Basanites generally have a microlitic porphyritic texture and are made up of olivine, clinopyroxene, feldspar and iron-titanium oxides. The phenocrystalline phase is characterized by phenocrysts of olivine, clinopyroxene and iron-titanium oxides (Figure 5(a)). All these phenocrysts are distributed in a mesostasis consisting of microlites of the same minerals, i.e. microcrystals of iron-titanium oxides, pyroxene, olivine and microlites of plagioclase.

The olivine phenocrysts (0.5 × 0.3 mm, 5 - 10 vol%) are euhedral to subhedral. They are corroded along the rims and sometimes in the cores (Figure 5(b)). Rare inclusions of iron-titanium oxides are observed in these phenocrysts. Some of them show corrosion gulfs in the cores, although they are not very pronounced. Other olivine crystals are serpentinized and exhibit a yellowish alteration product in the cracks (Figure 5(b)).

Clinopyroxene phenocrysts (1 × 0.4 mm, 10 - 15 vol%) are automorphic and occur in various forms: elongated, and sometimes rectangular (Figure 5(b)). Some clinopyroxene crystals are strongly cracked and occasionally show marked corrosion gulfs in the core and/or along the rims occupied by iron-titanium oxides (Figure 5(a)).

Phenocrysts of iron-titanium oxides (0.4 × 0.3 mm) are in very low percentage (less than 8 vol%). They are generally associated to the olivine and clinopyroxene phenocrysts (Figure 5(b)).

The mesostasis of the studied basalts consists of plagioclase and clinopyroxene microlites and microcrystals of olivine and iron-titanium oxides. No preferential orientation of the microlites is observed.

The iron-titanium oxide microcrystals are relatively abundant (5% - 10% vol%) in the matrix. They are in the form of dotted lines and fine dark squares (Figure 5(b)). Olivine microcrystals are the least represented in the mesostasis (less than 2% vol%). They are mostly observed around phenocrysts of the same phase.

Plagioclase microlites are the most abundant (10 - 15 vol%). They are elongated, appearing in the form of needles or rods of less than 1 mm for the most developed crystals. Clinopyroxene microlites (less than 0.5 mm, and 3 - 5 vol%) are lobate in the form of small elongated laths.

The thin sections of the trachyte samples have a classical trachytic texture. They are generally composed of phenocrysts of clinopyroxene, alkali feldspar, plagioclase and iron-titanium oxides. These phenocrysts are distributed in a microlitic matrix composed mainly of plagioclase, clinopyroxene and iron-titanium oxide microcrystals. Alkali feldspar phenocrysts (1.5 × 1 mm, 15% - 20% by volume of the rock) occur as elongated, automorphic platelets. They can reach 20% of the volume of the rock. They are sometimes cracked, destabilized Plagioclase phenocrysts (1.5 × 1 mm, 20% to 25% by volume of the rock) are automorphic and others have a xenomorphic appearance. The latter are sometimes corroded in their edges by mesostasis and contain numerous microcrystals of iron-titanium oxides in these edges (Figure 5(c)). Clinopyroxene phenocrysts (1.2 × 0.8 mm) occupy about 10% by volume of the rock. They are automorphic to subautomorphic, cracked and sometimes corroded at the edges by mesostasis. They contain inclusions of iron-titanium oxide crystals and are sometimes completely altered to a brownish product (Figure 5(c)).

The mesostasis of the trachytic lavas consists mainly of plagioclase, clinopyroxene and iron-titanium oxide microcrystals. The iron-titanium oxide microcrystals are less than 0.1 mm in size, and occupy about 10% by volume of the rock. They are present in the matrix and or as inclusions in plagioclase phenocrysts (Figure 5(d)). Alkali feldspar microlites are the most abundant in the matrix. They are finely crystallized and occur as small elongated needles or acicular rods. They are sometimes oriented and define the trachytic texture of the rock (Figure 5(c)). Clinopyroxene microlites are automorphic and less abundant (<2% by volume of the rock).

The phonolite lavas have a classic porphyritic microlitic texture (Figure 5(e)) with more or less hyalloclastic varieties. Alkali feldspar phenocrysts (1.5 × 0.8 mm and 15% - 20% by volume of the rock) are abundant in phonolites. They are automorphic, tabular. Other phenocrysts do not contain oxide inclusions but sometimes in the edges. The majority of phenocrysts have a cloudy, hazy appearance (Figure 5(f)). Clinopyroxene phenocrysts (1.3 × 0.8 mm, about 15% - 20% by volume of the rock) are automorphic. They are green and regularly corroded in the cores and sometimes in the rims by mesostasis. Clinopyroxene phenocrysts (1.3 × 0.8 mm, about 15% - 20% by volume of the rock) are automorphic. They are green and regularly corroded in the cores and sometimes in the rims by mesostasis. The iron-titanium oxide phenocrysts (1.2 × 0.9 mm, 5% - 10% by volume of the rock) are automorphic and very often corroded in the cores and rims.

The mesostasis of the phonolites consists mainly of alkali feldspar microlites, clinopyroxene and iron-titanium oxide microcrystals. The iron-titanium oxide microcrystals (less than 0.1 mm, about 10% by volume of the rock) are scattered in the matrix and sometimes included in clinopyroxene phenocrysts or fixed in the edges of alkali feldspar phenocrysts. They would come from the alteration and destabilisation of clinopyroxene phenocrysts. Alkali feldspar microlites (less than 0.1 mm) are the most abundant in the matrix (25% - 30% by volume of the rock). They are small acicular rods which show a preferential orientation in some places. They are mostly very finely crystallized. Clinopyroxene microlites are rare (1% to 3% by volume of the rock).

Chemical analyses of olivine crystals in the basanites are presented in Table 1.

The forsterite content of the phenocrysts in the Ngao Voglar basanites varies very little (78.4% to 78.9%). The Fo content of the olivine microcrystals shows a zonation with high contents in the core (Fo 78.9%) compared to the edges (Fo 75.4%). The lowest contents are found in the microcrystals of the mesostasis (Fo 73.0%). NiO contents are below 0.2 wt% and the lowest contents are found in basanites that outcrop as flows. CaO contents are high and range from 0.1 to 0.3 wt.% of oxides. Such high CaO contents characterize the olivines of alkaline lavas (Roeder & Emsly, 1970). They are different and high compared to those found in the spinel lherzolites of the mantle under Adamawa (Nkouandou & Temdjim, 2011) and some volcanic centers of the Cameroon Volcanic Line (Temdjim, 2012).

The chemical compositions of clinopyroxene crystals from the basanites (flows and Voglar massif) of the Beka region are presented in Table 2 and illustrated in Figure 6(a). The nomenclature used to designate them is that of the classification of Morimoto et al. (1988). Clinopyroxene crystals are generally diopside in phenocrysts (Wo47-En49-Fs5) and microlites (Wo46-En39-Fs14). CaO contents decrease and FeO contents increase from phenocrysts to microlites. TiO₂ contents are high and range from 2.44 to 3.17 wt% of oxides. Al₂O₃ contents are also

high and increase from the phenocrysts to the microlites (6.49% to 7.87%) as do Na₂O contents which range from 0.58% in the phenocrysts to 1.4% in the microlites.

In felsic lavas, i.e. trachytes and phonolites (Table 3), clinopyroxene crystals have a diopside composition (Figure 6(b)) The TiO₂ content of clinopyroxenes in all lavas varies from 0.53% to 2.48%.

The highest values are found in the clinopyroxene crystals of phonolites. Al₂O₃ contents reach 8.60% in phonolites compared to trachytes where they are between 2% and 2.38% by weight of oxides. The Na₂O content is high, ranging from 1.6% to 2.81% in the phonolites and compared to the content of the same oxide in the trachytes, which ranges from 1.5% to 1.8% by weight of oxides.

Some crystals have a high ZrO₂ content of 0.22%. The presence of cationic Al and Ti is assumed in the tetrahedral sites which contain less than 2% Si of the structural formula. The FeO content of clinopyroxene crystals increases slightly from the trachytes to the phonolite lavas. This increase is reflected in the ferrosilite contents which vary from 6% to 15% in the trachytes and from 10.7% to 19.4% in the phonolites (Table 3). The values of the Al^VI/Al^IV ratios are low and below 0.25.

The compositions of the plagioclase microlites in the basanites of the Beka area are presented in Table 4 and shown in Figure 7. These plagioclase crystals are calcic and generally have a labrador composition (An 59.4 - 62.1, Ab 37.7 - 39.2, Or 1.3 - 1.7) in the lavas sampled on the Voglar massif and is (An 61.9 - 63.5, Ab 35.3 - 37.1, Or 1.1 - 1.5) in the surrounding flows. BaO contents are generally very low and vary between 0.03% and 0.11% in the lavas, 0.11% in the basanite lavas and flows of Beka. Cationic Al and FeO₃ contents are assumed in tetrahedral sites so Si contents are less than 3% of the structural formula.

Chemical analyses of alkali feldspar crystals from felsic lavas are presented in Table 5. They have a composition of anorthose (Ab 54.0 - 72.4 Or 23.7 - 44.4 An 1.8 - 3.9) as shown in Figure 8. Some phenocrysts analyzed in phonolites have a composition that is close to that of sanidine (Or 56.6-Ab 40.6-An 2.7). Another analysis found in feldspar crystals of phonolites is that of oligoclase (Ab 40.6 Or 56.6 An 2.7) with anorthite content of 12.4%. anorthite content of 12.4%. The other compositions are: anorthose (Ab 54.0 Or 44.4 An 1.8), oligoclase (Ab 70.4 Or 18.4 An 11.2), and andesine (Ab 65.5 - 66.6 Or 9.7 - 10.4 An 23.7 - 24.2). FeO contents are relatively high in the trachytes (0.49%) compared to the phonolites (0.32%). Some anorthose crystals of the phonolites have high BaO contents (up to 2.84% by weight of oxides).

1) Titanomagnetites

Representative chemical analyses and structural formulae of titanomagnetite crystals are presented in Table 6. For basanite lavas, the composition of titanomagnetite is characterized by high TiO₂ contents of 20% to 23% by weight of oxides. Their FeO content is between 60% and 67% by weight of oxide. MgO contents are between 3% to 4% by weight of oxides to when the MnO content is low (strictly less than 1% by weight of oxide). Their Al₂O₃ contents are between 1% and 3% by weight of oxide. All these compositions are different and sometimes very high compared to those of the Titanomagnetites of the differentiated lavas (Table 7). In the latter, the TiO₂ contents are low and range from 7% to 11% by weight of oxides, their FeO contents are low compared to those of basanite.

of oxides, their FeO content is low compared to that of basalts and ranges between 32% and 35% by weight. 35% by weight. MgO contents are low (strictly less than 1% by weight) when MnO contents are high, between 4% and 7% by weight of oxide. Their molar contents in United States Pharmacopoeia (USP) are low compared to the Titanomagnetites of basanite lavas which are between 20% and 29% by weight of oxide.

2) Ilmenite

The chemical analysis and structural formula of the ilmenite crystal is presented in Table 6. The TiO₂ content is 43.52% by weight and the FeO content is 49.89% by weight. This composition is characterised by the high MgO content (3.46 wt% oxide) and low in Cr₂O₃ content (0.04% by weight of oxide) and that of MnO (strictly less than 1% by weight of oxide). The temperatures calculated from the empirical calibration of Sthomer, (1983) on the basis of the coexistence of ilmenite and titanomagnetite in basanites are 376% ± 50% ˚C. basanites are 376% ± 50% ˚C. The oxygen fugacity is high (Fo₂ = 10 - 5.7 atm) (Figure 9).

The geochemical compositions of the analyzed rocks are presented in Table 8 are used for classification, petrogenetic and geotectonic investigations. According to the TAS (Total Alkali vs Silica) diagram (Le Bas et al. 1986), the studied have compositions of basanite, trachyte and phonolite respectively. Basanite have SiO₂ contents of 41.66 wt% characteristic of ultrabasic lavas (SiO₂ < 45%). The phonolites have SiO₂ contents ranging from 56 - 58 wt%, lower than those of the Kapsiki plateau (SiO₂: 59.4%) (Ngounouno et al., 2006). TiO₂ contents are high in the Beka basanites (4.5 wt%) compared to those of Mt. Cameroon (3.5 wt%) (Wandji et al., 2009). They are significantly low in the differentiated lavas of the Beka region (0.48 - 0.63 wt%) and the Cameroon Volcanic Line (strictly less than 1%). Al₂O₃ contents are low in the volcanic formations (less than 15%) compared to the differentiated lavas of the two volcanic centres (between 15% and 21% by weight). In contrast, Fe₂O₃ content is high in the basanites (13.91% - 14.23%) and low in the differentiated formations of Beka (between 2% and 4%) and the Cameroon Volcanic Ligne (3% and 8% by weight) (Marzoli et al., 1999; Ngounouno et al., 2006). The MnO contents are strictly below 0.3% in all volcanic formations. The MgO contents are high in the volcanic formations (6 and 10 wt%). The highest value (9.28%) is found in the basanites of Mt. Cameroon. The same variations are observed for CaO contents ranging from 10.20% in the Beka and Mt. Cameroon basanites (11.18% by weight All the analyzed samples, likewise those from the Kapsiki Plateau (Ngounouno et al., 2000), Mt Cameroon (Wandji et al., 2009), Mt Bamenda (Kamgang et al., 2010) and the volcanic island of Sao Tome (Marzoli et al., 1999) plotted for comparison evidence alkaline affinities (Figure 10).

The trace elements compositional variations of the studied lavas are presented in Table 8. The transition elements have very low contents. Ni, Co and Cr range from 23.6 - 23.42 ppm; 43.12 - 42.12 ppm and 25.42 - 26.2 ppm respectively in the basalts and present lower values in the differentiated lavas.

In Figures 11(a)-(h), the binary diagrams of Ba, Nd, La, Rb and Zr versus silica do not show clear positive correlations in the lavas studied, indicating generally incompatible behavior throughout the series. Cr and Ni contentsare very low (<30 ppm) within all the studied samples, compared to the lavas of the Kapsiki Plateau in the Far North of Cameroon, with concentrations up to 326.3 ppm and 159.4 ppm respectively.

The elements Co, Sc, V, Cu, indicate positive correlations in the evolved terms, contrary to the basanite lavas where we rather observe negative correlations meaning compatible behaviours, their concentration decreases according to the evolution of the silica content Table 8. These same variations are observed in the Kapsiki Plateau, Mt Cameroon and Sao Tome Island volcanic lavas. Co, Sc, V, Cu, although not represented in this diagram, indicate positive correlations in the evolved terms, contrary to the basanite lavas where negative correlations are rather observed meaning compatible behaviours, their concentration decreases according to the evolution of the silica content Table 8. These same variations are observed in the lava of the Kapsiki Plateau, Mt Cameroon and the volcanism of Sao Tomé Island.

The Beka basanites like those of Mt. Cameroon are characterized by very strong negative anomalies in K and slight negative anomalies in P and Ti. Slight positive anomalies are observed in Nb, Ta, Sr and Eu in the Beka basanites. The Beka basanite spectrum is similar to those of the Cameroon Line. They show very strong negative anomalies in K, P and Ti. Positive anomalies are observed in Nb, Ta, Hf and Zr. Nb/Ta ratios are relatively constant (between 14 and 16) in all the Beka volcanic rocks (Figure 12).

The rare earth spectra normalised to the primitive mantle of (McDonough & Sun, 1995) are shown in Figure 13. They are steep and are characterized by High Light Rare earth contents that reach 90 to 300 times the mantle values anomalies are observed in all basanites and differentiated lavas except for the Eu anomaly in the Sao Tomé phonolite.

Beka domes are roughly conical in shape and strongly prismatic (Figure 3(a)). The conical shape and prismatic structure observed in lava outcrops are generally due to the cross-fracture emplacement and cooling process of highly viscous lavas, respectively. A similar mechanism has been proposed to explain the morphology and prismatic structure of the Atakor volcanic edifices (Girod, 1968). According to this mechanism, as the viscous magma rises, it begins to cool and thermal shrinkage cracks isolate polygonal prisms between them. These prisms, pushed as if through spinnerets into vertical columns tightly packed together, curl into divergent sheaves at the surface. The dome-cast massifs (Figure 3(b)) would therefore result from their placement on an inclined surface (Girod, 1968). This consideration explains well the dome-cast shape of the trachytic massif, which is built on the sloping side of a small cliff. The conical shape of the phonolite massif can be explained by the fact that this massif was built at the intersection of fractures (Figure 3(c)).

In the absence of isotopic analyses, the arguments presented for crustal contamination are speculative. The contamination of the Beka felsic and basaltic lavas is discussed here on the basis of their geochemical characteristics. The interaction of the magma with the rocks it passes through, during its magmatic ascent, such as basement rocks, can modify its composition. The influence of these traversed rocks can be detected through certain major element contents or trace element contents and ratios.

Contamination is sometimes suggested by the presence of basement enclaves and/or other rocks. The levels of major elements such as SiO₂ and TiO₂ can be used to determine whether contamination is present. The high TiO₂ (4.8% - 4.9%) and SiO₂ (41.57% - 41.91%) contents of basanites reflect their very negligible contamination. Their TiO₂ contents are similar to those of other uncontaminated lavas of the Cameroon Volcanic Line with a mantle origin similar to that of the OIBs (Arndt et al., 1993; Zhao et al., 1994; Ewart et al., 1998, 2004).

In general, the behaviour of certain trace elements, such as Sr, Zr, Ta and Nb are used to attest the involvement of crustal contamination (Cai et al., 2010). Very often crustally contaminated mafic rocks show negative Nb and Zr and positive Sr anomalies on the multi-element diagram (Cai et al., 2010). Also high LREE values with flat HREE spectra indicate crustal contamination of mafic rocks. The rare earth and multi-element spectra of the Beka volcanism show respectively a steep slope and the absence of systematic negative anomalies in Nb and Ta. This leads us to believe that the Beka formations and its surroundings would have suffered no or very negligible contamination.

In the Rb/Y vs. Nb/Y digram in Figure 14, part of the Beka rocks as well as the Mount Cameroon, Kapsiki Plateau formations are positioned within the field of uncontaminated alkaline basalt and felsic rocks. These results are in agreement with those of (Ngounouno et al., 2000, Déruelle et al., 2000; Tchop et al., 2020). A second part of the Beka and Kapsiki Plateau rocks are located between the field of uncontaminated formations and the field of basement rocks defined by (Kamgang et al., 2010) called “felsic and Precambrian basement of the Bamenda Mountains”. This second part remains close to the field of uncontaminated alkaline basaltic and felsic rocks. This leads us to conclude that the contamination was negligible and point to cryptic contamination.

Geochemical analyses of major, trace and rare earth elements show that SiO₂, Al₂O₃, Na₂O and K₂O contents are low in basalts compared to those of the same oxides in phonolites and trachytes. On the other hand, MgO, TiO₂, Fe₂O₃, CaO and to a lesser extent P₂O₅ contents are high in basalts compared to felsic lavas. The decrease in MgO, TiO₂, CaO and P₂O₅ contents reflects the crystallization of iron-titanium oxide, olivine and plagioclase in the basanites. The high contents of SiO₂, Na₂O, Al₂O₃ and K₂O in the differentiated lavas suggest the accumulation of alkali feldspar crystals in these rocks. Such a consideration is supported by the high values of alkali feldspar crystals in the differentiated lavas. The positive Zr and Hf anomalies could reflect the accumulation of feldspars in the Beka lavas and its surroundings.

Trace element data show that the transition element contents of the first series (Co, Ni, Cu and Zn) are very low compared to those of the primitive lavas which are directly derived from the partial melting of a mantle source (Green et al., 1973). Basanites have evolved through the crystallization of olivine crystals, plagioclase, and clinopyroxene, iron-titanium oxide crystals from an evolved parent magma. Such variations mark an evolution of the parent magma by the process of fractional crystallisation, as indicated in the other volcanic centers of the Cameroon Volcanic Line, for example the lavas of the Kapsiki Plateau, Mount Cameroon and Mt Bamenda.

In the lavas presently studied, the rare earths normalized to the early mantle (McDonough & Sun, 1995) are steeply dipping and characterized by high contents of light rare earths that reach 90 to 300 times the mantle values (Figure 13). These values indicate that the lava study is the result of a highly evolved mantle parent magma Toteu et al. (2004). The low values of heavy rare earth elements suggest the presence of garnet in the source of the Beka lavas. The rare earth element profiles of these lavas are tilted (Figure 13) and may indicate their derivation from a relatively low degree of partial melting from an enriched mantle source. The negative Eu anomalies indicate feldspar fractionation.

There is an increase in incompatible elements from Rb to Ta. These high incompatible element contents compared to the incompatible elements are an argument for a low partial melt rate of the source. Trace element spectra, normalised to the early mantle of (McDonough & Sun, 1995) of the Beka lavas and its surroundings in Figure 12, show correlations with each other. There is a parallelism with the trace element spectra of the Kapsiki plateaux, Mount Cameroon, Mt Bamenda and Sao Tome lavas. This would indicate a similarity in their geochemical characteristics.

In the compatible elements the contents progressively decrease attesting that the magma would have evolved with the fraction of a Cr and Ni rich mineral such as olivine or clinopyroxene. Magmas of intermediate or felsic composition have two main formation processes which are fractional crystallisation and assimilation. They can also be formed by the interaction of felsic and contemporary mafic magmas as suggested by Tchop et al. (2020); or partial melting of crustal material (Nkouandou et al., 2008; Ngwa et al., 2019), or partial melting of the metasomatised mantle wedge. The absence of mafic enclaves or mafic-felsic mixtures in the study area excludes the hypothesis of interaction of felsic and contemporary mafic magmas.