Petrologie du complexe alcalin sous-sature de Kokoumi (Cameroun)

Abstract The Cameroon Line was created by the rejuvenation, at the beginning of the opening of the Atlantic Ocean, of a Pan-African N070 degrees E fracture zone [Moreau et al., 1987], which acted as a huge lithospheric crack taping a hot asthenospheric zone [Deruelle et al., 1998; Marzoli et al., 2000]. The Kokoumi anorogenic pluton belongs to the E-W Garoua rift structure, which represents the easternmost extension of the Benue trough. The Garoua rift opened during the Neocomian-Lower Aptian ages [Benkhelil, 1988] through the rejuvenation of Pan-African normal faults. The rift subsided, was partially filled by conglomerates and sandstones, and the ensemble was folded in the Cretaceous period [Guiraud, 1993]. Post-Cretaceous faulting affected these sediments. Intrusion of the Kokoumi anorogenic complex through the Cretaceous sandstones was favoured by N-S, N070 degrees E, E-W and N135 degrees E faults and N030 degrees E extension [Moreau et al., 1987]. The Kokoumi complex was first described by Koch [1959]. It is composed of a plutonic gabbro-nepheline monzosyenite-nepheline syenite series and of lamprophyric dykes (monchiquites and camptonites). One trachyte dyke is also observed. The gabbros are olivine (Fo 70 )-, nepheline-, or kaersutite-bearing gabbros. They also contain Ti-Al-rich diopside, Ti-rich biotite, titanite, ilmenite, Ti-magnetite and apatite. The nepheline monzosyenites contain diopside, Fe-diopside, kaersutite, Fe-kaersutite, titanite and apatite. The nepheline syenites contain aegirine-augite, F-rich arfvedsonite and aenigmatite. Kaersutite and clinopyroxene predominate in the lamprophyres. Monchiquites and gabbros, camptonites and monzosyenites, display respective similar mineralogy. Monchiquites contain carbonate ocelli. The trachyte does not contain ferromagnesian minerals. For gabbros and monchiquites, equilibrium Fe-Ti oxide temperatures are between 650 and 750 degrees C (+ or -40 degrees C) and oxygen fugacities between 10 (super -15) and 10 (super -14) (+ or -0.5 X 10 (super -15) ) atmospheres, according to Spencer and Lindsley [1981]. Nepheline crystallized below 700 degrees C, according to Hamilton [1961]. All the rocks (except the trachyte) are nepheline normative (Ne 6 to Ne 40 ). Major and trace element distributions in MgO-element diagrams for the two series merge together into a single trend, from monchiquites to nepheline syenites. Nevertheless, the monchiquites trends have different slopes. We deduce the evolution from gabbros to nepheline syenites on the one hand and from monchiquites to camptonites on the other from primitive mantle normalized multi-element diagrams. Multi-element diagrams for the trachyte and the nepheline syenite are strictly similar. Patterns for Kokoumi gabbros are similar to those for basalts of the Kapsiki plateau [Ngounouno et al., 2000] and the Garoua rift [Ngounouno et al., 1997] with typical negative K and positive Zr and Ti anomalies. Patterns for nepheline monzosyenites display negative anomalies in Sr, P, Eu and Ti and those for nepheline syenites and trachyte display greater anomalies in these elements and Ba. Compared to gabbros, nepheline monzosyenites are enriched in all REE with a concave upward pattern and no Eu-anomaly. Nepheline syenites have a range of broadly similar REE patterns to nepheline monzosyenites with steep slope from La to Sm, strong Eu negative anomaly (Eu/Eu (super *) nearly equal 0.15) and heavy-REE spoon-shape. REE patterns for monchiquites, camptonites, and trachyte are respectively similar to those for gabbros, monzosyenites, and nepheline syenite. Initial Sr-isotope ratios of 0.7033 (recalculated from the measured ratios for an age of 39 Ma for plutonic rocks and 20 Ma for the lamprophyres and the trachyte) are similar to those obtained for basalts from the continental segment of the Cameroon Line [Halliday et al., 1988; Ngounouno et al., 2000; Demaiffe et al., unpubl.], whereas nepheline syenites and trachyte are distinctly more radiogenic with values between 0.7128 and 0.7251. Amphibole and whole-rock K-Ar analyses (table III) yield 39.0+ or -0.9 Ma and 36.6+ or -0.9 Ma respectively. Since amphibole is a reliable chronometer in K-Ar dating, we propose the first age as the probable time of emplacement of the gabbros. Whole-rock analysis of nepheline syenite 99 displays an age of 33.1+ or -0.5 Ma. Field and geochemical observations suggest that gabbros and nepheline syenite are cogenetic and hence contemporaneous.