Rocks of magmatic origin occur in the metasedimentary units in the external zone of the Dahomeyides belt. These greenstones are defined as metabasalts, serpentinites and prasinites associated with talcschists and chromitites [1] [2] [3]. They correspond to thrust outliers of intrusive volcanic bodies of voltaian ocean floor imbricated in the metasediments during the panafrican tectogenesis (600 ± 50 My).

Although reported on geological maps, the metamagmatites have been little studied except those of the Buem structural unit in the north of Benin [4] and the southeast of Ghana [5]. In the particular case of the Kantè serpentinites, the investigations of the Togolese Bureau of Mines and Geology [6] were carried out in the framework of search for the probable associated metal bearing indications. Recently, some data on chromiferous spinels were obtained by [7].

The purpose of this work is to characterize structural markers in the Kantè serpentinite outcrops, clarify their magmatic affinities and propose a geodynamic context of their emplacement. For this purpose, detailed field investigations to collect petro-structural data, thin sections for microfabric study and chemical analysis of thirteen (13) samples for major element content have been done.

The detailed cross-section in Figure 3 shows that the serpentinites form a big lenticular body tectonically enclosed in the schists. Contrary to the clearly outcropping serpentinites, the schistose country rock only occurs in deep gullies in an environment with wide-spreading debris of exsudation quartz. Thus, on the thrust sole or back of the serpentinites, sericite schists, or sericite chlorite schists exist in beds often associated with quartz vein boudins. Their cleavage corresponds to the N150˚ to N180˚ S1 schistocity plane with medium to high (55˚ to 80˚) east dips (Figure 3).

The serpentinite outcrops are distinguishable by their jagged aspect, their generally greenish grey colour and strong structuring (Figure 4). They are composed of three main rock facies: 1) a dark massive facies, rich in antigorite, and appearing as decimetric to metric bodins. 2) a lighter, brecciated facies with a yellowish green patina, a schisto-lenticular cleavage and rich in chrysotile fibers, and 3) a very finely schistified facies, with a purplish or ferruginous patina and belonging to the thrust sole of the entire serpentinite lens. All these facies are structured by a schistocity plane, S1, N130˚ to N160˚ with 45˚ to 55˚ NE dip, on which is superimposed a shear plane C, with low (15˚ to 35˚) NE dip (Figure 3). This flat shear plane signifies an overthrusting movement toward the west.

Under the microscope, all the rocks present a porphyroblastic texture, with relics of plagioclase, pyroxene and opaque minerals in an oriented groundmass with antigorite, chrysotile and talc in places (Figure 5). The plagioclasic porphyroblasts with strong deformation signs show undulatory polysynthetic twin and rolling extinction (Figure 5(a)). They appear in isolation or in clusters molded by the S1 schistocity and trapped between consecutive shear planes.

Small, rare ovoid relics of pyroxene occur in a state of advanced retromorphosis forming talc and serpentine (Figure 5(b)). Opaque clusters (magnetite, chromite and spinel, according to [7] are also molded by the S1 schistosity. Thus, all these relics are witness of the protolith involved in the panafrican tectogenesis to which should be associated the S1 and C plane development.

The serpentinites results (Table 1) show a very high loss on ignition (3.36 to 15.09 wt%). This high value of the loss on ignition indicates a very extensive alteration of the samples harvested. To reduce the effect of this alteration, the oxide contents of the major elements were recalculated relative to the sum of

these contents reduced to 100% without the loss on ignition for each sample. The alkalis (Na₂O and K₂O) appear completely leached. Despite the ultrabasic character (SiO₂: 45 - 58 wt%) of the analyzed samples, the CaO content remains low (0.02 - 0.06 wt%). The W17 sample with poor SiO₂ (43 wt%) content has a particular behavior compared to the rest of the samples. It is very poor in MgO (0.37 wt%) and is on the other hand very rich in Al₂O₃ (6.9 wt%), Fe₂O₃^t (47.49 wt%) with Mg/(Mg + Fe) = 0.4, in corundum (51.91 wt%) and magnetite (291.05 wt%) normative. The other samples have slightly varying oxide contents: Al₂O₃ (0.92 - 1.92 wt%), Fe₂O₃^t (7.15 - 10.05 wt%), MgO (39.51 - 44.17 wt%), TiO₂ (0.018 - 0.04 wt%). The ratio Mg/(Mg + Fe) is constant (0.99).

A positive linear correlation is well expressed in the Fe₂O₃^t = f(Fe₂O₃^t/MgO) diagram and suggests that the analyzed samples are indeed comagmatic (Figure 6(a)). Their Al₂O₃ content compared to the Al₂O₃/CaO ratio remains almost constant despite some disturbances probably related to the alteration (Figure 6(b)).

In the triangular classification diagram of Al-(Fe^t + Ti)-Mg volcanic rocks in cationic percentages of [27], the analyzed samples predominantly occupy the

field of komatiites with the exception of the W17 sample which occupies the field of tholeiitic rhyolites (Figure 7). Their low P₂O₅ content (0.1 wt%) compared to their Zr content (1130 - 3170 ppm) classifies them as tholeiitic basalts [28] (Figure 8). Moreover, the Zr-Ti diagram of [29] indicates that these rocks have the affinity of tholeiites of island arc which is confirmed in the Zr-Zr/Y diagram of [30], where samples are in the field of oceanic arc basalts (Figure 9). The geodynamic context of emplacement of these rocks would be an active margin as suggested by the Zr/Y-Ti/Y diagram [31] (Figure 10).

The Kantè serpentinous body is tectonically enclosed in schists of the occidental Atacora sub-unit. It includes rock facies with massive, brecciated, or schistose structure. From outcrop to microscopic examination, these rocks are distinguished by a strong deformational imprint materialized by an S1 schistocity plane and a flat C shear plane. The latter indicates a west vergence tangential movement involving the serpentinites.

Mineralogically, the rocks consist of an abundant serpentinous groundmass containing rare porphyroblasts of plagioclase, pyroxene and opaque clusters (magnetite, chromite and spinel). These porphyroblasts represent relics of a probable gabbroic to peridotitic protolith. In fact, by their chemical composition, the Kantè serpentinites are defined as ultrabasic rocks [7]. Their magmatic typology is that of komatites or tholeiithic basalts with ocean arc affinity, probably belonging to an active margin geodynamic context.

The geochemical characteristics of the Kante serpentinites reveal their particularities. In fact, these rocks are not comparable to the Tiélé volcanites that are assimilated to the MORB which belongs to a passive margin tectonic context [4]. They are also far from varied volcanites (pillows, and agglomerates associated with hawaiites, trachytes and phonolitic trachytes) represented in the Buem in southeast Ghana. According to [5], the latter were the products of a wide volcanic structure in the context of an oceanic rift zone. Moreover, the Kantè serpentinites differentiate themselves from their equivalents in the suture zone (particularly those of Monts Ahito-Meliendo, in the southwest of Togo) that are related to continental intraplate tholeiites derived from primitive mantellic magma [32].

For [7], the Kantè serpentinites could correspond to peridotitic cumulates comparable to stratiform complexes emplaced in an arc context. During the panafrican tectogenesis, such complexes were found in association with epizonal metasediments by “exhumation” related to reverse fault contacts or deep overthrusting.

The results presented here are obtained in the framework of Waxi 3 (West African Exploration Initiative) project. The authors seize this opportunity to express their gratitude to all the sponsors of this project that made possible to synthesize the geology of megasequences in the Volta basin and their equivalents in the Panafrican Dahomeyides belt. The authors also thank all the contributors, especially the collaboration of Mr. Anani AYITE of the Ghana Geological Survey Authority.

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

Tairou, M.S., Miningou, Y.M.W., Da Costa, Y.D. and Kwekam, M. (2022) Petrostructural and Geochemical Characteristics of the Metamagmatites in the External Zone of the Dahomeyides Belt: Case of the Kantè Serpentinites (Northern Togo). International Journal of Geosciences, 13, 779-792. https://doi.org/10.4236/ijg.2022.139039