age, source, and regional stratigraphy of the roraima supergroup

For permission to copy, contact [email protected] 2003 Geological Society of America 331

GSA Bulletin; March 2003; v. 115; no. 3; p. 331–348; 15 figures; 3 tables; Data Repository item 2003034.

Age, source, and regional stratigraphy of the Roraima Supergroup andRoraima-like outliers in northern South America based on

U-Pb geochronology

Joao Orestes Schneider Santos†

Departamento de Geologia, Companhia de Pesquisa de Recursos Minerais (CPRM), Avenida Andre Araujo 2160, Manaus,Amazonas, 69060-001, Brazil, and Universidade Federal do Rio Grande do Sul (UFRGS), Avenida Bento Goncalves 9500,Porto Alegre, Rio Grande do Sul, 91500-000, Brazil

Paul Edwin PotterGeology Department, University of Cincinnati, Cincinnati, Ohio 45221-0013, USA

Nelson Joaquim ReisSuperintendencia Regional de Manaus, Companhia de Pesquisa de Recursos Minerais (CPRM), Avenida Andre Araujo 2160,Manaus, Amazonas, 69060-001, Brazil

Leo Afraneo HartmannInstituto de Geociencias, Universidade Federal do Rio Grande do Sul (UFRGS), Avenida Bento Goncalves 9500, Porto Alegre,Rio Grande do Sul, 91500-000, Brazil

Ian Robert FletcherNeal J. McNaughtonCentre for Global Metallogeny, University of Western Australia, Nedlands 6907, Australia

ABSTRACT

New 207Pb/206Pb ion-microprobe datesfrom the basement underlying the RoraimaSupergroup, from sandstones and tuffswithin the supergroup, and from associatedmafic intrusions establish new temporalframework for this major stratigraphic unitin northern South America. Zircons fromthe tuffs within the supergroup yield a Pa-leoproterozoic age of 1873 6 3 Ma, Orosi-rian. The minimum age of the Roraima Su-pergroup was determined by U-Pbgeochronology using baddeleyite and zirconfrom two mafic sills (Avanavero magma-tism) and is 1782 6 3 Ma. Zircons of someRoraima-like outliers indicate that they arepost-Roraima in age and do not belong tothe supergroup. This conclusion is support-ed by the unconformity between the Matauıand Uaimapue Formations present in thePacaraima Plateau, which may represent ahiatus as long as 320 m.y. Thus, all thesandstones above the unconformity such asthe Matauı Formation, as well as outliers

†E-mail: [email protected].

such as the Serra Surucucus, Araca, andNeblina units, do not belong to thesupergroup.

Rocks of the Roraima Supergroup andpost-Roraima sandstones were deposited intwo separate but overlapping basins, each;1,200,000 km2 in area. The Roraima Su-pergroup represents fill in a foreland basinthat was derived mostly from the Trans-Amazon orogenic belt to the north andnortheast, whereas the fill in the post-Roraima Neblina successor foreland basinwas derived from both the Trans-Amazonand Tapajos-Parima orogenic belts to theeast and northeast. Although most of bothbasins are largely flat lying or gently de-formed, some of the westernmost outliers ofthe post-Roraima sandstones were de-formed by the far-field effects of the Sunsas(Grenvillian) orogen at 1.33 Ga (Ar-Ar inmuscovite).

These results are based on U-Pb deter-minations of nine samples, Ar-Ar step-heating plateau age of one muscovite sam-ple, and a complete review of many earlierRb-Sr and K-Ar earlier dates.

Keywords: Amazon craton, baddeleyite, di-amond, Guyana Shield, Roraima, U-Pbgeochronology, zircon.

INTRODUCTION

The Roraima Supergroup was deposited ina large Paleoproterozoic foreland basin innorthern South America and is a major sourceof diamonds (Reid, 1974; Baptista and Svis-ero, 1978; Gibbs and Barron, 1993).

This paper uses U-Pb SHRIMP (sensitive,high-resolution ion-microprobe) dating ofbaddeleyite and magmatic and detrital zircon,along with all other available radiometricdates, to evaluate the stratigraphy of the Pro-terozoic Roraima Supergroup and its possibleoutliers in northern South America. The su-pergroup covers a large area in adjacent partsof Brazil, Venezuela, Guyana, Suriname, andColombia (Fig. 1) and consists mostly of hor-izontal and gently dipping fluvial sandstones(quartz arenites and arkosic sandstones). Alsopresent are some minor conglomerates, shales,and ash-fall tuffs and some diamond-bearingbeds. Knowledge of the source of the dia-monds in the Roraima Supergroup is closely

332 Geological Society of America Bulletin, March 2003

SANTOS et al.

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Geological Society of America Bulletin, March 2003 333

RORAIMA SUPERGROUP AND RORAIMA-LIKE OUTLIERS IN NORTHERN SOUTH AMERICA

Figure 2. Location of the Roraima (inclined gray lines) and Neblina (parallel black lines)sedimentary basins in relation to the geologic provinces of the Amazon craton: Imataca(Archean); TA—Trans-Amazon (2.25–2.00 Ga); TP—Tapajos-Parima (2.1–1.88 Ga); Cen-tral Amazon (1.87–1.78 Ga); RN—Rio Negro (1.83–1.52 Ga); and SU—Sunsas (1.33–1.00Ga). Limits between provinces in hatched thick black lines. Arrows indicate the mainpaleocurrent directions (as measured in the field, Reis et al., 1990) and as inferred fromages of zircons. The schematic section A–B shows the relationship between the two basinsand the provinces of the craton.

related to better understanding of its stratig-raphy and underlying basement.

Understanding the stratigraphy of thesesandstone sequences in this large area hasbeen difficult, because they are unfossiliferousand occur in both a large preserved plateau;73,000 km2 in area, the Pacaraima Plateauor (Reis and Yanez, 2001), and in isolated out-liers over a much larger area of ;1,350,000km2, approximately four times larger thanFrance. For example, the southwesternmostoutlier of this group in the Traıra Mountainson the Brazil-Colombia border is 1800 kmdistant from the main body. Other reasons forconflicting opinions about the age and corre-lation of these units include (1) these rocksspan five countries, cropping out mostly in re-mote areas covered by jungle where access isdifficult and expensive; (2) some units in thewestern part of the study area were either fold-ed or metamorphosed by the Grenvillian orog-eny locally known as the Sunsas orogeny,1.33–0.98 Ga; and (3) previous use of geo-chronology (Rb-Sr and K-Ar) that is unreli-able in local circumstances has caused mis-understanding of outcrop stratigraphy.

There are three major stratigraphic ques-tions to ask about the Roraima Supergroup inthe Pacaraima Plateau and distant outliers thathave been correlated with it: (1) Do the rocksof the plateau represent one pulse of arena-ceous deposition or several? (2) What is therelationship of the undeformed outliers to themain body of sandstone in the Pacaraima Pla-teau? (3) Are the westernmost deformed out-liers (Tunuı Group) older than the typesection?

We integrate evidence from basement ge-ology, many years of stratigraphic and sedi-mentological study of the Roraima Super-group in all five countries, published articles,the distribution of diamonds in the super-group, isotopic dates, and eight new SHRIMPdates to answer these three key questions.

REGIONAL SETTING

Basement

The Roraima and Roraima-like depositswere laid down over a large area on the north-ern and northwestern Amazon craton (Fig. 2).This broad region comprises five geologicprovinces of the craton (Santos et al., 2000),which have different age and structural pat-terns: the Archean Imataca province and theProterozoic Trans-Amazon, Tapajos-Parima,Central Amazon, Rio Negro, and Sunsas prov-inces. The Imataca province is a small blockin the extreme north of the craton composed

of rocks of granulite (possibly 3700–3100Ma) and amphibolite (2800–2700 Ma) facies(Teixeira et al., 1999). The Trans-Amazonprovince is a granitoid-greenstone terrane gen-erated between 2.25 and 2.00 Ga (Gibbs andOlszewski, 1982; Cox et al., 1993; Santos etal., 2000), whose structural trend broadly par-allels the Atlantic coast from Venezuela,through the Guianas to Amapa State in Brazil.Tapajos-Parima, on the other hand, is thename of an orogenic belt with north-northwesttrend, which extends from the Parima region(northwest Roraima State) to the southeastinto the Tapajos and Alta Floresta regions ofPara and Mato Grosso States of Brazil (Santoset al., 2000). Four magmatic arcs that formedbetween 2.01 and 1.88 Ga have been recog-nized in the Tapajos-Parima orogenic belt. InRoraima State, the most prominent is best de-veloped in the Creporizao arc, which repre-sents a volcano-plutonic association of (1)calc-alkalic monzogranites and (2) rhyodacitesand andesites formed between 1990 and 1950Ma. The volcanic rocks are named Surumu inBrazil (Barbosa and Ramos, 1959), Iwokramain Guyana (Berrange, 1977), and Caicara inVenezuela (Mendoza, 1975); the associated

granitoids are named Pedra Pintada suite andRio Urubu suite in Brazil (Fraga et al., 1996)and South Savanna in Guyana (Barron, 1966).

The Rio Negro province is formed mostlyby collisional S- and I-type granitoids withages from ca. 1600 to 1520 Ma, which wereintruded into a poorly known basement of ca.1800 Ma (Santos et al., 2000). The Sunsasprovince—located in the westernmost Ama-zon craton—was the site of another collisionalorogeny, which is correlated to the Grenvillianorogeny of Laurentia (Sadowski and Betten-court, 1996). The Sunsas orogeny producedmany shear belts that cut older provinces andmay be the cause of the metamorphism thatreset many Rb-Sr and K-Ar isotope ages.These shear belts are referred as the K’Mudkuepisode (Barron, 1966), also locally called‘‘Nickerie’’(Suriname; Priem et al., 1971) and‘‘Orinoquense’’ (Venezuela; Bellizzia, 1972).The main zone of shearing is named theK’Mudku belt (Santos et al., 2000).

Roraima Supergroup

The rocks of the Roraima Supergroup con-sist principally of sandstones derived from the


SANTOS et al.

Trans-Amazonian Mountains to the north anddeposited in braided, deltaic, and shallow-marine environments (Reis et al., 1990) in aforeland basin. The supergroup is best known(Fig. 1) where it is continuous and covers73,000 km2 in the Pacaraima Plateau in thenortheastern corner of the Roraima State ofBrazil (Reis et al., 1990), the southeastern partof the Bolivar State of Venezuela (Reid,1972), and the northwestern part of Guyana(Keats, 1973).

The supergroup everywhere forms tabularplateaus, cuestas, and hogbacks that abruptlyrise above Paleoproterozoic basement. The gi-ant table mountains are locally called tepuis(Figs. 3A and 3B) and are the highest moun-tains of the Amazon craton (up to 3016 mhigh). Thicknesses range from 200 m in someoutliers to ;3000 m in the Pacaraima Plateau.Local sedimentological studies show that en-vironments of deposition range from alluvialfans to fluvial braided deposits (Fig. 3D) pluslacustrine, eolian, tidal, shallow-marine de-posits and even some shallow-water turbidites(Ghosh, 1985; Castro and Barrocas, 1986;Reis and Yanez, 2001), although sandy con-tinental deposits predominate (Fig. 4). Thewidespread marine (?) shale in the CuquenanFormation is the key to future studies of thesequence stratigraphy of the Roraima Super-group and certainly deserves much more in-vestigation, starting with a search for possiblemicrofossils. It is possible that this shale couldbe useful in future continental correlations.

Nomenclature and subunits of the RoraimaSupergroup have evolved since the work ofDalton (1912) and have been summarized forthe area along the Brazil-Venezuela border(Reis and Yanez, 2001). There, the supergroupis composed, from bottom to top, of the fol-lowing units (Fig. 4): Arai, Suapi (Uiramuta,Verde, Paure, Cuquenan, Quino), Uaimapue,and Matauı. The lower units dip 88 to 208 tothe north-northeast, whereas the top unit, theMatauı Formation, is flat lying (Fig. 3B). Thischange in dip has been interpreted in threeways: the result of a gradual attenuation ofdips into the center of the basin as it was filled(Bouman, 1959; Montalvao et al., 1975), theresult of border dips that were induced by pos-itive inversion during the K’Mudku episode(Reis et al., 1990), and the result of an uncon-formity (Ramos, 1956; Amaral, 1974; Santosand D’Antona, 1984; Alberdi and Contreras,1995).

Along the northern margin of the PacaraimaPlateau, the supergroup directly overlies most-ly 2.25–2.00 Ga crystalline Trans-Amazonianrocks (Pastora and Carichapo Groups in Ve-nezuela and the Barama and Mazaruni Groups

in Guyana), whereas along the plateau’s south-ern side, the underlying basement is composedchiefly of the Uraricaa, Surumu, and Pacarai-ma volcanic rocks (1.96 Ga; Schobbenhaus etal., 1994) and the Cuchivero Group (Figs. 2and 3F).

Thick dolerite sills, up to 400 m thick (San-tos and D’Antona, 1984), as well as doleritedike swarms are intrusive into the supergroupand its basement rocks. Since 1963, these sillsand 199 dikes in Guyana (Snelling, 1963; Mc-Dougall et al., 1963), Venezuela (Onstott etal., 1984), Brazil (Montalvao et al., 1975), andSuriname (Hebeda et al., 1973) have been iso-topically analyzed to determine the minimumage for the supergroup, but it has been diffi-cult to reconcile published ages with knownstratigraphic relationships.

OVERVIEW OF STRATIGRAPHIC ANDISOTOPIC STUDIES

Historical Development

Stratigraphic ideas about the Roraima Su-pergroup date back to 1875 (Table DR1).1 Thefirst to mention the Roraima were Brown andSawkings (1875), who described giant tablemountains of sandstone in southwestern Guy-ana. The name ‘‘Roraima Series’’ was intro-duced later by Dalton (1912) in Venezuela,who used the name of the highest (2870 m)table mountain in the region at the triple bor-der between Venezuela, Brazil, and Guyana.Since Dalton’s first studies, some 40 papershave been published on the stratigraphy, age,and depositional environments of the RoraimaSupergroup (Table DR1).

Another name, ‘‘Kaiteur Series,’’ was laterproposed by Connoly (1925) in Guyana, whoused the Kaiteur Falls on the Potaro River asa reference name; despite the lack of fossils,these rocks were thought to be Mesozoic inage probably because they are essentially flatlying. In Brazil, Paiva (1939) correlated theRoro-ima (the correct Indian name for ‘‘Ro-raima’’) sandstone with the Minas Series ofcentral Brazil and suggested a Neoproterozoicage (Torridonian). Ramos (1956) and Barbosaand Ramos (1959) were the first to recognizean unconformity in the Roraima deposits, con-sidering the upper part as Triassic (RoraimaFormation) and the lower part as Cambrian–Ordovician (Kaiteur Formation). Others whorecognized this unconformity include Amaral

1GSA Data Repository item 2003034, Table DR1,Nomenclature and Age of Roraima Unit, and TableDR2, SHRIMP Data, is available on the Web athttp://www.geosociety.org/pubs/ft2003.htm. Re-quests may also be sent to [email protected].

(1970), Braun and Ramgrab (1972), Santosand D’Antona (1984), Santos (1985), and Al-berdi and Contreras (1995). With time, therocks to which the term ‘‘Roraima’’ is appliedhave evolved from formational rank (Gansser,1954) to group (Reid, 1972) and to super-group rank (Pinheiro et al., 1988). The foldedand/or metamorphosed sandstone units to thewest of the Pacaraima Plateau also have beennamed the Cinaruco Formation (McCandless,1962), La Esmeralda (Cirivieux, 1966), TunuıGroup (Pinheiro et al., 1976), La Pedrera(Galvis et al., 1979), Araca (Fig. 3C) (Meloet al., 1994a), and Moriche (Cox et al., 1993);therefore, many local names abound for thesewidespread cover rocks. For this broad groupof widely scattered but similar deposits we usethe informal term ‘‘Roraima-like outliers’’ asproposed in the next section.

New Stratigraphic Interpretation

We correlate the sedimentary deposits atTepequem Mountain outlier in Roraima State,those at Makari Mountain in Guyana, and inthe Tafelberg-Emmatekken Mountains in Su-riname with the Roraima Supergroup, becauseall three have interlayered felsic tuffs, intru-sion of Avanavero-type gabbroic rocks (Nor-cross et al., 2000; Santos et al., 2002), and thepresence of detrital diamonds. However, themajority of outliers lack these criteria. Be-sides, there is a great deal of evidence thatthey may be much younger than the super-group, because they were deposited over amuch younger basement, the rocks of the RioNegro province (Santos et al., 2000). For thesesedimentary rocks, whose correlation to thesupergroup is weak or not well established, wepropose the term ‘‘Roraima-like outliers.’’ Ex-amples of these include the Neblina, Daraa,and Padre (Giffoni and Abraao, 1969; Pinhei-ro et al., 1976), Araca (Montalvao et al., 1975;Borges, 1987; Giovannini and Larizzatti,1994), Serra Surucucus (Montalvao et al.,1975), and Duida, Autana, Paru, and Sipapounits (Ghosh, 1985; Cox et al., 1993).

The Roraima-like outliers to the west andnorthwest, the Araca Formation (Fig. 3C) andthe Tunuı Group, display variable grades ofmetamorphism and folding and were supposedto be pre-Roraima and associated with Arche-an granitoid-greenstone terranes (Melo et al.,1994b; Costa and Hasui, 1997). Santos et al.(2000), however, demonstrated that the TunuıGroup is younger than 1.91 Ga on the basisof the age of a detrital-zircon population. Thegroup may be correlated to the Roraima Su-pergroup or to post-Roraima deposits. Its de-formation probably is related to the develop-



Figure 3. (A) Radar image showing two giant table mountains: R—Roraima, and C—Cuquenan. Both are underlain by the Matauı Formation. Borders of Brazil, Venezuela,and Guyana are outlined in white. (B) View of northern Cuquenan Mountain with cliffsof 1000 m of the Matauı Formation. Hidden in the talus is the contact with the UaimapueFormation. Photo taken from Roraima Mountain top. The top flat surface is ;2550 mhigh. (C) Radar image showing Araca Mountain (Araca Formation), also a table moun-tain, with location of sample CG30. (D) A typical quartz-pebble diamond-bearing con-glomerate of Arai Formation near Mutum mining camp in Roraima State, Brazil. (E)Orthogonal fractures in an ash-fall tuff of the Uaimapue Formation. (F) Typical outcropof Surumu Group rhyodacite with columnar jointing. Radar images are side-look airborneradar, band X, from Projeto Radar na Amazonia (Departamento Nacional da ProducaoMineral). Photographs D and E taken by geologist Persio Mandetta; geologic hammer(handle 32 cm long) for scale.

ment of shear belts in the Rio Negro provinceassociated with the Sunsas orogeny (1.33–1.10 Ga) to the west.

This relatively young age suggests thatthere are at least two different Proterozoic sed-imentary cover sequences that are distinct inboth time and location. We recognize one ba-sin for the rocks of the Roraima Supergroup—the Roraima Basin—and another successorbasin that includes both the rocks above theunconformity seen in the Pacaraima Plateauand the Roraima-like outliers. The name‘‘Neblina Basin’’ is proposed for the younger

basin, derived from Neblina Mountain (thehighest point in Brazil), a notable giant tablemountain sustained by post-Roraima sand-stones. Both basins cover large areas from Su-riname in the east and southeast to Colombiain the west, implying deposition over a mini-mum area of ;1,350,000 km2 (Fig. 2) andcovering four provinces of the Amazon craton(Santos et al., 2000).

Available paleocurrent data suggest a north-ern source for the Arai Formation of the Ro-raima Basin in Brazil (Reis et al., 1990) anda dominantly western source for most sand-

stones in the tepuis of the Neblina Basin (Ro-raima, Cuquenan, Ayuan, and Autana—Ghosh[1985]; Alberdi and Contreras [1995]). Morepaleocurrent study of the sandstones of thesetwo distinct basins would clarify their dis-persal systems and probably identify principalpathways (paleotectonic lows). Another ques-tion to be answered is geomorphic—what hascaused these two Proterozoic sand-rich basinsto survive as residual mountains, includingNeblina, with elevations up to 3016 m andmuch of the Pacaraima Plateau, with eleva-tions between 700 and 2870 m? Are all theresidual mountains related to rift-shoulderhighlands that formed when South Americaseparated from Africa? Or is their existencesimply an accident of post-breakup erosion?

Earlier Isotopic Studies

Several attempts to date the Roraima Su-pergroup were made starting in the 1960s, byusing the Rb-Sr and K-Ar methods. Its mini-mum age may be determined by the age of thethick intrusions of dolerite sills, related toAvanavero magmatism (Groeneweg and Bos-ma, 1969; Santos et al., 1980), that occur inthe supergroup rocks. The maximum age, onthe other hand, is established by the age of thesubjacent volcanic rocks of the SurumuGroup, and the age of sedimentation of thesupergroup (Fig. 3F) can be determined bydating the felsic tuffs of the Uaimapue For-mation (Fig. 3E) (Reid, 1972). Ages obtainedby these researchers are in disagreement, be-cause the younger doleritic sills consistentlyyield Rb-Sr and K-Ar ages (1805 Ma; Baseiand Teixeira, 1975) older than the Rb-Sr ages(1580 Ma; Pringle and Teggin, 1985) of theTafelberg-type pyroclastic rocks. A problemwhen using K-Ar ages is that there is com-monly excess 40Ar irregularly distributed inthe dolerites, especially around plagioclasecrystals (Hebeda et al., 1973). This excess40Ar causes dates to be determined that are tooold. The opposite effect can occur when usingRb-Sr—there can be a loss of radiogenic 87Sr(whole-rock open-system behavior), whichproduces lower 87Sr/86Sr ratios and youngerdates. The 40Ar enrichment in the doleritesprobably is the result of contamination duringtheir injection along a long path through rocksolder than 1.96 Ga (Surumu Group age,Schobbenhaus et al., 1994) into the Roraima.It has also been suggested that 87Sr loss is aresult of the K’Mudku episode at 1.25 Ga(Barron, 1966).


SANTOS et al.

Figure 4. Roraima Supergroup stratigraphy and depositional systems. Modified from Reisand Yanez (2001). Sampling sites (black dots) shown in approximate stratigraphicpositions.

METHODS

SHRIMP U-Pb

The techniques for SHRIMP (sensitive,high-resolution ion-microprobe) U-Pb isotopeanalyses and age determinations used in thisstudy have been described by Compston et al.(1984) and Smith et al. (1998). Following thecrushing and milling of fresh rock, its heavyminerals were separated by using commonprocedures: sieving, heavy liquids, and mag-netic separation. The selected grains werehandpicked from the final concentrate andmounted in a short epoxy cylinder togetherwith the standard fragments. The Pb/U ratiocalibration standard used for zircons was CZ3(565 Ma, 206Pb/238U 5 0.0928; Pidgeon et al.,1994) and that for baddeleyite was Phalabor-wa (2060 Ma; Heaman and Le Cheminant,

1993). Usually, a few hundred zircons wereselected and placed in the epoxy mount; how-ever, for baddeleyite, all crystals obtainedwere mounted. Each mount is composed ofthree to eight samples. All grains were pho-tographed under transmitted and reflectedlight. Scanning electron microscopy (SEM)was performed to produce backscattered-electron (BSE) and charge-contrast (CC) im-ages before SHRIMP work to select the bestareas to be analyzed and to interpret the zirconinternal structure.

The mounts were covered by a thin (;30A) layer of gold to assure uniform electricalconductivity. A primary beam of singly neg-atively charged O2

2, ;25 mm in width, wasused to generate positively charged secondaryions. Pb and U isotopes plus Zr2O were mea-sured directly by using an electron multiplierin ion-counting mode. Depending on the

thickness of the gold layer, a raster time ofbetween 1 and 2 min was used before analysisto remove the gold from the analysis area. Theindividual ages were determined from six suc-cessive scans of the mass spectrum. Data intables and figures are given with 1s precision,and the average ages reported in the text areweighted-mean 207Pb/206Pb ages with 95%confidence limits. Because all ages are olderthan 1.0 Ga, the 207Pb/206Pb ages are more pre-cise (lower error) when compared with the206Pb/238U ages. To assure more precise 207Pb/206Pb ages, the counting time of the 207Pb iso-tope (much longer half-life decay time) onSHRIMP was 30 s on each mass scan, where-as we used only 10 s for counting the 206Pbisotope. Parts of the data—samples RG34,NR531, CG8, and NR15—were reduced byusing the Krill software (Peter Kinny, CurtinUniversity of Technology, Perth, Australia,[email protected]) and plotted in con-cordia diagrams by using Isoplot/Ex (Ludwig,1999). Zircon data for samples RG184, SP1,HC165, HC377, and CG30 were reduced byusing SQUID software (Ludwig, 2001), andthe concordia and cumulative Gaussian plotswere prepared by using Isoplot/Ex (Ludwig,1999).

Laser Ar-Ar

Samples were irradiated in a VVR-c re-search nuclear reactor at the State ScientificCenter of the Karpov Physical-Chemical Re-search Institute of Obninsk, Kaluga region,Russia). The fast integral flux of neutrons was1.2 3 1018 n/cm2 at an energy of .0.3 MeV.Because of overirradiation, 10 months elapsedbefore the samples were analyzed.

The standard sample Bern-4 muscovite(Flish, 1982) was used as flux monitor. Mass-discrimination control was achieved by sys-tematic analyses of atmospheric argon duringthe work. The isotopic composition of Ar wasdetermined in a mass MI-1201 IG spectrom-eter coupled to a laser-ablation system that in-cludes an infrared impulse laser LTI-237/90and a Carl Zeiss LMA-1 microscope. The cra-ter diameters varied from 50 to 500 mm, butgenerally were ;300 mm.

All measured isotopic ratios were correctedfor mass discrimination, for radiogenic decay(for 37Ar and 39Ar), and for neutron-inducedreactions involving Ca. The correction factorsfor [36Ar/37Ar]Ca and [39Ar/37Ar]Ca were 2.64 31024 and 7.43 3 1024, respectively. The errorestimate for the 40Ar*/39ArK ratio was derivedby using the expression of Dalrymple andLamphere (1971). Uncertainties in the age



TABLE 1. PREVIOUSLY DETERMINED DATES FOR THE RORAIMA SUPERGROUP

References for ages Comment Age

Intrusive gabbros and volcanic rocksMcDougall et al. (1963), Guyana First date on dolerites intrusive into the Roraima

Formation (Kopinang, Kamarang, and Tumatumarisills)

Older than 2227 6 40 Ma (K-Ar) Older than 1954 6110 Ma (Rb-Sr isochron)

Snelling (1963) and McConnell et al. (1964), Guyana Mean of three whole-rock K-Ar (gabbro andhornfelses) dates

Older than 1626 6 81 Ma (K-Ar)

Snelling and McConnell (1969), Guyana Recalculation of the Rb-Sr dates of Snelling (1963) 1661 Ma (gabbros) and 1608 Ma (hornfels)Hebeda et al. (1973), Suriname Indication of excess radiogenic Ar in Avanavero

dolerites at Kabalebo River1671 6 28 Ma (Rb-Sr isochron; 87Sr/86Sri5 0.70436)

Montalvao et al. (1975), Brazil First date on intrusive gabbros, dating metamorphichost rocks at Pedra Preta

1868 6 73 Ma is age of the gabbro intrusion (Rb-Srisochron)

Fernandes et al. (1977), Brazil Established the age of felsic subvolcanic rocksintrusive into the Tunuı Group

1490 6 62* Ma (whole-rock Rb-Sr isochron; 87Sr/86Sri5 0.71021)

Onstott et al. (1984), Venezuela Ar-Ar method used to date Venezuelan diabase dikes(Guaniamo dike swarm)

1468 6 3 Ma (plagioclase) 1798 6 2 Ma (biotite)

Norcross et al. (2000), Guyana First precise U-Pb date on gabbro of Avanaveromagmatism at Omai

Older than 1794 6 2 Ma (U-Pb on baddeleyite)

Interlayered tuffsPriem et al. (1973), Suriname First date on interlayered ash-fall tuffs at Tafelberg 1655 6 20 Ma (Rb-Sr isochron; 87Sr/86Sri5 0.7075)Pringle and Teggin (1985), Venezuela First date on interlayered ash-fall tuffs at Quebrada

de Jaspe1580 6 86* Ma (Rb-Sr isochron; 87Sr/86Sri5 0.72078)

Gaudette and Olszewski (1985), Venezuela Another date on for ash-fall tuffs at Canaima andQuebrada de Jaspe

1731 6 49 Ma (Rb-Sr isochron; 87Sr/86Sri5 0.7082)

Basement rocksAmaral (1970), Brazil First K-Ar dates on subjacent Surumu volcanic rocks

in northeast RoraimaYounger than 1175 6 92 Ma (K-Ar)

Basei (1978), Brazil Age of the Surumu Group 1929 6 41* Ma (Rb-Sr isochron; 87Sr/86Sri5 0.7037)Priem et al. (1971), Suriname Age of volcanic Dalbana Formation and related

granitoids1873 6 42 Ma (Rb-Sr isochron; 87Sr/86Sri5 0.7056)

Hurley et al. (1973), Venezuela Age of the Cuchivero Group and Caicara Formation 1941 Ma (Rb-Sr isochron)

Gaudette and Olszewski (1985), Venezuela Age of the basement gneisses and granitoids(Macabana, Minicea, Cassiquiare, Padamo, andVentuari)

1823 6 15 and 1859 6 47 Ma (U-Pb). 1793 6 98Ma, 1758 6 87 Ma, 1846 6 65 Ma, 1805 6 60Ma, and 1803 6 71 Ma (Rb-Sr)

Schobbenhaus et al. (1994), Brazil First U-Pb age of the Surumu Group 1966 6 9 Ma (conventional U-Pb)Santos et al. (2000), Brazil Age of the Cauaburi Complex 1810 6 9 Ma (SHRIMP U-Pb)Tassinari et al. (1996), Brazil Age of the Cassiquiare Granite 1834 6 24* Ma (SHRIMP U-Pb)

Note: All radiometric ages prior to 1977 are recalculated according to the IUGS (International Union of Geological Sciences) decay constants (Steiger and Jager, 1977).*Recalculated error.

were calculated according to the procedure ofRoddick (1987).

GEOCHRONOLOGY

Previous Studies

We organized previous geochronologicalstudies of the Roraima Supergroup and relatedrocks in three ways—those that sought to es-tablish maximum and minimum ages andthose that dated the internal tuff layers.

Maximum AgesThe maximum age of the supergroup is de-

fined by the age of its basement. Along itsnorthern margin, the supergroup overliesmostly rocks of the Trans-Amazon orogenicbelt (2.25–2.00 Ga, Norcross et al., 2000; San-tos et al., 2000; Gibbs and Olszewski, 1982),whereas along its southern and northwesternborders, the supergroup was deposited over avolcano-plutonic assemblage composed most-ly of felsic volcanic rocks and syeno- andmonzogranites carrying four different names:Dalbana Formation in Suriname (Priem et al.,

1973), Surumu-Saracura in Brazil (Pinheiro etal., 1990), Caicara-Pacaraima in Venezuela,and Iwokrama and Burro-Burro Group inGuyana (Gibbs and Barron, 1993). The agesof these rocks were first measured by Rb-Sr,which gave dates in the 1925–1810 Ma range(Table 1). Basei and Teixeira (1975), for ex-ample, considered 1925 Ma (corrected accord-ing to the l 87Rb constant of 1.42 3 10211

yr21) as the age of the Surumu Group on thebasis of a Rb-Sr reference isochron. The firstU-Pb dating (Schobbenhaus et al., 1994) ofthese rocks yielded the older age of 1966 69 Ma for the Surumu Group (Fig. 3E) in Bra-zil, and this is now considered to be a maxi-mum age for the Roraima Supergroup.

In contrast, Roraima-like deposits are most-ly found in the Rio Negro province, whosebasement rocks are called the Cauaburi Com-plex in Brazil and their equivalents Macabana,Minicia, and Ventuari granitoids in Venezuela.In Brazil, the SHRIMP U-Pb ages for thebasement are 1834 6 24 Ma (CassiquiareGranite; Tassinari et al., 1996) and 1810 6 9Ma (Cauaburi Complex; Santos et al., 2000),whereas in Venezuela, Gaudette and Olszews-

ki (1985) published conventional U-Pb agesof 1823 6 15 Ma and 1859 6 47 Ma for theMinicea and Macabana Gneisses. The sameauthors also published some Rb-Sr isochronages for basement gneisses in the southernAmazonas Territory of Venezuela: 1793 6 98Ma (Atabapo Gneiss), 1758 6 87 Ma (Mini-cea Gneiss), 1846 6 65 Ma (MacabanaGneiss), 1805 6 60 Ma (Padamo Granite),1783 6 35 Ma (Cassiquiare Granite), and1803 6 71 Ma (Ventuari Quartz Diorite).These ages indicate that the Rio Negro prov-ince was formed in the 1849–1758 Ma timerange and that the overlying Roraima-like de-posits can be no older than this interval.

Direct Dating (Tafelberg-Type Ash-FallTuffs, the Uaimapue Formation)

The Uaimapue Formation has interlayeredash-fall tuffs (Fig. 3E), which yield the age ofthe middle sequence of the Roraima Super-group (see Table 1 and Fig. 4). These felsictuffs are red or green in color, very finegrained, and laminated to massive; they varyin thickness from a few centimeters to asmuch as 25 m and are rhyolitic in composition


SANTOS et al.

Figure 5. Concordia plot for basement rocks of Rio Negro province, sample CG8.

with abundant glass shards. They carry localnames such as the Tafelberg (Suriname), Uaila(Brazil), and Canaima and Quebrada de Jaspebeds (Venezuela), and—not surprising fromthe latter name—were originally reported inthe literature as jaspers (Gansser, 1954, 1974).

Several attempts were made in the past todate these tuffs by using the Rb-Sr method(Table 1). In Suriname, the tuff layers at Taf-elberg Mountain were dated by Priem et al.(1973), who published a whole-rock isochronyielding 1655 6 19 Ma. In Venezuela, eightsamples of the Canaima tuffs and one samplefrom the Quebrada de Jaspe (Santa Elena)were dated by Gaudette and Olszewski (1985),who obtained an age of 1747 6 49 Ma withMSWD (mean square of weighted deviates) 52.84. Another attempt with the Rb-Sr methodwas made by Pringle and Teggin (1985) on 16samples from Quebrada de Jaspe, which yield-ed a whole-rock reference isochron age of1579 6 18 Ma, but with MSWD 5 20, a highvalue. Finally, all the whole-rock Rb-Sr iso-chron ages of the felsic tuffs (1579, 1655, and1749 Ma) are too young and are in disagree-ment with the K-Ar ages of the post-Roraimadoleritic intrusions of Avanavero type (U-Pbage of 1794 6 4 Ma, Norcross et al., 2000;and Rb-Sr age of 1868 6 14 Ma, Basei,1975).

Minimum AgeThe minimum age of the supergroup is de-

termined by the age of the Proterozoic tholei-itic doleritic sills and dikes that cut it. Thesesills are thick, up to 450 m, and range in com-position from picrite to granophyre (Santos etal., 1980; Hawkes, 1966), although the dom-inant composition is that of an augite dolerite.These dolerite sills have several local names,but there is a consensus that most of them are

correlated to the Avanavero Dolerite Sill inSuriname (Hebeda et al., 1973; Santos et al.,1980; Gibbs and Barron, 1993). Several at-tempts (Table 1) to date the sills have beenmade since 1963 (Snelling, 1963) by the K-Ar method, and all yielded ages in a broadrange of ca. 2073–1497 Ma (Hargraves, 1968;Snelling, 1963; Gansser, 1974; McConnell etal., 1964). Most ages are older than the Rb-Srage of the Tafelberg-type tuffs (Priem et al.,1973; Olszewski and Gaudette, 1985), astratigraphic impossibility (Fig. 4). Even afew Rb-Sr dates reveal ages older than the fel-sic tuff ages, as, for example, the age of 18686 45 Ma for the hornfels produced by thePedra Preta Sill cutting the Cuquenan For-mation in Brazil (Montalvao et al., 1975; Bas-ei, 1975). The study of Hebeda et al. (1973)of the Avanavero Sill in Suriname used bothRb-Sr and K-Ar and demonstrated that the ap-parent older K-Ar ages are related to radio-genic 40Ar enrichment, which was possibly ac-quired during the K’Mudku episode (1250Ma). The Rb-Sr isochron age obtained by He-beda et al. (1973) for the gabbro-dolerites ofthe Avanavero Sill is 1659 6 27 Ma (cor-rected according to the l 87Rb constant of 1.423 10211yr21). By U-Pb methods, Santos andGaudette (1996, personal commun.) dated zir-cons collected from the Cotingo Sill in Braziland interpreted the obtained age of 1921 Maas the age of inherited zircons. The most ac-curate age available in the literature is 17946 4 Ma, which was published by Norcross etal. (2000), who studied an Avanavero-typedolerite intrusion at the Omai gold deposit inGuyana (U-Pb isotopes in baddeleyite).

New Results

We used U-Pb geochronology to clarify fivesubjects concerning the widespread Roraima

Supergroup and its Roraima-like deposits: (1)the age of the basement underlying the Rorai-ma to establish its maximum age; (2) a moreprecise age of the Tafelberg-type tuffs of theUaimapue Formation; (3) the age of the Cipoand Manga Brava Sills of Avanavero mag-matism to determine the Roraima minimumage; (4) the age of the western deformed rocks(Tunuı, Traıra, Araca), relative to those of thePacaraima Plateau; and (5) the provenance ofthe Roraima Supergroup and of the diamondsfound in the conglomerates near its base(which bears on the distribution of the Paleo-proterozoic kimberlites). Finally, we com-bined geochronology with available strati-graphic and sedimentological studies todevelop an overall model for the Roraima Su-pergroup, which we compare with other largePrecambrian psammitic units outside SouthAmerica.

Pre-Roraima RocksTo help define the maximum age of the Ro-

raima Supergroup, two samples of basementrocks were selected for dating: sample CG30is a tonalite of the Cauaburi Complex in theRio Negro province and sample RG34 is a fel-sic volcanic rock of the Surumu Group in theTapajos-Parima province (Fig. 1).

The oldest unit in the Rio Negro province,the Cauaburi Complex, was dated by Santoset al. (2000) at 1810 6 9 Ma (sample MS63,a tonalite). This age provides a maximum agefor the Neblina, Daraa, and Padre outliers(Fig. 1). Sample CG8 is another tonalite fromthe Cauaburi Complex, but was collected clos-er to Araca Mountain, also a table mountain,in the Marauia River basin to establish betterthe maximum possible age of its Roraima-likedeposits.

Only one population of zircon was recov-ered from sample CG8; all its crystals arelight pink, prismatic with sharp edges, and;120–250 mm in length. All have similar Ucontents (100–800 ppm) and Th/U ratios of0.45–0.99. Eleven age determinations wereperformed, and the results are nearly concor-dant (97% to 102%), except those from grainb10, which is only 94% concordant (TableDR2 [see text footnote 1]). Ten results (Fig.5) group at the weighted-mean 207Pb/206Pb ageof 1795 6 6 Ma (MSWD 5 1.20). The outlierzircon b.8–1 is younger (1749 6 8 Ma) andis not included in the mean age.

Sample RG34 is a Surumu Group rhyodacitecollected along the Uraricaa River to checkwhether the felsic volcanic basement has agesin the 1929–1810 Ma range (Rb-Sr results, seeTable 1) or is comparable to the U-Pb 1966 69 Ma age reported by Schobbenhaus et al.



Figure 6. Concordia plot for basement rocks of Tapajos-Parima province, sample RG34.

Figure 7. Cumulative probability Gaussian and histogram plot of detrital-zircon ages ofUiramuta quartz sandstone, Arai Formation, Roraima Supergroup, sample SP1.

Figure 8. Cumulative probability Gaussian and histogram plot of detrital-zircon ages ofAraca Formation muscovite quartzite, sample CG30.

(1994) in their U-Pb study. Zircons from sam-ple RG34 are pale brown and fractured andhave prismatic shape; most are zoned. No coresand rims were noticed in transmitted or reflect-

ed light or in BSE images, but the possibilityof internal age zonation was tested on grains 1and 12. In both crystals, the two ages measuredare in the same range (Table DR2 [see text

footnote 1]). Eighteen spot analyses were doneon 16 zircons. Grain a.15–1 is the most dis-cordant and the richest in common lead (1%)and is not included in the age calculation orconcordia plot (Fig. 6). The weighted-mean207Pb/206Pb age of the remaining 17 spot anal-yses on 15 grains is 1984 6 9 Ma (MSWD 51.9) (Fig. 6).

Ages of the Sources of the RoraimaSupergroup and Roraima-Like Deposits

To study the ages of the main sources to theRoraima and Roraima-like deposits, threesamples were collected. Sample SP1 is fromthe middle of the Arai Formation, a conglom-eratic sandstone collected near the UiramutaVillage (Fig. 1). Sample NR15 is from an im-mature conglomerate rich in cobbles and boul-ders of rapakivi granite collected from the Ser-ra Surucucus outlier. Sample CG30 is agreenschist-facies quartz sandstone sampled atthe top of the Araca Formation at AracaMountain.

Sample SP1 is sandstone very rich in zir-cons, of which many hundreds were recoveredfrom ,1 kg of sample. A more detailed effortwould be needed to study all the different zir-con populations of this sample, and only a re-connaissance (26 analyses; Table DR2 [seetext footnote 1]) was made here. Besides anArchean grain (a61, with 2715 6 18 Ma; 1s),one early to pre–Trans-Amazonian (a71, 23586 18 Ma; 1s), and three post–Trans-Amazonianzircons, all others are of Trans-Amazonian age(Fig. 7). Twenty-one zircons yielded a weighted-mean 207Pb/206Pb age of 2123 6 14 Ma (TableDR2 and Fig. 7), suggesting a dominantTrans-Amazonian source for the Arai Forma-tion. These results agree with previous U-Pbwork (Gaudette et al., 1997) that detected anearly Trans-Amazonian source (2171 6 16Ma) for the Arai Formation. The youngestpopulation of three zircons has an age of 19586 19 Ma and is interpreted to have been de-rived either from Surumu Group volcanicrocks and the plutonic equivalent Pedra Pin-tada granitoid suite.

All zircons from sample CG30, a metasand-stone, were abraded by clastic transport as isevident on grains 2, 12, and 25, all of whichare very well rounded. Twenty-six determi-nations were made on 20 detrital zircons (Ta-ble DR2 [see text footnote 1]). The interpre-tation of BSE images of five grains suggestedthe existence of two crystallization phases(older core and younger rim). This interpre-tation was confirmed in three grains (3, 7, and16), whereas crystals 18 and 19 have coresand rims with the same age. Four broadgroups of ages were obtained (Fig. 8). Two


SANTOS et al.

TABLE 2. SPOT-LASER Ar-Ar ANALYSES, SAMPLE CG30 MUSCOVITE

Grains 40Ar/39Ar 40Ar/36Ar 36Ar/39Ar(x 10–2)

37Ar/39Ar(x 10–2)

40Ar*/39ArK

(6 s)Age

(Ma 6 s)

1 core (0.5 mm) 46.24 6249 0.74 7.1 44.1 6 0.1 1317 6 41 rim (0.5 mm) 45.39 4728 0.96 7.5 42.6 6 0.1 1284 6 52 whole (0.5 mm) 45.84 6643 0.69 10.4 43.8 6 0.2 1311 6 63 whole (0.3 mm) 45.75 5942 0.77 5.8 43.5 6 0.2 1305 6 6

Note: All analyses were corrected for mass discrimination, decay (for 37Ar and 39Ar), interfering neutron-inducedreactions on calcium ([36Ar/37Ar]Ca5 2.64 3 10–4 and [39Ar/37Ar]Ca 5 7.43 3 10–4), and line blanks. J (fluenceparameter) 5 0.02440 6 0.00005.

TABLE 3. STEP-HEATING Ar-Ar ANALYSES, SAMPLE CG30

Stepno.

Temp(8C)

39Ar(%)

40Ar/39Ar 40Ar/36Ar 36Ar/39Ar(3 10–2)

37Ar/39Ar(3 10–2)

40Ar*/39ArK

(6 s)Age

(Ma 6 s)

1 550 4.0 50.54 3240 1.56 5.7 46.0 6 0.2 1308 6 62 600 16.1 48.95 9236 0.53 5.5 47.4 6 0.1 1337 6 43 650 21.3 49.05 8457 0.58 7.8 47.4 6 0.1 1336 6 44 700 23.1 48.83 6976 0.70 10.4 46.8 6 0.1 1325 6 45 800 6.5 40.02 4084 0.98 1.7 37.2 6 0.1 1120 6 56 900 19.6 48.90 8431 0.58 5.7 47.2 6 0.1 1334 6 47 1000 9.3 50.22 5643 0.89 7.0 47.6 6 0.2 1341 6 68* 1100 0.05 519.2 336.7 154.2 ;10 65 6 20 1650 6 3009* Fusion 0.1 164.6 364.2 45.2 ;10 31 6 9 1000 6 200

Note: All analyses were corrected for mass discrimination, decay (for 37Ar and 39Ar), interfering neutron-inducedreactions on calcium ([36Ar/37Ar]Ca 5 2.64 3 10–4 and [39Ar/37Ar]Ca 5 7.43 3 10–4), and line blanks. J (fluenceparameter) 5 0.02317 6 0.00005; mass 5 2.91 mg.

*Statistical meaning of steps 8 and 9 is negligible (released 39Ar 50.1%).

Figure 9. 39Ar/40Ar step-heating diagram for sample CG30 muscovites. The plateau age is1334 6 2 Ma.

Figure 10. U-Pb concordia plot for zirconsfrom conglomerate of the Serra SurucucusFormation, sample NR15.

results have Paleoarchean to Mesoarchean ages(3209 and 3126 Ma). Four Trans-Amazonianages lie between 2335 and 2221 Ma and arederived from zircons very poor in U, withlarge uncertainties of 41–29 m.y. Another zir-con has the middle Trans-Amazonian age of2130 6 19 Ma. Late Trans-Amazonian zirconsare present with the ages of 2048 6 13, 20236 15, 2017 6 12, and 2004 6 13 Ma.

The youngest zircons form the main groupof 13 grains (Fig. 8), which may representonly one population with 1915 6 8 Ma

(MSWD 5 0.78). The youngest zircon (c.20-1) has 1878 6 15 Ma (102% concordance)and defines the maximum age for the AracaFormation.

Muscovite was recovered from sampleCG30 to determine the Ar-Ar age of thegreenschist-facies metamorphism of the AracaFormation. Muscovite samples were irradiatedin April 1999 and analyzed in January 2000after radiation dissipation. A total of six spotswere performed on two muscovite flakes,three spots on each flake, from core to rim.

The spot-laser analyses yielded the ages of1317 6 4 Ma in the core and 1284 6 5 Main the rim of a muscovite crystal (Table 2).Spots covering both parts of the rim and corehave ages of 1311 6 6 Ma and 1305 6 6 Ma.

The step-heating analyses correspond tonine determinations following increasing tem-perature steps of 50 and 100 8C (Table 3). Thelast two steps (8 and 9) have little statisticalsignificance because they represent minimumamounts of released 39Ar (,0.1%). Individualstep results are 1308 6 6 Ma, 1337 6 4 Ma,1336 6 4 Ma, 1325 6 4 Ma, 1120 6 5 Ma,1334 6 4 Ma, and 1341 6 6 Ma. Six resultsrepresent 89.4% of the 39Ar released and cor-respond to the age of 1334 6 2 Ma (MSWD5 1.8) calculated by using Isoplot (Ludwig,1999). The cause of the dip at ;70% cumu-lative 39Ar is unknown (Fig. 9) and this fifthstep (1120 6 5 Ma) is not included in the agecalculation.

Sample NR15 is from an immature con-glomerate, and all its zircons have a well-preserved igneous shape with sharp edges andno evidence of transport. Crystals are relative-ly large (300–500 mm long), little fractured,clear, and light pink; they have no cores orevident zoning. No inherited zircons were de-tected, and all analyses constitute a singlepopulation of 20 grains (Table DR2 [see textfootnote 1]) with the grouped age (Fig. 10) of1552 6 6 Ma (MSWD 5 1.70). This is sim-ilar to the age of the rapakivi-textured graniteintruding the Serra Surucucus Formation(Santos and Reis Neto, 1982), which crops out120 m southeast of the conglomerate outcrop.

Ages of Interlayered Felsic Tuffs from theUaimapue Formation

Two samples of very fine ash-fall tuffs fromthe Uaimapue Formation were studied.

Sample HC165 comes from the dominant



Figure 11. U-Pb concordia plot for zircons from ash-fall tuff of the Uaimapue Formation,sample HC165.

Figure 12. U-Pb concordia plot for magmatic zircons of sample HC377 (Uaimapue ash-fall tuff).

red tuff of the Uaimapue Formation. Recov-ered zircons include several populations withdistinct shape and color. One grain of eachpopulation was selected for analysis (TableDR2 [see text footnote 1], Fig. 11), and eachgrain had a different age, suggesting a diver-sity of crustal contamination. All grains arepossibly inherited, but the youngest, g20(1862 6 15 Ma; 1s), is interpreted as mag-matic because it is a long prism (aspect ratio6:1) and its age is indistinguishable from themagmatic age of sample HC377 (1873 6 3Ma; 1s). Grain g5 is Paleoarchean (3341 6 6Ma, not shown in Fig. 11), and all other zir-cons (Table DR2 and Fig. 11) are Paleopro-terozoic (2166 6 4 Ma to 1862 6 15 Ma; 1s).

Sample HC377 is a green ash-fall tuff,which was searched for zircons having un-equivocal magmatic characteristics, and thus apopulation of small (,100 mm), clear, sharp-edged, and long (5:1 aspect ratio) prisms wasselected. Despite trying to avoid inherited zir-cons, one grain belongs to this group, yieldingthe age of 2013 6 13 Ma (Table DR2 [seetext footnote 1]). Twelve analyses from the re-maining 14 are in the same population (Fig.12), yielding ages of 1873 6 3 Ma (MSWD5 1.8).

Ages of the Avanavero-Type IntrusiveDolerites

Three rock specimens of the doleritic sillswere searched for baddeleyite and zircon.

Sample NR135 is a dolerite from the CotingoSill in Brazil, lacking both zircon and badde-leyite. Sample RG184 is from the lowermostsill (Manga Brava), which lacks baddeleyite,but has a small population of tiny zircons

(only four crystals were recovered). SampleNR531 is derived from the Cipo Sill, also inBrazil (Fig. 1), and revealed a small popula-tion of eight baddeleyite crystals. Four ofthem are too small or were not well enoughexposed in the epoxy disc to be analyzed, soonly four were analyzed. Two spots wereplaced in the larger two baddeleyite grains(h.1 and h.3), which were analyzed twice dur-ing two different SHRIMP sessions. The othertwo grains are smaller, and only one analysiswas performed on each (grains h.4 and h.5).The width of all grains is ;10–15 mm, soeven using a small spot size of 15 mm, allSHRIMP spots were placed partly over the ep-oxy. This circumstance may be the cause ofthe high common-lead content of grains h.1(average of 1.63%), h.4 (3.91%), and h.5(11.23%). The only grain with relatively low204Pb is h.3 (average of 0.41% in four deter-minations). To correct the common lead, weadopted the 208Pb method (Compston et al.,1984) instead of using measured 204Pb (fol-lowing Santos et al., 2002). We observedmuch larger uncertainties in the f206 valuescorrected by the 204Pb method (14.79% to87.67%) when compared to the f206 valuescorrected by the 208Pb method (1.78% to7.31%). (The f206 value is the ratio of com-mon 206Pb to total measured 206Pb, based onthe measured 204Pb in zircon or 208Pb in bad-deleyite.) Generally, the 208Pb method is more


SANTOS et al.

Figure 13. U-Pb concordia plot for baddeleyites of sample NR531 (Cipo Sill, Avanaveromagmatism).

Figure 14. U-Pb concordia plot for zircons of sample RG184 (Manga Brava Sill, Avan-avero magmatism).

reliable in minerals with low 232Th/238U ratios(Wingate and Compston, 2000), such asNR531 baddeleyites (Th/U 5 0.011–0.068,Table DR2 [see text footnote 1]).

The small size of the crystals precludestheir arrangement in similar orientation foranalysis to avoid or minimize the orientationeffect on the U-Pb sputtering and ionizationbehavior of baddeleyite (Wingate and Comps-ton, 2000). Because the 206Pb/238U ratios mayvary significantly up to 610% (Wingate andCompston, 2000), the size and shape of theerror polygons or ellipses in a common 207Pb/235U vs. 206Pb/238U concordia plot are meaning-less, in that they do not include this systematicuncertainty. However, that behavior does notaffect the 207Pb/206Pb age. The data reducedwith Krill software (Table DR2) was plottedon a conventional concordia diagram by usingIsoplot/Ex (Fig. 13), which yields the weight-ed mean 207Pb/206Pb age of 1787 6 14 Ma(MSWD 5 1.4; n 5 8). Spot analyses h.5–1and h.4–1 are not included in the concordiaplot (Fig. 13) or the age determination becausethey have very high common lead content(11.22% and 3.91%) and are highly discordant(126% and `%).

Zircons of sample RG184 are small, elon-gated prisms with sharp edges, up to 80 mmlong (aspect ratio 5:1 to 6:1). They show nointernal oscillatory zoning and are rich in U(550–2117 ppm). The shape, U content, andTh/U ratios of the grains, as well as the similarage determined in all of them, have led us toconsider that they were formed during the dol-erite crystallization and are not inherited. Thezircons are concordant (99% to 100% concor-

dant), and the Isoplot/Ex calculated weightedmean 207Pb/206Pb age is 1782 6 3 Ma (Fig.14). This age is comparable to the less precisebaddeleyite age of the Cipo Sill (1787 6 14Ma). The two ages are indistinguishable, so itis impossible to determine whether the upper-most Cipo Sill crystallized slightly after or be-fore the lowermost Manga Brava Sill.

DISCUSSION

Basement Ages

The age of the Marauia Tonalite of the Cau-aburi Complex is 1795 6 6 Ma, comparable

to the two previous U-Pb ages (1834 6 24Ma and 1810 6 9 Ma) published for the RioNegro province basement (Tassinari et al.,1996; Santos et al., 2000). This result con-firms that the Rio Negro province is not as oldas previously considered (Melo et al., 1994a),and together with other U-Pb results (Gaudetteand Olszewski, 1985; Santos et al., 2000; Tas-sinari et al., 1996), sets limits on the maxi-mum age of many Roraima-like outliers suchas Tunuı, Daraa, Padre, Neblina, and Araca inthe northwestern part of the Amazon craton.All these outliers are at least 95–51 m.y. youn-ger than the Roraima Supergroup (1873 6 3Ma) and thus cannot be correlated to it.

The age determined for the UraricaaRhyodacite (Surumu Group) is slightly olderthan the age of 1966 6 9 Ma published bySchobbenhaus et al. (1994), but both ages arecomparable to the ages of the Creporizao arc(Table DR2 [see text footnote 1]), the secondmagmatic arc of the Tapajos-Parima orogenicbelt (Santos et al., 2000). The older populationof 2027 6 16 Ma ages present in sampleRG34 is similar to the ages of the oldest rockspresent in the Tapajos-Parima province, suchas the Cuiu-Cuiu Suite in Para State (2033–2003 Ma, Santos et al., 2001) and the AnauaComplex in southeastern Roraima (2028 6 9Ma, Santos and Faria et al., 2002). In RoraimaState, the Creporizao arc rocks include vol-canic rocks of the Surumu Group and its plu-tonic equivalents, the Pedra Pintada suite(1990 6 10 Ma, J.O.S. Santos, 2002, personalcommun.), the Agua Branca monzogranite(1960 6 21 Ma, Almeida et al., 1997; 19726 7 Ma, J.O.S. Santos, 2002, personal com-



mun.), the Prainha meta-andesite (1949 6 6Ma, J.O.S. Santos, 2002, personal commun.),and the Rio Urubu suite granitoids (1921 615 Ma, Fraga et al., 1996). Thus, the RoraimaSupergroup in the Pacaraima Plateau overlies1.99–1.92 Ga rocks of the Creporizao arc ev-erywhere except to the north and northeast,where it overlies 2.25–2.00 Ga rocks of theTrans-Amazon province.

Source of Roraima Supergroup

Of the 27 grains analyzed from sample SP1(Arai Formation), 22 were derived from theTrans-Amazon belt to the north or other sourc-es not known or buried. Only one grain is pos-sibly derived from the known Mesoarcheansource to the northwest. These results are fullyconsistent with the paleocurrent evidence fora northern source (Reis et al., 1990). The pres-ence of zircons with ages of ca. 1.96 Ga (threegrains) shows that the underlying volcanicrocks of the Surumu-Cuchivero Groups or theplutonic rocks of the Pedra Pintada granitoidsuite may have contributed sand to this partof the Arai Formation. The ash-fall tuffs ofthe Uaimapue Formation all have inheritedgrains, which indicate either contaminationfrom older crustal material at depth or deri-vation from incorporated pre-Uaimapue sedi-mentary rocks. The second possibility is fa-vored by the presence of Trans-Amazonianand Paleoarchean zircons in sample HC165,whereas contamination with the underlyingSurumu-type volcanic rocks is supported bythe presence of zircons with ages of 1995 67 Ma, 1985 6 15 Ma, and 1950 6 25 Ma.

Age and Sources of Araca Formation andWestern Outliers

We use three lines of evidence to establishthe age of the Araca Formation—the ages ofits detrital zircons, the U-Pb age of its under-lying basement rocks, and the age of its meta-morphic muscovites based on 40Ar/39Aranalysis.

The sandstones of the outlier at AracaMountain are underlain by rocks of the RioNegro province and were correlated to the Ro-raima Supergroup (Borges, 1987; Santos,1984; Montalvao et al., 1975; Achao, 1974)until Melo et al. (1994a) introduced the nameAraca Formation and assigned to it an Arche-an age. They proposed the age on the basis ofdeformation in parts of the outlier that wassimilar to deformation of the Aguas ClarasFormation of the Archean Carajas province.The deformation and metamorphism presentin the southern and southeastern parts of Ar-

aca Mountain were interpreted by Melo et al.(1994a) to indicate deposition in a transpres-sional basin, despite the well-known mineralmaturity of its sandstones in outcrop.

SHRIMP dating of zircons from sampleCG30 from the upper part of Araca Mountainproved to be most instructive and revealed zir-cons derived from three main sources, consid-ering only the known occurrences. The largestgroup (Fig. 8) form ;60% of the sample andhave the post–Trans-Amazonian age of 19156 8 Ma (n 5 13; MSWD 5 0.78).

The youngest zircon (c.20–1) has an age of1878 6 15 (1s) and was probably derivedfrom rocks of the Tapajos-Parima province lo-cated 80 km to the east (Fig. 2) such as theRio Urubu Suite granitoids (1921 6 15 Ma;Fraga et al., 1996). However, being a singleanalysis of a grain that could have sufferedearly Pb loss, this value may represent a min-imum age. A second source, consisting of;30% of the zircons, has ages in the 2.33 Ga–2.05 Ga range and consists of two groups: (1)Five ages (2335–2130 Ma) are comparable tothe main zircon population of sample SP1 andare interpreted as having been derived fromthe oldest Trans-Amazon orogenic belt to thenortheast. (2) Four zircons have late Trans-Amazonian ages (2048, 2023, 2017, and 2004Ma), and they may have been derived eitherfrom late Trans-Amazonian granitoids, whichare abundant to the northeast in Venezuela andGuyana, but ;600 km away, or from the moreprimitive rocks of the Tapajos-Parima prov-ince located 150 km to the east and 250 kmto the north (Fig. 2). In addition, two detrital-zircon grains are Paleoarchean to Mesoar-chean in age (3209 6 7 and 3126 6 7 Ma;1s) and possibly were derived from the Ima-taca province in Venezuela to the north. Thus,the presence of Paleoarchean and Trans-Amazonian zircons in the Araca Formationsuggests that at least part of its source was tothe north and northeast (there are no knownrocks older than 2.01 Ga to the south, east, orwest in the Amazon craton). It is important tonote that no zircons derived from the 1.81 GaRio Negro province were found in the sample.Thus, the new zircon ages reported here sug-gest that the paleoslope of the basin in whichthe Araca Formation was deposited dippedmostly to the west and southwest.

Two other important conclusions can bedrawn from the ages of the zircons in the Ar-aca Formation. The youngest detrital zircon inthe Araca Formation is much older than theage of the basement of the Rio Negro province(1.81 Ga; Santos et al., 2000) and signals twoimportant facts: The Araca Formation cannotbe older than 1.81 Ga, and it was not derived

from underlying basement. To confirm this.1.81 Ga age, we sampled the CauaburiComplex (CG8), the oldest rocks of the RioNegro province, and obtained a similar age of1.795 Ma. Thus, we feel confident that the Ar-aca Formation is younger than the age of 1873Ma of the Roraima Supergroup. We also notethat there are many other well-known outliers(e.g., Neblina, Autana, Tunuı, and Sipapo) inthe Rio Negro province and thus it seems like-ly that they and the Araca Formation belongto a different and younger successor basinthan that of the Roraima Supergroup of thePacaraima Plateau (Fig. 2). This conclusionsupports observations of many previous work-ers (Table DR1 [see text footnote 1]), whohave suggested a regional unconformity in theupper part of the sandstones of the PacaraimaPlateau.

Metamorphism of Araca Formation

We tested the suggestion of Melo et al.(1994a) that the metamorphism of the AracaFormation was Archean in age by Ar-Ar dat-ing of its metamorphic muscovite. The age of1334 6 2 Ma obtained by 40Ar/39Ar step-heatingis slightly older than the classic ages referredto the K’Mudku episode (Barron, 1966),which are in the 1292–994 Ma range (Santoset al., 2001). In fact, the K’Mudku episode iswidespread and affects rocks of four distinctprovinces of the Amazon craton (Santos et al.,2000).

Barrios (1983) obtained three Rb-Sr whole-rock isochrons for the 1545 6 20 Ma Pargu-aza Granite Suite of southern Venezuela (zir-con U-Pb; Gaudette and Olszewski, 1985),and their Rb-Sr rejuvenated ages are 1297 610 Ma (San Pedro), 1352 6 11 Ma (Amazon-as and Suapure), and 1340 6 10 Ma (Ama-zonas). These values are much closer to theage of metamorphism of the Araca Formationand suggest that the deformation produced bythe K’Mudku episode is an intracratonic ex-pression—a far-field effect—of the Grenvilli-an collision along the northwestern and south-western margins of the Amazon craton, whichstarted before 1300 Ma. However, theK’Mudku episode is an extended deformation-al process that occurred at ca. 1240 Ma (Priemet al., 1973; Barron, 1966; Santos, 1999), butalso represents a much longer process withmore than one pulse of intense tectonic activ-ity. The Ar-Ar age of 1334 Ma and the Rb-Srages of Barrios (1983) suggest the existenceof an early event here called the ‘‘Aracaevent’’ of the K’Mudku episode of regionalshearing that affected zones of the Rio Negroprovince (Santos et al., 2000).


SANTOS et al.

Age of the Serra Surucucus Formation

The Roraima-like rocks at SurucucusMountain (Serra Surucucus) have been corre-lated to the Roraima Supergroup by many au-thors (Montalvao et al., 1975; Braun andRamgrab, 1972; Pinheiro et al., 1981; Santos,1985). The 1.52 Ga Surucucus Granite (Rb-Srisochron; Santos and Reis Neto, 1982) wasconsidered to be intrusive into the sedimen-tary sequence, which would therefore havebeen older than 1.52 Ga. However, Reis et al.(1991) found a bed of conglomerate near thebase of the sequence whose framework con-sists mostly of rock pebbles and cobbles de-rived from the Surucucus rapakivi granite. Theage of 1552 6 6 Ma for the main source ofthe Serra Surucucus Formation confirms thatthe conglomerates and sandstones of Surucu-cus Mountain are younger than 1.55 Ga anddo not correlate with the Roraima Supergroup.Quite possibly other nearby outliers such as atUrutanim, Uafaranda, and Melo Nunes alsohave the same age as the Serra Surucucus For-mation, i.e., are younger than 1.55 Ga.

Age of the Matauı Formation

The age of the top unit of the Roraima Su-pergroup, the Matauı Formation (Reid, 1972),has yet to be established because it lacks tufflayers and Avanavero-type dolerite intrusionswithin it. Several authors considered the pres-ence of a regional unconformity between thetop and the lower units of the Roraima Su-pergroup. Santos (1985) and Santos andD’Antona (1984) interpreted such discordancefrom aerial photography. Alberdi and Con-treras (1995) identified this unconformity inoutcrop in six stratigraphic sections studied atLema Mountains of eastern Venezuela. Thoseauthors introduced the informal name ‘‘Capasde Abaren’’ for the Matauı Formation of Reid(1972) and of Reis and Yanez (2001). The‘‘Capas de Abaren’’ unit directly overlies theequivalent of the Uaimapue Formation with itstypical ash-fall tuffs. Thus, everywhere in thePacaraima Plateau there is an unconformitybetween the Matauı Formation and the Uai-mapue Formation, leading us to remove theMatauı Formation from the Roraima Super-group (Fig. 4). How much younger could theMatauı Formation be? One possibility is thatthe Matauı Formation is related to other post-Roraima outliers, like the Serra SurucucusFormation, and thus could be at least 320 m.y.younger than the rocks below it.

Diamonds in the Arai Formation

A major unsolved question relating to thenorthern Amazon craton concerns the locationand age of the source rocks (kimberlites andlamproites) of the detrital diamonds mined inalluvium derived from the conglomerates ofthe Arai Formation (Fig. 3D) and other unitsof the Roraima Supergroup. The source of thediamonds in conglomerates of the RoraimaSupergroup is a quandary, because very fewProterozoic kimberlites and lamproites areknown in northern South America and westAfrica. Those that are known are either youn-ger than the Roraima or have a geographiclocation that seems to make improbable an in-terpretation that they were the diamonds’source. Hundreds of alluvial deposits havebeen mined since the end of the nineteenthcentury (only the more important districts areindicated in Fig. 15), but most of the miningareas are located in modern deposits derivedfrom nearby mature thin quartz-pebble con-glomerates of the Arai Formation (Fig. 3D) inthe lower part of the Roraima Supergroup (al-though not from its lowest, basal conglomer-ate, which commonly represents local alluvial-fan deposition). Some deposits are formed inconglomerates above the Arai Formation(Keats, 1973; Reis et al., 1990), but have lessimportance.

One possible source could be the GuaniamoKimberlites (Kaminski et al., 2000) of thenorthwest Amazon craton (Figs. 1 and 15).The Guaniamo diamond field was discoveredin 1969, but only in 1978 were kimberlitesinferred from studies of pyrope garnets (Bap-tista and Svisero, 1978). The kimberlites aremainly dikes and sills (Nixon et al., 1992),and about a dozen have been mapped (Meyerand McCallum, 1993).

Paleocurrent analyses (Reis et al., 1990) in-dicate a dominant north-to-south current di-rection during the deposition of the fluvialbraided deposits, which precludes the Guan-iamo kimberlites located in the extreme north-west as a source (Fig. 1). The Guaniamo kim-berlites are intrusive into granites and volcanicrocks of the Cuchivero Group, which estab-lishes the maximum age for the kimberlites as1.96 Ga. Nixon et al. (1992) proposed theGuaniamo Kimberlites as the source for theRoraima diamonds, on the basis of one Rb-Srwhole-rock isochron of 1732 6 82 Ma, which,however, probably is a rejuvenated age as isthe case for many other Rb-Sr dates in theAmazon craton (Santos et al., 2000). Nixon etal. (1992) cited a much younger and imprecise40Ar/39Ar age of 765–870 Ma (no plateau agedefined; nine steps with different ages) for the

late micaceous dikes of the Guaniamo Kim-berlites (Mg-rich mica from dike). Relevant tothis problem is that the Roraima-like rocks(Yavi, Coroba, Sipapo, Autana, and Moriche)closest to the Guaniamo Kimberlites (Fig. 1)are not diamond bearing. Thus, the Guaniamokimberlites are both much too young and alsohave an imperfect geographic location to bethe source for the Arai Formation diamonds.

Another approach to determining the sourceof the diamonds in the supergroup is to esti-mate the time of deposition of the conglom-erates of the Arai Formation. First, we tried todirectly date the Arai Formation by searchingfor diagenetic xenotyme overgrowths on itszircons (McNaughton et al., 1999), but with-out success.

As a second choice, we studied magmaticzircons from two of the lowest tuff beds ofthe Uaimapue Formation some 1200 m abovethe base of the diamond-bearing conglomer-ates. The age of the tuffs of the UaimapueFormation of the upper half of the RoraimaSupergroup is 1873 6 3 Ma. The age of theCuchivero Group is not well established, be-ing based on Rb-Sr data (1.9–1.75 Ga; Nixonet al., 1992). Post-Roraima ages for the Cu-chivero Group (younger than 1885 Ga) are un-likely, and possibly the group is even older, asindicated by U-Pb dating in its extension inBrazil (where it is called the Surumu Group).U-Pb ages of ca. 1.96 Ga are 100–200 m.y.older than the Rb-Sr ages obtained in the samerocks of the Surumu Group (Santos et al.,2001). Thus, Paleoproterozoic and post–Trans-Amazonian kimberlitic-lamproitic mag-matism in the northern Amazon craton oc-curred between 1966 6 9 Ma (the basementage) and 1885 6 5 Ma, the beginning of Ro-raima deposition.

West Africa kimberlites could be a source(Reid, 1974), but these are mostly Cretaceousin age (Nixon et al., 1992). Proterozoic sourc-es for diamonds in the West African Shieldare reported only locally in the Ivory Coast(Seguela) and Ghana (Fig. 15). In the IvoryCoast, diamonds are hosted by kimberlite andlamproite dikes, which have been dated by theRb-Sr method at 1.43 Ga (Bardet, 1974). Ifthis age is correct, these dikes are post-Roraimaand again must be disregarded as a source forthe Roraima diamonds. In Ghana, Proterozoicdiamonds occur in metamorphosed lower Bir-imian rocks older than 2.2 Ga (Cahen andSnelling, 1984). Besides the problem of con-flicting ages, paleocurrent data indicate a sed-imentary source to the north, and thus it isunlikely that what is now the southern West Af-rican craton was the source for the Roraima di-amonds. Reconstruction of the South America–



Figure 15. Distribution of kimberlites in West Africa and Amazon cratons in relation to the Trans-Amazon belt and the Roraima Basinin a Mesozoic reconstruction of the Gondwana continent (adapted from Fig. 5 of Meyer and McCallum [1993] and Fig. 4 of Pindell etal. [2000]).

Africa pre-Mesozoic Gondwana continent(Hurley et al., 1967; Feybesse et al., 1998;Rowley and Pindell, 1989; Pindell et al.,2000) indicates that the Ghana and IvoryCoast kimberlites-lamproites would have been;2000 km to the east-southeast of the Rorai-ma diamond deposits (Fig. 15).

A more likely source was situated to thenorth of the Roraima Basin, either inside theTrans-Amazon orogenic belt or farther north,in the now submerged Atlantic Ocean. TheAfrica–South America reconstruction is in-complete to the north of 68 N, where a missingblock termed ‘‘lost Atlantis’’ has been postu-lated (Rowley and Pindell, 1989; Pindell et al.,2000), combining terranes of Chiapas, Yuca-tan, Cuba, and south Florida (Fig. 15). How-ever, the crustal rocks of most such terranesare much younger, Neoproterozoic (Meyer andMcCallum, 1993; Pindell et al., 2000), and thediscontinuity between South America and Af-rica to the north of 68 N may only representan irregular geographic limit between theGondwana continent and the oceanic crust.Further evidence against either an Africansource or a ‘‘lost Atlantis’’ source is that atthe time of deposition of the Roraima Basin,the Trans-Amazon orogenic belt in the actual

border zone between South America and Af-rica was a mountain chain, so it seems un-likely (but not impossible, e.g., the Fraser Riv-er in Canada and the Brahmaputra inBangladesh) that a major river would havecrossed it. A now partly buried (by coastal-plain sediments) Trans-Amazon orogenic beltto the north of the Roraima Basin, betweenthe Paragua River to the west and the Maza-runi River to the east (Fig. 1), seems the morelikely source area for the diamonds of the Ro-raima Supergroup (Fig. 15).

In summary, our analysis suggests that themain source (primary or not) of the diamondsin the Arai Formation is probably within theTrans-Amazon orogenic belt to the north ofthe Pacaraima Plateau, even though such asource has yet to be found. Such a searchshould not only consider its outcrop belt, butalso where the orogenic belt is currently hid-den by overlapping Cenozoic sedimentary de-posits along the Atlantic coast and perhapseven under the northern part of the main Ro-raima Basin.

CONCLUSIONS

Our study, based on dating of eight sam-ples, has established 11 main conclusions

about the geochronology and provenance ofimportant Proterozoic units in northern SouthAmerica.

Roraima Supergroup

1. The best age estimate for all but the up-permost Roraima Supergroup is 1873 6 3 Ma(Orosirian), determined by U-Pb analyses ofzircons from a green ash-fall tuff of the Uai-mapue Formation, an age that is ;130 m.y.to ;300 m.y older than available Rb-Sr dateson these ash-fall tuffs.

2. Other Paleoproterozoic sedimentary cov-er rocks in the Amazon craton, such as theUrupi Formation in the northeastern Amazon-as State and the Palmares Formation in south-western Para State, have commonly been cor-related to the Roraima Supergroup. So, thesemay also be 130–300 m.y. older than previ-ously thought.

3. The ages of the Cipo and Manga BravaSills, which intrude the Roraima Supergroupin two different stratigraphic levels, are 17876 14 and 1782 6 3 Ma. These ages correlatewith the ages of other doleritic intrusions inGuyana (1794 6 3 Ma; Norcross et al., 2000)and in southwestern Para State (1778 6 9 Ma;


SANTOS et al.

Santos et al., 2001) and conclusively showthat Avanavero magmatism is Statherian andthat it was widespread in the Amazon cratonat the end of the Paleoproterozoic.

4. The main sources of the Arai Formationwere rocks of the Trans-Amazon orogenicbelt, as indicated by the ages of the main clas-tic population of zircons of 2123 6 14 Ma.The secondary sources were rocks with an ageof 1958 6 19 Ma; these may be either SurumuGroup volcanic rocks or its plutonic equiva-lents, the Pedra Pintada suite granitoids.

5. Inherited zircons in the Uaimapue redash-fall tuff suggest reworking in a sedimen-tary environment or contamination of thetuff’s magma with Trans-Amazonian (2170and 2130 Ma) and Paleoarchean (3345 Ma)detrital zircons of the lower Roraima Super-group and with magmatic zircons of the sub-jacent volcanic rocks of the Surumu Group(1985, 1950, and 1940 Ma).

Post-Roraima Stratigraphy

6. The Serra Surucucus Formation is Me-soproterozoic (Calymnian) and has a maxi-mum age of 1552 6 6 Ma. Consequently, thisformation is not related to the Roraima Su-pergroup, because it is at least 314 m.y. youn-ger. Many other isolated Roraima-like outlierssuch as the Neblina, Araca, Tunuı, Autana,Moriche, Duida, and Sipapo units overlyingthe Rio Negro province basement (1.810 Maand 1.795 Ma) may be correlated of the SerraSurucucus Formation and should be separatedfrom the Roraima Supergroup.

7. The Matauı Formation may correlate ei-ther to the Serra Surucucus or Araca Forma-tions, and all may be ;310 m.y. younger thanthe Roraima Supergroup. The unconformityseen in the Pacaraima Plateau between theflat-lying Matauı Formation and the underly-ing Uaimapue Formation indicates the exis-tence of two different, partly overlapping ba-sins. The fill in the older basin was largelyderived from the erosion of the Trans-Amazonorogenic belt, whereas that of the youngercame from both the Trans-Amazon belt andthe Tapajos-Parima orogenic belt.

8. The Araca Formation, with a youngestdetrital-zircon age of 1878 6 15 Ma (1s), isthus not Archean and was metamorphosedduring the first orogeny (Araca event, ;1334Ma) of the Sunsas orogenic cycle. The 1915Ma age of its main source is a typical age ofthe Tapajos-Parima province 80 km to theeast. Minor zircon populations indicate otherpossible sources to the north, from the Trans-Amazon orogenic belt (zircons of ca. 2200

Ma) and the Imataca province in Venezuela(zircons of 3209 and 3126 Ma).

9. The rocks of the Paleoproterozoic Rorai-ma Basin (Roraima Supergroup) and its suc-cessor Mesoproterozoic Neblina Basin (in-cluding the Neblina, Araca, Serra Surucucus,Matauı, and other units) remained buried fromProterozoic time until the Late Jurassic, whenthey were block uplifted during the Takutuevent.

Diamonds

10. Limits on the age of the Paleoprotero-zoic primary sources of the diamonds of theArai Formation are 2.25–2.02 Ga (the age ofthe Trans-Amazonian granitoid-greenstonerocks) and 1880 Ma (the age of the lower Ro-raima Supergroup).

11. The most probable source for the dia-monds found in the Roraima Supergroup isthe now partly buried (by coastal-plain sedi-ments) Trans-Amazon orogenic belt to thenorth of the Roraima Basin, between the Par-agua River to the west and the Mazaruni Riverto the east.

ACKNOWLEDGMENTS

This research was supported by the Conselho Na-cional do Desenvolvimento Cientıfico e Tecnologico(CNPq) and by Companhia de Pesquisa de RecursosMinerais (CPRM), Brazilian Geological Survey.Zircon and baddeleyite analyses were carried out ona sensitive, high–mass resolution ion microprobemass spectrometer (SHRIMP II), operated by a con-sortium consisting of the University of WesternAustralia (UWA), Curtin University of Technology,and the Geological Survey of Western Australia,with the support of the Australian Research Council.We thank CPRM’s geologists Sandoval Pinheiro andRaimundo Gato D’Antona for donation of two rocksamples and geographic details. Santos collected theisotopic data while a visiting doctoral student in theCentre for Global Metallogeny at the University ofWestern Australia. Paul Potter is indebted to Insti-tuto de Geociencias of the Universidade Federal doRio Grande do Sul (UFRGS), Brazil, for their sup-port and hospitality. Sincere thanks to Paul Renneand Ken Ludwig for their revisions and to MaryEberle for help on final editing.

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