Geochemistry and Petrogenesis of Basic and Ultrabasic Rocks Elogo Complex in Ivindo Archean Block (Congo Craton): Geodynamic Implications ()
1. Introduction
In western central Africa, the rocks of the Congo craton outcrop in Congo, Gabon, and Cameroon. In the northwest of the Congo, rocks of the Archean age form the Ivindo Archean Block (IAB) up to Gabon and the Ntem complex in southern Cameroon (Figure 1) [1] [2] .
The IAB is like other known archean cratons in various places in Africa and elsewhere (West African, Kalahari, Sao Francisco cratons), composed of various sequences of migmatitic granito-gneiss associated with greenstone belts of various dimensions. The granitoïdes are composed in this western part of the Congo craton of charnockites, tonalites, granodiorites, diorites, trondhjemites and syenogranites [3] [4] [5] [6] . In the IAB in Congo, greenstone belts are known in Badondo, Avima, Nabeba, Egbala mounts, and the Elogo region. The best-studied Belinga greenstone belt found in Gabon led to the definition of the Belinga group [4] . These belts are composed of ultrabasic rocks, amphibole-pyroxene rich rocks, amphibolite, chlorite-sericite-schists, talc schists, and banded iron formations. These formations are metamorphosed into greenschist and amphibolite facies. Greenstone belts are geological markers contributing to the geodynamic knowledge of the emplacement of known Archean cratons. They trace the magmatic properties of the mantle sources (composition, genetic temperature) up to 3.8 Ga [7] [8] [9] . In Congo, only some published studies are devoted to greenstone belts. The work often focuses on the BIF [2] [10] associated with these green rocks, which form iron deposits that are currently relatively well-mapped and are the subject of economic projects. The present study relates to the Elogo-Bamegod zone, where outcrops basic-ultrabasic rocks called the “Elogo Complex”. The work is focused on the petrology and geochemistry of the basic rocks sampled in these localities. The geochemistry of basic rocks allows the construction of affinities and magmatic sources and the evaluation of crust-mantle interactions to reconstruct the geodynamic context of their formation and related processes.
2. Geological Setting
The Elogo-Bamegod area represents one of the regions where outcrop rocks forming greenstone belts are distributed in the IAB (Figure 2(A)) attached to the Congo craton. It occupies the eastern portion of the greenstone belt of the locality and is overlain in major unconformity by the Paleoproterozoic to the Neoproterozoic cover of the Sembe-Ouesso basin. In the north of this locality, the granitoids are formed of charnockites, tonalites/granodiorites, and peraluminous, calc-alkaline trondhjemites, relatively rich in sodium, dated between 3.23 Ga and 2.7 Ga and placed in a conceptual model of continental crust evolution [5] .
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Figure 1. Simplified regional geological map of the Archean Congo-Gabonian Craton with the location of studied areas (IAB: Ivindo Archean Block) modified from [1] [2] .
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Figure 2. Cartographic maps: A—of Ivindo Archean Block (IAB) after [2] ; B—expanded area of the Elogo ultrabasic Complex modified from the Geological map of Ivindo basement of BRGM. Red points = tholeiitic basalts; blue points = basalts and calc-alkaline andesitic basalts, black points = calc-alkaline dacites.
In the Elogo area, granito-gneiss (Figure 2(B)) are composed of anatectic, migmatitic granites, diorites, microgranites, gneiss and migmatites in amphibolite to granulite metamorphic facies. The greenstone belt comprises metabasalts, amphibolites, undifferentiated basic and ultrabasic rocks, micaschists, quartzites, and BIF. They locally show East verging folds, accompanied by penetrative crenulation schistosity mainly oriented NNW-SSE, and often masking the flow schistosity. Further to the East, the formations are affected by the eastern vergence thrust. The Elogo-Bamegod zone is affected by a major shear-oriented E-W with locally associated shears more or less NNW-SSE to NNE-SSW.
3. Samples Material and Analytical Method
3.1. Samples Material
The greenstone samples were taken from the river banks, hills flanks, and artificial banks located at Elogo and Bamegod and mapped as Elogo complex. Thin sections were made on well selected samples.
3.2. Analytical Method
Geochemical analyses of major, trace and rare earth elements (REE) were done to evaluate the petrogenesis of rocks. Samples were analyzed in the ALS laboratory of Canada according to ALS protocol. Quality control is realized with international geostandards. Major, Ba, Cr, Sc, and trace elements are analyzed by ICP-AES. The rest of the trace and the rare earth elements were analyzed by ICP-MS. Total C and total S were determined using a LEXO analyzer.
4. Results
4.1. Macroscopic Description
The metabasite rocks outcrop widely in rivers (Ebeck, Maziézié, Melaba, Kampala, Guinée, Namoumédia), in some mountains, and on artificial slopes. They are composed of the most abundant amphibolites, chlorite schists, and epidotes. They appear in massive form with little penetrable schistosity, in the form of levels with strongly penetrable schistosity or gneissified and/or migmatized. Folds accompanied by flow schistosity (S1) and crenulation (S2) are locally visible (Figure 3). Fracture planes intersecting the schistosity are present and determine strongly developed shears-oriented E-W to N-S, sometimes NE-SW.
4.2. Microscopic Description
Twenty-eight (28) thin sections were made from samples of amphibolites, chlorite schists, and epidotes, which are studied under an optical microscope (Figure 4).
Amphibolites come in several compositions with the presence or absence of mineralogical varieties as well as their abundance. They are represented by samples Kamp 107 and 81, Mela 96 and Nam 43c. Sample Kamp 107 is composed of tremolite, brown hornblende, actinote, magnetite, and spinel with some chloritized or carbonatized hornblendes. The texture is nematoblastic. Sample Kamp 81 contains tremolite, brown hornblende, actinote, chlorite, epidote and opaque minerals with nematoblastic texture. Sample Mela 96 is made of brown and green hornblende, tremolite, actinote, epidote, chlorite and plagioclase with nematoblastic texture. Sample Nam 43c comprises porphyroblastic hornblendes disseminated in a microlitic matrix of biotite, epidote, chlorite, quartz, plagioclase, carbonate, and sulfide (Figure 4(A)).
Chlorite schists are often in contact with amphibolites and are represented in samples Maz 183b and Mela 101. Sample Maz 183b is composed of chlorite, muscovite, calcite, quartz, talc and opaque minerals with a lepido-granoblastic texture. Sample Mela 101 is composed of chlorite, calcite, quartz, cordierite, muscovite, rutile, sulpfide and hematite, with lepidoblastic texture. Epidotite is represented by samples Mela 103b and Bam 12. They were sampled in the locality of Bamegod. Sample Mela 103b is made of epidote, chlorite, calcite, cordierite, muscovite, plagioclases and rutile with lepido-granoblastic texture. Sample Bam 12 contains a large portion of the epidote cluster (pistachite) associated with actinote, tremolite, muscovite, zoisite, and opaque minerals in a granoblastic texture (Figure 4(B)).
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Figure 3. Examples of some basic rocks outcropping in Elogo-Bamegode area; A, B—Massive amphibolite with strong fracturation; C—Amphibolite with crenulation schistosity, D, E—Chlorite schists with crenulation schistosity, F—Altered epidote.
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Figure 4. Microscopic photography of amphibolites, chlorite schists, and epidotes from Elogo complex. A: Basic and ultrabasic amphibolites (Kamp 107 and 81, Mela 96), and hydrothermal circulation (Nam 43c). B: Basic and ultrabasic chlorite schists (Mela101, Maz 183) and epidotes (Mela 103b and Bam 12). Bt-biotite, Cal-calcite, Chl-chlorite, Crd-cordierite, Ep-epidote, Hbl-hornblende, Hem-hematite, Om-opaque, mineral, Mag-magnetite, Plplagioclase, Qtz-quartz, R-Qtz Quartz recrystallisation, Rt-rutile, Tr-tremolite.
4.3. Geochemistry and Petrology Classification
Twenty-eight (28) samples of amphibolites, chlorite schists and epidotes from the Elogo complex are analyzed. Their results are presented in Table 1. The SiO2 is comprised of between 37.50% and 68.40% values. Al2O3 varies from a low value of 2.74% to a high value of 15.60%. Fe2O3 is situated from 5.99% to 13.65%, MgO is comprised between 2.75% to 25.5%, CaO varies from 3.01% to 11.20%, Na2O is between 0.01% and 3.75% with one sample contains less of 0.01%, K2O has a value comprised between 0.01% and 2.31% with three samples showing less of 0.01%, MnO varies from 0.09 and 0.22, P2O5 is comprised between 0.01 and 0.40 and TiO2 is situated from 0.12% to 1.13%. The LOI is low (0.85%) to medium (1.13% to 1.53%) in a few samples and very high in most samples, reaching up to 16.4% and confirming a strong hydrothermal and/or metamorphic alteration observed in the field, despite the possible hydrothermal or metamorphism alteration Na2O + K2O vs. SiO2 diagram of TAS [11] plot the major rocks in sub alkaline tholeiitic basalt, a few samples in dacite and a rare sample in basaltic andesite (Figure 5(A)). In the AFM diagram of [12] , the samples are mostly in the tholeiitic series, although far from the line of separation of the calc-alkaline field. Two samples are, however, in this last field. In addition, all basic and ultrabasic rocks are plotted in the tholeiitic basalt field. (Figure 5(B)).
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Table 1. Composition of major elements (%) and traces (ppm) of basalts and dacites of Elogo complex.
The samples considered tholeiitic basalts (basic and ultrabasic rocks) have a ratio of FeOt/MgO = 0.34 à 2.13, A/NK = 4.03 à 10.22 and A/CNK = 0.66 à 1.52, low alkaline contents (Na2O + K2O = 0.02% to 3.46%), high MgO (5.36% to 26%) and Fe2O3 (8.94% to 13.65%). Those attributed to basalt and calc-alkaline andesitic basalt have (FeOt/MgO = 0.35 to 0.42), A/NK = 7.39 to 22.83; A/CNK = 0.25 to 0.74 with low alkaline Na2O + K2O (0.12% to 0.62%) content and high CaO (7.54% to 10.95%), MgO (20.2% to 24%) and Fe2O3 (8.5% to 10.45%) contents. The calc-alkaline dacite samples have FeOt/MgO (1.95 to 2.23), A/NK (2.48 to 2.74), and A/CNK (1.62 to 1.70), a moderate alkaline content Na2O + K2O (5.11% to 5.82%) compared to basalt, a low CaO (3.05% to 3.17%), MgO (2.75% to 3.21%) and of Fe2O3 (5.99% to 7.27%) contents. Therefore, the Th/Yb < 0.70 and Zr/Y < 3.9 ratios place the Elogo basalts in the field of tholeiites within the Zr/Y – Th/Yb diagram of [13] , where the dacites are plotted in the calc-alkaline domain (Th/Yb < 5 and Zr/Y < 7). A sample of andesitic basalt (Th/Yb = 0.43 and Zr/Y = 2.93) and two samples of dacite Th/Yb (0.60 and 0.68) and Zr/Y (2.81 and 3.27) are plotted in the transitional field between the tholeiitic series and calc-alkaline (Figure 5(C)). The A/NK vs. A/CNK plot diagram of [14] shows that tholeiitic basalts, basalts and calc-alkaline andesitic basalt are metaluminous and, however, calc-alkaline dacites are peraluminous (Figure 5(D)). Binary Harker plots of REE and trace elements contents correlated with MgO content for samples of the Elogo complex starting from tholeiitic basalts to calc-alkaline dacites (Figure 6). They show a positive evolution in Ni and in Cr and a negative evolution in Ba, Rb, Ce, Sr, Y, La, and Zr. Moreover, depending on the nature of the rocks, there is a varied behavior: 1) The tholeiitic basalts show an almost stable evolution for the Ba, La and Rb elements, a positive correlation for the Ni and Cr elements, a negative correlation for the Ce, Sr and Zr elements and dispersion for the Y element. 2) Basalt and andesitic basalt calc-alkalines show a negative correlation for the Ce, La, Sr, Y, and Zr elements, a positive correlation for the Ni element, a dispersion for the Cr, and a negative correlation to stable correlation for the Ba and Rb elements. 3) Dacites display a stable trend for Ni and a dispersive trend for the rest of the elements. The binary Harker diagrams of the traces elements and rare earth contents correlated this time with the Zr content show for all the rocks a positive correlation in Ba, Ce, La, Rb, Sr, Y, Nb and Ta, and a negative correlation in Ni, Co, Mg et Cr (Figure 7). However, depending on the rocks, the behavior varies according to the elements as in the above-mentioned: 1) The tholeiitic basalts show an almost stable evolution for the Ba, Ta, La and Rb elements, a positive correlation for the Ce element, and dispersion for the Nb, Sr, Y, Co, Cr, and Ni elements; 2) the basalts and the andesitic calc-alkaline basalts show a stable evolution for the Ta, a negative correlation for the Ni, a positive correlation for the Ba, Ce, Nb, Rb, Y, Sr, and La, a dispersion for the Co and the Cr; 3); Dacites still shows a stable trend for Ni, a negative to stable trend for the Cr and a dispersive trend for Ba, Ce, Co, La, Nb, Rb, Sr, Ta and Y.
The REE of the different rocks were normalized to the primitive mantle of [15] and a chondrite of [16] (Figure 8(A), Figure 8(B)). The tholeiitic basalts have LREE contents (La = 0.5 ppm to 8.1ppm and Ce = 0.8 ppm to 6.6 ppm) close to and lower than those of HREE ((Yb = 0.4 ppm to 2.24 ppm and Lu = 0.05 ppm to 0.37 ppm) and according to the samples an absent to weak fractionation between LREE [(La/Yb)n = 0.36 to 0.97; (Ce/Yb)n = 0.27 to 0.96] and HREE [(La/Yb)n = 1 to 2.15; and (Ce/Yb)n = 1.04 to 1.72)], a significant negative anomaly in Eu (Eu/Eu* = 0.25 to 0.99), and which are manifested by spectra with evolution almost horizontal.
The basalts and the andesitic calc-alkaline basalts have a horizontal to sub-oblique evolution. The calc-alkaline dacites show an equally oblique evolution. Normalizations to the primitive mantle of [17] and the chondrite of [18] (Figure 8(C), Figure 8(D)) show for the tholeiitic basalts the negative anomalies in Rb, Nb, Ba, K and Sr; and positive anomalies in Pb and Ta. The basalts and the andesitic calc-alkaline basalts show a negative anomaly in Nb and Ti and positive anomalies in Pb, Ta and U with enrichment in LILE (Cs, Th et U) and LREE (La and Ce).
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Figure 6. Binary Harker diagrams show mobile elements’ evolution, REE as a function of MgO. Red points = Tholeiitic basalts; blue points = basalts and andesitic basalts, black points = calc-alkaline dacites.
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Figure 7. Binary Harker diagrams showing the evolution of mobile elements and REE as a function of Zr. Red = Tholeiitic basalts; blue = basalts and calc-alkaline andesitic basalts, black= calc-alkaline dacites.
5. Discussion
Elogo complex is one of the metamorphic units forming the greenstone belts in the Ivindo Archean Block in the Congo. It is comparable to Belinga, well studied in Gabon [4] .
5.1. Petrogenesis
The metabasite rocks observed in Elogo and Bamegod are amphibolites, chlorite schists, and epidotes. The show LOI (Table 1) varied from low (0.85%) to medium (1.13% to 1.53%) and very high in most samples (16.4%). The high values testify to a post-genetic transformation linked to metamorphism or hydrothermalism. Retrograde metamorphic parageneses are marked by 1) chlorite-muscovite-talc-quartz-calcite; 2) chlorite-muscovite-cordierite-calcite-quartz-chalcopyrite; 3) epidote-actinolite-tremolite-muscovite-zoisite. Retrograde hydrothermal parageneses are characterized by relics of hornblende associated with carbonate, muscovite, and talc tremolite. Chemical analyses show that the rocks
have undersaturated to low saturated compositions and are low TiO2 < 2% according to [19] [20] [21] [22] [23] ; these samples are classed as a rock with low Ti (TiO < 2). Major element projection diagrams (Figure 5) place rocks in tholeiitic basalts, basalts and calc-alkaline andesitic basalts, and calc-alkaline dacites. The tholeiitic basalts, the basalts, and calc-alkaline andesitic basalts are metaluminous, and the dacites are peraluminous. The binary Harker diagrams in trace elements and REE vs. MgO (Figure 6) show a positive evolution in Ni and Cr and a negative evolution in Ba, Rb, Ce, Sr, Y, La, and Zr. On the other hand, those of the trace elements and REE vs. Zr (Figure 7) show a positive evolution in Ba, Ce, La, Rb, Sr, Y, Nb and Ta and rather negative in Ni, respectively Co and Cr. The behavior of trace and REE elements is identical to that observed in the greenstone belts of Rio das Velhas and Pitangui greenstone belts, São Francisco Craton, Brazil [24] .
5.2. Post-Genetic Alteration of Elogo Complex Metabasites
The retrograde metamorphism of amphibolite into chlorite-schists observed in the metabasites is accompanied by a significant hydrothermal circulation marked by a considerable variation in MgO contents (5.36% to 7.68%; 16% to 26%) within the basalts. The tholeiitic basalts and calc-alkaline dacites present negative anomalies in P, Rb, Ba, and Ce, and positive anomalies in Li, which symbolize this hydrothermal alteration (Figure 8). The retrograde metamorphism and hydrothermal alteration are considered controllers of the mobility of the LILE and REE [25] [26] [27] [28] . This mobility of the LILE continues without influencing the HFSE, HREE, Zr and Hf contents [27] [29] .
Works on the mobility of chemical elements in Precambrian rocks under the effect of metamorphism as well as during hydrothermal alteration show that major elements such as Al, Ti, Fe, P and HFSE, REE, except Eu and Ce, and transition metal Cr, Ni, Sc, V, Y are relatively immobile in greenschist and amphibolites facies and even under granulite facies conditions [30] - [35] . Eoarchean metavolcanic rocks from Isua in west Greenland show that rocks with Ce/Ce* [Ce/Ce* = Cecn/(Lacn*Prcn)1/2] < 0.9 or >1.11 exhibit REE mobility, while rocks with Ce/Ce* between 0.9 and 1.1 exhibits limited mobility of the REE [36] . Twelve (12) tholeiitic basalts samples show a strong negative Ce anomaly (Figure 8(A), Figure 8(B)) and a weak Ce/Ce* < 0.9 ratio between 0.24 and 0.89, marking a significant mobility of the LREE linked to an important hydrothermal circulation. Calc-alkaline dacites and some tholeiitic basalts without negative Ce anomalies are marked by a Ce/Ce* ratio between 0.91 and 1.18, revealing a low LREE mobility during hydrothermal circulation. Besides, negative Eu and Sr anomalies reveal a plagioclase accumulation [37] in tholeiitic basalts (Figure 8(A), Figure 8(C)).
5.3. Crustal Contamination in the Basalts and Dacites of Elogo Complex
Crustal contamination during the emplacement of igneous rocks can be assessed from the ratios between REE and HFSE as Nb/Yb, Th/Yb, Nb/La, La/Ta, (La/Yb)n, Y/Nb and Zr/Th known for the mantle and the continental crust [24] . The Th/Yb vs. Nb/Yb diagram that discriminates arc basalts at MORBs and OIBs, as well as crustal contamination [24] [38] [39] , displays low-contaminated tholeiitic basalts and calc-alkaline basalts (Th/Yb < 0. 59) in the arc field closer to MORBs; and highly contaminated calc-alkaline dacites (Th/Yb < 0.44) in the arc field also but further from the MORB field (Figure 9). REE/HFSE ratios, used to assess crustal contamination, are high in the crust and low in the mantle [24] . They are higher in calc-alkaline dacites (La/Nb = 1.55 to 5.89; La/Ta = 17 to 54.5) and in calc-alkaline andesitic basalts and basalts (La/Nb = 1.66 to 5.2; La/Ta = 13 to 26) than in tholeiitic basalts (La/Nb = 0.83 to 3.5; La/Ta = 4 to 26). REE/HFSE ratios reveal more marked contamination in calc-alkaline basalts and dacites than in poorly contaminated tholeiitic basalts. Thus, REE fractionation highlights contamination in tholeiitic basalts (La/Yb)n < 1.93 (0.98 to 1.92), calc-alkaline basalts (La/Yb)n < 1.66 (0.97 to 1.65), and more significant contamination in calc-alkaline dacites (La/Yb)n > 1.93 (1.97 to 1.98). The Zr/Th ratios of the crust (Zr/Th = 20) and the mantle (Zr/Th = 116) make it possible to trace the origin and contamination of the magma; they reveal crustal contamination when they are close to the Zr/Th ratio of the crust [24] [38] . Zr/Th ratios (122.22 to 300) higher than the Zr/Th ratio of the primitive mantle indicate an absence
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Figure 9. Geotectonic discrimination diagram of [38] using Nb/Yb vs. Th/Yb ratios to determine crustal contamination on the Elogo complex metabasite rocks. Red = tholeiitic Basalts; blue = basalts and calc-alkaline andesitic basalts, black = calc-alkaline dacites.
of contamination; on the other hand, Zr/Th ratios (63.23 - 106.66) lower than the Zr/Th ratio of the primitive mantle reveal in the tholeiitic basalts weak contamination as in the samples of calc-alkaline andesitic basalts (Zr/Th = 54.17 to 100). On the other hand, Zr/Th ratios (15.60 - 67) are lower than the mantle ratio but close to the continental crust Zr/Th ratio reveals significant contamination in the calc-alkaline dacites. Crustal contamination is also highlighted by positive Th-U-Pb anomalies in tholeiitic basalts, dacites, and calc-alkaline basalts and negative Nb-Ta anomalies in dacites (Figure 8(C), Figure 8(D)). However, the low contamination of tholeiitic basalts and calc-alkaline andesitic basalts is also characterized by their positive Ta anomalies and low Rb-Ba contents (Figure 8(C), Figure 8(D)).
5.4. The Composition of the Source Mantle of Basalts and Dacites of Elogo Complex
The composition of the source mantle and its depth for the rocks of the Elogo complex can be assessed by the ratios Al2O3/TiO2, CaO/Al2O3 and (Gb/Yb)PM [28] [40] . Mafic and ultramafic rocks are classified as Al-depleted when (Al2O3/TiO2)adj ≤ 16, (CaO/Al2O3)adj > 1.0, and (Gd/Yb)PM > 1 and Al-undepleted when the ratio (Al2O3/TiO2)adj > 20, (CaO/Al2O3)adj ≤ 1.0, (Gd/Yb)PM ≤ 1.0) [39] [41] [42] . The metabasites rocks of the Elogo complex (Figure 10) are mostly at Al2O3/TiO2 > 16 but between 16.5 and 35.12, with however many rocks at Al2O3/TiO2 < 20 and three tholeiitic basalts samples at Al2O3/TiO2 < 16 (12.45 to 14.48). The ratio CaO/Al2O3 is <1 in twelve tholeiitic basalt samples (0.52 à 0.97) and the calc-alkaline dacites (0.21 to 0.23). This ratio is >1 in ten samples of tholeiitic basalt (1.04 to 1.35) and basalts and calc-alkaline andesitic basalts (1.28, 1.68 and 3.99). The ratio (Gd/Yb)PM > 1, except for two basalt samples which have a ratio (Gb/Yb)PM < 1 (0.97 and 0.98). The Al2O3/TiO2, CaO/Al2O3 and (Gb/Yb)PM ratios show that part of the mantle is of the undepleted type and may contain garnet in the melting (deep source) and another part is of the depleted type (shallow source). The Th, Nb, Zr and Hf are used to determine the quality of the mantle source for the magma. The positive anomalies in Th and the flat evolution of Zr, Hf and Ti observed in the tholeiitic basalts indicate an undepleted mantle. The negative Th, Nb, Zr, and Hf anomalies reflect an origin linked to the depleted mantle in the tholeiitic basalts, calc-alkaline basalts, and calc-alkaline dacites [43] . The high MgO content helps identify melting temperatures [44] [45] . Tholeiitic basalt and calc-alkaline andesitic basalts with MgO between 16% and 26% characterize very high melting temperatures located
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Figure 10. The plot of metabasite rocks of Elogo complex in diagrams of [42] modified by [9] : A - Al2O3/TiO2 vs. (Gb/Yb)PM; B - CaO/Al2O3 vs. (Gb/Yb)PM. Tholeiitic basalts and dacites fall in undepleted source, and some tholeiitic basalts samples and three calc-alkaline andesitic basalts samples are in the garnet residue domain. Red = tholeiitic basalts; bleu = basalts and calc-alkaline andesitic basalts; black = calc-alkaline dacites.
from 1484, 09˚C to 1681, 77˚C and calc-alkaline dacites with contents between 2.75% and 3.21% showed low melting temperature located between 434.76˚C and 592.99˚C. Therefore, melting temperatures are calculated by [Tp(˚c) = 1463 + 12.74MgO − 2924/MgO] formula of [45] .
5.5. Geodynamic Context of Metabasite Elogo Complex
Most studied metabasites are characterized by negative Rb, Ba, Nb, Sr, and Eu anomalies. On the other hand, a few samples show positive anomalies in Ta and Sr, negative anomalies in Rb and Ba, and positive in Ta in the tholeiitic basalts and andesitic basalts calc-alkalines characterize an extensive tectonic context exempt from significant crustal contamination. Nb/Yb vs. Th/Yb diagram of [38] (Figure 9) plot samples in NMORB, EMORB and volcanic arc array. La/Yb vs. Nb/La diagram of [46] (Figure 11(A)) plots tholeiitic basalts in MORB and oceanic arcs domains, with one sample in a continental arc. The tholeiitic basalts and calc-alkaline andesitic basalts are located in the continental and oceanic arc domains. The dacites are situated in MORB, continental and alkaline arcs. The La/Yb vs. Th/Nb diagram of [46] (Figure 11(B)), on its side, places the tholeiitic basalts largely in oceanic arc domains with some samples in continental arcs field. The basalts and calc-alkaline andesitic basalts are plotted in continental and oceanic arc fields, and dacites are in alkaline continental and oceanic arc fields. All the projections show that the tholeiitic basalts have an oceanic character with an influence of arc domains, particularly the oceanic arc domain. These rocks’ anomalous negative in Nb and positive in Ta and free from a negative anomaly in Ti exclude the subduction context for their geodynamic context.
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Figure 11. Geotectonic discrimination diagrams of [46] applied to metabasites of Elogo complex. A—La/Yb vs Nb/La; B—La/Yb vs Th/Nb. Red = tholeiitic basalts; blue = basalts and calc-alkaline andesitic basalts; black = calc-alkaline dacites.
The positive Pb anomaly and the weak LREE depletion with a ratio (La/Yb)PM ~ 1 are geochemical signatures excluding a mid-ocean ridge tectonic context with a well-elaborated oceanic crust, but relatively favorable to an extensive continental rift-type tectonic context with a beginning of oceanization or a back-arc basin type tectonic context. The high ratios Nb/Th (2 to 10), Nb/U (1.82 to 26) and La/Ta (5 to 27) of tholeiitic basalts are characteristic of sources and /or remnants of divergent margin magmatism [47] . The diagram Nb/Yb vs. Th/Yb of [38] (Figure 9) plots calc-alkaline dacites in the contaminated volcanic back-arc domain. As for La/Yb vs. Nb/La and La/Yb vs. Th/Nb of diagrams of [46] (Figure 11), the basalts are in the domain of alkaline arcs and continental arcs. The association of the signature of NMORBs and island-arc basalts (Figure 11) within the basalts reveals a back-arc basin context [48] - [53] . Thus, the tholeiitic basalts of the Elogo complex were formed following an extensive geodynamic regime of the back-arc basin type. FeOT-MgO-Al2O3 triangular diagram of [54] (Figure 12(A)) plots all kinds of basalts in ocean ridge and floor and scatters dacites in four domains (orogenic, ocean Island, continental, ocean ridge and floor). The calc-alkaline andesitic basalts and dacites are not like the tholeiitic basalts in the domain of the ridge and the ocean floor but rather in the orogenic
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B
Figure 12. A—Geotectonic discrimination of metabasite Elogo complex in MgO-Al2O3-FeOT triangular diagram of [54] : All basalts are in ocean Ridge and Floor. Dacites are distributed in all domains. B—Basalts and dacites discrimination in the MORB field, compressive arc, and intracontinental rift field, according to [55] . MORB = Mid-Ocean Ridge Basalts, CRB = Continental Rift Basalts, IAB = Island Arc Basalts, OIB = Ocean Island Basalts, and BAB = Back arc basalts. The area circled in green in diagram B corresponds to the back-arc basins (BAB). Sample Red = tholeiitic basalts; bleu = basalts and calc-alkaline andesitic basalts; black = calc-alkaline dacites.
and continental domains. Indeed, in the ternary diagram of the ratios of trace elements and REE La/10, Y/15 and Nb/8 (Figure 12(B)) of [55] , the tholeiitic basalts of the Elogo complex are plotted in the rear basin oceanic arc field and at the transition between the BAB and N-MORB domain mainly (Figure 12(B)). Similarly, the signature of the calc-alkaline andesitic basalts and the dacites (positive anomalies in U, Th, K, Li, La, Pb, Ce and negatives anomalies in Nb and Ti) reflects crustal contamination and hydrothermal alteration as observed in thin sections. The geochemical signature of calc-alkaline andesitic basalts and dacites is characteristic of a compressive tectonic context affecting the back-arc basin in which the tholeiitic basalts were previously emplaced. The metabasites characterize two main tectonic phases: 1) a first extensive phase at the back-arc inducing a basin in which take place the tholeiitic basalts of intermediate type between NMORB and EMORB; 2) a second tectono-metamorphic phase compressing the basin; it would be associated with subduction accompanied by hydrothermal circulations.
The tholeiitic basalts (basic and ultrabasic rocks), except for a few samples, are comparable in MgO (16% to 26%), Al2O3 (4.7% to 15%), Co (44 to 113 ppm), Cr (1600 ppm to 3110 ppm) and in Ni (1060 ppm to 1540 ppm) to the basic-ultrabasic rocks to Belinga greenstone belt of Gabon [4] . In this last country, the greenstone belt is considered to have been emplaced during the Meso-Neoarchean extension (between 2870 Ma and 2750 Ma, the age of reworked zircons and the tectonometamorphic event that affects the greenstone of Belinga group). Calc-alkaline andesitic basalts and dacites, which present a geochemical signature of a compressive tectonic regime, closure of a back-arc basin by possible subduction would be related to this tectono-metamorphic event of [4] . Similarly, the relics of the greenstone belts of the Ntem complex in Cameroun, particularly the greenstone belt of Lolodof-Ngomezap dated to Mesoarchean (around 3000 Ma) by [56] , present a tholeiitic and calc-alkaline affinity like the basalts and dacites of Elogo.
6. Conclusions
Elogo complex’s amphibolites are related to tholeiitic basalts (SiO2 = 37.4 to 50.7%; FeOt/MgO = 0.34 to 2.13) metaluminous (A/NK = 4.03 to 102.22 and A/CNK = 0.66 to 1.52); chlorite schists and epidote rock are similar to calc-alkaline basalts and andesitic basalts (SiO2 = 46.1% to 47.4% and 53.1%; FeOt/MgO = 0.35 to 0.42 and 0.38 respectively) metaluminous (A/NK = 7.39 to 12.78 and 22.83; A/CNK = 0.55 to 0.74 and 0.25 respectively). Gneissic amphibolites are similar to calc-alkaline dacites (SiO2 = 65.2% to 66%; FeOt/MgO = 1.95 to 2.23) hyperaluminous (A/NK = 2.48 to 2.74 and A/CNK = 1.62 to 1.70). Trace element geochemistry of basalts and dacites reveals that:
v The retrograde chlorite-calcite-muscovite-talc-hematite paragenesis, negative Rb, Ba, Ce (Ce/Ce* < 0.7) and P anomalies, and positive Li anomalies in the basalts are typical of hydrothermal alteration.
v The low contamination of calc-alkaline tholeiitic and andesitic basalts is emphasized by a Th/Yb < 1 ratio, a positive Ta anomaly, and by low Rb-Ba contents compared to highly contaminated (Th/Yb > 1) and fractionated calc-alkaline dacites [(La/Yb)n > 1. 93] with high La/Nb (1.5 to 5.85) and La/Ta (17 to 54.5) ratios, Zr/Th (15.598 to 67) close to continental crust (Zr/Th = 20) and positive Th-U-Pb and negative Nb-Ta anomalies.
v The tholeiitic basalts have a depleted mantle source, including garnet in the melt residue [Al2O3/TiO2 = 12.45 to 14.48 and >16 (16.5 to 32.71); CaO/Al2O3 < 1 (0. 52 to 0.97) and >1 (1.04 to 1.35) and (Gb/Yb)PM > 1] whereas dacites have a shallow source in the spinel field [Al2O3/TiO2 > 16; CaO/Al2O3 < 1 and (Gb/Yb)PM > 1].
v The high MgO content in tholeiitic basalts (16% - 26%), in calc-alkaline basalts (20.2% - 24%), and low in calc-alkaline dacites (2.75% - 3.21%) mark very high melting temperatures of 1484.09˚c - 1681.778˚c and 1575.6˚c - 1646.9˚c, and low of 434.76˚c - 592.99˚c respectively. The difference in temperature and melting degree within basalts and between basalts and dacites characterize the variation in the depth of the depleted mantle source. Thus, the negative Th, Nb, Zr, and Hf anomalies of calc-alkaline basalts characterize a depleted mantle source, supported by the negative Th, Nb, and Zr anomalies of calc-alkaline dacites.
v The genesis of the basalts is explained by an extensive context that did not reach the MORB stage, justifying the positive Ta, negative Rb, Ba, and Nb anomalies associated with the lack of a negative Ti anomaly. A back-arc extension would explain well in the tholeiitic basalts the positive Ta and Li anomaly, the low Ti grades, the negative Rb and Ba anomalies, the horizontal evolution of the REE (La/Yb ~1) and the low grades in Zr and Hf.
v The geochemical signature of calc-alkaline andesitic basalts and dacites would be typical of a volcanic arc context in a subduction regime characterized by an enrichment in LILE and LREE defined by positive anomalies in Ba, U, Th, K, La, and Ce; and by a depletion in HFSE revealed by negative anomalies in Nb, Ta, and Ti.
The geochemical study reveals a back-arc extension inducing the genesis of poorly contaminated tholeiitic basalts, the extension of the complex was associated with the Meso-Neoarchean distension controlling the genesis of the Belinga group in Gabon and that of the relics of the Ntem complex rock belts in Cameroon. The distension is interrupted by the closure of the back-arc basin inducing the formation of calc-alkaline dacites and basalts in a subduction regime associated with the tectono-metamorphic event of the Belinga Group.
Acknowledgements
We thank MAC Congo for funding the field research and all the laboratory analyses. We also thank our research structure, the National Institute for Research in Exact and Natural Sciences (IRSEN) of Congo.