A major 2.1 Ga event of mafic magmatism in west Africa: An Early stage of crustal accretion

Abstract

Birimian terranes from West Africa (Mauritania, Senegal, Ivory Coast, Burkina Faso, Niger) comprise two major units: a dominantly mafic bimodal volcanic unit and a volcano‐detrital unit with mostly felsic to intermediate protolith. Stratigraphic relationships of these units are still a matter of debate but current work suggest that they both formed in a short time interval around 2.1 Ga. Widespread basaltic magmas from the bimodal unit have been analyzed for REE distributions and Sr‐Nd isotopes. Three Sm‐Nd isochrons on tholeiitic lavas were obtained at 2.229±0.042 Ga and initial εNd = 3.6±1.0 for Mauritania, 2.126±0.024 Ga and initial εNd = 2.9±0.7 for Burkina Faso, 2.063±0.041 Ga and initial εNd = 3.1± .0 for Eastern Senegal, data which compare with the age of 2.11±0.09 Ga and initial εNd = 2.1±1.8 obtained in Guyana by Gruau et al. (1985). Samples from other localities (Ivory Coast, Niger) give generally similar results. Although the variations of Sm/Nd ratios and the scatter of εNd(T) values from +1.2 to +4.3 preclude a single origin for these magmas, initial isotopic heterogeneities are unlikely to bias significantly the ages given by the isochrons which are in good agreement with U‐Pb zircon ages (Boher et al., 1989; unpublished data, 1990). Presence of lavas with frequent pillow structures and sediments virtually free of older recycled components suggests that Birimian terranes formed in ocean basins far from continental influence. The isotopic heterogeneities are not consistent with a MORB‐like mantle source. Most lavas are slightly depleted in LREE and inversion of the data through a melting model suggests 5–15 percent melting of a slightly depleted Iherzolite. Strong depletion (Burkina Faso) and slight enrichment (Senegal) are occasionally observed. With a noticeable trend of Ti enrichment with differentiation intermediate between that of MORB and IAT, the geochemical signature of Birimian basalts does not fit the best known geodynamic environments. Back‐arc or low‐Ti continental flood basalts provide a marginally good agreement but still face some difficulties. Oceanic flood basalts similar to those which form oceanic plateaus (e.g. in the Nauru basin) and later accreted to continents as allochtonous terranes represent the most acceptable modern analogue of many Proterozoic basalts. It is suggested that deep plumes piercing young lithosphere can generate huge amounts of tholeiites in a short time. Birimian basalts, like many Early Proterozoic basalts, may also be viewed as recent equivalents of the Archean greenstone belts. The modern komatiite of Gorgona Island is suggested to fit this model of intraplate volcanism. Although the 2.1 Ga magmatic event in West Africa has gone virtually unnoticed in the literature, it extends over several thousand kilometers and compares with the distribution of mantle‐derived magmatic activity in other major orogenic provinces (e.g. Superior). It shows that the growth rate of continents cannot be extrapolated from the data obtained solely from the best studied continent...

Rhyacian evolution of the eastern São Luís Craton: petrography, geochemistry and geochronology of the Rosário Suite

“…These results and the geochemical data reveal the juvenile nature of these rocks reinforcing the interpretation that this region may be part of the Rhyacian juvenile continental magmatic arc, that extends through other parts of São Luís Craton with correspondence in the northeast portion of the Amazonian Craton and in West Africa (Abouchami et al 1990, Boher et al 1992, Wright et al 1995, Hirdes et al 1996.…”

“…On the African side, several studies based on structural, geochemical, geophysical and geochronological data demonstrate the existence of Archean and dominantly juvenile Paleoproterozoic crust (Abouchami et al 1990, Boher et al 1992, Gasquet et al 2003. On the Brazilian side, the northern part of the Amazonian Craton and São Luís Craton have demonstrated geochronological and evolutionary similarities.…”

Neoarchean siliceous high-Mg basalt (SHMB) from the Taishan granite–greenstone terrane, Eastern North China Craton: Petrogenesis and tectonic implications

“…From a consideration of the appearance of komatiites and komatiitic basalts that represent the products of plume-derived magmatism in Archean greenstone terranes, some authors have suggested that accretion of mantle plume-related oceanic plateaux at convergent plate margins may have played an important role in the growth of the continental crust throughout Earth's early history (e.g., Abouchami et al, 1990;Desrochers et al, 1993;Stein and Hofmann, 1994;Condie, 1997Condie, , 2005Puchtel et al, 1998;White et al, 1999). Nonetheless, it is well documented that the Archean continental crust is composed primarily of tonalite-trondhjemite-granodiorite (TTG) granitic rocks (up to 80%; Jahn et al, 1981;Rudnick, 1995;Condie, 2005).…”

The geology of the gold deposits of Prestea gold belt of Ghana

“…Birimian volcanic gold belts generally extend for more than 100 km in length and up to 40 km in width and are separated by basins of isoclinally folded metasedimentary rocks averaging 90 km in width. The belts consist predominantly of metamorphosed tholeiitic to calc-alkaline basalts and form synclinoria that are made of variable proportions of metamorphosed basaltic lavas of bimodal composition comprising andesitic and dacitic pyroclastic rocks and rarely rhyolitic rocks (Abouchami et al, 1990).…”