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Sabat~, P., Marinho, M.M., Vidal, Ph. and Caen-Vachette, M., 1990. The 2-Ga peraluminous magmatism of the Jacobina-Contendas Mirante belts (Bahia, Brazil): Geologic and isotopic constraints on the sources.A 500-km north-south alignment of granitic intrusions cross-cuts the central part of the Silo Francisco craton. These late-to post-tectonic granitic bodies are emplaced between two Archean blocks (Jequi~ and Gaviio blocks) and cross-cut the Contendas Mirante volcano-sedimentary sequence and the metasedimentary rocks of the Serra de Jacobina. They are two-mica or muscovite-garnet-bearing peraluminous granites.Rb-Sr systematics show that these granites were emplaced during the Transamazonian orogeny ( ~ 1.9 Ga). The high initial STSr/86Sr ratios (0.706-0.748) and the very low £'Nd(t) ( --13 to -5) indicate a crustal origin. The possible sources are: ( 1 ) the Jequi6 block; (2) the Contendas Mirante sequence; and (3) the Gaviio block, including some early Archean domes [trondhjemitie-tonalitic-granodioritic (TTG), Boa Vista type ] which were tectonically emplaced within the volcano-sedimentary sequence.The currently available Rb--Sr and Sm-Nd data are not consistent with reworking of the 3.5-Ga TTG. The combination of field and isotopic constraints preclude the Jequi~ block and the Contendas Mirante sequence and favour instead the Gaviio block medium-grade terrains as the source for the peraluminous granitic line.The available data suggest that a continent-continent collision occurred during the Transamazonian orogeny, which followed subduetion/obduction marked by volcanism (are-tholeiitic, calc-alkaline and shoshonitic) and related plutonism. 0009-2541/90/$03.50 9 40 ° ~10 o J ~ 12 ° \ ALVADOR /,,,

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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.