In the Pan-African Dahomeyide belt, the basic to ultrabasic complexes of Dérouvarou (Benin), Kabyè-Kpaza, Djabatouré-Anié, Agou-Ahito (Togo), and Shaï or Akuse (Ghana) form a submeridian mountainous belt that marks the suture zone [1]-[5]. These complexes are considered to be highly metamorphosed metamagmatic assemblages composed of ultramafic rocks, gabbroic metacumulates, metagabbros, pyroxenites, amphibolites, and granulites [1]-[3][6]-[10]. The Djabatouré-Anié complex, located in central Togo, is composed of small units of basic to ultrabasic rocks [10][11]. Among the units that make up this complex is the Anié unit, located in the southeast of the complex and represented by small hills (inselbergs) oriented W-E. The work of [12] partially addressed the petrographic and structural characteristics of the inselbergs of the Anié unit. According to this work, the inselbergs of the Anié unit are composed mainly of metadiorites cut by pegmatite veins of highly variable directions. Within these inselbergs, tectono-metamorphic foliation is not very pronounced and varies in both direction and dip. Like the other units with which they form the Djabatouré-Anié complex, the lithostructural and geochemical characteristics remain undefined or poorly understood [10][12]. This contribution is therefore part of the updating of geological data for the Anié unit in terms of lithostructure, petrography, and geochemistry.

The Pan-African Dahomeyide belt [13] (Figure 1) is the result of a long process of convergence that led to the collision between the Benino-Nigerian shield and the southeastern margin of the West African craton at the end of the Neoproterozoic [14]-[17]. The southern segment of the trans-Saharan orogen comprises, in its frontal part, a submeridian belt of basic to ultrabasic massifs building the suture zone (Figure 1). This string of beads marks the boundary between the external zone to the west and the internal zone to the east.

The units of the external zone of the Dahomeyide are structured in stacks of layers and scales carried westward to the Volta Basin [15][18][19]. The externalmost nappes consist of various metasediments (sandstone-quartzite, hematite, metasilexite, schist, metaconglomerate, and metadiamictite), which are the lateral and tectono-metamorphic equivalents of the lower and middle megasequences of the Volta Basin [20][21]. These nappes form the structural units of Buem and Atacora (Figure 1) and tectonically support the nappes of the external orthogneiss units (Kara-Niamtougou unit; Kpalimé-Amlamé unit; Mô plain unit) [22][23]. The latter are considered to be evidence of the Eburnean substratum (2000 ± 200 Ma), which was largely remobilized by Pan-African thermo-tectonics [24]-[28].

The massifs in the suture zone (the best known of which are those of Dérouvarou, Kabyè-Kpaza, Djabatouré-Anié, Agou-Ahito, and Shai or Akuse (Figure 1) are identified as assemblages of eclogitic to granulitic nappes overlapping the external units [9][10][19]. They consist of various granulites and sometimes eclogites, associated with metasediments (mica schists, quartzites, and garnet and kyanite bearing gneisses), fragments of pyroxenites and carbonatites, and retromorphic equivalents (amphibolites, talcschists, and serpentinites) [2][29]. These massifs are evidence of the Dohomeyides subduction-collision

Figure 1. Simplified geological map showing the main structural domains of the Pan-African Dahomeyide belt and its foreland (from [13]; slightly modified) indicating the Anié unit.

process [4][10][26][30]-[35].

The Dahomeyide internal zone corresponds to the southern portion of the trans-Saharan metacraton [36]. This is the area on which the vast Benino-Nigerian peneplain is established. Its western front is roughly delimited by the string of massifs in the suture zone (Figure 1). This portion of the metacraton consists of a gneisso-migmatitic basement complex, bearing metavolcanic-sedimentary belts and more or less invaded by granitoids of Pan-African age [11][37]-[42].

The methodological approach of our study consists of a literature review, followed by fieldwork, laboratory analyses, data processing and analysis. The literature review allowed us to summarize previous work on regional geology and the geology of the study area. The field campaign allowed us to search for outcrops, collect samples, describe these outcrops and samples in situ in macroscopic terms, take structural measurements, and survey geological cross-sections. Sampling was carried out on rocks in situ using a geologist’s hammer. We georeferenced the outcrops and sample collection stations using a Garmin eTrex Legend H GPS device. A total of seventy (70) samples were collected, some of which were used to prepare twenty-five (25) thin sections for microscopic observation focused on microstructures and mineralogical compositions. The structural measurements collected were used to create rosettes using TectonicsFP software and to produce stereograms by projection onto a Wulff net (upper hemisphere). The summary geochemical study was based on chemical analysis data (major elements, trace elements, and rare earth elements) from five (05) carefully selected rock samples. These analyses were conducted at the scientific instrumentation center at the University of Granada (Spain). The analytical approach adopted is as follows:

1) Major elements oxides were determined with a Philips Magix Pro (Pw-2440) X-ray fluorescence (XRF) equipment after melting the rock sample in a solution with tetra lithium borate. The characteristic precision as determined from standards AN-G and BEN, was better than ±1.5% (relative error) for an analyte concentration of 10 wt.%. The iron content is expressed as FeO* total. The molar ratio MgO/(MgO+FeO*) is abbreviated Mg#. Zirconium was determined with the same instrument using the same glass beads with a precision better than ± 0.2% for 5 ppm Zr. Loss on Ignition (LOI) was determined by weight difference before and after ignition of samples in a furnace. In the diagrams, oxide concentrations are reported in an anhydrous (volatile free) basis.

2) Trace elements, except Zr, were determined by an Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) after HNO₃ + HF digestion of 0.1000 g of sample powder in a Teflon-lined vessel at 180˚C and 200 psi for 30 min, evaporation to dryness and subsequent dissolution in 100 ml of 4 vol.% HNO₃; the precision, as determined from standards PMS, WSE, UBN, BEN, BR and AGV run as unknowns, was better than ± 2% for analyte concentrations of 50 ppm and ± 5% for analyte concentrations of 5 ppm.

The results of the chemical analyses are shown in Table 1. The location of the samples analyzed is shown in Figure 2. The field and chemical analysis data were compiled in Excel and imported into various software programs (GCDkit, QGIS, etc.) for specific processing. For the processing of geochemical data, we normalized the major elements to 100% on an anhydrous basis.

Table 1. Composition in major elements (wt%), trace elements (ppm), and rare earth elements (ppm) of the Anié unit rocks.

Figure 2. Schematic map of the Anié area (modified from the geological map by [12]). 1: sericite and muscovite bearing schists; 2: feldspathic quartzites; 3: serpentinites; 4: biotite and amphibole bearing gneisses; 5: 2-mica bearing paragneisses; 6: biotite bearing micaschists; 7: 2-mica bearing gneisses; 8: granulites; 9: biotite and amphibole bearing metatexites; 10: biotite and amphibole bearing orthogneiss; 11: two-mica and amphibole bearing gneiss; 12: fine biotite and amphibole bearing metagranites; 13: biotite and garnet bearing migmatitic gneiss; 14: biotite and muscovite bearing metatexites; 15: biotite bearing metatexites; 16: biotite and muscovite bearing leptynites; 17: pyroxenites; 18: amphibolites; 19: leptynitic gneisses; 20: biotite bearing quartzites; 21: traces of the main Sn+1 foliation; 22: faults; 23: cross-section itinerary; 24: overlapping contact; 25: orientation of Sn+1 planes; 26: Ln+1 lineations; 27: sampling points.

In terms of lithostructure, the granulites of the Anié inselbergs mostly correspond to large hills, sometimes with metric to decametric benches or slabs intersected

Figure 8. Traces elements vs. SiO₂ diagram [43] for rocks from the Anié unit.

Figure 9. Spectra of rare earth elements normalized relative to the primitive mantle. (a) [44]; (b) [45].

Figure 10. Geochemical classification plot showing the type (a), magmatic affinities (b, c, and d), and geotectonic context (e and f) of the Anié unit. (a): Na₂O + K₂O vs SiO₂ [46]; (b): FeOt-(Na₂O + K₂O)-MgO [47]; (c): SiO₂ vs K₂O [48]; (d): A/NK vs A/CNK [49]; (e): MgO-FeOt-Al₂O₃ [50]; (f): TiO₂/Yb-Nb/Yb [51].

by quartz-feldspar veins and veinlets. They are tectonically encased in gneiss. Deformation marks are characterized by submeridian NNW-SSE to NE-SW planar Sn + 1 structures with moderate to low dips towards the east. The linear Ln + 1 structures generally dip towards the NE and the brittle structures are oriented WNW-ESE and N-S. The gneisses of the host rock show fractures oriented NW-SE and NNE-SSW. These major directions of brittle and semi-ductile deformation structures are coplanar with those of the Sn + 1 schistosity. The subhorizontal direction of the NE-dipping lineations observed in the Anié unit indicates dextral strike-slip movements associated with the ENE-WSW strike-slip zones [13][19]. The WNW-ESE orientation of the fractures is consistent with that of the Pan-African fractures, with those oriented N-S being the most recent, in accordance with the data provided by [1]. The strike-slip movements and submeridian orientation of the veins are characteristics of the Kandi shear zone, of which the Anié unit represents the southern extension [40][41]. The microstructures observed define three phases of deformation: a Dn phase (first phase of major tectonic deformation corresponding to the collision phase) highlighted by granulitization, a Dn + 1 phase (deformation phase following the Dn phase and corresponding to the tangential phase) corresponding to the main Sn + 1 foliation and associated with amphibolitization, and a Dn + 2 phase (deformation phase following phase Dn + 1 and corresponding to post-nappe folding) manifested by the folding of the main foliation [9][10][19][40][42].

Petrographic studies of the Anié inselbergs show that they are mainly composed of granulites. These are whitish to grayish granulites with a foliated structure and coarse foliation, and medium to fine grains. They are composed of plagioclase, prismatic amphibole, biotite, pyroxene, and quartz. They contain mesocratic basic enclaves. The different mineralogical associations of the samples described show the different metamorphic episodes that have affected these rocks. These granulites are characterized by the absence of garnet. This absence of garnet can be explained by the abundance of plagioclases in these granulites and the low-pressure conditions in which they were formed [2][10][11]. Amphibolization initiated the transformation of garnet-free granulites into amphibolites characterized by amphibole, plagioclase, and quartz paragenesis, indicating the transition from granulite to amphibolite facies [2][4][9][10]. The surrounding gneisses are grayish, with a foliated structure, sometimes augen, and a granoblastic texture composed of feldspar, quartz, biotite, muscovite, and amphibole.

The geochemical study of the Anié unit shows that the rocks in this unit are metaluminous and have calc-alkaline affinities. They are mainly granulites. The richness of these rocks in TiO₂ and P₂O₅ is consistent with the appearance of titanium and apatite in these rocks, respectively [27][52]. The depletion in Al₂O₃ and CaO suggests fractional crystallization of pyroxenes and calcic plagioclases in these rocks. Their poverty in FeOt and MgO indicates fractional crystallization of ferromagnesian minerals (pyroxene and amphibole) in these rocks [10][53]. Negative anomalies in Nb-Ta and Zr indicate the influence of crustal material recycling. The good linear correlation in [9] diagrams reflects the role of fractional crystallization in the differenciation process during the evolution of the parent magmas of the rocks in this unit. The negative anomalies in Nb-Ta and Zr observed in the granulites of the Anié unit are characteristic of calc-alkaline magmas in subduction zones [10][11][52]. The sloping aspect of the rare earth spectra is marked by low HREE fractionation and is thought to be associated with subduction. Geotectonic discrimination diagrams indicate that these rocks are calc-alkaline and derive from an OIB (oceanic island basalts) type mantle source that was subsequently modified in a subduction context [11][27][54]. This subduction resulted from submeridian thrusting that contributed to the formation of the Pan-African Dahomeyide belt [13].

It should be noted that the geochemical conclusions of this study are based on a limited number of samples (n = 5). Therefore, the proposed interpretations should be considered preliminary, calling for further research on a more extensive data set in order to better constrain the geological evolution of the Anié unit studied.

The lithostructural study of the Anié inselbergs revealed planar and linear structures expressing phases Dn, Dn + 1, and Dn + 2 of Pan-African tangential tectonics. The granulites of these inselbergs are tectonically encased in gneiss.

Petrographic studies of these inselbergs have identified granulites with a mineralogy of plagioclase, clinopyroxene, amphibole, biotite, and quartz. This paragenesis shows that these rocks underwent metamorphism in the granulite facies with retrogression in the amphibolite facies. The Anié unit therefore underwent tectonic-metamorphic evolution ranging from granulitization to retrograde metamorphism in the amphibolite facies.

Geochemical characterization shows that the rocks of the Anié unit are metaluminous and calc-alkaline. Inter-element variations indicate that these rocks evolved from a fractional crystallization process accompanied by crustal contamination. They were formed in an orogenic context. They are crustal basement rocks uplifted by Pan-African tectonics into crustal complexes. Geochemical trends clearly indicate their emplacement in a subduction context.

The rocks of the Anié unit show enrichment in LREE relative to HREE and sub-flat HREE spectra. Multi-element spectra show that most rocks are rich in mobile elements (Ba) and have negative anomalies in Nb-Ta and P. These characteristics are similar to those of rocks derived from an enriched mantle source and metasomatized during a subduction event.