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Geological Society of AmericaSpecial 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 MéxicoFred 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 diame-ter, located in the central part of the Mexican Volcanic Belt. It is one of seven calderasknown in the belt. Compared to those of the other calderas, the Amealco products areless evolved, and include only a minor volume of rhyolite. According to the stratigraphicrecord 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 andpyroclastic flows apparently of Plinian type. These events were followed by eruption offar-reaching surges and pyroclastic flows that deposited three widespread ignimbritesnamed Amealco I, Amealco II, and Amealco III. By about 4.7 Ma at least 77 km3 (Denserock equivalent, DRE) of trachyandesitic-trachydacitic magma were evacuated fromthe magma chamber and caused caldera collapse. After this climatic stage, pyroclasticactivity continued, probably as tephra fountains from ring-fracture vents, that eruptedpumice flows and fallouts that were accompanied by mud flows forming deposits oflocal 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 throughseveral ring-fracture vents, making up a DRE volume of 3.8 km3; the domes form thecaldera’s present topographic rim. At about 4.0 Ma, a modest-sized volcano had formedon the western flank of the caldera that erupted several trachyandesitic lava flows andfallout tephra (both lava and tephra deposits = 0.8 km3). Between 3.9 and 3.7 Ma, 10intracaldera lava domes were emplaced, accompanied by tephra eruptions that pro-duced relatively small deposits (volume not quantified) that were later reworked andredeposited as lake deposits within the caldera; five of these lava domes are trachy-andesitic (4.3 km3) and five are rhyolitic (2.4 km3). The central lava domes are inter-preted here as the viscous, gas-poor magma that usually erupts at the end of a calderacycle, 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 theemplacement of the central lava domes. These events include bimodal volcanism at3.7 Ma of basaltic-andesite lava from a volcano just north of the caldera rim (Hormigasvolcano) and emplacement of several rhyolitic lava domes to the southwest of thecaldera (Coronita rhyolite). At 2.9 Ma a rhyolite (obsidian) lava dome complex was
Aguirre-Díaz, G., and McDowell, F. W., 1999, Volcanic evolution of the Amealco caldera, central Mexico,in Delgado-Granados, H., Aguirre-Díaz, G., andStock, J. M., eds., Cenozoic Tectonics and Volcanism of Mexico: Boulder, Colorado, Geological Society of America Special Paper 334.
1
*Present address: Unidad de Investigacion en Ciencas de la Tierra, CampusJusiquilla, Universidad Nacional Autónoma de México, Apartado Postal 1-742,Centro, Querétaro, Qro., 7600 México.
INTRODUCTION
Relatively few calderas have been recognized within theMexican Volcanic Belt, a complex east-west–trending continen-tal volcanic province that crosses Mexico between lat 19° and21° N (Fig. 1). There are several tens of volcanic system com-plexes within the belt, but only seven have been identified ascalderas, including, from west to east, La Primavera, LosAzufres, Amealco, Mazahua, Huichapan, Aculco, Los Humeros,and Las Cumbres. From these, only the La Primavera and LosHumeros calderas, respectively at the extreme west and eastends of the Mexican Volcanic Belt, have been studied in detail(Mahood, 1980, 1981; Mahood and Drake, 1983; Mahood andHalliday, 1988; Ferriz and Mahood, 1984, 1987; Verma, 1984).Within the central sector, the Los Azufres complex has been thefocus of several studies, but its origin as a caldera is still debated(Cathelineau et al., 1987; Dobson and Mahood, 1985; Ferrariet al., 1993; Pradal and Robin, 1994).
This chapter focuses on a physical reconstruction of themajor volcanic events that formed the Amealco caldera. It isbased on the geologic mapping and detailed measurement of sev-eral stratigraphic sections that include the caldera products andthose of peripheral volcanoes. The field information is sup-plemented with stratigraphically controlled K-Ar ages and majorelement chemistry.
REGIONAL GEOLOGIC SETTING
The Amealco caldera is a Pliocene volcanic center in thecentral part of the Mexican Volcanic Belt (Fig. 1). The belt is acontinental margin volcanic arc that has been related to the sub-duction of the Cocos oceanic plate along the Middle Americatrench beneath southern Mexico (Urrutia-Fucugauchi and DelCastillo, 1977; Nixon, 1982; Suárez et al., 1990; Singh and Pardo,1993). The Mexican Volcanic Belt is a complex arc that can bedivided into sectors with their own characteristics, including vol-canic style, structure, morphology, age, and chemistry. At leastthree sectors can be recognized, the western, central, and eastern.The western sector is that between the western coast and theChapala lake (included); the central sector is between the Chapalalake and the Popocatepetl-Ixtaccihuatl north-south–trending vol-canic chain (both limits excluded); and the eastern sector isbetween this chain (included) and the eastern coast (Fig. 1).
Two main structural features characterize the centralMexican Volcanic Belt: the east-west–oriented graben system ofChapala-Cuitzeo-Acambay (Suter et al., 1991, 1995), also knownas the Chapala-Tula fault zone (Johnson and Harrison, 1990), andthe Taxco–San Miguel de Allende line (Demant, 1978; Nixonet al., 1987), or the Querétaro fracture zone (Johnson and Harri-son, 1990), which crosses the central part of the belt with a north-northwest–south-southeast orientation (Fig. 1), and which couldbe regarded as Basin and Range–style normal faulting. These tworegional 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 madeby Sánchez-Rubio (1978, 1984). Carrasco-Núñez (1988) alsostudied the Amealco caldera and provided a set of chemical dataon 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). Itssouthern portion was displaced by the Epitacio Huerta normalfault system, which bounds the Acambay graben on the north.These faults are part of the east-west–trending Chapala-Cuitzeo-Acambay graben system (Fig. 1). The north-northwest–south-southwest Taxco–San Miguel de Allende fault system is olderthan the Amealco caldera, and the east-west Acambay graben isyounger than the Amealco caldera, and has seismically activesegments (Suter et al., 1995). Representative schematic meas-ured sections of the Amealco caldera products are shown in Fig-ures 3 and 4. Locations of these and other measured sections inthe mapped area are shown in Figure 5.
The major ignimbrites of the Amealco Tuff are the AmealcoI, Amealco II, and Amealco III. Tephra 1 is the tephra beneath theAmealco I ignimbrite and includes Amealco zero, a minor butdistinctive ignimbrite only observed in the western deposits(Fig. 3). Tephra 2 is between the Amealco I and Amealco II, andtephra 3 is between the Amealco II and Amealco III (Fig. 3).K-Ar data are provided in Table 1. Representative major-elementchemical analyses of the Amealco caldera products and ofperipheral volcanism are provided in Table 2, and the total alkali-silica chemical classification is shown in Figure 6. All distancesmentioned are with reference to the center of the caldera.
It is inferred that the Amealco caldera was formed by
2 G. J. Aguirre-Díaz and F. W. McDowell
emplaced 15 km to the north of the caldera (El Rincón rhyolite). The last volcanicepisodes are represented by a 2.5 Ma andesitic lava dome (Garabato andesite) and a2.2 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, amajor west-northwest–south-southwest normal fault, which is part of the more regionalChapala-Cuitzeo-Acambay graben system. The Epitacio Huerta fault was active mostlyprior to 2.5 ± 0.3 Ma as the 2.5 Ma Garabato andesite was only slightly displaced by thisfault. Faulting apparently ended before 2.2 ± 0.1 Ma, because the El Comal scoria coneis not displaced in spite of its position on the trace of the Epitacio Huerta fault.
collapse, following the model of Smith (1979) and Smith andBailey (1968). However, it apparently did not undergo resurgenceafter collapse, although high-standing central lava domes wereemplaced toward the end of the caldera’s evolution.
Figure 7 shows the volcanic evolution of the Amealcocaldera. Eruptions are characterized as Plinian, vulcanian, orstrombolian, based on the observed qualitative characteristics ofthe deposits, rather than quantitatively, using grain-size and dis-tribution analysis.
K-Ar GEOCHRONOLOGY
We obtained 25 K-Ar ages (Table 1) for units that representthe entire range of the Amealco caldera activity, and include someunits of both precaldera and postcaldera volcanism. Some unitshave been dated with two or more samples from separate locali-ties. Except for minor rhyolite, volcanic units in the Amealcoregion lack potassium-rich phenocryst phases. For these samplesof Pliocene age we relied primarily upon glass or crystallinegroundmass to achieve reasonable precision. For some samples(samples Am-1, Am-179, Am-63, Am-81; Table 1) paired matrixand feldspar ages were obtained, with acceptable agreement inthree of four cases.
K-Ar analyses were performed in the Department of Geo-
logical Sciences of the University of Texas at Austin, in a labora-tory that has been commonly used for mid-Tertiary or older sam-ples. The techniques include flame photometry for K analysis andisotope dilution for Ar analysis using a Nuclide 3 in gas-sourcemass spectrometer. Details of the techniques can be found inAguirre-Díaz et al. (this volume).
Analytical uncertainties were mostly <5% of the age, andonly a few exceeded 8%. The worst case was 22% for Am-122, asample with high atmospheric argon content. For map units withmultiple age determinations, an assigned age is calculated from amean value that is weighted inversely in proportion to the preci-sion of the individual results.
VOLCANIC EVOLUTION OF THE AMEALCOCALDERA
Precaldera volcanic rocks (>4.7 Ma)
Most of the precaldera rocks are lava and pyroclastic rocksof intermediate or felsic composition. A K-Ar age of 5.7 Ma wasobtained from basaltic andesite (sample Am-67, Table 1) thatunderlies caldera products 28 km to the north-northeast of thecaldera, and a precaldera felsic ignimbrite 33 km to the westyielded a K-Ar age of 4.7 Ma (sample Am-84, Table 1).
Volcanic evolution of the Amealco caldera, central Mexico 3
Lake
VolcanoCalderaTown
Normal faults
O
22 N
20 N
18 N
16 N
105 W 103 W 101 W 99 W 97 W 95 W
105 W 103 W 101 W 99 W 97 W 95 W
22 N
20 N
18 N
16 N
M
Q
Mo
Ta
P
V
G
C
AMEALCOLP
H
LA
Miocene-Pliocene rocks
Pliocene-Quaternary rocks
100 km
Ma
ChapalaCuitzeo
LH
TSM
CCA
500 km
N
Amealco
MexicanVolcanic Belt
T
M E X I C O
Colima graben
Middle America Trench
Figure 1. Index map of major volcanoes,calderas, and major fault systems in theMexican Volcanic Belt. The Amealcocaldera is in the central portion of thebelt, at the intersection of the Taxco–SanMiguel de Allende (TSM) and Chapala-Cuitzeo-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 Pri-mavera, LA: Los Azufres, Ma: Mazahua,H: Huichapan, LH: Los Humeros. Modi-fied after Nixon et al. (1987). Inset showsregional index map of the Mexican Vol-canic Belt.
Plinian eruptions and associated pyroclastic flows (4.7 Ma)
The earliest activity related to the Amealco caldera occurredaround 4.7 Ma, when Plinian eruptions produced widespread pumicefallout and pyroclastic flows (e.g., Amealco zero ignimbrite; Figs. 3
and 7). This stage produced the earliest pyroclastic deposits of theAmealco Tuff that mainly consist of layered, white, medium- tocoarse-grained, angular, pumice lapilli. Thickness of the initial falloutdeposits is 3 m at a locality 25 km to the east, less than 1 m 15 km tothe west of the caldera, and practically zero 30 km to the west. They
4 G. J. Aguirre-Díaz and F. W. McDowell
0 2 5
km
Garabato
Molinos
AMEALCOLas Hormigasvolcano
Palomasvolcano
to Tepuxtepec
Tepuxtepecreservoir
Sierra El Rinc n
to Galindo
M
to Temascalcingo
SMT
100 00’W100 05’W100 10’W100 15’W
Tlc
Tch
Tsr
Tg
Tc
Tz
100 20’W
Garabato Andesite: hornblende andesite dome.2.5
Palomas Dacite: trachydacitic lavas and pyroclastic rocks.4.0
CALDERA UNITS
Campana Dacite: trachydacitic rim dome.
La Cruz Rhyolite: central domes; Tlc: La Cruz,Tch: Chiteje, Teb: El Barco.
Santa Rosa Andesite: trachyandesitic central domes;Tz: Zancudo,Tg: El Gallo, Tc: Capando, Tsr: Santa Rosa. Tm: La Mesa.
Spatter-lava cone.
Amealco Andesite: trachyandesitic rim domes.
Brick Pumice: pumice flow and fall, with epiclastic deposits.
Amealco tuff: trachyandesite-trachydacitic ignimbritesand interlayered surge, fallout and mud-flow deposits.
Reworked pumice and alluvium.
Intermediate (andesitic ?) lava flows and domes.
Silicic lava dome.
Town or village
Paved road
Unpaved road
Lake
River
Volcano
Normal fault
Caldera outline
PRECALDERA UNITSPOSTCALDERA UNITSAGE(Ma)
Quaternary
3.7
3.8
4.3
4.7
5.7
Coronita Rhyolite lava domes.
Hormigas Andesite: basaltic andesite lavas.3.7
3.9
4.3
Tm
Teb
cc
E
CG
S. Mateo
Tenango
LA
A il
El Comalvolcano
LomaLinda
120
100 20’W 100 15’W 100 10’W 100 05’W 100 00’W
120
El Comal cone and related andesitic lava flowand pyroclastic rocks.2.2
Huichapan Tuff.3.5
El Rinc n Rhyolite lava domes.2.9
Andesites, basaltic-andesites and related cones.Pliocene-Quaternary
EpitacioHuerta
SJ
EPITACIO HUERTAFAULT
Lerma river
2015’N
2010’N
to Coroneo
2005’N
2005
’N20
10’N
2015
’Nto
Acu
lco
Figure 2. Simplified geologic map of the Amealco caldera and peripheral volcanism. CC: Cerrito Colorado cone, E: ElEspía dome, SJ: San Juan Hedó, LA: Los Arcos, SMT: San Miguel Tlascaltepec, M: Mexquititlán, CG: Cañada de García.
36
42
49.8 mA
mea
lco
II
Amealco III
Am
ealc
o I18
24
30
covered
0
6
12
Am
ealc
o
Zer
o
Section 1
48
covered
32.5 m
Am
eal
co II
Am
eal
co II
IA
mea
l
co I
Section 2
40
50
60
70
73.5 m
0
10
20
30
Section 11
Am
ealc
o II
IA
mea
lco
IA
mea
lco
II
Am
ealc
o II
IA
mea
lco
II
31 m
24
18
12
Am
ealc
o I
6
0
Section 17
24
30
0
6
12
18
42
62 m
43
52
18
24
30
36
0
6
12
Section 13
Huichapan
tephra
Amealco III
Huichapanignimbrite
undifferentiated
Am
eal
co I
Am
eal
co II
covered
Ignimbrite withblack pumicesCoarse pumicefallout deposit
Fine pumicefallout deposit
Pumice flowdeposit
Ash-flow deposit
Finely layeredsurge depositsLithic-richsurge depositsBlack, uncollapsedpumice fragments
Black fiamme
White, uncollapsedpumice fragments
Accidental,angular lithics
Mud-flow deposit
MetamorphicrocksPaleosoil orvarved lake deps.
Vitrophyre ofignimbrite
Mafic lava flow
Felsic ignimbrite
Reworkeddeposit
Pumice veins
Weatheredpumice deposit
Monolithologicmatrix-free deposit
IntermediatelavaVesiculatedintermediate lava
Platy-jointedlava
BrecciatedlavaGlassylava
Undulatedpumice fall deposit
Lithics-richsurge deposit
Rounded lithics
Fluvial deposit
Recentalluvium
Recent talus
EXPLANATION
Figure 3. Representative measured sections of the Amealco Tuff. Locations are shown in Figure 5.
are also absent 28 km to the north-northeast of the caldera, and appar-ently 30 km to the south as well. Thus, paleowind transport of thePlinian column’s material may have been dominantly to the east.Apparently a column collapse caused the pyroclastic flow that formedAmealco zero ignimbrite, which is observed only to the west of thecaldera interlayered with pumice fall deposits (section 1, Fig. 3).
The Amealco zero ignimbrite is nonwelded, dark gray whenfresh, or red when weathered. It is mainly composed of blackpumice, 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 theAmealco zero yielded the most mafic composition of all theAmealco caldera products (SiO2 = 61.6 wt. 3°; this and subse-quent 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 inresponse to the eruption of voluminous and widespread pyroclas-tic flows and pumice fallouts that produced the major ignimbritesand interbedded tephra of the Amealco Tuff (Figs. 3 and 7).
The first voluminous pyroclastic flow deposited theAmealco I ignimbrite. This ignimbrite was preceded by erup-tion of surges, minor pyroclastic flows that deposited unweldedignimbrites, pumice fallouts, and mud flows. This pattern wasrepeated at least two more times to produce the tephra 2 andAmealco II ignimbrite, and the tephra 3 and Amealco III ign-imbrite (Fig. 3). Each of these cycles (tephra and major ign-imbrite) may have formed a transient caldera, which wasdestroyed by subsequent events, and/or was buried by youngerpyroclastic deposits. However, only one caldera structure isnow evident. Alternatively, it is possible that multiple collapseevents occurred in this single caldera (e.g., Druitt et al., 1994).The similar volumes of each cycle (major ignimbrite plus surgeand fall tephra) support the model of several calderas. The firsthypothetical caldera is related to a DRE volume of 21 km3
(Amealco zero, tephra 1, and Amealco I); the second caldera isrelated 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 volumeversus caldera diameter (Cas and Wright, 1987, p. 233), eachcycle 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,
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. Sectionsymbols are shown in Figure 3.
as indicated by paleosoils, irregular surfaces due to erosion, orthin layers of varved lake deposits (Fig. 3). These are particularlyevident after major pyroclastic flow eruptions that produced theignimbrites Amealco I, II, and III. Apparently, pauses occurredbetween the volcanic episodes that formed the Amealco Tuff.
On the basis of mapping, the Amealco Tuff has an estimatedminimum distribution of 2880 km2, and a total DRE volumeestimate of 77.7 km3. These are minimum volume estimatesbecause they do not include the intracaldera tuff, the distallapilli-ash fallouts, and the coignimbrite ash-cloud deposits,which were eroded away. A detailed description of the procedureto estimate the volume of the Amealco Tuff can be found inAguirre-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 sep-arate localities (Table 1). The range of ages is from 4.54 to4.74 Ma, and the weighted mean of 4.68 ± 0.10 Ma is chosen asthe best estimate. The mean is close to the ages for the three
glasses, which are more precise than those for the plagioclasesbecause of their higher potassium contents. The plagioclase agesare lower, but their errors overlap with both the weighted meanand the individual glass ages.
The bulk of the Amealco Tuff is trachyandesitic-trachy-dacitic in composition; most pumice fragments analyzed have aSiO2 range from 61.6 to 65.5 wt% (Table 2, Fig. 6). The mineral-ogy observed in the Amealco Tuff is consistent with this compo-sition, because it contains plagioclase, hypersthene, augite,ilmenite, titanomagnetite, accessory apatite, and rare olivine.
Mingling of glass is clearly evident in the Amealco Tuff. Themajor ignimbrites and the interlayered tephra deposits of theAmealco Tuff contain pumices with distinct compositions (docu-mented by electron-probe microanalysis; Aguirre-Díaz, 1993),and different colors (black, yellow, and white). Field and labora-tory observations (Aguirre-Díaz, 1991, 1993) have shown that thedifferent glasses are juvenile and are well mingled, even at themicroscopic scale (mingled shards). The different liquids wereejected simultaneously to form unzoned deposits that do notshow sorting of the different glasses. The eruptive mechanismthat caused these deposits could have been similar to that pro-posed by Sparks et al. (1977), in which the different magmascould have been simultaneously erupted after stirring, caused bymafic magma input at the base of a zoned subcaldera magmachamber. This mechanism must have occurred repeatedly to pro-duce the multiglass pyroclastic deposits of the Amealco Tuff, i.e.,the Amealco I, II and III.
Additional features indicate a complex eruptive history forthe Amealco Tuff. For example, fallout deposits consisting ofangular, coarse fragments of a single rock type, black glassy lava,are found intercalated in the sequence (section 1, Fig. 3). Thesedeposits could be interpreted as vulcanian-type eruptions thatdestroyed lava bodies, perhaps domes, that may have beenobstructing the vents. Thick stacks of pyroclastic surge deposits,that traveled as far as 28 km from source, indicate repeatedphreatomagmatic pulses of great explosivity (section 11, Fig. 3).Stacks of mud-flow deposits are also interbedded with ash-flowdeposits as far as 25 km from the source, and were common dur-ing the activity that produced the Amealco Tuff. Some of thesedeposits are lithic rich (20–35 vol% lithic material), others aremostly composed of sand-size material.
Brick Pumice, rim lava domes (Amealco Andesite), Campanadome (4.5–4.3 Ma)
Explosive activity continued in the Amealco caldera aftereruption of the Amealco Tuff. However, the pyroclastic materialissued during these events was less voluminous and more local-ized (Fig. 2). These pumice deposits were named the Brick Pum-ice by Sánchez-Rubio (1984). The Brick Pumice was emplacedin 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 ofthe caldera, named the Amealco Andesite (Fig. 7). The AmealcoAndesite was contemporaneous with the last events of deposition
Volcanic evolution of the Amealco caldera, central Mexico 7
Figure 5. Distribution of the Amealco Tuff based on the distribution ofthe major ignimbrites Amealco I, Amealco II, and Amealco III. Figureindicates location of the measured sections. H: Huimilpan; T: Tlalpu-jahua.
8 G. J. Aguirre-Díaz and F. W. McDowell
TABLE 1. K–Ar AGES OF AMEAICO CALDERA REGION
Unit Sample Location Mat.* K Weight Ar x 10-6 40Ar§ Age ±1σ∗∗ AssignedLat Long Age‡
(N) (W) (%) (g) (scc/g)† (%) (Ma) (Ma) (Ma ± 1σ)
Pre-Amealco Am-67 20°21’8” 100°6’50” Wr 1.27 0.611 0.269 33.2 5.69 0.35 5.69 ± 0.351.26 0.611 0.294 31.8
Am-84 20°10’33” 100°27’54” Fds 0.97 0.261 0.175 27.4 4.70 0.19 4.70 ± 0.190.99 0.327 0.183 38.5
Amealco Tuff Am-1 20°8’3” 100°18’10” Gl 4.64 0.263 0.854 18.5 4.74 0.15 4.68 ± 0.104.614.63
Am-1 Fds 0.53 0.316 0.094 19.9 4.54 0.280.53
Am-12 20°16’35” 100°9’7” Fds 0.46 0.301 0.082 13.1 4.55 0.400.46
Am-22 20°8’1” 100°18’8” Gl 3.62 0.432 0.640 35.9 4.71 0.143.53 0.277 0.664 31.13.563.53
Am-208 19°50’45” 100°11’15” Gl 2.90 0.606 0.542 34.4 4.71 0.192.91 0.278 0.524 38.82.86
Amealco And. Am-62 20°10’16” 100°10’17” Gms 2.00 0.668 0.342 58.0 4.42 0.21 4.31 ± 0.121.97
Am-58 20°10’13” 100°9’23” Wr 2.31 0.579 0.385 45.3 4.30 0.242.29
Am-46 20°6’38” 100°7’0” Wr 3.42 0.588 0.519 16.6 4.14 0.363.41 0.459 0.584 18.6
Am-195 20°7’54” 100°6’55” Wr 2.28 0.666 0.371 22.3 4.30 0.152.30 0.440 0.395 21.7
Campana Daci. Am-79 20°10’0” 100°15’22” Gl 3.79 0.618 0.618 42.6 4.31 0.10 4.31 ± 0.103.76 0.246 0.650 44.4
Palomas And. Am-41b 20°9’19” 100°16’8” Gms 0.58 0.546 0.096 16.9 3.96 0.40 3.96 ± 0.400.57 0.461 0.083 14.7
0.429 0.089 14.2La Cruz Rhyo. Am-63 20°10’4” 100°10’20” Gl 4.16 0.278 0.633 41.4 3.91 0.14 3.90 ± 0.14
4.16Am-63 Fds 1.38 0.284 0.209 19.8 3.89 0.27
1.37Sta. Rosa And. Am-61 20°8’49” 100°10’7” Gms 3.36 0.552 0.474 13.6 2.69 0.25 3.74 ± 0.25
3.31 0.681 0.478 13.03.303.32
Am-51 20°7’32” 100°9’43” Gms 2.82 0.723 0.374 19.2 3.79 0.262.72 0.353 0.423 20.22.69 0.494 0.416 17.3
Coronita Rhy. Am-179 20°7’7” 100°19’33” Gl 3.83 0.516 0.524 42.8 3.52 0.10 3.72 ± 0.273.823.83
Am-179 Fds 4.20 0.285 0.627 54.0 3.90 0.104.15 0.493 0.642 61.54.30
Hormigas And. Am-78 20°10’13” 100°12’31” Gms 1.14 0.533 0.174 32.3 3.70 0.39 3.70 ± 0.391.14 0.592 0.144 26.5
0.518 0.174 31.9Huichapan Tuff Am-81 20°8’38” 99°56’36” Gl 3.96 0.604 0.553 48.4 3.59 0.09 3.52 ± 0.16
3.96Am-81 Fds 3.58 0.274 0.443 23.3 3.36 0.21
3.52 0.414 0.482 58.03.543.50
Volcanic evolution of the Amealco caldera, central Mexico 9
TABLE 1. K–Ar AGES OF AMEAICO CALDERA REGION (continued - page 2)
Unit Sample Location Mat.* K Weight Ar x 10-6 40Ar§ Age ±1σ∗∗ AssignedLat Long Age‡
(N) (W) (%) (g) (cm3/g)† (%) (Ma) (Ma) (Ma ± 1σ)
El Rincón Rhy. Am-122 20°14’15” 100°14’25” Gl 4.86 0.664 0.555 4.5 2.92 0.44 2.92 ± 0.594.89
Garabato And. Am-18 20°6’0” 100°11’50” Wr 2.39 0.558 0.252 19.8 2.54 0.26 2.54 ± 0.262.37 0.588 0.218 29.0
El Comal And. Am-19b 20°5’59” 100°9’48” Gms 1.91 0.779 0.161 31.3 2.18 0.07 2.18 ± 0.071.90 0.647 0.163 24.9
*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 Ameaico Ignimbrites Rim Lava Domes Central DomesSample Am-39 Am-35 Am-22 Am-21 Am-255 Am-256 Am-133b Am-195 Am-53 Am-61Unit Zero Ame-1 Ame-II Ame-II Ame-III Ame-III Ame. And. Ame. And Santa Rosa Santa RosaLatitude 20°8’1” 20°8’1” 20°8’0” 20°8’5” 20°8’5” 20°21’2” 20°9’30” 20°7’54” 20°7’50” 20°8’49”Longitude 100°18’8” 100°18’8” 100°17’50” 100°17’55” 100°17’54” 100°6’6” 100°8’5” 100°6’55” 100°8’48” 100°10’7”Quadrangle F14C88 F14C86 F14C86 F14C86 F14C86 F14C76 F14C86 F14C86 F14C86 F14C86
SiO2 58.84 59.51 64.38 60.83 63.85 61.14 63.14 59.53 55.55 62.14TiO2 1.20 1.21 0.81 1.05 0.71 0.82 1.11 1.42 1.44 0.77Al2O3 15.95 15.97 15.46 15.72 15.25 14.42 15.64 16.09 17.24 15.87Fe2O3 3.53 3.21 1.79 2.40 2.58 2.08 1.95 2.98 3.30 2.10FeO 3.27 3.92 3.15 4.53 2.15 3.39 4.93 3.84 3.48 2.92MnO 0.13 0.12 0.09 0.12 0.09 0.12 0.13 0.12 0.11 0.09MgO 1.79 1.50 1.08 1.49 0.91 1.50 1.59 2.26 3.29 1.65CaO 4.57 4.02 2.78 4.24 2.73 3.94 3.87 4.87 6.75 4.24Na2O 3.85 4.34 4.67 5.24 3.36 3.94 4.90 4.39 3.96 4.04K2O 1.96 2.28 3.35 2.83 5.57 3.03 2.88 2.59 1.65 3.29P2O5 0.47 0.50 0.20 0.37 0.20 0.29 0.39 0.47 0.39 0.21H2O
+ 2.29 1.57 1.11 1.55 1.81 2.48 0.22 0.85 0.80 1.15H2O
- 0.59 0.44 0.15 0.15 0.53 0.52 0.02 0.34 1.02 0.31CO2 0.00 0.00 0.17 0.00 0.00 0.36 0.02 0.00 0.01 0.05
Total 98.43 98.58 99.19 100.51 99.74 98.03 100.78 99.75 99.01 98.83Mg# 49.4 40.6 37.9 37.0 43.1 44.0 36.5 51.2 62.8 50.2
CIPW Norm (wt. % volatile free)Q 17.99 15.37 16.57 8.83 16.88 17.20 12.34 11.59 8.77 15.29C 0.31 0.26Or 12.12 13.95 20.27 16.90 33.74 18.91 16.90 15.54 10.05 19.98Ab 34.10 37.99 40.45 44.85 29.19 35.29 41.21 37.74 34.69 35.12An 20.56 17.29 11.55 11.18 10.38 13.39 12.15 16.77 24.98 15.89WoDi 1.02 6.42 1.81 4.31 3.69 3.71 5.45 3.46Hy 6.05 6.74 5.43 5.44 2.30 5.35 7.89 6.42 7.41 5.11Mt 5.35 4.81 2.65 3.51 3.84 3.19 2.81 4.38 4.93 3.13Il 2.39 2.39 1.58 2.01 1.39 1.65 2.11 2.73 2.81 1.50Ap 1.14 1.20 1.58 0.86 0.46 0.72 0.90 1.11 0.93 0.51
Total 100.01 100.00 101.10 100.00 99.99 100.01 100.00 99.99 100.02 99.99TT index 64.20 67.32 77.29 70.58 79.82 71.39 70.45 64.88 53.51 70.39
of the Brick Pumice, because they occur interlayered in severalsites (sections 19 and 20, Fig. 4). The Amealco Andesite (3.8 km3
DRE) was emplaced as ring-fracture lava domes (Fig. 7), forminga continuous rim of trachyandesitic lava domes and associatedlava flows (SiO2 = 60.4–62.8 wt%, Table 2, Fig. 6) that formedthe topographic expression of the caldera (Fig. 2).
Several pumice flows consisting of coarse-grained, unsortedpumice deposits with a pumiceous matrix, and less frequently,pumice fallouts consisting of coarse-grained, sorted pumicedeposits 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 producedmassive, coarse-grained pumice flows that extended as far as17 km from the caldera. Fallouts probably were also part of theseeruptions, because pumice-fall deposits occur interlayered withthe pumice-flow deposits. Initially, pumice flow dominated theactivity, but pumice fallouts predominated at the end. The pumiceflows are thick next to the caldera (>15 m, no base exposed), butthickness decreases quickly with distance; they are on the orderof 2 m thick 2 or 3 km away from the rim.
Close to the caldera rim lavas are interlayered with thesetephra deposits (sections 19 and 20, Fig. 4). Farther (>2 km) fromthe caldera rim the pyroclastic deposits are interlayered with epi-clastic deposits (section 22, fig. 4). Both pyroclastic and epiclas-tic deposits are mapped as the Brick Pumice unit, which makesup 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 thecaldera, this dome is higher (more viscous?) and is chemically alittle more evolved (SiO2 = 66.8 wt%; Table 2, Fig. 6) than theother rim domes. The uncertainty in the K-Ar ages of both therim and Campana dome allows the Campana dome to be signifi-cantly later than the rim domes (Table 1), as the field relation-ships indicate.
Palomas volcano (4 Ma)
The Palomas volcano may have been already formed by4 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 anytime between 4.4 and 3.6 Ma. Palomas lavas overlie the BrickPumice to the south of the Palomas cone (Fig. 2).
Palomas activity included scoria flows and tephra and lava-flow eruptions with a trachydacitic composition (SiO2 = 65 wt%;Table 2, Fig. 6). These emissions gradually built a small com-posite cone that stands only 200 m above the adjacent lavaplateau. 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 theAmealco Tuff, making up a volume of 0.8 km3 (DRE). The lavaplateau extends to 8 km to the west of Palomas cone, where it isdisplaced by the Epitacio Huerta fault (Fig. 2).
Central lava domes: La Cruz Rhyolite and Santa RosaAndesite (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 inthe northern part of the Amealco caldera (Fig. 7). These domesformed the La Cruz Rhyolite, with a DRE volume of 2.4 km3.They are exogenous domes of Pelean type (Blake, 1990), havingsteep sides, crumble breccias, and block and deposits.
At 3.7 ± 0.25 Ma, several lava domes of trachyandesitic com-position (SiO2 = 57.1–63.9 wt%; Table 2, Fig. 6) were emplacedin the center of the caldera and formed the Santa Rosa Andesite(Fig. 7), with a total DRE volume of 4.3 km3. The uncertainty ofthe K-Ar ages overlap, but the ages suggest that the rhyoliticdomes were earlier than the intermediate lava domes, as the fieldrelationships indicate. The lavas of the undated trachyandesiticCapando dome (Fig. 2) overlie the rhyolite of the Chiteje de la
10 G. J. Aguirre-Díaz and F. W. McDowell
Figure 6. Total alkali-silica volcanic-rock classification diagram (Le Bas et al., 1986) of representa-tive Amealco caldera products and peripheral volcanism. Symbols in figure are: Amealco Tuff ign-imbrites: closed square; Amealco Andesite: open diamond; central lava dome (both rhyolite andtrachyandesite): open square; Palomas Dacite: plus sign; Campana Dacite: X; Hormigas Andesite:open circle; Garabato Andesite: triangle; Comal Andesite: closed circle; Coronita Rhyolite: closeddiamond. Abbreviations: b, basalt; ba, basaltic andesite; a, andesite; d, dacite; r, rhyolite; bta, basal-tic trachyandesite; ta, trachyandesite; td, trachydacite (see Table 2 for chemical data).
Caldera collapse(s?) associated to major ignimbrite eruptionsfrom ring-fracture vents, accompanied by pumice fallouts,pyroclastic surges, mud flows, and vulcanian eruptions(Amealco Tuff).
4.7 4.5Ma
Plinian eruptions from an inferred central ventproduced pumice-fall deposits, surges, and non-welded ignimbrites (e.g., Ignimbrite Zero).
Eruption from ring-fracture vents of coarse-grained pumiceflows and sparse pumice fallouts (Brick Pumice).
4.5 4.3 Ma
Emplacement of ring-fracture lava domes and related lavaflows with sporadic tephra eruptions (Amealco Andesite).
~ 4.3 Ma
~ 4.3 Ma
Pumice-flow and fallout activity persisted after theemplacement of the rim lava domes (Brick Pumice).Emplacement of the Campana dome.
Emplacement of the Palomas volcano adjacent to thecaldera margin. Eruption of lavas and pyroclastic rocks.
~ 4.0 Ma
Emplacement of trachyandesitic central lava domes (SantaRosa Andesite).
3.8 3.7 Ma
Eruption of felsic pyroclastic flows from a source east of theAmealco caldera (Huichapan caldera?) that produced theHuichapan Tuff.
3.5 Ma
3.7 Ma
Emplacement of Coronita Rhyolite lava domes west of thecaldera, and of basaltic andesite of Hormigas Volcano nextand to the north of the caldera rim.
Emplacement of rhyolitic central lava domes (La Cruz Rhyolite).
3.9 Ma
2.5 Ma
Emplacement of El Comal Cone on the fault trace that displacedthe caldera. Reworked material continue to fillthe tectonic depression south of the caldera.
2.2 Ma
Displacement of Amealco caldera by normal faulting along theEpitacio fault system. Emplacement of the Garabato lava dome.The dome was little displaced by faulting. Reworked materialfilled the tectonic depression south of the caldera.
~
~
~
~
Figure 7. Volcanic evolution of the Amealco caldera and peripheral volcanoes.
Cruz dome. The same arrangement is observed between the lavasof the Santa Rosa dome and El Barco domes (Fig. 2).
The relatively long time between emplacement of the rimdomes (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 magmachamber to undergo differentiation. It is suggested that a rhyoliticcap was formed at the top of the chamber as a result of this differ-entiation. This rhyolitic magma layer was underlain by trachy-andesitic magma, which likely made up the bulk of the magmachamber. Foundering of blocks above the already fractured andfaulted caldera roof may have squeezed out the viscous rhyoliticmagma and produced the rhyolitic lava domes of the La CruzRhyolite; as block foundering proceeded, the underlying trachy-andesitic magma was then squeezed out to produce the trachy-andesitic lava domes of the Santa Rosa Andesite.
Except for the emplacement of the central lava domes, noevidence of resurgence was observed. The negative evidenceincludes: (1) absence of a caldera moat, (2) absence of exposuresof the intracaldera facies of the Amealco Tuff, and (3) absence ofoutward tilting of the intracaldera layered pumice deposits of theBrick Pumice (section 26, Fig. 4). All these features would beexpected if the caldera had been “inflated” by resurgence; thus,there apparently was no resurgence in the Amealco caldera vol-canic evolution. The central lava domes are regarded as exog-enous. These lavas were viscous and did not flow far from thevent, but produced high-standing and steep-flanked domes.
Coronita Rhyolite and mafic Hormigas volcano (3.7 Ma)
Two rhyolitic domes were emplaced near the Amealcocaldera, 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 afeldspar 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, theweighted mean of the two results was used as the most repre-sentative age for this unit.
The Coronita Rhyolite is a much more evolved rock relativeto the Amealco caldera products (SiO2 = 75.9 wt%; Table 2,Fig. 6), and probably was derived from a magmatic system differ-ent from that of the Amealco caldera. Several similar rhyoliticdomes were emplaced around the Sierra Puruagua, which is alarge volcanic complex 15 km to the west-southwest of Amealcocaldera (Fig. 5). Thus, the magma chamber from which thesedomes originated was probably beneath the Sierra Puruagua vol-canic complex.
Also at 3.7 Ma, another small volcano, Hormigas, developednext to the caldera rim (Figs. 2 and 7), erupting a total DRE vol-ume of 0.8 km3. The volcano apparently only consists of olivine-bearing, andesitic lava flows (SiO2 = 55.8 wt%; Table 2, Fig. 6).Pyroclastic deposits are not observed. The Hormigas lavasreached as far as 5 km to the north of the vent. Similar to theCoronita Rhyolite, the products of the Hormigas volcano mayhave been derived from a different magmatic source than that ofthe Amealco caldera products (Aguirre-Díaz, 1993).
Huichapan Tuff (3.5 Ma)
The Huichapan Tuff is a voluminous and widespread felsicignimbrite with associated surge, ash-flow, and mud-flowdeposits that blanketed the eastern portion of the area at about3.5 Ma (Fig. 7). The total volume of this unit has not been esti-mated, but the ignimbrite can be followed for more than 50 km tothe east. The tuff probably originated from the Huichapan calderaby several major explosive eruptions. The caldera is about 40 kmto the east of the contact between the Amealco and HuichapanTuffs. This contact is about 20 km to the east of the Amealcocaldera. The ignimbrite is welded and has well-developed colum-nar jointing (the cover photo of Cas and Wright, 1987, is thisignimbrite). Because of its proximity to the Amealco caldera, thisignimbrite has been misinterpreted as an Amealco ignimbrite(Sánchez-Rubio, 1984). The ignimbrite of the Huichapan Tuff ispractically 100% glass, with sparse quartz and K-feldspar (both2–10 vol%). From both glass and feldspar, K-Ar ages of 3.6 ± 0.1and 3.4 ± 0.2 Ma, respectively, were obtained (Table 1). Theweighted mean of these ages, 3.5 ± 0.2 Ma, was used as the mostrepresentative age of this unit. The Huichapan Tuff may be cor-related with the San Francisco Tuff of Herrera and Milán (1981)and Milán et al. (1993) because of their similar physical aspectand mineralogy and same stratigraphic position. In addition, theHuichapan Tuff can be followed to the east from the contact withthe Amealco Tuff to a position very close to the Huichapancaldera, where it is buried by the lavas of the Nopala volcano,which postdates the San Francisco Tuff and was emplaced on theeastern rim of the Huichapan caldera.
El Rincón Rhyolite (2.9 Ma)
The Sierra El Rincón is a rhyolitic lava dome complex thatwas emplaced after the Amealco Tuff at 2.9 Ma (Table 1) and isabout 15 km north-northwest of the Amealco caldera (Fig. 2).The name of this unit is proposed as El Rincón Rhyolite. Therock is a perlitized obsidian. It is practically aphyric. The onlyphases observed were altered, mafic, euhedral crystals that onlymake 4% of the volume, still preserving a euhedral shape ofolivine. 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, itis an important peripheral unit. Both the Huichapan Tuff and ElRincón Rhyolite are useful to constrain the timing of theAmealco Tuff.
Caldera displacement by normal faulting, Garabato Dome,El Comal cone (3.7–2.2 Ma)
The southern portion of the Amealco caldera was displacedby normal faulting between 3.7 and 2.5 Ma, which are the ages ofthe central lava domes and the Garabato dome, respectively (seefollowing). Faulting took place along the south-facing EpitacioHuerta fault system, which changes orientation from east-west to
12 G. J. Aguirre-Díaz and F. W. McDowell
southeast-northwest from east to west (Figs. 1 and 2). The faultsystem is the northern boundary of the Acambay graben (Fig. 5).The displacement along this fault system must have been largerthan 200–250 m, the common height of the rim lava domes, asthe southern caldera margin is totally missing. To the east of theAmealco caldera there is another fault system with an orientationsimilar to that of the Epitacio Huerta fault system (Fig. 2), andwith scarps of 20–50 m, with opposite sense of movement (north-facing faults). We and other workers (Suter et al., 1991) proposethat this contrast in style of faulting may be due to an older struc-tural discontinuity with a northwest orientation, corresponding tothe Taxco–San Miguel de Allende fault system (or Querétarofracture zone, Fig. 1), which is overlain by the Amealco caldera.
At 2.5 Ma, Garabato dome was emplaced from a conduitapparently located along the Epitacio Huerta fault system. TheGarabato dome is hornblende andesite with an SiO2 content of62.6 wt% (Table 2, Fig. 6), and a DRE volume of 0.1 km3. Thedome was displaced by a normal fault, presumably part of thesame system. However, the fault displacement of the lavas of theGarabato dome is only 25 m (Sánchez-Rubio, 1984). Apparentlymost displacement of the caldera had already occurred when theGarabato dome was emplaced. Thus, faulting mostly occurredbetween 3.7 and 2.5 Ma, perhaps closer to 2.5 Ma.
The last volcanic episode in the Amealco caldera area wasthe development of the monogenetic El Comal scoria cone, at2.2 Ma, making up a DRE volume of 0.15 km3. It was formedpredominantly by strombolian eruptions. A lava flow breachedthe cone on its northeastern flank. The El Comal productsbecame chemically more evolved with time, from basaltic ande-site (56.3 wt% SiO2) to andesite (62 wt% SiO2) (Fig. 6). Thecone is in an advanced degree of erosion, but steep flanks ofscoria-fall deposits are preserved.
The cone was emplaced on the trace of the Epitacio Huertafault system (Figs. 2 and 7). As the cone is unfaulted, fault move-ment must be older than 2.2 Ma. Elsewhere, between EpitacioHuerta and Coroneo (Fig. 2), there are reports of historicalseismicity along this fault system that damaged the church ofCoroneo by the turn of this century (Urbina and Camacho, 1913).There is also field evidence for more recent faulting in the formof colluvium deposits in contact with the Amealco Tuff along theEpitacio Huerta fault system, near Epitacio Huerta (Suter et al.,1995). These facts suggest that the Epitacio Huerta fault tracemay be a system of several segments that have independent his-tories of movement.
CONCLUSIONS
The Amealco caldera is a well-preserved Pliocene volcaniccenter, 11 km in diameter, located in the central part of the Mex-ican Volcanic Belt. The caldera was formed by several discretevolcanic events: (1) eruption of 77 k3 of trachyandesitic-trachy-dacitic magma that deposited the Amealco Tuff and causedcaldera collapse between 4.7 and 4.5 Ma; (2) eruption from ringfracture vents of 2.4 km3 of magma as tephra that formed the
Brick Pumice unit; (3) eruption of 3.8 km3 of trachyandesiticmagma through several ring-fracture vents as lava domes at4.3 Ma (Amealco Andesite); (4) emplacement of 0.8 km3 oftrachydacitic magma at 4.3 Ma on the northwestern caldera rimas the Campana dome; (5) development of Palomas volcano at4.0 Ma, which represents 0.8 km3 of trachydacitic magma; and(6) emplacement of 10 lava domes in the center of the calderabetween 3.9 and 3.7 Ma, of which 5 are trachyandesitic (SantaRosa Andesite) with a magma volume of 4.3 km3, and 5 are rhyo-litic (La Cruz Rhyolite) with a magma volume of 2.4 km3.
After emplacement of the central lava domes volcanicactivity continued within and next to the caldera during the next1.6 m.y. These events are: (1) eruption of basaltic andesitemagma from Hormigas volcano at 3.7 Ma; (2) emplacement ofrhyolite lava domes, also at 3.7 Ma (Coronita Rhyolite); (3)emplacement of a 2.5 Ma hornblende andesite dome (GarabatoAndesite); and (4) the formation of the andesitic El Comalscoria 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 a3.5 Ma widespread felsic ignimbrite and related tephra deposits(Huichapan Tuff), and by a 2.9 Ma rhyolitic lava dome complex15 km north of the caldera (Sierra El Rincón).
The southern portion of the Amealco caldera was displacedat least 200–250 m by normal faulting along a segment of theEpitacio Huerta fault system. Faulting on this segment must haveoccurred between 3.7 and 2.5 Ma, and closer to 2.5 Ma, becausethe 2.5 Ma Garabato dome is displaced only 25 m by this fault-ing. At least in the vicinity of the caldera, faulting cannot be laterthan 2.2 Ma, the age of the El Comal cone, which is not dis-placed, although it is on the trace of the fault.
ACKNOWLEDGMENTS
This was greatly improved by comments on earlier versionsby Daniel S. Barker and Bruce N. Turbeville. We thanks ScottThieben, who provided assistance and supervision to gather thechemical analyses. This study was supported in part by endow-ments to Daniel S. Barker, the Geology Foundation of theDepartment of Geological Sciences of the University of Texas atAustin, a scholarship to Aguirre-Díaz from Dirección General deAsuntos del Personal Académico of Universidad NacionalAutónoma de México (DGAPA-UNAM), and by grant IN-106594 to Aguirre-Díaz from Programa de Apoyo a Proyectos deInvestigación e Inovación Tecnológica of DGAPA-UNAM. TheK-Ar laboratory at the University of Texas at Austin has beensupported by National Science Foundation grants EAR-8720380and EAR-9204635 to McDowell.
REFERENCES CITED
Aguirre-Díaz, G. J., 1991, Mixing of liquids in ignimbrites of Las AméricasFormation, Central Mexican Volcanic Belt: Eos (Transactions, AmericanGeophysical Union), v. 72, p. 578.
Aguirre-Díaz, G. J., 1993, The Amealco caldera, Querétaro, México: Geology,
Volcanic evolution of the Amealco caldera, central Mexico 13
geochronology, geochemistry, and comparison with other silicic centersof the Mexican Volcanic Belt [Ph.d. thesis]: Austin, University of Texas,401 p.
Aguirre-Díaz, G. J., 1996, Volcanic stratigraphy of the Amealco caldera and vicin-ity, central Mexican Volcanic Belt: Revista Mexicana Ciencias Geologícas,v. 13, p. 10–51.
Blake, S., 1990, Viscoplastic models of lava domes,in Fink, J. H., ed., Lava flowsand domes, IAVCEI Proceedings in Volcanology 2: New York, Springer-Verlag, p. 88–126.
Carrasco-Núñez, G., 1988, Geología y petrología de los campos volcánicos deLos Azufres (Mich.), Amealco y El Zamorano (Qro.) [tesis maestría]:México, D. F., Facultad Ingeniería, Universidad Nacional Autónoma deMéxico, 148 p.
Cas, R. A. F., and Wright J. V., 1987, Volcanic successions. Modern and ancient:London, Allen & Unwin, 528 p.
Cathelineau, M., Oliver, R., and Nieva, D., 1987, Geochemistry of volcanic seriesof the Los Azufres geothermal field (México): Geofísica Internacional,v. 26, p. 195–272.
Demant, A., 1978, Características del Eje Neovolcánico Transmexicano y susproblemas de interpretación: Universidad Nacional Autónoma de México,Instituto de Geología, Revista, v. 2, p. 172–187.
Dobson, P. F., and Mahood, G. A., 1985, Volcanic stratigraphy of the Los Azufresgeothermal area, Mexico: Journal of Volcanology and GeothermalResearch, v. 25, p. 273–287.
Druitt, T. H., Brenchley, P. J., Gökten,Y. E., and Francaviglia, V., 1994, Evolutionof the Acigöl caldera, Turkey: International Volcanolology Congress: Inter-national Association of Volcanology and Chemistry of Earth’s Interior(AVCE), Abstracts volume: Ankara, Turkey, Middle East Technical Uni-versity.
Ferrari, L., Garduño, V. H., Pasquarè, G., and Tibaldi, A., 1993, The Los Azufrescaldera, México: The result of multiple nested collapse: Reply: Journal ofVolcanology and Geothermal Research, v. 56, p. 345–349.
Ferriz, H., and Mahood, G. A., 1984, Eruption rates and compositional trendsat Los Humeros volcanic center, Puebla: Journal of GeophysicalResearch, v. 89, p. 8511–8524.
Ferriz, H., and Mahood, G. A., 1987, Strong compositional zonation in a silicicmagmatic system: Los Humeros, Mexican Volcanic Belt: Journal ofPetrology, v. 28, p. 171–209.
Herrera, F., and Milán, M., 1981, Estudio geológico de las zonas geotérmicas deYexthó, Pathé y Taxidó, estodos de Hidalgo y Querétaro: Informe técnicoC.F.E. 13–81 (inédito).
Johnson, C. A., and Harrison, C. G. A., 1990, Neotectonics in central Mexico:Physics of the Earth and Planetary Interiors, v. 64, p. 187–210.
Johnson, W. M., and Maxwell, J. A., 1981, Rock and mineral analysis (secondedition): Malabar, Florida, Robert E. Krieger Publishing Company, 489 p.
Le Bas, M. J., Le Maitre, R. W., Streckheisen, A., and Zanettin, B., 1986, A chem-ical classification of volcanic rocks based on the total alkali-silica diagram:J. Petrol. v. 27, p. 745-750.
Mahood, G. A., 1980, Geological evolution of a Pleistocene rhyolitic center—Sierra La Primavera, Jalisco, Mexico: Journal of Volcanology and Geo-thermal Research, v. 8, p. 199–230.
Mahood, G. A., 1981, Chemical evolution of a Pleistocene rhyolitic center—Sierra La Primavera, Jalisco, México: Contributions to Mineralogy andPetrolology, v. 77, p. 129–149.
Mahood, G. A., and Drake, R. A., 1983, K-Ar dating young volcanic rocks: Acase study of the Sierra La Primavera, Jalisco, México: Geological Soci-ety of America Bulletin, v. 93, p. 1232–1241.
Mahood, G. A., and Halliday, A. N., 1988, Generation of high-silica rhyolite:
A Nd, Sr, and O isotopic study of Sierra La Primavera, Mexican Neo-volcanic Belt: Contributions to Mineralogy and Petrology, v. 100,p. 183–191.
Milán, M.,Yáñez, C., Navarro, I., Verma, S. P., and Carrasco, G., 1993, Geologíay geoquímica de elementos mayores de la caldera de Huichapan, Hidalgo,México: Geofísica Internacional, v. 32, p. 261–276.
Nixon, G. T., 1982, The relationship between Quaternary volcanism in centralMexico and the seismicity and structure of subducted ocean lithosphere:Geological Society of America Bulletin, v. 93, p. 514–523.
Nixon, G. T., Demant, A., Armstrong, R. L., and Harakal, J. E., 1987, K-Ar andgeologic data bearing on the age and evolution of the Trans-Mexican Vol-canic Belt: Geofísica Internacional, v. 26, p. 109–158.
Pradal, E., and Robin, C., 1994, Long-lived volcanic phases at Los Azufres vol-canic center, Mexico: Journal of Volcanology and GeothermalResearch, v. 63, p. 201–216.
Sánchez-Rubio, G., 1978, The Amealco caldera: Geolological Society of AmericaAbstracts with Programs, v. 10, p. 145.
Sánchez-Rubio, G., 1984, Cenozoic volcanism in the Toluca-Amealco region,central Mexico [Master thesis]: London, Imperial College of Science andTechnology, University of London, 275 p.
Singh, S. K., Pardo, M., 1993, Geometry of the Benioff zone and state of stressin the overriding plate in central Mexico: Geophysical ResearchLetters, v. 20, p. 1483–1486.
Smith, R. L., 1979, Ash-flow magmatism,in Geological Society of AmericaSpecial Paper 180, p. 5–27.
Smith, R. L., and Bailey, R. A., 1968, Resurgent cauldrons,in Coats, R. R., Hay,R. L., and Anderson, C. A., eds., Studies in volcanology: Geological Soci-ety of America Memoir 116, p. 613–662.
Sparks, R. S. J., Sigurdsson, H., and Wilson, L., 1977, Magma mixing: A mecha-nism for triggering acid explosive eruptions: Nature, v. 267, p. 315-318.
Suárez, G., Monfret, T., Wittlinger, G., and David, C., 1990, Geometry of sub-duction and depth of the seismogenic zone in the Guerrero gap, Mexico:Nature, v. 345, p. 336–338.
Suter, M., Aguirre, G. J., Siebe, C., Quintero, O., and Komorowski, J. C., 1991,Volcanism and active faulting in the central part of the Trans-Mexican vol-canic belt,in Walawender, M. J., and Hanan, B. B., eds., Geologicalexcursions in southern California and Mexico: Department of GeologicalSciences, San Diego State University, p. 224–243.
Suter, M., Quintero, O., López-Martínez, M., Aguirre-Díaz, G., and Farrar, E.,1995, The Acambay graben: Active intra-arc extension in the Trans-Mexican Volcanic Belt, Mexico: Tectonics, v. 14, p. 1245–1262.
Thornton, C. P., and Tuttle, O. F., 1960, Chemistry of igneous rocks: I. Differen-tiation index: American Journal of Science, v. 258, p. 664–684.
Urbina, F., and Camacho, H., 1913, La zona megasísmica Acambay-Tixmadejé,Estado de México, conmovida el 19 de noviembre de 1912: Boletín delInstituto Geología de México, v. 32, 125 p.
Urrutia-Fucugauchi, J., and Del Castillo, L., 1977, Un modelo del Eje VolcánicoTransmexicano: Boletín de la Sociedad Geológica Mexicana, v. 38,p. 18–28.
Verma, S. P., 1984, Alkali and alkaline earth element geochemistry of LosHumeros caldera, Puebla, Mexico: Journal of Volcanology GeothermalResearch, v. 20, p. 21–40.
Verma, S. P., Carrasco-Núñez, G., and Milán, M., 1991, Geology and geo-chemistry of Amealco caldera, Qro., Mexico: Journal of VolcanologyGeothermal Research, v. 47, p. 105–127.
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