- Centro de Geociencias ::.. UNAM

Journal of Volcanology and Geothermal Research 146 (2005) 284 – 306
www.elsevier.com/locate/jvolgeores
Geology, geochronology and tectonic setting of late Cenozoic
volcanism along the southwestern Gulf of Mexico: The Eastern
Alkaline Province revisited
Luca Ferraria,T, Takahiro Tagamib, Mugihiko Eguchib, Ma. Teresa Orozco-Esquivela,
Chiara M. Petronec, Jorge Jacobo-Albarránd, Margarita López-Martı́neze
a
Centro de Geociencias, Universidad Nacional Autónoma de México, Campus Juriquilla, Qro. Apdo. Postal 1-742,
Centro, 76000 Querétaro, Qro., Mexico
b
Department of Geology and Mineralogy, Division of Earth and Planetary Sciences, Kyoto University, Japan
c
Dipartimento Scienze della Terra, Universitá degli Studi di Firenze, Italy
d
Instituto Mexicano del Petróleo, Mexico D.F., Mexico
e
Departamento de Geologı́a, Centro de Investigación Cientı́fica y Educación Superior de Ensenada, Ensenada, Baja California, Mexico
Received 22 March 2004; received in revised form 10 February 2005; accepted 28 February 2005
Abstract
A NNW-trending belt of alkaline mafic volcanic fields parallels the Gulf of Mexico from the U.S. border southward to
Veracruz state, in eastern Mexico. Previous studies grouped this volcanism into the so-called bEastern Alkaline ProvinceQ (EAP)
and suggested that it resulted from Gulf-parallel extensional faulting migrating from north to south from Oligocene to Present.
On the basis of new geologic studies, forty-nine unspiked K–Ar and two 40Ar–39Ar ages, we propose a new geodynamic model
for the volcanism along the southwestern Gulf of Mexico.
We studied in detail four of the six recognized fields of mafic alkaline volcanism in Veracruz state: 1) The lavas flows of
Tlanchinol area (7.3–5.7 Ma), 2) the Alamo monogenetic field and Sierra de Tantima (7.6–6.6 Ma), 3) the Poza Rica and
Metlatoyuca lava flows (1.6–1.3 Ma) and 4) the Chiconquiaco–Palma Sola area (6.9–3.2 Ma). Other two mafic volcanic fields
may represent the continuation of alkaline volcanism to the southeast: the Middle Miocene lavas at Anegada High, offshore port
of Veracruz, and the Middle to Late Miocene volcanism at the Los Tuxtlas.
The existence of major Neogene extensional faults parallel to the Gulf of Mexico (i.e., ~N–S to NNW–SSE) proposed in
previous works was not confirmed by our geological studies. Elongation of volcanic necks, vent alignment, and faults mapped
by subsurface data trend dominantly NE to ENE and NW to NNW. These directions are parallel to transform and normal faults
that formed during the Late Jurassic opening of the Gulf of Mexico. Ascent of mafic magmas was likely facilitated and
controlled by the existence of these pre-existing basement structures.
T Corresponding author. Tel.: +52 442 238 1104x177; fax: +52 442 238 1129.
E-mail address: luca@geociencias.unam.mx (L. Ferrari).
0377-0273/$ - see front matter D 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jvolgeores.2005.02.004
L. Ferrari et al. / Journal of Volcanology and Geothermal Research 146 (2005) 284–306
285
Coupled with previous studies, our data demonstrate the occurrence of three magmatic episodes in Veracruz: 1) A Middle
Miocene (~15–11 Ma) episode in southern Veracruz (Palma Sola, Anegada, and Los Tuxtlas); 2) A Late Miocene to Early
Pliocene (~7.5–3 Ma) pulse of mafic alkaline volcanism throughout the study region; and 3) A Late Pliocene to Quaternary
transitional to calc–alkaline volcanism in southern Veracruz (Palma Sola, Los Tuxtlas). Whereas the first and third episodes may
be considered part of the subduction-related Trans-Mexican Volcanic Belt, the second pulse of mafic alkaline volcanism has a
more complex origin. The absence of significant extensional faulting precludes a rift origin. We favor a model in which a
transient thermal anomaly and melting of the mantle was triggered by the tearing and detachment of part of the subducted slab.
D 2005 Elsevier B.V. All rights reserved.
Keywords: alkaline volcanism; tectonics; eastern Mexico; Gulf of Mexico; geochronology; late Cenozoic
1. Introduction
In the early days of the plate tectonics theory, the
occurrence of alkaline volcanism was generally
related to intra-plate tectonic settings distinct from
convergent plate boundaries. In recent decades,
however, alkaline volcanism has been recognized in
virtually every tectonic environment, including many
continental volcanic arcs worldwide (see Lange and
Carmichael, 1991, for a review). Alkaline volcanism
at convergent plate boundaries has been inferred to be
associated with slab-induced asthenospheric corner
flow (Toksöz and Bird, 1977), slab-window formation
(Dickinson and Snyder, 1979; Hole et al., 1995), slab
roll-back (Furlong et al., 1982), and combination of
slab windows and mantle plumes (e.g., Abratis and
Worner, 2001).
Cenozoic alkaline volcanism is widespread in
eastern Mexico but its relation with the southern
Mexico subduction zone (Fig. 1) is unclear. A roughly
north–south belt of Tertiary mafic alkaline volcanic
fields runs from the U.S. border to the southern
Veracruz State (Fig. 1), intersecting the subductionrelated Trans-Mexican Volcanic Belt (TMVB) (Ferrari
et al., 1999) in central Veracruz. Spanning over 1500
km in length, this zone of mafic volcanism constitutes
a prominent feature in the geology and geomorphology of eastern Mexico, which otherwise is dominated
by Mesozoic to early Tertiary marine and late Tertiary
nonmarine sedimentary successions. Robin (1976,
1982) defined the belt of mafic alkaline volcanism
as the bEastern Alkaline ProvinceQ (EAP hereafter)
and, based on a number of conventional K–Ar datings
and mostly major elements geochemistry, suggested
that it represented intraplate-type volcanism migrating
from north to south from Oligocene to Present. In the
Robin (1982) model, the EAP would be the result of
Gulf-parallel extensional faulting and would be
unrelated to the subduction of the Cocos plate;
however he reported that during the Pliocene the
products of alkaline volcanism alternated with arcrelated lavas of the eastern TMVB in the Veracruz
region. Subsequent geochemical and isotope studies
questioned this model, at least for the southernmost
part of the EAP. Besch et al. (1988) and LópezInfanzon (1991) show that the Chiconquiaco–Palma
Sola volcanic field has a geochemical imprint of fluids
from the subducting plate. Gómez-Tuena et al. (2003)
provided a more detailed petrologic study of the three
volcanic successions of this area. They interpret the
chemical and isotopic characteristics of the Neogene
volcanism at Palma Sola as controlled by variation in
time of the depth of the subducting Cocos slab.
Similarly Nelson et al. (1995) recognized a variable
subduction signal for the Los Tuxtlas volcanic field,
for which they suggest an analogy with lavas erupted
in back-arc settings in Japan and the Andes. Such data
would indicate that at least part of the EAP could
represent the continuation of the arc volcanism of the
TMVB toward the southeast (Fig. 1). If so, the mere
existence of a single volcanic province would be
under question. However, with the notable exception
of the Los Tuxtlas volcanic field (Nelson and
González-Caver, 1992), after the Robin (1982) synthesis the rest of the EAP has remained largely
unstudied geologically and geochronologically.
In an attempt to better define the time and space
evolution of the mafic alkaline volcanism in eastern
Mexico, and to understand its relation with the
TMVB, we undertook a geologic, geochronologic
and petrologic study of the southern half of this
province. In this paper we summarize the geologic
286
L. Ferrari et al. / Journal of Volcanology and Geothermal Research 146 (2005) 284–306
Fig. 1. Geodynamic map of Mexico showing the alkaline volcanism of eastern Mexico, the Trans-Mexican Volcanic Belt (adapted from Ferrari,
2004), and the present configuration of plates. The Eastern Alkaline Province of Robin (1982), includes the following volcanic fields: Sierra de
San Carlos (SC), Sierra de Tamaulipas (ST), Tlanchinol–Tantima–Alamo (TTA), Chiconquiaco–Palma Sola (CP), Anegada High (AH), and Los
Tuxtlas (LT). PV = Puerto Vallarta; Gdl = Guadalajara.
setting and present an extensive geochronologic study
of the products of mafic volcanism exposed in
Veracruz state. Our new fieldwork, 49 unspiked K–
Ar ages and two 40Ar/39Ar ages, indicate that
volcanism throughout the southern EAP commenced
pene-contemporaneously at ~7 Ma, peaked at different times, and, locally, continued until the Quaternary.
Together with a petrologic study (Orozco-Esquivel et
al., 2003 submitted) these data allow us to propose a
new model for the genesis of the late Miocene to
Pliocene alkaline volcanism in eastern Mexico.
2. Sampling and analytical procedure
Forty-six lavas were dated in an attempt to
elucidate the regional pattern of volcanism and the
main pulses of activity. Five intrusive rocks were
also dated to determine the initiation of magmatism
in the Palma Sola area. Our study focused on areas
without previous reported ages, or where previous
ages seemed to be inconsistent with the stratigraphy.
Tables 1 and 2 list the sampling localities and the
analytical data. Samples were collected from roadcuts or from large blocks broken from lava-flow
scarps using a sledgehammer. Most of the samples
are aphyric and none contains plagioclase phenocrysts. On the basis of visual observation, petrographic studies, and whole-rock water content (LOI),
the freshest rocks were selected for dating. For each
sample, about 100 g of fresh rocks were crushed in a
stainless steel mortar and sieved to 30-60 mesh. The
sieved samples were washed first with deionized
water and then with acetone in an ultrasonic cleaner
L. Ferrari et al. / Journal of Volcanology and Geothermal Research 146 (2005) 284–306
287
Table 1
Locations and K–Ar ages of samples dated in this study
Sample number
Site
Latitude (N)
Longitude (W)
Poza Rica–Metlatoyuca flows
EAP 1
3 km W of Poza Rica airport
EAP 2
Plateau bLa MesaQ, road from Santa Cruz
EAP 4
Road Lazaro Cardenas–Mecapalapa
EAP 6
W of Xicotepec
EAP 8
Lazaro Cardenas plateau
EAP 9
E of Emiliano Zapata
EAP 11
Plateau bLa PitaQ
EAP 12
Plateau bLa PitaQ
EAP 14
Plateau bCacahuatengoQ
EAP 15
Plateau bMetlatoyucaQ
20.595
20.508
20.512
20.315
20.464
20.554
20.902
20.856
20.815
20.807
97.475
97.556
97.822
97.869
97.710
97.519
97.861
97.862
97.902
97.854
58
232
389
562
362
123
116
127
131
227
1.53 F 0.03
1.62 F 0.05
1.31 F 0.03
1.43 F 0.03
1.64 F 0.06
1.53 F 0.08
1.61 F 0.10
1.60 F 0.06
1.39 F 0.14
1.40 F 0.05
Alamo volcanic field and Sierra de Tantima
EAP 10
Cerro Sombrerete
EAP 16
Cerro Cacalote
EAP 17
Neck near La Guasima
EAP 18
Neck south of Tepetzintla
EAP 19
Neck near Tierra Blanca
EAP 20
Cerro El Espinal
EAP 21
Cerro Tlacolula near Zapotal Espinal
EAP 22
Cerro El Cañón
EAP 23
Cerro Moralillo
EAP 24
Neck W of San Felipe
EAP 25
Cerro El Pelón
EAP 26
San Juan Otontepec, Sierra Tantima
EAP 27
S of Tantima village, Sierra Tantima
EAP 28
S of Tantima village, Sierra Tantima
20.902
21.086
21.110
21.152
21.154
21.056
21.040
21.050
21.189
21.162
21.200
21.223
21.324
21.324
97.813
97.826
97.833
97.838
97.920
97.982
97.971
97.913
97.814
97.792
97.856
97.944
97.833
97.833
147
303
308
404
308
149
156
163
239
202
365
473
650
560
7.56 F 0.15
7.02 F 0.16
6.66 F 0.12
6.69 F 0.41
6.74 F 0.14
7.33 F 0.13
7.11 F 0.16
7.32 F 0.15
7.12 F 0.14
7.12 F 0.14
9.04 F 0.16
6.91 F 0.11
6.57 F 0.12
6.75 F 0.09
Tlanchinol flows
EAP 30
EAP 31
EAP 33
EAP 36
21.069
21.075
21.023
20.941
98.253
98.538
98.610
98.687
470
715
1200
1473
2.82 F 0.16
5.72 F 0.13
7.33 F 0.13
7.30 F 0.13
19.620
19.667
19.668
19.660
19.700
19.735
19.811
19.843
19.839
19.777
19.767
19.766
19.934
19.922
19.882
19.670
96.758
96.772
96.775
96.739
96.674
96.667
96.824
96.816
96.703
96.658
96.621
96.559
96.664
96.608
96.590
96.415
1134
1426
1371
1270
1207
1152
1611
1326
367
760
815
585
418
231
586
141
2.04 F 0.04
3.18 F 0.06
3.25 F 0.06
1.97 F 0.04
3.22 F 0.06
14.65 F 0.32
3.37 F 0.06
3.38 F 0.06
15.62 F 0.50
6.93 F 0.16
3.85 F 0.07
3.50 F 0.07
4.03 F 0.07
3.53 F 0.05
3.27 F 0.04
7.48 F 0.13
Plateau Huautla
Plateau Zihuapiltepetl
SW of Huejuetla
S of Tlanchinol
Chiconquiaco–Palma Sola area
EAP 39
Alto de Tı́o Diego
EAP 41
Mafafas–Tepetlan
EAP 42
Mafafas–Tepetlan
EAP 43
Alto Lucero–Enrı́quez
EAP 44
El Madroño
EAP 45
South of Plan de las Hayas
EAP 46
Paz de Enrı́quez
EAP 47
Paz de Enrı́quez
EAP 48
Junique
EAP 49
La Esperanza
EAP 50
Plan de Las Hayas
EAP 51
Rı́o Vado
EAP 52
El Vencedor
EAP 53
Paso del Toro
EAP 54
Rancho NuevoJuan Martı́n
EAP 55
Cerro Cantera (Metates)
Altitude (m)
K–Ar age (Ma)
with error
(continued on next page)
288
L. Ferrari et al. / Journal of Volcanology and Geothermal Research 146 (2005) 284–306
Table 1 (continued)
Sample number
Site
Chiconquiaco–Palma Sola area
EAP 56
Mesa de 24
EAP 57
El Limón
EAP 58
Rı́o Actopan
EAP 59
Plan del Rı́o
to remove adhered fine grains. Phenocrysts were
removed from the fraction using a Frantz isodynamic
magnetic separator and handpicking to minimize the
possible influence of excess argon (Takaoka, 1989;
Matsumoto et al., 1989). An aliquot of each sample
was ground further by an automatic agate mortar for
potassium analysis.
Potassium concentrations were measured by flame
emission photometry, with lithium internal standard
and peak integration method (Matsumoto, 1989).
Analysis was performed at the Kyoto University
geochronology laboratory using a FP-33D flame
emission photometer, made by Hekisa Kagaku Co.,
Japan. About 100 mg of sample was used for each
analysis. See Matsumoto (1989) for the details of
handling procedures.
Precision and accuracy of the potassium determinations was established through the analysis, under
identical analytical conditions, of Geological Survey
of Japan (GSJ) standards JA-2 and JB-3 (Ando et al.,
1989; Matsumoto, 1989). The estimated precision and
accuracy of the sample data is thus ~1% (1 sigma),
which is propagated to determine the analytical
uncertainty of the K–Ar age.
Argon isotopic measurements were performed by
peak-height comparison method (Matsumoto et al.,
1989; Sudo et al., 1996). Analysis was made at the
Kyoto University geochronology laboratory with a VG
3600 mass spectrometer operated in the static mode,
connected to extraction and purification lines made by
Ayumi Co. Ltd., Japan. The extraction and analysis
system as well as the techniques used for argon
isotopic measurements are described by Sudo et al.
(1996). The standard air, calibrated using Sori 93
biotite standard (Sudo et al., 1998), and hot blank were
measured periodically during the experiments for
system sensitivity, mass discrimination and background corrections. To obtain the accurate content of
radiogenic 40Ar, we applied a mass-fractionation
Latitude (N)
Longitude (W)
19.712
19.646
19.497
19.402
96.547
96.540
96.583
96.647
Altitude (m)
564
429
222
342
K–Ar age (Ma)
with error
3.51 F 0.06
11.07 F 0.20
1.92 F 0.18
2.23 F 0.05
correction procedure (Matsumoto and Kobayashi,
1995) in cases where the uncorrected value deviated
from the actual value by N1%. If single-stage
fractionation is assumed, the difference between
corrected and uncorrected ages of a sample is less
than 1% when the air contamination is less than 16%.
Sample C-1 was analyzed by the conventional
K–Ar method in 1990 in the Instituto Mexicano del
Petroleo, Mexico city by J. Jacobo-Albarrán and the
result of the experiment is reported in Table 3.
All K–Ar ages were calculated using the isotopic
abundances and decay constants for 40K recommended by the IUGS Subcommission on Geochronology
(Steiger and Jäger, 1977; k e = 0.581 10 10 y 1,
k h = 4.962 10 10 y 1, 40K / K = 1.167 10 4).
Samples PS-99-21 and LH 1718 were dated by the
40
Ar–39Ar method at CICESE’s geochronology laboratory using a MS-10 mass spectrometer; details on
the methodology are given in Ferrari et al. (2002). The
samples were analyzed in duplicate using the stepheating technique. Five to six fractions were collected
between 950 and 1350 8C. The results are given in
Table 4 and Fig. 6. For sample PS-99-21, a
plagioclase concentrate was analyzed. Due to problems with the data-acquisition system, the first two
fractions collected on sample PS-99-21 (labeled 1 and
2 in the age spectra) are not considered reliable (see
Fig. 6). Nonetheless, the results of the duplicate
experiments yielded reproducible results with integrated ages of 12.5 F 1.6 Ma and 12.1 F 0.6 Ma. The
slightly bUQ-shaped age spectrum suggests the presence of excess argon, which is confirmed by the
isochron age of 10.9 F 0.8 Ma calculated from the
36
Ar/40Ar vs. 39Ar/40Ar correlation diagram with the
combined fractions of the duplicate experiments,
excluding fractions 1 and 2. We then take the isochron
age as our best estimate for the age of sample PS-9921. For sample LH 1718, two experiments were
performed using a biotite concentrate, and these
L. Ferrari et al. / Journal of Volcanology and Geothermal Research 146 (2005) 284–306
289
Table 2
Analytical data of K–Ar ages obtained with the unspiked sensitivity method at the Kyoto University geochronology laboratory
Sample
number
Weight
(g)
K2O
(wt.%)
Poza Rica and Metlatoyuca flows
EAP 1-3
3.8634
0.87
EAP 2-2
1.5552
1.24
EAP 4-3
3.0549
0.99
EAP 6-2
3.0000
1.00
EAP 8-2
1.5060
1.25
EAP 9-2
1.5237
0.80
EAP 11
0.7585
0.91
EAP 12-2
1.6327
0.89
EAP 14-2
1.4849
0.87
EAP 15-2
2.2867
0.86
38
Ar/36Ar
0.1866 F 0.0006
0.1880 F 0.0007
0.1855 F 0.0008
0.1866 F 0.0007
0.1883 F 0.0006
0.1870 F 0.0008
0.1873 F 0.0006
0.1859 F 0.0006
0.1880 F 0.0006
0.1878 F 0.0006
40
Ar/36Ar
40
Ar/36Ar
initial
40
Ar rad
(10 8 cm3 STP/g)
40
Ar atm
(%)
K–Ar age
(Ma)
561.1 F 0.9
380.2 F 0.6
426.2 F 0.7
455.2 F 0.7
367.8 F 0.6
349.1 F 0.6
329.4 F 0.5
352.2 F 0.6
319.3 F 0.5
362.8 F 0.6
294.6 F 1.9
298.8 F 2.2
291.2 F 2.5
294.6 F 2.2
299.8 F 1.9
295.8 F 2.6
296.9 F 1.9
292.5 F 1.9
299.0 F 1.9
298.3 F 1.9
4.290 F 0.072
6.485 F 0.206
4.199 F 0.101
4.595 F 0.094
6.647 F 0.217
3.966 F 0.209
4.723 F 0.294
4.599 F 0.166
3.903 F 0.383
3.865 F 0.132
52.5
78.6
68.3
64.7
81.5
84.7
90.1
83.0
93.6
82.2
1.53 F 0.03
1.62 F 0.05
1.31 F 0.03
1.43 F 0.03
1.64 F 0.06
1.53 F 0.08
1.61 F 0.10
1.60 F 0.06
1.39 F 0.14
1.40 F 0.05
582.3 F 0.9
443.2 F 0.7
877.7 F 2.3
332.6 F 0.5
468.2 F 0.7
739.4 F 1.2
486.1 F 0.8
514.9 F 0.8
519.4 F 0.8
537.6 F 0.9
419.5 F 0.7
416.1 F 0.7
294.1 F1.9
293.1 F1.9
295.5
298.1 F 2.0
291.6 F 1.9
295.5
298.2 F 2.4
298.9 F 1.9
292.8 F 1.9
295.2 F 1.9
295.1 F1.9
297.9 F 1.9
34.791 F 0.573
28.734 F 0.575
32.670 F 0.492
42.453 F 2.559
20.310 F 0.378
41.570 F 0.625
30.764 F 0.611
37.887 F 0.666
27.030 F 0.468
38.445 F 0.658
45.800 F 0.997
46.584 F 1.045
50.5
66.1
33.7
89.6
62.3
40.0
61.3
58.0
56.4
54.9
70.3
71.6
458.1 F 0.7
532.5 F 0.9
296.3 F 2.3
299.2 F 3.6
23.509 F 0.477
23.379 F 0.488
62.9
54.3
674.3 F 1.3
555.0 F 0.9
634.6 F 1.1
295.5
290.662 F 2.6
295.5
22.409 F 0.338
34.877 F 0.613
34.669 F 0.524
43.8
50.5
44.7
7.56 F 0.15
7.02 F 0.16
6.66 F 0.12
6.69 F 0.41
6.74 F 0.14
7.33 F 0.13
7.11 F 0.16
7.32 F 0.15
7.12 F 0.14
7.12 F 0.14
8.97 F 0.21
9.12 F 0.22
9.04 F 0.16
6.93 F 0.16
6.89 F 0.16
6.91 F 0.11
6.57 F 0.12
6.77 F 0.14
6.73 F 0.12
6.75 F 0.09
0.1866 F 0.0006
0.1875 F 0.0006
0.1869 F 0.0006
0.1869 F 0.0009
332.0 F 0.7
441.2 F 0.7
1391.2 F 2.2
788.0 F 1.4
294.6 F 1.9
297.4 F 1.9
295.5
295.5
8.761 F 0.486
18.335 F 0.373
27.943 F 0.419
39.249 F 0.590
88.7
67.4
21.2
37.5
2.82 F 0.16
5.72 F 0.13
7.33 F 0.13
7.30 F 0.13
Chiconquiaco–Palma Sola area
Intrusive rocks
EAP45
0.5311
1.19
EAP48
0.2580
1.26
EAP57
0.5114
2.13
0.1856 F 0.0008
0.1859 F 0.0006
0.1867 F 0.0011
517.5 F 1.4
367.0 F 0.6
1093.4 F 2.6
291.6 F 2.6
292.4 F 1.9
295.5
56.648 F 1.084
64.052 F 1.934
76.256 F 1.146
56.4
79.7
27.0
14.65 F 0.32
15.62 F 0.50
11.07 F 0.20
Chiconquiaco plateau
EAP39
2.2640
EAP41
1.5412
EAP42
1.5096
EAP43
1.5580
0.1844 F 0.0006
0.1842 F 0.0009
0.1865 F 0.0014
0.1849 F 0.0006
1393.7 F 4.2
1247.5 F 3.1
1090.4 F 2.3
548.9 F 0.9
295.5
295.5
295.5
289.4 F 2.1
22.951 F 0.345
26.760 F 0.402
21.654 F 0.325
14.346 F 0.245
21.2
23.7
27.1
52.7
2.04 F 0.04
3.18 F 0.06
3.25 F 0.06
1.97 F 0.04
Alamo volcanic field and Sierra de Tantima
EAP 10
0.7454
1.42
0.1864 F 0.0006
EAP 16
0.7704
1.27
0.1861 F 0.0006
EAP 17
0.7899
1.52
0.1875 F 0.0009
EAP 18-3
0.1642
1.96
0.1878 F 0.0006
EAP 19-2
1.5342
0.93
0.1856 F 0.0006
EAP 20-2
0.7748
1.75
0.1881 F 0.0007
EAP 21
0.7526
1.34
0.1878 F 0.0007
EAP 22-2
0.7708
1.60
0.1880 F 0.0006
EAP 23
0.7639
1.17
0.1860 F 0.0006
EAP 24-2
0.7416
1.67
0.1868 F 0.0006
EAP 25-2
0.5004
1.58
0.1868 F 0.0006
EAP 25-3
0.5096
1.58
0.1877 F 0.0006
Weighted mean
EAP 26-2
0.7524
1.05
0.1878 F 0.0006
EAP 26-3
0.8062
1.05
0.1876 F 0.0008
Weighted mean
EAP 27-2
1.2206
1.06
0.1860 F 0.0009
EAP 28-2
0.8074
1.59
0.1860 F 0.0006
EAP 28-3
0.8014
1.59
0.1836 F 0.0008
Weighted mean
Tlanchinol flows
EAP 30-2
0.5021
EAP 31
1.5292
EAP 33
1.5183
EAP 36-2
0.7924
0.96
0.99
1.18
1.66
3.49
2.61
2.06
2.20
(continued on next page)
290
L. Ferrari et al. / Journal of Volcanology and Geothermal Research 146 (2005) 284–306
Table 2 (continued)
Sample
number
Weight
(g)
Chiconquiaco plateau
EAP44
1.5655
EAP46
1.5550
EAP47
1.5525
EAP49
0.7649
EAP50
1.5932
EAP51
1.5079
EAP52
0.7760
EAP53
1.5378
EAP53
2.2857
Weighted mean
EAP54
2.3118
EAP54
3.0552
Weighted mean
EAP55
0.7567
EAP56
1.5621
EAP58
0.7976
EAP59
1.5277
K2 O
(wt.%)
38
Ar/36Ar
40
Ar/36Ar
1.26
1.27
1.60
1.45
1.62
1.20
2.02
1.05
1.05
0.1854 F 0.0013
0.1859 F 0.0014
0.1837 F 0.0015
0.1867 F 0.0006
0.1828 F 0.0014
0.1832 F 0.0007
0.1869 F 0.0020
0.1846 F 0.0016
0.1868 F 0.0012
881.2 F 2.4
737.1 F1.4
1055.9 F 2.1
442.1 F 0.7
1197.8 F 2.2
505.5 F 0.8
1129.2 F 3.9
888.4 F 2.4
881.3 F 1.6
0.86
0.86
0.1855 F 0.0008
0.1874 F 0.0007
2.63
1.30
0.67
2.82
0.1850 F 0.0006
0.1850 F 0.0016
0.1867 F 0.0006
0.1841 F 0.0006
40
Ar/36Ar
initial
40
Ar rad
(10 8 cm3 STP/g)
40
Ar atm
(%)
295.5
295.5
295.5
294.9 F 2
295.5
284.3 F 2.2
295.5
295.5
295.5
13.165 F 0.198
13.817 F 0.208
17.456 F 0.262
32.435 F 0.669
20.167 F 0.303
13.534 F 0.244
26.353 F 0.397
11.975 F 0.180
12.019 F 0.181
33.5
40.1
28.0
66.7
24.7
56.2
26.2
33.3
33.5
797.2 F 2.5
777.4 F 1.7
295.5
295.5
9.006 F 0.136
9.185 F 0.138
37.1
38.0
821.1 F1.3
922.7 F 2.5
316.6 F 0.7
425.9 F 0.7
295.5
295.5
294.8 F 1.9
286.9 F 1.9
63.674 F 0.957
14.685 F 0.221
4.155 F 0.392
20.239 F 0.415
36.0
32.0
93.1
67.4
Analytical error reported as 1r. bWeightQ indicates the amount of rock powder used for
yielded reproducible results, with statistically undistinguishable plateau ages defined by more than 90%
of the 39Ar released. The 37ArCa/39ArK diagram
mimics the bUQ-shaped age spectra. Our best age
estimate for sample LH 1718 is 4.0 F 0.1 Ma, taken
from the isochron age calculated with the combined
fractions of the two experiments performed with this
sample.
3. Geologic setting and geochronological results
In this section, we provide a geologic description
of the volcanic fields that compose the southern EAP
within the context of the ages obtained in this work as
well as those published previously. The new geochronological data are presented in Tables 1 and 2.
Major-element analyses were used to characterize the
chemical composition of the dated lavas (Fig. 2). The
volcanic fields are described below from north to
south.
K–Ar age
(Ma)
3.22 F 0.06
3.37 F 0.06
3.38 F 0.06
6.93 F 0.16
3.85 F 0.07
3.50 F 0.07
4.03 F 0.07
3.52 F 0.06
3.54 F 0.06
3.53 F 0.05
3.24 F 0.06
3.30 F 0.06
3.27 F 0.04
7.48 F 0.13
3.51 F 0.06
1.92 F 0.18
2.23 F 0.05
40
Ar measurements.
3.1. Sierra de Tantima and Alamo monogenetic
volcanic field
Sierra de Tantima is a 19-km long, 5-km wide, and
1320 m-high mountain with a marked NE alignment
that rises from the coastal plain of the Gulf of Mexico
(Fig. 3). At its center, it consists of a 700 m-thick
succession of mafic lavas flows with negligible dip
that cover early Tertiary sandstones and shales. The
flows are typically 2–10 m thick but, toward the top of
the succession, they can attain a thickness of up to 20
m. Lavas are aphyric to microporphyritic, olivine,
clinopyroxene and plagioclase being the most abundant minerals. Compositionally, they range from
basanite to hawaiite (Fig. 2). No ages were previously
available for this volcanic edifice, so we dated three
samples (EAP 26, 27 and 28) by the K–Ar unspiked
method. Ages range from 6.91 F 0.11 to 6.57 F 0.12
Ma (Table 2). Sample EAP 26 (6.91 F 0.11 Ma)
comes from the lower part of the succession in the
southwestern part of Sierra de Tantima. The other two
Table 3
Analytical data of K–Ar age obtained for core N-1 at well C-1 at Instituto Mexicano del Petróleo
Location
Offshore Villa Zempoala
Average % K
3.25
40
Ar* (mol/g)
4.14 10
11
40
K (mol/g)
8
9.73 10
The well is located east of Villa Zempoala (Fig. 5) at 96.3208 W, 19.3728 N.
40
Ar*/40K
% Rad
Age (Ma)
4.25 E 4
49
7.3 F 0.73
L. Ferrari et al. / Journal of Volcanology and Geothermal Research 146 (2005) 284–306
291
Table 4
40
Ar–39Ar results for samples from the Chiconquiaco–Palma area
Sample
t i(Ma)
t p(Ma)
t c(Ma)
(40Ar/36Ar)i MSWD/n J 10 3
PS 99-21
12.5 F 1.6
plagioclase
10 F 2
310 F 14
0.02 / 5
PS 99-21
12.1 F 0.6
plagioclase
11.0 F 0.9
304 F 6
0.04 / 5
10.9 F 0.8y 305 F 5
4.2 F 0.2 304 F 4
0.07 / 8
0.06 / 6
LH 1718
biotite
4.5 F 0.2 4.3 F 0.2
LH 1718
biotite
4.3 F 0.2 4.1 F 0.1
4.0 F 0.1
308 F 14
4.0 F 0.1y 307 F 3
0.08 / 6
Temp %
(8C)
39
4.97 F 0.03
950z
1000z
1150
1250
1350
4.97 F 0.03 850
1000
1150
1250
1350
21.3
12.9
21.4
28.7
15.7
13.0
18.7
23.0
29.5
15.8
2.07 F 0.04
1.8
35.2
19.9
22.1
19.7
1.3
3.5
35.3
23.8
32.1
4.7
0.6
850
1050
1125
1175
1225
1350
2.07 F 0.04 900
1050
1125
1175
1225
1350
Ar t(Ma)
37
ArCa / %
ArK
40
Aratm
39
14 F 5
12 F 6
11 F 2
12 F 2
15 F 3
14 F 2
11.8 F 1.2
11.3 F 1.0
11.8 F 0.8
12.7 F 1.7
10 F 4
4.5 F 0.2§
4.3 F 0.3§
4.3 F 0.3§
4.3 F 0.3§
11 F 5
8.8 F 1.6
4.2 F 0.2§
4.1 F 0.2§
4.0 F 0.2§
4.7 F 1.1§
6F9
9.4
5.8
2.8
3.7
9.8
9.6
7.8
2.9
3.8
9.4
84.9
70.7
73.6
75.4
87.7
88.9
68.8
66.6
68.0
82.4
2.0
0.09
0.06
0.04
0.24
6.3
1.4
0.07
0.06
0.04
0.9
1.3
94.0
66.1
40.8
27.7
38.1
96.2
92.8
62.3
36.7
25.1
52.8
95.4
0.21 / 12
PS 99-21: Candelaria gabbro (96.5218 W, 19.7928 N); LH 1718: diorite recovered from at about 1700 m below the surface in a well near Plan de
las Hayas (Fig. 5).
Analyses performed at CICESE’s geochronology laboratory, Mexico.
All errors are 1r. The abbreviations are: plg = plagioclase; bio = biotite; t i = integrated age; t p = plateau age with the error in J included;
t c = isochron age calculated from the 36Ar/40Ar vs 39 Ar/40 Ar correlation diagram; y = isochron age calculated with the combined fractions of the
duplicate experiments performed; z = fraction ignored in the isochron calculation (see text); §fractions used to calculate t p. Best age estimate is
given in bold typeface. The decay constants used, are those recommended by the IUGS (Steiger and Jäger, 1977). Formulae given in York et al.
(2004) was used to obtain the best straight line for the isochron calculation and its parameters.
samples (EAP 27 and 28) are stratigraphically higher
and yielded slightly younger ages (Table 2). The
narrow age range is consistent with the absence of
erosional discontinuity and/or paleosoils within the
lava flows.
The Alamo volcanic field is composed of at least
40 monogenetic volcanoes that surround Sierra de
Tantima (Fig. 3 and Table 5). Because of erosion, lava
flows are rarely preserved and a neck of massive lava
(volcanic vent) is the only evidence of the volcanic
structure. These volcanic necks have elevations
ranging from 100 to 300 m. Modal composition of
these rocks is very similar to the Sierra de Tantima
samples. However, they show a greater variation in
chemical composition, ranging from basanite to
phonotephrite (Fig. 2). Samples from ten necks
yielded ages that cluster in two time intervals:
6.66 F 0.12 to 6.74 F 14 Ma (samples EAP 17–19)
and 7.02 F 0.16 to 7.56 F 0.15 Ma (samples EAP 10,
EAP 16 and EAP 20–24) (Table 2). A neck close to
the southern edge of Sierra de Tantima yielded an
older age of 9.04 F 0.16 Ma (weighted mean of
duplicate experiments, sample EAP 25 in Table 2).
No radiometric ages were available previously for
this region. With the exception of the 9 Ma old neck
our ages clearly point to a relatively short volcanic
pulse spanning less than 900 ka. Initially, the volcanic
activity was weak and scattered, resulting in the
development of the Alamo monogenetic field. Later,
the output rate of volcanism apparently increased and
was able to sustain also more voluminous fissure
eruptions that eventually built Sierra de Tantima.
292
L. Ferrari et al. / Journal of Volcanology and Geothermal Research 146 (2005) 284–306
10
5
Quaternary
Poza Rica flows
8
Na2O + K2O wt.%
4
6
1
3
e
alin
Alk aline
k
l
a
Sub
Late Miocene – early Pliocene
Chiconquiaco plateau
4
Late Miocene
Alamo volcanic field
2
Sierra de Tantima
Tlanchinol flows
2
0
40
Late Pliocene
Chiconquiaco plateau
45
50
55
SiO2 wt%
Fig. 2. Total alkali–silica classification for the dated rocks (after Le Bas et al., 1986). Line dividing subalkaline and alkaline fields from Irvine
and Baragar (1971). Labels in fields are 1: Basanite; 2: Basalt; 3: Hawaiite/potassic trachybasalt; 4: Mugearite/shoshonite. The Late Pliocene
samples, with Na2O 2 b K2O, belong to the shoshonitic series (potassic trachybasalt and shoshonite). Major element analyses performed by
XRF by R. Lozano at Instituto de Geologı́a, UNAM, Mexico City.
Fig. 3. Geologic map of the Tlanchinol–Tantima–Alamo area showing the location and age of previous and new isotopically dated rocks.
L. Ferrari et al. / Journal of Volcanology and Geothermal Research 146 (2005) 284–306
293
Table 5
Vent elongation in the Alamo volcanic field
Name of vent
Latitude (N)
Longitude (W)
Cerro Las Borrachas
Cerro Quebracho
Cerro Pelón
Cerro El Borracho
Cerro El Trueno
Ojo de Brea
Cerro El Pelón
Neck Cerro Moralillo
W of San Felipe
Near Tierra Blanca
South of Tepetzintla
N of Loma El Repartidero
Near La Guasima
Cerro Tepenecuile W
Loma El Repartidero
Cerro Cacalote
Cerro Tepenecuile E
Near Tenango
Near La Mesilla
Cerro Artı́culo
Cerro La Cuatro
Cerro El Espinal
Cerro el Cañón
Cerro Chapopotal
Cerro Tlacolula
Cerro El Tepetate
Cerro La Mesa
S of Cerro Viejo
Cerro del Plumaje
Cerro Xococatl
Cerro Ayacaxtle
Cerro Tepecxtla
Cerro La Cruz
Cerro Aguacatepec
Cerro Tepoxteco
Cerro Ixcacuatitla
Cerro Sombrerete
Cerro Tzohuacale
Cerro Mirador
21.742
21.443
21.388
21.385
21.371
21.308
21.200
21.189
21.162
21.154
21.152
21.150
21.110
21.090
21.089
21.086
21.085
21.083
21.080
21.075
21.063
21.056
21.050
21.045
21.040
21.040
21.033
21.003
20.983
20.974
20.962
20.946
20.937
20.936
20.922
20.908
20.902
20.896
20.892
97.074
97.792
97.743
97.750
97.762
97.716
97.856
97.814
97.792
97.920
97.838
97.929
97.833
97.901
97.933
97.826
97.905
97.841
97.895
97.785
97.707
97.982
97.913
97.712
97.971
97.898
97.920
97.733
97.774
97.975
98.015
98.038
97.700
97.929
98.066
98.032
97.813
98.045
97.705
3.2. Tlanchinol flows
To the west of the Alamo volcanic field, the Sierra
Madre Oriental (SMOr) consists of a thick marine
succession of Mesozoic age involved in regional
NNW-trending folds and thrusts of Laramide age (Fig.
1). Several massive lava flows are exposed on the
eastern slopes of the SMOr between Tlanchinol and
Huejutla (Fig. 3). In the Tlanchinol area, these lavas
were studied by Robin and Bobier (1975) who
identified at least 22 basaltic flows emplaced in four
Sample number
EAP
EAP
EAP
EAP
EAP
Age (Ma)
Error
25
23
24
19
18
9.04
7.12
7.12
6.74
6.69
0.16
0.14
0.14
0.14
0.41
EAP 17
6.66
0.12
EAP 16
7.02
0.16
EAP 20
EAP 22
7.33
7.32
0.13
0.15
EAP 21
7.11
0.16
EAP 10
7.56
0.15
Elongation
320
320
58
49
323
312
17
318
Not
Not
43
Not
Not
22
Not
41
40
43
Not
315
338
40
55
335
45
Not
Not
345
Not
41
43
30
Not
45
Not
45
48
25
38
detectable
detectable
detectable
detectable
detectable
detectable
detectable
detectable
detectable
detectable
detectable
phases of activity. Cantagrel and Robin (1979) later
dated two samples near Molango at 6.5 and 7.4 Ma, as
well as a sample to the northeast of Tlanchinol at 7.3
Ma (Fig. 3).
The volcanic succession is thick as 250 m and the
lavas are aphyric to microporphyritic with olivine and
pyroxene phenocrysts. They range in composition
from alkali basalt to hawaiite (Fig. 2). In the highest
part of the SMOr, between Tlanchinol and Molango,
the flows are deeply eroded and in part covered by
early Pliocene silicic ash and pumice flow deposits
294
L. Ferrari et al. / Journal of Volcanology and Geothermal Research 146 (2005) 284–306
that become more abundant toward the southeast (Fig.
3). To the east, the mafic flows probably filled paleovalleys but now constitute WSW–ENE elongated
ridges because of their resistance to erosion (Fig. 3).
In the Huautla area, several flows coalesced to form
large bmesasQ with a thickness of several tens of
meters. In the Tlanchinol area we dated two samples
(EAP 33 and 36) from the lower part of the
succession, yielding nearly identical ages of 7.30 F
0.13 and 7.33 F 0.13 Ma, confirming the previous age
of Cantagrel and Robin (1979). We obtained an age of
5.72 F 0.13 Ma for a third sample (EAP 31), exposed
at lower elevation and closer to Huejutla, (Table 2).
Consisting with its more youthful morphology, the
Huautla flow yielded a younger age of 2.82 F 0.16 Ma
(EAP 30). This latter age is comparable with the
Atotonilco basaltic succession exposed to the south of
Zacualtipan (Fig. 3) for which Cantagrel and Robin
(1979) obtained ages of 2.6 and 2.4 Ma (south of the
limit of Fig. 3). Our new ages indicate that the
beginning of mafic volcanism in the Tlanchinol area is
contemporaneous with the early activity in the Alamo
volcanic field.
3.3. Poza Rica flows
Several massive mafic lava flows cover the eastern
slope of the SMOr west of Poza Rica (Fig. 4). Our
geological studies indicate that these lavas flowed for
over 90 km from the front of the SMOr west of
Huachinango to the coastal area of Poza Rica, about
2200 m topographically lower. Locally, the lavas
Fig. 4. Geologic map of the Tulancingo–Poza Rica area showing the location and age of previous and new isotopically dated rocks.
L. Ferrari et al. / Journal of Volcanology and Geothermal Research 146 (2005) 284–306
filled paleo-valleys, reaching a thickness of several
hundreds of meters. Although partly eroded, the flows
are almost continuous from Huachinango to Poza
Rica. Two other large flows are exposed northwest of
Poza Rica in the Metlatoyuca area (Fig. 4). In general,
all these flows are better preserved than those in the
Tlanchinol area. Lavas are variably porphyritic with
olivine and clinopyroxene as phenocrysts. They show
a small compositional range that span across the basalt
and hawaiite fields (Fig. 2).
No ages were available in the literature for these
rocks. We have dated six samples (EAP 1, 2, 4, 6, 8
and 9) at different elevations of the flows. All data
cluster in a narrow age range comprised between
1.64 F 0.06 and 1.31 F 0.03 Ma (Table 2). Four
samples (EAP 11, 12, 14 and 15) from the Metlatoyuca flows were also dated. The lower flow yielded
two ages, indistinguishable within the error, of ~1.6
Ma. The upper flow produced also two nearly
identical ages of ~1.4 Ma (Table 2). In light of these
results, the Metlatoyuca flows may represent part of
the Poza Rica flows that branched off toward the north
once the flows arrived in the coastal plain (Fig. 4).
The Poza Rica flows are probably equivalent to the
alkaline basalts described by Nelson and Lighthart
(1997) in the Tulancingo canyon (Fig. 4) at the base of
the Quaternary Las Navajas volcano. This correlation
is suggested by stratigraphic position and petrographic
similarity between the two successions. As a whole,
the flows in the Poza Rica area represent a major pulse
of alkaline volcanism in the eastern part of the
TMVB.
3.4. Chiconquiaco–Palma Sola area
Cenozoic volcanism in the Chiconquiaco–Palma
Sola region spans a longer time interval than the areas
to the north. The geology of the region is also more
complex. The region is located in a structural high
(the so-called bTeziutlán massifQ) where the Paleozoic
basement (polydeformed schists, volcano–sedimentary and intrusive rocks) is uplifted along a roughly
E–W-trending structure that breaks the continuity of
the coastal basins (López-Infanzón, 1991) and divide
the Tampico–Misantla basin from the Veracruz basin
to the south (Fig. 1). In the study area, these basement
rocks are found west of Altotonga (Fig. 5) and yielded
K–Ar ages of 269–252 Ma (López-Infanzón, 1991)
295
whereas 40Ar/39Ar experiments produced ages of 268
and 281 Ma (Iriondo et al., 2003). The basement rocks
are covered by the marine Mesozoic succession of the
SMOr and by Cenozoic siliciclastic rocks and igneous
successions (Fig. 5).
The Cenozoic magmatic activity in the region
consists of four groups of rocks: 1) middle to late
Miocene intrusive bodies of gabbroic to dioritic
composition, mainly exposed along the coast in the
Misantla and Palma Sola areas; 2) a latest Miocene to
early Pliocene alkaline basaltic plateau centered in the
Chiconquiaco area; 3) latest Pliocene shoshonitic lava
flows of the Alto Lucero–Actopan area; and 4) Late
Pleistocene to Holocene cinder cones with associated
lava flows, located mostly to the south of the
Chiconquiaco plateau (Fig. 5). López-Infanzón
(1991) also distinguishes a unit of lahars and basalts
between group 1 and 2. However, our field work
rather suggest that this unit consists of debris and
other erosional deposits, which locally may include
part of the overlying basalts.
The intrusive rocks often have a microporphyritic
or microcrystalline texture indicative of a shallow
depth of intrusion. They commonly display sulfur
mineralization and chlorite alteration and are locally
cut by mafic dikes. Previous workers dated these
rocks by conventional K–Ar methods. Cantagrel and
Robin (1979) obtained an age of 17 Ma for the
Laguna Verde microdiorite; Negendank et al. (1985)
report ages of 12.3 and 12.9 Ma for the Candelaria
gabbro (no errors and analytical data provided); and
López-Infanzón (1991) dated three plutons exposed
near Tenochtitlan and Junique at 13.0 F 1.0, 9.0 F 0.7,
and 6.2 F 0.6 Ma (Fig. 5). With the purpose of
confirming this rather large age interval, we dated
some of the plutonic bodies previously studied by
these workers. We were unable to find a sample of the
Laguna Verde microdiorite fresh enough for dating.
These rocks, also called bgreen formationQ by
Cantagrel and Robin (1979), are actually intensely
and pervasively chloritized; thus, we consider the
previously reported age of 17 Ma as unreliable. For
the Junique gabbro, previously dated at 6.2 Ma by
López-Infanzón (1991), we obtained an age of
15.62 F 0.5 Ma (sample EAP 48; Table 2), which
appears more consistent with the stratigraphy and
other ages of the intrusive bodies. A plagioclase
concentrate from the Candelaria gabbro was dated by
296
L. Ferrari et al. / Journal of Volcanology and Geothermal Research 146 (2005) 284–306
Fig. 5. Geologic map of the Chiconquiaco–Palma Sola area showing the location and age of previous and new isotopically dated rocks.
40
Ar/39Ar. The integrated ages obtained are in agreement with the published K–Ar ages (Negendank et al.,
1985). However, we prefer to take the isochron age of
10.9 F 0.8 Ma as our best estimate for this sample,
because of the possibility of excess argon as discussed
earlier. In addition, we dated two other samples (EAP
45 and 57) from hypoabyssal bodies exposed immediately below the overlying plateau basalts near Plan
de las Hayas and El Limón (Fig. 5). These rocks
yielded ages of 14.65 F 0.32 Ma and 11.07 F 0.2 Ma
respectively. In summary, the most reliable ages for
the intrusive rocks of Palma Sola range between 15.6
and 10.9 Ma. The only exception would be a tonalite
near the Tenochtitlan village (Fig. 5), for which
López-Infanzón (1991) reports an age of 9.0 Ma
age. This first phase of magmatism extended also in
the Gulf of Mexico as suggested by the recent
identification of a small eruptive center located
offshore west of Santa Ana (Santa Ana High, Fig.
5), for which seismic reflection data suggest an age
just younger than middle Miocene (Tim Wawrzyniec,
written communication, 2003).
An erosional unconformity and/or several tens of
meters of clastic deposits (blahars and basalts unitQ of
López-Infanzón, 1991) separate the intrusive rocks
from the fissural lava flows comprising the Chicon-
L. Ferrari et al. / Journal of Volcanology and Geothermal Research 146 (2005) 284–306
quiaco plateau, presently covering about 1700 km2.
The northern side of the plateau is highly dissected
whereas to the south the presence of capping
Quaternary volcanic rocks has impeded strong erosion. North of Chiconquiaco, the succession is up to
800 m thick, although it thins rapidly to the east, being
only some tens of meters in the coastal areas.
Individual flows are 10 to 50 m thick and are
generally separated by red soils or thin alluvial
deposits. Compositionally, the lavas span a large
range from basanite to benmoreite mostly with an
intra-plate type affinity (Fig. 2). Some rocks of this
succession, however, show a moderate subduction
signal, and their Pb isotope signature suggests the
involvement of melts from subducted sediments in
their genesis (Gómez-Tuena et al., 2003). South of
Palma Sola, a group of dacitic domes (Cerro Cantera,
Cerro Metates and associated domes) appear older
than the plateau as they diverted the basaltic flows
(Fig. 5). According to Gómez-Tuena et al. (2003),
melt derived from the subducted MORB is involved
in magma genesis; Cerro Cantera displays the highest
adakitic signature of the area.
Cantagrel and Robin (1979) dated the dacitic
dome of Cerro Cantera at 6.5 F 0.2 Ma and one
plateau basalt at 3.1 F 0.3 Ma. López-Infanzón
(1991) dated sixteen samples from several sections
on the Chiconquiaco plateau. Fourteen of them
yielded ages ranging between 6.0 F 0.6 and
2.2 F 0.2 Ma; two older ages are discussed below.
Our sampling strategy was designed to better constrain the reported age span of this volcanic episode
and to compare with some of the previously
published ages. A sample from Cerro Cantera yielded
an age of 7.48 F 0.13 Ma (EAP 55, Table 1),
somewhat older than the Cantagrel and Robin
(1979) age. The base of the plateau, sampled to the
NE of Plan de las Hayas, yielded an age of
6.93 F 0.16 Ma (EAP 49, Table 2), which is similar
to that obtained by López-Infanzón (1991) for one
basal lava southeast of Tenochtitlan (Fig. 5). Most of
the plateau succession, however, seems to have
formed in a narrower age range. In fact, eleven
samples taken at different stratigraphic levels within
the succession yielded ages between 4.0 and 3.2 Ma
(EAP 41, 42, 44–47, 50 and 56; Table 2). In
particular, two samples collected on elevation difference of nearly 300 m (EAP 46 and 47) along the road
297
from Chiconquiaco to Misantla yielded ages of about
3.4 Ma, indistinguishable within 1j error (Table 2).
Our datings also include isolated mafic flows towards
the coast in the El Bejuco area (EAP 52–54; Fig. 5),
confirming that these rocks are part of the Chiconquiaco plateau. Dioritic intrusives are also reported
at depth in boreholes drilled by PEMEX (Mexican
Oil Company) in the region, but doubts exist on
whether they are part of the basement complex or
belong to a younger magmatic episode. For this
reason, we dated two samples from these wells.
Biotite from a microdioritic dike from a well east of
Villa Zempoala (on the southeast corner of Fig. 5)
has been dated by conventional K–Ar at 7.3 F 0.7 Ma
(Table 3), which is consistent with the beginning of
the mafic volcanism of the Chiconquiaco plateau. A
diorite core (LH 1718) recovered at about 1700 m
below the surface in a well near Plan de las Hayas
(Fig. 5) yielded an isochron age of 4.0 F 0.1 by the
40
Ar/39Ar method on a biotite concentrate (Table 4).
Considering the good agreement of the two experiments and the nearly flat age spectrum obtained (Fig.
6), we interpret this age as a crystallization age
corresponding to the main phase of the Chiconquiaco
plateau volcanism. These two ages of hypoabyssal
bodies indicate the existence of intrusive equivalent
of the plateau at depth. Moreover, the presence of an
extensive, but unexposed, mafic intrusive complex
would also explain the prominent positive Bouguer
anomaly that this area displays in the gravimetric
map of Mexico (De la Fuente et al., 1994).
We regard with caution two ~9 Ma ages reported
by López-Infanzón (1991) for samples from the lower
part of the plateau. Sample LI 203, is reported as a
porphyritic trachydacite, a composition more akin to
the underlying rock unit. On the other hand, this
sample, with a 9.4 F 0.5 Ma age, is reported to have
been collected several tens of meters above two other
samples that yielded ages of 5.6 and 6.0 Ma, casting
doubts on its age and/or location. The other sample
(LI 100), according to López-Infanzón (1991), is a
hawaiite collected below a lahar and an unconformity,
thus it does not belong to the plateau. Therefore, our
new data suggest that the volcanic activity that built
the Chiconquiaco plateau began at around 7.0 Ma
(sample EAP 49, Table 2), although the main pulse of
volcanism seems to have occurred at the beginning of
the Pliocene.
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L. Ferrari et al. / Journal of Volcanology and Geothermal Research 146 (2005) 284–306
Fig. 6. 40Ar–39Ar age spectra and 37ArCa/39ArK diagram obtained for the samples LH 1718 and PS-99-21. Within the spectra, the errors are
given at the level of one standard deviation and are represented by the vertical width of the boxes. The analyses were performed by Margarita
López-Martı́nez at CICESE geochronological laboratory at Ensenada, Baja California, with the procedures described in Ferrari et al. (2002).
In the area between Alto Lucero and south of
Actopan (Fig. 5), several lava flows advanced to the
southeast, discordantly overlying the Chiconquiaco
plateau succession. These lavas are highly potassic
and can be classified as shoshonites (Fig. 2). In the
Alto Lucero area, we dated two samples (EAP 39
and 43) from lavas flows without visible vent
exposed at the top of the Chiconquiaco plateau.
These samples yielded nearly identical ages of
1.97 F 0.04 and 2.04 F 0.04 Ma (Table 2). For a
third sample (EAP 58) from a flow near Actopan, a
similar age of 1.92 F 0.18 Ma was obtained. Finally,
a lava flow at Plan del Rio, located about 10 km
south of Actopan (just south of the limit of Fig. 5)
yielded an age of 2.24 F 0.05 (EAP 59). These ages
are comparable with the 2.2 Ma age obtained by
López-Infanzón (1991) for a flow south of Chiconquiaco and the 1.9 Ma age reported by Cantagrel
and Robin (1979) for a flow 5 km to the east of
EAP 59.
L. Ferrari et al. / Journal of Volcanology and Geothermal Research 146 (2005) 284–306
The youngest activity in the Chiconquiaco–Palma
Sola region is represented by at least 20 cinder cones
and associated lava flows that in some cases display a
very young morphology. The vents are aligned in a
WSW–ENE direction between Perote and Palma Sola
(Fig. 5) and their lavas have basaltic to trachybasaltic
composition (Siebert and Carrasco-Núñez, 2002). In
the valley between Naolinco and Actopan (Fig. 5), a
welded ash flow tuff (not reproducible at the map
scale of Fig. 5) separates these rocks from the
underlying Chiconquiaco plateau succession. Siebert
and Carrasco-Núñez (2002) dated several of these
flows by the 14C method and obtained ages between
42,000 and 840 yr BP.
3.5. Neogene alkaline volcanism in southern Veracruz
The mafic volcanism of Chiconquiaco–Palma Sola
extends to the southeast into the submarine volcanism
at the Anegada High and the Los Tuxtlas volcanic
field. These two volcanic fields also bound the
299
Veracruz basin to the east (Jennette et al., 2003)
(Fig. 7). We describe briefly the geology of these two
volcanic areas, because they appear strictly related to
the Chiconquiaco–Palma Sola volcanic field.
3.5.1. Anegada volcanic center
The Anegada High is a positive structure where the
pre-Cenozoic basement is uplifted with a NNW trend.
The presence of submarine mafic volcanism in the
southeastern part of the Anegada high was initially
suggested by Moore and Castillo (1974), mostly on
aeromagnetic evidence and later corroborated by two
exploratory wells drilled by PEMEX offshore of the
Veracruz port (Anegada 2 and 3; Fig. 7). We obtained
several cores of these wells but the samples proved to
be too altered to be analyzed and dated. In thin
section, the samples show porphyritic structure with
olivine, clinopyroxene, and plagioclase phenocrysts.
Microprobe analyses of the clinopyroxenes indicate
an OIB affinity in the Ti vs. Ca + Na and Ti + Cr vs. Ca
diagrams, with values mostly overlapping the alkaline
Fig. 7. Regional geologic map of eastern Mexico showing the main Neogene volcanic episodes discussed in the text. Mid-late Miocene adakitic
TMVB as recognized by Gómez-Tuena et al. (2003). Apan = Apan volcanic field; CG = Cerro Grande; Popo = Popocatépetl; Izta = Iztaccihuatl;
Chich = Sierra Chichinautzin volcanic field; Pico = Pico de Orizaba; Cofre = Cofre de Perote. Crustal thickness (in kilometers) is from the
gravimetric study of Urrutia-Fucugauchi and Flores-Ruiz (1996). Boundary of the Tampico–Misantla and Veracruz basins from Jennette et al.
(2003).
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L. Ferrari et al. / Journal of Volcanology and Geothermal Research 146 (2005) 284–306
products of the Los Tuxtlas volcanics (JacoboAlbarrán et al., 1994). The seismic stratigraphy
calibrated at the Anegada 1 well indicates that the
volcanism occurred between 16 and 7 Ma (Jennette et
al., 2003), coeval with the early phase of magmatism
in the Palma Sola area and with the offshore
volcanism at Santa Ana High (Fig. 5).
3.5.2. Los Tuxtlas
The Los Tuxtlas volcanic field covers an area of
over 2200 km2 in southernmost Veracruz State. A
synthesis of previous studies (Nelson and GonzálezCaver, 1992; Nelson et al., 1995; Jacobo-Albarrán,
1997) indicates that volcanism may be divided into
two main episodes. An older episode began in late
Miocene (6.9 Ma, Nelson and González-Caver,
1992; 7.9 F 0.7 Ma, Jacobo-Albarrán, 1997) and
locally continued until ~1 Ma, producing hypersthene-normative alkali-basalts, hawaiites, mugearites
and benmoreites now exposed in remnants of large
cones, a plateau, and over 200 cinder cones. A
younger episode of calk–alkaline to transitional
affinity began around 3.3 Ma. Most of the products
of the younger episode were emplaced in five
calderas located in the southeastern part of the field
and other centers in the northwest of the field.
Although the oldest dated rocks are ~7 Ma, seismic
and borehole data indicate that volcanism in the Los
Tuxtlas has influenced the sedimentation of the
Veracruz basin since Middle Miocene (Jennette et
al., 2003). Therefore, the initial activity of the Los
Tuxtlas volcanism appears to be concurrent with the
first volcanic episode in the Palma Sola area as well
as that of the Anegada High.
4. Tectonic setting
From a tectonic point of view, the studied
volcanic fields are located in, or bound, two
Cenozoic coastal basins of the western Gulf of
Mexico that are well studied for their hydrocarbon
contents: the Tampico–Misantla basin and the
Veracruz basin (Figs. 1 and 7). The Tlanchinol and
Poza Rica flows were emitted from fissures along the
front of the SMOr, coincident with the western
boundary of the Tampico–Misantla basin. Sierra de
Tantima and the Alamo volcanic field are located
inside this basin and mostly overlie its filling.
Neogene volcanism of the Chiconquiaco–Palma
Sola, Anegada High, and Los Tuxtlas areas constitute the northern and eastern boundary of the
Veracruz basin respectively. Here, we discuss the
relation between the mafic alkaline volcanism and
the Neogene tectonics of the western Gulf of
Mexico.
4.1. Northern Veracruz and Hidalgo
Previous workers (Robin, 1976, 1982; Robin and
Tournon, 1978) associated the alkaline volcanism of
northern Veracruz and Hidalgo (Tlanchinol, Tantima
and Alamo) with NNW-trending extensional faulting
affecting most of the SMOr border. However, no
structural information was provided by these authors,
and in the geologic sections of Robin (1982) most of
the normal faults are drawn as inferred and do not cut
the surface. Although the dense vegetation precludes a
detailed structural study, our studies indicate that Late
Tertiary normal faults are not common in and around
most of the studied volcanic fields. Published geologic maps (Suter, 1990; Consejo de Recursos
Minerales, 1997) also show the border of the SMOr
as an Early Tertiary fold and thrust belt, only locally
affected by minor Late Tertiary normal faulting. In his
detailed geological survey of the Tlanchinol area,
Ochoa-Camarillo (1997) explicitly rejects the existence of the extensional fault system postulated by
Robin (1982). Moreover, the subsurface geology of
the Tampico–Misantla basin indicates that the compressive regime that built the SMOr lasted until late
Eocene times, but with only very minor deformation
affecting post Eocene strata (Eguiluz de Antuñano et
al., 2000).
Nevertheless, the emplacement of the relatively
primitive alkaline magmas of Tlanchinol, Tantima,
and Alamo is likely to have occurred through some
pre-existing crustal discontinuities. The flows of
Tlanchinol appear spatially associated with a major
basement fault system of probable Late Jurassic age.
In fact, the Proterozoic gneisses of the Huiznopala
Formation crop out at ~500 m elevation just 10 km to
the north of Molango (Fig. 3) (Ochoa-Camarillo,
1997; Lawlor et al., 1999), and disappear toward the
north-east due to the NNE Ixtlapala–Huiznopala
normal fault, which has a minimum displacement of
L. Ferrari et al. / Journal of Volcanology and Geothermal Research 146 (2005) 284–306
1600 m (Ochoa-Camarillo, 1997). Further to the east,
Precambrian rocks correlative with the Huiznopala
gneisses were cored at a depth of 2600 m in the
coastal plain of the Gulf of Mexico (Suter, 1990).
These normal faults developed in Late Jurassic as a
consequence of the southeast motion of the Yucatan
block during the initial opening of the Gulf of Mexico
(Pindell and Kennan, 2001).
The Sierra de Tantima and Alamo volcanic field lie
in a plain without any visible evidence of faulting.
Robin (1982) suggested that the peculiar NE alignment of Sierra de Tantima was due to the accumulation of lava flows in a paleo-valley whose sides were
later completely eroded. This explanation seems
highly unlikely, because the Sierra rises about 1300
m above the surrounding plain, plus a distance of over
35 km separates it from the other flows descending
from the SMOr (Fig. 3). In addition, lava flows that
built the Sierra do not show a NE dip, which would be
expected if they originated from the NNW-trending
front of the SMOr. Consequently, we suggest that the
NE elongation of Sierra de Tantima is the result of
eruption along a NE-trending fissure vent. This view
is corroborated by the analysis of vent elongation and
dike orientation in the Alamo volcanic field. In our
field work, we observe that many volcanic necks
show a preferential elongation. In four cases, the
feeding dikes were also exposed, with a direction
parallel to the neck elongation. As summarized in
Table 5 and Fig. 8, there are two main elongation
directions: necks in the southwestern part of the field
as well as Sierra de Tantima trend NE–SW, whereas
necks in the northeastern part of the field are
elongated in a NNW–SSE direction. The elongation
of a cone is generally considered to express the
underlying feeding fracture (e.g., Tibaldi, 1995). In
this context, the directions observed in the field may
reflect the presence of buried crustal structures.
Recent paleo-reconstructions of the opening of the
Gulf of Mexico (Pindell and Kennan, 2001) show that
Late Jurassic to Early Cretaceous paleotransforms that
allowed the southeastern motion of the Yucatan block
were oriented NNW, whereas normal faults bounding
continental blocks were oriented NE or ENE. We
conclude that the mafic volcanism in the Tampico–
Misantla basin (Tlanchinol, Alamo, Tantima) was fed
through old basement faults that were not necessarily
structurally active at the time of volcanism.
301
Fig. 8. Elongation direction (azimuth) of volcanic vents of the
Alamo Volcanic field (Table 5). Inset shows the possible orientation
of the principal underlying basement structures.
4.2. Southern Veracruz
The tectonics of the Veracruz basin is much more
complicated than those of basins to the North. The
mafic volcanism of this region (Palma Sola, Anegada,
and Los Tuxtlas) was included by Robin (1982) in the
EAP and considered as extension-related intraplate
volcanism. More recent studies by Besch et al. (1988),
Nelson et al. (1995), and Gómez-Tuena et al. (2003),
however, demonstrate that, unlike the volcanic fields
to the north, the volcanism in the Palma Sola and Los
Tuxtlas areas shows a variable signature from fluids
and melts from the subducted plate. In the Chiconquiaco–Palma Sola area, we could not find any clear
evidence of an episode of extensional faulting
concurrent with the alkaline volcanism. The main
tectonic feature of the region seems to be an old ENEtrending structure, as substantiated by the following
evidences. A 60-km long alignment of Quaternary
cinder cones trends ENE from the Cofre de Perote
area to the Candelaria area, in the coast (Fig. 5).
Along this alignment, near site EAP 50 (Plan de las
Hayas, Fig. 5), we measured several dikes feeding the
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early Pliocene Chiconquiaco plateau, which also
strikes ENE. Finally, at the eastern end of the
alignment lies the middle Miocene offshore volcanic
center of the Santa Ana High (see Previous section).
This volcanic center is located at the eastern end of an
ENE-trending left-lateral, strike-slip fault active until
the end of Middle Miocene (Tim Wawrzyniec, written
communication, 2003). Therefore, most of the volcanism in the Chiconquiaco–Palma Sola region seems to
be controlled by a major ENE-trending basement fault
system that acted as a preferential magma pathway
since Middle Miocene.
In the Los Tuxtlas volcanic field, a statistical
analysis of the distribution of cinder cones and
polygenetic centers shows the existence of a 65-km
long alignment with a 1208 (ESE) orientation,
expressed by at least 42 vents (Jacobo-Albarrán et
al., 1992; Jacobo-Albarrán, 1997). An array of
secondary alignments and satellite lineaments also
indicates that the main alignment could be the
expression of movements along a right-lateral basement structure (Jacobo-Albarrán, 1997). The age of
most recent activity of this fault system, however,
cannot be established from the field geology.
Additional information on the tectonics of the
region is provided by the subsurface geologic studies
in the Veracruz basin. Interpretation of extensive
seismic and borehole data indicate that contractile
and strike-slip deformations characterize the basin
during Miocene and Pliocene times (Jennette et al.,
2003). In particular, volcanic activity at both the
Anegada High and the Los Tuxtlas field was
controlled by a NW–SE trending fault system known
as the bAnton Lizardo trendQ, which has accommodated a right-lateral, strike-slip deformation since
middle Miocene times (Jennette et al., 2003) and still
shows strike-slip seismic activity (Suárez, 2000).
Jennette et al. (2003) also recognize a major change
in tectonics and sedimentation at about 7 Ma. At this
time, the uplift of the northern part of the basin (Palma
Sola area) triggered a major reorganization in the
regional sedimentation pattern. Intrabasinal faults
were reactivated mostly as strike-slip faults and broad
folds that affected the center of the basin. In summary,
the occurrence of mafic volcanism in the Veracruz
basin seems to have been guided mostly by preexisting strike-slip faults. Some of these faults may
have been active concurrently with the volcanism.
5. Discussion and conclusions
5.1. Regional evolution of volcanism
The geologic and geochronologic data presented in
this work, when integrated with previous studies, shed
light on the Neogene evolution of volcanism in
eastern Mexico (Fig. 7). Our synthesis of the available
data recognizes three main episodes of magmatism
east of Mexico City. The onset of the TMVB
magmatism can be placed in the Middle Miocene
(Ferrari et al., 1999). At this time, an E–W belt of
mostly intermediate calc–alkaline centers developed.
This belt includes polygenetic volcanoes of the Apan
volcanic field (Garcı́a-Palomo et al., 2002), the huge
Cerro Grande volcanic complex (Gómez-Tuena and
Carrasco-Núñez, 2000), and the intrusive and subvolcanic bodies of Palma Sola (Gómez-Tuena et al.,
2003; this work) (Fig. 7). According to Gómez-Tuena
et al. (2003), most of these rocks have an adakitic
signature (Fig. 7), presumably related to a period of
low-angle subduction that facilitated the melting of
the leading edge of the slab. The middle Miocene
TMVB may extend to the southeast, as expressed by
the offshore volcanism at Anegada High and the
initial activity at Los Tuxtlas (Jennette et al., 2003;
this work), although the geochemical affinity of these
rocks is poorly known.
A second eruptive episode is represented by a Late
Miocene to Early Pliocene (~7.5–3.0 Ma) pulse of
mafic alkaline volcanism located to the north and the
east of the previous episode (Fig. 7). Volcanism in the
northern part of the study region (Tlanchinol, Tantima, Alamo) seems to be unrelated to subduction
(Orozco-Esquivel et al., 2003; our unpublished data).
By contrast, lavas in the southern part of this mafic
alkaline belt (Palma Sola, Los Tuxtlas) show evidence
of a variable influx of fluids from the subducted slab
(Nelson et al., 1995; Gómez-Tuena et al., 2003).
Indeed, some of these alkaline to transitional rocks
have a moderate subduction signal and their Pbisotope signature suggests the involvement of melts
derived from subducted sediments in their genesis
(Gómez-Tuena et al., 2003). Thus, in one sense, it
could be considered as the extension of the TMVB.
However, the almost contemporaneous initiation of
this episode throughout the study region calls for a
common mechanism causing the magmatism.
L. Ferrari et al. / Journal of Volcanology and Geothermal Research 146 (2005) 284–306
In Late Pliocene and Quaternary times, subductionrelated volcanism dominates the region and is again
localized in the same position of the first episode of
mid-late Miocene age. In this framework, the ~7.5–
3.0 Ma pulse of mafic alkaline volcanism represents
an anomaly that cannot be easily reconciled with a
conventional subduction scenario, nor a consequence
of an extensional tectonics. The cause of this
volcanism is discussed below.
5.2. On the origin of the late Miocene to Early
Pliocene mafic alkaline volcanism
The Robin (1982) model implicitly supposes a
Neogene episode of extensional faulting capable of
producing mantle decompression and melting. In
previous discussion, however, we show that in the
study region significant extensional faulting was not
operative during the Neogene. Pre-existing crustal
structures were pathways for the ascent of mafic
alkaline melts, but they were not responsible for
melting of the mantle. Nelson et al. (1995)
explained the volcanism of the Los Tuxtlas as the
product of mantle melting induced by the subduction of the Cocos plate. Similarly, the presence of a
variable subduction signature in the volcanic succession of Palma Sola prompted Gómez-Tuena et al.
(2003) to interpret this volcanic field within the
context of increasing slab depth with time. Our
geochemical data (Orozco-Esquivel et al., 2003
submitted) also support the presence of a subduction
component in most of the lavas of the Chiconquiaco–Palma Sola region. Thus, subduction fluids
and melts may have played a role in lowering the
mantle solidus in southern Veracruz, which is also
within a few hundreds of kilometers from the
subduction zone. However, lavas from the northern
part of the study region (Tlanchinol, Tantima and
Alamo) show a clear intraplate character with no
subduction signal. On the other hand, the geochronologic data presented in this study, indicate
that the mafic alkaline pulse occurred at 7.5–6.5 Ma
in the north (Tlanchinol, Tantima, Alamo) and only
slightly after (~7 to 3 Ma) in the south (Palma Sola
and Los Tuxtlas areas), suggesting that a common
regional mechanism served to trigger melting of the
mantle. In addition, in the Chiconquiaco–Palma Sola
area, the 7–3 Ma alkaline rocks represent by far the
303
most voluminous volcanic episode, suggesting that
another mechanism apart from subduction enhanced
mantle melting.
One possible explanation is the onset of edgedriven convection at the Gulf of Mexico passive
margin. King and Anderson (1995, 1998) showed that
edge-driven, small-scale convection can be triggered
in the upper mantle by a discontinuity in the
lithospheric thickness, such as that between cratons
and oceanic basins. Cold downwelling is expected
beneath the discontinuity, whereas hot upwelling is
predicted at some hundreds of kilometers beneath the
thin oceanic crust. King and Ritsema (2000) showed
that this mechanism could account for hotspot
volcanism in the African and South America plates.
The state of Veracruz is situated in a zone of steep
gradients in crustal thickness variation dating back to
the late Jurassic. Such gradients may be suitable
candidates for the onset of small-scale convection.
The crust passes from 40 to 30 km between
Tlanchinol and Alamo and from 30 to 15 km beneath
the Palma Sola and Los Tuxtlas areas (Fig. 7). The
region of alkaline volcanism, however, coincides with
a predicted downwelling in the models of King and
Anderson (1998) rather than the uprise of hot upper
mantle. Therefore, this mechanism cannot explain the
latest Miocene onset of alkaline volcanism in the
study region.
Another possibility is that melting was triggered by
a relatively sudden increase in the mantle temperature
as a consequence of the detachment of the lower part
of the subducted slab. Ferrari (2004) proposed that the
deeper part of the subducted Cocos plate started to
detach at about 12.5 Ma in western Mexico, when
subduction ceased off southern Baja California. In his
model, the detachment propagated laterally to the east
because of the increasing slab pull of the still attached
part of the plate. The detachment produced the
opening of a slab window that would have been filled
by hot asthenosphere. The resulting increase of the
mantle temperature would have been the cause of the
eastward-migrating mafic pulse of volcanism widely
exposed to the north of the Plio–Quaternary TMVB
(Ferrari et al., 2000; Ferrari, 2004) (Fig. 1). In this
context, the 7.5–3 Ma alkaline volcanism described in
this work may represent the eastward and southeastward prolongation of this pulse. The hypothesized
slab loss can explain the abrupt begin of mafic
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L. Ferrari et al. / Journal of Volcanology and Geothermal Research 146 (2005) 284–306
volcanism at about 7.5 Ma throughout the study area.
In the northern part of the region, the volcanic pulse
was short and of limited volume. This is consistent
with a thermal origin for enhanced mantle melting
because this region was underlain by a dry mantle
unaffected by subduction since the Permian. By
contrast, in the southern part of the region, the mantle
had been already metasomatized by fluids and melts
of the subducted plate during the middle Miocene
episode of volcanism. This enabled a longer eruptive
pulse and more voluminous volcanism.
Acknowledgments
Our work was supported by a CNR (Italy)–
CONACyT (Mexico) bilateral grant, Centro de Geociencias, UNAM, and Department of Geology and
Mineralogy, Kyoto University (Japan). We acknowledge Miguel Angel Garcı́a G. and A. Susana Rosas
M., for their support with the 40Ar–39Ar analyses and
mineral separation of samples PS-99-21 and LH 1718.
We thank Gerardo Carrasco and Arturo Gómez-Tuena
for sharing their knowledge on the geology of the
Chiconquiaco–Palma Sola area. Arturo Gómez-Tuena
also lent his sample PS-99-21 for dating. Tim
Wawrzyniec kindly provided important information
on the offshore volcanic centers. Detailed review by
Robert Tilling greatly enhanced the clarity of the
original manuscript.
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