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The geochemical characteristics of some plutonic-volcanic complexes along the southern part of the Cameroon line
Journal of~91can Earth Sciences, Vol. 14, No. 2, pp. 255-266, 1992. Printed in Great Britain
0899-5362/92 $5.00+0.00 © 1992 Pe~amon Preu Ltd
The geochemical characteristics of some plutonic-volcanic complexes along the southern part of the Cameroon line E. NJONFANG*,P. KAMC;A~**, R. T. GItOGOMU**and F. M. TCHOUA** *F.coleNormale Sup~rieure, D6partement des Sciences Biologiques,B.P. 47, Yaound~, Cameroun **Facult6 des Sciences, D6partementdes Sciences de la Terre, B.P. 812, Yeound~, Cameronn (First received 3 April, 1991; revised version received 2 August, 1991) Abstract- About 60 volcenic-plutonicalkaline complexesof Tertiary age are located along the continental part of the Cameroon Line. This paper reports the chemicalnature and evolutionof six of them (Gwenfalabo,Nmmbaw,Nkogam, Nda AlL Bana and Koupe), all located in the southern part of this line. The geochemicaldata (major, trace and REE) of these six complexesreveal the following general behaviour: - differentiationfrom aprimary magma of intermediate compositionsituatedin the well-knownDaly gap. The absence of any discrimination between the differentcomplexesin geochemicaldiagrams suggeststhat they have a unique source. This model is defined by three co-magmatic rock-types: cumulative basic rocks composed of anorthositic gabbros, residual acidic rocks with abundant syenites and granites and primitive intermediate rocks mainly represented by monzodiorites and monzonites of the Ntumhaw complex; - emplacementof the acidicrocksby successivepulses, as illustratedin the Rb/SrversusSr diagramby threesubparallel trends, corresponding (with increasing Sr contents) to fayalite syenites, other syenites and granites respectively. From Rb/Sr isotopic data, the Nkogam granite is 67 Ma-old, while the fayalite syenite of the Mount Koupe is 46 - 50 Ma-old. Rb/Sr isotopic data of acidic formations of the other complexes and stable isotopes are needed.
GEOLOGICAL SETTING OF THE COMPLEXES STUDIED
INTRODUCTION The alkaline c o m p l e x e s of C a m e r o o n a r e situa t e d along t h e c o n t i n e n t a l p a r t of t h e C a m e r o o n Line (Fig. I). T h e y a r e essentially m a d e u p of large v o l c a n o e s (Cameroon, M a n e n g o u b a , B a m b o u t o ...) a n d of over 6 0 intrusive m a s s i f s c o m m o n l y referred to a s t h e "granites ultimes" (Lasserre, 1966). T h e y a r e generally a s s o c i a t e d to late volcanic f o r m a t i o n s a n d therefore, are called plutonicvolcanic c o m p l e x e s . G r a n i t e s a n d / o r s y e n i t e s cons t i t u t e t h e m a j o r r o c k types, while b a s i c (gabbros) a n d i n t e r m e d i a t e (diorites a n d monzonites) r o c k s are s u b o r d i n a t e . However, exceptions are f o u n d in t h e N t u m b a w c o m p l e x (Ghogomu, 1984), Westcentral C a m e r o o n , w h e r e i n t e r m e d i a t e r o c k s are p r e d o m i n a n t a n d in t h e M b o u t o u c o m p l e x ( J a c q u e m i n et al., 1981), North C a m e r o o n , w h e r e b a s i c r o c k s a r e a b u n d a n t . Detailed geochemistry, m i n e r a l o g y a n d isotopic s t u d i e s have b e e n perf o r m e d only for t h e M b o u t o u c o m p l e x ( J a c q u e m i n , 1981; J a c q u e m i n eta/., 1982; P a r s o n s et a t , 1986) a n d d a t a s h o w t h a t t h e r o c k s are of m a n t l e derivat i o n w i t h e v i d e n c e of c r u s t a l c o n t a m i n a t i o n . Similar r e s u l t s have b e e n e s t a b l I s h e d for m o s t of t h e o t h e r C a m e r o o n c o m p l e x e s (Cantagrel et a t , 1978; Lasserre, 1978). ThIs is also t h e c a s e of o t h e r c o m p l e x e s of t h e s a m e type, like t h e y o u n g e r g r a n i t e s of Nigeria (Van B r e e m a n et al., 1975; B o w d e n et al., 1987) a n d t h e Evisa c o m p l e x of Corsica (Bonin et al., 1978).
The Gwenfalabo (llalrnl) c o m p l e x Is situat e d b e t w e e n longitude 12" 22' a n d 12" 29' E a n d latitude 7" 42' a n d 7" 48' N. It h a s a n a r e a of a b o u t 2 0 0 k m 2 a n d is essentially m a d e u p of p l u t o n i t e s like g a b b r o s , diorites, m o n z o n i t e s , s y e n l t e s a n d granites, while b a s a n l t o i d s , b a s a l t s , rhyolltes, t r a c h y t e s , pantellerites a n d ignimbrites c o n s t i t u t e the volcanites (Ngonge, 1988). The N t u m b a w c o m p l e x o u t c r o p s a b o u t 15 k m SE of N k a m b e , in t h e M b a w plain (West Central Cameroon) on longitude 10" 54' E a n d latitude 6" 22' N. It Is m a d e u p of t w o J u x t a p o s e d c e n t r e s (cl, c2). The principal r o c k - t y p e s a r e monzodiorites, m o n z o n i t e s a n d q u a r t z - m o n z o n i t e s for cl a n d syenogranites, quartz-syenites, acid d y k e s a n d g r a n o p h y r e s for c2. T r a c h y t e a n d q u a r t z - p o r p h y r y d y k e s are present. C o u n t r y g r a n i t e - g a e i s s e s are partially covered b y b a s a l t s ( G h o g o m u , 1984; G h o g o m u et a t , 1989). The Nkoga__m c o m p l e x is l o c a t e d in t h e B a m o u n region (West Cameroon) b e t w e e n longit u d e s 10"38' a n d 10" 40' E a n d l a t i t u d e s 5" 35' a n d 5" 47' N. C u l m i n a t i n g at 2 2 6 8 m a b o v e s e a level. This c o m p l e x Is c o m p o s e d of t h r e e m a i n units: b a s a l t s , ignimbrites a n d p l u t o n i c r o c k s intruding a mlgmatitic b a s e m e n t (Kamgang, 1986). The Nda All c o m p l e x is s i t u a t e d in t h e Marnfe plain s o m e 32 krn SE o f t h e town, b e t w e e n
255
256
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8
The geochemical characteristics of some plutonic-volcanic complexes along the southern pan of the Cameroon line 257 longitude 9" 27' a n d 9" 33' E a n d latitude 5" 32' a n d 5" 37' N. Here, t h e plutonic rocks form a m a g m a t l c s e q u e n c e comprising: gabbros, dlorites, monzonltes a n d syenites to which are associated m a g m a tic breccias. The volcanic rocks include co-geneUc benmoreltcs, phonolttes a n d trachyte d o m e s which are posterior to co-magmatlc hawaflte a n d trachyte dykes. This complex is intrusive in t h e Mamfe s a n d s t o n e s except in the s o u t h w e s t border where t h e c o u n t r y rock ks granitic (NJonfang, 1986). The Balm complex is located 14 k m East of Bafang (West Cameroon). Plutonic rocks here include leucogabbros, monzodiorites, m o n z o n i t e s a n d granites, while volcanic rocks comprise interm e d i a t e a n d acidic types (mugearites, b e n m o r e i t e s a n d rhyolites) to which are associated welded tufts (Nana, 1988). The Koupe complex lies between longitude 4" 43' a n d 4" 52' E a n d latitude 9" 40' a n d 9" 47' N, covering a n area of about I 0 0 krn 2. It is mainly c o m p o s e d ofgabbros, syenites, microgranites a n d microsyenites i n t r u d i n g a n orthogneissic basem e n t complex. The volcanic activity essentially c o n s t i t u t e d of pyroclastic rocks a n d basaltic lava flows, ks relatively recent a n d ks f o u n d towards t h e periphery of t h e complex (Lamilen, 1989).
GEOCHEMISTRY Recent chemical d a t a of t h e complexes studied (table) are t a k e n from Ghogomu, 1984; Kamgang, 1986; NJonfang, 1986; Nana, 1988; Ngonge, 1988 a n d Lamilen, 1989. E a c h of these a u t h o r s s t u d i e d the b e h a v i o u r of t h e major a n d trace elements d u r i n g the e m p l a c e m e n t a n d evolution of the volcanic a n d plutonic formations of their respective complexes. All are alkaline m a g m a t i c complexes showing a peralkaline t e n d e n c y in the differentiated types. Fractional crystallization ks the m a i n m e c h a n i s m of differentiation. However, local dispersions often observed in the basic formations are attributed to their cumulative c h a r a c t e r while, for the other rocks, contamination and postmagmaUc p h e n o m e n a are very c o m m o n . We have plotted chemical d a t a (table} in some classical d i a g r a m s to observe possible relationships. Major elements: In t h e Na20 + K20 / SIO 2 diagram, (Fig. 2) the plutonic formations form two sets: t h e first one ks c o n s t i t u t e d of basic a n d intermediate rocks (SiO 2 < 54%) a n d the s e c o n d m a d e u p of acidic rocks (syenites a n d granites) with SiO 2 v a l u e s ranging between 60 a n d 77.5 %. These two sets are s e p a r a t e d by a small group of intermediate rocks with SIOz values ranging between 54 a n d 60 % {Fig. 2a). The volcanic formations s h o w a similar evolution except for t h e u n d e r s a t u r a t e d t r e n d in t h e differentiated types
(Fig. 2b) which occupies the d o m a i n of intermediate a n d evolved lavas of t h e oceanic sector of t h e C a m e r o o n Line (Fltton, 1987}. Similar results are observed in t h e TiO2/8iO 2 d i a g r a m (Fig. 3}. Here, basic a n d intermediate rocks fall in t h e d o m a i n s of New E n g l a n d - Q u e b e c (NEQ) dykes a n d p l u t o n s , a n d acidic r o c k s in t h a t of WhRe M o u n t a i n Magma Series (WMM8) plutons, which belong to the Mesozoic igneous province alkaline rocks of New E n g l a n d (McHone a n d Butler, !984). In the C a O / K 2 0 / N a 2 0 triangular diagram {Fig. 4), a c o n t i n u o u s evolution is observed from basic to acidic rocks m a r k e d by t h e progressive enrichm e n t in t h e alkalies: - basic rocks are calclc-sodlc, intermediate ones are calctc-sodlc-potasstc while acidic r o c k s are s o d i c - p o t a s s i c w i t h a m o r e potassic t e n d e n c y for t h e granites a n d a m o r e sodic t e n d e n c y for syenites. Nigglt's d i a g r a m (1923} modified by d a c o b s o n eta/. ( 1958} (Fig. 5a) ks often u s e d to illustrate the chemical relationship existing between the rocks of the Younger Granites of Nigeria. In this diagram, basic rocks s h o w a general s i m u l t a n e o u s e n r i c h m e n t in a l u m i n a a n d t h e alkalies, while in the detail, acidic rocks t e n d to show two m a g m a t i c trends: a n a l u m i n o u s to pera l u m i n o u s t r e n d a n d a lesser m a r k e d peralkaline trend. The Q-Fath-Color diagram (Fig. 5b} of De la Roche (1966) shows ordy two sets of rocks a n d not three as previously described by Tempier a n d Lasserre (1980): a first set c o m p o s e d of basic a n d intermediate rocks, while a s e c o n d set s h o w s a gradation from syenites to granites. Between these two s e t s are a few i n t e r m e d i a t e r o c k s from Gwenfalabo. Trace elements: The b e h a v i o u r of Rb a n d Sr in the rocks s t u d i e d is illustrated by the diagrams Rb=f(Sr) (Fig. 6a) a n d Rb/Sr=f(Sr) (Fig. 6b). I n t h e s e diagrams, despite a general dispersion of points, from the basic to t h e acidic types, there ks a positive correlation u p to the intermediate types where there ks a n inflection followed by a negative correlation towards t h e acidic rocks. Major basic rocks with a few syenites from Gwenfalabo are found at the inflection point. In t h e acidic types, three m a i n subparallel t r e n d s are found: the first one is mainly m a d e u p of the fayalite syenites of Koupe, t h e second g r o u p s t h e other syenites a n d the third one are c o n s t i t u t e d by t h e granites. The discrimination between t h e intermediate rocks of N t u m b a w a n d the rest of plutonites ks better s h o w n in the L a / Sm=llLa) (Fig. 7a) a n d La/Yb=f(La) (Fig. 7b) diagrams. The former shows a vertical gradation between the basic (La/Sm=4-5) a n d t h e acidic (LA/ Sm=4.5-5.5) plutonites, while t h e volcanites are dispersed. On t h e contrary, t h e latter shows a horizontal gradation (parallel to La) from the basic plutonites to the m o s t evolved volcanites passing t h r o u g h t h e acidic plutonites.
258
E. NJONFANG, P.
KAMGANG,
R. T. GHOC~MUand F. M. TCHOUA
Table. Major ( % wt) and trace (ppm) elements of plutonic and volcanic r o c k s of the complexes studied. Gabbros: 1-5(Koupe);6-9[NdaAl/}; 10-11 (Ba~: 1 2 - 1 6 (GwerLfa|mbo). D i o H t e s : 1 7 - 1 8 ( N d a A l i ) ; 19 ( B a n a ) ; 2 0 - 2 2 { G w e n f a l a b o ) ; 2 3 - 2 4 ( N t u m b a w ) . M o n z o n i t e s : 2 5 - 2 6 (NdaAli}; 2 7 - 2 9 ( G w e n f a l a b o } 3 0 - 3 4 ( N t u m b a w ) . S y e n i t e s : 3 5 - 4 1 {Koupe); 4 2 - 4 6 { N d a A l 0 . 4 7 (Bana): 48-52 (Gwenfalabo); 53-55 (Ntumbaw); 58-62 (Bana); 63-67 (Gwenfalabo); 68-72 (Nkogarn). Granites: 56-57 (Koupe); ;t111,+~ I I I I 1 ! 3 I 5 !1 I1 11 11 11 15 In 17 17 !77 ?!3 LI;] LDI] LII! h'IT! h!I1 h?I hlll JI! Y!I 111 Nil nil! 1111 D,~t ~i++: 52.11 11.11 41.10 ~3.11 51.31 !102 1.15 i.21 3.32 1.11 !.31 li~0,1 II+SI !1.!1 23.11 I K | ! 15.11 h:~3 5.45 :.41 7.11 I I . l ! 11.3t h0 I 45 I. i l laO ,is o.~= ~:~, ,:11 3:11 ld0 hD 12b
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The geochemical characteristics of some plutonic-volcanic complexes along the southern part of the Cameroon line 263 other evolving towards syen/tic a n d granitic compositions. Consequently, t h e p r i m a r y m a g m a t i c The probable e x i s t e n c e o f o n l y o n e m a g m a t i e liquid is n e a r t h a t of a n intermediate rock constit u t e d of aggregates of maflc minerals a n d plag/osonrce The b e h a v i o u r s of the major a n d trace ele- clase in a syenitic matrix. S u c h a r o c k w a s n a m e d m e n t s (except L a / S m a n d La/Yb) show no distinc- leopard-monzonite. The c u m u l a t i v e c h a r a c t e r of tion between t h e different complexes studied. This C a m e r o o n b a s i c r o c k s h a s b e e n d e s c r i b e d suggests t h a t they all have similar geochemical (NJonfang, 1986; Nana, 1988 a n d Ngonge, 1988). characteristics. However, Tempier a n d Lasserre This is illustrated a m o n g other t h i n g s b y average to (1980) interpreted the gap observed in t h e Q-Fath- high AI20s values (12-24%), close to basic anorthoColor d i a g r a m as two i n d e p e n d e n t m a g m a t i c sltlc rocks of t h e Air m a s s i f (Niger) a n d by positive sources (the one syenitic a n d t h e other basaltic) Eu-anomal/es. Also in s o m e intermediate rocks, evolving in a convergent way towards the granites. are f o u n d basic (kaersutlte a n d bytownite) a n d / o r Mineralogical s t u d i e s (Ghogomu, 1984; G h o g o m u evolved (Or~Ab4~n 5) mineral p h a s e s (NJonfang, etal., 1989; NJonfang, 1986; Nana, 1988; Ngonge, 1986). We therefore t h i n k of a divergent differen1988 a n d Lamilen, 1989) of t h e different com- tiation of alkaline suite from a m a g m a of interplexes s h o w a gradation from basic a n d inter- mediate composition. The presence of ultrabasic mediate types to syenitic a n d / o r granitic types. enclaves in alkaline plutonic complexes is indicaThese a u t h o r s have noted a c o n t i n u o u s evolution tive of a m a n t l e derivation a n d / o r a c c u m u l a t i o n of n a t u r e , texture a n d c h e m i s t r y of feldspars. from a primary basaltic m a g m a (Giret eta/., 1983). Plagioclase c o n s t i t u t e s a b o u t 98 % of the feldspars The very significant absence of ultrabasic types in basic rocks a n d decreases with increasing m a k e s the classical h y p o t h e s i s of crystallization a m o u n t s of alkali feldspar in intermediate rocks from a p r i m a r y alkaline liquid of basaltic composi(diorites to monzonites). Textures with alkali feld- tion unlikely. s p a r bordering or englobing plagioclase are freAfter Demaiffe a n d Hertogen (198 I), Lameyre et q u e n t a n d nearly replaced by alkali feldspar in a/. (1982} a n d Giret (1983), Bonin a n d Giret (1985) syenites a n d granites. Chemically, feldspars are estimated t h e composition of t h e p r i m a r y m a g m a calcic to calcic-sodic in basic rocks, increasingly tic liquid of s o m e alkaline complexes of Africa a n d sodic in intermediate rocks a n d sodic-potassic to Corsica, b a s e d on a n interpretation of the Daly potassic in differentiated rocks. This c a n also be gap. In Cameroon, rocks whose compositions are portrayed by t h e chemical evolution ofmafic mine- n e a r those of primary liquids are monzogabbros, rals (olivine, pyroxene, amphibole a n d biotite), monzonites a n d quartz-monzonites of Ntumbaw. characterized by a progressive e n r i c h m e n t of Fe 3÷ These are examplified by t h e following characterisa n d Na ÷at t h e d e t r i m e n t ofMg 2÷, Fe z* a n d Ca 2÷. We tics: Na20, K20, TIO=, Al203, P=O5, Rb a n d Sr contherefore t h i n k t h a t the gap observed is r a t h e r d u e t e n t s are almost c o n s t a n t a n d REE p a t t e r n s are to the absence of a solid solution between maflc identical (see Table 4, G h o g o m u etoL, 1989). These minerals (Colour pole) a n d feldspars (Fath pole). similarities are consistent with a composition n e a r The anorogenic complexes of Alr (Niger) a p p e a r as t h a t of a relatively non- or weakly modified primary a good example of t h e e m p l a c e m e n t of basic rocks m a g m a by differentiation m e c h a n i s m s . Similar from the differentiation of a basaltic m a g m a inde- observations were m a d e by S i m m o n s a n d Hedge p e n d e n t of those of t h e acidic suites. This h a s been (1978) for quartz-syenites of t h e Tertiary Stocks in suggested by Moreau (1982) a n d H u s c h (1982) to Colorado. The p r i n c i p a l e l e m e n t s , w h i c h are explain t h e two distinct sets of rock types obtained m a r k e r s of the degree of evolution of primary in t h e a-cfm-al diagram. S u c h a discrimination is liquids (MgO, Sr a n d Ni), are m o r e c o n c e n t r a t e d in a b s e n t here where there is a direct link between these rocks: 7.7% MgO, 1496 p p m Sr a n d 183 p p m basic, intermediate a n d acidic rocks. Ni for average values. A s a m p l e of a chilled m a r g i n h a s 343 p p m of Ni, therefore faning in the position An e x a m p l e o f d i v e r g e n t d i f f e r e n t i a t i o n from a of m a x i m u m variation of Ni in p r i m a r y m a g m a s p r i m i t i v e m a g m a o f i n t e r m e d i a t e c o m p o s i t i o n estimated by Allegre et aL (1977) to range between (Daly gap). 300 a n d 500 ppm. High Ni-values of 569 p p m , The possibility of a divergent differentiation from obtained in a picritic basalt of Pagalu, are rare only one m a g m a was evoked by Tempier a n d (Piper a n d Richardson, 1972). As in micromonzoLasserre (1980) in t h e Nda A1/complex. It is illus- gabbro of M e u g u e u r - M e u g u e u r in Niger (Moreau et trated here by t h e b e h a v i o u r of Sr in relation to Rb. a/., 1986) a n d in t h e monzonitic suite of the Studying t h e alkaline plutonic rocks of Kerguelen Tertiary Stocks in Colorado (Simmons a n d Hedge, Islands, Lameyre et al. (1982) revealed a diver- 1978), rocks of the N t u m b a w complex are also gence between two tendencies: the one of c u m u l a - characterized by t h e p r e s e n c e of Eu-negative tive c h a r a c t e r a n d of gabbroic nature, a n d t h e a n o m a l i e s with E u / E u * = 0 . 8 5 - 0 . 9 5 (Ghogomu, DISCUSSION - CONCLUSION
264
E. NJONFANG,P. KAMGANG,R. T. GHOGOMUand F. M. TCHOUA
1984). Most a u t h o r s , s u c h a s Barley a n d Schairer (1966), D u s c h e s n e a n d Demaiffe (1978), H a n s o n (1980), B o w d e n (1985), M o r e a u et a t (1986) a n d Key (1987), a t t r i b u t e d Eu-negative a n o m a l i e s to the r e s i d u a l n a t u r e of t h e melt after plagioclase fractionation. The p r e s e n c e of this a n o m a l y in t h e N t u m b a w intermediate r o c k s c o u p l e d with high MgO, Sr a n d Ni (for Ni c o n t e n t s , see Table 3, G h o g o m u et al., 1989), v a l u e s is similar to t h e chemical c h a r a c t e r i s t i c s of a leopard-monzonite.
An e v o l u t i o n b y fractional c r y s t a l l i z a t i o n The preceding c h a r a c t e r i s t i c s indicate t h a t the r o c k s s t u d i e d w o u l d b e t h e p r o d u c t s of a divergent differentiation of a p r i m a r y m a g m a of intermediate composition. The m o s t plausible p r o c e s s is t h e e m p l a c e m e n t in t w o s t a g e s a s s u g g e s t e d b y Demaiffe a n d H e r t o g e n (1981) in t h e anorthositic s u i t e s of Hidra in Norway. There are first c u m u l a tive b a s i c r o c k s p r o b a b l y p r o d u c e d t h r o u g h / n situ differentiation. T h e y are c h a r a c t e r i z e d b y positive a n d increasing E u - a n o m a l i e s a s a function of SiO 2, (Eu/Eu*= 1.1-1.86). The b a s i c r o c k s are therefore fractionated. The r e s i d u a l liquids evolved t o w a r d s the acidic types. Positive E u - a n o m a l i e s in t h e less differentiated t y p e s (SiO2<65%) s h o u l d be n o t e d here while t h e m o r e evolved r o c k s have negative a n o m a l i e s (Eu/Eu*=0.75-0.60). Differentiation is governed b y fractional crystallization. L a / Y b profile is parallel to t h e a b s c i s s a , w h i c h is very c h a r a c teristic and, according to D u s c h e s n e a n d Demaiffe (1978) a n d R o c k (1978), s h o w s the similar hygrom a g m a t o p h i l e b e h a v i o u r of La a n d Yb during differentiation. Their c o n t e n t s are at least two t i m e s higher in t h e intermediate r o c k s of N t u m b a w a n d are a t t r i b u t e d to t h e early crystallization of titanite a n d apatite a n d also to t h e relative a b u n d a n c e of biotite a n d amphibole. All t h e s e minerals are generally enriched in LREE with r e s p e c t to HREE (Nagasawa and Schnetzler, 1971; Hietanen, 1971; Arth et al., 1978; S i m m o n s a n d Hedge, 1978; Manetti et a t , 1979; H e n d e r s o n , 1980, 1984). If we admit, like Capaldi etal. (1987), t h a t L a / S m ratio indicates t h e degree of fractionation, t h e La/Sm=f(La) d i a g r a m displays a good example of a two p h a s e crystallization: a n early b a s i c p h a s e with a n L a / S m ratio lower t h a n t h a t of the r e s i d u a l acidic p h a s e . The p r e s e n c e in this residual p h a s e of t h r e e s u b p a r a l l e l t r e n d s corresponding to t h r e e s u c c e s s i v e different mineralogical a s s e m b l a g e s w o u l d s u g g e s t a n e m p l a c e m e n t in s u c c e s s i v e pulses. Isotopic d a t a (Rb/Sr) indicate t h a t N k o g a m granite is older (67 Ma-old) t h a n fayalite syenite of Koupe (46-50 Ma-old) (Lasserre, 1978; Kamgang, 1986; Caen-Vachette etal., 1987; Lamilen, 1989). They are therefore compatible with a depletion of Sr from granites to fayalite syenites. These results suggest a co-geneity
(unique source) for the c o m p l e x e s s t u d i e d while each c o m p l e x is co-magrnatic. However, R b / S r d a t a t i o n s in the acidic f o r m a t i o n s of t h e o t h e r c o m p l e x e s a n d stable i s o t o p e s are n e c e s s a r y to precise t h e s e interpretations. REFERERCE8
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Curie, Paris, 290 p. Giret, A., Bonin, B., Black, R., and Lameyre, J. 1983. Comments on some significant differences between plutonic and volcanic alkaline suites. Terra Cognlta
266
E. NJONFANG, P. KAMGANG, R. T. GHOC,OMU and F. M. TCHOUA
Simmons, E. C. a n d Hedge, C. E. 1978. Minor element a n d Sr-isotope g e o c h e m i s t r y of Tertiary Stocks. Colorado mineral belt. Contr. Min. Petrol. 67, 379396.
Templer, P. et Lasserre. M. 1980. G~ochlmie des massifs "ultlmes" du Cameroun: rapport entre l'~'voluUon magmaUque, l'~tge et la position g6ographlque. Comparalsons avec les "Younger Granites" d u Nigeria. Bu/L Soc. GeoL Ft. 7, XXII, 2, 203-21 I.
0899-5362/92 $5.00+0.00 © 1992 Pe~amon Preu Ltd
The geochemical characteristics of some plutonic-volcanic complexes along the southern part of the Cameroon line E. NJONFANG*,P. KAMC;A~**, R. T. GItOGOMU**and F. M. TCHOUA** *F.coleNormale Sup~rieure, D6partement des Sciences Biologiques,B.P. 47, Yaound~, Cameroun **Facult6 des Sciences, D6partementdes Sciences de la Terre, B.P. 812, Yeound~, Cameronn (First received 3 April, 1991; revised version received 2 August, 1991) Abstract- About 60 volcenic-plutonicalkaline complexesof Tertiary age are located along the continental part of the Cameroon Line. This paper reports the chemicalnature and evolutionof six of them (Gwenfalabo,Nmmbaw,Nkogam, Nda AlL Bana and Koupe), all located in the southern part of this line. The geochemicaldata (major, trace and REE) of these six complexesreveal the following general behaviour: - differentiationfrom aprimary magma of intermediate compositionsituatedin the well-knownDaly gap. The absence of any discrimination between the differentcomplexesin geochemicaldiagrams suggeststhat they have a unique source. This model is defined by three co-magmatic rock-types: cumulative basic rocks composed of anorthositic gabbros, residual acidic rocks with abundant syenites and granites and primitive intermediate rocks mainly represented by monzodiorites and monzonites of the Ntumhaw complex; - emplacementof the acidicrocksby successivepulses, as illustratedin the Rb/SrversusSr diagramby threesubparallel trends, corresponding (with increasing Sr contents) to fayalite syenites, other syenites and granites respectively. From Rb/Sr isotopic data, the Nkogam granite is 67 Ma-old, while the fayalite syenite of the Mount Koupe is 46 - 50 Ma-old. Rb/Sr isotopic data of acidic formations of the other complexes and stable isotopes are needed.
GEOLOGICAL SETTING OF THE COMPLEXES STUDIED
INTRODUCTION The alkaline c o m p l e x e s of C a m e r o o n a r e situa t e d along t h e c o n t i n e n t a l p a r t of t h e C a m e r o o n Line (Fig. I). T h e y a r e essentially m a d e u p of large v o l c a n o e s (Cameroon, M a n e n g o u b a , B a m b o u t o ...) a n d of over 6 0 intrusive m a s s i f s c o m m o n l y referred to a s t h e "granites ultimes" (Lasserre, 1966). T h e y a r e generally a s s o c i a t e d to late volcanic f o r m a t i o n s a n d therefore, are called plutonicvolcanic c o m p l e x e s . G r a n i t e s a n d / o r s y e n i t e s cons t i t u t e t h e m a j o r r o c k types, while b a s i c (gabbros) a n d i n t e r m e d i a t e (diorites a n d monzonites) r o c k s are s u b o r d i n a t e . However, exceptions are f o u n d in t h e N t u m b a w c o m p l e x (Ghogomu, 1984), Westcentral C a m e r o o n , w h e r e i n t e r m e d i a t e r o c k s are p r e d o m i n a n t a n d in t h e M b o u t o u c o m p l e x ( J a c q u e m i n et al., 1981), North C a m e r o o n , w h e r e b a s i c r o c k s a r e a b u n d a n t . Detailed geochemistry, m i n e r a l o g y a n d isotopic s t u d i e s have b e e n perf o r m e d only for t h e M b o u t o u c o m p l e x ( J a c q u e m i n , 1981; J a c q u e m i n eta/., 1982; P a r s o n s et a t , 1986) a n d d a t a s h o w t h a t t h e r o c k s are of m a n t l e derivat i o n w i t h e v i d e n c e of c r u s t a l c o n t a m i n a t i o n . Similar r e s u l t s have b e e n e s t a b l I s h e d for m o s t of t h e o t h e r C a m e r o o n c o m p l e x e s (Cantagrel et a t , 1978; Lasserre, 1978). ThIs is also t h e c a s e of o t h e r c o m p l e x e s of t h e s a m e type, like t h e y o u n g e r g r a n i t e s of Nigeria (Van B r e e m a n et al., 1975; B o w d e n et al., 1987) a n d t h e Evisa c o m p l e x of Corsica (Bonin et al., 1978).
The Gwenfalabo (llalrnl) c o m p l e x Is situat e d b e t w e e n longitude 12" 22' a n d 12" 29' E a n d latitude 7" 42' a n d 7" 48' N. It h a s a n a r e a of a b o u t 2 0 0 k m 2 a n d is essentially m a d e u p of p l u t o n i t e s like g a b b r o s , diorites, m o n z o n i t e s , s y e n l t e s a n d granites, while b a s a n l t o i d s , b a s a l t s , rhyolltes, t r a c h y t e s , pantellerites a n d ignimbrites c o n s t i t u t e the volcanites (Ngonge, 1988). The N t u m b a w c o m p l e x o u t c r o p s a b o u t 15 k m SE of N k a m b e , in t h e M b a w plain (West Central Cameroon) on longitude 10" 54' E a n d latitude 6" 22' N. It Is m a d e u p of t w o J u x t a p o s e d c e n t r e s (cl, c2). The principal r o c k - t y p e s a r e monzodiorites, m o n z o n i t e s a n d q u a r t z - m o n z o n i t e s for cl a n d syenogranites, quartz-syenites, acid d y k e s a n d g r a n o p h y r e s for c2. T r a c h y t e a n d q u a r t z - p o r p h y r y d y k e s are present. C o u n t r y g r a n i t e - g a e i s s e s are partially covered b y b a s a l t s ( G h o g o m u , 1984; G h o g o m u et a t , 1989). The Nkoga__m c o m p l e x is l o c a t e d in t h e B a m o u n region (West Cameroon) b e t w e e n longit u d e s 10"38' a n d 10" 40' E a n d l a t i t u d e s 5" 35' a n d 5" 47' N. C u l m i n a t i n g at 2 2 6 8 m a b o v e s e a level. This c o m p l e x Is c o m p o s e d of t h r e e m a i n units: b a s a l t s , ignimbrites a n d p l u t o n i c r o c k s intruding a mlgmatitic b a s e m e n t (Kamgang, 1986). The Nda All c o m p l e x is s i t u a t e d in t h e Marnfe plain s o m e 32 krn SE o f t h e town, b e t w e e n
255
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8
The geochemical characteristics of some plutonic-volcanic complexes along the southern pan of the Cameroon line 257 longitude 9" 27' a n d 9" 33' E a n d latitude 5" 32' a n d 5" 37' N. Here, t h e plutonic rocks form a m a g m a t l c s e q u e n c e comprising: gabbros, dlorites, monzonltes a n d syenites to which are associated m a g m a tic breccias. The volcanic rocks include co-geneUc benmoreltcs, phonolttes a n d trachyte d o m e s which are posterior to co-magmatlc hawaflte a n d trachyte dykes. This complex is intrusive in t h e Mamfe s a n d s t o n e s except in the s o u t h w e s t border where t h e c o u n t r y rock ks granitic (NJonfang, 1986). The Balm complex is located 14 k m East of Bafang (West Cameroon). Plutonic rocks here include leucogabbros, monzodiorites, m o n z o n i t e s a n d granites, while volcanic rocks comprise interm e d i a t e a n d acidic types (mugearites, b e n m o r e i t e s a n d rhyolites) to which are associated welded tufts (Nana, 1988). The Koupe complex lies between longitude 4" 43' a n d 4" 52' E a n d latitude 9" 40' a n d 9" 47' N, covering a n area of about I 0 0 krn 2. It is mainly c o m p o s e d ofgabbros, syenites, microgranites a n d microsyenites i n t r u d i n g a n orthogneissic basem e n t complex. The volcanic activity essentially c o n s t i t u t e d of pyroclastic rocks a n d basaltic lava flows, ks relatively recent a n d ks f o u n d towards t h e periphery of t h e complex (Lamilen, 1989).
GEOCHEMISTRY Recent chemical d a t a of t h e complexes studied (table) are t a k e n from Ghogomu, 1984; Kamgang, 1986; NJonfang, 1986; Nana, 1988; Ngonge, 1988 a n d Lamilen, 1989. E a c h of these a u t h o r s s t u d i e d the b e h a v i o u r of t h e major a n d trace elements d u r i n g the e m p l a c e m e n t a n d evolution of the volcanic a n d plutonic formations of their respective complexes. All are alkaline m a g m a t i c complexes showing a peralkaline t e n d e n c y in the differentiated types. Fractional crystallization ks the m a i n m e c h a n i s m of differentiation. However, local dispersions often observed in the basic formations are attributed to their cumulative c h a r a c t e r while, for the other rocks, contamination and postmagmaUc p h e n o m e n a are very c o m m o n . We have plotted chemical d a t a (table} in some classical d i a g r a m s to observe possible relationships. Major elements: In t h e Na20 + K20 / SIO 2 diagram, (Fig. 2) the plutonic formations form two sets: t h e first one ks c o n s t i t u t e d of basic a n d intermediate rocks (SiO 2 < 54%) a n d the s e c o n d m a d e u p of acidic rocks (syenites a n d granites) with SiO 2 v a l u e s ranging between 60 a n d 77.5 %. These two sets are s e p a r a t e d by a small group of intermediate rocks with SIOz values ranging between 54 a n d 60 % {Fig. 2a). The volcanic formations s h o w a similar evolution except for t h e u n d e r s a t u r a t e d t r e n d in t h e differentiated types
(Fig. 2b) which occupies the d o m a i n of intermediate a n d evolved lavas of t h e oceanic sector of t h e C a m e r o o n Line (Fltton, 1987}. Similar results are observed in t h e TiO2/8iO 2 d i a g r a m (Fig. 3}. Here, basic a n d intermediate rocks fall in t h e d o m a i n s of New E n g l a n d - Q u e b e c (NEQ) dykes a n d p l u t o n s , a n d acidic r o c k s in t h a t of WhRe M o u n t a i n Magma Series (WMM8) plutons, which belong to the Mesozoic igneous province alkaline rocks of New E n g l a n d (McHone a n d Butler, !984). In the C a O / K 2 0 / N a 2 0 triangular diagram {Fig. 4), a c o n t i n u o u s evolution is observed from basic to acidic rocks m a r k e d by t h e progressive enrichm e n t in t h e alkalies: - basic rocks are calclc-sodlc, intermediate ones are calctc-sodlc-potasstc while acidic r o c k s are s o d i c - p o t a s s i c w i t h a m o r e potassic t e n d e n c y for t h e granites a n d a m o r e sodic t e n d e n c y for syenites. Nigglt's d i a g r a m (1923} modified by d a c o b s o n eta/. ( 1958} (Fig. 5a) ks often u s e d to illustrate the chemical relationship existing between the rocks of the Younger Granites of Nigeria. In this diagram, basic rocks s h o w a general s i m u l t a n e o u s e n r i c h m e n t in a l u m i n a a n d t h e alkalies, while in the detail, acidic rocks t e n d to show two m a g m a t i c trends: a n a l u m i n o u s to pera l u m i n o u s t r e n d a n d a lesser m a r k e d peralkaline trend. The Q-Fath-Color diagram (Fig. 5b} of De la Roche (1966) shows ordy two sets of rocks a n d not three as previously described by Tempier a n d Lasserre (1980): a first set c o m p o s e d of basic a n d intermediate rocks, while a s e c o n d set s h o w s a gradation from syenites to granites. Between these two s e t s are a few i n t e r m e d i a t e r o c k s from Gwenfalabo. Trace elements: The b e h a v i o u r of Rb a n d Sr in the rocks s t u d i e d is illustrated by the diagrams Rb=f(Sr) (Fig. 6a) a n d Rb/Sr=f(Sr) (Fig. 6b). I n t h e s e diagrams, despite a general dispersion of points, from the basic to t h e acidic types, there ks a positive correlation u p to the intermediate types where there ks a n inflection followed by a negative correlation towards t h e acidic rocks. Major basic rocks with a few syenites from Gwenfalabo are found at the inflection point. In t h e acidic types, three m a i n subparallel t r e n d s are found: the first one is mainly m a d e u p of the fayalite syenites of Koupe, t h e second g r o u p s t h e other syenites a n d the third one are c o n s t i t u t e d by t h e granites. The discrimination between t h e intermediate rocks of N t u m b a w a n d the rest of plutonites ks better s h o w n in the L a / Sm=llLa) (Fig. 7a) a n d La/Yb=f(La) (Fig. 7b) diagrams. The former shows a vertical gradation between the basic (La/Sm=4-5) a n d t h e acidic (LA/ Sm=4.5-5.5) plutonites, while t h e volcanites are dispersed. On t h e contrary, t h e latter shows a horizontal gradation (parallel to La) from the basic plutonites to the m o s t evolved volcanites passing t h r o u g h t h e acidic plutonites.
258
E. NJONFANG, P.
KAMGANG,
R. T. GHOC~MUand F. M. TCHOUA
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The geochemical characteristics of some plutonic-volcanic complexes along the southern part of the Cameroon line 263 other evolving towards syen/tic a n d granitic compositions. Consequently, t h e p r i m a r y m a g m a t i c The probable e x i s t e n c e o f o n l y o n e m a g m a t i e liquid is n e a r t h a t of a n intermediate rock constit u t e d of aggregates of maflc minerals a n d plag/osonrce The b e h a v i o u r s of the major a n d trace ele- clase in a syenitic matrix. S u c h a r o c k w a s n a m e d m e n t s (except L a / S m a n d La/Yb) show no distinc- leopard-monzonite. The c u m u l a t i v e c h a r a c t e r of tion between t h e different complexes studied. This C a m e r o o n b a s i c r o c k s h a s b e e n d e s c r i b e d suggests t h a t they all have similar geochemical (NJonfang, 1986; Nana, 1988 a n d Ngonge, 1988). characteristics. However, Tempier a n d Lasserre This is illustrated a m o n g other t h i n g s b y average to (1980) interpreted the gap observed in t h e Q-Fath- high AI20s values (12-24%), close to basic anorthoColor d i a g r a m as two i n d e p e n d e n t m a g m a t i c sltlc rocks of t h e Air m a s s i f (Niger) a n d by positive sources (the one syenitic a n d t h e other basaltic) Eu-anomal/es. Also in s o m e intermediate rocks, evolving in a convergent way towards the granites. are f o u n d basic (kaersutlte a n d bytownite) a n d / o r Mineralogical s t u d i e s (Ghogomu, 1984; G h o g o m u evolved (Or~Ab4~n 5) mineral p h a s e s (NJonfang, etal., 1989; NJonfang, 1986; Nana, 1988; Ngonge, 1986). We therefore t h i n k of a divergent differen1988 a n d Lamilen, 1989) of t h e different com- tiation of alkaline suite from a m a g m a of interplexes s h o w a gradation from basic a n d inter- mediate composition. The presence of ultrabasic mediate types to syenitic a n d / o r granitic types. enclaves in alkaline plutonic complexes is indicaThese a u t h o r s have noted a c o n t i n u o u s evolution tive of a m a n t l e derivation a n d / o r a c c u m u l a t i o n of n a t u r e , texture a n d c h e m i s t r y of feldspars. from a primary basaltic m a g m a (Giret eta/., 1983). Plagioclase c o n s t i t u t e s a b o u t 98 % of the feldspars The very significant absence of ultrabasic types in basic rocks a n d decreases with increasing m a k e s the classical h y p o t h e s i s of crystallization a m o u n t s of alkali feldspar in intermediate rocks from a p r i m a r y alkaline liquid of basaltic composi(diorites to monzonites). Textures with alkali feld- tion unlikely. s p a r bordering or englobing plagioclase are freAfter Demaiffe a n d Hertogen (198 I), Lameyre et q u e n t a n d nearly replaced by alkali feldspar in a/. (1982} a n d Giret (1983), Bonin a n d Giret (1985) syenites a n d granites. Chemically, feldspars are estimated t h e composition of t h e p r i m a r y m a g m a calcic to calcic-sodic in basic rocks, increasingly tic liquid of s o m e alkaline complexes of Africa a n d sodic in intermediate rocks a n d sodic-potassic to Corsica, b a s e d on a n interpretation of the Daly potassic in differentiated rocks. This c a n also be gap. In Cameroon, rocks whose compositions are portrayed by t h e chemical evolution ofmafic mine- n e a r those of primary liquids are monzogabbros, rals (olivine, pyroxene, amphibole a n d biotite), monzonites a n d quartz-monzonites of Ntumbaw. characterized by a progressive e n r i c h m e n t of Fe 3÷ These are examplified by t h e following characterisa n d Na ÷at t h e d e t r i m e n t ofMg 2÷, Fe z* a n d Ca 2÷. We tics: Na20, K20, TIO=, Al203, P=O5, Rb a n d Sr contherefore t h i n k t h a t the gap observed is r a t h e r d u e t e n t s are almost c o n s t a n t a n d REE p a t t e r n s are to the absence of a solid solution between maflc identical (see Table 4, G h o g o m u etoL, 1989). These minerals (Colour pole) a n d feldspars (Fath pole). similarities are consistent with a composition n e a r The anorogenic complexes of Alr (Niger) a p p e a r as t h a t of a relatively non- or weakly modified primary a good example of t h e e m p l a c e m e n t of basic rocks m a g m a by differentiation m e c h a n i s m s . Similar from the differentiation of a basaltic m a g m a inde- observations were m a d e by S i m m o n s a n d Hedge p e n d e n t of those of t h e acidic suites. This h a s been (1978) for quartz-syenites of t h e Tertiary Stocks in suggested by Moreau (1982) a n d H u s c h (1982) to Colorado. The p r i n c i p a l e l e m e n t s , w h i c h are explain t h e two distinct sets of rock types obtained m a r k e r s of the degree of evolution of primary in t h e a-cfm-al diagram. S u c h a discrimination is liquids (MgO, Sr a n d Ni), are m o r e c o n c e n t r a t e d in a b s e n t here where there is a direct link between these rocks: 7.7% MgO, 1496 p p m Sr a n d 183 p p m basic, intermediate a n d acidic rocks. Ni for average values. A s a m p l e of a chilled m a r g i n h a s 343 p p m of Ni, therefore faning in the position An e x a m p l e o f d i v e r g e n t d i f f e r e n t i a t i o n from a of m a x i m u m variation of Ni in p r i m a r y m a g m a s p r i m i t i v e m a g m a o f i n t e r m e d i a t e c o m p o s i t i o n estimated by Allegre et aL (1977) to range between (Daly gap). 300 a n d 500 ppm. High Ni-values of 569 p p m , The possibility of a divergent differentiation from obtained in a picritic basalt of Pagalu, are rare only one m a g m a was evoked by Tempier a n d (Piper a n d Richardson, 1972). As in micromonzoLasserre (1980) in t h e Nda A1/complex. It is illus- gabbro of M e u g u e u r - M e u g u e u r in Niger (Moreau et trated here by t h e b e h a v i o u r of Sr in relation to Rb. a/., 1986) a n d in t h e monzonitic suite of the Studying t h e alkaline plutonic rocks of Kerguelen Tertiary Stocks in Colorado (Simmons a n d Hedge, Islands, Lameyre et al. (1982) revealed a diver- 1978), rocks of the N t u m b a w complex are also gence between two tendencies: the one of c u m u l a - characterized by t h e p r e s e n c e of Eu-negative tive c h a r a c t e r a n d of gabbroic nature, a n d t h e a n o m a l i e s with E u / E u * = 0 . 8 5 - 0 . 9 5 (Ghogomu, DISCUSSION - CONCLUSION
264
E. NJONFANG,P. KAMGANG,R. T. GHOGOMUand F. M. TCHOUA
1984). Most a u t h o r s , s u c h a s Barley a n d Schairer (1966), D u s c h e s n e a n d Demaiffe (1978), H a n s o n (1980), B o w d e n (1985), M o r e a u et a t (1986) a n d Key (1987), a t t r i b u t e d Eu-negative a n o m a l i e s to the r e s i d u a l n a t u r e of t h e melt after plagioclase fractionation. The p r e s e n c e of this a n o m a l y in t h e N t u m b a w intermediate r o c k s c o u p l e d with high MgO, Sr a n d Ni (for Ni c o n t e n t s , see Table 3, G h o g o m u et al., 1989), v a l u e s is similar to t h e chemical c h a r a c t e r i s t i c s of a leopard-monzonite.
An e v o l u t i o n b y fractional c r y s t a l l i z a t i o n The preceding c h a r a c t e r i s t i c s indicate t h a t the r o c k s s t u d i e d w o u l d b e t h e p r o d u c t s of a divergent differentiation of a p r i m a r y m a g m a of intermediate composition. The m o s t plausible p r o c e s s is t h e e m p l a c e m e n t in t w o s t a g e s a s s u g g e s t e d b y Demaiffe a n d H e r t o g e n (1981) in t h e anorthositic s u i t e s of Hidra in Norway. There are first c u m u l a tive b a s i c r o c k s p r o b a b l y p r o d u c e d t h r o u g h / n situ differentiation. T h e y are c h a r a c t e r i z e d b y positive a n d increasing E u - a n o m a l i e s a s a function of SiO 2, (Eu/Eu*= 1.1-1.86). The b a s i c r o c k s are therefore fractionated. The r e s i d u a l liquids evolved t o w a r d s the acidic types. Positive E u - a n o m a l i e s in t h e less differentiated t y p e s (SiO2<65%) s h o u l d be n o t e d here while t h e m o r e evolved r o c k s have negative a n o m a l i e s (Eu/Eu*=0.75-0.60). Differentiation is governed b y fractional crystallization. L a / Y b profile is parallel to t h e a b s c i s s a , w h i c h is very c h a r a c teristic and, according to D u s c h e s n e a n d Demaiffe (1978) a n d R o c k (1978), s h o w s the similar hygrom a g m a t o p h i l e b e h a v i o u r of La a n d Yb during differentiation. Their c o n t e n t s are at least two t i m e s higher in t h e intermediate r o c k s of N t u m b a w a n d are a t t r i b u t e d to t h e early crystallization of titanite a n d apatite a n d also to t h e relative a b u n d a n c e of biotite a n d amphibole. All t h e s e minerals are generally enriched in LREE with r e s p e c t to HREE (Nagasawa and Schnetzler, 1971; Hietanen, 1971; Arth et al., 1978; S i m m o n s a n d Hedge, 1978; Manetti et a t , 1979; H e n d e r s o n , 1980, 1984). If we admit, like Capaldi etal. (1987), t h a t L a / S m ratio indicates t h e degree of fractionation, t h e La/Sm=f(La) d i a g r a m displays a good example of a two p h a s e crystallization: a n early b a s i c p h a s e with a n L a / S m ratio lower t h a n t h a t of the r e s i d u a l acidic p h a s e . The p r e s e n c e in this residual p h a s e of t h r e e s u b p a r a l l e l t r e n d s corresponding to t h r e e s u c c e s s i v e different mineralogical a s s e m b l a g e s w o u l d s u g g e s t a n e m p l a c e m e n t in s u c c e s s i v e pulses. Isotopic d a t a (Rb/Sr) indicate t h a t N k o g a m granite is older (67 Ma-old) t h a n fayalite syenite of Koupe (46-50 Ma-old) (Lasserre, 1978; Kamgang, 1986; Caen-Vachette etal., 1987; Lamilen, 1989). They are therefore compatible with a depletion of Sr from granites to fayalite syenites. These results suggest a co-geneity
(unique source) for the c o m p l e x e s s t u d i e d while each c o m p l e x is co-magrnatic. However, R b / S r d a t a t i o n s in the acidic f o r m a t i o n s of t h e o t h e r c o m p l e x e s a n d stable i s o t o p e s are n e c e s s a r y to precise t h e s e interpretations. REFERERCE8
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The geochemical characteristics of some plutonic-volcanic complexes along the southern part of the Cameroon line
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