Strontium isotopic composition and trace element data bearing on the origin of Cenozoic volcanic rocks of the central and southern Andes

Journal of Volcanology and Geothermal Research, 2(1977)49--71

49

© Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

STRONTIUM ISOTOPIC COMPOSITION AND TRACE ELEMENT DATA B E A R I N G ON T H E O R I G I N O F C E N O Z O I C V O L C A N I C R O C K S O F T H E CENTRAL AND SOUTHERN ANDES

J. K L E R K X ,

S. D E U T S C H ,

H. P I C H L E R

and W. ZEIL

Mus~e Royal de l'Afrique Centrale, Tervuren (Belgium) Laboratoire de Min~ralogie et P~trologie, Universit~ Libre de Bruxelles, Bruxelles (Belgium) Mineralogisches Institut der Uniuersitiit Ti~bingen, T~bingen (Federal Republic of Germany) Institut fur Geologie und Paliiontologie, Technische Universit~t, Berlin (Federal Republic of Germany)

(Received July 7, 1976; revised and accepted December 14, 1976)

ABSTRACT Klerkx, J., Deutsch, S., Pichler, H. and Zeil, W., 1977. Strontium isotopic composition and trace element data bearing on the origin of Cenozoic volcanic rocks of the central and southern Andes. J. Volcanol. Geotherm. Res., 2: 49--71. The Cenozoic volcanic rocks of the southern Andes are characterized by low STSr/S6Sr ratios (0.7040--0.7045), which are consistent with an origin in the downgoing slab of oceanic lithosphere or the overlying mantle. These values are distinctly lower than those from corresponding rocks of the central Andes. The calc-alkalinerocks of the central Andes exhibit higher Sr isotopic values (0.705-0.713) and variable Rb/Sr ratios. Different explanations are possible for this behaviour as well as for the positive correlation between 8~Sr/S6Sr and Rb/Sr expressed in an apparent isochron of 380 + 50 m.y. It is postulated that these magmas result from a mixing process between a primary m a g m a with basaltic affinitiesand crustal material of relatively young age. A model is proposed for the generation of the "andesitic" m a g m a s of the central Andes by which crustal rocks of the upper part of the crust are added to the base of the crust by an accretionary process at the margin of the continent. Melts from these upper crustal rocks act as contaminants in "andesitic" magmas. The role of crustal material is stillmore significant in the generation of the ignimbritic magmas; they are considered to result from a two-stage melting process by which igneous rocks, belonging to a former stage of development of the Andes, are engulfed in the subduction zone, where they melt.

INTRODUCTION As w e l l - d o c u m e n t e d m o d e l s o f s u b d u c t i o n z o n e t e c t o n i c s b e c o m e available in volcanic arc regions and along c o n t i n e n t a l margins o f Pacific t y p e , m o d e r n m a g m a genesis in these regions can be discussed in the light o f these models.

50 Along active continental margins the plate tectonics model of magma genesis is less well demonstrated than with regard to island arcs, although recent studies have brought evident arguments for a close relation between volcanism and subduction processes within the Andean region (Deutsch et al., 1974~ McNutt et al., 1975; James, 1975; James et al., 1974, 1976). Nevertheless, differences both in bulk composition and in trace element composition exist between volcanics of different parts of the Andes, suggesting that magma generation processes are much more complicated in these regions than along island arcs. In the papers cited above the source region for the calc-alkaline magmas of the Andes is considered to be either the mantle above the plunging lithospheric slab or even the lower crust. Incorporation of important volumes of continental material is generally excluded by these authors. This study treats the Cenozoic volcanism of two different areas of the Andes, the central and southern regions. In contrast to the central Andean region (northern Chile, southwestern Bolivia, northwestern Argentina), which exhibits characteristics similar to those presented in the above-cited papers for neighbouring regions, a discrepancy appears with regard to the calcalkaline rocks of the southern Andes (central and southern Chile}. Taking into account petrographical arguments as well as the Sr isotopic composition and some trace element contents, we will discuss the alternative hypotheses for magma genesis, including crustal contamination. In trying to correlate the volcanism with the plate tectonics model, we have restricted our study, in a first approach, to the Cenozoic volcanism, as the models apply to present-day subduction and because in the light of our present knowledge, extrapolations to the past are still rather speculative. GEOLOGICAL SETTING

Central Andes In their central part (southern Peru, northern Chile, southwestern Bolivia and northwestern Argentina) the Andes are about 900 km wide, with a crustal thickness reaching about 70 km (Ocola et al., 1971). In t h a t area the Andean mountain belt comprises the following major tectonic units (from west to east): (1) The Coastal Range is developed between 18°S and about 43°S; its disappearance in the south coincides with the intersection of the East Chilean Rise with the continent. The Coastal Range consists of strongly folded Precambrian to Paleozoic metamorphic rocks which are intruded by the extensive Coastal Batholith {mostly tonalite and granodiorite). (2) The longitudinal Central Valley, which is filled by thick Quaternary continental sediments, separates the Coastal Range and the Andean Cordillera. This graben-like structure is developed over nearly the entire length of Chile

51

I

I I \ \ \

CA

\

13

\

)

C.

f

4~

/

I

/

V V

)

VC-V V

/

J ) ~

f~.

0

B

L

I

V

I

A

AREQUIPA v/

v

v

V

L~goPoopo'

ARICA

Rhyolite Formation UYUNI

Andesite = Formation

v

...J

v

//

ANTOFAGASTA

.7

(J -,v~ 0 I

150 I

~ /''~"

• TUCUMAN

300 km

oj 0 • LA RIOJA

Fig.1. Extension of the Rhyolite and "Andesite" Formation in the northern Andes.

52 between 18°S and 43°8. It is important to note that the longitudinal distribu~ tion of the Cenozoic Andean volcanism is related to the existence of the Central Valley. Between 27°S and 33°S, where the Central Valley is not developed, no Cenozoic volcanics occur. There is also a seismic gap between 26°S and 32°S. (3) The Western Cordillera, between 15°S and 27°S, is characterized by some hundreds of huge composite volcanoes which in that central part of the Andes reach altitudes of 5000--6880 m. The extreme height of the volcanoes is due to the fact that they rest on a plain with an altitude of about 3700-4200 m. This plain is formed by widespread sheets of mostly ignimbritic volcanics which cover an area of about 150,000 km 2 (Fig.l). This series of Late Miocene to Early Pleistocene acidic volcanics is called the Rhyolite or Ignimbrite Formation (Zeil and Pichler, 1967; Pichler and Zeil, 1972b; Fernandez et al., 1973). The composite volcanoes, perched on the ignimbrite series, are of Pleistocene to Holocene age and belong to the so-called "Andesite" or Strato-volcano Formation (Pichler and Zeil, 1969, 1972b; Fernandez et al., 1973). (4) The Altiplano of Bolivia (with an average height of about 3800 m) and its continuation to neighbouring northwestern Argentina, called Puna, separates the Western and Eastern Cordillera. This (up to 200 km wide) elevated depression forms a graben-like structure which is filled by thick series of mostly clastic sediments of Upper Cretaceous to Tertiary age. The basement consists mostly of slightly metamorphosed Paleozoic sandy shales and grey sandstones. Related to the orogenic movements during the Cenozoic, strong volcanic and plutonic activity occurred. Radiometric age determinations show that the volcanism continued almost uninterruptedly during the last 20 m.y. with its climax and widest distribution during the Pliocene (Kussmaul et al., 1975, 1977). Both the Rhyolite (Ignimbrite) and the "Andesite" (Strato-volcano) Formation participate in the morphological structure of the Altiplano. (5) The Eastern Cordillera or Paleozoic Andean Block is composed of strongly folded Paleozoic rocks which are intruded by large Miocene batholiths of intermediate plutonics. Cenozoic volcanic rocks occur in the western part of the Eastern Cordillera.

Southern Andes South of the zone w i t h o u t Cenozoic volcanic activity between 27°S and 33°S, the south Andean Cenozoic volcanic belt is developed. According to Vergara (1972) two units of Cenozoic volcanic belts parallel to each other can be distinguished in the southern Andes.

53

PETROCHEMICAL

•

CHARACTERISTICS

Central Andes

The volcanics of the Rhyolite (Ignimbrite) Formation range from alkalifeldspar rhyolites to quartz latiandesites with a concentration in the alkalifeldspar rhyolite, rhyolite and rhyodacite field of the Streckeisen doubletriangle (Pichler and Zeil, 1972b, p. 443). Average chemical compositions of north Chilean and south Bolivian ignimbrites are given in Table 1. The data show that the ignimbrites of northern Chile are somewhat more acidic than the stratigraphically corresponding volcanics in the adjoining Bolivia. The Bolivian ignimbrites are mostly rhyodacites, whereas those of northern Chile are alkali-feldspar rhyolites and rhyolites. The volcanics of the "Andesite" (Strato-volcano) Formation, which rest upon the ignimbrite sheets, are mostly quartz latiandesites, though also andesites, quartz andesites, quartz latites, dacites, rhyodacites and even rhyolites to alkali-feldspar rhyolites are found. Average chemical compositions are compiled in Table 1. TABLE 1 Average chemical composition of Cenozoic "andesitic" rocks from northern Chile, southern Bolivia, northwestern Argentina, the southern Andes (central and southern Chile and neighhouring Argentina) and of volcanics from the Rhyolite (Ignimbrite) Formation of northern Chile and southern Bolivia N Chile

S Bolivia

NW Argentina

S Andes

N Chile

S Bolivia

No. of analyses

62

77

43

97

60 ig.

21 ig.

SiO 2 A1203 Fe203 (total) MgO CaO Na20 IQO TiO2 P2Os

61.8 16.7 5.5 3.2 5.4 3.9 2.5 0.8 0.2

63.2 16.0 5.4 2.5 4.6 3.6 3.4 1.0 0.3

58.7 16.2 6.3 5.0 6.1 3.5 2.9 1.0 0.3

59.7 17.5 6.7 3.2 5.6 4.2 1.9 0.9 0.3

71.1 14.5 2.7 0.9 2.4 3.5 4.2 0.4 0.1

69.1 15.7 3.2 1.3 3.3 2.9 3.8 0.6 0.1

Colour index Sigma

15.5 2.4

18.0 2.8

23.2 2.9

19.3 2.4

7.4 2.2

12.9 1.9

Variations in the average chemical composition exist within different areas of the central Andes: thus, the volcanics of the south Bolivian "Andesite" Formation are more siliceous than those of the adjoining north Chilean region (Fernandez et al., 1973). Lavas with silica contents less than 60% are rare in southern Bolivia. Thus, most of the volcanics of the south Bolivian "Andesite"

54 Formation are rhyodacites and dacites. Therefore the term "Andesite" Formation, applied to the youngest Cenozoic volcanics of northern Chile, was replaced by the more appropriate name Strato-volcano Formation. In northwestern Argentina and in southern Peru K-rich lavas of shoshonitic c o m p o s i tion, corresponding mostly to slightly undersaturated olivine latites and latites, have been found (HSrmann et al., 1973; Lef~vre, 1973). These shoshonitic rocks differ from the "andesites" in their higher contents in alkalies, P2Os, MgO and Sr and Ba. Southern Andes

Andesites, quartz andesites and quartz latiandesites are the most abundant volcanic rocks in the Cenozoic volcanic belts of central and southern Chile. The bulk composition is somewhat more basic than in the central Andean volcanic area (cf. average chemical composition in Table 1). According to Katsui (1972) the south Andean volcanic zone is characterized by abundant high-alumina basalts. No Rhyolite (Ignimbrite) Formation is developed in the southern Andes. ANALYTICAL RESULTS The isotopic composition of Sr has been measured by thermo-ionisation on a rhenum filament (with titanium o x y d e as activator) on a TH5 Varian mass spectrometer from the "Centre Belge de G~ochronologie". Rb and Sr were determined by X-ray fluorescence by M. Delvigne (Mus6e Royal de l'Afrique Centrale, Tervuren). The analytical results are given in Table 2. The data are grouped corresponding to the different rock series within each investigated area and are listed within each group in order of increasing 87Sr/a6Sr ratio. K20 versus Si02 (Fig.2). The calc-alkaline "andesitic" lavas contain less than 65 wt.% SiO2 and present lower K20 values than the ignimbrites, which show SiO2 contents higher than 65%. Within the calc-alkaline rocks those from the southern Andes tend to contain lower K20 for an equal range of SiO2 contents compared to the volcanic rocks from the central Andes. In the other groups, which correspond to a west-east section through the central Andes, a slight increase in K20 is observed for rocks with similar SiO2 content from northern Chile towards southwestern Bolivia and northwestern Argentina, thus with increasing distance from the trench. This K--h relationship, previously established for continental margins and intra-oceanic magmatic arcs (Dickinson, 1975) and shown also for southern Peru (Lef~vre, 1973), is less well expressed here. Some rocks exhibit higher K20 values; they belong to a shoshonitic association that overlaps the calc-alkaline rocks in southwestern Bolivia and northwestern Argentina.

55

TABLE 2 87Sr/SeSr ratios and K, Rb, Sr, Y and Zr contents of Cenozoic volcanics from the central, s o u t h e r n and n o r t h e r n Andes. eTSr/S6Sr ratios for north Chilean "andesites" determined by C.E. Hedge (in Pichler and Zeil, 1972b, p. 447). Rb and Sr in central and southern Chilean andesites (re)determined by isotopic dilution; all other data obtained by X R F controlled by isotopic dilution Sample No.

Rock type

K (%)

Y (ppm)

Central and s o u t h e r n Chile Vi 24 olivine andesite Ll 56 andesite Vi 5 quartz latiandesite Os 51 andesite Ca 37 quartz andesite Vi 6 latiandesite

0.75 0.26 1.66 0.83 0.50 1.58

N o r t h e r n Chile 4 rhyodacite 8 latiandesite 18 quartz andesite 11 latiandesite 5 quartz latiandesite 2 quartz latiandesite 14 quartz latiandesite 15 quartz latiandesite 17 dacite 21 quartz latite 20 quartz latiandesite

2.49 1.33 0.91 1.41 2.16 1.66 1.66 1.49 2.08 2.57 1.74

56 16 17 28 14 22 30 15 11 25 26

Northwestern Argentina * 4/50 olivine latiandesite 4/57 latiandesite 4/69 olivine latite 4/28 olivine latite 4/30 quartz latiandesite

1.35 1.69 2.74 2.96 2.29

29 26 31 38 12

S o u t h w e s t e r n Bolivia * 11/1 latiandesite 16/3 quartz latiandesite 13]5 quartz latite 16/5 rhyodacite

1.45 1.97 3.15 3.58

S ou t h w e s t Bolivian ignim brites * 12/3 rhyodacite 2.92 13/4 rhyodacite 4.35 16/1 rhyodacite 2.90 N o r t h Chilean ignimbrites* 111 rhyodacite 231 rhyolite 245 rhyolite 214 rhyodacite 56 alkali rhyolite 53 alkali rhyolite 14 alkali rhyolite 324 alkali rhyolite

2.74 3.49 3.32 2.82 3.90 4.07 3.98 3.65

Zr (ppm)

Rb (ppm)

16.3 14.5 51.7 25 14 30.3

Sr (ppm)

Rb/Sr × 10 -2

K/Rb

sTSr/SeSr

398 458 285 319 372 364

4.1 3.2 18.1 7.8 3.8 8.3

460 179 380 332 357 520

0.7039 0.7040 0.7041 0.7042 0.7043 0.7043

88 28 21 40 65 47 72 29 64 155 98

618 734 482 622 654 655 506 773 676 418 464

14.3 3.8 4.3 6.4 9.9 7.1 14.2 3.7 9.4 36.9 21.0

283 475 433 353 332 353 231 514 328 166 178

0.7051 0.7053 0.7056 0.7058 0.7059 0.7065 0.7066 0.7067 0.7068 0.7070 0.7071

209 211 330 382 193

43 60 111 131 155

660 555 937 795 525

6.5 10.8 11.9 16.5 29.5

314 282 247 226 148

0.7055 0.7062 0.7066 0.7071 0.7088

37 29 29 27

137 156 161 267

47 92 143 194

763 424 574 375

6.2 21.7 24.9 51.7

309 214 220 184

0.7059 0.7077 0.7091 0.7133

15 23 20

111 163 159

166 162 150

254 424 318

65.4 38.2 47.2

176 269 193

0.7084 0.7114 0.7125

91 203 173 160 138 141 145 133

497 82 168 243 43 50 33 39

18.3 248 103 66 321 282 439 341

301 172 192 176 283 281 274 267

0.7053 0.7073 0.7071 0.7076 0.7085 0.7086 0.7099 0.7105

33

226

*Rb, Sr measured by X R F by M. Delvigne, Mus~e Royal de l'Afrique Centrale, Tervuren. Note: The standard Sr E and A yields on the same spectrometer T H 5 Varian aTSr/S6Sr= 0.70807.

56

"/. K 2O ÷ Andesites of the southern Andes • ,, Andesile" FormaUon of N - C h i l e ,, Andesite" Forma'[~or~ of $W-Botivia I ,, Andesite" Formatior~ of NW-Argenfina , ~ Ignimbriles of SW-E~olivia • Ignimbrites of N-Chile

~0-

14 •

o •

3O

•

•

it

•

e¸ ~_ g o

•

+ 50

°h S~0 2 5L.

58

82

66

70

7&

7g

Fig.2. K20 (wt.%) versus SiO2 (wt.%) for all the investigated samples. The K--h relationship is badly expressed.

Rb, Sr and Sr isotopic composition values. The southern and the central Andean rocks show clearly distinct contents both for Rb and St. In central and southern Chile both Rb and Sr are lower, corresponding also with a lower K content; Rb varies between 14 and 50 ppm, Sr between 285 and 450 ppm (Table 2). In southwestern Bolivia and northwestern Argentina, both values are higher and more dispersed, Rb varying between 40 and about 200 ppm, Sr between 425 and 900 ppm. Considering the Sr isotopic ratios, an obvious difference between both groups exists and a different behaviour within each group is evident: the calcalkaline rocks of the southern Andes exhibit low and well-grouped values, varying between 0.7039 and 0.7043 (Table 2). Higher values were obtaine:i for the calc-alkaline rocks of the central Andean area and the values are also much more dispersed: between 0.7051 and 0.7071 for the rocks of northern Chile (C.E. Hedge, in Pichler and Zeil, 1972b), between 0.7055 and 0.7088 for the Strato-volcano Formation of northwestern Argentina, whereas the southwest Bolivian lavas give values between 0.7059 up to 0.7133. No clear relation appears between the Sr isotopic ratios and the rock composition: even the most fractionated rocks do not exhibit the highest values; on the contrary, the lowest value obtained in the central Andean area comes from a rhyodacite. The ignimbrites show high Rb values but a relatively low Sr c o n t e n t (Table 2 and Fig. 3); their Sr isotopic ratios fall in the same range as the calc-alkaline rocks of northern Chile, varying between 0.7052 and 0.7091, whereas for southwestern Bolivia some samples reach values up to 0.7125. In the 87Sr/S~Sr versus Rb/Sr diagram, three clearly separated groups can be distinguished (Fig. 3): (1) The "andesitic" rocks of the southern Andes with a low Rb/Sr ratio

57

875r 1 86 Sr + Andesites of the southern Andes • ,,Andesite"Formation o~ N-Chile X ,,Andesitd:Forrnalion of SW-Bolivia ÷ . A n d e s i t e " Formalion of NW-Argenflna A Ignimbrite FormationotSW Bolivia • Ignimbrite Formation of N-Chile

0.71/, ×

A

0712

A 0.710

0.708

× 4= °

0706

0.704

Rb/Sr • 10.2 0702 100

200

300

400

500

600

Fig.3. STSr/S6Sr ratios versus Rb/Sr ratio including all the investigated samples of the northern and central Andes.

(between 0.02 and 0.20) and a low and nearly constant isotopic ratio (around 0.704). (2) A second group, comprising the bulk of the calc-alkaline rocks of the central Andes (northern Chile, southwestern Bolivia and northwestern Argentina), with higher 87Sr/8~Sr and Rb/Sr ratios varying over a wide range, b u t defining a positive correlation between both ratios. A similar relationship between STSr/S6Sr and the K20/Na20 ratios exists and a similar subdivision into three groups is defined. (3) The third group includes the ignimbrites of northern Chile: they exhibit much higher Rb/Sr ratios reaching up to 440, b u t corresponding with intermediate STSr/E~Sr ratios, varying from 0.705 to 0.709. A correlation similar to that expressed in the calc-alkaline rocks of the central Andes is less well expressed here; furthermore the samples with the lowest Rb/Sr ratios join the lower end of the trend defined by the second group. The ignimbrites of southwestern Bolivia show characteristics intermediate between the second and third groups: some rocks belong to the upper part of the trend defined by the second group, whereas one sample shows more affinities with the third group. DISCUSSION

The chemical and isotopic features show that the entire series of rocks can be divided in three groups with well-defined characteristics. As the most striking differences appear between the lavas from the southern Andes on the one hand and the central Andean region on the other, we will discuss the characteristics for each group separately.

58 Calc-alkaline rocks o f the southern A ndes

Owing to the fact that the investigated samples come from a restricted area and that they represent lavas erupted in recent times, they may be considered as a single group from a unique source. Geochemical data confirm this view: the amounts and ratios of the considered elements vary within a narrow range, minor variations of the Rb/Sr and K/Rb ratios being presumably related to differentiation processes during the ascent of the magma. The low STSr/S6Sr suggests a subcrustai origin. When compared with similar rocks from island arc regions, only small differences appear. For example, the Tonga volcanics have slightly lower isotopic ratios for similar Rb/Sr ratios (the mean difference is about 0.0005). On the other hand, the andesites and dacites of the Taupo volcanic zone of New Zealand are enriched in STSr (0.704--0.706) and show higher and more dispersed Rb/Sr ratios (Ewart and Stipp, 1968). Different explanations can be given for these small differences (such as different rates of weathering of the oceanic lithosphere which is involved in the subduction zone, differences in composition of the mantle) but the Sr isotopic data alone do n o t give sufficient evidence to support a definitive interpretation for these differences. Cenozoic volcanic rocks o f the central andes

Although most geochemical features of these series are distinct from the south Andean series, the predominant characteristics are the dispersed and relatively high Sr isotopic ratios. Particular attention has to be paid to the positive correlation between STSr/S6Sr and Rb/Sr ratios, which appears as a straight-line relationship in the "andesite" series of southwestern Bolivia and northwestern Argentina, and which is present also in the ignimbrites of northern Chile, but less well expressed. An apparent isochron (York, 1966) is defined by the "andesitic" series of southwestern Bolivia and northwestern Argentina, which would correspond to an age of 380 + 50 m.y. (2o), with an initial 87Sr/86Sr of 0.7046 (Fig. 4). This agrees well with data obtained for southern Peru by James et al. (1974), where the data relative to similar rocks fit on an isochron around 400 m.y. For the ignimbrites of northern Chile, the pseudo-linear relation between 87Sr/86Sr and Rb/Sr would define an apparent age of 24 +- 11 m.y. with {STSr/86Sr)0 = 0.706; however, as the samples are of different ages, this age is meaningless. In southern Peru, James et al. (1974) define an isochron, for similar formations, of 11.5 m.y. Explanations for such a correlation as expressed in the "andesitic" series of southwestern Bolivia and northwestern Argentina may consist of (1) smaU-scale inhomogeneity in the source material, (2) disequilibrium melting of mantle material, (3) contamination of homogeneous magma by different amounts of crustal material or eventually oceanic sediments. Each of these alternatives will be discussed separately.

59

Argentina ,, Bolivia: Calc- alkaline series Sr | ~ r 86

0.7131

-I-

0.712 0.7ti 0.710 0.7O9 0.708 0.707

0.706

0.705 ~~ / r 87/5r 86).=.- 0.7046. t 0.0002. 360

.320

./*80

.640

)' Rb = 1 ~.7 10-11y-1 .800

.960

1.12

1.28

1A4

160

RbB?//Sr86

Fig.4. Isochron defined by the calc-alkaline rocks of northwestern Argentina and southwestern Bolivia.

Small-scale inhomogeneity in the source material

Assuming the mantle is the source region for "andesitic" magmas, strong arguments are p u t forward by Hoffmann and Hart (1975) against a mantle which is n o t in chemical or isotopic equilibrium on a local scale. If, on the other hand, the magma originates in the continental crust, this would imply that the different rock types are melting products of individual parts of a petrochemicany heterogeneous complex which remained closed during the last 400 m.y., without isotopic homogeneization and also w i t h o u t isotopic or chemical equilibration between the different melts. The existence of such a complex which might have acted as a source for the considered magmas, cannot a priori be denied: melting of parts of the lower crust ("gabbroic layer", with a granulitic composition?) cannot be excluded, and we may assume that this lower crust is composed of different rock units with distinct Rb/Sr ratios. However, the relatively low radiogenic Sr content of the calc-alkaline lavas implies that the starting material of the lower crust has a low radiogenic Sr content. Furthermore, an age of 400 m.y. for the lower crust is rather surprising as Ordovician or even Precambrian rocks outcrop at the surface. We have no indications on the nature and the isotopic composition of the lower continental crust and consequently such a hypothesis remains speculative. Independently of this, melting of the lower crust without isotopic equilibration remains problematic.

60

Disequilibrium melting of mantle material Hoffmann and Hart (1975) consider that disequilibrium melting in an oceanic mantle is unlikely; nevertheless James (1975) favours this hypothesis for the generation of the Cenozoic volcanic association of southern Peru, considering that the arguments invoked by Hoffmann and Hart for the mantle beneath the oceans cannot be applied to the Andean volcanism. For this purpose he suggests that the magma of the central Andes was generated in the ancient continental lithosphere, remained solid over 400 m.y., which allows disequilibrium melting of ultramafic material containing phlogopite. Severe restrictions are still applied to the generation of andesite and related more-acid magmas from mantle peridotite: in a recent paper (Stern et al., 1975), it is even suggested that andesite can only originate as primary magma from mantle peridotite if the liquids are saturated or nearly saturated with water; these authors even question this last possibility. Water can be moving from the underlying oceanic plate into the overlying mantle, b u t then how can disequilibrium be maintained in the mantle during the melting processes? The hypothesis defended by James implies also that small amounts of phlogopite are incorporated in the magma, proportional to the degree of melting; this means that only a small amount of water is taken from the phlogopite; this is also against the results presented by Stern et al. In order to explain the variation of the STSr/86Sr ratio of the resulting magma, James considers that the degree of total melting is proportional to the amount of phlogopite melt, which implies that the bulk composition of the magma must vary as a function of the 87Sr/S6Sr ratio. To test this, we plotted the STSr/S6Sr ratios of the lavas from the central Andes, including the ignimbrites, versus the SiO~ content of these lavas. The figure (Fig. 5) obi 87Sr/865r

0714 i

x

A

o7~zt

,~

07~OJ

"i

07081

+

~

•

+

0706 ] i

•

.

•

+IQ< +

OTOL, i +

•

. *

• •

•

•

f

. °°

~ +

. ~

•

•

+

°loSi 02 50

55

60

65

Fig.5.8~Sr/a~Sr ratio versus SiO2 ( w t . % ) f o r all the investigated samples (same symbols as

for

Fig.3).

61 tained is very confusing: no correlation appears when the results are considered as a whole, although distinct domains appear for geographical groups of lavas. The ignimbritic lavas, although having a much higher SiO2 content, show Sr isotopic ratios in the same range as the rocks o f the "andesitic" suite. Consequently, independently from the process by which disequilibrium melting is possible, it fails to explain the Sr variation within the considered lavas of the central Andes.

Contamination of homogeneous magma by different amounts of crustal material or oceanic sediments Nature o f the primary magma. When considering the contents of some trace elements, the primary composition of the magma erupted in these regions has to be questioned. As a matter of fact, the contents of different trace elements show greater affinities with basaltic rocks than with andesites from island arcs. In order to account for the high Sr content of the lavas of southern Peru, James (1975) assumes that the Sr content of the clinopyroxene of the source regions is higher than the content normally measured in undepleted lherzolites. The same characteristics appear for the calc-alkaline rocks of the regions which are considered in this study: the Sr content falls in the range of 600--900 ppm, whereas for the south Andean rocks the variation range is between 300 and 450 ppm. The behaviour of other elements such as Ba, Cr and Ni (Siegers et al., 1969; Pichler and Zeil, 1972b) and also the Zr content, which is markedly higher than in island arc andesites (Table 2), points to a similar conclusion. These affinities with basaltic rocks accord with the results obtained b y Noble et al. (1975) for low-silica latiandesite from central Peru: based on a careful discussion of the chemistry of these rocks, they conclude that the magma is generated by a small degree of melting of ultramafic mantle material, b u t that the primary melt contains 52--53 wt.% SiO2. This corresponds to the idea that the primary melt of these regions has its own composition which is different from the andesitic melt of island arcs and closer to a basaltic magma, b u t nevertheless different from typical basaltic melts, especially concerning the higher SiO2 and lower K contents. This assumption is of course n o t valid for the shoshonitic rocks o f the central Andes. Contamination by oceanic sediments. Oceanic sediments acting as contaminants can be ruled out as their 8~Sr/8~Sr ratio does n o t exceed the isotopic composition of seawater (0.709) and consequently fails to explain the higher ratios obtained in the lavas. Contamination by crustal material. The high radiogenic Sr contents m a y be caused by contamination with crustal material. When considering the a m o u n t of contaminant, several authors (e.g. James et al., 1976) reject this hypothesis for the andesitic rocks, b u t sometimes retain it as a possibility for the genera-

62 tion of the ignimbritic rocks. However, there are arguments (Pichler and Zeil, 1972b) both from the field relations and from petrological data for a close relationship between the "Andesite" and the Rhyolite Formations: overlapping in time and space, also in rock composition, Sr isotopic ratios falling in the same range, etc. The ignimbrites cannot be considered as fractionation products of the andesitic magma; their eruption precedes the ~'Andesite" Formation and they have so much higher Rb/Sr ratios for a bulk composition which is not so different for both formations. On the other hand, their content in Zr and Y falls in the same range as for the andesites. Consequently a chemical relationship between both formations has to be explained by another process. There is geochemical evidence for crustal contamination in both series. The content in Zr is characteristic in both series, especially in the " A n d e s i t e " Formation: it is higher than in both basalts and in island arc andesites. Fernandez et al. (1973) also note that Zn is particularly high in the southwest Bolivian ignimbrites, which they consider as an indication for contamination of the melt by Zn-bearing crustal rocks. Other authors (Dupuy and Lef~vre, 1974; Andriambololona, 1976), investigating similar rocks of southern Peru, conclude that partial fusion of garnet granulite in the lower crust agrees with the distribution of Li, Rb, Ba, Sr and Cr, Co, Ni, V in the calc-alkaline lavas. Although they are aware that not all the features of the "andesitic" volcanism are explained by this process, they consider that participation of the crust in the genesis of the "andesites" is beyond doubt. The petrography of the rocks of both series, both in northern Chile and in southwestern Bolivia, shows several features which are interpreted (Pichler and Zeil, 1972b) as relics of highly metamorphosed rocks which survived melting: these concern the presence of labradoritic plagioclase phenocrysts with strongly corroded cores of sodic composition, fractured and corroded quartz phenocrysts with sometimes partial melting phenomena and also quartz aggregates with granoblastic texture, considered as relics of quartzites and gneisses. Starting from this assumption, we will test the hypothesis that the primary magma has been contaminated near its source or during its ascent in the crust by different amounts of crustal material, in order to explain the positive correlation between the STSr/86Sr and Rb/Sr ratios. On account on the thickness of the continental crust beneath the central Andes (more than 70 km; Ocola et al., 1971), we consider this contamination process as a mixing process between a basic magma and a magma of crustal origin, formed by anatexis of crustal material. Only the different rocks types which constitute the continental crust of the Andes have to be considered as contaminants, inferring in a first approach, on the rock formations which outcroup at the surface. From a series of rocks which represent widespread rock types of the Paleozoic and the Precambrian basement of the central Andes, we selected some rocks whose Rb and Sr contents are suitable for contamination and we determined their Sr isotopic composition. The results are presented in Table 3.

TABLE 3 Sr isotopic data on sedimentary and metamorphic rocks of South America Type

Sample No.

A

1

B

2

C

17

D

10

E

22

F

23

Rock type and location

K (%)

Na (%)

Rb (ppm)

shale (Paleozoic, between Chanaral and Taltal, N Chile) mieasehist (Paleozoic, between

2.92

1.26

146

Rb/Sr

'TSr/S~Sr

35.6

4.10

0.7486

2.67

0.94

158

79.7

1.98

0.7404

0.47

251

64.5

3.90

0.7855

3.15

114

373

0.304

0.7144

1.44

3.23

79

409

0.194

0.7190

2.88

2.79

127

702

0.182

0.7150

Chanaral and Taltal, N Chile) granite (Precambrian, Salar de Hombre4.22 Muerto, N W Argentina) greywacke (Middle Paleozoic, Cor3.49 dilleraClaudio Gay, N Chile) biotite gneiss (Precarnbrian ? , east of La Angostura, NW Argentina) biotite gneiss (Precambrian, east of La Angostura, NW Argentina)

Sr (ppm)

O~

64 Contamination models. Contamination models were tested taking into account the fact that the contamination or mixing of primary magma with continental material took place in a closed system, the variations of the Sr isotopic composition within the series implying that each rock type has to be considered as a distinct liquid. Computation of contamination models must consider n o t only the Sr isotopic composition, but also the total Sr content. As already pointed out by James et al. (1974), the high Sr content of the andesites of the investigated area imposes severe restrictions on contamination models: indeed, a low fraction of gneissic material with high STSr/86Sr ratio will easily equilibrate the Sr isotopic composition but the low Sr content of the contaminant will lower the total Sr content of the contaminated magma instead of raising it. The restrictions imposed on the S~Sr/S6Sr ratio of the contaminant (James et ai., 1974) are not valid if we consider that the less evolved rocks are already contaminated. In a first assumption, we used as parental magma the andesitic magma of the southern Andes whose characteristics were considered as: 87Sr/86Sr = 0.704; Sr = 400 ppm; Rb = 20 ppm. The different contaminants listed in Table 2, were added in different amounts to the parental magma in order to obtain the Sr isotopic composition of the different rock types of southwestern Bolivia and northwestern Argentina. The calculated Sr content, however, never corresponds to the actual Sr content of the rocks. This type of contamination model could only be applied to an open system, where fractionation may considerably m o d i f y the Sr content, but the meaning of the apparent isochron than becomes enigmatic. The difficulty resulting from the high Sr content of the contaminated magma induces us to consider the possibility of a parental magma whose Sr content is higher than that of the andesites. Such a parental magma could be an alkali basalt with STSr/86Sr = 0.703 and a Sr content of 1000 ppm. With this assumption, assimilation in a closed system applies for all type of contaminants but the amounts of contamination will vary considerably from one type to another. Reasonable values for the percentage of contamination are found for different types of contaminants and the calculated Sr c o n t e n t follows relatively closely the actual c o n t e n t of the different rock types of the series: an example is presented in Fig. 6. The amounts of contamination are not unreasonable for the bulk composition, varying respectively around 0.3 part of contaminant for 1 part of basalt in the andesites, 1 to 1 in the latiandesites and 2 to 1 in the rhyodacites. With regard to the ignimbrites, a primary origin of the magma seems to be ruled out by geochemical and isotopic arguments. On the other hand, petrographic data (Pichler and Zeil, 1972a, b) point to at least partial participation of crustal material. The high Rb c o n t e n t versus Sr in the ignimbrites possibly results from a source rock that was very poor in Sr, or from selective enrichment of Rb relative to Sr during the melting process. Rocks with similar Rb and Sr contents are widespread in the Andean crust, but the older ages of these Rb-rich

65

. ~I

Ij 6

Alkati basalt (87Sr~B 6 =0703 Sr:1000ppm) .gran,te(87Sr/ = 0.786~ Sr/--B°~Sl~rn) /~bSr J3~

~

SrB71SrB6

oTos 07b~ 0767 07'oB 0~09 0~10 0ill

0~12 0~13 0i14

Fig.6. Calculated ratio (x) of contaminant necessary to equilibrate the Sr isotopic composition of the calc-alkaline rocks of northwestern Argentina and southwestern Bolivia (dots). The numbers above the dots represent the actual Sr content; the framed numbers correspond to the calculated Sr content after contamination.

source rocks will imply a much higher 87Sr/86Sr ratio for the derived magma: this holds whatever the origin of the ignimbrites, partial or total fusion of crust or important amounts of contamination. Simple calculations, similar to those carried out for the andesitic magmas, fail to explain the low Sr content of the ignimbrites, even for an important degree of contamination of a primary andesitic or basaltic magma. As the ignimbrites cannot be derived directly from a primitive magma, or result from contamination or mixing with old crustal rocks, they must originate from a source of relatively y o u n g age with a moderate Sr isotopic ratio but also with a high Rb/Sr ratio; the Mesozoic intrusive rocks presently out~ cropping along the Andean belt present these characteristics (McNutt et al., 1975). Rocks of this type incorporated in the crust could generate the ignimbritic magmas by partial or total fusion. This would imply a two-stage model by which magmatic rocks, possibly generated at a subcrustal level, were afterwards engulfed in the subduction zone. Their melting product gave different magmas the characteristics of which present some affinities with rocks of crustal origin. GEOLOGICAL

IMPLICATIONS

In both cases, as well for the "andesites" as for the ignimbrites, we reach the conclusion that crustal material is involved in the genesis of the magmas; our models fit with material of the upper crust. We consider also that contamination during the ascent of the magma has to be excluded for different reasons and especially because, if it concerns total fusion, the contamination acts by assimilation of different proportions of the same type of crustal rocks, which seems unlikely. It looks more plausible that a mixing process was acting between a magma of crustal provenance and a subcrustal parental magma and that this process occurred near the source region of the parental magma. But the question is to know by what geological processes contamination or mixing can occur on such a large scale, and under what circumstances crustal rocks can be brought at such low levels within the lithosphere.

66 The peculiar structure of the lithosphere beneath the Andes may offer an answer: there is first the anomalous thickness of the crust beneath the central Andes; from geophysical evidence, James (1971) concluded that the crust of the central Andes has doubled in thickness over the past 50--100 m.y. On the other hand, structural evidence suggests that tensional rather than compres. sive stresses characterize the region of the volcanic arc (James et al., 1974). A classic model of tectogenesis cannot explain the unusual thickening of the crust and therefore massive volumes of material have to be added to the crust from beneath the Mohorovi~i6 discontinuity (James et al., 1974). As a mechanism for this process we may invoke an accretion model (Lomnitz, 1974): by this model, when a cooling piece of lithosphere sinks into the mantle, at the contact between two plates, the lithosphere on both sides sinks into the void created by the sinking block; one plate grows at the expense of the other and this growth can occur as a vertical thickening of the crust. Consequently superficial crustal material near the trench is accreted onto the continental block and can be brought to levels in the crust where melting is possible. This model would explain that superficial and eventually young rock formations have contributed as contaminants in the generation of the calcalkaline rocks and even as source material for the ignimbrites. This model applies only for the rocks of the northern part of the investigated area, whereas the generation of the rocks of the southern Andes is caused by processes similar to those acting for the origin of island arc andesites. This difference corresponds also with a difference in the structure of the crust: whereas the crust of the central Andes is more than 70 km thick, the crust becomes progressively thinner towards the south; near Santiago the depth of the Moho is about 50 km. Sufficient geochemical and isotopical data have now become available to sketch a general view of the Andes, in relation with the major structural features of the different regions. The region of the central Andes is characterized by calc-alkaline lavas with higher Sr isotopic ratios (this study for northern Chile, southwestern Bolivia, northwestern Argentina; James et al., 1974, for southern Peru). This corresponds with the largest development of the Andean belt (Fig. 7) and also with the m a x i m u m thickness of the continental crust. At the outer part of the belt, the andesites present 87Sr/86Sr ratios close to those of island arc andesites: 0.704 for the southern Andes as well as for some andesites of Ecuador (Pichler and Zeil, 1972b). This corresponds with regions where the thickness of the crust is less great. Other features distinguish the central Andean belt from its outer ends: both in Ecuador and in Columbia (M~gard, 1973; Pichler et al., 1974) as well as in southern Chile, at the other end of the belt, pre-orogenic basic volcanics occur, while the Peru-Chile trench, which borders the Andean belt in its central part, disappears at both extremities. This implies that different geological processes are responsible for the calcalkaline magma generation on one hand in the central Andes, and at the outer

67

Rb<50 Sr<400 0.704

,/

/

0705-

Rb>50 ~r>400

0.705 - 0.713~:~.( ' I 0.705 - 0 . 7 0 7 / ~ / 0.705 - 0.709"~ ~,~// /

Rb<50 Sr<400

Fi .7. Variations o f the 8~Sr/S~Sr ratio and the Rb and Sr contents of the calc-alkaline rocks along the Andean belt. Legend: 1 = pre-orogenic basic volcanism, 2 = extension of the ignimbritic volcanism, 3 = trench. The interrupted line delimits the Andean belt. Data for southern Peru from James et al. (1976), for Ecuador from C.E. Hedge in Pichler and Zeil (1972b).

extremities of the belt on the other, and that the central part is probably involved in a more advanced stage of tectonic evolution than the outside parts. CONCLUSIONS

The geochemical characteristics of recent calc-alkaline rock associations from the central and southern Andes show that the magma generation processes are different according to the tectonic evolution undergone by the continental margin: (1) The andesites of the southern Andes correspond closely to island arc andesites, although they present some peculiar geochemical characteristics; their isotopic composition indicates that they could be products of melting along the subducted oceanic lithosphere. (2) Continental material has participated in the genesis of the magmas of the "Andesite" Formation of the central Andes, where the rocks are characterized by higher Sr radiogenic contents; although it cannot be definitively excluded that the lower crust was involved in the magma generation processes, it seems more plausible that these lavas result from mixing between a sub-

68

crustal magma of basaltic affinity and upper crustal rocks that were added to the base of the crust, by an accretion process acting at the margin of the continent along the subduction plane. (3) The ignimbrites of the central Andes, although they have peculiar chemical properties, exhibit certain affinities with the "Andesite" Formation. They are considered to result from a two-stage melting process by which igneous rocks, belonging to a earlier stage of development of the Andes are engulfed in the subduction zone, where they melted to produce the ignimbritic liquid.

APPENDIX -- SAMPLE LOCATIONS

Central and southern Chile. Sample numbers correspond to those in Klerkx (1965, pp. 464--465). Major element analyses in that publication. Vi 5, Vi 6, Vi 24: Ca 37 : Os 51: L1 56:

Quartz latiandesitic, latiandesitic and olivine andesitic lavas from composite volcano Villarrica (2840 m) Quartz andesitic lava, composite volcano Calbuco (2015 m) Andesitic lava, composite volcano Osorno (2652 m) Andesitic lava, composite volcano Llaima (2554 m)

"Andesite" (Strato-volcano ) Formation in northern Chile. Sample numbers correspond to those in Pichler and Zeil (1969, pp. 877--880). Major element analyses in that publication, corresponding trace element data in Siegers et al. (1969, pp. 883--884). 20: Quartz latiandesite, south of Salar Plato de Sopa; 25°28'S, 68°44'W. 21: Quartz latite, east of Salar de Agua Amarga; 25°34'S, 68°49'W. 17 : Dacite, west of composite volcano Socompa; 24°28'S, 68°20'W. 15: Quartz latiandesite, southwest of composite volcano Socompa; 24°28'S, 68°18'W. 14: Quartz latiandesite, west of composite volcano Lascar, 3750 m; 23°22'S, 67°47'W. 2: Quartz latiandesite, composite volcano San Pedro; 21°53'S, 68°26'W. 5: Quartz latiandesite, northwest of composite volcano Irruputunco, 4000 m; 20°42'S, 68°35'W. 11: Latiandesite, north of Salar de Laco; 23°48'S, 67°47'W. 18: Quartz andesite, south of Monturaqui; 24°25'S, 68°27'W. 8: Latiandesite, north of Salar de Coposa; 20°35'S, 68°40'W. 4: Rhyodacite, northeast of Salar de Huasco, 4200 m; 20°17'S, 68°49'W.

"Andesite" (Strato-volcano) Formation in northwestern Argentina. Sample numbers correspond to those in Hbrmann et al. (1973, pp. 408--410). Major and trace element data in that publication. A 50 (4/50- 6): A 57 (4/57- 6): A 69 (4/69-11): A 28 (4/28- 2):

Olivine-bearing latiandesite, volcano Carachipampa, south of Antofagasta de la Sierra; 26°05'S, 67°13'W. Latiandesite, volcano Antofagasta de la Sierra; 26°06'S,67°14'W. Olivine latite (shoshonite), east flank of the fault near La Poma between San A n t o n i o de Los Cobres and Cachi; 24°51'S, 66°14'W. Olivine-bearing latite (shoshonite) (Pleistocene, 0.2 ± 0.15 m.y.; Schwab and Lippolt, 1976, pp. 701--702), Cerro Negro Chorillos, west of San Antonio de los Cobres; 24°15'S, 66°24T/.

69

A 30 (4/30-16):

Quartz latiandesite, Esquina Azul, 120 m, volcano Nevado Queva; 24°12'S, 66°34T¢.

Strato-volcano ("Andesite") Formation of southwestern Bolivia. Sample numbers correspond to those in Fernandez et al. (1973, pp. 158--163). Major and trace element data in that publication.

Latiandesite, Chiguana; 21°06'S, 67°54'W. Quartz latiandesite, Soniquera volcano; 21°58'S, 67°19T/. Quartz latite (shoshonite), Rondal lava belonging to the Lower Quehua

B 1/1: B 1/4: B RO:

Formation (Lower Miocene, 22.9 ± 0.9 m.y.; Kussmaul et ah, 1975, p. 115), road San Pablo de Lipez--Guadalupe, 4600 m; 21°51'S, 66°30'W. Biotite-bearing rhyodacite, volcano Uturuncu, about 4800 m; 22°15'S, 67°14'W.

B 1/5:

Ignimbrites of southwestern Bolivia. Sample numbers correspond to those in Fernandez et al. (1973, pp. 161--163). Major and trace element data in that publication.

Rhyodacite, Ignimbrite (Rhyolite) Formation (Pliocene, between

B 2/2:

9.7 ± 0.4 and 3.0 ± 0.2 m.y.; Kussmaul et al., 1975, pp. 115--117), Alota; 21°26'S, 67°36'W. Rhyodacite, Upper Quehua Formation (Upper Miocene; 14.6 -+ 0.6 m.y. ; Kussmaul et al., 1975, pp. 115--116), "cathedral's section", unit III, road San Pablo de Lipez--Guadalupe, about 4600 m; 21°46'S, 66°34'W. Rhyodacite, Ignimbrite (Rhyolite) Formation (Pliocene; between 9.7 ± 0.4 and 3 ± 0.2 m.y., Kussmaul et al., 1975, pp. 115--117), Soniquera; 21°53'S, 67°19'W.

B QS/4:

B 2/3:

Rhyolite (Ignimbrite) Formation of northern Chile: Sample numbers correspond to those in Zeil and Pichler (1967, pp. 61--63). Major element analyses in that publication, corresponding trace element data in EI-Hinnawi et al. (1969, p. 53). Isotopic K--Ar ages from biotites o f the Rhyolite F o r m a t i o n range between 12.6 -+ 0.5 and 2.3 m.y. (Late Miocene to Early Pliocene; cited in Pichler and Zeil, 1972b, p. 431).

C 111 (Z iii-12):

Rhyodacitic pumice, Quebrada de Batea, Collacagua; 20°32'S,

C C C C C C

Rhyolite, Laguna Lejia; 23°31'S, 67°42'W. Rhyolite, road Tumbre--Tocona, 3000 m; 23°18'S, 67°54'W. Alkali rhyolite, Amilca, northwest of Mamifia; 20°4'S, 69°15'W. Rhyodacite, east of Salar de Laco; 23°51'S, 67°26'W. Alkali rhyolite, Quebrada Lugaya near Mamifia; 20°5'S, 69°13'W. Alkali rhyolite, Quebrada north of (above) Salto Chico, oasis Pica;

68°42'W. 231 (Z 245 (Z 56(Z 214 (Z 53(Z 14(Z

231-18): 245-16): 56-6): 214-20): 53-4): 14-7):

20°29'S, 69°18'W. C 324 (Z 324- 2):

Alkali rhyolite, track Campamento Rio Seco--Arica, 2600 m; 18°22'S, 69°38'W.

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70

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