-
Earth and Planetao, Science Letters, 57 (1982) 47-62 47 Elsevier
Scientific Publishing Company, Amsterdam - Printed in The
Netherlands
[6]
A combined O, Sr, Nd, and Pb isotopic and trace element study of
crustal contamination in central Andean lavas,
I. Local geochemical variations
David E. James Carnegie Institution of Washington, Department of
Terrestrial Magnetism, 5241 Broad Branch Rd., N. ~ , Washington, DC
20015
(U.S.A.J
Received April 14, 1981 Revised version received October 5,
1981
Systematic variations in O, Sr, Nd, and Pb isotopic ratios and
in Sr, Nd, Pb, and Rb trace element abundances in central Andean
andesitic lavas are analyzed in terms of combined crustal
assimilation and fractional crystallization. The results of
integrated isotopic model calculations show that observed local
isotopic and trace element variations within the Arequipa and
Barroso volcanic groups of southern Peru are best fit by a simple
model involving shallow or intermediate depth crustal assimilation
of Charcani gneiss (or an equivalent rock) accompanied by
dominantly plagioclase fractional crystallization. A consequence of
this result is that the Rb-Sr pseudoisochrons reported for the
Arequipa and Barroso volcanics are wholly artifacts of the combined
processes of assimilation and plagioclase fractionation, as are all
of the Pb isotopic variations measured in the same lavas, already
interpreted by Tilton and Barreiro [14] to be the result of crustal
contamination. Isotopic and trace element compositions of the
primary magmas for the Arequipa and Barroso volcanic series are
obtained by correcting for assimilation and fractional
crystallization. The starting magmas are characterized over the
entire range of permissible primary compositions by high 87Sr/86Sr
ratios, low 143Nd/la4Nd ratios, and high Sr concentrations relative
to "normal" island arc andesites. These results appear to require
prior contamination of the magmas, but not involving plagioclase
fractionation. The uncertainty in Pb and O isotope ratios computed
for possible primary magmatic compositions is sufficiently large,
however, that it is not possible to preclude a subcrustal origin
for at least some of the regional variation in Sr and Nd isotopic
ratios.
I. Introduction
Insight into mountain building processes gained from plate
tectonics has provided the stimulus for a decade or more of
broad-ranging studies aimed at understanding the nature and
evolution of the Andean orogenic belt. With increased understand-
ing has come keener appreciation that the Andean system is
segmented into provinces with funda- mentally different geologic,
geochemical, and tectonic identities, and that results from one
province are not necessarily applicable to others. There are three
well-defined active Andean vol- canic provinces, generally termed
northern, central,
and southern, separated from one another by volcanically
inactive zones. Although each of the three active Andean volcanic
provinces has its own distinct geochemical "signature" [1], the
central Andean volcanics are remarkable for their anoma- lous
isotopic ratios and high abundances of large- ion-lithophile (LIL)
elements.
The origin of the unusual isotopic and trace element
compositions has generated a wide-ranging controversy, at the heart
of which is the issue of whether the compositional variations
observed are due to crustal contamination or are due instead to
compositional variations of the source (or are a combination of
both). Interest in the nature of
0012-821 X/82/0000-0000/$02.75 ;c~ 1982 Elsevier Scientific
Publishing Company
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48
Andean source material arises in part because of the potentially
large volumes of continental material available for subduction, and
the possible involvement of that material in subduction zone
melting. It is also in the Andean type arcs, how- ever, where
effects of crustal contamination (and possibly lithospheric mantle
contamination) are likely to be most important. Before any rational
conclusions can be drawn concerning the primary source of Andean
magmas, these various effects must be isolated.
The literature on central Andean volcanism in- cludes a
multitude of explanations for the origin of the isotopic and trace
element composition of the lavas: enriched continental lithosphere
[2,3]; magma mixing [4]; melting of lower crust [5]; subduction
zone melting of oceanic crust [6] and/or continentally derived
sediments [7,8]; and, of course, crustal contamination of "normal"
calc- alkaline magmas [9-14].
With each addition of new data in the past few years has come
increasing evidence for some form of crustal contamination. At
issue has been the nature of that contamination. James et al. [2]
showed that the high Sr abundances in the lavas of southern Peru
precludes simple bulk contamina- tion of the kind commonly modeled
in the litera- ture [15,16]. A similar conclusion was arrived at
from work on Argentinian lavas [11]. Relatively low 6t80 values in
the Peruvian lavas further limits the extent of crustal involvement
[7,8]. In an earlier attempt to reconcile the available data, I
proposed a petrogenetic model for magma genesis involving subducted
continental sediments [8]. Subsequent measurements of Pb isotopes
in these same lavas [14], however, indicate that subduction zone
melting of continental sediments cannot pro- vide a general
explanation for the geochemical variations observed.
In the past two years, considerable effort has been devoted to
developing a sensible model for crustal contamination [4,9,11-13].
The effort has been spurrred by a recognition that simple processes
such as two-component mixing or frac- tional crystallization are
not adequate to explain observed data, but that more physically
realistic models are required. Of particular importance has been
the development of a coupled assimilation-
fractional crystallization model in which the heat required to
melt assimilated wall rock is provided by the latent heat of
crystallization of the magma [13,17,18]. It has, of course, been
known since the time of Bowen that except for superheated mag- mas
assimilation must be accompanied by crystal fractionation. The
assimilation-fractional crys- tallization process seems not to have
been em- phasized, however, because assimilation of most common
rock types does not seriously alter major element chemistry or the
liquid line of descent for a differentiated series, although the
proportion of differentiates may vary [18]. In an important re-
cent paper, Taylor [18] showed that while assimi- lation-fractional
crystallization may not greatly affect the differentiation process
the consequences for trace element and isotope geochemistry can be
profound. In particular, the effects of combined
assimilation-fractional crystallization on isotope ratios and trace
element concentrations can differ markedly from those produced by
either two com- ponent mixing or fractional crystallization alone
[7,8].
This paper is the first of two papers whose purpose it is to
interpret the combined isotopic and trace element data for southern
Peru lavas in terms of the assimilation-fractional crystallization
model. This study utilizes a more complete data base than has been
available for previous work on central Andean crustal
contamination. Computer modeling is used to obtain model fits to
the com- bined isotopic and trace element data.
Included in this study are O, Sr, Nd, and Pb isotope ratios and
Sr, Nd, Pb and Rb abundances measured in the same rock samples from
the Arequipa and Barroso volcanic groups of southern Peru. The
southern Peru volcanics are notable in that they exhibit systematic
geochemical variations of two distinct types: a regional
geochemical vari- ation between volcanic groups of southern Peru,
and superimposed local geochemical variations within comagmatic
volcanic series.
The studies described in this paper are aimed at understanding
the nature and origin of the local geochemical variations. A second
paper will ex- amine the regional geochemical variations. The
primary purpose of the present paper is to demon- strate that local
geochemical variations within both
-
the Arequipa and Barroso volcanics can be mod- eled by a process
of assimilation of Charcani gneiss (or a rock of equivalent
composition) accompanied by dominantly plagioclase fractional
crystallization at comparatively shallow depths, possibly in a
magma chamber. Using this model, an attempt is made to define the
compositional bounds of the primary magmas from which the Arequipa
and Barroso volcanics were derived. Knowledge of the composition of
these primary magmas is essential to characterize the intrinsic
regional geochemical heterogeneity in southern Peru.
2. Analytical data
Isotopic and trace element data for the Are- quipa and Barroso
volcanics of southern Peru are summarized in Table 1. Sample
localities are shown in Fig. 1. The general geology and
geochemistry of the southern Peru volcanics have been discussed
elsewhere in some detail [2,8,19,20]. Geochemi- cally, the lavas
can be grouped regionally into at least five distinct series [8],
the most important of which are the Arequipa volcanics and the
Barroso volcanics (see Fig. 1). The Arequipa volcanics are all
younger than 1 m.y. and are essentially unal- tered, whereas the
Barroso volcanics are early Pliocene to Pleistocene in age [21] and
exhibit evidence of incipient low-temperature alteration. The
slight alteration in the Barroso volcanics ap- pears to have little
effect on radiogenic isotope ratios or trace element abundances,
but it may be a cause of greater scatter of ~80 values.
The radiogenic isotopic variations in the Are- quipa and Barroso
volcanics are shown in Fig. 2. 180 versus radiogenic isotope ratios
is shown in Fig. 3. While ~80 exhibits little correlation with
radiogenic isotope ratios, there are distinct inter- correlations
betwen radiogenic isotope ratios shown in Fig. 2. All of the
interisotopic correla- tions exhibit what appear to be continuous
trends which pass uniformly through both the Barroso and Arequipa
volcanics, as though the two volcanic series evolved from a single
parent magma inter- acting with a single contaminant. Isotopic
versus trace element variations within the Arequipa and
49
Barroso volcanics show, however, that the two series could not
have been derived from the same parent magma. This is shown in Fig.
4 where, for the purpose of illustration, 87Sr/86Sr ratios are
plotted against trace element abundances and in Fig. 5 where
8VSr/S6Sr ratios are plotted versus Rb/Sr ratios. Despite this
distinct contrast be- tween the two groups, the interisotopic
corre- lations between them are remarkably consistent, and seem to
suggest a prior stage of contamination in the lower Andean crust.
The Pb-Pb data in particular seem to demand this. More on this
question will be discussed in a subsequent paper.
Aspects of the combined isotopic and trace element data
important for modeling calculations include:
(1) Sr is the only trace element studied which decreases in
abundance with increasing 8VSr/86Sr. All other elements inlcuding
Nd, Pb, Rb, and Ba increase with increasing SVSr/S6Sr. This
confirms the dominant role of plagioclase in the magmatic
evolution, as the plagioclase partition coefficient for Sr is 1.8
[22], but for the other elements is 0.1 [22-24].
(2) Sr concentrations in the lavas are unusually high, relative
both to other trace element abun- dances and to Sr abundances in
"normal" island arc volcanics. The high Sr abundances are re-
flected in high Sr/Rb, Sr /Nd and Sr/Pb ratios relative to many
oceanic island arc lavas [25].
(3) The more isotopically "evolved" Arequipa volcanics are
characterized by lower average Rb, Nd, and Pb abundances and Rb/Sr
ratios, and higher Sr abundances and Sr/Nd, Sr /Pb and K /Rb ratios
than the more isotopically "primitive" Barroso volcanics.
(4) The Rb-Sr data (Fig. 5) define "pseudoiso- chrons" yielding
ages of 310 m.y. for the Barroso volcanics (correlation coeff ic
ient- 0.91) and 440 m.y. for the Arequipa volcanics (correlation
coeffi- cient = 0.82). Neither the Pb nor Nd-Sm data pro- duce
similarly unambiguous pseudoisochrons.
The coherence in the isotopic and trace element data for both
the Arequipa and Barroso volcanics provides a necessary condition
for modeling of the processes which gave rise to the observed
hetero- geneity. It bears emphasizing that the kind of analysis
presented here requires not only a corn-
-
TA
BLE
I
An
aly
tica
l da
ta.
Sa
mp
le
6 ~
80 a
87Sr/
S6Sr b
14
3N
d/t
44
Nd
c
20
4p
b/2
06
pb
d
20
7p
b/2
06
pb
a
Sr e
N
d e
P
b d
R
b e
No
. (%
0)
(pp
m)
(pp
m)
(pp
m)
(pp
m)
A re
quip
a volc
anic
s PE
24
7.2
7
0.7
0797
0.5
1217
0.0
5620
0.8
780
699.0
25.0
12.5
PE25
7.4
2
0.7
0765
657.0
PE26
7.2
8
0.7
0707
0.5
1221
0.0
5550
0.8
672
848.0
26.0
12.2
PE
27
7
.41
0.7
0718
848.0
PE
28
7
.25
0.7
0719
842.0
PE
29
7.4
2
0.7
07
11
856.0
PE
30
7.1
2
0.7
0709
849.0
PE
46
7
.01
0
.70
75
1
804.0
33.3
PE
47
6.9
0
0.7
0760
0.5
1229
0.0
5604
0.8
749
785.0
32.0
11.4
PE
48
6.8
3
0.7
0762
816.0
PE
49
7.0
5
0.7
0762
0.5
1220
0.0
5604
0.8
748
804.0
30.0
16.4
PE
79
7
.05
0.7
0749
780.0
PE
80
7.0
8
0.7
0742
753.0
PE81
6.8
7
0.7
0673
0.5
1227
799.0
24.0
PE
82
7.1
8
0.7
0733
0.5
1222
0.0
5561
0.8
670
763.0
13.4
PE83
6.9
8
0.7
0737
0.0
5572
0.8
694
730.0
16.2
Barr
oso
volc
anic
s PE128
7.9
2
0.7
0544
0.5
1235
675.0
26.0
PE
12
9
7.1
9
0.7
0647
0.5
1232
0.0
5471
0.8
534
614.0
0.0
22.1
P
EI3
0
7.4
2
0.7
05
51
0.5
1236
0.0
5467
0.8
523
653.0
25.6
10.7
P
EI3
1
7.1
8
0.7
0564
0.5
1224
0.0
5481
0.8
541
609.0
30.0
15.7
PE132
7.2
7
0.7
0644
579.0
PE
14
4
6.9
0
0.7
0644
0.5
1242
0.0
5508
0.8
581
590.0
25.7
15.9
P
EI4
5
6.9
9
0.7
0656
0.5
1227
0.0
5505
0.8
581
592.0
16.5
57.0
61.0
4
6.0
4
6.0
4
6.0
4
7.0
4
7.0
71.0
67.0
69.0
69.0
47.0
50.0
39.0
51.0
63.0
48
.0
79.0
53.0
78.0
97.0
84.0
86.0
a (~
lS O
. n
orm
ali
zed
to
NB
S N
o. 28 q
ua
rtz st
an
da
rd=
9.5
5%
0.
Re
sult
s fro
m Ja
me
s [8
].
b S
TSr/
S6Sr n
orm
ali
zed
to
E&
A s
tan
da
rd
SrC
O 3 =
0.7
08
00
a
nd
86
Sr/
/SS
Sr-
0.1
19
4.
20
un
ce
rta
inty
,'~;'1 0.704 I
500 600 700 800 Sr (ppm)
i i
0.-? 12 0.716 87S J86Sr
9.0
~ 8.0 o
oo 7.0
6 0 .5114 0.5'118
12.6 I0,0 '..:,
,,o.,.~6. . , \ '.ii;..
0,5124 AR2 % "~i""
~""'I " ' - -1 , , .... ~:~:AR2 - o , ,~ ~,0,po~,~o ,;
"-..:.:::::...,::~.:.i~.o, e .....
ARI I 0.5122 0.5126
143Nd/144Nd
9.0
~ 8.0 o
OO
7.0
6.0 0.5114 0.5118
I0.0
::;.. 0
i " _ -,~,~,o j \.":::o
0.5124 7 " ~ .~...
0,,2~,, ~0 2, ,o '~ ....... ' : ; "~ Nd (ppm) BA2 BA
h i
0~,~ ' 0.5.26 143Nd/144Nd
9.0
2 8.0 o
co 7 .0
6.0
~0.1~8. ', . ,,,,..' - , /ARZ .~;-~:--~r- /
:::L' 1 j / 12,6 ?ARI / 0.821 I i m ..:..,F
5 IO t5 20 / '3" I0.0 Pb (ppm) .."" ." /J.
..-II .-' / . ' - I I
..... ~_~" . i 7 ........ L , - -1~ ........ _..s
~ ~:';:';"'+" ~":'" ............ ,&'h2 ......... ~,RI i i /
i
0.84 0.86 0 .88 2o7pb/2O6pb
I0.0
9.0
o 8.0 oO 60
7.0 (
6.0
1 i
0.86 ' ' . . - 'S - - "
~o .8 , . / ? , , . / 083 ~10 15 20 25
Pb (ppm}
12.6 - , I I0.0
. / /
0 "
-
59
0.710 I I I I I
,,o 0 .708 Go
~ 0.706
O. 704 0
zo
'o Anl ~ ~ o~ BAt
0 60 80 I O0
AR2 ~
BA2 I I 1 I I
0 20 40 60 80 I00
Rb (ppm)
Fig. I 1. Assimilation-fractional crystallization mixing curves
for S7Sr/S6Sr versus Rb. Weight proportion of assimilated material
to weight of starting magma shown as tics on mixing lines. Some
data points not included in Table 1. Symbols as in Fig. 7.
End-member model compositions given in Table 2.
and Nd-Pb isotopic correlations shown in Fig. 9. The most
definitive relationships are shown by the Arequipa volcanics where
the Sr-Nd data form a cluster trending off to the right of the
mantle array. For the Barroso volcanics, the data lie within or
below the extension of the mantle array. Plots of Nd versus Pb
isotope ratios exhibit considerable scatter and no clear mixing
trend can be observed. Within the limits of scatter, however,
computed mixing curves are consistent with the observed data.
8180 values (measured in plagioclase separates) versus
radiogenic isotope ratios are shown in Fig. 10. In terms of
predicted mixing behavior, the lack of correlation between 8180 and
radiogenic iso- tope ratios in the volcanics is disappointing but
not surprising. A major cause of scatter in the oxygen data is
almost certainly the large variation in 8'80 of Peruvian gneisses
(7.5 to 12.6%o, D.E. James, unpublished data) which is uncorrelated
with radiogenic isotopic ratios in those rocks. The effect of the
scatter in 8180 of the contaminant is shown in Fig. 10, where
dashed (or dash-dot) mixing lines are indicated for ~180 c = 10.0%o
and dotted mixing lines for 818Oc of 7.5 and 12.6%o. Most of the
analytical data for the Arequipa and Barroso volcanics lie within
this range of values. The oxygen isotopic data demand at least some
degree of crustal contamination. Beyond that, however, 8180 values
in the volcanics seem to reflect primarily a lack of correlation
between 8 '80 and radiogenic isotope ratios in the assimi- lated
material. This lack of correlation effectively
precludes the use of oxygen isotopes for quantita- tive
modeling, although the oxygen isotope data are not inconsistent
with the model proposed here.
The variations in elemental abundances versus radiogenic
isotopic ratios are shown on insets in the plots in Fig. 10 and
also in Fig. 11. The observed correlations between isotope ratios
and element concentrations confirm that the dominant cumulate phase
must be plagioclase, with Ksr > 1.0 and KNd.eb,Rb --~ 0.1. In
general, the scatter in trace element concentrations appears to
reflect varying ratios of assimilation to fractional
crystallization. In the simplified model considered here, that
ratio is taken to be constant and, especially for the more evolved
end-members of the volcanic series, may be too low.
5. Discussion and conclusions
While there is considerable latitude and uncer- tainty in the
kind of modeling done here, the results presented in this paper
indicate that the Arequipa and Barroso volcanic series could well
have evolved by some process of assimilation and fractional
crystallization. The local isotopic and trace element variations
observed can be produced by assimilation of Precambrian crustal
material accompanied by dominantly plagioclase fractiona- tion.
Insofar as this model is correct, the composi- tion of primary
magmas for the two volcanic series lie within the range of
compositions given in Table 2. The following conclusions can be
drawn
-
60
from this study: (1) The Rb-Sr pseudoisochrons reported by
James et al. [2] for the Arequipa and Barroso volcanics are
entirely an artifact of the assimila- tion-fractional
crystallization process modeled here. Consequently, the
pseudoisochrons have no direct age significance and no relation to
the na- ture of the Andean mantle or to subduction zone
melting.
(2) The primary magmas for the Arequipa and Barroso volcanics
had 87Sr//86Sr ratios of at least 0.7062 and 0.7050, respectively,
and 143Nd/la4Nd ratios of probably no more than about 0.51255.
Primary compositions with 87Sr/86Sr ratios near 0.703 as suggested
by Harmon et al. [12] do not and cannot satisfy the southern Peru
data. Com- puted primary magma compositions ar.e well con- strained
by the observed isotopic correlations and are clearly outside the
range of compositions for "normal" island arc magmas.
(3) The Sr and Nd isotopic composition of the primary magmas and
their high Sr concentrations, together with the interisotopic
colinearity of the Arequipa and Barroso series suggests prior con-
tamination of the magmas by Precambrian material similar to that
exposed at the surface in southern Peru. That contamination would
have had to have taken place in the lower Andean crust (> 50 km)
where plagioclase is unstable. The results are suffi- ciently
ambiguous, however, that we cannot rule out a subcrustal origin for
some of the anomalous Sr and Nd isotopic compositions.
(4) Computed primary magma compositions are characterized by
very high Sr concentrations which are not reflected in uniformly
high concentrations of the other LIL elements studied here. As a
result, Sr/Nd ratios are anomalously high (34-80) rela- tive to
"normal" mantle or chondritic ratios of
18 [32], and Rb/Sr ratios are comparatively low, especially for
the Arequipa volcanics, relative to oceanic calc-alkaline volcanics
[33]. Sr/Pb ratios do not appear to be excessively high compared to
those measured in Marianas lavas [30], but they are on the high
side of ratios measured for other island arc lavas [25].
Although there is no a priori reason for choos- ing the more
"evolved" primary magma composi- tions (AR2 and BA2) over the more
"primitive"
compositions (AR1 and BA1), there are several reasons (albeit
none of them compelling) for pre- ferring the evolved compositions.
Firstly, they do not require as much magmatic differentiation be-
fore the first lavas are extruded; if the more primi- tive
compositions (AR1 and BA1) represented the primary magma, it is
surprising that no lavas close to that composition are found.
Secondly, the rela- tively small negative Eu anomalies in the more
isotopically primitive rocks of each volcanic series ([26], W.
Leeman, personal communication) seem to preclude large degrees of
plagioclase removal. Finally, models AR2 and BA2 require Pb con-
centrations of about 15 ppm in the contaminant, close to the
average concentration observed in the Charcani gneiss. If AR2 and
BA2 are appropriate primary magma compositions for the Arequipa and
Barroso volcanics respectively, then it appears that the primary
magmas reflect a prior crustal overprint. The nature and origin of
that overprint will be examined in a subsequent paper [34].
Whatever the precise composition of the primary mgmas, the
dominant role of plagioclase fractiona- tion in the evolution of
both volcanic series effec- tively precludes lower crustal (>50
km) con- tamination as a primary mechanism for producing the local
geochemical variations. An important conclusion arising from the
results presented here is that the central Andean geochemical data
can be modeled by straightforward and physically plausible
processes known to occur during volcanism--that is, assimilation of
wall rock and fractional crystallization, probably at relatively
shallow depths in a magma chamber. In an at- tempt to explain the
Sr isotopic and Sr concentra- tion data in these same volcanics,
Briqueu and Lancelot [9] suggested a process of selective con-
tamination through "collector zones of magma" in the crust. Their
suggestion has been reiterated more recently by Francis et al.
[10]. The magma collector zones are presumed to correspond to low-
velocity zones in the Andean crust as postulated by Ocola and Meyer
[35]. As there has been con- siderable confusion concerning the
significance of these "low-velocity zones" it seems appropriate to
emphasize that it has never been demonstrated that any significant
low-velocity zones exist in the Andean crust, despite the published
interpreta-
-
tions. The confusion arises because all Andean seismic profiles
for which low velocity zones have been inferred are unreversed
(i.e., there is a shot point at only one end of the observing line
of seismic stations). For such unreversed profiles, lateral changes
in geologic structure due to fault offsets or sharp changes in
lithology can produce seismic effects virtually indistinguishable
from those produced by low-velocity zones. When un- reversed
travel-time data are interpreted in terms of laterally homogeneous
(i.e., onion skin) earth structure, then travel-time anomalies
produced by lateral changes in structure are by necessity incor-
porated in (spurious) velocity-depth models which may contain
equally spurious crustal low- velocity zones. For the central
Andes, the structures so obtained do not agree with surface-wave
data or with much of the refraction data [36]. There is, in short,
no seismological basis for postulating zones of partial melting in
the Andean crust and hence there is no basis for attributing
geochemical effects to them. Results presented here show that such
models are not required to explain the data.
Finally, the results presented in this paper per- mit some
definite limits to be placed on the com- positions of possible
primary magmas for the Arequipa and Barroso volcanic series. These
com- positional limits serve as an essential basis for
characterizing the regional geochemical hetero- geneity in the
southern Peru lavas. In terms of radiogenic isotope ratios, the
Barroso volcanics exhibit consistently a more "primitive" character
than do the Arequipa volcanics. Yet in terms of trace element
abundances and abundance ratios, the Barroso volcanics appear to be
considerably more "continental" in character than the Arequipa
volcanics, with higher Nd, Pb, and Rb concentra- tions, lower Sr
concentrations, and, possibly, slightly higher 8180 values. It
remains to reconcile these contradictory characteristics of the
regional geochemical variations with self-consistent models of
petrogenesis and magmatic evolution of the central Andean
lavas.
Acknowledgements
Field studies for this work were supported by the National
Science Foundation and H.O. Wood
61
fund and were carried out in collaboration with Servicio
Geologico del Peru. I am indebted to Dr. Mark Feigenson for making
some of the Sr isotope and trace element analyses and to D. Diez de
Medina for point counts of the Arequipa and Barroso volcanics.
Oxygen isotope analysis were done on a mass spectrometer maintained
by Dr. T. Hoering of the Geophysical Laboratory of the Carnegie
Institution.
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