Valencia-Moreno et al., 2001

A chemical and isotopic study of the Laramide granitic belt of
northwestern Mexico: Identification of the southern edge
of the North American Precambrian basement
Martı́n Valencia-Moreno*
Estación Regional del Noroeste, Instituto de Geologı́a, UNAM, Apartado Postal 1039, Hermosillo, Sonora, México 83000
Joaquin Ruiz
Mark D. Barton
P. Jonathan Patchett
Lukas Zürcher
Damian G. Hodkinson
Geosciences Department, University of Arizona, Tucson, Arizona 85721, USA
Jaime Roldán-Quintana
Estación Regional del Noroeste, Instituto de Geologı́a, UNAM, Apartado Postal 1039, Hermosillo, Sonora, México 83000
ABSTRACT
Along the Laramide belt of northwestern Mexico, granitic rocks
of similar bulk composition show isotopic and trace element signatures that help to delineate the position of the southern edge of
the North American Precambrian basement. In the northern part,
the Laramide plutons (the ‘‘northern granites’’) intruded Proterozoic crystalline rocks and a thick Late Proterozoic through Paleozoic miogeoclinal cover of North American affinity. In the central part, the granitic bodies (the ‘‘central granites’’) were
emplaced into a sequence of Paleozoic eugeoclinal rocks overlain
by Late Triassic clastic units. The southern part of the belt (the
‘‘southern granites’’) intruded a less-known crust characterized by
middle to late Mesozoic island-arc–related volcanic and sedimentary rocks of the Guerrero terrane. Data from a suite of metaluminous to slightly peraluminous calc-alkalic granitic rocks along
the belt display north-to-south geochemical and isotopic variations,
which could correlate with the type of intruded basement. The
northern and central granites are characterized by strongly fractionated, light rare earth element (REE)–enriched patterns, which
display generally pronounced negative europium anomalies,
whereas the southern granites have lower total REE enrichments
and much flatter chondrite-normalized slopes displaying almost no
europium anomalies. Isotopic results also suggest regional variations, as shown by the following initial Sr and eNd ranges: 0.7070
to 0.7089 and24.2 to25.4, respectively, for the northern granites;
0.7060 to 0.7079 and23.4 to25.1 for the central granites; and
0.7026 to 0.7062 and20.9 to 14.2 for the southern granites. On
the basis of their isotopic similarities, the Proterozoic mafic to intermediate lower crust revealed by xenoliths from young volcanic
flows in southern Arizona and northern Mexico is interpreted as
*E-mail: [email protected].
a reasonable parental source for the northern and central granites;
however, mantle-derived melts are not excluded. The more primitive southern granites are interpreted to come from a source that
lacked Proterozoic basement. Instead, they were probably derived
by mixing of juvenile mantle melts with partial melts of the lower
parts of the Guerrero terrane. In general, the north-to-south compositional variations of the Laramide granitic rocks of northwestern Mexico reflect the crustal structure underneath the batholiths.
The Sr and Nd data indicate that the edge of the North American
Precambrian basement extends approximately southeastward from
the coastal batholith of central Sonora; then, about 200 km south
of Hermosillo in southern Sonora, the edge bends eastward and
continues to the east beneath the Sierra Madre Occidental volcanic
province.
Keywords: geochemistry, granitic rocks, Laramide, Mexico,
petrogenesis.
INTRODUCTION
An understanding of the geologic evolution of northwestern Mexico
is critical to defining the accretionary history of the southern margin
of the North American craton. The main difficulty in this regard is that
the Precambrian basement is only exposed in a few small windows
through the overlying Phanerozoic assemblages. The lack of basement
exposures, coupled with structural complications, has made the reconstruction of the geologic history of northwestern Mexico a complex
problem. Widespread Laramide (90–40 Ma) granitic rocks in northwestern Mexico occur along a northwest-trending belt that intruded
different geologic domains (Figs. 1 and 2). In the northern part of the
Laramide belt, the granitic rocks (termed here the ‘‘northern granites’’)
intruded an Early to Middle Proterozoic basement and overlying Late
Proterozoic and Paleozoic miogeoclinal rocks. In the central part of the
GSA Bulletin; November 2001; v. 113; no. 11; p. 1409–1422; 13 figures; 4 tables.
For permission to copy, contact [email protected]
q 2001 Geological Society of America
1409
VALENCIA-MORENO et al.
belt, the plutons (the ‘‘central granites’’) intruded a basement made up
of Paleozoic eugeoclinal sedimentary assemblages and Upper Triassic
continentally derived clastic deposits. In the southern part, the granitic
magmas (now the ‘‘southern granites’’) intruded accreted middle to late
Mesozoic island-arc–related volcanic and sedimentary sequences, apparently underlain by metamorphic rocks of the Late Triassic Sonobari
Complex (Anderson and Schmidt, 1983) and Paleozoic eugeoclinal
sedimentary rocks (Gastil et al., 1991). Given that granitic magmas can
carry information regarding their crustal source regions (e.g., Kistler
and Peterman, 1973; Zartman, 1974; Chappell and White, 1974; Taylor
and Silver, 1978; DePaolo, 1981a; Farmer and DePaolo, 1983, 1984;
DePaolo and Farmer, 1984), the present study involves using chemical
and Nd and Sr isotope compositions of Laramide granitic rocks in
northwestern Mexico to indirectly probe the nature of the lower crust
in this region.
Basement Configuration of Northwestern Mexico
Most of Mexico is underlain by accreted terranes (Campa and Coney, 1983; Coney and Campa, 1987; Sedlock et al., 1993), and only
part of northern Mexico is floored by autochthonous Precambrian
North American basement. Precambrian rocks, which have been considered to represent two distinct age provinces juxtaposed by a MidJurassic left-lateral fault named the Mojave-Sonora megashear (MSM)
(Fig. 1) (Anderson and Silver, 1979) are best exposed in northern Sonora. In northern and eastern Sonora, the Precambrian rock exposures
consist of strongly deformed greenschist-grade volcanic and sedimentary rocks deposited about 1.7 Ga ago (Anderson et al., 1980). These
rocks, which are best exposed near Cananea (Fig. 1), appear to correlate with the Mazatzal province of southern Arizona and New Mexico
(Fig. 1). Farther to the east, Precambrian rocks are rarely exposed but
likely underlie most of the northern half of the state of Chihuahua.
Grenville-age rocks (ca. 1.0 Ga) exposed in the Sierra del Cuervo near
Rancho Los Filtros in central Chihuahua (Fig. 1) (Blount, 1983) have
been recognized in wells (Campa and Coney, 1983), as tectonic blocks
in upper Paleozoic turbiditic sequences (Blount, 1983), and as xenoliths
in Cenozoic volcanic rocks (Ruiz et al., 1988; Rudnick and Cameron,
1991; Cameron et al., 1992). In northwestern Sonora, the Precambrian
crystalline basement consists of upper-amphibolite facies plutonic
rocks, schist, and feldspathic gneiss, which yielded U-Pb zircon isotope
ages between 1.7 and 1.8 Ga (Anderson et al., 1980). These rocks are
well exposed near the town of Caborca (Fig. 1), and according to the
model of Anderson and Silver (1979), they lie south and west of the
MSM (Fig. 1) forming part of a fragment of North America displaced
southward by ;800 km. The basement south and west of the megashear may correlate with the Mojave and/or the Yavapai provinces of
the southwest United States (Fig. 1). The Precambrian basement on
both sides of the megashear has been intruded by granites emplaced at
1.4 and 1.1 Ga (Anderson et al., 1980; Anderson and Schmidt, 1983).
Exposures of Late Proterozoic and Paleozoic miogeoclinal strata extend across northern Sonora into central Sonora (Stewart et al., 1984,
1990), but do not occur south of lat 298, slightly south of the city of
Hermosillo (Fig. 1). Most of the southern half of Sonora is underlain
by eugeoclinal sedimentary sequences consisting of a deformed pile of
Ordovician through Permian strata best exposed near Mazatán in central Sonora (Fig. 1) (e.g., Peiffer-Rangin et al., 1980; Noll, 1981; Poole
et al., 1991, 1995; Ketner and Noll, 1987; Radelli et al., 1987; Bartolini
et al., 1989; Stewart et al., 1990; Bartolini, 1993). Similar eugeoclinal
rocks have been also reported south of Isla Tiburón (Fig. 1) (Poole,
1993) as well as in other localities of eastern Baja California (Gastil
1410
Figure 1. Generalized map of the pre-Laramide basement distribution in northwestern Mexico and the southwestern United States.
MSM—Mojave-Sonora megashear, SAF—San Andreas fault,
Cb—Caborca, Cn—Cananea, H—Hermosillo, M—Mazatán, G—
Guaymas, N—Navojoa, EF—El Fuerte, Bt—Batopilas, PV—Piedras Verdes, IT—Isla Tiburón. The Paleozoic miogeoclinal-eugeoclinal boundary is modified from Stewart et al. (1990), and its
extension into the Baja California peninsula is from Gastil et al.
(1991). Proterozoic age domains adapted from Gehrels and Stewart
(1997). Line A–B is for the cross section shown in Figure 2.
et al., 1991). Geologic evidence suggests that these rocks were tectonically transported onto the miogeoclinal Paleozoic rocks between Middle Permian and Late Triassic time (Poole et al., 1991).
Granites in the southern part of the Laramide granitic belt intruded
rocks of the Guerrero terrane (Fig. 1). The Guerrero terrane (Campa
and Coney, 1983) consists of a sequence of Late Jurassic to middle
Cretaceous oceanic arc-related volcanic and associated sedimentary
rocks; the terrane was accreted to North America in Late Cretaceous
time (Coney, 1989). The Guerrero terrane has been best studied in
southern Mexico (e.g., Centeno-Garcı́a et al., 1993; Talavera-Mendoza
et al., 1993; Miranda-Gasca et al., 1993; Freydier et al., 1997), but it
extends north through Sinaloa (Ortega-Gutiérrez et al., 1979; Servais
et al., 1986; Freydier et al., 1995) and possibly southern Sonora (Roldán-Quintana et al., 1993). Any basement on which the northern part
of the Guerrero terrane rests is not known. However, such basement
may include the amphibolites and felsic to mafic gneisses of the Late
Triassic (?) Sonobari Complex, which is exposed west of El Fuerte in
northern Sinaloa (Fig. 1) (deCserna and Kent, 1961; Mullan, 1978;
Geological Society of America Bulletin, November 2001
A CHEMICAL AND ISOTOPIC STUDY OF THE LARAMIDE GRANITIC BELT OF NORTHWESTERN MEXICO
Figure 2. Simplified cross section A–B (Fig. 1) showing north-tosouth basement changes. To the north, the basement consists of
thick crystalline Precambrian crust overlain by Late Proterozoic
through Paleozoic miogeoclinal sedimentary strata. Movement on
the Mojave-Sonora megashear (MSM) is interpreted to have juxtaposed two different Proterozoic crustal domains. In the central
part, the Proterozoic basement is thinner and is overlain by eugeoclinal strata, which were transported northeastward onto the
miogeoclinal platform, constituting the Sonoran orogen (SO) (Poole
et al., 1995), and Upper Triassic sedimentary rocks of the Barranca
Group (BG). To the south, the Precambrian basement is missing,
and the crust is characterized by middle to late Mesozoic rocks of
the Guerrero terrane (GT), which is possibly underlain by Paleozoic eugeoclinal sequences and the metamorphic rocks of the Late
Triassic Sonobari Complex (SC) (see text for discussion).
Anderson and Schmidt, 1983). Deformed Paleozoic eugeoclinal strata
are also exposed near El Fuerte (Carrillo-Martı́nez, 1971; Mullan,
1978; Gastil et al., 1991), but their relationship to the eugeoclinal rocks
of central Sonora and eastern Baja California is uncertain.
THE LARAMIDE MAGMATIC BELT
After the demise of a Jurassic volcanic arc in north-central Sonora
and southern Arizona, arc activity shifted westward to the Pacific coast
(Coney and Reynolds, 1977; Damon et al. 1983a). Between 140 and
105 Ma, the arc activity was essentially stationary, sited along a belt
through southern California and Baja California (Silver and Chappell,
1988). After that, arc activity migrated slowly eastward and reached
what is now the western coast of Sonora and Sinaloa at ca. 100 to 90
Ma (Henry, 1975; Damon et al., 1983b). During the Laramide event
(90–40 Ma), the rate of convergence between North America and the
Farallon plate increased considerably (Coney, 1990), producing intense
calc-alkalic magmatism in southwestern North America. The age distribution of the igneous rocks suggests that during Laramide time, magmatic activity migrated eastward across the continental margin as a
result of flattening of the subducting slab (Coney and Reynolds, 1977;
Gastil, 1983; Dickinson, 1989). The eastward migration was coupled
with a progressive change in the magma composition from calc-alkalic
to more alkalic (Damon et al., 1981). At ca. 40 Ma, the rate of plate
convergence diminished and the arc migrated westward again, marking
the end of the Laramide orogeny (Coney and Reynolds, 1977; Damon
Figure 3. Sample location map showing the exposures of Late Cretaceous–early Tertiary plutonic rocks (shaded areas). Table 1 gives
the latitude and longitude as well as the ages of the samples whose
locations are shown here by the sample numbers without prefix
MV-.
et al., 1981). The end of the Laramide event was apparently followed
by a magmatic hiatus of ;6 m.y. duration. Volcanism resumed at ca.
34 Ma (McDowell and Clabaugh, 1979); middle Tertiary explosive
silicic eruptions built most of the Sierra Madre Occidental volcanic
province.
The Laramide magmatic rocks in northern Mexico are exposed along
a northwest-trending belt (Fig. 3) that can be compared in size with
the Sierra Nevada and the Peninsular Ranges batholiths (Damon,
1986). The exposures extend along most of the Pacific coast of Mexico.
They are widespread across much of northern Mexico, but especially
in Sonora and Sinaloa. They are considerably less common to the
south, although numerous plutons of this age occur in the states of
Jalisco, Colima, and Guerrero (Schaaf et al., 1995). Exposures of Laramide granitic rocks are concentrated in Sonora and Sinaloa, but also
occur in western and central Chihuahua (McDowell and Clabaugh,
1979; McDowell and Mauger, 1994; Bagby et al., 1981; Shafiqullah et
al., 1983) and western Durango (Clark et al., 1979; McDowell and
Clabaugh, 1979; Damon et al., 1983b). Most previous studies of the
Laramide plutonic rocks have focused on the related ore deposits, particularly in the southern extension of the porphyry copper belt of southwestern North America and northwestern Mexico (e.g., Damon et al.,
1983a, 1983b; Damon, 1986; Clark et al., 1979, 1988; Barton et al.,
1995). In addition, the ages of many Laramide plutons of northwestern
Mexico are known (Fredrikson, 1974; Henry, 1975; Gastil and Krummenacher, 1977; Damon et al., 1983a, 1983b), but the chemical and
isotopic compositions of these rocks have not been extensively studied.
In Baja California and Sonora and Sinaloa, R.G. Gastil (Gastil et al.,
1974; Gastil, 1983) observed three nearly parallel, roughly north-trend-
Geological Society of America Bulletin, November 2001
1411
VALENCIA-MORENO et al.
SAMPLES
Figure 4. Modal quartz–alkali feldspar–plagioclase (QAP) diagram
for Laramide granitic rocks of northwestern Mexico. Data for Batopilas and El Jaralito batholith from Bagby et al. (1981) and Roldán-Quintana (1991), respectively. NG, CG, and SG—northern,
central, and southern granites, respectively.
ing subbelts of batholithic rocks, which became progressively younger
and more alkalic to the east. Damon et al. (1983a) and Mead et al.
(1988) determined the Sr isotope composition in Laramide plutonic
rocks associated with several porphyry copper-molybdenum deposits
and tungsten-dominated skarn deposits along the northwestern Mexico
Laramide belt and thereby demonstrated that the Sr initial ratios in the
plutons increase regularly from south to north toward the North American craton.
The studied samples are medium- to coarse-grained granitic rocks
ranging from quartz diorite to granite, but hornblende-biotite granodiorites constitute by far the most abundant rock type in the batholiths
(Fig. 4, Table 1). No systematic petrographic differences were observed
through the belt, but more felsic compositions are found in some northern and central granites from batholiths of central Sonora, including
two-mica granites at Sierra El Jaralito (Roldán-Quintana, 1991) (Fig.
4). Because only a few samples of southern granites were studied, the
field of Laramide plutons from Batopilas in southwestern Chihuahua
(Fig. 1) from Bagby et al. (1981) is also shown in Figure 4. The latter
encompasses the southern granite samples of this study, but extends to
more mafic, quartz dioritic compositions (Fig. 4).
The studied rocks contain quartz, K-feldspar, plagioclase, biotite, and
hornblende. Clinopyroxene, sphene, apatite, and epidote occur as accessory phases. Mineral modes shown in Table 2 were obtained by
point-counting with a minimum of 600 points per sample. Quartz forms
large anhedral grains in proportions between 6% and 39%, whereas Kfeldspar constitutes about 8% to 31% of the rocks, commonly forming
patches up to 1 cm long, which display myrmekitic and microperthitic
intergrowths. Modal proportions of plagioclase vary from 35% to 65%,
showing compositions between An7 and An34. Biotite and hornblende
are the main mafic minerals, usually in combined proportions above
10%, although biotite is dominant in most rocks. Clinopyroxene appears in a few samples, particularly in the two less silica-rich rocks
MV-3 and MV-6, where it is found as large phenocrysts rimmed by
hornblende in proportions of 3.8% and 2.6%, respectively. Large euhedral honey-colored sphene and small apatite crystals are common in
most of the samples.
TABLE 1. SAMPLE LOCATION AND AGE
Sample
MV-1
MV-3
MV-4
MV-5
MV-6
MV-7
MV-9
MV-10
MV-11
MV-12
MV-14
MV-15
MV-17
MV-18
MV-SP
MV-19
MV-21
MV-22
MV-23
MV-25
MV-27
MV-28
MV-29
MV-30
A-163
59–96
SR-42
44–88
MV-25a
MV-25b
Locality
San Nicolás
San Javier
Tecoripa
Suaqui Grande
Rancho El Encino
Puerta del Sol
Cerro Bola
San Francisco
Granito Hermita
Hermosillo
Hermosillo
Hermosillo
Cruz Gálvez
Villa Pesqueira
Villa Pesqueira
Barita de Sonora
San Carlos
Rancho El Bayo
Bahı́a de Kino
Rancho La Salada
El Fuerte
El Fuerte–Choix
Tetaroba
Chinobampo
Sierra El Manzanal
Maycoba
La Reforma
Rancho El Encino
Sonobari Complex
Sonobari Complex
Rock type
Granodiorite
Qz diorite
Granodiorite
Granodiorite
Granodiorite
Granite
Granodiorite
Granodiorite
Granodiorite
Granodiorite
Granodiorite
Granodiorite
Granodiorite
Granodiorite
Qz monzonite
Granite
Tonalite
Granite
Granodiorite
Granite
Granodiorite
Qz monzonite
Tonalite
Tonalite
Granodiorite
Granodiorite
Qz monzonite
Granodiorite
Felsic gneiss
Mafic gneiss
Coordinates
(lat N)
(long W)
2882691099
2883690899
2883694999
2882191699
2985295499
2983490599
2883691699
2882992799
2884894999
2980694999
2980590299
2980390099
2885395199
2980694099
2981794399
2885395199
2785692999
2881294599
2884594999
2982693199
2682793699
2683694299
2682391599
2682394299
3083993099
288249
278309
298539
2681995299
2681590899
10981092199
10984490499
10985490799
10985793199
10982695499
11080190599
11080191499
11082890899
11083794399
11085691999
11085691099
11085695699
11180794399
10985894599
10985294899
10985491699
11180492399
11085893399
11185491999
11085990599
10883591599
10882392499
10882990699
10882195499
11081093099
1088389
1128249
1098279
10885394299
10885194599
Age
(Ma)
Method
Min.
Age
source*
56.7 6 0.2
62.0 6 1.7
62.0 6 1.7
58.8 6 1.6
59.6 6 1.3
57.0 6 3.0
62.0 6 1.7
62.9 6 1.5
62.9 6 1.5
64.1 6 1.4
64.1 6 1.4
64.1 6 1.4
64.1 6 1.4
58.0 6 3
58.0 6 3
62.0 6 1
83 6 2
76.9 6 2.8
83 6 2
64.1 6 1.4
57.2 6 1.2
57.2 6 1.2
57.2 6 1.2
57.2 6 1.2
67.9 6 0.2
63
91.2 6 2.1
59.6 6 1.3
220
220
Ar-Ar
K-Ar
K-Ar
K-Ar
K-Ar
U-Pb
K-Ar
K-Ar
K-Ar
K-Ar
K-Ar
K-Ar
K-Ar
U-Pb
U-Pb
U-Pb
K-Ar
K-Ar
K-Ar
K-Ar
K-Ar
K-Ar
K-Ar
K-Ar
Ar/Ar
K-Ar
K-Ar
K-Ar
U-Pb
U-Pb
Hb
Hb
Hb
Hb
Bi
Zr
Hb
Hb
Hb
Hb
Hb
Hb
Hb
Zr
Zr
Zr
Hb
Bi
Hb
Ser
Bi
Bi
Bi
Bi
Bi
Bi
Bi
Bi
Zr
Zr
1
2
2
2
2
4
2
2
2
2
2
2
2
4
5
6
6
6
3
3
3
3
3
7
8
9
2
10
10
*References: (1) Gans, 1997; (2) Damon et al., 1983a; (3) Damon et al., 1983b; (4) Anderson et al., 1980; (5) Poole et al., 1991; (6) Mora-Alvarez 1992; (7) GonzálezLeón et al., 2000; (8) McDowell, 1993; (9) Schmidt, 1975; (10) Anderson and Schmidt, 1983.
1412
Geological Society of America Bulletin, November 2001
Northern granite
Geological Society of America Bulletin, November 2001
31.72
78.78
7.99
30.29
7.03
0.95
6.64
0.85
5.95
1.23
3.30
0.42
2.91
0.35
178.41
7.67
0.43
8.48
172
309
821
14
18
8.90
16.50
2.93
7.92
4.67
19.5
15.7
44.4
8.3
9.6
0.0
2.0
0.6
20.1
20.1
48.2
7.3
1.6
0.0
2.1
0.5
99.37
0.98
67.1
0.49
15.8
1.09
2.05
0.06
0.93
2.89
4.13
3.82
0.11
0.90
27.6
30.9
37.6
3.1
0.0
0.0
0.6
0.3
13.20
1.03
70.9
0.24
15.2
0.62
1.18
0.02
0.34
2.35
3.86
3.86
0.07
0.65
106.76
208.37
15.50
43.64
8.09
0.82
7.74
1.06
5.99
1.28
3.56
0.45
3.17
0.47
406.90
23.69
0.32
7.72
214
229
837
117
20
8.46
32.35
2.72
5.90
21.73
28.42
54.42
5.13
17.84
3.81
1.10
3.24
0.62
3.96
0.81
2.19
0.27
1.95
0.27
124.03
10.25
0.97
1.91
71
360
833
171
8
1.60
5.83
0.79
4.61
17.86
27.70
57.81
5.70
20.09
4.01
1.03
3.46
0.62
3.64
0.67
1.85
0.24
1.73
0.25
128.78
11.29
0.86
2.52
100
343
794
102
7
2.93
6.08
5.42
3.66
23.31
18.43
48.00
5.07
18.03
4.76
1.31
3.18
0.50
2.56
0.42
1.10
0.13
0.98
0.14
104.61
13.16
1.04
2.43
129
548
803
111
5
2.26
4.35
0.41
3.05
16.09
114.87
225.62
18.38
47.31
6.80
0.71
5.51
0.63
3.60
0.79
2.27
0.36
2.46
0.32
429.63
32.89
0.36
4.31
116
447
792
230
13
9.72
80.22
3.30
4.63
23.72
33.91 21.89 135.82
23.48
68.37 51.07 259.53
37.35
6.90
6.50 21.85
5.59
22.18 22.12 56.12
18.49
4.12
3.71
7.17
2.25
0.86
0.78
0.52
0.29
3.84
3.40
5.16
1.51
0.58
0.56
0.77
0.19
3.35
3.38
4.38
1.13
0.78
0.64
0.98
0.21
2.16
2.00
2.87
0.63
0.25
0.28
0.48
0.09
2.01
1.80
2.99
0.72
0.31
0.20
0.35
0.06
149.63 118.33 499.00
92.00
11.86
8.54 31.97
22.88
0.67
0.68
0.27
0.48
3.65
4.36
3.20 N.D.
110
91
114
81
535
119
339
207
808
999
829
1628
136
79
167
133
13
20
9
8
5.30
2.91
3.72
1.32
7.84 13.50 24.28
8.35
3.30
1.16 N.D.
0.26
4.63
1.55
4.24
2.36
23.72 14.15 33.31
7.75
Central granite
24.2
24.9
39.6
6.6
0.0
0.0
2.1
2.6
24.1
16.6
44.6
11.9
0.6
0.0
1.6
0.6
6.3
4.1
65.0
13.5
4.1
3.8
2.8
0.3
66.3 66.5 66.1
0.48 0.48 0.46
15.6 15.3 14.9
1.77 1.64 1.62
2.31 2.15 2.12
0.06 0.07 0.06
1.93 1.93 1.83
4.16 4.29 3.74
3.09 3.21 2.84
3.14 3.23 3.96
0.12 0.11 0.10
1.06 0.69 1.04
20.0
20.1
34.5
14.1
8.5
0.0
2.6
0.3
34.11
62.58
6.89
28.44
5.11
0.34
5.49
0.84
4.87
1.06
3.03
0.44
3.53
0.11
156.83
6.80
0.20
10.82
166
193
520
882
34
14.22
32.78
N.D.
N.D.
11.00
51.57 22.59
114.96 45.49
11.80
4.40
43.74 15.23
7.14
2.56
0.88
0.74
6.08
1.93
0.84
0.32
5.34
2.19
1.00
0.49
3.13
1.10
0.44
0.17
3.44
1.08
0.48
0.14
250.83 98.43
10.54 14.67
0.41
1.03
17.67
8.24
314
33
245
67
641
804
206
248
33
9
16.36
2.93
58.60 21.35
3.25
0.48
8.41
3.47
22.14
9.16
19.79
49.14
5.47
20.42
4.54
0.88
4.63
0.79
4.33
0.89
2.36
0.29
1.89
0.28
115.70
7.36
0.60
2.03
53
438
637
142
11
2.39
5.15
4.77
4.03
20.13
MV-3
21.74
47.48
5.40
19.02
3.92
0.68
4.21
0.86
4.16
0.86
2.33
0.28
1.76
0.26
112.96
8.67
0.52
10.62
118
263
955
102
11
7.05
21.68
0.79
2.34
5.92
MV-4
26.2
9.6
46.6
14.3
1.1
0.0
1.6
0.6
24.2
21.3
36.7
11.5
4.5
0.0
1.5
0.3
Central granite
36.0
27.5
33.3
2.0
0.1
0.0
0.6
0.3
99.52
1.05
75.1
0.12
13.3
0.27
0.56
0.03
0.32
1.18
3.39
4.55
0.04
0.66
26.9
26.4
39.9
5.0
0.6
0.0
0.6
0.6
98.70
0.96
68.3
0.40
14.7
1.54
1.50
0.06
1.33
3.14
3.26
3.87
0.09
0.52
13.6
7.0
59.7
12.1
6.6
0.0
0.5
0.5
98.82
0.97
62.4
0.68
16.6
1.71
3.23
0.08
2.57
5.12
3.20
2.34
0.14
0.75
34.57 21.53 19.85
74.31 46.81 44.68
6.79
5.40
5.42
19.07 19.00 17.76
4.12
3.24
3.32
0.61
0.78
0.56
3.45
3.11
2.90
0.53
0.45
0.44
3.15
2.75
2.70
0.76
0.53
0.53
1.94
1.62
1.70
0.25
0.25
0.26
1.87
1.54
1.86
0.35
0.14
0.25
151.78 107.15 102.22
12.99
9.83
7.49
0.50
0.76
0.55
7.40
0.25
4.66
158
59
124
271
168
128
912
793
842
15
19
63
10
18
21
6.89
1.75 12.78
24.43
8.99 20.14
N.D.**
1.18
0.65
4.71
0.42
2.88
39.58
9.69 14.26
31.18
69.12
8.21
26.86
3.84
0.76
3.42
0.50
2.86
0.50
1.49
0.24
1.52
0.16
150.67
14.45
0.65
N.D.
73
150
882
58
18
7.74
32.57
1.10
1.79
12.35
MV-28
35.0
13.1
44.1
4.6
2.3
0.0
0.8
0.1
99.74
0.91
68.0
0.34
14.5
0.90
1.71
0.05
0.99
2.72
4.18
3.73
0.07
2.55
22.6
8.3
48.4
12.1
5.8
0.1
1.8
0.8
99.57
0.96
67.4
0.47
15.7
1.10
2.08
0.05
1.62
3.60
4.45
2.37
0.12
0.60
32.7
15.1
41.4
8.3
1.1
0.0
1.0
0.3
99.36
0.98
69.5
0.37
14.5
0.97
1.83
0.04
1.12
2.79
3.20
4.13
0.07
0.85
25.7
8.0
53.5
8.3
3.1
0.0
0.8
0.6
99.08
1.00
65.2
0.45
16.3
1.42
2.68
0.09
1.68
4.22
3.97
1.97
0.14
0.95
Sonobari Complex
27.0
10.5
52.1
7.3
1.1
0.1
0.6
1.3
98.96
0.98
66.8
0.55
15.9
1.14
2.15
0.04
1.29
3.33
4.79
2.12
0.15
0.70
9.41
22.10
2.84
10.77
2.21
0.77
2.38
0.43
2.54
0.54
1.52
0.21
1.52
0.25
57.50
4.35
1.03
N.D.
43
150
987
150
16
0.98
2.40
1.86
1.46
9.87
19.97
41.57
4.62
18.58
3.53
1.20
3.05
0.50
2.55
0.52
1.50
0.24
1.65
0.27
99.76
8.51
1.14
3.85
94.17
394.10
123.40
139.90
14.84
2.80
8.95
N.D.**
3.56
12.18
7.56
16.43
2.08
9.24
2.05
0.70
2.02
0.41
2.06
0.44
1.31
0.20
1.34
0.26
46.10
2.33
1.05
10.44
141.26
463.86
769.91
187.82
13.08
4.76
13.56
N.D.**
4.83
9.18
20.4
6.72
44.85
18.39
5.26
2.79
17.23
13.35
2.55
3.95
0.47
1.45
2.15
5.03
0.33
0.84
2.00
5.30
0.38
1.07
1.13
3.29
0.16
0.52
1.21
2.90
0.15
0.38
10.53
24.73
11.86
1.63
0.62
1.01
N.D.
N.D.
66.14
3.10
146
177
1421
131
120
28
14
34
1.85
0.43
8.04
0.48
0.74
0.76
2.00
0.12
11.57
3.51
MV-29 MV-30 Tam-1 Mal-74 MV-25a MV-25b
Southern granite
36.8
14.1
41.2
5.6
1.3
0.0
0.5
0.5
98.92
0.99
66.7
0.53
15.1
1.36
2.58
0.07
1.70
3.83
3.19
2.80
0.11
0.95
30.42
12.20
12.99
12.20
68.17
28.59
32.77
28.39
7.35
3.66
4.57
3.60
28.56
14.17
17.45
14.15
5.34
2.70
3.77
2.71
1.09
0.56
0.54
0.74
4.03
2.29
3.44
2.68
0.48
0.31
0.61
0.38
3.29
1.73
3.65
1.87
0.71
0.30
0.69
0.37
1.97
0.81
2.10
0.93
0.28
0.10
0.30
0.09
1.97
0.74
2.06
0.74
0.28
0.04
0.26
0.12
153.95
68.18
85.21
68.98
10.84
11.60
4.43
11.62
0.73
0.70
0.47
0.86
6.29 N.D.
N.D.
N.D.
162
48
116
48
402
178
141
184
897
1165
1132
1046
135
66
69
87
33
10
26
10
4.36
1.30
8.82
1.06
16.19
2.55
18.39
3.36
1.30
0.32
1.40
0.33
5.24
2.71
2.54
3.19
10.71
7.27
13.44
7.66
MV-9 MV-10 MV-11 MV-21 MV-22 MV-23 59–96 MV-27
23.27 20.56 16.87
55.65 49.68 36.25
5.69
5.69
3.77
17.00 23.09 13.55
3.25
5.15
3.34
0.74
0.85
0.51
2.99
4.17
2.87
0.48
0.69
0.51
3.02
4.82
3.43
0.66
1.09
0.74
1.89
3.03
2.02
0.22
0.39
0.29
1.79
2.26
2.02
0.30
0.44
0.32
116.95 121.91 86.50
9.15
6.41
5.88
0.73
0.57
0.51
5.98
8.51
4.11
135
31
31
299
60
29
767
651
933
109
269
147
11
18
16
6.66 12.00
4.91
14.41 36.59 12.16
3.69
1.96
0.85
3.99 10.04
6.02
15.59 14.69
6.14
MV-5
Southern granite
MV-5 MV-9 MV-10 MV-11 MV-21 MV-22 MV-23 MV-27 MV-28 MV-29 MV-30
98.92 99.54 98.89 100.01 99.60 98.77
0.97 0.94 0.88
0.97 0.92 0.95
67.4 65.8 56.0
0.53 0.54 0.83
14.4 15.5 17.4
1.59 1.35 3.01
1.88 2.87 4.24
0.07 0.06 0.12
1.27 1.78 3.99
2.60 4.09 7.06
3.23 3.26 3.01
4.41 3.46 1.86
0.10 0.12 0.20
1.47 0.71 1.17
MV-4
TABLE 3. SAMPLE TRACE ELEMENT COMPOSITIONS
33.3
20.5
34.3
9.4
1.5
0.0
0.5
0.6
99.30
1.07
71.1
0.32
14.7
1.10
1.48
0.06
1.17
2.51
3.62
3.01
0.10
0.47
MV-7 MV-12 MV-14 MV-15 MV-17 MV-18 MV-SP MV-19 MV-25 A-163 44–89 MV-1
25.3
12.3
47.8
8.3
4.8
0.0
1.3
0.1
98.03
0.95
64.0
0.54
15.6
1.36
2.85
0.09
2.03
3.92
3.63
3.06
0.19
0.77
Note: All values are given in ppm, except the two rows above the middle line, which represent normalized element ratios.
N.D.—no data.
La
Ce
Pr
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
eREE
LaN/YbN
Eu/Eu*
Cs
Rb
Sr
Ba
Zr
Y
U
Th
Ta
Hf
Nb2
MV-6
25.2
8.0
55.0
8.6
0.5
0.0
1.1
1.6
34.7
9.3
44.3
5.1
4.6
0.0
1.1
0.8
99.05
0.92
62.9
0.62
15.7
2.65
2.44
0.08
2.43
4.84
3.10
2.91
0.16
1.22
Northern grante
99.15
0.97
98.76
0.98
total
98.16 99.71 99.88
A/CNK
0.89 0.98
0.95
Modes (normalized to 100 %)
Q
18.3 29.2
38.8
F
11.0 16.8
21.3
P
49.9 43.9
37.0
Bi
14.5
6.5
1.6
Hb
1.0
1.3
0.5
Px
2.6
0.0
0.0
Op
2.1
1.8
0.6
Other
0.5
0.5
0.1
67.5
0.53
15.6
0.99
2.08
0.05
1.09
3.32
4.04
3.10
0.16
0.68
66.7
0.59
15.1
1.20
2.52
0.07
1.31
3.45
3.68
2.84
0.17
1.13
64.8
0.59
16.2
1.40
2.95
0.07
1.98
4.43
3.63
2.84
0.16
0.83
Major elements (wt%)
SiO2
61.7 69.0
0.75 0.44
TiO2
16.0 14.8
Al2O3
2.16 1.46
Fe2O3
FeO
2.82 1.55
MnO
0.09 0.05
MgO
2.15 1.30
CaO
4.35 2.78
3.78 3.22
Na2O
3.57 4.39
K2O
0.18 0.10
P2O5
LOI
0.61 0.62
MV-6 MV-7 MV-12 MV-14 MV-15 MV-17 MV-18 MV-SP MV-19 MV-25 A-163 MV-1 MV-3
TABLE 2. SAMPLE MAJOR ELEMENT AND MODAL COMPOSITIONS
A CHEMICAL AND ISOTOPIC STUDY OF THE LARAMIDE GRANITIC BELT OF NORTHWESTERN MEXICO
1413
VALENCIA-MORENO et al.
Figure 5. AFM diagram for Laramide granitic rocks from northwestern Mexico. The boundary between the calc-alkalic and tholeiitic series is according to Irvine and Baragar (1971). Data for
Batopilas from Bagby et al. (1981). A—Na2O 1 K2O, M—MgO,
F—Fe total as FeO. NG, CG, and SG—northern, central, and
southern granites, respectively.
Figure 6. K2O vs. SiO2 variation diagram for Laramide granitic
rocks of northwestern Mexico. The high-K–medium-K and medium-K–low K lines are according to Le Maitre et al. (1989). The
high-K–shoshonite boundary is according to Rickwood (1989).
Data for Batopilas from Bagby et al. (1981). NG, CG, and SG—
northern, central, and southern granites, respectively.
SAMPLE PREPARATION AND ANALYTICAL TECHNIQUES
the Universidad Nacional Autónoma de México by using a Finnigan
MAT 262 mass spectrometer. In this laboratory, the La Jolla standard
gave a mean 143Nd/144Nd of 0.511 881 6 21 for 1s deviation of 110
analyses.
The samples were disaggregated in the field, and fragments of 5 cm
size, free of weathered surfaces or crosscutting veinlets, were collected
for study. These fragments were washed with distilled water, dried, and
crushed in a steel jaw crusher to centimeter-sized pieces. A representative fraction of the crushed sample was powdered in an Al2O3 mill
for chemical analyses. Major element concentrations for most samples
were commercially analyzed by X-ray fluorescence (XRF) techniques
at the XRAL Laboratories (Ontario, Canada). Trace elements including
the rare earth elements La–Lu, in addition to Cs, Rb, Sr, Ba, Zr, Y, U,
Th, Ta, Hf, and Nb, were analyzed in a TS Sola inductively coupled
plasma–mass spectrometer (ICP-MS) at the University of Arizona. For
trace element analysis, ;50 to 100 mg of the powdered sample was
dissolved in distilled acids according to techniques adapted from Roberts and Ruiz (1989), but using 150 mL of a 10 ppm Re-In spike
solution. The run procedure consisted of a blank plus 2, 5, and 10 ppb
multielement calibration solutions prepared in 2% HNO3 from stock
analytical standards (SPEX high-purity), followed by eight unknowns
including a U.S. Geological Survey rock standard (RGM-1) and a sample blank. Single-element solutions of Ba, Ce, La, Nd, Sm, Eu, Gd,
Tb, Dy, Ho, Hf, and Ta were also run to estimate oxide production in
order to minimize element interference. A spreadsheet was used to
perform oxide corrections and internal standard adjustments to yield
the final rock concentrations in parts per million. Analytical uncertainties are described by Roberts and Ruiz (1989).
The Sr and Nd isotope compositions for some of the studied samples
were analyzed at the University of Arizona. Analyses were performed
by using a solid source VG 354 thermal ionization mass spectrometer
equipped with six Faraday collectors, following the analytical procedures described by Gleason et al. (1995). During the period of this
study, 36 analyses of the La Jolla standard gave a mean 143Nd/144Nd of
0.511 869 6 10 (errors refer to the uncertainty in the least significant
digits), and 69 analyses of the NBS-987 standard gave a mean 87Sr/
86
Sr of 0.710 218 6 11. The isotopic compositions for samples MV-5,
MV-21, MV-27, MV-30, and A-163 were analyzed in the Laboratorio
Universitario de Geoquı́mica Isotópica (LUGIS) of the Laboratory of
1414
RESULTS
Major Elements
The XRF results for major elements and the mineral modes for 25
studied samples are listed in Table 2. The silica content ranges from
56% to 75%, with a mean of 66%. On the basis of their relatively low
molar alkali ratios (Table 2), the samples are generally metaluminous
(A/CNK , 1.0, where A 5 Al2O3, C 5 CaO, N 5 Na2O, and K 5
K2O), but four of them are weakly peraluminous in composition (1.0
, A/CNK , 1.1). The samples follow a typical AFM calc-alkalic trend
(Fig. 5) and plot in the medium- to high-K calc-alkalic fields of the
K2O-SiO2 diagram (Fig. 6). Although the major element data do not
show systematic variations with the type of intruded crust, three of the
four samples of the southern granites from the Sinaloa batholith and
the granitic rocks from Batopilas display lower K2O/SiO2 ratios than
Laramide granitic rocks farther to the north.
Trace Elements
Concentrations of rare earth elements (REEs) and some other trace
elements are listed in Table 3. As for major elements, most analyzed
trace elements do not display preferential enrichments in any sample
group. However, the REEs appear to be more fractionated in the samples of the northern and central granites compared to those of the southern granites (Fig. 7). Total REE abundances range from 92 to 499 ppm
for the northern granites, from 82 to 154 ppm for the central granites,
and from 46 to 100 ppm for the southern granites (Table 3). The northern granites also tend to have higher (La/Yb)N ratios of ;7 to 24, and
larger negative europium anomalies (mean Eu/Eu* 0.56) (Table 3, Fig.
7A). The central granites have relatively flatter REE slopes displaying
slightly less pronounced negative europium anomalies (mean Eu/Eu*
Geological Society of America Bulletin, November 2001
A CHEMICAL AND ISOTOPIC STUDY OF THE LARAMIDE GRANITIC BELT OF NORTHWESTERN MEXICO
Figure 7. Chondrite-normalized REE abundances for Laramide
granitic rocks of northwestern Mexico according to the type of
underlying basement. The shaded area is the REE spectrum of the
northern granites. Samples were restricted to those with 62–69
wt% SiO2. Chondrite values are according to Anders and Grevesse
(1989). NG, CG, and SG—northern, central, and southern granites, respectively.
0.65) (Table 3, Fig. 7B). In contrast, the southern granites show much
flatter REE slopes with (La/Yb)N ratios of ;2 to 12, and they display
just minor europium anomalies (mean Eu/Eu* 0.87) (Table 3; Fig. 7C).
We note that in Figure 7 we have plotted chondrite-normalized REE
patterns for samples in each geographic area with 62%–69% SiO2, in
order to compare REE abundances in rocks with similar bulk
compositions.
Sr and Nd Isotope Data
The Sr and Nd isotope data for the studied samples are presented in
Table 4. Initial 87Sr/86Sr ratios range from 0.7070 to 0.7089 for the
northern granites, 0.7064 to 0.7073 for the central granites, and 0.7026
to 0.7062 for the southern granites. Although the range of 87Sr/86Sr
values of granites in southwestern North America is wider than that
shown by our samples, it is the north-to-south trend that is most important in our data. The Sr compositions of most of the Mexican Laramide granites are similar to those of Laramide plutons in southern
Arizona, Nevada, and California (Fig. 8). However, the samples from
Arizona display a higher mean initial Sr value between 0.708 and
0.710, and values reach up to 0.720. The granites from Nevada and
California show 87Sr/86Sr values mostly between 0.704 and 0.708, but
also extend to much higher ratios above 0.720. It is also interesting to
note that the Sr isotope compositions of the Laramide plutons of southwestern North America overlap the values determined for lower-crustal
xenoliths of mafic granulite and orthogneiss analyzed from southern
Arizona and northern Mexico (Ruiz et al., 1988; Esperanca et al., 1988;
Kempton et al., 1990; Rudnick and Cameron, 1991; and Cameron et
al., 1992) (Fig. 8).
The Nd isotope compositions show corresponding regional variations. More negative initial eNd values from 25.4 to 24.2 characterize
the northern granites, values between 25.1 and 23.4 are found in the
central granites, and more positive values from 20.9 to 14.2 are typical in the southern granites (Table 4). A histogram of the data reported
in this work plus compiled data (references given in Fig. 9 caption)
shows mean initial eNd values of about 25 in the northern granites and
23 for the central granites, whereas the southern granites form a distinct group with all but one of the samples having positive eNd values
(20.9 to 14.2) (Fig. 9). In a more regional context, the range of Nd
compositions is similar to that for the southern Arizona Laramide plutons, although the majority of the latter have more negative eNd values
between 26 and 212 (Fig. 9). The Late Cretaceous and Tertiary granites from Nevada and California show a wider range of compositions
that reach extremely negative initial values near 220 (Fig. 9). In order
to elucidate the source of the granitic melts and the possible implications of the crust beneath the batholiths, Figure 9 also shows the compositions of lower-crustal xenoliths from southern Arizona and northern Mexico (compiled from Ruiz et al., 1988; Esperanca et al., 1988;
Kempton et al., 1990; Rudnick and Cameron, 1991; and Cameron et
al., 1992), as well as the composition of exposed Proterozoic rocks in
Arizona and southern Nevada and California (Farmer and DePaolo,
1984; Nelson and DePaolo, 1985; Bennett and DePaolo, 1987). Figure
9 further shows that the xenoliths of mafic granulites and orthogneisses
cover the whole range of the initial eNd values observed in the northwestern Mexico granitic rocks. The paragneisses show more negative
eNd values, comparable to the values for the exposed Proterozoic rocks
in western United States.
Regional variations in the isotopic compositions of the Laramide
granitic belt from southern Arizona through northwestern Mexico are
strongly suggested by the Sr and Nd data (Fig. 10). The plutons in the
northern part of the belt have higher Sr ratios and lower eNd values
compared to plutons from the southern part of the belt. This generalization is particularly true for the samples located north of the apparent
position of the Paleozoic miogeoclinal-eugeoclinal boundary in central
Sonora (Figs. 1, 10), which can be loosely estimated from the position
of the southernmost known exposures of Paleozoic rocks in central
Sonora. Just south of this boundary, the central granites have isotopic
compositions similar to those of the northern granites, but their overall
range is slightly displaced to lower 87Sr/86Sr ratios and less negative
eNd values. The southern granites show an obvious shift to much lower
Sr ratios and to positive eNd values.
In a correlation diagram plotting Sr versus Nd, the southern granites
fall close to the bulk Earth composition, whereas the central and northern granites plot toward higher 87Sr/86Sr ratios and lower eNd values
Geological Society of America Bulletin, November 2001
1415
VALENCIA-MORENO et al.
TABLE 4. ISOTOPIC DATA OF LARAMIDE GRANITIC ROCKS FROM NORTHWESTERN MEXICO
Sample
Location
Northern granite
MV-6#
Puerto El Encino
MV-7
NE Puerta del Sol
MV-12
Hermosillo
MV-17
Cruz Gálvez
MV-19
Barita de Sonora
A-163**
Bacanuchi
Central granite
MV-1
San Nicolás
Tecoripa
MV-4#
MV-5**
Suaqui Grande
MV-9
Cerro Bola
Granito Hermita
MV-11#
MV-21
San Carlos
Southern granite
MV-27**
El Fuerte
MV-30**
Chinobampo
MAL-74
Malpica
TAM-1
Tameapa
SR-42
Santa Rosalı́a
Sonobari Complex
MV-25a
W of El Fuerte
MV-25b
W of El Fuerte
Sr/86Sr eSr†
initial
initial
Rb
(ppm)
Sr
(ppm)
87
Rb/86Sr*
measured
87
Sr/86Sr
measured
388
N.D.
81
111
101
166
265
N.D.
506
445
335
193
4.230
N.D.
0.463
0.720
0.869
2.486
0.7109
N.D.
0.709219 6
0.707640 6
0.709691 6
0.709876 6
181
155
139
164
231
77
357
341
334
326
293
398
1.468
1.316
1.198
1.453
2.282
0.557
0.708542 6
0.7076
0.707427 6
0.709149 6
0.7092
0.706583 6
16
48
43
134
23
57
178
150
408
371
442
0.780
0.829
0.950
0.177
0.371
0.705265
0.705782
0.703374
0.706393
0.706355
6
6
6
6
6
46
6
257
257
0.521
0.063
87
Sm
(ppm)
Nd
(ppm)
Sm/144Nd§
measured
147
Nd/144Nd
measured
143
Nd/144Nd eNd†
initial
initial
tDM
(Ga)
143
0.70729
N.D.
0.70880
0.70699
0.70893
0.70747
141
N.D.
162
136
164
142
7.04
5.10
3.96
4.13
2.53
7.63
36.66
25.16
22.35
21.40
15.05
41.29
0.1161
0.1226
0.1071
0.1166
0.1016
0.1117
0.512389 6 8
0.512381 6 6
0.512362 6 6
0.512331 6 7
0.512322 6 7
0.512345 6 25
0.512343
0.512336
0.512318
0.512283
0.512281
0.512295
24.2
24.5
24.6
25.3
25.4
25.0
1.03
1.11
0.98
1.12
0.98
1.05
40
0.70735
0.70640
0.70643
0.70787
0.70720
0.70593
141
129
128
149
139
122
4.96
N.D.
3.55
3.65
N.D.
3.714
24.81
N.D.
21.61
20.67
N.D.
18.69
0.1209
N.D.
0.0993
0.1067
N.D.
0.1202
0.512420 6 10
N.D.
0.512368 6 20
0.512342 6 8
N.D.
0.512423 6 17
0.512375
N.D.
0.512330
0.512298
N.D.
0.512358
23.7
N.D.
24.5
25.1
N.D.
23.4
1.03
N.D.
0.91
1.00
N.D.
1.01
41
38
21
21
21
0.70463
0.70511
0.70256
0.70624
0.70588
13
110
226
126
121
2.70
2.21
2.20
3.67
2.23
14.17
10.77
9.96
18.05
12.05
0.1151
0.1224
0.1337
0.1229
0.1119
0.512618 6 21
0.512566 6 20
0.512702 6 7
0.512771 6 7
0.512802 6 7
0.512575
0.512520
0.512593
0.512671
0.512735
10.2
20.9
11.7
13.2
14.2
0.67
0.48
0.67
0.81
0.39
0.708679 6 21
0.706353 6 21
0.70701
0.70615
139
127
2.88
3.87
18.50
12.99
0.0942
0.1801
0.512394 6 7
0.512829 6 7
0.512256
0.512565
21.8
14.2
0.84
0.98
23
34
21
42
34
31
Note: Samples A-163 and SR-42 kindly provided by Carlos González-León and Lucas Ochoa-Landı́n, respectively. N.D.—no data. Uncertainties shown are for the least
significant digit(s).
*Uncertainties in 87Rb/86Sr ratios ,1.0%. Sr ratios normalized to 86Sr/88Sr 5 0.1194.
†
eNd and eSr calculated by using present-day CHUR (chondritic uniform reservoir) values of 147Sm/144Nd 5 0.1966 and 143Nd/144Nd 5 0.512 638, and 87Sr/86Sr 5 0.7045
and 87Rb/86Sr 5 0.0827. Model ages (tDM)calculated from equation eNd(t) 5 0.25T2 2 3T 1 8.5 (DePaolo, 1981a).
§
Uncertainties in 147Sm/144Nd ratios , 0.5%. Nd ratios normalized to 146Nd/144Nd 5 0.7219.
#
Rb-Sr data recalculated from Damon et al. (1983b).
**Sm-Nd isotope data obtained in the LUGIS laboratory of the Universidad Nacional Autónoma de México in Mexico City. Element concentrations in samples MV-27,
MV-30, and Rb and Sr for A-163 are from ICPMS data.
(Fig. 11). The data for the central and northern granites overlap the
field of Laramide plutons from southern Arizona. The data for the
southern granites plot very close to the values determined for late Mesozoic island-arc rocks of the Guerrero terrane (Centeno-Garcı́a et al.,
1993; Freydier et al., 1997). Two samples of the Late Triassic(?) Sonobari Complex from northern Sinaloa, which may be part of the depositional basement of the northern part of the Guerrero terrane, were
studied here. One is a quartzofeldspathic gneiss having eNd(70 Ma) 523.8
and eSr(70 Ma) 5 153, and the other is an amphibolite with eNd(70 Ma) 5
13.9 and eSr(70 Ma) 5 127, both falling close to the values for the central
and southern granites (Fig. 11).
DISCUSSION
The REE and isotopic compositions of Laramide granitic rocks of
northwestern Mexico show clear north-to-south variations that suggest
that the sources of granitic rocks may vary regularly with geographic
position. This variation can be reproduced by the classical AFC (assimilation–fractional crystallization) modeling proposed by DePaolo
(1981c), using various mixtures of a high-eNd mantle melt enriched in
LREEs (light rare earth elements), together with lower-eNd felsic crust
(as in Farmer and DePaolo, 1983). However, given the compositional
heterogeneity observed in lower-crustal xenoliths from southern Arizona and northern Mexico (see references cited previously), we consider it unlikely that a compositionally uniform crustal end-member
was involved in the generation of the granitic magmas. Lang and Titley
(1998) suggested that the provinciality observed in the isotopic composition of the Laramide igneous rocks from Arizona reflected assimilation, or direct anatectic melting, of lower-crustal lithologies with
varying compositions and ages.
1416
The data do not allow clear choice among the various source possibilities for the granites, but do yield important clues. We surmise that
the REE patterns observed in the northern and central granites could
have been produced via melting of a LREE-enriched mafic to intermediate crust such as that indicated for the Precambrian lower-crustal
xenoliths of northern Mexico (see Roberts and Ruiz, 1989); however,
the involvement of mantle-derived melts cannot be excluded. In addition, plagioclase fractionation plus possible assimilation of more felsic upper-crustal materials may have enhanced the pronounced negative
Eu anomalies and increased the (La/Yb)N ratios observed in the northern granites (Fig. 7). The much flatter REE slopes with almost no Eu
anomalies seen in the southern granites suggest that either the crust
composition beneath the southernmost Sonora and Sinaloa area is notably different or the crust there was derived from mixtures involving
much larger proportions of mantle-derived magma. Also, it is interesting to note that the Jurassic(?)–Cretaceous volcanic and volcaniclastic
arc rocks of the Guerrero terrane display REE slopes that are closely
similar to those of the southern granites (see Centeno-Garcı́a et al.,
1993). This similarity may indicate either that they had a common
source or that the southern granites were produced by anatectic melting
of the Guerrero terrane assemblages.
The Sr and Nd isotope data also support this north-to-south regional
variation. The northern and central granites show initial 87Sr/86Sr and
eNd ranges that are about the same as those observed for the Laramide
granites from southern Arizona, suggesting similar source regions. The
bell-shaped distribution of the Nd data in southern Arizona shows a
lower limit that is slightly more negative, whereas most of the northern
granites and particularly the central granites are restricted to the more
positive side of the eNd bell (Fig. 9), possibly indicating minor but
progressive differences in the crustal structure. Figures 8 and 9 seem
Geological Society of America Bulletin, November 2001
A CHEMICAL AND ISOTOPIC STUDY OF THE LARAMIDE GRANITIC BELT OF NORTHWESTERN MEXICO
Figure 8. Sr isotope histograms for Late Cretaceous and early Tertiary granitic rocks from northwestern Mexico, southern Arizona,
Nevada, and California. Data for northwestern Mexico are from
this paper, Damon et al. (1983a, 1983b), Bagby et al. (1981), Mead
et al. (1988), Wodzicki (1995), and Schaaf et al. (1999). Data for
southern Arizona are from Lang and Titley (1998), Anthony and
Titley (1988), and Farmer and DePaolo (1984). Data for Nevada
and California are from Kistler and Peterman (1973), DePaolo
(1981a), Farmer and DePaolo (1983), and Asmerom et al. (1991).
The Sr compositions of crustal xenoliths from Arizona and northern Mexico, compiled from Ruiz et al. (1988), Esperanca et al.
(1988), Kempton et al. (1990), Rudnick and Cameron (1991), and
Cameron et al. (1992), are also shown. NG, CG, and SG—northern, central, and southern granites, respectively.
to imply that the compositions of granites in the ‘‘miogeoclinal’’ and
‘‘eugeoclinal’’ regions of Nevada and California—which are analogous
to the northern granites and central granites, respectively—display a
sharp break in composition, whereas the northern and central granites
overlap considerably. This difference may suggest that the surface expression of the Paleozoic miogeoclinal-eugeoclinal boundary in central
Sonora (Figs. 1, 2, and 10) does not reflect a major change in the
underlying crustal structure. Near this boundary, however, it seems that
the ocean basin received significant amounts of detritus reworked from
a distant Proterozoic source (Gehrels and Stewart, 1998), minimizing
the fact that the crystalline basement thins south away from the North
American craton. Moreover, this detrital input could also explain that
the studied samples with more negative eNd values occur on or slightly
south of the eugeoclinal-miogeoclinal boundary (Fig. 10).
Strongly peraluminous granites with very negative initial eNd values
of about 210 to 212 in southern Arizona and from 216 to 220 in
Nevada and California (Fig. 9) (Farmer and DePaolo, 1983, 1984) are
Figure 9. Nd isotope histograms for Late Cretaceous and early
Tertiary granitic rocks from southwestern North America, showing
the Nd composition of Precambrian crystalline rocks from Arizona,
Nevada, and California, and samples of deep-crustal xenoliths
from Arizona and northern Mexico. Data for northwestern Mexico
are from this paper, Wodzicki (1995), Schaaf et al. (1999), and
Espinosa (2000). Data for southern Arizona are from Lang and
Titley (1998), Anthony and Titley (1988), and Farmer and DePaolo
(1994). Data for Nevada and California are from DePaolo (1981a),
Farmer and DePaolo (1983), and Asmerom et al. (1991). Data for
Precambrian rocks are from Farmer and DePaolo (1984), Nelson
and DePaolo (1985), and Bennett and DePaolo (1987). Data for the
crustal xenoliths are compiled from Ruiz et al. (1988), Esperanca
et al. (1988), Kempton et al. (1990), Rudnick and Cameron (1991),
and Cameron et al. (1992). NG, CG, and SG—northern, central,
and southern granites, respectively.
considered to have been almost 100% derived from melting of a felsic
crust (Farmer, 1988), and thus they could represent crusts of different
age and/or composition. This possibility is also suggested by the exposed Precambrian rocks, which have eNd compositions concentrated
between 210 and 215 for Arizona and between 215 and 220 for
Nevada and California (Fig. 9). Because our samples are metaluminous
to slightly peraluminous (A/CNK , 1.1, Table 2), no comparison of
Geological Society of America Bulletin, November 2001
1417
VALENCIA-MORENO et al.
Figure 10. Regional Sr and Nd isotope variations for Laramide granitic rocks from southern Arizona across northwestern Mexico plotted vs. distance south of point A in Figure 1.
The Paleozoic miogeoclinal-eugeoclinal boundary has been roughly located as a reference at
;400 km south of A. The diagram shows the
oppositely evolving trends of Sr and Nd, which
suggests a north-to-south variation in which
more evolved signatures to the north are characterized by higher eSr and lower eNd and more
primitive isotope signatures to the south consist of lower eSr and higher initial eNd. Sample
SR-42, which is located near Santa Rosalı́a in
Baja California Sur (Fig. 3), was restored to
an approximate prerifting position. The position of the Precambrian basement edge is highly hypothetical and was configured for initial Sr and eNd values of 0.706 and23.4, respectively (the two dashed vertical lines). Data
sources for labeled localities: Cananea (Wodzicki, 1995), Batopilas (Bagby et al., 1981), Isla Tiburón and Caborca (Schaaf et al., 1999),
Piedras Verdes (Espinosa, 2000), and San Alberto mine (Mead et al., 1988). The isotopic ranges for the southern Arizona granites are
from Farmer and DePaolo (1984), Anthony and Titley (1988), and Lang and Titley (1998). NG, CG, and SG—northern, central, and
southern granites, respectively.
Figure 11. Sr vs. Nd isotope correlation diagram for samples of
Laramide granitic rocks from northwestern Mexico, showing the
fields of the Laramide intrusions from southern Arizona (Farmer
and DePaolo, 1984; Anthony and Titley, 1988; Lang and Titley,
1998) and the Late Cretaceous and Tertiary granites from Nevada
and California plotted according to the type of intruded crust and
denoted as ‘‘miogeoclinal’’ and ‘‘eugeoclinal’’ granites (DePaolo,
1981a; Farmer and DePaolo, 1983; Asmerom et al., 1991). Initial
epsilon values for the Sonobari Complex are corrected to 70 Ma.
Deep-crustal xenoliths were calculated for t 5 0 or 30 Ma according to source references cited in caption to Figures 8 and 9. The
data of the Guerrero terrane rocks are from Centeno-Garcı́a et al.
(1993) and Freydier et al. (1997), calculated for t 5 100 Ma and t
5 110 Ma, respectively. Pacific MORB shown for a depleted-mantle reference was adapted from Hofmann (1997). NG, CG, and
SG—northern, central, and southern granites, respectively.
1418
this type can be made to estimate the Nd isotope characteristics of the
crust in northwestern Mexico. However, one sample from Caborca in
northwestern Sonora (Fig. 1) yielded an initial eNd of about29 (Schaaf
et al., 1999), showing that the Caborca crust possibly resembles the
crust beneath southern Arizona. The main complication here is that
this granite was emplaced into a basement considered to be a crustal
fragment of North America laterally transported southeast, which may
represent part of the Yavapai and/or Mojave provinces (see Fig. 1 and
references in caption). It is worth noting here that the Laramide plutons
we have studied do not show any evidence to support or disprove
transport of this block along the megashear.
The clear correspondence, in both Sr and Nd compositions, of granites from southern Arizona and northwestern Mexico with the lowercrustal xenoliths in young volcanic rocks of the same general region
may suggest a genetic connection. Figures 8 and 9 seem to indicate
that pure melting of the lower-crustal rocks, samples of which occur
as xenoliths, could explain the whole spectrum of Sr and Nd isotope
signatures observed in the Laramide granitic rocks, without direct involvement of a mantle-derived end member. Figures 8 and 9 also suggest that among xenolith populations, mafic granulite and orthogneiss,
but not paragneiss, could represent the lower-crustal materials that produced the isotopic characteristics of the northwestern Mexican granites.
We think that this explanation for the origin of these rocks is the most
likely, but subduction-related mantle melts that partially melted the
lower crust and perhaps subordinately mixed with it could have also
been involved. The presence of an enriched ancient subcontinental lithospheric mantle with high initial 87Sr/86Sr ratios and negative eNd values, as suggested by isotope data in southern Nevada and southern
California (not shown in diagrams) (Farmer et al., 1989; Miller et al.,
2000), is not evident in the xenolith material from young volcanic
rocks of southern Arizona and northern Mexico.
The geochemical and isotopic compositions of the southern granites—characterized by flatter REE slopes, low 87Sr/86Sr ratios averaging
0.705, and mainly positive eNd values—are indicative of a distinct
source for this group of granites. Given these compositions, one possibility is that the southern granites were derived primarily from a
Geological Society of America Bulletin, November 2001
A CHEMICAL AND ISOTOPIC STUDY OF THE LARAMIDE GRANITIC BELT OF NORTHWESTERN MEXICO
LREE-depleted mantle melt with low Sr and high Nd isotope ratios,
which was subsequently contaminated by assimilation of materials
from a thick pile of accreted arc volcanic and volcaniclastic rocks of
the Guerrero terrane and its deep-seated foundations. Another possibility, allowed by the geochemical signatures, is that pure anatectic
melting of Guerrero terrane arc rocks gave rise to the southern granites
(Fig. 11). Actually, it seems unrealistic that melts for the southern
granites were generated at shallow levels, so that the volcanic assemblages themselves are not likely as a source. Instead, parental melts
for the southern granites (and perhaps also for the Guerrero terrane
volcanic arc rocks) could have been mainly generated in deeper parts
of the crust beneath the arc. The basement of the northern part of the
Guerrero terrane is not well known, but exposures in northern Sinaloa
suggest that it may include Paleozoic eugeoclinal and metamorphic
rocks of the Late Triassic Sonobari Complex. Simple inspection of the
Nd versus Sr relationships in Figure 11 suggests that pure mixing of
the two main members of the Sonobari Complex would generate a
reasonable trend for Nd, but because both samples are to the right of
the southern granites, the resulting composition cannot satisfy the Sr
isotope data. Thus, we think that asthenospheric mantle melt, which
partially melted and mixed with a crustal source like the Sonobari
Complex, would generate the Sr and Nd isotope compositions of the
southern granites. However, the southernmost sample of the southern
granites has a very low initial Sr (0.7026) for its eNd value of 11.7.
This finding may suggest either (1) the existence of an unknown crustal
component beneath southern Sinaloa that underwent strong Rb depletion or (2) that the Sr ratio was modified by hydrothermal alteration,
which is feasible because this sample comes from a porphyry copper
locality.
IMPLICATIONS FOR CRUSTAL STRUCTURE AND THE
SOUTHERN EDGE OF THE NORTH AMERICAN
BASEMENT
The nearly regular north-to-south geochemical and isotopic variations observed in the granites of northwestern Mexico could have been
achieved through progressive changes in crustal structure. In northwestern Mexico, the southern edge of the Precambrian basement trends
eastward, and the fringing eugeoclinal basin deepens to the south (Fig.
2), whereas in the western United States, the edge of the Precambrian
basement trends approximately southward and the eugeoclinal facies
lies to the west. In other words, in the United States, the Cordilleran
magmatic arc faced the edge of the North American Precambrian basement in a nearly parallel fashion, as revealed by the isotopic composition of Mesozoic and Tertiary granites of Nevada and California
(Kistler and Peterman, 1973; DePaolo, 1981a, 1984; Farmer and
DePaolo, 1983, 1984). In northwestern Mexico, however, the arc was
at ;908 to the edge of the Precambrian basement. This geometry suggests a considerably different scenario for the evolution of the Mexican
Laramide granitic magmas, a scenario dominated by a Precambrian
basement that becomes progressively thinner to the south until it disappears (Fig. 2). The northern and central granites have isotope characteristics that suggest involvement of a thick Precambrian crust, which
was apparently missing in the source region underlying the southern
granites. Nd(t)DM model ages from 0.9 to 1.1 Ga for the northern and
central granites, and from 0.4 to 0.8 Ga for the southern granites, also
suggest this possibility (Fig. 12, Table 3). However, if mantle-derived
melts were involved, as is more likely the case for the southern granites, juvenile Nd added would have lowered the model ages, making
them of little meaning. Moreover, the Sonobari Complex rocks yielded
Figure 12. Nd model ages (DePaolo, 1981a) of Laramide granitic
rocks from northwestern Mexico relative to the depleted mantle.
The thick solid line represents the depleted-mantle Nd evolution
curve from DePaolo (1981b). The plot shows two distinct ranges
of model ages, between 0.8 and 0.4 Ga for the southern granites
(SG) and an older range between 1.1 and 0.9 Ga for the northern
and central granites (NG and CG, respectively). See key for sample
numbers; locations in Figure 3.
Nd(t)DM model ages close to 1.0 Ga (Table 3), which probably suggests
that they were derived from reworked Proterozoic crust and would
perhaps explain the relatively old model age of 0.8 Ga yielded by one
of the studied samples of southern granites.
An important result of this study is that it provides useful information on the location of the southern edge of the North American Precambrian basement. The actual, tectonically disturbed surface position
of the Paleozoic miogeoclinal-eugeoclinal boundary in central Sonora
certainly indicates the southern limit of the exposures of Precambrian
rocks. However, the data suggest that the Precambrian basement continues for ;200 km farther south beneath the Phanerozoic rock assemblages (Fig. 13). More data are needed to trace this edge more accurately, but on the basis of the 0.706 initial Sr line proposed to contour
the edge of Precambrian continental basement in the southwestern
United States (Fig. 13; Kistler and Peterman, 1973, 1978), this boundary appears to be roughly located in southernmost Sonora near Sinaloa
(Fig. 10). An approximately east-west orientation in southern Sonora
is assumed by analogy to the miogeoclinal-eugeoclinal boundary. On
this basis, the Sr data suggest that the Precambrian continental basement edge extends north of Batopilas (Fig. 13). Then it bends north
somewhere just south of Guaymas (Fig. 1, sample MV-21 in Fig. 3)
and trends northwestward by the latitude of Isla Tiburón (Fig. 13). The
low Sr and relatively high Nd ratios of the sample from Isla Tiburón
(Schaaf et al., 1999) suggest that it lies west of the boundary, which
would imply a more abrupt western termination of the Precambrian
basement than to the south. If sample MV-21 effectively represents the
0.706 Sr line, an eNd value of about 23.4 would possibly indicate the
Nd counterpart for the Precambrian basement edge. Following this assumption, the southern position of this boundary would lie just north
of Navojoa (Fig. 1), on the basis of data from the Piedras Verdes
porphyry copper deposit (Espinosa, 2000) (Fig. 10).
Geological Society of America Bulletin, November 2001
1419
VALENCIA-MORENO et al.
Figure 13. Map of initial Sr ratios of
Laramide granites from northwestern Mexico, showing a hypothetical
location of the edge of Proterozoic
North America, based on the 0.706 Sr
line (see text for discussion). The line
in the central part represents the
boundary between the Paleozoic miogeoclinal (dotted) and eugeoclinal
(dark gray) facies (dashed where inferred). Horizontal lines for a possible displaced block along the Mojave-Sonora megashear (MSM). Lines
ruled down to left for the Guerrero
Terrane. The figure to the right indicates a possible connection with the
boundary of the Proterozoic crust in
western North America (from DePaolo and Farmer, 1984). Bt—Batopilas; Cb—Caborca; Cn—Cananea;
IT—Isla Tiburón; SA—San Alberto
mine. Data from this paper; Wodzicki, 1995 (Cn); Schaaf et al., 1999
(IT and Cb); Mead et al., 1988 (SA);
and Bagby et al., 1981 (Bt).
CONCLUSIONS
Laramide granitic rocks from northwestern Mexico show clear northto-south geochemical and isotopic variations, which can be explained
in terms of the differences in the source region. The granitic rocks can
be divided into a northern group, central group, and a southern group.
Lower eNd values in most northern and central granites correspond to
higher (La/Yb)N ratios, which may be consistent with derivation from
a LREE-enriched low-eNd parental magma possibly generated via melting of Precambrian lower crust of mafic to intermediate composition.
Such a lower crust is suggested by xenoliths in young volcanic rocks
in southern Arizona and northern Mexico. In contrast, the flatter REE
pattern and more primitive isotope compositions of the southern granites are interpreted to have been derived by mixing of LREE-depleted
mantle melts with anatectically melted crust underlying the Guerrero
terrane volcanic arc, which may have included the Late Triassic Sonobari Complex plus Paleozoic eugeoclinal rocks exposed in northern
Sinaloa. If we assume that initial 87Sr/86Sr and eNd values of 0.706
and23.4, respectively, indicate the edge of the North American Precambrian basement, the data provided in this study suggest that this
limit could be located in southernmost Sonora,;200 km south of Hermosillo (Fig. 1).
ACKNOWLEDGMENTS
Most of this contribution is part of Valencia-Moreno’s Ph.D. dissertation.
Funding for this research was provided from scholarship 84734 and project
I29887T granted by Consejo Nacional de Ciencia y Tecnologı́a (CONACYT)
to Valencia-Moreno. We thank the University of Arizona–Mining Industry Mexico Consortium, which provided the financial support for most of the field work
and chemical analyses. We are grateful to the Instituto de Geologı́a of the
Universidad Nacional Autónoma de México (UNAM), especially Dante Morán-
1420
Zenteno, Fernando Ortega-Gutiérrez, and Carlos González-León, for logistical
support through the Estación Regional del Noroeste. We also thank Mark Baker,
Claire Freydier, and Clark Isachsen for assistance with analyses. We express
our appreciation to G. Lang Farmer, Eric W. James, and Richard M. Tosdal for
helpful reviews and discussions that substantially improved the manuscript.
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