Palaeotectonic setting of the south-eastern Kédougou-Kéniéba Inlier, West Africa: new insights from igneous trace element geochemistry and U-Pb zircon ages

New U-Pb zircon ages and geochemistry from the eastern Kédougou-Kéniéba Inlier are presented and integrated with published data to generate a revised tectonic framework for the westernmost Birimian terranes. The Falémé Volcanic Belt and Kofi Series are highly prospective, hosting several multi-million ounce gold deposits and a significant iron ore resource, but remain under-researched. It is therefore important to constrain the fundamental geological setting. The igneous rocks of the eastern Kédougou-Kéniéba Inlier are dominantly of high-K calc-alkaline affinity, with fractionated REE patterns and negative Nb-Ta anomalies. The plutonic rocks in the Falémé Belt are dioritic to granodioritic in composition, with moderately fractionated REE patterns and metaluminous A/CNK signatures. Felsic, peraluminous granite stocks, dykes and plutons with fractionated REE patterns and negative Eu, Ti and P anomalies intruded both the Falémé Belt and Kofi Series. Albitisation masks the affinity of some units, although use of the Th-Co diagram shows that prior to albitisation, all igneous units belonged to the high-K calc-alkaline series. New U-Pb age data for the Boboti and Balangouma plutons indicate crystallisation at 2088.5 ± 8.5 Ma and at 2112 ± 13 Ma, respectively. Inherited zircons in the Boboti pluton indicate magmatic activity in the Falémé Belt at 2218 ± 83 Ma coincided with the oldest dated units in the Mako Belt to the West. Systematic changes in Dy/Yb, Sm/La, Nb/Zr, Rb concentration, Eu-anomaly and ɛNdt over ∼200 Ma reveal that the tectonic setting in the KKI evolved from a volcanic island arc environment to an active continental margin. Crustal thickening, as a result of a shift to collisional tectonic setting, combined with magmatic differentiation, led to the generation of peraluminous, granitic melts with a significant crustal component. A small suite of more basic intrusive and extrusive rocks on the eastern margin of the Dialé-Daléma basin are highly metaluminous and display limited LILE enrichment, with normalised HREE values close to unity. The Daléma igneous rocks may have formed in an extensional back arc, related to the arc system

Analyse petro-structurale d'un avant-pays metamorphique: influence du plutonisme tardi-orogenique Varisque sur l'encaissant epi a mesozonal de Vendee

SIGLEAvailable from INIST (FR), Document Supply Service, under shelf-number : T 79044 / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc

La construction sociale des espaces ruraux

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Les gras des viandes : des concepts aux déclinaisons sociales

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Archean evolution of the Leo Rise and its Eburnean reworking

Recent geological mapping in southeastern Guinea, supported by zircon dating, has called into question traditional understanding concerning the evolution of the Leo Rise. Gneiss dated at about 3540 Ma appears to constitute the earliest evidence for continental accretion within the Leo Rise. The existence of a Leonian depositional cycle at about 3000 Ma is confirmed, marked by volcanic and sedimentary rocks that can be correlated with the Loko Group in Sierra Leone. The span of ages (3244-3050 Ma) suggests that the Leonian cycle comprises different episodes whose respective chronology is as yet uncertain. Clearly distinct from the Leonian cycle, the Liberian cycle (∼2900-2800 Ma) is represented in Guinea by granite and migmatite (∼2910-2800 Ma), reflecting remobilization of the ancient Archean basement and deformation of the Leonian rocks; no deposition is associated with this cycle. After the Liberian, the Nimba and Simandou successions, containing Liberian detrital zircons, are assigned to the Birimian (∼2200-2000 Ma). Finally, Eburnean tectonism caused intense deformation of the Archean craton, accompanied by high-grade metamorphism and the intrusion of granite and syenite with ages between 2080 and 2020 Ma. The evolution of the Kénéma-Man domain, attributed to the cumulated effect of the Leonian and Liberian cycles, is thus in part Eburnean. We can suppose, therefore, that the NNE-SSW-trending structures attributed to the Liberian in Sierra Leone are, in fact, Eburnean. The Kambui Supergroup, also affected by this tectonism, should thus be assigned to the Birimian rather than the Liberian, which would explain its similarities with the Nimba and Simandou successions

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A 3.5 Ga granite-gneiss basement in Guinea: further evidence for early archean accretion within the West African Craton

A granite-gneiss formation (Guélémata Orthogneiss) was mapped on the northern and western slopes of the Mount Nimba hill range in Guinea. The original rocks were high-Al, low-Yb, medium to high-K granites similar to most Archean TTG. Analyses of U and

Archean evolution of the Leo Rise and its Eburnean reworking

International audienceRecent geological mapping in southeastern Guinea, supported by zircon dating, has called into question traditional understanding concerning the evolution of the Leo Rise. Gneiss dated at about 3540 Ma appears to constitute the earliest evidence for continental accretion within the Leo Rise. The existence of a Leonian depositional cycle at about 3000 Ma is confirmed, marked by volcanic and sedimentary rocks that can be correlated with the Loko Group in Sierra Leone. The span of ages (3244–3050 Ma) suggests that the Leonian cycle comprises different episodes whose respective chronology is as yet uncertain. Clearly distinct from the Leonian cycle, the Liberian cycle (∼2900–2800 Ma) is represented in Guinea by granite and migmatite (∼2910–2800 Ma), reflecting remobilization of the ancient Archean basement and deformation of the Leonian rocks; no deposition is associated with this cycle. After the Liberian, the Nimba and Simandou successions, containing Liberian detrital zircons, are assigned to the Birimian (∼2200–2000 Ma). Finally, Eburnean tectonism caused intense deformation of the Archean craton, accompanied by high-grade metamorphism and the intrusion of granite and syenite with ages between 2080 and 2020 Ma. The evolution of the Kénéma-Man domain, attributed to the cumulated effect of the Leonian and Liberian cycles, is thus in part Eburnean. We can suppose, therefore, that the NNE–SSW-trending structures attributed to the Liberian in Sierra Leone are, in fact, Eburnean. The Kambui Supergroup, also affected by this tectonism, should thus be assigned to the Birimian rather than the Liberian, which would explain its similarities with the Nimba and Simandou successions