Evolution of the early Proterozoic Boundiali-Bagoe Supracrustal Belt and associated granitic rocks, northern Cote d'Ivoire, West Africa.
The ca. 2.1 Ga Boundiali-Bagoo Supracrustal Belt is situated in the BaouM-Mossi domain of the
West African craton. The belt is composed of greenschist facies metavolcanics and metasediments
which comprise the Bagoo River Supergroup (BRS). It occurs in sheared contact with syn-tectonic
granitic gneisses and granites to the west (the Dyogo Granite Complex - DGC) and is intruded by
syn- and post-orogenic granitoids. Some 3,OOOkm2 of supracrustal and granite terrain has been
mapped at a scale of 1:100 000. The stratigraphy and structural history of BRS and DGC rocks and
the petrogenetic evolution of BRS metavolcanic and DGC granitic rocks are documented in the
The BRS is sediment-dominated and is characterised by abundant argillites and quartz wackes,
originally turbidites, and minor jasperoidal chemical sediments. The sediments separate four NNEtrending
units composed of subaqueous tholeiitic basalts and consanguineous gabbroic intrusives
locally, with andesites and extensive, proximal, subaerial andesitic volcaniclastic deposits. Major and
trace element geochemical data suggest that the basalts are all very similar: They are depleted in Ti,
P, 'h and Y and enriched in LILE relative to MORB and commonly have flat chondrite-normalised
REB patterns of ca. x10 abundance. These mafic metavolcanics are geochemically comparable to
modem immature, island arc and back-arc basin basalts. The geochemical and lithological
characteristics of the andesitic rocks are typical of mature, calc-alkaline island arc assemblages. The
south-western flank of the BRS is composed of coarse-grained, arkosic wackes with abundant
polymict paraconglomerate horizons that contain clasts of rhyolite, granite, granitic gneiss and
basalts. The siliciclastic sediments are fanglomerates and were originally deposited on a continental
margin or in a small, embryoniC fore-arc basin from a highland region to the west These sediments
are interbedded with a subaerial assemblage of calc-alkaline rhyolites and tholeiitic basaltic rocks.
Geochemical similarities between the rhyolites and the DGC suggest a co-genetic relationship. The
basalts interbedded with the fanglomerates are enriched in incompatible elements relative to basalts
elsewhere in the BRS, suggesting contamination of the continental margin.
Non-coaxial progressive bulk heterogeneous deformation (01) produced sinistral, strike-slip shear
zones and resulted in upright, NNE-striking, commonly mylonitic fabrics, in all but the post-tectonic
granitoids. Strain was partitioned into zones of progressive simple shear across which zones of
progressive pure shear (flattening) developed. This major DI deformation occurred in response to
oblique convergence of the BRS and DGC.
A plate tectonic model is proposed for the evolution of the BRS and DGC of the Boundiali region.
Immature arc-like rocks and marginal basin sediments became progressively accreted to a
continental margin during oblique subduction and basin closure. The proposed model suggests that
the basin developed by block faulting of a juvenile, "Dabakalian" (ca. 2.25 Ga) crust, although
isotopic data is not available to distinguish between this model and one involving terrane accretion
where no older basement is present
Fresh dolerites cross-cut the BRS and DGC rocks. They are quartz- and olivine-normative tholeiites
and were generated from a heterogeneous subcontinental lithospheric source. They are thought to be
Mesozoic in age and related to widespread dolerites and basalts that mark continental fragmentation
of the Gondwanaland supercontinent