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Tectonophysics 318 (2000) 71–98
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Stratigraphy, geochemistry and tectonic significance
of the Oligocene magmatic rocks of
western Oaxaca, southern Mexico
Barbara Martiny a, *, Raymundo G. Martı́nez-Serrano b,
Dante J. Morán-Zenteno a, Consuelo Macı́as-Romo a, Robert A. Ayuso c
a Instituto de Geologı́a, Universidad Nacional Autónoma de México, Apdo. Postal 70-296, Ciudad Universitaria,
04510 México, Distrito Federal, Mexico
b Instituto de Geofı́sica, Universidad Nacional Autónoma de México, Ciudad Universitaria, 04510 México, Distrito Federal, Mexico
c United States Geological Survey, National Center, Reston, VA 20192, USA
Received 26 August 1998; accepted for publication 30 August 1999
Abstract
In western Oaxaca, Tertiary magmatic activity is represented by extensive plutons along the continental margin
and volcanic sequences in the inland region. K–Ar age determinations reported previously and in the present work
indicate that these rocks correspond to a relatively broad arc in this region that was active mainly during the Oligocene
(~35 to ~25 Ma). In the northern sector of western Oaxaca (Huajuapan–Monte Verde–Yanhuitlán), the volcanic
suite comprises principally basaltic andesite to andesitic lavas, overlying minor silicic to intermediate volcaniclastic
rocks (epiclastic deposits, ash fall tuffs, ignimbrites) that were deposited in a lacustrine-fluvial environment. The
southern sector of the volcanic zone includes the Tlaxiaco–Laguna de Guadalupe region and consists of intermediate
to silicic pyroclastic and epiclastic deposits, with silicic ash fall tuffs and ignimbrites. In both sectors, numerous
andesitic to dacitic hypabyssal intrusions (stocks and dikes) are emplaced at different levels of the sequence. The
granitoids of the coastal plutonic belt are generally more differentiated than the volcanic rocks that predominate in
the northern sector and vary in composition from granite to granodiorite. The studied rocks show large-ion lithophile
element (LILE) enrichment ( K, Rb, Ba, Th) relative to high-field-strength (HFS ) elements (Nb, Ti, Zr) that is
characteristic of subduction-related magmatic rocks. On chondrite-normalized rare earth element diagrams, these
samples display light rare earth element enrichment (LREE) and a flat pattern for the heavy rare earth elements
(HREE ). In spite of the contrasting degree of differentiation between the coastal plutons and inland volcanic rocks,
there is a relatively small variation in the isotopic composition of these two suites. Initial 87Sr/86Sr ratios obtained
and reported previously for Tertiary plutonic rocks of western Oaxaca range from 0.7042 to 0.7054 and eNd values,
from −3.0 to +2.4, and for the volcanic rocks, from 0.7042 to 0.7046 and 0 to +2.6. The range of these isotope
ratios and those reported for the basement rocks in this region suggest a relatively low degree of old crustal
involvement for most of the studied rocks. The Pb isotopic compositions of the Tertiary magmatic rocks also show a
narrow range [(206Pb/204Pb)=18.67–18.75; (207Pb/204Pb)=15.59–15.62; (208Pb/204Pb)=38.44–38.59], suggesting a similar source region for the volcanic and plutonic rocks. Trace elements and isotopic compositions suggest a mantle
source in the subcontinental lithosphere that has been enriched by a subduction component. General tectonic features
in this region indicate a more active rate of transtensional deformation for the inland volcanic region than along the
* Corresponding author. Fax: +52-5-622-4317.
E-mail address: [email protected] (B. Martiny)
0040-1951/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved.
PII: S0 04 0 - 1 95 1 ( 9 9 ) 00 3 0 7- 8
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B. Martiny et al. / Tectonophysics 318 (2000) 71–98
coastal margin during the main events of Oligocene magmatism. The lower degree of differentiation of the inland
volcanic sequences, particularly the upper unit of the northern sector, compared to the plutons of the coastal margin,
suggests that the differentiation of the Tertiary magmas in southern Mexico was controlled to a great extent by the
characteristics of the different strain domains. © 2000 Elsevier Science B.V. All rights reserved.
Keywords: arc magmatism; geochemistry; Nd–Sr–Pb isotope ratios; Oaxaca, Mexico; Tertiary; transtension
1. Introduction
Tertiary magmatism of Paleocene to Miocene
age in southern Mexico is represented by extensive
outcrops of plutonic and volcanic rocks that form
part of the Sierra Madre del Sur and define two
broad belts approximately parallel to the Pacific
coast: the coastal plutonic belt and the inland
volcanic sequences ( Fig. 1). These rocks, together
with the latest Cretaceous magmatic rocks, often
represent the highest elevations of the Sierra Madre
del Sur (SMS ) and extend from the southern part
of the state of Jalisco to the Isthmus of
Tehuantepec area. The Tertiary magmatism
roughly displays a decreasing age trend from
Paleocene in Colima to Miocene in eastern Oaxaca.
The plutonic rocks along the continental margin
form a chain of intrusive bodies of different scales,
dominated by composite batholiths commonly cut
by silicic and mafic dike swarms. More discontinuous outcrops of lava flows, pyroclastic deposits
and hypabyssal intrusions make up the inland
volcanic sequences. Between approximately 100°W
and western Oaxaca, the magmatism tends to be
Oligocene in age, whereas to the west, it is predominantly Upper Cretaceous to Eocene. The exposure
of middle crustal plutonic rocks along the continental margin of southern Mexico, emplaced at
depths between 13 and 20 km, and the increasing
abundance of volcanic rocks of similar age in the
inland region indicate a relatively rapid uplift,
from 30 to 25 Ma before the present time, and
unroofing of the plutonic rocks (Morán-Zenteno
et al., 1998).
In northwestern Mexico, arc volcanism related
to Farallon–North American plate convergence
produced mid-Cretaceous to early Tertiary magmatic rocks, including the Oligocene to Miocene
silicic Upper Volcanic sequence of the coast-parallel NNE-trending Sierra Madre Occidental (SMO)
( Fig. 2) (e.g. McDowell and Clabaugh, 1979;
Damon et al., 1981; Ferrari et al., 1994, and
references therein). The Trans-Mexican Volcanic
Belt ( TMVB) crosses central Mexico from east to
west at about 19°N and is related to the subduction
of the Cocos and Rivera plates beneath the North
American plate. Volcanic activity of the TMVB
initiated at about 16 Ma and continues to this day
( Ferrari et al., 1994). These volcanic arc sequences
have an oblique distribution (16°) relative to the
Acapulco trench. Changes in the Tertiary magmatic activity in this region reflect a major reorganization of the tectonic plates adjacent to southern
Mexico involving the detachment and lateral displacement of the Chortis block (Malfait and
Dinkelman, 1972; Ross and Scotese, 1988;
Ratschbacher et al., 1991; Ferrari et al., 1994;
Herrmann et al., 1994; Schaaf et al., 1995). The
presence of mylonitic shear zones along the coastal
margin of Guerrero and Oaxaca ( Fig. 3), produced
during the detachment and subsequent eastward
displacement of the Chortis block, and the unusual
proximity of the coastal plutonic belt to the
Acapulco trench ( Fig. 1) support the interpretation of the truncated character of the continental
margin. The study of the distribution, geochronology and geochemical characteristics of the magmatism in southern Mexico is essential for
understanding the tectonic evolution of this region
during the Tertiary.
Previous studies of the plutonic rocks along the
Pacific coast of Oaxaca involve the along-the-coast
variations in geochemistry and geochronology
(Böhnel et al., 1992; Herrmann, 1994; Herrmann
et al., 1994; Schaaf et al., 1995; Hernández-Bernal
and Morán-Zenteno, 1996). There are also a few
stratigraphic and geochronologic studies for the
inland volcanic rocks (e.g. Salas, 1949; RuizCastellanos, 1970; Ferrusquı́a-Villafranca, 1970,
1976; Ferrusquı́a-Villafranca and McDowell, 1991;
B. Martiny et al. / Tectonophysics 318 (2000) 71–98
73
Fig. 1. Distribution of volcanic and plutonic rocks in southern Mexico with the study area marked. The inset shows state divisions
and geographical locations. J=Jalisco; M=Michoacan; G=Guerrero; O=Oaxaca; C=Chiapas; MC=Mexico City; IT=Isthmus
of Tehuantepec (modified from Morán-Zenteno et al., 1999).
Morán-Zenteno et al., 1998). Until now, there
have been no studies of the geochemistry of the
volcanic rocks of westernmost Oaxaca nor of the
regional geochemical and geochronologic patterns
of the magmatic rocks in this region. We therefore
focused our studies on an area in western Oaxaca
that crosses the Sierra Madre del Sur and includes
both intrusive and extrusive Tertiary rocks in order
to detect possible variations in the geochronological and geochemical patterns perpendicular to the
trench (Figs. 1 and 4).
In this paper, we examine the stratigraphy and
geochemistry of the magmatic rocks in western
Oaxaca in order to gain insight into the significance
of these rocks in the tectonic evolution of southern
Mexico during the Tertiary. Major and trace elements together with Sr, Nd and Pb isotopes as
well as isotopic dating have been used to address
this problem. The details of the petrogenesis of
these magmatic rocks will be a subject of a separate
paper and will include the determination of the
isotopic compositions of additional samples.
2. Regional geological setting
2.1. Basement rocks
The Tertiary magmatic rocks in western and
central Oaxaca are distributed in a region charac-
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B. Martiny et al. / Tectonophysics 318 (2000) 71–98
Fig. 2. Tectonic plates and major magmatic provinces of Mexico. IVS=inland volcanic sequences; CPB=coastal plutonic belt; SMO=
Sierra Madre Occidental; TMVB=Trans-Mexican Volcanic Belt. The inset shows the distribution of tectonostratigraphic terranes
for southern Mexico after Campa and Coney (1983). Abbreviations used in the inset are: G=Guerrero, Mi=Mixteca, O=Oaxaca,
X=Xolapa, J=Juárez, M=Maya terrane, SM=Sierra Madre, SMO=Sierra Madre Occidental and TMV=Trans-Mexico Volcanic
Axis, C=Coahuila.
terized by contrasting pre-Cenozoic tectonic and
stratigraphic settings. Three major tectonostratigraphic units have been recognized on the basis of
the petrotectonic associations and age of their
basement, namely the Mixteca, Oaxaca and
Xolapa terranes (Campa and Coney, 1983; Sedlock
et al., 1993) (Fig. 2 inset). The Tertiary volcanic
rocks of western Oaxaca cover metamorphic and
sedimentary units of the Mixteca terrane and probably the westernmost part of the Oaxaca terrane,
whereas along the coastal margin, Cretaceous and
Tertiary plutons are emplaced in metamorphic
rocks of the Xolapa terrane.
The basement of the Tertiary volcanic rocks in
the Mixteca terrane is represented by the Acatlán
Complex of Paleozoic age. This complex is formed
by a heterogeneous tectonic assemblage of
metamorphic units ranging from greenschist- to
eclogite-facies (Ortega-Gutiérrez, 1978, 1993). It
includes metasedimentary units of phyllites and
migmatites, as well as eclogite-facies micaceous
schists, gneisses and amphibolites, including ultramafic and serpentinitic rocks. It has been interpreted that a major part of this terrane was overthrust
in pre-Pennsylvanian time by the Grenville age
Oaxaca terrane and the Esperanza Granitoids are
found in the contact between these two terranes
(Sedlock et al., 1993). It is thought that the
Acatlán Complex is underlain by a Precambrian
basement that is tentatively considered to be
Grenvillian in age (Ortega-Gutiérrez et al., 1990).
Typical present-day 87Sr/86Sr and eNd values of
the Acatlán Complex range from 0.7153 to 0.7613
and −8.5 to −12, respectively, for the metasedimentary units and the granitoids of the Esperanza
Formation, whereas mafic components of the
eclogitic sequences have values ranging from
0.7058 to 0.7094 and +1.7 to +3.1 ( Yañez
et al., 1991).
To the east, the Tertiary volcanic rocks of the
central part of the state of Oaxaca cover the
Oaxaca terrane, which is characterized by a granu-
B. Martiny et al. / Tectonophysics 318 (2000) 71–98
Fig. 3. Map of south-central Mexico showing different Tertiary
deformation domains and indicating their age (modified from
Morán-Zenteno et al., 1999).
lite-facies metamorphic basement of Grenvillian
age (900–1100 Ma) (Ortega-Gutiérrez, 1981, 1993)
overlain by Paleozoic and Mesozoic sedimentary
sequences (Pantoja-Alor, 1970; Schlaepfer, 1970).
The metamorphic basement is mainly composed
of mafic and felsic gneisses, as well as metasedimentary rocks and charnockites (OrtegaGutiérrez, 1981, 1993; Ortega-Gutiérrez et al.,
1995). The present-day 87Sr/86Sr and eNd isotopic
values of the Oaxaca Complex generally range
from close to those of bulk earth to 0.717 (although
one paragneiss has a 87Sr/86Sr ratio of 0.750) and
from −9 to −12, respectively (Patchett and Ruiz,
1987; Ruiz et al., 1988a,b).
To the south, the Tertiary magmatic rocks of
western Oaxaca occupy the Xolapa terrane, which
is constituted by middle crustal, amphibolite-facies
75
metamorphic rocks, for which there are still uncertainties concerning the protolith ages. These rocks
are distributed along the continental margin of
eastern Guerrero and Oaxaca. The Xolapa terrane
includes mainly quartz-amphibolites, quartz-feldspathic gneisses, pelitic paragneisses and schists,
as well as some marble lenses and granulite facies
relicts (Ortega-Gutiérrez, 1981; Corona-Chávez,
1997; Tolson-Jones, 1998). There is a characteristic
occurrence of migmatites throughout most of the
Xolapa Complex that indicates different degrees
and conditions of anatexis. The present-day
87Sr/86Sr ratios reported up to now for the Xolapa
Complex range from 0.706 to 0.724 and eNd
values, from −12.4 to +2.5 (Morán-Zenteno,
1992). Undeformed Tertiary plutons of this complex, excluding the Acapulco intrusion that differs
in age and geochemistry from other plutons in the
region, display low 87Sr/86Sr ratios (0.7038–
0.7051), and positive eNd values (+0.5 to +3.7)
(Morán-Zenteno, 1992; Herrmann, 1994).
The Guerrero terrane lies farther west and has
a younger basement; it is characterized by Late
Cretaceous and Paleogene continental deposits
that unconformably overlie Mesozoic volcano-sedimentary units, the age and nature of which are
subject to controversy.
2.2. Tertiary tectonic features
The Tertiary tectonic features of the Oaxaca
region display a contrasting framework that is
suggestive of changing dynamic conditions in both
time and space. Most of the major Cenozoic
tectonic features indicate a different tectonic scenario with respect to that of central and northern
Mexico dominated by NNW–SSE extensional
faults for the Oligocene and Miocene. Although
the continuation of the Basin and Range province
to southern Mexico has been suggested on the
basis of the orientation and kinematics of some
structures (i.e. Oaxaca fault) (Henry and ArandaGomez, 1992), many other major features indicate
different dynamic conditions from that of central
and northern Mexico and include the deformation
associated with the Chortis block displacement
(Ratschbacher et al., 1991; Ferrari et al., 1994;
Nieto-Samaniego et al., 1995; Meschede et al.,
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B. Martiny et al. / Tectonophysics 318 (2000) 71–98
B. Martiny et al. / Tectonophysics 318 (2000) 71–98
1997; Tolson-Jones, 1998). Some of the normal
faults in southern Mexico, such as the Oaxaca
fault, have been reactivated several times. The
Oaxaca fault is a NNW-trending fault system, top
to the west, that delineates the eastern margin of
the Valley of Oaxaca (Centeno-Garcı́a, 1988;
Nieto-Samaniego et al., 1995; Alaniz-Álvarez et al.,
1996) ( Fig. 3). An early ( Triassic?) mylonitization
event along the Oaxaca fault is probably related
to the collision of the Mixteco-Oaxaca block
against the more eastern Maya terrane.
Reactivation occurred for the strike-slip phase of
this fault during the Jurassic (Alaniz-Álvarez et al.,
1996) and as a normal fault zone during the
Miocene ( Ferrusquı́a-Villafranca et al., 1988).
In the coastal region of Oaxaca and eastern
Guerrero, a series of shear zones with left lateral
and normal kinematics that trend roughly parallel
to the coast have been recognized. These shear
zones appear to display chronological differences
with later activity to the southeast (Fig. 3). South
of Tierra Colorada, Guerrero, a mylonitic zone
affecting the metamorphic rocks of the Xolapa
Complex is intruded by a felsic pluton that yielded
concordant U–Pb zircon ages ranging from 32.5
to 34.2 Ma (Herrmann et al., 1994). This mylonitic
zone has kinematic indicators of a normal-leftlateral oblique shear zone. North of Puerto
Escondido, in the Juchatengo area, a NW-trending
north to northeast-dipping mylonitic zone has been
recognized with shear criteria that indicate normal
fault kinematics ( Ratschbacher et al., 1991). North
of Huatulco and Puerto Angel, there is a welldefined EW trending shear zone, known as the
Chacalapa Fault, that is characterized by a subvertical anastomosing geometry and left-lateral
indicators. According to observations carried out
by Tolson-Jones (1998), the Huatulco intrusion
( U–Pb age of 29 Ma; Herrmann et al., 1994) is
affected by the crystal-plastic deformation of this
shear zone, and the mylonite is truncated by
granodiorite dikes, which yielded a 23.7±1.2 Ma
K–Ar age in hornblende.
77
In the inland region of western Oaxaca, the
distribution of the Tertiary volcanic rocks seems
to be controlled by a group of NNW–SSE-trending
faults that bound a series of down-thrown blocks
where interlayered volcanic and lacustrine
sequences accumulated (Fig. 4). In some cases, the
faults cut the Tertiary volcanic units, and in other
cases, the lava flows and pyroclastics overlap the
fault zones. This fact and the occurrence of dikes
emplaced in the faults are indicative of coeval
activity. The faults in this region display lateral,
vertical and oblique striae, and, based on this fact
and the regional distribution of the Mesozoic and
Tertiary units, Silva-Romo (in preparation) interpreted the Oligocene tectonic framework as an en
echelon left-lateral transfer fault system.
To the west of the study area, in the TaxcoHuautla region, the Tertiary volcanic rocks are
dominantly silicic and range in age from 38 to
27 Ma (Morán-Zenteno et al., 1998). The distribution of volcanism in this area does not seem to be
controlled, as in Oaxaca, by transtensional tectonic
features. In the Taxco region, an 800 m thick
sequence of rhyolitc ignimbrites and lava flows
overlaps a system of NW-trending subvertical
faults with a complex kinematic history including
normal and lateral displacements. The lower part
of the rhyolitic sequence, with K–Ar ages ranging
from 38 to 35.5 Ma, is affected by NNE-trending
lateral faults, indicating a deformation event contemporary with the volcanic activity (MoránZenteno et al., 1998).
Based on the analysis of outcrop-scale fault-slip
data, Meschede et al. (1997) interpreted that in
Tertiary times, there was an effective stress transmission across the plate margin represented by the
continental lithosphere of southern Mexico.
According to these authors, prior to 25 Ma, the
s axes of the stress field had a sub-vertical orienta2
tion, whereas the s was roughly parallel to the
1
oblique motion vector of the oceanic plate with
respect to North America. This interpretation does
not satisfactorily explain the characteristics of
Fig. 4. Schematic geological map of the study area in western Oaxaca showing Tertiary rock units, general structural features and
location of analyzed rocks. Numbers in parentheses refer to isotopic ages obtained in the present work and reported in other
studies (Table 1).
78
B. Martiny et al. / Tectonophysics 318 (2000) 71–98
major Tertiary tectonic features of the inland
region of Oaxaca and Guerrero, and additional
time-constraint observations seem to be required.
3. General stratigraphic features
The Tertiary-age inland volcanic sequences in
the westernmost part of the state of Oaxaca extend
over an area of approximately 4000 km2 in a region
known as the Mixteca Alta. Plutonic rocks crop
out to the south along the continental margin and
form part of the coastal plutonic belt of southern
Mexico. The general stratigraphic characteristics
of the Tertiary volcanic zone of western Oaxaca
permits its division into the northern sector, where
a thick pile of intermediate composition lavas and
autobreccias with interbedded tuffs are dominant,
and the southern sector where the sequence consists principally of volcaniclastic sequences
( Fig. 5).
3.1. Volcanic rocks of the northern sector
The northern sector includes the areas of
Huajuapan, Zapotitlán, Monte Verde, Chilapa and
Yanhuitlán (Fig. 4). The volcanic sequences in this
area are mostly Oligocene in age ( Table 1) and lie
on lower Tertiary age conglomerates or directly
on Mesozoic continental and marine sequences or
Paleozoic metamorphic rocks of the Acatlán
Complex. An andesitic hypabyssal intrusion,
emplaced in the reddish mudstones, sandstones
and tuffaceous beds of the Yanhuitlán Formation
( Fig. 5), yielded a hornblende K–Ar age of
40.5±1.7 Ma (sample CON-7, Table 1), which is
older than the other ages obtained for volcanic
rocks of this area. This age and a few additional
isolated Eocene ages that have been reported for
this area and the adjacent parts of the state of
Puebla (Grajales-Nishimura, pers. commun.) seem
to represent the earliest manifestations of Tertiary
magmatic activity in this region. There is no evidence that Eocene magmatism was widespread or
volumetrically important in this region since, up
to now, all the other volcanic units in western
Oaxaca have been dated as Oligocene ( Table 1)
and can be observed to rest directly on Paleozoic
or Mesozoic rocks.
The Oligocene volcanic sequence in the northern
sector can be divided into two general units. The
lower unit consists of pyroclastic (silicic to intermediate lithic and vitric ash fall tuffs) and epiclastic
deposits that were apparently deposited in a lacustrine fluvial environment. A 31.4±0.8 Ma K–Ar
age was determined for biotite of a silicic tuff
(sample CON-75, Table 1) north of Huajuapan,
but the potassium concentration in the biotite is
anomalously low, and therefore, this radiogenic
age probably does not represent the exact time of
emplacement. The magmas become more mafic in
the predominant upper unit, which consists of a
thick pile (>400 m in some areas) of up to 14 lava
flows and autobreccias of intermediate composition with interbedded tuffs in the lower part. The
lavas have a porphyritic or trachytic texture and
contain phenocrysts of clinopyroxene, iddingsitized olivine, hornblende or plagioclase. The presence of erosional remnants of volcanic vents in the
form of volcanic necks that are observed throughout this region suggests that these lavas were at
least partially produced by central volcanic structures. Widespread hornblende- or pyroxene-bearing hypabyssal intrusions (dikes and small stocks)
of intermediate composition that are emplaced at
different levels of the Tertiary sequences have been
recognized throughout western Oaxaca and the
adjacent parts of Puebla (e.g. Ferrusquı́aVillafranca, 1970; Ruiz-Castellanos, 1970). In the
study area, hornblende concentrates of a stock
and dike yielded K–Ar ages of 33.6±1.4 and
34.2±1.4 Ma, respectively (samples CON-8A and
CON-91, Table 1).
Several K–Ar age determinations for whole rock
samples of lavas and hypabyssal intrusions in this
region have also been reported elsewhere. Seven
whole-rock ages for the Zapotitlán-Huajuapan
area range from 32±1 to 29±1 Ma (GalinaHidalgo, 1996). The small variation between these
whole rock ages and the ages obtained in the
present study for mineral separates is probably
due to the different material dated. Farther east,
in the Tamazulapan–Yanhuitlán area, Ferrusquı́aVillafranca et al. (1974) and Ferrusquı́aVillafranca and McDowell (1991) report a K–Ar
Fig. 5. Composite stratigraphic columns for the volcanic sequences of western Oaxaca. (a) The northern sector includes the Huajuapan, Zapotitlán, Tamazulapan,
Chilapa, Monte Verde and Yanhuitlán areas. (b) The southern sector includes the areas of Tlaxiaco, S. M. Cuquila, Laguna de Guadalupe and NW of Chalcatongo.
B. Martiny et al. / Tectonophysics 318 (2000) 71–98
79
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B. Martiny et al. / Tectonophysics 318 (2000) 71–98
Table 1
Age determinations of Tertiary magmatic rocks in western Oaxaca1
Sample
Site
Longitude Latitude
W
N
Mineral
Northern sector
CON-7
CON-8A
CON-75
CON-91
FV69-180
Yanhuitlán
Huajuapan
N of Huajuapan
N of Huajuapan
N of Tamazulapan
97°23∞36◊
97°47∞16◊
97°41∞48◊
97°40∞52◊
97°34.8∞
17°34∞05◊
17°49∞43◊
18°04∞51◊
18°02∞36◊
17°42.8∞
Hornblende
Hornblende
Biotite
Hornblende
Biotite
FV69-182
E of Tamazulapan
Rock type
K (%) 40Ar*
(ppm)
Age
Age
determination (Ma)
Ref.a
97°25∞
17°34.8∞
Andesitic laccolith
Andesitic stock
Silicic tuff
Andesitic dike
Silicic tuff- Llano
de Lobos Fm.
Whole rock Yucudaac Andesite
Southern sector
CON-59A
L. de Guadalupe
CON-101
Tlaxiaco
97°51∞20◊
97°36∞45◊
17°11∞17◊
17°21∞37◊
Hornblende Silicic tuff
Biotite
Silicic tuff
0.484
7.732
0.001180
0.017810
K–Ar
K–Ar
34.8±1.4
32.9±0.9
a
a
Coastal plutons
CON-53
G-17
MS-28
MS-34
MS-35
MS-42
Mu20
Mx12
Mu9
97°58∞36◊
97°57∞01◊
97°45∞55◊
97°26∞44◊
97°49∞23◊
97°47∞24◊
98°03∞21◊
97°45∞07◊
96°38∞07◊
16°53∞27◊
16°10∞21◊
16°09∞49◊
16°00∞40◊
16°16∞38◊
16°15∞40◊
16°40∞53◊
16°09∞48◊
15°51∞00◊
Biotite
Biotite
Biotite
Biotite
Hornblende
Hornblende
Zircon
Zircon
Zircon
7.475
7.809
7.793
7.64
0.874
1.029
0.013330
0.0151
0.0133
0.0125
0.0018
0.0019
K–Ar
K–Ar
K–Ar
K–Ar
K–Ar
K–Ar
U–Pb
U–Pb
U–Pb
25.5±0.7
27.7±0.7
24.4±0.6
23.5±0.6
29.9±1.1
27.7±1.0
301
281
271
a
c
c
c
c
c
d
d
d
S. Ma. Zacatepec
Jamiltepec
Progreso
Rı́o Grande
Jamiltepec
Progreso
N of S.P. Amuzgos
NW of Progreso
NW of Pochutla
Granite
Granite
Granite
Granite
Tonalite
Granodiorite
Granodiorite
Tonalite
Granodiorite
0.360
0.458
5.662
0.496
6.67
0.001023 K–Ar
0.001076 K–Ar
0.012440 K–Ar
0.001187 K–Ar
0.012624 K–Ar
40.5±1.7
33.6±1.4
31.4±0.8
34.2±1.4
26.2±0.5
a
a
a
a
b
0.934
0.001980
K–Ar
28.9±0.6
b
a Ref. (= reference): a: this work; b: Ferrusquı́a-Villafranca et al. (1974) and Ferrusquı́a-Villafranca and McDowell (1991); c:
Hernández-Bernal and Morán-Zenteno (1996); d: Herrmann et al. (1994).
40Ar1=radiogenic 40Ar; l -(40K )=4.962×10−10/yr; (le+l∞e)=0.581×10−10; 40K/K=1.193×10−4 g/g
b
1 error not reported
whole-rock age of 28.9±0.6 Ma for the lavas of
the Yucudaac Andesite and a biotite K–Ar age
of 26.2±0.5 Ma for the Llano de Lobos
Formation, a volcaniclastic sequence composed of
rhyolitic to andesitic tuffs, welded ignimbrites and
epiclastic deposits.
In the inland volcanic area, an eastward decreasing age trend is seen between westernmost Oaxaca
and east-central Oaxaca where Miocene ages are
reported ( Ferrusquı́a-Villafranca et al., 1974;
Ferrusquı́a-Villafranca and McDowell, 1991). The
younger age obtained for the Llano de Lobos
Formation might also suggest a decreasing age
tendency from Huajuapan to Yanhuitlán, but this
trend is not clear. There is an apparent discrepancy
between the two ages from the TamazulapanYanhuitlán area since the Llano de Lobos
Formation is shown to underlie the Yucudaac
Andesite in the stratigraphic column presented by
Ferrusquı́a-Villafranca (1976). There are several
possible causes for this disparity in the ages, including the different material dated, possible reheating
of the tuff during the emplacement of the overlying
lavas or a lack of horizontal continuity within the
volcanic sequences and variations in the volcanic
stratigraphy so that the tuff dated is actually
younger than the lava dated (Ferrusquı́aVillafranca et al., 1974). We consider that, in order
to define the age trend between Huajuapan and
Yanhuitlán, it would be necessary to carry out
additional age determinations on mineral concentrates in this latter area.
3.2. Volcanic rocks of the southern sector
The southern sector includes the areas of
Tlaxiaco, northwest of Chalcatongo, and smaller
outcrops in the areas of Cuquila and Laguna de
Guadalupe of approximately 5 and 35 km2, respectively (Fig. 4). The Tertiary volcanic sequences in
B. Martiny et al. / Tectonophysics 318 (2000) 71–98
this sector unconformably overlie Mesozoic sedimentary sequences and Tertiary conglomerates
( Fig. 5). The conglomerates in the Tlaxiaco area
contain lithic fragments of volcanic and calcareous
rocks and siltstones and are generally <10 m thick.
The sequence in the southern sector is dominated
by intermediate to silicic volcaniclastic deposits of
epiclastic and pyroclastic origin that include ashfall tuffs and reaches a thickness of up to 300 m.
A biotite-bearing silicic ignimbrite caps this
sequence in Cuquila. The samples dated ( K–Ar)
from the southern sector are Oligocene in age:
34.8±1.4 Ma was obtained for hornblende in a
volcaniclastic rock and 32.9±0.9 Ma for biotite in
a silicic tuff (samples CON-59b and CON-101,
Table 1). Although extensive lavas were not recognized in the southern sector, abundant hypabyssal
intrusions, similar to those in the northern sector,
are emplaced in the volcaniclastic sequence. These
hypabyssal rocks display a porphyritic texture with
pyroxene or hornblende phenocrysts in a microlitic
plagioclase groundmass. The intrusions are dacitic
to andesitic stocks and dikes of varying dimensions. North and northeast of Tlaxiaco, several
lava flows extending over a distance of approximately 5 km and displaying a general NE–SW
trend, were recognized.
3.3. Granitoids of the coastal plutonic belt
In this paper, the intrusive rocks that are
exposed in the La Muralla–San Pedro Amuzgos
region in western Oaxaca ( Fig. 4) are referred to
as the La Muralla pluton. Similar plutonic rocks
are also observed throughout the coastal region,
including the areas of Jamiltepec, Progreso and
Rı́o Grande (Fig. 4) where they have been named
the Rı́o Verde batholith by Hernández-Bernal and
Morán-Zenteno (1996). The La Muralla pluton
appears to be an extension of the Rı́o Verde
batholith, and together, they form part of one of
the most extensive composite batholithic structures
in southern Mexico. The La Muralla pluton is
emplaced between two distinct terranes. At the
northern limit, these rocks intrude Paleozoic metamorphic rocks of the Acaltán Complex, whereas
81
along the southern limit, in the area of San Pedro
Amuzgos, this batholith is in contact with metamorphic rocks of the Xolapa Complex.
The plutons are medium-grained granodiorites
and granites containing biotite and/or hornblende
and are more highly differentiated than the inland
volcanic rocks, particularly the predominant upper
unit lavas of the northern sector. K-feldspar is
generally microcline and commonly displays perthitic intergrowths. Accessory minerals include
sphene, which sometimes occurs in large euhedral
to subhedral crystals, apatite, iron-oxides and
zircon. Abundant swarms of aplitic dikes of NW–
SE and NE–SW orientation intrude the plutonic
rocks, especially near the southern border of the
La Muralla pluton, south of Santa Marı́a
Zacatepec, as well as in the coastal region.
Isotopic ages of the plutons are only slightly
younger than those of the inland volcanic
sequences. In the present study, a biotite concentration yielded an K–Ar age of 25.5±0.7 Ma
(sample CON-53, Table 1) in a granite north of
Santa Marı́a Zacatepec, and Hernández-Bernal
and Morán-Zenteno (1996) report five K–Ar cooling ages in biotite and hornblende (Table 1) of the
Jamiltepec and Progreso areas that range from
29.9 to 23.5 Ma. U–Pb crystallization ages
obtained by Herrmann et al. (1994) for undeformed Tertiary age plutonic rocks from Pinotepa
Nacional to Huatulco range from 30 to 27 Ma.
4. Sample selection and analytical methods
Tertiary age volcanic and plutonic rocks as well
as basement rocks of western Oaxaca were collected during several work field trips to the area.
The different stratigraphic units of the volcanic
sequences as well as hypabyssal intrusions were
sampled in both the northern and southern areas.
From the coastal plutonic belt, samples were
obtained of the plutonic rocks in the La Muralla–
San Pedro Amuzgos area. Thin sections of more
than 150 sampled rocks were studied to classify
the rocks and select fresh samples for bulk chemical
analyses, K–Ar determinations and other geochemical studies.
82
B. Martiny et al. / Tectonophysics 318 (2000) 71–98
Table 2
Major and trace elements of volcanic and plutonic rocks from western Oaxaca
Sample:
CON-7
CON-9
CON-12
CON-14
CON-18
CON-20
CON-27
CON-28
CON-29a
CON-32
Lava
Lava
Lava
Hypabyssal
Lava
Lava
Lava
Lava
Northern sector
Hypabyssal
SiO
2
Al O
2 3
Fe O
2 3
MnO
MgO
CaO
Na O
2
KO
2
TiO
2
PO
2 5
L.O.I.
Total
Sr
Rb
Ba
Th
Nb
Zr
Hf
Y
Sc
Cr
Ni
Co
La
Ce
Pr
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
60.91
17.94
4.52
0.02
1.41
5.37
4.23
1.31
0.80
0.29
2.88
99.68
Lava
54.40
17.02
8.38
0.10
5.00
7.28
4.01
1.01
1.29
0.33
0.90
99.72
51.28
16.76
8.33
0.08
5.18
8.43
3.92
1.17
1.36
0.48
2.62
99.60
855
23
412
3
6
158
494
19
274
1
6
150
643
23
460
4
13
165
17
9
20
10
19
16
14
139
68
31
19
17
188
78
40
28
45
7
32
6
1.74
5
0.64
3
0.65
1.49
0.22
1.42
0.20
15
35
5
22
5
1.65
4
0.61
3
0.57
1.35
0.20
1.18
0.18
30
61
8
31
7
1.97
5
0.72
4
0.72
1.75
0.27
1.59
0.25
56.03
17.15
7.39
0.07
4.13
6.78
3.92
1.25
1.24
0.34
1.63
99.93
593
23
381
2
7
148
4
15
13
111
48
32
19
44
6
26
6
1.59
4
0.61
3
0.64
1.41
0.19
1.24
0.20
58.75
16.84
6.35
0.08
3.30
5.94
3.47
1.66
0.89
0.25
2.05
99.57
53.96
16.70
8.34
0.11
4.66
7.32
3.90
1.11
1.37
0.34
1.57
99.37
56.90
16.97
6.80
0.07
3.84
6.85
3.71
1.55
0.87
0.24
1.75
99.55
455
46
511
4
5
161
506
26
312
2
7
146
463
31
436
3
5
137
14
12
48
23
38
16
14
114
50
42
13
14
66
28
50
20
44
5
21
5
1.40
3
0.51
3
0.52
1.22
0.19
1.21
0.18
Major-element and Sc abundances were determined by inductively coupled plasma emission,
and all other trace elements by inductively coupled
plasma mass spectrometry (ICP-MS) in the analytical laboratories of the Centre de Recherches
17
40
5
23
5
1.61
4
0.61
3
0.62
1.46
0.21
1.32
0.19
17
37
5
19
4
1.24
3
0.47
2
0.53
1.21
0.18
1.13
0.18
54.82
18.24
6.11
0.08
4.03
7.28
3.19
1.20
0.90
0.33
3.78
99.96
817
47
335
2
5
128
3
16
13
29
14
23
18
38
5
22
4
1.38
4
0.53
3
0.63
1.44
0.21
1.40
0.21
51.54
17.71
8.85
0.12
5.57
7.87
4.02
0.81
1.34
0.31
1.74
99.88
484
14
219
1
5
130
3
16
15
214
101
37
12
30
4
19
5
1.47
4
0.57
3
0.65
1.43
0.22
1.35
0.21
53.36
16.80
8.21
0.11
5.98
7.87
3.74
1.00
1.24
0.32
1.34
99.96
459
21
309
2
6
139
17
16
208
74
45
16
37
5
21
5
1.58
4
0.61
3
0.68
1.52
0.23
1.41
0.23
Pétrographiques
et
Géochimiques,
Centre
National du Recherches Scientifiques, in Nancy,
France. For conventional mineral K–Ar measurements, rock was crushed and sieved to retain the
0.125–0.18 mm size fraction. Biotite was separated
83
B. Martiny et al. / Tectonophysics 318 (2000) 71–98
Table 2 (continued )
Sample: CON-35
CON-75
CON-77
CON-88
CON-90
CON-109
CON-141
CON-142
Northern sector
Lava
SiO
2
Al O
2 3
Fe O
2 3
MnO
MgO
CaO
Na O
2
KO
2
TiO
2
PO
2 5
L.O.I.
Total
Sr
Rb
Ba
Th
Nb
Zr
Hf
Y
Sc
Cr
Ni
Co
La
Ce
Pr
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
59.13
16.89
6.09
0.09
3.21
5.76
3.49
1.93
0.91
0.26
1.97
99.73
467
40
575
3
6
173
4
14
11
51
19
31
23
48
6
24
5
1.35
4
0.53
3
0.59
1.37
0.19
1.24
0.20
Tuff
67.00
14.70
1.75
0.02
1.09
2.79
3.78
2.20
0.32
0.08
6.14
99.87
275
131
631
5
6
144
4
11
5
14
4
24
16
31
3
12
2
0.89
2
0.31
2
0.42
1.09
0.16
1.17
0.18
CON-59b
CON-60a
Southern sector
Hypabyssal
53.03
16.85
8.33
0.11
6.29
7.50
3.79
0.93
1.26
0.32
1.42
99.83
474
15
315
1
5
132
3
15
15
202
83
34
16
36
5
21
5
1.51
4
0.56
3
0.59
1.39
0.20
1.29
0.18
Lava
51.41
17.50
8.98
0.07
4.91
7.73
3.64
0.83
1.32
0.31
3.18
99.88
441
15
213
1
5
115
3
16
15
224
85
25
13
28
4
19
4
1.51
4
0.57
3
0.66
1.55
0.22
1.43
0.21
Lava
52.68
17.43
9.15
0.10
5.57
7.67
3.90
0.75
1.49
0.31
0.82
99.87
474
13
211
1
5
130
3
17
15
181
71
33
13
30
4
20
5
1.69
4
0.62
3
0.65
1.54
0.23
1.45
0.21
with a shaking table, magnetically and with an
electronic mortar to separate the mica sheets and
free possible chlorite. Hornblende was separated
magnetically and with heavy liquids. Three of the
hornblende separates were acid-leached at room
temperature in an ultrasonic cleaner in 10% HF
Lava
Hypabyssal
Hypabyssal
58.55
16.75
7.37
0.09
3.86
6.45
3.35
1.76
1.12
0.22
1.83
101.36
63.65
16.27
4.55
0.07
2.08
4.91
3.60
2.26
0.68
0.14
2.27
100.47
65.66
16.33
4.06
0.05
1.84
4.34
3.79
1.82
0.63
0.13
3.19
101.83
Tuff
55.59
18.76
5.54
0.07
1.69
5.69
2.96
1.59
0.70
0.19
6.88
99.65
Hypabyssal
58.91
16.94
6.29
0.06
2.05
5.67
3.79
2.28
0.91
0.26
2.53
99.68
625
51
376
4
5
113
464
53
511
3
6
151
19
10
28
20
21
16
11
36
19
29
17
29
4
17
4
1.33
3
0.51
3
0.63
1.41
0.21
1.31
0.21
20
39
6
24
5
1.38
4
0.59
3
0.62
1.35
0.21
1.18
0.17
to remove other minerals adhered to the hornblende. Mineral concentrates of >99.5% purity
were prepared and were analyzed by Geochron
Laboratory Division of Krueger Enterprises, Inc.
87Sr/86Sr and 143Nd/144Nd ratios were measured
on a Finnigan MAT 262 mass spectrometer at
84
B. Martiny et al. / Tectonophysics 318 (2000) 71–98
Table 2 (continued )
Sample: CON-61a
CON-62
CON-70
CON-72
CON-101
CON-49b
CON-52
CON-53
CON-54
CON-56
Hypabyssal
Hypabyssal
Tuff
Intrusive
Intrusive
Intrusive
Intrusive
Intrusive
66.82
16.25
3.60
0.05
1.29
3.37
4.04
2.92
0.48
0.17
0.82
99.81
68.94
15.07
2.94
0.05
0.78
2.97
3.85
3.29
0.36
0.13
0.93
99.31
69.51
15.11
2.99
0.05
0.81
2.93
3.88
3.29
0.36
0.14
0.74
99.81
64.90
16.41
4.42
0.05
1.77
4.12
3.97
2.66
0.63
0.18
0.71
99.82
65.38
16.27
4.35
0.05
1.67
3.99
4.03
2.65
0.59
0.18
0.72
99.88
Southern sector
Hypabyssal
SiO
2
Al O
2 3
Fe O
2 3
MnO
MgO
CaO
Na O
2
KO
2
TiO
2
PO
2 5
L.O.I.
Total
Sr
Rb
Ba
Th
Nb
Zr
Hf
Y
Sc
Cr
Ni
Co
La
Ce
Pr
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
56.72
17.42
6.81
0.10
3.35
7.00
3.62
1.58
0.89
0.24
1.98
99.71
Ignimbrite
67.83
12.24
1.81
Traza
0.75
1.93
0.97
4.88
0.20
0.02
9.09
99.72
59.26
16.38
6.01
0.06
3.64
5.85
3.60
1.80
0.86
0.23
2.02
99.71
593
37
411
3
5
124
249
171
814
9
6
111
429
49
525
4
5
153
16
13
34
18
37
26
3
4
3
10
14
12
110
40
23
16
34
4
19
4
1.22
3
0.53
3
0.62
1.50
0.20
1.32
0.19
58
56
13
49
8
1.37
6
0.92
5
0.95
2.31
0.31
2.03
0.30
22
46
6
23
5
1.32
3
0.51
3
0.52
1.25
0.18
1.08
0.17
61.44
16.52
4.91
0.05
1.60
4.64
2.90
2.48
0.62
0.20
4.53
99.89
448
66
484
6
5
150
4
12
9
25
12
16
23
47
6
21
4
1.12
3
0.45
2
0.48
1.24
0.18
1.18
0.19
68.86
14.70
2.55
0.02
0.82
1.32
1.75
5.82
0.21
0.05
3.90
100.00
112
124
434
10
6
100
3
9
4
11
2
21
442
83
842
6
7
143
4
12
6
7
3
34
21
40
4
16
3
0.49
2
0.34
2
0.34
0.79
0.12
0.79
0.13
23
47
6
19
4
0.86
3
0.43
2
0.45
1.16
0.17
1.12
0.16
315
86
671
8
7
136
295
85
745
7
7
138
11
5
5
5
76
10
5
4
4
74
17
35
4
17
3
0.92
3
0.39
2
0.41
0.93
0.14
0.99
0.16
22
43
5
19
3
0.92
3
0.37
2
0.36
0.89
0.13
0.80
0.14
368
80
611
5
5
146
4
11
7
11
7
42
18
41
5
19
4
0.93
3
0.43
2
0.43
1.04
0.13
0.95
0.14
357
75
646
6
5
158
4
11
7
10
7
39
18
39
5
19
4
0.94
3
0.43
2
0.43
1.01
0.14
0.97
0.14
The following errors are reported: <3% per weight per cent for major elements and <10% per ppm for most trace elements.
Additional major-element abundances were obtained by X-ray fluorescence ( XRF ) at the University Laboratory for Isotope
Geochemistry (LUGIS), University of Mexico ( UNAM ). The precision of XRF is generally better than 1% for major elements.
Quality control in the CNRS and LUGIS laboratories is maintained with international standards.
LUGIS (Laboratorio Universitario Geoquı́mico
Isotópico), UNAM. Lead isotopic compositions
of HF-leached feldspar separates and whole rock
samples were determined on a Finnigan MAT 262
mass spectrometer at the United States Geological
Survey in Reston, Virginia. Procedural Pb blanks
B. Martiny et al. / Tectonophysics 318 (2000) 71–98
85
during this study were less than 200 pg and were
therefore negligible compared to the values measured in the samples.
5. Geochemical results
Major- and trace-element compositions were
determined in the Tertiary magmatic rocks of the
study area; major elements were obtained in 31
samples and trace elements in 28 ( Table 2). All
volcanic and plutonic rocks analyzed were classified on an anhydrous basis. Previous geochemical
studies carried out by Hernández-Bernal and
Morán-Zenteno (1996) on the Rı́o Verde batholith
include major and trace elements as well as Sr and
Nd isotope determinations of the Jamiltepec,
Progreso and Rı́o Grande intrusions.
5.1. Major- and trace-element geochemistry
The lavas are the least differentiated rocks of
the study area and vary from 53 to 61% SiO
2
(anhydrous basis). Hypabyssal rocks are generally
more evolved and range from 54 to 67%. Using
the classification system of Le Maitre (1989), the
lava flows range in composition from basaltic
Fig. 6. Total alkali — SiO for Oligocene age volcanic rocks of
2
western Oaxaca for the classification of nonpyroclastic rocks
after Le Maitre (1989). B=basalt, BA=basaltic andesite, A=
andesite; D=dacite, R=rhyolite, TB=trachybasalt, TBA=
basaltic trachyandesite, TA=trachyandesite, T=trachyte.
Division between alkaline and subalkaline fields from Irvine
and Baragar (1971). Crosses=lavas, open circles=hypabyssal
rocks.
Fig. 7. K O–SiO classification diagram after Peccerillo and
2
2
Taylor (1976). I=arc tholeiitic series; II=calc-alkaline series;
III=high-K calc-alkaline series; IV=shoshonitic series.
Western Oaxaca samples (this study): crosses=lavas, open circles=hypabyssal stocks and dikes, diamonds=tuffs. Open
squares=volcanic rocks from northeastern Guerrero (data
from Morán-Zenteno et al., 1998).
andesite to andesite and the hypabyssal intrusions,
from basaltic andesite to dacite ( Fig. 6); most
samples from both groups have medium-K
contents. The volcanic rocks from western Oaxaca
are characterized by being subalkaline ( Fig. 6)
with a calc-alkaline affinity. The K O contents of
2
these magmatic rocks are generally typical of the
normal calc-alkaline series based on Peccerillo and
Taylor (1976), as shown in Fig. 7. Pyroclastic
rocks are intermediate to felsic in composition.
Oligocene volcanic rocks from the NE Guerrero
area ( Taxco, Huautla and Buenavista areas) are
also more silicic than the intermediate lavas that
predominate in the northern sector of western
Oaxaca. In this part of Guerrero, the volcanic
sequences consist principally high-K rhyolitic to
dacitic ignimbrites and lava flows with no significant intermediate components (Morán-Zenteno
et al., 1998) ( Fig. 7).
86
B. Martiny et al. / Tectonophysics 318 (2000) 71–98
Fig. 8. Total alkali — SiO diagram after Cox et al. (1979) modified by Wilson (1989) for the classification of plutonic rocks. The
2
classification of granitoids of Tertiary age from the coastal plutonic belt of western Oaxaca is shown as: X in shaded area=plutonic
rocks of the La Muralla pluton (this study). Circles=Jamiltepec, triangles=Progreso and Rı́o Grande intrusions (data from
Hernández-Bernal and Morán-Zenteno, 1996).
The plutonic rocks analyzed in this study are
from the La Muralla–San Pedro Amuzgos region
and plot in the granodiorite and granite fields
(SiO =66–70%) using the classification of Cox
2
et al. (1979) modified by Wilson (1989) ( Fig. 8,
Table 2). Closer to the Pacific coast, the Rı́o
Grande and Progreso intrusions of the Rı́o Verde
batholith show a very similar composition, with
the exception of the Jamiltepec intrusion, which is
less differentiated. The data corresponding to the
La Muralla pluton and the Rı́o Verde batholith
straddle the boundary between the peraluminous
and metaluminous rocks using Shand’s index
(Maniar and Piccoli, 1989). These samples have
an A/CKN coefficient (Al O /CaO+K O+
2 3
2
Na O) of <1.1 (molar ratio) and relatively high
2
sodium contents (Na O>3.2%), corresponding to
2
I-type granites based on the classification of
Chappell and White (1974).
Variation diagrams for trace elements are shown
for the western Oaxaca samples in Fig. 9a and b.
Trace-element patterns of the coastal plutons and
the volcanic rocks of the inland region are similar
with enrichment in large-ion lithophile elements
(LILE) ( K, Rb, Ba, Th) relative to the HFS
elements (Nb, Ti±Zr) that are characteristic of
subduction-related magmatism (e.g. Gill, 1981;
Pearce, 1982, 1983; McCulloch and Gamble, 1991;
Saunders et al., 1991). Compared to the less
differentiated inland volcanic sequences, the plutons display more negative spikes for Ti and
P O , are more enriched in incompatible elements,
2 5
and show a greater depletion in compatible elements (Cr, Ni). The patterns for the immobile
elements (Nb, Zr, Hf, Ti Y and Yb) on variation
diagrams ( Fig. 9a and b) show more similarity to
that of intra-plate basalts than to MORB. This
and the enrichment in LILE suggest an enriched
mantle source in the subcontinental lithosphere
modified by subduction fluids, which have added
the more mobile elements (Rb, Ba, K ) (Pearce,
1983; Wilson, 1989). The inland volcanic sequences
have Ba/La ratios that vary from 15 to 25 and
La/Nb from 2 to 5 which is within the range that
is typical of calc-alkaline lavas from other convergent plate boundaries (Gill, 1981).
B. Martiny et al. / Tectonophysics 318 (2000) 71–98
87
Fig. 9. Trace-element variation diagrams in Tertiary magmatic rocks of western Oaxaca, MORB normalized using the values of
Pearce (1983). (a) Lavas and hypabyssal rocks of the northern and southern sectors. (b) Granitoids of the La Muralla pluton.
The rare earth element (REE ) abundances in
the samples of the inland volcanic sequences and
the coastal plutonic belt also show similar tendencies. Chondrite-normalized REE patterns display
light rare earth element enrichment (LREE; La–
Sm) and relatively flat patterns for the heavy rare
earth elements (HREE; Tb–Lu) ( Fig. 10a and b).
Although some granitoids display a very modest
negative Eu anomaly ( Fig. 10b), no significant
anomalies are observed in the volcanic rocks
88
B. Martiny et al. / Tectonophysics 318 (2000) 71–98
Fig. 10. Chondrite-normalized rare earth element data for Tertiary magmatic rocks of western Oaxaca normalized using the values
of Nakamura (1974). (a) Lavas and hypabyssal rocks. (b) Granitoids of the La Muralla pluton.
( Fig. 10a), indicating that plagioclase fractionation
was not significant. The plutonic rocks have
slightly lower HREE concentrations; (La/Lu)cn
ratios range from 6.0 to 13.6 in the lavas and
hypabyssal rocks, and from 11 to 16.7 in the
granitoids. La of the Tertiary rocks varies from 40
to 90 times chondrite and Lu, four to seven times
chondrite. The LREE correlate positively with
SiO in the Tertiary magmatic rocks although this
2
tendency is not displayed by the HREE.
89
B. Martiny et al. / Tectonophysics 318 (2000) 71–98
for the volcanic sequences, and from 0.7042 to
0.7044 for the granitoids in the La Muralla–San
Pedro Amuzgos area ( Table 3). eNd values
i
obtained for the Tertiary magmatic rocks of this
study area range from close to 0 to +2.6 ( Table 3,
Fig. 11). Considering the isotopic heterogeneity of
the crust in western Oaxaca, the narrow ranges
and generally low 87Sr/86Sr ratios and eNd values
i
near and mostly above that of bulk earth suggest
a relatively low degree of crustal contamination.
The Eocene age laccolith located in the eastern
5.2. Isotope geochemistry
The results of a first group of isotopic analyses
are presented here and consist of 12 Sr determinations, 10 for Nd and 9 for Pb ( Tables 3 and 4).
Despite the contrast in the degree of differentiation, the silicic coastal plutons and the intermediate
volcanic units of the northern volcanic sector show
similar isotopic features within narrow ranges.
Initial 87Sr/86Sr ratios of the Oligocene samples
are relatively low and range from 0.7042 to 0.7046
Table 3
Sr and Nd isotopic and chemical data: lavas, hypabyssal rocks and coastal plutons of western Oaxaca
Sample
no.
Rb
Sr
Sm
Nd
(87Sr/
(ppm) (ppm) (ppm) (ppm) 86Sr)
m
Lavas and hypabyssal intrusions
CON 7
21.7
918
5.75
(143Nd/
144Nd)
m
0.068
0.512715±19 0.1108 0.512686
1.50
1.95
0.512749±33 0.1414 0.512718
0.512712±40 0.1248 0.512684
2.17
1.44
2.41
1.76
0.512592 -0.37
0.512597 -0.29
0.512639 0.53
0.512726 2.28
-0.05
0.05
0.87
2.57
(eNd ) (eNd )
0
i
13.2
23.5a
22.3
35.5
38.4
48.6
15.2
535
646
671
508
548
490
556
5.16
5.74
6.16
4.72
4.96
4.64
4.88
22.1
27.8
27.2
23.4
25.5
23.8
22.4
La Muralla pluton
CON 52
86a
CON 53
85.2a
CON 54
79.3
358
338
420
3.45
3.69
4.03
17.8
19.1
19.9
0.704668±46
0.704677±40
0.704423±41
0.695
0.730
0.546
0.704372 0.512726±33 0.1170 0.512703
0.704366 0.512723±45 0.1166 0.512700
0.704190 0.512747±24 0.1225 0.512723
660
681
4
33
0.704339±59
0.704313±46
0.263
0.220
0.704287 0.512614±29 0.138
0.704270
0.51258
-0.2
480
450
677
352
2
4
24
23
0.704616 0.512617±40 0.118
0.704678 0.512651±33 0.113
0.704227
0.705387 0.512513±30 0.112
0.9
0.5
18
0.466
0.355
0.284
0.933
0.51264
0.51263
5
0.704701±36
0.704735±248
0.704271±34
0.70553±33
0.51249
-2.1
4
2
22
24
0.704997±41
0.705394±32
0.705444±41
0.704809±159
0.601
0.533
0.417
0.376
0.704905
0.705314 0.51247±34
0.70538 0.51247±51
0.704751
0.51245
0.51245
-3.0
-3.0
CON
CON
CON
CON
18
35
70
77
Rio Verde batholithb
Jamiltepec
503
50
504
47
Progreso
505
75
506
46
507
53
508
94
Rı́o Grande
509
72
510
62
511
54
512
68
413
437
470
651
0.071
0.105
0.096
0.202
0.203
0.287
0.079
0.703688
0.703688
0.704336
0.704536
0.704511
0.704626
0.704617
0.704553
0.704198
147Sm/ (143Nd/
144Nd 144Nd)
i
0.703727±37
0.703735±40
0.704371±60
0.704587±47
0.704557±33
0.704724±41
0.704715±45
0.704692±36
0.704236±36
CON 9
CON 14
31.4
87Rb/ (87Sr/
86Sr
86Sr)
i
0.512619±43
0.512623±20
0.512665±16
0.512755±19
0.1219
0.1178
0.1179
0.1316
0.090
0.090
1.72
1.66
2.13
2.02
1.96
2.41
Element concentrations obtained by isotope dilution.
a Obtained by ICP-MS. Measurements for the La Jolla Nd standard are 143Nd/144Nd=0.511885±27 and for the SRM-987 standard
are 87Sr/86Sr=0.710233±16. Initial eNd values and 87Sr/86Sr ratios were calculated at 30 Ma for the plutons, 34 Ma for the lavas
and hypabyssal intrusions, with the exception of CON-7, which was calculated at 40.5 Ma, and assuming a present-day
143Nd/144Nd (CHUR)=0.512638.
b Rı́o Verde batholith data from Hernández-Bernal and Morán-Zenteno (1996).
90
B. Martiny et al. / Tectonophysics 318 (2000) 71–98
Table 4
Pb isotopic compositions of Tertiary magmatic rocks and Precambrian basement
Rock type
206Pb/204Pb
207Pb/204Pb
208Pb/204Pb
Laccolith
Dike
Lava
18.679
18.720
18.714
18.669
15.592
15.605
15.608
15.587
38.457
38.523
38.532
38.442
Plutonic rocks of the La Muralla–San Pedro Amuzgos area
CON-52 WR
S. M. Zacatepec
Granite
CON-52 ksp
CON-53 WR
S. M. Zacatepec
Granite
CON-53 ksp
S. M. Zacatepec
CON-54 ksp
La Muralla
Granodiorite
18.749
18.720
18.706
18.696
18.703
15.618
15.623
15.587
15.594
15.615
38.588
38.580
38.487
38.481
38.545
Oaxaca Complex
CON-215 WR
CON-215 ksp
CON-336 WR
17.248
17.221
17.141
15.486
15.501
15.499
36.578
36.602
36.508
Sample number
Location
Lavas and hypabyssal rocks
CON-7 WR
Yanhuitlán
CON-20 WR
Huajuapan
CON-32 WR
Huajuapan
CON-32 plag
S of Oaxaca City
Nochixtlán–Oaxaca
Metagabbro
Metagabbro
Charnockite
Fig. 11. Sr–Nd isotopic initial ratios of Tertiary volcanic and plutonic rocks in southern Mexico and other Tertiary and Quaternary
magmatic provinces of Mexico. The shaded field represents the Tertiary age magmatic rocks of western Oaxaca: open circles: plutonic
rocks of the La Muralla pluton (this study); filled circles: lavas and hypabyssal rocks of the inland volcanic sequences (this study);
triangles: plutonic rocks of the Rı́o Verde batholith (data from Hernández-Bernal and Morán-Zenteno, 1996); squares: plutonic rocks
of the Jamiltepec and San Pedro Amuzgos areas (data from Herrmann, 1994). Other magmatic provinces: SMO: Sierra Madre
Occidental (SMO field includes data for the Upper Volcanic sequence of the northern SMO, with dashed lines enclosing typical
values, from Lanphere et al., 1980; Verma, 1984; Cameron and Cameron, 1985; Cameron et al., 1986; Smith et al., 1996). TMVB=
typical values of the Trans Mexican Volcanic Belt (data from Verma, 1983; Verma and Nelson, 1989). Coastal plutonic belt of
southern Mexico: M=Manzanillo, J=Jilotepec, Z=Zihuatanejo, and H=La Huacana (data from Schaaf, 1990); A=Acapulco (data
from Schaaf, 1990; Morán-Zenteno, 1992; Herrmann, 1994).
B. Martiny et al. / Tectonophysics 318 (2000) 71–98
91
Fig. 12. Plot of 207Pb/204Pb–206Pb/204Pb for feldspars and whole-rock samples of Tertiary magmatic rocks of western Oaxaca and
northeastern Guerrero, and Precambrian basement rocks; data for preliminary Paleozoic Acatlán Complex field are from Lopez
and Cameron (unpublished data) and Martiny et al. (1997). X=field for undeformed Tertiary plutons of the Xolapa terrane between
Acapulco and Huatulco from Herrmann et al. (1994). G=field for Tertiary volcanic rocks from NE Guerrero from Martiny et al.
(1997). Additional data for Oaxaca Complex field from Solari et al. (1998), Lopez et al. (in press) and Cameron et al. (submitted
for publication). Reference lines are the two-stage terrestial lead evolution curve (Stacey and Kramers, 1975), graduated at 250 Ma
intervals (SK ), and the Northern Hemisphere Reference Line (NHRL) (Hart, 1984).
part of the study area (sample CON-7), northwest
of Yanhuitlán, has a lower 87Sr/86Sr ratio of 0.7037
and could reflect less crustal involvement; this
sample has an eNd value of +2.0 ( Table 3).
i
Dacites and rhyolites from northeastern
Guerrero are of a similar age (30.5–38.2 Ma), and
in Taxco, for example, five samples analyzed have
higher initial 87Sr/86Sr ratios that range from
0.7052 to 0.7063 (Morán-Zenteno et al., 1998),
which could be explained by more crustal contamination or a more evolved crustal component. The
volcanic rocks analyzed from Taxco are near the
boundary between the Mixteca and Guerrero
terranes.
Sr and Nd ratios obtained by Hernández-Bernal
and Morán-Zenteno (1996) for the Rı́o Verde
batholith show more variation than the granitoids
analyzed in the present study ( Table 3, Fig. 11).
Tonalitic intrusions of the Jamiltepec area display
values similar to those of the La Muralla pluton
located farther inland, whereas the Progreso and
Rı́o Grande intrusions, located to the east of
Jamiltepec, present similar or higher 87Sr/86Sr
i
ratios and similar or lower eNd values ( Table 3).
i
Other plutonic rocks from this region reported by
Herrmann (1994) have similar Sr and Nd values.
Pb isotopic ratios of whole rocks and leached
feldspars of the Tertiary magmatic rocks of western
Oaxaca determined in this study display a relatively
restricted range, suggesting that the source of these
rocks is similar. On Pb isotope diagrams, the ratios
of the volcanic, hypabyssal and plutonic rocks
overlap and plot below the average Pb crust evolution curve of Stacey and Kramers (1975) ( Fig. 12).
The volcanic rocks of western Oaxaca display
present-day ratios of (206Pb/204Pb)=18.67–18.72,
(207Pb/204Pb)=15.59–15.61, and (208Pb/204Pb)=
38.44–38.53. The granitoids show similar ratios
of
(206Pb/204Pb)=18.70–18.75, (207Pb/204Pb)=
15.59–15.62, and (208Pb/204Pb)=38.48–38.59
( Table 4). The lead isotope range of the Tertiary
igneous rocks of the study area resembles that of
the orogene reservoir in the plumbotectonics model
of Doe and Zartman (1979).
In Fig. 12, the Tertiary magmatic rocks of western Oaxaca appear to define a steep mixing trend
between a mantle component and a 207Pb-rich
reservoir. Steep trends are typical of some subduc-
92
B. Martiny et al. / Tectonophysics 318 (2000) 71–98
tion-related rocks, such as the Aleutian, Cascades,
Mariana and Lesser Antilles arcs ( Kay et al., 1978;
Woodhead and Fraser, 1985; White and Dupre,
1986) and have been interpreted as due to the
incorporation of radiogenic Pb from subducted
sediments ( Hawkesworth et al., 1991). The more
silicic volcanic rocks from NE Guerrero are slightly
more radiogenic; in a 207Pb/204Pb vs. 206Pb/204Pb
diagram, they plot to the right of the Stacey and
Kramers curve and fall in a more scattered field
( Fig. 12) (Martiny et al., 1997). Pb ratios are also
reported in leached plagioclase feldspars of five
undeformed Tertiary granitoids of the Xolapa terrane (Herrmann et al., 1994) from the area that
extends from Acapulco to Pochutla (Fig. 12).
Compared to the magmatic rocks of the study
area, these plutons have similar 206Pb/204Pb ratios
and similar or slightly lower 207Pb/204Pb and
208Pb/204Pb ratios.
Pb isotopic compositions have been obtained for
the Precambrian and Paleozoic basement rocks in
the present study and by other workers who are
addressing problems related to the basement rocks
(Solari et al., 1998; Lopez et al., in press; Cameron
et al., submitted for publication). Whole rock
samples and feldspar separates from the igneous
units of the Precambrian Oaxaca Complex (metagabbro, metasyenite, charnockite, metagranite and
anorthosite) have present-day Pb isotope ratios that
are typical of Grenville age rocks [(206Pb/204Pb)=
16.95–17.55; (207Pb/204Pb)=15.47–15.54; (208Pb/
204Pb)=36.40–36.66 ] (Table 4; Solari et al., 1998;
Lopez et al., in press; Cameron et al., submitted
for publication). Preliminary Pb isotopic compositions of the Paleozoic Acatlán Complex units are
very scattered on a 207Pb/204Pb–206Pb/204Pb diagram (preliminary field shown in Fig. 12) and lie
above and to the right of the average Pb crust
evolution curve of Stacey and Kramers (1975)
(unpublished data from Lopez and Cameron;
Martiny et al., 1997).
6. Discussion
6.1. Space–time trends of magmatism
Stratigraphic and geochronologic evidence indicates that in western Oaxaca, a major magmatic
event commenced in Oligocene times (Table 1).
Oligocene magmatism in western Oaxaca and eastern Guerrero is coeval with the displacement of the
Chortis block along the Pacific margin of southern
Mexico (Herrmann et al., 1994; Schaaf et al., 1995)
and the consequent migration of the trench–trench–
transform triple junction that constituted the intersection between the North American, Farrallon and
Caribbean plates (Pindell et al., 1988; Ross and
Scotese, 1988; Herrmann et al., 1994; Schaaf et al.,
1995; Morán-Zenteno et al., 1996). The relationship
between the ages obtained for the mylonitic zones
parallel to the coast and the plutons support this
interpretation.
On a regional scale, along-the-coast magmatism
in southern Mexico during the Tertiary displays a
rough decreasing age trend from northwest to
southeast (Schaaf et al., 1995). The gradual extinction of magmatism along the coast, at least to the
east of the Zihuatanejo region, is directly related
to the passage of the triple junction (Herrmann
et al., 1994; Schaaf et al., 1995). In the inland
volcanic belt, certain differences are displayed in
this decreasing age trend, particularly by the
Miocene ages in the region between the Valley of
Oaxaca and Nejapa areas ( Ferrusquı́a-Villafranca
and McDowell, 1991) that lie north of the
Huatulco area where there are still Oligocene age
plutonic rocks (Schaaf et al., 1995).
In westernmost Oaxaca, the K–Ar and U–Pb
dates reported for plutonic rocks (30–23.5 Ma)
along the coast between Pinotepa Nacional and
Rı́o Grande are slightly younger than those of the
inland volcanic rocks (34.8–31.4 Ma) ( Table 1).
However, the K–Ar ages reported for the plutons
correspond to mineral cooling ages (biotite and
hornblende) and are not directly comparable to
the reported ages of the volcanic rocks. A comparison between the U–Pb ages of plutonic rocks (30
and 28 Ma) along the coast (Herrmann et al.,
1994) and K–Ar mineral ages obtained in this
study (34–31 Ma) for the volcanic rocks in the
Huajuapan–Tlaxiaco area indicates that the extrusive rocks are slightly older. None the less, the
reports of younger whole rock and mineral ages
present in the inland area (Ferrusquı́aVillafranca
and McDowell, 1991; Galina-Hidalgo, 1996) prevent us from confirming a southward migration of
the magmatism. Instead, we consider that the
B. Martiny et al. / Tectonophysics 318 (2000) 71–98
western Oaxaca magmatic rocks constituted a
broad arc parallel to the coast in Oligocene time
(~35 to ~25 Ma). In western Oaxaca, as well as
in northeastern Guerrero, magmatic activity ceased
in the late Oligocene and recommenced to the
north at about 16 Ma in the Trans-Mexican
Volcanic Belt. The magmatic gap at this longitude
was probably caused by changes in the geometry
of the subducted slab after the passage of the triple
junction (Morán-Zenteno et al., 1996). In central
and eastern Oaxaca, magmatism continued until
Miocene time ( Ferrusquı́a-Villafranca and
McDowell, 1991).
6.2. Geochemical patterns and variations
There are certain differences in the geochemical
behavior between the extensive inland volcanic
sequences of the predominant upper unit in the
northern sector and the Oligocene magmatic rocks
of other adjacent regions. The most evident difference is the degree of differentiation. In western
Oaxaca, the SiO contents of the magmatic rocks
2
increase towards the coast. Basaltic andesite to
andesitic compositions characterize the upper unit
in the northern volcanic sector, andesites and
dacites were identified in the southern volcanic
sector, and in the coastal plutonic belt, granites
and granodiorites are prevalent ( Table 2). An
exception is the Jamiltepec intrusion, the least
differentiated pluton within the Rio Verde batholith, which is of tonalitic composition (Fig. 8).
There is also a significant contrast between the
degree of differentiation of the inland volcanic
rocks of western Oaxaca and those of northeastern
Guerrero; in this latter region, the volcanic rocks
display rhyolitic to dacitic compositions, and intermediate units are not important ( Fig. 7).
The Sr and Nd isotopic compositions of the
intermediate lavas of the northern inland volcanic
sequence and the La Muralla pluton in western
Oaxaca have a restricted range, with relatively low
87Sr/86Sr ratios and eNd values from near 0 to
+2.6. There is a difference between the 87Sr/86Sr
ratios of these western Oaxaca rocks and the more
differentiated rocks of northeastern Guerrero, with
slightly higher and more heterogeneous 87Sr/86Sr
ratios observed in this latter region (MoránZenteno et al., 1999). The plutons along the coast
93
(Progreso and Rı́o Grande areas) also show more
variable isotopic compositions and have lower eNd
values and higher 87Sr/86Sr ratios ( Table 3). This
slightly greater range of Sr and Nd ratios in the
Tertiary plutons of the Xolapa terrane might be
the result of greater crustal assimilation during
magma ascent or assimilation of crust with a more
heterogeneous isotopic composition.
An even greater variability is seen in the Nd
isotopic composition of the Tertiary coastal plutons of the Guerrero terrane. These plutons, with
the exception of Puerto Vallarta (Schaaf et al.,
1995), have higher eNd values (+1 to +6.37)
(Schaaf, 1990; Böhnel et al., 1992) than the western
Oaxaca plutons (−3.0 to +2.6) ( Fig. 11, Table 3).
The reason for this difference is not clear. The
Guerrero terrane is part of a relatively young
crustal segment that was integrated with the North
American plate during the Mesozoic (CentenoGarcı́a et al., 1993), whereas the Tertiary magmatic
rocks in western Oaxaca have an older basement.
This difference could be explained by a lithospheric
mantle that is more enriched in a subduction
component in western Oaxaca than in Guerrero
or by assimilation of crust with variable isotopic
signatures.
Although the western Oaxaca Tertiary magmatic rocks were emplaced in Precambrian–
Paleozoic basement, the 87Sr/86Sr ratios are low,
and eNd values range from −3.0 to +2.6
( Table 3). These eNd values are similar to those
displayed by the mid-Tertiary ignimbrites and lavas
of the Upper Volcanic sequence in the northern
Sierra Madre Occidental, where they range from
−1.8 to +4.1, although 87Sr/86Sr ratios show a
larger range (0.7038–0.710) (Lanphere et al., 1980;
Verma, 1984; Cameron and Cameron, 1985;
Cameron et al., 1986; Smith et al., 1996).
Pb isotopic compositions of the magmatic rocks
in western Oaxaca, as with Sr and Nd isotopic
ratios, show a very narrow range ( Table 4, Fig. 12)
although these rocks vary from intermediate to
acidic compositions. This suggests a similar source
and evolution for these rocks. The distribution of
data from Tertiary rocks of the study area on a
207Pb/204Pb–206Pb/204Pb diagram appears to define
a steep mixing line with a narrow range. The
possible mixing end members cannot have been
conclusively identified with the data available up
94
B. Martiny et al. / Tectonophysics 318 (2000) 71–98
to now, although the general trend of the data
suggests a mantle source contaminated with a
207Pb-rich component. The 207Pb-rich component
could correspond to the influence in the mantle
wedge of fluids derived from the subduction zone
or assimilation of the Acatlán Complex. It is not
possible to identify the isotopic composition of a
subduction component by establishing an analogy
with the present-day sediments in the Acapulco
trench. The continental source for the trench sediments was most likely different in the early
Oligocene since the Chortis block must have been
involved, and extensive exposures of Oligocene
batholiths were lacking.
The preliminary data available up to now for
the Paleozoic Acatlán Complex indicate that the
Sr, Nd and Pb isotopic composition is highly
variable ( Yañez et al., 1991; Martiny et al., 1997;
unpublished data from Lopez and Cameron). It
seems that any significant degree of assimilation
would have resulted in a greater dispersion of the
data for the Tertiary magmatic rocks, although we
do not completely discard assimilation of the
Acatlán Complex. The Pb isotopic composition of
igneous units of the Oaxaca Complex ( Table 4,
Fig. 12) indicates that these units of the
Precambrian basement were not incorporated into
the Tertiary magmatic rocks to any notable degree.
There are several indications that fractional
crystallization is probably the most important process of magma differentiation for the western
Oaxaca magmatic rocks analyzed in this study.
Assuming a similar source for these rocks and
given the heterogeneity of the basement rocks in
this region, the narrow range of Sr, Nd and Pb
isotope ratios, particularly for the northern volcanic sector and La Muralla pluton, indicates a
low degree of crustal contamination. Coherent
linear trends with little scatter for unaltered
samples on variation diagrams of major oxides
and trace elements vs. SiO could, thus, be
2
explained by fractional crystallization. None the
less, the probability of a low degree of assimilation
cannot be discarded.
6.3. Relationship between tectonics and magmatism
The cause of the greater differentiation of the
plutons along the coast is not completely under-
stood but does not appear to be related to the
presence of a thick crust since this region was
affected by transtension even before the Oligocene
magmatism. In the Huajuapan–Tlaxiaco region,
thick sequences of lacustrine-fluvial volcaniclastic
deposits and volcanic rocks accumulated in NNW–
SSE-trending basins at the time of volcanic activity
and the presence of oblique, lateral and vertical
striae in fault planes reflect the extensional environment for this region. We consider that the greater
differentiation of the coastal plutons is related to
a lower extensional deformation at the time of the
magmatism with respect to the inland regions and
the greater volume of magma involved in this zone.
Structural observations in the Chacalapa shear
zone and the relationship with the Oligocene intrusions of the coastal region suggest that, previous
to the magmatism, extension was the main component of deformation, whereas the strike-slip component dominated afterwards ( Tolson-Jones,
1998). The peak of magmatism in the coastal
region of Oaxaca seems to have occurred during
the transition between these two strain regimes.
We have documented that the volcanic rocks in
this region, especially the predominant upper volcanic unit of the northern sector, are less
differentiated than the coastal plutons ( Table 2).
The northern sector volcanic sequences are also
less differentiated than the volcanic rocks of the
Taxco region, where Oligocene volcanism is associated with NNE-trending strike-slip faults and no
significant extensional features have been recognized (Morán-Zenteno et al., 1998). In central and
southeastern Oaxaca, the occurrence of abundant
silicic ignimbrites also suggests a lower rate of
extension. Silicic rocks reported in the Oaxaca
fault zone seem to have occurred in an extensional
region, but since the Oaxaca fault is an old feature
with different episodes of reactivation, the extension rate at the time of silicic magmatism is
uncertain.
In arc regions, deformation and stress fields
influence the generation and the ascent of magmas,
which, in turn, is regulated by buoyancy and
thermal effects (e.g. Singer et al., 1989; Apperson,
1991; Takada, 1994). Since lithosphere extension
in arc regions affects the level to which a magma
body ascends, it will also have an effect on the
degree and type of differentiation (crustal contami-
B. Martiny et al. / Tectonophysics 318 (2000) 71–98
nation and crystal fractionation) (Burkart and
Self, 1985; Glazner and Ussler, 1989). For example, in the central Aleutian arc, basaltic lavas are
associated with the degree of intra-arc extension
along the volcanic axis that modified the thermal
and density structure of the lithosphere (Singer
and Myers, 1990). In the currently active extensional Trans-Mexican Volcanic Belt, AlanizÁlvarez et al. (1998) found a correlation between
the type of volcanism (monogenetic vs. polygenetic) and the strain rate. Monogenetic volcanism
tends to be more mafic and is associated with
faults having a higher strain rate. In western
Oaxaca, a correlation also seems to exist between
the extensional strain rate and the general degree
of differentiation.
7. Conclusions
(1) K–Ar age determinations of igneous rocks
in western Oaxaca indicate that volcanic and plutonic activity occurred during the Oligocene (~35
to ~25 Ma); the volcanic sequences crop out in
the inland region, whereas the plutonic rocks are
found along the coast. These rocks form part of
an extensive magmatic arc in southern Mexico
that roughly displays a decreasing age trend from
Paleocene in Colima to Miocene in eastern Oaxaca.
(2) Although age determinations reported in
this work for the western Oaxaca region appear
to indicate that the coastal plutonic rocks are
slightly younger than the inland volcanic
sequences, other data reported previously give no
clear indication of a southward migration for the
magmatism and, instead, suggest a broad magmatic arc parallel to the coast during the
Oligocene.
(3) In general, the SiO content of the Tertiary
2
magmas of western Oaxaca increases from the
inland region toward the coast. In the northern
sector of western Oaxaca, magmatism began with
volcanic activity of acidic to intermediate composition that produced a lower unit of epiclastic deposits, ash fall tuffs and ignimbrites overlain by a
predominant upper unit of basaltic andesite to
andesitic lavas and autobreccias. In the southern
volcanic sector dacitic to andesitic compositions
are predominant with insignificant amounts of
95
more mafic magmatism. The volcanic sequences in
both sectors are intruded by hypabyssal bodies
that vary in composition from dacite to basaltic
andesite. The coastal plutonic belt is even more
differentiated and is composed principally of granitic to granodioritic plutons.
(4) The trace-element concentration of the magmatic rocks in western Oaxaca is characteristic of
arc-related magmas. The relatively low Sr ratios
and eNd ratios near that of bulk earth as well as
i
the general low variability of Sr, Nd and Pb
isotope ratios, especially for the inland volcanic
region and the La Muralla pluton, suggest a low
degree of crustal contamination. The narrow range
of isotopic ratios, which are more radiogenic compared to depleted mantle, indicate the subcontinental lithospheric mantle contaminated by a
subduction component as a probable source.
(5) The degree of differentiation in the magmatic rocks in western Oaxaca seems to have been
influenced by the different strain domains in the
region. The higher degree of differentiation of the
plutons along the coastal area and their slightly
greater crustal contamination, compared to the
intermediate volcanic sequences that are dominant
in the northern volcanic sector, seem to be related
to the lower extensional deformation in the
coastal area.
Acknowledgements
Financial support by the National Council of
Science and Technology (CONACyT ) (project
3361 T9309) in Mexico is gratefully acknowledged.
Pb isotopic determinations were made possible by
student grants received by one of the authors
(Barbara Martiny) given by the Program of
Financial Aid for Graduate Studies (PADEP) at
UNAM and the Geological Society of America
(Howard T. Sterns Fellowship Award). The
authors wish to thank G. Silva-Romo, S.A. AlanizÁlvarez, Á. F. Nieto-Samaniego and R. Lopez for
discussion and helpful comments; P. Schaaf and
J.J. Morales-Contreras for assistance with the analytical aspects of the isotopic determinations; R.
Lozano-Santacruz for the XRF determinations;
M. Reyes-Salas for SEM analyses in the evaluation
of some of the samples for isotopic determinations;
96
B. Martiny et al. / Tectonophysics 318 (2000) 71–98
L. Alba-Aldave and T. Hernández-Treviño for
assistance in the field; A. Victoria-Morales for
providing the charnockite of the Oaxaca Complex;
and J.T. Vazquez for some of the sample preparations. We also thank K. Cameron and F.W.
McDowell for their helpful reviews. Additional
unpublished data for the preliminary Acatlán
Complex Pb isotope field are from R. Lopez, K.
Cameron and F. Ortega.
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