Macaubas Group Research Articles

Provenance of the Ediacaran Salinas Formation (Araçuaí Orogen, Brazil): Clues from lithochemical data and zircon U-Pb (SHRIMP) ages of volcanic clasts

Abstract Salinas Formation occurs in a large region of the Northern Aracuai orogen, Southeastern Brazil. It includes turbiditic wackes (> 10% matrix) to arenites (< 10% matrix), pelites and clast-supported conglomerates, metamorphosed from the biotite zone of the greenschist facies to the sillimanite zone of the amphibolite facies. Salinas Formation lies unconformably on the top of or in tectonic contact with the Macaubas Group, and hosts Cambrian granitic intrusions dated between 540 and 500 Ma. Aiming to unravel sediment provenances for the Salinas basin, we present a detailed lithochemical (45 samples) study on low-grade rocks preserved from the regional deformation, which are found in the type area of the Salinas Formation. In addition, we compare them with deformed and more metamorphic rocks of similar composition but located in other basin sectors. The lithochemical data indicate limited chemical weathering in the sediment sources, good correlations with the mineralogical compositions in respect to the variable amounts of metamorphic minerals typical of pelitic (micas, garnet, and other peraluminous silicates) and psammitic (feldspars, quartz) fractions. The main provenances of sedimentary protoliths are clearly related to continental magmatic arc and active continental margin environments. U-Pb (SHRIMP) analyses performed on zircon grains from clasts of intermediate to felsic volcanic rocks, extracted from a clast-supported metaconglomerate, yield concordant zircon Pb206 /U238 ages from ca. 579 Ma to ca. 697 Ma, with most of the ages in the interval of 587 to 630 Ma. This indicates that the main primary sediment source is the Rio Doce magmatic arc (630-580 Ma), in very good agreement with the arc-related lithochemical signature. Eleven zircon crystals yield a Concordia age of 620 ± 10 Ma, representing an important Ediacaran volcanic episode in the tectonic evolution of the Aracuai orogen.

The Rio Pardo salient, northern Araçuaí orogen: an example of a complex basin-controlled fold-thrust belt curve

ABSTRACT: The Rio Pardo salient, the large antitaxial curve described by the Aracuai fold-and-thrust belt along the southeastern edge of the Sao Francisco craton, is one of the most prominent and one of the least studied features of the Brasiliano Aracuai-West Congo orogenic system (AWCO). In addition to the Archean/Paleoproterozoic basement, the salient is comprised of metasedimentary rocks mainly from the Neoproterozoic Macaubas Group and the Salinas Formation. Its western limb occupies a portion of the Espinhaco ridge, where the NS-trending structures of the Aracuai belt progressively curve NE and E, thereby defining the hinge zone along the Serra Geral on the Minas-Bahia boundary. The eastern limb is NW-trending and marked by a major shear zone. In models postulated to generate the AWCO through the closure of the Neoproterozoic Macaubas basin, this large curve plays a critical kinematic role. Yet, in spite of this, its development is still not fully understood. How did this curve originate? Which factors controlled its generation? Our field study performed in the northern Aracuai orogen characterized the kinematic picture of the salient, and led to a model that addresses these questions. The results we obtained indicate that the Rio Pardo salient developed in response to four deformation phases. The contractional D1 and D2 phases are coaxial and responsible for a craton-directed tectonic transport along the salient’s outer arc, which is coupled with an overall southward motion of the inner arc, thereby giving rise to a rather complex kinematic picture. Furthermore, structures of the D1/D2 phases define a zigzag pattern with alternating NE- and NW-trending segments along the salient’s leading edge. Along the NE-trending segments, the metasedimentary rocks are thrust northwestwards on top of the craton basement, while along the NW-trending segments, the supracrustal rocks are displaced along dextral to reverse-dextral transpressional shear zones located on the basement/cover contact. Structures of the D3 phase, which are well developed in the hinge zone, record a final WSW-ENE contraction, which was responsible for rotation of the preexistent fabric elements around NNW-trending axes and the enhancement of the salient curvature. The D4 phase is extensional and is recorded by two large-scale structures, the Chapada Acaua and Tingui normal shear zones, as well as by the normal-sinistral reactivation of the Itapebi strike-slip shear zone that marks the salient eastern limb. We interpret the initiation of the Rio Pardo salient during the collisional 565-575 Ma D1/D2 phases essentially as a primary arc that is mainly controlled by the geometry of the Macaubas precursor basin. The thickened internal portion of the Rio Pardo salient was affected by extensional tectonism at c. 530 Ma, and is recorded by the D4 deformation phase, which is currently ascribed to the extensional collapse of the Aracuai-West Congo orogen.

Registro e análise de estruturas relacionadas com a tectônica extensional do rifteamento Macaúbas na Serra Central, norte de Minas Gerais

Na região norte da faixa Araçuaí, no domínio fisiográfico do Espinhaço Central, existem registros de bacias sedimentares superpostas instaladas sobre o embasamento do cráton São Francisco. Nesse contexto, a morfoestrutura da Serra Central, norte de Minas Gerais, é sustentada por uma sucessão siliciclástica do Supergrupo Espinhaço superposta a oeste por diamictitos da base do Grupo Macaúbas, representando os registros de rifteamento superpostos ao final do Esteniano e do Toniano Superior, respectivamente. No bordo oeste da Serra Central, são descritas estrias de média a alta obliquidade desenvolvidas sobre os planos de acamamento da sucessão do Grupo Sítio Novo, incompatíveis com a tectônica compressional Brasiliana, que afeta essas rochas. Essas estrias são associadas a degraus e definem vetores de deslizamento interestratais indicativos de movimentação normal a normal destral. Associados a essas lineações, ocorrem veios que também apontam para a atuação de um campo de esforço extensional, com componente destral associada. A análise cinemática e dinâmica dessas estruturas permitiu a interpretação de um regime tectônico distensivo de natureza transtrativa destral e, possivelmente, relacionado ao evento de rifteamento Macaúbas. Nesse cenário, o bloco de capa regional do rifte Macaúbas, representado pela sucessão esteniana do Grupo Sítio Novo, foi deformado sob a atuação de um cisalhamento antitético, com desenvolvimento de um rollover associado. No flanco frontal do rollover, as camadas experimentaram significativa magnitude de rotação, revelando elevado mergulho próximo à borda de falha e favorecendo o desenvolvimento do deslizamento interestratal.

Interação tectônica entre bacias sucessoras no domínio externo do Orógeno Araçuaí: estudo de caso da região de Planalto de Minas, Minas Gerais

O Supergrupo Espinhaço Meridional e o Grupo Macaúbas, em Minas Gerais, são as maiores expressões geotectônicas dos ciclos de fragmentação sucessivos que atingiram o paleocontinente São Francisco-Congo desde o Estateriano até o Criogeniano. O mapeamento geológico em escala 1:25.000 da região de Planalto de Minas, localizada no centro-norte do estado, revelou a ocorrência da Formação São João da Chapada (ca. 1.7 Ga, Supergrupo Espinhaço) diretamente sobreposta por uma sequência metavulcano-sedimentar toniana do Grupo Macaúbas (ca. 889 Ma), aqui designada de Formação Planalto de Minas. A integração geológica-geofísica revelou ainda que esta área permaneceu como um alto estrutural do final do Estateriano até o Toniano. A existência do alto estrutural de Planalto de Minas é o reflexo do complexo arranjo estrutural de rifteamentos sobrepostos, com formação e interação de um meio-gráben, ou sistema ramificado de meio-grabens, que exerceram importante controle tectono-estratigráfico na arquitetura — formação e evolução — da bacia Macaúbas.

Evidência de vida gregária em paleotocas atribuídas a Mylodontidae (preguiças-gigantes)

This article presents evidence of gregarious behavior in paleoburrows attributed to giant sloths. Paleoburrows are biogenic structures in the shape of galleries and halls excavated by extinct giant mammals that lived in the Pliocene and Pleistocene. In the case discussed here, they were excavated in rocks composed by neoproterozoic metadiamictites with hematite of the Macaubas Group (Nova Aurora Formation). The rocks, of glacial origin, underwent metamorphism, were strongly cemented by hematite and exhibit high hardness. In the north of Minas Gerais, were identified new paleoburrows distributed in a grouped manner, located in valleys with steep slopes embedded in geological faults. In each group, the paleoburrows are arranged on both sides of the valley, facing at each other. At the walls and roofs of the paleoburrows, there are hundreds of claw marks, forming parallel double marks and intertwined marks. Silicone molds of the marks were compared with hand bones of Valgipes sp. (Mylodontidae) deposited in the collection of PUC-MG; the size and morphology of the double marks allowed to attribute these paleoburrows to digging mylodontids (two-toed giant sloths). At the lateral walls of the paleoburrows we identified several polished surfaces of elliptical shape, each one with a diameter of approximately 2.4 m. These polished surfaces aroused due to the abrasive action of the giant sloths´ fur at their resting place. A single paleoburrow may show from 1 to 3 of these elliptical surfaces, suggesting a gregarious behavior of the sloths, who slept together in these spaces.

Thermobarometry and electron-microprobe Th-U-Pb monazite dating in garnet metapelites from the Capelinha Formation, Araçuaí Orogen, Brazil

The Capelinha Formation (Macaubas Group) consists of a lower quartzitic unit with metamafic intercalations and an upper metapelitic sequence. It occurs in a complex tectono-metamorphic sector of the Aracuai orogen, where post-collisional collapse-related structures superimposed collisional structures. The garnet-bearing assemblages started crystallization in the collisional deformation stage that formed the main regional foliation around 570 Ma. Garnet porphyroblasts display a well-developed growth zonation of Fe-Mg-Ca-Mn and show, from core to rim, increasing almandine and pyrope contents in contrast with decreasing grossular and spessartine contents. Mineral relations and microstructures provide criteria for local equilibria and a structurally controlled application of geothermobarometers based on cation exchange and net transfer reactions. The P-T values calculated from cores to rims of garnets, aligned along clockwise trends, resulted in increasing temperatures (from 500 oC up to 620 oC) under decompression conditions (from 8.0 kbar to 4.5 kbar). The Th-U-Pb dating of homogeneous monazites by electron microprobe revealed a recrystallization period at around 490 - 480 Ma. These ages can be related to the tectono-thermal event associated with the gravitational collapse, constraining the youngest time limit for metamorphic processes in the Aracuai orogen.

The puzzle assembled: Ediacaran guide fossil Cloudina reveals an old proto-Gondwana seaway

ABSTRACTDuring the Ediacaran the Clymene Ocean separated the Lau-rentia, Amazonia, and Rio Apa cratons from several landmasses to the west forming the proto-Gondwana supercontinent. However, no clear evidence about the existence of Ediacaran epeiric seas over those landmasses has been found. Here we report and discuss the discovery of the Ediacaran guide fossil Cloudina sp. associated with other metazoan body and trace fossils in the Bambui Group (central eastern Brazil). The Ediacaran age of the Bambui Group and the paleogeographic position of Cloudina -bearing successions in Brazil, Antarctica, Namibia, and Argentina suggest a scenario of ocean con-nectivity among coeval intracratonic basins of South America, Africa, and Antarctica at the end of Neoproterozoic time. The new fi nding epitomizes one of the most important paleontological discoveries ever made in South America, helping to solve an old paleogeographic puz-zle of the Gondwana supercontinent.INTRODUCTION The Bambui Group crops out in central eastern Brazil, and is one of the most studied Neoproterozoic sedimentary successions in South America. It comprises an extensive, fl at-lying cratonic cover in erosional contact with Paleoproterozoic and Archean basement rocks of the Sao Francisco craton. In its eastern and westernmost occurrences, the Bam-bui Group is deformed and metamorphosed by the Brasilia and Aracuai mobile belts, respectively (Fig. 1A). The regional tectonostratigraphy has been interpreted as refl ecting deposition in a foreland basin related to Gondwanan collisions involving the Sao Francisco craton (Pimentel et al., 2011).The Bambui Group (BG) overlies rocks of the Macaubas Group and Jequitai Formation and the Carrancas conglomerate, units supposedly de-posited during Sturtian glaciation. The unit is 700–1000 m thick (Misi et al., 2007) and comprises limestones and dolomites of the Sete Lagoas For-mation at the base that grade upward to shales and siltstones of the Serra de Santa Helena Formation, followed by marls, siltstones, limestones, and sandstones of the Lagoa do Jacare, Serra da Saudade, and Tres Marias Formations (Fig. 1B).The precise age of the BG is not well established, precluding accurate correlation with other supposedly coeval successions such as the Corumba Group (Brazil), Itapucumi (Paraguay), Arroyo del Soldado (Uruguay), Si-erras Bayas (Argentina), and Nama Group (Namibia). The presence of

Monazita em veios de quartzo da Serra do Espinhaço Meridional(MG): mineralogia, idades LA-ICP-MS e implicações geológicas

Due to the discovery of monazite occurrences in an uncommon geologic setting, in quartz veins cutting rocks of the Southern Espinhaco Range, a study was carried out on the geology and mineral chemistry of these deposits in three localities ( Buenopolis, Olhos d’Agua and Diamantina) , as well as to make U-Pb laser ablasion MC-ICP-MS geochronological determinations in the Diamantina and Olhos d ’A gua samples. In Buenopolis, monazite specimens are common as twin crystals, occurring in colluvial gravel deposits in a contact zone between the Espinhaco Supergroup with the Macaubas Group. The Olhos d’Agua occurrence is in metric oval-shaped pockets within metapelitic rocks of the Macaubas Group, where large crystals have been discovered. In Diamantina, millimeter-size monazites were studied as broken grains recovered in a colluvial deposit near the Caete-Mirim river. Certainly in the first and in the third cases, such grains came from the abundant regional quartz veins. Electron microprobe analyses showed similar oxide values for the three deposits, but with some chemical specific aspects. The samples have been identified as monazites-(Ce), locally Nd-rich; the Diamantina and Olhos d’Agua deposits seem to have some chemical similarities, although in general there are no absolute resemblance between the two deposits. The U-Pb results showed an older age of ~491 My that probably indicates the crystallization age of the mineral, and a complete extension between 490-440 My that could mean a later reheating periods. In geological terms, such ages can signify the last events related to the Brasiliano Cycle in the region.

A Formação Salinas, Orógeno Araçuaí (MG): história deformacional e significado tectônico

The Salinas Formation, made up of metasandstones, metapelites and metaconglomerates, occurs mainly in the surroundings of the town of Salinas, northern Minas Gerais. In this region, rocks of the Salinas Formation lie unconformably on top of the Macaubas Group on the west, and are intruded by Neoproterozoic and Cambrian granites on the east. Four generations of deformational structures were documented in this region, three of them affecting the Salinas rocks. The structures of the oldest phase, D D, representing an episode of syn-sedimentary deformation, include folds, faults and convolute bedding. The subsequent phases, D1, D2 and D G are related to the development of the Aracuai orogen. The D1 phase, represented by folds, shear zones, and a whole series of small-scale elements, took place during the Brasiliano event (585 to 560 Ma) and was associated to regional metamorphism in the green schist to amphibolite facies conditions. The D2 phase, corresponding to the extensional collapse of the orogen (520 to 500 Ma), is recorded by fold trains that cascade down to ESE. These structures are associated to a crenulation cleavage and normal-sense shear zones, which affect only the metasediments of the Macaubas Group. The D G phase, related to the emplacement of granite bodies, caused folding and contact metamorphism in the Salinas rocks. The nature, age, deformation history and tectonic setting of Salinas Formation in the Aracuai orogen point out to a deposition in a syn-collisional basin - a flysch basin -, developed between a passive margin and an orogenic front.

A sequência pré-glacial do Grupo Macaúbas na área-tipo e o registro da abertura do rifte Araçuaí

The Macaubas Group on the Macaubas river basin (MG) and the register of the rifting phase of the Aracuai Basin. This paper presents the sedimentary evolution and a new stratigraphy sug- gested by the rift phase of the Macaubas Group and Espinhaco Supergroup of the Aracuai orogen, located at the Macaubas River basin (Minas Gerais State). The Espinhaco Supergroup is formed by the Resplandecente Formation (aeolian depositional system). Paleocurrent data indicate that the source area of the Resplandecente Formation was the Sao Francisco craton region. The Macaubas Group represents the precursor basin of the Neo- proterozoic Aracuai orogen and the pre-glacial sedimentation is represented by two units: Matao and Duas Barras formations. The basal unit of the Macaubas Group - Matao Formation - is formed by lower breccia and conglom- erates and represents deposition under unstable conditions, covered by a thick package of fine sandstone (shallow marine transgressive sediments). Detrital zircon grains dated at ca. 1.16 Ga (U-Pb SHRIMP) limit the maximum age of the Matao Formation. Duas Barras Formation represents pre-glacial marine sediments. Both Sao Francisco craton and Congo craton could be the source for Duas Barras sediments.

Glaciação de montanha em faixas orogenéticas Brasilianas no Craton do São Francisco? Uma revisão e uma proposta

Neoproterozoic sedimentary deposits, partially glaciogenic, related to Brasiliano orogenic belts surrounding the Sao Francisco Craton are described and interpreted under a new perspective. The occurrences include: Brasilia Belt: diamictites of the Sambura Formation, diamictites occurring at the base of the Vazante Group (Santo Antonio do Bonito Formation), diamictites at the top of the Vazante Group, diamictites of the Cristalina Dome, diamictites of Formosa, Padre Bernardo, Nova Roma and Minacu localities in Goias state, and diamicites of Campos Belos, in Tocantins state. Aracuai Belt: Nova Aurora Formation, in the upper section of the Macaubas Group, Salinas Formation in Minas Gerais, Salobro Formation in Bahia. Sergipano Belt: diamictites of the Palestina Formation, in Sergipe state. Rio Preto Belt: diamictites of the Canabravinha Formation, in Bahia state. Ribeira Belt: the upper section of the Andrelândia Basin, in Minas Gerais state. We review these glaciogenic deposits which had previously been either related to the Sao Francisco glacial event, not considered to be glaciogenic or simply not known. Further, we suggest the presence of a mountain related glacial event, directly related to the tectonic evolution of the different belts of the Brasiliano orogeny. Many occurrences of iron formation and/or phosphorite are registered in these sediments partially glaciogenic and attributed to the Ediacarian. This geological scenario in Brazil is compatible with african examples.

EVOLUÇÃO GEOQUÍMICA E PETROGÊNESE DE CHARNOCKITOS ÍGNEOS NO CINTURÃO MÓVEL COSTEIRO DO BRASIL

Rochas máficas e félsicas com hiperstênio são importantes componentes da zona de transição entre o cráton do São Francisco e o Cinturão Móvel Costeiro Neoproterozóico no extremo sul do Estado da Bahia. Os charnockitóides possuem teores 54 a 70% de SiO2, correspondendo de termos intermediários a ácidos. A geoquímica revela um caráter metaluminoso e uma assinatura cálcio-alcalina/álcali-cálcica, compreendendo rochas ricas em Ti, P, Zr, Ba e ETR, ligeiramente enriquecidas em Fe e K e pobres em Mg, Al e V, sendo variavelmente empobrecidas em Ca, se comparadas com rochas granitóides de teor de sílica similar. Possuem teores altos de ETR, com padrões moderadamente a fortemente fracionados para os termos mais básicos e ácidos. A interpretação petrogenética baseia-se na(s): (i) natureza dos contatos, os quais mostram que as rochas félsicas intrudiram nas rochas máficas; (ii) texturas magmáticas das rochas charnockíticas félsicas em contraste com texturas inequivocamente metamórficas dos membros finais máficos; (iii) mineralogia predominantemente anidra; e (iv) trends homogêneos em muitos diagramas de variação, além do caráter geoquímico não-depletado das rochas félsicas; sugerindo que magmas tipo C são produzidos pela fusão de fontes máficas a intermediárias de alto-K, nas quais a hornblenda foi previamente desidratada. O baixo conteúdo de CaO e alta razão Ca/Sr dos magmas tipo A é uma evidência que suporta plagioclásio residual, mas não hornblenda, nas rochas fontes da região.

GEOCRONOLOGIA U-Pb (SHRIMP) E Sm-Nd DE XISTOS VERDES BASÁLTICOS DO ORÓGENO ARAÇUAÍ: IMPLICAÇÕES PARA A IDADE DO GRUPO MACAÚBAS

The age and stratigraphic position of the basaltic greenschists of the Rio Preto valley, located in the western part of the Aracuai Belt ( ca . 60 km north of Diamantina, Minas Gerais) were a matter of controversy in the geologic literature, because several authors correlated them to the Neoproterozoic Macaubas Group but others to the lower Espinhaco Supergroup (ca. 1.7 Ga). However, detailed studies demonstrate that these greenschists represent an interplay of submarine basaltic volcanism, volcanoclastic sedimentation and fire fountaining, and that they belong to the Chapada Acaua Formation of the Macaubas Group (Gradim et al ., 2005). Geochemical studies indicate that the greenschist protoliths evolved in a continental intraplate environment. Zircon crystals were separated from a greenschist sample and analyzed by the U-Pb SHRIMP method. A Sm-Nd whole-rock isotopic analysis was obtained from the same sample. The Sm- Nd model age of ca . 1.52 Ga suggests that the greenschist protoliths are younger than the magmatism of the Espinhaco rift. Most analyzed zircon crystals show features of detrital grains. The older ages indicate zircon grains inherited from the Archean- Paleoproterozoic basement and from magmatic rocks of the Espinhaco rift. The younger U-Pb values constrain the maximum age of the greenschist protoliths at ca. 1,16 Ga.

Xistos verdes do alto Araçuaí, Minas Gerais: vulcanismo básico do rifte neoproterozóico macaúbas

Greenschists ( s.s. ) of the upper valley of the Aracuai River were grouped in four lithofacies, based on preserved macroscopic textures: i) schists with pillow structures and columnar joints, ii) banded schists, iii) breccias, and iv) schists with no primary structures. An interpretation of environments and processes involved in the formation of these lithofacies indicates interplay between submarine basaltic volcanism, volcaniclastic sedimentation and fire fountaining. The litho-structural framework indicates normal contact between the greenschists and the metasedimentary rocks of the Chapada Acaua Formation. U-Pb SHRIMP dating constrains the maximum age of the protoliths at ca. 1.1 Ga. The Sm-Nd model age of ca . 1.5 also corroborates that the greenschists are younger than the magmatism of the Espinhaco rift. The greenchists have tholeiitic protoliths, a within-plate basalt signature and inherited zircon crystal from older felsic rocks. However, a Nd(900 Ma) slightly positive (+ 0.23) and samples with oceanic signature suggest that the basic magma migrated trough a thinned continental crust, during the late rift stage. All data sets support the correlation between these rocks and the Rio Preto Member of the Chapada Acaua Formation (Macaubas Group).

A Formação Salinas na área-tipo, Ne de Minas Gerais: uma proposta de revisão da estratigrafia da Faixa Araçuaí com base em evidências sedimentares, metamórficas e idades U-Pb shrimp

The Salinas Formation has been considered to be a stratigraphic unit of the Macaubas Group. This group comprises the rift to passive margin sequences of the precursor basin of the Neoproterozoic Aracuai Orogen, eastern Brazil. However, new roadcuts show extensive and spectacular outcrops with very well-preserved sedimentary structures, allowing detailed studies in the type-locality of this formation, located in the Salinas town and surroundings, northeast Minas Gerais State. In its type-locality, the Salinas Formation consists of graywacke, pelite and clast-supported conglomerate, metamorphosed in the greenschist facies. The sedimentary lithofacies are grouped into three facies association (shelf, slope and deep-sea), indicating sedimentation from shelf deposits to deep-water turbidites. The shelf sedimentation was influenced by storm-wave during deposition. The slump and deep-sea deposits were generated by gravitational flows and high- to low-concentration turbidity currents. Shelf sandstone, clast-supported conglomerate and proximal to distal turbidites outline a submarine fan system. U-Pb SHRIMP data from detrital zircons of graywacke samples indicate a maximum sedimentation age of ca . 568 Ma. Thus, the Salinas Formation is much younger than the Macaubas Group, and represents late orogenic deposits ( ca . 568- 500 Ma). The distal, passive margin unit of the Macaubas Group is now called Ribeirao da Folha Formation ( ca . 800 Ma). Keywords : Salinas Formation, Macaubas Group, Aracuai Belt, sedimentary facies, depositional environment, U-Pb geochronology

SERRA DO ESPINHAÇO MERIDIONAL: UM ORÓGENO DE COLISÃO DO MESOPROTEROZÓICO

The Serra do Espinhaco Meridional (SdEM) is built mostly by Paleo- and Mesoproterozoic rocks of the homonymous supergroup. This supergroup includes sedimentary and volcanic rocks of the Guinda Group deposited between 1750 and 1700 Ma in a continental environment. The rift breakup generated a passive continental margin recorded by sedimentary and volcanic rocks (including banded iron formations) of the Serro Group.Tec tonic inversion occurred between 1.5 and 1.3 Ga. Syntectonic crustal stacking resulted in the formation of sedimentary throughs in fr ont of the orogenic belt, initially as a foredeep basin which is represented by deposits of the Conselheiro Mata Group, and later as foreland basins along the margins of the belt. During Late Mesoproterozoic times (about 1050 Ma) this orogenic belt underwent a mountain glaciation, recorded by the glaciogenic deposits of the Macaubas Group along the borders of the SdEM. After an episode of intracontinental magmatism (Pedro Lessa Suite) the SdEM together with a large region of the adjoining Sao Francisco craton, was covered by an epicontinental sea and, by that time, carbonate and pelitic rocks of the Bambui Group were deposited. The SdEM underwent uplift during the Late Neoproterozoic, causing local gravitional collapse of units of the Bambui Group at the western border of this orogenic belt.

Constraining the age of Neoproterozoic glaciation in eastern Brazil: first U-Pb (shrimp) data of detrital zircons

Detrital zircons extracted from a diamictite-bearing formation of the Macaubas Group, located within the Aracuai Belt (Eastern Brazil), were dated using a Sensitive High-Resolution Ion Microprobe (SHRIMP). The youngest reliable measurement sets the upper limit for the age of the glaciation at 950 Ma. The main group of zircons (1000-950 Ma) is a good evidence of a magmatic episode, interpreted as related to the rift stage of the Aracuai-West-Congo basin system.

Neoproterozoic oceanic remnants in eastern Brazil: Further evidence and refutation of an exclusively ensialic evolution for the Araçuaí–West Congo orogen

The Aracuai (eastern Brazil) and West Congo (southwestern Africa) belts are counterparts of the same Neoproterozoic orogen located between the Sao Francisco and Congo cratons. The Macaubas Group represents a major passive margin sequence and is a key unit for interpreting the evolution of that orogen. The Salinas Formation is the distal rock assemblage of the Macaubas Group and consists of a deep-sea sand-mud sequence, and a volcanic-sedimentary unit called the Ribeirao da Folha facies. The latter includes metamorphosed volcanic-exhalative sediments associated with ocean-floor basalts (amphibolites). The magmatic protoliths of these amphibolites crystallized at about 816 ± 72 Ma (Sm-Nd whole-rock isochron, ϵ Nd(t) =+3.8 ± 0.2). Regional metamorphism reached the amphibolite facies at about 630 Ma (Rb-Sr whole-rock isochron), when slabs of ultramafic rocks were tectonically emplaced over the Ribeirao da Folha facies. We consider this volcanic-sedimentary facies and the coeval slabs of ultramafic rocks to be remnants of a branch of the Adamastor-Brazilide ocean. The extensive occurrence of syntectonic to late tectonic calc-alkalic granitoids along the internal domain of the Aracuai belt implies that a reasonably large amount of ocean crust was consumed, via an east-dipping subduction zone, during formation of the Aracuai–West Congo orogen.

AS TAFROGÊNESES SUPERPOSTAS ESPINHAÇO E SANTO ONOFRE, ESTADO DA BAHIA: REVISÃO E NOVAS PROPOSTAS

The built up of the Espinhaco rift in the states of Bahia and Minas Gerais, took place along NS and NW-SE faults. In the State of Bahia the initial phase of the taphrogeny was marked by the Rio dos Remedios volcanism, dated at about 1750 Ma (U-Pb), and followed by the deposition of more than 5,000 meters of continental and shallow marine sediments. These units represent the Espinhaco supergroup in the northern part of the Espinhaco range and in the Chapada Diamantina plateau. A second extensional phase at the beginning of the Neoproterozoic, between 1000 and 900 Ma, lead to a new rifting episode in the area of the Espinhaco supergroup and its basement. Old faults of the former aborted Espinhaco rift were reactivated. A triple junction was developed in the southeastern margin of the Sao Francisco craton. In the lesser active arm of this junction the Santo Onofre rift was formed, penetrating the site of the Espinhaco supergroup in the State of Bahia. Along this neoproterozoic rift continental and marine sediments including turbiditic lobes were deposited. These units are represented by the redefined Santo Onofre group, which outcrops along the strike of the Espinhaco supergroup. The southern extension of the redefined Santo Onofre group is represented by the Macaubas group, in the upper Pardo river, northern Minas Gerais. Both southernly located limbs of the triple junction are represented by the assimetric Macaubas rift, which opens to the east to form a passive continental margin - the Aracuaf basin. In the Chapada Diamantina plateau, a discordance at the basis of the Morro do Chapeu formation, is interpreted to be genetically related to the deposition of the redefined Santo Onofre group. The Morro do Chapeu formation represents therefore also a neoproterozoic unit. On the other hand, the Tombador and Caboclo formations are mesoproterozoic platform sediments (∼1200-1000 Ma), probably related to a geodynamic event of grenvillian age.The closing of the Macaubas basin during the Brasiliano orogenesis, lead to the formation of the Aracuai belt and to the deformation of the Santo Onofre group along the so called Paramirim deformation corridor in the interior of the Sao Francisco craton. This deformation involved also the Espinhaco system and its basement.

GEOLOGIA DO DOMO DE CRISTALINA, GOIÁS

The Cristalina dome is located in sout heastern of the State of Goias and is made up of rocks of the Canastra, Parana and Macaubas Groups. Phyllites of the Canastra Group were thrust from west to east upon rocks of the Macaubas Group. Thus a sequence of allochtonous phyllites Canastra lies in contact with tillites and varvites Macaubas, in obvious tectono-metamorphic contrast, interpreted as an overthrust fault or nappe. The phyllites of the Canastra Group display folds related to three phases of deformation while metarhythmites of the Paranoa Group display folds to one phase of deformation only, thus defining their relative stratigraphic positions. The tillites and varvites of the Macaubas Group provide evidence for the existence of an Upper Protetorozoic (950 Ma) glaciation.