The Cameroon Line: A Review 275

9 The Cameroon Line: A Review 14 , '., B. Deruelle, C. Moreau, C. Nkoumbou, R. Kambou, J. Lissom, E. Njonfang, i l\ .. GZ R. T. Ghogomu and A. Nono ~ Mh Mb.. ..:;-..~.~:av .S.Nyp 1 Introduction 'or 8 Nn G<;>. ..M Since the work of VonBaumann (1887)the Cameroon Line (Fig. 1)is known as a ma- .:::::::::::: . G~F..rM ~. jor geological feature in Central Africa. The Cameroon Line is an alignment of o '. ~D : :... N oceanic and continental volcanic massifs and of anorogenic plutonic complexestrend- ~ S ing N 30° from Pagahi Island to Lake Chad (Geze 1941). It is now considered ..Da~~l"" '!fii1tJ (Moreau et al. 1987b) as a Pan-African lineament more or less permanently re- ::::::::::::-:.. 'MD Adarnawa 6'; juvenated from the Late Precambrian to the Present. The oceanic segment of the D Cameroon Line is composed of the four volcanic islands of the Gulf of Guinea (Pagahi, formerly Annob6n, SiloTome,Principe and Bioko, formerly Fernando P6o). RH .~:]I\;~;~:.' Its continental segment is represented by the volcanic massifs of Mount Cameroon, ~€3":'::::~~ Rumpi, Manengouba, Bambouto, Oku and the volcanic outpourings of the Benue J Valley(Westof Garoua) and of the Kapsiki Plateau and by more than 60 anorogenic 4 "N~B ..- -...... plutonic complexesfrom Mount Koupe to Waza. Extensions of the Line to Ascension E"k.. ~/ \ CHAD (Gouhier et al. 1974) or to Saint-Helena (lYrrell 1934; Furon 1953, 1968; Vincent '0\ .':'::::'::'.tt: / I 1970b) for the oceanic segment, or to Tibesti (Furon 1953, 1968;Vincent 1970b) or G u I f ...... \ lJ I~ ' Southeastern Libya (Tempier and Lasserre 1980) for the continental one have been ./ ," \ " ) suggested. The Cameroon Line has also been described as a Y-shaped zone (Fitton I -, 1980, 1983) with the trunk represented by a line from Pagahi to Mt Oku and the "\ ' o f /' I" branches by the Adamawa and the Biu plateaux. /" I/- / I / A. I From a geomorphological point of view,the Cameroon Line is composed of a suc- / Y , cession of horsts and grabens. The horsts are represented by the four islands of the I I C.A.R. I 0 \ Gulf of Guinea and the continental areas of high relief (the massifs of Mount G u o CAMEROON \ Dv \ E.G. \ \I / 0 \1 --_(- - - - I o 0 ~~:-I I-,'--_J . S.T.p.~ J Fig. 1. Sketch map of the Cameroon Line (see upper inset for location in Africa: W.A. C. West <:J " / African Craton; C. C. Congo Craton; K. C. Kalahari Craton, and lower inset for location in Central ~ GABON'~ C. Africa, C. Congo; C.A. R. Central African Republic; E. O. Equatorial Guinea; N Niger; S. T. P. Silo .100km, ,V I Tome and Principe). Cainozoic volcanic rocks (dotted areas) are indicated conjointly with anorogenic ---I ring-complexes. From South to North, anorogenic ring complexes (Roman type) large letters for the 8 massifs studied in the present work (Chap. 4, 1 to 8): B Bana; NA Nda Ali; N Ntumbaw; MD Mayo Darle; 0 Guenfalabo. and small letters for the other massifs; K Koupe; Nl Nlonako; No Namboe; S Sabri; B Bonhari; 00 Gounguel; F Fourou; Nn Nan; M Mana; T Tchegui; P PoIi; Ny Nyore; Ko Kokoumi; Mn Mouhour; Gr Grea; WWaza; volcanoes (italic type) large letters for the volcanoes studied in the present work (Chap. 5, 1 to 16): E Etinde; RH Rumpi Hills; BP Bamoun Plateau; BV Benue Valley; K Kapsiki Plateau, and small letters for the other massifs: TM Tchabal Mbabo; D Djinga; N Nganha; Biu Biu Plateau. Legends of Figs. 1 and 2 are complementary r 276 Ring Complexes and Related Structures The Cameroon Line: A Review 277

Cameroon, Rumpi, Manengouba, Bambouto, Mbam and Oku) which alternate with Gongols I grabens (Kumba, Tombel, Mbo, Ndop, Tikar Plain). The topographical expression of ___")" I_I " Yola t"'~' / the northern part of the Cameroon Line (northwards of the Oku massif and the Tikar ' " Plain) is less pronounced. The Cameroon Line intersects the Adamawa direction ,J I _ Garoua', / ___ I ~ "~o ~ north of Oku Massif. ".' ,;. t,., " ,.' ' '", The Adamawa plateau represents a large horst limited to the north by the great 8 e ,r ./ , Adamawa (also named Ngaoundere) fault zone and to the south by the Djerem-Mbere (\ basin. e " ,. ," I A remote sensing analysis of the continental part of the Cameroon Line and the II- ~ ...1'" \ Adamawa plateau accompanied by a study of geological maps has been used to show \ _" ,_J " ",J , " O&t1'&"'&N ;.;-;; , ,~ gaoundere that the Cameroon Line is a major structural feature and not a mere alignment of / ' /" -Banyo subvolcanic and volcanic massifs, ",., Ok ~ \ , ~J~ ~.,l tA,,8 ""fl!!.~ Ndo''''- "Mbam" ".,PIa;. oj."" ~ / S '.f'.. dJ .,. ,r . ~~.amb"'-"uto /\ - } ->-.. , ~ \ 2 Various Interpretations of the Cameroon Line ~ . }j.">M' ' \ .. ~T' anengouba " .~~ fu~~ " \ " Numerous interpretations have been given to the Cameroon Line: 4 ~~ c ameroon I

A) It has been considered as a volcanic alignment made of a succession of horsts and grabens (Geze 1943, and references therein). Epeirogenic uplift along the Line (Black and Girod 1970) has induced normal and reverse faulting (Fig, 2) G u I f (Deruelle 1982; Deruelle et al. 1983a). This alignment has been considered as J-- """"''''''" a unit independent of the Adamawa structure (Fail et al. 1970; Gouhier et al. 1974). B) The southern part of the Cameroon Line (from Pagalu to the Oku massif) and 0~princiPe the Adamawa Plateau have been considered by some authors as a whole and a f named "Cameroon Line itself" (Fig. 3) (Burke et al. 1970;Cornacchia and Dars 1983); the northern part of the Line (from Oku to Lake Chad) being excluded A N from this definition. o ~sao Tome C) Another point of view is that the Cameroon Line is the southernmost geosuture of a Pelusium megashear system which is supposed to extend from the Amazon G u Basin to Anatolia (TInkey)via the Atlantic equatorial fracture zone and across ,100km, Africa from the Gulf of Guinea to the Nile Delta (Neev et al. 1982). 1 Similarly, the Cameroon Line and the Adamawa fracture zone have been sup- Pagalu 8 12 posed to be the more or less well-fitted prolongation of the Santonian transform fault (north of Ascension transform fault) at the beginning of the opening of the Fig. 2. The Cameroon Line, succession of horsts (Roman type) and grabens and rifts (italic type) South Atlantic Ocean (Fig. 4) (Sibuet and Mascle 1978; Sykes 1978). They have (after Deruelle et al. 1983b). The Adamawa horst, the Djerem Mbere graben and the Yola-Garoua, also been supposed to be the prolongation of the Ascension transform fault at Gongola and Benue rifts are also indicated Present time (Cornacchia and Dars 1983). Nevertheless, all these interpretations are doubtful and uncertain for correlations beyond the Santonian (Figs. 14, 15 and 11 in Sibuet and Mascle 1978). Mascle 1976)or the Patos (BenkhelilI986) faults (NE Brazil) before the opening The transform faults of the Atlantic Ocean appear to extend into the African con- of the Atlantic Ocean and is thus considered as a major Pan-African lineament tinent, not along the Cameroon Line, but rather through the Adamawa fracture (Fig. 5) as has been recognized in Nigeria (Ajakaiye et al. 1986). zone (Vincent 1968, 1970b; Le Marechal and Vincent 1970;Mascle 1976). This D) The Cameroon Line has also been interpreted as a volcanic and subvolcanic align- last waspropagated by the Pernambuco (De Almeida and Black 1967;Louis 1970; ment resulting from hot-spot activity (Tchoua 1974a; Duncan 1981;Morgan 1982; 278 Ring Complexes and Related Structures The Cameroon Line: A Review 279

Fig. 3. Hypothetical propagation of the Cameroon Line with the Adamawa-Yade shear zone (after Cornacchia and Dars 1983). Sedimentary cover, 1 Cainozoic s.l; 2 Mesozoic; Basement; 3 Upper and Late Precambrian; 4 Lower and Medium Precambrian; Eruptive rocks; 5 Cainozoic [l]]]DImiJ.. volcanism; 6 Granitic batholiths; 7 Upper 3 4 5 6 7 Precambrian dolerites

D m....:.: ;,:.;;: m 's:'.'':;;;i ODD 0 1 2 3 4 5 6 7

Fig. 5. Reconstitution of the African - South American block at the beginning of the opening of the South Atlantic Ocean (after Louis 1970). Sanaga fault, after Benkhe1il1986. The propagation of the Pernambuco fault (NE Brazil) by the Ngaoundere (Adamawa) fault is noteworthy (De Almeida and Black 1967). 1 Cainozoic-Mesozoic; 2 Paleozoic; 3 Upper Precambrian; 4 Pan-African; 5 Old cratons (1, 2 and 3, respectively West-African, Sao Francisco and Congo cratons); 6 rifts; 7 thrusts

Van Houten 1983).Moreover, various hypothetical motions of the African plate over a Cameroon hot-spot have been formulated (Fig. 6). E) Following the hypothesis that "the Cameroon-Adamawa zone is a new ridge/rift Africa feature where separation has not yet taken place and where a rift valley form has not yet developed" (Burke et al. 1970), an inventive model has been proposed which considers the Cameroon Line as a unique exampleof a rift systemproduced actively by a thermal anomaly in the asthenosphere. This model is essentially based upon the approximate shape and size of the Benue trough and the Cameroon Line (continental segment) - Adamawa horst - Biu Plateau. In this model, "the 'Y~shaped hot zone in the asthenosphere which would have lain beneath the Benue trough in the Cretaceous became displaced (relative to the lithosphere) so that it now lies beneath Cameroon and the Gulf of Guinea" (Fit- Fig. 4. Location of the Cameroon Line and the Adamawa fault zone on a reconstruction of the South ton 1980, 1983) (Fig. 7). In the view arguing for the rift model, numerous lavas American and African continents at the end of Santonian (Anomaly 34, 79 Ma) (after Sibuet and of the Cameroon Line have been erroneously qualified as transitional (Fitton Mascle 1978) 1980, 1983) despite of their alkaline character.

~ 280 Ring Complexes and Related Structures The Cameroon Line: A Review 281

All the above interpretations have been recently discussed and are subject to con- troversy (Reyre 1984;Benkhe1il1986;Moreau et al. 1987b). The most widely accepted structural explanation is that the Cameroon Line is the product of rejuvenation of a Pan-African N 70° fracture zone at the beginning of the opening of the Atlantic Ocean (Moreau et al. 1987b). This model is not inconsistent with "the harpoon effect that occurs when there is a reversal in the sense of movement in a pre-existing fault system" (Black et al. 1985).

3 A New Tectonic Model for the Cameroon Line

A new tectonic model for the Cameroon Line has been recently proposed (Moreau Fig. 6. Hypothetical motions of et al. 1987b) based upon remote sensing and autocorrelation analysis combined with African hotspots since 200 Ma - from Sierra Leone to St a detailed review of all available geological studies. Helena, after Duncan (1981); Remote sensing reveals four networks of rectilinear lineaments: a N 70° or : from Abakaliki to St Adamawa trend, aN 135° or Upper Benue trend, a N-S or Pan-African trend, and Helena, after Morgan (1982); an E-W trend restricted to north Cameroon (Fig. 4 in Moreau et al. 1987b). The N - - -: from Hoggar to 30° or Cameroon Line trend appears to be a dependent direction of the submeridian Cameroon, after Van Houten one. (1983) The autocorrelation analysis is a method which can be applied to the study of the surface distribution of subvolcanic and volcanic complexesin well-defined magmatic provinces such as the Cameroon Line (see Moreau et al. 1987b). For this area, an elongated elliptical graph with a major axis oriented N 37° (Fig. 9a in Moreau et al. 1987b) was obtained. The comparison of the main fracture trends observed on satellite imagery with those obtained in the autocorrelation analysis confirms the prominent role of fractur- ing (or stress distribution) in the emplacement of magmatic complexes. The new tectonic model takes into account several geological aspects:

first, only a tension oblique to a transcurrent fault zone (Riedel model) fits with all available geological data and with the observed structural elements; second, only large-scale lineaments can play the role of shear zones. In the present case, N-S, N 70°, N 135° and E-W lineaments should be considered (see Fig. to in Moreau et al. 1987b). The N 37° and N 50° lineaments, common in the central spindle of the autocorrelation graph, are also considered.

Fig. 7. The correlation between The N 135° trend does not appear to be sufficiently important to represent a shear the "Y'~shape of the zone capable of creating megatension gashes. When the Riedel model is applied to Cameroon Line - Adamawa other directions it is only consistent with a sinistral shear zone along the N 70° trend and Biu plateaus apd of the Benue trough has been inter- to explain the main fractural directions known in Cameroon and adjacent areas. The preted (Fitton 1980, 1983) as theoretical model (Fig. 8a) givesT = N 25°, R = N 55°, R' = N 175°,P = N 85° and the result of a reverse at = N 40° and a3 = N 130°. The N 25° tension gash direction is a fair fit with the clockwise rotation of the Cameroon Line and represents a major structure associated with anorogenic African plate 80 to 70 Ma ago by 7° about a pole in Sudan, magmatism characterized first by subvolcanic intrusives then by eruptive volcanic ac- above an hypothetical uplift of tivity. A transitional regime can explain the emplacement of ring complexes,implying the asthenosphere. Inset loca- a rejuvenation of the N 70° fracture zone; in more detail (Fig. 8b), an "en echelon" tion in Africa arrangement is suggested for the Cameroon Line including the two models proposed The Cameroon Line: A Review 283 282 Ring Complexes and Related Structures N The geochronological data for these anorogenic massifs (Gouhier et al. 1974;Can- A .;;.' tagrel et al. 1978b; Lasserre 1978;Tempierand Lasserre 1980;Dunlop 1983),indicate I @ I ~'1 that their emplacement occurred from the Paleocene (66Ma) to the Early Oligocene I / I I (33 Ma). Radiometric data are only available for 10 of the 60 massifs. Thus no space- 7/~ / time relationships can be established, nor can evident migration be deduced as has 1/ /R ;- / been recently proposed (N'ni et al. 1986).At best, it is possible to argue for two main /,/ / /./ I phases of emplacement around 60 and 40 Ma, but many more age determinations /: I would be necessary to establish a space-time migration. :'- ...;' / I P Some of these massifs have been recently studied and a brief description of them Y / . / --=----- (from south to north) is given now. For Rumpi Hills; see 5.8. =-- ;. /./1. X / / "/ .... 1/ .../ ~"'" I I ,// / / a; ,. I / 4.1 Mount Bana / L. / ~ ~,4i The volcanic-plutonic massif of Bana (Nana 1988) intrudes the Pan-African I /? j

..//1<0/ metamorphic basement and the plateau basalts of probable Eocene-Oligocene age. ,.. The massif has a circular outline and a diameter of 6 km (Dumort 1968; Tchoua 1974a; Moreau et al. 1987b). The early volcanic suite comprises mugearites, ben- N A moreites, rhyolites and welded rhyolitic tuffs. Plutonic rocks are leucogabbros, mon- zodiorite, monzonite and granite with peraluminous or peralkaline affinities. Rb/Sr ~ / whole rock radiometric data give an age of 30 Ma (Lasserre 1978). : &... ! "N"V':': Fig. 8 a. Sinistral N 70° shear; T tension I '2\"1(... gashes; Rand R' conjugate shear faults; 4.2 Nda Ali J-..~.I:.~...... " P symmetrical to R; X and Z maximum -- -:.. ." ...... stretching and shortening axes; 0'3and at, .~.:.. ~._---- tensional and compressive principal stress The Nda Ali massif is located in the Mamfe basin. It intrudes the Okoroba granite axes. b Interpretation of autocorrelation and Cretaceous sedimentary rocks and is structurally controlled by N 135° and :::'h..::~~:l~ graph for the continental part of the .. Ki. I Cameroon Line (after Moreau et al. N 170° faults (Njonfang 1986) (Fig. 9). .1..'" : 1987b); A major axis of ellipse; thick lines Nda Ali is made of plutonic and volcanic rocks, with a so-called normal distribu- [p>I ': secondary alignments of points; dotted tion. Gabbroic cumulates with internal dips lamination represent the first magmatic / :1 lines potential wrench faults (N 70°) phase. They are cut by a syenitic formation having a hook-shape elongated relief. The plutonic suite is alkaline and is followed first by dykes of hawaiite and trachyte which are comagmatic with the plutonic suite, 'then by flows of benmoreite, phonolite and for intracontinental basins (Lavilleand Harmand 1982;Benkhelil and Robineau 1983; trachyte which represent the late volcanic series. These volcanic rocks may be Popoff 1988a). For discussion, see Sect. 6. cogenetic with the plutonic series. Plagioclase is present in basic and intermediate rocks, with a regular decrease in anorthite content from An 73 to An 10. The alkali feldspars are common in syenitic 4 Anorogenic Ring-Complexes rocks and have a composition located in the thermal gap. Feldspar chemistry suggests More than 60 anorogenic ring-complexes crop out along the Cameroon Line on its a hypersolvus sequence. Only clinopyroxene is present; calcic pyroxenes are found in continental segment from Mount Cameroon to Lake Chad. Their spatial distribution basic and intermediate rocks, whereas calci-sodic and sodic pyroxenes appear in the is slightly different from north to south. The complexes are isolated in the northern syenites.Four types of amphibole have been observed in the Nda Ali complex: magne- part of the Cameroon Line, and relativelymore abundant in the middle part, between sian cummingtonite in the basic rocks, edenite in the basic and intermediate rocks, Foumban and Banyo (see Figs. 1 and 2). All the anorogenic ring-complexes occur calci-sodic and sodic amphiboles in the syenites. within a large ellipsoidal envelope, curved in the middle part, trending N 50° in the Petrological and geochemical data (Thble1) suggest two divergent differentiation trends from a mantle-derived undersaturated magma (Njonfang 1986).All the rocks southern part and N 25° in the northern part (Fig. 3 in Moreau et al. 1987b). No large anorogenic plutonic massifs, such as those in Niger (Moreau 1982)or Nigeria, are pre- have alkaline characteristics with a peralkaline trend in the most evolved composi- sent but only small complexes which average 5 to 10km in diameter. tions. T 284 Ring Complexes and Related Structures The Cameroon Line: A Review 285 I Table 1 (continued)

A Rumpi Hills

W.R. o-Gb a-Gb SD S No. ND23 NK40 NK42 NK28

Q N Or 4.78 9.80 15.88 26.27 Ab 20.37 31.26 48.51 51.69 t An 37.73 26.66 8.39 3.14 Ne 5.45 4.86 3.16 2.08 Co Ac Di 13.85 9.78 8.21 6.75 Hy 01 7.81 8.84 7.50 4.89 Mt 1.65 1.26 1.50 1.11 II 5.40 3.96 3.23 2.06 Ap 1.79 1.75 1.03 0.57 D.I. 30.6 45.9 67.6 80.0 E2I ... .".- @:l . 0 8 9 1 2 3 4 5 6 7 2km B Nda Ali Fig. 9. Geological map of Nda Ali (after Njonfang 1986). 1Okoroba granite; 2 gabbro; 3 intermediary plutonic rock; 4 syenite; 5 basaltic dyke; 6 benmoreite; 7 trachytic lava; 8 phonolite; 9 breccia W.R. Gb q-SM a-Gb mGb M-S mGr q-S S S No. NA72 NA185 NA186 NA77 NA520 NA102 NA670 NA22 NAZI

SiOz 43.25 49.30 51.63 47.13 52.38 69.43 66.15 63.35 65.21 Table 1. Chemical analyses (major elements in wt07o;trace elements in ppm) and CIPW norms of TiOz 4.83 2.66 1.43 3.26 2.42 0.52 0.44 0.66 0.66 plutonic and volcanic representative rocks of intrusive anorogenic "massifs. Rock types (R. T.) as Alz03 13.06 17.40 23.04 15.49 16.15 15.27 15.38 17.28 15.89 follows: D diorite; Gb gabbro; Gr granite; M monzonite; MD monzodiorite; MS monzosyenite; N FeZ03 14.79 9.69 5.21 11.97 10.35 1.81 5.61 4.00 4.08 norite; S syenite; SGr Syenogranite; SM syenomonzonite; a anorthoclase; 0 olivine; q quartz; (m FeO micro). Data sources: Rumpi Hills: Nkoumbou 1990, Nda Ali: Njonfang 1986; Ntumbaw: Ghogomu MnO 0.21 0.18 0.09 0.23 0.26 0.03 0.16 0.22 0.14 1984; Mboutou: Jacquemin 1981; and CIPW norms; Parsons et al. 1986; Golda Zuelva: Jacquemin MgO 6.08 3.46 1.50 3.57 2.64 1.61 0.20 0.44 0.33 1981. CIPW norms are from the authors above and were not recalculated, except for Golda Zuelva CaO 10.67 8.88 9.77 6.26 5.89 0.49 0.86 0.43 0.16 where FeO =0.85 (peO + Fe,O,); 22 G2: norm includes NaSi: 0.94 NazO 3.10 3.31 4.72 4.61 5.19 2.18 5.38 6.30 6.42 KzO 1.06 2.83 0.84 2.93 2.98 5.49 4.55 5.00 4.98 A Rumpi Hills PzOs 0.78 1.25 0.63 1.21 1.05 0.43 0.12 0.32 0.26 L.O.I. 1.98 0.29 0.23 2.27 0.73 1.27 0.79 0.86 0.90 W.R. o-Gb a-Gb SD S Total 99.81 99.25 99.09 98.93 100.04 98.53 99.64 98.86 99.03 No. ND23 NK40 NK42 NK28 Q 32.46 12.05 4.62 5.16 Or 6.26 16.70 4.96 17.29 17.59 32.40 26.86 29.51 29.39 SiOz 45.76 49.55 55.34 59.83 Ab 17.16 27.98 39.89 27.47 38.70 18.43 45.47 18.43 45.47 TiOz 2.84 2.08 1.70 1.08 An 18.59 24.26 39.19 12.93 11.98 3.48 0.65 Alz03 20.63 19.40 16.56 16.77 Ne 4.90 6.23 2.86 FeZ03 8.75 8.64 7.92 5.89 FeO Co 5.75 0.33 1.50 Ac 1.75 MnO 0.18 0.17 0.19 0.12 Di 8.81 MgO 3.54 2.84 1.10 24.12 9.66 4.30 8.69 1.95 5.89 8.14 6.18 5.49 CaO 12.07 8.79 4.27 2.57 Hy 1.00 01 12.96 10.33 10.47 NazO 3.60 4.76 6.43 6.57 5.30 12.47 Mt 2.79 1.83 0.98 2.61 1.94 0.34 1.22 0.86 0.89 KzO 0.81 1.66 2.69 4.45 II 9.19 5.06 2.72 6.20 4.60 0.99 0.84 1.26 1.26 PzOs 0.82 0.80 0.47 0.26 1.70 2.96 1.49 2.86 2.49 0.90 0.26 0.76 0.96 L. 0.1. 0.81 1.17 2.38 0.72 Ap Total 99.90 99.86 99.90 99.36 D.I. 28.3 44.7 44.9 50.9 59.1 83.3 84.4 87.3 88.4

....l TI 286 Ring Complexes and Related Structures The Cameroon Line: A Review 287

Table 1 (continued) Table 1 (continued) I B B Nda Ali - NdaAIi W.R. H Ph W.R. Ob q-SM a-Ob mOb M-S mOr q-S S S B T No. NA72 NA185 NA186 NA77 NA520 NAI02 NA670 NA22 NA21 -No. NA131' NA241 NA272 NAI13 Rb 17 70 67 103 80 85 II 5.25 2.00 0.70 0.17 Sr 1283 1176 969 164 129 20 Ap 1.35 0.55 Ba 971 1148 1324 96 1963 D.I. 36.9 73.2 84.4 92.5 V 152 110 <10 <10 13 Cr 7 <10 <10 <10 <10 <10 Co 46 63 10 <10 13 Y 13.42 46.65 59.38 La 21.97 73.50 84.90 C Ntumbaw Ce 51.26 153.70 169.16 Nd 24.86 75.08 76.49 W.R. MD M q-M q-S SOr R Sm 5.29 14.43 14.83 No. NS8 NS3 NSI T5 NS12 H2 Eu 2.88 4.63 3.36 Od 4.54 11.84 12.37 Si02 52.74 53.44 56.34 63.53 64.22 76.06 Dy 2.67 9.14 10.88 Ti02 1.14 1.16 1.17 0.83 0.93 0.26 Er 1.40 4.43 5.45 AIP3 14.42 13.80 14.61 14.55 13.43 10.61 Yb 0.80 3.38 4.39 Fe203 7.80 7.51 7.03 6.80 6.06 3.30 Lu 0.17 0.60 0.73 FeO MnO 0.13 0.12 0.11 0.13 0.09 tr MgO 7.43 7.33 6.70 0.99 0.07 tr CaO 6.77 6.04 5.37 2.46 1.66 tr 3.24 2.88 3.30 4.81 4.13 4.22 B Nda Ali Nap K20 2.93 3.57 4.20 3.52 4.61 4.79 0.68 0.55 0.55 0.12 0.57 tr W.R. H B Ph T P20S L. 0.1. 1.93 2.40 0.77 1.60 3.23 0.77 NA131' NA241 NA272 NAI13 No. - Total 99.20 98.80 100.16 99.36 98.92 100.07 Si02 45.62 53.87 56.91 66.08 Q 12.55 14.88 35.76 Ti02 2.76 1.05 0.37 0.09 Or 17.29 21.07 24.79 20.78 23.41 28.27 AI203 14.76 19.52 20.81 17.47 Ab 27.38 24.34 27.69 40.65 40.65 27.84 Fe203 11.00 5.49 3.08 2.68 An 16.15 14.27 12.65 7.72 4.54 FeO Ne MnO 0.17 0.15 0.14 0.04 Co MgO 7.73 1.08 0.49 0.07 Ac 1.43 CaO 8.03 2.91 1.27 - Di 10.54 9.80 8.37 3.18 6.26 Na20 3.41 6.75 7.87 7.15 Hy 13.77 19.65 19.47 9.39 12.26 4.47 K20 2.11 4.28 5.11 4.52 01 6.62 2.10 1.06 P20S 0.62 0.25 - - Mt 1.70 1.64 1.53 1.48 1.32 0.72 L.O.I. 3.26 3.44 3.33 1.74 II 2.17 2.21 2.23 1.58 1.77 0.50 Total 99.47 98.79 99.38 99.84 Ap 1.61 1.30 1.33 0.28 1.35 Q 5.34 D.I. 44.7 45.4 52.7 74.0 79.3 91.9 Or 12.45 25.05 30.06 26.68 Ab 19.18 37.43 39.73 60.43 An 18.74 10.29 6.40 Ne 5.23 10.69 14.56 Co 0.82 Ac Di 13.90 2.13 Hy 4.09 01 17.88 5.67 3.69 Mt 2.08 0.93 0.69 0.51 288 Ring Complexes and Related Structures The Cameroon Line: A Review 289

Table 1 (continued) Table 1 (continued)

D Mboutou E Golda Zuelva

o-Gb Gr W.R. Gb Gb GbN q-SGb q-SD q-S q-SD Q 5.08 14.16 30.46 0.51 0.57 31.18 M161 MI44 Mt05 M85 M72 I Or No. M162' M120 M3 M159 3.60 9.98 13.99 14.40 25.73 25.91 6.85 22.67 24.67 24.14 Ab 21.73 34.85 38.12 48.17 37.27 33.30 20.94 27.89 42.17 30.86 72.47 Si02 33.19 42.90 49.77 50.92 60.34 63.17 37.71 53.84 An 29.56 20.54 8.21 9.70 9.67 0.30 18.99 12.21 9.33 Ti02 7.23 5.69 1.84 3.08 1.50 0.64 8.64 2.52 0.28 Ne 0.45 0.17 6.01 14.51 I AI203 9.90 14.76 23.14 14.96 15.68 17.08 9.53 15.32 Co 0.08 0.20 1.85 6.34 0.81 Fe203 10.54 8.40 2.45 4.96 4.20 8.02 Ac 1.88 3.63 0.38 FeO 16.66 6.26 3.43 6.52 3.26 2.32 9.52 I Di 20.73 14.68 18.30 5.33 16.09 10.42 7.40 1.61 0.02 MnO 0.34 0.13 0.02 0.09 0.15 0.03 0.04 0.08 Hy 8.76 to.59 9.10 6.89 16.27 10.92 7.63 MgO 10.68 6.63 2.39 4.86 2.12 0.39 13.42 3.24 0.27 01 13.13 11.46 3.74 17.27 CaO 7.53 11.36 11.68 8.04 3.84 2.14 11.56 6.40 0.21 Mt 2.47 2.17 2.07 1.53 0.94 0.77 2.74 2.08 1.75 Na20 1.78 2.32 3.74 3.81 5.24 6.06 1.02 4.18 4.41 11 7.59 5.54 4.60 2.63 1.90 0.30 6.98 3.71 1.96 0.34 0.10 0.14 0.43 1.35 2.88 4.11 0.12 3.14 5.32 K20 D.I. 25.8 45.0 52.1 67.7 77.2 89.7 33.8 51.1 67.4 86.2 P20S L. 0. I. 0.43 0.39 0.47 0.08 0.33 0.87 0.23 0.69 0.99 Total 98.38 98.98 99.36 98.67 99.54 98.66 99.81 99.38 99.67 Q 0.45 2.12 9.09 6.96 2.85 25.60 Or 0.59 0.83 2.54 7.98 17.02 24.29 0.71 18.56 31.44 Ab 10.61 19.63 31.65 32.24 44.34 51.28 8.63 35.37 37.32 An 18.73 29.45 45.09 19.74 to.77 7.27 21.08 13.77 1.04 4.3 Ntumbaw Co Ac The Ntumbaw massif crops out over an area of 12 km2 (Fig. to). The massif is com- Ne 2.41 Di 14.99 20.94 10.21 16.04 6.49 2.82 28.24 14.00 posed of two juxtaposed centres (I and II) intrusive in the Pan-African basement. Hy 6.80 2.28 7.43 2.57 1.28 1.54 1.58 0.67 These centres are partly covered by recent basaltic lava flows (Ghogomu 1984; 01 21.61 0.07 13.17 Ghogomu et al. 1989). Mt 15.28 4.11 3.55 7.19 6.09 2.68 5.77 4.66 0.48 Centre 1 is composed of coarse-grained porphyritic rocks subdivided into a succes- He 5.57 4.04 3.13 0.48 sion of three main petrographic facies: monzodiorite, monzonite and quartz mon- 11 13.73 10.81 3.50 5.85 2.85 1.22 16.41 4.79 0.53 zonite (Thble 1). The essential minerals are andesine-oligoclase, orthoclase, augite, D.I. 13.6 20.9 34.2 42.3 71.5 82.5 9.3 54.8 94.4 amphibole and occasional hypersthene. Centre II comprises two petrographic types, quartz syenite and syenogranite, both cutting the rocks of Centre I. The quartz-syenite is composed of orthoclase, oligoclase, quartz, biotite, actinolite and ferroaugite; the syenogranite is made of or- E Golda Zuelva thoclase, oligoclase, quartz, ferroaugite, ferrorichterite and biotite. The last magmatic events are represented by trachytic and quartz-feldspathic porphyry dykes. W.R. o-Gb MGb Md M q-S Gr H M BE R The Ntumbaw rocks present a continuous compositional variation of the plagioclase No. 12GZ 91GZb 91GZa 8GZ 4771 G1.80 G59.80 3GZ 035.80 22GZ from An 40 in monzodiorites to An 13 in syenogranites. Orthopyroxene (En 55-64, Si02 45.85 51.02 54.16 59.61 65.13 73.59 44.18 53.28 58.49 73.88 Fs 42 - 35, Wo 3 -1) is only present in the rocks of centre I. Clinopyroxene composition Ti02 3.99 2.91 2.42 1.38 1.00 0.16 3.67 1.95 1.03 0.18 varies from augite in monzodiorite to ferroaugite in syenogranite. The amphiboles are 15.89 AI203 16.20 12.99 15.57 15.59 11.54 14.45 14.06 16.15 to.43 restricted to calcic amphiboles and calci-sodic amphiboles. The continuous evolution Fe203 13.06 11.48 10.98 8.10 4.95 4.08 14.48 11.02 9.28 5.00 FeO of the minerals of the Ntumbaw rocks is in accordance with their cogenetic origin from MnO 0.20 0.18 0.19 0.18 0.06 0.07 0.19 0.22 0.16 0.05 a common magmatic source of subalkaline to mildly alkaline nature. MgO 5.98 4.10 4.10 1.29 0.91 0.38 6.27 3.19 0.94 0.28 The distribution of major and trace elements in the lava series is well explained by CaO 11.03 7.70 6.09 3.21 1.95 0.06 7.76 4.96 3.60 0.37 mineral fractionation. Thus, plagioclase, augite, biotite and apatite fractionation are Na20 2.67 4.16 4.51 5.70 4.41 3.94 3.79 3.30 4.99 4.38 respectively correlated with Eu, Cr, Ba and P20S distributions. The chondrite nor- K20 0.61 1.69 2.37 2.44 4.36 4.39 1.16 3.84 4.18 4.09 P20S malized rare earth element values vary between 200 and 320 (La) and 6 and 25 (Lu). L. 0. I. 1.27 1.17 1.61 2.32 1.34 1.26 3.83 3.64 0.86 0.67 The Ntumbaw massif is an atypical anorogenic ring complex with more than 900/0 Total 99.54 99.85 98.68 99.43 99.52 99.47 99.78 99.46 99.68 99.33 of the rocks having an intermediate composition (52 < Si02 % < 65). -----L 290 Ring Complexes and Related Structures The Cameroon Line: A Review 291 4.5 Guenfalabo

The Guenfalabo volcano-plutonic massif results from two volcano-plutonic cycles (Ngonge 1988).An early gabbro-monzonite-granite suite is followed by late syenite- granite porphyry suite. Both suites have associated volcanic equivalents. Plagioclase, salite, oxides, apatite occur in the gabbros and plagioclase, alkali feldspars, augite, pargasitic hornblende, biotite, titanite, apatite, Fe':fi-oxides and zircon Occur in the monzodiorites and in the monzonites. Alkali feldspars and quartz are abundant in the syenites and granites. Sodic amphibole and aegirine are the mafic minerals in the more evolved granites.

4.6 Mboutou

With a diameter of 6 km, the Mboutou massif crops out about 100km southwest of Golda Zuelva. Its age is of 55-60 Ma (Jacquemin et al. 1982). The geological map of the Mboutou massif is not well established; two overlapping centres occur, the southern centre being younger than the northern one. Considerable petrological infor- mation is available (Jacquemin 1981;Jacquemin et al. 1981;Parsons et al. 1986).The ~~LLJ~~~I~~l1JfJ~r;-:1f'+'""':;1r.;-:":1~~r++1~m:TI D 1 2 3 4 5 6 7 8 9 10 main rock types are: layeredgabbros composed of troctolitic gabbro, noritic melagab- bro, anorthositic leucogabbro having a good inward dipping lamination and later Fig. to. Geological map of Ntumbaw complex (after Ghogomu 1984). 1 basement; 2 monzodiorite; quartz syenodiorite, quartz syenite and granite. Minor hawaiite and potassic ben- 3 monzonite; 4 quartz-monzonite; 5 quartz-syenite; 6 quartz-biotite syenite; 7 syenogranite; 8 quartz- moreite dykes cut the plutonic rocks. feldspar porphyry dyke; 9 trachyte; 10 basalt Feldspars have a continuous compositional range from An 85 in the melagabbro to An 1 in the quartz-syenite and forms an entirely hypersolvus sequence. Salite and calcic augite and Fo 78- Fo 62 olivines have restricted compositional ranges. Evolved 4.4 Mayo Dade rocks contain calcic to ferroan-amphiboles (magnesio-hornblende and ferro-edenite) with minor biotite showing progressive iron enrichment. The Mayo Darle complex is elliptical (12x5 km) and elongated NE. It intrudes the Mineralogical and geochemical data (Thble1) show that the rocks belong to a very Pan-African basement and consists of quartz-syenite, biotite- and sodic amphibole- mildly alkaline series.The more evolvedmembers (which are insignificant in terms of granite, granite porphyry and rhyolitic volcanic rocks containing benmoreite volume) show some mineralogical calc-alkaline characteristics probably due to in- xenoliths. Rb-Sr isotopic ages are scattered from 73 Ma (syenite) to 63 and 54 Ma troduction of water in residual magma batches (Parsons et al. 1986). Fractionation (Lasserre 1978), 58 Ma (Gazel et al. 1963)and 55 Ma (granite, Nguene 1982;Nguene of olivine, Fe-Ti-oxides,Ca-rich pyroxenes, plagioclase, amphibole, biotite and alkali and Norman 1985). A K-Ar age of 49.5Ma has also been obtained for the granite feldspar are deduced from transition and incompatible element distribution (Jac- (Cantagrel et al. 1978b). quemin 1981). At the northern centre, values such as 0180 (5.6-6.2), 87Sr/86Sr Mayo Darle is the only mineralized massif known in the Cameroon Line. Tin (0.7030-0.7045) and 207Pb/204Pb(15.60-15.64) support a mantle origin for the ini- mineralization (2.3km2 areal extent) occurs as a stockwerk of veinlets developed tial magmas with an extensive magma-crust interaction at the southern centre (0018 along conjugated fractures in the biotite granite. Veinminerals are quartz, cassiterite, up to 7.0, 87Sr/86Sr:0.7050-0.7097, 207Pb/204Pb:15.70-16.65) (Jacquemin et al. zinnwaldite and minor topaz. Fluid inclusions indicate temperatures of 200° to 600°C 1982). The evolution of the magmas was dominated by fractional crystallization of and salinity from 20 to 65 eq. wtOJoNaCI with periodic boiling at more than 385°C. a mildly alkaline basaltic magma, with progressive interaction of crustal contamina- A saline fluid phase was present in the Mayo Darle granite during or soon after crystallization but there is no evidence that this fluid was responsible for the tion during the differentiation process (Jacquemin et al. 1982). mineralization. It has been suggested (Nguene and Norman 1985) that fluids emanating from a later intrusion leached the tin from the Sn-bearing minerals which 4.7 Golda Zuelva could have thus constituted the source of Sn-mineralization. Golda Zuelva is one of the northernmost Cameroon ring complexes.It is circular with a diameter of about 14km and intruded the Pan-African basement at 66 Ma BP (Jac- quemin et al. 1982). J 292 Ring Complexes and Related Structures The Cameroon Line: A Review 293

Petrographic studies (Jacquemin 1981; Jacquemin et al. 1982)reveal a succession Ikm 5"37' of plutonic and volcanic rocks. The former consist of troctolitic gabbros, mon- , (j zodiorite, amphibole-biotite monzosyenite and sodic-amphibole granite, while the lat- I I ter are hawaiite, potassic mugearite, potassic benmoreite and alkaline rhyolite. , ,, Petrology and geochemistry (Thble1) of the rocks reveal an alkaline suite ending I in peralkaline affinity. Low initial 87Sr/86Sraround 0.7020:t0.0012 suggests a mantle origin for the magmas and a low crustal contamination. These results also demon- strate that the plutonic and volcanic series are comagmatic. .('>~ -", ~{~~t@ 4.8 Other intrusive anorogenic massifs

Little is known about the other alkali-syenite, alkali-granite massifs. Among them, Koupe, Nlonako, Namboe, Sabri, Bonhari, Gounguel, Fourou, Mana, Poli, Tchegui, Nyore, Kokoumi, Mouhour, Grea and Waza are the most prominent. o

4.9 Conclusion .. N No regional chemical evolution of the Tertiary ring-complexesof the Cameroon Line has been observed; they present a great chemical similarity with the Nigerian- t '>Tortuga Fig. 11. Geological map of Pagalu (after Cornen and ring-complexes(Tempierand Lasserre 1980).The alkaline character of all the massifs Maury 1980). 1 basalt; 2 trachyte; 3 basaltic volcanic is confirmed by the mineralogy and the geochemistry of their rocks. The clinopyrox- ashes; 4 palagonitic breccia ene composition varies from augite, titanaugite and salite (in the basic and intermedi- ate rocks) to ferroan-augite, calci-sodic and sodic phases (in the acid rocks). Hedenbergite has not been observed. Major and trace elements distribution (especiallyrare earth elements) show typical produced palagonitic breccia which crops out around the island; (2)basaltic flow pile alkaline trends for all the massifs. Isotopic geochemistry suggests a mantle origin for covered the major part of the island; the oldest flow has been dated as 18.4Ma (Piper the massifs and possible traces of crustal contamination during their magmatic evolu- and Richardson 1972). Numerous dykes related to the following period intersect the tion. pile and havebeen dated as 5.3 Ma (Cornen and Maury 1980);a large pyroclasticcone was built upon the layered basaltic flows; (3) trachytic plugs cut the cone and have been dated as 3.9 Ma (Cornen and Maury 1980); (4) more recently, the volcano 5 Volcanism erupted to the north (2.6 Ma, Piper and Richardson 1972)and to the south and on the islet of Tortuga. The Cameroon Line comprises eight major volcanic regions (with large strato- The lava series developed a potassic alkaline trend with basanites, hawaiites, volcanoes) and small monogenic volcanoes scattered in five other areas. tristanites and trachytes (Cornen and Maury 1980)(Thble2). The basaltic lavas con- In WesternCameroon (Manengouba, Mounts Bambouto), the lavas have been clas- tain forsteritic olivine rich in calcium which is typical of alkaline compositions and sified into three series(Geze 1953):lowerblack, medium white and upper black. How- occur together with chromium-poor salite. The trachytes contain phlogopitic mica. ever, detailed fieldwork has not yet been carried out that permits to confirm this Kaersutite phenocrysts occur in hawaiites and tristanites and ferroan-pargasite in classification which appears to be oversimplified. trachytes. Labradorite microphenocrysts are found in the hawaiites while andesine or potassic-oligoclase phenocrysts occur occasionally in the benmoreites and trachytes. Phenocrysts of sanidine or anorthoclase are present in the trachytes. 5.1 Pagahi Increase in the La/Yb and Zr/Y ratios without an increase in Y and Yb contents from hawaiites to benmoreites is consistent with crystal fractionation of kaersutite. The southernmost island of the Gulf of Guinea is the top of a strato-volcano (Fig. 11) The lava series may have originated from basaltic (basanitic) magma by crystal frac- about 5300m high which is built on the oceanic crust (Gorini and Bryan 1976). Its tionation involving mainly kaersutite (2/3), clinopyroxeneand plagioclase (Liotard et altitude reaches 813m. The volcano was progressivelybuilt: (1) submarine volcanism al. 1982). 294 Ring Complexes and Related Structures The Cameroon Line: A Review 295

Table 2. Chemical analyses (major elements in wt07o;trace elements in ppm) and CIPW norms of Table 2 (continued) representative lavas of the Cameroon Line (CIPW norms are after the authors referred below and A were not recalculated). The lavas (except nephelinites of Etinde) have been classified here according Pagalu to their D. I. value (Thornton and Tuttle 1960): D.I <20, pBa picritic basalt; 20phonolites, trachytes and alkali-rhyolites 4 7 9 have been distinguished from their petrographic characteristics. Data sources: Pagalu: ANI5, Fitton II 6.07 (1987) 4, 7 and 9, major elements and CIPW norms, Cornen and Maury (1980), trace elements, 5.43 2.81 1.36 1.99 Liotard et al. (1982) (4, 7 and 9 respectively A16, A12 and A17 in Liotard et al. 1982; 9: Di includes Ap 2.56 1.32 0.38 D.I. Wo 1.62; Siio Tome: 1: Assun~ao (1957), other data: Fitton (1987) (ST72 and STt9: basanites, 21.1 25.2 75.8 91.4 STtl0: phonolitic tephrite, after Fitton (1987); Principe: P18, P31 and P12, Fitton (1987) (PI8: Li 7 13 16 olivine nephelinite, P31: transitional basalt, P12: tristanite, after Fitton 1987), P33 and P5: Fitton Rb 37 26 99 136 and Hughes (1977), 12: Cotelo Neiva (1957) in Barros (1960), 12: LOI includes H20+ 1.18, H20- Cs 0.27 and CI 0.43, P5: CIPW norm includes NaSi: 2.77; Bioko: Deruelle and Kambou (in prep.); 0.35 0.24 0.52 Sr 779 Mount Cameroun, Deruelle et al. 1987a; Etinde: M146, M163: Mouafo (1988); other data: Deruelle, 900 961 669 Ba 516 557 unpubl., N: nephelinite, h: hauyne, I: leucite, n: nosean, S10 and M163: Di includes Wo 0.60 and 1033 1365 Sc 25 23 1.40 respectively; Rumpi Hills: Nkoumbou (1990). Manengouba: 3, chemical data: Jeremine (1941), 4 1 V 268 CIPW norm: 10 in Jeremine (1943), 4531, 4831 and 4202: Tchoua (1974a) (4531: basalt after Tchoua 253 79 23 Cr 588 1974a), C65 and C63: Fitton (1987) (C65: transitional basalt, after Fitton 1987),4531 and 4202, LOI 390 12 4 Co 56 includes H20+ 0.15 and 1.40 respectively; 4531, 4831 and 4202, LOI includes HP- 0.45,0.10 and 10 1 Ni 569 280 1.20 respectively; Bambouto: 5218, 5873, 4233 and 4758, Tchoua (1974a), other data, Fitton (1987) 13 3 Cu 43 56 (5873: basalt, after Tchoua 1974a and C116: trachyphonolite, after Fitton 1987) 5281, 5873, 4233 8 1 Zn 118 130 and 4758, LOI includes H20+ 0.60, 2.25, 0.90 and 0.80 respectively, 5873, 4233 and 4758, LOI in- 101 106 Y 30 33 34 49 cludes HP- 0.45,0.55 and 0.10 respectively; Oku Massif: C89, C85 and C87, Fitton (1987), JA, Zr Gouhier et al. (1974) (C89: transitional basalt, after Fitton 1987, C85 and C87, CIPW norm includes 253 253 633 897 Nb NaSi 1.17 and 2.29 respectively; Garoua Upper Benue valley and Kapsiki plateau, Deruelle, unpubl., 57 59 97 116 NC4P and NC1A: CIPW norm includes 1.02 and 0.51 Co respectively Hf 5.3 13.8 18.9 Ta 3.4 6.9 10.9 La A Pagalu 58 50.8 87.4 123.3 Ce 90 106.1 165.0 247.0 W.R. Ba Ba Be T Nd 43 No. AN15 4 7 9 Sm 10.8 8.1 14.3 Eu 3.6 3.2 4.1 Tb Si02 41.67 43.08 56.87 63.52 1.3 1.1 1.6 Ti02 3.12 2.81 1.46 0.71 Dy Yb AIP3 10.62 11.58 17.35 18.03 1.9 2.6 3.7 Lu Fe203 13.78 4.65 3.94 1.99 0.3 0.4 0.6 FeO 7.94 2.16 0.88 Th 5.6 19.2 20.0 MnO 0.19 U 1.7 1.7 4.5 MgO 15.52 11.84 1.57 0.12 CaO 9.70 11.23 4.04 1.65 Na20 2.36 3.10 5.90 6.52 K20 0.85 1.32 4.97 5.93 B Siio Tome P20S 0.82 1.06 0.55 0.16 L.O.I. 0.62 1.56 0.89 0.54 W.R. pBa Ba H M B T T Total 99.23 100.17 99.39 100.05 No. 1 ST72 ST19 ST73 ST110 STtO ST84 Q 0.68 Or 5.12 7.93 29.79 35.23 Si02 41.69 43.80 44.25 48.54 53.64 57.39 61.66 Ab 10.74 6.10 40.45 55.44 Ti02 3.47 3.17 3.81 2.65 1.63 0.24 0.26 An 16.27 14.03 6.28 2.44 AI203 14.57 14.06 15.30 17.42 18.83 21.70 18.63 Ne 5.25 11.14 5.52 Fe203 4.13 12.46 12.95 9.93 6.41 2.20 3.41 Lc FeO 8.99 Ac MnO 0.14 0.22 0.21 0.22 0.16 0.15 0.24 Di 21.87 28.78 8.64 2.29 MgO 7.74 8.31 5.63 3.51 1.76 0.11 0.16 Hy CaO 11.44 10.62 9.50 8.16 4.96 1.24 0.94 01 27.00 21.87 3.01 Na20 2.89 3.61 4.76 5.38 7.28 9.43 7.93 Mt 5.68 2.21 2.21 2.19 K20 0.79 1.41 2.15 2.45 3.65 6.02 5.46

J 296 Ring Complexes and Related Structures The Cameroon Line: A Review 297

Table 2 (continued) -Table 2 (continued) B Silo Tome C Principe

W.R. pBa Ba H M B T T W.R. Ba Ba H Be Ph Ph No. 1 ST72 ST19 ST73 ST110 ST10 ST84 No. P18 P31 P33 P12 12 P5

0.04 0.03 P20s 0.69 1.03 1.02 0.93 0.44 Si02 37.01 47.71 47.95 53.98 55.76 54.38 L. 0.1. 3.54 0.54 0.04 0.34 0.55 1.19 0.99 Ti02 4.15 2.94 3.33 1.08 0.20 0.21 Total 100.08 99.23 99.61 99.53 99.31 99.72 99.70 Al203 10.88 14.07 15.94 19.54 20.99 22.04 Fe203 14.28 11.90 4.48 4.80 2.22 1.63 Q FeO 6.93 0.80 Or 5.00 8.52 12.86 14.72 21.97 36.19 32.76 0.70 MnO 0.21 0.16 0.17 0.13 0.02 0.14 Ab 12.18 15.05 14.33 26.48 34.35 29.02 50.12 MgO 12.24 8.15 4.52 1.42 0.73 An 24.19 18.39 14.21 16.40 8.03 0.23 CaO 13.19 8.00 8.01 5.11 2.23 0.94 Ne 6.75 8.76 14.35 10.66 15.33 7.26 8.86 3.10 3.05 3.85 5.39 9.56 11.67 Lc Nap 1.55 1.67 1.91 4.41 5.59 5.10 Ac 1.58 1.47 K20 1.42 0.67 1.04 0.24 0.21 0.05 Di 29.19 22.92 21.67 15.08 11.57 5.26 3.98 PPs L.O.I. 1.62 1.60 1.50 4.08 1.88 1.57 Hy Total 99.95 99.92 99.63 100.17 100.19 99.49 01 11.35 12.58 7.51 5.24 1.91 0.02 1.59 4.07 2.63 0.11 0.66 Mt 6.03 5.14 5.30 Q II 6.69 6.15 7.33 5.10 3.14 0.46 0.51 Or 10.13 11.50 27.19 32.99 31.06 Ap 1.68 2.49 2.44 2.25 1.07 0.10 0.07 Ab 26.43 33.24 35.23 24.33 24.44 I 0.1. 24.0 30.1 41.5 51.9 71.7 92.5 91.7 An 11.53 20.31 20.96 16.79 I Ne 14.63 6.73 28.44 34.22 Li Lc 7.40 Rb 47 60 64 114 223 200 Ac 1.14 2.00 Cs Di 24.95 12.77 10.24 6.60 8.26 3.89 1284 1128 110 19 Sr 1130 1113 Hy 8.66 1.86 70 Ba 718 728 870 845 154 I, 01 19.33 9.42 8.32 2.69 1.60 1.10 0 0 Sc 18 16 9 4 Mt 5.94 4.92 5.01 2.02 V 237 234 173 79 15 0 II 8.12 5.73 6.44 2.13 0.38 0.40 381 33 23 5 4 4 Cr Ap 3.46 1.62 2.52 0.60 0.46 0.11 Co Ni 171 43 19 6 2 1 1 0.1. 22.0 36.6 44.7 69.2 85.8 89.7 Cu 48 41 21 7 0 0 Zn 114 133 120 117 114 174 Li I 377 Y 33 36 38 27 13 43 Rb 40 38 43 180 Zr 336 472 599 751 965 1414 Cs Sr 1297 625 933 1115 73 Nb 92 99 133 116 82 264 I Hf Ba 987 439 758 Ta SC 26 21 18 2 La 79 83 105 79 76 199 V 307 231 56 Cr 231 14 Ce 157 163 197 148 95 338 ,I 338 Nd 66 72 79 52 18 88 Co Sm Ni 193 167 6 11 2 Eu Cu 56 42 4 Tb II Zn 108 112 125 87 188 Dy Y 35 29 33 20 19 Yb Zr 387 327 393 719 1904 Lu Nb 118 49 35 119 103 Th 17 11 18 17 34 39 I Hf U Ta La 103 43 58 68 96 Ce 203 88 136 111 102 Nd 85 41 63 36 7 Sm

j 298 RingComplexesand RelatedStructures I The Cameroon Line: A Review 299

Table 2 (continued) -Table 2 (continued) C Principe I D Bioko Be Ph Ph W.R. W.R. Ba Ba H Ba H H H P33 P12 12 P5 No. 10 No. P18 P31 I lS IG lK Cr Eu 359 387 29 17 Co Tb 125 23 41 10 Dy Ni 316 18 22 12 Yb Cu 120 10 10 9 Lu Zn Th 10 5 26 Y U Zr 414 442 197 Nb Hf 8.8 8.3 5.5 Ta 8.5 4.9 5.1 D Bioko I La 99.3 64.8 55.7 Ce 152.0 34.6 35.5 W.R. Ba H H H Nd IG lK No. 10 lS I Sm 11.3 9.14 7.29 Eu 4.50 3.39 2.75 54.07 45.39 SiOz 45.25 44.94 Tb 1.39 1.08 0.91 2.38 3.12 TiOz 2.88 3.18 Dy 16.29 16.60 AIP3 13.28 16.31 Yb 10.43 10.17 Fez03 13.14 13.80 Lu FeO Th 9.82 9.33 5.60 0.19 MnO 0.17 0.24 0.14 U 2.57 2.23 1.43 3.66 MgO 9.89 4.27 2.82 CaO 10.14 9.50 7.01 8.38 3.41 5.09 Nap 2.64 4.27 2.24 3.08 KP 0.71 2.08 0.96 E PzOs 0.87 1.11 0.56 I Etinde L.O.I. 1.25 0.37 1.04 2.18 Total 100.22 100.07 100.39 98.82 W.R. N h-N N I-N In-N In-N I No. M146 S19 S15 S13 S10 M163 Q 4.43 4.19 12.28 13.22 18.18 Or 39.40 39.40 40.02 39.79 45.82 46.25 Ab 21.21 15.17 28.82 12.62 SiOz 4.60 3.59 3.47 3.66 1.21 1.06 22.28 19.19 22.52 13.36 TiOz An 13.01 16.42 16.14 15.86 20.72 19.46 Ne 0.60 11.34 16.48 AlP3 6.07 11.00 12.17 11.54 6.61 4.22 Lc Fez03 FeO 7.59 2.18 Ac MnO 0.19 0.32 0.28 0.33 0.37 0.36 Di 17.32 7.22 18.21 18.16 8.08 4.08 0.89 1.16 15.23 MgO 3.98 4.50 Hy CaO 15.52 12.28 12.11 12.30 6.66 6.15 01 13.14 7.67 22.44 2.22 4.12 4.38 5.61 7.60 7.80 Mt 2.61 1.97 1.92 Nap 2.48 1.22 0.92 3.28 3.72 5.54 6.22 II 6.05 4.53 5.94 KzO 5.48 0.68 1.35 1.25 1.22 0.21 0.16 Ap 2.42 1.22 2.10 PzOs 1.90 L. 0.1. 1.43 6.59 2.21 0.74 4.35 4.04 D.I. 26.0 38.8 46.5 47.9 Total 100.01 99.97 99.81 98.85 99.98 99.06 Li Q Rb 22 48 49 69 Or 5.43 14.66 Cs Ab 6.53 16.68 Sr 784 1181 753 1662 An 21.93 23.59 14.70 7.12 6.08 Ba 406 792 632 963 Ne 10.17 15.83 20.06 25.70 34.81 35.46 Sc Lc 5.65 15.18 17.22 12.57 17.29 V 228 225 242 208 Ac 0.43 300 Ring Complexes and Related Structures I The Cameroon Line: A Review 301

Table 2 (continued) Table 2 (continued) E Etinde -F Mount Cameroon W.R. N h-N N I-N In-N In-N W.R. pBa Ba H M No. M146 S19 S15 S13 S10 M163 No. 5M 2A 1B lA

Di 41.23 23.64 30.89 38.14 21.42 23.75 MnO 0.19 0.20 0.19 0.19 Hy MgO 14.79 6.22 4.60 3.67 01 9.50 6.78 6.44 2.15 CaO 10.27 12.05 8.71 7.59 Mt 2.58 2.08 2.30 2.18 1.25 0.99 Na20 2.26 3.63 5.00 4.91 0.81 II 8.75 6.83 6.60 6.96 2.30 2.02 Kp 1.35 2.13 1.93 0.51 0.56 0.89 Ap 1.49 2.95 2.73 2.66 0.46 0.35 P20S 0.77 L. 0. I. 0.34 0.88 0.43 0.12 42.9 64.1 67.4 D.I. 15.8 27.3 35.2 Total 100.06 99.62 99.29 99.30 Li Q Rb 71 62 98 106 200 201 Or 4.78 7.97 12.57 11.39 Cs Ab 11.77 6.73 22.86 36.80 1153 4342 2102 2453 4190 6596 Sr An 18.32 20.73 16.84 18.19 589 2488 3493 Ba 695 506 1366 Ne 3.97 12.98 10.51 2.55 Sc Lc V 379 329 349 220 Ac Cr 25 24 24 <10 Di 23.58 29.29 16.92 11.97 Co 29 20 <10 <10 Hy Ni 32 <10 13 <10 01 29.38 12.46 9.37 9.19 Cu 32 75 115 173 Mt 2.44 2.67 1.78 1.66 Zn II 4.85 6.32 5.37 4.79 Y Ap 1.21 1.33 2.11 1.82 Zr 313 914 Nb I D.I. 20.5 27.7 46.0 50.7 Hf Li Ta Rb 21 36 47 51 La Cs 0.50 0.63 Ce Sr 576 964 1147 970 Nd Ba 272 443 634 593 Sm Sc 15.4 14.6 Eu V 271 353 Tb Cr 945 222 53 40 Dy Co 59 34 25 23 Yb Ni 420 179 14 10 Lu Cu 66 114 Th Zn U Y 32.5 Zr 490 600 Nb Hf 9.7 11.5 Ta 8.9 8.7 La 60.9 91.0 90.4 F Mount Cameroon i Ce 123 156 154 Nd 65.2 W.R. pBa Ba H M Sm 11.7 9.7 9.7 No. 5M Eu 4.33 4.43 2A IB lA I 3.42 Tb 1.30 1.33 Si02 44.80 43.98 47.90 51.16 Dy 7.10 3.20 2.44 Ti02 2.55 3.32 2.82 2.52 Yb 2.22 11.31 Lu 0.32 AIP3 15.03 16.70 16.83 10.7 12.91 Th 10.2 Fe203 14.16 10.27 9.61 2.82 2.99 FeO U 302 Ring Complexes and Related Structures The Cameroon Line: A Review 303

Table 2 (continued) r Table 2 (continued)

G Rumpi Hills 1 - H Manengouba Ba H Be Ph T T R W.R. H W.R. H H BE CN38 NK52 C63 T T No. NK21 ND37 NK9 CN45 CN44 CN31 No. 4531 C65 4831 3 4202 44.64 46.56 49.34 56.24 56.93 60.46 66.08 75.70 Si02 Q 1.52 Ti02 3.03 2.72 2.72 0.56 0.52 0.51 0.23 0.12 8.04 14.28 Or 7.69 8.07 18.02 23.66 AIP3 13.89 16.59 15.48 18.68 19.04 18.96 16.76 12.06 26.69 25.14 Ab 20.55 29.70 43.21 53.67 13.65 12.67 10.56 6.32 5.31 4.53 3.61 1.15 49.25 48.17 Fe203 An 13.33 19.36 FeO 13.90 0.03 7.23 Ne 6.96 1.05 MnO 0.21 0.24 0.19 0.23 0.20 0.18 0.20 0.03 Lc 0.04 MgO 8.70 4.20 3.65 0.92 0.83 0.58 0.09 Ac CaO 10.24 9.33 7.44 3.37 2.72 1.19 0.29 0.07 Di 26.22 12.95 7.18 10.74 Na20 3.02 4.20 4.89 6.01 8.43 7.11 6.21 4.28 Hy 12.51 0.54 1.07 1.59 2.64 4.33 4.22 5.27 4.94 3.90 1.96 0.50 K20 01 6.65 7.12 7.65 0.58 0.82 0.98 0.26 0.21 0.11 0.06 0.05 P20S Mt 11.16 5.05 3.64 L. 0. I. 0.86 0.76 1.44 2.84 1.30 0.92 1.39 2.44 2.10 3.25 3.39 II 4.56 4.29 3.80 4.75 Total 99.89 99.68 99.33 99.76 99.71 99.82 99.86 99.84 1.52 0.95 Ap 1.66 0.95 1.55 0.83 0.34 3.00 8.05 34.99 Q D.I. 35.2 Or 6.32 9.39 15.58 25.56 24.91 31.11 29.16 23.02 37.7 62.3 78.9 84.0 87.6 Ab 15.54 22.32 28.74 41.96 39.35 49.14 52.49 36.17 Li An 21.18 21. 72 12.50 11.22 1.67 4.27 1.05 0.02 Rb 31 87 Ne 5.41 7.14 6.82 4.72 17.29 5.93 Cs Lc I Sr 429 856 Co 0.82 0.80 Ba 458 701 Ac Sc 23 9 Di 21.10 16.05 15.00 3.28 9.00 0.79 V 193 91 Hy 6.00 1.88 Cr 275 22 01 19.71 12.79 9.75 7.21 3.67 5.53 Co Mt 2.58 2.39 1.99 1.19 1.00 0.86 0.68 0.22 Ni 197 24 II 5.76 5.17 5.17 1.07 0.99 0.97 0.44 0.23 Cu 57 14 Ap 1.27 1.79 2.14 0.57 0.46 0.24 0.13 0.11 Zn 122 106 I Y D.I. 27.3 38.9 51.2 72.3 81.5 86.2 89.7 94.2 29 32 Zr 299 643 Nb 47 107 Hf Ta H Manengouba I La 41 76 Ce 64 148 W.R. H H C63 BE T T I Nd 33 58 No. 4531 C65 4831 3 4202 Sm Eu Si02 44.50 50.14 53.99 60.00 63.34 63.95 I Tb Ti02 2.40 2.24 1.98 2.50 0.82 0.50 Dy AI203 12.80 14.21 17.03 14.80 18.16 14.55 I Yb Fe203 7.70 12.40 8.95 3.15 2.18 6.25 Lu FeO 8.65 2.75 2.03 1.35 Th 4 13 MnO 0.15 0.18 0.17 0.15 0.08 0.15 I U MgO 6.40 7.94 3.30 2.20 0.54 0.20 CaO 10.10 7.60 5.39 3.20 1.62 0.50 Na20 3.95 3.48 5.29 6.35 5.78 5.70 K20 1.30 1.35 3.02 4.00 4.49 4.25 PPs 0.70 0.40 0.65 0.35 0.06 0.65 L.O.I. 0.60 0.08 0.22 0.10 1.27 2.60 Total 99.25 99.80 99.98 99.55 100.37 100.65 -- -L The Cameroon Line: A Review 305 304 Ring Complexes and Related Structures Table 2 (continued) Table 2 (continued) I Bambouto Bambouto - T W.R. Ba Ba H T TP Ph W.R. Ba Ba H T TP Ph T No. 5218 CII2 5873 CI04 CII6 4233 No. 5218 CII2 5873 CI04 CII6 4233 4758 - 4758 Sm 68.56 58.19 56.60 64.70 Si02 41.05 43.85 47.60 Eu 0.35 0.91 0.70 0.70 Ti02 5.50 3.10 4.10 Tb 13.08 19.50 20.15 16.10 Alp) 15.25 13.65 18.80 4.39 2.35 2.35 Dy Fe20) 7.20 12.52 3.10 5.74 2.05 1.75 Yb FeO 7.50 6.55 Lu 0.13 0.21 0.10 0.20 MnO 0.15 0.19 0.30 Th I 30 6 0.05 0.80 0.95 1.40 MgO 7.25 9.21 3.85 U CaO 11.50 10.55 6.20 0.44 2.88 2.30 0.60 5.87 7.77 8.50 6.15 Nap 2.75 3.47 3.90 4.26 4.75 5.00 K20 1.25 1.47 1.65 4.81 0.20 0.30 0.15 P20S 0.70 1.03 0.65 0.01 L. 0.1. 0.60 0.19 2.70 0.60 0.84 1.45 0.90 J Oku massif Total 100.70 99.21 99.95 99.64 99.93 100.10 100.00 W.R. H H R R 16.18 6.74 Q I No. C89 JA C85 C87 Or 7.39 8.82 9.76 28.79 25.47 28.10 29.57 51.98 Ab 12.22 14.09 32.96 41.10 45.74 38.49 I Si02 46.62 47.90 72.70 71.13 1.52 An 25.54 17.66 28.87 5.83 2.76 I Ti02 3.18 2.43 0.37 0.68 Ne 5.98 8.56 11.27 18.08 AI2O) 16.86 16.88 10.74 12.56 Lc Fe20) 12.50 13.15 5.69 3.31 Ac 4.69 FeO Di 21.32 23.07 0.85 1.92 6.13 5.53 0.46 I MnO 0.19 0.23 0.09 0.03 3.77 Hy 7.96 5.72 MgO 5.56 4.52 0.04 0.04 1.56 0.41 01 5.78 14.19 3.14 I CaO 8.50 6.43 0.12 0.02 3.41 Mt 8.71 5.14 4.49 1.79 3.41 Na20 3.39 4.69 4.72 5.68 1.33 II 10.45 6.00 7.79 0.68 1.74 1.33 K20 1.37 1.94 4.57 5.26 0.71 0.35 Ap 1.66 2.48 1.54 0.01 0.48 P20S 0.86 0.00 0.09 L.O.I. 1.10 1.84 0.22 0.66 0.1. 25.6 30.5 42.7 80.1 82.5 84.7 88.2 Total 100.13 100.01 99.26 99.46 23.18 I Q 29.11 Li Or 8.23 11.12 27.40 31.57 32 133 101 Rb I Ab 29.18 26.72 30.18 35.83 Cs I An 27.26 19.46 Sr 1242 6 1549 Ne 7.10 Ba 874 31 2501 Lc Sc 23 0 0 Ac 4.64 2.70 V 232 0 2 I Di 0.51 Cr 3 8.11 10.04 459 3 6.26 3.04 Co Hy 4.37 I 01 9.49 15.18 Ni 148 3 2 Cu Mt 5.14 5.10 43 0 0 0.71 1.31 Zn II 6.15 4.56 96 300 75 0.01 0.21 I Ap 2.07 Y 26 112 49 Zr 190 1372 558 0.1. 37.4 44.9 86.7 90.1 Nb 56 251 152 Li Hf I 172 207 Ta Rb 22 Cs La 43 249 116 Sr 1005 1276 4 15 Ce 89 398 183 Ba 517 510 4 80 Nd 46 188 73 -..-- 306 Ring Complexes and Related Structures The Cameroon Line: A Review 307

Table 2 (continued) Table 2 (continued)

J Oku massif K Benue valley Kapsikiplateau

W.R. H H R R W.R. Ba R Ba T R NC4P NC4L NCtD NC1F No. C89 JA C85 C87 -No. NC1A Sc 19 0 2 Lc V 182 98 0 0 Co 1.02 0.51 Cr 46 55 4 5 Ac Co 29 Di 22.89 24.32 2.06 Ni 29 20 1 1 Hy 5.21 7.40 Cu 22 <10 0 0 01 20.34 20.21 7.08 Zn 98 141 261 Mt 2.51 0.73 2.46 1.01 0.90 Y 28 26 165 II 5.46 0.82 6.35 0.19 0.23 Zr 254 1405 2071 Ap 1.66 0.11 2.25 0.31 0.28 255 Nb 46 379 D.I. 30.6 89.5 27.4 83.7 85.0 Hf Ta Li La 46 20 325 Rb 33 159 35 149 424 Ce 94 18 659 Cs 0.42 1.46 0.43 Nd 48 19 267 Sr 963 68 1705 238 0 Sm Ba 711 591 506 712 0 Eu Sc 28.7 0.28 0.35 Tb V 223 24 Cr 225 <10 264 18 0 Dy Yb Co 50 <10 54 1 1 Lu Ni 209 <10 247 0 0 Th 2 38 39 Cu 27 <10 U Zn Y 30.57 33.62 Zr 393 1019 2083 Nb Hf 8.2 23.8 76.5 K Benue valley Kapsiki plateau Ta 6.5 17.1 75.4 La 77.85 100.12 58.0 135.0 52.3 W.R. Ba R Ba T R Ce 138.91 157.70 108 200 149 No. NC4P NC4L NCtD NCIF NCIA Nd 57.10 57.04 Sm 11.37 10.04 9.9 10.0 18.5 Eu 0.5 Si02 43.01 65.66 42.59 59.13 71.06 3.28 1.73 4.0 3.0 Tb 3.7 Ti02 2.87 0.43 3.34 0.10 0.12 1.3 1.1 Al203 12.96 17.20 12.79 18.44 11.23 Dy 6.21 5.61 Yb 2.07 Fe203 13.30 3.87 13.00 5.35 4.74 1.76 3.20 1.53 16.0 FeO Lu 0.24 0.52 MnO 0.18 0.08 0.22 0.16 0.06 Th 6.9 22.5 86.7 MgO 9.54 tr 10.23 0.93 0.17 CaO 9.41 0.37 10.48 1.32 0.40 Na20 4.03 5.97 3.46 7.40 3.72 K20 1.24 5.37 1.39 4.70 3.87 P20S 0.76 0.05 1.03 0.14 0.13 5.2 Sao Tome L.0.1. 1.66 1.12 0.99 1.77 3.92 Total 98.96 99.42 100.12 99.52 99.44 Sao Tome is a complex strato-volcano about 5000 m high. It rises from the abyssal Q 7.39 30.68 plain to an elevation of 2024 m above sea level. The island is made essentially of Or 7.32 8.20 27.74 22.84 31.70 basaltic lavas; phonolitic plugs and necks are abundant in the southern part of the Ab 10.93 50.46 7.40 48.21 31.44 An 13.61 1.51 15.26 3.24 1.14 island (Fig. 12). The basaltic lavas rest upon Cretaceous quartzose sandstones with Ne 12.71 7.77 metamorphic minerals (Hedberg 1969) which crop out at the centre of the island 308 Ring Complexes and Related Structures The Cameroon Line: A Review 309

6"45' The lava series is the result of differentiation of material from mantle source by fractional crystallization of olivine, augite, plagioclase, hornblende, magnetite and apatite (Fitton 1987).

5"3 Principe

This island rests upon the oceanic floor at a depth of about 3000m and culminates at 948 m above s.l . on Pico do Principe. Basaltic rocks predominate in the north, whilst phonolites and tephrites are common in the south (Fig. 13).The lavas rest upon palagonitic breccias which contain blocks of tholeiitic basalt and represent a sub- marine phase in the evolution of the island. The basaltic lavas have been subdivided (Fitton and Hughes 1977) into an older lava series (basalts, hawaiites) intruded by numerous dykes of basaltic lavas, and a younger lava series (basanites, nephelinites). The lavas of both series have been overlain by tristanites, phonolites and trachy- phonolitic lavas in the centre and in the south of the island. Nine lava samples have been dated (Dunlop and Fitton 1979)and gaveK-Arradiometric ages around 30.4Ma (tholeiites from the palagonitic breccia) and 4.9 Ma (trachy-phonolitic suite). The basaltic lavas of the older lava series contain phenocrysts of olivine and

(\ titanaugite plus phenocrysts and microphenocrysts of plagioclase and titano- $ 0 Co 0 N N 0" . Skm , c;J t Fig.12. Geologicalmap of Sao t 6"30' 1 Tome (after Cotelo Neiva 1956a). 1 basalt; 2 trachyte; 1°40 D [§]}...... IIIIillIII B 3 phonolite; 4 sedimentary rock; 1 2 3 4 5 5 unmapped zone " " ... , $ (Mitchell-Thome 1970 and references herein). Palagonitic tuffs and pillow-lavas p.' characterize submarine eruptions and are overlain by younger subaerial lavas. Trachy- phonolitic plugs cut the basaltic material, but the centre of the island has not been described yet. The oldest dated rocks are trachytes (15.7Ma, Grunau et al. 1975). Other lavas gaveages between 13.2Ma and Recent (Hedberg 1969;Macedo et al. 1977 in Dunlop 1983;Dunlop 1983). ' The mineralogy of the basaltic lavas is characteristic for an alkaline lava series and comprises phenocrysts of olivine, augite and magnetite in a microlitic groundmass of labradorite-bytownite, augite, ferric oxides and accessory titanite (Mitchell-Thome 1970). Hornblende phenocrysts have been found in some lavas (Cotelo Neiva 1954). The phonolitic lavas contain hauyne or sodalite phenocrysts; barkevicite has been seen in some trachytes (Assun9ao 1957). From basalts to phonolites and trachytes (Thble2) a continuum exists with no Daly gap (see Fig. 5 in Fitton 1987). The basaltic lavas are all nepheline normative (data Fig. 13. Geological map of Principe (after ' .'" ITill/.. ,.,'. from Boese 1912; Assun9ao 1957; Cotelo Neiva and De Albuquerque 1962; Fitton Cotelo Neiva 1956b). 1 basalt; 2 trachyte; D 1987). 3 phonolitic lava; 4 laterite 1 .3 4 310 RingComplexesand RelatedStructures I The Cameroon Line: A Review 311 magnetite; the groundmass contains microlites of the same minerals exceptolivine. In I addition, alkali feldspar, nepheline and apatite occur in the groundmass of the 3°45' Malabo younger lava series (Fitton and Hughes 1977). The phonolitic lavas are porphyritic with a microlitic groundmass. The dominant phenocrysts are nepheline, sanidine, aegirine-augite and barkevicite, titanite and magnetite with possible sodalite and Atlantic plagioclase (Cotelo Neiva 1956b; Fitton and Hughes 1977). The basaltic lavas (MgO> 6 wtOTo)are nepheline normative (Thble2); their Y/Nb ratios (see, in Table2, data from Fitton and Hughes 1977)are less than 1. 87Sr/86Sr ratios of the older lava series are in the range 0.7030-0.7037 (Dunlop 1983). o c e 8 n

5.4 Bioko

The island rests upon the continental shelf less than 35 km from the mainland. It con- sists of three amalgamated strato-volcanoes (Fig. 14). Pico Santa Isabel occupies the northern part of the island and is the highest volcano (3008m). Pico Biao (San Joa- quim) and San Carlos volcanoes are adjacent and form the southern part of the island. Pico Biao has a small crater (0.4km diameter) occupied by a lake; San Carlos has a large caldera (2.5 km diameter). Numerous fresh pyroclastic cones, mostly on Bloko Pico Santa Isabel and between this volcano and Pico Biao testify to recent activity. a Historical eruptions have been reported in 1898 (Piper and Richardson 1972), 1903 and 1923(Enciclopedia Universal Ilustrada 1924).The ages of the dated lavas are not older than 1.1Ma (Hedberg 1969), but the island is probably of the same age as neighbouring Mt Cameroon. N Only basaltic lavas are known on the island (Fuster Casas 1954).They are picrites (with abundant peridotite nodules), alkali basalts, hawaiites and rare kaersutite-bear- 10km t ing mugearites (Deruelle and Kambou 1988).One phonolite has been reported (Boese 1912), but the sample came from an erratic block on the shore and may have been S030' 8"45' brought as ballast by a vessel from Principe or Siio Tome. All the lavas with a differentiation index (D.I.) less than 35 are nepheline normative Fig. 14. Geological sketch map of Bioko (after Deruelle and Kambou 1988). Recent cinder cones are (Thble2). The picrites contain large olivine xenocrysts (with more than 2750ppm Ni) indicated. The whole island is made of lavas of basaltic type. Contours in metres and some basalts contain large centimetric augite xenocrysts (with more than 3000ppm Cr, 50ppm Co and 35ppm Sc). The xenocrysts have been considered as mantle melting residua and the basaltic lavas which contain these xenocrysts as close- the abundance of a thick surface mulch. Nevertheless, some outcrops of fresh lavas ly representative of primitive magmas. in vertical cliffs were inaccessible; sampling was completed along streams and on the beach. No basaltic xenoliths were found in the nephelinites of Mount Etinde. Blocks and 5.5 Mount Etinde pebbles of nephelinites wereobserved in pyroclastic deposits (alkali basalts of Mount Cameroon) at Batoke beach and at Etome. Five K-Arages (whole rocks and nepheline Mount Etinde is located on the southwestern flank of Mount Cameroon. It appears separate) for Etinde lavas (Le Marechal 1976; Dunlop 1983;Fitton 1987) are in the as a steep-sided volcano rising to 1715m of altitude. The lavas of Mount Etinde cover range 0.065-1.10 Ma and a further determination (haiiyne separate) gave 6.30Ma. Mount Cameroon lavas; they mostly flowed towards the Atlantic Ocean although Thus the activity at Etinde and Mount Cameroon is probably partly contem- without reaching it. Most of the Etinde lavas are of nephelinitic type (Fig. 15).Various poraneous (see above for ages of Mt Cameroon lavas). transects through the summit of the massif were made by two of us (N. C. and D.B) Seven petrographic facies have been identified (Mouafo 1988) among the Etinde despite exceptional difficulties due to the steep slope of the volcano (around 65°) lavas: nephelinitess.I., haiiyne-nephelinites,leucite-nephelinites,leucite-nosean-nephe- and the high density of the rain forest. Systematic in-situ sampling of unweathered linites, nosean-garnet nephelinites, melilite-nephelinites, haiiynophyres. These lavas lavas was obtained notwithstanding the meteoritic alteration of the lava surfaces and contain phenocrystsof euhedral nepheline (Ne 74, Ks22, Q 4) which havealso been de- 312 Ring Complexes and Related Structures The Cameroon Line: A Review 313

The nephelinites of Etinde contain 360/0to 46% of silica and 30/0to 15% of alkalis, with a D.I. ranging between 16 and 72 (Thble2). They are rich in titanium and phosphorus. Incompatible elements are concentrated in these lavas (Sr: 1000 to 8500ppm; Ba: 500 to 3000ppm; Zr: 300 to 1200ppm; REE: see Fig. 25). N t 5.6 Mount Cameroon

It is a volcanic mountain or a volcanic horst rather than a volcano (Deruelle et al. 4°4' 1987a). This mountain is also known for its seismic activity (Fairhead 1985). It culminates at 4090m. The oldest lavas rest upon a Precambrian metamorphic base- ment covered with Cretaceous to Recent sediments. More than 140 pyroclastic cones are scattered along the great axis of the massif (Fig. 16). Only two radiometric datings on Mt Cameroon lavas have been published up to now and they give ages younger than 1Ma (Hedberg 1969).Nineteen paleomagnetic determinations on lavas of the massif correspond to the Brunhes epoch and are younger than 0.7 Ma (Piper and Richardson 1972). Nevertheless, and without con- sidering the classification of the lavas into lower black, middle white and upper black series (Geze 1943),the oldest lavas are estimated to be of Late Miocene age (Vincent 1971). Historical eruptions have been recorded in 1800-1815, before 1835, 1838-1839, between 1845and 1865, 1852, 1865, 1866, 1868, 1909, 1922, 1925, 1954, 1959and 1982(Deruelle 1982;Deruelle et al. 1983b, 1984, 1987a; Fitton et al. 1983). As in the case of Bioko, the Mount Cameroon lava series is limited to picrites, basalts, hawaiites and basic mugearites. Nevertheless, the lavas described up to now \ have only been sampled on the southwestern, southern and southeastern flanks of the \ massif as well as in the summit area devoid of forest. Atlantic . The Fo contents (Fo 75- 85) of the olivine phenocrysts (cores) are rather constant Ocean ..Batoke~ ~ in the picrites and basalts, whereas they decrease in the microphenocrysts from picrites (Fo 80) to hawaiites (Fo 60). The crystallization temperatures deduced from olivine-liquid equilibrium decrease from 12650to 11150C from picrites to hawaiites. The oxides are Ti-magnetite and ilmenite. They have high MgO contents (3% -6%) 0 122]...... [II] '" fEE]...... EZ3 1 2 3 4 5 6 7 .8 9 which indicate low oxygenfugacities during crystallization. The phenocrysts of pyrox- ene are diopsidjc augites in the picrites, salite in the basalts, salite and augite in the Fig. 15. Geological map of Etinde. 1 basaltic lavas of Mount Cameroon; 2 nephelinitic tuff; 3 nepheli- hawaiites. They all contain low chromium and high titanium and sodium (N'ni 1984). nite; 4 nosean-leucite nephelinite; 5 hauyne-nephelinite; 6 melilite-nephelinite; 7 leucite-nephelinite; Microlites of labradorite are found in the picrites. Phenocrysts of labradorite and of 8 hauynophyre; 9 nosean-garnet nephelinite (after Nkoumbou, in prep.); .& summit (altitude: 1713 m) bytownite are scarce in the basalts and more abundant in the hawaiites. The mugearites contain microphenocrysts and microlites of alkali feldspar. The pyroxene appears before the plagioclase. Ferro-kaersutite phenocrysts, phlogopite microlites scribed as triclinic twinned crystals (Esch 1901)or as an orthorhombic phase (Tilley and sporadic small phenocrysts of nosean (Mesch 1916)appear in the mugearites. 1953), slender prisms of zoned titaniferous salite (WO48 - 52, En 25- 35, Fs 27 -13) All the lavas are nepheline normative (Thble2). The steady decrease in CaO with which may reach 5cm in length, titanomagnetite and apatite prisms, and Nb-Sr- and increasing differentiation is striking. This decrease is dependent of continuous augite REE- rich perovskite (Smith 1970). Hatiyne, nosean and leucite phenocrysts are fractionation all along the series. MgO contents decrease strongly in the picrites and sporadic. In the hatiynophyres, the phenocrysts of hatiyne or nosean (4% to 12% in the basalts since forsteritic olivine fractionates. Iron distribution depends upon oxide volume) coexist with destabilized kaersutite phenocrysts. Olivine (Fa83) is present in fractionation. Plagioclase fractionation only becomes effectivein the hawaiites, when hatiyne- and leucite-nephelinites.Zoned melanite (up to 17.2% of TiO~, more or less D.I. > 40. The trend of the REE is purely an alkaline one. REE-rich titanite (Smith 1970),Sr-melilite(Fitton and Hughes 1981),aenigmatite and biotite may belong to the mineraiogic;u assemblages of the nephe1inites. 314 Ring Complexes and Related Structures The Cameroon Line: A Review 315 5.7 Kumba graben

N BetweenMount Cameroon and the Rumpi Hills, near Kumba, three pyroclastic cones with large maars (more than 2 km diameter) of phreatomagmatic origin are located. Abundant, large (up to 0.50 m), peridotite xenoliths are found in the scoriae on the t slopes of Barombi Lake volcano.

5.8 Rumpi Hills

The Rumpi Hills are located 80 km north of Mount Cameroon. They cover an area of about 1500km2 mostly in a forest reserve devoid of car tracks and are of really difficult access. They culminate at 1768m at Rata Mount. The Rumpi Hills (Nkoumbou 1990)correspond to an intrusive-extrusivemagmatic unit which cuts and rests upon the pluto-metamorphic basement of Pan-African age (Fig. 17).

5km

O ~ .i/~ ~ ~ 1 234 -:--:--- r::::::::::3 -=--=------Fig. 17. Geological map of the Rumpi Hills (after Nkoumbou 1990). 1 basaltic lavas; 2 trachytic lavas; D ~ g.----- . ~ 3 phonolitic lavas; 4 rhyolitic lavas; 5 Cainozoic sedimentary rocks; 6 Pan-African intrusive and 5 6 7 metamorphic basement; 7 Cainozoic intrusive rocks 1 -2 .3 4 . Fig. 17. Geological map of the Rumpi Hills (after Nkoumbou 1990). 1 basaltic lavas; 2 trachytic lavas; 3 phonolitic lavas; 4 rhyolitic lavas; 5 Cainozoic sedimentary rocks; 6 Pan-African intrusive and metamorphic basement; 7 Cainozoic intrusive rocks 316 Ring Complexes and Related Structures The Cameroon Une: A Review 317

Plutonic rocks are mostly olivine gabbros, anorthositic gabbros, monzonites and 5.10 Manengouba syenites.Microgranular facies are present as radial dykes around the plutonic massifs. In gabbros, olivine crystals show weak zonation from cores (Fo 80)to rims (Fo 60) and This is a large strato-volcano of 25 km diameter at 2420m altitude. It is built upon form assemblages with titaniferous salite or calcic augite crystals. Kaersutite resulting a Cambrian-Precambrian granitic and gneissic basement. The massif has been af- from an incomplete transformation of diallage is observed. Titaniferous biotite sur- fected by two nested calderas. Various small pyroclastic cones with two crater-lakes rounds titano-magnetite. Hedenbergite and hedenbergite-aegirine coexist with biotite are built inside the youngest caldera. Numerous cognate xenoliths have been found in syenites. Plagioclase crystals decrease in modal abundance (750/0to 7%) and An inside that caldera (Tchoua 1974b). More than 20 adventivepyroclastic cones are scat- content (An 75 to An 10) from gabbros to syenites, whereas alkali feldspars show a tered on the flanks of the massif. The geomorphology of the massif has been mapped concomitant reverse distribution. The major element distribution (Thble1) cor- (Morin, pers. comm.). The age of a trachytic dome 12km northwest of the centre of responds to an incomplete undersaturated sodic alkaline series. Fractional crystalliza- the massif is 11.8Ma. The lavas of the Manengouba give ages up to 1.5Ma (Gouhier tion may be the dominant process of differentiation of the plutonic series(Nkoumbou et al. 1974;Dunlop 1983). 1990). The lavas are basalts, hawaiites (with olivine, augite, zoned plagioclase and horn- The volcanic stratigraphy shows, from the bottom to the top, a basal series of alkali blende phenocrysts sometimes recrystallized into rhonite, Jeremine 1943), trachytes olivine basalts, trachytes and phonolites followed by series of alkali olivine basalts, (in parts with barkevikite, Jeremine 1941), rhyolitic obsidians (Tchoua 1970) and hawaiites, benmoreites, trachytes, phonolites and rhyolites (Thble2). rhyolites. The lavas inside the inner caldera are olivine-augite basalts. Nevertheless, Kinked olivinexenocrysts (Fo 90) and limpid olivine phenocrysts (Fo 80)are present chemical analyses from the Mount Manengouba (Thble2) that have been published in the basalts. Anorthoclase and sanidine occur in benmoreites, phonolites, trachytes up to now are not of basalts, their D.I. being at least higher than 35 (Tchoua 1974a; and rhyolites. In the trachytes, early kaersutite undergoes a centripetal transformation Fitton 1987). into augite and titanomagnetite, while arfvedsonite coexists with aegirine and aenigmatite. Some trachytes contain fayalite and AI-rich ferri-annite. The occurrence of aegirine, aenigmatite and arfvedsonite in the phonolites, trachytes and rhyolites 5.11 Bamoun plateau denotes the peralkaline or agpaitic character of the more evolved volcanic rocks of the lava series. About 20 strombolian cones are scattered in the Noun plain at the south of the Ba- The two series of lavas correspond to classical sodic alkaline types devoid of moun plateau and northeast of Mount Manengouba. They are similar to those of the mugearites. Major and trace elements distribution (Thble2) indicate that the differen- Tombel plain. On 15August 1984,a lethal gas burst issued from a submerged 98-m tiation of the series may be interpreted by the fractional crystallization of olivine, deep crater in Lake Monoun, killing 37 people; emission of C02 was attributed to calcic and titaniferous salite, titanomagnetite, kaersutite, biotite and arfvedsonite volcanic exhalation from vents within the crater (Sigurdsson et al. 1987). The lavas from a peridotite mantle-derived magma. of these cones are alkali basalts (Wandji 1985)with a classical mineralogy. Salic rocks are present in the massifs of Mbam and Mbapit (Weecksteen1957)and are associated with basalts (20Ma old, Caen-Vachetteet al. 1987)in the Nkogam massif (Kamgang 5.9 Tombel plain 1986).Mbapit is a larger volcano on the plateau resting upon a Pan-African plutonic and metamorphic basement. It is constituted by peraluminous and soda-potash The Tombel plain is a graben (20x 70 km) which extends between Mount Cameroon rhyolites. These lavas are partly covered by pyroclastic and scarce flows of basaltic and Mount Manengouba. More than 70 small (100m high) generally monogenic lavas with xenoliths of prior hawaiites. In the rhyolites, quartz and perthitic anor- strombolian cones of Tertiary to Recent age are erected and scattered upon the thoclase are dominant. Minor sodic amphibole and Fe-Ti-oxidesare present. Some metamorphic substratum. Some of them have emitted scarce basaltic lava flows basalts show olivine, clinopyroxene and amphibole phenocrysts in a microlitic around and upon the "ultime" syenitichorsts of Nlonako and Koupe (Lasserre 1978). groundmass of labrador and augite (Tchoua 1972). Basaltic scoriae, lapilli and ashes of the volcanic cones show vertical and lateral grad- ed beddings. Blocky basaltic lava flows have originated barrier lakes (Tchoua 1971). The mineralogy of the lavas is typical of the alkaline series. The lavas are vesicular 5.12 Mounts Bambouto basalts with olivine, sporadic pyroxene and scarce bytownite or labradorite phenocrysts scattered in a groundmass of plagioclase, augite, magnetite and glass. This massif culminates at 2679m and rests upon a metamorphic basement at 1400m These lavas often contain metamorphic and plutonic xenoliths from the basement. of altitude. The massif has been described as a hawaiian shield volcano deeply Scarce hypersthene crystals (Tchoua 1971) (xenocrysts?) have been reported. dissected by two episodes of caldera collapse (Tchoua 1974a). The fault zones are picked out by the trachyphonolitic emissions. Numerous ignimbrites (quartz- trachytes), which may be related to the formation of the caldera, crop out in a zone of 40 km diameter around the caldera (Tchoua 1968, 1973). To date, no geological 318 Ring Complexesand Related Structures The Cameroon Line: A Review 319

map of this massif has been published: nevertheless,a geomorphological map is avail- of Garoua (Koch 1959, Ngounouno, in prep.). Their age is post-Cretaceous as they able (Morin, pers. comm.). Trachyticactivity occurred between 22.7 and 15.9Ma and have displaced and metamorphosed the Garoua sandstones attributed to the Upper basaltic activity between 14.9and 5.8 Ma (Gouhier et al. 1974;Dunlop 1983).Early Cretaceous. Similar plugs in the Upper Benue valleyin Nigeria have given K/Ar ages basaltic lava series have been described (Geze 1943, 1953; Gouhier et al. 1974). between 22 and 11Ma (Grant et al. 1972). The basaltic lavas are olivine-phyric and The basaltic lavas are restricted to olivine-phyric basalts and hawaiites; the salic monchiquite with phenocrysts of augite, barkevikite and olivine is also present. lavas are quartz-trachytes and trachy-phonolites (Thble2). Aegirine-bearing trachytes are the commonest evolved lavas (Table2).

5.13 Oku massif 5.15 Kapsiki Plateau

The Oku massif is a large volcanic group composed of four major adjacent strato- The volcanic effusions are located between 10°25' and 10°00' of lat. Nand 10°35' and volcanoes: Mount Oku, Mount Babanki, Nyos and Nkambe massifs. 10°40' of long. E, on the metamorphic basement of the Kapsiki plateau which is part- Mount Oku (Lissom, in prep.) is a tentacular strato-volcano. It culminates at ly coveredby LowerCainozoic sandstones (Dresch 1952).Basaltic and trachy-rhyolitic 3011m above sea leveland rests upon an intrusive basement. Initially basaltic lavas volcanisms are more or less contemporaneous (27 to 35 Ma; Gouhier et al. 1974; were erupted (Peronne 1969);they are now very altered. Trachytic lavas then erupted Dunlop 1983). The basaltic lavas appear as large flows or small domes, and the and can be seen as xenoliths in later flows. They werefollowed in time by voluminous trachy-rhyolitic lavas as 60 plugs more or less deeply eroded to form spectacular, up rhyolitic and trachytic ignimbrite deposits which may reach more than 1000m in to 100m high, spines. The basaltic lavas are olivine basalts and the differentiated thickness. These ignimbrites werein turn partially covered by younger trachytic lavas lavas are alkali, aegirine quartziferous, biotite-bearing or amphibole quartziferous- and volcanic tuff and breccia. Finally, this last formation was overlain to the north trachytes and peralkaline or hypersiliceous rhyolites and trachytic breccias (Gouhier by summital basaltic lava flows rich in peridotite and pyroxenite nodules, and by 1978); these lavas contain around 100/0of alkalies (Thble2). rhyolitic and phonolitic lavas to the south. This last event occurred when severalsmall basaltic pyroclastic cones, with explosive craters, erupted in Present times. The Oku Lake occupied one of these craters. 5.16 Other Volcanic Regions in Cameroon Three stratigraphically uncorrelated lavas of Mount Oku massif have given K/Ar ages 17Ma (hawaiite,Gouhier et al. 1974)and 22 and 23 Ma (rhyolite and ignimbritic The Adamawa plateau is covered by large post-Cretaceous highly lateritized basaltic rhyolite respectively,Dunlop 1983). lavas (Guiraudie 1955; Lasserre 1961). Two large basaltic-trachy-phonolitic strato- The basalts contain phenocrysts of forsteritic olivine and augite disseminated in a volcanoes (Ngan'ha: Nono 1987;Djinga: Ezangono, in prep.) are known. Numerous matrix of plagioclase,augite and Fe-Ti-oxides.The trachytic lavas contain phenocrysts trachy-phonolitic plugs associated with basaltic lava flows and necks are observed in of sanidine and sporadically of hedenbergite, fayalite, arfvedsonite, richterite and the Tchabal Mbabo (2460m above sea level)area (Nono 1987). Similarly, numerous aenigmatite (Lissom, in prep.). trachy-phonolitic plugs associated with basaltic lava flows (Deruelle et al. 1987b) Little has been published about the geochemistry of the lavas of Mount Oku; it has of Miocene-Pliocene age (Gouhier et al. 1974)are known to the north and east of been estimated that they form a strongly bimodal suite (Fitton 1987). Ngaoundere. Recent strombolian cones (Temdjim 1986) are located South of Mount Babanki is located southwest of Mount Oku. It is a strato-volcano of 15km Ngaoundere. diameter which emitted basaltic and trachytic lavas. Its summital area is occupied by explosive craters. 5.17 Conclusion The Nyos massif is located northwest of Mount Oku and is composed of several cones with large explosive craters in places occupied by lakes (Njopi, Lwi and The petrography of the lavas of the Cameroon Line is typical of the alkaline series. Nyi = Nyos) infamous for the gas explosion of August 1986, the magmatic origin of All the distinct terminology of the alkaline assemblages is present in the Cameroon which has not yet been demonstrated (Freeth 1987;Tchoua 1987). The lavas of this Line: picrites, alkali-basalts, hawaiites, mugearites, benmoreites, phonolites, trachytes massif were emitted as flows and pyroclasts of basaltic nature. and alkali-rhyolites. As can be deduced from the authors' fieldwork and from the The Nkambe massif is located northeast of Mount Oku and is made of basaltic and trachytic lavas. observation of the geological maps of the volcanic massifs of the Cameroon Line (Fig. 11 to 17), the basaltic lavas are largely predominant over the trachy-phonolitic ones. Some volcanic massifs are exclusively composed of basaltic lavas (Mount 5.14 Garoua Upper Benue Valley Cameroon, the three volcanic massifs of Bioko Island); on the contrary, phonolitic lavas are commoner than basaltic ones in the south of Principe Island, and trachytic 1\venty five trachytic plugs (from less than 1km to more than 4 km diameter) and lavas are abundant at Mount Oku and in the Garoua Upper Benue valleyand rhyolitic eight basaltic outpourings are numbered in the Upper Benue valleyin Cameroon, west lavas at the Kapsiki Plateau. 320 Ring Complexes and Related Structures The Cameroon Line: A Review 321

Alk 6 Discussion About the Nature of the Cameroon Line

Among the various interpretations of the Cameroon Line, the "rift system" hypothe- 14 sis has been particularly favoured in the last 10 years (Fitton and Hughes 1977; Dunlop and Fitton 1979;Dunlop 1983;Fitton 1983, 1987;Halliday et al. 1988).This hypothesis is founded on the similarity in shape between the Benue trough and the .. Fig. 18a, b. SiOralkalies geographical distribution of the post-Cretaceous volcanic rocks of Cameroon and of diagram for the lavas of the .. Eastern Nigeria (Fig. 7). Following the same idea, numerous lavas have been in turn .. Cameroon Line. . Pagalu 8 ... (data from Cornen and qualified as transitional by these authors. Nevertheless, no structural, min.eralogical \. .. .. Maury 1980 and Fitton or geochemical data have been put forward by them to prove their "rift system" hy- ~\.t ...... 1987); . Sao Tome (data pothesis. On the contrary, in the light of recent structural analyses on the Benue .4 e.~ from Mitchell-Thome 1970 trough as well as on the Cameroon Line (BenkhelilI986; Moreau et al. 1987b) this ~...... and Fitton 1987);.. Prin- last appears to result from the rejuvenation of the Adamawa fault zone, a Pan-African ... tt~~... .. cipe (data from Mitchell- 2 . Thome 1970 and Fitton fracture reworked during Albian-Aptian times and which could have initiated "en 1987); 8 Bioko (data from echelon" mega-tension gashes within the Cameroon Line during a Cainozoic left- a Deruelle and Kambou 1988 40 60 Si~X lateral transcurrent movement. and Mitchell-Thome 1970); In WestAfrica, close relationships exist betweenpost-Hercynian tectonics and deep Alk *' Etinde (data from Mouafo, in prep.; Fitton structures of the Pan-African mobile zones (Moreau et al. 1987b; Guiraud et al. 1987) 1987; Deruelle, unpubl.); possibly inherited from.Proterozoic events. The mylonites of the Adamawa fault zone 14 o D Mount Cameroon (data are of Pan-African age (Koch 1953;Cornacchia and Dars 1983).The continental seg- from Deruelle et al. 1987a; ment of the Cameroon Line (from Mt Cameroon to Lake Chad) should be considered Fitton 1987; Geze 1943; as a Pan-African palaeosuture. The alignement of synkinematic leucogranites along o Jeremine 1943); b. Rumpi o · Hills (data from Nkoumbou, the N 300 trend and the alignment of older (around 630 Ma, Toteu et al. 1986, 1987) o in prep.; Geze 1943); granites according to the same trend are noteworthy. . o ~ Manengouba (data from The Cretaceous reworking of the accidents in the Cameroon-Nigeria provinces has 8 o Tchoua 1974a; Fitton 1987); been diversely interpreted. Geophysical studies seem to reveal that the now inactive o Bambouto (data from Tchoua 1974a; Fitton 1987); West-African Rift system [Benue trough - Gongola rift and Yola (Garoua) rift] is o Mount Oku (data from most typical of the early stage development in passive continental margin structures Fitton 1987). Limits between (Bermingham et al. 1983), whereas the active East-African Rift is a candidate for the alkaline (above) and developing into a plate margin (Browne and Fairhead 1983; Fairhead and Okereke 2 subalkaline (below) fields after Macdonald and Kat- 1987).The high topography surrounding the Benue rift system is caused by low-densi- b ty, low-velocityupper mantle (Dorbath et al. 1986). Conversely, structural studies 40 60 sura 1964 (dashed line) and Miyashiro 1978 (full line ) (Benkhelil 1986;Benkhelil and Robineau 1983;Moreau et al. 1987b) favour the ex- istence of a series of intraplate pull-apart basins created by intracontinental transcur- rent movements during Cretaceous times, instead of a rift system. Peridotitic xenoliths are commonly found in the lavas of the islands of the Gulf of Likewise,extensivepetrological studies of the Cameroon Line volcanicand plutonic rocks do not allow the conclusion that the magmatism develops a transitional trend Guinea (Annob6n, Bioko) as wellas in those of the continental part of the Cameroon as observed in the East-African Rift. Line (Mount Cameroon, Kumba, Mount Oku). Olivine is only found as phenocrysts or microphenocrysts and never appears in the In the anorogenic plutonic massifs of the Cameroon Line that havebeen consistent- matrix of the lavas. Clinopyroxene develops a complete series from calcic-augite to ly studied (see above), the clinopyroxene evolution always corresponds to alkaline ferroan-augite. Hypersthene is found only as xenocrysts. suites; amphibole trends (Fig. 19) are similar to those of other alkaline massifs. The geochemistry of the lavas of the Cameroon Line clearly indicates their alkaline Geochemical data, especially rare earth element trends are characteristic of alkaline affinity (Fig. 18).The lavas of the oceanic segment as well as those of the continental series. Nevertheless, controverse resulting from misinterpretation of published data arises concerning the transitional character of some peculiar suites. For example, segment present similar geochemical element distributions (Fitton and Dunlop 1985). about the occurrence of hypersthene-bearing gabbros in the Mboutou complex (Par- The lava series of the two segments commonly present a Daly gap. sons et al. 1986)these authors clearly explain that "Mboutou was, at the outset, only .. very mildly alkaline". They never conclude, however,to a primitive transitional nature 322 Ring Complexes and Related Structures The CameroonLine:A Review 323 Ca+AIIV Ne Q

D...n E 1M .... 3 o ------Fig. 19. Compositional trend of am- 8. ~1fi!J.00 0 8 DO ~ 8 phiboles of Nda Ali plutonic rocks . . (number of cations on the basis of 23 *: . oxygens. The dashed line suggests the .~8~ breakdown of Ca-AI rich amphiboles l .8 8 0 o ... L when Nap + K20/AlP3 < 0.9, (after 8 tJ8 Or Giret et al. 1980). E and L respectively . D8 .. -IlL ...... early and late magmatic stages; o basic rocks; . intermediate rocks; 7 9 Si+Na+K 11 * syenites s. s.; . quartz-syenites

Wo

, . 01 Fig. 21. Ne-Ol-Di-Hy-Q normative diagram for basaltic lavas of the Cameroon Line (with wtOJoMgO less than 6 or D.I. of Thornton and Thttle 1960, less than 35). Same symbols as in Fig. 18 except *: transitional basaltic lavas of Ethiopia (Barberi et al. 1975b). (Data from Pagalu, after Cornen and Maury 1980 and Fitton 1987; from Sao Tome and Principe, after Fitton 1987; from Bioko, after Deruelle and Kambou 1988; from Mount Cameroun, after Deruelle et al. 1987 a and Fitton 1987, and from Manengouba, Bambouto and Oku, after Fitton 1987); tholeiites of Iceland: 0 (data from lakobsson et al. 1978)

interaction of the alkaline mantle-derived magma with the granitic envelope rocks, as indicated by 0-, Sr- and Pb-isotopic geochemistry (Jacquemin et al. 1982). The mineralogic, petrographic and geochemical characteristics of the lavas of the En Fs Cameroon Line are typical of alkaline lava series. Pigeonite as well as hypersthene phenocrysts are unknown in these lavas and chro- Fig. 20. Comparison between the compositions of pyroxenes of Principe (.) (data from Fitton and Hughes 1977, Fig. 7), Bioko (*) (data from Deruelle and Kambou 1988), and Mount Cameroon (.) mian spinel has never been found in the basaltic units; augite is rich in calcium (data from Deruelle et al. 1987b) and those of the main transitional lavas of Ethiopia (*:) (data from (Fig. 20)and alumina compared to that in the transitional series of Ethiopia or Kenya. Barberi et al. 1972a) and Kenya (0) (data from Bellieni et aI. 1981) All the lavas of the Cameroon Line have alkali contents typical of the alkaline series (Fig. 18). The contents are higher than 2.5% at 45% Si02 and higher than 9% at 60% Si02. The basalts are richer in potassium than the transitional basalts of the of the magma. Naturally, they emphasize that in the Mboutou "more evolved East-African Rift. All the basaltic lavas of the Cameroon Line with a differentiation members embarked a line of evolution with some calc-alkaline characteristics". But index (Thornton and Thttle 1960)less than 35 and/or MgO higher than 6 wtOJo(except they demonstrate that orthopyroxene never coexists with olivine and is not in for some rather weathered samples) contain normative nepheline and are not to be equilibrium with clinopyroxene on the two pyroxene surfaces. They also specify that compared with basalts of the transitional series of Ethiopia or Zaire (Fig. 21) which in terms of volume, at the present exposed level, the syenodioritic and more evolved usually contain normative hypersthene. The hypersthene normative lavas of the members of the Mboutou complex are insignificant and that they possibly appear in Cameroon Line are not basalts s.s. but hawaiites, mugearites and other, more dif- residual pockets of magma probably involving introduction of water from the ferentiated, lavas. envelope rocks. So the transitional and even calc-alkaline line of evolution they em- Numerous discriminating relationships of trace elements havebeen proposed to dis- barked on is not a genuine characteristic of the magma source; it results from later tinguish the tholeiitic from the alkaline series. According to them, the basalts of the 324 Ring Complexes and Related Structures The Cameroon Line: A Review 325 , , r Transitional o ......

CO L>.O @) , , , 0.3 0.5 0.7 2 y YNb Fig. 24. Distribution of Y/Nb ratios in basaltic lavas (see Fig. 21 for their definition) of the oceanic (0) and continental (C) sectors of the Cameroon Line. The full line corresponds to transitional basaltic rocks (after Pearce and Cann 1973). Same symbols and data source as in Fig. 21 and 22

Fig. 22. Ti-Zr-Y.diagram for basaltic lavas (see Fig. 21 for their definition) of the Cameroon Line. Same data sources and symbols as in Fig. 21; other data from Pagalu, after Liotard et al. (1982). Fig. 25. Coryell-Winchester 100 f::. Rumpi Hills (data after Nkoumbou 1990); . transitional basaltic lavas of Zaire (data after diagram for lavas of Mount Kampunzu et al. 1986). WPB within-plate basalts; LKT low-K tholeiites; OFB ocean-floor basalts; Cameroon (D, hatched CAB calc-alkaline basalts (after Pearce and Cann 1973) area), data after Deruelle et al. (1987a) compared to transitional lavas of Ethiopia (*, dotted area) data after Barberi et al. (1975b). Also indicated are transitional lavas of Zaire (.), data Alkali-basal t after Karnpunzu et al. 1986, s . 10 and the trend ( ) for Tholeiites 3 @

thus plot near the zirconium pole of the within-plate basalts. The Nb-Ti02 diagram 1 10 100 Nbppm is better discriminative (Fig. 23) than the previous one due to the niobium content of Fig. 23. TiOrNb diagram for basaltic lavas (see Fig. 21 for their definition) of the Cameroon Line. the alkali basalts which have more than 50 ppm Nb. Three basaltic lavas from Mt Same symbols and data source as in Fig. 21. The full line delimits the field of tholeiitic lavas and the Oku, Manengouba and Principe with 46, 47 and 49 ppm of niobium respectivelyhave dashed line the field of alkaline lavas (after Hekinian 1982). . transitional basalts o(Zaire (data after Kampunzu et al. 1986) been qualified as transitional basalts (Fitton 1987),in spite of the fact that they are either plagioclase-phyric or have a differentiation index (36.6< D.I. < 37.8) of hawaiites (Fitton 1987).Nevertheless, such Nb contents are higher than those found Cameroon Line, without exception, are typical within-plate basalts. This fact appears in transitional lavas from Afar or from Zaire and in tholeiitic lavas. Moreover, the clearly in the Ti-Zr-Ydiagrams (Fig. 22). As the samples represented in this figure are Y/Nb ratio of the basaltic lavas of the Cameroon Line is lower than 1 (Fig. 24), both not primitive (they may contain less than 4070MgO and may have D.I. higher than for the lavas of the continental and oceanic segments. All the lavas have higher light 35), some of them havehigh zirconium content relativeto undifferentiated basalts and rare earth element contents than transitional lavas (Fig. 25). Lastly, in the Th-Hf-Th 326 Ring Complexes and Related Structures The Cameroon Line: A Review 327

The authors are grateful for the logistic support during fieldwork in Cameroon pro- vided by the Faculty of Sciences (Deans: Profs. M. Bopelet, then S.M. EnD Belinga, then G. Valet)University of Yaounde,the University Centre of Ngaoundere (Dir. Prof. E. Ndomgang) and the Centre of Geology and Mines of Garoua (Dir. B. Nyobe). The local authorities and the populations are gratefully acknoWledged for their hospitality. Financial support for the chemical analyses came from the French Embassy in Cameroon (Conseiller Culturel, C. Montandreau), the Laboratoire de Petrologie (Dir. Prof. G. Rocci) Universite de Nancy I~ Nancy, the Laboratoire de Petrologie Mineralogique (Dir. Prof. Danielle Velde)Univ. Pierre et Marie Curie, Paris, the In- stitute for Geological and Mining Research (Dir. Soba Djallo) Yaounde,the Faculty of Sciences (Dean Prof. G. Valet) Univ. of Yaounde and the Centre de Recherches Petrographiques et Geochimiques (K. Govindaraju and J. Leterrier), Nancy. Research grant from Elf Serepca, Douala, for studies of Mount Etinde and the Rumpi Hills (C.N.) is gratefully acknowledged. The authors are indebted to G. Rocci, C. Coulon, R. Black, A. B. Kampunzu, Felici- ty E. Lloyd, L. Aguirre and to other anonymous reviewersfor constructive comments on an earlier draft of this paper.

Fig. 26. Th.:Ja-Hf diagramm for the basaltic lavas (see Fig.21 for their definition) of the Cameroon Line. 0 Mont Cameroun (data after Deruelle et al. 1987a); . Bioko (after Deruelle, in prep.); EEEtinde (Deruelle, unpubl.). Transitional basaltic lavas from Ethiopia <*; data from Barberi et al. 1975b) are indicated. A and B tholeiitic field; C alkaline field and D calc-alkaline field (after Wood et aI. 1979)

diagram (Fig. 26) all the analyzed basaltic lavas of the Cameroon Line plot inside the alkaline field (Wood et al. 1979). Weconclude that the magmatism of the Cameroon Line is entirely alkaline in char- acter and not transitional as has been repeatedly advanced (Fitton and Hughes 1977; Dunlop and Fitton 1979; Fitton 1983, 1987).

Acknowledgements. The present study was initiated in Cameroon during stays (B.D., C. M.) at the Department of Earth Sciences of the Faculty of Sciences(Heads: Profs. E. Tchoua, then S.M. Eno Belinga) of the University of Yaounde under the auspices of the French Cameroonian Scientific and Technical Cooperation. Fieldwork in Equatorial Guinea was possible thanks to the French Embassy and to the Depar- tamento de Minas e Hidrocarburos (Dir. H. Bahamondes). Petrological and geochemical studies were carried out in France during stays in the Laboratoire de Petrologie (Dir. Prof. G. Rocci)Universite de Nancy I, Nancy (R.T.G., E. N., A. N., C. N.), the Laboratoire de Petrologie Mineralogique (Dir. Prof. Danielle Velde)Universite Pierre et Marie Curie, Paris (A. N., C. N., J. L.), the Laboratoire de Magmatologie et Geochimie Inorganique et Experimentale (Dir. Prof. A. Jambon) Universite Pierre et Marie Curie, Paris (B.D.), the Laboratoire Pierre Sue (Dir. Prof. M. Treuil), Saclay, France (R. K.).

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