Archean crust in the Rio de la Plata Craton, Uruguay — SHRIMP U–Pb zircon reconnaissance geochronology

Journal of South American Earth Sciences 14 (2001) 557±570

www.elsevier.com/locate/jsames

Archean crust in the Rio de la Plata Craton, Uruguay Ð SHRIMP U±Pb zircon reconnaissance geochronology LeÂo A. Hartmann a,*, NeÂstor Campal b, JoaÄo Orestes S. Santos c, Neal J. McNaughton d, Jorge Bossi b, Alejandro Schipilov b, Jean-Michel Lafon e a

Instituto de GeocieÃncias, Universidade Federal do Rio Grande do Sul, Avenida Bento GoncËalves, 9500; 91500-000 Porto Alegre, Rio Grande do Sul, Brazil b CaÂtedra de Geologia, Facultad de Agronomia, Universidad de la RepuÂblica, Avenida GarzoÂn, 780, Montevideo, Uruguay c Brazilian Geological Survey (CPRM), Rua Banco da ProvõÂncia, 105; 90840-030 Porto Alegre, Rio Grande do Sul, Brazil d Centre for Global Metallogeny, Department of Geology and Geophysics, The University of Western Australia, Nedlands 6907, Western Australia, Australia e Centro de GeocieÃncias, Universidade Federal do ParaÂ, BeleÂm, ParaÂ, Brazil Accepted 1 June 2001

Abstract Two Archean units were detected in the center of the Nico PeÂrez Terrane of the Rio de la Plata Craton in Uruguay, the La China Complex, a deformed granite±greenstone belt and the Las Tetas Complex, a deformed sedimentary platform cover. Reconnaissance dating of zircons from three samples by U±Pb sensitive high-mass resolution ion microprobe (SHRIMP) indicates that the two units are amongst the oldest Archean continental remnants in South America. The La China metatonalite has zircons with igneous cores (3.41 Ga) with metamorphic rims between 3.10 and 2.7 Ga. A muscovite quartzite from the Las Tetas Complex has detrital zircons with ages between 3.26 and 3.14 Ga, while the main source of the Las Tetas metaconglomerate was dated by zircon at about 2.76 Ga. The interpretation is that a ,3410 Ma greenstone belt was thrust-stacked with a 2.7 Ga carbonate±quartzite±pelite±conglomerate platform cover, so that granite±greenstone piles are positioned on top of cover rocks. The La China greenstone belt has intercalations of subvertical decameter-sized layers of talc±chlorite±tremolite schists with ma®c and ultrama®c amphibolites, a presumed volcanic komatiite/basalt sequence. The Las Tetas platform cover is a layered sequence of metamorphosed limestones, quartzites, conglomerates and pelites. Stromatolites occur in the carbonate sequence, whereas the pelites are now staurolite schists. The characterization of Archean continental remnants in the center of the Nico PeÂrez Terrane requires a reevaluation of the internal structure of the Rio de la Plata Craton. Important consequences emerge from this investigation for the understanding of the oldest environments in the South American crust and for the reconstruction of Archean paleocontinents. q 2001 Elsevier Science Ltd. All rights reserved. Keywords: SHRIMP; Sedimentary; Metamorphism

Resumo Unidades arqueanas foram identi®cadas na parte central do Terreno Nico PeÂrez, CraÂton do Rio de la Plata, Uruguai, o Complexo La China e o Complexo Las Tetas, respectivamente granito±greenstone belt deformado e cobertura plataformal sedimentar deformada. A datacËaÄo U± Pb de zircoÄes de 3 amostras por `sensitive high-mass resolution ion microprobe' (SHRIMP) mostra que essas unidades arqueanas encontramse entre os remanescentes continentais mais antigos da AmeÂrica do Sul, mesmo que a geocronologia tenha sido de caraÂter preliminar. Os zircoÄes do metatonalito La China tem nuÂcleos magmaÂticos (3.41 Ma) e bordas metamoÂr®cas (3.10 e 2.7 Ga). Um muscovita quartzito do Complexo Las Tetas tem zircoÄes detrõÂticos com idades entre 3.26 e 3.14 Ga, ao passo que a fonte da sedimentacËaÄo do meta-conglomerado Las Tetas foi datada em zircoÄes em torno de 2.76 Ga. A interpretacËaÄo da evolucËaÄo do bloco arqueano e o empilhamento por falhamentos de empurraÄo de um greenstone belt de ,3410 Ma sobre uma cobertura plataformal (2.76 Ga) constituõÂda de carbonatos±quartzitos±pelitos± conglomerados. O greenstone belt La China possui intercalacËoÄes de camadas decameÂtricas subverticais formadas por talco±clorita±tremolita xistos e an®bolitos ma®cos e ultrama®cos, provavelmente uma sequeÃncia basaÂltica±komatiõÂtica. EstromatoÂlitos ocorrem nos calcaÂrios da cobertura * Corresponding author. Tel.: 155-51-316-6370; fax: 155-51-319-1811. E-mail addresses: [email protected] (L.A. Hartmann), [email protected] (J.O.S. Santos), [email protected] (N.J. McNaughton), [email protected] (J. Bossi), [email protected] (J.-M. Lafon). 0895-9811/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S 0895-981 1(01)00055-4

558

L.A. Hartmann et al. / Journal of South American Earth Sciences 14 (2001) 557±570

Las Tetas, ao passo que os pelitos foram transformados em estaurolita xistos. A caracterizacËaÄo de remanescentes continentais arqueanos na parte central do Terreno Nico PeÂrez exige a re-avaliacËaÄo da estrutura interna do CraÂton do Rio de la Plata. Importantes consequeÃncias emergem desta investigacËaÄo no tocante ao entendimento dos ambientes mais antigos da crosta sul-americana e aÁ reconstrucËaÄo dos paleo-continentes arqueanos. q 2001 Elsevier Science Ltd. All rights reserved. Palavras chave: SHRIMP; Sedimentar; MetamoÂr®cas

1. Introduction The earliest Archean remnants of the Earth are close to 4.0 Ga, because the ®rst 500 million years of Earth history have been erased by meteorite impact, erosion at the surface and tectonic recycling of the lithosphere. By 3.0 Ga, continental crust was well developed as seen in the Barberton greenstone belt of South Africa, the AmõÃtsoq and Itsaq complexes in Greenland, part of the Slave Province in Canada and the Pilbara Block of Western Australia (e.g. Lumbers and Card, 1991; Rogers, 1996; Barley and Pickard, 1999; Nutman et al., 1999; Bowring and Williams, 1999). The search for remnants of the ancient (.3.0 Ga) continents is intensive, because they may clarify the natural conditions prevalent on Earth during these early times, including primitive life. In South America, Precambrian rocks older than 3.0 Ga have been identi®ed in a few terrains (Fig. 1) after intense search based on zircon geochronology (Nutman and Cordani, 1993; Teixeira et al., 1998; Dantas et al., 1998; Brito Neves, 1999; Brito Neves et al., 1999). The Campo Belo gneiss dome of Bahia state in Brazil is considered the oldest (close to 3.4 Ga) continental fragment preserved from younger orogenies (Nutman and Cordani, 1993; Martin et al., 1997). The oldest detrital zircon in South America (Table 1) was dated with SHRIMP from the Andorinhas Supergroup, CarajaÂs Province (,3.67 Ga; Macambira et al., 1998). Zircon crystals older than 3.0 Ga were identi®ed in the Sete Voltas tonalite (3466 ^ 4 Ma; Nutman and Cordani, 1993) and in the QuadrilaÂtero FerrõÂfero (3539 ^ 34 Ma; Machado et al., 1996), both in the SaÄo Francisco Craton. Old zircon xenocrysts were dated with SHRIMP in southern Brazilian granites (3.23 Ga; Remus et al., 1999; Silva et al., 2000) and in Patagonia (3.41 Ga; Rolando et al., 1999). Greenstone belts are known from several places in South America, mostly Brazil, but they are restricted in age to the Neoarchean and Paleoproterozoic. A Paleoproterozoic example is the Paso Severino belt in Uruguay (Hartmann et al., 2000a). The oldest rocks in the Rio de la Plata Craton are considered Paleoproterozoic (Bossi et al., 1993b, 1998; Campal and Schipilov, 1999; Cordani et al., 2000; Basei et al., 2000) and the few Archean rocks known were metamorphosed during the Trans-Amazonian cycle (Hartmann et al., 1999). The scarcity of isotopic data in Uruguay led to a geological and geochronological project by the Faculdad de Agronomia, Universidad de la Republica, Uruguay, the Universidade Federal do Rio Grande do Sul, and the Geolo-

gical Survey of Brazil. This geological and geochronological reconnaissance work identi®ed a large area of exposed Archean crust, not affected by the Trans-Amazonian orogeny (,2.0 Ga). This paper reports the results and interpretations, based on 54 SHRIMP II (sensitive high-mass resolution ion microprobe) U±Pb analyses on zircons from three rocks and complementary Pb evaporation zircon dating. The Archean ages obtained require major reevaluation of previous understanding concerning the geotectonic evolution of the southern portion of the Brazilian Shield.

2. Geology The southern portion of the Brazilian Shield has three major Precambrian geotectonic units in Uruguay (Bossi et al., 1998): the Piedra Alta Terrane on the west, the Nico PeÂrez Terrane in the center and the Cuchilla Dionisio along the Atlantic coast (Fig. 2). The Piedra Alta Terrane (Fig. 2 and Table 2) of Paleoproterozoic age (Hartmann et al., 2000a) was stabilized at 1.78 Ga (Teixeira et al., 1999). Thousands of E±W ma®c dikes cut this terrane (Teixeira et al., 1999) but are absent from the eastern Nico PeÂrez Terrane, which is an indication of the allochthonous origin of the two terranes relative to each other. The Nico PeÂrez Terrane (Fig. 2) contains the Valentines granulites in the north, the La China Complex and Las Tetas Complex in the center, and the Fuente del Puma Formation in the south. The Nico PeÂrez Terrane is limited on its east by the Sierra Ballena subvertical shear zone. The Nico PeÂrez and Piedra Alta Terranes constitute the Rio de la Plata Craton in Uruguay (Hartmann et al., 2000a). The Cuchilla Dionisio Terrane was considered a Neoproterozoic mobile belt that de®ned the eastern limits of the craton. We focus our study on the central part of the Nico PeÂrez Terrane, because it contains Paleoarchean crust and some of the oldest rocks identi®ed in South America, presently reported and previously unrecognized (Cordani and Sato, 1999; Brito Neves et al., 1999; Hartmann et al., 2000b). The Nico PeÂrez Terrane has ®ve major geological units exposed (Figs. 2 and 3). In the north, the Valentines Granulitic Complex includes tonalites, trondhjemites, perthite granites, pyroxenites and sillimanite gneisses. Magmatic protoliths of a tonalite and a perthite granite were dated at 2.6 Ga, while the high-grade metamorphic event was dated at 2.2 Ga (L.A. Hartmann, unpubl., zircon U±Pb SHRIMP). Two contrasting geological units are thrust-stacked in this region (Figs. 3 and 4), namely a greenstone belt and gneissic

L.A. Hartmann et al. / Journal of South American Earth Sciences 14 (2001) 557±570

559

Fig. 1. Major geotectonic units of South America. Location of oldest rocks and zircons indicated; ages by SHRIMP U±Pb isotopes in zircon, plus one conventional U±Pb zircon dating.

unit and a platform cover. The La China Complex contains ma®c and ultrama®c rocks and tonalites with some thin metachert layers, strongly deformed and metamorphosed in the amphibolite facies. Some of the ultrama®c rocks are interpreted as metamorphosed harzburgites, dunites and komatiites; serpentinites and talc schists are common. The complex is in contact with the granulites to the north (Campal and Schipilov, 1999) through the Sierra de Sosa dextral shear zone (Figs. 3 and 4). The platform unit is the

Las Tetas Complex, made up from bottom to top of metaconglomerates, quartzites, muscovite±tourmaline gneisses, staurolite±garnet mica schists, marbles and calc-silicate rocks; no volcanic intercalations are known. We suggest that the La China Complex was the basement for the deposition of the Las Tetas Complex, but intense thrust stacking renders inconclusive the relationships of the two units. The regional extent of the Las Tetas Complex is under investigation, but the unit is continuous at least for 110 km from

560

L.A. Hartmann et al. / Journal of South American Earth Sciences 14 (2001) 557±570

Table 1 Precambrian geotectonic units of Uruguay Geotectonic unit

Stratigraphic unit

Structure

Lithologies

Metamorphism

PiriaÂpolis foreland basin

Arroyo del Soldado Group, ,0.6 Ga Arroyo Grande Belt, ,2.1 Ga

Carbonate and clastic sedimentary and K-rich volcanic rocks Schists, volcanic, and sedimentary rocks

Subgreenschist facies

Piedra Alta Terrane; includes 1.8 Ga ma®c dike swarm

Discordant over basement, mostly in troughs E±W foliation

San Jose Belt, 2.07 Ga

E±W foliation

Montevideo Belt, 2.07 Ga

E±W foliation

Valentines Complex, 2.6±2.0± 0.6 Ga

Complex; E±W foliation dominant

La China Complex, 3.4±2.7 Ga

Thrust stacking La China: subvertical layers

Las Tetas Complex, 3.3±3.1± 2.7 Ga

Las Tetas: two schistosities, S2 subhorizontal Complex (poorly known)

Paso Severino ma®c/acid volcanics; Isla Mala Krich granites Amphibolites, schists, intrusive rapakivi granites Ma®c and acid granulite, perthite granite, pyroxenite, BIF Talc±chlorite±tremolite schist, hornblendite, amphibolite Muscovite (fuchsite) quartzite, staurolite schist, conglomerate, marble Limestones, phyllites, quartzites, and basalts Gneisses, schists, granites, migmatites

Nico Perez Terrane; includes 0.6 Ga ma®c dikes

Cuchilla Dionisio Terrane

Fuente del Puma Formation, age? Gneisses, granites, 2.0±0.75± 0.6 Ga

Subhorizontal foliation, NE subvertical foliation

Maria Albina village in the north to the town of Minas in the south. Quartzite units from the Las Tetas Complex may reach hundreds of meters in thickness. The diversity of stromatolites in the unit (LLH-C, LLH-S and SH-V types of Logan et al., 1964) indicates a minimum age of deposition in the Mesoproterozoic (Gaucher et al., 1996, 1998). The thrust stacking of the La China Complex and Las Tetas Complex produced several rock units, such as muscovite gneisses with the foliation parallel to the thrust planes, talc-rich ultrama®c rocks in 5±30 m-long, 10 m-thick bodies, massive serpentinites with associated rodingites, and S-type granites with muscovite, garnet and scheelite. Muscovites from the granites were dated by Ar±Ar at 1253 ^ 32 Ma (C. Cingolani, verbal comm. in Campal and Schipilov, 1999). To the south of the La China Complex (Fig. 2), the volcano-sedimentary Fuente del Puma Formation has limestones, phyllites, quartzites and basalts. The grade of metamorphism increases from subgreenschist facies on the east to amphibolite facies on the west; higher grade rocks are

Amphibolite facies dominant Amphibolite facies Amphibolite facies Granulite facies, amphibolite facies Amphibolite facies Amphibolite facies Subgreenschist to amphibolite facies Amphibolite facies dominant, also granulite facies

also known as the Carape Formation. No isotopic ages are available for the Fuente del Puma Formation, and the geological relationships with the Las Tetas Complex are unknown. The Arroyo del Soldado Group (Figs. 2 and 3) is discordant over the previous units in the Nico PeÂrez Terrane and is entirely sedimentary, composed of sandstones, conglomerates, limestones and pelites. The rocks are metamorphosed in subgreenschist facies in the west, but the grade of metamorphism increases eastward to greenschist facies. The rocks have abundant microfossils from the Vendian± Cambrian limit (Gaucher et al., 1996, 1998). The Valentines, La China and Las Tetas Complexes, the Sierra de Sosa shear zone and the thrust planes were intruded by a dike swarm containing hundreds of 2±3 mthick andesites and basalts. Age of intrusion may be 581 ^ 13 Ma (Bossi et al., 1993a; Rivalenti et al., 1995), determined by K±Ar in biotite from a thermal aureole. Due to intense deformation, the La China and Las Tetas Complexes now form a coherent unit. The ®rst deformation

Table 2 Selected previous geochronology in the Precambrian of Uruguay Terrane

Belt

Rock sample

Method

Age (Ma)

Reference

Piedra Alta

San JoseÂ

Nico Perez

Ma®c dikes La China

Granodiorite Monzogranite Granodiorite Diabase Muscovite quartzite

Zircon U±Pb SHRIMP Zircon U±Pb SHRIMP Zircon U±Pb conventional 40 Ar± 39Ar, hornblende 40 Ar± 39Ar, muscovite

2074 ^ 6 2065 ^ 9 2088 ^ 12 1727 ^ 10 1253.1 ^ 32.2

Hartmann et al. (1999) Hartmann et al. (1999) Preciozzi et al. (1999) Teixeira et al. (1999) Campal and Schipilov (1999)

L.A. Hartmann et al. / Journal of South American Earth Sciences 14 (2001) 557±570

561

still preserve internal lithological continuity over distances of about 10 km. A third deformation folded the subhorizontal thrust planes into open folds, as seen on the geological map (Fig. 3). 3. Geochronology

Fig. 2. Major geotectonic units of Uruguay. Location of Archean La China Complex and Las Tetas Complex indicated. Inset shows location in South America.

produced a ®ne-scale penetrative foliation or schistosity, superposed by a less penetrative foliation or schistosity spaced 0.5 mm as shown in many rocks, particularly in muscovite quartzites. The second foliation is subhorizontal in many places, but may be subvertical elsewhere. Both deformational events were accompanied by amphibolite facies metamorphism Ð feldspar became ductile in tonalities, and staurolite formed in metapelites. The La China greenstone belt and the Las Tetas platform cover thus became a coherent basement complex; original relationships are mostly blurred or erased, although the two units

The Precambrian of Uruguay (Table 3) has been mostly studied by Rb±Sr and K±Ar geochronology (Bossi et al., 1993b). It was understood that the Piedra Alta Terrane is of Paleoproterozoic age, although an Archean age was not discarded. The Nico PeÂrez Terrane was considered Archean in the north, due to the presence of granulites. Equivalent rocks 100 km farther to the north, in Brazil, were dated by Hartmann et al. (1999) at 2.55 Ga (magmatism) and 2.03 Ga (metamorphism), but the southern part of the Nico PeÂrez Terrane was considered late Mesoproterozoic in age because of 1.2 Ga Ar±Ar ages obtained on muscovite (C. Cingolani, verbal inf. in Campal and Schipilov, 1999). The Cuchilla Dionisio Terrane is believed to be Neoproterozoic with Mesoproterozoic inheritance (Preciozzi et al., 1999). U±Pb zircon dating with SHRIMP showed the Isla Mala granitic suite in the Piedra Alta Terrane to be 2.07 Ga (Hartmann et al., 2000a), while Rb±Sr and Ar±Ar geochronology displayed stabilization ages of the terrane at about 1.78 Ga (Teixeira et al., 1999). Three samples from the Nico PeÂrez Terrane were selected for SHRIMP dating, because they may constrain the magmatic, sedimentary and metamorphic evolution of the terrain. Sample preparation, charge contrast (CC) images, cathodoluminescence (CL) images and SHRIMP II analyses at Curtin University of Technology followed the procedures described by Hartmann et al. (1999) and Watt et al. (2000). Errors are given at 1s . Sample 1 is a metatonalite from the La China Complex, collected at coordinates 33832 0 10 00 S, 54855 0 45 00 W, on the side of a bridge at the head of the La China Creek, about 2 km east of Zapican town (Fig. 3). It has irregular, centimeter-thick, dark (amphibole-rich) and light (quartz and feldspar) gray bands and strong lineation. The mineralogy consists of quartz, 40% by volume, plagioclase 1 sericite, 31%, amphibole, 25%, biotite, 2%, opaque minerals, 1%, accessory minerals (apatite, zircon, epidote, chlorite), 1%. Plagioclase composition is An25.2±44.5 (eight analyses by electron microprobe) in sample 1. Grain size is about 0.5 mm but has bimodal distribution about 1 and 0.1 mm; the larger grain size is preserved in magmatic crystals, while the smaller grains are metamorphic. The amphibole is a tschermakite; a representative of six electron microprobe analyses (wt%, total ˆ 6) is: 42.79 SiO2, 0.23 TiO2, 16.06 Al2O3, 8.23 MgO, 9.69 CaO, 0.22 MnO, 18.60 FeO, 1.35 Na2O, 0.54 K2O, 0.47 Cl, 98.20 total; Cr2O3, NiO and F not detected. Zircon is abundant Ð 10 crystals in one thin section, and about 200 crystals were recovered for analyses. Some are dark brown, large (200±400 mm), and

562

L.A. Hartmann et al. / Journal of South American Earth Sciences 14 (2001) 557±570

Fig. 3. Geological map of core of Archean complexes. Location of two dated samples shown; sample 3 is to the southeast of the map area. Line A±A 0 indicates cross-sectional position of Fig. 4.

L.A. Hartmann et al. / Journal of South American Earth Sciences 14 (2001) 557±570

563

Fig. 4. Cross-section A±A 0 of La China Complex and Las Tetas Complex. Location of cross section shown in Fig. 3.

equidimensional (grains 11, 15, 13 and 14) whereas others are small (50±200 mm), prismatic, and clear (grains 7, 8, 12 and 16). All grains are rounded due to amphibolite facies metamorphism dissolution and reprecipitation. Cores and rims are prominent in most grains (Figs. 5 and 6). Sample 2 is a muscovite quartzite from the Las Tetas Complex, collected at coordinates 33845 0 19 00 S, 0 00 54851 54 W (Fig. 3). The quartzose unit has pelitic bands. Two schistosities are present, S1 intensely deformed by subhorizontal S2, and open folds are a late structural feature. A quartz-rich, coarse-grained rock was selected for zircon separation. The rock is zircon-poor Ð only about 60 grains were recovered from 1 kg of rock. All zircons are small (20±80 mm), rounded, and most show cores and rims. However, most of the grains, are highly fractured and metamict, and only nine were suitable for SHRIMP analyses. Sample 3 is a metaconglomerate from the Las Tetas Complex, collected at coordinates 33815 0 36 00 S, 0 00 55813 11 W, to the south of the mapped area. The rock is compositionally mature; both matrix and framework are composed almost exclusively by quartz. The quartz clasts are well-rounded pebbles and cobbles, strongly deformed, some elongated 5:1. The rock is also zircon poor, and only about 50 grains were recovered from 1 kg of rock, but they are less metamict than sample 2 zircons. They are dark brown and yellowish brown, mostly rounded to subhedral fragments. SHRIMP dating (Fig. 7(a) and Table 3) of 26 spots on 15 zircon crystals of the metatonalite, sample 1, yielded an array of data from 3443 ^ 16 to 2535 ^ 8 Ma, but the largest concentration of ages is between ,3410 and 3094 Ma (24 ages). The U contents of the zircon crystals differ between rims and cores: cores are very poor in uranium (19±72 ppm, average 34), and the rims have 93±

339 ppm of U (average 204 ppm). The Th/U ratios (Fig. 8) are very informative, because the cores of crystals and crystals without structure of core and rim have ratios between 0.43 and 1.22, typical of magmatic zircons from tonalites (e.g. Hartmann et al., 2000b). The rims and portions of crystals without structure of core and rim have very low ratios (0.01±0.04), which are commonly ascribed to metamorphic compositions (Vavra et al., 1996, 1999). The concordant analysis of 2721 ^ 7 Ma is interpreted as a younger M2 metamorphic event, which is broadly correlated to the main source of the Las Tetas metaconglomerate (2.76 Ga). The scattered ages between 3443 and 3094 Ma from sample 1 are dif®cult to interpret. The seven dated metamorphic rims in this group have ages between 3181 and 3094 Ma indicating an older M1 metamorphic event at about 3.10 Ga. The magmatic cores have ages between 3448 and 3281 Ma, apparently composing three groups of ages at 3404 ^ 8 …n ˆ 3†; 3340 ^ 19 Ma …n ˆ 5†; and 3281 ^ 17 Ma …n ˆ 3†: Two hypotheses are considered for the generation of this array of data. One possibility is that the youngest magmatic population (3.28 Ga) may correspond to the age of crystallization of the tonalite and the older ages, up to 3404 Ma, and represent inherited magmatic grains from the source of the magma. The other possibility is the partial resetting of magmatic zircon crystals, 3408 Ma, during metamorphism M1 at 3.10 Ga. We prefer this second hypothesis because the partial resetting of zircon isotopic compositions during a younger metamorphic event has been documented elsewhere (Friend and Kinny, 1995; Hartmann et al., 1999). In our interpretation, the oldest age of magmatic zircons (3408 ^ 16 Ma) represents the age of the magmatism, and the array of ages to 3.10 Ga is due to partial resetting during amphibolite

0.10 0.02 0.50 0.06 0.96

f 206a

Sample 3, metaconglomerate, Las Tetas Complex d1-1 24 22 0.89 17 d1-2 25 27 1.07 18 d10-1 32 29 0.92 20 d10-2 35 31 0.89 23 d11-1 31 37 1.17 22

Pb (ppm)

0.23 0.13 0.13 0.12 0.19 0.17 0.10 0.14 0.26 0.17

Th/U

Sample 2, muscovite quartzite, Las Tetas Complex a.1-1 73 34 0.46 58 a.2-1 93 49 0.53 77 a.3-1 99 56 0.56 75 a.3-2 157 90 0.57 117 a.4-1 138 95 0.69 111 a.5-1 122 61 0.50 96 a.6-1 193 91 0.47 134 a.7-1 194 135 0.70 126 a.8-1 116 52 0.45 92 a.9-1 207 289 1.39 128

Th (ppm) 0.03 0.00 0.13 0.10 0.00 0.00 0.04 0.03 0.95 0.00 0.30 0.10 0.03 0.05 0.41 0.03 0.04 0.00 0.22 0.00 0.20 0.26 0.07 0.00 0.70 0.11

U (ppm)

Sample 1, metatonalite, La China Complex b1-1 168 7 0.04 105 b2-1 54 0 0.01 37 b3-1 93 19 0.21 67 b4-1 195 6 0.03 126 b5-1 26 26 1.00 25 b6-1 46 52 1.12 43 b10-1 306 9 0.03 142 b10-2 72 50 0.69 59 b11-1 21 10 0.48 16 b11-2 200 6 0.03 130 b13-1 26 24 0.91 23 b13-2 153 5 0.03 105 b14-1 339 14 0.04 185 b14-2 33 29 0.87 29 b15-1 71 18 0.25 48 b15-2 176 6 0.04 119 b16-1 36 21 0.57 28 b7-1 22 27 1.21 23 b7-2 19 20 1.06 19 b7-3 29 35 1.22 29 b7-4 41 28 0.70 32 b8-1 25 22 0.86 22 b9-1 205 4 0.02 139 b9-2 22 17 0.76 20 b9-3 23 11 0.48 18 b9-4 22 23 1.05 21

Spot

0.5570 0.5630 0.5140 0.5380 0.5320

0.6610 0.6820 0.6280 0.6170 0.6430 0.6550 0.5880 0.5390 0.6660 0.4970

0.5810 0.6500 0.6302 0.6023 0.7335 0.6879 0.4588 0.6601 0.6476 0.6024 0.6977 0.6368 0.5288 0.6873 0.5895 0.631 0.6321 0.7278 0.7700 0.7190 0.6170 0.6692 0.6324 0.6974 0.6680 0.6900

Pb b/ 238U

206

15 13 13 10 13

13 13 11 11 11 12 10 9 12 8

9 11 10 9 15 13 7 12 15 9 15 10 8 14 10 10 13 16 13 9 7 15 10 15 10 10

^

Pb b/ 235U

14.520 15.240 13.800 14.170 13.810

23.880 24.620 21.020 20.820 23.200 23.040 20.290 18.460 23.570 18.110

18.950 21.810 22.027 20.300 27.909 25.952 10.608 24.335 23.580 20.218 26.218 21.079 13.672 25.828 20.272 20.537 23.012 28.856 31.300 27.730 22.040 25.547 20.699 27.714 23.070 27.310

207

0.550 0.431 0.474 0.313 0.485

0.508 0.504 0.422 0.385 0.435 0.441 0.363 0.333 0.463 0.331

0.311 0.412 0.414 0.326 0.669 0.540 0.170 0.480 0.699 0.325 0.748 0.350 0.212 0.621 0.394 0.338 0.563 0.720 0.670 0.428 0.323 0.728 0.333 0.693 0.538 0.500

^

0.1889 0.1962 0.1948 0.1909 0.1884

0.2622 0.2617 0.2429 0.2446 0.2617 0.2551 0.2503 0.2483 0.2569 0.2645

0.2366 0.2433 0.2535 0.2445 0.2759 0.2736 0.1677 0.2674 0.2641 0.2434 0.2725 0.2401 0.1875 0.2725 0.2494 0.2361 0.2641 0.2876 0.2947 0.2798 0.2593 0.2769 0.2374 0.2882 0.2504 0.2870

Pb/ 206Pb

207b

43 28 42 18 42

18 16 16 12 13 15 11 12 15 13

11 15 19 11 27 21 8 19 45 11 38 12 8 28 19 11 31 28 31 19 21 42 10 29 39 25

^

0.2311 0.2919 0.2373 0.2346 0.2921

0.1217 0.1372 0.1518 0.1521 0.1831 0.1315 0.1232 0.1511 0.1128 0.1720

0.0095 0.0013 0.0634 0.0062 0.2536 0.2830 0.0070 0.1720 0.1110 0.0072 0.2241 0.0072 0.0106 0.2239 0.0640 0.0095 0.1650 0.3201 0.2677 0.3145 0.2006 0.2195 0.0041 0.1993 0.1243 0.2771

Pb b/ 206Pb

208

89 58 88 32 92

25 22 23 18 19 20 13 16 22 19

9 1 23 9 35 29 9 26 80 8 67 12 6 44 25 12 46 46 57 31 38 75 9 32 76 44

^

Table 3 SHRIMP U±Th±Pb data on zircons (C ˆ core, I ˆ intermediate, and R ˆ rim, CZ3 zircon standard used for calibration ( 206Pb/ 238U ˆ 0.0928, 572 Ma))

2733 2795 2783 2750 2728

3260 3257 3139 3150 3257 3217 3187 3174 3228 3274

3097 3141 3207 3149 3340 3327 2535 3291 3271 3142 3321 3120 2721 3321 3181 3094 3271 3404 3443 3362 3243 3345 3103 3408 3187 3401

Age c

207

Pb/ 206Pb

38 23 35 15 37

11 10 10 8 8 9 7 8 9 8

8 10 12 7 15 12 8 11 27 7 22 8 7 16 12 8 18 15 16 10 13 23 7 16 25 14

^

104 103 96 101 101

100 103 100 98 98 101 94 88 102 79

95 103 98 96 106 101 96 99 98 97 103 102 101 102 94 102 97 104 107 104 95 99 102 100 104 99

Conc d

R C C R C

C R C R C R C C C C

R R R R C C R C C R C R R C R R C C C I R C R C R C

Spot position

564 L.A. Hartmann et al. / Journal of South American Earth Sciences 14 (2001) 557±570

d

c

b

31 84 208 40 50 38 42 38 43 19 91 91 37

Th (ppm)

1.03 1.27 1.58 1.11 1.28 1.23 1.08 0.59 0.51 0.72 1.46 1.44 0.69

Th/U 21 47 95 24 28 22 25 45 54 17 46 46 34

Pb (ppm) 0.67 0.21 0.00 0.62 0.41 0.02 0.35 0.67 0.20 0.45 0.46 0.12 0.14

f 206a

Pb.

204

0.5380 0.5350 0.5160 0.5170 0.5380 0.5460 0.5140 0.5750 0.5570 0.5330 0.5450 0.5380 0.5390

Pb b/ 238U

206

f 206 ˆ (common 206Pb)/(total measured 206Pb) based on measured All isotopic ratios are radiogenic lead. Uncertainties are 1s . Conc ˆ concordance, as 100{t‰ 206 Pb=238 UŠ=t‰207 Pb=206 PbŠ}:

30 66 131 36 39 31 38 65 83 26 62 63 54

d12-1 d13-1 d14-1 d16-1 d16-2 d2-1 d3-1 d4-1 d4-2 d6-1 d7-1 d7-2 d8-1

a

U (ppm)

Spot

Table 3 (continued)

14 11 9 12 10 14 12 12 11 15 11 9 12

^

Pb b/ 235U

14.180 14.360 13.790 13.850 14.120 14.440 13.650 17.240 16.900 14.410 14.370 14.250 13.910

207

0.493 0.364 0.260 0.450 0.321 0.478 0.434 0.434 0.369 0.584 0.376 0.271 0.377

^ 0.1911 0.1948 0.1937 0.1941 0.1902 0.1918 0.1927 0.2175 0.2202 0.1960 0.1913 0.1921 0.1871

Pb/ 206Pb

207b

41 26 12 41 21 38 38 25 23 52 30 14 28

^ 0.2622 0.3323 0.4322 0.2930 0.3374 0.3371 0.2864 0.1405 0.1285 0.1808 0.3788 0.3816 0.1751

Pb b/ 206Pb

208

88 57 32 89 46 83 81 45 36 107 70 32 53

^ 2751 2783 2774 2778 2744 2757 2765 2962 2972 2793 2753 2760 2717

Age c

207

Pb/ 206Pb

36 22 10 35 18 32 32 19 17 43 25 12 24

^

101 99 97 97 101 102 97 99 96 99 102 101 102

Conc d

C C C C R C C R C C R C C

Spot position

L.A. Hartmann et al. / Journal of South American Earth Sciences 14 (2001) 557±570 565

566

L.A. Hartmann et al. / Journal of South American Earth Sciences 14 (2001) 557±570

Fig. 5. CC (A Ð grain b9-1, C Ð b14-1) and CL (B,D) images of zircon crystals of metatonalite, sample 1. Paleoarchean ages (Ma) of magmatic, zoned cores and two ages of homogeneous, Mesoarchean and Neoarchean, metamorphic rims evident. Th/U ratios are also shown. Background of images was blackened with computer software.

facies metamorphism. This interpretation is supported by the core position of the oldest spots, their U and Th compositions and high Th/U ratios, all compatible with magmatic crystallization. The oldest age of 3443 ^ 16, is not used because it is the most reversely discordant result in the sample. Ten SHRIMP spots on nine zircon crystals (Fig. 7(b) and Table 4) from the Las Tetas Complex muscovite quartzite, sample 2, have scattered 207Pb/ 206Pb ages from 3274 to 3139 Ma. The U content of the zircon crystals shows no correlation with Th contents; the contents of both elements are low in the analyzed zircon crystals. Th/U ratios are about 0.5, variable and without correlation with age, but suggest lack of metamorphic recrystallization for the zircons. The number of zircon crystals analyzed is small for the full understanding of the sedimentary rock sources, but some useful information was obtained through preliminary grouping of age populations. The oldest ages are about 3262 ^ 14 Ma …n ˆ 4† and may re¯ect a source comparable in age to sample 1 (3.4 Ga), while the youngest ages are about 3145 ^ 4 Ma, which is close to the metamorphic M1 age of sample 1. Other age groupings are 3222 ^ 5 Ma and a discordant and doubtful age of 3181 ^ 6 Ma. This indicates a complex geological evolution of the region in the 3.26±3.14 Ga time interval. Eighteen SHRIMP spots on 13 zircon crystals (Fig. 7(c) and Table 3) from the Las Tetas metaconglomerate, sample

Fig. 6. Backscattered electron (BSE) and CL images of metatonalite, sample 1, and its zircon crystals. A and B Ð rock texture, showing greenschist facies alteration; zircon crystal in A (zr) is the same as in C and E; zircon crystal in B (zr) is the same as in D and F. C±F Ð rounded zircon crystals; cores and rims are clear on the images, and are interpreted as formed during the 3.41 magmatic crystallization (cores) of the tonalite and the 3.1 amphibolite facies metamorphic recrystallization (rims). zr ˆ zircon, ilm ˆ ilmenite, qz ˆ quartz, amp ˆ amphibole, chl ˆ chlorite, ap ˆ apatite, pl ˆ plagioclase, ep ˆ epidote.

L.A. Hartmann et al. / Journal of South American Earth Sciences 14 (2001) 557±570

567

Fig. 7. Concordia diagrams of three dated samples. A Ð rims are blank, cores are patterned. Estimated magmatic age in black. Older, reversely discordant spot, striped. B Ð four possible age groups of detrital zircons and C Ð two detrital zircon populations: main population blank and minor in gray.

3, show one concordant age group of 2762 ^ 8 Ma …n ˆ 16† and two older ages at 2968 ^ 12 Ma. The two groups correspond to the ages of two different source-rocks. The U and Th contents of the zircon crystals are very low and have no correlation. The main population has higher Th/U ratios (average ,1.1) than the older small population (0.55). Both the U and Th contents and the Th/U ratios indicate less-evolved granitic rocks as the source of the clastic zircon crystals. The main source is close to 2.7 Ga, in agreement with M2 metamorphism interpreted in sample 1.

4. Conclusions A continental remnant of Archean age is identi®ed in the Nico PeÂrez Terrane of the Rio de la Plata Craton from preliminary U±Pb SHRIMP and Pb±Pb geochronology of zircon. Ages obtained in the La China Complex can be interpreted to show that tonalite magmatism at 3.41 Ga was followed by metamorphism at about 3.10 Ga and the deposition of a platform sedimentary cover, the Las Tetas Complex, at approximately

568

L.A. Hartmann et al. / Journal of South American Earth Sciences 14 (2001) 557±570

Table 4 Lead isotopic data by zircon single-crystal evaporation from La China Complex, metatonalite, sample 1 Zircon

Temp.

Ratio number

206

Crystal 1

1450 1500 1500 1500 1550 1450 1500 1450 1500 1550 1450 1500 1550 1450 1500 1450 1500 1550

88 86 52 82 90 14 88 48 52 88 16 88 88 34 86 20 88 86

58,824 58,824 22,222 66,667 166,667 . 1000,000 41,667 41,667 100,000 125,000 58,824 30,303 20,408 100,000 500,000 166,667 333,333 333,333

Crystal 2 Crystal 3 Crystal 4 Crystal 5 Crystal 6 Crystal 7 Crystal 8

a

Pb/ 204Pb

208

Pb/ 206Pb

2s

207

Pb/ 206Pb

2s

207

Pb a/ 206Pb a

2s

Age (Ma), 2s

0.00982 0.01021 0.01978 0.00856 0.00929 0.01018 0.02387 0.02227 0.01219 0.02297 0.01302 0.01510 0.12382 0.01101 0.02638 0.03296 0.10735 0.17784

13 47 39 9 12 13 19 56 8 30 9 7 145 8 194 264 425 122

0.22898 0.23770 0.22663 0.21608 0.23479 0.22261 0.24301 0.19490 0.22775 0.23881 0.20629 0.23488 0.26278 0.20921 0.23380 0.23063 0.25666 0.26718

87 34 43 32 40 103 49 37 73 42 56 36 48 123 97 291 80 42

0.22880 0.23752 0.22613 0.21590 0.23474 0.22261 0.24275 0.19467 0.22764 0.23873 0.20609 0.23452 0.26222 0.20910 0.23378 0.23056 0.25662 0.26717

88 34 44 32 41 103 50 37 67 43 56 37 49 110 96 293 78 42

3044 3104 3025 2951 3085 3000 3138 2782 3036 3112 2875 3084 3260 2899 3078 3056 3226 3290

6 2 3 2 3 7 3 3 5 3 4 2 3 9 7 20 5 2

3104 3025

2 3

3085

3

3138

3

3112

3

3260

3

3078

7

3290

2

Common lead correction using Stacey and Kramers (1975) Pb composition.

2.7 Ga. The La China ma®c±ultrama®c volcanism may have been synchronous with the 3.41 Ga tonalite injection, but currently no evidence has been encountered to demonstrate this (we will continue to investigate this relationship). The whole sequence of tonalite, ma®c±ultrama®c volcanism and platform sedimentary cover was deformed into open folds at approximately 2.7 Ga (on the basis of one rim analysis). Zircon geochronology does not show any evidence of younger deformation of the region, although previous Ar±Ar geochronology on muscovites about 1.2 Ga indicates some Mesoproterozoic deformation of the Archean block. The central part of the Nico PeÂrez Terrane is, therefore, a deformed Archean greenstone belt with sedimentary platform cover. Our data support the hypothesis of an allochthonous terrane collage between the Piedra Alta and the Nico PeÂrez Terranes, because the ages are 2.07 Ga in the Piedra Alta Terrane (Hartmann et al., 2000a) and 3.41±2.7 Ga in

the Nico PeÂrez Terrane. It is unlikely that the two terranes were formed in the same relative position they are today because the strong Paleoproterozoic event of the Piedra Alta Terrane did not leave an overprint in the investigated central part of the Nico PeÂrez Terrane. The Piedra Alta and Nico PeÂrez Terranes constitute the Rio de la Plata Craton in Uruguay, but the limits of the craton are yet to be de®ned because deformation ages are poorly constrained in the eastern Cuchilla Dionisio Terrane. The Rio de la Plata Craton is a major unit of the Precambrian of South America, important for reconstruction of paleo-continents (Unrug, 1997). We show that this Neoproterozoic craton consists of a complex arrangement of terranes of variable ages spanning from the Paleoarchean (3.41 Ga) to the Neoarchean (2.7 Ga) in the La China remnant block, contrasting with predominantly Paleoproterozoic ages (2.07 Ga) in the Piedra Alta Terrane. Stromatolites from the Las Tetas carbonate platform may yield clues to the evolution of life, while the La China greenstone belt is a window into mantle processes in the Archean.

Acknowledgements

Fig. 8. Age vs. Th/U ratios of zircons from the metatonalite, sample 1. Magmatic ratios are 0.8±1.0, metamorphic ratios are lower than 0.05.

Ion microprobe U±Pb isotopic analyses of zircon crystals were performed with the `sensitive high-mass resolution ion microprobe' (SHRIMP II) operated jointly by Curtin University of Technology, The University of Western Australia and Geological Survey of Western Australia, with the support of the Australian Research Council. Paul Potter is thanked for advice and English review. JSAES reviewer C.M. Fanning is thanked for important contributions to the manuscript.

L.A. Hartmann et al. / Journal of South American Earth Sciences 14 (2001) 557±570

Appendix A After the SHRIMP isotopic results were known, another metatonalite sample was collected in the same outcrop of sample 1, within 10±20 cm of the ®rst sample, to produce independent age information by zircon Pb±Pb evaporation method (Kober, 1987). Analyses on zircon crystals from this replicate sample 1 were performed at the geochronological laboratory of Universidade Federal do ParaÂ, BeleÂm, ParaÂ, Brazil. Isotope analyses were performed on a Finnigan MAT262 mass spectrometer in dynamic mode using the ion counting detector. 207Pb/ 206Pb ratios were corrected by a mass discrimination factor of 0.12% (0.03 determined by repeated analyses of NBS-982 Pb standard. Analyses with 206 Pb/ 204Pb ratios lower than 2500 were eliminated. Common lead corrections were done using the Stacey and Kramer's (1975) model. Weighted mean and errors on the ages were calculated following Gaudette et al. (1998). The ages are presented at the 2 sigma level (Table 4). Pb±Pb evaporation ages older than 3.02 Ga were con®rmed in eight zircon crystals from sample 1. The analyses show that the ages increase with temperature, and thus only the highest heating steps are considered for the age calculation of each crystal. Ages range between 3290 and 3025 Ma and represent either different generations of zircon growth or different degrees of discordance of older zircons. The 207Pb/ 206Pb evaporation values are within the range of the ages obtained by SHRIMP and indicate a complex history for the metatonalite evolution. Values at about 3.10 Ga may represent the age of a ®rst metamorphic episode as suggested by the SHRIMP II results, but no evidence of a second metamorphic event at ca. 2.7 Ga was found in the analyzed crystals. The 208Pb/ 206Pb ratios are between 0.009 and 0.178, which correspond roughly to the observed values for the zircons dated by SHRIMP. No 208 Pb/ 206Pb values of about 0.28±0.32 were obtained by Pbevaporation analyses, in good agreement with the lack of Pb-evaporation ages at about 3.4 Ga. The hypothesis that the high 208Pb/ 206Pb and corresponding high Th/U ratios are related to the oldest 3.4 Ga magmatic zircons is therefore reinforced. The Pb±Pb evaporation method relies on simultaneous evaporation of both cores and rims of zircon crystals, therefore leading to intermediate ages between the magmatic 3.4 Ga and the metamorphic 3.1 Ga ages, but closer to the metamorphic ages because of predominance of the largest volume and highest U contents of the metamorphic rims. References Barley, M.E., Pickard, A.L., 1999. An extensive, crustally-derived, 3325± 3310 Ma silicic volcanoplutonic suite in the eastern Pilbara Craton: evidence from the Kelly Belt, McPhee Dome and Corunna Downs Batholith. Precambrian Research 96, 41±62. Basei, M.A.S., Siga Jr, O., Masquelin, H., Harara, O.M., Reis Neto, J.M., Preciozzi Porta, F., 2000. The Dom Feliciano Belt and the Rio de la

569

Plata Craton: tectonic evolution and correlation with similar provinces of southwestern Africa. In: Cordani, U.G., Milani, E.J., Thomaz Filho, A., Campos, D.A. (Eds.). Tectonic Evolution of South America. 31st International Geological Congress, Rio de Janeiro, pp. 311±334. Bossi, J., Campal, N., Civetta, L., Demarchi, G., Girardi, V.A.V., Mazzucchelli, M., Negrini, L., Rivalenti, G., Fragoso Cesar, A.R.S., Sinigoi, S., Teixeira, W., Piccirillo, E.M., Molesini, M., 1993a. Early Proterozoic dike swarms from western Uruguay: geochemistry, Sr±Nd isotopes and petrogenesis. Chemical Geology 106, 263±277. Bossi, J., Preciozzi, F., Campal, N., 1993b. Predevoniano en el Uruguay. Terreno Piedra Alta, vol. 1. Instituto del Libro, Montevideo, Uruguay 92pp. Bossi, J., Ferrando, L.A., MontanÄa, J., Campal, N., Morales, H., Gancio, F., Schipilov, A., Sprechman, P., PinÄeyro, D., 1998. Carta Geologica del Uruguay a escala 1/500.000. Geoeditores SRL, Montevideo. Bowring, S.A., Williams, I.S., 1999. Priscoan (4.00±4.03) orthogneisses from northwestern Canada. Contributions to Mineralogy and Petrology 134, 3±16. Brito Neves, B.B., 1999. AmeÂrica do Sul: Quatro fusoÄes, quatro ®ssoÄes e o processo acrescionaÂrio andino. Revista Brasileira de GeocieÃncias 29, 379±392. Brito Neves, B.B., Campos Neto, M.C., Fuck, R.A., 1999. From Rodinia to Western Gondwana: an approach to the Brasiliano-Pan African Cycle and orogenic collage. Episodes 22, 55±166. Campal, N., Schipilov, A., 1999. The eastern edge of the Rio de la Plata craton: a history of tangential tectonics. In: Sinha, A.K. (Ed.). Basement Tectonics. Kluwer, Amsterdam, pp. 33±48. Cordani, U.G., Sato, K., 1999. Crustal evolution of the South American Platform, based on Nd isotopic systematics on granitoid rocks. Episodes 22, 67±173. Cordani, U.G., Sato, K., Teixeira, W., Tassinari, C.C.G., Basei, M.A.S., 2000. Crustal evolution of the South American Platform. In: Cordani, U.G., Milani, E.J., Thomaz Filho, A., Campos, D.A. (Eds.). Tectonic Evolution of South America. 31st International Geological Congress, Rio de Janeiro, pp. 19±40. Dantas, E.L., Hackspacher, P.C., Van Schmus, W.R., Brito Neves, B.B., 1998. Archean accretion in the SaÄo Jose do Campestre massif, Borborema Province, Northeast Brazil. Revista Brasileira de GeocieÃncias 28, 221±228. Friend, C.R.L., Kinny, P.D., 1995. New evidence for protolith ages of Lewisian granulites, northwest Scotland. Geology 23, 1027±1030. Gaucher, C., Sprechmann, P., Schipilov, A., 1996. Upper and Middle Proterozoic fossiliferous sedimentary sequences of the Nico PeÂrez Terrane of Uruguay: lithostratigraphic units, paleontology, depositional environments and correlations. Neues Jahrbuch fuÈr Geologie und Paleontologie 199, 339±367. Gaucher, C., Sprechmann, P., MontanÄa, J., 1998. New advances on the geology and paleontology of the Vendian to Cambrian Arroyo del Soldado Group of the Nico PeÂrez Terrane of Uruguay. Neues Jahrbuch fuÈr Geology und PalaÈeontologie, Monatshefte 2, 106±118. Gaudette, H.E., Lafon, J.M., Macambira, M.J.B., Moura, C.A.V., Scheller, T., 1998. Comparison of single ®lament Pb evaporation/ionization zircon ages with conventional U±Pb results: Examples from the Precambrian of Brazil. Journal of South American Earth Sciences 11, 351±363. Hartmann, L.A., Leite, J.A.D., McNaughton, N.J., Santos, J.O.S., 1999. Deepest exposed crust of Brazil Ð SHRIMP establishes three events. Geology 27, 947±950. Hartmann, L.A., PinÄeyro, D., Bossi, J., Leite, J.A.D., McNaughton, N.J., 2000a. Zircon U±Pb SHRIMP dating of Palaeoproterozoic Isla Mala granitic magmatism in the Rio de la Plata Craton, Uruguay. Journal of South American Earth Sciences 13, 105±113. Hartmann, L.A., Leite, J.A.D., Silva, L.C., Remus, M.V.D., McNaughton, N.J., Groves, D.I., Fletcher, I.R., Santos, J.O.S., Vasconcellos, M.A.Z., 2000b. Advances in SHRIMP geochronology and their impact on understanding the tectonic and metallogenic evolution of southern Brazil. Australian Journal of Earth Sciences 47, 829±844.

570

L.A. Hartmann et al. / Journal of South American Earth Sciences 14 (2001) 557±570

Kober, B., 1987. Single-grain evaporation combined with Pb 1 emitter bedding for 207Pb/ 206Pb-age investigations using thermal ion mass spectrometry, and implications for zirconology. Contributions to Mineralogy and Petrology 96, 63±71. Logan, B.W., Rezak, R., Tappan, H., 1964. Classi®cation and environmental signi®cance of algal stromatolites. Journal of Geology 72, 62±83. Lumbers, S.B., Card, K.D., 1991. Chronometric subdivision of the Archean. Geology 19, 958±959. Macambira, M.J.B., Lafon, J.-M., Pidgeon, R., 1998. Crescimento crustal arqueano registrado em zircoÄes de sedimentos da regiaÄo de Rio Maria, ProvõÂncia CarajaÂs, ParaÂ. In: XL Brazilian Geological Congress, Belo Horizonte, pp. 55. Machado, N., Schrank, A., Noce, C.M., Gauthier, G., 1996. Ages of detrital zircon from Archean±Paleoproterozoic sequences: Implications for greenstone belt setting and evolution of a Transamazonian foreland basin in QuadrilaÂtero FerrõÂfero, SE Brazil. Earth and Planetary Science Letters 141, 249±276. Martin, H., Peucat, J.J., SabateÂ, P., Cunha, J.C., 1997. Crustal evolution in the early Archean of South America: example of the Sete Voltas Massif, Bahia State, Brazil. Precambrian Research 82, 35±62. Nutman, A.P., Cordani, U.G., 1993. SHRIMP U±Pb zircon geochronology of Archean granitoids from the Contendas±Mirante area of the SaÄo Francisco Craton, Bahia, Brazil. Precambrian Research 63, 179±188. Nutman, A.P., Bennett, V.C., Friend, C.R.L., Norman, M.D., 1999. Metaigneous (non-gneissic) tonalites and quartz-diorites from an extensive ca. 3800 Ma terrain south of the Isua supracrustal belt, southern West Greenland: Constraints on early crust formation. Contributions to Mineralogy and Petrology 137, 364±388. Preciozzi, F., Masquelin, H., Basei, M.A.S., 1999. The Namaqua/Grenville Terrane of eastern Uruguay. In: Proceedings of South American Symposium on Isotope Geology, 2nd, Cordoba, Servicio Geologico Minero Argentino, pp. 338±340. Remus, M.V.D., McNaughton, N.J., Hartmann, L.A., Koppe, J.C., Fletcher, I.R., Groves, D.I., Pinto, V.M., 1999. Gold in the Neoproterozoic juvenile Bossoroca volcanic arc of southernmost Brazil: isotopic constraints on timing and sources. Journal of South American Earth Sciences 12, 349±366. Rivalenti, G., Mazzucchelli, M., Molesini, M., Petrini, R., Girardi, V.A.V., Bossi, J., Campal, N., 1995. Petrology of the Late Proterozoic ma®c

dikes in the Nico PeÂrez region, central Uruguay. Mineralogy and Petrology 55, 239±263. Rogers, J.J.W., 1996. A history of continents in the past three billion years. Journal of Geology 104, 91±107. Rolando, A.P., Hartmann, L.A., Macambira, M.B., Santos, J.O.S., Fernandez, R., Etcheverry, R., Schalamuk, I.A., McNaughton, N.J., 1999. Isotopic constraints on the Pb, Zn, Cu, Au metallogeny and Andean crustal evolution in the Lago Fontana region, Patagonia. In: Proceedings of South American Symposium on Isotope Geology, 2nd, Cordoba, Servicio Geologico Minero Argentino, pp. 491±494. Silva da, L.C., Hartmann, L.A., McNaughton, N.J., Fletcher, I.R., 2000. Zircon U±Pb SHRIMP dating of a Neoproterozoic overprint in Paleoproterozoic granitic-gneissic terranes, southern Brazil. American Mineralogist 85, 649±667. Stacey, J.S., Kramers, J.D., 1975. Approximation of terrestrial lead isotope evolution by a two-stage model. Earth and Planetary Science Letters 26, 207±221. Teixeira, W., Cordani, U., Nutman, A.P., Sato, K., 1998. Polyphase Archean evolution in the Campo Belo metamorphic complex, Southern SaÄo Francisco Craton, Brazil: SHRIMP U±Pb zircon evidence. Journal of South American Earth Sciences 11, 279±289. Teixeira, W., Renne, P.R., Bossi, J., Campal, N., D'Agrella Filho, M.S., 1999. 40 Ar± 40Ar and Rb±Sr geochronology of the Uruguayan dike swarm, Rio de la Plata Craton and implications for Proterozoic intraplate activity in western Gondwana. Precambrian Research 93, 153±180. Unrug, R., 1997. Rodinia to Gondwana: the geodynamic map of Gondwana Supercontinent assembly. GSA Today 7, 1±6. Vavra, G., Gebauer, D., Schmid, R., Compston, W., 1996. Multiple zircon growth and recrystallization during polyphase Late Carboniferous to Triassic metamorphism in granulites of the Ivrea Zone (Southern Alps): an ion micriprobe (SHRIMP) study. Contributions to Mineralogy and Petrology 122, 337±358. Vavra, G., Schmid, R., Gebauer, D., 1999. Internal morphology, habit and U±Th±Pb microanalysis of amphibolite-to-granulite facies zircons: geochronology of the Ivrea Zone (Southern Alps). Contributions to Mineralogy and Petrology 134, 380±404. Watt, G.R., Grif®n, B.J., Kinny, P.D., 2000. Charge contrast imaging of geological materials in the environmental scanning electron microscope. American Mineralogist 85, 1784±1794.