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www.elsevier.com/locate/jvolgeores
Journal of Volcanology and Geotherm
Sr, Nd and Pb isotope and geochemical data from the Quaternary
Nevado de Toluca volcano, a source of recent adakitic magmatism,
and the Tenango Volcanic Field, Mexico
Raymundo G. Martınez-Serranoa,*, Peter Schaaf a, Gabriela Solıs-Pichardob,
Ma. del Sol Hernandez-Bernalb, Teodoro Hernandez-Trevinoa,
Juan Julio Morales-Contrerasa, Jose Luis Macıasa
aUniversidad Nacional Autonoma de Mexico, Instituto de Geofısica, Laboratorio Universitario de Geoquımica Isotopica (LUGIS),
Ciudad Universitaria, Mexico D.F. 04510, MexicobUniversidad Nacional Autonoma de Mexico, Instituto de Geologıa, Laboratorio Universitario de Geoquımica Isotopica (LUGIS),
Ciudad Universitaria, Mexico D.F. 04510, Mexico
Received 24 November 2003; accepted 22 June 2004
Abstract
Volcanic activity at Nevado de Toluca (NT) volcano began 2.6 Ma ago with the emission of andesitic lavas, but over the past
40 ka, eruptions have produced mainly lava flows and pyroclastic deposits of predominantly orthopyroxene–hornblende dacitic
composition. In the nearby Tenango Volcanic Field (TVF) pyroclastic products and lava flows ranging in composition from
basaltic andesite to andesite were erupted at most of 40 monogenetic volcanic centers and were coeval with the last stages of
NT. All volcanic rocks in the study area are characterized by a calc-alkaline affinity that is consistent with a subduction setting.
Relatively high concentrations of Sr (N460 ppm) coupled with low Y (b21 ppm), along with relatively low HREE contents and
Pb isotopic values similar to MORB-EPR, suggest a possible geochemical adakitic signature for the majority of the volcanic
rocks of NT. The HFS- and LIL-element patterns for most rocks of the TVF suggest a depleted source in the subcontinental
lithosphere modified by subduction fluids, similar to most rocks from the Trans-Mexican Volcanic Belt (TMVB). The isotopic
compositions are similar for volcanic rocks of NT and TVF regions (87Sr/86Sr: 0.703853–0.704226 and 0.703713–0.704481;
qNd: +4.23–+5.34 and +2.24–+6.85; 206Pb/204Pb: 18.55–18.68 and 18.58–18.69; 207Pb/204Pb: 15.54–15.62 and 15.56–15.61;208Pb/204Pb: 38.19–38.47 and 38.28–38.50, respectively), suggesting a MORB-like source with low crustal contamination.
Metamorphic xenoliths from deeper continental crust beneath NT volcano show isotopic patterns similar to those of Grenvillian
rocks of north-central Mexico (87Sr/86Sr: 0.715653–0.721984, qNd: –3.8 to –7.2, 206Pb/204Pb: 18.98–19.10, 207Pb/204Pb: 15.68–15.69, 208Pb/204Pb: 39.16–39.26 and Nd model age (TDM) of 1.2–1.3 Ga). In spite of a thick continental crust (N45 km) that
underlies the volcanoes of the study area, the geochemical and isotopic patterns of these rocks indicate low interaction with this
crust. NT volcano was constructed at the intersection of three fault systems, and it seems that the Plio–Quaternary E–W system
0377-0273/$ - s
doi:10.1016/j.jv
* Correspon
E-mail addr
al Research 138 (2004) 77–110
ee front matter D 2004 Elsevier B.V. All rights reserved.
olgeores.2004.06.007
ding author. Tel.: +52 55 56 22 40 28; fax: +52 55 55 50 24 86.
ess: [email protected] (R.G. Martınez-Serrano).
R.G. Martınez-Serrano et al. / Journal of Volcanology and Geothermal Research 138 (2004) 77–11078
played an important role in the ascent and storage of magmas during the recent volcanic activity in the two regions. Chemical
and textural features of orthopyroxene, amphibole and Fe–Ti oxides from NT suggest that crystallization of magmas occurred at
polybaric conditions, confirming the rapid upwelling of magmas.
D 2004 Elsevier B.V. All rights reserved.
Keywords: Geochemistry; isotopes; volcanic rocks; Adakites; Nevado de Toluca; Mexico
1. Introduction
Nevado de Toluca (NT) volcano and the Tenango
Volcanic Field (TVF) belong to the Trans-Mexican
Volcanic Belt (TMVB) in central Mexico (Fig. 1).
This is one of the best-studied volcanic zones
because of its accessibility and position next to the
cities of Mexico and Toluca. The TMVB is
considered to be a continental magmatic arc that
transects central Mexico with an almost E–W
orientation, from the Pacific Ocean to the Gulf of
Mexico. Activity in this volcanic arc apparently
started at about 16 Ma (Ferrari et al., 1994) and
continues until today. The TMVB is 1200 km long,
and can be divided into three regions on the basis of
petrological, tectonic and volcanological character-
istics (Pasquare et al., 1988 and references therein).
The western region consists of alkaline and calc-
alkaline volcanic rocks at the Colima–Chapala–Tepic
junction. The central region is composed of extensive
monogenetic volcanism and by higher stratovolca-
noes with predominantly calc-alkaline compositions.
The eastern region is characterized by the presence
of some dacitic-rhyolitic stratovolcanoes and mono-
genetic volcanic fields with alkaline–calc-alkaline
composition. Volcanism in the TMVB has been
associated with the subduction of the Cocos and
Rivera plates beneath the North America plate (Fig.
1) (Robin, 1976, 1982; Demant, 1978, 1981; Pal et
al., 1978; Nixon, 1982; Negendank et al., 1985;
Besch et al., 1987; Nixon et al., 1987; Verma and
Nelson, 1989; Ferrari et al., 1999; Wallace and
Carmichael, 1999 and others). However, Mooser
(1972), Cebull and Shurbet (1987), Marquez et al.
(1999) and Verma (1984, 1987, 1999, 2000) pro-
posed that the TMVB is the result of a combination
of several tectonic processes including crustal frac-
turing, a continental rifting scenario associated with
an upwelling mantle, and production of ocean island
basalts (OIB) by a propagating rift opening from
west to east that is related to the effects of a mantle
plume.
Many geological, geochemical, geophysical and
volcanological studies have been carried out on the
TMVB during the past 35 years. In spite of these
studies, little attention has been focussed on the
geochemical and isotopic variations existing in the
rock sequences of stratovolcanoes or monogenetic
volcanic products. Many geochemical and isotopic
(Sr, Nd and Pb) data exist in all regions along the
TMVB. However, most active volcanoes lack detailed
geochemical and isotopic characterization of their
main magmatic events.
In the present study, geochemical and isotopic
characterization of the main products of Nevado de
Toluca and the Tenango Volcanic Field was carried
out in order to improve the understanding of the
chemical evolution of these volcanic structures.
Geochemical data for the Nevado de Toluca and
Tenango Volcanic Field products are then used to
test models accounting for compositional variations
in the source region, contamination and mixing of
magmas. These results can be used to further study
volumetric discharges, cyclicity of eruptions and
tectonic models.
2. Geological setting
NT is the fourth highest peak in Mexico (4680 m
asl) and a thick sequence of several hundreds meters
of Mesozoic and Tertiary metamorphic, carbonate
and volcanic sequences from the Guerrero Block
(Johnson and Harrison, 1990) underlies the volcanic
structures in the study area. Stratigraphic and
petrographic descriptions of these older sequences
are summarized by Garcıa-Palomo et al. (2000,
2002). In these papers and references therein, the
authors propose that NT volcano was constructed at
the intersection of three complex fault systems of
Fig. 1. Location of the TMVB in central Mexico (RFZ=Rivera Fracture Zone, EPR=East Pacific Rise, MAT=Middle America Trench,
CP=Cocos Plate, C=Ceboruco, Co=Colima, P=Paricutın, NT=Nevado de Toluca, Po=Popocatepetl, Pi=Pico de Orizaba, SM=San Martın Tuxtla,
C=Chichon and T=Tacana volcanoes) and the study area. The Tenango Volcanic Field is in the westermost part of the Sierra Chichinautzin. This
was characterized by Bloomfield (1975), Martin del Pozzo (1989), Marquez et al. (1999) and others. Stratovolcanoes are indicated as triangles
and important volcanic cones are shown by white squares. T=Tenango, TC=Tres Cruces, TX=Texontepec, X=Xitle, TA=Tabaquillo,
CH=Chichinautzin, P=Pelado, D=Dos, Iz=Iztaccıhuatl and Po=Popocatepetl.
R.G. Martınez-Serrano et al. / Journal of Volcanology and Geothermal Research 138 (2004) 77–110 79
different ages, orientations and kinematics. These
fault systems are Taxco-Queretaro with a NNW–
SSE orientation, San Antonio with a NE–SW
orientation and Tenango with an E–W direction
(Fig. 2). This last fault system controls the position
of monogenetic volcanoes in the TVF. The fault
systems have coexisted since the late Miocene
(Garcıa-Palomo et al., 2000). Fig. 2 shows a
generalized geologic map and locations of samples
included in this study.
Fig. 2. Schematic geologic map of Nevado de Toluca and the Tenango Volcanic Field with location of samples (modified from Macıas et al.,
1997). Inset shows distribution of the main fault systems (H=horst and G=graben).
R.G. Martınez-Serrano et al. / Journal of Volcanology and Geothermal Research 138 (2004) 77–11080
R.G. Martınez-Serrano et al. / Journal of Volcanology and Geothermal Research 138 (2004) 77–110 81
2.1. Nevado de Toluca volcano
Cantagrel et al. (1981) proposed that volcanic
activity at NT started some 1.5 Ma with the emplace-
ment of andesitic lava flows that constructed the
primitive volcano (bPaleo-NevadoQ). However,
recently published K–Ar ages from some andesitic
lavas suggest that the volcanic activity started at 2.6
Ma (Garcıa-Palomo et al., 2002). The volcanic
activity at NT between 1.5 Ma and 100 ka was
volcaniclastic, according to Cantagrel et al. (1981). A
thick volcaniclastic sequence of debris avalanches,
lahars and fluvial deposits on the southern flanks of
the volcano give evidence that bPaleo-NevadoQ was
Fig. 3. Composite stratigraphic sequences of NT and, in italics, the TVF. A
Macıas et al. (1997), 2. Bloomfield (1974), 3. Bloomfield and Valastro (197
et al. (2003) and 7. Caballero et al. (2001).
destroyed at least twice by failure of the volcanic
structure (Capra and Macıas, 2000). Macıas et al.
(1997) and Garcıa-Palomo et al. (2002) presented a
detailed stratigraphic description of the main units
emplaced during late Pleistocene and Holocene time
at NT (Fig. 3):
– A debris-avalanche deposit (DAD1), up to 15 m
thick, composed of blocks showing jigsaw-fit
structures, embedded in an indurated coarse sandy
matrix, overlies a paleosoil and a thick sequence
of epiclastic deposits from bPaleo-NevadoQ.Blocks of porphyritic gray juvenile dacite and
red altered dacite from the volcanic structure are
lso indicated are the samples analyzed in this study. Ages from: 1.
7), 4. Cantagrel et al. (1981), 5. Garcıa-Palomo et al. (2002), 6. Arce
R.G. Martınez-Serrano et al. / Journal of Volcanology and Geothermal Research 138 (2004) 77–11082
present. DAD1 spreads south to a distance of 55
km from the volcano.
– The Pilcaya and Mogote debris flow deposits (PDF
and MDF, Capra and Macıas, 2000) were
emplaced on a pale brown paleosoil. The PDF
deposit shows blocks of gray porphyritic dacite,
red altered dacite, green altered andesite, basalt and
schist from the local basement, all embedded in a
coarse sandy matrix. A block-and-ash flow deposit
that covers the PDF and MDF deposits yielded a14C age of 37 ka (Macıas et al., 1997). Thus, the
age of these debris flow deposits must be N37 ka.
– A thick, pink pumice-rich pyroclastic flow deposit
(PPF) with at least four units was emplaced
around NT. A radiocarbon age obtained by Macıas
et al. (1997) from a tree trunk within the deposit
indicated 42 ka. Outcrops of this unit are scarce
and no samples for geochemical studies were
available.
– The first pumice-fall deposit, dated between 36
and 39 ka, was emplaced on the northern slope of
NT (5 km from the summit, Garcıa-Palomo et al.,
2002). It consists of an alternating sequence of
pumice-fall, pyroclastic-surge and pyroclastic-
flow deposits (3.5 m thick).
– Two violent eruptions at NT produced large
magmatic explosions that destroyed old dacitic
central domes and excavated the present-day
crater. The explosions produced two block-and-
ash flow (BAF in Fig. 3) deposits at 37 and 28 ka,
with a similar maximum thickness of 35 m
(Macıas et al., 1997). Garcıa-Palomo et al.
(2002) identified four BAF deposits that consist
almost entirely of gray porphyritic juvenile dacitic
clasts with minor amounts of pumice, glassy
dacitic lithic clasts and red oxidized dacitic clasts
from the volcanic structure set in ash matrix.
– A fallout deposit with inverse grading, denomi-
nated Lower Toluca Pumice (LTP), covers the
BAF deposits. The LTP is clast-supported with
62% pumice, 27% lithic clasts and 11% crystals
for the entire deposit (Bloomfield et al., 1977). In
the present study, we observed abundant ochre
dacitic pumice fragments with hornblende and
orthopyroxene, lesser amounts of gray dense
juvenile dacite, altered dacitic clasts and meta-
morphic fragments (xenoliths) such as gneiss,
schists and phyllites from the local basement.
Isolated crystals of euhedral amphibole, pyroxene
and feldspar are disseminated in this deposit. A
thin ash-flow deposit and a dark-brown paleosoil
dated at 24.3 ka (Bloomfield and Valastro, 1977)
overlie the LTP.
– A younger gray BAF overlies the LTP. It differs
strikingly from the older BAF because it has a
more radial distribution around the volcano. This
unit consists of a gray cross-bedded pyroclastic-
surge deposit overlain by two massive gray block-
and-ash flow units made of block-sized lithic
fragments in a coarse ash matrix with a total
thickness of 10 m (Garcıa-Palomo et al., 2002).
The age of this deposit is uncertain, although
Caballero et al. (2001) assumed an age N14 ka.
– The Middle Toluca Pumice (MTP; Cervantes et
al., 2004) consists of a complex sequence of three
fallout layers, two pyroclastic-surge deposits, and
two massive pumice-rich pyroclastic-flow depos-
its (7 m thick), the reason for which it was first
dubbed as the White Pumice Flow (WPF). The
age of the MTP was defined through 14C dates of
charcoal found inside the pyroclastic-flow depos-
its at 12.1 ka (Garcıa-Palomo et al., 2002).
– Pumice-fall deposits, a pyroclastic-flow, and
pyroclastic-surge beds, with a minimum age of
11 ka (14C, Bloomfield and Valastro, 1974, 1977)
(Fig. 3) were referred to by these authors as the
Upper Toluca Pumice (UTP). Macıas et al. (1997)
and Arce et al. (2003) refined the stratigraphy of
this deposit, that in some places reaches a total
thickness of about 30 m. These authors recognized
four pumice-fall layers that contain abundant pink
pumice, banded pumice, gray juvenile dacite
clasts and altered andesitic lithics, within an ash
matrix. Pyroclastic-surge deposits with cross-
stratification, dunes and antidunes are intercalated
in the sequence. The volcanic products of the UTP
cover an area of 2000 km2 with a minimum
estimated volume of 8 km3 with an age of 10.5 ka
(14C) (Arce, 2003). The UTP event ended with the
emplacement of a dacitic dome in the Nevado de
Toluca crater.
– Gray cross-stratified pyroclastic-surge deposits
and pyroclastic-flow deposits dated by Macıas et
al. (1997) at 3.3 ka (14C) represent the most recent
volcanic event at NT, and indicate that activity has
continued into Holocene time.
R.G. Martınez-Serrano et al. / Journal of Volcanology and Geothermal Research 138 (2004) 77–110 83
2.2. Tenango Volcanic Field
The TVF represents the westernmost part of the
Sierra Chichinautzin (Fig. 1) described by Bloomfield
(1975), Martin del Pozzo (1989), Marquez et al.
(1999, 2001) and others. The Sierra Chichinautzin
consists of nearly 220 Quaternary monogenetic
volcanic cones with an E–W general structural
orientation. This volcanic province is bracketed by
NT to the west, Popocatepetl volcano to the east and
by the Mexico Basin in the north (Fig. 1). The TVF
(Fig. 2) is composed of more than 40 volcanic
monogenetic cones and associated lava flows with
ages from 8 to N38.5 ka BP (Bloomfield, 1975).
Cinder cones, lava cones and effusive fissural lava
flows with andesite and basaltic-andesite composi-
tions dominate in the TVF. The cone density in the
TVF is 0.5/km2 but locally reaches 1/km2; the mean
cone height is 650 m.
The cinder cones consist of dark gray to brick-red
scoria and ash fragments with diameters of 0.5–7 cm,
in layers 3–15 cm thick and dips from 208 to 268.Well-sorted scoria and a small proportion of bombs
are the main ejecta products of the cinder cones. Thin
lenses of lava are present in some cone sequences.
Bloomfield (1975) described the presence of at least
220 separate beds of black ash in some cinder cones
that indicate repeated short eruptive pulses.
The lava cones are made up of angular to
subangular lava blocks with sizes of 1–30 cm, that
form dip layers of 258. Although scoria fragments are
rare in these volcanoes, thin lenses (35 cm thick) are
occasionally observed in lavas. The effusive fissure-
fed lava flows are the most recent volcanic events
(b8.5 ka, Bloomfield, 1975) in the TVF. These lava
flows have the greatest length (~8 km long) and
volume of those in the area. The fissure-fed lava flows
are mainly aa-type and in minor proportion appear as
pahoehoe and lava blocks. A complex set of faults and
fractures with a similar E–W orientation to that
observed in the Sierra Chichinautzin seems to control
the distribution of volcanic cones in the TVF as well
(Fig. 2). Mooser and Maldonado-Koerdell (1961) and
Bloomfield (1974) have presented the relationships
between these structures and the monogenetic activity,
and Garcıa-Palomo et al. (2000) proposed that this
fault system is a continuation of the older Chapala-
Tula Fault Zone that has been reactivated during
Pleistocene–Holocene times. Fig. 3 shows the relative
ages of volcanic events for the TVF on the basis of
radiocarbon age data and morphologic characteriza-
tion of cones developed by Bloomfield (1975). We
used the nomenclature proposed by Bloomfield
(1975) to describe the volcanic cone ages: PLV1c40
ka, PLV2c30 ka, PLV3 from 18.6 to 21.9 ka and
HVb8.5 ka (Fig. 3). These data suggest that volcanic
events in the TVF have occurred simultaneously with
the emplacement of some pyroclastic deposits at NT.
3. Analytical methods
A representative suite of volcanic products (N100
samples) was obtained on the flanks of NT and
around the TVF (Figs. 2 and 3). Pumice, lava and
scoria fragments were sampled considering their
stratigraphic position. Thin sections were studied to
assess the mineralogy and petrography of the rocks
and fresh samples were selected for bulk chemical
analyses and Sr, Nd and Pb isotopic determinations.
Metamorphic xenolith fragments from the LTP were
handpicked for geochemical and isotopic studies.
Major-elements and Sc abundances were determined
by inductively coupled plasma-emission spectro-
scopy, and all other trace elements by inductively
coupled plasma mass spectrometry (ICP-MS) at the
analytical laboratories of the Centre de Recherches
Petrographiques et Geochimiques, Nancy, France
(SARM, 2003). Sr, Sm, Nd and Pb isotopic ratios
of whole rock samples were measured using a
Finnigan MAT 262 thermal ion mass spectrometer
at LUGIS (Laboratorio Universitario de Geoquimica
Isotopica), UNAM. The spectrometer is equipped
with a variable multicollector system (eight Faraday
cups) and all measurements were done in static
mode. Rb isotope ratios were measured with an NBS
type single collector mass spectrometer (Teledyne
Model SS-1290). Rb, Sr, Sm and Nd samples were
loaded as chlorides on double rhenium filaments and
measured as metallic ions. Lead samples were loaded
with a mixture of silica gel+phosphoric acid. Sixty
isotopic ratios were determined for Rb, Sr, Sm and
Nd, and 100 for Pb on each sample. Elements were
separated using standard ion-exchange methods.
Total procedure blanks during analyses of these
samples were less than: 1 ng Rb, 10 ng Sr, 1 ng
R.G. Martınez-Serrano et al. / Journal of Volcanology and Geothermal Research 138 (2004) 77–11084
Sm, 20 ng Nd and 300 pg Pb. More than 350
analyses of pyroxene, amphibole, olivine, Fe–Ti
oxides and feldspar from lava samples and juvenile
fragments from pyroclastic deposits were carried on
an automated CAMECA SX100 electron microprobe
(University of Barcelona, Spain). An acceleration
voltage of 15 kV was used. The excitation current
varied from 8 to 10 nA and the counting time was 10
s. The maximum analytical error in major oxides is
estimated to be less than 3%.
4. Results
4.1. Mineral studies and petrographic characteristics
Sixty samples from NT and the TVF were studied
petrographically, and a subset of them were analyzed
by electron microprobe. Table 1 shows modal
Table 1
Modal mineral assemblages (vol.%) of selected NT and TVF lava and py
Sample Phenocrysts Groundmass
Plag
(An32–54)
Horn Opx Cpx Oliv Qtz Plag
(Ab32–54)
B
Nevado de Toluca
NT9 4 3 2 0 0 0 1 0
NT12 7 4 2 0 0 0 1 0
NT13 7 5 2 0 0 0 1 0
NT6 21 5 3 0 0 0 20 1
NT8 4 2 2 0 0 0 2 0
NT33 7 4 2 0 0 0 2 0
NT14 7 6 1 0 0 0 0 1
NT10 11 5 3 0 0 0 5 0
NT11 20 7 4 0 0 0 40 1
NT15 24 5 3 1 0 1 25 2
NT17 20 3 3 1 0 1 15 1
NT22 20 4 2 0 0 1 15 1
NT24 25 7 4 3 0 1 50 0
NT29 30 3 2 4 0 0 41 0
NT30 27 3 3 5 0 0 50 0
Tenango Volcanic Field
TEO1 2 0 3 5 1 5 56 0
TEP1 0 0 0 7 2 0 50 0
RMS7 0 0 2 6 1 0 67 0
RMS8 0 0 2 5 2 0 75 0
RMS9 0 0 2 5 2 0 73 0
JAJ1 20 0 2 2 0 1 66 T
Plag=Plagioclase, Horn=hornblende, Opx=orthopyroxene (hypersthene),
Biot.=biotite, Oxides=Fe–Ti oxides, Zr=zircon, Ap=apatite, C. mins=clay
mineralogical analyses and petrography of the major-
ity of the samples. Lava, pumice and other pyroclastic
samples from NT display porphyritic textures, with a
predominantly dacitic composition. However, ande-
sites have been observed in the earliest volcanic
events of bPaleo-NevadoQ and also in some pyroclas-
tic materials such as the LTP. Lava and pyroclastic
products show different proportions of crystals, 50
and 10 vol.%, respectively. Phenocrysts of plagio-
clase, amphibole, orthopyroxene (hypersthene) and
rare biotite appear in two sizes in all samples, as
macrophenocrysts (1–2.5 mm) and phenocrysts (b1
mm), although plagioclase and some pyroxene appear
also as microlites in the glassy groundmass. Macro-
phenocrysts of hornblende, plagioclase, and biotite
from dacitic lavas commonly show reaction rims. The
petrography of most NT rocks is very similar. Phase
abundances, without considering vesicular porosity,
range as follow: plagioclase (oligoclase–andesine)
roclastic samples
iot Oxides Other phases Glass Total Petrography
1 0 89 100 Dacitic pumice
1 0 85 100 Dacitic pumice
1 Zr, Ap=T 83 100 Dacitic pumice
2 0 48 100 Andesitic tuff
2 0 88 100 Andesitic pumice
2 0 83 100 Dacitic pumice
1 0 84 100 Dacitic pumice
1 C. mins=T 75 100 Dacite
1 0 27 100 Dacite
1 Zr, Ap=T 38 100 Dacite
11 Zr, Ap=T 45 100 Dacite
7 0 50 100 Dacite
4 0 7 100 Andesite
5 Xen=8 7 100 Andesite
5 0 7 100 Andesite
5 0 25 100 Andesite
2 0 39 100 Basaltic-andesite
2 0 22 100 Andesite
4 0 12 100 Basaltic-andesite
3 0 15 100 Basaltic-andesite
2 0 7 100 Andesite
Cpx=clinopyroxene (augite-diopside), Oliv=olivine, Qtz=quartz,
minerals, T=traces, Xen=xenoliths.
R.G. Martınez-Serrano et al. / Journal of Volcanology and Geothermal Research 138 (2004) 77–110 85
between 4 and 50 vol.%, hypersthene between 2 and 5
vol.%, hornblende between 2 and 7 vol.%, Fe–Ti
oxides b2 vol.% and biotite appears in traces (b1
vol.%). The glassy matrix is very abundant (N30
vol.%). A typical characteristic of most pumices and
dacitic lavas from NT is the presence of orthopyrox-
ene (hypersthene) and the absence of clinopyroxene,
although some dacitic flows display rare phenocrysts
of augite. This petrographic characteristic is rarely
observed in rocks of the TMVB.
Rocks from the TVF show a predominantly
andesitic composition, although basaltic andesites have
also been observed. All samples from this field show
aphanitic textures with rare disseminated phenocrysts
(~1 mm) of plagioclase, pyroxene and olivine (Table
1). Phase abundances are: plagioclase in microlites ~55
vol.%, andesitic glass between 10 and 40 vol.%, ortho-
and clinopyroxene ~7 vol.%, Fe–Ti oxides ~3 vol.%
and olivine ~1%. Some TVF samples exhibit xenoliths
of dioritic composition. Minor amounts of quartz (b1
vol.%) exist in some TVF samples and in some NT
andesites. Olivine and xenocrystic quartz coexist in
some TVF samples with textural evidence of disequi-
librium. This is a common characteristic observed in
some Chichinautzin samples. Marquez and De Ignacio
(2002) considered these assemblages as the result of
magma mixing, whereas Siebe et al. (2004) proposed
that similar mineralogy and textures are consistent with
normal fractional crystallization processes and the
assimilation of some wall rock material in the Sierra
Chichinautzin volcanic field.
Plagioclase phenocrysts and microlites in NT
samples show normal and reverse zoning, twinning
and sieve textures. Some phenocrysts show clear
evidence of multiple periods of dissolution and
growth. In the TVF, plagioclase appears mostly as
microlites, but rare phenocrysts with reaction rims are
also observed. Phenocrysts and microlites show
similar compositions (An32–54) in NT samples,
although reverse zoning leads to more An-rich rims
(An55–58) in some dacitic rocks. Plagioclase in some
TVF andesites displays a more rich An (An54–60)
content than in NT rocks. For comparison, plagio-
clases from two important pumice events of Popoca-
tepetl volcano show values more rich in An than those
from our study area (An60–80, Siebe et al., 1999).
Olivine is commonly present in TVF basaltic
andesites as disseminated phenocrysts or as rare
glomerophenocrysts associated with clinopyroxene
and minor plagioclase. The crystals vary from
subhedral to anhedral in shape and skeletal forms
are also observed. Analyses of olivine show homoge-
neous compositions (Fo84–86). Some crystals display
evidence of disequilibrium conditions such as coronas
of reaction rims of clinopyroxene. Iddingsite as
secondary alteration of the olivine is observed in
some rocks.
In some dacitic NT samples, clinopyroxene is
present as rare subhedral phenocrysts (~1 mm) with
reaction rims of orthopyroxene and amphibole, and
the composition ranges from salite to diopside-augite
with minor proportions of endiopside (Wo41–47,
En41–50, Fs5–14). Clinopyroxene is absent in most
NT pumices and other pyroclasts. In TVF rocks
clinopyroxene appears as subhedral to anhedral
phenocrysts (~1 mm), commonly small isolated
crystals in the groundmass or in minor proportions
as corona reaction rims on quartz. Twinning and
zoning are rarely observed, and disequilibrium
conditions of crystallization can been inferred from
the presence of reaction borders in some samples.
Most analyses are augite to diopside, although nearly
30% of the analyses fall within the endiopside field.
Clinopyroxene found in quartz corona reaction rims
shows compositions of Wo38–45, En45–51, Fs8–14 and
very low Al2O3 concentrations (0.29–0.77 wt.%),
whereas isolated crystals disseminated in the same
rock sample display similar compositions (Wo35–45,
En46–53, Fs7–11) but different Al2O3 concentrations
(1.44–4.0 wt.%). Clinopyroxene compositions of NT
and TVF are very similar to values observed in rocks
of Popocatepetl volcano (Siebe et al., 1999).
Orthopyroxene is very common in NT pumice and
lava samples, and a minor phase in TVF rocks. At NT,
it is subhedral to euhedral with several indications of
reaction with the groundmass and sometimes it is
associated with amphibole. Important pumice-fall
deposits such as the UTP and LTP, with wide aerial
distributions in central Mexico contain only orthopyr-
oxene as a ferromagnesian mineral. Macrocrysts and
phenocrysts of hypersthene display zoned borders,
and compositional ranges of En70–78 for cores and
En55–76 for rims. Orthopyroxene shows relatively
variable compositions in dacitic NT lavas, ranging
from En84–90 to En60–67. In TVF rocks, orthopyroxene
is present in microcrystals (b0.4 mm) disseminated in
Table 2
Major oxide and trace element abundances of selected rocks from Nevado de Toluca (A=andesite, B-A=basaltic andesite, D=dacite, D-P=dacitic pumice,
A-P=andesitic pumice, T=tuff)
Sample NT4 NT11 NT15 NT17 NT22G NT22R NT24 NT25 NT29 NT30 NT6
Rock D D D D D D D D A A A-P
Long. W 99847.41V 99839.77V 99845.37V 99845.9V 99846.49V 99846.49V 99846.49V 99846.49V 99847.99V 99848.09V 99847.41VLat. N 19813.36V 19811.12V 1986.37V 1986.95V 18851.29 18851.29 18851.29 18851.29 1988.28V 1988.59V 19813.16V
(wt.%)
SiO2 64.88 65.98 64.87 64.25 63.83 64.05 63.41 63.13 57.41 61.93 59.95
TiO2 0.65 0.64 0.62 0.63 0.70 0.69 0.62 0.70 0.90 0.75 0.70
Al2O3 16.76 16.44 16.57 16.51 16.76 16.66 15.75 16.05 18.47 17.08 18.56
Fe2O3 4.32 4.29 4.31 4.50 4.72 4.66 4.85 4.97 6.77 5.46 4.77
MnO 0.06 0.05 0.05 0.05 0.06 0.06 0.08 0.06 0.09 0.08 0.07
MgO 1.75 1.72 1.79 2.67 1.99 1.98 4.00 3.29 3.23 2.65 2.01
CaO 4.20 4.10 4.15 4.71 4.51 4.43 4.56 5.06 6.26 5.03 4.64
Na2O 4.35 4.41 4.37 4.26 4.27 4.19 4.05 4.25 3.92 4.36 4.28
K2O 1.94 2.01 1.94 1.98 2.02 2.05 2.16 2.02 1.76 1.87 1.28
P2O5 0.17 0.17 0.18 0.20 0.18 0.18 0.17 0.25 0.23 0.18 0.18
LOI 0.80 0.04 1.02 0.13 0.82 0.91 0.75 0.15 0.86 0.47 3.44
Total 99.88 99.85 99.87 99.89 99.86 99.86 100.40 99.93 99.90 99.86 99.88
Trace elements (ppm)
V 71 70 62 77 84 81 71 84 145 113 83
Cr 23 23 29 83 21 22 161 125 20 45 29
Co. 7.56 7.52 8.81 11.33 10.18 9.90 18.21 14.47 17.83 14.74 9.25
Ni 6 b5 21 40 11 10 104 69 8 16 6
Cu 8 7 10 19 13 15 23 19 13 15 8
Zn 71 70 75 76 81 76 70 74 84 78 78
Rb 36.14 37.25 40.70 35.39 37.55 37.87 38.09 32.87 35.22 39.72 19.68
Sr 543 541 527 694 548 560 629 843 678 514 597
Y 14.74 14.73 13.61 14.35 15.42 15.69 14.22 14.36 21.15 22.69 15.72
Zr 149 142 160 160 140 138 135 149 152 163 150
Nb 4.23 4.19 4.97 4.30 4.19 4.12 4.03 4.60 4.24 4.07 4.29
Ba 481 464 513 511 430 413 521 578 397 409 410
La 14.41 14.63 16.64 17.53 13.33 13.11 17.06 22.66 18.30 19.35 13.62
Ce 30.51 30.77 35.10 38.81 28.78 27.18 35.23 47.65 37.61 38.24 31.18
Pr 3.84 3.93 4.47 4.99 3.91 3.69 4.63 6.51 5.40 5.16 4.02
Nd 15.55 15.49 18.45 19.33 15.62 14.89 18.07 24.69 22.69 20.95 15.82
Sm 3.33 3.50 3.86 4.03 3.42 3.33 3.64 5.22 5.29 4.66 3.61
Eu 1.08 1.09 1.17 1.25 1.12 0.96 1.10 1.41 1.49 1.20 1.18
Gd 2.78 3.06 3.22 3.13 3.11 2.89 2.96 3.64 4.38 4.14 3.21
Tb 0.44 0.43 0.43 0.48 0.44 0.42 0.43 0.51 0.64 0.63 0.49
Dy 2.57 2.64 2.50 2.57 2.54 2.53 2.19 2.71 3.73 3.65 2.77
Ho 0.51 0.53 0.48 0.49 0.53 0.50 0.45 0.50 0.73 0.77 0.56
Er 1.34 1.33 1.26 1.32 1.39 1.33 1.26 1.35 1.96 2.05 1.52
Tm 0.22 0.20 0.19 0.20 0.21 0.20 0.18 0.18 0.28 0.30 0.21
Yb 1.33 1.34 1.20 1.23 1.39 1.37 1.25 1.21 1.87 2.10 1.59
Lu 0.22 0.21 0.20 0.19 0.21 0.22 0.19 0.19 0.30 0.32 0.26
Hf 3.77 3.72 4.31 3.99 3.48 3.47 3.37 3.99 3.93 4.48 4.17
Ta 0.38 0.38 0.44 0.37 0.34 0.34 0.37 0.39 0.38 0.37 0.42
Th 3.74 3.82 4.02 3.70 3.35 3.04 4.23 4.15 4.18 4.78 4.29
U 1.47 1.47 1.51 1.34 1.52 1.43 1.71 1.60 1.42 1.59 1.48
Ta/Yb 0.28 0.28 0.37 0.30 0.25 0.25 0.30 0.32 0.20 0.18 0.26
Hf/U 2.57 2.53 2.84 2.98 2.29 2.42 1.97 2.49 2.78 2.81 2.81
Zr/U 101 96 106 120 92 96 79 93 107 102 101
U/Ta 0.39 0.38 0.38 0.36 0.45 0.47 0.40 0.39 0.34 0.33 0.35
Th/Hf 0.99 1.03 0.93 0.93 0.96 0.88 1.26 1.04 1.06 1.07 1.03
Ba/Zr 3.24 3.28 3.20 3.18 3.07 3.00 3.87 3.87 2.61 2.52 2.73
Ba/La 33.35 31.73 30.85 29.12 32.27 31.49 30.50 25.50 21.68 21.15 30.06
La/Nb 3.41 3.49 3.35 4.07 3.18 3.19 4.24 4.92 4.32 4.76 3.18
Th/Ta 9.95 10.01 9.12 9.92 9.75 8.89 11.38 10.69 11.08 12.94 10.32
R.G. Martınez-Serrano et al. / Journal of Volcanology and Geothermal Research 138 (2004) 77–11086
Sample NT7 NT9 NT14 NT13 NT8 NT10 NT12 NT31 NT32 NT33
Rock D D-P D-P D-P A-P D D-P D D D
Long. W 99839.77V 99839.77V 99839.05V 99841.37V 99839.77V 99839.77V 99841.37V 99845.9V 99845.35V 99845.66VLat. N 19811.12V 19811.16V 1982.76V 1986.33V 19811.12V 19811.12" 1986.33V 1986.95V 1987.17V 19813.37V
(wt.%)
SiO2 63.26 62.35 63.30 63.76 55.59 64.69 63.46 64.90 66.48 62.02
TiO2 0.58 0.65 0.58 0.62 0.68 0.64 0.61 0.61 0.64 0.64
Al2O3 17.39 17.03 16.52 16.54 18.61 16.66 16.53 15.96 16.25 16.93
Fe2O3 4.10 4.34 4.16 4.13 4.82 4.32 4.21 4.16 3.95 4.01
MnO 0.05 0.05 0.06 0.05 0.05 0.05 0.05 0.07 0.07 0.06
MgO 1.78 1.79 1.83 1.69 2.08 1.79 1.75 2.44 1.65 1.70
CaO 4.28 4.28 4.19 4.18 4.44 4.32 4.22 4.41 4.12 4.05
Na2O 4.29 4.21 3.96 4.35 3.31 4.29 4.32 4.31 4.47 4.14
K2O 1.69 1.75 1.89 1.90 1.28 2.06 1.87 2.00 1.98 1.78
P2O5 0.21 0.18 0.16 0.17 0.21 0.15 0.19 0.17 0.15 0.20
LOI 2.25 3.24 3.22 2.46 8.79 0.89 2.65 0.62 �0.04 4.21
Total 99.88 99.87 99.87 99.85 99.86 99.86 99.86 99.65 99.72 99.74
Trace elements (ppm)
V 66 63 70 62 89 78 64 68 64 55
Cr 32 29 31 28 37 27 28 162 72 69
Co. 7.80 8.21 8.19 7.98 9.29 8.38 8.50 9.49 6.79 7.33
Ni 7 9 13 10 12 6 10 38
Cu 9 10 6 6 14 6 7 21 13 32
Zn 72 79 69 76 75 75 84 78 64 75
Rb 30.15 37.78 39.44 39.60 25.23 37.54 39.43 39.26 41.09 36.86
Sr 569 557 553 547 553 559 541 608 482 470
Y 15.18 14.31 14.83 13.47 16.30 14.67 13.42 15.12 15.25 14.80
Zr 167 163 123 159 173 138 160 143 125 150
Nb 4.99 5.08 4.14 4.91 4.93 3.89 4.89 4.21 4.06 4.80
Ba 528 524 444 528 381 433 505 543 457 531
La 17.70 16.57 12.90 16.62 15.19 12.50 16.18 16.93 12.41 16.42
Ce 32.69 37.56 28.61 35.25 29.36 27.77 33.93 36.16 26.12 31.43
Pr 5.01 4.61 3.86 4.47 4.65 3.58 4.31 4.29 3.13 4.11
Nd 19.77 18.58 15.27 18.92 19.14 14.77 17.38 18.85 13.58 17.51
Sm 3.99 3.86 3.36 3.91 4.42 3.31 3.84 3.87 3.09 3.75
Eu 1.27 1.28 1.01 1.23 1.24 1.02 1.17 1.20 1.01 1.17
Gd 3.52 3.25 3.23 3.21 3.51 2.71 3.02 3.59 3.12 3.64
Tb 0.52 0.52 0.47 0.46 0.57 0.44 0.46 0.51 0.48 0.51
Dy 2.79 2.54 2.45 2.30 3.02 2.43 2.43 2.74 2.69 2.77
Ho 0.51 0.46 0.50 0.46 0.54 0.46 0.45 0.50 0.51 0.49
Er 1.36 1.26 1.41 1.21 1.52 1.31 1.09 1.46 1.52 1.35
Tm 0.21 0.19 0.21 0.16 0.21 0.21 0.17 0.21 0.22 0.19
Yb 1.46 1.31 1.40 1.19 1.49 1.35 1.10 1.38 1.44 1.25
Lu 0.22 0.19 0.22 0.17 0.24 0.21 0.18 0.20 0.22 0.18
Hf 4.34 4.34 3.58 4.27 4.21 3.43 3.83 3.35 3.02 3.50
Ta 0.48 0.46 0.39 0.45 0.42 0.35 0.42 0.30 0.30 0.40
Th 4.76 4.05 3.46 4.02 4.03 3.55 3.88 4.20 3.76 4.19
U 1.69 1.49 1.60 1.65 1.35 1.42 1.49 1.38 1.43 1.42
Ta/Yb 0.33 0.35 0.28 0.38 0.28 0.30 0.38 0.21 0.21 0.29
Hf/U 2.57 2.91 2.24 2.58 3.11 2.40 2.57 2.43 2.11 2.47
Zr/U 99 109 77 96 128 97 107 104 87 105
U/Ta 0.35 0.37 0.46 0.41 0.34 0.40 0.38 0.33 0.38 0.34
Th/Hf 1.10 0.93 0.97 0.94 0.96 1.03 1.01 1.25 1.25 1.19
Ba/Zr 3.16 3.23 3.61 3.33 2.20 3.14 3.16 3.79 3.67 3.55
Ba/La 29.81 31.65 34.40 31.79 25.06 34.61 31.20 32.09 36.87 32.32
La/Nb 3.55 3.26 3.12 3.39 3.08 3.21 3.31 4.02 3.05 3.42
Th/Ta 9.92 8.86 8.81 8.98 9.67 10.10 9.25 14.42 12.43 11.57
(continued on next page)
R.G. Martınez-Serrano et al. / Journal of Volcanology and Geothermal Research 138 (2004) 77–110 87
Major oxide and trace element abundances of selected rocks from Tenango Volcanic Field (A=andesite, B-A=basaltic andesite, D=dacite)
Sample JAJ1 TEO1 STA1 ESP1 TEP1 RMS2 RMS3 RMS4 RMS5 RMS7
Rock A A A A A B-A B-A B-A A A
Long. W 99833.47V 99836V 99826.97V 99826.06V 99826.29V 99823.88V 99824.26V 99825.26V 99829.57V 99831.11VLat. N 1986.91V 1985.76V 1989.80V 1984.23V 1983.5V 19813.35V 19811.78V 1988.91V 1987.95V 1987.04V
(wt.%)
SiO2 59.52 59.78 60.46 60.60 58.74 52.71 54.76 54.62 59.42 59.47
TiO2 0.79 0.72 1.01 0.75 0.78 0.89 1.32 1.20 0.91 0.75
Al2O3 15.99 16.42 16.55 16.41 16.58 15.21 16.02 16.33 16.21 15.66
Fe2O3 6.08 5.77 6.42 5.73 6.06 7.24 7.81 7.69 6.01 6.05
MnO 0.09 0.07 0.10 0.08 0.09 0.11 0.11 0.11 0.08 0.09
MgO 4.42 4.50 3.93 4.38 4.92 8.21 5.80 6.24 4.36 5.24
CaO 6.06 6.06 5.51 5.90 6.41 8.15 6.97 7.05 5.73 6.12
Na2O 3.73 3.91 4.14 4.12 4.05 3.59 4.34 3.96 4.39 4.06
K2O 2.03 1.71 1.90 1.67 1.58 2.17 1.63 1.22 1.71 1.62
P2O5 0.25 0.19 0.31 0.25 0.26 0.62 0.49 0.41 0.31 0.27
LOI 0.95 0.76 �0.02 0.02 0.46 0.60 0.43 0.90 0.60 0.40
Total 99.91 99.89 100.31 99.91 99.93 99.50 99.68 99.73 99.73 99.73
Trace elements (ppm)
V 124 123 109 107 118 147 131 134 100 117
Cr 170 119 120 198 230 353 197 262 144 254
Co. 33.22 33.85 43.45 37.92 36.07 28.20 25.30 24.60 17.30 19.90
Ni 50 55 52 90 105 198 117 81 81 113
Cu 11 17 19 24 27 34 23 19 21 23
Zn 87 82 95 82 90 81 78 81 72 74
Rb 33.21 27.53 44.02 36.50 34.04 29.80 28.40 21.60 32.60 39.10
Sr 611 649 448 588 623 1310 761 646 583 591
Y 20.15 15.42 23.33 16.94 17.96 26.50 25.50 21.70 17.90 18.10
Zr 150 139 247 181 185 192 208 176 174 150
Nb 4.94 3.83 9.15 5.23 5.11 4.27 14.40 10.20 8.14 4.85
Ba 532 379 538 497 527 1246 563 405 493 562
La 21.73 12.88 25.06 22.71 23.77 59.51 31.80 22.27 21.82 22.94
Ce 49.62 30.54 55.55 51.60 54.15 133.80 67.95 49.05 44.29 47.46
Pr 6.51 3.88 6.85 6.43 6.62 17.70 8.79 6.40 5.71 6.17
Nd 27.47 15.96 26.74 24.70 27.27 77.97 38.14 27.94 24.22 26.88
Sm 5.77 3.43 5.35 4.84 5.17 14.24 7.62 5.86 4.83 5.43
Trace elements (ppm)
Eu 1.55 1.09 1.45 1.40 1.59 3.81 2.18 1.71 1.49 1.54
Gd 4.21 2.84 4.31 3.84 4.32 10.11 6.18 4.80 4.17 4.23
Tb 0.62 0.44 0.67 0.53 0.59 1.24 0.88 0.72 0.59 0.62
Dy 3.44 2.58 3.89 3.13 3.27 5.72 4.66 4.18 3.36 3.39
Ho 0.65 0.48 0.77 0.53 0.61 1.01 0.98 0.87 0.71 0.74
Er 1.70 1.21 1.92 1.44 1.66 2.35 2.31 2.03 1.73 1.76
Tm 0.25 0.22 0.31 0.21 0.24 0.32 0.35 0.33 0.25 0.25
Yb 1.72 1.32 2.00 1.33 1.47 2.12 2.38 2.05 1.75 1.67
Lu 0.30 0.20 0.31 0.23 0.25 0.31 0.35 0.30 0.24 0.25
Hf 3.55 3.27 5.39 4.04 4.28
Ta 0.51 0.39 0.91 0.55 0.50
Th 4.15 2.85 4.67 4.37 4.42
U 1.57 0.95 1.30 1.34 1.30
Ta/Yb 0.30 0.29 0.45 0.41 0.34
Hf/U 2.27 3.43 4.15 3.01 3.30
Zr/U 96 145 190 135 142
U/Ta 0.38 0.34 0.28 0.31 0.29
Th/Hf 1.17 0.87 0.87 1.08 1.03
Ba/Zr 3.54 2.73 2.18 2.74 2.85 2.71 2.30 2.83 3.75
Ba/La 24.95 29.41 21.47 21.88 22.20 20.94 17.70 18.19 22.59 24.50
La/Nb 4.40 3.36 2.74 4.35 4.65 2.21 2.18 2.68 4.73
Th/Ta 8.14 7.39 5.14 8.02 8.78
Table 2 (continued)
R.G. Martınez-Serrano et al. / Journal of Volcanology and Geothermal Research 138 (2004) 77–11088
Sample RMS8 RMS9 RMS10 MX84 MX33 MX47 MX49 MX46 MX85 MX45
Rock B-A B-A B-A A A A A A A A
99827.09V 99823.56V 99823V 99827.19V 99826.69V 99828.66V 99831.46V 99825.91 99836.7V 99826.04VLat. N 19810.31V 1989.3V 1989.78V 1980.96V 19813.42V 1987.76V 1986.51V 1984.5V 1983.70V 1986.05V
(wt.%)
SiO2 54.77 53.97 53.92 56.70 61.04 61.25 61.63 60.67 60.12 63.93
TiO2 1.22 1.29 1.30 0.82 0.75 0.80 0.80 0.77 0.72 0.69
Al2O3 16.34 16.30 16.30 16.07 15.92 15.97 16.01 16.12 16.22 16.08
Fe2O3 7.88 7.96 8.12 6.54 5.59 5.43 5.58 5.72 5.59 4.52
MnO 0.11 0.12 0.12 0.10 0.09 0.09 0.09 0.09 0.09 0.07
MgO 5.92 6.14 6.50 6.07 4.91 4.37 4.00 4.69 4.33 2.41
CaO 7.42 7.58 7.50 6.90 5.62 6.02 5.69 6.08 5.78 4.65
Na2O 4.02 4.13 4.14 3.99 4.38 4.25 4.39 4.24 4.10 4.51
K2O 1.23 1.36 1.35 1.41 1.56 1.70 1.86 1.62 1.78 2.03
P2O5 0.38 0.47 0.47 0.24 0.19 0.26 0.26 0.24 0.17 0.22
LOI 0.46 0.36 �0.03 0.12 �0.01 �0.01 �0.01 �0.01 0.63 0.02
Total 99.75 99.68 99.69 98.96 100.04 100.13 100.30 100.23 99.53 99.13
Trace elements (ppm)
V 138 144 148 119 101 99 105 102 115 79
Cr 220 221 250 262 193 167 145 200 92 58
Co. 24.60 24.80 26.10 43.90 48.40 51.50 56.70 54.90 43.20 70.60
Ni 63 77 89 126 106 76 67 87 18
Cu 25 25 27 35 21 21 19 22 11
Zn 84 84 82 69 63 67 71 66 62
Rb 22.30 24.30 23.70 34.00 26.00 51.00 44.00 35.00 20.00 54.00
Sr 692 679 685 545 465 661 571 573 633 477
Y 23.70 24.80 24.70 18.00 17.00 18.00 18.00 17.00 15.00 16.00
Zr 173 195 194 149 137 176 174 169 124 185
Nb 9.73 12.30 12.50
Ba 431 491 494 495 389 587 528 529 398 604
La 23.93 26.89 27.30 21.60 14.90 28.10 25.10 24.60 14.20 24.80
Ce 51.99 58.26 59.25 50.00 37.00 61.00 60.00 58.00 30.00 51.00
Pr 6.86 7.78 7.85
Nd 30.04 33.64 33.38 24.00 18.00 31.00 27.00 28.00 14.00 25.00
Sm 6.34 7.20 7.34 4.69 3.85 5.66 5.14 4.88 3.55 4.39
Trace elements (ppm)
Eu 1.95 2.17 2.14 1.53 1.21 1.49 1.65 1.57 1.20 1.32
Gd 5.18 5.45 5.63
Tb 0.77 0.81 0.84 0.60 0.60 0.60 0.60 0.60 0.50 0.60
Dy 4.40 4.47 4.62
Ho 0.93 0.94 0.96
Er 2.26 2.12 2.26
Tm 0.33 0.30 0.32
Yb 2.23 2.20 2.30 1.83 1.74 1.76 1.98 1.82 1.48 1.64
Lu 0.33 0.36 0.36 0.23 0.26 0.26 0.28 0.27 0.22 0.25
Hf
Ta 0.40 0.50 1.20 0.60 0.90 0.60
Th 3.30 2.70 4.30 4.00 3.60 4.90
U 1.10 1.10 1.20 1.70 1.40 1.40
Ta/Yb 0.22 0.29 0.68 0.30 0.49 0.37
Hf/U 3.64 3.09 4.33 2.59 3.00 3.64
Zr/U 135 125 147 102 121
U/Ta 0.33 0.41 0.28 0.43 0.39 0.29
Th/Hf 0.83 0.79 0.83 0.91 0.86 0.96
Ba/Zr 2.49 2.52 2.55 3.32 2.84 3.34 3.03 3.13 3.21 3.26
Ba/La 18.01 18.26 18.10 22.92 26.11 20.89 21.04 21.50 28.03 24.35
La/Nb 2.46 2.19 2.18
Th/Ta 8.25 5.40 3.58 6.67 4.00 8.17
R.G. Martınez-Serrano et al. / Journal of Volcanology and Geothermal Research 138 (2004) 77–110 89
Fig. 4. (a) Total alkalis (Na2O+K2O) vs. SiO2 diagram for volcanic rocks of NT and the TVF (after Le Maitre et al., 1989). Division between
alkaline and subalkaline fields from Irvine and Baragar (1971). (b) Analyzed samples displaying a typical calc-alkaline trend in the AFM
diagram, A=Na2O+K2O, B=Fe2O3T and C=MgO.
R.G. Martınez-Serrano et al. / Journal of Volcanology and Geothermal Research 138 (2004) 77–11090
the groundmass, with compositions of En70–88, similar
to that observed in orthopyroxene from Popocatepetl
pumices.
Amphibole is relatively abundant in NT rocks as
subhedral to euhedral macrocrysts and phenocrysts,
and in some samples this phase becomes resorbed by
the magma. Amphiboles in dacitic NT rocks are
pargasitic (nomenclature of Leake, 1978), whereas in
UTP and LTP amphiboles fall within the fields of
pargasitic–hornblende and edenitic–hornblende. For
comparison, some amphiboles from Popocatepetl
volcano show compositions that fall in the fields of
pargasite and ferroan–pargasite. In the case of
Popocatepetl and NT volcanoes, hornblende may be
used to identify the volcanic source of regional
pumice deposits.
Quartz abundance is less than 1 vol.% in some
andesitic and dacitic lavas from NT and the TVF. It is
observed as rare phenocrysts with corona reactions of
radiating augite–diopside, indicating disequilibrium.
Fe–Ti oxides are present in all NT and TVF
rocks as rare phenocrysts and as a groundmass
phase. In TVF samples, opaque microlites (1–6
vol.%) of titanomagnetite and ilmenite occur in the
matrix as euhedral–subhedral crystals, b0.7 mm in
size and randomly distributed. In NT rocks, titanif-
erous magnetite is present as sub- to anhedral
phenocrysts (0.1–0.5 mm) and as a groundmass
phase. Magnetite comprises from 1 to 2 vol.% of
lavas and ilmenite is much less abundant. Some-
times, opaque oxides are present as inclusions or as
reaction borders on amphibole.
Glass is in most cases the most voluminous
groundmass phase in NT rocks. This phase displays
a dominant rhyolitic composition, regardless of bulk
lava composition (Arce, 2003). In TVF samples, glass
is also abundant in the matrix with an andesitic
predominant composition.
The LTP is a clast-supported pumice-fall
deposit with 20% of metamorphic and igneous
lithics derived from the local basement. Metamor-
phic xenoliths show diameters varying from 0.5 to
1.5 cm. Handpicking of these metamorphic frag-
ments under a polarizing binocular microscope
Fig. 5. Variation diagrams for (a) SiO2 (wt.%), (b) Al2O3 (wt.%), (c) CaO (wt.%), (d) TiO2 (wt.%), (e) K2O (wt.%) and (f) Cr (ppm) as a
function of MgO (wt.%). Samples NT24 and NT25 represent blocks from an avalanche deposit of NT (see text for discussion). Black circles
represent compositions of lavas from three volcanoes oriented E–W in the TVF.
R.G. Martınez-Serrano et al. / Journal of Volcanology and Geothermal Research 138 (2004) 77–110 91
Fig. 6. Trace-element diagrams for (a) NT and (b) TVF rocks. Primitive mantle normalized using the values of Sun and McDonough (1989).
R.G. Martınez-Serrano et al. / Journal of Volcanology and Geothermal Research 138 (2004) 77–11092
allowed us to group them into two types: (1)
green low-grade metaigneous schists with chlorite,
epidote and plagioclase along with some phyllite
fragments; (2) high-grade gneiss and some meta-
sedimentary schists.
4.2. Major and trace element results
Table 2 shows major and trace element concen-
trations of NT and TVF rocks analyzed in this work.
Previous chemical data for these areas have been
R.G. Martınez-Serrano et al. / Journal of Volcanology and Geothermal Research 138 (2004) 77–110 93
obtained by Bloomfield (1974, 1975), Macıas et al.
(1997) and Verma (1999), and in some parts of this
paper are cited for comparison. Chemical classifica-
tion of rocks is given in the alkalis vs. SiO2 diagram
in Fig. 4a (Le Maitre et al., 1989). The early
products of NT are andesites (SiO2 from 58 to 62.31
wt.%), whereas the later ones are mainly dacites
(SiO2 from 63 to 67 wt.%) with minor andesites (e.g.
LTP). The TVF rocks have compositions ranging
from basaltic andesites to andesites (53–61 wt.%
SiO2) with rare dacites and basaltic trachy-andesites.
All volcanic rocks in the study area belong to the
subalkaline series as defined by Irvine and Baragar
(1971) (Fig. 4a). They lack iron enrichment, dis-
Fig. 7. Variation diagrams for (a) Th vs. Th/Hf, (b) U/Ta vs. Z
playing a typical calc-alkaline trend in the AFM
diagram of Fig. 4b. This is equivalent to the low- to
medium-Fe suite proposed by Arculus (2003). These
features are consistent with their subduction setting.
Chemical data from Macıas et al. (1997) and Verma
(1999) show patterns similar to our data for rocks of
NT and the western Sierra Chichinautzin. Bloom-
field’s (1975) chemical data show important dis-
persion in values of major elements for the TVF
rocks (Fig. 4a).
NT rocks show a decrease in SiO2 from 67 to
57 wt.% with increasing MgO contents (1.5–4
wt.%). TVF rocks display a similar trend, but with
lower SiO2 concentrations (61–54 wt.%) and higher
r/U and (c) Th/Hf vs. U/Ta of the NT and TVF rocks.
Fig. 8. Chondrite-normalized rare earth element data for (a) NT and (b) TVF rocks, using the values of Nakamura (1974).
R.G. Martınez-Serrano et al. / Journal of Volcanology and Geothermal Research 138 (2004) 77–11094
MgO (4–7 wt.%) (Fig. 5a). Al2O3 contents are also
characteristic in each volcanic zone. NT rocks
display values from 16.2 to 20.5 wt.% with very
little variation in MgO, whereas the TVF rocks
have a narrow range of Al2O3 (15.8–16.7, Fig. 5b).
CaO and TiO2 concentrations in most NT and TVF
R.G. Martınez-Serrano et al. / Journal of Volcanology and Geothermal Research 138 (2004) 77–110 95
samples are in the range of island volcanic arcs and
correlate positively with MgO (Fig. 5c,d), whereas
K2O correlates negatively with MgO (Fig. 5e). Data
from Macıas et al. (1997) and Verma (1999) show
similar patterns for these elements. However, TiO2
Table 3
Sr, Nd and Pb present-day isotopic compositions for NT and TVF rocks
Sample Agea Rock type 87Sr/86Sr
F1j 143Nd/144Nd
F
Nevado de Toluca volcano
NT15 3300 dacite 0.704150 46 0.512907 1
NT17 3300 dacite 0.703868 43 0.512912 2
NT31 3300 dacite 0.703912 46 0.512887 1
NT6 10,500 andes. pumice 0.703958 36 0.512899 2
NT9 10,500 daci. pumice 0.704226 56 0.512884 2
NT13 10,500 daci. pumice 0.704205 52 0.512886 2
NT12 10,500 daci. Pumice 0.704208 44 0.512876 2
NT14 12,040 daci. pumice 0.703853 48 0.512893 1
NT8 24,000 andes. pumice 0.704039 40 0.512896 1
NT33 24,260 andes. pumice 0.704211 46 0.512855 2
NT4 28,000 dacite 0.703952 40 0.512870 2
NT7 28,000 dacite 0.704019 40 0.512872 2
NT11 37,000 dacite 0.703940 43 0.512901 2
NT10 37,000 dacite 0.703965 55 0.512903 2
NT22G 43,000 dacite 0.703887 44 0.512888 1
NT22R 43,000 dacite 0.703899 43 0.512890 1
NT24 dacite block 0.703702 31 0.512980 2
NT25 dacite block 0.703776 43 0.512945 2
NT29 1,500,000 andesite 0.703923 45 0.512862 3
NT30 1,500,000 andesite 0.703959 38 0.512876 2
NT32 dacite 0.704030 40 0.512832 2
Tenango Volcanic Field
TEO1 8500 andesite 0.703728 48 0.512878 1
TEP1 8500 andesite 0.704165 46 0.512906 1
RMS7 8500 andesite 0.704320 46 0.512948 1
MX84 8500 andesite 0.704141 38 0.512946 1
MX33 8500 andesite 0.703994 45 0.512937 1
MX85 8500 andesite 0.703713 41 0.512831 2
ESP1 19,530 andesite 0.704065 41 0.512901 1
RMS8 19,530 basal. andesite 0.703877 54 0.512989 2
RMS10 19,530 basal. andesite 0.704040 43 0.512957 2
CUATL1 19,530 andesite 0.704005 45 0.512938 2
SIL1 19,530 andesite 0.704167 39 0.512954 1
STA1 30,000 andesite 0.704481 52 0.512753 1
JAJ1 30,000 andesite 0.704028 50 0.512972 1
RMS2 40,000 andesite 0.704338 40 0.512981 2
Isotopic composition for La Jolla Nd standard are 143Nd/144Nd=0.511880F(1j, n=220), for NBS981 standard 206Pb/204Pb=16.89F0.04%, 207Pb/20
standard deviation correspond to the last two digits.a Relative age (years) proposed after data of Bloomfield (1974), Blo
(1997), Garcıa-Palomo et al. (2002) and Arce et al. (2003). daci.=dacitic,
concentrations in some TVF samples (RMS3,
RMS4, RMS8, RMS9 and RMS10; Table 2) are
unusually high (1.2–1.32 wt.% TiO2) for subduc-
tion-zone magmas and more typical of intraplate-
type mafic alkalic rocks. Similar concentrations
1j qNd 206Pb/204Pb
1 S.D.
(%)
207Pb/204Pb
1 S.D.
(%)
208Pb/204Pb
1 S.D.
(%)
7 +5.25 18.573 0.016 15.544 0.017 38.225 0.018
5 +5.34 18.576 0.014 15.556 0.015 38.256 0.015
6 +4.86 18.590 0.019 15.570 0.023 38.310 0.028
8 +5.09 18.577 0.020 15.572 0.020 38.298 0.022
3 +4.80 18.610 0.048 15.587 0.065 38.374 0.087
2 +4.84 18.603 0.017 15.579 0.018 38.342 0.021
1 +4.64 18.607 0.018 15.583 0.021 38.358 0.025
8 +4.97 18.593 0.020 15.585 0.021 38.354 0.023
9 +5.03 18.598 0.027 15.569 0.027 38.314 0.030
2 +4.23 18.635 0.034 15.617 0.035 38.475 0.038
4 +4.53 18.557 0.020 15.548 0.025 38.217 0.026
9 +4.56 18.612 0.018 15.581 0.018 38.359 0.020
0 +5.13 18.554 0.012 15.545 0.013 38.206 0.012
9 +5.17 18.580 0.018 15.576 0.017 38.312 0.018
7 +4.88 18.553 0.017 15.567 0.017 38.192 0.017
8 +4.92 18.555 0.015 15.546 0.016 38.200 0.016
1 +6.67 18.587 0.016 15.570 0.017 38.295 0.016
2 +5.99 18.596 0.019 15.578 0.021 38.332 0.020
8 +4.37 18.677 0.024 15.594 0.027 38.431 0.032
1 +4.64 18.611 0.080 15.568 0.019 38.315 0.018
4 +3.78 18.576 0.020 15.570 0.023 38.294 0.027
7 +4.68 18.581 0.020 15.562 0.025 38.275 0.028
7 +5.23 18.671 0.024 15.595 0.023 38.454 0.023
9 +6.05 18.635 0.020 15.578 0.022 38.369 0.025
9 +6.01
8 +5.83
9 +3.76
9 +5.13 18.679 0.034 15.607 0.039 38.496 0.051
1 +6.85 18.659 0.021 15.595 0.020 38.433 0.020
4 +6.22 18.667 0.025 15.575 0.025 38.386 0.026
1 +5.85 18.689 0.021 15.599 0.019 38.472 0.021
5 +6.16
9 +2.24
5 +6.52 18.616 0.027 15.575 0.028 38.346 0.031
3 +6.69
22 (1j, n=116), for the SRM987 standard 87Sr/86Sr=0.710234F184Pb=15.42F0.06% and 208Pb/204Pb=36.50F0.09%. Values for 1j
omfield and Valastro (1977), Cantagrel et al. (1981), Macıas et al.
andes.=andesitic, basal.=basaltic.
R.G. Martınez-Serrano et al. / Journal of Volcanology and Geothermal Research 138 (2004) 77–11096
have been determined in other lava samples from
the Sierra Chichinautzin volcanic field (Marquez et
al., 1999; Siebe et al., 2004).
Samples NT24 and NT25 (black squares in Figs.
5 and 7) show a slightly different chemical compo-
sition in comparison to most NT rocks. These
samples are blocks of the debris avalanche deposit
DAD1 south of NT (Fig. 3). Therefore, it is very
likely that the source of these blocks is not NT. On
the other hand, although sample NT29 shows an
andesitic composition, the presence of small xen-
oliths (b8 vol.%) in its matrix seems to affect its
chemical patterns, shifting them away from the
average trend of NT rocks (Figs. 4, 5 and 7).
Samples RMS4, RMS8, RMS9 and RMS10 were
obtained from lava flows produced by three E–W
aligned TVF monogenetic cones with similar ages
(~21 ka) (Fig. 2). Major and trace element concen-
trations for these samples are very similar (black
Table 4
Sr, Nd and Pb present-day isotopic compositions for xenoliths found in th
found in the La Goleta Tertiary volcanic field by Elıas-Herrera et al. (199
Sample NT35PB NT35PV NT35EC NT3
Rock type White phyllite Green phyllite Chlorite schist Gne87Rb/86Sr 5.92 2.94 0.46 6.87Sr/86Sr 0.721098 0.716177 0.705731 0.
F1j 38 39 38 38147Sm/144Nd 0.109 0.127 0.144 0.143Nd/144Nd 0.512281 0.512351 0.512693 0.
F1j 17 18 18 20
qNd �6.96 �5.60 +1.07 �7.
F1j 0.33 0.35 0.35 0.
Rb (ppm)b 111.71 115.72 31.88 89.
Sr (ppm)b 54.70 113.90 199.50 40.
Sm (ppm)b 4.57 3.28 3.04 3.
Nd (ppm)b 25.24 15.64 12.76 18.
TDM (Ga)c 1.27 1.42 1.02 1.206Pb/204Pb 18.984 19.021 18.864 19.
1 S.D. (%) 1.035 0.048 0.131 0.207Pb/204Pb 15.631 15.690 15.632 15.
1 S.D. (%) 1.031 0.052 0.131 0.208Pb/204Pb 39.114 39.162 38.713 39.
1 S.D. (%) 1.039 0.067 0.131 0.
a Data from Elıas-Herrera et al. (1998).b Concentrations obtained by isotope dilution.c Depleted mantle model ages calculated using 147Sm/144Sm=0.2
uncertainties (1j) of 87Rb/86Sr=F2.0% and of 147Sm/144Nd=F2.5%. Relati
laboratory are F4.5%, F1.8%, F3.2% and F2.7%, respectively.
circles in Fig. 5), suggesting that the same magmatic
source fed these vents.
Relatively high contents of compatible elements
are observed in rocks of the TVF, such as Cr 92–353
ppm, Ni 50–198 ppm and Co 17–56 ppm. These
relatively high values, coupled with Mg#
(MgO�100/(MgO+Fe2O3)) between 34 and 53, and
SiO2 compositions indicate that magmas of this
region were produced by fairly primitive melts. In
contrast, dacitic pumice and lava flows of NT
volcano show lower concentrations of these trace
elements (Cr 20–83 ppm, Ni 0–20 ppm and Co 6–11
ppm) and Mg# 28–37. The TVF rocks exhibit a
positive correlation between Cr (Fig. 5f), Ni and
MgO, but no similar correlation is observed in rocks
of NT (Fig. 5f). These results indicate the importance
of crystal fractionation processes in melts from the
TVF. However, element ratios such as Th/Hf vs. Th
(Fig. 7a) indicate that partial melting processes have
e Lower Toluca Pumice of NT volcano and for high-grade xenoliths
8)
5EM NT35EN NT35PN PEP2a PEP3a
iss Schist Phyllite Gneiss Gneiss
34 6.25 4.02 – –
721887 0.717112 0.715653 – –
37 32 – –
111 0.109 0.114 0.130 0.138
512268 0.512442 0.512279 0.512296 0.512263
18 20 – –
22 �3.82 �7.00 �6.67 �7.32
39 0.35 0.39 – –
66 124.88 125.28 – –
95 57.89 90.17 – –
47 4.53 5.73 4.99 4.63
90 25.18 30.42 23.17 20.17
32 1.04 1.34 1.54 1.79
036 19.100 19.025 – –
022 0.062 0.075 – –
694 15.679 15.675 – –
136 0.074 0.076 – –
256 39.224 39.223 – –
154 0.099 0.075 – –
14 and 143Nd/144Nd=0.51316 (Goldstein et al., 1984). Relative
vely reproducibility (1j) for Rb, Sr, Sm and Nd concentrations at the
Fig. 9. Sr vs. qNd present-day isotopic ratios for NT and TVF rocks. Inset shows isotopic composition of metamorphic xenoliths found in the
LTP and data of the Guerrero terrane from Talavera-Mendoza et al. (1995). Data of Colima volcano from Verma and Luhr (1993), data of
Popocatepetl from Schaaf et al. (2002, submitted for publication), and average isotopic data of DSDP 487 and 488 Cocos plate sediments,
Mexico from Verma (2000).
R.G. Martınez-Serrano et al. / Journal of Volcanology and Geothermal Research 138 (2004) 77–110 97
also played an important role in the composition of
magmas from the study area.
Variation diagrams for trace elements are shown
in Fig. 6a,b. Trace-element patterns of NT and
TVF samples are very similar, with enrichment in
the large-ion lithophile elements (LILE) relative to
the high-field-strength element (HFSE). This
enrichment is typical of calc-alkaline volcanic arcs.
All rocks show negative Nb anomalies as well as
several other negative anomalies (P, Ta and Ti) that
are also characteristic of subduction-related magma-
tism (e.g. Gill, 1981; Pearce, 1983; Walker et al.,
2001). The patterns of the immobile elements (Nb,
Zr, Hf, Ti, Y and Yb) on the variation diagrams
and the enrichment in LILE suggest a depleted
mantle source in the subcontinental lithospheric
mantle modified by subduction fluids, which have
added the more mobile elements (Rb, Ba, K and
maybe Pb) (Pearce, 1983; Wilson, 1989). NT rocks
display little chemical variation for the major and
trace elements such as Y (11–17 ppm), Hf (3.4–4.5
ppm), Zr (123–180 ppm) and Nb (3.9–5.0 ppm). In
contrast, TVF rocks show a wide variation Y (15–
25 ppm), Hf (3.3–5.4 ppm), Zr (124–219 ppm) and
Nb (3.8–14.4 ppm) (Table 2). These HFSE data,
along with some ratios of incompatible elements
such as Zr/U vs. U/Ta and U/Ta vs. Th/Hf (Fig.
7b,c), may suggest that NT products were produced
by a relatively homogeneous magmatic source,
whereas the volcanic rocks from the TVF were
Fig. 10. Plot of 207Pb/204Pb–206Pb/204Pb for whole-rock samples of NT and the TVF. Reference lines are the two-stage terrestrial lead evolution
curve (Stacey and Kramers, 1975), graduated at 250 Ma intervals (SK), and the Northern Hemisphere Reference Line (NHRL) (Hart, 1984). Pb
isotopic data for the Paleozoic Acatlan complex are from Martiny et al. (2000). Data for the Oaxaca Complex field from Solari et al. (1998).
MORB-EPR data are from PETDB (2002), Pacific Ocean Sediments (POS) from Church and Tatsumoto (1975) and Hemming and McLennan
(2001). Pb isotopic data of DSDP 487 and 488 Cocos plate sediments, Mexico from Verma (2000).
R.G. Martınez-Serrano et al. / Journal of Volcanology and Geothermal Research 138 (2004) 77–11098
probably derived from a more heterogeneous
source.
The rare earth element (REE) abundances in the
NT and TVF samples show similar trends. Chondrite-
normalized REE patterns display light rare earth
element enrichment (LREE, La–Sm) and relatively
bflatQ patterns for the heavy rare earth elements
(HREE, Tb–Lu). However, NT rocks show the lowest
concentrations of HREE in comparison to Popocate-
petl, Sierra Chichinautzin (e.g. Siebe et al., 2004), and
the TVF rocks. No Eu anomalies are observed (Fig.
8a,b) in rocks of the study area, indicating that
plagioclase fractionation was not significant. This is
consistent with Na2O and Al2O3 concentrations that
slightly increase with SiO2 concentrations, indicating
that plagioclase crystallized late and was not substan-
tially removed during ascent to the surface. Dacites of
NT show slightly lower REE concentrations in
comparison to andesites and basaltic andesites of the
TVF. La/Lucn ratios range from 7 to 12 in NT dacites,
from 7 to 13 in NT and TVF andesites, and from 12 to
14 in TVF basaltic andesites. In addition, NT and TVF
rocks show a general decrease in incompatible
elements as SiO2 increases. Verma (1999, 2000),
Marquez et al. (1999), and Marquez and De Ignacio
(2002) noticed that the suite of rocks from the Sierra
Chichinautzin and other sites of the TMVB display a
trend opposite of that most frequently observed at
many volcanoes in continental arcs, in which abun-
dances of incompatible trace elements (HFSE and
REE) typically increase with increasing SiO2. They
proposed that mixing between OIB magma types and
a lower-crustal component of dacitic composition
produced lavas of the Sierra Chichinautzin. However,
R.G. Martınez-Serrano et al. / Journal of Volcanology and Geothermal Research 138 (2004) 77–110 99
Siebe et al. (2004) suggested that the trace element
patterns in the Sierra Chichinautzin could be
explained by polybaric assimilation and fractional
crystallization (AFC; DePaolo, 1981) processes. In
our case, petrographical and chemical data of NT and
TVF rocks suggest that the presence of some mineral
phases such as hornblende, Fe–Ti oxides, and minor
clinopyroxene can explain the fractionation of these
trace elements as an AFC process.
4.3. Isotopic results
Isotopic analyses of Sr, Nd and Pb are given in
Table 3 for volcanic samples of NT and the TVF
and in Table 4 for the metamorphic xenoliths found
in the LTP. Despite the contrast in the petrographic
and chemical compositions between NT and TVF
rocks, they show similar isotopic values. Present-day87Sr/86Sr ratios range from 0.70385 to 0.70423 for
NT rocks and from 0.70371 to 0.70448 for TVF
rocks. qNd values generally range from +3.8 to +5.3
for NT rocks and from +2.2 to +6.8 for the
monogenetic field (Table 3 and Fig. 9). The narrow
range of isotopic data, with generally low 87Sr/86Sr
ratios and qNd values mostly above that of bulk
Earth that are within the mantle array, suggest
relatively low crustal contamination of the magmas.
In contrast, metamorphic xenoliths representing the
basement under NT show variable isotopic ratios
(Table 4 and Fig. 9) and can be grouped in two
lithologic types. The first type includes gneiss,
mica-schist, and some phyllites with 87Sr/86Sr
present-day values from 0.71565 to 0.72189 and
qNd from �3.8 to �7.2. The second type, formed
by green schists, is less radiogenic (87Sr/86Sr=
0.70573 and qNd=+1.1). Sr and Nd isotopic ratios,
obtained by Schaaf et al. (2002, submitted for
publication), for pumices and lavas from Popocate-
petl show more variation than the volcanic rocks
analyzed in the present study (Fig. 9), indicating a
larger interaction of these magmas with the crust.
Pb isotopic data of lava and pumice from the study
area display a relatively narrow range, suggesting a
similar source for these rocks. In Fig. 10, the ratios of
volcanic rocks of NT and TVF overlap and plot below
the average crust model curve of Stacey and Kramers
(1975). Pb isotopic ratios for NT rocks vary as follows:206Pb/204Pb=18.55–18.68, 207Pb/204Pb=15.54–15.62
and 208Pb/204Pb=38.19–38.47, whereas for the TVF
the range of isotopic ratios is 206Pb/204Pb=18.58–
18.69, 207Pb/204Pb=15.56–15.61 and 208Pb/204
Pb=38.28–38.50. Volcanic rocks from the study area
appear to define a steep mixing trend between a mantle
component such as the MORB of the Pacific Ocean
(PETDB, 2002) and a 207Pb-rich reservoir. This 207Pb-
rich component might correspond to the influence of
fluids derived from the subduction zone in the mantle
wedge or perhaps it is represented by addition of
continental crust. Pb isotopic data for the metamorphic
xenoliths from the basement also show two different
groups (Fig. 10). Most gneiss, mica-schist and phyllite
samples exhibit radiogenic values (206Pb/204Pb=
18.98–19.10, 207Pb/204Pb=15.68–15.69 and 208Pb/204Pb=39.16–39.26), whereas the green schists display
less radiogenic values (206Pb/204Pb=18.86, 207Pb/204Pb=15.63 and 208Pb/204Pb=38.71). These isotopic
data lie above and to the right of the average
Pb crustal evolution curve of Stacey and Kramers
(1975).
5. Discussion
5.1. Petrographic patterns
Stratigraphic studies in the area are difficult
because several lava flows and pyroclastic products
with similar petrographic compositions were erupted
and distributed over wide areas in a short period of
time (between ~40 and 3 ka). Considering these
complications, we used petrographic, mineralogical
and geochemical data for rocks, and also 14C and K–
Ar age determinations (age data of Bloomfield,
1975; Cantagrel et al., 1981; Macıas et al., 1997;
Garcıa-Palomo et al., 2002) to understand the
magmatic evolution of the NT and TVF regions.
The mineral association of NT products changed
from phenocrysts of plagioclase–clinopyroxene–
orthopyroxene–hornblende in an andesitic glassy
matrix for the events of bPaleo-NevadoQ, to dacitic
pumices, lavas and domes with phenocrysts of
plagioclase–hornblende–orthopyroxene. The presence
of lavas with only orthopyroxene as phenocrysts is
relatively uncommon in TMVB rocks. Volcanoes
such as Popocatepetl, Pico de Orizaba and Iztaccı-
huatl and most monogenetic volcanoes from TMVB
Fig. 11. Chemical and isotopic variations over time for rocks of NT and the TVF. Measured isotopic error is indicated for each sample. Data ages
are taken from Fig. 3.
R.G. Martınez-Serrano et al. / Journal of Volcanology and Geothermal Research 138 (2004) 77–110100
show the presence of two pyroxene types (clino- and
orthopyroxene) in andesitic–dacitic rocks and pumi-
ces. The distribution of most pumice and lava
erupted by NT could be evaluated by identifying
the pyroxene types present in the volcanic products
of central Mexico.
R.G. Martınez-Serrano et al. / Journal of Volcanology and Geothermal Research 138 (2004) 77–110 101
Paricutın volcano (McBirney et al., 1987) and
some of the other volcanoes in the TMVB show
similar pyroxene characteristics. Activity at Paricutın
began with the eruption of an olivine-bearing basaltic
andesite (55 wt.% SiO2) and ended with a hypersthene
andesite when 85% of the magma volume had been
discharged (McBirney et al., 1987). These authors
suggest that the absence of Ca-rich pyroxene on their
liquidus could be attributed to a loss of water after
differentiation by fractional crystallization and crustal
contamination of the magma had taken place.
The chemical compositions of Fe–Ti oxides from
some UTP samples were determinated by Arce (2003)
and in the present study. These chemical data were
used to calculate preliminary temperatures of magma
crystallization. Based on the ilmenite–titanomagnetite
geothermometer of Anderson and Lindsley (1998) and
the mineral reformulation model of Stormer (1983),
crystallization temperatures of 815–852 8C for the
magma of UTP were calculated. Only titanomagnetite
crystals in equilibrium conditions with the matrix
were considered for the temperature calculations.
Fig. 12. Sr isotopic variations vs. differentiation index (SiO2) for
volcanic rocks of NT and the TVF. Isotopic data from Popocatepetl
and Pico de Orizaba from Schaaf et al. (2002, submitted for
publication).
Hydrothermal experiments using a UTP sample
were conducted by Arce (2003). This author con-
structed P–T stability fields for the main mineral
phases with water pressures going from 150 to 200
MPa, equivalent to depths around 4–6 km below the
volcano. Most NT samples show petrographic and
chemical characteristics similar to those of the dacitic
rocks described by Blundy and Cashman (2001) for
the Mount St. Helens volcano. Based on the composi-
tional variations in natural and experimental glasses
from this volcano, these authors proposed a model for
magmatic crystallization of plagioclase, amphibole,
orthopyroxene and Fe–Ti oxides at pressures between
300–400 MPa (6–8 km depth) and 11 MPa (550 m
depth). Crystallization of magma at NT could follow a
polybaric process over 4 km depth as was suggested
by Blundy and Cashman (2001) for the Mount St.
Helens volcano.
Petrographic characteristics of the TVF rocks are
very similar to sample descriptions reported by
Marquez et al. (2001), Marquez and De Ignacio
(2002) and Siebe et al. (2004) for the Sierra
Chichinautzin rocks. The most common rocks in
this region are andesites, basaltic andesites, some
basalts and minor dacites with a predominant calc-
alkaline character. Nonetheless, some mafic rocks in
the Sierra Chichinautzin bear resemblances to OIBs,
as noticed by Marquez et al. (1999), Wallace and
Carmichael (1999) and Verma (2000). In the TVF,
some lavas are characterized by the coexistence of
olivine and xenocrystic quartz with evidence of
disequilibrium, as mentioned above. The presence
of these xenocrysts has been interpreted as some
type of magma mixing (Marquez et al., 2001;
Marquez and De Ignacio, 2002). However, as was
mentioned by Blatter and Carmichael (1998) in
andesites from Zitacuaro, in the central part of the
TMVB, there is no evidence in the TVF of
contemporaneous silicic magmas that contain quartz
phenocrysts that could explain the presence of
xenocrysts in andesites. In addition, no petrographic
evidences of magma mixing such as reverse zoning,
dissolution structures and mottled groundmass,
observed at the neighbouring Iztaccıhuatl (Nixon,
1988), were identified in NT and TVF lavas. The
occurrence of partially digested sandstone and
granodiorite fragments provides evidence of the
assimilation of some crustal material in the TVF.
R.G. Martınez-Serrano et al. / Journal of Volcanology and Geothermal Research 138 (2004) 77–110102
5.2. Geochemical patterns
The most evident difference between NT and TVF
rocks is the degree of differentiation. Volcanic rocks at
NT are more silicic than in the TVF (57–67 and 54–61
wt.% SiO2, respectively). In addition, geochemical
patterns of some trace elements (Fig. 7) and isotopic
data (Table 3) suggest that volcanic rocks of the two
regions were produced by magmas derived from
similar sources. Nonetheless, Sr and Nd isotopic data
are relatively more variable in rocks of the TVF than
at NT, suggesting a slightly different magmatic source
or crustal contamination.
Fig. 11 displays SiO2, V and Y concentrations, and
Sr and Nd isotopic variations over time for NT and the
TVF. The time scale was constructed according to the
stratigraphic column of Fig. 3. The most noticeable
aspects of this figure are the sharp decreases of SiO2
and the corresponding increases of V, Y, MgO and
Fe2O3t (data not shown for the oxides) for rocks of
NT during at least four periods of activity. The first
one occurs at 2.6 Ma (andesites and dacites were
reported by Bellotti et al., 2003, but no chemical data
are available), the second one at 1.5 Ma, the third one
at 25 ka and the last one at 10.5 ka. These sharp
chemical variations could be related to the input of
new less silicic magma into the NT magma reservoir.
TVF rocks only show a slight increase in SiO2 and
corresponding decreases in the other elements with
time (e.g. V and Y). Sr isotopic values for most rocks
of NT are homogeneous; although two peaks are
observed at 25 and 10.5 ka. These variations could
represent mafic replenishments of the magmatic
system that also led to greater crustal interactions.
TVF rocks display a scattered distribution of Sr
isotopes with time, indicating different amounts of
magma interaction with the crust during each mag-
matic event. Nd isotopic values for rocks of NT seem
to be less sensitive to variations over time, because all
rocks show values between 0.51283 and 0.51291.
TVF rocks show a slight decrease in 143Nd/144Nd over
time, interpreted as progressively stronger interaction
of magmas with the crust.
A diagram of Sr isotope ratios vs. SiO2 differ-
entiation index (Fig. 12) is used to characterize AFC
processes. For NT rocks SiO2 concentrations are
nearly similar (57–64 wt.%) with a range of Sr
isotopic values from 0.70385 to 0.704230, suggesting
a slight positive correlation for them. The correlation
of the data confirms the existence of assimilation and
crystal fractionation processes in NT lavas. TVF rocks
do not show any correlation between SiO2 wt.% and
Sr isotopic compositions. A rather scattered distribu-
tion of the Sr isotopic values is observed for rocks of
the monogenetic field (Fig. 12). However, assimila-
tion and crystal fractionation can also be inferred for
these rocks.
The nature of continental crust in the study area is
not known because a thick volcanic pile of the TMVB
covers the older rocks. However, it is assumed that
rocks from the Upper Cretaceous Guerrero terrane
(Campa and Coney, 1983) underlie the region. This
terrane is considered a major tectonic accretion of an
Upper Jurassic–Lower Cretaceous intraoceanic vol-
canic arc complex, essentially devoid of an old sialic
basement, onto the continental framework of Mexico.
Geochemical and isotopic data for typical sedimentary
and volcanic sequences from this terrane suggest that
these rocks were formed in a complex fossil island
arc-trench system similar to the present-day western
Pacific island arc system (Mendoza and Suastegui,
2000). Present-day Sr and Nd isotopic values range
from 0.70403 to 0.70473 and 0.51267 to 0.51282,
respectively, for volcanic rocks of this Mesozoic
terrane (Talavera-Mendoza et al., 1995). Nevertheless,
some geological and geochemical data (Elıas-Herrera
et al., 1998; Elıas-Herrera and Ortega-Gutierrez,
2000) suggest involvement of an old continental crust
in the Pre-Cretaceous magmatic evolution of the
Guerrero terrane. Gneiss xenoliths found by these
authors in a Tertiary volcanic field (La Goleta), 50 km
SW of the study area are characterized by an orogenic
low-pressure granulite facies metamorphism. These
xenoliths have present-day 143Nd/144Nd values of
0.51230 and 0.51226 (qNd=�6.7 and �7.3) and Nd
model ages (TDM) of 1.54 and 1.79 Ga (Elıas-Herrera
et al., 1998). These data are similar to values of
Grenvillian rocks in Mexico; therefore, it is very
likely that in the vicinity of the study area an old
continental crust covered by the Mesozoic Guerrero
terrane could exist.
The petrographic characteristics of gneiss and
some schist xenoliths found in the LTP are similar
to those described by Elıas-Herrera et al. (1998).
However, green schist xenoliths show characteristics
similar to the metamorphic volcanic andesitic sequen-
R.G. Martınez-Serrano et al. / Journal of Volcanology and Geothermal Research 138 (2004) 77–110 103
ces described by Talavera-Mendoza et al. (1995) for
the Guerrero terrane. This type of xenolith is relatively
abundant in the LTP. Sr and Nd isotopic data obtained
in the present work for representative xenoliths are
listed in Table 4 and displayed in Fig. 9. Isotopic data
are relatively variable but are mainly similar to values
obtained by Elıas-Herrera et al. (1998) for their
granulitic xenoliths. Green schist xenoliths (sample
NT35EC, Table 4) display values similar to those of
the volcanic rocks of the Guerrero terrane (Talavera-
Mendoza et al., 1995). Therefore, it is very probable
that the metamorphic xenoliths present in the Lower
Toluca Pumice were scavenged from different crustal
depths. The high Rb/Sr ratios of the gneiss, phyllites
and some schists might indicate a highly evolved
sedimentary crust. In addition, present-day qNd values(between �3.8 and �7.2) and Nd model ages (TDM
between 1.04 and 1.42 Ga) for most of the analyzed
xenoliths also show typical values of Grenvillian
rocks in Mexico, supporting the existence of a
recycled older continental crust. Therefore, isotopic
characteristics of xenoliths from the NT deposits
appear to confirm the presence of a Pre-Mesozoic
sialic basement under the southern border of the
TMVB.
Considering the possible effect of crustal contam-
ination on the magmas of the study area along with
the geochemical and isotopic data of the continental
crust, represented by the metamorphic xenoliths
analyzed here, it seems plausible that any degree of
assimilation would have resulted in a greater dis-
persion of the isotopic and trace element data in
volcanic rocks. However, Sr, Nd and Pb isotopic
compositions of the volcanic rocks of the Toluca area
plot in a restricted range (Figs. 9 and 10), excluding
an important role for crustal contamination in the
generation of the chemical and isotopic compositions
observed in the volcanic rocks. Therefore, fractional
crystallization could represent the most important
process of magma differentiation in the Toluca
volcanic rocks. Coherent linear trends with little
scattering of samples in the variation diagrams of
major and trace elements vs. SiO2 or MgO could,
thus, be explained by fractional crystallization. This
does not preclude that continental crust could slightly
influence the compositions of the volcanic rocks.
Volcanic rocks from other volcanoes of the central-
eastern TMVB, such as Pico de Orizaba and
Popocatepetl, show more variable isotopic composi-
tions (e.g. qNd ranges from �2 to +2 and from +3 to
+5, respectively, Schaaf et al., submitted for publica-
tion). These isotopic data suggest a more significant
interaction of magmas with the continental crust than
observed for NT and the TVF. The thickness (50 km
average) of the crust under the central-eastern part of
the TMVB is similar to that suspected beneath NT and
the TVF (Urrutia-Fucugauchi and Flores-Ruiz, 1996).
However, the weak interaction of magmas with
continental crust beneath the study area could be
explained by the presence of important fault systems
(Garcıa-Palomo et al., 2000) that facilitate magma
ascent through the crust. Three complex fault systems,
of different ages, orientations and kinematics, inter-
sect at NT volcano (Fig. 2). These authors propose
that at least three main deformation events affected
central Mexico since the late Miocene. During the
early Miocene, an extensional phase with a Basin and
Range deformation style occurred in northern Mexico
and produced NW–SE and NNW–SSE horsts and
grabens south of NT. In the middle Miocene, a
transcurrent event generated NE–SW faults with two
movements: (a) left-lateral strike-slip displacement
and (b) normal faulting. The latest deformation event
started during the late Pliocene and involved oblique
extension accommodated by E–W right-lateral fault-
ing that changed to normal faults. It is clear that
during the last tectonic event, the majority of the
volcanic eruptions in the study area and also in other
areas of the central TMVB were produced, indicating
a narrow relationship between E–W extensional
faulting and magma ascent. Recent laboratory experi-
ments (Hall and Kincaid, 2001) have indicated that
the interaction between buoyantly upwelling diapirs
and subduction-induced flow in the mantle creates a
network of low-density, low-viscosity conduits
through which buoyant flow is rapid (b30 ka).
5.3. Relationship between tectonics and magmatism
(magma source)
The petrologic origin and evolution of the TMVB
has been intensely debated recently. Two composi-
tionally contrasting suites of rocks have been recog-
nized in this volcanic region on the basis of
geochemical data: (1) calc-alkaline rocks with high
LILE/HFSE ratios that have been associated with a
Fig. 13. Sr/Y vs. Y discrimination diagram between adakites and typical arc calc-alkaline compositions (after Drummond and Defant, 1990).
R.G. Martınez-Serrano et al. / Journal of Volcanology and Geothermal Research 138 (2004) 77–110104
subduction environment; and (2) alkaline or transi-
tional rocks with low LILE/HFSE ratios and compo-
sitions similar to OIB. The simultaneous existence of
these two suites of rocks in the volcanic arc has been
explained by means of two main divergent hypoth-
eses. Some authors (e.g. Wallace and Carmichael,
1999; Luhr, 1997; and others) considered that the
predominantly calc-alkaline character of rocks and the
production of the OIB magma-types could be
explained by the generation of magmas in a sub-
duction setting along the Middle America Trench.
However, Verma (1999, 2000), Sheth et al. (2000),
Marquez et al. (1999) and Marquez and De Ignacio
(2002) regard the rocks of the central part of the
TMVB as produced by magma mixing between the
OIBs and a lower-crustal component of dacitic
composition. They deny a direct relationship between
subduction processes and the genesis of the TMVB.
Because these two divergent points of view have been
widely discussed in several recent works such as
Verma (2000), Ferrari et al. (2001), Marquez and De
Ignacio (2002) and Siebe et al. (2004), it is not our
aim to extend here its discussion. On the basis of our
petrographic, geochemical and isotopic data of NT
and the TVF, along with the present-day geodynamic
facts between the North America and Cocos plates
(summarized by Siebe et al., 2004), we propose that
magma generation in the study area is related to a
subduction environment.
Geochemical and isotopic data for NT and TVF
rocks, particularly LILE/HFSE ratios, suggest that
these rocks were produced in a typical subduction
environment where a depleted-mantle source
(MORB-type) was modified by different proportions
of fluids or melts from the subducted lithosphere.
Rocks of NT and the TVF show narrow ranges in Pb
isotopic values, suggesting similar sources. Most
rocks of NT and some from the TVF fall in the DM
(depleted mantle) field (206Pb/204Pb from 18.58 to
18.69 and 207Pb/204Pb from 15.54 to 15.61), repre-
sented by the EPR-MORB (Fig. 10), evoking partial
fusion of the oceanic slab. Pb isotopic data from NT,
the TVF, and some rocks from the Sierra Chichinaut-
zin (Verma, 1999) define a steep mixing line with a
R.G. Martınez-Serrano et al. / Journal of Volcanology and Geothermal Research 138 (2004) 77–110 105
narrow range (Fig. 10), where the possible end
members could be the MORB-EPR and the fluids of
the Pacific Ocean Sediment type (Church and
Tatsumoto, 1975; Plank and Langmuir, 1998; Hem-
ming and McLennan, 2001; Verma, 2000). Indeed, the
positive anomalies of Ba and Pb and also the high
values of some element ratios (e.g. Ba/Zr and K/Nb)
shown by the volcanic rocks (NT and the TVF)
confirm that fluids and/or melts from the subducted
slab contributed to magma genesis.
Melting of subducted hydrous basaltic crust
(MORB) is argued to produce magmas with a
distinctive chemical signature known as adakites
(Kay, 1978; Defant and Drummond, 1990; Martin,
1999; Garrison and Davidson, 2003). The major and
trace element characteristics used to classify rocks as
adakites are SiO2 contents between 63 and 70 wt.%,
high Sr concentrations (N400 ppm) coupled with low
Y (b19 ppm) and Sr/Y ratios N20. Typical adakites are
phenocryst-bearing volcanic rocks with compositions
of hornblende andesite to dacite, and basalts are
systematically lacking. Adakites commonly show a
low HREE content that is interpreted as reflecting the
presence of garnetFhornblende in the residual source
after partial melting. Most rocks of NT and two
Fig. 14. Ba/Zr vs. 87Sr/86Sr diagram showing the results of a mixing mod
(Hemming and McLennan, 2001).
samples (TEO1 and MX85, Fig. 2) from the western-
most part of the TVF display relatively high Sr and
low Y concentrations (460–694 and 14–21 ppm,
respectively) (Fig. 13). These rocks also show other
characteristics such as relatively low HREE contents
and Pb isotopic values similar to MORB-EPR,
suggesting a possible adakitic signature for them.
Typical models for the generation of adakites require
the subduction of a young (b5 Ma) and warm oceanic
lithosphere, where temperatures in the slab rise above
the solidus of wet basalts at high pressures, producing
melts (Defant and Drummond, 1990). However, some
authors such as Gutscher et al. (2000) have suggested
that most of the known Pliocene–Quaternary adakites
are paradoxically related to subduction of N10 Ma
lithosphere. They proposed an unusual mode of
subduction known as flat subduction, occurring in
~10% of the world’s convergent margins, that can
produce the temperature and pressure conditions
necessary for the fusion of moderately old oceanic
crust (e.g. Chile, Ecuador and Costa Rica subduction
regions). In central Mexico, it was generally believed
that the Cocos plate was subducting at a constant dip
angle N258. However, seismic data show that the
subducted Cocos slab is subhorizontal beneath south-
el between EPR-DM (White et al., 1987) and bulk Pacific sediment
R.G. Martınez-Serrano et al. / Journal of Volcanology and Geothermal Research 138 (2004) 77–110106
central Mexico (Suarez et al., 1990; Singh and Pardo,
1993; Pardo and Suarez, 1995). A similar geometry
has been observed in regions of central Peru and Chile
where it was designated as a bflat-slabQ. Pardo and
Suarez (1995) inferred that the 80–100 km depth
contours of the subducted slab lie beneath the
volcanic front of the TMVB in central Mexico.
However, the position of the subducted slab can not
be confirmed due to the paucity of earthquake
hypocenters along the volcanic front.
In southwestern Japan, Morris (1995) described the
existence of adakitic magma in at least two Quater-
nary volcanoes, Daisen and Sambe, that are associated
with the volcanic arc. In this area, no intermediate-
and deep-focus earthquakes have been identified,
similar to the central part of the TMVB. However,
this author proposes that melting of the Philippine Sea
plate beneath southwestern Japan can explain the
presence of the adakitic magmas. In addition, he
argued that slab melting can account for the aseis-
micity in the area, because the plate behaves as a
plastic body rather than a solid. In central Mexico the
behavior of the subducted plate could be similar.
Although adakitic signatures can be identified in
rocks of NT and in some rocks from the TVF,
suggesting a melting process of the subducted
oceanic crust, some authors (Garrison and Davidson,
2003) proposed that similar geochemical patterns can
be obtained by melting another basaltic source such
as the lower continental crust. Distinction between
these two sources of magma is nontrivial, and
requires integrated investigations of regional geo-
chemistry along with tectonic and geophysical data.
On the basis of geochemical and isotopic data, we
propose that melts of the subducted oceanic crust
contributed to the magmas erupted at NT and the
western part of the TVF. Melting of the lower
continental crust under NT could produce magmas
with higher isotopic variations, similar to values
observed in metamorphic xenoliths analyzed here.
However, the geochemical data attest to minor
interaction of magmas with the continental crust,
but interaction of adakitic magma with the mantle
during its passage to the surface could produce
fractionation of some elements (Y and Sr/Y ratios).
This is the first time that Quaternary adakitic
magmas have been identified in the central part of
the TMVB. Recently, adakitic signatures have also
been determined on the eastern section of the TMVB
but in Miocene volcanic rocks (Gomez-Tuena et al.,
2003).
The Valle de Bravo Volcanic field (VBVF) is also
located in the central part of the arc front of the
TMVB. It is bracketed by NT to the east and the
Zitacuaro Volcanic Complex to the west. Lava ages
from this volcanic field range from 300 to b10 ka
(Blatter et al., 2001) and the main rock compositions
are andesite and dacite. In addition we have noticed
an adakitic signature for some of the samples of the
VBVF based on rock compositions and Sr and Y
concentrations reported by Blatter and Carmichael
(2001). In fact, this adakitic composition was also
suggested by Aguirre-Diaz et al. (2003) for rocks
from the same region. Therefore, it is very likely that
the presence of adakites at NT is not a local
phenomenon but a regional characteristic of the
volcanic arc in central Mexico. However, more
detailed geochemical and isotopic studies are neces-
sary in the VBVF to further evaluate this hypothesis.
On the basis of some trace element ratios and
isotopic data of the studied rocks indicating partic-
ipation of slab components and mantle melts, a two-
component mixing model is proposed to explain the
genesis of NT and TVF rocks. This model is
characterized by the presence of a depleted mantle
(DM) source and a subduction component with
different fluid/melt ratios. The mixing calculation
assumes that the subducted component and the DM,
represented by the composition of a MORB-like
composition (White et al., 1987), are the two end
members of the model. The results of modeling
isotopic values and trace element ratios are displayed
in Fig. 14. The volcanic rocks plot above the mixing
line between the DM source and the subduction
component. With variable fluid/melt proportions the
mixing line shifts upward. The amount of subduction
component involved in the genesis of NT volcanic
rocks is estimated to be around 15–20%.
6. Concluding remarks
Understanding magma genesis and petrological
processes in the TMVB might be a relatively
difficult task because neighboring volcanic centers,
such as NT and the TVF, present particular geo-
R.G. Martınez-Serrano et al. / Journal of Volcanology and Geothermal Research 138 (2004) 77–110 107
chemical and isotopic patterns that can not be
generalized for the whole volcanic province.
Detailed geochemical and isotopic studies allowed
us to determine two slightly different magmatic
sources for the two areas. Most rocks of NT and
some from the TVF can be related to an adakite
magma source that was slightly modified by its
passage into the mantle wedge. It is feasible that
Quaternary adakitic magmas in the central part of
the TMVB are a common phenomenon associated
with melting of subducting slab. On the other hand,
most TVF magmas show typical calc-alkaline
patterns, where the mantle wedge was modified by
different fluid/melt proportions derived from the
subducted slab. Melting of the Cocos plate beneath
south-central Mexico could explain the distinctive
chemistry of NT volcano and the aseismic nature of
this region.
Although a thick continental crust (~50 km) has
been inferred by geophysical data in the study area,
there is no strong evidence for partial melting of the
lower continental crust having produced the magmas.
Isotopic compositions (Sr, Nd and Pb) suggest a
MORB-slab melting source for the rocks. In addition,
metamorphic xenoliths in the area suggest the
presence of an older continental crust (Nd model
age N1.0 Ga).
Mechanisms of melt generation at subduction
zones and transfer to the surface still remain uncertain.
However, laboratory experiments have indicated that
rapid ascent (b30 ka) of magmas is possible. It is
important to remember that during the Late Pleisto-
cene (~40 ka) a distensive tectonic event produced the
E–W normal fault system in the study area. Most
volcanic eruptions of NT and the TVF were produced
during the last 40 ka and probably were controlled by
this last fault system. Therefore, this last tectonic
event favored the rapid ascent of magmas to the
surface and it can explain the low crustal contami-
nation observed and the presence of typical mineral
associations indicating the ascent and crystallization
of magmas following polybaric processes.
Acknowledgements
Financial support by the National Council of
Science and Technology (CONACYT) (project
32330-T) in Mexico is gratefully acknowledged. The
authors wish to thank Jose Luis Arce for assistance in
the field, and Giovanni Sosa and Benjamin Domı-
nguez for assistance in the field, mechanical prepara-
tion of rock samples and in the analytical aspects of the
isotopic determinations. We are also grateful to
Barbara Martiny for revision and comments on the
English. Editorial handling by Margaret Mangan and
reviews by Jim Luhr and Alvaro Marquez were very
helpful and are greatly appreciated.
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