A new metamorphic constraint for the Eburnean orogeny from Paleoproterozoic formations of the Man...

Precambrian Research 123 (2003) 47–65

A new metamorphic constraint for the Eburnean orogeny fromPaleoproterozoic formations of the Man shield (Aribinda and

Tampelga countries, Burkina Faso)

Pierre Debata,∗, Serge Nikiémab, Alain Merciera, Martin Lompoc,Didier Béziata, François Bourgesa, Martin Roddaza, Stefano Salvia,

Francis Tollona, Urbain Wenmengaca Laboratoire de Minéralogie et Cristallographie, UMR 5563, Université Paul Sabatier,

39 Allées Jules Guesde, 31400 Toulouse Cedex, Franceb BHP-BILLITON Ltd., Cape Town, South Africa

c Département de Géologie, FAST, Université de Ouagadougou, Ouagadougou, Burkina Faso

Received 7 June 2001; accepted 18 January 2003

Abstract

Progressive metamorphic zones developed in pelites around the Aribinda granite, Burkina Faso, indicate contact metamorphismat 5–6 kbar. Biotite, chloritoid, staurolite–kyanite zones are distributed concentrically around the Aribinda pluton on its northernmargin but are displaced by later thrust faults in the south of the Aribinda area and in the nearly Tampelga area. These metamorphicand structural characteristics provide new constraints for the geodynamical evolution of the Eburnean orogeny in the North part ofthe Baoulé-Mossi domain. They show the leading part of the granitoid emplacement in the thermic structuration of the greenstonebelts and the post-metamorphic character of the tangential tectonic. In Burkina Faso, the Eburnean orogeny underwent: (1) initialgreenschist-facies metamorphism; (2) emplacement of granitoids at mid- deep-crustal levels leading to unusual medium pressuremetamorphism; (3) later thrusting.© 2003 Elsevier Science B.V. All rights reserved.

Keywords: Paleoproterozoic; Birimian; Eburnean; Staurolite–kyanite assemblage; Contact metamorphism; Burkina Faso

1. Introduction

The Paleoproterozoic Birimian Group (Junner,1954) of the Man shield (Fig. 1a) forms a major part ofthe West African Craton (Bessoles, 1977), at the east-ern and northern boundaries of the Archaean Liberiancratonic nucleus. The Birimian terranes, which makeup the Baoulé-Mossi domain, consist of narrow sed-

∗ Corresponding author. Fax:+33-5-6155-8250.E-mail address: [email protected] (P. Debat).

imentary basins and linear and arcuate volcanic beltsintruded by several generations of granitoids (Leubeet al., 1990; Pons et al., 1995; Hirdes et al., 1996;Doumbia et al., 1998). They correspond to a periodof accretion during the 2.1–2.0 Ga Eburnean orogeny(Bonhomme, 1962; Abouchami et al., 1990; Liégeoiset al., 1991; Boher et al., 1992; Taylor et al., 1992).

The lithostratigraphic succession of these Paleo-proterozoic formations, although previously much de-bated (e.g.Milési et al., 1992; Feybesse and Milési,1994; Hirdes et al., 1996; Pouclet et al., 1996) is now

0301-9268/03/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved.doi:10.1016/S0301-9268(03)00046-9

48 P. Debat et al. / Precambrian Research 123 (2003) 47–65

Fig. 1. Simplified geological map of (a) the Man shield (modified fromBessoles, 1977and Feybesse and Milesi, 1994) showing thelocation of the Djibo and the Kaya–Goren greenstone belts. (b) The Djibo greenstone belt (modified fromDucellier, 1963andHottin andOuedraogo, 1982) showing the location of the Aribinda area (A).

considered to be well established. The Birimian crusts.l. comprises the following lithologies, from bottomto top (Hirdes et al., 1996; Pouclet et al., 1996): (i) am-phibolite and gneissic rocks representing deep crustal

levels; (ii) a thick sequence of basalts, locally pillowed,as well as dolerites and gabbros, all displaying a tholei-itic composition, interlayered with immature detritalsediments and limestones; (iii) a detrital sedimentary

P. Debat et al. / Precambrian Research 123 (2003) 47–65 49

pile (volcanoclastics, turbidites, mudstones and car-bonates) including interbedded calk-alkaline volcanicsended by a coarse clastic sedimentary sequence be-longing to the Tarkwaian Group (Hirdes et al., 1996;Oberthür et al., 1998).

During the Eburnean orogeny, the volcanic andsedimentary rocks were subjected to crustal shorten-ing associated with lower greenschist-facies regionalmetamorphism (Bessoles, 1977; Liégeois et al., 1991;Taylor et al., 1992; Hirdes et al., 1996; Vidal et al.,1996; Doumbia et al., 1998; Oberthür et al., 1998;Béziat et al., 2000and others). Locally, however, ev-idence of medium amphibolite-facies metamorphismcan be observed in the vicinity of granitoid plutons,and is marked by the presence of the kyanite–stauroliteassemblage. The occurrence of this mineral pair wasfirst described byJunner (1935, 1940)in Ghana andhas been recognized within the Birimian and Tarkwa-ian metasediments in all areas of the Baoule-Mossidomain, with the exception of the Kedougou-Keniebainlier (Junner, 1935, 1940; Woodfield, 1966; Moonand Mason, 1967; Bessoles, 1977; Eisenlohr andHirdes, 1992; Ledru et al., 1994; Feybesse and Milési,1994; Nikiéma, 1992; Pons et al., 1995).

The processes by which these staurolite–kyaniteassemblages developed remain controversial.Junner(1935, 1940), Woodfield (1966), andMoon and Mason(1967)suggest that kyanite and staurolite formed bycontact metamorphism, based on the evidence thatthese minerals are localized around granitic batholiths.Pons et al. (1995)interpret that the kyanite–stauroliteassemblages resulted from deep-level metamorphism,and were subsequently dragged up with an ascendingcrystallized pluton. On the other hand,Ledru et al.(1994)andFeybesse and Milési (1994)consider thatthe late-tectonic development of staurolite and kyaniteresulted from crustal thickening associated with thrusttectonism. Thus, it appears evident that understandingthe significance of this relatively high pressure assem-blage is fundamental in the definition of the charac-teristics of the Eburnean orogeny.

This paper presents the results of a field and lab-oratory investigation in two areas of Burkina Faso(Aribinda and Tampelga countries), West Africa.We intend to determine the metamorphic process(es)leading to local development of the staurolite–kyaniteassemblage within the Paleoproterozoic greenschist-facies formations. In addition, we define the relation-

ships between metamorphic assemblages and thethrust structures present in the area. We put forwardthe hypothesis that the staurolite–kyanite assemblageresults from contact metamorphism, in agreementwith Junner (1935, 1940), Woodfield (1966), Moonand Mason (1967), Eisenlohr and Hirdes (1992), thusproviding a new insight towards the understanding ofthe Eburnean orogeny.

2. Metamorphism

The characteristics of the amphibolite-facies assem-blages, particularly of the staurolite–kyanite assem-blage, have been determined in the Aribinda area,which is part of the Djibo greenstone belt, located innorthern Burkina Faso near the Mali border (Fig. 1a).

2.1. Lithology and structure

The Djibo greenstone belt (Fig. 1b) consists ofa calk-alkaline series (ultramafics, gabbro, basalt,andesite, rhyolite) associated with pyroclastic, vol-canoclastic and sedimentary rocks (conglomer-ate, quartzite, pelite) and rimmed by granodioriticbatholiths (Ducellier, 1963). The series, which under-went greenschist-facies metamorphism, is intrudedby granodioritic and granitic plutons, including theAribinda granite (Nikiéma, 1992) (Fig. 2). TheAribinda pluton maps out as a roughly EW-trendingelliptical body (30 km long, 9 km wide) (Fig. 1b).The Aribinda pluton is an alkaline granite char-acterized by an agpaitic index varying from 0.78(Table 1, analysis 1) to 0.92 and by the presence offerro-pargasite (Table 2, analysis 1) (Leake et al.,1997) with Na + K = 1. It belongs to the alkalinegranitoid group described byDucellier (1963), whichconsist of relatively small intrusive bodies emplacedwithin Paleoproterozoic sediments across the entireMan shield (Lacroix, 1926; Bessoles, 1977; Hirdeset al., 1987; Leube et al., 1990; Léger et al., 1992;Morel and Alinat, 1993; Cheilletz et al., 1994; Ponset al., 1995; Yobou and Bonin, 1996; Doumbia et al.,1998). Isotopic data give contrasting cooling ages:Rb–Sr ages of 2081± 25 and 1968± 42 Ma were cal-culated for K-rich granitoids in Ghana (Hirdes et al.,1987), while for the Dolbel alkaline granitoid in NigerLéger et al. (1992)determined a Rb–Sr isochron at


Fig. 2. Geological map of the Aribinda area (fromNiki ema, 1992). Locations ofFigs. 3 and 9are also shown.

2137± 126 Ma. However,Bessoles (1977), Leubeet al. (1990), Taylor et al. (1992), Eisenlohr andHirdes (1992), Morel and Alinat (1993), andDoumbiaet al. (1998), consider that this alkaline series is lateto post-orogenic, thus, emplaced after the basin- andbelt-type granitoids.

The northern contact between the Aribinda plutonand the country rock is an intrusive one, whereasto the south it shows a late-stage thrust relation-ship (Nikiéma, 1992). In its northern part (Fig. 3),the Aribinda pluton is surrounded by packages ofmetagabbro and metabasalt intercalated with 10–100 m thick metasedimentary horizons (metapeliteand -conglomerate and -volcano sediments). Thiscontact is well exposed north of the village of Pem(Fig. 3 point A) where we could study a continue tra-verse across the edge of the pluton into its immediatesurrounding rocks.

Except on the peripheral zone, the Aribinda granitehas an apparent overall homogeneous and isotropicmagmatic structure. All minerals including quartz areundeformed and the biotite and amphibole-rich en-claves show subcircular to subrectangular shapes. In

the margin of the pluton (200–300 m away from thecontact) the granite shows a weak planar fabric definedby a preferred orientation of biotite flakes, amphibolelaths and by the elongation of the mafic enclaves. Nearthe contact, mineral orientation is stronger. Quartzforms large lenticular and ovoid grains with serratededges and displays undulose extinction. Most miner-als show evidence for high- to moderate-temperaturesolid-state transformations (rigid rotation of feldspars,pressure shadow zones, recrystallization features),which are recorded by the minerals as microstruc-tures such as described inPaterson et al. (1989)and Bouchez et al. (1992). In this contact zone theenclaves are strained and display oblate ellipsoidalshape. The shape of the enclaves suggests a low vis-cosity contrast with the granite and is consistent withthe plastic behaviour of the host rock at the end ofmagma crystallization or shortly after that.

A significant and progressive change in intensityand direction of deformation can be observed in thecountry rock when approaching the pluton. For in-stance (Fig. 3), 500 m away from the granite the meta-conglomerate has a foliation trending 60–70◦N and

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Table 1Representative whole-rock chemical analyses

Analysis 1 2 3 4 5 6 7 8 9 10 11

Metamorphiczone

Biotite zone Chloritoid zone Kyanite–staurolite zone

Rock type Aribindagranite

MetapeliteA16

Metavolcanosediment 9.5

Metasediment9.2

MetasedimentA11

MetabasaltA5

MetasedimentS1∗

MetasedimentS2∗

MetasedimentS4∗

MetasedimentY52∗

MetasedimentK7

SiO2 71.94 53.73 59.48 47.58 51.61 50.81 75.26 47.88 52.70 80.01 81.09Al2O3 14.36 21 14.02 28.37 21.50 14.44 13.05 30.31 33.34 13.76 15.87Fe2O3 2.51 13.68 12.69 10.31 14.20 11.28 2.71 3.38 3.51 3.19 1.75MnO 0.05 0.22 0.21 0.02 0.14 0.19 0.06 0.07 0.01 0.01 <d.l.MgO 0.53 0.79 1.75 0.27 1.94 8.47 0.45 0.48 0.10 <0.01 <d.l.CaO 1.67 0.25 6.50 0.15 0.28 10.33 0.06 0.07 0.05 0.02 <d.l.Na2O 4.28 0.51 2.83 0.82 0.47 1.87 0.06 0.58 0.84 <0.01 <d.l.K2O 3.82 5.47 0.22 7.12 4.49 0.16 4.31 9.18 3.08 0.96 <d.l.TiO2 0.26 0.98 1.14 1.33 1.33 0.66 0.19 2.27 2.62 0.36 0.42P2O5 0.06 0.21 0.28 0.10 0.15 0.15 0.02 0.04 0.02 0.04 0.09LOI 0.42 3.4 0.76 3.77 3.79 2.05 2.39 4.37 2.10 0.93 0.54

Total 99.9 100.24 99.88 99.84 99.99 100.41 98.56 98.63 98.37 99.28 99.80

1: Aribinda granite; 2–8: northern aureole of the Aribinda granite. 2: volcanic metasediment with the quartz–garnet–hastingsite–plagioclase–chlorite–biotite assemblage; 3: metasediment with thequartz–muscovite–chloritoid–garnet assemblage; 4: metasediment with the quartz–muscovite–chloritoid assemblage; 5: metabasite with the amphibole–plagioclase assemblage; 6 and 7: metasedimentwith the quartz–kyanite–muscovite–ilmenite assemblage; 8: metasediment with the quartz–muscovite–kyanite–staurolite–chloritoid assemblage; 9: metaarkose with the quartz–muscovite–kyaniteassemblage sampled at the southern aureole of the Aribinda granite; 10: metaquartzite with the quartz–kyanite–ilmenite assemblage sampled at the Tampelga area.

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Analysis 1 2 3 4 5 6 7 8 9

Mineral Amphibole Garnet Staurolite

Host rock Aribindagranite A22



MetabasiteA5

MetasedimentA21


MetasedimentA11

MetasedimentS4

MetasedimentA13

SiO2 39.65 37.23 38.16 48.21 37.15 36.91 36.78 27.91 27.89TiO2 0.54 0.42 0.14 0.11 0.03 0.08 0.05 0.58 0.55Al2O3 10.20 20.52 18.94 9.74 20.86 20.42 20.28 55.25 55.66Cr2O3 0.00 0.00 0.00 0.16 0.00 0.07 0.00FeO 25.90 22.53 24.79 13.59 24.75 34.18 34.31 13.93 11.82MnO 1.93 0.20 0.30 0.26 8.42 3.07 5.07 0.00 0.04MgO 4.43 3.32 1.86 12.45 0.18 1.39 1.05 0.72 0.45CaO 10.54 10.89 11.01 12.25 9.91 4.63 3.34 0.03 0.02Na2O 1.96 2.29 1.20 0.73 0.04 0.00 0.00 0.00 0.02K2O 1.75 0.45 0.61 0.13 0.04 0.00 0.00 0.00 0.00ZnO 0.50 1.84

Total 97.73 97.85 97.01 97.37 101.37 100.75 100.88 98.92 98.29

23 ox. 23 ox. 23 ox. 24 ox. Si+ Al = 25.53Si 6.376 5.648 5.881 6.993 5.907 5.946 5.953 7.659 7.616Ti 0.065 0.048 0.016 0.012 0.003 0.010 0.006 0.120 0.113Al 3.910 3.878 3.869 17.871 17.914Al(IV) 1.624 2.352 2.119 1.007Al(VI) 0.310 1.317 1.321 0.659Cr 0.000 0.000 0.000 0.018 0.000 0.009 0.000Fe3+ 0.212 0.578 0.590 0.272 0.287 0.104 0.125Fe2+ 3.270 2.281 2.605 1.317 3.003 4.502 4.519 3.197 2.699Mn 0.263 0.026 0.039 0.032 1.134 0.419 0.695 0.000 0.009Mg 1.062 0.751 0.427 2.691 0.042 0.334 0.253 0.294 0.183Ca 1.816 1.795 1.844 1.904 1.689 0.799 0.579 0.009 0.006Na 0.611 0.683 0.364 0.204 0.012 0.00 0.000 0.000 0.011K 0.359 0.088 0.122 0.024 0.007 0.000 0.000 0.000 0.000Zn 0.101 0.371Fe/(Fe+ Mg)

Na/(Na + K) Xan0.76 0.75 0.88 0.37 0.98 0.93 0.95 0.92 0.94

1: Amphibole from the Aribinda granite; 1 and 3: amphibole of metavolcano sediment characterized by the biotite–amphibole–garnet–plagioclase assemblage in the biotite zone; 4: metabasiteamphibole in the staurolite–kyanite zone; 5: garnet associated with biotite and plagioclase in volcanic metasediments of the biotite zone; 6: garnet associated with biotite, amphibole, andplagioclase in the volcanic metasediments of the biotite zone; 7: garnet associated with chloritoid in metasediments of the chloritoid zone; 8 and 9:staurolite associated with chloritoid andkyanite (8), and only with kyanite (9) in metasediments of the kyanite–staurolite zone; 10: chloritoid associated with staurolite and kyanite in metasediments of the staurolite–kyanite zone;11 and 12: chloritoid associated with garnet and muscovite (11) and only with muscovite (12) in metasediments of the chloritoid zone; 13: biotite associated with garnet in metasedimentsof the biotite zone; 14: biotite associated with amphibole and garnet in volcanic metasediments of the biotite zone; 15 and 16: plagioclase associated with amphibole, biotite and garnet involcanic metasediments of the biotite zone; 17: muscovite of metasediments of the biotite zone; 18 and 19: muscovite of metasediments of the kyanite–staurolite zone.


Fig. 3. Geological map (a) and cross-section (b) of the northern aureole of the Aribinda granite.

dipping 50◦NW which is defined by quartz and biotitepreferred orientation and by the elongation of the peb-bles. Near the pluton, this foliation becomes a pene-trative schistosity which rotates and becomes parallelto the limit of the granite and its planar fabric. Theschistosity, in the metasediments near the pluton, isdefined by a strong orientation of muscovite lamellaeand of quartz grains. However, the abundant kyaniteand staurolite porphyroblasts remain undeformed andunoriented. Similarly garnet and chloritoid have eu-hedral textures and contain inclusion trails of schisto-sity.

The peripheral zone of the Aribinda granite and itsimmediate country rock are characterized by the in-

crease of the plastic deformation commonly observedin granitic intrusions (Pitcher and Berger, 1972; Bate-man, 1985; Ramsay, 1989; Pons et al., 1991, 1992,1995; Nyman et al., 1995). This deformation is gen-erally interpreted to be related to the final expansion(ballooning) of the pluton (Bateman, 1985; Ramsay,1989; Pons et al., 1991, 1992, 1995).

2.2. Petrographic features of the metamorphic zones

On the basis of index-mineral distribution in themetasedimentary and, to a lesser extent, in the meta-volcanic rocks, the contact zone area of the plutoncan be divided into three zones, going towards the


Fig. 4. Chemical composition of the metasediments plotted on aHerron (1988)scheme.

granite: a biotite zone, a chloritoid zone, and astaurolite–kyanite zone (Fig. 3a and b).

The biotite zone occurs at a minimum distance ofabout 750 m from the pluton contact; its maximumextent could not be defined because of poor expo-sure. Rock types defining the biotite zone includemetapelite, metavolcano sediments and metaconglom-erate. The metapelite (Table 1, analysis 2), shale in theHerron’s classification (Fig. 4), is composed mainlyof dark-brown biotite, quartz, muscovite and chlorite(Table 3, sample A16). The metavolcano sedimentsplot in the Fe-shale field ofHerron’s classification(1988) (Fig. 4) and show high Fe2O3 and CaO con-tents (12.69 and 6.5%, respectively) (Table 1, analysis3). They are typified by the assemblage quartz, pla-

Table 3Petrography of investigated rocks

Minerals A16 9.5 A21 9.2 A11 S1 S2 S4 Y52 K7

Quartz a b a a a a b b a a

Muscovite b a a a a a a

Biotite a b a b

Chlorite b b

Plagioclase a b

Amphibole a

Epidote b

Garnet b a b

Chloritoid b b b

Staurolite a a

Kyanite a a a a a

Ilmenite b b b b b b b b

Magnetite

a Majors minerals.b Minor minerals.

gioclase, garnet, chlorite, biotite, blue-green amphi-bole (Table 3, sample 9.5) (Fig. 5a). The amphibolehas a hastingsite composition according to the clas-sification ofLeake et al. (1997)(Table 2, analyses 2and 3). Poikiloblastic garnet encloses quartz, biotiteand hastingsite (Fig. 5a), whereas randomly-orientedeuhedral hastingsite crystals overprint a preexistingfoliation defined by chlorite flakes and quartz grains(Fig. 5b). The conglomerate is composed of elon-gated quartzite pebbles (10–30 cm for the long axis)included in a matrix consisting of biotite, garnet,plagioclase (An 35–50), epidote and quartz (Table 3,sample A21). Garnet forms small euhedral grains, par-tially surrounded by small, randomly-oriented biotiteflakes.

The chloritoid zone is 80–100 m wide, and is con-stituted by intercalations of metapelite and metabasicrocks. The composition of the metapelites classi-fies them as shales inHerron’s (1988)plot (Fig. 4).In addition, they are characterized by high Fe2O3(14.2–10.31%) and K2O (4.49–7.12%) contents(Table 1, analyses 4 and 5). The metapelites are typ-ified by two assemblages (Table 3, samples 9.2 andA11): (i) muscovite, quartz, magnetite and chloritoidoccurring as small, randomly-oriented flakes gatheredin isolated ovoid spots (Fig. 5c); and (ii) muscovite,quartz, garnet, chloritoid, garnet porphyroblasts andcoarse, randomly-oriented chloritoid laths concen-trated in thin layers alternating with muscovite- andquartz-rich layers (Fig. 5d).


Fig. 5. (a) Volcanic metasediment characterized by the biotite–hastingsite–garnet (Grt)–chlorite (Chl) assemblage; (b) volcanic metasedimentwith randomly oriented porphyroblasts of hastingsite including an earlier foliation (fo); (c) metasediment characterized by the associationof muscovite (Ms) with randomly oriented chloritoid laths; (d) metasediment characterized by the chloritoid (Cld)–garnet assemblage.

The staurolite–kyanite zone is about 400 m wide.It is comprised of metabasic rocks and two metased-iments namely, metaarkose and -pelite. The metaba-sic rocks correspond to metabasalt and metagabbroshowing the characteristic calk-alkaline affinities ofthe upper Birrimian sequence (Table 1, analysis 6).Metabasic rocks show a well-developed metamor-phic assemblage consisting of randomly-orientedmagnesio-hornblende flakes (Table 2, analysis 4)(Leake et al., 1997) associated with albitized pla-gioclase, chlorite and epidote. Metaarkose (Fig. 4)outcropping as thin beds (5–15 m) has high SiO2and K2O contents (75.26 and 4.31%, respectively)and low Al2O3 (13.05%;Table 1, analysis 7). It ismade up of quartz, muscovite and kyanite but lacksstaurolite and chloritoid (Table 3, sample S1). Themetapelite (shale in Herron’s classification;Fig. 4)has high Al2O3 content (30.31–33.4%;Table 1,analyses 8 and 9) and contains the common criticalmineral assemblage (Table 3, samples S2 and S4)quartz, muscovite, kyanite, staurolite, ilmenite andtourmaline, plus chloritoid in the upper level of thezone (Fig. 6a). Textures are typically granoblastic

(Fig. 6a and b) with a fine-grained matrix made up ofquartz and muscovite enclosing randomly-distributedeuhedral porphyroblasts of staurolite, kyanite and,locally, chloritoid. Kyanite and chloritoid occur aselongated prisms while staurolite grains exhibit acoarse prismatic habit. Kyanite is intergrown withchloritoid, although more commonly with staurolitein a manner similar to that described byWenk (1980).Staurolite and chloritoid may also form intergrowths.In these rocks, kyanite, staurolite and chloritoid ap-pear to be in equilibrium without any evidence ofmutual replacement (Fig. 6a and b). Presence of theKy–St–Cld assemblage is unusual, as it is unknownin contact metamorphic aureoles. This mineral as-semblage has been observed in regional metamor-phism context (Droop, 1985; Droop and Harte, 1995;Ballèvre et al., 1989; Chauvet et al., 1992), althoughthese authors report that garnet is always present,and that either staurolite or chloritoid mostly occuras small inclusions within garnet. A stable relation-ship among these phases has also been describedfrom the chloritoid–staurolite quartzite of the Cloughformation, Moosilauke quadrangle (New Hampshire)


Fig. 6. (a) Metasediment characterized by the staurolite (St)–kyanite (Ky)–chloritoid assemblage; (b) metasediment characterized byrandomly oriented staurolite and kyanite porphyroblasts; the pattern “en faisceau” of the kyanite porphyroblasts is significative of staticcrystallization; (c) a view of the southern contact of the Aribinda pluton showing a south-dipping mylonitic foliation (mfo) developedin the kyanite-bearing quarzite which overlies the Aribinda granite (Arg); (d) mylonitic foliation developed in kyanite-bearing quartzitesampled at the southern contact of the Aribinda granite; kyanite porphyroclasts are oriented along the mylonitic surfaces.

by Rumble (1971). In this area (New Hampshire),kyanite, staurolite and chloritoid are associated withmuscovite, quartz and opaques, thus mimicking theassemblage found in the metapelite of the Aribindaaureole. The fact that chloritoid, or evidence for hispresence (e.g. replacement textures), is absent frommost of the staurolite–kyanite zone may be due to anunsuitable chemical composition of the host rock.

2.3. Mineral chemistry

Mineral compositions were analyzed on a CAMECACAMEBAX electron microprobe at the Paul SabatierUniversity (Toulouse) under the following operat-ing conditions: 15 kV accelerating voltage, 10–20 nAbeam current and 10 m beam size. Representativeanalyses are listed inTable 2; iron is expressed asFeO for the calculation of mineral formulae exceptfor garnet and amphibole, for which it is listed Fe2O3.

All ferromagnesian minerals from the metamorphicaureole are characterized by an unusually Fe-rich com-

position (Fig. 7). Garnet in biotite-bearing rocks of thebiotite zone is an almandine (∼52%) with high spes-sartine (∼20%) and grossular (∼25–28%) components(Table 2, analysis 5). Garnet in hastingsite-bearingrocks is predominantly an almandine (70–75%) rel-atively rich in grossular (17–18%), with a significant

Fig. 7. An AFM diagram showing mineral compositions of theexceptional association Ky+ St + Cld, and of a rock containingthis association (projection from Ms, Qtz and H2O in excess).


spessartine component (6–7%) and low pyrope (3%)(Table 2, analysis 6). Garnet associated with chlori-toid in metapelite is also an almandine (75%) but islower in spessartine (∼10%) and in grossular (8–10%)(Table 2, analysis 7).

Staurolite has a similar composition in chloritoid-bearing (S4) and in chloritoid-absent (A13) assem-blages, and is characterized by the same exceptionally-high Fe contents (XFe = 0.94;Table 2, analyses 8 and9, andFig. 7) (seeDeer et al., 1991, for comparison).

Chloritoid, whether in equilibrium with garnet andmuscovite in the chloritoid zone (A11) or with mus-covite alone (9.2) or with kyanite and staurolite in thehigher grade zone (S4), is characterized by a high Fecontent (XFe = 0.90; Table 3, analyses 10, 11, and12; Fig. 7).

Biotite in equilibrium with muscovite in themetapelite of the biotite zone (A16) is characterizedby anXFe = 0.75. Biotite in equilibrium with garnet(A21) is richer in Fe than that belonging to the asso-ciation hastingsite–garnet (9.5) (XFe = 0.86 against0.70, respectively;Table 2, analyses 13 and 14).

Plagioclase in the conglomerate matrix (A21) andin metavolcano sediment (9.5) of the biotite zone hasan homogeneous andesine composition (An 37–50;Table 2, analyses 15 and 16).

Muscovite, a major phase in metapelite from thechloritoid and the kyanite+ staurolite zones, con-tains a relatively high and homogeneous paragoniticcomponent throughout the zones but shows a signif-icant decrease in its celadonite content (expressed asSi/2–3) in rocks closer to the pluton, evolving fromabout 0.17 to 0.07 (Table 2, analyses 17, 18, and 19).This chemical evolution is known to be commonlyrelated to a metamorphic grade increase, and morespecifically to rising temperature (e.g.Mather, 1979;Miyashiro, 1973; Miyashiro and Shido, 1985).

Hastingsite, specific to the metavolcano sedimentsof the biotite zone (9.5), does not show importantchemical variations and is characterized by similarlyhigh XFe values (=0.75–0.88) as the associated min-erals (Table 2, analyses 2 and 3).

2.4. P–T conditions

The metapelites are nearly homogeneous in com-position across the different metamorphic zones, i.e.biotite zone (analysis 2,Table 1), chloritoid zone

(analysis 5,Table 1), and kyanite+ staurolite zone(analysis 8,Table 1), that have been mapped out inthe Aribinda pluton. The sequence of mineral assem-blages may therefore be attributed to a progressiveincrease in metamorphic grade towards the bound-ary with the granite. This inference is also supportedby the chemical evolution of the ubiquitous mus-covite, i.e. decrease in its celadonite component (seeabove). Diagnostic features for high-pressure con-ditions characterizing this metamorphic evolutioninclude absence of andalusite and sillimanite and astable coexistence of kyanite+ staurolite+ chloritoidin high-grade rocks. OverallP–T conditions can beestimated from diagnostic assemblage. However, thatestimate is limited by the following constraints:

1. No mineral reaction evidence has been observed inany rock types throughout the different zones. Fur-thermore, it is worth noting that biotite, which is acommon mineral of pelitic associations, is poorlyrepresented as a reactant or a product in metapelitesshowing chloritoid and/or staurolite–kyanite asso-ciations.

2. Existing peliticP–T grids usually depict rocks withbulk compositions characterized by Al and Fe con-tents significantly lower than those recorded in theAribinda metapelitesA ≈ 0.25–0.30 andXFe =0.67, compared toA = 0.77 andXFe = 0.9, re-spectively, whereA = (Al2O3 − 3K2O)/(Al2O3 −3K2O + FeO+ MgO) andXFe = Fe/(Fe+ Mg).

3. Lack of suitable mineral assemblages, such as inthe staurolite–kyanite zone, or unsuitable com-positions of the mineral involved (i.e. garnet toorich in Mn and Ca, or amphibole too rich in Fe)prevent from using available thermobarometricmethods.

Petrogenetic studies, integrating the effects ofminor components, especially MnO, have recently re-fined, with some controversies, the phase relations oflow to medium grade metapelite (Spear and Cheney,1989; Wang and Spear, 1991; Droop and Harte, 1995;Mahar et al., 1997). Nevertheless, these studies can-not be applied to the unusual sequence of assem-blages found in the Aribinda metapelites which, in thehigh grade rocks, contain biotite and are commonlychlorite- and garnet-free. It is well known that the Aland Fe components in pelitic bulk compositions ex-ert a major influence on mineral stability (Spear and


Cheney, 1989; Wang and Spear, 1991; Droop andHarte, 1995; Mahar et al., 1997). Notably, the partic-ular composition of the Aribinda pelites may explainwhy in the Aribinda metapelites garnet (Mn-rich) doesnot persist over the chloritoid zone, while for a com-mon pelitic bulk composition the presence of MnOusually extends the stability field of garnet-bearing as-semblages towards lower and higher grade conditions.On the other hand, the very highXFe of these rocks(cf. Table 1) in addition to high Al contents promotedthe development of chloritoid-bearing assemblages(Spear and Cheney, 1989; Droop and Harte, 1995;Mahar et al., 1997). Therefore, in our petrogeneticgrid we can only consider garnet-free assemblagesobserved in metapelite characterized by low MnOcontents (XMn < 0.006).

The associations observed in the staurolite–kyanitezone are representative of the highest grade condi-tions experienced by the metamorphic rocks aroundthe Aribinda granite. The stability of the assemblageskyanite+ staurolite+ chloritoid suggests that, for apressure in excess of 4 kbar (Fig. 8), peak temperaturewas over 520◦C (above the Cld+As → St+Qtz+Vreaction defined in the KFASH system;Guowei et al.,

Fig. 8. A P–T grid showing selected mineral reaction curves in the KFASH system (a and b bySpear and Cheney, 1989; c by Guoweiet al., 1994) and in the KFMASH system (d byGuowei et al., 1994and Mahar et al., 1997). Aluminium silicate triple point (e) afterHoldaway (1971). Dashed lines are isopleths of Si4+ content in phengites afterMassonne and Schreyer (1987). The arrow illustrates theinferred metamorphic evolution within the Aribinda aureole, going towards the pluton.

1994), but lower than 570◦C (below the reactioncurve St+ Chl+ Ms → Bt + As+ Qtz+ V; Guoweiet al., 1994; Mahar et al., 1997). White mica inmetapelites from the biotite and chloritoid zones hasSi4+ contents of 3.17 and 3.13 p.f.u. (based on 11oxygens), respectively, indicating, because of the lackof K-feldspar in the limiting assemblages, a minimumpressure of 4 kbar (Massonne and Schreyer, 1987) foran arbitrary fixed temperature range of 400–450◦C(Fig. 8). In average, the Si4+ contents of white mica(=3.11) suggest a minimum pressure of 3.5 kbar(Fig. 8), while the lowest values inferred from thepresence of kyanite are of 4 and 5 kbar for the upperand the lower parts of the same zone, respectively (seeabove).

If we assume the generally accepted temperature forthe biotite occurrence of about 370–400◦C (e.g.Ferry,1984; Yardley, 1989), we can reasonably estimate thatthe whole range of temperatures experienced by themetavolcanics and the metapelites around the Aribindagranite was ca. 370–570◦C, with pressures of about4.5–5.5 kbar (Fig. 8). These conditions underline un-usually high-pressure conditions in the metamorphicaureole of a granitic intrusion.


3. Metamorphism and thrusting

The relationships between the amphibolite-faciesmetamorphism and thrusting have been observed inrocks surrounding the southern part of the Aribindagranite and in the Tampelga area.

3.1. South Aribinda

The country rock to the south of the Aribinda gran-ite is affected by thrust faults trending 80–150◦N anddipping 20–40◦S to SW (Ducellier, 1963; Nikiéma,1992) (Fig. 3). One of these thrust faults marks thesouthern limit of the Aribinda pluton, as opposed tothe northern limit which is intrusive in nature. Fivekilometers east of the Belahouro village, this contactshows up on a small EW-trending elliptical outcropbound by south-dipping faults (Fig. 9a and b). Thedominant rock types are kyanite-bearing metaquartziteand metaconglomerate. The metaquartzite has agrainsize ranging from fine to coarse, and classi-fies as wacke inHerron’s (1988)scheme (Fig. 4).It displays similar chemical characteristics as themetaarkose outcropping in the northern Aribinda

Fig. 9. Map and cross-section of the thrust structure defining the southern contact between the Aribinda granite and its surrounding;location in Fig. 3.

aureole (Table 1, analysis 10), notably, high SiO2(80.01%) and low Al2O3 (13.76%) contents. It is es-sentially composed (Table 3, sample Y52) of quartz,muscovite, randomly distributed kyanite porphyrob-lasts and magnetite. The metaconglomerate occurs asintercalations within metaquartzite unit and is madeup of fine-grained quarzite pebbles, which measure5–10 cm and are included within a medium-grainedquartz–muscovite–kyanite matrix. Immediately tothe south of the kyanite-bearing quartzite, crop outmetasediments containing quartz, biotite, epidote,and opaque minerals. The southern extension ofthese rocks could not be defined exactly, due to poorexposure. A kilometer south of the biotite-bearingmetasediments crop out metapelite and metabasaltcontaining greenschist-facies mineral assemblages.

The contacts between the various units are markedby N-verging thrust faults. These faults caused over-thrusting of the kyanite-bearing quartzite over thegranite and of the biotite-bearing metasedimentsover the kyanite-bearing quartzite (Fig. 9b). The al-lochthonous series are characterized by a specificfoliation trending parallel to the thrust faults (Fig. 6c).Further away from the granite, this foliation is


superimposed onto the pre-existing greenschist-faciesfoliation and is axial plane to asymmetric folds thatindicate a northward displacement (Nikiéma, 1992).In the kyanite-bearing quartzites, a mylonitic folia-tion obliterates all earlier structures. This foliation isdefined by the orientation of partially recrystallizedquartz grains and of muscovite and broken kyaniteporphyroclasts (Fig. 6d). The mylonitic foliation isaccompanied by an intense N–S-trending lineationdefined by mineral constituents as well as by thelong axes of conglomerate pebbles. This stretchinglineation attitude is consistent with the N-vergence ofthrusting. Deformation intensity is heterogeneous in-side the thrust sheets. It reaches a maximum towardsthe major thrust surfaces, and particularly in the zonesof contact between quartzite and granite. This is evi-denced by tighter foliation planes and recrystallizationof quartz crystals. Planolinear structures are also welldeveloped at the top of the autochthonous granite.They are defined by high excentricity of the lenticulargrains of quartz which are entirely recrystallized andby the stretching of ferromagnesian minerals and offeldspars. These intracrystalline mylonitic structuresare low-temperature solid-state structures, accord-ing to the criteria given byPaterson et al. (1989)and Bouchez et al. (1992). They are fundamentallydifferent from the high-temperature solid-state mi-crostructures developed within the northern edge ofthe Aribinda granite.

In the thrust sheet, mylonitic foliation is deformedby small wavelength folds which are best developedin the frontal part of a nappe. These folds are asym-metrical, and have an E–W axial plane gently inclinedto the south to subhorizontal, indicating the samereverse-northward displacement as the thrust faults.

The structural evidence presented above suggeststhat, in the southern edge of the Aribinda pluton,thrusting is a late event which developed after em-placement of the granitic pluton and crystallizationof the kyanite–muscovite assemblage. It follows thatthe metamorphic event which led to the formation ofkyanite-bearing rocks is not related to thrusting.

3.2. Tampelga

In the Zitenga–Korsimoro area (50 km north ofOuagadougou, greenstone belt of Kaya–Goren, cf.Fig. 1a) around the Tampelga village (Fig. 10a), a

metasedimentary outcrop forms an ENE–SSW elon-gated shape. These rocks are surrounded by two differ-ent granitoid units: a granitoid complex (layered gra-nodiorites, tonalite, etc.) to the south and a K-feldsparmegacrysts granite to the north. The metasedimentsconsist of kyanite-bearing and muscovite-bearingunits. Kyanite-bearing rocks can be subdivided intotwo types: (i) a kyanite-bearing quartzite charac-terized by a high SiO2 content (81.09%) and noK2O (Table 1, analysis 11), and by a very simpleassemblage: quartz, kyanite, ilmenite (Table 3, sam-ple K7); (ii) a kyanite- and muscovite-bearing whitequartzite with an appreciable K2O content (2.70%).Granitoids and metasediments are associated withan imbricate fan structure defined by a series ofreverse faults trending ENE–WSW and dipping tothe SE (Fig. 10b). From SE to NW, these reversefaults caused the following imbrication of units, fromtop to bottom: granitoid complex, kyanite-bearingquartzite, muscovite-bearing quartzite, and K-feldsparmegacryst granite. This structural pattern has beendescribed byFeybesse and Milési (1994)and wasinterpreted as synchronous to the early tectonic event(D1 Eburnean deformation ofMilési et al., 1992andFeybesse and Milési, 1994). However, we do notagree with this interpretation because we considerthat, contrary to the interpretation ofFeybesse andMilési (1994), the southern granitoid complex andthe kyanite-bearing quartzite do not represent thebasement (Dabakalian Group) with respect to thelow-grade Birimian sediments. The granitoid complexwhich is emplaced over the metasediments actuallybelongs to the TTG complex defined in Ivory Coastby Doumbia et al. (1998)and dated to Birimian age(Hirdes et al., 1996; Doumbia et al., 1998). Thrustsin the Tampelga area are younger than the Birimiangranitoid emplacement. Furthermore, the metamor-phic assemblages are deformed by the mylonitic fo-liation associated with the reverse faulting; therefore,development of the Ky–St assemblage is not contem-poraneous to the tangential structures but earlier.

To summarize, both in the Tampelga and in theAribinda areas, thin-skinned thrust sheets with smalldisplacements relate to a late-tectonic tangential phasewhich has been otherwise recognized in other areasof the Man shield. In the Ivory CoastSiméon et al.(1992) ascribe it to the last event of the Eburneanorogeny. However, this interpretation is questionable,


Fig. 10. (a) Geological map of the Zitenga–Korsimoro–Tampelga area. (b) Cross-section of the thrust sheet east of the Tampelga village;the cross-section is located in a.

as it cannot be excluded that thrusting represents aPan-African structure, and thrusting affects the alka-line granites (e.g. the Aribinda granite) whose em-placement seals the Eburnean orogeny (e.g.Bessoles,1977; Taylor et al., 1992; Morel and Alinat, 1993;Doumbia et al., 1998).

4. Discussion and conclusion

Amphibolite-facies assemblages, particularly thekyanite–staurolite mineral pair, occur in Paleoprotero-zoic formations surrounding the northern part of theAribinda granite. In this area, metasedimentary units


show a succession of metamorphic zones, with no rec-ognized tectonic discontinuity between the granite andits surrounding rocks nor between the different meta-morphic zones. These amphibolite-facies assemblagesare superimposed onto regional greenschist-faciesassemblages, and define, going toward the pluton,three metamorphic zones: a biotite-, a chloritoid-,and a kyanite–staurolite-zone. The succession ofthese zones indicates a progressive and continuousmetamorphic-grade increase towards the granite.

Some rocks typically display a granoblastic tex-ture. Euhedral porphyroblasts are randomly oriented,and a number of them (biotite, hastingsite, garnet,chloritoid) overprint an earlier regional foliation. Allminerals are in mutual equilibrium, with no texturalevidence of reaction. The euhedral habit of the por-phyroblasts and the lack of reaction features suggest aquasi-instantaneous isobaric temperature increase. Asindicated in previous sections, we interpret these datato indicate development of a high pressure (5–6 kbar)contact metamorphic aureole in the northern part ofthe Aribinda granite.

Several authors have already pointed out thatamphibolite-facies assemblages developed in Paleo-proterozoic formations may have resulted from con-tact metamorphism (Junner, 1935, 1940; Woodfield,1966; Moon and Mason, 1967; Eisenlohr and Hirdes,1992). However, none of these studies includes a sys-tematic metamorphic analysis, nor defines the mineralpararageses or metamorphic zones.

Recently, John et al. (1999)described regionalamphibolite-facies metamorphism in southeasternGhana, based on mineral-chemical data and geother-mometry. They suggest that the entire Ashanti beltunderwent epidote–amphibolite-facies metamor-phism (T = 500–650◦C andP = 5–6 kbar) beforeexperiencing retrograde greenschist-facies condi-tions. This amphibolite-facies metamorphic eventaffected tholeiitic metabasite and belt granitoidsat the regional scale before being overprinted byretrograde greenschist-facies metamorphism. Theamphibolite-facies metamorphism described byJohnet al. (1999)cannot be equivalent to the Ky–St–Cldmetamorphism restricted to enclaves within grani-toids (Pons et al., 1995) or to their contact zoneswith the country rock (Junner, 1935, 1940; Moon andMason, 1967; Woodfield, 1966; this paper) inthe upper part of the Birimian pile. In addition,

this local scale metamorphic event overprints thegreenschist-facies regional metamorphism.

Kyanite-bearing assemblages unambiguously asso-ciated with contact metamorphism are rare. Accordingto Pattison and Tracy (1991), the only two occur-rences of stable kyanite related to contact metamor-phism are the Kwoiek aureole in British Columbia(Hollister, 1969a,b) and the Caplongue aureole in theFrench Massif Central (Delor et al., 1984). There-fore, the northern aureole of the Aribinda granite is,to our knowledge, the third world occurrence of ametamorphic-assemblage sequence corresponding tothe facies series of type 4 (i.e. facies where kyaniteoccurs above the staurolite isograd in the granitic aure-ole) defined byPattison and Tracy (1991). This aureoleshows close similarities to the sequence described inthe Caplongue aureole, where biotite, garnet, stauro-lite and kyanite zones developed around a granodioritepluton (Delor et al., 1984). However, in the Caplongueaureole close to the contact with the granodiorite,kyanite occurs above the staurolite isograd and nee-dles of fibrolitic sillimanite have been observed in onekyanite-zone sample, whereas in the highest-graderocks of the Aribinda aureole staurolite and kyaniteare in equilibrium and sillimanite is absent.

The relationships between metamorphism and tec-tonism have been established by linking observationsfrom both the northern and southern aureoles of theAribinda granite. To the north, the pluton forms anintrusive contact with the surrounding rocks while thesouthern surrounding of the Aribinda granite, as wellas the Tampelga area, are dominated by thrust faults.To the south of the Aribinda granite these faults tran-sect and displace the structural pattern constituted bythe granite and its aureole, and the mylonitic foliationassociated with the faults contains broken kyaniteand muscovite porphyroclasts. Amphibolite-faciesassemblages clearly overprint the regional foliationdefined by the greenschist-facies assemblages, whichoccurs as relicts and inclusion trails within chloritoid,garnet and hastingsite porphyroblasts. Thrust fault-ing unambiguously post-dates development of theamphibolite-facies assemblages and emplacement ofgranitoids.

The relationship established in this paper betweenamphibolite-grade assemblages and contact meta-morphism, as well as the definition of a chronologybetween metamorphic and structural events in two


areas of Burkina Faso, provide new constraints forthe geodynamical evolution of the North part of theBaoulé-Mossi domain during the Eburnean orogeny(2.0 Ga). Our data show that, in Burkina Faso, theKy–St assemblages developed independently to thetangential tectonic that overprinted this area, butrather from contact metamorphism. Our interpreta-tion differs from that ofFeybesse and Milési (1994)and of Ledru et al. (1994), who suggest that kyan-ite development in the Paleoproterozoic formationsof the whole Baoulé-Mossi domain is a result ofprograde regional metamorphism under increasingpressure associated with progressive crustal thick-ening (D1 thrust phase ofFeybesse and Milési,1994; Ledru et al., 1994; Milési et al., 1992). On theother hand, our interpretation is close to that ofPonset al. (1995)considering that, in the North part ofthe Baoulé-Mossi domain (North and Central Burk-ina Faso and also Niger), the granite emplacementstake a prominent part in the tectonic and Sthermicstructuration of the Birimian crust. In this region, thegeodynamical context appears dominated by juve-nile granitic magmatic activity and shows, as alreadypointed out byVidal et al. (1996)andDoumbia et al.(1998) in Ivory Coast, close similarities to Archaeangeodynamical environments, where staurolite andkyanite-bearing assemblages commonly occur aroundgranite batholiths (e.g.Bouhallier et al., 1993; Collinsand Van Kranendonk, 1999).

Acknowledgements

This study was supported by the Campus project“Evolution crustale et minéralisations aurifères dansle Birimien du Burkina Faso.” The authors gratefullyacknoledge Pierre Toé (Resolute West Africa Ltd.)for his help in the field and Paule Eychenne for thedrafting. We thank M. Ballèvre for the references heprovided us. This manuscript benefited from a con-structive review by Simon Harley. Reviews by A.Kröener et S. Muhango are also acknowledged.

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A new metamorphic constraint for the Eburnean orogeny from Paleoproterozoic formations of the Man...

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