Petro-Geochemistry Contraints of Côte d’Ivoire North-East Plutonites: Implications for Eoeburnean Magmatism of Baoulé-Mossi Domain (Southern of West African Craton)

Plutonites are part of the set of crystallines compose the Precambrian base-ment. Their study is capital because it allows participating in the debates on the geodynamic evolution of the West African craton. This study concerns granitoids and dioritoids of the North-East of Côte d’Ivoire located in the Man-Leo ridge structured by Eoeburnean magmatism. The petrographic and geochemistry constraints make it possible to say that these rock formations have evolved by the process of fractional crystallization. They have a hybrid origin (mantle and crust), metaluminous to peraluminous highlighted by geochemistry. Concerning the geotectonic context, these plutonites of the Baoulé-Mossi domain are emplacement in a context of subduction more precisely in a context of volcanic arc.


Introduction
The West African Craton stabilized at ca. 1900 Ma and includes two principal Precambrian domains: the Réguibat Rise to the North and the Leo-Man Rise to the South (Rocci, 1965). The Leo-Man Rise is composed of an Archean cratonic nucleus in the southwest, the so-called Kéména-Man domain (KMD), surrounded by some Paleoproterozoic belts in the north and east forming the Baoulé-Mossi low-grade Birimian (ca. 2.20-2.13 Ga) and Tarkwaian (ca. 2.10-2.09 Ga) sediments and plutono-volcanic rocks, which emplaced during a major juvenile crustforming event, referred as the Eburnean orogeny and dated between ca. 2250 and 1980 Ma (Allibone et al., 2002;Feybesse et al., 2006;Pouclet et al., 2006;Vidal et al., 2009;Baratoux et al., 2011). Volcanic rocks' geochemical signature is compatible with the evolution from the tholeiitic oceanic crust or oceanic plateau to juvenile calc-alkaline volcanic island arcs (Abouchami et al., 1990;Leube et al., 1990;Boher et al., 1992;Baratoux et al., 2011). Eoeburnean intrusions dominate the eastern portion of the Baoulé-Mossi domain and during the Eburnean period, intrusions are mainly concentrated across the western portion. This study allows constraining the source and the geotectonic context of these plutonites. It is therefore a contribution to the Paleoproterozoic crustal evolution.

Local Geological Context
Located in the Paléoproterozoic field, geological formations in the Bondoukou region were structured during Eburnean orogenesis. The very open petrographic range of geological formation in the Bondoukou region and the complexity of structural phenomena make their study difficult. From a lithological point of view, the study area is covered by a complex set of geological formations. There are two major distinct fields: the quaternary domain (Holocene) and the Paleoproterozoic domain. Training in the quaternary field is the most recent in the Bondoukou region and has been deposited precisely at the Holocene . They come from the alteration of the preexisting parent rock. There are two types: -vases and sands: these are observable fluvial sediments only along rivers.
These sediments have a wide lithological variety: more or less coarse sands, vases, and clays, fine and very fine silts .
-Lateritic cuirass: Laterites refer to all indurated materials which constitute the soils, the superficial horizons as well as the deep horizons of the alteration profiles . These often red or brown materials consisting of aluminum hydroxides and/or iron, come from the alteration of the rock without transport of minerals (residual rocks). These formations cover almost the surface of the study area and can be subdivided into three rock formations: -Tarkwaïen formations; -volcanic and sedimentary formations; -Intrusive formations.

Tarkwaien Formations
This template has been tailored for output on the custom paper size (21 cm × 28.5 cm). The Tarkwaïen name is its origin in Ghana, named after the Ghanaian locality "Tarkwa". The arguments taken from the work carried out on the Tarkwaïen (Touré, 2007) are most often divergent. According to Touré (2007), the main reason for disagreements lies essentially in the existence of very little ra-B. R. Kouassi et al. diometric data on the formations attributed to Tarkwain. The Tarkwaïen ensemble of the Bondoukou region is essentially made up of conglomerate (met in the region of Duakouamé and Zanzan) and Cinérite (in the region of Takoutou and Siago). The presence of sandstone, quartz and Andesites is also reported by (Touré, 2007). The Tarkwaïen formations are based on the other Paleoproterozoic formations (Siméon et al., 1995), notably metavolcanites in the north, metasediments to the south and intrusive rocks (granodiorites, tonalites).

Volcanic and Sedimentary Formations
It is an intimate association of various volcanic rocks, pyroclastic rocks (fine conglomerates, tufs, etc.), clay sedimentary rocks and essentially chemical rocks (quartzite) . So, this is a very complex formation for several geological units of a different petrographic nature. This set outcrops in the southern and northern regions of the study area with extensions to the East and West. The undifferentiated volcano-sedimentary shales the most encountered in this set constitute the large massif inside which is the whole sequence of volcanic and sedimentary hectometric lenses. Thus, for volcanic sets, metadolerites and metamicrogabbros which are massive rocks, with an isotropic appearance have a grainy and porphyroid texture with plagioclase or ferromagnesians. They are outcrop in several places in the formations of volcano-sedimentary schists. The amphibole and chlorite schists north of the Bondoukou granodiorite are darkcolored rocks and have a conglomeratic appearance with brecciated or angular elements . On the north and west flanks of the Bondoukou granodiorite, outcrops metaandesites with porphyric microlithic texture. Besides these formations which occupy relatively large surfaces, there are also metagabbros, metarhyolites with a vitreous appearance. The sedimentary rocks are represented by calco-chlorito-schists who appear west of the Bondoukou granodiorite. It has a conglomerate aspect with more or less rounded and coarse elements. Small lenticles of quartz shale flip on the northern flank of the Bondoukou granodiorite. Volcano-sedimentary complexes are well marked in morphology and are present in all intrageosynclines. These formations were very studied everywhere in Côte d'Ivoire, given their genetic link with manganese mineralization, one of the first to work on the Birimien brought lithostratigraphic and individualized details within a volcano-sedimentary complex of detrital formations (quartzites, arkoses, sandstones) and terms plutono-volcanic (Andesites, gabbros, amphibolites).

Intrusive Formations
The intrusive formations in the Bondoukou region are the syntectonic granitoids and the post-tectonic intrusive granites (Tempier, 1969).

Syntectonic Granitoids
In these formations, undifferentiated metatonalites and metagranodiorites are the most widespread and are outcrop to the east, north and west of the study area. These formations with granonematoblastic texture, consisting of more or less sericitized oligoclase, xenomorphic quartz as well as Hornblende and green biotite . Within the formations, we find: -to the northeast: biotite metagranites which are medium-fine grain granites, light color containing both biotite and muscovite.
-to the northwest: granites and monzogranites with biotite. These fairly dark facies consist of more or less chloritized plagioclase and green biotite ; -to the west: a granodioritic facies whose rocks are melanocrats, these are biotite gran odiorites and/or hornblende with grainy texture with a percentage of plagioclase which dominates on that of the microcline.

Post-Tectonic Granitoids
Post-tectonic (discordant granites of Bondoukou type) which are outcrop northeast of the study area circumscribed around the city of Bondoukou, are essentially made up of biotite granodiorites and/or porphyroid texture. They cause contact metamorphism on the volcanic rocks. Most often isotropic in the central areas of the massif, their periphery presents ferromagnesians (biotite and hornblende) which are phenocrystals of plagioclase and individualize with them a mineral and/or high dip stretch to the NNW . These rocks with melanocratic facies, give the impression of having been put in place after all folding and all metamorphism. With the work of (Touré et al., 1987), the traces of "Burkinian" orogenesis were highlighted in the Bondoukou region by the Rb-Sr age at 2166 ± 49 mA (on whole rock) of the porphyroid granodioritic facies of Bondoukou. This facies is a particular type of discordant and post-tectonic intrusive formation, hence the name of intrusive granite of the Bondoukou type attributed to the similar formations encountered in other regions of the country (Mankono, Dabakala, etc.). According to Touré (2007), the discordant granitoids of Bondoukou cannot be considered as younger than Baoulé type granitoids.

Analytical Methods
To obtain the results that will be presented in this study, we used two main methods.
The methodology consisted in observing and describing the different lithologies, either with the naked eye or using a magnifying glass on the ground. This phase was accompanied by samples. The samples collected allowed for the making of thin sections in the geochemical and geochronology laboratory of The University of Rennes 1. Their study was done in the geology, mineral and energy resources (LGRME) of the UFR Earth Sciences and the Mineral Resources (UFR STRM) of Félix Houphouët Boigny University of Abidjan-Cocody, Côte d'Ivoire.
This phase makes it possible to come out for each facies of rocks: color, texture, alteration levels, constitutive minerals (primary and secondary) and if possible, their proportions, the presence of sulfides and veins or venules of quartz and carbonate and sericite feldspars, and chlorite. Biotite is also present with Zircon inclusions and it shows on certain sections an alteration in chlorite. Preserved plagioclases are also observed, as well as rare sulfides and oxides.

Granodiorite
The granodiorites are massive, weakly deformed and show medium to coarse grains (Figure 2(B)). They are made up of quartz, feldspaths, amphibole and biotite. These rocks often contain granophyres made up of quartz and feldspar; as well as enclosures of mafic rocks. Under the microscope, they show a grainy texture and are made up of Feldspaths minerals generally pseudomorphosed in sericite, epidote and carbonates. We also observe green hornblende, chlorite, preserved and zoned plagioclases, as well as rare sulfide.

Tonalite
On the outcrop, the tonalite are medium and more or less mesocratic grains (Figure 2(C)). Macroscopically, we observe quartz crystals, feldspars, amphiboles and biotites. Microscopic observation, they show a grainy texture and are made up of microcline minerals, quartz, and orthoses. Feldspar crystals (plagioclase and/or alkaline) are often altered in carbonates, epidote and sericite. The observed chlorite comes from the pseudomorphosis of minerals of biotite and green hornblende. Rare sulphides as well as oxides are observed.

Gabbro
The Gabbros are outcrop in the form of massifs, melanocrats, with a grainy texture ( Figure 2(D)). The minerals observed are plagioclases and ferromagnesians. Under the microscope, the gabbros have a porphyric texture with pyroxene and green hornblende phenocrystals. Pyroxene crystals have generally undergone uralitization. Plagioclases are generally pseudomorphosed in carbonates (calcite ± dolomie). Gabbros also contain chlorite, epidote, sericite, sulfides and oxides. We also observe quartzo-feldspathetic venules often associated with epidot crystals.

Diorite
The facies of diorite are mesocrats, of average granulometry and generally in enclaves of granitoids (Figure 2(E)). They are dominated by two minerals of Plagioclase and green hornblende. Under the microscope, these rocks show a grainy texture. They are generally made up of green hornblendes and plagioclase; and accessory epidote, chlorite and sericite. So, let's note the presence of rare sulfides and oxides.

Geochemistry
The results of intrusive analyzes are recorded in Tables 1-3. Inserted into the classification diagram of (Middlemost, 1994) applied to plutonites the samples are distributed from the fields of Tonalites to granites via the granodiorites (Figure 3(A)) as well as gabbros and diorites (Figure 4(A)). Reported to the Journal of Geoscience and Environment Protection    Wright et al. (1969); (C) Diagram of Shand (1922) and (D) Diagram of Chappell & White (1974) applied to the granitoids of the Bondoukou-Tanda region.  Shand (1922) and (D) Diagram of Chappell & White (1974) applied to the granitoids of the Bondoukou-Tanda region.
diagram of (Wright et al., 1969) (Figure 3(B) and Figure 4(B)), the intrusive of the Bondoukou region shows a chimism generally identical to that of sub-alkaline rocks. However, it should be noted that the TB 07 granodiorite sample as well as the granites TA 07 and TA 48 show alkaline chemicals. The diagram of (Shand, 1922) (Figure 3(C) and Figure 4(C)), indicates that these rocks are metaluminous to pearluminous. The granitoides and dioritoides of Bondoukou region are type I (Figure 3(D)) and unweathered rocks (Figure 4(D)).
Their TiO 2 value is very low (0.09% to 0.18%). The granites define a calco-alkaline chemical (Figure 3(B)). The REE contents (ΣREE) of the granites are between 61.24 ppm and 68.10 ppm. The spectrum of the REE of these rocks normalized to chondrites according to (Sun & Mc. Donough, 1995), presented to (   co-alcaline series (Figure 3(B)). The REE contents (ΣREE) of the granodiorites are between 75.72 ppm and 125.50 ppm. The rare earth spectrum of these normalized rocks in chondrites according to (Sun & Mc.donough, 1995)

Magmatic Evolution and Source of Magma
The intrusives of the Bondoukou-Tanda region are granites, granodiorites, tonalites, gabbros and diorites. The petrographic study highlights evidence of intense hydrothermal alteration undergone by these facies. These are mainly sericitization, chloritization and epidotation induced by epizonal metamorphism like the intrusives of the Baoulé-Mossi domain (Vidal et al., 2009;Baratoux et al., 2011). Still on the petrographic level, we have noticed that the granodiorites express an extreme poverty in potassium feldspars which reflects their placement during deformation. These rocks evolved by fractional crystallization, hence their enrichment in Rb and their depletion in Sr. The Ba and Sr anomalies in the spectrum suggest that feldspars are present in the residual liquid and the negative P ano-maly is the result of apatite fractionation. The high Ba contents compared to the Sr show that there was an important fractionation of plagioclase in the rock. The meta-aluminous to peraluminous trends in terms of the chemistry of these intrusives clearly show that they issued a hybrid source (mantle-crust). Structured by Eoeburnean magmatism, these massifs are similar to the geological formations of Boromo-Houndé and Banfora in Burkina Faso (Parra-Avila et al., 2018). They are also calc-alkaline and highly enriched in LREE. Indeed, for ( Barbarin, 1990), granitoids with these characteristics have a hybrid origin (mantle and crustal). According to Rogers & Hawkesworth (1989), Sigmarsson et al. (1990), the presence of residual garnet and/or hornblende explains the low Yb contents of the magmas. The (La/Yb) N ratios between 0 and 40 and Yb N levels between 0 and 5 ppm observed in these samples support this assertion. This is also explained by the fact that it is type I. The low TiO 2 contents of the gabbros analyzed resemble those of the plutonic rocks of the magmatic arcs (Pearce & Cann, 1973), but are different from the basalts of those of the intra-plaque arrangements, which often maintain the high TiO 2 concentrations (>2%). The rare earth elements spectrum of Bondoukou-Tanda region intrusives is generally show negative to almost zero anomalies in europium, with however very rare slightly positive anomalies. Indeed, for Martin (1999), the absence of an anomaly in Eu would come from the fusion of a hydrated subducted oceanic crust in a moderately deep subduction zone which reacted with the mantle at the base of the continental crust. Cerium anomalies are common to modern arc magmas but can also result from post-magmatic alterations such as the dominant circulation of hydrothermal fluids (Abouchami et al., 1990;Sylvester & Attoh, 1992).

Tectonic Implication
Figure 6(A) shows that the granitoides of the Bondoukou-Tanda region are generally put in place in a context of volcanic arc, according to the diagram of Pearce et al., 1984. In the diagram the/yb-yb of Martin (1986), in the Figure  6(b), these rocks are generally located in the common field with archaean TTG and rocks of the island arcs. This position of these rocks shows that they have a certain affinity for the Archaean TTG. This shows a possibility of mixing due to the existence of a small archaean inherited component. Proponents of the arctype setting across the Baoulé-Mossi domain have identified whole-rock elemental compositions which are commonly associated with modern arc environments. These include negative Nb and Ta anomalies and variable degrees of enrichment of the LILE (Arculus et al., 1999). Recent studies have identified subduction-related geochemical signatures across Burkina Faso, Ghana, Guinea and Mali (Fontaine et al., 2017;Lambert-Smith et al., 2016;Masurel et al., 2017;Petersson et al., 2016). Furthermore, these rocks exhibit a highly fractionation spectrum in the HREE portion, which is typical of most subduction zones. The intrusives of the Bondoukou-Tanda region show an enrichment in lithophilic elements (LILE), a depletion in HFSE elements, as well as a clear negative anomaly in Nb-Ta. According to (Doumbia et al., 1998), these characteristics observed on these intrusives imply that they would have been emplacement in active continental margins. Significant negative Nb anomalies also indicate the role played by titaniferous and/or amphibole phases (Martin, 1999). Positive Ta and negative Nb, K, Ti and P anomalies clearly indicate a subduction environment more precisely in arc type.

Conclusion
The Bondoukou-Tanda intrusives are divided into four main facies which are granites, granodiorites, tonalites, gabbros and diorites. These formations of the Baoulé-Mossi domain structured by the Eburnean orogenic mega cycle are affected by hydrothermalism, particularly sericitization, chloritization and epidotation which characterize the epizonal metamorphic facies which facies is frequent at the level of the rocks of the West African craton. These resulting from the Eoeburnean magmatism have evolved by the process of fractional crystalliza-tion also confirmed by geochemical data. Regarding their source, the intrusives of the study area have a hybrid origin, hence their chemistry, which varies from metaluminous to peraluminous. Concerning the geotectonic context, the arguments of geochemistry show that these formations define an emplacement by the mechanism of subduction more precisely in an arc context as the majority of the rocks of the West African craton.