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Magnetic Fabrics of the Miocene Ignimbrites from West-Cameroon: Implications for Pyroclastic flow Source and Sedimentation

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Magnetic Fabrics of the Miocene Ignimbrites from West-Cameroon: Implications for Pyroclastic flow Source and Sedimentation Journal of Volcanology and Geothermal Research 203 (2011) 113–132 Contents lists available at ScienceDirect Journal of Volcanology and Geothermal Research journal homepage: www.elsevier.com/locate/jvolgeores Magnetic fabrics of the Miocene ignimbrites from West-Cameroon: Implications for pyroclastic flow source and sedimentation M. Gountié Dedzo a,⁎, A. Nédélec b, A. Nono a, T. Njanko a, E. Font c, P. Kamgang d, E. Njonfang e, P. Launeau f a LGE, Department of Earth Sciences, University of Dschang, B.P. 67 Dschang, Cameroon b UMR5563-LMTG-OMP, University of Toulouse-CNRS-IRD, 14 Av. Edouard-Belin, 31400 Toulouse, France c IDL-FUL, Instituto Dom Luis. Universidade de Lisboa, 1749-016 Lisboa, Portugal d Department of Earth Sciences, University of Yaoundé I, B.P. 812 Yaoundé, Cameroon e Laboratoire de Géologie, Ecole Normale Supérieure, Université de Yaoundé I, B.P. 47, Yaoundé, Cameroon f UMR-CNRS 6112, Laboratoire de Planétologie et Géodynamique, Université de Nantes, rue de la Houssinière, 44322 Nantes, France article info abstract Article history: The Miocene ignimbrites of Mounts Bambouto and Bamenda located in the central part of Cameroon Volcanic Received 24 September 2010 Line are generally made of welded and non-welded massive lapilli tuff and lithic breccias. These discontinuous Accepted 18 April 2011 deposits cover a total area of 180 km2 with thickness ranging from 25 to 200 m. The different facies contain Available online 5 May 2011 several lithic fragments of mainly trachytic nature. The devitrified matrix of the welded ignimbrites is constituted by sanidine, anorthoclase, quartz, plagioclase, clinopyroxene, biotite, Fe–Ti oxides and devitrified Keywords: fi AMS ammes. Anisotropy of magnetic susceptibility (AMS) is used to characterize magnetic fabrics and to provide fl fl magnetic mineralogy an estimate of ow direction of each ignimbrite sheet. Magnetic mineralogy results from different ow units Miocene ignimbrite show that titanomagnetite, titanohematite, maghemite and goethite with grain size ranging from coarse MD caldera to very fine SP are the main magnetic carriers of these ignimbrites. Inferred transport directions based on the Western-Cameroon AMS data and field indicators show that Bambouto caldera is the source of main pyroclastic deposits of Mount Bambouto. In southwestern Mount Bamenda, Santa-Mbu caldera or Bambouto caldera constituted the probable emission center of Mbengwi, Bamenda and Mbu ignimbrite sheets, whereas magnetic fabrics of Bambili, Sabga and Big Babanki ignimbrites demonstrate that these deposits were emitted from a northeastern source, namely Oku vent in Mount Oku. A small number of subvertical AMS fabrics correspond to rocks possibly modified by an elutriation process. © 2011 Elsevier B.V. All rights reserved. 1. Introduction (Moundi et al., 2007) and Mount Bangou between 44.7 and 43.1 Ma (Fosso et al., 2005), and is still active at Mount Cameroon (1999 and The Cameroon Volcanic Line (CVL), a 1600 km long mega-shear 2000 eruption). zone in central Africa, shows a characteristic alignment of volcanoes, Most of the CVL volcanics are made up of mafic and felsic lavas. anorogenic complexes and grabens. This chain of Tertiary to recent Ignimbritic flow deposits are scarce and mainly observed in the generally alkaline volcanoes extends over more than 900 km across central part of the CVL, especially in Mounts Bambouto and Bamenda Cameroon from the Bui and Adamawa plateaux in the north to Mt (Fig. 1c). Other small flow deposits are present in Nkogam massif and Cameroon and Equatorial Guinea southwestwards (in the gulf of in Mount Oku. Pyroclastic flow deposits, ranging from non-welded to Guinea, Fig. 1a, b). These volcanoes are separated by plains, low areas welded ignimbrites, form an important component of rock formations corresponding to collapsed grabens, namely: Tombel, Mbo and Noun in the studied massifs of Mounts Bambouto and Bamenda. (Fitton, 1987; Deruelle et al., 1991, 2007; Nkouathio et al., 2002, 2008; Some of our understanding of pyroclastic flow behavior has been Itiga et al., 2004; Fosso et al., 2005). It continues seawards for a further gained by observations of recent pyroclastic eruptions; e.g. (i) 1998 and 700 km through the Atlantic Islands of Bioko, Principe, São Tomé and 2003 Soufriere Hills volcano eruptions in the Carrribean (Hart et al., Pagalu. 2004; Edmonds et al., 2006), (ii) 2006 and 2007 eruptions of Mt Etna in The volcanism along the CVL begun during the Eocene with the Italy (Behncke et al., 2008; Behncke, 2009) (iii) 2006 pyroclastic flows emplacement of the Bamoun plateau between 51.8 and 46.7 Ma on Mt Merapi in Java, Indonesia (Charbonnier and Gertisser, 2008). On the other hand, most of our understanding of the mechanisms and flow ⁎ Corresponding author. Tel.: +237 75 08 85 86. dynamics of pyroclastic density currents comes from examination of the E-mail address: [email protected] (M. Gountié Dedzo). deposits they left behind. Detailed examination of the facies architecture 0377-0273/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jvolgeores.2011.04.012 114 M. Gountié Dedzo et al. / Journal of Volcanology and Geothermal Research 203 (2011) 113–132 Fig. 1. a) Location map of the Cameroon Volcanic Line (CVL); the main geologic features of Africa are indicated. b) The main volcanic centers of the Cameroon Volcanic Line. Central Cameroon Shear Zone according to Ngako et al. (2006); fracture zones following Lee et al. (1994) and Ballentine et al. (1997). c) Digital elevation model (DEM) of Mounts Bambouto and Bamenda and the close surroundings; the different calderas are also indicated. and particle fabric of pyroclastic flow deposits gives insight into their Anisotropy of magnetic susceptibility (AMS) measurements can be transport and deposition. Determination of flow directions in ignim- used for determining lineations and foliations in ignimbrites. This brites is often uneasy. In some cases, flow directions have been method detects the alignment of magnetic minerals in the ignimbrite, successfully determined from textural indicators. These include the and has been used in many studies to evaluate flow directions in use of imbricated logs (Froggatt et al., 1981), as well as orientation of pyroclastic deposits in order to determine vent location or, more glass shards, crystals, pumice, and lithic fragments (Elston and Smith, recently, to provide information about transport and depositional 1970; Suzuki and Ui, 1982, 1983, 1988; Potter and Oberthal, 1987; Ui et process involved at different distances from the vent (Ellwood, 1982; al., 1989; Buesch, 1992; Seaman and Williams, 1992). Imbrication of Incoronato et al., 1983; Knight et al., 1986; Wolff et al., 1989; pumice and other features indicate not only the flow direction but also McDonald and Palmer, 1990; Hillhouse and Wells, 1991; Palmer, et al., the sense of motion (Kamata and Mimura, 1983; Suzuki and Ui, 1988). 1991; Fisher et al., 1993; Ort, 1993; Bear et al., 1997; Cagnoli and These methods, however, are time consuming and the features are Tarling, 1997; Le Pennec et al., 1998; MacDonald et al., 1998; Ort et al., commonly absent in specific outcrops. 1999, 2003; Palmer and MacDonald, 1999; Le Pennec, 2000; Wang M. Gountié Dedzo et al. / Journal of Volcanology and Geothermal Research 203 (2011) 113–132 115 et al., 2001; Porreca et al., 2003; Alva-Valdivia et al., 2005; Giordano of outcrops due to their general re-covering by more recent basaltic et al., 2008; Petronis and Geissman, 2008). flows or due to their rapid erosion, makes difficult the determination of the corresponding emission centers. Therefore, systematic AMS deter- 1.1. Aim of study mination was the selected method, here used for the first time in the CVL. AMS results, combined to magnetic mineralogy and image analysis Determination of flow directions in ignimbrites of Bambouto and studies, help to connect discontinuous Bambouto and Bamenda flow Bamenda massifs is difficult, because the fabric in these deposits is deposits to their sources and to discuss magnetic fabric acquisition and visually nearly isotropic in most outcrops. In addition, the discontinuity emplacement mode of these ignimbrites. Fig. 2. Geologic sketch maps of a) Mounts Bambouto and b) Bamenda showing the sampled stations and locations of dated samples. 116 M. Gountié Dedzo et al. / Journal of Volcanology and Geothermal Research 203 (2011) 113–132 2. Geological setting et al., 2001; Nzolang et al., 2003). Mt Bamenda culminates at 2621 m (Bambili Lake borders) and is characterized by two elliptic calderas After Mounts Cameroon and Manengouba, Mount Bambouto (Fig. 1c): Santa-Mbu caldera (6km×4km) and Lefo caldera constitutes the third most important volcano (in volume) of the CVL (4 km×3 km) (Gountié Dedzo et al., 2009), whose basements are with Mount Mélétan (2740 m) as its highest point (Figs. 1c, 2a). It is essentially made of trachytic domes that are also abundant on the made up of volcanic products dated from 21.12 Ma to 0.5 Ma, and external versant of the massif. According to Kamgang et al. (2007, 2008), comprising basalts, trachytes, phonolites, rhyolite and ignimbritic felsic and intermediate lavas (27.40–18.98 Ma) are made of mugearites, deposits constituted by various facies (Nono et al., 2003, 2004). benmoreites, trachytes, rhyolites. Mafic lavas (basanites, basalts, and Geochemical, mineralogical and crystallographic data (Marzoli et al., hawaiites) are dated from 17.4 Ma to the present; rhyolitic ignimbrite 2000; Saviulo et al., 2000; Njonfang and Nono, 2003)showgenetic flow deposits are inserted between the granito-gneissic basement at relationships between different petrographic types. The Mount Bam- their floor and the lateritized old basalts on top. bouto caldera (Fig. 1c) is a dissymmetric roughly elliptic depression (13×8 km), which opens to the west of the volcano. On the southeastern side, it shows subvertical walls that rise 1300 m above 3. Field observations and petrography of ignimbrites its floor made of trachytic and phonolitic domes and dome-flows.
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