Depuis le Pléistocène inférieur, le complexe volcanique de Los Azufres alterne l'émission de magmas différenciés - rhyolites et dacites - et basiques. Compte tenu de cette longue histoire, du caractère "cyclique" des phases magmatiques, et de l'âge récent (26 000 à 38 000 ans) des dernières ignimbrites, on peut penser que l'activité éruptive, liée à un (ou plusieurs) volumineux réservoir(s) de magma différencié à faible profondeur, n'est pas terminée, en particulier au sud du complexe, où se situe le champ géothermique et où vivent plus de 200 000 personnes. (Résumé d'auteur

Pradal, E.

Robin, Claude

Name not available

Late pleistocene ignimbrites at Los Azufres volcanic center, Mexico : implications for the present magmatic cycle

Horizon / Pleins textes

Late Pleistocene ignimbrites 
at Los Azufres Volcanic tenter., Mexico. 
Implications for the present 
magmatic cycle 
Evelyne PRADAL(l), Claude ROBIN(2) 
Abstract: Since Lower Pleistocene, the Los Azufres voicanic tenter alternates the emission of acidic and basic 
magmas. Considering this long history, the “cyclic” pattern of the magmatic phases and the recent age of the last 
ignimbrites (26000 to 38000 yearsl, one cari assume that the volcanic activity related to one - or more - 
voluminous body(ies) of differentiated magma (rhyolite) at shallow depth has not yet ended, especially in the southern 
area of the complex, where the geothermal field is Iying and where more than 200000 persons live in. 
Keywords: Ignimbrites - Late Pleistocene - Magmatic cycles - Caldera - 
Basic magmas - Volcanic activity - Volcanic hazards. 
Résumé : Les ignimbrites Pléistocène terminal du complexe volcanique de 
Los fkzufres. Implications pour le cycle magmatique actuel. Depuis /e Pléistocène 
inférieur, le complexe volcanique de Los Azufres alterne l’émission de magmas différenciés - rhyolites et dacites- 
et basiques. Compte tenu de cette longue histoire, du caractère « cyclique )J des phases magmatiques, et de l’âge 
récent (26000 à 38000 ans) des dernières ignimbrites, on peut penser que /‘activité éruptive, liée à un (ou plusieurs) 
volumineux réservoir(s) de magma différencié à faible profondeur, n’est pas terminée, ‘, en particulier au sud du 
complexe, où se situe le champ géothermique et où vivent plus de 200000 personnes. 
Mots-clés : Ignimbrites - Pliocène tkminal - Cycles magmatiques - Caldera- 
Magmas basiques - Activité volcanique - Risques éruptifs. 
INTRODUCTION 
Located 200 km west of Mexico City, the Los 
Azufres geothermal field has been extensively 
studied (CAMACHO y PALACIOS, 1979; AUMENTO y 
GUTIERREZ, 1980; DE LA CRUZ et a/., 1982; DOBSON, 
1984; DOBSON and MAHOOD, 1985). This geothermal 
field lies on the southern part of a volcanic 
complex (Fig. 1 et 2) interpreted as a large caldera 
(PRADAL et ROBIN, 1985; PRADAL, 1990). This paper 
deals with the age of the main volcanic sequences 
emitted over the whole complex. Since Lower 
Pleistocene, the volcanic’ activity alternates mafic 
(andesites and/or basalts) and differenciated pro- 
ducts (rhyolites and/or dacites). New ages from 
Late Pleistocene ignimbrites suggest that the last 
period of activity which started about 0.6 Ma ago 
has probably not ended. 
MAIN VOLCANIC UNITS 
Basement 
Oligocene and Miocene andesite lavas may be 
observed in the Sierra Mil Cumbres and as inliers 
within the graben of Cuitzeo. The largest outcrop 
in the studied area is the Sierra Santa Ines (Fig. 2). 
(1) Ecole des Mines, 30319 A/&s, France. 
(2) Orstom, UR 13, BP A5, Nouméa, New Caledonia. 
Géodynamique 6 (2), 1991 : 201-205 201 
E. PRADAL, C. ROBIN 
Fig.1. - Schematic ‘structural map from Morelia to Maravatio. 1: Geothermal field; 2: Approximate limits of the Los Azufres Sierra; 
3: Extent of the Los Azufres volcanic complex. 
Esquisse structurale dù secteur Morelia-Maravatio. 1: Champ gkothermique; 2 : Limites approximatives de la Sierra Los Azufres; 
3 : Extension du complexe volcanique. 
This structure together with the andesites in the 
Sierra Los Azufres represents the remnants of 
andesitic series from Miocene to Upper Pliocene. 
Ages ranging from 13 to 3 m.y. have been 
obtained on these rocks (AUMENTO y GUTIERREZ, 
1980; SILVA-MORA, 1979; PRADAL, 1990). 
Ignimbrites 
They consist of six rhyolitic sequences, the origin 
of which is the ring fracture zone (PRADAL, 1990). 
The Tarandacuaro (TA) and Ucareo (UC) ignimbrites 
are widely distributed on the eastern, northern 
and north-western sides of the complex (Fig. 2). 
They consist of poorly to densely welded tuffs 
which are locally associated with pumice air-fall 
and surge deposits. Many domes participate to 
this rhyolitic phase. 
The age of the Ucareo rhyolites (ignimbrites and 
domes) is well defined between 1.39 4 0.03 Ma. 
A problem subsists concerning the age of the 
earlier TA ignimbrites: two different dates (4.5 f 
0.1 and 3.4 t- 0.1 Ma) have been obtained on the 
same ignimbrite sampled in two localities (PRADAL, 
1990). Such a difference is probably due to the 
heterogeneity of the samples since they contain 
202 
a few percent of andesite xenoliths. Thus, the 
age is probably less than 3.4 Ma. Considering 
remains of Elephas Imperator leidy within the 
underlying sediments which refill the graben 
(GARDUNO, 19871, PRADAL (19901 proposes a Late 
Pliocene or Lower Pleistocene age for the TA 
ignimbrite. 
The San Pedro Jacuaro @PJ) sequence of rhyolites 
crops out mainly on the southern side of Sierra 
Los Azufres. North of San Pedro Jacuaro, several 
10 to 20 m thick ash flow deposits from this 
sequence are well exposed in quarries. The Pueblo 
Viejo (PV) sequence, up to 60 m thick, lies on the 
western part of the complex (Fig. 2). These two 
series are associated with air fall deposits and 
domes. Ages from the SPJ and PV rhyolites range 
from about 1.1 to 0.8 Ma (Tabl. 1). 
The most important group from the Upper Pleisto- 
cene ignimbrites is the Cieneguillas group (CI) 
which partly filled the eroded PV deposits. Domes 
extruded near the ring fracture and on the south- 
western slope of the complex belong to this 
group. Two of them (Cerros Carpintero and El 
Bosque) were dated = 0.15 Ma (0.15 + 0.05 and 
0.14 IL 0.02 Ma) by DOBSON (1984). However, 
our data show that some extrusions -e.g. Cerro 
Géodynamique 6 (Z), 1991 : 201-205 
Magmatic cycle of ignimbrites (Mexico) 
Acambaro .L _ _ 
b’ . . * . . . .* 
Maravatio 
=Mess Llano grande 
Fig.2. - Geological sketch map of the Los Azufres volcanic complex. For clarity, domes belonging to the UC, SPJ, PV and CI 
sequences have not been represented. 
TA: Tarandacuao; UC: Ucaréo; SPJ: San Pedro Jacuaro; PV: Pueblo Viejo; CA: Acambaro; Cl: Cieneguillas. 
Carte géologique simplifiée du complexe volcanique de Los Azufres. Par souci de clarté, les dômes des formations UC, SPJ, PV 
et CI n’ont pas été représentés. 
(ignimbrites and domes) N 
Dacites 
f 
Andesites 
and basalts 
Others domes 
(rhyolites) - k-an,, 
[E~SPJ [Rd-i-~ m 
Guangoche (Fig. 2) and their pyroclastic products South of Acambaro, the AC ash and pumice flow 
are much more recent (26.000 to 29.000 years; deposit covers at least 20 km*. It reaches 25 m 
Tabl. Il). This supposes that the Cl group corres- in thickness. Obsidian-rich levels in the deposits 
ponds to a long phase of rhyolitic magmatism indicate that they were emplaced as several flows. 
which is still going on. The volume of the CI This ignimbrite is 36 000 + 1400 years old (Tabl. II). 
rhyolites is estimated at 10 12 km3. The AC deposits cari be followed from the 
Géodynamique 6 (2). 1991 : 201-205 203 
E. PRADAL, C. ROBIN 
TABLE I 
Summary of K-Ar dates. Sources of data. 
1: SILVA MORA (1979); 2: AUMENTO y GUTIEREZ (1980); 3: DOBSON and MAHOOD (1985); 4: PRADAL (1990). 
Datations K-Ar. Origine des données. 
Sample and locality Coordinates Material K% Ar 40’ Ar Atm 
ncib % 
CIENEGUIIlAS 
Rhyolite Mesa El carpintero lcv43’15 - 19”48’05 
Rhyolite 
glass 3.92 - 3.93 81.7 
Mesa El Basque lOO”42’27 - 19”47’41 biotite 5.51 98.5 
Rhyolite Cerro El Rosario 100’42’52 - 19’45’32 biotite 5.69 - 6.03 96.8 
DANTE DOMES Cerro San Andres 100°37’30 - 19’46’38 plagiocl. 1.5 1.55 - 95.5 
Cerro Mozo 100043’55 - 19’51’25 WR 1.46 0.050 95.4 
LA cALAaA2A Andesite AZ 130 10035’60 - 19062’ WR 1.14 0.047 89.3 
~&XE Acre Basic Andesite AZ 124 loO”44’50 - 19”50’30 WR 1.87 0.103 87.2 
Basic Andesite AZ 74 100’48’45 - 19’48’25 WR 1.71 0.096 90.3 
P~EBLO VIWO - SAN PEDRO JUACUARO 
Rhyolite AZ 157 Agua Fria 100°3S’30 - 19’47’20 WR 
Rhyolite Geothermal field lOOn10’22 - 19’48’46 glass , 
3.82 0.201 88.0 
3.87 - 3.90 49.7 
Rhyolite Geothermal field 100=39’46 - 19”46’58 WR 3.73 - 3.87 87.0 
Rhyolite Geothermal field lOOO39’26 - 19’56’21 glass 3.88 - 3.91 47.1 
Obsidian 122 San Pedro Jucuaro 100Q3!3’30 - 19”44’00 WR 3.95 0.285 84.2 
Lohu GRANDE Dacite AZ 128 lOO”43’30 - lS”52’30 WR 1.94 0.165 87.2 
Jcareo 
qhyolite AZ 121 Zinapacuaro 100’50’50 - lS”51’30 WR 3.84 0.347 61.8 
qhyolite AZ 11 Ucareo 100’41’55 - 19-53’25 WR 3.97 0.360 67.9 
qhyoiite AZ 82 Cerro Cuate lOO”45’25 - lS”50’30 WR 3.57 0.343 77.1 
(4) Analyses realized at the K-Ar laboratory of Clermont Ferrand University by C. JAMOND, using new decay constants, 
Age (My) 
0.14 +/-0.02 (3) 
0.15 0.05 +/- 131 
0.30 0.07 +/- (31 
0.33 +/- 0.07 (31 
0.50 +/- 0.10 (41 
0.60 0.05 +/- (41 
0.79 +/- 0.06 (4) 
0.81 +/- 0.08 (4) 
0.76 0.06 +/- (4) 
0.84 0.02 +/- (3) 
0.93 0.04 +/- (31 
1.03 0.02 +/- (3) 
1.04 +/- 0.06 (41 
1.22 d- 0.09 (4) 
1.30 0.03 +/- (41 
1.31 0.04 +/- (4) 
1.39 0.05 +/- 14) 
Acambaro plain up a zone marked by large obsidian 
veins and small rhyolitic extrusions close to the 
form a plateau of about 70 km* , south of Zinape- 
southern edge of the Sierra Santa Ines. 
cuaro (Fig.2). Two samples have been dated at 
- 0.8 - 0.75 Ma. 
Dacitic domes 
They are less numerous than the rhyolitic domes. 
A large dacitic extrusion in the north-western part 
of the complex was dated 1.22 f 0.09 Ma (Lama 
Grande, Tabl. 1) and seems to follow the Ucareo 
series of ignimbrites and domes. The major dacitic 
phase belongs to the emplacement of Cerros 
Mozo and San Andres, from 0.6 to 0.3 Ma. 
(ii) - about 0.6 my ago, andesites and basalts from 
cerro La Calabaza partly covered the eastern edge 
of the complex and spred out over 120 km2 with 
a maximum thickness of about 300 m. 
TABLE II 
Mafic lava flows 
Cl4 ages of ignimbrite deposits from the Cieneguillas 
and Acambaro sequences. 14C analyses 
by the centre des faibles radioactivités (CFR). 
Gif-sur-Yvette. France. 
Basaltic volcanism occurred between the eruption 
of the lower ignimbrites (TA and UC) and the 
intermediate rhyolite sequences (PV and SPJ 
ignimbrites) as indicated by small volume of lava 
flows in the western part of the complex and 
scoria reworked by the PV ignimbrites. 
Three additional series of basalts and andesites 
occurred during the Pleistocene development: 
(i) - in the west, olivine basalts and andesite lava 
flows with a total thickness of 100 to 120 m 
Datations au carbone 14 des coulées cendre-ponceuses 
de Cieneguillas et Acambaro. Analyses réalisées 
au centre des Faibles Radioactivités 
de Gif-sur-Yvette (France). 
Volcanic sequence Localitv Age (years B.P.) 1 
CIENEGUILLAS 
AZ88 lOO”45’20 W - lS”45’50 N 28,000 +/- 650 
AZ120 lOO”45’ W - 19”42’40 N 26,800 +/- 900 
AZ 136 lOO”43’45W - 19O41’55 N 26,700 +l- 450 
ACAMBARO 
AZ126 lOO”44’3OW - lS”59’20 N 36,300 +/- 400 
l 
204 Géodynamique 6 (2). 1991 : 201-205 
Magmatic cycle of ignimbrites (Mexico) 
(iii) - on the south-east slopes, the Mesa Lano rhyolites. Another cycle took place between 2: 1 
Grande (100 km*) consits of a pile of basaltic flows and 0.75, 0.8 Ma.; it began with the emission of 
which commonly reaches 150 m in thickness. Its the SPJ and PV ignimbrites and ended with the 
age is inferred the same age as the La basaltic andesites on the western side (Zapote 
Calabaza series. Alto, Tabl. 1). 
Numerous undated - almost presumably recent - 
scoria cones occur outside the complex. Some of 
them are scattered above the basalt sequences 
and the andesites of La Calabaza. 
A period of quiescence from - 0.75 Ma to = 0.6 Ma 
preceeded the voluminous series of mafic lava 
flows which gave rise to the La Calabaza and 
Llano Grande sequences. During a long lasting 
phase of nearly continuous activity, this mafic 
volcanism grades into dacites (cerro San Andres 
and cerro Mozo, 0.5 to 0.3 Ma) and then rhyolites 
(ignimbrites and domes of the Cl and AC groups). 
Considering the volume of the CI and AC rhyolites 
(10-12 km3 and l-2 km3 respectively) and the 
recent age of the last ignimbrites, one cari assume 
that the Upper Pleistocene volcanic activity corres- 
ponds to progressive differentiation in one, or 
more, voluminous body(ies) of magma which evo- 
lution has not yet ended. Consequently, volcanic 
hazards remain permanent threat to the complex 
area, specially to the southern part, where more 
than 200000 persons live in. 
VOLCANIC HAZARDS 
The previous data suggest that volcanic risks 
remain threat, specially to the northwestern and 
southern area. On the north-western part of the 
complex, Acambaro city (about 80 000 inhabitants) 
is built on the Acambaro ash and pumice flow 
deposits and more than 200000 people live in the 
southern part of the complex, where the recent 
rhyolitic domes and ash flows tuffs (Cieneguillas 
unit) deposits lie on (Fig. 2). According to this, it 
seems that a detailed mapping of the volcanic 
hazards should be realized in a next field work. 
CONCLUSION 
Since = 1.4 my, the Los Azufres volcanic complex 
alternated acidic and mafic volcanisms. As an 
example of “magmatic cycle”, dacitic lavas (Lama 
Grande) and then basalts followed the Ucareo 
Aknowledgements 
Field work was supported by the Centre National de la 
Recherche Scientifique (CNRS). We are grateful to the centre 
d’Etudes Mexicaines et Centraméricaines (CEMCA, Mexico) for 
providing a 4 WD car. We also thank the Comision federal de 
Electricidad, Departamento de Geotermia for warm welcome 
in Morelia and Dr. V.H. Garduno for constructive comments 
at field. 
Manuscrit accepté par le Comité de rédaction le 25 sep- 
tembre 1992. 
BIBLIOGRAPHIE 
AUMENTO (F.) y GUTIERREZ (A.), 1980. - Geochronologia 
de Los Azufres; Michoacan. Int. Report, Comision Federal 
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CAMACHO (F.) y PALACIOS (N.M.), 1979. - Geologia de /a 
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DE LA CRUZ (V.), AGUILAR (J.), ORTEGA (D.) y SANDOVA 
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Géodynamique 6 (2). 1991 : 201-205 205 


Late pleistocene ignimbrites at Los Azufres volcanic center, Mexico : implications for the present magmatic cycle

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