Crustal Recycling of Metamorphic Basement: Late Palaeozoic Granitoids of Northern Chile (~22 S). Implications for the Composition of the Andean Crust

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1 JOURNAL OF PETROLOGY VOLUME 40 NUMBER 10 PAGES Crustal Recycling of Metamorphic Basement: Late Palaeozoic Granitoids of Northern Chile (~22 S). Implications for the Composition of the Andean Crust F. LUCASSEN 1, G. FRANZ 1, M. F. THIRLWALL 2 AND K. MEZGER 3 1 FACHGEBIET PETROLOGIE, TECHNISCHE UNIVERSITÄT BERLIN, ERNST REUTER PLATZ 1, BERLIN, GERMANY 2 DEPARTMENT OF GEOLOGY, ROYAL HOLLOWAY UNIVERSITY OF LONDON, EGHAM TW20 0EX, UK 3 MINERALOGISCHES INSTITUT, UNIVERSITÄT MÜNSTER, CORRENSSTRASSE 24, MÜNSTER, GERMANY RECEIVED AUGUST 20, 1998; REVISED TYPESCRIPT ACCEPTED APRIL 30, 1999 Upper Palaeozoic silicic magmatism is widespread in the Central Andes, but its origin is poorly constrained. We investigated wholerock chemical and isotopic composition of Upper Palaeozoic granitoids and their Early Palaeozoic high-grade country rocks in the Chilean Coastal Cordillera and Precordillera at ~22 S, in comparison with an Upper Cretaceous granitoid. The age of the Late Palaeozoic granitoids from a Rb Sr isochron of ~300 Ma is consistent with K Ar cooling ages of hornblende and biotite. Similar major and trace element patterns as well as Nd and Pb isotopic composition of Upper Palaeozoic granitoids and gneisses point to a source of the granitoids that is similar to the gneisses at outcrop. Sr isotope ratios of the Upper Palaeozoic granitoids are less radiogenic than those in many of the gneisses. We propose a stratification of the Early Palaeozoic crust with a Rb-deficient granulitic mid lower crust, resulting in less radiogenic Sr compared with the upper crust, based on the interpretation of the P T t history and isotopic composition of the Lower Palaeozoic metamorphic basement and of the isotopic composition of the Late Palaeozoic granitoids and younger magmatic rocks. Nd isotopic composition is identical in lower and upper crust and in crustal melts from the Late Palaeozoic to Recent. The Cretaceous granitoid evolved from partial melts of a mantle-derived source with con- siderable contamination by the old crustal component. The crust that formed in the Early Palaeozoic is the major source of material for the Cenozoic tectonic thickening of the Andean crust. KEY WORDS: Central Andes; crustal composition; crustal recycling; granitoid magmatism; isotopic composition INTRODUCTION The magmatic arc in the Puna Altiplano plateau of the Central Andes rests on a continental crust of a maximum thickness of ~70 km. Crustal thickening is mainly at- tributed to shortening of the crust and not to juvenile magmatic additions from the subduction zone [see review by Allmendinger et al. (1997)]. This implies a major contribution of pre-mesozoic material to the present thick crust, because Mesozoic juvenile additions to the crust are focused in the Mesozoic magmatic arcs mainly in the Chilean Coastal Cordillera (Fig. 1, e.g. Scheuber et al., 1994; Lucassen et al., 1996a), which is not involved in the crustal thickening. The last major reorganization of the crust before the Cenozoic was in the Early Palaeozoic orogeny between ~500 Ma, the age of peak meta- morphism, and ~400 Ma, the age of the final exhumation (Damm et al., 1990, 1994; Lucassen et al., 1996b, un- published data, 1998; Becchio et al., 1997). The widespread Upper Palaeozoic granitoid magmatism starting at ~300 Ma was assigned to arc magmatism (e.g. Brown, 1991; Bahlburg & Hervé, 1997) or to melting of the crust at a passive margin [Breitkreuz & Zeil (1984) and references therein]. The metamorphic history of the pre- Mesozoic crust was studied in detail before (Lucassen et al., 1999a, unpublished data, 1998; summary of previous work given by Damm et al., 1990, 1994; Miller et al., 1994). This study focuses on the composition of both the high-grade metamorphic basement and granitoid intrusions along a traverse from the Chilean Coastal Corresponding author. luca0938@mailszrz.zrz.tu-berlin.de Oxford University Press 1999

2 JOURNAL OF PETROLOGY VOLUME 40 NUMBER 10 OCTOBER 1999 Cordillera to the Precordillera at ~22 S. Granitoids During the Mesozoic a dramatic change in magmatism are used widely to monitor crustal composition and occurred, from mainly felsic crustally derived to gabbroic geodynamic settings. Surprisingly, the granitoids pre- dioritic compositions with mantle signatures and varying viously assigned to an arc setting are crustal melts from amounts of crustal contamination (Rogers & Hawkes- a source very similar to the high-grade basement without worth, 1989; Pichowiak, 1994; Lucassen & Franz, 1994, traceable influence of mantle-derived material. A comparison and references therein). The centre of Jurassic of the isotopic data from northern Chile with Cretaceous magmatism, the Coastal Cordillera, com- those of the Lower Palaeozoic high-grade basement of prises ~70% intrusive or volcanic rocks at the surface NW Argentina (R. Becchio, unpublished data, 1998) (Scheuber et al., 1994), and gravity and seismic velocity shows the regional relevance of our findings. We propose data indicate the continuation of the prevailing mafic a stratification of the Early Palaeozoic crust with a magmatic crust to depths of at least 20 km (Götze et al., Rb-deficient granulitic mid lower crust resulting in less 1994; Wigger et al., 1994). All magmatic activity has been radiogenic Sr isotope ratios, but with identical Sm Nd attributed to an active continental margin setting with ratios and Nd isotope ratios based on the P T t history a tectonic regime of prevailing normal extension to and the isotopic composition of the basement, the isotopic transpression during the Mesozoic (e.g. Scheuber et al., composition of the granitoids and the geological history 1994; Dallmeyer et al., 1996). Mantle-derived magmatism of the area. This hypothesis is supported by the isotopic with considerable contributions from crustal melts and composition of various crustal or contaminated melts generation of crustal melts (ignimbrites) continued from Mesozoic to Recent. throughout the Cenozoic (e.g. Francis et al. 1989; Ort et al., 1996; Wittenbrink, 1997). The Cenozoic Andean subduction zone is tectonically dominated by crustal shortening and thickening, and has been the subject of GEOLOGICAL SETTING numerous studies. Extended periods of magmatism in the Central Andes Little is known about the Late Palaeozoic magmatism occurred from Late Precambrian to Early Palaeozoic (21 26 S), and previous studies did not consider the (~ Ma; e.g. Damm et al., 1990, 1994; Rapela et possible source compositions or the effects of conal., 1990; Coira et al., 1999) and from Late Palaeozoic tamination by the pre-silurian basement. The study (~300 Ma) to Recent (Rogers & Hawkesworth, 1989; presented here focuses on the intrusions in the southern Brown, 1991; Scheuber et al., 1994). The early period Sierra de Moreno (Fig. 1). All but one of the intrusions was contemporaneous with widespread low-pressure (4 7 are of Late Carboniferous to Early Permian age. Included kbar) and high-temperature ( C) metamorphism in this study is an Upper Cretaceous pluton in the in northern Chile and NW Argentina (Lucassen et al., southernmost Sierra de Moreno, to compare differences 1996b, unpublished data, 1998; Becchio et al., 1997). in the possible state of the crust and the sources of the These magmatic metamorphic events formed the baseitoids different magmatic pulses. Additional Palaeozoic granment for the next period of magmatism. There is no in the Caleta Loa area of the Coastal Cordillera record of magmatic activity in northern Chile from and at the western slope of Sierra de Limón Verde were Silurian time (Damm et al., 1990, 1994) until the renewed sampled to compare the intrusions on a regional scale onset of magmatism in Late Carboniferous time. The (Fig. 1). Previous investigations of these rocks are limited post-silurian history starts with the deposition of clastic to isotopic dating (Damm et al., 1990; Maksaev, 1990). Devonian Lower Carboniferous sediments in a passive We also investigated isotope chemistry of representative margin setting, and a hypothetical landmass in the west samples from the high-grade basement (Fig. 1). The area has been proposed to explain depositional features (Bahl- of investigation is covered by the geological maps No. 3 burg & Breitkreuz, 1991; Bahlburg, 1993; Bahlburg & Tocopilla (preliminary), No. 58 Calama, No. 51 Quill- Hervé, 1997). Late Palaeozoic Triassic magmatic rocks agua, No. 40 Ollagüe (all published by Servicio Nacional are mainly felsic and form a broad belt from the Coastal de Geología y Mineria, Santiago, Chile). Granitoid in- Cordillera to the Altiplano (Fig. 1; Berg et al., 1983; trusions of similar age occur between Taltal and Chañaral Brown, 1991; Breitkreuz & Zeil, 1994). Their chemistry (Fig. 1) in the Coastal Cordillera (Berg et al., 1983; Brown, has been variously interpreted as a result of melting of 1991), the Precordillera (Cordillera Domeyko; Smoje & the crust at a passive margin [Breitkreuz & Zeil (1984) Marinovic, 1994) and in the Altiplano (Brown, 1991). and references therein] or as a result of interaction of mantle-derived melts from a subduction zone with the crust (Brown, 1991). The latter interpretation implies the RESULTS transition from a passive to an active margin in Late Petrography and field relations Carboniferous time (Bahlburg, 1993; Bahlburg & Hervé, 1997). In Sierra de Moreno we distinguish a northern Palaeozoic intrusive complex at Cerro Negro separated by meta- 1528

3 LUCASSEN et al. CRUSTAL RECYCLING AND COMPOSITION OF ANDEAN CRUST Fig. 1. Distribution of Upper Palaeozoic magmatism in northern Chile and sample locations. The geological sketch map shows the distribution of Early to Late Palaeozoic rocks (modified after: Bahlburg & Breitkreuz, 1991; Breitkreuz & Zeil, 1994; Breitkreuz & Van Schmus, 1996). Only dated Late Palaeozoic intrusions are considered (references in: Berg & Baumann, 1985; Maksaev, 1990; Brown, 1991; Breitkreuz & Zeil, 1994; Smoje & Marinovic, 1994; this work). The Sierra de Moreno forms part of the Precordillera. The Jurassic to Early Cretaceous arc is restricted to the Coastal Cordillera. morphic rocks from a southern complex referred to here Palaeozoic metamorphic basement (Lucassen et al., 1996b, as SM (north) and SM (south) and a Cretaceous intrusion unpublished data, 1998) and in the north also intruded in the southernmost part at Qda. de Barreras (Fig. 1). presumed Devonian Early Carboniferous sediments When the Upper Palaeozoic intrusions are referred to (Fig. 1; Bahlburg & Breitkreuz, 1991). as a group, the term Palaeozoic granitoids is used. Granitic and granodioritic tonalitic compositions can Representative analyses are given in Table 1. The Palaeozoic be easily distinguished in the field by their colour index. granitoids intruded the high-grade Lower The northern plutons comprise granite granodiorite, 1529

4 JOURNAL OF PETROLOGY VOLUME 40 NUMBER 10 OCTOBER 1999 Table 1: Major and trace element abundances in intrusions and gneisses Cretaceous granitoids Palaeozoic granitoids (SM north) Sample: 4/7 4/9 4/12 4/10 4/14 4/32 4/404 4/36 4/37 4/414 SiO Al 2 O TiO Fe 2 O MgO CaO Na 2 O K 2 O P 2 O MnO LOI Total Ba Cr Cu Ga Hf Nb Ni Pb Rb Sc Sr Th U V Y Zn Zr La Ce Pr Nd Sm Eu Gd Dy Ho Er Yb Lu whereas the rocks from the southern block are granite tonalite. Granodiorites tonalites are less voluminous than the more evolved granite at SM (north) and are sub- ordinate at SM (south). At SM (north) the granite intruded into the granodiorite, but at SM (south) the different rock types may have been coeval, and pods and dykes of 1530

5 LUCASSEN et al. CRUSTAL RECYCLING AND COMPOSITION OF ANDEAN CRUST Palaeozoic granitoids (SM north) SM (south) Sample: 4/29 4/23 4/25 4/417 4/35 4/27 3/299 4/5 4/65 4/45 SiO Al 2 O TiO Fe 2 O MgO < CaO Na 2 O K 2 O P 2 O MnO LOI Total Ba Cr Cu Ga Hf Nb Ni Pb Rb Sc Sr Th U V Y Zn Zr La Ce Pr Nd Sm Eu Gd Dy Ho Er Yb Lu granodiorite tonalite interacted with the unsolidified granite. Also, a few hornblende-rich cumulates were found. Mineral parageneses and fabrics of the granitoids are very similar at SM (north) and SM (south). Major phases are plagioclase, hornblende, biotite, quartz and potassic 1531

6 JOURNAL OF PETROLOGY VOLUME 40 NUMBER 10 OCTOBER 1999 Table 1: continued SM (south) Limon Verde Caleta Loa (west) Caleta Loa (east) Sample: 4/84 4/88 5/15 5/19 5/17 5/36 5/39 5/40 5/51 5/49 SiO Al 2 O TiO Fe 2 O MgO CaO Na 2 O K 2 O P 2 O MnO LOI Total Ba Cr Cu Ga Hf Nb Ni Pb Rb Sc Sr Th U 7 8 V Y Zn Zr La Ce Pr Nd Sm Eu Gd Dy Ho Er Yb Lu feldspar, with generally increasing proportions of the last disappears and biotite is rare. Common minor phases three minerals with increasing SiO 2 content. In the most are opaque minerals, titanite, apatite, zircon and mon- evolved compositions (SiO 2 > 70 wt %) hornblende azite, and in the more evolved granitoids magmatic Ce- 1532

7 LUCASSEN et al. CRUSTAL RECYCLING AND COMPOSITION OF ANDEAN CRUST Chilean gneisses Average values Sample: 4/354 3/291 3/287 3/354 5/29 3/306 3/307 n = 69 n = 35 n = 39 SiO Al 2 O TiO Fe 2 O MgO CaO Na 2 O K 2 O P 2 O MnO LOI Total Ba Cr Cu < Ga Hf Nb Ni Pb Rb Sc Sr Th U V Y Zn Zr La Ce Pr Nd Sm Eu Gd Dy Ho Er Yb Lu Average values of Palaeozoic granitoids (n = 69), Chilean gneisses (n = 35) and Argentine gneisses [n = 39; R. Becchio (unpublished data, 1998)]. REE by ICP; all other elements by XRF. Measured by isotope dilution. 1533

8 JOURNAL OF PETROLOGY VOLUME 40 NUMBER 10 OCTOBER 1999 epidote. Garnet is present as a minor phase in the evolved minerals was observed in the sediments with contact compositions at SM (north) and at Caleta Loa (west). aureoles of some hundred metres to ~1 km. Chlorite formed in many samples after biotite and/or The high-grade basement comprises abundant gneisses hornblende. Sericitization and/or saussuritization reach and migmatites (quartz plagioclase biotite ± garnet ± variable extent in the different samples and are commonly K-feldspar). Common minor phases are zircon, mag- restricted to the core region or parts of the zoned plasame netite, ilmenite and apatite. Both rock types have the gioclase. Grain sizes generally increase from the mafic compositional range and are referred to in the text to the evolved compositions. The minerals are never as gneisses. Rare minor intercalations (decimetre to metre strongly deformed and all rocks have a magmatic fabric. size) of quartzite and calcsilicate point to a sedimentary In some coarse-grained samples small rims of re- protolith, but granitoid orthogneisses were also found. crystallized or primary smaller grains occur around quartz Aluminous compositions with quartz cordierite and feldspar. Mineral alignment is not found at the plagioclase micas garnet aluminosilicate are rare in the microscopic scale. All macroscopic alignment, for exing <5% of the metamorphic rocks, and are former dykes basement of northern Chile. Amphibolites are rare, form- ample, of feldspar laths, is in magmatic flow textures. The Upper Cretaceous granitoid in the southermost or minor volcanic intercalations. Peak metamorphic con- Sierra de Moreno (Fig. 1) is intruded into Cretaceous ditions are of high-t and low-p type at ~ C and and Jurassic sediments in the west (Bogdanic & Espinoza, 4 7 kbar, and migmatization is widespread in these rocks. 1994), Palaeozoic granitoids in the east and the Lower Metamorphism occurred at ~500 Ma, and subsequent Palaeozoic metamorphic rocks in the southeast. The uplift and erosion was finished at ~400 Ma with the pluton is mainly granodiorite to monzonite with few fairly rocks close to the present erosion surface. More details of the geology and metamorphic history of the highmafic compositions. The latter have mainly plagioclase, grade basement in northern Chile and NW Argentina biotite and clinopyroxene that can be replaced by hornhave been given by Damm et al. (1990, 1994), Lucassen blende. Primary hornblende is found in rocks with >59 et al. (1994, 1996b, 1999a, unpublished data, 1998), Miller wt % SiO 2. In the most evolved rocks biotite is the only et al. (1994) and Becchio et al. (1997). Fe Mg mineral and the proportions of potassic feldspar and quartz increase. Ductile deformation is not found in the rocks. Geochronology At Sierra de Limón Verde (Fig. 1) a granodiorite pluton intrudes migmatites for which no isotopic dating Seven samples of the Palaeozoic granite from SM (north is available, but which are very similar to those from and south; sample 4/5 excluded) define an isochron of ± 2 5 Ma, mean squared weighted deviation Sierra de Moreno [Baeza (1984), including a geological (MSWD) = 4 3 (Fig. 2, Table 2) with Sr i = map). At Caleta Loa (Fig. 1), the western pluton intrudes ± 53, calculated according to the method of York (1969) into migmatites of Lower Palaeozoic metamorphic rocks. using a 0 5% error (1σ) on the 87 Rb/ 86 Sr ratios and the Apart from local shear zones of unknown age, the relinternal precision on the 87 Sr/ 86 Sr ratios in the calculation. atively homogeneous pluton is undeformed. The eastern The MSWD could be further improved to 2 9, if the pluton intruded into Devonian Lower Carboniferous external reproduceability of 0 003% from the Sr standard sediments (Bahlburg & Breitkreuz, 1991). All but one is used as error on the 87 Sr/ 86 Sr ratios in the calculation. sample plot into the granite field. The appearance of the The ~300 Ma age is interpreted as age of intrusion and rocks is different from the western pluton, with the coincides with a concordant U Pb zircon age of 298 ± development of large (centimetre size) K-feldspar mega- 1 5 Ma from a granodioritic sample from Sierra de crysts. Their size and frequency varies, but they are Limón Verde (Fig. 1; Damm et al., 1990). present in most samples. The K Ar ages (Table 3) are considered as cooling In general, the intrusions are undeformed and no ages. Ages of 283 ± 6 Ma for biotite (3/299) from a foliation planes or preferred orientations developed. Con- granite sample and 281 ± 9 Ma for hornblende (3/300) tacts between the intrusions and the country rocks are from a diorite sample of SM (south) coincide. The K Ar primary or overprinted by post-jurassic Recent brittle ages from SM (north) are 301 ± 8 Ma for biotite (4/ faults. The contact relations of the Upper Palaeozoic 417) from a granite and 332 ± 7 Ma for biotite (4/36) intrusions with Palaeozoic sediments (maximum thickness from a granodiorite. The difference in age between ~3000 m, Bahlburg & Breitkreuz, 1991) and of the granite and granodiorite might be significant, though Cretaceous intrusion with Jurassic Cretaceous sediments sample 4/36 plots on the Rb Sr isochron (Fig. 2), because point to high levels of intrusion (<5 km) at all locations. the intrusions are in contact and the granite contains This is consistent with the absence of primary muscovite, dioritic xenoliths. In the Rb Sr isochron it is not possible because muscovite is not stable at pressures below ~2 to distinguish between slightly older and younger samples kbar. Contact metamorphism with the formation of new if the Rb/Sr ratio is low. 1534

9 LUCASSEN et al. CRUSTAL RECYCLING AND COMPOSITION OF ANDEAN CRUST Table 2: Sm, Nd, Rb, Sr contents, isotope ratios and t DM model ages 147 Sample Location Sm Nd Sm/ 143 Nd/ 144 Nd 143 Nd/ 144 Nd t 87 DM Rb Sr Rb/ 87 Sr/ 86 Sr 144 (ppm) (ppm) Nd measured 300 Ma (Ga) (ppm) (ppm) 86 Sr measured Palaeozoic gneisses 3/354 Caleta Loa ± ±11 4/ ± ±5 3/382 Mejillones ± ±10 4/76 Sierra de Moreno ± ±11 3/ ± ±22 3/ ± ±10 4/ ± ±22 4/ ± ±12 4/ ± ±22 5/29 L. Verde ± ±10 3/ ± ±10 3/ ± ±11 6/160 Antofalla ± ±21 4/164 H. Muerto ± ±12 6/120 S. Quilmes ± ±10 Metabasite 3/379 Mejillones ± ±12 3/303 L. Verde ± ±32 6/ ± ±11 Palaeozoic granitoid 4/23 Sierra de Moreno ± ±11 4/ ± ±11 4/ ± ±11 4/ ± ±9 4/ ± ±11 4/ ± ±11 3/ ± ±9 4/ ± ±

10 JOURNAL OF PETROLOGY VOLUME 40 NUMBER 10 OCTOBER 1999 Table 2: continued 147 Sample Location Sm Nd Sm/ 143 Nd/ 144 Nd 143 Nd/ 144 Nd t 87 DM Rb Sr Rb/ 87 Sr/ 86 Sr (ppm) (ppm) Nd measured 300 Ma (Ga) (ppm) (ppm) Sr measured Palaeozoic granitoid 5/19 L. Verde ± ±10 5/40 C. Loa (w) ± ±12 5/49 C. Loa (e) ± ±34 Cretaceous granitoid 80 Ma 4/6 Sierra de Moreno ± ±11 4/ ± ±12 4/ ± ±9 4/ ± ±11 t DM model ages calculated according to Goldstein et al. (1984); Rb, Sr contents by XRF (granitoid samples at RHBNC, University of London; all others at TU-Berlin); the assumed error on the 87 Rb/ 86 Sr is 0 5% 1σ; other errors are 2σ. Sm, Nd determined by ICP; all others by isotope dilution. 1536

11 LUCASSEN et al. CRUSTAL RECYCLING AND COMPOSITION OF ANDEAN CRUST on biotite is 310 ± 7 Ma. A similar age of 318 ± 6Ma on biotite of the same intrusion further to the south was reported by Maksaev & Marinovic (1980). A granite sample from Caleta Loa (west, 5/40) has a K Ar age on biotite of 236 ± 5 Ma. Skarmeta & Marinovic (1981) reported K Ar ages on biotite from the same intrusion of 322 ± 5 and 320 ± 5 Ma, however. We speculate that our sample was partially reset by the widespread Jurassic Cretaceous igneous activity in the relevant magmatic arcs. All samples used for K Ar dating plot on or close to the Rb Sr isochron (Fig. 2). We conclude in combination with the contact relations of the granitoids with deformed Devonian Upper Carboniferous sedi- Fig. 2. Rb Sr isochron of Upper Palaeozoic granitoid from Sierra de Moreno (Φ; for data see Table 2; sample 4/5 excluded). Samples ments that all samples belong to the same period of 5/19, 5/40 and 5/49 are from different plutons, and are not included Late Palaeozoic igneous activity between 330 and 270 in the calculation. Sample 4/36 yielded a K Ar age of ~332 Ma. Ma also known from other occurrences of granitoids in Errors are 2σ. the Precordillera (Brown, 1991; Smoje & Marinovic, 1994) and in the Coastal Cordillera (Berg et al., 1983; At Sierra de Limón Verde K Ar ages (5/19) are 270 Berg & Baumann, 1985). ± 15 Ma for hornblende and 273 ± 8 Ma for biotite Hornblende from the Cretaceous pluton in SM (south) from granite. The K Ar age at Caleta Loa (east, 5/49) yielded an Upper Cretaceous K Ar age of 82 ± 6Ma Table 3: K Ar ages Sample location K 2 O 40 Ar Age (wt %) (nl/g) (Ma) Sierra de Moreno (north) 4/ " granite ± " biotite 4/ " granodiorite ± " biotite Sierra de Moreno (south) 3/ " granite ± " biotite 3/300 diorite ±9 hornblende Qda. Barreras 4/ " granodiorite ± " hornblende Sierra de Limón Verde 5/ " granodiorite ± " biotite hornblende ±15 Caleta Loa (west) 5/ " granite ± " biotite Caleta Loa (east) 5/ " granite ± " biotite Analysis by Krueger Enterprise Inc.; all others at University of Göttingen. 1537

12 JOURNAL OF PETROLOGY VOLUME 40 NUMBER 10 OCTOBER 1999 (4/10; Table 3). This cooling age, together with the Table 4: Lead isotope composition of observation of intrusive contacts into the Cretaceous (Bogdanic & Espinoza, 1994) country rocks, is clear feldspar separates evidence for a Mesozoic intrusion age. Sample 206 Pb/ 204 Pb 207 Pb/ 204 Pb 208 Pb/ 204 Pb Palaeozoic gneisses Whole-rock chemical composition 3/ Major elements 3/ The granitoids and the gneisses from the basement have 4/ a wide range of chemical composition (Figs 3 and 4; Palaeozoic granitoids Table 1). The Palaeozoic granitoids of Sierra de Moreno 3/ and Sierra de Limón Verde range from 50 to 78 wt % 5/ SiO 2, whereas at Caleta Loa all but one sample have 4/ SiO 2 > 70 wt %. The Cretaceous granitoid has a restricted 5/ range of wt % SiO 2. All Palaeozoic granitoid Cretaceous granitoid samples are subalkaline, whereas those from the Cre- 4/ taceous pluton are alkaline (Fig. 3b, d, g). For these rocks K 2 O is slightly higher than Na 2 O in many samples and they can be classified as shoshonitic; alkalis and SiO 2 show no systematic variation. Most of the Palaeozoic the high La N /Yb N (>40); however, other samples from rocks are mildly peraluminous (Fig. 3h) with A/CNK = other plutons low in Y have low LaN /Yb N ratios (see 1 1 1, the Caleta Loa plutons are more aluminous (A/ below). CNK = 1 2), and many of the Cretaceous samples are metaluminous (A/CNK = 0 9 1). The differences in major element composition between the Palaeozoic gran- Rare earth elements itoids from different localities are minor and roughly The rare earth elements (REE) of the granitoids and the follow the common trends of magmatic differentiation basement gneisses are generally similar in their range of within the scatter of the values that are typical for rocks element contents and their patterns apart from effects of with varying proportions of cumulates (Fig. 3). The magmatic differentiation in the granitoids (Fig. 5, basement gneisses are similar in composition and com- Table 1). Within the Palaeozoic granitoids, there is a positional trend to the Palaeozoic granitoids, and most systematic variation of contents and patterns with SiO 2 samples plot in the range of the less evolved granitoids content at different locations. At SM (north) the less (Fig. 3). The average composition of 35 gneiss samples evolved rocks with SiO 2 < 70 wt % have a slightly from northern Chile (Tables 1 and 4) matches the com- negative Eu anomaly, whereas the evolved rocks > 70 position of upper-crustal rocks as greywackes (Taylor & wt % SiO2 have a pronounced negative Eu anomaly, McLennan, 1985). lower L(light)REE contents and rather variable H(heavy) REE contents (Fig. 5a). The latter rocks show a Nd Sm Trace elements plateau as a result of a high Sm/Nd ratio compared The similarity between the basement gneisses and the with the other samples (sample 4/417 contains garnet). Palaeozoic granitoids is also evident from the trace eleshow In contrast, at SM (south) (Fig. 5b) the evolved rocks ment contents (Table 1; selected element variation dialower higher REE contents, and the less evolved rocks grams in Fig. 4), for example, Ba, V, Sr and Y. The REE contents with a positive Eu anomaly. These Cretaceous granitoids are mostly similar to the Palaeozoic observations, in line with the CaO and Sr variation of granitoids, but Sr is on average significantly higher. V these rocks (Figs 3c and 4c) point to fractionation and and TiO 2 are positively correlated (Fig. 4b), as are other accumulation, respectively, of plagioclase in the respective transition metals. Ba behaves rather irregularly, though magmas. The granitoids from Sierra de Limón Verde it is in many localities positively correlated with K 2 O. and Caleta Loa (east) show the same features as those The samples from the Cretaceous pluton and those from from Sierra de Moreno; the samples from Caleta Loa Caleta Loa, however, show no correlation. Rb (not (west) show a very steep pattern with La N /Yb N of shown) is positively correlated with K 2 O in all rock types, The La N /Yb N ratio in most of the other granitoids (12 but not with other incompatible elements such as P. Y samples) varies between two and nine, and three samples is not correlated with SiO 2 (Fig. 4d) in general and within the distinct groups. Low Y at Caleta Loa (west) could be related to the presence of garnet in the source seen in have a ratio of 11, 13 and 15. The high La N /Yb N ratio for Caleta Loa west points to garnet that is present in the samples or zircon involvement in the source or during 1538

13 LUCASSEN et al. CRUSTAL RECYCLING AND COMPOSITION OF ANDEAN CRUST Fig. 3. Major element distribution in Cretaceous granitoids (n = 16), Palaeozoic granitoids from SM (north, n = 24), SM (south, n = 24), Limón Verde (n = 9), Caleta Loa (west, n = 5), Caleta Loa (east, n = 6) and the gneisses (n = 35). (g) shows the division between alkaline and subalkaline from Wilson (1989); in (h) the A/CNK index is corrected for apatite. fractionation not evident at the other locations. The (Fig. 5d) show no systematic variation with SiO 2 content. basement gneisses have a La N /Yb N ratio between five Their patterns are slightly steeper than those of the and nine. REE patterns of the Cretaceous granitoids Palaeozoic granitoids, with La N /Yb N of

14 JOURNAL OF PETROLOGY VOLUME 40 NUMBER 10 OCTOBER 1999 Fig. 4. Trace element distribution in Cretaceous granitoids (n = 16), Palaeozoic granitoids from SM (north, n = 24), SM (south, n = 24), Limón Verde (n = 9), Caleta Loa (west, n = 5), Caleta Loa (east, n = 6) and the gneisses (n = 35). Symbols are as in Fig. 3. Sr, Nd and Pb isotopic composition of granitoids and gneisses We analysed 11 samples from the Upper Palaeozoic granitoids, four from the Cretaceous granitoid and 12 samples from the gneisses (Table 2). Additionally, we included samples from the basement in NW Argentina (our data; R. Becchio, unpublished data, 1998) for regional comparison. We also included Lower Palaeozoic mafic, mantle-derived rocks (Table 2 and Damm et al., 1990), Mesozoic intrusive and volcanic rocks and lower- crustal xenoliths (Rogers & Hawkesworth, 1989; Lucassen & Franz, 1994; Lucassen & Thirlwall, 1998; Lucassen et The Argentine Lower Palaeozoic metamorphic base- ment is similar in composition, but comprises more evolved pelitic metasedimentary rocks, as seen by the comparison of the average composition of gneisses from northern Chile and NW Argentina (Table 1, Fig. 6). Lower Ca and Sr compared with the Chilean gneisses point to less plagioclase, and the higher K and Rb to more clay minerals and micas in the protoliths, in accordance with the interpretation of more abundant pelitic protoliths in NW Argentina. Apart from these differences the element patterns of Chilean and Argentine gneisses are similar. al., 1999b; our unpublished data, 1998) and Cenozoic volcanic rocks ( Francis et al., 1989; Ort et al., 1996; Wittenbrink, 1997) in our study to constrain the isotopic composition of mantle sources and the possible con- tamination of the Cenozoic volcanic rocks by the pos- tulated crustal source. For Sr and Nd, whole-rock samples were selected (Table 2), whereas for Pb determination feldspar separates were prepared (Table 4). ( For analytical technique, see the Appendix.) The 87 Sr/ 86 Sr ratios and ε Nd of Palaeozoic granitoids and those basement gneisses with low Rb/Sr ratios cover a similar range at 300 Ma (Fig. 7a). The differences in the 143 Nd/ 144 Nd 300Ma ratio are small (range for the gneisses including those with high 87 Sr/ 86 Sr ratio; for the granitoids; Fig. 7a). The basement in NW Argentina has a similar Nd isotopic composition at 300 Ma; however, the 87 Sr/ 86 Sr ratios of many samples are more radiogenic compared with those of granitoids and basement of northern Chile, as a result of the generally higher amounts of metapelitic material with higher Rb contents (Fig. 7a). Recalculated to 500 Ma, the mean age of high-grade metamorphism and isotope homogenization, both areas show the same range of isotopic composition (Fig. 7b). The Early Palaeozoic amphibolites cluster at ε Nd +5 (Fig. 7a), consistent with the interpretation that they are not metasediments, but metamorphosed igneous dykes or volcanic intercalations. 1540

15 LUCASSEN et al. CRUSTAL RECYCLING AND COMPOSITION OF ANDEAN CRUST Fig. 5. REE distribution of granitoids from Sierra de Moreno north (a) and south (b), Sierra de Limón Verde and Caleta Loa (c), and the Cretaceous granitoid (d) normalized to chondrite (Evensen et al., 1978 ). The SiO 2 (wt %) is indicated in the legend together with the sample number. The shaded area shows the range of REE in gneisses. Fig. 6. Comparison of the average compositions of Palaeozoic granitoids, Chilean gneisses and Argentine gneisses (R. Becchio, unpublished data, 1998) normalized to the average upper crust (Taylor & McLennan, 1981). REE analysis was by inductively coupled plasma (ICP) on a subset of samples; all other elements were analysed by X-ray fluorescence (XRF) (Table 1). A group of high-sio 2 granites (samples 4/23, 4/25, 4/417) from SM (north) has unusually high 147 Sm/ 144 Nd ratios (~0 15, Table 2) compared with other granitoids and the gneisses (~ ), and higher ε Nd (Fig. 7a). However, their 143 Nd/ 144 Nd 300Ma ratios are similar to those of the other samples (Fig. 7a). High Sm/Nd ratios from granites compared with their protoliths are known from other areas (e.g. Harris, 1996; Ayres & Harris, 1997) and ascribed to the preferred dissolution of high-ree phases, preferably monazite and apatite, in the anatectic melt. The 143 Nd/ 144 Nd might be also displaced compared with the bulk source if the isotopic equilibration of the source occurred much earlier than the melting (e.g. Ayres & Harris, 1997) and the radiogenic growth of 143 Nd continued in the relevant mineral. The concept could be valuable for our samples; however, a quantitative evaluation is not possible without knowledge of the mineral compositions in the source. The three samples are highly fractionated, with relatively low REE abundances (Table 1), and crystallization of respective phases could also fractionate the REE (very minor garnet and monazite are present; see Nd Sm plateau in the REE pattern of the three samples in Fig. 5a). The 143 Nd/ 144 Nd 300Ma ratio of for sample 5/ 40 indicates the same isotopic composition of the source for the granitoid at Caleta Loa (west) as for the other 1541

16 JOURNAL OF PETROLOGY VOLUME 40 NUMBER 10 OCTOBER 1999 Fig. 8. SiO 2 content of Palaeozoic and Cretaceous granitoids plotted against their ε Nd (recalculated to their age of intrusion). The strong variation of SiO 2 at rather constant ε Nd points to fractionation of the granitoids or to different degrees of partial melting of the same source. ratios ( ) than the Palaeozoic granitoids, as a result of the influence of a mantle or mantlederived source (Fig. 7b; Table 2). The ε Nd of the granitoids at time of intrusion is not correlated with SiO 2 of the rocks (Fig. 8). This indicates that in both the Upper Palaeozoic and the Cretaceous granitoid fractionation of a homogeneous parental magma or different amounts of partial melting of the Fig. 7. ε Nd and Sr isotope ratios of Lower Palaeozoic metamorphic same source can be the reason for the variation of SiO 2 basement and magmatic rocks from Late Palaeozoic to Recent. (a) Sr isotope ratios, ε content. Magma mixing of two sources, such as a mantle Nd and 143 Nd/ 144 Nd (inset in figure; all data recalculated to 300 Ma, the age of granitoid intrusion) of Upper Palaeozoic granitoids and a crustal component, is unlikely in the Palaeozoic compared with Lower Palaeozoic basement gneisses from Chile cover granitoids. The magma of the Cretaceous intrusion posa similar field of composition, including three samples (labelled high sibly is a mixture of a depleted mantle source magma, SiO 2 -granites ) with distinctly higher Sm/Nd (see text and Table 2). The gneisses from Argentina (R. Becchio, unpublished data, 1998) are as defined by the mafic rocks of the area (Fig. 7b), and more radiogenic in Sr. Lower Palaeozoic amphibolites cluster in the a source similar to the Early Late Palaeozoic crust, depleted mantle field [additional data from Damm et al. (1990)]. (b) 87 as presented by Lower Palaeozoic gneisses and Upper Sr/ 86 Sr ratios and ε Nd of Mesozoic igneous rocks are recalculated to the time of intrusion or extrusion, compared with the present-day Palaeozoic granitoids (see discussion below). The very isotopic ratios of Cenozoic volcanic rocks and Palaeozoic basement small spread in isotopic composition and the absence gneisses and granitoids. Also included are Cretaceous mantle (re- of correlation between ε Nd and SiO 2 indicate a wellcalculated to 80 Ma) and lower-crustal xenoliths (present-day values). Most of the Mesozoic magmatic rocks and mantle xenoliths plot homogenized magma. close to values that are typical for depleted mantle compositions; Pb isotopic ratios from feldspar separates of the gneisses the Cretaceous granitoid and other Cretaceous igneous rocks are and of the granitoids are also similar (Fig. 9a, b). 206 Pb/ contaminated by the crust. Cenozoic andesites and ignimbrites point 204 to a crustal component with low Pb, 207 Pb/ 204 Pb and 208 Pb/ 204 Pb ratios plot above or 87 Sr/ 86 Sr ratios, similar to the gneisses at 500 Ma (stippled vertical lines indicate their Sr i at 500 Ma, the age near the Pb ore development line (Stacey & Kramers, of metamorphism). We propose that the basement at the outcrop level 1975). This is in agreement with other Precambrian to continues to the middle and lower crust, where it is depleted in Rb as Palaeozoic basement rocks from the Central Andes. Pb a result of an increasing grade of metamorphism with increasing depth at 500 Ma. Such Rb-depleted gneiss type source would have similar isotope ratios are not correlated with Nd or Sr isotope present-day isotope ratios to the gneisses in the upper crust recalculated ratios (not shown). The similarity of the values for all to 500 Ma. (Data sources: Francis et al., 1989; Rogers & Hawkesworth, rocks is consistent with our hypothesis of a homogeneous 1989; Ort et al., 1996; Wittenbrink, 1997; Lucassen & Thirlwall, 1998; Lucassen et al., 1999b; our unpublished data, 1998.) crustal source. This source is different from the Proterozoic Arequipa Massif and transitional between the fields for young volcanic rocks of the northern and Palaeozoic granitoids. However, the 147 Sm/ 144 Nd ratio of southern Central Volcanic Zone (Wörner et al. 1994), reflects garnet in the source (Table 2). in general agreement with the assumption of a more The Cretaceous granitoid at 80 Ma has lower 87 Sr/ radiogenic Pb source of contamination for the Cenozoic 86 Sr ratios ( ) and higher 143 Nd/ 144 Nd volcanic rocks south of 21 S. 1542

17 LUCASSEN et al. CRUSTAL RECYCLING AND COMPOSITION OF ANDEAN CRUST Fig. 9. Lead isotopic ratios of feldspar separates (Fsp) of Lower Palaeozoic gneisses from Chile and Argentina, compared with those of the Upper Palaeozoic and Cretaceous granitoids, together with previous whole-rock data of the basement between 17 and 21 S (Wr; Tosdal, 1996), the Precambrian Arequipa Massif (southern Peru; Tilton & Barreiro, 1980) and the Cenozoic volcanic rocks north and south of 21 S (Wörner et al., 1994; Aitcheson et al., 1995). Palaeozoic and Mesozoic granitoids and basement from our study plot in the same field, which is transitional between the southern and northern lead provinces from the Cenozoic volcanic rocks, but strongly different from the Precambrian gneisses of the Arequipa Massif. The continuous line is the reference line for Pb-ore development (Stacey & Kramers, 1975). DISCUSSION Source of the Palaeozoic granitoids Isotope composition Ratios of radiogenic isotopes are widely used to hy- pothesize about possible source rocks of igneous rocks. Nd isotope ratios, recalculated to 300 Ma (Fig. 7a), of all Palaeozoic granitoids are similar and in the same range as those of the Lower Palaeozoic gneisses from northern Chile and NW Argentina. Sm Nd-depleted mantle model ages of the gneisses (Table 2) have an average of ~1 65 ± 0 2 Ga with a range of Ga. The Palaeozoic granitoids average is 1 58 ± 0 2 Ga with a range of Ga, exclusive of samples 4/23, 4/25, 4/417 and 5/40 (all Palaeozoic granitoids yield 1 60 ± 0 2 Ga). The Sr isotope ratios (at 300 Ma, Fig. 7a) are low for a crustal source, though similar to the samples of the high- grade metamorphic basement with low Rb/Sr ratios. We propose as a hypothesis a source for the granitoids similar to the currently exposed country rocks at the intrusion level and refer to this as the gneiss-type source. The low Sr ratios of the granitoids are interpreted considering the metamorphic history of the basement. The P T conditions of the Early Palaeozoic metamorphism with widespread migmatization at ~500 Ma indicate a midcrustal level of km at temperatures of upper amphibolite lower granulite facies. Therefore it can safely be assumed that the Early Palaeozoic lower crust (the possible source for the Upper Palaeozoic granitoids) had experienced a granulite facies metamorphism. Rb depletion in rocks of granitoid composition is common during granulite facies metamorphism (e.g. Rudnick & Presper, 1990) and this depletion explains the lack of radiogenic Sr. Direct evidence of possible Rb depletion and conservation of low Sr isotope ratios in the lower crust is given by lower-crustal felsic xenoliths from the Cretaceous Salta Rift of NW Argentina, which have the same Nd isotopic composition as the Early Palaeozoic metamorphic basement (Fig. 7b; Lucassen et al., 1999b). The more heterogeneous Sr 300Ma isotopic composition of the basement gneisses, compared with the granitoids, is due to the fact that the Rb/Sr of the gneisses varies at a scale of cm because of the layered structure of the rocks, with biotite-rich and biotite-poor layers. This could be already the case in the sedimentary protolith of the gneisses, with enrichment of clay minerals in layers. A prominent contribution of a mantle source to the granites could not be excluded from the isotope composition, if an enriched mantle with ε Nd of 5 and a 87 Sr/ 86 Sr ratio of is assumed, similar to, for example, the enriched mantle under South Africa [e.g. Nixon (1987) and references therein]. This is unlikely for the Andes, however, as all known mafic rocks are strongly depleted (Fig. 7b). Furthermore, the granitic magmatism is voluminous (including the Permian SiO 2 -rich volcanic rocks; see Fig. 1) and fractionation of a major volume of granitic magma from a basaltic parental magma would leave a considerable gabbroic residue of the order of 6 10 times the granites volume (Cox, 1993). There is no indication for such rocks from the geophysical data on the crustal structure (Götze et al., 1994; Wigger et al., 1994) and such rocks are also absent in an exposed lower- crustal section of Permian Triassic age at Sierra de Limón Verde (Lucassen et al., 1999a). The Pb isotope ratios confirm the hypothesis of the gneiss-type source, similar to the Early Palaeozoic base- ment (Fig. 9). Compared with the data given by Tosdal (1996) for a variety of basement rocks from the area between ~17 and 21 S, they are rich in 206 Pb and 207 Pb, and intermediately radiogenic in 208 Pb. This type of basement can be clearly distinguished from the less radiogenic 206 Pb and 207 Pb, and high radiogenic 208 Pb in 1543