Petrographie Des Volcanites Et Plutonites De La Partie Sud Du Sillon Volcano-Sedimentaire De Toumodi-Fetekro (Cote D’ivoire)

The southern part of Toumodi-Fètêkro greenstone belt is located in the Center - Southeast of Ivory Coast. Petrographic study of volcanic and plutonic rocks shows three units. The first unit is composed of basaltic to rhyolitic lavas which imply effusive character. Then we have volcanosedimentary unit composed of pyroclastic formations (lapilli tuff, breccia, ash deposit and ignimbrites) and the pillow-lavas. Indeed, the presence of this last shows clearly that an explosive volcanism and a submarine effusive volcanism have occurred during during the setting of Toumodi-Fètêkro belt. Plutonic unit is constituted of gabbroic to granitic rocks. Sericite, chlorite, epidote observed in these rocks are consistent with the impacts of greenschist facies metamorphism. The rocks of the southern part of the Toumodi-Fètêkro greenstone belt are formed in a subduction context rather than in oceanic plateaus context because of the old inheritance, sometimes of Archean age, found somewhere in theBirimiandomain. The lithologies of the southern part of Toumodi-Fètêkro meet elsewhere in the other Birimian greenstone belts. Also, these lithologies are affected by a hydrothermal alteration due to the abundant veins of quartz, carbonates, sericite, chlorite, epidote, sulphides and oxides. However, volcanic show in some places amphibolit facies metamorphism

Pétrographie et géochimie des granitoïdes d’Issia (Centre-Ouest de la Côte d’Ivoire)

Les granitoïdes de la partie Sud du batholite de Ferkessédougou s’étendent ou couvrent la région d’Issia où ils sont représentés par une série de roches allant des granites à deux micas à biotite dominante aux granites à deux micas à muscovite dominante. Cette étude rapporte de nouveaux résultats pétrographiques et géochimiques des granites de la région d’Issia. Sur le plan pétrographique, ils se répartissent en trois séries granitiques à savoir les granites G1, G2 et G3. Les granites G1 sont caractérisés par une abondance de la biotite par rapport à la muscovite et ont subi une migmatisation. Les granites G2 sont marqués par une abondance de minéraux de feldspaths potassiques présents sous forme de porphyres. Les granites G3 quant à eux se distinguent par une dominance de la muscovite, suivie de la tourmaline et sont pourvus de filons de pegmatites d’épaisseurs centimétriques à métriques. Sur le plan de la chimie, les granites G1 sont faiblement peralumineux (1˂A/CNK˂1,16), les granites G2 sont moyennement peralumineux (1,14˂A/CNK˂1,18) et les G3 fortement peralumineux (A/CNK˃1,19). Les caractères pétrographiques et géochimiques rattachent les granites G1 au type I et les granites G2 et G3 au type S.   The granitoids of the southern part of the Ferkessédougou batholith located in the Issia region are represented by a series of rocks ranging from two-mica granites predominantly biotite to two-mica granites predominantly muscovite. This study reports new petrographic and geochemical results from granites in the Issia area. Petrographically, they are distributed in three (3) granitic series, namely the G1, G2 and G3 granites. G1 granites are biotitebearing and have undergone a migmatization. The G2 granites are marked by K-feldspar minerals as porphyries G3 granites are distinguishable by the abundance of muscovite and tourmaline and are associated with pegmatite veins of centimeter to metric thickness. Chemically, G1 granites are weakly peraluminous (1˂A / CNK˂1.16), G2 granites are moderately peraluminous (1.14˂A / CNK˂1.18) and G3 granites are strongly peraluminous (A / CNK˃1.19). These petrographic and geochemical characteristics link the G1 granites to type I granites whereas G2 and G3 granites show similarities to type S

A tectonic model for the juxtaposition of granulite- and amphibolite-facies rocks in the Eburnean orogenic belt (Sassandra-Cavally domain, Côte d’Ivoire)

The Sassandra-Cavally (SASCA) domain (SW Côte d’Ivoire) marks the transition between the Archean Kenema-Man craton and the Paleoproterozoic (Rhyacian) Baoule-Mossi domain. It is characterized by the tectonic juxtaposition of granulite-facies and amphibolite-facies rocks. Migmatitic grey gneisses, garnet–cordierite–sillimanite migmatitic paragneisses and garnet–staurolite-bearing micaschists reached peak pressure conditions ranging from ∼ 6.6 kbar at 620 °C to ∼ 10 kbar at 820 °C. These conditions are associated with the first recorded deformation D1 and correspond to a Barrovian geothermal gradient of ∼ 25 °C/km. Subsequent exhumation, associated with a second deformation D2, was marked by decompression followed by cooling along apparent geothermal gradients of ∼ 40 °C/km. A D3 deformation phase is marked by folding and local transposition of the regional S1/S2 foliation into E-W trending shear zones. LA-ICP-MS U–Pb dating of monazite, which displays complex internal structures, reveals four age groups correlated to textural position of monazite grains and analytical points: (1) rare relictual zones yield dates at the Archean-Paleoproterozoic transition (ca. 2400–2600 Ma); (2) a cluster of dates centered at ca. 2037 Ma on grains aligned along the S2 foliation of the migmatitic grey gneiss, attributed to D2; (3) a cluster of dates centered at ca. 2000 Ma, and (4) dates spreading from ca. 1978 to 1913 Ma, documented for the first time in the West African Craton monazites. The ages of the latter two groups are similar to the ones identified in the Guiana Shield, and could be attributed to a disturbance by fluids, to a periodic opening of U–Pb system or to an episodic crystallization of monazite during slow cooling lasting several tens of Myrs. These data allow to propose a model for the tectonic evolution of the SASCA domain at the contact between the Rhyacian Baoule-Mossi domain and the Archean Kenema-Man nucleus whereby crustal thickening is achieved by crustal-scale folding and is followed by and concomitant with lateral flow of the thickened partially molten crust accommodated by regional transcurrent shear zones. This combination of crustal thickening controlled by tectonic forces and gravity-driven flow leads to the juxtaposition of granulite- and amphibolite-facies rocks

Géochronologie et Géochimie des Formations Archéennes et Protérozoïques de la Dorsale de Man en Côte d'Ivoire. Implications pour la Transition Archéen-Protéozoïque

Mémoires de Géosciences Rennes, N° 73, 290 p. ISBN : 2-905532-72-6The "Dorsale de Man" (Western Africa, Ivory Coast) is divided into two domains: Archean to the West and Proterozoic to the East, separated by the transcurrent Sassandra Fault. This situation gives a unique opportunity for studying the ArcheanProterozoic transition. In the Archean domain, Sm-Nd model ages show that mantellic differentiation took place between 3.2 and 3.5 Ga. Oldest zircon inherited age of 3.2 Ga, already indicates a crustal recycling (ENd=-0.5) at this time. The major magmatic and metamorphic event, the Liberian event (2.8 Ga), is recorded in this domain. Datation of mineraI assemblages (zircon, monazite, gamet, sphene) indicates an important recycling of the Archean rocks during the Birimian event (i.e. 2.1 Ma). The earliest influence occurs at around 2250 ± 30 Ma and the major one at around 2100 ± 40. Partial and total resetting of V-Pb and Sm-Nd geochronometers indicate high temperature conditions during the Birimian event. Mafic granulites record a clockwise P-T path with an early high pressure episode (850 ± 30°C and Il ± 1 kbar) followed by a near-isothermal decompression (up to 720°C ± 30 and 7 ± 1 kbar). Biotite-bearing migmatitic gneisses from the Logoualé area give monazite age at 2074 ± 7 Ma. Nd model ages and inherited cores within zircons strongly argue in favour of an Archean origin for these gneisses. The thermal flow generated during the ascension of Birimian magmas could be the cause of the recycling of the Archean domain. In the Proterozoic do main (Baoulé-Mossi domain), sorne Archean contamination is inferred because of the presence of inherited zircons. This means that the Baoulé-Mossi domain was generated not very far from the Archean continent. This leads us to support a model involving rifting of an Archean protocontinent for the building of the Proterozoic domain of the "Dorsale de Man".L'Ouest de la Côte d'Ivoire se caractérise par la juxtaposition d'un domaine Archéen (Kenema-Man) et d'un domaine Protérozoique (Baoulé-Mossi), respectivement situés à l'Ouest et à l'Est de la faille du Sassandra. Elle offre donc l'opportunité d'une étude de la transition entre l'Archéen et le Paléoprotérozoique. Dans le domaine Archéen, les roches se différencient du manteau entre 3,3 et 3,5 Ga (âge modèle Nd). Les formations granulitiques les plus anciennes sont datées à 3050 ± 10 Ma (âge monozircon). Des zircons hérités dans des paragneiss indiquent un héritage continental, au minimum à 3,2 Ga. Ce domaine est marqué, comme l'ensemble de l'Archéen de la dorsale de Man, par l'événement magmatique et métamorphique majeur qu'est le Libérien (Camil, 1984; Kouarnelan et al., 1994, 1995). La datation des assemblages de minéraux a permis de mettre en évidence un recyclage important des formations archéennes durant l'événement Birimien. Ce recyclage s'effectue dans des conditions de ra assez élevées (700 à 800°C), vu la remise à zéro partielle ou totale des géochronomètres U-Pb et Sm-Nd. Il est essentiellement localisée au Sud de la faille de Man-Danané où prédominent des gneiss migmatitiques à biotite. Une étape pré90ce survient vers 2250 ± 30 Ma. L'influence majeure a lieu vers 2100 ± 40 Ma avec (1) la décompression preSCIu'isotherrne des granulites archéennes de haute pression (850 ± 30°C et 10 ± 1 kbar) vers des conditions de plus basse pression (720°C ± 30 et 7 ± 1 kbar) et (2) la fusion de sédiments pour donner des gneiss migmatitiques à biotite (monazite à 2074 ± 7 Ma). L'âge modèle Nd à 3,2 Ga-et la présence de coeurs hérités dans les zircons montrent l'origine archéenne de ces formations. Ces deux étapes correspondent à l'histoire de l'évolution de la croûte birimienne reconnue dans le domaine Protérozoique (s.l.). Le domaine Protérozoïque de la Côte d'Ivoire est constitué d'ensembles plutonovolcaniques et sédimentaires juvéniles ( Abouchami et al., 1990, Boher et al., 1992). L'histoire précoce de ces ensembles débute vers 2220 ± 8 Ma (âge monozircon). La phase majeure d'accrétion magmatique se situe à 2094 ± 6 Ma (âge monazite) avec la formation de nombreux massifs de leucogranite. Une phase intermédiaire, de migmatisation, à 2150 ± 10 Ma (âge monozircon) semble caractériser le socle granito-gneissique. Le domaine situé entre la faille du Sassandra et la longitude 6°W, que nous dénommons zone de transition, montre des évidences de contamination de croûte archéenne (âge modèle Nd intermédiaire et zircon hérité). Au Sud de ce domaine (domaine SASCA), nous avons mis en évidence l'existence d'un segment de croûte archéen (3,15 Ga), dont les caractères géochimiques indiquent qu'elle a été très peu remobilisée. La décompression isotherme des formations archéennes au Birimien et la contamination des magmas d'âge Birimien par la croûte archéenne permet d'envisager soit un modèle de collision, soit un modèle de rifting. Un modèle de collision ne semble pas correspondre car si une rétrogradation est évidente dans le domaine Archéen, aucune évidence de progradation n'est par contre observée dans la zone de transition. Aussi, J'existence de nappe de chevauchement, dans l'Archéen, est incertaine (Delor et al., 1994). De plus, les linéations minérales observées sur les bordures est et sud-est du massif granulitique de Man indiquent un mouvement en faille normale. Le modèle de rifting semble s'appliquer beaucoup mieux à cette région de la dorsale de Man. A un amincissement d'un continent archéen au Paléoprotérozoique, succède l'océanisation et la formation du domaine Baoulé-Mossi par underplating eVou transpression d'un magma de composition globalement intermédiaire. La présence de la granodiorite de Toulépleu, intrusive en plein domaine archéen à 2100 ± 10 Ma est une manifestation de la fragilisation de l'Archéen

Archean inheritance in the juvenile Paleoproterozoic (Birimian) domain of Rise Man : Geochemical and geodynamical implications

International audienceThe juvenile character of Birimian rocks of the Baoulé-Mossi domain is well established and the Birimian occupies a specific place in the evolution of the continental lithosphere between 2.5 and 1.9 Ga with a paroxysm at 2.1 Ga. Thus, this juvenile character has led many authors to evoke a tectonic context as oceanic plateaus, very far from Archean continent for the genesis of the Baoulé-Mossi domain, without any contamination. For others, the genesis of the Baoulé-Mossi domain is made with a high interaction of the Archean continent, particularly in a rifting context. Recent works based on thermo-mechanical models stipulate that the exhumation of metasediment lying on TTG basement is triggered by folding/shortening and gravitational instabilities in volcanic island arcs environment. Our geochemical and geochronological data obtained in the Transition Zone (TZ) of the Baoulé-Mossi domain indicate the existence of Archean components inheritance in the basement gneisses, the granitoids and the volcanites. The εNd are often negative or fairly positive (εNd = ±1) and the TDM model ages are sometimes older than 2500 Ma. Mixing model calculations carried out on the samples would indicate an average contribution of Archean crust from 5 to 20% in the Birimian magmatism, leading to a decrease of εNd (2.1Ga) from +5.5 to +2.5 on average. This contribution could reach or pass 30% for the Dabakalian rocks and in this case, the εNd (2.1Ga) can be smaller than -2. In-situ zircon dating (SIMS and ICPMS-LA) and evaporated zircon age (TIMS) highlight effectively Archean inheritance (2.6 to 2.9 Ga) in Dabakalian tonalitic gneiss (2.25 Ga), partially remobilized during Birimian sensu stricto event (2.08 Ga). The present results in the TZ and those obtained in the Eglab (Reguibat Rise) lead us to support a rifting model of an Archean protocontinent for the genesis of the Baoulé-Mossi domain. This rifting was probably occurring during Archean at ca 3.0-2.7 Ga with oceanization. The recycling of the Archean crust during the rifting will contaminate later the juvenile Dabakalian and Birimian magmas at respective rates of 30 and 15%. More thought must be given on the Archean crustal contribution in the genesis of the Birimian mineralization. Up to now, no more study seems to reveal Archean relics in Birimian mineralization. Investigations should be initiated in the Birimian ore deposits located In the Kenema-Man domain to highlight such interaction

The oldest rock of Ivory Coast

International audienceThe tonalitic gneiss of Balmer (TGB), in the SASCA area of south-western Ivory Coast, previously dated at 3141 ± 2 Ma using the single zircon evaporation method, is regarded as a relic of Archean rock within the Paleoproterozoic (Birimian) formation of the West African Craton (WAC). We present new geochronological data for the TGB using the laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) method. We obtain a U–Pb age of 3207 ± 7 Ma for abundant zircons extracted from the tonalitic gneiss, and interpret this age as that of the magmatic protolith because of the igneous-type homogeneous zircon population. Certain magmatic zircon edges and some round zircons define an upper intercept age of 3155 ± 17 Ma which could represent overgrowths during gneissification. It appears that the TGB was not affected by the events posterior to its genesis, i.e. the Liberian (2.9–2.7 Ga) and Eburnean (2.4–2.0 Ga) events. Additionally, the TGB proves to be a juvenile Leonian rock, as indicated by the Nd model age of 3456 Ma, and could also constitute the protolith of the granulitic grey gneisses and charnockites of the Man area, which are 150–400 Ma younger

Re-evaluation of Leonian and Liberian events in the geodynamical evolution of the Man-Leo Shield (West African Craton)

International audienceThe Leonian (3400–3100 Ma) and Liberian (2850–2700 Ma) events are important magmatic and tectono-metamorphic events in the Archean domain of the Man-Leo Shield in the southern West Africa Craton. The older Leonian event was associated with crust formation, which was variably reworked during the younger Liberian event. In the Man domain of western Côte d’Ivoire, zircon in-situ dating by LA-ICPMS of Granulitic Grey Gneisses (GGGs), the Mangouin charnockite and the Lagoulalé augen orthogneiss allows for the re-evaluation of magmatic and metamorphic activity related to the Leonian and Liberian events. We report here our findings that the GGGs represent the oldest rocks in the study area and formed during the Leonian event. The ages recorded in the GGGs in the Man area are 3156±24, 3098±19, 3093±49 and 3019±53 Ma. The ages of 3156±24 and 3098±19 Ma represent Leonian juvenile magmatism while those of 3093±49 and 3019±53 Ma represent Leonian metamorphism. The ages of the Mangouin charnockite and the Lagoulalé augen orthogneiss are 2798±8 Ma and 2795±9 Ma, respectively, and are interpreted to record peak metamorphic conditions during the Liberian event. Zircon from the augen orthogneiss includes an inherited core with an age of 3121±37 Ma, which corresponds to Leonian GGG ages. This inheritance indicates that the Mangouin charnockite and the Lagoulalé augen orthogneiss were derived either partially or completely from Leonian formations. A previously constrained Sm–Nd model age of 3250 Ma for the charnockite and augen orthogneiss agrees with such a process. The majority of Th/U is higher, indicating a magmatic origin for most zircons. The Leonian and Liberian events are proposed as two distinct events in Man-Leo Shield

Chromite, mg-ilmenite and priderite as indicators minerals of diamondiferous cretaceous kimberlites and lamproites from Côte d'Ivoire (West Africa)

Seguela kimberlites are characterized by xenocrysts chromites with elevated contents of TiO2 (0.6-5%) due to reaction with the host rock and Magmatic. Chromites associated with diamonds have high levels of Cr 2O3 (usually > 55% wt), MgO (9-12%wt) and Al 2O3 (12% wt). The Cr content in xenocryst chromites is pressure dependent and thus indicative of diamond potential. Two groups characterized Seguela kimberlites chromites analyses. The first group is characterized by high contents of MgO (> 8.7 wt%) and Cr2O 3 (>57.8 wt.%). They contain more than 0.8 wt% TiO2. The Cr-Ti chromites are phenocrysts crystallized from TiO2-rich kimberlitic magmas derived from lithospheric mantle source. The second group present elevated contents of TiO2 (1-5%wt) intermediate Cr 2O3 (30%wt) and relatively low Al2O3 (<3%). The first group kimberlitic chromites are useful in diamond exploration. Chromites are associated with Mg-rich-ilmenite (Fe3+/Fe 2+ < 0.6), in kimberlite which is an important mineral indicator in diamond research. Priderite (TiO2 > 75%wt) with high K/(K+Ba > 0.8) ratio and jeppeyite with elevate BaO content (> 16%wt) and badeleyite (ZrO2> 75% wt) are characteristics minerals of Seguela olivine lamproite. These signatures allow to supose the geotectonic hypothesis of within plate continental Stratiform or Alpin (MORB) complex suggest by chromites Cr-spinels and spinels studies. Kimberlites contain diamonds with a large range in size, varying from microdiamonds (< 1 mm) weighting on average about 1 mg (0.005 carats) up to large diamonds 27 carats.29 page(s

The Logoualé Band: A large Archean crustal block in the Kenema-Man domain (Man-Leo rise, West African Craton) remobilized during Eburnean orogeny (2.05 Ga)

International audienceThe Archean domain of the Man Rise was strongly remobilized during Eburnean coincidentally with the genesis of the Baoulé-Mossi domain (Birimian). This remobilization has allowed the recycling of Leonian and Liberian formations to generate a large gneissic crust, represented by the Logoualé Band. Zircons dates by laser ablation (LA-ICP-MS) of two samples of biotite-bearing pink gneisses of the Logoualé Band yielded ages of 2709 ± 15 and 2804 ± 11 Ma, confirming the Archean age of the Logoualé Band formations. The Eburnean tectono-metamorphic event in the Logoualé Band has totally reset both U-Th-Pb chronometer with an average age at 2050 ± 16 Ma in recrystallized zones of monazites, and Sm-Nd chronometer of garnets with an age at 2053 ± 15 Ma. Non-recrystallized zones of monazites give an average age at 2712 ± 16 Ma. We propose that the Logoualé Band rocks were originally sediments deposited in some protocratonic rift-type basins. During Eburnean, these sediments were buried, underwent high-grade metamorphism and exhumed in a tectonic context dominated by transcurrent motion. The structural setting of banded iron formations (iron deposits), which are abundant in the Logoualé Band, would date back to the Eburnean

Metamorphism and U-P Dating of the SASCA domain (South-West Ivory coast)

International audienceThe Sassandra-Cavally (SASCA) domain (SW Ivory Coast) is located in the transition zone between the Paleoproterozoic and Archean domains (Fig. 1). It is characterized by the coexistence of high-grade and low-grade metamorphic units and the existence of Archean relics in the Birimian formations. Heterogeneous gneiss formations comprising migmatites, orthogneisses, garnet-cordierite-sillimanite-garnet metasediment and staurolite-bearing micaschists are affected by three main deformation phases (D1-D3). Thermobarometric calculations using Theriak-Domino (de Capitani, 1994) suggest a retrograde evolution from the granulite facies to the amphibolite facies, from Grand Bereby to Sassandra. The metamorphism in the paragneisses of Grand Bereby and in the orthogneisses of San-pedro with quartz-plagioclase-biotite-garnet-sillimanite-ilmenite and quartz-garnet-biotite-ilmenite partial fusion, respectively, evolves at the transition of granulite and amphibolite facies with a maximum pressure of 10 kbar and a temperature of 820°C. It seems to characterize a decompression period in the granulite facies followed by cooling to the amphibolite facies with an apparent geothermal gradient of 40°C/km. However, the evolution of staurolite-bearing micaschistes occurred mainly in amphibolite facies with PT conditions corresponding to 3.5-6.6 kbar and 550-620°C. During this evolution, two metamorphic stages are distinguished: a prograde stage and a retrograde stage. The prograde path evolves according to a gradient of 30°C/km up to a peak of metamorphism estimated at 6.6 kbar-620°C in the amphibolite facies for the sample of Kounoukou. This path would correspond to a burial comparable to a thickening of the crust up to about 14 km in depth under an apparent geothermal gradient of 39°C/km. At this P-T path, the rock passes through a cooling phase with low decompression evolving according to a geothermal gradient of 40°C/km. This decompression would be associated with a cooling phase recorded in staurolite-bearing micaschist. U-Pb in situ dating of monazites gives ages of 2044 ± 11 Ma, 2003 ± 11 Ma, 1973 ± 11 Ma in the orthogneiss to quartz-garnet-biotite-ilmenite and 2000 ± 10 Ma for staurolite-bearing micaschists. These ages correspond to metamorphic ages in the SASCA domain and their meaning will be further investigated.Keywords