ba-Aldavea, J. Solea, A. Iriondob,1

bearing dacitic to andesitic lava flows. The age (33 to 32Ma) and isotopic signatures of these lava flows indicate a resurgent event

related with the input of more primitive magmas into the magma chamber.

Journal of Volcanology and Geothermal Retectonic lineaments that are part of a regional strike-slip system, active at the time of the caldera formation. We interpret that the

NW tectonic structures defined zones of weakness that accommodated the caldera collapse in the northeastern and southwestern

segments of the caldera structural margin.

D 2004 Elsevier B.V. All rights reserved.

Keywords: collapse caldera; resurgent caldera; strike-slip tectonics; ignimbrite; mega-breccia; MexicoThe rectilinear northeastern and southwestern segments of the structural margin of the caldera correspond to NW-trendinga Instituto de Geologa, Universidad Nacional Autonoma de Mexico, Ciudad Universitaria, Mexico D.F. 04510, MexicobDepartment of Geological Sciences, University of Colorado at Boulder, Campus Box 399, Boulder, CO 80309-0399, USA

Accepted 26 April 2004

Abstract

The Tilzapotla caldera constitutes the first discovery of a major Tertiary collapse volcanic structure south of the Mexican

Volcanic Belt. Although it is spatially associated with silicic ignimbrites in a region relatively distant from the extensive

ignimbritic province of the Sierra Madre Occidental (SMO), it is among the largest collapse calderas documented in Mexico. The

caldera is defined by a 33 24 km semi-elliptical structure that encircles the largest exposures of the Tilzapotla ignimbrite andcorresponds to the structural margin rather than the topographic rim. A central uplifted block limited by NW-trending faults is the

main indication of a resurgent stage. The caldera structural margin is surrounded by extensive exposures of Cretaceous marine

sequences that structurally define a broad elliptical dome (45 35 km) originated in the first stage of the caldera evolution.There is evidence showing that the 34 Ma Tilzapotla ignimbrite represents the climatic event of the caldera collapse. It is

constituted by a massive sequence of crystal vitric tuff with conspicuous euhedral biotite and abundant quartz. The intra-caldera

facies is intercalated with mega- and meso-breccias of limestone and anhydrite fragments derived from the slumping of the caldera

wall during the caldera collapse. The overlying sequence includes post-collapse ignimbrites as well as amphibole and pyroxeneD.J. Moran-Zentenoa,*, L.A. AlA major resurgent caldera in southern Mexico: the source of the

late Eocene Tilzapotla ignimbrite0377-0273/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.jvolgeores.2004.04.002

* Corresponding author. Tel.: +52-5-616-0557; fax: +52-5-550-

6644.

E-mail address: dantez@servidor.unam.mx

(D.J. Moran-Zenteno).1 Present address: Centro de Geociencias, Universidad Nacional

Autonoma de Mexico, Campus Juriquilla, Queretaro, Qro., 76230

Mexico.www.elsevier.com/locate/jvolgeores

search 136 (2004) 971191. Introduction

Although the Tilzapotla caldera is a volcanic struc-

ture with a remarkable semi-elliptical expression in

satellite images (Fig. 1), it was not recognized as a

major volcanic structure until recently (Moran-Zenteno

et al., 1998), probably due to the paucity of studies

D.J. Moran-Zenteno et al. / Journal of Volcanology and Geothermal Research 136 (2004) 9711998focused on the Tertiary volcanic rocks in southern

Mexico. The caldera is spatially related to a discontin-

uous silicic volcanic cover distributed in southern

Morelos and northern Guerrero states, in the northern

Sierra Madre del Sur (Fig. 2A and B). It represents the

first discovery of a major collapse caldera south of the

Mexican Volcanic Belt and it is among the largest

reported in Mexico. Because of its age and depth of

erosion, expressed in an inverted relief, this volcanic

Fig. 1. Landsat Thematic Mapper (TM) image of the Tilzapotla caldera ar

ignimbrite, Ts = El Salto lava flows, Th = hypabyssal rocks, Gd =Coxcatla

structural margin and main tectonic lineaments are indicated. The area wh

line. Satellite image is provided by Industrias Penoles Mining Company.center clearly displays features of a ring fault zone. The

fact that the volcanic zone is surrounded by broad

exposures of Cretaceous marine rocks makes the fea-

tures related with the caldera collapse more prominent

(Figs. 1 and 3).

Ignimbrites associated with the Tilzapotla caldera

are part of a discontinuous dissected belt of Tertiary

volcanic rocks that extends for about 600 km from the

states of Michoacan to Oaxaca (Moran-Zenteno et al.,

ea. Tz = Tilzapotla ignimbrite, Tr =Rodarte ignimbrite, Tg =Gallego

n granodiorite intrusion, Km=marine Cretaceous rocks. The caldera

ere the elliptical dome is recognizable is encircled by a finer dashed

D.J. Moran-Zenteno et al. / Journal of Volcanology and Geothermal Research 136 (2004) 97119 991999). Volcanic rocks of this belt range in composi-

tion from basaltic-andesite to rhyolite. Coeval bath-

oliths are broadly exposed along the exhumed

continental margin of southern Mexico. Both, the

Tertiary plutonic and volcanic belts represent the wide

magmatic arc of the Sierra Madre del Sur. It was

originated during subduction episodes along the Pa-

cific margin previous to, and in part contemporary,

with the margin truncation attributed to the displace-

ment of the Chortis block (Ross and Scotese, 1988;

Pindell et al., 1988; Ratschbacher et al., 1991; Herr-

mann et al., 1994; Schaaf et al., 1995; Moran-Zenteno

et al., 1999). The Tertiary magmatism of the Sierra

Madre del Sur is partially coeval with the major

episodes of Oligocene ignimbrite volcanism of the

northern and southeastern Sierra Madre Occidental

(McDowell and Clabaugh, 1979; Nieto-Samaniego et

al., 1999, Ferrari et al., 2002; Aranda-Gomez et al.,

2003). The region where the Tilzapotla caldera is

Fig. 2. (A) Sketch map of the central part of the northern Sierra Madre del

Cenozoic tectonic features in the region. (B) Distribution of the ignimbritic

distribution of the outflow sheet remnants of the Tilzapotla ignimbrite.located is dominated by silicic volcanic rocks that

appear to be the southern extension of the flare-up of

the Sierra Madre Occidental, where several collapse

calderas have been reported (Fig. 2A) (McDowell and

Clabaugh, 1979; Swanson and McDowell, 1984).

The Tertiary volcanic rocks of the study region

were first described by Fries (1960, 1966) and De

Cserna and Fries (1981), who described the sequence

in terms of Tilzapotla Rhyolite and overlaying

Buenavista Andesite or Buenavista Group. They

interpreted one of the probable sources of the Tilza-

potla Rhyolite as located south of the village of

Tilzapotla, without specifying the nature and the

precise position of the volcanic center. According to

these authors, the Tilzapotla Rhyolite includes a

series of pyroclastic flows ranging in composition

from dacite to rhyolite (although this unit was iden-

tified as a pyroclastic sequence, they used the term

rhyolite). They applied this name even to units

Sur showing the distribution of Tertiary volcanic rocks and the main

rocks attributed in this study to the Tilzapotla caldera, including the

D.J. Moran-Zenteno et al. / Journal of Volcanology and Geothermal Research 136 (2004) 97119100cropping out in distant volcanic zones (i.e. Taxco

region) and displaying some significant lithologic

differences. In this paper, we use the informal denom-

ination of Tilzapotla ignimbrite instead of Tilzapotla

Rhyolite to avoid confusion. Since the volcanic units

overlying the Tilzapotla ignimbrite in the caldera area

are more diverse and spatially variable than Fries

originally supposed, in this paper different lithostrati-

graphic units are informally proposed, although Bue-

navista Group name can be preserved to include most

of them. In a regional reconnaissance study of the

stratigraphy and petrology of the volcanic rocks in the

Taxco-Huautla region, Moran-Zenteno et al. (1998)

identified three different major volcanic centers (i.e.

Taxco, Tilzapotla-Buenavista and Huautla) (Fig. 2B).

They recognized the Tilzapotla and Buenavista vol-

canic zones as a part of a laterally continuous volcanic

cover defining a semicircular structure. The Taxco

volcanic field is located about 50 km northwest of the

Tilzapotla area and is characterized by a volcanic

sequence that includes ignimbrites and rhyolitic lava

Fig. 3. Geologic map of the Tilzapotla caldera area. Distribution of Meso

Recursos Minerales (Rivera-Carranza et al., 1998). Sections AB and Cflows that were originated from ca. 38 to 32 Ma. The

Huautla volcanic field is located to the east of the

Tilzapotla caldera (Fig. 2B) and is represented by a

sequence of lava flows and pyroclastic deposits that

overlie outflow volcanic deposits similar to those in

the Tilzapotla-Buenavista region (Fries, 1966; Moran-

Zenteno et al., 1998).

In this paper, we present stratigraphic, structural

and geochemical data of the volcanic sequence of the

Tilzapotla-Buenavista area that confirm the existence

of a large resurgent caldera as the source of the

Tilzapotla ignimbrite. These data lead to an interpre-

tation of its volcanic evolution and its relationship

with the Tertiary tectonic structures.

2. Caldera structure and related tectonic features

The Tilzapotla caldera can be recognized in satel-

lite images as a semi-elliptical structure with a major

axis of about 33 km and a minor axis of 24 km (Figs.

zoic units was based on the geologic map published by Consejo de

D presented in Fig. 5 are indicated as solid lines.

1, 3 and 4) that encircles a laterally continuous and

thick volcanic sequence. The elongated shape of the

caldera is defined by NW-trending lineaments form-

ing its NE and SW boundaries. The most conspicuous

lineament is the SW boundary of the caldera that

extends from Huitzuco to Tlaxmalac (Los Amates

fault). Although less defined, the northeastern limit

is also expressed by a NW-trending lineament passing

north of Tilzapotla (Fig. 4). The southeastern bound-

ary of the caldera displays a well-defined arcuate

segment that coincides with a subvertical contact

between beds of Cretaceous marine rocks and the

intra-caldera ignimbrite. The western boundary of the

caldera, around and north of Buenavista, has a more

irregular outline.

Cretaceous marine beds surrounding the caldera

margin structurally delineate an elliptical dome

(45 35 km) whose contour, in the eastern and south-eastern segments, is semi-parallel to the caldera margin

and oblique to the near north-trending pre-existing

Laramide structures (Figs. 1 and 4). The interference

between the elliptical dome and the Laramide fold belt

can be recognized by nearly opposite plunging folds

south and north of the caldera (Fig. 4).

There are clear indications that the elliptical shape

of the caldera corresponds to the structural margin

ence

. Fau

ce be

D.J. Moran-Zenteno et al. / Journal of Volcanology and Geothermal Research 136 (2004) 97119 101Fig. 4. Structural sketch map showing the caldera margin, the resurg

segments with direct evidence of the position of the structural margin

nets, lower hemisphere. Plunges that portray the structural interfereninset. Solid and open circles indicate plunges measured in marine beds diblock and the related tectonic lineaments. Solid lines indicate those

lt data and striations from selected localities are shown in equal area

tween the dome structure and the Laramide folds are shown in thestributed south and north of the caldera, respectively.

rather than the topographic rim. The relatively deep

level of erosion allows a steep contact to be observed

between the thick ignimbritic sequence and the pre-

caldera rocks, that in some segments of the eastern

and southeastern boundary corresponds with vertical

and lateral faults (Fig. 4). Additionally, the occurrence

of collapse meso- and mega-breccias, as well as lag

breccias along the conspicuous rectilinear and arcuate

segments of the margin, support this interpretation.

Due to the relatively deep level of erosion, there is no

evidence of a recognizable topographic rim. In the

rectilinear southern segment of the structural margin,

east of Huitzuco, the contact between the in-fill

ignimbrites and host limestone is a subvertical fault

with oblique and subhorizontal striations that suggest

reactivation after the collapse (Fig. 4). At the south-

eastern arcuate segment, near Quetzalapa, lag breccias

within the Tilzapotla ignimbrite indicate the proximity

of a volcanic vent. The occurrence in this area of

hydrothermal alteration zones and sulfide deposits,

related to porphyritic sub-volcanic rocks (Rivera-

Carranza et al., 1998) (La Mina AuPb mine), is also

suggestive of the nearness of the structural margin. At

the southwestern rectilinear margin, there are sulfide

vein deposits (Huitzuco Hg District) and hydrother-

mal alteration zones associated with the caldera vol-

canic activity. At the northeastern limit of the caldera,

the proximity of the structural margin is indicated by

the occurrence of lag breccias and collapse mega-

blocks near Tilzapotla. There are also fault segments

with left lateral to dip-slip kinematic indicators affect-

ing ignimbrites, collapse breccias and Cretaceous

rocks (Fig. 4). The presence of sub-volcanic bodies,

north and south of Buenavista (Fig. 3), is also indic-

ative of the structural margin in this area.

Differential erosion in the caldera area produced an

inverted relief, with higher elevations for the top of

volcanic in-fill sequences than for the surrounding

Mesozoic rocks. This is also due to the uplift related

to the resurgence (Fig. 5). Differences in elevations

of the contact between the collapse ignimbrite and

overlying volcanic units delineate a resurgent block

that occupies more than a half of the caldera area

(Figs. 3 and 5). The block is bordered by NW-trending

lineaments located north of Huitzuco and south of

Tilzapotla, and can be recognized in aerial photo-

potla

ange

D.J. Moran-Zenteno et al. / Journal of Volcanology and Geothermal Research 136 (2004) 97119102Fig. 5. Schematic NS and EW trending sections through the Tilza

The ring fault projection is shown vertical for convenience. Note chuplift of the central block.caldera. Vertical scale is exaggerated to enhance resurgence features.

s in altitude of Tilzapotla and Rodarte ignimbrites produced by the

middle part of the of the Tilzapotla ignimbrite. There is

a previously reported KAr date of 31.9F 1 Ma for a

D.J. Moran-Zenteno et al. / Journal of Volcanology and Geothermal Research 136 (2004) 97119 103graphs and satellite images. The northwestern and

southeastern edges of the block seem to coincide with

the caldera margin. The southwestern block boundary

corresponds to a NW-trending fault zone with oblique

to vertical striations and kinematic indicators of a

normal component (Fig. 4). A series of dikes and

groups of volcanic necks of dacitic composition are

intruded along the shear zone. The low position of

post-collapse ignimbrites to the south, defines a moat

between the caldera margin and the uplifted block.

The northeastern bounding fault of the uplifted block

is mostly covered by lava flows and related hypabys-

sal intrusions attributed to the resurgence and can be

inferred by the abrupt change in the elevation of the

base of the post-collapse sequence (Figs. 3, 4 and 5).

The NE and SW rectilinear boundaries of the

Tilzapotla caldera are nearly parallel with regional

NW-trending tectonic lineaments recognized in the

region (Fig. 2A). These lineaments mainly correspond

with left-lateral faults active in late Eocene time

(Alaniz-Alvarez et al., 2002; Moran-Zenteno et al.,

2003). The southwestern rectilinear boundary of the

Tilzapotla caldera is collinear with the NW-trending

Los Amates fault, located west of the caldera (Fig. 4).

This feature is also near collinear with the Tetipac fault

(Fig. 2A), which is one of the most striking regional

tectonic features and extends more than 50 km north-

west of the city of Taxco (Rivera-Carranza et al., 1998;

Alaniz-Alvarez et al., 2002). The Tetipac Fault and

other near parallel structures (i.e. Chichila and Tux-

pan) show evidence of a complex kinematic evolution

which includes the reactivation of pre-Eocene struc-

tures. Los Amates fault zone is characterized by

subvertical fault planes with a complex kinematic

history with preserved vertical to oblique striations

(Figs. 3 and 4). Based on structural observations and

age data, Alaniz-Alvarez et al. (2002) concluded that

most of the regional NW-trending structures had left

lateral displacement in late Eocene time (38 to 33 Ma).

Rivera-Carranza et al. (1998) and Fitz-Daz (2001)

reported a NW-trending fault located 10 km southwest

of the caldera ring. This fault hosts a wide (20 to 40 m)

pyroclastic dike very similar in composition and min-

eralogy, to the Tilzapotla ignimbrite. The occurrence

of left-lateral faults affecting the dike is suggestive of

the contemporaneity between the strike-slip tectonics

and the volcanic activity of the caldera. Other NWfault segments recognized northwest and south of thebiotite concentrate of the intra-caldera Tilzapotla ig-

nimbrite (Alba-Aldave et al., 1996; Moran-Zenteno et

al., 1999) (sample SOL5), but replicate analyses of the

K content of this sample provided a corrected date ofcaldera display left lateral to oblique kinematic indi-

cators (Fig. 4). These faults also affect the volcanic

rocks of the caldera indicating that strike-slip faulting

continued after the caldera formation.

3. Volcanic stratigraphy

The most extensive volcanic cover related to the

Tilzapotla caldera is continuously distributed within

the caldera, over an area of 700 km2. Outcrops of the

outflow facies are discontinuously distributed to the

northeast and south of the caldera (Figs. 2B and 3).

Due to the regional dissection, the outflow facies

represent less than 30% of the total area of the volcanic

cover and only incomplete sections are preserved.

The base of the volcanic sequence is not exposed

within the structural margin of the Tilzapotla caldera. A

maximum exposed thickness of 1500 m, including the

caldera forming ignimbrite and lava flows of the resur-

gence, has been estimated for the central segment of the

caldera. Pyroclastic and lava flows were grouped based

on lithological similarities or when contrasting flow

units of a continuous sequence could not be separated

due to scale restrictions (Figs. 3 and 6). The outflow

volcanic sequence has a maximum preserved thickness

of 50 m in the proximal facies. Ash fall deposits are

preserved only where they lie between ignimbrites. Pre-

collapse pyroclastic deposits were only observed in

restricted outcrops near the eastern ring segment (km

147.5, highway 95). They are represented by a 3-m-thick

layer of altered ash fall tuff that underlies the extra-

caldera facies of the Tilzapotla ignimbrite.

KAr, RbSr and ArAr dates from the volcanic

units of the Tilzapotla caldera obtained in this study are

listed in Tables 1A, 1B and 1C. KAr dates for the

whole volcanic sequence range from 35.5 to 32.6 Ma,

whereas those for the ignimbrites representative of the

climatic event range from 35 to 34 Ma. A 34.26F 0.09Ma ArAr date was calculated from individual anal-

yses of 21 sanidine grains (Table 1B, Fig. 7) from the35.1F1 Ma. This corrected date is more compatible

stern

ness o

D.J. Moran-Zenteno et al. / Journal of Volcanology and Geothermal Research 136 (2004) 97119104with the well-defined group of dates obtained in this

study from the Tilzapotla ignimbrite and from the

overlying volcanic rocks.

In the Taxco region, the sequence included within

the Tilzapotla unit by De Cserna and Fries (1981)

lacks some characteristic features of the Tilzapotla

unit in the Buenavista-Tilzapotla area, namely, an

Fig. 6. Generalized composite stratigraphic sections of the central ea

caldera section observed south of Valle de Vazquez. Since the thickabundant crystal content and the presence of conspic-

uous euhedral biotite. This fact and the presence of lag

breccias, as well as lava flows and sub-volcanic rocks

in the Taxco area, are suggestive of a source close to

this area rather than a distal facies of the Tilzapotla

caldera ignimbrite. Age inferences of De Cserna and

Fries (1981) for the Tilzapotla Rhyolite were based

on dates carried out in the Taxco area (35.5F1.2 and36.9F 1.3 Ma for sanidine and whole rock fractionsof the same sample, respectively), but not in the

Tilzapotla area. Additionally to the ignimbrites dated

by De Cserna and Fries (1981), Alaniz-Alvarez et al.

(2002) reported KAr ages for ignimbrites and rhy-

olite lavas of the Taxco area, showing that the most

voluminous silicic volcanism in this area occurred

between 32 and 31 Ma.

3.1. Pre-caldera rocks

Volcanic rocks of the Tilzapotla caldera uncon-

formably overlie deformed Cretaceous marinesequences that crop out widely in the region. The

most extensive outcrops correspond to the platform

limestone beds of the Albian-Cenomanian Morelos

Formation and Aptian-Albian evaporitic beds of the

Huitzuco Formation (Fries, 1960, 1966; De Cserna

et al., 1980; Hernandez-Romano et al., 1997; Her-

nandez-Romano, 1999). There are also extensive

and the western zones of the Tilzapotla caldera, as well as the extra-

f volcanic units is variable, those indicated are only representative.outcrops of the terrigenous beds of the Turonian-

Maestrichtian Mexcala Formation. The most impor-

tant tectonic structures affecting these units are

NNW- to NNE-oriented folds and NW and NS

regional lateral and normal-oblique faults (Fig. 2A).

Cretaceous rocks are unconformably overlain by

PaleoceneEocene fluvial deposits of the Balsas For-

mation cropping out northwest and southwest of the

caldera (Fig. 3).

3.2. Volcanic rocks associated with the caldera

collapse

3.2.1. Tilzapotla ignimbrite

3.2.1.1. Intra-caldera facies. The intra-caldera fa-

cies of the Tilzapotla ignimbrite is represented by a

massive sequence of dacitic, moderately to densely

welded tuffs. It includes several pyroclastic flow

units with similar petrographic characteristics and

poorly defined contacts among them. In the south-

Rock

ignim

ignim

ignim

D.J. Moran-Zenteno et al. / Journal of Volcanology and Geothermal Research 136 (2004) 97119 105Table 1A

KAr and ArAr and RbSr dates

Sample Location Mineral

Tilzpotla ignimbrite

Sol 5 99j11V57U biotite18j19V33U

Sol 9 99j10V58U biotite18j21V15U

Tz25-98 99j14V54U biotiteeastern part of the caldera, an exposed thickness of

600 m has been estimated for this unit. Despite the

relatively deep incision of the drainage, the base of

the unit is not exposed within the ring fault.

Typically, the Tilzapotla ignimbrite is represented

by a vitric-crystal tuff with a crypto- to microcrys-

talline groundmass of quartz and plagioclase that

includes ghosts of spherulites and small crystals of

FeTi oxides and zircon. Although most of the

18j03V24UTz145-01 99j24V30U biotite ignim

10j33V51USOL 2 99j10V39W sanidine ignim

18j22V33W

Rodarte ignimbrite

Tz187-01 99j13V34U plg ignim18j26V32U

Hypabyssal and

El Salto lava flows

Tz4-99 99j72V42U sanidine rhyol18j43V45U

Tz17-99 99j17V06U plg ande18j26V37U

Tz18-99 99j17V61U plg ande18j26V41U

Tz 63-02 99j17V10W plg ande

18j27V05WTz 62-02 99j17V09W plg ande

18j17V09W

Sample Location Mineral Rb (p

Coxcatlan granodiorite

Bv 21 99j27V29W biotite 693Bv 21 18j29V47W WR 103

KAr and ArAr dates obtained in this study for different volcanic unit

Geoqumica Isotopica (LUGIS) at the National University of Mexico (UNa Data in Table 1B.b Data in Table 1C.40Ar* (mol/g) K (%) Age (Ma)

brite 4.389 10 10 7.14 35.1F1.0

brite 3.85110 10 6.45 34.1F1.1

brite 4.022 10 10 6.62 34.7F 1.0groundmass seems to have been originally vitroclas-

tic, there are no preserved fractions of unaltered

glass. The phenocryst fraction includes quartz, bro-

ken plagioclase, minor sanidine, and conspicuous

euhedral biotite. The lithic fraction is dominated

by fragments of crypto-crystalline texture and, in a

minor proportion, by porphyritic lava and sub-vol-

canic fragments. Phenocrysts range from 15 to 50

(vol.) %, being quartz and plagioclase the most

brite 3.843 10 10 6.40 34.3F 1.5

brite Single crystal

ArAr datea34.26F 0.1

brite 0.154 10 10 0.27 32.6F 2.5

ite 2.742 10 10 4.41 35.5F 1.0

site 0.307 10 10 0.51 34.4F 1.4

site 0.298 10 10 0.52 32.8F 1.6

site ArAr plateau

ageb32.75F 0.1

site ArAr isochron

ageb33.43F 0.1

pm) Sr (ppm) 87Sr/86Sr Age (Ma)

10 0.793522

193 0.705937 32.18F 1

s. KAr dates were carried out in the Laboratorio Universitario de

AM). ArAr analytical data of sample Sol 2.

grains

40

3.

3.

3.

3.

3.

3.

3.

3.

3.

4.

4.

4.

4.

4.

4.

4.

4.

D.J. Moran-Zenteno et al. / Journal of Volcanology and Geothermal Research 136 (2004) 97119106Table 1B49Ar39Ar laser total fusion single-crystal age data for 21 sanidine

Tilzapotla ignimbrite

Unit 39Ark (mol) Radiogenic yield (%)

Tilzapotla, 1.90e 14 97.399j10.6548V, 3.09e 14 87.218j22.5526V 2.16e 14 97.7

2.59e 14 97.13.96e 14 91.74.22e 14 97.13.67e 14 98.34.15e 14 97.14.34e 14 97.62.23e 14 98.53.17e 14 98.33.36e 14 97.93.80e 14 89.11.86e 14 95.42.29e 14 96.43.84e 15 74.31.53e 14 95.2abundant components. The size of phenocrysts of the

intra-caldera facies reaches up to 4 mm, while the

size of lithic fragments is highly variable, depend-

ing on the position with respect to the caldera

structural margin. Lag breccias, including vitrophy-

ric fragments altered to zeolites, were observed in

Tilzapotla and Quetzalapa areas. Two characteristic

lithic components are anhydrite and limestone frag-

ments, derived from the surrounding Cretaceous

sequence.

The Tilzapotla ignimbrite conformably underlies

the pyroclastic rocks of the Rodarte ignimbrite. KAr

and ArAr dating of samples collected in three

different localities yielded ages ranging from 34.3 to

35.3 Ma (Tables 1A, 1B and 1C, Fig. 7). KAr dates

in biotite concentrates (35.1F1 and 34.1F1.4 Ma)are undistinguishable, within the error, from ArAr

dates obtained from individual sanidine crystals

(34.3F 0.09 Ma). Two additional KAr determina-

1.73e 14 96.8 4.9.96e + 15 83.4 4.

2.25e 14 79.6 4.3.08e 14 99.6 4.

Analyses in italics are not used to calculate the weighted mean age.

ArAr analytical data of samples Tz 63-02 and Tz 62-02. ArAr dates w

Denver (see Appendices A and B for analytical procedures).of sample Sol 2, J = 0.004784F 0.25%

Ar*/39Ark K/Ca K/Cl Age (MaF 1r)

975 60.46 88.89 34.06F 0.07979 75.70 53.22 34.10F 0.08984 60.75 96.53 34.14F 0.06984 66.62 91.16 34.14F 0.06991 73.96 109.29 34.20F 0.06994 66.31 111.23 34.23F 0.05996 65.49 120.19 34.24F 0.05997 67.93 91.41 34.25F 0.05998 66.09 120.48 34.26F 0.05001 70.22 92.94 34.29F 0.06002 73.10 125.31 34.29F 0.05002 75.64 104.6 34.30F 0.06005 68.97 76.98 34.32F 0.07008 69.16 117.79 34.35F 0.07010 65.23 51.73 34.36F 0.07016 3.80 47.30 34.42F 0.26017 70.22 48.85 34.42F 0.08tions of the extra-caldera facies yielded 34.3F 1.5 and34.7F 0.9 Ma.

3.2.1.2. Extra-caldera facies. Due to the prolonged

erosion in the region since the caldera formation, the

extra-caldera facies of the Tilzapotla ignimbrite is

exposed in relatively isolated outcrops showing

incomplete sections. Distal outcrops are distributed

east and south of the caldera, reaching distances up

to 36 km from the caldera margin (Fig. 2B).

Proximal sections of the extra-caldera facies are

exposed near Amacuzac, north of Buenavista, and

east and south of the caldera margin. South of

Amacuzac, ignimbrites are exposed continuously

from the margin to a distance of 8 km to the

northwest (Fig. 3). In this area, the extra-caldera

facies unconformably overlies pre-caldera conglom-

erates of the Balsas Formation and has a maximum

thickness of 50 m, that gradually decreases to the

017 41.05 120.05 34.42F 0.06028 60.86 90.09 34.51F 0.13030 60.46 99.01 34.53F 0.11075 75.36 134.59 34.91F 0.05

Weighted mean

age = 34.26F 0.09 Ma

ere carried out at the Thermochronology Laboratory of the USGS

k 10

1305

1521

3535

2325

3559

2245

teps

4868

7529

9966

0456

3340

2592

8751

eter

D.J. Moran-Zenteno et al. / Journal of Volcanology and Geothermal Research 136 (2004) 97119 107Table 1C

ArAr samples Tz 63-02 and Tz 62-02

Unit/location T (jC) % 39Arof total

Radiogenic

yield (%)

39Ar

(mol

TZ 63-02, plagioclase, J = 0.004751F 0.25%, wt. = 239.3 mgEl Salto, 900 9.6 80.1 0.05

18j27.090V, 1000 22.8 88.0 0.1299j17.166V 1100 28.8 91.4 0.15

1200 24.9 81.5 0.13

1300 13.8 75.4 0.07

Total gas 100.0 84.9 0.53

90.36% of gas on plateau in 1000 through 1300 s

TZ 62-02, andesite matrix, J = 0.004755F 0.25%, wt. = 241.1 mgEl Salto, 750 9.5 91.5 0.34

18j27.137V, 850 38.5 98.2 1.3999j17.144V 900 24.0 98.2 0.86

950 14.6 96.8 0.53

1000 7.0 93.9 0.25

1100 6.4 89.3 0.23

Total gas 100.0 96.5 3.62

Ages calculated assuming an initial 40Ar36Ar = 295.5F 0.Ages of individual steps do not include error in the irradiation paramnorth. In the caldera margin, there is an abrupt

increase in the ignimbrite thickness to f 600 m. Abiotite concentrate from this area yielded a KAr date

of 34.3F 1.5 Ma. North of Buenavista, the extra-caldera facies of the Tilzapotla ignimbrite contains

abundant fragments of limestone (f 30%) and minorvolcanic and sub-volcanic fragments in a matrix-sup-

ported structure.

The best preserved sections of relatively distal

facies underlie lava flows of the Huautla volcanic

center, east of the Tilzapotla caldera (Fig. 6). South of

Valle de Vazquez, 12 km from the caldera margin, the

section is constituted by three slightly welded ignim-

brite units. An ash fall layer separates the two upper

flow units. The three layers are crystal-rich and

contain phenocrysts of euhedral biotite. A 34.2 Ma

ArAr date in biotite was preliminary reported by

Campa-Uranga et al. (2002) for an ignimbrite sample

collected in this area. The most distal outcrop of the

Tilzapotla ignimbrite was found 36 km south of the

caldera margin at km 200 of highway 95, leading to

Acapulco (Fig 2B). A 34.7F 1 Ma KAr age obtainedfrom a biotite concentrate of this ignimbrite (Table 1A)

No error is calculated for the total gas age.12)

40Ar*/39Ark Apparent

K/Ca

Apparent

K/Cl

Apparent age

(MaF 1r)

3.959 0.07 1949 33.62F 0.223.861 0.06 3069 32.80F 0.083.826 0.06 2356 32.50F 0.173.856 0.06 623 32.75F 0.243.865 0.06 453 32.82F 0.143.860 0.06 1786 32.78

Plateau age = 32.75F 0.11Isochron age = 32.52F 0.36

3.970 2.38 1438 33.74F 0.054.036 1.47 0 34.29F 0.033.954 0.67 0 33.60F 0.063.972 0.45 6792 33.75F 0.044.053 0.42 2999 34.44F 0.064.074 0.46 1296 34.62F 0.064.004 1.08 1422 34.03

No plateau Isochron

age = 33.43F 0.13

J.supports its relationship with the caldera collapse

event.

3.2.1.3. Collapse breccia. Collapse breccia deposits

were identified mainly in the eastern and southeastern

boundaries of the caldera (Fig. 3). They are repre-

sented by discontinuous exposures of breccia accumu-

lations interlayered with the Tilzapotla ignimbrite, as

well as relatively isolated mega-blocks. Exposed sec-

tions of meso-breccia interlayered with pyroclastic

beds, located at the eastern ring fault zone, have a

minimum thickness of 100 m. The thickest sections

crop out along the deep cuts of highway 95, near the

locality of Coaxintlan. At the southeastern segment of

the caldera ring, near the village of Quetzalapa, there

are accumulations of recrystallized limestone and

anhydrite breccia, as well as blocks of marble, several

meters in length, embedded in the intra-caldera ignim-

brite (Fig. 8). At the northern segment of the ring fault

zone, there are also large blocks of limestone and

anhydrite, embedded in Tilzapotla ignimbrite. A gyp-

sum quarry in Tilzapotla corresponds to one of the

largest collapse blocks contained in the ignimbrite.

Fig. 7. Graphic representations of ArAr data obtained from samples: (a) Sol 2 of the Tilzapotla ignimbrite; (b) Tz 62-02 and (c) Tz 63-02 of the

El Salto lava flows. The Sol 2 age was obtained from the total fusion of 21 single crystals of sanidine. See data in Tables 1B and 1C and

analytical procedures in Appendix A.

D.J. Moran-Zenteno et al. / Journal of Volcanology and Geothermal Research 136 (2004) 97119108

bearing andesites and comprises subordinated interca-

he me

D.J. Moran-Zenteno et al. / Journal of Volcanology and Geothermal Research 136 (2004) 97119 1093.3. Post-collapse volcanic rocks

Post-collapse rocks overlying the Tilzapotla ig-

nimbrite are exposed mainly at high elevations in the

eastern and south-central parts of the caldera, al-

though there are some preserved outcrops at lower

elevations in the southern moat zone (Figs. 3 and 5).

They include recognizable ignimbrite flow units and

debris flow deposits with irregular contacts among

them, probably due to fluvial erosion. Based on

recognizable lithological characteristics the sequence

was divided in three informal units: Rodarte ignim-

Fig. 8. Blocks of limestone embedded in the Tilzapotla ignimbrite of t

ring fault.brite, Rodeo formation and Las Mesas formation.

The Rodarte ignimbrite is represented by a vitroclas-

tic ignimbrite sequence that includes several flow

units, ranging from nonindurated to moderately

welded, and contains pumice fragments and biotite

phenocrysts. La Mesas formation is constituted by a

sequence of conglomerate layers and debris flow

deposits. The Gallego formation overlies the Rodarte

ignimbrite and is formed by a thick sequence of

densely welded rheomorphic ignimbrites, vitrophyre

flow units and dacitic lava flows and contains

phenocrysts of plagioclase, sanidine, biotite and

quartz. The presence of some tilted segments of this

sequence dipping toward the caldera margin, at the

eastern boundary of the uplifted block, is indicative

of their emplacement previous to the resurgence. No

suitable material was found for dating this unit, but

its age is constrained by the overlaying El Saltoformation (3432 Ma) and the underlying Tilzapotla

ignimbrite (3534 Ma).

3.4. Resurgence-related volcanic rocks

Lava flows and hypabyssal rocks overlying and

intruding the Tilzapotla ignimbrite and the post-col-

lapse units are distributed in different areas of the

caldera margin and in the uplifted central block. Lava

flows (El Salto formation) range in composition from

hornblende bearing dacites to ortho- and clinopyroxene

ga-breccia interval from localities at the southeastern segment of thelations of debris flow deposits. KAr and ArAr ages

of this sequence range from 34.4 to 32.8Ma (Tables 1A

and 1C). Hypabyssal rocks form dikes and volcanic

necks emplaced mainly near the caldera margin and

along the faults limiting the uplifted block. Although

hypabyssal rocks have a wide range in composition,

from andesites to rhyolites, their spatial relationships

with El Salto formation are, in most cases, suggestive

of being their feeding dikes. There are also coarse-

grained intrusions of biotite bearing granodiorite locat-

ed near the western margin of the caldera (Coxcatlan

and Buenavista localities), although the ArAr age

reported for the Buenavista intrusion (35.9F 0.5Ma;Meza-Figueroa et al., 2003) does not support a con-

nection with the resurgence stage.

On the eastern margin of the intra-caldera area,

exposures of conglomerates and volcanic debris flow

deposits include some thin layers of altered greenish

Table 2

Major element chemical analyses

Sample SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5 LOI Total

El Salto lava flows

Bv20 57.38 1.17 17.87 7.57 0.11 3.63 6.85 3.43 1.38 0.29 1.06 100.74

Tz17-98 62.14 0.65 17.00 5.30 0.08 2.53 5.22 3.66 1.94 0.16 1.49 100.13

Tz18-98 63.00 0.97 15.7 5.62 0.192 2.17 4.99 3.2 2.11 0.159 1.82 99.94

Tz19-98 61.87 0.73 16.79 5.56 0.10 2.44 5.21 3.81 1.79 0.18 1.54 100.03

Tz20-98 59.91 0.63 17.24 5.98 0.10 2.72 5.61 3.85 2.01 0.16 1.97 100.17

Tz21-98 60.39 0.68 18.08 6.34 0.06 1.29 4.21 3.49 2.45 0.15 2.53 99.67

Tz101-00 61.09 0.95 18.11 6.75 0.05 1.41 5.29 3.53 1.65 0.25 1.67 100.70

Tz171-01 61.33 0.86 16.95 5.78 0.05 2.06 5.01 3.20 2.21 0.32 2.10 99.86

Tz193-01 62.73 0.69 16.83 5.40 0.07 2.25 4.94 4.16 2.04 0.19 0.90 100.20

Tz57-02 60.66 0.65 17.51 5.72 0.09 2.60 5.77 4.27 1.51 0.20 1.27 100.25

Tz77-02 60.73 0.78 17.73 6.69 0.08 1.30 5.00 3.96 2.07 0.26 1.74 100.34

Tz80-02 61.09 0.60 16.32 6.24 0.12 2.41 5.17 3.61 1.95 0.16 2.77 100.44

Tz62-02 59.52 0.76 17.35 6.36 0.13 2.29 5.79 4.28 2.05 0.25 1.62 100.40

Tz17a-02 57.26 0.72 18.11 6.83 0.10 3.29 5.25 4.28 1.66 0.20 1.83 99.53

Tz190b-01 64.96 0.60 16.46 4.88 0.14 1.01 4.05 4.03 2.77 0.17 1.17 100.24

Tz190c-01 64.26 0.60 16.33 4.75 0.05 1.20 3.97 3.39 2.85 0.16 2.61 100.17

Hypabyssal and plutonic rocks

Tz4-99 69.39 0.38 14.47 3.41 0.087 0.636 2.42 2.63 4.71 0.09 2.44 100.68

Tz135-01 60.33 0.82 17.38 6.23 0.11 2.60 5.50 3.61 1.65 0.22 1.49 99.93

Tz136-01 70.74 0.29 14.56 2.57 0.07 1.11 2.42 3.52 3.59 0.11 1.10 100.07

Tz75-02 60.79 0.71 16.89 6.20 0.10 2.24 5.35 4.09 2.05 0.22 1.59 100.23

Tz28-03 55.16 0.73 17.97 6.77 0.18 2.55 6.48 3.71 2.11 0.19 3.86 99.71

Bv12 61.35 0.44 17.35 5.30 0.10 1.88 3.40 3.53 3.28 0.15 4.57 101.35

Bv14 65.69 0.46 14.43 4.49 0.05 1.55 3.05 2.59 1.55 0.17 6.36 100.37

Bv17 64.84 0.41 16.81 4.95 0.06 0.69 2.78 3.35 2.84 0.13 3.16 100.02

Bv21 66.32 0.64 15.53 4.76 0.09 1.39 3.44 3.54 3.38 0.11 1.17 100.36

Tz48-99 68.24 0.51 14.33 4.96 0.06 0.92 3.39 2.10 4.01 0.10 2.20 100.80

Tz126-01 69.17 0.36 15.57 3.92 0.02 0.35 2.46 4.00 3.19 0.08 1.23 100.35

Tz46b-02 67.36 0.54 14.29 4.41 0.05 1.75 3.14 3.45 3.53 0.11 1.50 100.13

Tilzapotla ignimbrite

SOL2 64.09 0.49 14.06 3.47 0.04 1.04 5.84 3.54 2.74 0.07 5.39 100.75

SOL5 68.32 0.48 14.34 3.59 0.02 1.40 3.67 1.99 4.14 0.05 4.68 102.68

Hz1 65.56 0.55 14.97 4.61 0.06 1.71 2.35 2.49 5.04 0.06 3.85 101.25

Hz2 67.74 0.55 14.84 4.28 0.11 1.55 3.03 3.37 3.55 0.07 1.89 100.96

Hz3 64.79 0.52 14.53 4.71 0.07 1.70 3.01 2.29 4.82 0.06 4.63 101.12

Tz25-98 68.50 0.37 15.03 3.28 0.036 0.58 2.7 3.02 5.251 0.085 1.00 99.86

Tz49-99 63.32 0.46 13.67 3.98 0.04 0.60 5.12 0.52 5.41 0.09 7.15 100.30

Tz105-00 67.48 0.53 14.97 4.54 0.08 1.49 3.78 3.23 3.74 0.17 0.77 100.77

Tz112-00 64.01 0.42 13.29 3.92 0.07 1.06 5.43 0.22 3.76 0.09 8.30 100.57

Tz112V-00 65.37 0.46 14.69 3.63 0.09 0.40 4.36 0.33 4.12 0.11 7.20 100.75Tz145-01 69.11 0.40 14.66 3.56 0.04 1.19 2.13 2.84 4.05 0.08 2.20 100.25

Rodarte ignimbrite

Tz53-99 70.84 0.39 12.49 2.28 0.075 0.976 2.97 2.84 3.99 0.078 3.11 100.04

Tz185-01 70.07 0.56 15.51 3.05 0.05 0.43 1.75 4.33 4.10 0.05 0.60 100.50

Tz187-01 69.83 0.40 14.73 3.78 0.05 0.56 2.52 2.66 4.55 0.05 1.10 100.22

Tz188-01 68.65 0.58 15.67 3.39 0.06 0.05 2.26 4.31 3.87 0.12 0.70 99.66

D.J. Moran-Zenteno et al. / Journal of Volcanology and Geothermal Research 136 (2004) 97119110

ash fall tuff. The most extensive outcrops are distrib-

uted near Coaxintlan (Fig. 3).

4. Major element and isotope geochemistry

some scattering within the dacite field, probably due to

some degree of alkali mobilization (Fig. 9). Most

differentiated rocks are related to the Tilzapotla ignim-

brite and post-collapse pre-resurgence sequences. In-

termediate rocks are mainly related to lava flows of the

resurgence. Sub-volcanic rocks display a more variable

Table 2 (continued)

Sample SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5 LOI Total

Gallego formation

Bv1 69.05 0.29 14.7 2.35 0.06 0.78 2.44 3.49 3.48 0.11 3.71 100.46

Tz53-99 70.84 0.391 12.49 2.28 0.08 0.98 2.97 2.84 3.99 0.08 3.11 100.04

Tz182-01 66.42 0.64 16.35 5.14 0.04 0.43 3.09 3.46 3.31 0.16 1.20 100.24

Tz17b-02 72.88 0.15 12.72 1.52 0.05 0.40 1.36 3.24 3.91 0.06 3.84 100.13

Tz17c-02 74.37 0.16 12.99 1.36 0.04 0.51 0.94 2.38 4.75 0.03 1.74 99.27

Tz27-03 66.73 0.66 15.28 4.85 0.05 1.09 2.90 3.35 3.22 0.16 2.10 100.34

Whole rock chemical composition of representative samples of the volcanic sequence in the study area. Analyses were carried out by XRF at the

Laboratorio Universitario de Geoqumica Isotopica (LUGIS) at UNAM.

D.J. Moran-Zenteno et al. / Journal of Volcanology and Geothermal Research 136 (2004) 97119 111Representative whole-rock chemical composition of

the volcanic rocks of the Tilzapotla caldera is presented

in Table 2. SiO2 content ranges from 57 to 76 (wt.) %,

with a dominance of rocks containing more than 60%

SiO2. Because of the lack of unaltered pumice or glass

fragments in the Tilzapotla ignimbrite, its composition

was estimated from whole rock analyses of samples

composed mainly of juvenile material. Hydrothermal

alteration zones, as well as accessory and accidental

fragments, were avoided to obtain the best approxima-

tion of the magma composition. Nonetheless, in the

TAS diagram the data of Tilzapotla ignimbrite displayFig. 9. TAS diagram showing the composition of representative samples ofcomposition ranging from andesite to rhyolite. Nor-

malized alkalis and silica data presented in the TAS

diagram (Fig. 9) show the compositional variation of

the entire sequence indicating a subalkaline affinity.

Biotite is a characteristic accessory mineral in rhyolitic

to dacitic rocks, whereas hornblende is common in

dacitic and andesitic lava flows of the resurgence.

Ortho- and clinopyroxene are common in andesites

with the lowest silica content.87Sr/86Sri in the sequence ranges from 0.7034 to

0.7066 (Table 3), with the youngest lava flows having

the lowest ratios (0.70340.7037), and the collapsedifferent volcanic units of the Tilzapotla caldera. See data in Table 2.

brite

2r

D.J. Moran-Zenteno et al. / Journal of Volcanology and Geothermal Research 136 (2004) 97119112Table 3

Sr, Nd and Pb isotopic data of selected samples of the Tilzapotla ignim

Sample Rb Sr Rb/Sr 87Rb/87Sr

87Sr/86Srand post-collapse ignimbrites the highest ratios

(0.70550.7066). Hypabyssal rocks display more var-

iable 87Sr/86Sri values (0.70370.7053). Although

Tilzapotla ignimbrite samples were carefully selected

for isotopic analyses, avoiding accessory lithic frag-

Lava flows of the resurgence

Tz 17-98 53 531 0.1 0.289 0.703773 FTz 18-98 67 457 0.15 0.424 0.703890 FTz 20-98 37 616 0.06 0.174 0.703534 F

Ignimbrites

Sol 2 113 255 0.44 1.161 0.707132 FTz 25-98 197 227 0.87 2.511 0.706688 FTz 53-99 131 133 0.98 2.85 0.707227 F

Hypabyssal rocks

Bv12 98 374 0.26 0.758 0.704097 FBv17 89 471 0.19 0.547 0.704044 FTz 4-98 223 141 1.58 4.576 0.707560 FTz 48-99 194 213 0.91 2.635 0.706480 FTz 136-01 143 208 0.69 1.989 0.705518 F

Sample 206Pb/204Pb 207Pb/204Pb 208Pb/204Pb

Lava flows of the resurgence

Tz-17 plag 18.699 15.605 38.516

Tz-17 WR 18.733 15.604 38.515

Tz-18 plag 18.657 15.593 38.445

Tz-18 WR 18.798 15.644 38.626

Tz-20 plag 18.668 15.590 38.451

Tz-20 WR 18.675 15.590 38.440

Hypabyssal and Rodarte ignimbrite

Bv-17 plag 18.745 15.615 38.602

Bv-17 WR 18.789 15.599 38.557

Tz-53 WR 18.970 15.650 38.811

Tz-4 sanidine 18.779 15.644 38.696

Tz-4 WR 18.933 15.653 38.822

The Sr, Nd and Sm isotope ratios were determined using a Finnigan MAT26

at the Isotope Geochemistry Laboratory (LUGIS) of the National Unive

collection mode. Analysis of Rb were carried out using a single collector N

and measured as metallic ions. Values of 2r(m) (2r(m) = 2rabsMn) were calcand 20 for Sm. The measured 87Sr/86Sr values were normalized to an 86Sr/88

of 0.7219. The 87Sr/86Sr of the NIST-SRM987 and 143Nd/144Nd of the La

n= 237) and 0.511878F 21 (F 1rabs, n= 134). Pb samples were loaded w100 individual measurements. Laboratory mean values of standard NIST-N208Pb/204Pb = 36.51 0.07%, n= 23) (0.13% fractionation per mass unit). P

Relative uncertainties for 87Rb/86Sr was F 2% and for 147Sm/144Nd Fabundances was F 4.5%, F 1.8%, F 3.2%, and F 2.7%, respectively. To0.2719 ng for Sm, 1.0522 ng for Nd. Chemical blank for Pb was 94, lava flows of the resurgence and hypabyssal rocks of the study area

mean87Sr/86Sri

Sm Nd 147Sm/144Nd

143Nd/144Nd

2rmean143Nd/144Ndiments of limestone and secondary calcite precipitates,

the possible influence of carbonate impurities in the Sr

isotopic signatures cannot be completely discarded.

Given the 87Sr/86Sr obtained (0.707320.70735) and

the Sr abundance range (200500 ppm) for Creta-

13 0.703634 5.97 26.58 0.133 0.512858 F 5 0.51282810 0.703685 5.67 28.14 0.122 0.512820 F 6 0.51279310 0.703450 3.59 17.04 0.130 0.512900 F 6 0.512871

15 0.706586 7.66 32.63 0.142 0.512571 F 4 0.5125399 0.705476 5.99 27.85 0.130 0.512586 F 5 0.51255715 0.705851 8.49 43.19 0.118 0.512588 F 5 0.512562

17 0.703731 2.40 10.46 0.133 0.512778 F 5 0.51274815 0.703780 10.96 51.59 0.128 0.512769 F 12 0.51274011 0.70535 8.73 39.01 0.135 0.512631 F 5 0.5126019 0.705208 6.41 30.43 0.127 0.512598 F 4 0.51257011 0.704558 4.13 18.74 0.106 0.512580 F 5 0.512556

2 mass spectrometer equipped with eight faraday collectors, installed

rsity of Mexico. The isotopic measurements were made in a static

BS mass spectrometer. Rb, Sr, Sm and Nd, were loaded as chlorides

ulated from 60 individual isotopic determinations for Rb, Sr and Nd

Sr value of 0.1194, and those of 143Nd/144Nd to an 146Nd/144Nd ratio

Jolla-standard throughout this study were 0.710235F 18 (F 1rabs,ith a mixture of silica gel and phosphoric acid and runs consisted of

BS981 (Pb) (206Pb/204Pb = 16.89 0.04%, 207Pb/204Pb = 15.43 0.05%,

b isotope ratios presented in table are present-day values.

1.5% (1r). Relative reproducibility (1r) for Rb, Sr, Sm and Ndtal procedure blanks were 0.150.59 ng for Rb, 0.5731 ng for Sr,

pg.

tios o

the L

D.J. Moran-Zenteno et al. / Journal of Volcanology and Geothermal Research 136 (2004) 97119 113ceous limestone, the differences observed between the

Tilzapotla ignimbrite and the resurgence lava flows are

greater than expected from the sole influence of the

small amount of carbonate impurities acquired during

the eruption and later remobilizations. Differences

were also observed between Nd isotopic signatures

of the collapse and post-collapse ignimbrites (143Nd/144Ndi = 0.512560.51263) with respect to those of

the lava flows of the resurgence (143Nd/144Ndi =

0.512790.51287; Table 3, Fig. 10). Analyzed hyp-

abyssal rocks display two groups of 87Sr/86Sri and143Nd/144Ndi values indicating isotopic affinity with

both ignimbrites and lava flows of the resurgence.

Although common Pb isotopic signature in feldspars is

less sensitive to the influence of limestone impurities

Fig. 10. 87Sr/86Sri 143Nd/144Ndi diagram showing initial isotopic ra

Initial ratios were calculated for t = 34Ma. Analyses were carried out induring the eruption and later fluid remobilization, Pb

isotopic ratios carried out in plagioclase concentrates

of hypabyssal rocks (206Pb/204Pb = 18.77918.745)

and lava flows (206Pb/204Pb = 18.65718.699) display

recognizable differences (Table 3). This variability

suggests the input into the magma chamber of more

primitive magmas related to the resurgence stage.

5. Discussion

5.1. Regional stratigraphic and tectonic implications

The geochronology and distinctive features in the

petrography of the Tilzapotla ignimbrite allow to

differentiate it from other silicic ignimbrites in the

region. Although there is an overlap in the ages of thevolcanic sequences of the Tilzapotla and the Taxco

regions, they display significant mineralogic differ-

ences. These differences, as well as the occurrence of

lava flows and sub-volcanic rocks and some diachron-

ism in the peaks of volcanism in both areas (3534

Ma in Tilzapotla and 3132 Ma in the Taxco),

confirm the existence of two different volcanic cen-

ters. Stratigraphic relationships of the extra-caldera

Tilzapotla ignimbrite, with overlying extensive andes-

itic lava flows and pyroclastic deposits, on the eastern

flank of the Huautla range (Valle de Vazquez-China-

meca sector) indicate a younger volcanic center lo-

cated to the east, as was previously recognized by

Fries (1960), although, at present, there are no avail-

able geochronologic data to constrain the age range of

f representative samples of volcanic rocks of the Tilzapotla caldera.

aboratorio Universitario de Geoqumica Isotopica (LUGIS), UNAM.the volcanic sequences of the Huautla volcanic center.

Ages obtained from the Tilzapotla ignimbrite to-

gether with geochronologic data from the Taxco

volcanic field indicate that silicic volcanism in the

northern Sierra Madre del Sur is partially coeval with

the ignimbrite volcanism in the northern Sierra Madre

Occidental (SMO), where main episodes occurred

between 38 and 28 Ma (McDowell and Clabaugh,

1979; Aranda-Gomez et al., 2003). In the southern

Sierra Madre Occidental and the Mesa Central

regions, north of the Mexican Volcanic Belt, the

Tertiary silicic volcanism is slightly younger, ranging

in age from 30 to 21 Ma (Nieto-Samaniego et al.,

1999; Ferrari et al., 2002).

The Tilzapotla caldera represents a remarkable vol-

canic feature, not only for its size but also for its

tectonic framework. The rectilinear northeastern and

D.J. Moran-Zenteno et al. / Journal of Volcanology and Geothermal Research 136 (2004) 97119114southwestern boundaries are coincident with Tertiary

strike-slip faults active in late Eocene time. It has been

recognized that the shape and structure of the collapses

and resurgences are often controlled by pre-existing

normal faults (i.e. Lipman, 1984; Acocella et al., 2004),

but there are few reports of collapses accommodated by

vertical displacement along segments of coeval strike-

slip faults (i.e. Acocella et al., 2002). According to

Alaniz-Alvarez et al. (2002), extension in overstep

zones in this region favored the emplacement of

voluminous silicic magma. In the Taxco area, the initial

stages of magmatism occurred synchronous with left

lateral displacement of regional NW-trending faults

that induced NNW extension and subsidence (Alaniz-

Alvarez et al., 2002). Left-lateral displacement in faults

of the northeastern and southwestern segments of the

Tilzapotla caldera structural margin (Fig. 4) could have

produced extension that favored magma ascent to the

upper crust in the caldera area. Extension could have

occurred in two possible scenarios. In the first case,

extension is restricted to the termination of the two

faults. In the second case, extension is produced in the

releasing overstep by interaction of the two faults. Due

to the younger volcanic cover in the Huautla area, it is

not possible to determine the length of the northeastern

lineament toward the east to support the second sce-

nario. There is no preserved evidence of subsidence

associated with extension in the Tilzapotla caldera area,

previous to the tumescence stage. The volume of

ascending magma could have accommodated the ex-

tension inhibiting subsidence, as has been documented

in some extension zones (Parsons et al., 1998). Col-

lapse calderas in extension zones, associated with

strike-slip systems, have also been documented in the

central Andes (Riller et al., 2001).

Eocene strike-slip tectonics, in this region and in a

great part of southern Mexico (Moran-Zenteno et al.,

1999), contrasts with the regional OligoceneMiocene

coaxial extension associated with ignimbrites and large

collapse calderas in the Sierra Madre Occidental and

with other major calderas in the world (Lipman, 1984,

1997; Aguirre-Daz and McDowell, 1993; Aguirre-

Daz and Labarthe-Hernandez, 2003). In the Sierra

Alquitran region, 150 km farther south, there is also a

volcanic collapse structure that seems to have evolved

in a similar tectonic scenario (Moran-Zenteno et al.,

2003). Other silicic volcanic centers such as Taxco andHuautla evolved in step-overs of left lateral faultssegments, but no collapse calderas have been identified

yet (Alaniz-Alvarez et al., 2002).

5.2. Volcanic evolution

5.2.1. Initial tumescence stage

The distribution of the Cretaceous Morelos/Huit-

zuco beds, bordering the caldera in a higher position

with respect to the surrounding areas, is the main

indication of a tumescence stage related to the caldera

evolution. Due to the pre-existing folding, at the east

and southeast of the caldera, the structural attitude of

Cretaceous beds does not clearly portray the flanks of

an antiform. Nevertheless, fold plunges define a struc-

tural interference produced by the doming process

(Fig. 4). The northwest side of the elliptical dome

extends about 10 km from the caldera margin, indi-

cating an incomplete collapse with respect to the

doming produced by ascending magma (Fig. 1). This

also is suggested by the presence, in this area, of the

Coxcatlan intrusion (Fig. 3), which probably was an

apophysis of the magma chamber. Evidence of the first

stages of volcanism of the Tilzapotla caldera previous

to the collapse is fragmentary. In several areas, the

direct contact of the outflow facies of the collapse

ignimbrite over the pre-volcanic marine and fluvial

sequence is suggestive of the removal of initial ash fall

deposits by active erosion. At only few localities, east

of the caldera, a thin ( < 3 m) altered layer of ash fall

pyroclastic material was preserved below the first

ignimbrite accumulations. The small 35 Ma granodi-

orite intrusion close to Buenavista could also be a

manifestation of pre-collapse magmatism. Northeast

and south of the caldera the Tilzapotla ignimbrite

unconformably overlies Cretaceous limestone and an-

hydrite, as well as tilted beds of the Balsas Formation.

5.2.2. Collapse stage

The collapse stage and the first episodes of volu-

minous ash flows can be documented in the massive

intra-caldera sequence of the Tilzapotla ignimbrite and

by the presence of large blocks of marine limestone

and anhydrite, embedded in the ignimbrite near the

margin. Outflow facies of the Tilzapotla ignimbrite

extended a minimum distance of 36 km from the

caldera margin, but the general distribution area of

the ignimbrite and ash fall deposits cannot be deter-mined given the erosion of most of the extra-caldera

D.J. Moran-Zenteno et al. / Journal of Volcanology and Geothermal Research 136 (2004) 97119 115cover. Based on similar crystal abundance and com-

position, at least three main flow units and one ash fall

deposit, associated with the collapse ignimbrite, were

recognized in the extra-caldera sequences preserved

east of the caldera, in the Valle de Vazquez-Chinameca

sector (Fig. 6). The massive character of most of the

intra-caldera facies of the Tilzapotla ignimbrite pre-

clude the estimation of the number of the individual

flow unit deposits and speaks for their emplacement in

a short time interval. Slumping episodes of the caldera

inner wall are depicted mainly in the uppermost

stratigraphic levels of the ignimbrite, where they are

intercalated with meso- and mega-breccia lenses. Nor-

mal faults, near parallel to the structural margin,

cutting the breccia deposits in the caldera margin are

indicative of minor episodes of subsidence reactiva-

tion, after the main deposition episodes of intra-caldera

ignimbrite and breccias.

The size of the caldera and the structural relation-

ships along its margin are suggestive of an overall

piston subsidence of the caldera floor. Structural

discontinuities associated with NW-trending linea-

ments seem to have defined the nearly rectilinear

northeastern and southwestern segments of the caldera

structural margin (Fig. 4). There are indications in the

region of left-lateral displacement along NW-trending

faults, in late Eocene time (Alaniz-Alvarez et al.,

2002). Temporally and locally NW-trending faults,

limiting the Tilzapotla caldera, had a vertical compo-

nent associated with the collapse. Fault plane kine-

matic indicators, affecting the Tilzapotla ignimbrite,

show that the left-lateral displacement continued after

the collapse in the caldera area.

5.2.3. Volume estimation

The estimation of the total volume of the Tilzapotla

ignimbrite is restricted by the incomplete exposure of

the intra-caldera facies and the poorly preserved

outflow sheet. The maximum exposed thickness of

the intra-caldera facies, near the southeastern segment

of the caldera ring fault, is about 600 m. If we assume

a conservative thickness of 1000 m and a caldera area

of 550 km2 defined by the ring fault zone, we obtain a

minimum volume of 550 km3 for the intra-caldera

facies. This volume includes the host rock breccia

accumulations derived from the slumping and caving

of the caldera walls. The contrasting topography at thetime of the caldera formation and the erosion effects tothe original thickness make difficult a realistic esti-

mation of the outflow sheet volume. The preserved

outflow exposures suggest that they could have been

continuously distributed in an area of about 4500 km2.

The thickness of the preserved outflow sheet varies

from 50 m, near the northwestern caldera ring, to 5 m

in the more distal outcrops. Assuming an average

thickness of 10 m for the outflow sheet, a volume of

45 km3 can be estimated for the outflow facies. Given

these conservative assumptions and taking into ac-

count the caldera sizevolume correlation inferred by

Smith (1979), the total figure of 600 km3 must be

considered a minimum.

5.2.4. Post-collapse and resurgent stages

Ash flow units of the Rodarte ignimbrite display

evidence of post-collapse and pre-resurgence volca-

nism. Erosive contacts among pyroclastic flow units

are indicative of a reduction in the volcanic activity

following the major ash flow events related to the

caldera collapse. In the Mesa del Rodarte area, ignim-

brite layers are in a subhorizontal position but to the

south, at the margin of the uplifted block, they are

tilted to the northeast. This fact and the higher

topographic position of equivalent ignimbrite layers

over the central block are indicative of pre-resurgence

emplacement. The remnants of conglomerate and

agglomerate sequences of the Salitre formation over

the central block are indicative of fluvial and debris

flow accumulations, coeval with volcanism and pre-

vious to the resurgence stage. There are no remnants

of lacustrine sediments preserved within the caldera.

Lava flows of the El Salto formation are mainly

distributed on topographic highs in the central area of

the caldera, but they also display a continuous distri-

bution to lower topographic positions. The elongated

shape of the lava flows flanked by older units and the

intercalation of auto-breccias and debris flow depos-

its, suggest that they descended along relatively steep

narrow canyons.

The gradual change in composition of the resur-

gence lava flows of the Salto formation from dacitic to

more intermediate indicates the extrusion of magma

coming from deeper levels of a zoned magma cham-

ber or the input of new magma. The relatively large

difference in isotopic signatures between the collapse

ignimbrites and lava flows of the resurgence, as wellas the variability in isotopic ratios of hypabyssal

cides with the ring fault zone.

the El Salto Formation and one of the Tilzapotla

ignimbrite (Sol-2) were dated with 40Ar39Ar geo-

D.J. Moran-Zenteno et al. / Journal of Volcanology and Geothermal Research 136 (2004) 971191166. Conclusions

The distribution of volcanic features and thickness

variations, with respect to a large-scale elliptical

structure, indicate that the Tilzapotla ignimbrite is

associated to the climatic caldera-forming event.

An initial tumescence stage in the area produced a

NW-trending elliptical dome causing structural inter-

ference with pre-existing NNE to NNW folds. After

the collapse, the caldera underwent a resurgent pro-

cess characterized by the uplift of a central block

limited by NW-trending faults.

The NW-oriented elliptical structure was defined

by the tectonic control of coeval regional left-lateral

faults that coincide with the northeastern and south-

western margins of the caldera. The caldera floor

collapse was accommodated by temporal subvertical

displacements of these fault segments.

Differences in Sr, Nd and Pb isotopic signatures,

between collapse ignimbrites and volcanic rocks of

the resurgence, suggest that the latter are related to the

input of new more primitive magma to the magmarocks, favors the assumption of the chamber replen-

ishment by more primitive magma.

The most conspicuous indication of resurgence is

the NW elongated uplifted block in the central part of

the caldera (Figs. 4 and 5). The outline of the

resurgent block does not completely coincide with

the ring fault zone as in other resurgent calderas

characterized by piston subsidence (i.e. Lindsay et

al., 2001). The rectilinear SW and NE margins of the

uplifted block are near parallel to the major axis of the

caldera, but in an inner position relative to the

structural margin. The regional tectonic fabrics of

NW-trending regional faults, imposed by strike-slip

tectonics, seemed to have controlled the elongated

shape of the block (Lipman, 1984). Pre-existing

fractures in the caldera floor were probably the cause

of a block uplift style rather than a better defined

dome, involving the whole floor of the caldera. Most

lava flows and hypabyssal bodies have a source

associated to the resurgent block boundaries. Lava

flows and hypabyssal intrusions of the caldera are

located where the resurgence block boundary coin-chamber that produced the caldera resurgence.chronology (Fig. 7 and Tables 1B and 1C). The 250

180 Am size fractions of the andesite samples wereleached in 10% hydrochloric acid to remove second-

ary calcite. Phenocrysts were removed from the vol-

canic matrix using heavy liquids and magnetic

separation techniques. Plagioclase was also separated

for andesite sample TZ-63-02. A sanidine mineral

separate from the ignimbrite was produced using

magnetic separation, heavy liquids and handpicking

to a purity of >99%. The mineral and volcanic matrix

separates were washed in acetone, alcohol, and deion-

ized water in an ultrasonic cleaner to remove dust and

then re-sieved by hand using a 120-Am sieve.Approximately 250 mg of volcanic matrix and 10

mg of sanidine were packaged in copper and alumi-

num capsules, respectively, and sealed under vacuum

in quartz tubes. The samples were then irradiated for

20 h (package KD31) in the central thimble facility at

the TRIGA reactor (GSTR) at the U.S. GeologicalAcknowledgements

We are grateful to Gerardo Aguirre Diaz for

valuable suggestions and fieldwork assistance during

the first stage of this study. We also acknowledge

Enrique Gonzalez and Barbara Martiny for helpful

discussions and assistance during fieldwork. Fred

McDowell, Zinzuni Jurado and Ken Rubin made

helpful suggestions that greatly improved the manu-

script. The following persons provided support in

sample analyses, fieldwork and preparation of figures

and diagrams: Margarita Reyes, Patricia Giron,

Rufino Lozano, Peter Schaaf, Rodolfo Corona,

Gabriela Sols, Juan Julio Morales, Sol Hernandez,

Gabriela Guzzy, Esther Leyva, Ahiram Monter and

Armando Alcala. Financial support came from the

Direccion General de Asuntos del Personal Academ-

ico, UNAM (Grant PAPIIT IN104802) and resources

from the Instituto de Geologa, UNAM.

Appendix A. 40Ar39Ar geochronology

A.1. Sample preparation and analysis

Two andesite samples (Tz-63-02 and Tz-62-02) ofSurvey, Denver, CO. The monitor mineral used in the

al. (1988).

All samples were analyzed at the U.S. Geological

Aguirre-Daz, G.I., Labarthe-Hernandez, G., 2003. Fissure ignim-

brites: fissure-source origin for voluminous ignimbrites of the

D.J. Moran-Zenteno et al. / Journal of Volcanology and Geothermal Research 136 (2004) 97119 117Survey Thermochronology laboratory in Denver, CO.

The volcanic matrix separates were analyzed on a VG

Isotopes, Model 1200 B Mass Spectrometer fitted

with an electron multiplier using the 40Ar39Ar

step-heating method of dating. Sanidine grains were

analyzed using a MAP 216 mass spectrometer fitted

with an electron multiplier using the 40Ar39Ar laser

fusion method of dating. Twenty-one individual

grains of sanidine were fused with a CO2 laser. For

additional information on the analytical procedure, see

Kunk et al. (2001).

The argon isotopic data for the andesites were

reduced using an updated version of the computer

program ArAr* (Haugerud and Kunk, 1988). The

sanidine isotopic data was reduced using the computer

program Mass Spec (Deino, 2001). We used the decay

constants recommended by Steiger and Jaeger (1977).

Table 1B shows 40Ar39Ar step-heating data for the

andesites and includes the identification of individual

step, plateau, and total gas ages. Total gas ages repre-

sent the age calculated from the addition of all of the

measured argon peaks for all steps in a single sample.

The total gas ages are roughly equivalent to conven-

tional KAr ages. No analytical precision is calculated

for total gas ages. Plateau ages are identified when three

or more contiguous steps in the age spectrum agree in

age, within the limits of analytical precision, and

contain more than 50% of the 39Ar released from the

sample. Table 1B shows the 40Ar39Ar laser total

fusion data and includes individual total fusion age

analyses and the weighted mean age for the sample.

Appendix B. KAr geochronology

After drying at 110j C overnight, the mineralfractions were split in two parts, one for K determi-

nations and the other for Ar measurement. K was

obtained following the method of Sole and Enriquepackage was Fish Canyon Tuff sanidine (FCT-3) with

an age of 27.79 Ma (Kunk et al., 1985; Cebula et al.,

1986) relative to MMhb-1 with an age of 519.4F 2.5Ma (Alexander et al., 1978; Dalrymple et al., 1981).

The type of container and the geometry of samples

and standards are similar to that described by Snee et(2001). Briefly, 100 mg of sample were fused withSierra Madre Occidental and its relationship with Basin and

Range faulting. Geology 31, 773776.

Aguirre-Daz, G.J., McDowell, F.W., 1993. Nature and timing of

faulting and synextensional magmatism in the Southern Basin

and Range, central-eastern Durango, Mexico. Geol. Soc. Amer.

Bull. 105, 14351444.

Alaniz-Alvarez, S.A., Nieto-Samaniego, A.F., Moran-Zenteno,

D.J., Alba-Aldave, L.A., 2002. Rhyolitic volcanism in extension

zone associated with strike-slip tectonics in the Taxco region,

southern Mexico. J. Volcanol. Geotherm. Res. 118, 114.

Alba-Aldave, L.A., Reyes-Salas, M., Moran-Zenteno, D., Angeles-

Garca, S., Corona-Esquivel, R., 1996. Geoqumica de las rocas

volcanicas terciarias de la region de Taxco-Huautla. Memoria

del VII Congreso Nacional de Geoqumica San Luis Postos,

Mexico, Actas INAGEQ 2, 3944.

Alexander Jr., E.C., Mickelson, G.M., Lanphere, M.A., 1978.

Mmhb-1: a new 40Ar/39Ar dating standard. In: Zartman, R.E.

(Ed.), Short papers of the fourth international conference, geo-

chronology, cosmochronology, and isotope geology. U.S. Geo-

logical Survey Open-File Report 78-701, pp. 68.

Aranda-Gomez, J.J., Henry, C.D., Luhr, J.F., McDowell, F.W.,

2003. Cenozoic volcanic-tectonic development of northwesternlithium metaborate + lithium tetraborate. The fused

pearls were measured with a Siemens 3000 spectrom-

eter calibrated against several international standards

prepared in the same way. Results were accurate

within 1% (1r) or better.Argon was measured by isotope dilution (38Ar

tracer) with a VG1200B noble gas mass spectrometer

operated in static mode. Samples were fused with a

double vacuum tantalum furnace. After fusion, sam-

ples were purified with a cold finger, and two SAES

getters, one operated at 400j C and the other at roomtemperature. Eight series of measurements of each

mass were made sequentially and extrapolated to gas

introduction time. Signal was acquired with a Faraday

collector. Variation coefficients for 40Ar and 38Ar are

generally below 0.1% and below 0.5% for 36Ar. All

analyses were made at Laboratorio Universitario de

Geoqumica Isotopica (LUGIS), UNAM.

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D.J. Moran-Zenteno et al. / Journal of Volcanology and Geothermal Research 136 (2004) 97119 119

A major resurgent caldera in southern Mexico: the source of the late Eocene Tilzapotla ignimbriteIntroductionCaldera structure and related tectonic featuresVolcanic stratigraphyPre-caldera rocksVolcanic rocks associated with the caldera collapseTilzapotla ignimbriteIntra-caldera faciesExtra-caldera faciesCollapse breccia

Post-collapse volcanic rocksResurgence-related volcanic rocks

Major element and isotope geochemistryDiscussionRegional stratigraphic and tectonic implicationsVolcanic evolutionInitial tumescence stageCollapse stageVolume estimationPost-collapse and resurgent stages

ConclusionsAcknowledgements40Ar-39Ar geochronologySample preparation and analysisK-Ar geochronologyReferences