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VII SSAGI South American Symposium on Isotope Geology Brasilia, 25th-28th July 2010

Isotopic approach of the basement/cover boundary in Arcos region (Brazil):

New evidence of a Neoproterozoic glaciation in the Sao Francisco basin

Matheus Kuchenbecker~, Marty BabinskiZ, Antonio Carlos Pedrosa-Soares3, Leonardo

Lopes-Silva4, Felipe Pimenta4, Mateus Gomes RossiS a Paulo Henrique Amorim Diasl

1 — CPMTC e Programa de Pos Graduagao em Geologia RegionaUIGC/UFMG — Av. Antonio Carlos, 6627, Pampulha, Belo Horizonte/MG —Brasil. CEP: 31270-901. E-mail: [email protected] 2 — CPGeo/IGC/USP —Rua do Lago, 562, Cidade Universitaria, Sao Paulo/SP -Brasil. CEP: 05508-080 E-mail: [email protected] 3 —CPMTC/IGC/iJFMG — Av. Antonio Carlos, 6627, Pampulha, Belo Horizonte/MG —Brasil. CEP: 31270-901. E-mail: [email protected] 4 — Lhoist do Brasil - Av. Jorge dias de Oliva, 3301, Centro,S~o Jose da LapalMG —Brasil. CEP: 33350-000. E-mail: [email protected], [email protected] 5 — Gradua~ao em Geologia/IGC/USP -Rua do Lago, 562, Cidade Universiti;ria, Sao Paulo/SP -Brasil. CEP: 05508-080. E-mail: [email protected]

INTRODUCTION

The Arcos region (Fig. 1), located in the central-western portion of Minas Gerais State, is known in the geological literature by the extensive occurrence of carbonatic rocks of the Bambui Group. In this region, many limestone quarries are mined to produce cement lime.

In this study, we present the first isotopic data obtained from rock samples collected from two boreholes drilled by a mining company in the Arcos region. These boreholes allowed to carry out petrographical, geochemical and isotopic detailed studies of a 175m thick section free of weathering alteration, which includes the granitic basement and ten sedimentary units.

REGIONAL GEOLOGY

The basement of the southern Sao Francisco Craton consists of an Archean block surrounded by a Paleoproterozoic (Rhyacian) orogenic belt (Noce et al. 2000). In Arcos region, the basement comprises the Divinopolis and Campo Belo complexes, which consist of orthogneisses, amphibolites and meta-ultramafic rocks, with minor schists and quartzites, intruded by gabbronorites, granitoids and gabbros (Fernandes &Carneiro 2000, Oliveira & Carneiro 2001). On these complexes lies the Neoproterozoic sedimentary rocks of the Sao Francisco basin, which cover a large part of the Craton. This basin includes a basal diamictite-bearing unit (named Carrancas and Jequitai formations) covered by a thick petite-carbonate succession (the Paraopeba Subgroup of the Bambui Group). In the southeast portion of the basin, C and O isotopic studies suggest a glacial origin for the basal diamictites, which are covered by the Sete Lagoas cap carbonate (Vieira et al. 2007). Babinski et al. (2007) obtained a Pb-Pb isochronic age of 740 f 22 Ma for this cap carbonate and correlated it to the end of the Sturtian glaciation, one of the most intriguing glacial events of Earth's history.

Kuchenbecker et al. Isotopic approach of the basement/cover boundary in Arcos region (Brazil): New evidence of a Neoproterozoic glaciation in the Sao Francisco basin

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VII SSAGI South American Symposium on Isotope Geology Brasilia, 25th-28th July 201Q

450000 7760000

774500 433000

(Basement ®Pelitic Unit - Calcipelites, ■ carbonatic Unit ~C 1` Studied Quarry Gnaiss and granitoids pelites, limestones Limestones J~

Figure I -Location map showing the Arcos region in the Sao Francisco Craton. (I) Basement (> 1.8 Ga); (II) Sao Francisco basin: (III) Phanerozoic cover; FA - Ara~uai Belt; FR - Ribeira Belt; FB -Brasilia Belt; FRP -

Rio Preto and Riacho do Pontal belts; FS - Sergipano Belt

PETROGRAPHY AND STRATIGRAPHY The basement and ten sedimentary units were observed in the studied boreholes. The

basement is represented by a fine- to medium-grained, dark green to gray, granodiorite, which shows a discrete foliation. This rock consists of quartz, plagioclase, microcline and biotite as essential minerals, and monazite, sphene, zircon and garnet, as accessory minerals. Epidote and carbonate record an incipient hydrothermal alteration.

The lowermost sedimentary unit 1 is represented by the c. 0.5 m thick diamictite, with sub-rounded to angular granules to cobbles of granitic rocks, carbonate, siltite, quartz and calcite. These clasts are supported by a greenish matrix of fine sand and mud.

The diamictite is covered by the unit 2, a 9 m thick layer of light gray, impure calcarenite, with sub-rounded to angular granule- to sand-size clasts of quartz, mica, granite and cloritite. Centimetric intercalations of gray mudstone often occur in the unit 2.

Unit 3 is strikingly carbonatic, consisting of a 8 m thick succession of gray calcilutite to fine calcarenite with some horizons of carbonatic shale. Cross lamination occurs at the bottom of the unit and, at the top, millimetric-size aragonite fans are clearly observed. The carbonatic content of the unit 3 decreases to the top, where intercalations of green marl become abundant and record a gradational contact with the unit 4.

A 12 m thick succession of green marl and mudstone characterizes the unit 4. Both contacts of this unit are gradational and marked, at the bottom and top, by the occurrence of centimetric layers of white to rose carbonate.

The unit 5 is a 30 m thick succession of massive, light gray calcarenite with widespread styloliths.

Unit 6 consists of a 31 m thick pile of grey calcarenite with alternating lamellar intercalations and remarkable occurrence of microbial lamination.


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1 VII SSAGI South American Symposium on Isotope Geology Brasilia, 25th-28th Juiy 2010

The 17 m thick unit 7 is similar to unit 6, but shows some centimetric to decimetric intraclastic horizons, which show elongated, sub-rounded, micritic clasts, generally oriented along the bedding.

The unit 8 is defined by the sharp decrease of lamellar horizons, giving place to a 14 m thick interval with frequent granulometric variations from calcarenite to calcissiltite.

A 21 m thick homogeneous pile of massive, fine- to medium-grained, locally intraclastic, oolitic to pisolitic calcarenite characterizes the unit 9.

At the top of the boreholes occurs the unit 10, a thick layer of dolarenite with some stromatolitic structures.

The lithofacies succession suggests atransgressive-regressive cycle with the onset of a new transgressive event at the top. The lower sedimentary units also suggest significant change of source areas during the beginning of the basin filling. The aragonite fans are similar to those found in other occurrences of the Sete Lagoas Formation.

METHODS Samples were collected along the whole section, in intervals of 5 m or less,

accordingly to the rock heterogeneities. All isotopic analyses were carried out at the CPGeo-USP, following the standard analytical procedures. Zircon crystals were separated from the basal granodiorite and dated by U-Pb ID-TIMS technique. Each carbonate sample was detailed studied on thin section before the isotopic analysis. The non-altered portions of the carbonate slices were sampled using a microdrilling equipment, and C and O isotopes were determined on the recovered powder.

RESULTS An Archean age of 2653 f 34 Ma was obtained from U-Pb (TIMS) analyses of five

zircon fractions selected from the granodiorite sample, the basement of the Sao Francisco basin.

The C and O isotopic results suggest three subdivisions, with different isotopic behavior, of the carbonatic succession. At the base, units 2 and 3 show negative values of S13CppB, ranging from -5.4 to -0.7 960, with only two samples presenting weakly positives values (+0,1 and +0,4 960). In this same interval, the S~ gOppB varies significantly, ranging from -13.3 to -6.2 960. The units 4 to 9 show a relatively stable isotopic behavior, Wlth S13Cppg values ranging from +0,4 to +1.9 960, and S180ppB from -9 to -6 %o in the most part of the section. Finally, the unit 10 shows a strong positive trend of S13Cpps values reaching up to

+7,3 96o at the top of the succession. In this interval, S'gOppB data show less negative values, reaching -3.4 %o, the highest value of the section.

DISCUSSION

The Archean age of the basement granodiorite is coherent with the regional context of

this portion of the Sao Francisco Craton. This granodiorite may represent an intrusion

correlated with the granitic unit of the Campo Belo Complex (Fernandes & Carneiro 2000).


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VII SSAGI South American Symposium on Isotope Geology Brasilia, 25th-28th July 2010

The negative excursion of the 813C values in the lower portion of the carbonate pile,

associated with the presence of aragonite fans and the transgressive trend showed by the

lithofacies succession, suggests that the basal portion of the carbonatic column is a cap

carbonate. Similar features were used to recognize cap carbonates elsewhere (Kaufman et al.

1991, Hoffman et al. 1998), and also in other parts of the Sao Francisco Basin (Vieira et al.

2007). Despite the differences between Arcos carbonates and the typical carbonates of the

Sete Lagoas Formation, a chronologic correlation can be suggested based on the stratigraphic

position in relation to the basal diamictite and granitic basement. Therefore, the basal

diamictite and its cap carbonate can be tentatively correlated with the Sturtian glacial event.

The positive S13C excursion in the upper part of the column starts at an important

litologic limit, which exhibits a dolomitic collapse breccia, considered to be the maximum

regression surface (Kuchenbecker et al. in press). This excursion may represent a regional

discordance within the Bambui Group, as also suggested by previous studies (Vieira 2007,

Zalan &Romeiro-Silva 2007, Rodrigues 2008).

REFERENCES

Babinski M., Vieira L.C., Trindade R.I.F. 2007. Direct dating of the Sete Lagoas cap carbonate (Bambui Group, Brazil) and implications for the Neoproterozoic glacial events. Terra Nova, 19, 401 06.

Fernandes R. A. & Carneiro M.A. 2000. O Complexo Metamorpio Campo Belo (Craton Sao Francisco Meridional): unidades litodemicas a evolu~~o tectonica. Revista Brasileira Geociencias, 30: 671-678.

Hoffman P.F., Kaufman A.J., Halverson G.P. 1998. Comings and goings of global glaciations on a Neoproterozoic tropical platform in Namibia. GSA Today, 8, 1-9.

Kaufman A.J., Hayes J.M., Knoll A.H., Germs G.J.B. 1991. Isotopic composition of carbonates and organic carbon from upper Proterozoic successions in Namibia: Stratigraphic variation and the effects of diagenesis and metamorphism. Precambrian Research, 49, 301-327.

Noce C.M., Teixeira W., Qurim~neuer J.J.G., Martins,V.T.S. a Bolzachini E. 2000. Isotopic signatures of paleoproterozoic granitoids from the southern Sao Francisco Craton and implications forthe evolution of the Transamazonian Orogeny. Journal of South America Earth Sciences, 13, 225-239.

Oliveira A.H. & Carneiro M.A. 2001. Campo Belo Metamorphic Complex: Tectonic evolution of an Archean sialic crust of the southern Sao Francisco Craton in Minas Gerais (Brazil). Anais da Academia Brasileira de Ciencias, 73(3): 397-415.

Rodrigues J.B. 2008. Proveniencia de sedimentos dos grupos Canastra, Ibia, Vazante a Bambui — Um estudo de zirco"es detriticos e Idades Modelo Sm-Nd. Tese de Doutorado, Instituto de Geociencias, UnB.

Vieira, L.C., Trindade, R.LF., Nogueira, A.C.R. and Ader, M., 2007. Identification of a Sturtian cap carbonate in the Neoproterozoic Sete Lagoas carbonate platform, Bambui Group, Brazil. Comptes Rendus Geoscience, 339, 240-258.

Zahn, P. V. & Romeiro-Silva, P.C. 2007. Proposta de mudan~a significativa na coluna estratigr~fica da Bacia do Sao Francisco. In: 14°. Simposio de Geologia de Minas Gerais e 10°Simposio de Geologia do Sudeste, p. 79.


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