Volcanic evolution of the Amealco caldera, central Mexico

Geological Society of America
Special Paper 334
1999
Volcanic evolution of the Amealco caldera, central Mexico
Gerardo J. Aguirre-Díaz*
Estación Regional del Centro, Instituto de Geología, Universidad Nacional Autónoma de México,
Apartado Postal 376, Guanajuato, Guanajuato, 36000 México
Fred W. McDowell
Department of Geological Sciences, University of Texas at Austin, Austin, Texas 78713, United States
ABSTRACT
The Amealco caldera is a well-preserved Pliocene volcanic center, 11 km in diameter, located in the central part of the Mexican Volcanic Belt. It is one of seven calderas
known in the belt. Compared to those of the other calderas, the Amealco products are
less evolved, and include only a minor volume of rhyolite. According to the stratigraphic
record and K-Ar data, the inferred volcanic history of the Amealco caldera is as follows.
Caldera-related activity started ca. 4.7 Ma with eruptions of pumice fallout and
pyroclastic flows apparently of Plinian type. These events were followed by eruption of
far-reaching surges and pyroclastic flows that deposited three widespread ignimbrites
named Amealco I, Amealco II, and Amealco III. By about 4.7 Ma at least 77 km3 (Dense
rock equivalent, DRE) of trachyandesitic-trachydacitic magma were evacuated from
the magma chamber and caused caldera collapse. After this climatic stage, pyroclastic
activity continued, probably as tephra fountains from ring-fracture vents, that erupted
pumice flows and fallouts that were accompanied by mud flows forming deposits of
local extent. Both tephra and mud-flow deposits make up a DRE volume of 2.35 km3.
This was followed by 4.3 Ma trachyandesitic lava domes that were emplaced through
several ring-fracture vents, making up a DRE volume of 3.8 km3; the domes form the
caldera’s present topographic rim. At about 4.0 Ma, a modest-sized volcano had formed
on the western flank of the caldera that erupted several trachyandesitic lava flows and
fallout tephra (both lava and tephra deposits = 0.8 km3). Between 3.9 and 3.7 Ma, 10
intracaldera lava domes were emplaced, accompanied by tephra eruptions that produced relatively small deposits (volume not quantified) that were later reworked and
redeposited as lake deposits within the caldera; five of these lava domes are trachyandesitic (4.3 km3) and five are rhyolitic (2.4 km3). The central lava domes are interpreted here as the viscous, gas-poor magma that usually erupts at the end of a caldera
cycle, and thus may mark the end of the volcanic evolution of the Amealco caldera.
Volcanic activity continued adjacent to the caldera for at least 1.6 m.y. after the
emplacement of the central lava domes. These events include bimodal volcanism at
3.7 Ma of basaltic-andesite lava from a volcano just north of the caldera rim (Hormigas
volcano) and emplacement of several rhyolitic lava domes to the southwest of the
caldera (Coronita rhyolite). At 2.9 Ma a rhyolite (obsidian) lava dome complex was
*Present address: Unidad de Investigacion en Ciencas de la Tierra, Campus
Jusiquilla, Universidad Nacional Autónoma de México, Apartado Postal 1-742,
Centro, Querétaro, Qro., 7600 México.
Aguirre-Díaz, G., and McDowell, F. W., 1999, Volcanic evolution of the Amealco caldera, central Mexico, in Delgado-Granados, H., Aguirre-Díaz, G., and
Stock, J. M., eds., Cenozoic Tectonics and Volcanism of Mexico: Boulder, Colorado, Geological Society of America Special Paper 334.
1
2
G. J. Aguirre-Díaz and F. W. McDowell
emplaced 15 km to the north of the caldera (El Rincón rhyolite). The last volcanic
episodes are represented by a 2.5 Ma andesitic lava dome (Garabato andesite) and a
2.2 Ma andesitic scoria cone on the southern margin of the caldera (El Comal andesite).
The southern portion of the caldera was displaced by the Epitacio Huerta fault, a
major west-northwest–south-southwest normal fault, which is part of the more regional
Chapala-Cuitzeo-Acambay graben system. The Epitacio Huerta fault was active mostly
prior to 2.5 ± 0.3 Ma as the 2.5 Ma Garabato andesite was only slightly displaced by this
fault. Faulting apparently ended before 2.2 ± 0.1 Ma, because the El Comal scoria cone
is not displaced in spite of its position on the trace of the Epitacio Huerta fault.
INTRODUCTION
Relatively few calderas have been recognized within the
Mexican Volcanic Belt, a complex east-west–trending continental volcanic province that crosses Mexico between lat 19° and
21° N (Fig. 1). There are several tens of volcanic system complexes within the belt, but only seven have been identified as
calderas, including, from west to east, La Primavera, Los
Azufres, Amealco, Mazahua, Huichapan, Aculco, Los Humeros,
and Las Cumbres. From these, only the La Primavera and Los
Humeros calderas, respectively at the extreme west and east
ends of the Mexican Volcanic Belt, have been studied in detail
(Mahood, 1980, 1981; Mahood and Drake, 1983; Mahood and
Halliday, 1988; Ferriz and Mahood, 1984, 1987; Verma, 1984).
Within the central sector, the Los Azufres complex has been the
focus of several studies, but its origin as a caldera is still debated
(Cathelineau et al., 1987; Dobson and Mahood, 1985; Ferrari
et al., 1993; Pradal and Robin, 1994).
This chapter focuses on a physical reconstruction of the
major volcanic events that formed the Amealco caldera. It is
based on the geologic mapping and detailed measurement of several stratigraphic sections that include the caldera products and
those of peripheral volcanoes. The field information is supplemented with stratigraphically controlled K-Ar ages and major
element chemistry.
REGIONAL GEOLOGIC SETTING
The Amealco caldera is a Pliocene volcanic center in the
central part of the Mexican Volcanic Belt (Fig. 1). The belt is a
continental margin volcanic arc that has been related to the subduction of the Cocos oceanic plate along the Middle America
trench beneath southern Mexico (Urrutia-Fucugauchi and Del
Castillo, 1977; Nixon, 1982; Suárez et al., 1990; Singh and Pardo,
1993). The Mexican Volcanic Belt is a complex arc that can be
divided into sectors with their own characteristics, including volcanic style, structure, morphology, age, and chemistry. At least
three sectors can be recognized, the western, central, and eastern.
The western sector is that between the western coast and the
Chapala lake (included); the central sector is between the Chapala
lake and the Popocatepetl-Ixtaccihuatl north-south–trending volcanic chain (both limits excluded); and the eastern sector is
between this chain (included) and the eastern coast (Fig. 1).
Two main structural features characterize the central
Mexican Volcanic Belt: the east-west–oriented graben system of
Chapala-Cuitzeo-Acambay (Suter et al., 1991, 1995), also known
as the Chapala-Tula fault zone (Johnson and Harrison, 1990), and
the Taxco–San Miguel de Allende line (Demant, 1978; Nixon
et al., 1987), or the Querétaro fracture zone (Johnson and Harrison, 1990), which crosses the central part of the belt with a northnorthwest–south-southeast orientation (Fig. 1), and which could
be regarded as Basin and Range–style normal faulting. These two
regional fault systems intersect south of the city of Querétaro.
The Amealco caldera is at this intersection (Fig. 1).
LOCAL GEOLOGICAL SETTING
The first formal reference to the Amealco caldera was made
by Sánchez-Rubio (1978, 1984). Carrasco-Núñez (1988) also
studied the Amealco caldera and provided a set of chemical data
on the volcanic rocks that was published by Verma et al. (1991)
together with numerous isotopic (Sr and Nd) analyses.
The Amealco caldera is 11 km in diameter (Fig. 2). Its
southern portion was displaced by the Epitacio Huerta normal
fault system, which bounds the Acambay graben on the north.
These faults are part of the east-west–trending Chapala-CuitzeoAcambay graben system (Fig. 1). The north-northwest–southsouthwest Taxco–San Miguel de Allende fault system is older
than the Amealco caldera, and the east-west Acambay graben is
younger than the Amealco caldera, and has seismically active
segments (Suter et al., 1995). Representative schematic measured sections of the Amealco caldera products are shown in Figures 3 and 4. Locations of these and other measured sections in
the mapped area are shown in Figure 5.
The major ignimbrites of the Amealco Tuff are the Amealco
I, Amealco II, and Amealco III. Tephra 1 is the tephra beneath the
Amealco I ignimbrite and includes Amealco zero, a minor but
distinctive ignimbrite only observed in the western deposits
(Fig. 3). Tephra 2 is between the Amealco I and Amealco II, and
tephra 3 is between the Amealco II and Amealco III (Fig. 3).
K-Ar data are provided in Table 1. Representative major-element
chemical analyses of the Amealco caldera products and of
peripheral volcanism are provided in Table 2, and the total alkalisilica chemical classification is shown in Figure 6. All distances
mentioned are with reference to the center of the caldera.
It is inferred that the Amealco caldera was formed by
Volcanic evolution of the Amealco caldera, central Mexico
105 W
103 W
99 W
101 W
97 W
95 W
22 N
22 N
LP
AMEALCO
TSM
G
Q
CCA
Chapala
Cuitzeo
20 N
C
Colima graben
Mo
LA
Ma
H
T
LH
20 N
P
M
V
Ta
18 N
18 N
100 km
O
Middle America Trench
16 N
16 N
105 W
103 W
99 W
101 W
97 W
500 km
Miocene-Pliocene rocks
Pliocene-Quaternary rocks
Normal faults
Lake
Volcano
Caldera
Town
3
MEXICO
95 W
Figure 1. Index map of major volcanoes,
calderas, and major fault systems in the
Mexican Volcanic Belt. The Amealco
caldera is in the central portion of the
belt, at the intersection of the Taxco–San
Miguel de Allende (TSM) and ChapalaCuitzeo-Acambay (CCA) fault systems.
Towns: G: Guadalajara, C: Colima, Mo:
Morelia, Q: Querétaro, T: Toluca, M:
Mexico City, P: Pachuca, V: Veracruz, O:
Oaxaca, Ta: Taxco. Calderas: LP: La Primavera, LA: Los Azufres, Ma: Mazahua,
H: Huichapan, LH: Los Humeros. Modified after Nixon et al. (1987). Inset shows
regional index map of the Mexican Volcanic Belt.
Amealco
N
Mexican
Volcanic Belt
collapse, following the model of Smith (1979) and Smith and
Bailey (1968). However, it apparently did not undergo resurgence
after collapse, although high-standing central lava domes were
emplaced toward the end of the caldera’s evolution.
Figure 7 shows the volcanic evolution of the Amealco
caldera. Eruptions are characterized as Plinian, vulcanian, or
strombolian, based on the observed qualitative characteristics of
the deposits, rather than quantitatively, using grain-size and distribution analysis.
K-Ar GEOCHRONOLOGY
We obtained 25 K-Ar ages (Table 1) for units that represent
the entire range of the Amealco caldera activity, and include some
units of both precaldera and postcaldera volcanism. Some units
have been dated with two or more samples from separate localities. Except for minor rhyolite, volcanic units in the Amealco
region lack potassium-rich phenocryst phases. For these samples
of Pliocene age we relied primarily upon glass or crystalline
groundmass to achieve reasonable precision. For some samples
(samples Am-1, Am-179, Am-63, Am-81; Table 1) paired matrix
and feldspar ages were obtained, with acceptable agreement in
three of four cases.
K-Ar analyses were performed in the Department of Geo-
logical Sciences of the University of Texas at Austin, in a laboratory that has been commonly used for mid-Tertiary or older samples. The techniques include flame photometry for K analysis and
isotope dilution for Ar analysis using a Nuclide 3 in gas-source
mass spectrometer. Details of the techniques can be found in
Aguirre-Díaz et al. (this volume).
Analytical uncertainties were mostly <5% of the age, and
only a few exceeded 8%. The worst case was 22% for Am-122, a
sample with high atmospheric argon content. For map units with
multiple age determinations, an assigned age is calculated from a
mean value that is weighted inversely in proportion to the precision of the individual results.
VOLCANIC EVOLUTION OF THE AMEALCO
CALDERA
Precaldera volcanic rocks (>4.7 Ma)
Most of the precaldera rocks are lava and pyroclastic rocks
of intermediate or felsic composition. A K-Ar age of 5.7 Ma was
obtained from basaltic andesite (sample Am-67, Table 1) that
underlies caldera products 28 km to the north-northeast of the
caldera, and a precaldera felsic ignimbrite 33 km to the west
yielded a K-Ar age of 4.7 Ma (sample Am-84, Table 1).
G. J. Aguirre-Díaz and F. W. McDowell
100 20’W
100 15’W
100 10’W to Galindo
100 05’W
100 00’W
20 15’N
Sierra El Rinc n
0
2
5
20 15’N
4
km
120
to Coroneo
cc
Las Hormigas
volcano
AMEALCO
120
Palomas
volcano
Tch
Tc
Epitacio
Huerta
S. Mateo
20 10’N
A il
Tlc
to Aculco
20 10’N
SJ
E
Tsr
Tg
EPITACIO HUERTA
FAULT
Teb
SMT
Tm
Tenango
Tz
CG
Garabato
20 05’N
20 05’N
Loma
Linda
El Comal
volcano
LA
Molinos
Lerma river
M
Tepuxtepec
reservoir
100 20’W to Tepuxtepec
AGE
(Ma)
Quaternary
PlioceneQuaternary
100 15’W
100 05’W to Temascalcingo
100 10’W
POSTCALDERA UNITS
PRECALDERA UNITS
Silicic lava dome.
Reworked pumice and alluvium.
Andesites, basaltic-andesites and related cones.
2.2
El Comal cone and related andesitic lava flow
and pyroclastic rocks.
2.5
Garabato Andesite: hornblende andesite dome.
100 00’W
5.7
2.9
El Rinc n Rhyolite lava domes.
3.5
Huichapan Tuff.
3.7
Hormigas Andesite: basaltic andesite lavas.
3.7
Coronita Rhyolite lava domes.
3.8
Santa Rosa Andesite: trachyandesitic central domes;Tz: Zancudo,
Tg: El Gallo, Tc: Capando, Tsr: Santa Rosa. Tm: La Mesa.
3.9
La Cruz Rhyolite: central domes; Tlc: La Cruz,
Tch: Chiteje, Teb: El Barco.
4.0
Palomas Dacite: trachydacitic lavas and pyroclastic rocks.
Intermediate (andesitic ?) lava flows and domes.
Caldera outline
Normal fault
Volcano
CALDERA UNITS
Paved road
Unpaved road
Spatter-lava cone.
4.3
Campana Dacite: trachydacitic rim dome.
4.3
Amealco Andesite: trachyandesitic rim domes.
Town or village
Lake
River
Brick Pumice: pumice flow and fall, with epiclastic deposits.
4.7
Amealco tuff: trachyandesite-trachydacitic ignimbrites
and interlayered surge, fallout and mud-flow deposits.
Figure 2. Simplified geologic map of the Amealco caldera and peripheral volcanism. CC: Cerrito Colorado cone, E: El
Espía dome, SJ: San Juan Hedó, LA: Los Arcos, SMT: San Miguel Tlascaltepec, M: Mexquititlán, CG: Cañada de García.
Plinian eruptions and associated pyroclastic flows (4.7 Ma)
The earliest activity related to the Amealco caldera occurred
around 4.7 Ma, when Plinian eruptions produced widespread pumice
fallout and pyroclastic flows (e.g., Amealco zero ignimbrite; Figs. 3
and 7). This stage produced the earliest pyroclastic deposits of the
Amealco Tuff that mainly consist of layered, white, medium- to
coarse-grained, angular, pumice lapilli. Thickness of the initial fallout
deposits is 3 m at a locality 25 km to the east, less than 1 m 15 km to
the west of the caldera, and practically zero 30 km to the west. They
EXPLANATION
Intermediate
lava
Vesiculated
intermediate lava
49.8 m
Platy-jointed
lava
Brecciated
lava
Glassy
lava
Undulated
pumice fall deposit
62 m
Amealco III
52
42
Huichapan
undifferentiated
73.5 m
tephra
43
42
covered
Lithics-rich
surge deposit
Amealco III
48
Huichapan
ignimbrite
70
60
32.5 m
24
co II
Ameal
24
24
Amealco III
Ameal
50
Ameal
24
Amealco I
30
co II
30
Recent talus
Ignimbrite with
black pumices
Coarse pumice
fallout deposit
Fine pumice
fallout deposit
31 m
co I
30
Amealco II
36
Ameal
36
Fluvial deposit
Recent
alluvium
co III
Amealco II
Amealco III
Rounded lithics
40
Pumice flow
deposit
Ash-flow deposit
Finely layered
surge deposits
Lithic-rich
surge deposits
Black, uncollapsed
pumice fragments
Black fiamme
12
18
12
12
30
Amealco II
covered
12
18
co I
Ameal
covered
18
Amealco I
18
20
White, uncollapsed
pumice fragments
Accidental,
angular lithics
Vitrophyre of
ignimbrite
Monolithologic
matrix-free deposit
Mud-flow deposit
Mafic lava flow
Felsic ignimbrite
6
6
6
0
0
0
10
Zero
0
Amealco
Amealco I
6
0
Weathered
pumice deposit
Metamorphic
rocks
Paleosoil or
varved lake deps.
Reworked
deposit
Pumice veins
Section 1
Section 2
Section 13
Section 17 Section 11
Figure 3. Representative measured sections of the Amealco Tuff. Locations are shown in Figure 5.
6
G. J. Aguirre-Díaz and F. W. McDowell
Figure 4. Representative measured sections of the Brick Pumice and Amealco Andesite caldera units. Locations are shown in Figure 5. Section
symbols are shown in Figure 3.
are also absent 28 km to the north-northeast of the caldera, and apparently 30 km to the south as well. Thus, paleowind transport of the
Plinian column’s material may have been dominantly to the east.
Apparently a column collapse caused the pyroclastic flow that formed
Amealco zero ignimbrite, which is observed only to the west of the
caldera interlayered with pumice fall deposits (section 1, Fig. 3).
The Amealco zero ignimbrite is nonwelded, dark gray when
fresh, or red when weathered. It is mainly composed of black
pumice, with minor amounts of lithics in a pumiceous matrix.
The physical aspect of this deposit is like a reddish scoria flow.
The Amealco zero yielded a minimum dense rock equivalent
(DRE) volume of 0.75 km3. Whole-rock pumice analysis of the
Amealco zero yielded the most mafic composition of all the
Amealco caldera products (SiO2 = 61.6 wt. 3°; this and subsequent values of SiO2 are normalized volatile free; Table 2, Fig. 6).
Caldera formation and emplacement of the Amealco Tuff
(4.7–4.5 Ma)
Between 4.7 and 4.5 Ma caldera collapse occurred in
response to the eruption of voluminous and widespread pyroclastic flows and pumice fallouts that produced the major ignimbrites
and interbedded tephra of the Amealco Tuff (Figs. 3 and 7).
The first voluminous pyroclastic flow deposited the
Amealco I ignimbrite. This ignimbrite was preceded by eruption of surges, minor pyroclastic flows that deposited unwelded
ignimbrites, pumice fallouts, and mud flows. This pattern was
repeated at least two more times to produce the tephra 2 and
Amealco II ignimbrite, and the tephra 3 and Amealco III ignimbrite (Fig. 3). Each of these cycles (tephra and major ignimbrite) may have formed a transient caldera, which was
destroyed by subsequent events, and/or was buried by younger
pyroclastic deposits. However, only one caldera structure is
now evident. Alternatively, it is possible that multiple collapse
events occurred in this single caldera (e.g., Druitt et al., 1994).
The similar volumes of each cycle (major ignimbrite plus surge
and fall tephra) support the model of several calderas. The first
hypothetical caldera is related to a DRE volume of 21 km3
(Amealco zero, tephra 1, and Amealco I); the second caldera is
related to a DRE volume of 32 km3 (Amealco II and tephra 2);
and the third caldera is related to a DRE volume of 24 km3.
According to an empirical relationship of ignimbrite volume
versus caldera diameter (Cas and Wright, 1987, p. 233), each
cycle should have produced a caldera of about 11 km diameter
(for 21 to 32 km3), which is the actual caldera diameter.
Several unconformities are present within the Amealco Tuff,
Volcanic evolution of the Amealco caldera, central Mexico
Figure 5. Distribution of the Amealco Tuff based on the distribution of
the major ignimbrites Amealco I, Amealco II, and Amealco III. Figure
indicates location of the measured sections. H: Huimilpan; T: Tlalpujahua.
as indicated by paleosoils, irregular surfaces due to erosion, or
thin layers of varved lake deposits (Fig. 3). These are particularly
evident after major pyroclastic flow eruptions that produced the
ignimbrites Amealco I, II, and III. Apparently, pauses occurred
between the volcanic episodes that formed the Amealco Tuff.
On the basis of mapping, the Amealco Tuff has an estimated
minimum distribution of 2880 km2, and a total DRE volume
estimate of 77.7 km3. These are minimum volume estimates
because they do not include the intracaldera tuff, the distal
lapilli-ash fallouts, and the coignimbrite ash-cloud deposits,
which were eroded away. A detailed description of the procedure
to estimate the volume of the Amealco Tuff can be found in
Aguirre-Díaz (1993, 1996).
The age of the Amealco Tuff is derived from five K-Ar ages
(three on glass and two on plagioclase separates) from four separate localities (Table 1). The range of ages is from 4.54 to
4.74 Ma, and the weighted mean of 4.68 ± 0.10 Ma is chosen as
the best estimate. The mean is close to the ages for the three
7
glasses, which are more precise than those for the plagioclases
because of their higher potassium contents. The plagioclase ages
are lower, but their errors overlap with both the weighted mean
and the individual glass ages.
The bulk of the Amealco Tuff is trachyandesitic-trachydacitic in composition; most pumice fragments analyzed have a
SiO2 range from 61.6 to 65.5 wt% (Table 2, Fig. 6). The mineralogy observed in the Amealco Tuff is consistent with this composition, because it contains plagioclase, hypersthene, augite,
ilmenite, titanomagnetite, accessory apatite, and rare olivine.
Mingling of glass is clearly evident in the Amealco Tuff. The
major ignimbrites and the interlayered tephra deposits of the
Amealco Tuff contain pumices with distinct compositions (documented by electron-probe microanalysis; Aguirre-Díaz, 1993),
and different colors (black, yellow, and white). Field and laboratory observations (Aguirre-Díaz, 1991, 1993) have shown that the
different glasses are juvenile and are well mingled, even at the
microscopic scale (mingled shards). The different liquids were
ejected simultaneously to form unzoned deposits that do not
show sorting of the different glasses. The eruptive mechanism
that caused these deposits could have been similar to that proposed by Sparks et al. (1977), in which the different magmas
could have been simultaneously erupted after stirring, caused by
mafic magma input at the base of a zoned subcaldera magma
chamber. This mechanism must have occurred repeatedly to produce the multiglass pyroclastic deposits of the Amealco Tuff, i.e.,
the Amealco I, II and III.
Additional features indicate a complex eruptive history for
the Amealco Tuff. For example, fallout deposits consisting of
angular, coarse fragments of a single rock type, black glassy lava,
are found intercalated in the sequence (section 1, Fig. 3). These
deposits could be interpreted as vulcanian-type eruptions that
destroyed lava bodies, perhaps domes, that may have been
obstructing the vents. Thick stacks of pyroclastic surge deposits,
that traveled as far as 28 km from source, indicate repeated
phreatomagmatic pulses of great explosivity (section 11, Fig. 3).
Stacks of mud-flow deposits are also interbedded with ash-flow
deposits as far as 25 km from the source, and were common during the activity that produced the Amealco Tuff. Some of these
deposits are lithic rich (20–35 vol% lithic material), others are
mostly composed of sand-size material.
Brick Pumice, rim lava domes (Amealco Andesite), Campana
dome (4.5–4.3 Ma)
Explosive activity continued in the Amealco caldera after
eruption of the Amealco Tuff. However, the pyroclastic material
issued during these events was less voluminous and more localized (Fig. 2). These pumice deposits were named the Brick Pumice by Sánchez-Rubio (1984). The Brick Pumice was emplaced
in the interval between 4.6 ± 0.1 Ma (the age of Amealco Tuff)
and the 4.3 Ma emplacement of lava domes that form the rim of
the caldera, named the Amealco Andesite (Fig. 7). The Amealco
Andesite was contemporaneous with the last events of deposition
8
G. J. Aguirre-Díaz and F. W. McDowell
TABLE 1. K–Ar AGES OF AMEAICO CALDERA REGION
Unit
Pre-Amealco
Amealco Tuff
Sample
Location
Lat
Long
(N)
(W)
Mat.*
Am-67
20°21’8” 100°6’50”
Wr
Am-84
20°10’33” 100°27’54”
Fds
Am-1
20°8’3”
Gl
100°18’10”
Am-1
Fds
Am-12
20°16’35” 100°9’7”
Fds
Am-22
20°8’1”
Gl
Am-208
19°50’45” 100°11’15”
Gl
Am-62
20°10’16” 100°10’17”
Gms
Am-58
20°10’13” 100°9’23”
Wr
Am-46
20°6’38” 100°7’0”
Wr
Am-195
20°7’54” 100°6’55”
Wr
Campana Daci.
Am-79
20°10’0” 100°15’22”
Gl
Palomas And.
Am-41b
20°9’19” 100°16’8”
Gms
La Cruz Rhyo.
Am-63
20°10’4” 100°10’20”
Gl
Amealco And.
100°18’8”
Am-63
Sta. Rosa And.
Coronita Rhy.
Fds
Am-61
20°8’49” 100°10’7”
Gms
Am-51
20°7’32” 100°9’43”
Gms
Am-179
20°7’7”
Gl
100°19’33”
Am-179
Fds
Hormigas And.
Am-78
20°10’13” 100°12’31”
Gms
Huichapan Tuff
Am-81
20°8’38” 99°56’36”
Gl
Am-81
Fds
K
Weight
Ar x 10-6
40Ar§
Age
±1σ∗∗
(%)
(g)
(scc/g)†
(%)
(Ma)
(Ma)
Assigned
Age‡
(Ma ± 1σ)
1.27
1.26
0.97
0.99
4.64
4.61
4.63
0.53
0.53
0.46
0.46
3.62
3.53
3.56
3.53
2.90
2.91
2.86
2.00
1.97
2.31
2.29
3.42
3.41
2.28
2.30
3.79
3.76
0.58
0.57
0.611
0.611
0.261
0.327
0.263
0.269
0.294
0.175
0.183
0.854
33.2
31.8
27.4
38.5
18.5
5.69
0.35
5.69 ± 0.35
4.70
0.19
4.70 ± 0.19
4.74
0.15
4.68 ± 0.10
0.316
0.094
19.9
4.54
0.28
0.301
0.082
13.1
4.55
0.40
0.432
0.277
0.640
0.664
35.9
31.1
4.71
0.14
0.606
0.278
0.542
0.524
34.4
38.8
4.71
0.19
0.668
0.342
58.0
4.42
0.21
0.579
0.385
45.3
4.30
0.24
0.588
0.459
0.666
0.440
0.618
0.246
0.546
0.461
0.429
0.278
0.519
0.584
0.371
0.395
0.618
0.650
0.096
0.083
0.089
0.633
16.6
18.6
22.3
21.7
42.6
44.4
16.9
14.7
14.2
41.4
4.14
0.36
4.30
0.15
4.31
0.10
4.31 ± 0.10
3.96
0.40
3.96 ± 0.40
3.91
0.14
3.90 ± 0.14
0.284
0.209
19.8
3.89
0.27
0.552
0.681
0.474
0.478
13.6
13.0
2.69
0.25
0.723
0.353
0.494
0.516
0.374
0.423
0.416
0.524
19.2
20.2
17.3
42.8
3.79
0.26
3.52
0.10
0.285
0.493
0.627
0.642
54.0
61.5
3.90
0.10
0.533
0.592
0.518
0.604
0.174
0.144
0.174
0.553
32.3
26.5
31.9
48.4
3.70
0.39
3.70 ± 0.39
3.59
0.09
3.52 ± 0.16
0.274
0.414
0.443
0.482
23.3
58.0
3.36
0.21
4.16
4.16
1.38
1.37
3.36
3.31
3.30
3.32
2.82
2.72
2.69
3.83
3.82
3.83
4.20
4.15
4.30
1.14
1.14
3.96
3.96
3.58
3.52
3.54
3.50
4.31 ± 0.12
3.74 ± 0.25
3.72 ± 0.27
Volcanic evolution of the Amealco caldera, central Mexico
9
TABLE 1. K–Ar AGES OF AMEAICO CALDERA REGION (continued - page 2)
Unit
Sample
Location
Lat
Long
(N)
(W)
Mat.*
El Rincón Rhy.
Am-122
20°14’15” 100°14’25”
Gl
Garabato And.
Am-18
20°6’0”
Wr
El Comal And.
Am-19b
20°5’59” 100°9’48”
100°11’50”
Gms
K
Weight
Ar x 10-6
40Ar§
Age
±1σ∗∗
(%)
(g)
(cm3/g)†
(%)
(Ma)
(Ma)
Assigned
Age‡
(Ma ± 1σ)
4.86
4.89
2.39
2.37
1.91
1.90
0.664
0.555
4.5
2.92
0.44
2.92 ± 0.59
0.558
0.588
0.779
0.647
0.252
0.218
0.161
0.163
19.8
29.0
31.3
24.9
2.54
0.26
2.54 ± 0.26
2.18
0.07
2.18 ± 0.07
*Material used for K-Ar analyses: Gl = glass; Wr = whole rock; Gms = groundmass; Fds = feldspar.
†Scc/g = standard cubic cm/gram.
§ 40Ar = radiogenic argon content of sample, in percent of total 40Ar.
**Errof of age at one sigma, see text for details.
‡Assigned Age = weighted mean of the different ages obtained for a particular unit.
40K/K = 1.167 x 10-4 moles/mole; λβ = 4.963 x 10-10/yr; λε + ε = 0.581 x 10-10/yr.
TABLE 2. REPRESENTATIVE WHOLE-ROCK CHEMICAL ANALYSES
Product
Sample
Unit
Latitude
Longitude
Quadrangle
Ameaico Ignimbrites
Am-22
Am-21
Ame-II
Ame-II
20°8’0”
20°8’5”
100°17’50” 100°17’55”
F14C86
F14C86
Am-39
Zero
20°8’1”
100°18’8”
F14C88
Am-35
Ame-1
20°8’1”
100°18’8”
F14C86
SiO2
TiO2
Al2O3
Fe2O3
FeO
MnO
MgO
CaO
Na2O
K2O
P2O5
H2O+
H2OCO2
58.84
1.20
15.95
3.53
3.27
0.13
1.79
4.57
3.85
1.96
0.47
2.29
0.59
0.00
59.51
1.21
15.97
3.21
3.92
0.12
1.50
4.02
4.34
2.28
0.50
1.57
0.44
0.00
64.38
0.81
15.46
1.79
3.15
0.09
1.08
2.78
4.67
3.35
0.20
1.11
0.15
0.17
Total
Mg#
98.43
49.4
98.58
40.6
CIPW Norm (wt. % volatile free)
Q
17.99
15.37
C
0.31
0.26
Or
12.12
13.95
Ab
34.10
37.99
An
20.56
17.29
Wo
Di
Hy
6.05
6.74
Mt
5.35
4.81
Il
2.39
2.39
Ap
1.14
1.20
Total
TT index
100.01
64.20
100.00
67.32
Rim Lava Domes
Central Domes
Am-133b
Am-195
Am-53
Am-61
Ame. And. Ame. And Santa Rosa Santa Rosa
20°9’30”
20°7’54”
20°7’50”
20°8’49”
100°8’5”
100°6’55” 100°8’48” 100°10’7”
F14C86
F14C86
F14C86
F14C86
Am-255
Ame-III
20°8’5”
100°17’54”
F14C86
Am-256
Ame-III
20°21’2”
100°6’6”
F14C76
60.83
1.05
15.72
2.40
4.53
0.12
1.49
4.24
5.24
2.83
0.37
1.55
0.15
0.00
63.85
0.71
15.25
2.58
2.15
0.09
0.91
2.73
3.36
5.57
0.20
1.81
0.53
0.00
61.14
0.82
14.42
2.08
3.39
0.12
1.50
3.94
3.94
3.03
0.29
2.48
0.52
0.36
63.14
1.11
15.64
1.95
4.93
0.13
1.59
3.87
4.90
2.88
0.39
0.22
0.02
0.02
59.53
1.42
16.09
2.98
3.84
0.12
2.26
4.87
4.39
2.59
0.47
0.85
0.34
0.00
55.55
1.44
17.24
3.30
3.48
0.11
3.29
6.75
3.96
1.65
0.39
0.80
1.02
0.01
62.14
0.77
15.87
2.10
2.92
0.09
1.65
4.24
4.04
3.29
0.21
1.15
0.31
0.05
99.19
37.9
100.51
37.0
99.74
43.1
98.03
44.0
100.78
36.5
99.75
51.2
99.01
62.8
98.83
50.2
16.57
8.83
16.88
17.20
12.34
11.59
8.77
15.29
20.27
40.45
11.55
16.90
44.85
11.18
33.74
29.19
10.38
18.91
35.29
13.39
16.90
41.21
12.15
15.54
37.74
16.77
10.05
34.69
24.98
19.98
35.12
15.89
1.02
5.43
2.65
1.58
1.58
6.42
5.44
3.51
2.01
0.86
1.81
2.30
3.84
1.39
0.46
4.31
5.35
3.19
1.65
0.72
3.69
7.89
2.81
2.11
0.90
3.71
6.42
4.38
2.73
1.11
5.45
7.41
4.93
2.81
0.93
3.46
5.11
3.13
1.50
0.51
101.10
77.29
100.00
70.58
99.99
79.82
100.01
71.39
100.00
70.45
99.99
64.88
100.02
53.51
99.99
70.39
10
G. J. Aguirre-Díaz and F. W. McDowell
Figure 6. Total alkali-silica volcanic-rock classification diagram (Le Bas et al., 1986) of representative Amealco caldera products and peripheral volcanism. Symbols in figure are: Amealco Tuff ignimbrites: closed square; Amealco Andesite: open diamond; central lava dome (both rhyolite and
trachyandesite): open square; Palomas Dacite: plus sign; Campana Dacite: X; Hormigas Andesite:
open circle; Garabato Andesite: triangle; Comal Andesite: closed circle; Coronita Rhyolite: closed
diamond. Abbreviations: b, basalt; ba, basaltic andesite; a, andesite; d, dacite; r, rhyolite; bta, basaltic trachyandesite; ta, trachyandesite; td, trachydacite (see Table 2 for chemical data).
of the Brick Pumice, because they occur interlayered in several
sites (sections 19 and 20, Fig. 4). The Amealco Andesite (3.8 km3
DRE) was emplaced as ring-fracture lava domes (Fig. 7), forming
a continuous rim of trachyandesitic lava domes and associated
lava flows (SiO2 = 60.4–62.8 wt%, Table 2, Fig. 6) that formed
the topographic expression of the caldera (Fig. 2).
Several pumice flows consisting of coarse-grained, unsorted
pumice deposits with a pumiceous matrix, and less frequently,
pumice fallouts consisting of coarse-grained, sorted pumice
deposits without matrix, were erupted through ring fracture vents
(Fig. 7). The pumice flow and fall eruptions may have been
“foam fountains” of sub-Plinian type, because they produced
massive, coarse-grained pumice flows that extended as far as
17 km from the caldera. Fallouts probably were also part of these
eruptions, because pumice-fall deposits occur interlayered with
the pumice-flow deposits. Initially, pumice flow dominated the
activity, but pumice fallouts predominated at the end. The pumice
flows are thick next to the caldera (>15 m, no base exposed), but
thickness decreases quickly with distance; they are on the order
of 2 m thick 2 or 3 km away from the rim.
Close to the caldera rim lavas are interlayered with these
tephra deposits (sections 19 and 20, Fig. 4). Farther (>2 km) from
the caldera rim the pyroclastic deposits are interlayered with epiclastic deposits (section 22, fig. 4). Both pyroclastic and epiclastic deposits are mapped as the Brick Pumice unit, which makes
up a total DRE volume of 2.4 km3.
The Campana dome was also emplaced at about 4.3 Ma
(0.8 km3, DRE). Although it also forms part of the rim of the
caldera, this dome is higher (more viscous?) and is chemically a
little more evolved (SiO2 = 66.8 wt%; Table 2, Fig. 6) than the
other rim domes. The uncertainty in the K-Ar ages of both the
rim and Campana dome allows the Campana dome to be significantly later than the rim domes (Table 1), as the field relationships indicate.
Palomas volcano (4 Ma)
The Palomas volcano may have been already formed by
4 Ma (Fig. 7). However, the large uncertainty (±0.4 m.y., Table 1)
of this K-Ar age allows the formation of the Palomas volcano any
time between 4.4 and 3.6 Ma. Palomas lavas overlie the Brick
Pumice to the south of the Palomas cone (Fig. 2).
Palomas activity included scoria flows and tephra and lavaflow eruptions with a trachydacitic composition (SiO2 = 65 wt%;
Table 2, Fig. 6). These emissions gradually built a small composite cone that stands only 200 m above the adjacent lava
plateau. Most of the Palomas lavas flowed to the west of the vent
(Fig. 2), and formed a blocky lava (aa type) plateau on top of the
Amealco Tuff, making up a volume of 0.8 km3 (DRE). The lava
plateau extends to 8 km to the west of Palomas cone, where it is
displaced by the Epitacio Huerta fault (Fig. 2).
Central lava domes: La Cruz Rhyolite and Santa Rosa
Andesite (3.9–3.7)
At 3.9 ± 0.1 Ma, central lava domes of rhyolitic composition
(SiO2 = 70.4–72.6 wt%; Table 2, Fig. 6) were emplaced mostly in
the northern part of the Amealco caldera (Fig. 7). These domes
formed the La Cruz Rhyolite, with a DRE volume of 2.4 km3.
They are exogenous domes of Pelean type (Blake, 1990), having
steep sides, crumble breccias, and block and deposits.
At 3.7 ± 0.25 Ma, several lava domes of trachyandesitic composition (SiO2 = 57.1–63.9 wt%; Table 2, Fig. 6) were emplaced
in the center of the caldera and formed the Santa Rosa Andesite
(Fig. 7), with a total DRE volume of 4.3 km3. The uncertainty of
the K-Ar ages overlap, but the ages suggest that the rhyolitic
domes were earlier than the intermediate lava domes, as the field
relationships indicate. The lavas of the undated trachyandesitic
Capando dome (Fig. 2) overlie the rhyolite of the Chiteje de la
~
3.9 Ma
Plinian eruptions from an inferred central vent
produced pumice-fall deposits, surges, and nonwelded ignimbrites (e.g., Ignimbrite Zero).
4.7 4.5Ma
Emplacement of rhyolitic central lava domes (La Cruz Rhyolite).
3.8 3.7 Ma
Caldera collapse(s?) associated to major ignimbrite eruptions
from ring-fracture vents, accompanied by pumice fallouts,
pyroclastic surges, mud flows, and vulcanian eruptions
(Amealco Tuff).
4.5 4.3 Ma
Emplacement of trachyandesitic central lava domes (Santa
Rosa Andesite).
3.7 Ma
Eruption from ring-fracture vents of coarse-grained pumice
flows and sparse pumice fallouts (Brick Pumice).
~ 4.3 Ma
Emplacement of Coronita Rhyolite lava domes west of the
caldera, and of basaltic andesite of Hormigas Volcano next
and to the north of the caldera rim.
3.5 Ma
Emplacement of ring-fracture lava domes and related lava
flows with sporadic tephra eruptions (Amealco Andesite).
~ 4.3 Ma
Eruption of felsic pyroclastic flows from a source east of the
Amealco caldera (Huichapan caldera?) that produced the
Huichapan Tuff.
2.5 Ma
Pumice-flow and fallout activity persisted after the
emplacement of the rim lava domes (Brick Pumice).
Emplacement of the Campana dome.
~ 4.0 Ma
Displacement of Amealco caldera by normal faulting along the
Epitacio fault system. Emplacement of the Garabato lava dome.
The dome was little displaced by faulting. Reworked material
filled the tectonic depression south of the caldera.
2.2 Ma
Emplacement of the Palomas volcano adjacent to the
caldera margin. Eruption of lavas and pyroclastic rocks.
Emplacement of El Comal Cone on the fault trace that displaced
the caldera. Reworked material continue to fill
the tectonic depression south of the caldera.
Figure 7. Volcanic evolution of the Amealco caldera and peripheral volcanoes.
12
G. J. Aguirre-Díaz and F. W. McDowell
Cruz dome. The same arrangement is observed between the lavas
of the Santa Rosa dome and El Barco domes (Fig. 2).
The relatively long time between emplacement of the rim
domes (4.3 ± 0.1 Ma) and that of the central domes (3.9 ± 0.1 Ma)
was sufficient for the magma remaining in the subcaldera magma
chamber to undergo differentiation. It is suggested that a rhyolitic
cap was formed at the top of the chamber as a result of this differentiation. This rhyolitic magma layer was underlain by trachyandesitic magma, which likely made up the bulk of the magma
chamber. Foundering of blocks above the already fractured and
faulted caldera roof may have squeezed out the viscous rhyolitic
magma and produced the rhyolitic lava domes of the La Cruz
Rhyolite; as block foundering proceeded, the underlying trachyandesitic magma was then squeezed out to produce the trachyandesitic lava domes of the Santa Rosa Andesite.
Except for the emplacement of the central lava domes, no
evidence of resurgence was observed. The negative evidence
includes: (1) absence of a caldera moat, (2) absence of exposures
of the intracaldera facies of the Amealco Tuff, and (3) absence of
outward tilting of the intracaldera layered pumice deposits of the
Brick Pumice (section 26, Fig. 4). All these features would be
expected if the caldera had been “inflated” by resurgence; thus,
there apparently was no resurgence in the Amealco caldera volcanic evolution. The central lava domes are regarded as exogenous. These lavas were viscous and did not flow far from the
vent, but produced high-standing and steep-flanked domes.
Coronita Rhyolite and mafic Hormigas volcano (3.7 Ma)
Two rhyolitic domes were emplaced near the Amealco
caldera, just south of San Mateo (Fig. 2), at about 3.7 Ma (Fig. 7).
This age is the weighted mean of a glass age of 3.5 Ma and a
feldspar age of 3.9 Ma (Table 1). There is no overlap of errors,
but there is no reason to discard either of the two; thus, the
weighted mean of the two results was used as the most representative age for this unit.
The Coronita Rhyolite is a much more evolved rock relative
to the Amealco caldera products (SiO2 = 75.9 wt%; Table 2,
Fig. 6), and probably was derived from a magmatic system different from that of the Amealco caldera. Several similar rhyolitic
domes were emplaced around the Sierra Puruagua, which is a
large volcanic complex 15 km to the west-southwest of Amealco
caldera (Fig. 5). Thus, the magma chamber from which these
domes originated was probably beneath the Sierra Puruagua volcanic complex.
Also at 3.7 Ma, another small volcano, Hormigas, developed
next to the caldera rim (Figs. 2 and 7), erupting a total DRE volume of 0.8 km3. The volcano apparently only consists of olivinebearing, andesitic lava flows (SiO2 = 55.8 wt%; Table 2, Fig. 6).
Pyroclastic deposits are not observed. The Hormigas lavas
reached as far as 5 km to the north of the vent. Similar to the
Coronita Rhyolite, the products of the Hormigas volcano may
have been derived from a different magmatic source than that of
the Amealco caldera products (Aguirre-Díaz, 1993).
Huichapan Tuff (3.5 Ma)
The Huichapan Tuff is a voluminous and widespread felsic
ignimbrite with associated surge, ash-flow, and mud-flow
deposits that blanketed the eastern portion of the area at about
3.5 Ma (Fig. 7). The total volume of this unit has not been estimated, but the ignimbrite can be followed for more than 50 km to
the east. The tuff probably originated from the Huichapan caldera
by several major explosive eruptions. The caldera is about 40 km
to the east of the contact between the Amealco and Huichapan
Tuffs. This contact is about 20 km to the east of the Amealco
caldera. The ignimbrite is welded and has well-developed columnar jointing (the cover photo of Cas and Wright, 1987, is this
ignimbrite). Because of its proximity to the Amealco caldera, this
ignimbrite has been misinterpreted as an Amealco ignimbrite
(Sánchez-Rubio, 1984). The ignimbrite of the Huichapan Tuff is
practically 100% glass, with sparse quartz and K-feldspar (both
2–10 vol%). From both glass and feldspar, K-Ar ages of 3.6 ± 0.1
and 3.4 ± 0.2 Ma, respectively, were obtained (Table 1). The
weighted mean of these ages, 3.5 ± 0.2 Ma, was used as the most
representative age of this unit. The Huichapan Tuff may be correlated with the San Francisco Tuff of Herrera and Milán (1981)
and Milán et al. (1993) because of their similar physical aspect
and mineralogy and same stratigraphic position. In addition, the
Huichapan Tuff can be followed to the east from the contact with
the Amealco Tuff to a position very close to the Huichapan
caldera, where it is buried by the lavas of the Nopala volcano,
which postdates the San Francisco Tuff and was emplaced on the
eastern rim of the Huichapan caldera.
El Rincón Rhyolite (2.9 Ma)
The Sierra El Rincón is a rhyolitic lava dome complex that
was emplaced after the Amealco Tuff at 2.9 Ma (Table 1) and is
about 15 km north-northwest of the Amealco caldera (Fig. 2).
The name of this unit is proposed as El Rincón Rhyolite. The
rock is a perlitized obsidian. It is practically aphyric. The only
phases observed were altered, mafic, euhedral crystals that only
make 4% of the volume, still preserving a euhedral shape of
olivine. The obsidian is a peraluminous, high-silica rhyolite
(SiO2 = 75.3 wt%, sample Am-122, Table 2). Although this unit,
as the Huichapan Tuff, was not erupted by the Amealco caldera, it
is an important peripheral unit. Both the Huichapan Tuff and El
Rincón Rhyolite are useful to constrain the timing of the
Amealco Tuff.
Caldera displacement by normal faulting, Garabato Dome,
El Comal cone (3.7–2.2 Ma)
The southern portion of the Amealco caldera was displaced
by normal faulting between 3.7 and 2.5 Ma, which are the ages of
the central lava domes and the Garabato dome, respectively (see
following). Faulting took place along the south-facing Epitacio
Huerta fault system, which changes orientation from east-west to
Volcanic evolution of the Amealco caldera, central Mexico
southeast-northwest from east to west (Figs. 1 and 2). The fault
system is the northern boundary of the Acambay graben (Fig. 5).
The displacement along this fault system must have been larger
than 200–250 m, the common height of the rim lava domes, as
the southern caldera margin is totally missing. To the east of the
Amealco caldera there is another fault system with an orientation
similar to that of the Epitacio Huerta fault system (Fig. 2), and
with scarps of 20–50 m, with opposite sense of movement (northfacing faults). We and other workers (Suter et al., 1991) propose
that this contrast in style of faulting may be due to an older structural discontinuity with a northwest orientation, corresponding to
the Taxco–San Miguel de Allende fault system (or Querétaro
fracture zone, Fig. 1), which is overlain by the Amealco caldera.
At 2.5 Ma, Garabato dome was emplaced from a conduit
apparently located along the Epitacio Huerta fault system. The
Garabato dome is hornblende andesite with an SiO2 content of
62.6 wt% (Table 2, Fig. 6), and a DRE volume of 0.1 km3. The
dome was displaced by a normal fault, presumably part of the
same system. However, the fault displacement of the lavas of the
Garabato dome is only 25 m (Sánchez-Rubio, 1984). Apparently
most displacement of the caldera had already occurred when the
Garabato dome was emplaced. Thus, faulting mostly occurred
between 3.7 and 2.5 Ma, perhaps closer to 2.5 Ma.
The last volcanic episode in the Amealco caldera area was
the development of the monogenetic El Comal scoria cone, at
2.2 Ma, making up a DRE volume of 0.15 km3. It was formed
predominantly by strombolian eruptions. A lava flow breached
the cone on its northeastern flank. The El Comal products
became chemically more evolved with time, from basaltic andesite (56.3 wt% SiO2) to andesite (62 wt% SiO2) (Fig. 6). The
cone is in an advanced degree of erosion, but steep flanks of
scoria-fall deposits are preserved.
The cone was emplaced on the trace of the Epitacio Huerta
fault system (Figs. 2 and 7). As the cone is unfaulted, fault movement must be older than 2.2 Ma. Elsewhere, between Epitacio
Huerta and Coroneo (Fig. 2), there are reports of historical
seismicity along this fault system that damaged the church of
Coroneo by the turn of this century (Urbina and Camacho, 1913).
There is also field evidence for more recent faulting in the form
of colluvium deposits in contact with the Amealco Tuff along the
Epitacio Huerta fault system, near Epitacio Huerta (Suter et al.,
1995). These facts suggest that the Epitacio Huerta fault trace
may be a system of several segments that have independent histories of movement.
CONCLUSIONS
The Amealco caldera is a well-preserved Pliocene volcanic
center, 11 km in diameter, located in the central part of the Mexican Volcanic Belt. The caldera was formed by several discrete
volcanic events: (1) eruption of 77 k3 of trachyandesitic-trachydacitic magma that deposited the Amealco Tuff and caused
caldera collapse between 4.7 and 4.5 Ma; (2) eruption from ring
fracture vents of 2.4 km3 of magma as tephra that formed the
13
Brick Pumice unit; (3) eruption of 3.8 km3 of trachyandesitic
magma through several ring-fracture vents as lava domes at
4.3 Ma (Amealco Andesite); (4) emplacement of 0.8 km3 of
trachydacitic magma at 4.3 Ma on the northwestern caldera rim
as the Campana dome; (5) development of Palomas volcano at
4.0 Ma, which represents 0.8 km3 of trachydacitic magma; and
(6) emplacement of 10 lava domes in the center of the caldera
between 3.9 and 3.7 Ma, of which 5 are trachyandesitic (Santa
Rosa Andesite) with a magma volume of 4.3 km3, and 5 are rhyolitic (La Cruz Rhyolite) with a magma volume of 2.4 km3.
After emplacement of the central lava domes volcanic
activity continued within and next to the caldera during the next
1.6 m.y. These events are: (1) eruption of basaltic andesite
magma from Hormigas volcano at 3.7 Ma; (2) emplacement of
rhyolite lava domes, also at 3.7 Ma (Coronita Rhyolite); (3)
emplacement of a 2.5 Ma hornblende andesite dome (Garabato
Andesite); and (4) the formation of the andesitic El Comal
scoria cone at 2.2 Ma.
In addition to the peripheral volcanism next to the caldera,
the Amealco Tuff is overlain 20 km to the east of the caldera by a
3.5 Ma widespread felsic ignimbrite and related tephra deposits
(Huichapan Tuff), and by a 2.9 Ma rhyolitic lava dome complex
15 km north of the caldera (Sierra El Rincón).
The southern portion of the Amealco caldera was displaced
at least 200–250 m by normal faulting along a segment of the
Epitacio Huerta fault system. Faulting on this segment must have
occurred between 3.7 and 2.5 Ma, and closer to 2.5 Ma, because
the 2.5 Ma Garabato dome is displaced only 25 m by this faulting. At least in the vicinity of the caldera, faulting cannot be later
than 2.2 Ma, the age of the El Comal cone, which is not displaced, although it is on the trace of the fault.
ACKNOWLEDGMENTS
This was greatly improved by comments on earlier versions
by Daniel S. Barker and Bruce N. Turbeville. We thanks Scott
Thieben, who provided assistance and supervision to gather the
chemical analyses. This study was supported in part by endowments to Daniel S. Barker, the Geology Foundation of the
Department of Geological Sciences of the University of Texas at
Austin, a scholarship to Aguirre-Díaz from Dirección General de
Asuntos del Personal Académico of Universidad Nacional
Autónoma de México (DGAPA-UNAM), and by grant IN106594 to Aguirre-Díaz from Programa de Apoyo a Proyectos de
Investigación e Inovación Tecnológica of DGAPA-UNAM. The
K-Ar laboratory at the University of Texas at Austin has been
supported by National Science Foundation grants EAR-8720380
and EAR-9204635 to McDowell.
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