Journal ofthe Geological Society, London, Vol. 153, 1996, pp. 5G7-510, 3 figs. Printed in Northern Ireland

3.5 Ga old terranes in the West African Craton, Mauritania

A L A I N P O T R E L ' , J E A N J . P E U C A T ' , C . MARK FANNING' , BERNARD

A U V R A Y 2 , J E A N P . B U R G ' & C H R I S T I A N E C A R U B A 4

'Ge'osciences Rennes (UPR CNRS 4661), 35042 Rennes Cedex, France

'Research School of Earth Sciences, Australian National University, Canberra ACT0200, Australia

.'Geologisches Institut, ETH Zentrurn Sonneggstrasse, 5 CH-8092 Zurich, Switzerland U R A 1279 Universitt de Nice-Sophia Antipolis,

Pare Valrose, 06108 Nice Cedex, France

Evidence for early Archaean relicts in the northern part of the Wer it African Craton are provided from ion-microprobe (SHRIMP) U-Pb zircon and Sm-Nd whole-rock ages in the West Reguibat Rise of Mauritania. The U-Pb ages range between 3515 f 15 Ma and 3422 f 10 Ma for concordant zircons from an orthogneiss which has also been affected by a 2.7Ga old metamorphic event. Nd model ages calculated for adjacent metasediments range between 3.9 and 3.5 Ga, suggesting that these supracrustal rocks may be derived from the erosion of a similar early Archaean crust. Zircon ages and Nd depleted-mantle model ages provide the first evidence for an early Archaean crustal growth, at least 3.5-3.6 Ga old, in the West African Craton.

Keywords: West Africa, lower Archaean. zircon. U-Pb, Sm-Nd.

Early Archaean terranes (>3.3 Ga) are well known in the southern part of Africa. Various formations in the Kaapvaal craton, the Limpopo belt and the Zimbabwe craton have been dated between 3.3 and 3.6 Ga, (e.g. Hamilton et al. 1979: Jahn et al. 1982: Kroner & Compston 1988: Armstrong et al. 1990: see also references in Kroner & Tegtmeyer 1994).

In contrast, only a few and poorly constrained geochronological studies indicate early Archaean ages in northern and western Africa. The oldest zircon ages known are close to 3.2-3.3 Ga as in the Man and Tuareg shields (Fig. 1) since the Nd model ages are as high as 3.3-3.5 Ca. (Kouamelan et al. 1995: Peucat er al . 1996).These results strongly suggest the occurrence of early Archaean rocks in northern and western Africa, but no samples have been dated conclusively at more than c. 3.1-3.2 Ca. In order to constrain the occurrence of such ancient relicts in the West African Craton, we performed a geochronological study of the Amsaga area (Reguibat Rise, Fig. l ) , using both the SHRIMP ion-microprobe and the Sm-Nd isotopic techniques.

Geological setting. The Amsaga terrane (Fig. 2) is bounded to the south by the Pan-African Mauritanides (600 Ma) and

overlain to the east by the undeformed Proterozoic sedimentary Atar succession of Proterozoic age. The Amsaga terrane itself is comprised of three major lithological units: (1) charnockitic gneisses (qtz + kf + p1 + opx + biot f cpx f amph): (2) paragneisses: leptynites and metapelites (qtz + kf + pl + cord + grt + sill f biot f graph); (3) migmatitic orthogneisses of trondjhemite and granodior- ite composition (qtz + kf + pl + biot f opx f cpx) infolded with volcaniclastic belts (p1 + qtz + amph f opx f cpx f biot).

All these formations have been affected by a granulite-facies metamorphism. Phase relationships in the paragneisses and thermo-barometric calculations indicate a clockwise P-T- t path with peak conditions near 800 f SO "C and 5 f 1 kbar for the high-grade metamorphism. Zircon dates of 3.0Ga are interpreted as the age of the magmatic protoliths of the charnockites (Potrel 1994).

Two late-tectonic but post-granulite facies intrusions (the Touijenjert granite and the Iguilid gabbro, Fig. 2) cross-cut the granulitic gneisses and the migmatitic protoliths, corresponding to the youngest magmatic events recorded in the area. Zircon dating of the Touijenjert granite and the Sm-Nd mineral isochron of the Iguilid gabbro yield ages close to 2.7 Ga, (Auvray et al. 1992; Potrel 1994). This age probably marks the end of the high-grade metamorphic event (Potrel 1994).

Analytical procedures and samples. The SHRIMP analytical procedure used here is similar to that described by Compston ef al. (1992). 29 spots were analysed on 19 individual zircons (100-250 Fm in size) from a granitic orthogneiss sample AG41 (Fig. 2). Two populations of zircon are distinguished: (1) transparent elongate euhedral grains with optical zoning, and (2) round to ovoid euhedral to subhedral grains, a few showing optical zoning. None of the zircons show evident inherited cores, complex internal structures or overgrowths.

Sm-Nd analyses were performed on the same orthogneiss (AG41) and on four paragneisses. Three of the paragneisses (AG173. AG89, AG82) were collected in the northern part of the area, the fourth sample (AG190) in the eastern part, within the volcaniclastic belt (Fig. 2). Sm contents were determined by isotope dilution. using a Cameca TSN 206 mass-spectrometer. Nd contents and isotopic compositions were measured using a Finnigan MAT 262 mass-spectrometer. Isotopic ratios were normalized to '4hNd/'J4Nd = 0.7219. The reported values (Table 2) have been adjusted to La Jolla Nd standard = 0.51 1860. During the period of data acquisition. 13 replicates of the same standard gave 0.51 1822 f 0.000021 ( 2 ~ ) . In-run precision of measurements is given at the 95% confidence level. Errors are 0.5% for l " S ~ n / ' ~ ~ N d ratios. Isotopic Nd ratios are given with 2 m uncertainties. &Nd values are calculated using a '"Sm/'"Nd ratio of 0.1967 and a '43Nd/'44Nd ratio of 0.512638 for the present-day chondritic uniform reservoir (CHUR). Model ages (TDM) are calculated with respect to a depleted mantle with a eNd(0) value of +10, isolated from the CHUR since 4.55 Ga and following a linear evolution.

Results and discussion. Orthogneiss AG41. The SHRIMP U-Pb zircon analyses (Table 1 ) for orthogneiss AG41 yield a scatter in 2'1'Pb/Z')hPb ratios greater than analytical uncertainty. The majority of the spot analyses are concordant within la uncertainty and the corresponding Z'17Pb/2'KPb ages range from 3515 f 15 ( l a ) Ma to 3422 * l0 (1q) Ma (Fig. 3, note analyses 4.1 and 12.2 are significantly discordant and not shown). The discordant analysis, (grain

507

508 A. POTREL E T A L .

Fig. 1. Geological sketch map of the West African Craton. Tuareg and Nigerian shields and location of the Amsaga in the southwestern part of the Reguibat Rise (modified from Haddoum et al. 1994).

4.1) has a 2'"Pb/2'"Pb age (3446*21 Ma, Table 1) in the same range as the concordant analyses and its discordance is interpreted as resulting from recent loss of radiogenic Pb. The other significantly discordant analysis, (grain 12.2) has a younger 2''7Pb/2(K'Pb age (3319 f 9 Ma, Table 1) and appears to have suffered no zero-age radiogenic Pb loss.

Despite the presence of two morphologically distinct zircon populations, the results are relatively homogeneous (Table 1). The two populations have the same order of magnitude in U and Th contents and similar U/Th ratios of about 0.5. The 2"7Pb/2'"Pb ages overlap and each has an oldest age of c. 3500 Ma (grain spots 3.1 and 12.1, Fig. 3). The Nd depleted mantle age of this orthogneiss (3.6Ga, Table 2) is in agreement with the range of 207Pb/2'"Pb ages recorded on zircons.

The scattered 2"7Pb/2'"Pb ages do not permit a precise estimate of the emplacement age for the magmatic protolith of the orthogneiss AG41, even if most of the 207Pb/206Pb ages are close to 3460 * 10 Ma.

The zircons may consist of a heterogeneous population of both older inherited crystals and younger zircons crytallized from the protolith itself. This hypothesis accounts for the two zircon populations but not the absence of inherited cores, the chemical homogeneity or the lack of systematic age difference between the two populations. It is possible that the protolith has an age of the order of 3.50Ga, with a second generation of zircon at around 3.46Ga.

Furthermore, the gneiss may have been affected by a thermal event causing early radiogenic lead loss. A second analysis was carried out on grains 3,4, 6,7 and 13, as well as three analyses of grains 12 and 17 (Table l ) , but the narrow

Fig. 2. Sketch map of the Amsaga area (modified from Badre 1967) and sample locations (orthogneiss AG41 in bold).

r 21'1W

20'70'

20'50

20'10'

age difference is not significant and does not confirm or rule out this hypothesis. However, if such an event occurred, it must have happened early after the emplacement of the protolith as suggested by the concordance of the analyses. Whatever the cause of the scatter, the emplacement of the magma occurred between c . 3515 and 3422 Ma.

The paragneisses. The Sm-Nd results of the paragneisses from the northern zone (samples AG82, AG89, AG173) are reasonably homogeneous, with low '47Sm/'"Nd ratios (0.0742-0.0949) and TDM ages between 3.6 and 3.5Ga (Table 2). However sample AG190 from the eastern zone (Fig. 2) shows a higher ratio (0.1550) and an older TDM age of 3.9 Ga.

Taking the uncertainties into account, the model ages of the metasediments from the northern part of the area are similar to that of the orthogneiss AG41. Hence, we conclude that these sediments could have been derived from the erosion of such a basement. Furthermore, numerous studies (e.g. Allkgre & Rousseau 1984; Miller & O'Nions 1984: Goldstein et al. 1985; Dia et al. 1990) have demonstrated that sediment Nd model ages in the Archaean are typically close to their stratigraphic ages. This is particularly applicable to the three samples from the northern part of the Amsaga area, since they have unusually low '47Sm/'"Nd ratios, (Table 2). In contrast, the Nd depleted mantle model age of sample AG190 is less constrained due to its higher '47Sm/'"Nd ratio. Such a ratio could be the result of a two-stage evolution, but it is not known whether it reflects a perturbation of the Sm/Nd system during metamorphism or the mixing of felsic and basic materials. Alternatively it

3 . 5 G a T E R R A N E S I N M A U R I T A N I A 509

Table 1. Ub-Pb resulls for zircons ion probe from sample AF41

1.1 ov 1.2 ov 2.1 ov 3.1 ov 3.2 ov 4.1 ov 4.2 ov 5.1 ov 6.1 ov 6.2 ov 7.1 el 7.2 el 8.1 ov 9.1 el

10.1 ov 11.1 ov 12.1 el 12.2 el 12.3 el 13.1 ov 13.2 ov 14.1 ov 15.1 ov 16.1 ov 17.1 ov 17.2 ov 17.3 ov 18.1 ov 19.1 ov

150 186 149 180 142 579 269 301 164 1 54 218 187 280 220 240 218 352 347 342 200 272 230 113 137 213 177 174 234 205

63 80 63 86 60

279 136 1 54 80 74

104 90

128 103 97 96

149 93

190 107 126 116 88 98

107 92 87

124 109

0.42 0.43 0.43 0.48 0.42 0.48 0.51 0.5 1 0.49 0.48 0.48 0.48 0.46 0.47 0.40 0.44 0.42 0.27 0.56 0.53 0.46 0.51 0.78 0.72 0.50 0.52 0.50 0.53 0.53

130 162 130 161 120 300 235 268 143 129 191 163 234 192 207 189 30 1 148 281 173 224 193 106 125 189 158 1.50 206 188

0.000083 0.000024 0.0000 1 7 0.000063 0.000249 0.000569 0.000383 0.000037

bd 0.000654 0.00001 1 0.000289 0.000009 0.000013 0.000007 0.000016 0.000005 0.000813 0.000639 0.000028 0.000227 0.000001 0.000194 0.000338 0.000174 0.000105 0.000070 0.000270 0.000278

0.10 0.03 0.02 0.07 0.30 0.67 0.45 0.04

<0.01 0.77 0.01 0.34 0.01 0.02 0.01 0.02 0.01 0.96 0.76 0.03 0.27 0.00 0.23 0.40 0.21 0.12 0.08 0.32 0.33

0.7160 f 0.016 0.7147 f 0.017 0.7188 f 0.014 0.7237 f 0.015 0.6987 f 0.017 0.41 18 f 0.029 0.7053 f 0.016 0.7219 f 0.015 0.7079 f 0.014 0.6907 f 0.018 0.7181 f 0.014 0.7193 f 0.019 0,6840 f 0.017 0.7113 f 0.014 0.7130 f 0.015 0.7093 f 0.015 0.6979 f 0.014 0.3510 f 0.008 0.6550 f 0.017 0.6937 f 0.014 0.6566 f 0.018 0.6788 f 0.014 0.7296 f 0.020 0.7202 f 0.020 0.7216 f 0.019 0.7205 f 0.018 0.7047 f 0.016 0.7173 f 0.020 0.7461 f 0.020

29.29 f 0.75 29.57 f 0.73 29.52 f 0.70 30.82 f 0.72 28.72 f 0.76 16.77 f 1.22 29.07 f 0.69 29.43 f 0.63 29.18 f 0.66 27.88 f 0.77 28.80 f 0.63 29.1 1 f 0.82 28.20 f 0.83 29.23 f 0.70 29.22 f 0.66 29.49 f 0.66 29.45 f 0.71 13.17 f 0.31 26.84 f 0.74 29.02 f 0.67 27.02 f 0.79 28.37 f 0.61 29.71 f 0.90 29.35 f 0.87 29.90 f 0.84 29.66 f 0.77 28.50 f 0.70 29.30 f 0.85 30.39 f 0.85

0.2967 f 0.003 0.3001 f 0.001 0.2978 f 0.003 0.3089 f 0.003 0.2981 f 0.002 0.2953 f 0.004 0.2989 f 0.001 0.2957 f 0.001 0.2990 + 0.002 0.2928 f 0.002 0.2909 f 0.002 0.2936 f 0.002 0.2991 f 0.004 0.2980 f 0.003 0.2972 * 0.002 0.3016 f 0.001 0.3061 f 0.003 0.2722 f 0.002 0.2972 f 0.002 0.3034 f 0.002 0.2985 f 0.002 0.3032 f 0.001 0.2953 f 0.003 0.2956 f 0.002 0.3005 f 0.002 0.2986 f 0.002 0.2933 f 0.001 0.2963 f 0.002 0.2954 f 0.001

3453 f 16 3471 f 7 3459 f 16 3515 f 15 3461 f 12 3446 f 21 3465 f 6 3448 f 7 3465 f 12 3432 f 11 3422 f 10 3437 f 9 3465 f 21 3460 f 16 3456 f 11 3478 f 8 3501 f 16 3319 f 9 3456 f 8 3487 f 12 3462 f 12 3486 * 7 3446 f 16 3447 f 11 3473 f 8 3463 f 8 3435 f 8 3451 f 10 3446 f 7

101 100 101 100 99 65 99

1 02 100 99

102 102 97

100 100 99 98 58 94 97 94 96

103 101 101 101 100 101 104

bd, no 2"4Ph detected. Pb* radiogenic lead. % Conc: concordance of U/Pb ratios. %f2"'Ph: percent of total 2'HPb that is nonradiogenic, based on measured zo4Ph. ov. euhedral to subhedral ovoid zircons: el, euhedral elongate zircons. Uncertainies are given at one sigma level.

0 8 0

0 75

0 70

0 65

1; AMSAGA

Orthogneiss AG 41

34222 10 Ma (la) A

v 3501 f 16 Ma (la)

Fig. 3. Enlarged concordia plot of SHRIMP U-Pb zircon analyses for sample AG41. Analyses plotted as Icer ror boxes. Shaded concordant analyses indicate the range in 2r)7Pb/2'KPh ages recorded.

could also represent the contribution of an older component.

Conclusions. We present evidence for the existence of continental crustal relicts in the West African Craton from at least c. 3.5 Ga. This early Archaean crust was probably affected by a thermal event prior to 3.4 Ga, as shown by the scattering of the concordant SRHIMP U-Pb data. This early Archaean nucleus is bounded by 3.0 Ga old gneisses. Both relics and gneisses were metamorphosed at c. 2.7 Ga (Auvray et al. 1992; Potrel 1994). The 3.5 Ga zircon ages and the Nd depleted mantle model ages represent the first well documented evidence for an early Archaean crustal accretion event in the West African Craton. The present results, suggest that work should be carried out to locate further occurences of early Archaean crust in West Africa

Field expenses were supported by French-Mauritanian cooperation grant (IRIM). We would like thank the two reviewers, B.M. Jahn, E. Hallot, M. Stasiuk and A. Dia for constructive criticism of the manuscript. An early draft of this manuscript was edited and corrected for English style by M.S.N. Carpenter (Professional

510 A . P O T R E L E T A L .

Table 2. Sm-Nd isotopic data for Amsaga whole-rock samples

Nature Sample no. Sm Nd 147Sm/144Nd 14’Nd/144Nd Error e TDM

(ppm) (PPm) xlO-h T=OMa Ma

Orthogneiss AG 41 10.13 62.14 0.099 0.510376 6 -44.2 3638 Paragneiss AG 82 2.03 13.06 0.094 0.510394 5 -43.8 3481 Paragneiss AG 89 3.99 25.42 0.095 0.5 10290 9 -45.8 3631 Paragneiss AG 173 3.86 31.43 0.074 0.509864 4 -54.2 3560 Paragneiss AG 190 4.34 17.15 0.153 0.511592 5 -20.4 3875 Paragneiss AG 190 4.53 17.86 0.153 0.51 1595 4 -20.4 3880

Scientific Translator). Correspondence to J.J. Peucat (e-mail: peucat@univ-rennes1.fr).

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Received S January 1996; revised typescript accepted 13 February 1996. Scientific editing by Cordon Taylor.