Crustal growth in West Africa at 2.1 Ga

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 97, NO. B1, PAGES 345-369, JANUARY 10, 1992

Crustal Growth in West Africa at 2.1 Ga

MURI•L BOHER, 1'2 WAFA ABOUCHAMI, 1'2 ANNIE M/CHARD, • FP. ANClS ALBAREDE, 2's AND NICHOLAS T. ARNDT n

Birimian (-2.1 Ga) terranes in the West African craton are a mixture of highly metamorphosed volcanic, sedimentary and plutonic rocks and low grade metavolcanics and metasediments. The volcanic units contain thick, commonly pillowed, tholeiitic basalts overlain by pelagic sediments cherts and black shales. The sedimentary units are characterized by an abundance of clastic turbiditic sedimeliiS. Andesites and calc-alkaline felsic volcanics occur at uppermost stratigraphic levels and as dykes. Field relationships between the volcanic and sedimentary units remain a matter of debate. Calc-alkaline and local alkaline granites, which intruded in distinct pulses and occasionally are related to transcurrent tectonics, represent almost half of the Birimian terranes. New isotopic work on the highly metamorphosed units greatly improved the chronology for the Birimian crust. The age of the early Dabakalian event is precisely defined by a U-Pb zircon age at 2186 + 19 Ma, while Rb-Sr and Sm-Nd methods give ages of 2i62 + 19 Ma and 2141 + 24 Ma, respectively. A Sm-Nd garnet-whole rock age of 2153 + 13 Ma suggests that metamorphism culminated at about the same time. In contrast, the most precise zircon U-Pb and Sm-Nd data for the more widespread Birimian terranes (sensu stricto), from this study and from the literature, cluster between 2.12 and 2.07 Ga. The major evolution of the Birimian crust apparently lasted less than 50 Ma. Isotopic evidence indicates that Birimian granitoids contain a negligible component of Archcan crust: •Nd(2.1 Ga) values are positive and similar to those of Birimian basalts, crustal residence times are shorter than 200 Ma, U-Pb ages for detrital zircons from clastic sediments range from 2098 + 11 Ma to 2125 + 17 Ma, while granite chemistry and Nd isotopic characteristics are unrelated. Only very locally in Guinea is there isotopic evidence of interaction between Birimian felsic magmas and the Archcan rocks from the Man craton. In accord with Abouchami et al.'s (1990) suggestion that Birimian basalts represent oceanic plateaus, the present data argue that the protolith of much of the West African continent was created around 2.1 Ga in an environment remote from Archean crust. Intrusion of calc-alkaline magmas into the tholeiitic units suggests that island arcs formed on top of the assumed oceanic plateaus which then collided with the Man Archcan craton. Taking the Birimian formations from the Guyana shield into account, the minimum crustal growth rate at 2.1 Ga is about 1.6 km3/a, some -60% higher than the present growth rate. Birimian crust growth at 2.1 Ga is reminiscent of Archcan processes but contrasts with 1.7 - 1.9 Ga crust formation in the North Atlantic continent which generally involved significantly more interaction with older continental crust. A comparison of the Birimian crustal growth rate with the average crustal growth rate over the Earth history implies that a large part of the Birimian crust has been recycled into the mantle or incorporated into younger orogenic segments. This apparent deficit in the crustal budget is even more dramatic for the Archcan crust.

INTRODUCTION

Extreme models of continental evolution support a relatively uniform growth rate through geological time [Hurley and Rand, 1969], or continent formation during the Early Archeart followed by steady state recycling of continental material through the mantle [Armstrong, 1968]. Enhanced crustal growth during the Archeart and Early.Proterozoic seems supported by geological and geochemical evid9nce [Moorbath, 1977; McCulloch and Wasserburg, 1978; O'Nions et al., 1979; Veizer and Jansen, 1979; Alldgre et al., 1980]. Alldgre and Rousseau [1984] interpreted Nd isotopic data on Australian shales as indicating a predominance of crustal growth over reworking of preexisting crustal material during the period preceding 2 Ga and a converse trend in the late Proterozoic and Phanerozoic. Michard et al. [1985], however, demonstrated continuing input of mantle material to the modern sedimentary mass. Gurnis and Davies [1986] also suggested that preferential erosion of elevated Phanerozoic crust could result in an apparent peak of crustal growth at 2-3 Ga.

'Centre de Recherches P•trographiques et G6ochimiques, Vandoeuvre, France.

aUniversit• de Nancy I, Vandoeuvr•, France. •Ecole Nationale Superieur• de G6ologie, Vandoeuvr•, France. •ax Planck Institut fdr Chemic, Mainz, Germany.

Copyright 1992 by the American Geophysical Union.

Paper number 91JB01640. 0148-0227/92/91JB-01640505.00

Gastil [1960] pointed out that radiometric ages cluster within rather well-defined orogenic periods (3.8 - 3.5, 2.9 - 2.6, 1.9 - 1.6, 1.2 - 0.9, 0.6 - 0 Ga) that are widely separated by longer intervals of apparent quiescence. In North America, patterns of isotopic ages have been related by Hurley et al. [1962] to episodic centripetal continental growth. They were further refined by numerous authors, e.g. Hoffman and Bowring [1984], Nelson and DePaolo [1985], and are also found in Australia [McCulloch, 1987] and throughout the North Atlantic continents [Patchett and Arndt, 1986]. These episodes may reflect major pulses of crust-mantle segregation. Alternatively, if crustal growth is continuous, but unevenly distributed over the surface of the Earth, those episodes could be viewed as artifacts related to oversampling some areas, notably North America and Europe. Evidence that huge piles of tholeiitic basalts and rhyolites erupted all over West Africa during a major episode at 2.1 Ga is barely acknowledged in the literature, which underlines the risk of missing major events in poorly surveyed areas [Abouchami et al., 1990].

The processes by which continental crust is produced, and the nature of material extracted from the mantle to produce the proto-crust, remains controversial, particularly for Archeart and Proterozoic terranes. Taylor [1967] suggested that continental crust grew through time by accretion of islands arcs. Tonalite-trondhjemite gneiss, a major component of all Precambrian shields, is widely held to represent the metamorphic equivalents of arc plutonic rocks (see reviews by Glikson [1979], and Weaver and Tarney [1984]). In the North Atlantic continents, Condie [1982] and Patchett and Arndt [1986] also argued for crustal growth via accretion of island

345

346 BOHER ET AL.: CRUSTAL GROWTH IN WEST AFRICA

arcs. Formation of typical continental crust with felsic rocks likely requires more than one magmatic step [Arth and Hanson, 1975; Glikson, 1979]: granites and rhyolites, in particular, probably form by remelting of thick layers of basalts, gabbros or their volcaniclastic and metamorphic equivalents. Although orogenic magmatism may produce the large volumes of felsic

al., 1969; Vachette, 1964; Bassot and Caen-Vachette, 1984] from the Birimian domain, which covers immense stretches of continental crust in Mauritania, Algeria, Mali, Senegal, Guinea, Ivory Coast, Burkina Faso, Niger and Ghana (Figure 1), documented a period of intense orogenic and magmatic •ictivity around 2.1 Ga. Recent Sm-Nd work by Abouchami et

rocks typical of continental crust, by remelting the al. [1990] showed that over the same area thick piles of downgoing/subducting basaltic crust [Arth, 1979] or the lower tholeiitic lavas Were also emplaced during a 100-Ma period layers of thickened island arc crust, it has been suggested that also around 2.1 Ga ago. This age is consistent with Sm-Nd data Precambrian crustal growth results from processes unrelated to by Gruau et al. [1985] on mafic lavas from the Guyana shield. modem plate tectonics [Taylor and McLennan, 1985]. More The 2.1 Ga age has special relevance because it falls in a period specifically, the mafic continental protolith has been assigned of apparent quiescence, from 2.6 to 1.9 Ga, in the North to orogenic volcanism [Taylor, 1967], rift/ocean island America-Europe regions [Windley, 1973, 1984; Nelson and magmas underplated below preexisting continents [Lambert, 1981; KrOner, 1984], continental flood basalts [Cox, 1980; Arndt and Goldstein, 1989], or oceanic flood basalts [Abouchami et al., 1990].

The West African craton provides new constraints on the manner of crustal growth. Early chronological data [Ledent et

DePaolo, 1985; Patchett and Arndt, 1986] and Australia [McCulloch, 1987], and, furthermore, follows the period of major crust growth in the late Arcbean [O'Nions et al., 1979; KrOner, 1984; Taylor' and McLennan, 1985].

The present study, a companion to the paper by Abouchami et al. [1990] on Birimian tholeiites, reports Sm-Nd isotopic

20 ø

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Fig. 1. Geological map of West Africa with Precambrian domains: the Leo and Reguibat rises, the Kedouõou-Kenieba inlier.

BOItER El' AL.: CRUSTAL GROWTH IN WEST AFRICA 347

data on Birimian felsic plutonics, volcanics and volcaniclastic sediments, together with U-Pb data on zircons from a variety of granitic and sedimentary rocks. Certain samples were analyzed for Rb-Sr systematics. The study was designed deliberately as an investigation of a large area of West Africa so as to focus on broad rather than local features of Birimian formations. The

first goal of the project was to date the sequence of events that led from basaltic protolith, through major orogenesis, to the final episodes of granite emplacement and metamorphism that produced a stable continental crust. The second goal was to determine the extent of involvement of preexisting (Archean) continental material in Birimian crust formation in order to constrain the mode and tectonic site of crustal accretion.

GEOLOGY AND PREVIOUS GEOCHRONOLOGICAL WORK

In the view of early authors [e.g. Bessoles, 1977], most Precambrian crust in West Africa (Figure 1) formed during three major orogenic events: Liberian (3.0-2.5 Ga), Eburnian (2.5 to 1.8 Ga) and Pan-African (ca. 0.6 Ma). The term of Birimian refers to formations postdating the Liberian which formed during, or were affected by, the Eburnian event. Archean and Proterozoic formations are present in two large areas, the Reguibat rise in the North and the Leo (Guinea) rise in the south. The Leo rise is divided into an Archean portion, the Man shield, in the west and a Birimian portion, the Baoule-Mossi domain, in the east. Between the major rises, two western inliers (Kayes and Kedougou-Kenieba) suggest continuity of the Proterozoic basement below the Late Precambrian and Early Phanerozoic basin of Taoudeni. In both the Reguibat and Man rises, Archean and Birimian formations are separated by an important shear zone (called the Sassandra Fault in Ivory Coast and the Zednes Fault in Mauritania). Some authors suggest that there is one shear zone which extends under the Taoudeni Basin

[Caen-Vachette, 1988]. However, Archean formations are missing in the Precambrian inliers of Kedougou-Kenieba and Kayes [Bassot, 1963; Bassot and Caen-Vachette, 1984] where continuity between the Man shield and the Amsaga domain would be expected. Similarly, the Guiana shield has also been divided into a Late Archean (2700 Ma) region, the Imataca craton in eastern Venezuela, and Early Proterozoic formations affected by the Trans-Amazonian orogenic event (2000-2200 Ma) in the Guianas [Caen-Vachette, 1988]. Paleomagnetic reconstructions [Onstott and Hargraves, 1981] suggest that West Africa and Guiana shields formed a single domain in the Early Proterozoic.

Geochronological work in West Africa up to 1984 has been reviewed by Cahen et al. [1984], and later work by Lernoine et al. [1985] and Caen-Vachette [1988]. Archean ages, largely from orthogneisses and charnockites, are reported in the western parts of the Reguibat and Leo rises [Hurley et al., 1971; Vachette and Bronner, 1975; Beckinsale et al., 1980; Carnil et al., 1983, 1984]. In the Man shield and the western part of the Reguibat rise (Amsaga), two orogenic cycles dated at 3.00 Ga (Leonian) and 2.75 Ga (Liberian) are distinguished. Eburnian ages were obtained mainly from the eastern part of the two rises as well as in the Kedougou-Kenieba and Kayes inliers, but Eburnian reheating apparently locally imprinted mineral ages from Archean formations [Cahen et al., 1984]. Ages in the Reguibat rise (2.04 - 1.75 Ga) are somewhat younger than in the Kedougou-Kenieba inlier and the Baoule- Mossi domain (2.180 - 1.920 Ga). Rb-Sr ages spread over a large range from 1920 to 2200 Ma.

Small high-grade terranes containing the metamorphosed equivalents of basalts, rhyolites, granites and quartzites are found in the Baoule-Mossi domain in regions remote from Archean cratons. Because of deformation much more pervasive and the metamorphic grade much higher than in other B irimian formations, these series, which have been described in Ivory

Coast by Arnould [1961] and in Burkina Faso by Hottin and Ouedraogo [1975], were considered to be of Arcbean [Arnould, 1961; Tagini, 1971] or Early Birimian ages [Bard, 1974]. Lemoine et al. [1985] coined the name of "Dabakalian" for these early formations. Preliminary U-Pb results of Lemoine [1988] on zircons from Dabakala (central Ivory Coast) assign an age of 2144 + 6 Ma to these units while a Sm-Nd isochron on basic dykes suggests a consistent although less precise age of 2140 + 22 Ma. Field relationships between Dabakalian and Birimian units are not precisely known.

Birimian formations sensu stricto consist of volcanic and

volcaniclastic series intruded by various generations of granitic rocks. Two principal units were identified in Ghana by Junner [1935, 1940] [see also Bates, 1955, 1956]. The first is largely metasedimentary and contains Fe-Mn deposits and subordinate volcanics. In Ghana, Leube et al. [1990] divide the metasedimentary rocks into volcaniclastics, turbidites and argillites. The second unit consists of thick sequences of tholeiites interlayered with carbonates and immature detrital sediments. Presence of pillows, cherts, and black shales and one reported occurrence of polymetallic Mn-rich nodules [D. Regnoult and P. Barbey, personal communication, 1990] hint at a pelagic environment. Calc-alkaline rocks are commonly found as dykes or as lavas in the uppermost stratigraphic levels [Zonou et al., 1985; Deschamps et al., 1986; Dia, 1988]. The stratigraphic relationship between the two units remains controversial. For Junner [1940], the metasedimentary unit represents the Lower Birimian and the volcanic unit the Upper Birimian, a view adopted by Bates [1955], Lernoine et al. [1985] and supported by Mildsi et al. [1986] and Ledru et al. [1989; 1991] on structural evidence. Arnould [1959], Tagini [1971], Bassot [1963] and Bertrand et al. [1989], on the other hand, held the opposite opinion, namely that the tholeiitic volcanics underlay the metasedimentary series. Finally, Leube et al. [1990] assume a lateral variation between the two units. Birimian formations are intruded by prekinematic, synkinematic to postkinematic granites representing locally up to 50 percent of surface rocks, as in the Baoule-Mossi domain.

Birimian formations have been affected by major orogenesis, the "Eburnian" event, which was dated at-2.1 Ga by Bonhornrne [1962] in a Rb-Sr study of posttectonic granites. Structural studies have shown the existence of two stages of deformation in both the Baoule-Mossi domain [Ternpier, 1969; Bard, 1974] and the Reguibat rise [Barbey, 1975]. Regional metamorphism reaches greenschist facies but granitic intrusions, which are particularly numerous in the Baoule-Mossi domain, apparently were responsible for large- scale static reheating up to the amphibolite facies when temperatures of about 550øC and pressures around 5 kbar prevailed (M. Boher et al., unpublished data, 1990).

In spite of a wealth of Rb-Sr geochronological data on Birimian rocks, precise age determinations are provided only by rare U-Pb zircon data. There are few Rb-Sr isochrons that meet statistical criteria ruling out magma mixing or other disturbances that may have biased the results. In Mali, in the North of the Baoule-Mossi domain, zircon U-Pb dating on calc- alkaline granites [Lidgeois et al., 1991] and a few undisturbed Rb-Sr isochrons agree well and indicate an age of 2.073 - 2.095 Ga. Younger Rb-Sr ages on zircon-dated granites provide evidence for large-scale heating which lasted some 100 Ma after granite emplacement. Lidgeois et al. [1991] conclude that the whole sequence of magmatism, erosion, sedimentation, deformation and metamorphism lasted no longer than 30 - 40 Ma.

Casanova [1973] showed that Early Birimian granites (which, at that time, he considered to be of Liberian age) have trondhjemitic affinities whereas synkinematic to postkinematic plutons are more K-rich and essentially calc-


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Fig. 2. Sampling sites: local names mentioned in text, tables and figures. (Top): Reguibat Rise, (bottom): Leo Rise.

BOHER ET•AL.: CRUSTAL GROWTH IN WEST AFRICA 349

alkaline (granodiorites, granites and rare leucogranites). Applying Chappel! and White's [1974] terminology, the vast majority of Birimian granites could be termed I type. Vidal and Alric [1987] in Ivory Coast, Ledru et al. [1989] in the Kedougou-Kenieba inlier, and D. Dupuis et al. (personal communication, 1990) in the Liptako area (Niger) suggest that eraplacement of synkinematic to postkinematic granitic plutons is associated with transcurrent tectonics. A few alkaline granites are post-kinematic [Arnould, 1961; Tagini, 1971; Gamsord, 1975] and rare rapakivi textures are reported in the Reguibat domain [Deschamps et al., 1986]. Granitic plutons contain inclusions resembling the deformed Dabakalian formations and the Birimian tholeiitic and

metasedimentary series [Wenmenga, 1986; P. Barbey, personal communication, 1990].

SAMPLING

Samples from the Reguibat rise included prekinematic and synkinematic granites, and postkinematic granites (including rapakivi granites). These rocks, which had previously been dated as 2.4 - 1.9 Ga old by Rb-Sr methods (M. Caen-Vachette, personal communication, 1988) were analyzed for Sm-Nd and reanalyzed for Rb-Sr. Samples from the Kedougou-Kenieba inlier (Mako and Dialle-Dalema series), included granodiorites and biotite-granites dated between 2180 + 80 and 1950 + 50 Ma by Rb-Sr [Bassot and Caen-Vachette, 1984]. Eburnian

Sampling included Birimian clastic sediments (greywaclces, sandstones and schists, metamorphosed to greenschist facies) from both the metasedimentary and the volcanic units at various localities in the Eastern Reguibat rise, the Baoule- Mossi domain the Kedougou-Kenieba inlier (Figure 2).

High-grade metamorphic rocks (metasediments, metaigneous rocks, and migmatites) from the following provenance were also analyzed: (1) Mauritania: areas of Tsalabia, Oudian Kharroub, Alous Tmar, Imourene; (2) North Burkina Faso: the Bouroum-Bamga-Yalogo area; (3) Central Ivory Coast: the Yaoure area. The samples from the last two localities come from to the Dabakalian formations of the

Baoule-Mossi domain.

ANALYTICAL TECHNIQUES

Major and trace element abundances (including rare earth elements) were obtained by Ion Coupled Plasma [Govindaraju and Mevelle, 1987].

Rb-Sr analytical techniques have been described by Alibert et al. [1983]. In felsic rocks, a large fraction of rare earth elements (REE) is hosted by accessory minerals refractory to acid dissolution, hence a fluxing method was used instead. About 100 mg of powdered sample were mixed with ultra-pure Li-metaborate in 1:3 proportions, then poured into a Pt-Au-Ir crucible and mixed 147Sm/150Nd spike added. After gentle

granitic rocks came from various settings scattered over the drying, the mixture was melted at around 1000øC for 30 min. whole Baoule-Mossi domain. They consist essentially of The resulting glass was dissolved in 20cm 3 of 1N HC1 and REE granodiorites and some of their enclaves, biotite-granites from Burkina Faso (Ouahigouya, Boulsa, Bouroum), Niger (Liptako), Ivory Coast and Guinea (Kerouane and Kankan) and 2-mica leucogranites from Ivory Coast. The degree of deformation ranged from strongly gneissose to virtually undeformed. Rb-Sr data exist on most of these rocks [Tourd et al., 1987; Vachette and Ouedraogo, 1978; Gamsord, 1975].

co-precipitated with iron hydroxide. The hydroxide was centrifuged, rinsed in distilled water, then redissolved in concentrated HC1. REE were separated from Fe by passing the solution through an anion exchange column. The next steps of Sm-Nd separation are described by Michard et al. [1985]. Nd chemistry blanks (beyond the fusion step onwards) are of about 10 pg. As Li-metaborate does not form a melt in the absence of

Grain Characters Pb, ppm U, ppm TABLE 1. U-Pb Analysis of Zircon Populations

206pb/204pb 207*pb/235U 206'pb/238 u 207'pb/206' pb Apparent Ages, Ma

Yaourd (LI3), Gneis

B 247 1048 9042 3.8642 > 100ram C 225 995 7752 3.7269 > 75ram 245 1139 11442 3.4894 NM 243 927 12048 4.3690 M 261 1406 6601 2.8693

0.21798 0.12857 1606 1271 2079 0.21106 0.12807 1577 1234 2071 0.20046 0.12625 1524 1177 2046 0.24371 0.13002 1710 1403 2098 0.16907 0.12309 1374 1007 2002

Kedoug ou-Kenieba, Loulo, Sandstone

RM (67) 8.6 80.2 120 1.6017 0.09819 0.11831 971 604 1939 M (67) 12.4 99.3 366 1.8975 0.11395 0.12077 1080 696 1968 E(67) 9.3 45.8 247 2.8312 0.16449 0.12483 1364 982 2026 BM (36) 17.3 165 152 1.4374 0.08512 0.12248 905 527 1993 YNM (46) 18.2 85.2 667 3.4445 0.19420 0.12864 1515 1144 2080

Kedougou-Kenieba, Keniebandi, Sandstone

> 150ram (HL 127) 50 164 222 4.8088 0.26634 0.13095 1786 1522 2111 > 100ram (HL 127) 64.3 242 277 4.2045 0.23369 0.13049 1675 1354 2105 >45ram (HL 127) 54.1 195 320 4.4427 0.24646 0.13074 1720 1420 2108 R(HL 126) 70.7 239 211 4.7037 0.26010 0.13116 1768 1490 2114 G(HL 126} 12.7 42.6 136 4.7816 0.26626 0.13025 1782 1522 2101 Common lead composition used for blank correction: 206pb/204pb = 18.70; 207pb/204pb = 15.63 [Stacey and Kramers, 1975]. Measured average for Pb blanks is 180 pg. U and Pb concentrations are accurate to within +1.5 percent and 207*pb/206*pb ratios are given with error bars of 0.3 percent. Nine analyses of NBS 983 give these average values: 206pb/204pb = 2580 + 10, 207pb/206pb = 0.07123 + 0.00006 and 208pb/206pb = 0.013628 + 0.000012. E = elongate, M = magnetic and NM = nonmagnetic, R = red, B = brown, Y = yellow, C = clear. Size fractions in micrometers (45-100-150 for Keniebandi and 50-75-100 for Yaoure gneiss). Sample numbers in parentheses.


silicate, an upper limit of the total procedural blank has been estimated by fusing 100 mg of optically pure natural quartz crystals. This amount of silica + flux was found to contain 3 ng of Nd, which amounts to about one thousandth of the Nd commonly processed in sample analysis.

Zircons were dissolved in small teflon bombs with HF and HNO3 [Krogh, 1973]. Two separate attacks were made on a same fraction, one for determination of Pb isotopic composition and the other for determination of U and Pb concentrations using a mixed 208pb/235U spike. Separate dissolutions were found to result in more reproducible U/Pb ratios than liquid aliquots. The chemical procedure is essentially identical to that of Manhes et al. [1978]. Most of the data were obtained on a Cameca 206 SA mass spectrometer, and a few on a Finnigan-Mat 262 mass spectrometer equipped with six collectors. SHRIMP ion microprobe analyses were made using the procedure described by Compston et al. [1986]. Correction of SHRIMP data was made using observed 204pb, and the revised value of 206pb/238U of the standard zircon SL3, taken as 0.0928 equivalent to an age of 572 Ma, was used for the determination of Pb/U ratios.

Ages were calculated with the method of Minster et al. [1979]. Concordia intercepts and their errors were calculated from the least squares straight-line parameters through a Newton quadrature scheme. Errors on ages and intercepts are propagated 2-sigma errors.

0.76

0.72

87Sr '/86Sr ....... Gneisses and Leptynites

from Yalogo

IT= 2162:1::19 Ma MSWD = 7

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0.5130

0.5126

0.5122

0.5118

0.5114

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0.20 0.22

(a)

(b)

RESULTS

High-Grade Gneisses (Dabakalian)

Six fractions of rounded and zoned zircons from a Yaoure gneiss sample (Central Ivory Coast) give discordant U-Pb values. A well-defined linear array was obtained with an upper intercept at 2186 + 16 Ma, a lower intercept at 261 + 26 Ma, and a Mean Square Weighted Deviation of 0.65 (Table 1, Figure 3).

Metaigneous and metasedimentary felsic gneisses from the Bouroum-Yalogo area (Burkina Faso) give a Rb-Sr whole rock errorchron with an age of 2162 + 19 Ma, a low initial isotopic ratio of 0.70098 + 4 and a MSWD of 7 (Figure 4a). Sixteen samples give a Sm-Nd errorochron age of 2140 _+ 32 Ma with a MSWD of 1.4 and an initial 143Nd/144Nd ratio of 0.509990 + 31, which corresponds to an œNd(T) value of 2.4 (Figure 4b). Combination with the five amphibolite samples from the same area reported by Abouchami et al. [1990] results in a slightly better defined age of 2141 _+ 24 Ma (MSWD = 1.2) and an initial 143Nd/144Nd ratio of 0.509988 + 25 (eNd(T) = 2.4). On two samples, internal (garnet-whole rock) Sm-Nd isochrons

o.518 Yalogo gneisses •

0.516

0.514 / - T = 2107 _:1:93 Ma 147 Srn / 144 Nd

0.512 ' ' ' ' ................. 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.65 0.70

(c)

Fig. 4. (a) Rb-Sr whole rocks isochron for the high-grade metamorphic rocks from the Bouroum-Yalogo area (North of Burkina Faso). (b) Whole-rock Sm-Nd errorchron for high-grade metamorphic formations from the Baoule-Mossi domain (Yaoure and Bouroum-Yalogo areas). (c) Garnet-whole rock Sm-Nd isochrons for Yalogo gneiss samples B21 and G2b.

(Figure 4c) give metamorphic ages of 2107 + 93 Ma and 2153 + 13 Ma.

0.4 206p'l•/23•U ' Baoule Mossi

0.3 G neiss(Ll•• . .

0.1 • MSWD =0.65 O i I I I I I I

0 2 4 6 8

Fig. 3. Concordia diagram for zircon populations in the gneiss sample L13 from the Yaoure area in the Baoule-Mossi domain (Ivory Coast).

Clastic Sediments

Detrital zircons from the Loulo tourmalinized sandstone in

the metasedimentary unit of the Kedougou-Kenieba inlier have variable morphology and color. The U-Pb data are very discordant and define a linear array with an upper intercept at 2098 + 11 Ma and a lower intercept at 89 _+ 9 Ma (MSWD = 16) (Figure 5a). Zircons from the same sample were analyzed on the SHRIMP ion microprobe at ANU in Canberra (Table 2). Ten crystal cores were concordant and gave a mean 207pb/206pb age of 2093 + 7 Ma, which is indistinguishable from the conventional results (Figure 5b). One grain was discordant and plotted on the discordia line defined by the conventional data.

Deltaic deposits from Keniebandi in the same region, but assigned by Mildsi et al. [1986] to the volcanic unit, locally contain accumulations of heavy minerals. The U-Pb array of zircons in the Concordia diagram of Figure 6 gives an upper

BOHER E'r AL.: CRUSTAL GROWTH IN WEST AFRICA 351

0 2 4 6 8

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0.2

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207 PI•/235 U i , i

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(a) 0.3

(b)

Fig. 5. (a)Concordia diagrams on zircons from B1 tourmalinized sandstone from Loulo (Kedougou-Kenieba) populations data. (b) SHRIMP data.

intercept at 2125 + 27 Ma and a lower intercept at 64 + 100 Ma with a MSWD of 0.44.

Sr isotopes do not yield precise chronological information because of initial isotopic heterogeneities and open system behavior upon prolonged weathering. Assuming an initial 87Sr/86Sr ratio of ~ 0.701 similar to the initial ratio at 2.1 Ga of the basalts analyzed by Abouchami et al. [1990], the samples with highest Rb-Sr ratios give consistent model ages of 2.1 to 2.3 Ga (Table 3). In the Sm-Nd diagram, 40 granitic rocks from the Baoule-Mossi domain define an errorchron at

2200 + 20 Ma, an initial 143Nd/144Nd ratio of 0.509916 + 14 (œNd(T) = 2•5) and a MSWD of 2.8 (Figure 7a). Excluding the easternmost Gamaye granite, 30 samples from the Kedougou- Kenieba inlier give a Sm/Nd errorchron age of 1945 + 52 Ma

0.2

0.1

206 p'l•/23• U .... ' Sandstone from

Keniebandi

1 T=2125 + 27 Ma MSWD = 0.44

I

I I •7 p•/,235U- 0 2 4 6 8

Fig. 6. Concordia diagram on zircon populations from layers rich in heavy minerals from a B2 deltaic sandstone (Keniebandi) in the Kedougou-Kenieba inlier.

with an initial 143Nd/144Nd ratio of 0.510179 + 38 (eNd(T) = 1.2) and a MSWD of 2.2 (Figure 7b). The poor alignment of data from samples from the Reguibat rise does not allow a significant age to be obtained.

Sm-Nd and Rb-Sr Isotope Geochemistry

The Sm-Nd data are reported in Table 4 and the œNd(2.1) values shown in the histogram form in Figure 8. Most samples have positive œNd(2.1) values with an average of +2.1. Exceptions are the Birimian granites from Guinea, which are emplaced in supposedly Archean formations from the Man craton and have œNd(2.1) values equal to or less than zero. Volcaniclastic sediments show slightly higher •Nd(2.1) values

with an average of +3.4. The œNdvalues at T=2.1 Ga of the high-grade Arcbean formations from the Man craton analyzed by M. Boher et al. (unpublished data, 1990) are shown for comparison and are significantly less radiogenic (-9 to -18) than those of the B irimian rocks.

Crustal residence ages (TDM) may be calculated using diverse models of depleted mantle evolution. Models by Ben Othman et al. [1984], Nelson and DePaolo [1985], Goldstein et al. [1984], McCulloch [1987] and Albardde and Brouxel [1987] give ages which vary by several hundreds of Ma, even for crustal rocks with low 147Sm/144Nd. The model mantle calculated by Abouchami et al. [1990] on the basis of B irimian samples (Figure 9), tums out to be quite similar to the earlier model of Ben Othman et al. [1984], and when this model is used, most Birimian granites have crustal residence ages (TDM)

TABLE 2. SHRIMP Ion Probe Analysis of Sinl•le ,Zircons from the B 1 Tourmalinized Sandstone in the Kedoul•ou-Kenieb 9 Inlier Sample U/ppm Pb*/ppm 206/204 207/235 206/238 207/206 Age 7/35 Age 6/38 1.1 49 21 763.24 6.508 0.38044 0.12407 2047 2078

3.1 145 60 2028.19 6.913 0.38357 0.13071 2100 2093

4.1 204 41 875.58 2.872 0.17367 0.11996 1375 1032

5.1 44 17 479.66 6.415 0.36111 0.12885 2034 1987

6.1 89 38 1119.78 6.193 0.35620 0.12610 2003 1964

7.1 60 28 972.10 6.775 0.37590 0.13072 2082 2057

8.1 57 23 274.88 6.332 0.37594 0.12216 2023 2057

9.1 187 72 2212.00 6.641 0.36633 0.13149 2065 2012

10.1 45 19 465.35 6.740 0.38400 0.12730 2078 2095

11.1 47 20 760.34 6.933 0.37187 0.13522 2103 2038

Pb* corresponds to radiogenic Pb.

Age7/6

2016

2108

1956

2082

2044

2108

1988

2118

2061

2167

352 BOHER ET AL.: CRUSTAL GROWTtt IN WEST AFRICA

!

Localization

Burkin•a Faso Yalogo Burkina Faso Yalogo

Burkina Faso Yalogo

Burkina Faso Yalogo

Burkina Faso Yalogo

Burkina Faso Yalogo

Burkina Faso Yalogo

Burkina Faso Yalogo

Burkina Faso Bamga

Burkina Faso Bamga

Burkina Faso Bamga

Burkina Faso Bamga

Burkina Faso Bamga

BurkSna Faso Bamga Burkina Faso Bouroum

Burkina Faso Bouroum

Burkina Faso Bouroum

TABLE 3. Sr Isotopic Compositions in Birimian Felsic Samples Samples Nature Rb Sr 87Rb/86Sr 87Sr/86Sr+20 (87Sr/86Sr•.l +IiVI

High-Grade Metamorphic Rocks

Glf leptynite 25.4 21.4 3.4320 0.812664 + 69 0.7088 + 51 2.26 + 0.03 Glg leptynite 26.9 14.9 5.2350 0.864269 + 54 0.7058 + 78 2.17 + 0.03 LOBN leptynite 7.8 46.1 0.4885 0.710777 + 46 0.69599 + 70 1.35 + 0.03 L2b leptynite 22.8 18.1 3.6360 0.808555 + 32 0.6985 + 54 2.06 + 0.03 B8 leptynite 1.24 54.2 0.0661 0.702994 + 38 0.70099 + 10 2.87 + 1.27 G2b leptynite 1.57 55.4 0.0819 0.703605 + 41 0.70113 + 10 2.77 + 0.62

G3b gneiss 4.09 59.4 0.1990 0.706313 + 38 0.70029 + 30 1.86 + 0.1

G3d gneiss 5.79 59.5 0.2810 0.704884 + 49 0.69638 + 40 0.78 + 0.03

B21 trondhjemite 1 1.2 109 0.2968 0.709958 + 39 0.70097 + 40 2.15 + 0.07

164b leptynite 33.6 385 0.2520 0.710256 + 39 0.70263 + 40 2.71 + 0.11

314bl quartzite 30.3 77.6 1.1301 0.735177 + 37 0.7010 + 17 2.11 + 0.04

197'c amphibologneiss 3.42 1176 0.0084 0.701891 + 36 0.70164 + 3 -

197'd amphibologneiss 27.7 781 0.1025 0.704896 + 35 0.70179 + 20 3.36 + 0.46

BN91 trondhjemite 8.93 177 0.1455 0.705082 + 37 0.70068 + 20 2.01 + 0.16

261b leptynite 1.43 77.9 0.0530 0.703320 + 39 0.70172 + 10 - 66a metarhyolite 99.2 99.3 2.8856 0.790180 + 36 0.7028 + 43 2.15 + 0.03

66cl metarhyolite 51.2 330 0.4486 0.714590 + 36 0.70101 + 70 2.14 + 0.05

Kedougou- Dalema HL 104 rhyodacite Kenieba


Kedoug•- Dalema HL 108 rhyodacite Kenieba


Ivory Coast Yaoure D4M12 volcanite

Ivory Coast Yaoure 6 volcanite

Volcanics

1.71 24.5 0.2012 0.706052 + 36 0.69996 + 30 1.72 + 0.09

3.58 45.3 0.2283 0.706002 + 37 0.69909 + 30 1.44 + 0.06

0.78 78.9 0.0287 0.705380 + 38 0.70451 + 10

2.86 64.6 0.1280 0.706026 + 36 0.70215 + 20 3.26 + 0.31

54.6 330 0.4782 0.716746 + 40 0.70227 + 70 2.33 + 0.06

0.7259 196 1.6340 0.754889 + 40 0.7054 + 24 2.30 + 0.04

Granites

Kedougou- Dalema HL71 microgranite 3.06 141 0.0629 0.704309 + 37 0.70241 + 10 9.77 + 5.24 Kenieba

Kedougou- Dalema HL 96 microgranite 16.1 187 0.2500 0.709068 + 29 0.70150 + 40 2.34 + 0.09 Kenieba

Kedougou- Dalema HL 123 microgranite 76.2 354 0.6209 0.718818 + 37 0.70002 + 90 2.01 + 0.04 Kenieba

Niger Liptako D 89-5 enclave 86.9 658 0.3812 0.713211 + 41 0.70167 + 60 2.28 + 0.06

Niger Liptako D 89-4 granodiorite 70.7 991 0.2064 0.707859 + 34 0.70161 + 30 2.45 + 0.12

Niger Liptako D 89.76 aplite 63.5 1024 0.1792 0.707117 + 35 0.70169 + 30 2.57 + 0.15

Guinea Kankan G 9* granodiorite 71.2 525 0.3920 0.714151 + 18 0.70229 + 60 2.39 + 0.06 Guine•i Kankan G 10' enclave 81.5 497 0.4742 0.716740 + 41 0.70239 + 70 2.35 + 0.06

Guinea Kankan G 11' aplite 42.5 522 0.2351 0.709727 + 67 0.70261 + 40 2.76 + 0.12 Reguibat Ghillaman MAS 138 granite 249 155 4.6480 0.833885 + 43 0.6932 + 69 1.99 + 0.03

Reguibat Ghallaman MAS 134 granite 130 256 1.4750 0.745932 + 59 0.7013 + 22 2.12 + 0.03

Reguibat Ghallaman MAS 265 granite 573 82.6 20.10 1.261891 + 61 0.653 + 30 1.94 + 0.03 Reguibat Archeouat MAS76 rapakivi granite 231 162 4.1250 0.809848 + 48 0.6850 + 61 1.83 + 0.03

Reguibat Archeouat MAS 213 rapakivi granite 269 96.9 8.0240 0.925038 + 48 0.682 + 12 1.94 + 0.03 Reguibat Archeouat MAS 215 rapakivi granite 376 38.8 28.08 1.589 + 137 0.739 + 42 2.19 + 0.03

Reguibat A'/n ben Till IM 237 granite 155 210 2.1440 0.760553 + 49 0.6957 + 32 1.93 + 0.03

Reguibat A'in ben Tili IM 315 granite 241 139 5.0010 0.847695 + 57 0.6963 + 74 2.04 + 0.03

Reguibat A'/n ben Tili MAS 219 granite 297 111 7.7530 0.922291 + 69 0.688 + 12 1.98 + 0.03

87Rb/86Sr ratios are known at 1.5 percent. Analytical errors for 87Sr/86Sr ratios are expressed in 2-sigmas. Twenty nine standard analyses give an average value for E and A of 0.708054 + 12. Model ages (TDM) calculated by assuming a linear evolution of the depleted mantle fror• 0.699 to 0.7025 over 4.5 Ga. Measurement on Cameca 206 SA except (asterisk entries) on Finnigan Mat 262 in static mode.

BOHER ET AL.: CRUSTAL GROWTtt IN WEST AFRICA 353

Localization

Burkina Faso

Burkina Faso

Burkina Faso

Burkina Faso

Burkina Faso

Burkina Faso

Burkina Faso

Burkina Faso

Burkina Faso

Burkina Faso

Burkina Faso

Burkina Faso

Burkina Faso

Burkina Faso

Burkina Faso

Burkina Faso

Ivory Coast Reguibat Reguibat Reguibat Reguibat Reguibat

Kedougou-K Kedougou-K Kedougou-K Kedougou-K Kedougou-K Kedougou-K Kedougou-K Kedougou-K Kedougou-K Burkina Faso

Burkina Faso

Burkina Faso

Niger Niger Niger

Ivory Coast Ivory Coast Ivory Coast Ivory Coast Ivory Coast Ivory Coast Ivory Coast Ivory Coast Ivory Coast Ivory Coast Guinea

Guinea

Reguibat

BoaFoam

Boaroum

Bamga Bamga Bamga Bamga Bamga Yalogo Yalogo Yalogo Yalogo Yalogo Yalogo Yalogo Yalogo Yalogo Yaoure

Tsalabia

Tsalabia

Ghallaman

Ghallaman

Ghallaman

Mako

Mako

Mako

Mako

Dalerna

Dalerna

Dalerna

Dalerna

Dalerna

Ouahigouya Ouahigouya Ouahigouya Liptako Liptako Liptako Yaoare

Yaoare

YaouIE

YaoalE

YaoalE

Boundiali

Comoe

Comoe

Comoe

Comoe

Siguiri Siguiri Tsalabia

TABLE 4. Nd Isotopic Compositions of Birimian Felsic Samples Nature Sm Nd 147Sm/ 143Nd/144Nd

144Nd High-Grade Metamorphic Roc•

Dabakalian

66a

66c

164b

314b

197'c

197'd

B21

Glf

Glg LOBN

L2b

B8

G2b

LWRT

G3b

G3d

L13*

MAS 157b

MAS 196

MAS 272

MAS 241

MAS 279

Birimian

M35

M8

M36

40E39'

HL57

HL121

HL122

HL126

HL127

5448

5450

5454*

AS 93*

AS 161'

D 113'

J16bis*

S161'

AC 36c*

AC 37c*

J38

MB 5*

AN 406*

NZ 504*

NZ 505*

BO 101'

G24'

G31*

MAS 259c

metarhyolite 5.40 metarhyolite 4.89

leptynite 4.92 quartzite 8.30 amphibologneiss 11.16 amphibologneiss 7.33 gneiss 4.63 leptynite 10.22 leptynite 8.16 leptynite 8.51 leptynite 9.47 leptynite 8.14 leptynite 6.52 leptynite 7.79 gneiss 2.50 gneiss 4.32 gneiss 3.31 migmatite 3.88 leptynite 7.16 gneiss 4.30 leptynite 1.121 leptynite 8.89

Sediments

27.90 0.1177 0.511719 + 27

23.86 0.1247 0.511742 + 37

22.45 0.1333 0.511812 + 37

34.59 0.1460 0.512071 + 27

68.64 0.0990 0.511334 + 27

37.49 0.1191 0.511649 4- 25

16.48 0.1710 0.512414 + 31

36.70 0.1697 0.512343 + 26

28.97 0.1714 0.512453 + 30

30.27 0.1710 0.512490 + 30

34.15 0.1687 0.512345 + 28

26.29 0.1880 0.512651 ñ 43

19.62 0.2023 0.512829 + 23

25.75 0.1840 0.512490 + 29

7.94 0.1915 0.512708 + 29

13.56 0.1898 0.512647 + 29

17.34 0.1161 0.511627 + 11

24.82 0.0952 0.511321 + 27

45.78 0.0952 0.511354 + 21

23.30 0.1123 0.511592 + 25

6.09 0.1120 0.511367 + 30

41.09 0.1316 0.511926 + 24

6.80

pelite 1.71 3.78

schist 3.77

sandstone 4.60

quartzite 4.37 schist 4.23

sandstone 5.53

sandstone 3.18

argilo-schist 9.77 argilo-schist 16.40 argilo sandstone 0.77 metasiltite 2.26

phyllade 2.75 schist 3.07

1.40

1.83

black shale 3.90

black shale 2.18 black-shale 7.08

conglomerat 3.57 greywacke 4.51 greywacke 3.48

greywacke 5.32 greywacke 4.02

pelite 3.7 5 pelite 1.082 schist 4.00

30.07 0.1375 0.512028 + 20

5.01 0.2081 0.512940 + 25

19.02 0.1209 0.511681 + 25

18.62 0.1233 0.511774 + 16

25.89 0.1081 0.511576 + 25

25.47 0.1043 0.511473 + 23

24.57 0.1047 0.511485 + 26

31.37 0.1072 0.511582 + 26

22.91 0.1013 0.511652 + 25

58.44 0.1017 0.511622 + 28

93.98 0.1062 0.511643 + 25

3.55 0.1327 0.511860 + 28

10.03 0.1368 0.512172 + 19

13.40 0.1249 0.511856 + 8

14.64 0.1276 0.511698 + 19

4.88 0.1742 0.512552 + 10

5.90 0.1884 0.512653 4- 10

18.63 0.1272 0.511751 4- 5

10.65 0.1247 0.511627 4- 26

29.18 0.1476 0.512139 4- 21

20.65 0.1052 0.511412 4- 15

25.81 0.1063 0.511480 4- 29

18.53 0.1143 0.511579 + 21

27.34 0.1184 0.511620 4. 15

23.59 0.1037 0.511429 4. 14

21.60 0.1057 0.511425 + 14

5.48 0.1201 0.511632 4. 18

21.98 0.1107 0.511538 4- 30

3.4 + 0.6

2.0 + 0.7

1.0 + 0.7

2.6 4- 0.6

0.9 + 0.5

1.7 +0.5

2.6 4- 0.6

1.5 4- 0.6

3.2 4- 0.6

4.1 +0.6

1.8 + 0.6

2.6 + 0.9

2.2 ñ 0.5

0.5 4- 0.6

2.8 + 0.6

2.1 4.0.6

2.0 + 0.3

1.7 ñ 0.5

2.3 4.0.4

2.4 4-0.5

-2.0 4- 0.6

3.7 +0.5

4.1 4-0.4

2.8 + 0.6

1.8 4.0.5

3.0 4- 0.4

3.2 + 0.5

2.2 4- 0.5

2.3 + 0.5

3.6 + 0.5

6.5 + 0.5

5.8 + 0.6

5.0 + 0.5

2.1 4-0.6

7.1 4-0.4

4.1 4-0.2

0.3 4- 0.4

4.4 + 0.4

2.5 4. 0.5

1.5 4. 0.3

-0.3 4. 0.5

3.5 + 0.5

0.8 4- 0.3

1.8 4- 0.6

1.6 4- 0.3

1.3 + 0.3

1.5 + 0.3 0.9 4-0.3

1.0 + 0.4

1.8 4-0.6

2.14 4. 0.16

2.27 + 0.19

2.39 4. 0.22

2.25 4- 0.24

2.31 + 0.14

2.29 4- 0.17

2.36 + 0.39

2.58 4- 0.40

2.23 + 0.37

2.05 + 0.35

2.50 4- 0.38

2.60 + 0.68 -

3.33 4- 0.68

2.63 4- 0.76

2.97 + 0.75

2.25 + 0.16

2.25 + 0.14

2.20 + 0.13

2.22 + 0.16

2.57 + 0.18

2.11 4- 0.18

2.07 +0.19 -

2.28 + 0.17

2.18 + 0.17

2.15 + 0.15

2.22 + 0.14

2.21 4- 0.14

2.12 4- 0.14

1.91 4- 0.12

1.96 4- 0.13

2.01 4- 0.13

2.28 + 0.20

1.75 4- 0.16

2.07 4- 0.16

2.44 4- 0.20

1.97 4- 0.35

2.64 4- 0.66

2.33 4- 0.19

2.49 + 0.19

2.14 4- 0.23

2.33 4- 0.15

2.25 4- 0.15

2.29 + 0.16

2.32 + 0.17

2.27 4. 0.14

2.32 4. 0.15

2.34 + 0.17

2.26 4- 0.16

Localization

Kedougou-K Kedougou-K Kedougou-K Kedougou-K Kedougou-K Kedougou-K Kedougou-K Kedougou-K Burkina Faso

Burkina Faso

Ivory Coast Ivory Coast Ivory Coast Ivory Coast Ivory Coast Ivory Coast Ivory Coast Reguibat Reguibat

Kedougou-K Kedougou-K Kedougou-K Kedougou-K Kedougou-K Kedougou-K Kedougou-K Kedougou-K Kedougou-K Kedougou-K Kedougou-K Kedougou-K Kedougou-K Kedougou-K Kedougou-K Kedougou-K Kedougou-K Kedougou-K Kedougou-K Kedougou-K Kedougou-K Kedougou-K Kedougou-K Kedougou-K Kedougou-K Kedougou-K Kedougou-K Kedougou-K Kedougou-K Kedougou-K Kedougou-K Kedougou-K Kedougou-K Niger Niger Niger Niger Niger Niger

DsJelna

Dalema

Dalema

Dalema

Dalema

Malco

Mako

Mako

Bouroum

Bouroum

¾aoure

Yaoure

Yaoure

Yaoure

Yaoure

Yaoure

Yaoure

Tsalabia

Tsalabia

Mako

Mako

Kakadian

Kakadian

Kakadian

Kakadian

Kakadian

Kakadian

Mako-Dialle

Mako-Dialle

Mako-Dialle

Mako-Dialle

Mako-Dialle

Boboti

Boboti

Boboti

Boboti

Boboti

Boboti

Boboti

Saraya Saraya Saraya Saraya Saraya Saraya Gainaye Dalema

Dal•ma

Dal•ma

Dalema

Dalema

Liptako Liptako Liptako Liptako Liptako Liptako

Sample

Birimian

HL12

HL15

I-IL 97

HL103

HLlll

R5

M23

T139

BN 208

BN 230

J 196

L219A

D4M12

V201 F

V2015

6

V22D4

MAS 206

MAS 17lb

Birimian

87L

86624

9208

9201

9205

9225

9212

9211

9461

9462

9463

9464

9466

9452

9457

9459

946O

9110

9111

9113

8667

8668

8669

8670

8672

9451

9109(2) HL71

HL123

DJ*

W17*

Y2C*

D 89-5

D51

D 89-76*

D 89-4

D60

D19

Nature

madesite

madesite

rhyodacite rhyodacite rhyodacite madesite

tuff

andesitic tuf

lamprophyre madesite

rhyodacite rhyolite rhyodacite rhyodacite rhyodacite basalte

volcanite

granodiorite pegmatite diorite

granodiorite granodiorite diorite

granodiorite granite granodiorite tonalite

granodiorite granodiorite granodiorite granodiorite granodiorite monzogranite monzogranite diorite

monzodiorite

tonalite

leucogranite leucogranite leucogranite leucogranite tonalite

tonalite

granite microgranite microgranite microgranite

granodiorite granite granite enclave

granite Aplite granodiorite granite granite

TABLE 4. (continued)

Volcanics

147Sm / 144Nd

4.25 22.02 0.1174

2.30 6.83 0.2047

6.12 35.72 0.1043

5.50 38.30 0.0874

3.41 17.53 0.1184

3.65 19.24 0.1155

1.21 3.89 0.1893

0.89 3.19 0.1697

2.42 10.06 0.1461

1.79 8.62 0.1266

7.29 44.63 0.0994

3.40 20.41 0.1014

3.14 15.48 0.1234

2.34 21.00 0.0678

2.85 17.07 0.1016

3.15 16.21 0.1182

5.99 31.61 0.1153

1.92 10.84 0.1078

13.24 75.50 0.1067

Granites

8.39

18.19

12.29

19.94

15.28

11.03

18.75

19.62

27.73

14.36

28.39

12.61

11.91

23.18

39.87

35.3O

32.42

29.22

36.52

42.93

17.93

2O.32

11.25

28.62

39.19

5O.9O

32.O5

26.60

28.00

32.02

29.17

14.54

20.40

88.65

57.49

2.94

38.59

11.08

15.64

1.62

3.95

2.71

3.50

2.62

2.58

3.34

3.61

4.9O

2.68

6.64

2.45

2.39

3.88

6.65

6.37

5.6O

5.33

6.54

7.57

3.97

3.13

2.82

6.02

6.O9

7.96

4.3O

4.45

4.93

4.83

5.12

2.61

3.52

16.07

6.91

0.24

6.81

2.20

2.52

0.1175

0.1321

0.1339

0.1068

0.1043

0.1425

0.1083

0.1118

0.1074

0.1136

0.1420

0.1180

0.1223

0.1017

0.1015

0.1098

0.1052

0.1110

0.1089

0.1073

0.1348

O.O938

0.1520

0.1280

O.0946

O.O952

0.0816

0.1017

0.1070

0.0918

0.1068

0.1092

0.1050

0.1103

0.0731

0.0491

0.1074

0.1208

0.0981

'143Nd / 144Nd

0.511674 + 26

0.51239O + 27

0.511486 + 24

0.511290 + 31

0.511638 + 24

0.511676 + 35

0.512720 + 24

0.512595 + 24

O.511979 + 11

0.511701 + 26

0.511427 + 22

0.511375 + 23

0.511543 + 23

0.511366 + 27

0.511425 + 23

O.511519 + 2O

0.511594 + 23

0.511590 + 36

0.511505 i 24

0.511648 + 28

0.5118O7 + 25

0.511841 + 23

0.511551 + 25

0.511518 + 25

0.512050 + 25

O.511 645 + 29

0.511550 + 25

0.511569 + 21

O.511626 + 23

0.512010 + 25

0.511719 + 27

0.511745 + 22

0.511526 + 25

O.511523 ñ 21

0.511597 ñ 30

0.511564 + 29

0.511586 ñ 26

0.511542 ñ 22

O.511513 4- 26

O.511912 ñ 35

0.511423 + 25

0.512152 + 32

0.511751 + 27

0.511404 + 20

0.511354 + 25

0.511105 ñ 24

0.511453 ñ 25

0.511529 ñ 26

0.511346 ñ 26

0.511565 + 13

0.511546 + 15

0.511487 + 18

0.511507 + 26

0.511026 + 21

0.510603 + 26

O.511479 + 22

0.511657 + 22

O.511315 ñ 25

•d(2.1)

2.6 + 0.5

-7.0 + 0.6

2.5 + 0.5

1.4 + 0.6

1.6 + 0.5

3.2 + 0.7

3.6 + O.5

6.5 + 0.5

0.8 + 0.5

0.6 4- 0.5

2.6 + 0.4

1.1 +0.5

-1.9 + O.5

10.0 + 0.5

2.0 + 0.5

-0.7 + 0.4

1.6 + 0.5

3.6 + 0.7

2.1 +0.5

2.1 ñ0.6

1.2 + 0.5

1.4 +0.5

3.1 ñ0.5

3.1 +0.5

3.2 + 0.5

4.5 + 0.6

1.7 +0.5

3.3 +0.4

2.7 +0.5

2.5 ñ 0.5

3.3 + O.6

2.7 +0.5

4.0 ñ 0.5

4.0 ñ 0.4

3.2 + 0.6

3.8 + 0.6

2.6 ñ O.5

2.3 + O.5

2.2 ñ O.5

2.5 + 0.7

4.1 +0.5

2.6 + 0.7

1.2 ñ 0.6

3.5 ñ0.4

2.4 +0.5

1.2 ñ O.5

2.5 + 0.5

2.6 + O.5

3.1 +O.5

3.3 + 0.3

2.3 ñ 0.3

2.3 +0.4

1.3 + O.5

1.9 + O.4

0.1 ñ0.5

1.5 + 0.5

1.3 ñ O.5

0.8 ñ0.5

2.20 4- 0.16

-

2.20 4- 0.14

2.14 4- 0.12

2.29 4- 0.17

2.16 4- 0.16

2.19 4- 0.61

1.59 4- 0.27

2.48 + 0.25

2.40 + 0.19

2.18 + 0.14

2.30 + 0.14

2.60 + 0.21

1.76 + 0.09

2.23 + 0.14

2.49 + 0.18

2.29 + 0.16

2.12 ñ 0.15

2.22 ñ 0.15

2.25 + 0.17

2.37 ñ 0.22

2.35 + 0.21

2.16 + 0.14

2.15 + 0.14

2.18 ñ 0.22

2.05 ñ 0.14

2.27 +0.16

2.14 ñ 0.14

2.19 + 0.16

2.25 + O.22

2.14 + 0.16

2.20 ñ 0.17

2.09 ñ 0.13

2.09 ñ 0.13

2.15 + 0.15

2.11 + 0.14

2.20 ñ 0.15

2.22 ñ 0.15

2.23 + 0.15

2.23 + 0.21

2.08 + 0.12

2.27 ñ O.26

2.35 + 0.19

2.12 ñ 0.13

2.20 ñ 0.13

2.26 + 0.12

2.19 + 0.14

2.2O + 0.15

2.15 + 0.13

2.14 + 0.14

2.22 + 0.15

2.21 + 0.14

2.30 ñ 0.16

2.21 + 0.11

2.28 + 0.20

2.28 + 0.15

2.32 + 0.18

2.31 ñ 0.14

Localization

Niger Burkina Faso

Burkina Faso

Burkina Faso

Burkina Faso

Burkina Faso

Burkina Faso

Burkina Faso

Burkina Faso

Burkina Faso

Burkina Faso

Burkina Faso

Burkina Faso

Burkina Faso

Burkina Faso

Burkina Faso

Burkina Faso

Burkina Faso

Burkina Faso

Ivory Coast

Ivory Coast

Ivory Coast Ivory Coast Ivory Coast

Ivory Coast Ivory Coast

Ivory Coast Ivory Coast Ivory Coast

Ivory Coast Ivory Coast Ivory Coast Ivory Coast

Ivory Coast Ivory Coast Gmnea

Guinea

Gmnea

Guinea

Gutnea

Guinea

Reguibat Reguibat Reguibat Reguibat Reguibat Reguibat Reguibat Reguibat Reguibat Reguibat Reguibat Reguibat Guiana

Guiana

Guiana

Guiana

Liptako D65' Bouroum BN 71A

Bouroum BN 46

Bouroum BN 69B

Bouroum BN 75

Koupela 6867 Koupela 6868 Koupela 6869 Zorgho 6890 Zorgho 6894 Zorgho 6895 Ouidi 6875

Nanga-Yarce 5430 Nanga-Yarce 5431 Nanga-Yarce 5433 Zogore 5440 Zogore 5443 Bidi 5444

Bidi 5446

Ferke CI-2

Ferke CI-4

Ferke CI-8

Ferke CI-10

Bouake y3 Bouake y7c Boundiali MB 8*

Bondoukou STlb

Bondoukou ST 34b

Bondoukou ST 35b

Bondoukou BK 16

Bondoukou ST 44b

Bondoukou ST 7

Yaoure G8*

Yaoure D292'

Yaoure B 328*

Kerouane D 192

Kerouane D 169

Kerouane D 200

Kankan G 1 *

Kankan G9*

Kankan G 10*

Ghallaman MAS 134

Ghallaman MAS 138

Ghallaman MAS 265

Kareth MAS 229

Kareth MAS 179'

Kareth MAS 180

Archeouat MAS 76

Archeouat MAS 215

Archeouat MAS 213

A'/n ben Till IM 237

A'/n ben Till MAS 219

A'/n ben Till IM 315

Degrad Roche S 20 Degrad Roche S 542 Degrad Roche H104

Del•rad Roche H 945

ß

Nature

diorite

trondhjemite granophyre granophyre granite granite granite granite granite granite

gramte

gramte

gramte

granite

gramte

gramte

gramte

gramte

granodiorite granite granite granite granite granite granite enclave

enclave

enclave

granodiorite granodiorite granodiorite granite granite granite

monzogranite

gneiss granite granite granodiorite enclave

granite

granite granite

granite granite

granite

rapakiwi granite rapakiwi granite rapakiwi granite granite gramte

granite

tonalitc

tonalitc

tonalRe

tonalitc

TABLE 4. (continued)

' •n Nt 147Sm/ '143Nd/144Na end(2.1) 144Nd

2.58 18.60 0.0844 0.511132 _+ 19 0.9 _+ 0.4 2.28 + 0.12

10.55 34.72 0.1849 0.512598 ñ 25 2.0 ñ 0.5 2.60 ñ 0.59

10.43 36.03 0.1761 0.512526 ñ 20 3.4 ñ 0.5 2.21 ñ 0.40

10.05 33.26 0.1839 0.512565 ñ 24 2.4 ñ 0.5 2.71 ñ 0.58

10.66 36.38 0.1783 0.512485 ñ 26 2.0 ñ 0.6 2.60 ñ 0.49

3.65 20.80 0.1068 0.511401 ñ 26 0.1 ñ 0.5 2.38 ñ 0.16

6.09 35.85 0.1034 0.511369 ñ 22 0.4 ñ 0.5 2.35 ñ 0.15

6.23 29.27 0.1295 0.511770 + 25 1.2 ñ 0.5 2.36 ñ 0.20

7.42 49.48 0.0912 0.511196 + 25 0.3 ñ 0.5 2.33 ñ 0.13

2.67 19.59 0.0830 0.511083 + 24 0.3 ñ 0.5 2.32 ñ 0.13

4.12 30.50 0.0822 0.511128 ñ 25 1.4 + 0.5 2.25 ñ 0.12

3.86 22.47 0.1046 0.511387 + 29 0.5 ñ 0.6 2.35 + 0.15

2.10 13.96 0.0915 0.511284 ñ 26 2.0 ñ 0.5 2.22 ñ 0.13

10.97 66.22 0.1008 0.511379 ñ 24 1.3 ñ 0.5 2.28 + 0.14

6.66 32.07 0.1264 0.511658 ñ 28 -0.2 ñ 0.6 2.48 ñ 0.20

12.20 82.24 0.0902 0.511252 + 20 1.7 + 0.4 2.24 ñ 0.13

12.70 113.86 0.0678 0.510977 ñ 21 2.4 ñ 0.4 2.18 + 0.11

4.95 23.44 0.1286 0.511805 + 29 2.1 + 0.6 2.26 + 0.19

4.13 24.33 0.1033 0.511388 + 26 0.8 + 0.5 2.32 + 0.15

2.74 15.68 0.1062 0.511439 ñ 25 1.0 + 0.5 2.31 + 0.15

3.00 12.95 0.1410 0.511931 + 30 1.3 + 0.6 2.40 + 0.23

1.55 8.95 0.1051 0.511413 + 25 0.8 + 0.5 2.33 + 0.15

5.08 26.45 0.1169 0.511581 + 22 0.9 ñ 0.5 2.35 ñ 0.17

1.06 5.40 0.1196 0.511610 + 26 0.7 + 0.6 2.37 + 0.18

3.58 21.66 0.1005 0.511406 + 24 1.9 + 0.5 2.24 + 0.14

5.90 28.32 0.1267 0.511739 + 36 1.4 + 0.7 2.33 + 0.19

9.10 46.32 0.1195 0.511725 ñ 25 3.0 + 0.5 2.17 ñ 0.16

30.79 179.39 0.1044 0.511438 + 26 1.5 + 0.5 2.27 ñ 0.15

6.11 31.13 0.1194 0.511670 + 24 2.0 + 0.5 2.26 + 0.17

2.06 11.30 0.1109 0.511551 ñ 28 2.0 + 0.5 2.25 ñ 0.15

0.37 2.64 0.0846 0.511115 + 28 0.5 + 0.6 2.31 + 0.13

2.97 17.62 0.1027 0.511428 + 25 1.8 + 0.5 2.25 + 0.14

3.28 18.75 0.1064 0.511485 + 10 1.9 + 0.5 2.25 ñ 0.15

4.29 26.22 0.0996 0.511415 + 13 2.3 ñ 0.5 2.21 + 0.14

2.23 13.41 0.1014 0.511416 ñ 20 1.9 __+ 0.4 2.24 ñ 0.14

12.89 67.80 0.1157 0.511349 ñ 24 -3.3 ñ 0.5 2.70 ñ 0.19

4.75 27.29 0.1059 0.511382 ñ 22 0.0 + 0.5 2.39 ñ 0.16

7.84 50.84 0.0938 0.511133 ñ 25 -1.6 __+ 0.5 2.47 ñ 0.15

3.60 29.31 0.0747 0.510968 + 21 0.4 + 0.4 2.30 + 0.12

2.40 18.25 0.0799 0.511058 ñ 6 0.7 + 0.7 2.29 + 0.12

0.89 5.34 0.1013 0.511377 + 20 1.1 + 0.7 2.29 ñ 0.15

5.19 37.23 0.0848 0.511255 ñ 25 3.2 + 0.5 2.14 ñ 0.12

3.36 23.84 0.0857 0.511199 + 25 1.9 ñ 0.5 2.22 + 0.12

10.42 59.05 0.1074 0.511464 + 25 1.2 + 0.5 2.30 + 0.15

5.31 31.47 0.1026 0.511427 + 21 1.8 + 0.4 2.25 ñ 0.14

0.95 6.60 0.0879 0.511196 + 23 1.2 + 0.5 2.27 + 0.13

1.22 8.31 0.0893 0.511298 ñ 30 2.8 + 0.6 2.16 ñ 0.13

6.21 41.65 0.0907 0.511336 __+ 23 3.2 + 0.5 2.14 + 0.12

11.60 66.04 0.1068 0.511520 + 18 2.5 + 0.4 2.20 + 0.14

10.84 60.65 0.1088 0.511545 ñ 19 2.4 + 0.4 2.21 + 0.15

10.98 68.79 0.0971 0.511416 ñ 22 3.0 ñ 0.5 2.15 ñ 0.13

6.03 32.21 0.1139 0.511588 + 20 1.9 + 0.4 2.26 + 0.16

6.93 46.36 0.0910 0.511310 ñ 25 2.6 + 0.5 2.18 ñ 0.13

5.69 36.90 0.0938 0.511269 + 25 1.1 + 0.5 2.29 + 0.14

4.74 31.46 0.0917 0.511281 ñ 31 1.9 + 0.6 2.23 + 0.13

5.13 33.20 0.0940 0.511235 + 31 0.3 + 0.6 2.34 + 0.14

4.81 31.31 0.0935 0.511262 + 25 1.0 + 0.5 2.29 + 0.14

Nd analyzed as metal. 147Sm/144Nd ratios are known at 0.5 percent. Analytical errors for 143Nd/144Nd ratios are expressed in 2-sigmas. Standard analyses give an average value for La Jolla of 0.511834 + 17 (12 runs) using the Cameca 206 SA mass spectrometer, and 0.511851 + l0 (98 runs) using the Finnigan Mat 262 mass spectrometer. end values are calculated at 2.1 Ga, relative to the present-day chondritic values of 143Nd/144Nd = 0.512638, 147Sm/144Nd = 0.1967. Model ages (TDM) calculated using the depleted mantle evolution of Ben Othman et al. [1984]. Measurement on Cameca 206 SA except (asterisk entries) on Finnigan Mat 262 in static mode. Kedougou-K = Kedougou-Kenieba

356 BOILER ET AL.: CRUSTAL GROWTH IN WEST AFRICA

0.5128

0.5124

0.5120

0.5116

0.5112

0.5108

(a)

143,d'/14•, ,c• ........ • Baoule Mossi Granites r• J

o _ 0.06 0'510•. 04 0.08 0.10 0.12 0.14 0.16 0.18 0.20 0.0g 0.10 0.11 0.12 0.13 0.14 0.15 0.16

0.5120

Kedougou-Kenieba Granites ••r O

0.5116

. ,-• 945 + 52 Ma II O 0.5114 I • ENd(T)- 2.5 I

147 Sm / 144 Nd 0.5110 , , , , , , , ,. , , , . , i ,

0.08

Fig. 7. Sm-Nd isochron diagram for Birimian granites (a) Baoule-Mossi domain and (b) Kedougou-Kenieba inlier. A few samples have not been included in the calculation of the errorchron ages.

falling in the range 2.05 - 2.48 Ga (Figure 10). Rare older and +1.6 (+ 0.8) in the Baoule-Mossi area. The corresponding crustal residence ages (up to 2.39 Ga) are found only for variations for TDM are 2.21 (+ 0.02), 2.19 (+ 0.04), and 2.28 Birimian granites from Guinea (Kerouane), which are (+ 0.04) Ga (Figure 11). These differences are not significant at associated with the Archcan domain. For certain gneissic rocks from the Baoule-Mossi domain and particularly in the Yalogo segment, meaningful TDM cannot be obtained because their 147Sm/144Nd ratios are too similar to the depleted mantle value. With two exceptions (AS 93 in the Liptako and S161 in Yaoure), the TDM values of the sediment samples fall in the range 1.91-2.49 Ga, with samples with tNd(2.1) > +4 yielding the youngest ages.

No systematic relationship between tNd(2.1) or TDM with the distance to the Archcan eraton is observed, as in Saskatchewan [Chauvel et al., 1987] and the western United States [Bennett and DePaolo, 1987]. Subtle regional variations of œNd(2.1) and TDM may exist (Figures 8 and 10) but cannot be resolved at the 95 percent confidence level. Average granite ENd(2.1) values change from +2.3 (+ 0.4; ls errors) in the Reguibat rise to +2.6 (+ 0.6) in the Kedougou-Kenieba inlier,

the 95 percent confidence level. Although Sr isotopic compositions in many samples are

too radiogenic for a significant 87Sr/86Sr ratio to be calculated at 2.1 Ga, most precisely determined values fall in the range 0.7010 - 0.7025 (Table 3). Conspicuous exceptions are two felsic lavas from Yaoure (Ivory Coast). If samples with either poorly determined initial 87Sr/86Sr ratios or less radiogenic than the depleted mantle (~0.701) are discarded, the correlation with •Nd(2.1) values is weak and steep for most samples, although a few felsic lavas extend the trend significantly towards an enriched component (Figure 12).

Granite Geochemistry

Major and trace elements have been measured in several granites from the studied area. Most samples present both La and Gd enrichment relative to Yb (Figure 13). The Dabakalian

E Ga)

-15 -10 -5 0 5 10

High Grade Rocks

= Birimian / . Archean

Ill I .... I .... I .... I I I .... I' i

i

i

i

i

i

,,,[ .... [ .... [, i i •1, i

i

i

i

i

I -" ="-W .

"'1 .... I" I .... ! I "1" -15 -10 -5 0 5 10

.! Guyana shield "'1 .... I .... I .... I .... I .... I'

I

I

==w Reguibat Rise "'1 .... I .... I ........ I .... I'

I

I

= El'.-. == ........ Leo Rise '" I .... I .... I" ' ";':" •T" ?" I .... I .... I .... I .... I .... I .... I'

I

I

I

I .

I L Kedougou- I .• Kenieba Inlier

, .n.. , '-'i--"J- "'1 .... I .... I .... I .... I .... I .... I .... I .... I .... I .... I .... I'

-15 -10 -5 0 5 10 -15 -10 -5 0 5 10

Volcan ics Sediments Granites

Fig. 8. Distribution of ENd(2.1 Ga) in Birimian areas (Kcdougou-Kenieba, Baoulc-Mossi and Rcguibat rise) for high-grade metamorphic rocks, intermediate to fclsic volcanics, granites and sediments. Solid squares: Birimian samples from Africa; open squares: Archcan samples from the Man shield (M. Boher et al., unpublished data, 1990). Also shown: Trans- Amazonian granite samples from Guiana (R. Capdevila et al., unpublished data, 1990).

BOILER E'r AL.: CRUSTAL GROWTH IN WEST AFRICA 357

12

10

i

•--'Nd (T) i i i

(• Ben Othman et al. (1984) •) Goldstein et al. (1984) (•) Nelson & DePaolo (1985) (•) McCulloch (198 7) (• Albar&de & Brouxel (1987) (•) Parabolic Fit to Depleted

Mantle-Derived Samples

Crustal Evolution ]/ 147Sm / 144Nd ~ 0.11 T (Ga)

0 I I I I I

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5

Fig. 9. Various literature models of depleted-mantle evolution. Heavy curve: second degree model based on depleted samples with 147Sm/144Nd ratios higher than 0.19. Note the excellent agreement with the model of Ben Othman et al. [1984] (dashed curve). Evolution of typical crust material (arrow) shows a change of- - 1 epsilon unit per 100 Ma.

sample L13 from Yaoure has a flat heavy REE pattern. Eu anomaly is extremely variable but decreases with the calcium/ alkali element ratio (Figure 14). Extreme REE enrichment is occasionally present in an alkaline granite from Reguibat (5443) and an inclusion from the Bondoukou granite (ST 34b) while flat REE patterns typical of aplitic granites are found for the B idi granite.

DISCUSSION

Zircon U-Pb ages from this work (2185 Ma) and from Lernoine [1988] (2145 Ma) for Ivory Coast agree with the

geological evidence that the Dabakalian metamorphic formations represent the earliest rocks of the B irimian cycle [Ternpier, 1986]. Single zircon ages on early deformed gneisses (2194 Ma) extend this evidence to the Kedougou- Kenieba inlier (A. Dia et al., unpublished data, 1990). The climax of the amphibolite facies metamorphism which affected the Dabakalian formations is dated in the Yalogo area by the most precise Sm-Nd internal isochron at 2.15 Ga.

Population U-Pb data do not exist on felsic magmatic zircons from the Kedougou-Kenieba inlier. A. Dia et al., (unpublished data, 1990) obtained single zircon ages of 2.079

TDM

.! .

I

I

I

I

I

i

.ll I lomb 2 2.5 3

High Grade Rocks

II

Guyana shield

Reguibat Rise

" "== ..... ,=,'?, I , - Birirnian .... m .... '1" '"m ........ • .... I .... • ........... Kedougou-Kenieba 2 2.5 3 2 2.5 3 2 2.5 3 Inlier

-- Archean

Voicanics Sediments Granites

Fig. 10. Distribution of crustal residence ages (TDM) in Birirnian areas (Kedougou-Kenieba, Baoule-Mossi and Reguibat rise) relative to the depleted-mantle evolution of Ben Othman et al. [1984]. See Figure 8 for symbol key. Crustal residence ages with error larger than 250 Ma have been not been represented.


•5•Hercvniart I•I•ufitamdes

•Pan African

•Proterozoic

•?,•Archean • Fault .-'--' Thrust faul(

-- 2.3-.• 3.3 1.0 ..... '08 •2.0 2 4 ..... 11 1.5ii:::::'0.61.,

.

.iiiiiiii:':'l? 1.3 1.9

"2.3

Fig. 11. Geographical distribution of œNd(2.1 Ga) values in samples from West Africa. Data for French Guiana samples (R. Capdevila et al., unpublished data, 1990) given for reference. No correlation is found between the position and œNd(2.1 Ga) value. Only few samples from the boundary between Archean and Birimian terranes show a contribution from Archean continental material.

and 2.091 Ga for the prekinematic to synkinematic granites of Kakadian and Sandikounda. J. Y. Calvez et al. (unpublished data, 1990) found that single zircons from andesires and felsic tuff in the volcanic unit have 207pb/206Pb ages of 2.07 and 2.12 Ga, respectively. The present zircon U-Pb ages from the Loulo (2098 + 11 Ma) and Keniebandi (2125 + 27 Ma) sediments cannot therefore be distinguished from the magmatic 4.0 ages. Zircon population U-Pb ages reported by Lidgeois et al. [1991] on rhyodacites (2098 + 5 Ma), synkinematic granites 3.0 (2074 + 9 Ma) from south Mali and by Lemoine [1988] on synkinematic granites from central Ivory Coast (2090 + 4 Ma) 2.0 suggest that felsic magmatic activity and pene- contemporaneous erosion/sedimentation are confined in a 1.0 narrow time interval. The Sm-Nd errorchrons on felsic rocks

0.0

yield little useful chronometric information. Although Sm-Nd isochron ages on the metatholeiites from Burkina Faso (2126 -1.0 + 24 Ma) published by Abouchami et al. [1990] are not precise enough to establish age relationships between the metasedimentary and volcanic units, they strongly indicate that the bulk of post-Dabakalian magmatic activity, i.e., the -3.0 main Birimian episode, was restricted to a time interval of about 50 Ma (2126 - 2074 Ma) or less, in agreement with -4.o previous suggestions by Feybesse et al. [1989] and Lidgeois et al. [ 1991 ]. -s.0

Whatever the depleted mantle reference, the crustal residence -o.0 ages of felsic plutonic rocks are older than emplacement ages by no more than 100 to 200 Ma, which indicates both a short -7.o crustal history and very little contribution from Archean continents. Furthermore, œNd(2.1 Ga) values of most Birimian granitoids are almost indistinguishable from those of tholeiites in the same area [Abouchami et al., 1990], which suggests that both mafic and continental rocks had ultimately the same mantle source. Huge stretches of continents were therefore created from a mantle protolith in a very short period of time.

An Archean crustal component is nevertheless present in the few late felsic lavas from Yaoure that have radiogenic Sr and unradiogenic Nd. These samples may attest either to subduction of sediments from distant Archean cratons or to the

presence of Archean terranes in lower crustal layers. Open system behavior of Sr in basalts made it impossible to identify a Birimian "mantle array" for Sr and Nd isotopes [Abouchami et al., 1990]. The correlation for granitic rocks (Figure 10) can be accounted for in two different ways: (1) the Archean component as that present in some felsic lavas exists in all granite samples, although in much smaller proportions; (2) the mantle-derived protolith is heterogeneous and its isotopic properties can be represented by a mixture between a depleted mantle component (DM in Figure 10) and an enriched component which may be characterized by the rare Low Rare Earth Elements-enriched basalts described by Abouchami et al. [1990].

The vast majority of sediments also have œNd(2.1 Ga) and TDM values that do not support their derivation from an Archean craton. The same is true for all zircons but one Pb-Pb

ages obtained with the evaporation technique (J. Y. Calvez et al., unpublished data, 1990). Ion probe (this work) failed to find Archean zircons in B irimian sediments. It appears quite unlikely therefore that B irimian sedimentation took place in the vicinity of Archean cratons. The subordinate contribution of the Archean continental crust to the Birimian protolith is supported by field and geochemical evidence. The predominance of pillowed tholeiites with geochemical characters of oceanic flood basalts [Abouchami et al., 1990], and the presence of cherts, black shales and turbidires rich in volcanic clasts, suggest that Birimian sediments formed in deep water and in a pelagic environment. The usually small fraction of detrital quartz in turbidires provides further support for this interpretation.

Exceptions to this general pattern exist only in the vicinity of Kerouane (south Guinea) where œNd(2.1 Ga) values as low as

œ1•Jd(2.1 •a) ' '

' ::'.".•-•- Bidmian basalt field

I

I

I

I

Bulk Earth

.o.o , I , , 0.700 0.704 0.708 0.712

Fig. 12. Plot of œNd (2.1 Ga) versus (87Sr/86Sr)2.1 for Birimian felsic rocks. Error bars are calculated assuming an uncertainty of 100 Ma on the emplacement age. LREE enriched basalts (stars) and basaltic lavas (stippled field) from Aboucharni et al. [1990]. DM: contemporaneous depleted mantle from Ben Othman et al. [1984].

(87Sr/86Sr) 0 I

0,716

granites gneisses and leptynites

BOILER ET AL.: CRUSTAL GROWTH IN WEST AFRICA 359

IVORY COAST BURKINA FASO lOOO

lOOO

ß , ß i i i . i ,

La Ce Nd Sm Eu (•d Dy Er Yb Lu (a)

ø'--ø•-"'o.,,,,.,.,•.BONDOUKO U

1000 ...... , ..... . • * -Zo•gho

BOULSA ß Koul:•la

I ß ! ,

La Ce Nd Sm Eu (•d E•y Er Yb Lu (b) 10000

OUAHIGOUYA ß Nanga-Yarc• x Bidi

1OOO ' o Zogorc•

I ,

I La Ce Nd Sm Eu Gd Dy Er Yb Lu L; (•e ' N•I ' S•n E'u (•d ' 6y ' I•r 4b •u

KEDOUGOU - KENIEBA (SENEGAL)

1OOO I ..... • ......... 1OOO ,, ............... 100• • 1•

O.1 ...............

La Ce Nd Sm Eu Gd Dy Er Yb Lu (c) La Ce Nd Sm Eu Gd Dy Er Yb Lu (d) I 1• ...............

a [ !' Post-t•onic granges •• •• •U/N• •a K•rouan• •KO ß KaYdian granites ooE ' J

La • Nd Sm Eu Gd • •r Yb Lu I La Ce Nd Sm Eu Gd Dy Er Yb Lu Fig. 13. Chondrite-normalized REE patterns of some Birimian granitic rocks from West Africa. (a) Ivory Coast; stippled field of rhyodacites from R. Fabre et al. (unpublished data, 1990), (b) Burkina Faso, (c) Niger (top) and Guinea (bottom), (d) Eastern Senegal. Chondrite concentrations from Evensen et al. [1978].

-3.3 and TDM as old as 2700 Ma are indicative of the involvement of Arcbean basement in the genesis of Birimian granites, and near Ziemougoula in Northwestern Ivory Coast where similar data have been obtained (M. Boher et al., unpublished data, 1990) A few felsic lavas and sediments from Yaoure and Mako have lower œNd(2.1 Ga) values and older TDM. They may imply that ancient crustal fragments were locally involved in magma genesis and sedimentation, although, as a general rule, this involvement has been limited.

The absence of correlation between Sr and Nd isotopes (Figure 12) makes it difficult to identify the contribution of well-defined source components. However, comparison of major element data with isotopic arrays may help understand the role of mixing processes in granite genesis and crustal evolution. As far as the few analyzed granites are concerned, major element systematics are much simpler than isotopic patterns (Table 5a - 5c). For instance, SiO2, CaO and K20 are simply related and sample points define a nearly perfect plane


2.0

1.5

1.0

El

0.5 I• a

0.0

0.0

oo ID

[]

o

ooo •:•oo [] o o o

o

0.5

C3 []

C.,aO (Na2.0+K20)

I i , ß i I i i . i

1.0 1.5 2.0

holding for granites distributed over a wide area can result from either variable conditions of melting and differentiation or from mixing magmas of different origin. Magma mixing has been repeatedly observed on the field mainly in the form of marie inclusions in granites (e.g., Bard [1973], in Ivory Coast) and deduced from trace element arguments by A. Dia et al. (unpublished data, 1990) for the Kakadian batholith, in the west of the Kedougou-Kenieba inlier. When projected on the SiO2-CaO-K20 plane, the •:Nd(2.1 Ga) values show no indication of a relationship with the granite chemical composition, which shows that the end-members of the granitic series are not isotopically distinct.

Gd/Yb fractionation relative to chondrites suggests that garnet was present in the source of most Birimian granites. The covariation of the Eu anomaly and CaO/(Na20+K2 ¸) ratio (Figure 14) may be indicative of magma differentiation with plagioclase removal, or, alternatively, that low-Ca granites were generated in the alkali feldspar stability field. Two samples from the Bondoukou granite (Ivory Coast) show a heavy REE pattern concave upwards suggestive of amphibole fractionation.

Fig. 14. Variation of the Eu anomaly in granitic rocks with their A CRUSTAL GROWTH MODEL calcium/alkali element ratio. This diagram suggests plagioclase removal. The present work and the companion paper on Birimian

marie rocks by Aboucharni et al. [1990] provide a series of in this tridimensional space: comparison of the actual SiO 2 consistent geochemical features useful in the discussion of content with the value obtained by fitting a plane through the models of crustal growth during the Proterozoic: data (Figure 15) demonstrates the quality of the fit between 55 1. The mafic continental protolith is dominated by abyssal and 75 percent $iO 2. That sort of very simple relationship flood tholeiites emplaced at large distance from the continents.

Bobøti2 9457 Boboti2 9460

Boboti2 9452

Boboti2 9113

Saraya2 8672

Saraya2 9451 Saraya2 8667 Saraya2 8668 Saraya2 8669

Saraya2 8670 Kakadianl 9201

Kakadianl 9205

Kakadianl 9208

Kakadianl 9211

Kakadianl 9212

Kakadianl 9225

Gainaye2 9109 Mako 1 9461

Mako 1 9463

Mako 1 9464

Dalema2 HL 71

Dalema2 HL 96

Dalerna2 HL 123

Dalema2 Yatia

Dalema2 W17

Dalema2 DJ

TABLE 5a. Concentration Data on

SiO2 A!203 Fe203 * MnO MgO CaO Na20 K20 'TiO2 p205 PF Toal i

60.71 16.27 5.74 0.06 2.36 4.45 4.31 3.86 0.79 0.28 0.88 99.71

60.92 18.77 2.30 - 1.38 6.03 8.34 0.36 0.43 0.16 0.80 99.49 61.69 15.55 5.23 0.06 3.50 3.44 4.64 3.59 0.58 0.24 1.48 100.00 59.53 14.91 6.83 0.07 5.30 4.97 3.45 2.87 0.72 0.38 0.63 99.66 66.43 14.29 3.90 0.04 2.27 2.76 3.54 3.93 0.55 0.20 0.84 98.75

64.67 14.25 4.32 0.05 2.50 2.90 3.09 4.41 0.66 0.25 1.09 98.19

71.67 15.23 1.22 0.02 0.32 0.96 3.55 5.31 0.18 0.44 0.94 84.61

71.83 14.55 1.46 ø 0.42 1.27 4.11 4.53 0.21 0.13 1.39 99.90

72.81 15.13 0.83 0.01 0.24 0.68 3.91 4.83 0.10 0.27 0.80 99.61

71.06 14.86 1.62 0.01 0.39 0.93 3.34 5.50 0.27 0.28 0.80 99.06

67.43 14.78 3.45 0.05 1.47 2.95 4.16 3.54 0.45 0.18 0.59 99.05 69.81 14.71 2.49 0.03 0.93 2.31 4.62 3.04 0.34 0.17 0.58 99.03 65.90 14.78 6.05 0.08 1.95 4.25 3.97 0.83 0.58 0.20 1.43 100.02

64.7i 15.93 4.83 0.08 1.92 3.89 4.19 3.25 0.45 0.28 0.64 100.17 64.96 15.86 4.66 0.07 1.87 4.59 4.33 1.50 0.38 0.22 1.17 99.61

50.42 17.04 8.99 0.13 6.54 9.88 2.66 0.69 0.66 0.30 1.89 99.20

71.95 15.03 1.58 - 0.44 1.28 3.82 5.12 0.30 0.20 0.57 100.29

61.65 13.88 5.25 0.06 5.04 4.33 3.79 2.87 0.56 0.29 1.30 99.02

70.43 14.06 3.54 0.05 1.18 3.27 4.00 1.37 0.39 0.19 1.27 99.75

68.17 15.42 2.52 0.03 1.20 3.84 4.83 0.58 0.27 0.16 1.59 98.61

59.25 16.98 1.06 0.O1 1.72 5.17 9.16 0.14 0.38 0.22 5.73 99.82

70.10 16.O7 2.15 - 0.69 0.72 8.16 0.48 0.40 - 1.08 99.85 67.81 15.26 3.83 0.04 2.25 2.33 4.59 2.06 0.45 - 1.12 99.74

70.24 •5.33 1.95 0.01 0.63 1.60 5.27 3.27 0.27 0.19 0.90 99.66 66.04 15.48 3.04 0.06 1.10 3.16 5.85 1.51 0.30 0.19 3.07 99.80

63.46 15.41 5.66 0.06 2.45 3.79. 4.23 3.60 0.71 0.26 0.67 100.30

Major elements in percent, trace elements in parts per million measured by ICP with error of + 10 percent (K. Govindaradju, analyst). (Fe203*) = total Fe203.

* Correspond to enclave samples. For the Kedougou-Kenieba inlier, two areas have been distinguished: (1) Mako and (2) Diale-Dalema area.

BOILER E'r AL.: CRUSTAL GROWI'll IN WEST AFRICA 361

2. Calc-alkaline plutons intrude the volcanic and together in the following typical sequence of geodynamic sedimentary rocks [Deschamps et al., 1986; Mildsi et al., events (Figure 16): 1986; Bassot, 1987]. 1. Plume-related tholeiites are emplaced in an oceanic basin

3. Detritus from Archean continents comprises a negligible to form thick plateaus which become covered by abyssal fraction of the sediments. sediments (cherts and black shales).

4. The whole orogenic process, which formed extremely 2. Overloading of the oceanic crust by thick piles of basalts large volumes of new crust, took place in circa 100 Ma. initiates subduction and, locally, triggers some calc-alkaline

5. The protolith of the felsic magmas had a fairly short volcanic activity. Huge sequences of turbidites accumulate crustal history. downslope.

A model of ensialic tectonics (e.g., intracratonic rift) 3. Plateaus collide with the Archean continents. As developed by Condie [1982], Francis et al. [1983] and Kr6ner suggested by Feybesse et al. [1990], the northern boundary of [1984] for some Archean and Proterozoic orogens has been the Man craton in Guinea may represent a collision zone applied by Hastings [1982] to the Birimian terranes of West within which Archean basement is reworked in Birimian Africa. A rift in an Archean craton is doubtful because rift granites. Granites formed by melting of amphibolites and sediments would show a strong isotopic contribution of volcaniclastic sediments are emplaced in association with ancient crustal material unless the rifted continent is extremely transcurrent faults: these faults, which separate terranes with young. If a rift had formed in a young Dabakalian continent, different lithologies and induced severe deformation [Vidal and the contribution of such a continental crust to Birimian rocks Guibert, 1984; Zonou, 1987; Lemoine, 1988], may represent would not be easily detected from its isotopic and the lateral extrusion of the collided material. The existence of chronological characteristics. However, high-grade at least two successive phases of crust formation in circa 100 Dabakalian formafi•ons have not been preserved as a large Ma (Dabakalian and Birimian) suggests that the sequence of tectonic unit and are preserved only as small slices in the large- events described above may be recurrent over a short period of scale Eburnean tectonics. Although systematic field studies are time. required to document this point, the existence of a large Large segments with essentially juvenile character are not Dabakalian continent seems to us to be unlikely. The time common in the Lower Proterozoic: the 1.8-Ga-old Front Range scale and, to some extent, lithologies assumed for the Birimian in Colorado [DePaolo, 1981], the margins of the Reindeer Lake crust formation events may resemble somewhat those inferred zone of Saskatchewan [Chauvel et al., 1987], the Early for the juxtaposed Ancient Gneiss Complexes and Greenstone Proterozoic granulites fr6m Central Australia [Windrim and Belts from Barberton [Kr6ner and Todt, 1988]. McCulloch, 1986] contrast markedly with relatively

Given that the whole Birimian orogen was probably located unradiogenic Nd and longer crustal residence times commonly in the oceanic realm, the features listed above can be brought observed in the North Atlantic continent at 1.7 - 1.9 Ga

Birimian Granites, Kedougou-Kenieba

Ba Co Cr Cu Nb Ni Rb Sc Sr V Y Zn Zr La Ce •d Sm Eu Gd Dy Er Yb Lu 653 37 87 68 12 37 176 12 372 79 19 50 256

176 34 18 23 13 37 ll 9 384 21 17 ll 221

737 33 124 17 8 33 123 13 348 92 15 39 204 28.7 56.3 19.5 4.0 1.1 3.2 2.1 1.2 1.2 0.3

681 53 266 32 9 85 97 18 605 116 18 80 148 21.1 48.9 35.4 5.0 1.1 2.7 2.5 0.8 0.8 -

610 65 103 23 9 47 186 10 378 59 13 54 181

637 117 137 23 13 118 246 9 382 76 16 46 274

377 57 - 17 9 11 357 4 93 6 9 73 86

684 48 - 28 9 7 232 2 197 15 - 46 134

277 43 5 - 9 - 282 2 87 - - 52 47 8.6 19.0 8.5 2.2 0.3 1.9 0.8 0.3 0.2 -

297 60 7 - 10 6 352 4 87 7 9 71 128 30.6 69.4 29.8 6.6 0.4 4.4 1.9 0.7 0.5 0.2

682 34 51 54 8 21 166 8 403 52 13 56 179

745 57 35 11 7 16 93 5 467 32 8 50 125 21.1 45.5 16.1 3.3 0.7 2.6 1.5 1.0 0.7 0.3

379 37 36 43 - 26 19 14 362 95 15 76 142 12.6 27.9 12.5 3.2 0.9 2.8 2.5 1.4 1.2 0.3

781 38 49 18 17 15 112 10 635 74 16 66 147 22.4 47.3 20.6 4.2 0.9 3.3 2.5 1.5 !.4 0.3

787 47 38 31 6 17 48 13 715 76 17 72 132 19.6 40.9 !8.0 3.8 1.0 3.2 2.6 1.6 1.5 0.3

297 144 213 90 - 388 19 28 436 178 15 66 50

835 118 13 8 11 25 325 2 276 15 - 51 191

628 52 318 26 7 113 87 14 598 97 14 59 125 29.3 59.1 26.3 5.2 1.3 3.5 2.3 1.2 1.1 0.2

370 57 26 55 12 26 40 8 207 44 23 63 145 20.9 46.3 20.7 5.3 1.0 4.5 3.8 1.9 1.6 0.2

289 39 28 18 6 20 19 6 593 37 10 29 94 13.6 28.7 12.6 2.8 0.7 2.2 1.7 0.9 0.9 0.2

121 65 45 5 8 10 3 7 141 35 11 10 129

166 66 70 15 11 43 14 6 196 73 13 - 169

702 69 136 16 7 41 72 9 395 67 10 43 168

972 56 8 9 6 10 90 3 733 26 - 41 128 24.4 49.4 21.3 4.0 1.0 2.6 1.3 0.7 0.4 0.1

802 48 27 10 18 19 36 7 651 49 7 39 122 18.6 37.8 16.0 3.1 0.8 2.2 1.6 1.1 0.8 0.1

534 23 84 25 6 27 205 11 375 73 18 55 279 33.9 69.0 28.8 5.8 1.2 4.4 3.4 2.1 1.7 0.2


Zogorel 5440 Zogorel 5443 Bidil 5444

Bidil 5445

Bidil 5446

Nanga-Yarcel 5430

Nanga-Yarcel 5431

Nanga-Yarcel 5433

Koupela2 6867

Koupela2 6868

Koupela2 6869 Ouidi2 6875

Zorgho2 6890

Zorgho2 6894

Zorgho2 6895 Liptako3 D89-43

Liptako3 D89-41

Liptako3 D89-39

Liptako3 D89-4

Liptako3 D89-5'

Liptako3 D89-76

Liptako3 D89-77 Liptako3 D19

Liptako3 D60

Liptako3 D51

Bouake4 g3

Bouake4 g7c Ferke4 CI-2

Ferke4 CI-4

Ferke4 CI-8

Bondoukou 4 $T35b*

Bondoukou 4 $T34b*

Bondoukou 4 ST lb*

Bondoukou 4 ST 44b

Bondoukou 4 ST 49

Bondoukou 4 ST 26

Bondoukou 4 ST 7

Bondoukou 4 ST 6

Bondoukou 4 BK 16

Yaoure 4 G8

Yaoure 4 D 292

Kerouane5 D 169

Kerouane5 D 192a

Kerouane5 D 200

Kankan5 G9

Kankan5 G 10*

Kankan5 G 11

TABLE 5b. Concentration Data on

SiO2 AI203 Fe203* Mno Mg ¸ CaO Na20 K20 TiO2 P205 PF 76.14 12.00 1.88 0.03 0.04 0.38 3.60 4.84 0.10 0.08 0.63 75.55 11.83 2.58 0.05 0.08 0.68 3.58 4.57 0.20 0.10 0.32

76.29 12.07 1.45 0.01 0.08 0.59 3.52 4.76 0.08 0.08 0.39

75.06 13.14 1.22 0.02 0.02 0.89 4.45 4.37 0.10 0.06 0.57

73.54 13.64 1.67 0.03 0.26 1.18 3.62 5.12 0.17 0.15 0.66

71.06 14.91 2.06 0.02 0.64 2.20 4.34 3.16 0.27 0.17 0.44

68.75 14.71 3.72 0.06 0.85 2.20 4.07 4.30 0.65 0.30 0.78

67.40 14.93 3.27 0.08 0.91 1.21 3.35 6.44 0.39 0.22 0.63

66.71 14.21 3.27 0.05 2.24 3.39 4.62 2.66 0.30 0.24 0.67

63.69 14.36 5.30 0.06 2.77 3.47 3.30 4.72 0.72 0.35 1.05

60.29 14.08 7.33 0.17 4.50 5.33 4.05 2.61 0.50 0.34 0.94

64.77 16.23 3.90 0.05 1.47 3.27 4.55 3.39 0.44 0.32 0.90

72.24 !4.11 1.50 0.02 0.20 1.13 4.08 4.69 0.17 0.12 0.54

71.43 14.48 1.60 - 0.30 1.38 4.05 4.79 0.20 0.16 0.81

70.75 14.21 2.12 0.02 0.56 1.89 4.37 3.97 0.40 0.19 0.43

61.46 16.28 6.08 0.08 2.82 5.75 4.33 1.12 0.56 0.27 0.76

54.32 15.18 9.91 0.15 5.57 7.78 3.50 1.33 0.80 0.34 1.08

47.53 14.06 13.17 0.22 8.00 11.80 2.15 0.65 0.85 0.17 1.31

56.54 15.76 7.58 0.10 4.90 6.24 4.08 2.16 0.79 0.43 1.09

49.76 11.91 11.00 0.19 8.78 9.16 1.76 3.90 1.03 1.04 0.94

74.54 13.76 0.30 - - 1.08 3.20 5.19 - 0.06 0.56

53.46 13.06 8.58 0.22 7.98 8.53 2.66 3.34 0.55 0.28 0.81

70.52 14.88 2.75 0.03 0.79 2.79 5.00 1.62 0.29 0.17 0.96

64.84 16.38 5.00 0.07 1.67 4.57 4.58 1.41 0.52 0.22 0.83

67.15 15.85 3.45 0.03 1.33 3.59 5.05 0.88 0.40 0.22 1.99

75.21 13.00 0.51 0.00 0.20 0.72 3.65 5.00 0.04 0.11 0.58

72.06 14.14 1.23 0.02 0.27 0.98 3.70 5.33 0.11 0.14 0.72

66.71 14.98 3.72 0.06 2.00 3.75 3.97 2.20 0.39 0.17 1.50

72.24 14.94 1.10 0.00 0.22 0.76 3.89 5.16 0.10 0.20 0.77

72.90 14.60 1.83 0.02 0.48 2.07 4.49 2.82 0.20 0.15 0.71

61.17 15.41 6.91 0.08 2.45 5.47 4.82 0.81 0.65 0.38 1.57

52.09 14.07 8.80 0.12 4.75 9.33 4.50 1.20 1.29 1.37 1.63

59.70 15.48 7.05 0.12 3.52 5.54 4.69 0.93 0.60 0.35 1.46

76.46 12.88 0.96 0.01 0.02 0.53 4.50 3.84 0.05 0.12 0.43

77.05 12.75 0.75 0.01 0.02 0.43 4.19 4.12 0.05 0.11 0.47

69.56 15.11 2.54 0.03 0.9I 2.65 5.12 1.77 0.27 0.17 0.85

67.43 14.71 3.69 0.05 1.52 3.50 4.19 1.82 0.38 0.20 1.13

66.62 14.85 4.12 0.05 1.79 3.40 4.01 2.00 0.40 0.20 1.49

68.96 14.83 4.00 0.05 1.45 3.54 4.33 1.46 0.39 0.17 1.26

63.02 15.78 5.05 0.07 2.20 4.08 4.17 3.18 0.48 0.32 1.25

66.79 15.73 3.08 0.03 1.16 2.91 5.22 2.27 0.38 0.26 1.14

57.29 14.86 6.53 0.07 2.54 9.08 2.79 4.45 0.61 0.40 1.64

61.50 15.19 6.01 0.07 2.52 3.70 3.13 5.66 0.48 0.40 0.83

73.40 14.05 1.73 0.00 0.22 0.46 3.54 4.57 0.22 0.20 1.35

67.42 15.96 3.00 0.02 0.81 3.22 4.69 2.33 0.32 0.22 0.88

58.69 !6.46 6.98 0.11 3.54 5.50 4.50 1.76 0.86 0.39 1.01

71.90 15.25 0.44 0.00 0.14 2.88 4.66 2.18 0.05 0.14 0.74

Major elements in percent, trace elements in parts per million measured by ICP with error of + 10 percent (K. Govindaradju, analyst). (Fe203*) = total Fe203. * Correspond to enclave samples. For the Baoule-Mossi domain, samples from Burkina Faso: Ouahigouya (1) and Boulsa (2), Niger (3), Ivory Coast (4) and Guinea (5).

99.72

99.54

99.32

99.90

100.04

99.27

100.39

98.83

98.36

99.79

100.14

99.29

98.80

99.20

98.91

99.51

99.96

99.91

99.67

99.47

98.69

99.47

99.80

100.09

99.94

99.02

98.70

99.45

99.38

100.27

99.72

99.15

99.44

99.80

99.95

98.98

98.62

98.93

100.44

99.60

98.97

100.26

99.49

99.74

98.87

99.80

98.38

[Patchett et al., 1981; Patchett and Bridgwater, 1984; Nelson and DePaolo, 1985; Patchett and Arndt, 1986; Bennett and DePaolo, 1987] and in northern and western Australia [McCulloch, 1987]. In contrast, TDM close to depositional ages and positive •Nd(T) are quite common in Arcbean felsic

rocks [e.g., Shirey and Hanson, 1986]. This isotopic evidence, together with occasional presence of komatiites and predominance of I-type granites, indicate that the Birimian from West Africa has many characteristics of Arcbean terranes. The present study shows that even after the major periods of

BOILER ET AL.: CRUSTAL GROWTH IN WEST AFRICA 363

Birimian Granites, Baoule Moss/ ,

Ba Co Cr Cu Nb Ni Rb Sc Sr V Y Zn Zr La Ce Nd Sm Eu Gd Dy Er Yb Lu 647 - 180 60 9 7 105 5 24 - 27 65 216 65.4 146.2 66.5 11.4 1.2 7.4 4.6 2.5 2.3 0.4

2659 - 154 53 9 8 68 10 109 - 21 62 350 268.7 534.2 120.5 18.6 3.8 13.4 5.0 2.8 2.5 0.8

62 - 218 68 7 15 191 2 22 - 17 50 99 19.7 51.7 25.0 5.5 0.2 4.1 3.2 1.6 1.4 0.2

19 8 186 40 19 4 277 1 5 - 48 68 60 3.2 13.5 5.3 4.0 0.1 4.0 5.8 3.7 4.0 0.7

559 - 173 45 11 13 225 3 155 12 18 163 154 37.2 66.8 25.2 4.8 0.6 3.7 3.1 2.0 1.9 0.3

787 6 164 20 - 18 146 3 370 20 - 45 155 24.7 43.8 14.7 2.6 0.6 1.8 0.8 0.5 0.4 0.1

1140 11 172 32 31 10 218 7 267 45 59 73 396 116.1 161.7 82.1 15.3 2.5 11.0 7.9 4.8 4.9 0.8

811 - 159 13 24 17 345 12 193 19 49 86 293 48.4 93.6 36.6 8.5 0.7 7.4 5.7 3.5 2.7 0.5

1674 6 134 9 - 50 67 8 1180 33 10 57 106 21.9 46.4 21.6 4.1 1.0 3.0 1.7 0.9 0.8 0.1

908 19 137 40 14 53 212 12 529 98 20 46 290 47.4 91.2 40.8 7.7 1.5 5.6 3.9 2.2 1.8 0.2

552 24 214 177 9 52 110 21 452 122 24 101 107 15.7 47.6 30.6 6.7 1.2 4.8 4.0 2.1 2.1 0.3

1487 - 35 17 7 21 99 7 994 48 8 60 128 21.2 44.7 22.0 4.0 1.1 2.7 1.4 0.6 0.5 0.2

762 - - 6 6 6 155 3 181 6 7 54 120 46.5 82.3 26.8 4.5 0.6 2.9 1.3 0.6 0.5 0.1

1266 5 5 11 3 10 121 2 333 12 - 34 147 40.0 65.0 20.4 3.2 0.7 1.9 0.7 0.6 0.3 -

1198 - 9 21 - 10 114 3 507 28 6 64 215 47.9 86.5 30.4 4.6 1.0 2.7 1.1 0.5 0.4 -

618 97 57 12 5 24 26 16 865 108 14 72 126 24.6 57.4 24.7 4.7 1.3 3.5 2.2 1.2 1.0 0.2

553 74 180 371 - 58 30 31 710 174 22 108 132 19.8 60.9 32.1 6.7 1.8 5.5 3.5 1.8 1.6 0.3

111 92 186 164 - 117 12 54 195 286 18 84 24 3.7 18.8 5.1 2.0 0.8 2.4 2.7 1.7 1.7 0.4

801 78 223 54 9 63 67 22 1018 128 21 82 167 35.2 83.9 37.5 7.2 1.9 5.5 3.6 1.9 1.6 0.3

3180 89 509 192 - 121 92 32 650 169 34 115 291 75.5 184.5 85.8 16.6 3.4 12.5 6.9 3.1 2.6 0.5

3991 147 10 - 9 - 74 1 1095 8 - - 33 11.3 20.4 3.4 0.5 0.7 0.5 0.1 0.1 0.1 -

2980 67 561 169 9 161 51 24 998 125 24 109 94 37.6 108.8 49.4 9.4 3.5 6.9 4.4 2.3 2.2 0.4

485 117 11 7 9 6 36 5 530 31 7 49 108 18.6 41.5 15.7 2.9 0.8 2.0 1.2 0.6 0.5 0.1

621 98 23 37 5 9 34 10 714 78 11 65 113 12.5 31.8 13.2 3.0 0.9 2.4 1.7 0.9 0.8 0.2

566 75 20 5 - 9 18 6 1095 39 - 66 120 45.0 86.1 29.1 4.2 1.1 2.8 0.9 0.4 0.3 -

160 240 722 8 5 90 282 1 59 6 - 17 27 ..........

583 196 239 41 12 15 312 3 170 9 8 28 96 ..........

739 120 156 28 7 25 65 9 517 61 10 45 103 ..........

288 131 73 20 5 11 191 3 108 8 6 39 70 ..........

875 - 8 - - 8 93 3 457 18 6 46 109 ..........

142 18 146 251 7 33 29 18 459 113 21 73 162 21.5 55.9 32.6 7.0 1.4 5.3 3.6 1.9 1.7 0.2

213 32 121 311 15 99 30 15 963 162 35 113 295 124.7 323.5 185.5 32.8 5.8 20.1 8.0 2.9 1.8 0.3

956 25 167 205 6 75 31 21 510 124 25 92 128 32.5 71.6 46.3 10.0 2.0 7.2 4.4 2.2 1.8 0.2

16 - 104 11 - 10 164 1 11 5 - 20 53 5.7 11.1 2.8 0.6 0.1 0.5 0.3 0.3 0.5 0.1

35 - 104 11 - 7 150 2 19 - - 21 50 11.8 15.9 4.6 0.9 0.2 0.6 0.4 0.3 0.4 0.2

306 11 168 32 - 18 46 5 504 35 9 56 99 16.1 34.5 14.6 2.7 0.7 2.0 1.1 0.6 0.5 0.1

341 13 124 31 5 31 40 8 386 57 10 50 115 19.2 39.1 16.5 3.1 0.8 2.2 1.6 0.9 0.9 0.2

361 13 151 50 - 53 51 9 332 62 9 58 124 19.1 41.2 16.3 3.3 0.9 2.4 1.5 0.8 0.8 0.2

305 9 254 27 - 39 27 8 479 53 10 48 141 14.7 34.7 11.2 2.6 0.7 2.4 1.5 1.0 0.8 0.3

931 14 173 65 - 32 86 10 770 89 10 64 129 23.2 46.5 19.4 3.9 1.0 2.9 2.0 1.4 1.0 0.1

762 6 133 16 4 19 64 6 803 39 6 55 142 28.8 59.5 26.9 5.2 1.2 3.3 1.8 1.1 0.6 0.1

1329 109 55 404 13 36 206 18 734 97 19 87 149 38.2 74.2 31.3 5.8 1.4 4.8 3.4 1.9 1.7 0.2

1731 75 65 34 18 28 194 19 488 87 41 82 370 76.7 144.1 69.0 13.6 1.8 10.6 7.6 3.9 3.6 0.5

605 48 8 18 26 10 314 3 151 10 26 29 156 87.4 143.7 55.4 9.2 1.0 6.8 5.2 2.7 2.4 0.3

1087 105 13 11 - 9 65 2 542 38 - 47 125 50.0 79.0 20.1 2.8 1.0 1.8 0.7 0.6 0.2 -

452 70 96 43 - 50 75 14 527 108 13 102 141 28.0 59.5 27.7 5.5 1.5 4.2 2.6 1.6 1.0 0.1

1146 99 - 11 - 7 41 1 541 - - 21 23

crustal growth at the end of the Archean, vast areas of recycled component in felsic rocks older than 3.0 Ga cannot be continental crust could still be extracted from the mantle, with taken as a strong evidence against widespread early Archean little involvement of the preexisting continents. Transposing continents. this conclusion to the earliest stages of continental growth The Birimian differs drastically from the well-investigated suggests that the lack of Nd and Sr isotopic evidence for a 1.7-1.9 Ga crust in the North Atlantic continent: the eNd(T)

364 BOItER LeT AL.' CRUSTAL GROWTH IN WEST AFRICA

Ghallarnan MAS 157b Ghallarnan MAS 134

Gha!laman MAS 138

Ghallaman MAS 265

Ghallaman MAS 268

Kareth MAS 140

Kareth MAS 141

Kareth MAS 144

Kareth MAS 179

Kareth MAS 180

Kareth MAS 229

Archeouat MAS 76

Archeouat MAS 80

Archeouat MAS 84

Archeouat MAS 167

Archeouat MAS 168

Archeouat MAS 211

Archeouat MAS 213

Archeouat MAS 215

Ain ben Tili AL 58

AYn ben Tili IM 203

AYn ben Tili IM 237

AYn ben Tili IM 315

A'fn ben Tili IM 322

AYn ben Tili MAS 219

TABLE 5c. Concentration Data on

SiO2 A1203 Fe203* MnO MgO CaO Na20 K20 TiO2 PF Total 70.71 13.56 2.00 0.03 0.59 0.54 2.79 7.03 0.28 0.69 98.39

71.76 14.07 2.26 0.04 0.69 1.51 3.15 4.72 0.30 0.91 99.41

72.14 14.65 1.51 0.03 0.39 1.23 3.75 4.64 0.17 0.77 99.28

70.12 15.02 2.07 0.08 0.85 0.65 3.29 5.52 0.39 0.86 98.85

66.82 14.69 4.46 0.05 1.42 1.73 3.19 4.44 0.88 1.02 98.70

70.12 15.35 2.49 0.09 0.76 2.70 4.68 2.38 0.26 0.37 99.20

70.91 15.57 2.04 0.04 0.72 2.47 4.98 2.06 0.22 0.63 99.64

69.35 14.83 2.75 0.05 1.07 2.26 4.06 3.65 0.34 0.75 99.11

69.56 16.00 1.98 0.08 0.48 3.17 5.05 1.48 0.18 0.89 98.87

70.95 16.00 2.14 0.10 0.65 3.12 5.46 1.44 0.20 0.50 100.56

62.58 16.21 5.70 0.06 2.27 4.91 4.34 1.93 0.67 0.76 99.43

72.71 13.10 1.98 0.03 0.29 1.24 3.52 5.11 0.30 0.82 99.10

74.13 12.48 0.65 0.03 0.21 1.09 5.41 5.15 0.12 0.86 100.13

74.28 12.89 1.77 0.03 0.11 0.85 3.12 5.38 0.21 0.67 99.31

76.53 12.28 1.30 0.08 0.30 0.40 3.80 4.70 0.11 0.48 99.98

74.05 12.67 0.54 0.03 0.16 0.93 3.16 5.54 0.12 0.92 98.12

73.75 12.65 2.49 0.09 0.69 0.90 3.79 4.88 0.26 0.30 99.80

72.06 13.23 2.48 0.09 0.36 0.93 3.48 5.24 0.26 0.60 98.73

75.64 11.61 0.39 0.03 0.23 0.76 3.02 5.48 0.17 0.83 98.16

72.05 14.00 2.65 0.08 0.60 1.35 3.92 4.40 0.32 0.34 99.71

74.54 13.50 1.20 0.10 0.56 0.75 3.40 5.24 0.14 0.44 99.87

69.18 14.87 3.49 0.09 0.65 1.33 4.09 4.43 0.52 0.40 99.05

74.26 13.91 1.12 0.04 0.16 0.49 3.42 5.85 0.09 0.29 99.63

72.25 13.44 3.05 0.07 0.25 0.98 3.36 5.70 0.22 0.39 99.71

74.17 12.93 1.81 0.03 0.19 1.00 3.77 4.78 0.20 0.60 99.48

Major elements in percent, trace elements in parts per million measured by ICP with error of + 10 percent (K. Govindaradju, analyst). (Fe203*) = total Fe203. •' Data from M. Deschamps (personal communication, 1989), X ray fluorescence.

\o \

\

\

o ¸\

o ENd(2'1) 0 04-5 03-4 02-3 01-2 OO-1 o <0

, \ ,

\ o '6'

\

\

\ o

o

0 o¸CD c

øo C \ o

0 0 ¸ o \ 0

\ o o 0

i i

/ Si02 measured i i i i

• 60 70 80 \ 0

\ 0 0 \

,o

i o

\

i i

0 1 2 3 4 5 6 7

percent CaO

Fig. 15. K20 versus CaO plot of the Birimian granitic rocks from West Africa. Inset: regression if SiO 2 versus CaO and K20 data: SiO2(adjusted ) = 84.3% - 1.59% K20 - 3.92% CaO. SiO 2 contents (percents on dashed lines), K20 and CaO are

extremely well correlated with each other but show no correlation with ;Nd(2.1 Ga).

BOHER Er AL.: CRUSTAL GROWTtt IN WEST APRICA 365

Bh-hnian Granites, Reõuibat

Ba Nb Rb Sr Y Zr

1021 7 161 154 13 184

1087 14 132 264 14 222

715 16 253 170 11 152

352 3 555 81 13 211

575 18 314 186 28 353

370 3 64 448 5 106

794 10 132 400 17 153

627 3 29 773 3 106

668 4 34 818 5 105

1171 9 46 674 19 183

634 24 234 188 32 213

396 12 337 74 65 186

127 31 411 37 57 103

La Ce Nd Sm Eu Gd Dy Er Yb 31.87 64.26 25.35 4.5 0.69 3.12 1.99 1.1 1.14

527 13 275 109 61 212

147 16 389 36 72 240

580 24 210 210 38 174

652 11 266 152 15 123

1061 21 159 236 38 362

485 13 245 147 24 79

1022 15 243 215 92 230

299 24 309 118 44 140

histogram of Figure 17 shows that the 1.7-1.9 Ga crust is generally less radiogenic than B irimian by 1 epsilon unit, that it has a significantly larger relative dispersion, and that its crustal residence times (TDM - emplacement age) are 100 to 200 Ma longer. These two major crust formation events differed significantly, either in terms of the age of the crustal protolith [Nelson and DePaolo, 1985] or in the proportion of Archcan crust [Patchett and Bridgwater, 1984]. The 1.7-1.9 Ga crust formed close to large continental segments and sediment mixing [Patcherr and Bridgwater, 1984] or continent underplating [Etheridge et al., 1987] may account for the incorporation of Archcan material in the Proterozoic rocks. In contrast, the earlier Birimian crust apparently formed remote from ancient continents. Whether this contrast is a major feature of the thermal evolution of the Earth or whether it

results from a biased sampling of crustal formation events is unclear. Detailed investigations of 2.2 - 1.7 Ga old terranes in other localities (Brasil, Congo Craton, India, Australia) may shed further light on this intriguing issue.

As most of the Birimian crust has a juvenile character, the rate of Birimian crustal growth can be estimated with some confidence. The actual outcrops of preserved Birimian formations cover some 0.9 x 10 6 km 2 in Africa and probably 0.3 x 106 km 2 in the Guyana shield [Caen-Vachette, 1988]. Their probable extension under the Taoudeni Basin, which is fringed by Birimian rocks in most directions, and the Voltaian Basin, brings the total figure to-3 x 106 km2. Assuming that the crust was -30 km thick and that crustal growth took place in 120 Ma, i.e. uniformly for Dabakalian and Birimian events, suggests a crustal growth rate of 0.75 km3/a. If most (-90%)

crustal growth is concentrated over the B irimian episod sensu stricto, crustal growth took place at a rate of 1.6 km3/a. These figures do not represent extremely fast growth rates since, neglecting any recycling, the total continental mass (-0.8 x 1010 km 3 using the estimates of Turcotte and Schubert [1982]) would grow from the mantle in 4 Ga at a mean rate of 2 km3/yr. In fact, the higher limit of the Birimian crustal growth is only 60 percent higher than the modem growth rate estimated by Reymer and Schubert [1984] to be ~1.0 km 3. Therefore the mean crustal growth rate inferred for Birimian crust formation is slower than the mean crustal growth rate over the Earth history. Since creation of juvenile crust is likely to take place during orogenic episodes, one would expect the Birimian world-wide crustal growth rates to exceed largely this mean crustal growth rate. Therefore a significant fraction of the Birimian crust may be (1) still to be identified in poorly known areas, (2)recycled into the mantle, (3) incorporated into younger crustal segments.

The same argument extends to the Archcan crustal segments: assuming a crustal growth rate identical to the mean value over the Earth history, 14 x 108 km 3 of continental crust would be produced from 3.5 to 2.7 Ga, i.e., about 20 percent of the present-day crustal volume. Inspection of the geological map of any continent leaves no doubt that this proportion is a gross overestimate of the Archcan realm. Emphasizing higher- than-average crustal growth rates in the Archcan would further strengthen the point that most of the ancient crust has been recycled into the mantle or is now part of much younger crustal segments. Drawing conclusions about the evolution of the continental crust from the restricted surface of Early Archcan


• • +_1 . Continental Crust [."?.?.it... Oceanic Crust [/i• :!:i:!:!:i:i:!:it Lithospheric Mantle ..... ----•--::•• Asthenospheric Mantle

++++++++,• 4- 4- 4- 4- 4- 4- 4- 4- ../.:../......-...•...../...../...../............/...../............-..:.9...9...9......•.•.•. ++++++ ++•'}.: . _, 4- 4- 4- 4- 4- 4- 4- 4- :_-..'.'_-.:•-.':/-;:_-./,• "• ;.. 4-4-4-4-4-4-4-4-4-

4-4-4-4-4-4-4-4- 4-4-4-4-4-4-4-4-4-• 4-4-4-4-4-4-4-4-

•,_._•jj_ Pillowed Tholeiites Volcaniclastic Sediments

• Calc-alkaline Magmas '•oø_o_øo• Late Detrital Formations

x ß '.. '.. '.. '.-. ".: ".-. ".: ".-. "'.-. ".-. ".-. ".: ".-. ".-. ".; .'.• +,,+++• ...... •-•%-x. '• :,.2..:: ;•.....'.•.; ;...;;}.. :..• .... :..., .... :...., .... :.-...... •.... •.;. ,.... •:.... ,._ ... •...... ::.... •..... •'•:•- 2/• ============================================================================================================================

::;:.... .• .• • .' % ::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: ;..;::;::::.. ß • % ,,, :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::

.... ----- ......... __-:-:e--•:• :- •-:-_--: ..... ------=--=--==•--_-:--_-_--_--_--_--_--_---_ =-.-..............,..,.,.•.• ,.•., ,.•, ,.e.... ........ •.- ...... :- - -_-_-_- ........... --

Fig. 16. A model for the growth of Birimian crust. The first stage is characterized by plume activity and abundant pillowed basalts with geochemical characters close to those of modem oceanic plateaus. Next stage is characterized by calc-alkaline magmatism related to intraoceanic subduction beneath the plateaus. Last stages correspond to the collision of the plateaus with Archean continental nuclei [Feybesse et al., 1989].

outcrops, in particular about whether an older than 3.9 Ga crust was present, rests entirely on the rare leftovers spared by the whims of otherwise mighty processes of erosion and subduction.

CONCLUSIONS

New age dating suggests that the Birimian terranes of West Africa formed during two distinct events and that the entire process lasted a relatively short period of time. The early highly metamorphosed Dabakalian formations probably formed between 2.19 and 2.14 Ga ago and were metamorphosed at the end of this time interval. The most precise zircon U-Pb and Sm-Nd data from literature [Feybesse et al., 1989; Abouchami et al., 1990; Li•geois et al., 1991] date Birimian terranes sensu stricto at 2.12 - 2.07 Ga, which suggests that the major evolution of the Birimian crust was completed in less than 50 Ma.

Nd isotopic evidence indicates that B irimian granitoids were not derived from Archeart crust: œNd(2.1 Ga) values are positive and similar to these of Birimian basalts, and crustal residence

ages are less than 200 Ma. U-Pb ages for detrital zircons from clastic sediments support their derivation from pene- contemporaneous protoliths. In agreement with the suggestion of Abouchami et al. [1990], the present data argue for the Birimian crust being created around 2.1 Ga in an environment remote from Archean crust influence. Taking the Birimian formations from the Guyana shield into account, a minimum crustal growth rate at 2.1 Ga is about 1.6 km3/yr, which is 60 percent higher than the modern growth rate of about 1.0 km3/yr estimated by Reymer and Schubert [1984].

Birimian crustal growth processes at 2.1 Ga resemble the processes that produced Archean granite-greenstone terranes but differ from mid-Proterozoic crust formation events which generally involve more crustal recycling. A comparison of the Birimian crustal growth rate with the crustal growth rate averaged over the Earth history implies that a large part of the B irimian crust has been recycled into the mantle or incorporated into younger orogenic segments. This apparent deficit in the crustal budget is even more dramatic for the Archean crust.

BOILER ET AL.: CRUSTAL GROW]'H IN WEST AFRICA 367

North Atlantic Craton (1.7- 1.9 Ga)

œ.• (m)

ß ß

Bill

r'l

ll

11 Ill

• ommm [] Dmmm o_Dmmmm •mmmmm

•mmmmm •mmmmm •mmmmmmmm •mmmmmmmm

•mmmmmmm

TDM -Templ

-15 -10 -5 0 5 10 I • i i i I i m i i I , i , i I i i i m m .... i ,

mmmmm mmmmm mmmmm

mmmm mmmm

mmm

mm

,me mm me mm ,mm

West African Craton g'--

(2.1- 2.2 ea) m

ß Magmatic Rocks

[] Sedimentary Rocks

1

i

ii II mmm mmm• mmic_

m mmmm mmmmmon mmmmmm mmmmmm• mmmmmmD mmmmmmD mmmmmmD

,m,m,m, mm, m, ,mq,, o 5o0 lOOO I,,,, I , , , , I mmmmmmm mmm mmmm mmmm m mmmm m

mmm mm mmm mmm mmm mmm mmm mmm mmm mmm mmm mmm mm m• mmm mmm mmm mmm

r m m m m m m m m m m m m m m m

m

Fig. 17. Comparison between Nd isotopic data from the 1.7 - to 1.9- Ga-old rocks from the North Ariantic craton and 2.1-Ga-old Birimian rocks from West Africa (felsic rocks only). All TDM calculated from the same depleted mantle model by Ben Othman et al. [1984]. Solid squares: felsic magmatic rocks; open squares: sediments. The 1.7 - to 1.9-Ga crust is less radiogenic than Birimian by 1 epsilon unit with a significantly larger relative dispersion. Its crustal residence times (TDM - emplacement age) are 100 to 200 Ma longer.

Acknowledgments. We are grateful to J.P. Mil6si and G. Rocci for the strong initial impetus given to the Birimian project. We would like to thank all the colleagues who contributed samples for this study: M. Caen-Vachette, for several Birimian localities, G. Bronner, G. Rocci and M. Deschamps for Mauritania, A. Dommanget, G. Alric, R. Fabre, J. L. Feybesse, H. Maluski, S. Tour6 for Ivory Coast, M. Rossi, and S. Zonou for Burkina Faso, P..Barbey and A. Pouclet for Niger. Institut National des Sciences de l'Univers grants 88-3835 and 89-3827 and Bureau de Recherches G6ologiques et MiniSres grant 50-8015 have made that work possible. Field work was made possible in Senegal and Mali by A. Dommanget, H. Lapierre, J.P. Milesi, in Ivory Coast by S. E. Monsieur le Ministre de la D6fense, S. E. Monsieur le Ministre des Mines, A. Diabi, S. Tour6, R. Fabre, J. L. Feybesse, P. Morel, in Guinea by R. Capdevila and the UNESCO staff. Help in the laboratory by Danielle Dautel and Zaza Leroy and maintenance of the mass spectrometer by J. C. Demange were highly appreciated. Discussions at different stages of this work with C. Alibert, G. Alric, P. Barbey, J. P. Bassot, J. M. Bertrand, R. Capdevila, M. Deschamps, A. Dia, A. Dommanget, E. Deloule, R. Fabre, J. L. Feybesse, V. Johan, H. Lapierre, P. Ledru, J. Leterfier, D. Lowe, J.P. Mi16si, A. Pouclet, G. Rocci, M. Rossi, J. M. Stussi, M. Vidal, S. Zonou, are gratefully acknowledged. Reviews by Lou Derry, Steve Galer, Jon Patchett and Jan Veizer helped improve the manuscript. REE analyses could be obtained thanks to the skills of K. Govindaradju and his group. C. R. P. G. Contribution 0885.

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W. Abouchami, and M. Boher, Centre de Recherches Prtrographiques et G6ochimiques, Universit6 de Nancy I, BP 20, 54501 Vandoeuvre Cedex, France.

F. Albar•de, and A. Michard, Centre de Recherches Prtrographiques et G6ochimiques, Ecole Nationale Suprrieure de G6ologie, 54501 Vandoeuvre Cedex, France.

N. T. Arndt, Max Planck Insitut far Chemie, Saarstrasse 23, Postfach 3060, D-6500 Mainz, Germany.

(Received January 28, 1991; revised June 3, 1991;

accepted June 17, 1991.)

Crustal growth in West Africa at 2.1 Ga

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