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1 Transpressional granite-emplacement model: structural and 1 2 2 magnetic study of the Pan-African Bandja granitic pluton (West- 3 3 Cameroon) 4 5 a,b, a,c d e f a 6 4 Yakeu Sandjo A. F. *, Njanko T. , Njonfang E. , Errami E. , Rochette P. , Fozing E. 7 8 5 a Laboratory of Environmental Geology, The University of Dschang, P.O. Box 67, Dschang, 9 10 6 Cameroon 11 12 b 13 7 Ministry of Water Resources and Energy, DR/C, Gas and Petroleum Products Service, P.O. 14 15 8 Box: 8020 Yaoundé, Cameroon 16 17 c 18 9 Ministry of Scientific Research and Innovation, DPSP / CCAR, P.O. Box 1457, Yaoundé, 19 20 10 Cameroon 21 22 d 23 11 Laboratory of Geology, ENS, The University of Yaoundé I, P.O. Box 47, Yaoundé, 24 25 12 Cameroon 26 27 13 e Department of Geology, Faculty of Sciences, The Chouaïb Doukkali University of El Jadida, 28 29 30 14 P.O. Box: 20, 24000 El Jadida, Morocco 31 32 15 f CEREGE UMR7330 Aix-Marseille Université CNRS, 13545 Aix-en-provence, France 33 34 35 16 Corresponding author: Yakeu Sandjo A. F. ([email protected]) 36 37 38 17 39 Abstract 40 41 18 The Pan-African NE-SW elongated Bandja granitic pluton, located at the western part of the 42 43 19 Pan-African belt in Cameroon, is a K-feldspar megacryst granite. It is emplaced in banded 44 45 46 20 gneiss and its NW border underwent mylonitization. The magmatic foliation shows NE-SW 47 48 21 and NNE-SSW strike directions with moderate to strong dip respectively in its northern and 49 50 22 central part. This mostly ferromagnetic granite displays magnetic fabrics mostly carried by 51 52 53 23 magnetite and characterized by (i) magnetic foliation with best poles at 295/34, 283/33, and 54 55 24 35/59 respectively in its northern, central and southern part and (ii) a subhorizontal magnetic 56 57 58 25 lineation with best line at 37/8, 191/9 and 267/22 respectively in the northern, central and 59 60 26 southern part. Magnetic lineation shows a “S” shape trend that allows to (1) consider the 61 62 63 64 1 65 27 complete emplacement and deformation of the pluton during the Pan-African D2 and D3 1 2 28 events which occurred in the Pan-African belt in Cameroon and (2) reorganize Pan-African 3 4 5 29 ages from Nguiessi Tchakam et al. (1997) compared with those of the other granitic plutons 6 7 30 in the belt as: 686 ± 17 Ma (Rb/Sr) for D1 age of metamorphism recorded in gneiss; and the 8 9 10 31 period between 604-557 Ma for D2-D3 emplacement and deformation age of the granitic 11 12 32 pluton in a dextral ENE-WSW shear movement. 13 14 15 33 Keywords 16 17 34 Granite; AMS; Pan-African; Bandja; Cameroon 18 19 20 21 35 1. Introduction 22 23 36 Spatial and temporal relationships between granites and regional tectonic structures suggest 24 25 37 granite emplacement during compression rather than extension in many convergent orogenic 26 27 28 38 belts (Hutton, 1997; Brown and Solar, 1998). Granite emplacement and deformation during 29 30 39 regional tectonic events is a challenging study, because granitic rocks do not always develop 31 32 33 40 mesoscopic scale deformation fabrics. However, microstructural studies of granites can help 34 35 41 to identify magmatic or solid-state deformation fabrics (Simpson, 1985; Paterson et al., 1989; 36 37 38 42 Bouchez et al., 1992). 39 40 43 Detailed lithotectonic study of the Pan-African orogeny (mainly along the N-S, N30°E and 41 42 44 N70°E shear zones) in Cameroon (Fig. 1) reveals that it is marked by the emplacement of 43 44 45 45 many plutonic massifs of granitic composition. The petrology and mesoscopic to microscopic 46 47 46 structural features of a few granites have been studied (the Ngondo plutonic complex, the 48 49 50 47 west-Tibati pluton, the two-mica granites from the Nkambé area and the Fomopéa granitic 51 52 48 pluton (Fig. 2)) respectively by Tagné-Kamga (2003), Njanko et al. (2006), Tetsopgang et al. 53 54 55 49 (2006) and Kwékam et al. (2010)). However, their tectonic evolution remains poorly 56 57 50 understood, due to the lack of detailed structural data (e.g. anisotropy of magnetic 58 59 60 51 susceptibility study on rocks). Indeed, the application of the anisotropy of magnetic 61 62 63 64 2 65 52 susceptibility (AMS) to the study of granitoids has led to a better understanding of the 1 2 53 tectonic history of plutons elsewhere and recently in Cameroon (Kankeu et al., 2009; Njanko 3 4 5 54 et al., 2010; Kankeu et al., 2012; Dawaï et al., 2013). Moreover, AMS is a widely accepted 6 7 55 technique and a number of studies have investigated the relationship between AMS fabric and 8 9 10 56 strain (Hrouda and Janak, 1976; Borradaile, 1988; Rochette et al., 1992; Tarling and Hrouda, 11 12 57 1993; Borradaile and Henry, 1997; Mamtani et al., 1999). 13 14 58 The aim of this paper is to combine field structures, microstructures and magnetic data to 15 16 17 59 reconstruct the kinematic of emplacement of the Bandja Pan-African pluton and show its role 18 19 60 in the Pan-African history of the west Cameroon. 20 21 22 23 61 2. Regional geological setting 24 25 26 62 2.1 The Pan-African belt in Cameroon 27 28 63 The Pan-African orogenic belt of Cameroon (Fig. 1) is one of the two main domains (together 29 30 64 with the Archaean Ntem Complex) proposed by previous research works for the Precambrian 31 32 33 65 basement rocks in Cameroon. It is characterized by medium- to high-grade metamorphism, 34 35 66 polycyclic deformation, possible reworking of older crust and generation of new granitic 36 37 38 67 magmas. The Pan-African orogenic belt is also known as Pan-African North Equatorial Fold 39 40 68 Belt (Nzenti et al., 1988) or Pan-African Belt of Central Africa (PBCA; Penaye et al., 1993; 41 42 69 Toteu et al., 2001). 43 44 45 70 According to Ngako and Njonfang (2011), PBCA is a three-plate collision belt involving 46 47 71 three major landmasses. The model proposed by these authors includes: the Sao-Francisco- 48 49 50 72 Congo Craton (SFCC), the Eastern Saharan Block (ESB) and the West African Craton 51 52 73 (WAC) (Fig. 1a). Collision in the north-south direction involved ESB and the northern margin 53 54 55 74 of the SFCC originally differentiated into a basin and a range province. This collision 56 57 75 generated an indent and intense deformation in the Cameroon domain (PBCA), following 58 59 76 penetration of the Saharan rigid prong into the SFCC active margin between 640 and 580 Ma 60 61 62 63 64 3 65 77 (Ngako and Njonfang, 2011). Successive tectonic events recorded in this active margin 1 2 78 involved: (i) crustal thickening, (ii) left- and right-lateral conjugate wrench movements, and 3 4 5 79 (iii) right-lateral wrench movements in the N70°E to EW direction. According to Ngako and 6 7 80 Njonfang (2011), structural and kinematic data show that, although collision between the 8 9 10 81 Trans-Sahara active margin and the WAC started at ca 630 Ma, deformation in north-western 11 12 82 Cameroon was mostly controlled by a NNE-SSW stress direction until ca 590 Ma. Further 13 14 83 deformation in this domain is recorded by a regional clockwise rotation compatible with a 15 16 17 84 NW-SE stress direction suggesting that the late tectonic event was the only one controlled by 18 19 85 the prominent active role of the WAC. 20 21 22 86 PBCA comprises three lithotectonic domains (Fig 1b): the northern Cameroon domain, the 23 24 87 central Cameroon domain and the southern Cameroon domain. 25 26 27 88 The northern Cameroon domain or Poli Group, also known as west Cameroon domain (WCD 28 29 89 (Fig. 1b); Toteu et al., 2004; Penaye et al., 2006) is limited to the south by the Buffle Noir- 30 31 90 Mayo Baléo (BNMB) shear zone. It is characterized by Meso- to Neo-proterozoic volcano- 32 33 34 91 sedimentary basins that were deformed and metamorphosed into schists and high-grade 35 36 92 gneisses (Nzenti et al., 1988). It represents an early Neoproterozoic back-arc basin formed 37 38 39 93 between 830 and 665 Ma (Toteu et al., 1990; Penaye et al., 2006; Toteu et al., 2006), that 40 41 94 includes: detrital and volcaniclastic deposits, metavolcanics (tholeitic basalts and calc-alkaline 42 43 44 95 rhyolites), and pre-, syn- to late-tectonic calk-alkaline intrusions (diorites, granodiorites and 45 46 96 granites). These intrusions, emplaced between 660 and 580 Ma (Toteu et al., 2001), form a 47 48 49 97 NNE-SSW corridor. 50 51 98 The central Cameroon domain, to which belongs the Bandja pluton, also known as Adamawa 52 53 99 domain, is located between the BNMB shear zone to the North and the Sanaga shear zone 54 55 56 100 (SSZ) to the South (Fig. 1b). It includes huge Pan-African batholiths and large scale 57 58 59 60 61 62 63 64 4 65 101 Paleoproterozoic remnants that were emplaced and metamorphosed during the Pan-African 1 2 102 tectonic evolution. Massifs are often made up of syn-tectonic granitoids. 3 4 5 103 The southern Cameroon domain is limited to the north by the Sanaga fault and to the south it 6 7 104 thrusts onto the Congo Craton. This domain represents a syn-tectonic basin which deposition 8 9 10 105 age is younger than 625 Ma (U-Pb dating of detrital zircons, Toteu et al., 2006). It includes 11 12 106 meta-sediments (shales, greywackes, minor quartzites, dolomites and evaporites) and pre- to 13 14 107 syn-tectonic alkaline to transitional intrusions represented by pyroxenites, pyroxene- 15 16 17 108 amphibole and pyroxene-plagioclase rocks (Nzenti, 1998); scarce outcrops of serpentinized 18 19 109 ultramafic rocks associated with gabbros, diorites and mafic dykes are also found (Seme 20 21 22 110 Mouangue, 1998; Nkoumbou et al., 2006). All these rocks were metamorphosed in the HP- 23 24 111 HT granulite facies at ca 620 Ma (U-Pb zircon age by Penaye et al., 1993) prior to 25 26 27 112 exhumation and thrusting over the Congo craton. 28 29 113 Many authors have proposed models to explain the emplacement of granitoids within the Pan- 30 31 114 African belt. These models evoke the transpressional context (Njiékak et al. (2008) for the 32 33 34 115 Batié granitic complex, Djouka Fonkwé et al. (2008) for the Bafoussam granitoids, Kankeu et 35 36 116 al. (2009) for the Lom basin granites) as well as the transtensional context (Nguiessi 37 38 39 117 Tchankam et al. (1997) for the Bandja magmatic complex, Tagné Kamga (2003) for the 40 41 118 Ngondo plutonic complex, Tchameni et al. (2006) for the Ngaoundéré Pan-African granitoids, 42 43 44 119 and Kwékam et al. (2010) for the Fomopéa Pan-African plutonic complex) for granitoids 45 46 120 emplacement. 47 48 49 50 121 2.2 Geology and geochronology of the area 51 52 122 The west Cameroon region (Fig. 2) is underlined by pre-, syn- to post-tectonic granitoids that 53 54 123 have been subdivided into three main types, based on their relationships with orogenic 55 56 57 124 deformational phases: (1) 630-620 Ma pre- to syn-D1 granitoids; (2) 600-580 Ma syn-D2 58 59 125 granitoids; and (3) 550 Ma post-orogenic granitoids (U/Pb ages; Toteu et al., 2001). Using 60 61 62 63 64 5 65 126 CHIME (chemical Th-U-total Pb isochron method) on zircon and allanite crystals in rocks, 1 2 127 Tetsopgang et al. (1999, 2008), in the Nkambé granitoids, determined an age of 532±35 Ma 3 4 5 128 on allanite and 523±45 Ma on zircon in the biotite hornblende orthogneiss and an age of 6 7 129 569±12 Ma in hornblende-biotite granite. Tagné Kamga (2003) in the Ngondo plutonic 8 9 10 130 complex suggests and older continental crust during the genesis of the magma with little or no 11 12 131 addition of mantle derived magma to the crust for the 600 Ma granitoids. Tchameni et al. 13 14 132 (2008) obtained ca. 615-575 Ma (Th-U-Pb age on monazite) on the Ngaoundéré granitoids 15 16 17 133 and suggested a differentiation of a mafic magma from an enriched sub-continental 18 19 134 lithospheric mantle with possible crustal assimilation. Njiékak et al. (2008) determined in the 20 21 22 135 Bangou-Batié granitic pluton 632-602 Ma (U-Pb on zircon) and proposed a syn-kinematic 23 24 136 emplacement of the pluton along a crustal scale NNE to ENE trending strike-slip shear zone. 25 26 27 137 Kwékam et al. (2010) obtained U-Pb ages of 621-613 Ma on the Fomopéa granitic complex 28 29 138 and proposed a model whereby linear lithospheric delamination occurred along the Central 30 31 139 Cameroon Shear Zone (CCSZ) in response to post collisional transpression. This 32 33 34 140 delamination induced the partial melting of the mantle and old lower crust. According to these 35 36 141 latter authors, the emplacement of numerous post collisional Neoproterozoic plutons along the 37 38 39 142 CCSZ during the Pan-African orogeny indicates that this process was of paramount 40 41 143 importance. Recent research works of Dawai et al. (2013) and Mosoh Bambi et al. (2013) 42 43 44 144 respectively for the Guider quartz-syenite (Northern-Cameroon) and Ekomédion granites 45 46 145 (Southwestern Cameroon) determined a U-Pb age of 593 Ma on zircon in quartz-syenite and 47 48 49 146 578 Ma in granites. Mosoh Bambi et al. (2013) concluded for a crustal origin of the magma. 50 51 147 These granitoids, crosscut by gabbroic dykes (Kwékam et al., 2010), are partially covered 52 53 148 with basaltic and trachytic lavas. Early data from the granitoids yielded ages in the 550–700 54 55 56 149 Ma range (Nguiessi Tchakam and Vialette, 1994; Djouka Fonkwé et al., 2008; Njiékak et al., 57 58 150 2008; Kwékam et al., 2010). Recent Th–U–Pb monazite dating in the Bafoussam region by 59 60 61 62 63 64 6 65 151 Djouka Fonkwé et al. (2008) yielded 558-564 Ma emplacement ages of two–mica granitoids. 1 2 152 The metamorphic host rocks, orthogneiss (hornblende–biotite gneiss) and amphibolites were 3 4 5 153 reworked during the Pan-African orogeny (Njiékak et al., 2008; Kwékam et al., 2010). 6 7 154 The Bandja granitic pluton has NE-SW elongated shape with a long axis around 25 km and a 8 9 2 10 155 short axe not more than 6 km (Fig. 3). Its outcrops cover roughly 80 km . Outcrops are of 11 12 156 variable quality and granites often correspond to topographic elevations. Outcrops of the 13 14 157 gneissic basement are scarce, and contacts with the pluton are generally lacking. 15 16 17 158 Dumort (1968) represents the Bandja granitic pluton as a sheet of magmatic rock that 18 19 159 comprises a porphyritic facies at its northeastern part whereas its southwestern part is made of 20 21 22 160 anatexitic rocks (Fig. 2). It is stuck between the Fomopéa granitic pluton (Njanko et al., 2010; 23 24 161 Kwékam et al., 2010) and the Bangou-Baham porphyritic granitic pluton. It is affected at its 25 26 27 162 western border by the ductile Fotouni-Fondjomekwet mylonitic zone (FFMZ; Njanko et al., 28 29 163 2010). The Bandja granitic pluton is dominantly porphyritic in nature. According to Nguiessi 30 31 164 Tchakam et al. (1997), it comprises two rock groups: (i) a highly deformed group, including 32 33 34 165 mylonites (called orthogneiss and charnokites) and banded gneiss and (ii) a less deformed 35 36 166 group including diorite, monzonite, granodiorite, alkali feldspar megacryst granite and fine 37 38 39 167 grained granite so-called Bandja porphyritic granite. In the field, this latter petrographic unit 40 41 168 is not really very well organized and distinctively mappable; then those rocks are regrouped 42 43 44 169 under alkali feldspar megacryst granites. 45 46 170 According to Nguiessi Tchakam and Vialette (1994) and Nguiessi Tchakam et al. (1997), 47 48 49 171 monzonite and granites from Bandja granitic pluton are metaluminous. The geochemical 50 51 172 behaviour is consistent with (i) a common origin of these rocks and (ii) a potassic calc- 52 53 173 alkaline magmatism of orogenic context, close to the syn-collisional to post-collisional 54 55 56 174 domain (Pearce et al., 1984; Lagarde, 1992). The emplacement of orthogneiss is dated with an 57 58 175 age of 686±17 Ma (Rb/Sr) (Lamilen, 1989); an age of 640±15 Ma (U/Pb, Pb/Pb on zircons) 59 60 61 62 63 64 7 65 176 dates the emplacement of charnockites; and an age of 557±8 Ma (Rb/Sr) dates the 1 2 177 emplacement of K-feldspar megacryst granites (Nguiessi Tchakam and Vialette, 1994). These 3 4 5 178 ages are similar to those of syn-tectonic rocks of the Pan-African belt (604 Ma; Penaye et al., 6 7 179 1993) and give a good relation between different Pan-African phases of deformation: 687-633 8 9 10 180 Ma (earlier-D1), 604-580 Ma (syn-D2) and late at 557 Ma (Penaye et al., 1993). 11 12 13 181 3. Materials and methods 14 15 182 In this study, we use classical field methods and anisotropy of magnetic susceptibility (AMS) 16 17 18 183 method. 19 20 184 Oriented cylinder samples were drilled, directly in the field, from 88 different sites (Fig. 3) 21 22 23 185 distributed among the granitic pluton, the mylonitic zone and the banded gneiss of the country 24 25 186 rocks: 64 sites in feldspar megacryst granites, 14 sites in mylonites and 10 sites from banded 26 27 187 gneiss. A total of 363 specimens have been analyzed in this study. 28 29 30 188 In classical field method, the petrography is carried out using thin sections from remaining 31 32 189 oriented cores sampled for AMS study. Structural elements and kinematic indicators (folds, 33 34 35 190 foliations, stretching lineations, S-C structures ...) are recognizable at a metric or centimetric 36 37 191 scale and can be used to infer the kinematic during the emplacement. These data were 38 39 40 192 statistically analyzed using Stereonet software (Allmendinger's Stereonet program) with 41 42 193 conventional techniques. 43 44 194 Measurement of AMS is today, a structural analysis powerful tool (Bouchez, 1997, 2000) to 45 46 47 195 determine petrofabric orientation in rocks, even in rocks that are visually isotropic. AMS 48 49 196 principles are described in various research works (Borradaile and Henry, 1997; Bouchez, 50 51 52 197 1997, 2000). We therefore present here elements to understand the emplacement of Bandja 53 54 198 magmatic complex. 55 56 57 199 Two or three oriented cores were collected at each site. Each core yielded at least two 58 59 200 cylindrical samples, hence providing an average of four specimens per site. Each individual 60 61 62 63 64 8 65 201 sample is 22mm in length and 25mm in diameter, the standard size for magnetic 1 2 202 measurements. AMS measurements were then performed on a Kappabridge susceptometer 3 4 -4 5 203 KLY-3S (Agico Ltd, Czeck Republic); working at a low alternating field (±4 x 10 T, 920Hz) 6 7 204 with a sensitivity of about 2 x 10-7 SI, allowing anisotropy discrimination below 0.2% over a 8 9 10 205 wide range of susceptibility. The orientations of the principal axes Ki (i = 1 to 3) of the 11 12 206 average AMS ellipsoid for a given site are obtained by a classical eigenvector calculation 13 14 207 between the individual AMS measurements Kij (specimens j = 1 to 4). The magnetic 15 16 17 208 susceptibility is measured for different orientations of the cylindrical specimen and is used to 18 19 209 calculate the intensity of the three principal axes of the magnetic susceptibility ellipsoid K1, 20 21 22 210 K2, and K3. AMS is a second-rank tensor expressed by its principal eigenvectors and 23 24 211 eigenvalues Kmax=K1 > Kint=K2 > Kmin=K3, respectively representing the maximum, 25 26 27 212 intermediate and minimum axes of the magnetic susceptibility ellipsoid. 28 29 213 The long axis of the ellipsoid K1 defines the magnetic lineation and the short axis K3 defines 30 31 214 the pole of the magnetic foliation (the plane formed by K and K axes). In plutonic rocks, the 32 1 2 33 34 215 magnetic fabric is usually coaxial with the mineral fabric. When a granite has a so called 35 36 216 “paramagnetic” behaviour, K3 represents the pole of the foliation marked by the preferred 37 38 39 217 orientation of the main Fe-bearing silicate mineral (namely biotite and amphibole), and K1 40 41 218 represents the biotite lineation defined as the axis of rotation, or zone axis, of the biotite 42 43 44 219 foliation (Bouchez, 2000). It is worth to notice that, in these rocks, very small quantity of 45 46 220 ferromagnetic material may be present as tiny inclusions in silicate minerals (Pignotta and 47 48 49 221 Benn, 1999). According to Grégoire et al., (1998), when a granite has a ferrimagnetic or 50 51 222 ferromagnetic (sensu lato) behaviour (i.e magnetite-bearing granites), K3 defines the pole of 52 53 223 the average flattening plane and K1 the elongation direction of the shape preferred orientation 54 55 56 224 of magnetite. Therefore, the foliations and lineations obtained by magnetic methods can be 57 58 225 used as indicatives of the mineral fabrics. 59 60 61 62 63 64 9 65 226 The relationship between tensor axes, normalized by means of Jelinek’s method (1978, Table 1 2 227 1) was studied using: (i) the anisotropy degree P’ according to Jelinek (1981) which shows 3 4 5 228 the intensity of the preferred orientation of minerals, and (ii) the shape parameter, T = 6 7 229 (2ln(K2/K3)/ln(K1/K3))-1), which varies between T= -1 (prolate ellipsoids) and T= +1 (oblate 8 9 10 230 ellipsoids). 11 12 231 Information on magnetic mineralogy is of crucial importance in rocks magnetic studies 13 14 232 (Petrovsky and Kapicka 2006). Standard analytical methods and sensitive thermomagnetic 15 16 17 233 analyses are used to determined temperature dependence of magnetic parameters as basis of 18 19 234 determination of magnetic second order phase transition temperatures. Although limited by 20 21 22 235 several drawbacks, the most serious being thermally induced transformations of the original 23 24 236 minerals, this method provides useful information about the presence of magnetite minerals, 25 26 27 237 and additional knowledge on the prevailing grain size distribution or degree of substitution. 28 29 238 According to Tarling and Hrouda (1993), the contribution of magnetic mineralogy to rock 30 31 239 susceptibility is very important in magnetic anisotropy investigation because the minerals 32 33 34 240 show different behaviour in different geological situations. One of the methods for the 35 36 241 resolution of the rock susceptibility into its ferromagnetic and paramagnetic components is 37 38 39 242 based on the investigation of the temperature variation of susceptibility. The thermomagnetic 40 41 243 curves give information about the temperature changes of susceptibility in the initial part of 42 43 44 244 the thermomagnetic curves of the ferromagnetic minerals (Hrouda et al. 1997). Although the 45 46 245 susceptibility temperature variation can be used in principle for the susceptibility resolution 47 48 49 246 into ferromagnetic and paramagnetic components, the application must be made with care, 50 51 247 because of the possible problems concerning the susceptibility versus temperature relationship 52 53 248 for the ferromagnetic minerals. 54 55 56 249 To ground the interpretation of AMS results, it is necessary to know which minerals are 57 58 250 carrying the susceptibility and in which proportion (e.g. Rochette et al., 1992; Benn et al, 59 60 61 62 63 64 10 65 251 1999). Namely, hysteresis loops were used to estimate the proportion carried by paramagnetic 1 2 252 minerals and estimate the type and grain size of the ferromagnetic minerals. Thermomagnetic 3 4 5 253 analyses further allowed to determine the Curie point of the ferromagnetic minerals 6 7 254 (following Petrovski and Kapicka, 2006). 8 9 10 255 Samples were measured for AMS (1) in the laboratory of magnetism at the University 11 12 256 Chouaïb Doukkali of El Jadida in Morocco and (2) in the “Laboratoire de Géologie– 13 14 257 Minéralogie-Pétrophysique et Tectonique” of the University of Ouagadougou in Burkina- 15 16 17 258 Faso. Thermomagnetic analyses were obtained using a CS2 apparatus coupled to the KLY-3S 18 19 259 Kappabridge instrument (Agico, Czech Republic) in the Laboratoire Géosciences 20 21 22 260 Environnement Toulouse (GET; France) and hysteresis analyses were carried out using a 23 24 261 Micromag VSM on 1 cc cubes in CEREGE UMR6635 Aix-Marseille Université CNRS 25 26 27 262 Europole de l'Arbois Aix-en-Provence (France). 28 29 263 The raw magnetic data for each of the 88 sites concerning the study area are reported in table 30 31 264 1. 32 33 34 35 265 4. Results 36 37 38 266 4.1 Petrography 39 40 267 K-feldspar megacryst granites are the main petrographic component of the pluton. They are 41 42 268 greyish or leucocratic and pinkish, and coarse- to medium-grained to fine-grained (average 43 44 45 269 grain size <3.8mm). They contain phenocrysts of alkali feldspar (Fig. 4b, c, e, f) responsible 46 47 270 for a noticeable mineral preferred orientation acquired during the magmatic stage or later. In 48 49 50 271 this post-magmatic case, porphyroclasts of feldspar are moulded in a fine-grained matrix, 51 52 272 sometimes with quartz ribbons showing evidences of dynamic recrystallization. Other 53 54 55 273 minerals are quartz, plagioclase and variable amount of hornblende, biotite and accessory 56 57 274 minerals as zircon, apatite, sphene, epidote and oxides. Myrmekite are generally abundant at 58 59 60 61 62 63 64 11 65 275 the contact between plagioclase and K-feldspar (Fig. 4b). The textures are granoblastic 1 2 276 heterogranular to mylonitic (Fig. 4h). 3 4 5 277 Banded gneiss outcrop at Bania and Babouantou. Rocks are dark grey in colour and 6 7 278 composed of clinopyroxene, pargasitic and hastingsitic hornblende, biotite and plagioclase. 8 9 10 279 Accessory minerals are zircon, apatite, sphene, allanite and oxides. 11 12 280 Mylonites are found at the northwestern part of the study area (Fondjomekwet and Fotouni). 13 14 281 They are greyish, or sometimes leucocratic, banded, fine-grained, with quartz, K-feldspar, 15 16 17 282 biotite, plagioclase, garnet, muscovite, cordierite, sillimanite and monazite. Their mineral 18 19 283 composition is indicative of a metapelite protolith that underwent mylonitization under mid- 20 21 22 284 pressure amphibolite facies conditions. These mylonites are the one described by Njanko et 23 24 285 al. (2010) in the FFMZ. Beside mylonites, Njanko et al. (2010) have described, in the FFMZ, 25 26 27 286 protomylonites on the southeastern border of the Fomopéa granitic pluton. They belong to the 28 29 287 pluton body and are medium- to coarse-grained with quartz, K-feldspar, plagioclase and 30 31 288 chlorite, corresponding to the deformation of the plutonic rocks in greenschist facies 32 33 34 289 conditions. 35 36 37 38 290 4.2 Structures 39 40 291 4.2.1 Mesostructural observations 41 42 292 Structures observed in the field are foliations, S/C structures, stretching lineation and folds. 43 44 45 293 Foliation is observed in banded gneiss, granites and mylonites (Fig. 5). 46 47 294 In banded gneiss, the foliation is the result of alternation of white (quartz and feldspar rich) 48 49 50 295 and dark (biotite and amphibole rich) layers. It varies between N05°E and N19°E strikes with 51 52 296 low to moderate dips (23° – 37°) towards the SE or NW and shows best poles at 289/53, 53 54 55 297 277/66, and 82/62 (Fig. 5). 56 57 298 In granite, the magmatic foliation is characterized by preferred orientation of minerals as 58 59 299 quartz, feldspar, biotite and amphibole. It varies between N16°E and N54°E strikes with 60 61 62 63 64 12 65 300 moderate to strong dips towards the SE and shows best poles at 296/23, 313/35, 299/43 and 1 2 301 297/40. This magmatic foliation shows NE-SW strike direction in the northern part of the 3 4 5 302 pluton and NNE-SSW strike directions in its central part (Fig. 5). The foliation in banded 6 7 303 gneiss seems to deflect to fit the elliptical geometry of the Bandja granitic pluton. 8 9 10 304 In mylonites, the mylonitic foliation shows homogeneous NE-SW strikes with moderate to 11 12 305 very strong dips towards NW or SE. Best poles are at: 302/45 in Fotouni, 318/30 and 317/2 in 13 14 306 Fondjomekwet (Fig. 5). 15 16 17 307 S/C structures are observed in the Fotouni-Fondjomekwet mylonitic zone and on the western 18 19 308 border of the granitic pluton and scattered in the pluton body. In the mylonitic zone, they are 20 21 22 309 consistent with a dextral sense of motion, also confirmed by typical sigma-type 23 24 310 porphyroclasts and their crystallization tails. In granite, it is not easy to determine the sense of 25 26 27 311 the motion. 28 29 312 Stretching lineation is observed and measured in the Fotouni-Fondjomekwet mylonitic zone 30 31 313 where it is marked by quartz and biotite stretched crystals on the mylonitic foliation planes. It 32 33 34 314 shows NE-SW azimuth with low plunges (<10°; best lines at 49/5 and 56/9; Fig. 5). 35 36 315 The morphology of the folds points to asymmetric fold shape with SE vergence that have 37 38 39 316 developed long western limbs and thin short eastern limbs with dominant Z-folds on long 40 41 317 limbs. Fold axes (best line at 35/6) and mineral stretching lineations are subhorizontal and 42 43 44 318 strike NE-SW as mylonitic foliation that dips moderately to strongly towards the NW or SE. 45 46 47 319 4.2.2 Microstructural observations 48 49 320 In accordance with modern structural studies in granites (Paterson et al., 1989; Bouchez et al., 50 51 52 321 1992) detailed microstructural observations have been performed in rocks. Samples from 53 54 322 Bandja feldspar megacryst granite show submagmatic to solid-state deformation 55 56 57 323 microstructures. Submagmatic deformation microstructures are marked by (i) feldspar 58 59 324 microcracks filled with quartz and (ii) myrmekite crystals (Fig. 4b) developed on the border 60 61 62 63 64 13 65 325 of K-feldspar megacrystals. The solid-state deformation microstructures are marked by quartz 1 2 326 crystals that have strong undulatory extinction attesting to intracrystalline deformation. Some 3 4 5 327 samples show chessboard texture in quartz (Fig. 4g, h) in which square subgrains with 6 7 328 boundaries parallel to both prism and basal planes attest the deformation at high temperature 8 9 10 329 (Blenkinsop, 2000). Deformation twins in feldspar, S-C structures observed in granites (Fig. 11 12 330 4d, f) and mylonites (Fig. 4g) and kinked biotites are noted. Besides, ductilely deformed 13 14 331 feldspar, recrystallized feldspars are also observed. All the above microstructures imply that 15 16 17 332 granite developed intense deformation in the solid-state and at rather high temperatures. 18 19 20 333 21 4.3 Magnetic mineralogy 22 23 334 4.3.1 hysteresis 24 25 335 Fifteen representative 1 cc samples (for about 1cm3 volume) have been measured (Table 2 26 27 28 336 and Fig.11). The ferromagnetic contribution was calculated by subtracting the high field 29 30 337 slope, mostly paramagnetic, to the hysteresis curve that combines both ferromagnetic (sensu 31 32 338 lato) and paramagnetic contributions. Hysteresis loop shapes and parameters are typical of 33 34 35 339 magnetite or other spinel oxides. The comparison of high field susceptibility (Khf) to low 36 37 340 field susceptibility (Ko), measured on the same sample with the same orientation on the 38 39 40 341 Kappabridge, allows to quantify the relative contribution of paramagnetic minerals to Ko. 41 42 342 Alternatively, to overcome the calibration problem between VSM and Kappabridge, we 43 44 45 343 calculate a model Ko based on saturation magnetization (Ms) and Khf following Benn et al., 46 47 344 1999. Both methods lead to similar paramagnetic relative contribution. This contribution 48 49 345 varies from 47 to 94% for Ko<1000 µSI, except one sample, AY59, with minor paramagnetic 50 51 52 346 contribution. Still the ferromagnetic contribution is significant in all samples, except AY76, 53 54 347 for which AMS must be entirely carried by iron-bearing silicates. For the other samples as the 55 56 57 348 anisotropy of magnetite may be much stronger than the one of silicates, it is still likely that 58 59 349 AMS is carried by magnetite even if paramagnetic contribution to Ko is dominant. For 60 61 62 63 64 14 65 350 samples with Ko>1000 µSI, the paramagnetic contribution is minor (up to 20% on two 1 2 351 samples) to negligible. The loop parameters reflect the state of the magnetic domains in 3 4 5 352 magnetite. A plot of the saturation ratio (Mrs/Ms; Mrs is the remanence at saturation and Ms 6 7 353 the magnetization at saturation) versus the hysteresis ratio (Bcr/Bc where Bcr is the remanent 8 9 10 354 coercive force and Bc the coercive force), or Day’s diagram (Day et al., 1977; Fig. 11), 11 12 355 provides an indication of the size of the magnetite grains. Our specimens fall into the PSD 13 14 356 (pseudo-single domain) and MD (multi-domain) fields. Once transposed into grain sizes, this 15 16 17 357 indicates that magnetite ranges between 0.1µm (PSD) and > 1.0µm (MD). Samples AY27, 18 19 358 AY79 and AY61 likely point toward the mixture of hematite and magnetite contribution to 20 21 22 359 remanence. However, the contribution of hematite to susceptibility in these samples must be 23 24 360 negligible based on the low specific susceptibility of hematite compared to magnetite. 25 26 27 361 Progressively, samples were heated up to 700°C and cooled to room temperature. 28 29 362 Thermomagnetic curves of high susceptibility samples (Fig. 10) have quite similar shapes, 30 31 363 suggesting a similar type of magnetic mineralogy for all parts of the intrusion. Most curves 32 33 34 364 show a significantly higher susceptibility on cooling [not for samples AY10 and AY67 in fig. 35 36 365 10a and 10b], indicative of neoformation of magnetic minerals starting around 300-400°C 37 38 39 366 (probably maghemite converted to hematite). They all show a Curie point around 580°C with 40 41 367 a sharp decrease in susceptibility in all the heating curves, corresponding to the Curie 42 43 44 368 temperature of pure magnetite. The remaining susceptibility above 600°C is likely a 45 46 369 combination of paramagnetic contribution and residual magnetite signal due to the use of a 47 48 49 370 too large sample mass resulting in part of the sample being cooler than temperature probe. 50 51 371 Therefore it is confirmed that pure magnetite is the main carrier in our high susceptibility 52 53 372 samples. 54 55 56 57 373 4.4 AMS data 58 59 60 61 62 63 64 15 65 374 The AMS analysis gives well-defined magnetic fabrics for all sites in Bandja magmatic 1 2 375 pluton and its country rocks (banded gneiss) as well as in mylonites (Table 1 and Fig. 6). 3 4 5 6 376 4.4.1 Magnetic susceptibility 7 8 377 The bulk magnetic susceptibility magnitude (Km) varies from 51 µSI to 35836 µSI (Table 1; 9 10 378 Fig. 9a) suggesting, in the study area, the presence of both (i) dominant ferromagnetic 11 12 13 379 behaviour sensu lato (Km > 500 µSI) dominated by the signal of magnetite that represents 14 15 380 72% of sites and (ii) dominant paramagnetic behaviour (Km < 500 µSI) probably dominated 16 17 381 by the signal of iron-bearing silicates (Rochette, 1987; Bouchez, 2000) that represents 28% of 18 19 20 382 the sites. Km values range from 62 µSI to 35836 µSI in feldspar megacryst granites, 21 22 383 indicating 79% of sites with ferromagnetic behaviour and 21% of sites with paramagnetic 23 24 25 384 behaviour (Fig. 9b). The lowest value belongs to site AY41 situated at the western border of 26 27 385 the pluton in contact with the mylonites and the highest value belongs to site AY70 nearby the 28 29 30 386 NNE-SSW fracture in the northeastern part of the pluton. In banded amphibole gneiss, Km 31 32 387 varies from 119 µSI to 22463 µSI. Km values range from 51 µSI to 13131 µSI in mylonites 33 34 35 388 indicating 50% of sites with ferromagnetic behaviour. 36 37 389 In spite of the fact that fresh samples were obtained in the fields, both paramagnetic and 38 39 390 ferromagnetic behaviours are identified in the same petrographic unit and sometimes in the 40 41 42 391 same site, indicating an uneven distribution of magnetite. There is no particular organization 43 44 392 of both behaviours in the granitic pluton. 45 46 47 48 393 4.4.2 Anisotropy degree 49 50 394 The anisotropy degree P’ varies from 1.02 to 1.95 for all sampled sites (Table 1, Fig. 10a). In 51 52 395 granite, P’ varies from 1.02 to 1.95. The very high values of P’ are located in the northeastern 53 54 55 396 part of the pluton. In mylonites P’ varies from 1.06 to 1.65. The site TJ3, that shows the very 56 57 397 low value of Km (53 µSI) also shows low value of P’ (1.14). In banded gneiss P’ varies from 58 59 398 1.09 to 1.78. The kink in P’ versus Km plot (Fig. 8c) confirms that the paramagnetic 60 61 62 63 64 16 65 399 contribution becomes important below Km= 1000 µSI, with P’ around 1.1, following 1 2 400 Rochette et al. (1992). The continuous increase of P’ versus Km for Km>>1000 µSI may 3 4 5 401 indicate that anisotropy of magnetite is enhanced by interaction (Gregoire et al., 1998). 6 7 402 P’ values lower than 1.15 represents 73% of sites in feldspar megacryst granites, 20% of sites 8 9 10 403 in banded gneiss and 53% of sites in mylonites (Fig. 10b). According to Rochette et al., 11 12 404 (1992); Bouchez (1997, 2000), P’ values lower than 1.15 may correspond to paramagnetic 13 14 405 dominant rocks (also characterized by Km values below 1000 µSI), although ferromagnetic 15 16 17 406 dominant rocks may also show low values due to low preferred orientation of magnetite 18 19 407 grains. 20 21 22 408 The sites AY70 and AY71 that show the highest values of Km also show the highest values of 23 24 409 P’ and are concentrated along a fracture at the northeastern part of the pluton. In all 25 26 27 410 petrographic units, the site that shows the highest value of Km also shows the highest value of 28 29 411 P’, but sites that show the lowest value of Km do not show the lowest value of P’. 30 31 32 33 412 4.4.3 Shape parameter (T) 34 35 413 The Jelinek’s (1981) shape parameter (T) of the AMS ellipsoid is presented in table 1 and 36 37 414 figure 11. About 12% of sites show oblate shape ellipsoid while 5% of sites show prolate 38 39 415 shape ellipsoid and 83% of sites a triaxial shape ellipsoid. In mylonites, T indicates 7% of 40 41 42 416 sites with an oblate shape ellipsoid, 13% of sites with a prolate shape ellipsoid and 80% of 43 44 417 sites with a triaxial shape ellipsoid. In feldspar megacryst granites, T indicates 8%, 2% and 45 46 47 418 90% respectively and in banded gneiss, T indicates 20% of sites with oblate shape ellipsoid 48 49 419 and 80% of sites with triaxial shape ellipsoid. The obtained T parameter correlates well with 50 51 52 420 the AMS projection diagrams. It indicates either K1 as axis of K2 and K3 in a prolate shape 53 54 421 ellipsoid, or K3 as axis of K1 and K2 in an oblate shape ellipsoid, or the grouping together of 55 56 422 axis K , K or K in a triaxial shape ellipsoid. 57 1 2 3 58 59 60 423 4.4.4 Magnetic foliations 61 62 63 64 17 65 424 70% of sites in banded gneiss show magnetic foliation dips between 0° and 30°, 10% between 1 2 425 30° and 60° and 20% between 60° and 90°. 3 4 5 426 General orientations of magnetic foliations in the Bandja feldspar megacryst granite allow to 6 7 427 distinguish three different domains (Fig. 12) called domain I in the northeastern part of the 8 9 10 428 pluton, domain II in its central part and domain III in its southern part. 24% of sites show 11 12 429 magnetic foliation dips between 0° and 30°, 24% between 30° and 60° and 52% between 60° 13 14 430 and 90°. Domains I shows magnetic foliation projection diagrams striking NE-SW with a best 15 16 17 431 pole at 295/34 and domain II shows magnetic foliation projection diagrams striking NNE- 18 19 432 SSW with a best pole at 283/33 while domain III shows a NW-SE striking direction with best 20 21 22 433 at 35/59. 23 24 434 In mylonites, azimuths and plunges of magnetic foliations are well organized in NE-SW strike 25 26 27 435 and NW or SE dipping direction with moderate to very high values. The best pole is at 129/6. 28 29 436 79% of sites show magnetic foliation dips between 60° and 90° while 21% of sites show dips 30 31 437 between 30° and 60°. 32 33 34 35 438 4.4.5 Magnetic lineations 36 37 439 In banded gneiss and mylonites, magnetic lineations indicate very homogeneous low plunges 38 39 440 (90% and 87% of sites respectively) with moderate plunges (30°-60°) poorly represented 40 41 42 441 (10% and 13% respectively). 43 44 442 In feldspar megacryst granite, K1 presents mostly low plunges (0°-30° for 75% of sampled 45 46 47 443 sites; Fig. 13). Moderate plunges represent 20% of sites while strong plunges (60°-90°) 48 49 444 represent 5% of sites. Magnetic lineations show different behaviours in the three previous 50 51 52 445 domains as magnetic foliation. Domain I shows a NE-SW trending magnetic lineations (best 53 54 446 line at 37/8) while domain II shows N-S (best line at 191/9) trending magnetic lineations and 55 56 447 domain III, E-W trending magnetic lineations (best line at 267/22). These trending magnetic 57 58 59 448 lineations point to an S form trail as magnetic foliation trends. 60 61 62 63 64 18 65 449 5. Interpretation 1 2 3 450 5.1 Structures and microstructures 4 5 451 Figure 5 indicates magmatic foliation plane strikes varying from the northern to the southern 6 7 452 part of the pluton with moderate to strong dips towards the SE. The Bandja feldspar 8 9 10 453 megacryst granite shows weakly deformed to protomylonitic microstructures. Thin sections 11 12 454 exhibit megacrysts surrounded by anhedral aggregates of quartz. However, feldspar 13 14 15 455 megacryst microcracks filled with quartz and/or feldspar are likely related to the submagmatic 16 17 456 stage and are indicative of melt migration during deformation (Bouchez et al., 1992). 18 19 457 Microstructural observations point to the development of the pluton in sub-solidus conditions 20 21 22 458 and during a continuum from high to low-temperature conditions. Quartz crystals have strong 23 24 459 undulatory extinction with chess-board texture in which square subgrains with boundaries 25 26 27 460 parallel to both prism and basal planes and stepped grain boundaries attest the deformation at 28 29 461 high temperature (Mainprice et al, 1986). Feldspars show deformation twins indicating that 30 31 32 462 they underwent deformation and recrystallization above 450-500°C (Simpson (1985) and 33 34 463 McCaffrey et al. (1999) and kinked biotites are noted. Myrmekite, which has been inferred as 35 36 464 a deformation induced texture (Simpson and Wintsch, 1989) and microcline twinning in K- 37 38 39 465 feldspar (Vernon, 2000) are also present in granite. S-C structures in the whole pluton witness 40 41 466 either dextral, either sinistral transcurrent tectonics that occurred in granite or along the 42 43 44 467 northwestern border of the pluton. It is not really possible to dissociate submagmatic to solid- 45 46 468 state deformation in the whole pluton and all samples seem to illustrate solid state or 47 48 49 469 mylonitic deformations as: crystal plastic deformation/recrystallization of feldspar, 50 51 470 myrmekite, S-C structures and crystal plastic deformation of quartz, from ribbons of equant 52 53 54 471 polygonal grains to dynamically recrystallized grains. A continuous deformation from 55 56 472 submagmatic to solid-state and mylonitic deformation is recorded in the Bandja granitic 57 58 473 pluton. 59 60 61 62 63 64 19 65 474 Njanko et al. (2010) in the neighbouring area of the Bandja granitic pluton described three 1 2 475 phases of deformation: a D1 phase responsible of the emplacement of biotite hornblende 3 4 5 476 granite (BHG) magma with NNE-SSW stretching direction that reflect the tectonic strain at 6 7 477 that time; a N-S sinistral D2 phase which correspond to the emplacement of the biotite 8 9 10 478 monzogranite (BmG) and a younger dextral D3 phase linked to the functioning of Fotouni- 11 12 479 Fondjomekwet Mylonitic Zone (FFMZ), that is the mylonitic zone of the northwestern border 13 14 480 of the Bandja granitic pluton. 15 16 17 18 481 5.2 Magnetic data and emplacement of granite 19 20 482 According to the thermomagnetic analysis, K-T curves indicate the presence of magnetite as 21 22 23 483 main susceptibility carrier event if certain low susceptibility samples reveal the paramagnetic 24 25 484 component. The predominance of magnetite in samples allows to envisage that the high 26 27 485 ferromagnetic behaviour of the granitic unit is closely linked to the presence of magnetite MD 28 29 30 486 in rocks. Sites that show very high magnitude of magnetic susceptibility also show very high 31 32 487 values of the anisotropy degree (P’ >1.1). On the other hand, sites with very low magnitude of 33 34 35 488 magnetic susceptibility do not always present low values of the anisotropy degree. This 36 37 489 indicates that the P’ parameter (usually the quantitative strain intensity indicator) is also 38 39 40 490 influenced by the presence of magnetite in form of PSD or MD grains in rocks than by the 41 42 491 strain itself. This is supported by the semi-logarithmic diagram of the magnetic susceptibility 43 44 492 Km versus anisotropy degree P’ (Fig. 10c) where samples show a hyperbola plot. According 45 46 47 493 to Archanjo et al. (1994), P’ depends weakly on Km occasionally, suggesting that rock- 48 49 494 composition controls P’. 50 51 52 495 Magnetic fabric axes are parallel to the mineral fabric axes and these axes recorded the strain 53 54 496 (flattening plane) to which the magma was subjected after its complete crystallization (high- 55 56 57 497 temperature solid-state microstructures). The magnetic foliation and lineation in the three 58 59 498 domains of the granitic pluton show different orientations resulting in a sigmoidal path. 60 61 62 63 64 20 65 499 According to Njanko et al. (2010), the sigmoidal trajectories of magnetic lineation observed 1 2 500 in biotite monzogranite of the Fomopéa granitic pluton correspond to a sinistral sense of 3 4 5 501 motion linked to the D2 phase of deformation recorded in this neighbouring area. This 6 7 502 sinistral D2 event predates the dextral D3 phase recorded in the FFMZ. 8 9 10 11 503 5.3 Discussion on age and geodynamic implication 12 13 504 The geological history of the Bandja granitic pluton should take into account: (i) different 14 15 505 ages obtained by Nguiessi Tchakam et al. (1997), (ii) the available Pan-African ages and the 16 17 18 506 evolution of the Pan-African belt in Cameroon. According to Njiékak et al., (2008) structural 19 20 507 and geochronological data from the Batié pluton (BP), in the continuity north of the Bandja 21 22 23 508 granitic pluton, the migmatitic complex of Foumbot (MCF), and the Bangoua metagranitoid 24 25 509 complex (BMC) are provided to put constraints on the tectonic of the N30°E strike segment 26 27 510 of the belt. BMC is dated 638 ± 2 Ma (U-Pb age on zircon) and the BP is dated 602± 1.4 Ma 28 29 30 511 (U-Pb age on zircon). Tagné Kamga (2003) dates the Ngondo plutonic complex (also 31 32 512 emplaced along N30°E strike shear zone) at around 600 Ma. These plutons are emplaced 33 34 35 513 during the Neoproterozoïc D2 Pan-African event. In the same order, Kwékam et al. (2010) 36 37 514 determined, on the Fomopéa granitic pluton, ages: 622 – 613 Ma for the biotite hornblende 38 39 40 515 granitoid corresponding to D1 Pan-African event (Njanko et al., 2010); 613-590 Ma for the 41 42 516 biotite granitoid and corresponding to the D2 Pan-African event (Njanko et al., 2010). The 43 44 517 Rb-Sr ages obtained for the Bandja granites (557 ± 18Ma) are strikingly younger than the U- 45 46 47 518 Pb ages obtained on neighbouring plutons and seem to correspond to the noticeable 48 49 519 perturbation of the Rb–Sr isotopic system, hence the 572 ± 48 Ma age obtained by Kwékam et 50 51 52 520 al. (2010) using the same method. The S shape of the magnetic foliation and lineation trends 53 54 521 in the Bandja pluton can allow us to consider the emplacement of the pluton in dextral shear 55 56 57 522 context. 58 59 60 61 62 63 64 21 65 523 Sure enough, Ngako et al. (2003, 2008) and Ngako and Njonfang (2011) bring up the problem 1 2 524 of left lateral wrench movement (613-585 Ma) in the Pan-African belt and the sinistral shear 3 4 5 525 event (613-590 Ma) accompanying the emplacement of granitoids in the belt. This lateral 6 7 526 wrench motion develops the Buffle Noir Mayo Baleo (BNMB) shear zone (Fig 1b), the Mayo 8 9 10 527 Nolti shear zone (MNSZ), the Godé-Gourmaya shear zone (GGSZ) in the northern part of the 11 12 528 belt and certainly the Rocher du Loup shear zone (RLSZ) along the southwestern coast of 13 14 529 Cameroon. It appears as one of the three main tectonic events related to Pan-African collision 15 16 17 530 and post-collision evolution identified in Cameroon after the crustal thickening initiated at 18 19 531 630 Ma (Ngako and Njonfang, 2011). According to Nguiessi Tchakam and Vialette (1994) 20 21 22 532 and Nguiessi Tchakam et al. (1997) the Bandja granitic pluton units (charnockite and K- 23 24 533 feldspar megacryst granite) are completely emplaced between 604 – 557 Ma. These ages can 25 26 27 534 be reorganized as: (i) 686±17 Ma, age of metamorphism of banded gneiss, basement of K- 28 29 535 feldspar megacryst granite. This age is close to 687-633 Ma (earlier-Dl) in the belt; (ii) 604- 30 31 536 557 Ma, as period of emplacement of the K-feldspar megacryst granite and its deformation 32 33 34 537 through the dextral Pan-African D3 last event recorded in the FFMZ on the western part of the 35 36 538 pluton. This reorganization seems, in accordance with the tectonic evolution of the PBCA in 37 38 39 539 Cameroon, to emplace the Bandja granitic pluton in a tectonic context initiated from the 40 41 540 sinistral shear event of the Pan-African D2 to the dextral Pan-African D3 shear event with the 42 43 44 541 functioning of the Central Cameroon Shear Zone (CCSZ; Ngako et al., 2003; Ngako et al., 45 46 542 2008; Njonfang and Ngako, 2011) and gives a good correlation with different tectonic events 47 48 49 543 described in the Pan-African belt: crustal thickening and subsequent N-S to NNE-SSW 50 51 544 sinistral wrench movement (630 – 530 Ma) and right lateral wrench movement (ca 585 – 540 52 53 545 Ma). 54 55 56 546 According to Njiékak et al. (2008), the intrusion of the Batié porphyritic granite was dated at 57 58 547 602±1.4 Ma. Njanko et al. (2010) dated the D2 N-S striking sinistral deformation bands 59 60 61 62 63 64 22 65 548 between 613 – 590 Ma that integrate the 602±1.4 Ma (U-Pb on zircons; Njiékak et al. (2008)) 1 2 549 on Batié porphyritic granite. The period ca 602 – 590 Ma corresponds to the late Pan-African 3 4 5 550 tectonomagmatic stage of the Batié area (Njiékak et al., 2008) and adjacent southern Bandja 6 7 551 region. Magma emplacement of the Bandja feldspar megacryst granite is likely coeval with 8 9 10 552 movements along transcurrent dextral shear zones (Fig. 14). 11 12 13 553 6. Conclusion 14 15 554 The major conclusion of our work is that Bandja granitic pluton has a close relationship with 16 17 18 555 granitic pluton of Fomopea and Batié pluton. This relationship can be guided through new 19 20 556 results from the systematic study of magnetic fabric and microstructures that help to 21 22 23 557 understand the emplacement of the Bandja pluton. AMS of Bandja pluton is controlled by 24 25 558 ferromagnetic minerals that typically include MD magnetite. Moreover continuous low field 26 27 559 thermomagnetic curves (K-T curves) confirm magnetite as the main susceptibility carrier. The 28 29 30 560 shape and orientation of the AMS ellipsoids defined a dominant triaxial shape ellipsoid. The 31 32 561 organization of T parameter diagram (T vs P’) correlates with the AMS projection diagrams 33 34 35 562 of K1, K2 and K3. The quantitative strain intensity indicator P’ is also linked to the magnetite 36 37 563 distribution in rocks than to the strain itself. The fabrics of the Bandja granitic pluton were 38 39 40 564 acquired at the solid state deformation at high temperatures. Magnetic foliation and magnetic 41 42 565 lineation trend a S shape and allow to conclude that the complete emplacement and 43 44 566 deformation of the pluton extend from the Pan-African D event to the dextral ENE-WSW 45 2 46 47 567 Pan-African D3 shear event occurred in the Pan-African belt in Cameroon and related to the 48 49 568 functioning of the Bandja N30°E strike–slip shear zone that shows a dextral sense of motion. 50 51 52 53 569 Acknowledgements 54 55 570 We thank Mbakop N. and Petchou Z. who provided financial supports for (i) field trips and 56 57 58 571 (ii) the first author stay in the laboratory of magnetism at the University Chouaïb Doukkali of 59 60 572 El Jadida, Morocco. We also thank Philippe Olivier (GET, France) for thermomagnetic 61 62 63 64 23 65 573 analyses and curves; its comments have greatly improved this manuscript. 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Geoscience 11 12 699 338 1167-1175. 13 14 700 Nzenti J P, Barbey P, Macaudière J and Soba D 1988 Origin and evolution of the Late 15 16 17 701 Precambrian high-grade Yaounde gneiss (Cameroon); Precambrian Research 38 91–109. 18 19 702 Nzenti J P 1998 Neoproterozoic alkaline meta-igneous rocks from the Pan-African north 20 21 22 703 equatorial fold belt (Yaoundé, Cameroon): biotitites and magnetic rich pyroxenites; 23 24 704 Journal of African Earth Sciences 26 37–47. 25 26 27 705 O'Reilly W 1984 Rock and mineral magnetism; Blackie, Glasgow, 230 p. 28 29 706 Paterson S R, Vernon R H and Tobisch O T 1989 A review for the identification of magmatic 30 31 707 and tectonic foliations in granitoids; Journal of Structural Geology 11 (3), 349-363. 32 33 34 708 Pearce J A, Harris N B W and Tindle A G 1984 Trace elements discrimination diagrams for 35 36 709 the geotectonic interpretation of granite rocks; J, Petrol 25 956-983 37 38 39 710 Penaye J, Toteu S F, Van Schmus W R and Nzenti J P 1993 U-Pb and Sm-Nd preliminary 40 41 711 geochronologic data on the Yaoundé series, Cameroon: re-interpretation of the granulitic 42 43 44 712 rocks as the suture of a collision in the “Centrafrican” belt; C. R. Acad. Sci. Paris 317 45 46 713 789-794. 47 48 49 714 Penaye J, Kröner A, Toteu S F, Van Schmus W R and Doumnang J C 2006 Evolution of the 50 51 715 Mayo Kebbi region as revealed by zircon dating: an early (ca. 740 Ma) Pan-African 52 53 716 magmatic arc in south-western Chad; Journal of African Earth Sciences 44 530–542. 54 55 56 717 Petrovsky E and Kapicka A 2006 On determination of the Curie point from thermomagnetic 57 58 718 curves; Journal of Geophysical Research III, B12S27, doi: 10.1029/2006JB004507. 59 60 61 62 63 64 29 65 719 Pignotta G S and Benn K 1999 Magnetic fabrics of the Barrington passage pluton, Meguma 1 2 720 Terrane, Nova Scotia: a two-stage fabric history of syntectonic emplacement; 3 4 5 721 Tectonophysics 307 75–92. 6 7 722 Rochette P, Jackson M and Aubourg C 1992 Rock magnetism and the interpretation of 8 9 10 723 anisotropy of magnetic susceptibility; Reviews of Geophysics 30 209–222. 11 12 724 Seme Mouangue A 1998 Géochimie, métamorphisme et métallogénie des formations 13 14 725 ultrabasiques du secteur de Lomié (Sud-Est Cameroun) ; Thèse de Doctorat de 3è cycle 15 16 17 726 Université de Yaoundé Cameroun 150p. 18 19 727 Simpson C 1985 Deformation of granitic rocks across the brittle–ductile transition; Journal 20 21 22 728 of Structural Geology 7 503–511. 23 24 729 Simpson C and Wintsch R P 1989 Evidence for deformation induced K-feldspar replacement 25 26 27 730 by myrmekite; Journal of Metamorphic Geology 7 261–275. 28 29 731 Tagné-Kamga G 2003 Petrogenesis of the Neoproterozoic Ngondo Plutonic complex 30 31 732 (Cameroon, west central Africa): a case of late collisional ferro-potassic magmatism; 32 33 34 733 Journal of African Earth Sciences 36 149–171. 35 36 734 Talla V 1995 Pan-African Granitic Massif of Batié (west-Cameroon): Petrology Petrofabrics- 37 38 39 735 Geochemistry. Unpublished 3rd cycle Doct. thesis Univ. Yaounde I Cameroon 144 p. 40 41 736 Tarling D H and Hrouda F 1993 The Magnetic Anisotropy of Rocks. Chapman and Hall, 42 43 44 737 London 217p. 45 46 738 Tetsopgang S, Anami M and Njonfang E 2006 Petrology of highly evolved Pan-African two- 47 48 49 739 mica granites from the Nkambé area, West Cameroon; Journal of African Earth Sciences 50 51 740 46 305-317. 52 53 741 Tetsopgang S, Suzuki K and Adachi M 1999 Preliminary CHIME dating of granites from the 54 55 56 742 Nkambé area, northwestern Cameroon, Africa; Journal of Earth and Planetary Sciences 57 58 743 Nagoya University 46 57-70. 59 60 61 62 63 64 30 65 744 Toteu S F, Macaudière J, Bertrand J M and Dautel D 1990. Metamorphic zircons from 1 2 745 northern Cameroon: implications for the Pan-African evolution of central Africa; 3 4 5 746 Geologisch Rundschau 79 777–786. 6 7 747 Tchameni R, Pouclet A, Penaye J, Ganwa A A and Toteu S F 2006 Petrography and 8 9 10 748 geochemistry of the Ngaoundéré Pan-African granitoids in Central North Cameroon: 11 12 749 Implications for their sources and geological setting; Journal of African Earth Sciences 13 14 750 44 511–529. 15 16 17 751 Toteu S F, Van Schmus W R, Penaye J and Michard A 2001 New U– Pb and Sm–Nd data 18 19 752 from north-central Cameroon and its bearing on the pre-Pan African history of central 20 21 22 753 Africa; Precambrian Research 108 45–73. 23 24 754 Toteu S F, Penaye J and Poudjom Djomani Y 2004 Geodynamic evolution of the Pan-African 25 26 27 755 belt in central Africa with special reference to Cameroon; Canadian Journal of Earth 28 29 756 Sciences 41 73-85. 30 31 757 Toteu S F, Penaye J, Deloule E, Van Schmus W R and Tchameni R 2006 Diachronous 32 33 34 758 evolution of volcano-sedimentary basins north of the Congo craton: insights from U-Pb 35 36 759 ion microprobe dating of zircons from the Poli, Lom and Yaounde Groups (Cameroon); 37 38 39 760 Journal of African Earth Sciences 44 428-442. 40 41 761 Vernon R H 2000 Review of microstructural evidence of magmatic and solid-state flow; 42 43 44 762 Electronic Geosciences 5 2. 45 46 763 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 31 65 764 Table captions 1 2 3 765 Table 1. Summary of magnetic scalar and directional data on the Bandja granitic pluton. K1, 4 5 6 766 K2 and K3 are the maximum, intermediate and minimum susceptibility intensities respectively; 7 8 767 Km = (K1+K2+K3)/3 is the mean magnetic susceptibility; P’ is the anisotropy degree (Jelinek, 9 10 11 768 1981); T = (2ln(K2/K3)/(ln(K1/K3))-1) is the Jelinek’s shape parameter (Jelinek, 1981); N is 12 13 769 the number of analyzed specimens; Dec: declination in degrees; Inc: inclination in degrees. 14 15 770 16 17 18 19 771 Table 2. Hysteresis parameters for a selection of representative one cc samples, together with 20 21 772 low susceptibility parameters. High and low field susceptibilities are expressed in volumic 10- 22 23 6 24 773 units using a fixed density of 2.7. Ms is in Am2/kg. Model low field susceptibility is derived 25 26 774 from Ms using theoretical Ms/K ratio for MD magnetite. 27 28 29 775 30 31 776 Figure captions 32 33 34 35 777 Fig. 1. (a) Geological sketch map of Central-North Africa (western Gondwana) (Ngako et al., 36 37 778 2008; redrawn and re-interpreted from Küster and Liegeois, 2001); 1 = Craton; 2 = Inferred 38 39 40 779 Craton; 3 = Pan-African remobilization; 4 = Neoproterozoic juvenilme crust; (b) Pan-African 41 42 780 structural map of Cameroon (Ngako et al., 2008; modified and re-interpreted from Toteu et 43 44 781 al., 2001). Large grey arrow represents syn-D1-3 regional main stress direction. Thick lines = 45 46 47 782 shear zone (SZ): BSZ = Balché SZ; BNMB = Buffle Noir-Mayo Baléo; CCSZ = Central 48 49 783 Cameroon SZ; GGSZ = Godé-Gormaya SZ; MNSZ = Mayo Nolti SZ; RLSZ = Rocher du 50 51 52 784 Loup SZ; SSZ = Sanaga SZ. 53 54 55 785 Figure 2. West-Cameroon geological map showing the Bandja granitic pluton and 56 57 58 786 neighbouring (Fomopéa and Batié-Bangou) plutons (Dumort, 1968). 59 60 61 62 63 64 32 65 787 Figure 3. Geological map of the study area showing the sampled sites. 1 2 3 788 Figure 4. Field photographs and microphotographs (crossed polars) of microstrutures in 4 5 6 789 feldspar megacryst granite: (a) A view of Bandja granitic pluton from Bandja main road; (b) 7 8 790 submagmatic microstructure (AY20), noted (i) the microfault in feldspar filled with quartz 9 10 11 791 and (ii) myrmekite abundance; (c) Feldspar megacryst granite from Bakoven (AY14). Noted 12 13 792 the stretchted granodioritic xenolith; (d) Incipient solid-state deformation (AY7); (e) 14 15 793 Magmatic foliation (S) due to the preferred orientation of K-feldspar megacrysts in granite; 16 17 18 794 (f) S-C sinistral microstructure showing (i) the rotation of feldspar megacryst and (ii) biotite 19 20 795 crystal folded in “S” shape (AY32); (g) localized sinistral microshear zones (AY7); (h) 21 22 23 796 mylonitic texture (AY1). Bt: biotite, Hbn: hornblende, Kfs: K-feldspar, Myr: myrmekite, Pl: 24 25 797 plagioclase, Qtz: quartz, S: foliation, C: shear planes. 26 27 28 29 798 Figure 5. Field foliations and lineations map showing lower hemisphere projection diagrams 30 31 799 for foliation poles (contour intervals = 2%) and stretching lineation (circle with tail). 32 33 34 35 800 Figure 6. Projection diagrams (lower hemisphere) for AMS data; box represents Kmax, 36 37 801 triangle represents Kint and circle represents Kmin; Full symbols (box, triangle and circle) 38 39 40 802 represent the averages. 41 42 43 803 Figure 7. K-T curves (susceptibility vs temperature) for representative samples (AY10, AY67, 44 45 804 AY62, AY27, AY21, AY16). 46 47 48 49 805 Figure 8. (a) Mrs/Ms versus Hcr/Hc (hysteresis ratios; Day et al. (1977)) diagram of 50 51 806 representative samples in K-feldspar megacryst granite, with boundaries of single domain 52 53 54 807 (SD), pseudo-single-domain (PSD), and multi-domain (MD) after Day et al. (1977) and 55 56 808 Dunlup (1986), (b) Example of hysteresis loops for samples with paramagnetic (AY76), PSD 57 58 59 809 (AY75) and MD (AY77) behaviour. 60 61 62 63 64 33 65 810 Figure 9. (a) Contour map of the magnetic susceptibility (Km in µSI unit) of the Bandja 1 2 811 granitic pluton; (b) Susceptibility histograms. 3 4 5 6 812 Figure 10. (a) Contour map of the anisotropy degree (P’) of the Bandja granitic pluton; (b) 7 8 813 Anisotropy degree histogram; (c) P’ vs Km semi-log diagram. 9 10 11 12 814 Figure 11. T vs P’ diagram 13 14 15 815 Figure 12. Magnetic foliation map and projection diagrams for foliation poles (lower 16 17 18 816 hemisphere, contour intervals = 2%). 19 20 21 817 Figure 13. (a) Magnetic lineation map and projection diagrams of K1 AMS parameter (lower 22 23 24 818 hemisphere, contour intervals = 2%); (b) lineation plunges histogram. 25 26 27 819 Figure 14. Sketch showing the tectonic history of the Bandja granitic pluton. FGP = Fomopéa 28 29 820 Granitic Pluton; BGP = Bandja Granitic Pluton; FFMZ = Fotouni-Fondjomekwet Mylonitic 30 31 32 821 Zone. 33 34 822 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 34 65 Figure

Figure 1

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Figure 2

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Figure 6 (continued)

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Figure 9

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Figure 10

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Figure 11

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Figure 12

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Figure 14

15 Table

Table 1. Summary of magnetic scalar and directional data on the Bandja granitic pluton. K1, K2 and K3 are the maximum, intermediate and minimum susceptibility intensities respectively; Km = (K1+K2+K3)/3 is the mean magnetic susceptibility; P’ is the anisotropy degree (Jelinek, 1981); T = (2ln(K2/K3)/(ln(K1/K3))-1) is the Jelinek’s shape parameter (Jelinek, 1981); N is the number of analyzed specimens; Dec: declination in degrees; Inc: inclination in degrees.

Mean AMS parameters Mean eigenvectors -6 Site Lat N Long E N Km (10 ) P' T K1 K2 K3 (°) (°) (µSI) Dec (°) Inc (°) Dec (°) Inc (°) Dec (°) Inc (°) Mylonites TJ1 5.3152 10.2322 4 406 1.18 0.06 207 7 314 68 115 22 TJ3 5.3166 10.2375 8 51 1.14 0.33 38 2 307 42 130 48 TJ4 5.3158 10.2322 4 1241 1.32 0.12 215 9 329 69 124 19 TJ14 5.32 10.2133 4 692 1.21 0.02 227 6 330 66 135 23 TJ20 5.2997 10.2327 4 13131 1.65 0.09 235 9 142 81 145 2 TJ21 5.303 10.2361 4 7285 1.64 0.22 22 2 117 70 289 20 TJ25 5.3347 10.2094 4 954 1.13 0.15 25 54 334 43 302 19 TJ26 5.328 10.2038 3 547 1.14 0.76 205 11 33 78 298 3 TJ28 5.3302 10.1922 4 1026 1.22 0.26 52 7 118 63 315 25 TJ30 5.2727 10.1897 4 337 1.06 0.20 220 15 316 49 127 50 TJ33 5.2811 10.1755 3 350 1.17 0.07 51 12 172 69 318 19 AY55 5.2397 10.1905 4 278 1.10 -0.64 39 3 136 73 307 17 AY56 5.2566 10.1722 4 322 1.10 -0.21 40 4 306 52 133 37 AY57 5.2827 10.2125 4 311 1.09 -0.60 53 4 204 86 323 2

Banded gneiss AY43 5.2252 10.255 4 219 1.40 0.15 35 14 129 23 277 66 AY44 5.2072 10.2497 4 740 1.11 -0.34 182 1 91 37 273 53 AY45 5.2208 10.2616 4 247 1.18 -0.06 12 4 104 17 287 74 AY47 5.2288 10.2625 4 119 1.63 0.09 21 0 112 15 290 75 AY48 5.2275 10.2716 4 22463 1.78 0.66 1 6 105 67 269 22 AY49 5.2152 10.2722 4 390 1.09 0.37 176 3 82 25 274 65 AY50 5.2122 10.2747 4 12833 1.61 0.26 54 1 143 3 288 86 AY51 5.2122 10.2747 4 1100 1.16 0.51 142 1 44 16 232 77 AY52 5.2080 10.2680 4 2236 1.34 0.11 173 7 44 24 300 79 AY79 5.1777 10.1833 6 1109 1.23 0.79 181 36 9 54 272 4

K-feldspath megacryst granites Domain I AY17 5.2675 10.2269 4 1081 1.08 -0.39 33 6 297 84 296 7 AY18 5.2666 10.22 4 658 1.13 0.07 207 4 117 11 287 79 AY23 5.2569 10.2086 4 1466 1.10 0.26 75 35 200 43 322 32 AY24 5.2605 10.2127 4 1120 1.09 0.36 76 55 239 45 322 10 AY25 5.2597 10.2166 4 1015 1.05 -0.05 145 49 357 40 263 10 AY26 5.2641 10.2208 4 1251 1.06 -0.46 210 4 317 76 116 15 AY27 5.2658 10.213 4 1227 1.17 0.50 105 1 195 12 6 78 AY28 5.2722 10.2244 4 994 1.06 -0.27 81 43 322 34 211 32 AY29 5.3066 10.2666 4 787 1.14 -0.35 52 18 175 62 309 20 AY30 5.3022 10.2647 4 671 1.07 0.27 18 33 227 54 117 14 AY31 5.2858 10.2483 4 2725 1.15 -0.16 208 6 93 77 300 13 AY32 5.2919 10.2602 4 2784 1.19 -0.17 210 7 86 78 301 10 AY33 5.2816 10.2444 4 161 1.06 -0.15 234 7 112 75 326 12 AY34 5.2816 10.2444 4 620 1.05 -0.09 204 6 111 41 301 50 AY35 5.2813 10.2455 4 802 1.06 -0.33 48 16 142 14 270 69 AY36 5.2794 10.2394 4 99 1.08 0.05 56 16 36 28 177 62 AY37 5.2697 10.2475 4 446 1.07 0.51 144 5 53 5 281 84 AY38 5.275 10.2308 4 2413 1.07 -0.02 49 28 181 50 299 24 AY39 5.2813 10.225 4 477 1.10 0.45 39 7 146 67 306 22 AY40 5.2741 10.2177 4 512 1.10 0.39 29 4 126 65 297 25 AY41 5.2858 10.23 4 62 1.11 0.22 31 5 133 67 299 24

AY42 5.2922 10.24 4 359 1.08 0.58 31 35 175 50 288 18 AY58 5.308 10.3066 4 1343 1.11 0.03 190 51 340 34 79 14 AY59 5.3105 10.3027 4 2089 1.14 0.50 211 62 3 27 101 5 AY60 5.3058 10.303 4 4527 1.16 0.23 91 14 283 71 181 11 AY61 5.3083 10.2961 4 684 1.14 0.32 2 3 228 86 92 3 AY62 5.3069 10.2933 6 4657 1.30 0.46 185 26 231 56 84 20 AY63 5.3183 10.2897 4 5625 1.37 0.40 65 26 85 35 358 1 AY64 5.3044 10.2802 4 6832 1.35 0.13 217 11 123 21 333 67 AY65 5.3086 10.2819 4 290 1.08 -0.04 111 1 21 14 212 76 AY66 5.3005 10.2822 4 527 1.05 0.31 317 25 66 34 199 45 AY67 5.2727 10.2655 5 9469 1.38 0.36 207 19 112 14 351 67 AY68 5.2755 10.2636 4 17295 1.33 0.38 290 34 159 43 37 26 AY69 5.278 10.2583 5 15511 1.58 -0.24 301 53 77 25 301 53 AY70 5.2777 10.2602 4 35836 1.95 0.22 29 4 251 84 119 4 AY71 5.2841 10.263 6 18196 1.73 0.12 216 10 38 81 306 1 AY72 5.2913 10.2641 5 662 1.07 0.44 7 23 98 1 189 67 AY73 5.2877 10.25 4 486 1.07 -0.24 90 62 230 22 327 18 AY74 5.2608 10.2525 4 4129 1.22 0.57 243 52 340 5 74 38 AY80 5.2602 10.2252 4 598 1.07 0.62 275 52 12 5 107 33 AY81 5.2533 10.218 4 768 1.11 0.35 235 77 251 12 55 36 Domain II AY1 5.2447 10.2166 4 120 1.06 -0.50 13 12 104 14 243 14 AY2 5.2436 10.2227 4 3104 1.18 0.20 56 10 166 64 323 30 AY3 5.2363 10.2133 4 381 1.07 -0.30 18 4 113 43 276 46 AY4 5.2327 10.2102 4 376 1.07 -0.04 189 1 101 69 278 34 AY5 5.2283 10.213 4 598 1.10 0.40 196 2 100 69 289 21 AY6 5.2269 10.2102 4 362 1.10 0.00 195 17 8 74 105 1 AY7 5.2258 10.2083 4 153 1.04 0.12 179 19 291 48 76 36 AY8 5.225 10.2047 4 2254 1.13 0.12 207 4 117 11 267 78 AY9 5.2213 10.2016 4 2555 1.20 0.10 189 13 50 74 281 10 AY10 5.2133 10.1961 4 3339 1.20 -0.02 193 6 52 83 283 5 AY11 5.2275 10.203 4 2353 1.19 0.02 197 8 104 22 305 67 AY12 5.2355 10.1997 4 5320 1.21 -0.36 176 26 79 14 324 60 AY13 5.2336 10.1911 4 4638 1.18 -0.32 177 23 82 10 328 65 AY14 5.23 10.1977 4 708 1.03 0.08 226 85 328 13 220 17 AY15 5.2375 10.2063 4 322 1.06 -0.17 196 6 103 27 301 62 AY16 5.2402 10.2052 4 1493 1.13 -0.26 203 4 313 78 112 11 AY19 5.2208 10.1933 3 946 1.07 -0.75 181 3 274 64 90 26 AY20 5.2177 10.1925 3 1412 1.03 -0.18 338 9 69 21 233 69 AY21 5.2297 10.1872 4 772 1.02 -0.24 173 42 27 51 291 17 Domain III AY75 5.2086 10.1808 5 887 1.03 -0.50 290 34 129 63 36 31 AY76 5.1986 10.1788 4 561 1.04 -0.22 228 8 121 66 31 24 AY77 5.2077 10.1883 4 3370 1.05 -0.16 348 24 145 64 171 81 AY78 5.1911 10.1997 5 611 1.11 0.35 263 10 153 4 350 86

Table 2. Hysteresis parameters for a selection of representative one cc samples, together with low susceptibility parameters. High and low field susceptibilities are expressed in volumic 10-6 units using a fixed density of 2.7. Ms is in Am2/kg. Model low field susceptibility is derived from Ms using theoretical Ms/K ratio for MD magnetite.

Site m Ms Mrs/Ms Bcr/Bc Bcr Bc Ko µSI Khf Ko model %para %para model

AY27 1.57 37.5 0.06 7.3 47.1 6.5 1070 208 849 19 24 AY35 1.58 11.6 0.13 2.4 24.0 10.0 724 491 690 68 71 AY59 1.50 31.3 0.03 8.9 34.8 3.9 832 137 672 16 20 AY61 1.26 118.3 0.09 4.3 45.0 10.4 2330 83 2105 4 4 AY62 1.76 168.3 0.01 9.9 11.9 1.2 4375 148 3024 3 5 AY63 1.35 748.3 0.03 4.7 12.1 2.6 23019 131 12918 1 1 AY70 1.36 626 0.06 2.6 7.5 2.9 23552 700 11397 3 6 AY71 1.51 522 0.01 7.0 8.4 1.2 14722 396 9317 3 4 AY73 1.49 528 0.02 12.1 20.6 1.7 12572 232 9255 2 3 AY75 1.44 23.5 0.25 2.6 73.0 28.0 860 468 869 54 54 AY76 1.63 2.3 0.08 2.7 29.0 10.8 556 521 560 94 93 AY77 1.44 134.4 0.02 8.1 23.5 2.9 3000 254 2551 8 10 AY78 1.46 47.9 0.04 7.9 38.6 4.9 1172 239 1058 20 23 AY79 1.57 22.9 0.17 5.0 93.0 18.6 944 447 838 47 53 AY80 1.53 13.4 0.04 7. 7 46.0 6.0 496 308 537 62 57