1 Introduction
The ‘Cameroon Hot Line’ (CHL) is the unique example on Earth of an active intraplate alkaline tectono-magmatic alignment simultaneously developed into both oceanic and continental domains. Oriented N30°E, it crosses the Gulf of Guinea and Cameroon on more than 2000 km, from Pagalu Island to Lake Chad (Fig. 1). The CHL is segmented by numerous N70°E shear zones (fracture zones) of Pan-African age (Fig. 2). These shear zones (for example, the Adamawa, also called Central Africa Shear Zone, and the Sanaga faults) and their extensions into the South-American continent (Patos, Pernambuco) before the opening of the South Atlantic Ocean traverse both the continental crust and the upper mantle down to a depth of 190 km [16,17]. They induced the development of oceanic fracture zones transverse to the mid-ocean ridge during the opening of the Atlantic Ocean in the Cretaceous. The morphology of the African coast, from the Gulf of Guinea to the Cape Palmas (Fig. 2) is directly related to these zones.
The Cameroon hot line (CHL) is a N30°E-trending alignment that extends from Pagalu Island (Gulf of Guinea) to Lake Chad: it consists of oceanic and continental volcanoes and of anorogenic plutonic complexes (black circles, small italics). From a structural point of view, the continental sector is a succession of horsts and grabens (graben names in large italics). Mantle-derived xenolith occurrences are indicated by empty circles. Lower inset: Cainozoic volcanic rocks of the Cameroon Line (black) and of the other provinces (Adamawa and Biu plateau, grey). Upper inset: African cratons after [30].
Fig. 1. La ligne chaude du Cameroun s’étend de l’île de Pagalu au lac Tchad, entre les cratons Ouest-Africain et du Congo (cartouche supérieur, d’après [30]). En domaine continental, c’est une succession de horsts et de grabens (en grand italique), où sont installés les volcans. Les principaux plutons cénozoïques anorogéniques sont indiqués (petits ronds noirs, petit italique). Les gisements de nodules de roches ultramafiques sont indiqués par des petits ronds blancs. L’Adamaoua et le plateau de Biu sont deux autres entités volcaniques distinctes de la ligne chaude du Cameroun (en grisé, cartouche inférieur).
The Cameroon Hot Line trends N30°E (thick grey lines). Its continental sector is segmented by Pan-African shear zones in Africa propagating as fracture zones into the Atlantic Ocean (data after [12,18,19,37,39,53,58]). Its northern extension towards Lake Chad is tentative only. Dotted lines in the Congo craton correspond to Proterozoic dykes of doleritic composition.
Fig. 2. La ligne chaude du Cameroun, orientée N30°E, est recoupée par des décrochements continentaux (orientés N70°E), qui se prolongent en zones de fractures en domaine océanique (voir cartouche, données d’après [12,18,19,37,39,53,58]). L’extension de la LChC vers le nord (lac Tchad) est supposée. Les lignes pointillées dans le craton du Congo correspondent à des fractures soulignées par des dykes d’âge Protérozoïque.
Among the numerous hypotheses that have been proposed to explain the structure and the formation of the CHL (for a comprehensive review and a discussion, see [12]), the most widely accepted structural explanation is that the CHL would be a succession of ‘en-échelon’ mega-tension gashes resulting from reworking during Aptian–Albian times of the N70°E shear zones at the beginning of the opening of the Central Atlantic Ocean [39]. The reworking of shear zones could be linked to the occurrence of hot lines in the asthenospheric mantle [5].
Numerous hypotheses have also been put forward to explain the characteristics of the magmatism along the CHL and to locate its source.
It has been proposed that the CHL originated from a sublithospheric ‘hot zone’ periodically fed and melted by a deep mantle plume head [33]. But the absence of a simple and regular age distribution (Fig. 3), which is expected in the case of a plume (hotspot trail), and the occurrence of a deep hot zone (400 km) overlaid by a cold shallow one [2] do not correspond to what is observed at Hawaii, paradigm of plume, and leads to discard this hypothesis [38].
Age distribution of volcanic and plutonic rocks from the Cameroon hot line (data from [13] and references herein, and [7,20,32,35,38,43,46–49,56]).
Fig. 3. Distribution des âges des roches volcaniques et plutoniques de la ligne chaude du Cameroun. Source des données : [7,20,32,35,38,43,46–49,56].
More recently, the CHL has been considered as a hot line resulting from lithospheric cracks (N30°E) tapping a sublithospheric mantle [38]. The hotline hypothesis has been supported by deep-imaging seismic and gravity studies along the oceanic sector of the CHL [37]. Speculations for hotlines in the Atlantic part of the African plate have also been proposed to explain the occurrences of other oceanic rises (Walvis, Cape Verde–Canary…) as well as on the continental sector, but with the limitation that the presence of cratons does not permit crustal upwelling [37].
2 Geology
2.1 The volcanoes
The oceanic sector of the CHL (Fig. 1) is composed of six major volcanoes, one on each of Pagalu, São Tomé and Principe Islands and three (Santa Isabel, Biao and San Carlos) on Bioko Island. The continental sector is composed of Mt Etinde, Mt Cameroon, Mts Rumpi, Mt Manengouba, Mts Bambouto, Mt Mbam, and Mt Oku, which are built upon horsts that alternate with grabens (Kumba, Tombel, Mbo, Ndop, Tikar plain) where monogenic cones predominate. Whether the volcanism of the Cretaceous Benue rift west of Garoua and of the Kapsiki plateau is related or not to the CHL is still debated. The 69-Ma-old rhyolite flows and domes southwest of Lake Chad [71] would possibly be the northernmost occurrence of the CHL. Historical volcanic activity has been reported [12,14,65,68, and references herein] at Pico Santa Isabel (Bioko) in 1898, 1903, and 1923, and at Mt Cameroon, where six eruptions are known between 1868 and 2000. Massive outgassing of CO2 occurred at Lakes Monoun (1984) and Nyos (1986), killing nearly 2000 persons and a large part of the cattle. The volcanism of Adamawa (distributed according to the N70°E trend) and of the Biu plateau (distributed according to the north–south Gongola rift fracturation) does not belong to the CHL, as it is presently defined.
2.2 The anorogenic ring-complexes
Because of erosion, more than 60 anorogenic ring-complexes crop out along the continental segment of the Cameroon Line. They are more abundant in the southern part of the continental sector. Their ages are comprised between 6 and 70 Ma [38]. All the complexes have a typical alkaline character [12]. Trachytes and rhyolites are commonly associated to plutonic ring complexes and the Kokoumi massif is the only one that includes significant volumes of lamprophyres [46].
3 Petrography
3.1 Nomenclature, geographical distribution of the lavas
All the rocks of the CHL (except the nephelinite series of Mt Etinde) are typically alkaline. The lithological sequence of differentiated alkaline series is complete and includes basaltic lavas (alkali basalts – and picrites –, hawaiites), intermediate lavas (mugearites and benmoreites), and felsic lavas (phonolites, trachytes, and alkali rhyolites) (Fig. 4). The lavas have been classified (see Fig. 7) according to their D.I. [70] and to their mineralogy (occurrence of hauyne, nosean or sodalite in the mode of phonolites, absence of feldspathoids in the mode of trachytes, and occurrence of glass and occasional quartz phenocrysts in rhyolites). Nevertheless, a gap separates the basaltic lavas from the felsic ones in both oceanic and continental sectors. Trachytes are more abundant in the continental sector and rhyolites are absent from the oceanic one. Phonolites are more alkaline and less silica-rich in the oceanic sector. Although difficult to quantify, the volume ratio between felsic and basaltic lavas increases from the ocean–continent boundary region (Mt Cameroon and Bioko volcanoes are only composed of basalts) to the northern and southern extremities of the line.
Total alkalis–silica (TAS) diagram for the lavas of the Cameroon hot line. Nephelinites from Mt Etinde are indicated separately. Data from [12] and references herein and from [9,11,14,22,28,35,36,42–44,55,62,63,65,71] and unpublished data.
Fig. 4. Distribution des laves des secteurs océanique et continental de la ligne chaude du Cameroun dans le diagramme SiO2–(Na2O + K2O). Le domaine des néphélinites du Mt Étindé a été présenté séparément. Source des données : [12] et références citées [9,11,14,22,28,35,36,42–44,55,62,63,65,71], et données non publiées.
Sr-isotope initial ratio variations vs Differentiation index (D.I.) [70] for the differentiated lavas of the Cameroon hot line. Phonolites, trachytes and rhyolites have D.I. > 80. The arrows indicate the results of contamination by a continental crust, of mixings between basaltic and evolved magmas, and of fractional crystallization (same data sources as for Fig. 6).
Fig. 7. Variations de la composition isotopique initiale du Sr en fonction de l’indice de différenciation I.D. [70] pour les suites différenciées de la ligne du Cameroun. Les phonolites, trachytes et rhyolites ont des I.D. > 80. Les flèches indiquent les effets : de la contamination par la croûte continentale, des mélanges magmatiques entre magmas évolués et magmas basaltiques, et de la cristallisation fractionnée. Source des données : voir Fig. 6.
3.2 Alkaline lavas s.s.
The basaltic lavas have seriate to porphyritic textures. Basalts contain phenocrysts of olivine and augite in a microlitic matrix of plagioclase, augite, and Ti-magnetite. Hawaiites contain abundant plagioclase phenocrysts. Mugearites contain rare phenocrysts of plagioclase, augite, Fe–Ti oxides, and of amphibole destabilized into clinopyroxene and oxides, and xenocrysts of sanidine, quartz, apatite, and biotite. Benmoreites contain feldspar and biotite phenocrysts and plagioclase and/or quartz xenocrysts. The felsic lavas have a microlitic porphyritic texture. Phonolites contain phenocrysts of sanidine, aegirine-augite, brown hornblende (kaersutite, hastingsite), Ti-rich aenigmatite, Fe–Ti oxides, nepheline and hauyne (or nosean or sodalite) in a matrix of microlites of sanidine, aegyrine–augite, and Fe–Ti oxides. Microphenocrysts of titanite and apatite may also occur. Trachytes contain mostly phenocrysts and microlites of K-felspar (sanidine, anorthoclase). Aegyrine-augite, Ti-rich aenigmatite, brown hornblende (hastingsite, pargasite), richterite, arfvedsonite, biotite, Fe–Ti oxides, occasional titanite, apatite, and zircon, and exceptional fayalite, may also occur in phenocrysts and/or microlites. Rhyolites have hyaloporphyritic to hyalopilitic textures; some of them are ignimbrites, pumices, or obsidians. They contain phenocrysts of sanidine–anorthoclase and occasional quartz, arfvedsonite, richterite, magnetite, ilmenite, aegyrine–augite, biotite, and zircon in a matrix of microlites of K-felspar and glass. In some samples, ghosts of fayalite phenocrysts [57] are commonly replaced by quartz and alkali feldspar.
3.3 ‘Transitional’ lavas
Basaltic lavas (with D.I. > 32) close to hawaiites have been described at Mts Mbam [40], Bangou [20], and Bana [31], and qualified of transitional, or transitional towards tholeiites. These lavas are older than Miocene, strongly weathered (occurrence of epidote, carbonates, chlorite). They commonly erupted along fissures and are located in zones where high-temperature N70°E fracturation predominates [52,53]. Olivine phenocrysts are rare (Fo55–65) or absent, while plagioclase phenocrysts are abundant (mode > 20%): hence the name ‘plagioclase basalts’ [31]. Pigeonite never occurs. These lavas do not present the mineralogical characteristics typical of tholeiitic lavas erupted in extension domains. Nevertheless, in Cameroon, dykes and sills of basaltic lavas (ages between 43 and 87 Ma) with typical continental tholeiitic character [45] occur in Cretaceous east–west semi-grabens of Mayo Oulo–Léré and Babouri–Figuil. These lavas do not belong to the CHL.
3.4 Nephelinitic lavas
The lavas from Mt Etinde [56] are peralkaline. They are melanephelinites, nephelinites sensu stricto and varied nephelinites (that contain one or more of the following minerals: nosean, melilite, perovskite, garnet, aenigmatite, leucite, and felspar) and hauynophyres.
3.5 Lamprophyres
Some alkaline lamprophyres (camptonites and monchiquites) occur at Mt Cameroon [50] and in the northern part of the continental sector of the CHL, in the Benue valley, at the Kokoumi ring-complex [46] and at Tchircotché [47,49]. They are contemporaneous with the nearby alkali basalts. Mt Cameroon camptonites contain olivine, clinopyroxene (diopside core), plagioclase, and Ti-magnetite phenocrysts in a matrix of plagioclase, amphibole, clinopyroxene, and magnetite microlites. Monchiquites from Tchircotché have a hyaloporphyritic texture with phenocrysts of olivine, clinopyroxene (diopside), amphibole (kaersutite) and/or microphenocrysts of biotite in an analcitic matrix of Fe–Ti oxides and apatite microlites, where feldspars may occur. These lamprophyres contain abundant (mode up to 8%) carbonate–analcite–glass ocelli.
3.6 Plutonic rocks
Ring-complexes are essentially made of sometimes layered (=cumulates) gabbroic rocks (with olivine and/or clinopyroxene) associated with syenites (with riebeckite), nepheline syenites, alkali granites and peralkaline granites (with riebeckite) [12 and references herein, 27,29,32,46,51,60]. Not all these rocks are systematically present in all ring complexes.
3.7 Ultramafic xenoliths
Numerous occurrences of ultramafic xenoliths have been found in basaltic lavas all along the CHL (Fig. 1): six sites at São Tomé [6], two at Santa Isabel (Bioko) [13], and one in each of the other sites: Mt Cameroon [15], Barombi–Mbo (Kumba) [67], Lake Enep (Oku) [34,61], Nyos (Wum) [41,68] and Liri (South of the Kapsiki plateau) [66]. The rocks are mostly spinel lherzolites; some pyroxenites occur at São Tomé and at Mt Cameroon. Spinel [23] and garnet [69] lherzolites are also present in Adamawa.
4 Geochemistry and isotope geochemistry
Except the nephelinites of Mt Etinde, the silica–alkalis diagram (Fig. 4) points out the Daly gap between basaltic and felsic alkaline lavas. Mugearites and benmoreites are very scarce (15 and 13 samples respectively, among more than 400 samples for the whole CHL). In the oceanic sector, phonolites are poor in silica and trachytes are very scarce. Rhyolites only occur in the continental sector.
The basalts from the oceanic and continental sectors have similar trace element compositions (Fig. 5). These basalts are quite similar to St Helena basalts, although the latter are slightly richer in uranium. Although basalts from Adamawa do have patterns similar to those of CHL basalts, they have, by contrast, higher trace element contents (particularly from Rb to Nb).
Primitive mantle-normalized multi-element diagram for basalts of the Cameroon Hot Line (data from [9,25,44]). Primitive mantle after [26]. Basalts from Mt Cameroon and from the extremities of the continental (Kapsiki) and oceanic (Pagalu) sectors of the CHL are considered to be representative of the whole line. Basalts from St Helena [8] and Adamawa [54,59] are also indicated for comparison.
Fig. 5. Diagramme normalisé (manteau primitif d’après [26]) de basaltes de la ligne chaude du Cameroun. Ont été sélectionnés comme échantillons représentatifs, les basaltes du mont Cameroun, et ceux provenant des extrémités des secteurs continental (Kapsiki [48]) et océanique (Pagalú [25]) de la LChC [9]. Sont indiqués, pour comparaison, des basaltes représentatifs de Sainte-Hélène [8] et de l’Adamaoua [54,59]. Données d’après [1].
Some lavas are considered as transitional or tholeiitic transitional according to their high Y/Nb ratios (>0.9) and to the gentle slope of their REE patterns (La/Yb)N < 12) when compared to alkali basalts (Y/Nb < 1 and (La/Yb)N > 12). The studied lavas of Mt Bana are not true basalts, but instead hawaiites with high SiO2 (47.4–51.6%) and Al2O3 (>20%) contents and very low Ni contents (<20 ppm). Moreover, they are strongly weathered (sum of major element oxides often <98 wt%, sometimes <96 wt% [31]). Lavas from Mts Bangou [20] and Mbam [40] have the same characteristics. At Mt Bangou, the Ti–Zr–Y concentrations are typically in the range of alkaline lavas. The transitional character cannot be attributed to these lavas.
The nephelinites (Mt Etinde) are characterized by their high Sr, Ba, Zr contents (higher than 10000, 3500, and 1100 ppm, respectively), by high LREE/HREE ratios (22 < (La/Yb)N < 48), and by high volatile contents (>0.6% CO2; >8% H2O and >2.0% SO3).
Lamprophyres of the CHL have systematically lower MgO (<8%), Cr (<60 ppm) and Ni (<75 ppm) contents than the basalts, but their contents in incompatible elements are by far higher than those of basalts. Their normalized patterns (22 < La/Yb)N < 32) are steeper than those of basalts (14 < La/Yb)N < 28).
Basaltic lavas from both continental and oceanic sectors have identical Sr and Nd isotope compositions [9,24,25,33–36,43,44,62]. Their (87Sr/86Sr)i and vary between 0.70299 and 0.70343, and +1.7 and +7.0, respectively. Megacrysts (mostly of clinopyroxene) have similar isotope compositions [34]. Similarly, Hf isotope compositions are identical for both sectors of the CHL (mean ≈ +1.95 [3]). When compared to the atmosphere 3He/4He ratio (Ra), the helium isotope compositions of the CHL lavas show contrasting results: they are either considered as homogeneous around 6.3 × Ra all along the CHL [4], or they increase from 4 to 6 × Ra at the continent–ocean boundary to ∼8 × Ra towards the northern and southern extremities of the CHL, reaching there values close to those of HIMU-type basalts [1]. Lead isotope compositions present significant regional variations [24,25,33]. Basalts from Pagalu, the southernmost island, are the least radiogenic (206Pb/204Pb: 19.01; 208Pb/204Pb: 38.83), whereas basalts from volcanoes located near the continent–ocean boundary (Bioko, Mt Etinde, and Mt Cameroon) are strongly more radiogenic (206Pb/204Pb: 20.52; 208Pb/204Pb: 40.34). By contrast, 207Pb/204Pb ratios do not vary significantly all along the CHL. Osmium isotope ratios (187Os/188Os) are lower in the oceanic sector (0.128–0.188) than in the continental one (0.145–0.557) [21]. Measurements of 238U–230Th–226Ra disequilibria for lavas from historical eruptions of Mt Cameroon are well distinct for each eruption from 1909 to 2000, and do not show any evolution with time [72].
Phonolites have Sr–Nd–Pb isotope signatures similar to those of basaltic lavas. The same applies for trachytes from the oceanic sector. The whole lava series (from basalts to trachytes) of Principe [25] does not show any significant isotope variation. In the continental sector, some trachytes and rhyolites have initial Sr ratios higher (up to 0.715, Fig. 7) and ɛNdi lower (from +3.22 to +0.32) than basalts.
Lamprophyres have Sr and Nd isotope compositions [47,48,50] close to those of basalts, even if (87Sr/86Sr)i values are slightly higher (0.70329–0.70387) and ɛNdi slightly lower (+2.45 to +2.65) than those of basalts.
5 Discussion
5.1 Hot spot, hot line?
Following the hypothesis of the presence of a fossil hot spot at the continent–ocean boundary [24], it has been suggested that the magmatic activity along the CHL could be related to an active plume with a HIMU (μ = 238U/204Pb) signature initiated in the deep mantle and that this plume would be currently expanding at the asthenosphere–lithosphere boundary, entraining ambient materials represented by a mixture between depleted (DM) and enriched (EMI) mantle components [33]. As there is no hot-spot trail (regular age progression along the line), the classical stationary hot-spot model can be discarded.
An alternative model to explain volcanic alignments is that of the ‘hot line’. First proposed for the volcano-tectonic alignment that extends from Easter Island to San Felix and San Ambrosio islands in the eastern Pacific Ocean [5], the hot-line hypothesis has been proposed for the CHL volcano-tectonic alignment [13], and sustained with seismic and gravimetric data in the oceanic domain [37]. The development of a hot line is accompanied by a crustal uplift in the oceanic and continental sectors. Other hot lines have been proposed for the Atlantic Ocean (Walvis, Cap-Vert, Canaries…) and the Africa continent [37]. They would represent the surface expression of asthenospheric convective rolls that could produce lithospheric uplift, decompression melting, and accumulation of melts. Upwelling could also cause the reactivation of pre-existing (Pan-African) lithospheric fractures. The emplacement of numerous volcanoes (from 40 Ma ago to Present) and of numerous plutonic ring-complexes (from 70 to 30 Ma ago) along the CHL appears to be strongly controlled by these lithospheric fractures.
5.2 Nature of the mantle source, magma genesis, and magma differentiation
5.2.1 Sublithospheric source
The geochemical and isotopic similarities between the CHL oceanic and continental basalts attest that the continental crust did not play any role in the magma genesis and that the source is not of lithospheric origin. Indeed, the oceanic sector of the CHL is built upon a young oceanic lithosphere of Upper Cretaceous age, while the continental lithosphere is much older and of Pan-African age (>500 Ma). The Sr–Nd–Hf isotope compositions imply that the lithospheric contribution is negligible. Osmium isotope ratios [21] and data from U–Th–Ra disequilibria [72] also suggest an asthenospheric origin for the basaltic magmas of the CHL.
Nephelinites from Mt Etinde have a deeper source than basaltic magmas. They erupted at the very limit between the ocean and the continent, where fracturation (fractures at the continent-ocean boundary and regional N30°E and N70°E zones) is more intense and deeper.
Lead isotope variations require relatively recent U/Pb and Th/U fractionations (∼125 Ma) in the upper mantle [24].
5.2.2 FOZO, HIMU?
Geochemical characteristics and Pb isotopic compositions of basalts have been initially interpreted [24,33] as suggesting a HIMU-type mantle source similar to that of St Helena, the influence of the HIMU component decreasing on 400 km, northwards and southwards, from the continent–ocean boundary [24]. It is noteworthy that St Helena is located on the southwards extension of the CHL (Fig. 2, inset). Nevertheless, in Pb–Pb, Pb–Sr and Sr–Nd isotope diagrams (Fig. 6), all the data for the basalts (and nephelinites) of the CHL define broad domains encompassing the recently redefined [64] FOZO (FOcal ZOne). The CHL domains are generally outside the typical HIMU field. Therefore, CHL basaltic magmas do not result from the mixing between the Atlantic MORB component (DM) and HIMU. The isotopic dataset suggests that both DM and FOZO were involved in the genesis of CHL alkaline series, as shown for the majority of alkali basalts from oceanic islands. Moreover, a component with a radiogenic Pb signature distinct from HIMU is also needed. The differentiated lavas of the CHL derived from the evolution of these basaltic magmas by crystal fractionation of olivine, clinopyroxene, Fe–Ti oxides, plagioclase, and apatite (e.g., [35,44,48]).
(87Sr/86Sr)i–(143Nd/144Nd)i, (208Pb/206Pb)i–(87Sr/86Sr)i and (206Pb/204Pb)i–(208Pb/204Pb)i diagrams for basalts of the Cameroon hot line (data after [24,25,33–36,44,46,47,49,62] and unpublished). FOZO (Focal Zone) and HIMU fields after [64]. Data from St Helena [8] and N-MORBs from the Central Atlantic Ocean [64 and ref. herein] are also presented for comparison. The basalts of the Cameroon hot line plot above the line between St Helena basalts and Atlantic N-MORBs.
Fig. 6. Diagrammes (87Sr/86Sr)i–(143Nd/144Nd)i, (208Pb/206Pb)i–(87Sr/86Sr)i et (206Pb/204Pb)i–(208Pb/204Pb)i pour les laves de la « ligne du Cameroun » (données d’après [24,25,33–36,44,46,47,49,62] et données non publiées). Les champs de Sainte-Hélène [8] et des réservoirs mantelliques FOZO (FOcal ZOne), HIMU et des N-MORBs (basaltes de l’Atlantique central) sont indiqués (données d’après [64] et références citées).
5.2.3 Metasomatic effects
Evidence of metasomatism in the mantle source of the CHL parental magmas has been deduced from mantle xenoliths, on the one hand, and from lamprophyres and nephelinites, on the other hand.
Metasomatized peridotites are likely to be representative of enriched subcontinental lithosphere, the commonly accepted metasomatic agent being partial melts that form a network of thin veins lacing mantle peridotite beneath zones of recent volcanism and that induce the crystallization of K-rich amphibole [15,34].
The occurrence of amphibole (pargasite) in upper-mantle xenoliths (spinel lherzolites, websterites, wehrlites, pyroxenites) at Mt Cameroon [15], Mt Bambouto [36], Mt Oku [34], and Nyos [68] suggests that the lithospheric mantle beneath the CHL has been metasomatized. The presence of fluid inclusions and interstitial carbonates in amphibole-free mantle xenoliths from São Tomé [6] is another evidence of metasomatic fluid activity. Moreover, the low Al2O3 (<4 wt%) and high CaO (21–24 wt%) contents of clinopyroxenes, their REE enrichment and also the relatively high fO2 (above the FMQ buffer) in dunites [6] in São Tomé xenoliths [6] are also suggestive of a metasomatic influence by carbonatitic fluids. LREE enrichment in clinopyroxenes and amphiboles from mantle xenoliths [67,68] points to a cryptic metasomatic event by fluids enriched in those elements.
The lamprophyres of the CHL (monchiquites from Kokoumi [46] and Tchircotché [49] and camptonites from Mt Cameroon [50]) have high incompatible element contents and contain hydrated (kaersutite, biotite) and/or carbonate minerals, suggesting that they originate from a volatile-rich infra-lithospheric metasomatized peridotite mantle. The various Mt Etinde nephelinites contain pockets of carbonates or interstitial carbonates, indicating that the mantle source from which the nephelinitic magmas derive has probably been metasomatized by liquids of carbonatitic affinities [56]. It is interesting to note that lamprophyres and nephelinites are clearly related to domains of lithosphere extension such as continental rift (Benue) and Atlantic Ocean opening zone (Mt Etinde, Mt Cameroon).
The isotopic characteristics of the various xenoliths from the continental sector of the CHL suggest that they could represent fragments of sub-continental lithosphere that formed during the Pan-African orogeny and were subsequently enriched during metasomatism at the time of early break-up of Gondwana [34,68].
5.2.4 Silica-rich evolved lavas: crustal contamination?
Whilst the basaltic magmas have not been contaminated during their ascent through the continental crust, their differentiation is accompanied by a significant crustal contamination, as demonstrated by their Sr–Nd isotope compositions (e.g., Mts Bambouto [35], Benue valley [48] Kapsiki plateau [44]). The magmatic evolution from basalt to phonolite can be modelled by crystal fractionation without crustal contamination [44,48]. On the contrary, the evolution to trachyte and rhyolite requires a crustal component, as indicated by their high (87Sr/86Sr)i and lowvalues. A contamination (up to 10 wt% [48]) by a continental crust (87Sr/86Sr ≈ 0.750) allows to explain the high 87Sr/86Sr values of some trachytes and rhyolites (Fig. 7).
At Tchabal Nganha (Adamawa), (87Sr/86Sr)i values regularly increase with increasing Rb and SiO2 contents [59], suggesting assimilation coupled with crystal fractionation process (AFC) [10].
5.2.5 Magma mixings
Intermediate lavas (mugearites and benmoreites) have mineralogical characteristics (simultaneous presence of two populations of feldspar phenocrysts, occurrence of xenocrysts) typical of both basaltic and evolved magmas, suggesting that they can originate through mixing between these two magma types (Fig. 7). Nevertheless, considering the very limited number of mugearites and benmoreites observed in the CHL when compared to ubiquitous basalts and hawaiites, on the one hand, and to phonolites, trachytes and rhyolites, on the other hand, this process (which tends to reduce the Daly gap) must have been very limited.
6 Conclusions
The Cameroon Hot Line (CHL) is a N30°E magmato-tectonic megastructure of Central Africa, and the paradigm of active hot lines on Earth, a concept originally defined for the Easter chain in the Pacific Ocean. It stretches on both the oceanic and continental domains on more than 2000 km, from the Gulf of Guinea, Central Atlantic Ocean, to Lake Chad. This line is marked by numerous volcanoes and plutonic ring-complexes of Cainozoic to Recent age. As there is no regular age progression along the line, the classical plume model can be discarded. Nevertheless, the volcanic and plutonic rocks display the typical petrological and geochemical signature of alkaline intraplate magmatism, the Mt Etinde nephelinites being peralkaline.
Geophysical studies in the oceanic sector suggest that the volcanic alignment could be related to a deep-mantle hot line. Lithospheric uplift induced decompression melting and the upwelling of melted material provoked the reactivation of pre-existing (Pan-African) N70°E lithospheric fracture zones that previously affected the African and South-American continents at the beginning of the opening of the central Atlantic Ocean.
The parental basaltic magmas have similar Sr and Nd isotopic composition in both oceanic and continental sectors of the line, which implies a common sublithospheric mantle source. These magmas have been enriched during their ascent through the lithospheric mantle that has been metasomatized by volatile-rich (carbonatitic) fluids. In isotopic diagrams, CHL lavas define broad domains encompassing FOZO, but not HIMU. Besides DM, a radiogenic Pb component, distinct from HIMU, is needed to explain the observed range of isotopic composition. Differentiation of basaltic magmas to trachytes and rhyolites in the continental domain was accompanied by significant crustal contamination, whereas evolution to phonolites has not been affected.
Nowadays, the volcanic activity is dominated by eruptions of basaltic lavas (Bioko Island, Mt Cameroon) and gases (Lakes Monoun, Nyos) and concentrated in the central zone of the line, where both the N30°E and N70°E fracturations are by far more intense.
Acknowledgements
The French ‘Ministère de la Coopération’ is acknowledged for providing I.N. with grants for stays in France in the ‘Laboratoire de magmatologie et de géochimie inorganique et expérimentale, université Pierre-et-Marie-Curie’, Paris, and to B.D. for financially support the field work in Cameroon. Stays of B.D. in Cameroon have been substantially supported by the Faculty of Sciences of the University of Ngaoundéré. The isotopic measurements at the ‘Laboratoire de géochimie isotopique’ (U.L.B., Brussels) were supported by FNRS Grants to D.D. The authors are grateful to J. Touret for fruitful discussion. Contribution IPGP No. 2264.