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The Ntumbaw Complex, NW Cameroon: An atypicalanorogenic ring complex of intermediate composition
Journal of African Earth Sciences, Vol.8, No. 1, pp. 1-9,1989
Printedin GreatBritain
0731-7247/89$3.00+ 0.00 MaxwellPexgamonMacmillanpie
The Ntumbaw Complex, NW Cameroon : an atypical anorogenic ring complex of intermediate composition R. GHOGOMU*,C. MOREAU**, W. L. BROWN + and G. ROCCI ÷÷ *Departement des Sciences de la Terre, Facult~ des Sciences, Yaound~, Cameroon **Departement de Geologic, Facult6 des Sciences, Dakar-Farm, S~n~gal ÷CRPG-CNRS B.P.20, 54501 Vandoeuvre-l~s-Nancy Cedex, France ÷+Laboratoirede P~trologie, Facult~ des Sciences B.P. 239 54506 Vandoeuvre-l~s-Nancy Cedex, France Abstract- The Ntumbaw anorogenic ring complex, NW Cameroon, is composed of two intersecting centres intruded into the Precambrian metamorphic basement and partly hidden by basalt flows. It covers an area of about 12 krn2 and, despite lack of radiometric dating, is similar to the other anorogenic complexes of Tertiary age along the Cameroon Line. The rocks range from monzodiorite, monzonite and quartz monzonite for centre 1 to quartz syenite, syenogranite and granophyre for centre 2. The two centres are inversely zoned petrographically with the most acid rocks towards the periphery of both. The chemistry of major, trace and rare-earth elements shows that the rocks of both centres are subalkaline to alkaline, cogenetic, and that their variation was governed by fractional crystallization. More than 90% of this complex is of intermediate composition with silica values between 52 and 65%. Such rocks are often absent from anorogerdc complexes, their compositions falling in the Daly Gap.
INTRODUCTION AND GEOLOGICAL SETTING
T h e N t u m b a w c o m p l e x is o n e of o v e r s i x t y h i g h - l e v e l c o m p l e x e s in C a m e r o o n r e f e r r e d to a s t h e . g r a n i t e s u l t i m e s . ( L a s s e r r e , 1966). T h e y e x t e n d f r o m E t i n d e to W a z a (Fig. la), a d i s t a n c e of a b o u t 1 0 0 0 k m a l o n g a N 3 0 ° E l i n e a m e n t k n o w n a s t h e C a m e r o o n Line, b u t o n l y a few of t h e s e m a s s f f s h a v e b e e n s t u d i e d in detail: M " o o u t o u , G o l d a - Z u e l v a ( J a c q u e m i n , 1981; P a r s o n s e t a l . , 1986), N t u m b a w ( G h o g o m u , 1984), N ' d a All (Njonfang, 1986) o r a r e u n d e r s t u d y b y a F r e n c h - C a m e r o o n i a n t e a m (Poll, T c h e g u i , E t i n d e , M t s R u m p t s , N a m b o e , F o u r o u g a i n etc.). T h e N t u m b a w c o m p l e x is s i t u a t e d o n t h e n o r t h e a s t e r n f r i n g e of t h e B a m e n d a H i g h l a n d s (longit u d e 10°54'E, l a t i t u d e 6°22'N) a b o u t 15 k m s o u t h - e a s t of N k a m b e , N o r t h W e s t P r o v i n c e . P r e v i o u s s t u d i e s of t h i s c o m p l e x a r e r a r e . P e r o n n e (1969) p r o d u c e d a geological m a p of Wum-Banyo and mapped this complex as an elliptical s y e n i t i c p l u t o n . F r o m L a n d s a t p h o t o g r a p h s , M o r i n (1980) s h o w e d t h a t c i r c u l a r fractures affected some Younger Granites, such as N t u m b a w a n d S a b o n g a r i . A c c o r d i n g to h i m , t h e t y p i c a l l a n d f o r m s (high relief, a b r u p t c o n t a c t s , cliffs a n d c i r c u l a r a p p e a r a n c e ) a r e a r e s u l t of regional tectonics. R e c e n t l y , t h e geology, c h e m i s t r y a n d p e t r o l o g y of t h i s c o m p l e x of a r e a - 1 2 k m 2 w e r e s t u d i e d
( G h o g o m u , 1984) a n d it w a s s h o w n t h a t it c o n s i s t s of two c o g e n e t i c c e n t r e s ( c e n t r e I a n d c e n t r e 2) w i t h > 9 0 % of t h e r o c k s of i n t e r m e d i a t e c o m p o s i t i o n (silica v a l u e s b e t w e e n 52-65%).
PETROGRAPHY
T h e r o c k t y p e s of t h e N t u m b a w c o m p l e x w e r e d e s c r i b e d in d e t a i l b y G h o g o m u (1984), w h o s h o w e d t h a t t h e two i n t e r s e c t i n g c e n t r e s of t h e c o m p l e x are i n t r u s i v e i n t o t h e r o c k s of t h e granito-gneissic basement complex of Nseh and N s o p a n d p a r t l y m a s k e d b y t h e late b a s a l t flows of N t u m b a w (Fig. lb). C e n t r e 1 (C i), t h e o l d e r of t h e two, o c c u p i e s t w o - t h i r d s of t h e c o m p l e x , is circular in form and comprises monzodiorlte, monzonite and quartz monzonite; basalt dykes a n d f r e q u e n t q u a r t z v e i n s o c c u r . C e n t r e 2 (C2) is c o m p o s e d of m o r e d i f f e r e n t i a t e d r o c k s w i t h a b o r d e r facies. T r a c h y t e a n d q u a r t z - f e l d s p a r porphyry dykes also occur. For both centres, the different rock types occur in a r c u a t e o r c i r c u l a r a r e a s w i t h p r o g r e s s i v e v a r i a t i o n f r o m o n e r o c k t y p e to a n o t h e r . T h e p e t r o g r a p h y , relative m o d a l a b u n d a n c e a n d m i n e r a l o g y of r e p r e s e n t a t i v e s a m p l e s of t h e s e r o c k s are g i v e n in T a b l e 1. T h e r e is c o n t i n u o u s v a r i a t i o n in t h e t e x t u r e a n d m i n e r a l o g y of t h e r o c k s . F o r c e n t r e 1 t h e r o c k s are m o s t l y c o a r s e grained porphyritic and dark-grey, while those
2
R. GHOOOMUet al. 14
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Fig. 1 a) Location of the anorgenic complexes in Cameroon showing the position of the Ntumbaw complex (A = volcanic rocks, B = younger granites, C = sedimentary rocks, D = basement complex).
of centre 2 are m e d i u m to fine-grained a n d m u c h m o r e leucocratic. E a c h centre s h o w s inverse zoning, the m o r e evolved r o c k s occurring tow a r d s the periphery. The m o r e or less contin u o u s variation of the m o d a l c o m p o s i t i o n of t h e r o c k s from centre i to centre 2 is m a r k e d b y a n increase in the p e r c e n t a g e of alkali feldspar a n d quartz a n d a d e c r e a s e in t h a t of plagioclase (Fig. 2). A m o n g the minerals, plagioclase d e c r e a s e s in An c o n t e n t from An a7 in the monzodiorites to An isin the s y e n o g r a n i t e s (Fig. 3). This progressive d e c r e a s e is c o n s i s t e n t with the fractionation of m o r e calcic minerals from a less evolved m a g m a . T h e s e t r e n d s in texture, m o d e s and mineral c h e m i s t r y s u g g e s t t h a t b o t h c e n t r e s belong to the s a m e m a g m a t i c suite with centre
2 having b e e n formed j u s t after t h e final s t a g e s of centre i. MAJOR, TRACE AND RARE-EARTH ELEMENT CHEMISTRY The w h o l e - r o c k c h e m i c a l d a t a are listed in Table 2 {for sampling a n d analytical m e t h o d s see Ghogomu, 1984). Locations of r e p r e s e n t a t i v e s a m p l e s are given in Fig. 1. The variation of t h e s e e l e m e n t s (as s h o w n on H a r k e r diagrams, Ghogomu, 1984, Fig. 41) reveals several trends: AI20 a, CaO, total iron, MgO a n d TiO 2 d e c r e a s e with differentiation suggesting t h a t fractionation of f e r r o m a g n e s i a n m i n e r a l s a n d m o r e calcic plagioclase occurred. Only K20 a n d Na20 s h o w
The Ntumbaw
Complex, NW Cameroon
3
Fig. lb) Petrographicmap of the Nntmbaw complex (1 = granito-gneissicbasementcomplex, 2 = monzodiorite, 3 = monzohite, 4 = quartzmonzonite, 5 = quartz syenite, 6 = quartz-biotite syenite, 7 = syenogranite, 8 = granophyre,9 = machytic dyke, 10 = quartz-feldsparporphyry dyke, 11 = tzachyte dome, 12 = basaltflow). Rock Type NS4, monzodiorite
NS6, monzonite
NS7, quartz monzonite
T1,T5, quartz syenite
NS 12, syenogranlte NS 13, basalt dyke T l l , trachyte dyke H2, quartz-feldspar porphyry
Texture~ colour coarse to medium grained, porphyritic (dad-grey feldspars) coarse grained porphyritic (darkgrey feldspars) coarse grained porphyritic, light-grey
medium to fine-grained, light-grey medium to fine-grained, miarolitic, light-greyish, green amphibole needles fine-grained porphyritic, greenish black sanldlne fine-grained porphyritic, pyromeride at contact, greenish-grey fine to medium-grained, greenish brown
Phenocrysts andesine (52%) perthile (20%) augite (11%) andesine-oligoclase (42%) orthoclase (24%) calclc ollgoclase (31%) orthoclase (32%) quartz (6%)
Groundmass hypersthene (2.4%) biotite (7.5%) amphibole (2.3%) quartz (2.3%) augite (10%) amphibole (5%) blollte (6.8%) hypersthene (2.8%) quartz (3.2%) amphibole (7.8%) augile (7.8%) hypersthene (1.5%)
orthoclase (43%) oligoclase (24%) quartz (10%)
biolite (4.5%) actinolite (4.2%) ferroaugite 3.8%) opaques (3.8%)
orthoclase (44%) ollgoclase (21%) quartz (19%)
katophorite-ferrorichterite (5.0%) opaques (5.5%), biotite (2.5%) ferroaugile (1.8%) brown hornblende augite opaque
plagioclase olivine transformed to serpentine and Fe oxide sanidine spherulites aegirine-augite diopside opaques quartz orthoclase plagloclase An 26
plagioclase fayalite opaques opaques green amphibole
Table 1 - Petrographic characteristics of selected rocks of the Ntumbaw complex.
Accessories apatite zircon magnetite rutile apatite sphene zircon opaques apatite zircon serieite damourlte chlorite apatite sphene chlorite calcite sericite apatite sphene
alkali feldspar
biotite clinopyroxene
4
R.GHoGOMtJ et al.
well in t h e so-called Daly G a p of t h e s e m a s s e s .
o •
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Fig. 2 Modal composition of the rocks of the Ntumbaw complex on a QA-P diagram (Streckeisen, 1976) (I = monzodiorite, 2 = monzonite, 3 = quartz monzonite, 4 = quartz syenite, 5 = syenogranite).
a positive correlation with silica d u e to the late c o n c e n t r a t i o n of alkali feldspar. The r o c k s of this complex, although p r o b a b l y n o t all representing liquids, are alkaline according to the limits proposed b y Macdonald and Katsura (1964) a n d Miyashiro (1978). The r o c k s from N t u m b a w are more a l u m m o u s (Fig. 5) t h a n t h o s e of the Nigerian Younger Granites ( J a c o b s o n et al., 1958). C o m p a r e d to t h e r o c k s of the Air province (Husch, 1982; Moreau, 1982). r o c k s from N t u m b a w lie b e t w e e n t h e b a s i c m e m b e r s (gabb r o s a n d anorthosites) a n d the salic ones, fitting
T r a c e - e l e m e n t a n a l y s e s are given in Table 3. Ba a n d Sr s h o w a negative correlation with silica since t h e s e e l e m e n t s were p r e s u m a b l y c a m o u flaged b y Ca in t h e calcic m i n e r a l s (plagioclase a n d augite) f r a c t i o n a t e d from t h e m a g m a . Rb s h o w s a slight positive correlation with differentiation a n d is t h o u g h t to b e i n c o r p o r a t e d in potassic minerals (Heier a n d A d a m s , 1964; Lange et al., 1966) s u c h a s K-feldspar a n d micas, w h i c h also s h o w a general c o n c e n t r a t i o n with differentiation. In s o m e anorogenic c o m p l e x e s like Liruei, Nigeria (Butler et al., 1962), high Rb v a l u e s ( - 1 6 0 0 ppm) are linked to cassiterite mineralization; N t u m b a w with Rb v a l u e s ranging from 7 2 - 2 4 0 p p m s e e m s to b e sterile. For t h e t r a n s i t i o n e l e m e n t s (except Co) a strong negative correlation with differentiation exists. S u c h a t r e n d h a s b e e n d e m o n s t r a t e d b y Treuil (1973), Treufl a n d J o r o n ( 1975,1976) as a m a r k e r of fractional crystallization. C o n c e n t r a t i o n s of REE are given in Table 4; c h o n d r i t e - n o r m a l i z e d REE p a t t e r n s are s h o w n on Fig. 6. All t h e r o c k s are rich in R E E with a preferential e n r i c h m e n t in LREE. - L R E E / H R E E ratios are high a n d d e c r e a s e with differentiation. L a / Y b v a l u e s v a r y from 66 in the monzodiorites to 12.7 in t h e acid d y k e s portraying a s h a r p negative trend. a negative Eu a n o m a l y o c c u r s a n d i n c r e a s e s with differentiation, as r e s u l t of t h e fractional crystallization of plagioclase from t h e m a g m a . -
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Fig. 3 A) Composition of the feldspars on an Ab-An-0r dia£Tam (symbols as in Fig. 2). B) Chemical variation of the plagioclases : a = syenogranites, b = quartz syenites, c = quartz monzonites, d = monzonites, e -- monzodiorites.
The Ntumbaw Complex, NW Cameroon
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Fig. 5 a-cfm-al diagram (same symbols as in Fig. 4).
Fig. 6 Combination of all REE patterns for the rocks o f the w h o l e complex. a = plutonites o f C1, b = plutonites of C2, c = volcanites o f C1 and C2.
centrc l mom,~bbm
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Rb Sr Cu Ni Cr V Co
centre 2
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facies average
NS4
NS8
R3
NS6
NS3
TI0
ST5
NSI
NS7
2852 75 1301 23 198 501 : 209 13
2699 93 1598 16 175 431 150 77
3016 96 1671 12 198 470 157 51
3073 108 1556 20' 222 551 217 30
3086 104 1535 14 175 438 177 21
3032 115 1571 18 175 432 158 75
2850 104 1543 15 206 480 177 44
2495 131 1383 25 164 420 198 28
2084 2810 109 126 1306 1239 20 68 166 343 400 612 164 185 54 44
r25
Ca
2.4 4.3
2.43 4.7
2.6 4.7
2.9 4.6
2.96 4.2
3.09 4.4
2.87 3.2
3.49 3.8
3.1 3.9
3.1 5.1
K/Rb Rb/Sr O/Nil 8r/Ca B~¢K B~/Sr
320 0.058 2.53 0.030 0.119 2.192
261 0.058 2.46 0.034 0.111 1.689
271 0.057 2.37 0.036 0.116; 1.805 i
267 0.069 2.48 0.034 0.105 1.975
285 0.067 2.50 0.037 0.104 2.010
269 0.073 147 0.038 0.098 1.930
276 0.066 2.33 0.048 0.099 1.847
266 0.095 2.56 0.036 0.072 1.804
285 0.083 2.41 0.033 0.067 1.596
246 0.102 1.78 0.02.4 0.091 2.28
K
2799 105 1473 28 183 451 178 44 I 2.87 4.2 276 3.072 2.46 ).036 ).095 1.85
quartzsycni¢
sycnogranite
average
aciddykes
NSI0
T5
Tlb
NSa
NSI1
NSI2
H2
H4
TII
992 76 323
1040 88 322
1014 121 252
1290 1140 92 113 484 202
1145 123 197 <10 13 14 39 21
33 242 20
27 227 19
37 149 17
753 139 191 <10 <10 29 42 35
2.92 1.98
2.92 1.7
2.92 1.6
3.2 2.0
3.47 1.1
3.83 1.0
3.96
3.74
3.73
3.41 1.04
307 0.56
311 0.62
164 12.1
165 11.9
250 8.76
267 3.96
~..020 0.034 5.644
1.08 0.020 0.03O 5.812 1.65
1.42
2.~8
0.024 3.54
384 0.24
332 241 348 0 . 2 7 3 0.429 0.188 1,75 -4.8 0.016 0.019 0.018 0.024 0.034 0.036 0.035 0.040 3.090 3.23 "3.60 2.64 --
Table 3 - Trace-element (,ppm) ,and major-element (weight %) abundances.
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R.GHOOOMUet al. DISCUSSION AND CONCLUSION
C h a y e s {1963) s h o w e d t h a t p u b l i s h e d analyses of oceanic-island volcanic rocks all over t h e world c l u s t e r into two m a i n groups, one basaltic a n d the o t h e r trachytic. He convincingly dem o n s t r a t e d the rarity of lavas in the range 53% < SiO 2 < 57%, the ,Daly Gap,. Several a u t h o r s (Harris, 1963; Macdonald, 1963; Carm, 1968; Baker, 1968) have a t t e m p t e d to explain t h i s gap as d u e to a bias in s a m p l e collection. Le Maitre (1968) a n d G a s s a n d Mallick (1968) p o i n t e d out t h a t the rarity of i n t e r m e d i a t e lavas could be due to the eruptive process, s u c h t h a t t r a c h y a n d e s i tic m a g m a formed at d e p t h w a s not e r u p t e d at the s u r f a c e as readily as more basic a n d acid m a g m a . B r y a n (1964) a n d Wilkinson (1966) have shown, however, t h a t silica gaps are characteristic of differentiated i n t r u s i o n s , w h i c h provide some of the b e s t evidence of fractional crystallization. On the o t h e r h a n d , Wyllie (1963) p r e s e n t e d m o d e l s where s u c h silica g a p s c a n be p r o d u c e d d e p e n d i n g on the g e o m e t r y of the p h a s e diagram, implying t h a t fractional crystallization m i g h t n o t be a prerequisite. According to Macdonald e t al. (1970), the rarity of s u c h rocks in alkaline provinces is linked to the high ratio of acid to basic lavas, the acid lavas being too v o l u m i n o u s to be considered residual liquids of fractional crystallization of the observed volume of basalts. C h a y e s (1977) a n d Clague (1978) s h o w e d t h a t this statistical bimodality could be due to the choice of the f r a c t i o n a t i o n index. Chemical p a r a m e t e r s like SiO 2, CaO a n d the T h o r n t o n a n d Tuttle index a s s u m e d to vary directly with increasing crystal f r a c t i o n a t i o n m i g h t not always be reliable. Incompatible elem e n t s like p h o s p h o r u s (Clague a n d B u n c h , 1976) a n d z i r c o n i u m (Clague, 1978) m a y provide a more quantitative m e a s u r e of fractionation in t h e liquid d e s c e n t from b a s a l t to trachyte. Anorogenic complexes are generally of alkaline type with plutonic a n d subvolcanic rocks ranging from gabbro, m o n z o n i t e a n d syenite to granite, while volcanic rocks range from b a s a l t to t r a c h y t e a n d rhyolite (Bonin, 1982). In t h e s e complexes t h e r e is a l m o s t always a gap b e t w e e n the gabbros a n d syenites, which in some complexes is partly occupied b y hybrid rocks whose a c t u a l composition does not c o r r e s p o n d to t h a t of a m a g m a . E x a m p l e s of s u c h h y b r i d rocks include : t h e monzodiorites, dioritic breccias a n d granodiorites rich in xenolith f r a g m e n t s of the Belknap complex U.S.A. (Model, 1936); - the m a g m a t i c breccias of the Sara-Fier complex, Nigeria (Turner, 1963); the m o n z o - a n o r t h o s i t e s a n d m a g m a t i c
breccias of Ofoud a n d Taguei, Niger (Moreau, 1982; Moreau e t al., 1987). The N t u m b a w complex is atypical b e c a u s e over 90% of its rocks are of i n t e r m e d i a t e composition r a n g i n g from monzodiorite to syenogranite with silica v a l u e s b e t w e e n 52 a n d 65%. Hybrid rocks have not b e e n observed in this complex. Petrography, m i n e r a l o g y a n d geochem i s t r y show t h a t the rocks are cogenetic with fractional crystallization, t h e principal process of t h e i r formation; the rocks fall a l m o s t entirely in the ,Daly Gap, of o t h e r complexes. Unfortunately, basic a n d u l t r a b a s i c r o c k s a n d chilled facies are a b s e n t so t h a t t h e c o m p o s i t i o n of t h e p r i m a r y m a g m a is u n k n o w n , as is t h e extent of the possible c r u s t a l or m a n t l e origin for the i n t e r m e d i a t e rocks. REFERENCES
Baker, I. 1968. Intermediate oceanic volcanic rocks and the ,Daly Gap,. Earth Planet. ScL Lett. 4. 103106 Bonin, B. 1982. Les granites des complexes annulaires. Manuel et methodes, n°4, B.R.G.M. Orleans, 183 p. Bryan, W.B. 1964. Relative abundance of intermediate members of the oceanic basalt-trachyte association : evidence from Clarion and Soccoro Islands, Revfllagigedo Islands, Mexico. J. Geophys Res. 69, 3047-3049. Butler, J.R., Bowden, P. and Smith, A.Z. 1982. K/Rb ratios in the evolution of the Younger Granites of Northern Nigeria. Geochem. Cosmochem. Acta 26, 89-100. Cann, J.R., 1968. Bimodal distribution of rocks from volcanic islands. Earth Planet. ScL Lett. 4, 479-480. Chayes, F. 1977. The oceanic basalt-trachyte relation in general and in the Canary Islands. Amer. Mineral. 62, 666-671. Clague, D.A. 1978. The oceanic basalt-trachyte association : an explanation of the Daly Gap. J. GeoL U.S.A. 88, 724-743. Clague, D.A. and Bunch, T.E. 1976. Formation of ferrobasalt at East Pacific mid-ocean spreading centres. J. Geophys. Res. 81, 4247-4256. Gass, I.G. and Mallick, D.I.J. 1968. Jebel Khariz : an Upper Miocene strato-volcano of commenditic affinity on the South Arabian Coast, Bull. Volcan. 32-1, 33-88. Ghogomu. R.T. 1985. Geology, geochemistry and petrology of the Ntumbaw anorogenic complex, Cameroon : an example of a ring complex of intermediate composition. Unpublished doctoral thesis, University of Nancy I. 143 p. Harris, P.G. 1963. Comments on a paper by F. Chayes. ,Relative abundance of intermediate members of the oceanic basalt-trachyte association~. J. Geophys. Res, 88, 5103-5107. Heier, K.S. and Adams, J.A.S. 1964. The geochemistry of the alkali metals. Phys. Chem. Earth.. $, 253381.
The Ntumbaw Complex, NW Cameroon Husch, J.M. 1982. Geology, petrology a n d geochemistry ofanorthositic a n d associated rocks, Republic of Niger. Ph. D. thesis, Princeton 231 p. (unpublished). J a c o b s o n , R.R.E. MacLead, W.N. and Black, R. 1958. Ring complexes in the Younger Granite province of Northern Nigeria. Mem. Geol. Soc. Lond. I, 70 p. J a c q u e m i n , H. 1981. Contribution a r e t u d e g6ochimique des elements en trace a la petrogen6se des complexes anorog6niques (Nord Carneroun). These de 3eme cycle Nancy I, 197 p. Lange, I.M. Reynolds. R.C., Lyons, S.B. 1966. K / R b ratios in coexisting K-feldspars a n d biotites from s o m e New England granites a n d metasediments. Chem. Geol. I, 317-328. Lasserre, M. 1966. Confirmation de l'existence des g r a n i t e s t e r t i a i r e s a u C a m e r o u n , (Afrique Equatoriale). Bu//. B.R.G.M., 3, 141-148. Le Maitre, R.W. 1968. Chemical variations between a n d within volcanic rock series - a statistical approach. J. Petrol. 9, 220-252. Macdonald, G.A. 1963. Relative a b u n d a n c e of intermediate m e m b e r s of the oceanic basalt-trachyte association - a discussion. J. Geophys. Res. 68, 5100-5102. Macdonald, G.A. a n d Katsura, T. 1964. Chemical composition of Hawaiian lavas. J. Petrol. 5, 82-133. Macdonald, R., Bailey, D.K., Sutherland, D.S. 1970. O v e r s a t u r a t e d peralkaline glassy trachyte from Kenya. J. Petrol. I I , 507-517. Miyashiro, A. 1978. Nature of the alkalic rock series.. Contr. Miner. Petrol. 66, 91-104. Modell. D.A. 1936. Ring-dyke complex of the Belknap Mountains, New Hampshire. Bull. Geol. Soc. Am. 47, 126-144. Moreau, C. 1982. Les complexes annulaires anorogenique & suite anorthositique de rAIr Central et Septentrional (Niger). These Doct. d'Etat Univ. Nancy I 356 p. Moreau, C., Brown, W.L. a n d Karche, J.P. 1987. Monzo-anorthosite from the Taguel ring-complex, Air, Niger : a hybrid rock with c u m u l u s plagioclase a n d an infiltrated granitic i n t e r c u m u l u s liquid ? Contr. Miner. Petrol. 95, 32-43.
Morin, S. 1980. Apport des images l a n d s a t & la c o n n a i s s a n c e de la s t r u c t u r e des h a u t e s terres de r O u e s t C a m e r o u n . Geogr. Phys. Dept.. Geogr. Yaounde, 181-196. Njonfang. E. 1986. Un exemple de complexe plutovolcanique tertlaire a roches b a s i q u e s et intermediaires : le m a s s i f de Nda Ali, S u d - O u e s t du Cameroun. Unpublished thesis, University of Yaounde, C a m e r o u n , 171 pp. Parsons, I., Brown, W.L. a n d J a c q u e m i n , H. 1986. Mineral chemistry and crystallization conditions of the Mboutou layered gabbro-syenite-granite complex, North Cameroon. Jour. Petrol. 27, 1305-1329. Peronne, Y. 1969. Notice explicative s u r la feuille Wum-Banyo. Dir. Min. Geol. C a m e r o u n . 59 p. Streckeisen, A.L. 1976. To each plutonic rock its proper name. Earth Sci. Rev. 12, I, 1-34. Treuil, M. 1973. Criteres p6trologiques, g6ochimiques et s t r u c t u r a u x de la gen6se et de la diff6renciation des m a g m a s basaltiques - exemple de l'Afar. These Doct. d'Etat. Orleans. Treuil, M. a n d Joron, J.L. 1975. Utflisation des eILm e n t s h y g r o m a g m a p h i l e s p o u r la simplification de la mod61isation quantitative des p r o c e s s u s m a g m a tiques. Exemple de l'Afar et de la dorsale m6dio aflantique. Soc. Ital. Miner. Petrol. Y ~ X I , 125-174. Treuil, M. and Joron, J.L. 1976. Etude geochimique des elements en trace d a n s le m a g m a t i s m e de rAfar. Implication p6trog6netique et c o m p a r a i s o n avec le m a g m a t i s m e de l'Islande et de la dorsale m6dioaflantique. Afar depression of Ethiopia. Pilger et R6sler (eds.) Stuttgart, E. Schweizerbarts'she Ver lags-buchhandlung, 2 . 2 6 - 7 9 . Turner, D.C. 1963. Ring s t r u c t u r e s in the Sara-Fier complex, Northern Nigeria. Geol. Soc. London Quart. Jour. 119, 345-366. Wilkinson, J . F . G . 1966. Residual glasses from some alkali basaltic lavas from New South Wales. Mineralog. Mag. 35, 847-860. Wyllie, P.J. 1963. Effects of the changes of slope on liquidus a n d solidus p a t h s in the s y s t e m diopsideanorthite-albite. Min. Soc. Am. Spec. Pap. I, 204-212.
9
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0731-7247/89$3.00+ 0.00 MaxwellPexgamonMacmillanpie
The Ntumbaw Complex, NW Cameroon : an atypical anorogenic ring complex of intermediate composition R. GHOGOMU*,C. MOREAU**, W. L. BROWN + and G. ROCCI ÷÷ *Departement des Sciences de la Terre, Facult~ des Sciences, Yaound~, Cameroon **Departement de Geologic, Facult6 des Sciences, Dakar-Farm, S~n~gal ÷CRPG-CNRS B.P.20, 54501 Vandoeuvre-l~s-Nancy Cedex, France ÷+Laboratoirede P~trologie, Facult~ des Sciences B.P. 239 54506 Vandoeuvre-l~s-Nancy Cedex, France Abstract- The Ntumbaw anorogenic ring complex, NW Cameroon, is composed of two intersecting centres intruded into the Precambrian metamorphic basement and partly hidden by basalt flows. It covers an area of about 12 krn2 and, despite lack of radiometric dating, is similar to the other anorogenic complexes of Tertiary age along the Cameroon Line. The rocks range from monzodiorite, monzonite and quartz monzonite for centre 1 to quartz syenite, syenogranite and granophyre for centre 2. The two centres are inversely zoned petrographically with the most acid rocks towards the periphery of both. The chemistry of major, trace and rare-earth elements shows that the rocks of both centres are subalkaline to alkaline, cogenetic, and that their variation was governed by fractional crystallization. More than 90% of this complex is of intermediate composition with silica values between 52 and 65%. Such rocks are often absent from anorogerdc complexes, their compositions falling in the Daly Gap.
INTRODUCTION AND GEOLOGICAL SETTING
T h e N t u m b a w c o m p l e x is o n e of o v e r s i x t y h i g h - l e v e l c o m p l e x e s in C a m e r o o n r e f e r r e d to a s t h e . g r a n i t e s u l t i m e s . ( L a s s e r r e , 1966). T h e y e x t e n d f r o m E t i n d e to W a z a (Fig. la), a d i s t a n c e of a b o u t 1 0 0 0 k m a l o n g a N 3 0 ° E l i n e a m e n t k n o w n a s t h e C a m e r o o n Line, b u t o n l y a few of t h e s e m a s s f f s h a v e b e e n s t u d i e d in detail: M " o o u t o u , G o l d a - Z u e l v a ( J a c q u e m i n , 1981; P a r s o n s e t a l . , 1986), N t u m b a w ( G h o g o m u , 1984), N ' d a All (Njonfang, 1986) o r a r e u n d e r s t u d y b y a F r e n c h - C a m e r o o n i a n t e a m (Poll, T c h e g u i , E t i n d e , M t s R u m p t s , N a m b o e , F o u r o u g a i n etc.). T h e N t u m b a w c o m p l e x is s i t u a t e d o n t h e n o r t h e a s t e r n f r i n g e of t h e B a m e n d a H i g h l a n d s (longit u d e 10°54'E, l a t i t u d e 6°22'N) a b o u t 15 k m s o u t h - e a s t of N k a m b e , N o r t h W e s t P r o v i n c e . P r e v i o u s s t u d i e s of t h i s c o m p l e x a r e r a r e . P e r o n n e (1969) p r o d u c e d a geological m a p of Wum-Banyo and mapped this complex as an elliptical s y e n i t i c p l u t o n . F r o m L a n d s a t p h o t o g r a p h s , M o r i n (1980) s h o w e d t h a t c i r c u l a r fractures affected some Younger Granites, such as N t u m b a w a n d S a b o n g a r i . A c c o r d i n g to h i m , t h e t y p i c a l l a n d f o r m s (high relief, a b r u p t c o n t a c t s , cliffs a n d c i r c u l a r a p p e a r a n c e ) a r e a r e s u l t of regional tectonics. R e c e n t l y , t h e geology, c h e m i s t r y a n d p e t r o l o g y of t h i s c o m p l e x of a r e a - 1 2 k m 2 w e r e s t u d i e d
( G h o g o m u , 1984) a n d it w a s s h o w n t h a t it c o n s i s t s of two c o g e n e t i c c e n t r e s ( c e n t r e I a n d c e n t r e 2) w i t h > 9 0 % of t h e r o c k s of i n t e r m e d i a t e c o m p o s i t i o n (silica v a l u e s b e t w e e n 52-65%).
PETROGRAPHY
T h e r o c k t y p e s of t h e N t u m b a w c o m p l e x w e r e d e s c r i b e d in d e t a i l b y G h o g o m u (1984), w h o s h o w e d t h a t t h e two i n t e r s e c t i n g c e n t r e s of t h e c o m p l e x are i n t r u s i v e i n t o t h e r o c k s of t h e granito-gneissic basement complex of Nseh and N s o p a n d p a r t l y m a s k e d b y t h e late b a s a l t flows of N t u m b a w (Fig. lb). C e n t r e 1 (C i), t h e o l d e r of t h e two, o c c u p i e s t w o - t h i r d s of t h e c o m p l e x , is circular in form and comprises monzodiorlte, monzonite and quartz monzonite; basalt dykes a n d f r e q u e n t q u a r t z v e i n s o c c u r . C e n t r e 2 (C2) is c o m p o s e d of m o r e d i f f e r e n t i a t e d r o c k s w i t h a b o r d e r facies. T r a c h y t e a n d q u a r t z - f e l d s p a r porphyry dykes also occur. For both centres, the different rock types occur in a r c u a t e o r c i r c u l a r a r e a s w i t h p r o g r e s s i v e v a r i a t i o n f r o m o n e r o c k t y p e to a n o t h e r . T h e p e t r o g r a p h y , relative m o d a l a b u n d a n c e a n d m i n e r a l o g y of r e p r e s e n t a t i v e s a m p l e s of t h e s e r o c k s are g i v e n in T a b l e 1. T h e r e is c o n t i n u o u s v a r i a t i o n in t h e t e x t u r e a n d m i n e r a l o g y of t h e r o c k s . F o r c e n t r e 1 t h e r o c k s are m o s t l y c o a r s e grained porphyritic and dark-grey, while those
2
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Fig. 1 a) Location of the anorgenic complexes in Cameroon showing the position of the Ntumbaw complex (A = volcanic rocks, B = younger granites, C = sedimentary rocks, D = basement complex).
of centre 2 are m e d i u m to fine-grained a n d m u c h m o r e leucocratic. E a c h centre s h o w s inverse zoning, the m o r e evolved r o c k s occurring tow a r d s the periphery. The m o r e or less contin u o u s variation of the m o d a l c o m p o s i t i o n of t h e r o c k s from centre i to centre 2 is m a r k e d b y a n increase in the p e r c e n t a g e of alkali feldspar a n d quartz a n d a d e c r e a s e in t h a t of plagioclase (Fig. 2). A m o n g the minerals, plagioclase d e c r e a s e s in An c o n t e n t from An a7 in the monzodiorites to An isin the s y e n o g r a n i t e s (Fig. 3). This progressive d e c r e a s e is c o n s i s t e n t with the fractionation of m o r e calcic minerals from a less evolved m a g m a . T h e s e t r e n d s in texture, m o d e s and mineral c h e m i s t r y s u g g e s t t h a t b o t h c e n t r e s belong to the s a m e m a g m a t i c suite with centre
2 having b e e n formed j u s t after t h e final s t a g e s of centre i. MAJOR, TRACE AND RARE-EARTH ELEMENT CHEMISTRY The w h o l e - r o c k c h e m i c a l d a t a are listed in Table 2 {for sampling a n d analytical m e t h o d s see Ghogomu, 1984). Locations of r e p r e s e n t a t i v e s a m p l e s are given in Fig. 1. The variation of t h e s e e l e m e n t s (as s h o w n on H a r k e r diagrams, Ghogomu, 1984, Fig. 41) reveals several trends: AI20 a, CaO, total iron, MgO a n d TiO 2 d e c r e a s e with differentiation suggesting t h a t fractionation of f e r r o m a g n e s i a n m i n e r a l s a n d m o r e calcic plagioclase occurred. Only K20 a n d Na20 s h o w
The Ntumbaw
Complex, NW Cameroon
3
Fig. lb) Petrographicmap of the Nntmbaw complex (1 = granito-gneissicbasementcomplex, 2 = monzodiorite, 3 = monzohite, 4 = quartzmonzonite, 5 = quartz syenite, 6 = quartz-biotite syenite, 7 = syenogranite, 8 = granophyre,9 = machytic dyke, 10 = quartz-feldsparporphyry dyke, 11 = tzachyte dome, 12 = basaltflow). Rock Type NS4, monzodiorite
NS6, monzonite
NS7, quartz monzonite
T1,T5, quartz syenite
NS 12, syenogranlte NS 13, basalt dyke T l l , trachyte dyke H2, quartz-feldspar porphyry
Texture~ colour coarse to medium grained, porphyritic (dad-grey feldspars) coarse grained porphyritic (darkgrey feldspars) coarse grained porphyritic, light-grey
medium to fine-grained, light-grey medium to fine-grained, miarolitic, light-greyish, green amphibole needles fine-grained porphyritic, greenish black sanldlne fine-grained porphyritic, pyromeride at contact, greenish-grey fine to medium-grained, greenish brown
Phenocrysts andesine (52%) perthile (20%) augite (11%) andesine-oligoclase (42%) orthoclase (24%) calclc ollgoclase (31%) orthoclase (32%) quartz (6%)
Groundmass hypersthene (2.4%) biotite (7.5%) amphibole (2.3%) quartz (2.3%) augite (10%) amphibole (5%) blollte (6.8%) hypersthene (2.8%) quartz (3.2%) amphibole (7.8%) augile (7.8%) hypersthene (1.5%)
orthoclase (43%) oligoclase (24%) quartz (10%)
biolite (4.5%) actinolite (4.2%) ferroaugite 3.8%) opaques (3.8%)
orthoclase (44%) ollgoclase (21%) quartz (19%)
katophorite-ferrorichterite (5.0%) opaques (5.5%), biotite (2.5%) ferroaugile (1.8%) brown hornblende augite opaque
plagioclase olivine transformed to serpentine and Fe oxide sanidine spherulites aegirine-augite diopside opaques quartz orthoclase plagloclase An 26
plagioclase fayalite opaques opaques green amphibole
Table 1 - Petrographic characteristics of selected rocks of the Ntumbaw complex.
Accessories apatite zircon magnetite rutile apatite sphene zircon opaques apatite zircon serieite damourlte chlorite apatite sphene chlorite calcite sericite apatite sphene
alkali feldspar
biotite clinopyroxene
4
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well in t h e so-called Daly G a p of t h e s e m a s s e s .
o •
s
•
4
A
3
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2
Q
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Fig. 2 Modal composition of the rocks of the Ntumbaw complex on a QA-P diagram (Streckeisen, 1976) (I = monzodiorite, 2 = monzonite, 3 = quartz monzonite, 4 = quartz syenite, 5 = syenogranite).
a positive correlation with silica d u e to the late c o n c e n t r a t i o n of alkali feldspar. The r o c k s of this complex, although p r o b a b l y n o t all representing liquids, are alkaline according to the limits proposed b y Macdonald and Katsura (1964) a n d Miyashiro (1978). The r o c k s from N t u m b a w are more a l u m m o u s (Fig. 5) t h a n t h o s e of the Nigerian Younger Granites ( J a c o b s o n et al., 1958). C o m p a r e d to t h e r o c k s of the Air province (Husch, 1982; Moreau, 1982). r o c k s from N t u m b a w lie b e t w e e n t h e b a s i c m e m b e r s (gabb r o s a n d anorthosites) a n d the salic ones, fitting
T r a c e - e l e m e n t a n a l y s e s are given in Table 3. Ba a n d Sr s h o w a negative correlation with silica since t h e s e e l e m e n t s were p r e s u m a b l y c a m o u flaged b y Ca in t h e calcic m i n e r a l s (plagioclase a n d augite) f r a c t i o n a t e d from t h e m a g m a . Rb s h o w s a slight positive correlation with differentiation a n d is t h o u g h t to b e i n c o r p o r a t e d in potassic minerals (Heier a n d A d a m s , 1964; Lange et al., 1966) s u c h a s K-feldspar a n d micas, w h i c h also s h o w a general c o n c e n t r a t i o n with differentiation. In s o m e anorogenic c o m p l e x e s like Liruei, Nigeria (Butler et al., 1962), high Rb v a l u e s ( - 1 6 0 0 ppm) are linked to cassiterite mineralization; N t u m b a w with Rb v a l u e s ranging from 7 2 - 2 4 0 p p m s e e m s to b e sterile. For t h e t r a n s i t i o n e l e m e n t s (except Co) a strong negative correlation with differentiation exists. S u c h a t r e n d h a s b e e n d e m o n s t r a t e d b y Treuil (1973), Treufl a n d J o r o n ( 1975,1976) as a m a r k e r of fractional crystallization. C o n c e n t r a t i o n s of REE are given in Table 4; c h o n d r i t e - n o r m a l i z e d REE p a t t e r n s are s h o w n on Fig. 6. All t h e r o c k s are rich in R E E with a preferential e n r i c h m e n t in LREE. - L R E E / H R E E ratios are high a n d d e c r e a s e with differentiation. L a / Y b v a l u e s v a r y from 66 in the monzodiorites to 12.7 in t h e acid d y k e s portraying a s h a r p negative trend. a negative Eu a n o m a l y o c c u r s a n d i n c r e a s e s with differentiation, as r e s u l t of t h e fractional crystallization of plagioclase from t h e m a g m a . -
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Fig. 3 A) Composition of the feldspars on an Ab-An-0r dia£Tam (symbols as in Fig. 2). B) Chemical variation of the plagioclases : a = syenogranites, b = quartz syenites, c = quartz monzonites, d = monzonites, e -- monzodiorites.
The Ntumbaw Complex, NW Cameroon
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Fig. 6 Combination of all REE patterns for the rocks o f the w h o l e complex. a = plutonites o f C1, b = plutonites of C2, c = volcanites o f C1 and C2.
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Rb Sr Cu Ni Cr V Co
centre 2
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NS4
NS8
R3
NS6
NS3
TI0
ST5
NSI
NS7
2852 75 1301 23 198 501 : 209 13
2699 93 1598 16 175 431 150 77
3016 96 1671 12 198 470 157 51
3073 108 1556 20' 222 551 217 30
3086 104 1535 14 175 438 177 21
3032 115 1571 18 175 432 158 75
2850 104 1543 15 206 480 177 44
2495 131 1383 25 164 420 198 28
2084 2810 109 126 1306 1239 20 68 166 343 400 612 164 185 54 44
r25
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2.4 4.3
2.43 4.7
2.6 4.7
2.9 4.6
2.96 4.2
3.09 4.4
2.87 3.2
3.49 3.8
3.1 3.9
3.1 5.1
K/Rb Rb/Sr O/Nil 8r/Ca B~¢K B~/Sr
320 0.058 2.53 0.030 0.119 2.192
261 0.058 2.46 0.034 0.111 1.689
271 0.057 2.37 0.036 0.116; 1.805 i
267 0.069 2.48 0.034 0.105 1.975
285 0.067 2.50 0.037 0.104 2.010
269 0.073 147 0.038 0.098 1.930
276 0.066 2.33 0.048 0.099 1.847
266 0.095 2.56 0.036 0.072 1.804
285 0.083 2.41 0.033 0.067 1.596
246 0.102 1.78 0.02.4 0.091 2.28
K
2799 105 1473 28 183 451 178 44 I 2.87 4.2 276 3.072 2.46 ).036 ).095 1.85
quartzsycni¢
sycnogranite
average
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NSI0
T5
Tlb
NSa
NSI1
NSI2
H2
H4
TII
992 76 323
1040 88 322
1014 121 252
1290 1140 92 113 484 202
1145 123 197 <10 13 14 39 21
33 242 20
27 227 19
37 149 17
753 139 191 <10 <10 29 42 35
2.92 1.98
2.92 1.7
2.92 1.6
3.2 2.0
3.47 1.1
3.83 1.0
3.96
3.74
3.73
3.41 1.04
307 0.56
311 0.62
164 12.1
165 11.9
250 8.76
267 3.96
~..020 0.034 5.644
1.08 0.020 0.03O 5.812 1.65
1.42
2.~8
0.024 3.54
384 0.24
332 241 348 0 . 2 7 3 0.429 0.188 1,75 -4.8 0.016 0.019 0.018 0.024 0.034 0.036 0.035 0.040 3.090 3.23 "3.60 2.64 --
Table 3 - Trace-element (,ppm) ,and major-element (weight %) abundances.
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R.GHOOOMUet al. DISCUSSION AND CONCLUSION
C h a y e s {1963) s h o w e d t h a t p u b l i s h e d analyses of oceanic-island volcanic rocks all over t h e world c l u s t e r into two m a i n groups, one basaltic a n d the o t h e r trachytic. He convincingly dem o n s t r a t e d the rarity of lavas in the range 53% < SiO 2 < 57%, the ,Daly Gap,. Several a u t h o r s (Harris, 1963; Macdonald, 1963; Carm, 1968; Baker, 1968) have a t t e m p t e d to explain t h i s gap as d u e to a bias in s a m p l e collection. Le Maitre (1968) a n d G a s s a n d Mallick (1968) p o i n t e d out t h a t the rarity of i n t e r m e d i a t e lavas could be due to the eruptive process, s u c h t h a t t r a c h y a n d e s i tic m a g m a formed at d e p t h w a s not e r u p t e d at the s u r f a c e as readily as more basic a n d acid m a g m a . B r y a n (1964) a n d Wilkinson (1966) have shown, however, t h a t silica gaps are characteristic of differentiated i n t r u s i o n s , w h i c h provide some of the b e s t evidence of fractional crystallization. On the o t h e r h a n d , Wyllie (1963) p r e s e n t e d m o d e l s where s u c h silica g a p s c a n be p r o d u c e d d e p e n d i n g on the g e o m e t r y of the p h a s e diagram, implying t h a t fractional crystallization m i g h t n o t be a prerequisite. According to Macdonald e t al. (1970), the rarity of s u c h rocks in alkaline provinces is linked to the high ratio of acid to basic lavas, the acid lavas being too v o l u m i n o u s to be considered residual liquids of fractional crystallization of the observed volume of basalts. C h a y e s (1977) a n d Clague (1978) s h o w e d t h a t this statistical bimodality could be due to the choice of the f r a c t i o n a t i o n index. Chemical p a r a m e t e r s like SiO 2, CaO a n d the T h o r n t o n a n d Tuttle index a s s u m e d to vary directly with increasing crystal f r a c t i o n a t i o n m i g h t not always be reliable. Incompatible elem e n t s like p h o s p h o r u s (Clague a n d B u n c h , 1976) a n d z i r c o n i u m (Clague, 1978) m a y provide a more quantitative m e a s u r e of fractionation in t h e liquid d e s c e n t from b a s a l t to trachyte. Anorogenic complexes are generally of alkaline type with plutonic a n d subvolcanic rocks ranging from gabbro, m o n z o n i t e a n d syenite to granite, while volcanic rocks range from b a s a l t to t r a c h y t e a n d rhyolite (Bonin, 1982). In t h e s e complexes t h e r e is a l m o s t always a gap b e t w e e n the gabbros a n d syenites, which in some complexes is partly occupied b y hybrid rocks whose a c t u a l composition does not c o r r e s p o n d to t h a t of a m a g m a . E x a m p l e s of s u c h h y b r i d rocks include : t h e monzodiorites, dioritic breccias a n d granodiorites rich in xenolith f r a g m e n t s of the Belknap complex U.S.A. (Model, 1936); - the m a g m a t i c breccias of the Sara-Fier complex, Nigeria (Turner, 1963); the m o n z o - a n o r t h o s i t e s a n d m a g m a t i c
breccias of Ofoud a n d Taguei, Niger (Moreau, 1982; Moreau e t al., 1987). The N t u m b a w complex is atypical b e c a u s e over 90% of its rocks are of i n t e r m e d i a t e composition r a n g i n g from monzodiorite to syenogranite with silica v a l u e s b e t w e e n 52 a n d 65%. Hybrid rocks have not b e e n observed in this complex. Petrography, m i n e r a l o g y a n d geochem i s t r y show t h a t the rocks are cogenetic with fractional crystallization, t h e principal process of t h e i r formation; the rocks fall a l m o s t entirely in the ,Daly Gap, of o t h e r complexes. Unfortunately, basic a n d u l t r a b a s i c r o c k s a n d chilled facies are a b s e n t so t h a t t h e c o m p o s i t i o n of t h e p r i m a r y m a g m a is u n k n o w n , as is t h e extent of the possible c r u s t a l or m a n t l e origin for the i n t e r m e d i a t e rocks. REFERENCES
Baker, I. 1968. Intermediate oceanic volcanic rocks and the ,Daly Gap,. Earth Planet. ScL Lett. 4. 103106 Bonin, B. 1982. Les granites des complexes annulaires. Manuel et methodes, n°4, B.R.G.M. Orleans, 183 p. Bryan, W.B. 1964. Relative abundance of intermediate members of the oceanic basalt-trachyte association : evidence from Clarion and Soccoro Islands, Revfllagigedo Islands, Mexico. J. Geophys Res. 69, 3047-3049. Butler, J.R., Bowden, P. and Smith, A.Z. 1982. K/Rb ratios in the evolution of the Younger Granites of Northern Nigeria. Geochem. Cosmochem. Acta 26, 89-100. Cann, J.R., 1968. Bimodal distribution of rocks from volcanic islands. Earth Planet. ScL Lett. 4, 479-480. Chayes, F. 1977. The oceanic basalt-trachyte relation in general and in the Canary Islands. Amer. Mineral. 62, 666-671. Clague, D.A. 1978. The oceanic basalt-trachyte association : an explanation of the Daly Gap. J. GeoL U.S.A. 88, 724-743. Clague, D.A. and Bunch, T.E. 1976. Formation of ferrobasalt at East Pacific mid-ocean spreading centres. J. Geophys. Res. 81, 4247-4256. Gass, I.G. and Mallick, D.I.J. 1968. Jebel Khariz : an Upper Miocene strato-volcano of commenditic affinity on the South Arabian Coast, Bull. Volcan. 32-1, 33-88. Ghogomu. R.T. 1985. Geology, geochemistry and petrology of the Ntumbaw anorogenic complex, Cameroon : an example of a ring complex of intermediate composition. Unpublished doctoral thesis, University of Nancy I. 143 p. Harris, P.G. 1963. Comments on a paper by F. Chayes. ,Relative abundance of intermediate members of the oceanic basalt-trachyte association~. J. Geophys. Res, 88, 5103-5107. Heier, K.S. and Adams, J.A.S. 1964. The geochemistry of the alkali metals. Phys. Chem. Earth.. $, 253381.
The Ntumbaw Complex, NW Cameroon Husch, J.M. 1982. Geology, petrology a n d geochemistry ofanorthositic a n d associated rocks, Republic of Niger. Ph. D. thesis, Princeton 231 p. (unpublished). J a c o b s o n , R.R.E. MacLead, W.N. and Black, R. 1958. Ring complexes in the Younger Granite province of Northern Nigeria. Mem. Geol. Soc. Lond. I, 70 p. J a c q u e m i n , H. 1981. Contribution a r e t u d e g6ochimique des elements en trace a la petrogen6se des complexes anorog6niques (Nord Carneroun). These de 3eme cycle Nancy I, 197 p. Lange, I.M. Reynolds. R.C., Lyons, S.B. 1966. K / R b ratios in coexisting K-feldspars a n d biotites from s o m e New England granites a n d metasediments. Chem. Geol. I, 317-328. Lasserre, M. 1966. Confirmation de l'existence des g r a n i t e s t e r t i a i r e s a u C a m e r o u n , (Afrique Equatoriale). Bu//. B.R.G.M., 3, 141-148. Le Maitre, R.W. 1968. Chemical variations between a n d within volcanic rock series - a statistical approach. J. Petrol. 9, 220-252. Macdonald, G.A. 1963. Relative a b u n d a n c e of intermediate m e m b e r s of the oceanic basalt-trachyte association - a discussion. J. Geophys. Res. 68, 5100-5102. Macdonald, G.A. a n d Katsura, T. 1964. Chemical composition of Hawaiian lavas. J. Petrol. 5, 82-133. Macdonald, R., Bailey, D.K., Sutherland, D.S. 1970. O v e r s a t u r a t e d peralkaline glassy trachyte from Kenya. J. Petrol. I I , 507-517. Miyashiro, A. 1978. Nature of the alkalic rock series.. Contr. Miner. Petrol. 66, 91-104. Modell. D.A. 1936. Ring-dyke complex of the Belknap Mountains, New Hampshire. Bull. Geol. Soc. Am. 47, 126-144. Moreau, C. 1982. Les complexes annulaires anorogenique & suite anorthositique de rAIr Central et Septentrional (Niger). These Doct. d'Etat Univ. Nancy I 356 p. Moreau, C., Brown, W.L. a n d Karche, J.P. 1987. Monzo-anorthosite from the Taguel ring-complex, Air, Niger : a hybrid rock with c u m u l u s plagioclase a n d an infiltrated granitic i n t e r c u m u l u s liquid ? Contr. Miner. Petrol. 95, 32-43.
Morin, S. 1980. Apport des images l a n d s a t & la c o n n a i s s a n c e de la s t r u c t u r e des h a u t e s terres de r O u e s t C a m e r o u n . Geogr. Phys. Dept.. Geogr. Yaounde, 181-196. Njonfang. E. 1986. Un exemple de complexe plutovolcanique tertlaire a roches b a s i q u e s et intermediaires : le m a s s i f de Nda Ali, S u d - O u e s t du Cameroun. Unpublished thesis, University of Yaounde, C a m e r o u n , 171 pp. Parsons, I., Brown, W.L. a n d J a c q u e m i n , H. 1986. Mineral chemistry and crystallization conditions of the Mboutou layered gabbro-syenite-granite complex, North Cameroon. Jour. Petrol. 27, 1305-1329. Peronne, Y. 1969. Notice explicative s u r la feuille Wum-Banyo. Dir. Min. Geol. C a m e r o u n . 59 p. Streckeisen, A.L. 1976. To each plutonic rock its proper name. Earth Sci. Rev. 12, I, 1-34. Treuil, M. 1973. Criteres p6trologiques, g6ochimiques et s t r u c t u r a u x de la gen6se et de la diff6renciation des m a g m a s basaltiques - exemple de l'Afar. These Doct. d'Etat. Orleans. Treuil, M. a n d Joron, J.L. 1975. Utflisation des eILm e n t s h y g r o m a g m a p h i l e s p o u r la simplification de la mod61isation quantitative des p r o c e s s u s m a g m a tiques. Exemple de l'Afar et de la dorsale m6dio aflantique. Soc. Ital. Miner. Petrol. Y ~ X I , 125-174. Treuil, M. and Joron, J.L. 1976. Etude geochimique des elements en trace d a n s le m a g m a t i s m e de rAfar. Implication p6trog6netique et c o m p a r a i s o n avec le m a g m a t i s m e de l'Islande et de la dorsale m6dioaflantique. Afar depression of Ethiopia. Pilger et R6sler (eds.) Stuttgart, E. Schweizerbarts'she Ver lags-buchhandlung, 2 . 2 6 - 7 9 . Turner, D.C. 1963. Ring s t r u c t u r e s in the Sara-Fier complex, Northern Nigeria. Geol. Soc. London Quart. Jour. 119, 345-366. Wilkinson, J . F . G . 1966. Residual glasses from some alkali basaltic lavas from New South Wales. Mineralog. Mag. 35, 847-860. Wyllie, P.J. 1963. Effects of the changes of slope on liquidus a n d solidus p a t h s in the s y s t e m diopsideanorthite-albite. Min. Soc. Am. Spec. Pap. I, 204-212.
9