Syllabus Review, Sci. Ser. 3 (2012): 11 - 23

N SYLLABUS REVIEW E S Science Series

RESEARCH ARTICLE- GEOSCIENCES

Dynamic and evolution of the Mounts and by study of ignimbritic deposits (West-, )

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 , 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 , 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 (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 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, , 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 , , and ; 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

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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 , , , 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 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, 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.

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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. (2007) (K/Ar method) and Fitton and Dunlop (1985) (K/Ar method).

Fig. 2. Digital elevation model (DEM) of Mounts Bambouto and Bamenda showing the different calderas (Shuttle Radar Topography Mission; SRTM, NASA).

Fig. 5: a) and b) Geological cross-sections of the Mounts Bambouto caldera (modified after Zangmo et al., 2011); c) Geological cross-section of the southern slope of Mounts Bambouto with a part of caldera; d) and e) Geological cross-sections of the Mounts Bamenda calderas (modified after Zangmo et al., 2011)

Field observations and petrography

The ignimbrites of Mounts Bambouto

Fig. 3. Geologic sketch maps of Mounts Bambouto (a) and Bamenda (modified after Kamgang et al., 2010) (b). The ignimbrites outcrop discontinuously on about 17% of the massif (Fig. 3a) representing approximately 135 km2 for a total volume estimated at 13.5 km3. This volume is really much important (> 20 km3) because these formations are covered in the South of the massive by generally lateritized basalts. In the lower zone of the massif, they lie on the metamorphic basement, whereas in the upper zone, they cover the trachytes. The different facies which are essentially massive lapilli tuff (mlT, Branney and Kokelaar, 2002), were Fig. 4. a) SE border of Mounts Bambouto caldera. b) NE identified in many localities (Dschang, border of Lefo caldera constituted by subvertical prismatic Baranka, Mbeng, Lepo, Nzemla I and Nzemla trachyte II); the massive lithic breccia facies (mlBr, Methodology and analytical methods Kokelaar and Branney, 2002) outcrop at Mbou. The largest outcrop covers an area of about 17 The methodology of this work is essentially km long and 3 to 3.5 km wide with thickness based on field observations, thin sections ranging from 10 to 120 m. These ignimbrites studies and bibliographic review. Nature of have high aspect ratio of about 1.5 x 10-2 to 3.2 feldspars, the most dominant mineral phase, x 10-2. Indeed, the general shape of an was investigated in GET (Géosciences ignimbrite sheet may, apart from its volume,

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be simply and quantitatively described by the the majority of the enclaves with accessorily aspect ratio which is defined as the ratio of those of trachytes, vitrophyre and granitic average sheet thickness to the diameter of a basement rocks (Table 1). The ashy matrix with circle that covers the same area as the sheet non-devitrified glass shards is made of alkali (Walker, 1983, Wilson et al. 1995; Freundt et al., feldspar (15%), quartz (5%), plagioclase (2%), 2000). The ratios of the ignimbrites range from oxides (2%) and biotite (1%). < 10-5 (low ratio) to > 10-2 (high aspect ratio). In the field, according to the degree of welding The ignimbrites of Lepo, Nzemla I and II we distinguish the welded and non-welded Nzemla ignimbrites. The ignimbrite sheet, which extends from southwest of Bangang to Fongo-Tongo (Fig. 3a) The welded ignimbrites covers an area of about 50 km2 with a thickness ranging between 45 to 90 m; it is constituted by The welded ignimbrites are abundantly three main outcrops: Lepo, Nzemla I and II represented in the southern slopes of the Nzemla. Bambouto massif. They are classified according The ignimbrite of Lepo is a mlT facies lying on to the density, the degree of welding and the a granitic basement and covered in places by color of the rock. The nature and ratio of lithic lateritized basalts (Fig. 3a). It consists of a fragments vary from one facies to another simple cooling unit represented by two flow (Table 1). units. The lowermost light gray unit has a vitroclastic and eutaxitique texture The ignimbrites of Dschang and Mbeng characterized by stretched and distorted The ignimbrites of Dschang (18.1 Ma; Youmen devitrified fiammes (10-15%), which are et al., 2005) outcrop in sheets in the preferentially oriented. The upper dark grey valley on about 9 km2 with a maximum unit is less rich in fiammes (<10%). thickness ranging between 80 and 120 m. Its The ignimbrite of Nzemla I is represented by a mlT facies consists of a simple cooling unit dark gray mlT facies constituted only by one made of two flow units overlay by basalt flow flow unit covered by a mlBr facies with limited (Fig. 3a and 5c). The fiammes with ovoid to extension. The texture is marked by eutaxitique lenticular shapes represent 5-20% of the rock. fiammes (5%) with unidirectional orientation. The welded ignimbrites of Mbeng, identified The ignimbrite of Nzemla II is characterized by along the roadcut (Fig. 6a) are covered a whitish mlT facies which consists of one flow successively by a first paleosol, non welded unit covered by a non-welded mlT overlay by ignimbrites, rhyolitic flow, a second paleosol basalt flow (Fig. 3a). The fiammes (15-20%) are and finally the second deposit of non welded often black and generally has lenticular shapes. ignimbrites. The thickness and the area The lithic fragments of different ignimbrites are covered by this facies cannot be directly essentially trachytic (10-20%). Enclaves of known. Its mlT facies also consists of two flow granite, scoriae, ignimbrite and vitrophyre are units representing one simple cooling unit. The less represented (Table 1). Devitrified matrix fiammes represent 5-10% of the rock. (50-90%) are made up of alkali feldspar (10- The enclaves of rocks, mostly trachytic are 35%; sanidine and anorthoclase: Ab66-44Or55-31; more abundant in the upper units (10-15%). Table 2), plagioclase (1%; oligoclase: Ab81- Enclaves of granites, vitrophyre and fragments 86An13-16), quartz (2-5%), biotite (2%), oxides (1- of fossilized wood entirely or partially charred 2%) and clinopyroxene (1%). are less represented (Table 1). The matrix is made of alkali feldspar (15-25% sanidine and The ignimbrites of Baranka anorthoclase: Ab68-59Or37-31; Fig. 4b, Table 2), The welded mlT ignimbrites of Baranka (15.28- plagioclase (1 -2%), quartz (3-5%), biotite (2%), 15.5 Ma; Youmen et al., 2005) occupy the oxides (2%), clinopyroxene (1%). uppermost zone of the massif (Fig. 3a); it covers intermittently the south-western rim of The ignimbrite of Mbou the caldera and the bottom of the depression, The ignimbrite of Mbou (Fig. 6c) is a mlBr representing a total surface of about 10 km2. facies represented by one flow unit and covers The true thickness of the deposits (> 30 m) is an area of 5 km2 with a thickness not exceeding difficult to estimate because of the hilly relief 25 m. Black scoriae (20-25%) (Fig. 6b) constitute and dense vegetation cover. Several ignimbritic

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deposits phases are present with intercalation The welded ignimbrites of trachytic and basaltic flows (Fig. 7a). The welded ignimbrites outcrop as sheets, Enclaves of these ignimbrites are mainly metric and decimetric blocks balls. The rock is trachytes (10-20%) with accessorily those of massive and compact with variable color ignimbrites, charred wood and vitrophyre depending of the type of the facies. The (Table 1). The microscopic observations show proportions of lithic fragments and minerals identical mineralogy in different flow unit and are much less important (Table 1) in the consists of alkali feldspar (15% sanidine and ignimbrites of Mt Bamenda compare with anorthoclase: Ab64-61Or37-35; Table 2), quartz those of Mt Bambouto. (<5%), plagioclase (2%), oxides (2%), biotite (1%), clinopyroxene (1%). The ignimbrites of Bamenda, Mbu, The ignimbrites of Bamenda (11 km2), Mbu (3 The non-welded ignimbrites km2) and Mbengwi (12 km2) have thickness ranging between 50 and 200 m and lie on a The non-welded ignimbrites are volcanic tuff basement rock made of granite and syenite and also belong to mlT facies. These deposits (Fig. 3b). The welded mlT facies of Bamenda cover about 60 km2 of the massif (Fig. 3a). The has two light gray cooling units covered by rocks outcrop in the lowermost zone of the mlBr facies while that of Mbu is represented by massif, Southwest of Dschang, North of Mbeng a single dark gray cooling unit covered by a and surrounding localities. In the uppermost non-welded mlT above, which lie a mlBr facies zone, it occupies the major part of the caldera with very limited expansion (Fig. 7b). The where they are inserted in places in the entire unit of Mbengwi are made of Brlm facies trachytic flows and the welded ignimbrites. overlay in the SW by basalt flow (Fig. 3b). The Petrography of tuffs is similar to those of fiamme (5-10%) are predominantly black in welded ignimbrites. Trachytic lithic fragments Bamenda and generally flattened, giving to the (average diameter: 7 x 5 cm) are more rock a eutaxitique texture. Lithic fragments (10- abundant (50-60%); however, they reach 15%) of mlT facies are mostly trachytes, several meters in some cases (sharp fragments rhyolites and granites (Table 1); in brecciated of trachyte and ignimbrites measuring 4.5 x 6 part of the deposit, their proportions reach 30 m at Fotang in the Mts Bambouto caldera). to 40%. The matrix is made of alkali feldspar These lithics are similar to co-ignimbritic (5-10%; sanidine: Ab62-58Or42-38; Table 2), quartz breccias generally associated with subsidence (1-3%), oxides (1-3%), plagioclase (1-2%), related to the formation of calderas. biotite (1 -2%) and clinopyroxene (1%). Accretionary lapilli (up to 10%), consisting of agglomerated ash are also present in all non- The ignimbrites of Bambili, Sabga and Big welded mlT with variable size (0.6 to 2.5 cm in Babanki diameter). Ashy fine particles of the matrix The ignimbrites Bambili (6 km2), Big Babanki (3 represent 25-30% of these formations. km2) and Sabga (5 km2) form small outcrops (Fig. 3b) with maximum thickness between 70 The ignimbrites of Mounts Bamenda and 120 m. The remnant blocks (up to 6.5 x 11 m) of ignimbrites are also observed at Bambili The ignimbrites of Mounts Bamenda (Fig. 3b) (Fig. 6d). They lie on a basement rocks made of constitute about 7.5% of the rocky outcrops of granite, schist and migmatite. The ignimbrite 2 the massif representing approximately 45 km of Bambili is a welded mlT with trace of 3 with a volume estimated at 6.3 km . As in the rubefaction in places, characterized by two case of Mounts Bambouto, ignimbrites lie on a cooling units (dark-grey and whitish) whereas granito-gneissic basement and are covered by the ignimbrite of Big Babanki is represented lateritized basalt. The different facies have been only by a whitish unit; the two outcrops are identified at Bamenda, Bambili, Big Babanki, covered in places by a mlBr facies. The Babanki Tungo, Mbengwi and on inner edges ignimbrite of Sabga is also a welded mlT and bottoms of the Santa and Mbu calderas. It constituted by one dark-gray unit covered by - are also high aspect ratio ignimbrites (2.77 x 10 one light grey unit. The fiammes (30-40%) are 2 -2 to 7.23 x 10 ). generally distorted in the basal parts of the outcrops. Enclaves of rocks (10-15%) are

represented by trachytes, rhyolites, vitrophyre,

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ignimbrites and granites (Table 1); they depend of the degree of compaction and their represent 65 to 70% in mlBr facies of Bambili position in the deposit sequences. Some of where there are mainly enclaves of black them are very flattened, distorted (Bambili and vitrophyre. The matrix is much devitrified and Lepo) and preferentially oriented, giving to the is made up of alkali feldspar (5-10%; sanidine: rock a eutaxitic texture. Devitrification is very Ab58-57Or57-41; Table 2), quartz (2-3%), advanced in most fiammes (Fig. 6f,i) and is plagioclase (1-3%), oxides (1-3%), biotite (1%) characterized by the formation of and clinopyroxene (1%) (Fig. 6e). quartzofeldspathic fibres with radial disposition in relation to the axial channel The non-welded ignimbrites (axiolitic structure). When the channel is absent, the fibres have a spherulitic The non-welded ignimbrites outcrop in the arrangement; they constitute in this case the Santa-Mbu caldera on about 2 km2, at Santa spherulites (Fig. 6f,ii). Splinters of glass, Coffee and Ndzah in the Lefo caldera on about products the bursting of bubble coming from 3 km2. In the Lefo caldera, they lie on a pumice are numerous and generally have a basement rock consisting of micaschist covered devitrification on their borders. successively by trachytic and basaltic flow (Fig. 7c). Their exact thickness (> 20 m) is difficult to Enclaves of rock formations in ignimbrites assess because of the abundant vegetation and uneven terrain. As in the Bambouto massif, the In both massifs, enclaves of trachyte are the rock is very powdery and consists of enclaves most abundant comprising mostly fragments (20-25%) mainly trachytic in nature with minor of juvenile lava, having a typical trachytic proportion of ignimbrite, rhyolite, granite and structure marked by the preferential obsidian. The average size of these enclaves is 3 orientation of the laths of alkali feldspar (about x 2.4 cm; few enclaves reached 1.5 x 2.5 m. As 75%). in Mount Bambouto, they have characteristics Enclaves of rhyolite are abundant in the close to co-ignimbritic breccias. The brecciated zones and their porphyritic texture accretionary lapilli are poorly represented is made of quartz (5%), alkali feldspar (10-15%) (<5%) probably due to the very advanced state and a dark grey glass. of alteration in the deposits. In the ashy matrix, The enclaves of vitrophyre or obsidian are there is presence of quartz, feldspar and relict shiny black and are also juvenile lava of pumices. fragments; they are most represented in mlBr The ignimbritic volcanism of the CL began in facies of Bambili and Mbu. In welded mlT, Mounts Bamenda by non-welded deposits in their proportions are generally low. The the Lefo caldera (> 22 Ma; Fig. 7c) followed by vitreous mass is formed in some cases of alkali those of the Santa-Mbu caldera (> 20.6 Ma; Fig. feldspar (< 2%). 7b). The ignimbrites are younger in Mounts Enclaves of granites are observed in Bambouto; the first deposits are observed in ignimbrites of Dschang, Nzemla II Baranka Southern lower slope of the volcano, in and Bambili. Those of Dschang are very flat Dschang ocality (18.1 Ma). Three episodes of (Fig. 6g) and show signs of melting at their ignimbritic deposits are identified in the edges. Their texture is granular or Bambouto caldera; each deposit lie on a microgranular and the mineralogy is made up trachytic flow: the first episode (approximately of quartz, alkali feldspar, microcline, biotite 15.5 Ma) is the most important in terms of and plagioclase (Fig. 6h). volume of deposit; the second episode is dated The enclaves of ignimbrites are present in at 15.28 Ma; the latter younger than 12.93 Ma, Bambili, Mbu, Mbengwi, Lepo and Baranka. is essentially represented by non-welded Despite their advanced state of alteration, deposit. trachytic enclaves and fiammes are still identifiable. Eutaxitic texture is still observable The fiammes in most cases. The enclaves of scoriae identified at Nzemla I, The fiammes are flat pumices present in almost Baleveng Mbengwi are dark and very porous; all welded ignimbrites. Their abundance in their presence in ignimbrites which lie on different sites is very variable (5-60%). Their basement rock, strongly suggests that volcanic shapes display a wide range of geometry and activity in the Mount Bambouto began with an

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explosive strombolian phase (Nono et al ., and Sparks 1984; Heiken and McCoy 1984; 2004). Scandone 1990; Palladino and Simei, 2005). So Microphotograph showing the presence of it is the presence of large volumes of spherulite in the matrix of Bambili ignimbrites. pyroclastic rocks, in particular ignimbrites g) Presence of flat enclave of granite in widely widespread on the slopes, edges and Dschang ignimbrite. h) Microphotograph of inside the Bambouto and Bamenda calderas, Dschang ignimbrite showing an enclave of that provides the main argument for the granite with their mineral association. Qtz: interpretation in favour of the collapse after quartz; San: sanidine; Anor: anorthoclase; Bt: highly explosive eruptions, as in the case of the biotite; Mi: microcline Las Canadas caldera in the Canary Islands (Marti et al. 1994; Aubert and Keiffer, 1998) and Discussion and conclusion Vulsini caldera in Italy (Palladino and Simei, 2005). The current estimated volume of the Mounts Bambouto and Bamenda have the ignimbritic deposits of Mounts Bambouto (13.5 characteristics of shield volcanoes where the km3) and Bamenda (6.3 km3), is in fact below felsic lavas are most abundant compare to the the actual volume (Gountié Dedzo et al., basaltic lavas. The presence of ignimbritic 2011b). Indeed, in addition to the erosion that deposits in their summit area associated to the has greatly reduced the volume of the less calderas is consistent with their important consolidated deposits since about 13 Ma (age of contribution to the genesis of these calderas the youngest ignimbritic deposit), large areas (Wood, 1984; Lipman, 1997, Roche and Druitt, of ignimbrites were covered by recent basaltic 2001). and trachytic lavas inside and outsite the As it is widely reported in the volcanological calderas (Fig. 3 and 5). literature, major explosive eruptions often occur as causes and/or effects of caldera collapses (e.g., Smith and Bailey 1968; Druitt

Table 1. Proportions of the main components of Mounts Bambouto and Bamenda ignimbrites. Total glass represents glass shards and glass of the matrix, pumices and fiammes. Proportions of crystals in mineralogy correspond to free crystals in the matrix

Locality Total glass Devitrification Mineralogy Lithic fragments 10-15% KFs, 3% Qtz, 2% Pl, 2% Ox, 5-10% trachyte, 2% granite, Dschang 60-65% 30-60% 20-25% 10-15% 2% Bt, 1% Cpx. 2-5% vitrophyre, 1% rhyolite 15-25% KFs, 5% Qtz, 1% Pl, < 2% 10-15% trachyte, < 5% rhyolite, 2% carbonized Mbeng 40-45% 15-20% 25-35% 15-25% Ox, 2% Bt, 1% Cpx. wood, 2% granite, 1% vitrophyre 15 % KFs, 5% Qtz, 2% Pl, 20-25% scoriae, 5%trachyte, Mbou 40-45% 5% 25% 30-35% 2% Ox, 1% Bt. 3% vitrophyre, 2% granite. 10-20% trachyte, 2% granite, 10-25% KFs, 2% Qtz,

Bambouto Lepo 35-50% 60-90% 20-30% 15-20% 2% ignimbrite, 1% scoriae, 1% Pl, 2% Ox. 1% vitrophyre. 10-15% KFs, < 1% Pl, 10% trachyte, 2% granite, 2% scoriae, Nzemla I 60-70% 60-80% 15-25% 15% Mounts 2% Qtz, 1% Ox, 1% Cpx. 1% vitrophyre. 30-35% KFs, 5% Qtz, 15% trachyte, 5% granite, Nzemla II 30-35% 50-70% 40-45% 25% 1% Pl, 2% Bt, 2% Ox. 3% carbonized wood, 2% rhyolite 15% KFs, < 5% Qtz, 2% Pl, 10-20% trachyte, < 3% ignimbrite, Baranka 50-60% 30-40% 25% 15-25% 2% Ox, 1% Bt, 1% Cpx. 1% carbonized wood, < 1% vitrophyre. 10% KFs, 2% Qtz, 1% Pl, Bamenda 75% 10-15% 15% 10% 8% trachyte, 2% rhyolite, 1% granite. 1% Bt, 1% Ox.

5% KFs, 2% Pl, 1% Ox, 5-10% trachyte, 2% granite, Mbu 75-80% 30% 10% 10-15% 1% Qtz, 1% Cpx 3% ignimbrite. 10% KFs, 3% Qtz, 2% Pl, 15% trachyte, 2% ignimbrite, 1% scoriae, Mbengwi 55% 5% 20% 25%

2% Bt, 3% Ox. 1% vitrophyre, 1% carbonized wood. Bamenda 5-10% KFs, 2% Qtz, < 3% Pl, 3% Ox, 5% trachyte, 2% vitrophyre, 2% Bambili 70-80% 50-95% 10-20% 10% 1% Bt, < 1% Cpx. ignimbrite, 1% granite. 5% KFs, 3% Qtz, 1% Pl, 10% trachyte, 3% rhyolite, 1% granite, Mounts Big Babanki 75% 5-10% 10% 15% 1% Ox. 1% vitrophyre. 5-10% trachyte, 3% vitrophyre, Sabga 70-85% 5-10% 5-15% 5-10% KFs, 2% Qtz, 2% Pl, 1% Ox. 10-15% 1% rhyolite, 1% granite

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Table 2. Representative microprobe analyses of feldspars from selected ignimbrite samples from Mounts Bambouto and Bamenda.

Sanidine Anorthoclase Oligoclase Localisation Mounts Bambouto Mounts Bamenda Mounts Bambouto Dscanhg Nz I Nz II Baranka Lepo Bambili Mbgenwi Dschang Nz I Nz II Baranka Lepo Lepo Sample No. GM2 GM7 GM12 GM19 GM13 GM26 GM32 GM2 GM7 GM12 GM19 GM13 GM14 Analysis No 4 12 5 36 43 57 19 24 25 26 28 2 20 6 35 42 54 20 21 22

SiO2 67,21 67,96 66,38 67,94 67,33 66,51 67,14 67,8 67,02 67,71 68,03 67,81 67,47 66,17 66,64 66,51 65,83 66,83 65,82 67,12 TiO2 0,1 0,06 0,09 0,00 0,00 0,00 0,18 0,00 0,11 0,00 0,01 0,02 0,08 0,13 0,09 0,42 0,62 0,23 0,00 0,00 Al2O3 19,25 18,77 19,67 18,73 19,32 18,94 18,88 18,69 18,79 18,50 18,26 18,73 19 19,69 19,24 17,03 16,62 18,56 21,86 19,60 FeO 0,18 0,2 0,27 0,25 0,32 0,42 0,34 0,29 0,25 0,56 0,80 0,37 0,38 0,07 0,10 2,03 2,71 0,56 0,04 0,24 MnO 0,00 0,01 0,00 0,00 0,00 0,00 0,00 0,00 0,00 0,00 0,00 0,02 0,00 0,08 0,00 0,03 0,00 0,00 0,04 0,00 MgO 0,00 0,00 0,00 0,00 0,00 0,00 0,00 0,00 0,00 0,00 0,00 0,00 0,00 0,00 0,00 0,23 0,30 0,00 0,04 0,01 CaO 0,42 0,26 0,87 0,14 0,38 0,30 0,31 0,14 0,16 0,05 0,00 0,13 0,27 1,19 0,64 0,44 0,59 0,07 2,79 3,41 BaO 0,00 0,00 0,05 0,09 0,00 0,00 0,00 0,00 0,00 0,00 0,03 0,09 0,00 0,36 0,23 0,00 0,00 0,00 0,10 0,00

Na2O 6,89 7,08 6,12 6,78 6,85 7,17 6,78 6,52 6,61 7,21 6,23 7,47 7,31 6,61 7,92 7,44 7,35 7,64 10,37 9,66 K2O 6,66 6,63 7,05 7,03 6,43 6,53 7,26 7,27 7,17 6,86 7,56 6,27 5,98 6,62 5,72 5,92 6,06 6,14 0,18 0,47 Total 100,71 100,97 100,50 100,96 100,62 99,89 100,89 100,71 100,11 100,89 100,93 100,91 100,49 100,92 100,58 100,4 100,08 100,03 101,25 100,52

Si 2,98 3,01 2,96 3,01 2,99 2,98 2,99 3,01 3,00 3,01 3,02 3,00 2,99 2,95 2,97 2,99 2,97 2,99 2,87 2,94 Ti 0,00 0,00 0,00 0,00 0,00 0,00 0,01 0,00 0,00 0,00 0,00 0,00 0,00 0,00 0,00 0,01 0,02 0,01 0,00 0,00 Al 1,01 0,98 1,03 0,98 1,01 1,00 0,99 0,98 0,99 0,97 0,96 0,98 0,99 1,03 1,01 0,90 0,88 0,98 1,12 1,01 Fe3+ 0,01 0,01 0,01 0,01 0,01 0,02 0,01 0,01 0,01 0,02 0,03 0,01 0,01 0,00 0,00 0,08 0,10 0,02 0,00 0,01 Mn 0,00 0,00 0,00 0,00 0,00 0,00 0,00 0,00 0,00 0,00 0,00 0,00 0,00 0,00 0,00 0,00 0,00 0,00 0,00 0,00 Mg 0,00 0,00 0,00 0,00 0,00 0,00 0,00 0,00 0,00 0,00 0,00 0,00 0,00 0,00 0,00 0,02 0,02 0,00 0,00 0,00 Ca 0,02 0,01 0,04 0,01 0,02 0,02 0,02 0,01 0,01 0,00 0,00 0,01 0,01 0,06 0,03 0,02 0,03 0,00 0,13 0,16 Ba 0,00 0,00 0,00 0,00 0,00 0,00 0,00 0,00 0,00 0,00 0,00 0,00 0,00 0,01 0,00 0,00 0,00 0,00 0,00 0,00 Na 0,59 0,61 0,53 0,58 0,59 0,62 0,59 0,56 0,57 0,62 0,54 0,64 0,63 0,57 0,68 0,65 0,64 0,66 0,88 0,82 K 0,38 0,37 0,40 0,40 0,36 0,37 0,41 0,41 0,41 0,39 0,43 0,35 0,34 0,38 0,33 0,34 0,35 0,35 0,01 0,03

Ab 59,89 61,11 54,45 59,04 60,66 61,61 57,81 57,29 57,9 61,37 55,59 64,02 64,16 56,87 65,80 64,26 63,02 65,20 86,22 81,49 An 2,02 1,24 4,28 0,67 1,85 1,45 1,46 0,68 0,77 0,23 0,02 0,62 1,31 5,66 2,94 2,08 2,81 0,33 12,82 15,90 Or 38,09 37,65 41,27 40,28 37,49 36,94 40,73 42,03 41,32 38,4 44,38 35,36 34,53 37,47 31,27 33,65 34,17 34,47 0,97 2,61

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Fig. 6. a) Roadcut with ignimbrite outcrop in Mbeng locality. b) Microphotograph of Mbeng ignimbrite showing devitrified matrix and multiple fragments of minerals. c) Presence of multiple enclaves of scoriae in Mbou ignimbrite. d) Remnant blocks of Bambili ignimbrites with enclave of trachyte (i) and vitrophyre (ii). e) Microphotograph of Sabga ignimbrite with crystals and rock inclusions. f) (i) Microphotograph showing eutaxitic textutre of the devitrified fiammes in Bambili ignimbrites; (ii) Microphotograph showing the presence of spherulite in the matrix of Bambili ignimbrites. g) Presence of flat enclave of granite in Dschang ignimbrite. h) Microphotograph of Dschang ignimbrite showing an enclave of granite with their mineral association. Qtz: quartz; San: sanidine; Anor: anorthoclase; Bt: biotite; Mi: microcline

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Fig. 7. Synthetic stratigraphic sections in Bambouto caldera (a), Santa-Mbu caldera (b) and Lefo caldera (c). The ages are from Youmen et al. (2005) (Ar/Ar method), Kamgang et al. (2007) (K/Ar method) and Fitton and Dunlop (1985) (K/Ar method).

The high aspect ratio (1.5 x 10-2 to 7.23 x 10-2) avalanches at high temperatures. The reflects the concentrated character of ignimbrites of Mounts Bambouto and pyroclastic flows at the origin of the formation Bamenda are probably considered to result of ignimbrites of Mounts Bambouto and from the first process, because in addition to Bamenda. The important proportion of rocky evidence of violent explosions, there is a lithic fragments (10-35%) in most ignimbritic presence of juvenile lava fragments resulting facies and those of fragmented minerals (40- from the explosion of magma. 85%) reflects the violence of eruptions The presence of specific facies in pyroclastic (Branney and Kokelaar, 1991) that governed deposits can be assumed as indicators of the the emplacement of ignimbritic formations. formation of a caldera. Large volumes of ash The accretionary lapilli found in the calderas associated to deposits of pumice and lithic and surroundings of these massifs, are deposits fragments compared to co-ignimbritic breccias of pyroclastic falls in hydrated conditions and observed in calderas here studied are generally are setting up during the late phases of interpreted to mark the onset of caldera by eruption after the collapse of the top of the collapse (Druitt and Sparks, 1982 Walker, 1985; magma reservoir (Sigurdsson et al. 1995; Druitt and Bacon, 1986, Mellors and Sparks, Palladino and Simei, 2005). These lapilli are 1991; Perrotta and Scarpati, 1994, Rossi et al., coming from hydromagmatic eruptions 1996). These co-ignimbritic breccias rich in characterized by strong explosions pulverizing coarse lithic fragments are proximal facies the overlying bedrock during the confined around and inside the vents and are transformation liquid-water vapor (Bardintzeff, setting up during the collapse of the roof of the 2006). magma reservoir (Branney and Kokelaar, 2002; Three main processes can usually cause Palladino and Simei, 2005). pyroclastic flows (Sparks et al. 1978; Druitt, Elements of dynamics characterized by 1992, Calder et al., 1999): the collapse of a subvertical ramparts and subcircular to Plinian column on itself (Saint Vincent Type in elliptical shapes reflecting the geometry of Guadeloupe) resulting from an explosive magma chambers (Zangmo Tefogoum et al., disintegration of magma and rock in a volcanic 2011), show that calderas may have formed by chimey; the destruction of a lava dome collapse. In both massive collapses were associated with a laterally directed explosion preceded by violent explosions that firstly (Pelee type in Martinique); or the collapse of a accelerated emptying of magma reservoirs and summital dome of viscous lava by simple also facilitated the faulting causing subsidence. gravity (Merapi type in Indonesia), causing Recent studies were more over shown that

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