A phlogopite-amphibole-plagioclase bearing websterite from Guinadji volcano (Adamawa Plateau, Northern Cameroon): evidence of a cumulate origin and crustal event

The GJI 12 xenolith is rounded, ≈10 cm in diameter, and free of alteration. It is an olivine-free websterite characterized by interlocking pyroxenes with clinopyroxene modally prevailing over orthopyroxene (50.8% vs 44.2%), and containing accessory phlogopite, amphibole, plagioclase and rutile, while iron oxides (opaque minerals) occur interstitially; spinel has not been observed so far. This rock is partly moderately recrystallized. Ortho- and clinopyroxene are mostly cracked. The texture of this websterite is cumulative with medium and equant grains (Fig. 2a), the grain size is relatively homogeneous (0.5–1.5 mm). The grain borders are curvilinear, rarely straight and devoid of reaction products except scarce points where the pyroxenes are in contact with phlogopite or where clinopyroxene is in contact with plagioclase. The light brown melts (Fig. 2b,c) are infiltrations of host basalt.

Figure 2.

Photomicrographs of the studied websterite. (a) Cumulative texture. (b) Interstitial phlogopite. (c) Edenite as rim around cpx. (d) Three aspects of opx. Dashed lines represent their triple junctions. hbi, host basalt infiltration, op, opaque minerals; Phl, phlogopite.

Phlogopite, pleochroic brown and sub-euhedral to euhedral, occurs as long rods of approximately 0.5 mm. It does not appear to be perfectly in equilibrium with the mantle paragenesis but rather in an interstitial form: it is mainly found intercalated between orthopyroxene grains, sometimes also between orthopyroxene and clinopyroxene.

Orthopyroxene in the thin section under the optical microscope is mainly sub-euhedral (sometimes euhedral). It occurs in three aspects (Fig. 2d):

• –

  An appearance without cleavage, strongly cracked, 0.7–1.5 mm in size, sometimes reaching 2 mm when stretched. It is the largest orthopyroxene. It often shows kink-bands;

• –

  A parallel cleavage appearance of medium to strong relief, slightly cracked, with a size between 0.7 mm and 0.1 mm;

• –

  A spongy appearance, mostly without cracks, with folded and very tight cleavages with a size varying between 0.7 mm and 0.1 mm.

When these three grains are side by side, they show triple junctions of 120°. A few grains <0.5 mm are intercalated between the clinopyroxene grains.

Rutile occurs in the form of very small grains (around 15 μm, Fig. 3a) developing either alongside amphibole or alongside the phlogopite and sometimes it is found with both minerals between the pyroxenes.

Figure 3.

BSE images of the studied websterite. (a) Association phlogopite-amphibole-rutile. (b) Amphibolitization of clinopyoxene. (c) Amphibole elongated exsolution lamellae following clinopyroxene cleavage. (d) Plagioclase as intercumulus phase. (e) Image showing that amphibole and phlogopite are of metasomatic origin. (f) Clinopyroxene remants as inclusion in plagioclase. sul, sulfides.

Amphibole (0.2–0.3 mm), sub-euhedral, is intrestitial and always associated with clinopyroxene, sometimes forming a rim around the latter (Fig. 2c and 3b) or as elongated exsolution lamellae within clinopyroxene (Fig. 3c). Amphibole does not exhibit reaction rims with other crystallized metasomatic minerals or dark oxidized parts, unlike amphibole breakdown by decompression or incongruent melting (Ntaflos et al., 2017). Moreover, upon contact with iron oxide, its border becomes rectilinear, unlike when it is in contact with the clinopyroxene (which shows a mainly curvilinear contour).

Plagioclase (around 200 μm, Fig. 3c,d, and f) appears in interstitial areas between clinopyroxene and orthopyroxene. It is mainly anhedral, rarely sub-euhedral. Sometimes, this phase has clinopyroxene relics in inclusions.

Clinopyroxene is sub-euhedral to euhedral, slightly cracked and cleaved more or less intensely. Traces of deformation are almost absent. It is often found as relics in plagioclase and it is often rimmed by amphibole (Fig. 3d,e, and f).

The whole rock chemical composition of the sample GJI12 is shown in Table 1. The websterite has a Mg# of 0.79 and is characterized by low alkalis (Na₂O = 0.58 wt% and K₂O = 0.12) and by relatively high TiO₂ = (0.25 wt%).

A striking feature of sample GJI12 is the high concentration of vanadium (V = 248 ppm). The chondrite-normalized REE pattern (using values from Sun & McDonough, 1989) exhibit a relatively flat pattern and a slight depletion in heavy rare earth elements (HREE) (Fig. 4a). Notably, there is no significant Eu anomaly, as the Eu concentration lies close to the general REE trend. The multi-element diagram (Fig. 4b) reveals distinct geochemical anomalies. Pronounced positive anomaly is observed in Ba. Positive anomalies are also observed in Nd and Sm, while negative anomalies are evident in Nb, Sr, and Y.

Figure 4.

Chondrite normalized REE (a) and spider diagram (b) of the Guinadji volcano websterite; normalization values are from Sun and McDonough (1989). REE, rare earth elements.

Representative chemical analyses of mineral phases of the websterite are given in Tables 2–5.

Clinopyroxenes (En_{43.88–45.13}Wo_{45.90–46.91}Fs_8.92–9.28) plot according to the Morimoto et al. (1988) classification within the diopside field (Fig. 5). This cpx is Fe-rich (Mg# = 0.83–0.86) and Al₂O₃ contents vary between 3.07 wt% and 4.50 wt% (Al-rich diopside). The Cr₂O₃ contents are low and vary slightly among the crystal (0.31–0.40 wt.%). The Na₂O and TiO₂ contents are also low (0.45–0.53 wt.% and 0.35–0.56 wt.%, respectively). The correlation is positive between Mg# and CaO, while it is negative between Mg# and Al₂O₃ (Fig. 6a,b).

Figure 5.

Pyroxene composition from the studied websterite plotted on the En-Wo-Fs ternary diagram after Morimoto et al. (1988).

Orthopyroxenes (En_{75.98–76.65}Wo_1.21–1.76Fs_{21.74–22.45}) plot within the enstatite field of Morimoto et al. (1988) (Fig. 5). This opx shows a lower Mg# (0.77–0.79) than coexisting cpx. Cr₂O₃, Al₂O₃, CaO and TiO₂ contents are low respectively 0.13–0.25 wt%, 2.05–2.78 wt%, 0.61–0.89 wt% and 0.10–0.14 wt%. The correlation is positive between Mg# and CaO, whereas no clear correlation is observed between Mg# and Al₂O₃ (Fig. 6c,d).

Figure 6.

Clino- and orthopyroxene mineral chemistry plots. (a) CaO (wt%) versus Mg# content of clinopyroxene, (b) Al₂O₃ versus Mg# for clinopyroxene, (c) CaO (wt%) versus Mg# content of clinopyroxene, (d) Al₂O₃ versus Mg# for clinopyroxene. Plagioclase-bearing olivine websterite from Serri et al. (1988), Layered websterite from Wilson and Chaumba (1997), Abyssal crustal cumulate pyroxenite after Dantas et al. (2007); Mantle websterites from Rogkala et al., 2017; Garnet websterites from Lu et al., 2018, Befang websterite from Tedonkenfack et al. (2021).

Detailed electron microprobe analysis (EMPA) data and calculated structural formulae for representative phlogopite are presented in Table 3. The analyzed micas are K-phlogopite (see Khedr & Arai, 2016). Based on the International Mineralogical Association scheme, phologopite compositions lie close to the siderophyllite and eastonite boundary (Fig. 7a) and show Mg# of 0.86. Element concentrations do not show large variation. The crystal is homogeneous throughout the xenolith (Table 3).

Figure 7.

(a) Chemical composition of phlogopite on the Mg/(Fe + Mg) versus Al^tot classification diagram of Rieder et al. (1998). Red circles represent chemical compositions of phlogopite. (b) Amphibole classification diagram after Leake et al. (1997). The blue field represents amphiboles described by Pintér et al. (2015) and Njombie Wagsong et al. (2018). TFA, Tetra-Ferri-Annite; TFP, Tetra-Ferri-Phlogopite.

We have classified the amphibole compositions by calculating a.p.f.u. of Si and Mg/Fe + Mg on a diagram Si versus Mg# following the recommendation of Leake et al. (1997) (Fig. 7b). Mineral formulae were calculated using the AMPH16 program of Preston and Still (2001). All of the amphibole analyses plot in the edenite field, which is a rare in upper mantle xenoliths. The analyzed grains are calcic (Ca/[Ca + Na] a.p.f.u ≥ 0.75 with slightly higher values of [Na + K] a.p.f.u: ≈0.8) and are of magnesian affinity with Mg# values ranging between 0.82 and 0.83. This amphibole is slightly ferrous (FeOt = 6.19–6.43 wt%).

Representative plagioclase compositions are given in Table 4. This plagioclase is identified by its medium calcic nature (An_{58.30–61.47}) and has a slightly low sodium content (Ab_{35.71–-38.75}): it is a labradorite ([Na_0.36–0.39Ca_0.59–0.62] [Si_2.38–2.42Al_1.56–1.60]O₈). Compared to the plagioclase (labradorite and andesine) described by Dautria and Girod (1986) in the lhe Iherzolites sampled in the same study area, this plagioclase is more potassic (Or₃ against Or_0.4).

Cumulate rocks typically form through the precipitation of solid crystals from a fractionating basaltic magma. The studied websterite provides evidence of a cumulate origin.

First, the predominance of early-crystallizing phases such as orthopyroxene and clinopyroxene supports a cumulate nature. These two minerals are arranged in such a way as to show an interlocking texture characteristic of cumulates (Fig. 2a). They are therefore the minerals forming the cumulus phase. Between them, plagioclase is very often found filling the very small intervals left after their crystallization (Fig. 3d); the latter thus constitutes the intercumulus phase. Phlogopite and amphibole are metasomatic minerals that make up the late-stage mineral assemblage. The low alkalis and the relative high FeO and MgO show that the source of the cumulates was presumably a tholeiitic magma. In addition, the whole rock low Mg# (0.79) as well the absence or olivine and spinel preclude an ultramafic origin.

The compostion of clinopyoxene in the websterite studied also attests to this cumulate origin. Mantle clinopyroxenes are typically Mg-rich (high in Mg# which is often around 0.9), since they are the residues after partial melting of upper mantle peridotites while clinopyroxene from pyroxenite cumulate have been precipitated from a basaltic magma.

The fact of having small orthopyroxene grains intercalated between the clinopyroxene grains suggests that clinopyroxene was the first to crystallize, the second was orthopyroxene and the third was plagioclase.

As shown in Figure 6, this websterite departs significantly from mantle websterites and is comparable to crustal cumulate pyroxenites (Fig. 8). The calculated equilibration temperatures (1195–1208°C) and pressures (4.7–5.9 kbar) indicate that the websterite crystallized under high-temperature conditions at mid-to lower-crustal depths (approximately 15–20 km).

Figure 8.

Whole-rock CaO–Al₂O₃–MgO plot of the Guinadji volcano websterite (references of the fields after Selim et al., 2025).

Cumulates with similar whole rock composition and estimated temperatures between 950°C and 1000°C have been reported by Marchev et al. (2006).

The varied nature of ultramafic xenoliths sampled in the alkaline basalts of Adamawa and the major element compositions of their minerals highlight a great heterogeneity of the mantle below this plateau. The GJI 12 xenolith differs in mode and phase composition from peridotite and pyroxenite xenoliths from the same volcanic field. For example, compared to clinopyroxene in the olivine-websterite and websterite xenoliths of Nyos (Temdjim, 2012) and Oku (Tedonkenfack et al., 2021), its clinopyroxene is Fe-rich and Al₂O₃-poor. Also, orthopyroxene has been less magnesian than orthopyroxene in the olivine-websterite described by Temdjim (2012). So far, pargasite is the amphibole described along the CVL (e.g. Njombie Wagsong et al., 2018; Nkouandou & Temdjim, 2011; Pintér et al., 2015; Temdjim et al., 2004a) while the amphibole described in this sample is edenite (Fig. 7b). Compared to the rare amphibole crystals already described in Adamawa (e.g. Njombie Wagsong et al., 2018), this amphibole is less aluminous (<13 wt% [10.52–11.49 wt%]) and less sodic (<4 wt% [1.75–1.81 wt%]) but has a high K₂O (>0.8 wt% [1.57–1.71 wt%]) contents.

As previously shown, this websterite is of cumulative origin and does not contain olivine or spinel as do many pyroxenite xenoliths from the CVL (Tamen et al., 2015; Temdjim et al., 2004a); however, importantly, it does contain phlogopite and some grains of amphibole. The presence of phlogopite and amphibole provides clear evidence that the studied websterite is modally metasomatized. Both hydrous phases contain F indicating that they formed at the same time from the same metasomatic agent. The amphibole is always associated with clinopyroxene but never found included in it. It is often found in clinopyroxene as elongated exsolution lamellae following its cleavage (Fig. 3c). These amphibole textures support a reaction-replacement origin where clinopyroxene is replaced as rods along cleavage planes at crystal margins (resulting in pseudo-interstitial texture; Fig. 3b). The markedly low Al^VI and Na (M4, Table 3) are evidence for a low-pressure formation of the amphibole (Niida & Green, 2000). Best (1974) attributed this feature, in specimens from Grand Canyon (Arizona), to a reaction between intercumulus melt and cumulus pyroxene grains. As previously stated, amphibole shows no evidence of destabilization resulting from decompression or in situ partial melting processes. This also support a reaction-replacement origin. The formation of amphibole through melt-clinopyroxene reaction is a well-documented and long-recognized process in cumulate rocks (Smith, 2014), mantle pyroxenites (Dantas et al., 2009; Francis, 1976) and peridotitic mantle affected by metasomatism (Coltorti et al., 2004). This development of amphibole through alteration of anhydrous phases in pyroxenites is a response to the presence of a hydrous melt enriched in volatile components and not critically undersaturated in silica. Figure 7b shows that this amphibole is an edenite (sodic amphibole). Such type of amphibole (with tremolite) forms when ultramafic rocks interacts with water-rich crustal fluids (Yang, 2003). The presence of a high concentration of K and F in this sodic amphibole (Table 3) is also indicative of the influence of crustal-derived fluids.

Based on the petrographic characteristics (Fig. 2b,c), phlogopite is interstitial and sometimes associated with amphibole between pyroxenes, more particulary near clinopyroxene. This shows that phlogopite in the studied websterite crystallized after the coexisting orthopyroxene and clinopyroxene, directly from the fluid phase during amphibole-associated hydrothermal metasomatism and did not attain chemical equilibrium with primary minerals.

Given its composition, phlogopite formation requires the supply of K, Mg and Al. Bonadiman et al. (2021) showed that phlogopite coexisting with amphibole suggests an influx of K(Na)-OH-bearing melts. This attests that metasomatic fluid that affected the websterite was rich in alkalis, mainly K. The abundance of fluorine in the phlogopite (2.65–3.14 wt%, Table 3) and amphibole also confirms the richness in volatiles of the metasomatic fluid. The metasomatic agent would thus be K–F-rich crustal-derived fluids responsible for the marginal amphibolitization of clinopyroxene and the formation of phlogopite.

Because of their silica and volatile-rich compositions, these melts may be saturated in titanium oxides even at relatively low concentrations of Ti (Karmalakar et al., 2005). This characteristic is confirmed by the frequent occurrence of rutile side by side with OH⁻ bearing minerals (Fig. 3a) and also by the high TiO₂ contents in phlogopite and amphibole (respectively 3.56–3.69 wt% and 1.64–1.81 wt%; Table 3) while they are very low in primary minerals (<0.6 wt% Table 2). Such type of K-melts with Ti has already been described in the xenoliths of Kutch (India) by Karmalakar et al. (2005) and of northern Great Xing’an Range (Nuomin volcanic field, China) by Sui et al. (2014). In the Kutch xenoliths, the exsolutions of rutile in the orthopyroxenes have been interpreted as products of K-metasomatism while the enrichment of K and Ti in Nb metasomatic melt was associated with recycled crustal materials.

The estimated unusual high temperature at relatively low pressure could be an indication of a steep geothermal gradient, which evidently is attributed to the existence of a plume underneath the Cameroon Line (Burke, 2001). As mentioned above, the phlogopite and amphibole are characterized by relatively high F contents. Also, the Ba concentration is high and shows a prominent positive anomaly in the chondrite-normalized spider diagram (Fig. 4b). The orthopyroxene and clinopyroxene cannot account for the high light REE (LREE) and low HREE abundances in the chondrite normalized REE patterns. Instead, the metasomatic phase phlogopite, which is also the repository for the high Ba content, and amphibole are the main contributors to the LREE enrichments in the websterite. Evidently, the metasomatic, fluid-rich melts might have been derived from the granitoids of the Adamawa Plateau.