Crustal growth of the central-eastern Paleoproterozoic domain ... - UFPa

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

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Crustal growth of the central-eastern Paleoproterozoic domain, SW Amazonian craton: 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, Brazil Companhia de Pesquisa de Recursos Minerais, Av. Dr. Freitas, 3645, 66095-110 Belém, PA, Brazil c Programa de Pós-graduação em Geologia e Geoquímica, Universidade Federal do Pará, Caixa Postal 8608, 66075-110 Belém, PA, Brazil d Universidade Federal do Paraná, Departamento de Geologia, Centro Politécnico, Caixa Postal 19001, 81531-990 Curitiba, PR, Brazil e Mineração Rio do Norte S.A., Porto Trombetas, PA, Brazil b

a r t i c l e

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Article history: Received 2 August 2007 Accepted 6 February 2009

Keywords: Trans-Amazonian cycle Zircon Nd isotopes Amazonian craton Paleoproterozoic

a b s t r a c t The Trans-Amazonian cycle was an important rock-forming event in South America, generating voluminous juvenile and reworked fractions of continental crust. The Bacajá domain, in the southern sector of the Maroni-Itacaiúnas Province in the Amazonian craton, is an example of the Trans-Amazonian terranes adjacent to the Archean Carajás block. Zircon Pb-evaporation and whole-rock Sm–Nd analyses were carried out on representative samples of six lithological units, and allowed the proposal of a comprehensive tectonic-magmatic evolutionary sequence for the central and eastern parts of this domain, from the Neoarchean to the Rhyacian. Gneisses with ages of ca. 2.67 and 2.44 Ga are the oldest rocks recorded in the region, 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 supracrustal rocks in the Amazonian craton. It was probably part of an island arc/ocean floor accreted to a craton margin. Rhyacian granitogenesis lasted for ca. 140 My (2.22–2.08 Ga), marking different stages of the Trans-Amazonian cycle. The first stage is represented by continental arc granitoids formed by melting of Archean crust at 2.22–2.18 Ga. The second is characterized by the production of juvenile material between 2.16 and 2.13 Ga. The third and final stage at ca. 2.08 Ga is represented by a large volume of granitoids originated from either juvenile material or reworked crust during compressive stresses. Nd isotopes reveal that juvenile rocks dominated in the northern part of the domain, whereas those formed from reworked crust predominate in the south. The present-day configuration of the Bacajá domain results from collision against the Archean Carajás block at the end of the Trans-Amazonian cycle. Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction The geotectonic model of evolution suggesting that the Amazonian craton (Guiana and Central Brazil shields) represents a collage of Proterozoic belts or geochronological provinces surrounding Archean nuclei was first presented in the seventies (e.g. Amaral, 1974; Cordani et al., 1979). Presently, this model is considered to be the most appropriate to explain the main general features of the craton, and has been updated by several authors (Lima, 1984; Teixeira et al., 1989; Tassinari and Macambira, 1999, 2004; Tassinari et al., 2000; Dall’Agnol et al., 2000; Santos et al., 2000, 2006). The division of the craton into provinces (Fig. 1) mainly takes into account the geochronology of the regional basement, as well as general geological and geophysical features (e.g. Tassi* Corresponding author. Tel.: +55 91 3201 7483; fax: +55 91 3246 2323. E-mail address: [email protected] (M.J.B. Macambira). 0895-9811/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsames.2009.02.001

nari and Macambira, 1999). The boundaries between these geochronological provinces are key areas for understanding the growth of the craton and of the provinces themselves, which have their own geochronological, tectonic and lithological characteristics. The Trans-Amazonian cycle was an important rock-forming event in the South American Platform (e.g. Cordani and Sato, 1999). The southern part of Maroni-Itacaiúnas Province, which is the Bacajá domain, is a special example of the Trans-Amazonian terranes since it makes contact with the Archean Carajás block (Fig. 1), included in the Central Amazonian Province (e.g. Tassinari and Macambira, 2004). Mapping projects carried out by CPRM resulted in conflicting proposals for the location of the boundary between the Archean and Paleoproterozoic domains (e.g. Santos, 2003; Faraco et al., 2005; Santos et al., 2006). Apart from this question, it is also important to take into account the internal structure, composition and evolution of the provinces themselves.

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

60º W

50º W 60º W

Amazonian craton

Guyana Suriname French Guiana

Venezuela Colombia

Guia

na sh i

ield

Atlantic Ocean

Guiana Shield

Ecuador

0º

Amazon Basin

ield il sh Braz Central tralBrazil Shiel Cen

Peru

Brazil

g

Pacífic Ocean

b (Iricoumé)

Solimões Basin

500 km

Bolivia

Atlantic Ocean

Amazon Basin

0º

c b (Xingu)

a

10º S

a - Carajás block b - Xingu-Iricoumé block c - Bacajá domain

500 km

Geochronological Provinces Neoproterozoic belt

Sunsás (1.25-1.0 Ga)

Ventuari-Tapajós (1.95-1.8 Ga)

Rondoniano-San Ignacio (1.5-1.3 Ga)

Maroni-Itacaiúnas (2.2-1.95 Ga)

Rio Negro-Juruena (1.8-1.55 Ga)

Central Amazonian (> 2.5 Ga)

Figure 2

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.

This work presents new isotope data (Pb-evaporation on zircon and whole-rock Sm–Nd) for rocks cropping out in the central and eastern parts of the Bacajá domain, Pará state, in order to better characterize the age and origin of these rocks. Additionally, we hope to clarify the formation and evolution of the southernmost part of the Maroni-Itacaiúnas Province and its nature, whether by juvenile accretion, or by reworking of the rocks involved in the Trans-Amazonian cycle.

2. Regional geological setting According to recent studies (e.g. Tassinari et al., 2000; Tassinari and Macambira, 2004; Santos et al., 2000, 2006), Archean terranes constitute the southeasternmost part of the Amazonian craton (Central Amazonian Province), and are surrounded by Proterozoic provinces, which become progressively younger southwestwards (Fig. 1). Tassinari and Macambira (2004) defined the Central Amazonian Province as the oldest continental crust of the craton, which was not affected by the Trans-Amazonian cycle. Following Tassinari and Macambira (1999), Dall’Agnol et al. (1999a), and Tassinari

et al. (2000), Tassinari and Macambira (2004) divided the province into two segments: the Carajás and the Xingu-Iricoumé blocks (Fig. 1). The first comprises a 3.00–2.85 Ga granite-greenstone basement covered, in its northern part, by a ca. 2.76 Ga volcanosedimentary sequence hosting the most important mineral deposits (Cu, Fe, Au, Mn etc.) of the craton. All the Archean rocks of the Carajás block have TDM(Nd) ages between 3.2 and 2.86 Ga. The Xingu-Iricoumé block is a NW–SE segment located in the central part of the craton, and is partially covered by Phanerozoic sedimentary rocks of the Amazon basin. It represents the least studied part of the Amazonian craton. Paleoproterozoic granitoids and volcanic rocks, which dominate in this block, are largely covered by sedimentary sequences. Geochronological data for the regional basement 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 basement is based on a few TDM(Nd) ages of the Paleoproterozoic granitoids and volcanic rocks, which were probably formed by melting of the basement. The Maroni-Itacaiúnas Province (2.2–1.95 Ga) borders the northeastern and northern parts of the Central Amazonian


Province. It was formed during the Trans-Amazonian orogenic cycle, but several Archean inliers are recognized within the Paleoproterozoic rocks. The province is characterized by widespread exposures of greenschist to amphibolite facies metavolcanic and metasedimentary units, as well as by granulitic and gneissic-migmatitic terranes. Apart from the two provinces described above, Tassinari and Macambira (2004) revised some province boundaries, while maintaining others proposed in previous works (Fig. 1), which are: Ventuari-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). Santos et al. (2000) suggested other names and limits for the geochronological provinces of the Amazonian craton, which were also updated in recent publications (e.g. Santos et al., 2006). The Maroni-Itacaiúnas Province can be divided into several domains according to their geological features and geographical distribution. The Bacajá domain (Fig. 2) borders the northern part of the Carajás block (Central Amazonian Province). Its northern part is covered by rocks of the Amazon basin, and its eastern part by the Grajaú basin and the Neoproterozoic Araguaia belt. The domain extends westwards parallel to the southern margin of the Amazon basin, and is covered here by the Paleoproterozoic volcanic rocks of the Central Amazonian Province. The Bacajá domain is comparatively less well studied than the Carajás block. Its central-eastern part, the object of this work, is composed of deformed granitoids, granulites, gneisses and the Três Palmeiras and São Manoel greenstone belts which are discussed below.

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as in the Itatá Amphibolite, and in the Bacajá Micaschist. According to Faraco et al. (2005), the Três Palmeiras greenstone belt encompasses the last two units. Siderian granitoids are grouped into the Jacaré Complex, whereas Rhyacian granitoids are represented by the Valentin Complex, as well as the Felício Turvo and Bacajá granites. The map presented by Faraco et al. (2005) will be used as the geological background of this study. In spite of some divergences in relation to our data regarding rock classification, and the locations of the contacts between the lithological units, we maintain this map because it is the most recent and the most complete available. For this reason, in this work geographical references to sample locations are preferred, rather than the geological units and contacts proposed by Faraco et al. (2005). Barros et al. (2007) studied a NW–SE oriented area, parallel to the BR230 road in the northeastern part of the Bacajá domain, and described monzogranites and granodiorites, with subordinate tonalites, syenogranites and scarce quartz diorites. These rocks are rather homogeneously deformed at the regional scale, with foliations striking N60 W and WNW–ESE. Primary subvertical and flat-lying igneous layering are transposed to high-temperature secondary foliations and mylonite zones. According to these authors the development of these structures was controlled by progressive deformation under decreasing temperatures, characterizing the syntectonic emplacement of these granitoids during regional shortening. Taking into consideration the age of the granitoids (2076 ± 6 Ma, Pb-evaporation zircon age), they proposed an evolution related to a continental arc environment developed during soft amalgamation of continental plates at the end of the Trans-Amazonian cycle.

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) produced the first geological map of the region when, based only on K–Ar and Rb–Sr data, it was speculated that the Trans-Amazonian cycle had affected older rock units. Later on, Jorge João et al. (1987) and Santos et al. (1988) studied the northwestern part of this region. Their investigation recognized several lithostratigraphic units such as: 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 Paleoproterozoic reworking of gneisses, as well as juvenile additions represented by the mafic metavolcanic rocks of the Iriri-Xingu region. Some local studies were carried out on the central and the western parts of the Bacajá domain (Fig. 2). In the central part in the Manelão gold mine, Souza et al. (2003) and Souza and Kotschoubey (2005) described the poly-metamorphic regional basement of the Xingu Complex (Silva et al., 1974), and the São Manoel volcanosedimentary sequence, both of them intruded by the Felício Turvo Granite. For the northwestern part of the domain, in the Iriri-Xingu area, Vasquez et al. (2008) and Santos (2003) presented new Pbevaporation and U–Pb SHRIMP zircon data for granitoids and gneisses which indicated ages between 2.50 and 2.07 Ga. Faraco et al. (2004, 2005) reviewed the geology of the eastern Bacajá domain (Fig. 2) and proposed new lithostratigraphic units which are usually elongated along NW–SE and WNW–ESE trends. These are: the Direita Granulitic Suite composed by foliated quartz-feldspar granulites; the calc-alkaline Bacajaí Charnockitic Complex; the Ipiaçava Kinzigitic Complex including rocks with garnet, biotite and sillimanite; the Rio Preto Piriclasite represented by tholeiitic to calc-alkaline mafic granulites formed at high temperature and pressure; and the Cajazeiras Enderbitic Complex comprising calc-alkaline granulites. Metavolcano-sedimentary rocks were included in the Misteriosa and São Manoel groups, as well

4. Analytical methods Zircon from seven samples, and 15 whole-rock samples from the central-eastern part of the Bacajá domain were analyzed by Pb-evaporation and by Sm–Nd methods, respectively, at the Isotope Geology Laboratory of the Federal University of Pará (ParáIso), Brazil, using a Finnigan MAT 262 mass spectrometer. For the Pb-evaporation technique (Kober, 1986, 1987), zircon crystals were concentrated by conventional methods of heavy mineral separation, and then were hand-picked. In this technique, the individual zircon grain is encapsulated in the Re-filament used for evaporation, which was placed directly in front of the ionization filament. Both filaments are introduced into the mass spectrometer. The evaporation filament is heated to evaporate the Pb from the zircon, and the Pb liberated is condensed on the cold ‘‘ionization” filament. Three evaporation steps, each of a maximum of 5 min, are performed at 1450, 1500 and 1550 °C. After each evaporation step, the temperature of the ionization filament is raised to the point of Pb emission, and the isotopic measurements are dynamically made with the ion counter of the instrument. The intensities of the emission of each Pb isotope were measured in one cycle by peak stepping through the 206–207–208–206–207– 204 mass sequence for five mass scans, defining one data block with eight 207Pb/206Pb ratios. Five blocks are usually recorded for each evaporation step. The weighted 207Pb/206Pb mean for each block is corrected for common Pb using appropriate age values derived from the two-stage model of Stacey and Kramers (1975), and the corrected block is used for sample age calculation. Blocks yielding a 204Pb/206Pb mean above 0.0004, and those that scatter more than two standard deviations (2r) from the mean age value are discarded. The calculated age for a single zircon grain and its error, according to Gaudette et al. (1998), is the weighted mean and standard error of the accepted blocks of data. The same procedure is adopted to calculate the age for a rock sample from a set of cogenetic grains. The ages are presented with 2r error.

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

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For the Sm–Nd analysis, a mixed 150Nd–149Sm spike is added to ca. 100 mg of rock powder and attacked with HF + HNO3 in Teflon vials inside PARR containers at 150 °C for one week. After evaporation, new additions of HF + HNO3 are made, the solutions are dried, followed by dissolution with HCl (6 N), drying, and finally dissolution with HCl (2 N). After the last evaporation, the REE are separated 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 exchange chromatography (Dowex AG1-X4 resin) using a mixture of HNO3 (7 N) and methanol. The isotopic measurements are statically acquired using the Faraday cups of the mass spectrometer, and Nd data are normalized to a 146Nd/144Nd ratio of 0.7219. Procedure blanks were