Marcel Auguste Dardenne

Dolomitization and isotope stratigraphy of the Vazante Formation, Sao Francisco Basin, Brazil

The Vazante Formation consists of approximately 1700 m of mainly microbial mats and stromatolitic reefal lenses that were deposited on shallow marine platform and have been entirely dolomitized. Samples representing different dolomite generations were taken from three boreholes covering the entire spectrum of the Vazante carbonates. Dolomites can be classified, based on petrography and geochemistry, into four generations ranging in crystal size between 4 m (almost micritic) and 3 mm and occurring as both replacements and cements. The Sr/Ca molar ratios, calculated for the dolomitizing fluid (0.0006–0.0138), suggest a contribution from a non-marine, possibly meteoric, water component. The 18 Oa nd 13 C values vary from − 0.1 to − 14.3‰ (PDB) and 0.2 to − 2.3‰ (PDB), respectively. Fluid-inclusion study suggests that dolomitization must have commenced at a temperature lower than about 50 °C. A mixing-zone model of dolomitization for Dolomites I–III satisfies the constraints from elemental chemistry, 18 O, 87 Sr/ 86 Sr and fluid inclusions. The petrographic and chemical criteria of the latest generation, Dolomite IV, reflect conditions of deep burial environment at temperatures above 120–130 °C. The 13 C values show only small variations at the base of the formation, followed by a major negative plunge ( 4‰) at the top, this plunge correlated with the Sturtian glacial phase. In contrast to C-isotopes, only few samples may have retained their near-primary 87 Sr/ 86 Sr values of 0.70614–0.70734. The lowest 87 Sr/ 86 Sr value (0.70614), from fibrous cement in the upper part of the formation, correlates with the negative 13 C shift and is also consistent with the Sr-isotope signature proposed for the glacial Sturtian seawater. The 34 S values, obtained from sulfates trapped in carbonates, range between 10.8 and 16.9‰ with a jump to 21.3‰ in the overlying formation. All isotope signals are within the range suggested for the early Neoproterozoic seawater, but the Precambrian baseline is poorly known and additional work is required to confirm this tentative assignment.

Carbon and oxygen isotope profiles across Meso-Neoproterozoic limestones from central Brazil: Bambuı́ and Paranoá groups

We present carbon and oxygen isotopic data on carbonates along three profiles of the Neoproterozoic Bambui group in central Brazil. This unit covers an area of more than 300 000 km2 and comprises carbonate–silicliclastic sediments at the base that grade into siliciclastic sediments towards the top. The Bambui group overlies by unconformity the Paranoa group, which consists mostly of siltstone, quartzite and minor limestone. The data presented here improve the stratigraphic correlation within the Bambui basin and show that it evolved in an environment significantly different from that of the Paranoa basin. Our data show large fluctuations of δ13CPDB in limestones from the Bambui Group (from +0.8 to +13.5‰) in all the three studied areas. Some of these fluctuations represent stratigraphic markers that can be used as a chronostratigraphic tool within a basin scale. This observation is relevant considering the lack of fossil record and other stratigraphic markers in Neoproterozoic sequences. We also present the first isotopic profiles along the Paranoa–Bambui transition, which shows that the δ13CPDB values grade from +1.0‰ in the Paranoa group, to +2.6‰ in the lower portion of the Bambui group, increasing up to +12‰ in the upper part of this unit. Based on our carbon isotope data, as well as other geological, mineralogical and Nd isotope studies, we argue that the sediments of the Paranoa group were deposited on an open platform that was fully connected to the ocean. On the other hand, the sediments of the Bambui group were deposited in an epicontinental sea and during a tectonic inversion in a foreland basin at about 790–600 Ma. This unit displays an increased amount of clastic sediments upwards. We argue that the high carbon isotope values observed in limestones and marlstones from the Bambui group are correlated to worldwide high carbon isotope values reported for the Neoproterozoic. However, we also point out that novel marine conditions induced by the tectonic inversion of the basin may also have contributed to increase the carbon isotopic composition of the Bambui carbonates.

Proterozoic Age of the Bambui Group in Brazil

Conophyton aff. C. metula Kirichenko is abundant in dolomites of the Paraopeba Formation (Bambui Group) north of Vazante, Minas Gerais, Brazil. That suggests a middle Riphean (Proterozoic) age for the Bambui Group of this region. Such an age, in the range of 950 to 1,350 m.y., and probably closer to the younger number, is consistent with the probable ages of other now known Bambui stromatolites, as well as with the sparse and equivocal radiometric data available. It is older, however, than the Silurian age given on the 1960 geologic map of Brazil and the “upper Eocambrian” age now generally accepted.

The Ribeira fluorite district, southern Brazil

The origin and evolution of different ore deposits grouped in the same district are often complex and may involve inheritance from crustal or mantle geochemical anomalies, remobilization of former ore deposits and a polyphase hydrothermal history. Localized in a Proterozoic basement in the Parana state, the Ribeira fluorite district is such an example composed of three deposit types with distinct geological and geochemical characters. Emplaced at different periods from the late Proterozoic to the Cretaceous, they are roughly aligned along a belt nearly 10 km in width and 50 km in length, the southern boundary of which is a transcurrent fault. Two main ore facies are present: (1) microcrystalline ore (< 0.1 mm grains) and (2) macrocrystalline ore (with a grain size of several millimetres). The former results from the replacement of metalimestones or internal karstic sediments and the latter from microcrystalline ore dissolution and pore precipitation or recrystallization. At least two different groups of source rocks can be proposed for the trapped REE in CaF2: (1) fluorite samples associated with the Mato Preto carbonatitic rocks display a slightly negative ɛNd compatible with a mantle source and a REE pattern with the higher ΣREE and La/Yb ratio in the district; (2) other fluorites have a strongly negative ɛNd (− 14 to − 20) which indicates a crustal source. That fluorine and REE have the same source is possible in strata-bound and fracture-filling deposits, but is doubtful at Mato Preto, the only economic fluorite deposit associated with carbonatite rocks in Brazil. This occurrence within a Precambrian fluorite belt suggests that remobilization of a former strata-bound deposit was a more significant metallogenic process than magmatic differentiation.

ORIGEM E EVOLUÇÃO TECTÔNICA DA BACIA SANFRANCISCANA

This paper presents data about the origin and tectonic evolution of the Phanerozoic cover of the Sao Francisco Craton, defmed as the Sanfranciscana Basin. The tectonic compartmentation allowed the subdivision in the Abaete (south portion) and the Urucuia sub-basins (middle-north portion). The stratigraphy and sedimentology of the Phanerozoic successions are as follows: Santa Fe Group (Permo-Carboniferous) - subdivided into the Floresta and Tabuleiro formations. Areado Group (Early Cretaceous) comprising the Abaete, Quirico and Tres Barras forraations. Mata da Corda Group (Late Cretaceous) composed by the Patos and Capacete formations. Urucuia Group (Late Cretaceous) composed by the Posse and Serra das Araras formations. Chapadao Formation (Cenozoic) represents the sandy, unconsolidated, recent covers of talus, residual or alluvium origin. Paleozoic epyrogenesis characterized the onset of tectonism in the Sanfranciscana Basin. During the Mesozoic the basin has undergone tectonic reactivations finishing with neotectonic activity in Cenozoic times. The origin of the basin, as well as its tectonic evolution had been controlled by the Sao Francisco Craton marginal fold-and-thrust belts (Brasilia and Aracuai belts), by the South Atlantic opening (rift phase) and by transform oceanic fractures (drift phase).

The role of shoshonitic and calc-alkaline suites in the tectonic evolution of the Carajás District, Brazil

Abstract In the Carajas District, the Bahia Prospect and Grao Para volcanosedimentary sequences, both of Archean age, have been the object of extensive petrochemical studies for major, trace, and rare earth elements. In spite of the widespread spilitization that affected the volcanic mafic rocks, the petrochemical nature of the mafic volcanism may still be assessed on the basis of the less mobile elements like P, Cr, Zr, Nb, Ti, and REE. The Bahia Prospect basalts have a petrochemical affinity with Phanerozoic island-arc calc-alkaline basalts. The Grao Para basalts and basaltic andesites, on the other hand, have a petrochemical affinity with Phanerozoic basalts of a mature island arc environment. The petrochemical data presented here lead to a new geotectonic interpretation of the area, where the volcanic sequences are genetically linked to Archean subduction processes.

Geologia do Grupo Paranoá na porção externa da Faixa Brasília

The Paranoa Group represents a psamo-pelitic-carbonated succession deposited in shelf conditions. The stratigraphy of this important lithostratigraphic sequence of the Brasilia Fold Belt was originally proposed by letter-code designation that includes 11 units with the following stratigraphic stacking: SM, R1, Q1, R2, Q2, S, A, R3, Q3, R4 and PC. The present paper formalizes these units and proposes the following denomination to the formations, named from base to top: Ribeirao Sao Miguel, Corrego Cordovil, Serra da Boa Vista, Serra Almecegas, Serra do Parana, Ribeirao Picarrao, Ribeirao do Torto, Serra da Meia Noite, Ribeirao Contagem, Corrego do Sansao and Corrego do Barreiro. The deposition of the Paranoa Group is considered to have been processed in the Mesoproterozoic (1,542 to 1,042 Ma). This age interpretation is supported by the stratigraphic position (as it occurs over the post-rift phase sediments of the Arai Group and under slates and carbonates of the Bambui Group), by the presence of conical stromatolite (conophyton) and by isotopic data. The sedimentation was controlled by transgressive-regressive cycles, including grading and traction processes, debris flows, tides, waves, storms and basin bottom paleogeography. The sediments were affected by low-grade metamorphism and the deformation resulted in the formation of monoclines, different styles of folds (chevrons, in box and cylindrical ones) and folding interference resulting in domes and structural basins. The regional deformation of the Paranoa Group is mainly controlled by the Parana and Rio Maranhao thrust systems and by the Ribeirao Sao Miguel shear belt.

Chapter 48 Neoproterozoic successions of the São Francisco Craton, Brazil: the Bambuí, Una, Vazante and Vaza Barris/Miaba groups and their glaciogenic deposits

Abstract The Neoproterozoic successions of the São Francisco Craton are primarily represented by the Bambuí and Una groups, deposited in cratonic epicontinental basins, and by the Vazante and Vaza Barris/Miaba groups, which accumulated on passive margins on the edges of the craton. The epicontinental basins comprise three megasequences: glaciogenic, carbonate platform (marine) and dominantly continental siliciclastics. Possible correlative sequences are observed in the passive margin deposits. At least two major transgressive–regressive sea-level cycles occurred during the evolution of the carbonate megasequence, which lies above glaciomarine diamictites of probable early Cryogenian (i.e. Sturtian) age. C, O, Sr and S isotope trends from analyses of well-preserved samples, together with lithostratigraphic observations, provide reasonable correlations for most of the Neoproterozoic successions of the São Francisco Craton. The 87Sr/86Sr record of these successions, ranging from 0.70769 to 0.70780, supports the proposed correlation with the Bambuí, Una and Vaza/Barris successions, and with the basal units of the Vazante Group. In addition, C-isotope positive excursions ranging from +8.7 to +14‰ and negative excursions from –5.7 to –7‰ VPDB in the Bambuí, Una and Vaza-Barris successions provide key markers for correlations. The precise ages of the sedimentation in these successions remains a matter of debate, but organic shales of two units of the Vazante Group have been dated by Re–Os techniques in two different laboratories, both yielding Mesoproterozoic ages. The Neoproterozoic and Mesoproterozoic successions preserve significant glaciogenic deposits.

Chapter 3 The São Francisco Palaeocontinent

Abstract The Sao Francisco and the Congo cratons represent sectors of a Neoproterozoic palaeocontinent preserved from the Brasiliano-Pan African orogeny, recorded in their marginal belts. The boundaries between the Sao Francisco Craton and the surrounding belts are marked by intensive folding and overthrusting. In the interior of the craton, Neoproterozoic sedimentary covers are horizontal or gently deformed, forming isolated basins. The term Sao Francisco Basin includes Palaeo- and Mesoproterozoic successions but in this chapter it is restricted to the Neoproterozoic sedimentary successions of the Sao Francisco Supergroup, characterised by a glaciogenic unit at the base and an argillaceous–carbonatic–arkosic unit at the top. The Neoproterozoic evolution of the Sao Francisco Craton, including its marginal belts and sedimentary cover, is reviewed. The lithostratigraphy, Neoproterozoic mineralisations (lead, zinc, fluorine, barium and phosphates) and geochronological and stable isotope studies are also presented. The geological record of glacial events and stable isotope data of related cap carbonates are also discussed.

Chapter 51 The glaciogenic Jequitaí Formation, southeastern Brazil

Abstract Glaciogenic deposits of the Jequitaí Formation (Fm.) are well exposed along the margins of the Serra do Cabral on the São Francisco Craton, southeastern Brazil. The Jequitaí Formation is thin (0–150 m thick), lenticular and overlies the Espinhaço Supergroup on a discrete unconformity. Sandstones show subglacial erosional structures such as grooved and striated pavements oriented ENE–WSW. The Jequitaí Fm. consists of massive and stratified diamictites with granules, pebbles and boulders of gneiss, granite, quartzite and carbonate. At the base, the diamictites are massive, whereas the upper part contains many alternating beds of clast-rich and -poor diamictites. They also contain discontinuous, fine-grained sandstones and a few laminated siltstone–mudstone intercalations. This diamictite association indicates glaciomarine sedimentation. The Jequitaí Fm. covers the São Francisco cratonic domain and its equivalent extends eastward over the Araçuaí fold belt where it is part of the metasedimentary Macaúbas Group, a thick Neoproterozoic unit with metadiamictites, quartzites and schists. The diamictite–turbidite association of the Macaúbas Group was deposited on the border of the Pan-African–Brasiliano rift as gravity flows.