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Origin and Evolution of the Formation of the Nyong Series in the Western Border of the Craton

Ndema Mbongue J. L., Ngnotue T., Ngo Nlend C. D., Nzenti J. P.*, Cheo Suh E.

Laboratory of Petrology and Structural Geology, University of Yaoundé I, P.O. Box 3412, Cameroon

Abstract: The Nyong series at Edéa and Eseka area is located in the western edge of the Congo Craton and comprise three distinguish rock units namely: metasedimentary rocks unit (schists, garnet-rich micaschist); meta-igneous rocks unit (pyroxene-rich gneiss, garnet-rich charnockitic gneiss, charnockitic gneiss, biotite-rich gneiss, amphibole and biotite-rich gneiss, garnet and amphibole-rich gneiss, amphibolite, pyribolite, pyrigarnite, garnet-rich amphibolite) and a unit of the rocks resulting from the melt (migmatite, TTG) displaying quartzo-feldspathic segregation arising from either in situ partial melting or injection along dykes or ductile shear zones. The meta-igneous rocks derived from (i) intermediate to basic tholeiitic rocks with high TiO₂ (0.6-3.47%) contents compatible with the extensive orogenic domain and (ii) calc-alkaline protolith display high FeO*/MgO (1.5-3.31) ratios which is in accordance with the typical domain of collisional orogeny. The chemical patterns of metasedimentary rocks are those of shale. The average Nb/Y (0.004) ratio and the fractioned REE patterns suggest that the contribution of alkaline vulcanite and a continental environment can be envisaged for these metasedimentary rocks.

Keywords: nyong series, extensive orogenic domain, domain of collisional orogeny, shale, continental environment

Received April 22, 2014; Revised April 24, 2014; Accepted April 27, 2014

Citation: Ndema Mbongue J. L., Ngnotue T., Ngo Nlend C. D., Nzenti J. P., Cheo Suh E., Origin and Evolution of the Formation of the Nyong Series in the Western Border of the Craton. Journal of Geosciences and Geomatics, 2 (2). 62-75. January 2014. doi:10.12691/jgg-2-2-4

1. Introduction

The Nyong series, one of the main block of the Congo Craton is situated in the western border of the Congo Craton in Cameroon [1,2]. The nature and the origin of the western of Congo Craton are not very well constrained. As a result, questions about the older events remain incompletely answered. To characterise the western border better, an area of 500 km² around Edéa and Eséka was mapped and petrologically and geochemically analyzed. The study area represents an Archean/Proterozoic lower crustal segment in which unaltered magmatic and metamorphic are well exposed. Because little data exist on the rock types in the western border of the Congo Craton, the main objectives of this study is to elucidate the nature, the origin, the geological significance and crustal evolution for a better understanding of the western border of the Congo Craton.

2. Geological Setting

The Congo craton is a large sub-circular mass of about 5711000 km² in area, and has a diameter of about 2500 km, comprising Archaean crust, early to mid-Proterozoic fold belt and Proterozoic cover [3]. Archaean formations form a ring of Archaean crusts of diverse sizes, (in clockwise succession from the south: Kasai–Angolan composite, Chaillu-Ntem complex, Gabon and Cameroon, Bouca, Bomu–Kibalian, Tanzania and Zambian), surrounding the unusually large, (1000–1200 km), sub-circular central late Proterozoic to Phanerozoic-ﬁlled Congo Basin [4]. The northwestern margin of the Archaean Congo craton in Southern Cameroon is represented by the Ntem complex [4,5,6], which is bordered in the north by the Yaoundé Group (e.g. [7,8,9,10]), of the Pan-African orogenic belt in Central Africa (Figure 1). The Ntem Complex is divided into two main structural domains: the Nyong series, to the northwest end, and the Ntem series, in the south-central area. The Ntem series is dominated by massive and banded plutonic rocks of the charnockite suite and by intrusive tonalites, trondhjemites and granodiorites. Some of these bodies were previously dated at ca. 2.9 Ga [2,11,12]. The igneous plutons contain large xenoliths of supracrustal rocks interpreted as remnants of greenstone belts and dated at ca. 3.1 Ga [13]. The studied areas, Edéa and Eséka (Figure 1 & Figure 2) belong to the Nyong series. This series consists of metasedimentary and metavolcanic rocks, as well as syn-to late-tectonic D₂ granitoids and syenites [1]; and displays three groups of ages [2,12,14,15]: (1) Archaean U-Pb ages (2500–2900 Ma) were obtained from detrital zircons in metasedimentary rocks, and presumably magmatic zircons in charnockites and migmatitic gneisses; (2) Palaeoproterozoic U-Pb zircon ages of ca. 2050 Ma corresponding to a thermometamorphic event associated with an important crustal melting and emplacement of some granite and syenite massifs, and (3) a Neoproterozoic U-Pb zircon age of 626 ± 26 Ma corresponding to the metamorphic effect of Pan-African thrusting. This unit includes some Archaean parts of the Ntem Complex that were reworked during a Palaeoproterozoic event, and new Palaeoproterozoic material that was accreted to the Archaean craton.

Figure 1. Geological map of south-west after Maurizot et al. [2], modified by Shang et al. [3] showing the studied areas (Edéa and Eséka)

Figure 2. Geological map of Edéa and Eséka areas, modified from [2]

3. Analytical Methods

Fresh rock samples were collected from which more than 50 thin sections of metasediments, meta-igneous and migmatite were prepared and petrographically investigated. 52 representative samples (34 from Edéa, 18 from Eséka) were selected for chemical analysis. Major elements were analysed by inductively coupled plasma atomic emission spectrometry (ICP-AES), and trace element by inductively coupled plasma mass spectrometry (ICP-MS) on a VG-Plasma Quad STE ICP mass spectrometer at the Institute of Mineralogy and Geochemistry of the University of Lausanne in Switzerland. The samples were dissolved in a Teflon pressure bomb, using a 1:1 mixture of HF and HClO₄ at 180°C, and then taken up in HNO₃ solution with an internal standard. After dissolution in HF-HClO₄, the samples were taken up in a mixture of HNO₃, 6N HCL and HF and diluted. These solutions were measured within 24 hours after dilution, to prevent absorption of high field strength elements (HFSE) on the sample bottle. Analysis uncertainties are currently better than 1% for major elements and 5-10% for trace element concentrations. Analysis precision for rare earth elements is estimated at 5% for concentrations > 10 ppm and 10% when lower. Representative analyses of different rock types are given in Table 1, Table 2, Table 3 and Table 4. Geochemical data were computed and the different and discriminate diagrams in the various plots displayed in Figure 4 to Figure 7.

Table 1. Major (wt.%) and trace (ppm) element analyses of representative garnet-rich micaschist from Edéa area

Table 2. Major (wt. %) and trace (ppm) element analyses of representative pyroxene-rich gneiss, garnet-richcharnockitic gneiss and biotite-rich gneiss from Edéa area

Table 3. Major (wt.%) and trace (ppm) element analyses of representative amphibole and biotite-rich gneiss, charnockitic gneiss, pyribolite and amphibolite from Edéa area

Table 4. Major (wt.%) and trace (ppm) element analyses ofrepresentative garnet and amphibole-rich gneiss, pyrigarnite and garnet-rich amphibolite from Eséka area

4. Results

4.1. Rock Lithology

The Edéa and Eséka areas are made up of three distinguish rock units. The first unit is of sedimentary parentage, the second of clear igneous origin and the third from the melting. All these three units contain quartzo-feldspathic segregations arising from either in situ partial melting or injection along dykes or ductile shear zones. However detail observations and sampling were essentially carried out by mapping. Mineral abbreviations are given by Kretz [16].

4.1.1. The Metasedimentary Unit

This unit is composed of garnet-rich micaschist and schist. Garnet-rich micaschist are the most important types and outcrop as flagstones (Figure 3 a). They are fine to medium-grained with alternating millimetre quartzo-feldspathic and ferromagnesian layers. Granoblastic microstructures prevail in this rock type but flaser and mylonitic ones are frequently observed. Garnet-micaschist are composed of biotite (8-10%), muscovite (10-20%), garnet porphyroblasts (15-20%), quartz (25-30%), plagioclase (20-25%), accessories mineral are monazite and oxide (Figure 3 b).

Figure 3. (a)Flat outcrop of garnet bearing micaschist; (b) Photomicrograph of garnet-bearing micaschist displaying garnet porphyroblast containing biotite and muscovite as inclusions. (c)Detail outcrop view of pyroxene-rich gneiss. Note thethin alternation of ferromagnesian and quartzo-feldspathic layers. (d)Photomicrograph of pyroxene-rich gneiss showing almond-shaped orthopyroxene crystal displaying a rim composed of Qtz + Pl + Ox. (e)Outcrop view of garnet-rich charnockitic gneiss; (f)Orthopyroxenemegacrystal displaying corona of Qtz + Pl in garnet-bearing charnockitic gneiss; (Qtz: quartz; Pl: plagioclase;Grt: garnet; Opx: orthopyroxene; Ms: muscovite; Bt: biotite; Ox: opaque oxide)

4.1.2. The Meta-Igneous Unit

The meta-igneous rocks consist of pyroxene-rich gneiss, charnockitic-rich gneiss, pyribolite, amphibolite, amphibole and biotite-rich gneiss, biotite-rich gneiss, garnet-amphibole-rich gneiss, garnet-rich amphibolite and pyrigarnite. Granoblastic microstructures prevail in all rock types, although flaser and mylonitic ones are also commonly observed.

Pyroxene-gneiss (Figure 3 c) occurs as dome and block. They are fine to medium-grained, dark-gray colored rocks displaying alternating millimetre to decimetre ferromagnesian and quartzo-feldspathic layers. They display granoblastic and corona microstructures and composed of quartz (28-30%), plagioclase (15-18%), almond-shaped orthopyroxene (20-22%) showing corona composed essentially of Qtz + Pl and Qtz + Pl + Ox (Figure 3 d); hornblende (< 9%), biotite (15-17%), ovoid and kelyphitic garnet (11-13%) crystals displaying symplectitic association of Hbl + Bt + Qtz + Pl. Accessories mineral are oxides.

Garnet-charnokitic gneiss (Figure 3 e) are medium-grained and outcrop as dome. They display coloured minerals (quartz and feldspath are blue-yellowish in colour). The microstructures are granoblastic although rims are observed on some minerals. These rocks consist of quartz (37-40%), plagioclase (< 7%), almond-shaped orthopyroxene crystals (18-20%) displaying corona of Qtz + Pl + Px ± Bt, biotite (13-15%) (Figure 3 f), garnet porphyroblasts (23-25%) surrounded by Qtz + Pl + Bt-rich rim. Monazite and oxides are accessories minerals.

Garnet and amphibole-rich gneiss outcrop as flagstone and blocks; they are composed of hornblende (15-18%) partly replacing pyroxene and displaying transformation in quartz and biotite, quartz (20-22%), plagioclase An₂₆₋₃₁, (20-21%), K-feldspar (< 5%), orthopyroxene (< 10%), garnet (10-12%), biotite (18%). Accessories are zircon and apatite.

Pyrigarnite occurs as blocks and as boudins, they display granoblastic and corona microstructures. They are made up of quartz (17-20%), plagioclase An₂₆₋₃₁ (13-15%), K-feldspar (< 10%), orthopyroxene (27-30%) with rim of Qtz + Pl; hornblende (< 5%), garnet porphyroblasts (22-25%) with rims of Qtz + Kfs, Pl + Px, Pl + Qtz. Accessories are apatite, rutile and oxides.

Garnet-rich amphibolite occurs as blocks, boudins and flagstone and display granoblastic and corona microstructure. They composed of quartz (17-20%), amphibole (27-30%) partly replacing pyroxene; kelyphitic garnet (22-25%) displaying Pl + Qtz-rich rim and symplectitic association of Hbl + Px; pyroxene (< 12%), plagioclase An₂₄₋₂₆ (12-15%). Accessories are apatite, zircon and oxides.

4.1.3. The Migmatites and TTG

Rocks resulting from the melting are composed of migmatites and TTG (tonalites, trondhjemites and granodiorites) suit. Migmatites are the most dominant type and occur as flagstones. They composed of quartz (27-30%), plagioclase (25-30%), perthitic K-feldspar (7-10%), amphibole (<10%) displaying transformation of biotite; garnet (< 10%) showing Pl + Qtz-rich rim; biotite (5-10%), pyroxene (12-15%). Accessories are zircon and oxides. TTG rocks are well known in the Congo Craton (e.g. [1,2,3,6,17,18,19,20,21]).

4.2. Whole-Rock Chemistry and Nature of the Protoliths

Chemical composition of the studied rocks is presented in Table 1 to Table 4. We used the Fe₂O₃t + TiO₂ +CaO vs. Al₂O₃ whole rocks diagram of De la Roche [22] to determine the protolith of the diﬀerent petrographic type. In this diagram, meta-igneous define a chemical trend parallel to the igneous rocks origin, while metasediments plot in the shale ﬁeld (Figure 4). These metashale are similar to other shale documented in the literature (e.g. [23-30]).

Figure 4. Fe₂O₃t + TiO₂ +CaO vs. Al₂O₃ diagram of De la Roche showing the whole rock chemical composition of the studied rocks

Figure 5. Distinctive variationdiagrams for the metasedimentary unit. (a) Pot of TiO₂ vs. Al₂O₃, the solid line corresponds to the TiO₂/Al₂O₃ ratio for clay afterGoldschmidt ; (b) PAAS normalised-REE patterns

4.2.1. Metasedimentary rocks

Garnet-rich micaschist display all the features of pelitic rocks with FeO, MgO, K₂O and TiO₂ closely correlated to Al₂O₃ (Table 1; Figure 5 a). All these elements are anticorrelated with silica suggesting that these gneisses were composed of a quartz-clay mixture. Their average composition corresponds closely to those of Post-Archean shale [25]. Their TiO₂ content (0.76-0.82%) and average TiO2/Al2O3 (0.04) ratio are similar to those of clay (0.040 according to Goldschmidt, [31] 1954; 0.03-0.05 according to Taylor and McLennan, [25], similar to average terrigeneous and correspond to values given for continental or near-shore argillaceous and arenaceous sediments [32]. The overall chemical patterns are closed to the Neoproterozoic metashales of Yaoundé [7,33,34] and Paleoproterozoic metashales of Banyo [26]; Kekem [35], and Bossangoa-Bossembele in Central African Republic [29]. Trace element contents are high suggesting that these rocks are distinct from passive and active margin greywackes [36], but have some similarities with alkaline rocks. The average Nb/Y ratio (0,004) and the fractionated REE (601-1170 ppm) elements suggest that these metasediments could be derived from reworking of alkaline volcanic rocks. Their REE patterns are fractionated, with a high positive Eu anomaly (Figure 5 b).

4.2.2. Meta-igneous Rocks

According to the silica contents, pyroxene-rich gneiss are intermediate rocks (55 < SiO₂ <61.3%). These chemical compositions are those of quartz-diorite (Figure 6). They are rich in Al₂O₃, Fe₂O₃, MgO, Na₂O, Na₂O + K₂O, Ba, Rb but poor in K₂O, CaO, Sr (Table 2). TiO₂ contents and FeO*/MgO ratios varies from 0.35 to 0.76% and 1.35 to 3 respectively. The iron enrichment with the values of titanium in these rocks is comparable to those of the tholeiitic series. Also, CaO/TiO₂ (3.19-10) and Al₂O₃/TiO₂ (22-47.86) ratios correspond closely to those of oceanic tholeiite [37,38]. However Y/Nb (2.15-241.35) ratios > 1 are similar to those of tholeiite ([39] Pearce and Cann, 1973). Plotting in the TiO₂ vs. FeO*/MgO diagram [40] (Figure 6), pyroxene-bearing gneiss fall in the tholeiites field. Their REE patterns (Figure 7 a) are very strongly fractionated (LaN/YbN = 6.67-353.47); LREE enriched (CeN/SmN = 0.81-2.15), HREE enriched (Gd/YbN = 1.56-2.36) and displays a high positive Eu anomaly (Eu/Eu* = 0.88-17.61). All these features are due to the accumulation of pyroxenes. Their overall trace element patterns (Figure 7 b) show Ba, Th, Nb, Sr, Zr, Yb anomaly, indicating after Thompson et al. [42] their crustal and mantle origin.

Figure 6. TiO₂ vs. FeO*/MgO diagram of Miyashiro [40] showing calc-alkaline and tholeiitic characters of meta-igneous rocks from Edéa and Eséka areas

Figure 7. Chemical patterns for meta-igneous rocks from Edéa area. (a, c, e) Chondrite-normalised REE patterns.Normalized values after Evensen et al. [41]; (b, d, f) Chondrite-normalised trace element patterns. Normalised values after Thompson et al. [42]

Garnet-bearing charnokitic gneiss are slightly scattered in composition (Table 2). These rocks are silica-rich with a total range of SiO₂ between 66.23 to 72%; these chemical compositions are those of granite (Figure 4). They display distinctly higher CaO, Na₂O, Ba, Sr, REE and low TiO₂, MgO, K₂O contents and the total alkali concentrations varies from 3.32-5.65%. Ba/Sr, Ba/Rb and K/Rb ratios are similar to those observed in continental calc-alkaline igneous suites [43-61]. These rocks display the characteristics of calc-alkaline series (Figure 6). Their REE patterns (Figure 7 c) are very strongly fractionated (LaN/YbN = 96.8-598) and resemble those of pyroxene-gneisses; LREE enriched (CeN/SmN = 0.7-2), HREE enriched (Gd/YbN = 0.9-3) and display a high positive Eu anomaly (Eu/Eu* = 9.8-15). Their overall trace element patterns (Figure 7 d) show Ba, Th, Nb, Sr, P, Zr, Ti, Yb anomalies, indicating after Thompson et al. [42] their crustal and mantle origin.

Amphibole and biotite-rich gneiss have the composition of gabbro and quartz-diorite rocks (Figure 4). They are deficient in silica (49.13 to 53.36%) and rich in K₂O (≈ 4%). They also have high FeO and MgO contents and exceptionally high P₂O₅ and REE contents (Table 3). The Figure 6 shows that these rocks have tholeiitic affinity. The REE patterns (Figure 7 e) are strongly fractionated (LaN/YbN = 7.32-23) with high LREE enrichment (CeN/SmN = 1.36-2.41), HREE depletion and negative Eu anomaly (Eu/Eu* = 0.44-1.15). Trace element distribution patterns display Ba, Th, Nb, Sr, Sm, Ti, Tm anomalies (Figure 7 f). All these chemical features are similar to those of ultramafic and mafic varieties of alkaline rocks [7,62,63,64,65,66]. Garnet-bearing amphibolite and pyrigarnite have the composition of gabbroic rocks (Figure 6) with a silicate content ranging from 46 to 49% and from 47 to 53% respectively (Table 4). The higher FeO and MgO contents together with high Cr and Ni contents and with constant Y and Nb contents suggest that their more mafic composition is related to accumulation of pyroxene rather than to a less differentiated nature. TiO₂ (0.56-1.09%) contents varies with FeO (12-15.5%) contents, these variations are similar to those of magmatic tholeiitic series. The REE pattern (Figure 8 a & Figure 8 b) are sub-flat and fractionated (LaN/YbN = 0.75-8.15) with à LREE enrichment of 10 to 100 times chondritic values (CeN/SmN = 0.83-6.33), and a weak negative Eu anomaly (Eu/Eu* = 0.61-1.2). This suggests that they could be of transitional or alkaline affinity [52]. Trace element distribution patterns of garnet-bearing amphibolites display Ba, Th, Nb, Sr, Ti, Tm (Figures 8c & 8d), indicating after Thompson et al. [42] their crustal and mantle origin; and Rb, Nb, La, Sr, Sm, Ti for pyrigarnites anomalies (Figure 8 d), indicating their crustal origin [42].

Figure 8. Chemical patterns for meta-igneous rocks from Eséka area. (a, c, e) Chondrite-normalised REE patterns.Normalized values after Evensen et al. [41]; (b, d, f) Chondrite-normalised trace element patterns. Normalised values after Thompson et al. [42]

5. Discussion

The main lithology of Edéa and Eséka area corresponds to (1) a metasedimentary unit rocks make up of garnet-bearing micaschist and schist with chemical patterns have similarity with Neoproterozoic and Paleoproterozoic metashale. Schist are well known in the neighboring series: Yaoundé series [7,67], Mbalmayo-Bengbis- Ayos series [7,67,68]; (2) a meta-igneous unit rocks comprise pyroxene-rich gneiss, garnet-rich charnockitic gneiss, charnockitic gneiss, amphibole and biotite-rich gneiss, biotite-rich gneiss, pyribolite, amphibolite, garnet and amphibole-rich gneiss, garnet-rich amphibolite and pyrigarnite; these rocks display chemical patterns similar to those of the Pan-African and Paleoproterozoic rocks in central Cameroon regions such as the Maham III [60] and Banyo mafic meta-igneous rocks [26,66]. All the major and trace element patterns of metasediments have the composition of shale and similar to Post-Archean shale [25] and close to those of continental or near-shore argillaceous sediments. The average Nb/Y (0.004) ratio and the fractioned REE patterns suggest that the contribution of alkaline vulcanite and a continental environment can be envisaged for these metasediments.

Metasedimentary rocks extend to the Yaoundé series [7,30,69] and Ayos-Mbalmayo-Bengbis series [7,68] which is largely dominated by the Neoproterozoic metasedimentary sequence; the protoliths of these series were deposited in a passive margin environment at the northern edge of the Congo Craton. The lithological assemblage constituting the Yaoundé series clearly corresponds to a depositional environment related to an emerged continent and thus a derivation of the sediments from the Congo Craton seems likely.

The investigated Edéa and Eséka meta-igneous rocks exhibit petrographical and chemical compositions characteristic of intermediate to basic tholeiitic and calc-alkaline rocks [40] derived from igneous protoliths varying from granites to gabbros. The calc-alkaline rocks display high FeO*/MgO (1.5-3.31) ratios. The tholeiitic rocks are characterised by high TiO₂ (0.6-3.47%) contents and their chemical compositions correspond to those of continental tholeiite [25,70,71,72], with Th/Ta ratios and Nb relatively high (Table 2, Table 3 and Table 4), characterizing the tholeiites linked to rifting [73,74,75,76,77]. The calc-alkaline affinity characterizes the compressive orogenic domain while the tholeiitic affinity characterizes the distensive orogenic domain. This double affinity of the source (calc-alkaline/tholeiite) indicates a double paleoenvironment: calc-alkaline affinity indicates thickening environment while tholeiitic affinity characterizes extensive environment. This result is similar to those obtained in the central domain of the Pan-African fold belt, where Pan-African calc-alkaline rocks (biotite-amphibole gneiss, orthogneiss, S-C mylonitic granites, leucogranites) and Paleoproterozoic tholeiitic rocks (garnet-amphibole gneisses, banded amphibolites, garnet-amphibolites) are recorded in Tonga region [78].

The nature of the igneous source can be constrained using the geochemical signatures of the meta-igneous rocks. The REE and multi-elements patterns (Figure 7 and Figure 8) suggest genetic processes involving both participation of the crust and the mantle [42]. The recent work of Tanko Njiosseu [78] also recorded crustal and mantle origin for the meta-igneous rocks of Tonga located in the central domain. This allows us to conclude that, the Nyong series and central domain of Pan-African fold belt represent an ancient continental domain which has been reworked. The rocks of these domains have been affected by thrusting, and this thrust continues towards the East, forming the Oubanguides Nappe in the Republic of Central Africa and towards the NE Brazil in the Borborema province.

The high contents of LREE (10 to 100 times Chondrite values) in these rocks could be related either to the enrichment of their source materials in LREE or to the presence of mineralization fluids. Moreover the spider diagrams characteristically display negative anomalies for Ba, Th, Nb, Sr, P, Zr, Ti, Yb. These anomalies result either from the low content of these elements in the source, or their retention in the residue during partial melting.

6. Conclusions

1. The Nyong series comprises a metasedimentary unit rocks make up of garnet-bearing micaschist and schist, and main meta-igneous unit rocks (pyroxene-rich gneiss, garnet-rich charnockitic gneiss, charnockitic gneiss, amphibole-biotite-rich gneiss, biotite-rich gneiss, pyribolite, amphibolite, garnet-amphibole-rich gneiss, garnet-rich amphibolite and pyrigarnite).

2. Metasediments have the composition of shale and close to those of continental or near-shore argillaceous sediments. Meta-igneous rocks exhibit the compositions of intermediate to basic tholeiitic and calc-alkaline rocks derived from continental igneous protoliths varying from granites to gabbros.

3. The Study area has experienced the compressive and the tholeiitic orogenic domain indicatives of a duality paleo-environment: thickening environment and distensive environment.

4. The REE and multi-elements patterns are in accordance with genetic processes involving both participation of the crust and the mantle.

5. The recent work in Central domain of the Panafrican North Equatorial Fold Belt [26,56,78] which also documented crustal and mantle origin for the meta-igneous rocks allows us to conclude that, the western border of the Congo craton and central domain of the fold belt represent an ancient continental domain which has been reworked during the later events (Pan-African orogeny for the central domain and probably Paleoproterozoic orogeny for the Nyong series). The rocks of these domains were thrust towards the South and towards the East, forming the Oubanguides Nappe in the Republic of Central Africa and towards the NE Brazil in the Borborema province.

Acknowledgements

The data presented here form a part of the first author’s Ph. D thesis supervised by J.P. Nzenti and E. Suh Cheo. Thanks are due to We gratefully acknowledge the Institute of Mineralogy and Geochemistry of the University of Lausanne (Switzerland) for providing facilities for whole rock geochemical analyses. Thanks are due to Dr Ganno Sylvestre (University of Yaoundé 1) for the comments and suggestions of earlier version of the manuscript.

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