Crustal growth of the central-eastern Paleoproterozoic domain, SW Amazonian craton: Juvenile accretion vs. reworking

Journal of South American Earth Sciences 27 (2009) 235–246

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Journal of South American Earth Sciences

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Crustal growth of the central-eastern Paleoproterozoic domain, SW Amazoniancraton: Juvenile accretion vs. reworking

Moacir José Buenano Macambira a,*, Marcelo Lacerda Vasquez b, Daniela Cristina Costa da Silva c,Marco Antonio Galarza a, Carlos Eduardo de Mesquita Barros d, Julielson de Freitas Camelo e

a Laboratório de Geologia Isotópica – Para-Iso, Instituto de Geociências, Universidade Federal do Pará, Caixa Postal 8608, 66075-110 Belem, PA, Brazilb Companhia de Pesquisa de Recursos Minerais, Av. Dr. Freitas, 3645, 66095-110 Belém, PA, Brazilc Programa de Pós-graduação em Geologia e Geoquímica, Universidade Federal do Pará, Caixa Postal 8608, 66075-110 Belém, PA, Brazild Universidade Federal do Paraná, Departamento de Geologia, Centro Politécnico, Caixa Postal 19001, 81531-990 Curitiba, PR, Brazile Mineração Rio do Norte S.A., Porto Trombetas, PA, Brazil

a r t i c l e i n f o

Article history:Received 2 August 2007Accepted 6 February 2009

Keywords:Trans-Amazonian cycleZirconNd isotopesAmazonian cratonPaleoproterozoic

0895-9811/$ - see front matter � 2009 Elsevier Ltd. Adoi:10.1016/j.jsames.2009.02.001

* Corresponding author. Tel.: +55 91 3201 7483; faE-mail address: [email protected] (M.J.B. Macambi

a b s t r a c t

The Trans-Amazonian cycle was an important rock-forming event in South America, generating volumi-nous juvenile and reworked fractions of continental crust. The Bacajá domain, in the southern sector ofthe Maroni-Itacaiúnas Province in the Amazonian craton, is an example of the Trans-Amazonian terranesadjacent to the Archean Carajás block. Zircon Pb-evaporation and whole-rock Sm–Nd analyses were car-ried out on representative samples of six lithological units, and allowed the proposal of a comprehensivetectonic-magmatic evolutionary sequence for the central and eastern parts of this domain, from the Neo-archean to the Rhyacian. Gneisses with ages of ca. 2.67 and 2.44 Ga are the oldest rocks recorded in theregion, and probably represent remnants of island and continental arcs. The Três Palmeiras succession,emplaced between 2.36 and 2.34 Ga, hosts gold deposits and represents the first record of Siderian supra-crustal rocks in the Amazonian craton. It was probably part of an island arc/ocean floor accreted to a cra-ton margin. Rhyacian granitogenesis lasted for ca. 140 My (2.22–2.08 Ga), marking different stages of theTrans-Amazonian cycle. The first stage is represented by continental arc granitoids formed by melting ofArchean crust at 2.22–2.18 Ga. The second is characterized by the production of juvenile materialbetween 2.16 and 2.13 Ga. The third and final stage at ca. 2.08 Ga is represented by a large volume ofgranitoids originated from either juvenile material or reworked crust during compressive stresses. Ndisotopes reveal that juvenile rocks dominated in the northern part of the domain, whereas those formedfrom reworked crust predominate in the south. The present-day configuration of the Bacajá domainresults from collision against the Archean Carajás block at the end of the Trans-Amazonian cycle.

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1. Introduction

The geotectonic model of evolution suggesting that the Amazo-nian craton (Guiana and Central Brazil shields) represents a collageof Proterozoic belts or geochronological provinces surrounding Ar-chean nuclei was first presented in the seventies (e.g. Amaral,1974; Cordani et al., 1979). Presently, this model is considered tobe the most appropriate to explain the main general features ofthe craton, and has been updated by several authors (Lima, 1984;Teixeira et al., 1989; Tassinari and Macambira, 1999, 2004; Tassi-nari et al., 2000; Dall’Agnol et al., 2000; Santos et al., 2000,2006). The division of the craton into provinces (Fig. 1) mainlytakes into account the geochronology of the regional basement,as well as general geological and geophysical features (e.g. Tassi-

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x: +55 91 3246 2323.ra).

nari and Macambira, 1999). The boundaries between thesegeochronological provinces are key areas for understanding thegrowth of the craton and of the provinces themselves, whichhave their own geochronological, tectonic and lithologicalcharacteristics.

The Trans-Amazonian cycle was an important rock-formingevent in the South American Platform (e.g. Cordani and Sato,1999). The southern part of Maroni-Itacaiúnas Province, which isthe Bacajá domain, is a special example of the Trans-Amazonianterranes since it makes contact with the Archean Carajás block(Fig. 1), included in the Central Amazonian Province (e.g. Tassinariand Macambira, 2004). Mapping projects carried out by CPRM re-sulted in conflicting proposals for the location of the boundary be-tween the Archean and Paleoproterozoic domains (e.g. Santos,2003; Faraco et al., 2005; Santos et al., 2006). Apart from this ques-tion, it is also important to take into account the internal structure,composition and evolution of the provinces themselves.

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g

500 km

AtlanticOcean

AmazonBasinSolimões

Basin

50º W

0º

10º S

60º W

500 km

0ºAtlanticOcean

PacíficOcean

AmazonBasin

GuyanaSuriname

Venezuela

Colombia

Peru

Bolivia

Ecuador

Brazil

Central Brazil Shiel

Guiana Shield

French Guiana

s ield

Cent l Brazil hra

uG iana si eldhi

60º W

Amazonian craton

Neoproterozoic belt

Maroni-Itacaiúnas (2.2-1.95 Ga)

Ventuari-Tapajós (1.95-1.8 Ga)

Rondoniano-San Ignacio (1.5-1.3 Ga)

Sunsás (1.25-1.0 Ga)

Rio Negro-Juruena (1.8-1.55 Ga)

Geochronological Provinces

Central Amazonian (> 2.5 Ga)

b

a

c

b

(Xingu)

(Iricoumé)

Figure 2

a - Carajás block

b - Xingu-Iricoumé block

c - Bacajá domain

Fig. 1. Sketch map showing the geochronological provinces of the Amazonian craton (based on Tassinari and Macambira (2004)) and the location of the study area.

236 M.J.B. Macambira et al. / Journal of South American Earth Sciences 27 (2009) 235–246

This work presents new isotope data (Pb-evaporation on zirconand whole-rock Sm–Nd) for rocks cropping out in the central andeastern parts of the Bacajá domain, Pará state, in order to bettercharacterize the age and origin of these rocks. Additionally, wehope to clarify the formation and evolution of the southernmostpart of the Maroni-Itacaiúnas Province and its nature, whetherby juvenile accretion, or by reworking of the rocks involved inthe Trans-Amazonian cycle.

2. Regional geological setting

According to recent studies (e.g. Tassinari et al., 2000; Tassinariand Macambira, 2004; Santos et al., 2000, 2006), Archean terranesconstitute the southeasternmost part of the Amazonian craton(Central Amazonian Province), and are surrounded by Proterozoicprovinces, which become progressively younger southwestwards(Fig. 1). Tassinari and Macambira (2004) defined the Central Ama-zonian Province as the oldest continental crust of the craton, whichwas not affected by the Trans-Amazonian cycle. Following Tassi-nari and Macambira (1999), Dall’Agnol et al. (1999a), and Tassinari

et al. (2000), Tassinari and Macambira (2004) divided the provinceinto two segments: the Carajás and the Xingu-Iricoumé blocks(Fig. 1). The first comprises a 3.00–2.85 Ga granite-greenstonebasement covered, in its northern part, by a ca. 2.76 Ga volcano-sedimentary sequence hosting the most important mineral depos-its (Cu, Fe, Au, Mn etc.) of the craton. All the Archean rocks of theCarajás block have TDM(Nd) ages between 3.2 and 2.86 Ga. The Xin-gu-Iricoumé block is a NW–SE segment located in the central partof the craton, and is partially covered by Phanerozoic sedimentaryrocks of the Amazon basin. It represents the least studied part ofthe Amazonian craton. Paleoproterozoic granitoids and volcanicrocks, which dominate in this block, are largely covered by sedi-mentary sequences. Geochronological data for the regional base-ment are not available, but it has been considered to be of pre-‘‘Trans-Amazonian” age (>2.5 Ga) (Tassinari and Macambira,2004). The Archaean age for the rarely exposed metamorphic base-ment is based on a few TDM(Nd) ages of the Paleoproterozoic grani-toids and volcanic rocks, which were probably formed by meltingof the basement.

The Maroni-Itacaiúnas Province (2.2–1.95 Ga) borders thenortheastern and northern parts of the Central Amazonian

M.J.B. Macambira et al. / Journal of South American Earth Sciences 27 (2009) 235–246 237

Province. It was formed during the Trans-Amazonian orogenic cy-cle, but several Archean inliers are recognized within the Paleopro-terozoic rocks. The province is characterized by widespreadexposures of greenschist to amphibolite facies metavolcanic andmetasedimentary units, as well as by granulitic and gneissic-mig-matitic terranes.

Apart from the two provinces described above, Tassinari andMacambira (2004) revised some province boundaries, while main-taining others proposed in previous works (Fig. 1), which are: Ven-tuari-Tapajós (1.95–1.8 Ga), Rio Negro-Juruena (1.8–1.55 Ga),Rondonian-San Ignacio (1.55–1.3 Ga) and Sunsas (1.3–1.0 Ga). San-tos et al. (2000) suggested other names and limits for the geochro-nological provinces of the Amazonian craton, which were alsoupdated in recent publications (e.g. Santos et al., 2006).

The Maroni-Itacaiúnas Province can be divided into several do-mains according to their geological features and geographical dis-tribution. The Bacajá domain (Fig. 2) borders the northern part ofthe Carajás block (Central Amazonian Province). Its northern partis covered by rocks of the Amazon basin, and its eastern part bythe Grajaú basin and the Neoproterozoic Araguaia belt. The domainextends westwards parallel to the southern margin of the Amazonbasin, and is covered here by the Paleoproterozoic volcanic rocks ofthe Central Amazonian Province. The Bacajá domain is compara-tively less well studied than the Carajás block. Its central-easternpart, the object of this work, is composed of deformed granitoids,granulites, gneisses and the Três Palmeiras and São Manoel green-stone belts which are discussed below.

3. Geology of the central-eastern part of Bacajá domain

Few studies have been carried out in the eastern Bacajá domain.The RADAM project (Silva et al., 1974; Issler et al., 1974) producedthe first geological map of the region when, based only on K–Arand Rb–Sr data, it was speculated that the Trans-Amazonian cyclehad affected older rock units. Later on, Jorge João et al. (1987) andSantos et al. (1988) studied the northwestern part of this region.Their investigation recognized several lithostratigraphic units suchas: the Bacajaí Granulite, the Três Palmeiras Metamorphic Suite(greenstone belt), the Anapu Granodiorite, the Oca Granodiorite,and the João Jorge Granite. The second study cited presented Rb–Sr data, and suggested that the domain was formed by Paleoprote-rozoic reworking of gneisses, as well as juvenile additionsrepresented by the mafic metavolcanic rocks of the Iriri-Xinguregion.

Some local studies were carried out on the central and the wes-tern parts of the Bacajá domain (Fig. 2). In the central part in theManelão gold mine, Souza et al. (2003) and Souza and Kotschoubey(2005) described the poly-metamorphic regional basement of theXingu Complex (Silva et al., 1974), and the São Manoel volcanosed-imentary sequence, both of them intruded by the Felício TurvoGranite. For the northwestern part of the domain, in the Iriri-Xinguarea, Vasquez et al. (2008) and Santos (2003) presented new Pb-evaporation and U–Pb SHRIMP zircon data for granitoids andgneisses which indicated ages between 2.50 and 2.07 Ga.

Faraco et al. (2004, 2005) reviewed the geology of the easternBacajá domain (Fig. 2) and proposed new lithostratigraphic unitswhich are usually elongated along NW–SE and WNW–ESE trends.These are: the Direita Granulitic Suite composed by foliatedquartz-feldspar granulites; the calc-alkaline Bacajaí CharnockiticComplex; the Ipiaçava Kinzigitic Complex including rocks with gar-net, biotite and sillimanite; the Rio Preto Piriclasite represented bytholeiitic to calc-alkaline mafic granulites formed at high temper-ature and pressure; and the Cajazeiras Enderbitic Complex com-prising calc-alkaline granulites. Metavolcano-sedimentary rockswere included in the Misteriosa and São Manoel groups, as well

as in the Itatá Amphibolite, and in the Bacajá Micaschist. Accordingto Faraco et al. (2005), the Três Palmeiras greenstone belt encom-passes the last two units. Siderian granitoids are grouped into theJacaré Complex, whereas Rhyacian granitoids are represented bythe Valentin Complex, as well as the Felício Turvo and Bacajá gran-ites. The map presented by Faraco et al. (2005) will be used as thegeological background of this study. In spite of some divergences inrelation to our data regarding rock classification, and the locationsof the contacts between the lithological units, we maintain thismap because it is the most recent and the most complete available.For this reason, in this work geographical references to samplelocations are preferred, rather than the geological units and con-tacts proposed by Faraco et al. (2005).

Barros et al. (2007) studied a NW–SE oriented area, parallel tothe BR230 road in the northeastern part of the Bacajá domain,and described monzogranites and granodiorites, with subordinatetonalites, syenogranites and scarce quartz diorites. These rocks arerather homogeneously deformed at the regional scale, with folia-tions striking N60 W and WNW–ESE. Primary subvertical andflat-lying igneous layering are transposed to high-temperature sec-ondary foliations and mylonite zones. According to these authorsthe development of these structures was controlled by progressivedeformation under decreasing temperatures, characterizing thesyntectonic emplacement of these granitoids during regionalshortening. Taking into consideration the age of the granitoids(2076 ± 6 Ma, Pb-evaporation zircon age), they proposed an evolu-tion related to a continental arc environment developed during softamalgamation of continental plates at the end of the Trans-Amazo-nian cycle.

4. Analytical methods

Zircon from seven samples, and 15 whole-rock samples fromthe central-eastern part of the Bacajá domain were analyzed byPb-evaporation and by Sm–Nd methods, respectively, at the Iso-tope Geology Laboratory of the Federal University of Pará (Pará-Iso), Brazil, using a Finnigan MAT 262 mass spectrometer.

For the Pb-evaporation technique (Kober, 1986, 1987), zirconcrystals were concentrated by conventional methods of heavy min-eral separation, and then were hand-picked. In this technique, theindividual zircon grain is encapsulated in the Re-filament used forevaporation, which was placed directly in front of the ionizationfilament. Both filaments are introduced into the mass spectrome-ter. The evaporation filament is heated to evaporate the Pb fromthe zircon, and the Pb liberated is condensed on the cold ‘‘ioniza-tion” filament. Three evaporation steps, each of a maximum of5 min, are performed at 1450, 1500 and 1550 �C. After each evap-oration step, the temperature of the ionization filament is raised tothe point of Pb emission, and the isotopic measurements aredynamically made with the ion counter of the instrument. Theintensities of the emission of each Pb isotope were measured inone cycle by peak stepping through the 206–207–208–206–207–204 mass sequence for five mass scans, defining one data blockwith eight 207Pb/206Pb ratios. Five blocks are usually recorded foreach evaporation step. The weighted 207Pb/206Pb mean for eachblock is corrected for common Pb using appropriate age values de-rived from the two-stage model of Stacey and Kramers (1975), andthe corrected block is used for sample age calculation. Blocks yield-ing a 204Pb/206Pb mean above 0.0004, and those that scatter morethan two standard deviations (2r) from the mean age value arediscarded. The calculated age for a single zircon grain and its error,according to Gaudette et al. (1998), is the weighted mean and stan-dard error of the accepted blocks of data. The same procedure isadopted to calculate the age for a rock sample from a set of coge-netic grains. The ages are presented with 2r error.

Fig. 2. Geological map of the central-eastern Bacajá domain (based on Faraco et al. (2005)) with location of dated samples.



For the Sm–Nd analysis, a mixed 150Nd–149Sm spike is added toca. 100 mg of rock powder and attacked with HF + HNO3 in Teflonvials inside PARR containers at 150 �C for one week. After evapora-tion, new additions of HF + HNO3 are made, the solutions are dried,followed by dissolution with HCl (6 N), drying, and finally dissolu-tion with HCl (2 N). After the last evaporation, the REE are sepa-rated from other elements by cation exchange chromatography(Dowex 50WX-8 resin) using HCl (2 N) and HNO3 (3 N). After that,Sm and Nd were separated from the other REE by anion exchangechromatography (Dowex AG1-X4 resin) using a mixture of HNO3

(7 N) and methanol. The isotopic measurements are statically ac-quired using the Faraday cups of the mass spectrometer, and Nddata are normalized to a 146Nd/144Nd ratio of 0.7219. Procedureblanks were <100 pg for Sm and <400 pg for Nd. The La Jolla Ndstandard yielded a 143Nd/144Nd ratio of 0.511844 ± 22 (2r) basedon four analyses. The crustal residence ages were calculated usingthe model of De Paolo (1988) for the depleted mantle (TDM).

Age calculation was done using the software Isoplot (v.2.49) ofLudwig (2001) and others developed in the Pará-Iso.

5. Zircon ages

Seven rock samples from different igneous and metaigneouslithological units of the central-eastern Bacajá domain were datedby the zircon Pb-evaporation technique. The results revealed ageswithin a ca. 0.6 billion years interval. Combined with the fieldand petrographic data, as well as with previous geochronological

Table 1Zircon Pb-evaporation isotopic data from rocks of the central-eastern part of the Bacajá doxxx/yyy is the total isotopic ratios measured (xxx) and used (yyy) in age calculation.

Sample/grain Ratios 204Pb/206Pb ±2r (208Pb/206Pb)c

MDM03/3 24/62 0.000009 4 0.13606MDM03/5 34/62 0.00002 4 0.12147MDM03/6 40/114 0.000005 4 0.15041MDM03/9 106/114 0.000013 2 0.13313MDM03/10 66/88 0.000071 47 0.15042

270/440MDM01/1 16/54 0.000248 26 0.18409MDM01/2 34/44 0.000019 4 0.16612MDM01/12 40/100 0.000027 4 0.16481MDM01/13 32/70 0.000024 3 0.14657

122/268MDM07C/1 30/38 0.000099 11 0.08815MDM07C/2 36/72 0.000019 7 0.15248MDM07C/4 32/66 0.000019 3 0.13579MDM07C/7 34/34 0.000052 7 0.10401MDM07C/8 62/86 0.000017 13 0.12469MDM07C/10 8/8 0.000021 2 0.11289

202/304MJ36/1 32/64 0.000034 4 0.20205MJ36/2 38/62 0.000027 2 0.21637MJ36/5 34/70 0.000011 4 0.24534

104/196MCM18/4 40/54 0.000062 8 0.11353MCM18/8 36/48 0.000118 22 0.11915MCM18/9 72/72 0.000039 15 0.10852MCM18/10 20/20 0.000076 76 0.09662MCM18/11 62/62 0.000039 3 0.07653MCM18/12 34/42 0.000014 5 0.10268

264/298MDM02/2 16/24 0.000349 37 0.10198MDM02/3 32/62 0.000154 2 0.13494MDM02/4 16/46 0.00004 18 0.09334MDM02/6 8/12 0.000133 2 0.12484

72/144MCM58/1 40/78 0.000110 12 0.35182MCM58/2 38/54 0.000041 13 0.17094MCM58/4 20/50 0.000053 6 0.07279

98/182

(207Pb/206Pb)c and (208Pb/206Pb)c = ratio corrected for common Pb.

results, the rock units can be separated into six groups represent-ing different stages of the magmatic and tectonic evolution of theBacajá domain. Although the rocks show different degrees of re-crystallization and deformation, the general features of the zircongrains, the similarity with some previous results, and the accuracyof the new ages allow them to be interpreted as the crystallizationages of the grains and, consequently, the emplacement ages of thebodies. These results will be discussed below, from the oldest tothe youngest rock units.

5.1. Tonalitic gneiss

Sample MDM03A was collected at the Manelão gold mine lo-cated on the WNW–ESE Bacajá transcurrent shear zone (Fig. 2). Itis an orthogneiss included by Souza et al. (2003) and Souza andKotschoubey (2005) in the Xingu Complex (Silva et al., 1974). Itis a light gray, fine to medium-grained, banded hornblende-biotitetonalitic gneiss. Under the microscope, this gneiss shows polygonalgranoblastic quartz-feldspar arrays and lepido-nematoblasticaggregates of mafic minerals. Hornblende and epidote are subordi-nate, and apatite, titanite, zircon and opaque are accessoryminerals.

Selected zircon crystals are prismatic, bipyramidal, light brownto colorless, semitransparent, and show few inclusions, fracturesand metamictization features. Some crystals have rounded edgesand are sometimes drop-shaped. Five crystals were analyzed yield-ing individual ages varying from 2674 to 2664 Ma, and a mean ageof 2671 ± 3 Ma (Table 1, Fig. 3). The 40 blocks and 270 isotopic

main. Only results included in the age calculation are presented. In the Ratios column,

±2r (207Pb/206Pb)c ±2r Age (Ma) ±2r

35 0.18177 55 2669.5 5.031 0.18158 23 2667.5 2.157 0.18122 37 2664.3 3.481 0.18208 44 2672.3 3.9134 0.18225 17 2673.8 1.6

207Pb/206Pb mean age= 2671.2 2.6117 0.15922 69 2447.6 7.469 0.15899 54 2445.2 5.86 0.15842 23 2439.1 2.434 0.15809 24 2435.6 2.5

207Pb/206Pb mean age= 2438.5 3.963 0.15055 32 2352.4 3.738 0.15114 26 2359.1 3.043 0.15136 41 2361.7 4.695 0.15123 56 2360.2 6.337 0.15118 16 2359.6 1.866 0.15159 53 2364.2 6.0

207Pb/206Pb mean age= 2359.0 2.369 0.13681 39 2187.5 4.958 0.13714 35 2191.7 4.564 0.13712 2 2191.4 2.5

207Pb/206Pb mean age= 2190.8 2.171 0.13395 31 2150.6 4.0103 0.13362 97 2146.3 12.7103 0.13447 4 2157.4 5.2181 0.13311 63 2139.6 8.258 0.13381 24 2148.8 3.113 0.13411 65 2152.7 8.4

207Pb/206Pb mean age= 2153.9 3.873 0.12924 68 2088.0 9.357 0.12902 21 2084.9 2.838 0.12945 66 2090.8 8.9174 0.12832 58 2075.5 7.9

207Pb/206Pb mean age= 2084.7 4.039 0.12492 18 2079.2 4.445 0.12822 36 2073.4 4.967 0.12856 57 2078.8 7.8

207Pb/206Pb mean age= 2076.9 3.0

Zircon2346

Age

[Ma]

MDM07C/10

MDM07C/8

MDM07C/7MDM07C/4

MDM07C/2

MDM07C/1

2350

2354

2358

2362

2366

2370

2374

Age = 2359 ± 2 MaMSWD = 3.5

MDM-07Cbox heights are 2

Age

[Ma]

Zircon

MCM18/12MCM18/11

MCM18/10

MCM18/9

MCM18/8

MCM18/4

2125

2135

2145

2155

2165

Mean = 2154 ± 4 MaMSWD = 3.2

MCM-18box heights are 2

MDM03/10

MDM03/9MDM03/6

MDM3/5

MDM3/3

2658

2662

2666

2670

2674

2678

Zircon

Age

[Ma]

Mean = 2671 ± 3 MaMSWD = 5.6

MDM-03box heights are 2

Zircon

Tonalitic gneiss

MDM01/13

MDM01/12

MDM01/2

MDM01/1

2430

2434

2438

2442

2446

2450

2454

2458

Age

[Ma]

Age = 2439 ± 4 Ma MSWD = 5.7


Quartz-monzodioritic gneiss

Metandesite

MJ36/5

MJ36/2

MJ36/1

2181

2183

2185

2187

2189

2191

2193

2195

2197

Zircon

Age

[Ma]

Age = 2191 ± 2 MaMSWD = 1.1

MJ-36box heights are 2

Monzogranite

Granodiorite

MCM58/1

MCM58/2

MCM58/4

2066

2070

2074

2078

2082

2086

2090

Age = 2077 ± 3 MaMSWD = 1.7

box heights are 2

Zircon

Age

[Ma]

MCM-58Granodiorite

MDM02/6

MDM02/4

MDM02/3

MDM02/2

2060

2070

2080

2090

2100

Zircon

Age

[Ma]

Age = 2085 ± 4 Ma MSWD = 2.6


Granodiorite

σ

σσ

σ σ

σσ

Fig. 3. Zircon Pb-evaporation age diagrams for rocks from the central-eastern part of Bacajá domain.



ratios, mostly obtained at the highest evaporation temperature(1550 �C), show a very homogeneous pattern, yielding a well-de-fined mean age.

5.2. Quartz-monzodioritic gneiss

Sample MDM01 is a gneissic quartz-monzodiorite collected atBelmonte (Fig. 2), where the outcrop is intruded by a pink mediumto coarse-grained leucogranite (possibly a leucossome vein?). Thegneiss is fine to medium-grained, light gray and banded, with gra-noblastic and lepidoblastic textures. The fabric is marked by elon-gated crystals of quartz and feldspar. Hornblende and biotite aresubordinate, and the accessory minerals are apatite, zircon andallanite.

Zircon crystals are prismatic, semitransparent, light brown tocolorless, and show fractures and metamictization features. Differ-ent types of inclusions are present: long and colorless (fluid?),round, dark brown, irregular etc. The analytical results for fourgrains yielded a mean age of 2439 ± 4 Ma Table 1, Fig. 3) from 17blocks and 122 isotopic ratios. Crystal # 6 was rejected since ithas a slightly older age (2457 ± 5 Ma) and most likely representsan inherited grain.

A similar early Siderian age (2440 ± 7 Ma obtained by Pb-evap-oration on zircon) was reported by Vasquez et al. (2005) for an en-clave of quartz dioritic gneiss hosted by a porphyroclasticgranodiorite with an age of 2215 ± 2 Ma obtained by Pb-evapora-tion on zircon. The granodiorite is exposed south of Brasil Novoon the western bank of the Xingu River (Fig. 2). Additionally, San-tos (2003) presented an age of 2491 ± 7 Ma (U–Pb SHRIMP) forinherited zircon crystals included in the Brasil Novo Tonalite(2182 ± 7 Ma) (Fig. 2).

5.3. Três Palmeiras metandesite

Mafic to intermediate metavolcanic rocks, metatuffs, and asso-ciated tonalites and diorites are exposed in the eastern part of theTrês Palmeiras greenstone belt. These rocks are cut by gold-bearingquartz veins with arsenopyrite, pyrite and chalcopyrite related toNW–SE shear zones. Sample MDM07C, from the Zé Meneses goldmine (Fig. 2), is a metandesite with porphyroclasts of plagioclaseand hornblende, in a microgranular biotite and quartz groundmass.Epidote, zircon, apatite and titanite are accessory minerals.

Zircon crystals from sample MDM07C are prismatic, bipyrami-dal, but with slightly rounded edges. They are pale brown color,translucent to transparent, and have few inclusions and fractures.Radial cracks and long inclusions parallel to the c-axis are observedin some grains. Of the crystals selected for isotopic analysis, eightwere used to calculate an age of 2359 ± 2 Ma (Table 1, Fig. 3),which was obtained from 220 isotopic ratios of 33 blocks.

Two other rocks from Bacajá domain furnished similar ages byU–Pb SHRIMP on zircon (Fig. 2): a tonalite (2313 ± 9 Ma, Faracoet al., 2005), intruded into the Jacaré Complex and located closeto the town of Novo Repartimento, and a metatonalite(2338 ± 5 Ma, Vasquez et al., 2008), intruded into the Três Palme-iras greenstone belt.

5.4. 2.19 Ga old monzogranite

Sample MJ36 was also collected at Belmonte (Fig. 2) and is amonzogranite of medium grain size and pale gray to pink color,which cuts the MDM01 metaquartz-monzodiorite gneiss. The rockshows hipidiomorphic granular to granoblastic texture with mod-erate to incipient mylonitic foliation, and is locally banded. Anti-perthite is present and amphibole and biotite are the maficminerals. Accessory minerals are allanite and zircon; muscoviteseems to be an alteration product.

Zircon crystals from sample MJ36 selected for analysis haverounded edges, few inclusions and fractures, and are pale brownto colorless, transparent to translucent and show faint oscillatoryzoning. From seven grains analyzed, only three crystals emitted en-ough Pb for isotopic measurements to be useful in the age calcula-tion (Table 1, Fig. 3). They yielded a mean age of 2191 ± 2 Ma from104 isotopic ratios distributed in 15 blocks.

Santos (2003) obtained a similar age of 2182 ± 6 Ma (zircon U–Pb SHRIMP) for a tonalite exposed near Brasil Novo, on the leftmargin of the Xingu River in the northwestern part of the study re-gion (Fig. 2).

5.5. 2.15 Ga old granodiorite

In Belo Monte, at the eastern margin of the Xingu River (Fig. 2),granodiorites are common. They host E–W quartz-feldspathicveins, which are folded, giving them a gneissic structure. SampleMCM18 is a medium- to fine-grained leucogranodiorite havinggranular to granoblastic, locally cataclastic textures and incipientfoliation. Biotite and hornblende are the main mafic minerals,and titanite, apatite, zircon and allanite are accessory minerals.

Zircon crystals show rounded edges, light pink color, and aretranslucent to transparent, with few inclusions and fractures. Theyhave oscillatory zoning suggesting an igneous origin. The isotopicresults for six grains yielded the mean age of 2154 ± 4 Ma calcu-lated from 264 ratios in 39 blocks (Table 1, Fig. 3).

Vasquez et al. (2008) presented a U–Pb SHRIMP zircon age of2133 ± 10 Ma for a sheared tonalite from the northwestern partof the region, on the east bank of the Xingu River (Fig. 2). This rockcontains 2340 Ma-old inherited zircon crystals. Additionally, aquartz monzodiorite intruded into the Três Palmeiras greenstonebelt at the Galo gold mine was dated at 2160 ± 3 Ma.

5.6. 2.08 Ga old monzogranite and granodiorite

A coarse-grained biotite leucogranodiorite (sample MCM58)showing N70 W sub-horizontal magmatic foliation is exposedapproximately 15 km NE from Novo Repartimento (Fig. 2).

Zircon grains from sample MCM58 are euhedral, pale to darkbrown, forming short prisms with few inclusions or cracks, andhaving weak oscillatory zoning. From seven grains selected for iso-topic analysis, only three emitted enough Pb to be considered inthe calculation of a mean age of 2077 ± 3 Ma (Table 1, Fig. 3), ob-tained from 19 blocks encompassing 140 isotopic ratios.

Barros et al. (2007) analyzed a sample (MCM55b) of titanite–biotite granodiorite, collected 8 km NW from Novo Repartimento(Fig. 2). This granodiorite is considered to be a variety of theMCM58 leucogranodiorite. Four zircon crystals yielded an age of2076 ± 6 Ma by the Pb-evaporation method. A fifth grain indicatedan age of 2110 ± 11 Ma, which ‘‘probably corresponds to an inher-ited grain from an early stage of the long magmatic event associ-ated with the Maroni-Itacaiúnas Province”.

A foliated, medium-grained, pale pink monzongranite (sampleMDM02) is exposed between Belmonte and the Manelão gold mine(Fig. 2). In this monzogranite, varietal minerals are biotite andmuscovite, while zircon, allanite and apatite are accessoryminerals.

Zircon crystals are frequently euhedral, form long, colorless topale brown prisms with few inclusions and fractures, and faintoscillatory zoning. Some grains have rounded edges, radial cracksand metamictization features. Four zircon grains provided isotopicresults suitable for age calculation and yielded a mean age of2085 ± 4 Ma from 72 isotopic ratios divided into 10 blocks (Table1, Fig. 3).

The petrographic features and age of sample MDM02 are verysimilar to those described by Souza and Kotschoubey (2005) for


the Felício Turvo Granite (2069 ± 6 Ma, Pb-evaporation method), atthe Manelão gold mine. In light of additional mapping undertakenby Faraco et al. (2005), it is possible to confirm that both samplesbelong to the same elongated igneous body.

Similar ages to those discussed above were obtained for graniticrocks from the Bacajá domain, on the left margin of the Xingu River(Fig. 2); for example, the Belo Monte monzogranite yielded a U–PbSHRIMP age for zircon of 2086 ± 6 Ma (Santos, 2003). To the south-west, Vasquez et al. (2005) described an inequigranular biotitemonzogranite with an age of 2077 ± 2 Ma (Pb-evaporation on zir-con) crosscutting a 2215 ± 2 Ma old porphyroclastic granodiorite.

At the eastern end of the Bacajá domain (Fig. 2), Faraco et al.(2005) reported a U–Pb SHRIMP zircon age of 2114 + 35/�33 Mafor a granodiorite of the Valentim Complex.

6. Whole-rock Sm–Nd isotopic data

Except for sample MCD58, all samples dated by the Pb-evapora-tion technique were also analyzed by the Sm–Nd method. Analyseson eight additional samples were also carried out. The results aregiven in Table 2 and Fig. 4. There is a direct relationship betweenthe number of samples analyzed and the estimated area of expo-sure of each unit in the study area. In consequence, there are moreresults for the younger groups compared with the older ones. Sam-ple MDM07A, from the same gold mine where metandesiteMDM07C was collected, corresponds to a metadiorite with amphi-bole and biotite. Taking into consideration the field relationships,and that it is isotopically similar to the metandesite MDM07C(see Table 2), it was considered to be coeval with the metandesite,and also to belong to the Três Palmeiras greenstone belt.

The crustal residence ages were calculated using the De Paolo(1988) model for the depleted mantle (TDM), whereas eNd(T)(CHUR)values were calculated using the zircon ages obtained in this study.Where the ages are not available, the sample was correlated withone of the lithological units of the Bacajá domain, taking into ac-count similarities of petrographic features and geographical distri-bution. Its age was assumed to be that of the unit.

Since metandesite MDM07C is composed of different fragmentsfrom drill-core samples, two of them (C1 and C2) were analyzed inorder to check heterogeneity in the sequence. The Sm and Nd con-tents of the sample set range from 2 to 9 ppm and 12 to 71 ppm,

Table 2Whole-rock Sm–Nd isotopic results from the central-eastern part of the Bacajá domain.

Sample Sm (ppm) Nd (ppm) 147Sm/144Nd 143Nd/144Nd (±

2.67 Ga Archean tonalitic gneissMDM03 3.47 16.32 0.12845 0.511571 (22)2.44 Ga Siderian gneissesMDM01 13.05 71.42 0.11048 0.511104 (17)2.36 and 2.31 Ga Siderian metandesites and metadioritesMDM07C1 2.50 12.19 0.12412 0.511549 (23)MDM07C2 2.84 13.76 0.12485 0.511476 (31)MDM07A 2.47 11.95 0.12490 0.511549 (16)2.22–2.18 Ga Rhyacian granitoidsMJ36 8.16 52.20 0.09448 0.510774 (15)MJ37 7.08 45.07 0.09492 0.510912 (12)2.16 to 2.13 Ga Rhyacian granitoidsMCM18 2.37 17.60 0.08140 0.511013 (32)2.09 to 2.07 Ga Rhyacian granitoidsMCM27 9.04 53.42 0.10234 0.511132 (10)MCM54 1.95 12.63 0.09319 0.511266 (26)MCM55 6.13 44.93 0.08248 0.511047(22)MCM56 25.19 171.07 0.08903 0.510957 (08)MDM02 3.90 25.49 0.09257 0.510806 (14)MDM04 7.44 38.16 0.11787 0.511376 (16)MDM08 5.32 23.51 0.13691 0.511804 (13)MDM09 2.44 19.61 0.07506 0.510642 (28)

* Estimated age.

respectively, with the lower contents in the mafic to intermediaterocks from the Três Palmeiras greenstone belt. An exception issample MCM56 (hornblende-biotite granodiorite), with 25 ppmSm and 171 ppm Nd.

Nd TDM model ages for the sample set range from 2.25 to2.93 Ga and can be divided into two groups with Paleoproterozoicand Archean model ages. The samples with Paleoproterozoic modelages are dominated by 2.08 Ga monzogranite to granodiorite. Aparticular aspect of this group is that all the samples were collectedin the northern part of the area, especially along the BR-230 road.Samples from all other lithological units belong to the group withArchean model ages. The higher eNd(T) values are close to zero andcorrespond to samples of the group with Paleoproterozoic modelages (eNd(T) from �0.60 to +0.83) and also to the mafic to interme-diate rocks of the Três Palmeiras greenstone belt (eNd(T) from –0.87to +0.78). A special case is represented by sample MDM03 (Arche-an tonalitic gneiss), with one sample showing the highest eNd(T)

value (+2.7). Apart from these samples, the others have negativeeNd(T) values ranging from �2.9 to �8.3, typical of rocks with acrustal origin.

In summary, taking into account the division into lithologicalunits and the Nd isotopic data, the samples from the study areacan be divided into the following geochronological and isotopicgroups:

1. Paleoproterozoic monzogranite and granodiorite (2.08 and2.15 Ga) with Paleoproterozoic Nd TDM ages (2.25–2.47 Ga)and eNd(T) close to zero, between �0.60 and +0.83;

2. Paleoproterozoic monzogranite to granodiorite and quartz-monzodioritic gneiss (2.08–2.44 Ga) with Archean Nd TDM ages(2.57–2.93 Ga) and negative eNd(T), between �8.33 and �2.9;

3. Paleoproterozoic mafic to intermediate rock (2.36 Ga) withArchean Nd TDM ages (2.56–2.71 Ga) and eNd(T) close to zero,between �0.87 and +0.78, and

4. Archean tonalitic gneiss (2.67 Ga) with Archean Nd TDM age(2.65 Ga) and positive eNd(T) (+2,66).

7. Discussion

Table 3 summarizes the geochronological data available forigneous and metaigneous rocks from central-eastern Bacajá

2r) f (Sm/Nd) eNd(0) t(zircon) (Ga) eNd(t) TDM (Ga)

�0.3470 �20.81 2.67 2.66 2.65

�0.4383 �29.92 2.44 �2.90 2.89

�0.3690 �21.24 2.36 0.78 2.56�0.3653 �22.67 2.36 �0.87 2.71�0.3650 �21.24 2.36 * 0.55 2.58

�0.5197 �36.36 2.19 �7.63 2.93�0.5174 �33.67 2.19 * �5.04 2.76

�0.5862 �31.70 2.15 0.21 2.35

�0.4797 �29.38 2.08 �4.25 2.63�0.5262 �26.76 2.08 0.83 2.25�0.5807 �31.04 2.08 * �0.60 2.33�0.5474 �32.79 2.08 * �4.12 2.57�0.5294 �35.74 2.08 �7.89 2.84�0.4008 �24.62 2.08 * �3.63 2.67�0.3040 �16.27 2.08 * �0.34 2.47�0.6184 �38.94 2.08 * �6.55 2.66

1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4

T(Ga)

-50

-45

-40

-35

-30

-25

-20

-15

-10

-5

0

5

10

15

DM

CHUR

Carajás Block

MCM18

MJ36MJ37

MCM54MCM55MCM56

MDM04

MDM09

MDM02MCM27

MDM08

2.08 Ga monzogranite to granodiorite

2.19 Ga monzogranite

2.15 Ga granodiorite

Trans-Amazonian rocks

1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4

(Nd)

-50

-45

-40

-35

-30

-25

-20

-15

-10

-5

0

5

10

15

DM

CHUR

Carajás Block

2.67 Ga tonalitic gneiss

MDM01

MDM03

MDM07C1MDM07C2MDM07A

2.36 Ga metandesite and metadiorite

2.44 Ga quartz-monzodioritic gneiss

Pre-Trans-Amazonian rocks

T(Ga)

εA B

Fig. 4. eNd vs. time diagram from the central-eastern part of Bacajá domain: A – Pre-Trans-Amazonian rocks, and B – Trans-Amazonian rocks. Field of Archean rocks fromCarajás block is also plotted (see text for references).


domain, including those obtained in this work. Only zircon analy-ses (U–Pb SHRIMP and Pb-evaporation) were considered in orderto better constrain the timing of the magmatic events. The rocksare Neoarchean and Paleoproterozoic (Siderian and Rhyacian peri-ods). Taking into consideration the ages of the rocks and their geo-logical setting, the following magmatic events, and their mostprobable tectonic settings may be recognized in the Bacajádomain:

1. Archean tonalitic gneiss, whose protolith was formed at ca.2.67 Ga, occurs at the Manelão gold mine, and is included inthe Xingu Complex (Souza and Kotschoubey, 2005). At the sam-pling site, evidence that clarifies the origin and the genetic rela-tionship of this gneiss with the surrounding rocks was notobserved. Since it is the oldest rock in the domain, adjacent

Table 3Geochronological data available for zircon from igneous and metaigneous rocks of the cen

Rock type/lithological unit/sample Area

2.67 Ga Archean tonalitic gneissTonalitic gneiss/MDM03A Manelão gold mineSiderian gneisses of 2.44 GaQuartz-dioritic gneiss Brasil NovoQuartz-monzodioritic gneiss/MDM01 Belmonte village2.36 and 2.31 Ga Siderian metatonalites and metandesitesMetandesite/Três Palmeiras/MDM07C Zé Meneses gold minePorphyroclastic metatonalite Bacajá RiverTonalite/Jacaré Complex Novo Repartimento2.22–2.18 Ga Rhyacian granitoidsPorphyroclastic granodiorite Brasil NovoMonzogranite/MJ36 BelmonteTonalite Brasil Novo2.16–2.13 Ga Rhyacian granitoidsQuartz monzodiorite Galo gold mineLeucogranodiorite/MCM18 Belo MonteSheared tonalite Xingu River-Brasil NovoGranodiorite/Valentim Complex Novo Repartimento2.09–2.07 Ga Rhyacian granitoidsBelo Monte Monzogranite Belo MonteLeucogranodiorite/MCM58 Novo RepartimentoGranodiorite Novo RepartimentoFelício Turvo Granite/MDM02 Manelão gold mineFelício Turvo Granite Manelão gold mineMonzogranite Xingu River-Brasil Novo

References: 1 – this work; 2 – Santos (2003); 3 – Vasquez et al. (2005); 4 – Vasquez et a

rocks intrude or cover it (Souza and Kotschoubey, 2005). Corre-lation with Archean rocks in the adjacent Carajás block in theCentral Amazonian Province seems unlikely. In the Carajásblock, the rocks are older (3.00–2.76 Ga), and have generallyhigher Nd TDM ages, mainly 2.9–3.20 Ga (e.g. Olszewski et al.,1989; Sato and Tassinari, 1997; Dall’Agnol et al., 1999b; Teixe-ira et al., 2002; Rämö et al., 2002; Galarza, 2002; Rolando andMacambira, 2003; Barros et al., 2004), whereas the Manelãomine gneiss is clearly juvenile. Unless new results show thatthese rocks played a significant role in the evolution of the Bac-ajá domain, a possible working hypothesis is that this gneiss isjust a small fragment of older crust, trapped during the accre-tion of arcs which probably generated the Bacajá domain duringthe Paleoproterozoic. Its isotopic characteristics suggest thatthe protolith was an island-arc or TTG suite. Another Archean

tral-eastern of the Bacajá domain.

Zircon age (Ma) magmatic/inherited Method Ref.

2671 ± 3 Pb-evaporation 1

2440 ± 7 Pb-evaporation 32439 ± 4 Pb-evaporation 1

2359 ± 2 Pb-evaporation 12338 ± 5 U–Pb SHRIMP 42314 ± 9 U–Pb SHRIMP 5

2215 ± 2/2524 ± 5 Pb-evaporation 32191 ± 2 Pb-evaporation 12182 ± 6/2491 ± 7 U–Pb SHRIMP 2

2160 ± 3 U–Pb SHRIMP 42154 ± 4 Pb-evaporation 12133 ± 10/2340 U–Pb SHRIMP 42114 +35/-33 U–Pb SHRIMP 5

2086 ± 6 U–Pb SHRIMP 22077 ± 3 Pb-evaporation 12076 ± 6/2110 ± 11 Pb-evaporation 62085 ± 4 Pb-evaporation 12069 ± 6 Pb-evaporation 72077 ± 2 Pb-evaporation 3

l. (I 2008); 5 – Faraco et al. (2005); 6 – Barros et al. (2007); 7 – Souza et al. (2003).


gneiss is exposed at the western end of the Bacajá domain,south of Uruará, outside the area showing in Fig. 2. It is a2503 ± 10 Ma old tonalitic gneiss with inherited crystals datedat 2581 ± 6 Ma (U–Pb SHRIMP on zircon, Santos, 2003).

2. Siderian granitoids crystallized at ca. 2.44 Ga and were latertransformed into quartz-dioritic gneiss (Brasil Novo area) andquartz-monzodioritic gneiss (Belmonte area). At the Brasil Novoarea, the quartz-dioritic gneiss is a xenolith included in Rhya-cian granitoids. Apart from the Nd results for the Belmontegneiss, which suggest an origin from melting of older Archeancrustal rocks (Table 2), their situation is similar to that of theArchean gneiss of Bacajá domain, i.e., there is no clear evidenceto speculate about the origin of these rocks. The hypothesis sug-gested here that these Siderian gneisses are simply small rem-nants of older continental crust (in a continental arc?) needsconfirmation.

3. Siderian metatonalites, metadiorites and metandesites (2.36–2.31 Ga, Table 3) are associated with the Três Palmeiras green-stone belt and with the Jacaré Complex, in the northwesternand northeastern parts of the study area (Fig. 2), respectively.According to Jorge João et al. (1987), the compositions of theTrês Palmeiras mafic rocks range from island-arc tholeiite toMORB. On the other hand, the Nd isotopes (Table 2) denote acommon mantle source, and no or very little crustal contamina-tion for both metandesite and metadiorite. This, added to char-acteristics previously described, is indicative of an island-arcenvironment. In spite of the absence of conclusive evidence tocharacterize the tectonic setting of the Três Palmeiras green-stone belt and surrounding gneiss and metagranitoid units, itis thought that remnants of an oceanic floor are present, ashas been suggested for the northern part of Maroni-ItacaiúnaProvince – Guiana shield (Vanderhaeghe et al., 1998). Thesesupracrustal rocks in the Bacajá domain, together with the2.44 Ga gneisses, are the first Siderian rocks reported in theAmazonian craton, and represent a unique feature that con-trasts with other domains of the Maroni-Itacaiúnas Province.The Jacaré Complex (Novo Repartimento area) was describedby Faraco et al. (2005) as a 2313 ± 9 Ma old (SHRIMP age for atonalite) association of protomylonitic monzogranite, met-amonzogranite, metatonalite, metagranogranodite, tonaliteand metasienogranite. However, Barros et al. (2007) reportedthe age of 2076 ± 6 Ma for a granodiorite from the same area,in agreement, therefore, with the age of 2077 ± 3 Ma reportedhere for another granodiorite sample (Fig. 2, Table 3). OurSm–Nd results (Table 2) suggest the participation of a juvenilecomponent in the northern part of the region, corroboratingthe homogeneity of the rock types as suggested by Barroset al. (2007). Since tonalite is a subordinate rock type in thearea, the hypothesis that Siderian rocks have restricted occur-rences remains to be tested; they might represent remnantsof plutonic rocks associated with the Três Palmeiras greenstonebelt.

4. Rhyacian granitoids are widespread in the northern part of theBacajá domain and are represented by different rocks types.Tonalite, granodiorite and monzogranite were emplacedroughly in this sequence during an igneous event which lastedfor approximately 140 My. Few detailed petrographic andstructural studies have been carried out on the Bacajá domain(Vasquez et al., 2005; Barros et al., 2007), and the observationthat the older rocks are more deformed than the younger onesneeds confirmation.

Despite the scarcity of data, it is possible to trace a parallel withother better studied domains of the Maroni-Itacaiúnas Province. Inthis way, the Rhyacian granitoids can be separated into sub-groups, which may correspond to different stages (or orogenies:

Santos, 2003) of the tectono-magmatic evolution of the Trans-Amazonian cycle, as already suggested for the northern part ofthe province, the Guiana shield (e.g. Vanderhaeghe et al., 1998; De-lor et al., 2003; Santos, 2003; Rosa-Costa et al., 2006).

Monzogranite, tonalite and quartz monzodiorite showing hipid-iomorphic granular to granoblastic texture with moderate to incip-ient mylonitic foliation, locally banded, are recognized in the BrasilNovo and Belmonte areas. They were intruded between 2.22 and2.18 Ga into older continental crust, as indicated by their Nd isoto-pic compositions (Nd TDM ages = 2.9 and 2.8 Ga; eNd(T) = �5.0 and�8.3) and by the presence of inherited zircon grains (2491 ± 7 Maand 2524 ± 5 Ma, see Table 3). The evidence corroborates the tec-tonic setting as a continental arc at the margin of an Archean con-tinent, representing, therefore, the fist stage of the Trans-Amazonian cycle.

The period between 2.16 and 2.13 Ga is characterized by theemplacement of tonalite, quartz monzodiorite and granodioritein the northwestern part of the region. They cross cut the TrêsPalmeiras greenstone belt, which had been already accreted tothe continental margin. A particular feature of this phase, whichcontrasts with the previous, is the presence of juvenile material(Table 2). In fact, this period corresponds to the main granitogene-sis in the Guiana shield, similarly correlated with the evolution of acontinental arc (e.g. Vanderhaeghe et al., 1998; Delor et al., 2003;Rosa-Costa et al., 2006).

The rocks formed in the central-eastern Bacajá domain duringthe short period of time from 2.09 to 2.07 Ga mainly comprisegranodiorites, monzogranites with subordinate syenogranites(Felício Turvo Granite), and charnockites with preserved igneoustextures. They predominate in the northern part of the study re-gion, where Barros et al. (2007) reported a belt of calc-alkaline I-type granitoids. Nd isotopes allowed the classification of theserocks into two groups: granitoids with Paleoproterozoic Nd TDM

ages (2.25–2.47 Ga) and eNd(T) close to zero, between �0.60 and+0.83, and granitoids with Archean Nd TDM ages (2.57–2.84 Ga)and eNd(T) essentially negative, between �7.9 and �3.6. These datalead to the proposal that both juvenile and Archean reworkedcrusts are the sources for the last magmatic products of theTrans-Amazonian cycle. Some degree of mixing generated inter-mediate Nd TDM ages values.

8. Conclusions

Although a Paleoproterozoic evolution for the Bacajá domainwas proposed several decades ago (Amaral, 1974; Cordani et al.,1979), its geology is still very poorly known, especially when com-pared with that of the adjacent Carajás block. Over the last years,efforts of teams from the Federal University of Pará and CPRM-Bra-zilian Geological Survey have improved the knowledge about thedomain, allowing the tracing of parallels with other areas of theMaroni-Itacaiúnas Province, especially those in the Guiana shield.Our new geological and isotopic results combined with previousdata lead to the proposal of a comprehensive multi-stage evolutionfor the eastern part of the Bacajá domain, starting during the Neo-archean and ending at the Rhyacian.

Neoarchean tonalitic gneiss (2671 ± 3 Ma) included into theXingu Complex is the oldest rock recorded in the Bacajá domain.Due to its composition and juvenile nature, it probably representsa remnant of an island arc or TTG suite and marks an early stage ofcrust formation. Another probable remnant is represented by Side-rian gneisses crystallized at ca. 2.44 Ga. Contrasting with the Neo-archean gneiss, protholiths of these rocks represent reworkedcontinental crust, most likely in a continental arc. Despite theuncertainty about the origin of these gneisses, it is evident thatthese rocks cannot be correlated with the Archean rocks of the


Carajás block, which are older and originated from reworking of aca. 2.9–3.2 Ga crust.

The Três Palmeiras greenstone succession forms NW–SE beltsaffected by shear zones and hosting gold deposits. Its volcano-plu-tonic association, emplaced between 2.36 and 2.34 Ga, representsthe first Siderian supracrustal rocks recorded in the Amazoniancraton. This succession is surrounded by younger continentalrocks, suggesting that it was part of a probable island arc/oceanicfloor accreted to the continental margin, a hypothesis corroboratedby its chemical composition and juvenile origin indicated by Ndisotopes.

Rhyacian granitoids were intruded at different times during aninterval of ca. 140 My (2.22–2.08 Ga), marking at least three stagesof the Trans-Amazonian cycle. In general, the granitoids related tothe younger stages are chemically more evolved and less de-formed. The first stage is represented by granitoids produced at2.22–2.18 Ga by melting of the Archean crust in a probable conti-nental arc setting. The second encompasses 2.16–2.13 Ga oldgranitoids which display a larger juvenile component in their ori-ginal magmas. Finally, the third stage (2.09–2.07 Ga) was mainlycharacterized by the emplacement of granodiorites, monzogranites(Felício Turvo Granite), and charnockites produced from melting ofeither juvenile or reworked crust during soft collisions.

The present-day configuration of the lithological units of thearea investigated suggests that the collision of the Bacajá domainagainst the Archean Carajás block occurred during the Trans-Ama-zonian cycle. Although only a few Nd analyses are available, espe-cially in the northern part of the domain, the Nd TDM ages andeNd(T) reveal that juvenile rocks dominated in the north, whereasin the south, approaching the boundary with the Carajás block, areworked crust predominates. No clear contribution of the Carajásblock was observed on the rocks of the Bacajá domain, althoughmixing processes could mask the evidences of this inheritance.

Acknowledgments

This work was supported by CNPq (Grant 467104/00-0), CTMin-eral/FINEP 01/2001 project, CPRM–Geological Survey of Brazil andPará-Iso Laboratory/UFPA. R. Florencio is acknowledged for techni-cal support during analytical work at UFPA. The manuscript wassubstantially improved with the constructive contributions of thereviewers, as well as the English language review of I. McReath.This paper is a contribution to PRONEX/CNPq (Proj. 103/98, Grant662103/1998-0).

References

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Crustal growth of the central-eastern Paleoproterozoic domain, SW Amazonian craton: Juvenile accretion vs. reworking