GondwanaResearch

Gondwana Research, V. 8, No. 2, pp. 143-161.© 2005 International Association for Gondwana Research, Japan.ISSN: 1342-937X

Geochemistry and Crustal Evolution of Volcano-sedimentarySuccessions and Orthogneisses in the São Gabriel Block,Southernmost Brazil – Relics of Neoproterozoic Magmatic Arcs

K. Saalmann1*, M.V.D. Remus2 and L.A. Hartmann2

1 Geologisch-Paläontologisches Institut, J.W. Goethe-Univ. Frankfurt, Senckenberganlage 32-34, D-60054 Frankfurt am Main,Germany, E-mail: saalmann@em.uni-frankfurt.de

2 Instituto de Geociências, Universidade Federal do Rio Grande do Sul, Caixa Postal 15001 CEP 91501-970, Porto Alègre, RS,Brazil, E-mail: afraneo@if.ufrgs.br; marcus.remus@ufrgs.br

* Corresponding author

(Manuscript received June 29, 2004; accepted December 15, 2004)

Abstract

Precambrian metaplutonic rocks of the São Gabriel block in southernmost Brazil comprise juvenile Neoproterozoiccalc-alkaline gneisses (Cambaí Complex). The connection with associated (ultra-)mafic metavolcanic andmetasedimentary rocks (Palma Group) is not well established. The whole complex was deformed during the Brasilianoorogenic cycle. Both metasedimentary and metavolcanic rocks as well as metaplutonic rocks of the Cambaí Complexhave been sampled for geochemical analyses in order to get constraints on the tectonic setting of these rocks and toestablish a tectonic model for the São Gabriel block and its role during the assembly of West-Gondwana. The majorelement compositions of the igneous rocks (Palma Group and Cambaí Complex) indicate a subalkaline character; mostorthogneisses have a calc-alkaline chemistry; many metavolcanic rocks of the Palma Group show signatures of low-Ktholeiitic volcanic arc basalts. Trace element data, especially Ti, Zr, Y, Nb, of most igneous samples from both the lowerPalma Group and the Cambaí Complex indicate origin at plate margins, i.e., in a subduction zone environment. This iscorroborated by relative enrichment in LREE, low contents of Nb and other high field strength elements and enrichmentin LILE like Rb, Ba, and Th. The data indicate the possible existence of two suites, an oceanic island arc and a continentalarc or active continental margin. However, some ultramafic samples of the lower Palma Group in the western SãoGabriel block indicate the existence of another volcanic suite with intra-plate character which possibly represents relicsof oceanic island basalts (OIB). Trace element data indicate contributions from andesitic to mixed felsic and basic arcsources for the metasedimentary rocks. The patterns of chondrite- and N-MORB-normalized spider diagrams resemblethe patterns of the igneous rocks, i.e., LILE and LREE enrichment and HFS depletion. The geochemical signatures ofmost igneous and metasedimentary samples and their low (87Sr/86Sr)t ratios suggest only minor contribution of oldcontinental crust.

A geotectonic model for the São Gabriel block comprises east-ward subduction and following accretion of an intra-oceanic island arc to the eastern border of the Rio de la Plata Craton at ca. 880 Ma, and westward subduction beneaththe newly formed active continental margin between ca. 750 and 700 Ma. The São Gabriel block represents relics of anearly Brasiliano oceanic basin between the Rio de la Plata and Kalahari Cratons. This ocean to the east of the Rio de laPlata Craton might be traced to the north and could possibly be linked with Neoproterozoic juvenile oceanic crust in thewestern Brasília belt (Goiás magmatic arc).

Key words: Geochemistry, Neoproterozoic, arc magmatism, Brasiliano orogenies, São Gabriel block.

Introduction

Juvenile rocks are key areas for the identification oforogenic sutures and hence are important for theinvestigation of plate-tectonic processes during theamalgamation of continents. Open questions concerningthe formation of West-Gondwana during theNeoproterozoic Pan-African/Brasiliano orogenic cycleinclude the occurrence and width of former oceanic basins,

the identification of suture zones, timing and kinematicsof collisional events as well as the number and distributionof involved cratonic fragments.

Neoproterozoic Brasiliano metamorphism andmagmatism in South America is dominated by intensereworking and melting of older crustal material,nevertheless some occurrences of Neoproterozoic juvenilerocks have been identified in the Brasília belt (Pimentel

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and Fuck, 1992; Junges et al., 2002), in the Ribeira belt(Tassinari et al., 2002) and in southern Brazil (Babinskiet al., 1996) (Fig. 1b).

Precambrian rocks in southernmost Brazil display acomplex tectono-metamorphic evolution since theArchean (Hartmann et al., 1999; Da Silva et al., 2000;Chemale, 2000; Hartmann et al., 2000). Major crustalaccretion occurred during the 2.2–2.0 Ga Trans-Amazonianorogeny which represents the most important orogenicevent in southern Brazil (Hartmann et al., 2000).In southernmost Brazil, juvenile Neoproterozoic igneousrocks (ca. 750–700 Ma) have been identified and datedby Babinski et al. (1996) and Leite et al. (1998). Theyoccur in the western part of the Shield and intrude avolcano-sedimentary succession comprising ultramaficand mafic metavolcanic and associated metasedimentaryrocks. The volcano-sedimentary succession has beeninterpreted to represent, at least in parts, as relics ofArchean or Palaeoproterozoic greenstone belts (Hartmannand Nardi, 1983; Jost and Hartmann, 1984; Koppe andHartmann, 1988; Remus et al., 1993, 1999; Hartmann etal., 1999) whereas other authors suggest a Neoproterozoicage (Fernandes et al., 1992; Strieder et al., 2000). Recentlypublished Sm-Nd analyses give Meso- to NeoproterozoicTDM model ages (Chemale, 2000; Saalmann et al., 2005)and hence favor the latter interpretation. These rocks arekey units for a consistent plate tectonic model for theNeoproterozoic Brasiliano orogenic cycle which has notexisted up to now.

This paper presents geochemical and isotopic data ofthe volcano-sedimentary succession as well as fromplutonic rocks of the Cambaí Complex sampled withindifferent areas of the São Gabriel block in order to getconstraints on the stratigraphic relation between theCambaí Complex and the volcano-sedimentary succession,the tectonic setting of the latter, and their significance forthe assembly of West-Gondwana.

Geologic Setting

Precambrian rocks of southernmost Brazil can besubdivided into a number of major tectono-stratigraphicunits (Jost and Hartmann, 1984, Soliani, 1986, Babinskiet al., 1996) (Fig. 1c):

The Dom Feliciano belt in the E is characterizedby different types of ca. 600 Ma granitoids derivedfrom extensive Neoproterozoic crustal reworking ofPalaeoproterozoic (2.2–2.0 Ga) basement gneisses(Encantadas gneisses) (Babinski et al., 1996, 1997;Da Silva et al., 1999; Hartmann et al., 2000; Da Silvaet al., 2000).

The Porongos belt to the W consists of metavolcanicand metasedimentary rocks of possible Meso- or

Neoproterozoic age with some exposures ofPalaeoproterozoic gneisses (Encantadas gneisses)(Chemale et al., 1995).

Juvenile Neoproterozoic rocks are restricted to theSão Gabriel Block in the W. It is bounded to the south bythe Taquarembó block; the northern and western partsare covered by Phanerozoic successions of the Paraná basin(Fig. 2). To the east, the belt is tectonically juxtaposed tothe Porongos belt. The São Gabriel block is intrudedby ca. 600 Ma old undeformed, post-orogenic granites(e.g., Lavras granite, Jaguarí granite in Fig. 3).

The southern part of the shield, part of the Rio de LaPlata Craton, contains the Taquarembó block whichcomprises the Palaeoproterozoic Santa Maria Chicogranulites (Hartmann, 1998; Hartmann et al., 1999).

The Camaquã basin is a late to post-orogenic fault-bounded basin filled with very low-grade tounmetamorphosed volcano-sedimentary successioninterpreted as molasse deposits.

Three Neoproterozoic Brasiliano events have beenrecognized in southern Brazil (Hartmann et al., 2000;Da Silva et al., 2000). The oldest Neoproterozoic event,the ca. 880 Ma Passinho event documents the firstsubduction activity preserved in a metadiorite south ofLavras do Sul. It is followed by the 750–700 Ma SãoGabriel event, comprising the intrusion of and CambaíComplex, and the ca. 620 Ma Dom Feliciano event.

Stratigraphic Subdivision and Age of the SãoGabriel Block

The stratigraphy of the São Gabriel block comprisesmainly two units, (1) metamorphic volcano-sedimentarysuccessions of the Palma Group and (2) the CambaíComplex (Fig. 3).

The age of the Passo Feio Formation surrounding theCaçapava batholith (Fig. 2) is ambiguous and correlationwith other formations elsewhere within the São Gabrielblock is not established. The formation comprisesmetapelites, chloritoid schists and metarenites intercalatedwith metamorphosed (ultra-)mafic rocks. The rock units,however, do not constitute one unique stratigraphicsequence so that the Passo Feio Formation is a compositeunit, which also comprises Trans-Amazonian basementunits. Schists in the northern parts, considered in this study,however, contain Neoproterozoic zircons (Remus et al.,2000) and thus were deposited after the Trans-Amazonianorogeny.

The Palma Group is a volcano-sedimentary succession,comprising ultramafic and mafic metavolcanic rocksintercalated with schists and gneisses of probablemetasedimentary origin. The Palma Group can besubdivided into a lower and an upper unit with the former

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comprising the Cerro do Ouro and Cambaizínho formations(Fig. 3). The Cerro do Ouro Formation comprises mainlymetabasalts, amphibolites and ultramafic rocks likevarious magnesian schists and serpentinites (Remuset al., 1993). The metavolcanic rocks are interleaved withamphibole gneiss, chert layers, quartzite and minormarble. The latter two form elongate lenses ormegaboudins. The Cambaizínho consists of paragneisses,quartzite, metapelite and amphibole gneiss. Themetasedimentary rocks are intercalated with magnesianschist, serpentinite, and amphibolite. The volume ofmetasedimentary rocks is higher with lower amounts ofmafic metavolcanic rocks than in the Cerro do OuroFormation. A Neoproterozoic age for the lower PalmaGroup is indicated by 0.6–1.3 Ga Nd TDM model ages andpositive εNd(t) values of both the metavolcanic andmetasedimentary rocks suggesting that the latter weremainly derived from juvenile igneous rocks formed shortly

before whereas the input from old continental crust wasminor (Saalmann et al., 2005).

In the western parts of the São Gabriel block, the lowerPalma Group was intruded by granites (Sanga do Jobimgranite and equivalents) which occur as SW-NE oriented,elongated lenticular bodies with thicknesses varying fromseveral meters to hundreds of meters cross-strike. Theyintruded parallel to the foliation of the country rocks andtheir fabrics indicate a synkinematic emplacement(Saalmann, 2004). A zircon Pb-Pb evaporation age of697±3 Ma was obtained (Remus et al., 2001), and positiveεNd(t) values of +5.2 (Babinski et al., 1996) indicate ajuvenile source.

The upper Palma Group contains the Campestre Formationwhich comprises low-grade metamorphic volcanic andvolcanoclastic rocks like andesites, dacites, intermediatetuffs and tuffitic rocks, resembling equivalents in modernisland arcs. The formation also comprises metasandstones

Fig. 1. (a) 500 Ma reconstruction of Gondwana with Neoproterozoic mobile belts (modified from Trompette, 1994 and Unrug, 1997); note theposition of the Dom Feliciano belt (dark grey) and the adjacent São Gabriel block (black). (b) Map showing the distribution of cratons andNeoproterozoic Brasiliano belts in Brazil (modified from Alkmin et al. 2001). Cratons: A–Amazon, RP–Rio de la Plata, SFC–São Francisco-Congo. Belts: BP–Borborema province, CD–Chapada Diamantina, DF–Dom Feliciano belt, NB–Northern Brasilia belt, P–Paraguai belt,PC–Paramirin corridor, R–Ribeira belt, RPB–Riaco do Pontal belt, S–Sergipano belt, SB –Southern Brasília belt. (c) Geologic map with thetectono-stratigraphic domains in Rio Grande do Sul and Uruguay (modified from Fernandes et al., 1992; Chemale et al., 1995, Chemale, 2000).

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and metapelites, partly derived from tuffitic protoliths.Magmatic zircons of the Campestre Formation yield U-Pbages of about 753±2 Ma and 757±17 Ma interpreted ascrystallization ages (Machado et al., 1990; Remus et al.,1999). The succession is commonly interpreted as the volcanicpart of a magmatic arc (Koppe and Hartmann, 1988;Chemale et al., 1995; Babinski et al., 1996; Hartmann etal., 1999, 2000) probably related to the juvenile dioritesand tonalites of the Cambaí Complex (Hartmann et al.,1999). Positive εNd(t = 750 Ma) values and 770–900 MaTDM model ages resemble the data for the Cambaí Complexand lower Palma Group (Saalmann et al., 2005).

The Cambaí Complex consists of juvenile metadiorites,metatonalites and metatrondhjemites of several kilometersin width and tens of kilometers along strike. Deformationand metamorphic overprint of the plutonic rocks haveled to formation of gneisses. They are truncated bydifferent generations of dykes and veins of trondhjemitic,granitic and pegmatitic composition. The plutonic rockshave calc-alkaline chemical signatures interpreted ashaving been formed in a magmatic arc environment(Chemale et al., 1995; Babinski et al., 1996). They intrudedthe units of the lower Palma Group. U-Pb conventional

and SHRIMP ages of the Cambaí Complex are about750–700 Ma, like 750±16 Ma old tonalites from thesouthwestern São Gabriel Block (Leite et al., 1998) and704±13 Ma diorites from Vila Nova area (Babinski et al.,1996). The Passinho metadiorite to the south of the Lavrasgranite (Fig. 2) shows an age of 879±14 Ma (Leite et al.,1998) (Fig. 3).

The Santa Zélia granite, exposed in the westernmostparts of the Palma area (Fig. 2), is a late-tectonic intrusion.It was also affected, at least at its margins, by shearingduring intrusion and cooling (Saalmann, 2004) andcontains a subhorizontal stretching lineation comparablewith structures found in the adjacent country rocks. Itsage is unknown, however, its synorogenic formationindicates that it is older than the ca. 600 Ma post-tectonicgranitoids in this area. It is only weakly deformed and itpost-dates the Cambaí Complex indicating that it isyounger than 700 Ma. In contrast to all other units in theSão Gabriel block, the granite displays a negative εNd (t)for a proposed intrusion age of about 660 Ma and a higherTDM model age (1.4 Ga) which suggests melting orconsiderable incorporation of older crustal material(Saalmann et al., 2005).

Fig. 2. Geologic map of the São Gabriel block (modified from Remus et al., 2000).

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Structural Evolution of the São Gabriel Block

Three Neoproterozoic Brasiliano events have beenrecognized in southern Brazil (Hartmann et al., 2000;Da Silva et al., 2000): the ca. 880 Ma Passinho event, the750–700 Ma São Gabriel event and the ca. 620 Ma DomFeliciano event.

In the São Gabriel, four deformation events related tothe Brasiliano orogenic cycle, D1 to D4, the first threecharacterized by ductile deformation, can bedistinguished. D1 and D2 only occur in the lower PalmaGroup (Cerro do Ouro and Cambaizínho formations). Thelayering and banding S1 of the lower Palma Group rockswas formed during the first deformation phase D1. Isoclinalfolding of S1 took place under amphibolite faciesmetamorphic peak conditions during D2 associated withSE-directed thrusting. The third deformation phase D3

represents the first deformation that affected the rocks of

the Cambaí Complex and the Sanga do Jobim granite andequivalents. In the schists, deformation took place mainlyunder greenschist to lower amphibolite faciesmetamorphic conditions. In the Cambaí Complex as wellas in the Sanga do Jobim granite, D3 is characterized byright-lateral shearing along SW-NE oriented shear zoneswhich show a width of dm to several meters and can behundreds of meters along-strike. Fabrics and structure ofthe Cambaí rocks reflect a deformational evolution startingwith magmatic flow fabrics followed by subsolidus andsolid-state high to low temperature deformation, andascent, emplacement and solidification of the juvenileplutonic rocks was accompanied by dextral movements(Saalmann, 2004). Magmatic and metamorphic layering,fold axes and the lineation of the plutonic rocks have thesame orientation like S3, F3 and L3 of the country rocks(lower Palma Group. Hence, emplacement and deformation

Fig. 3. Scheme showing the stratigraphy of the São Gabriel block. Age and stratigraphic relationship of the Passo Feio Formation are not established.The northern parts of this composite unit contain 800 Ma zircons indicating a Neoproterozoic depositional age. The metasedimentary andmetavolcanic Cambaizinho and Cerro do Ouro formations represent Meso- to Neoproterozoic units. The Passinho dorite has an age of ca.879 Ma. 750–700 Ma calc-alkaline diorites adn tonalites of Cambaí Complex intrude the lower Palma Group. The Campestre formationprobably represents the volcanic counterpart of the magmatic arc within the Cambaí Complex so that stratigraphic boundaries are diachronous.The Santa Zélia granite formed in the western part of the Palma region and intruded during late tectonic stages. The lower successions ofthe Camaquã (molasses) basin are deformed in contrast to the upper parts. The Lavras granite and the Caçapava granite are post-tectonicintrusions with ages of 595 Ma and 560–540 Ma, respectively.

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of the plutonic rocks took place during D3. Thedeformation was partitioned into a transcurrentcomponent controlled by synkinematic melt ascent, anda contractional component dominating the deformationin the country rocks (Saalmann, 2004). D3 is also the firstdeformation of the Campestre formation (upper PalmaGroup). It is characterized by SE-vergent folding.Cleavage-bedding intersection lineations and fold axesstrike SW-NE and correspond to F3 and L3 in the lowerPalma Group. The lithologic units were tectonicallyjuxtaposed by dm to several meters thick ductile shearzones related to D3 leading to a SE-vergent ductile thrust-stack (Fig. 4). Thrusting was accompanied by SW-NEoriented strike-slip shearing. D3 represents the São Gabrielevent and hence, the age of the ca. 750–700 Masynkinematic Cambaí Complex gives the time frame forthis deformation phase.

The last deformation phase D4 took place underretrograde conditions and is characterized by semi-brittleSE-directed thrusting, and local brittle reactivation of older(D3) shear zones. Thrust faulting led to imbrication,kinking and thrust-related folding of the rocks. D4

deformation is localized and restricted to the fault zones.

Geochemistry

In order to constrain the source and tectonic setting of SãoGabriel Block, samples of metasedimentary, metavolcanic

and granitic rocks of the Palma Group from differentstratigraphic units and areas were sampled. Samples oflate-tectonic intrusions of the last main ductile shearingevent (D3) are represented by the Santa Zélia granite.

Thirty-two whole-rock samples of 5–10 kg have beencrushed and powdered. Chemical analyses of majorelements for 13 samples were obtained by XRF (BondarClegg laboratories, Canada), trace elements for 32 sampleshave been analyzed with ICP-MS (Bondar Clegglaboratories, Canada) or XRF (Geoscience Institute,Universidade Federal do Rio Grande do Sul, UFRGS, PortoAlegre). Rb-Sr isotope data for 21 samples were obtainedat the Isotopic Geology Laboratory of the UniversidadeFederal do Rio Grande do Sul (UFRGS), Porto Alegre,Brazil. For Sr isotope analyses, 21 whole-rock sampleswere powdered and dissolved using distilled HF, HNO3,and HCl in teflon digestion vessels. Rubidium andstrontium were then separated and purified by passingthe leachate once through a Teflon column. The massspectrometer analyses were performed at the IsotopicGeology Laboratory of the Universidade Federal do RioGrande do Sul (UFRGS), Porto Alegre, Brazil. Analyses ofsample and standard solutions consist of ±130 total ratios;the average measured 87Sr/86Sr ratio for the NBS-987standard was 0.71026±11.

Samples and whole-rock geochemical analyses ofmajor and trace elements are given in table 1 and table 2.Table 3 lists Rb-Sr results.

Fig. 4. Schematic interpretative section through the São Gabriel block. SE-ward oblique thrust-stacking already started during D2 (and possiblyduring D1). Final juxtaposition of the different stratigraphic units occurred during D3 ductile shearing including dextral strike-slip shear aswell as reverse dip slip. D4 brittle SE-directed thrust faulting locally reactivated D3 shear zones. Two major “nappe” units (a western andeastern unit) can be distinguished, each further subdivided by thrust units. The structure is bordered and cut to the east by later shear zonessurrounding the 562–540 Ma Caçapava batholith.

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Table 1. List of samples. Major element chemical analyses.

Igneous rocks

The composition metavolcanic rocks of the PalmaGroup, excluding the ultramafic rocks, ranges from basaltand basaltic andesite; volcanic and volcanoclastic rocksof the Campestre formation have andesitic and daciticcompositions. The major element compositions of theigneous rocks (Palma Group and Cambaí Complex)indicate a subalkaline character; and most rocks have acalc-alkaline chemistry; some belong to low-K-series.

Pearce and Cann (1973) have used the Y/Nb ratio toidentify the petrologic character of basaltic rocks,i.e., alkalic or tholeiitic. The Palma Group metavolcanicsamples have a Y/Nb ratio indicating tholeiitic ortransitional character, only few samples (a tonalite of theCambaí Complex and a metabasalt in the Bosoroca belt)have low Y/Nb ratios of alkaline rocks. This is consistentwith the spread of data in the Ti-Cr discrimination diagram(Fig. 5a) plotting the low-K field of volcanic arc basalts,though many samples plot near the transition line.

The diagram plotting Ti versus Zr of metavolcanic rocks(Fig. 5b) illustrates that nearly all samples plot the fieldsfor island arc tholeiite and calc-alkaline basalt. In theZr/Y-Ti/Y diagram after Pearce and Gale (1977) nearlyall metabasaltic rocks of the Palma Group plot the fieldfor plate margin basalt, which comprises both volcanicarc basalts and MORB (Fig. 6c). Zr and Y can be used forfurther discrimination by plotting Zr/Y vs Zr (Pearce andNorry 1979) (Fig. 6a). This diagram shows great dispersion.The important point is that no samples plot the field B forclear MORB and that most samples plot the field ofcontinental arc, with some of them the field for oceanicarc. The latter comprise some ultramafic rocks of the Cerrodo Ouro Fm and an andesitic rock of the Palma area. It isinteresting to note that other samples of ultramafic rocksof the Cerro do Ouro Formation plot the field for continental

Cambaizínho Formation Bossoroca Belt Cambaí Complex Campestre Fm SantaBR- BR- BR- BR- BR- BR- BR- BR- BR- BR- BR- Zélia g.

147/1 126/6i 126/10 137/2 138/2 125/2 126/3 126/7 148/1 101/1 107/1 BR-amph. schist schist meta- amph. meta- meta- meta- meta meta- meta- 123/2gneiss basalt gneiss diorite diorite quartz- quartz- andesite andesite granite

diorite diorite

SiO2 65.77 70.16 52.47 54.67 52.72 54.36 51.06 69.17 72.5 60.25 54.83 73.8TiO2 0.55 0.33 0.8 1.16 0.98 1.69 0.29 0.35 0.65 0.5 0.55 0.19Al2O3 16.28 14.87 16.43 15.56 15.79 16.51 6.08 17.86 11.75 16.33 17.98 13.44Fe2O3 2.97 2 7.21 7.79 9.15 8.03 9.46 2.66 3.86 4.95 6.29 1.73MnO 0.05 0.15 0.22 0.47 0.16 0.12 1 0.04 0.2 0.07 0.44 0.04MgO 1.18 0.76 5.78 2.53 6.91 3.87 16.33 0.99 0.87 3.11 1.74 0.41CaO 3.78 1.42 9.27 10.65 7.67 5.23 11.37 2.57 2.3 6.7 13.64 1.19Na2O 4.37 3.85 3.76 3.2 3.25 4.05 0.75 4.05 3.14 3.68 1.85 3.64K2O 1.67 3.66 1.21 1.51 0.9 1.83 0.16 2.71 1.41 1.47 0.21 4.29P2O5 0.16 0.09 0.14 0.3 0.19 0.56 0.15 0.11 0.27 0.15 0.12 0.06Cr2O3 1.97 1.8 1.5 1.2 1.25 2.66 1.05 1.48 1.64 1.97 1.88 0.48Total 98.78 99.1 98.81 99.07 99 98.91 97.86 99 98.61 99.2 99.53 99.29

arc. Some samples of the upper Palma Group, the Bossorocabelt and a Cambaí Complex sample plot the field forwithin-plate basalt.

Zr/Nb serves as a discriminant to distinguish oceanicisland basalts (OIB) which have low ratios (<10). Thesamples display a wide range of Zr/Nb ratios varyingbetween ca. 2 and ca. 50, though most samples haveZr/Nb ratios above 10, with the exception of twometabasalts of the Bossoroca belt, two ultramafic samplesand a tonalite sample of the Cambaí Complex of the Palmaarea. Plotting Zr/Nb vs. Y/Nb (Fig. 7a) most samplesspread over the left side of the diagram with lower Y/Nbratios transitional to the compositions of average OIB andE-MORB, however, they show varying and higher Zr/Nbof rocks in subduction zone settings. Rocks of island arcsand active continental margins show high Th/Yb ratiosdue to subduction-zone fluids which are enriched in Th(Wilson, 1989). Plotting Th/Yb vs Nb/Yb (Fig. 7b) thesamples of this study plot above the mantle array even atthe enriched end. This is in accordance with samples fromactive continental margins (some Andean volcanic samplesplotted for reference). Some samples shift to lowerNb/Yb ratios and are transitional to oceanic island arcs(Aleuten arc samples plotted for reference).

Rare earth element patterns (REE) corroborate theabsence of clear N-MORB indicated by the discriminationdiagrams. Normalized to chondrite they exhibitenrichment of light REE (LREE) (Fig. 8). Some samplesof the Cambaí Complex resemble with OIB, regardingLREE, however, they differ in stronger depletion in HREE.Many samples plot between OIB and E-MORB: they areless enriched in LREE than OIB but more than MORB.However, these samples as well do not show the flat HREEpattern of the reference rocks but a trough-like shape,and this pattern fits best with REE pattern of Andean-

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Table 2. Trace element analyses.

Cambaizinho Formation Cerro do Ouro Formation

BR-146/1 BR-147/1 BR-126/1 BR-126/3 BR-126/6 BR-126/10 BR-127/2 BR-131/1 BR-150/1 BR-109/1 BR-101/4amph. Gt-gneiss amph. amphi schist amph. quartzite quartzite Mg-schist Mg-schist serpentinitegneiss gneiss bolite gneiss

Ce 29 47 - 7 39 18 - - 4 - -Eu 1.2 0.9 - 0.5 0.6 1 - - - - -La 13 25 10 4 28 9 - 18 - - 8Lu 0.3 0.1 - 0.1 0.1 0.2 - - 0.1 - -Nd 18 20 - 7 17 11 - 12.63** 3.99** - -Sc 18.2 8.2 8 10.7 5 17.5 - 6 8 - 10Sm 4.1 3.31** - 1.4 2.7 2.8 0.2 3.3** 0.21** - -Tb 1 1 - - - - - - - - -Th 1.5 4.9 - - 8 0.8 - 5.85* 1.89* 1.03* 3.56*U - - - - 2 - - - - - -Yb 2 1 - 1 1 1 - - 1 - -Hf 3 3.9 - - 3.4 2.1 - 5.1* 0.46* 0.38* 2.72*Rb* 7 38 - - 83 43 - 49.8* 1 2.22* 16Ag 0.7 - - 8.7 - - - - - - -Cu 6 29 39 6 21 5 - 11 11 4 2Pb 3* 9* 1037 3278 18 7 - 17 3 4 48Zn 78 43 458 2109 31 80 - 68 64 33 137Mo 2 - 1 10 2 - 1 2 - 2 4Ni 72 24 37 913 20 87 1 19 1244 1682 51Co 42 14 66 52 11 - - 10 62 81 36Cd - - 65.9 47.9 - - - - - - -As - - - 259 - - - - - - 19Fetotal 6.7 2.2 2.64 7.34 1.45 5.29 0.09 2.01 5.52 4.2 5.22Mn 1105 359 2096 7739 1064 1688 7 891 978 608 1084Te - - - - - - - - - -Ba 54 800.9* 242 346 1787 339 - 633 9 - 197Cr 137 40 106 720 82 108 35- 45 618 950 143V - 47 35 68 29 109 3- 28 57 - 49Sr 621* 537* 389 54 316.82* 501* - 279 24.58* 3.86* 13Y 27.67* 11.71* 22 10 11 14 3 26 11.4* - 12Ga - - - - 13 - - 13 - - -Li - - 11 21 18 14 - 28 4 - 56Nb 3 4.49* - - 5.53* 7 - 7 - - 8Ta - - - - - 0.07* - 2.58* - - 1.51*Ti 6600 3200 2700 1400 2200 4100 - 3100 400 - 3300Zr 122.21* 179.39* 90 22 94 26 - 114 5.74* - 64

Cerro do Ouro Bossoroca Belt Passo Fei North Formation Formation BR-137/1 BR-137/2 BR-138/1 BR-138/2 BR-138/3 BR-139/1 BR-167/1 BR-168/1 BR-168/2BR-111/1 BR-112/3 Cl-schist metabasalt amphi amph. metatuff metap Cl-schist meta meta

serpentinite Mg-schist bolite gneiss sammite sandstone sandstone

Ce - - 25 41 - - - 17 - 19 -Eu - - 0.8 1.6 - - - - - 0.9 -La - 6 11 20 9 5 22 8 - 9 26Lu - - 0.3 0.4 - - - - 0.1 0.3 -Nd 0.02** 3.19** 10.03** 22 - - 18.54** 9 0.74** 13 -Sc - 12 4.7 15.2 28 25 25 1.5 24.6 18.5 -Sm 0.005** 0.88** 2.35** 5.1 - - 3.90** 1.8 0.22** 3 -Tb - - - - - - - - - 1 -Th 0.06* 2* 6.8 3.8 0.95* - 1.88* 1.5 0.13* 1.8 -U - - 2 - - - - 1 - - -Yb - - 2 2 - - - 1 1 2 -Hf - 1.7* 3.9 4.9 4.15* - 2.11* 1.8 1.2* 2.7 -Rb* 0.66* - 4 - 28* - 30 - 74 - -Ag - - - - - - 0.6 - 0.6 - -Cu 2 14 2 37 43 100 89 5 26 192 31Pb 2 0.3* 15* 5 1.51* 7 2.31* 18 28 6 6Zn 45 83 33 92 109 72 80 8 42 89 13Mo 1 - 2 2 2 3 3 - 3 1 -

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Table 2. Contd.

Ni 2379 1256 5 44 64 35 135 10 195 28 8Co 80 58 5 27 67 42 57 1 30 29 11Cd - - - 1 - - - - - - -As 5 9 - - - - - - 46 - -Fetotal 4.67 6.31 1.08 5.44 9.68 6.44 8.34 0.42 4.77 90 1.28Mn 700 1243 389 3456 1414 1156 1576 654 992 888 276Te - - - - - - - - - - -Ba 10 36 147.38* 1090 104 213 258 505 135 737 904Cr 229 852 26 62 63 61 162 15 286 39 70V 18 75 15 90 212 158 267 4 95 127 27Sr 3 20 138* 382 639.25* 357 380 177 210.45* 152 388Y 0.4* 17 19.86* 22 23 19 18 9 6.11* 15 -Ga 0.1* 9.95* - - - - - 15 8.06* - 16Li 3 7 4 6 4 11 8 3 40 15 3Nb - 7 4.62* 11 22 10 34 6 5 6 -Ta - - - - - - - - - - 9Ti - 2700 1700 6900 1300 5400 9600 200 800 4000 2800Zr - 33 174.96* 24 133.14* 54 102.69* 38 8.36* 60 13

Campestre Formation Cambaí Complex Santa Zélia g.BR-31/2 BR-32/2 BR-107/1 BR-116/1 BR-114/1 BR-101/1 BR-125/2 BR-126/7 BR-148/1 BR-123/2phyllite phyllite andesite phyllite phyllite andesite metatonalite metatonalite metatonalite granite

Ce 23 44 7 12 - 27 82 59 33 56Eu 1 1 0.7 0.6 - 0.6 1.9 0.8 1.2 0.5La 11 20 4 6 - 13 40 33 15 26Lu 0.3 0.4 0.2 0.3 - 0.2 0.2 0.2 0.6 0.8Nd 12.56** 20.02** 7 9 9.86** 13 36 21 15.89** 19Sc 19.8 10.4 14.4 26.4 29 12.3 13.5 5.5 15.3 3.3Sm 3.17** 4.47** 1.8 2.3 2.87* 2.4 7 3.6 3.75** 4.5Tb 1 1 - - - - 1 - 1 1Th 2.3 5.1 - 0.9 0.9* 3.6 3.8 11 3 32U - 2 - - - - 1 2 - 8Yb 2 3 1 2 - 1 1 1 4 5Hf 2.9 4.7 - 2.1 2.1* 2.5 4.8 4.3 4.8* 4.6Rb* 66 68* 2.51 31 31 - - - 40 209.66Ag - - - - - - - - - -Cu 26 29 4 46 68 52 64 22 18 8Pb 5.42* 11.47* 4 7 6 15 7 20 7.68* 18Zn 86 56 59 79 136 42 98 48 66 29Mo - 1 2 - - 2 2 9 1 2Ni 64 54 10 24 19 68 45 19 8 11Co 28 13 16 - 15 26 - 16 13 -Cd - - - - - - - - - -As - 6 - - - 21 - 7 - -Fetotal 5.58 2.45 4.32 5.01 5.35 3.48 5.9 1.95 2.84 1.26Mn 1040 787 3075 404 624 500 882 326 1460 320Te - - - - - - - - - -Ba 444.42* 971.1* 77 301 261 762 854 1247 471.44* 444Cr 131 101 42 42 63 127 64 109 42 99V 93 34 108 87 125 74 126 40 51 14Sr 232* 224 258 90 152 406 938 493 266* 143.07Y 25.75* 20 10 17 17 12 15 15 31.56* 37Ga - 10 - 11 14 - - 16 11 14Li 42 7 4 74 66 11 40 19 15 73Nb 1.48* 3.81* - - - 6 17 6 2.08* 13Ta - - - - - - - - -Ti 4500 3000 3400 3700 4100 3100 9700 2300 3800 1200Zr 131 196.99* 16 32 34 55 69 125 196.13** -

* XRF ** mass spectrometry

Cerro do Ouro Bossoroca Belt Passo Fei North Formation Formation BR-137/1 BR-137/2 BR-138/1 BR-138/2 BR-138/3 BR-139/1 BR-167/1 BR-168/1 BR-168/2BR-111/1 BR-112/3 Cl-schist metabasalt amphi amph. metatuff metap Cl-schist meta meta

serpentinite Mg-schist bolite gneiss sammite sandstone sandstone

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Table 3. Rb-Sr analyses.

type active continental margins. This can best be seen onprimitive mantle-normalized diagrams (Fig. 9). The Andeansamples also display a wide range of REE expressed indifferent degrees of LREE enrichment. Spider diagramsnormalized to chondrite (Fig. 9) show a trough at Nb whichis a typical feature of volcanic-arc basalts. The pattern fitswell with that of the central and northern volcanic zonein the Andes. Two ultramafic rocks of the lower Palmagroup in the Palma area show higher Nb contents andthus more strongly resemble alkalic oceanic island basaltswhich have a peak at Nb reflecting an enriched source.Normalization to N-MORB highlights the nature of thesubduction zone component. All samples are depleted inZr, Ti, Y, Yb relative to MORB. The Cambaí Complex,(ultra-)mafic rocks of the lower Palma Group and volcanicrocks of the Campestre Formation also show negative Smanomalies. In addition, Hf is depleted in both lower andupper Palma Group rocks. Low contents of high fieldstrength elements are typical for arc basalts or rocks fromactive continental margins and thus for subduction-related

magmas. The spider diagrams also highlight enrichmentof Rb, Ba, Th, and, except in the Bossoroca belt, of Sr.

Regarding Rb-Sr analyses pf the igneous rocks,(87Sr/86Sr)t values spread between 0.7000 to 0.70489. The(87Sr/86Sr)t values of the magnesian schists andserpentinites vary most, with values up to 0.708, which isinterpreted as being result from younger disturbance ofthe Rb-Sr system, e.g., during metasomatism and associatedfluid flow to have mobilized Rb and Sr shifting the analysesto unrealistic values. Disturbance of the Rb-Sr isotopicsystem cannot be excluded for the other samples as wellalthough they do not spread like the ultramafic rocks andthe values fit well with the trace element and other isotopicdata. The Santa Zélia granite differs from the othersamples also in the Sr isotopes. Its (87Sr/86Sr)t values of0.7068 and 0.708 are considerably higher than those ofthe igneous and metasedimentary rocks indicatingsignificant contribution from old continental crust. Thisis also illustrated in the εNd(t) vs (87Sr/86Sr)t diagram(Fig. 10). Due to their positive εNd(t) values and low initial

sample* 87Rb/86Sr error 87Sr/ error age(t) 87Sr/(SD abs) 86Sr(present) (SD abs) 86Sr(t)

BR-104/01 0.8499 0.000131 0.7091 0.000237 750 0.7BR-107/01 0.8022 0.000067 0.7088 0.000154 700 0.7008BR-109/01 0.0381 0.000008 0.7078 0.000153 700 0.7074BR-111/01 0.3638 0.000024 0.7127 0.000246 800 0.7085BR-112/03 0.7854 0.078543 0.7109 0.0012 800 0.7019BR-114/01 0.5151 0.051506 0.7093 0.001 800 0.7034BR-126/06 0.7629 0.07629 0.7101 0.0023 800 0.7014BR-126/07 0.2486 0.00002 0.7057 0.00011 750 0.7031BR-126/10 0.2424 0.024243 0.7056 0.0009 800 0.7028BR-31/02 0.8035 0.080353 0.7096 0.001 700 0.7016BR-32/02 0.807 0.080701 0.712 0.0014 700 0.7039BR-131/01 0.4804 0.048036 0.7077 0.0015 800 0.7022BR-137/01 0.0819 0.008187 0.7045 0.001 800 0.7036BR-138/01 0.1161 0.011613 0.7048 0.0018 800 0.7035BR-138/03 0.2129 0.021291 0.7069 0.0009 700 0.7048BR-167/01 0.1187 0.000008 0.7168 0.000085 800 0.7154BR-146/01 0.0273 0.002729 0.7036 0.0008 750 0.7033BR-147/01 0.1999 0.019987 0.7053 0.001 800 0.703BR-148/01 0.4247 0.042474 0.7077 0.0009 700 0.7035BR-150/01 0.1149 0.01 0.7042 0.000097 800 0.7029BR-123/02 4.149 0.000035 0.7447 0.000227 620 0.708BR-123/02 4.2817 0.003812 0.7447 0.000101 620 0.7068

* samples in italic = igneous samples, others = metasedimentaryrocks Table: Average of ±130 isotopic values, 1,0 V of ionic intensityfor 88Sr and multicolector with 86Sr in the axial colector. Analyses ofsample and standard solutions consist of ±130 total ratios; the averagemeasured 87Sr/86Sr ratio for the NBS-987 standard was 0.71026±11.Normalization to 86Sr/88Sr = 0,1194, fitted to bias with base on theSrCO3 NBS-987, using 87Sr/86SN = 0,71025 and correction in order ofthe presence of spike. NBS values during analises were 0,71026+11.Initial 87Sr/86Sr values have been calculated by using the formula:(87Sr/86Sr)t = (87Sr/86Sr)measured – 87Rb/86Sr(eλ × t -1), where t = proposedor measured age of the rock, λ = 1.42 × 10-11.

Fig. 5. (a) Ti-Cr plot (Pearce, 1986) for basic rocks illustrating theirlow-K tholeiitic volcanic arc basalts to transitional nature.(b) Variations in Ti and Zr content in metavolcanic rocks of thelower and upper Palma Group. Fields for Mid Ocean Ridge basalt(MORB), island arc tholeiite (IAT), calc-alkaline basalt (CAB),ocean island tholeiite (OIT), and ocean island alkali basalt (OIA)from Pearce and Cann (1973) and Floyd and Winchester (1975).

(b)

(a)

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Fig. 6. (a) + (b) Discrimination diagrams for basalts based on Zr/Y-Zrvariations (Pearce and Norry, 1979). A–volcanic arc basalt,B–MORB, C–within-plate basalt, D–MORB + volcanic arc basalt,E–MORB + within-plate basalt. (c) Different enrichment of Ti andZr in within plate basalts and plate margin basalts (Zr/Y-Ti-Y afterPearce and Gale, 1977).

87Sr/86Sr ratios the rocks with the exception of the SantaZélia granite plot the field of the mantle array.

Metasedimentary rocks

Th and Sc are considered to be immobile the Th/Scratio is an excellent provenance indicator (Taylor andMcLennan, 1985). Sc is a good tracer of a mafic sourcewhereas Th is enriched in felsic rocks. Nearly all samplesfrom both the lower and upper Palma Group showrelatively low to moderate Th/Sc values except a quartziticschist and a gneiss of the lower Palma Group in theBossoroca belt and Palma area, respectively, which haveTh/Sc ratios >1. This could be explained by recycling ofsediments as these samples represent rocks with highermaturity. The low Th/Sc ratios of the majority of thesamples indicate a provenance from a mafic source.

In the Ti/Zr vs La/Sc diagram (Fig. 11) themetasedimentary samples plot the fields of oceanic islandarc source and active continental margin source. Mostigneous samples plot the field of oceanic islands, some inthe fields for continental island arc and active continentalmargin. This suggests significant contributions from amafic source to many sedimentary samples. On a plotLa/Th vs. Hf (Fig. 11) the data fall close to the fields ofan acidic and andesitic arc source suggesting a mixedfelsic/basic source, possibly an active continental marginsetting. Some minor contribution from a mafic source to

at least some sedimentary samples cannot be excluded aswell. The data also show derivation from two volcanicsuites, an oceanic arc and an active continental margin.

The patterns of chondrite- and N-MORB-normalizedspider diagrams resemble the patterns of the igneousrocks. The metasedimentary rocks as well showenrichment in Rb, Th, and Ba and a trough at Nb (andTa). This suggests source rocks of a subduction-relatedsetting. The patterns would also follow the trace of uppercrustal rocks except of the stronger depletion in Nb whichsupports a strong subduction-related source component.

Most metasedimentary rocks have low (87Sr/86Sr)t

values ranging between 0.7014 and 0.739. This corroboratesthe trace element data indicating derivation of thesediments from mafic to intermediate igneous rocks.A sample of the Passo Feio North Formation has aconsiderably higher (87Sr/86Sr)t of 0.7154. This samplealso shows differing εNd(t) values and a TDM model age of2.57 Ga (Saalmann et al., 2005) suggesting Archeancontinental crust source rocks.

Discussion

Tectonic setting

In discrimination diagrams, most igneous samples plotthe fields of plate margin basalt, i.e., oceanic or continentalisland arc and active continental margin. None of the

(c)

(b)

(a)

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samples shows clear N-MORB signatures. A subductionzone setting for most rocks is corroborated by the REEand multi-element patterns showing relative enrichmentin LILE and LREE and a trough-like shape of HREE. Thepositive spikes of LILE can be interpreted as consequenceof components added to the mantle source of the basaltsby subduction zone fluids which are enriched in theseelements. Variable LREE enrichment is typical for islandarc and active continental margin rocks, and patterns ofsome samples shifting to OIB or E-MORB could beexplained with a back-arc basin environment which isgeochemically transitional between OIB and MORB. Lowcontents of immobile HFS elements (Y, Hf, Zr, Ti, Nb, Ta)and especially the trough at Nb are characteristic features

of rocks formed in a subduction zone environment. Insubduction zones, the sediments on the subducting platemight contribute to the partial melts. Presence of arecycled sediment component in basaltic rocks can beinferred from the Pb/Ce as they contain much higherconcentrations of Pb than MORB and OIB. TypicallyCe/Pb is greater than 0.3 in sediments and <0.4 in MORB.The igneous samples of this study show Pb/Ce ratiosbetween 0.08 and 0.6 suggesting existence of a recycledsediment component in some rocks.

Hence, most rocks were very likely related to amagmatic arc setting at either an intra-oceanic arc or activecontinental margin. The existence of both is suggested bythe distribution of metabasaltic samples of the lowerPalma Group in the fields for continental and oceanic arcin figure 6b. The oceanic arc samples are mainly from thePalma area and include ultrabasic rocks and ametaandesite. It is possible that they are related to the ca.880 Ma Passinho diorite (Leite et al., 1998) which isexposed to the south of the Palma area (Fig. 2).

Although most samples favor a subduction zoneenvironment, some samples of the lower Palma Groupshow signatures of intra-plate oceanic islands (Fig. 6a, b).These are ultramafic rocks of the Palma area andmetabasaltic rocks of the Bossoroca belt. The latter showchemical signatures transitional to OIB or E-MORB. Theyare, however, associated with the Cambaí Complex andthe upper Palma Group in the Vila Nova region (Fig. 2)which clearly show a calc-alkaline geochemistry andsignatures of an active continental margin. This suggeststhat the trace element concentration of the samplesprobably was also influenced by petrogenetic mechanismslike melting processes, metasomatism by fluids,fractionation and different amounts of contamination andor mixing. The shift to a stronger intra-plate character ofthese samples might be a secondary feature caused bythe subduction zone environment. It could be explainedby early dehydration of the slab allowing melting of less-metasomatized components in the mantle wedge resultingin the increase of the intraplate character (Harry andGreen, 1999). This can also be attributed to a tonalitesample of the Cambaí Complex as well as to otherplate-margin rocks displaying a transitional geochemistry(Figs. 6, 7a, 10). Nevertheless, an intra-plate origin forthese rocks cannot be excluded. Moreover, few samplesof ultramafic rocks in the Palma area show clear signaturesof OIB regarding trace element concentrations and a peakat Nb. They therefore probably represent relics of intra-plate rocks preserved in the western part of the São Gabrielblock. The existence of intra-plate ultrabasic rocks couldalso explain the komatiitic geochemistry of some rocks inthis area (e.g., Remus et al., 1993). The rocks mightrepresent either oceanic plateaux basalts (OIB) or mantle

Fig. 7. (a) Zr-Nb versus Y/Nb diagram with end member concentrationsof N-MORB, E-MORB and OIB (values taken from Sun andMcDonough, 1989) as well as compositions of samples of theCentral Volcanic Zone (CVZ) of the Andes (values taken fromDavidson et al., 1990 and Tormey et al., 1991). (b) Th/Yb versusNb/Yb plot to discriminate between subduction-related basaltsand oceanic basalts derived from MORB or OIB sources.Increasing Th/Yb ratios indicate increasing influence ofsubduction components and crustal contamination. Aleutenback-arc data taken from Kay (1980).

(a)

(b)

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plumes related to continental break-up leading to theopening of an oceanic basin. Subsequent closure of thisbasin would have generated the arc magmas of the SãoGabriel block. It is interesting to note that the intra-platesamples also display high TDM model ages of >900 Ma(Saalmann et al., 2005). However, it cannot be deducedfrom the data whether the samples formed in an intra-oceanic arc or continental break-up setting.

Trace element concentrations indicate island arc andcontinental arc sources for the metasedimentary rocks of

the lower and upper Palma Group. Low Th concentrationsrelative to Sc point to significant contribution from a maficsource. The REE patterns and spider diagrams are similarto the patterns of the igneous rocks displaying the samerelative enrichment of LILE and depletion in Nb and hence,the rocks have inherited the subduction zone geochemicalsignature of their source rocks. Therefore, the sedimentarybasin formed in the vicinity of a magmatic arc and thedetritus was shed from the arc rocks. This is corroboratedby the isotope geochemistry of the metasedimentary

Fig. 8. Rare Earth Element patterns of different lithologies and formations of the Palma Group and Cambaí Complex normalized to chondrite anddepleted mantle (DM). Normalizing values after Sun and McDonough (1989).

metasedimentary rocks

metasedimentary rocks

(a)

(b)

Campestre formation

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Fig. 9. Chondrite and N-MORB normalized spider diagrams of different igneous lithologies and formations of the Palma Group and CambaíComplex. Normalizing values after Sun and McDonough (1989).

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samples which like the metavolcanic samples show low87Sr/86Sr values (Table 3, Fig. 10) and positive εNd(t)(Saalmann et al., 2005) indicating that the main sourceof the metasedimentary rocks were Neoproterozoic

juvenile magmatic rocks formed shortly before whereascontribution from old continental crust was minor.

The late tectonic Santa Zélia granite formed duringcessation of D3 and thus, at the end of the mainductile orogenic deformations in the São Gabriel block.Its (87Sr/86Sr)t values are higher than the values of themetasedimentary rocks. The granite represents the firstrock in the São Gabriel rocks with slightly negative εNd(t)values (Saalmann et al., 2005). The data indicatecontribution to the melt from a source which ischaracterized by relatively high 87Sr/86Sr and evolved Ndisotopy. This source most probably is an old cratonic crustsuggesting juxtaposition and mobilization of oldcontinental crustal blocks at the end of large-scale dextraltranspressional event which characterizes D3.

Plate tectonic model

A subduction zone environment explains many featuresobserved in the São Gabriel block like the geochemistryof the rocks, the juxtaposition of metasedimentary andultra-basic metavolcanic rocks as well, as the preservationof intra-plate rocks which probably became scraped offthe subducting slab and were subsequently tectonicallyjuxtaposed with the arc rocks.

First subduction of oceanic crust in the São Gabriel blockassociated with formation of a juvenile magmatic isdocumented in the ca. 879 Ma Passinho metadiorite (Leiteet al., 1998) exposed in the southern parts of the São Gabriel

Fig. 10. εNd(t) versus (87Sr/86Sr)0 isotope correlation diagram withrelative positions of incompatible element depleted andenriched mantle reservoirs. All samples from the Palma group(metavolcanic and metasedimentary rocks) and CambaíComplex plot the upper left quadrant, only the Santa Zéliagranite plots the “enriched” quadrant showing a source withcontribution of continental crust.

Fig. 11. Sediment discrimination diagrams. Left: Ti/Zr versus La/Sc diagram (after Bhatia and Crook, 1986). Right: La/Th versus Hf (after Floydand Leveridge, 1987).

(87Sr/86Sr)t

εNd

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Fig. 12. (a) Proposed model for the geodynamic evolution of the São Gabriel block. Stage I east-dipping subduction and formation of the 0.9 GaPassinho intra-oceanic arc (Passinho event) followed by accretion to the eastern margin of the Rio de la Plata Craton; stage II west-dippingsubduction below an active continental margin consisting of the Rio de la Plata Craton and the previously accreted Passinho arc (São Gabrielevent); stage III collision with the Encantadas passive margin. (b) Schematic reconstruction of the Neoproterozoic plate tectonic evolutionbased on the model in a) as well as from literature data (modified and combined from Brito Neves et al., 1999 and Pimentel et al., 1999).

block. Some metavolcanic samples and metasedimentaryrocks of the lower Palma Group very likely were associatedto this intra-oceanic arc. The second major tectono-magmatic event is the intrusion of the 750-700 Ma CambaíComplex and its volcanic counterparts in the CampestreFormation (upper Palma Group) (São Gabriel event,Hartmann et al., 2000). They are exposed mainly in theVila Nova region further east (Fig. 2). Suturing of juvenilerocks with little amounts of reworked old continental crustis typical for accretionary orogens. Accretion took placeat the eastern margin of the Rio de la Plata Craton. Verylikely, more or less continuing progressive accretionoccurred between ca. 880 and 700 Ma. Field studies pointto SE-directed thrusting during D2, dextral transpressionassociated with SE-vergent thrust-stacking during D3 andbrittle overprint during D4. The resulting overall geometryof a SE-vergent thrust stack with juxtaposed metasedimentaryand metavolcanic as well as plutonic rocks is suggestedto correspond, at least in parts, to an accretionary wedgeabove a west-dipping subduction zone.

The Encantadas gneisses forming the basement of thePorongos and Dom Feliciano belts to the east of the SãoGabriel block are suggested to represent anothercontinental block or microcontinent which may have beeneither part of the Kalahari or Congo Craton or representsa continental fragment of the Rio de la Plata Craton whichwas separated during the opening of the oceanic basin.

The Brasiliano tectonic evolution starts with creationof an intra-oceanic arc (Passinho arc, ca. 880 Ma)represented by the Passinho diorite by subduction ofoceanic crust and following accretion of this arc to theeastern border of the Rio de la Plata Craton (Fig. 12,stage I). The intra-oceanic island arc formed due toeastward subduction and the eastern border of the cratonwas a passive margin until it arrived in the subductionzone. Detritus of this island arc was shed into thedepositional basin of sedimentary rocks. Subsequentsubduction (between ca. 850–700 Ma) occurred to thewest beneath the newly formed active continental marginconsisting of the Rio de La Plata Craton and the previously

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attached Passinho island arc (Vila Nova arc) (Fig. 12, stageII). The derivation of parts of the metasedimentary rocksof the lower Palma Group from 0.75–1.0 Ga juvenile sourcerocks and their Sm-Nd isotopic signature was due todeposition in the fore-arc region with only minor amountsof sedimentary input shed from the old craton but insteadpredominant input from the previously accreted Passinhoarc. The volcanics of the Campestre Formation (upperPalma Group) represent the volcanic parts of the Cambaímagnatic arc and the volcanoclastic and metasedimentaryrocks possibly were deposited in the back-arc region withmajor input from exposed parts of the magnatic arc. Thefinal stage of the São Gabriel event at ca. 700 Ma couldrepresent the collision of the Rio de la Plata Craton withthe Porongos/Encantadas passive margin (Fig. 12, stageIII). This remains speculative, however, because of thelack of age determinations in the Porongos belt. Collisioncould also have occurred during the Dom Feliciano event(630–600 Ma). Anyway, oceanic crust was clearly formedbetween the Rio de la Plata Craton and the Encantadasblock and thus, the São Gabriel block represents thecollision and suture zone.

Regional implications

Three main gravimetric and aeromagnetic anomaliesof deep sources and large lateral extent have beenidentified in Rio Grande do Sul (Costa, 1998): the SãoGabriel Suture separates the high-grade gneisses andgranulites in the west from the juvenile lower grademetamorphic rocks of the São Gabriel block (Fig. 1c). TheCaçapava Suture is a magnetic anomaly and is interpretedas marking the closure of a Brasiliano oceanic basin (Costa,1998). This would correspond to the basin containing theNeoproterozoic juvenile metavolcanic and metasedimentaryrocks presented in this paper. The gravimetric andmagnetometric Porto Alegre Suture is thought to markthe boundary between the Rio de la Plata and KalahariCratons; however, the pre-Brasiliano basement on bothsides of this anomaly consists of Trans-AmazonianEncantadas gneisses (Da Silva et al., 2000; Hartmann etal., 2000) and thus does not separate two exotic terranes.We suggest that the suture between the Rio de la PlataCraton and the Kalahari Craton is represented by a broadcollision zone comprising the relics of a number ofBrasiliano magmatic arcs. The Caçapava Suture wouldseparate two different cratonic blocks (Rio de La Plataand Encantadas block).

In Brazil, juvenile Neoproterozoic oceanic crust can alsobe found in Goiás magmatic arc located in the westernBrasília belt (Pimentel and Fuck, 1992; Junges et al.,2002). The Brasília belt resulted from the collision of thenorthern part of the Rio de la Plata Craton with thewestern São Francisco Craton. Both areas contained

magmatic arcs formed due to subduction of oceanic crustand subsequent collision. Like in the São Gabriel block,the first event is related to the docking of a magmatic arc(Mara Rosa Arc, ca. 850 Ma), and the second eventcomprises development of wide juvenile arc magmatismbetween ca. 780 and 700 Ma (e.g., Campos Neto, 2000;Junges et al., 2002). The final ocean closure occurred atca. 630–600 Ma in the Southern Brasilia belt (Brito Neveset al., 1999; Pimentel et al., 1999), and this event mightbe correlated with the Dom Feliciano event in southernBrazil.

Possibly the two relics of oceanic basins might havebeen linked and represent one major oceanic realm to theE and NE of the Rio de La Plata Craton, although aconnection between the two areas remains speculative asthe juvenile rocks cannot be traced directly in the fieldbecause of Phanerozoic cover rocks. Anyway, theyrepresent the collision zones due to subduction of oceaniccrust between the Rio de la Plata Craton the Encantadasblock and São Francisco Craton, respectively.

Acknowledgments

Field work was partially supported by travel grants ofthe Herrmann-Willkomm-Stiftung, Frankfurt a.M. Visitand work in the geochronology lab at UFRGS, PortoAlegre, was made possible by financial support from theGerman Academic Exchange Council (DeutscherAkademischer Austauschdienst, DAAD). Thanks to CNPq(Proc. 477875/03-3) for partial support of field work. Wethank F. Bitencourt for supplying geological maps of thearea. Offer of a car for field work by the CPGq-Geoscienceinstitute of UFRGS is highly appreciated. All the help bymany colleagues is gratefully acknowledged. We appreciateconstructive reviews of S.P. Neves, P.C. Hackspacher andan anonymous referee which improved the manuscript.

References

Alkmin, F.F., Marshak, S. and Fonseca, M.A. ( 2001) AssemblingWest Gondwana in the Neoproterozoic: clues from the SãoFrancisco Craton region, Brazil. Geology, v. 29, pp. 319-322.

Babinski, M., Chemale, F. Jr., Hartmann, L.A., Van Schmus, W.R.and Da Silva, L.C. (1996) Juvenile accretion at 750-700 Main southern Brazil. Geology, v. 24, pp. 439-442.

Babinski, M., Chemale, F. Jr., Van der Schmus, W.R.,Hartmann, L.A. and Da Silva, L.C. (1997) U-Pb and Sm-Ndgeochronology of the Neoproterozoic granitic-gneissic DomFeliciano Belt, southern Brazil. J. South Amer. Earth Sci., v.3-4, pp. 263-274.

Bathia, M.R. and Crook, K.A.W. (1986) Trace elementcharacteristics of graywackes and tectonic settingdiscrimination of sedimentary basins. Contrib. Mineral.Petrol., v. 92, pp. 181-193.

Brito Neves, B.B., Campos Neto, M.C. and Fuck, R.A. (1999)

160

Gondwana Research, V. 8, No. 2, 2005

K. SAALMANN ET AL.

From Rodinia to Western Gondwana: an approach to theBrasiliano-Pan African cycle and orogenic collage. Episodes,v. 22, pp. 155-166.

Campos Neto, M.C. (2000) Orogenic Systems from SouthwesternGondwana. An approach to Brasiliano-Pan African Cycle andorogenic collage in southeastern Brazil. In: Cordani, U.G.,Milan, E.J., Thomaz Filho, A. and Campos, D.A. (Eds.),Tectonic evolution of South America, Rio de Janeiro,pp. 335-365.

Chemale, F. Jr. (2000) Evolução Geológica do Escudo Sul-Rio-Grandense. In: Holz, M. and De Ros, L.F. (Eds.), Geologia doRio Grande do Sul, Porto Alegre, pp. 13-52.

Chemale, F. Jr., Hartmann, L.A. and Da Silva, L.C. (1995) Stratigraphyand tectonism of the Brasiliano Cycle in southern Brazil.Communs Geol. Surv. Namibia, v. 10, pp. 151-166.

Costa, A.F.U. (1998) geophysical modeling and testing of thestructure of the lithotectonic associations in theSul-Riograndense Shield, southern Brazil. Pesquisas, v. 25,pp. 75-76.

Da Silva, C., Hartmann, L.A., McNaughton, N.J. and Fletcher,I.R. (1999) SHRIMP U/Pb zircon timing of Neoproterozoicgranitic magmatism and deformation in the Pelotas Batholithin southernmost Brazil. Int. Geol. Rev., v. 41, pp. 531-551.

Da Silva, C., Hartmann, L.A., McNaughton, N.J. and Fletcher,I.R. (2000) Zircon U/Pb SHRIMP dating of a Neoproterozoicoverprint in Paleoproterozoic granitic-gneissic terranes,southern Brazil. Amer. Min., v. 85, pp. 649-668.

Davidson, J.P., McMillan, N.J., Moorbath, S., Wörner, G.,Harmon, R.S. and Lopez-Escobar, L. (1990) The Nevadosde Payachata volcanic region (18°S/69°W, N. Chile) II.Evidence for widespread crustal involvement in Andeanmagmatism. Contrib. Mineral. Petrol., v. 105, pp. 412-432.

Fernandes, L.A.D., Tommasi, A. and Porcher, C.C. (1992)Deformation patterns in the southern Brazilian branch ofthe Dom Feliciano Belt: A reappraisal. J. South Amer. EarthSci., v. 5, pp. 77-96.

Floyd, P.A. and Leveridge, B.E. (1987) Tectonic environment ofthe Devonian Gramscatho basin, south Cornwall: frameworkmode and geochemical evidence from turbidite sandstones.J. Geol. Soc. London, v. 144, pp. 531-542.

Floyd, P.A. and Winchester, J.A. (1975) Magma-type and tectonicsetting discrimination using immobile elements. Earth Planet.Sci. Lett., v. 27, pp. 211-218.

Harry D.L. and Green N.L. (1999) Slab dehydration and basaltpetrogenesis in subduction systems involving very youngoceanic lithosphere. Chemical Geol., v. 160, pp. 309–333.

Hartmann, L.A. (1998) Deepest exposed crust of Brazil -geochemistry of Paleoproterozoic depleted Santa Maria Chicogranulites. Gondwana Res., v. 1, pp. 331-341.

Hartmann, L.A. and Nardi (1983) Contribução à geologia daregião oeste do Escudo Sul-Riograndense. In: Simpósio Sul-Brasileiro de Geologia, Porto Alegre, SBG, pp. 9-18.

Hartmann, L.A., Nardi, L.V.S., Formoso, L.L., Remus, M.V.D., DeLima, E.F. and Mexias, A.S. (1999) Magmatism andmetallogeny in the crustal evolution of Rio Grande do SulShield, Brazil. Pesquisas, v. 26, pp. 45-63.

Hartmann, L.A., Leite, J.A.D., Da Silva, L.C., Remus, M.V.D.,McNaughton, N.J., Groves, D.I., Fletcher, I.R., Santos, J.O.S.and Vasconcellos, M.A.Z. (2000) Advances in SHRIMPgeochronology and their impact on understanding thetectonic and metallogenic evolution. Australian J. Earth Sci.,

v. 47, pp. 829-844.Jost, H. and Hartmann, L.A. (1984) Provincia Mantiqueira–

Sector Meridional. In: Almeida, F.F.M. and Hasui, Y. (Eds.),O Pre-Cambriano do Brasil, Rio de Janeiro, 378p.

Junges, S.L., Márcio Martins Pimentel, M.M. and de Moraes, R.(2002) Nd isotopic study of the Neoproterozoic Mara RosaArc, Central Brazil: implications for the evolution of theBrasília Belt. Precambrian Res., v. 117, pp. 101-118.

Kay, R.W. (1980) Volcanic arc magmas: implications of a melting-mixing model for element recycling in the upper mantlesystem. J. Geol., v. 88, pp. 497-522.

Koppe, J.C. and Hartmann, L.A. (1988) Geochemistry of theBossoroca greenstone belt, southernmost Brazil. Geochim.Brasil., v. 2, pp. 167-174.

Leite, J.A.D., Hartmann, L.A., McNaughton, N.J. and ChemaleF.Jr., (1998) SHRIMP U/Pb zircon geochronology ofNeoproterozoic juvenile and crustal-reworked terranes insouthernmost Brazil. Int. Geol. Rev., v. 40, pp. 688-705.

Machado, N., Koppe, J.C. and Hartmann, L.A. (1990) A LateProterozoic U-Pb age for the Bossoroca Belt, Rio Grande doSul, Brazil. J. South Amer. Earth Sci., v. 3, pp. 87-90.

Pearce, J.A. (1986) A ‘‘users guide’’ to basalt discriminationdiagrams. In: Tarney, J. (Ed.), Oceanic basalts. Blackie,Glasgow.

Pearce, J.A. and Cann, J.R. (1973) Tectonic setting of basicvolcanic rocks determined using trace element analyses.Earth Planet. Sci. Lett., v. 19, pp. 290-300.

Pearce, J.A. and Gale, G.H. (1977) Identification of ore-deposition environment from trace element geochemistryof associated igneous host rocks. Geol. Soc. Spec. Pub, v. 7,pp. 14-24.

Pearce, J.A. and Norry, M.J. (1979) Petrogenetic implicationsof Ti, Zr, Y and Nb variations in volcanic rocks. Contrib.Mineral. Petrol., v. 69, pp. 33-47.

Pimentel, M.M. and Fuck, R.A. (1992): Neoproterozoic crustalaccretion in Central Brazil. Geology, v. 20, pp. 375-379.

Pimentel, M.M., Fuck, R.A. and Botelho, N.F. (1999) Granitesand the geodynamic history of the Neoproterozoic Brasíliabelt, Central Brazil: a review. Lithos, v. 46, pp. 463-483.

Remus, M.V.D., Hartmann, L.A. and Formoso, M.L.L. (1993) Ospadrões de Elementos Terras Raras (ETR) e a afinidadegeoquímica komatiítica dos xistos magnesianos e rochasassociadas do Complexo Cambaizínho, São Gabriel-RS. Rev.Brasil. de Geosci., v. 23, pp. 370-387.

Remus, M.V.D., Macambira, M.B., Hartmann, L.A. and Beilfuss, M.(2001) Crustal evolution of granitoids and gneisses fromthe Cambaizinho Belt, Southern Brazil: review, new zirconPb-Pb evaporation ages and Pb-Nd-Sr isotopes. III SouthAmer. Symp. on Isotope Geol., Pucon, Chile. CD-ROM.

Remus, M.V.D, LHartmann. L.A., McNaughton, N. J., Groves, D.I.and Fletcher, I.R. (2000) The link between hydrothermalepigenetic copper mineralization and the Caçapava graniteof the Brasiliano cycle in southern Brazil. J. South Amer.Earth Sci. 13, pp. 191-216.

Remus, M.V.D., McNaughton, N.J., Hartmann, L.A., Koppe, J.C.,Fletcher, I.R., Groves, D.I. and Pinto, V.M. (1999) Gold inthe Neoproterozoic juvenile Bossoroca volcanic arc ofsouthernmost Brazil: isotopic constraints on timing andsources. J. South Amer. Earth Sci., v. 12, pp. 349-366.

Saalmann, K. (2004) Deformation of the Neoproterozoic juvenileCambaí magmatic arc complex and syntectonic granites

161

Gondwana Research, V. 8, No. 2, 2005

NEOPROTEROZOIC MAGMATIC ARCS, BRAZIL

during the Brasiliano orogenic cycle in southern-most Brazil.Z. dt. Geol. Ges., v. 154, pp. 557-578.

Saalmann, K., Hartmann, L.A., Remus, M.V.D., Koester, E. andConçeição, R. V. (2005) Sm-Nd isotope geochemistry ofmetamorphic volcano-sedimentary successions in the SãoGabriel block, southernmost Brazil: evidence for theexistence of juvenile Neoproterozoic oceanic crust to theeast of the Rio de la Plata Craton. Precambrian Res. v. 136,pp. 159-175.

Soliani Jr., E. (1986) Os dados geocronológicos do Escudo Sul-rio-grandense e suas implicações de ordem geotectônica.Master thesis Instituto de Geosciências, Universidade SãoPaulo, 425p.

Strieder, A.J., Roldao, D.C. and Hartmann, L.A. (2000) ThePalma Volcano-Sedimentary Supersuite, PrecambrianSul-Riograndense Shield, Brazil. Intern. Geol. Rev., v. 42,pp. 984-999.

Sun, S.S. and McDonough, W.F. (1989) Chemical and isotopicsystematics of oceanic basalts: implications for mantlecomposition and processes. In: Saunders, A.D. and Norry,M.J. (Eds.), Magmatism in ocean basins. Geol. Soc. London,Spec. Pub., v. 42, pp. 313-345.

Tassinari, C.C.G., Munhá,J.M.U., Ribeiro, A. and Correia, C.T.(2002) Neoproterozoic oceans in the Ribeira Belt(southeastern Brazil): The Pirapora do Bom Jesus ophioliticcomplex. Episodes, v. 24, pp. 245-251.

Taylor, S.R. and McLennan, S.M. (1985) The continental crust:its composition and evolution. Blackwell, Oxford, 312p.

Tormey, D.R., Hickey-Vargas, R., Frey, F.A., Lopez-Escobar, L.(1991) Recent lavas from the Andean volcanic front (33 to42°S); interpretations of along-arc compositional variations.Geol. Soc. Amer., Spec. Paper, v. 265, pp. 57-77.

Wilson, M. (1989) Igneous petrogenesis. Unwyn Hyman,London, 466p.