Mineralogy and Magmatic Processes of Cenozoic Intraplate Alkaline Volcanic Rocks of Bafang and Its Environs (Cameroon Volcanic Line, West Africa) ()
1. Introduction
The generation and the evolution of magma are known to be two stages related to the formation of igneous rocks. The Cameroon Volcanic Line (Figure 1(A)) is the unique tectonomagmatic feature all over the world [1] which the origin is still debated. Mineralogical studies of basaltic rocks provide informations for the better understanding of petrogenitic processes developed in the Cameroon Volcanic Line (CVL). Here we present mineral chemistry of alkaline volcanic rocks of Bafang and its environs. These volcanic rocks are basanites, alkali basalts, hawaiites and mugearites [2]. According to [2], the basaltic lavas of Bafang and its environ are alkaline volcanic rocks with Na (Na2O/K2O = 1.5 - 3.0) and also Ne-Ol-Di-Hy normative. According to % MgO and their mantle sources, these basaltic rocks are subdivided into two groups: the high MgO basalts (HMg-B) and the low MgO basalts (LMg-B). The HMg-B is made of samples BAF 22, BAF 34, BAF 42, BAF 42 and BAF 44 with MgO (9.12 wt% - 11.81 wt%) and the LMg-B made of samples BAF 2, BAF 3, BAF 5a, BAF 10, BAF 11, BAF 13, BAF 15, BAF 18, BAF 33, BAF 36, BAF 37, BAF 38, BAF 40, BAF 41, BAF 43 and BAF 46 with MgO (2.38 wt% - 6.69 wt%). Mineral chemistry is used in this study to give new constraints into magmatic processes beneath the volcanic region of Bafang and its environs.
2. Geological Setting
The basaltic lavas from Bafang and its environs (Figure 1(B)) belong to the Bamileke plateau located in the West Cameroon Highlands (Figure 2). The study erea is 5˚4'21" to 5˚13'38" North and 10˚8'17" to 10˚19'38" East and covers surface of 500 km2 for the culminant point at 1300 m high (Figure 1(B)). It is bounded to the north by Monts Bambouto [6], Bamenda ( [7] [8] [9] [10] ) and Oku ( [11] [12] ), to the south-west by Manengouba Mont ( [13] ), to the east by Monts Bana ( [14] [15] [16] ) and Bangou ( [17] ) and to the west by Mbo Plaines (Figure 2). The basaltic lavas of Bafang and its environs cover granito-gnessitic basement rocks ( [14] [15] [18] ) and poured out during three volcanic eruptions in the upper Miocene (10 to 6 Ma [2] [19] ).
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Figure 1. Study area. (A) Location of the Bafang area (Red square) along the Cameroon Volcanic Line (adapted after [3], completed). Location of seamounts after [4]. Inset, bottom right is after [5]. (B) Geological map of Bafang and its environs.
3. Analytical Methods
3.1. Major and Trace Element Analyses
Whole-rock major elements were measured on the Horiba Jobin-Yvon Ultima 2
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Figure 2. Simplified geological map of the studied area and location of the samples in the West Cameroon Highlands (after [20], modified).
ICP-AES at the IUEM (European Institute for Marine Studies, Pôle de Spectrométrie Océan, Brest, France). The detailed description of the analytical procedure is given in [21]. Major elements were determined from an H3BO3 solution, boron being used as internal standard for ICP-AES analysis. For major elements, relative standard deviation was 1% for SiO2 and 2% for the other major elements, except for low values (<0.50 wt%), for which the absolute standard deviation is ±0.01 wt%. Trace element concentrations were measured with a Thermo Element2 HR-ICP-MS in Brest (France), after a repeated HF-HClO4 digestion, and HNO3 dilutions (see [22] for details). The repeated analysis of the international standard BCR2 demonstrated an external reproducibility better than 5% - 10% depending on the element and concentration.
3.2. Minerals Analysis
The polished thin sections of the different rock-types were observed under an Olympus BH2-HLSH microscope. The minerals of the different thin sections were observed at magnifications ×5, ×10, ×25 and ×50. Microprobe data of basaltic lavas of Bafang and its environs of Bafang were obtained using a Cameca SX-100 automated electron microprobe at Service Microsonde Ouest of Brest, France. Chemical analyzes (“Université de Bretagne Occidentale-Brest, France”) of major elements were performed using an electron microprobe with beams of: 10 and 40 μm, 15 kV accelerating potential and 10 - 12 nA current and 6 s counting time per element (see [23] ) for analytical details]. Errors considered for these analyzes are between 5% and 10% of measured values < 1% and between 1% and 5% of the measured values > 1%. The main mineralogical data are given in Table 1.
4. Results
4.1. Field Observations and Petrography
The alkaline basaltic lavas of Bafang and its environs occur as flow, prismatic, bowl and bloc lavas generally in the summit and the side of the hill as well as in the water flows. Prismatic lavas are found in the localities of Kotchou and Njenfa. They are thick lavas with collonades form of 2 m high (Figure 3(A)) observed in some places. Prismatic lavas are hexagonal to pentagonal with 5 to 30 cm in diameter and deliminated by tensional cracks (Figure 3(B)). At Kotchou and Bana, prismatic lavas are exhumed horizontally in the soil and recover by one to two meter of soils. At Njenfa and Bafang, prismatic lavas dip to the south west and to the south indicating two directions of flowing (Figure 3(C) and Figure 3(D)). Oblic prismatic lavas dipping to the south west are qualified to false collonades whereas vertical prismatic lavas dipping to the south are true collonades. All the basaltic lavas from Bafang and its environ are mesocrate, melanocrate to holomelanocrate with phorphyritic to aphyritic and sometime fluidal textures (Figure 4). Some samples in thin section show mineral inclusions and zonations. Theses textural features are observed with olivines and clinopyroxene crystals exhibiting oxide (Figure 4(A')) and olivines inclusions respectively. Zonations
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Figure 3. Prism and different colonnades in the craft quarrying of Kotchou and Njenfa. (A) colonnades of olivine and pyroxene basalts in the craft quarrying of Kotchou (N05˚09'55.9'', E10˚17'46.8'' 1610 ± 3 m), showing prisms with many faces and buried in the soil; (B) colonnades of olivine and pyroxene basalt in the craft quarrying of Njenfa (N05˚08'33.1'', E10˚09'38.6'' 1197 ± 3 m), showing different types of prisms with many faces (trigonal, pentagonal and hexagonal) and more than 2 m length; (C) disposition of olivine and pyroxene basalt false collonnades in the craft quarryinf Njenfa; (D) disposition of olivine and pyroxene basalt true (vertical arrow) and false (oblic arrow) colonnades in the craft quarryinf Njenfa. Arrow indicate the disposition of colonnades whereas white dashed line indicate the length.
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Table 1. Main mineralogical characteristics for alkaline lavas of Bafang and its environs. φ = phenocryst; µφ = microphenocryst; µ = microcryst or microlite.
are observed with olivine, clinopyroxene and plagioclase phenocrysts (Figure 4(C')). The decrease of olivine and clinopyroxene phenocrysts is also observed from basanites to mugearites (Figure 4).
4.2. Mineralogy
Seven (07) thin sections of representative samples (Two HMg-B: BAF 22 and BAF 42 and five LMg-B: BAF 2, BAF 3, BAF 18, BAF 36 and BAF 37) have been chosen for microprobe analysis in order to determine chemical compositions of
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Figure 4. Photomicrographs and microscopic images of representative samples of alkaline basaltic lavas from Bafang and its environs. (A) and (B) Fields observation of HMg-B; (C) and (D) Fields observation of LMg-B. (A') and (B') microscopic images of microlitic porphyric texture of HMg-B. (C') and (D') microscopic images of microlitic porphyric and aphyric textures of LMg-B. These observations are in cross and under-polarized light. Pl: plagioclase; Cpx: clinopyroxene; Ol: olivine; Op: Opaque mineral.
different mineral phases. One hundred and eight chemical analyses have been performed on phenocrysts, microphenocrysts and microcrysts of olivine, clinopyroxene, oxide, plagioclase and anorthoclase in these samples (Tables 2-5). In these seven thin sections of samples analysed one is from basanite, three from alkali basalt, one from hawaiite and two from mugearite.
4.2.1. Olivine
Olivine crystals analysed (Table 2) are characterized by their higher content in MgO relative to FeO, synonymous with their forsterite composition (Fo86-75) in HMg-B. But in LMg-B, olivines in hawaiite and mugearite have higher content in FeO relative to MgO, synonymous with their fayalite composition (Fo38-53). The range of olivine composition is shown in Figure 5. In this Figure 5, olivines of HMg-B are chrysolite and hyalosiderite; and those of LMg-B are chrysolite,
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Figure 5. End-member compositional variation in olivine of alkaline basaltic lavas from Bafang and its environs.
hyalosiderite and hortonolite. Olivine compositions in HMg-B range from Fo67 to Fo87 with 0.17 wt% - 0.54 wt% CaO and 0.15 wt% - 0.51 wt% MnO. In the LMg-B, olivines composition range from Fo38 to Fo86 with 0.15 wt% - 0.70 wt% CaO and 0.18 wt% - 1.41 wt% MnO. In HMg-B CaO shows negative correlation while NiO display positive correlation (Figure 6). CaO in olivine phenocrysts display positive correlation in core and negative in rim contrary to NiO (Figure 6(E) and Figure 6(F)) whereas in LMg-B CaO and NiO variations with Fo are contrasted (Figure 7). CaO and NiO contents in the studied basaltic lavas are in the range generally obtained in alkali basalts from the Cameroon volcanic Line ( [1] [10] [24] [25] [26] ). In these rocks, olivine phenocrysts are generally more magnesian and exhibit zoning, with Fe enrichment from core (Fo81-87) to rim (Fo73-74), suggesting equilibrium at ~1150˚C - 1200˚C ( [27] [28] ).
4.2.2. Clinopyroxene
Structural formular have been calculated according to [29]. Results are shown in Table 3. In the binary plot Q-J (Q = Ca + Mg + Fe2+; J = 2Na) of [29] (Figure 8(A)), pyroxenes in the studied mafic lavas in the area of Bafang fall in the Quad (Ca-Mg-Fe) domain. They are Ca-Mg-Fe pyroxenes regarding thier relative contents of Ca2Si2O6 (Wo), Mg2Si2O6 (En) et Fe22+Si2O6 (Fs). In the ternary plot (Wo-En-Fs; Figure 8(B)) of [29], these pyroxenes are clinopyroxenes and mainly augite (Wo44.9-43.6En41.0-37.0Fs18.3-14.4) and diopside (Wo51.9-45.0En41.4-33.2Fs16.4-10.7) in compositions.
Structural formula indicates Si permanent gap in the tetraedric site T and the presence of 85% of ferriferous iron in the octaedric sites. Oxides in the clinopyroxene in the erea of Bafang are TiO2 (0.3 à 5.4%wt), Al2O3 (1.0 - 9.6%wt), Na2O
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Figure 6. Relations between Fo and CaO, and between Fo and NiO of olivines in basanites and alkali basalt of HMg-B of Bafang. Black circle (basanites), White circle (alkali basalt), Green (phenocryst), Red (micro-phenocryst), Blue (microcryst), Orange (core), Pink (rim).
(0.4 - 1.8%wt), FeO (5.7 - 14.2%wt), MgO (10.2 - 14.6%wt), CaO (19.8 - 23.3%wt), Cr2O3 (0 - 0.66%wt) and MnO (0.1 - 0.5%wt).
Chemical compositions of the representative clinopyroxene are shown in a binary and ternary plot of the Wo-En-Fs system (Table 3; Figure 8). According to binary plot (Figure 8(A)), pyroxene of Bafang are Ca-Mg-Fe rich. In ternary plot (Figure 8(B)), pyroxene in alkaline volcanic rocks of Bafang are clinopyroxene and have compositions ranging from diopside (Wo51.9-45.0En41.4-33.2Fs16.4-10.7) to augite (Wo44.9-43.6En41.0-37.0Fs18.3-14.4). Diopsides are more abundant than augites. But in basanite, augites are predominent.
Clinopyroxene in HMg-B range from Wo51.2En41.0Fs18.3 to Wo43.8En35.5Fs11.9. TiO2 (0.31 - 5.39 wt%), Al2O3 (2.55 - 8.29 wt%), Na2O (0.42 - 1.79 wt%) and CaO (19.76 - 23.45 wt%) abundances are fairly high and various indicating the alkali
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Figure 7. Relations between Fo and CaO, and between Fo and NiO of olivines in alkali basalts, hawaiites and mugearites of LMg-B of Bafang and its environs. White circle (alkali basalt), Black star (hawaiites), Grey circle (mugearite), Green (phenocryst), Red (micro-phenocryst), Blue (microcryst), Pink (rim).
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Figure 8. Nature and nomenclature of clinopyroxenes in the alkaline basaltic lavas of Bafang and its environs. (A) Position of clinopyroxenes of alkaline basaltic lavas of Bafang in the Q-J diagramme of [29]. (B) Position of clinopyroxenes of alkaline basaltic lavas of Bafang in the Wo-En-Fs diagramme of [29]. Symbols as in Figure 5.
nature of these pyroxenes [31]. Clinopyroxenes in some samples show both concentric and zoning. Zoning Cpx phenocrysts show differences in TiO2, Al2O3, FeO and MgO contents in different sectors (Table 3). Some of the larger phenocrysts are Na2O and FeO rich but low in Al2O3, MgO and TiO2 (Table 3). Al/Ti ratios are low in the rim (2.78 - 3.49) and high in the core (4.99 - 13.10).
Cpx in LMg-B range from Wo51.9En40.7Fs15.7 to Wo43.6En33.2Fs12.6. TiO2 (0.53 wt% - 3.25 wt%), Al2O3 (0.97 wt% - 4.89 wt%), Na2O (0.53 wt% - 0.72 wt%) and CaO (21.77 wt% - 21.96 wt%). These compositions shows that HMg-B Cpx are rich than LMg-B Cpx.
Clinopyroxenes with zoning in alkaline basaltic lavas of Bafang show two
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Table 3. Representative microprobe analyses of clinopyroxene for alkaline lavas of Bafang and its environs. Wo = Ca (wollastonite). En = Mg (enstatite). Fs = Fe2 + Fe3 + Mn (ferrosilite). Mg# = 100*Mg/(Mg + Fe2+), where Fe2+ is calculated after [30].
evolution (Figure 9): in one side the enrichment in FeO and MgO from rim to core (Figure 9(C) and Figure 9(E)) and in another side the enrichment of TiO2 and CaO from core to rim (Figure 9(B) and Figure 9(D)), with the decrease of Al2O3 and Na2O from core to rim (Figure 9(A) and Figure 9(F)).
Clinopyroxene crystals core in basanite BAF 22 and in alkali basalt BAF 36 are more Mg-rich than the rim. Ti is higher in the rim compared to the core (Table 3). This observation is also made with the Aliv. Notice that Mn contents are lower from rim to core. Zoning clinopyroxenes are rim Fe3+ rich than the core. Alvi and Cr are higher from core to rim. The cores in phenocrysts are Si rich (1.71 - 1.79 a.p.f.u.) compared to the rim (1.70 - 1.74 a.p.f.u.). In addition the Al/Ti ratio is low at the rim (2.78 - 3.49) and high in the core (3.63 - 13.10).
4.2.3. Fe-Ti Oxides
Structural formula of oxides has been calculated according the [32] method and the percentage of ulvospinel according method [33]. Results are listed in Table 4. In all the studied lavas oxides are represented by phenocrysts, microphenocrysts and microcrysts. All these oxides are magnetite and belong to the
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Figure 9. Relations between Al2O3, TiO2, FeO, Na2O, MgO and SiO2 of zoning Cpx in alkaline basaltic lavas of Bafang and its environs. Black circle (basanites). White circle (alkali basalt). Blue (core). Pink (rim).
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Table 4. Representative microprobe analyses of Fe-Ti oxide from alkaline lavas from the Bafang and its environs. USP = Uvӧspinel; Sp = Spinel; Mt = Magnetite.
titanomagnetite serie forming the solid solution between magnetite (Fe3O4) and ulvospinel (Fe2TiO4) (Figure 10). These oxides are FeO high (65.30 wt% - 72.90 wt%) if regarding the TiO2 (19.93 wt% - 26.17 wt%) contents. Their chemical
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Figure 10. Position of oxide opaque of alkaline basaltic lavas from Bafang and its environs in TiO2-FeO-Fe2O3 diagram of [34]. Symbols as in Figure 5.
compositions are close to ulvospinel (Usp74.84-90.19Mt5.23-22.94Sp1.77-7.12) (Figure 10). The TiO2 (19.93 wt% - 26.15 wt%), MnO (0.56 wt% - 0.79 wt.%), FeO (62.85 wt% - 73.37 wt%), Na2O (0.00 wt% - 0.07 wt%), K2O (0.00 wt% - 0.02 wt%) and P2O5 (0.00 wt% - 0.02 wt%) contents decrease whereas the MgO (1.11 wt% - 5.07 wt%), CaO (0.00 wt% - 0.22 wt%) et Cr2O3 (0.00 wt% - 0.58 wt%) contents increase. The Al2O3 (1.32 wt% - 4.75 wt%) contents are relatively constant.
4.2.4. Feldspars
Representative analyses of feldspars are listed in Table 5 and plotted in Figure 11. Feldspars in HMg-B are only plagioclase (An67.97-47.94Ab40.41-30.43Or11.65-1.60). In LMg-B, feldspars are plagioclase (An64.75-15.84Ab69.19-34.63Or20.59-1.51) and anorthoclase (Ab68.11-61.20Or33.87-20.91An10.98-4.93). Plagioclases in HMg-B are labradorite (An67.97-59.30Ab38.74-30.43Or2.57-1.60) and andesine (An47.94Ab40.41Or11.65). In LMg-B, plagioclases are labradorite (An64.75-51.96Ab44.98-33.72Or3.06-1.51), andesine (An45.44-31.82 Ab62.66-51.79Or5.52-2.77) and oligoclase (An26.65-15.84Ab69.19-63.57Or20.59-8.55). Plagioclases are more abundant than anorthoclase. Only two microcrystals analyzed correspond to anorthoclase in mugearite.
The CaO content (10.32 wt% - 14.02 wt% and 1.06 wt% - 13.46 wt% for HMg-B and LMg-B respectively) shows a wide range and the anorthite content ranges from 47.94 wt% to 67.97 wt% in HMg-B and from 4.93 wt% to 64.75 wt% in LMg-B. The Al2O3 and FeO contents show wide compositional ranges with 28.88 wt% - 31.11 wt%; and 0.57 - 0.93 wt in HMg-B. The values of Al2O3 (20.01 wt% - 30.27 wt%) and FeO (0.15 wt% - 1.34 wt%) in LMg-B in contrary show narrow compositional ranges.
The zoning observed on the plaglioclase in the alkali basalt BAF 36 indicates decreasing in Al and Ca and increasing in Na and K from rim to core (Figure 12).
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Figure 11. Position of feldspars of alkaline basaltic lavas Bafang in An-Ab-Or diagram of [35]. Symbols as in Figure 5.
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Figure 12. Relations between Al2O3, CaO, K2O, Na2O, and SiO2 of zoning plagioclases in alkaline basaltic lavas of Bafang and its environs. Blue (core), Pink (rim).
This depletion causes at the end reverse zoning illustrated by the passage in some crystal from andesite (Ab55-59An37-42Or2-4) composition in core to labrador (An54-59Ab39-44Or1-2) composition in rim.
5. Discussion
5.1. Fractional Crystallization
Many petrographical, mineralogical and geochemical features favor the fact that fractional crystallization is the main process responsable in the variation of compositions and the lithology diversity of basaltic lavas from area of Bafang. In the same order, the samples have highly variable compatible element contents. For example, Cr ranges from 1.6 to 515 ppm and Ni from 1.1 to 261 ppm, and their content decreases with increasing Mg# [2], indicating that fractionation of olivine and clinopyroxene are the main factors controlling the compatible element abundances. Ni, Cr and Co contents in the most mafic lavas are generally smaller than the values assumed for primary magmas (Ni: 300 - 400 ppm; Cr: 300 - 500 ppm; Co: 50 - 70 ppm; e.g., [36] [37] ), but are still high enough to indicate that for these samples the fractionation was limited.
Factional crytallazation process can be visible petrographically for the basaltic lavas from Bafang. This evolution by fractional crystallization [38] of studied rocks is corroborated by: the oxide inclusions observed in olivine, clinopyroxene and plagioclase (Figure 4); parallelism observed in the REE patterns [2], the variations in chemical compositions of phenocrysts fom core to rim (Figure 6, Figure 7, Figure 9, Figure 11; Tables 2-5) and the correlations observed in the Harker diagrammes [2] are also consistent with the fractional crystallization. The Mg# values vary from 86 to 65 in the HMg-B and from 53 to 38 in the LMg-B [2]. These values indicate that olivine in the HMg-B are more magnesian (Fo87-65, with Fo = [Mg/(Mg + Fe2+). In contrary in the LMg-B, they are more ferrouferous (Fo53-38). The variations observed with Mg# from HMg-B to HMg-B also suggest fractional crystallization. In other hand, clinopyroxene crystals in studied lavas display the decreasing of Ti + Aliv from basanites to mugearites suggesting that fractional crystallization occur at variable pressures ( [39] [40] ). Their Mg# range from 63.35 to 89.52 and their TiO2 and Al2O3 contents vary between 0.31 wt% and 5.39 wt% and 0.97 wt% and 9.57 wt%, respectively. Ti, Al and both elements increase with iron enrichment (Table 3). Their increasing Ti with decreasing Mg# reflects the normal fractionation trend (e.g. [41] ). The Cr2O3 contents can reach 0.5 wt% but it sharply decreases with decreasing Mg# (Table 3). Their Ti/Al ratios (0.08 - 0.51) are higher, while the Alvi/Aliv ratios (0.00 - 0.53) are lower. These ratios imply that they could have crystallized under various-pressure conditions (e.g. [42] [43] ). Based on their slightly increasing Ti/Al ratios during crystallization, they could have precipitated under continuously decreasing pressure. They could have been characterized by a significantly higher crystallization rate compared with the olivine phenocrysts (as suggested by Figure 9).
The ages obtained on the alkaline basaltic lavas of Bafang (10.46 - 6.27 Ma, [2] suggest that the geochemical diversity observed is explained by two distinctives magmatic series: basanite to basanite and hawaiite to mugearite.
The process of fractional crystallization has been modeling by using mass balance of sums square of major elements ( [2] according to [44], after the calculation sheet of petro-mode. The model series have been calculated according to the process described by [45]. The trend showing the evolution of studied HMg-B were tested as well as the trend of LMg-B. The results are reported in Tables 6-8. The first stage of crystallisation is that of basanite among them (BAF 22 à BAF 44), after a removal of 52.61% clinopyroxene + 20.55% plagioclase
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Table 6. Least-square mass balance calculations (major elements) for derivation by factional crystallization of basanite from the basanite parent (BAF 22) to the basanite daughter (BAF 44). ∑r2 = sum of residual squares.
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Table 7. Least-square mass balance calculations (major elements) for derivation by factional crystallization of hawaiite to mugearite from the hawaiite parent (BAF 2) to the mugearite daughter (BAF 3). ∑r2 = sum of residual squares.
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Table 8. Least-square mass balance calculations (major elements) for derivation by factional crystallization of hawaiite to mugearite from the hawaiite parent (BAF 2) to the mugearite daughter (BAF 41). ∑r2 = sum of residual squares.
+ 14.31% olivine + 12.54% de magnetite, thus the average of mineral fractionation of 11.5%. The second stage is that of crystallization from hawaiite to mugearite (BAF 2 to BAF 3 and BAF 2 to BAF 41) after the removal of (20.30% - 22.43%) clinopyroxene + (61.85% - 63.35%) plagioclase + (14.14% - 17.85%) magnetite, thus the average of mineral fractionation of 13.5% and 9.7%.
5.2. Magma Storage and Recharge
Before erupting at the surface, magma is often (although not always) stored in subsurface reservoirs, where chemical differntiation produces a variety of compositional products. The differences in bulk composition among analyzed basaltic lavas from Bafang and its environ [2], could be due to the short residence in the magma chambers. In fact, basaltic lavas display a large among of lava types from more basaltic (basanite) to less basaltic (mugearite). The distinction among these lavas could indicate different magma chambers from where they derived. Similarities can also be seen in mineral chemistry (Table 2, Table 3 and Table 5; Figure 6, Figure 7, Figure 9 and Figure 12) and optical appearance in thin sections with olivines, clinopyroxenes and plagioclases (Figure 4(A') and Figure 4(C')) showing zonations. Taking into account these features mentioned above and combining them with temperature and pressure results from studied basaltic lavas, it is here seem plausible to suggest that there are magma storage which favor fractional crystallization.
During these periods of storage, interaction between injected primary melts and the shallow level reservoirs, could be suggested to cause chemical differentiation observed in olivine, clinopyroxene and plagioclase phenocrysts (Table 2, Table 3 and Table 5; Figure 6, Figure 7, Figure 9 and Figure 12) and probably their textural feature such as zonation (Figure 4(A') and Figure 4(C')). Zoned phenocrystals preserve fingerprints of magmatic processes over their life time ( [46] [47] ). Perturbations in composition, temperature and pressure can cause crystal zonation [48]. The textural feature and chemical composition of zoned crystals in the basaltic lavas from Bafang and its environs can be used to discriminate between different magmatic processes responsible for the zonation. Zoned crystals were subdivided into two categories: (i) concentric reverse-zoned (with Mg and Ni-rich cores and Fe, Mn and Ca-rich rims in olivines and clinopyroxene; as well as Al and Ca in plagioclase; Table 2, Table 3 and Table 5); (ii) concentric normal zoned (Fe, Mn and Ca-rich cores and Mg and Ni-rich rims; Table 2 and Table 3). There is a relation between the style of zoning and the nature of the magmatic perturbation involving different magmas [48]. Normal zoning observed in studied lavas may be a consequence of rapid crystallization whereas reverse zoning can be produced as crystals resident in the chamber are subjected to heating by the intruding magma induced by recharge process [48].
6. Conclusion
Alkaline basaltic lavas of Bafang with Ne-Ol-Di-Hy normative are derived from low degree of partial melting of garnet peridotite and pyroxenite of lithospheric mantle. These rocks are mesocrate, melanocrate and holomelanocrate exhibiting phorphyritic, aphyritic and fluidal textures. They are made of olivine, pyroxene, oxides and feldspar. Olivines are magnesian and ferrouferous and are represented by chrysolite, hyalosierite and hortonolite. Pyroxenes are Ca-Mg-Fe clinopyroxenes represented by diopside and augite. Oxides are magnetite represented by ulvospinel in the titanomagnetite serie. Feldspars are plagioclase and anorthoclase. Plagioclases are calcic and sodic represented by labrador, andesine and oligoclase. The differentiation from basalts and basanites to mugearites is accounted by the removal of olivine, Clinopyroxene and oxides. The chemical variation from core to rim or rim to core of phenocrysts analyzed from the studied lavas in this work may indicate a less evolved primary magma and that of shallower chamber after the recharge.
Acknowledgements
We thank IRD (Institut de Recherche pour le Développement) for logistic support during field work in Cameroon.