Geological context of the Boumnyebel talcschists (Cameroun): Inferences on the Pan-African Belt of Central Africa Le contexte géologique des talcschistes de Boumnyebel (Cameroun) : implications pour la chaîne panafricaine d'Afrique centrale

International audienceThe talcschists of the Boumnyebel area (southern Cameroon) form ≤ 30 m thick discontinuous layers within a Pan-African nappe unit (Yaoundé group), which includes, at the base, muscovite + biotite ± garnet micaschists associated with amphibolites and pyroxenites, and, at the top, muscovite + biotite + garnet + kyanite micaschists locally associated with marble and amphibolites. The metamorphic peak (not, vert, similar650 °C/9.5 kbar; ca. 620 Ma) postdates nappe emplacement. Isograds are in normal position, micaschists passing downwards to migmatites in the northwestern part of the area studied. The rock types in the lower part of this nappe suggest active margin environments with detrital input from a nearby continental crust (arc or back-arc context)

New Data on the Genesis and Evolution of the Primitive Magmas of Mount Cameroon: Contribution of Melt Inclusions

Mount Cameroon is a Plio-Quaternary volcanic massif, without a centralcrater, made up of more than 140 pyroclastic cones. It is one of theactive volcanoes of the Cameroon Volcanic Line. Mount Cameroon meltinclusions are found in microdroplets trapped in the early minerals (olivines)from the pyroclastic products. The analysis of these melt inclusions allowedus to find primitive liquids compared to lavas. Major elements study ofthe magmatic inclusions, trapped in the most magnesian olivines (Mg#84-86) of Mount Cameroon revealed “primitive” liquids of basanite and alkalibasalt type with variable composition compared to the much more uniformbasalts of the magmatic series of Mount Cameroon. The study of thesetrapped liquids shows that: (1) the original primitive lavas did not undergothe process of evolution by FC, but rather underwent fundamentally (orexclusively) the process of partial melting; (2) the emitted lavas, evolvedessentially by FC; (3) the variations in the trace element contents of theprimitive liquids directly reflect a variation in the rate of partial melting ofa homogeneous mantelic source. The very high La/Yb ratios of the MountCameroon melt inclusions (> 20) characterize a garnet lherzolite source.Spectra of the melt inclusions show a negative anomaly or depletion in K,Rb and Ba as those of HIMU. The “primitive” liquids and lavas of MountCameroon represent a co-genetic sequence formed by varying degrees ofpartial melting of a source considered as homogeneous

I/ Etude géologique des Monts Roumpi : un ensemble plutonique et volcanique de la "Ligne du Cameroun". II/ Données pétrologiques sur les néphélinites du Mont Etinde (Cameroun)

Rumpi Hills designate a mosaic of hills and valleys (1500 km2) at 80 km north of Mount CaMeroon. They are constituted of plutonic and volcanic rocks which cut or layon a metamorphic and granitic basement. Satellite and Radar remote sensing indicate that the N300E lineaments prevail on N70°E, N130°E or submeridian ones. 25 K-Ar datations yield ages fram 576-566 Ma (amphibolites)to 531 Ma (granites) for the basement, from 145 to 10 Ma for late-stage plutonic rocks and from 10 to less than 1 Ma for the volcanic rocks.The paragneisses and the ortho-amphibolites and -hornblendites have suffered from granulitic metamorphism followed by an intense retromrphosis. This latter is synchronous to the intrusion of post- tectonic subalkaline granites and alkaline syenites. Late-stage plutonicrocks are predominantely olivine-gabbros and leucogabbros, the remaining being syeno-gabbros, -diorites, syenites and syenite porphyries. Micropegmatitic Quartz in an olivine-gabbro reflects minor crustal contamination. In syenites, early crystallized phlogopite is unstable. Primocryst and phenocryst repartition and the variations of mineral and rock compositionsare characteristic of an alkaline suite. The differentiation was controled by fractionalcrystallization involving olivine, clinopyroxene, Fe-Ti oxide, then plagioclase and apatite. The volcanic stratigraphy shows a basal series comprising basic alkaline rocks with associated peralkaline salic plugs and dykes. This series is overlain by a series of alkaline basic and kaersutite- bearing salic rocks. Magnesium-rich olivine and salite are classically found in basanites,basalts and hawaiites. In peralkaline trachytes, Fe-olivine appears only as phenocrysts, 80dic pyroxene, Fe-micas, titanomagnetite, aenigmatite. anorthoclase and sanidine constitute phenocrysts and microlites. The above ferromagnesian minerals are euhedral in rhyolites and dendritic in phonoliteswhere olivine is absent. In addition, phonolite groundmasse comprises anorthoclase, sanidine, nosean, sodalite and analcite. Electron microprobe analyses show that olivine is iron-rich and manganiferous, annite contains 30% of the ferriannite Molecules. pyroxene has up to 5.6 wt% and arfvedsonite up to 2 wt% Zr02. Nepheline is silica-rich and alkali-deficient. The chemical compositions of all the Minerals are induced by cOMplex substitutions controlled by variations inmagma compositions and oxygen fugacity, decreasing temperature and the effect of magmatic volatiles. From kaersutite-bearing trachytes to phonolites, pyroxene show a peculiar variation from hedenbergite to salite with the increasing volume of the destabilized kaersutite. The variations of phenocrysts species and compositions, major, trace and rare earth element distributionsand a quantitative modelling show that the differentiation of the two series was controlled by fractional crystallization involving olivine, clinopyroxene, titanomagnetite, feldspars, kaersutite (and biotite).Hawaiites are singled out of the basalt -kaersutite-bearing phonolite series. These hawaiites and associated silica-oversaturated rocks are presumed to resultfrom the partial melting of the roof of the chamber of mantle-derived basic -.gmas during intrusions. Mount Etinde is located on the southern flank of Mount Cameroon and appears as a steep-sidedvolcano with the highest peak (1715 m) flanked by two summits NW (1510 m) and NE (1580 ml. The whole massif is cut up by deep cliffs which correspond to the remaining of breached crater walls of late eruptions of Mount Cameroon. Mount Etinde is built up of nephelinites, nephelinitic tufs,melilite- or garnet-nosean nephelinites, Sr-mlilite or aenigmatite-hyalophane leucitonephelinites, haüyne-nephelinites and haüynophyres. Sr-melilite-aegirine leucitonephelinites comprise resorbed salite crystals. Melanite garnet and (occasionaly) nepheline show reverse zoning. Sr-meliliteis sodium-rich. Aenignatite is titanium-rich. Apart from haüynophyres, perowskite is ubiquitous. Mineral habitus and compositions are controlled by the phases appearing on the liquidii. The lavas of Mount Etinde are characterized by high volatile contents, a positive correlation of rare earth element and magnesiue contents, high strontium (up to 10540 ppm) and baryum (up to3500 ppm) contents. The lavas belong to two series in which early prevailing fractional crystallization competed with mineral-liquid reactions and element transfers by fluids. Mass balance equations indicate an important fractionation of feldspathoids at the late stage of differentiation. This has diminished the increasing peralkalinity index rate. The mantle source, chemically heterogeneous,was enriched in light rare earth elements. The nephelinitic magmas formed at great depth, but no relics of high pressure fractionation have been found.This testifY that magma stopped in a less deep Mantle magma chamber (40-30 km) where early crystallized minerals have reequilibrated.Les monts Rumpi (sud-ouest Cameroun) sont constitués de roches plutoniques et volcaniques qui regroupent et reposent sur un substratum métamorphique et granitique. La géochronologie potassium-argon a été réalisée sur toutes les séries de roches. La série plutonique alcaline et ultime (gabbros-diorites-syénites) s'est différenciée par cristallisation fractionnée. La stratigraphie montre deux séries volcaniques allant des laves basiques alcalines aux laves felsiques hyperalcalines puis aux laves felsiques à kaersitite. La minéralogie, la géochimie et les équations de balance de masse indiquent une différenciation par cristallisation fractionnée. La source mantellique était enrichie en terres rares légères. Le montetinde a été cartographié et daté. Les caractéristiques pétrographiques, minéralogiques et géochimiques de ces laves nephelinitiques montrent qu'elles appartiennent à deux séries ou la cristallisation fractionnée a été, sporadiquement, perturbée par des réactions minéral-liquide et par des transferts d'éléments par des fluides. La source mantellique, chimiquement hétérogène, était enrichie en terres légères et était localisée en grande profondeur

Etude geologique des monts Roumpi: un ensemble plutonique et volcanique de la "lignes du Cameroun" - Donnees petrologiques sur les nephelinites du Mont Etinde (Cameroun)

SIGLEINIST T 74631 / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc

Le contexte géologique des indices de talc de la région de Boumnyebel (Chaîne panafricaine d'Afrique Centrale, Cameroun)

De nouveaux levers cartographiques et une étude pétrologique ont permis de redéfinir la lithologie de la région de Boumnyebel et sa signification sur la chaîne panafricaine d'Afrique centrale. Deux unités sont distinguées. Une première unité d orthogneiss et amphibolites paléoprotérozoïques affleure au sud et sud-ouest de la région d étude. Elle est recouverte par une seconde unité panafricaine (nappe) constituée vers la base de micaschistes à grenat, amphibolites, métagabbros, hornblendites et pyroxénites, et vers le haut de micaschistes à grenat-disthène localement associés à des marbres et amphibolites ; des talcschistes s'intercalent en niveaux discontinus (=30 m) entre ces deux ensembles panafricains. La nature des faciès de la partie inférieure de la nappe suggère l'existence d'une marge active à proximité d'une croûte continentale (environnement d'arc ou d'arrière-arc), ceux de la partie supérieure suggérant plutôt un apport de sédiments en provenance d'une croûte continentale soumise à l'érosion. Le métamorphisme des formations panafricaines, de faciès amphibolite, atteint la fusion (migmatites) au nord-est de la région étudiée. Le pic métamorphique (syn-D2) est estimé à ~ 650- 670C / ~ 9.5 kbar vers 620 Ma (datation Th-U-Pb sur monazite) et les isogrades sont en position normale. Ceci suggère que le pic thermique est postérieur à la mise en place de la nappe. Soulignons, enfin, que l'abondance des roches mafiques et ultramafiques de nature tholéïtique dans la partie inférieure de la série repose la question de l'existence d'une marge active au Nord du craton du Congo durant le Néoprotérozoïque.New field work and petrological studies in the Boumnyebel area has led us to redefine rock types and discuss their bearing on the evolution of the Pan-African fold belt of central Africa. Two units are distinguished. A Palaeoproterozoic unit consists of orthogneisses and amphibolites restricted to the south and southwest of the area studied. It is overlain by a Pan-African unit (nappe) comprised in its lower part of garnet micaschists associated with amphibolites, metagabbros, hornblendites and pyroxenites, and towards its top of garnet + kyanite micaschists locally associated with marble and amphibolites; talcschists occur as discontinuous =30 m thick layers mainly above the lower micaschists. The rock types in the lower part of this nappe suggest active margin environments with detrital input from a nearby continental crust (arc or back-arc context), whereas in the upper part they rather suggest detrital input from a nearby continental crust submitted to erosion. Metamorphism in the Pan-African formations corresponds to amphibolite facies conditions, reaching anatexis to the NW of the area studied (migmatites). The metamorphic peak (syn-D2) is estimated at ~ 650-670C / ~ 9.5 kbar at ca. 620 Ma (monazite Th-U-Pb data), and the isograds are in normal positions. These data suggest that the thermal peak postdate nappe emplacement. We emphasize that the presence in significant proportions of mafic and ultramafic rocks of tholeiitic affinity in the lower part of the series suggests the existence of an active margin to the North of the Congo craton during the Neoproterozoic.NANCY1-Bib. numérique (543959902) / SudocSudocFranceF

Characterization for Ceramic Applications of Clay Materials from Lembo Deposit, Mount Bana (Cameroon)

International audienceThe mineralogical composition of 10 different samples, coming from Lembo (Western Cameroon) has been discussed and estimated in order to seek their ceramic applications. The following analyses have been used: chemical analysis, X-ray diffraction, TGA, DTA and infrared spectroscopy. The results reveal that all the samples are suitable for ceramic applications. In particular, the compositions of samples LB3 and LB9 are very close to those of the mixtures used for the manufacture of ceramic products presenting sintering ability and mechanical properties. Sample LB3 (illite content, 34+/-4 %) could be used for the manufacture of low firing temperature products. Iron-poor sample LB9 (kaolinite, quartz and microcline in proportions approx. 1/3, 1/3 and 1/3) could be used for high firing temperature products. However, the composition variability of the studied samples could generate complication for industrial exploitation of the deposit