1. Introduction
Paleoproterozoic rocks of the Man shield Fig. 1a, referred to as the Birimian Group Junner,
1954, form a major part of the West African Craton Bessoles, 1977, to the east and north of
the Archean Liberian cratonic nucleus Camil, 1984. The Birimian terrains form narrow sedi-
mentary basins and linear or arcuate volcanic belts intruded by various generations of granitoids
Leube et al., 1990; Pons et al., 1995; Hirdes et al., 1996. They correspond to a period of accretion
around 2.1 Ga Abouchami et al., 1990; Boher et al., 1992; Taylor et al., 1992 during the 2.1 – 2.0
Ga
Eburnean orogeny
Bonhomme, 1962;
Lie´geois et al., 1991. The supracrustal sequence was folded and metamorphosed under green-
schist-facies conditions. The lithostratigraphic succession of the Birim-
ian greenstone belt formations, although previ- ously much debated e.g. Feybesse and Milesi,
1994; Hirdes et al., 1996; Pouclet et al., 1996, now seems to be well established. The Birimian
greenstone belt succession consist of a thick se- quence of basalt, locally pillowed, as well as do-
lerites,
and gabbros
displaying a
tholeiitic composition, interlayered with immature detrital
sediments and carbonates, overlain by a detrital sedimentary pile volcanoclastics, turbidites, mud-
stones and carbonates, including interbedded calc-alkaline volcanics references in Sylvester and
Attoh, 1992; Hirdes et al., 1996; Pouclet et al., 1996.
On the other hand, the processes of growth of the Proterozoic continental crust has been the
subject of much speculation: island-arc accretion, intracontinental rifting and magmatic underplat-
Fig. 1. Simplified geological sketch maps. a Man shield from Bessoles, 1977; b Boromo greenstone belt from Feybesse et al., 1990; c Poura district from the modified BRGM-BUVOGMI map, 1983; d Loraboue´ area.
Fig. 1. Continued
ing and overplating or continent-continent colli- sion e.g. Taylor and McLennan, 1985; Condie,
1992. In the Man shield, these processes have been characterized essentially from geochemical
and isotopic data on the volcanic sequence of the Birimian mafic suite which is, in fact, made up of
volcanic rocks but also of plutonic complexes ultrabasites and gabbros sometimes outcropping
over wide areas, e.g. the ultrabasic bodies from the eastern border of the Mako series, Eastern
Senegal Bassot, 1966; Debat et al., 1984; N’Gom, 1995 and from the Ashanti belt in Ghana Loh
and Hirdes, 1996. Because of the absence of radiometric age dates and the structural disloca-
tion, the relations between such intrusive com- plexes and their enclosing volcanic rocks remain
ambiguous Mile´si et al., 1989; N’Gom, 1995; Hirdes et al., 1996.
From a study of the mafic rocks of the Loraboue´ area in the Boromo greenstone belt and
using mineralogical, petrological and chemical data, the aim of this paper is to establish evidence
for a cogenetic and coeval intrusive-extrusive suite and to characterize the conditions of crystalliza-
Fig. 1. Continued
tion so that to discuss the possible environments of formation of the series. This volcanic and
plutonic assemblage may represent: 1. an association of basalt and layered intrusions,
2. all of the crustal members of an ophiolitic suite,
3. the plutonic roots of the lower levels and the coeval volcanic rocks of an island arc.
2. Geology
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