Cerro Grande volcano: the evolution of a Miocene stratocone in the

Transcripción

Tectonophysics 318 (2000) 249–280
www.elsevier.com/locate/tecto
Cerro Grande volcano: the evolution of a Miocene stratocone
in the early Trans-Mexican Volcanic Belt
Arturo Gómez-Tuena a, *, Gerardo Carrasco-Núñez b
a Instituto de Geologı́a, Universidad Nacional Autónoma de México, Cd. Universitaria, Coyoacán 04510, Mexico, DF
b Unidad de Investigación en Ciencias de la Tierra, Instituto de Geologı́a, UNAM, Campus Juriquilla, Apdo. Postal 1-742, Querétaro,
Qro. 76230, 04510, Mexico
Received 4 July 1998; accepted for publication 30 August 1999
Abstract
Cerro Grande volcano is a well-preserved, low-angle composite volcano that evolved between 11 and 9 Ma. It
represents the beginning of volcanism in the eastern sector of the early Trans-Mexican Volcanic Belt. Extensive field,
stratigraphic and petrologic work revealed a complex volcanic evolution that can be summarized in six principal
stages. (I ) construction of a shield-like lava cone; (II ) generation of unusual lithic-rich ignimbrites, apparently related
to powerful eruptions that caused rapid and significant vent erosion — these eruptions could have resulted from
shallow magma–water interactions; (III ) eruption of peripheral fissural lava flows following a NNW–SSE-trending
fissure system; (IV ) period of dome growth and explosive collapse; ( V ) short period of repose followed by open-vent
explosive magmatic and hydromagmatic eruptions; ( VI ) radial eruption of lava flows, related to a ring-fissural system.
Geochemistry data of late Miocene volcanics show a continental arc magmatic origin, and show that crystal
fractionation contributed strongly to magma differentiation. Several lava flows were erupted with a persistent NNW–
SSE-trending orientation, and the entire late Miocene geologic record is affected by highly dislocated normal faults
with the same orientation; this may indicate that late Miocene volcanism was under tectonic control. Finally, late
Miocene volcanics, and the inception of the early Trans-Mexican Volcanic Belt, could be related to a shallow
subduction angle influenced by a significant increase in the convergence rate along the Middle American Trench at
about 20 Ma. © 2000 Elsevier Science B.V. All rights reserved.
Keywords: hydromagmatic eruptions; late Miocene; lithic-rich ignimbrites; Mexico; Trans-Mexican Volcanic Belt; volcanic evolution
1. Introduction
The evolution of the Trans-Mexican Volcanic
Belt ( TMVB) as a distinctive geologic entity has
been a long-standing geological problem for the
last three decades. However, in the last few years,
a substantial amount of information on the space–
time distribution of volcanic rocks, and the
* Corresponding author. Fax: +52-5-622-4326.
E-mail address: [email protected] (A. Gómez-Tuena)
recognition of geometric variations in the subduction system, has provided important insights into
the origin of the magmatic arc. However, it is
surprising that very few systematic studies have
been written about the evolution of individual
volcanoes in Mexico, and even fewer about ancient
volcanic centers. Miocene volcanism is particularly
important and needs to be well documented
because of its implications on our understanding
of the inception of the TMVB. In this regard, the
basic geologic investigations, such as mapping,
stratigraphy and petrology, must be achieved in
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 1 4- 5
250
A. Gómez-Tuena, G. Carrasco-Núñez / Tectonophysics 318 (2000) 249–280
order to obtain a better framework of the temporal
and spatial variations of the arc and a better
understanding of crustal evolution in Mexico.
Cerro Grande volcano is located in La Sierra
de Tlaxco mountain range, in east-central Mexico
( Fig. 1). Geologically, it is situated at the intersection of the TMVB with the Mesozoic Sedimentary
Fold and Thrust Province (Ortega-Gutiérrez et al.,
1992). Effusive, explosive and volcaniclastic material associated with the activity of Cerro Grande
volcano occupies almost 2000 km2 ( Fig. 2). The
most conspicuous geomorphologic feature is a
4.5 km in diameter and 650 m deep caldera-like
structure truncated in its NE flank by a NW–SEtrending normal fault ( Fig. 2).
Previous work in Cerro Grande volcano includes
a regional geologic map with a general stratigraphy
(Carrasco-Núñez et al., 1997), two technical reports
from the local electricity company (CFE) (Garduño
et al., 1985; Carrasco-Núñez et al., 1995), and a
regional geological recognition (López-Hernández,
1995). Here, we present a comprehensive study on
the evolution of Miocene volcanism and discuss its
implications on the early development of the TMVB.
An interesting aspect from the volcanological
point of view was found in a distinctive stratigraphic
level of Cerro Grande volcano. This horizon displays
a highly indurated lithic-rich pyroclastic rocksequence that is apparently related to powerful
volcanic eruptions that caused rapid and significant
vent erosion. The mechanism of formation of such
eruptions is still not completely understood. This is
because the only well-documented similar deposit,
known as the Roque Nublo Formation in the Island
of Gran Canaria (Spain), has been interpreted in
many different ways (Anguita, 1973; Brey and
Schmincke, 1980; Perez-Torrado et al., 1995).
Nevertheless, recent studies in this rock-succession
showed that its lithologic character can be associated
with intense hydromagmatic explosions (PerezTorrado et al., 1997). We conclude that a similar
phenomenon played a significant role in the evolution of Cerro Grande volcano.
2. Geologic setting: the Trans-Mexican Volcanic
Belt
The TMVB is a continental magmatic arc
1000 km long and between 20 and 200 km wide
that extends from the Pacific Coast to the Gulf of
Mexico ( Fig. 1). The magmatic arc crosses central
Mexico from east to west, delimited by the 19 and
21° N parallels, with an oblique (~16°) distribution relative to the Middle American Trench
(MAT ). This particular orientation led to the
definition of the arc as the Trans-Mexican Volcanic
Belt because it is clearly transversally emplaced
over most of the NW–SE-trending Mexican geologic provinces (Ortega-Gutiérrez et al., 1992).
Several historic volcanic eruptions (Simkin and
Siebert, 1994), active faulting (Suter et al., 1991,
1995a,b) and high heat flow (Polak et al., 1985;
Prol-Ledezma and Juárez, 1985) have been recognized as prominent features of its activity.
Over a century, several hypothesis had been
proposed for the origin and the striking nonparallelism of the volcanic arc relative to the trench
(Humboldt, 1808; De Cserna, 1958; Mooser, 1972;
Gastil and Jensky, 1973; Johnson and Harrison,
1989). Now, it is generally accepted that the
TMVB is related to geometric variations in the
subducting angle of the Cocos and Rivera Plates
underneath the North American Plate ( UrrutiaFucugauchi and Del Castillo, 1977; UrrutiaFucugauchi and Böhnel, 1987; Pardo and Suárez,
1993, 1995).
The petrologic spectrum of the TMVB is dominated by calc-alkaline andesites and dacites
(Aguilar-y-Vargas and Verma, 1987). However,
several alkaline volcanic rocks have been extruded
in the eastern and western limits of the province
(Cantagrel and Robin, 1979; Luhr and Carmichael,
1985; Negendank et al., 1985; Besch et al., 1988;
Nelson and González-Caver, 1992; Luhr, 1997).
The connection between these two contrasting
petrogenetic suites is still in debate. It has been
suggested that: (1) they related to the same subduction process along the MAT (Negendank et al.,
1985; Besch et al., 1988; Nelson and GonzálezCaver, 1992); (2) they are the product of fusion
of different mantle reservoirs ( Verma and Nelson,
1989; Luhr, 1997); and (3) at least on the eastern
portion of the arc, they represent two distinct
petrogenetic provinces ( Thorpe, 1977; Cantagrel
and Robin, 1979).
Structurally, the TMVB cannot be visualized as
a uniform entity. Several studies based on the
Fig. 1. Regional localization of Cerro Grande Volcano, major tectonic lineaments and regional geology of the TMVB. Modified from Aguirre-Dı́az et al. (1997).
251
252
directions of faulting and stress fields divide the
province in three main sectors subject to different
tectonic styles: Western, Central and Eastern
( Fig. 1) (Demant, 1978; Pasquaré et al., 1988).
The brittle deformation features (fractures and
faults) are better organized and defined in the
western and central sectors, where tectonic extension have been coeval with volcanism (Luhr, 1997
and references therein). Nevertheless, the structural features of the eastern sector are not easily
recognized, either because they have been obscured
by erosion or recent volcanism, or because the
crust in this sector responded differently to similar
stress fields. These differences can also be related
in some extent to the nature of basement rocks. It
has been suggested that the TMVB is emplaced
over boundaries of ancient terranes that have
undergone episodic reactivations after accretion
(Sedlock et al., 1993; Ortega-Gutiérrez et al., 1994)
and that their tectonic limits may play an important role in magma ascent and emplacement
(Alaniz-Alvarez et al., 1998). Indeed, the crustal
thickness increases from the coasts to the continental interior reaching nearly 50 km beneath the
eastern sector of the TMVB (Molina-Garza and
Urrutia-Fucugauchi, 1993; Urrutia-Fucugauchi
and Flores-Ruiz, 1996). Therefore, the variation
in composition, age and basement structure over
which the arc is emplaced may be related to
differences in crustal response to a stress field,
which would significantly influence magma ascent,
differentiation paths and emplacement conditions.
The structural framework of the eastern sector
is of relevance here because it may have an important influence in the distribution of volcanic vents
(Cantagrel and Robin, 1979; Negendank et al.,
1985; Höskuldsson and Robin, 1993; AlanizAlvarez et al., 1998). Fault population analyses
are only documented in a few technical reports
from Comisión Federal de Electricidad (Garduño
253
et al., 1985; Carrasco-Núñez et al., 1995; LópezHernández, 1995) and in some regional geologic
recognitions (Negendank et al., 1985; Pasquaré
et al., 1986). The volcanic rocks of this sector
commonly lie over late Cretaceous–Early Tertiary,
NNW–SSE-trending, laramidic folds and thrusts
of the Mesozoic Sedimentary Folds and Thrusts
Province (Ortega-Gutiérrez et al., 1992). Toward
the northern and eastern limit of the eastern sector,
greatly fragmented and dislocated NNW–SSEtrending normal faults cut late Miocene volcanic
rocks, but apparently, they do not affect postPliocene rock sequences (Garduño et al., 1985;
Negendank et al., 1985; Pasquaré et al., 1986;
Carrasco-Núñez et al., 1995, 1997; LópezHernández, 1995). Moreover, several late Miocene
volcanic vents were clearly erupted along NNW–
SSE-trending fissures (i.e. Teziutlán andesite
( Yañez-Garcia, 1980) and the Terrenate group and
Crestón andesite (Carrasco-Núñez et al., 1997 and
this paper). These faults, and the alignment of
volcanic structures in the same direction, may
indicate the existence of a late Miocene tectonic
extension period coeval with volcanism in the
eastern TMVB. The NNW–SSE-trending structures are also present in other areas of the TMVB
(e.g. the Taxco–San Miguel fault system in Fig. 1),
and they have usually been related to a southern
protraction of the Basin and Range style of deformation (Pasquaré et al., 1988; Henry and ArandaGómez, 1992; Suter et al., 1995a; Jansma and
Lang, 1997). However, a clear definition of its
temporal and spatial extent has not been achieved
due to the lack of a comprehensive study on the
tectonics of the eastern TMVB.
East from the Taxco–San Miguel fault system
(e.g. south of the Mexico City Basin, Fig. 1), a set
of ENE–WSW-trending normal faults cut
Pliocene–Quaternary volcanic rocks. Although
this system is not well defined eastwards, the NE–
Fig. 2. Geologic map of Cerro Grande volcano modified from Carrasco-Núñez et al. (1997). Rock-unit abbreviations: Qdpa=
pyroclastic and alluvial deposits, undivided; Qix=Xaltipan ignimbrite; Qgt=Tlaxco group; Qatc=Tres Cerros andesite; Tacl=Cruz
de Leon andesite; Tsvn=Nocayoco volcano-sedimentary sequence; Tiba=Ixtacamaxtitlán block-and-ash-flow deposit; Tac=El
Crestón andesite; Tgc=Cuyoaco group; Toi=Oriental lithic rich ignimbrite; Tacg=Cerro Grande andesite; Tgtl and Tgtu=Terrenate
group; Tdpc=Coyoltepec pyroclastic deposits, undivided; Tsbj=Benito Juárez pyroclastic sequence; Ti=Tertiary intrusives, undivided; J-K=Mesozoic sedimentary rock sequence, undivided.
254
SW alignment of cinder cones and minor fractures
suggest its presence (Pasquaré et al., 1988;
Carrasco-Núñez et al., 1997). Therefore, it is
apparent that a second period of tectonic extension
acted in the eastern TMVB in close relation with
volcanism since the Pliocene age.
The recognition that large stratovolcanoes in
the TMVB are aligned in an approximately N–S
direction led to several interpretations on the relation of volcanism with tectonic style (Cantagrel
and Robin, 1979; Negendank et al., 1985;
Höskuldsson and Robin, 1993; Siebe et al., 1993).
Recently, Alaniz-Alvarez et al. (1998) proposed
that the large stratovolcanoes are located over N–
S-trending faults with slow slip rates, while monogenetic volcanism is emplaced over E–W-trending
normal structures with higher slip rates. This
hypothesis implies that these two almost orthogonal fault systems (NNW–SSE and ENE–WSW )
have been simultaneously active since the late
Miocene. However, this interpretation has been
challenged by several authors in the lack of
unequivocal evidence supporting the tectonic control of volcanism (Contreras and Gómez-Tuena,
1999; Suter, 1999). Furthermore, as noted above,
at least in the northeastern sector of the TMVB
where the present study was performed, the NNW–
SSE system only affects the late Miocene geologic record.
3. Samples and procedures
Geologic mapping was performed using: a
1:100 000 scale LANDSAT satellite image, 1:50 000
scale aerial photographs and 1:50 000 scale topographic maps. Geologic contacts and stratigraphic
sections were later verified and measured during
several field seasons from 1995 to 1997. Up to 250
rock samples were collected and described from
outcrops during field work. Petrographic thin sections of nearly 150 samples were used to characterize stratigraphic rock units.
Whole-rock major-element geochemistry was
obtained in the laboratories of the Instituto de
Geologı́a ( UNAM ) by wavelength-dispersive
X-ray fluorescence spectroscopy ( XRF ). Sr and
Nd isotope determinations were obtained by mass
spectrometry in the laboratories of the Instituto
de Geofı́sica ( UNAM ). Trace-element analyses of
whole-rock samples were obtained by ICP-MS at
the Lamont-Doherty Earth Observatory of
Columbia University.
Whole-rock lava samples for geochemistry were
mechanically crushed and pulverized in a Herzog
mortar. However, rocks as heterogeneous as the
Oriental lithic-rich ignimbrites ( Toi, see below for
descriptions) are difficult to sample and analyze
for petrological and geochemical studies due to
the high induration of the deposits, their lithicrich character that blur the recognition of the
juvenile material, and the high degree of alteration
of the glassy-juvenile components to secondary
minerals. For these reasons, an attempt to recognize the composition was carried out selecting
relatively homogenous samples from different sites
and stratigraphic positions. These samples were
mechanically crushed in a ceramic mortar and the
juvenile components separated by hand-picking
using a binocular microscope. The separated juvenile components were pulverized and analyzed by
XRF. Major element chemistry results showed
that the extensive alteration of the glass could not
be avoided (see below).
Pyroclastic rock units of Cerro Grande volcano
are invariable highly indurated. Thus, a conventional grain-size separation analysis is not possible
because a mechanical desegregation of the rock
will inevitably fragment the components.
Therefore, an estimation of the grain size distribution was obtained in the field using a 64 mm grid
mounted in a 1 m2 wood frame (see Fig. 4b). A
volume proportion of the >64 mm components
and surrounding matrix was estimated with a
methodology similar to that used for point-counting in thin-section petrography.
4. Geology of Cerro Grande volcano
A general description of the stratigraphic units,
areas and rock volumes that crop out in the Cerro
Grande area can be found in Carrasco-Núñez et al.
(1997). Table 1 and Fig. 2 summarize the geology
of the map area. This study focuses on the evolu-
255
Table 1
Chronostratigraphic units of the Cerro Grande volcanic fielda
Unit
Absolute and
relative ages
(Ma)
Type of deposit
Composition
Area (km2)Interpretation
average thickness (m)volume (km3)
Plio-Quaternary
volcanics (Qgt,
Qatc, Qix, Qdpa)
0.45±0.09b,
0.49±0.07b
Cinder cones, lava
flows, ignimbrites
Basalt to rhyolite
Undivided. See
Carrasco-Núñez
et al. (1997).
Strombolian
eruptions, lava flows
and caldera collapse
ignimbrites
Cruz de León
andesite (Tacl )
9.0±0.4b
Lava flows and
minor pyroclastic
flows
Andesite to dacite
140-280-39.2
Ring-fissural lava
flows
Pyroclastic fall and
surges. Fluviolacustrine sediments
Dacite to rhyolite.
Polymictic
conglomerates,
sands and clays
120-200-24
Open-vent magmatic
and hydromagmatic
explosive eruptions
Nocayoco volcanoLate Miocene
sedimentary sequence
( Tsvn)
Ixtacamaxtitlán
block-and-ash-flow
deposit (Tiba)
9.2±0.3c
Block-and-ash-flow
deposit
Dacite
45-30-1.35
Dome explosive
collapse
El Crestón andesite
( Tac)
9.7±0.5b
Lava flows with
auto-breccias
Andesite
175-400-70
Fissural lava flows
probably related to
a NW–SE fault
system
Cuyoaco Group
( Tgc)
Oriental lithic rich
ignimbrite (Toi)
8.9±0.4b,
10.5±0.4d
Late Miocene
Andesite
120-400-48
Basaltic-andesite to
dacite
40-70-2.8
Cerro Grande
andesite (Tacg)
Terrenate group
( Tgtl, Tgtu)
11.0±0.6b
Columnar jointed,
massive lava flows
Pyroclastic flows,
falls, surges and
minor lahars
Blocky to slabjointed lava flows
Lava flows with
auto-breccias and
minor
volcaniclastics
Andesite to dacite
140-400-56
Fissural and central
lava flows
Intense magmatic
and hydromagmatic
explosive eruptions
Shield-like volcano
Basaltic-andesite to
dacite
48-250-12
Volcanic cones
emplaced along a
NW–SE-trending
lineament
Lacustrinepyroclastic units
( Tsbj, Tdpc)
Late Miocene?
Lacustrine
sediments
Clay-rich sediments
and minor
pyroclastics
Undivided. See
Carrasco-Núñez
et al. (1997).
Ponds and lakes
occupying small
depressions
Tertiary intrusives
( Ti)
31–14.5c
Plutonic rocks
Granitoids
Undivided. See
Carrasco-Núñez
et al. (1997).
Roots of the ancient
TMVB volcanic arc?
Basement
sedimentary rocks
(J–K )
Jurassic–
Cretaceous
Platform and basin
sediments
Mainly calcareous
and minor
terrigenous
Undivided. See
Carrasco-Núñez
et al. (1997).
Shallow marine
environment
Late Miocene
a Cerro Grande volcano rock units are shown in bold.
b Carrasco-Núñez et al. (1997).
c This study.
d Yañez-Garcia (1980).
256
tion of Cerro Grande volcano based on a detailed
description of the Miocene stratigraphic units.
The oldest exposed rocks are highly deformed
marine calcareous and terrigenous sedimentary
rocks of Mesozoic age (J–K, Fig. 2 and Table 1).
In several places, these rocks are intruded by
granitoids (Ti; Fig. 2 and Table 1) of unknown
age, but similar rocks were dated in nearby areas
between 31 and 14.5 Ma ( Yañez-Garcia, 1980).
These dates suggest a long period of intrusions
that may represent the roots of volcanic centers in
the early TMVB. However, the lack of a comprehensive study in these rocks prevents further interpretations about their origin and their relation to
surrounding volcanism.
During the Cenozoic, the area was affected by
two different volcanic phases that occurred in the
late Miocene and Pleistocene ages. Contemporaneous with the Miocene volcanism, several
lacustrine basins and fluvial drainage generated
thick (e.g. 45 m in section A3, Fig. 2), stratified
sedimentary rock sequences commonly interbedded with pyroclastic deposits. These units ( Tsbj
and Tdpc; see Fig. 2 and Table 1) were described
by Carrasco-Núñez et al. (1997), but their relation
to Cerro Grande evolution is unclear.
From the early Pliocene to Pleistocene, a period
of volcanic repose very likely occurred in the Cerro
Grande area as no volcanic rocks of these ages
were identified. At about 0.5 Ma, a NE–SW-trending monogenetic volcanic field was emplaced
around the Miocene volcanic structures (Fig. 2
and Table 1, Qgt, Qatc). Regionally, caldera-collapse ignimbrites from Los Humeros Caldera
( Fig. 2; Qix) filled valley depressions in the center
of the map area. Several ash flows and pyroclastic
falls were deposited over the Miocene sequences
( Fig. 2; Qdpa). Some of these deposits are probably related to the activity of La Malinche stratovolcano (about 30 km south from Cerro Grande,
Fig. 1),
257
4.1. Late Miocene stratigraphic record
4.1.1. Terrenate group (Tgtl and Tgtu)
The Miocene volcanic phase started as lava flow
fields, and minor pyroclastic and volcaniclastic
rocks, erupted from three different vents located
along a NW–SE-trending lineament (Fig. 2; El
Conejo, Pilancón and Tlacoxolo vents). The
Terrenate group is divided into two members,
lower Terrenate ( Tgtl ) and upper Terrenate
( Tgtu). The Tgtl member consists of stratified,
gray to dark-gray lava flows and auto-breccias
with a composition ranging from two-pyroxenebearing basaltic andesites to andesites with similar
mineralogy (SiO =56.5–62%). The Tgtu member
2
comprises a thick sequence of gray to brown, slabjointed, porphyritic augite-bearing andesitic lava
flows (SiO =61.6%) that commonly show vertical
2
jointing in the flow fronts.
Although there is no isotopic date for these
rocks, field evidence indicates that the Tgtl lavas
erupted contemporaneously or slightly earlier than
the base of Cerro Grande volcano (~11 Ma).
Lava flow fronts from El Crestón andesite
(~9.7 Ma) and from Cruz de León andesite
(~9 Ma) were clearly deflected by the Pilancón
volcanic structure, hence being older. The Tgtl
rock-unit overlies lacustrine sediments of the Tsbj
sequence at site S-2 (Fig. 2).
4.1.2. Cerro Grande andesite (Tacg)
The initial volcanic phase of Cerro Grande
volcano is composed of a ~400 m thick sequence
of blocky to slab-jointed lava flows. Composition
of these rocks vary from low-silica andesites
(SiO =58%) to dacites (SiO =67%). The
2
2
lowermost unit is a gray, massive to blocky, porphyritic andesitic lava-flow, which contains plagioclase, augite, and magnetite phenocrysts. The
middle part is composed of gray to brown, subhorizontally slab-jointed porphyritic andesites with
Fig. 3. Representative stratigraphic columns of the Oriental lithic rich ignimbrite (Toi). Letters beside columns depict different units
and sample labels. Note that sections are drawn at different scales due to variable thicknesses. This rock unit represents a radical
change in Cerro Grande’s eruptive behavior from effusive volcanism to powerful hydromagmatic eruptions (see text and Stage II
for details).
258
phenocrysts of plagioclase, augite and magnetite
with traces of chlorite, hematite and clay minerals.
The uppermost lava flow unit is composed of gray
to brown, massive to blocky, porphyritic to subophitic hornblende-bearing dacites with less augite
and magnetite.
The geographic distribution of this rock
sequence is apparently very wide, but it is usually
covered by later volcanism and lacustrine sedimentation. Lava flows related to this volcanic
phase crop out at nearly 30 km from the vent, but
the base is not exposed ( Fig. 2, site 6), and its
association with a central vent cannot be unambiguously defined. The base of this unit usually rests
over highly deformed Mesozoic sedimentary rocks
in the northern ( Fig. 2, site 31) and western sectors
( Fig. 2, site 34). A whole-rock andesitic sample
from the base of the sequence was K–Ar-dated at
11.0±0.6 Ma (Fig. 3, site J-8) (Carrasco-Núñez
et al., 1997).
4.1.3. Oriental lithic rich ignimbrite (Toi)
Carrasco-Núñez et al. (1997) named Ixtacamaxtitlán Breccias a pyroclastic rock-sequence
with two different members ( Tbil and Tbiu)
that apparently had a close relation in their
stratigraphic record and genesis. Further investigations indicated that each member must be separated into two different stratigraphic units because
they were clearly generated by distinct eruptive
mechanisms, different magma compositions, and
are separated by a period of intense erosion
and lava-flow emplacement (Gómez-Tuena and
Carrasco-Núñez, 1997). Therefore, in this work,
the Ixtacamaxtitlán breccias lower member ( Tbil )
of Carrasco-Núñez et al., (1997) was renamed as
Oriental lithic rich ignimbrite ( Toi), and the upper
member ( Tbiu) as Ixtacamaxtitlán block-and-ashflow deposit ( Tiba). Also, the stratigraphic correla-
259
tion scheme of Carrasco-Núñez et al. (1997) is
modified (Fig. 2 and Table 1).
The Oriental lithic rich ignimbrite ( Toi) is a
major rock unit widely distributed around the
volcano. It is mainly composed of highly indurated, unwelded, massive, polymictic, pyroclastic
breccia-type deposits [sensu Fisher (1961) and
Schmid (1981)]. The thickness of the breccia-type
depositional units ranges from 1 to 4 m near the
crater rim (Figs. 2 and 3, sections 2 and 8), to 2–
16 m in distal parts ( Figs. 2 and 3, section 4).
They are normally massive, occasionally showing
either normal or inverse grading in large lithic
clasts (Fig. 3). Degassification pipes are absent or
poorly preserved, although some horizons show a
red thermal alteration. Breccia-type deposits
always show sharp planar, non-erosive contacts
with underlying units, even if they are composed
of non-indurated material. Breccia units are usually separated by thinly laminated, well-sorted
pumice-rich fall layers or lithic-rich surge deposits
( Fig. 4a). These horizons can be traced laterally
for tens of meters, but they cannot be unambiguously verified between outcrops. In distal areas,
there are some reworked, laharic horizons near the
top of the stratigraphic section ( Figs. 2 and 3,
section 4).
Although a conventional grain-size analysis was
impossible because of the highly indurated nature
of the deposits, a field estimation was performed
using a 64 mm grid (Fig. 4b, see above). Lithic
clasts with a diameter >64 mm (i.e. blocks) constitute between 10 and 45% of the total rock volume.
If the complete grain-size spectra is considered,
lithic clasts comprise between 25 and 80% by
volume. Juvenile fragments >64 mm across constitute less than 5% of the deposit and are typically
less than 5 cm in diameter. A field estimation
suggests that juvenile clasts comprise between 10
Fig. 4. Field and microscope photographs of the Toi. (a) Lithic-rich surge deposit divides two breccia-type layers in section 8 ( letters
represent layers in Fig. 3). (b) Breccia-type horizon ‘f ’ in section 8. The photograph shows the 1 m2 wood frame with a 64 mm grid
used in field grain-size estimations. (c) Microphotograph (10×) of juvenile material with low vesicularity of breccia-like layer ‘a’ in
section 4 (see Fig. 3). A large normally zoned plagioclase (Plag), and palagonitization of the glassy material (Dv), can be observed.
(d ) Microphotograph (10×) of pumice fragment from layer 4d’3 in section 4 (see Fig. 3), showing well-developed subspherical vesicles
(v), resorbed plagioclase phenocrysts (plag) and altered glassy matrix. (e) Microphotograph (10×) of lithic-rich surge deposit layer
h-2 in section 8 (See Fig. 3). Non-vesiculated glassy material (gl ) occupies interstitial spaces between lithic fragments (L).
260
and 35% of the total rock volume. These constituents are always enclosed by a criptocrystalline,
devitrified glassy matrix (10–40%).
Lithic clasts are often angular to subrounded,
and range in size from a few millimeters to 2 m
across. Although they are heterogeneous in composition, a two-pyroxene porphyritic andesite is
the most abundant component. Hornblende-bearing dacites and augite-bearing andesites are also
present in minor proportions. No accidental lithologies were found in the deposits (i.e. limestones
or plutonic rocks). Therefore, it is apparent that
lithics fragments have a cognate character, derived
from the walls of the eruptive vent at a shallow
depth.
The principal juvenile material is composed of
yellow, and sometimes banded yellow–brown, twopyroxene-bearing pumice with angular to subrounded morphologies. However, some hornblende-bearing pumice is observed at the base of
sections 2 and 8 in pyroclastic-fall sequences. Also,
some dark-brown, ‘bread-crusted’ to ‘cauliflower’shaped andesitic bombs were observed in breccia
layers of section 8. In thin-sections, the glassy
matrix from breccia layers exhibit a strong devitrification to palagonite and clay minerals ( Fig. 4c).
This alteration accounts for the induration of the
deposits and represents the principal factor that
obscures the magma composition in the wholerock chemical analyses (see below). Juvenile fragments also display variable degrees of vesicularity
( Fig. 4c–e). Juvenile material from breccia-type
layers usually does not exhibit vesicles (Fig. 4c),
although some fragments with a low vesicularity
are present. Juvenile fragments in fall deposits
commonly exhibit well-developed subespherical
vesicles ( Fig. 4d ). However, juvenile material from
pyroclastic surge deposits usually does not show
vesicles, and the glassy matrix commonly fills
interstitial spaces between lithic fragments
( Fig. 4e).
The most representative stratigraphic sections
of the Toi can be found at the caldera rim (Sites
8 and 2, Figs. 2 and 3), at the center of the
structure (Site 11, Figs. 2 and 3) and 30 km SE
from the vent (Site 4 and 6, Figs. 2 and 3). Fig. 3
depicts a stratigraphic scheme of these sections.
The initial eruptive stage emplaced a series of
thinly laminated pyroclastic surge and fall horizons
that incorporated a few accessory lithics (sections
2 and 8; Fig. 3). In both sites, surge deposits show
the same phenocryst suite: plagioclase, orthopyroxene, clinopyroxene and traces of hornblende. After
this initial phase, a series of breccia layers, and
interbedded pumice-rich fall deposits or lithic-rich
surge horizons, were emplaced around the volcano.
The mineralogy of these rocks is very similar:
plagioclase and two-pyroxenes. However, the base
of the section 11, northeast from the vent, is
composed of breccia-type layers with only normally zoned plagioclase phenocrysts, overlaid by
lithic-rich ignimbrites with plagioclase and twopyroxenes. Therefore, it is possible that the base
of section 11 was the second magma batch
extruded from a different portion of the chamber.
The following blasts might be regarded as the
climactic phase of the eruption where the main
portion the chamber was extruded as volumetric
breccia layers, and associated falls and surges, with
very similar juvenile material containing plagioclase and two pyroxenes.
The base of the Toi commonly overlies andesitic
lava flows of the Cerro Grande andesite ( Tacg;
~11 Ma; sites 8 and 6) and is covered by 50–70 m
thick fluvial conglomeratic horizons that underlie
the Ixtacamaxtitlán block-and-ash-flow deposit
( Tiba; ~9.2 Ma, site 38). Andesitic lava flows of
the El Crestón andesite ( Tac; ~9.7 Ma; sites 4
and 6), and from the Cruz de León andesite ( Tacl;
~9 Ma; sites 8 and R-4), directly overlie the Toi.
Although no isotopic dates were obtained directly
from the Toi because of its alteration and lithicrich character, stratigraphic relations showed that
it is directly constrained between lava flows of 11
and 9.7 Ma. However, no stratigraphic relations
were verified between the Toi and the Cuyoaco
group ( Tgc; 10.5–8.9 Ma). For this reason, it is
either possible that old lava flows of the Cuyoaco
group entirely covered the Toi or that the Toi was
never emplaced toward the eastern sector of the
map area. As it is apparent that the Toi had a
wide and radial distribution, the former possibility
seems the most favorable explanation. Therefore,
the Toi may reflect a single eruptive phase of a
zoned magma chamber as no interbedded lava
flows or recognizable paleosoils were identified
within the sequence.
4.1.4. Cuyoaco group (Tgc)
The Cuyoaco group is composed of gray to
brown, two-pyroxene porphyritic andesitic lava
flows with ophitic and microlitic textures
(SiO =62–63%), and some hornblende-bearing
2
porphyritic dacites (SiO =67–70%). The lava
2
flows are commonly massive, with a persistent
columnar jointing and are usually affected by a
intense hydrothermal alteration. These rocks were
erupted from different volcanic vents that are
currently deeply eroded, but they appear aligned
in the N–S direction. They were dated by YañezGarcia (1980) at 10.5±0.4 Ma ( K–Ar whole rock,
site D-1) and by Carrasco-Núñez et al. (1997) at
8.9±0.4 Ma ( K–Ar whole rock, site X-4). These
dates imply a long period of effusive activity for
the emplacement of this group.
4.1.5. El Crestón andesite (Tac)
This unit is composed of thick, massive, porphyritic andesitic lava flows, and associated autobreccias, that form a 23 km long NW–SE-trending
elongated ridge. The composition varies from fairly
homogeneous two-pyroxene-porphyritic andesites
to a two-pyroxene-hornblende bearing dacites
(SiO =56–64%). These lavas were clearly related
2
to a NW–SE fissure system, as no eruptive centers
were identified. Also, the distribution of lava flow
fronts implies that they were extruded from a
fissural axis (Fig. 2). This rock unit directly overlies the Terrenate group ( Tgtl-Tgtu), the Cerro
Grande andesite ( Tacg) and the Ixtacamaxtitlán
lithic rich ignimbrites ( Toi). A whole-rock andesitic sample from Site G-4 was K–Ar dated at
9.7±0.5 Ma (Carrasco-Núñez et al., 1997).
4.1.6. Ixtacamaxtitlán block-and-ash-flow deposit
(Tiba)
This rock unit is exposed toward the northern
and northeastern flanks of the volcano ( Fig. 2,
Sites D-9, 16, 19, 20, and 38). It is composed of
white to light gray and pink, highly indurated,
poorly sorted, stratified block-and-ash-flow deposits with a homogenous, non-vesicular, biotite-hornblende bearing porphyritic dacite as the principal
261
clast component (SiO =65%). Hornblende and
2
biotite euhedral phenocrysts are often 0.5 cm in
size. Individual strata vary in thickness between 2
and 10 m, but the layer thickness decreases as the
distance from the vent increases ( Fig. 5). In general, individual block-and-ash-flow layers are
poorly sorted, with a sandy matrix at distal sites,
but showing a clast-supported, blocky-sized character at proximal positions. Large clasts up to 2 m
in diameter are common, and are usually located
at the top of the deposits forming inverse graded
horizons (Fig. 6). Lithic clasts are angular at proximal sites, but at distal positions, larger clasts are
often well rounded. Some blocks contain radially
arranged cooling joints, which show that they were
emplaced as hot blocks. Individual block-and-ashflow layers are often divided by a planar or undulated, thinly laminated, sandy-sized, ash-cloud
surge deposit ( Figs. 5 and 6). Very well preserved
degassing pipes are observed in all outcrops
( Fig. 6), but they are not continuous through
different layers, indicating that the deposits were
emplaced from several discrete pyroclastic flows.
The initial eruptive stage apparently emplaced
an interbedded sequence of well-sorted, pumicefall and surge deposits with a composition of the
juvenile material similar to that of the block-andash-flow deposits. These layers are only observable
at the base of site 38 (Fig. 5), the only outcrop
where the base is exposed. These layers are overlaid
by massive, 40 m thick, block-and-ash-flow
deposits.
This unit rests over 50–70 m thick fluvial conglomeratic horizons (Fig. 5, site 38), and is overlaid by the Nocayoco volcano-sedimentary rocksequence ( Tsvn) and by the Cruz de León andesite
( Tacl ). A pure biotite concentrate, separated from
a fresh 50 kg dacitic block collected in Site 16
( Figs. 2 and 5), was K–Ar-dated at 9.2±0.3 Ma
in Geochron Laboratories.
4.1.7. Nocayoco volcano-sedimentary sequence
(Tsvn)
This unit has a conspicuous morphology toward
the northern and northwestern sectors of the volcano forming white-colored, stratified, relatively
flat plateaus. It is composed of ~650 m thick,
moderately indurated, fluvial conglomeratic hori-
262
Fig. 5. Representative stratigraphic columns of the Ixtacamaxtitlán block-and-ash-flow deposit ( Tiba). The distance from the vent
increases from left to right (see Fig. 2 for site locations). The base of the sequence is only exposed in section 38. A pure biotite
concentrate separated from a fresh block collected from the top of section 16 was K–Ar dated at 9.2±0.3 Ma. This rock unit
represents the product of a central dome explosive collapse (see text and Stage IV for details).
zons, interbedded with laminated sandy and clayey
layers of lacustrine origin with sparse conglomeratic lenses. At the base of the sequence, the conglomeratic layers are considerably more abundant.
They usually contain different clasts of older volcanic sequences, but calcareous and terrigenous
Mesozoic rocks are also present. The sedimentary
units are often interbedded with pumice-rich, pyroclastic fall and base-surge horizons that have a
general tendency to become more abundant toward
the upper 250 m of the sequence ( Fig. 2, sites 27
and 28). The juvenile material of the pyroclastic
263
4.1.8. Cruz de León andesite (Tacl)
This unit is formed by blocky to slab-jointed
andesitic to dacitic lava flows, associated breccias,
and a very minor proportion of highly weathered
pyroclastic flow-deposits. Two main lava compositions were identified: a light-gray, hornblendeaugite-bearing porphyritic dacite (SiO =65%),
2
and a gray, slightly porphyritic to microlitic and
subophitic textured, two-pyroxene bearing andesite (SiO =58%). The lava-breccias are commonly
2
associated with slab-jointed lava flows, and show
a poorly sorted arrangement of rounded to subrounded clasts up to 1.2 m in size ( Fig. 2, site 8).
This volcanic-succession constitutes the external
structure of Cerro Grande volcano as low-angle
lava flows radially distributed from the crater-rim.
This unit rests over the Toi ( Fig. 2, Site 8) and
over the Tsvn (Fig. 2, Site 32). An andesitic wholerock lava flow sample from the top of the sequence
was K–Ar-dated as 9.0±0.4 Ma (Fig. 2, Site R-1)
(Carrasco-Núñez et al., 1997).
5. Geochemistry
5.1. Major-element chemistry
Fig. 6. Top of section 16 of the Tiba showing degassing pipes
at the base of the deposit, ash cloud layers separating each
block and ash flow horizon, and a laminated hyperconcentrated-laharic deposit separating two block-and-ash-flow
deposits at the top. See geologist for scale.
deposits is a fairly homogeneous hornblende-bearing dacite (SiO =66–68%). Fall deposits are usu2
ally well-sorted, with abundant highly vesiculated
pumice fragments and minor andesitic lithics. The
base surges are usually planar-bedded and occasionally cross-bedded, with well-rounded, lowvesiculated pumice fragments and minor lithics.
The base of this unit directly overlies the Tiba
(~9.2 Ma; Fig. 2, site 16) and is covered by the
Cruz de León andesites (Tacl; ~9 Ma). No isotopic date is available for this unit, but stratigraphic relations reflect that it was deposited
during an interval of about 0.2 Ma, in which
volcanic repose and fluvio-lacustrine deposition
was replaced by volcanic explosive activity.
Whole-rock compositions of Cerro Grande volcanic rocks and peripherally emplaced Miocene
vents are shown in Table 2. Rock types found, in
order of decreasing abundance, are andesite, dacite
and basaltic-andesite. No basaltic or rhyolitic
rocks were identified in the Miocene geologic
record ( Fig. 7). The rock compositions define a
subalkaline magmatic trend in a total alkalis vs.
silica diagram, located below the Irvine and
Baragar (1971) subdivision. When plotted on an
AFM diagram (not shown), most samples define
a calc-alkaline trend, and are mainly located in
the medium-K field on a K O–SiO plot (not
2
2
shown). However, whole-rock juvenile samples
from the Toi depart from this trend displaying a
FeO1 enrichment and a low K O character. It is
2
apparent that this variation is associated with an
alkali depletion due to alteration of the glassy
material.
Major-element variation diagrams of SiO vs.
2
TiO , Fe O1 , CaO and MgO exhibit well-defined
2
2 3
95-S5
Tgtl
Lava
19° 27.3∞
97° 53.0∞
56.52
0.8
18.64
7.02
0.09
2.8
8.59
3.61
1.74
0.19
100
1.63
2.04
44.14
CG-2e
Toi
Pumice
19° 34.8∞
97° 50.0∞
54.62
1.04
23.65
9.16
0.05
1.65
6.16
3.29
0.31
0.06
100
–
4.5
26.25
Sample:
Unit:
Material
Lat N
Lon W
SiO
2
TiO
2
Al O
2 3
Fe O1
2 3
MnO
MgO
CaO
Na O
2
KO
2
PO
2 5
Total
HO
2
LOI
Mg #
Sample:
Unit:
Material
Lat N
Lon W
SiO
2
TiO
2
Al O
2 3
Fe O1
2 3
MnO
MgO
CaO
Na O
2
KO
2
PO
2 5
Total
HO
2
LOI
Mg #
Pumice
19° 33.3∞
97° 37.9∞
58.39
0.85
21.31
7.24
0.08
3.37
5.88
2.24
0.59
0.06
100
–
5.4
47.99
CG-8m6
Toi
Lava
19° 26.8∞
97° 53.5∞
61.95
0.76
17.37
6.12
0.05
2.05
5.38
3.78
2.34
0.2
100
0.48
1.25
39.91
95-S3A
Tgtl
M Glass
19° 22.1∞
97° 37.9∞
58.97
0.88
19.35
6.95
0.04
3.8
6.67
2.61
0.58
0.15
100
–
7
51.97
CG-4b∞
Toi
Lava
19° 30.0∞
97° 53.2∞
61.63
0.78
16.65
6.13
0.09
2.95
5.39
3.68
2.51
0.19
100
0.41
1.26
48.8
95-S8
Tgtu
Pumice
19° 22.1∞
97° 37.9∞
59.19
0.94
18.87
7.83
0.06
3.67
6.73
2.11
0.45
0.15
100
–
6.1
48.17
CG-4c∞
Toi
Lava
19° 33.6∞
97° 50.6∞
58.35
0.8
17.38
6.66
0.09
3.65
7.33
3.64
1.82
0.26
100
0.53
0.86
52.06
95-J8
Tacg
Bomb
19° 33.3∞
97° 37.9∞
59.75
0.7
19.9
5.89
0.08
4.68
6.26
2.13
0.57
0.04
100
–
4.8
61.13
CG-8o
Toi
Lava
19° 22.3∞
97° 38.6∞
58.65
0.92
18.16
7.54
0.13
2.43
6.77
3.86
1.38
0.16
100
–
1.06
38.99
CG-6a
Tacg
M Glass
19° 33.3∞
97° 37.9∞
59.82
0.66
18.57
5.54
0.09
3.76
7.06
2.99
1.36
0.14
100
–
2.57
57.32
CG-8L
Toi
Lava
19° 34.8∞
97° 50.0∞
58.92
0.77
18.33
6.44
0.1
3.24
6.26
3.82
1.98
0.14
100
–
0.67
49.91
CG-2a
Tacg
Pumice
19° 34.0∞
97° 49.5∞
63.56
0.8
17.29
7.68
0.05
2.97
5
1.74
0.82
0.1
100
–
7.42
43.35
CG-11E3
Toi
Lava
19° 22.3∞
97° 38.6∞
59.17
1.05
17.81
8.04
0.1
1.99
5.72
4.32
1.58
0.23
100
0.81
0.92
32.95
95-C5b
Tacg
Table 2
Major-element data of Cerro Grande volcanics and surrounding Miocene ventsa
Lava
19° 34.1∞
97° 40.3∞
62.11
0.72
18.14
5.38
0.07
2.07
5.49
4.29
1.57
0.16
100
0.14
0.82
43.3
95-X7
Tgc
Lava
19° 33.3∞
97° 37.9∞
59.2
0.75
17.72
6.37
0.07
3.58
6.39
3.61
2.14
0.2
100
–
1.42
52.66
CG-8p
Tacg
Lava
19° 43.0∞
97° 46.6∞
62.28
0.72
18.4
5.19
0.07
1.78
5.46
4.44
1.46
0.2
100
–
0.97
40.45
95-L10
Tgc
Lava
19° 33.9∞
97° 51.5∞
59.45
0.79
18.77
6.39
0.11
2.46
5.89
4.41
1.59
0.13
100
–
1
43.21
CG-9
Tacg
Lava
19° 36.2∞
97° 42.5∞
62.87
0.77
19.14
5.49
0.05
1.31
4.32
4.17
1.71
0.17
100
0.08
2.09
32.08
95-X4D
Tgc
Lava
19° 34.8∞
97° 50.0∞
59.83
0.72
17.6
5.92
0.09
3.44
6.32
3.61
2.27
0.2
100
–
1.08
53.47
CG-2c
Tacg
Lava
19° 42.7∞
97° 38.4∞
67.38
0.43
17.33
3.04
0.06
1.16
4.59
4.59
1.27
0.14
100
–
0.81
42.93
95-M6
Tgc
Lava
19° 38.0∞
97° 49.4∞
62.66
0.68
18.69
4.74
0.04
1.42
5.7
4.17
1.73
0.17
100
–
0.71
37.25
CG-31
Tacg
Lava
19° 29.6∞
97° 44.0∞
70
0.62
15.51
4.93
0.08
1.17
2.23
2.38
2.93
0.15
100
1.59
2.9
31.87
95-B2a
Tgc
Lava
19° 34.8∞
97° 50.0∞
62.85
0.74
16.09
5.7
0.07
2.59
5.85
4.05
1.88
0.18
100
–
1.07
47.37
CG-2b
Tacg
Lava
19° 22.3∞
97° 39.8∞
56.34
0.81
18.12
7.5
0.11
3.94
8.32
3.42
1.18
0.26
100
0.43
0.64
50.97
95-C3a
Tac
Lava
19° 34.1∞
97° 48.4∞
63.12
0.59
17.71
4.76
0.05
2.29
5.3
4.16
1.92
0.1
100
–
1
48.78
CG-15
Tacg
Lava
19° 25.1∞
97° 43.0∞
57.23
0.89
18.66
6.96
0.11
3.3
7.31
3.69
1.59
0.25
100
0.09
1.01
48.39
95-G4A
Tac
Lava
19° 36.0∞
97° 46.1∞
67.11
0.39
17.36
2.99
0.05
1.46
4.8
4.83
0.89
0.11
100
0.31
0.77
49.21
95-D7
Tacg
264
Lava
19° 34.0∞
97° 49.5∞
57.32
0.99
17.7
7.67
0.12
4
7.3
3.44
1.25
0.21
100
–
0.81
50.78
95-R3B
Tacl
Lava
19° 34.2∞
97° 53.1∞
58.51
0.72
18.39
6.85
0.1
3.28
6.43
4.13
1.43
0.15
100
0.49
1.05
48.69
Material
Lat N
Lon W
SiO
2
TiO
2
Al O
2 3
Fe O1
2 3
MnO
MgO
CaO
Na O
2
KO
2
PO
2 5
Total
HO
2
LOI
Mg #
Sample:
Unit:
Material
Lat N
Lon W
SiO
2
TiO
2
Al O
2 3
Fe O1
2 3
MnO
MgO
CaO
Na O
2
KO
2
PO
2 5
Total
HO
2
LOI
Mg #
Lava
19° 33.3∞
97° 37.9∞
59.08
0.74
18.14
6.45
0.08
3.09
6.61
3.96
1.63
0.21
100
–
1.23
48.72
CG-8a
Tacl
Lava
19° 22.1∞
97° 37.9∞
58.39
0.88
17.77
6.91
0.12
3.4
7.4
3.7
1.16
0.26
100
–
1.09
49.35
CG-4j
Tac
Lava
19° 33.6∞
97° 53.6∞
59.49
0.55
19.17
5.77
0.09
2.61
6.55
4.13
1.5
0.15
100
–
0.4
47.31
JCG44
Tacl
Lava
19° 29.1∞
97° 48.3∞
63.38
0.62
17.91
5.22
0.08
1.62
5.5
3.96
1.53
0.18
100
0.62
1.08
38.03
95-A6
Tac
Lava
19° 36.5∞
97° 50.8∞
59.51
0.73
18.69
6.59
0.08
2.85
6.17
3.73
1.5
0.15
100
0.29
0.69
46.13
95-R1
Tacl
Lava
19° 31.0∞
97° 48.5∞
63.71
0.63
17.57
5.01
0.07
2.63
4.58
3.55
2.09
0.16
100
0.15
1.44
50.93
95-H5
Tac
Lava
19° 34.0∞
97° 51.0∞
59.57
0.75
18.66
6.28
0.08
2.76
6.24
4.13
1.36
0.18
100
–
0.91
46.53
CG-10
Tacl
Lava
19° 27.9∞
97° 50.9∞
63.77
0.62
17.44
5.02
0.09
1.77
5.49
4.02
1.58
0.19
100
0.96
1.26
41.18
95-A5a
Tac
Lava
19° 33.3∞
97° 37.9∞
59.7
0.78
18.46
6.4
0.1
2.64
5.81
4.08
1.91
0.13
100
–
1.26
44.95
CG-8c
Tacl
Lava
19° 34.6∞
97° 46.5∞
63.86
0.72
17.86
4.92
0.06
1.73
4.72
4.2
1.74
0.2
100
0.68
1.67
41
95-J5
Tac
Lava
19° 31.4∞
97° 51.7∞
60.32
0.65
17.53
5.76
0.09
3.83
6.61
3.67
1.41
0.14
100
–
0.77
56.81
JCG36
Tacl
Lava
19° 34.1∞
97° 47.0∞
64.42
0.61
17.52
4.96
0.05
1.54
4.66
4.08
2.02
0.14
100
0.37
1.18
38.13
95-H6A
Tac
Lava
19° 33.4∞
97° 55.0∞
60.39
0.51
18.93
5.53
0.09
2.85
6.36
3.83
1.38
0.13
100
–
1.24
50.54
JV5
Tacl
Block
19° 37.5∞
97° 48.6∞
65.02
0.65
17.06
4.18
0.07
2.04
4.73
3.69
2.42
0.13
100
–
1.37
49.2
CG-16
Tiba
Lava
19° 32.8∞
97° 52.2∞
60.79
0.63
17.57
5.56
0.08
3.53
6.43
3.85
1.42
0.14
100
–
0.96
55.72
JCG37
Tacl
Pumice
19° 39.4∞
97° 48.5∞
65.6
0.58
18.4
4.35
0.07
2.18
4.34
2.16
2.15
0.17
100
–
6.59
49.81
CG-27
Tsvn
Lava
19° 37.0∞
97° 49.2∞
62.02
0.63
17.53
5.34
0.06
2.71
6.12
3.69
1.77
0.13
100
–
1.16
50.09
JCG40
Tacl
Pumice
19° 36.7∞
97° 49.2∞
66.1
0.49
18.31
3.66
0.07
1.54
4.18
2.97
2.54
0.14
100
1.31
4
45.42
95-001
Tsvn
Lava
19° 37.7∞
97° 52.6∞
62.62
0.6
17.4
5.44
0.08
2.56
5.21
3.87
2.06
0.15
100
0.45
0.97
48.3
95-E5
Tacl
Pumice
19° 34.2∞
97° 48.6∞
66.78
0.52
17.86
3.73
0.07
1.02
4.36
3.29
2.26
0.11
100
–
3.6
35.09
CG-14e
Tsvn
a LOI: loss of ignition; total iron as Fe O1 ; FeO1 recalculation FeO1=FeO+(Fe O × 0.89981); Mg #=100(Mg2+/(Fe2++Mg2+).
2 3
2 3
CG-11a
Tac
Sample:
Unit:
Table 2 (continued )
Lava
19° 38.3∞
97° 52.0∞
64.55
0.68
17.33
5.17
0.13
1.43
4.37
3.59
2.6
0.15
100
–
4.3
35.45
CG-32
Tacl
Pumice
19° 39.4∞
97° 48.5∞
67.91
0.51
18.53
3.75
0.05
1.18
3.63
2.5
1.85
0.08
100
3.25
5.75
38.46
95-Q2E
Tsvn
Lava
19° 34.6∞
97° 50.9∞
65.36
0.64
18.01
4.86
0.05
0.57
3.24
4.59
2.53
0.13
100
–
1.66
18.93
CG-7
Tacl
Lava
19° 37.5∞
97° 52.8∞
58.33
0.92
17.63
6.89
0.07
3.01
7.18
3.66
2.04
0.27
100
–
1.09
46.35
CG-33
Tacl
265
266
Fig. 7. Total alkalis-silica diagram after Le Maitre (1989).
Classification is grounded on the weight per cent of major elements recalculated to 100% anhydrous. The thick line marks
the subdivision of volcanic rocks into alkaline and subalkaline
fields (Irvine and Baragar, 1971). Rock-unit abbreviations as
in Fig. 2. Note alkali depletion in the Toi, probably due to
intense alteration of the glassy components (see text for details).
Data are given in Table 2.
negatively correlated linear trends (Fig. 8). Al O
2 3
and MnO values display a slight decrease with
increasing SiO , but a linear trend is not clearly
2
observed. Positive correlation trends are roughly
defined for Na O and K O in lavas, but low values
2
2
of alkalis in pyroclastic samples from the Toi and
Tsvn rock-units are clear. Na O/K O ratios vary
2
2
from 10.6 to 2.1 in Toi samples, and from 1.0 to
1.5 in Tsvn samples. The high values in
Na O/K O ratio and low alkali content in the Toi
2
2
reflect the fact that alteration is always present,
obscuring the magma composition involved in this
particular rock sequence. The Tsvn sequence shows
a somewhat lower degree of alteration.
The composition of lavas and pyroclastic units
does not show a systematic variation with stratigraphy. Although pyroclastic rocks from Tiba and
Tsvn have a dacitic composition, a correlation
between rock chemistry and eruptive style cannot
be postulated since lava flows from other units
have similar compositions. Moreover, the rock
chemistry from the Toi is comparable to that of
lava flows from other units, albeit it exhibits an
evident loss of alkalis due to alteration.
5.2. Trace-element chemistry and Sr and Nd isotopes
Trace-element concentrations in whole-rock
lava and pyroclastic rocks (Table 3) are used as
indicators of magma differentiation in samples that
span the major-element chemistry variation, and
the stratigraphic record of Cerro Grande volcano.
Incompatible trace-element concentrations (e.g.
Nb, Hf, La, Th) show a general increase with
increasing differentiation, using Rb as the differentiation index ( Fig. 9). Sr exhibits a general
decrease in abundance in more evolved rocks,
presumably due to plagioclase control during crystal fractionation. The correlation of these trace
elements with Rb suggests that compositional
trends of these elements were influenced by the
same magmatic processes. Thus, it is likely that
crystal fractionation of the observed mineral
phases (Plag+Opx+Cpx±Hbl ) contributed
strongly to magma differentiation.
The chondrite-normalized trace-element data
for Cerro Grande volcanics (Fig. 10a) have similar
shapes and abundances, exhibiting depletions in
the high field strength elements (HFSE ) Nb, Ta
and Ti relative to large ion lithophile elements
(LILE) Rb, Ba, K, and Sr and the light rare earth
elements (LREE). This is a typical pattern of trace
elements in arc magmas.
REE patterns for the Cerro Grande suite
( Fig. 10b)
exhibit
LREE
enrichment
(La/Sm =3.53–2.02), and relatively flat but
N
enriched HREE patterns, Tb/Yb =1.54–1.22,
N
Yb =10.91–5.73. Negative Eu anomalies are
N
weak with Eu/Eu1=0.85–0.98. Low positive Eu
anomalies are observed in two andesitic samples
with Eu/Eu1=1.064–1.066. These values suggest
that plagioclase played an important role either as
a cumulate or as separation product during crystal
fractionation. A moderately defined convexupward mid-REE pattern in some samples indicates that the amphibole fractionation occurred to
some extent.
Sr and Nd isotope determinations were made
for only one sample of the Ixtacamaxtitlán blockand-ash-flow deposit (site 16; Fig. 3). Sample
CG-16 has an 87Sr/86Sr initial ratio of 0.703930
and e =1.21. These data fall within the field of
Nd
the TMVB ( Verma, 1983; Verma and Verma,
1986).
In summary, the major- and trace-element characteristics are typical of a calc-alkaline arc volcanic
series. These include the relative enrichments of
Fig. 8. Harker variation diagrams of Cerro Grande whole-rock samples. Data are given in Table 2. Rock-unit abbreviations as in Fig. 2.
267
268
Table 3
Trace-element data of Cerro Grande volcanics
Sample:
Unit:
95-J8
Tacg
CG-6a
Tacg
CG-2a
Tacg
CG-31
Tacg
CG-8L
Toi
CG-11a
Tac
CG-4j
Tac
CG-16
Tiba
CG-8a
Tacl
JCG44
Tacl
CG-8a
Tacl
CG-7
Tacl
Material
Lat N
Lon W
Pb
Sn
W
Mo
Sb
Rb
Cs
Ba
Sr
Tl
Li
Ta
Nb
Hf
Zr
Y
Th
U
La
Ce
Pr
Nd
Sm
Eu
Gd
Tb
Dy
Ho
Er
Yb
Lu
Be
Lava
19° 33.6∞
97° 50.6∞
6
1.11
0.71
0.71
0.07
29
0.44
427
1030
0.13
7.47
0.37
6.2
3.07
116
21
3.2
0.9
22.9
49.85
6.43
26.55
5.08
1.47
4.57
0.65
3.55
0.69
1.85
1.81
0.27
1.67
Lava
19° 22.3∞
97° 38.6∞
5
0.99
0.89
1.34
0.26
33
1.22
434
373
0.15
10.68
0.36
5.9
3.28
148
25
2.59
0.81
14.91
32.35
4.24
18.28
4.2
1.21
4.36
0.69
4.09
0.83
2.34
2.4
0.36
1.3
Lava
19° 34.8∞
97° 50.0∞
7
0.94
1
1.05
0.14
34
1.3
432
775
0.15
9.81
0.37
5.8
3.85
154
19
3.77
1.12
18.24
38.44
5.39
22.46
4.68
1.3
4.1
0.58
3.13
0.64
1.76
1.7
0.25
1.62
Lava
19° 38.0∞
97° 49.4∞
6
0.63
0.65
0.89
0.09
23
0.35
395
853
0.07
7.59
0.37
5.9
3.71
141
14
2.33
0.74
16.67
35.33
4.51
18.76
3.58
1.17
3.18
0.46
2.58
0.49
1.31
1.26
0.19
1.53
M Glass
19° 33.3∞
97° 37.9∞
6
0.9
1.85
1.27
0.17
19
0.83
300
889
0.17
7.56
0.27
4.2
3.76
129
15
2.4
0.75
13.77
29.95
4.06
17.21
3.62
1.1
3.31
0.48
2.73
0.54
1.48
1.51
0.23
1.41
Lava
19° 34.0∞
97° 49.5∞
4
0.97
0.66
0.97
0.15
30
0.76
324
513
0.15
10.22
0.45
7.9
3.48
162
26
2.59
0.73
17.56
39.18
5.16
22.17
4.84
1.31
4.67
0.71
4.15
0.84
2.35
2.35
0.34
1.33
Lava
19° 22.1∞
97° 37.9∞
5
0.68
0.57
0.74
0.07
13
0.42
348
989
0.09
5.76
0.31
5.9
2.95
123
24
1.84
0.52
16.83
39.72
5.45
24.23
5.12
1.51
4.82
0.72
4.02
0.83
2.31
2.34
0.35
1.37
Block
19° 37.5∞
97° 48.6∞
4
0.7
1.06
1.2
0.19
33
1.98
372
696
0.2
8.68
0.55
8.1
2.74
114
15
2.75
0.83
18.3
36.54
4.56
18.18
3.49
1.05
3.14
0.46
2.53
0.49
1.35
1.33
0.21
1.49
Lava
19° 33.3∞
97° 37.9∞
7
1.04
1.33
1.67
0.19
39
1.6
394
471
0.17
9.81
0.42
6.7
3.63
154
20
2.68
0.99
15.71
32.13
4.16
17.34
3.79
1.1
3.77
0.57
3.34
0.68
1.91
1.95
0.29
1.75
Lava
19° 33.6∞
97° 53.6∞
6
0.73
1.36
1.15
0.11
19
0.28
376
787
0.08
9.63
0.23
3.8
3.12
114
15
2.17
0.73
13.01
27.02
3.55
14.98
3.08
1.05
2.99
0.44
2.49
0.5
1.42
1.48
0.23
1.45
Lava
19° 33.3∞
97° 37.9∞
7
1.04
1.33
1.67
0.19
39
1.6
394
471
0.17
9.81
0.42
6.7
3.63
154
20
2.68
0.99
15.71
32.13
4.16
17.34
3.79
1.1
3.77
0.57
3.34
0.68
1.91
1.95
0.29
1.75
Lava
19° 34.6∞
97° 50.9∞
10
1.24
1.04
2.05
0.23
61
1.71
622
356
0.4
14.37
0.61
9
5.35
203
25
5.18
1.87
32.58
34.84
7.7
29.61
5.69
1.54
5.14
0.75
4.12
0.79
2.2
2.23
0.32
2.67
LILE and depletions of HFSE relative to the REE.
Depletion of HFSE relative to LILE is considered
a common geochemical characteristic of arc
magmas and is generally attributed to modifications of the upper mantle source region by interaction with slab-derived fluids (Gill, 1981; Tatsumi
and Eggins, 1995). Moreover, Nb and Ta depletion
is commonly cited as a characteristic signature of
subduction-related rocks that have suffered contamination of a crustal component (Rollinson,
1993). It is interesting to note that, under this
criterion, the volcanic rocks exhibit a strong deple-
tion for both elements, suggesting that the magmas
might be contaminated to some extent by a crustal
component. Nevertheless, the contamination signature is not clearly observed in the isotope geochemistry data; thus, in this regard, no definite
conclusion can be made.
6. Evolution of Cerro Grande volcano
The field, stratigraphic and geochemical data
presented above are consistent with a volcanic
Fig. 9. Trace-element variation vs Rb (ppm) as the differentiation index. Data are given in Table 3. Rock-unit abbreviations as in Fig. 2. Trace-element replicate analyses
(n=2) of the US Geological Survey AGV-1, AGV-2, BCR-1, BHVO-1, BIR-1 were used as standards for precision and accuracy. Lamont-Doherty internal references
115In, 169Tm and 209Bi were used as monitors of instrumental drift.
269
270
Fig. 10. Multi-element plots representative of Cerro Grande volcanics. (a) Chondrite-normalized trace-element diagram; (b) chondritenormalized REE diagram. Rock compositions are depicted as SiO %. Normalization values from Nakamura (1974) and Thompson
2
(1982). Filled diamond=Tacg; open square=Toi; filled circle=Tac; cross=Tiba; filled triangle=Tacl. Data are given in Table 3.
evolution of Cerro Grande volcano, and peripheral
late Miocene volcanic vents, divided into six principal stages ( Fig. 11).
6.1. Stage I. Construction of a shield-like volcano
(~11 Ma)
The landscape that prevailed before emplacement of volcanic rocks consisted of NW–SE mountain ranges made up of folds and trusts of
Mesozoic sedimentary rocks. The presence of stratified lacustrine deposits in the map area ( Fig. 2,
Tsbj and Tdpc) indicates that small ponds and
lakes occupied intermountain depressions before
the inception of volcanic activity in the late
Miocene.
Volcanism apparently began as andesitic lava
flows were erupted from NW–SE-trending vents
or fracture zones, forming the Terrenate group
volcanics. Cerro Grande volcano initiated its activity at about 11 Ma as a mainly effusive volcano
with very minor pyroclastic activity ( Fig. 11, Stage
I ). Lava flows were radially distributed from the
central sector of the map area forming a low-relief
271
Fig. 11. Schematic evolution of Cerro Grande volcano. Arrows indicate the approximate transverse direction of the sections. Rockunit abbreviations as in Fig. 2. See text for description of stages.
272
shield-like volcano. Fluvio-lacustrine sedimentation apparently had a close relation with volcanic
activity as lava flows continuously modified the
landscape, and new stream networks and sedimentary basins developed.
The persistent alignment of the initial volcanic
structures in a NW–SE direction, and the later
dislocation of the volcanic rocks by normal faults
with the same orientation [see Fig. 2 and CarrascoNúñez et al. (1997)], strongly suggest that volcanism and tectonic deformation was coeval in the
area during late Miocene times. Furthermore, the
recognition that most late Miocene, arc-related,
volcanic structures in other areas of central Mexico
are in close association with NNW–SSE normal
faults ( Yañez-Garcia, 1980; Pasquaré et al., 1988;
Suter et al., 1995b) may imply that this tectonic
phase allowed the ascent and emplacement of
volcanic rocks on a regional scale.
6.2. Stage II. Major phreatomagmatic and
magmatic explosions (11–10.5 Ma)
After the initial effusive phase, Cerro Grande
drastically changed its eruptive behavior and
entered an intense explosive activity that generated
voluminous pyroclastic and volcaniclastic rocks
around the volcano (Fig. 11, Stage II ). This rock
sequence, here named Oriental lithic rich ignimbrites ( Toi), marks a long-standing eruptive phase
that can be easily traced in the stratigraphic record
of Cerro Grande volcano. The principal characteristics of this rock-sequence are depicted by: (1) an
unusual cognate-lithic-rich, polymictic, matrixsupported, massive breccia-type deposits, with a
close association with lithic-rich base-surge and
pumice-rich fall deposits; (2) a persistent lowvesicularity and strong devitrification of the juvenile material in breccia-type and surge deposits;
and (3) similar eruptive and emplacement mechanisms regardless of the magma composition. These
features, together with a radial distribution of the
deposits bordering the crater rim and a maximum
run-out distance of 30 km SE from the vent,
indicate that the Toi breccia-like deposits were
produced by deposition from pyroclastic flows.
Therefore, as the term ‘breccia’ has no genetic
implication, such deposits can be better classified
as non-welded, lithic-rich ignimbrites, following
the term proposed by Perez-Torrado et al. (1997).
The high vesicularity of the juvenile material is
a consequence of volatile separation from the
magma and is a common feature of plinian or
subplinian eruptions in which a positive buoyant
eruptive column develops ( Fisher and Schmincke,
1984; Cas and Wright, 1987). However, the low
vesicularity of the juvenile material may indicate
a rather brittle fragmentation of the magmatic
component, implying that volatile separation is
inhibited (Zimanowski et al., 1997). Low vesicularity in juvenile material of pyroclastic deposits is
often cited as a common feature of hydromagmatic
eruptions (Sheridan and Wohletz, 1983; Wohletz,
1983; Lorenz, 1986, 1987). Hydromagmatic eruptions result from explosive interaction of magma
with external water (surface or ground water), as
a response to a physical phenomena experimentally
observed and often referred to as a molten–fuel–
coolant interaction (Zimanowski et al., 1997). This
is a common feature in geologically active environments where groundwater is usually located in
hydraulically active zones of structural weakness
(Lorenz, 1987) and is the most efficient volcanic
mechanism for converting thermal energy into
kinetic energy ( Wohletz, 1983, 1986; Lorenz, 1987;
Zimanowski et al., 1997).
The differences in vesicularity of the juvenile
material between layers (i.e. surge, fall and flow)
of the Toi indicate that a complex mixture of
fragmentation and eruption mechanisms, from
pure magmatic to hydromagmatic, operated
during eruption. The low vesicularity in juvenile
material from breccia layers and surge deposits
clearly accounts for a hydromagmatic fragmentation mechanism. In contrast, highly vesiculated,
pumice-rich fall deposits between breccia layers
resulted from the progressive fall of positively
buoyant columns in the same eruptive cycle. Thus,
once a hydromagmatic reaction developed, an
intense devastation of the host rock opened a new
vent. Ejection of the cap rock may have promoted
a pressure relaxation into system and a sudden
volatile exsolution from the magma. This process
influenced the development of vertical eruptive
columns that gave rise to well-vesiculated, pumicerich fall deposits. Therefore, the Toi depicts a
repetitive eruptive cycle in which hydromagmatic
explosions coexisted with buoyantly rising eruptive
columns.
The cognate lithic-rich character of the brecciatype deposits indicates that the magmatic thermal
energy was efficiently transformed into kinetic
energy to strongly devastate and erode the preexistent shield-like volcano. The absence of accidental
basement rocks into the lithic-rich ignimbrites
implies that the magma–water interaction took
place at shallow depths into the volcanic edifice.
The variation in magma composition (as depicted
by mineralogy) within sections and between
outcrops accounts for a compositionally zoned
magma chamber. Nevertheless, it seems that
magma composition had a limited influence on
eruption mechanisms as no variations in eruptive
style were identified among different rock compositions. The whole-rock geochemistry of the Toi is
characterized by a conspicuous alkali depletion
due to alteration (Figs. 7 and 8), a distinct feature
of this unit even when compared with other pyroclastic units of Cerro Grande volcano. Therefore,
the high induration and alteration of the deposits
may mean that the hydromagmatic eruptions generated voluminous, high-density pyroclastic flows
that were unable to separate the water vapor
efficiently from the glassy components during
transport.
Hence, the overall data indicate that the
Oriental Lithic-Rich Ignimbrite was mainly the
consequence of powerful hydromagmatic, probably high-intensity vulcanian-type eruptions, that
occurred at shallow depth into the preexistent
shield-like volcano. The high explosibility was
acquired by the ascent of magma-batches from a
compositionally zoned magma chamber into a
highly fractured and water-saturated host rock. It
seems possible that the late Miocene regional stress
field promoted the brittle fracture of rocks and
favored the development of water-saturated zones
of structural weakness into the volcanic edifice.
6.3. Stage III. Peripheral fissural lava flows (10.5–
9.7 Ma)
After emplacement of the Toi, a period of
intense effusive activity very likely occurred in
273
adjacent areas of Cerro Grande volcano ( Fig. 11,
Stage III ). Rock units related to this volcanic
phase covered a wide extension towards the southern and eastern portions of the map area; here
named the Cuyoaco group ( Tgc) and El Crestón
andesite ( Tac). This period is characterized by the
emplacement of voluminous and relatively homogeneous andesitic lava flows with auto-brecciated
surfaces that were clearly erupted from NNW–
SSE-trending fissure system.
6.4. Stage IV. Dome formation and explosive
collapse (~9.2 Ma)
During the emplacement of the Cuyoaco group
volcanics ( Tgc), and apparently after the eruption
of the El Crestón andesite ( Tac), a homogenous
biotite-hornblende bearing a dacitic dome grew
inside Cerro Grande’s crater (Fig. 11, Stage IV-a).
Although no relicts of this dome were identified in
the study area (they may have been covered by
subsequent volcanic units or completely destroyed
by explosive activity), the voluminous, and compositionally homogeneous, block-and-ash flow
deposits of the Tiba toward the northern and
northeastern flanks of the Cerro Grande crater
reflect that they were derived from an explosively
directed collapse of a central dome ( Fig. 11,
Stage IV-b).
The explosive activity began with the generation
of phreatomagmatic explosions and the development of small positively buoyant columns (see
Fig. 5, section 38). This initial vent-opening phase
probably caused a pressure instability in the magmatic system and the generation of climactic explosive bursts in the dome. The fine ash surge and
fall layers between single block-and-ash flow
deposits indicate that a buoyant, elutriated, fine
ash cloud eventually decoupled from the main
dense pyroclastic flows. These layers, together with
the presence of hyperconcentrated horizons and
lahars at distal positions, and the truncation of
degassing pipes by subsequent flow-units, clearly
indicate that the entire sequence is the product of
several discrete explosive bursts in the central
dome system.
274
6.5. Stage V. Volcanic repose and open-vent
explosive eruptions (9.2–9.0 Ma)
After dome formation and explosive collapse,
Cerro Grande experienced a period of volcanic
tranquillity in which a fluvial-lacustrine environment developed in a very extensive area. Up to
400 m of conglomerates and clay-rich horizons
deposited in this period forming the base of the
Tsvn sequence ( Fig. 11, Stage V-a). After this
period of repose, Cerro Grande entered a new
explosive phase that generated a thick, hornblendebearing, pumice-rich pyroclastic-rock sequence.
The sequence is composed of an interstratification
of pyroclastic fall and base-surge deposits, with no
associated flows, that were preferentially distributed to the northern and western sectors of the
volcano ( Fig. 11, Stage V-b).
Well-sorted pumice-fall deposits were probably
formed by the progressive fall of unstable positively buoyant eruptive columns. In contrast, basesurge deposits were very likely emplaced from
turbulent, low concentration pyroclastic currents.
The sharp contacts between different deposits in
all outcrops indicate that a lateral transition
between them is unlikely. Thus, the relation
between these two contrasting pyroclastic emplacement conditions in the same eruptive cycle indicates that they were derived from two different
fragmentation mechanisms in the volcanic conduit.
It is possible that the ascent of a homogenous
dacitic magma, with a high volatile content, developed intense buoyant eruptive columns that were
later preferentially deflected downwind toward the
northern and western flanks of the volcano.
Sudden and repetitive access of external water
(either surface or phreatic) into the erupting
magma could have generated highly energetic
hydromagmatic eruptions that produced turbulent
base surges. Furthermore, a distinct lack of cognate or accessory lithic material in the deposits
means that the Tsvn pyroclastic sequence was the
product of shallow open-vent explosive eruptions.
6.6. Stage VI. Ring-fissural induced lava-flows
(~9 Ma)
Shortly after the emplacement of the Tsvn pyroclastic sequence, an intense period of effusive activ-
ity recorded the last volcanic stage of Cerro
Grande’s geologic history ( Fig. 11, Stage VI ).
Radially distributed lava flows were emplaced
around the former vent, probably influenced by a
ring fissure system. Stratigraphic evidence indicates
that these rocks were emplaced about ~0.2 Ma
after the emplacement of the last explosive phase
( Tsvn). Hence, they could not have been affected
by a central roof-collapse event as a response to
the evacuation of a large volume of pyroclastic
material. Therefore, a period of slumping of unstable crater walls, NW–SE-trending normal faulting
and a long-standing erosion were probably responsible for the crater widening and the construction
of a caldera-like structure, as pointed out by
Lipman (1997).
7. Discussion: tectonic implications of late Miocene
volcanism and the inception of the TMVB
The relation of ancient volcanism in the TMVB
with the pacific plate tectonics environment is still
not well understood. Therefore, we will discuss the
regional implications of late Miocene volcanism of
the TMVB and provide a possible tectonic scenario
for the inception of the volcanic arc.
Although several hypothesis have been proposed for the space–time relations of the magmatic
arc (Mooser, 1972; Demant, 1978; Cantagrel and
Robin, 1979; Demant, 1981; Robin and Cantagrel,
1982; Venegas et al., 1985; Nixon et al., 1987), it
has been reported recently that the TMVB
acquired a geometric configuration similar to the
present configuration since the middle to late
Miocene (Ferrari et al., 1994a, 1999). This configuration was apparently the consequence of a
counterclockwise migration of the near trench,
NW–SE-trending, early Oligocene to early
Miocene, silicic Sierra Madre Occidental to the E–
W-trending andesitic TMVB. Even though the
isotopic age database approach (Ferrari et al.,
1994a, 1999) portrays the global space–time distribution of volcanic rocks, it does not provide
information on the geologic evolution of individual
volcanic centers. Indeed, only a few volcanoes of
the TMVB have been studied in detail and even
fewer late Miocene volcanic centers. Three late
Miocene andesitic stratovolcanoes have been
studied in the northern portion of the central
sector (Fig. 1): El Zamorano volcano ~10 Ma
(Carrasco-Núñez et al., 1989), Palo Huérfano volcano ~11 Ma (Perez-Venzor et al., 1996) and La
Joya volcano ( Valdéz-Moreno and Aguirre-Dı́az,
1996). Ferrari et al. (1994b) reported the existence
of a widespread calc-alkaline basaltic plateau
emplaced between 10 and 6 Ma in the central part
of the TMVB. Toward the eastern sector, the
oldest reported volcanic rocks are those from
Cerro Grande volcano, and the so-called Alsaseca
andesite with ~11 Ma ( Yañez-Garcia, 1980).
Several studies along the present Middle
American Trench (MAT ) have verified that mechanisms of subduction erosion (Mercier de Lépinay
et al., 1997) and subduction accretion ( Karig et al.,
1978; Moore and Shipley, 1988) have been taking
place since the late Miocene, confirming the existence of an active subduction margin since that
time. However, at least two independent lines of
evidence suggest that the MAT acquired its present
configuration much earlier since the late Oligocene
(Schaaf et al., 1995; Morán-Zenteno et al., 1996).
Thus, it seems that the migration of volcanism
from the Sierra Madre Occidental to the TMVB
occurred when the MAT was already intensively
active.
The reasons why the volcanic activity migrated
from the near trench NW–SE-trending silicic arc
to the intermediate rocks of the E–W-trending
TMVB by the middle or late Miocene are not yet
understood, nor sufficiently explored. However, if
the MAT had its present configuration since the
late Oligocene, a major reorganization in the geometry of convergence, subduction rate and angle
must account for this migration (Morán-Zenteno
et al., 1997; Ferrari et al., 1999). Major plate
reorganizations in the Pacific occurred at 25 Ma,
between 12.5 and 11 Ma and between 6.5 and
3.5 Ma (Mammerickx and Klitgord, 1982;
Londsdale, 1991). Convergence rates between the
Farallon plate and its remnants (Guadalupe, Cocos
and Rivera) with the North American plate had a
step-down decrease from the mid-Eocene
(160 mm/yr) to the late Oligocene (80 mm/yr), but
increased to 120 mm/yr by the early Miocene, at
~20 Ma ( Engebretson et al., 1985). Pardo and
Suárez (1995) pointed out that the current active
275
front of the TMVB is located above an irregular
slab, but volcanism is manifested at the surface
when it reaches 100 km deep, regardless of the
angle of subduction. A depth of ~100 km is a
common feature of many volcanic arcs and is
usually accounted for by the break-up of amphibole and the release of fluids from the slab into
the mantle wedge (Gill, 1981; Tatsumi and Eggins,
1995). If magma generation by the late Miocene
was induced when the subducted plate reached a
similar depth, a very shallow angle of the subduction must be taken into consideration to generate
volcanic rocks as far as ~500 km from the MAT
(i.e. Cerro Grande volcano is located at 440 km
from the trench). Exactly which was the angle of
subduction at that time is unknown; furthermore,
it may have had significant variations along its
length as it is observed today. However, the major
reorganization in the Pacific plates at about 25 Ma,
and the high convergence rate at about 20 Ma,
may be closely related to the shallowing of the
subduction angle and the migration of volcanism
toward its E–W location in central Mexico.
A plausible tectonic scenario for this hypothesis
may start at 20 Ma with a convergence rate of
120 mm/yr and a subduction angle of ~11°
( Fig. 12a). The subducted slab may have reached
100 km beneath the North American plate in
4.25 Ma (Fig. 12b). Therefore, the tectonic conditions for magma generation at a horizontal distance of ~500 km from the trench were already
reached at 15.75 Ma. Certainly, there is the need
to allow some time for magma generation and
eruption. Halliday et al. (1989), for example, estimate that the silicic magma at Glass Mountain
may have had a residence time of 0.7 Ma, whereas
other models in arc magmas posit transit times
from 8000 to 50 000 a for melt generation, transport, fractionation, and eruption (Reagan et al.,
1987; Gill and Condomines, 1992). In any case,
these may represent favorable tectonic conditions
for magma to be emplaced in the TMVB by the
middle to late Miocene.
8. Conclusions
The Cerro Grande volcano is a major structure
that marks the beginning of volcanism in the
276
Fig. 12. Interpretative configuration of Mexico’s subduction
zone for the Miocene. The inception of TMVB in the middle
to late Miocene may be the result of a lower subduction angle,
probably influenced by a faster convergence rate along the
MAT. Volcanic rocks with ages ~20 were emplaced with an
orientation parallel to the trench ( Ferrari et al., 1999) and
extended to northwestern Oaxaca (Ferrusquı́a-Villafranca et al.,
1988). By the middle to late Miocene, the volcanic arc had a
predominant E–W orientation and reached nearly 500 km from
the MAT (see text for examples of volcanic centers). EPR: East
Pacific rise. See text for details on plate velocities and angles.
eastern sector of the Trans-Mexican Volcanic Belt
by the late Miocene. Extensive geologic mapping
and stratigraphic studies show that Cerro Grande
volcano followed a complex geologic evolution
that can be summarized in six principal stages,
during a 2 Ma time span (11–9 Ma). The initial
volcanic phase in Cerro Grande volcano apparently constructed a mainly andesitic shield-like
volcano (Stage I ). This period was followed by a
radical change in Cerro Grande’s eruptive behavior, here depicted by the Toi sequence and Stage
II. This period produced voluminous lithic-rich
ignimbrites and associated pyroclastic fall and
surge deposits around the volcano. The unusual
lithic-rich character of the ignimbrites, together
with significant differences in the characteristics of
the juvenile components, indicates that they were
related to intense, shallow depth, hydromagmatic
eruptions that caused significant devastation in the
preexistent shield-like edifice. This explosive stage
was followed by an intense period of effusive
activity in many parts of the map area (Stage III ),
and by the construction of a compositionally
homogenous dome inside the Cerro Grande’s
former crater (Stage IV-a). This dome eventually
acquired an explosive termination forming voluminous block-and-ash-flow deposits on a wide area
(Stage IV-b). After this stage, Cerro Grande
entered a period of volcanic repose on which a
fluvio-lacustrine environment persisted (Stage
V-a). Shortly thereafter, this period of volcanic
repose was followed by intense open-vent magmatic and hydromagmatic eruptions (Stage V-b).
The last eruptive phase in Cerro Grande’s geologic
history was an intense period of lava flow emplacement, probably related to a ring-fissural system
(Stage VI ).
Effusive volcanism was by far the most common
volcanic phenomena. Several lava flows and conspicuous volcanic vents in Cerro Grande area show
a persistent alignment in a NNW–SSE direction.
Furthermore, the entire late Miocene sequence,
including the NE flank of Cerro Grande’s crater,
was affected by highly dislocated NW–SE-trending
normal faults. These features strongly suggest that
the late Miocene volcanic record was related to a
period tectonic extension. However, volcanism was
not a continuous phenomenon since the late
Miocene. From the early Pliocene to the
Pleistocene, a period of volcanic tranquillity was
likely to occur. Volcanism resumed in the
Pleistocene with the development of NE–SWtrending cinder cones, that were peripherally
emplaced from the Cerro Grande massif.
Major-element geochemistry data show that
Cerro Grande volcanics, and peripherally
emplaced Miocene vents, have fairly homogeneous
intermediate compositions. No basaltic or rhyolitic
rocks were identified in the geologic record. Major-
and trace-element geochemical trends indicate that
they have a co-magmatic origin and that crystal
fractionation played a significant role in magma
differentiation. Trace-element results also show
that Cerro Grande volcanics exhibit typical continental magmatic arc patterns related to modifications of a mantle source region by the interaction
with slab-derived fluids. Although it seems possible
that these magmas were later modified by a crustal
component, no definite conclusion was provided
by the isotopic data.
Several lines of evidence indicate that the beginning of the TMVB as a distinctive geologic feature
occurred during the middle to late Miocene
( Ferrari et al., 1994a, 1999) as the product of
subduction along a trench that had its present
configuration since the late Oligocene (Schaaf
et al., 1995; Morán-Zenteno et al., 1996). These
temporal and tectonic constraints strongly suggest
that the migration of volcanism from the NW–
SE-trending Sierra Madre Occidental to the E–Wtrending TMVB must be related to variations in
the Pacific plate tectonics arrangement. Therefore,
it is very likely that this migration can be associated
with the 25 Ma plate reorganization in the Pacific
(Mammerickx and Klitgord, 1982; Londsdale,
1991) that later influenced an increase in the
convergence rate along the MAT ( Engebretson
et al., 1985). These constraints led us to envisage
a tectonic scenario in which the 120 mm/yr peak
in the subduction rate at 20 Ma produced a shallower subduction angle along the MAT and thus
the migration of volcanism toward an inland position with inception of the TMVB by the middle
to late Miocene age.
Acknowledgements
Our deep thanks to Laura Lozano and Jorge
Rivera who provided invaluable help in long field
seasons. Rufino Lozano, Patricia Girón, Peter
Schaaf and Gabriela Solı́s (LUGIS-UNAM ) are
thanked for their help in major-element and isotopic determinations. We gratefully acknowledge
support in trace-element determinations from
Charlie Langmuir and Kerstin Lehnert (LDEOColumbia). We thank MikeAbrams (JPL) for pro-
277
viding us with the satellite image of Cerro Grande
volcano. Special thanks to the people of San
Francisco Ixtacamaxtitlán who kindly sheltered us
in their homes. We are grateful to Fernando
Ortega-Gutiérrez and Luca Ferrari for discussions
and suggestions during the preparation of this
article. The manuscript was greatly improved by
thoughtful reviews of Martı́n Barajas and an anonymous reviewer. This research was funded by a
GSA student research grant (No. 5830-6) to A.
Gómez-Tuena, and partially by Gerencia de
Proyectos Geotermoeléctricos (Comisión Federal
de Electricidad ) and Instituto de Geologı́a,
UNAM. A. Gómez-Tuena gratefully acknowledges support from DGAPA-UNAM and
Fundación-UNAM during his graduate studies.
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Cerro Grande volcano: the evolution of a Miocene stratocone in the

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