Journal of African Earth Sciences 95 (2014) 63–76

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Journal of African Earth Sciences

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Structural study of the Foréké-Dschang trachytic dome(Mount Bambouto, West Cameroon): An anisotropy of magneticsusceptibility (AMS) approach

http://dx.doi.org/10.1016/j.jafrearsci.2014.02.0141464-343X/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding author at: Ministère de la Recherche Scientifique et del’Innovation, DPSP/CCAR, B.P. 1457, Yaounde, Cameroon. Tel.: +237 77173742.

E-mail address: tnjanko@gmail.com (T. Njanko).

B.E. Bella Nké a, T. Njanko a,b,⇑, M. Kwékam a, E. Njonfang c, S. Naba d, K.J. Tcheumenak a, M. Gountié e,P. Rochette f, A. Nédélec g

a Laboratoire de Géologie de l’Environnement (LGE), Université de Dschang, B.P. 67, Dschang, Cameroonb Ministère de la Recherche Scientifique et de l’Innovation, DPSP/CCAR, B.P. 1457, Yaounde, Cameroonc Laboratoire de Géologie, Ecole Normale Supérieure, Université de Yaoundé I, B.P. 47, Yaounde, Cameroond Département de Géologie, Université de Ouagadougou, BP 7021, Ouagadougou 03, Burkina Fasoe Ecole Normale Supérieure, Université de Maroua, Maroua, Cameroonf UMR 6635-CEREGE, 13545 Aix-en-Provence, Franceg UMR 5563-GET-OMP, Université Paul-Sabatier, 14 Avenue Edouard-Belin, 31400 Toulouse, France

a r t i c l e i n f o a b s t r a c t

Article history:Received 25 September 2012Received in revised form 21 January 2014Accepted 26 February 2014Available online 12 March 2014

Keywords:AMSForéké-Dschang trachytic domePan-AfricanTertiaryCameroon

The Tertiary trachytic dome of Foréké-Dschang is located on the southwestern slope of Mount Bambouto.This dome, made of porphyritic and microlitic lavas which enclose sanidine, pyroxene and oxides, isdated at 16–8.8 Ma and was emplaced in gneisses belonging to the Pan-African basement. The meanmagnetic susceptibility magnitude Km values of the trachytic dome show a ferromagnetic behavior in79% of the stations, likely due to the presence of more or less maghemitized titanomagnetite based onthermomagnetic and hysteresis data.

The Foréké-Dschang dome is elliptical and strikes NE–SW, parallel to a border fracture. The Pan-Africanhost gneisses display N70� striking and steeply dipping foliations crosscut by the dome. Some Tertiarysinistral reworking of these foliations may have induced the opening of a local tension gash in agreementwith the regional model suggesting that the N30� Cameroon Volcanic Line opened as a mega-tension gashwith respect to the N70� Adamawa fault zone. Two petrographic types are recognized in the trachyticdome, aphyric (domain I) or porphyritic (domain II). The magnetic foliations show an outward-dippingconcentric pattern in domain I. A flat lying zone with the highest magnetic anisotropy (18%) is regardedas overlying the vent. The organization of magnetic fabrics suggests that domain II was emplaced beforedomain I. As a whole, the Foréké-Dschang trachytes represent a multilobate dome emplaced in a pulsa-tory manner.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

The Cameroon Volcanic Line (CVL; Fig. 1), a 1600 km longtectono-magmatic feature in central Africa, shows a characteristicalignment of volcanoes, anorogenic complexes and grabens. It con-sists of a chain of Tertiary to recent generally alkaline volcanoesextending over more than 900 km across Cameroon from Mt Cam-eroon and Equatorial Guinea southwestwards (in the gulf of Gui-nea Fig. 1c) to the Bui and Adamawa plateaux or to the Dandiregion in Chad (Ganwa et al., 2008) in the north. These volcanoes

are separated by flat low areas corresponding to collapse grabens,namely: Tombel, Mbo and Noun grabens (Fitton, 1987; Déruelleet al., 1991, 2007; Nkouathio et al., 2002, 2008; Itiga et al., 2004;Fosso et al., 2005). The volcanic alignment continues seawardsfor a further 700 km through the Atlantic Islands of Bioko, Principe,Sao Tomé and Pagalu.

According to Moundi et al. (2007), the volcanism along the CVLbegun during the Eocene with the emplacement of volcanic rocksthat formed the Bamoun plateau between 51.8 and 46.7 Ma andMount Bangou between 44.7 and 43.1 Ma (Fosso et al., 2005).Volcanic activity is still active at it southwestern part (MountCameroon: 1999 and 2000 eruptions).

The Cameroon Volcanic Line, in its continental part, is made upmainly of mafic and felsic lavas. In the Mount Bambouto, felsic

Fig. 1. Geological map of the CVL. (a) Location of Cameroon in Africa and the different cratons; (b) CVL presenting the extension of volcanic lavas; (c) CVL, a succession ofhorsts (roman type) and grabens and rifts (italic type), the Adamawa horst, the Djérem-Mbéré graben, and main volcanic and plutonic complexes are also indicated (afterDéruelle et al., 1991).

64 B.E. Bella Nké et al. / Journal of African Earth Sciences 95 (2014) 63–76

lavas predominate and include abundant trachytes and minor rhy-olites. Trachytes generally have a domal shape, more or less ellip-tical. The trachytic dome of Foréké-Dschang, located SW of mountBambouto, is one of many small intrusive domes in the CVL (Fig. 1).It consists of three aligned hills striking N47�E. Most of theoutcrops are characterized by the scarcity of macroscopically mea-surable magmatic fabrics in the field, rendering difficult the deter-mination of the relationships with the gneissic basement andhampering the building of an emplacement model.

Anisotropy of magnetic susceptibility has been applied on thetrachytic dome of Foréké-Dschang in order to reveal the internalmagmatic flow pattern and to discuss possible relationships withthe country rocks. Indeed, numerous AMS studies of mafic and fel-sic intrusive and extrusive rocks have demonstrated that this tech-nique efficiently describes flow and strain fabrics (e.g. Tarling andHrouda, 1993; Bouchez, 2000). However AMS studies of trachyticor other viscous lava domes are scarce (Arbaret et al., 1993;

Canon-Tapia and Castro, 2004; Morgan et al., 2008; Závada et al.,2009b) and AMS was only used once for the study of volcanic rocksin Cameroon so far, namely to locate the emission centers of theignimbrites of Mounts Bambouto and Bamenda (Gountié Dedzoet al., 2011).

2. Geological setting

2.1. The Pan-African belt in Cameroon

The Neoproterozoic Pan-African belt of Cameroon is a part ofthe Central African Pan-African belt. It is bordered to the northby the Saharan Metacraton (Abdelsalam et al., 2002) and to thesouth by the Congo Craton.

Previous work on the Central Africa Pan-African belt focusedmainly on the tectonic evolution of the belt, and the generalaspects of associated magmatic activity (e.g. Nédélec et al., 1986;

B.E. Bella Nké et al. / Journal of African Earth Sciences 95 (2014) 63–76 65

Toteu et al., 1987; Toteu, 1990; Barbey et al., 1990; Ekwueme et al.,1991; Penaye et al., 1993; Ngako et al., 2003, 2008; Njanko et al.,2006, 2010; Nsifa et al., 2013). Post-collisional evolution is charac-terized by the development of a shear zone network enabling todivide the Pan-African belt of Cameroon into three lithostructuraldomains, namely the northern, central and southern domains.The study area belongs to the central domain. The central domain(located between the Sanaga SZ and the Buffle Noir–Mayo Baléo(BNMB) shear zone: Fig. 1c) includes Palaeoproterozoic metasedi-ments and orthogneisses (Penaye et al., 2004) intruded by wide-spread syn- to late- Pan-African granitoids mainly of crustalorigin with high-K calc-alkaline affinities (Soba et al., 1991;Nguiessi-Tchankam et al., 1997; Toteu et al., 2001; Tagne-Kamga,2003; Nzolang et al., 2003; Njanko et al., 2006; Kwékam et al.,2010). Several of these intrusions were emplaced in spatial relationwith the major CCSZ (Central Cameroon Shear Zone: Fig. 1c, alsonamed Adamawa shear zone), that is characterized by a dominantN70�E orientation in the central domain. The CCSZ is partly coveredby volcanics in the study area.

The whole Pan-African basement rock constitutes the mainbasement of the CVL, active since the Cenozoic.

2.2. The Cameroon Volcanic Line (CVL)

The CVL (Fig. 1b and c) extends for ca 2000 km from Pagaluisland to lake Chad with a N30� orientation. It is made up of an

Fig. 2. (a and b) Outcrop and microphotograph of thin sections of gneiss. (c) A view of tw(d) outcrop contact between trachyte and gneisses (coin diameter: 2.7 cm); (e and f) thin sapatite, Bt: biotite, Cpx: clinopyroxene, Kfs: K-feldspar, Ox: oxide, Qtz: quartz, Sa: sanid

oceanic (Sao Tomé-Principe and Bioko) and a continental (MountsEtinde, Cameroon, Manengouba, Bambouto, Oku, Ngaoundéré, Buiand Mandara mountains) sectors (Fig. 1c), and has been activesince the Cenozoic. The oceanic part trends from Saint Hélène(Vincent, 1970) or from island Ascension (Gouhier et al., 1974)to the gulf of Guinea. More than 60 plutonic complexes were em-placed in the continental sector. These plutonic complexes aresmall in size (mostly 5–10 km in diameter) and are mainly consti-tuted of granites or syenites, to which subordinate intermediateand basic rocks are sometimes associated (Njonfang et al., 1992;Njonfang and Moreau, 2000). Bimodal volcanoes are usuallyaligned and associated with the plutonic complexes. Mafic lavascomprise basanites, alkali basalts, hawaiites and mugearites andthe silicic lavas comprise benmoreites, trachytes, phonolitesand/or rhyolites (Nono et al., 1994; Marzoli et al., 2000; Ngou-nouno et al., 2000; Nkouathio et al., 2002; Kagou Dongmo et al.,2010). The southern part of the continental sector of the CVLcomprises a series of horsts and grabens bounded by N20–30�striking normal faults.

Two different hypotheses have been suggested for the forma-tion of the CVL, either as a rift or as the rejuvenation of an olderPan-African shear zone. Moreau et al. (1987) regarded the CVL asa megatension gash striking N30�, possibly opened due to somesinistral reactivation of the N70� Adamawa shear zone at the endof Cretaceous times.

o summits of Foréké-Dschang trachytic dome towards the NE from the SW summit;ections of aphiric trachyte (e) and porphyric trachytes (f); S: magmatic foliation. Ap:ine, Zrn: zircon.

66 B.E. Bella Nké et al. / Journal of African Earth Sciences 95 (2014) 63–76

3. Geology and geochronology of Foréké–Dschang area

The rock types constituting the Foréké–Dschang area includePan-African gneisses, granodiorites and granites of thePrecambrian basement and much younger trachytes (Fig. 2)(Dumort, 1968; Fosso et al., 2005) that crosscut the gneisses.

Gneisses, the most abundant basement rock type, are divided intwo types: banded amphibole gneisses (Fig. 2a) and migmatiticamphibole gneisses. They crop out in form of flagstone in Litieu.Grey in color, the gneisses are medium- to fine-grained (average si-ze < 1.5 mm) with a preferred orientation of their minerals and agranoblastic texture (Fig. 2b). They are composed of biotite, horn-blende, plagioclase, K-felsdpar and quartz. Accessory minerals aresphene, zircon, apatite and oxides.

The granodiorites are grey to dark grey in color and containabundant plagioclase and K-feldspar. They also show a preferredorientation of minerals acquired during the magmatic stage. Otherminerals are plagioclase, hornblende, biotite, quartz and accessoryminerals (sphene, zircon, apatite and oxides).

Granites are greyish and pinkish, fine- to medium-grained. Theycontain phenocrysts of perthitic alkali feldspar showing noticeablemineral preferred orientation. Other minerals are quartz, plagio-clase and variable amount of hornblende, biotite and oxides. Themicrostructures are magmatic heterogranular to granoblastic. Por-phyroclasts of feldspar are moulded in a fine-grained matrix,sometimes with quartz ribbons showing evidences of dynamicrecrystallization.

Trachytes crop out in domal shape and dyke (Fig. 2c and d)clearly cross-cutting the gneissic banding (Fig. 2d) at several places(e.g., at Litieu). Sometimes trachytes contain enclaves of granodior-ites, granites and gneisses. The dome is elongated and composed ofthree summits aligned NE-SW (respectively at 1000 m, 1200 m and

Fig. 3. Geological map of Foréké-Dschang sca

1420 m from the SW to the NE; Figs. 2c and 3). The trachytes areaphyric or porphyritic respectively depending of the absence orthe presence of sanidine phenocrysts. The southwestern part ofthe dome is characterized by low altitude (<1000 m) and is calledhere root domain or domain I. This domain is lithologically charac-terized by predominance of aphyric trachytes (Fig. 2e) while thenortheastern part of the dome (domain II) is characterized by por-phyritic trachytes (Fig. 2f). The rock is grey to light grey in colorand mainly composed of K-feldspar (sanidine) in form of stickand phenocrysts in a matrix made of oxides and scarce crystalsof plagioclase in the mesostase. Micro- or pheno-crysts of sanidinedisplay a preferred orientation indicating the direction of lava flowduring the emplacement.

Other trachytic domes along the CVL were dated using K–Ar andAr–Ar methods. Bambouto trachytic domes show ages varying be-tween 16 and 8.8 Ma (Tchoua, 1974; Fitton and Dunlop, 1985;Marzoli et al., 1999; Youmen et al., 2005; Kagou Dongmo et al.,2010), whereas Bamenda trachytic domes have ages ranging from27 to 12 Ma (Kamgang et al., 2007) and those of mount Manen-gouba have been dated at 0.7 Ma (Kagou Dongmo et al., 2010).Therefore, emplacement of the Fokéré-Dschang dome located in-between likely occurred during the 16–8 Ma time interval.

4. Structural field data

In the field, the main structure of the Pan-African country rockssurrounding the trachyte dome is characterized by steeply dippingE–W to ENE–WSW foliations, with subhorizontal ENE–WSWtrending lineations and fold axes (respectively at 72/22 and 247/3) as shown in Fig. 4. Locally, this main structure is identified asresulting from dextral transcurrent tectonics reworking an earlier(D1) flat foliation. It is therefore regarded as D2 in the studied area.

rp dome showing the sampling stations.

B.E. Bella Nké et al. / Journal of African Earth Sciences 95 (2014) 63–76 67

At site NJ10, the D2 structure is crosscut by small N–S strikingsinistral vertical shear zones, therefore appearing as resulting froma third phase D3.

The general orientation of the D2 foliation planes are clearly ob-lique with respect to the general strike of the trachytic dome. In thetrachyte body, it is not easy to determine the direction of lava flowin spite of the presence of sanidine phenocrysts. At last, brittlefractures mainly strike N40� to N60�, as illustrated by the mainfracture running along the southwestern border of the trachyticdome (Fig. 3).

5. Materials and methods

Today, measurement of anisotropy of magnetic susceptibility(AMS) is an efficient method to determine petrofabric orientationin rocks, even in rocks that are visually isotropic (e.g. Bouchez,2000). The universal stage measurements of crystallographicpreferred orientation of sanidine crystals in trachytic dome are ex-tremely tedious due to the fine to ultrafine grain size of sanidineprisms resulting in small number of measurements (Jancušková

Fig. 4. Projection diagrams for foliation poles, stretching lineations and fold axe

et al., 1992). Consequently, the interpretation of fabric patterns isdifficult if not impossible. The AMS technique has been used as arapid and extremely sensitive indicator of the preferred orientationof magnetic minerals in all rock types in the past 20 years, (see Tar-ling and Hrouda, 1993). This method is widely used as a standardtechnique to assess the flow patterns of deep-seated magmaticrocks. However, the AMS has been used less frequently in volcanicrocks than in plutonic rocks, even though its potential is very high,because the preferred orientation of minerals in these rocks is of-ten so weak that it can hardly be investigated by any other method.The AMS technique provides the fabric ellipsoid shape and orienta-tion for magnetic minerals only. However, it has been demon-strated that the magnetic fabric is representative of the wholemineral fabric in plutonic rocks (Archanjo et al., 1995: Grégoireet al., 1998), as well as in volcanic rocks, either of basaltic or rhy-olitic composition (Knight and Walker, 1988; Gountié Dedzoet al., 2011).

Oriented cylinder samples were taken from 35 different stations(Fig. 3) distributed among two mean lithological formations: 16stations from gneiss, 19 stations from trachytes. Two or three

s of the gneissic country rocks (lower hemisphere, contour intervals = 2%).

68 B.E. Bella Nké et al. / Journal of African Earth Sciences 95 (2014) 63–76

oriented cores were collected per station. Each core yielded at leasttwo cylindrical samples, hence providing two (or more) samplesper station. A total of 160 samples were analyzed in this study.Each individual sample is 22 mm in length and 25 mm in diameter,the standard size for magnetic measurements. Samples named NJwere measured for AMS in LMTG (Laboratoire des Mécanismes etTransferts en Géologie; Toulouse, France) and those named BNwere measured in (Laboratory of Petro-Physic of University ofBurkina Faso) using a Kappabridge susceptometer (KLY-3, Agicoand (MFK1-FA) both working at a low alternating inductive field(4.10�4 T, 920 Hz) with a sensitivity of about 2p10�7 SI, allowinganisotropy discrimination below 0.2% over a wide range of suscep-tibility. AMS is a second-rank tensor expressed by its principaleigen vectors and eigen values Kmax = K1 > Kint = K2 > Kmin = K3,respectively representing the maximum, intermediate andminimum axes of the magnetic susceptibility ellipsoid. The bulkmagnetic susceptibility magnitude is given by Km = (K1 + K2 + K3)/3. The long axis of the ellipsoid, K1, defines the magnetic lineationand the short axis, K3, defines the pole of the magnetic foliation(the plane formed by K3 and K2 axes). When a rock has a so-called‘‘paramagnetic’’ behavior, K3 represents the pole of the foliationmarked by the preferred orientation of the main Fe-bearing silicatemineral, which is often biotite, and K1 represents the biotitelineation defined as the axis of rotation, or zone axis, of the biotite

Table 1Anisotropy of low-field magnetic susceptibility for the trachytic dome of Foréké-Dschang.respectively; Km = (K1 + K2 + K3)/3 is the mean magnetic susceptibility; L% is the linear ani(F% = 100 � [(K2/K3) � 1]), P% is the total anisotropy percentage (P% = 100 � [(K1/K3) � 1])declination in degrees; Inc: inclination in degrees.

Site Long (E) Lat (N) Mean AMS parameters

(�) (�) Km (lSI) L% F% P%

TrachytesBN15 10.0071 5.393 999 1 1 2BN16 10.0074 5.3934 3813 1 5 6BN17 10.0175 5.3992 1660 0 6 6BN19 10.0162 5.3976 6884 1 5 6BN20 10.0122 5.3931 4103 1 3 4BN21 10.0153 5.3976 7034 6 6 13BN22 10.0112 5.3993 5168 1 6 7BN23 10.0111 5.3993 4344 2 5 7NJ24 10.0156 5.4019 5152 3 6 9NJ25 10.0122 5.4003 2715 0 8 9BN32 10.0051 5.3936 949 0 3 4BN33 10.0072 5.3929 449 1 1 1BN34 10.0077 5.3934 205 0 1 1NJ04 10.0081 5.3906 445 4 14 18NJ18 10.0097 5.3906 740 1 1 2NJ17 10.0067 5.3914 495 1 14 16BN60 10.015 5.4008 4086 3 4 6NJ12 10.0144 5.3958 5675 2 3 5NJ13 10.0142 5.3994 3668 1 5 5

GneissBN18 10.0176 5.3965 92 3 12 15BN24 10.0015 5.3939 445 8 12 21NJ01 10.0131 5.3853 1796 0 5 5NJ02 10.0131 5.3847 687 5 10 16NJ03 10.0139 5.3878 359 1 10 11NJ05 10.0028 5.3953 583 3 3 6NJ06 10.0064 5.3997 415 2 17 19NJ07 10.0139 5.4058 481 2 6 8NJ10 10.0194 5.3964 280 3 8 12NJ14 10.0175 5.3931 454 3 16 19NJ15 10.0139 5.3908 610 2 11 13NJ16 10.0228 5.405 449 2 3 5NJ20 10.0011 5.3881 448 2 2 4NJ21 10.0058 5.3881 493 0 9 9NJ22 10.0111 5.405 439 1 21 22NJ23 10.0147 5.3906 127 2 2 4

foliation (Bouchez, 2000). The thermomagnetic curves were ob-tained using a CSL/CS-3 apparatus coupled to the KLY-2 or MFK1instrument (Agico, Czech Republic), from �200 �C to 700 �C. Hys-teresis measurements were performed up to 1 T on 12 representa-tive trachyte samples on the Micromag VSM in CEREGE (Centre deRecherche et d’Enseignement de Géosciences de l’Environnement;Aix-en-Provence, France), to define further the magnetic mineral-ogy and measure the high field susceptibility (slope fitted above0.7 T). As their volume is uneasy to estimate precisely we preferredto normalize to mass and compute a volume susceptibility basedon an arbitrary density of 2500 kg/m3. These samples were alsomeasured with the MFK1.

The raw magnetic data for each of the 35 stations concerningthe Foréké-Dschang trachytic dome and its gneissic basement rockare in Table 1, successively giving the station, the geographicallocation, the main AMS parameters such as: bulk susceptibilitymagnitude (Km), the linear anisotropy percentage (L% = 100 �(L � 1) with L = K1/K2), the planar anisotropy percentage(F% = 100 � (F � 1) where F = K2/K3), the total anisotropy percent-age (P% = 100 � (P � 1) with P = K1/K3), the T-shape parameter ofJelinek (1978) (T = 2ln (K2/K3)/ln (K1/K3)) � 1); the declinationand inclination of maximum, intermediate and minimum eigen-vectors, i.e. K1 (magnetic lineation), K2 and K3 (pole to the magneticfoliation).

K1, K2 and K3 are the maximum, intermediate and minimum susceptibility intensitiessotropy percentage (L% = 100 � [(K1/K2) � 1]); F% is the planar anisotropy percentage; T = [2ln(K2/K3)/(ln(K1/K3)] � 1 is the Jelinek’s shape parameter (Jelinek, 1981); Dec:

Mean eigen vectors

T K1 K2 K3

Decl Incl Dec Incl Decl Incl

0.33 92 13 336 25 233 630.44 180 39 310 38 65 280.93 8 14 252 59 100 210.65 322 51 34 9 145 390.47 264 7 360 42 166 480.02 333 2 10 71 125 840.7 231 15 329 18 117 660.53 200 10 110 3 5 790.43 245 25 341 12 170 620.95 136 7 228 18 25 700.46 270 57 12 17 112 250.41 26 59 269 17 169 290.42 352 32 295 27 71 460.54 237 13 38 76 146 40.14 3 50 264 40 160 370.85 150 46 183 44 316 430.16 71 49 55 67 169 70.17 75 18 259 12 328 400.78 163 8 53 7 256 22

0.54 63 7 327 42 157 460.36 229 22 126 31 358 480.99 48 44 250 44 149 110.35 231 49 61 41 327 50.78 190 25 285 12 39 620.03 257 5 120 83 347 50.79 36 60 282 13 186 270.57 241 65 89 23 354 100.44 218 65 89 23 342 150.69 198 63 77 20 356 260.69 218 43 90 9 321 140.13 203 53 64 44 340 290.1 206 37 303 8 44 520.93 261 2 88 88 351 00.92 51 70 236 21 146 20.16 260 58 70 32 163 5

B.E. Bella Nké et al. / Journal of African Earth Sciences 95 (2014) 63–76 69

6. Magnetic susceptibility and magnetic mineralogy

6.1. Magnetic susceptibility

The magnetic susceptibility magnitudes of the whole collection,expressed by Km in SI units, vary greatly ranging from 204 lSI to7034 lSI in trachytes and from 93 lSI to 1796 lSI in gneisses(Table 1; Fig. 5a). These values indicate the presence of both (1)paramagnetic behavior (Km usually less than 500 lSI) dominatedby the signal of iron-bearing silicates (Rochette, 1987; Bouchez,2000) that represents 44% of the stations and (2) ferromagneticbehavior (Km > 500 lSI) dominated by the signal of the magnetitethat represents 56% of the stations. Ferromagnetic and paramag-netic behaviors appear in the same lithological group and some-times, in the same station, indicating an uneven distribution offerromagnetic minerals in some samples. In the trachytic dome,79% of the stations are ferromagnetic and 21% of the stations areparamagnetic (Fig. 5b).

It should be noted that stations with magnetic susceptibility mag-nitudes less than 1000 lSI belong to the southwestern part (called theroot domain or Domain I) of the dome, and the station with thehighest susceptibility magnitude (7034 lSI) is found in domain II.

6.2. Magnetic mineralogy: susceptibility vs. temperature andhysteresis

Thin sections allow observing different types of opaque miner-als, in form of (1) free intercrystalline fine- to coarse-grained oxi-des in the mesostase (Fig. 6a–c); (2) fine- to medium-grainedoxides included in sanidine and clinopyroxene (Fig. 6d and e).

Fig. 5. (a) Susceptibility (Km) map in 10�3 SI of Foréké Dschang scarp;

Xenomorphic crystals arise from destabilisation of mineral likeclinopyroxene, plagioclase and sanidine (Fig. 6b–f).

Most of the samples show thermomagnetic curves (K–T curves:normalized susceptibility vs. temperature) that are irreversibleduring cooling. Representative examples of these curves are shownin Fig. 7 for porphyric samples of the dome. Progressively, sampleswere heated up to 700 �C and cooled to room temperature. In thetrachyte group, most of these curves have a regular shape in bothheating and cooling cases and they display a decrease in the inten-sity of susceptibility at 550–580 �C indicating the presence of lowTi titanomagnetite as the main carrier of the magnetic susceptibil-ity. This is consistent with the high susceptibility values and min-eral compositions witnessing the oxidized nature of the magmas. Aslight decrease at 350 �C in the heating curve suggests the presenceof some maghaemite. The low temperature curve on BN19 does notexhibit the Verwey transition typical of pure unoxydized magne-tite (Martin-Hernandez and Hirtz, 2001; Peters and Dekkers,2003). Hysteresis data (Fig. 8 and Table 2) obtained in the trachytesamples confirmed the presence of a low coercivity mineral(Bc 6 7 mT, Bcr 6 26 mT) with domain state at the transition be-tween multidomain and large pseudosingledomain (0.04 6Mrs/Ms 6 0.12, 2.3 6 Bcr/Bc 6 5.1). High field susceptibilities vary be-tween 70 and 360 lSI (average 190 lSI). Most samples thus havetheir low field susceptibility dominated by titanomagnetite, theparamagnetic contribution being at most 35% and below 20% on10 out of 12 samples. A good correlation appears between low fieldparameters Km and P at the scale of the site and on the one cc sam-ple, indicating fine scale homogeneity of the magnetic properties.Magnetic grain-size appears smaller in domain I.

In the gneisses, the K–T curves are quite different from those oftrachytes. Cooling curves indicate a higher susceptibility than the

values >5 � 10�3 SI is shaded; (b) susceptibility histogram of Km.

Fig. 6. Microphotographs of thin sections of trachytes showing magnetite crystals (a) in the mesostase with a crystal of sanidine; (b and d) in equilibrium with theplagioclase, sanidine and clinopyroxene; (c and d) in phenocrystals of plagioclase (c) and sanidine (d); (e) in xenocryst in clinopyroxene; (f) phenocrystal in matrice; Mt:magnetite.

Fig. 7. K–T curves (susceptibility vs. temperature) for representative porphyric trachyte samples at high temperature (NJ11, NJ12, NJ24), and temperature dependence ofmagnetic susceptibility in the range between �200 and 700 �C for BN19. Note the magnetite with a Curie temperature around 595 �C for all samples.

70 B.E. Bella Nké et al. / Journal of African Earth Sciences 95 (2014) 63–76

Fig. 8. Examples of hysteresis curve for the most and least ferromagnetic trachyte samples.

Table 2Rock magnetic parameter from hysteresis measurements of circa 1 cc sample. Mean low field susceptibility (Km in lSI) and anisotropy ratio P measured on the MFK1, comparedto the average values obtained on the same site from standard samples. High field susceptibility (Khf in lSI) and percent of ‘‘paramagnetic’’ susceptibility (Khf/Km � 100).Hysteresis parameters with saturation magnetization Ms in mAm2/kg and remanent coercive field Bcr in mT. First four samples are from domain I.

Sample Km (Sample) P (Sample) Km (Site) P (Site) Khf %Para Ms Mrs/Ms Bcr/Bc Brc

BN15B1 1225 1.02 918 1.02 231 17 31 0.067 3.28 12BN16A1 1182 1.02 810 1.02 239 19 28 0.068 3.42 12BN19B2 7595 1.06 7274 1.05 188 2 388 0.041 3.97 13BN21A2 7250 1.14 7107 1.15 356 5 362 0.053 4.44 26BN23A1 5988 1.07 4039 1.07 74 1 197 0.058 3.75 13BN32D1 1679 1.06 1593 1.05 330 18 75 0.123 2.40 16BN33B 540 1.02 451 1.01 203 35 11 0.121 2.31 12NJ11 4374 1.06 4086 1.06 134 3 182 0.088 3.05 18NJ12 6287 1.08 5675 1.05 128 2 363 0.036 5.15 21NJ18 490 1.04 740 1.02 190 34 10 0.121 2.53 11NJ24 5435 1.09 5152 1.09 113 2 284 0.055 3.96 19NJ25 2867 1.09 2715 1.09 87 3 132 0.057 3.80 17

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susceptibility obtained during heating, indicating the creation ofmagnetite during heating period. Cooling or heating curves arenot on the T axis after the Curie temperature of magnetite, suggest-ing that there is still a susceptibility carrier in the rocks (hematitefor example) (see Fig. 7).

6.3. Anisotropy rate of magnetic susceptibility (P%)

The total anisotropy percentage (P%) in the trachytic dome var-ies from 1% to 18% (Table 1 and Fig. 10). The maximum valuesseems to be high compared to basaltic and rhyolitic flows or dikes(e.g. Tarling and Hrouda, 1993; Rochette et al., 1999) but are sim-ilar to what is observed in phonolites and trachytes (Morgan et al.,2008), indicating a rather large strain induced by high viscosity. Itshould be noted that trachyte shows a positive correlation between

Km and P(%) with two trends (Fig. 10c) that correspond to eachlithological domain. However, the samples that have the highestKm yield a high value (13%) of P% but not the highest. The highestanisotropies correspond to samples of NJ04 and NJ17 which haveP% very high (18% and 16% respectively), and susceptibility magni-tude indicating a paramagnetic behavior (445 lSI and 495 lSIrespectively). Inspite of these high values, the other samples inthe root domain have P% ranging 1–6%.

The Jelinek shape parameter (T) has positive values in the stud-ied area. These values are quite similar to those of trachyte Mesaintrusion (Morgan et al., 2008) where negative values representless than 13% of stations. In the trachyte dome, shape parameteris most characterizing by plano–linear fabrics ellipsoids 58%(�0.5 < T < 0.5) and planar ellipsoids 42% (T > 0.5; Fig. 9). Theserelatively high anisotropies and triaxial to planar fabrics are typical

Fig. 9. Jelinek shape parameter (T) vs. susceptibility (Km) for trachytes samples associated to the ellipsoid shapes and the corresponding orientation diagrams of AMS dataanalyzed in this work (K1: box; K2: triangles and K3: circles; average axis are filled). Noted T > 0 for all samples.

Fig. 10. (a) Anisotropy percentage (P% = 100 � [(K1/K3) � 1]) map of Foréké Dschang. Values >6% are shaded; (b) anisotropy percentage histogram for trachytes; (c): P% vs. Kmdiagram.

72 B.E. Bella Nké et al. / Journal of African Earth Sciences 95 (2014) 63–76

of high viscosity volcanic rocks that registered a pronounced min-eral fabric during cristallization (Canon-Tapia and Castro, 2004;Gountié Dedzo et al., 2011).

6.4. Directional data

The orientations of K1, K2 and K3 with respect to the north asdeclinations (azimuths) and inclinations (plunges) are given inTable 1. K3 represent the magnetic foliation pole and it is generallybetter defined than K1 which represent magnetic lineation as can

be seen also in the lower hemisphere projections for each analyzedsites presented in Fig. 11. Nevertheless, magnetic fabrics are well-defined in most cases.

6.4.1. Magnetic foliationMagnetic foliation is generally parallel to the foliation measured

in the field for the gneiss or under the microscope for trachytes(Fig. 12). Lower hemisphere projection for the foliation in gneissshows a best pole at 340/9, corresponding to steep foliationplanes striking ENE–WSW (Fig. 12a). The trachytic dome of

Fig. 11. Lower hemisphere projections of all AMS data encountered in Foréké-Dschang scarp with; squares, triangles and circles represent K1, K2 and K3 respectively; filledsymbols represent means of K1, K2 and K3.

B.E. Bella Nké et al. / Journal of African Earth Sciences 95 (2014) 63–76 73

Fig. 12. Magnetic foliation map, foliation trajectories and projection diagrams of magnetic foliation poles (lower hemisphere, contour intervals 2%); shaded symbolsrepresent the magma emplacement pulses; (b) histogram of magnetic foliation in the dome.

Fig. 13. (a) Magnetic lineation map and projection diagrams of magnetic lineation (lower hemisphere, contour intervals 2%); shaded symbols represent the magmaemplacement pulses (b) histogram of magnetic lineation in the dome.

74 B.E. Bella Nké et al. / Journal of African Earth Sciences 95 (2014) 63–76

Fig. 14. Interpretative cross-section of the trachytic dome.

B.E. Bella Nké et al. / Journal of African Earth Sciences 95 (2014) 63–76 75

Foréké-Dschang displays 64% of stations with magnetic foliationdip angles between 0� and 60� and 36% of stations between 60�and 90� (Fig. 12b). In domain I (Fig. 12a) of the dome, magnetic foli-ations display a concentric structure mainly dipping outward. It isworth to note that the flattest foliation correspond to the highestanisotropy. In domain II, foliations also appear concentric, butplunge mainly inward. Magnetic foliation in gneisses is crosscutby the dome magnetic fabrics.

6.4.2. Magnetic lineationMagnetic lineation has a predominant NE–SW direction in

gneiss with the best line at 225/42 (Fig. 13a). In the trachytic dome,magnetic lineations show a radiating pattern in domain II. In do-main I, they are either parallel or perpendicular to the elongateNE–SW shape of the dome, with dominantly shallow plunges.

7. Discussion and interpretation

7.1. Fracture opening to feed the trachyte dome

The gneissic basement of the trachytic dome has been affected byductile deformation during the Pan-African orogeny and mainly dis-play the so-called D2 structure characterized by steep ENE–WSWstriking foliations evidenced by field and magnetic data. Mildly togently plunging lineations and fold axes together with kinematicindicators point to a dextral transcurrent nature for this D2 event.Three mean phases of Pan-African deformation have been recog-nized near Fomopéa, 25 km northwest of the Foréké-Dschang dome.In Fomopéa region, where gneisses are very similar to those of thestudied area, Njanko et al. (2010) recognized three Pan-Africandeformation events, respectively responsible for a flat-lying folia-tion (D1), discrete N–S striking sinistral shear zones (D2) and a broadNE-striking steep dextral shear zone (D3). The D3 structures near Fo-mopéa and the D2 structures in Foréké-Dschang are recognized asresulting from the same dextral transcurrent tectonics at the south-western end of the CCSZ. Fractures striking at ca N40� to 50�E per-mitted the ascent and emplacement of the trachytic magma. Theyare oblique with respect to the main D2 foliations of the country-rocks. We suggest that they opened as local tension gashes, possiblyas a consequence of sinistral reworking of the main N70� D2 struc-tures. This hypothesis is similar to the model proposed at a broaderscale for the whole CVL by Moreau et al. (1987).

7.2. Internal fabrics in the dome and magma emplacement model

The AMS directional data (magnetic foliation and magneticlineation) reveals the building of the trachytic dome. Domain Iwas emplaced as a nearly circular dome with outward dipping

foliations. The most anisostropic and flat magnetic foliation likelyindicates the position of the feeding conduit at depth (Fig. 14), inagreement with the analog models of Závada et al. (2009a). Thefabrics of domain II are also evocative of a rather concentric fabric,but they are cut by the younger domain I structures. A possiblevent close to domain I is also suggested by high anisotropy and flatfoliation (Fig. 14). Inward dipping foliations and gently plunginglineations more or less parallel to the long axis of the domain sug-gest that the prophyritic magma began to outflow towards the NE.

We can then suggest that the trachytic magma was emplaced astwo successive batches, that built two different lobes in theForéké-Dschang dome.

8. Conclusion

The present study provides new results from the study of fieldand magnetic fabrics that help to understand the emplacementof the Foréké-Dschang trachytic dome in a tension gash resultingfrom a possible Cenozoic reactivation of the Pan-African N70�ECCSZ. Magnetite has been determined as the main susceptibilitycarrier in the studied rocks. Using the scalar and directional AMSdata and petrographic observations, it has been possible to drawstructural maps of the Foréké-Dschang dome revealing its bilobateinternal fabric, corresponding to the successive emplacement ofthe porhyritic and then the aphyric trachyte magmas.

Acknowledgements

Fieldwork was possible thanks to technical assistance of IRD-CORUS project ‘‘Concentrations minérales en Afrique’’ of M. Jesselland J.L. Bouchez from GET (Toulouse-France). Njuéya Kopa A. isacknowledged for field participation. We are gratefull to the anon-ymous reviewers for their constructive comments.

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