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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)

Abstract : 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 compositions are characteristic of an alkaline suite. The differentiation was controled by fractional crystallization 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 phonolites where 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 in magma 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 distributions and 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 result from 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-sided volcano 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-melilite is 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 to 3500 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.
Mots-clés : Géologie Néphélinite
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Charles Nkoumbou. 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). Sciences de la Terre. Université Henri Poincaré - Nancy 1, 1990. Français. ⟨NNT : 1990NAN10460⟩. ⟨tel-01754364⟩

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