Petrologie du complexe alcalin sous-sature de Kokoumi (Cameroun)
Petrologie du complexe alcalin sous-sature de Kokoumi (Cameroun)
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Petrologie du Complexe alcalin sous-sature de Kokoumi（喀麦隆）
DOI：
10.2113/172.6.675
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发表时间：
2001
影响因子：
--
通讯作者：
R. Montigny
中科院分区：
文献类型：
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作者：
I. Ngounouno;C. Moreau;B. Déruelle;Daniel Demaiffe;R. Montigny
喀麦隆线是在大西洋开放之初，泛非N 070度E断裂带的复兴形成的[Moreau等人，1987年]，它充当了一个巨大的岩石圈裂缝，覆盖了一个热的软流圈区[Deruelle等人，1998; Marzoli等人，2000年]。Kokoumi非造山岩体属于东西向Garoua裂谷构造，代表贝努埃海槽的最东端延伸。Garoua裂谷在Neocomian-Lower Aptian时代[Benkhelil，1988]通过泛非正断层的再生而打开。裂谷下沉，部分被砾岩和砂岩填充，整体在白垩纪被折叠[Guiraud，1993]。后白垩纪断层影响了这些沉积物。Kokoumi非造山杂岩侵入白垩纪砂岩受到N-S、N 070度E、E-W和N135度E断层和N 030度E延伸的影响[Moreau等人，1987年]。Kokoumi复合体首先由Koch [1959]描述。它由深成的辉长岩-霞石二长正长岩-霞石正长岩系列和层状岩墙（二长岩和斜长岩）组成。还观察到一条粗面岩脉。辉长岩是橄榄石（Fo 70）-，霞石-，或钾霞石轴承辉长岩。富钛铝透辉石、富钛黑云母、钛铁矿、钛铁矿、钛磁铁矿和磷灰石。霞石二长正长岩中含有透辉石、铁透辉石、钾霞石、铁钾霞石、钛铁矿和磷灰石。霞石正长岩中含有霓辉石、富氟钠铁闪石和霓霞岩。钾霞石和单斜辉石在层状岩石中占优势。二长岩与辉长岩、斜长岩与二长正长岩具有相似的矿物学特征。Monchiquites含有碳酸盐ocelli。粗面岩不含铁镁矿物。根据Spencer和Lindsley [1981]的研究，辉长岩和二长岩的平衡Fe-Ti氧化物温度在650 ~ 750 ℃（+或-40 ℃）之间，氧逸度在10（超-15）~ 10（超-14）（+或-0.5 X 10（超-15））大气压之间。根据汉密尔顿[1961]的研究，霞石在700摄氏度以下结晶。除粗面岩外，其余岩石均为霞石标准岩（Ne 6 ~ Ne 40）.主元素和微量元素分布在MgO元素图的两个系列合并成一个单一的趋势，从二长岩霞石正长岩。然而，monchiquites趋势有不同的斜率。根据原始地幔标准化多元素图解，我们一方面推断出辉长岩到霞石正长岩的演化，另一方面推断出二长岩到斜长岩的演化。粗面岩和霞石正长岩的多元素图解严格相似。Kokoumi辉长岩的模式与Kapsiki高原玄武岩的模式相似[Ngounouno等人，2000]和加鲁阿裂谷[Ngounouno等人，1997]具有典型的负K异常和正Zr和Ti异常。霞石二长正长岩的Sr、P、Eu、Ti异常模式为负异常，霞石正长岩和粗面岩的Sr、P、Eu、Ti异常模式为负异常，Ba异常模式为负异常。与辉长岩相比，霞石二长正长岩的稀土元素全部富集，呈凹上型，无Eu异常。霞石正长岩具有与霞石二长正长岩相似的稀土元素配分模式，具有从La到Sm的陡斜率、强Eu负异常（Eu/Eu（super *）约为0.15）和重稀土勺形。二长岩、斜长岩和粗面岩的稀土配分模式分别与辉长岩、二长正长岩和霞石正长岩相似。初始Sr同位素比值为0.7033（根据深成岩39 Ma和纹层岩和粗面岩20 Ma的测量比值重新计算），与喀麦隆线大陆段玄武岩获得的比值相似[Halliday等人，1988年; Ngounouno等人，2000; Demaiffe等人，unpubl.]，而霞石正长岩和粗面岩的放射成因明显更强，其值在0.7128和0.7251之间。角闪石和全岩K-Ar分析（表III）分别得出39.0+或-0.9 Ma和36.6+或-0.9 Ma。角闪石是K-Ar定年中可靠的计时器，我们建议辉长岩的可能侵位时间为第一个年龄。霞石正长岩99的全岩分析显示年龄为33.1 ± 0.5Ma。野外和地球化学观测表明，辉长岩和霞石正长岩是同生的，因此是同生的。
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.