This study focuses on the metabasite rocks of the Nemba Complex of the Mayombe belt, an African segment of Araçuaï-West Congo Orogen (A-WCO) extending from the southwest of Gabon to the northwest of ...This study focuses on the metabasite rocks of the Nemba Complex of the Mayombe belt, an African segment of Araçuaï-West Congo Orogen (A-WCO) extending from the southwest of Gabon to the northwest of Angola. These metabasite rocks outcrops are in southwestern Congo along the Loukounga river. The Nemba complex is of Neoproterozoic age and represents the lower part of the west congolian Supergroup. The objective of this study is to constrain the geodynamic context of the Nemba complex from the petrology and geochemistry of the metabasites sampled in the Loukounga River. The observed rocks are composed of amphibolites, metagabbros, epidotites and greenschists and are affected by folding accompanied by flux schistosity and crenulation schistosity. Geochemical analyzes show that the rocks have a basic to ultrabasic chemical composition with SiO<sub>2</sub> contents between 41.85% and 58.23%. The geochemical composition of the major and traces elements shows that the rocks are basalts, basaltic andesites and andesites. The magma shows enrichment in LREE, LILE and depletion in HREE and HFSE. The multielement spectra show negative anomalies in Nb-Ta, Ti and a relatively low Nb/La ratio which characterize a lithospheric source contaminated by continental crust. Traces elements discrimination plots show that Loukounga metabasites are emplaced in intraplate geodynamic context like that associated with the basalts of the trap-types continental shelves and are possibly derived from mantle plumes contemporaneous with or slightly prior to magmatism.展开更多
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 West Congo Belt (WCB) is one of many Proterozoic orogens considered to be mobile belts adjacent to the margins of Archean cratons. This orogen underlies the western margin of the Congo craton in central Africa and...The West Congo Belt (WCB) is one of many Proterozoic orogens considered to be mobile belts adjacent to the margins of Archean cratons. This orogen underlies the western margin of the Congo craton in central Africa and outcrops from south-west Gabon to north-west Angola. In the Republic of Congo, the Mayombe Chain in south-west has preserved a geological record of Proterozoic to early Paleozoic rocks that have been investigated to understand the evolution of the West Congo Belt within the regional tectonic context. Investigation into lithostratigraphy and structural architecture of the Mayombe Chain has been done by earlier workers since the 19th century. However, the lack of consistent and diverse geochronological data induced several interpretations of its geodynamic setting. The chain was previously thought to be the result of more than two orogenic cycles, and subsequently to be the result of a single orogenic cycle. Recent petrographic, geochronological, geochemical data and paleogeographic reconstructions allow redefining the tectonic context and the main lithostratigraphic subdivisions of the Mayombe Chain. It is currently recognized that the Mayombe Chain consists of three major tectonostratigraphic domains: Western, Central and Eastern, which are prolongated by the Niari-Nyanga foreland basin. These lithounits seem to have been formed through a complex history that included: (i) Late Paleoproterozoic sedimentation, magmatism and metamorphism between 2110 and 1970 Ma related to the Eburnean tectono-thermal event;(ii) Neoproterozoic rifting magmatism and sedimentation;and (iii) Pan-African tectono-thermal event between 620 and 496 Ma leading to the amalgamation of western Gondwana. In this contribution, we review existing literature on this part of the West Congo Belt and summarize the current knowledge of its geologic setting with a focus on structure, lithostratigraphy and metamorphism. We aim to show how current research, with multidisciplinary approaches, changed the interpretation of the Mayombe with consideration to the Araçuaí-West Congo Orogenic system.展开更多
· AIM: to determine the frequency and causes of blindness in diabetic Africans. ·METHODS: The study was a cross-sectional survey carried out among known black diabetics consecutively admitted at the Teaching...· AIM: to determine the frequency and causes of blindness in diabetic Africans. ·METHODS: The study was a cross-sectional survey carried out among known black diabetics consecutively admitted at the Teaching Hospital, University of Kinshasa, between 2005 and 2007. Examination methods included interviewer -administered structured question - naire, eye examinations (visual acuity, tonometry, funduscopy), and fasting plasma glycaemia test. ·RESULTS: Of the 227 patients examined, 15.9% had blindness. Univariate analyses showed significant association between female, severity of diabetic retinopathy, Mayombian ethnic group, use of insulin treatment, low intake of vegetables, diabetic nephropathy, open angle glaucoma and blindness in all diabetics. After logistic regression, only diabetic nephropathy, use of insulin treatment, macular oedema, Mayombian ethnic group and vegetables low intake were the independent risk factors of blindness in all diabetics. However, after logistic regression in the sub -group with diabetic retinopathy, only open angle glaucoma and proliferative diabetic retinopathy were the independent determinants of blindness.·CONCLUSION: The majority of the causes of blindness in these diabetic Africans are avoidable. It is recommended that appropriate diabetes care, nutrition education, periodic eye examination and laser photocoagulation facilities should be provided for treating diabetics in sub-Saharan Africa. ·展开更多
文摘This study focuses on the metabasite rocks of the Nemba Complex of the Mayombe belt, an African segment of Araçuaï-West Congo Orogen (A-WCO) extending from the southwest of Gabon to the northwest of Angola. These metabasite rocks outcrops are in southwestern Congo along the Loukounga river. The Nemba complex is of Neoproterozoic age and represents the lower part of the west congolian Supergroup. The objective of this study is to constrain the geodynamic context of the Nemba complex from the petrology and geochemistry of the metabasites sampled in the Loukounga River. The observed rocks are composed of amphibolites, metagabbros, epidotites and greenschists and are affected by folding accompanied by flux schistosity and crenulation schistosity. Geochemical analyzes show that the rocks have a basic to ultrabasic chemical composition with SiO<sub>2</sub> contents between 41.85% and 58.23%. The geochemical composition of the major and traces elements shows that the rocks are basalts, basaltic andesites and andesites. The magma shows enrichment in LREE, LILE and depletion in HREE and HFSE. The multielement spectra show negative anomalies in Nb-Ta, Ti and a relatively low Nb/La ratio which characterize a lithospheric source contaminated by continental crust. Traces elements discrimination plots show that Loukounga metabasites are emplaced in intraplate geodynamic context like that associated with the basalts of the trap-types continental shelves and are possibly derived from mantle plumes contemporaneous with or slightly prior to magmatism.
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 West Congo Belt (WCB) is one of many Proterozoic orogens considered to be mobile belts adjacent to the margins of Archean cratons. This orogen underlies the western margin of the Congo craton in central Africa and outcrops from south-west Gabon to north-west Angola. In the Republic of Congo, the Mayombe Chain in south-west has preserved a geological record of Proterozoic to early Paleozoic rocks that have been investigated to understand the evolution of the West Congo Belt within the regional tectonic context. Investigation into lithostratigraphy and structural architecture of the Mayombe Chain has been done by earlier workers since the 19th century. However, the lack of consistent and diverse geochronological data induced several interpretations of its geodynamic setting. The chain was previously thought to be the result of more than two orogenic cycles, and subsequently to be the result of a single orogenic cycle. Recent petrographic, geochronological, geochemical data and paleogeographic reconstructions allow redefining the tectonic context and the main lithostratigraphic subdivisions of the Mayombe Chain. It is currently recognized that the Mayombe Chain consists of three major tectonostratigraphic domains: Western, Central and Eastern, which are prolongated by the Niari-Nyanga foreland basin. These lithounits seem to have been formed through a complex history that included: (i) Late Paleoproterozoic sedimentation, magmatism and metamorphism between 2110 and 1970 Ma related to the Eburnean tectono-thermal event;(ii) Neoproterozoic rifting magmatism and sedimentation;and (iii) Pan-African tectono-thermal event between 620 and 496 Ma leading to the amalgamation of western Gondwana. In this contribution, we review existing literature on this part of the West Congo Belt and summarize the current knowledge of its geologic setting with a focus on structure, lithostratigraphy and metamorphism. We aim to show how current research, with multidisciplinary approaches, changed the interpretation of the Mayombe with consideration to the Araçuaí-West Congo Orogenic system.
文摘· AIM: to determine the frequency and causes of blindness in diabetic Africans. ·METHODS: The study was a cross-sectional survey carried out among known black diabetics consecutively admitted at the Teaching Hospital, University of Kinshasa, between 2005 and 2007. Examination methods included interviewer -administered structured question - naire, eye examinations (visual acuity, tonometry, funduscopy), and fasting plasma glycaemia test. ·RESULTS: Of the 227 patients examined, 15.9% had blindness. Univariate analyses showed significant association between female, severity of diabetic retinopathy, Mayombian ethnic group, use of insulin treatment, low intake of vegetables, diabetic nephropathy, open angle glaucoma and blindness in all diabetics. After logistic regression, only diabetic nephropathy, use of insulin treatment, macular oedema, Mayombian ethnic group and vegetables low intake were the independent risk factors of blindness in all diabetics. However, after logistic regression in the sub -group with diabetic retinopathy, only open angle glaucoma and proliferative diabetic retinopathy were the independent determinants of blindness.·CONCLUSION: The majority of the causes of blindness in these diabetic Africans are avoidable. It is recommended that appropriate diabetes care, nutrition education, periodic eye examination and laser photocoagulation facilities should be provided for treating diabetics in sub-Saharan Africa. ·
