Gondwana Research 21 (2012) 466–482

Contents lists available at ScienceDirect

Gondwana Research

j ourna l homepage: www.e lsev ie r.com/ locate /gr

Ages (U–Pb SHRIMP and LA ICPMS) and stratigraphic evolution of theNeoproterozoic volcano-sedimentary successions from the extensionalCamaquã Basin, Southern Brazil

Liliane Janikian a,⁎, Renato Paes de Almeida b, Antonio Romalino Santos Fragoso-Cesar b,Veridiana Teixeira de Souza Martins b, Elton Luiz Dantas c, Eric Tohver d,Ian McReath b, Manoel Souza D'Agrella-Filho a

a Instituto de Astronomia, Geofísica e Ciências Atmosféricas, Universidade de São Paulo, Rua do Matão, 1226, Cidade Universitária CEP 05508–080, Brazilb Instituto de Geociências, Universidade de São Paulo. Rua do Lago, 562, Cidade Universitária CEP 05508–080, Brazilc Instituto de Geociências, Universidade de Brasília, Campus Universitário Darcy Ribeiro, Brasilia-DF, Brazild Tectonics Special Research Center, University of Western Australia, Perth, WA, Australia

⁎ Corresponding author. Tel.: +55 11 38767260.E-mail address: lijanikian@yahoo.com.br (L. Janikian

1342-937X/$ – see front matter © 2011 International Adoi:10.1016/j.gr.2011.04.010

a b s t r a c t

a r t i c l e i n f o

Article history:Received 26 January 2011Received in revised form 16 February 2011Accepted 5 April 2011Available online 6 May 2011

Keywords:EdiacaranCamaquã BasinBom Jardim GroupAcampamento Velho FormationDepositional systemStratigraphic evolution

During the Ediacaran, southern Brazil was the site of multiple episodes of volcanism and sedimentation,which are best preserved in the 3000 km2 Camaquã Basin. The interlayered sedimentary and volcanic rocksrecord tectonic events and paleoenvironmental changes in a more than 10 km-thick succession. In thiscontribution, we report newU–Pb and Sm–Nd geochronological constraints for the 605 to 580 Ma Bom JardimGroup, the 570 Ma Acampamento Velho Formation, and a newly-recognized 544 Ma volcanism. Depositionalpatterns of these units reveal the transition froma restricted, fault-bounded basin into awide, shallow basin. Theexpansion of the basin and diminished subsidence rates are demonstrated by increasing areal distribution andcompressed isopachs and increasing onlap of sediments onto the basement to the west. The Sm–Nd isotopiccomposition of the volcanic rocks indicates mixed sources, including crustal rocks from the adjacent basement.Both Neoproterozoic and Paleoproterozoic sources are indicated for the western part of the basin, whereas onlythe older Paleoproterozoic signature can be discerned in the eastern part of the basin.

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

1. Introduction

A series of small fault-bounded relicts of sedimentary basins occurfrom southeastern Brazil to southern Uruguay, and expose unmeta-morphosed sedimentary and volcanogenic successions of Ediacaran toEarly Paleozoic age (see Fragoso-Cesar et al., 2000; Sánchez-Bettucciet al., 2001, 2004, 2009; Oyhantçabal et al., 2007; Almeida et al., 2010).These successions record the tectonic and paleoenvironmental evolu-tion of the region during a period of important global changes andregional geotectonic events, which spans from the aftermath of theextreme glaciations of the Neoproterozoic to the Cambrian explosion ofmetazoans. Themost complete and best preserved of these occurrencesis the Camaquã Basin (Fig. 1), which hosts the well-dated CamaquãSupergroup (Ediacaran to Early Cambrian), exposed in the south-centralregion of the Rio Grande do Sul State (Southern Brazil).

The stratigraphic successions of the Camaquã Basin were depositedbetween ca. 610 and 535 Ma (Janikian et al., 2008; Almeida et al., 2010)and are frequently related to the main phase of the Brasiliano Orogeny,

).

ssociation for Gondwana Research.

which is considered tohaveoccurredbetween650and550 Ma(Cordaniet al., 2000). These previous models consider distinct Neoproterozoictectono-magmatic events in Southern Brazil: an early one, consisting ofjuvenilemagmatismand terrane accretion fromca. 900 to700 Ma, and alater one, forming the Pelotas Batholith and the collisional DomFeliciano Belt between 600 and 550 Ma (e.g. Babinski et al., 1996;Silva et al., 1999; Chemale, 2000; Gastal et al., 2005a,b). This generalscenario masks some controversy over the early stages in the history ofthe basin, alternately considered to record an arc-related environment(e.g. Fragoso-Cesar et al., 1984; Fernandes et al., 1992; Gresse et al.,1996) or a foreland basin (e.g. Gresse et al., 1996; Chemale, 2000; Gastalet al., 2005a) generated by continent–continent collision (Fernandeset al., 1995; Leite et al., 2000). Other authors interpret a syn- to post-collisional strike-slip setting for the basin (e.g. Oliveira and Fernandes,1992; Brito Neves et al., 1999; Sommer et al., 2006).

More recent work indicates an extensional tectonic regime for theformation of the Camaquã basin (e.g. Fragoso-Cesar et al., 2000, 2001;Almeida, 2001, 2005; Fambrini, 2003, 2005; Janikian et al., 2003, 2005;Pelosi and Fragoso-Cesar, 2003; Leitão et al., 2007; Almeida et al., 2009,2010) and demonstrates that the recognized compressional deforma-tion is limited to post-depositional strike-slip events (Almeida, 2005).Broadly speaking, the stratigraphic evolution for the volcano-

Published by Elsevier B.V. All rights reserved.

Fig. 1. A— Pre-Ediacaran and Ediacaran–Eocambrian successions of the RioGrande do Sul State (Brazil) and Ediacaran–Cambrian volcanic and sedimentary units of Uruguay (modifiedfrom Fragoso-Cesar, 1991); RPC — Rio de La Plata Craton, DFB — Dom Feliciano Belt (sensu Fragoso-Cesar, 1991), PB — Pelotas Batolith, RVT — Rio Vacacaí Terrane. B — CamaquãSupergroup Units, located at the south-central part of Rio Grande do Sul State (modified from Fragoso-Cesar et al., 2000; Janikian et al., 2005).

467L. Janikian et al. / Gondwana Research 21 (2012) 466–482

sedimentary units of the Camaquã Supergroup, based on stratigraphicmeasurements, detailed geological mapping (Janikian, 2004), corrobo-rate an extensional origin for the Camaquã Basin between ~605 and544 Ma. The interpretation of the geotectonic setting of the events ofsubsidence, deformation, volcanism and related granitic emplacementfor the Camaquã Basin as awhole is discussed elsewhere (Almeida et al.,2010).

To put constrains in the magmatic phases of each formation, wepresent new U–Pb zircon ages for the acid volcanogenic units of theCamaquã Supergroup and integrate these ages with previouslyobtained data from the Bom Jardim Group and the AcampamentoVelho Formation (Janikian et al., 2008). We also performed a Sm–Ndisotopic study on the volcanic rocks, in order to test the hypothesis ofdifferent basement sources or contaminants in the genesis of volcanicrocks from distinct parts of the basin. Based on our results, wedistinguish a younger acid volcanism, previously considered as part ofthe Acampamento Velho Formation.

2. The basement of the Camaquã Basin

The basement of the Camaquã Basin is composed of Proterozoicmetamorphic and intrusive rocks that crop out in a structural highsurrounded by Paleozoic and Mesozoic sedimentary rocks of theParaná Intracratonic Basin (Fig. 1A). The Precambrian geology hasbeen interpreted differently by various authors, with a conflictingnomenclature proposed for the different cratonic units, suspectterranes, and fold belts (e.g. Soliani, 1986; Chemale et al., 1995; Frantzet al., 1999; Chemale, 2000; Saalmann et al., 2005). The followingdiscussion considers the main divisions proposed by Fragoso-Cesar(1991), as shown in Fig. 1A.

The basement of the Camaquã Basin comprises three maincomponents: (i) the northern portion of the Rio de La Plata Craton,represented by Paleoproterozoic rocks of the Santa Maria ChicoGranulite Complex; (ii) the eastern Dom Feliciano Belt, includingPaleoproterozoic gneissic and granitic rocks covered by Neoproterozoic

468 L. Janikian et al. / Gondwana Research 21 (2012) 466–482

lowgrademetasedimentary rocks andminormetavolcanics (e.g. Cerro doAlemão Metarhyolite, Jost, 1981), as well as voluminous Neoproterozoicgranitoids (Pelotas Batholith); and (iii) the Rio Vacacaí Terrane,comprising calc-alkaline orthogneiss (Cambaí Complex), ophiolites andmetavolcano-sedimentary successions of a Neoproterozoic (ca. 900 Ma to700 Ma) intra-oceanic island-arc terrane accreted during the BrasilianoOrogeny. Published isotopic data for these units (Table 1) support ajuvenile origin for the Neoproterozoic magmatism of the Rio VacacaíTerrane,with TDMmodel ages of 0.86 Ga to 1.03 Ga (Babinski et al., 1996).TDM model ages of 2.0 Ga to 3.2 Ga were obtained for the Dom FelicianoBelt (Chemale, 2000), and of ca. 2.6 Ga for the SantaMaria Chico Complex(Mantovani et al., 1987).

3. Tectonic setting of Camaquã Supergroup

The Camaquã Supergroup (Fragoso-Cesar et al., 2000; Janikian etal., 2003) comprises five units (Fig. 1B), from base to top: the MaricáGroup (marine and fluvial clastic deposits, also minor acid volcanicsaccording to Borba et al., 2008), the Bom Jardim Group (basic tointermediate volcanic rocks and continental clastic successions), theAcampamento Velho Formation (mainly acid extrusive and pyroclas-tic rocks), the Santa Bárbara Group (post-volcanic continentalsiliciclastic successions) and the Guaritas Group (alluvial and aeoliansuccessions), intruded by the Rodeio Velho Intrusive Suite. Each ofthese units represents a distinct episode of tectonic subsidence withdifferent depocenters and basin configurations, now exposed in threesub-basins separated by post-depositional basement highs (Fig. 1B). Ayounger acid volcanism identified in the Dom Pedrito region reveals avolcanogenic event distinct from the Acampamento Velho Formation.

Superposed deformational events caused brittle faulting, mostly ofstrike-slip and normal nature (e.g. Almeida, 2005), as well as blocktilting and fault-drag folds. Spatial frequency of faults is higher in thelower stratigraphic units of the basin, but even in the Maricá and BomJardim Groups, the concentration of structures in discrete zones ofintense deformation and the absence of regional metamorphismenable stratigraphic studies in fault-bounded blocks.

Different tectonic interpretations of the Camaquã Basin are basedmainly on the characterization of the deformational structures and on

Table 1Published isotopic data from units of the Camaquã Basin and its metamorphic basement. (

(2005a,b); (4)Machado et al. (1990); (5)Babinski et al. (1996); (6)Hartmann (1998); (7)Manzircon (SHRIMP) method; ♦U–Pb zircon method; • whole-rock Rb–Sr method; ■ Pb–Pb me

Unit Rock

Camaqu ã Super-group Acampa-mento VelhoFormation

Acid volcanic rocks

Andesitic to basaltic rocks

Tuffs and welded tuffsRhyolitic flows

Bom Jardim Group(Hilário Fm.)

Basic and intermediate volcanic(southern part of the Eastern)

TrachyandesiteBasaltic trachyandesiteMetarhyolites(4)

Rio Vacacaí Terrane Metamorp hic Supracrus Metatuff//Basalt

Cambaí Complex Dioritic gneiss, metatonalite,pegmatite and metadiorite

Rio de La Plata Craton Santa Maria Chico Complex Basic Granulite TrondhjemitcGranulite

Dom Felician o Belt Encatada s Complex Metatonalites/PegmatitesCerro do AlemãoMetarhyolites

Metarhyolities**

geochemical data for the volcanic successions and related intrusiverocks. The high-K2O basic to intermediate volcanic rocks of the HilárioFormation of the Bom Jardim Group were identified as calc-alkalineand alkaline by Roisenberg et al. (1983), and later reinterpreted assubduction-related shoshonitic magmatism (e.g. Nardi and Lima,1985). Recently, these volcanic rocks have been considered to be theproduct of partial melting of a lithosphericmantlemodified by previoussubduction (Nardi and Lima, 2002). The acid and basic volcanic rocks ofalkaline affinity of the Acampamento Velho Formationwere regarded asorogenic by Roisenberg et al. (1983), and recently have been ascribed tothe post-collisional stage of this orogeny (Sommer et al., 2006) and tolate topost-orogenic environments byvarious authors (e.g.Wildner andNardi, 1999; Nardi and Lima, 2002; Almeida et al., 2003, 2005).

The sedimentary record of the Camaquã Basin has also been thesubject of divergent geotectonic interpretations, being considered asanorogenic by Issler (1982), as the infill of a molassic foredeep (e.g.Fragoso-Cesar et al., 1984; Issler, 1985; Gresse et al., 1996), asthickened crust rift successions (Nardi and Lima, 1985), as retro-arcbasin successions (Fragoso-Cesar, 1991) and as late-orogenic strike-slip basin successions (e.g. Oliveira and Fernandes, 1992). Recentworks based on detailed stratigraphic, provenance, paleocurrent andstructural analyses of all units of the Camaquã Basin indicate that thebasin formed in an extensional environment as part of an intraplaterift (e.g. Fragoso-Cesar et al., 2000, 2001; Almeida, 2001, 2005;Fambrini, 2003, 2005; Janikian et al., 2003, 2005; Pelosi and Fragoso-Cesar, 2003; Leitão et al., 2007; Almeida et al., 2010). Provenanceanalysis of several distinct stratigraphic levels (Almeida, 2005; Leitãoet al., 2007) reveals sources for alluvial fan deposits which wherespatially variable but constant in each site through time, suggestingthat the basin-border faults had no recognizable syn-depositionalstrike-slip movement. Therefore the strike-slip deformation thatsupports the hypothesis of a strike-slip origin for the basin (e.g.Sommer et al., 2006) is clearly post-depositional (Almeida et al.,2010). The voluminous volcanism and the tectonic subsidence ofseveral thousands of meters are not consistent with an extensionalcollapse model (Almeida et al., 2010) and suggest active rifting withno clear connection with the previous Brasiliano Orogeny (Fragoso-Cesar et al., 2003).

1) Chemale (2000); (2a)Almeida et al. (2003); (2b)Almeida et al. (2005); (3)Gastal et al.tovani et al. (1987); (1) Chemale (2000); **reworked during Neoproterozoic; ♠U–Pbthod.

Crystallization Age(Ma)

εNd (t) TDM (Ga)

♦573±18(1) −9.34 and−9.37(1)(t=570 Ma)

1.7 to 1.9(1)

– −2.97 to −10.31(2a,b)

(t=550 Ma)1.11 to 1.78(2a,b)

– −7.2 to −9.8(2a,b) (t=550 Ma) 1.33 to 1.92(2a,b)

• 545.1±12.7(2) −5.36 to −9.56(2a)

(t=550 Ma)1.35 and 2.17(2a,b)

rocks Estimated ~592 e573(1)

−1.52 and −2.34(1)

(t=590 Ma)1.2(1)

−8.98(1) (t=590 Ma) 1.8(1)

Estimated 599(3) −8.96(3) (t=599 Ma) 1.97(3)

−2.32(3) (t=599 Ma) 1.42(3)

♦ 753±2(4) – –

– +7.8//5.2(5) (t=750) 0.42//– −1.32 to+3.73(3) (t=750 Ma) 1.66 to 1.83(3b)

■ 704±13(5) +2.8 to +4.5(5) (t=700 Ma) 0.86 to 1.03 (5)

♦2.55 Ga (6) +2.6(7) and +3.0(6) (t=2.6 Ga) 2.60 to 2.67(7)

♠2263±18/2363±6 – 2.0 and 3.2(1)

♠ 783±6 (8) −9.63 to −12.53(1)

(t=780 Ma)2.01 to 2.22(1)

469L. Janikian et al. / Gondwana Research 21 (2012) 466–482

4. Methods

The applied methods include facies description and depositionalsystem analysis of the volcano-sedimentary units of the CamaquãSupergroup, based on the models published for both siliciclastic (e.g.Reading, 1986, 1996; Walker and James;, 1992) and volcanogenicsuccessions (e.g. McPhie et al., 1993). Isotopic analyses were carriedout in different stratigraphic successions of the Bom Jardim Groupand the Acampamento Velho Formation, based on Sm–Nd and U–Pbmethods.

New U–Pb zircon ages for four samples are presented, obtainedfrom 2 pebbles from conglomerates that underlie the volcaniclasticrocks of the Acampamento Velho Formation (samples SE-01 and SE-08), one sample of pyroclastic rock from the Taquarembó Plateauregion (sample DPM-155) and one sample of hypabissal rhyolite fromthe Bom Jardim Region (sample BJL-31). Zircon grains were datedthrough U–Pb laser ablation at Universidade de Brasília (samplesDPM-155 and BJL-31— Table 2), following the procedure described byBühn et al. (2009), and SHRIMP at the John de Laeter Centre forGeochronology, Perth (samples SE-01 and SE-08— Table 2), followingthe analytical methods described by Smith et al. (1998). U–Pb ageswere calculated using BR266 standard with 14–18 standard spots peranalytical session and an external standard mean error of b1.5%.

Ten Sm–Nd whole-rock analyses were performed at the Geochro-nological Research Centre of the Geoscience Institute of São PauloUniversity, Brazil. The Sm and Nd concentrations were determined byisotope dilution with a combined spike tracer and the chemicalseparations were made using two-column technique, described bySato et al. (1995). The isotope ratios were measured on VG-354multicollector and single collector mass spectrometers. The laborato-ry blanks for the chemical procedure during the period of analysisyielded maximum values of 0.4 ng for Nd and 0.7 ng for Sm. Theaverage 143Nd/144Nd value (and the 2σ standard deviations) obtainedfor the La Jolla standard was 0.511857 (46). The Sm–Nd and TDMmodelages and εNd(t), valueswere calculated usingDePaolo (1981, 1988) andDePaolo et al. (1991) model parameters. Two-stage TDM model ages(DePaolo et al., 1991) were calculated for these samples that show Sm–

Nd fractionations outside the normal range (0,54N=Sm/NdN=0,34),according to Sato (1997) and constants byMillisenda et al. (1994). Theycame from the Caçapava do Sul Granite and volcanic rocks from BomJardim Group and Acampamento Velho Formation.

Given the abundance of normal and strike slip faults and relateddrag folds in the studied area, stratigraphic measurements andcorrelation were preceded by detailed geological mapping (scale of1:50,000) of each area of occurrence of the Bom Jardim Group(Janikian, 2004), including the recognition of major structures and

Table 2New isotopic results from the Bom Jardim Group, and the Acampamento Velho Formation, izircon TIMS method; ♥Ar-Ar in plagioclase method; ♠U-Pb zircon - SHRIMP method; ▪ U-P(1995).

Unit (T0=xMa) Sample-rock Age (Ma) Sm (pp

Caçapava do Sul GraniteT0=550

GR01-Granite ♠2400, 561±6 and 541±11(C) 0.87

Subvolcanic bodiesT0=550

BJL54-Rhyolite ♦547.2±9.1(B) 3.12BJL31-Rhyolite ○ 572.2±6.5 4.52BJL111-Andesite ♦571±69(B) 14.34

Younger Acid Volcanism. DPM155-lapilli tuff ○ 544.2±5.5 –

Acampamento Velho Fm.T0=570

KBU58-Rhyolite – 8.94LRA303-Rhyolite – 9.05LRA305-Rhyolite ♦574±7(A) 8.44LRA392-Ignimbrite – 2.96SE01-Rhyolite clast ♠579±13 –

SE08-Rhyolite clast ♠569±2.4 –

Hilário Fm. T0=590 LLA544-Andesite – 8.04LLA01-Andesite ♥588±7(A) 8.09

their displacements. Stratigraphic data was collected in the lessdeformed areas, and correlation took into account the nature anddisplacements of the identified faults.

5. The Bom Jardim Group

The Bom Jardim Group includes both epiclastic sedimentarysuccessions and thick volcanic intercalations enabling geochronologicaldating of several stratigraphic levels (Janikian et al., 2008). U–Pb ages ofzircon and Ar–Ar ages of plagioclase of specific stratigraphic levels inthe Bom Jardim Group indicate an evolution from ca. 605 Ma to 580 Mafor this unit (Janikian et al., 2008). The classic lithostratigraphic modelproposed by Ribeiro (1966) and Ribeiro and Lichtemberg (1978)included all sedimentary units of the Bom Jardim Group in the Arroiodos Nobres Formation, with all conglomerates being attributed to theVargas Member and all fine sandstones and rhythmites to theMangueirão Member, regardless of stratigraphic position. This lack ofstratigraphic control and the fact that some of the geographic terms(such as Vargas and Arroio dos Nobres) were references to areas farfrom the type-area of the unit, bringing correlation uncertainties to thedefinitions, lead Janikian et al. (2003, 2005) to revise the lithostrati-graphy of the Bom Jardim Group. This newer model is adopted in thepresent work.

The basal unit, Cerro da Angélica Formation (Janikian et al., 2003),records the early stages of basin development, when the basin wasrestricted to the area of the Central Camaquã Sub-basin and boundedby normal faults. The Hilário Formation (Ribeiro and Lichtemberg,1978) records the main volcanic phase, when the basin expandedwestwards and the volcanic centers were subaerial, with subaqueoussedimentation prevailing at the depocenter. The Picada das GraçasFormation (Janikian et al., 2003) was deposited after the maintectono-volcanic events, being probably related to post-rift thermalsubsidence (Figs. 2A and 3).

5.1. Cerro da Angélica Formation

The Cerro da Angelica Formation is restricted to the centralCamaquã Sub-basin, where it ranges in thickness from ca. 1500 malong its northernmost occurrences in the northern part of thecentral sub-basin, to ca. 1700 m at the southern portion of thecentral sub-basin. The lower part of the unit is characterized bysandy and conglomeratic alluvial-fan deposits dominated bysheet-floods (Fig. 4A), with local occurrence of aeolian facies(Fig. 4B), grading upwards to lacustrine prodelta and delta frontdeposits (Fig. 4C and D). Peperite formed by the intrusion of basicmagma into unlithified sediment is also present at the lower part.

ncluding younger volcanism from Dom Pedrito region. *TDM (Ga) double-stage; ♦U-Pbb zircon – LA ICPMS method; (A)Janikian et al. (submitted);(B)Janikian (2004); (C)Leite

m) Nd (ppm) 147Sm/144Nd 143Nd/144Nd fsm/Nd TDM (Ga) εNd(0) εNd (t)

3.14 0.1675 0.511568 −0.15 2.49* −20.88 −18.80

20.56 0.0918 0.511511 −0.53 1.92 −21.99 −14.6317.62 0.1551 0.511831 −0.21 2.06* −15.74 −12.8276.96 0.1127 0.511639 −0,43 2.12 −19.59 −13.37– – – – – – –

34.31 0.1576 0.512000 −0.20 1.84* −12.44 −9.5745.86 0.1193 0.511845 −0.39 1.93 −15.47 −9.8135.78 0.1426 0.512670 −0.28 0.83* 0.62 4.5810.04 0.1785 0.511897 −0.09 2.09* −14.45 −13.12– – – – – – –

– – – – – – –

42.55 0.1143 0.512170 −0.42 1.34 −9.13 −2.9341.73 0.1173 0.512167 −0.40 1.38 −9.20 −3.22

Fig. 2. Composite stratigraphic section of the Bom JardimGroup and the Acampamento Velho Formation showing siliciclastic and volcanic deposits, and the position of analyzed samples.

Fig. 3. Stratigraphic evolution of the volcano-sedimentary successions of the Camaquã Basin. A) Cerro da Angélica Formation: fluvial and sheet-flood alluvial-fan deposits that gradeupwards into lacustrine prodelta and delta front deposits, and lobe and channel deposits of subaqueous fans; B) Hilário Formation constituted by volcanic (basalts, latite-basalts,latites and andesites) and pyroclastic rocks, extruded in subaqueous environments in the Central Camaquã Sub-basin (CCSb) and in subaerial environments in theWestern CamaquãSub-basin, and rhythmic successions of prodelta deposits in the CCSb; C) Picada das Graças Formation: prodelta deposits, related to the last stages of tectonic activity, and D) the deltafront and river-dominated delta deposits, related to progradation under lower subsidence rates during the thermal subsidence phase. E) Acampamento Velho Formation with fluvialdeposits, effusive and pyroclastic acid rocks, intruded by mafic bodies. F) Alluvial deposits, effusive and pyroclastic acid rocks from Dom Pedrito region.

470 L. Janikian et al. / Gondwana Research 21 (2012) 466–482

471L. Janikian et al. / Gondwana Research 21 (2012) 466–482

Fig. 4. A — Sheetflood deposits of the Cerro da Angélica Formation in the southern area of the Central Camaquã Sub-basin. Conglomeratic sandstone and gravel couplets formed bysheetfloods in the upper part of the sheetflood section. B — Medium-grained sandstones with large-scale cross stratification. Aeolian deposits of the Cerro da Angélica Formation,southern part of the Central Camaquã Sub-basin. C and D — Prodelta deposits of the Cerro da Angélica Formation from the southern (C) and northern (D) parts of the CentralCamaquã Sub-basin. These are mudstones and fine-grained plane bedded and ripple cross-laminated sandstones, commonly showing liquefaction structures. E and F— Scour-basedturbiditic channels (E), filled with stratified conglomerates and sandstones (F), Cerro da Angélica Formation, northern part of the Central Camaquã Sub-basin.

472 L. Janikian et al. / Gondwana Research 21 (2012) 466–482

The intermediate succession is composed of rhythmic sandstones,siltstones and mudstones cut by channel-shaped bodies of pebblysandstones, interpreted as lobe and channel deposits of sub-lacustrine fans, respectively (Fig. 4E and F). These deep waterdeposits are overlain by rhythmic sandstones and siltstones,interpreted as distal deltaic deposits. The delta front depositsof the southern portion of the central Camaquã Sub-basin(CCSB) are intruded by a granite dike that was dated by singlegrain U–Pb method in zircon (Janikian et al., 2008), giving an ageof 600.5±2.4 Ma.

5.2. Hilário Formation

The Hilário Formation is approximately 1200 m thick in thenorthern part of the Central Camaquã Sub-basin, and over 2500 mthick in the southern part of the Western Camaquã Sub-basin in thetype-area of the unit (Janikian et al., 2008). It is constituted byvolcanic rocks of predominantly basic and intermediate composition(basalts, latite-basalts, latites and andesites), extruded in subaqueousenvironments in the Central Camaquã Sub-basin and in subaerialenvironments in theWestern Camaquã Sub-basin. Related pyroclastic

473L. Janikian et al. / Gondwana Research 21 (2012) 466–482

rocks are also present (pyroclastic breccias, lapilli-tuffs and coarse-grained lithic and vitric tuffs), formed by primary pyroclastic flowprocesses in the Western Camaquã Sub-basin (Fig. 5), and bysecondary gravity flows (including water-settled vitric tuffs) in theCentral Camaquã Sub-basin. In the Central Camaquã Sub-basin, thesevolcanogenic rocks are interbedded with rhythmic successions ofpelites and fine-grained sandstones deposited in a lacustrine prodeltaenvironment.

The great difference in thickness between the volcanic andpyroclastic successions of the two regions suggests that the westernregion was closer to the edge of the basin and to the volcanic centerswhile the central region was nearer to the basin depocenter, with a

Fig. 5. Pyroclastic rocks of the Hilário Formation in the Western Camaquã Sub-basin, formpolarizers, C and D with crossed polarizers. A— Crystal-rich tuff with some glass shards (arrorich tuff with devitrification textures; D — Lapilli matrix of pyroclastic breccia, showing micclasts.

consequently smaller contribution of volcanic flows in a lacustrineprodelta environment. The volcanic rocks in both sub-basins have acrystallization age of about 590 Ma (Ar–Ar in plagioclase and U–Pb inzircon ages, Janikian et al., 2008).

5.3. Picada das Graças Formation

The Picada das Graças Formation overlies the Hilário Formation inthe Central Camaquã Sub-basin, where it is approximately 1800 mthick. In the northern part of the Western Camaquã Sub-basin, theformation is ca. 700 m thick and was deposited directly over theMaricá Group (Figs. 2B and 3). The lower portion of the unit is

ed in subaerious environments. A–D microscopic photographs, A and B with parallelw); B— Lithic tuff with basic rock fragments (L) and feldspar crystals (fd); C— Crystal-rospherulites (arrow). E and F — pyroclastic breccias with sub-angular to sub-rounded

474 L. Janikian et al. / Gondwana Research 21 (2012) 466–482

characterized by the local occurrence of medium to coarse fluvialsandstones (Fig. 6A), passing laterally to rhythmic layers of fine-grained sandstones, siltstones and mudstones, deposited in lacustrineprodelta environments (Fig. 6B and C), overlain by thick conglomer-atic and sandy successions of fluvial and delta front environments(Fig. 6D and E). The prodelta deposits are thought to represent thesedimentary response to the continued basin widening, related to the

Fig. 6. A—Medium to coarse-grained sandstones with trough cross stratification from the lowSub-basin. B and C — Prodelta deposits of the Picadas das Graças Formation in the WesterConvoluted fine-grained sandstones. D and E—Deltaic deposits of the Picada das Graças Formgrading upward to plane-bedded conglomerates (E).

last stages of tectonic activity, and the fluvial and delta front depositsmay be related to progradation under lower subsidence rates duringthe thermal subsidence phase.

In the Central Camaquã Sub-basin, the upper part of the Picada dasGraças formation comprises of pebbly sandstones and sandstone–siltstone successions of a river-dominated deltaic environment. Thisupper portion of the Picada das Graças Formation marks the

er levels of the Picada das Graças formation in the northern part of the Central Camaquãn Camaquã Sub-Basin. B — Sand–mud heterolithic bedding and climbing ripples. C —

ation in the nothern part of the Central Camaquã Sub-basin. Massive conglomerates (D)

Fig. 7. A and B— U–Pb (SHRIMP) zircon ages of rhyolites from pebbles of the fluvial conglomeratic deposits from the lower Acampamento Velho Formation in theWestern CamaquãSub-basin. A — sample SE-01. B — sample SE-08. C — U–Pb (TIMS) zircon age from hypabissal rhyolite from the Central Camaquã sub-basin.

475L. Janikian et al. / Gondwana Research 21 (2012) 466–482

progradation of river-dominated deltas with the absence of proximaldeposits fed from border-faults or sedimentary successions related tothe uplift of tectonic highs. Thus, the upper levels of the Picada dasGraças Formation may reflect the post-rift stage of the basin. A tuffbed with a crystallization age of 580±3.6 Ma (U–Pb – SHRIMPmethod – in zircon, Janikian et al., 2008) was found in the CentralCamaquã Sub-basin, intercalating rhythmic lacustrine prodeltadeposits.

6. The Acampamento Velho Formation

The Acampamento Velho Formation (Ribeiro and Fantinel, 1978)occurs mainly in the Western Camaquã Sub-basin. It is constituted byconglomeratic deposits and mainly volcanoclastic and eruptive rocks

formed essentially in subaerial environments (Fig. 2B). Previousworks describe the geochemical and lithologic characteristics of theunit (e.g. Wildner et al., 1994; Zerfass et al., 2000; Almeida et al., 2002,2003; Sommer et al., 2003; Janikian et al., 2005). Almeida et al. (2002)obtained geochemical data for the Acampamento Velho Formation,considering “a model similar to intra-plate or extensional basalts”.

The unit was formed in a much wider basin than the underlyingBom Jardim Group units and progressively onlaps the Maricá Groupand basement onto the west (Figs. 1B and 3). The AcampamentoVelho Formation is overlain by the thick, essentially siliciclastic unitsof the Santa Bárbara Group in an angular unconformity (Almeida,2001, 2005).

The lower succession of the Acampamento Velho Formation(previously considered as part of the Picada das Graças Formation

Table 3U/Pb SHRIMP data for a rhyolitic clast from the fluvial deposits of the Acampamento Velho Formation (sample SE-01).

(1) (2) (3) (1) (1) %

206Pb/238UAge

206Pb/238UAge

206Pb/238UAge

207Pb/206PbAge

208Pb/232ThAge

Discordant Total238U/206Pb

±% Total207Pb206Pb

±% (1)238U/206Pb⁎

±% (1)207Pb⁎

/206Pb⁎

±% (1)207Pb⁎

/235U

%

206PbcppmU

ppmTh

232Th/238U

ppm206Pb⁎Spot

SE01-2

0.60 0.72 571.8±9.8

575±10

578±11

398±73

517±15

−30 10.72 1.8 0.05954 1.3 10.78 1.8 0.0547 3.2 0.699

SE01-8

0.53 0.74 572.2±9.7

573.4±9.9

579±11

504±42

513±15

−12 10.72 1.8 0.06169 1.3 10.77 1.8 0.0573 1.9 0.734

SE01-3

0.54 1.07 578±11

580±11

584±13

479±58

549±14

−17 10.6 1.9 0.0611 1.7 10.65 1.9 0.0567 2.6 0.733

SE01-7

0.32 1.08 580.7±9.8

581.4±10

585±12

543±53

559±14

−6 10.57 1.8 0.06098 1.2 10.61 1.8 0.0584 2.4 0.759

SE01-5

3.51 0.18 583±10

588±10

591±10

252±170

270±98

−57 10.2 1.8 0.08017 1 10.57 1.8 0.0512 7.6 0.668

SE01-6

0.60 0.63 585.3±9.9

588±10

589±11

466±58

552±15

−20 10.46 1.8 0.06123 1.2 10.52 1.8 0.0563 2.6 0.738

SE01-1

1.56 0.54 606±9.6

607.1±9.8

608±11

549±37

582±13

−9 9.99 1.7 0.07125 0.37 10.15 1.7 0.05851 1.7 0.795

SE01-4⁎

0.45 1.04 1708±35

1713±39

1713±40

1659±37

1676±49

−3 3.281 2.3 0.1058 1.2 3.296 2.3 0.1019 2 4.26

Errors are 1-sigma; Pbc and Pb⁎ indicate the common and radiongenic portions, respictively.Error in standard calibration was 0.45% (not included in above errors but required when comparing data from different mounts).(1) Common Pb corrected using measured 204Pb.(2) Common Pb corrected by assuming 204Pb/238U–207Pb/235U age-concordance.(3) Common Pb corrected by assuming 206Pb/238U–208Pb/232Th age-concordance.⁎Data point not used in calculating age.

476 L. Janikian et al. / Gondwana Research 21 (2012) 466–482

by Janikian et al., 2005, 2008) is composed of fluvial conglomeratesdeposits that gradually pass into fluvio-deltaic sandstones andsiltstones (Fig. 2B). These deposits are overlain by pyroclastic flows,with a thickness of approximately 100 m, of moderately to highlywelded coarse-grained vitric, lithic and crystal tuffs. These tuffs aregradually overlain by 15 m of lapilli tuffs in massive tabular beds, with

Table 4U/Pb SHRIMP data for a rhyolitic clast from the fluvial deposits of the Acampamento Velho

(1) (2) (3) (1)

206Pb/238UAge

206Pb/238UAge

206Pb/238UAge

207Pb/206PbAge

Spot %206Pbc

ppmU

ppmTh

232Th/238U

ppm206Pb*

se08-3 0.37 713 2648 3.84 56 562.5±7.3

563±7.4

598±19 538±32

se08-9 0.41 403 175 0.45 31.8 564.5±7.6

565.5±7.7

567.3±8.2

509±54

se08-4 0.33 496 84 0.18 39.2 566.1±7.5

567±7.6

567.9±7.7

516±55

se08-1 0.36 463 939 2.09 36.7 567±7.5

568.1±7.6

580±11 504±36

se08-2 1.16 709 1813 2.64 56.9 569.1±7.4

570.7±7.5

583±13 482±54

se08-6 0.20 539 108 0.21 43 570.5±7.5

571.2±7.6

572.1±7.7

535±33

se08-10 0.34 546 141 0.27 43.8 573.6±7.5

574.7±7.7

575.8±7.8

512±39

se08-7 0.17 555 166 0.31 45 580.2±7.6

580.6±7.7

581.7±7.9

561±29

se08-11* 0.15 453 71 1.16 36.7 580.3±7.7

581±7.8

581.6±7.9

539±34

se08-5 0.09 1598 2262 1.46 131 587.4±7.4

588.3±7.5

595.8±9.5

538±14

se08-8* 0.41 319 108 0.35 26.5 594.1±8.1

596±8.3

596.9±8.6

492±60

Errors are 1-sigma; Pbc and Pb⁎ indicate the common and radiongenic portions, respictivelError in standard calibration was 0.34% (not included in above errors but required when co(1) Common Pb corrected using measured 204Pb.(2) Common Pb corrected by assuming 206Pb/238U–207Pb/235U age-concordance.(3) Common Pb corrected by assuming 206Pb/238U–208Pb/232Th age-concordance.⁎Data point not used in calculating age.

some levels being moderately to highly welded. Tuff-breccias occurabove the lapilli tuffs, comprising an approximately 200 m thicksuccession dominated by coarse-grained pyroclastic deposits com-posed of lithic fragments (mainly tuffs and acid lava flows rocks). Theupper part of the Acampamento Velho Formation is formed by acidvolcanic rocks, mainly rhyolites, up to 150 m in thickness. The

Formation (sample SE-08).

(1) %

208Pb/232ThAge

Discordant Total238U/206Pb

±% Total207Pb/206Pb

±% (1)238U//206Pb⁎

±% (1)207Pb⁎

/206Pb⁎

±%

533.2±7.6

−4 10.93 1.3 0.06126 0.77 10.97 1.4 0.05821 1.5

522±20 −10 10.88 1.4 0.0608 1 10.93 1.4 0.0574 2.4

492±40 −9 10.86 1.4 0.06033 0.93 10.9 1.4 0.0576 2.5

536.4±8.2

−11 10.84 1.4 0.06028 0.93 10.88 1.4 0.05733 1.7

546.7±8.1

−15 10.71 1.3 0.06622 0.73 10.83 1.4 0.0567 2.4

518±19 −6 10.78 1.4 0.05979 0.88 10.81 1.4 0.05815 1.5

517±19 −11 10.71 1.4 0.0603 0.88 10.75 1.4 0.0575 1.8

547±13 −3 10.6 1.4 0.0602 0.86 10.62 1.4 0.05883 1.3

524±24 −7 10.6 1.4 0.0595 0.97 10.62 1.4 0.05824 1.5

555.2±7.6

−8 10.47 1.3 0.05895 0.5 10.48 1.3 0.05822 0.66

542±22 −17 10.32 1.4 0.06037 1.3 10.36 1.4 0.057 2.7

y.mparing data from different mounts).

477L. Janikian et al. / Gondwana Research 21 (2012) 466–482

uppermost 50 m of this unit comprise reworked lapilli tuffs formed bygravity flows (Fig. 2B). This unit is intruded by thick subvolcanic basicigneous bodies.

U–Pb analyses in zircons from rhyolites of the AcampamentoVelho Formation from the Ramada Plateau region (Fig. 1B) indicatecrystallization ages of 573±18 Ma (Chemale, 2000) and 574±7 Ma(Janikian et al., 2005, 2008). Aminimum age of 572±3 Ma for the unitwas obtained in a granitic intrusion that cuts the rhyolites in theTaquarembó Plateau (Gastal and Lafon, 2001).

New U–Pb SHRIMP dating of zircons from two pebbles of rhyoliticcomposition from the conglomeratic fluvial deposits of the lowersuccession of the Acampamento Velho Formation yielded crystalliza-tion ages of 579±13 Ma (sample SE-01— Fig. 7A, Tables 3 and 4) and569±2.4 Ma (sample SE-08 — Fig. 7B, Table 4). These data indicate avolcanic source of similar age to the overlying rhyolites.

A rhyolite dike found at the Bom Jardim region yield an U–Pb ageof 572.2±6.5 Ma (sample BJL-31, obtained by LA ICPMS — Fig. 7C,Table 5), being the only evidence of the Acampamento VelhoFormation magmatism in the Central Camaquã Sub-basin.

7. The younger acid volcanism

The successions of the Camaquã Supergroup that crop out in theTaquarembó Plateau (Figs. 1B and 8A) begin with conglomerates andsandstones, which are overlain by pyroclastic rocks and finallyeruptive rhyolites. The lower sedimentary succession is approximate-ly 130 m thick, being composed of massive conglomerates rhythmi-cally intercalated with massive coarse sandstones. Pebblecompositions are dominantly acid, with 90% of oxidized vitricpyroclastic rocks and 10% of effusive acid rocks. Considering theregularity and tabular shape of the strata, these deposits areinterpreted as of fan-delta-front origin. Massive conglomeratesoccur in the upper 30 m of the succession, interbedded with fine totabular medium-grained sandstones with through and tabular cross-stratification. Fine-grained, rippled sandstones with mud drapes are

Table 5U/Pb TIMS data for a hypabissal rhyolite of the Acampamento Velho Formation (BJL-31 sam

Spot Ratio6/4

Ratio7/6

1 s(%)

2 s(%)

Ratio6/8⁎

1 s(%)

2 s(%)

Ratio7/5⁎

3-z1-3A40 6816 0.060 0.9 1.3 0.095 4.6 4.7 0.7904-z2 15,345 0.060 0.8 1.0 0.097 4.6 4.6 0.8065-z3-1a40(33,34) 72,968 0.059 0.8 0.8 0.094 4.6 4.6 0.7676-z4(1a19) 2365 0.059 0.8 1.1 0.093 4.6 4.6 0.7619-z5 61,607 0.059 0.8 0.8 0.091 4.6 4.6 0.74410-z6-14a38(19,30) 81,290 0.061 0.8 0.9 0.091 4.8 5.4 0.76711-z7-4a40(23,24) 86,920 0.061 0.8 0.9 0.095 4.6 4.6 0.79912-z8-1a40(20,21) 2325 0.060 0.8 0.8 0.098 4.6 4.7 0.871

16-z10-1a37 42,286 0.068 1.5 2.7 0.098 4.6 4.6 0.91117-z11-1a39 56,265 0.060 0.8 0.9 0.097 4.6 4.6 0.80718-z12-a22 2258 0.132 0.8 0.8 0.390 4.6 4.6 7.10021-z13 27,251 0.137 0.8 0.8 0.411 4.6 4.6 7.793

23-z15 57,923 0.135 0.7 0.8 0.411 4.6 4.6 7.67024-z16 2238 0.061 0.8 0.9 0.102 4.7 4.8 0.85627-z17 21,711 0.065 1.0 1.5 0.099 4.6 4.7 0.88628-z18-1a40(26,27) 20,956 0.062 1.0 1.5 0.095 4.6 4.7 0.81129-z19-128 47,054 0.059 0.8 0.8 0.094 4.6 4.7 0.75930-z20 2207 0.059 0.9 1.1 0.092 4.6 4.6 0.75536-z22-1a20 29,911 0.130 0.8 0.9 0.310 4.6 4.8 5.56337-z23 30,519 0.228 2.4 4.7 0.039 5.5 7.6 1.23938-z24-1a20 12,895 0.060 0.8 1.1 0.103 4.6 4.7 0.84530-z25 2165 0.059 1.0 1.6 0.093 4.6 4.7 0.76545-z25 9798 0.060 0.8 1.0 0.095 4.6 4.6 0.77846-z26-6a39 113,787 0.123 0.8 0.8 0.372 4.6 4.6 6.31844-z28-1a40(8-10,20-22-32-34) 148,194 0.125 0.9 1.2 0.353 4.6 4.8 6.09145-z29 2161 0.059 1.0 1.5 0.093 4.6 4.6 0.760

also present in this upper succession, which is interpreted as theproduct of bed-load-dominated rivers.

The acid volcanogenic succession is more than 130 m thick in theTaquarembó Plateau. It begins with an 11 m thick succession of metrictabular beds of interstratified massive and laminated coarse tuff, withoxidized intervals. These facies are interpreted as ash-fall deposits.Overlaying the coarse tuffs are metric tabular beds of welded massivelapilli tuffs and ignimbrites, constituting an 8 m thick succession. Weinterpret these facies as pyroclastic flow deposits, formed in subaerialenvironments. More than 110 m of the upper portion of thisformation consists of eruptive acid rocks, principally porphyriticrhyolites with flow structures.

We obtained a concordant age of 544.2±5.5 Ma in zircon from thesample DPM-155 by LA ICMPS (Fig. 8B, Table 6) in lapilli tuffs of theTaquarembó Plateau, interpreted as crystallization age. Our resultreveals that this succession is considerably younger than theAcampamento Velho Formation.

Previous works considered this unit as part of the AcampamentoVelho Formation (e.g. Wildner and Nardi, 1999, 2002; Sommer et al.,2006). A similar age of 549.3±5 Ma, was obtainedwith U–Pb SHRIMPmethod by Sommer et al. (2005) in rhyolitic hypabyssal rocks fromthe Ramada Plateau region, attesting the regional character of thisyounger magmatism.

These younger acid volcanic rocks are spatially and chronologicallydistinct from the other volcanogenic units of the Camaquã Supergroup(Fragoso-Cesar et al., 2000; Almeida, 2005; Janikian et al., 2008), andwere formed after the Acampamento Velho Formation and probablybefore the Santa Bárbara Group.

Based onmineralogical and trace andmajor element data from thebimodal mildly alkaline magmatism of the Acampamento VelhoFormation, Sommer et al. (2006, p. 585) recognized two magmaticsequences (both in the Taquarembó and the Ramada plateaus) thatwere referred to as High-Ti–P basalts–rhyolites and Low-Ti–P basalts–rhyolites. The compositional differences were “attributed to differentmelt fractions from a dominantly lithospheric mantle, previously affectedby subduction” (Wildner and Nardi, 2002; Sommer et al., 2006).

ple).

1 s(%)

2 s(%)

Age7/6

1 s(Ma)

2 s(Ma)

Age6/8

1 s(Ma)

2 s(Ma)

Age7/5

1 s(Ma)

2 s(Ma)

Rho

4.7 4.8 604.8 19.7 27.9 587.6 25.9 26.2 591.1 21.1 6.6 1.04.7 4.7 606.8 17.9 22.6 598.7 26.3 26.4 591.1 21.2 6.8 1.04.7 4.7 582.8 16.5 17.7 576.5 25.4 25.4 600.4 20.6 6.4 1.04.7 4.7 577.7 4.8 6.1 573.8 26.4 26.5 577.8 20.8 21.1 1.04.7 4.7 568.0 16.4 17.4 563.9 24.9 25.0 574.6 6.2 6.2 1.04.9 5.5 646.6 16.6 18.5 561.0 25.9 29.3 564.7 6.6 6.6 1.04.7 4.7 641.4 17.0 19.8 584.2 25.7 25.9 578.3 6.7 6.7 1.04.7 4.7 609.6 4.6 4.9 605.6 28.0 28.3 596.0 22.0 22.0 1.0

4.8 5.4 855.7 30.9 55.9 601.2 26.4 26.6 657.5 23.4 7.8 0.94.7 4.7 615.2 16.8 19.2 506.6 26.3 26.4 600.5 21.2 6.8 1.04.7 4.7 2126.3 16.1 17.2 2121.6 97.6 97.8 2124.0 42.9 43.0 1.04.7 4.7 2194.4 13.2 14.1 2221.4 86.5 86.8 2207.4 42.0 65.2 1.0

4.7 4.7 2170.3 13.1 13.5 2217.5 86.4 86.7 2193.1 41.9 64.2 1.04.7 4.9 631.4 5.0 5.8 627.3 29.2 30.3 628.2 22.7 23.6 1.04.7 5.0 770.0 20.9 32.0 609.1 26.9 27.5 644.3 22.6 7.5 1.04.7 4.9 665.7 20.7 31.1 586.6 25.9 26.1 603.1 21.4 6.8 1.04.7 4.8 555.3 16.7 18.1 577.7 25.6 26.1 573.2 20.6 6.4 1.04.7 4.8 573.8 4.9 6.5 570.2 26.3 26.4 570.9 21.0 21.3 1.04.7 4.9 2098.2 13.9 16.2 1742.3 70.9 72.9 1910.4 40.5 46.8 1.06.0 8.9 3038.8 39.2 75.7 249.1 13.4 18.6 818.5 33.8 11.3 0.84.7 4.9 591.3 18.1 23.4 630.3 27.8 28.5 621.9 21.9 7.1 1.04.7 5.0 579.7 5.9 9.2 576.1 26.7 27.3 576.9 21.6 22.8 0.94.7 4.7 585.4 4.8 6.0 584.4 26.9 27.1 588.6 20.8 21.1 1.04.7 4.7 2004.3 13.3 13.8 2037.4 80.5 80.8 2021.0 40.9 52.9 1.04.7 4.9 2029.2 15.4 20.9 1950.5 78.2 80.7 1988.9 41.3 51.4 1.04.7 4.9 576.4 5.6 8.5 573.3 26.4 26.6 573.9 21.6 22.3 1.0

Fig. 8. A — Composite stratigraphic section of the sedimentary and volcanigenic successions of the younger acid volcanism from Dom Pedrito region in the Taquarembó Plateau(Western Camaquã Sub-basin). B — U–Pb (LA-ICPMS) zircon age of lapilli tuff of the younger acid volcanism from Dom Pedrito region.

478 L. Janikian et al. / Gondwana Research 21 (2012) 466–482

Considering these geochemical data and the gochronologicalconstraints, we propose the distinction of two acid volcanic eventspossibly related to the recurring activation of extensional structures.

The first event is identified with the Acampamento Velho Formation,with an age of ca. 570 Ma (e.g. Chemale, 2000; Gastal and Lafon, 2001;Janikian et al., 2008), and the second with an age of ca. 545 Ma,

Table 6U/Pb LA ICPMS data for a lapilli tuff deposit of the younger volcanism from Dom Pedrito region (DPM-155 sample).

Spot Ratio6/4

Ratio7/6*

1 s(%)

2 s(%) Ratio6/8*

1 s(%)

2 s(%)

Ratio7/5*

1 s(%)

2 s(%)

Age7/6

1 s(Ma)

2 s(Ma)

Age(6/8)

1 s(Ma)

2 s(Ma)

Age7/5

1 s(Ma)

2 s(Ma)

Rho

03-z1 6208 0.061 1.0 1.2 0.091 2.0 2.1 0.765 2.3 2.4 642.0 21.3 26.4 560.8 10.9 11.3 577.1 9.9 10.7 0.904-z2 3502 0.059 1.1 1.5 0.091 2.0 2.1 0.738 2.3 2.6 561.6 6.0 8.2 561.2 11.4 11.8 561.2 10.5 11.7 0.805-z3 5706 0.154 0.9 0.9 0.453 2.1 2.2 9.641 2.2 2.4 2395.9 15.4 15.6 2407.2 41.2 44.2 2401.1 20.6 21.9 0.908-z4 15,505 0.063 1.1 1.7 0.094 2.1 2.4 0.824 2.4 2.9 719.7 24.2 35.2 581.3 11.7 13.1 610.3 10.9 13.2 0.809-z5 4157 0.062 1.0 1.2 0.088 2.0 2.2 0.752 2.3 2.5 663.6 21.1 25.9 546.0 10.7 11.4 569.3 9.9 10.9 0.910-z6 2297 0.058 1.0 1.2 0.084 2.0 2.2 0.668 2.3 2.5 520.0 5.2 6.4 519.2 10.6 11.3 519.4 10.2 11.2 0.913-z7 6384 0.155 0.9 1.0 0.427 2.0 2.1 9.154 2.2 2.3 2407.1 15.5 16.2 2292.3 38.9 40.0 2353.6 20.3 20.9 0.914-z8 2957 0.058 1.0 1.2 0.087 2.0 2.1 0.703 2.2 2.4 541.9 5.3 6.4 540.5 10.9 11.1 540.7 9.9 10.4 0.915-z9 2179 0.058 2.0 3.6 0.088 2.0 2.1 0.707 2.8 4.2 542.8 10.6 19.5 543.3 11.0 11.4 11.4 13.5 19.9 0.518-z10 1728 0.058 3.1 6.1 0.079 2.9 4.7 0.930 4.3 7.7 1322.9 60.6 117.4 590.3 13.9 22.4 22.4 21.0 37.6 0.620-z12 2957 0.059 0.9 1.0 0.089 2.0 2.1 0.716 2.2 2.4 550.2 5.2 5.8 548.2 11.1 11.6 11.6 9.8 10.4 0.923-z13 8770 0.112 1.3 2.0 0.299 2.1 2.4 4.635 2.5 3.1 1836.1 22.7 35.6 1688.6 31.4 35.8 35.8 20.5 26.0 0.824-z14 5048 0.060 1.0 1.1 0.089 2.0 2.1 0.743 2.2 2.4 614.4 20.9 24.8 551.5 10.7 10.9 10.9 9.7 10.2 0.9

479L. Janikian et al. / Gondwana Research 21 (2012) 466–482

recognized in the Taquarembó Plateau and possibly related to thesubvolcanic rhyolites of the Ramada Plateau region, dated by Sommeret al. (2005) at 549 Ma.

8. Sm–Nd results and discussions

Two Sm–Nd isotopic analyses of andesites of the Hilário Formationfrom the Western Camaquã Sub-basin suggest mixing of Neoproter-ozoic and Paleoproterozoic sources, with TDM model ages of 1.38 Ga(sample LLA01C) and 1.34 Ga (sample LLA544), and presentingnegative εNd (t) values, around −3.22 and −2.93, respectively.(εNd (t) calculated considering the crystallization age of 590 Ma).Previous isotopic Sm–Nd data for the volcanic rocks for the HilárioFormation (Bom Jardim Group) reveal TDM model ages of 1.2 Ga and1.8 Ga, with values of ε(590) from −1.52 to −8.98 (Chemale, 2000,see Table 1), interpreted as the result of varied proportions of crustalcontamination of a juvenile mantle source. Mesoproterozoic terraneswere reported in Uruguay, from Punta del Este Terrane (Basei et al.,2000), Cuchilla Dionisio-Pelotas and Nico Pérez terranes (Bossi andGaucher, 2004) and from the Rocha Group (Basei et al., 2005).Nevertheless, no Mesoproterozoic event has so far been identified inthe south portion of Brazil (Leite et al., 2000; Gastal et al., 2005a,b),and therefore the hypothesis of mixed sources to explain theMesoproterozoic model ages is favored.

Fig. 9. εNd(t) evolution in time for the inves

Sm–Nd isotopic analyses of samples representative of theAcampamento Velho Formation (samples LRA392, LRA305, LRA303e KBU58) reveal extremely negative values of εNd (t) at −9.57 to−13.12, for To=574 Ma, and TDM between 1.84 and 2.09 Ga,suggesting a Paleoproterozoic source region. These values clearlyindicate the contribution of the Paleoproterozoic basement even insamples located in the western part of the basin, where only theNeoproterozoic Rio Vacacaí Terrane crops out. Similar results for theAcampamento Velho Formation obtained by Chemale (2000) andAlmeida et al. (2003, 2005) (see Table 1) show values of ε(Nd) from−2.97 to −10.31 and TDM model ages of 1.11 Ga to 2.17 Ga,interpreted as a significant crustal contribution.

One sample (LRA305) presents positive εNd (t) values at +4.58and TDM model age around 0.83 Ga (Table 2). This could possiblyindicate a juvenile origin, although the TDM model age of 0.83 Ga(Table 2 and Fig. 9) suggests contribution from the Rio VacacaíTerrane. Similar values have been obtained for rocks from this terrane,specifically from the Cambaí Complex (Babinski et al., 1996). Thus, thevolcanic rocks of the Acampamento Velho Formation have signaturesattributed to different proportions of contributions of the Paleopro-terozoic basement and the Neoproterozoic Rio Vacacaí intra-oceanicisland-arc terrane, corroborating the model of obduction of the latterover the Paleoproterozoic basement of the Rio de La Plata Craton andthe western border of the Dom Feliciano Belt (Fragoso-Cesar, 1991).

tigated rocks and their possible sources.

480 L. Janikian et al. / Gondwana Research 21 (2012) 466–482

Contamination by Paleoproterozoic crust is also inferred for thesub-volcanic andesites that intrude the lower Picada das GarçasFormation in the Central Camaquã Sub-basin (sample BJL-111), whichshow TDM model age at 2.12 Ga (Table 2) and negative εNd (t) valuesat−13.37 and also, in the Central Camaquã Sub-basin, where youngerintrusive bodies of 547.2±9.1 Ma (Janikian, 2004), cut the volcanicrocks of the Hilário Formation, presenting TDM model ages between2.06 Ga (sample BJL-31B) and 1.92 Ga (sample BJL-54), respectively.

The Caçapava do Sul Granite, which is considered as comagmaticto the studied volcanic rocks (e.g. Chemale, 2000) yielded the oldestTDM (2.49 Ga) of all samples and the most negative εNd (−18.80),suggesting Archean contamination.

Fig. 9 compares the εNd evolution of the investigated rocks withpossible sources (Rio Vacacaí terrane, Santa Maria Chico Complex andDom Feliciano Belt). In this scenario, it is clearly seen thatAcampamento Velho Formation and the younger intrusive bodiesare related to the Dom Feliciano rocks (Pinheiro Machado suite andEncantadas Gneisses), with the exception of the sample with positiveεNd (LRA305), which probably has some influence from Rio Vacacaíterrane. The Hilário Formation evolution is plotted just between thePaleoproterozoic (Dom Feliciano) and Neoproterozoic rocks evolu-tion, indicating the mixed source or contamination of heterogeneouscrust. The Caçapava Granite is the only unit with Archeancontamination.

9. Conclusions

The nature and timing of the tectonic processes that occurred afterthe main collisional events of the Brasiliano orogenic system in SouthAmerica are an important and still unsolved question in the history offormation of Gondwana. In this context the Ediacaran Bom JardimGroup and Acampamento Velho Formation (Camaquã Basin — SEBrazil) constitute a rare record of the first stages of basin formation(dated between ca 605 and 570 Ma), revealing basin-forming eventsof a period that elsewhere records only plutonic activity.

The Bom Jardim Group (~605 to 580 Ma) is constituted by threeunits. The lower Cerro da Angélica Formation records the initialextensional stage of the basin, when the basin was restricted to thearea of the Central Camaquã Sub-basin and limited by normal faults(Janikian et al., 2003, 2005). The onset of major volcanic activity wasaround 590 Ma, represented by the Hilário Formation. At this time thebasin expanded to the west, where the volcanic centers probablydeveloped in a subaerial environment, while the depocenter was stillin the Central Camaquã Sub-basin, in a lacustrine environment. Afterthe volcanic events, the basin expanded even more, with no evidenceof proximal border faults in the upper levels of the Picada das GraçasFormation, which is probably related to post-rift thermal subsidence(Fig. 3).

The Acampamento Velho Formation records a second volcanicevent that occurred at 574–570 Ma and is characterized by thedominance of acid rocks formed essentially in subaerial environmentsand occupying a wider area than the previous units (Figs. 1B and 3).New geochronological data reveals a third volcanic event, whichoccurred at approximately 544 Ma and overlies the basement rocks ina much wider area (Figs. 1B and 3).

Whole-rock Sm–Nd isotopic studies reveal a large contribution ofidentifiable crustal units of Neoproterozoic and Paleoproterozoic agein the origin of the volcanic rocks of the Camaquã Supergroup (Fig. 9).A range of ε Nd from −2.9 to −18.8 for the western occurrences ofthe Acampamento Velho and Hilario Formations is observed, withTDMS ranging from Paleoproterozoic to Neoproterozoic. We interpretthe Mesoproterozoic model ages as reflective of mixing of materialfrom different source regions. In the eastern occurrences the sourceswere exclusively Paleoproterozoic, indicating that the basin overliesPaleoproterozoic basement in the east and a Neoproterozoic terranein the west. The less negative values and younger TDM model ages for

the Hilário Formation may reflect mixing of the two distinct crustalunits of the basement of the area where the samples were collected,the Neoproterozoic Rio Vacacaí juvenile terrane (sensu Fragoso-Cesar,1991) and the Paleoproterozoic basement. This interpretation isconsistent with the fact that the only sample with positive εNd(+4.55), an acid volcanic rock from the Acampamento VelhoFormation, shows a TDM model age (0.83 Ga) approximately 250 Maolder than the crystallization age of the unit, but very similar to theTDM of the Rio Vacacaí Terrane. Thus, the apparently juvenile sourceand the Neoproterozoic model age of this sample may also beinterpreted as a heritage of the previous orogenic event.

The evolution from a small basin bounded by normal faults to ashallower and wider basin onlapping older units and the basementtowards the west strongly suggests a transition from an active,extensional basin to regional subsidence due to thermal decay. Thespatial distribution of probable source regions for the crustal inputinto the volcanic rocks reveals two distinct basin domains withcontributions from both of the main geological provinces of thebasement, the Neoproterozoic Rio Vacacaí juvenile terrane and thePaleoproterozoic northern part of the Rio de La Plata Craton.

Acknowledgments

We thank FAPESP – Fundação de Amparo à Pesquisa do Estado deSão Paulo – for graduate scholarships and research grants (03/12802-2, 05/53522-8, 05/57939-0, 09/53362-1), and the technical staff of thegeochronological labs of the Geosciences Institute, São PauloUniversity.

References

Almeida, R.P., 2001. Evolução tectono-sedimentardaFormaçãoSantaBárbaranaSub-baciaCamaquã Ocidental, RS. Master dissertation. São Paulo University. 160 p.

Almeida, R.P., 2005. Tectônica e sedimentação dos grupos Santa Bárbara (Neoproterozóico,RS), Guaritas (Cambriano, RS) e Caacupé (Ordoviciano, Paraguai Oriental): trêsexemplos de bacias do intervalo entre a Orogenia Brasiliana e o estabelecimento dassinéclises cratônicas paleozóicas da América do Sul. Ph.D thesis, São Paulo University.

Almeida, D.P.M., Zerfass, H., Basei, M.A., Petry, K., Gomes, C.H., 2002. The AcampamentoVelho Formation, a Lower Cambrian bimodal volcanic package: geochemical andstratigraphics studies from the Cerro do Bugio, Perau and Serra de Santa Bárbara(Caçapava do Sul, Rio Grande do Sul, RS–Brazil). Gondwana Research 5, 721–733.

Almeida, D.P.M., Borba, A.W., Chemale Jr., F., Koester, E., Conceição, R.V., 2003. Isotopicsignature of the Acampamento Velho and Rodeio Velho volcanic successions fromthe Camaquã Basin, Southern Brazil. IV South American Symposium on IsotopeGeology: Short Papers, pp. 491–494.

Almeida, D.P.M., Conceição, R.V., Chemale-Jr, F., Koester, E., Borba, A.W., Petry, K., 2005.Evolution of heterogeneous mantle in the Acampamento Velho and Rodeio Velhoevents, Camaquã Basin, Southern Brazil. Gondwana Research 8, 479–492.

Almeida, R.P., Janikian, L., Fragoso-Cesar, A.R.S., Marconato, A., 2009. Evolution of a riftbasin dominated by subaerial deposits: the Guaritas Rift, Early Cambrian, SouthernBrazil. Sedimentary Geology 217, 30–51.

Almeida, R.P., Janikian, L., Fragoso-Cesar, A.R., Fambrini, G.L., 2010. The Ediacaran toCambrian rift system of Southeastern South America: tectonic implications. TheJournal of Geology 118, 145–161.

Babinski, M., Chemale-Jr, F., Hartmann, L.A., Van Schmus, W.R., Silva, L.C., 1996. Juvenileaccretion at 750–700 Ma in southern Brazil. Geology 24, 439–442.

Basei, M.A.S., Siga Jr., O., Masquelin, H., Harara, M., Reis Neto, J.M., Preciozzi, P.E., 2000. TheDom Feliciano Belt of Brazil and Uruguay and its foreland domain, the Rio de La PlataCraton. In: Cordani, U.G., Milani, E.J., Thomaz Filho, A., Campos, D.A. (Eds.), Tectonicevolution of South America, International Geological Congress, 31, pp. 311–334.

Basei, M.A.S., Frimmel, H.E., Nutman, A.P., Preciozzi, F., Jacob, J., 2005. A connectionbetween the Neoproterozoic Dom Feliciano Belt (Brazil/Uruguay) and Gariep(Namibia/South Africa) orogenic belts — evidence from a reconnaissanceprovenance study. Precambrian Research 139, 195–221.

Borba, A.W., Mizusaki, A.M.P., Santos, J.O.S., McNaughton, N.J., Ono, A.T., Hartmann, L.A.,2008. U–Pb zircon and 40Ar–39Ar K-feldspar dating of syn-sedimentary volcanismof the Neoproterozoic Maricá Formation: constraining the age of foreland basininception and inversion in the Camaquã Basin of southern Brazil. Basin Research 20,359–375.

Bossi, J., Gaucher, C., 2004. The Cuchilla Dionisio Terrane, Uruguay: an allochthonousblock accreted in the Cambrian to SW-Gondwana. Gondwana Research 7, 661–674.

Brito Neves, B.B., Campos Neto, M.C., Fuck, R., 1999. From Rodinia to eastern Gondwana:an approach to the Brasiliano–Pan African cycle and orogenic collage. Episodes 22,155–166.

Bühn, B., Pimentel, M.M., Matteini, M., Dantas, E.L., 2009. High spatial resolutionanalysis of Pb and U isotopes for geochronology by laser ablation multi-collector

481L. Janikian et al. / Gondwana Research 21 (2012) 466–482

inductively coupled plasma mass spectrometry (LA-MC-ICP-MS). Anais daAcademia Brasileira de Ciências 81, 99–114.

Chemale Jr., F., 2000. Evolução geológica do Escudo Sul-rio-grandense. In: Holz, M., DeRos, L.F. (Eds.), Geologia do Rio Grande do Sul, pp. 13–52.

Chemale Jr., E., Hartmann, L.A., Silva, L.C., 1995. Stratigraphy and tectonism of theBrasiliano Cycle in southern Brazil. Communication Geology Surveys Namibia 10,151–166.

Cordani, U.G., Sato, K., Teixeira, W., Tassinari, C.C.G., Basei, M.A.S., 2000. Crustalevolution of the South American Platform. In: Cordani, U.G., Milani, E.J., Thomaz, A.,Campos, D.A. (Eds.), Tectonic Evolution of South America, International GeologicalCongress, 31, pp. 19–40.

DePaolo, D.J., 1981. Nd isotopic studies; some new perspectives on Earth structure andevolution. Eos, Transactions, America Geophysics Union 62 (14), 137–140Washington, DC, United States.

DePaolo, D.J., 1988. Neodymium Isotope Geochemistry: An Introduction. Springer-Verlag ed., New York.

DePaolo, D.J., Linn, A.M., Schubert, G., 1991. The continental crustal age distribution;methods of determining mantle separation ages from Sm–Nd isotopic data andapplication to the Southwestern United States. Lawrence Berkeley Lab., BerkeleyCent. Isot. Geochem., Berkeley, CA, United States. Journal Geophysical Research, B,Solid Earth and Planets 96, 2071–2088.

Fambrini, G.L., 2003. O Grupo Santa Bárbara (Neoproterozóico III) da Bacia do Camaquã,Rio Grande do Sul. Ph.D. thesis, São Paulo University, 293 p.

Fambrini, G.L., Janikian, L., Almeida, R.P., Fragoso-Cesar, A.R.S., 2005. O Grupo SantaBárbara (Neoproterozóico III) na sub-bacia Camaquã Central, RS: sistemasdeposicionais, paleogeografia e implicações tectônicas. Revista Brasileira Geociên-cias 35, 227–238.

Fernandes, L.A.D., Tommasi, A., Porcher, C.C., 1992. Deformation patterns in thesouthern Brazilian branch of the Dom Feliciano Belt: a reappraisal. Jounal of SouthAmerica Earth Sciences 5, 77–96.

Fernandes, L.A.D., Menegat, R., Costa, A.F.U., Koester, E., Porcher, C.C., Tommasi, A.,Kraemer, G., Ramgrab, G.E., Camozzato, E., 1995. Evolução tectônica do CinturãoDom Feliciano no Escudo Sul-rio-grandense: parte I. Uma contribuição a partir doregistro geológico. Revista Brasileira Geociências 25, 351–374.

Fragoso-Cesar, A.R.S., 1991. Tectônica de Placas no Ciclo Brasiliano: as orogenias dosCinturões Dom Feliciano e Ribeira no Rio Grande do Sul. Ph.D. thesis, São PauloUniversity, 366 p.

Fragoso-Cesar, A.R.S., Lavina, E.L., Paim, P.S.G., Faccini, U.F., 1984. A Antefossa Molássicado Cinturão Dom Feliciano no Escudo do Rio Grande do Sul. CONGRESSOBRASILEIRO DE GEOLOGIA, 33, Rio de Janeiro, RJ, Brasil, SBG 7, pp. 3272–3283.

Fragoso-Cesar, A.R.S., Fambrini, G.L., Almeida, R.P., Pelosi, A.P.M.R., Janikian, L.,Riccomini, C., Machado, R., Nogueira, A.C.R., Saes, G.S., 2000. The Camaquãextensional basin: Neoproterozoic to early Cambrian sequences in southernmostBrazil. Revista Brasileira Geociências 30, 438–441.

Fragoso-Cesar, A.R.S., Fambrini, G.F., Riccomini, C., Janikian, L., Almeida, R.P., Pelosi,A.P.M.R., Machado, R., 2001. Estruturas induzidas por abalos sísmicos na SeqüênciaSanta Bárbara (Neoproterozóico III-Eocambriano), Bacia do Camaquã, RS: oexemplo do Passo da Capela. Revista Brasileira Geociências 31, 155–162.

Fragoso-Cesar, A.R.S., Fambrini, G.L., Paes de Almeida, R., Pelosi, A.P.M.R., Janikian, L.,2003. A Bacia Camaquã: Um sistema intracontinental anorogênico de rifts doNeoproterozóico III — Eopaleozóico no Rio Grande do Sul. I Encontro Sobre aEstratigrafia do Rio Grande do Sul: Escudos e Bacias. Boletim de Resumos, pp.139–144.

Frantz, J.C., Botelho, N.F., Pimentel,M.M., Potrel, A., Koester, E., Teixeira, R.S., 1999. Relaçõesisotópicas Rb–Sr e Sm–Nd e idades do magmatismo granítico no Rio Grande do Sul:evidências de retrabalhamento de crosta continental paleoproterozóica. RevistaBrasileira Geociências 29, 227–232.

Gastal, M.C.P., Lafon, J.M., 2001. Novas idades 207Pb/206Pb e geoquímica Nd/Sr paragranitóides shoshoníticos e alcalinos das regiões de Lavras do Sul e Taquarembó,RS. Congr. Bras. Geoqu., 7, Curitiba…SBGP Anais CD-Room, no 094. 7 pp.

Gastal, M.C.P., Lafon, J.M., Hartmann, L.A., Koester, E., 2005a. Sm–Nd isotopiccompositions as a proxy for magmatic processes during the Neoproterozoic ofthe southern Brazilian shield. Journal of South American Earth Sciences 18,255–276.

Gastal, M.C.P., Lafon, J.M., Hartmann, L.A., Koester, E., 2005b. Sm–Nd isotopicinvestigation of Neoproterozoic and Cretaceous igneous rocks from southernBrazil: a study of magmatic processes. Lithos 82, 345–377.

Gresse, P.G., Chemale Jr., F., Silva, L.C., Walravens Hartman, L.A., 1996. Late- to post-orogenic basins of the Pan-African-Brasiliano collision orogen in southern Africaand southern Brazil. Basin Research 8, 157–171.

Hartmann, L.A., 1998. Deepest exposed crust of Brazil — geochemistry of Paleoproter-ozoic depleted Santa Maria Chico granulites. Gondwana Research 1, 131–141.

Issler, R.S., 1982. Evento geodinâmico Brasiliano — fechamento de oceano e colisãocontinental dos crátons Rio de la Plata e Dom Feliciano: granitos a duas micas eofiolitos. CONGRESSO BRASILEIRO DE GEOLOGIA, 32, Salvador, BA, Brasil, SBG 1, pp.24–29.

Issler, R.S., 1985. Bacia Periférica Camaquã-Itajaí: elemento tectônico desenvolvido pelatectônica de placas. SBG, II Simpósio Sul-Brasileiro de Geologia, Atas…, 2, pp.184–198.

Janikian, L., 2004. Seqüências deposicionais e evolução paleoambiental do Grupo BomJardim e da Formação Acampamento Velho, Supergrupo Camaquã, Rio Grande doSul. Ph.D. thesis, São Paulo University. 189 p.

Janikian, L., Almeida, R.P., Fragoso-Cesar, A.R.S., Fambrini, G.L., 2003. Redefinição doGrupo Bom Jardim (Neoproterozóico III) em sua área-tipo: litoestratigrafia,paleogeografia e significado tectônico das sucessões vulcano-sedimentares doSupergrupo Camaquã, RS. Revista Brasileira Geociências 33, 349–362.

Janikian, L., Almeida, R.P., Fragoso-Cesar, A.R.S., Corrêa, C.R.A., Pelosi, A.P.M.R., 2005.Evolução paleoambiental e seqüências deposicionais do Grupo Bom Jardim eFormação Acampamento Velho (Supergrupo Camaquã) na porção norte da Sub-Bacia Camaquã Ocidental. Revista Brasileira Geociências 35, 245–256.

Janikian, L., Almeida, R.P., Trindade, R.I.F., Fragoso-Cesar, A.R.S., D'Agrella-Filho, M.S.,Dantas, E.L., Tohver, E., 2008. The continental record of Ediacaran volcano-sedimentary successions in southern Brazil and its global implications. Terra Nova20, 259–266.

Jost, J.A.D., 1981. Geology and metallogeny of the Santana da Boa Vista region, SouthernBrazil. Ph.D. thesis, University of Georgia, Athens.

Leitão, A.C.F., Janikian, L., Almeida, R.P., Fragoso-Cesar, A.R.S., Figueiredo, F.T., 2007.Proveniência de arenitos da Formação Cerro da Angélica (Grupo Bom Jardim,Ediacarano do RS) na porção sul da Sub-Bacia Camaquã Central e suas implicaçõestectônicas. Revista Brasileira de Geociências 37, 677–692.

Leite, J.A.D., 1995. Datação Schrimp U/Pb em zircões e o exemplo de dois corposgraníticos contrastantes no Escudo Sul-Riograndense. SBG/Núcleo RS, VI SimpósioSul-Brasileiro de Geologia/I Encontro de Geologia do Cone Sul, Porto Alegre.Boletim de Resumos Expandidos, pp. 5–12.

Leite, J.A.D., Hartmann, L.A., Fernandes, L.A.D., McNaughton, N.J., Soliani Jr., E., Koester,E., Santos, J.O.S., Vasconcelos, M.A.Z., 2000. Zircon U–Pb SHRIMP dating of gneissicbasement of Dom Feliciano Belt, southernmost Brazil. Journal of South AmericanEarth Sciences 13, 739–750.

Machado, N., Koppe, J.C., Hartmann, L.A., 1990. A Late Proterozoic U–Pb age for theBossoroca Belt, Rio Grande do Sul, Brazil. Journal of South American Earth Sciences2, 87–90.

Mantovani, M.S.M., Hawkesworth, C.J., Basei, M.A.S., 1987. Nd and Pb isotope studiesbearing on crustal evolution of southeastern Brazil. Revista Brasileira Geociências17, 263–268.

McPhie, J., Doyle, M., Allen, R., 1993. Volcanic textures: a guide to the interpretation oftextures in volcanic rocks. Center for Ore Deposits and Exploration Studies.University of Tasmania. . 198 pp.

Millisenda, C.C., Liew, T.C., Hofmann, A.W., Köhler, H., 1994. Nd isotopic mapping of theSri Lanka basement: up date and additional constraints from Sr isotopes.Precambrian Research 66, 95–110.

Nardi, L.V.S., Lima, E.F., 1985. A Associação shoshonítica de Lavras do Sul, RS. RevistaBrasileira Geociências 15, 139–146.

Nardi, L.V.S., Lima, E.F., 2002. Omagmatismo shoshonítico e alcalinodaBacia doCamaquã–RS. In: Holz, M., De Ros, L.F. (Eds.), Geologia do Rio Grande do Sul, pp. 119–131.

Oliveira, J.M.M.T., Fernandes, L.A.D., 1992. Bacias molássicas brasilianas, mito ourealidade? SBG/UNISINOS, I Workshop Sobre as Bacias Molássicas Brasilianas, SãoLeopoldo. Boletim de Resumos Expandidos, pp. 97–105.

Oyhantçabal, P., Siegesmund, S., Wemmer, K., Frei, R., Layer, P., 2007. Post-collisionaltransition from calc-alkaline to alkaline magmatism during transcurrent deforma-tion in the southernmost Dom Feliciano Belt (Braziliano–Pan-African, Uruguay).Lithos 98, 141–159.

Pelosi, A.P.M.R., Fragoso-Cesar, A.R.S., 2003. Considerações estratigráficas e paleogeo-gráficas do Grupo Maricá (Neoproterozóico III), Rio Grande do Sul. RevistaBrasileira Geociências 33, 137–148.

Reading, H.G., 1986. Facies, In: Reading, H.G. (Ed.), Sedimentary Environments andFacies, 2nd Edition. , pp. 4–19.

Reading, H.G., 1996. Sedimentary Environments: Processes, Facies and Stratigraphy.688 pp.

Ribeiro, M., Fantinel, L.M., 1978. Associações petrotectônicas do Escudo Sul-Riograndense:I Tabulação e distribuição das associações petrotectônicas do Escudo doRio Grande doSul. Ihneríngia, Série Geologia, Porto Alegre 5, 19–54.

Ribeiro, M., Lichtemberg, E., 1978. Síntese da Geologia do Rio Grande do Sul. Anais doXXX Congresso Brasileiro de Geologia 6, 2451–2463.

Ribeiro, M., Bocchi, P.R., Figueiredo Filho, P.M., Tessari, R.I., 1966. Geologia daQuadrícula de Caçapava do Sul, Rio Grande do Sul. Boletim 127, Rio de Janeiro,DNPM/DFPM. 232 p.

Roisenberg, A., Loss, E.L., Altamirano, J.A.F., Ferreira, A.C., 1983. Aspectos petrológicos egeoquímicos do vulcanismo Pré-cambriano — Eopaleozóico do Rio Grande do Sulcom base nos elementos maiores. Atas do I Simpósio Sul-Brasileiro de GeologiaPorto Alegre–RS, pp. 271–285.

Saalmann, K., Hartmann, L.A., Remus, M.V.D., Koester, E., Conceição, R.V., 2005. Sm–Ndisotope geochemistry of metamorphic volcano-sedimentary successions in the SãoGabriel Block, southernmost Brazil: evidence for the existence of juvenileNeoproterozoic oceanic crust to the east of the Rio de la Plata craton. GondwanaResearch 8, 143–161.

Sánchez-Bettucci, L., Cosarinsky,M., Ramos, V., 2001. Tectonic setting of the Late ProterozoicLavalleja Group (Dom Feliciano Belt), Uruguay. Gondwana Research 4, 395–407.

Sánchez-Bettucci, L., Oyhantçabal, P., Preciozzi, F., Loureiro, J., Ramos, V.A., Basei, M.A.S.,2004. Mineralizations of The Lavalleja Group (Uruguay), A NeoproterozoicVolcano-Sedimentary Sequence. Gondwana Research 7, 745–751.

Sánchez-Bettucci, L., Koukharsky, M., Pazos, P.J., Stareczek, S., 2009. Neoproterozoicsubaqueous extrusive–intrusive rocks in the Playa Hermosa Formation in Uruguay:regional and stratigraphic significance. Gondwana Research 16, 134–144.

Sato, K., 1997. The double stage Sm–Nd model age and aplications to Brazilian platformrocks. An. South Amer. Symp. Isotope Geol., 1. Campos do Jordão, Brasil, pp. 286–288.

Sato, K., Tassinari, C.C.G., Kawashita, K., Petronilho, L., 1995. OMétodoGeocronológico Sm–

Ndno IG/USP e suas aplicações. Anais da Academia Brasileira de Ciências 67, 313–336.Silva, C., Hartmann, L.A., McNaughton, N.J., Fletcher, I.R., 1999. SHRIMP U/Pb zircon

timing of Neoproterozoic granitic magmatism and deformation in the PelotasBatholith in southernmost Brazil. International Geological Review 41, 531–551.

Smith, J.B., Barley, M.E., Groves, D.I., Krapez, B., Mc-Naughton, N.J., Bickle, M.J., Chapman,H.J., 1998. The Sholl shear zone, West Pilbara: evidence for a domain boundary

482 L. Janikian et al. / Gondwana Research 21 (2012) 466–482

structure from the integrated tectonostratigraphic analysis. SHRIMP U–Pb dating andisotopic and geochemical data of granitoids. Precambrian Research 88, 143–171.

Soliani Jr., E., 1986. Os dados geocronológicos do Escudo Sul-Riograndense e suasimplicações de ordem geotectônica. Ph.D thesis, São Paulo University.

Sommer, C.A., Lima, E.F., Nardi, L.V.S., Liz, J.D., Pierosan, R., Waichel, B.L., 2003.Stratigraphy of the Acampamento Velho Alloformation in the Ramada Plateau, VilaNova do Sul region, RS. I Encontro Sobre a Estratigrafia do Rio Grande do Sul:Escudos e Bacias. Porto Alegre, pp. 105–110.

Sommer, C.A., Lima, E.F., Nardi, L.V.S., Figueiredo, A.M.G., Pierosan, R., 2005. Potassic andLow- and High-Ti mildly alkaline volcanism In the Neoproterozoic Ramada Plateau,Southermost Brazil. Journal of South American Earth Sciences 18, 237–254.

Sommer, C.A., Lima, E.F., Nardi, L.V.S., Liz, J.D., Waichel, B.L., 2006. The evolution ofNeoproterozoic magmatism in Southernmost Brazil: shoshonitic, high-K tholeiiticand silica-saturated, sodic alkaline volcanism in post-collisional basins. Anais daAcademia Brasileira de Ciências 78, 573–589.

Walker, R.G., James, N.P., 1992. Facies Models and Response to Sea-level Change.Geological Association of Canada Geotext, p. 1.

Wildner, W., Nardi, L.V.S., 1999. Características geoquímicas e petrogenéticas dovulcanismo neoproterozóico do sul do Brasil— Platô do Taquarembó–RS. I Simpósiosobre Vulcanismo e Ambientes Associados, Boletim de resumos, p. 30.

Wildner, W., Nardi, L.V.S., 2002. Características geoquímicas e de posicionamentotectônico do magmatismo neoproterozóico aflorante no Platô do Taquarembó.Revista Brasileira Geociências 32, 169–184.

Wildner, W., Sander, A., Lopes, R.C., 1994. Estudo petrológico e litoquímico de umaparcela do vulcanismo ácido cambriano do Rio Grande do Sul — FormaçãoAcampamento Velho. Pesquisas 21, 47–57.

Zerfass, H., Almeida, D.P.M., Gomes, C.H., 2000. Faciology of the Acampamento VelhoFormation volcanic rocks (Camaquã Basin) in the region of Serra de Santa Bárbara,Cerro do Perau and Cerro do Bugio (Municipality of Caçapava do Sul-RS). RevistaBrasileira Geociências 30, 375–379.