Éditeur(s) : HAL CCSD Geological Society of London
Résumé : The Borborema and Benin–Nigeria provinces of NE Brazil and NW Africa, respectively, are key areas in the amalgamation of West Gondwana by continental collision during the Brasiliano/Pan-African orogenies. Both are underlain by complex basement: Nigeria has c. 3.05 Ga Archaean crust but no known Palaeoproterozoic rocks >2.0 Ga; in NE Brazil, 2.6–3.5 Ga Archaean rocks form small cores within Palaeoproterozoic gneiss terrains affected by plutonism at c. 2.17 Ga. Both regions exhibit Late Palaeoproterozoic (c. 1.8 Ga) rift-related magmatism and metasedimentary sequences overlying the basement. The Seridó Group of NE Brazil (<0.65 Ga) is similar to the Igarra Sequence in SW Nigeria. The Ceará Group, which may date back to c. 0.85 Ga, is a passive margin deposit on crust thinned during initiation of an oceanic domain. In both provinces, basement and sedimentary cover were involved in tangential tectonics that resulted in crust-thickening by nappe-stacking associated with closure of this ocean. Frontal collision between c. 0.66 and 0.60 Ga later evolved to an oblique collision, generating north–south continental strike-slip shear zones at c. 0.59 Ga. In NE Brazil, the main Pan-African suture is probably buried beneath the Parnaíba Basin. The Transbrasiliano Lineament, interpreted as the prolongation of the Kandi–4°50 Lineament in Hoggar, may represent a cryptic suture.
West Gondwana: Pre-cenozoic correlations across the south atlantic region

hal-00412234
https://hal.archives-ouvertes.fr/hal-00412234
DOI : 10.1144/SP294.4

Éditeur(s) : HAL CCSD Elsevier
Résumé : The Borborema Province, in the NE of Brazil, is a rather complex piece in the Brazil-Africa puzzle as it represents the junction of the Dahomeyide/Pharusian, Central African, Aracuai and Brasilia fold belts located between the West-African/Sao Luis, Congo/Sao Francisco and Amazonas craton. The correlation between the Dahomeyides from W-Africa (Ghana, Benin, Togo, and Mali) and the Borborema Province involves the Medio Coreau and Central Ceara domains. The inferred continuation of the main oceanic suture zone exposed in the Dahomeyides of W Africa is buried beneath the Phanerozoic Parnaiba Basin in Brazil (northwest of the Medio Coreau domain) where some high density gravity anomalies may represent hidden remnants of an oceanic suture. In addition to this major suture a narrow, nearly continuous strip composed of mainly mafic pods containing relics of eclogite-facies assemblages associated with partially migmatized granulite-facies metapelitic gneisses has been found further east in the NW Borborema Province. These high pressure mafic rocks, interpreted as retrograded eclogites, are located between the Transbrasiliano Lineament and the Santa Quiteria continental arc and comprise primitive to evolved arc-related rocks with either arc- or MORB-type imprints that can indicate either deep subduction of oceanic lithosphere or roots of continental and oceanic magmatic arcs. Average peak P-T conditions under eclogite-facies metamorphism (T=770 degrees C and P = 17.3 kbar) were estimated using garnet-clinopyroxene thermometry and Jd content in clinopyroxene. Transition to granulite-facies conditions, as well as later widespread re-equilibration under amphibolite facies, were registered both in the basic and the metapelitic rocks and suggest a clockwise P-T path characterized by an increase in temperature followed by strong decompression. A phenomenon possibly related to the exhumation of a highly thickened crust associated with the suturing of the Medio Coreau and Central Ceara domains, two distinct crustal blocks separated by the Transbrasiliano Lineament.
ISSN: 1342-937X

hal-00413584
https://hal.archives-ouvertes.fr/hal-00413584
DOI : 10.1016/j.gr.2009.01.003

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The Dahomey belt in Togo and Benin is an important segment of the larger West Gondwana Orogen. Here, we review the geodynamic evolution of the Dahomey belt and discuss new U–Pb and Lu–Hf zircon data in light of similar data previously acquired on the geologically related Northern Borborema Province, in NE Brazil. Eighteen samples from different tectonic settings and regions within the belt were collected for zircon isotopic investigation. Passive margin deposits of the Atacora Structural Unit and lower units of the Volta Basin have detrital zircon signatures compatible with the flanking West Africa Craton. The arc-related magmatism resulted from the east-dipping subduction of the Goiás-Pharusian oceanic lithosphere and is represented by a variety of granitoids emplaced in the Benino-Nigerian Shield between 670 and 610 Ma. These granitoids were mainly sourced from crustal reservoirs with subordinate juvenile input. Detrital zircon ages from syn-orogenic deposits in Benino-Nigerian Shield suggest that arc development could have started as early as 780 Ma. The main period of melting in the internal part of the belt, the Benino-Nigerian Shield, is related to crustal thickening and occurs only ca. 30 m.y. after initiation of the continental collision, marked by the ca. 610 Ma ultra-high pressure (UHP) metamorphism recorded at Lato Hills. Foreland development represented by the upper units of the Volta basin developed soon after continental collision and persisted with the development of the west-verging thrust front synchronously with the main period of crustal melting due to collision at ca. 580 Ma. The subvertical Transbrasiliano Lineament in South America, that corresponds to the Kandi Lineament in Africa, provides a present-day fit between NW Africa and NE Brazil. Restoration of the movement of the Transbrasiliano-Kandi Lineament (strike-slip plate boundary) places the Dahomey belt and Borborema Province (NE Brazil) along the same section of the West Gondwana Orogen. This configuration would explain some of the misfits previously discussed in the literature and aligns the UHP eclogites in Togo and NE Brazil.
ISSN: 0301-9268

hal-01358584
https://hal.archives-ouvertes.fr/hal-01358584
DOI : 10.1016/j.precamres.2016.01.032

Éditeur(s) : HAL CCSD Société géologique de France
Résumé : International audience
New geodynamical data from West Africa bring consistent informations on the pre-Mesozoic reconstruction within a large area running from the western Sahara to the Colombian cordillera. These new data support a Neoproterozoic Ocean (WANO) between the Amazonian (AMC) and West African (WAC) cratons previously to the Iapetus and Rheic oceans. We delineate 31 blocs detached from the surrounding three continents: NAC (North American Craton), AMC and WAC. 7 came from the WAC margin, 7 from the NAC, 6 from the AMC and 11 from an intermediate volcano sedimentary domain (COB) built on a 1200-1000 Ma oceanic crust. These imbricated blocks formed a tight mosaic by the Hercynian/Alleghanian tectonic event which gave way to the Pangea super-continent. But, during the Atlantic Ocean opening these blocks began to move. They were separated by new oceanic basins. However, previously to the Pangea, blocks from the COB domain formed two sets of garlands located on the northwestern Gondwana margin. The northern one moved to the North until the Silurian to collide the NAC (Taconic tectonic event) meanwhile the southern one remains on the Gondwana margin. All together were gathered by the Carboniferous/Permian time. Then, the framework for the opening of the Atlantic Ocean was not totally disconnected from the "Variscan" collage and many variscan weakness zones were re-used as initial breaking zones. Beyond this tectonic impact, the pre-mesozoic assemblage allows us to compare this "Caribbean" island arc with another one: the Indonesian "Banda" arc. Thus, West Africa is a geological key area for correlations between the Caribbean, the Appalachian, the Brazilian "Nordeste" and the West European domains and for the understanding of the Atlantic Ocean opening process.
EISSN: 1777-5817

hal-00842747
https://hal.archives-ouvertes.fr/hal-00842747
DOI : 10.2113/gssgfbull.184.1-2.5