evolution of heterogeneous mantle in the acampamento velho...

GondwanaResearch

Gondwana Research, V. 8, No. 4, pp. 479-492.© 2005 International Association for Gondwana Research, Japan.ISSN: 1342-937X

Evolution of Heterogeneous Mantle in the Acampamento Velhoand Rodeio Velho Volcanic Events, Camaquã Basin,Southern Brazil

Delia del Pilar M. de Almeida1,*, Rommulo V. Conceição2, Farid Chemale Jr.2,Edinei Koester2, André W. de Borba2 and Karla Petry3

1 PPGeo, Universidade do Vale do Rio dos Sinos (UNISINOS), Av. Unisinos, 950 Cristo Rei, São Leopoldo, CEP 93022-000, RS,Brazil

2 Laboratório de Geologia Isotópica, UFRGS, Caixa Postal 15001, CEP 91509-900, Porto Alegre, RS, Brazil3 Graduate student in Geology – CNPq/UNISINOS, Brazil* Corresponding author: E-mail: [email protected]

(Manuscript received March 1, 2005; accepted June 15, 2005)

Abstract

The Camaquã Basin, developed during the last phases of the Brazilian/Pan-African Orogeny and was filled with athick volcano-sedimentary succesion, in which two volcanic events of alkaline affinity are represented by the AcampamentoVelho Alloformation and the Rodeio Velho Member. The Acampamento Velho Alloformation records a bimodal eventwith a lower association of mafic flows and an upper association of felsic pyroclastic rocks and flows. It was formedduring extension, after the subduction of the Adamastor oceanic plate beneath the Rio de La Plata continental plate atthe end of the Neoproterozoic III. The second event, the Rodeio Velho Member, represented by mafic flows, intrusionsand piroclasts, took place during overall extensional tectonism, probably in the middle Ordovician. Rb, Sr, Sm, and Ndisotopic measurements were carried out on samples from both units. Regardless the event they represent, all the samplesdisplay negative values for epsilon Nd, ranging from �2.97 to �10.31 for the Acampamento Velho Alloformation andfrom �8.39 to �13.92 for the Rodeio Velho Member. The initial 87Sr/86Sr ratios vary from 0.706 to 0.707 and from0.704 to 0.707 for the Acampamento Velho Alloformation and Rodeio Velho Member, respectively. Mafic flow deposits inboth units show a preferential enrichment in Ba relative to Th. Flow samples from the Rodeio Velho Member also displaya distinctive enrichment in the Ba/Th ratio, without a change in the initial Sr, compared to the mafic flow deposits fromthe Acampamento Velho Alloformation, which show a slight enrichment in those ratios. As for the Acampamento VelhoAlloformation, the mafic lavas could be a mixture of depleted mantle-derived basalts plus 20% to 30% of crustalcontamination by sediment (probably Neoproterozoic arkosic quartzites). The formation of a magmatic chamber and theseparation of the magma into two fractions gave rise initially to the mafic rocks at the base of the Acampamento VelhoAlloformation The other magma fraction underwent a significant enrichment in crustal component before the felsicrocks of this Alloformation were formed. The flows from the Rodeio Velho Member originated in a distinct magmachamber, with EM I characteristics that was much more enriched in incompatible elements and depleted in radiogenic Sr.

Key words: Volcanic rocks, Brasiliano/Pan-African orogeny, isotopic signature, crustal contamination, Camaquã Basin.

Introduction

The Neoproterozoic/Early Paleozoic transition insouthern Brazil was marked by the last phases of theBrasiliano/Pan-African Cycle (900�540 Ma, Chemale Jr.,2000). These include the development of a series of fault-bounded troughs which occupied one preferentialsubsiding locus called the Camaquã Basin (Paim et al.,2000). The basin was filled with a thick (ca. 11 km)package of sedimentary rocks deposited in marine (at thebase) and desertic (at the top) environments.

The volcanic events of the Camaquã Basin arerepresented by: (a) the Neoproterozoic rhyolitic, calc-alkaline lava flows and pyroclastic rocks interlayeredwith the Maricá Formation (Santos et al., 1978); (b) theNeoproterozoic (ca. 580 Ma) Hilário Formation withbasaltic-andesitic composition and shoshonitic affinity(Lima and Nardi, 1998); (c) the Cambrian AcampamentoVelho Formation (Cordani et al., 1974), with intermediate/basaltic lavas at the base (lower mafic succesion) andrhyolitic pyroclastic rocks and lava flows (upper felsic

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succession) at the top (Almeida et al., 2002a), both ofalkaline affinity; and (d) the Ordovician Rodeio VelhoMember, which encompasses highly vesicular, basaltic andandesitic lava flows of alkaline affinity (Almeida et al.,2000).

The isotopic signature of the volcanic successions canprovide important information on the processes of magmageneration. An integrated analysis of major and traceelements (including REE), coupled with Sr and Ndisotopes of the Acampamento Velho Alloformation (AVAf)and the Rodeio Velho Member (RVM), was carried out inorder to discuss the magma generation and an eventualcontamination by crustal components. Such data areimportant to broaden the understanding of the tectonicsetting in which these magmatic events took place, as wellas their significance in the evolution of the Camaquã Basinand the final stages of the Brasiliano/Pan-African Cyclein southern Brazil.

Stratigraphy and Evolution of the CamaquãBasin

The volcanic and sedimentary deposits of the CamaquãBasin are located in south-central Rio Grande do Sul State,southern Brazil. They are part of the Precambrian Sul-rio-grandense Shield (Fig. 1), a major geotectonic blockthat represents the southern portion of the MantiqueiraProvince (Hasui et al., 1975; Almeida et al., 1981) andthat records the events of the Brasiliano/Pan-African Cycle(Porada, 1979). This block was active in this regionbetween 900 and 540 Ma (Chemale Jr., 2000). ChemaleJr. (2000) recognized two phases of eastward subductionduring the evolution of the Brasiliano/Pan-African Cyclein southern Brazil: an island-arc (900�800 Ma) and acontinental-arc (800�700 Ma) phase. Between 650 and540 Ma, westward subduction took place, followed bythe collision between the Rio de la Plata and the Kalaharicontinents (respectively in the west and in the east),and the consequent assembly of southwestern Gondwana(Fig. 2).

The Camaquã Basin has been related to transcurrentmotion along NE-SW-trending structures by Wernicket al. (1978), Oliveira and Fernandes (1992), and Beckel(1992). It has also been interpreted as an anorogenic riftsystem, with no evidence of transcurrent effects duringits inception, and thus post-dating the orogenic events(e.g., Fragoso-César et al., 2003), or as a foreland retroarcbasin related to the latest orogenic events of the Brasiliano/Pan-African Cycle (Gresse et al., 1996; Hartmann et al., 1998;Chemale Jr., 2000; Paim et al., 2000; Basei et al., 2000)The latter interpretation is the most widely accepted, andhence adopted in the present work.

In terms of stratigraphy, five major unconformity-bounded units (Allogroups) have been defined by Paimet al. (2000): the Maricá, Bom Jardim, Cerro do Bugio,Santa Bárbara and Guaritas Allogroups (Fig. 1 andTable 1). They reflect the progressive continentalizationof the Camaquã Basin. These units can be interpreted asthe evolution of temporally and spatially restricted basins,evolved from a foreland retro-arc into a rift (or hemi-graben) tectonic setting (e.g., Paim et al., 2000).

The Maricá Allogroup (or Maricá Formation, as definedby Leinz et al., 1941) is a 2,500 m-thick sedimentarypackage with coastal deposits, including fluvial to shallowmarine sediments with scarce volcanic contribution(Leites et al., 1990). The Bom Jardim Allogroup(ca. 4,000 m-thick) is characterized by large volumes ofandesitic rocks with shoshonitic affinity (the HilárioFormation) (Lima and Nardi, 1998; Wildner et al., 2002),coeval with the intrusion of the Lavras do Sul IntrusiveComplex (592±5 Ma, according to Remus et al., 1997).The sedimentary succession (the Arroio dos NobresFormation) comprises brown-colored, alluvial sandstones,siltstones and conglomerates (Oliveira and Fernandes,1992). The Cerro do Bugio Allogroup (>1,000 m-thick)is composed of volcanic rocks at the base (AcampamentoVelho Alloformation) and alluvial conglomerate depositsof the Santa Fé Alloformation at the top (Paim et al.,2000). The Santa Bárbara Allogroup (or Santa BárbaraFormation, as defined by Robertson, 1966) comprises red-colored conglomerates, sandstones and siltstonesdeposited under semi-arid conditions in alluvial fan,braided fluvial, fan-deltaic and lacustrine environments.These deposits are organized in three depositionalsequences throughout the 2,500 m-thick sedimentary pile(Borba and Mizusaki, 2003). The Guaritas Allogroup (theGuaritas Formation of Goñi et al., 1962) is lithologicallysimilar to the Santa Bárbara Allogroup. The former,however, bears widespread eolian deposits and thebasaltic/andesitic Rodeio Velho Member at the base ofthe 1,300 m-thick package.

This work aims at the characterization of theAcampamento Velho Alloformation and the Rodeio VelhoMember, which are part of the Bom Jardim and theGuaritas Allogroups, respectively. These units are describedin detail below.

Petrography, Geochemistry and AgeConstraints of the Acampamento VelhoAlloformation and Rodeio Velho Member

Acampamento Velho Alloformation (AVAf)

The earliest references to acid volcanic rocks insouthern Brazil are in Leinz et al. (1941), who described

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�tuffs, vitrophyres, feldsites and quartz-porphyries�unconformably overlying the Maricá Formation. This unitwas named the Ramada Rhyolite by Robertson (1966)and the Acampamento Velho Formation by Cordani et al.(1974). It has been traditionally considered as exclusivelyacid in composition, but detailed geological mapping inthe Cerro do Bugio area (west of Caçapava do Sul town)

revealed the existence of a basalt/andesite unit at the baseof the 1,000 m package (Zerfass and Almeida, 1997;Almeida et al., 1998a, 2002a; Zerfass et al., 2000, 2001).This occurrence led Almeida et al. (1998a, 2002a),Zerfass and Almeida (1997) and Zerfass et al. (2001) topropose that the AVAf actually represents a bimodalvolcanic unit, with a lower mafic and an upper felsic

Fig. 1. Simplified geologic map of the Camaquã Basin (after Paim et al., 2000), showing the sampled areas.

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succession. The lower mafic succession consists ofspillitized, gray-colored, porphyritic andesite-basalts andandesites, with a total maximum thickness of ca. 350 m.These rocks contain phenocrysts of altered plagioclase(Ab100) and clinopyroxene replaced by white micas,carbonates, chlorite and oxides. The pylotaxitic matrix iscomposed of plagioclase and clinopyroxene microcrysts,and replaced by chlorite and carbonates. Poorly-sorted,basaltic/intermediate volcaniclastic breccias (with fragments>5 cm in diameter) are interlayered with the lava flows.Geochemical analyses showed moderate sodium, alkalinecharacter (high La/Yb and Eu/Sm ratios) and the absenceof a significant Eu negative anomaly. These rocks bear

negative anomalies of P and Ti, and enrichment in Ce, Thand Zr contents (Almeida et al., 1998a, 2002a).

The upper felsic succession encompasses rhyoliticrocks, with lapilli-tuffs, tuffs and welded tuffs at the base,and pink-colored, banded lava flows at the top of thevolcanic package. The lapilli-tuffs contain microcrystallinebasaltic-intermediate-acidic lithoclasts, cuspate and platyvitroclasts, and crystaloclasts of quartz, sanidine andplagioclase in a silicified ash matrix. Tuffs and weldedtuffs present crystaloclasts of quartz, microperthiticsanidine, anorthoclase, albitized plagioclase andphylossilicates in a matrix composed of shards and pumice,with eutaxitic texture, concoidal and perlitic fractures and

Table 1. Some stratigraphic concepts for the Camaquã Basin.

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fiamme. Connate volcanic lithoclasts with spheruliticmatrix are also present. Rhyolite lava flows show abruptcontact with the tuffaceous rocks. They are pink-coloredand homogeneous, with porphyritic texture (quartz andmicroperthitic sanidine phenocrysts in a quartz-feldsparmatrix). The rocks in the upper felsic succession are highlysiliceous (average SiO2 73.09% for tuffs, 77.2% for rhyoliteflows), with variable negative Eu anomalies and strongalkaline affinity. However, the REE patterns in the upper

Fig. 2. Evolution model for the Sul-Riograndense Shield during the end of Neoproterozoic.

felsic and in the lower mafic successions are similar, witha clear parallelism between the spectra of each group andwithin groups, except for the strong negative Eu anomalyseen in the felsic rocks (Fig. 3a, b). The behavior of REEand of some trace elements suggests a common magmaticsource. The Nb/Zr and Y/Zr ratios show differentevolutionary trends for the mafic and felsic successions(Fig 4a, b), reinforcing the bimodal character of thismagmatism. Trace element data and the spidergram show

(a) Phase III (650–595 Ma)

(b) Phase IV (595–540 Ma)

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that the rocks are co-magmatic. The ryholites show abehavior similar to S-type granites, formed by crustalmelting or basaltic melt with intense crustal assimilation(Almeida et al., 1998a, 2002a).

Regarding the geodynamic setting, the Zr/Nb ratiosaround or greater than 10 for the felsic rocks (9.7 for

tuffs, 13.8 for welded tuffs and 13.4 for lava flows), alongwith the behavior of some trace elements, suggest anextensional setting for both groups of rocks (lower maficand upper felsic succession). Based on thesecharacteristics, Almeida and others (2002a) suggested thatthe Acampamento Velho volcanic event represents the final

Fig. 3. Chondrite-normalized REE diagram of Taylor and MacLennan (1985) (in Almeida et al., 2002a) for the AVAf. (a) Lower Mafic Association;(b) Upper Felsic Association, showing a similar pattern for both associations, except for the strong negative Eu anomaly in the Upper FelsicAssociation rocks.

Fig. 4. Pearce and Norry (1979) diagrams, showing two distinct trends of evolution for Lower Mafic Association and Upper Felsic Association forthe AVAf (in Almeida et al., 2002a). (a) Zr versus Nb and (b) Zr versus Y. Symbols same as in figure 3.

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phases of subduction of the Adamastor oceanic crustbeneath the Rio de la Plata Craton, right before thecollision between the Kalahari and the Rio de la Plataplates, with extension taking place in a retro-arc setting.

The first geochronological investigation on the age ofthe AVAf rhyolites was carried out by Cordani et al. (1974),followed by Sartori (1978) and Teixeira (1982).Those results were compiled by Soliani Jr. (1986), anda 529±4 Ma Rb-Sr whole-rock reference isochron(R0= 0.7057) was presented. Another Rb-Sr dating wascarried out by Almeida et al. (1996, 2002a), in the rhyoliticflows of the Cerro do Bugio region and the dykes intrudingthe Maricá Formation. Two whole-rock isochronswere obtained: 545.1±12.7 Ma (R0= 0.70932) and546±12.9 Ma (R0 = 0.71454). Chemale Jr. (2000)obtained a U/Pb zircon age of 573±18 Ma, εNd (t=570 Ma)of �9.34 and �9.37, and TDM model ages of 1.7 to 1.9 Ga.Sommer et al. (2003) used SHRIMP U/Pb dating in elevenzircon crystals from rhyolites of the Vila Nova do Sulregion, which yielded a 549.3±5 Ma age. Therefore, allthe ages obtained for this Alloformation so far belong inthe late Neoproterozoic III.

Sm-Nd data from acid rocks from the Vila NovaBelt show values of εNd ranging from �12.46 to �12.57(for t = 570 Ma) and TDM from 1.87 to 2.32 Ga.They suggest crustal contribution in the generation of theacid volcanism of AVAf (Chemale Jr., 2000). Almeida et al.(1998a, 2002a) also pointed out the significant crustalcontribution in this volcanic event. Flow and tuff rocks fromthe AVAf yielded εNd values between �8 and �20. Modelages between 1.5 and 1.7 Ga indicate a paleoproterozoicage of the crust (Almeida et al., 2002b, 2003b).

Rodeio Velho Member (RVM)

The last volcanic manifestations in the Camaquã Basinare contemporaneous to and interlayered with thealluvial and eolian deposits of the Guaritas Allogroup.They were formed during the development of atranstensive, NE-SW-oriented rift system (the Guaritashemi-graben). These rocks are referred to as the RodeioVelho Member (RVM) (Ribeiro et al., 1966). They consistof at least three successive, highly vesiculated, andesitelava flows, approximately 100 m-thick. Silva Filho (1996)and Fragoso-César et al. (1999) emphasized the intrusivecharacter of this group of rocks, and suggested that theyare in fact an intrusive suite, and thus post-date theGuaritas deposits. Field observations, however, indicatea contemporaneity between the volcanism and thedeposition of alluvial facies. The development ofpepperites, pillow-like structures and other featuressuggest interaction between hot lava and wet, poorlyconsolidated sediments (e.g., Lima et al., 2002). Accordingto Almeida et al. (2003b), the RVM is contemporaneous

to or slightly younger than the Pedra Pintada Alloformation,and slightly older than the Varzinha Alloformation of theGuaritas Allogroup. It could also be contemporaneous tothis unit, especially in the east part of the Camaquã Basin.Sediment-lava interaction features within the GuaritasAllogroup suggest the possible contemporaneity of events.

According to Almeida et al. (1998b, 2000), the RVM ischaracterized by andesitic lava flows with aa and pahoehoestructures, pyroclastic deposits and intrusions.Petrographically, these rocks are andesites, subalkalinebasalts and trachyandesites, with glomeroporphyriticplagioclase phenocrysts (as well as relics of pyroxene andolivine), opaque and accessory minerals (apatite, zircon)in a pylotaxitic, vitrophyric or ophitic matrix withplagioclase, pyroxene and olivine. Intersertal glass isrecrystallized (forming spherulites) or altered. Pyroclasticrocks are stratified and vary from ash-flow tuffs tolapillites, brecciated and poorly sorted. Shards and fiammeattest to the pyroclastic character. Plagioclase andeuhedral quartz crystalloclasts (<2%) are dispersed inthe tuffaceous, partially glassy matrix.

Geochemical analyses of the RVM carried out in samplesfrom three volcanic cones (lava flows) and other regionsof the basin (Fig. 1) display a SiO2 content between 46.84and 65.53% (average 53.01%) (Almeida et al., 2000). Traceelement and REE signatures (Fig. 5a) suggest an alkalinecharacter related to the intraplate continental basalts fieldin the Wood (1980) diagram (Almeida et al., 2000). This isconfirmed by the N-MORB normalized spidergram (Sun andMcDonough, 1989, Fig. 5b) and the Zr/Y versus Zr diagram(Pearce and Norry, 1979). These rocks probably wereformed by fractionated crystallization, as indicated by thegood correlation between pairs of incompatible elements,such as Ce versus Sm and La, and Zr versus Nb and Y.

The formation of the RVM is probably related to thelast magmatic manifestations of the Brasiliano/Pan-African Cycle, following the final collision between theRio de la Plata and Kalahari continental masses (Almeidaet al., 2003a). However, other authors (e.g., Fambrini, 1998;Fragoso-César et al., 1999, 2000; Lopes et al., 1999;Paim et al., 2000) interpreted this event as the beginningof a new cycle. It would represent the initial rifting stageof a completely consolidated crust (the Gondwanasupercontinent or South American Platform), precursorto the formation of the Paraná Basin in the Paleozoic-Mesozoic.

Regarding the age of the volcanic event, Hartmannet al. (1998) referred to a poorly constrained U/PbSHRIMP zircon (one single grain) that yielded an age of470±19 Ma (Meso-Ordovician). On the other hand,Chemale Jr. (2000) obtained Sm/Nd model ages (TDM)of 1.6 to 1.9 Ga, suggesting a modified-mantle origin forthese rocks. The RVM event was probably the source of

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Table 2. Chemical analyses of some major and trace elements and isotopic results for Nd, Sr, and Sm. Major elements in %; trace elements in ppm. * - flows; **- shallow intrusives; #- tuffs;##- welded tuffs. The geochemical analyses were performed at the Activation Laboratories (ACTLAB), Canada, with the Argonium Plasma Spectrometry (ICP) technique.The isotopic analyses were carried out at the Laboratory for Isotopic Geology (LGI) at the Institute of Geosciences at the Federal University of Rio Grande do Sul (UFRGS).

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heat and hydrothermal solutions for Cu (Ag, Au), Pb andZn (Cu, Ag) mineralizations in the Camaquã Mines (Lima,1998; Lima et al., 2001). Hydrothermal illites collectedin those mines also yielded K-Ar ages around 465 Ma(Bonhomme and Ribeiro, 1983).

Rb, Sr, Sm and Nd Isotopic data

Analytical procedures

Samples for radiogenic isotope analyses were spikedwith mixed 87Rb plus 84Sr, and 149Sm plus 150Nd tracers,and processed through standard dissolution procedureswith HF, HNO3 and HCl in Teflon vials and heated untilcomplete material dissolution. A column using cationicAG-50W-X8 (200�400 mesh) resin developed in order toseparate Rb, Sr and rare earth elements (REE). Separationof Sm and Nd was accomplished through the use of anionicLN-B50-A (100�200 µm) resin. Each sample was dried toa solid and loaded onto the appropriate filament (singleTa for Rb, Sr, Sm, Pb, and triple Ta-Re-Ta for Nd) with0.25N H3PO4 (Rb, Sr, Sm and Nd). The samples wererun in a multi-collector VG Sector 54 thermal ionizationmass spectrometer at the Laboratório de Geologia Isotópicaof the Universidade Federal do Rio Grande do Sul (LGI-UFRGS)in a static mode. Nd and Sr ratios were normalized to146Nd/144Nd = 0.7219 and 88Sr/86Sr = 0.1194, respectively.Measurements on the NIST standard NBS-987 yielded87Sr/86Sr = 0.710260±0.000014, while on the La Jollastandard, 143Nd/144Nd = 0.511859±0.000010. Total blankaverages were <150 pg for Sr and Sm, and <750 pg for Nd.

All the errors listed in table 2 correspond to 1σ963;of the block mean (1 block = 10 isotope ratios). They arepresented in table 2 as standard deviation (SD) for Rb/Sr

and in ppm for Sm/Nd isotopic system. Errors for the87Rb/86Sr and 147Sm/144Nd are ±1% (SE% � standard errorpercent), and for 87Sr/86Sr and 143Nd/144Nd are <0.0025%(SE%�standard error percent, or smaller (based oninteractive sample analysis and spike).

Results

The isotopic results obtained in this study are shown intable 2 and figures 6, 7, 8, and 9. Samples of the lowermafic succession of the AVAf show Rb contents between12.1 and 82.9 ppm (average 32.3 ppm) and Sr contentsbetween 23.5 and 703.4 ppm (average 309.4 ppm).Measured 87Sr/86Sr ratios in these rocks range from0.70755 to 0.73125, while initial ratios, calculated for550 Ma, are between 0.70616 and 0.71103. The Smcontent varies from 5.02 to 13.74 ppm, and the Nd rangesbetween 22.9 and 78.2 ppm. Measured 143Nd/144Nd ratiosconcentrate between 0.51199 and 0.51216, resultingon εNd(0) from �9.34 to �16.66 and εNd (t = 550 Ma) of�2.97 to �10.31. TDM model ages lie between 1.11 and1.78 Ga.

Tuffs and welded tuffs of the upper felsic associationof AVAf display Rb values of 91.6 to 167.0 ppm (mean126.1 ppm), and 36.2 to 98.8 ppm for Sr (mean 55.6 ppm),compatible with acidic (potassic) volcanics. The measured87Sr/86Sr ratios are high, between 0.74597 and 0.77196,while the initial values range from 0.7014 to 0.7133.They present low Sm and Nd contents (0.8�9.5 ppm and4.1�43.8 ppm, respectively), and 143Nd/144Nd ratios of0.51180 to 0.51205. The εNd(0) lies between �11.93and �16.44, while the εNd at the time of crystallization(t = 550 Ma) ranges from �7.2 to �9.8. The TDM model

Fig. 5. Diagrams plots for the RVM. (a) Multi-elements spider diagram normalizing the values of Sun and McDonough (1989), (b) Chondrite-normalized REE diagram (Nakamura 1977) (in Almeida et al., 2000).

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ages for the particulate fraction of AVAF are 1.33 to1.92 Ga.

The rhyolitic lava flows of the upper felsic associationof AVAf show even higher Rb values (100.3 to 176.7 ppm,mean 137.6 ppm) compared to Sr (13.1 to 55.6 ppm, mean39.8 ppm), with very high measured as well as initial87Sr/86Sr ratios, respectively 0.77186 to 0.93947 and0.7014 to 0.7218. The 143Nd/144Nd ratios ranges from0.51181 to 0.51215, corresponding to εNd(0) values of�7.14 to �16.25, εNd (for t = 550Ma) of �5.69 to �8.80and TDM ages between 1.35 and 2.17 Ga.

The results for the RVM show low Rb (4.5 to 91.0 ppm,mean 31.0 ppm) and high Sr contents (310.1 to1203.25 ppm, mean 733.3 ppm), with low Sr isotopicratios (0.70526 to 0.71040 for measured, and 0.70467to 0.70774 for initial 87Sr/86Sr calculated for 470 Ma.).The 143Nd/144Nd values concentrate between 0.51165 and0.51193, corresponding to strongly negative εNd(0) of�13.89 to �19.36, εNd (t = 470Ma) of �8.39 to �13.92,and TDM model ages of 1.50 to 1.96 Ga.

Discussion and Conclusions

According to Almeida et al. (1998a, 2002a), the bimodalvolcanism of the Acampamento Velho Alloformation (AVAf)is characterized by the presence of minor basic and majoracid rocks, with overall absence of rocks with SiO2 contentbetween 54% and 67%. Originally alkaline, basic magmamoves upward through deep fractures under extensional

conditions. At a certain point of its ascending trajectory,the magma separates into two fractions, resulting in abimodal event.

Plots of initial 87Sr/86Sr ratio against 143Nd/144Nd andεNd (Fig. 6) show different signatures for mafic lavas ofthe RVM and mafic lavas of the AVAf and felsic rocks fromAVAf. The mafic lavas of the RVM show a variable radiogenicNd and εNd(t) to a nearly constant radiogenic Sr (Fig. 6a).The opposite occurs with the mafic lavas of the AVAf. Thedata in figure 6b display a negative correlation betweenradiogenic Sr and εNd for the samples from the AVAf.

All lavas, whichever unit they belong to, show anegative εNd in t or in the present time. The meaning ofnegative εNd values in acid lavas is not always easilyaddressed. It could be related to crust-derived lavas,crustal contamination of basaltic lavas duringdifferentiation, or mantle-derived, metasomatic lavas.However, negative εNd values in basalts are usually relatedto the latter two processes. Crustal contamination, eitherin acid or mafic lavas generally also affects the Rb/Srsystem, increasing the amount of radiogenic Sr andconferring the rocks a high 87Sr/86Sr initial ratio. The maficlavas of AVAf and RVM display εNd ranging from �2.97 to�10.31 and from �8.39 to �13.92 respectively, and aninitial 87Sr/86Sr ratio from 0.706 to 0.707 and from 0.704to 0.707. These initial 87Sr/86Sr ratios are consistent withrocks derived from a Neoproterozoic depleted mantle,represented by the Cambaí Complex rocks (Babinski et al.,1996), plotted in figure 7. The negative correlation

Fig. 6. Diagram for the isotopic data obtained at the volcanic rocks from the Acampamento Velho Alloformation and Rodeio Velho Member:(a) 144Nd/143Nd versus initial 87Sr/86Sr, (b) εNd versus initial 87Sr/86Sr, showing the different signatures for the mafic lavas of RVM and AVAfand the felsic ones from AVAf.

Symbols:

Acampamento Velho Allormation:

Flows

Tuffs and Welded tuffs

Flows: Lower Mafic Association

Upper FelsicAssociation

Rodeio Velho Member

X Andesite/basalt flows

87Sr/86Sr (i) 87Sr/86Sr (i)

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between initial 87Sr/86Sr ratio and SiO2 content observedin figure 8a, along with the constancy of the SiO2 contentdespite εNd values (Fig. 8b) for AVAf and RVM mafic lavasratify the idea that crustal contamination was not significantduring the differentiation of these rocks, in the early stagesof differentiation (at least until these rocks have reachedaround 55 wt.% of SiO2 content). Crustal contaminationor crustal origin is assumed for rocks with SiO2 contenthigher than 55 wt.%. It can also explain the occurrence ofacid lavas and pyroclastic material in the AVAf (Fig. 8a, b).A plot of initial 87Sr/86Sr against Ba/Th ratios (Fig. 9)shows strong enrichment in radiogenic Sr and Th in theacid rocks, which could be related to a crustal component.

In order to better constrain the possible sources of theAVAf and RVM lavas, juvenile, Neoproterozoic depletedmantle derived rocks (the Cambaí Complex) andNeoproterozoic crustal rocks (arkosic quartzites, taken as

representative of the average composition of the crust,and meta-pelagic sediments) were plotted with AVAf andRVM samples (Fig. 7). The isotopic ratios and compositionsof these rocks were calculated for 550 Ma, and mixturelines were calculated based on two-end-member models.From the analysis of figure 7, the AVAf mafic lavas couldbe regarded as a mixture of depleted mantle-derived basaltplus 20% to 30% of crustal contamination. The crustalmaterial matches best Neoproterozoic arkosic quartzitesrather than pelagic sediments The evolution trend of theAVAf felsic lavas displays an increase in crustal contamination.

The mafic lavas of the RVM, however, cannot be easilyexplained by crustal contamination of depleted mantle-derived magma. The composition of these lavas requestan end member highly enriched in radiogenic Nd, butimpoverished in radiogenic Sr. An enriched mantle type I(EM I according to Zindler and Hart, 1986) is such areservoir, well established for Phanerozoic rocks with thesame trend as RVM (Fig. 7).

The differences between AVAf and RVM mafic samplescan be highlighted using Ba/Th ratio (Fig. 9). Both maficlavas show preferential enrichment in Ba relative to Th.However, the mafic lavas of the RVM show distinctiveenrichment in the Ba/Th ratio (due to its very highBa content, up to 2695 ppm) without a change in the87Sr/86Sr initial ratio. The mafic lavas of the AVAf, on theother hand, show a weak enrichment in Ba/Th and87Sr/86Sr initial ratios. Such behavior suggests that theRodeio Velho Member originated from a depleted mantle,re-enriched in highly incompatible elements such as Ba.

Regarding the Nd model ages (Table 2) we can see themafic associations of the AVAf and of the RVM with valuesraging from 1.1 to 1.8 Ga and 1.5 to 2.0 Ga, respectively.These model ages point to also for a modified mantle atend of Brasiliano Cycle, in post-orogenic tectonic setting

Fig. 7. 143Nd/144Nd versus initial 87Sr/86Sr diagram at the volcanic rocksfrom the Rodeio Velho Member and the flows fromAcampamento Velho Alloformation, showing the field for crustalcontamination. Symbols same as in figure 6.

Fig. 8. Diagram for the isotopic data obtained at the volcanic rocks from the Acampamento Velho Alloformation and Rodeio Velho Member:(a) initial 87Sr/86Sr versus SiO2 , (b) εNd(t) versus SiO2. Symbols same as in figure 6.

490



(during the Upper Neoproterozoic to Eopaleozoic), butthe AVAf have some juvenile component with alreadymodified mantle since it displays somewhat youngermodel ages and lesser negative εNd values. On other side,the Nd model age for the felsic association of the AVAfare from 1.3 to 2.8 Ga (Table 2). This information coupledwith lower εNd negative values of AVAf felsic associationwhen compared to those of the AVAf mafic associationsuggest at least two alternatives, namely, either (1) thefelsic magma is a melting of a lower crust with a dominantPaleoproterozoic age or mixed Brasiliano-older crust, or(2) the felsic magma may be derived from the mixture ofmafic magma and a crustal enrichment component asdescribed below.

In this case the existence of different sources in themantle during the Neoproterozoic magmatism in theCamaquã Basin could be connected with a sedimentarycomponent. The subduction process could be responsiblefor the 20 to 30% of contamination by sediments, probablyNeoproterozoic arkosic quartzites, and the establishmentof a reservoir. The generated magma at some pointinitiated its ascension under conditions of high oxygenfugacity. At a certain stage, this magma separated intotwo fractions: a mafic portion, from which the mafic lavasof the AVAf originated, and another one, enriched incrustal component, which gave rise to the felsic rocks ofthe AVAf. The rocks belonging to the RVM seem to haveoriginated from a different reservoir, much more enrichedin incompatible elements and depleted in radiogenic Sr,with EMI I characteristics.

Acknowledgments

The authors wish to thank FAPERGS (Grant No. 01/0881-5)for its financial support. The authors are grateful to theUniversidade do Vale do Rio dos Sinos (UNISINOS) for

the scholarship granted to the geology undergraduatestudent Ricardo Medeiros de Freitas, who collaboratedwith figure formatting.

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evolution of heterogeneous mantle in the acampamento velho...

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