The Burkinian orogenic cycle, precursor of the Eburnian orogeny in West Africa

Abstract

In the best known areas of the West African craton structural, petrographic, and geochronological data provide a distinction between two separate units of the Lower Proterozoic in West Africa. A lower unit was metamorphosed from low to medium grade around 2170 Ma by horizontal shearing. This unit is mainly composed of dominantly basic bimodal magmatic rocks, and some trondhjemitic to tonalitic anatectic gneisses, and locally mesozonal mica schists known in Ivory Coast as the ‘Kounoukou formation’ which has been dated as 2183 Ma old. An upper unit, which frequently begins with polygenic conglomerates, also shows important bimodal vulcanism, but in contrast to the lower unit is dominantly silicic. This unit is affected by lower grade conditions of metamorphism and is weakly deformed. These two units correspond to the classic Eburnian I/Eburnian II succession. However, a clearer distinction is possible, with the recognition of two successive orogenic cycles. In the basal part of each of the two units a major episode of tholeiitic magmatism is evidence for separate periods of lithospheric thinning and fracturing. This was followed by the deposition of various lithological sequences, then by one or more tectonometamorphic events. Later uplift led to the emplacement of anorogenic subvolcanic granitic massifs which are displayed occasionally as ring complexes. This suggests that ‘Eburnian I’ is actually an independent orogenic cycle appearing in the West African formations between 2400 and 2150 Ma, for which we propose the name ‘Burkinian cycle’. This cycle has affected the lower magmatic and sedimentary formations, which we designate Dabakalian. This implies a restricted time span for the Eburnian cycle from 2100–2150 to 1800 Ma. The stratigraphic term ‘Birimian’ is applied to the sedimentary and magmatic formations of the upper unit. This scheme proposed for the Lower Proterozoic in West Africa is probably applicable to other Lower Proterozoic terranes in Africa.

The relative and absolute chronology of strato-tectonic events in the Gorom-Gorom granitoid terrane and Oudalan-Gorouol belt, northeast Burkina Faso

“…The metamorphic conditions attained granulite facies. These rocks are similar in composition to orthogneisses described by Lemoine et al (1990) in Cote d'Ivoire and Burkina Faso, Pons et al (1995) in Niger, Hirdes et al (1996) in Cote d'Ivoire, Gueye et al (2008) in the KKI, and De Kock et al (2009) in Ghana, where they are consistently described as basement rocks. However, there is a considerable variation in the ages of these gneissic complexes across the WAC.…”

Petrological and geochronological constraints on lower crust exhumation during Paleoproterozoic (Eburnean) orogeny, NW Ghana, West African Craton

“…A polycyclic orogenic evolution was suggested based on field relationships reflecting poly-phased structural-metamorphic fabrics (Arnould, 1961;Bard & Lemoine, 1976;Ledru et al, 1991). Following this model, the gneissic basement supposedly pre-dating the Birimian series underwent high-grade metamorphism during an early orogenic event (Tempier, 1986;Lemoine et al, 1990;Tshibubudze et al, 2009;Hein, 2010) which is termed 'Eoeburnean' De Kock et al, 2011Perrouty et al, 2012). After the deposition of Birimian sediments, the Paleoproterozoic domains were tectonically assembled against the Archean nucleus during the Eburnean orogeny (Kouamelan et al, 1997), in a predominantly transcurrent tectonic regime (e.g.…”

Crustal growth in West Africa at 2.1 Ga

“…Because of deformation much more pervasive and the metamorphic grade much higher than in other B irimian formations, these series, which have been described in Ivory Coast by Arnould [1961] and in Burkina Faso by Hottin and Ouedraogo [1975], were considered to be of Arcbean [Arnould, 1961;Tagini, 1971] or Early Birimian ages [Bard, 1974]. Lemoine et al [1985] coined the name of "Dabakalian" for these early formations. Preliminary U-Pb results of Lemoine [1988] on zircons from Dabakala (central Ivory Coast) assign an age of 2144 + 6 Ma to these units while a Sm-Nd isochron on basic dykes suggests a consistent although less precise age of 2140 + 22 Ma.…”