Gondwana Research 19 (2011) 84–99

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Time frame of 753–680 Ma juvenile accretion during the São Gabriel orogeny,southern Brazilian Shield

L.A. Hartmann a,⁎, R.P. Philipp a, J.O.S. Santos b, N.J. McNaughton c

a Instituto de Geociências, Universidade Federal do Rio Grande do Sul, Av. Bento Gonçalves, 9500; 91501-970 Porto Alegre, Rio Grande do Sul, Brazilb RedStone Resources, 110 East Parade, East Perth 6004 WA, Australiac John de Laeter Centre of Mass Spectrometry, Applied Physics, Curtin University of Technology, GPO Box U1987, Perth WA 6845, Australia

⁎ Corresponding author.E-mail address: leo.hartmann@ufrgs.br (L.A. Hartma

1342-937X/$ – see front matter © 2010 International Adoi:10.1016/j.gr.2010.05.001

a b s t r a c t

a r t i c l e i n f o

Article history:Received 13 July 2009Received in revised form 30 April 2010Accepted 4 May 2010Available online 20 May 2010

Keywords:Cambaí ComplexGeochronologyNeoproterozoicJuvenileBrazilian ShieldSão Gabriel orogeny

The time frame of the three main geological events in the Neoproterozoic Cambaí Complex, juvenile SãoGabriel belt in the southern Brazilian Shield is established by integrating field mapping, back-scatteredelectron imaging and sensitive high-resolution ion microprobe (SHRIMP II) U–Pb dating of 96 zircon crystalsfrom nine granitic and metasedimentary rock samples. The three events are: (1) voluminous flat-lyingparagneisses (Cambaizinho Complex) and orthogneisses (Vila Nova gneisses) between 735 and 718 Ma, (2)tonalite–trondhjemite association (Lagoa da Meia-Lua Suite) between 710 and 690 Ma, and (3) lategranodiorite intrusions (Sanga do Jobim Suite) at 680 Ma. An additional older volcanic event (CampestreFormation) was dated at 753 Ma. These results are most significant for the reconstruction of WestGondwana.

© 2010 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

1. Introduction

Understanding the time frame of Precambrian evolution is essentialfor the reconstruction of accretionary belts and evolution of super-continents. Detailedfieldmapping clarifies the relative ages of geologicalunits, such as the sequential intrusion of granitic rocks and shear zones,but robust geochronology is required for the reliable determination ofthe absolute ages of rock crystallization. Orogenic cycles commonly lastfor several hundred million years, the duration of a full Wilson Cycle ofoceanic crust generation and consumption (Condie, 1997; Condie et al.,2009); sucha cycle evolves fromaccretionary orogenies in the beginningto a great final collision of continental blocks. The Brasiliano Cycle (900–550 Ma) in the southern Brazilian Shield includes the accretionary SãoGabriel orogeny (735–680 Ma) and the collisional Dom Felicianoorogeny (650–550 Ma) (Hartmann et al., 2000; Heilbron and Machado,2003;Heilbronet al., 2004) andextends into southeasternBrazil (Silva etal., 2005; Borba et al., 2006; Schmitt et al., 2008; Borba et al., 2008). Wepresently focus on the main accretionary events because of theirimportance for understanding the crustal evolution of this southwesternportion of Gondwana Supercontinent.

Neoproterozoic juvenile terranes are extensive (N1000 km wide)in the Arabian–Nubian Shield, but less extensive in South America. Alarge (about 500 km long) belt occurs in central Brazil, the Goíás Arc

nn).

ssociation for Gondwana Research.

(Pimentel and Fuck, 1992; Laux et al., 2005; Matteini et al., 2010) anda smaller (about 100 km long) belt in southernmost Brazil, the SãoGabriel belt (Babinski et al., 1996) where the existence of Meso/Neoproterozoic juvenile oceanic crust and island arc rocks formedduring the Brasiliano orogenic events was demonstrated by Saalmannet al. (2005b). All three terranes were formed between 800 and550 Ma, but accretion of granitic rocks to the crust occurred atdifferent ages. The main peak in NE Africa was 750–700 Ma and incentral Brazil spread between 900 and 700 Ma. In the São Gabriel belt,previous investigations (e.g., Hartmann et al., 2000; Saalmann et al.,2005a, b, c, 2006a, b 2007) indicate an age peak near 750–700 Ma, theyoungest intrusion occurring at about 704 Ma. The knowledge of theages of magmatic and metamorphic events is essential for thereconstruction of the evolution of West Gondwana, as seen inprevious investigations (e.g., Vaughan and Pankhurst, 2008).

Because tonalite–trondhjemite–granite (TTG) associations are mostsignificant for the evolution of juvenile continental crust (Philipp et al.,2008; Senshu et al., 2009) and because detailed geochronologicalinvestigations can define the timing of generation of different portionsof the juvenile crust, we concentrated field and laboratory (sensitivehigh-resolution ion microprobe — SHRIMP II) investigations on theCambaí Complex, southern Brazilian Shield (Fig. 1). We selected theSanga do Jobim, Vila Nova and Palma regions (Fig. 1), because fieldrelationships indicate that the rocks are particularly suitable forunderstanding the timing of events in this segment of WesternGondwana. Field mapping and petrography indicate that the juvenileterrane is constituted by three main units, namely (1) amphibolite

Published by Elsevier B.V. All rights reserved.

Fig. 1. Regional geological map of western Rio Grande do Sul shield. Inset shows location of studied area; areas in black are cratons (older than 1.0 Ga), gray area is the Brasiliano cyclemobile belt and in white is the Andean orogen.

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facies, voluminous, flat-lying gneisses of mostly sedimentary (Cambai-zinho Complex ) and plutonic (Vila Nova gneisses, Cambaí Complex)derivation, (2) less deformed, diorite–tonalite–trondhjemite rocks(Lagoa da Meia-Lua Suite, Cambaí Complex), and (3) intrusive, littledeformed, granodiorite and tonalite plutons (Sanga do Jobim Suite,Cambaí Complex). The intrusion of the granitic rocks was intimatelyassociated with the development of the NE-trend, steep dipping shearzones. Greenschist and amphibolite facies volcanic belts also occur. Ninekey rock samples were dated by SHRIMP using 96 zircon crystals; thesewere supported by back-scattered electron images of all the crystals.

From these observations we delimit precisely the timing ofaccretionary events related to the consolidation of SupercontinentGondwana in its southwestern margin.

2. Geology and samples

The São Gabriel belt is located in the southern portion of theBrazilian Shield (Fig. 1), which includes all Precambrian rocks exposedat the surface in South America. The shield contains juvenile graniticand volcanic rocks of the Neoproterozoic Brasiliano Cycle in the region(Babinski et al., 1996). The concept of Brazilian Shield is distinct fromthe usage of South American Platform, because the shield does notinclude the cover intracratonic basins. The geology of the belt (Fig. 1)has been investigated for several decades (e.g., Jost and Hartmann,1984; Chemale, 2000; Hartmann et al., 2007), including sensitive highresolution ion microprobe (SHRIMP) age determinations of zirconsfrom many rocks (e.g., Hartmann et al., 2000). Several mappingprojects were undertaken by senior undergraduate students ofUniversidade Federal do Rio Grande do Sul; the reports remainunpublished but can be accessed in the libraries (Bitencourt et al.,1996, 1997, 2001, 2002, 2004). Structural relationships and a platetectonic model for the evolution of the São Gabriel belt, in the contextof the Brasiliano Cycle, are presented by Saalmann (2004) andSaalmann et al. (2005a, b, c, 2006a, b). These relationships constitutethe base of the present field and geochronological investigation.Mantle and crustal evolution in the São Gabriel belt and surroundinggeotectonic units are interpreted by Gastal et al. (2005a, b) using Ndand Sr isotopes as a proxy for the processes.

The SãoGabriel belt has a complex arrangement of geological units,as seen on the geological map (Figs. 1 and 2) and stratigraphic column(Figs. 3 and 4); see also Appendix A. Thrusting to the SE further

complicates the geological structure of the belt (Figs. 3 and 5). A largestep forward in the understanding of the geological evolution of thebelt was made by Saalmann (2004) and Saalmann et al. (2005a, b, c)and is used as the reference for this investigation. Particularly sig-nificant is the division of the Palma Group into a lower part (Cerro doOuro Formation) and an upper part (Campestre Formation); the lowerpart has evidence of deformations D1 and D2, not observed in theupper part, which only displays D3 and D4 deformations (Figs. 3 and5). The rocks of the Cambaizinho Complex also have evidence of D1and D2 deformations. Also most significant is the observation that thegranitic rocks of the Cambaí Complex (Lagoa da Meia-Lua Suite) wereintruded coeval with the upper Palma Group (only the D3 and D4structures of the Palma Group are present in the Lagoa da Meia-LuaSuite. The Vila Nova gneisses (Cambaí Complex) show structuresformed during D1 and D2. The granitic rocks were deformed insubvertical shear zones,whereas the samedeformational event causedflattening in the country-rock schists of the Palma Group.

2.1. Cambaizinho Complex

The Cambaizinho Complex, composed mostly of paragneisses, iscoevalwith the lower PalmaGroup and has a strong flat-lying foliationdeveloped under amphibolite facies conditions. Amphibolite-facies,flat-lying orthogneisses (Vila Nova gneisses) are also coeval, but areincluded in the Cambaí Complex, because this stratigraphic unitincludes all granitic rocks in the São Gabriel belt. The CambaizinhoComplex includes mostly paragneisses and has the oldest rock unit inthe São Gabriel belt. These paragneisses (sample RL1) occur in manyoutcrops as xenoliths in the orthogneisses, indicating that they are theoldest unit from field relationships.

The paragneisses have mostly quartzo-feldspathic composition,interpreted as meta-arkoses, and have a significant volume of inter-calated lenses of pelitic gneisses (garnet–biotite, staurolite–garnetand plagioclase–quartz–muscovite–biotite assemblages), quartzites,calcitic and dolomitic marbles, and calc–silicate gneisses. These lensesof paragneisses (Cambaizinho Complex) are segmented and form roofpendants (tens to hundreds of meters long) in the dominant dioritic–tonalitic–trondhjemitic orthogneisses (Vila Nova gneisses). In thewesternmost portion of the São Gabriel belt, the paragneisses forma continuous body of 80 km length from NW Palma region, passingthrough Passo do Ivo and reaching the Cambaizinho creek, the type-

Fig. 2. Geological map of Vila Nova region (many authors; most recently Bitencourt et al., 2001). Location of eight studied samples indicated; only one sample is located outside themap area.

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locality of the Cambaizinho Complex. Some slivers of other rocksare also intercalated in the paragneisses; these are mostly mafic–ultramafic rocks such as magnesian schists, serpentinites, and

Fig. 3. Stratigraphic description of stud

minor deformed peridotite, gabbro, norite, troctolite and anorthosite.All these rocks were deformed and recrystallized during orogenicactivity.

ied area (Saalmann et al., 2005a).

Fig. 4. Summary of stratigraphic names and ages (Saalmann et al., 2005a).

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The main exposure of quartz–feldspar paragneisses occurs in theRufino Farias region and still has remnants of sedimentary layeringand the S1 metamorphic banding followed by three main foldingepisodes. The sedimentary layering is identified in continuous andcompositionally distinct layers each typically with regular alternationof grain size; this is observed particularly in the quartz-feldspargneisses, quartzites and marbles.

The two oldest deformational events generated flat-lying struc-tures, the S1 metamorphic banding and its transposition into S2. F2folds are isoclinal and are preserved as recumbent, rootless folds. Thiswas followed by F3 subvertical transcurrent faults, leading to therefolding of the previously formed structures and generating normal,dipping anticlines and synclines. This third deformational phaseformed the regional structure marked by NE-trending foliation,dipping to NW and SE. A last F4 phase is seen in refolding of F3 foldaxes, with formation of axial fracture cleavage oriented NW-SE.

The paragneisses from the Cambaizinho Complex and theorthogneisses from the Cambaí Complex (Vila Nova gneisses) were

Fig. 5. NW–SW cross-section of studi

metamorphosed during two events in the middle to the upperamphibolite facies (M1 and M2). These conditions were attainedduring development of foliations S1 and S2. A third M3 is related tosteep dipping shear zones and occurred in variable conditions fromgreenschist to low amphibolite facies.

2.2. Orthogneisses (Vila Nova gneisses, Cambaí Complex)

This is the dominant unit in the Cambaí Complex. Steep shearzones also delimit the occurrence of orthogneisses (samples SL2,SL19), which are exposed in an elongated, extensive NE–SW unit fromSanga do Jobim to Sanga do Velocindo about 20 km. These two creeksare located respectively to the west and to the east of the town of VilaNova do Sul. The best exposures of orthogneisses are situated to theeast of Rufino Farias village and in the beds of the following creeks:Sanga do Jobim, Cambaí, Laranjeiras, and Sanga do Velocindo, all in theVila Nova do Sul region. The unit extends 50 km to the SW into theeastern part of Palma region.

ed area (Saalmann et al., 2005a).

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The orthogneisses (Vila Nova gneisses, Cambaí Complex)are delimited to the west by magnesian schists and serpentinites.These are cut by younger bodies of diorite–tonalite–trondhjemite–granodiorite (Lagoa da Meia-Lua Suite) and granodiorite (Sanga doJobim Suite), both belonging to the Cambaí Complex. The gneissesform lens-shaped and tabular bodies; tonalites are dominant, butdiorites and trondhjemites are also present. The trondhjemiticgneisses form thin (1–50 mm) bands in the tonalites, whereas thediorites are thicker (10–200 cm). Banding is regular, continuous andranges in thickness from millimeters to centimeters. It may showintense mineral orientation and biotite–hornblende schlieren. Someprimary magmatic, plutonic textures are preserved, such as blasto-equigranular hipidiomorphic texture and blasto-poikilitic texture,particularly by prismatic plagioclase.

The four deformational events found in the paragneisses are alsopresent in the orthogneisses, namely S1 metamorphic banding suc-ceeded by three main folding episodes. Granoblastic plagioclase andquartz predominate in bands, which are intercalated with nemato-blastic to lepidoblastic biotite and hornblende bands. The main min-eral assemblage is plagioclase–quartz–hornblende–biotite–epidote,indicative of middle amphibolite facies metamorphic conditions. Inthe least deformed portions, the gneissic structure changes into awell-marked foliation of biotite and hornblende.

2.3. Lagoa da Meia-Lua Suite (Cambaí Complex)

This suite is part of the Cambaí Complex and consists of tonalite,diorite, granodiorite and trondhjemite. The granitic rocks (datedsamples RL6, RL12, RL15 and SL6) were intensely deformed in ductileconditions and range from tabular bodies with either subhorizontal orsubvertical foliation tomoremassive structure. In general, the graniticbodies are elongated NE–SW parallel with the metamorphic foliationof the paragneisses and orthogneisses.

The foliation is marked by the orientation of biotite and hornblende,but mylonitic textures developed in high strain portions; these portionsdisplay consistent mineral stretching and partial recrystallization ofplagioclase, K-feldspar and quartz. There is also formation of chlorite,epidote and white mica. In a few outcrops, the rocks are banded andhave intercalation of decimeter to meter-thick bands of diorites andtonalites. Granitic bodies of the Lagoa da Meia-Lua Suite are mostlyconcordant with S3 foliation of the orthogneisses, although some arelocally discordant.

2.4. Sanga do Jobim Suite (Cambaí Complex)

This suite includes the youngest granitic bodies of the CambaíComplex judging from field relations and is composed (dated samplesSL4 and RL4) of tabular, concordant granitic bodies and intrusive,elliptical bodies with variable composition from diorite and tonalite togranodiorite. The suite includes the Capivara diorite (Garavaglia et al.,2002), the Sanga do Jobim granodiorite and the Cerca de Pedratonalite. Texture is equigranular, medium to coarse grained andprincipal minerals are plagioclase and quartz, with some biotite andhornblende, in addition to minor K-feldspar, epidote, titanite, zirconand apatite. Its structure is massive in most of the granitic bodybut there is some mineral orientation along the contacts with theother granites; protomylonitic to mylonitic textures occur in a fewexposures.

The nine dated samples from the Cambaí Complex and Cambai-zinho Complex (Figs. 6 and 7; description in Appendix A) wereselected from key exposures in the Vila Nova region (Fig. 1). Onedated sample (RL1) is from the Cambaizinho Complex and twoorthogneiss samples (SL19 and SL2) are from the Cambaí Complex.Judging from field relationships, all three samples are representativeof the oldest events, because they have flat-lying foliation, gneissicstructure, middle to upper amphibolite facies metamorphism, and

granoblastic texture. Four dated samples (RL6, RL12, RL15, and SL6)belong to the main granitic unit (Lagoa da Meia-Lua Suite) that formsthe juvenile crust. The two additional samples (SL4 and RL4) are fromthe youngest granitic intrusions (Sanga do Jobim Suite) based on fieldrelationships — magmatic structures and textures, little shear-zonedeformation.

This sampling, integratedwith previous studies, forms the basis fora reliable time frame for the evolution of the São Gabriel orogeny(Cambazinho and Cambaí Complexes), the granitic and medium-grade metasedimentary portion of crust formed and deformed duringthe São Gabriel orogeny. The three main geological structuresobserved in the field can thus be dated — (1) flat-lying, amphibolitefacies para- and orthogneisses, (2) volumetrically dominant, shear-zone related diorite–tonalite–trondhjemite, and (3) late-tectonicgranodiorite plutons.

3. SHRIMP U–Pb zircon geochronology

The overall time frame of rock formation and deformation isstrongly bimodal in the southern Brazilian Shield (Hartmann et al.,2000, zircon/SHRIMP; Tickyj et al., 2004, monazite/electron micro-probe) with one age peak at 2260–2000 Ma, the Trans-AmazonianCycle, and another at 800–550 Ma, the Brasiliano Cycle, with fewdatable Archean rocks. Both orogenic cycles have similar evolution,starting with juvenile accretion of granitic rocks (2260–2100 Ma and800–700 Ma) and ending with collision of continental plates (2100–2000 Ma and 630–590 Ma). The intervening time period (2000–800 Ma) corresponds to the position of the southern Brazilian Shieldinside Columbia Supercontinent (Hartmann, 2002; Rogers andSantosh, 2009; Santosh et al., 2009; Ramos et al., 2010).

The discovery of the juvenile terrane in the São Gabriel beltwas based on Nd isotopes and widespread occurrence of tonalites,trondhjemites and granodiorites, and included the TIMS dating ofzircon crystals from a diorite at 704 Ma (Babinski et al., 1996).Machado et al. (1990) had already established the magmatic age(zircon TIMS) of a rhyolite from the terrane at 753 Ma. Similar ages(zircon SHRIMP) were obtained by Leite et al. (1998) in theMantiqueiras section of the Cambaí Complex. This timing wassimplified and extensively used as 750–700 Ma for the delimitationof the São Gabriel orogeny (e.g., Hartmann et al., 2000; Saalmann et al.,2005a, b, c). These previous zircon geochronological investigations arepresently re-evaluated and integrated with new zircon SHRIMPgeochronology of nine rocks to establish the time frame of the SãoGabriel orogeny of juvenile accretion in the São Gabriel belt.

Ninety six zircon crystals were separated from nine rock samplesby crushing andmilling 5–10 kg of each rock followed by heavy liquidand magnetic methods at the laboratories of Universidade Federal doRio Grande do Sul. The crystals were mounted on an epoxy disc,polished to half their thicknesses and carbon coated for backscatteredelectron imaging at the University of Western Australia. The mountwas repolished and gold coated for SHRIMP II U–Pb isotopicdeterminations at Curtin University of Technology, Western Australia(Smith et al., 1998). Data reduction used the SQUID software (Ludwig,2001) and plots were prepared with Isoplot/Ex (Ludwig, 1999).

Our geochronological investigation included the study of theinternal structure of the zircon crystals by electronic imaging, back-scattered electrons (BSE), prior to isotopic study (Figs. 8–10), as donein many similar studies (e.g., Ali et al., 2009; Chen et al., 2010). Allzircon crystals display complex internal structure with two dominantstructural domains. One domain is the magmatic portion (commonlyhomogeneous and dark grey in BSE images) and the other is themetamorphic (shear-zone related) portion that is commonly brightgrey in BSE image. In the following discussion, the light grey portionsare considered the product of alteration by regional metamorphism ofthe dark gray portions, which are taken as the original magmaticcomposition of the crystals. This is based on extensive observation of

Fig. 6. Field photos of some of the studied units. (a) Gt–bt gneiss, Cambaizinho Complex (sample RL1), (b) Orthogneiss, Vila Nova gneisses, Cambaí Complex (sample SL19),(c) Tonalite (sample SL6) and trondhjemite pegmatite (sample RL6) from the Lagoa da Meia-Lua Suite, Cambaí Complex, and (d) Cerca de Pedra granodiorite (sample SL4), Sanga doJobim Suite, Cambaí Complex.

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internal structure, chemical and isotopic compositions of zircons fromSouth America (e.g., Hartmann et al., 2000; Silva, 2006).

Zircon crystals from orthogneisses (Fig. 8) have complex internalstructures. Titanite crystals from sample SL2 are rounded to embayed,have anhedral shapes, and range in size from 100 to 200 μm. They alsohave large quartz inclusions and narrow metamict seams. Thefractures in the crystals end as they reach lighter (in BSE) parts ofthe crystal (interpreted as younger, recrystallized zircon). Crystalsfrom samples RL1 and SL19 are similar.

Fig. 7. Photomicrographs of some of the studied samples. (a) Sample RL1, gt–bt gneiss, Cam(c) Sample RL6, trondhjemite, Lagoa da Meia-Lua Suite, Cambaí Complex, and (d) Sample R

The zircon crystals from the main magmatic stage (samples RL6,RL12, RL15, and SL6) are all prismatic, size between 100 and 300 μm,aspect ratio 3:1 to 5:1 (Fig. 9). The crystals are mostly euhedral, butrounding is omnipresent and ranges from pronounced to angular.Mineral inclusions are common, and may include apatite. All crystalsshow some fractures in the dark grey portions (magmatic), and theseare sealed in many places by light grey zircon (metamorphic).

The imaged zircon crystals from sample RL6, trondhjemite BR290,have an aspect ratio of 4:1, lengths near 200 μm(Fig. 9a, b) and euhedral

baizinho Complex, (b) Sample SL19, orthogneiss, Vila Nova gneisses, Cambaí Complex,L4, Sanga do Jobim Tonalite, Sanga do Jobim Suite, Cambaí Complex.

Fig. 8. Back-scattered electron images of zircon crystals from sample SL2, orthogneiss, Vila Nova gneisses, Cambaí Complex. Black circles indicate local and size of SHRIMP analyses.Analyses number and age shown.

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external faces, although some rounding is observed caused by meta-morphism. Inclusions are present, probably apatite. Darkest patchesaremetamict,whicharepresent in several crystals; these are restricted toan original euhedral zone of the crystal. Radial and crosscutting fracturesarepresent inmany crystals. Althoughcomplex, their internal structure isdivided into dark grey and light grey portions. The light grey portionsoccur anywhere in the crystals, in the core, mantle and rim. It isnoteworthy that fractures originally established in the dark grey portionwere sealed during recrystallization of zircon in the light grey portion, aprocessdescribedas fracture sealingbyHartmannet al. (1997), processesdescribed by Geisler et al. (2007) and Harley et al. (2007).

Sample RL12, the Santa Zélia granite, has zircon crystals 100–200 μm in length and an aspect ratio 3:1 with many euhedral faces(Fig. 9). Mineral inclusions are common, and some are probablyapatite. Some very dark portions are observed, both irregular orfollowing approximately the internal euhedral structure, and areinterpreted as metamict portions. Some crosscutting and radialfractures are also observed. The light grey portions occur mostly inthe cores of crystals, but are also observed in the mantle and rims.Fractures are sealed by recrystallization (light grey in BSE) of theoriginal magmatic zircon (dark grey in BSE).

RL15, the Buriti meta-tonalite, has zircons with an outstandinginternal structure displaying dark grey cores surrounded by light greyrims (Fig. 9). The crystals are 100–200 μm long, with an aspect ratio of2:1. Although magmatic euhedral faces are present in some crystals,rounding is a common feature in many crystals. Crosscutting fracturesare also present; several fractures are restricted to the dark grey(magmatic) core of the crystal and sealed by metamorphic recrystal-lization (light grey portion in BSE) of the magmatic rims. Some of thefractures well-marked in the dark grey portion have faint extensionsinto the light grey portions, a result of partial sealing of the fractureduring metamorphism.

In sample SL6, the tonalite BR290, the zircon crystals are 200–300 μm long, aspect ratio 5:1, and have euhedral external faces withvery little rounding (Fig. 9). Few crosscutting fractures are observed inthe dark grey portions which are sealed in places by light grey zircon.The recrystallized (light grey in BSE) zircon forms large bands,irregular patches and narrow rims on the zircon.

Zircons from the two samples (SL4 — Cerca de Pedra granodioriteand RL4 — Sanga do Jobim granodiorite) are from little-deformed,well-defined intrusive bodies, and are similar in the two rocks(Fig. 10). The crystals are 150–200 μm long, have an aspect ratio of4:1, are euhedral, and have irregular to planar metamict portions(very dark in BSE). Some fractures crosscut the crystals, well-markedin the dark grey in BSE (magmatic) portions and faint to nonexistantin the light grey (metamorphic) portions. The fainting of the fracturetrace into the metamorphic portion is due to relative intensity offracture sealing. Dark grey cores are preserved in a few crystals, butalteration was very intense so that the light grey in BSE portionspredominate and occur everywhere in the crystal.

The significance of the U–Pb ages of the zircon crystals (Fig. 11,Table 1) is better understood when the ages are associated with the U,Th contents and Th/U ratios of the zircon (Fig. 12, Table 1). Insequence, the three oldest, most deformed rocks (SL19, SL2, and RL1)are described first, followed by the main magmatic stage (samplesRL6, RL12, RL15, and SL6) and then the two samples from the waningstage of granitic magmatism in the arc (SL4 and RL4).

In sample SL19, the Vila Nova gneiss (tonalite), nine analyses ofnine crystals reveal low U contents near 100 ppm and low Th contentsof 50 ppm resulting in Th/U ratios near 0.5 (variation from 0.28 to0.69). In sample SL2, the Vila Nova gneiss (diorite), seven analyses inseven crystals reveal higher U near 400 ppm and Th near 450 ppmresulting in high Th/U ratio (near 1.2) with very small variation (1.16–1.23). In sample RL1, garnet–biotite gneiss, 17 analyses in 10 crystalsdisplay highly variable U 45–673 ppm and Th 0–772 ppm. The Th/Uratios are accordingly highly variable (0.00–1.18). The samples fromthe main magmatic stage (RL6, RL12, RL15, and SL6) belong to theLagoa da Meia-Lua Suite, and are representative of the magmatismthat was responsible for the largest volume of granitic magma thatbuilt the juvenile magmatic arc. Sample RL6, trondhjemite (Lagoa daMeia-Lua Suite), studied by nine analyses in nine crystals has high U ofabout 500 ppm (156–917 ppm) and variable Th (ranging from 20 to974 ppm). The Th/U ratios are consequently variable ranging from0.13 to 1.10. Sample RL12, the Santa Zélia granite (Lagoa da Meia-LuaSuite), was studied in six crystals and has high U contents between296 and 1363 ppm, mostly high Th contents which varies between 52

Fig. 9. Back-scattered electron images of analyzed zircon crystals. (a, b) Sample RL6, trondhjemite BR290, Lagoa da Meia-Lua Suite, Cambaí Complex, (c, d) Sample RL12, Santa Zéliagranite, Lagoa da Meia-Lua Suite, Cambaí Complex, (e, f) Sample RL15, Buriti meta-tonalite, Lagoa da Meia-Lua Suite, Cambaí Complex, and (g, h) Sample SL6, tonalite BR290, Lagoada Meia-Lua Suite, Cambaí Complex. Black circles indicate SHRIMP analysis position; spot number and age (Ma) shown.

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and 757 ppm. The Th/U ratios are near 0.50 but vary between 0.20 and0.86.

Sample RL15, the Buriti meta-tonalite (Lagoa da Meia-Lua Suite),has 16 analyses in 10 crystals and mostly high U contents between202 and 1389 ppm although one analysis is only 41 ppm. Th contentsare mostly low 6–17 ppm, but some higher values were also obtained(26–160 ppm). This sample shows a special feature, because Th/Uratios are bimodal, many are near 0.02 (metamorphic) and a few arenear 0.5 (magmatic). In the BSE images, the best-preserved core is inFig. 9e, f, because it is dark grey and homogeneous. This core also hasthe highest Th/U ratio of this sample (Fig. 12). The age of analysis b.1-2 is 766±14 Ma and is taken as the magmatic age of the Buriti meta-

tonalite. The other cores are not as homogeneous and display somealteration by light grey portions (Fig. 9). This alteration is interpretedas causing partial resetting of the original magmatic age and loweringof the Th/U ratio (Fig. 12).

Sample SL6, the tonalite BR290, is intercalated with sample RL 6,the BR290 trondhjemite, has six analyses in five crystals and Ucontents near 400 ppm (variation between 155 and 671 ppm) and Thnear 100 (variation between 32 and 361 ppm). The Th/U ratio variesbetween 0.17 and 0.81.

The two analyzed samples from the waning stages of graniticintrusion in the juvenile terrane (SL4 and RL4) are part of the Sanga doJobim Suite, a late-tectonic granitic phase in the juvenile magmatic

Fig. 10. Back-scattered electron images of analyzed zircon crystals, Sanga do Jobim Suite, Cambaí Complex. (a, b) Sample SL4, Cerca de Pedra granodiorite, (c, d) Sample RL4, theSanga do Jobim granodiorite.

Fig. 11. Concordia plots of all nine analyzed samples.

92 L.A. Hartmann et al. / Gondwana Research 19 (2011) 84–99

Table 1U–Pb zircon SHRIMP isotopic data from Cambaí Complex (and related Cambaizinho Complex) samples.

Spot U Th Th 4f206 Isotopic ratios Ages

207Pb 208Pb 206Pb 207Pb 208Pb 207Pb 206Pb Disc.

ppm ppm U (%) 206Pb 206Pb 238U 235U 232Th 206Pb 238U %

RL1, Garnet–biotite gneiss (Cambaizinho Complex)h.1-1 79 25 0.32 1.09 0.06438±6.3 0.1104±2.3 0.1344±1.29 1.1933±6.5 0.0364±11.9 754±134 813±10 −8h.1-2 75 22 0.30 0.61 0.06781±6.9 0.1028±2.5 0.1375±1.33 1.2857±7.0 0.0412±13.1 863±142 831±10 4h.2-1 268 62 0.24 0.14 0.13484±0.5 0.0695±1.0 0.3937±1.01 7.3189±1.1 0.1086±2.1 2162±9 2140±18 1h.3-1 82 0 0.00 0.65 0.05909±2.2 0.0161±6.7 0.0934±1.16 0.7606±2.5 – 570±47 576±7 0h.3-2 72 0 0.00 0.42 0.05879±2.5 0.0129±10.5 0.0956±1.70 0.7748±3.0 – 559±51 589±10 0h.3-3 45 4 0.09 1.90 0.05900±3.2 0.0528±7.5 0.1055±1.51 0.8573±3.5 – 565±69 642±10 −11h.3-4 39 0 0.00 1.94 0.06130±3.5 0.0283±21.0 0.0960±1.62 0.8111±3.9 – 650±74 591±10 −2h.5-1 135 36 0.27 0.68 0.13160±1.5 0.1001±3.4 0.3478±1.20 6.3111±2.0 0.1074±6.4 2119±27 1924±20 9h.6-1 113 71 0.65 0.03 0.18843±0.8 0.1815±0.9 0.5312±1.20 13.8007±1.4 0.1470±1.6 2729±12 2746±27 −1h.7-1 72 41 0.58 1.14 0.05922±7.5 0.2016±2.2 0.0920±1.37 0.7511±7.6 0.0285±6.6 575±163 567±7 1h.8-1 153 67 0.45 0.34 0.06648±2.5 0.1454±2.2 0.1302±1.20 1.1936±2.8 0.0398±3.7 822±52 789±9 4h.8-2 106 38 0.37 0.73 0.06870±3.2 0.1375±2.5 0.1308±1.16 1.2385±3.4 0.0431±4.5 890±67 792±9 11h.9-1 213 169 0.82 0.38 0.06287±2.2 0.2555±1.0 0.1201±0.85 1.0412±2.3 0.0365±1.8 704±46 731±6 −4h.9-1 96 67 0.72 0.34 0.13774±1.2 0.2097±1.4 0.3864±1.17 7.3387±1.7 0.1083±2.4 2199±20 2106±21 4h.10-1 143 99 0.71 0.53 0.05562±4.3 0.2224±5.0 0.0836±1.03 0.6414±4.5 0.0249±5.7 437±96 518±5 −18h.10-2 300 2 0.00 0.30 0.06031±1.2 0.0053±11.5 0.0926±0.78 0.7699±1.4 – 615±25 571±4 0h.12-1 673 772 1.18 0.28 0.06271±1.2 0.3514±0.5 0.1146±0.88 0.9913±1.5 0.0335±1.1 698±25 700±6 0

SL19, orthogneiss (Vila Nova gneisses, Cambaí Complex)e.1-1 78 33 0.44 0.00 0.06178±1.94 0.1430±2.16 0.1166±1.03 0.9940±2.19 0.0348±2.83 668±41 711±7 −6e.2-1 133 64 0.50 0.00 0.06247±5.13 0.1497±1.56 0.1184±1.22 1.0208±5.18 0.0321±7.40 692±99 721±8 −4e.3-1 114 48 0.44 0.20 0.06312±2.81 0.1308±1.83 0.1195±1.10 1.0401±3.01 0.0348±3.63 712±60 728±8 −2e.5-1 112 50 0.46 0.08 0.06222±1.79 0.1409±1.78 0.1173±1.00 1.0066±2.05 0.0374±3.40 682±38 715±7 −5e.7-1 102 40 0.41 0.29 0.06352±2.55 0.1284±2.29 0.1216±1.15 1.0649±2.80 0.0357±3.03 726±54 740±8 −2e.9-1 152 101 0.69 0.00 0.06510±2.22 0.2070±1.34 0.1185±0.66 1.0636±2.32 0.0367±2.05 777±47 722±4 7e.15-1 94 25 0.28 0.00 0.06397±1.57 0.0870±3.17 0.1160±0.72 1.0231±1.73 0.0353±1.93 741±33 707±5 4e.16-1 67 26 0.41 0.27 0.06733±3.68 0.1158±2.55 0.1179±1.10 1.0945±3.84 0.0405±4.03 848±77 718±8 15e.19-1 101 50 0.51 0.04 0.06349±1.72 0.1500±1.83 0.1190±1.03 1.0414±2.00 0.0366±2.17 725±36 725±7 0

SL2, orthogneiss (Vila Nova gneisses, Cambaí Complex)e.1-1 300 350 1.21 0.15 0.06402±1.08 0.0635±1.71 0.1211±0.26 1.0686±1.11 0.0061±3.11 742±23 737±2 0.8e.2-1 574 647 1.16 0.13 0.06338±0.75 0.0698±1.77 0.1190±0.17 1.0401±0.76 0.0069±2.36 721±16 725±1 −0.6e.3-1 314 364 1.20 0.12 0.06423±1.11 0.0585±2.14 0.1213±0.22 1.0738±1.13 0.0057±3.63 749±23 738±2 1.5e.5-1 338 402 1.23 0.33 0.06314±1.17 0.0531±5.73 0.1224±0.31 1.0658±1.21 0.0046±6.98 713±25 745±2 −4.4e.6-1 367 435 1.23 0.39 0.06467±1.49 0.0613±2.04 0.1210±0.25 1.0791±1.51 0.0053±5.13 763±31 736±2 3.5e.7-1 593 674 1.17 0.11 0.06367±0.80 0.0767±1.55 0.1195±0.17 1.0487±0.82 0.0076±2.21 731±17 727±1 0.4e.8-1 184 212 1.19 0.35 0.06322±2.05 0.0628±1.40 0.1211±0.35 1.0554±2.08 0.0057±6.09 716±44 737±2 −2.9

RL15, Buriti meta-tonalite (Lagoa da Meia-Lua Suite, Cambaí Complex)b.1-1 591 9 0.02 0.03 0.0897±1.39 0.0051±5.15 0.1146±1.08 0.9783±1.40 0.0320±11.5 669±20 702±6 −6b.1-2 42 26 0.65 0.00 0.0900±1.28 0.2016±3.59 0.1279±2.34 1.0633±4.42 0.0399±4.3 614±81 776±17 −26b.2-1 922 15 0.02 0.00 0.0891±1.53 0.0052±4.12 0.1129±1.03 0.9780±1.25 0.0352±11.3 703±15 699±6 0b.3-1 850 14 0.02 0.08 0.0900±1.28 0.0051±4.40 0.1134±1.03 0.9793±1.28 0.0233±11.1 697±16 687±6 −2b.4-1 719 12 0.02 0.10 0.0891±1.53 0.0052±4.73 0.1117±1.06 0.9637±1.33 0.0470±13.9 694±17 689±6 1b.5-1 664 13 0.02 0.05 0.0900±1.28 0.0064±5.97 0.1092±1.07 0.9461±1.38 0.0304±6.8 702±18 675±6 3b.5-2 691 12 0.02 0.02 0.0891±1.53 0.0053±5.32 0.1139±1.05 0.9862±1.34 0.0330±13.4 701±17 699±6 0b.6-1 268 129 0.50 0.05 0.0900±1.28 0.1516±1.45 0.1215±1.09 1.0944±1.75 0.0368±1.9 785±29 739±8 6b.7-1 668 11 0.02 0.06 0.0891±1.53 0.0055±4.83 0.1154±1.11 1.0063±1.39 0.0292±12.2 716±18 703±6 0b.8-1 202 83 0.42 0.26 0.0900±1.28 0.1250±1.78 0.1208±1.18 1.0495±2.05 0.0343±2.4 708±35 735±8 −4b.8-2 884 13 0.01 0.06 0.0891±1.53 0.0044±4.72 0.1142±1.03 0.9820±1.27 0.0440±13.6 686±16 691±6 1b.9-1 1168 17 0.01 0.04 0.0900±1.28 0.0047±3.93 0.1137±1.01 0.9762±1.32 0.0432±10.7 683±18 694±6 0b.10-1 267 6 0.02 0.05 0.0891±1.53 0.0076±7.00 0.1283±1.28 1.1327±2.34 0.0477±26.0 742±41 778±9 −5b.11-1 278 160 0.59 0.20 0.0899±1.36 0.1801±1.27 0.1230±1.08 1.0714±1.80 0.0364±1.8 715±30 748±8 −5

RL6, trondhjemite BR-290 (Lagoa da Meia-Lua Suite, Cambaí Complex) — intercalated with SL-6 tonalitek.1-1 611 534 0.90 0.02 0.06246±0.55 0.2725±0.91 0.1080±0.77 0.9299±0.95 0.0325±1.20 690±12 661±5 4k.2-1 373 59 0.16 0.08 0.06239±0.76 0.0491±1.42 0.1131±0.81 0.9729±1.12 0.0329±2.14 687±16 691±5 0k.3-1 917 974 1.10 0.01 0.06263±0.48 0.3497±0.77 0.1076±0.76 0.9289±0.90 0.0343±1.08 696±10 659±5 5k.4-1 368 116 0.33 0.11 0.06238±1.02 0.1040±1.41 0.1106±0.81 0.9512±1.31 0.0346±2.05 687±22 676±5 2k.5-1 251 80 0.33 0.03 0.06248±1.76 0.1043±1.78 0.1150±1.10 0.9903±2.08 0.0361±2.72 690±38 701±7 −2k.6-1 912 806 0.91 0.02 0.06275±0.72 0.2796±1.49 0.1125±0.89 0.9733±1.14 0.0344±1.73 700±15 687±6 2k.8-1 655 296 0.47 0.04 0.06274±0.55 0.1462±0.56 0.1091±0.76 0.9435±0.94 0.0344±0.99 700±12 667±5 5k.10-1 156 20 0.13 0.11 0.06237±2.02 0.0386±3.76 0.1131±1.26 0.9730±2.38 0.0346±5.59 687±43 691±8 −1k.11-1 271 102 0.39 0.09 0.06235±1.05 0.1220±3.42 0.1120±0.85 0.9625±1.35 0.0346±3.65 686±22 684±5 0

RL12, Santa Zélia granite (Lagoa da Meia-Lua Suite, Cambaí Complex)c.2-1 384 319 0.86 0.04 0.06322±1.17 0.2534±0.96 0.1157±1.01 1.0082±1.55 0.0762±4.70 716±25 706±7 1c.3-1 530 399 0.78 0.03 0.06281±0.96 0.2378±0.84 0.1157±0.95 1.0016±1.35 0.0744±1.74 702±20 706±6 −1c.3-2 534 101 0.20 0.14 0.06309±1.27 0.0631±1.80 0.1161±0.97 1.0100±1.60 0.0718±1.88 711±27 708±7 0c.4-1 383 233 0.63 0.20 0.06299±1.39 0.1896±1.26 0.1122±1.07 0.9747±1.76 0.0716±1.55 708±30 686±7 3c.4-2 383 155 0.42 0.05 0.06331±1.08 0.1275±1.54 0.1191±1.01 1.0393±1.47 0.0711±1.72 719±23 725±7 −1

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93L.A. Hartmann et al. / Gondwana Research 19 (2011) 84–99

Table 1 (continued)

Spot U Th Th 4f206 Isotopic ratios Ages

207Pb 208Pb 206Pb 207Pb 208Pb 207Pb 206Pb Disc.

ppm ppm U (%) 206Pb 206Pb 238U 235U 232Th 206Pb 238U %

c.9-1 165 52 0.33 0.13 0.06285±1.99 0.0974±2.70 0.1146±1.36 0.9932±2.41 0.0716±1.55 703±42 699±9 1c.9-2 1363 757 0.57 0.03 0.06227±0.67 0.1766±1.39 0.1158±0.86 0.9942±1.09 0.0711±1.72 683±14 706±6 −3c.9-3 1125 569 0.52 0.06 0.06230±0.75 0.1571±0.72 0.1149±0.87 0.9866±1.15 0.0725±0.86 685±16 701±6 −2c.9-4 553 115 0.22 0.03 0.06329±0.95 0.0637±1.88 0.1149±1.00 1.0027±1.38 0.0723±5.40 718±20 701±7 2c.10-1 296 160 0.56 0.53 0.06255±2.16 0.1606±1.35 0.1170±1.15 1.0091±2.45 0.0732±2.60 693±46 713±8 −3c.11-1 308 245 0.82 0.17 0.06191±1.60 0.2541±1.06 0.1154±1.07 0.9851±1.92 0.0735±1.32 671±34 704±7 −5

SL6, tonalite BR290 (Lagoa da Meia-Lua Suite, Cambaí Complex), intercalated with RL-6 trondhjemiteg.3-1 414 273 0.68 0.01 0.06178±1.22 0.0668±2.35 0.1115±0.44 0.9496±1.29 0.0338±1.05 667±26 681±3 −2.2g.4-1 460 361 0.81 0.06 0.06287±1.18 0.0823±5.75 0.1137±0.47 0.9856±1.27 0.0345±0.96 704±25 694±3 1.4g.7-1 218 66 0.31 0.25 0.06275±1.99 0.0531±1.26 0.1120±0.71 0.9702±2.10 0.0330±2.90 701±42 685±5 2.4g.9-1 155 32 0.21 0.14 0.06413±2.98 0.2067±0.84 0.1134±0.82 1.0027±3.09 0.0341±7.16 746±63 692±5 7.2g.10-1 470 119 0.26 0.04 0.06312±0.99 0.2471±0.75 0.1139±1.45 0.9914±1.76 0.0356±5.95 712±21 695±10 2.3g.10-2 671 112 0.17 0.03 0.06219±0.94 0.0970±1.75 0.1137±0.46 0.9752±1.04 0.0344±1.85 681±20 694±3 −2.0

RL4, Sanga do Jobim tonalite (Sanga do Jobim Suite, Cambaí Complex)g.1-1 672 107 0.16 0.04 0.06327±0.82 0.0516±1.54 0.1134±0.92 0.9896±1.23 0.0362±1.80 717±18 693±6 2g.2-1 724 176 0.25 0.03 0.06322±0.81 0.0752±1.25 0.1097±0.91 0.9561±1.22 0.0331±1.55 716±17 671±6 2g.3-1 489 68 0.14 0.20 0.06266±1.32 0.0443±2.10 0.1120±0.97 0.9674±1.63 0.0313±4.27 697±28 684±6 4g.4-1 1419 220 0.16 0.10 0.06223±0.71 0.0505±1.07 0.1105±0.87 0.9483±1.12 0.0334±1.95 682±15 676±6 2g.7-1 979 393 0.41 0.07 0.06261±0.84 0.1276±1.36 0.1111±0.88 0.9587±1.22 0.0345±1.73 695±18 679±6 2g.9-1 8939 1687 0.20 0.01 0.06251±0.23 0.0589±0.38 0.1246±0.80 1.0740±0.83 0.0374±0.91 692±5 757±6 1g.13-1 685 175 0.26 0.14 0.06194±1.06 0.0819±1.26 0.1096±0.93 0.9357±1.41 0.0328±2.03 672±23 670±6 2g.14-1 904 177 0.20 0.08 0.06162±0.84 0.0631±1.21 0.1128±0.89 0.9587±1.22 0.0344±1.86 661±18 689±6 2g.16-1 980 91 0.10 0.05 0.06241±0.73 0.0287±1.75 0.1109±0.89 0.9545±1.15 0.0320±2.13 688±16 678±6 2

SL4, Cerca de Pedra granodiorite (Sanga do Jobim Suite, Cambaí Complex)g.1-1 66 15 0.23 0.23 0.06005±2.66 0.0778±1.99 0.1167±0.72 0.9663±2.75 0.0364±5.10 605±58 712±5 −17.5g.1-2 339 101 0.31 0.16 0.06188±1.44 0.0918±1.88 0.1107±0.82 0.9442±1.66 0.0318±2.81 670±31 677±5 −1.0g.2-1 172 578 0.35 −0.01 0.06324±0.81 0.1099±1.02 0.1118±0.49 0.9746±0.94 0.0354±1.13 716±17 683±3 4.6g.2-2 874 232 0.27 0.03 0.06254±0.42 0.0892±0.50 0.1143±0.29 0.9858±0.52 0.0370±0.71 693±9 698±2 −0.7g.4-1 46 19 0.29 0.33 0.05972±4.37 0.0901±2.22 0.1124±0.86 0.9257±4.45 0.0324±7.86 594±95 687±6 −15.7g.5-1 136 62 0.47 0.06 0.06327±1.23 0.1490±1.03 0.1103±0.56 0.9626±1.35 0.0349±1.38 717±26 675±4 5.9g.6-1 175 35 0.21 0.17 0.06094±1.70 0.0695±1.37 0.1115±0.46 0.9369±1.76 0.0350±3.43 637±37 682±3 −7.0g.8-1 207 80 0.40 0.16 0.06154±1.41 0.1277±0.88 0.1078±0.40 0.9150±1.47 0.0335±1.81 658±30 660±2 −0.3g.8-2 817 158 0.20 0.04 0.06165±0.69 0.0578±1.48 0.1126±0.70 0.9573±0.98 0.0321±1.86 662±15 688±5 −3.9g.8-3 1020 218 0.22 0.06 0.06267±0.74 0.0661±1.34 0.1124±0.66 0.9715±0.99 0.0330±1.88 697±16 687±4 1.4g.9-1 468 97 0.21 −0.11 0.06424±0.74 0.0700±0.83 0.1197±0.43 1.0603±0.85 0.0406±1.57 750±16 729±3 2.8g.10-1 5011 621 0.13 0.00 0.06278±0.27 0.0379±0.76 0.1228±0.66 1.0631±0.72 0.0364±1.01 701±6 747±5 −6.6g.11-1 572 181 0.33 0.02 0.06278±0.65 0.1046±1.91 0.1167±0.36 1.0098±0.74 0.0371±1.97 701±14 711±2 −1.5g.12-1 773 206 0.27 0.06 0.06218±0.88 0.0846±1.32 0.1107±0.68 0.9489±1.11 0.0335±1.80 680±19 677±4 0.5

Notes: Isotopic ratios errors in %.All Pb in ratios are radiogenic component. Most are corrected for 204Pb and some for 208Pb (metamorphic, Th-poor grains or rims).disc. = discordance, as 100−100{t[206Pb/238U]/ t[207Pb/206Pb]}.f206=(common 206Pb)/(total measured 206Pb) based on measured 204Pb.Uncertainties are 1σ.

RL12, Santa Zélia granite (Lagoa da Meia-Lua Suite, Cambaí Complex)

94 L.A. Hartmann et al. / Gondwana Research 19 (2011) 84–99

arc. In sample SL4, the Cerca de Pedra granodiorite, 14 analyses in 10crystals show variable U contents — two analyses are 46 ppm and66 ppm, one analysis is 5011 ppm, and many vary between 136 and1020 ppm. Th contents also vary considerably — two analyses are 15and 19 ppm, one is 621 ppm, and many are between 35 and 578 ppm.The Th/U ratio varies less extensively between 0.13 and 0.35. SampleRL4, Sanga do Jobim granodiorite, nine analyses in nine crystals dis-play high U contents (293–980 ppm, one analysis at 1419 ppm andone at 8939 ppm. Th content is low in one analysis (14 ppm) and highin many analyses (68–393 ppm), one exception at 1687 ppm.

The U–Pb isotopic spot ages obtained by SHRIMP from the 96analyses in nine rock samples from the juvenile terrane are 207Pb/206Pbages for the Archean and Paleoproterozoic and 206Pb/238U ages forthe Neoproterozoic. Only four individual spot ages are older thanNeoproterozoic: one is Archean—2729±12 Ma (spot h.6-1) and threeare Paleoproterozoic — e.g., 2162±9 Ma (spot h.2-1, sample RL1), allfour old ages were obtained in zircons from sample RL1, the garnet–biotite gneiss.Most zircons from this paragneiss areNeoproterozoic, asare the ages of the 92 other analyses. The spot ages from the ninesamples are concentrated in the time interval 778–660 Ma, but a feware near 830 Ma or 520 Ma; both oldest and youngest Neoproterozoic

ages are from sample RL1. The geochronology of each rock sample isdescribed in sequence.

Zircons from sample SL19, Vila Nova orthogneiss, have an agespread between 728 and 707 Ma. The concordia age of the sample is718±2 Ma (Fig. 11), interpreted as the metamorphic age of the rock.Th/U ratios of the zircons are high, near 0.5; metamorphic composi-tions commonly remain high in tonalite zircons.

Sample SL2 is an orthogneiss (diorite) from the Vila Nova gneisses(Cambaí Complex), and has an intercept age of 735±7 Ma, inter-preted as the magmatic age, and two younger ages near 725 Ma,probably resetting by metamorphism without evidence for lead-loss.

Sample RL1, a metasedimentary rock from the CambaizinhoComplex, has an intercept age of 579±6 Ma, interpreted as the ageof shear-zone metamorphism of this sample. One Archean crystal ispresent, two Paleoproterozoic crystals and several crystals with agesnear 800 Ma.

The intercept age of sample RL6, trondhjemite BR290, is 694±5 Ma (Fig. 11). As seen on the BSE images, the spots were analyzedeither on the dark (magmatic) or light (metamorphic) grey portion ofthe crystal. Because the age difference of the analyses from the twoportions is not significant, we consider that magmatism and shear-

Fig. 12. Age versus Th/U diagrams of all nine studied samples; magm. = magmatic, met. = metamorphic.

95L.A. Hartmann et al. / Gondwana Research 19 (2011) 84–99

zone metamorphism occurred within a short time-period. This time-period is delimited by the error of the intercept age (∼10 m.y.).

Sample SL6, tonalite BR290, yields a concordia age of 690±2 Mawhich overlaps with sample RL6; both occur in alternating bands inthe same outcrop and have similar ages. This is in agreement withfield and petrographic evidence that indicates that this outcrop is partof a magmatic body, the banding corresponding to magmatic flow.Babinski et al. (1996) dated three zircon crystals (conventional) fromthe same outcrop, from the tonalite band, and obtained an age of704 Ma. Re-evaluation of the data shows that the correct interpreta-tion is to use the age of the most concordant analysis, which yields695 Ma for the magmatic age. This age is in agreement with theSHRIMP data presently reported.

Babinski et al. (1996) determined the age of zircon from a dioriteband (704 Ma, TIMS U–Pb geochronology) in this same outcrop fromwhich samples RL6 and SL6 were collected. This age is presently re-evaluated as 695 Ma±5 Ma, because three zircon crystals wereanalyzed by TIMS (683, 695 and 712 Ma) and this age correspondsto the most concordant (2%) analysis. The three ages cannot begrouped, and one of the two remaining analyses is not taken intoconsideration because the crystal is much smaller than the other two.The third analysis is more discordant (6%). The recalculated age(695 Ma±5 Ma; Babinski et al., 1996) is here interpreted as themagmatic age of the rock, and is nearly identical with the SHRIMPages of samples RL6 (694±5 Ma) and SL6 (690±2 Ma) presentlyreported.

The concordia age of sample RL12, Santa Zélia granite, is 704±3 Ma (Fig. 11). Altered and unaltered portions of the crystals yieldsimilar ages, so the emplacement and shear-zone deformation agesmust be similar, within the error (∼6 m.y.).

Sample RL15, the Buriti meta-tonalite, yields an intercept age ofanalyses that have Th/U ratios near 0.02 and are therefore of

metamorphic compositions. This intercept age of 696±5 (n=12;MSWD=1.6) Ma is thus interpreted as a metamorphic age related toshear-zone deformation. Three analyses have higher Th/U ratios near0.50 and also older ages of 750±6 Ma (Fig. 12), interpreted as themagmatic age of the tonalite. The crustal residence age of this sampleis therefore at least 54 m.y., a large time interval in comparison withthe other samples from the Cambaí Complex.

Sample SL4, the Cerca de Pedra granodiorite, has a concordia age of682±1 Ma, interpreted as the magmatic age of the sample; severalolder crystals are inherited. Magmatic crystallization and shear-zonealteration occurred within 2 m.y.

The concordia age of sample RL4, the Sanga do Jobim tonalite, is680±2 Ma and interpreted as the magmatic age of the rock. Alteredand unaltered portions of the crystals yield the same age and occurredwithin the error (±4 m.y.) of the analyses.

4. Discussion and conclusions

The dating of 96 zircon crystals from nine rocks established thetime frame for the evolution of the São Gabriel orogeny, particularlythe granitic Cambaí Complex and metasedimentary CambaizinhoComplex in the Vila Nova region of the southern Brazilian Shield(Fig. 13). Nd isotopic geochemistry indicates the beginning of crustalformation at 1300 Ma (Saalmann et al., 2005a, b, c). However, theseNd model ages may be due to the mixing of younger magmas andsediments with older crust. The presence of older crust was envisagedby Saalmann et al. (2005a, b, c), because the geochemistry of themagmatic rocks is comparable to rocks in continental margin arcs. Forinstance, volcanic and granitic rocks have low to medium K2Ocontents between 0.2 and 3.0 wt.% in the SiO2 interval 55–71 wt.%(Ruy P. Philipp, unpublished data). The presence of Archean andPaleoproterozoic zircons in a paragneiss (sample RL1) confirms that

Fig. 13. Summary of major orogenic events during the São Gabriel orogeny.

96 L.A. Hartmann et al. / Gondwana Research 19 (2011) 84–99

the São Gabriel orogeny was established on active continental margin(Andean type) represented by the la Plata Craton. An overview ofmagmatic and metamorphic ages obtained in zircons in the presentstudy is given in Table 2.

Deep crustal levels are exposed as amphibolite-facies paragneisses(Cambaizinho Complex) and orthogneisses (Vila Nova gneisses, CambaíComplex); the intense deformation that generated this flat-lyingfoliation occurred at ∼720–730 Ma (Remus et al., 1999). The para-gneisses resemble a deformedpassivemargin,whichwaspresent on themargin of the continent before this 730 Ma deformation occurred buttheymay also represent paleo-turbidites (trench)deposits. The volcanicrocks of the Campestre Formation formed at about 753 Ma and weresituated closer to the surface and only deformed at a later time (699 Ma,Remus et al., 1999). Orthogneisses dated at 735 Ma are included in theCambaí Complex.

The main crust-forming event in the region was the melting andintrusion of the orthogneisses protoliths (Vila Nova gneisses). Thiswas followed by the intrusion of diorite, tonalite, trondhjemite andgranodiorite of the Lagoa da Meia-Lua Suite (Cambaí Complex). Thisevent lasted about 15 m.y. from 705 to 690 Ma. This interval isreasonable for the construction of a volcanic arc. The intense shear-zone metamorphism that affected the suite occurred during the sametime span, because themagmatic andmetamorphic ages of zircons arethe same within error. The syntectonic nature of these granitic rocks

Table 2Ages and classification of the nine samples dated by SHRIMP II in this investigation; sampl

Sample number Rock description Stratigraphic unit

RL1 Garnet–biotite gneiss Cambaizinho ComplexSL19 Orthogneiss Vila Nova gneisses, CambSL2 Orthogneiss Vila Nova gneisses, CambRL15 Meta-tonalite Buriti meta-tonalite, LagoRL6 Trondhjemite BR-290 — intercalated

with SL6 tonaliteLagoa da Meia-Lua Suite,

RL12 Granite Santa Zélia granite, LagoaSL6 Tonalite BR290 — intercalated with

RL6 trondhjemiteLagoa da Meia-Lua Suite,

SL4 Granodiorite Sanga do Jobim tonalite, SRL4 Tonalite Sanga do Jobim tonalite, SBabinski et al. (1996) Diorite BR290 — intercalated with SL6

and RL6Lagoa da Meia-Lua Suite,

reported by Saalmann et al. (2005a,b,c) is now confirmed by zircondating.

Granitic intrusions late in the shear-zone deformational event areincluded in the Sanga do Jobim Suite and were formed at 680 Ma;minor deformation occurred within the error of the analyses.

The São Gabriel orogeny thus started with andesitic volcanism(Campestre Formation) at 753 Ma, followed by intrusion of orthog-neisses protoliths (Vila Nova gneisses, Cambaí Complex) and theirdeformationwith the passivemargin (Cambaizinho Complex) at 720–730 Ma and a major phase of juvenile granitic intrusions (Lagoa daMeia-Lua Suite, Cambaí Complex). The granitic intrusions of the Sangado Jobim Suite (Cambaí Complex) occurred at 680 Ma in the waningstages of the orogeny. The crust remained stable for 50 m.y., becausethe oldest events of the Dom Feliciano orogeny occurred at 630 Maand affected the São Gabriel belt by minor intrusion of granitic rocksand formation of the Camaquã basin.

The collision of several microcontinents and oceanic terranesduring the assembly of West Gondwana (e.g., Alkmim et al., 2001;Saalmann et al., 2006a) generated voluminous volcanic and graniticrocks. The robust dating of geological events in West Gondwana isrelevant for investigations in peripheral domains (e.g., Bueno et al.,2009; Silva Filho et al., 2010; Zeh and Gerdes, 2010). This makes thepresent investigation most significant because we have now estab-lished the time frame of volcanism, deformation and granitic rockinjection during the juvenile São Gabriel orogeny. Volcanism occurredat 753 Ma, followed by the main collisional event at 719 Ma, a majorpost-collisional event of tonalite–trondhjemite intrusion at 705–690 Ma, and ending with the injection of granodiorites at 680 Ma.

Some of the most significant results of this investigation can besummarized as follows.

1. The São Gabriel orogeny occurred between 753 and 680 Ma, asregistered in the Campestre Formation, Cambaizinho Complexand Cambaí Complex of the Palma and Vila Nova regions,southern Brazilian Shield.

2. The largest volume of juvenile granitic rocks was generated anddeformed early in the orogeny (Vila Nova gneisses) — 719–735 Ma.

3. A significant granitic event (Lagoa da Meia-Lua Suite) occurredbetween 705 and 690 Ma.

4. Early deformation occurred at 719–735 Ma generating flat-lying,amphibolite facies paragneisses, the Cambaizinho Complex, andorthogneisses, the Vila Nova gneisses, of the Cambaí Complex.

5. The final stages of the orogeny had intrusions of granodiorites at680 Ma, the Sanga do Jobim Suite.

6. Cores of zircon crystals (Buriti meta-tonalite) are dated at 776 Ma,but their significance requires additional investigations.

7. Andesitic volcanism occurred earlier (753 Ma) in the CampestreFormation and was little deformed.

e dated by Babinski et al. (1996) by TIMS included.

Magmatic age, Ma Metamorphic age, Ma

2729–830, inherited 579±6aí Complex 718±2 718±2aí Complex 735±7 ∼725a da Meia-Lua Suite, Cambaí Complex ∼740 703±7Cambaí Complex 694±5 694±5

da Meia-Lua Suite, Cambaí Complex 704±3 704±3Cambaí Complex 690±2 690±2

anga do Jobim Suite, Cambaí Complex 682±1 682±1anga do Jobim Suite, Cambaí Complex 680±2 680±2Cambaí Complex 695±5 695±5

97L.A. Hartmann et al. / Gondwana Research 19 (2011) 84–99

8. In several juvenile rocks, magmatic crystallization and metamor-phic recrystallization of zircon occurred in a short time interval ofb5 m.y.

9. The passive margin, the Cambaizinho Complex paragneisses, wasdeformed together with the voluminous juvenile granitic rocks(Vila Nova gneisses).

10. Flat-lying foliation and associated amphibolite facies metamor-phism occurred at 719–735 Ma.

11. The São Gabriel belt stabilized tectonically at about 680 Ma; it wasonly subjected to the intrusion of younger granites (e.g., São SepéGranite) and shear zone deformation.

Acknowledgements

The Brazilian Government supported this investigation throughCAPES (Coordenação de Aperfeiçoamento de Pessoal de NívelSuperior) — a post-doctoral scholarship to JOSS for the laboratorywork in Australia, and CNPq (Conselho Nacional do DesenvolvimentoCientífico e Tecnológico) — financial support for the use of laboratorytime through the project “Solução de problemas científicos e tecnoló-gicos na geologia do sul do Brasil com uso de tecnologia avançada”,Coordinated by LAH. Part of thefinancial support came from the project:“Evolução composicional e estrutural deminerais estratégicos do sul doBrasil”, PRONEX-FAPERGS/CNPq, coordinated by LAH. CNPq offeredresearch scholarships to LAH and RPP. The sensitive high resolution ionmicroprobe (SHRIMP II) is operated jointly by Curtin University ofTechnology, The University of Western Australia and the GeologicalSurvey ofWesternAustralia,with the support of theAustralian ResearchCouncil. Paul E. Potter is acknowledged for assistance with themanuscript. We acknowledge the significant contributions to theimprovement of the paper by GR reviewer V.A. Ramos.

Appendix A. Sample location and description

All nine rock samples were collected in the western portion of theRio Grande do Sul shield, in the São Gabriel belt. Sample RL1 belongsto the Cambaizinho Complex. The other eight samples belong to theCambaí Complex. The GPS coordinates of the sample collection sitesare:

Sample

Coordinates

SL19

0783454 S, 6639835 W SL2 0786347 S, 6638802 W RL1 0215397 S, 6637598 W RL6 0220377 S, 6639330 W RL12 0767448 S, 6608636 W RL15 0787414 S, 6639442 W SL6 0220377 S, 6639330 W SL4 0229895 S, 6636784 W RL4 0787414 S, 6639442 W

Sample RL1, biotite paragneiss (Cambaizinho Complex), Vila Nova region

Dark-coloured rock showing continuous, irregular banding (mil-limeter-thick) defined by alternating bands rich in biotite or quartz–biotite–muscovite–plagioclase. Mineral content is: quartz 40–45 vol.%, biotite 30–35, plagioclase 15–20, garnet 3–4, opaques 1–2, zircon tr,monazite tr. The texture is coarse (0.5–1.0 mm), granoblastic withbiotite and muscovite lineation. Intercalations of calk-silicate gneisses(diopside–quartz–plagioclase) occur as continuous, little boudinagedbands, thickness 15–35 cm. Subhorizontal banding refolded byrecumbent and open to closed, normal folds.

Sample SL19, orthogneiss (tonalite, Vila Nova gneisses, Cambaí Complex)

This orthogneiss is banded, with dominant felsic bands (2–5 cm)rich in quartz and plagioclase with interlobate to poligonal grano-blastic texture. The dark mafic bands (0.5–2.0 cm thick) aresubordinate in volume, made up of hornblende and biotite withcoarse grained, nematoblastic and lepidoblastic texture. Overall, thesample has plagioclase 55–60%, quartz 20–25%, biotite 10–15%,hornblende 3–5% and opaques 1–2%.

Sample SL2, biotite–hornblende orthogneiss (diorite, Vila Nova gneisses,Cambaí Complex)

This paragneiss is banded, with dominant dark mafic bands (5–12 mm thick) made up of hornblende–plagioclase–quartz with coarsegrained, nematoblastic texture. The felsic bands (0.5–2.0 mm) aresubordinate in volume, rich in quartz with poligonal, granoblastictexture but also containing sub-bands rich in quartz and prismatic,subhedral to euhedral plagioclase (trondhjemite composition).Overall, the sample has hornblende 60–65 vol.%, plagioclase 10–15%,quartz 10–15%, garnet 2–4% and opaques 2%.

Sample RL6, trondhjemite BR290 (Lagoa da Meia-Lua Suite, CambaíComplex)

Rock collected in a road side cut, known locally as the traditionaloutcrop of Vila Nova “gneisses”, in direct contact in the outcrop withsample SL6. Coarse grained (5–8 mm) trondhjemite, light grey,foliated with incipient orientation of biotite. It is part of a tabularbody crosscutting the dark grey, tonalitic rocks (sample SL6), and hasthickness 15–25 cm, folded and boudinaged; axial surface of folds isconcordant with the regional foliation of other units in the outcrop.Modal composition is plagioclase 55–60 vol.%, K-feldspar 5–7%, quartz25–30%, biotite 3–5%, opaques tr, zircon tr, apatite tr, sericite tr,chlorite 1–2%. Texture is heterogranular (blasto-inequigranular), with80 vol.% plagioclase porphyroclasts with equidimensional, prismaticshape (3.0–5.5 mm). The matrix is recrystallized and composed ofquartz, plagioclase, K-feldspar, and is granoblastic, medium grained(0.2–0.6 mm); mirmeckite occurs. The matrix is not continuous but israther distributed around the plagioclase porphyroclasts.

Sample RL12, Santa Zélia granite (Lagoa da Meia-Lua Suite, CambaíComplex)

Sample collected behind the Estância Santa Zélia main house, Palmaregion. It is a medium grained (2–3 mm) granite, pink, little altered,massive. Approximate mineral composition is K-feldspar 70–75 vol.%,plagioclase 3–5, quartz 25–30, biotite tr, opaques tr, zircon tr, apatite tr,muscovite tr, chlorite 1–2, epidote tr. Rock is foliated with alignment ofquartz, texture is hypidiomorphic inequigranular, medium to coarsegrained (3–6 mm). K-feldspar is prismatic, subhedral.

Sample RL15, Buriti tonalite (Lagoa da Meia-Lua Suite, Cambaí Complex)

Outcrop located on BR-290 highway, to the NW of the bridge overSanga do Jobim creek. Medium grained (3–5 mm) tonalite, light grey,foliatedwithmagmatic orientation of biotite. Texture is hypidiomorphicequigranular, medium to coarse grained (3–6 mm). Plagioclase isprismatic, subhedral to euhedral.

Sample SL6, tonalite BR290 (Lagoa da Meia-Lua Suite, Cambaí Complex)

Biotite tonalite, gneissic banding with light grey bands 1–2 mmthick of trondhjemitic composition, intercalated with dark grey bands1–2 cm thick rich in biotite. Strong foliation marked by biotite

98 L.A. Hartmann et al. / Gondwana Research 19 (2011) 84–99

orientation. Texture is inequigranular, granoblastic, coarse grained,and the mineral assemblage is plagioclase, quartz and biotite.

Sample SL4, Cerca de Pedra granodiorite (Sanga do Jobim Suite, CambaíComplex)

The granodiorite has diffuse foliation, massive, some orientation ofbiotite and feldspar. The texture is heterogranular with few K-feldsparmegacrysts (subhedral to anedral, 1.5–2.5 mm, rich in inclusions ofbiotite, opaques and plagioclase) and plagioclase megacrysts (equi-dimensional, prismatic, euhedral to subhedral, 1.5 mm size, polysyn-thetic twinning). The matrix is equigranular, fine to medium grained(0.6–1.2 mm), and has quartz, plagioclase, biotite and titanite.Plagioclase is unaltered.

Sample RL4, Sanga do Jobim tonalite (Sanga do Jobim Suite, CambaíComplex)

Outcrop located on BR-290 highway, on the side of bridge over theSanga do Jobim creek. The sample is medium grained (1–2 mm), grey,foliated with orientation of biotite. The texture is hypidiomorphic,granular.

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