Petrography and Geochemical Characterization of Dolerites from Figuil (Northern Cameroon) and Léré (Southwestern Chad)

This work presents the petrographic and geochemical 
data of the dolerite dykes crosscutting the 
Pan-African basement of Figuil (North-Cameroon) and Lere (South-West 
Chad) in order to approach their petrogenesis and their emplacement context. 
Two groups of dolerites have been highlighted by petrographic and geochemical studies. These groups were 
discriminated by their TiO2, Fe2O3, 
REE, Ba, Nb, Zr, La and Hf contents which are relatively higher in group I; 
group II, on the other hand, has higher MgO, Mg#, Sc, Ni and Cr contents. The 
mineralogical assemblage of these dolerites is made up by plagioclases, 
pyroxenes, olivine, oxides, amphibole, biotite and sometimes pyrite, calcite, 
apatite, epidote and chlorite. The behaviour of the major and trace elements suggest 
that studied dolerites have an evolution dominated by fractional 
crystallization. Most dolerite samples show higher REE concentrations and 
(La/Yb)N > 8.7, (Tb/Yb)N > 1.9 and Dy/Yb > 2 
ratios characterizing a garnet-bearing mantle. The difference in incompatible 
elements between the two groups is explained by the degree of partial melting 
of the same source which becomes more important over time. Low (Ce/Yb)N values (3.3 - 11.58) also suggest relatively 
low partial melting degree of the source. Fractional crystallization 
process was possibly combined with minor crustal contamination as shown by 
enrichment of Th/Yb from group II to Group I that might be due to turbulent 
magma emplacement. The chemical compositions of these dolerites are similar to 
that of continental tholeiites with slightly moderate negative Nb-Ta anomalies which are attributed to 
crustal contamination of magmas. As other dolerites of Cameroon, 
continental tholeiitic signature of the studied dolerites is evidenced in 
geotectonic discrimination diagrams with Group II dolerite compositions falling 
within the field of tholeiitic basalts and group I within the field of alkali 
basalts.


Introduction
Mafic dyke swarms provide the most complete record of short-lived, mantlegenerated magmatic events through time and space [1] [2]. The crystallization of basaltic magmas can also occur on the subsurface in the form of sills and dolerite dykes. These intrusions are often formed and located in areas of high orogenic activity (shear and collision zones) and are intrinsically linked to the dynamics of mountain ranges. Intrusive magmatism generally exploits synchronous fracturing systems and may also follow pre-existing reactivated systems with networked organizations [3]. Their study is therefore fundamental for understanding the geological history. In the Pan-African Central African Fold Belt (CAFB), petrographic, geochemical and geochronological data were obtained from several magmatic intrusions (e.g. [4]- [9] amongst others). The Dolerite investigated in the present study is located in the northern domain of CAFB in Cameroon and southwest of Chad. Its preliminary geochemical data of some samples in Cameroon domain of CAFB were presented in Ngounouno et al. [10]. Geochemical studies of the dolerites of Chad have so far not been carried out, as well as comparisons of those of neighboring Cameroon. The aim of this study is to characterize, constraint the petrogenesis and define geotectonic context of the Figuil (Cameroon) and Léré (Chad) dolerites using new major, trace and REE compositions data.

Regional Geological Setting and Geological Background of the Study Area
The Central African Fold Belt is one of the most important tectonic units of Precambrian central Africa. It has been formed by the collision between three main cratonic blocks: the Congo craton, the West African craton and the Saharan metacraton during the West Gondwana assembly [11] [12]. During this collision, the margins of the cratons were fragmented and remobilized by the metacratonization process. Sub-blocks are delimited by large shear zones and are characterized by metacratonic and non-metacratonic domains [13]. It is an orogenic mega-belt that crosses several territories (Nigeria, Cameroon, Central African Republic, Chad). This Belt has the characteristics of a collision chain [14], with external nappes of regional extension, high-pressure granulite metamorphism, intense migmatization, regional-scale stalls and the presence of molasse deposits [14].
The emplacement of intrusions, which very often outcrop into dykes and sills, is  [15] [16].
In southwestern Chad Republic and in northern Cameroon (Figure 1) [21]. It is an assemblage of Phanerozoic sediments covering a Precambrian substratum dominated by intrusive tonalitic batholith in amphibole gneissic complexes [22].
Cretaceous sediments are deposited in two basins (   [27]. The dolerites are therefore characterized by an impressive system of dykes emplacement strongly linked to the tectonic events that affected these regions.
The ages of these doleritic intrusions are not exactly well-known. However, similar basalts, microgabbros and dolerites sampled eastwards in Chad, in the same basin, gave K-Ar ages between 43 ± 2 and 87 ± 3 Ma [28]. Furthermore, structural field observations show that the dykes are older than the regional Late Eocene compressive event [26] [29].

Sampling and Analytical Methods
Ten (10)  rences. Oxide concentration was calculated from the determined elemental concentration and the result was reported in that format. Loss on ignition (LOI) was measured by weight difference after ignition at 1000˚C. To certify data quality (95% confidence level) and to calibrate the equipment for optimal precision, a replicate, standard and blank was measured. For the major oxides, the analytical 4. Results

Petrography and Structural Characteristics
The studied dolerite dikes are vertical to subvertical and striking N10˚ -175˚E; their width and length generally range from 5 to 40 m and from 0.02 to 4 km respectively ( Table 3

Major Elements
With the exception of a sample PletM2 (which is a trachy-andesite), all the rocks exhibits a basaltic composition which are basanite tephrite and basalt in the TAS classification of Le Bas et al. [30] (Figure 4) [31] show that dolerites are essentially alkaline (Figure 4).
The four groups of dolerites previously defined during petrographic descriptions are subdivided into two geochemical groups. Group I consists of calcite and pyrite-bearing dolerites and pyrite-bearing dolerites with enclave of basement rocks; Group II comprises amphibole and titanite-bearing dolerites and pyrite-bearing dolerites. In the AFM diagram of Ivrine and Baragar [31], almost all the analyzed samples are classify in tholeiitic series; only one sample of group II dolerite (PletM2 which seem to be contaminated) belong to calc-alkaline series ( Figure 5). This is confirmed in TiO 2 versus Y/Nb diagram of Floyd and Winchester [32] where the majority of dolerites samples are plotted in the field of continental tholeiite ( Figure   6). The two groups of dolerites are also clearly discriminated by their MgO      Figure 8). In each group, the contents of Sc, Ni, Cr   Normalizing values data are from [33]. The gray part of the diagrams represents dolerites of Mayo Oulo-Lere [10], Biden [61], Maham, Kendem, Dschang, Mnajo and Banganté [6] [60] and of Mbaoussi [7] which are added for comparison.

Trace Elements
anomalies in Ba, Zr and Th for dolerites of group I (Figure 9(b)). There is a positive anomalies in K, Zr, Hf and negative anomalies in Ti, Th and Sm for group II dolerites (Figure 9(d)). The dolerites of group II also display negative to slightly positive Nb-Ta anomalies (Nb/Nb* = 0.20 -1.23); these negative anomalies are most important in group II dolerite (Figure 9(d), Table 2).

1) Fractional crystallization and crustal contamination
The contents of Ni, Cr and Co for dolerites from Léré and Figuil are in equilibrium with mantle peridotites [34] or far from the probable composition of melts equivalent to primitive mantle (Ni: 300 -400 ppm, Cr: 300 -500 ppm, Co; 50 -70 ppm, Mg#: 68 -72, [35] [36]. These characteristics suggest that they underwent extensive fractional crystallization from parental magmas either en route to the surface or in magma chambers [37]. The observed behavior of trace elements and major element oxides of dolerites are reliable with the crystallization of Fe-Ti oxide, clinopyroxene, and plagioclase which are the most important mineral phases. The constant decrease in Fe 2 O 3 t and TiO 2 when plotted against MgO (Figure 7(a), Figure 7 International Journal of Geosciences the differentiation of magma. This is also reliable with the absence or slight Sr or Eu depletion or negative anomalies even in the most differentiated samples ( Figure  9). Nevertheless, the negative correlation of SiO 2 content with MgO observed in group II dolerites may be linked to the presence of plagioclase as seen in the thin section (FcvM1, Figure 2(h)). But this plagioclase is a xenocrystal and is therefore not linked to doleritic magma. The inconsiderable negative correlation of Na 2 O and K 2 O with MgO associated to large variation of Y/Nb (0.71 -3.59) and nearly constant Ba/La (12.37 -19.44) demonstrate that there are no crystallization of alkali feldspar [38] [39]. Low amounts of Σ REE (100.9 -252.4 ppm) combined with slightly negative or positive Eu anomaly is attributed to the crystallization of amphibole, apatite and/or plagioclase [40]. Nb negative anomaly in most dolerite samples of group II (Figure 10) could evidence the presence of amphibole in the metasomatized mantle [41].  (Figure 8(j)), produce almost linear depicting for fractional crystallization and high gradient lines showing partial melting [42]. Fractional crystallization process was possibly combined with minor crustal contamination as shown by enrichment of Th/Yb from group II to group I that might be due to turbulent emplacement ( Figure 11, [43]). This has been observed in other continental tholeiites worldwide [44]- [51]. Through assimilation and fractional crystallization processes, magmas progression en route to the surface, evolving through fractionation, would have assimilated fragments of continental crust. Figure 10. Nb/Y vs. Rb/Y after Cox and Hawkesworth [51] and Leeman and Hawkesworth [53] showing basement samples relative to the dolerite compositions. Data for Precambrian basement are from Sep Nlomngan et al. [40].  [53] can also be used to monitor the impact of the crustal assimilation over fractional crystallization on magma compositions. The dolerites of Figuil and Léré have low Rb/Y ratios (0.45 -1.24) compare to high values (2.54 -7.64) of this ratio in Pan-African granitoids of this area, suggesting a limited effect of crustal contamination ( Figure 10). Only one sample of dolerite of group II (PletM2) seem to be really contaminated because it is in the field of basement rocks. The group 1 dolerites which are most far away from the granitoids field appear to be the least contaminated. This point of view is supported by the presence of more marked Nb-Ta negative spikes in group II dolerites (Figure (9d)) suggesting somewhat significant crustal contamination [54].
2) Mantle source and melting The studied doleritic magma migrated through continental Pan African granitoid and thus the origin of the dolerite from the melting of the continental crust cannot be excluded. Nevertheless, incompatible trace elements ratios, principally Rb/Zr and Ba/Nb (LILE/HFSE) are greater in the Pan-African granitoids rocks from Boula Ibi (North Cameroun) (0.12 -4.13 and 28.11 -245.18, respectively; [40]) than those of Léré and figuil dolerites (0.05 -0.11 and 14.02 -24.1) and therefore preclude the derivation of the studied dolerites directly from the melting of the continental crust. During anatexis of the continental crust, LILE are more compatible than HFSE, this is the reason why Melting of rocks from the continental crust preserve or increase rations of LILE/HFSE [55] [56] [57]. Low (Ce/Yb) N values (3.3 -11.58) suggest fairly high partial melting degree of the source. International Journal of Geosciences

Conclusion
Dolerites of Figuil and Léré are alkaline and exhibit basaltic compositions which are basanite tephrite and basalt; only one sample is a trachy-andesite. Based on the mineralogical composition and geochemical studies, two distinct groups of dolerites are identified: 1) the group of pyrite calcite bearing dolerites and pyrite bearing dolerites with alkali feldspars rich basement rock enclaves and 2) the group of pyrite bearing dolerites and amphibole titanite bearing dolerites. The mineralogical assemblage of these dolerites of alkaline composition consists of plagioclases, pyroxenes, oxides, amphibole, apatite, biotite and sometimes pyrite and calcite. The two groups of dolerites are also discriminated by their TiO 2 , Fe 2 O 3 , REE, Ba, Nb, Zr, La and Hf contents which are relatively higher in group I than in group II. Group II, on the other hand, has higher MgO, Mg#, Sc, Ni and Cr contents compare to Group I. Figuil and Léré dolerites comprise two cogenetic groups and constitute a series evolving by fractional crystallization with some contribution of the continental crust. With higher concentrations of incompatible elements and LREE, group I dolerites resulted from group II through continuous crystallization of its mineral phases. The higher REE contents and (La/Yb) N > 8.7, (Tb/Yb) N > 1.9 and Dy/Yb > 2 ratios characterize a garnet-bearing mantle. The continental tholeiitic signature of the studied dolerites is highlighted in geotectonic discrimination diagrams and also confirms the tholeiitic nature of Cameroon dolerites.