Gondwana Research 21 (2012) 422–438

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Gondwana Research

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Juvenile contribution of the Neoproterozoic Rio Negro Magmatic Arc (Ribeira Belt,Brazil): Implications for Western Gondwana amalgamation

Miguel Tupinambá a,⁎, Monica Heilbron a, Claudio Valeriano a, Rubem Porto Júnior a,b,Fátima Blanco de Dios a,c,d, Nuno Machado e,1,Luiz Guilherme do Eirado Silva a, Júlio Cesar Horta de Almeida a

a TEKTOS, Geotectonics Research group, UERJ Rio de Janeiro State University, Rua S. Francisco Xavier, 524/4006-A. Rio de Janeiro RJ, Brazil 20550-900b Petrology research group, Universidade Federal Rural do Rio de Janeiro. Instituto de Agronomia, Departamento de Geociências. BR-465,Km 7. Seropédica. Rio de Janeiro RJ Brazil 23890-000c Petróleo Brasileiro SA, PETROBRAS. Av. República do Chiled Petróleo Brasileiro SA, PETROBRAS., 330/24th Torre Leste Rio de Janeiro RJ Brazil 20031–170e GEOTOP, Université du Québec à Montréal, Canada

⁎ Corresponding author at: Faculdade de Geologia, UERio de Janeiro, R. S. Francisco Xavier, 524 s. A4016. Mar20550–900. Tel.: +55 21 2334 0533#210; fax: +55 21

E-mail addresses: tupi@uerj.br, tupinambamiguel@g1 In memoriam.

1342-937X/$ – see front matter © 2011 International Adoi:10.1016/j.gr.2011.05.012

a b s t r a c t

a r t i c l e i n f o

Article history:Received 4 January 2011Received in revised form 3 May 2011Accepted 16 May 2011Available online 6 June 2011

Keywords:Western GondwanaRibeira beltNeoproterozoicMagmatic arcTectonics

The ca. 790–600 Ma Rio Negro Complex (RNC) of the Ribeira belt (Brazil) consists of a plutonic portion of amagmatic arc built by the E-vergent subduction of the ESE border of the São Francisco paleoplate during theamalgamation of Western Gondwana.The plutonic series comprises low- to medium-K granitoids (ca. 790–620 Ma) and high-K granitoids andshoshonite rocks (ca. 610–605). The age span of 185 m.y. is suggestive of a long history of arc-relatedmagmatism, continuously or not in time. The Nd isotopic signatures of the RNC consist of εNd(t) ratios from−3 to+5 for the medium-K series shoshonite series and from−14 to−3 for the younger high-K group. Thistime-dependent trend of Nd isotopes is indicative of progressive maturity of the arc over time. The sameevolution is indicated by Sr data, as the medium-K rocks have 87Sr/86Sr initial ratiosb0.705 while the high-Krocks yield values between 0.705 and 0.710. The predominance of intermediate rocks over mafic onessuggests an initial intra-oceanic to transitional stage, possibly developed in a distal portion of a passivemargin, such as the Japanese arc, evolving to a more developed, differentiated felsic rock associations.The role of transform fault zones, such as the Luanda shear zone, is emphasized in order to explain theconsumption of a wide oceanic plate in the inner portion of Western Gondwana.

© 2011 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

1. Introduction

At the onset of Western Gondwana amalgamation, east-vergingsubduction was in course along the eastern and southern margins ofthe São Francisco–Congo proto-continent (Ribeira and Araçuaí belts)and along the western margin (Coastal terrane) of the Angola-Kasaipaleo-plate, presently in the Kaoko Belt (Heilbron et al. 2008; Babinskiet al., 2011; Tohver et al., 2011). The active margin and the arc-relatedrocks of this episode are well preserved both in the Ribeira belt and inthe Coastal terrane of Africa.

RJ. Universidade do Estado doacanã. Rio de Janeiro RJ, Brazil2334 0533#214.mail.com (M. Tupinambá).

ssociation for Gondwana Research.

The Ribeira belt (Fig. 1) extends for almost 1400 km along theAtlantic coast of SE-Brazil (Almeida et al., 1981; Campos Neto, 2000;Trouw et al., 2000; Heilbron et al., 2000, 2004a,b, 2008). Its Africancounterpart, in Angola and Namibia, is represented from north tosouth by the West Congo belt, the Angola craton and the Kaoko belt(Goscombe et al., 2003, 2005a,b; Gray et al., 2006; Goscombe andGray, 2007, 2008).

Rio Negro Complex is the plutonic portion of a 790–600 Mamagmatic arc (foliated tonalites, granodiorites, granites and gabbros)that presently crops out in the central segment of the Ribeira Belt(Tupinambá et al., 2000a; Heilbron and Machado 2003).

The Rio Negro Arc is located in the central portion of westernGondwana between large cratonic blocks (Fig. 1). Despite the internallocation within the agglutinating supercontinent, subduction of alarge oceanic lithospheric plate is suggested by great longitudinalextension and protracted history (Heilbron and Machado, 2003;Heilbron et al., 2008).

Published by Elsevier B.V. All rights reserved.

Fig. 1. a) Western Gondwana map, cratonic blocks in black; b) tectonic map of SE Brazil and SW Africa in the pre-South Atlantic (Cretaceous) reconstruction of the Gondwanasupercontinent (simplified from Heilbron et al. 2008). 1, Post-Cambrian sedimentary basins; 2, cratons (SF, São Francisco; Co, Congo; LA, Luis Alves; RP, Rio de la Plata; AN, Angola;KA, Kalahari); 3, Mesoproterozoic units; 4, cratonic cover; 5, reworked cratonic margins, including basement and Neoproterozoic passive margins; 6, intra-continental West CongoBelt; 7, Brasília (Bb) and São Gabriel (Sgb) belts; 8, Apiaí terrane; 9, Paraíba do Sul, Embú and Curitiba terranes; 10, magmatic arcs (RN, Rio Negro; PA, Paranaguá; PE, Pelotas; WT,Western terrane); 11, Cabo Frio terrane.

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In order to investigate the tectonic setting of the Rio Negro Arc inthe Rio de Janeiro State, new U–Pb zircon ages and geochemical/isotopic (Nd, Sr) data from the Rio Negro Complex are presented.Discussions on the role of the arc in the context ofWestern Gondwanaamalgamation and comparisons withmodern arc associations are alsoaddressed.

2. Tectonic subdivision of Ribeira belt

Themajor tectonic framework of the Ribeira belt (Fig. 2) comprisesseveral tectono-stratigraphic terranes (Howell, 1989): the Occidental,Paraíba do Sul-Embú, Oriental (Serra do Mar Micro plate) and CaboFrio terranes in Rio the Janeiro and southern Espírito Santo states(Northern and Central Ribeira belt, Heilbron et al., 2004a); andSocorro, Apiaí, Embú, Curitiba and Luiz Alves terranes in São Paulo andParaná states (Southern Ribeira belt, Campos Neto, 2000). Theseterranes are limited either by thrust faults or dextral transpressiveshear zones.

The accretionary history of the belt is characterised by complexdiachronous docking of Neoproterozoic magmatic arcs and older

cratonic fragments, throughout the southern and south-easternsectors of the São Francisco paleocontinent. Terranes and microplateswere progressively accreted during four major tectonic episodes:

a) The oldest episode (ca. 650–630 Ma) is recorded only in thesouthern segment of the Ribeira belt (Fig. 2). This event resulted fromthe accretion of the Socorro terrane as NE–E verging nappes (CamposNeto, 2000; Trouw et al. 2000) in the context of the development ofthe southern Brasília belt (Valeriano et al., 2008).

b) The second tectonic episode occurred between ca. 605 and580 Ma and resulted in the docking of the Paraíba do Sul and Embúterranes. This event overprints the southern segment of the Brasíliabelt.

c) The third collisional event (580 Ma) is related to the docking ofthe Oriental terrane, which contains the Neoproterozoic Rio NegroArc – against the São Francisco paleocontinent. The collisional suturezone, referred as the Central Tectonic Boundary (CTB), separates theOriental terrane from the lower Occidental terrane (Figs. 2 and 3). TheCTB (Almeida et al., 1998, Almeida, 2000) is a folded shear zone(Fig. 3) which shows a complex long-term structural evolutiondeveloped under high temperature conditions. Mylonitic fabrics and

Fig. 2. (a) Tectonic map of central Ribeira Belt and interference zone with the Brasília Belt (Heilbron et al. 2008). 1, Phanerozoic cover; 2, Upper Cretaceous alkaline plutons; 3, 640–610 Ma east-verging Brasília Belt; 4–6, São Francisco Craton (SFC): 4, Paleoproterozoic to Archean basement; 5, cratonic sedimentary cover (pelitic to carbonate); 6, Mesoproterozoicto Neoproterozoic metasediments of the autochthonous domain; 7–14, terranes of Ribeira Belt; 7, Andrelândia and 8, Juiz de Fora domains of Occidental terrane of the Ribeira Belt; 9,Paraíba do Sul terrane; 10, Embú terrane; 11, Neoproterozoic magmatic arc and 12, Neoproterozoic metasedimentary successions of the Oriental terrane; 13, Cabo Frio terrane; 14,Apiaí terrane. CTB, Central Tectonic Boundary; CFT, Cabo Frio Thrust; APSZ, Além Paraíba Shear Zone. (b) Geological section A–B (enlarged 4x).

Fig. 3. Studied areas along the Rio Negro Complex in theOriental Terrane, Ribeira Belt: south-western (a), central (b), north-eastern (c) and Trajano deMoraes (d). Sample Locations:diamond symbols, Nd and Sr isotopes; open symbols, U–Pb zircon ages. Geological map of the Oriental Terrane simplified from Heilbron and Machado (2003). 1: Metasediments ofthe Cambuci klippe; 2: metasediments and amphibolites of the Italva klippe; 3: arc-related granitoids of the Rio Negro Complex; 4: metasediments of the Costeiro domain;5: syn-collisional granitoids; 6: late to syn-collisional granitoids; 7: post-collisional granitoids; 8: Paleoproterozoic orthogneisses and metasediments of the Cabo Frio Terrane.

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intrusive granite bodies of multiple generations are observed andused as relative chronological markers for deformational andmagmatic events.

d) The youngest tectonic episode (ca. 520 Ma) resulted in the dockingof the Cabo Frio terrane(Machado et al., 1996, Schmitt et al., 2004).

3. The Rio Negro Complex

In the central segment of the belt, along Rio de Janeiro state, theOriental terrane comprises arc-related rocks distributed into threestructural domains (Heilbron and Machado, 2003): a) the Serra daBolívia Complex, at the basal Cambuci domain (Tupinambá et al.,2007); b) the Rio Negro Complex, at the Costeiro domain (Tupinambáet al., 2000b); and c) the Serra da Prata unit, at the Italva domain(Peixoto and Heilbron 2010). The first two associations intruded highgrade pelitic to psammitic metasedimentary rocks while the Serra daPrata unit is interlayered with marbles and MORB-type amphibolites.

The Rio Negro Complex extends for more than 300 km in a NE–SWdirection, ranging in width from only a few to almost 40 km (Fig. 3).The RNC consists of orthogneisses of plutonic origin that wereemplaced within the high grade metasedimentary units of the SãoFidélis group, in the Costeiro domain of the Oriental terrane(Tupinambá et al. 2007). The São Fidélis group comprises a basalunit of garnet-sillimanite-biotite gneisses, locally bearing cordierite,containing lenses of feldspathic quartzites, marbles and calc-silicaterocks; and an upper unit of banded garnet gneisses with lenses ofquartzites, sillimanite schists and homogeneous biotite (hornblende)gneisses.

The geological mapping and sampling were focused in four areasand cross-sections (insets in Fig. 3): a) the northeastern area betweenNova Friburgo and Cantagalo towns; b) the central area in the Rio de

Fig. 4. Plutonic rocks from the Rio Negro Complex. a) coarse grained tonalite with gneissic folnorth of Nova Friburgo town; b) field relations between quartz-diorite (black) and tonalitegneissic foliation of the granitic type, road cut near Angra dos Reis; d) sharp contact betweMoraes town.

Janeiro city; c) the southwestern area, along the Angra dos Reis bay;d) the easternmost area, around the town of Trajano de Moraes.

In these areas the RNC comprises several plutonic associations,each onewith distinctive geochemical signatures: a) low tomedium-Kassociation, predominating in the northeastern area and also occurringin the central and southwestern areas; b) high-K association, in thecentral and south-western areas; c) shoshonitic association in theeasternmost segment of the complex.

3.1. The low to medium-K plutonic association

This association contains quartz-diorite, tonalite, granodiorite andtrondhjemite. Tonalite is the most widespread rock-type of thisassociation, frequently associated with quartz-diorite in both outcropand map scales (Fig. 4b).

The quartz-diorite is a weakly foliated fine grained melanocraticrock. Mafic minerals are represented by deep green and eventuallypoikiloblastic hornblende and by brown biotite flakes. Plagioclase(oligoclase) is the only feldspar, while quartz occurs as tiny intra-granular lenses. Titanite and apatite occur as accessory minerals.Textural features suggestive of magmatic origin are euhedralplagioclase and isolated hornblende and biotite crystals parallel tothe weak foliation.

Tonalites are coarse grained andmesocratic with a spotted texture,due to scattered planar biotite and hornblende aggregates (Fig. 4a).These rocks usually presentmafic microgranular centimetric to metricsize enclaves, with sub-milimetric hornblende crystals.

Trondhjemitic rocks are rare. They are leucocratic, mediumgrained rocks, weakly foliated, normally with less than 5% biotiteand rare hornblende. They occur as metric lenses within the pre-viously described association. These rocks can easily be misinter-preted as the younger Cambrian leucogranites that intrude the Rio

iation due to the parallelism of mafic mineral centimetre clots – outcrop at Rio Grandina,(white) near the city of Sumidouro; c) microcline megacrystals recrystallized along theen biotite rich (deep grey) and K-feldspar rich (light grey) rock types near Trajano de

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Negro Complex rocks, due to their similar textures and geometries.Under the microscope, besides quartz, K-feldspar and sodic plagio-clase, they present deep brown biotite flakes and a higher proportionof accessory zircon.

Granodiorites are represented by mesocratic to leucocratic coarsegrained rocks. Regarding the mafic phases, they contain higherproportions of modal hornblende than of biotite. Both main maficminerals occur as anhedral agglomerates aligned with the foliation.Subhedral plagioclase is the dominant feldspar, while microcline andorthoclase are present in the granoblastic matrix. Larger granoblasticgrains of quartz are widespread. Anhedral titanite is the typicalaccessory mineral frequently associated with hornblende. Alterationof feldspars and biotite respectively resulted in sericite and chlorite.

3.2. The high-K plutonic association

This high-K plutonic associationwas firstly described by Fernandes(2001) in the southwestern segment of the RNC, along the Ilha GrandeBay (Fig. 3). It contains quartz monzodiorite, quartz monzonite,granodiorite and granite.

The quartz-monzodiorites and quartz-monzonites are fine tomedium grained massive deep grey plutonic rocks, generally crosscut by a network of thin aplitic veins. Centimetric to decimetricdioritic to quartz-dioritic elliptical enclaves are common. Deep greeneuhedral hornblende is the main mafic mineral from quartz-monzodiorites. Subhedral and highly fractured titanite associatedwith ilmenite, and apatite are common accessory minerals. Deepbrown biotite with titanite, apatite and zircon inclusions is thecommon mafic mineral from quartz-monzonites. Plagioclase (oligo-clase) is the main feldspar in both rock types, and is easy to find relictmagmatic zoning in short, tabular subhedral crystals. K-feldspar(microcline) is distributed in the granoblastic matrix and as mega-crystals. Myrmekitic intergrowths are found along contacts withplagioclase. Anhedral quartz with undulose extinction is restricted tointergranular spaces in the granoblastic matrix.

Granodiorites display migmatitic features with a deep greymesosome and concordant or discordant leucosome veins. Thepresence of K-feldspar mega-crystals enclosed by a weakly foliatedfine-grained gray matrix renders the rock a prominent augen gneissicstructure (Fig. 4c). Major phases are subhedral microcline, quartz,plagioclase and biotite. The mafic mineral association of thegranoblastic matrix is composed of highly pleochroic green horn-blende (with corroded borders) and a deep brown biotite, withapatite and zircon inclusions. Plagioclase and quartz constitute thefelsic mineral association of the matrix. Subhedral prismatic plagio-clase (oligoclase) grains are rich in hornblende, titanite, apatite andzircon inclusions with borders partially corroded by quartz. Myrme-kite intergrowths are common along plagioclase and microclinecontacts. Quartz is found as polygonal aggregates or large anhedralquartz crystals. The common accessory mineral is titanite, alwaysassociated with an opaque mineral (ilmenite?), besides apatite andzircon. Microcline sericitization and plagioclase saussuritization iswidespread and some anhedral muscovite flakes were observed in themost altered feldspars. Enclaves of diorite to quartz-diorite arecommonly present in this rock.

Granitic rocks are medium to fine grained, weakly foliatedgneisses. Major phases are quartz, K-feldspar and plagioclase, withina granoblastic texture, with biotite as the mafic phase. Zircon andapatite are common accessory minerals.

Fig. 5. Classification, variation and tectonic discrimination diagrams for rocks of the Rio Negr2001. a) alkaline×subalkaline character (a) and AFM tholeiitic×calc-alkaline series (b) diagdiagram; (d), (e), (f): classification diagrams from Middlemost (1985) and LeMaitre (198Figs. 5–7: green symbols: basic rocks; magenta squares: alkaline rocks of Trajano de Moraes arocks; pink squares: throndhjemites; orange symbols: medium-K calc alkaline rocksmore de(not belonging to the Rio Negro Complex).

3.3. Shoshonitic association

This association is constituted by two main rock types: biotite richdiorites and leucocratic granites (Fig. 4d). The biotite-rich gneisses aremelanocratic to mesocratic, fine- to medium-grained schistosegneisses, locally called Trajano de Moraes Unit (Sad and Dutra,1988). They are mainly dioritic in composition, with local porphyr-oblastic texture (mafic mineral clots). Porphyroblastic hornblendegranodiorites with biotitic schlieren crosscut the diorites. Their high-Kcontent is due to the large amount of biotite and amphibole (20–50%),while microcline is not a frequent mineral. The magmatic origin ofthese rocks is revealed by zoned, twinned and fractured plagioclaseand by orthopyroxene relict crystals. Magnetite, ilmenite, chalcopy-rite, zircon and apatite are the accessory minerals. The greyleucocratic rocks are microcline-rich granodiorites and graniticleucocratic migmatites, the Crubixais Unit of Sad and Dutra (1988).Plagioclase (An35), quartz, reddish biotite and garnet are the othermain constituent minerals. Zircon and opaque minerals are theaccessory minerals.

3.4. Basic rocks

Basic sills are common in the RNC (Rosier 1965, Matos et al. 1980,Junho et al. 1998, Tupinambá 1999). The northeasthern area wassampled for these rocks. In that area field relations suggest that thegabbroic rocks are intrusive in the tonalitic rocks of the low tomedium-K association.

The largest gabbroic pluton is located near the Bom Jardim town,Rio de Janeiro State (UTM coordinates: 23 K 771555; 7553729). Fieldmapping revealed a 1700 m thick concordant sheet (dipping 20–30°NNW) interlayered with the tonalite gneiss and surrounding para-gneisses. hornblende gabbronorite predominates, with brown biotiteand green hornblende megacrystals, both containing exsolution ofopaqueminerals. Clinopyroxene is usually surrounded by hornblende,while orthopyroxene crystals show subhedral tabular habit. Plagio-clase with andesine (An47) composition is always antiperthitic, andlocally zoned. Apatite is the typical accessory mineral, and titanite israrely found. Epidote and chlorite occur as secondary minerals.Intergranular contacts suggest cumulate textures, with an intercu-mulus matrix composed of a mosaic of quartz and subhedralplagioclase, with isolated orthopyroxene crystals. The other layers ofthe complex are constituted by hornblende tonalite, leucogranodior-ite, trondhjemite, graphic granite and magmatic breccia.

Pale to deep green poikilitic hornblende is the main megacryst.Biotite is the other mafic mineral, occurring as brown to reddishflakes. Short tabular plagioclase crystals are highly calcic (An58–66)and when zoned, labradorite is surrounded by andesine. Minorplagioclase crystals are also found in the granoblastic matrix.Microcline (associated with quartz) is only present in the inter-granular spaces. Clear and anhedral quartz crystals usually make upfrom 15 to 25% of the rock. Titanite is the typical accessory mineral,and may occur as inclusions in hornblende and biotite, as isolatedcrystals or as aggregates. Apatite occurs as inclusions in hornblende,biotite and plagioclase.

4. Geochemical data

The geochemical data presented in this paper results from acompilation of more than one hundred samples from several master

o Complex. Data extracted from Tupinambá et al., 2000b; Porto Júnior, 2004; Fernandes,rams from Irvine and Baragar (1971); c) Shand's Index from Maniar and Piccoli (1989)9); g) tectonic discrimination diagram of Batchelor and Bowden (1985). Symbols forrea; purple symbols: high-K calc alkaline rocks; yellow symbols medium-K calc alkalinepleted in alkalis; light blue symbols: syn-collisional leucogranites from the studied areas

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and PhD theses developed along the last 10 years by researchers ofthe TEKTOS- Geotectonics Research Group and partners (Tupinambáet al., 2000a; Fernandes, 2001; Porto Júnior, 2004). All geochemicalanalyses were performed by the Activation Laboratories Ltd.(ACTLABS), Ancaster, Canada. Major element geochemistry is illus-trated in diagrams of Fig. 5. Chemical data allow the subdivision of theRio Negro Complex into three groups and a set of basic rocks. The firstone, represented by magenta squares, is an alkaline group very rich inK, related to rocks of Trajano de Moraes region. The other two groupscontain calc-alkaline rocks, the first one is rich in alkaline elements(Na+K), characterizing a high-K calc-alkaline group (represented bypurple color), with monzodiorites, monzonites, syenites, granodio-rites and granites. The second one is a medium- K calc-alkaline group,represented by symbols with yellow color, and comprises diorites,quartz diorites, tonalites and granodiorites. Only two rocks (pinksquares) are trondhjemites. Some rocks of the medium K calc-alkalinegroup are more depleted in alkalis and are represented as orangesymbols in the diagrams of Figs. 5 and 6.

The basic rocks do not form a homogeneous group. One samplefrom a large gabbroic body in the Serra do Mar range in Rio de JaneiroState, displaying picritic composition is tholeiitic. The other basicrocks display transitional to alkaline character and are classified astrachybasalts and basanites.

The Harker diagrams of Fig. 6 also indicate different geochemicalgroups. In the K2O×SiO2 diagram the samples are separated in low-,medium-, high-K and shoshonitic groups. The high-K calc-alkalinegroup displays higher contents of K2O, TiO2, P2O5, MgO, HFS elements(Y, Nb), U, including very high contents of Th and Zr, and higher Zr/Nband La/Yb ratios than the medium-K calc-alkaline group. The Harkerdiagrams also indicate that the latter group may be subdivided intomore than two suites, reflecting different sources and/or contamina-tion processes. The medium-K calc-alkaline group displays highercontents of Ca and Co. The only two trondhjemitic samples arehighlighted in the Na2O×SiO2 diagram.

The majority of the rocks from the Trajano de Moraes regiondisplay distinctive shoshonitic geochemical character (magentasymbols on the diagrams).

For the calc-alkaline groups, all the tectonic diagrams using majoror trace elements, point to arc-related environments (Fig. 5f–h). Asexpected, the shoshonitic group suggests a late-tectonic emplace-ment. The basic rocks are very heterogeneous and only one point toIAT environment.

The rare earth elements (REE) diagrams of Fig. 7 also helpdiscriminate the different groups of arc related rocks. Some samplesof the medium-K group show flatter patterns (Fig. 7a) than theother rocks of the group. The rest of the group displays fractionatedpatterns of light REE elements with weak negative Eu anomaly(Fig. 7b and c). Nevertheless, some rocks of the southwestern area(Rio de Janeiro/Ilha Grande bay) display more enriched patterns.The high-K group yields more fractionated patterns when comparedto the other groups (Fig. 7e and f). Samples from the Serra do Marrange display more enriched and fractionated patterns. The samefractionated and enriched patterns are by the shoshonitic group(Fig. 7d).

This geochemical subdivision corroborates the previous associa-tions described in the field and petrographic observations. Thegeographic distribution of the geochemical groups of the RNC(Fig. 3) is remarkable. As described in the previous item, the low- tomedium-K calc-alkaline group is the only association at thenorthwestern tip of the Complex and extends to the Central area.On the other hand, the high-K calc-alkaline group crops out in thenortheastern area, is common at the central area and predominates inthe southwestern segment, at the Ilha Grande bay. The shoshoniticrocks were found only in the region of Trajano de Moraes. Thisdistribution suggests a geochemical polarity, compatible with asoutheast- or south-verging subduction polarity.

5. U–Pb geochronology

5.1. Previous data

The first U–Pb data been reported for rocks of the Central RibeiraBelt (Cordani et al., 1973) were later interpreted as Neoproterozoiccrystallization ages of the RNC (Tupinambá et al. 1997). Tupinambáet al. (1998) obtained a discordia with an upper intercept at 634+/−10Ma for a foliated RNC tonalite. Additionally, three Pb–Pbzircon ages for leucogneisses interlayered with the RNC of 599+/−5,588+/−9 and 589+/−6 Ma, were obtained by Tupinambá (1999).Heilbron andMachado (2003) have obtained twomore TIMSU–Pb agesfor the RNC. A hornblende-bearing foliated tonalite cropping out in thenortheastern portion yielded a discordant age of 633+/−5 Ma. Atonalitic banded gneiss from a quarry in western Rio de Janeiro cityyielded an upper intercept at 790+/−3 Ma, interpreted as thecrystallization age of the body. Because of the time span of thedevelopment the RNC given by ages from ca. 790 to 620 Ma., Heilbronand Machado (2003) argued for a protracted period of subduction andconsequently a large oceanic space between blocks of WesternGondwana in SE Brazil and west Africa.

5.2. New U–Pb data

5.2.1. Analytical methodsThree samples weighting about 30 kg each were crushed and

milled under clean conditions and concentrated for heavyminerals bymanual panning and bromoform liquid at the LGPA, LaboratórioGeológico de Processamento de Amostras, Rio de Janeiro StateUniversity. The ensuing procedures were performed at the GEOTOP,Centre de Recherche en Géochimie et en Géodynamique, Universitédu Québec à Montréal, Canada. Zircon was separated into magneticand diamagnetic fractions using a Frantz separator. Zircon grains freeof alteration, inclusions and fractures were handpicked for analysis.Following the general procedure of Krogh (1982), most of theanalyzed zircon fractions were abraded during between 12 and 30 husing air pressure of 2 to 3 psi in order to eliminate grain borders withrecent lead loss and improve age concordance. Even though the timespent in air abrasionwas lower than those previously used in Braziliansamples by Machado et al. (1996) and Heilbron and Machado (2003),the samples yielded concordant to sub concordant analytical results.Mineral dissolution, chemical extraction of U and Pb, and massspectrometric analyses in a VG-mass spectrometer were carried outfollowing the procedures described in Machado et al. (1996). Totalprocedural blanks average was 15 pg for Pb and 2 pg for U. Theuncertainties in isotopic ratios presented in Table 1 were calculatedwith an error propagation programwhich takes into consideration theanalytical precision of the measured isotopic ratio. Regressions werecalculated and concordia diagrams were plotted using the Isoplotsoftware (Ludwig, 2003). Errors are represented at 1σ level but allages are calculated for the 95% confidence interval.

5.2.2. ResultsThree samples (RN-1, RN-2 and R-14) from the Rio Negro

Complex were collected in the northeastern area of Fig. 3, alongtwo geological cross-sections in the central segment of the RibeiraBelt described by Tupinambá et al. (2000a). In this area, the RioNegro Complex is intruded by the Serra dos Órgãos batholith (Silvaet al. 2003) and the gneissic foliation gently dips to NW (Tupinambáet al. 2003). The samples belong to the quartz monzodiorite-quartz-monzonite-granodiorite-granite association, and their geochemicalsignatures place them in the high-K group. The isotopic data ispresented in Table 1.

5.2.2.1. Sample RN-1. This sample was collected at the km 70 of theBR-040 highway (in the Juiz de Fora direction, nearby the Petrópolis

Fig. 6. Harker variation diagrams for the Rio Negro magmatic series. The boundaries between low-, medium-, high-K and shoshonitic series in the K2O vs. SiO2 diagram as Picci and Taylor (1976) and Stern et al. (2003). Symbols and datasources as Fig. 5.

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rilo

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College), at the UTM coordinates 23 K 687298/7517480. The 150 mlong road cut exposes non-foliated metadiorite and strips of a foliatedmetagranite (sample RN-1) and some aplite veins. The sample is aleucocratic coarse-grained metagranite with foliation marked by

Fig. 7. REE diagrams normalized for chondrite values (Sun and McDonough, 1989). Symbd: shoshonitic group; (e) and (f) High-K group; g) basic rocks; h) syn-collisional leucogran

alignment of biotite clots. The felsic mineral assemblage containsperthite (as isolated crystals or associated with microcline), largeanhedral quartz grains and small quantity of plagioclase. Myrmekite isfrequently found along contacts between plagioclase and K-feldspar.

ols and data sources as Fig. 5. (a), (b) and (c): samples from the medium-K group;ites.

Table 1U–Pb data for rocks of the Rio Negro Complex.

No. SampleMineral

Concentrations Atomic ratios

Weight (mg) U (ppm) Pb rad. (ppm) Pb com. (pg) 206Pb/204Pb 208Pb/206Pb 206Pb/238U +/− 207Pb/235U +/− 207Pb/206Pb +/−

(1) (2) (2) (3) (4) (5) (5) (6) (5) (6) (5) (6)

Gray gneiss (sample R-14)R14-1 Z (1) 0.024 61.7 6.0 9 1003 0.138 0.0949 0.17 0.78071 0.68 0.05994 0.62R14-2 Z (1) 0.022 98.5 10.1 11 1253 0.177 0.0964 0.17 0.80566 0.32 0.06064 0.26R14-3 Z (1) 0.013 85.1 8.6 22 324 0.198 0.0933 0.16 0.78572 0.40 0.06111 0.33

Quartz-diorite (sample RN-2)RN-2-1 Z (1) 0.015 423.5 41.0 24 1669 0.103 0.0968 0.18 0.79865 0.27 0.05985 0.19RN-2-2 Z (1) 0.056 120.1 13.4 30 1336 0.318 0.0945 0.16 0.77955 0.24 0.05983 0.18RN-2-3 Z (1) 0.053 443.6 37.1 39 3108 0.139 0.0811 0.47 0.64945 0.48 0.05806 0.13RN-2-4 Z (1) 0.032 115.5 13.3 3 7425 0.324 0.0968 0.16 0.7961 0.34 0.06009 0.28

Granite (sample RN-1)RN-1-1 Z (1) 0.024 437.9 41.2 9 6730 0.083 0.0958 0.19 0.79106 0.21 0.05990 0.11RN-1-2 Z (3) 0.032 141.8 14.7 12 2333 0.198 0.0959 0.19 0.80614 0.24 0.06095 0.15RN-1-3 Z (1) 0.015 423.7 39.7 8 4861 0.079 0.0958 0.15 0.79435 0.21 0.06017 0.13RN-1-4 Z (1) 0.007 361.9 33.1 4 3643 0.112 0.0907 0.19 0.74872 0.35 0.05987 0.29RN-1-5 Z (1) 0.027 461.8 44.3 68 1103 0.132 0.0936 0.36 0.76284 0.45 0.05924 0.25

Notes: (1) Mineral: Z=zircon. (2) Concentrations are known to 20% for weights below 20 μg. (3) Total common Pb present in analysis corrected for Pb in spike (4) Measured ratio,corrected for fractionation only. (5) Ratios corrected for spike, fractionation, blank, and initial common Pb.(6) Standard error of the mean in% at the 1-sigma level (7) Discordance inpercent. Conc. means concordant analysis. Maximum total blanks for zircon analyses are 15 pg for Pb and 2 pg for U; Isotopic composition of initial common Pb was calculated usingthe two-stage model of Stacey and Kramers (1975).

Fig. 8. ID-TIMS concordia diagrams for orthogneisses from the Rio Negro magmatic arc.a) granitic orthogneiss RN-1; b) dioritic orthogneiss RN-2.

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Brown biotite in large isolated flakes is the only mafic mineral.Muscovite and chlorite crystals are products of feldspar and biotitealteration. The accessoryminerals are apatite, zircon, rutile and pyrite.Titanite grains were not found either in thin sections or in heavymineral concentrates. Zircon is included in biotite and feldspar asshort or long prismatic (2 to 3:1) crystals. The long prismatic grainsare present in many of the electromagnetic fractions but containnumerous inclusions and were not analyzed. Colorless and cleanequant or short prismatic grains are present in the diamagneticfractions and were analyzed in the five following runs: RN-1-1 isrepresented by one large and limpid equant grain not completelyabraded and picked from the non-magnetic M0 fraction; RN-1-2 isrepresented by two equant grains, abraded during 6hs (2 psi) andpicked from the M0 magnetic fraction; RN-1-3 is represented by onelimpid equant grain abraded during 6 h 40' (2 psi) and picked fromthe M1 magnetic fraction; RN-1-4 represented by one clean rosecolored flat equant grain, abraded during 11 h 30' (2 psi) and pickedfrom the same population and magnetic fraction (non-magnetic M0)of the RN-1-1; RN-1-5 is represented by one large equant grainshowing one colorless inclusion in its center, abraded during 11 h 30'(2 psi)+10hs (3 psi) and picked from the same population andmagnetic fraction (non-magnetic M0) of the RN-1-1.

Three fractions yielded a discordia with upper intercept at ca.607 Ma, interpreted as the crystallization age and a concordantfraction at ca. 592+/−58 Ma is regarded as related to the mainmetamorphic overprint (Fig. 8a).

5.2.2.2. Sample RN-2. The sample was collected in a road cut of the BR-040 highway at the UTM coordinates 23 K 692983/7534002. Itcontains at least four rock types: a medium to fine grained quartzdiorite with small granitic aplites (sample RN-2) crosscut byleucogranitic and porphyritic gneisses and isotropic granite. Thesample is a melanocratic medium to fine grainedmeta- quartz diorite.In thin section it presents a granoblastic texture with brownish greenhornblende crystals, isolated reddish brown biotite flakes, plagioclaseand quartz. Big interstitial metamorphic titanite crystals, zircon andapatite are accessory minerals. Zircon is always short and equant andis included in plagioclase. The electromagnetic heavy mineralseparation yielded fractions full of titanite and apatite but poor inzircon grains. The sample was analyzed in the four following runs:RN-2-1 is represented by one limpid equant grain abraded during 12 h30' (2 psi), picked from the M0 magnetic fraction; RN-2-2 is

Table 2Rb-Sr isotope data from Rio Negro Complex.

Sample name Geochem. group Coord X 23 kUTM Coord Y 23 kUTM Rb ppm Sr ppm 87Rb/86Rb 87Sr/86Sr (m) Error×10-6

CBB L-02 C AK 557652 7458368 159 535 0.861 0.719330 7PG 2A AK 654337 7451873 140 1190 0.341 0.711580 9Q15 AK 661645 7463123 136 139 2.839 0.732960 8MS 98 AK 675081 741011 149 151 2.861 0.727390 6CBB L-02B AK 557652 7458368 145 415 1.012 0.718130 6CBC quarry AK 574155 7465775 22.39 152.3 0.427 0.732340 6RML-13 AK 553160 7471011 265 266 2.891 0.733346 8IGL AK 580136 7453237 110 936 0.340 0.709750 9PG 8D AK 653746 7451517 170 918 0.536 0.710090 6MS 14D AK 675090 7463821 35 479 0.212 0.711650 5PBG 3 MK 657920 7468030 136 302 1.304 0.714130 7PF 30 MK 659264 7456741 145 520 0.808 0.717100 6VM-Nm-02A MS 658127 7467871 78 406 0.556 0.708460 8PG-9A MK 653383 7451281 196 900 0.631 0.714110 6PG 8E MK 653746 7451517 155 1390 0.323 0.711540 5PRGF 4 C MK 651758 7454607 67 300 0.647 0.710800 10JD 02 MK 696500 7539330 100 397 0.729 0.714230 5DB-TUP-8A MK 742700 7554750 52.48 735.69 0.205 0.712590 9DB TUP 8C MK 742700 7554750 64 663 0.280 0.712850 8DB-TUP-8o,r MK 742700 7554750 43.43 736.08 0.171 0.708800 8DB-TUP-8F MK 742700 7554750 56.90 535.50 0.308 0.709520 14DB-TUP-8I MK 742700 7554750 65.10 467.90 0.403 0.710470 15SAP NM 19A MK 793937 7592522 3.902 17.501 0.645 0.706460 8SAP M19B MK 793937 7592522 11.251 45.084 0.722 0.706970 7SAPNM19C MK 793937 7592522 26 408 0.184 0.709420 6PST 2D MK 638885 7457058 251 500 1.454 0.713970 7PTA 55 MK 663250 7462794 73 358 0.592 0.736570 7PRT 2A MK 661645 7458642 58 912 0.184 0.707440 5PT-04a MK 658127 7467919 55 267 0.596 0.708460 7DB TUP 33A MK 748122 7560254 0.632 2.956 0.619 0.707490 12DB-FR-34A MK 747654 7560178 11.2 423.62 0.077 0.70848 1DB-TUP-30D MK 756093 7560235 48.61 347.86 0.405 0.70927 1CO-TUP-7B MK 766132 7546072 54.53 244.15 0.647 0.71438 1PTA 30 MK 663468 7463860 112 446 0.727 0.709260 6PBG 4 M 658260 7468102 58 596 0.282 0.704200 7AG MBG M 566030 7449075 107 221 1.402 0.716460 6DB I 10 M 748300 7559100 25 215 0.337 0.711900 5

Notes: HK, High-K group; MK, Medium-K group; M-mafic (basic) rocks. (m) –measured isotope ratio; (i) initial isotope ratio calculated to 630 Ma, except sample VN-NM-2, with anU–Pb age of 790 Ma (Heilbron and Machado 2003).

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represented by half of a thick prismatic grain with a small darkinclusion, abraded during 12 h 30' (2 psi), picked from the M0magnetic fraction; RN-2-3 is represented by half of a thick limpidprismatic crystal, abraded during 11 h 20' (2 psi), picked from the M1magnetic fraction; RN-2-4 is represented by one large equant grainwith a colorless inclusion, abraded during 10 h (3 psi), picked fromthe M0 magnetic fraction.

Unfortunately the data is very dispersed, but a 207Pb/206Pb age ofca. 607 Ma and a concordant fraction of 590+/− 9 Ma point tosimilar results to those of sample RN-1, suggesting the same timeintervals for both crystallization and metamorphic overprint thatresulted in the migmatization of the diorites.

5.2.2.3. Sample RN-14. The sample was collected in a road cut of theBR-116 federal road at the UTM coordinates 23 K 725634/7536229. Itis a medium to fine grained mesocratic granodiorite gneiss. Brownishbiotite is the main mafic mineral, as hornblende is seen only in scarcesmall crystals. Plagioclase is the main feldspar, with orthoclase beingthe only K-feldspar. Intergranular myrmekitic pockets are common.Titanite is the main accessory mineral, followed by zircon. Zircons areequant grains or short prisms.

Three single zircon fractions were analyzed: M-1/2 is representedby a short prismatic grain with no inclusions, abraded during9 h30 min, 2.5 psi; M0 is represented by a large rose colored grainwith a light colored mineral inclusion; M0 is represented by a rosecolored equant small grain without inclusions.

The three fractions analyzed are discordant, in spite of the time ofabrasion, and did not yield a discordia. However, all the 207Pb/206Pb

ages point to the 626–643 Ma interval, similarly to the publisheddata for the RNC (Tupinambá et al., 2000b, Heilbron and Machado,2003).

6. Nd and Sr isotopic data

6.1. Analytical procedures

All isotopic analyses were conducted by isotope dilution thermalionization mass spectrometry (ID-TIMS) at the GeochronologyLaboratory of the University of Brasilia. Between 50 and 100 mg ofpreviously powdered rock samples were mixed with a 149Sm–150Ndspike and dissolved with HF-HNO3 in closed Savillex vials or Parr-type Teflon bombs. Separation of Sr and REE as a group was firstlyperformed in ion-exchange columns using the Bio-Rad AG50W-X8200–400 mesh resin. Separation of Sm and Nd then followed thetechnique of Richard et al. (1976), modified by Gioia and Pimentel(2000), using columns loaded with HDEHP (di-2-ethylhexeylphosphoric acid) supported on Teflon resin. Sm, Nd and Sr sampleswere then evaporated and loaded onto Re filaments for massspectrometry in a double filament assembly for Sm and Nd andsingle assembly for Sr.

Isotopic analyses were carried out using a Finningan MAT-262multi-collector mass spectrometer in static mode. All 143Nd/144Ndratios were normalized to the 146Nd/144Nd ratio of 0.7129. Ndprocedure blanks were smaller than 100 pg, and the method describedby)results in the following values for the international rockstandards BCR-1 (143Nd/144Nd=0.512647±8; Nd=28.73 ppm;

Fig. 9. Rb and Sr isotopic values for the Rio Negro Complex rocks. The shoshonitic groupwas not sampled. Key isotopic ratios (0.705 and 0.710) are emphasized in dashed lines.Data from Table 2. a) distribution of Initial 87Sr/86Sr (630 Ma) along the sample set;b) 87Rb/86Sr versus initial 87Sr/86Sr (630 Ma).

Fig. 10. Sm and Nd isotopic values for the magmatic series of the Rio Negro Complex.Data from Table 3. a) Measured Sm and Nd isotopic ratios; b) εNd (T) parameter versusTdm model ages.

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Sm=6.66 ppm), BHVO-1 (Nd=24.83 ppm; Sm=6.2 ppm). Theobtained 143Nd/144Nd ratio for the La Jolla reference material is0.511835±14. Typical 2σ errors for 87Sr/86Sr are below 0.017%.

6.2. Sr isotopic data

Whole-rock samples from two geochemical groups and from basicrocks of the RNC were analyzed for Rb and Sr isotopes (Table 2). Mostof the samples (24) belong to the medium-K group whilst 9 samplesbelong to the high-K group. The shoshonitic group is not represented,and only 3 samples from the group of basic rocks were analyzed.

The 87Sr/86Sr isotopic ratios, when calculated to the maincrystallization age of the Rio Negro Complex (630 Ma), are distributedalong two ranges (Fig. 9): low isotopic ratios (b0.705) and normalcrustal ratios, between 0.705 and 0.710. The low isotopic ratios arewell registered in the medium-K group, with mantle-like 87Sr/86Srvalues. Only one sample of the high-K group is within this low isotopicrange (Fig. 9). According to this data set, this group is characterized bymagmatic crustal Sr isotopic ratios.

The medium-K group presents 87Rb/86Sr ratios below 1.0 (Fig. 10).This low chemical range is associated with the predominance ofintermediary rocks in this plutonic series. The scarcity of acidic rocksexplains the absence of high K and Rb concentrations. On the otherside, Ca-rich intermediate rocks present higher Sr contents.

The high-K group shows a wider spread of 87Rb/86Sr ratios (0.2 to2.98) attributed to an expandedmagmatic series with higher contentsof K and Rb without increasing the 87Sr/86Sr ratio (Fig. 10). Two high-K samples yielded 87Sr/86Sr ratios around 0.730 but present 87Rb/86Srratios below 1.0. Although the orthogneisses from the RNC frequentlyshow leucosome bands generated during the 580–560 Ma M1metamorphic event (Machado et al. 1996), the low 87Rb/86Sr ratiosexclude this hypothesis. Probably their high 87Sr/86Sr was caused bythe assimilation of metasedimentary country rocks.

6.3. Nd Isotopic data

A set of 8 shoshonitic (S) samples, 15 high-K (HK), 19 medium-K(MK) and 3 mafic-basic (M) samples had their Sm and Nd isotopesanalyzed by ID-TIMS (Table 3).

A relatively wide range of Sm–Nd isotope compositions isobserved, with measured 143Nd/144Nd ratios within the rangebetween 0.5114 and 0.5126, and 147Sm/144Nd ratios between 0.08and 0.16 (Fig. 10a). A negative correlation between εNd(t) and Tdmmodel ages in all four geochemical groups (S, HK, MK, M) indicatesdifferent degrees of continental crust contamination (Fig. 10b).Sample populations of the shoshonite and medium-K groups plotinto a juvenile field in the εNd(t) and Tdm diagram when compared tothe high-K group of samples. Variation of εNd(t) within the same rangeof Tdm in the high-K group suggests post-magmatic Sm/Nd fraction-ation, possibly due to partial melting and leucosome generationduring M1 metamorphic event (Machado et al. 1996).

The distribution of Tdm model ages spans from 0.99 Ga to 2.47 Ga,but with three important modes at ca. 1.0, 1.3 and 1.8 Ga, the latter

Table 3Sm–Nd analytical results for the Rio Negro Arc orthogneisses.

Sample name Geochem. group Coord X UTM Coord Y UTM Sm ppm Nd ppm 147Sm/144Nd 143Nd/144Nd Std±1 s 143Nd/144Nd (t)

TM4 S 802636 7554390 11.2 60.4 0.1122 0.512319 6 0.511852076TM7 S 802754 7551448 12.5 71.0 0.1063 0.511958 6 0.511515629TM9 S 802081 7550784 8.6 50.6 0.1032 0.511789 11 0.51135953TM10 S 798442 7544564 12.9 55.1 0.1421 0.512233 6 0.511641646TM11 S 802754 7551448 5.9 32.5 0.1102 0.511914 6 0.511455399TM14 S 799519 7548605 8.5 52.7 0.0974 0.512064 5 0.511658666TM16 S 799519 7548605 9.4 57.5 0.0991 0.512058 6 0.511645592TM18 S 799519 7548605 9.1 64.5 0.0854 0.511924 5 0.511568605RCM 01 AK 683380 7504767 11.3 63.9 0.1069 0.511670 4 0.511225132RCM 7 AK 700715 7541831 14.7 87.3 0.1018 0.511632 6 0.511208356RCM 4 C AK 692809 7533582 5.0 24.7 0.1225 0.511955 6 0.511445212TM15 AK 799519 7548605 29.4 215.0 0.0826 0.511438 5 0.511094257RCM 5 AK 694876 7537390 3.2 16.4 0.1174 0.512093 8 0.511604436RCM 10 AK 729042 7551637 7.4 37.2 0.1209 0.511918 7 0.51141487CBB L-02C AK 557652 7458368 10.3 47.2 0.1325 0.512025 7 0.511473597PG 2A AK 654337 7451873 19.5 122.4 0.0964 0.511610 19 0.511110651Q15 AK 661645 7463123 4.1 23.7 0.1053 0.512063 17 0.511517549MS 98 AK 675081 741011 4.1 27.8 0.0900 0.511909 7 0.511442803CBB L-02B AK 557652 7458368 14.5 82.3 0.1064 0.511830 5 0.511387213CBC quarry AK 574155 7465775 22.4 152.3 0.0889 0.511706 17 0.51133604RML-13 AK 553160 7471011 10.0 63.3 0.0954 0.511760 6 0.51136299IGL AK 580136 7453237 18.7 120.9 0.0933 0.511594 5 0.511205729PG 8D AK 653746 7451517 19.9 136.7 0.0878 0.511556 16 0.511101199PBG 3 MK 657920 7468030 2.7 14.3 0.1151 0.512361 31 0.511764785PF 30 MK 659264 7456741 17.3 107.8 0.0968 0.511551 17 0.511049579TM6 MK 801213 7552400 11.6 60.0 0.1172 0.512213 6 0.511725268TM13 MK 799519 7548605 10.9 56.4 0.1167 0.512238 6 0.511752349RCM 4B MK 692809 7533582 5.0 26.2 0.1144 0.511928 6 0.51145192RCM 11 MK 725833 7555858 5.6 25.8 0.1301 0.512004 6 0.511462584RCM 6A MK 695162 7538270 6.9 34.8 0.1201 0.511783 5 0.5112832RCM 2B MK 687298 7517480 10.2 46.5 0.1328 0.511739 5 0.511186348VM-Nm-02A MS 658127 7467871 5.6 23.2 0.1457 0.512465 16 0.511710278PG-9A MK 653383 7451281 2.5 15.9 0.0952 0.511652 13 0.511158867PG 8E MK 653746 7451517 3.2 22.6 0.0867 0.511572 29 0.511122897PRGF 4 C MK 651758 7454607 2.2 9.9 0.1329 0.512159 13 0.511470582ID 02 MK 696500 7539330 6.4 30.2 0.1271 0.511885 7 0.511356069DB TUP 8C MK 742700 7554750 7.6 38.9 0.1178 0.511936 7 0.511445771SAP NM 19A MK 793937 7592522 3.9 17.5 0.1348 0.512395 28 0.511834025PRT 2A BMK 661645 7458642 14.8 88.2 0.1017 0.511752 18 0.511225197SAP M19B BMK 793937 7592522 11.3 45.1 0.1509 0.512514 7 0.511886024PST 2D MK/S 638885 7457058 9.3 39.0 0.1435 0.512300 11 0.511556674PTA 55 MK/AK 663250 7462794 8.8 47.8 0.1116 0.511961 2 0.511382915PBG 4 M 658260 7468102 8.6 37.7 0.1376 0.512571 15 0.511858236TM5 M 802427 7554119 8.1 41.7 0.1172 0.512408 7 0.511920268

Notes: S, shoshonitic group; HK, high K group; MK, medium-K group; M, mafic (basic) rocks; BMK, basic medium-K; MK/S transitional between shoshonitic and medium-K groups;MK/AK, transitional between high-K and medium-K groups; (i) initial isotope ratio calculated to 630 Ma, except sample VN-NM-2, with an U–Pb age of 790 Ma (Heilbron andMachado 2003).

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one being the most expressive (Fig. 11a). The Mesoproterozoic modelages modes probably represent mixture between juvenile andbasement isotopic compositions, and the older on at 1.8 Ga representsthe age of basement complexes that contaminated the juvenilemagmatic arc rocks.

Although the εNd(t) parameter spans from−14 up to+5 (Fig. 11b),with the largest mode at −7, there is a large proportion of juvenilesamples with positive or slightly negative values (down to −2).

7. Discussion

The magmatic rocks of the RNC comprise a very importantcomponent in the evolution of the Ribeira belt. Taking into accounta possible continuation to the north towards the G1 metaplutonicseries in the Araçuaí Belt (Pedrosa-Soares et al., 2001; Heilbron et al.,2008; Wiedemann et al. 2002), the roots of the magmatic arc areexposed along 1000 km from southern Bahia to São Paulo states. Thejuvenile character of the Rio Negro Magmatic Arc is also revealed byfield relations with country rocks. As only the plutonic part of the arcwas preserved, the basement of the arc is exposed along its margins. Itis made up by high grade pelitic gneisses at the bottom withprogressive increasing on immature meta-arenite layers to the top of

the sequence (Tupinambá et al. 2007). Thin centimetric layers of Mn-rich and calc-silicate rocks suggest a distal environment with somevolcanic interaction. Few marble boudins were also found, indicatingsupply of a nearby carbonate platform.

In spite of the large geographic extension, no Paleoproterozoic orolder basement rocks were found in the arc domain. On the otherhand, detrital zircons from the metasedimentary country rocks(Heilbron and Machado 2003, Schmitt et al., 2004, Valladares et al.,2008) suggest a complex mixture of source areas, ranging fromArchean, Paleoproterozoic, Mesoproterozoic (as the major modalsource) and even coeval to the arc itself.

The geochemical and Sr and Nd isotopic data presented herereinforce the juvenile contribution of the arc to the continental crust.The geochemical data (Figs. 5 and 7) indicate that the orthogneissesof the RNC display signatures of typical modern magmatic arcs(Wilson, 1989; Thorpe et al., 1981) such as the intra-oceanicmagmatic arcs of the southwest Pacific (Ewart, 1982), and theMesozoic magmatic province of northern Chile (Lucassen et al.,2006). In Brazil, rocks of the Rio Negro Arc are similar in compositionto the Neoproterozoic Mara Rosa Magmatic Arc in the Brasília belt(Pimentel and Fuck, 1992) and to the Galiléia Suite in the Araçuaíbelt (Nalini et al. 2005).

Fig. 12. a) Sr evolution diagram of several Paleoproterozoic basements in the CentralRibeira Belt and the ranges of initial 87Sr/86Sr of the Rio Negro Complex.Paleoproterozoic Sr data from Heilbron (1993), Fonseca (1994) and unpublisheddata. Initial 87Sr/86Sr ratios from Neoproterozoic juvenile arcs in South America: GoiásMagmatic Arc, Brasília Belt (Pimentel and Fuck 1994); São Gabriel Arc, Dom FelicianoBelt (Saalmann et al., 2005). Mantle evolution fromMoorbath and Taylor (1981). b) Ndevolution diagram from Rio Negro Magmatic Arc compared with the Goiás Arc (BrasiliaBelt) and with the São Francisco Craton Archean to Paleoproterozoic basement. Nd datafrom Goiás Arc and from the São Francisco Craton in Pimentel and Fuck (1992).

Fig. 11. Nd isotopic values of the Rio Negro Complex rocks, data from Table 3:a) histogram of neodymium Tdm model ages; b) histogram of εNd (T) parameter.

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When compared to the Sr evolution of the surrounding Paleopro-terozoic basement complexes of the Ribeira Belt (Fig. 12a), the RioNegro Sr isotopes signature rules out any important crustal partici-pation in the magmatic evolution. The Sr ratios of the Rio NegroMagmatic Arc are similar to or even lower (Fig. 12a) than thoserecorded in other South American juvenile Neoproterozoic arcs, suchas the Goiás (Pimentel and Fuck, 1994) and the São Gabriel (Saalmannet al., 2005) magmatic arcs.

In the Nd evolution diagram of Fig. 12b, rocks from the RNCpresent evolution lines that are not linked to Archean-Paleoproter-ozoic rocks from the basement of the São Francisco Craton. In thesame diagram, the cluster of lines representing the RNC partlyoverlaps the evolutionary trend of the juvenile rocks from the MaraRosa Arc in the Brasilia Belt (Pimentel and Fuck 1992). UpperMesoproterozoic to lower Paleoproterozoic TDM model ages from theRio Negro Complex, with more positive εNd(t) values, also suggestcontribution from older components, in contrast with the Sr isotopicdata. Since no basement rocks were yet found within the arc terrane(Oriental Terrane) the crustal component may have originated fromthe assimilation of the enclosing metasedimentary rocks.

The diversity of the plutonic groups, ranging from low- to medium-K, high-K and shoshonitic, also suggest a long, complex history of arcmagmatism. This hypothesis is corroborated by the U–Pb ages,characterized by three age intervals: ca. 790 Ma for the low- tomedium-K group, 630–620 Ma for themedium-K group (Tupinambá etal., 2000b; Heilbron and Machado 2003) and 610–605 Ma for the high-K group (this work). The evolution of the magma composition, fromless to more evolved rocks with time, indicates progressive construc-tion of the arc, changing from a more primitive or either intra-oceanicsetting to a cordilleran environment. The predominance of intermedi-ate rocks over basalts suggests an initial intra-oceanic to transitionalstage, possibly reflecting the distal portion of a passive margin, such asthe Japan arc, tomore evolved and differentiated rock assemblages. TheSr and Nd isotopic compositions, along with the εNd(t) values, rangingfrom −3 to +5 in the medium-K and shoshonitic groups, to −14 to−3 in the high-K group, also point to the progressive maturity of thearc with time (Fig. 13). The Sr isotopic signature also indicates that a

part of the medium-K group displays lower 87Sr/86Sr initial ratios(b0.705) when compared with the high-K group (N0.705).

Finally, a compositional zoning was detected within the arcterrane. The diagrams of Fig. 13 and the map of Fig. 3 show that thepresent geographical position of the geochemical groups is controlledby the longitude of the collected samples. In spite of somegeographical superposition, the high-K group predominates to thewest and persists in the central and northeastern area. Medium-Krocks are concentrated in central and northeastern area, but are notpresent in western area and are scarce in the eastern area. Theshoshonitic group is only present in the eastern area.

Variations along the main axis of an arc is known in active islandarcs, as the Izu-Bonin-Mariana arc (Stern et al. 2003), where low tomedium-K provinces are intercalated with a shoshonitic province.They reflect the complexity of the magmatic processes below the arc,

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such as variation in the depth and degree of partial melting, magmamixing, fractionation and assimilation (Kearey et al., 2009).

8. Conclusions

Five Neoproterozoic magmatic arcs were described at the centralsector of Western Gondwana: the Goiás arc in Brasilia Belt (Pimenteland Fuck 1992; Pimentel et al., 2000; Valeriano et al., 2008), theGaliléia suite in Araçuaí Belt (Pedrosa-Soares et al., 2001), the SãoGabriel arc (Saalmann et al., 2005; Hartmann et al., 2010) and thePelotas batholith in the Dom Feliciano Belt (Philipp et al., 2002) on theBrazilian side. In Africa, ca. 650–635 Ma arc-related granitoid rocks ofthe Western domain of the Kaoko belt have been described(Goscombe et al., 2005a; Gray et al., 2006; Goscombe and Gray,2007, 2008). Ages range from ca. 900 Ma in the Goiás arc to ca. 595 Main the Galiléia suite. The juvenile contribution was solely reported inGoiás and São Gabriel arcs, both located in a peripheral position withrespect to the São Francisco Congo block, and therefore are regardedas representative for intra-oceanic settings and for the subduction oflarge oceanic lithosphere (Cordani et al. 2003a,b).

Fig. 13.Geographic distribution of the Rio Negro Complex geochemical groups and theirisotopic signatures. a) initial 87Sr/86Sr ratios versus longitude; b) εNd (T) parameterversus longitude. Location of the areas in Fig. 3.

On the other hand, the Rio Negro magmatic arc at Ribeira belt islocated in the central portion of western Gondwana, between twolarge cratonic areas, São Francisco and Congo (Fig. 14). Nevertheless,the acquired geological, geochemical and isotopic data suggest thatthe subduction history began in an oceanic setting.

Between 790 and 630 Ma the low- to medium-K rocks of the RNCpresent strong mantle signatures (low Sr isotopic ratios and positiveto slightly negative εNd(t)) without assimilation of continental crust,except its own metasedimentary basement.

The results indicate that during 160 m.y. an oceanic lithosphericplate was consumed. Using conservative plate motion velocities(1 cm/yr) a 1300 km wide oceanic plate was consumed. Fastervelocities (10 cm/yr) imply a consumption of a 13000 km wideoceanic plate, comparable to the present Pacific Plate.

In a context of an interior oceanic environment (at the place whereRibeira Belt is positioned in Western Gondwana) the large time spandetected for subduction in the Ribeira belt results in a typical geologicalspace-problem. A partial solutionwas proposed byAlkmim et al. (2006)with a “nutcracker” mechanism that uses escape tectonics to accom-modate continental blocks in a relatively small space. During the pre-collisional phase their model predict stagnant plate velocities of 0,3 km/yr and a subduction angle of 45o that support the consumption of anoceanic plate 500 km wide in 150 m.y. of subduction.

To explain the existence of a much larger oceanic plate (1300 to13,000 km) Heilbron et al. (2008) pointed to the existence ofimportant transform fault zones, such as the Luanda shear zone(Fig. 17). These structures may have functioned as arc-arc transformzones and their kinematic history may hide large offsets. In thecartoon of the Fig. 14a, the Luanda shear zone is responsible for thedisplacement of the Neoproterozoic magmatic arc between theRibeira and Araçuaí belts. As the fault linked an extinct ridge and atrench, the fault length may decrease with time if the subduction ratewas faster than the spreading rate. Therefore, the present-day lengthof the Luanda shear zone (and its probable extension in the RibeiraBelt) could hide a much larger displacement.

Alternatively, the Luanda shear zone could have acted as a tearfault, in this particular case a STEP (Subduction-Transform-Edge-Propagator) fault (Wortel et al. 2009). At the curving endings ofmagmatic arcs, such as in the Mediterranean region and in Tonga-Fiji,vertical tearing of subducted lithosphere and slab detachment createtear faults (Fig. 14b, I). The subduction then propagates along thesefaults (STEP faults), promoting trench retreat and slab roll-back. TheSTEP faults extend throughout the whole interval of the magmatism,causing vertical thickening of the arc.

Acknowledgements

M. Tupinambá thanks CAPES for his post-doctoral grant (BE 3025-04-0). Field work, geochemical and isotopic analyses were supportedby FAPERJ, CNPq, FINEP and Petrobras grants to M. Heilbron. Theauthors thank the technical staff of UERJ's preparation lab (LGPA). Wealso thank Dr. E. Tohver and two anonymous referees for theiraccurate comments. We gratefully acknowledge the guest editors ofthis volume for inviting us to write this contribution.

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