Fractional Crystallization and Crustal Contamination of Doleritic and Trachytic Dykes Crosscutting the Cretaceous Sedimentary Basins from Figuil (North Cameroon) and Léré (South-Western Chad): Geodynamic Implications

Magmatism in the Cretaceous sedimentary basins of the Figuil and Léré re-gions constitutes one of the fundamental parameters in the reconstruction of the history of the Cretaceous sedimentary basins. The main objective of this paper is to constrain the petrogenetic processes of hypovolcanic rocks and to determine their geodynamic context of emplacement. The petrographic study of mafic hypovolcanic and trachytic rocks was carried out under a polarizing microscope on thin sections. For the geochemical study, the major oxides and some trace elements were analyzed by ICP-AES. Trace and rare earth elements were analyzed by ICP-MS. The dolerites of the Cretaceous sedimentary basins are composed of dykes of amphibole bearing dolerites, biotite and pyroxene bearing dolerite, pyroxene bearing dolerites and Dy/Yb > 2 (2.32 - 3.50) ratios of most samples, which suggests melting in a garnet-bearing mantle. Concerning the geodynamic context of the studied rocks, doleritic samples are classified as within-plate tholeiite and volcanic arc basalt, and within-plate alkali basalts.


Introduction
Magmatic intrusions usually made up of basic and ultrabasic rocks generally exploit fracturing networks to set up (Srivastava, 2011;Silpa et al., 2021;Huang et al., 2021). The geochemical characteristics of these magmatic intrusions provide, on the one hand, elements in the reconstruction of regional geodynamic contexts, synchronous for their establishment and, on the other hand, information on the source zones of the magmas (Halls, 1987). In Central Africa, magmatic intrusions of a doleritic nature are represented by two subsets: the dolerites of the extension basins and the so-called continental tholeiite dolerites. Dolerites from extension basins have been identified in Anambra in Nigeria (Coulon et al., 1996) and in northern Cameroon, in the region of Mayo Oulo-Léré and Babouri-Figuil . The formation of basic to intermediate intrusions which very often outcrop in dykes and sills is considered to be a direct consequence of tectonic events that have affected the Pan-African basement (Toteu et al., 1987(Toteu et al., , 1990. In Cameroon, mafic dykes have been studied in southern Cameroon in many localities: 1) Biden in the south-east of Ngaoundéré ; 2) Mayo Oulo-Léré ; 3) Banganté, Maham, Kendem, Dschang, Bangoua and Manjo (Tchouankoué et al., 2012(Tchouankoué et al., , 2014; 4) Mbaoussi (Nkouandou et al., 2016); 5) Likok (Nkouandou et al., 2015); 6) Mongo in Central Chad (Nkouandou et al., 2017); 7) Temte (Poli) in North Cameroon (Atour et al., 2020); 8) Figuil and Léré (Far North Cameroon and SW of Chad) where these dykes intersect the Pan-African basement (Klamadji et al., 2020). In the Cretaceous sedimentary basins of the study area, straddling Cameroon and Chad (Figuil and Léré), basic intrusions of a doleritic nature have so far not been the subject of a detailed petrological study. The main objective of this study is to constrain the petrogenetic processes of these hypovolcanic rocks and to determine their geodynamic context using major and trace elements' data.
lidian facies of the Lower Cretaceous whose history is linked to the formation of the Bénoué ditch. The main geological units in the area include the Precambrian basement dated Meso to Neo-Proterozoic between −700 to −1000 Ma (Dawaï, 2014); a thick sedimentary layer dated from the Lower Cretaceous which lies in discordance on the Precambrian basement and post-Pan-African magmatic occurrences (Schwoerer, 1965). The magmatic rocks of Léré and Figuil intersected a basement of Middle to Upper Proterozoic age, covered with sediments of Lower Cretaceous age which were deposited in the two basins of 1000 km 2 surface (Schwoerer, 1965). The sedimentary cover formations (Figure 1 (Bouyo et al., 2015)). TBF: Tcholliré-Banyo fault; NP: Neoproterozoic; PP: Paleoproterozoic.
ists. In Figuil (North Cameroon), bituminous shales occupy a graben which continues east of Lake Léré and develops on Cameroonian territory.
The Lamé series is made up of formations of marine and continental origin from the Upper Cretaceous (Albian-Cenomanian) (Maurin & Guiraud, 1990).
The formations consist of coarse conglomerates surmounted by arkoses, sandstones, limestone sandstones, marls, clays and lenses with lumachelles of lagoon origin. The conglomerates found along the border of the basin, north of Pala contain pebbles and boulders up to 30 cm in diameter. The limestones appearing mainly between Baoaré and Louga, near the Cameroon border, provided molluscs (Gastropods). It is crosscut by veins and laccolites of basalt and olivine bearing dolerites.

Sampling and Analytical Methods
A total of seven (7) samples were collected from the different dykes outcropping in the area. They were then carefully cleaned and, labeled. The preparation of thin sections was realized at Key laboratory Coalbeb Methane Resource and Reservoir Training in China. These samples were then also sent to Bureau Veritas Mineral Laboratories in Vancouver, Canada for major and trace element geochemical analyzes according to the techniques described by Klamadji et al. (2020). Rock powder of each sample (0.2 g) was added to lithium metaborate/lithium tetraborate flux (0.90 g), well mixed and fused in a furnace at 1000˚C. The resulting melt was then cooled and dissolved in 100 ml of 4% nitric acid and 2% hydrochloric acid.
This solution was then analyzed by a combination of ICP-MS (Inductively Coupled Plasma-Mass Spectrometry) and ICP-AES (inductively-coupled plasma-atomic emission spectrometry) to determine major and trace element compositions of the samples. The obtained results were corrected for spectral inter-element interferences. 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 uncertainties were about 0.01 wt%, apart from  Table 1 and Table 2.

Petrography
The hypovolcanic and volcanic dykes that are the subject of this work are dolerites and trachytes with length varying from 0.38 to 9.5 km and width ranging M. N. Klamadji et al.    Inclusions of accessory minerals such as apatite and titanite have been observed in some sections.

Pyroxene-Bearing Dolerites
The pyroxene bearing dolerites (PD) of Tchintchou Golombé outcrop in the form of a rectilinear dyke oriented N30˚ -60˚E (75 to 100 m width and >5000 m long) with blocks of centimetric (10 to 24 cm) to metric (1.2 × 2.8 m) in diameter. The PD have a sub-ophitic texture (Figure 3(b)) and are made up of plagioclase, clinopyroxene, alkali feldspars, quartz, opaque minerals, epidote and chlorite. Damourite also occurs as a secondary mineral of plagioclase alteration ( Figure 3(b)).

Biotite and Pyroxene-Bearing Dolerites
The

Major Elements Geochemistry
The geochemical analyzes of the dolerite and trachyte samples are reported in Table 2.
Using the TAS diagram (Le Bas et al., 1986), dolerites are mainly basaltic, except the FmsM1 sample which is placed in the trachyte fields ( Figure 4) AS suggested by the Th/Ta ratio (1.40 -1.66) (Cabanis & Thieblemont, 1988), these dolerites belong to the continental tholeiite series. In fact, in the AFM diagram (Irvine & Baragar, 1971), all the dolerite samples are placed in the tholeiitic domain ( Figure 5(a)) except for the trachyte sample which is found in the calc-alkaline field. Figure 5
in LILE (Rb, Ba, Th) compared to HFSEs (Hf, Zr, Y). The dolerites show negative anomalies in U, Th, Ce and Nd, and positive anomalies in Ba,K, Nb and Sr.

Fractional Crystallization
The major and trace elements systematics together with the petrographic observations of the studied samples suggest important level of continuous crystallization of mineral phases from their parental magmas. For example, the low contents of (Ni < 122 ppm), (Co < 43 ppm), MgO (average of 6.02%) and Mg# (<58) testify that the parental magmas would have undergone a significant fractionation of ferromagnesian minerals (Arth, 1976;Frey et al., 1978;Xu et al., 2001) within the magma chamber or as they ascend to the surface of the earth's crust.

Crustal Contamination
In the Th/Yb vs. Ta/Yb diagram of Pearce (1982) (Figure 10 Pearce (1982) applied to studied dolerites; CA: calc-alkaline, SHO: shoshonite, TH: tholeiitic basalts. The vectors S, C, W and F refer to subduction zone, crustal contamination, intra plate fractionation, and fractional crystallization respectively according to Pearce (1982). (b) Nb/Y vs. Rb/Y diagram after Cox & Hawkesworth (1985) and Leeman & Hawkesworth (1986) showing basement samples relative to the dolerite compositions. Data for Precambrian basement are from Isseini et al. (2012). uncontaminated lavas. The low effect of contamination by crustal fragments can be also evidenced in the Rb/Y vs. Nb/Y diagram of Cox & Hawkesworth (1985) and Leeman & Hawkesworth (1986) (Figure 10 (Sun & McDonough, 1989;Hofmann et al., 1986;Hollanda et al., 2006), and far from the continental crust that has a low value of the ratio Nb/U (4.4 -25) and high values of the ratios of La/Nb (1.6 -2.6) and Th/Nb (0.24 -0.88) according to Rudnick et al. (2003). These features indicate a negligible effect of crustal contamination. The positive Nb-Ta anomalies (Nb/Nb*: 1.04 -1.25) in all dolerites also signify the nonexistence of contamination by crustal materials.

Mantle Source and Melting
Given that the magma migrated through continental basement rocks, consequently the origin of the dolerites and trachyte from the melting of the conti- The nature of doleritic and trachytic magma source is supported by their (Tb/Yb)N > 1.9 (1.91 -3.79) and Dy/Yb > 2 (2.32 -3.50) ratios of most samples, which suggests melting in a garnet-bearing mantle (Wang et al., 2002;Jung et al., 2006). The normalizing values are of Sun & MCDonough (1989

Conclusion
The chemical study of studied dykes shows that mafic samples (dolerites) are predominantly basaltic in composition, while the felsic one is trachytic. Based on the mineralogical geochemical compositions, three groups of dolerites are identified: the pyroxene bearing dolerites (PD) and biotite and pyroxene bearing dolerites (BPD) are sub-alkaline while amphibole bearing dolerites (AD) and trachyte are alkaline. The rocks of the studied area have an evolution dominated by a fractional process with negligible impact of the crustal contamination characterized by low Rb/Y ratios for dolerites (0.36 -0.97) and high values of Rb/Y for the Pan-African granitoids' samples (1.95 -4.01). The nature of doleritic and trachytic magma sources is supported by their (Tb/Yb)N > 1.9 (1.91 -3.79) and Dy/Yb > 2 (2.32 -3.50) ratios of most samples, which suggests melting in a garnet-bearing mantle. Regarding the geodynamic and geotectonic context of the studied rocks, doleritic samples are classified as 1) Within-plate tholeiite and volcanic arc basalt; 2) Within-plate alkali basalts.
In the future, we plan to increase the number of analyzes in these dolerite formations in order to better characterize them geochemically. Isotopic data will be needed to better characterize the source of these rocks. In order to insert the dolerite dykes studied in the context of CPAC, radiogenic dating will be also necessary.