolitic
composition with high silica (~75%) and alkalis
(~6%) and low alumina (~11%) and very low Ca
(<0.5%) contents. In the rhyodacitic end member (P02–15 and P02–20), plagioclases are more
sodic (An40 to An27) than those in the dacites and
orthopyroxene more iron-rich (En51–47; Fig. 3).
Rhyolites
Rhyolites of the pre-Pinacate sequences
are almost aphyric. They contain microcrysts
(<200 µm) of Na-sanidine (Or43–52) as the principal
phase. Honey-colored fayalite (Fa96–98) and green
iron-rich ferrohedenbergite (Deer et al., 1978b;
Table 2) are the other components (<50 µm).
Peralkaline rhyolites [with (Na + K)/Al >1] have
an agpaitic texture characterized by radiate intergrowths of arfvedsonite (Fe- and Na-rich amphibole) and aenigmatite (Deer et al., 1978a). In one
Geological Society of America Bulletin, May/June 2008
695
Vidal-Solano et al.
A
40
30
20
Basalts
Dacites
Rhyolites
10
En
10
20
30
40
50
An
B
30
. .
nd
70
80
90
Fs
JR97-23
JR98-48
JR97-9
JR98-20
JR97-28
JR99-79
P02-17
P02-15
JR99-83
P02-20
JR97-19/JR98-23/JR98-31
•
P02-13A/14
lite
Pe
mi ralk
xe ali
d r ne
oc rh
ks yo
dri
Ce
rro
La
rra
Sie
rra
Sie
sa
ros
lle
Su
tam
Ba
Sa
50
Ce
rro
vu
k
ote
70
nP
ed
ro
90
60
JR99-82
•
10
Ab
Ab
25
50
75
Or
Figure 3. Pyroxene (A) after Morimoto et al. (1988) and feldspar compositions (B) for representative lavas of the pre-Pinacate volcanic
sequences.
sample from Vidrios Viejos (JR99–82), fayalite
microcrysts are enclosed by late crystallizing
xenomorphic arfvedsonite (Vidal-Solano, 2005).
In sample JR98–31A, aegirine is present as green
pleochroic microcrysts amongst the quartz + Kfeldspar association of the flow planes. Quartz is
never present as phenocrysts in these lavas. This
distinctive mineral association characterizes
comendite-type, high-silica rhyolites (Sutherland, 1974; Mahood, 1980).
40
Ar/ 39Ar Geochronology
Until now, the chronology of the pre-Pinacate volcanic successions was only established
by field relations (Vidal-Solano et al., 2005).
40
Ar/ 39Ar age determinations have been performed to clarify the chronology of the volcanic sequences. Nine samples (three basalts, two
dacites, and four rhyolites) were collected in different places from the study area (Fig. 1), and 12
date analyses were obtained from mineral grain
696
separates (plagioclase and hornblende) and were
whole rock dated. The integrated and plateau
dates for the 40Ar/ 39Ar step-heating analyses are
reported in Table 3; age spectra are illustrated in
Figure 4 (for the complete 40Ar/ 39Ar step-heating result analyses, and correlation diagrams,
see Data Repository Appendix B [ footnote 1]).
For the purposes of this paper, a plateau date is
obtained when the apparent date of at least three
consecutive steps, consisting of a minimum of
30% of the 39ArK released, agree within 2σ errors
with the integrated date of the plateau segment.
Errors on the age spectrum and isotope-correlation diagrams represent the analytical precision
at ±2σ level.
Three plagioclase separates from basalts were
dated. Two yield apparent “argon-loss” spectra
characterized by the increase of the age with the
increasing of the temperature steps, thus yielding a maximum plateau date for the remaining
50%–60% of the spectrum. Two plagioclase
separates (samples 91–30 and JR98–21B) come
from tilted mesas located at the northern end
of the study area (Fig. 1). Sample JR98–21B
gives a climbing date spectra starting at 12.50
± 1.81 Ma, with a maximum date of 19.00
± 0.86 Ma at the highest power increment. Sample 91–30 shows a saddle-shape spectra with a
maximum date at 20.07 ± 2.17, at the highest
power increments, consistent with its correlation date at 19.87 ± 2.45. The third plagioclase
separate (sample JR97–23) taken at the base of
the basaltic sequence of Cerro San Pedro, gives
a disturbed spectrum with a minimum date at
11.77 ± 2.91 Ma at the low power increments
climbing to a maximum date at the highest
power increments at 20.64 ± 1.70 Ma.
The dated dacitic samples come from (1) a
plagioclase + two pyroxenes lava dome at Sierra
Suvuk, and (2) a dacitic lava flow containing
fresh amphiboles from Cerro Ladrilleros (Fig. 1).
The age spectrum of plagioclase from Sierra
Suvuk (JR98–20) is disturbed and presents an
“argon-loss” spectrum. The spectrum yielded a
Geological Society of America Bulletin, May/June 2008
Tectonomagmatic evolution of NW Mexico
plateau date at 13.53 ± 1.24 Ma corresponding
to 66.5% of the total degassed 39Ar, and its lowtemperature (T) steps show a minimum date at
5.27 ± 2.50 Ma. Hornblende from Cerro Ladrilleros (JR99–83) yields a plateau date of 12.04
± 1.37 Ma calculated for the last four steps.
The rhyolitic rocks are generally aphyric;
therefore, whole-rock samples were used for
dating. Obsidian collected at the base of Cerro
Picú (P03–27, Fig. 1) yields an “argon-loss”
spectrum with an integrated age of 14.70
± 0.15 Ma, and a climbing date spectra starting
at 11.60 ± 1.74 Ma with a plateau date at 15.30
± 0.16 Ma. The rhyolitic sample from Lomas
del Norte (JR98–23, Fig. 1) has a maximum
date at 14.23 ± 0.15 Ma corresponding to 90.7%
of the total degassed 39Ar, with a first step starting at 11.10 ± 1.96 Ma. An obsidian nucleus
(commonly referred to as “Apache tears” in SW
Arizona and NW Sonora; Shackley, 2005) from
Vidrios Viejos, sample P03–22, yields a reproducible spectra with a maximum date at 14.27
± 0.87 (obs) and 14.15 ± 1.15 Ma (obs-HCl,
same sample after HCl leaching). Also, it shows
well-defined plateau dates at 12.08 ± 0.62 Ma
and 11.98 ± 0.62 Ma, respectively, at the low-T
steps. Finally, an obsidian sample (P02–8 obs),
from a small outcrop west of Cerro San Pedro
(Fig. 1), and its associated pumice layer (P02–8
wr) give concordant and reproducible plateau
dates at 12.16 ± 0.07 Ma and 12.05 ± 0.07 Ma,
respectively (Fig. 4; Table 3). Another experiment was done for the same sample after HCl
leaching to verify that the low apparent age
in step 2 (corresponding to high Ca/K) that
could be related to calcite present in the perlitic
fractures. For this sample, an excellent 12.10
± 0.10 Ma plateau age was obtained with all
the steps, and this age was concordant with the
12.30 ± 0.38 Ma correlation age.
TABLE 2. SELECTED MICROPROBE ANALYSES
OF SPECIFIC MINERALS FROM THE
PERALKALINE RHYOLITES
Geochemistry
Most of the samples analyzed are fresh as
shown by H2O content less than 2% (Table 1). In
the total alkali-silica (TAS) diagram (Fig. 2A),
the data set shows a relative continuum among
the basaltic, dacitic, and rhyolitic groups. However, on the K2O versus SiO2 diagram (Fig. 2B),
differences are apparent within the basalt group.
A clear shift in the K2O component is observed
between the dacites and the rhyolites, with the
rhyodacites P02–15 and P02–20 lying in an
intermediate position.
Abundances in Ni, Cr, Co, Sr, and Ba are
highly variable in the mafic lavas (Table 1).
Incompatible multielement patterns normalized
to the primitive mantle of Sun and McDonough
(1989) and rare-earth elements (REE) spectra
normalized to chondrites (Boynton, 1984) discriminate four subtypes of mafic lavas (Fig. 5).
Type 1 corresponds to sample 91–30, a tilted
basaltic mesa at the northern boundary of the
area. This basalt is slightly enriched in light
rare-earth elements (LREE) [(La/Yb)N = 2.28]
and displays a relatively flat pattern on the
mantle-normalized multielement diagram with
a pronounced positive peak in Pb. Basalts forming the scattered outcrops located at the eastern limit of the studied area belong to Type 2.
They are more enriched in LREE [(La/Yb)N =
15–17] and present a slight negative anomaly in
Eu and flat, heavy rare-earth elements (HREE).
Their multielement spectra are enriched in the
most incompatible elements and characterized
by (1) positive peaks in Pb and Ba, (2) slight
negative anomalies in Ti and P, and a more
pronounced anomaly in Nb-Ta. Types 3 and 4
consist of basalts and basaltic andesites from
Cerro San Pedro and Sierra Batamote, respectively. Their REE patterns are slightly enriched
Sample no.
analysis no.
SiO2
FeO
MnO
MgO
CaO
TiO2
Total
Fa
FAYALITE
JR98-23A
JR99-82
3
4
10
14
70
71
28.97 28.36 28.72 29.03 28.60 28.74
69.45 68.42 69.61 70.20 69.72 69.73
2.17 2.09 2.00 2.03 2.94 2.89
0.49 0.51 0.57 0.48 0.07 0.04
0.25 0.26 0.26 0.33 0.29 0.29
0.03 0.04 0.03 0.02 0.02 0.04
101.36 99.69 101.18 102.09 101.63 101.72
0.98 0.98 0.98 0.99 0.99 0.99
Sample no.
Analysis no.
SiO2
Al2O3
FeO
MnO
MgO
CaO
Na2O
K2O
TiO2
Total
ARFVEDSONITE
JR99-82
66
67
72
74
48.64 49.27 48.50 48.39
0.20 0.24 0.25 0.13
36.82 35.40 37.09 36.75
1.04 0.98 1.04 1.18
0.16 0.16 0.05 0.05
3.01 2.54 2.67 2.66
6.84 7.13 6.98 7.08
1.52 1.41 1.27 1.57
0.23 0.26 0.24 0.24
98.45 97.40 98.08 98.03
Sample no.
Analysis no.
SiO2
Al2O3
FeO
MnO
MgO
CaO
Na2O
K2O
TiO2
Total
AEGYRINE
JR98-31A
51
58
51.60 52.47
0.18 0.18
27.73 27.22
1.42 0.68
0.08 0.05
3.32 2.12
11.50 12.06
0.00 0.04
2.14 3.24
97.97 98.06
Sample no.
Analysis no.
SiO2
Al2O3
FeO
MnO
MgO
CaO
Na2O
K2O
TiO2
Total
63
39.81
0.60
43.22
0.78
0.04
0.78
6.60
0.02
7.87
99.75
FERROHEDENBERGITE
JR99-82
58
59
60
61
47.30 47.66 47.41 47.32
0.16 0.16 0.14 0.16
31.85 30.92 31.11 32.14
1.00 0.96 1.00 0.97
0.12 0.14 0.14 0.08
16.59 16.76 16.60 15.60
2.20 2.52 2.79 2.80
0.03 0.03 0.04 0.03
0.57 0.54 0.79 0.53
99.82 99.69 100.02 99.63
AENIGMATITE
JR99-82
65
68
75
76
40.07 40.16 40.49 39.41
0.61 0.25 0.34 0.61
42.10 44.59 41.84 42.57
0.90 0.94 0.89 0.78
0.03 0.01 0.04 0.05
0.65 0.25 0.57 0.82
6.64 6.59 7.10 7.02
0.02 0.04 0.07 0.03
8.23 6.55 8.21 8.02
99.26 99.40 99.54 99.33
TABLE 3. 40AR/39AR RADIOMETRIC AGES OF THE PRE-PINACATE MIOCENE VOLCANIC SEQUENCES
Plateau
Sample no. Rock type
Locality
Mineral or Laboratory Integrated Error Correlation Error MSWD 40Ar/ 36Ar Error
Lat/Long
run
(initial)
2σ
date (Ma)
whole rock
date (Ma) 2σ date (Ma) 2σ
Dacite
1.01
14.93
6.67 1.18 267.26 210.12
13.53
JR98-20
Sierra Suvuk
Pl
L705
10.72
31° 45' 37.45'', 113° 20' 59.16''
Dacite
1.21
12.02
JR99-83
Cerro Ladrilleros
Hb
R714
11.34
2.48 0.77 294.87 90.93
12.04
31° 44' 8.73'', 113° 18' 0.24''
PO3-27
0.15
14.16
14.70
1.82 0.61 288.85 73.44
15.30
Obsidian
Cerro El Picu
Obs
R726
31° 41' 38.08'', 113° 8' 11.29''
JR98-23
0.24
Wr
R715
13.94
n/a
n/a
n/a
n/a
n/a
14.23
Rhyolite
Lomas del Norte
31° 54' 32.45'', 113° 11' 44.54''
0.58
13.27
PO3-22
Vidrios Viejos
Obs
R727
13.15
71.03 0.07 291.26 2394.39 14.27
Obsidian
31° 51' 15.74'', 113° 13' 46.76'' Obs-HCl
12.77
0.62
n/a
n/a
n/a
n/a
n/a
14.15
R756
PO2-8
Rhyolite
R730
11.75
0.10
12.16
0.08
1.4
270.25 138.27
12.16
N Batamote
Obs
R743
12.11
0.11
12.30
0.38 4.45 235.36 109.34
12.10
31° 42' 31.45'', 113° 14' 47.75'' Obs-HCl
Wr
R716
12.04
0.08
11.93
1.72 5.01 333.27 1991.22 12.05
91-30
Mesa Norte
Pl
712
23.93
7.44
19.87
2.45 1.03 298.91
58.7
20.07
Basalt
31° 7' 47.16'', 113° 51' 36.58''
JR98-21
Basalt
Road n°2
Pl
R713
16.03
0.70
n/a
n/a
n/a
n/a
n/a
19.00
31° 52' 13.24'', 113° 55' 51.81''
JR97-23
Pl
R711
16.81
1.59
9.89
9.32 1.69 299.26
421
20.64
Basalt
San Pedro
Lower sequence
31° 44' 24.38'', 113° 14' 12.22''
Geological Society of America Bulletin, May/June 2008
Error Volume
2σ 39Ar (%)
1.24 54.1
1.37
63.9
0.16
58.4
0.15
90.7
0.87
1.15
0.07
0.10
0.07
2.17
48.8
36.6
93.1
97.1
90.3
47.1
0.86
42.7
1.70
56.7
697
Vidal-Solano et al.
Ca/K
10
Apparent Age (Ma)
60
Ca/K
2
10
25
10
Maximum date
Maximum date
2
40
20
Maximum date
1
20
15
B
0
1
10
91-30 (Pl)
JR98-21B (Pl)
JR97-23 (pl)
5
-20
5
Integrated date = 16.03 ± 0.70 Ma
Maximum date = 19.00 ± 0.86 Ma
Integrated date = 14.79 ± 9.06 Ma
Maximum date = 20.07 ± 2.17 Ma
-40
0
0
20
60
40
80
100
0
20
60
40
80
100
0
Integrated date = 16.81 ± 1.59 Ma
Maximum date = 20.64 ± 1.70 Ma
0
20
40
60
80
100
100
100
Ca/K
Ca/K
Ca/K
1
10
10
Apparent Age (Ma)
100
100
100
Ca/K
0.1
Plateau
Maximum date
Plateau
15
15
15
10
10
10
JR98-20 (pl)
5
JR99-83 (hb)
5
Integrated date = 11.34 ± 1.21 Ma
Plateau = 12.04 ± 1.37Ma
Integrated date = 11.42 ± 0.92 Ma
Maximum date = 15.45 ± 1.37 Ma
20
60
40
20
80
Ca/K
1
15
10
10
10
JR98-23 (wr)
60
80
0
20
60
40
80
Integrated date = 12.77 ± 0.62 Ma
100
0
0.1
20
20
20
15
15
15
10
10
Integrated date = 11.75 ± 0.10 Ma
Plateau date = 12.16 ± 0.07 Ma
Integrated date = 12.04 ± 0.08 Ma
Plateau date = 12.05 ± 0.70 Ma
60
39
80
Cumulative % ArK Released
100
0
10
P02-8 (obs-HCl)
P02-8 (obs)
5
40
8
10
P02-8 (wr)
20
6
Ca/K
0.1
0
4
Ca/K
0.1
0
2
1
1
Ca/K
Apparent Age (Ma)
P03-22 (obs-HCl)
5
Integrated date = 13.15 ± 0.58 Ma
100
1
5
Ca/K
P03-22 (obs)
5
Integrated date = 13.94 ± 0.24 Ma
Plateau date = 14.25 ± 0.15 Ma
40
80
Maximum date
14.15 ± 1.15 Ma
15
20
60
00.1
15
0
40
Maximum date
14.27 ± 0.87 Ma
Maximum date
14.25 ± 0.15 Ma
0
20
0.1
00.1
5
Plateau = 15.30 ± 0.16 Ma
integrated date = 14.70 ± 0.15 Ma
0
80
Ca/K
0.1
0.1
Apparent Age (Ma)
60
40
P03-27 (obs)
5
0
20
60
40
80
100
5
0
Integrated date = 12.11 ± 0.11 Ma
Plateau date = 12.10 ± 0.10 Ma
0
39
Cumulative % ArK Released
20
40
60
80
100
Cumulative % 39ArK Released
Figure 4. 40Ar/39Ar age spectra for the different pre-Pinacate volcanic sequences (see Table 3). Pl—plagioclase;
hb—hornblende; obs—obsidian; wr—whole rock; obs-HCl—obsidian washed in hydrochloride acid.
698
Geological Society of America Bulletin, May/June 2008
Tectonomagmatic evolution of NW Mexico
A
100
B
200
100
100
400
30
20
200
100
10
10
10
Rock / Chondrites
Rock / Primitive mantle
100
100
10
20
10
30
1
20
100
10
10
La
10
Pr
Ce
1
Rb Th Nb K Ce Sr Nd Hf Eu Dy Ho Lu
Ba U Ta La Pb P Zr Sm Ti
Y Yb
Nd
Sm Gd
Dy
Er
Yb
Eu
Tb Ho Tm
Lu
Type 4 (Batamote)
Type 3 (San Pedro)
Type 2 (Tres Mosqueteros)
Type 1 (91-30)
Figure 5. Chondrite-normalized, rare-earth element (REE) abundances (A) and primitive mantle-normalized trace-element patterns (B) for
selected mafic lavas for the pre-Pinacate sequences. Normalizing values for the REE after Boynton (1984), and from Sun and McDonough
(1989) for the incompatible elements.
in LREE [(La/Yb) N = 5–7]. Sierra Batamote
spectra are more enriched in LREE than those
of Cerro San Pedro lavas (Fig. 7). The multielement patterns are relatively flat but more
enriched than that of Type 1 basalts.
Dacitic lavas display a regular increase in
LREE [(La/Sm) N = 3.7–4.9], a strong negative
anomaly in Eu, and irregular and variable patterns
for the HREE (Fig. 6). Rhyodacites (P02–15 and
P02–20) have a more regular and enriched REE
spectra. Differences with the dacites are also
apparent on the multielement diagram. Rhyodacites are enriched in all the elements but at the
same time present more pronounced negative
anomalies in Ti, P, Sr, and Nb-Ta.
Rhyolites are enriched in LREE and present
a large negative anomaly in Eu and a flat HREE
pattern. These rocks display spiky trace-element
patterns due to marked negative anomalies in
Ti-Eu, P-Sr, and less significant ones in Nb-Ta
and Ba. A progressive evolution is observed
among the rhyolites (ex P02–8 and P02–11), and
like the rhyodacites relatively high Ba contents,
whereas peralkaline lavas (ex JR97–19 having
ac in the norm) are characterized by high Zr but
low Ba contents and an overall enrichment in
all the elements excluding Sr and P (Tables 1
and 4).
Sr-Nd-Pb Isotopic Compositions
Sr and Nd isotopic compositions were determined on 16 samples—eight basalts and eight
differentiated lavas (Table 5). The pre-Pinacate
lavas display a large degree of isotopic heterogeneity on the εNd versus 87Sr/86Sr (Fig. 7). The
four types of mafic lavas, defined by their chemistry and multielement patterns, are distributed
along the mantle array but plot in quite different
fields. The basalts from Sierra Batamote (Type
4) have the lowest Sr ratios (0703–0.704) and
positive εNd (+4 to +6). The lavas from the eastern limit of the study area (Type 2) have, on the
opposite, the highest Sr (~0.707) and the lowest
εNd values (−5 to −7). Type 1 (sample 91–30)
and Type 3 (Cerro San Pedro) basalts have isotopic compositions similar to those inferred for
Bulk Silicate Earth (BSE). Lead isotopic compositions on mafic rocks show a limited range
(Table 6). The higher ratios correspond to the
mafic rocks Types 1 and 2, and the lowest ratio
corresponds to the mesa basalts of Sierra Batamote (Type 4, sample P02–17).
Dacites and rhyolites have isotope ratios identical to those of the mafic lavas. Dacites from
Sierra Suvuk and Cerro Ladrilleros have similar 87Sr/86Sr ratios (0.7045–0.7046) but different
εNd (+1.3 and +1.5 for Cerro Ladrilleros and
+3 for Sierra Suvuk). Rhyodacite PO2–15 has
a higher εNd for identical Sr values (Table 5).
Rhyolites exhibit the widest range of εNd and Sr
ratios. There are two groups (Fig. 7)—lavas that
have negative εNd (−2.3 to −0.6) and extremely
high Sr (up to 0.7585) and rhyolites that have
positive εNd; the rhyolite (P02–8) has relatively
low Sr ratios (0.7068), whereas the peralkaline
Geological Society of America Bulletin, May/June 2008
699
Vidal-Solano et al.
B
A
100
100
100
10
10
20
10
400
1
5
100
Rock vs. Chondrites
Rock vs. Primitive mantle
200
10
1
100
50
20
10
5
100
1
La
10
Pr
Ce
1
0.1
Rb Th Nb K Ce Sr Nd Hf Eu Dy Ho Lu
Ba U Ta La Pb P Zr Sm Ti
Y Yb
Sm Gd
Dy
Er
Yb
Nd
Eu
Tb Ho Tm
Lu
Peralkaline rhyolite (JR97-19)
Rhyolite (P02-8)
Rhyolites (12 Ma )
Rhyolites (15-14 Ma )
Rhyodacites (P02-15 & P02-20)
Andesites and dacites
Figure 6. Chondrite-normalized, rare-earth element (REE) abundances (A) and primitive mantle-normalized, trace-element patterns (B)
for selected dacitic and rhyolitic lavas. Normalized values are after Boynton (1984) for the REE and Sun and McDonough (1989) for the
incompatible elements.
10
6
DM
t
an
M
Type 4 (Sierra Batamote)
y
εNd
ra
ar
3
Type 3 (Cerro San Pedro)
Array
0
Basalts
le
le
Mant
5
Pinacate volcanic rocks
0
Type 2 (PI97-24 and JR98-21)
Type 1 (91-30)
-3
0.703
BSE
0.704
0.705 0.706
Peralkaline rhyolites (JR97-19)
12 Ma Rhyolite (P02-8)
0.758
-5
14-15 Ma Rhyolites (JR99-74 and JR98-23)
Rhyodacites (P02-15)
Dacites
-10
0.700
0.705
0.710
0.715
87
700
0.720
0.725
86
Sr/ Sr
Geological Society of America Bulletin, May/June 2008
Figure 7. εNd versus 87Sr/86Sr
isotope diagram for selected prePinacate volcanic rocks. DM—
Depleted mantle; BSE—Bulk
Silicate Earth from Zindler and
Hart (1986); Pinacate volcanic
rocks from Lynch et al., 1993.
Sample no.
TABLE 5. Sr AND Nd ISOTOPE RATIOS OF SELECTED SAMPLES FROM THE PRE-PINACATE MIOCENE SEQUENCES
143
87
87
147
Rb
Sr
Type
Locality
Sm
Nd
Sm/144Nd
ε (Nd)i
Sr/86Sr
Rb/86Sr
(87Sr/86Sr)i
Nd/144Nd
(ppm)
(ppm)
(ppm)
(ppm)
PI97-24
K BA
P
9.2
50.5
0.512334
0.112066
-5.73
38.6
954
0.707255
0.112808
0.707226
JR98-21
K BA
no. 2
7.4
37.0
0.512357
0.113029
-5.56
31.4
926
0.707672
0.094545
0.707648
JR98-29
K BA
TM
8
40.0
0.512272
0.113027
-6.96
35
940
0.707997
0.103818
0.707970
91-30
Thol B
2,5
8
0.512696
0.192249
1.15
7
200
0.705327
0.097563
0.705301
JR97-23
Trans thol B
SP
6.4
26
0.512682
0.151432
0.97
26.2
448
0.705900
0.163029
0.705854
PI97-33
Trans thol BA
SP
6.5
26.5
0.512664
0.150896
0.58
24.2
465
0.705933
0.145079
0.705908
P02-6
Trans thol B
B
9.2
38.5
0.512885
0.147014
4.89
15.1
519
0.704077
0.081091
0.704063
P02-17
Trans thol B
B
6.8
27.6
0.512977
0.151080
6.68
10.4
457
0.703381
0.063424
0.703370
0.704523
JR98-20
D
S
4
18.5
0.512778
0.133018
3.02
49
522
0.704571
0.261644
1.33
51.8
526
0.704610
0.274493
0.704559
JR97-1
D
L
2.7
13
0.512700
0.127772
1.47
51.4
481
0.704682
0.297857
0.704635
JR99-83
D
L
2.6
12.5
0.512708
0.127961
P02-15
RD
L
5.4
25.7
0.512840
0.129267
4.04
81.6
191
0.704922
1.190849
0.704719
JR99-74
R
P
9.1
35
0.512513
0.159944
-2.37
164
51
0.760333
9.012481
0.758541
JR98-23
R
LN
9.4
42.5
0.512600
0.136064
-0.63
243
12
0.725892
56.561870
0.714646
JR97-19
R
VV
12
55.5
0.512804
0.133019
3.34
137
12
0.718326
31.865007
0.712896
P02-8
R
B
5.5
27.7
0.512764
0.122153
2.57
129
71
0.707667
5.065813
0.706804
Note: Abbreviations: K BA—potassium rich basaltic andesite; thol B—tholeiitic basalt; Trans thol B—transitional-tholeiitic basalt and basaltic andesite;
Trans alk B—transitional-alkalic basalt; D—dacite; RD, rhyodacite; R—rhyolite. Other abbreviations—same as in Table 1. For sample localities,
see Vidal-Solano (2005).
39
44.98
49.56
11.68
19.55
20.19
-6
-15.84
39
44.98
49.56
-15.84
11.68
19.55
20.19
-6
ε (Sr)i
TABLE 4. INDUCTIVELY COUPLED PLASMA–ATOMIC EMISSION SPECTROSCOPY TRACE-ELEMENT ANALYSES
Sample no. 91-30 PI97-24 JR98-21 JR97-23 PI97-33 JR97-27 P02-17 JR98-2 P02-06 PI97-16 JR98-48 JR98-20 JR97-1 JR99-83 P02-20 P02-15 JR99-74 JR98-23 JR98-25 JR97-19 JR99-82 P02-8
Rock type
BA
BA
B
B
D
D
B
B
B
B
B
D
D
D
D
R
R
R
R
R
D
R
B
Locality
P
C#2
SP
SP
SP
B
S
S
S
L
L
L
P
LN
LN
VV
VV
B
L
B
34
36
Rb
7
38.6
31.4
26.2
24.2
10.4
15
15.1
49
52
51
189
243
244
141
137
129
46
88
82
Sr
200
954
926
448
465
346
457
553
519
727
669
522
526
481
191
47
18
12
13
11
71
182
Ba
425
756
779
100 1105
933
596
599
293
612
678
733
761
841
928
11
121
38
131
84
807
913
Co
26.0
30.6
11.5
8.5
47.5 28.0
28.0
31.0
30.5
40.8
28.5
7.0
5.0
5.0
2.3
2.7
0.5
<0.5
<0.5
<0.5
1.6
<0.5
5
Cu
100
15
20
10
10
30
34
25
<5
<5
<5
<5
5
15
<5
<5
<5
<5
<5
<5
25
Ni
65
30
20
10
5
150
110
35
48
35
36
5
5
5
<5
<5
<5
<5
<5
23
5
<5
V
258
95
70
245
220
155
180
240
240
175
358
60
55
45
6
<5
40
15
10
5
10
10
85
60
Zn
100
90
90
140
115
70
122
110
130
70
70
50
67
120
55
70
120
120
49
69
Zr
72
231
228
252
291
265
166
224
279
138
166
206
130
128
336
273
195
197
657
634
223
325
Y
36.0
35.5
36.5
40.8
21.5 29.0
22.5
35.2
43.0
14.5
16.5
20.5
12.5
30.2
30.0
61.5
63.0
66.0
79.5
71.5
13.5
30.9
Nb
4.0
14.0
8.0
16.0
16.0
16.0
3.4
26.0
3.7
6.0
10.0
12.0
7.0
7.0
3.2
3.3
33.0
28.0
29.0
45.0
45.0
3.3
Cs
0.1
0.3
0.4
0.2
0.2
0.2
0.1
0.1
0.2
0.6
0.7
0.9
0.9
1.2
1.5
1.4
1.7
3.0
1.7
0.8
1.8
2.8
Th
<1
7
4
1
4
2
1
1
2
<1
2
2
2
1
9
7
24
26
15
9
15
10
Ta
<0.5
0.5
<0.5
0.5
0.5
0.5
1.6
1.5
1.7
0.4
0.4
0.5
0.4
0.4
1.6
1.5
2.0
2.0
2.0
3.0
3.0
1.6
U
<0.5
<0.5
1.5
1.0
<0.5
1.0
0.5
<0.5
0.7
0.4
0.5
0.5
0.5
1.0
2.3
2.3
6.0
6.0
3.5
0.5
3.1
4.0
Pb
30
10
25
5
25
10
5
10
<5
10
5
20
15
10
25
35
45
30
15
17
11
13
7
4
Hf
2
7
5
6
6
5
7
7
4
6
4
4
9
8
9
18
18
7
9
11
La
44.0
35.0
7.00 69.5
27.0
27.0
27.5
22.6
35.5
21.5
20.5
24.5
20.5
36.2
38.7
38.5
62.5
66.0
75.0
67.0
18.5
45.3
Ce
57.5
78.5
78.2
14.5 139.5
91.5
57.0
58.5
51.8
43.0
40.5
48.5
37.0
73.0
73.8
83.5
128.5
133.5
155.0
138.0 83.6
34.5
Pr
1.7
6.3
6.1
6.2
7.0
9.0
10.0
4.5
4.6
4.9
3.5
3.8
7.9
8.1
9.4
12.5
13.1
15.3
14.9
9.0
13.9
9.7
Nd
8.00 50.5
37.0
38.5
26.0
26.5
24.0
27.6
37.0
17.5
18.0
18.5
13.0
25.7
25.7
35.0
42.5
45.5
55.5
54.0
12.5
27.7
Sm
9.2
2.5
9.2
7.4
6.4
6.5
5.5
6.8
8.8
3.8
3.5
4.0
2.7
2.6
5.2
5.4
9.1
9.4
10.3
12.0
11.3
5.5
Eu
1.00
2.0
2.0
1.4
2.2
2.6
2.5
1.1
1.0
1.0
0.6
0.6
0.8
0.9
0.1
0.3
0.3
1.2
1.1
0.5
2.1
1.8
Gd
3.7
8.0
6.5
7.3
7.1
6.2
6.9
9.5
8.5
3.8
3.6
4.2
2.4
2.4
4.9
5.1
10.5
10.4
13.7
10.6
5.4
9.9
Tb
1.3
0.6
1.0
0.9
1.2
1.2
1.0
1.1
1.4
0.5
0.6
0.6
0.4
0.4
0.8
0.8
1.9
1.8
1.8
2.2
2.1
0.8
Dy
3.4
3.7
6.8
5.9
6.0
6.9
7.5
8.1
2.4
2.7
3.1
1.8
2.3
5.0
5.1
10.4
10.1
10.2
13.1
11.6
5.3
4.8
Ho
0.8
1.0
0.8
1.3
1.3
1.3
1.4
1.7
1.6
0.5
0.6
0.7
0.4
0.5
1.1
1.1
2.2
2.3
2.9
2.4
1.1
2.1
Er
4.5
2.2
2.8
2.5
3.7
3.7
3.9
3.9
4.3
1.4
1.6
2.1
1.4
1.2
3.2
3.4
5.6
6.5
7.0
8.9
7.6
3.5
Tm
0.3
0.4
0.3
0.5
0.5
0.6
0.5
0.6
0.6
0.2
0.3
0.4
0.2
0.2
0.5
0.5
1.0
1.0
1.0
1.4
1.2
0.5
Yb
2.2
2.9
2.1
3.5
3.6
3.9
3.4
4.0
3.7
1.4
1.7
2.1
1.4
1.7
3.2
3.3
6.5
6.7
8.9
8.3
3.3
6.3
Lu
0.6
0.3
0.4
0.3
0.5
0.5
0.6
0.5
0.6
0.2
0.3
0.3
0.1
0.2
0.5
0.5
0.9
1.0
1.0
1.3
1.2
0.6
Note: Abbreviations—same as in Table 1. For sample localities, see Vidal-Solano (2005).
P02-11
R
B
125
73
805
1.5
261
5
10
40
218
30.9
3.3
0.9
15
1.6
3.0
19
7
39.8
81.8
8.2
25.4
5.1
0.5
5.1
0.8
5.2
1.1
3.4
0.5
3.3
0.5
Tectonomagmatic evolution of NW Mexico
Geological Society of America Bulletin, May/June 2008
701
Vidal-Solano et al.
TABLE 6. LEAD ISOTOPE RATIOS OF SELECTED SAMPLES
FROM THE PRE-PINACATE MIOCENE SEQUENCES
206
Sample no.
Type
Locality
U
Pb
Th
Pb/204Pb 207Pb/204Pb 208Pb/204Pb
(ppm) (ppm) (ppm)
PI97-24
K BA
P
1.5
10
7
19.13
15.67
38.95
JR98-21
K BA
no. 2
2.7
13
51.8
19.23
15.69
38.94
91-30
Thol B
0.4
30
0.9
19.23
15.67
38.87
JR97-23
Trans thol B
SP
0.4
5
1
19.01
15.66
38.84
PI97-33
Trans thol BA
SP
0.4
5
1
19.01
15.66
38.83
P02-17
Trans alk B
B
0.5
4
2
18.66
15.57
38.25
JR99-83
D
L
1
10
1
18.86
15.63
38.62
P02-15
RD
L
2.3
13
10
18.88
15.63
38.61
P02-8
R
B
3.1
30
15
18.94
15.65
38.75
Note: Abbreviations—same as in Table 5. For sample localities, see Vidal-Solano (2005).
rhyolite (JR97–19) has about the same εNd
value but much higher radiogenic Sr (0.7128).
TECTONIC AND PETROGENETIC
IMPLICATIONS
Age and Tectonic Significance of the PrePinacate Volcanic Successions
In northwestern Mexico, two basaltic events
related to major extensional processes have
been recognized. The first one is represented by
ca. 30 Ma continental flood basalts in the northern Sierra Madre Occidental plateau (Montigny
et al., 1987; Cameron et al., 1989; Demant et al.,
1989), the second, related to Basin and Range
tectonics, corresponds to ca. 20 Ma basalts intercalated in continental deposits of the Báucarit
Formation (Cochemé et al., 1988; Paz-Moreno,
1992; Vidal-Solano, 2005). Until now, the volcanic evolution of the pre-Pinacate event in the
Pinacate Volcanic Field was not well known. In
general the stratigraphy of the Miocene volcanic rocks (Vidal-Solano, 2001; Vidal-Solano et
al., 2005) has been divided into three main rock
types but without absolute ages: (1) basalts and
basaltic andesites from Sierra Batamote, Cerro
San Pedro, Cerro Picú, and Cerro Tres Mosqueteros; (2) andesitic and dacitic domes and lava
flows at the Sierra Suvuk and Cerro Ladrilleros;
and (3) silicic volcanic rocks (rhyolitic domes
and pyroclastic flow deposits) with frequent
obsidian facies, in the Lomas del Norte and Vidrios Viejos areas (Fig. 1). One of the major problems in establishing the chronostratigraphy of
the pre-Pinacate sequence has been the fact that
its outcrops are isolated and its volcanic units are
not directly in contact. To try to resolve this difficulty, a 40Ar/39Ar geochronology was obtained to
clarify the chronology of the volcanic events.
(1) Basalts
A common problem with dating basaltic lavas
on plagioclase is that the Ar gas content restricts
the number of steps because this mineral has
low K contents (Ortega-Rivera, 2003; Schulze
et al., 2004). Nevertheless, we decided to date
these isolated mafic outcrops lying directly on
702
the crystalline basement even though the age
spectra (Fig. 4) for the three basaltic samples
(91–30, JR98–21, and JR97–23) might be disturbed due to their low K content, as was the
case. The age spectra in general show maximum
dates of ca. 20 Ma that are interpreted as the
minimum ages of the basaltic rocks. The minimum dates from the age spectra at ca. 12 Ma
may correspond to a later reheating volcanic
event. Moreover, although the climbing nature
of the spectra could be interpreted as excess Ar,
inherited Ar, or Ar loss, because basaltic volcanism related to typical Basin and Range tectonic
extension appears in Sonora at ca. 20 Ma, and
in view of the fact that a later volcanic event has
been recognized in the area, we favor the latter case and therefore consider the maximum
40
Ar/39Ar age determinations on the plagioclases
to be a good estimate for the onset of the volcanic activity in the Pinacate area.
(2) Dacites
The dacitic samples dated come from (1) a
plagioclase + two pyroxene lava domes at
Sierra Suvuk and (2) a dacitic lava flow containing fresh amphiboles from Cerro Ladrilleros
(Fig. 1). Although the plagioclase age spectrum
from Sierra Suvuk (JR98–20) is disturbed at
the low-T steps and presents an “argon-loss”
spectrum, we believe that the last step yields a
plateau age for the dome emplacement at 13.53
± 1.24 Ma. The reason is that basaltic subhorizontal lava flows are capping the summit of the
Sierra Suvuk and are correlated with the basic
volcanism cropping out at the top of the Cerro
San Pedro and dated at 12.61 ± 0.27 Ma (Lynch,
1981). The basic Suvuk valley volcanic unit
(described by Vargas-Gutierrez, 2006) is represented by subhorizontal basaltic lava flows, and
dikes found at the level of the actual valley, on
the southeast flank of Sierra Suvuk, represent
the latest activity in the area. Although we do not
have enough geological evidence, we think that
the first step of this sample at 5.27 ± 2.50 Ma
could date this event.
The hornblende dated from Cerro Ladrilleros (JR99–83) yields a plateau date of 12.04
± 1.37 Ma calculated for the last four steps. This
age represents the final volcanic fluidal phase of
activity at Cerro Ladrilleros that locally caps a
peralkaline pumice layer that is related to the
pyroclastic index level located below the basaltic mesa north of Cerro San Pedro and between
basalt flows at Sierra Batamote (Fig. 8A).
(3) Rhyolites
The age of silicic volcanism was constrained at ca. 14–15 Ma as indicated by three
different rhyolitic sample spectra maximum
ages (P03–27, JR98–23, and P03–22). Even
though their spectra are disturbed at the lower
temperature steps at ca. 12 Ma, we have interpreted a 14–15 Ma date as the minimum age of
rhyolitic emplacement since the three samples
have the same ages at the highest temperature
steps despite the fact that each was collected
several km apart from spatially well distributed
localities, i.e., the base of Cerro Picú, Lomas del
Norte, and Vidrios Viejos (Fig. 1).
The obsidian nucleus from Vidrios Viejos
(samples P03–22 and P03–22 HCl, ca. 14 Ma)
shows well-defined plateau dates (12.08
± 0.62 Ma and 11.98 ± 0.62 Ma) at the lower
temperature steps, and the rhyolites from Lomas
del Norte and Cerro El Picú show also first steps
at 11.10 ± 1.96 Ma and 11.60 ± 1.74 Ma (samples JR98–23 and P03–27, respectively) consistent with the age of a later reheating bimodal
volcanic event in the area, corresponding to the
basalts and rhyolites that crop out on Sierra Batamote and Cerro San Pedro.
The vitric rhyolites (P02–8 obs and P02–8 wr)
from the small outcrop west of Cerro San Pedro
and its associated pumice layer that are found
intercalated between basalts of the Sierra Batamote and Cerro San Pedro, were previously
thought to be stratigraphically and geochemically related to the only other ca. 14–15 Ma
obsidian outcrop recognized in the area (Lomas
del Norte and Vidrios Viejos). Our new concordant and reproducible 40Ar/39Ar plateau dates
of 12.16 ± 0.07 Ma and 12.05 ± 0.07 Ma have
facilitated setting them apart as two different volcanic events (P02–8 obs and P02–8 wr,
respectively, Fig. 4; Table 3). The oldest event is
related to the onset of rhyolitic volcanism, and
the youngest is related to basic volcanism at the
tops of the Cerro San Pedro dated by K/Ar at
12.61 ± 0.27 Ma (Lynch, 1981) and the Sierra
Batamote. With our new 40Ar/39Ar ages in the
obsidians and its associated pumice layer (P02–
8 wr), we can define a new regional stratigraphic
marker at ca. 12 Ma due to their widespread distribution across the area.
The geochronological data obtained on
representative volcanic samples from the prePinacate sequences lead to the following conclusions: (1) the oldest volcanic episode is
Geological Society of America Bulletin, May/June 2008
Tectonomagmatic evolution of NW Mexico
represented by basaltic rocks that have been
emplaced in a short time interval during the
early Miocene (ca. 20 Ma, samples 91–30,
JR98–21B, and JR97–23); (2) the dacitic and
rhyolites rocks from different localities erupted
contemporaneously during the middle Miocene (samples JR98–20, JR99–83, P03–27,
JR98–23, P03–22, and P02–8); (3) the dacitic
and rhyolitic episode spans a time interval of
ca. 3 Ma (15–12 Ma,); and (4) the last volcanic
event is shown by the concordant and reproducible 40Ar/39Ar plateau dates at ca. 12 Ma
of the rhyolite P02–08 samples. Also, the first
steps from all the oldest samples have recorded
this event; these steps have been reset (see
Fig. 4), at ca. 12 Ma. This thermal anomaly
corresponds to a bimodal volcanism associated
with crustal extension in the region.
On the flank of the basaltic mesa located
north of Cerro San Pedro (Fig. 1), a landslide
gives access to the material present below the
scree-covered slope (the white spot labeled “A”
in Fig. 8). At this point, a pyroclastic sequence
correlated with the Batamote rhyolites (VidalSolano, 2005) was documented; it overlies a
poorly sorted detrital succession containing
floated pumice. This reveals that the acidic lavas
were emplaced in tectonically controlled basins,
which were locally occupied by lakes. The mesa
basalts directly cap the pyroclastic and sedimentary sequences. Therefore, they also have
a middle Miocene age. This is consistent with
a K/Ar age of 12.61 ± 0.27 Ma obtained on a
basaltic sample from the summit of Cerro San
Pedro (Lynch, 1981).
In summary, the chronostratigraphy of the
volcanic sequences allows us to distinguish
two main volcanic events: (1) a lower Miocene sequence, consisting of mostly basalts and
basaltic andesites, which has been later affected
by extensional tectonics, and (2) a middle
Miocene volcanic succession. In addition, we
have defined a regional stratigraphic marker at
ca. 12 Ma corresponding to the widespread rhyolitic pumice. A relatively long period of quiescence (ca. 10 Ma) occurs in the region after
the Miocene events since the renewed volcanic
activity that built the Pinacate shield volcano
took place only during the Quaternary.
Mixing between Calc-Alkaline and
Peralkaline Lavas
The pyroclastic level located below the basaltic mesa north of Cerro San Pedro (samples
P02–13A and P02–14, Fig. 3), is of particular
interest, because it clearly demonstrates mixing between the dacitic and peralkaline magmas
(Fig. 9). Moreover, the presence of crystal clots
with calcic plagioclase, partly resorbed olivine
Figure 8. Basaltic mesa north of Cerro San Pedro, showing the outcrop (A)
where mixing between calc-alkaline and peralkaline magmas was observed.
Figure 9. Thin section showing mixing between a white peralkaline liquid
(Na-sanidine + green ferrohedenbergite + fayalite) and a brown dacitic liquid
(plagioclase + orthopyroxene + amphibole).
and clinopyroxene overrun by amphibole, indicates that a basaltic magma was also involved
and has probably triggered the pyroclastic eruption. The regular trend of clinopyroxene compositions from a Ca- and Mg-rich toward a Fe-rich
end member (Fig. 3A) can be explained by the
interference of basalt with the dacitic liquid in
the reservoir. On the other hand, orthopyroxene
composition evolves from an Fe-rich end member in equilibrium with the dacite to a more magnesian type that probably crystallizes after the
intrusion of the basalt. Finally, the slight enrich-
ment in Ca observed for the ferrohedenbergite
and the sodic sanidine in the peralkaline rocks is
the result of mixing with the dacitic liquid. Rhyolites are enriched in LREE and present a large
negative anomaly in Eu and a flat HREE pattern.
Evidence for extensive feldspar fractionation in
these liquids comes from the Eu anomaly and
very low Sr abundances. The regular evolution
of Y and Zr versus Nb and the distribution of
dacites and rhyolites on the Ba/Nb versus Nb
diagram (Fig. 10) seem to correspond to simple
mixing between a dacitic (represented by sample
Geological Society of America Bulletin, May/June 2008
703
Vidal-Solano et al.
Rock vs. Primitive mantle
700
Zr
500
300
100
Ba vs. Nb
100
90%
1
Rhyodacite P02-20
Basalt JR97-27
80%
0.1
60%
70%
30%
5
Figure 11. Chemical similarities
between the acidic and basic middle Miocene lavas lying north of
Sierra Batamote.
10
60
20
10
Nb
Rhyolite P02-8
Dacites
Rhyolites
Peralkaline rhyolite JR97-19
Rhyolite P02-8
Rb Ba Th U Nb Ta K La Ce Pb Sr P Nd Zr Hf SmEu Ti Dy Y Ho Yb Lu
100
Peralkaline rhyolite JR97-19
Dacite JR97-1
Rhyodacites P02-15
and P02-20
Figure 10. Modeling of the rhyodacite as the
result of mixing between the dacitic and the
peralkaline rhyolitic magmas. Empty triangle—dacites; gray triangle—dacite JR97–1;
black triangle—rhyodacites (P02–15 and
P02–20); empty circle—rhyolites; gray circle—rhyolite P02–8; black circle—peralkaline rhyolite JR97–17.
JR97–1) and a peralkaline end member (sample
JR97–19). Such a mixing between peralkaline
and calc-alkaline liquids has also been observed
in the Quaternary peralkaline comenditic caldera complex of La Primavera, near Guadalajara
(Mahood et al., 1985). Moreover, some kind of
petrogenetic link does exist between the 12 Ma
mesa basalt and the differentiated rocks as documented by similar patterns shared by the rhyodacite, the rhyolite, and the basalt from Cerro San
Pedro on the multielement diagram (Fig. 11).
Petrogenesis of the Pre-Pinacate Volcanic
Sequences
Source of the Basaltic Lavas
Abundances in compatible trace elements
and Mg-numbers in the mafic lavas show that all
the basalts are differentiated liquids (Mg# <65,
Ni <200, and Cr <250 ppm). The basalt Type 1
(sample 91–30, ca. 20 Ma), has a flat REE pattern with a tholeiitic character that is supported
by low potassium contents. Meanwhile, the
basalt Type 2 consists of basaltic andesites dated
at ca. 19 Ma with enriched multielement spectra
that have features generally expected for subduction-related magmas. Finally, basalt Types 3 and
4, the basalts and basaltic andesites from Cerro
San Pedro and Sierra Batamote, respectively,
show REE patterns that have a weak negative
anomaly in Eu, indicating the involvement of
plagioclase during fractional crystallization, or
704
100
partial melting of a source region in which plagioclase is residual. These basaltic rocks from
Cerro San Pedro and Sierra Batamote show a
transitional character between tholeiitic and
alkaline magmas.
Sr, Nd, and Pb isotope data and trace-element
behavior are commonly used to decipher the
source of the basaltic magmas, but a possible
role of crustal contamination must first be discarded. Because K and P behave incompatibly
during fractional crystallization, and the continental crust is potassium rich, the P/K ratio of
mafic rocks plotted against SiO2 and/or isotopic
compositions is a good indicator of crustal contamination (Carlson and Hart, 1987; Farmer et
al., 1995). Since most of the pre-Pinacate basalts
have P/K ratios >0.3, samples show minimal
contamination; thus their isotopic compositions
most likely reflect the diverse mantle sources
from which they were derived. The diagrams
that combine Sr and Nd isotopes with 206Pb/204Pb
ratios (Fig. 12) emphasize the enriched character of most of the pre-Pinacate lavas, which lie
well above the Northern Hemisphere Reference Line of Hart (1984). They also support
the existence of three different kinds of basalt
at the Pinacate area, as shown previously with
the major- and trace-element diagrams. These
basalts also illustrate an overall evolution with
time from a Nd-poor and Sr-enriched source
(enriched-mantle [EM] 2 type), toward a midocean ridge basalt (MORB)-type end member.
The ca. 20 Ma potassium-rich basaltic andesites
(Type 2) from the eastern limit of the study area
exhibit LREE enriched patterns, high Sr and Sri
isotope ratio (≥0,707), low εNd values (<−4),
and high 208Pb/204Pb and 207Pb/204Pb ratios for
any given 206Pb/204Pb. These values are analogous to those for the lithospheric mantle-derived
early Miocene basalts, well known in southern
Nevada and westernmost Arizona, or to the Miocene basaltic andesites from the Mojave Desert
located east of longitude 116° W (Fig. 13).
Miller et al. (2000) interpreted these basalts as
derived from a Precambrian lithospheric mantle
source. Proterozoic basement at the Pinacate
region is not conspicuous. For Nourse et al.
(2005), the Precambrian crystalline rocks in
northwestern Sonora and southwestern Arizona
constitute the southwestward limit of the Proterozoic basement, composed of the Mojave,
Yavapai, and Mazatzal crustal provinces, and
the Caborca block. As the region experienced
extension during Tertiary time, the North American basement extended farther west, resulting
in the present-day distribution of the crust, but
the Proterozoic North American mantle did not.
Consequently, the low εNd values and Pb isotopic characteristics (Bennett and DePaolo, 1987;
Wooden et al., 1988) of the easternmost basalts
were likely derived from Precambrian mantle
lithosphere; hence, this mantle could be, most
likely, associated with “Mojavia.”
Alkaline volcanic rocks and spinel-lherzolite nodules from the Pinacate volcano record
the presence of asthenospheric depleted-mantle
source. Quaternary Pinacate basalts, likewise
basalts from the southwestern USA, have Sri
values between 0.70312 and 0.70342, and εNd
between +5.0 and +5.7 (Lynch et al., 1993).
Because the middle Miocene Type 4 basalts
from Sierra Batamote (samples P02–6 and
P02–17) have comparable Sr and Nd isotopic
values, and a mildly alkali nature (Figs. 7 and
13), they probably also derive from an asthenospheric mantle source. Tholeiitic basalt (91–30)
and transitional basaltic andesites (Types 1 and
3) can be explained by simple mixing of an
enriched and a depleted mantle source. Such a
change with time, from a shallow lithospheric to
deeper asthenospheric mantle source, has been
interpreted in the Basin and Range province as
the result of convective thinning and extension
of the lithosphere (Fitton et al., 1991; Kempton
et al., 1991; Leeman and Harry, 1993; Hawkesworth et al., 1995; DePaolo and Daley, 2000;
Paz Moreno et al., 2003). The shift in mantle
signature (from Type 2 to Type 4), could also
be an expression of a major geologic boundary,
i.e., the western limit of the Proterozoic North
American lithospheric mantle that has been
located in the Mojave Desert toward the north
Geological Society of America Bulletin, May/June 2008
Tectonomagmatic evolution of NW Mexico
15.9
16
MORB
RL
NH
8
• Nd
207Pb/204Pb
12
15.7
Figure 12. Conventional lead
isotope diagrams, εNd versus
206
Pb/204Pb and 87Sr/86Sr versus
206
Pb/204Pb for selected pre-Pinacate volcanic rocks. Symbols—
same as in Figure 7. MORB and
EM2 fields after Rollinson (1993).
Field of data for the middle Miocene peralkaline and calc-alkaline magmas from southeastern
Nevada (Scott et al., 1995) are
shown for comparison. NHRL
(northern hemisphere reference
line)—the average Pb array for
oceanic basalts (Hart, 1984).
4
0
15.5
-4
-8
EM2
H
R
L
15.3
N
0.707
0.706
87Sr/86Sr
208Pb/204Pb
39
38.5
0.705
0.704
0.703
38
MORB
0.702
37.5
18
18.5
19
19.5
20
18
18.5
19
19.5
20
206Pb/204Pb
206Pb/204Pb
Calc-Alkaline Magmatism
Dacitic lavas from Sierra Suvuk and Cerro
Ladrilleros plot in the mantle array; they have
low and uniform 87Sr/86Sr ratios but variable εNd
values close to BSE (Fig. 7). Their Sr and Nd
isotopes, coupled with 206Pb/204Pb ratios close to
the P02–17 sample, suggest a major contribution
from a largely depleted mantle source (Fig. 12).
However, high concentrations in Pb and negative anomalies in Nb-Ta on the multielement
diagram indicate a weak subduction component
in their mantle source.
0.713
AFC
CF
0.711
ε Nd
0.709
0.707
0.705
10
5
0
87Sr/86Sr
(Miller et al., 2000). The slight but systematic
increase of the Sr ratios with increasing silica
content and decreasing P/K ratios (Fig. 13) can
be interpreted as the result of fractional crystallization and assimilation of Precambrian upper
crust (AFC process) of parental basalts deriving
from different mantle sources.
-5
-10
Origin of Peralkaline Magmas
The origin of high-silica peralkaline liquids
has been strongly debated during past decades.
The most generally accepted explanation is that
they were derived from transitional basalts,
through fractional crystallization coupled with
crustal assimilation (Barberi et al., 1975; Gasparon et al., 1993; Mungall and Martin, 1995;
Civetta et al., 1998; Peccerillo et al., 2003). An
alternative to the AFC model involves a strong
crustal control (Black et al., 1997; Trua et al.,
CF
AFC
-15
-20
46
49
52
55
58
61
64
67
70
73
76
79
SiO2
Figure 13. Variation of Sr isotopic ratios and εNd versus SiO2 weight percent. Symbols—
same as in Figure 7 for the pre-Pinacate rocks. Small closed circles correspond to lavas of
the Quaternary Pinacate Volcanic Field (Lynch et al., 1993). Small black stars and empty
stars correspond respectively to the lavas located west and east of longitude 116° W in the
Mojave Desert (Miller et al., 2000). See text for discussion. The large gray star corresponds
to the average composition of the Proterozoic lower crust (Miller et al., 2000).
Geological Society of America Bulletin, May/June 2008
705
Vidal-Solano et al.
1999) or the remelting at depth of basaltic or gabbroic material (Lowenstern and Mahood, 1991;
Bohrson and Reid, 1997). Middle Miocene mafic
and evolved lavas of the pre-Pinacate sequences
share common Sr, Nd, and Pb isotopic signatures. Rhyolites plot close to the transitional
basalts on the εNd versus 206Pb/204Pb diagram,
but are displaced toward higher Sr values on the
87
Sr/86Sr versus 206Pb/204Pb diagram (Fig. 12).
Relatively constant εNd and highly variable Sr
isotope ratios show that the opening of the RbSr system occurred in an upper crustal reservoir.
The high Sr ratios of some rhyolites imply a
high radiogenic contaminant, which is certainly
consistent with the Precambrian upper crust
(Faure, 2001). This is, among others, an argument indicating that rhyolites were produced by
open system differentiation of more primitive
magmas. Given that rhyolites show evidence for
extensive feldspar fractionation and that high
Sr isotopic ratios indicate assimilation of upper
crustal material, partial melting at depth of a
mafic precursor must be able to generate magmas of intermediate compositions, before final
fractionation in the upper crust. Such a process
of two stages is unlikely to have occurred in the
Pinacate area, because intermediate trachytic
compositions are not represented. Higher lead
isotope ratios and εNd of the middle Miocene
pre-Pinacate magmas, compared to sequences
of the same age from southeastern Nevada
(Scott et al., 1995), show that parental magmas
derived from an asthenospheric mantle source
rather than from a lithospheric one. Therefore, if
peralkaline magmatism is indeed a good marker
of upper crustal evolution, its isotopic signatures also reflect the nature of the mantle source
(Scott et al., 1995; Edwards and Russell, 2000;
Miller et al., 2000). Moreover, Vidal-Solano et
al. (2007) found that peralkaline ignimbrites
erupted during middle Miocene times either in
central Sonora, or in the Puertecitos area, in Baja
California, and recognized that they are a good
geodynamic marker for the structural evolution
of the Gulf of California rift system. They have
also proposed that this volcanic episode has petrochemical characteristics clearly different from
those of the other Miocene volcanic sequences
related to the proto-Gulf of California, thus indicating a change in the mantle source.
Furthermore, because the Sonoran peralkaline rhyolites have low Sr contents, even a weak
assimilation of a highly radiogenic contaminant,
such as the Precambrian crust, could rapidly
raise the Sr isotopic ratios. Therefore, higher
87
Sr/86Sr ratios from peralkaline rhyolites are
related to upper crustal Proterozoic contribution,
in agreement with the final stage of differentiation of these liquids in a shallow magma chamber. Decreasing εNd with increasing Sr isotopic
706
ratios implies a low εNd wall-rock contaminant
during residence in the upper crust (Tegtmeyer
and Farmer, 1990). The ca. 12 Ma rhyolites have
higher εNd compared to the ca. 14–15 Ma rhyolites, not reflecting evolution under open system
conditions, but instead a different fractionation
path most likely related to a more alkalic basaltic parent.
Tectonic Significance
Some peculiar features seem to control the
development of silicic peralkaline magmatism
(Bohrson and Reid, 1997)—a mildly extensional
tectonic setting, the stagnation of magmas in a
shallow reservoir, and parental basalts of transitional to mildly alkali composition. Middle Miocene peralkaline volcanic rocks occurred in the
southwestern United States after a long period
of subduction-related magmatism (Best et al.,
1989). Their distribution from Nevada to California (Scott et al., 1995; Miller et al., 2000; Perkins and Nash, 2002), in most cases, coincides
with the Sri = 0.708 Line (Kistler and Peterman,
1973) and/or the εNd = −7 Line (Farmer and
DePaolo, 1983), defined as an isotopic boundary that marks the western edge of the Precambrian crystalline basement. Recently, Miller et
al. (2000) redefined Sri = 0.706 Line as the limit
of the Precambrian North American mantle. PrePinacate silicic magmatism (15–12 Ma) constitutes the southernmost extension of the North
American middle Miocene peralkaline province.
A close spatial and temporal tie exists between
peralkaline magmatism and crustal extension in
these regions (Scott et al., 1995). A thin lithosphere and asthenospheric upwelling is required
to form peralkaline magmatism. Palinspastic reconstruction of the region shows that the
middle Miocene volcanism at the Pinacate area
coincides closely, in time and space, to the proposed incremental expansion of a growing slab
window of the Farallon slab gap (Severinghaus
and Atwater, 1990; Dickinson, 1997). The setting for continental rift magmatism in the Pinacate area, thus, is constrained by the possibility
for a sub-slab mantle to ascend through the slab
window. Finally, sporadic emplacement of contemporaneous calc-alkaline volcanism shows
that remnant parts of a subduction-modified,
supra-slab mantle persisted during this time.
CONCLUSIONS
Based on age criteria, two volcanic sequences
have been identified at the Pinacate area, east of
the main Quaternary volcanic field: (1) a lower
Miocene basaltic volcanic sequence (ca. 20 Ma)
and (2) middle Miocene volcanic sequences
(ca. 12–15 Ma) composed of calc-alkaline andesites and dacites, high-silica rhyolites (evolving
toward peralkaline liquids), and mesa basalts
with transitional alkali character. Sr, Nd, and Pb
isotopes reveal different sources for the Miocene
basalts. The easternmost outcrops have signatures
indicating an old Precambrian lithospheric mantle
source, whereas toward the west, the basalts have
tholeiitic to transitional characteristics in relation
to the mixing of lithospheric and asthenospheric
components. The mildly alkali character of the
middle Miocene basalts shows a greater influence
of the asthenospheric component. This evolution
of the isotopic signatures, in space and time, indicates that: (1) the volcanic activity was located
over a major lithospheric boundary that is the
limit of the North American Craton, and (2) the
lithosphere was progressively thinned toward
the west so that huge volumes of alkali basalts
could easily access the surface during the Quaternary, thus building the Pinacate Volcanic Field.
Contemporaneous eruption of calc-alkaline and
peralkaline magmas occurred during the middle
Miocene in the pre-Pinacate area. Moreover,
mineralogical and chemical evidence clearly
supports mixing between the two liquids. Isotope signatures show that the calc-alkaline dacites were differentiated from basalts that in turn
were derived from a depleted mantle source only
slightly modified by subduction components. The
rhyolites are the result of fractional crystallization
of transitional basalts and slight contamination
with the Precambrian crust in a shallow reservoir.
Chemical modeling shows that peralkaline rhyolites are related to slightly higher assimilation
during residence in the upper crust but also to a
change in the mantle source of the parent basalt.
For the chemical and isotopic characteristics of
the rhyodacites, the model requires the complex
interaction of three components (dacite, rhyolite,
and basalt) providing evidence for the evolution
of the acidic liquids in a shallow reservoir under
open-system conditions. The progressive change
in the source of the magmatism observed for the
lower and middle Miocene pre-Pinacate lavas
can be convincingly related to the development
of a slab window behind the volcanic front and
is related to the tectonic evolution of the western
margin of the North American Craton. Moreover,
the more voluminous and primitive lavas that further appear in the Pinacate Volcanic Field, related
to a greater degree of melting and an easy access
to the surface, reveal the presence of a thin lithosphere during the Quaternary.
ACKNOWLEDGMENTS
This study is part of the Ph.D. thesis of the senior
author at the Université Paul Cézanne (Aix-Marseille
3). These four years of doctoral work were funded
by Consejo Nacional de Ciencia y Tecnología
(CONACYT) and Société française d’exportation
des ressources éducatives (SFERE) (129313/168910)
Geological Society of America Bulletin, May/June 2008
Tectonomagmatic evolution of NW Mexico
and by a research grant from CONACYT (4891005-3584-T) to F.A. Paz-Moreno. Sampling and mapping were carried out from 1997 to 2002 with the
financial support of the Departamento de Geología
de la Universidad de Sonora. Thanks to M.O. Trensz
(ICP-AES analyses, Centre Européen de Recherche et
d’Enseignement des Géosciences de l’Environnement
[CEREGE]) to J-C. Girard (thin-section preparation),
and C. Merlet (electron microprobe), Institut des Sciences de la Terre, de l’Environnement et de l’Espace de
Montpellier (ISTEEM). École Normale Supérieure de
Lyon (ENS Lyon) supported Pb isotope facilities. Funding for the 40Ar/ 39Ar analytical work was provided by
a research grant from CONACYT (33100-T) to M.A.
Ortega-Rivera, and by Natural Sciences and Engineering Research Council of Canada (NSERC) Master of
Fine Arts (MFA) and Discovery grants to J.K.W. Lee.
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