U–Pb Geochronology of Paleoproterozoic Rocks in the Southern Part of the Kedougou-Kéniéba Inlier, Senegal, West Africa: Evidence for Diachronous Accretionary Development of the Eburnean Province

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U
Pb Geochronology of Paleoproterozoic Rocks in the Southern Part of the Kedougou-Ke´nie´ba Inlier, Senegal, West Africa: Evidence for Diachronous Accretionary Development of the Eburnean Province /

W. Hirdes a,*, D.W. Davis b a b

Bundesanstalt fu¨r Geowissenschaften und Rohstoffe (BGR), Stilleweg 2, D30655 Hannover, Germany Department of Geology, Royal Ontario Museum, 100 Queen’s Park, Toronto, Ont. M5S 2C6, Canada Received 17 April 2001; accepted 22 May 2002

Abstract The Kedougou-Ke´nie´ba Inlier (KKI) in eastern Senegal is the westernmost exposed part of the Paleoproterozoic (Birimian/Eburnean) Baoule´-Mossi domain on the West African craton. Regional mapping indicates that the southern, relatively well preserved part of the KKI contains two northeast trending volcanic belts, the Mako and Fale´me´ belts with associated Na-rich granitoid stocks. These belts are separated by the Diale-Dalema metasedimentary basin, containing the extensive syn- to late-kinematic K-rich Saraya batholith and other basin-type plutons. A similar pattern of northeast trending volcanic belts and sedimentary basins is also seen in Ghana and eastern Coˆte d’Ivoire, i.e. in the eastern part of the Baoule´-Mossi domain, as well as in central Coˆte d’Ivoire where, however, primary lithological relationships seem to be more obliterated by later tectono-thermal overprint. U
/Pb ages were determined on single zircon and monazite from rocks in the KKI to investigate its relationship with the rest of the Baoule´-Mossi domain. A rhyolite from the Fale´me´ belt gave a zircon age of 20999/4 Ma. Granitoid rocks from the Fale´me´ (2) and Mako (1) volcanic belts possess zircon ages of 20829/1, 20809/1 and 20769/3 Ma, respectively. Reversely discordant monazites from a muscovite-bearing phase of the Saraya batholith in the Diale-Dalema basin yielded an interpreted igneous age of 20799/2 Ma and a metamorphic age of 20649/4 Ma. Detrital zircons from a quartz wacke in the Diale-Dalema basin all gave the same age within error, defining an average age of 21659/1 Ma for the source. Inherited zircon, 21559/34 Ma in age, was seen in the felsic flow from the Fale´me´ belt while the Mako belt granitoid rock contains inheritance at least as old as 21229/5 Ma. These data indicate that volcanic belts in the KKI are substantially younger than the 2150
/2190 Ma Birimian volcanic and associated plutonic rocks of the eastern Baoule´-Mossi domain, despite their geological similarities. Syn- to late-kinematic basin-type plutons in the KKI are also younger than lithologically similar 2090
/2115 Ma old plutons in Ghana and eastern Coˆte d’Ivoire (eastern part of the Baoule´-Mossi domain). These relationships are consistent with previous suggestions that Paleoproterozoic rocks of the Baoule´-Mossi domain may be divided into an older eastern subprovince and an approximately 50
/100 Ma younger western subprovince. They also show that

* Corresponding author. Tel.: /49-511-643-2326; fax: /49-511-643-3689 E-mail addresses: [email protected], [email protected] (W. Hirdes). 0301-9268/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 1 - 9 2 6 8 ( 0 2 ) 0 0 0 8 0 - 3

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supracrustal and intrusive Paleoproterozoic rocks in both subprovinces as well as over the entire domain developed diachronously from east to west. The data support an accretionary tectonic model for Paleoproterozoic growth of the West African craton. # 2002 Elsevier Science B.V. All rights reserved. Keywords: U
/Pb geochronology; Paleoproterozoic; Volcanic belt; Birimian-Eburnean province; Diachroneity; Senegal

1. Introduction Paleoproterozoic supracrustal and intrusive rocks are widespread on the southern part of the West African craton. They make up the Baoule´Mossi domain, which is in fault contact with an Archean domain to the west. The formation of these Paleoproterozoic supracrustal rocks and associated synvolcanic and syn- to late-kinematic intrusives marks a major juvenile crust-forming event that is loosely referred to as the ‘Eburnean orogeny’. Geochronological studies to date indicate that these rocks formed over a maximum time interval of 2.27
/2.05 Ga (Abouchami et al., 1990; Ama-Salah et al., 1996; Boher et al., 1992; Bossie`re et al., 1996; Davis et al., 1994; Davis, reported in Loh and Hirdes, 1999; Davis, reported in Lu¨dtke et al., 1999; Doumbia et al., 1998; Hirdes and Davis, 1998; Hirdes et al., 1992, 1996; Kouamelan, 1996; Kouamelan et al., 1997; Lie´gois et al., 1991; Taylor et al., 1988, 1992). The Paleoproterozoic supracrustal rocks in West Africa occur generally as volcanic belts with predominantly tholeiitic lavas, and intervening sedimentary basins with felsic volcaniclastics, wackes, argillites and chemical sediments. Until recently these rocks have been grouped together as the Birimian Supergroup (excluding the volumetrically little important conglomerates and sandstones of the so-called Tarkwaian Group which represent erosion products of the Birimian Supergroup). However, Hirdes et al. (1996) suggested that the Birimian/Eburnean province can be divided into an eastern and a western subprovince possibly separated by a regional lineament known as the Ouango-Fetini shear in Coˆte d’Ivoire (Fig. 1): . The eastern subprovince which covers Ghana (where the original type locality for the Birimian is located), eastern Coˆte d’Ivoire and the

eastern part of Burkina Faso typically displays approximately 2150
/2190 Ma belt volcanism and coeval belt plutonism. Evidence for younger volcanism is so far absent. . The western subprovince (central and western Coˆte d’Ivoire, western Burkina Faso, Guinea, Mali) is characterized by younger, approximately 2105 Ma old volcanic belts and coeval belt plutons. Rocks of the 2150
/2190 Ma time span are at least locally present as various gneisses, and possibly form a substratum for the younger volcanic rocks. Hirdes et al. (1996) suggested that the term ‘Bandamian’ instead of ‘Birimian’ be used for the 2105 Ma old supracrustal rock suite of the western subprovince. The term ‘Eburnean’ would then continue to refer to the major tectono-thermal event affecting both 2150
/2190 Ma old Birimian rocks and the 2105 Ma old Bandamian rocks at approximately 2090
/2100 Ma. The structural style and composition of Paleoproterozic terrains in West Africa greatly resemble Archean greenstone terrains. As with the Archean, the tectonic processes that formed these rocks have been a subject of debate. Broadly, two schools of thought have emerged. One sees the Eburnean as an accretionary orogeny, the result of collisions of island arcs and oceanic plateaux with an Archean craton. This is suggested to have been part of a much larger accretionary event that involved fragments of Precambrian crust now preserved over large parts of Africa and South America (Abouchami et al., 1990; Boher et al., 1992; Davis et al., 1994; Feybesse and Mile´si, 1994; Hirdes et al., 1996; Ledru et al., 1994). The other school emphasizes the control of transcurrent tectonics and the lack of clear evidence for collisional processes. This view suggests a largely autochthonous evolution either in an oceanic domain that differentiated to produce continental crust or in

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Fig. 1. Subdivision of the Paleoproterozoic Baoule´-Mossi domain/Birmian-Eburnean province into an (older) eastern subprovince and a (younger) western subprovince divided by the Ouango Fitini shear zone (after Hirdes et al., 1996), position of the KKI and study area.

association with Archean basement (Bassot, 1987; Doumbia et al., 1998; Kouamelan, 1996; Pouclet et al., 1996; Vidal et al., 1996). Variations on these models admit the presence of subduction processes but argue for small scale extensional and accretionary processes rather than large-scale collision (Ama-Salah, 1996; Vidal and Alric, 1994). An important constraint on models would be to establish whether there is a diachroneity in geological development across the craton. This would be predicted by an accretionary model. Such

diachroneity has been argued by Hirdes et al. (1996) and by Feybesse and Mile´si (1994). The objective of this paper is to document regional geological relationships and U-Pb ages from rocks in the southern part of the Kedougou-Ke´nie´ba Inlier (KKI), Senegal, which represents the westernmost part of Paleoproterozoic terrane in West Africa. These data will be used to test the suggested subprovince division as well as the hypothesis from previous field work and geochronology (Davis et al., 1994; Hirdes et al., 1996;

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Hirdes and Nunoo, 1994) that the region was affected by southeast to northwest directed Eburnean crust formation events.

2. Geology of the southern part of the KedougouKe´nie´ba Inlier (KKI) The southwestern half of the KKI is situated in Senegal whereas its northeastern part is in Mali (Fig. 2). The KKI is bounded on its western side by the Bassarides, a mobile belt thought to be Panafrican in age, and on all other sides by flatlying Neoproterozoic sediments. Previous studies on the geology of the KKI (e.g. Debat et al., 1984; Bassot, 1987; Ngom, 1989; Pons et al., 1992; Dia, 1993a,b; Dia et al., 1993, 1997) have distinguished a volcanic Mako Series, a volcano-sedimentary, pelitic and siliciclastic dominated Diale Series, and a volcano-sedimentary Dalema Series which is said to have a relatively high proportion of carbonate sediments. No consensus as to the stratigraphic and/or paleogeographic relationship between these series exists. For the Mako Series an island-arc origin was assumed by Dia et al. (1997). The clastic rocks of the Diale-Dalema Series were interpreted by Ledru et al. (1991) to have been deposited in an intra-cratonic basin. The latter authors claimed that the sediments of the Diale-Dalema Series underlie and are older than the Mako Series maintaining that the former have been overprinted by two tectonic phases and the latter only by one phase, in accordance with their conceptions for the Birimian elsewhere on the craton (B1-Lower Birimian, B2-Upper Birimian, see also Mile´si et al., 1989). However, this could not be confirmed by our work. Up to present, intrusive rocks of the KKI have been divided into K-dominated, extensive ‘syntectonic’ basin-type plutons as well as small, Nadominated, unfoliated belt-type intrusions, said to be ‘posttectonic (e.g. 1:500 000 Geological Map of Senegal; BRGM, 1962). Elsewhere in the Baoule´Mossi domain, however, in particular in Ghana and eastern Coˆte d’Ivoire, ample evidence has been given that such belt-type intrusions are synvolcanic and coeval with belt volcanism, and that they

always predate basin-type plutonism, in several cases by some tens of millions of years (Hirdes et al., 1992, 1996; Lu¨dtke et al., 1998, 1999). The authors carried out field work and sampling along traverses through the southern portion of the KKI within the scope of a research project on base metal mineralization in the West African craton. The southern portion is tectonically less disturbed than the KKI’s structurally more complex northern part where intensive deformation has often obliterated primary spatial relationships between individual rock units. Major geological units in the southern part of the KKI display a generally NE
/SW trending pattern of parallel volcanic belts separated by a sedimentary basin */ strikingly similar to the parallelism and even-spacing of volcanic belts encountered in the Birimian of Ghana and eastern Coˆte d’Ivoire. The Paleoproterozoic terrane in the KKI is characterized by (from NW to SE, Figs. 2 and 3): . the NW
/SE-trending Mako volcanic belt, . the Diale-Dalema sedimentary basin which is centrally intruded by the extensive Saraya pluton, . the NW
/SE-trending Fale´me´ volcanic belt which, due to poor exposure, has only been recognized as an entity due to recent exploration activities, and which is incomplete due to the fact that it is cut at its eastern side by the N
/S-trending Senegalo-Mali fault, . the post-Eburnean, N
/S-trending SenegaloMali graben east of the Senegalo-Mali fault, . unspecified (probably Birimian) basin sediments of the so-called Kofi Series (almost exclusively on Malian territory). The Mako volcanic belt appears to consist largely of basaltic, frequently carbonate-altered flow rocks and minor intercalated volcaniclastics, ultramafic (pyroxenitic) subvolcanic intrusions and numerous relatively small, massive biotite and amphibole-bearing TTG granitoids which resemble the synvolcanic, comagmatic belt-type granitoids of Ghana and eastern Coˆte d’Ivoire. The boundary of the Mako belt with the adjacent Diale-Dalema sedimentary basin is partly transi-

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Fig. 2. Lithological sketch map of the southern part of the KKI (Senegalese side, partly modified after BRGM, 1962) showing sample localities.

tional and characterized by interlayering of metavolcanics and basin metasediments. Consequently, volcanics in the Mako and Fale´me´ belts and

volcaniclastics and sediments in the Diale-Dalema basin are interpreted to be lateral facies equivalents of approximately similar age. On the north-

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Fig. 3. Schematic cross section through southern part of the KKI; radiometric age data in Ma. Dips of lithological contacts are exaggerated towards the vertical. True dips are generally in the range 60
/908.

western side of the Mako belt, west and northwest of the shear-zone hosted Sabadola gold mine, an area of sediments that probably represents another basin is present. However, primary relationships are unclear due to the presence of the Kakadian batholith and the adjacent Panafrican mobile belt. The Dalema and the Diale Series of metasediments are interpreted by the present authors as parts of the same sedimentary basin which is centrally intruded by an extensive basin-type granitoid, the Saraya batholith. The basin sediments include volcaniclastic lithologies, some of which contain lapilli-sized fragments, quartz-bearing wackes, argillites and carbonates. Seemingly, wackes are more widespread towards the boundaries of the basin. All sedimentary rocks are isoclinally folded. Folds are upright or slightly overturned to the SE. The Saraya batholith interpreted to be syntectonic, is generally foliated and medium-grained and consists of several coalescent plutons. According to Pons et al. (1992), 90% of the Saraya batholith consists of biotitemuscovite adamellite-granite. Its central part, in the vicinity of Saraya town, contains a U-bearing episyenite. The Fale´me´ volcanic belt was apparently not recognized by previous workers due to poor

exposure and extensive lateritization of the terrane. According to results of ongoing exploration work, volcanic lithologies in the form of pillowed andesite and occasional felsic flows are widespread. In addition, volcaniclastic rocks, chemical sediments (cherts, manganiferous sediments) and wackes are present in the realm of the belt/basin boundary, suggesting a proximal volcanic environment. The supracrustal rocks of the Fale´me´ volcanic belt are intruded by relatively small stocks of Na-dominated belt-type granitoids of dioritic to granodioritic composition some of which are spatially associated with major skarn-type iron deposits. The NE
/SW trending Fale´me´ volcanic belt is cut on the east by the regionally important N
/S trending Senegalo-Malian fault, which bounds a narrow (B/5 km width), post-Eburnean graben. There has apparently been major displacement along the eastern boundary fault of this SengaloMalian graben or one of its parallel faults, since the terrane to the east of the Senegalo-Malian graben consists of Birimian-type basin sediments (referred to as the Kofi Series in Mali). Within the Birimian sediments of the Kofi Series, tourmalinitization, mainly alongside N
/S trending faults and stockworks, has taken place. On strike towards the

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north, on Malian territory, tourmalinites are associated with gold mineralization at the Loulo gold deposit. The Series is intruded by several granitoid stocks, partly converted into albitite (Hurtubise/Schwartz, personal communication). Further towards the east, these rocks are concealed beneath flat-lying Neoproterozoic sediments.

3. Geochronology 3.1. Previous dating Several geochronological studies have been carried out in the southern part of the KKI (south of ca. 13810? latitude). Results include Rb/Sr whole rock ages for the Saraya basin type granitoid of 19739/33 and 20089/35 Ma (Bassot and CaenVachette, 1984) and of 20089/16 Ma (Ndiaye et al., 1997). A similar Rb/Sr age of 19899/28 Ma was reported for the Boboti granitoid (Bassot and Caen-Vachette, 1984). These probably reflect regional cooling or low temperature remobilization and ought to be considered as minimum ages. A Sm/Nd whole rock isochron age of 20639/41 Ma was determined for basalt of the Mako volcanic belt by Abouchami et al. (1990), and a Pb/Pb whole rock (volcanic) isochron age of 21959/118 Ma was given for the ‘Mako volcanic
/plutonic complex’ by Dia (1988). Calvez et al. (1990), also in Mile´si et al. (1989)) reported zircon Pb/Pb evaporation ages between 20969/8 and 21659/10 Ma for detrital zircons of sediments of the Dalema Series as well as zircon Pb/Pb ages of 20709/10 and 20729/9 Ma for an andesite dyke cutting these sediments. For the northern part of the KKI, where primary lithological relationships have been obscured through intense structural overprint, Dia et al. (1997) reported zircon Pb/Pb ages of 21949/4 and 22029/6 Ma in gneisses of the ‘Sandikounda amphibolite
/gneiss complex’, 21589/8 Ma in a layered gabbro of the ‘Sandikounda layered plutonic complex’, and a U/Pb zircon age of 21279/6 Ma for quartz diorite of the ‘Laminia pluton’. It is, however, not clear whether this pluton belongs to

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the belt-type- or basin-type-category, or neither one of these. 3.2. Analytical methods U
/Pb analyses were carried out at the Earth Sciences Department, Royal Ontario Museum, Toronto, Canada. Crushing and mineral separation were performed using standard procedures. Exterior surfaces of all selected zircon grains were removed by air abrasion (Krogh, 1982). Weights of mineral fractions were estimated by eye. Estimated weights should be accurate to about 9/50%. This affects only U and Pb concentrations, not age information, which depends only on isotope ratio measurements (Table 1). Dissolution of zircon, using HF, and monazite, using HCl, and subsequent chemistry also followed standard procedures (Krogh, 1973), with small columns (0.05 ml anion exchange resin) to minimize blank. No chemistry was performed on zircons with estimated weights of less than 5 mg. Pb and U were loaded together on Re filaments using silica gel and analyzed with a VG354 mass spectrometer using a Daly collector in pulse counting mode. 3.3. Results Results of U
/Pb isotopic analyses are given in Table 1 (errors at 2s) and ages are summarized in Table 2. Analyses are plotted on concordia diagrams on Figs. 4
/9. Error ellipses are plotted at 2s. Ellipses are numbered with reference to analysis numbers on Table 1. Average age errors are given at 95% confidence levels. In most cases, ages were calculated from the concordia intercept of a best fit line forced through the origin (equivalent to averaging the 207Pb/206Pb ages of the data) using the program of Davis (1982). Probabilities of fit would be expected to be around 50% on average for random data with correctly chosen analytical errors. 207Pb/206Pb age errors are quoted at 2s. Sample locations are given on Fig. 2. 3.3.1. HIR00-139: felsic flow, Fale´me´ volcanic belt Flow banded rhyolite occurs in the vicinity of Betakili village close to the Fale´me´ river, inter-

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Table 1 U
/Pb isotopic data for zircon and monazite from Paleoproterozoic rocks in Senegal Number

Fraction

Weight (mg)

HIR00-139 Felsic flow, Fale´me´ volcanic belt, Senegal 1 1 Ab zr, spr 0.0005 2 1 Ab zr, eq 0.0015 3 1 Ab zr, spr 0.0005 4 1 Ab zr, eq 0.0004

U (ppm)

29 45 48 27

Th/U

PbCom (pg)

207/204

206/238

2s

207/235

2s

207/206 age (Ma)

2s

1.84 1.13 0.34 0.77

43.23 205.2 239.5 60.24

0.4005 0.3843 0.3803 0.3852

0.0035 0.0015 0.0021 0.0042

7.419 6.906 6.811 6.888

0.179 0.036 0.043 0.125

2155.4 2102.3 2096.4 2093.6

33.7 5.9 6.1 22.4

HIR00-146 Quartz-rich wacke, Diale-Dalema sedimentary basin, Senegal 1 1 Ab zr, spr 0.008 49 0.63 2 1 Ab zr, spr, incl 0.006 28 0.56 3 1 Ab zr, lpr, rod incl 0.003 16 0.45 4 1 Ab zr, eq 0.008 52 0.58 5 1 Ab zr, eq 0.005 34 0.50 6 1 Ab zr, lpr, incl 0.006 27 0.64

0.56 0.41 0.46 0.56 1.76 8.59

2421 1440 373.8 2548 336.5 79.48

0.3986 0.3973 0.3967 0.3963 0.3862 0.3937

0.0011 0.0012 0.0025 0.0009 0.0011 0.0017

7.427 7.400 7.387 7.375 7.186 7.300

0.022 0.023 0.049 0.019 0.027 0.077

2166.1 2165.0 2164.6 2163.9 2163.3 2157.4

2.0 1.8 4.2 1.6 3.5 14.8

HIR00-143 Tonalitic belt-type granitoid, Fale´me´ volcanic belt, Senegal 1 1 Ab zr, lpr, incl 0.003 103 0.58 2 1 Ab zr, eq, melt incl 0.003 92 0.60 3 1 Ab zr, eq 0.002 118 0.58

0.92 0.61 0.37

1077 1436 2024

0.3801 0.3806 0.3801

0.0009 0.0012 0.0009

6.753 6.757 6.748

0.018 0.023 0.017

2082.4 2081.0 2081.0

2.1 2.3 1.9

614.6 4334 14185 6567

0.3777 0.3802 0.3634 0.3766

0.0013 0.0009 0.0012 0.0011

6.704 6.745 6.449 6.681

0.026 0.019 0.023 0.021

2080.5 2080.1 2080.1 2079.8

2.4 1.5 1.9 2.2

0.3897 0.3640 0.3812 0.3855 0.3791 0.3807 0.3743

0.0023 0.0010 0.0016 0.0014 0.0026 0.0017 0.0014

7.083 6.584 6.893 6.947 6.722 6.743 6.625

0.043 0.021 0.031 0.028 0.074 0.042 0.031

2122.4 2113.8 2113.5 2107.2 2079.0 2077.0 2075.7

5.0 2.7 1.7 2.3 13.5 7.4 4.2

0.4318 0.3862 0.3821 0.3835 0.3799 0.3832

0.0014 0.0011 0.0009 0.0009 0.0009 0.0011

7.683 6.857 6.773 6.750 6.679 6.733

0.034 0.022 0.018 0.017 0.019 0.021

2085.2 2081.1 2078.3 2065.9 2064.0 2062.8

4.9 2.1 2.3 1.6 1.7 1.6

HIR00-110 Granodioritic belt-type granitoid, Fale´me´ volcanic belt, Senegal 1 1 Ab zr, eq 0.001 192 0.69 1 2 1 Ab zr, spr 0.003 203 0.67 0.44 3 1 Ab zr, spr 0.002 1169 0.60 0.49 4 1 Ab zr, lpr 0.0015 1366 0.63 0.97 HIR00-147 Granodioritic belt-type granitoid, Mako volcanic belt, Senegal 1 1 Ab zr, spr, melt incl 0.002 53 0.45 2 1 Ab zr, lpr 0.003 55 0.42 3 1 Ab zr, lpr, incl 0.003 69 0.34 4 1 Ab zr, spr 0.004 22 0.39 5 1 Ab zr, lpr, incl 0.0006 30 0.29 6 1 Ab zr, lpr 0.0008 47 0.33 7 1 Ab zr, lpr 0.0010 81 0.3

0.99 0.73 0.51 0.38 0.77 0.83 0.93

HIR00-145 Granitic basin-type granitoid, Diale-Dalema sedimentary basin, 1 1 Ab mon, best grain 0.0015 217 74.34 2 1 Ab mon, incl 0.005 1092 9.62 3 1 Ab mon 0.003 1164 11.62 4 1 mon, eq, incl 0.003 3686 6.88 5 1 Ab mon, crk 0.002 1041 30.85 6 1 mon, eq, incl 0.002 3924 4.11

Senegal 5.42 12.97 7.83 1.76 4.74 2.38

367.8 702.8 1307 755.5 90.12 158.0 282.4 230.2 1358.1 1420.7 19737 1379.3 10341

Zircons are colorless, equant and free of inclusions unless noted otherwise. Ab-abraded; zr-zircon grain; mon-monazite grain; eq-equant; spr-short prismatic; lpr-long prismatic; incl-inclusions; crk-cracks. PbCom-Common Pb, assuming all has blank isotopic composition. Th/U calculated from radiogenic 208Pb/206Pb ratio and 207Pb/ 206Pb age assuming concordance. Percent Disc-per cent discordance for the given 207Pb/206Pb age. Uranium decay constants are from Jaffey et al. (1971).

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0.61 0.56 1.16 0.63

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bedded with volcaniclastic rocks. This sample yielded abundant rutile but only a small amount of zircon, generally as tiny short and long prismatic grains that are multifacetted (i.e. having a large number of high order crystal faces). Many of the crystals have a whitish appearance, which often indicates the presence of alteration. Four tiny colorless zircons were analyzed individually. Errors are relatively large because of the small size of the samples. Three of the grains gave concordant or near-concordant data whose 207Pb/206Pb ages overlap and define an average of 20999/4 Ma (Fig. 4). A fourth datum is older at 21559/34 Ma and shows that the sample contains inheritance. Given the evidence for inheritance, the 20999/4 Ma age is strictly an older limit but the agreement of the three younger data indicates that it is probably the true age of eruption. 3.3.2. HIR00-146: quartz-rich wacke, DialeDalema sedimentary basin This sample was from a particularly quartz-rich layer in Diale-Dalema basin sediments. It was collected on the road between Binbou and Kossanto villages, about 3 km SSE of the latter. Only a small amount of zircon could be recovered. The grains are mostly euhedral, stubby, multifacetted crystals but a few show slight rounding. Six abraded single detrital zircons gave data with overlapping 207Pb/206Pb ages (Fig. 5). The average age is 2164.79/0.9 Ma. This likely gives the age of the principal source rock and is a maximum age estimate for deposition of the sediments. 3.3.3. HIR00-143: tonalitic belt-type granitoid, Fale´me´ volcanic belt This quasi-circular, massive pyroxene tonalite body, which is spatially associated with the Boto iron deposits, was sampled 0.5 km SE of Boto village. A moderate amount of zircon was recovered from this sample. Most grains are shortprismatic and multifacetted. Many are cloudy and some may contain cores. Three single abraded zicons gave concordant data with overlapping 207 Pb/206Pb ages (Fig. 6). The average age is 2081.59/1.1 Ma and likely represents crystallization of the pluton.

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3.3.4. HIR00-110: granodioritic belt-type granitoid, Fale´me´ volcanic belt This sample of pyroxene
/amphibolite
/biotite granodiorite was obtained from the massive belttype ‘Boboti granitoid’ on the track between Nafadi village and Boboti village. It yielded a moderate amount of zircon. The grains are mostly long prismatic or flat crystals with dominant loworder faces. Data from four single abraded zircons show variable discordance but all 207Pb/206Pb ages are within error and define an average of 2080.29/ 0.9 Ma (Fig. 7). This is likely the age of emplacement of the granitoid rock. 3.3.5. HIR00-147: granodioritic belt-type granitoid, Mako volcanic belt This small stock of medium grained, massive, biotite-amphibole granodiorite is intruded into mafic flow rock in the vicinity of Mamakono village. The sample yielded a fairly small amount of zircon most of which is highly cracked. The grains are mostly short to long prismatic and multifacetted. Inheritance proved to be common in this sample. 207Pb/206Pb ages of concordant data range from 2122 to 2077 Ma (Fig. 8). However, the three youngest data have overlapping 207Pb/206Pb ages with an average of 20769/3 Ma. Given their agreement, the three youngest data likely define the intrusion age of the pluton, or at least give an older age estimate. 3.3.6. HIR00-145: granitic basin-type granitoid, Diale-Dalema sedimentary basin This sample, collected near Binbou village (northeastern part of the regionally important Saraya batholith), was from a foliated, 558-trending muscovite-biotite granite. The sample yielded only a few cracked zircons, which appear rounded or contain cores. There is a small amount of monazite in the magnetic fractions, which is mostly highly altered. A few relatively clear monazite grains were picked, in some cases breaking off altered parts, and analyzed. Two unabraded grains gave reversely discordant data (analyses 4 and 6 in Table 1 and Fig. 9) with 207 Pb/206Pb ages of about 2064 Ma, although the errors do not quite overlap. Four other grains were abraded gently to remove their outside layers.

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Table 2 Summary of U/Pb ages of Paleoproterozoic rocks from Senegal Sample

HIR00-139, HIR00-146, HIR00-143, HIR00-110, HIR00-147, HIR00-145, tary basin

Felsic flow, Fale´me´ volcanic belt Quartz-rich wacke, Diale-Dalema sedimentary basin Tonalitic belt-type granitoid, Fale´me´ volcanic belt Granodioritic belt-type granitoid, Fale´me´ volcanic belt Granodioritic belt-type granitoid, Mako volcanic belt Granitic basin-type granitoid, Diale-Dalema sedimen-

Age (Ma)

Probability of fit (%)

Number of data

20999/4 2164.79/0.9 2081.59/1.1 2080.29/0.9 2076.39/3.2 20799/2, igneous

33 47 55 98 87 17

3 6 3 4 3 3

1

3

20649/4, metamorphic

Fig. 4. U
/Pb concordia diagram showing zircon data points from flow-banded rhyolite (HIR00-139) of the Fale´me´ volcanic belt, Kedougou-Ke´nie´ba Inlier, Senegal.

Fig. 5. U
/Pb concordia diagram showing zircon data points from a quartz wacke (HIR00-146) of the Diale-Dalema sedimentary basin, Kedougou-Ke´nie´ba Inlier, Senegal.

These grains were cleaned in an ultrasonic bath with water. The normal cleaning procedure, using dilute nitric acid, was avoided so as to minimize the possibility of leaching. However, these grains also gave reversely discordant data. The clearest, most pristine-looking grain proved to be the lowest in uranium but gave a datum that is 13% above concordia (analysis 1). The data appear to fall into two age clusters. The three older data give a marginal fit to a line (17% probability of fit) that intersects concordia at 20799/2 Ma. The lower concordia intercept is almost within error of 0 Ma.

Averaging the 207Pb/206Pb ages of the other three data gives an age of 20649/4 Ma but with a low probability of fit. The analyses show a wide range of Th/U ratios that are not correlated with age. U
/Pb data from monazites sometimes show slight amounts of reverse discordance. In Phanerozoic samples, this can be caused by large amounts of 230Th being incorporated during crystallization (because of the high Th/U partition coefficient of monazite), which decays to form excess 206Pb. In this case, the 207Pb/235U age is unaffected and the data point is raised vertically above concordia.

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Fig. 6. U
/Pb concordia diagram showing zircon data points from a tonalitic belt-type granitoid (HIR00-143) of the Fale´me´ volcanic belt, Kedougou-Ke´nie´ba Inlier, Senegal.

This effect is corrected in the age calculations by using the calculated Th/U of the monazite and an assumed Th/U of the magma (4.2, the mantle average). Excess 206Pb from this source is negligible for these Precambrian samples because of the

Fig. 8. U
/Pb concordia diagram showing zircon data points from a granodioritic belt-type granitoid (HIR00-147) of the Mako volcanic belt, Kedougou-Ke´nie´ba Inlier, Senegal.

93

Fig. 7. U
/Pb concordia diagram showing zircon data points from a granodioritic belt-type granitoid (HIR00-110) of the Fale´me´ volcanic belt, Kedougou-Ke´nie´ba Inlier, Senegal.

large amount of accumulated normal radiogenic 206 Pb. Reverse discordance in Precambrian monazites is usually along a line through the origin of concordia (i.e. the 207Pb/206Pb age is unaffected)

Fig. 9. U
/Pb concordia diagram showing monazite data points from a granitic basin-type granitoid (Saraya batholith)(HIR00145) of the Diale-Dalema sedimentary basin, KedougouKe´nie´ba Inlier, Senegal.

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and may be due to recent U loss, perhaps even by leaching during laboratory washing, or lack of equilibration between spike and sample. The latter possibility is considered unlikely since spike was added at the beginning and there was no evidence of a residue after dissolution. Monazite can potentially show complex age systematics due to intragrain growth episodes during different igneous and metamorphic events. This was shown for Archean gneisses in Cote d’Ivoire that were remobilized during the Eburnean orogeny (Cocherie et al., 1998). Different growth episodes can potentially be distinguished by imaging polished grains and dated by microprobe techniques, or by physical separation of core and overgrowths. Unfortunately, in this case the small quantity of fresh material (portions of only a few monazite grains) precluded such a detailed analysis. The approximate consistency of 207Pb/206Pb ages for the two groups of data, combined with geochronology of other rocks in the region, suggests that 20799/2 Ma likely represents intrusion of the pluton and magmatic crystallization, while the best interpretation for the 20649/4 Ma age seems to be that it represents a period of growth of metamorphic monazite. The latter age would be an older limit on metamorphism if the populations have been mixed. Both ages approximately overlap with the youngest age of monazite obtained by Cocherie et al. (1998) for migmatitic gneiss in Coˆte d’Ivoire by Kober evaporation. This is 20749/7 Ma, although the Pb loss pattern obtained from isotope dilution data suggested a true age of about 2030 Ma for high grade metamorphism.

4. Discussion 4.1. Diachroneity of geologic development The hypothesis that the Baoule´-Mossi domain can be divided into an eastern subprovince displaying approximately 2150
/2190 Ma old Birimiam belt volcanism and coeval belt plutonism, and a western subprovince characterized by approximately 2100 Bandamian volcanic belts and

corresponding plutonism (Hirdes et al., 1996) is further strengthened by the present study. The measured 20999/4 Ma extrusion age of the Fale´me´ belt rhyolite flow, and a Sm/Nd whole rock isochron age of 20639/41 Ma for basalt of the Mako belt (Abouchami et al., 1990) are much younger than volcanism in the eastern Baoule´Mossi domain. This has been directly dated at 21589/1 Ma in Burkina Faso (Davis, 2000) and 21899/1 Ma in Ghana (Hirdes and Davis, 1998). More extensive geochronology on belt-type plutons in Ghana, eastern Coˆte d’Ivoire and Niger gives minimum ages for volcanism in the range 2150
/2180 Ma (Hirdes et al., 1992, 1996; AmaSalah et al., 1996). Thus far, approximately 2100 Ma volcanic ages are found only to the west of the Ouango Fitini shear zone in Coˆte d’Ivoire where a felsic flow in the Ouango-Fitini volcanic belt was dated at 21049/2 Ma (Hirdes et al., 1996) and rhyolitic ignimbrite in the Fe´te´kro volcanic belt gave 21059/ 1 Ma (Leake, 1992). The present data indicate that c. 2100 Ma old belt volcanism characteristic of the western subprovince of the Baoule´-Mossi domain (Hirdes et al., 1996) extends from the Ouango Fitini shear zone in Coˆte d’Ivoire to the KKI in Senegal. Presently available data also reinforce the suggestion of a westerly progressing development within and between subprovinces. Belt granitoid plutons of the eastern subprovince appear to be 2172 Ma or older in Ghana while 2152 Ma granitoids are found on the western edge of the subprovince (Hirdes et al., 1996). The 20999/4 Ma old rhyolite from Senegal is slightly younger than the felsic extrusive rocks dated near the eastern edge of the western subprovince (Ouango Fitini and Fe´te´kro belts). Belt-type plutons from Senegal are now precisely dated in the range 2082
/2076 Ma, and are distinctly younger than belt-type plutons dated in the range 2113
/2103 Ma from the Ouango Fitini belt in eastern Coˆte d’Ivoire. There is also evidence for rocks of the c. 2150
/ 2200 Ma episode throughout the western domain, mainly in the form of gneisses but occasionally as volcanics and plutons, as well as evidence for rocks from a approximately 2125
/2135 Ma time span (i.e. belonging to neither one of the above de-

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scribed episodes) (Dia et al., 1997; Doumbia et al., 1998; Kouamelan, 1996; Davis reported in Lu¨dtke et al., 1998). Intrusion ages of basin granitoids, which appear to have formed syn-late kinematically with respect to Eburnean deformation over the entire Baoule´Mossi domain, as well as available age constraints on metamorphism, also suggest a northwest-directed younging tendency. The 20799/2 Ma age for magmatic crystallization of the Saraya granitoid in the Diale-Dalema sedimentary basin is the youngest basin-type granitoid age so far reported from the Baoule´-Mossi domain. Older basin-type leucogranites were dated at 20949/6 Ma in central Coˆte d’Ivoire by Kouamelan (1996), while similar granitoid rocks in the eastern part of the Baoule´Mossi domain (Ghana) yield ages from 2088 to 2117 Ma (Hirdes et al., 1992; Davis et al., 1994). These data imply that the Eburnean tectonothermal event affected the Baoule´-Mossi domain diachronously from SE to NW over a time span of at least 8 million years. In the KKI this event overlaps with belt-type granitoid plutonism, in contrast to the eastern Baoule´-Mossi domain where belt-type plutons are 35
/90 million years older than basin-type plutons. Regional metamorphism as expressed by the growth of metamorphic monazite affected the Saraya batholith at 20649/4 Ma at the earliest and thus at least in this specific case is clearly postdeformational. Monazite and titanite from west to central Coˆte d’Ivoire records metamorphism at 20809/10 Ma (Kouamelan, 1996), while metamorphic titanite in gneiss from eastern Coˆte d’Ivoire gave 21009/3 Ma (Hirdes et al., 1996). Regional-metamorphically grown monazites further east from Ghana have been dated at 21029/1 Ma (Davis, reported in Loh and Hirdes, 1999). A potential complication to metamorphic age patterns is the possibility of obtaining younger ages on rocks that have been exhumed from deep crustal levels. Such a downward younging trend in metamorphic ages has been found in Archean cratons (Krogh, 1993). This might explain indications of metamorphic ages extending down to c. 2030 Ma in gneisses associated with the more deeply exhumed Archean craton in western Coˆte d’Ivoire (Kouamelan, 1996; Cocherie et al., 1998).

95

The spatial distribution pattern of metamorphic ages in medium to high level Birimian rocks indicates approximately 35 million years age gap between regional metamorphism in the eastern part of the Baoule´-Mossi domain (Ghana) and the KKI, which suggests diachroneity of the Eburnean tectono-thermal event as well. 4.2. Constraints on tectonic models The Paleoproterozoic terrane in the southern part of the KKI is lithologically similar to Birimian terranes in the eastern part of the Baoule´-Mossi domain (Ghana, central and eastern Coˆte d’Ivoire), despite the fact that there is a considerable age difference of almost 100 million years between the two. As in Ghana and eastern Coˆte d’Ivoire, the southern part of the Paleoproterozoic terrane of the KKI is characterized by at least two quasi-parallel, SW
/NE-trending volcanic belts separated by basin sediments, where the volcanic belts are intruded by relatively small stocks of Na-rich, ‘synvolcanic’ TTG granitoids, and the sedimentary basin is occupied by an extensive K-rich batholith. Also, similar deformational-metamorphic relationships are encountered in both the eastern and western areas of the Baoule´-Mossi domain where Eburnean deformation post-dates and has affected belt volcanics, synvolcanic belt granitoids and basin sediments, and is itself quasi-contemporaneous and/or postdated only by basin granitoid intrusions. Contrary to the situation encountered in Ghana, Tarkwaian grabens and the corresponding conglomeratic sediment infill have not been detected in the two volcanic belts present in the southern KKI. However, this might be due to the lack of detailed mapping, because the occurrence of Tarkwaian rocks is also very minor in some of the Ghanaian volcanic belts (e.g. Leube et al., 1990). Any tectonic scenario must explain the apparently diachronous juxtaposition of these granitegreenstone terrains which, on the basis of extensive Nd isotopic studies, originally formed in an oceanic environment (Abouchami et al., 1990). It must also explain the role of Archean rocks to the west, which may have underthrust the Paleoproterozoic rocks at a late stage of Eburnean devel-

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opment (Feybesse and Mile´si, 1994). This pattern suggests a tectonic scenario involving progressive docking of terranes that resulted in amalgamation of Birimian supracrustal and coeval belt granitoid rocks against a growing continental mass to the southeast of the Baoule´-Mossi domain. This process may have culminated with the collision of this mass with the Liberian Archean terrane, now exposed as a large-scale structural window on the western side of the Baoule´-Mossi domain. In this case, the metasedimentary belts could represent the remains of either accretionary prisms or foreland basins. All available data from detrital zircons indicate that deposition of Birimian sediments was largely derived from but post-dates the early ‘type’ Birimian magmatism. This suggests a syndeformational foreland basin, rather than a synvolcanic accretionary prism environment. The source and timing of basin sedimentation are of key importance for testing this model. The diachroneity of ages from the igneous rocks is apparently not reflected in the provenance of sediments from the basins. As mentioned, throughout the Baoule´-Mossi domain these appear to have sourced similar-aged terranes made up predominately of rocks equivalent to the ‘type’ Birimian: 2150
/2200 Ma juvenile volcanic and granitoid rocks. For example, detrital zircon from the investigated quartz-wacke in the Diale-Dalema basin shows a uniform 2165 Ma age. Although a more complex pattern might be obtained from a larger number of analyses, these results suggest a focussed (proximal?) source. Their uniformity contrasts with the wide age range found for detrital zircons from metasedimentary rocks in the eastern Baoule´-Mossi domain, where even a limited sampling of detrital zircon reflects most of the age span of Birimian igneous activity (Davis et al., 1994). Gneisses within the 2150
/2200 Ma age range have been reported by Dia et al. (1997) towards the north of the investigated portion of the KKI (e.g. Sandikounda amphibolite
/gneiss complex). Thus, they may form a substratum for some of the younger volcanic belts that was uplifted and locally eroded into the Diale-Dalema basin. This is also suggested by 21559/34 Ma zircon inheritance found in the 20999/4 Ma rhyolite from the Fale´me´ volcanic belt as well as

by somewhat younger inheritance in the Mako belt granodiorite. However, detrital zircon as young as 20969/8 Ma has also been reported in this basin by Calvez et al. (1990), in addition to the measured 2165 Ma grains, pointing to much younger deposition and partial derivation from nearby ‘Bandamian’ volcanic belts where extrusive volcanism has been dated at 20999/4 Ma. Transitional field relationships between sediments and volcanic rocks both in the KKI and in Ghana also suggest proximal derivation of sediments from rocks in neighboring volcanic belts, and characterize a dynamic paleoenvironment as expressed by various, rapidly changing volcanic and sedimentary facies. Rare younger zircons from metasedimentary rocks suggest that basin sedimentation also progressed from east to west across the Baoule´-Mossi domain. Doumbia et al. (1998) obtained a youngest age of 21079/7 Ma on detrital zircon from the Bandama sedimentary basin in central Coˆte d’Ivoire. Although this can only be considered an older age limit on deposition, it is younger than the youngest detrital zircon thus far dated from the Bambela basin to the northeast (21269/1 Ma, Davis, reported in Lu¨dtke et al., 1999), which is in turn younger than the youngest detrital zircon found in the Kumasi basin near the eastern margin of the Baoule´-Mossi domain (2135/5 Ma; Davis et al., 1994). The Kumasi basin is cut by a 21169/2 Ma old pluton, thus establishing that orogenic sedimentation was occurring in the eastern Baoule´-Mossi domain earlier than the youngest 20969/8 Ma detrital zircon dated in the KKI. The prevalence of 2150
/2200 Ma Birimian-aged detritus in locally derived sediments throughout the Baoule´-Mossi domain suggests that these older rocks must have been uplifted, transported and eroded before or during the younger c. 2100 Ma Bandamian phase of volcanic activity. Perhaps the older parts of an accretionary complex (i.e. rocks of the Birimian Supergroup sensu stricto) were transported westward during terrane collisions and largely eroded to form foreland basins, locally acting as basement to younger igneous rocks. Deep transpressional and transtensional faults would have dissected this complex and may have channeled the younger generation of magmas, as

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discussed by Pouclet et al. (1996). Differential uplift along these faults and erosion might expose a complex pattern of different aged rocks that partially preserves the overall belt-like pattern resulting from the accretionary process. A similar model of a magmatically active accretionary orogen has been proposed to explain the belt-like distribution of metavolcanic and metasedimentary terranes in the Archean Superior province of Canada (Card, 1990; Davis, 1998). Orogens pass through stages of both crustal compression and extension. An apparent feature of Archean-type orogens, to which the Eburnean belongs, is the ‘hot’, magmatically active character of the crust and mantle throughout this process, which tends to obliterate evidence of its earlier stages. Resolving the debate between ‘fixist’ and ‘mobilist’ models may only be possible by understanding geologic patterns on a craton-wide scale. An important test of the proposed scenario will be to more tightly constrain ages for the commencement of regional deformation and the deposition of sedimentary rocks in basins across the Baoule´Mossi domain to see whether they confirm or contradict present evidence for northwest-directed assembly of continental crust.

5. Conclusions New age data on zircon and monazite from Paleoproterozoic rocks in the KKI in eastern Senegal show that this area contains the youngest volcanic rocks, as well as the youngest belt and basin-type granitoid plutons yet dated from the Baoule´-Mossi domain. This provides support for the proposed division of the Baoule´-Mossi domain into an eastern subprovince dominated by relatively old (2150
/2190 Ma) Birimian supracrustal rocks and belt-type plutons and a western subprovince dominated by younger (2080
/2110 Ma) Bandamian volcanics and associated plutons. While Birimian-aged rocks in the eastern subprovince show little or no evidence for interaction with older (Archean) rocks, some Bandamian rocks in KKI show zircon inheritance from Birimian-aged sources.

97

Detailed age relationships within and between subprovinces indicate a general northwest-directed diachroneity in crustal development and regional deformation. This supports an accretionary tectonic model for the Eburnean orogeny of West Africa.

Acknowledgements The present paper was prepared within the scope of a sectoral research project on base metal mineralization in the Birimian of West Africa, founded by the German Ministry for Economic Cooperation and Development (BMZ). The authors thank Professor A. Dia, Director of the Direction des Mines et de la Ge´ologie in Dakar, for his enthusiastic and constructive support of the field operation in Senegal and for fruitful discussions. L. Sy and M. Diouf of DMG are thanked for their guidance and companionship in the field. Special thanks go to D. Bray, E. Hurtubise, D. Boisvert and A. Die´me of IAMGold for their outstanding willingness to share ideas and for meticulously organized field trips in the Fale´me´ belt, as well as generous logistical support. The preparation of some of the illustrations by W. Weinmann of BGR is gratefully acknowledged. Technical assistance in geochronology by Galina Amelina, Mark Smethurst, Tom Pestaj, and Raivo Tahiste is also acknowledged. The paper benefitted considerably from careful reviews by J.J. Peucat, W.R. Van Schmus, and an anonymous reviewer.

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