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Géologie et pétrologie de l'Archéen de Guinée : une contribution régionale à la formation de la croûte continentale

Abstract : The formation of the continental crust is one of the main topics of geosciences and, in this debate, the geologist plays a major role allowing the knowledge of the composition of the Earth surface and timing of the events responsible for the building of the crust. Once achieved the space and time framework of a given " piece " of continental crust, the main problems concern both the processes by which the crustal matter is created and those by which this matter differentiates, which may include chemical (magmatism, hydorthermalism) and physical (déformation, metamorphism) processes. This memoir is an attempt to reconstruct the mechanisms reponsible for the building and differentiation of the Archean crust of the south-western part of W-Africa from the data collected during different mapping projects of the BRGM in eastern and centre Guinea (south-east Guinea, 1998-1999 ; north-east Guinea, 1998-1999 ; centre Guinea, 2001-2003). My approach is essentially magmatologic with the aim of deciphering the origin of magmas and their mechanisms of differentiation. For this aim, the method used is mainly systematic and based on a comparison between the geochemical signatures of Archean rocks and those of recent rocks, as well as on a measure of the trace element fractionation of the sources of Archean magmas relative to a primordial mantle and to the sources of recent magmas. In the south-western part of W-Africa, the formation and differentiation of the Archean craton (the Kénéma-Man domain) has resulted from four major stages : 1/ Paleoarchean (~ 3.5 Ga) ; 2/ Leonean (~ 3.1-2.95 Ga) ; 3/ Liberian (~ 2.9-2.8 Ga) ; 4/ Late-eburnean (~ 2.1-2 Ga). The Paleoarchean stage is marked by the emplacement of moderately potassic granitoids with the common geochemical signatures of magmas derived from the high-P melting of subducted basic materials (" slab melts "). A the same time, basic rocks were actually subducted, eclogitised and stored in the subcontinental mantle and those rocks reappeared during the Mesozoic (~ 100-200 Ma) as enclaves in kimberlites from Sierra Leone (Barth et al., 2002b). Most of them are deeply contaminated by the kimberlitic magma. The geochemical signatures of the Paleoarchean granitoids suggest that they resulted from the low degree partial melting of a primordial basaltic protolith, and rare (one or two) uncontaminated eclogites actually show the geochemical signature required for that protolith. The Leonean stage resulted in the emplacement of basaltic (greenstone) to felsic (gneiss, granitoids), and rarely ultramafic, magmatic rocks, and some sedimentary rocks (quartzites, Fe-quartzites, paragneiss ...). The main point is the clearly orogenic signature of some of the basaltic rocks which suggests the working of an Archean " cold " (or at least " mild ") subduction comparable to recent ones. In addition, the signatures of some of the basic magmatic rocks suggests the persistence of a primordial mantle below the Archean craton during the Leonean. Finally, the intermediate to felsic magmatic rocks appear to have been largely contaminated by the Paleoarchean crust. In Guinea as in Ivory Coast (Kouamélan et al., 1977a-b), the Liberian appears as a major stage of crustal differentiation rather than one of crustal accretion. Some rare basic rocks witness the persistence of a primordial mantle as that time, but more than 99% of the Liberian magmatism is composed of felsic rocks. The most abundant are moderately to highly potassic granitoids with high contents of incompatible elements (Th, LREE ...), and they are often closely associated with migmatites. Amongst the granitoids, charnockite constitutes a petrologically important, despite volumetrically restricted, type, which attests for high-T metamorphic conditions in the lower crust probably responsible for the Liberian event. This event is interpreted as a generalized stage of partial melting of the lower crust under water-undersaturated conditions. Such conditions enable the partial melting at a limited degree of the Paleoarchean to Leonean felsic rocks and therefore a marked enrichment of the magmas in incompatible elements. It is suggested that the Liberian thermic event could have resulted from an injection of mafic magmas at the base of the Paleoarchean to Leonean crust but this question should required further investigations. The last " orogenic " event suffered by the craton is a partial remobilization during the late Paleoproterozoic (Late-Eburnean event) associated with strike-slip tectonics probably caused by the collision between the newly formed Birimian crust (~ 2.2-2.1 Ga) and the Archean craton. Shortly before this event, juvenile ultramafic (komatiites) to mafic (tholeiites) lavas emplaced along the margin of the craton. Their geochemical signatures suggest the involvement of both a primordial mantle and a Th-depleted constituent thought to resulted from the recycling of Paleoarchean to Leonean subducted slabs. Finally, as suggested by its mean surface composition, the Archean upper continental crust of W-Africa appears slightly more felsic and less mafic than the average present upper continental crust (Taylor and MacLennan, 1985). Nevertheless, these slight differences cannot account for some major differences between the geochemical signatures. In particular, the Archean crust of W-Africa is four-times poorer in Ta and Nb than the average upper continental crust, but it is significantly more HREE-fractionated and LREE-enriched. Is is also 1.5- to 2-times richer in Th and Ba. Thus, the Archean events have resulted in the production of an upper continental crust with a granodioritic composition very close to the average upper continental crust, but with a specific and more " orogenic " and " slab-melt-like " signature
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Denis Thiéblemont. Géologie et pétrologie de l'Archéen de Guinée : une contribution régionale à la formation de la croûte continentale. Géochimie. Université de Bretagne occidentale - Brest, 2005. ⟨tel-00830519⟩



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