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Journal of South American Earth Sciences 31 (2011) 345e357

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Journal of South American Earth Sciences

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The tectonic evolution of the Neoproterozoic Brasília Belt, central Brazil, basedon SHRIMP and LA-ICPMS UePb sedimentary provenance data: A review

Márcio M. Pimentel a,*, Joseneusa B. Rodrigues b, Maria Emilia S. DellaGiustina c, Sergio Junges c,Massimo Matteini c, Richard Armstrong d

a Instituto de Geociências, Universidade Federal do Rio Grande do Sul, CP 15001, 91501-970 Porto Alegre, BrazilbCPRM e Geological Survey of Brazil, Brasília, Brazilc Instituto de Geociências, Universidade de Brasília, Brasilia 70910-900, BrazildResearch School of Earth Sciences, Australian National University, Canberra, Australia

a r t i c l e i n f o

Article history:Received 15 September 2010Accepted 22 February 2011

Keywords:Brasilia BeltSediment provenanceZircon UePbTectonic evolution

* Corresponding author. Tel.: þ55 51 99211861.E-mail address: marcio.pimentel@ufrgs.br (M.M. P

0895-9811/$ e see front matter � 2011 Elsevier Ltd.doi:10.1016/j.jsames.2011.02.011

a b s t r a c t

The Brasília Belt is a Neoproterozoic orogenic belt in central Brazil, developed between the Amazon, SãoFrancisco-Congo and Paranapanema cratons. It consists of a thick sedimentary pile, made up of severalstratigraphic units, which have been deformed and metamorphosed along the western margin of the SãoFrancisco Craton during the Brasiliano orogenic cycle. In the western part of the belt, a large, juvenilemagmatic arc is exposed (the Goiás Magmatic Arc), consisting of calc-alkaline plutonic suites as well asvolcano-sedimentary sequences, ranging in age between ca. 860 and 650 Ma. Regional-scale, west-dipping thrusts and reverse faults normally mark the limits between the main stratigraphic units, andclearly indicate tectonic transport towards the east. The age of deposition and tectonic significance of thesedimentary units comprising the Brasília Belt have been a matter of continuous debate over the lastthree decades. In the present paper, recent provenance data based on LA-ICPMS UePb ages of detritalzircon grains from several of these units, are reviewed and their significance for the age of deposition ofthe original sediments and tectonic evolution of the Brasília Belt are discussed.

The Paranoá, Canastra and the Vazante groups, in the central part of the Belt, have detrital zircongrains with ages older than ca. 900 Ma and are interpreted as representative of the passive marginsequence deposited on the western margin of the São Francisco Craton. On the other hand, samples fromthe Araxá and Ibiá groups have a much younger population of Neoproterozoic zircon grains, as young as650 Ma, and have been interpreted as syn-orogenic (fore-arc?) deposits. The Bambuí Group, exposed inthe easternmost part of the belt and covering large areas of the São Francisco Craton also has youngzircon grains and is interpreted, at least in part, as the foreland basin of the Brasília Belt.

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1. Introduction

The Brasília Belt is a large and well preserved Neoproterozoicorogenic belt in central Brazil formed by the convergence betweenthe Amazon, São Francisco-Congo and Paranapanema paleo-continents. It consists mainly of: (i) a thick sequence of metasedi-mentary and sedimentary rocks, in its eastern section, depositedand deformed along thewesternmargin of the São Francisco-CongoCraton (SFCC), (ii) the Goiás Massif, interpreted as an allochthonoussialic block consisting mainly of the Archaean terrains of the Goiás-Crixás area, (iii) the metamorphic core of the orogen, known asthe Anápolis-Itauçu granulite complex; very similar rock associa-tions are exposed in the so-called Uruaçu Complex, to the north

imentel).

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(DellaGiustina et al., 2009), and (iv) a large exposure of Neopro-terozoic juvenile volcanic/plutonic associations, the GoiásMagmatic Arc (Pimentel and Fuck, 1992) (Fig. 1). The supracrustalassociations in the southern part of the belt aremarked by regional-scale thrust sheets presenting eastward vergence, towards the SãoFrancisco-Congo platform. Another important structural feature ofthe Brasília Belt is the extensive NNE-trending strike-slip shear zonesystem roughlymarking the limits between the Brasilia Belt and theAraguaia and Paraguay fold belts, to the west. These linear featuresare known as the Transbrasiliano Lineaments.

Of great relevance to understand the tectonic evolution of theBrasília Belt, as well as the provenance patterns of the sedimentaryrockunits is theGoiásMagmatic Arc. This underlies a large area alongthe western part of the orogen and consists mainly of: (i) calc-alka-line metaplutonic suite made of tonalites-granodiorites recordingtwo main periods of intrusion (860e800 Ma and 660e610 Ma),

Fig. 1. Main tectonic features and components of the Brasília Belt.

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(ii) metavolcano-sedimentary sequences of calc-alkaline nature,ranging in age from ca. 900 Ma to 620 Ma, (iii) post-tectonic graniteintrusions forming large bimodal complexes, emplaced between ca.600 and 500Ma.Most of these rocks present juvenile character witha strong mantle input, as indicated by the positive eNd(T) values andTDM model ages mostly between 0.9 and 1.2 Ga (Pimentel and Fuck,1992).

The sedimentary rock units of the Brasília Belt are grouped intoseven main stratigraphic units: the Paranoá, Canastra, Ibiá, Araxá,Vazante, Serra da Mesa and Bambuí groups (for a review seeValeriano et al., 2008 and Sial et al., 2009) (Fig. 2). In most previousmodels these sequences, except for the Bambuí Group, have beeninterpreted as passive margin deposits of the São Francisco paleo-continent (e.g.Trompette, 1994; Valeriano et al., 2004, 2008; andreferences therein), although the provenance work carried out byPimentel et al. (2001) using the SmeNd isotopic systematics, andmore recently by Rodrigues et al. (2010), have proposed an alter-native view, according to which several of these units are syn-orogenic (fore-arc) deposits and the Bambuí Group representsa foreland basin.

The depositional age and tectonic setting of these supracrustalunits have been a matter of debate for decades. The intense defor-mation and tectonic transport, especially in the southern part of thebelt, the absence of fossils and of interlayered volcanic rocks havebeen important obstacles for the determination of accurategeochronological data and of the original tectonic setting of depo-sition. In the present study we review provenance data for sedi-mentary rocks of the Brasília Belt, especially recent data obtained bySHRIMP and LA-ICPMS in order to help understanding the tectonicevolution of the belt and to constrain the depositional age of the

original sediments. Data for the Paranoá, Canastra, Vazante, Ibiá,Araxá and Bambuí Groups, as well as for exposures of high grademetasediments of the Anápolis-Itauçu and Uruaçu complexes arereviewed and their significance for models of tectonic evolution ofthe belt is discussed.

2. Geology of the supracrustal units of the Brasília Belt

The supracrustal part of the Brasília Belt is made of severallithostratigraphic units, which are mostly separated from eachother by regional thrusts and reverse faults (for a review seeDardenne, 2000; Valeriano et al., 2008 and Sial et al., 2009). Theseunits are the Paranoá, Canastra, Vazante, Ibiá, Araxá, Serra da Mesaand Bambuí groups (Fig. 2). The Jequitaí and Cubatão diamictites,exposed just below the Bambuí and Ibiá groups, respectively, havebeen investigated in previous studies and will also be discussedhere. Also, the significance of the high grade metasedimentaryrocks (sillimanite-garnet gneisses) exposed in the Anápolis-Itauçuand Uruaçu complexes will be re-assessed (Piuzana et al., 2003a;DellaGiustina et al., 2009).

In the sections below we present the geological/stratigraphiccharacteristics of these sedimentary/metasedimentary rock units.

2.1. Paranoá Group

In the Brasília area, the Paranoá Group is over 1400 m thick andits stratigraphic organization has been studied in detail by Faria(1995). These sedimentary rocks comprise a mature siliciclasticsedimentary pile including thick quartzite layers, with intercalationof metasiltstones and minor lenses of limestones and dolostones.

Fig. 2. Simplified geological map of the Brasília Belt (after Dardenne, 2000).

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This unit has been divided into nine lithostratigraphic units,beginning with a paraconglomerate, followed by transgressive andregressive siliciclastic dominated cycles, ending with pelites anddolostones containing Conophyton metulum Kirichenko stromato-lites (Cloud and Dardenne, 1973). Available geochronological andmicrofossil data for the Paranoá Group point to an age between ca.1170e950 Ma and a source region in the Paleoproterozic sialicbasement of this craton as suggested by the direction of paleo-currents (Guimarães, 1977; Pimentel et al., 2001). The 50-m-thickSãoMiguel paraconglomeratemarks the base of the Paranoá Group,lying on an erosive unconformity over the 1.77 Ga old Araí Group,and marking the rift phase of the Paranoá basin. The conglomerateis overlain by rhythmites with mudcracks and evaporite layers,which are typical of tidal to supratidal environments. These arefollowed by marine rhythmites and quartzites deposited in

a platformal environment dominated by tidal currents. The sedi-ments in the upper portion of the Paranoá Group display featuresindicating more varied environments, reflecting important fluctu-ations of the sea level. In this section, deeper water pelites alternatewith tidal rhythmites and quartzites, storm rhythmites, limestonesand stromatolitic dolomites. Arkoses and quartzites at the upperpart of the Paranoá sequence are chemically similar to passivemargin clastic sediments (Guimarães, 1997).

2.2. Canastra Group

This stratigraphic unit comprises a typical platformal associa-tion of psammitic and pelitic metasediments, with some carbonaticintercalations. Phyllites and quartzites are the most common rocks.The basal part of the group (Serra do Landim Formation) is made

Fig. 3. Summarized stratigraphic column of the Bambuí Group (from Sial et al., 2009).

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of chlorite-rich calc-phyllite or calcschist. Towards the top, theParacatu and the Chapada dos Pilões formations are formed bycarbonaceous phyllites and quartzites, representing a coarsening-upward sequence formed by a regressive megacycle. This is madeof deep water sediments, grading into turbidites deposited ona continental slope by gravitational currents, and to top platformaldeposits with hummocky structures and cross stratification, indi-cating sediment transport from east to west. The Canastra Group ismore commonly considered to be the lateral equivalent of theParanoá Group (Dardenne, 2000). The relationships between theCanastra, Araxá, and Ibiá groups are not clear due to intensetectonic imbrication between these units (Fig. 2).

2.3. Araxá Group

This is dominantly made of micaceous quartzites and micas-chists including calcschists, chlorite � muscovite schists, biotitegarnet schists, staurolite schists, and feldspathic schists, with a fewparagneiss and marble intercalations. The internal stratigraphy ofthe group is poorly known due to intense deformation withdevelopment of low angle thrust sheets. Volcanic rocks associatedwith the Araxá micaschists are observed in many areas. These areamphibolites, meta-andesites and rhyolites (Pimentel et al., 1992;Valeriano and Simões, 1997). Peraluminous granites dated bet-ween ca. 720 and 640 Ma are also common in some areas. In theeastern part of the Araxá Group, the ca. 0.79 Ga Maratá Sequencerepresents a narrow strip of rhyolitic/granitic rocks interlayeredwith Araxá metasediments. This ca.100 km long narrow unit wasinitially interpreted as volcanic (e.g. Pimentel et al., 1992) and laterinterpreted as highly deformed syn-tectonic intrusions. In the latterinterpretation, therefore, the UePb zircon age of 0.79 Ga (Pimentelet al., 1992) would represent a minimum age for the deposition ofthe Araxá sediments. More recently, amphibolites intercalatedwiththe Araxá schists near Goiânia have been dated at ca. 0.8 Ga(SHRIMP UePb zircon data of Piuzana et al., 2003b). The traceelement characteristics of these amphibolites are, however, verysimilar to MORBs and they have been interpreted as slices of oceanfloor tectonically emplaced within the Araxá metasediments. Insummary the stratigraphic relationships between these metaig-neous rocks and the Araxá metasediments are still far from clearand remain controversial due to the tectonic imbrications comm-only observed within the Araxá Group. Another important featureassociatedwith the Araxá rocks is the presence of a large number ofsmall lenses of serpentinites, amphibolites and talc schists, locallywith podiform chromite deposits, representing a long, roughly NeSophiolitic mélange (Strieder and Nilson, 1992).

2.4. Ibiá Group

The Ibiá Group has been traditionally divided into two forma-tions: (i) the Cubatão Formation, at the base, which lies on anerosive unconformity over the Canastra Group and consists ofa thick diamictite unit, and (ii) the Rio Verde Formation, at the top,which is made of deep water calcschists and calc-phyllites with finelayers of quartzite.

The Cubatão diamictite has been correlated with the JequitaíDiamictite, while the rhythmic phyllites are considered to beequivalent to the metasediments of the Araxá Group. Geochemicalfeatures of the phyllites in the southern part of the belt indicateprovenance from a primitive island arc source (Seer, 1999).

2.5. Vazante Group

This group consists of a thick marine pelite-carbonate sequenceexposed in a ca. 200 km NeS narrow strip roughly between the

towns of Unaí and Vazante, in tectonic contacts with the CanastraGroup in the west, and with the Bambuí Group, in the east. It hasbeen divided into seven formations; from base to top these are theRetiro, Rocinha, Lagamar, Serra do Garrote, Serra do Poço Verde,MorrodoCalcário andLapa formations. ThebasalRetiro Formation ismadeof quartzite, conglomerate, diamictite and slate. Thediamictiterepresents debris flows deposited in glacio-marine environment, inrelatively deep waters, under the influence of gravitational currents(Dardenne, 2000). It grades upwards to the Rocinha Formation,a rhythmic sandyandpelitic sequence. At the top it consists of a thicksequence of slate and metasiltstone which grades upwards tocarbonate- and pyrite-bearing shales with fine phosphatic layers,which locally form important phosphate deposits (e.g. Rocinha andLagamar deposits). The Lagamar Formation starts at the base witha psamo-pelitic unit with conglomerate, quartzite, metasiltstone,and slate, followed by dolomitic breccia and stromatolitic dolomite.The Serra do Garrote Formation is formedmainly by slates with finequartzite intercalations. The Serra do Poço Verde and Morro doCalcário formations are dominantly carbonatic with oolitic andoncolitic dolarenite facies. The upper Lapa Formation is formed bycarbonaceous phyllite, carbonatic metasiltstone, dolomite lensesand quartzite layers.

The age of the Vazante Group is controversial. The Conophyton-type stromatolites indicate a very broad time interval (ca. 1.35e0.9Ga; Cloud and Dardenne, 1973), suggesting correlation with theParanoá Group. On the other hand, the occurrence of diamictites inthe base of the sequence, very similar to those found in the JequitaíFormation, favours a correlation with the Bambuí Group. An extradiamictite layer is also exposed in the upper formation of thegroup.C- and Sr-isotope data for the upper Lapa Formation sug-gested correlation of this group with the Rasthof Formation in

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Namibia, which is constrained to be younger than ca. 750Ma (Azmyet al., 2006). New ReeOs isotope data from the shales associatedwith the upper diamictites of the Vazante Group yield radiometricages between 993� 46 and 1100� 77Ma (Azmy et al., 2008) whichhave been interpreted as representative of the depositional age ofthe sequence.

2.6. High grade metasediments e Anápolis Itauçu and Uruaçucomplexes

High grade metasediments (including UHT granulites) areexposed in two main areas within the Brasilia Belt: the largeAnápolis-Itauçu Complex, in the southern part of the belt, and theUruaçu Complex, a small exposure of high grade supracrustal rockswithin lowgrademetasediments of the Serra daMesaGroup (Fig. 2).

In both cases, the supracrustal association includes sillimanite-garnet gneiss, garnet quartzite, and minor marble, associated withamphibolites layers and intruded by a number of Neoproterozoicgranites (Piuzana et al., 2003a; DellaGiustina et al., 2009). They havebeen traditionally interpreted as exposure of old continental base-ment to the Brasília Belt metasediments, however detrital zircondata presented by Piuzana et al. (2003a) and by DellaGiustina et al.(2009) have demonstrated that they were deposited during theNeoproterozoic and represent high grade equivalents of the BrasíliaBelt sedimentary rocks. Metamorphic zircon ages of ca. 650 Mareported by Piuzana et al. (2003a) have been interpreted as repre-sentative of the main metamorphic phase of the orogen, possiblyrelated to continental collision.

Fig. 4. Distribution of TDM values for different sedimentary units of the Brasília Belt (datSubmitted for publication).

2.7. Bambuí Group and Jequitaí Diamictite

The Bambuí Group is the most important Neoproterozoic sedi-mentary unit in central Brazil occupying all the eastern side of theBrasilia Belt and covering large areas of the São Francisco Craton.The general stratigraphic column of this group is shown in Fig. 3.It overlies the Paranoá Group from which it is separated by anunconformity marked by glacial diamictites of the Jequitaí Forma-tion. The age of this glacial event remains a matter of controversy.This diamictite consists of clast-supported rocks (quartzite, granite,gneiss, limestone, dolostone and siltstone) with clay-rich greenish-grey matrix, and minor siltstone and sandstone lenses. This unit isexposed in discontinuous outcrops in central Brazil and overliesboth the Paranoá rocks and the sialic basement (Karfunkel andHoppe, 1988; Uhlein et al., 1999) and represents the expression ofa glacial episode that occurred in a wide area of the São Franciscocraton and Brasília Belt.

The end of this glaciation was followed by the necessary con-ditions for the development of a marine environment and thebeginning of the deposition of the pelitic-carbonatic sediments ofthe base of the Bambui Group (Misi and Kyle, 1994). According toSantos et al. (2000), the sediments of the Bambuí Group weredeposited on an epicontinental sea influenced by tectonic move-ments in the Brasília Belt and by restricted marine conditions. TheBambuí sediments comprise three regressive megacycles. Each ofthese megacycles begins with a fast marine transgression of reg-ional amplitude, associated with a sudden subsidence of the basinevidenced by deep pelitic marine facies, passing to shallow-plat-form facies and tidal to supratidal facies.

a compiled from Pimentel et al., 2001; Valeriano et al., 2004; Rodrigues et al., 2010,

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From base to top, these megacycles are:Megacycle I is pelitic-carbonatic, corresponding to the top of the

the Sete Lagoas Formation (Fig. 3), showing a coarsening-upwardsequence with dark grey to black calcilutites in the basal portionpassing to limestones and dolostones at the top. Vieira et al. (2007)have interpreted this change in lithological association as an im-portant limit of depositional sequences within the Sete LagoasFormation.

Megacycle II is pelitic-carbonatic, formed by the Serra de SantaHelena Formation, essentially pelitic, indicating a sudden andgeneralized subsidence of the basin, followed by the Lagoa doJacaré Formation, characterized by the sedimentation of dark greyplatformal limestones deposited in an environment dominated bystorms and tidal currents.

Megacycle III is pelitic-sandy, represented by the Serra da Sau-dade Formation, which is a pelitic sequence deposited in deepplatform environment with episodic influence of storms and bythe Três Marias Formation, predominantly arkosian, deposited inshallow-platform environment dominated by storm currents withepisodic tidal to supratidal facies.

The age of sedimentation of the Bambuí Group has beena matter of discussion and controversy for many years. Sr and Pbisotopic data reported by Parenti-Couto et al. (1981) suggested

Fig. 5. SmeNd provenance patterns for the metasediments of the Goiás Magmatic Arc,Bambuí and Paranoá groups (data from Pimentel et al., 2001; Dantas et al., 2001;Junges et al., 2002; Laux et al., 2005).

depositional ages of ca. 600Ma. Similarly, previous RbeSr and KeArdata on shales reported by Thomaz Filho et al. (1998) haveproduced ages between ca. 640 Ma for the Sete Lagoas Formationand 540 Ma for the Três Marias Formation.

More recently,the cap dolostones at the base of the Sete Lagoasthat overlies Jequitaí diamictites have been dated at ca 740 Ma(PbePb isochron on carbonates; Babinsky et al., 2007). This data,associated with C and Sr isotopic data for the carbonates supporta Sturtian age for the Jequitaí glaciation.

3. Sediment provenance

3.1. SmeNd isotopes

During the last ten years, several studies have focused on theinvestigation of the provenance of detrital sediments of theBrasília Belt, in order to assess information on their maximumdepositional ages and on their tectonic setting of theirformation.

The earlier provenance studies (e.g. Pimentel et al., 2001; Dantaset al., 2001) used the SmeNd isotopic systematics of fine-grainedsedimentary and metasedimentary rocks as a first-order indicatorof the nature of their continental sources. This was followed bya number of other studies involving SHRIMP and LA-ICPMS UePb

Fig. 6. Detrital zircon provenance patterns for high grade metasediments of theBrasília Belt (Piuzana et al., 2003a and DellaGiustina et al., 2009).

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geochronology of detrital zircon (e.g. Pimentel et al., 2002; Piuzanaet al., 2003b; Valeriano et al., 2004; Matteini et al., 2008; Rodriguesand Pimentel, 2008; Rodrigues et al., 2008, 2010, Submitted ForPublication).

The study by Pimentel et al. (2001) revealed important differ-ences between the provenance patterns of the original sedimentsof the several geological units of the Brasília Belt. For instance, thesedimentary rocks of the Paranoá and Canastra groups have TDMmodel ages ranging between ca. 2.0 and 2.5 Ga, which is compatiblewith sources within the Paleoproterozoic terrains of the São Fran-cisco-Congo craton (Fig. 4). These sequences were, therefore, on thebasis of SmeNd provenance patterns, interpreted as representativeof passive margin associations of the Brasília Belt. On the otherhand, the fine-grained sedimentary rocks of the Ibiá and Araxágroups, in the southern part of the Brasília Belt showed a clearbimodal character with one mode presenting model ages betweenca. 0.9 Ga and 1.4 Ga, and the other one with TDM values between1.7 Ga and 2.1 Ga. (Fig. 4) This clearly indicates the participation ofmuch younger sources in the origin of these sediments. Pimentelet al. (2001) interpreted that this young source might be

Fig. 7. Provenance patterns of the Paracatu and Chapada dos Pilões fo

represented by the Neoproterozoic magmatic arc whosemetaigneous rocks have TDM model ages typically between 0.9 and1.2 Ga (see Pimentel and Fuck, 1992; Dantas et al., 2001; Jungeset al., 2002; Laux et al., 2005). (Fig. 5).

Noteworthy in the SmeNd provenance patterns of the BrasíliaBelt are:

(i) Two main source areas seem to have contributed to the orig-inal sedimentary basins, an older one (Paleoproterozoic/Archean), most probably within the São Francisco Craton, anda younger (Neoproterozoic), represented by the Goiás Mag-matic Arc, to the west;

(ii) The provenance pattern of the Bambuí Group sediments, withTDM model ages between ca. 1.4 and 1.9 Ga is intermediatebetween the two main sources above, indicating that itrepresents a mixture of the different sources that providedsediments to the different basins constituting the Brasília Belt(Fig. 5);

(iii) The clear difference between the provenance patterns of theBambuí and Paranoá groups (Fig. 5).

rmations of the Canastra Group (data for Rodrigues et al., 2010).

Fig. 8. Provenance patterns for the Ibiá (ICPMS) and Araxá (SHRIMP) groups (datafrom Rodrigues et al., 2010).

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3.2. SHRIMP and LA-ICPMS data

In this section we review some of the published UePb zircondata for sediments and metasediments of the Brasília Belt andcompare them with the SmeNd isotopic data.

3.2.1. High grade metasedimentsSamples of garnet (�sillimanite) gneisses from both high grade

terrains in the central part of the Brasília Belt have been investi-gated by Piuzana et al. (2003a) and DellaGiustina et al. (2009) usingthe SHRIMP and LA-ICPMS methods, respectively. In both cases thedetrital zircon provenance patterns display very important contri-butions from Neoproterozoic (900e640 Ma) with minor inputsfrom Paleoproterozoic sources (Fig. 6) suggesting source areasdominantly within the Goiás Magmatic Arc.

3.2.2. Paranoá and Canastra GroupsThe LA-ICPMS UePb data for detrital zircon grains from the

Canastra Group (Fig. 7) show important contribution from Meso-and Paleoproterozoic sources for the original sedimentary rocks. Aphyllite sample from the basal Serra do Landim Formation hasa dominant Paleoproterozoic population, with ages rangingbetween 2.07 and 2.25 Ga,. Some zircon grains have Mesoproter-ozoic ages and the youngest concordant grain has the age of1079 � 45 Ma (Rodrigues et al., 2010). Quartzite samples of theParacatu Formation have a larger proportion of Mesoproterozoicgrains, compared with the basal formation, whereas in the upperChapada dos Pilões Formation the Mesoproterozoic group becomesdominant again. The youngest age peak observed for these rocks isca. 1040 Ma, which represents, therefore the upper age limit fordeposition of the Canastra Group.

The provenance of the Paranoá Group has been investigatedby Matteini et al. (2010). These authors investigated six samplesof the different lithofacies of the Paranoá Group and identified atleast five main group of detrital zircon grains. The Paleoproter-ozoic age population, with ages ranging from ca. 2.0 to 2.2 Ga isby far the most important and is ubiquitous in all the analyzedsamples. A mesoproterozoic population at w1.57 Ga is wellregistered in some samples and a Late Paleoproterozic populationat w1.78e1.80 Ga is also evident. A less abundant neoarcheanpopulation are represented in most samples. Finally, a scatteredPaleoarchean population is represented only in some of thesamples representing the upper part of the Paranoá Group(Matteini et al., 2010).

The late Mesoproterozoic population which is evident in theCanastra Group is not seen in the Paranoá rocks. Although thesource of these Mesoproterozoic zircon grains remain unclear,the detrital zircon age distributions of the Canastra and Paranoásedimentary rocks are compatible with erosion of sources withinthe São Francisco-Congo Craton, and deposition in a passive marginsetting.

3.2.3. Araxá and Ibiá GroupsProvenance of samples from the Araxá and Ibiá samples have

been investigated in the Anápolis and Vazante areas, respectively,in the central part of the Brasília Belt (Piuzana et al., 2003b;Rodrigues et al., 2010) and in the southern area of belt, in theAraxá nappe (Valeriano et al., 2004). Araxá and Ibiá samples fromthe central part of the belt have a clear dominance of detritalmaterial derived from Neoproterozoic sources contrasting withsediments from the Canastra and Paranoá groups, discussed above(Fig. 8). The presence of an important populationwith ages rangingfrom ca. 640 to 900 Ma suggests that the juvenile rocks from theGoiás Magmatic Arc, to the west, represent the main sources of theoriginal sediments.

This is compatible with the SmeNd isotopic characteristics ofthese rocks as demonstrated by Pimentel et al. (2001), indicatingthat the original sediments are not deep water equivalents ofthe Canastra Group or Paranoá groups, as suggested in previousmodels. The abundant presence of Neoproterozoic zircon grains,as young as ca. 630 Ma, demonstrate that these units weredeposited in syn-orogenic basins, most probably in a fore-arcsetting.

The data reported by Valeriano et al. (2004), however, do notreveal the presence of young, Neoproterozoic zircon grains insamples of the Araxá and Ibiá groups. In fact, in the southern part ofthe Brasília Belt the provenance of these sedimentary rocks is verysimilar to that of the Canastra Group and, therefore, all the threeunits have been interpreted as representative of a passive marginsequence.

3.2.4. Vazante GroupProvenance patterns of the several units of the Vazante Group

have been investigated by Rodrigues et al. (2008) and Rodrigueset al. (Submitted for publication) and are shown in Fig. 9. Thelowermost formations display a clear dominance of Paleoproter-ozoic source areas and subordinate Mesoproterozoic sources. Theyoungest population identified in these rocks is ca. 0.94 Ga,observed in the Rocinha formation and establishes an upper agelimit for the deposition of the original sediments of the base of thesequence. In the upper Morro do Calcário and Lapa formations, the

Fig. 9. Provenance patterns for the Vazante Group (data from Rodrigues et al., Submitted for publication).

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Mesoproterozoic zircon population is dominant and the youngestpeaks are of ca. 1.2 Ga, in consonance with ReeOs isochron data ofca. 0.99e1.1 Ga presented by Azmy et al. (2008).

The provenance patterns of the Vazante Group are not verydifferent from those of the Canastra Group and both can be,therefore, interpreted as parts of the original passive marginsequence in the southern part of the continental platform of theBrasília Belt.

3.2.5. Jequitaí and Cubatão DiamictitesThese are the most important glacial deposits associated with

the Neoproterozoic sedimentary sequences in the Brasília Belt. TheCubatão Formation forms the basal part of the Ibiá Group andthe Jequitaí Diamicitite lies directly under the limestones of the theSete Lagoas Formation of the Bambuí Group.

Both diamictites show important detrital components derivedfrom Paleo and Mesoproterozoic sources, with a few early Neo-proterozoic zircon grains (ca. 0.85 Ga). Paleoproterozoic detritalzircon grains are clearly more abundant in the Jequitaí diamictiteswhen compared with the Cubatão rocks (Fig. 10) The obviousabsence of grains younger than 0.85 Ga suggests that these rocksrepresent “Sturtian” glacial events as previously suggested byBabinsky et al. (2007) based on a PbePb whole-rock isochron forcap carbonates associated with the Jequitaí rocks.

Fig. 10. Provenance patterns for diamictites of the Ibiá Group (Cubatão Formation e

ICPMS data) and of the Jequitaí Formation e SHRIMP data.

One interesting aspect of these patterns is that they contrastwith the provenance patterns of the detrital rocks lying just abovethe glacial deposits. Both in the Ibiá and in the Bambuí Group(see below) the rocks overlying the diamictites have abundantNeoproterozoic zircon grains, as young as 620 Ma old, indicatinga sudden change in provenance, with the input of an importantcontribution from Neoproterozoic sources in the overlying SerraVerde Formation (Ibiá Group) and Sete Lagoas Formation (BambuíGroup).

3.2.6. Bambuí GroupA large number of UePb zircon analyses has been carried out by

Rodrigues (2008) for all the six formations of the Bambuí Group. Inall of them, the presence of a large number of Neoproterozoicdetrital material is the main feature of the provenance patterns. InFig. 11, the patterns for the sample from the upper part of the SeteLagoas Formation is shown. It displays almost only Neoproterozoiczircon grains, which are as young as 620 Ma. The fluvial uppermostTrês Marias Formation also shows a dominant Neoproterozoicpopulation, indicating that these rocks are the product of erosion ofthe Brasília Belt itself, with very little contribution from olderbasement. The data indicate that the deposition of the BambuíGroup took place towards the end of the Neoproterozoic and

Fig. 11. Provenance patterns for a pelite sample of the basal Sete Lagoas Formation(ICPMS data) and for an arkose of the upper Três Marias Formation of the BambuíGroup (SHRIMP data).

Fig. 12. Tectonic sub-division of the supracrustal units of the Brasília Belt according to their most likely depositional setting.

M.M. Pimentel et al. / Journal of South American Earth Sciences 31 (2011) 345e357 355

roughly post-dates the peak metamorphism registered in the oro-gen. Therefore, it is likely that most of the Bambuí Group representsa foreland basin with respect to the evolution of the Brasília Belt, aspreviously suggested by Chang et al. (1988) and Parenti-Couto et al.(1981).

There seems to be a contradiction, however, with the PbePbisochron age of ca. 0.74 Ga reported by Babinsky et al. (2007) forcarbonates at the base of the Sete Lagoas Formation. This data hasbeen interpreted as the depositional age of the sequence. However,the much younger zircon grains dated from a pelitic rock samplefrom the upper Sete Lagoas, indicate that there is an importantgap/unconformity within the Sete Lagoas Formation separating thelower limestone-rich part of the sequence, from the dolostone-richupper section. Although the physical evidence for such unconfor-mity has not been reported yet, its existence has been suggested inprevious studies using the CeO isotopic data, which show an

important shift in isotope composition towards the top of the SeteLagoas Formation (Santos et al., 2000). Therefore, the basal part ofthe Sete Lagoas Formation might well represent the cap carbonatesequence of the Sturtian Jequitaí diamictite, however, its upperportion ismuchyounger and there should be a longdepositional gapbetween the lower and theupper parts of the Sete Lagoas Formation.

4. Conclusions e tectonic implications of the provenancedata

The compilation of the available provenance data based onUePb dating of detrital zircon grains allows some first-orderconclusions regarding the nature and tectonic setting of the diff-erent lithostratigraphic sequences of the Brasília Belt.

Thedata compiledanddiscussedhere contradict previousmodelsaccording to which most of these units represent different parts or

M.M. Pimentel et al. / Journal of South American Earth Sciences 31 (2011) 345e357356

different stages of evolution of a passive margin sequence depositedalong the continental shelf of the São Francisco-Congo Craton.

Based, therefore, on the provenance data discussed here, theBrasília Belt maybe subdivided into:

(i) A Neoproterozoic magmatic arc with small volcano-sedi-mentary sequences inwhich the detrital sediments are almostentirely derived from erosion of the arc rocks themselves(Goiás Magmatic Arc; I in Fig. 12);

(ii) The syn-orogenic basins, comprising the Araxá, Ibiá andpossibly part of the Serra da Mesa groups, which surround themetamorphic core of the orogen represented by the Uruaçuand Anápolis-Itauçu high grade complexes (II in Fig. 12); allsediments comprising this zone of the orogenwere deposited,metamorphosed and deformed between ca. 650 and 630 Ma;

(iii) The passive margin sequences, represented mainly by theParanoá, Canastra and Vazante groups and maybe by thesouthern sectors of the Araxá and Ibiá groups (III in Fig. 12);the depositional age of these sequences remain unclear butmust be younger than ca. 1.0 Ga;

(iv) The foreland basin, deposited after ca. 0.62 Ga, represented bymost of the Bambuí Group in the eastern part of the belt andcovering large areas of the São Francisco Craton (IV in Fig. 12).

Acknowledgements

The authors would like to thank CNPq for continuous financialsupport for field and laboratory work.

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