Antiquity of the Rı́o de la Plata craton in Tandilia, southern Buenos Aires province, Argentina

Journal of South American Earth Sciences 16 (2003) 5–13 www.elsevier.com/locate/jsames

Antiquity of the Rı´o de la Plata craton in Tandilia, southern Buenos Aires province, Argentina R.J. Pankhursta,b,*, A. Ramosc, E. Linaresc a British Antarctic Survey, Cambridge, UK NERC Isotope Geosciences Laboratory, British Geological Survey, Nottingham NG12 5GG, UK c INGEIS, CONICET. Pabello´n INGEIS, Ciudad Universitaria. 1428 Buenos Aires, Argentina

b

Abstract Rb– Sr and Sm – Nd whole-rock data for granitoids and orthogneisses from the western part of the Sierras Septentrionales of the southern Buenos Aires province yield an errorchron of 2009 ^ 71 Ma (initial 87Sr/86Sr ¼ 0.7041, MSWD ¼ 69) and an isochron of 2140 ^ 88 Ma (initial 143Nd/144Nd ¼ 0.50977), respectively. As in previous investigations, the Rb– Sr data are clearly disturbed, but the Sm– Nd isochron may record the age of emplacement of igneous precursors. These results reaffirm that this region is the southern extension of the crystalline basement of the Rı´o de la Plata craton. The Sm – Nd age, though not very precise, is slightly older than previously demonstrated but consistent with most recent U – Pb studies of the craton exposed in Uruguay and Brazil. Crust-derived Sm– Nd model ages averaging 2620 ^ 80 Ma indicate that, though the principal rock-forming events were Paleoproterozoic, a Late Archaean prehistory is possible. However, the data place strict constraints on the nature and intensity of post-2000 Ma activity in this area, which seems to be confined to tholeiitic dyke emplacement and hydrothermal reactivation. q 2003 Elsevier Science Ltd. All rights reserved. Keywords: Continental crust formation; Geochronology; Neodymium isotopes; Proterozoic; Transamazonian orogeny

1. Introduction The Rı´o de la Plata craton, which crops out in both Argentina and Uruguay, is thought to be one of the main Precambrian shield areas of the South American continent. However, it is the least well exposed, with rather uncertain limits, especially to the west. It also differs from most others in that, until very recently, no rocks significantly older than , 2 Ga had been identified. Almeida et al. (1973) define the Rı´o de la Plata craton as constituting crystalline gneisses and granitoids beneath the Parana´ basin, older than and unaffected by the Brasiliano (Neoproterozoic) orogenic cycle. Prior to 2000, most geochronological evidence came from K –Ar and Rb – Sr data, supported by few Sm –Nd data from Uruguay (Sato, 1988; Cordani and Sato, 1999). Recent U –Pb studies in Uruguay (Hartmann et al., 2000; Halls et al., 2001; Hartmann et al., 2001) and southernmost Brazil (Leite * Corresponding author. Address: NERC Isotope Geosciences Laboratory, British Geological Survey, Nottingham NG12 5GG, UK. Fax: þ44-115-936-3302. E-mail addresses: [email protected] (R.J. Pankhurst), adriana@ ingeis.uba.ar (A. Ramos), [email protected] (E. Linares).

et al., 2000) have provided much more precise and definitive data. General stratigraphic reviews have been provided by Dalla Salda et al. (1988), Dalla Salda (1999), In˜iguez Rodrı´guez (1999) and Cingolani and Dalla Salda (2000). In addition to the predominant granitic gneisses and migmatites, other lithologies, such as schists, marbles, and mafic and ultramafic igneous rocks, indicate a complex crustal evolution, thought to involve both subduction and continental collision. Neoproterozoic –Early Paleozoic sedimentary rocks cover much of the crystalline basement of the craton. To the south of the fairly extensive outcrops in southern Uruguay, rocks assigned to this craton are known from Martin Garcı´a Island in the Rı´o de la Plata estuary (Linares and Latorre, 1968; Dalla Salda, 1981a), several boreholes in the plains of northeastern Argentina (Russo et al., 1979), and the Sierras Septentrionales of the southern Buenos Aires province. The last of these areas, also referred to geologically as Tandilia (Dalla Salda et al., 1988; Dalla Salda, 1999; Cingolani and Dalla Salda, 2000), is the most extensive Precambrian outcrop in Argentina. There is no evidence for a comparable cratonic basement farther south—the basement rocks of the Sierras de la Ventana are Neoproterozoic – Cambrian, according to Rapela et al.

0895-9811/03/$ - see front matter q 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0895-9811(03)00015-4

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(2001)—and Tandilia is usually taken as marking the southernmost limit of the Rı´o de la Plata craton. Teruggi and Kilmurray (1980), Dalla Salda (1999) and Cingolani and Dalla Salda (2000) have described the petrology, structure, and inferred evolution of Tandilia. The chronology and geochemistry of dykes cutting the granitoids have been described by Teixeira et al. (1999, 2001) and Iacumin et al. (2001). More than 50 granitoid samples from this region have been dated by K – Ar and Rb – Sr methods, with results ranging from 1550– 2190 Ma (for a complete listing, see Linares and Gonza´lez, 1990; Linares, 2001); one of the aims of this paper is to test whether this spread of ages reflects continued granite magmatism or merely subsequent thermal

effects. A Rb –Sr whole-rock study of granitoids from several localities near Tandil (Varela et al. 1988) also yielded variable results, ranging from 1770 ^ 80 Ma (Sierra Alta de Vela) to 2154 ^ 28 Ma (Tandilia quarry, Cerro Albio´n). At the western end of this range, in the region between Olavarrı´a and Azul (Fig. 1a), there are many small hills and quarries that contain outcrops of the basement granitoids and gneisses. Halpern and Linares (1970a,b) and Halpern et al. (1970) present Rb –Sr whole-rock data for samples from the Cerro Redondo quarry (1792 ^ 50 Ma) and quarries near Olavarrı´a (1947 ^ 44 Ma). Both values were recalculated with l87Rb ¼ 1.42 £ 10211 a21.

Fig. 1. (a) Sketch map of the Sierras Septentrionales, showing major outcrops and the study area near Olavarrı´a. The inset shows the regional position of this map. (b) Location map of the quarries from which the samples were analyzed.

R.J. Pankhurst et al. / Journal of South American Earth Sciences 16 (2003) 5–13

A new set of samples from the same area (San Nicola´s, Villa Mo´nica, and Cerro Redondo in the Sierras Bayas area; El Penal and Cenit near the town of Sierra Chica; Cerro Sotuyo to the south of Olavarrı´a; and El Peregrino near Chillar, south of the city of Azul) was collected by Linares and Ramos and is the subject of this paper.

2. Petrography The rocks have the textural characteristics of metamorphic rocks, with a marked degree of deformation. However, in most cases, they are clearly derived from igneous granitoids and classified as such according to their predominant mineralogy. In compositional terms, they vary from quartz – diorite to granite. In the granitoid rocks, compositional gradation among mafic, intermediate, and granitic types is general, most notably between granitic phases with different colors. However, amphibolite enclaves and thin metamorphosed basic dykes cutting the granitoids are occasionally observed. The few units described as migmatites have a mafic melanosome with amphibole, presumably derived from relict pyroxene, and invasive leucosome. Plagioclase feldspar (oligoclase-andesine) is commonly replaced by porphyroblastic microcline and myrmekite. The textures that result from recrystallization of the quartzo-feldspathic material show a gradual transition between granular and granoblastic. Accessory minerals are zircon, apatite, titanite, allanite, and magnetite. Further details appear in Appendix A.

3. Analytical methods Fourteen samples from the localities indicated in Fig. 1b were selected as representative, on the basis of

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the petrographical study, and prepared as whole-rock powders. Major elements (Table 1) were mostly determined by X-ray fluorescence at INGEIS and are considered accurate to within 3% of the reported concentrations; Na2O and MgO were determined by atomic absorption spectroscopy. Analyses of Sm, Nd, 143Nd/144Nd, and 87Sr/86Sr ratios were carried out on a VG 354 multicollector mass spectrometer, following standard techniques, at the NERC Isotope Geosciences Laboratory (for details, see Pankhurst et al., 1992; Rapela et al., 1992). Sm and Nd concentrations were determined by isotope dilution using a mixed spike, and the resultant Sm/Nd ratios are considered accurate to ^ 0.1% ð1sÞ: The corresponding uncertainties for the 143 Nd/144Nd and 87Sr/86Sr ratios are estimated as 0.005 and 0.01%, respectively. Rb/Sr ratios were determined by X-ray fluorescence at the British Geological Survey and are considered accurate to ^ 0.5%. The values obtained for the NBS 987 and La Jolla standards were 0.710235 ^ 0.000042 and 0.511835 ^ 0.000018, respectively. Seven of the samples were also analyzed by the Rb –Sr method at INGEIS using X-ray fluorescence, as described previously, and isotope analysis on a VG-54R double collector mass spectrometer, following standard techniques. The isotope data appear in Table 2. Isochron fits were assessed using Isoplot/Ex (Ludwig, 2000), and errors are reported at the 2s level.

4. Geochemical characteristics The major element geochemical data (Table 1) indicate that these are meta-igneous rocks of calc-alkaline composition. The granitoids are generally metaluminous but with a tendency to peraluminosity in more acid varieties. Normative corundum is less than 2%, similar to the typical

Table 1 Geochemical data for analyzed samples of Olavarrı´a granitoids VM10 Major elements (%) 56.55 SiO2 TiO2 1.7 Al2O3 14.93 Fe2O3T 8.4 MnO 0.13 MgO 2.1 CaO 5.95 Na2O 2.8 K2O 3.45 P2O5 0.71

VM12

VM13

SO –S1

SO–S8

SO– T5

78.07 0.15 12.96 1.49 0.02 0.25 0.58 2.75 5.85 0.02

70.12 0.32 16.91 3.03 .08 0.77 3.54 3.18 4.41 0.11

66.79 0.53 16.28 4.67 0.07 1.75 4.3 2.84 4.8 0.18

78.11 0.05 13.41 0.51 0.02 0.11 0.44 1.85 7.07 0.03

66.27 0.58 15.75 3.38 0.09 1.33 3.76 3.42 2.78 0.12

Trace elements (ppm) Rb 82 158 Sr 628 248 Sm 24 3.5 Nd 148 30

108 501 5.7 30

125 510 6.7 45

206 438 1.6 15

89 567 5 40

SO –T8

50.19 1.42 14.66 10.24 0.32 6.25 8.73 2.75 4.22 0.84 170 1044 23 136

CR1

78.5 0.12 13.24 1.54 0.02 0.25 0.77 3.44 3.97 0.08 125 102 6.5 32

CR5

73.9 0.24 13.42 2.98 0.03 0.52 1.39 3.58 4.21 0.17 85 148 11 64

CR6

52.85 1.26 15.09 9.38 0.3 5.84 6.85 4.12 1.39 0.2 25 152 7.4 34

SCH2

73.65 0.43 14.12 3.78 0.04 0.7 2.36 3.13 4.33 0.11 153 236 12 75

SCH6

75.19 0.1 13.56 1.03 0.02 0.25 0.93 2.9 4.94 0.02 180 55 2.5 8

CP2

76.52 0.27 14.16 2.09 0.02 0.48 1.1 3.1 4.8 0.12 217 182 7 47

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R.J. Pankhurst et al. / Journal of South American Earth Sciences 16 (2003) 5–13

Table 2 Rb –Sr and Sm– Nd isotope data for Olavarrı´a granitoids Villa Mo´nica

Cerro Sotuyo

Sample

VM10

VM12

VM13

SO –S1

SO –S8

SO– T5

SO– T8

Composition† Sm (ppm) Nd (ppm) 147 Sm/144Nd 143 Nd/144Nd 1Ndt Tchur TDM a Rb (ppm) Sr (ppm) Rb/Sr 87 Rb/86Sr 87 Sr/86Sr e Srt 87 Rb/86Srb 87 Sr/86Srb

mig 23.81 147.82 0.0973 0.511150 21.7 2275 2606 82 628 0.1304 0.3776 0.715799 32 0.3680 0.71543

G

Gd

Sd

G

T

3.48 30.19 0.0697 0.510755 21.8 2252 2613 158 248 0.6371 1.8520 0.756082 242 1.7998 0.75600

5.74 30.01 0.1155 0.511351 22.8 2407 2680 108 501 0.2156 0.6246 0.721732 8 0.6121 0.72160

6.73 44.84 0.0907 0.511033 22.2 2301 2639 125 510 0.2451 0.7103 0.724579 11 0.6736 0.72326

mig 23.14 135.67 0.1031 0.511285 20.7 2196 2531 170 1044 0.1628 0.4716 0.717171 10 0.4602 0.71660

Cerro Redondo

1.65 15.20 0.0656 0.510659 22.6 2293 2665 206 438 0.4703 1.3652 0.741044 243

Sierra Chica

5.11 40.03 0.0771 0.510821 22.6 2308 2666 89 567 0.1563 0.4525 0.716563 10

San Nicola´s

El Peregrino

Sample

CR1

CR5

CR6

SCH2

SCH6

SN14

CP2

Composition† Sm (ppm) Nd (ppm) 147 Sm/144Nd 143 Nd/144Nd 1Ndt Tchur TDM a Rb (ppm) Sr (ppm) Rb/Sr 87 Rb/86Sr 87 Sr/86Sr e Srt 87 Rb/86Srb 87 Sr/86Srb

G

Gd 10.92 64.15 0.1029 0.511204 22.2 2323 2639 85 148 0.5736 1.6674 0.755131 25

enc

Gd 12.17 74.61 0.0986 0.511172 21.7 2270 2600 153 236 0.6483 1.8852 0.759148 213 1.8524 0.75994

G

Gd

Gb

2.46 7.70 0.1932 0.512468 22.4 7542 2652 180 55 3.3028 9.8161 0.986300 2263 9.1049 0.98230

1.97 13.71 0.0866 0.511010 21.5 2247 2591 75 349 0.2160 0.6260 0.722078 12

6.88 47.05 0.0884 0.511026 21.7 2261 2603 217 182 1.1923 3.4806 0.799494 2140

6.49 31.70 0.1238 0.511524 21.7 2321 2604 125 102 1.2255 3.5795 0.805159 2102

7.45 34.24 0.1315 0.511651 21.4 2299 2578 25 152 0.1618 0.4688 0.720679 61

Notes†: G ¼ granitic, Gd ¼ granodioritic, Sd ¼ syenodioritic, T ¼ tonalitic, Gb ¼ granitic blastomylonite, mig ¼ banded diatexitic migmatite with predominant pyroxene-bearing melanosome, and enc ¼ mafic enclave; Tchur is the inferred age of separation from a chondritic reservoir a TDM is the inferred age of formation of parental continental crust (for the calculation, see DePaolo et al., 1991) b Data determined at INGEIS.

metaluminous series of subduction-related batholiths. Together with the predominance of hornblende as the main mafic phase and the absence of peraluminous minerals, this indicates that the parent magmas were Itype according to the classification of Chappell and White (1974), that is, derived by remelting of preexisting mafic igneous source rocks. According to White and Chappell’s (1977) model, the mafic enclaves, rich in hornblende and lacking peraluminous minerals, represent modified relict source material (restite), and compositional variation is ascribed to the selective separation of felsic melt from residual mafic restite. However, other authors (e.g. Vernon, 1990; Didier and Barbarin, 1991) emphasize the evidence of textural, chemical, and isotopic disequilibrium between

mafic enclaves and host granitoids, which generally suggests an interaction between a more mafic primary magma and crustal melts. In the tectonic discrimination diagram of Batchelor and Bowden (1985) (Fig. 2), the majority of the compositions plot in subgroup 2 (pre-plate collision, destructive plate margin, subduction regime) with a trend toward subgroups 6 and 7 (syn-collision or anatectic melting zone, postorogenic granites). These processes could all be involved as part of a sequence of events beginning in a subduction-related environment and culminating with intense deformation and anatexis during a subsequent collisional phase. However, the same geochemical trend extends in the other direction to the mafic enclaves and could equally imply

R.J. Pankhurst et al. / Journal of South American Earth Sciences 16 (2003) 5–13

Fig. 2. Geochemical parameter plot (Batchelor and Bowden, 1985) to discriminate granite tectonic settings. Data are given in Table 1.

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more consistent and define an acceptable isochron (MSWD ¼ 1.4) with an age of 2140 ^ 88 Ma. This suggests that the majority of the rocks were derived from cogenetic igneous parents. If the mafic enclave samples SO-T8 and CR-6 are excluded as those least likely to have a cogenetic relationship with the granites, the MSWD is reduced to an even more acceptable value of 0.85, and the corresponding age becomes 2127 ^ 68 Ma. In either case, the Sm –Nd age of the protolith granitoids of the region, close to 2100 Ma, is older than many of the Rb –Sr age estimates. The initial 143Nd/144Nd ratio of 0.50977 ^ 0.00006, which is insensitive to the inclusion of the enclave data, corresponds to an initial 1Ndt value of 2 1.9 ^ 0.6, which implies that the granitoid magmas were either derived from or contaminated by older crustal material at the time of their emplacement. This is consistent with the model of a preexisting igneous crustal

a coherent geochemical and petrogenetic process for the whole suite, such as variable partial melting of a simple mafic (tholeiitic) source or simple mixing between a tholeiitic magma and a uniformly evolved crustal composition.

5. Isotope data The Rb –Sr data do not yield an acceptable isochron. Those obtained at NIGL scatter about an errorchron of 1970 ^ 94 Ma, with an initial 87Sr/86Sr ratio of 0.7045 ^ 0.0012 and an MSWD ¼ 77 (Fig. 3a). The subset corresponding to the analyses at INGEIS has a more restricted range of Rb/Sr ratios but gives a comparable scatter at 2083 ^ 130 Ma, with an initial 87Sr/86Sr ratio of 0.7034 ^ 0.0014 and an MSWD ¼ 44. Although we do not have adequate control on interlaboratory bias, the entire data set combined provides values of 2009 ^ 71 Ma, 0.7041 ^ 0.0009, and 69, respectively. If the scatter is attributed to variable initial 87Sr/86Sr ratios (Model 3 fit), the ages are not significantly changed and the estimated age uncertainties are reduced. However, the probability that the scatter is mostly due to the disturbance of the whole-rock Rb – Sr system during metamorphism, alteration, or weathering means that this solution is doubtful. The ages derived are comparable to the previous result of Halpern and Linares (1970b), recalculated as 1947 ^ 44 Ma using the current convention for the decay constant of 87Rb, but the new data show much more scatter. We do not consider that the Rb – Sr whole-rock method has yielded a reliably defined age for the crystallization of the igneous precursors of these rocks. The Sm – Nd system is considered much more resistant to disturbance and more likely to yield reliable protolith ages, though the more limited natural range of Sm/Nd ratios in granitic rocks limits the precision that can be obtained in age estimates. The data obtained in this study (Fig. 3b) are much

Fig. 3. Rb –Sr and Sm–Nd isochron plots for the data listed in Table 2. See the text for interpretation (n.b. the grey square for Sierra Chica in the Rb –Sr plot is the only INGEIS analysis visually distinguishable from the NIGL data).

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R.J. Pankhurst et al. / Journal of South American Earth Sciences 16 (2003) 5–13

source region. Sm – Nd model ages for derivation from the continental crust, calculated according to the method of DePaolo et al. (1991), range from 2530– 2680 Ma, with a mean of 2620 Ma, approximately 200 Ma older than the commonly reported depleted mantle model ages, as calculated by DePaolo (1981). Because it is likely that these evolved granitic rocks were derived only indirectly from mantle sources, with an earlier stage of crustal differentiation, 2600– 2700 Ma is considered a maximum possible age for the original extraction of continental crust from the mantle in this part of the craton.

6. Discussion Early geochronological data obtained from the Rı´o de la Plata craton basement gave ages ranging from ca. 2200– 600 Ma, ascribed to orogenic cycles from early Transamazonian to Brasiliano. Many of these results are only available in conference literature. The oldest K – Ar ages from Tandilia are ca. 2200 –2000 Ma (Hart et al., 1965; Halpern et al., 1970). Other data vary between 2000 –1500 Ma (Hart et al., 1965; Linares and Latorre, 1968; Teruggi et al., 1974, 1988), but most are younger, often less than 1000 Ma (Hart, 1966; Cazaneuve, 1967; Linares and Gonzalez, 1990). For amphibolite samples from Martı´n Garcı´a Island, Linares and Latorre (1968) indicate K – Ar biotite ages of 1950 ^ 200 and 1750 ^ 100. Subsequently, Dalla Salda (1981a) obtained three K – Ar amphibole ages averaging 2065 Ma (^ 100 Ma), as well as younger amphibole and muscovite K –Ar ages of ca. 1870, 1600, and 1120 Ma. The first Rb – Sr data for Tandilia were presented by Halpern and Linares (1970a,b) who gave a calculated age of 1947 ^ 44 Ma for rocks in the area studied herein. Granites and microgranites from the San Miguel quarry at Cerro Queseria, east of Tandil, gave an age of 2120 ^ 10 Ma (Halpern et al., 1970; Umpierre and Halpern, 1971). Varela et al. (1988) presented a more comprehensive study with Rb – Sr data for several areas in the Sierras Septentrionales and cited ages of 2154 ^ 28, 2001 ^ 60, 1971 ^ 398, and 1770 ^ 88 Ma. The range and pattern of such data have been interpreted by Teruggi and Kilmurray (1980) and Dalla Salda (1981b) as representing a series of Paleo to Mesoproterozoic events. These events were believed to begin with sedimentation, mafic volcanism, and nappe formation in the 2200 – 2000 Ma interval; followed by intense deformation, metamorphism, and migmatization in Mesoproterozoic times (ca. 1800 Ma); and then granite emplacement, folding, and low grade events during the Neoproterozoic. Ramos (1985) suggests, for the region of Tandı´l, a major tectonometamorphic event at 2200 –2000 Ma, mafic dyke emplacement at 1700 – 1600 Ma, and locally important granitization at 1300 – 1100 Ma. Dalla Salda et al. (1988, 1992), Varela et al. (1988) and Cingolani and Dalla Salda (2000) have distinguished two main intrusive episodes:

granitoids related to continental collision in the interval 2154– 1971 Ma and postcollisional anatexis at 1770 Ma. Low grade mafic rocks and metacherts in the basement west of Tandı´l have been cited by Teruggi et al. (1988) and Ramos et al. (1990) as evidence for Mesoproterozoic subduction related to the amalgamation of Tandilia with the rest of the Rı´o de la Plata craton. The oldest undeformed metasedimentary cover (La Tinta Group) is Neoproterozoic; Bonhomme and Cingolani (1980) and Cingolani and Bonhomme (1982) report a Rb – Sr age of 793 ^ 32 Ma for pelitic intercalations in dolomite. The general metamorphic grade of the cover is very low (anchizone), and postdepositional regional heating cannot have been a factor in the observed isotope systems of the craton. These interpretations must be reassessed in light of U – Pb zircon and Sm – Nd data recently published for the northern areas of the craton, new Ar –Ar data for dykes in Tandilia, and the data presented here. Precise U –Pb zircon ages have been obtained from exposures of the Rı´o de la Plata craton in the Piedra Alta terrane of western Uruguay; relatively undeformed granites intruding volcanosedimentary rocks of the San Jose´ belt have provided ages of 2088 ^ 12 Ma (conventional U –Pb analysis by Preciozzi et al., 1999a), 2065 ^ 9 Ma, and 2074 ^ 6 Ma (SHRIMP analyses by Hartmann et al., 2000). Preciozzi et al. (1999a) also present Sm – Nd model ages of 2060– 2440 Ma for these granites, which must be younger than their host metamorphic rocks. Leite et al. (2000) analyze an orthogneiss from the Dom Feliciano belt of Rio Grande do Sul, southernmost Brazil, using the U – Pb SHRIMP method on zircons. Igneous cores indicate a mean crystallization age of 2078 ^ 13 Ma, though a few analyzed spots indicate inheritance up to 2200 Ma in age. Low U/Th metamorphic embayments and overgrowths on the igneous zircons give ages of 800 – 600 Ma, interpreted by Leite et al. (2000) as representing a metamorphic response to shear zone formation during Brasiliano events. However, their work indicates a Paleoproterozoic basement evolution similar to that of the craton. Most recently, Cingolani et al. (2002) present U – Pb SHRIMP zircon magmatic ages for granitoids of the Azul and Tandil areas of 2051 ^ 3 to 2228 ^ 6 Ma, with inherited ages of ca. 2185 –2657 Ma. (These authors also report 1Ndt values for analyzed granitoids that range from 2 1.3 to 2 2.9, with one leucogranite at þ 1.2). Thus, including the results presented here, there is strong evidence for major granite magmatism in three widely spaced areas of the Rı´o de la Plata craton at ca. 2100 Ma, with possible older crust formation as far back as 2600 Ma. The pre2100 Ma history of some host rocks of the granites has been confirmed by Cingolani et al. (2002) and Hartmann et al. (2001). The latter authors obtain U – Pb SHRIMP evidence for 3.41 Ga zircon growth in a greenstone belt lithology in the Nico Pere´z terrane of Uruguay and a 3.26 – 3.14 Ga detrital zircon in a nearby 2.76 Ga conglomerate. In the same area, east of the Sarandi del Yi shear zone, Preciozzi et al. (1999b)

R.J. Pankhurst et al. / Journal of South American Earth Sciences 16 (2003) 5–13

deduce evidence of events younger than 2000 Ma, but some of these eastern terranes may not have amalgamated with the Piedra Alta terrane until the Brasiliano cycle (i.e. during the late stages of Gondwana formation). The coherence of the Sm –Nd data presented here for the Olavarrı´a granitoids and gneisses underlines the uniformly early Paleoproterozoic formation of the parent material of these rocks from the mantle. The data indicate little contribution from younger mantle-derived sources, which might be expected if differentiation of the crystalline basement of the craton had reached a climax at 1800 Ma and continued actively until ca. 1700 Ma or later. One of the localities sampled (Cerro El Peregrino) lies within an important mylonite zone ascribed by Dalla Salda et al. (1992) to the collisional phase of the Transamazonian orogeny. In the Uruguayan exposures of the craton, the only demonstrated Mesoproterozoic magmatic event is the emplacement of totally undeformed mafic dykes that trend ENE between 348– 348300 S. For these dykes, Campal et al. (1991) and Bossi et al. (1993) report K –Ar and Rb – Sr whole-rock ages of 1700 – 1800 Ma. More recently, they have yielded concordant 40Ar – 39Ar plateau ages of 1727 ^ 10 and 1725 ^ 10 Ma (Teixeira et al., 1999). Halls et al. (2001) also refer to a concordant U – Pb baddeleyite age of 1790 ^ 5 Ma. These ages coincide with a K –Ar age of 1750 ^ 50 Ma for a dolerite at Estancia La Paulina, near Tandil (Teruggi et al., 1974). Teixeira et al. (1999) interpret the emplacement of the mafic dykes as associated with a failed Mesoproterozoic rift. The dating of the dykes has been extended recently to Tandilia, where biotites from hornfelses adjacent to early calc-alkaline (andesite –rhyolite) sources southeast of Azul have yielded 40 Ar – 39Ar ages of 1974 ^ 24 and 2007 ^ 24 Ma (Teixeira et al., 2001). Initial 1Nd values of 2 3 to 2 4 (Iacumin et al., 2001) indicate slightly more crustal contamination or prehistory than do the granites analyzed herein (mostly 2 1 to 2 2). These dykes are essentially contemporaneous with granite emplacement during the restricted orogenic activity now ascribed to the Transamazonian orogeny. Plagioclase from later tholeiitic dykes indicate much younger K –Ar and 40Ar – 39Ar ages of 800 –1100 Ma, which is considered to reflect hydrothermal activity (Teixeira et al., 2001).

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the mantle as long ago as 2600 Ma. These results are in full agreement with more precise U – Pb dating of the craton in western Uruguay and southernmost Brazil, as well as with recent data presented for Argentina by Cingolani et al. (2002), which also indicate a relatively short-lived Transamazonian orogeny with major granite formation, metamorphism, and postorogenic granite emplacement by 2070 Ma. Some Rb –Sr errorchrons, including the 2009 ^ 71 Ma obtained herein, are concordant with this conclusion within analytical error or may indicate resetting during a subsequent collisional phase. We conclude that, during the interval 2200– 1700 Ma, only closed-system reworking (deformation, metamorphism, and anatexis) occurred with no further contribution of the granitoids to crustal growth, at least in the Olavarrı´a-Azul and Tandil regions. The significance of Rb –Sr whole-rock ages less than 2000 Ma is no longer clear; they may just reflect partial open system behavior following the rock-forming events at 2050– 2100 Ma. Subsequent deformation, metamorphism, and even local melting of the basement may have extended until Mesoproterozoic times, according to Rb – Sr and K – Ar data, but involved little further addition of mantlederived magma, apart from the emplacement of a mafic dyke suite at ca. 1730 Ma. This conclusion must be contrasted with evidence for the reactivation of the eastern border of the craton in Uruguay (east of the Piedra Alta terrane), where Hartmann et al. (2002) have dated Neoproterozoic granites at 762 ^ 8 Ma (with 2.0 Ga inheritance), 633 ^ 8 Ma, and 571 ^ 8 Ma. These periods of igneous activity occurred during the Brasiliano cycle, broadly contemporaneous with the deposition of the La Tinta Group in Tandilia.

Acknowledgements We thank Luis Dalla Salda and Miguel Angel Basei for their helpful reviews of the manuscript. This work is a contribution to the International Geological Correlation Programme Project 436 (Pacific Gondwana Margin).

Appendix A. Petrography of the analyzed samples 7. Conclusions A.1. Villa Mo´nica quarry The parent magmas of the granitoid gneisses studied here, from the crystalline basement at the western end of the Sierras Septentrionales, were emplaced in a convergent regime related to subduction 2140 ^ 88 Ma ago, according to Sm – Nd whole-rock data. The igneous parent magmas were of rather uniform Nd-isotopic composition and probably derived in part from preexisting crustal rocks. The material that constituted the source region underwent its primary differentiation from

VM10. Amphibolitic migmatite with invasive granitic leucosome. Composed of brown hornblende, biotite, quartz, and plagioclase. There is important recrystallization of microcline. Accessory minerals are apatite, zircon, and opaque minerals. VM12. Granitoid of granitic composition with porphyroblasts of perthitic microcline, quartz, a small amount of acidic plagioclase with myrmekite, and highly altered relict

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R.J. Pankhurst et al. / Journal of South American Earth Sciences 16 (2003) 5–13

mafic minerals. Accessory minerals are apatite, zircon, titanite, and allanite. VM13. Granitoid of granodioritic composition with porphyroblasts of microcline and plagioclase on a groundmass of quartz. There is relict amphibole and biotite, with zircon and opaque minerals.

recrystallized over a groundmass of quartz, K-feldspar, and plagioclase. Interstitially, there are relicts of altered mafic minerals associated with opaque minerals. Accessory minerals are apatite and zircon.

A.2. Cerro Sotuyo

CP2. Blastomylonite of granitic composition with microcline recrystallized over a groundmass of quartz. The mafic minerals are highly deformed and replaced by opaque minerals. Apatite and zircon are present as accessory phases.

SO – S1.Granitoid of synodioritic composition, containing pyroxene associated with biotite and opaque minerals. The plagioclase is oligoclase – andesine with myrmekite. Microcline forms porphyroblasts with a graphic texture over a groundmass of quartz. Apatite and zircon. SO – S8. Granitoid of granitic composition, highly deformed, with porphyroblasts of microcline and quartz over a groundmass of quartz and alkali-feldspar. Interstitial relict mafic minerals are chloritized and altered to opaque minerals. SO – T5. Granitoid of tonalitic composition with porphyroblasts of plagioclase, alkali-feldspar, and quartz. The relict mafic minerals are green amphibole, biotite, and pyroxene, associated with opaque minerals. Apatite and zircon. SO – T8. Pyroxenitic migmatite with invasive granitic material. Green pleiochroic pyroxene is associated with strongly orientated dark biotite. Diffuse banding, defined by recrystallization of feldspar and quartz over a groundmass of quartz. Apatite, zircon, and abundant opaque minerals. Apparently a migmatized mafic enclave. A.3. Cerro Redondo CR1. Granitoid of granitic composition (no thin section). CR5. Granitoid of granodioritic composition with porphyroblasts of perthitic microcline and recrystallized plagioclase over granoblastic quartz and feldspar. Pyroxene is replaced by brown hornblende and chloritized biotite associated with opaque minerals. Zircon and apatite. CR6. Dark relict enclave (no thin section). A.4. Sierra Chica SCH2 (El Cenit). Granitoid of granodioritic composition with porphyroblasts of recrystallized microcline over quartz, microcline with graphic texture, and myrmekitic plagioclase. Relict mafic minerals are completely altered and deformed. SCH6 (El Penal). Granitoid of granitic composition with porphyroblasts of quartz and microcline with patchy perthites in a groundmass of quartz, feldspar, and plagioclase. Graphic texture and myrmekite are seen. There are relicts of amphibole, highly chloritized and altered to opaque minerals. A.5. San Nicola´s SN14. Granitoid of granodioritic composition with deformed porphyroblasts of microcline and quartz,

A.6. Cerro El Peregrino

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