Francisco Craton Research Articles

Archean crustal evolution in the Southern São Francisco craton, Brazil: Constraints from U-Pb, Lu-Hf and O isotope analyses

In this study we present U–Pb and Hf isotope data combined with O isotopes in zircon from Neoarchean granitoids and gneisses of the southern São Francisco craton in Brazil. The basement rocks record three distinct magmatic events: Rio das Velhas I (2920–2850Ma), Rio das Velhas II (2800–2760Ma) and Mamona (2750–2680Ma).The three sampled metamorphic complexes (Bação, Bonfim and Belo Horizonte) have distinct εHf vs. time arrays, indicating that they grew as separate terranes. Paleoarchean crust is identified as a source which has been incorporated into younger magmatic rocks via melting and mixing with younger juvenile material, assimilation and/or source contamination processes. The continental crust in the southern São Francisco craton underwent a change in magmatic composition from medium- to high-K granitoids in the latest stages, indicating a progressive HFSE enrichment of the sources that underwent anatexis in the different stages and possibly shallowing of the melting depth. Oxygen isotope data shows a secular trend towards high δ18O (up to 7.79‰) indicating the involvement of metasediments in the petrogenesis of the high potassium granitoids during the Mamona event. In addition, low δ18O values (down to 2.50‰) throughout the Meso- and Neoarchean emphasize the importance of meteoritic fluids in intra-crustal magmatism.We used hafnium isotope modelling from a compilation of detrital zircon compositions to constrain crustal growth rates and geodynamics from 3.50 to 2.65Ga. The modelling points to a change in geodynamic process in the southern São Francisco craton at 2.9Ga, from a regime dominated by net crustal growth in the Paleoarchean to a Neoarchean regime marked by crustal reworking. The reworking processes account for the wide variety of granitoid magmatism and are attributed to the onset of continental collision.

Paleogeography of the Congo/São Francisco craton at 1.5 Ga: Expanding the core of Nuna supercontinent

The Congo/São Francisco (C/SF) craton, one of the largest cratons in Proterozoic paleogeography, has been lacking reliable paleomagnetic data for the supercontinent Nuna interval (ca. 1600–1300Ma). Here we provide a new paleomagnetic key pole for this craton from recently dated mafic dykes in the Curaçá (1506.7±6.9Ma) region of Brazil. The characteristic remanent magnetization (ChRM) direction D=070.6°, I=54.0° (k=22.1 and a95=13.1°) corresponds with a paleomagnetic pole at 10.1°N, 009.6°E (K=15.6, A95=15.8°), which places C/SF craton in moderate paleolatitudes at the time of remanence acquisition. Primary nature of the paleomagnetic remanence is supported by a baked-contact test. A similar ChRM direction was obtained for four Mesoproterozoic mafic intrusions in Chapada Diamantina region. The new pole, only from Curaçá, for C/SF allows us to reconstruct the extended core of the supercontinent Nuna at 1.5Ga. Based on coeval 1.5Ga and 1.38Ga magmatism in Baltica, Siberia and C/SF, we favor the position where Southwest Congo is reconstructed against present South-Southeast (S-SE) Baltica. We explore two alternative 1.5Ga reconstructions of Nuna’s core. In both of them Baltica and Laurentia are shown in the well-defined NENA (Northern Europe North America) fit, together with Siberia in a tight fit to northern Laurentia. In reconstruction option A, more traditional fit of Amazonia with Baltica is shown, modified from the geologically based SAMBA (South AMerica BAltica) model to accommodate paleomagnetic data. In this option, however, West Africa must be extricated from SAMBA because C/SF has taken its place. For reconstruction option B, Amazonia is shifted to lie adjacent to NE Laurentia and West Baltica. In both options SW Congo is reconstructed against S-SE Baltica, but in option B there is a tighter fit between them, and there is a better match with our new paleomagnetic data for C/SF. In either option, separation of C/SF from Baltica and Siberia probably occurred at 1.38Ga, the age of pronounced mafic magmatism throughout this sector of Nuna.

A complete Wilson Cycle recorded within the Riacho do Pontal Orogen, NE Brazil: Implications for the Neoproterozoic evolution of the Borborema Province at the heart of West Gondwana

The Riacho do Pontal Orogen (RPO) is part of the extensive Brasiliano/Pan-African orogenic system in northeastern South America (Borborema Province). This region is crucial for understanding the Neoproterozoic evolution of West Gondwana because it occupies a strategic position between three of its most important constituents, namely the São Francisco/Congo, Amazon and West Africa cratons. Based on lithostratigraphic, lithochemical, geochronological and isotopic data, a five-stage model for the geodynamic evolution of the RPO and adjacent areas in the Borborema Province is proposed: (1) rift phase – Development of a triple junction rift system at ca. 900–820Ma ago, probably related to the emplacement of a mantle plume and leading to intense mafic–ultramafic magmatism of the Brejo Seco layered intrusion (ca. 900Ma). This was followed by the development of a continental rift system testified by the Paulistana Complex metavolcanosedimentary sequence (ca. 882Ma); (2) drift phase – With continued lithospheric thinning, the rift system further evolved to a broad passive margin in the northern São Francisco craton edge, represented by the Barra Bonita Formation platformal sediments. The drift phase culminated in the development of true oceanic crust, represented by the Monte Orebe ophiolite, around 820–630Ma ago; (3) convergence phase – At circa 630–620Ma, onset of subduction in the Borborema Province caused the inversion of the basins, obduction of oceanic crust slices, and sedimentation of the Mandacaru Formation syn-orogenic greywackes; (4) collisional phase – Continental collision between the São Francisco craton (lower plate) and the Pernambuco-Alagoas block (upper plate) occurred around 620–590Ma, with the stacking of the Casa Nova nappes upon the lower plate, crustal thickening, deformation, metamorphism, melt generation and intrusion of the syn-collisional Rajada Suite two-mica granites; (5) lateral escape phase – The collisional stage was followed by a lateral escape tectonics stage around 590–530Ma, that generated the western branch of the E–W trending Pernambuco Shear Zone, which truncates the northern part of the orogen and displaces its counterparts in the Transversal Zone in a dextral-sense shear. This phase was accompanied by extensive alkaline magmatism in the form of the Serra da Aldeia/Caboclo suites. Thus, the RPO represents a complete plate tectonic cycle that spans the whole Neoproterozoic and preserve excellent time markers for both the break-up and dispersion of the Rodinia Supercontinent fragments and for the re-amalgamation of those fragments in West Gondwana. The interpretation of a complete Wilson Cycle in this area challenges current views of the Borborema Province as a coherent block since Paleoproterozoic times (part of the Atlantica Supercontinent); suggesting, instead, a dynamic setting where multiple plates interacted to form the heart of West Gondwana.

Intraplate stress field in South America from earthquake focal mechanisms

We present an updated compilation of earthquake focal mechanisms in Brazil together with focal mechanisms from the sub-Andean region (mainly from global CMT catalogs). All earthquakes in the sub-Andean region show reverse (majority) or strike-slip faulting mechanisms. Focal mechanisms in Brazil show reverse, strike-slip and normal faulting. Focal mechanisms of nearby earthquakes in the same tectonic environment were grouped and inverted for the stress tensor. In the sub-Andean region, stresses are compressional, as expected, with the principal major compression (S1) roughly E-W, on average. A slight rotation of S1 can be observed and is controlled by the orientation of the Andean plateau. In the sub-Andean region, the intermediate principal stress (S2) is also compressional (i.e., larger than the lithostatic pressure, Sv), a feature that is not always reproduced in numerical models published in the literature. In mid-plate South America stresses seem to vary in nature and orientation. In SE Brazil and the Chaco-Pantanal basins, S1 tends to be oriented roughly E-W with S2 approximately equal to S3. This stress pattern changes to purely compressional (both SHmax and Shmin larger than Sv) in the São Francisco craton. A rotation of SHmax from E-W to SE-NW is suggested towards the Amazon region. Along the Atlantic margin, the regional stresses are very much affected by coastal effects (due to continent/ocean spreading stresses as well as flexural effects from sediment load at the continental margin). This coastal effect tends to make SHmax parallel to the coastline and Shmin (usually S3) perpendicular to the coastline. Few breakout data and in-situ measurements are available in Brazil and are generally consistent with the pattern derived from the earthquake focal mechanisms. Although numerical models of global lithospheric stresses tend to reproduce the main large-scale features in most mid-plate areas, the S1 rotation from ∼E-W in SE Brazil to SE-NW in the Amazon region are not well explained by the current numerical models. This means that the observed stress pattern in mid-plate South America should provide new insights into upper mantle dynamics, distinct from current global convection models.

Uranium irradiation history of carbonado diamond; implications for Paleoarchean oxidation in the São Francisco craton

Abstract Carbonado is a porous polycrystalline diamond rock found in central African and Brazilian placer deposits. It contains unsupported radiogenic isotopes of He, Ne, Kr, Xe, and Pb. Here we show that these, and the radiation-related defects introduced to the diamond structure, are a result of uranium precipitation, with no isotopic or spectroscopic evidence of Th enrichment. The daughter products are unsupported due to Proterozoic U remobilization. Combining existing carbonado Pb isotope data with recent studies of the geochronology of the tectonic evolution of the São Francisco craton (eastern South America) reveals that the most likely scenario is Paleoarchean uranium enrichment of carbonado, followed by Mesoproterozoic uranium dissolution. Under all possible scenarios, the carbonado radiation damage history requires U mobilization in the Mesoarchean or late Paleoarchean. This is consistent with recent studies of South Africa and India Mesoarchean paleosols, which also show evidence for local oxygen activity greater than that of the Archean atmosphere and ocean. While those studies rely on whole-rock trace element and transition metal stable isotope measurements, this combination of crystallographic defects, sedimentary geochronology, and radiogenic isotopes supports the same conclusions of nonmarine, near-surface Archean oxygen enhancement.

The Carrancas Formation, Bambuí Group: A record of pre-Marinoan sedimentation on the southern São Francisco craton, Brazil

The Carrancas Formation outcrops in east-central Brazil on the southern margin of the São Francisco craton where it comprises the base of the late Neoproterozoic Bambuí Group. It is overlain by the basal Ediacaran cap carbonate Sete Lagoas Formation and was for a long time considered to be glacially influenced and correlative with the glaciogenic Jequitaí Formation. New stratigraphic, isotopic and geochronologic data imply that the Carrancas Formation was instead formed by the shedding of debris from basement highs uplifted during an episode of minor continental rifting. Reddish dolostones in the upper Carrancas Formation have δ13C values ranging from +7.1 to +9.6‰, which is a unique C isotopic composition for the lowermost Bambuí Group but similar to values found in the Tijucuçu sequence, a pre-glacial unit in the Araçuaí fold belt on the eastern margin of the São Francisco craton. The stratigraphic position below basal Ediacaran cap carbonates and the highly positive δ13C values together indicate a Cryogenian interglacial age for the Carrancas Formation, with the high δ13C values representing the so-called Keele peak, which precedes the pre-Marinoan Trezona negative δ13C excursion in other well characterized Cryogenian sequences. Hence, The Carrancas Formation pre-dates de Marinoan Jequitaí Formation and represents an interval of Cryogenian stratigraphy not previously known to occur on the southern margin of São Francicso craton. Documentation of Cryogenian interglacial strata on the São Francisco craton reinforces recent revisions to the age of Bambuí Group strata and has implications for the development of the Bambuí basin.

Chemostratigraphy of the lower Bambuí Group, southwestern São Francisco Craton, Brazil: insights on Gondwana paleoenvironments

ABSTRACT: The Bambuí Group, the most extensive carbonate-siliciclastic cover on the São Francisco craton, has been a matter of debate because of its potential correlations to global glacial events. Unfortunately, most available chemostratigraphic data came from samples collected on surface rock exposures, ever susceptible to the aggressive chemical weathering that characterizes the southeastern Brazil. On the other hand, we present here high-resolution chemostratigraphic studies based on C, O and Sr isotopic data from 53 samples collected along a weathering-free, continuous, 175 m thick sedimentary succession. This succession was recovered by borehole drilling in the southwestern São Francisco craton, where occur the Carrancas and Sete Lagoas formations, the lowermost units of the Bambuí Group. The drill cores reveal extremely irregular contacts between the basal diamictite and its basement, an Archaean foliated granodiorite. Geochronological and sedimentological data strongly suggest that the diamictite represents a lodgement till. This glaciogenic deposit is covered by a limestone succession which starts with impure carbonates showing aragonite pseudomorph fans and thin bands of black shale. The limestone pile grades to a marl-mudstone interval, which turns to a carbonate with biological components, succeeded by stromatolitic dolomite at the top. C and O isotopic signatures (referred to V-PDB) allow to the subdivision of the lower carbonate-pelite section into three intervals. The first isotopic interval corresponds to a cap carbonate, and displays negative values of δ13C (c . -4‰), and a large oscillation of the δ18O (-6 to -15‰). The Interval II shows a striking homogeneity in δ13C and δ18O, around 1‰ and -7‰, respectively. At the top, Interval III shows a large positive excursion of the δ13C (up to 8‰) and δ18O (-8 to -3‰) values. Unaltered 86Sr/87Sr ratios range from 0.7075 to 0.7077, mainly at the top of the section. The geochemistry of the carbonates is controlled by their terrigenous content (mostly quartz and clay minerals) which is concentrated in the lower units. Samples free of terrigenous contamination show Y/Ho ratios ranging from 25 to 50, suggesting a freshwater input during carbonate deposition. It is concluded that the diamictite has a glaciogenic origin and is covered by a cap carbonate. This pair has been identified along the basin and is related to one of the main Neoproterozoic glaciations. Discrepancy between the 86Sr/87Sr values and the global variation curves can be related to freshwater input during the carbonate deposition. Based on the regional tectonic context, the Bambuí Basin may have been a restricted marine basin, totally or partially surrounded by mountain ranges within Gondwana, in the Neoproterozoic/Paleozoic boundary. In its early stages, the sedimentation was influenced by a global glacial event, whose melting phase was responsible by freshwater input in the basin. The gradual rise of the temperature was followed by an increase of the biological activity. Finally, a sudden increase in the biological activity could have been driven by paleogeographic changes caused by the active tectonic.

Paleoproterozoic continental crust generation events at 2.15 and 2.08 Ga in the basement of the southern Brasília Orogen, SE Brazil

New zircon U-Pb LA-ICP-MS data combined with Nd-Hf isotopes and whole-rock geochemistry provide constraints on timing and tectonic setting of Paleoproterozoic continental crust generation events in the basement of the Neoproterozoic southern Brasília Orogen, SE Brazil. The Paleoproterozoic basement, designated as the Pouso Alegre Complex, comprises mainly metatexitic tonalitic to granodioritic orthogneisses with lenses and boudins of metamafic rocks. Minor porphyroclastic granite bodies also occur. Two well-defined crystallization ages groups were recognized at 2.14–2.15Ga (6 samples; weighted average of 2146.7±6.6Ma) and 2.07–2.08Ga (5 samples; weighted average of 2078.7±6.7Ma). Inheritance of older zircon grains is almost absent. Whole-rock Nd data show juvenile signatures with TDM ages between 2.16 and 2.37Ga associated with positive ɛNd(t) values from +0.16 to +2.85. Zircon Hf LA-ICP-MS analysis yields positive ɛHf(t) values from +1.9 to +8.7. Whole-rock geochemical analyses show a wide range of SiO2 contents from 52 to 76wt% with felsic types predominating over mafic ones. The analyzed samples show high concentrations of Rb, Ba, Th, U and low Nb/La ratios, which are characteristics of subduction zone magmas and most of the samples show continental arc affinities. The geochemical signatures associated with the predominance of felsic rocks suggest a continental arc margin or an evolved accreted oceanic arc as the favored tectonic setting for the Pouso Alegre Complex. The Pouso Alegre Complex is interpreted as the southernmost and youngest recognized part of an arc complex emplaced at the southern edge of the São Francisco paleo-continent during the Paleoproterozoic. This part of the arc complex was deeply reworked by the collisional events related to the Neoproterozoic southern Brasília Orogen and its cratonic counterpart is the Mineiro Belt at the southern São Francisco craton. The Pouso Alegre Complex and the juvenile suites of the Mineiro Belt represent a major Paleoproterozoic continental crust generation event between 2.35 and 2.08Ga. This time period has not been recognized for extensive continental crust generation and preservation globally. However, it appears to be a major period of continental crust generation and preservation at the southern edge of the São Francisco paleo-continent.

U-Pb baddeleyite dating of the Proterozoic Pará de Minas dyke swarm in the São Francisco craton (Brazil) – implications for tectonic correlation with the Siberian, Congo and North China cratons

U–Pb baddeleyite ages demonstrate the presence of three dyke generations within the NW-trending Pará de Minas swarm (São Francisco craton, South America), dated at ca. 1795 Ma (1798 ± 4, 1791 ± 7 and 1793 ± 18 Ma), ca. 1710 Ma (1702 ± 13 and 1717 ± 11 Ma) and 766 ± 36 Ma. These ages have implications for the São Francisco-Congo (SF-Congo) craton within the Rodinia and Columbia supercontinent configurations. At 1795 Ma, the Rio de la Plata craton (1790 ± 5 Ma Uruguayan dykes), Amazonian craton (1790–1780 Ma Avanavero LIP), Sarmatia (1789 ± 3 Ma Tomashgorod dyke), North Australia (1790 ± 4 Ma Hart dolerites) and the North China craton (1790–1770 Ma Taihang-Xiong’er event) could potentially have been “next-door neighbours” to the SF-Congo craton. Possible matches at 1710 Ma are the 1750–1700 Ma Bilyakchan-Ulkan complex and coeval dykes in the Siberian craton and the 1730 Ma Miyun dyke swarm in NCC. Given large uncertainties, the 766 ± 36 Ma age may be compared with the age of the 790 Ma Niquelândia complex in adjacent Goias state, the 799 Ma Gannakouriep dykes in the Kalahari craton, the 780 Ma Gunbarrel LIP of western Laurentia, the 760 Ma Mount Rogers LIP of eastern Laurentia and the 755 Ma Mundine Well event (western Australia). We propose a reconstruction in which the SF-Congo is attached to North China and Siberia, which existed from 1790 to 1380 Ma when SF-Congo and North China broke away from Siberia, and until 930 Ma when North China separated from SF-Congo.

Structural Characterization of Outcrop-Scale Superposed Folding in the Kimbi Area (NW Cameroon): Implications for the Tectonic Evolution of the Pan-African North Equatorial Fold Belt

A study of predominantly folding structures was carried out on a number of outcrops in the Kimbi area. The main aim of this study is to provide a detailed structural characterization of the three folding events (F1, F2-F3) recognized in this area, in order to improve our understanding of the deformation mechanisms that controlled the tectonic evolution of the Pan-African North Equatorial Fold belt. The F1 folding is marked by decimetre-sized isoclinal folds with S1 schistosity parallel to the axial plane of F1 folds. The F2-F3 folding is superposed on the F1 older folds. Geometric shapes of F2-F3 folds typically form Ramsay’s type-3 interference patterns. The complex geometry of these folds suggests that they formed by shearing linked to the Central Cameroon shear zone (CCSZ). The Kimbi area superposed F2-F3 folds have been interpreted as the result of progressive deformation dextral transpressive deformation caused by the oblique convergence between the Congo/Sao Francisco craton and the Eastern Saharan block.

Disclosing the Paleoarchean to Ediacaran history of the São Francisco craton basement: The Porteirinha domain (northern Araçuaí orogen, Brazil)

This geochronological and isotopic study focuses on one of the Archean-Paleoproterozoic basement domains of the São Francisco craton reworked in the Araçuaí orogen, the Porteirinha domain, Brazil. It also includes a thorough compilation of the U–Pb geochronological data related to the adjacent Archean and Rhyacian terranes from the São Francisco craton and Araçuaí orogen. The main target of this study is the TTG gneisses of the Porteirinha complex (Sample 1). The gneiss dated at 3371 ± 6 Ma unraveled a polycyclic evolution characterized by two metamorphic overprinting episodes, dated at 3146 ± 24 Ma (M1) and ca. 600 Ma (M2). The former (M1) is so far the most reliable evidence of the oldest metamorphic episode ever dated in Brazil. The latter (M2), in turn, is endemic in most of the exposed eastern cratonic margin within the Araçuaí orogen. Whole-rock Sm–Nd analysis from the gneiss provided a slightly negative εNd(t3370) = - 0.78 value, and a depleted mantle model (TDM) age of 3.5 Ga, indicating derivation mainly from the melting of a ca. 3.5 Ga tholeiitic source. Sample 2, a K-rich leuco-orthogneiss from the Rio Itacambiriçu Complex, was dated at 2657 ± 25 Ma and also presents a ca. 600 Ma M2 overprinting M2 age. The other two analyses were obtained from Rhyacian granitoids. Sample 3 is syn-collisional, peraluminous leucogranite from the Tingui granitic complex, showing a crystallization age of 2140 ± 14 Ma and strong post-crystallization Pb*-loss, also ascribed to the Ediacaran overprinting. Accordingly, it is interpreted as a correlative of the late Rhyacian (ca. 2150-2050 Ma) collisional stage of the Mantiqueira orogenic system/belt (ca. 2220-2000 Ma), overprinted by the Ediacaran collage. Sample 4 is a Rhyacian post-orogenic (post-collisional), mixed-source, peralkaline, A1-type suite, with a crystallization age of 2050 ± 10 Ma, presenting an important post-crystallization Pb*-loss related to Ediacaran collision. The focused region records some of the oldest magmatic and metamorphic events ever found in the São Francisco craton basement, a ca. 3.4 Ga TTG generation and ca. 3.1 Ga metamorphism, as well as two younger orogenic events: the Rhyacian (2.15–2.05 Ga), accretionary event, correlated to the Mineiro belt, and the Ediacaran Brasiliano overprinting, event imposed by the Araçuaí orogen.

Crustal evolution of the northern Brasília Belt basement, central Brazil: A Rhyacian orogeny coeval with a pre-Rodinia supercontinent assembly

The Brasília thrust and fold belt developed in the course of Neoproterozoic Brasiliano orogeny, during the convergence of São Francisco, Amazonian and Paranapanema cratons, in an early phase of Gondwana agglutination. A Paleoproterozoic terrane cropping out in its northern segment represents the basement of the belt and corresponds to a magmatic arc. The arc developed during Rhyacian, possibly on the margin of an Archean-Paleoproterozoic block that is today represented by the São Francisco craton. The Paleoproterozoic arc evolution starts following a calc-alkaline trend between 2.30 and 2.16Ga, changing from a typical juvenile to crustal contaminated terrane with slightly negative ɛNd(t) values, as revealed by tonalites and granodiorites of Conceição do Tocantins Suite. By c. 2.17–2.15Ga, a collisional event took place, triggering crustal melting and generating peraluminous granites. These syn-collisional granites present strongly negative ɛNd values and belong to Aurumina Suite. Natividade, Almas-Conceição do Tocantins and Cavalcante-Arraias domains within the basement do not represent distinct tectonic blocks. The first two are areas where mainly plutonic arc rocks crop out and in the last one, syn-collisional granites are the most common lithology. Syn-collisional granites characterize granitogenesis produced by collision of another landmass against São Francisco landmass. The arc development is contemporary with collisional orogens that culminated in a conjectural Paleoproterozoic pre-Rodinia supercontinent. Since Rhyacian, these rocks are part of the landmass that would become the São Francisco craton and compose Brasília Belt basement.

The Archean–Paleoproterozoic evolution of the Quadrilátero Ferrífero (Brasil): Current models and open questions

The Quadrilátero Ferrífero is a metallogenic district (Au, Fe, Mn) located at the southernmost end of the São Francisco craton in eastern Brazil. In this region, a supracrustal assemblage composed of Archean greenstone and overlying Neoarchean–Paleoproterozoic sedimentary rocks occur in elongated keels bordering domal bodies of Archean gneisses and granites. The tectonomagmatic evolution of the Quadrilàtero Ferrífero began in the Paleoarchean with the formation of continental crust between 3500 and 3200 Ma. Although this crust is today poorly preserved, its existence is attested to by the occurrence of detrital zircon crystals with Paleoarchean age in the supracrustal rocks. Most of the crystalline basement, which is composed of banded gneisses intruded by leucogranitic dikes and weakly foliated granites, formed during three major magmatic events: Rio das Velhas I (2920–2850 Ma), Rio das Velhas II (2800–2760 Ma) and Mamona (2760–2680 Ma). The Rio das Velhas II and Mamona events represent a subduction-collision cycle, probably marking the appearance of a modern-style plate tectonic regime in the Quadrilátero Ferrífero. Granitic rocks emplaced during the Rio das Velhas I and II events formed by mixing between a magma generated by partial melting of metamafic rocks with an end member derived by recycling gneissic rocks of older continental crust. After deformation and regional metamorphism at ca. 2770 Ma, a change in the composition of the granitic magmas occurred and large volumes of high-K granitoids were generated.The ca. 6000 m-thick Minas Supergroup tracks the opening and closure of a basin during the Neoarchean–Paleoproterozoic, between 2600 and 2000 Ma. The basal sequence involves continental to marine sediments deposited in a passive margin basin and contain as a marker bed the Lake Superior-type Cauê Banded Iron Formation. The overlying sediments of the Sabará Group mark the inversion of the basin during the Rhyacian Minas accretionary orogeny. This orogeny results from the collision between the nuclei of the present-day São Francisco and Congo cratons, generated the fold-and thrust belt structure of the Quadrilátero Ferrífero. Afterwards, the post- orogenic collapse resulted in the deposition of the Itacolomi Group and in the genesis of the dome-and-keel structure.In this paper, we review current knowledge about the 1500 Ma long-lasting tectonomagmatic and structural evolution of the Quadrilátero Ferrífero identifying the most compelling open questions and future challenges.

Synkinematic emplacement of the magmatic epidote bearing Major Isidoro tonalite-granite batholith: Relicts of an Ediacaran continental arc in the Pernambuco–Alagoas domain, Borborema Province, NE Brazil

The Neoproterozoic Major Isidoro batholith (∼100 km2), composed of metaluminous to slightly peraluminous magmatic epidote-bearing tonalite to granite, is part of the Águas Belas–Canindé composite batholith, which intruded the Pernambuco–Alagoas Domain of the Borborema Province, northeastern Brazil. These rocks contain biotite, amphibole, titanite and epidote that often shows an allanite core as key mafic mineral phases. K-diorite mafic enclaves are abundant in this pluton as well as are amphibole-rich clots. The plutonic rocks are medium-to high-K calc-alkaline, with SiO2 varying from 59.1 to 71.6%, Fe# from 0.6 to 0.9 and total alkalis from 6.1 to 8.5%. Chondrite-normalized REE patterns are moderately fractionated, show (La/Lu)N ratios from 13.6 to 31.8 and discrete negative Eu anomalies (0.48–0.85). Incompatible-element spidergrams exhibit negative Nb–Ta and Ti anomalies.This batholith was emplaced around 627 Ma (U–Pb SHRIMP zircon age) coevally with an amphibolite-facies metamorphic event in the region. It shows Nd-model age varying from 1.1 to 1.4 Ga, average εNd(627Ma) of −1.60 and back-calculated (627 Ma) initial 87Sr/86Sr ratios from 0.7069 to 0.7086. Inherited zircon cores that yielded 206Pb/238U ages from 800 to 1000 Ma are likely derived from rocks formed during the Cariris Velhos (1.1–0.9 Ga) orogenic event. These isotopic data coupled with calculated δ18O(w.r.) value of +8.75‰ VSMOW indicate an I-type source and suggest a reworked lower continental crust as source rock. A granodioritic orthogneiss next to the Major Isidoro pluton, emplaced along the Jacaré dos Homens transpressional shear zone, yielded a U–Pb SHRIMP zircon age of 642 Ma, recording early tectonic movements along this shear zone that separates the Pernambuco–Alagoas Domain to the north, from the Sergipano Domain to the south. The emplacement of the Major Isidoro pluton was synkinematic, coeval with the development of a regional flat-lying foliation, probably during the peak of metamorphism related to the convergence/contractional deformation of the São Francisco craton and Pernambuco–Alagoas block during the Brasiliano Orogeny.

Generation of ca. 900–870 Ma bimodal rifting volcanism along the southwestern margin of the Tarim Craton and its implications for the Tarim–North China connection in the early Neoproterozoic

Large scale rift related volcanic suites are widely accepted as the evidence of the initial breakup of supercontinents and thus can be used to reconstruct the relative position of the cratons. Numerous early Neoproterozoic volcanic rocks have been identified in the Tiekelik Belt, southwestern part of the Tarim Craton. The Sailajiazitage volcanic rocks in the Tiekelik Belt are composed of a suite of bimodal volcanics and related pyroclastic rocks. The geochemical characteristics resemble those of continental flood basalts (CFBs) formed in the shallower mantle dominated by spinel–lherzolite with EM2 affinities. The rhyolites have been dated at 896±11Ma at the base, 881±14Ma at the middle and 872±8Ma at the top by U–Pb method on zircon. They erupted in an intraplate rift environment and are interpreted to have formed by crustal melting. Hf isotopes of individual dated zircon grains show a wide range of initial εHf values, from strongly negative (down to −28) to highly positive (up to +12.8), indicating a mixture of mantle- and crustal-derived materials. The bimodal volcanic suite is overlain by the Bochatetage Formation sediments, deposited sometime after 797±12Ma. An identical ca. 900–870Ma age for rift-related volcanic rocks indicate Neoproterozoic regional extensional processes in the southern margin of the Tarim–North China, which allow us to speculate on the correlation of the Tarim and North China cratons during the Neoproterozoic. This voluminous bimodal magmatism resembles the rift-related magmatism in the North China, West African, Congo and São Francisco cratons of age ca. 925–870Ma, suggesting that the Tarim Craton was located close to these cratons during early Neoproterozoic, or alternatively, that such early Neoproterozoic rift magmatism was widespread over large parts of Rodinia.

Thin lithosphere between the Amazonian and São Francisco cratons, in central Brazil, revealed by seismic P-wave tomography

Thin lithosphere between the Amazonian and São Francisco cratons, in central Brazil, revealed by seismic P-wave tomography

The late Neoproterozoic Sierra de las Ánimas Magmatic Complex and Playa Hermosa Formation, southern Uruguay, revisited: Paleogeographic implications of new paleomagnetic and precise geochronologic data

A new paleomagnetic study was carried out on 15 sites of the Sierra de las Ánimas complex, exposed in SE Uruguay. A paleomagnetic remanence was isolated at six sites, and this direction is likely to be pre-tectonic and primary. Mean directions from five of these sites are consistent with the SA2 paleomagnetic pole, which is based on a previous paleomagnetic study of the complex. A new paleomagnetic pole was computed for compiled results from the Sierra de las Ánimas complex (SAn, 12.2°S, 258.9°E, A95: 14.9°). A geochronological study based on U/Pb (SHRIMP) dating of magmatic zircons was also carried out on three samples of the Sierra de las Animas Complex, yielding a grand mean age of 578±4Ma. The new SAn paleomagnetic pole anchors the Early Ediacaran segment of the Apparent Polar Wander Path for the Río de la Plata craton. Consistency of the new pole with other Ediacaran poles from the Congo-São Francisco craton suggests a coherent Central Gondwana by this time. These data also support a wide Clymene Ocean between the conjoined Río de la Plata–Congo-São Francisco blocks and Western Gondwana, i.e., Amazonia–West Africa. Additional sampling (sixteen samples) was carried out on the basal levels of the glaciogenic Playa Hermosa Formation, exposed in the same area. These samples retain a paleomagnetic component consistent with the previously reported data, confirming the original pole position. U–Pb (SHRIMP) dating of detrital zircons reveal a bimodal distribution of Paleoproterozoic (around 2.1Ga) and late Neoproterozoic ages. The youngest zircons are interpreted as pre-dating to penecontemporaneous with deposition and points to a ≤594Ma age for deposition of the older levels of the Playa Hermosa Formation. A minimum age of deposition is set by the 578±4Ma age of the Sierra de las Ánimas magmatism. The low inclination of magnetization indicates that low-latitude (13.0°+9.5°/−5.5°), Gaskiers-aged glacial deposits are preserved atop the Rio de la Plata craton.

Plate tectonic settings for Precambrian basic rocks from Brazil by multidimensional tectonomagmatic discrimination diagrams and their limitations

Multidimensional discrimination diagrams (2006–2011) for basic and ultrabasic igneous rocks were applied to Precambrian rock suites from the Amazonian and São Francisco cratons, and the Tocantins Province, Brazil, to infer their possible tectonic settings. The chosen study cases in the Amazonian craton include the ca. 3.0 Ga metabasalts of the Identidade greenstone belt, 1.87–1.80 Ga Parauapebas anorogenic basalt-rhyolite dikes, 1.86–1.82 Ga Rio Branco anorogenic gabbro-basalt association, ca. 1.76–1.74 Ga Aripuanã and Teles Pires intracratonic basalt-felsic volcanic associations, and 1.76–1.74 Ga Jamari and 1.60–1.53 Ga Serra da Providência arc-related gabbroic rocks. In the São Francisco craton, we selected 1.48 Ga arc-related amphibolites of the Rio Capim greenstone belt, continental mafic dikes of Uauá (2.6 Ga), Curaçá and Chapada Diamantina (1.5 Ga), and Espinhaço (ca. 1.0 Ga). In the Tocantins Province, ca. 3.0 Ga komatiites associated with oceanic basalts of the Crixás and Guarinos greenstone belts were studied. The application of the diagrams generally provided consistent results with the authors’ proposed tectonic settings based on field relationships and geochemical data. The exceptions are some within-plate (continental) mafic dikes and basalts for which our diagrams do not work well. For comparison, we also used two ternary and two bivariate traditional discrimination diagrams for the data from the Amazonian craton, whose results were poorer than the newer multidimensional diagrams.

Structural analysis of the Rio Preto fold belt (northwestern Bahia / southern Piauí), a doubly-vergent asymmetric fan developed during the Brasiliano Orogeny

The Rio Preto fold belt borders the northwestern São Francisco craton and shows an exquisite kilometric doubly-vergent asymmetric fan structure, of polyphasic structural evolution attributed exclusively to the Brasiliano Orogeny (∼600-540 Ma). The fold belt can be subdivided into three structural compartments: The Northern and Southern compartments showing a general NE-SW trend, separated by the Central Compartment which shows a roughly E-W trend. The change of dip of S2, a tight crenulation foliation which is the main structure of the fold belt, between the three compartments, characterizes the fan structure. The Central Compartment is characterized by sub-vertical mylonitic quartzites, which materialize a system of low-T strike slip shear zones (Malhadinha – Rio Preto Shear Zone) crosscutting the central portion of the fold belt. In comparison to published analog models, we consider that the unique structure of the Rio Preto fold belt was generated by the oblique, dextral-sense interaction between the Cristalândia do Piauí block to the north and the São Francisco craton to the south.

New evidence of an Ediacaran age for the Bambuí Group in southern São Francisco craton (eastern Brazil) from zircon U–Pb data and isotope chemostratigraphy

Extensive carbonate–siliciclastic successions of the Bambuí Group, which overlie Neoproterozoic glaciogenic diamictites, cover most of the southern São Francisco craton (eastern Brazil). This group records sedimentation in a foreland setting related to the diachronic orogenic processes that formed the Brasília and Araçuaí marginal belts. The lowermost unit of the Bambuí Group, the Sete Lagoas Formation, comprises two shallowing-upward sequences of carbonate rocks with subordinated pelitic intercalations, overlying the glaciogenic diamictites in the southern São Francisco Craton. This study combines isotope chemostratigraphy (C, O, Sr) and U–Pb dating of zircon detrital grains retrieved from marls of the Sete Lagoas Formation. The basal sequence comprises low organic matter limestones and dolostones with δ13C values around 0‰, positioned above cap carbonates dated at around 740Ma (Pb–Pb whole-rock isochron). The U–Pb ages obtained for this sequence show several age peaks between 1270–870Ma and 625–550Ma. The upper sequence includes dark limestones with δ13C values as high as +10‰, best preserved 87Sr/86Sr ratios of around 0.7075 and U–Pb ages ranging from 625Ma to 550Ma. Our geochronological data suggest that the Araçuaí orogen is the main source of sediment for the Sete Lagoas Formation, and the youngest zircon population sets the maximum depositional age for its upper part at around 557Ma. This suggests that the studied section of the Sete Lagoas Formation is not related to either the Sturtian or the Marinoan glacial events. Also, the 87Sr/86Sr ratios obtained from Sete Lagoas carbonates contrast with Sr evolution curves available in the literature, especially with those for the Ediacaran–Cambrian limit, when ratios higher than 0.7080 would be expected. The same discrepancy is reported for other Ediacaran carbonate successions, pointing to local disturbances in Sr composition of marine basins rather than global processes. Interbasinal correlations and blind dating based on isotope chemostratigraphy should proceed carefully, especially for Ediacaran marine deposits located on the inner parts of large palaeocontinental regions, such as those found in western Gondwana.