Syllabus Review, Sci. Ser. 3 (2012): 11 - 23 N SYLLABUS REVIEW E S Science Series RESEARCH ARTICLE- GEOSCIENCES Dynamic and evolution of the Mounts Bamboutos and Bamenda calderas by study of ignimbritic deposits (West-Cameroon, Cameroon Line) Gountié Dedzo M.1, Njonfang E.2, Nono A.3, Kamgang P.4, Zangmo Tefogoum G.5, Kagou Dongmo A.3, Nkouathio D.G.3 1Department of Life and Earth Sciences, Higher Teachers’ Training College, University of Maroua, P.O. Box 55, Maroua, Cameroon. [email protected]; 2Laboratory of Geology, Higher Teachers’ Training College, University of Yaoundé I, P.O. Box 47, Cameroon; 3Department of Earth Sciences, University of Dschang, P.O. Box 67, Dschang, Cameroon, 4Department of Earth Sciences, University of Yaoundé I, P.O. Box 812, Yaoundé, Cameroon, 5Higher Institute of Sahel, University of Maroua, P.O. Box 45, Maroua, Cameroon. Received: 13 September 2012 / Revised: 03 December 2012 / Accepted: 05 December 2012 Ecole Normale Supérieure, Université de Yaoundé I, Cameroun Abstract The studies realized in Bambouto and Bamenda volcanoes highlight the volumetric importance of ignimbritic formations that cover about 180 km2 of the massifs with thickness between 150 and 200 m. Their massive lapilli tuff (mlT) and massive lithic breccias (mlBr) facies are made up of various natures of lithic fragments (mainly trachytic) and have same mineralogy made up of alkali feldspar (sanidine and anorthoclase), quartz, plagioclase, clinopyroxene, biotite and oxide. The abundance of fragmented minerals (40-85%), accretionary lapilli and co-ignimbritic breccias in the caldera and its surroundings reflects the highly explosive character of eruptions that have presided the setting up of these rocks and therefore favored collapses which are at the origin of the calderas formation. The structural, petrographic and morphological features show that these calderas belong to explosive and collapse types. Keywords: Cameroon Line, Mounts Bambouto, Mounts Bamenda, ignimbrites, calderas Résumé Les études menées dans les Monts Bambouto et Bamenda mettent en exergue l’importance volumétrique des formations ignimbritiques qui couvrent environ 180 km2 des massifs avec des puissances atteignant 150 à 200 m. Leurs faciès tuf de lapilli massif (Tlm) et brèches de lithiques massifs (Brlm) possèdent en plus de fragments lithiques de nature variée (essentiellement trachytique), une minéralogie identique faite de feldspaths alcalins (sanidine et anorthose), quartz, plagioclase, clinopyroxène, biotite et oxyde. L’abondance de minéraux fragmentés (40-85%), de lapillis accrétionnés et de brèches co-ignimbritiques dans les caldeiras et leurs abords traduit le caractère très explosif des éruptions ayant présidé à la mise en place de ces roches et donc favorisé les effondrements à l’origine de la formation des caldeiras. Les éléments structuraux, pétrographiques et morphologiques montrent que ces caldeiras sont de types d’explosion et d’effondrement. Mots-clés: Ligne du Cameroun, Monts Bambouto, Monts Bamenda, ignimbrites, caldeiras. Introduction Atlantic Ocean to Lake Chad (Fig. 1). It is segmented by N70°E Central Cameroon Shear Cameroon alkaline volcanism is related to a Zone (Fig 1) along the volcanism of Adamawa. major fracture direction N30°E. These fractures The volcanism along the CL seems to have are confined to a band of about 100 km wide started during the Eocene with the and more than 1600 km long known as the emplacement of the Bamoun plateau between Cameroon Line (CL) or Cameroon Hot Line 51.8 and 46.7 Ma (Moundi et al., 2007) and (Déruelle et al., 2007). This line is characterized Mount Bangou between 44.7 and 43.1 Ma by alignment of oceanic and continental (Fosso et al., 2005), and is still active at Mount volcanic massifs, and anorogenic plutonic Cameroon (1999 and 2000 eruption). complexes extending from Pagalu island in the Gountié Dedzo et al. / Syllabus Review, Sci. Ser. 3, 2012 : 11 - 23 Fig. 1. a) Location map of the Cameroon Volcanic Line (CVL). b) The CL (diagonal grey area) with the main volcanic centres and the plutonic complexes (In Nkouathio et al., 2008). Central Cameroon Shear Zone according to Ngako et al. (2006); Fracture zones following Lee et al. (1994) and Ballentine et al. (1997). The products of this volcanism are mainly and Bamenda in relation with the caldera basalt, trachyte, phonolite and rhyolites; genesis. ignimbritic deposits are found only in the continental part of the CL, particularly in the Geological setting Mounts Bambouto and Bamenda. Other small deposits (< 10 km2) are also reported in Mounts Bambouto and Bamenda belong to the Nkogam massif (Kamgang, 1986), and in most important geomorphologic system in the Mount Oku (Dunlop, 1983 and Lissom, 1991). region called West-Cameroon Highlands The purpose of this work is to constrain the (Morin, 1988). ignimbritic deposits of the Mounts Bambouto 12 Gountié Dedzo et al. / Syllabus Review, Sci. Ser. 3, 2012 : 11 - 23 Mounts Bambouto Mounts Bambouto are the third largest volcano Mounts Bamenda (800 km2) of the CL after Mounts Cameroon Mounts Bamenda (600 km2), which constitute and Manengouba. This massif is situated in volumetric importance the fourth largest between longitudes 09°57'E and 10°15'E and volcano of the CL, are the NE extension of the latitudes 05 °27'N and 05 °48'N. Mount Mélétan Mounts Bambouto with which they do not (2740 m) is the highest point of the massif. have clear limit. This massif lies between Volcanic products of the massif are made up of longitudes 10°00'E and 10°30'E and latitudes basalts, trachytes, phonolites, rhyolites and 05°45'N and 06°10'N and culminates at 2621 m ignimbrites. Their ages ranged from 21.12 Ma at Lake Bambili. Petrographic and geochemical to 0.50 Ma (Gouhier et al. 1974; Tchoua, 1974 ; studies (Kamgang et al. 2007; 2008; 2010) show Dunlop, 1983; Fitton and Dunlop, 1985; that the Mounts Bamenda consist of basanites, Marzoli et al. 1999; 2000; Youmen et al., 2005 ; basalts, hawaiites, mugearites, benmoreites, Nkouathio et al. 2008; 10, Kagou Dongmo et al., trachytes, rhyolites and ignimbrites. these 2010). rhyolitic ignimbrite overlays basement rock The lower part of Bambouto massif is (constituted by granite and gneiss) and are composed of basaltic rocks that cover in places covered by laterized basalt. The felsic lavas are thick ignimbritic flow deposits with various most abundant than intermediate types facies (Gountié Dedzo, 2002, 2004; Nono et al., (mugearites and benmoreites). The radiometric 2003, 2004; Gountié Dedzo et al., 2011a). In the dating gives ages ranging from the current to upper and middle parts, felsic rocks (trachyte, 17.4 Ma for the basaltic lavas and from 18.98 phonolite, rhyolite and ignimbrite) Ma to 27.40 Ma for the felsic lavas (Kamgang et predominate and represent 60-65% of the al., 2007, 2008). massif. The Miocene ignimbrites (12.7 to 18.1 Ma) (Gouhier et al. 1974; Tchoua, 1974; Marzoli Mounts Bambouto and Bamenda calderas et al. 1999; Youmen et al., 2005) that lie on The Mounts Bambouto caldera (Fig. 2 and 3a) granitic basement have been considered as the is located in the uppermost zone of the massif first products of volcanic activity of the massif and is an asymmetrical depression with a (Tchoua, 1973). The discovery of enclaves of roughly elliptical shape of about 13 km from basaltic scoriae in some of these ignimbritic West to East and 8 km from North to South. It formations lying on basement rock helped has on its southeast side, subvertical walls (Fig. highlight an early volcanic stage which is 4a) rising up to 1300 m above its floor bristling strombolian and pre-ignimbritic in the south of with trachytic and phonolitic domes and flow- the massif (Nono et al., 2004). domes. These walls are lowered from East to A synthetic revision of the volcanic story of West until it disappears at the opening of the Mount Bambouto is proposed as follows by caldera. Kagou et al. (2010) and Zangmo Tefogoum et The Mounts Bamenda are characterized by the al. (2011). The first stage (precaldera stage), ca. presence of two calderas (Fig. 2 and 3b) of 21 Ma, corresponds to the building of the initial smaller dimensions: the calderas of Santa-Mbu basaltic shield volcano and characterized by (6 x 4 km) and Lefo (4 x 3 km). Their floor the tumescence of the volcanic shield due to located respectively at elevation of 550 m and magma injection giving rise to several annular 400 m is mainly composed of trachytic domes, fissures observed in the whole volcano. The which are also abundant on the external slopes second stage (syncaldera stage), from 18.5 to of the massif (Fig. 4b). 15.3 Ma, is marked by the collapse of the The calderas of these volcanic massifs are caldera linked to the pouring out of ignimbritic covered by felsic rocks; such as trachyte, rhyolites and trachytes. Zangmo Tefogoum ignimbrite, phonolite and rhyolite. These rocks (2007) has shown that the model of formation uneven the calderas morphology; they are of this caldera is comparable to that of Cole et found in prismatic lavas on the caldera rims al. (2005). The third stage (postcaldera stage), and in the protrusions on the caldera floor (Fig. from 15 to 4.5 Ma, renews with basaltic effusive 5). Therefore, the slopes are very diverse in activity, together with post-caldera extrusions several directions; the “V” shaped valleys are of trachytes and phonolites. The 0.5 Ma Totap found in the different directions of the basaltic effusive activity could indicate the calderas. beginning of a fourth phase. 13 Gountié Dedzo et al. / Syllabus Review, Sci. Ser. 3, 2012 : 11 - 23 Environnement Toulouse) laboratory by electron microprobe analyses on CAMECA SX50 apparatus operating at the usual conditions. The ages are from Youmen et al. (2005) (Ar/Ar method), Kamgang et al.