Precambrian ultramafic rocks, such as peridotites, komatiites and associated rocks, have been extensively studied worldwide for their Ni and Cr content but also for platinum group elements (Begg et al., 2010; Groves & Bierlein, 2007; Naldrett, 1999). The vast majority of mantle peridotites are associated with komatiitic lava flows, or plutonic layered intrusions (Cameron, 1978; Maier, 2005; Mathison & Ahmat, 1996; McCallum, 1996; Myers, 1976; Rosengren et al., 2005). In Côte d’Ivoire, notably in the Archean domain of the Man Ridge, mafites and ultramafites were first described and mapped by Bolgarsky (1950). Several years later, Ouattara (1998) and Gouedji (2014) have conducted research in the regions of Biankouma and Marabadiassa respectively, and Allialy (2006) in the Seguéla region. The study conducted by Gouedji (2014) revealed that the Samapleu intrusion was of Paleoproterozoic age. Furthermore, in the Paleoproterozoic domain, the Alékro ultramafics show MORB signatures (Ouattara, 1998). On the basis of isotopic arguments, it has been demonstrated that the rocks in the Séguéla region belong to the Archean-Paleoproterozoic transition zone of the West African Craton (Kouamelan, 1996). The Séguéla region is also well known for its kimberlite field. Allialy (2006) has compared the kimberlites of this region with those already known in the sub-region and resulted in the finding of small, well-preserved kimberlitic diatremes. However, even if the kimberlites of Séguéla are the focus of detailed studies (e.g., Allialy, 2006; Allialy et al., 2011; Allialy et al., 2017), the fact remains that there is very little knowledge about the peridotites found in the region. Consequently, there is a need for a geochemical study to contribute to a better understanding of these rocks in this region.

Côte d’Ivoire belongs to the West African Craton (WAC) that stabilised 1.9 Ga ago, precisely at the Man Dorsal, the southern part of this craton (Bessoles, 1977). It is located on the Leo Ridge. On the basis of new geochronological data published by Boher et al. (1992), Kouamelan (1996), Kouamelan et al. (1997) and Doumbia (1997), Côte d’Ivoire is made up of four geological zones: the Archean zone, the transitional zone (Archean-Birimian transition), the Paleoproterozoic zone (or Birimian zone located east of the Sassandra fault) and the sedimentary basin (located in the southern part of the country). The Séguéla region in west-central Côte d’Ivoire falls within the Archean-Proterozoic transitional zone to the east of the Archean zone and the Sassandra Fault, and on the western edge of the Paleoproterozoic zone sensu stricto. It is known for its diamond field fed by kimberlite and lamproite dykes. These dykes, undeformed and unmetamorphosed, cross two granite plutons separated by small panels of amphibolite belonging to the Birimian formations (Figure 1). They are monzonitic granites with biotite and hornblende showing their differentiated pegmatitic and aplitic dome and porphyroid rims (Allialy, 2006). Their N170˚ orientation is controlled by the structured contact between the Archean and Paleoproterozoic lithospheres of the West African craton.

This work was carried out in two steps, a fieldwork and a laboratory stage. All these different stages are described in the following sections.

Crystallochemistry has shown that the Séguéla peridotites consist of the following mineral paragenesis: olivine, pyroxene, amphibole and spinel. Forsterite is more abundant in olivine, orthopyroxene is represented by enstatite, clinopyroxene by diopside and amphiboles by tschermakite, pargasite, actinote-tremolite and magnesio-hornblende. The mineral chemistry of the peridotites, particularly with olivine (89% < Fo < 91%), Opx and Cpx (negative correlation between Al₂O₃ and Mg# contents) indicates a mantle origin by fractional crystallisation. This correlates with the research of Zaki Khedr et al. (2013) in the Waki Sarami ophiolite and Gouedji (2014) found the Biankouma region. Furthermore, the forsterite contents of the olivines (89% - 91%) and the Mg# of the orthopyroxenes (84% - 96%), indicate crystallisation from a magnesian magma. The presence and abundance of amphiboles in the peridotites is indicative of the water-rich nature of this magnesian magma. These results support those of Allialy et al. (2017), and are similar to those of Gouedji (2014) and Ouattara (1998) obtained on the ultramafic and mafic intrusions of the Yacouba complex (Biankouma region). Moreover, the chemistry of the Séguéla peridotites is similar to that of the Beiyan peridotites in northern China (Hao et al., 2012). However, the Cr# ratios of the Séguéla spinels are slightly lower than those of Beiyan, with values ranging from 7.3% to 44.3%. The signature of spinels with XFe³⁺ < 20% is indicative of crystallisation under low oxygen fugacity. These conditions could be due to the early crystallisation of these spinels and sometimes even before the olivine in which they are often embedded. These results corroborate those of Allialy (2006) and Ouattara (1998) in the Biankouma region.

These facies are comparable to silica-rich komatiitic basalts (Arndt et al., 1977; Cattell & Taylor, 1990). The distribution of major elements in the peridotites suggests that the lherzolites, dunites and harzburgites are probably the result of a single primitive magma that gradually evolved. As per (Pearce & Cann, 1973), rock samples with low levels of TiO₂ (<2%) are consistent with magmatic arc rocks, but are different from intraplate basalts, which often have high levels of TiO₂ (>2%). Indeed, peridotites have TiO₂ levels below 2%. This corroborates the studies done by Ouattara (1998) and Gouedji (2014) in the Biankouma region.

Characteristics such as negative anomalies (Ta and Nb) and light rare earth enrichments are the main characteristics of the low-Ti peridotites found mostly in the Chinese northeast craton (Hao et al., 2012), and in the Oman ophiolite (Nicolle, 2014). The negative Ta-Nb anomalies observed in this study, although characteristic of a subduction context, are, together with the enrichment in Ba, Rb, Th, characteristic of a mantle plume. These observations are similar to those made by Gouedji (2014) in the Biankouma region, Côte d’Ivoire.

The present study has highlighted the petrographic background, crystallochemistry and lithogeochemistry of the Séguéla peridotites in order to propose a geodynamic model of setting. Based on the results, the Séguéla peridotites are composed of lherzolites, dunites and harzburgites. The minerals within the peridotites are essentially olivine, most often associated with serpentinisation, pyroxene, amphibole, spinel and phlogopite. The crystallochemistry of these rocks indicates high levels of forsterite (89% - 91%) which proves that they have crystallised from a magnesian magma and the presence of actinote-tremolite, tschermakite, pargasite, and magnesiohornblende would indicate the water richness of this magma and the spinels show a signature of the mid-oceanic rifts. Pyroxene species in the Séguéla peridotites are mainly enstatites and diopsides. Based on their geochemistry, these rocks show calc-alkaline chemistry suggesting a single magmatic source. The spectra of the peridotites show overall enrichments in lithophilic elements and light rare earths compared to heavy rare earths and HFSEs, associated with a negative Ta-Nb anomaly that is not very marked, which proves that these rocks were emplaced in a back-arc context, i.e., a mantle enriched by fluids from a subduction zone. This could also be the result of interaction between the mantle and magmas in a subduction zone context. The enrichments of light rare earths compared to heavy rare earths would therefore indicate a metasomatic episode. The negative cerium anomalies observed would reflect the interaction of these rocks with sea water, those in K would result from a post-settlement alteration phenomenon and the depletion in certain elements such as Na, Ca and Al indicates that these elements existed in small quantities in the mantle.

The authors declare no conflicts of interest regarding the publication of this paper.

Ephrem, A. M., Kagou, D., Nestor, H. N., Nicaise, K. A., & Yacouba, C. (2022). Geochemical Signatures of Séguéla Peridotites in the West African Craton. Journal of Geoscience and Environment Protection, 10, 100-116. https://doi.org/10.4236/gep.2022.1012008