pb isotopes in central andes

8/10/2019 Pb Isotopes in Central Andes

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Pb isotopes define basement domains of the Altiplano,

central Andes

S. J. Aitcheson Department of Earth Sciences, University of Oxford, Parks Road, Oxford OX1 3PR, United Kingdom

R. S. Harmon U.S. Army Research Office, P.O. Box 12211, Research Triangle Park, North Carolina 27709

S. Moorbath Department of Earth Sciences, University of Oxford, Parks Road, Oxford OX1 3PR, United Kingdom

A. Schneide r TVX GOLD, 11 Septiembre 2353, Santiago, Chile

P. Soler ORSTOM, Departement Terre-Oce an-Atmosphere, 213 rue Lafayette, 7 5480 Paris cedex 10, France

E. Soria-Escalante Department of Earth Sciences, University of Oxford, Parks Road, Oxford OX1 3PR, United Kingdom

G. Steele EMICRUZ (COMSUR/RTZ JV), Casilla 4326, La Paz, Bolivia

I. Swainbank Natural Environment Research Council Isotope Geosciences Laboratories, Keyworth, Nottingham NG12 5GG,

United Kingdom

G. Worner Geochemisches Insti tut, Universitat Gottingen, D-37077 Gottingen, Germany

ABSTRACT

Detailed Pb isotopic maps of the central Andes, based on 345

(163 previously published, 182 new) analyses of ores, volcanic rocks,

and their host rocks, elucidate the gross structure of the base-

ment and reveal that several isotopicallydistinct basement domains

are juxtaposed in this region. The data clearly show that most of the

Pb in central Andean igneous and ore samples is derived from the

local basement, including Pb in ore deposits of the Bolivian tin belt.Some of the isotopic domain boundaries correspond to geologic

structures and the residual gravity pattern, as well as to metallo-

genic boundaries such as the western edge of the Bolivian tin

belt.

INTRODUCTION

The 300 2000 km Altiplano-Puna plateau dominates thephysiography of the central Andes. It has an average elevation of 3.8km and overlies exceptionally (up to 70 km) thick continental crustat the great bend in the Andes at lat 18 S. Here, Cenozoic volcanicrocks associated with the subduction of the Nazca plate beneath thewestern margin of South America form an arc several hundred ki-lometres wide. The Altiplano, or northern part of the central An-

dean plateau, is now an elevated, low-relief, internally drained basinsituated between the volcanic front to the west and the fold-and-thrust belt of the Eastern Cordillera. Over much of the centralAndes, and especially on the Altiplano, young volcanic and sedi-mentary deposits obscure the basement rocks and so inhibit ourunderstanding of the regional crustal structure. Proterozoic gneissesof the Arequipa massif crop out to the northwest of the Altiplanoon the Peruvian coast (Shackleton et al., 1979; Dalmayrac et al.,1977), and Proterozoic rocks of the Brazilian craton (e.g., Litherlandet al., 1989) occur to the northeast of the Eastern Cordillera. Else-where, however, published geological and geochronological infor-mation on the basement is limited to a few inliers in northern Chileand northwest Argentina (e.g., Damm et al., 1990; Pacci et al., 1980;Kay et al., 1993) and to one drill-core sample from northwesternBolivia (Lehmann, 1978).

Pb isotopic compositions of crustal rocks can vary enormouslybecause of differences in their ages or U-Th-Pb fractionation his-tories. Different basement provinces can develop distinct isotopiccharacteristics and may, therefore, be distinguishable on the basis oftheir Pb isotopic compositions, even in the absence of other geologicinformation. Such a situation occurs in the Altiplano where, al-though there are few exposures of the basement rocks, Pb from

these rocks has reached the surface at many locations in magmasand ore-forming fluids through crustal-contamination processes.

In this study we have compiled all 163 previously published Pbisotopic data for ores, volcanic rocks, and country rocks for the areain the central Andes from lat 16 to 24S and from long 64 to 70W.We also present new Pb isotopic data from a further 182 samples (32basement or host rocks, 100 igneous rocks, and 50 ores). We show

that the Pb isotopic composition of ores and igneous rocks is ofcrustal origin, and we use this information to infer the existence ofdifferent basement blocks beneath the Altiplano area and to locatethe boundaries between these blocks.

Previous studies of Pb isotopes in the central Andes (e.g., Til-ton and Barreiro, 1980; Tilton et al., 1981; Puig, 1988) consideredmainly petrogenesis of the volcanic rocks and sources of ore Pb byusing Pb-Pb isotope plots. All of these studies suggested that the Pbin the igneous rocks and ores was a mixture of mantle-derived Pband an additional component from the continental crust. Accordingto Puig (1988), varying proportions of these components in the hostrocks led to isotopic variations that were inherited directly by theores. Some workers (e.g., McNutt et al., 1979; Macfarlane et al.,1990) also considered subducted (metalliferous) sediments to be an

important Pb source for ores and igneous rocks.Macfarlane et al. (1990) established that the central Andes

contained several geographic provinces with different ore Pb iso-topic compositions. Their survey area was much larger but lessdensely sampled than in this study, and their suggested Pb isotopicboundaries differ substantially from those defined here. Worner etal. (1992) used Pb isotopic data from volcanic rocks along the vol-canic front of northern Chile to show that a marked change incrustal Pb isotopic composition occurred at 19.5S, with less ra-diogenic Pb to the north and more radiogenic Pb to the south. Kayet al. (1993) observed Pb isotopic provinciality in the area from 28to 33S, with relatively radiogenic Pb in the main Cordillera volcanicrocks and less radiogenic Pb in those to the west (Precordillera) andto the east (Sierras Pampeanas). They attributed this pattern partlyto basement Pb components and partly to subcrustal Pb sourcessuch as subducted sediment.

SAMPLES

Volcanic rocks of Miocene-Holocene age were sampledthroughout the study area (Fig. 1). Most are of andesite-rhyolitecomposition, but they also include members of the back-arc sho-shonite suite. K-feldspar separates were obtained from several Mes-

Data Repository item 9529 contains additional material related to this article.

Geology; June 1995; v. 23; no. 6; p. 555558; 3 figures. 555


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front eastward to Lago Poopo and southward to the Salar de Uyuniat 19.5S. It is characterized by relatively nonradiogenic Pb, i.e., lowvalues of 206Pb/204Pb (18.3), 207Pb/204Pb (15.62), and 208Pb/204Pb (38.5).

2. The Eastern Cordillera domain has more radiogenic Pb, i.e.,higher values of Pb isotopic ratios (206Pb/204Pb 18.6, 207Pb/204Pb 15.64, and 208Pb/204Pb 38.9) and extends from Lago Poopoeastward into the Eastern Cordillera. At Lago Poopo this domain isseparated from the northern Altiplano domain to the west by asharp, north-trending boundary that appears to be offset south ofLago Poopo. At Lake Titicaca the boundary between the two do-

mains is sharp and trends northwest.3. The southern Altiplano domain is isotopically similar to the

Eastern Cordillera domain except that it has slightly lower 208Pb/204Pb ratios (38.538.9). Its 207Pb/204Pb ratios are all at the low endof the range for the Eastern Cordillera. The boundary between thenorthern and southern Altiplano domains is a broad transition zonebetween 19.5S and 21S, where both radiogenic and nonradiogenicPb is present.

4. The Jurassic coastal domain forms a belt parallel to, andwest of, the volcanic front and is characterized by relatively radio-genic Pb. There are too few data to identify precisely the locationand nature of the boundary between it and the Altiplano domains.

The contours in Figure 1 are hand-drawn. The isotopic domainboundaries change little if a smaller contour interval is used.

Figure 2 plots 208Pb/204Pb vs. 206Pb/204Pb data for ore depositsand volcanic rocks from the first three domains enumerated belowand superimposes the compositional fields measured directly fromthe basement or country rocks in each area. Figure 2 shows that ineach area the basement rocks have a distinctly different Pb isotopiccomposition and that the ore and volcanic data reflect the localbasement composition.

INTERPRETATION AND DISCUSSION

The close match between the known basement compositionand that of local igneous and ore samples strongly supports a crustalorigin for most of the Pb in these samples. The main control on Pbisotopic compositions of the ores and volcanic rocks is clearly the

composition of the basement. In our opinion the Pb isotopic pro-vinciality displayed by these samples could not be inherited from themantle source of the magmas. Sharp province boundaries from themantle are unlikely to survive long-lived magmatic processes andmantle convection, and crustal contamination would rapidlyobscureany Pb isotopic heterogeneity in the primary magmas. In fact, it iswidely accepted that Andean igneous rocks are significantly con-taminated by continental material (e.g., Hildreth and Moorbath,1988; Davidson et al., 1990; Worner et al., 1992), and models in-volving as little as 10%20% of contamination of the magmaticsystems of the Altiplano area indicate that the crustal Pb component

would still be very large (75%) in the erupted liquids (Aitchesonand Forrest, 1994, and unpublished; Worner et al., 1988). Thecrustal Pb component must, then, be large in the ore deposits also,even if the ore Pb and ore-forming fluids were all exsolved frommagma and no Pb was scavenged by fluids directly from the crust.

For all of these reasons we regard the Pb isotopic compositionof ores and volcanic rocks in this area as approximately representingthe composition of the underlying basement. We interpret the dif-ferent Pb isotopic domains of the central Andes (Fig. 3) inferredfrom the Pb isotopic maps (Fig. 1) as representing distinct basementdomains of different age and composition. We suggest that the iso-topic domain boundaries reflect the positions and character (i.e.,whether abrupt or transitional) of real geologic boundaries betweenthe different crustal blocks.

The abrupt boundary between the Eastern Cordillera (EC inFig. 3) domain and the Altiplano domains (NA and SA in Fig. 3)could be a major fault and apparently coincides with the Copaca-bana fault zone at Lake Titicaca and with the Coniri fault betweenLa Paz and Oruro. The broad transition zone (TZ in Fig. 3) betweenthe northern and southern Altiplano domains contains both radio-genic and nonradiogenic crust and might represent a single south-dipping thrust zone (e.g., Worner et al., 1992) or a more complexzone in which the two types of crust are tectonically interleaved. Onthe Bouguer gravity map of the Altiplano (Cady, 1992), the trend ofelongate gravity anomalies changes from north-northwest north ofthe Salar de Uyuni to northeast south of the Salar de Uyuni, sug-gesting that a change in orientation of crustal structures occurswhere the Pb isotopic data define the transition zone. The positions

of minor Pb isotope anomalies, such as A in Figure 3, also coincidewith small gravity anomalies.

The basement of the northern Altiplano, though relatively non-radiogenic, nevertheless has higher 206Pb/204Pb and 207Pb/204Pb ra-tios than the measured compositions (Tilton and Barreiro, 1980) of

Figure 2. 208Pb/204Pb vs. 206Pb/204Pb plot for volcanic rocks

and ores of central Andes, classified by geographic area.

For comparison, compositions of basement rocks from the

different areas are also shown (as fields). Note distinctly

different isotopic composition of each geographic group

and how it reflects known composition of local basement

rocks. Less radiogenic Eastern Cordillera samples, which

overlap with northern Altiplano field, come from north of

19S and from close to Argentinian frontier (see text for

discussion). Jurassic coastal domain data (omitted for

clarity) overlap mainly with southern Altiplano data.

Figure 3. Map showing

m a i n c r u s t a l d o m a i n

boundar ies of centr al

Andes inferred from Pb

isotopic maps in Figure

1 . N A no r t he r n A l t i -p l a no d o m ai n , S A

southern Altiplano do-

m a i n , E C E a st e r n

Cordillera domain, TZ

transition zone, JCD

J u r a s s i c c o a s t a l d o -

m a i n . A s m a ll P b

isotopic anomaly that

cor r esponds t o sm all

gravity anomaly.

GEOLOGY, June 1995 557


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Arequipa massif gneisses to the northwest, and so it is uncertain ifthe northern Altiplano basement is part of the same massif. How-ever, we note that Nd crustal extraction ages ( tDM, DePaolo, 1981)of1.9 Ga from northern Altiplano basement samples (Aitchesonand Moorbath, 1992, unpublished data) are similar to the protolithage inferred by Dalmayrac et al. (1977) for Arequipa massif gran-ulites from U-Pb zircon upper-intercept ages. The northern Alti-plano basement is overlain locally by Cretaceous and Tertiary sed-imentary rock containing much more radiogenic Pb, which isisotopically similar to that found in the Eastern Cordillera. Thiscoincidence may reflect deposition in the Altiplano basin of detritus

from the east. Several less radiogenic samples occur north of 19Sin the Eastern Cordillera and may reflect a minor sedimentary com-ponent derived from the northern Altiplano. A few less radiogenicsamples also occur close to the Bolivia-Argentina frontier, as notedpreviously by Macfarlane et al. (1990). Kay et al. (1993) reportedless radiogenic Pb isotopic compositions for Miocene volcanic rocksand Precambrian basement of the Argentinian Precordillera andSierras Pampeanas. Evidently these data document yet another dis-tinctive Pb isotopic basement domain to the south of our study area,which may be the source of the nonradiogenic Pb component in oursamples on the Bolivia-Argentina frontier.

The boundary between the Eastern Cordillera and both Alti-plano isotopic domains also corresponds to the western limit oftin-tungsten mineralization in Bolivia (e.g., Ahlfeld and Schneider-

Scherbina, 1964, Fig. 43). This correspondence suggests that tinmineralization is linked to the bulk chemical composition of theEastern Cordillera crust, despite its apparent lack of a primary tinanomaly; perhaps melts of the relatively carbon-rich Paleozoic rocksthere had a low oxygen fugacity, leading to tin enrichment in themagma during crystal fractionation. The Pb in the Eastern Cordil-lera ore deposits is isotopically indistinguishable both from Pb in thelocal crust and generally also from Pb in likely mantle sources. Thus,the presence or absence of a large mantle Pb component in theseores cannot be demonstrated by using Pb isotopic data from theEastern Cordillera alone.

CONCLUSIONS

Pb isotopic compositions of central Andean igneous rocks,

basement samples, and ores show overlapping values in each areaand show pronounced provinciality that can be mapped out on alarge scale. These Pb isotopic provinces are interpreted as distinctbasement domains of different age and composition. Boundariesbetween these domains follow large-scale structural trends and inplaces may be controlled by crustal-scale faults. Pb ore is derivedmainly from the crust either by direct scavenging from local base-ment or from igneousfluids whosePb was acquired mainly by crustalcontamination of the parent magmas.

ACKNOWLEDGMENTSSupported by Oxford University and the Natural Environment Re-

search Council (grant GR/3/7679 to Moorbath) (Aitcheson and Moorbath),ORSTOM (UR1H-TOA Departement) (Soler), Carl Duisberg Foundation(Bayer), Germany (Schneider), and the German Science Foundation (grant

Wo 362/5-1,2,3 to Worner). We thank G. Anthes, G. Carlier, J. Entenmann,M. Fornari, A. Forrest, A. Heumann, L. Hoke, L. Kennan, S. Lamb, and A.Medina for assistance with sampling and A. Medina, M. Cheatham, W.White, R. Goodwin, and C. Fry for technical assistance. We thank B. Leh-mann for providing a drill-core sample (A2) of the northern Altiplano base-ment. C. R. Stern and M. Dungan provided helpful reviews. This study wasmade possible by the cooperation of the Bolivian Geological Survey(GEOBOL) and Universidad Mayor de San Andres (La Paz, Bolivia) andforms part of International Geological Correlation Programme Project 345.

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Manuscript received October 21, 1994Revised manuscript received March 13, 1995Manuscript accepted March 20, 1995

558 GEOLOGY, June 1995Printed in U.S.A.

pb isotopes in central andes

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