Bonhomme, M.G.

Fornari, Michel

Laubacher, Gérard

Sebrier, M.

Vivier, G.

Horizon / Pleins textes

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I 
1 Journal ofsouth American Earth Sciences. Vol. I ,  No. 2. pp. 179-183. 1988 
Printed in Great Britain 
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0895-981 1/88 53.00 + 0.00 
Pergamon Prcss plc 
New Cenozoic K-Ar ages on volcanic rocks from the 
eastern High Andes, southern Peru 
M. G. BONHOMME', M.'F"NXËd2~G. mUhÄGgER', M. sEBRIER4, and G. VIVIER' 
1- -- 1 2 -  
'Institut Dolomieu, UA 69 CNRS. rue M. Gignoux, 38031 Grenoble, France; 20RSTOM, 213 rue La 
Fayette, 75480 Paris Cedex 10, France; 'ORSTOM and Centre Gdologique et  &ophysique, USTL, 34060 
Montpellier, France; 4Laboratoire de Gdologie Dynamique Interne, Universith de Paris Sud, 91405 Orsay 
Cedex, France 
t 
I 
(Received for publication September 1987) 
Abstract-New K-Ar age's of Cenozoic volcanic rocks from the southeastern Peruvian Andes, where 
geochronologic data are still scarce, are presented in this paper. North of Lake Titicaca, the new data 
provide evidence of various late Oligocene to Pliocene volcanic pulses. In the Crucero intramontane basin 
(14"205,70W), basaltic andesites, dacites, rhyolites, and ash-flow tuffs interbedded in fluvial and lacu- 
strine sediments yield ages ranging from 25 to 22 Ma, characterizing a late Oligocene to early Miocene 
volcanic pulse. In the same area, widespread acidic ash-flow tuffs and rhyolites, with ages ranging between 
17 and 12 Ma, demonstrate that volcanism was also active during mid- to late Miocene times. Finally, the 
younger ages show that volcanism was still active during the mid-Pliocene and Pleistocene in the south- 
eastern Peruvian Andes. 
R e s u m e n S e  presentan nuevas edades radiocronol6gicas K-Ar de rocas volcknicas cenomicas de los 
Andes Surorientales del Perú de donde se conocen todavia pocos datos geocronológicos. Al norte del Lago 
Titicaca, estas nuevas dataciones demuestran la existencia de varios pulsos de actividad volchica entre el 
Oligoceno superior y el Pleistoceno. En la cuenca intramontañosa de Crucero (14"20'S, "OW), andesitas 
bashlticas, dacitas, riolitas e ignimbritas intercaladas en sedimentos aluviales y lacustres, han dado edades 
con rango de 25 hasta 22 Ma, lo que evidencia una fase de intenso volcanismo durante el Oligfceno tardio al 
Mioceno temprano. En la misma zona, grandes mesetas de ignimbritas y riolitas, con edades comprendidas 
entre 17 y 12 Ma, atestiguan de una fuerte actividad volchica durante el Mioceno medio a tardío. Por fin, 
edades recientes comprueban una actividad volcánica significativa en los Andes Surorientales del Pera 
durante el Plioceno y el Pleistoceno. 
I 
INTRODUCTION 
ANDEAN magmatism in the Eastern Cordillera of the 
Central Andes is volumetrically minor compared to 
that in the Western Cordillera and i t  has received 
little attention in the past. Recently, however, met- 
allogenic, petrographic, and geochemical investiga- 
tions have provided new information on this very 
unusual magmatism. Thus, a crustal origin has been 
nism from northern Bolivia and southeastern Peru 
(Clark et al., 1983; Kontak et al., 1984) and for the 
late Miocene and Pliocene ignimbritic volcanism of 
southeastern Peru (Herrera et al., 1984; Kontak and 
Pichavant, 1984; Noble et al., 1984). 
The genesis of these crustal derived magmas is of 
fundamental interest. Nevertheless, the relationship 
of their genesis with the Pacific subducting plate (the 
trench axis of which is approximately 500 km from 
the Eastern Cordillera at present) and/or with other 
tectonic processes remains uncertain. This magma- 
tism is also of great economic interest because of its 
enriched lithophile elements (Noble et al., 19841, 
some of which - such as tin, tungsten, and zinc - 
have mining potential (Clark et al., 1984). 
* 
1 
1 ,.
* proposed for the Oligo-Miocene peralumilous Pluto-- . 
i 
In this context, new K-Ar f.adiochronologica1 data 
from Tertiary and Pleistocene volcanics of the south- 
eastern Peruvian Andes are presented (Fig. l ) ,  from 
which area only limited data were previously avail- 
able. This study dated primarily volcanic rocks of 
acidic composition; however, some basalts of latest 
Oligocene to earliest Miocene age provide the first 
indication of a mantle-derived magmatism contem- 
poraneous with crustal-derived magmatism. 
METHODS 
The new radiochronologic data are presented in 
Table 1. Procedures used for sample preparation, 
potassium determinations, argon extraction-purifica- 
tion, and mass isotopic analyses were slightly modi- 
fied from those of Bonhomme et al. (1975). Isotopic 
and decay constants used were compiled by Steiger 
and Jager (1977). Error was calculated according to 
the method of Mahood and Drake (1982). Argon 
calibration used the G1-O standard sample (Odin, 
19821, with a value of 24.80k0.14 nVg (lu). 
Some of the studied samples showed very low 
radiogenic argon content with respect to total argon 
01.;ai UNI fonds Documentaire 
n 
n 
Fig. 1. Cenozoic and Pleistocene magmatism in the eastern High Andes of Peru: a) main volcanic outliers, b) position and numeration ofthe study samples. 
4 
b 
i 
i New Cenozoic K-Ar ages on volcanic rocks from the eastern High Andes, southern Peru 181 
concentration. The effect of this on the reliability of 
the dating is discussed below. 
RESULTS AND GEOLOGIC 
RELATIONSHIPS 
The Crucero-Ananea Region (North of Lake Titicaca) 
Volcanic rocks from the Cayconi Formation. The 
Cayconi Formation, type section near Crucero (Lau- 
bacher et al., in press), consists of alluvial reddish 
sandstones and conglomerates, and palustrine light 
colored shales and sandstones. These sediments are 
interbedded with volcanic rocks of basaltic and basalt 
andesitic, dacitic, rhyodacitic, and rhyolitic composi- 
tion. The Cayconi Formation unconformably overlies 
a basement composed of Paleozoic, Permo-Triassic, 
and Cretaceous rocks that have been deformed at 
least three times: i )  during the Paleozoic, i¿) during 
the Permian and i i i )  during the Eocene (Laubacher, 
1978; Laubacher et al., in press). The age of the last 
phase is not well defined, but it is probably late 
Eocene (Noble et al., 1974). The K-Ar ages obtained 
here on eight rock samples definitively places this 
volcanism at the OligoceneMiocene boundary (Odin, 
1982) - i.e., between 25 and 22 Ma. 
Sample MS82008 must be discussed in more de- 
tail. The K-Ar isotopic age of this sample is 18.8 
k1.5 Ma, which is some 6 Ma younger than samples 
MS82007 and MS82009, which were collected a short 
L 
b 
! 
I distance from MS82008 and apparently in the same ? 
volcanic sequence. The radiogenic argon content of 
MS28008 is not significantly lower than that of the 
two other samples; therefore, this young age cannot 
be attributed to analytical error. In  contrast, a 
'strong alteration of the rock is'observed in thin sec- 
tion and is confirmed by a H 2 O  content of 10% 
(Laubacher et al., in press). This alteration may be 
caused by hydrothermal transformation, indicated by 
the intense seritization of the feldspars. As the ana- 
lysis was performed on the whole rock, it may be that 
the datum 18.8 Ma has no geologic meaning and is an  
intermediate value between the age of cooling (25 to 
22 Ma) and the time of alteration (younger than 18 
Ma). Examples of similar behavior for neighboring 
regions are reported by Soler and Bonhomme (1988). 
Palca X I  sample. The Paka XI sample was col- 
lected from a small rhyolitic stock (-1 hectare of 
extension) near Cerro Pucaorco, approximately 2 km 
to the north of the Palca XI mine (Fig. 1). Four kilo- 
meters to the south of the mine, another rhyolitic 
stock -.similar in composition but larger (Injoque et 
al.,  1983) - forms the Cerro Choquene. This stock 
intrudes Cretaceous sandstones, shales, and conglo- 
merates of probable mid-Tertiary age that correlate 
with the Cayconi Formation. A middle to late Ter- 
tiary age was therefore suggested for these stocks 
(Laubacher, 1978). The mined vein, mineralized 
with W-Zn-Ag-Sn-Cu, is emplaced in a large NNW- 
SSE zone located between these two stocks. Unpub- 
lished reports suggest a genetic relationship between 
the mineralization and the rl&olitic stocks. The Pal- 
ca XI rhyolite sample of the Cerro Pucaorco has been 
dated at roughly 12.5 Ma, confirming previous field 
observations, but it is somewhat older than the 8.3 
Ma age reported by Clark etlal., (1984) for minera- 
lization of the Palca XI mine itself. Both are approxi- 
mately coeval with late Miocene ignimbrites of Ma- 
l 
; 
I 
Table 1. Location, petrographic, and K-Ar isotopic data for the samples studied. 
"Arrad 
Petrographic Fraction K20 'OArtot "Arrad Age 
Samples and Location Definition Analyzed (%) (%I (nVg) (Ma ? lo) 
Crucero Region 
L 
h. MS82003 
MS82006 
r! 
MS82007 
MS82008 
MS82009 
MS82012 
MS82015 
MF271801 
14"15'S, 70'04W 
14'34's. 69"50W 
14"33'S, 69"47W 
14"32'S, 69"46W 
14"32'S,69"46W 
14"15'S, 70'07W 
14"13'S, 70'08W 
14"17'S, 70'03W 
Palaca JI 14"42'S, 69"41 W 
PO232 14°40'S,69029W 
CUSCO Region 
Ms82021 13"378,71°42W 
Ms82022 13"35'S, 71'43W 
rhyodacitic tuff 
rhyolitic tuff 
andesite 
andesite 
andesite 
rhyolite 
rhyolite 
andesite 
rhyolite 
acidic tuff 
latite 
latite 
F K  11.49 
V 4.47 
B 8.88 
R 2.56 
R 2.07 
R 1.97 
FK 10.90 
FK 10.19 
R-I 1.32 
R-JI 1.32 
FK 7.57 
B 8.80 
V 6.19 
R 3.77 
R 3.89 
98.1 
61.1 
82.4 
54.2 
48.3 
39.1 
96.8 
97.2 
20.9 
24.5 
81.5 
78.5 
27.6 
8.99 
18.18 
8.77 
3.32 
6.75 
2.058 
1.263 
1.574 
7.84 
7.41 
1.016 
0.954 
3.10 
3.44 
0.76 
0.850 
0.074 
23.5 f 0.3 
22.9 f 0.6 
23.5 f 0.4 
24.8 f 0.7 
18.8 f 1.5 
24.6 f 1.3 
22.2 f 0.2 
22.4 f 0.3 
23.7 f 1.8 
22.3 f 0.7 
12.7 f 0.6 
12.1 f 0.3 
3.79 (n.d.) 
7.2 f 0.5 
0.59 f 0.25 
Analytical procedure and constants from Lavenu et al. (in press). R, whole mck; V, glass; B, biotik KF, K-feldspar. 
182 M. G. BONHOMME, M. FORNARI, G.LAUBACHER, M. SEBRIER, and G. VIVIER 
cusani, for which Noble el al. (1984) reported an age 
of 9.4 Ma. A mid-Miocene age of -16 Ma (Laubacher 
et al., in press) was obtained from an ignimbritic ash- 
flow sample collected approximately 20 km north of 
the Paka XI mine. This mid- to late Miocene mag- 
matism is coeval with many other magmatic and 
mineralized occurrences (see references in Soler and 
Bonhomme, 19881, with ages ranging between 14 Ma 
and 7 Ma. 
Tuff P0232. Sample PO232 was collected from the 
Cerro Viscachani 3 km to the northeast of Ananea 
(Fig. 1). It  is partially a reworked tuff, interbedded 
in the palustrine part of the Arco-Aja Formation 
defined approximately 20 km north of Ananea near 
the hmdet  afAncac212. Field observati~ns have süg- 
gested a Pliocene to early Pleistocene age for the 
Arco-Aja Formation (Fornari et a l . ,  1982) and the 
3.79 Ma age of the interbedded tuff dates the lacu- 
strine formation as Pliocene. The Arco-Aja Forma- 
tion, therefore, may be considered as a time equiva- 
lent of the La Paz Formation (Lavenu et al., 1984; 
Lavenu et aZ.,in press; Laubacher et al., 1984). 
2) The Pliocene age of the Arco-Aja Formation is 
demonstrated by the 3.79 M a  date of an  interbedded 
tuff. Therefore, the Arco-Aja and La Paz Formations 
(Ballivian et al., 1978) -both of Pliocene age - can 
be correlated. The tuff of the Arco-Aja Formation is 
approximately coeval with the Chijini Toba dated at 
3.27 Ma (Clapperton, 1979) and with the youngest 
Macusani ignimbritic ash flow dated at 4.2 Ma 
(Barnes et al., 1970). These data confirm that, during 
Pliocene times, volcanism was very active on the 
southwestern edge of the Eastern Cordillera of sou- 
thern Peru and northern Bolivia. 
3) The Pleistocene age of the volcanites of the Cuz- 
co area is partly confirmed. 
Acknowledgements-Fieldwork was carried out in 1980 and 1982 
under a cooperative scientific agreement between INGEMMET 
(Peru) and ORSTOM (France). We are indebted to ORSTOM Lima 
and ATP Geodynamique II (INAG, Paris) for field assistance and 
support. ORSTOM (Paris) and the Institut Dolomieu, UA 69 
(CNRS) Grenoble supported radiochronologic dating. 
The Cuzco Region 
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New cenozoic K-Ar ages on volcanic rock from the eastern high Andes, southern Peru

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