Whole-rock major elements were measured on the Horiba Jobin-Yvon Ultima 2

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.

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.

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

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).

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

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.

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 (Fo_87-65, with Fo = [Mg/(Mg + Fe²⁺). In contrary in the LMg-B, they are more ferrouferous (Fo_53-38). 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 + Al^iv 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 Al^vi/Al^iv 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

+ 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%.

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].