Strontium isotope evidence for crustal contamination of calc-alkaline volcanic rocks from Cerro Galan, northwest Argentina

Earth and Planetary Science Letters, 48 (1980) 257-267 © Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

257

[6]

STRONTIUM ISOTOPE EVIDENCE F O R CRUSTAL CONTAMINATION OF CALC-ALKALINE VOLCANIC ROCKS FROM CERRO GALAN, NORTHWEST ARGENTINA P.W. FRANCIS

1, R.S. THORPE

l, S. MOORBATH 2, G.A. KRETZSCHMAR 1, M. HAMMILL 1

l Department of Earth Sciences, The Open University, Milton Keynes, MK7 6AA (U.K.) 2 Department of Geology and Mineralogy, University o f Oxford, Parks Road, Oxford, OX1 3PR (U.K.)

Received November 2, 1979 Revised version received March 14, 1980

The Pampean Ranges of northwest Argentina are a basin-and-range tectonic province with a late Precambrian to Paleozoic basement and extensive Miocene-Recent talc-alkaline volcanism. The volcanoes include the large resurgent Cerro Galan caldera, and Recent scoria cones and lava flows. Miocene-Recent volcanic rocks of basalt to dacite composition from the Cerro Galan area exhibit a range of Rb/Sr ratios of 0.043-1.092 and initial 87Sr/86Sr ratios of 0.7057-0.7115 with a clear positive correlation between 87Sr/86Sr and 87Rb/86Sr, indicating an apparent age of ca. 130 Ma. This relationship is interpreted to indicate that the Sr isotope variation in the Cerro Galan volcanic rocks results from mixing of a mantle-derived component with low 87Sr/a6Sr (<0.7057) and high Sr (>700 ppm) with a crustal component characterized by higher 87sr/a6sr (>0.7115) and lower Sr (<240 ppm). It is concluded that the mixing is best explained as a result of a small degree of selective crustal Sr contamination (ca. 10%) of a range of subsequently erupted magmas produced largely by fractional crystallization within the continental crust. We propose that the mantle-derived end-member is derived by partial melting of sub-Andean mantle with an 87St / 86Sr ratio of ca. 0.704, and that such an Sr isotope ratio characterizes the source region for calc-alkaline volcanic rocks throughout the Andes.

1. Introduction Several independent lines o f evidence indicate that Andean andesite magmas are dominantly mantlederived. These include the restriction o f active volcanic zones to areas underlain b y a relatively thick wedge of mantle [1,2], Sr and Nd isotope characteristics similar to those o f intra-oceanic island arcs [3,4] and the massive continental thickening since the Jurassic which cannot be explained by crustal shortening alone [5,6]. However, there has been much discussion o f the extent to which continental crust has been involved in the petrogenesis o f Andean andesites. The higher 87sr/a6Sr and lower 14aNd/144Nd ratios o f north Chilean andesites compared with those of Ecuador and south Chile might reflect contamination with ancient crustal rocks [3,7,8] or might reflect the isotope characteristics o f the subcontinental mantle source [4,6]. This paper presents new Rb, Sr and Sr

isotope data for volcanic rocks from the Cerro Galan area of northwest Argentina and argues that t h e isotope systematics demonstrate the important r61e o f selective crustal contamination in the petrogenesis of these calc-alkaline magmas.

2. Geological

setting

The Cerro Galan caldera is located on the eastern edge o f the Andean cordillera in an area characterized by basin-and-range tectonics known as the "Pampean Ranges" o f Argentina (Fig. 1). The basement consists o f high-grade metamorphic and acid intrusive rocks with a cover of Paleozoic and younger sedimentary rocks [9]. Although the metamorphic rocks are widely considered to be of late Precambrian to Paleozoic age, isotopic dates are poorly defined. Scattered Rb-Sr dates between 600 and 500 Ma have been

258

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.OOk-L / Fig. 1. The location of Cerro Galan in the Miocene-Recent volcanic belt of the Central Andes. reported [10], and a number of K-Ar dates fall within the range 500-400 Ma [11,12]. These Paleozoic dates are interpreted as representing Precambrian rocks which were metamorphosed and experienced intrusive igneous activity during the Lower Paleozoic. Between the Middle Silurian and beginning of the Cretaceous, the Pampean Ranges formed a stable continental block subjected to erosion [9]. Since the Cretaceous, some 6000 m of terrestrial sediments have been deposited, but no detailed stratigraphic work has been carried out on these sediments. In the Late Tertiary, major vertical movements of fault blocks and concomitant calcalkaline volcanism commenced, and has continued to the present. In the Salar De Cauchari area, Schwab and Lippolt [13] have shown that vertical fault movement must have been initiated prior to 11.8 Ma ago, and that widespread deposition of clastic sediments had ceased by about 7 Ma ago. In adjacent areas of north Chile and southwest Bolivia, widespread volcanism appears to have been initiated about 20 Ma ago and the same is probably true for northwest

Argentina [14]. Geophysical data for the region are sparse, but the map of James [15] suggests that the crustal thickness may be about 50 kin. The Cerro Galan caldera is a large resurgent caldera similar in morphology and in broad tectonic setting to the Valles caldera of New Mexico [ 16]. The caldera was first identified by Friedman and Heiken [17] from Spacelab imagery, and independently by Francis and Baker [18] from LANDSAT imagery and subsequently described by them [19]. The caldera and adjacent volcanic centres appear to be related to, and controlled by, major north-south faults which enclose a relatively flat-floored rift [19, fig. 1 ]. Preliminary geological and geochronological studies around the caldera show that the basement rocks in the area consist of varied schists and gneisses of both sedimentary and igneous origin and including migmatic varieties, of presumed Late Precambrian age [12,20]. These are overlain by a cover of folded Ordovician arenaceous and pelitic sediments mapped by Ace~olaza et al. [21] as the Falda Cienega Formation which are in turn unconformably overlain by Tertiary continental sediments known as the Sijes Formation. Deposition of these clastic sediments was probably related to the initiation of rift faulting. The earliest dated volcanic activity in the area was that giving rise to the Beltran Formation of Acefiolaza et al. [21]. A prominent component of this formation is Cerro Beltran, a large eroded andesitic composite volcano located on the western rift fault which was active about 10 Ma ago (G.A. Kretzschmar, unpublished data). Other andesite lavas and small dacite ignimbrites were erupted at about 7 Ma ago. Large-volume ignimbrite sheets were erupted from the caldera about 5 Ma ago, and were followed by resurgence of the centre of the caldera. The ignimbrites in the area have been grouped collectively by Acefiolaza et al. [21 ] as the Toconquis Formation. The youngest activity in the area was the eruption of small acid extrusions from sources located on the caldera ring fractures, and of basaltic andesites on the floor of the rift, outside the caldera. The cones and flows of the rift floor are youthful morphologically, and are grouped collectively as the Incahuasi Formation by Aceffolaza et al. [21 ] ; some analytical data on these rocks have been presented by Hormann et al. [221.

259

3. Analytical results Volcanic rocks from the Cerro Galan caldera and adjacent centres within the rift range from basalt through basaltic andesites and andesite to dacite in composition using the classification of Peccerillo and Taylor [23]. The analysed samples are all lavas except sample G64 and G67 which are dacite clasts from an ignimbrite and are assumed to be representative of the magma composition. There is no petrographic evidence, such as partly fused xenoliths of basement rocks in the lavas, to suggest contamination of the magmas by crustal material. Fifteen representative samples have been analysed for Rb, Sr and 87Sr/ 86Sr ratios and the results are listed in Table 1. Sr isotope ratios were determined on a VG Micromass 30 mass spectrometer, using chemical and mass spectrometric techniques described elsewhere [24]. All Sr isotope ratios have been normalised to a value of 87Sr/86Sr = 0.70800 for the Eimer and Amend standard SrCO 3. Rb and Sr were determined by a precise

X-ray fluorescence technique [25]. Using measured K-Ar ages and ages inferred from stratigraphic consideration, the initial 8~Sr/a6Sr ratios have been calculated and are included in Table 1. The samples show a range of initial 87Sr/a6Sr ratios from 0.70569 to 0.71152 and a range of Rb/Sr ratios from 0.043 to 1.092. Both ratios show a wider range than for other calc-alkaline lavas from the Central Andes 87Sr/86Sr = 0 . 7 0 5 - 0 . 7 0 8 [3,6,26]. The basalt and basaltic andesite lavas have similar initial Sr isotope ratios to those of andesite lavas in north Chile and southern Peru [3,4,6,8,26]. However, the initial Sr isotope ratios of the andesites and dacites are higher respectively than those we have reported for north Chilean andesites [3,4] and dacite-rhyolite ignimbrites [27]. The major feature of the new data is the clear positive correlation of 87Sr/86Sr with 87Rb/a6Sr (and hence Rb/Sr) (Fig. 2). Such positive correlations, or "pseudo-isochrons", were identified by James et al. [6] for andesites and dacite lavas from two volcanic groups in southern Peru (the "Arequipa"

TABLE 1 Analyses of volcanic rocks from the Cerro Galan rift area of northwestern Argentina Sample No. Rock G3 G4 G26 G30 G33 G35 G37 G44 G49

andesite basaltic andesite dacite dacite basaltic andesite andesite basaltic andesite

Age (Ma) *

Sr ** (ppm)

Rb/Sr

87Rb/a6sr t

87Sr/86Srt t

(87Sr/86Sr)0

434

0.269

0.779

0.70953 ± 5

0.70935

661

0.094

0.272

0.70597 ± 5

0.70597

259 327

0.828 0.595

2.397 1.723

0.71099 _+6 0.70985 -+6

0.71023 0.70927

47.7

548

0.087

0.252

0.70660 -+7

0.70660

91.4

442

0.207

0.599

0.70852 ± 4

0.70842

(0)

44.5

1048

0.043

0.124

0.70723 ± 6

0.70723

7.1 (0) 9.9 10.4 (0) 5.1

Rb ** (ppm) 117 62.2 214 195

(0) (0)

51.7 58.8

645 698

0.080 0.084

0.232 0.243

0.70627 ± 6 0.70569 ± 8

0.70627 0.70569

G55

andesite basalt basaltic andesite

(6)

75.1

477

0.157

0.454

0.70731 ± 7

0.70722

G56 G57 G58 G67 G64

dacite andesite dacite dacite dacite

(6) (0) (6) (6) (6)

184 88.6 225 269 253

317 470 238 246 236

0.578 0.189 0.945 1.092 1.073

1.673 0.547 2.736 3.162 3.107

0.70909 0.70890 0.71155 0.71177 0.71142

0.70890 0.70890 0.71136 0.71152 0.71117

±5 _*5 _*6 _+6 ±6

* K-At age determinations (G.A. Kretzschmar). Figures in parentheses indicate estimated age from stratigraphic considerations. ** Determined by X-ray fluorescence. Mass absorption coefficients estimated from Compton scattering, concentration data about ±5%. t Average precision +1%. t t Within-run precision quoted as 20 level.

260 0712 O67 58°

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037

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Fig. 2. Plot of 87Sr/S6Sr against 87Rb/STSr for volcanic rocks from the Cerro Galan area. The numbers are the sample numbers listed in Table 1.

and "Barroso" volcanics). The points with higher Rb/ Sr ratios plotted in Fig. 1 define an apparent "age" of ca. 130 Ma for volcanic rocks dated at younger than ca. 12 Ma, but we do not attribute any age significance to this. We examine the significance of the relationship for the petrogenesis of the Cerro Galan volcanic rocks below.

4. Discussion Correlations of initial 87Sr/S6Sr with STRb/SeSr have been reported for a wide range of volcanic rocks and have been reviewed by James et al. [6] and Brooks et al. [28]. These authors claimed that such correlations were established within the mantle where they date a fractionation event. However, as pointed out by Pankhurst [29] there are many difficulties with such a hypothesis and in many cases such correlations are more plausibly explained by mixing of components with contrasted trace elements and isotope characteristics, so that the positive correlation has no age significance. Such mixing might reflect simple mixing of two magmas; partial melting of material from mixed sources; mixing of a magma with a partially melted crustal component; or more complex models in which individual components are mixed ([30], cf. [8]).

A more elaborate mixing model to account for the characteristics of Peruvian andesites was advanced by James [31 ]. The andesites studied by James had 87Sr/ 868r = 0.705-0.708 (cf. [3]) and 6180 = +5.9 to +6.3% [31,32]. James argued that the Sr isotope data appeared to require contamination on a much larger scale than indicated by the oxygen isotope data and that the isotope characteristics could not be explained by "crustal contamination of a parental magma of "normal" Sr isotopic composition (i.e. 87Sr/S6Sr = 0.704)" [31, p. 574]. In view of the significant partition coefficients of Sr into minerals in possible crustal contaminants, James also rejected selective contamination. However, in view of clear isotopic evidence for involvement of sialic material, James concluded that "the isotopic and trace element composition of the Central Andean volcanics can plausibly be explained by partial melting at depth of subducted greywacke and altered ocean basalt, and subsequent reaction of that melt with overlying mantle material to yield andesitic magma" [31, p. 563]. We cannot accept that subduction of sedimentary material plays a significant role in the petrogenesis of Central Andean andesites. Kulm and Schweller [33] have reviewed the characteristics of the Andean continental margin and divided the margin into three provinces: Province I (6-19.5°S), Province 2 (19.527°S) and Province 3 (27-45°S). They argue that the occurrence of long prominent benches on the lower continental slope of sedimentary Provinces 1 and 3 indicates that there are regions where "accretion has occurred along the outer continental margin over the long term" [33, p. 296]. They also argue that some of the geological characteristics of Province 2, such as the absence of small development of benches on the continental slope, and the absence or small size of sedimentary basins on the continental shelf, might be taken to indicate consumption of material in the trench, or even of the continental margin. The geological evidence for the consumption and subduction of material from the continental margin is equivocal; if it has occurred, it can only have taken place to a rather small extent. The subduction of continental sediments seems even less likely. Most of the material eroded from the Andean cordillera is transported to the east, away from the trench, and the amount entering the trench itself is a direct func-

261 tion of the annual rainfall in coastal regions [34,35]. Since the trench off the Central Andes lies adjacent to the Atacama desert, the driest in the world, and since geomorphological and geochronological studies show [36,37] that similar climatic conditions have prevailed over at least the past 20 Ma, we consider that the characteristics of the trench described by Kulm and Schweller reflect climatic conditions. It is most unlikely that subducted continental sediments have contributed to the petrogenesis of andesites in the central volcanic zone of the Andes. We consider that there is no simple ingredient which can be mixed with andesite magma to produce the observed isotopic compositions. It appears more likely that Central Andean andesites are derived from an isotopically nearly homogeneous mantle and are subsequently modified by selective contamination with crustal components. Our arguments are developed below. Pushkar et al. [38] have outlined the formal characteristics of Sr mixing models and distinguished closed-system (bulk contamination) and open-system (selective contamination) models. Where mixing occurs between components of contrasted 87Sr/S6Sr ratio and Sr content well-defined relationships result [30,38,39]. These include a hyperbolic negative correlation of initial 87Sr/S~Sr, corresponding to a linear positive correlation of initial 87Sr/86Sr with 1/Sr (see Faure [39, p. 99]). For such a model to be applied to the Cerro Galan lavas, the mixing must be between an end-member with relatively low s 7Sr/S6Sr (<0.7057) and with high Sr (ca. 700 ppm), and an end-member with relatively high 878r/86Sr (>0.7115) and with low Sr (ca. 240 ppm). Further constraints on the identities of the two end-members can be established by reference to Andean volcanic rocks elsewhere and to the regional ecology. The low-STSr/a6Sr, high-Sr end-member represented by the Cerro Galan basalt and basaltic andesites (initial 87Sr/86Sr = 0.705-0.706) has higher initial 875r/a6Sr ratios than Ecuadorian basaltic andesites [3] and basalts and basaltic andesites from south Chile ([26], 87Sr/a6Sr = 0.704) but similar ratios to north Chilean and south Peruvian andesites (STSr/S6Sr = 0.705-0.708 [3,6,26,31]). The endmember might therefore be a mantle-derived basic magma with Sr isotope characteristics like those of basalts and basaltic andesites in Ecuador and south

Chile and we comment further on this below. The high-STSr/S6Sr, low-Sr end-member has characteristics which match those of pre-Cenozoic metamorphic rocks exposed within the Central Andes. For such rocks 87Sr/86Sr and Sr analyses have been presented by Munizaga et al. [40], James et al. [6], Cobbing et al. [41], Lehmann [42], Halpern and Fuenzalida [43], Klerkx et al. [26] and Shackleton et al. [44]. Rb and Sr contents for a range of metamorphic and igneous rocks have also been reported by Rapela and Shaw [45]. Many of these basement rocks have aTSr/S6Sr = 0.71-0.78 and Sr contents below 300 ppm and such characteristics broadly match those of the high-STSr/S6Sr, low-Sr component of the Cerro Galan lavas. Closed-system mixing models have been calculated between basalt G49 (Sr = 698 ppm; STSr/S6Sr = 0.7057) and various crustal rocks and minerals from the sources above. A mixing model between G49 and a crustal rock with Sr = 150 ppm and STSr/a~Sr = 0.718, representative of common values within these data, is shown in Fig. 3 (line A). Because of the high aTSr/S6Sr ratio and low Sr content of this crustal component, the mixing model has a hyperbolic form which shows good agreement with the isotope data. The data in Fig. 3 are shown as a plot of STSr/S6Sr against 10S/Sr in Fig. 4. In this figure the hyperbolic mixing curve plots as a straight line (A). Andesites 3, 35 and 56 and basaltic andesite 37 lie significantly away from the mixing lines in Figs. 3 and 4 but are not otherwise distinguished on geochemical or geological grounds from other samples. Although the isotope characteristics of the Cerro Galan lavas do not contradict a closed-system, bulkmixing model, there are several obstacles in accepting such an explanation. Firstly, in the closed-system model the Sr contents of the lavas are determined entirely by mixing of components with different Sr contents. The model illustrated in Fig. 3 shows that up to 80% of the Sr in the acid rocks could be of crustal origin. The proportion of crustal Sr is relatively insensitive to the STsr/a6Sr of the crustal contaminants. For example, if the STSr/a6Sr of the crustal component is increased from 0.715 to 0.740, the maximum proportion of crustal Sr required to generate the Sr isotope characteristics of the acid rocks is only decreased from about 80% to 60%. A bulk-mixing model therefore requires that the bulk

262 0.712 ;6

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0.706

) 49 I

0

I

200

I

I

I

I

400

600

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800

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Sr (ppm)

Fig. 3. Plot of STSr/S6Sr against Sr (ppm) for volcanic rocks from the Cerro Galan area. Curve A is a closed-system mixing model between basalt G49 (87Sr/S6Sr = 0.70569, Sr = 698 ppm) and a crustal component with 87Sr/86Sr = 0.718 and Sr = 150 ppm. The numbered open circles on A are the percentages of crustal component involved. The dashed curve B is an open-system mixing model involving 10% contamination of a magma with 87Sr/S6Sr = 0.704 by a crustal component with 87Sr/86Sr = 0.78 and Sr = 200 ppm. The numbers against the solid circles are the sample numbers listed in Table 1. See text for further discussion.

compositions of the volcanic rocks are determined by the proportions of mantle-derived andesites and crustal contaminant, the latter dominantly the compositions of the acid rocks. However, field and petrographic evidence for involvement of such large proportions of crustal rocks is totally lacking and we consider that the proportion of crustal assimilation suggested by the bulk.mixing closed-system model is implausible. A second problem with a closed-system mixing model is that such a model would yield uniform systematic relationships between the chemical characteristics of the end-members (see Langmuir et al. [30]). Such simple relationships do not characterize our major and trace element data (G.A. Kretzschmar, unpublished). For example, while correlations of 87Sr] 8~Sr with 1/Sr are statistically highly significant, correlations o f STSr/S6Sr with other trace elements,

including incompatible elements such as Zr, are not significant. We therefore consider that, in spite of the formal correspondence of the Sr isotope data with a closed-system bulk-mixing model, the latter is not appropriate to explain our data. Finally, we have argued elsewhere that the major process involved in formation of dacite and rhyolite lavas is fractional crystallization [7,27,46]. Such a model is consistent with the compositional similarities between andesite lavas and ignimbrites and their overlapping Sr isotope compositions in north Chile [27]. A fractional crystallization model also accounts for the high Rb/Sr ratios and low Sr contents, and characteristics of REE patterns of dacite-rhyolite ignimbrites in comparison with andesites [27]. In view of the similar compositional characteristics of the Cerro Galan lavas and ignimbrites we consider that the overall chemical characteristics of the acid

263

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rocks reflect fractional crystallization rather than melting/mixing processes. In view of the problems posed by closed-system, bulk-mixing models, we now consider the "open-system" model (cf. [39]). This model assumes that the range of magma compositions (including Sr contents) result from fractional crystallization (cf. [27]) and that each magma is contaminated to a similar degree with crustal strontium. In such a model the Sr contents of the magmas are determined largely by fractional crystallization and only to a smaller extent by contamination. Such open-system mixing produces 87Sr/S6Sr-Sr relationships'analogous to those characteristics of "closed-system" mixing. For example, a model involving a uniform degree of contamination of magmas with fixed 87Sr/a6Sr and varying Sr contents by a contaminant with fixed Sr and 87Sr/a6Sr yields a hyperbolic negative correlation of 87sr/a6sr with Sr. Such a model can account for the Sr isotope characteristics of the Cerro Galan lavas. In contrast to the

situation for closed-system conditions, the termination of the 87Sr/a6Sr-Sr hyperbolic negative correlation is unconfined, but asymptotically approaches the specified Sr isotope ratio and Sr content. An opensystem contamination model showing the effect of 10% contamination of a parent magma of 87Sr/S6Sr = 0.704 (q.v. later) with continental crust with 87Sr/ 865r = 0.78, Sr = 200 ppm, is shown in Fig. 4. Such a model provides an approximation to the data similar to the "closed-system" model outlined earlier. An "open-system" contamination model can therefore account for the Sr isotope relationships of the Cerro Galan magmas by invoking a small, but uniform degree of contamination (ca. 10%). This is con. sistent with the formation of the range of magmas by fractional crystallization, and also accounts for the lack of correlation between aTSr/S6Sr and other trace elements. This is because, after such selective contamination, such correlations would only be observed between 87Sr/S6Sr and elements which were correlated with Sr in the fractionated magmas before

264 contamination. A contamination process, acting after fractional crystallization of mantle-derived andesite magmas, can provide the best explanation of the Sr isotope characteristics of the Cerro Galan lavas. We now consider the nature of the mantle source for parent magmas of the Cerro Galan rocks.

5. Sr isotope characteristics of the sub-Andean mantle Accepting that the Sr isotope characteristics of the Cerro Galan volcanic rocks reflect mixing between crustal and mantle-derived components, it is necessary to consider to what extent the Sr isotope compositions (ca. 0.705-0.707) of the basalt and basaltic andesites are representative of the sub-Andean mantle source of Andean andesites. For this, we refer to published and unpublished data for Andean volcanic rocks and make the following points: (1) It was noted earlier that active Andean volcanism occurs in areas underlain by a relatively thick mantle wedge. Diffusion data for trace elements and isotopes indicate that such presumed partially molten mantle will tend to equilibrate locally within geologically short periods of time (less than l0 s to 104 years [47,48]). Under such conditions, the mantle-derived end-member would be relatively homogeneous, and variations in 87Sr/86Sr ratios within small areas such as the San Pedro-San Pablo volcanoes in north Chile (initial aTSr/86Sr = 0.7058-0.7072) probably result from interaction of andesite magma with crustal rocks. If this interpretation of the variation is correct, then it implies that even the lowest aTSr/a6Sr ratios of the north Chilean andesites and the Cerro Galan basalts and basaltic andesites might have been increased by contamination with a crustal component. (2) In several andesitic provinces, for example Mexico and the Lesser Antilles as well as south Chile and Patagonia, oversaturated andesites and undersaturated basaltic rocks have similar initial aTSr/a6Sr ratios, generally in the range 0.7035-0.7045 [4,49-51]. In Ecuador our unpublished data show that a hauyne-tephrite from Sumaco volcano has an initial aVSr/a6Sr = 0.70395 +- 6 (sample AW 587) which falls virtually within the range of initial a 7Sr/ 865r ratios reported for Ecuadorian andesites [3]. It appears that a range of contrasted volcanic rocks

might be derived from a relatively homogeneous mantle source with 87Sr/a6Sr = 0.7035-0.7045. In contrast to these provinces, Sr isotope data for volcanic rocks from western Bolivia show that an undersaturated basalt has an initial 87Sr/a6Sr ratio of 0.70414 +- 7, but that the andesites and dacites have 87Sr/a6Sr = 0.7055-0.7064 [7]. The simplest explanation of these data is that Central Andean andesites are derived from a source region capable of producing basaltic magma, with 87Sr/a6Sr of 0.70350.7045, but that such magmas interact with continental crust during slow ascent. Such interaction has a greater effect on andesites due to the generally lower Sr content in comparison with basaltic rocks. (3) For the Andes as a whole there is a clear regional relationship between initial 87Sr/a6Sr ratio and crustal character and thickness similar to that documented by Kistler and Peterman [52] and Armstrong et al. [53] for the western U.S.A. Thus, published data for Ecuadorian andesites [3] as well as published and unpublished data for central and south Chile andesites (south of 35°S [26] ; D6ruelle and Moorbath, unpublished) have low initial 87Sr/a6Sr ratios in the range 0.7035-0.7043 (mostly towards the lower end) which are independent of rock composition. These areas are all underlain by relatively thin continental crust [7, Fig. la] and Precambrian rocks appear to be absent. In contrast, the Central Andean province (south Peru, north Chile, west Bolivia) is underlain by thicker crust which may include an Archaean or Proterozoic component [41,42,44]. Volcanic rocks for this area have characteristicaUy high and variable initial aTSr/a6Sr ratios [3,7,26]. Although the crustal structure below Cerro Galan is poorly known, the geological affinities are clearly with the Central Andean volcanic province and the widespread regional correlation between initial aTSr/a6Sr and crustal structure is considered to provide strong circumstantial evidence for interaction between magmas derived from a mantle source region with a7Sr/S6Sr = 0.7035-0.7045 and continental crust. (4) For the Cerro Galan volcanic rocks we have argued that the Sr isotope systematics can only be explained by mixing of mantle-derived and crustal components. However, there is no petrographic evidence for contamination by crustal material. This paradox has been discussed for several provinces by

265

Carter et al. [54], Thorpe and Francis [46], Moorbath and Thompson [55] and Faure et al. [56]. These workers conclude that contamination cannot be explained by bulk assimilation but must be due to selective contamination resulting from such processes as crustal dehydration, crustal disequilibrium melting, or addition of anatectic crustal melts. Since the process is selective and may be relatively insensitive to the 875r/86Sr ratio of the contaminant (see earlier) the amounts of contamination may be very much less than the values derived for simple bulk mixing (Fig. 3). Briqueu and Lancelot [8] may well be correct when they state that "the variation of the isotopic composition of these (southern Peru) lavas, as well as 87Sr/S6Sr vs. 1/S~Sr diagrams form the basis for a model involving processes of fractional crystallization combined with mixing and addition of radiogenic Sr that originated in continental crust and was transported by a fluid phase". This scavenging fluid phase may originate from the break-down of hydrous minerals in buried crustal rocks during massive influx of magmatic heat [57]. For the above reasons there is no compelling reason to regard the Sr isotope characteristics of the Cerro Galan basaltic andesites as representative of a sub-Andean mantle source region. Rather points (1) to (4) favour a model in which these basic magmas are derived from a mantle source region with 87Sr/ 86Sr --- 0 . 7 0 3 5 - 0 . 7 0 4 5 , similar to the mantle source of the Ecuadorian and south Chilean basalts and basaltic andesites. Partial melting of such mantle produced the parental magma for the andesites and these experienced fractional crystallization and contamination during uprise through thick, ancient continental crust. Further fractional crystallization during storage within the crust was responsible for formation of the range of andesite-dacite magmas and selective contamination after formation of these magmas was responsible for the Sr isotope relationships described.

Acknowledgements We thank R. Goodwin and M. Humm for skilled technical assistance. The isotope work at Oxford and field work in the Andes is supported by the Natural Environment Research Council.

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