Insights into the tectonomagmatic evolution of NW Mexico: Geochronology and geochemistry of the Miocene volcanic rocks from the Pinacate area, Sonora

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691

ABSTRACT

Miocene volcanic rocks in the Pinacate area, Sonora, record a progressive change in the source of magmatism induced by asthe-nospheric upwelling and lithospheric thin-ning. 40Ar/39Ar age data, mineral chemistry, and major- and trace-element contents allow the identifi cation 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 peral-kaline liquids. Sr, Nd, and Pb isotope ratios reveal different sources for the Miocene basalts. The easternmost basalts have signa-tures indicating a Precambrian lithospheric mantle source, while the westernmost tho-leiitic 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-

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 indi-cates 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. Corre-lation 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 magma-tism can be related to the development of a slab window during the Miocene.

Keywords: Volcanism, Mexico, geochronol-ogy, petrology, geochemistry, isotopes.

INTRODUCTION

The Pacifi c coast of northwestern Mexico has been a convergent plate boundary since at least the mid-Cretaceous. In Sonora, subduction-related magmatism is represented by batholitic

granitoids between 90 and 40 Ma toward the west (Damon et al., 1983; Richard et al., 1989; McDow-ell et al., 1997, 2001; Valencia-Moreno et al., 2001). Meanwhile, toward the east, the subduc-tion-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; Dem-ant et al., 1989; Cochemé and Demant, 1991). From Miocene to present, as the Farallon plate fragmented and subduction under North Amer-ica 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 exten-sion has disrupted the Sierra Madre Occiden-tal 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 morphol-ogy (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 Pacifi c–North America plate and the rift system of the Gulf of California ( Atwater, 1989; Stock

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 andPétrologie Magmatique, Université Paul Cézanne (Aix-Marseille 3), Case Courier 441, 13397 Marseille Cedex 20, France

Alain DemantPétrologie Magmatique, Université Paul Cézanne (Aix-Marseille 3), Case Courier 441, 13397 Marseille Cedex 20, France

Francisco A. Paz MorenoDepartamento 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-RiveraEstación Regional del Noroeste, Instituto de Geología, Universidad Nacional Autónoma de México, Apdo. Postal 1039, 83000 Hermosillo, Sonora, México

James K.W. LeeDepartment of Geological Sciences and Geological Engineering, Queen’s University, Kingston, Ontario K7L 3N6, Canada

*E-mail: [email protected] †Deceased

GSA Bulletin; May/June 2008; v. 120; no. 5/6; p. 691–708; doi: 10.1130/B26053.1; 13 fi gures; 6 tables; Data Repository item 2008046.

Vidal-Solano et al.

692 Geological Society of America Bulletin, May/June 2008

Trail

MEXICO (SONORA)

E.U.A. (ARIZONA)

to SL Río Colorado

toSonoyta

to Pto. Peñasco

6

113° 15’

******************************************************************

3520

000

3530

000

290000

32°

00’

113° 15’

31°

45’

31°

45’

Quaternary Pinacate Volcanic Field

Basalt

Rhyolite

Dacite and andesiteRoad

VP 1

7

2 3 4

5

86

PV

HERMOSILLO

Altar desert

12

3

4

5

7

8

8

5 Km

P02-8

2

P03-27

P03-22

JR98-21B

Río

Son

oyt

aJR98-23

JR97-23

JR99-83

JR98-20

P02-15and

P02-20

to Sonoyta

1 outcrop

91-30

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—Vid-rios Viejos; 8—Lomas del Norte; PV—Pinacate Volcano (also called Santa Clara volcano).

Tectonomagmatic evolution of NW Mexico

Geological Society of America Bulletin, May/June 2008 693

and Hodges, 1989; Stock, 2000; Oskin et al., 2001; Oskin and Stock, 2003).

The Pinacate Volcanic Field lies at the north-ern end of the Gulf of California in an arid region that is part of the Altar desert. The vol-canic fi eld is composed of: (1) a Quaternary volcanic shield (Santa Clara volcano of Lynch, 1981); (2) hundreds of scoria or spatter cones covering the fl anks of the shield; (3) well-pre-served 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 (defi ned 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 Qua-ternary 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, dis-cuss their petrogenesis, elucidate the correlation between geochemical signatures and tectonic evo-lution of the western margin of the North Ameri-can Craton, and show that a progressive change in the source of magmatism can be related to the development of a slab window during the Mio-cene, we present this study. New 40Ar/39Ar ages, a summary of the mineral chemistry of the dif-ferent 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 stratig-raphy of the Miocene volcanic rocks (Vidal-Solano, 2001, 2005). According to their mor-phology and petrologic affi nity, three main rock types have been distinguished. (1) Sierra Batamote and Cerro San Pedro are volcanic mesas composed of basalts and basaltic andes-ites; Cerro Picú and Cerro Tres Mosqueteros located eastward are smaller and isolated mafi c outcrops lying directly over crystalline base-ment. (2) Sierra Suvuk and Cerro Ladrilleros correspond to andesitic and dacitic domes and lava fl ows with more rugged shape. (3) Silicic volcanic rocks (rhyolitic domes and pyroclastic fl ow 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 signifi cant obstacle to establishing the stra-tigraphy in the region arises from the fact that the Miocene outcrops are dispersed and not directly in contact due to Late Tertiary exten-sional tectonics and recent covering by alluvial fan deposits and Quaternary sand dunes.

RESULTS

All the analytical methods for Mineral chem-istry, 40Ar/39Ar geochronology, and geochemis-try are described in the GSA Data Repository Appendix A section.1

Petrography and Mineral Chemistry

The three groups of lavas recognized in the fi eld are clearly delimited in the total alkalis-silica diagram (Le Bas et al., 1986; Le Maitre, 1989). The mafi c lavas (group 1) have composi-tions ranging from 48% to 57% silica (Fig. 2). Most of these lavas are olivine to quartz norma-tive basalts or basaltic andesites. Samples from Cerro Picú and Cerro Tres Mosqueteros have higher alkalis (mostly K

2O) and fall therefore

in the fi eld 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, fi ve types of mafi c lavas are distinguished on Figure 2A. The mafi c 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 fi elds on the K

2O versus SiO

2 diagram (Fig. 2B). These lavas

K2

O

40 75706560555045

2

4

10

8

6

Na

2O

+ K

2O

SiO2

R

M

H

DABAB

1

987

65

432

757065605550SiO2

1

2

3

4

5

medium K

high K

Low K

Shoshonites

A B

Figure 2. (A) Total alkalis-silica diagram for the pre-Pinacate volcanic sequences; fi elds 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.

1GSA Data Repository Item 2008046, geochro-nological 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 [email protected].

Vidal-Solano et al.


contain plagioclase and orthopyroxene phe-nocrysts 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 con-tents (Table 1). They plot therefore in an inter-mediate position between the dacites and the rhyolites (Fig. 2A).

The rhyolites (group 3) have high sodium and potassium contents and relatively low alu-mina. As a result, some of them show a peral-kaline signature [with (Na + K)/Al >1] also indicated by the presence of normative acmite. All the high-silica rocks (>72% SiO

2) classify

as comendites (Macdonald, 1974) on the Al2O

3

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.

Basalts and Basaltic AndesitesOlivine in the basalts and basaltic andesites

displays relatively large compositional varia-tions. Fresh olivines (Fo

82 to Fo

75) 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 chromium-bearing spinels are enclosed in olivine phe-nocrysts in the basalts from Cerro Tres Mos-queteros (JR98–30). Microcrysts (<200 µm) from the matrix range in composition from Fo

65 to Fo

44. Plagioclase, the dominant phase,

has compositions in the range An63–38

. Ca-rich phenocrysts (An

75–70) in sample JR97–28 cor-

respond 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 (An

63–60). Clino-

pyroxene displays distinct evolutionary paths on the En-Wo-Fs classifi cation diagram (Mori-moto et al., 1988). The fi rst 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 wol-lastonite 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 dis-tribution of the analyses due to complex sequen-tial growth and sector zoning, a common feature in the subophitic textures (Hall et al., 1986). In

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71

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0.46

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0.26

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84

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1 55

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5 62

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Rb

(ppm

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32

9 25

22

28

50

9

18

14

18

18

S

r 19

5 88

4 83

0 79

1 36

7 42

5 43

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1 12

29

1008

13

40

155

675

690

615

476

266

307

665

398

375

424

310

346

268

322

Co

46

30

27

24

48

49

37

38

35

52

40

35

41

40

35

28

27

28

27

Cu

103

36

42

33

47

21

26

51

52

39

40

33

12

40

30

29

29

31

29

Cr

244

208

121

134

110

54

58

118

157

87

129

85

40

121

116

101

100

107

137

Ni

162

115

63

105

64

30

30

40

65

56

59

31

17

54

58

39

40

48

57

V

196

154

164

123

218

223

231

165

157

325

239

231

258

247

236

158

153

152

161

Zn

98

86

69

77

81

119

120

77

85

90

75

85

96

78

80

63

60

61

63

Zr

28

8 17

3 43

9 12

9 24

0 22

8 24

8 26

4 21

7 17

9 28

6 27

9 19

4 20

0 14

2 14

5 13

6 13

8 Y

31

22

32

27

37

34

38

38

34

29

41

40

31

29

20

21

20

20

Nb

16

8

20

11

18

18

16

17

29

34

28

29

34

34

14

15

15

14

(con

tinue

d)



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 titanomag-netite also crystallizes in JR97–9 and P02–17 samples. Such minerals are typically late crys-tallizing phases in the basalts.

Andesitic and Dacitic LavasThe differentiated rocks are slightly more

porphyritic than the mafi c lavas (5%–10% phe-nocrysts). Plagioclase is by far the most abun-dant mineral either as phenocrysts or as micro-lites 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 An

78 to

An33

, and it is even more calcic (up to An82

) in Cerro Ladrilleros dacites. However, these high-Ca phenocrysts are partly resorbed. Likewise, sieve textures in Na-rich crystals are also evi-dence for disequilibrium. Feldspar in equilib-rium with the host dacitic magma hence has a limited compositional range of An

65 to An

40.

Orthopyroxene is the most common ferro-magnesian phase in the dacitic lavas. Mg-rich orthopyroxene (En

75–58) is observed in the less

evolved dacite (JR98-48) with high Mg-number [100 × Mg/(Mg + Fe)]. In the other dacites, ortho-pyroxene has homogeneous compositions in the range En

61–50 (Fig. 3). In some samples, augite

(Wo44–40

En40–32

Fs18–25

) is also present. Orthopy-roxene-clinopyroxene pairs in equilibrium give crystallization temperatures in the range 1000° to 940 °C (Wells, 1977; Lindsley, 1983). In the other samples, amphibole, which classifi es as pargasite hornblende (Leake, 1978), accompa-nies 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 mem-ber (P02–15 and P02–20), plagioclases are more sodic (An

40 to An

27) than those in the dacites and

orthopyroxene more iron-rich (En51–47

; Fig. 3).

RhyolitesRhyolites of the pre-Pinacate sequences

are almost aphyric. They contain microcrysts (<200 µm) of Na-sanidine (Or

43–52) 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 inter-growths of arfvedsonite (Fe- and Na-rich amphi-bole) and aenigmatite (Deer et al., 1978a). In one

TA

BLE

1. M

AJO

R-

(WT

%)

AN

D T

RA

CE

-ELE

ME

NT

(P

PM

), IN

DU

CT

IVE

LY C

OU

PLE

D

PLA

SM

A–A

TO

MIC

EM

ISS

ION

SP

EC

TR

OS

CO

PY

AN

ALY

SE

S O

F T

HE

PR

E-P

INA

CA

TE

MIO

CE

NE

VO

LCA

NIC

SE

QU

EN

CE

S (

cont

inue

d)

S

ampl

e no

. JR

99-7

8 JR

99-8

0 P

I97-

16

JR98

-48

JR99

-81

JR97

-30

JR98

-20

JR99

-79

JR98

-14

JR97

-1JR

99-8

3P

02-2

0P

02-1

5 JR

99-7

4JR

98-6

8JR

98-2

6JR

98-2

3JR

98-2

5JR

97-1

9JR

99-8

2P

02-8

P02

-11

Loca

lity

S

S

S

S

S

S

S

S

S

L L

L L

P

P

LN

LN

LN

VV

V

V

B

B

SiO

2 (w

t %)

57.4

9 59

.66

61.1

8 64

.15

64.5

4 65

.24

65.5

8 66

.91

67.0

9 67

.79

68.8

6 69

.11

69.9

6 74

.33

76.0

3 71

.97

74.8

3 76

.07

74.2

5 75

.89

74.7

973

.49

TiO

2 0.

98

1.12

0.

80

0.66

0.

67

0.62

0.

58

0.60

0.

60

0.43

0.

37

0.30

0.

31

0.08

0.

03

0.10

0.

10

0.10

0.

46

0.20

0.

19

0.15

A

l 2O3

17.7

9 17

.12

17.7

1 16

.75

16.3

9 15

.35

15.8

0 15

.91

15.6

4 14

.92

15.3

6 14

.23

14.3

1 11

.55

13.0

2 11

.69

12.1

2 12

.53

11.4

1 11

.23

12.8

812

.90

Fe 2

O3

1.75

2.

77

1.47

1.

21

1.55

1.

01

0.85

2.

31

1.89

2.

95

0.83

1.

40

1.55

0.

89

0.99

1.

48

1.15

0.

50

1.55

2.

11

0.84

1.

13

FeO

4.

31

3.31

3.

52

2.35

2.

44

2.76

2.

78

1.55

1.

89

0.27

1.

98

1.58

1.

46

0.56

0.

07

0.15

0.

38

0.98

1.

91

1.43

1.

13

0.51

M

nO

0.11

0.

10

0.09

0.

07

0.07

0.

07

0.07

0.

07

0.07

0.

02

0.05

0.

07

0.06

0.

02

0.04

0.

02

0.02

0.

02

0.06

0.

07

0.03

0.

03

MgO

3.

82

2.59

2.

85

1.68

1.

90

1.46

1.

25

1.09

1.

20

1.24

1.

18

0.35

0.

37

0.03

0.

05

0.10

0.

16

0.05

0.

01

0.08

0.

13

0.12

C

aO

7.11

5.

92

5.72

4.

65

4.69

5.

10

4.12

3.

46

3.71

3.

75

3.20

1.

73

1.98

0.

78

0.93

0.

80

1.04

0.

58

0.42

0.

45

0.74

1.

56

Na 2

O

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.

26

3.33

3.

82

4.33

4.

07

5.35

3.

78

4.14

3.

95

K2O

0.

87

1.23

1.

53

1.54

1.

38

2.00

1.

94

1.94

2.

30

2.35

1.

87

3.06

3.

33

3.16

3.

79

4.83

4.

56

4.80

4.

70

3.65

4.

52

4.34

P

2O5

0.26

0.

31

0.26

0.

21

0.23

0.

20

0.21

0.

20

0.20

0.

13

0.13

0.

08

0.09

0.

03

0.33

0.

03

0.04

0.

04

0.02

0.

05

0.03

0.

04

H2O

+

0.93

1.

50

1.29

1.

34

1.37

1.

98

1.84

1.

53

0.59

0.

84

2.13

0.

16

0.17

4.

18

0.42

3.

61

0.60

0.

28

0.44

0.

27

0.27

0.

16

H2O

- 0.

09

0.12

0.

14

0.92

0.

19

0.08

0.

19

0.11

0.

23

0.31

0.

05

1.97

0.

67

0.24

0.

09

0.23

0.

14

0.06

0.

01

0.02

0.

01

0.83

Tot

al

99.2

4 99

.71

100.

16

99.1

4 99

.04

100.

64

99.8

0 99

.34

99.4

1 99

.65

99.5

0 98

.84

98.8

7 99

.11

99.1

2 98

.83

99.4

7 10

0.08

10

0.59

99

.23

99.7

099

.21

Q

z 10

.13

13.8

5 16

.08

24.2

3 24

.42

17.5

4 20

.30

28.2

9 24

.14

21.9

5 32

.39

24.5

4 24

.77

43.0

1 40

.80

30.7

8 30

.33

32.1

7 29

.31

37.2

3 30

.79

30.6

9 O

r 5.

23

7.40

9.

15

9.38

8.

36

11.9

7 11

.71

11.7

2 13

.77

14.0

8 11

.34

18.6

8 20

.05

19.7

0 22

.69

30.0

1 27

.26

28.4

1 27

.70

21.7

7 26

.83

26.0

8 A

b 32

.11

34.1

3 30

.83

31.5

0 31

.40

40.9

1 39

.69

31.6

7 34

.30

39.9

1 30

.32

41.9

6 39

.76

29.1

1 28

.55

34.0

0 37

.08

34.5

0 32

.45

32.3

0 35

.21

34.0

0 A

n 29

.74

25.7

8 27

.17

22.5

2 22

.46

14.7

7 17

.15

16.3

5 17

.46

13.0

8 15

.52

8.38

8.

68

3.90

2.

71

0.51

0.

17

1.75

1.96

3.

23

4.73

A

c

1.

43

N

s

2.

58

Mg#

57

.89

48.5

9 55

.48

50.8

5 51

.20

45.7

5 42

.75

38.8

8 41

.44

47.3

0 47

.82

20.7

0 21

.53

4.46

9.

92

12.5

0 19

.32

6.89

0.

64

4.84

12

.74

14.2

7 A

I

0.76

0.

74

0.99

1.

00

0.95

1.

22

0.91

0.

91

0.87

Rb

(ppm

) 23

27

32

39

37

43

52

47

47

79

74

16

4 25

8

137

126

116

119

Sr

563

520

690

638

640

485

475

441

469

487

449

170

180

51

12

6 12

7

12

14

64

70

Ba

530

567

823

808

787

811

752

819

854

867

807

832

849

55

132

15

130

39

136

80

734

750

Co

21

19

14

12

12

11

8 9

9 9

7 4

4 2

2 2

1 1

2 3

3 3

Cu

23

18

10

7 7

6 32

4

6 7

4 7

12

12

2 2

2 1

3 3

4 4

Cr

87

23

23

18

18

15

3 6

8 24

14

27

18

3

2 4

4 1

11

4

21

Ni

10

3 14

1

1 3

2 <

1 11

12

1

14

5 2

<1

1 2

3 3

<1

7

V

171

119

86

67

70

49

48

46

51

44

42

14

16

21

67

3 32

2

4 5

6 18

Z

n 59

51

78

63

54

67

66

53

51

62

41

54

50

10

4 63

71

46

58

12

0 10

0 43

30

Z

r 12

6 18

1 14

4 18

5 17

2 21

2 21

5 23

0 21

8 14

5 14

0 32

7 33

4 28

8 18

5 25

7 20

6 21

8 70

8 67

1 22

2 23

7 Y

17

21

16

17

15

22

21

23

21

14

13

29

30

67

17

78

63

65

84

77

30

30

N

b 7

19

7 16

13

13

10

18

11

8

9 20

20

50

16

35

26

27

47

74

17

19

Not

e: A

bbre

viat

ions

: P—

Cer

ro E

l Pic

ú; C

#2—

Roa

d no

. 2; T

M—

Cer

ro T

res

Mos

quet

eros

; SP

—C

erro

San

Ped

ro; B

—S

ierr

a B

atam

ote;

S—

Sie

rra

Suv

uk; L

—C

erro

Lad

rille

ros;

LN

—Lo

mas

del

Nor

te;

VV

—V

idrio

s V

iejo

s. S

ampl

es d

ated

(P

03–2

7 an

d P

03–2

2) c

orre

spon

d to

JR

99–7

4 an

d JR

97–1

9, r

espe

ctiv

ely.

For

sam

ple

loca

litie

s, s

ee V

idal

-Sol

ano

(200

5).

Vidal-Solano et al.


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 + K-feldspar association of the fl ow planes. Quartz is never present as phenocrysts in these lavas. This distinctive mineral association characterizes comendite-type, high-silica rhyolites (Suther-land, 1974; Mahood, 1980).

40Ar/ 39Ar Geochronology

Until now, the chronology of the pre-Pina-cate volcanic successions was only established by fi eld relations (Vidal-Solano et al., 2005).

40Ar/39Ar age determinations have been per-formed to clarify the chronology of the volca-nic sequences. Nine samples (three basalts, two dacites, and four rhyolites) were collected in dif-ferent places from the study area (Fig. 1), and 12 date analyses were obtained from mineral grain

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-heat-ing 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 39Ar

K released, agree within 2σ errors

with the integrated date of the plateau segment. Errors on the age spectrum and isotope-correla-tion 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 yield-ing 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. Sam-ple 91–30 shows a saddle-shape spectra with a maximum date at 20.07 ± 2.17, at the highest power increments, consistent with its correla-tion 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 fl ow 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

40

40

302010

10

20

30

En Fs80706050 90

JR97-23

JR97-28

JR97-9

P02-17

JR99-83

P02-15

P02-20

JR98-20

JR98-48

JR99-79

P02-13A/14

JR99-82

JR97-19/JR98-23/JR98-31

..

•

•

Or25 50 75Ab

An

Cer

ro L

adril

lero

s

Sier

ra S

uvuk

Pera

lkal

ine

rhyo

lites

and

mix

ed ro

cks

10

30

50

70

90

Ab

Sier

ra B

atam

ote

Cer

ro S

an P

edro

Basalts Dacites Rhyolites

A

B

Figure 3. Pyroxene (A) after Morimoto et al. (1988) and feldspar compositions (B) for representative lavas of the pre-Pinacate volcanic sequences.



plateau date at 13.53 ± 1.24 Ma corresponding to 66.5% of the total degassed 39Ar, and its low-temperature (T) steps show a minimum date at 5.27 ± 2.50 Ma. Hornblende from Cerro Ladril-leros (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 fi rst step start-ing 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 repro-ducible 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-defi ned 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 experi-ment 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.

Geochemistry

Most of the samples analyzed are fresh as shown by H

2O 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. How-ever, on the K

2O versus SiO

2 diagram (Fig. 2B),

differences are apparent within the basalt group. A clear shift in the K

2O 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 mafi c 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) dis-criminate four subtypes of mafi c 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 fl at pattern on the mantle-normalized multielement diagram with a pronounced positive peak in Pb. Basalts form-ing the scattered outcrops located at the east-ern 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 fl at, 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, respec-tively. Their REE patterns are slightly enriched

TABLE 2. SELECTED MICROPROBE ANALYSES OF SPECIFIC MINERALS FROM THE

PERALKALINE RHYOLITES FAYALITE Sample no. JR98-23A JR99-82 analysis no. 3 4 10 14 70 71 SiO2 28.97 28.36 28.72 29.03 28.60 28.74FeO 69.45 68.42 69.61 70.20 69.72 69.73MnO 2.17 2.09 2.00 2.03 2.94 2.89 MgO 0.49 0.51 0.57 0.48 0.07 0.04 CaO 0.25 0.26 0.26 0.33 0.29 0.29 TiO2 0.03 0.04 0.03 0.02 0.02 0.04 Total 101.36 99.69 101.18 102.09 101.63 101.72Fa 0.98 0.98 0.98 0.99 0.99 0.99 ARFVEDSONITE Sample no. JR99-82 Analysis no. 66 67 72 74 SiO2 48.64 49.27 48.50 48.39 Al2O3 0.20 0.24 0.25 0.13 FeO 36.82 35.40 37.09 36.75 MnO 1.04 0.98 1.04 1.18 MgO 0.16 0.16 0.05 0.05 CaO 3.01 2.54 2.67 2.66 Na2O 6.84 7.13 6.98 7.08 K2O 1.52 1.41 1.27 1.57 TiO2 0.23 0.26 0.24 0.24 Total 98.45 97.40 98.08 98.03 AEGYRINE FERROHEDENBERGITE Sample no. JR98-31A JR99-82 Analysis no. 51 58 58 59 60 61 SiO2 51.60 52.47 47.30 47.66 47.41 47.32Al2O3 0.18 0.18 0.16 0.16 0.14 0.16 FeO 27.73 27.22 31.85 30.92 31.11 32.14MnO 1.42 0.68 1.00 0.96 1.00 0.97 MgO 0.08 0.05 0.12 0.14 0.14 0.08 CaO 3.32 2.12 16.59 16.76 16.60 15.60Na2O 11.50 12.06 2.20 2.52 2.79 2.80 K2O 0.00 0.04 0.03 0.03 0.04 0.03 TiO2 2.14 3.24 0.57 0.54 0.79 0.53 Total 97.97 98.06 99.82 99.69 100.02 99.63 AENIGMATITE Sample no. JR99-82 Analysis no. 63 65 68 75 76 SiO2 39.81 40.07 40.16 40.49 39.41 Al2O3 0.60 0.61 0.25 0.34 0.61 FeO 43.22 42.10 44.59 41.84 42.57 MnO 0.78 0.90 0.94 0.89 0.78 MgO 0.04 0.03 0.01 0.04 0.05 CaO 0.78 0.65 0.25 0.57 0.82 Na2O 6.60 6.64 6.59 7.10 7.02 K2O 0.02 0.02 0.04 0.07 0.03 TiO2 7.87 8.23 6.55 8.21 8.02 Total 99.75 99.26 99.40 99.54 99.33

TABLE 3. 40AR/39AR RADIOMETRIC AGES OF THE PRE-PINACATE MIOCENE VOLCANIC SEQUENCES Sample no. Rock type Locality Mineral or Laboratory Integrated Error Correlation Error MSWD 40Ar/ 36Ar Error Plateau Error Volume Lat/Long whole rock run date (Ma) 2σ date (Ma) 2σ (initial) 2σ date (Ma) 2σ 39Ar (%)JR98-20 Dacite Sierra Suvuk Pl L705 10.72 1.01 14.93 6.67 1.18 267.26 210.12 13.53 1.24 54.1 31° 45' 37.45'', 113° 20' 59.16'' JR99-83 Dacite Cerro Ladrilleros Hb R714 11.34 1.21 12.02 2.48 0.77 294.87 90.93 12.04 1.37 63.9 31° 44' 8.73'', 113° 18' 0.24'' PO3-27 Obsidian Cerro El Picu Obs R726 14.70 0.15 14.16 1.82 0.61 288.85 73.44 15.30 0.16 58.4 31° 41' 38.08'', 113° 8' 11.29'' JR98-23 Rhyolite Lomas del Norte Wr R715 13.94 0.24 n/a n/a n/a n/a n/a 14.23 0.15 90.7 31° 54' 32.45'', 113° 11' 44.54'' PO3-22 Obsidian Vidrios Viejos Obs R727 13.15 0.58 13.27 71.03 0.07 291.26 2394.39 14.27 0.87 48.8 31° 51' 15.74'', 113° 13' 46.76'' Obs-HCl R756 12.77 0.62 n/a n/a n/a n/a n/a 14.15 1.15 36.6 PO2-8 Rhyolite N Batamote Obs R730 11.75 0.10 12.16 0.08 1.4 270.25 138.27 12.16 0.07 93.1 31° 42' 31.45'', 113° 14' 47.75'' Obs-HCl R743 12.11 0.11 12.30 0.38 4.45 235.36 109.34 12.10 0.10 97.1 Wr R716 12.04 0.08 11.93 1.72 5.01 333.27 1991.22 12.05 0.07 90.3 91-30 Basalt Mesa Norte Pl 712 23.93 7.44 19.87 2.45 1.03 298.91 58.7 20.07 2.17 47.1 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 0.86 42.7 31° 52' 13.24'', 113° 55' 51.81'' JR97-23 Basalt San Pedro Pl R711 16.81 1.59 9.89 9.32 1.69 299.26 421 20.64 1.70 56.7 Lower sequence 31° 44' 24.38'', 113° 14' 12.22''

Vidal-Solano et al.


App

aren

t Age

(M

a)

Integrated date = 16.03 ± 0.70 MaIntegrated date = 14.79 ± 9.06 Ma

Integrated date = 13.94 ± 0.24 Ma

Plateau date = 12.05 ± 0.70 Ma

Integrated date = 11.75 ± 0.10 MaPlateau date = 12.10 ± 0.10 Ma

0

5

10

15

20

Cumulative % 39ArK Released

P02-8 (wr)

Ca/K


0 20 40 60 80 100

App

aren

t Age

(M

a)

0

5

10

15

20

0.1


P02-8 (obs)

Ca/K

0 20 40 60 80 1000

5

10

15

20

0.1


P02-8 (obs-HCl)

Ca/K

0 20 40 60 80 100

Plateau date = 12.16 ± 0.07 MaIntegrated date = 12.11 ± 0.11 Ma

11

0.1

1

0

5

10

15

0.1

1

P03-27 (obs)

Ca/K

Plateau = 15.30 ± 0.16 Maintegrated date = 14.70 ± 0.15 Ma

20 40 60 80

Plateau

10

100

JR99-83 (hb)

Ca/K

Plateau = 12.04 ± 1.37Ma

20 40 60 80

Plateau

JR98-20 (pl)

20 40 60 80

Maximum date = 15.45 ± 1.37 Ma

Ca/K

100

10

Maximum date

0

5

10

15

P03-22 (obs)

Ca/K

Plateau date = 14.25 ± 0.15 Ma

0 20 40 60 80 100

App

aren

t Age

(M

a)

5

10

15

00.1

0.1

JR98-23 (wr)

Ca/K


0 20 40 60 80 100

Maximum date14.27 ± 0.87 Ma

0.1

1

P03-22 (obs-HCl)5

10

15

00.1

0.1Ca/K


0 2 4 6 8 10


10

100

JR97-23 (pl)

Maximum date = 20.64 ± 1.70 Ma0 20 40 60 80 100

Maximum date2

0

5

10

15

20

25

Ca/K Ca/K

0

5

1

1

2

10

100

JR98-21B (Pl)

B


0 20 40 60 80 100

Maximum date

60

40

20

0

-20

-40

10

100

91-30 (Pl)


0 20 40 60 80 100

App

aren

t Age

(M

a)

Maximum date

Ca/K

5

10

15

5

10

15


Integrated date = 11.34 ± 1.21 MaIntegrated date = 11.42 ± 0.92 Ma


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.



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 mul-tielement patterns are relatively fl at 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. Rhyoda-cites 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 pre sent a large negative anomaly in Eu and a fl at HREE pattern. These rocks display spiky trace- element patterns due to marked negative anomalies in Ti-Eu, P-Sr, and less signifi cant 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 deter-mined on 16 samples—eight basalts and eight differentiated lavas (Table 5). The pre-Pinacate lavas display a large degree of isotopic hetero-geneity on the εNd versus 87Sr/86Sr (Fig. 7). The four types of mafi c lavas, defi ned by their chem-istry and multielement patterns, are distributed along the mantle array but plot in quite different fi elds. 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 east-ern 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 iso-topic compositions similar to those inferred for Bulk Silicate Earth (BSE). Lead isotopic com-positions on mafi c rocks show a limited range (Table 6). The higher ratios correspond to the mafi c rocks Types 1 and 2, and the lowest ratio corresponds to the mesa basalts of Sierra Bata-mote (Type 4, sample P02–17).

Dacites and rhyolites have isotope ratios iden-tical to those of the mafi c lavas. Dacites from Sierra Suvuk and Cerro Ladrilleros have simi-lar 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

Type 1 (91-30)Type 3 (San Pedro)Type 4 (Batamote) Type 2 (Tres Mosqueteros)

Roc

k / C

hond

rites

10

2030

10

20

200

100

400

10

20

10

30

100

200

100

LaCe

PrNd

SmEu

GdTb

DyHo

ErTm

YbLu

10

100

Roc

k / P

rimiti

ve m

antle

100

10

10

100

1

RbBa

ThU

NbTa

KLa

CePb

SrP

NdSmZr

Hf EuTi

DyY

HoYb

Lu1

100

10

A B

Figure 5. Chondrite-normalized, rare-earth element (REE) abundances (A) and primitive mantle-normalized trace-element patterns (B) for selected mafi c lavas for the pre-Pinacate sequences. Normalizing values for the REE after Boynton (1984), and from Sun and McDonough (1989) for the incompatible elements.

Vidal-Solano et al.


A B100

10

10

100

1

1

100

10

0.1

1

10

100

200

100

400

100

10

5

20

5

1

10

20

50

LaCe

PrNd

SmEu

GdTb

DyHo

ErTm

YbLu

Roc

k vs

. Prim

itive

man

tle

Roc

k vs

. Cho

ndrit

es

Andesites and dacites

Rhyolites (12 Ma )

Rhyolites (15-14 Ma )

Rhyodacites (P02-15 & P02-20)

Peralkaline rhyolite (JR97-19)

Rhyolite (P02-8)

RbBa

ThU

NbTa

KLa

CePb

SrP

NdSmZr

Hf EuTi

DyY

HoYb

Lu

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.

Mantle A

rray

Dacites

12 Ma Rhyolite (P02-8)

Peralkaline rhyolites (JR97-19)

Rhyodacites (P02-15)

14-15 Ma Rhyolites (JR99-74 and JR98-23)

-10

-5

0

5

10

0.700 0.705 0.710 0.715 0.720 0.725

-3

0

3

6

0.703 0.704 0.705 0.706

DM

BSE

Mantle array

87Sr/86Sr

εNd

0.758

Type 1 (91-30)

Type 4 (Sierra Batamote)

Type 2 (PI97-24 and JR98-21)

BasaltsPinacate volcanic rocks

Type 3 (Cerro San Pedro)Figure 7. εNd versus 87Sr/86Sr isotope diagram for selected pre-Pinacate volcanic rocks. DM—Depleted mantle; BSE—Bulk Silicate Earth from Zindler and Hart (1986); Pinacate volcanic rocks from Lynch et al., 1993.



TA

BLE

4. I

ND

UC

TIV

ELY

CO

UP

LED

PLA

SM

A–A

TO

MIC

EM

ISS

ION

SP

EC

TR

OS

CO

PY

TR

AC

E-E

LEM

EN

T A

NA

LYS

ES

Sam

ple

no.

91-3

0 P

I97-

24 J

R98

-21

JR97

-23

PI9

7-33

JR

97-2

7P

02-1

7JR

98-2

P02

-06

PI9

7-16

JR98

-48

JR98

-20

JR97

-1 J

R99

-83

P02

-20

P02

-15

JR99

-74

JR98

-23

JR98

-25

JR97

-19

JR99

-82

P02

-8P

02-1

1R

ock

type

B

B

B

B

B

A

BA

B

B

B

D

D

D

D

D

D

D

R

R

R

R

R

R

R

Lo

calit

y

P

C#2

S

P

SP

S

P

B

B

B

S

S

S

L L

L L

P

LN

LN

VV

V

V

B

B

Rb

7 38

.6

31.4

26

.2

24.2

46

10

.4

15

15.1

34

36

49

52

51

88

82

18

9 24

3 24

4 14

1 13

7 12

9 12

5 S

r 20

0 95

4 92

6 44

8 46

5 34

6 45

7 55

3 51

9 72

7 66

9 52

2 52

6 48

1 18

2 19

1 47

18

12

13

11

71

73

B

a 10

0 11

05

933

596

599

425

293

612

678

756

779

733

761

841

913

928

11

121

38

131

84

807

805

Co

47.5

28

.0

28.0

31

.0

30.5

26

.0

40.8

28

.5

30.6

11

.5

8.5

7.0

5.0

5.0

2.3

2.7

<0.

5 0.

5 <

0.5

<0.

5 <

0.5

1.6

1.5

Cu

100

15

20

10

10

30

34

25

25

<5

5 <

5 <

5 <

5 5

15

<5

<5

<5

<5

<5

<5

261

Ni

150

110

65

30

20

35

48

35

36

10

5 5

5 5

<5

5 <

5 <

5 <

5 <

5 <

5 23

5

V

220

155

180

240

240

175

358

245

258

95

70

60

55

45

6 10

<

5 40

15

10

5

10

10

Zn

100

90

90

140

115

70

122

110

130

85

60

70

70

50

69

67

120

55

70

120

120

49

40

Zr

72

265

166

231

228

252

224

279

291

138

166

206

130

128

325

336

273

195

197

657

634

223

218

Y

21.5

29

.0

22.5

36

.0

35.5

36

.5

35.2

43

.0

40.8

14

.5

16.5

20

.5

12.5

13

.5

30.2

30

.0

61.5

63

.0

66.0

79

.5

71.5

30

.9

30.9

N

b 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 3.

3 C

s 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 2.

8 1.

7 3.

0 1.

7 0.

8 1.

8 0.

9 T

h <

1 7

4 1

1 4

2 <

1 2

2 1

2 2

1 9

10

7 24

26

15

9

15

15

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

1.6

U

<0.

5 1.

5 1.

0 <

0.5

<0.

5 1.

0 0.

5 <

0.5

0.7

0.4

0.5

0.5

0.5

1.0

2.3

2.3

4.0

6.0

6.0

3.5

0.5

3.1

3.0

Pb

30

10

25

5 5

10

<5

10

5 25

10

20

15

10

11

13

25

35

45

30

15

17

19

H

f 2

7 5

6 6

7 5

7 7

4 4

6 4

4 9

9 11

8

9 18

18

7

7 La

7.

00

69.5

44

.0

27.0

27

.0

27.5

22

.6

35.5

35

.0

21.5

20

.5

24.5

20

.5

18.5

36

.2

38.7

38

.5

62.5

66

.0

75.0

67

.0

45.3

39

.8

Ce

14.5

13

9.5

91.5

57

.5

57.0

58

.5

51.8

78

.5

78.2

43

.0

40.5

48

.5

37.0

34

.5

73.0

73

.8

83.5

12

8.5

133.

5 15

5.0

138.

0 83

.6

81.8

P

r 1.

7 13

.9

9.7

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 8.

2 N

d 8.

00

50.5

37

.0

26.0

26

.5

24.0

27

.6

37.0

38

.5

17.5

18

.0

18.5

13

.0

12.5

25

.7

25.7

35

.0

42.5

45

.5

55.5

54

.0

27.7

25

.4

Sm

2.

5 9.

2 7.

4 6.

4 6.

5 5.

5 6.

8 8.

8 9.

2 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

5.1

Eu

1.00

2.

1 1.

8 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 0.

5 G

d 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 9.

9 10

.5

10.4

13

.7

10.6

5.

4 5.

1 T

b 0.

6 1.

0 0.

9 1.

2 1.

2 1.

0 1.

1 1.

4 1.

3 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 0.

8 D

y 3.

4 4.

8 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

5.2

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

2.1

2.2

2.3

2.9

2.4

1.1

1.1

Er

2.2

2.8

2.5

3.7

3.7

3.9

3.9

4.3

4.5

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

3.4

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 0.

5 Y

b 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.

3 6.

5 6.

7 8.

9 8.

3 3.

3 3.

3 Lu

0.

3 0.

4 0.

3 0.

5 0.

5 0.

6 0.

5 0.

6 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 0.

5

Not

e: A

bbre

viat

ions

—sa

me

as in

Tab

le 1

. For

sam

ple

loca

litie

s, s

ee V

idal

-Sol

ano

(200

5).

TA

BLE

5. S

r A

ND

Nd

ISO

TO

PE

RA

TIO

S O

F S

ELE

CT

ED

SA

MP

LES

FR

OM

TH

E P

RE

-PIN

AC

AT

E M

IOC

EN

E S

EQ

UE

NC

ES

Sam

ple

no.

Typ

e Lo

calit

y S

m

Nd

143 N

d/14

4 Nd

147 S

m/14

4 Nd

εε (N

d)i

R

b S

r 87S

r/86S

r 87R

b/86S

r (87

Sr/

86S

r)i

ε (

Sr)

i

(ppm

) (p

pm)

(p

pm)

(ppm

)

P

I97-

24

K B

A

P

9.2

50.5

0.

5123

34

0.11

2066

-

5.73

38

.6

954

0.70

7255

0.

1128

08

0.70

7226

39

JR

98-2

1 K

BA

no

. 2

7.4

37.0

0.

5123

57

0.11

3029

-

5.56

31

.4

926

0.70

7672

0.

0945

45

0.70

7648

44

.98

JR98

-29

K B

A

TM

8

40.0

0.

5122

72

0.11

3027

-

6.96

35

94

0 0.

7079

97

0.10

3818

0.

7079

70

49.5

6 91

-30

Tho

l B

2,

5 8

0.51

2696

0.

1922

49

1.15

7

200

0.70

5327

0.

0975

63

0.70

5301

11

.68

JR97

-23

Tra

ns th

ol B

S

P

6.4

26

0.51

2682

0.

1514

32

0.97

26

.2

448

0.70

5900

0.

1630

29

0.70

5854

19

.55

PI9

7-33

T

rans

thol

BA

S

P

6.5

26.5

0.

5126

64

0.15

0896

0.

58

24.2

46

5 0.

7059

33

0.14

5079

0.

7059

08

20.1

9 P

02-6

T

rans

thol

B

B

9.2

38.5

0.

5128

85

0.14

7014

4.

89

15.1

51

9 0.

7040

77

0.08

1091

0.

7040

63

-6

P02

-17

Tra

ns th

ol B

B

6.

8 27

.6

0.51

2977

0.

1510

80

6.68

10

.4

457

0.70

3381

0.

0634

24

0.70

3370

-

15.8

4 JR

98-2

0 D

S

4

18.5

0.

5127

78

0.13

3018

3.

02

49

522

0.70

4571

0.

2616

44

0.70

4523

39

JR

97-1

D

L

2.7

13

0.51

2700

0.

1277

72

1.33

51

.8

526

0.70

4610

0.

2744

93

0.70

4559

44

.98

JR99

-83

D

L 2.

6 12

.5

0.51

2708

0.

1279

61

1.47

51

.4

481

0.70

4682

0.

2978

57

0.70

4635

49

.56

P02

-15

RD

L

5.4

25.7

0.

5128

40

0.12

9267

4.

04

81.6

19

1 0.

7049

22

1.19

0849

0.

7047

19

-15

.84

JR99

-74

R

P

9.1

35

0.51

2513

0.

1599

44

-2.

37

164

51

0.76

0333

9.

0124

81

0.75

8541

11

.68

JR98

-23

R

LN

9.4

42.5

0.

5126

00

0.13

6064

-

0.63

24

3 12

0.

7258

92

56.5

6187

0 0.

7146

46

19.5

5 JR

97-1

9 R

V

V

12

55.5

0.

5128

04

0.13

3019

3.

34

137

12

0.71

8326

31

.865

007

0.71

2896

20

.19

P02

-8

R

B

5.5

27.7

0.

5127

64

0.12

2153

2.

57

129

71

0.70

7667

5.

0658

13

0.70

6804

-6

Not

e: A

bbre

viat

ions

: K B

A—

pota

ssiu

m r

ich

basa

ltic

ande

site

; tho

l B—

thol

eiiti

c ba

salt;

Tra

ns th

ol B

—tr

ansi

tiona

l-tho

leiit

ic b

asal

t and

bas

altic

and

esite

; T

rans

alk

B—

tran

sitio

nal-a

lkal

ic b

asal

t; D

—da

cite

; RD

, rhy

odac

ite; R

—rh

yolit

e. O

ther

abb

revi

atio

ns—

sam

e as

in T

able

1. F

or s

ampl

e lo

calit

ies,

se

e V

idal

-Sol

ano

(200

5).

Vidal-Solano et al.


rhyolite (JR97–19) has about the same εNd value but much higher radiogenic Sr (0.7128).

TECTONIC AND PETROGENETIC IMPLICATIONS

Age and Tectonic Signifi cance of the Pre-Pinacate Volcanic Successions

In northwestern Mexico, two basaltic events related to major extensional processes have been recognized. The fi rst one is represented by ca. 30 Ma continental fl ood basalts in the north-ern 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 inter-calated in continental deposits of the Báucarit Formation (Cochemé et al., 1988; Paz-Moreno, 1992; Vidal-Solano, 2005). Until now, the vol-canic evolution of the pre-Pinacate event in the Pinacate Volcanic Field was not well known. In general the stratigraphy of the Miocene volca-nic 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 Mosquet-eros; (2) andesitic and dacitic domes and lava fl ows at the Sierra Suvuk and Cerro Ladrilleros; and (3) silicic volcanic rocks (rhyolitic domes and pyroclastic fl ow deposits) with frequent obsidian facies, in the Lomas del Norte and Vid-rios Viejos areas (Fig. 1). One of the major prob-lems 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 diffi -culty, a 40Ar/39Ar geochronology was obtained to clarify the chronology of the volcanic events.

(1) BasaltsA 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 mafi c outcrops lying directly on

the crystalline basement even though the age spectra (Fig. 4) for the three basaltic samples (91–30, JR98–21, and JR97–23) might be dis-turbed 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 mini-mum 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 volca-nism 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 lat-ter case and therefore consider the maximum 40Ar/39Ar age determinations on the plagioclases to be a good estimate for the onset of the volca-nic activity in the Pinacate area.

(2) DacitesThe dacitic samples dated come from (1) a

plagioclase + two pyroxene lava domes at Sierra Suvuk and (2) a dacitic lava fl ow contain-ing 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 subhori-zontal lava fl ows 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 repre-sented by subhorizontal basaltic lava fl ows, and dikes found at the level of the actual valley, on the southeast fl ank of Sierra Suvuk, represent the latest activity in the area. Although we do not have enough geological evidence, we think that the fi rst step of this sample at 5.27 ± 2.50 Ma could date this event.

The hornblende dated from Cerro Ladril-leros (JR99–83) yields a plateau date of 12.04 ± 1.37 Ma calculated for the last four steps. This

age represents the fi nal volcanic fl uidal phase of activity at Cerro Ladrilleros that locally caps a peralkaline pumice layer that is related to the pyroclastic index level located below the basal-tic mesa north of Cerro San Pedro and between basalt fl ows at Sierra Batamote (Fig. 8A).

(3) RhyolitesThe age of silicic volcanism was con-

strained 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 inter-preted 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-defi ned 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 fi rst steps at 11.10 ± 1.96 Ma and 11.60 ± 1.74 Ma (sam-ples JR98–23 and P03–27, respectively) con-sistent with the age of a later reheating bimodal volcanic event in the area, corresponding to the basalts and rhyolites that crop out on Sierra Bat-amote 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 Bata-mote and Cerro San Pedro, were previously thought to be stratigraphically and geochemi-cally related to the only other ca. 14–15 Ma obsidian outcrop recognized in the area (Lomas del Norte and Vidrios Viejos). Our new concor-dant 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 differ-ent 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 defi ne a new regional stratigraphic marker at ca. 12 Ma due to their widespread dis-tribution across the area.

The geochronological data obtained on representative volcanic samples from the pre-Pinacate sequences lead to the following con-clusions: (1) the oldest volcanic episode is

TABLE 6. LEAD ISOTOPE RATIOS OF SELECTED SAMPLES FROM THE PRE-PINACATE MIOCENE SEQUENCES

Sample no. Type Locality U (ppm)

Pb (ppm)

Th (ppm)

206Pb/204Pb 207Pb/204Pb 208Pb/204Pb

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).



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 Mio-cene (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 repro-ducible 40Ar/39Ar plateau dates at ca. 12 Ma of the rhyolite P02–08 samples. Also, the fi rst 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 fl ank 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 (Vidal-Solano, 2005) was documented; it overlies a poorly sorted detrital succession containing fl oated 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 sedi-mentary 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 Mio-cene 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 defi ned a regional stratigraphic marker at ca. 12 Ma corresponding to the widespread rhy-olitic pumice. A relatively long period of qui-escence (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 basal-tic mesa north of Cerro San Pedro (samples P02–13A and P02–14, Fig. 3), is of particular interest, because it clearly demonstrates mix-ing between the dacitic and peralkaline magmas (Fig. 9). Moreover, the presence of crystal clots with calcic plagioclase, partly resorbed olivine

and clinopyroxene overrun by amphibole, indi-cates that a basaltic magma was also involved and has probably triggered the pyroclastic erup-tion. The regular trend of clinopyroxene compo-sitions 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 mem-ber in equilibrium with the dacite to a more mag-nesian 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. Rhy-olites are enriched in LREE and present a large negative anomaly in Eu and a fl at 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

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).

Vidal-Solano et al.


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 cal-dera 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 docu-mented by similar patterns shared by the rhyoda-cite, 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 LavasAbundances in compatible trace elements

and Mg-numbers in the mafi c 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 fl at REE pat-tern 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 subduc-tion-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

partial melting of a source region in which pla-gioclase 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 fi rst be dis-carded. Because K and P behave incompatibly during fractional crystallization, and the conti-nental crust is potassium rich, the P/K ratio of mafi c rocks plotted against SiO

2 and/or isotopic

compositions is a good indicator of crustal con-tamination (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 refl ect 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 charac-ter of most of the pre-Pinacate lavas, which lie well above the Northern Hemisphere Refer-ence 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 mid-ocean 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 Sr

i

isotope ratio (≥0,707), low εNd values (<−4), and high 208Pb/204Pb and 207Pb/204Pb ratios for any given 206Pb/204Pb. These values are analo-gous to those for the lithospheric mantle-derived early Miocene basalts, well known in southern Nevada and westernmost Arizona, or to the Mio-cene 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 Pro-terozoic basement, composed of the Mojave, Yavapai, and Mazatzal crustal provinces, and the Caborca block. As the region experienced extension during Tertiary time, the North Amer-ican 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 isoto-pic 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-lherzo-lite nodules from the Pinacate volcano record the presence of asthenospheric depleted-mantle source. Quaternary Pinacate basalts, likewise basalts from the southwestern USA, have Sr

i

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 astheno-spheric 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; Hawkes-worth 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

Roc

k vs

. Prim

itive

man

tle

0.1

1

10

100

Rb Ba Th U Nb Ta K La Ce Pb Sr P Nd SmZr Hf Eu Ti Dy HoY Yb Lu

Peralkaline rhyolite JR97-19Rhyodacite P02-20

Basalt JR97-27 Rhyolite P02-8

Rhyolite P02-8

Dacite JR97-1

Dacites Rhyolites

Peralkaline rhyolite JR97-19

Rhyodacites P02-15 and P02-20

Zr

100

300

500

700

Nb

20

60

100

Ba

vs. N

b

90%

80%

70% 60%30%

10 1005

Figure 11. Chemical similarities between the acidic and basic mid-dle Miocene lavas lying north of Sierra Batamote.

Figure 10. Modeling of the rhyodacite as the result of mixing between the dacitic and the peralkaline rhyolitic magmas. Empty trian-gle—dacites; gray triangle—dacite JR97–1; black triangle—rhyodacites (P02–15 and P02–20); empty circle—rhyolites; gray cir-cle—rhyolite P02–8; black circle—peralka-line rhyolite JR97–17.



(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 crystal-lization and assimilation of Precambrian upper crust (AFC process) of parental basalts deriving from different mantle sources.

Calc-Alkaline MagmatismDacitic 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 nega-tive anomalies in Nb-Ta on the multielement diagram indicate a weak subduction component in their mantle source.

Origin of Peralkaline MagmasThe 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; Gasp-aron 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.,

NHRL

NHRL

15.3

15.5

15.7

15.9

37.5

38

38.5

39

18 18.5 19 19.5 20

208 P

b/20

4 Pb

206Pb/204Pb

207 P

b/20

4 Pb

MORB

-8

-4

0

4

8

12

16

0.707

0.702

0.703

0.704

0.705

0.706

EM2

MORB

87S

r/86

Sr

18 18.5 19 19.5 20

206Pb/204Pb

•Nd

0.705

0.707

0.709

0.711

0.713

46 49 52 55 58 61 64 67 70 73 76 79

SiO2

CF

AFC

-20

-15

-10

-5

0

5

10

CF

AFC

87S

r/86

Sr

εNd

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).

Figure 12. Conventional lead isotope diagrams, εNd versus 206Pb/204Pb and 87Sr/86Sr versus 206Pb/204Pb for selected pre-Pina-cate volcanic rocks. Symbols—same as in Figure 7. MORB and EM2 fi elds after Rollinson (1993). Field of data for the middle Mio-cene peralkaline and calc-alka-line 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).

Vidal-Solano et al.


1999) or the remelting at depth of basaltic or gab-broic material (Lowenstern and Mahood, 1991; Bohrson and Reid, 1997). Middle Miocene mafi c and evolved lavas of the pre-Pinacate sequences share common Sr, Nd, and Pb isotopic signa-tures. Rhyolites plot close to the transitional basalts on the εNd versus 206Pb/204Pb diagram, but are displaced toward higher Sr values on the 87Sr/86Sr versus 206Pb/204Pb diagram (Fig. 12). Relatively constant εNd and highly variable Sr isotope ratios show that the opening of the Rb-Sr 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 argu-ment 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 mafi c precursor must be able to generate mag-mas of intermediate compositions, before fi nal 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 signa-tures also refl ect 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 pet-rochemical characteristics clearly different from those of the other Miocene volcanic sequences related to the proto-Gulf of California, thus indi-cating a change in the mantle source.

Furthermore, because the Sonoran peralka-line 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 87Sr/86Sr ratios from peralkaline rhyolites are related to upper crustal Proterozoic contribution, in agreement with the fi nal stage of differentia-tion of these liquids in a shallow magma cham-ber. Decreasing εNd with increasing Sr isotopic

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 rhyo-lites, not refl ecting evolution under open system conditions, but instead a different fractionation path most likely related to a more alkalic basal-tic parent.

Tectonic Signifi canceSome 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 transi-tional to mildly alkali composition. Middle Mio-cene 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 Cali-fornia (Scott et al., 1995; Miller et al., 2000; Per-kins and Nash, 2002), in most cases, coincides with the Sr

i = 0.708 Line (Kistler and Peterman,

1973) and/or the εNd = −7 Line (Farmer and DePaolo, 1983), defi ned as an isotopic bound-ary that marks the western edge of the Precam-brian crystalline basement. Recently, Miller et al. (2000) redefi ned Sr

i = 0.706 Line as the limit

of the Precambrian North American mantle. Pre-Pinacate silicic magmatism (15–12 Ma) consti-tutes 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 litho-sphere and asthenospheric upwelling is required to form peralkaline magmatism. Palinspas-tic reconstruction of the region shows that the middle Miocene volcanism at the Pinacate area coincides closely, in time and space, to the pro-posed incremental expansion of a growing slab window of the Farallon slab gap (Severinghaus and Atwater, 1990; Dickinson, 1997). The set-ting for continental rift magmatism in the Pina-cate area, thus, is constrained by the possibility for a sub-slab mantle to ascend through the slab window. Finally, sporadic emplacement of con-temporaneous calc-alkaline volcanism shows that remnant parts of a subduction-modifi ed, supra-slab mantle persisted during this time.

CONCLUSIONS

Based on age criteria, two volcanic sequences have been identifi ed at the Pinacate area, east of the main Quaternary volcanic fi eld: (1) a lower Miocene basaltic volcanic sequence (ca. 20 Ma) and (2) middle Miocene volcanic sequences (ca. 12–15 Ma) composed of calc-alkaline andes-ites 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 infl uence of the asthenospheric component. This evolution of the isotopic signatures, in space and time, indi-cates 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 Qua-ternary, 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. Iso-tope signatures show that the calc-alkaline dac-ites were differentiated from basalts that in turn were derived from a depleted mantle source only slightly modifi ed 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 rhyo-lites 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 fur-ther 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 litho-sphere 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)



and by a research grant from CONACYT (489100-5-3584-T) to F.A. Paz-Moreno. Sampling and map-ping were carried out from 1997 to 2002 with the fi nancial 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 Sci-ences de la Terre, de l’Environnement et de l’Espace de Montpellier (ISTEEM). École Normale Supérieure de Lyon (ENS Lyon) supported Pb isotope facilities. Fund-ing 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 Engineer-ing Research Council of Canada (NSERC) Master of Fine Arts (MFA) and Discovery grants to J.K.W. Lee.

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Insights into the tectonomagmatic evolution of NW Mexico: Geochronology and geochemistry of the Miocene volcanic rocks from the Pinacate area, Sonora

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