Carbon and oxygen isotope geochemistry of Ediacaran outer platform carbonates , Paraguay Belt , central Brazil

After the late Cryogenian glaciation the central region of Brazil was the site of extensive deposition of platformal carbonates of the Araras Group. This group includes a basal cap carbonate sequence succeeded by transgressive, deep platform deposits of bituminous lime mudstone and shale. Facies and stratigraphic data combined with carbon and oxygen isotopic analyses of the most complete section of the transgressive deposits, exposed in the Guia syncline, were used to evaluate the depositional paleoenvironment and to test the correlation of these deposits along the belt and with other units worldwide. The studied succession consists of 150 m thick tabular beds of black to grey lime mudstone and shale with predominantly negative δCPDB values around –2.5 to −1◦/◦◦ . The δCPDB profile of Guia syncline shows a clear correlation with the upper portion of Guia Formation in the Cáceres region, about 200 km to the southwest. The δCPDB profile of the Araras Group is comparable with δCPDB profiles of Ediacaran units of the southern Paraguay Belt, western Canada, and the Congo and Kalahari cratons. Moreover, facies distribution, stratigraphy and the carbon isotopic profile of the Araras Group match the middle Tsumeb Subgroup in Namibia, which reinforces the Ediacaran age assigned to the Araras Group.


INTRODUCTION
The transition zone between the southern margin of the Amazon Craton and the northern portion of the Paraguay Belt is an extensive thrust-and-fold belt formed during the final stages of Brasiliano-Pan-African orogenesis (ca.540-500 Ma, Almeida 1984, Trompette 2000, Alvarenga et al. 2000), as a result of the convergence and collision of the Amazônia (West), São Francisco-Congo (East) and Paraná (South) blocks (Almeida 1984, Basei and Brito Neves 1992, Alkmim et al. 2001).It is inter-Fig.1 -Geologic map of Paraguay Belt with location of the Guia region and detailed geologic section of Guia syncline (after Almeida 1964) with location of studied profiles.genic diamictites and includes the first well-documented cap carbonate sequence in South America (Nogueira et al. 2003).Paleomagnetic data indicate a low paleolatitude (22+6/−5 • ) for the Amazon block, right after deposition of the diamictite (Trindade et al. 2003).The post-glacial cap carbonate was succeeded by an impressive transgression recorded by a thick accumulation of bituminous lime mudstone and shale, discontinuously distributed for more than 400 km in the region, and considered as deep platform deposits (Nogueira et al. 2003).The most complete section of these deposits occurs in the Nossa Senhora da Guia mine, preserved in an asymmetric syncline with NE-SW-oriented axis, located 30 km northwest of Cuiabá (Fig. 1).Facies and stratigraphic data combined with carbon and oxygen analysis were used to evaluate the depositional paleoenvironment and to test the correlation of these deposits along the belt and with other units worldwide.

STRATIGRAPHY AND AGE
Three major lithostratigraphic units may be distinguished in the region (Fig. 2), from base to top: the Puga Formation (diamictite, siltstone with dropstones); the Araras Group (dolostone, limestone, sandstone); and Alto Paraguay Group (sandstone, mudstone and calca- renite).The laterally extensive Araras Group is interpreted as platform carbonate deposits, as suggested by its homogeneous, predominantly deep-water lithofacies exposed on the craton and in the Paraguay Belt.The stratigraphic succession of the Araras Group includes megacycles, cycles and event beds distributed in most of 600 m (composite section) with distinct isotopic signatures (Nogueira et al. 2007).In particular, the cap carbonate sequence (Hoffman and Schrag 2002) is characterized by the presence of the Puga cap carbonate, predominance of deep-water deposits, CaCO 3 -oversaturation events, and δ 13 C with predominantly negative values (Nogueira et al. 2003).
The carbonate rocks of the Araras Group have not yet been dated radiometrically, nor have tuffs or volcanic rocks been identified.Reported Rb-Sr ages for mudrocks of the overlying Alto Paraguay Group (569 ± 20 Ma, Cordani et al. 1978, Bonhomme et al. 1982;and 660 ± 60 Ma, Cordani et al. 1985) are open to debate.The Rb-Sr age of the São Vicente Granite (483 ± 8 Ma, Almeida and Mantovani 1975), an anorogenic granite intruding the Cuiabá Group (a lateral equivalent of the studied units of the Araras Group), provides a minimum age for the sedimentary succession of the Paraguay Belt.The age of the Araras Group has been inferred in part on the basis of lithostratigraphic correlation with carbonate rocks of the Corumbá Group, which crop out about 500 km south of the study area.Nevertheless, this correlation is speculative because facies, sedimentary environments, chemostratigraphy of carbonates and fossils are different (P.C. Boggiani, unpublished data, Gaucher et al. 2003, Alvarenga et al. 2004, Figueiredo et al. 2006).Although lithologically similar, diamictites assigned to the Puga Formation in its type area (southern Paraguay Belt) and in the northern part of the belt may be different stratigraphic units with different ages.Recent descriptions of more than one glacial horizon both in the Corumbá Group (Boggiani et al. 2004) and in the northern Paraguay Belt (Serra Azul Formation, Figueiredo et al. 2004, 2006, Alvarenga et al. 2007) make lithostratigraphic correlations more complex.The Corumbá Group is notable for the occurrence of Cloudina, an index fossil of the latest Neoproterozoic, and considered contemporaneous with the Ediacara biota (Germs 1972, Grant 1990, M.F.Zaine, unpublished thesis, Zaine and Fairchild 1985, 1992, Gaucher et al. 2003).The Araras and Corumbá groups overlain glacial deposits.

SAMPLING AND ANALYTICAL METHODS
Sampling for C-and O-isotope analyses were performed on 38 fine-grained limestone samples, collected along 150 m of two stratigraphic sections of the Guia Formation, organized in a composite stratigraphic profile representative of the proposed stacking for the Araras Group (Fig. 3).As the carbonate succession of this area was affected by low-grade metamorphism, homogeneous samples with little diagenetic or metamorphic alteration were selected, although some dolomitized samples were also analyzed.Fractured, mineral-filled and weathered zones were avoided.Samples were micro-drilled with a 1 mm drill.
Carbon isotope analyses were performed at the Stable Isotope Laboratory (LABISE) of the Department of Geology, Federal University of Pernambuco, Brazil.CO 2 gas was extracted from powdered carbonate in a high vacuum line after reaction with 100% phosphoric acid at 25 • C for one day (three days were allowed when dolomite was present).The CO 2 was analyzed in a double inlet triple collector SIRA II mass spectrometer following cryogenic cleaning.The results are reported in conventional notation in per mil ( • / •• ) relative to the VPDB (Vienna -PDB or Pee Dee Belemnite) standard.VPDB is a scale recognized by the National Institute for Standards and Technology (NIST) of USA that is used for reporting relative abundances of 13 C or 18 O via the delta notation.The uncertainties of isotope measurements were 0.1% for carbon and 0.2% for oxygen, based on multiple analyses of an internal laboratory standard (BSC, Borborema skarn calcite).

THE OUTER PLATFORM DEPOSITS IN THE GUIA SYNCLINE
The sedimentary succession in the Guia syncline consists of tabular beds of black to grey lime mudstone and shale 150 m in thickness (Fig. 4).Silt and fine sand are disseminated in the limestones and pyrite crystals are found locally.These rocks are disposed in compositionally uniform and laterally extensive beds for dozens of meters in the Nossa Senhora da Guia mine, but certainly extends for more than 200 km to the southwest, reaching the region of Cáceres (Figs. 1, 2).The main sedimentary structure in these rocks is the even parallel lamination but cross-lamination occurs locally associated with thin beds of intraclastic packstone.This succession overlies a diamictite with abundant silty-matrix or gradedpebbly siltstone.The first meters of limestone above the diamictite are dolomitized and exhibit a pinkish color.A level of intensely dolomitized breccia with clasts of fine limestone occurs at the top of section 2 (Fig. 5).
The deep platform setting is indicated by the predominance of low energy carbonate facies (laminated lime mudstone) and suspension deposits (shale).Their distribution in monotonous, laterally extensive beds for more than 200 km, suggest deposition into an ample deep platform (Pfeil andRead 1980, Coniglio andJames 1990).The dark color of lime mudstone associated with bituminous and pyritous shale indicate anoxic conditions responsible for the accumulation and preservation of organic matter.Weak to moderate currents caused ripple migration.Locally brecciated limestone beds were accumulated within the slope setting (Coniglio and Dix 1992).
The position of the Guia syncline, more than 200 km far from the craton, suggests that the studied succession represents the most distal part of Araras carbonate platform.

C AND O ISOTOPES
The δ 13 C PDB values for the carbonate succession of the Guia syncline (Fig. 3) are interpreted as reflecting original seawater composition (δ 18 O PDB > −10 • / •• ), which is corroborated by available Mn/Sr < 2 values (Alvarenga et al. 2004).In section 1 δ 13 C PDB values are uniform, ranging from -1 to −2.5 • / •• .These negative values indicate that the low metamorphic overprint did not affect the original δ 13 C PDB values (Fig. 3).Contrarily, in section 2 (Fig. 3) dolomitization may have modified slightly the δ 13 C PDB values that reach −4 • / •• , as indicated by δ 18 O PDB values lower than −10 • / •• .Strontium isotopic ratios up to 0.71151 in dolostones of the upper part of section 2 were related to low-grade metamorphism (Alvarenga et al. 2004).The isolated occurrence of bituminous limestone and shales in the Guia syncline has been previously positioned as a unit of the Cuiabá Group (Alvarenga et al. 2004), but our data points to a correlation of facies, paleoenvironmental significance and C and O isotopic values between the Cáceres and Guia regions (Fig. 3).The δ 13 C PDB profile at the Guia syncline shows a clear correlation with the upper portion of Guia Formation of Cáceres region, indicating that the succession in the Guia syncline belongs to the Araras Group (Figs. 2 and 3).The present-day distribution of this limestone within a syncline is related to relief evolution forming an isolated testimony.
Another observation concerns the relationship between pink dolostone and diamictite observed in the Guia syncline.This relationship is very similar to that observed in cap dolostones, but is still open to debate mainly because the dolomite is secondary (dolomitized limestone).Moreover, although no single cap carbonate develops the full set of anomalous sedimentary structures observed worldwide above glacial diamictites, this pink dolostone does not exhibit any of them.Thus, no evidence is available to support the interpretation that the lower part of the section of the Guia syncline is a cap carbonate.
It is difficult to estimate the elapsed time of erosion that represents the transgressive boundary between pebbly siltstone and the limestone, but it is possible to consider that the Guia Formation represents a long-term transgression deposit, not influenced by late Cryogenian glaciation.

REGIONAL AND GLOBAL CORRELATIONS
The δ 13 C PDB profile for more than 600 m of the Araras Group, with negative values around 0 • / •• , is comparable with δ 13 C PDB profiles of post-late Cryogenian units of the Kalahari and Congo cratons, western Canada and southern Paraguay Belt (Nogueira et al. 2007).It is worth mentioning the similarities between the stratigraphy and the carbon isotopic profile of the Araras Group in Amazonia and the middle Tsumeb Subgroup in northern Namibia (Halverson et al. 2005).The similarity of facies associations with this Namibian succession is the base for tentatively correlate the Araras Group carbonate rocks to the post-Ghaub time-interval.

CONCLUSIONS
(1) The carbonate succession in the Guia syncline is represented by limestone and shale deposited in an outer platform setting; (2) the clear comparison of facies, paleoenvironment and carbon isotopes with the Guia Formation of the Araras Group in Cáceres region indicate that the studied section in the Guia syncline belongs to the upper part of the Guia Formation; (3) evidence that the pinkish dolostone beds overlying glaciogenic pebbly siltstone in the Guia syncline comprise a cap carbonate has yet to be found; (4) the Guia Formation represents a first-order transgressive deposit postdating the late Cryogenian glaciation; (5) the composite δ 13 C PDB profile of the Araras Group shows the same pattern of other post-Ghaub units worldwide; the similarity of facies associations with the Namibia succession lead us to assign the Araras Group carbonate rocks to the post-Ghaub (Ediacaran) time-interval.Palavras-chave: isótopos de carbono e oxigênio, Ediacarano, Grupo Araras, Formação Guia, Cráton Amazônico, glaciação.

Fig. 2 -
Fig. 2 -Stratigraphic units of northern Paraguay Belt (after Nogueira et al. 2007, modified).The inferred ages based in correlation with Marinoan glaciation and Rb/Sr age for Sepotuba Formation are debatable.

Fig. 3 -
Fig.3-Isotopic correlation of outer shelf platform deposits of Cáceres and Guia regions.The δ 13 C PDB profiles are perfectly comparable and the measured sections in the Guia syncline can be correlated to the top of Guia Formation.

Fig. 5 -
Fig. 5 -Dolomitized breccia with clasts of fine limestone at the top of the section, Nossa Senhora da Guia Mine, Guia syncline.