Elson P. Oliveira

Proterozoic links between the Borborema Province, NE Brazil, and the Central African Fold Belt

Abstract The Congo (CC) and the São Francisco (SFC) cratons were joined at about 2.05 Ga; northern parts of Palaeoproterozoic basement subsequently underwent extension at about 1 Ga, forming intracratonic basins. Neoproterozoic metasedimentary rocks in these basins yield detrital zircons as young as 630 Ma. The Brasiliano and Pan-African (c. 620–580 Ma) assembly of West Gondwana extensively altered this system. The Sergipano domain occurs north of the SFC, and the comparable Yaoundé domain occurs north of the CC. Crust north of the Sergipano domain comprises the Pernambuco–Alagoas (PEAL) domain. The NE–SW-striking Tcholliré–Banyo fault in Cameroon may extend southwestwards between the PEAL and Sergipano domains, defining northern limits of abundant SFC/CC basement. The Adamawa–Yadé domain in Africa does not appear to extend into Brazil. The Transverse domain of Brazil is a collage of Palaeoproterozoic crustal blocks, the 1.0 Ga Cariris Velhos orogen (CVO), late Neoproterozoic basins, and Brasiliano granites. The CVO extends ENE for more than 700 km in Brazil, but eastern continuation into Africa has not been identified. North of the Transverse domain contiguous c. 2.15 Ga gneisses comprise basement of Rio Grande do Norte and Ceará domains, which continue eastwards into western Nigeria and western Sahara.

Development of symmetrical and asymmetrical fabrics in sheet-like igneous bodies: the role of magma flow and wall-rock displacements in theoretical and natural cases

Abstract Two orientations of fabrics are recognised in vertical dykes: a symmetric orientation associated with magma injected into non-deforming wall-rocks and an asymmetric orientation usually associated with external non-coaxial shearing, or magma injected into an active fault. The notion of two fabric arrangements stems from two planes of symmetry, the dyke symmetry plane (DSP) and the fabric symmetry plane (FSP). This paper uses conceptual models to simulate all possible orientations of magma flow indicators that might be produced within a vertical dyke emplaced under coeval internal and external stresses. The effect of magmatic buoyancy-related stresses are portrayed in terms of magma flow velocities (MFV) and the external non-coaxial tectonic-related stresses in terms of wall displacement velocities divided by two (WMV/2). The tectonic setting is that of a sinistral transcurrent fault. Using these assumptions, five cases are proposed: MFV⪢WDV/2, MFV>WDV/2, MFV=WDV/2, MFV

Radiating 1.0 Ga Mafic Dyke Swarms of Eastern Brazil and Western Africa: Evidence of Post-Assembly Extension in the Rodinia Supercontinent?

Abstract Several mafic dyke swarms of similar composition and age (tholeiite- ca.1.0 Ga) occur on both sides of the Atlantic Ocean in eastern South America and western Africa. When assembled to their pre-drift position in the Mesozoic, the Brazilian coastal dyke province of Bahia, and the African dykes in Cameroun (Ebolowa suite) and Congo (Comba and Sembe-Ouesso provinces) define a giant radiating pattern (1200 km × 800 km) similar to other dyke swarms elsewhere associated with large-scale continental rifting. Magma flow indicators of the Brazilian dykes and branching propagation styles of their African counterparts indicate that the dyke conduits were fed with magmas diverging from a source beneath the long axis of the Meso-Neoproterozoic West-Congolian Basin in Africa. There, MORB-like metabasalts have been described in the La Bikossi Group of the Mayombian Supergroup. Whether the rifting event and intrusion of dyke swarms were triggered or not by a mantle plume beneath part of the Rodinia subcontinental lithosphere remain to be confirmed.

Evolução do segmento norte do orógeno Itabuna-Salvador-Curaçá: cronologia da acresção de arcos, colisão continental e escape de terrenos

U-Pb and Pb-evaporation zircon ages, together with whole-rock Sm-Nd and Pb-Pb data, are used to unravel the tectonic evolution of the northern segment of the Itabuna-Salvador-Curaca Orogen, northeastern Sao Francisco Craton. The results of this geochronologic study indicate that oceanic crust and island arc sequences accreted at 3.3 Ga, forming the Mundo Novo Greenstone Belt, and that between 2.2 - 2.12 Ga the Rio Itapicuru and Rio Capim Greenstone Belts were formed in a similar fashion. Age data and Nd isotopes also suggest that Andean-type continental margins are likely to have formed in the span of time 3.08 - 2.98 Ga, giving rise to the TTG belts of Retirolândia and Jacurici river, and to the Caraiba Complex between 2.69 - 2.63 Ga. Around 2109 Ma, continent-continent collision closed the Rio Itapicuru volcano-sedimentary basin, as inferred from the syntectonic emplacement of the Itareru Tonalite between the Rio Itapicuru Greenstone Belt to the east and the banded gneisses of the Serrinha Block to the west. This collision culminated in a major shift from frontal or oblique convergence to orogen-parallel block-escape about 2084 Ma ago and continued at least till 2039 Ma. During this event several elongated granitic and syenitic bodies were emplaced in the continental crust. The Itiuba Syenite, dated at 2084 Ma and stretching over 150 km east of the Caraiba Complex, is an example of one of these intrusive bodies.

Fission track analysis of apatites from São Francisco craton and Mesozoic alcaline-carbonatite complexes from central and southeastern Brazil

Abstract Thermal histories on seven Brazilian apatites were obtained by fission track analysis using Monte Carlo simulations. The apatites were collected from two distinct geotectonic provinces. One group, originated from Sao Francisco craton, represents a typical cratonic domain with Proterozoic and Eopaleozoic rocks and yielded Permo-Triassic ages (counted since the instant when temperature was low enough so that the damage produced by fission tracks in apatite started to be preserved). The common thermal history accepted by all samples of this group is a linear cooling from ~90 to ~25 °C for the last 240 Ma, in agreement with present day thermal gradient and denudation rates. The other group, from Mesozoic alkaline-carbonatite complexes, in central and southeastern Brazil, yielded Cretaceous ages, close to those of the intrusions. For the samples of central Brazil, fission track analysis suggests a slow cooling from ~95 ° to ~85 °C between 90 and 60 Ma ago, followed by a faster cooling from ~85 ° to ~27 °C for the last 60 Ma. Otherwise, two trends exist for the samples of southeastern Brazil. The primary one is an increase in temperature from ~75 ° to ~95 °C, which occurred between 140 and 60 Ma ago. In this period, there is also another trend: a cooling from ~100 ° to ~80 °C. However, both trends are followed by a common thermal history during the last 60 Ma: a cooling from approximately ~80 ° to ~25 °C.

Return to Rodinia? Moderate to high palaeolatitude of the São Francisco/Congo craton at 920 Ma

Abstract Moderate to high palaeolatitudes recorded in mafic dykes, exposed along the coast of Bahia, Brazil, are partly responsible for some interpretations that the São Francisco/Congo craton was separate from the low-latitude Rodinia supercontinent at about 1050 Ma. We report new palaeomagnetic data that replicate the previous results. However, we obtain substantially younger U–Pb baddeleyite ages from five dykes previously thought to be 1.02–1.01 Ga according to the 40Ar/39Ar method. Specifically, the so-called ‘A-normal’ remanence direction from Salvador is dated at 924.2±3.8 Ma, within error of the age for the ‘C’ remanence direction at 921.5±4.3 Ma. An ‘A-normal’ dyke at Ilhéus is dated at 926.1±4.6 Ma, and two ‘A-normal’ dykes at Olivença have indistinguishable ages with best estimate of emplacement at 918.2±6.7 Ma. We attribute the palaeomagnetic variance of the ‘A-normal’ and ‘C’ directions to lack of averaging of geomagnetic palaeosecular variation in some regions. Our results render previous 40Ar/39Ar ages from the dykes suspect, leaving late Mesoproterozoic palaeolatitudes of the São Francisco/Congo craton unconstrained. The combined ‘A-normal’ palaeomagnetic pole from coastal Bahia places the São Francisco/Congo craton in moderate to high palaeolatitudes at c. 920 Ma, allowing various possible positions of that block within Rodinia.

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

Abstract 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.

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.

Application of 55 multi-dimensional tectonomagmatic discrimination diagrams to Precambrian belts

Fifty-five new multi-dimensional diagrams, mostly based on log-ratio transformations, were used to decipher tectonic settings for 17 cases of Precambrian belts in Canada, the USA, Poland, Finland, Jordan, Democratic Republic of Congo and Zambia, China, and India. The results of different sets of diagrams for basic-ultrabasic, intermediate, and acid magmas were generally internally consistent. Possible reasons for some inconsistencies may be related to the use of a sample group of mixed ages because of their uncertainties, extreme element mobility caused by metamorphism especially of high-grade type, analytical data quality, different petrogenetic processes for basic to acid magmas such as mantle versus crustal origin, and some deficiencies that still exist in the multi-dimensional diagrams. To partly overcome the last problem, one new multi-dimensional diagram is proposed and used to discriminate mid-ocean ridge and oceanic plateau settings.

Nature and Evolution of the Archean Crust of the São Francisco Craton

We overview the Archean tectonic framework the Sao Francisco craton based on geologic constraints, integrated geochronologic interpretation and isotopic-geochemical evidence of basement rocks. U–Pb provenance studies of Archean and Paleoproterozoic supracrustal sequences are also used to provide additional inferences about the geodynamic scenario. The Archean rocks crop out mainly in two large areas in the southern and northern portions of the craton, surrounded and/or in tectonic contact with Paleoproterozoic orogenic belts. The ancient substratum is essentially composed of medium- to high-grade gneissic-migmatitic rocks including TTG suites and coeval granite-greenstone associations that collectively provide an isotopic record as old as 4.1 Ga. The combined U–Pb and Sm–Nd TDM age peaks coupled with U–Pb inherited ages in detrital zircons from the supracrustal sequences indicate that very ancient continental crust (˃3.5 Ga) exist, particularly in the northern portion of the craton. Mesoarchean events are episodic between 3.6–3.3 and 3.2–2.9 Ga, as for the Neoarchean (2.8–2.6 Ga) in both cratonic portions. This isotopic record indicates a protracted Archean history for the Sao Francisco craton, highlighted by peculiar tectonic-metamorphic histories of the basement rocks. From a tectonic point of view the compiled data concur with a diachronic evolution from Paleo- to Neoarchean times by means of juvenile accretion/differentiation events characterized by multiple TTG plutonism in genetic association with greenstone belts, coupled with partial melting events of earlier-formed material. All ancient basement complexes and/or continental blocks assembled diachronically during the Late Neoarchean by convergence-related processes akin to plate dynamics. Late-tectonic K-rich granitoids, mafic-ultramafic complexes and mafic dikes collectively mark the Neoarchean thickening and final cratonization of the continental crust.