Bonhomme, M.G.

Fornari, Michel

Laubacher, Gérard

Sebrier, M.

Vivier, G.

Horizon / Pleins textes

J w n d  of Suulli Anurkaii Errrrh Sckncer, Vol. I ,  No. 2, pp. 179-1113, 1988 
~. Prinkd in Grcnl Britain 
0895.911 I/BM S3.W + 0.00 . Pccgomon I'rcss plc 
New Cenozoic K-Ar ages on volcanic rocks from the 
eastern High Andes, southern Peru 
' 
M. G. BONHOMME", M. POR NA RI^, G. LAUßACkIER3, M. SEßrzlER", and G. VIVIER' 
'Institut Dolomieu, UA 69 CNRS, rue M. Gignoux, 38031 Grenoble, France; 20RSTOM, 213 rue Lu 
Fayette, 75480 Puris Cedex 10, France; 30~1STOM and Centre Cbologiquo et. Gbophysiquu, USTL, 34060 
Montpellier, Prunco; 4Loboruhire de GOologie Dynuniiquc Inturne, UiiiversiM de Puris Sud, 91405 Orsuy 
Cedex, I'runcc 
(Received for pulrlìcation September 1987) 
Abulract-New I<-Ar ugcs ol' Cenozoic, volcunic rocks from the southoustcrn Peruviun Andes, whcre 
geoclironologic dutu Ure slill scurcc, urd presented in lhis puper. Norlh of Luke Titicucu, Lho now d u h  
provide evidence of various lute Oligocene to Pliocene volcunic pulses. I n  lhe Crucero intrumontane busin 
(1lro20'S, 70V), busultic andesites, ducites, rhyolites, und ush-flow tuffs interbedded in fluviul und lucu- 
strine sediments yield uges ranging from 25 to 22 Mu, churuclerizing u lute Oligocene IA curly Miocene 
volcanic pulse, In the sume area, widespreud acidic ush-flow tuKs and rhyolites, with uges rungiiig between 
17 and 12 Mu, demonstrute tha t  volcunism was ulso ncLivc during mid- to I u b  Miocene times. E'inully, the 
younger ages show thul  volcunism wus slill uctive during lhe mid-Pliocene und Pleishcenc in  lie soulh- 
eastern Peruviun Andes. 
ReuumcnCe  prescntnn nuevas edudes radiocronol6gicus K-Ar de rocus volshnicas cenozoicus de los 
Andee Surorientales del Perd de donde so conocun toduvh pocos datos gcocronol6gicos. AI norm del Lugo 
Titicaca, estas nuevas dubciones demuestran Ia existenciu de varios pulsos de uctividud volchica onlre cl 
Oligoceno superior y el Pleistoceno. En  lu cuenca intraniontañosa de Crucero (14"20'S, 70"W), undesitas 
b u d t i c a s ,  ducitau, riolitas e ignimbritas inkrculudus en sediinenlos aluviules y lucuslres, hun dudo cdudes 
con rango de 25 hastu 22 Mu, lo que evidencia una fuse de intenso volcunismo durunte el Oligoccno turdlo u1 
Mioceno tenipruno. En lu misinn zonu, grandes mesetas de ignimbritas y riolitcis, con edudes cornprendidus 
untra 17 y 12 Mu, utestiguun de  unu fuerte actividad volch ico  durunte el Mioceno medio u tnrd[o. Por Lin, 
edadea recientes comprueban una uctividad volcdnicu significutivu en los Andes Suroricnlulas del Perd 
durante el Plioceno y el Pleistoceno. 
c; - 
, 
INTRODUCTlON 
ANDEAN magmatism in the Eastern Cordillera of the 
Central Andes is volumelirically minor compared to 
that  in the Western Cordillera and it 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 lieen 
proposed for the Oligo-Miocene peralumilous pluto- 
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 el al., 1984; Kontak and 
Pichavant, 1984; Noble et al., 1984). 
The genesis of these crustal derived magmas is of 
fundamental inlerest. 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 e l  al., 1984). 
. 1 
' 1 ' 
I 
In this context, new K-Ar radiochronological dala 
from Tertiary and Pleistocene volcanics of the south- 
eastern Peruvian Andes are presented (Fig. 11, from 
which area only limited data &re 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 magmalisni conlem- 
poraneous with cruslal-derived magmatism. 
METH O DS 
The new radiochronologic data are  presented in  
Table 1. Procedures used for sample preparation, 
polassium determinalions, argon exlraclion-purifïca- 
tion, and mass isotopic analyses were slightly modi- 
fied from those of Bonhomme e l  al. (1975). lsolopic 
and decay constants used were compiled by Steiger 
and Jager (1977). Error was calculntecl according lo  
the method of Mahood and Drake (1982). Argon 
calibration used the G1-0 standard sample (Odin, 
1982), with a value of 24.80k0.14 nl/g ( l o ) .  
Some of the s ludicd samplds showed very l o w  . 
radiogenic argon conlcnt with respgct to total argon 
1 9 FE!, 
.t / 
G\ + 
O 5 0  km 
................... .. .......-........ ........._. _... ................... 
1-1 .~ , ................ 
* b   ........ * -... .... 
................. :. :::::::i.':.:::::: cl ................ 
Fig. 1. Cenozoic and Pleistocene magmatism in the  eastern High Andes of Peru: a)mainvolcanic outliers, b) position and numeration oÏthestudy samples. 
. .  
n 
n 
P 
? 
s 
*i 
i i 
' -2 ~ New Cenozoic K-Ar ages on volcanic rocks from the eastern High Andes, southern Peru 181 
concentration. The eflect of this on the reliability of 
the dating is discussed below. 
RESULTS AND GEOLOGIC 
RELATIONSHIPS 
T h e  Crucero-Ananea Region (Norlh ofLake 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 conglomerales, and paluslrine light 
colored shales and sandstones. These sediments are 
interbedded with volcanic rocks of basallic 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, ìi) during 
the Permian and iii) during the Eocene (Laubacher, 
1978; Laubacher el al., in press). The age of the last 
phase is not well defined, b u t  i t  is probably late 
Eocene (Noble el al., 1974). The K-Ar ages obtained- 
here on eight rock samples definitively places this 
volcanism a l  the Oligocene/Miocene boundary (Odin, 
1982) - i.e., between 25 and 22 Ma. . 
SamplGMMS82008 must be discussed in more de- 
' tail. The K-Ar isotopic age of this sample is 18.8 
f1.5 Ma, which is some 6 Ma younger than samples 
MS82007 and MS82009, which were collected a short 
distance 'from MS82008 and apparently in the same 
volcanic sequence. The radiogenic argon contcnt of 
MS28008 is not significantly lower than thal of the 
two other samples; therefore, this young age cannot,, 
- 
, 
; 
be attributcd to ilnalytical error. In  C O I I ~ ~ M ~ ,  a 
strong alteration of the rock is observed in  thin sec-. 
tion and is confirmed by a 1320 conlent of 10% 
(Lauhacher e t  al., in press). This alteration may be 'j, 
caused by hydrothermal transforma tion, indicailed by 
the intense seritization of the feldspars. As the ana- 
lysis was performed on the whole roclc, i t  may be that 
the datum 18.8 Ma has no geologic meaning and is a n  
intermediate value belween 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). 
, 
Paka  X I  sample. The Palca XI sample was col- 
lected from a small rhyolitic stoclc (-1 hectare of 
extension) near Cerro Pucaorco, approximately 2 km , ' . 
to the north of the Palca XI mine (Pig, 1). Four Itilo- 
meters to thc south of Lhe mine, another rhyolitic 
stock - similar in  composition but larger (Injoque e t  
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 stoclcs 
(Laubacher, 1978). The mined vein, mineralized 
with W-Zn-Ag-Sn-Cu, is emplaced in a large NNW- 
SSE zone locat'ed between these two stoclrs. Unpub- 
lished reports suggest a genetic relationship between 
the mineralization and the rhyolitic stoclrs. The Pal- 
ca XI rhyolite sample of the Cerro Pucaorco has been 
dated a t  roughly 12.5 Ma, confirming, previous field 
observations, bu1 it is somewhat older than the 8.3 
Ma age reported by Clark et al., (1984) €or minera- 
lization of the Palca XI mine iLself. Both are approxi- 
mately coevallwith late Miocene ignimbrites of Ma- 
Tab le  l. Location, petrographic,  and  K-Ar isotopic dntn for the sainples studied. 
''Arrud I '  
Potragrnphic Frac t ion  X20 'OArl,,l "Ar,,d A !Xe 
Samples  and Location Definition : Annlyzed (%) (%I (nllg) (Ma i 10) 
Crucero Region 
MS82003 
MS82006 
MS82007 
' MS82008 
MS82009 
MS82012 
MS82015 ' 
MF271801 
Palaca Il 
PO232 
Cusco Region 
MS82021 
MS82022 
14'15'S, 70"04W 
14"34'S, 69"50W 
14"33'S, 69'47%' 
14"32'S,69"46%' 
14'32'S, 69'46W 
14"15'S, 70"07W 
14'13'S, 70"08W 
14'17'S, 70"03%' 
14"42'S, 69'41'W 
14"40'S, 69"29W 
13"37'S, 71'42713 
13'36'S, 71'43W 
rhyodacitic tuff 
rhyolitic tuff 
andesite 
andesi te '-- 
andesi te 
rhyolite 
rhyolite 
andesite 
rhyolite 
acidic tuff 
latite '' 
latite 
FK 11.49 98.1 
V 4.47 61.1 
13 8.88 82.4 
R 2.56 54.2 
R 2.07 48.3 
R 1.97 39.1 
FK 10.19 97.2 
R-I 1.32 20.9 
R-Il 1.32 24.5 
FK 7.57 81.5 
B 8.80 78.5 
V 6.19 27.6 
FK 10.90 96.8 
. 1 
8.77 
3.32 
6.75 
2.058 
1.263 
1.574 
7.41 
1.016 
0.954 
3.10 
3.44 
0.76 
7.84 
R 3.77 8.99 0.850 
R 3.89 18.18 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 I 1.3 
22.2 f 0.2 
22.4 f 0.3 
23.7 St 1.8 
22.3 f 0.7 
12.7 St 0:s 
12.1 10.3 
3.79 (n.d.) 
7.2 f 0.5 
0.59 k 0.25 
Analytical procedure and constants fromLavenu et al. (in press). R, whole rock; V, glass; B, biotite; KF, IC-feldspar. 
, 182 M. G. BONHOMME, M. FORNARI, G.LAUBACHER, M. SEBRIER, and G. VIVIER 
cusani, €or which Noble et al. (1984) reported an age 
of 9.4 Ma. A mid-Miocene age of -16 Ma (Laubacher 
et  al., in press) was.obtained from a n  ignimbritic ash- 
flow sample collected approximately 20 km north of 
the Palca 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. 
TuffP0232. Sample PO232 was collected from the 
Cerro Viscachani 3 lrm to the northeast of Ananea 
(Fig. 1). It is partially a reworked tuff, interbedded 
in the palustrine par t  of the Arco-Aja Formation 
defined approximately 20 km north of hnanea near. 
the hamlet of Ancocala. Field observations have sug- 
gested a Pliocene to early Pleistocene age for the 
Arco-Aja Formation (Fornari et al.,  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 e l  al.,in press; Laubacher et al,, 1984). 
The Cuzco Region 
/ 
, 
Lalite MS82021. Sample MS82021 was collected 
on the saythern edge of the Rumicola volcano, which 
marks the eastern termination of the Cuzco Valley 
approximately 20 km to the southeast of Cuzco (Pig. 
1). Field observations suggest a Quaternary age for 
this volcanism; thus, the 7.2 Ma isotopic determina- 
tion (see Table 1) might not be valid. This whole-rock 
sample contains only about 9% radiogenic argon, 
which result was obtained in  replicate analyses, I n  
these conditions, a n  excess of "Ar may be suspected. 
Similar excesses of radiogenic argon in biotites from 
tuffs of Lhe Pliocene La Paz Formation have'already 
been measured. This interpretation may be valid for 
sample MS82021, which is also rich in biotite. 
' 
Latite MS82022. Sample MS82022 came from the 
Huambutillo -volcano located at the confluence of the 
Vilcanota River and the stream in the Cuzco Valley, 
which volcanic center is regarded as more or less con- 
temporaneous with the Rumicola. Thus, the 0.59 Ma 
age (see Table l), even if reported with a large un- 
certainty, seems to be móre satisfactory - the per- 
cent radiogenic argon content here beini 'high 
enough to be regarded as correctly supporting the 
age. k. 
CONCLUSIONS 
This study has  led to the following conclusions: 
1) A basic to acidic volcanism oí' latest Oligocene to 
earliest Miocene age is dated in the Crucero Basin. I t  
is contemporaneous with peraluminous plutonism al- 
ready reported by Clarlset aL(1983). 
2) The Pliocene age OP the Arco-Aja Pormation is 
demonstrated by the 3.79 Ma date of a n  interbedded 
tuff. Therefore, the Arco-Aja and La Paz Formations 
(Ballivian e t  al., 1978) - both of Pliocene age - can 
be correlated. Tho tuf€ of lhe Arco-Aja Formation is 
approximately coeval with the Chijini Toba daled at 
3.27 Ma (Clapperton, 1979) and with the youngest 
Macusani ignimbritic ash [low dated a t  4.2 Ma 
(Barnes et al., 1970). These data colzfirin 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. 
Acknowledgemenfs-Fieldwork wus curried out in 1980 und 1982 
under u cooperutive scientific agreement between INGEMMET 
(Peru) und ORSTOM (Frunce). We nre indebtod to ORSTOM Limn 
and ATP Geodynumique 11 (INAC, Paris) for field ussistance und 
support. ORSTOM (I'uris) und the  Ins l i lu t  Dolomieu, UA 69 
(CNRS) Grenoble supported rudiochronologic duting. ' 
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New cenozoic K-Ar ages on volcanic rocks from the estern high Andes, southern Peru

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