Origin and evolution of the late precambrian high-grade Yaounde Gneisses (Cameroon)

Precambr~an Research, 38 (1988) 91-109 Elsevier Science Publishers B V, Amsterdam - - Printed in The Netherlands

91

ORIGIN AND EVOLUTION OF THE LATE PRECAMBRIAN HIGHGRADE YAOUNDE GNEISSES (CAMEROON) j p N Z E N T I ~, p BARBEy1,2, j M A C A U D I E R E 23 and D SOBA ~ ILaborato~re de Petrolog~e, Unwers~te de Nancy I, BP 239, 54506 Vandoeuvre les Nancy Cedex (France) 2Centre de Recherches Pdtrographtques et Gdochtmtques, BP 20, 54501 Vandoeuvre les Nancy Cedex (France) %aboratotre de Pdtrolog~e, E N S G, BP 452, 54001 Nancy Cedex (France) ~Inst~tut de Recherche G~olog~que et Mm~re, BP 4110, Yaound~ (Cameroon) (Recewed March 17, 1987, rewsmn accepted September 7, 1987)

Abstract Nzenti, J P , Barbey, P , Macaudmre, J and Soba, D, 1988 Ongm and evolutmn of the late Precambrian high-grade Yaound~ gnelsses (Cameroon) Precambrian Res, 38 91-109 The Yaound~ series is composed of low to high-grade garnet-bearing schists and gnelsses belonging to the PanAfrican North-equatorial fold belt The high-grade rocks consist of kyanlte-garnet gneisses and garnet-plagmclase gnelsses containing layers of calcsilicate rocks, marbles, quartzites and magnetite-rich orthopyroxenltes The chemical patterns of these rocks are those of a sedimentary sequence of shales and greywackes and minor dolomite-rocks, dolomltlC marls, evaporltic beds, quartzltes and Iron-rich sediments Some volcanogenlc greywackes may have been of alkaline affinity This sequence was probably deposited in shallow-water, near-shore, semi-barred conditmns which can be related either to an intracontinental dlstenslve environment or to a passive margin at the northern edge of the Congo craton The Yaound~ gneisses were deformed during two mare tectomc phases the first phase (D~) is marked by a layering ( S o - S 1) resultmg from tectonic transposition, the second one (D2) corresponds to tangentml tectonics with isochnal folds and flat-lying shear zones associated with a Se schistosity and a Le hneatlon, a third phase (late-D2) is marked by the development of low temperature mylonitlc shears Scarce wrench faults indicate later E-W compression The D1-D2 transltmn is characterized by the emplacement of maflc and ultramafic rocks and by the development of highpressure granuhte facies condItmns (T=750-800°C, P = 10-12 kbar) These peak metamorphic conditions triggered a widespread migmatlzatmn dated at 565_+22 Ma and coeval with the De event The gradual decrease of the metamorphic conditions toward the south and the existence of a syn-De, retrograde, Inverse metamorphic gradient, suggest that the overall structure of the Yaound~ series is a large tectonic nappe thrust onto the Congo craton A geotectonic scheme for the North-equatorial fold belt is discussed

Introduction

The North-equatorial fold belt, previously called the mobile belt of central Africa, runnmg E - W to the north of the Congo craton from the Republic of Central Africa to Cameroon and extending probably farther west into Brazil, remains poorly known as yet although it constl-

0301-9268/88/$03 50

tutes a key-structure of the Pan-Afrman orogeny After the model of Kroner (1977), this fold belt has generally been considered as an mtracontmental orogen without slgmficant accretion (e g Bessoles and Trompette, 1980) More recently, Poldevm (1983), Ball et al (1984 ), Jegouzo (1984) and Nzentl et al (1984) considered that the southern part of the fold

© 1988 Elsevier Science Publishers B V

92 belt, overthrustmg the Congo craton, was charactemzed by horizontal tectonic regimes assumed to be related to continental colhsmn In Northern Cameroon, recent data clearly suggest that crustal accretion occurred during the Pan-African orogeny andthatthetectono-metamorphlc evolutmn is compatible with a colhsion (Toteu et al, 1987) Nevertheless, some important questions arisen by this colhsional model are still a matter of debate and more partlcularly the positron of the suture remains unknown In the southern part of the belt (Fig la,b), the following units have been distinguished from south to north (Bessoles and Lasserre, 1977, Lasserre and Soba, 1979) (1) a unit (Ntem complex) of high-grade metamorphic and igneous rocks (amphlbohtes, charnockltes, gabbros, granodmntes and syenltes) containing remnants of greenstones and banded iron formatmns; whole-rock Rb-Sr radmmetrm dating give Archaean ages for both charnockltes (2850___65 Ma) and granodlorites (2790_+ 120 Ma), (2) a u m t of low to medmm-grade schists of sedimentary o n g m (Mbalmayo series), charactemzed by a northward increasing metamorphic gradient and passing m c o n t m m t y to, ( 3 ) a umt of garnet-bearing mica schists and gnelsses (Yaound~ series) metamorphosed during the Pan-African orogeny (565 + 22 Ma), the metamorphic conditions increase northwards and reach a maximum at the latltude of Yaoundg (granuhtes and mlgmatites) and then decrease farther north (Nzentl et al, 1984, J Mvondo, personal commumcatmn, 1985, Nedelec et al, 1986) The Ntem complex corresponds to the northern part of the Congo craton and the Mbalmayo and Yaound~ series belong to the edge of the North-equatorial fold belt This study is concerned with the highest grade rocks of the Yaound6 series constituting the core of a large-scale nappe structure New chemical, structural and petrogenet~c data are presented The origin and the tectono-meta-

morphm evolution of the Yaound~ series and their bearing on a geodynamlc model are discussed.

Petrography The Yaound~ series are made up of two distinct rock units The first unit is of sedimentary parentage and the second of clear ~gneous ongin Both contain quartzo-feldspathm segregations amsmg from either in situ partial melting or injection along dykes or ductile shear zones Mlgmatization and related rocks will be described elsewhere All these rocks crop out in mselberg rehefs surrounding Yaound~ However, detailed observations and sampling were essentially made in quarrms located m this town or m its close vicinity (Fig lc) The metased~mentary unit This u m t is predominantly composed of kyamte-garnet gneisses and garnet-plagloclase gnelsses m which lenses of marbles and calc-slhcate rocks are interspersed, occasionally accompanied with quartmtes and magnetite-rich orthopyroxemtes Granoblastm textures prevail in all rock types but flaser and mylomtm ones are frequently observed Kyanlte-garnet gnelsses and garnet-plagioclase gne~sses are medium to coarse-grained layered rocks with alternating mflhmetre to centlmetre thick alumina-rich and quartzo-feldspathlc layers This layering is enhanced by migmatlzatlon Kyamte-garnet gnelsses are the most abundant type They are composed of quartz (25-30%), perthitlc K-feldspar (15-20%), plagloclase An3o_38 (20-30%) occasionally antiperthltlc, zoned garnet porphyroblasts (15-20%), biotite (5-10%) and kyamte (2-5%) The occurrence of garnet-plagioclase gnelsses is more restricted They are composed of quartz (30-40%), plagmclase An4o_5o (40-50%), garnet (10-20%) and bmtlte (5-10%) Accessories are graphite, rutlle, zlr-

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20km

Fig 1 Geologmal setting of Yaoundd area (a) Structural trends of Cameroon (after JSgouzo, 1984 and Dumont, 1986), (b) geologmal sketch map of the Pan-Afmcan North-equatomal fold belt m the Yaound$ area (after Champetmr de Rlbes and Aubague, 1956 and Nedelec et al, 1986), symbols 1 - Congo craton, 2 - Mbalmayo series, 3 and 4 - Yaound~ seines (garnetbearing mma schists and mlgmatltm gnelsses, respectwely), (c) samphng quarmes con and m o n a z l t e T h e s e gnelsses grade into k y a m t e - b l o t ] t e - g a r n e t - m u s c o v l t e schists with decreasing m e t a m o r p h i c grade Marbles and calc-slhcate rocks occur as dis-

c o n t i n u o u s layers, m o s t l y b o u d m a g e d m k y a n l t e - g a r n e t gnetsses Marbles are characterized b y a z o n e d structure m a d e up of two reaction coronas with the surroundmg gnelsses T h e core

94 ~s the most frequently composed of both calcite and dolomite (90%), dmpslde, pargaslte and accessory phlogoplte, more restricted types are composed of ferroan dolomite (60%), calcite (5%), chnopyroxene (25%) and ohvme (10%) rimmed with chnopyroxene. The innermost corona is composed essentmlly of dmpslde, calcite, dolomite and pargasltm hornblende The outermost corona ~s made up of a symplectltlc assocmtmn of pargasltm and edemtm hornblende, bmtlte and plagmclase (An34_45), and is frmged toward the pehtlc gneiss by a thin bmt~te-nch n m Three mare types of calc-sdmate rocks are mmeraloglcally distinguished These are m decreasing order of abundance (1) quartz (15-45%), scapohte (30-50%), plagmclase ( < 30% ), chnopyroxene (10%), garnet (10%) and magnetite, calcite and sphene as accessories, (2) scapohte ( 50% ), chnopyroxene (30%), pargasltlc hornblende ( < 5 % ) , plagmclase ( < 10% ), quartz (5-10%), magnetite, pyrite and sphene, (3) dolomite (30%), calcite (10-15%), chnopyroxene (30%), scapohte (10%), quartz (5-10%) and sphene Retromorphlc zolz~te is frequently present m addltmn to the above assemblages Pecuhar rock types are found assocmted with calc-sfllcate rocks and marbles These are (1) amphlbohtes composed of pargasltlc hornblende (40-50%), dmpslde ( 20-30% ), scapol~te (5-20%), plagmclase (10-15%), garnet ( < 5% ), quartz ( < 10% ), bmt~te, sphene and calcite, (2) garnet-bearing quartz~tes composed of quartz (40-70%), garnet (5-30%), ferros~hte (0-10%), minor bmt~te, graphite and rutile, and (3) magnetite-rich orthopyroxemtes composed of ferrosd~te (40%), quartz (30%), magnetite (15%), garnet (15%), and minor apat~te and probably equivalent to eulys~tes The meta-tgneous unit

Pynclas~tes constitute the dominant rock type and occur, more partmularly, as a large body intruding the metased~mentary umt m the

town of Yaoundd They are fine to medmmgrained, dark coloured rocks displaying a weak layering They are composed of plagloclase An3o_35 (35-50%), chnopyroxene (10-25%), garnet ( < 1 0 % ) , biotite (10%) and quartz (5%) Accessories are magnetite, apatlte and zircon Some of them contain abundant retromorphlc polklloblastlc hornblende ( < 20% ) replacing pyroxenes whereas some others contam a slgmficant proportion of allamte, sphene and apatlte These pynclasltes contam locally cent~metre s~zed mclusmns of pyroxemtes A few other rock types, with unusual mineralogy, occur as dykes that cross-cut the regmnal fohatlon or that have been rejected along ductile shear-zones. These rocks are characterized by a very high content of accessory phases and are reminiscent of alkahne rocks They comprise (1) garnet pyrlbohtes made up of chnopyroxene (40-45%), hornblende (25-30%) partly replacing pyroxene, bmtlte (10-15%), plagmclase An45_5o (10%), garnet ( < 5 % ) , quartz ( < 5% ) and apatlte, allamte, zircon, sphene and magnetite, (2) pyroxemtes composed either of orthopyroxene (60-70%), garnet (15-20%), magnetite (15-20%), apatlte (5%) and quartz ( < 2 % ) or of chnopyroxene (40%), bmtlte (30%), garnet (10%), quartz (10-15%), sphene, apatlte and mrcon ( < 10% ) and scarce carbonates, and (3) bmtlte-nch rocks composed predominantly of bmtlte (80-90%), plagmclase (5-10%), quartz ( < 2% ), garnet ( < 2% ), apatlte and allamte (5-10%) Whole-rock chemistry and nature of the protoliths Samples free of mlgmatlzatlon were analysed for major and trace elements Major and trace (except REE) elements were determined by XRF spectrometry, but Na by atomic absorption, at the Umverslty of Nancy (Govmdaraju and Montanan, 1978). Analytical uncertain-

95 ties are currently better than 1% for major elements and at 5 or 10% for trace element concentratmns higher or lower than 20 ppm, respectively Rare earth elements (REE) were determined by emission spectroscopy (ICP) at CRPG, Nancy (Gowndaraju, 1980) Analytical preclsmn is estimated at 5% when chondrlte-normahzed concentratmns are > 10 and at 10% when they are lower REE are normal~zed to chondnte according to the values of Jahn et al (1980)

Metasedtmentary rocks Kyamte-garnet gneisses display all the features of pehtlc rocks with FeO, MgO, K20 and T102 closely correlated to A1203 (Table I and Fig 2a) All these elements are antlcorrelated with silica suggesting that these gnelsses were composed of a quartz-clay mixture Their average composition corresponds closely to those of post-Archaean shales (Taylor and McLennan, 1985 ) Their T102 content (0 98-1 29% ) and T102/A1203 ratms (0 063-0 075) are high w,th respect to those of clay (0 040 according to Goldschmldt, 1954), but correspond to values g,ven for continental or near-shore argillaceous and arenaceous sed,ments (Mlgdlsov, 1960) The trace element contents (Table I) are those of shales Their REE patterns are fractIonated, with a shght negative Eu anomaly (Fig 2d) and closely resemble the NASC (Hask,n et al, 1968) Garnet gnelsses are more scattered in composition (Table I) They display distinctly higher CaO, Na20 and TI02 and lower K20 contents than kyanlte-garnet gnelsses but have similar major element correlation patterns therefore suggest,ng that they can be regarded as metasedlments containing a s~gnificant proportion of clay (F,g 2a) However, the very high T102/A1203 ratios of many samples (0 062-0 130, 0 092 on average) are not those of clay-inch sediments but could indicate that they have been contaminated by a basra to intermediate volcanic component (Spears and

Kanans-Sotmou, 1976) Most samples have low to moderate slhca and high T102 contents and K20/Na20 ratios < 1 and resemble quartzpoor volcanogemc greywackes (Taylor and McLennan, 1985) Their T1, Zr, Nb and Y contents are much higher than those from active or passive margin greywackes (Bhatla and Crook, 1986) The average TI/Zr, Nb/Y ratios and the fractlonated REE patterns of sample MM5 and MM13 (Fig 2d) suggest that these greywackes could be derived from reworkmg of alkaline volcanic rocks Calc-slhcate rocks and marbles are characterlzed by high CaO and MgO contents (Table II) The fact that these two elements are anticorrelated to A1203, S102 and T102 suggests that they were associated in a carbonate phase ( dolomlte) Their TIO2/A12Q ratios (Fig 2a) identical to those of shales clearly indmate the presence of a clay fraction The mixing of a clay fractmn and dolomite is clearly wslble on an A1-Ca-Mg ternary plot (Fig 2c), therefore corroborating that calc-sfllcate rocks and marbles were dolomltlc marls and dolomite-rocks Trace element contents remain low except for Sr which is fairly well correlated to CaO (Fig 2b) The sample CY6 (pyrite-bearing scapollte-rock ) is characterized by very high CaO and Sr contents, relatively high Na20 content and high Na20/CaO ratio with respect to the other calc-sfllcate rocks This qmte peculiar composltmn could correspond to that of an evapomtm layer (Ramsay and Davldson, 1970, Serdyuchenko, 1975, Jarousse et al, 1978)

Meta-~gneous rocks Pyrlclasltes and pynbohtes have the composition of Intermediate rocks (Table III) with a silica content ranging from 53 7 to 60 4% and a rather constant alumina content (14 5% ) The higher FeO and MgO contents of the less slhcarich samples (GS1 and OL5) together with high Cr and NI contents and with constant Y and Nb contents suggest that the,r more mafic composition ~s related to accumulation of pyroxene

-

136 156 231 6 48

below detectlon limit

Zr Nb Y La Ce Nd Sm Eu Gd DY Er Yb Lu

Sr

Rb 108 177 236 14 43 38 13 77 16 34 81 7 83 156 705 703 3 89 436 0 67

99 49

6085 16 96 10 03 0 18 3 78 142 156 3 32 127 0 49

58 74 17 96 1107 0 21 3 84 141 144 3 52 129 023 0 95

9991

2 16bRT

1 NY9a

124 169 214 12 34 34 92 74 14 34 44 792 184 6 29 6 32 3 44 3 93 0 59

99 43

63 46 16 81 8 20 0 15 2 74 123 188 3 63 1 12 0 52

3 MB13a

Garnet-kyamte gnelsses

Total

MnO MgO CaO Na,O K,O TIO* P,O, Ll

ALO, FedA

SIOz

Sample

110 162 218 8 27 4164 87 53 39 36 800 155 6 86 6 68 3 74 4 18 067 86 143 239 9 35 33 93 68 27 32 74 6 66 137 5 80 5 72 3 20 3 52 0 57

99 80

67 14 15 64 761 0 14 2 53 095 125 2 61 098 0 20

66 27 17 38 699 0 13 2 44 0 71 1 12 3 24 1 19 0 12 99 96

5 MS18

4 MM16

88 169 206 10 37 36 73 75 03 34 51 702 135 604 5 80 3 29 3 64 061

91 567 414 19 24 72 20 144 70 74 55 14 46 3 55 10 68 6 84 2 76 2 38 0 37

8 682 512 13 7 75 54 135 44 52 18 7 74 2 11 4 60 132 0 35 0 13 004

74 287 264 9 35 96 660 278 9 12

58 534 339 15 28 71 702 414 16 24

38 189 220 14 30

99 67

99 80 9997

10020

7144 12 52 5 87 008 186 2 97 2 38 0 85 104 066 67 35 15 93 490 0 05 1 14 4 70 3 29 064 135 0 45 99 86

13 NZ2 12 MM13

99 23

99 74

99 83

0 35

11 NKZl 65 10 1433 8 16 0 14 344 2 95 2 43 2 06 105 0 31

10 NZ12 64 57 1674 5 42 007 223 4 14 3 01 193 1 16 0 75

53 96 17 81 10 39 0 16 3 26 7 62 2 15 106 2 31 0 51

52 83 18 68 10 28 0 18 3 89 7 36 2 27 176 2 14 -

67 65 14 89 805 0 23 2 23 0 87 109 3 20 099 0 11 0 21

9 MM12b 5851 17 87 8 49 0 16 3 10 5 76 2 80 1 18 163 0 36

8 MM18

I MM5

6 MM20a

Garnet-plaeoclase gnelsses

Major (wt % ) and trace (ppm ) element analyses of representative garnet-kyamte gnelsses and garnet-plagoclase gnelsses

TABLE I

97

% TIO 2

3

0~

(~

3O

20-

°°

2J I

% CaO

(~)

•

• Oo~ ~

5

[

2

0

10

3

15

10

i~

20

0

200

'

400

Sample / Chondnt e 3r,~

(~

blONTj ILL

~Mg

MM5

101

~--o--e

I

'

i! Ca

MM13

a Nd Eu Dy LIu F--I i I i i ~11 Ce S~m Gd Er fb

Fig 2 Dlstmctwe vanatmn dmgrams for the metasedlmentary umt (a) Plot of T102 vs A1203,the sohd hne corresponds to the TIO2/A1203ratm for clay (Goldschrmdt, 1954), (b) CaO-Sr dmgram, (c) A1-Ca-Mg ternary plot (Jarousse et al, 1978), (d) REE patterns Symbols open circles-- kyamte-garnet gnelsses, sohd circles= garnet-plagnoclase gnelsses, open triangles = calc-sfllcate rocks and marbles rather than to a less differentiated nature Their R E E patterns (Fig 3a) are fractmnated (LaN/YbN----7-18.4), light R E E enrmhed ( LaN/SmN----2-3.9) and display a negative Eu anomaly (Eu/Eu* = 0 7-0 8) Their overall trace element patterns (Fig 3b) suggest that they could be of transitional or alkaline affinity Orthopyroxemtm cumulates are characterlzed by high FeO, P205, Zr, Nb and REE, but low MgO contents (Table III) The R E E pattern of sample MM11 (Fig. 3a) is fractlonated (LaN/YbN = 14 16) with a L R E E enrichment of 200 times chondntm values (LaN/ SmN----217) and a weak negative Eu anomaly (Eu/Eu* = 0 83 ) Much more pecuhar is the chemical composltmn of biotite-rich rocks They are deficient m sdma and rich in K20 (up to 7% ) so that they contain feldspatholds m

their norm They also have high FeO and MgO contents and exceptionally high P20~ and R E E contents The R E E pattern of sample M M 1 5 d (Fig 3a) is very strongly fractlonated (LaN/YbN----682), with high L R E E enrmhment, H R E E depletion and negative Eu anomaly ( E u / E u * = 0 41) All these chemical features are slmdar to those of ultramaflc and mafic varletms of alkahne rocks

Structural evolution The Yaoundd formations show two main deformation events associated with metamorphism Mlgmatlzatlon developed mainly durmg the second one The markers of the deformation history can be analysed as follows

98 T A B L E II Major (wt % ) a n d trace ( p p m ) element analyses of representative calc-sflmate rocks a n d marble Sample

Calc-sihcate rocks 1 CY6

2 MS3

Marble 3 MS7

4 MM3a

5 MS9

6 MM3b

7 CY7

8 MS23

9 MS22b

$10~ Al20~ Fe20~ MnO MgO CaO Na~O KzO T~O~ P20~ L 1

46 19 10 76 10 29 0 16 8 02 17 88 1 84 0 28 0 50 0 43 3 32

48 83 9 59 8 26 0 15 13 35 14 83 1 26 0 89 0 62 0 01 2 30

50 85 4 50 6 48 0 07 16 54 16 01 0 64 0 48 0 17 0 02 4 36

51 03 16 87 13 40 0 26 5 96 8 09 1 29 0 98 1 38 tr 0 52

58 37 12 07 9 72 0 13 4 61 9 54 1 31 0 41 0 70 0 75 2 43

59 16 16 42 14 39 0 62 2 18 1 95 1 22 2 63 1 34 tr 0 91

60 91 11 56 22 10 0 53 2 20 1 91 0 06 0 29 0 64 tr 0 76

70 49 11 07 3 43 0 05 5 15 4 74 1 93 1 51 0 19 0 03 1 28

38 12 4 86 5 79 0 15 15 28 19 73 0 70 0 46 0 26 0 30 14 19

Total

99 67

100 09

100 12

99 78

100 04

99 91

100 20

99 87

99 84

Rb Sr Zr Nb Y

19 376 78

18 55 48

24 57 23

48 150 155 21 42

15 171 81

80 103 253 18 49

68 24 168 17 46

89 110 145

14 208 26

Fohatmn So~S1 This surface ~s conspicuous in peht~c gne~sses that show a composltmnal banding generally parallel to the plane of phyllosflicate ormntatmn This plane is defined by bmtlte and graph~te flakes, kyanite prisms, platy quartz and rutfle needles The schlstoslty moulds garnet porphyroblasts with internal fabrics S~ m their cores and hmpld rims (Ball et al., 1984) Some of these fabrics are S-shaped but never show strong rotational features as described m zones of intense shearing (Schoneveld, 1977; Powell and Vernon, 1979) Some others are nearly straight or have the shape of tight lsochnal folds or look hke stram-shp structures All these S, probably represent the first stages m the deformatron of So and hence show that the present So/S~ banding originated partly by tectomc transpomtmn of So In the garnet-kyamte mma schists, garnet porphyroblasts do not show such complex structures, S, is nearly similar to S~ It

must be due to the dlachronlsm in the growth of garnet which appeared earher m the higher grade rocks. This fohatlon does not seem to be present m the rocks of ~gneous ongm (see later, $2 structures) The distribution of So/S1 planes from the mare quarrms around Yaound~ (Fig lc, Fig 4a) shows that they are roughly in zone around a N 35-40°E, 10-15°NE axis and the girdle presents two maxima The first one represents planes gently d~ppmg towards the NE, and the second one represents planes strongly dipping toward the NW As the outcrops are very scattered ~t is difficult to know the size and the exact shape of the large-scale folds which now affect So/S1 The true nature of the DI phase is unknown as it exhibits neither strong rotatmnal fabmcs nor a clear mineral hneatmn associated with the S~ planes At the end of the first deformatlonal event (D1), at the peak of metamorphism, the ~'aoundd region underwent an episode of mlg-

99 TABLE III Major (wt % ) and trace (ppm) element analyses of representative meta-lgneous rocks Sample

Pynclasltes

Pynbohtes

Pyroxemtes

Bmt~te-nch rocks

1

2

3

4

5

6

7

8

9

GS1

NG6

PC1

OL5

MM7

MMll

MM25

NZ9b

EKD3

10 MM15d

$102 AI20~ Fe20~ MnO MgO CaO Na20 K20 T102 P20~ L1

53 73 14 59 9 67 0 18 7 37 6 36 2 49 2 90 1 18 0 32 0 80

59 96 14 53 8 40 0 18 4 76 4 75 2 54 2 58 1 07 0 24 0 37

60 40 14 40 7 56 0 15 4 82 4 94 2 76 2 79 1 08 0 21 0 53

55 02 13 23 8 76 0 16 10 49 7 27 0 98 2 05 0 78 0 24 0 89

57 80 16 49 8 04 0 14 4 86 6 43 1 95 1 64 1 27 0 41 1 00

49 90 10 98 11 89 0 47 1 71 8 41 0 43 2 51 1 88 1 96 1 87

43 85 4 26 45 03 0 47 1 71 3 26 0 08 0 03 0 25 0 79 0 92

42 90 16 31 16 13 0 35 12 06 3 71 0 43 4 74 0 19 0 07 1 04

46 45 15 31 14 12 0 21 8 81 1 87 1 11 7 62 2 41 0 82 0 98

45 92 18 38 8 93 0 18 5 54 6 16 2 57 4 66 2 55 2 89 1 35

Total

99 59

99 38

99 64

99 87

100 03

99 63

99 73

99 70

99 71

99 13

Rb Sr Zr Nb Y N1 Cr La Ce Nd Sm Eu Gd Dy Er Yb Lu

104 444 94 12 29 150 373 25 21 59 81 36 01 7 60 1 74 6 03 4 79 2 32 2 35 0 37

112 371 195 9 30 93 249 36 38 69 66 34 46 6 76 1 46 5 71 4 15 2 20 1 95 0 33

113 416 240 8 29 89 244 48 70 97 72 42 74 7 57 1 50 5 68 4 02 1 91 1 75 0 26

106 55 145 6 20

70 656 272 10 27

164 162 483 10 40

30 125 81 28 53

206 32 110 16 46 173 798

253 160 209 15 23 141 277

386 588 653

62 41 148 25 86 83 17 56 4 16 13 50 8 36 3 56 2 91 0 44

11

537 37 978 44 375 69 56 54 5 65 32 23 5 95 1 89 0 52 0 13

m a t l z a t l o n w h i c h c e r t a , n l y c o n t r i b u t e d to the d e v e l o p m e n t o f t h e D2 p h a s e T h e f i r s t t r a c e o f

T h e i r d i p r a r e l y e x c e e d s 30 ° a n d is n e a r l y c o n stant m a quarry It can be southward ( Fig 4b)

mlgmatlzatlon has been a 'lit par lit' developm e n t o f g r a n m c m a t e r i a l w h , c h will b e f o l d e d w i t h t h e So/S1 s u r f a c e T h e n g r a n i t i c l i q u i d s h a v e b e e n p r o d u c e d a n d e m p l a c e d all a l o n g t h e D2 e v e n t

or m o r e o f t e n n o r t h w a r d ( F i g 4 c , d ) T h e s p a c i n g o f t h e s h e a r z o n e s v a n e s f r o m 1 0 - 1 5 c m for the t i g h t e r ones to some m e t r e s or m o r e Lo-

C2 shear zones

T h e d e f o r m a t i o n m e c h a n i s m is t y p i c a l l y p l a s t i c flow, b u t t h e a m o u n t of r e c o v e r y is extremely variable from place to place m the same

C2 s h e a r z o n e s a p p e a r as f l a t - l y i n g d u c t i l e z o n e s w h i c h s h e a r e d t h e So/S1 p l a n e so t h a t t h e upper block always moved toward the south

cally, it gives t h e g n e l s s e s a s t r o n g t e c t o n i c b a n d i n g by t r a n s p o s i t i o n of the p r e v i o u s foliation

shear zone Th,s progressive variation m the c o n d i t i o n s o f d e f o r m a t i o n is a l s o s h o w n b y t h e

100 [

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Eu

Sm

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Lu

k Rb

La Nb

Sr Ce

Sm Nd

Zr

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Yb

Fig 3 Chemlcal patterns for meta-lgneous rocks (a) Chondnte-normahzed R E E patterns, (b) Chondrlte-normallzed trace element patterns (Thompson et al, 1983) Symbols open squares = pyrlclas]tes,sohd triangle= orthopyroxemte (sample MM11), open trmngle--blotlte-mch rocks (sample M M I 5 d

N

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/

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,-3

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Fig 4 Structural dlagrams from the Yaound~ area ( Schmldt net, lower hemmphere) (a) whole area So/St schlstos]typlanes, (b) M e y o n M e y e m e , (c) Nkometou, (d) Olembe Symbols I - S0/SI, 2 - $2,3 - C2, 4 - late C2, 5 - F2, 6 - L2, 7 - b o u d m a g e

101

presence of late shears, cutting at low angle across the former ones They developed m mesozonal conditions and are associated with dark fine-grained mylonItes instead of coarser recrystallized rocks (Fig 4c,d) These late shears (C2L) could be considered as a third deformation phase but as they display the same sense of motion and the same flat-lying position as C2 they can be interpreted as resulting from the same stress field under decreasing metamorphic climate The shear planes are underlined by Injectmns of granitic liquids The field relations show clearly that migmatization began earlier than shearing, accompanied it, but stopped before it ( late shears cutting across mlgmatized zones) Some injections of basic magmas along shear planes have been found locally Other structural features are strongly linked to C2 planes as a response to bulk shearing One can particularly mention the S2 schlstoslty, the L2 hneatlon and the F2 mesoscopic folds

$2 schtstostty The S 2 planes are inequally distributed They are mostly visible (1) as an axial plane schistosity for the F2 folds (Fig. 5a), (2) in some mlgmatitic layers injected along C2 surfaces and located obliquely to their walls (Fig 5b), (3) in the metabasites and cutting across migmatitic dykes In the gnelsses they show low dips and their relation with C2 is in perfect agreement with a southward thrusting ( Fig 6b,c). The angle between $2 and C2 is often clearly visible and can reach 25-30 °, indicating locally moderate shear values At a microscopic scale, $2 is marked by elongated shape of quartz crystals, slabs of sliced garnets, prisms of reoriented kyamte and biotite flakes It may show late retromorphm condltions (garnet rimmed by elliptic spots of $2 oriented biotite ) In the metabasites a flat-lying surface Sm is microscopically expressed mainly by the omentation of pyroxenes, amphiboles, quartz and

biotite Its macroscopic expression is uneven, as shown by the examination of the right-angle walls of the quarry of Ngoa Ekele. On the E - W wall one can note ovoid domains devoid of marked schistosity and moulded by strongly sheared and schistosed zones which give, at a decametric scale, the appearance of a boudinage with a roughly E - W stretching direction The same aspect is also visible on the N-S wall of the quarry but the stretching is stronger and parallel to a roughly N-S hneatton in the Sm plane This bearing is confirmed by the egg-box ('en d6me et cuvette') structure of the floor of the quarry (Fig 6a and 7a) So the N 10-30°E direction is the X-axis and the N 100-120°E direction is the Y-axis of finite strain which is also extensive The whole domain is cut-across by several generations of granitic veins A single vein can be straight or gently folded in the ovoid domains and completely sheared and tightly folded in the surrounding zones with a strong Sm axial plane schistosity All these observations lead us to correlate Sm of the metabasites with $2 of the surrounding rocks The same conclusion can be drawn from the contact between gneisses and metabasltes on the outcrops of the Congress Hall.

L2 hneatton This hneation is conspicuous and nearly perfectly oriented in the whole region It has a N 30 °E azimuth and a very low plunge, it is linked to C2 shears or lies in the Se schlstosity and shows all the characters of a mineral and stretching hneatIon

The F~ folds The F2 folds are clearly visible at the mesoscopIc scale They are strongly dissymmetric and show a west vergency in Yaoundd and an east one in Nkometou (NE of Yaoundd) They possess an axial plane schlstoslty which is typically $2 and their axes are roughly parallel to L2 (Fig 4b,c,d) These folds show far better

102

Fig 5 (a) Axialplane schmtosltyfor F2 foldsand (b) obhqueschlstosltym rejectionmlgmatltes reormntatlon by shearing than those of the southern region (Fig 7b) According to the regmnal distribution of S0-S1, which can remain nearly flat (Ball et al, 1984) but is often very steep (Fig 4a), the axes ofmegascoplc folds are nearly parallel to the stretching hneatlon They must be large-scale sheath folds (Coward, 1981, Mattauer, 1981 ) even if the typmal noses of such folds have never been observed On the whole, there is a broad synmlgmatm event which begins by melting, and then develops folding and flat-lying shears as melting fol-

lows on. The stretching hneatlon is always undoubtedly hnked to this event The attitude of the $1 sch~stoslty at the end of the D2 phase with respect to metamorphic lsograds shows that the metamorphism during D2 is of an inverted type Some vertical shear fractures developed under the same thermal conditions as the last flatlying shears, but they indicate an E - W dlrectmn of shortening. They represent probably very early traces of the Sanaga and Adamaoua stmke-shp systems

103

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F~g 6 Mesoscopm and megascopm structural features (a) Structures m pynclas~tes and contact w~th m~gmat~t~c gnelsses (Ngoa Ekele), (b) structures m m]gmat~tm gne~sses from an area of steep fohatlon (Meyon Meyeme), (c) synoptm crosssectmn across the Mbalmayo and Yaound~ series showing the metamorphm patterns ranging from mverse to normal and the b-type tectomcs gradmg to a-type one from south to north Symbols Gn = Gne~sses, y = Gramtm melts, y,g = Pegmat~tm and aphtm vems, PC = Pyr]clas~tes, P ~ = Sch~stosed pynclas~tes 1 - Mbalmayo serms, 2 - Yaound4 serms, 3 - Pynclas~tes, 4 Congo craton -

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Fig 7 Structural diagrams (a) Ngoa-Ekele quarry, (b) Mbalmayo schists (from Nedelec et al, 1986) Same symbols as m Fig 4

104 P - T conditions of m e t a m o r p h i s m

Mineral assemblages Three successive mineral assemblages have been dlstmgmshed m kyamte-garnet gnelsses and related mlgmat:tes. The older assemblage consists of Bt + Ky + Qtz + Rt + Gr (abbrevlatmns of mineral phases according to Kretz, 1983) This association is preserved in garnet porphyroblasts as mclusmns trails wh:ch mimic the early schlstoslty (S,) The highest grade assemblage developed under granuhte facies condltmns during the transition between the D1 and De deformatmnal events and was assocmted w~th mc~p~ent partml melting This assemblage is Qtz + Kfs + Pl(An3o_3s) + Grt + Ky + Rt + Gr + Bt Although bmt~te flakes appear m most cases to be secondary phases, the stability of bmt~te m the h:ghest grade pehtlc assemblages cannot be disregarded The retrograde assemblages developed with increased partial melting during the D2 deformatlonal event Th~s retrogression corresponded mainly to the widespread growth of bmtlte at the expense of garnet m the m~gmatltm rocks, leading to Bt + Ky + Kfs + P1 + Qtz assemblages These bmtltes are alumma-rmh (1 20 < A1TM< 1 25 ) with mtermedmte XMg values (0 64-0 68) The late stages are character:zed by the local growth of sflhmamte needles ( Qtz + Kfs + P1 + Sfl assemblages) and finally by the development of Ms + Qtz + Kfs + P1 assemblages Garnet porphyroblasts of pehtlc gne:sses are on the whole almandme (56-66 mol %), pyrope ( 22-36 mol % ) sohd solutions w:th low Cagarnet ( < 9 % ) and spessartme ( < 3 % ) components These porphyroblasts d~splay a strong optical and chemmal zoning characterized by the succession of several zones alternately hmpld and mclusmn-nch All the garnets display an inclusion-free hmpld n m Several zoning profiles were determined All are characterized

by decreasing MgO and increasing FeO contents from core to rim, with a s:gnificant mcrease of the Fe/Mg atomic ratio m the n m Frequently, FeO increases more rapidly m the turb:d zones than m the hmpld ones (sample NY-3, Fig 8) MnO and CaO display a similar behavlour and decrease m some profiles, whereas they increase m some others (sample CY-1, F~g. 8) In the other rock types, the highest grade assemblages are mainly. (1) Scp(MeTo_77) + Pl(Anso_7o) + DI + Qtz + Grt + Spn + Cal + Dol + Hbl m calc-slhcate rocks and marbles, (2) Opx (XFe = 0 70-0 90) + Qtz + Mag + Grt (XFe=075-0 86, XMg~---020, Xca=0 05-0 10, XMn<0 03) m quartzltes and orthopyroxemtes, (3) P1 (An3o_35) + Cpx(XFe-~0 10-0 15) + Grt(XFe=0 49-0 52, XMg=0 27-0 32, Xca=0 18, XMn<0 02) +Qtz m pynclas~tes The retrograde assemblages correspond essentially to the growth of hornblende and bmt:te at the expense of garnets and pyroxenes. In calc-slhcate rocks, hornblende is frequently zoned w~th a core composed of pargas~tic or magnes:o-hornblende and a m m of edemtm or actmohtm hornblende. In pyriclasltes and pyrlbohtes, the amphiboles are mainly tschermak:tic, pargaslt:c or magneslo-hornblendes whereas bmtttes have variable XMg (0 65-0 80)

Thermobarometry Mineral compos~tmns were determined w:th an automated CAMEBAX electron microprobe (Department of Mmro-analyses, Umversity of Nancy) operating at 15 kv, 10 nA, using the MBXCOR correction procedure (Henoc and Tong, 1978) As recasted, ferric Iron always remares very low and does not slgmficantly affect precision of P - T estimates (currently + 50°C and + 1 kbar) Hence microprobe analytmal data have been used with total iron as Fe 2+

Grt-Bt-Ky-Pl-Qtz assemblages B:ot:tes and garnet from the pehtm gnelsses have, respect:vely, low T1 and Ca and Mn con-

105

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Fig 8 Examples of zoning profiles m garnets from metapehtes tents allowing use of the Grt-Bt thermometer of Ferry and Spear (1978) Temperature estimates fall in the range 730-840°C, except for a lower value at 650 °C Pressures obtained from the Grt-Ky-PI-Qtz assemblages, using the calibration of Newton and Haselton (1981) with the garnet sohd solution model of Ganguly and Saxena (1984), are compatible with the stability of kyamte and are estimated at 9-13 kbar for the above temperature range (Fig 9a) P-T varmtmns have been estimated from zoning profiles of garnet using the calculation procedure of Spear and Selverstone (1983), assuming that garnets were m equlhbrmm with P1-Kfs-Bt-Ky-Qtz The results show a sigmfmant increase m temperature but only a slight increase m pressure (Fig 9a) This could indicate that the growth of garnet occurred near or after the pressure maximum was achmved

Grt-Cpx-Pl-Qtz-Hbl assemblages Pressures have been estimated from the Grt-Cpx-P1-Qtz barometer of Newton and Haselton (1981) as modified by Ralth et al (1983), and temperatures from the Grt-Cpx and Grt-Hbl thermometers cahbrated by Powell (1985) Pressures and temperatures estimated from the Grt-Cpx-PI-Qtz-Hbl assemblages from pyrlclasltes fall m a more restricted range ( T = 690-750°C, P= 10 5-12 5 kbar) than those from metapelitm gnelsses. The same assemblages from pynbohtes give pressures between 11 and 12 kbar and temperatures between 620 and 780°C (Fig. 9b) As Grt-Hbl assemblages are syn-D2 retrograde parageneses, the latter temperature data can be cons~dered as minimum estimates for the peak metamorphic conditions

106

15~ P(kb

(~)

600

,'

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P (kb)

ii ,,

/, ,

15~

,

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800

6OO

800

T-I~*

Fig 9 SynthetmP-T dmgrams (a) P-T fmld estimatedfrom metapehtmassemblages (dashedhnes) and from garnet zoning (blackarrow), (b) P-T field estimatedfrom maficassemblages Sflhmanlte-kyamtecurve (sohd hne) from Holdaway (1971) Grt-O_px assemblages Grt-Opx thermometry using the cahbratmn of M Rmth and K R Lal (personal commumcatmn, 1986) gives a temperature range of 650-800 ° C, similar to those obtained from other assemblages In conclusmn, the different P - T estimates from the varmus assemblages are consistent w~th each other and mdmate that the P - T cond~tmns of the highest grade stage were approximately 800°C and 11 kbar These values, although shghtly higher, are in agreement with those (735°C and 10 kbar) prewously published by Ball et al (1984)

Discussion All the major and trace element patterns of kyamte-garnet gne~sses are hke those of contmental or near-shore argillaceous sediments Most of garnet-gneiss samples have the chemistry of volcanogenlc greywackes Their trace element patterns clearly suggest that these greywackes are distract from pasmve and active margin greywackes (Bhatm and Crook, 1986) but have some slmflantms with alkahne rocks Therefore, the contnbutmn of alkahne volcan~tes to these greywackes and hence a continental environment can be reasonably envisaged The calc-sflmate rocks and marbles whmh were dolomltlc marls and dolomite-rocks require shallow-water, semi-barred deposltmnal condltmns (Velzer, 1973) This is further corroborated by the presence of possible evapontlc

layers Finally, the presence of quartzltes and iron-rich sediments (magnetite-rich orthopyroxemtes) strongly favours the proximity of a weathered and eroded continental crust Such a shallow-water, near-shore sedimentary sequence can be related to either a passive margin or, more hkely, an mtracontmental dlstenslve basra ( to account for a possible alkahne affimty of the volcanogemc greywackes), at the northern edge of the Congo craton Th~s interpretation can be extended to both the lower grade Yaoundd and Mbalmayo series, which show no major hthologmal dlscontmmtms with the high-grade gnelsses (Nedelec et al., 1986). So, there is no evidence of any volcano-sedimentary successmn related to active margins just north of the Congo craton, but only a series deposited before any convergent plate enwronment had been set up As previously suggested by Ball et al (1984), the tectomc evolutmn of the Yaoundd gne~sses is probably monocychc and results from a single tectono-metamorphm event as supported by the apparent continuity between the different mineral assemblages The first stage (D1) corresponded to prograde h~gh-pressure recrystalhzatmns and to the development, partly by tectomc tranpomtmn, of a composltmnal banding The condltmns of metamorphism culminated at the end of this D1 deformatmnal event with high-pressure granuhte facms assemblages and mclpmnt partml melting ( T ~ 800 ° C, P ~ 11 kbar) There are no proofs of tangentml tectomcs during D1 as there

107

is no strong hneatmn linked to S1 and a lack of slgmficant rotatmnal features during this stage This could be accounted for by the late growth of garnets, 1 e after the pressure increase and thus at the end of the D1 tectomc movements The subsequent evolutmn (De) is marked (1) by the emplacement of several ultramafic to mafic bodies, (2) by increased m sltu partial melting and injection of quartzo-feldspathlc anatectm melts along De shear zones, and (3) by strong tangential movements toward the south leading to the formatmn of a large tectomc nappe overthrust onto the Congo craton (Fig 6c) The P - T path deduced from zoning profiles of garnets suggests that partial melting and the emplacement of the nappe occurred near or after the maximum pressures and temperatures were achmved The D1 lsograds were folded and reverted during the De phase The paroxysmm assemblages were overprinted by a retrograde metamorphism increasing from the sole of the nappe (low-grade assemblages in and close to the Congo craton) to its core (h~ghgrade amphlbohte assemblages in the Yaoundd area) and corresponding to an reverse, syn-De, northward metamorphic gradient (Nedelec et al, 1986, Fig 6c) The tangentml movements continued later by low temperature flat-lying mylomtlc shears and the tectomc events ended by moderate E-W shortening accommodated by rare stnke-shp faults The successive mineral assemblages and related structures in mlgmatltes (1 e shear zones lubricated by anatectm magma) clearly suggest that the De event is that of the tectomc surge of the Yaoundd seines according to the process described by Holhster and Crawford (1986) The age of the tangential tectomcs can be estimated at 565 + 22 Ma (whole-rock Rb/Sr lsochron of Lasserre and Soba, 1979 ) and could be time-equivalent with the De event m Northern Cameroon (Toteu et al, 1987) These results compared with recent data on Northern Cameroon (Toteu et al, 1987) suggest that two geotectomc domains w~th con-

trastmg patterns can be distinguished within the North-equatorial fold belt The southern domain extends to the south of the Adamaoua mylomtm line and is separated from the Congo craton by a major thrust plane concealed by late normal faults (ManguelleDlcoum et al, 1986) This unit is composed schematically of reworked Archaean and early Proterozotc formations of the Congo craton (Nyong and Lokundje series) m the west, of low to high-grade platform sediments (Mbalmayo and Yaoundd series) m its central part and of syn- to late-tectonic plutomc rocks in the east Geophysical data show that the Congo craton extends beneath the Yaound~ series up to the latitude of Yaound~ (Colhgnon, 1968, Dumont, 1986) therefore suggesting that an important portion of the fold belt is underlain by the craton The hthologlcal assemblage constltutmg the Yaound~ series clearly corresponds to a deposltlonal environment related to an emerged continent and thus a derivation of the sediments from the Congo craton seems hkely It must be also noted that the Yaound~ and Mbalmayo series are only weakly affected by the late vertical shears related to the Adamaoua strike-slip system - - which develops mainly in the inner part of the mobile zone - therefore corroborating the important northwards extension of the craton The overall patterns of this domain are strongly dependent on the presence of an older continental crust and obviously ~t can be consldered as the northern margin of the Congo craton Consequently, an eventual suture is necessarily located north of it This umt seems to be equated farther east in the Republic of Central Africa, where the platform sediments of the Bakouma regmn also show decreasing metamorphic grade and recumbent folds with a dlrectmn of movement towards the Congo craton (Poldevm et al, 1980/1981), and where Pan-African granuhtes with a N-S stretching llneatlon have been recently recognized (Pin and Poldewn, 1987) The northern domain extends in North Ca-

108 m e r o o n and Nigeria b e y o n d the A d a m a o u a hne T h i s d o m a i n c o m p r i s e s essentially high-grade m e t a m o r p h i c rocks ( a m p h l b o h t e s , m e t a s e d l m e n t s , o r t h o g n e l s s e s ) a n d s y n t e c t o m c plutonic rocks r e f e r r e d to as ' C o m p l e x e de base' a n d a v o l c a n o - s e d x m e n t a r y s u c c e s s m n recryst a l h z e d u n d e r low to m e d m m - g r a d e c o n d l t m n s , called 'Serms m t e r m ~ d m l r e s ' (Bessoles a n d Lasserre, 1977) T o t e u et al (1987) have rec e n t l y shown t h a t this d l s t m c t m n is no longer suitable T h e y also show t h a t the e a r h e r volc a m c stage r e c o r d e d in t h e P o h f o r m a t m n suggests e x t e n s m n a l e n v i r o n m e n t a n d crustal throning and that the calc-alkahne plutomsm a n d the whole t e c t o n o - m e t a m o r p h ~ c e v o l u t m n are c o m p a t i b l e w~th a c o l h s m n H o w e v e r , t h e r e ts evidence nexther o f a s u t u r e zone n o r of remn a n t s of o c e a m c m a t e r m l So, t h e y enwsage two possible geodynamxc settings" c o n v e r g e n t plates or m t r a c r a t o m c basins Anyway, t h e i r d a t a clearly suggest t h a t , at the opposite to t h e s o u t h e r n p a r t o f t h e belt, th~s u m t ~s a zone of crustal a c c r e t m n even if the relative v o l u m e of accreted m a t e r m l is still debatable ( T o t e u et a l , 1987) T h e A d a m a o u a r e g m n whmh is t h e transltxon b e t w e e n t h e n o r t h e r n m a r g i n o f t h e Congo craton and the zone of Pan-Afmcan a c c r e t m n could be of a great i n t e r e s t m d e c i p h e r i n g t h e t r u e nat u r e of the N o r t h - e q u a t o r m l belt

Acknowledgements T h i s work is p a r t of a pro3ect of the Geological a n d M m m g R e s e a r c h I n s t i t u t e (Yaoundd, C a m e r o o n ) Financial s u p p o r t from the F r e n c h M m l s t b r e des R e l a t i o n s e x t g n e u r e s ( C o n v e n t m n U m v e r s l t d de Y a o u n d d - I N P L o r r a i n e / U m v e r s l t d de N a n c y I) is also a c k n o w l e d g e d T h a n k s to J M B e r t r a n d for l m p r o w n g the m a n u s c r i p t C R P G C o n t n b u t m n N o 726

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