Geochemistry and origin of volcanic rocks of the Andes (26°–28°S)

Contrib. Mineral. Petrol. 63, 113-128 (1977) Contributions to Mineralogy and Petrology �9 by Springer-Verlag 1977

Geochemistry and Origin of Volcanic Rocks of the Andes (26~176

Jaroslav DostaP, Marcos Zentilli 2, Juan C. Caelles 3, and Alan H. Clark ~ 1 Department of Geology, Saint Mary's University, Halifax, Nova Scotia, Canada B3H 3C3 2 Department of Geology, Dalhousie University, Halifax, Nova Scotia, Canada B3H 3J5 3 Cominco Ltd., 200 Granville Square, Vancouver, British Columbia, Canada V6C 2R2 4 Department of Geological Sciences, Queen's University, Kingston, Ontario, Canada K7L 3N6

Abstract. Mesozoic to Recent volcanic rocks from a transect of the Central Andes between latitudes 26 ~ and 28 ~ South in northern Chile and Argentina show chemical and temporal zonation with respect to the Peru-Chile trench. Jurassic to Eocene lavas occur closer to the trench and are comparable to calc-alkaline rocks of island arcs. Eastwards they are followed by Miocene to Quaternary sequences of typical continental margin calc-alkaline rocks which have higher contents of K, Rb, Sr, Ba, Zr, and REE and also higher K/Na and La/Yb ratios. The rocks occurring farthest f rom the trench have shoshonitic affinities. The distribution of major and trace elements is consistent with a model in which magmas were derived by anatexis of an upper mantle source already enriched in LILE and located above the descending oceanic slab. It is suggested that the chemical variations across the volcanic belt reflect systematic changes in the composit ion of the magmas due to a decreasing degree of partial melting with increasing depth, and probably also due to the heterogeneity of the source materials.

Introduction

The Central Andes between 26 ~ and 28 ~ South in northern Chile and Argentina (Fig. 1) contains one of the most complete records of igneous activity available for the western margin of South America. This provides a good opportunity to study the nature of the magmatism associated with the interaction of converg- ing oceanic and continental lithosphere. In this area, " A n d e a n " magmat ism started possibly as early as the Triassic and was almost continuous from the Jurassic to the Quaternary. A series of cycles of volcanism took place along partially superimposed longitudinal belts (Clarke and Zentilli, 1972). Jurassic volcanism, although more intense in the belt nearest the present coast, extended more than 100 km inland. The axis of magmat ism migrated eastward at a rate increasing f rom ca. 0.6 mm/yr, in the Early Mesozoic to ca. 1.10 ram/yr. in the mid-Tertiary. In the Miocene, an abrupt extension of magmatic activity

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Geochemistry of Volcanic Rocks of the Andes 115

deep into the continent resulted in the magmatic eruptions at distances of up to 250 km to the east of the former inner boundary of the orogen, giving rise to a volcanic belt many times broader than those formed earlier. In the Upper Pliocene and the Pleistocene, a rapid westward rejuvenation of volcanism led to the formation of stratovolcanic complexes located near the continental divide.

The volcanic rocks of the area have been described by Segerstrom (1968) and Zentilli (1974), while the geology of the Argentinian part of the transect has been summarized by Caminos (1972). McNutt et al. (1975) have discussed the Sr isotopes of the volcanic and plutonic rocks of the area.

The purpose of this paper is to present geochemical data on the volcanic rocks of the transect of the Central Andes in northern Chile and Argentina (latitudes 26~ ~ South) and to discuss their petrogenetic implications.

Analytical Methods

The contents of major and trace elements (Rb, Sr, Ba and Zr) have been determined by X-ray fluorescence and those of Ni, Co, Cr and V by emission spectroscopy. The rare-earth elements (REE) were determined by an instrumental neutron activation technique using BCR-1 as a standard. The precision of the data obtained by X-ray fluorescence and neutron activation is generally better than 10% while for the determinations by emission spectroscopy, the precision is better than 20%. The accuracy of the trace element data may be estimated from the analysis of the standard rock W-1 (Tables 1 and 2).

Geochemistry

The average chemical compositions of volcanic rocks from the transect 1 are given in Table 1. These rocks, ranging in age from Middle Jurassic to Quater- nary, cover most of the age span of volcanic activities associated with the Andean orogeny between latidudes 26 ~ and 28 ~ South. According to their age and geographic position, the rocks have been divided into five groups (Fig. 1). Group 1 comprises rocks of Jurassic to Early Cretaceous age collected about 160-230 km from the Peru-Chile trench; these rocks are intercalated with shal- low-marine and continental sediments. The rocks of t h e second group, with ages ranging from Late Cretaceous to Eocene, were collected 180-230 km from the trench. The last three groups include rocks of Miocene to Quaternary age. The rocks of group 3 crop out 230 to 260 km from the trench, those of the fourth group 260-360 km and those of the fifth group 480 530 km from the trench. Rocks of groups 2 through 5 are subaerial volcanics.

Major Elements

The content of SiO2 (on an anhydrous basis) in the rocks of group 1 varies within a narrow range, with the majority of rocks having 53-55% S i O 2. Although the S i O 2 content of most rocks of group 2 varies between 53 and 62%, this

1 Complete chemical analyses and locations of the samples may be obtained on request from the authors

Tab

le l

. A

vera

ge C

om

po

siti

on

of

Vol

can

ic S

uite

s of

th

e T

ran

sect

Gro

up

1

Gro

up

2

Gro

up

3

Gro

up

4

Gro

up

5

W-1

SiO

2 (

wt%

) 53

.52

50.9

1 54

.54

61.2

9 60

.68

64.2

6 61

.08

63.7

1 51

.18

61.3

8 63

.31

TiO

2 1.

19

1.08

0.

89

0.62

0.

80

0.56

0.

86

0.77

1.

25

0.73

0.

68

A12

03

18.1

8 19

.54

18.1

7 18

.04

18.5

4 17

.24

17.0

2 17

.12

17.1

4 17

.86

17.3

9 F

eOt

9.06

9.

02

8.73

5.

29

4.64

3.

84

5.44

3.

92

8.92

5.

56

4.39

M

gO

4.

32

4.82

3.

81

2.65

2.

21

1.64

2.

71

1.76

4.

24

1.81

2.

03

CaO

7.

87

8.87

7.

41

4.16

6.

14

4.81

5.

83

4.59

10

.06

4.39

5.

21

Na2

0

4.18

3.

80

4,63

4.

90

3.98

4.

24

3.75

3.

77

3.35

4.

28

2.9

8.

K2

0

1.12

1.

04

1.25

1.

35

1.81

1.

94

2.64

2.

78

3.11

3.

53

3.46

F

eOt/

Mg

O

2.10

1.

87

2.29

2.

00

2.10

2.

34

2.01

2.

23

2.10

3.

07

2.16

K

20

/Naz

O

0.27

0.

27

0.27

0.

28

0.45

0.

46

0.70

0.

74

0.93

0.

82

1.16

Rb

(pp

m)

18

15

21

25

66

80

108

107

80

130

87

Sr

396

434

491

563

860

758

545

572

512

690

510

Ba

370

420

508

474

815

718

605

724

556

948

628

Zr

157

70

99

118

215

184

225

226

141

210

152

Ni

26

25

22

19

10

22

16

35

23

Co

26

23

17

11

16

11

42

33

30

V

368

317

145

93

117

103

208

110

Cr

50

20

11

10

10

9 45

38

88

30

6

K/R

b

517

575

494

448

228

201

203

216

322

225

330

Rb

/Sr

0.04

5 0.

035

0.04

3 0.

044

0.07

7 0.

106

0.19

8 0.

187

0.15

6 0.

188

0,17

1 B

a/R

b

21

28

24

19

12

9 6

7 7

7 7

No.

of

sam

ple

s 5

2 2

2 3

5 8

4 2

3 3

22

181

201 96

47

110

~J

.=_


Group I o Group 2 z~ Group 3

/ \ Group 4 Group 5

/ \

is.~ %%% / / / ~ �9 �9 o ""

\ / / . ~ Y

/'"i>'" \ / ." \

K20

Fig. 2. AFM diagram for volcanic rocks of the transect. Dashed lines represent the limits of the hypersthenic (calc-alkaline) series of Kuno (1968)

group also includes rhyolites with g i O 2 >68%. In the rocks of the third and fourth groups, S i O 2 ranges from 59 to more than 70%, while in group 5, it varies from 50 to 69%.

Most of the analyzed volcanics have mineralogical and chemical characteristics of calc-alkaline rocks. The exceptions are the rocks of group 5 which have a Kz O/Na20 ratio close to unity, a feature typical of shoshonites (Jakes and White, 1972), and a high content of K20 similar to that of alkalie basalts. In comparison with alkali basalts, the rocks of group 5 have a different mineralogy and lower abundances of TiO 2 and Zr. The volcanic rocks of the transect do not show significant " i ron enrichment" and fall on the A F M diagram (Fig. 2) into the field of the hypersthenic (calc-alkaline) rock-series of Kuno (1968).

Volcanic rocks of the transect show distinct spatial and temporal chemical variations. Among the major elements, the variation is clearly displayed by potassium (Fig. 3); for a given S i O 2 content, K 2 0 regularly increases with distance from the trench. The same also applies to the KzO/Na20 ratio, although its variation is smaller than that across typical island arcs (Table 1). The increase of this ratio is due to the increase of K20 since the content of Na20 does not change significantly .with either S i O 2 o r geographic position.

According to the geochemical criteria of Jakes and White (1972) for dis- tinguishing between calc-alkaline rocks of continental margins (Andean-type)

118 J. Dostal et al.

Table 2. Rare Earth Element Abundances (in ppm) in Volcanic Rocks of the Transect

Group 1 Group 2 Group 3

DZ-1 DZ-2 DZ-3 DZ-4 DZ-5 DZ-6 DZ-7 DZ-8 DZ-9

SiO2(wt%) 53.19 53.98 54.00 50.27 55.08 60.77 60.97 62.73 63.20 La(ppm) 13.9 15.9 8.16 9.17 15.7 19.2 25.1 22.4 24.1 Ce 35.i 40.4 20.2 23.8 36.1 43.6 57.3 51.0 54.4 Sm 4.86 5.72 3.43 3.82 4.92 3.74 4.67 3.88 4.85 Eu 1.37 1.50 1.11 1.19 1.30 1.23 1.35 1.17 1.41 Tb 0.667 0.892 0.502 0.625 0.747 0.586 0.539 0.433 0.620 Yb 2.70 3.72 2.09 2.61 3.11 1.46 1.26 0.952 1.42 Lu 0.452 0.615 0.343 0.445 0.547 0.216 0.178 0.140 0.205

and island arcs, it appears (Table l) that the rocks of groups 1 and 2 are similar to island arc volcanics, while the rocks of groups 3 and 4 are typical continental margin andesites. The presence of island arc volcanic rocks in a continental margin environment is, however, not unique. These suites were reported e.g. f rom the Cascade Range of Nor th America (Condie and Swenson, 1973) and southern Chile (Lopez-Escobar et al., 1974). It is of interest that, as in southern Chile, the island arc calc-alkaline volcanic rocks of the studied transect are restricted to areas with relatively thin continental crust where the distance to the underlying seismic zone is probably small.

K~/o

3 x Group 1 0 Group 2 /k Group 3 �9 Group 4 �9 Group 5

I i I I I

5'2 s8 6o 68 Si02 ( ~'o )

Fig. 3. Variation of K versus SiO 2 in the volcanic rocks of the transect. Vertical bars correspond to 1 standard deviation


Group 4 Group 5

DZ-10 DZ-I I DZ-12 DZ-13 DZ-14 DZ-15 DZ-16 DZ-17 W-1

62.42 61.70 65.01 61.46 50.87 51.48 61.24 45.6 44.0 35.8 38.9 25.4 26.8 42.4

102 96.9 68.6 86.3 59.6 63.2 84.9 8.00 7.61 5.39 6.35 6.04 5.77 7.02 1.62 1.87 1.65 1.75 1.73 1.67 1.86 0.738 1.03 0.888 0.921 0.8t6 0.806 0.860 2.18 3.34 2.56 2.78 2.35 2.17 2.08 0.341 0.527 0.380 0.422 0.366 0.341 0.317

63.63 29.9 69.9

6.46 1.63 0.804 2.51 0.419

10.6 23.9

3.66 1.06 0.638 2.06 0.345

10

5

3

2O0

100'

50' z

20

o I0

5 m~

A Group i ~ DZ-2 . . . . 0Z-I

Group 2 ~ DZ-5 . . . . DZ-4

DZ-3

- . 2 : 2 .~ . . . . . . . -o . . . . . . . . . . . . . . . ~ - -e

B Group 3 ~ DZ-6 a---~ 0Z-7 ,~-'- ~. DZ-8

- - . ' . ~ ~....z~ DZ-9 '".ii'-~.-..'~.'~.'~.'~.'~.~. Group 4 ~ DZ-IO ~_ ' ~ J " : . i ~ o---~ DZ-l l

" DZ-13

............ 222 . . . . :}

C L.... Group 5 ~ DZ-14

I00 ; ",,. w---g DZ-15 ~'-':-'._-..'. -.. ,-.- -, DZ-16

30i5010" ~ ~ ' " " " ' " ; ' > ~ " ,,-.-..,, DZ-17

La Co Sm Eu Tb Yb Lu

Fig. 4A-C. Chondrite-normalized REE abundances in volcanic rocks of the transect


Trace Elements

The average abundances of several trace elements in the studied volcanic rocks are given in Table 1. The contents of Rb, Ba, Sr and Zr in the rocks of the transect are similar to those of comparable rocks of other orogenic areas (Jakes and White, 1972; Noble et al., 1975). The rocks of groups 1 and 2 of a given SiO2 content are lower in Rb, Ba, Sr and Zr than the rocks occurring farther from the trench. The abundances of these elements in group 5 do not differ significantly from those of groups 3 and 4. The K/Rb ratio of rocks of groups 1 and 2 is distinctly higher than that of groups 3, 4 and 5. In the calc-alkaline rocks (groups 1, 2, 3 and 4), the Rb/Sr ratio increases while the Ba/Rb ratio decreases with distance from the trench. The ratios for group 5 are similar to those of group 4.

The REE abundances of representative samples of the rocks from the transect are given in Table 2 and are plotted normalized to chondrites (Frey et al., 1968) in Figure 4. The REE patterns of groups 1 and 2 show a moderate enrichment in light REE (LREE) and only small fractionation of heavy REE (HREE), features which are typical of calc-alkaline rocks of island arcs (Taylor et al., 1969; Gill, 1970; Yajima et al., 1972; Lopez-Escobar et al., 1974). Ande- sites of groups 3 and 4 are strongly enriched in LREE and have distinctly fractionated HREE. Their REE distribution is similar to that of continental margin andesitic rocks (Noble et al., 1975; Thorpe et al., 1976). For a given SiO2 content, the rocks of group 4 have higher absolute contents of REE than the rocks of group 3. The REE concentrations and abundance patterns of group 5 are comparable to those of shoshonites from Fiji (Gill, 1970) and New Guinea (Jakes and Gill, 1970).

Among the volcanic rocks of the transect, there is a compositional gap between andesitic rocks and ignimbritic rhyolites. It is possible that many of the rhyolites are not directly related to andesites and resulted from partial fusion of upper crustal material (Lefevre et al., 1976; Zeil and Pichler, 1967).

Petrogenesis of the Volcanic Rocks

According to the major and trace element compositions, three rock associations can be distinguished. The first association, represented by groups 1 and 2, is closely comparable to the calc-atkaline rocks of the island arcs (Jakes and White, 1972) and is situated closest to the trench. The content of some incompat- ible trace elements (Ba, Sr) in these rocks is, however, somewhat higher than the average abundances given by Jakes and White (1972). The second association, as exemplified by groups 3 and 4, corresponds to typical calc-alkaline rocks of the continental margins. The third association occurring farthest from the trench consists of rocks of shoshonitic affinities.

The geochemical differences among the three associations cannot be explained by low-pressure fractionation involving the present phenocryst phases. These differences probably reflect either various conditions under which magmas were formed or different source materials (Dickinson and Hatherton, 1967). Some possible processes which may account for the genesis and the observed


chemical variations of the volcanic rocks from the Andean transect are evaluated in the light of the trace element, particularly REE, data. The models examined were recently reviewed by Boettcher (1973) and Ringwood (1974) and include: 1. Anatexis of the lower crust; 2. Fractionation of basaltic magma by the crystallization of amphibole, or partial melting of amphibolite; 3. Fractionation of basaltic magma by the crystallization of garnet and clinopyroxene, or anatexis of quartz eclogite; 4. PartiMmelting of hydrous upper mantle peridotite.

Anatexis of the Lower Crust

An origin of the Andean andesitic rocks by partial melting of the lower crust has been suggested by Pichler and Zeil (1969, 1972) and Fernandez et al. (1973). Furthermore, Andriambololona (1976) has recently shown that the distribution of transition elements in andesites of southern Peru is consistent with their derivation from garnet granulites. However, there are a number of data which argue against this process (c.f. Ringwood, 1974). They include the occurrence of similar calc-alkaline rocks in some island arcs without pre-existing old continental crust (Gorshkov, 1962; Taylor et al., 1969) and spatial chemical variations and their relationship with the subduction zones (Hatherton and Dickinson, 1969). Likewise on the basis of changes in the thickness of the Andean crust since Mesozoic time, James et al. (1976) concluded that a lower crustal origin for the volcanic rocks is not likely. In the light of these arguments the model will not be discussed further. It is possible, however, that the rising magma was contaminated by crustal material to a small degree (McNutt et al., 1975).

Amphibole- Controlled Fractionation

Partial melting of subducted oceanic crust metamorphosed to amphibolite can yield an andesitic liquid (Green and Ringwood, 1968; Holloway and Burnham, 1972; Allen et al., 1975). This model could probably be applied only to the rocks of groups 1 and 2, as the subduction zone under other volcanic centres is too deep to be in the amphibolite facies (Zentilli, 1974). Assuming that the REE composition of such a subducted oceanic crust is similar to ocean-ridge basalts, the partial fusion model of REE for the amphibolite facies assemblage is shown in Figure 5A. The melt produced by amphibolite fusion has higher contents of HREE and lacks a LREE enrichment in comparison with the Andean rocks. The derivation of the volcanics by amphibolite fractionation would require a source material with a REE distribution similar to that of the resulting rocks. A similar discrepancy for southern Chilean andesites led Lopez-Escobar et al. (1974) to reject the amphibolite model.

The partial melting of ocean-ridge tholeiitic amphibolite is also difficult to reconcile with the low content of Ni in the more basic rocks of groups 1 and 2. The liquid produced by anatexis of amphibolite would have a significantly higher Ni content (DeLong, 1974) while the low SiO2 content of the rocks of groups 1 and 2 preclude extensive fractional crystallization of olivine


3D

t * 3

20 t - -

g

1(] t ~

0

<

20

6

A ~ F : g 4 ECLOGITE . - - . F=O.7

,,__. F = 0.44 1 AMPHIBOLITE . . . . F = O. 74 �9 ..... NAZCA PLATE THOLEIITE

B x Spinel peridotite �9 Garnet peridotite

r , . F= ~3.2

. . . . ~ z--=,~-222 . . . . . . . . . . . . . .

F = 0.3 . . . . .

........ - ~ i ' i ~

i

Ce Nd Sm Eu Dy Er Yb

Fig, 5, A Chondrite-normalized REE contents of melts produced by parlial melting of oceanic tholeiites in amphibolite ( i ) and eclogite (1) facies using equation 15 of Shaw (1970) and the partition coefficients of Arth and Hanson (1975) for plagioclase and amphibole and of Kay and Gast (1973) for the other minerals. REE contents of the parental source are those of the average Nazca Plate tholeiite (Lopez-Escobar et al., 1974). Mineral percentages and the degree of partial melting (F) after Green and Ringwood (1968). Amphibolite: parent -62 amph., 20cpx., 2 ol., 10 plg., 6 qtz.; melt with F=0.74 77 amph., 3 cpx,, 13 plg., 7 qtz. ; melt with F =0.44~68 amph., 4cpx., 16 plg., 12qtz. Quartz eclogite: Parent -55cpx, 35 grt., 10 qtz.; melt with F=0.70-50 cpx., 36 grt., 14 qtz. ; melt with F=0.40-52 cpx., 22 grt., 26 qtz. B Chondrite-normalized REE contents of melts produced by partial melting (F=0.1, 0,2 and 0.3) of spinel peridotite (x) and garnet peridotite (e) using equation 15 of Shaw (1970) and the partition coefficients of Kay and Gast (1973). The REE content of the upper mantle was taken as twice that of chondrites (Kay and Gast, 1973; Frey and Green, 1974). Mineral percentages of the upper mantle after Shaw (1972). Spinel peridotite: parent -55 ol,, 25 cpx., 15 opx., 5 sp.; melt 35 ol., 50 cpx., 15 opx. Garnet peridotite: parent-55 ol., 20 cpx., 15 opx., 10 grt.; melt ~40 ol., 25 cpx., 15 opx., 20 grt

a n d c I i n o p y r o x e n e f r o m such a mel t . S imi l a r a r g u m e n t s can a lso be i n v o k e d aga ins t the d e r i v a t i o n o f the vo l can i c s by f r a c t i o n a l c rys t a l l i za t ion o f low-s i l i ca

a m p h i b o l e f r o m tho le i i t i c basal ts , a p rocess sugges ted by G r e e n a n d R i n g w o o d

(1968) a n d H o l l o w a y a n d B u r n h a m (1974).


Eclogite-Controlled Fractionation

Anatexis in the subduction zone of the ocean crust transformed to quartz eclogite appears to be a viable mechanism for the genesis of calc-alkaline volcanic rocks (Green and Ringwood, 1968; Green, 1972). A calculation of a partial melting model for REE, taking the composition of the Nazca plate tholeiite as representative of the eclogite source, is shown in Figure 5A. Although the liquid produced by partial melting of quartz eclogite has an abundance of HREE comparable to the rocks of groups 3, 4, and 5 (Fig. 4), the chondrite- normalized LREE depletion of typical ocean-ridge basalts is retained in the hypothetical liquid (Fig. 5A). Thus, the simple eclogite model cannot account for the LREE enrichment in Andean volcanic rocks if the source material has the composition of ocean-ridge tholeiite. Furthermore, as garnet is the only major rock-forming mineral which preferentially retains the HREE relative to an equilibrium melt, the flat HREE patterns of the rocks of groups 1 and 2 indicate that garnet did not play a significant role in the petrogenesis of these rocks.

Arguments similar to those of Gill (1974) and DeLong (1974) show that the eclogite model cannot explain the high abundances of K, Rb and Ba. and low content of Ni in the rocks of the transect. Even if the basaltic source material had the composition of altered basalts, quartz eclogite is still not a suitable source (Gill, 1974). On the basis of major and trace element geochemistry, Noble et al. (1975) and Thorpe et al. (1976) have also discarded the eclogite model for andesites from southern Peru and northern Chile. Likewise, the available trace element data are not compatible with the high-pressure eclogite fractional crystallization of ocean-ridge tholeiite.

Partial Melting of Hydrous Upper Mantle

Recent experimental evidence suggests that the melting of hydrous upper mantle peridotite could have played an important role in the genesis of andesitic magmas (Kushiro et al., 1972; Mysen et al., 1974; Mysen and Boettcher, 1975). The andesitic melt produced by the anatexis of peridotite is, however, expected to have a significantly higher content of Cr and Ni (Lopez-Escobar et al., 1974) than that of the Andean volcanics (Table 1). The observed abundances of transition elements would require that the partial melting was followed by fractional crystallization of olivine and spinel as suggested by Nicholls and Ringwood (1973).

Assuming that the volcanic rocks of the transect were derived from the upper mantle peridotite with a chondritic REE pattern, the relatively flat patterns of HREE in the rocks of groups 1 and 2 exclude the presence of garnet as either a crystallizing or residual phase during their genesis. Conversely, the fractionated HREE patterns of the rocks occurring farther from the trench requires the presence of garnet. Figure 5B shows that the partial melting of spinel peridotite could produce a liquid with HREE abundances similar to


those of the rocks of groups 1 and 2, while the liquid formed by the anatexis of garnet peridotite has a HREE distribution comparable to those of groups 3, 4 and 5. Although the partial melting of peridotite can account for HREE, it cannot explain the LREE enrichment and the high content of K, Rb and Ba in Andean volcanic rocks unless very low degrees of melting (<5%) are invoked (Thorpe et al., 1976). Subsequent fractional crystallization of olivine, spinel, plagioclase and pyroxenes would also not provide a sufficient LREE enrichment. As these minerals have low REE partition coefficients (Schnetzler and Philpotts, 1970; Arth and Hanson, 1975), their fractionation leads mainly to the increase of the absolute REE content without a distinct change of their patterns.

Discussion

The spatial and temporal chemical zonation of the volcanic rocks of the Andean transect indicates the close relationship of volcanism with the Benioff zone. The geochemistry of these rocks, however, is not consistent with the single stage models suggested for the origin of andesitic rocks such as simple melting of typical ocean-ridge basalts (in amphibolite or eclogite facies, altered or un- altered) or of peridotite believed to be representative of the upper mantle. The volcanics are enriched in elements such as K, Rb, Ba, and LREE in comparison with the calculated melts. Likewise, extensive fractional crystallization involving garnet and clinopyroxene (Thorpe et al., 1976) does not explain the geochemical characteristics of volcanic rocks, since some rocks have SiO2 contents of only about 50-53%. The crustal contributions to the magmas also cannot account for the large-ion-lithophile element (LILE) abundances in the Andean andesitic rocks, as the Sr isotopes indicate that the rocks had not been significantly affected by the upper crustal contamination (James et al., 1976; McNutt et al., 1975). Furthermore, the maximum thickness of the crust is found in the regions where the rocks of groups 3 to 4 occur while the rocks of group 5 have highest 8~Sr/86Sr ratio.

Thus the high content of LILE in volcanic rocks of the transect may suggest their derivation from a source already enriched in LILE. The two most likely sources are: a) enriched subducted oceanic crust, and, b) enriched upper mantle. Regarding the enriched oceanic crust, the subduction of deep sea pelagic sediments with oceanic lithosphere could lead to the enrichment of LILE. The increase of LILE away from the trench would require that the contribution of sediments increases with the progressive downward movement of melting, yet the sediment layer is probably the first material to melt (Oxburgh and Turcotte, 1970). Isotope studies (Church, 1973) also indicate that the sediment contribution to calc-alkaline magmas was very limited (< 2%~ and was probably insufficient to account for the observed enrichment. For example, Noble et al. (1975) have shown that the contribution of at least 10% of the Ba-rich oceanic clay to a typical ocean-ridge tholeiite would be needed for the generation of a liquid with 500 ppm of Ba by 50% of partial melting. In addition, none


of these mechanisms would explain the low content of minor transition elements in volcanic rocks.

A more suitable source appears to be enriched upper mantle overlying the Benioff zone. Nicholls (1974) and Ringwood (1974) have recently proposed a multi-stage process, where partial melting of subducted oceanic crust in eclogite mineralogy yields rhyodacite-rhyolite magmas which rise into and react with the overlying peridotite mantle producing "enr iched" pyroxenite-peridotite bodies. These less dense masses rise diapirically and partially melt to form calc-alkaline magmas. On the other hand, James et al. (1976) have argued that the upper mantle overlying the Benioff zone under the South American continent is isotopically and geochemically heterogeneous and enriched in dispersed elements due to the addition of material percolated from the astheno- sphere. The continental lithospheric mantle, which is rigid and cannot circulate, probably moves with the continent and chemically evolves through time. Conse- quently, its older parts, which occur progressively farther from the present trench would be expected to be more enriched in radiogenic strontium and lithophile elements. Miyashiro (1974) has also hypothesized that degassing of a deep mantle would lead to the addition of alkalies into the upper mantle wedge overlying the descending slab while Best (1975) has suggested that hydrous fluids released from the descending oceanic lithosphere could enrich the overlying upper mantle in LILE by scavenging and zone refining. All these processes are not mutually exclusive and each might have participated during the long evolution of lithosphere below the South American continent.

Although geochemical characteristics of the volcanic rocks of the transect seem to require their derivation from an "enr iched" source, the heterogeneous upper mantle is probably not the sole reason for the systematic variations with respect to the distance from the trench and the age of the rocks. The consistent time/space pattern of igneous activities and the systematic variations of LILE in the volcanic rocks of the transect can also be interpreted as indicative of progressively deeper magma generation. The high content of Sr and the absence of Eu anomalies in basaltic and andesitic rocks could reflect their generation at a depth where plagioclase was not a stable phase. The flat HREE patterns of the rocks of groups 1 and 2 exclude the participation of garnet in their genesis and are consistent with their origin in the spinel stability field. On the other hand, the HREE depletion in the other volcanic rocks from the transect suggests that garnet played a role in the petrogenesis of these volcanics and is compatible with their derivation from garnet peridotites. Thus, the higher content of LILE in groups 3, 4 and 5 in comparison with groups 1 and 2 also correlates with the greater depth and higher pressure of magma generation within the upper mantle wedge. As Miyashiro (1974) has pointed out the increase in pressure alone might not lead to the increase of the content of LILE in the melt. The LILE increase could be a result of the decreasing degree of partial melting with increasing depth. During the partial melting of peridotite, LILE are strongly partitioned into a liquid phase and the decrease of the degree of anatexis would lead to a nearly proportional increase of LILE in the melt (Gast, 1968; Miyashiro, 1974). As the degree of partial melting of peridotite decreases with the decrease of the water content (Green, 1973),


it can be argued that progressive dehydration of the subducted oceanic slab would lead to lower water fluxes and thus to the decrease of the degree of partial melting with the increasing depth of magma generation.

Jakes and White (1972) have suggested that shoshonitic rocks from island arc environments might have been formed by a degree of upper mantle anatexis as low as 5%. Such a small extent of partial melting for the rocks of group 5 appears to be reasonable although it is difficult to evaluate it in the light of the uncertainties of the composit ion of an "enr iched" upper mantle. The model calculation of Kay and Gast (1973) shows, however, that the REE distribution is consistent with such a process.

The low content of transition elements, especially Ni and Cr, in the volcanic rocks indicates that after leaving the source, magmas underwent a separation of a limited amount of olivine _+ spinel _+ clinopyroxene probably at depths greater than 35 km (Thorpe et al., 1976). In addition, the abundant phenocrysts in the volcanic rocks of the transect suggest low-pressure fractionation.

Conclusions

The volcanic rocks from this transect of the Andes in Chile and Argentina are representative of igneous activity associated with active continental margins. The rocks show a distinct chemical and temporal zonation with respect to the present Peru-Chile trench. The older rocks (Jurassic to Eocene) occur closer to the trench and are comparable to calc-alkaline rocks of island arcs. Eastwards, they are followed by younger sequences (Miocene to Quaternary) of calc-alkaline rocks typical of continental margins, while farthest f rom the trench, the rocks have shoshonitic affinities. The geochemistry of the rocks cannot be explained by the single-stage models suggested for the origin of volcanic rocks of orogenic belts, such as partial melting of subducted oceanic lithosphere (amphibolite or eclogite) or of upper mantle peridotite. The data are consistent with a model in which magmas were derived by anatexis of an upper mantle source already enriched in LILE and located above the descending oceanic slab. The spatial chemical variations across the volcanic belt probably reflect primary systematic changes in the composition of the magmas due to a decreasing degree of partial melting with increasing depth and possibly also due to the heterogeneity of the source materials. During the ascent to the surface, magmas were affected by a varying degree of fractional crystallization and possibly by limited crustal contamination.

Acknowledgements. The study has been supported by the National Research Council of Canada

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Received February 21 / Accepted March 30, 1977

Geochemistry and origin of volcanic rocks of the Andes (26°–28°S)

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