Neoproterozoic granulite facies metamorphism and coeval granitic magmatism in the Brasilia Belt, Central Brazil: regional implications of new SHRIMP U–Pb and Sm–Nd data

Precambrian Research 125 (2003) 245–273

Neoproterozoic granulite facies metamorphism and coeval graniticmagmatism in the Brasilia Belt, Central Brazil: regionalimplications of new SHRIMP U–Pb and Sm–Nd data

Danielle Piuzanaa, Márcio Martins Pimentela,Reinhardt A. Fucka,∗, Richard Armstrongb

a Instituto de Geociˆencias, Universidade de Bras´ılia, Brasılia DF 70910-900, Brazilb Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200, Australia

Received 30 May 2002; accepted 7 March 2003

Abstract

New SHRIMP U–Pb zircon ages combined with Sm–Nd isotopic characteristics of granulites and associated granitic rocks ofthe Anápolis–Itauçu Complex in the central-southern part of the Brasılia Belt are presented and discussed in this study. Igneouscrystallization ages obtained in zircon grains of orthogranulite and granites vary between 760 and 650 Ma. Growth of new zirconat ca. 650–640 Ma dates the high-grade metamorphism. Zircon cores from paragranulites and granites give U–Pb ages between2.0 and 0.8 Ga.

TDM model ages of the granulitic rocks fall into two age intervals: 2.3–1.9 and 1.7–1.4 Ga.εNd(T) values are negative, varyingbetween−9.29 and−1.42. SHRIMP U–Pb ages of zircon cores indicate that the granulite sedimentary protoliths were depositedafter 800 Ma ago, contradicting previous models which assigned Paleoproterozoic or Archean ages for these rocks. Their Ndisotopic signature indicate that Paleoproterozoic sources (within the São Francisco Craton?), as well as younger sources, suchas the Goiás Magmatic Arc, contributed to the sediment infilling of the former basin.

The intrusive granites display Nd model age pattern similar to that of the granulitic rocks, with mostTDM model ages rangingbetween 1.45 and 1.2 Ga.εNd(T) values are between−2.61 and−7.96, indicative of assimilation of older material by the originalmagma. SHRIMP U–Pb data for the granite intrusions indicate that granitoids metamorphosed under amphibolite facies showstrong inheritance (between 2.1 and 0.8 Ga) and magmatic(?)/metamorphic age at ca. 660–650 Ma. The syn-granulite faciesgranitoid ANA 1 gave magmatic and metamorphic ages of ca. 650 Ma, without any discernible inheritance. The growth of newzircon occurred at the same time of the high-grade metamorphism, probably related with the collision event between the GoiásMagmatic Arc to the west–southwest and the São Francisco Craton, to the east.

The data suggest that the Anápolis–Itauçu Complex, instead of representing the ancient sialic basement of the supracrustalrock units of the Brasılia Belt, constitutes a Neoproterozoic high-grade metamorphic complex related with the evolution of theBrasiliano orogen.© 2003 Elsevier Science B.V. All rights reserved.

Keywords:Anápolis–Itauçu; Granulite; Brasılia Belt; U–Pb SHRIMP; Sm–Nd; Neoproterozoic

∗ Corresponding author. Fax:+55-61-3474062.E-mail addresses:[email protected] (D. Piuzana),

[email protected] (M.M. Pimentel), [email protected] (R.A. Fuck).

1. Introduction

The Brasılia Belt is a large Neoproterozoic orogenformed along the western margin of the São Francisco

0301-9268/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved.doi:10.1016/S0301-9268(03)00108-6

246 D. Piuzana et al. / Precambrian Research 125 (2003) 245–273

Fig. 1. Geological sketch map of the Brasılia Belt (after Fuck et al., 1994).

D. Piuzana et al. / Precambrian Research 125 (2003) 245–273 247

Craton, in Central Brazil. It comprises: (i) a thickMeso-Neoproterozoic metasedimentary tectonic pileshowing eastward tectonic displacement (for a reviewseePimentel et al., 2000a; Dardenne, 2000); (ii) alarge Neoproterozoic juvenile arc in the west (GoiásMagmatic Arc); and (iii) a micro-continent (or al-lochthonous sialic terrain) formed by Archean rockunits (the Crixás-Goiás granite-greenstones) and asso-ciated Proterozoic formations (Almeida et al., 1981;Fuck et al., 1993, 1994; Pimentel et al., 2000a,b)(Fig. 1).

During the last decade, important progress hasbeen achieved in our understanding of the tectonicevents which constructed the Brasılia Belt. However,several aspects of the geological evolution of the oro-gen still remain obscure. One of them is the tectonicsignificance and age of the extensive granulite ter-rains underlying large areas along the central axis ofthe belt.

In the northern section of the Brasılia Belt, gran-ulitic rocks exposed in the Barro Alto, Niquelandiaand Cana Brava mafic–ultramafic layered complexes(Fig. 1) are better known due to several detailedmapping projects carried out during the 1980s and1990s, and also due to the relatively large amount ofgeochronological data obtained both by SHRIMP andconventional U–Pb methods, as well as by Sm–Ndmineral and whole rock isochrons. The isotope dataindicate that these Paleo- to Mesoproterozoic layeredcomplexes were affected by high-grade metamor-phism between ca. 740 and 790 Ma and were subse-quently metamorphosed at lower grade at ca. 600 Ma(Ferreira Filho et al., 1994; Suıta et al., 1994; Correiaet al., 1996, 1997; Ferreira Filho and Pimentel, 2000;Pimentel et al., 2001c).

On the other hand, very little is known about thegranulites in the southern part of the Brasılia Belt,included in the Anápolis–Itauçu Complex (Fig. 1).Rocks of the Anápolis–Itauçu Complex include fel-sic hypersthene-bearing granulite, sillimanite–garnetgneiss, quartzite and carbonate-rich rocks, as wellas granulitic mafic–ultramafic intrusions and a largenumber of granite intrusions, some of which havealso been metamorphosed under granulite facies con-ditions. The high-grade rocks are in tectonic contact,low- and high-angle shear zones, with greenschist toamphibolite facies Neoproterozoic metasedimentaryrocks of the Araxá Group (Figs. 1 and 2).

Early studies have considered the rocks of theAnápolis–Itauçu Complex to be dominantly Archean(Danni et al., 1982; Marini et al., 1984; Wolff, 1991;Lacerda Filho and Oliveira, 1995; Winge, 1995),constituting the sialic basement of metasedimentaryrocks of the Brasılia Belt. However, this view hasbeen challenged recently byPimentel et al. (1999)andFischel et al. (1999), based on reconnaissance Ndisotope studies. Preliminary Sm–Nd data have indi-cated that the Nd isotopic signature and metamorphicages of the Anápolis–Itauçu felsic granulites, Araxámetasedimentary rocks, and intrusive granites are allvery similar, with a large proportion of model agesin the interval between ca. 1.1 and 1.5 Ga (Fischelet al., 1998, 1999; Pimentel et al., 1999). This ledPimentel et al. (1999)andFischel et al. (1999)to con-sider that, at least, part of the felsic granulites of theAnápolis–Itauçu Complex could represent high-gradeequivalents of the Araxá metasedimentary rocks, andthat the granites might represent the product of partialmelting of these metasediments.

The absence of geochronological data and the needto understand the tectonic environment of formationof the granulites, granites and metasediments in thecentral-southern part of the Brasılia Belt has encour-aged us to investigate their age relationships andisotopic characteristics in a greater detail, aiming tocontribute to the better understanding of the tectonicevolution of the Brasılia Belt.

2. The Brasılia Belt

The Brasılia Belt represents the central/eastern partof a large Neoproterozoic orogenic zone known asthe Tocantins Province (Almeida et al., 1981), whichresulted from the convergence and collision of threemajor continental blocks at the end of the Neopro-terozoic: the Amazon Craton to the west, the SãoFrancisco Craton to the east, and the ParanapanemaCraton, presently covered by Phanerozoic rocks ofthe Paraná Basin, to the south.

The eastern part of the Brasılia Belt is formedby several sedimentary/metasedimentary rock units(Paranoá, Canastra, Ibiá, Araxá, Vazante and BambuıGroups) (for a recent review, seeDardenne, 2000;Pimentel et al., 2000a, 2001b). These are progres-sively more intensely deformed and metamorphosed


Fig. 2. Geological sketch map of the Anapolis–Itauçu Complex (modified afterAraujo, 1994; Lacerda Filho and Oliveira, 1995).


towards the west, reaching amphibolite facies condi-tions in the central part of the belt. They show cleartectonic vergence towards the São Francisco Craton,with nappes and thrusts being very well developedespecially in the southern part of the Brasılia Belt(Campos Neto and Caby, 2000; Dardenne, 2000; Seeret al., 2000, 2001).

The westernmost part of the Brasılia Belt consists ofa large Neoproterozoic juvenile arc (Goiás MagmaticArc) formed by arc-type volcano-sedimentary rocksand tonalite/granodiorite gneisses (Fig. 1) (Pimenteland Fuck, 1992; Pimentel et al., 1991, 1997). Arc mag-matism started ca. 930–900 Ma ago in the Arenópo-lis area, Western Goiás, and ca. 860 ma ago in MaraRosa, Northern Goiás, with the emplacement of veryprimitive calc-alkaline volcanic rocks and associatedtonalites/granodiorites (εNd values between ca.+3.0and+6.0, andTDM values mostly between ca. 0.8 and1.1 Ga;Pimentel et al., 1991, 1997, 2000b; Pimenteland Fuck, 1992). Geochemical and isotopic data fromPimentel (1991)and Pimentel et al. (1997)suggestthat the original magmas formed above intraoceanicsubduction zones in a multi-arc system. Pre-collisionalcalc-alkaline igneous activity lasted until ca. 640 Ma,and the main metamorphic episode occurred at ca.630–610 Ma, as indicated by U–Pb titanite and Sm–Ndgarnet ages (for a review, seePimentel et al., 2000a).Detrital metasediments (feldspar–garnet schist and bi-otite schist) in the Mara Rosa and Arenópolis arcshaveTDM model ages between 0.9 and 1.2 Ga, indi-cating derivation mostly from erosion of the igneousrocks in the magmatic arc, supporting the intraoceanicarc model (Pimentel and Junges, 1997; Pimentel et al.,1997, 2001c).

In the central part of the Brasılia Belt is the GoiásMassif, represented by: (i) Archean greenstone beltsand TTG orthogneisses; (ii) Paleoproterozoic or-thogneisses largely covered by younger supracrustals;(iii) and Paleo- Mesoproterozoic mafic–ultramaficlayered complexes of Barro Alto, Niquelandia,and Canabrava and associated Mesoporeterozoicvolcano-sedimentary sequences. The eastern mar-gin of the Goiás Massif is marked by a gravimetricanomaly typical of suture zones, and it has been,therefore, interpreted as an allochtonous block (orblocks) amalgamated to the Brasılia Belt during theNeoproterozoic (Brito Neves and Cordani, 1991; Fucket al., 1994; Pimentel et al., 2000b).

One distinctive structural feature in the central partof this orogen is the Pireneus Syntaxis (Araújo Filho,1999), a strong bend in the main structural trends,which is marked by a WNW–ESE lineament runningfor hundreds of kilometers (Fig. 1). This feature hastraditionally been used to subdivide the Brasılia Beltinto the northern and southern segments, each with dis-tinct structural patterns (Marini et al., 1984; Fonsecaand Dardenne, 1995; Araújo Filho, 1999). The South-ern Brasılia Belt is marked by major low-angle struc-tures such as nappes and thrusts indicating tectonictransport towards the east, whereas in the northernpart of the belt, low-angle structures are less commonand the supracrustal units are less deformed. Otherrelevant aspects, exclusive to the southern part of thebelt are the presence of: (i) an extensive ophioliticmélange associated with metapelitic rocks of theAraxá Group (Drake Jr., 1980; Strieder and Nilson,1992), exposed along a roughly NS belt to the southof Goiania, and (ii) a number of peraluminous granitebodies intrusive into metapelites of the Araxá and Ibiágroups (Pimentel et al., 1992, 1999; Seer et al., 2001).

3. The Anápolis–Itauçu Complex

The Anápolis–Itauçu Complex (Fig. 1) underlies alarge area in the central-southern part of the BrasıliaBelt and consists of a NW–SE elongated (260 km×70 km) exposure of high-grade rocks and granitesseparated from the metasedimentary rocks of theAraxá Group by poorly exposed low- and high-angleshear zones (Fig. 2). It comprises: (i) granulite fa-cies gabbro and pyroxenite intrusions and tonaliteand granodiorite bodies, and (ii) paragranulites repre-sented mainly by garnet- and sillimanite-bearing alu-minous paragneiss associated with Fe- and Mn-richbanded formation, sillimanite–garnet quartzite, andgrossular-scapolite-diopside marble.Araújo (1994)and Winge (1995)assume that these rocks were de-rived from pelitic and greywacke sediments, althoughWolff (1991) argues that part of them are the resultof recrystallization of leucosome material derived bypartial melting of sedimentary components. In somelocalities these rocks contain cordierite, sapphirineand spinel with rutile and zircon as accessory min-erals.Moraes et al. (2002)suggested that the min-eral assemblages containing sapphirine and quartz in


equilibrium are indicative of ultra-high-temperature(UHT) metamorphism, which may have reached ca.1150◦C and >10 kbar. Retrograde reactions texturesin these rocks indicate near-isobaric cooling paths.

The Araxá Group, one of the main componentsof the Southern Brasılia Belt, comprises a sequenceof micaschist, quartzite, and carbonate-bearing schistwith minor paragneiss, carbonaceous schist, andmetachert. Small mafic–ultramafic bodies, tecton-ically interlayered with the metasediments of theAraxá Group, occur mainly to the north of Abadianiaand to the south of Goiania. They are part of theophiolitic mélange identified in the region (Drake Jr.1980; Strieder and Nilson, 1992), which extends inthe SSE direction for more than 200 km within themetasediments of the Araxá Group.

To the southwest, metasediments of the AraxáGroup are in tectonic contact with Anápolis–Itauçugranulites and with the Silvania Volcano-SedimentarySequence through a transcurrent shear zone. Furtherto the southwest, metasediments were thrusted ontothe Anápolis–Itauçu granulites. Regional structuraldata available in the literature are only preliminaryand do not help to clarify the relationships betweenthe high-grade rocks and the lower grade metased-iments of the Araxá Group. The apparent predom-inance of high- to moderate-dipping shear zonesseparating these units, however, seem to suggest thatthis relationship is not exactly the same as that de-scribed in the southern part of the Brasılia Belt, wherehigh-grade rocks were clearly thrusted over lowergrade metasediments (e.g. the Passos Nappe and theTres Pontas granulites), as observed byCampos Netoand Caby (2000).

Volcano-sedimentary sequences such as theSilvania and Rio do Peixe sequences (Fig. 2), compris-ing amphibolite, felsic metavolcanic rocks and micas-chist, were recognized and mapped as discontinuousstrips along the contacts between the Anápolis–ItauçuComplex and the Araxá Group (Lacerda Filho andOliveira, 1995; Araújo, 1994). Recent SHRIMPU–Pb and Sm–Nd results have shown that SilvaniaSequence volcanic rocks and associated JurubatubaGranite crystallized during the Paleoproterozoic, ca.2.1 Ga ago (Fischel et al., 2001), and therefore belongto the basement of the Brasılia Belt.

The Anápolis–Itauçu granulites and Araxá Groupmetasediments were intruded by a number of variably

deformed and metamorphosed granite bodies. In thewestern part of the complex, north of Bonfinópolis andeast of Goiania, they occur as small elongated bod-ies which are slightly to strongly deformed. Some ofthese medium to coarse-grained granites were meta-morphosed under granulite facies conditions. Theynormally show granoblastic texture and are composedof mesoperthitic and perthitic feldspar, quartz, reddishbiotite and hypidiomorphic garnet crystals, commonlyincluding sillimanite needles; opaque minerals, zircon,and monazite are accessory minerals. One granite sam-ple containing sillimanite and garnet (ANA 1) meta-morphosed in the granulite facies, was investigated byFischel et al. (1998)using the Sm–Nd method. Themineral isochron, including analyses of garnet, mon-azite, and biotite concentrates, indicated the age of633± 28 Ma, interpreted as the age of the high-grademetamorphism. SHRIMP U–Pb age of ca. 630 Ma wasalso reported byTassinari et al. (1999)for metamor-phic zircon from the Interlandia quarry granulite, ca.20 km to the north–northwest of Anápolis.

In the northwestern part of the Anápolis–ItauçuComplex two post-metamorphic syenitic to alkaligranite intrusions, emplaced into orthogneisses andvolcano-sedimentary sequences, were dated usingconventional and SHRIMP U–Pb (Pimentel et al.,2001a). The U–Pb data revealed magmatic ages of624± 10 Ma and 618± 4 Ma for these bodies. In-herited cores indicating Paleo- and Mesoproterozoic207Pb/206Pb ages (Pimentel et al., 2001a) and Sm–NdTDM model ages of ca. 1.4 Ga suggest contaminationwith older crust.

To the southeast of the Anápolis–Itauçu Complex,granite intrusions and related felsic volcanics asso-ciated with the Araxá and Ibiá groups have beeninterpreted as syn-tectonic, possibly collision relatedigneous rocks (e.g. Maratá Sequence) with a crystal-lization age of ca. 794±10 Ma (Pimentel et al., 1992).

4. Isotopic results

4.1. Analytical procedures

4.1.1. SHRIMP U–PbApproximately 10 kg samples of orthogranulite

(ANA 318), paragranulite (ANA 259, 230), sapphi-rine-garnet granulite (ANA 279 or PT 62 inMoraes


et al., 2002) and granite (ANA 1, 30, and 239) wereselected from a much larger sampling carried out inthe study area (Fig. 2). Rock samples were initiallycrushed to cm-sized fragments using a jaw crusher.The fragments were then ground, in small batches, ina tungsten carbide disk mill.

Heavy mineral concentrates were obtained usinga DENSITEST® table. The concentrates were thenpassed through a Frantz isodynamic magnetic sep-arator to obtain a pure zircon fraction. At least 100representative zircon grains from each sample werehand-picked from this fraction under a binocular mi-croscope, mounted in a 1 in. diameter epoxy diskwith standard RSES zircon crystals SL13+ AS3 orSL13+FC1 and sectioned approximately in half. Themount surface was then polished to expose the graininteriors and photographed at 150× magnificationin reflected and transmitted light. Cathodolumines-cence (CL) images were obtained in order to revealinternal structures of the zircon grains. Metamorphiccrystals show homogeneous internal structure or typ-ical sector zoning, whereas zircon crystals formedfrom “granitic” melts display thin oscillatory zoning.The internal features of the crystals revealed by CLimages, together with transmitted and reflected lightmicroscopic characteristics and Th/U ratios of thegrains, were used as criteria to distinguish betweenmetamorphic and igneous zircon grains.

Ion microprobe analyses were carried out usingSHRIMP I and II at the Research School of EarthSciences, Australian National University, Canberra,Australia. SHRIMP analytical methods and data treat-ment followed those described byWilliams (1998)and Williams and Meyer (1998). Common lead cor-rections were performed using theCumming andRichards (1975)model.

Uncertainties reported in tables are given at the 1�level, and final ages are quoted at 95% confidencelevel. The data have been processed using SQUID andIsoplot/Ex software (Ludwig, 1999, 2000). Isotopicdata are inTables 1–4 and 7–9, sample locations areshown inFig. 2, and brief petrographic descriptionsfor the samples investigated are inAppendix A.

4.1.2. Sm–NdEighteen granulite samples were analyzed. One

mineral isochron (whole rock and garnet) was obtainedfor garnet gneiss sample, ANA 26. Nd isotopic data

for the granites refer to samples collected to the south-east of Bonfinópolis (ANA 30 and 239). Samples fromother intrusions (ANA 1, 60A, 90, 163, 199) were alsoobtained and investigated. A Sm–Nd mineral isochron(whole rock and garnet) was obtained for ANA 30.

Sm–Nd isotopic analyses followed the method de-scribed inGioia and Pimentel (2000)and were carriedout at the Geochronology Laboratory of Universidadede Brasılia. Whole rock powders (ca. 50 mg) weremixed with149Sm–150Nd spike solution and dissolvedin Savillex capsules. Sm and Nd extraction of wholerock and garnet samples followed conventional cationexchange techniques, using teflon columns containingLN-Spec resin (HDEHP—di-ethylhexil phosphoricacid supported on PTFE powder). Sm and Nd sampleswere loaded on Re evaporation filaments of doublefilament assemblies and the isotopic measurementswere carried out on a multi-collector Finnigan MAT262 mass spectrometer in static mode. Uncertaintiesfor Sm/Nd and143Nd/144Nd ratios are better than±0.2% (2�) and ±0.003% (2�), respectively, basedon repeated analyses of international rock standardsBHVO-1 and BCR-1.143Nd/144Nd ratios were nor-malized to 146Nd/144Nd of 0.7219 and the decayconstant (λ) used was 6.54 × 10−12. TDM valueswere calculated usingDe Paolo’s (1981)model.Isochron ages were calculated using Isoplot/Ex soft-ware (Ludwig, 1999). Sm–Nd data are inTables 5and 6and sample locations are shown inFig. 2.

4.2. U–Pb and Sm–Nd results

4.2.1. Orthogranulite (ANA 318)This corresponds to medium-grained metatonalite

which has been metamorphosed under granulite faciesconditions. It contains zircon grains which show welldeveloped euhedral to prismatic oscillatory zoning,typical of magmatic crystals (Fig. 3A). Rims with lowluminescence are observed around the magmatic crys-tals (Fig. 3B and C). A subpopulation of zircon grainslacking oscillatory zoning was also identified, andprobably represents metamorphic crystals (Fig. 3D).

Twelve analyses of the magmatic crystals yieldedconcordant results and indicate the concordia ageof 760 ± 9 Ma (MSWD = 1.3) (Fig. 4) which isinterpreted as the crystallization age of the igneousprotolith. Analyses of the low luminescence rimsand metamorphic grains show very high U contents

252D

.P

iuza

na

et

al./P

reca

mb

rian

Re

sea

rch1

25

(20

03

)2

45

–2

73

Table 1Summary of SHRIMP U–Pb data for sample ANA 318

Grainspot

% 206Pbca ppm

UppmTh

232Th/238U

ppm206Pb∗

206Pb/238UAgea

207Pb/206PbAgea

% Discordant 238U/206Pb∗a

± %

207Pb∗/206Pb∗a

± %

207Pb∗/235Ua

± %

206Pb∗/238Ua

± %Errorcorrection

1.1 r 0.19 753 9 0.01 65.1 617± 9.9 619.0± 25 0 9.95± 1.7 0.0604± 1.2 0.837± 2.1 0.1005± 2 0.8221.2 c 0.10 363 42 0.12 38.9 758± 12 833.0± 48 9 8.02± 1.7 0.0668± 2.3 1.150± 2.9 0.1248± 2 0.5882.1 r – 1835 34 0.02 150 586± 9.0 668.0± 12 12 10.5± 1.6 0.0618± 0.6 0.810± 1.7 0.0951± 2 0.9412.2 c 0.10 295 38 0.13 31.6 757± 12 782.0± 29 3 8.02± 1.7 0.0653± 1.4 1.121± 2.2 0.1246± 2 0.7803.1 0.07 825 87 0.11 91.5 782± 12 777.0± 16 −1 7.75 ± 1.6 0.0651± 0.8 1.158± 1.8 0.1290± 2 0.9033.2 c 0.41 285 57 0.21 30.9 762± 13 725.0± 51 −5 7.97 ± 1.8 0.0630± 2.4 1.098± 3 0.1254± 2 0.6094.1 c 0.11 306 95 0.32 33.6 774± 12 835.0± 23 7 7.83± 1.7 0.0669± 1.1 1.178± 2 0.1276± 2 0.8356.1 r 13.97 2493 5272 2.18 382 919± 14 1258.0± 59 27 6.53± 1.6 0.0825± 3 1.744± 3.4 0.1532± 2 0.4777.1 r 0.28 728 182 0.26 71.4 696± 13 745.0± 34 7 8.78± 2 0.0641± 1.6 1.007± 2.5 0.1139± 2 0.7807.2 c 0.89 935 223 0.25 89.9 678± 14 551.0± 140 −23 9.01± 2.2 0.0586± 6.5 0.896± 6.9 0.1110± 2 0.3168.1 m – 1976 104 0.05 176 635± 10 685.0± 12 7 9.66± 1.6 0.0623± 0.5 0.890± 1.7 0.1036± 2 0.9499.1 c 0.26 902 575 0.66 114 879± 13 808.0± 20 −9 6.84 ± 1.6 0.0661± 1 1.331± 1.9 0.1461± 2 0.863

10.1 r 0.07 2123 66 0.03 199 668± 10 684.0± 14 2 9.16± 1.6 0.0623± 0.6 0.938± 1.7 0.1092± 2 0.92910.2 c 0.06 548 248 0.47 58.2 751± 12 802.0± 27 6 8.09± 1.7 0.0659± 1.3 1.122± 2.1 0.1236± 2 0.79311.1 c – 321 73 0.24 33.6 741± 12 844.0± 21 12 8.21± 1.7 0.0672± 1 1.128± 2 0.1218± 2 0.85612.1 r 0.18 2464 84 0.04 241 695± 11 737.0± 14 6 8.79± 1.6 0.0639± 0.7 1.002± 1.7 0.1138± 2 0.92312.2 c 0.81 345 68 0.20 36.7 747± 12 774.0± 68 4 8.14± 1.7 0.0650± 3.2 1.100± 3.6 0.1228± 2 0.46413.1 m 0.05 2609 150 0.06 256 696± 11 715.7± 10 3 8.77± 1.7 0.0632± 0.5 0.994± 1.7 0.1140± 2 0.96514.1 m 0.14 1005 160 0.16 88.5 628± 10 616.0± 25 −2 9.77 ± 1.6 0.0604± 1.2 0.852± 2 0.1023± 2 0.81615.1 m – 1831 167 0.09 192 744± 11 776.0± 10 4 8.18± 1.6 0.0651± 0.5 1.097± 1.7 0.1223± 2 0.95816.1 c – 616 93 0.16 64.1 738± 11 815.0± 19 9 8.25± 1.6 0.0663± 0.9 1.107± 1.9 0.1212± 2 0.87717.1 0.05 1949 104 0.06 209 758± 11 733.0± 9 −3 8.01 ± 1.6 0.0638± 0.4 1.097± 1.7 0.1248± 2 0.96818.1 r 0.13 1691 85 0.05 156 657± 10 626.0± 13 −5 9.31 ± 1.6 0.0606± 0.6 0.897± 1.7 0.1074± 2 0.94018.2 c 0.06 344 70 0.21 37 759± 12 805.0± 25 6 8 ± 1.7 0.0660± 1.2 1.136± 2.1 0.1249± 2 0.82119.1 r 0.21 1317 31 0.02 124 667± 10 652.0± 30 −2 9.17 ± 1.6 0.0614± 1.4 0.922± 2.1 0.1090± 2 0.760

Errors are 1�; Pbc and Pb∗ indicate the common and radiogenic portions, respectively. Error in standard calibration was 0.57% (not included in above errors but required when comparing datafrom different mounts). r: rim; c: core; m: metamorphic grain.

a Common Pb corrected using measured204Pb.

D.

Piu

zan

ae

ta

l./Pre

cam

bria

nR

ese

arch

12

5(2

00

3)

24

5–

27

3253

Table 2Summary of SHRIMP U–Pb zircon data for sample ANA 259

Grainspot

% 206Pbc U(ppm)

Th(ppm)

232Th/238U

206Pb∗

(ppm)

206Pb/238Uagea

207Pb/206Pbagea

% Discordant 207Pb∗/206Pb∗a

± %

207Pb∗/235Ua

± %

206Pb∗ /238Ua

± %Errorcorrection

1.1 c 0.79 508 330 0.67 59.9 827± 21 955± 41 13 0.07090± 2 1.338± 3.3 0.1368± 2.7 0.8001.2 0.10 216 22 0.11 19 627± 13 636± 31 1 0.06092± 1.4 0.858± 2.7 0.1022± 2.3 0.8452.1 c – 178 107 0.62 22.2 873± 43 765± 56 −14 0.06470± 2.7 1.295± 5.9 0.1451± 5.3 0.8942.2 0.05 282 80 0.29 25.4 643± 14 603± 26 −7 0.05999± 1.2 0.868± 2.6 0.1049± 2.2 0.8793.1 0.26 89 78 0.91 7.95 636± 15 525± 150 −21 0.05790± 6.6 0.828± 7.1 0.1037± 2.5 0.3523.2 0.06 191 20 0.11 15.6 588± 13 606± 49 3 0.06010± 2.2 0.790± 3.2 0.0954± 2.3 0.7194.1 – 430 52 0.12 38.3 636± 13 602± 23 −6 0.05994± 1.1 0.857± 2.5 0.1037± 2.2 0.8985.1 c 0.19 260 167 0.66 29.6 803± 17 850± 24 6 0.06739± 1.1 1.232± 2.5 0.1326± 2.3 0.8935.2 0.64 215 55 0.26 17 566± 12 707± 42 20 0.06300± 2 0.796± 3 0.0917± 2.3 0.7626.1 0.03 504 74 0.15 44.3 628± 13 605± 20 −4 0.06005± 0.9 0.847± 2.4 0.1023± 2.2 0.9257.1 0.05 86 94 1.13 7.57 627± 15 569± 54 −10 0.05900± 2.5 0.831± 3.5 0.1021± 2.4 0.7028.1 0.19 270 17 0.07 23.9 632± 14 578± 59 −9 0.05930± 2.7 0.842± 3.5 0.1030± 2.3 0.6399.1 c 2.96 657 222 0.35 65 701± 15 1291± 130 46 0.08390± 6.5 1.329± 6.9 0.1148± 2.3 0.3339.2 0.32 246 61 0.26 22.4 645± 14 572± 59 −13 0.05910± 2.7 0.858± 3.5 0.1052± 2.3 0.637

10.1 0.10 154 74 0.50 13.8 640± 14 669± 38 4 0.06180± 1.8 0.889± 2.9 0.1043± 2.3 0.79710.2 0.03 298 77 0.27 27.3 651± 14 605± 33 −8 0.06004± 1.5 0.879± 2.7 0.1062± 2.2 0.82311.1 0.26 332 20 0.06 28.6 615± 14 678± 26 9 0.06212± 1.2 0.858± 2.7 0.1002± 2.4 0.89311.2 0.20 258 63 0.25 22.8 629± 13 630± 35 0 0.06073± 1.6 0.858± 2.8 0.1025± 2.2 0.80712.1 0.32 278 22 0.08 24.5 630± 14 691± 31 9 0.06250± 1.5 0.884± 2.7 0.1026± 2.3 0.84112.2 0.12 344 53 0.16 31.7 655± 15 636± 29 −3 0.06092± 1.4 0.899± 2.7 0.1070± 2.4 0.86913.1 c 8.56 206 142 0.71 25.3 803± 25 1156± 320 31 0.07800± 16 1.430± 16 0.1327± 3.4 0.20513.1 0.17 254 94 0.38 36.9 1001± 25 934± 150 −7 0.07020± 7.3 1.630± 7.8 0.1680± 2.7 0.34613.2 0.72 110 72 0.68 9.75 632± 14 747± 50 15 0.06420± 2.4 0.912± 3.4 0.1030± 2.4 0.70714.1 – 332 3 0.01 31.3 672± 14 662± 29 −1 0.06166± 1.4 0.934± 2.6 0.1099± 2.3 0.85612.3 0.11 323 2 0.01 32.1 703± 15 633± 49 −11 0.06080± 2.3 0.966± 3.2 0.1152± 2.2 0.70315.1 0.62 251 105 0.43 24.3 686± 15 750± 110 9 0.06420± 5.1 0.994± 5.6 0.1122± 2.3 0.40316.1 0.64 327 47 0.15 30.4 655± 14 561± 60 −17 0.05880± 2.8 0.868± 3.5 0.1070± 2.2 0.62816.2 c 1.82 388 127 0.34 48.2 866± 18 1163± 43 26 0.07860± 2.2 1.558± 3.1 0.1437± 2.2 0.71517.1 1.16 276 57 0.21 25.3 646± 14 658± 110 2 0.06150± 5 0.894± 5.5 0.1054± 2.2 0.40718.1 0.35 615 22 0.04 59.3 684± 14 696± 28 2 0.06264± 1.3 0.966± 2.6 0.1119± 2.2 0.85619.1 c 3.09 199 158 0.82 49.5 1629± 41 1906± 29 15 0.11670± 1.6 4.620± 3.3 0.2874± 2.8 0.87119.2 – 150 8 0.05 18.1 845± 19 574± 110 −47 0.05920± 5.1 1.143± 5.7 0.1400± 2.4 0.42920.1 2.09 100 88 0.91 9.35 657± 16 833± 150 21 0.06690± 7.3 0.990± 7.7 0.1074± 2.5 0.32220.2 2.02 181 59 0.34 16.7 644± 14 667± 100 3 0.06180± 4.9 0.896± 5.4 0.1051± 2.3 0.424

Errors are 1�; Pbc and Pb∗ indicate the common and radiogenic portions, respectively. Error in standard calibration was 0.70% (not included in above errors but required when comparing datafrom different mounts). c: inherited core.


254D

.P

iuza

na

et

al./P

reca

mb

rian

Re

sea

rch1

25

(20

03

)2

45

–2

73


Grainspot

% 206Pbc U(ppm)

Th(ppm)

232Th/238U

206Pb∗

(ppm)

206Pb/238Uagea

207Pb/206Pbagea

% Discordant Total238U/206Pb± %

Total207Pb/206Pb± %

207Pb∗/206Pb∗a

± %

207Pb∗/235Ua

± %

206Pb∗/238Ua

± %Errorcorrection

1.1 0.38 216 104 0.50 19.5 642.2± 18.0 599± 55 −29 10.81± 2.3 0.06640± 2.4 0.0660± 4.5 0.842± 5 0.0924± 2.3 0.4542.1 0.96 161 28 0.18 14.3 629.7± 17.6 477± 119 −36 12.23± 2 0.11270± 0.91 0.0642± 5.1 0.6810± 5.4 0.0770± 2 0.3683.1 1.40 145 10 0.07 11.9 578.5± 16.9 616± 199 9 8.81± 2 0.06789± 1.1 0.0646± 2.3 1.007± 3.1 0.1130± 2 0.6554.1 0.47 157 82 0.54 13.9 627.3± 17.3 564± 85 −22 7.18± 3.4 0.07850± 3 0.0753± 3.4 1.4390± 4.9 0.1387± 3.4 0.7055.1 0.35 247 9 0.04 21.8 628.0± 17.0 652± 52 −3 9.6 ± 2 0.06383± 1.1 0.0604± 2.7 0.8640± 3.3 0.1038± 2 0.6046.1 0.60 192 72 0.39 17.5 646.3± 18.0 568± 72 25 9.23± 2.2 0.07120± 3.7 0.0682± 4 1.0150± 4.6 0.1079± 2.2 0.4777.1 0.16 152 16 0.11 13.5 633.8± 17.5 658± 69 8 10.76± 3.3 0.06180± 0.75 0.0605± 1.2 0.7730± 3.5 0.0928± 3.3 0.9427.2 0.46 184 94 0.52 16.8 646.4± 12.8 – – 8.89± 2 0.06893± 0.8 – – 0.1124± 2 –8.1 0.28 202 70 0.36 18.3 645.2± 12.6 – – 9.65± 2 0.06240± 0.97 – – 0.1034± 2 –8.2 1.13 274 22 0.08 25.0 643.4± 12.7 – – 14.09± 2.8 0.06754± 1.1 – – 0.0704± 2.8 –9.1 2.21 192 100 0.54 17.7 642.8± 13.3 – – 9.46± 2 0.06260± 1 – – 0.1053± 2 –9.2 3.93 115 47 0.42 10.4 622.9± 13.4 – – 10.48± 2 0.06373± 1.1 – – 0.0949± 2 –

10.1 1.22 179 71 0.41 16.4 644.3± 12.8 524± 112 −231 12.43± 2 0.10350± 1.5 0.0489± 7.3 0.5050± 7.6 0.0749± 2 0.26311.1 c 0.27 536 49 0.09 64.9 848.7± 15.8 1812± 108 −23 9.72± 2.3 0.06519± 1.4 0.0575± 3.8 0.8070± 4.4 0.1019± 2.3 0.52511.2 0.22 190 9 0.05 17.8 665.8± 13.0 628± 51 7 9.17± 2.1 0.06924± 1.4 0.0631± 4.1 0.9430± 4.6 0.1083± 2.1 0.45812.2 0.57 241 91 0.39 21.2 624.1± 12.2 531± 90 −4 9.58 ± 2 0.06239± 0.57 0.0603± 1 0.8660± 2.2 0.1042± 2 0.89213.1 c 0.30 413 53 0.13 47.9 814.3± 15.2 1104± 40 −5 9.46 ± 2 0.06262± 0.72 0.0604± 1.4 0.8770± 2.4 0.1054± 2 0.815

Errors are 1�; Pbc and Pb∗ indicate the common and radiogenic portions, respectively. Error in standard calibration was 0.74% (not included in above errors but required when comparing datafrom different mounts). c: inherited cores.


D.

Piu

zan

ae

ta

l./Pre

cam

bria

nR

ese

arch

12

5(2

00

3)

24

5–

27

3255


Grainspot

%206Pbca

U(ppm)

Th(ppm)

232Th/238U

206Pb∗(ppm)

206Pb/238Uagea

206Pb/238U ageb

206Pb/207Uagea

%Discordant

Total238U/206Pb± %

Total207Pb/0.6Pb± %

207Pb∗/0.6Pb∗a

± %

207Pb∗/235Ua

± %

206Pb∗/238Ua

± %

Errorcorrection

1.1 1.09 52 49 0.97 4.69 639.0± 11 639.0± 11 639 ± 190 0 9.480± 1.7 0.0699± 2 0.0610± 9 0.877 ± 9.2 0.1043± 1.9 0.2021.2 c 0.27 221 12 0.06 21.1 677.8± 9.3 679.4± 9.5 609 ± 51 −11 8.990± 1.4 0.0624± 1 0.0602± 2.4 0.920± 2.8 0.1109± 1.4 0.5202.1 0.71 79 89 1.17 7.25 652.1± 9.7 652.8± 9.9 622 ± 63 −5 9.330 ± 1.6 0.0663± 2 0.0605± 2.9 0.888± 3.3 0.1065± 1.6 0.4703.1 0.55 74 78 1.09 6.58 634.5± 9.6 635.2± 9.8 602 ± 57 −5 9.610 ± 1.6 0.0644± 2 0.0600± 2.6 0.855± 3.1 0.1034± 1.6 0.5164.1 0.56 66 61 0.95 5.72 615.0± 10 612.0± 10 752 ± 130 18 9.940± 1.7 0.0689± 2 0.0643± 6.2 0.887± 6.4 0.1001± 1.7 0.2695.1 0.57 57 52 0.95 4.84 605.2± 9.9 603.0± 10 728 ± 72 17 10.100± 1.7 0.0683± 2 0.0636± 3.4 0.863± 3.8 0.0984± 1.7 0.4486.1 c 0.22 510 133 0.27 88.3 1181.0± 18 1122.0± 19 2002 ± 28 41 4.960± 1.7 0.1250± 2 0.1231± 1.6 3.414± 2.3 0.2011± 1.7 0.7356.2 0.93 60 24 0.42 5.49 650.0± 11 653.0± 11 459 ± 150 −42 9.350± 1.7 0.0637± 2 0.0562± 6.8 0.821± 7 0.1060± 1.7 0.2507.1 0.46 61 49 0.84 5.52 647.0± 11 646.0± 11 708 ± 88 9 9.430± 1.7 0.0668± 2 0.0630± 4.1 0.917± 4.5 0.1056± 1.7 0.3848.1 0.32 36 23 0.68 3.15 629.0± 12 630.0± 12 599 ± 160 −5 9.730 ± 1.9 0.0625± 4 0.0599± 7.5 0.846± 7.7 0.1025± 2 0.2549.1 0.31 95 79 0.85 8.88 661.8± 9.6 662.6± 9.8 625 ± 54 −6 9.220 ± 1.5 0.0631± 2 0.0606± 2.5 0.903± 2.9 0.1081± 1.5 0.5219.2 – 50 42 0.87 4.72 675.0± 11 672.0± 12 779 ± 69 13 9.090± 1.8 0.0621± 3 0.0652± 3.3 0.992± 3.7 0.1104± 1.8 0.479

10.1 0.34 92 60 0.67 8.6 662.0± 13 665.0± 13 565 ± 96 −17 9.210± 2 0.0617± 2 0.0589± 4.4 0.880± 4.8 0.1082± 2 0.415

Errors are 1�; Pbc and Pb∗ indicate the common and radiogenic portions, respectively. Error in standard calibration was 0.61% (not included in above errors but required when comparing data from different mounts). c:inherited core.

a Common Pb corrected using measured204Pb.b Common Pb corrected by assuming206Pb/238U–207Pb/235U age-concordance.


Table 5Sm–Nd isotopic data for granulites from Anapolis–Itauçu Complex and associated granites

Sample Sm (ppm) Nd (ppm) 147Sm/144Nd 143Nd/144Nd TDM (Ga) εNd(0) εNd(650)

Anapolis–Itauçu granulitesAna 225 5.743 21.766 0.1595 0.512225 (13) 2.299 −8.06 −4.98Ana 25 8.315 42.526 0.1188 0.511848 (11) 1.905 −15.41 −8.96Ana 59 9.308 54.684 0.1029 0.511856 (36) 1.620 −15.25 −7.48Ana 26 6.988 35.147 0.1208 0.511865 (16) 1.918 −15.08 −8.79Ana 27 7.799 39.212 0.1202 0.511837 (31) 1.951 −15.62 −9.29Ana 275 5.186 24.415 0.1284 0.511927 (44) 1.927 −13.87 −8.22Ana 261 2.88 12.43 0.1401 0.511965 (20) 2.226 −13.13 −5.11Ana 259 5.458 20.406 0.1616 0.511952 (13) – −13.38Ana 270 8.12 43.57 0.1127 0.511998 (11) 1.569 −12.48 −5.52Ana 230 9.79 48.046 0.1232 0.512021 (16) 1.682 −12.05 −5.95Ana 120 15.96 85.40 0.1130 0.512059 (10) 1.481 −11.29 −7.17Ana 309 6.164 26.030 0.1431 0.512060 (21) 2.108 −11.3 −6.92Ana 318∗ 4.971 20.697 0.1452 0.512063 (23) 2.08 −11.22 −6.30Ana 156 5.19 17.91 0.1751 0.512163 (10) – −9.27Ana 93 8.674 39.008 0.1344 0.512274 (09) 1.464 −7.10 −1.93Ana 86 5.260 23.951 0.1328 0.512273 (15) 1.437 −7.13 −1.82Ana 277 4.973 23.321 0.1289 0.512277 (10) 1.365 −7.0 −1.42Ana 101 6.894 28.392 0.1468 0.512314 (12) 1.651 −6.33 −2.19

GranitesAna 1 15.79 137.7 0.069 0.511779 (05) 1.32 −16.8 −6.16Ana 199 13.364 86.871 0.0930 0.511887 (10) 1.45 −14.65 −6.05Ana 90 14.470 109.529 0.0798 0.511972 (25) 1.216 −12.99 −3.29Ana 30 3.443 14.825 0.1404 0.512004 (12) 2.154 −12.12 −7.71Ana 163 8.967 46.305 0.1171 0.511892 (17) 1.807 −14.55 −7.96Ana 60 A 8.735 49.113 0.1075 0.512125 (20) 1.308 −10.01 −2.61Ana 239 8.401 27.884 0.1821 0.512443 (31) – −3.80

The numbers in parenteses are 1� error in the last two digits of the isotopic ratio.∗ εNd calculated for the time of crystallization (ca. 760 Ma).

Table 6Sm–Nd mineral isochrons from granulite of Anapolis–Itauçu Complex and associated granite

Sample Sm (ppm) Nd (ppm) 147Sm/144Nd 143Nd/144Nd TDM (Ga) εNd

Anapolis–Itauçu granulite/granite

Ana 26 (WR) 6.988 35.147 0.1208 0.511865 (16) 1.918 −9.23: εNd(603)Ana 26 (gt) 11.550 20.908 0.3339 0.512711 (25)

Ana 1 (WR) 15.79 137.7 0.069 0.511779 (05) 1.32 −7.0: εNd(625)Ana 1 (mona) 9.320 80.72 0.070 0.511756 (05)Ana 1 (gt) 9.584 8.117 0.714 0.514429 (05)Ana 1 (bio) 4.020 28.31 0.085 0.511808 (10)

Ana 30 (WR) 3.443 14.825 0.1404 0.512004 (12) 2.154 −7.92 εNd(630)Ana 30 (gt) 13.193 14.342 0.5560 0.513709 (07)

The numbers in parenteses are 1� errors in the last two digits of the isotopic ratio. WR: Whole-rock; gt: garnet; mona: monazite; and bio:biotite.


Fig. 3. (A) CL images of orthogranulite ANA 318 zircon grains. (B)–(D) show detail morphology and internal features of zircon grains,Th/U ratio and Pb–U ages. Arrow indicates grain number and circle indicates analyzed spot.

(753–2.609 ppm) and low Th contents (9–167 ppm)resulting in low Th/U values (0.01 and 0.06) (Table 1).They show varied degrees of lead loss and yielded anupper intercept age of 638± 37 Ma (MSWD = 4)(Fig. 4), interpreted as indicative of the high-grademetamorphic event.

Analyses 7.1 and 12.1 most probably representmixed ages between the older and younger groups andwere not, therefore, included in the age calculations.Analyses 9.1 and 6.1 have older206Pb/238U ages andindicate some degree of assimilation of older materialby the original magma, a not unexpected result con-sidering the Sm–NdTDM model age of 2.08 Ga andεNd(760) of−6.3 obtained for this sample.

4.2.2. Paragranulites (ANA 259 and 230)Zircon grains in paragranulite ANA 259 (garnet–

sillimanite–feldspar rock) are typically rounded de-trital grains and are rimmed by metamorphic zircon

overgrowths (Fig. 5A). Individual metamorphic crys-tals are also present. Internal structure of the grainsare shown in CL images inFig. 5C and D.

Thirty-four spot analyses were carried out on 20zircon crystals (Table 2). Rims have lower Th/U ratios,although some cores show equally low Th/U ratios. Insuch cases, ages of the cores are similar to those ofthe rims and both grew during the high-grade meta-morphic event.

The analyses produced a very complex age patternmost probably due to a combination of inheritancewith recent and/or early Pb-loss, associated withthe high-grade metamorphism. The best result formetamorphic zircon grains and rims is given by 21concordant spot analyses indicating the concordiaage of 639± 9 Ma (MSWD= 1.4, Fig. 6) similar tothat calculated for metamorphic overgrowths in theorthogranulite. This may be interpreted as the bestestimate for the age of the high-grade metamorphism.


Fig. 4. Concordia diagram of zircon analyses from felsic orthogranulite ANA 318. Data-point error ellipses are 68.3% confidence level(light grey ellipse: metamorphic rims and grains; black ellipses: mix analyses; striped ellipses: cores).

Analyses of crystals with very high U contents (whichsuffered severe Pb-loss) and also those with high com-mon Pb contents were excluded from age calculation.

Cores of detrital zircon grains which yieldedconcordant or nearly concordant analyses have206Pb/238U ages between ca. 800 and 950 Ma. Theyounger cores represent, therefore, an upper age limitfor the deposition of the original sediments, clearlyindicating that the original sedimentary protolith isNeoproterozoic.

Garnet gneiss ANA 230 contains two distinct zir-con populations (Fig. 5B): (i) grains with zonedcores surrounded by homogeneous metamorphic rimsand (ii) metamorphic grains without internal zona-tion. Most grains are metamorphic, and only a fewof them show older cores. U contents vary between115 and 536 ppm and Th contents between 9 and104 ppm (Table 3). Grains 11 and 13 show discordantcores with206Pb/238U ages of ca. 815 Ma (Table 3)(Fig. 7). Analyses of the metamorphic grains are con-

cordant, however they display a rather large spread of206Pb/238U ages, rendering low probability of con-cordance. Therefore, we consider that the weightedmean206Pb/238U age of 640±8 Ma (MSWD= 0.68)for metamorphic zircon grains and rims representsthe best estimate for the age of the high-grade event.

4.2.3. Ultra-high-temperature granulite (ANA 279)One sample of UHT granulite (Fig. 2) was inves-

tigated. This sample has a mineral assemblage typi-cal of UHT granulite with quartz, plagioclase, biotite,orthopyroxene, cordierite, garnet, spinel, sillimanite,sapphirine and rutile (Moraes et al., 2002).

Twelve analyses in 10 zircon grains were carriedout, two in cores (analyses 1.2 and 6.1) and 10 in meta-morphic grains or rims (Table 4, Fig. 8). Cores yielded206Pb/238U ages of 1.18 and 0.68 Ga. Analyses ofmetamorphic zircon produced a coherent concordantpopulation with a concordia age of 650± 10 Ma(MSWD = 1.3, 95% confidence level), excluding


Fig. 5. (A) CL images of zircon grains from paragranulite ANA 259. (B) CL images of zircon grains from paragranulite ANA 230. (C,D) show detail of cores and rims from ANA 259 zircon. Circle indicates analyzed spot, Th/U ratio and Pb–U ages.

core 6.1 and two very discordant analyses (4.1 and5.1). Despite the small number of zircon grains andcores of this sample, we observed that they are similarto those of samples ANA 230 and 259. Therefore, weimply that UHT granulite protolith is also of sedimen-tary origin and that it must be of Neoproterozoic age.

Nd model ages of sillimanite–garnet gneisses ofthe Anápolis–Itauçu Complex fall into two intervals:1.9–2.3 Ga and 1.4–1.7 Ga (Table 5). Granulites witholder Nd model ages are mainly exposed to the south-west of Anápolis, while younger model ages are morefrequent in rock samples from outcrops to the north-west of that city (Fig. 2).

The TDM model age values are similar to datareported previously for high-grade aluminous rocksexposed to the north of Anápolis withTDM modelages between 1.2 and 1.5 Ga (Sato, 1998; Pimentelet al., 1999). The relatively young model age values

of some high-grade metasedimentary rocks indicatethat Neoproterozoic source areas were involved. Thisis in agreement with the SHRIMP U–Pb data, whichalso point to Neoproterozoic sources participatingin the filling of the former sedimentary basin wherethe granulites protoliths were deposited in mid Neo-proterozoic (Cryogenian) times. Combined with theage of high-grade metamorphism reported in thisstudy, the ages of detrital zircon grains brackets thesedimentation between ca. 800 and 650 Ma.

Garnet–whole rock Sm–Nd data for felsic granuliteANA 26 (Table 6) indicate the age of 606± 30 Ma(Fig. 9), here interpreted as indicative of the closureage of the isotopic system in garnet in this rock.

4.2.4. Granites (ANA 1, 239 and 30)Although unambiguous cross-cutting relationships

have not been observed between granite ANA 1 and


Fig. 6. Tera-Wasserburg diagram for analytical data of paragranulite ANA 259. Data-point error ellipses are 68.3% confidence level (greyellipse: metamorphic rims and grains; transparent ellipse: inherited cores).

Fig. 7. Concordia diagram for paragranulite ANA 230. During this analytical session the207Pb peak was lost for five analyses, and theseare not plotted here. The206Pb∗ /238U data are, however, included inTable 3. Data-point error ellipses are 68.3% confidence level.


Fig. 8. Concordia diagram for paragranulite ANA 279. Data-point error ellipses are 68.3% confidence level.

Fig. 9. Sm–Nd mineral isochron for paragranulite ANA 26 withanalyses in whole rock and garnet.

the enclosing granulites, it has been interpreted as in-trusive into the high-grade rocks. However, it has alsobeen strongly deformed and metamorphosed to gran-ulite facies, and represents, therefore, a syn-granulitefacies intrusion. Zircon is either prismatic with chiselto round terminations or equant, rounded grains. CLimages of zircon clearly show two distinct types ofgrains: single metamorphic crystals and grains with athinly zoned magmatic core rimmed by zircon withirregular patchy zoning (Fig. 10). Twenty-seven spotson 22 zircon grains were investigated (Table 7). Th/Uratios vary from 0.31 to 0.94 for metamorphic grainsand this interval is similar to that of metamorphic rimsaround zoned zircons (between 0.99 and 0.24). Coresof zoned grains have higher Th/U ratios than meta-morphic grains and rims, varying between 0.11 and2.89 (Table 7).


Fig. 10. CL images of zircon grains from granite ANA 1. Circle indicates analyzed spot.

Most of the data for this sample cluster aroundthe concordia, with a number of discordant analysesfalling away from the main group showing variablePb-loss (Fig. 11). Although all the data seem to lie ona discordia for which an upper intercept age of 643±14 Ma (MSWD= 1.14; probability of fit= 0.29) canbe calculated, more precise and accurate concordia

ages can be calculated from the subsets of concordantdata from cores and metamorphic zircon grains. Thir-teen of the analyses of metamorphic overgrowths andgrains yield a concordia age of 655±10 Ma (MSWD=1.4; Fig. 11). Six of the seven analyses of igneouscores give a similar result, with a concordia age cal-culated at 657± 11 Ma (MSWD= 0.62). Therefore,

D.

Piu

zan

ae

ta

l./Pre

cam

bria

nR

ese

arch

12

5(2

00

3)

24

5–

27

3263


Grainspot

% 206Pbca U

(ppm)Th(ppm)

232Th/238U

206Pb∗

(ppm)

206Pb/238Uagea

207Pb/206Pbagea

% Discordant 207Pb∗/206Pb∗a

± %

207Pb∗/235Ua

± %

206Pb∗/238Ua

± %Errorcorrection

1.1 0.34 467 242 0.54 39.6 605.0± 12 599± 53 −1 0.05990± 2.4 0.812± 3.2 0.0984± 2.1 0.6502.1 0.70 511 487 0.98 40.1 560.0± 11 664± 74 16 0.06170± 3.5 0.772± 4 0.0908± 2 0.5113.1 – 202 111 0.57 18.9 669.0± 13 719± 39 7 0.06330± 1.8 0.954± 2.8 0.1093± 2.1 0.7504.1 3.60 97 60 0.64 8.42 597.0± 14 833± 290 28 0.06680± 14 0.900± 14 0.0971± 2.4 0.1704.2 0.35 283 84 0.31 26.1 655.0± 13 577± 40 −14 0.05930± 1.9 0.874± 2.8 0.1070± 2 0.7395.1 0.80 490 224 0.47 39.2 569.0± 11 626± 56 9 0.06060± 2.6 0.772± 3.3 0.0923± 2 0.6125.2 – 432 95 0.23 37.3 619.0± 12 697± 30 11 0.06268± 1.4 0.871± 2.5 0.1008± 2 0.8146.1 0.77 316 158 0.52 30.3 677.0± 14 531± 80 −27 0.05800± 3.6 0.885± 4.2 0.1107± 2.2 0.5136.2 0.00 471 196 0.43 41.9 635.0± 12 625± 20 −2 0.06059± 0.95 0.865± 2.2 0.1036± 2 0.9027.1 1.92 183 67 0.38 15.4 592.0± 12 498± 150 −19 0.05710± 6.9 0.758± 7.2 0.0961± 2.1 0.2977.2 0.14 558 305 0.56 43.7 561.0± 12 579± 50 3 0.05930± 2.3 0.744± 3.2 0.0910± 2.2 0.6868.1 0.12 312 290 0.96 29 661.0± 13 622± 38 −6 0.06050± 1.8 0.902± 2.7 0.1080± 2 0.7518.2 – 279 63 0.23 25.1 644.0± 14 688± 45 6 0.06240± 2.1 0.904± 3.1 0.1051± 2.3 0.7459.1 2.27 514 53 0.11 47.5 645.0± 12 629± 72 −3 0.06070± 3.3 0.881± 3.9 0.1053± 2 0.512

10.1 0.05 183 166 0.94 16.8 652.0± 13 651± 73 0 0.06130± 3.4 0.900± 4 0.1064± 2.1 0.52011.1 0.84 276 145 0.54 24.8 635.0± 12 600± 74 −6 0.05990± 3.4 0.854± 4 0.1035± 2 0.50912.1 0.16 213 97 0.47 19.4 647.0± 13 609± 30 −6 0.06015± 1.4 0.876± 2.5 0.1056± 2 0.82913.1 0.05 355 479 1.40 33.4 670.0± 13 667± 32 0 0.06179± 1.5 0.933± 2.5 0.1096± 2 0.79814.1 0.82 104 66 0.66 9.68 660.0± 14 490± 150 −35 0.057± 7 0.847± 7.3 0.1079± 2.2 0.30215.1 0.07 246 81 0.34 23.2 671.0± 14 606± 38 −11 0.06010± 1.8 0.909± 2.8 0.1097± 2.1 0.77116.1 0.37 230 128 0.57 21.9 674.0± 13 690± 61 2 0.06250± 2.8 0.949± 3.5 0.1102± 2.1 0.59417.1 0.10 426 209 0.51 40.2 672.0± 13 695± 31 3 0.06262± 1.5 0.949± 2.5 0.1099± 2 0.80418.1 2.59 526 747 1.47 38.4 513.5± 9.8 747± 73 31 0.06410± 3.5 0.733± 4 0.0829± 2 0.49919.1 1.48 686 1913 2.88 63.7 652.0± 12 651± 48 0 0.06130± 2.3 0.901± 3 0.1065± 2 0.65820.1 0.08 682 78 0.12 64.5 673.0± 13 628± 22 −7 0.06069± 1 0.921± 2.2 0.1100± 2 0.88721.1 – 175 32 0.19 16.1 655.0± 13 675± 32 3 0.06202± 1.5 0.914± 2.5 0.1069± 2.1 0.81122.1 – 144 62 0.44 13.2 653.0± 13 694± 57 6 0.06260± 2.7 0.919± 3.4 0.1066± 2.1 0.617

Errors are 1�; Pbc and Pb∗ indicate the common and radiogenic portions, respectively. Error in standard calibration was 0.48% (not included in above errors but requiredwhen comparing data from different mounts).



Fig. 11. Concordia plot of all analyses from the granite ANA 1.The unfilled error ellipses represent discordant data not includedin the calculation of the concordia ages. Dark grey ellipse: cores;light grey ellipse: metamorphic rims and grains.

Fig. 12. (A) CL images of zircon grains from granite ANA 239. Arrow indicates grain number. (B, C) CL images with Pb–U ages ofcores and rims.

ages of igneous cores and metamorphic rims and crys-tals are identical within uncertainty, suggesting thatmetamorphic and igneous zircon crystallized roughlyat the same time. This means that granites such asANA 1 were emplaced into the lower crust, approx-imately at the same time when the country-rocksexperienced high-grade metamorphic conditions.

Samples ANA 239 and 30 are part of a 7 km diam-eter intrusion located to the southeast of Bonfinópolis(Fig. 2). Sample ANA 239 is from the central part ofthe intrusion, and ANA 30 (white to greenish granite,with garnet, plagioclase and sillimanite) is locatedclose to the contact with the granulitic country rocks(Fig. 2). Zircon grains from ANA 239 are clear to yel-lowish and form stubby prismatic crystals. CL imagesshow irregular compositional zoning. One importantfeature of these grains is the presence of concentricfractures between cores and rims and radial fracturesin the rims. The grains can be divided into: (i) roundedgrains with inherited prismatic or rounded cores; (ii)long prismatic grains with round terminations and

D.

Piu

zan

ae

ta

l./Pre

cam

bria

nR

ese

arch

12

5(2

00

3)

24

5–

27

3265


Grainspot

U(ppm)

Th(ppm)

Th/U Pb∗(ppm)

204Pb/206Pb f206 (%) Measured ratios Radiogenic ratios Ages (Ma)

206Pb/238U 207Pb/206Pb 206Pb/238U 206Pb/238U ±2.1 201 123 0.61 31 0.000982 1.07 7.106± 0.18 0.0815± 0.0012 0.1392± 0.0036 840 202.2 133 44 0.33 14 0.000100 0.18 9.757± 0.28 0.0624± 0.0014 0.1023± 0.0030 628 173.1 186 50 0.27 20 0.000010 0.02 9.256± 0.24 0.0629± 0.0010 0.1080± 0.0028 661 164.1 166 48 0.29 18 0.000087 0.16 8.837± 0.24 0.0608± 0.0009 0.1130± 0.0031 690 185.1 154 107 0.69 24 0.000261 0.31 6.982± 0.17 0.0717± 0.0009 0.1428± 0.0035 860 205.2 187 52 0.28 20 0.000157 0.28 9.397± 0.24 0.0638± 0.0009 0.1061± 0.0027 650 165.3 876 30 0.03 88 0.000006 0.01 9.197± 0.23 0.0614± 0.0004 0.1087± 0.0027 665 166.1 158 102 0.65 27 0.000076 0.13 6.336± 0.19 0.0718± 0.0009 0.1576± 0.0048 944 276.2 263 38 0.15 24 0.000002 0.12 10.356± 0.28 0.0623± 0.0008 0.0965± 0.0026 594 157.1 112 78 0.69 23 0.000010 0.02 5.407± 0.15 0.0745± 0.0014 0.1849± 0.0052 1094 297.2 138 45 0.33 15 0.000335 0.60 9.220± 0.25 0.0626± 0.0009 0.1078± 0.0029 660 178.1 210 97 0.46 30 0.005110 9.17 6.635± 0.21 0.1365± 0.0016 0.1369± 0.0044 827 258.2 222 39 0.18 25 0.000216 0.38 8.606± 0.44 0.0638± 0.0008 0.1158± 0.0059 706 349.1 63 31 0.50 9 0.000321 1.85 6.821± 0.22 0.0764± 0.0018 0.1439± 0.0047 867 279.2 228 44 0.19 24 0.000010 0.02 9.008± 0.24 0.0618± 0.0011 0.1110± 0.0029 679 17

10.1 230 37 0.16 25 0.000001 0.83 9.033± 0.44 0.0681± 0.0011 0.1098± 0.0053 671 3110.2 175 45 0.26 18 0.000131 0.23 9.750± 0.26 0.0629± 0.0013 0.1023± 0.0028 628 1611.1 209 99 0.47 34 0.000189 0.32 6.562± 0.18 0.0737± 0.0007 0.1519± 0.0041 912 2311.2 198 44 0.22 21 0.000213 0.38 9.125± 0.24 0.0636± 0.0011 0.1092± 0.0029 668 1712.1 179 44 0.24 21 0.000876 2.67 8.021± 0.23 0.0832± 0.0011 0.1213± 0.0035 738 2013.1 165 45 0.27 18 0.000007 0.01 9.199± 0.26 0.0624± 0.0013 0.1087± 0.0030 665 1813.2 158 122 0.77 30 0.000638 1.09 5.889± 0.15 0.0808± 0.0008 0.1680± 0.0043 1001 2414.1 318 23 0.07 32 0.000012 0.02 9.242± 0.23 0.0614± 0.0008 0.1082± 0.0027 662 1614.2 484 379 0.78 83 0.000566 0.98 6.571± 0.16 0.0765± 0.0008 0.1507± 0.0036 905 2015.1 99 35 0.35 11 0.000010 0.02 9.311± 0.26 0.0614± 0.0013 0.1074± 0.0030 658 1716.1 71 34 0.48 11 0.000077 0.13 6.822± 0.21 0.0718± 0.0015 0.1464± 0.0046 881 2616.2 400 28 0.07 41 0.000079 0.14 9.140± 0.23 0.0621± 0.0006 0.1093± 0.0027 668 1617.1 72 33 0.46 14 0.000331 0.56 5.508± 0.15 0.0842± 0.0013 0.1806± 0.0051 1070 2817.2 403 32 0.08 36 0.000321 0.49 10.333± 0.24 0.0653± 0.0006 0.0963± 0.0023 593 1318.1 145 48 0.33 16 0.000093 0.17 9.065± 0.26 0.0625± 0.0012 0.1101± 0.0031 673 1819.1 161 55 0.34 16 0.000042 0.07 9.793± 0.27 0.0624± 0.0011 0.1020± 0.0028 626 1720.1 1031 28 0.03 101 0.000280 0.50 9.393± 0.34 0.0660± 0.0005 0.1059± 0.0039 649 2320.2 112 89 0.79 21 0.000544 0.93 5.959± 0.18 0.0800± 0.0010 0.1663± 0.0050 992 27

(1) Uncertainties given at the 1� level. (2) f206 denotes the percentage of206Pb that is common Pb. (3) Correction for common Pb made using the measured204Pb/206Pbratio. (4) For % concordance, 100% denotes a concordant analysis.


prismatic cores (Fig. 12C); and (iii) metamorphiccrystals. Metamorphic rims have also been identified(Fig. 12A and B).

Thirty-three spots on 20 zircon crystals from ANA239 were analyzed (Table 8). Twelve zircon grainsshow inherited cores with206Pb/238U ages between824 and 1094 Ma (Fig. 13). Metamorphic crystals andmetamorphic rims yielded concordant analyses fromwhich the concordia age of 653± 13 Ma can be cal-culated (Fig. 13).

In general, zircon grains from sample ANA 30have high U content in central areas, grading into

Fig. 13. Concordia diagram for granite ANA 239. Data-point error ellipses are 68.3% confidence level (grey ellipse: metamorphic rimsand grains; striped ellipse: inherited grain).

outer zones with lower U content. The grains havebeen metamorphosed, with physical evidence beingthe metamorphic rounding of edges and tips and thesmoothing out of any initial magmatic zoning. Th/Uratios vary from core to rim, but this is mainly a con-sequence of changing U contents. The low Th/U insome central areas should not be used, therefore, asevidence of metamorphic growth (Fig. 14).

Eighteen spots on 10 zircon crystals from ANA30 were analyzed (Table 9). Cores were analyzed inspots 3.1, 4.1, 6.2 and 7.1 indicating inheritance fromdifferent sources. The oldest core is Paleoproterozoic


Fig. 14. CL images of zircon grain from granite ANA 30. Arrow indicates grain number and circle indicates analyzed spot.

(ca. 2.1 Ga), two cores have Mesoproterozoic ages (ca.1.5–1.6 Ga) and one is late Mesoproterozoic (1.0 Ga).Determination of the magmatic/metamorphic age ofthe rock is complicated by inheritance, Pb-loss (re-cent as well as probable early Pb-loss associated withhigh-grade metamorphism), and by the breakdownof the U–Pb calibration caused by extremely high Ucontents measured in some areas (e.g. analysis 6.2).Although concordant or semi-concordant, the analyti-

Fig. 15. Concordia diagram for granite ANA 30. Data-point error ellipses are 68.3% confidence level.

cal results did not meet the requirements which allowthe calculation of a concordia age and the weightedmean206Pb/238U age of 664± 7 Ma (MSWD= 1.3;probability of fit = 0.25) (Fig. 15) was, therefore,considered to be the best estimate for the igneous crys-tallization and coeval metamorphism of this granite.

Nd isotopic data for the granites (Table 5) revealPaleo- and Mesoproterozoic model ages. A Paleopro-terozoic TDM was obtained in the granite exposed

268D

.P

iuza

na

et

al./P

reca

mb

rian

Re

sea

rch1

25

(20

03

)2

45

–2

73


Grainspot

% 206Pbca U

(ppm)Th(ppm)

232Th/238U

206Pb∗

(ppm)

206Pb/238Uagea

207Pb/206Pbagea

208Pb/232Thagea

% Discordant Total238U/206Pb± %

207Pb∗/206Pb∗a ± %

207Pb∗/235Ua ± %

206Pb∗/238Ua ± %

Errorcorrection

1.1 3.08 105 61 0.60 10.2 667.0± 11 643± 210 700± 54 −4 8.890± 1.6 0.0611± 9.7 0.919± 9.8 0.1090± 1.7 0.1781.2 0.01 3499 44 0.01 316 645.0± 31 634± 19 724± 60 −2 9.500± 5 0.0609± 0.9 0.883± 5.1 0.1053± 5 0.9842.1 0.32 171 84 0.51 15.7 653.2± 9.7 549± 89 628± 28 −19 9.350± 1.5 0.0585± 4.1 0.860± 4.4 0.1066± 1.6 0.3553.1 – 64 41 0.66 20.7 2055.0± 29 2006± 35 2155± 51 −2 2.665± 1.6 0.1234± 1.9 6.390± 2.5 0.3754± 1.6 0.6463.2 0.88 429 44 0.11 40.8 671.3± 9.2 485± 58 538± 74 −38 9.030± 1.4 0.0568± 2.6 0.860± 3 0.1098± 1.4 0.4854.1 1.12 73 30 0.43 11.2 1052.0± 37 1021± 120 981± 89 −3 5.580± 3.8 0.0732± 5.9 1.790± 7 0.1773± 3.9 0.5494.2 0.12 294 54 0.19 25.8 626.2± 8.8 618± 37 614± 26 −1 9.790± 1.5 0.0604± 1.7 0.850± 2.3 0.1020± 1.5 0.6485.1 0.10 136 75 0.57 13 677.0± 10 579± 57 686± 20 −17 9.020± 1.6 0.0593± 2.6 0.905± 3 0.1107± 1.6 0.5195.2 0.75 390 25 0.07 35.5 644.2± 8.9 568± 65 431± 130 −13 9.440± 1.4 0.0590± 3 0.855± 3.3 0.1051± 1.5 0.4396.1 – 241 130 0.56 61.4 1674.0± 22 1621± 35 1759± 47 −3 3.375± 1.5 0.0998± 1.9 4.081± 2.4 0.2965± 1.5 0.6246.2 0.01 7927 93 0.01 768 688.9± 8.9 627.2± 5.0 678± 20 −10 8.870± 1.4 0.0607± 0.2 0.943± 1.4 0.1128± 1.4 0.9866.3 0.56 273 55 0.21 26.2 679.6± 9.7 588± 74 638± 55 −15 8.940± 1.5 0.0596± 3.4 0.913± 3.7 0.1112± 1.5 0.4007.1 0.33 154 52 0.35 36 1551.0± 31 1467± 53 1574± 63 −6 3.666± 2.2 0.0920± 2.8 3.450± 3.6 0.2719± 2.2 0.6287.2 0.71 269 46 0.18 25.3 665.0± 10 612± 78 557± 71 −9 9.130± 1.6 0.0602± 3.6 0.903± 4 0.1087± 1.6 0.4118.1 0.45 2867 45 0.02 264 653.8± 8.6 617± 20 319± 180 −6 9.330± 1.4 0.0604± 0.9 0.889± 1.7 0.1067± 1.4 0.8288.2 1.03 134 62 0.48 12.7 668.0± 10 376± 130 598± 37 −78 9.070± 1.6 0.0541± 5.6 0.814± 5.8 0.1092± 1.6 0.2789.1 – 158 66 0.43 14.9 670.3± 9.9 715± 43 710± 20 6 9.140± 1.5 0.0632± 2 0.955± 2.6 0.1096± 1.6 0.605

10.1 5.14 96 30 0.33 9.31 658.0± 12 651± 260 638± 120 −1 8.830± 1.7 0.0613± 12 0.910± 12 0.1074± 1.9 0.153

Errors are 1�; Pbc and Pb∗ indicate the common and radiogenic portions, respectively. Error in standard calibration was 0.76% (not included in above errors but requiredwhen comparing data from different mounts).



Fig. 16. Sm–Nd mineral isochron for granite ANA 30 yielded with whole rock and garnet.

to the southeast of Bonfinópolis (ANA 30;TDMof 2.15 Ga). ThisTDM model age is Paleoprotero-zoic, ca. 150 Ma younger thanTDM model agesfound in samples of the Jurubatuba Granite whichTDM values vary between 2.3 and 2.4 Ga (Fischelet al., 2001). Mesoproterozoic model ages (ca.1.2–1.3 Ga) are observed in granites exposed betweenAnápolis and Goiania (ANA 1, 163), to the northwestof Anápolis (ANA 90, 199) and to the north of BelaVista de Goiás (ANA 60A).εNd(T) values for thesesamples vary between−2.60 and−7.71 (Table 5).The age pattern and Nd isotopic characteristics arevery similar to those observed for the granulites(Table 5). The U–Pb and Sm–Nd data allow to inferthat the sources of magmas that gave origin to thesegranites are probably a mixture of juvenile Neo-proterozoic material and possibly Paleoproterozoiccontinental crust, such as the Silvania Sequence andJurubatuba Granite (Fischel et al., 2001), or alter-natively, Neoproterozoic sediments containing Neo-,Meso- and Paleoproterozoic components, such as theparagranulite protoliths.

Granite ANA 30 yielded a garnet–whole rock ageof 625± 16 Ma (Table 6, Fig. 16), ca. 30 Ma youngerthan the age obtained in metamorphic zircon grains.

5. Discussion

Isotopic data obtained in this work are summa-rized in Table 10. SHRIMP U–Pb results indicatethat the high-grade metamorphism recorded in theAnápolis–Itauçu Complex took place around 650 Ma.Within uncertainty, this is similar to previous resultsusing different isotopic methods in widely differ-ent metamorphic rocks of the same or other unitsof the Brasılia Belt. Therefore, it appears that theAnápolis–Itauçu Complex is in fact the core of themetamorphic complex in the central axis of the beltand does not represent its basement as assumed inearlier studies. On the other hand, we note that thisgranulite terrain is ca. 130 Ma younger than high-grade metamorphism recorded in the Niquelandia,Barro Alto and Canabrava mafic–ultramafic lay-ered complexes of the northern part of the BrasıliaBelt.

The data presented here also stress that protoliths ofthe Anápolis–Itauçu high-grade rocks are of Neopro-terozoic age. The orthogranulite (ANA 318) sampledin the vicinities of Anápolis represents a former felsicplutonic rock intruded at ca. 760 Ma ago. The intru-sion is coeval with calc-alkaline magmatic activity


Table 10Geochronological data for granulites of Anapolis–Itauçu Complex and associated granites

Rock unit Crystallizationage (Ma)

Metamorphicage (Ma)

Inheritance(Ma)

TDM (Ga) εNd (T)

Anapolis–Itauçu ComplexParagranulite ANA 26 606± 30a 1.92 −8.79UHT granulite ANA 279 650± 10b 680 and 1180Paragranulite ANA 259 640± 8b 800 to 950Paragranulite ANA 230 640± 10b Ca. 815 1.68 −5.95Orthogranulite ANA 318 759± 9b 638 ± 37b 2.08 −6.30

GranitesANA 1 643 ± 14b 655 ± 10b

633 ± 28a 1.32 −6.16

ANA 30 664 ± 7b 1000 to 2100

625 ± 16a 2.15 −7.71

ANA 239 653± 13b 824 to 1094

Inheritance was obtained by206Pb/238U ages in zircon cores.a Sm–Nd whole rock–garnet age.b 206Pb/238U zircon ages.

within the Goiás Magmatic Arc in SW Goiás, as wellas with magmatism comprising the Maratá Sequencein Southern Goiás (Pimentel et al., 1992). The Sm–Ndisotopic signature of ANA 318 reveals that its originalmagma was derived from or strongly contaminatedwith Paleoproterozoic crustal material. Paragranulitesgive TDM model ages, and U–Pb zircon core ages,between 2.0 and 0.8 Ga, indicating that their pro-toliths have been deposited after ca. 800 Ma ago.Nd isotopic signatures of these rocks indicate thatthey had Paleoproterozoic sources. However, youngersources such as the Goiás Magmatic Arc, were alsoshedding sediment into the basin where protolithsof the Anápolis–Itauçu paragranulites originallyaccumulated.

The new U–Pb data show some of the granitoidsshow strong inheritance pattern (between 2.1 and0.8 Ga) and magmatic(?)/metamorphic age at ca.660–650 Ma. The granulite facies granite ANA 1 gavemagmatic/metamorphic ages of ca. 650 Ma, withoutany discernible older inheritance. The absence ofinherited zircons could be explained, in this case,by complete dissolution of inherited minerals in theAl-rich, high-temperature melt. In ANA 1, the growthof igneous and metamorphic zircon occurred roughlyat the same time, and this magmatic/high-grade meta-morphic event may be related with the collision be-tween continental areas, such as the Goiás Magmatic

Arc to the west–southwest and the São FranciscoCraton, to the east.

6. Conclusions

The new U–Pb and Sm–Nd data discussed in thisstudy allow some relevant conclusions to be put for-ward regarding the nature and tectonic significanceof the Anápolis–Itauçu granulites and intrusive grani-toids:

(i) Granitoids investigated are all Neoproterozoic,ranging in age from ca. 760 to 650 Ma. Theolder, pre-metamorphic, granitoid emplacementevent is coeval with magmatic activity in theGoiás Magmatic Arc, further to the west. Con-trasting with the Goiás Magmatic Arc rocks,however, this early magmatic event in theAnápolis–Itauçu Complex is not totally juve-nile, and original magmas were emplaced intoand contaminated with older (possibly Paleo-proterozoic) continental crust;

(ii) Felsic granulites have detrital zircon populationswhich indicate Proterozoic sources, ranging inage from ca. 2.0 to 0.8 Ga. Therefore, the datashow that the original sediments were depositedduring the Neoproterozoic. Young sediment


sources may be attributed to the NeoproterozoicGoiás Magmatic Arc (0.93–0.64 Ga) presentlyexposed to the west and southwest of theAnápolis–Itauçu Complex, and Paleoprotero-zoic sources are widely distributed to the east,constituting a large part of the São Franciscocraton;

(iii) Neither the inheritance found in the granites,nor detrital zircon grains in the metasedimentarygranulites point to the involvement of Archeansources; thus, not only the original granitoidand sedimentary rocks are not Archean, buttheir sources or contaminants are not Archeaneither.

(iv) The peak of high-grade metamorphism in theAnápolis–Itauçu Complex occurred betweenca. 650–640 Ma, approximately 150–100 Malater than the high-grade metamorphic eventidentified in the Barro Alto, Cana Brava andNiquelandia mafic–ultramafic layered com-plexes in the northern part of the Brasılia Belt.Sm–Nd isotopic data in garnet from granulitesamples suggest that closing temperature of Ndisotopes in garnet was reached ca. 20–30 Maafter zircon was closed for Pb diffusion.

(v) Voluminous granitic magmatism, present-ing a general peraluminous nature, in theAnápolis–Itauçu Complex suggests importantmelt generation in the lower crust, concomitantwith granulite-facies metamorphism. The gran-itoids have Sm–Nd isotopic patterns that arenot much different from that observed for thefelsic granulites, withTDM model ages in theinterval between ca. 1.37 and 2.15 Ga andεNd(T) between−2.7 and−8.1, suggesting that themagmas are the product of remelted granulitesand older sialic crust. This is also supported bythe inheritance pattern observed in zircon grainsfrom the granitoids.

(vi) The Nd isotopic compositions of the granuliticrocks investigated are also similar to those ob-served for Araxá Group metasediments of theBrasılia Belt in Central-southern Goiás, support-ing previous suggestions that the paragranulitesmight represent high-grade equivalents of someof the Brasılia Belt metasediments;

(vii) The Anápolis–Itauçu Complex represents,therefore, the core of a metamorphic complex

associated with the Brasiliano orogeny, and notthe exposure of ancient sialic basement.

Acknowledgements

This work was supported by CNPq (grant200520/01-6 to DP; grants 52.2269/95-8, 42.0081/99-2to RAF) and FAP-DF (grant 193.000.068/96) to RAF.We thank Simone Gioia and Sérgio Junges for assis-tance in the Laboratory of Geochronology of Univer-sidade de Brasılia and Renato de Moraes is thankedfor supplying zircon concentrate of sample ANA 279.Drs. R. Caby and U. Cordani are thanked for theirthorough and careful reviews of the manuscript.

Appendix A

ANA-318-homogeneous biotite-bearing granuliteshowing typical granoblastic texture and weak fo-liation. It is made of plagioclase, quartz, biotite,K-feldspar and orthopyroxene (hypersthene) and rep-resents a metamorphosed tonalitic intrusion.

ANA 259 and ANA 230-These are very similarpetrographically and correspond to medium grained,grey-colored, strongly foliated, garnet gneisses. Theyare made of quartz, plagioclase, K-feldspar, garnet, sil-limanite, and biotite. Their mineralogical compositionand association with calc-silicate rocks and quartzitesuggests derivation from a pelite or greywacke.

ANA-279-granulite (UHT) presenting granoblastictexture, made of quartz, plagioclase, biotite, orthopy-roxene, cordierite, garnet, spinel, plagioclase, biotite,sapphirine and rutile.

ANA-1-medium grained, gray-colored foliatedgranite (s.s.), with locally preserved porphyritic tex-ture. It is made of quartz, plagioclase, K-feldspar(orthoclase), garnet, biotite and sillimanite. Zirconand monazite are abundant accessories.

ANA-30 and ANA-239-form a single intrusivebody of granitic composition. ANA-30 represents awhite to greenish medium-grained facies, with quartz,K-feldspar (orthoclase), plagioclase, garnet, and silli-manite. ANA-239 is a coarse-grained, red-coloured,porphyritic facies made of quartz, K-feldspar, plagio-clase (oligoclase), biotite and garnet.


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Documents

Neoproterozoic granulite facies metamorphism and coeval granitic magmatism in the Brasilia Belt, Central Brazil: regional implications of new SHRIMP U–Pb and Sm–Nd data