Monatélé Region As in the Bafia and Yaounde Groups Within the Central
Total Page:16
File Type:pdf, Size:1020Kb
Sciences, TechnologiesSciences, Technologies et Développement, & Développement, Volume 15, ISSN pp1-15, 1029 Septembre- 2225 2014 http://www.univ-douala.com/sdt/ Evidence of the ~NE-SW extension in the Sa’a - Monatélé Region as in the Bafia and Yaounde groups within the Central African Fold belt (Cameroon): Implication for the Southern Cameroon Neoproterozoic extension Simon Pierre Mbola Ndzana 1&2 , Joseph Mvondo Ondoa 2* , Sébastien Owona 3, Jean Paul Sep Nlongang 1, Jean Bosco Olinga 1, Paul Bilong 2 1Institute for Geological and Mining Research, Yaoundé, P.O. Box: 4110 Nlongkak, Yaoundé, Cameroon, 2University of Yaoundé I, Faculty of Science, Department of Earth Sciences, P.O. Box: 812 Yaoundé Cameroon, 3University of Douala, Faculty of Science, Department of earth Sciences. P.O. Box. 24157, Cameroon. Received : April 2013 Révision received: August 2013 Accepted: August 2013 Available online: September 2014 Abstract The Sa’a -Monatélé Region within the Neoproterozoic North Equatorial Fold Belt located between the Bafia and Yaounde groups, highlights a polyphase D 1- D4 Pan-African deformation that emplaced the Yaounde tectonic nappe transported top-to the SSW onto the Congo Craton and responsible for its actual geometry. D1 is a compression tectonic phase dominated by the simple shear type, represented by S 0/1 foliation and F 1 folds. D 2 is a general flattening under a pure shear regime associated to the exhumation of D 1 nappe due to the NE-SW extension; responsible for L2 lineations, B 2 boudins, F 2 folds, S 0/1/2 and S 2 foliations in metapelites and metagranitoids, respectively and C 2 shear planes. D 3 mainly an ~E-W shortening and a ~NE-SW extension, emplaced F 3 meso- and large-scale folds that form tectonic units of the Yaounde nappe. D 4 was an N-S to NE-SW shortening, perpendicular to the D 3 once and represented by F4 large-scale folds. C 3 shear zones as the NE-SW Central Cameroon shear zone was emplaced under the same r egime. The Sa’a -Monatélé Region may correspond mostly accordingly to a thrusting zone than a real tectonic suture between the Bafia and Yaounde groups. In comparison, D 1-D4 stages show similar geometry and kinematics in Bafia and Yaounde groups. Both groups are composed of similar rock types made of low- to high grade metapelites, amphibolites and metagranitoids with comparable volcano-sedimentary origin for metapelites and amphibolites; Archean to Proterozoic (Nd model ages 3500-2100 Ma) in metapelites, crustal rocks with minor mantle contributions origin for metagranitoids; the Neoproterozoic as the same maximum deposit age fixed at ~1600 Ma (U/Pb-, Pb/Pb-detrital zircons) as well as a comparable syntectonic granitoids emplaced between ~625 – 600 Ma (U/Pb-, Pb/Pb- zircons); an analogous Pan-African orogeny (650-542 Ma) with the retrograde phase from the granulite to amphibolite facies that ranges 616 – 540 Ma (Sm/Nd-garnet, EMP-monazite, Rb/Sr- whole rock - biotite ± muscovite). The admitted Bafia and Yaounde groups that outline the similar litho-chemistry nature and origin, a comparable Pan-African thermotectonic evolution can therefore be considered as a single and a same litho-chrono-thermo-structural unit: the Yaounde Group. ISSN 1029 –2225©2014 Sciences, Technologies et Développement os Key words : Pan-African tectonics, Sa’a -Monatélé Region, Yaoundé Group, North Equatorial Pan-African Fold Belt, Cameroon. Résumé La région de Sa’a -Monatélé située au cœur de la chaine néoprotérozoïque nord équatoriale située entre les groupes de Bafia et de Yaoundé, met en relief une déformation panafricaine polyphasée D 1-D4, responsable de la mise en place la nappe de Yaoundé transportée vers le SSW sur le Congo Craton et sa géométrie actuelle. D 1 est une phase compressive dominée par un cisaillement simple, représentée par la foliation S 1 et les plis P 1. D 2 est un aplatissement général en régime de cisaillement pure associée à une extension NE-SW provoquant l’exhumation de la nappe mise en place au cours de D 1 ; responsable des linéations L 2, des boudins B 2, des plis P2, les foliations S 0/1/2 et S 2 respectivement dans les metapelites et les métaplutonites et des plan de cisaillement C2. D 3 dominée par un raccourcissement ~E-W et une extension ~NE-SW, a mis en place des plis P 3 méso- et cartographiques formant les unités tectoniques de la nappe de Yaoundé. D 4 un raccourcissement N-S à NE-SW perpendiculaire à celui de D 3, représentée par des plis P 4 cartographiques. Les cisaillements C 3 à l’instar du Cisaillement Centre Camerounais ont été mis en place sous le même régime. La région de Sa’a -Monatélé correspondrait donc beaucoup plus à une zone de chevauchement qu’à une zone de suture entre les groupes de Bafia et de Yaoundé. En comparaison, D 1-D4 sont des déformations similaires dans les groupes de Bafia et de Yaoundé. Les deux groupes possèdent des types lithologiques identiques que sont de métapelites de faible à haut degrés métamorphiques et des amphibolites, d’origine volcano -sedimentaire ainsi que des métagranitoïdes d’origine crustale archéenne à protérozoïque (âges Nd 3500-2100 Ma) avec des contributions mantelliques ; une même période maximale de dépôts, le Néoprotérozoïque, fixée à ~1600 Ma (U/Pb-, Pb/Pb-zircons détritiques) ainsi qu’une période comparable de mise en place des granitoïdes syntectoniques fixée entre ~625 – 600 Ma (U/Pb-, Pb/Pb-zircons); une orogénèse panafricaine analogue (650-542 Ma) dont la phase rétrograde du faciès des granulites à celui des amphibolites est fixée entre 616 – 540 Ma (Sm/Nd-grenat, EMP-monazite, Rb/Sr-roche totale - biotite ± muscovite). Les groupes de Bafia et de Yaounde qui admettent une origine et une nature litho-géochimique similaires, une évolution thermotectonique panafricaine semblable peuvent être considérées en une seule et même unité litho- chrono-thermo-structurale: Le groupe de Yaoundé. ISSN 1029 –2225©2014 Sciences, Technologies et Développement os Mots clés : Tectonique panafricaine, Région de Sa’a -Monatélé, Groupe de Yaoundé, Chaine panafricaine nord équatoriale, Cameroun. 1. Introduction such as pertrographical, structural, mineralogical, geothermo- 1The world is comprised of cratons, mobile zones and barometrical, geochemical and geochronological. Phanerozoic covers differentiated into lithostructural units (Fig. Lithostructural units such as Bafia, Yaounde, Mbalmayo 1a, b). These lithostructural units have been unfortunately, Mbengbis, Ayos, Lower Nyong in South Cameroon were mainly defined by pioneers without a maximum of criterion typified as “ series ” (e.g. Lasser e et Soba, 1979; Noizet, 1982, Nédélec et al., 1986), term dedicated for chronostructural Corresponding author: [email protected] ones. 1 Mbola et al., Sciences, Technologies et Développement ( Septembre 2014 ), Volume 1 6, 1-15 Sciences, Technologies & Développement, ISSN 1029 - 2225 Figure 1. (a) Geological sketch of the West-Central Africa and South America connection with cratonic masses, Paleo- and Neoproterozoic provinces of Pan-Gondwana belt in a Pangea reconstruction modified from Castaing et al. (1994) and Ngako et al. (2003). CC: Congo Craton; KC: Kalahari craton; SFC: Sao-Francisco Craton; TK: Tanganyika Craton; WAC: West African Craton; CMR: Cameroon; CAR: Central African Republic; EG: Equatorial Guinea. CCSZ: Central Cameroon Shear Zone; SF: Sanaga Fault. Dashed lines mark the country boundaries. (b) The southern Cameroon geological map (modified after Ngako et al. , 2003; Ngnotue et al. , 2000; Njonfang et al. , 2008; Nzenti et al. , 2006; Toteu et al. , 2006): SCSG: Southern Cameroon Super group; NC: Ntem complex; NyC: Nyong Complex; YG: Yaounde Group, DG: Dja Group, YoG: Yokadouma Group, SOG: Sembe-Ouesso Group; SF: Sanaga fault. Black arrows indicate Nyong nappe transport toward the East onto to NC. White arrows indicate Yaounde nappe transport toward the SSW onto to NC and NyC. The location of study area is shown (c). 11 Mbola et al., Sciences, Technologies et Développement ( Septembre 2014 ), Volume 1 6, 1-15 Sciences, Technologies & Développement, ISSN 1029 - 2225 Figure 2. Geological sketch of the Yaounde Group modified from Champetier de Ribes and Aubague, 1956; Weecksteen, 1957, Owona et al. (2011a, 2013). NEFB: North Equatorial Fold belt; SCSG: Southern Cameroon Supergroup; WCAFB: West Central African Fold Belt; CC: Congo Craton. When talking of lithostructural units, these denominations Bafia and Yaounde as groups. These have been previously have to be replaced by “Complex”, “Group”, “Formation” typified as two Precambrian units without a clear identified or according to the international union of the geologist society mapped suture zone in the Sa’a -Monatélé Region (SMR), (IUGS) classification (Neville, 1976). In this work, we consider between both groups (Fig. 1c; Noizet, 1982). Recent 3 Mbola et al., Sciences, Technologies et Développement ( Septembre 2014 ), Volume 1 6, 1-15 Sciences, Technologies & Développement, ISSN 1029 - 2225 geochemical and geochronological data show that the Bafia a sub- to horizontal pure shear regime, is characterised by the and Yaounde groups derived from Neoproterozoic volcano- S0/1/2 foliation in metapelites and S 2 foliation in metaplutonites, sedimentary protoliths and experienced the Pan-African the L 2 lineations, the B 2 boudins, the F 2 asymmetric, isoclinal pressure-temperature-deformation-time (P-T-d-t) evolution folds and C2 shear planes. The D 3 deformation phase is a ~E- (Nzenti et al. 1988; Mvondo et al., 2003; Mvondo et al., 2007; W transcurent tectonics regime that induced F 3 meso- and Tchakounté et al. 2007; Ganwa et al., 2008; Owona et al., megafolds that form structural units of the Yaounde nappe 2011b). The SMR, poorly known, is located at 4°10’ - 4°35’’ N (Mvondo et al. 2003; 2007; Mvondo Ondoa, 2009; Owona et and 11°10’ - 11°35’ E, between the above Bafia and Yaounde al. 2011a). This nappe is dissected by C 3 shear zones as the groups (Fig.