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

Abstract

Magmatism in the Cretaceous sedimentary basins of the Figuil and Léré regions 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 trachytes. The dykes are in the order of 20 to 100 m wide by several kilometers long and oriented from N23˚E to N90˚E. The textures of these rocks are sub-ophitic to intergranular for dolerites and trachytic for evolved rocks (trachytes). The geochemical study shows that the dolerites are basaltic in composition with alkaline to subalkaline character. The sampled dykes have an evolution dominated by fractional with the minor impact of the crustal assimilation 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.

Major, trace elements and Sr–Nd–Pb isotopes systematics of mafic dykes from the Figuil (northern Cameroon) and Léré (southwestern Chad) areas

“…This primary magma generally has high abundances of MgO (>10 wt %), Mg# (68-72), Co (50-70 ppm), Ni (300-400 ppm) and Cr (300-500 ppm) contents (Frey et al, 1978;Jung & Masberg, 1998). The (2) and ( 3): dolerites of the same study area (Klamadji et al, 2020(Klamadji et al, , 2021.…”

“…All these geological formations are intruded in places by mafic and felsic dykes and sills (421.3-43 Ma;Aka et al, 2018;Ngounouno et al, 2001;Tchouankoue et al, 2014) due to the rejuvenation of Pan-African faults (Maurin & Guiraud, 1990, 1993. In the localities of Figuil and Léré, the dykes indifferently intersect the Precambrian basement and Cretaceous sediments (Isseini et al, 2012;Klamadji et al, 2020Klamadji et al, , 2021Figure 2a,b).…”

“…The opening of the Southern Atlantic may have widened the process and led to the opening and collapse of the Benue Trough in Nigeria and the formation of the CL. Basalt dykes studied here could be a part of the igneous system that predated the formation of the Benue Trough and could thus reflect internal adjustments F I G U R E 2 (a) Geological map of the studied area and location of the samples (modified fromKlamadji et al, 2021). (b) Cross-sections across the lithotectonic units outcropping in the Figuil-Léré area.…”

“…The samples are all classified in tholeiitic series in the AFM (A: Na 2 O + K 2 O; F: FeOt; M: MgO) and FeOt vs. (FeOt/MgO) diagrams (Figure6d,e). When taking into account data from the literature(Klamadji et al, 2020(Klamadji et al, , 2021 in the analysis of oxide trends, the plots of MgO against other major elements of DPB display a negative correlation with TiO 2 , Na 2 O, P 2 O 5 and Fe 2 O 3 (Figure 7a,b,f), positive correlation with Al 2 O 3 and CaO (Figure 7f,g) and no clear correlation with K 2 O and SiO 2 (Figure 7c,e). Concerning the dykes cross-cutting the sedimentary Cretaceous basins (DGCSB: DCSB and FGG), there is a negative and positive correlation for Al 2 O 3 and Fe 2 O 3 , respectively (Figure 7f,h) and no real clear correlation with other oxides.…”

“…Data ofKlamadji et al (2020Klamadji et al ( , 2021 are also include in calculations. Abbreviations: DGCSB, dolerite and gabbro of Cretaceous sedimentary basin; DPB, dolerite of Pan-African basement.…”