Journal of African Earth Sciences 79 (2013) 1–9

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Journal of African Earth Sciences

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Geochemical characteristics and U–Pb zircon LA-ICPMS ages of granitoidsfrom the Pan-African Dahomeyide orogen, West Africa

Kodjopa Attoh a,1, Scott Samson b, Yao Agbossoumondé c,⇑, Prosper M. Nude d, Jennifer Morgan a

a Department of Earth & Atmospheric Sciences, Cornell University, Ithaca, NY 14853, USAb Department of Earth Sciences, Syracuse University, Syracuse, NY 13244, USAc Département des Sciences de la Terre, Université de Lomé, Lomé, Togod Department of Earth Sciences, University of Ghana, Legon, Ghana

a r t i c l e i n f o a b s t r a c t

Article history:Received 1 August 2011Received in revised form 27 July 2012Accepted 30 September 2012Available online 17 October 2012

Keywords:I-type granitoidPaleoproterozoicU–Pb zircon agePan-African orogenWest Africa

1464-343X/$ - see front matter � 2012 Elsevier Ltd.http://dx.doi.org/10.1016/j.jafrearsci.2012.09.015

⇑ Corresponding author.E-mail addresses: sdsamson@syr.edu (S. Sam

(Y. Agbossoumondé), pmnude@ug.edu.gh (P.M. Nude)1 In memory of Kodjopa Attoh who spent several d

Dahomeyides. He initiated this collaboration, and paswas being prepared.

The Dahomeyide orogen in southeastern Ghana and adjoining parts of Togo and Benin records the sutureof West African craton (WAC) into northwest Gondwana. The suture zone is characterized by distinctivehigh-pressure mafic rocks that mark the collision of WAC with exotic blocks to the east. Granitoids to theeast of the suture zone, which are postulated to be juvenile crust representing the arc terrane that formedduring subduction and oceanic closure, were investigated. We compiled a map of granitoid gneisses inGhana, Togo, and Benin which allows us to distinguish between two main rock types: (1) migmatiticgneisses with biotite as the dominant mafic mineral and (2) dioritic gneiss characterized by hornblendeas the main mafic mineral and with occasional garnet. These rocks types are separated by a shear zoneinferred to be a splay of Kandi shear zone, a segment of the Trans-Saharan shear zone that extends for�2500 km from the Sahara to the Gulf of Guinea. The granitoids are characterized by I-type signatures,display variable amounts of LREE enrichment, Eu depletion and Sr/Y ratios that are consistent with a sub-duction origin. The migmatitic gneiss unit, within a zone of highly strained granitoids (straight gneiss)that forms a sliver just east of the suture zone, yielded U–Pb zircon ages ranging from 615 to 589 Ma. Thisage range is similar to the metamorphic crystallization age of the suture zone rocks. The zircons analyzeddisplay concentric zoning (in CL images) and are inferred to be magmatic. On the other hand, zirconseparates from the dioritic gneiss, with grains also showing igneous zoning in CL images, yielded U–Pbzircon crystallization ages ranging from 2.19 to 2.14 Ga. These are the first U–Pb zircon ages to documentPaleoproterozoic rocks in the postulated juvenile terrane, west of the Kandi fault zone. Nd model ages ofrocks in the suture zone are also consistent with the existence of widespread involvement of older crustin the Pan-African collision zone, similar to the isotopic data from the correlative Medio Coreau domain ofthe northwest Borborema province in Brazil.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction interpreted to have resulted from easterly subduction of the rifted

The Dahomeyide orogen, which occurs in southeastern Ghana,Togo, and Benin, is the southern segment of the Pan-AfricanTrans-Saharan belt (TSB). The TSB defines the eastern margin ofthe West African craton (WAC) and extends for >2500 km fromthe Sahara to the Gulf of Guinea (Caby, 1987). It formed duringthe Pan-African orogenic cycle that resulted in the assembly ofnorthwest Gondwana (Hoffman, 1991; Cordani et al., 2003; Tohveret al., 2006). In southeastern Ghana and adjoining parts of Togo andBenin the Dahomeyide is a well-organized orogen that is

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son), yagboss@tg.refer.org.ecades on understanding thesed on while this manuscript

margin of WAC (Affaton et al., 1991; Agbossoumondé et al., 2004;Attoh and Nude, 2008). As such, the area provides the opportunityto shed light on tectonic processes during Gondwana assembly. Forexample, the granitoid complex to the east of the suture zone ispostulated to be juvenile crust representing the roots of volcanicarc complex which is also recognized to the north in Mali (Bergeret al., 2009, 2011). Here, we present the geochemical characteris-tics of this granitoid terrane, and explore the extent to which itwas predominantly juvenile based on U–Pb zircon geochronology.

We have compiled a map of granitoid gneisses in Ghana, Togoand Benin (Fig. 1), sampled representative units, and carried outmajor and trace element chemical analyses of these samples. Inaddition, we obtained U–Pb zircon ages of selected rocks by LA-ICPMS analyses. These data are used to evaluate the hypothesisof new crustal production during the assembly of Gondwana asrecorded in the Dahomeyide orogen.

Fig. 1. Geological Map of the Dahomeyide Orogen in southeastern Ghana and adjoining parts of Togo and Benin compiled from published maps (after Sylvain et al., 1986;Castaing et al., 1993; Attoh et al., 1997).

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2. Geologic setting and field relations

Fig.1 is a geologic map of the Dahomeyide Pan-African colli-sion belt in southeastern Ghana, southern Togo and Benin com-piled from published maps (Sylvain et al., 1986; Castaing et al.,1993; Attoh et al., 1997). It shows the principal lithologies ofthe orogen, beginning from the west with the deformed marginof the WAC. Rocks of the WAC margin include 2.1 Ga granitoids(Agyei et al., 1987; Agbossoumondé et al., 2007) now deformedinto proto-mylonites and it is cover rocks (Atacora quartzites)deposited on the rifted passive margin, which together comprisecraton-verging nappes. These are bounded to the east by distinc-tive high-pressure (HP) mafic rocks that form the suture zoneunit (Attoh, 1998; Agbossoumondé et al., 2001; Attoh andMorgan, 2004) and mark the zone of collision of WAC with

presumed exotic blocks to the east. Granitoids to the east of thesuture zone are thus postulated to include juvenile crust repre-senting the arc terrane that formed during easterly subductionaccompanying oceanic closure. Two main types can be distin-guished among the granitoid gneisses (Fig. 1): (i) Dioritic gneisscharacterized by hornblende as the main mafic mineral and occa-sional garnet (Fig. 2A) and (ii) Migmatitic gneiss with biotite asthe dominant mafic mineral (Fig. 2B). The migmatites alsoinclude highly-strained granitoids (straight gneiss, Fig. 2C) withmuscovite and are in tectonic contact with the suture zone rocks.The dioritic gneiss is bounded by a shear zone to the west, in-ferred to be a splay of Kandi fault, which is the southern contin-uation of the 4�500E Saharan shear zone (Attoh and Brown, 2009).Fresh granitoids samples for geochemistry and geochronologywere obtained from quarries as well as outcrops in the area.

Fig. 2. Field photographs of representative samples of the Dahomeyide granitoids. A. Garnet–biotite dioritic gneiss from Agbelouve quarry in southern Togo (sample RT72). B.Migmatitic gneiss with biotite as the mafic phase from southeastern Ghana. C. A sample of the straight (granitic) gneiss with biotite from southeastern Ghana. D. K-feldsparphenocrysts in quartz-monzonite (sample DB01) as enclave in isotropic granite from Tré, Dassa area in central Benin.

Table 1Major and trace element composition of selected granitoid samples.

Samples DAH 19 DAH 20 AK 26 DZ 28 DD 44 AH 47 RT 72 HV 1 DB 01

SiO2 68.1 67.9 56.06 66.14 68.6 67.82 64.2 72.01 69.7TiO2 0.22 0.42 1.02 0.43 0.53 0.37 0.69 0.31 0.58Al2O3 17.2 15.7 20.19 15.37 15.5 16.23 15.4 14.6 14.7FeO 1.5 2.4 5.09 4.02 2.2 1.87 3.9 nd ndFe2O3 0.61 0.95 nd nd 2.84 nd 5.18 2.11 3.24MnO 0.05 0.06 0.09 0.05 0.06 0.03 0.08 0.02 0.03MgO 0.49 1.3 2.29 1.74 0.97 0.75 0.9 0.92 0.66CaO 3.86 4.45 4.76 4.74 2.67 2.84 3.01 3.18 2.3Na2O 6.74 4.88 4.55 4.3 4.21 4.99 4.14 4.43 3.14K2O 0.89 1.13 3.5 1 3.07 2.47 3.54 1.24 4.86P2O5 0.08 0.16 0.47 0.12 0.12 0.11 0.2 0.08 0.2H2O nd nd nd nd 0.4 nd 0.6 nd ndLOI 0.31 0.54 nd nd 0.6 nd 0.7 0.44 0.47Total 99.74 99.35 98.02 97.91 100.77 97.48 101.24 99.4 99.8Nb 9.60 7.70 63 10.08 32 2 17Zr 613 178.2 200 15.88 666 163 207Y 24.15 13.99 15 5.93 30 2.6 21Sr 229.5 175.9 1054 492 224.5 567 585 708 267Ba 89.8 821 3034 456 421.2 802 2310 528 1150Rb 112 15.3 162 71.02 50 414 191Hf 2.8 8.98 12.5 3.33 5.67 2.83 16.3 4 8Ta 0.55 2.34 0.914 2.28 2.66 1.32 0.606 <0.5 0.8Th 5.39 14.7 4.49 0.95 17.82 4.33 5.92 15 22.3U 1.23 2.29 1.28 0.02 3.47 0.99 1.24 0.22 1.33Sc 3.17 6.16 6.98 8.99 3.82 1.31 9.49 0Cr 10.44 14.7 6.38 79.1 11.56 4.67 13.70 20 20Ni 5.67 3.61 6.24 22.61 3.93 2.31 7.96 <0.5 <0.5La 11.9 56.18 44.63 12.79 38.41 18.8 77.38 62.3 90.40Ce 23.49 116.9 90.40 28.96 80.01 31.91 167.26 100 178Nd 11.25 55.32 42.13 15.87 28.62 16.54 77.51 27.9 69.7Sm 2.57 10.48 7.28 2.70 6.34 2.61 10.82 2.60 10.80Eu 0.73 1.04 1.86 0.81 0.68 0.61 2.18 0.53 1.54Tb 0.32 1.21 0.84 0.26 0.71 0.23 1.06 0.19 1.05Yb 1.42 4.22 1.81 0.65 1.52 0.53 3.30 0.3 1.6Lu 0.2 0.53 0.25 0.08 0.18 0.07 0.45 0.11 0.34

K. Attoh et al. / Journal of African Earth Sciences 79 (2013) 1–9 3

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3. Chemical characteristics of granitoids

Representative major and trace element concentrations arelisted in Table 1. Normative feldspar compositions of the granitoidsindicate sodic affinities with the majority of samples plotting in thegranodiorite and tonalite fields (Fig. 3A). The trondjemitic sampleis an aplitic rock whereas the only Qtz-monzonite (DB01) sampleis from the undifferentiated unit in Dassa, Benin (Fig. 2D). The plotof Al saturation index (Al2O3 vs. SiO2) shows that the rocks straddlemetaluminous–peraluminous boundary (Fig. 3B). The most

Fig. 3. Chemical characteristics of Dahomeyide granitoids. Symbols used are the same forthe migmatitic unit whereas AK26, DD44 and AH47 are granodiorites from the same unitGhana, whereas the sample RT72 is Garnet–Biotite dioritic gneiss from Agbelouve quarryfrom Tré, Dassa area in central Benin. A. Normative An–Ab–Or classification diagram (h(1965) are shown in faint lines. B. SiO2 vs. ASI diagram of Clarke (1992), showing(FeOt + MgO) vs. SiO2 wt.% classification of Frost et al. (2001). The rocks plot mainly in t

aluminous samples come from the migmatite unit, but sampleswith high Al values, such as HV1 which is from the dioritic unit,are also peraluminous, consistent with the occurrence of garnetin the mineral assemblage. In general the granitoids have moretypical I-type signature rather than S-type chemical characteristics(Fig. 3B). A plot FeOt/(FeOt + MgO) vs. SiO2 (Fig. 3C) shows that thegranitoids are largely magnesian and fall in the field of Cordillerangranitoids in the classification of Frost et al. (2001).

The granitoids were also analyzed for REE and other trace ele-ments (Ba, Hf, Sc, Ta, Cr, Ta, Th, U, Cs, Y, Sr) by neutron activation

all the figures. DAH19 and DAH20 are respectively aplitic, pegmatitic samples fromin southeastern Ghana. Samples DZ28 and HV1 are dioritic gneiss from southeasternin southern Togo. Sample DB01 is quartz-monzonite as enclave in isotropic granite

eavy lines) showing the compositions of the rocks. The original fields of O’Connorthe I-type characteristics of the rocks. ASI = Al/(2�Ca � 1.678�P + Na + K). C. FeOt/he magnesian pluton field.

K. Attoh et al. / Journal of African Earth Sciences 79 (2013) 1–9 5

analyses except for two samples which were analyzed by ICP-MS.The results are listed in Table 1. A chondrite-normalized REE plotis shown (Fig. 4A). From the REE patterns, two types of granitoidscan be distinguished: (i) those with moderate-strong Eu depletionanomalies (RT72, DD44, DAH20) and moderate LREE enrichmentand (ii) those with depleted REE contents, with relatively flat REEsignatures (AH47, DZ28, DAH19) and strong HREE-depletion. TheLREE-enriched samples (RT72, AK26, DD44, DAH20) are moder-ately steep in chondrite–normalized plots with (La/Yb)N < 16.8and (La/Sm)N � 4.6 to 3.5. The REE patterns show an overlap ofthe chemical characteristics of the migmatites and tonalitic dior-ites and are interpreted as those of island arc magmatic suites.

With Sr/Y between 5 and 100 (Fig. 4B), except HV1, the grani-toids plot in Sr/Y vs. Y space that are consistent with subductioncomponents (Huang et al., 2010). In primitive-mantle normalizedmulti-element plot (Fig. 4C), the rocks show depletions (negativeanomalies) in U, Nb, Ta, P, Zr and Ti. The depletions in Ta, Nb, Baof the migmatites represented by DD44, and dioritic gneiss

Fig. 4. Trace element characteristics of granitoids: A. Chondrite-normalized REEplot (after Nakamura, 1974) of the granitiods. B. Sr/Y vs. Y plot of the granitoids. C.Primitive-mantle normalized multi element plot of the granitoids.

represented by DZ28, are typical of arc rocks. Thus, although theDahomeyide granitoids display some major element characteris-tics of TTG-rocks they apparently preserve overall I-type magmaticsignatures typical of the roots of magmatic arcs.

4. U–Pb zircon analyses and results

4.1. LA-MC-ICPMS analytical methods

Individual zircon crystals and age standards were aligned in agrid pattern on tape. A 2.5 cm ring was then added and BuehlerEpothin� low viscosity epoxy was poured in the ring coveringthe zircon crystals. After 24 h of curing (�40 �C), the epoxy mountswere ground to expose the interiors of the crystals using 2000 gritsandpaper. A final polish using 1 lm aluminum oxide was thenperformed. Each zircon crystal was imaged by backscatter-electron(BSE) and cathodoluminescence (CL) to reveal internal detailsincluding potential cores and rims. Fig. 5 shows selected CL imagesand one BSE image of zircons analyzed. These images, which showconcentric zoning in the zircons, are interpreted to be magmatic.

U–Pb dating of zircon was performed by laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICPMS) using the Isoprobe-P

�instrument at the LaserChron Center

at the University of Arizona (see Gehrels et al., 2008 for details).The laser spot size was either 25 or 35 lm depending upon crystalsize and if core + rim analyses were performed. Common Pb correc-tions were made by measuring 204Pb and assuming common Pbisotopic composition as modeled by Stacey and Kramers (1975).U/Pb and 206Pb/207Pb fractionation is calibrated relative to frag-ments of a Sri Lanka zircon standard (SL-1) with an age of564 ± 3.2 Ma (2r). This standard is also used to calibrate the Uconcentration and U/Th of unknowns, and the accuracy of thosemeasurements is estimated at �20%. Decay constants (238k =9.8485 � 10�10, 235k = 1.55125 � 10�10) and isotopic abundance(238U/235U = 137.88) follow that proposed by Steiger and Jager(1977). Analyses that are >30% normally discordant (as determinedby 206Pb�/238U and 207Pb�/206Pb� dates) or >5% reversely discordantare not reported. All dates were calculated using the program ISO-PLOT (Ludwig, 2003). The reported dates for crystals younger than900 Ma are 206Pb�/238U dates and for crystals older than 900 Ma are207Pb�/206Pb� dates, unless otherwise noted. All reported zircon iso-topic data are given in Table 2 (Supplementary data).

4.2. Results and interpretation

Zircons from samples DZ28, HV1, AH47 and KM1 were ana-lysed. The first three samples are representative of the tonaliticto granodioritic plutons, whereas KM1 is a migmatite with uncer-tain protolith. Results of the isotopic analyses are listed in Table 2.All dates listed and displayed in figures are given with 2runcertainties.

4.2.1. Sample DZ28Most of the zircon crystals from DZ28 show simple oscillatory

zoning, however some grains appear to have distinct cores andrims in CL images (Fig. 5). For most of the crystals the laser spotwas positioned in the center of the zircon. The majority of theseanalyses yield 207Pb�/206Pb� dates between 2.2 and 2.1 Ga. Combin-ing 24 analyses from 21 different crystals yields an upper interceptwith Concordia of 2155 ± 30 Ma (MSWD = 0.6) with a poorlyconstrained lower intercept of 656 ± 470 Ma (Fig. 6A). If only theeleven concordant analyses are considered then the best estimateof the emplacement age of this granitoid is 2161 ± 14 Ma(MSWD = 0.31) (Fig. 6B). For one of the zircon crystals that ap-peared to be zoned in a CL image a rim analysis was performed.

Fig. 5. CL and back-scattered electron images of selected zircons grains analyzed from the granitoids.

6 K. Attoh et al. / Journal of African Earth Sciences 79 (2013) 1–9

The U–Pb date is concordant, within error, and yields a 206Pb�/238Udate of 524 ± 38 Ma (Fig. 6A inset). This date is considered to markthe time of new zircon growth. Our preferred interpretation is thatthe rim most likely had a magmatic, rather than metamorphic,origin because (1) it has high U/Th ratio of 1.9, whereas metamor-phic zircon often has very low U/Th ratios, and (2) the U/Th ratio isvery similar to many of the 2 Ga zircon cores from this sample.Regardless of origin, the date is thought to represent the minimumtiming of new zircon growth because small amounts of Pb�-losscannot be resolved with the current data.

4.2.2. Sample HV1Sample HV1, collected from the dioritic units, also contains zir-

con displaying oscillatory zoning. Analyses of six zircon crystalsfrom this unit have an upper intercept with Concordia of2105 ± 56 Ma (Fig. 6C) and a lower intercept within error of mod-ern Pb�-loss. If these same analyses are forced through the origin,the upper intersection with Concordia is 2102 ± 42 Ma(MSWD = 0.03). In either case, forced or unforced regression, theupper intercepts are within error of the best estimate of the crys-tallization age of DZ28.

Four additional zircon crystals analyzed from HV1 yield olderdates, ranging from 2.47 to 2.25 Ga (Table 2). These crystals areinterpreted as xenocrysts, suggesting the presence of older grani-toids beneath the exposed level of the dioritic plutons. Finally,two zircon rims on dated �2.1 Ga cores were analyzed, as was anadditional zircon core. These analyses yielded substantially

younger 206Pb�/238U dates (Table 2). If the three analyses are com-bined they yield a weighted mean 206Pb�/238U date of571.9 ± 9.7 Ma (Fig. 6D). This late Neoproterozoic date is consid-ered to reflect the timing new zircon growth.

4.2.3. Sample AH47Only five zircon analyses were obtained for sample AH47. Three

concordant zircon crystals yield a weighted mean 206Pb�/238U dateof 587 ± 9.8 Ma (Table 2). Two additional crystals yield youngerdates, but because they are not concordant, it is likely they havesuffered Pb� loss. Although the data are too sparse to view587 Ma as a robust estimate of the age of emplacement of AH47,the date is broadly consistent with other estimates for ages of unitswithin the suture zone (e.g. Affaton et al., 2000; Attoh et al., 2007and Refs. therein).

4.2.4. Sample KM1Zircons were also analyzed from the migmatite unit sample

KM1 (Table 2). Three analyses have overlapping 206Pb/238U datesproducing a weighted mean of 575 ± 9.8 Ma (Fig. 6E). However,the total range in 206Pb/238U dates is 623–521 Ma and thus a ro-bust estimate of the protolith age of the migmatite is not yet con-strained. However, the range of these dates for this unit is ingeneral agreement with the estimates of the timing of deforma-tion within the suture zone (Attoh et al., 2007 and Refs. therein).This age range also straddles the metamorphic recrystallizationage of the suture zone rocks (Attoh et al., 2007 and Refs. therein)

Fig. 6. Concordia diagrams of selected zircons: A. All zircon cores from sample DZ28 (sampled from Dzodze area, southeastern Ghana). Inset diagram: concordant age of azircon rim. B. Concordia diagram for DZ28 showing only the concordant zircon analyses. C. Concordia diagram for sample HV1 (sampled from southeastern Ghana) showingonly the 2.1 Ga zircon crystals. D. Concordia diagram showing Neoproterozoic ages of zircon rims in sample HV1. E. Concordia diagram showing the average age of575 ± 9.8 Ma obtained on zircon from the migmatite sample KM1 (sampled from southeastern Ghana).

K. Attoh et al. / Journal of African Earth Sciences 79 (2013) 1–9 7

and represent the principal evidence for Pan-African crustal pro-duction in the Dahomeyide granitoid terrane. Thus the migma-tites separate the Paleoproterozoic crust represented by the

dioritic rocks to the east from the Pan-African rocks of the WACmargin which apparently formed during Neoproterozoic Gondw-ana assembly.

Fig. 7. Granitoid crystallization ages compared with model ages in the Dahomeyide suture zone. Explanations to abbreviations and key references: PN32 and PN41 arecarbonate-rich rock and nepheline syenite samples respectively from southeastern Ghana (Nude et al., 2009). Mt Lato is an eclogite sample from the suture zone in southernTogo (Bernard-Griffiths et al., 1991). KB21 and AD18 are HP mafic granulites from southeastern Ghana (Attoh, 1998). YM13 and A2 are HP mafic granulites from the AgouIgneous Complex in southern Togo (Agbossoumondé et al., 2004). TO135 is a mafic granulite from the Kabyè massif in northern Togo (Duclaux et al., 2006).

8 K. Attoh et al. / Journal of African Earth Sciences 79 (2013) 1–9

5. Discussion

The geochemical characteristics of the granitoids summarizedin Figs. 3 and 4 represent the first of such data in the area. Allthe granitoids show negative anomalies in Sr, P, and Ti and areenriched in most lithophile elements (up to 600 times chondrite).Sample DD44, representative of the migmatites, a rock resultingfrom the remelting of WAC rocks, is depleted in Ba and Ta suggest-ing an active margin source, however, it is significantly enriched inRb, Ce, Nb which may be the result of fractional crystallization.Tonalitic granitoids DZ28 and HV1 also display negative Nb anom-alies which is consistent with either a crustal component in thesource of their magmas or that the magmas were generated in aPan-African island arc. However, the overlap of the trace elementcontents, especially REE, preclude the derivation of the migmatitesfrom the tonalitic granitoids. Thus the tonalitic, hornblende-bear-ing granitoids may represent a fragment of exotic lithospherebounded by cryptic sutures to the migmatites to the west andthe Kandi fault to the east (Fig. 1).

Previous evidence for older crust in the Dahomeyide suturezone include Nd-model ages and are summarized in Fig. 7. Forexample, eclogite from the suture zone (Mt Lato, Togo) gave a Ndmodel age of 1150 Ma (Bernard-Grifiths et al., 1991), high-pressuremafic granulites from Ghana gave model Nd ages of 940 Ma (Attohand Schmitz, 2005) as do those from Kabyè massif, Togo, whichyielded Nd-model age of 1000 Ma (Duclaux et al., 2006). TheseNd model ages may suggest the existence of widespread oldercrust in the Pan-African suture zone, but this is first robustevidence for Paleoproterozoic crust in the presumed arc terraneof the Dahomeyides in southeastern Ghana and adjoining part ofTogo. The Nd model ages likely represent mixing between juvenileca 600 Ma crust and preexisting ca 2100 Ma basement, which mayinclude the dioritic gneisses identified here. To the east of the Kan-di shear zone, the Nigeria shield is also characterized by Paleopro-terozoic crust, reworked during the Pan-African orogeny (Arthaudet al., 2008; Dada, 2008).

The Dahomeyide granitoid ages are similar to recent geochro-nological data from the correlative Medio Coreau domain of thenorthwest Borborema Province in northeast Brazil (Fetter et al.,2003; Santos et al., 2008). There, rocks of the Santa Queteria conti-nental arc record crystallization ages between 665 and 630 Ma, aresurrounded by a basement of juvenile Paleoproterozoic crust(2.1–2.3 Ga) and yield Nd model ages of ca 1.2 Ga. Lower intercept

U–Pb concordia ages and Nd mineral isochron ages of 550–570 Maall testify to Pan-African/Brasiliano collision events in theBorborema.

The presence of Paleoproterozoic crust east of the establishedDahomeyide suture zone suggests the assembly of NorthwestGondwana involved the accretion of older (pre-600 Ma) crust. Sucholder crust, it has been suggested, may be evidence for the involve-ment of the Saharan craton in the collision zone (Duclaux et al.,2006; Ngako et al., 2008). If the Saharan craton and other exoticcratonic blocks were involved in the assembly of the Dahomeyideand the adjacent Nigerian shield, it should provide further justifi-cation for correlations with the Hoggar shield as well as BorboremaProvince. In the Hoggar, the terranes of various ages are juxtaposedalong NS shear zones (Caby, 2003; Liégeois et al., 2003). Similarly,in the Borborema, Archean and Palaeproterozoic terranes are su-tured along Neoproterezoic shear zones that project into West Afri-ca. It is worthwhile to note that while both the Paleoproterozoicand the Pan-African granitoid rocks preserve arc geochemical sig-natures, it is evident that Paleoproterozoic hornblende granitoidsare not the eastern extension of the WAC basement. Rather, thefield relations suggest that the splay of Kandi shear zone betweenthe Pan-African migmatites and dioritic gneiss represents a crypticsuture similar to the terrane boundaries in the Tuareg shield (Lié-geois et al., 2003).

Acknowledgments

This study represents a product of the collective research of theauthors and their colleagues. Fulbright Scholar grant partly sup-ported this project (YA), while Department of Geology, Universityof Lomé also funded field work.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.jafrearsci.2012.09.015.

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