Introduction

Mount Cameroon is the highest volcanic massif within the continental segment of the Cameroon Volcanic Line (CVL) (Fig. 1). This huge stratovolcano culminates at 4,095 m. The CVL is made up of 12 main volcanic centres with ages ranging from 51.8 Ma to the Present (Fitton and Dunlop 1985; Ménard et al. 2002; Fosso et al. 2005; Déruelle et al. 2007; Moundi et al. 2007). The most recent eruptions were those of 1999 and 2000 on Mount Cameroon (Déruelle et al. 2000; Wandji et al. 2001; Suh et al. 2003), and the emanation of lethal gas from Lake Monoun in 1984 and Lake Nyos in 1986 (Kusakabe et al. 1989; Wandji et al. 1994). Among the ca. 140 strombolian cones that are scattered over the Mount Cameroon area, a hundred are located on the southern, south-eastern and south-western plunging towards the Atlantic Ocean flanks of the volcano.

Fig. 1
figure 1

a Location map of the Cameroon Volcanic Line (CVL). The main geologic features of Africa are indicated. b Location of Mount Cameroon and other main volcanic centres along the Cameroon Volcanic Line (modified according to Nkouathio et al. 2008). COB Continental Ocean Boundary. Dashed lines are limits between the CVL segments: ocean, COB and continent

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Ultramafic xenoliths (dunites, wehrlites, and clinopyroxenites) were collected on one of the southern strombolian cones, at an elevation of 500 m near Batoke (Fig. 2). Similar xenoliths (wehrlite and clinopyroxenite, 15 cm in diameter) have already been discovered in pyroclastic deposits (alkaline basalt) of another strombolian cone at 3,000 m elevation on the northwest flank of Mount Cameroon (Déruelle et al. 2001; Ngounouno et al. 2001).

Fig. 2
figure 2

Sampling map. Recent lava flows are represented in black and the years of eruption are indicated, including the most recent of 1999 and 2000. Stars indicate lava samples and ages are indicated in brackets. Filled square indicates xenoliths and the host basanite (this study) located 500 m above the sea level. Empty square indicates xenoliths collected at 3,000 m above sea level (Déruelle et al. 2001). Elevations are indicated. Roads are represented by thick, discontinuous lines

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Other outcrops with xenoliths have already been identified in Cameroon. Dautria and Girod (1986) described lherzolite xenoliths from a Quaternary volcano at Dibi, Northern Adamawa, near the rift of Ngaoundere, containing 63–81 vol.% olivine, 12–18 vol.% orthopyroxene, 6–16 vol.% clinopyroxene, and moreover spinel and plagioclase. Nana (2001) and Temdjim et al. (2004) studied peridotite xenoliths in recent alkali basalts at Lake Nyos: these are mostly lherzolites and harzburgites with scarce olivine-websterites. Teitchou et al. (2007) observed typical lherzolites in the Kumba Plain near Mount Cameroon, with 58–69 vol.% olivine, 14–26 vol.% orthopyroxene, 11–17 vol.% clinopyroxene, and 1–5 vol.% spinel. Lee et al. (1996) studied xenoliths (harzburgites and lherzolites) from several places in Cameroon. Princivalle et al. (2000) studied the clinopyroxenes of lherzolites from Lake Nji. Caldeira and Munha (2002) discussed the petrology of ultramafic nodules from Sao Tomé Island: these included harzburgites and lherzolites, dunites, orthopyroxenites as well as clinopyroxenites and olivine clinopyroxenites (see below).

The aim of this study is to describe the petrographical, mineralogical and geochemical character of the xenoliths and their relations to their host lavas, in order to determine their origin. The interpretation of the associated magmatic process will allow us to propose a model for the petrogenetic evolution of the basanites and then their relations with xenoliths.

Geological framework

Mount Cameroon is a part of the CVL tectonic structure which is the consequence of a series of parallel faults oriented N30 (Burke and Whiteman 1973). From the distribution of the cones scattered on the massif, three major tectonic axes that control the volcanic activity could be identified: the Batoke axis (030° to 040°), the Debundscha axis (060° to 070°) and the Limbe axis (140° to 150°).

Mount Cameroon is a large stratovolcano intercalated between the Tombel graben to the north and the Douala basin to the south. Its elliptical base measures 50 km in length by 35 km in width. Morphologically, it looks like a horst with prominent boundary faults that can be traced along breaks of slopes (Déruelle 1982). Its basement (Pan-African granites and gneisses) is covered by Cretaceous to Quaternary sediments (Dumort 1968; Tsafack et al. 2007).

Different types of volcanic activity have been identified:

  • Important effusive eruptions produced lava flows that tend to be thick, chaotic. Many are bounded by levees, that are natural banks of chilled material channelling the lava into a self-made channel (Francis 1993);

  • Important explosive eruptions alternated with effusive eruptions. Pyroclastic deposits are several meters thick, sometimes more. As a result, intercalated massive lava flows and thick tephra layers have progressively built the stratovolcano. Statistically, pyroclastic deposits have built more than the half of the volcanic massif;

  • Moderate explosive activity has built numerous strombolian cones; today, about 140 cones are scattered on the whole massif;

  • Exceptional hydromagmatic activity is responsible for two maars: one is located at Debundscha and the other is the maar of Mount Bomana in the sedimentary basement at the North West flank of the volcano (Fig. 2).

Mount Cameroon lavas are mainly basanites (60%), alkali basalts (20%), hawaiites (12%) and less common mugearites (8%). Moreover, camptonites, which are a type of lamprophyre, composed mainly of plagioclase and brown hornblende, have been described by Ngounouno et al. (2006).

The xenoliths described in this paper were collected from a small strombolian cone at an elevation of 500 m, between 4°02′ and 4°04′ North and between 9°08′ and 9°10′ East on the Limbe axis. This cone has a height of about 70 m and a diameter of 600 m. Blocks of xenolith-rich basanites are abundant within 300 m of the base.

Analytical methods

Lavas and xenoliths have been analysed at the Centre de Recherches Pétrographiques et Géochimiques (CRPG) at Nancy, France. Major element analyses have been made by ICP-OES (Inductively Coupled Plasma - Optical Emission Spectrometry) and trace element analyses by ICP-MS (Mass Spectrometry). During preparation of the samples, about 300 mg of powder have been treated with LiBO2 and dissolved in HNO3. Five international geostandards have been used (basalt BR, diorite DRN, serpentinite UBN, anorthosite ANG and granite GH, Carignan et al. 2001).

Minerals have been analysed with CAMECA SX 50 or SX 100 (15 kV, 10 nA) electron microprobes at Université Pierre et Marie Curie, Paris VI, France. Kα lines were used. Standards are diopside for Si, Ca and Mg, Fe2O3 for Fe, MnTiO3 for Ti and Mn, Cr2O3 for Cr, albite for Na, orthoclase for K and Al, counting times are 10 s for Pk (peaks) and 10 s for Bg (background), with a 5 µm defocused beam.

Isotopic analyses were made by D. Demaiffe at the Laboratoire de Géochimie Isotopique, Université Libre de Bruxelles, Belgium, with a Micromass VG 54 multicollector thermal-ionisation mass spectrometer. Sr isotopic ratios are corrected for mass fractionation with normalization to 86Sr/88Sr = 0.1194. Replicate analyses of the NBS 987 standard yielded an average 87Sr/86Sr value of 0.710255 ± 10. Nd isotopic data are corrected for mass fractionation by normalization to ratio 146Nd/144Nd = 0.7219. Replicate analyses of the La Jolla Nd standard gave an average 143Nd/144Nd value of 0.511854 ± 8.

Whole rock K-Ar ages were determined at the Laboratoire de Géochronologie, Université de Bretagne Occidentale, Brest, France, following the analytical procedure detailed in Bellon and Rangin (1991). The samples were crushed and sieved to 0.30–0.15 mm grain size. Alteration products were eliminated by sifting, washing in distilled water and passing through the ultra-sound cleaner at the Université Paris-Sud Orsay. The grained fraction was used for argon extraction by high frequency heating under high vacuum. Isotopic composition of argon and concentration of radiogenic argon (40Ar*) were measured with mass spectrometry. Part of the granular fraction was crushed to a powder in an agate mill. K contents of whole rock were measured by atomic absorption spectrometry on acid solutions realized after a chemical attack of the powders in sealed polycarbonate bottles with hydrofluoric acid (HF) at 70°C during 12 h. Constant values of Steiger and Jäger (1977) have been used for age calculations. Uncertainties have been calculated using equations of Mahood and Drake (1982) and are given at 1 σ.

Two samples, TM198 and TM407 were K-Ar dated, using an unspiked technique described by Charbit et al. (1998). This technique differs from the conventional isotope dilution method in that argon extracted from the sample is measured in sequence with purified aliquots of atmospheric argon at the same working gas pressure in the mass-spectrometer. This allows percentages of radiogenic argon (40Ar*/40ArT) as small as 0.14% to be measured (Scaillet and Guillou 2004). Combined with a high within-run signal stability (maximum ratio drift around 1‰), this permits individual samples of 40–50 ka basalt containing 2 wt.% K2O to be dated with an analytical precision of ±2 ka on a single-run basis (2σ). Groundmass was separated from fresh samples following the methods of Guillou et al. (1998). Phenocrysts and xenocrysts which are potential carriers of extraneous 40Ar were eliminated using magnetic, gravimetric, and visual hand picking. Replicate unspiked K-Ar age determinations were done on the microcrystalline groundmass of the two samples. The determination of K was carried out by atomic absorption with a relative precision of 1%. Argon was extracted by radio frequency heating of 1.0–2.5 g of sample, then transferred to an ultra-high-vacuum glass line and purified with titanium sponge and Zr-Ar getters. Isotopic analysis were performed using a 180°, 6 cm radius mass spectrometer with an accelerating potential of 620 V. The atmospheric correction is monitored via two separate measurements of atmospheric argon for each sample. A first atmospheric argon aliquot is measured at the same gas pressure as the sample, and serves as an isotopic reference for the determination of its radiogenic content under identical mass-discrimination conditions. The second aliquot measured is a calibrated volume of atmospheric Ar, used to convert beam intensities to atomic abundances. As both reference aliquots (isotopic and manometric) are atmospheric in composition, they provide a cross check on the radiogenic composition of the sample. Periodic cross-calibration of zero-age standards precisely constrains the mass-discrimination to within ±0.5‰ on the 40Ar/36Ar ratios. The manometric calibration is based on periodic, replicate determinations of international dating standards of known K-Ar age using the same procedure for the unknowns as described in Charbit et al. (1998). This allows the total 40Ar content of the sample to be determined with a precision of about ±0.2% (2 σ). Uncertainties for the K and Ar data are 1 σ analytical only, and consist of propagated and quadratically averaged experimental uncertainties arising from the K, 40Ar (total), and 40Ar* determinations. Uncertainties on the ages are given at 2 σ.

Petrography and mineralogy of xenoliths

  1. (1)

    Host lava

The host lava outcrops as discontinue blocks of porphyritic vesicular basanites. The basanites are rich in euhedral phenocrysts and angular xenocrysts of olivine (chrysolite) and diopside and contain xenoliths. Resorption of some chrysolite xenocrysts has reduced them to skeletal shape.

The matrix contains fine microcrysts of chrysolite, clinopyroxene, Fe-Ti oxides and plagioclase. Glass is rare. Reaction rims between the host basanitic liquid and the xenocrysts resulting from re-equilibration are not observable under the microscope, but are revealed by electron microprobe analyses.

Olivine phenocrysts of the host lava have chemical compositions of chrysolite, decreasing from Fo81 in the cores to Fo75–80 at their rims. The microcrysts have compositions of Fo75 which are similar to those at the rim of phenocrysts. Hyalosiderite is found in other mafic lavas of the Mount Cameroon. The clinopyroxenes of the host basanite are diopsides rich in TiO2 (2 to 4 wt.%) and CaO (22 wt.%) but differ from fassaites described by Wandji et al. (2000) in the Noun Plain. They have compositions of Wo49–46En36–42Fs10–15 and their Mg# = Mg/(Mg+Fe) vary from 71 to 80. Their Cr2O3 contents are about 0.06 wt.% for the phenocrysts and 0.12 wt.% for the xenocrysts. The oxides are titanomagnetites with TiO2 contents varying from 19 to 23 wt.% and contain a high percentage of ulvöspinel component (52 to 62 wt.%). FeOt contents range from 61.8 to 65.6 wt.%.

  1. (2)

    Chrysolite xenocrysts

Chrysolite xenocrysts in the host basanite have compositions ranging from Fo81–84 in their cores to Fo75–83 at their rims. Electron microprobe analyses show significant re-equilibration between the chrysolite xenocrysts and the host basanitic liquid (Table 1). It is marked by decreasing Mg2+ and Ni+ and by an increase of all the others cations such as Fe2+, Mn2+, Ca2+, Si4+ in the xenolith rim. The exchanges of Fe2+/Mg2+ between chrysolite xenocrysts and the liquid are the most important. Rims of xenocrysts are richer in Fe2+ than the cores (increase of 2.06%) as well as Si, Mn (increase of 66.7%), and Ca and poorer in Mg, Ni (Table 1).

  1. (3)

    Xenoliths

Table 1 Evidence for re-equilibration of chrysolite xenocryst with host basanite from element zoning profiles
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Three types of xenoliths are found in the basanite: dunites, wehrlites and clinopyroxenites. They are 0.5 to 4 cm wide and 1 to 5 cm in length. They contain three mineral phases: olivine (chrysolite), clinopyroxene (diopside) and spinel (Tables 2, 3 and 4).

  1. 3. a.

    Dunites

Table 2 Representative microprobe analyses of chrysolites from the host basanite and xenoliths
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Table 3 Representative microprobe analyses of diopsides from the host basanite and xenoliths
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Table 4 Representative microprobe analyses of microcrysts of spinels from the host basanite and xenoliths
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More or less rounded dunite xenoliths measure 3 to 5 cm in diameter. They are composed (visually) of 91% chrysolite, 7% diopside and 2% spinel (Fig. 3). Dunites have a porphyroclastic texture (sample TM190b, Fig. 4a) with typical kink-bands in chrysolites, that suggest that some degree of deformation has affected these rocks. The size of their grains ranges from 0.8 to 1.6 mm for the chrysolites and from 2 to 4 mm for the diopsides. Under the microscope, the contact between the xenolith and host basanite has a rusty colour attributable of alteration.

Fig. 3
figure 3

Modal composition (vol.%) of studied xenoliths compared with some other xenoliths founded in Cameroon, in a olivine - orthopyroxene - clinopyroxene triangle. Classification after Streckeisen (1976)

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Fig. 4
figure 4

Texture of the xenoliths. Each picture corresponds to a sector of 5 x 3,8 mm. ol = olivine (chrysolite), cpx = clinopyroxene, sp = spinel. a porphyroclastic texture (dunite, TM190b); b porphyroclastic texture (wehrlite, TM190e); c mosaïc equigranular texture (clinopyroxenite, TM190d)

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Chrysolites (Fo83–84) have the same composition as the cores of the xenocrysts. Diopsides have compositions of Wo47–46En43–44Fs9–10 (Fig. 5). Spinels are mostly interstitial between grains of chrysolite, and some are as inclusion in chrysolites. Cr2O3 contents are about 20 wt.%, for Al2O3 contents of 10 wt.%, and MgO contents of 7 wt.%; Cr# (= Cr / Cr + Al) and Mg# are respectively 57 to 58 and 31 to 32. FeOt contents vary between 52 and 54 wt.% (Fig. 6, Table 4).

  1. 3. b.

    Wehrlites

Fig. 5
figure 5

Clinopyroxene compositions, plotted in the Wo-En-Fs diagram (according to Morimoto 1988). Diamond: host basanite, triangle: dunite, circle: wehrlite, square: clinopyroxenite

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Fig. 6
figure 6

Diagram for oxides contents vs. Cr2O3 of spinels (same symbols as Fig. 5)

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Wehrlites are composed of a granular assemblage (0.4–2 mm) of crystals of chrysolite (78%), diopside (21%) and spinel (1%). They show granular and porphyroclastic textures (sample TM190e, Fig. 4b). The olivines are cracked. The clinopyroxenes occupy spaces between chrysolite minerals. Spinels are found mostly as inclusions in chrysolite and rarely occupy the interstices between the grains.

Chrysolites are Fo81. CaO contents of the chrysolites of wehrlites (0.2–0.3 wt.%) are nearly the same as those of the dunites (0.1–0.4 wt.%) and are higher (Table 2) than those from the mantle (0.05–0.1 wt.%) (Lee et al. 1996). This probably reflects an equilibrium environment under a lower pressure (Caldeira and Munha 2002). Clinopyroxenes (Wo47–45En43–46Fs8–9) of the wehrlites are almost identical to those analyzed in dunites (Fig. 5). They are diopsides with Mg# values similar to those of chrysolites (81–84), except in the case of some diopsides that have relatively high Mg# values of 85 to 86. They are Cr-rich, with a Cr2O3 contents reaching 1.2 wt.%. Spinels have relatively constant compositions in terms of FeOt (41.9 to 43.3 wt.%), Cr2O3 (22.7 to 25.3 wt.%), MgO (6.2 to 9.7 wt.%) and Al2O3 (16 to 19 wt.%). Cr# and Mg# values are 45 to 50 and 40 to 43 respectively (Fig. 6, Table 4).

  1. 3. c.

    Clinopyroxenites

Clinopyroxenite xenoliths have sizes ranging from 0.5 to 1 cm. They are composed of 95% diopside, 4% chrysolite and 1% titanomagnetite and have a mosaic equigranular texture according to Mercier and Nicolas (1975) (sample TM190d, Fig. 4c). The mineral sizes are 0.4 to 0.7 × 0.3 to 0.6 mm. No hydrous minerals such as amphibole, phlogopite, or apatite are present.

Chrysolites are Fo78–81. Diopsides (Wo48–47En40–41Fs10–11) (Fig. 5) of the clinopyroxenites are Al2O3-rich (up to 7.2 wt.%) and contain up to 2.3 wt.% TiO2 (Table 3). These clinopyroxenes (Mg# = 78 to 79) are very different from those of typical mantle origin (Mg# = 91 to 93) described elsewhere along the CVL (Nana 2001). Spinels are interstitial titanomagnetites. Their FeOt contents are between 58 and 60 wt.% (Fig. 6, Table 4). Their low Cr2O3 contents (4.5 wt.% Cr2O3) are one of the major difference that distinguish the spinels of clinopyroxenites from those of dunites (19 to 21 wt.% Cr2O3) and wehrlites (22 to 25 wt.% Cr2O3). Mg# and Cr# have almost constant values of nearly 25.

Geochemistry of the series

Twenty-six samples of lavas and two xenoliths from the Mount Cameroon have been analyzed. Nine lava analyses have been selected (Table 5) as representative of the alkaline series: three basanites including the host of the xenoliths, two alkali basalts, two hawaiites, and two mugearites. In the TAS diagram (Le Maitre 2002), most of the lavas plot within the basanite, basalt and hawaiite fields (DI < 45) (Fig. 7). The most differentiated lavas (hawaiites and mugearites) were mainly sampled above 2,000 m. The analyzed xenoliths are one dunite and one wehrlite (Table 5).

Fig. 7
figure 7

Total alkalies versus silica plots for the studied lavas (according to Le Maître 2002). Full diamond: host basanite

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Table 5 Whole rock chemical composition of the host lava compared to the other representative lavas from Mount Cameroon and of two xenoliths (TM190b dunite and TM190e wehrlite) (analyses CRPG, Nancy, France)
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Basic lavas have typical characteristics of an alkaline series. TiO2 contents are high (2.4 to 3.4 wt.%), Na2O contents range between 2.9 and 4.7 wt.% and K2O between 1.1 and 2.1 wt.%. Note that losses on ignition are far lower than 2 wt.%, corresponding to the upper limit for fresh, unaltered igneous rocks according to Le Bas et al. (1986).

The chemical variations of the lavas are characterized by progressive increase in SiO2, Na2O, K2O and Al2O3 and decreasing MgO, CaO and Fe2O3 with progressive differentiation (Fig. 8a). Transition element contents (Cr, Ni, Co) decrease with decreasing MgO content in agreement with fractionation of mafic minerals such as olivine (chrysolite) and diopside in the less evolved magmas (Fig. 8b). Fe2O3, TiO2 and V contents are nearly constant and then decrease as differentiation progresses, which attests the crystallization of Fe-Ti oxides occurred during an intermediate stage. Incompatible element contents (K, Rb, Zr, Y) increase with differentiation, while Sr remains roughly constant. The host basanite is characterized by a low Mg# ratio = 100 Mg / (Mg + Fet) of 56.9 and total alkali of 4.8 wt.%. The CIPW normative composition of this basanite, calculated for Fe2O3/FeO ratio of 0.2 according to Middlemost (1989), reveals 12.4 wt.% of nepheline and a DI of 26.8.

Fig. 8
figure 8

a Major element contents (wt.%) of lavas vs. MgO (wt.%) contents, b Trace element contents (ppm) of lavas vs. MgO (wt.%) contents

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The bulk analysis of the dunite (TM190b, Table 5) is very close to those of the olivines (Table 2) which are the main constituents of that rock, as exemplified by very high Ni and Cr contents. The bulk composition of the wehrlite (TM190e, Table 5) reflects its modal analysis. This is graphically illustrated (Fig. 9) by a wehrlite composition that plots between those of its olivines and clinopyroxenes. CIPW norm calculation (for Fe2O3/FeO ratio of 0.2) for the wehrlite gives 46.1 wt.% of forsterite and 42.7 of diopside and a small amount of oxides, feldspars and feldspathoids, that is to say a ratio (percentage) of 51.9 wt.% of olivine against 48.1 wt.% of diopside. These values are somewhat different from the modal composition (respectively 79 and 21 wt.%) estimated by eye while modal minerals are not pure end-members. Moreover these differences could be linked to some heterogeneities and the small size (0.4–2 mm) of the xenoliths. Mg# ratios are respectively 84.7 and 83.7 for the dunite and the wehrlite. Note that Ni and Cr contents of the wehrlite are about half of those of the dunite.

Fig. 9
figure 9

Comparative bulk rock xenolith and mineral compositions plotted in a MgO vs SiO2 diagram: wehrlite (open star) and its clinopyroxenes (open circle) and olivines (open square); dunite (full star) and its olivines (full square)

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The trace-element patterns of the studied lavas are all parallel, indicating that the magmas are cogenetic (Fig. 10), and that fractional crystallization was an important process. Note that the dunites and wehrlites are poorer in normalized REE than their host lava and all others lavas of the volcano (i.e. Lu normalized value of 0.15 and 0.63 respectively in the dunite and wehrlite, 4.58 in the host lava). Their lower REE contents and low (Nb/Y) ratios emphasise that they have cumulative origin. These alkaline liquids could have been derived from partial melting of a garnet- rich lherzolite in the upper mantle beneath the Cameroon Volcanic Line as witness by the fractionation of the HREE (Tb, Y, Yb). Then they crystallised at lower pressures the dunites and wehrlites.

Fig. 10
figure 10

Primitive mantle-normalised trace element patterns for lava samples and xenoliths from Mount Cameroon. Normalising values from McDonough and Sun (1995). Sample numbers are the same than in Table 5 (TM190b is dunite and TM190e is wehrlite). The nine lava samples have very similar patterns; below are the patterns of wehrlite (square) and of dunite (diamond)

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In view of the very young ages of the lavas (see below), the measured Sr and Nd isotopic ratios (results obtained by D. Demaiffe at the Laboratoire de Géochimie isotopique, Université Libre de Bruxelles) can be used as initial ratios. The 87Sr/86Sr ratios fall in a narrow range (0.703344 and 0.703198 for basanite TM198 and basalt TM1 respectively) as the 143Nd/144Nd ratios (0.512773 and 0.512851). These values are similar to those obtained for the lavas of the whole CVL (Halliday et al. 1988) and for most minerals of xenoliths (Lee et al. 1996) and are typical of mantle-derived magmas.

K-Ar ages

Whole rock dating of seven lavas coming from different areas of Mount Cameroon (located in Fig. 2) have been carried out by the K-Ar method: five samples have been dated on whole rock at the Université of Brest and two samples on groundmass at CNRS Gif-sur-Yvette (Table 6). These are the first seven K-Ar data on this volcano.

Table 6 K-Ar ages of some lavas from Mount Cameroon (Whole rock, Université de Bretagne Occidentale, Brest; Groundmass, CNRS, LSCE, Gif-sur-Yvette)
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The ages range from 2.83 ± 0.11 to 0.00 ± 0.09 Ma. The host basanite (TM190a) of the xenoliths has been dated at 0.73 ± 0.08 Ma, in the range of the other ages (Table 6). The ages obtained on groundmass are recent (20 ka and 29 ka). Some ages are older than those of Hedberg (1968) who proposed an age of Mount Cameroon of less than 1 Ma.

In fact, Mount Cameroon grew in two major phases:

  1. (1)

    during the Miocene, which is only known as interstratified basaltic relics in the Burdigalian sediments of the Douala Basin (Gèze 1943, Reyre 1966). The other volcanics of this phase have been covered by the more recent products during important recent activity;

  2. (2)

    after a long (12–17 Ma) period of rest, a new period of activity started during the middle Pliocene, at nearly 2.8 Ma (Piacenzian) until recent times (ages of 29 to 0 ka) and continues today. Five eruptions (1838, 1852, 1865, 1866, 1868) are noted during the 19th century and seven eruptions (1909, 1922, 1954, 1959, 1982, 1999, 2000) during the 20th century.

The regular and high activity of Mount Cameroon explains the important volume of the volcanic edifice (the highest of the whole Cameroon Line) as well as the weak differentiation of the lavas.

Interpretation and discussion

This study has led the following results:

  1. (1)

    The host lava is a vesicular, porphyritic basanite, which contains olivine (chrysolite) and diopside as euhedral phenocrysts as well as angular xenocrysts, and chrysolite, diopside, titanomagnetite and plagioclase as microcrysts in the matrix.

The whole rock K-Ar age of the host basanite is 0.73 ± 0.08 Ma (Table 6). Six others lavas have been dated from 2.83 ± 0.11 to 0.00 ± 0.09 Ma. These seven age determinations are the first K-Ar data for Mount Cameroon; they show that Mount Cameroon has been active for the last three millions years, and remains active as shown by important recent eruptions.

The 87Sr/86Sr and 143Nd/144Nd ratios fall in a narrow range (0.703198 to 0.703344 and 0.512851 to 0.512773 respectively) typical of mantle origin. The Sr-Nd isotopic compositions of Mount Cameroon lavas show that crustal contamination was negligible during ascent and differentiation of the magma.

  1. (2)

    Three types of xenoliths have been found as inclusions in the host basanite: dunites, wehrlites and clinopyroxenites. They contain three main mineral phases: chrysolite, diopside and spinel. Few such xenoliths have been described from Cameroon. Conversely, xenoliths of spinel lherzolites and olivine websterites are common along both the oceanic and the continental sectors of the Cameroon Line (Déruelle et al. 1991; Lee et al. 1996; Temdjim et al. 2004). Recently, Teitchou et al. (2007) describe xenoliths that are lherzolites containing 14–26% of orthopyroxene in Kumba Plain, Cameroon.

    1. (i)

      Mg# of chrysolites in the xenoliths range from Fo83–84 in the dunites to Fo79 in the clinopyroxenites while Mg# of chrysolite phenocrysts of their host lava range between 75 and 81. These values are far lower than those (91 to 94) of olivines of mantle xenoliths of the CVL that are interpreted as residues of partial melting. In the same way, CaO contents in the chrysolites of the dunites (0.1–0.4 wt.%) and the wehrlites (0.2–0.3 wt.%) are greater than those of chrysolites of mantle origin (0.05–0.1 wt.%) (Lee et al. 1996). This probably indicates equilibration under lower pressure.

    2. (ii)

      Diopsides of clinopyroxenites are rich in Al2O3 (up to 7.2 wt.%) and have TiO2 contents up to 2.3 wt.%. These values are typical of the alkaline series (Kushiro 1960). AlIV/AlVI ratios are quite high (5 to 10.5) in the clinopyroxenes of the host basanite but lower (1.2 to 4.9) in the clinopyroxenes of the xenoliths (Table 3).

    3. (iii)

      Spinels found in the xenoliths are mostly interstitial and xenomorphic, probably because they crystallized late. They are always Fe- and Ti-rich (40–65 wt.% FeOt and 4–23 wt.% TiO2) vs. Mg-poor (3–10 wt.% MgO), sometimes Cr-rich (20–25 wt.% Cr2O3, except in those of the clinopyroxenites with 4.5 wt.%) (Table 4, Fig. 6) and contain 9–19 wt.% Al2O3. Cr/Cr+Al ranges from 0.05 to 0.58 (Table 4). In this way they are quite different from spinels of typical mantle peridotite, that are usually interstitial, but Cr- and Al-richer, with Al2O3 contents quite high (>30 wt.%) according to Babkine et al. (1966).

    4. (iv)

      All these xenoliths lack orthopyroxene, which is a strong indication that they crystallized from an alkaline magma. Moreover, Mg# ratios of respectively 84.7 and 83.7 for the dunite and the wehrlite are typical of the alkaline series and lower than values of more than 90 that are typical of the tholeiitic series.

    5. (v)

      Geothermometry calculations based on the chemical composition of olivine (White 2005) give temperatures on the order of 1200°C for the chrysolite phenocrysts of the host basanite. Moreover, the equilibration temperatures of xenoliths have also been estimated using Mg/Fe2+ partitioning between olivine and spinel (Fabriès 1979). The estimated late equilibrium temperatures range between 690°C and 700°C for the dunite and between 726°C and 765°C for the wehrlite. These temperatures are similar (750–850°C) to those obtained on ultramafic xenoliths (spinel lherzolites with minor dunites and wehrlites) from Tahiti (Tracy 1980).

  2. (3)

    These results enable us to discuss three hypotheses for the origin of these xenoliths:

    1. (i)

      they are the solid residues of a mantle of lherzolitic composition, which are common along the CVL (Nana 2001).

    2. (ii)

      they may be products (cumulates) of fractional crystallization of a liquid of similar chemical composition as the host basanite.

    3. (iii)

      or they may have crystallized (cumulates) from an earlier magma and were picked up by a younger magma on his way to the surface.

Mg# ratios of dunite and wehrlite of 83–85 are significantly lower than those of lherzolites (89–90) described by Teitchou et al. (2007), which appear to be similar to those of primitive mantle (88.7–90.1 according to GERM (Geochemical Earth Reference Model) internet site: http://www.earthref.org).

The presence of interstitial chromian spinel (Cr# between 45 and 58) with relatively high TiO2 contents (between 4 and 7 wt.%) in the dunite and wehrlite, exclude the hypothesis (i) that the xenoliths are the solid residue of mantle. According to Cr# Sp vs Mg# Ol discriminating compositions of peridotites (Arai 1992), the dunite and the wehrlite analyses strongly depart from the OSMA (Olivine-Spinel Mantle Array) (Fig. 11).

Fig. 11
figure 11

Cr# of spinel versus Mg# (Fo) of olivine for dunites and wherlites compared to values for intraplate basalts (tholeiites and alkali basalts) and OSMA (Olivine-Spinel Mantle Array) (following Arai 1992)

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More generally, the characteristics of Mount Cameroon xenoliths differ of those resulting from a solid residual mantle as reported by Caldeira and Munha (2002) on Sao Tome Island (Mg# > 90 and NiO > 0.36 wt.% for olivines, Mg# > 90 and TiO2 < 0.06 for clinopyroxenes), by Temdjim et al. (2004) at Lake Nyos (Mg# of 89–91 for olivines) and by Teitchou et al. (2007) in the xenoliths of Kumba Plain in Cameroon (Mg# of 89–90 for olivines, AlIV/AlVI between 0.6 and 0.8 for the clinopyroxenes). In the same way, diopsides of the clinopyroxenes have Mg# of 78 to 79, very different from Mg# values of 91 to 93 of clinopyroxenes of typical mantle origin described elsewhere along the CVL (Nana 2001). The higher values of AlIV/AlVI (1.2 to 4.9) in the clinopyroxenes of the xenoliths of Mount Cameroon compared to the equilibrium of the clinopyroxenes of the mantle peridotites of Kumba Plain close to Mount Cameroon indicate that they were emplaced at a lower pressure. Spinel of dunites of Mount Cameroon have Cr# and Mg# of respectively 20 and 30, against 9 to 14 and 74 to 78 in Kumba.

Chemical compositions of mineral phases of the dunites and wehrlites are systematically different than those of the basanitic host: (a) olivines are more magnesian, (b) AlIV/AlVI ratios are systematically lower in the clinopyroxenes of the xenoliths, and (c) spinels are richer in Al. This indicates that they were crystallized from an early alkaline magma (hypothesis iii). Reversely, chemical compositions of spinels of clinopyroxenites are very close to those of the host basanite (Fig. 6). Moreover there is a significant gap between the spinel compositions of dunite and wehrlite on one hand and of clinopyroxenite and host basanite on the other hand. In the same way, olivines of dunites (Fo83–84) and wehrlites (Fo81–84) are nearly the same, and are more magnesian than those of clinopyroxenites (Fo78–81) and host basanite (Fo75–81).

Clinopyroxenite xenoliths are therefore cumulates formed during an early fractional crystallization which may have occurred before the crystallization of the phenocrysts of the host basanite, typically of alkaline affinities. Dunite and wehrlite xenoliths seem to be cumulates that have crystallized from a previous more primitive batch of alkaline magma, with more or less different physical and chemical characteristics, that crystallised beneath the surface and were entrained in a younger eruption of magma of a different composition.

Such a mechanism of emplacement of xenoliths (cumulate type) had been discussed by Vilminot (1965) in Devès Massif, France, Babkine et al. (1966) and Upton et al. (2000) at Réunion Island (Table 7), and Giannetti (1982) at Roccamonfina volcano, Italy. The higher value of Mg# (78 to 84) of olivines in the xenoliths and the lower value (75 to 81) in the phenocrysts show that the xenoliths have crystallized at higher temperatures than the phenocrysts. In the same way, the ratio AlIV/AlVI in the diopsides of the xenoliths is lower than those of the diopsides of the host basanite, suggesting that the xenoliths crystallized under higher pressures (Wilkinson 1975; Wilshire and Shervais 1975). The pressure in the magma chamber has been calculated at 1.0–1.3 GPa (10–13 kbar) using the clinopyroxene-liquid thermobarometer of Putirka et al. (2003) for the host basanite. This corresponds to a depth of 37 to 44 km according to the relationship between depth and pressure: depth (km) = 3.02 P(kbar) + 5 assuming a crustal thickness of 30 km with density of 2.8 and a mantle density of 3.3 for depths more than 30 km (Scarrow and Cox 1995).

Table 7 Comparison of whole rock chemical analyses of a xenolith of dunite (1), wehrlite (2) from Mount Cameroon (this study), and of three other ultramafic rocks (cumulates): dunites from Réunion Island (3) (Babkine et al. 1966) and (4) (Upton et al. 2000), pyroxenite from Rocher du Lion, Devès Massif, France (5) (Vilminot 1965). l.d. limit of detection
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Déruelle et al. (2001) and Ngounouno et al. (2001) report the presence of frozen basaltic liquids as veins in the xenoliths of wehrlites and pyroxenites sampled at 3,000 m above sea level on the North West of the Mount Cameroon. They propose a mantle metasomatism hypothesis to explain the relatively iron-rich olivine (Fo82–85) according to Bodinier et al. (1991). However, there is no evidence for metasomatism in the xenoliths of dunites, wehrlites and clinopyroxenites sampled during this work on the south side of the volcano as such veins have not been found.

  1. (4)

    Main concluding remarks

Petrographic studies show that the Mount Cameroon lavas are essentially mafic and mildly differentiated and form a typical alkaline series.

Dunites, wehrlites and clinopyroxenites are the three petrographic types of xenoliths found in a host basanite.

According to respective Mg# = Mg/(Mg+Fe) ratios and to the occurrence of a gap between mineral compositions, we propose a two stages formation for these xenoliths: (1) dunites and wehrlites and (2) clinopyroxenites from host basanites, all in an alkaline context, as has been described from Mount Cameroon and confirmed by the lack of orthopyroxene.