The West Congo Belt contains in its rocks of Neoproterozoic age from Nemba complex outcropping in the Moumba River. This West Congo belt is made up of a crustal segment of the Arcuaï-West Congo orogen which exten...The West Congo Belt contains in its rocks of Neoproterozoic age from Nemba complex outcropping in the Moumba River. This West Congo belt is made up of a crustal segment of the Arcuaï-West Congo orogen which extends from southwest Gabon to the northeast of Angola. This study aims to constrain the geochemical signature Nemba complex of West Congo belt from the petrograhic and geochemical study on the whole rock. The petrographic data from this study show the Moumba metabasites are made up of amphibolites, metagabbros, epidotites and greenschists interstratified in the Eburnean metasediments and affected by mesozonal to epizonal metamorphism characterized by the retromorphosis of intermediate amphibolite facies minerals into greenschist facies. Whole-rock geochemical data indicate that these metabasites are continental flood basalts (CFB) of basic nature and transitional affinity emplaced in intraplate context. These continental flood basalts are generated from magma originating from a significantly enriched shallow mantle plume and this magma then contaminated by the continental crust during their ascent. The reconstruction of tectonic signature suggests that West Congo belt would result from closure of an ocean basin with subduction phenomena. This collision would be marked by the establishment of ophiolite complex. We show that this model is incompatible with the CFB nature of metabasites and the orogenic evolution of Neoproterozoic. It does not seem that we can evoke a genetic link with a subduction of oceanic crust, because the paleogeography of Neoproterozoic (Rodinia) is marked by intracontinental rifts linked to opening of Rodinia. We therefore suggest the non-existence of ophiolitic complex in western Congo belt and reject the collisional model published by certain authors. We confirm the currently available intracontinental orogen model.展开更多
The Elogo complex is a greenstone belt portion located on the Eastern edge of the Archean Congo craton at the junction with the Paleoproterozoic to Neoproterozoic Sembe Ouesso basin. This study was carried out on this...The Elogo complex is a greenstone belt portion located on the Eastern edge of the Archean Congo craton at the junction with the Paleoproterozoic to Neoproterozoic Sembe Ouesso basin. This study was carried out on this complex to determine the context of the placement of basaltic rocks. Metaluminous tholeiitic basalts (basic and ultrabasic), calc-alkaline basalts, andesitic basalts, and peraluminous calc-alkaline dacites represent greenstones. Tholeiitic and calc-alkaline basalts come from deep enriched and depleted mantle sources, including garnet in fusion residues [Al<sub>2</sub>O<sub>3</sub>/TiO<sub>2</sub> > 16 (16.5 to 35.12) and in some samples between 12.45 to 14.48;CaO/Al<sub>2</sub>O<sub>3</sub> 1 (1.04 to 1.35) in ten samples and (Gb/Yb)<sub>PM</sub> > 1]. The calc-alkaline dacites come from a shallow depleted mantle source [Al<sub>2</sub>O<sub>3</sub>/TiO<sub>2</sub> > 16;CaO/Al<sub>2</sub>O<sub>3</sub> 1]. Tholeiitic and calc-alkaline basalts have a negative Rb, Ba, Ce, and Nb anomaly without negative Ti anomaly, positive Ta, Pb anomalies, and a lack of significant REE [(La/Yb)n = 0.36 to 0.97 and 1 to 2.15;(Ce/Yb)n = 0.27 to 0.96 and 1.04 to 1.72, respectively] fractionation. High Nb/Th (2 to 10) and Nb/U (1.82 to 26) ratios and low La/Ta (5 to 27) ratios are characteristic of divergent margin magmatic sources. Tholeiitic and calc-alkaline basalts correspond to an extensive back-arc basin-type tectonic setting. Calc-alkaline andesitic basalts and dacites show positive Ba, U, Th, K, La, Ce, Pb, and Li anomalies and negative Nb, Ta, and Ti anomalies reflecting crustal contamination and hydrothermal alteration in a compressive tectonic context as a volcanic arc in a subduction regime marking the interruption of the meso-neoarchean Elogo’s opening. Elogo’s opening and closing are probably associated with the emplacement of the greenstone of the meso-neoarchean Gabon Belinga group and the relics of the Mesoarchean greenstones of the Cameroun Ntem complex.展开更多
文摘The West Congo Belt contains in its rocks of Neoproterozoic age from Nemba complex outcropping in the Moumba River. This West Congo belt is made up of a crustal segment of the Arcuaï-West Congo orogen which extends from southwest Gabon to the northeast of Angola. This study aims to constrain the geochemical signature Nemba complex of West Congo belt from the petrograhic and geochemical study on the whole rock. The petrographic data from this study show the Moumba metabasites are made up of amphibolites, metagabbros, epidotites and greenschists interstratified in the Eburnean metasediments and affected by mesozonal to epizonal metamorphism characterized by the retromorphosis of intermediate amphibolite facies minerals into greenschist facies. Whole-rock geochemical data indicate that these metabasites are continental flood basalts (CFB) of basic nature and transitional affinity emplaced in intraplate context. These continental flood basalts are generated from magma originating from a significantly enriched shallow mantle plume and this magma then contaminated by the continental crust during their ascent. The reconstruction of tectonic signature suggests that West Congo belt would result from closure of an ocean basin with subduction phenomena. This collision would be marked by the establishment of ophiolite complex. We show that this model is incompatible with the CFB nature of metabasites and the orogenic evolution of Neoproterozoic. It does not seem that we can evoke a genetic link with a subduction of oceanic crust, because the paleogeography of Neoproterozoic (Rodinia) is marked by intracontinental rifts linked to opening of Rodinia. We therefore suggest the non-existence of ophiolitic complex in western Congo belt and reject the collisional model published by certain authors. We confirm the currently available intracontinental orogen model.
文摘The Elogo complex is a greenstone belt portion located on the Eastern edge of the Archean Congo craton at the junction with the Paleoproterozoic to Neoproterozoic Sembe Ouesso basin. This study was carried out on this complex to determine the context of the placement of basaltic rocks. Metaluminous tholeiitic basalts (basic and ultrabasic), calc-alkaline basalts, andesitic basalts, and peraluminous calc-alkaline dacites represent greenstones. Tholeiitic and calc-alkaline basalts come from deep enriched and depleted mantle sources, including garnet in fusion residues [Al<sub>2</sub>O<sub>3</sub>/TiO<sub>2</sub> > 16 (16.5 to 35.12) and in some samples between 12.45 to 14.48;CaO/Al<sub>2</sub>O<sub>3</sub> 1 (1.04 to 1.35) in ten samples and (Gb/Yb)<sub>PM</sub> > 1]. The calc-alkaline dacites come from a shallow depleted mantle source [Al<sub>2</sub>O<sub>3</sub>/TiO<sub>2</sub> > 16;CaO/Al<sub>2</sub>O<sub>3</sub> 1]. Tholeiitic and calc-alkaline basalts have a negative Rb, Ba, Ce, and Nb anomaly without negative Ti anomaly, positive Ta, Pb anomalies, and a lack of significant REE [(La/Yb)n = 0.36 to 0.97 and 1 to 2.15;(Ce/Yb)n = 0.27 to 0.96 and 1.04 to 1.72, respectively] fractionation. High Nb/Th (2 to 10) and Nb/U (1.82 to 26) ratios and low La/Ta (5 to 27) ratios are characteristic of divergent margin magmatic sources. Tholeiitic and calc-alkaline basalts correspond to an extensive back-arc basin-type tectonic setting. Calc-alkaline andesitic basalts and dacites show positive Ba, U, Th, K, La, Ce, Pb, and Li anomalies and negative Nb, Ta, and Ti anomalies reflecting crustal contamination and hydrothermal alteration in a compressive tectonic context as a volcanic arc in a subduction regime marking the interruption of the meso-neoarchean Elogo’s opening. Elogo’s opening and closing are probably associated with the emplacement of the greenstone of the meso-neoarchean Gabon Belinga group and the relics of the Mesoarchean greenstones of the Cameroun Ntem complex.
