Magmatism of the Beka Volcanic Massifs (Cameroon Volcanic Line, West-Central Africa): New Petrographical and Mineralogical Data ()
1. Introduction and Geological Setting
The Adamawa plateau belongs to the Cameroon Volcanic Line. The relationship between the CVL and the Adamawa Plateau remains complex. However, the origin of the Cameroon Volcanic Line (CVL) remains the subject of much debate. Several hypotheses have been put forward on this subject: 1) According to the model of Freeth (1979) and Browne & Fairhead (1983), a rapid rotation 20 Ma ago between 8˚ and 9˚ of the African plate would have induced an inflection of the CVL which extends from the Gulf of Guinea and progressively curves from N30˚E to N70˚E on the Adamawa plateau. 2) Fitton (1983) suggested a 7˚ rotation of the African plate from a pole in Sudan 80 Ma ago (Santonian), which would have cut the lithosphere of the Benue trough and moved it to the present geographical location of the CVL. 3) According to Ashwal & Burke (1989), they would be related to the reactivation of the Pan-African crust, and the orientation of the dyke swarms could result from the reworked Pan-African fault network of the Adamawa Plateau (Moreau et al., 1987) during several Ordovician (450 Ma) and Jurassic episodes; 4) Fairhead & Okereké (1988) suggested the development of the West and Central African rift systems. 5) Ngangom et al. (1983) mentioned the development of the Cretaceous Djerem and Mbere basins of northern Cameroon. These hypotheses are plausible from a structural point of view, but remain difficult to establish from a petrological point of view. Based on field, mineralogical and geochemical investigations, the present study aims to highlight the nature, genesis and determines the magmatic relation between the observed Beka lavas. The signification of these lavas in the context of CVL is also debated by comparing with other volcanic occurrences of this line.
The Adamawa Plateau (Figure 1) is a pan-African metamorphic and plutonic basement horst bounded to the north by the Adamawa Fault and to the south by the Djerem and Mbere faults (Dumont, 1987). It is intensely affected by a network of major N70˚E faults locally masked by Cenozoic basaltic flows (Moreau et al., 1987). The main fault that crosses it, named the Adamawa Fault, represents one of the structures that cross Africa from West to East (Cornacchia & Dars, 1983). This area was the site of significant Mid-Pliocene volcanic activity (Gouhier et al., 1974; Oustriere, 1984; Deruelle et al., 1987; Nkouandou et al., 2008). Volcanic eruptions have created high massifs such as Tchabal Nganha (Nono et al., 1994), Tchabal Djinga (Ménard et al., 1998) and Tchabal Mbabo (Fagny et al., 2020). Effusive flows of alkaline basalts have contributed to the formation of uneven relief on the plateau to the north and the east of Ngaoundere town (Nkouandou et al., 2010). Other petrographic (Nono et al., 1994; Temdjim et al., 2004; Nkouandou et al., 2010), geophysical (Poudjom Djomani et al., 1997) and structural (Dumont et al., 1987; Cornacchia & Dars, 1983;
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Figure 1. (a) Location of the study area (black rectangle) in the Adamawa Plateau in relation to the Cameroon Volcanic Line. (b) Main geological features in the Adamawa Plateau. The Djerem graben, the main volcanic and plutonic complexes are from (Déruelle et al., 2007) as well as the Central Cameroon Shear Zone is after Ngako et al. (2006) and Central African shear zone (Guiraud et al., 1992). The Cratons fields are after (Kampunzu & Popoff, 1991). WAC: West Africa Craton; CC: Congo Craton; CT: Tanzania Craton; KC: Kalahri Craton.
Ganwa et al., 2008) studies have also helped characterise the Adamawa basement.
The Adamawa Plateau basement is intensely dissected by the Pan-African faults trending N70˚E (Moreau et al., 1987), which were remobilised during the Albian-Aptian period, exposing numerous types of volcanic rocks: basanite, basalt, hawaiite, mugearite, benmoreite, trachyte, rhyolite, phonolite. The differentiated lavas show peralkaline affinities (Nkouandou et al., 2008, 2015; Fagny et al., 2012).
The volcanic formations in the vicinity of Ngaoundere have been grouped into three successive emission series (Guiraudie, 1955; Lasserre, 1961) and in accordance with the observations of (Gèze, 1943) in western Cameroon: 1) the ancient basaltic series of terminal Cretaceous (Lasserre, 1961); 2) the intermediate trachytic and phonolitic-dominated series of Tortonian period and 3) the recent basaltic series.
The trachytic and phonolitic dominated series is represented by 35 necks, domes and dome flows of differentiated lavas associated with basaltic flows of Mio-Pliocene basalt age (10.0 to 7.0 ± 0.2 Ma, Temdjim et al., 2004; Nkouandou et al., 2008, 2010) and differentiated lavas (11.39 ± 0.03 to 9.28 ± 0.03, Marzoli et al., 1999). The basalts contain peridotite nodules. Two large strato-volcanoes; Tchabal Nganha (Nono et al., 1994; Fagny et al., 2016) and Djinga-Tadorgal (Mbowou et al., 2010) consist of basaltic, trachytic, phonolitic flows and volcanic breccias, crossed by numerous trachytic and phonolitic necks. One basalt and two trachytes have been dated K-Ar at 7.2 - 7.9 and 7.9 - 9.8 (±0.2) Ma, respectively (Gouhier et al., 1974).
The Adamawa Plateau was formed during the Tertiary period and then raised up to 1 km from the surrounding areas (Okereke, 1988). It is largely covered by large basaltic and basalt-andesitic volcanic outpourings of mainly Tertiary age, which spill over into the middle part of the southern trough (Le Maréchal & Vincent, 1971). The plateau is then also presented as a volcanic horst about 200 km wide, bounded to the north and south, as mentioned above, by pan-African faults, generally oriented N70˚E. Depending on the author, the volcanism of the Adamawa Plateau remains a matter of debate. It either belongs to the Cameroon Volcanic Line (CVL) or is rather related to pan-African fault replay, resulting from the reactivation of the N70˚E sinistral trans-tensional shear zone, at the beginning of the opening of the central Atlantic Ocean in the Aptian-Albian age (Moreau et al., 1987) and the separation of Africa and South America (Gouhier et al., 1974; Aka et al., 2009).
2. Analytical Methods
After field investigations, fresh representative samples were selected and six thin sections were prepared at the GEOPS Laboratory (Geosciences-Environment Laboratory of the University of Paris Saclay) in France. Mineralogical analyses (olivine, clinopyroxene, plagioclase and iron-titanium oxides) of the studied lavas were carried out with the SX100 microanalyser at the department of Pierre and Marie Curie University, Paris XI. The analytical conditions are specific to the minerals and take into account their structure, their stability under the electron beam and the elements sought.
Whole rock (major, trace and rare earth elements) geochemical analyses were carried on representative lavas using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry) and ICP-MS (Inductively Coupled Plasma-Mass Spectrometry) at the laboratory of the CRPG of Nancy. After each sample, the grinder was systematically cleaned with compressed air. These analyses were carried out on 0.2 g of rock powder and the analytical accuracies varied between 0.04% and 0.1% for major elements and 0.1 to 0.5 ppm for trace and rare earth elements. The loss on ignition was determined by the weight difference after ignition at 1000˚C.
3. Results
3.1. Geological Setting of the Beka Area
Rocks within the Beka area comprise basanites, trachytes and phonolites (Figure 2). The basanite lavas outcrop as flows and as domes, at an altitude of 1272 m, with a height of 62 m and a diameter of approximately 270 m (Figure 3(a)). The basanite dome consists of the first few meters of the base, centimetric (35 to 80 cm) to metric (1 to 1.3 m) blocks and centimetric balls (15 to 80 cm) (Figure 3(a)). The blocks have rough surfaces with coarse centimetric to metric pyroxene crystals comparable to charcoal platelets. The balls, on the other hand, have a smooth surface and present rare pyroxene and olivine crystals on the surface. The upper part consists entirely of highly prismatic lavas, some of which are in the form of lauzes.
Trachytes occur as a 300 m × 150 m dome, culminating at an altitude of 1280 m with a height of 110 m above the basement (Figure 3(b)). The lava is dark green to greyish when ongoing alteration. It is covered by an approximately (1.5 to 5 cm) whitish to light yellow weathered patina. The fresh matrix has a pisolitic structure characterized by dark concretions consisting of mineral aggregates of variable diameter (0.5 to 1.5 cm).
The phonolite massif is a roughly conical dome, with steep slopes and almost circular base (Figure 3(c)). It rises to an altitude of 1314 m, and is 12 m high above the basement with a diameter of about 300 m. The massif is strongly prismatic. The prisms, mechanically degraded, show intense fracturing, due to climatic variations. The lava is characterized by the presence of dark grey spherical balls with diameter of 0.5 - 2 cm (Figure 3(c)).
The Beka volcano was developed on a slightly south-facing plateau and is represented by the conical and elongated domes and dome flows respectively.
The WNW-ESE cross section (Figure 4), based on the geological map of the study area (Figure 2), also enabled us to define the main morphological features of the Beka area. The landscape is made up of conical domes and hills whose
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Figure 2. (a) Location map of the Cameroon Volcanic Line (CVL); (b) Main geological features of Africa and alkaline volcanic complexes of CVL. Central Cameroon Shear Zone is after Ngako et al. (2006); (c) Simplified geological map of the studied area with sampling sites.
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Figure 3. Panoramic view of the Beka outcrops and its surroundings, (a) Prismatic volcanic massif in its upper part; (b) Trachytic flow dome; (c) Phonolite massif.
slopes can easily reach 7%. The slopes have large incisions which have been heavily carved by torrents and unevenly shaped by slabs, balls and blocks of granitic basement.
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Figure 4. Sketch of the geological section illustrating the emplacement of Beka volcanism, based on the geological and topographic maps of the study area.
3.2. Petrography
3.2.1. Basanites
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.
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Figure 5. Microphotographs of thin sections of representative samples from the study area: (a) Destabilised phenocrysts of olivine in basanites; (b) Microlitic porphyritic texture of basanites; (c) Destabilised sanidine crystals in its core; (d) Trachytic texture of trachytic lavas; (e) and (f) Microlitic porhyric texture of phonolitic lavas.
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.
3.2.2. Trachytes
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).
3.2.3. Phonolites
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).
3.3. Mineralogy
3.3.1. Olivine
Chemical analyses of olivine crystals in the basanites are presented in Table 1.
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Table 1. Chemical analysis of olivine crystals in Beka basanites.
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).
3.3.2. Clinopyroxene
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
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Figure 6. (a) Chemical composition of clinopyroxene crystals in basanites; (b) Composition of clinopyroxene crystals in felsic lavas in a Wo-En-Fs triangular diagram.
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Table 2. Chemical analyses of clinopyroxene crystals in basanite lava.
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 AlVI/AlIV ratios are low and below 0.25.
3.3.3. Plagioclase
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.
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Figure 7. Plagioclase crystal composition of basaltic lavas in the Or-Ab-An.
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Table 3. Chemical analyses of clinopyroxenes in felsic lavas.
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Table 4. Chemical analyses of plagioclase crystals in basaltic lavas.
3.3.4. Alkali Feldspars
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).
3.3.5. Iron Titanium 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.
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Figure 8. Alkali feldspar crystal composition of felsic lavas.
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Table 5. Chemical analysis of alkali feldspar crystals in felsic lavas.
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Table 6. Chemical analysis of Titanomagnetites and ilmenite from basalts.
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Table 7. Chemical analysis of Titanomagnetite in felsic lavas.
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).
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Figure 9. Composition of Fe-Ti oxide crystals in the TiO2-FeO-Fe2O3 diagram.
3.4. Geochemistry
3.4.1. Major Element Variations
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).
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Table 8. Chemical analyses and CIPW standards of lava from the Beka area.
3.4.2. Trace Elements
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).
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Figure 12. Multi-element patterns of trace element analyses from Beka volcanism normalized to the primitive mantle (McDonough & Sun, 1995). Those from Mt. Cameroon and Sao Tome are shown for comparison.
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.
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Figure 13. primitive mantle normalized REE diagram of the Beka volcanic rocks (McDonough & Sun, 1995). Those from Mt. Cameroon and Sao Tomé are shown for comparison.
4. Discussion
4.1. Development of Beka Volcanism and Magmatic Evolution
The present work shows that the Beka volcanism developed a series of formations consisting of domes sensu stricto, conical domes and dome flows characterised by basanites, trachytes and phonolites. The study of the different processes and mechanisms that led to magmatic differentiation and petrographic diversity will give us an idea of the different petrogenetic processes involved in the formation of these rocks. These processes and mechanisms are generally characterised by fractional crystallisation, partial melting, magma mixing or crustal contamination. In the course of the discussion, the lavas of Beka and its surroundings are compared with those of other volcanic formations of the Cameroon Volcanic Line such as the lavas of Mount Cameroon, Mt Bamenda, and the Kapsiki Plateau and Sao Tomé. The latter, studied by several authors, are of mantle origin and result from weak partial melting of a deep asthenospheric mantle (Déruelle et al., 2000; Ngounouno et al., 2000; Marzoli et al., 1999; Ngounouno et al., 2006; Wandji et al., 2009; Kamgang et al., 2010, Tchop et al., 2020).
The mineralogical study of the lavas of the Beka region reveals the evolution of the chemical compositions of microlite phenocrysts from relatively primitive lavas (basanite) to differentiated lavas. Fo contents decrease from phenocrysts (Fo 79%) to microlites (Fo 75%). (Fo 75%). Iron contents increase relatively from clinopyroxenes in basanites to those in felsic lavas. The TiO2 and MgO contents of the iron-titanium oxides decrease from the basanites to the phonoliths, contrary to those of Fe and Mn. All these changes may reflect the differentiation of the parent magma in the series the driving force behind this differentiation would be the process of fractional crystallisation.
This hypothesis is supported petrographically by the presence of basanite enclaves in the trachytes, which suggest the origin of all the lavas in the same mantle environment.
The low values of the AlVI/AlIV ratios of clinopyroxene crystals in the Beka lavas (less than 0.25) suggest the equilibrium pressures of suggest the relatively low equilibrium pressures of crystallization (Wass, 1979). The TiO2 content of clinopyroxene crystals is high (3.17 wt%) indicating low crystallization temperature (Ferguson, 1977) and a low oxygen fugacity during crystallization (Nielsen, 1979). The mineralogical composition of the Beka volcanism is similar to that of lavas reported in other volcanic environments of the Adamawa plateau (Nono et al., 1994; Nkouandou et al., 2010; Temdjim et al., 2004) and the enttire CVL (e.g Kamgang et al., 2008; Wandji et al., 2009; Marzoli et al., 1999).
4.1.1. Laying of the Lavas
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)).
4.1.2. Crustal Contamination
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.
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Figure 14. Basanite alkaline magmas are often enriched in Nb and then have relative low Rb/Nb. On contrary, crustal rocks and melts derived there from generally have higher Rb/Nb ratios (Weaver & Tarney, 1984). In Nb/Y vs Rb/Y plot, Cox & Hawkesworth (1985), show that melts could be separated into contaminated and uncontaminated The first one have higher Rb/Y ratios and low Nb/Y values, and the second ones are positively and steeper slope.
4.1.3. Fractional Crystallisation
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.
4.1.4. Petrogenesis and Origin of Magmas
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.
5. Conclusion
New petrographical and mineralogical data in relation with the magmatism of the Beka volcanic massifs situated at the Ngaoundere Plateau, in the northeastern part of the CVL, consists of alkaline basalts and basanites capped by trachytes and phonolitic flows. The most recent volcanism in this area consists of cinder cones aligned in a WNW-ESE direction, sometimes producing small lava flows. They are dominantly monogenetic and alternate with more evolved polygenetic volcanoes (e.g. Kapsiki plateau, Mts Bamenda).
In details, the Beka volcanism consists of basanite, trachyte and phonolite domes. The shape of the outcrops was acquired during their cooling and according to the topography. The lavas belong to the same magmatic series of an alkaline signature. The basanites have microlitic porphyrytic textures and consist of olivine, diopside, and titanomagnetite crystals. The felsic lavas have generally trachytic textures and are composed of anorthose, sanidine, oligoclase and andesine, slightly iron-rich diopside and titanomagnetite. All the lavas are nepheline normative and originate from depleted mantle source. The lavas belong to the same magmatic series, and show an alkaline and probably uncontaminated nature. They evolved by fractional crystallization after partial melting. The geochemical characteristics point that these lavas would come from a low melting rate from an enriched mantle source. The felsic lavas are thought to have evolved by fractional crystallization of basanite magma. All these lavas present geochemical characteristics close to those of other regions of the Cameroon Volcanic Line.
These first data on the magmatism of Beka, allowed us to present preliminary results on the zone. Several other data are being collected for a better geochemical, isotopic and geochronological characterization of these formations.
Acknowledgements
We sincerely thank Prof Nkouandou Oumarou Faarouk for his support by providing chemical analyses.