intrusive rocks and tectono-metamorphic evolution of the mako paleoproterozoic belt (eastern...

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Journal of African Earth Sciences 50 (2008) 88–110

Intrusive rocks and tectono-metamorphic evolution of the MakoPaleoproterozoic belt (Eastern Senegal, West Africa)

Mamadou Gueye a,*, Papa Malick Ngom b, Mouhamadane Diene a, Yaouba Thiam a,Siegfried Siegesmund c, Klaus Wemmer c, Sabine Pawlig d

a Institut des Sciences de la Terre, Universite Cheikh Anta Diop, BP 16508, Dakar-Fann, Senegalb Departement de Geologie, Universite Cheikh Anta Diop Dakar, Senegal

c Geoscience Center, Georg-August University, Goldschmidtstras. 3, 37077 Gottingen, Germanyd Nu Instruments Limited, Unit 74, Clywedog Rd South, Wrexham Industrial Estate, Wrexham LL13 9XS, United Kingdom

Received 1 April 2006; received in revised form 14 February 2007; accepted 13 September 2007Available online 26 October 2007

Abstract

The Kedougou Kenieba Inlier (KKI) (Paleoproterozoic of Eastern Senegal) is a portion of the West African Craton (WAC) contain-ing a granite-greenstone terrain that experienced three distinct periods of magmatic activity, peaking at 2200, 2160–2130 and 2100–2070 Ma. In the Inlier, Paleoproterozoic granitoids and large-scale transcurrent shear zones are spatially associated, suggesting a geneticlink between magma bodies and shear zones. Granitic intrusions are associated with all the volcanic episodes and phases of deformation,and have been used to constrain the age of many of these events. Our structural data and deformation sequence indicate that the MakoGreenstone Belt and the adjacent granitoid complexes have undergone a multi-phase evolutionary history that is spread over a prolongedperiod. The available geochronological data and field studies allowed classification of the granitoids of the KKI into four generations GI,GII, GIII and GIV.

The current data suggest that the oldest rocks in the KKI, the Badon granites (2198 ± 2 Ma) and the tonalitic gneisses from Tonkouto(2200–2198 Ma) (GI), could be correlated with an early Birimian magmatic event. The gneisses, crystallized at depth, record the earliestdeformation and in contrast to other tonalites, do not appear to have intruded volcanic rocks. The second manifestation of magmatismwas intrusion of mafic diorite – the Gabbro Sandikounda Layered Igneous Complex type (GII) and development of the Laminia Kaou-rou Plutonic Complex (LKPC) (2160–2130 Ma). These bodies pre-date or are sometimes synchronous with a major deformational epi-sode, and may, therefore, have formed very early in convergent Birimian orogenesis.

The third major peak of magmatic activity occurred after the above major episode with the development of the oval shaped Diom-balou and Bouroumbourou plutons (GIII). The orientation of these plutons parallel to the regional strike of the schistosity indicatesstructural control on granite emplacement. Eburnean magmatism was terminated in the Mako Belt following compressional Eburneandeformation, with the emplacement of the Tinkoto, Mamakono plutons (GIV) in the east of the complex and continued in the Diale–Dalema supergroup with the syntectonic emplacement of the Saraya batholith. Garnitiferous granites of crustal derivation wereemplaced in the final period of extensional activity around 2080 Ma.

Field observations suggest the early plutons of the complex granitic (Kakadian) batholith intruded during convergent deformationwhereas later igneous activity accompanied regional orogen-parallel extension, followed by exhumation. In the Mako Belt, thickeningof the crust was proposed to have caused heating and the ‘apparent diapiric rise’ of the Diombalou and Bouroumbourou plutons.� 2007 Elsevier Ltd. All rights reserved.

Keywords: West Africa; Senegal; Paleoproterozoic; Mako belt; Granitoids

1464-343X/$ - see front matter � 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.jafrearsci.2007.09.013

* Corresponding author.E-mail address: [email protected] (M. Gueye).

mailto:[email protected]

M. Gueye et al. / Journal of African Earth Sciences 50 (2008) 88–110 89

1. Introduction

Paleoproterozoic Birimian terrains form a major part ofthe West African Craton (WAC) (Bessoles, 1977) (Fig. 1)and are coeval with the emplacement of large granitoid plu-tons (Leube et al., 1990; Cheilletz et al., 1994; Pons et al.,1995). Plutonic igneous rocks comprise one third of theexposed rocks in the WAC. Although these rocks havebeen subject to few detailed investigations, a large litholog-ical range is recognised, including texturally diversetonalite to granodiorite, gabbro to gabbronorite, musco-vite-bearing plutons, associated with migmatites, and dis-

Fig. 1. The Birimian terrane of the West African Craton (WAC) with the posi(2003). (1) Limit of the WAC. (2) Post-Paleozoic cover. (3) Late ProterozoiPaleoproterozoic basement. Hatched zone with diagonal lines indicated the po

tinctive granitic plutons (Boher et al., 1992; Hirdes et al.,1996; Pons et al., 1992, 1995; Lompo et al., 1995; Ludtkeet al., 1999; Egal et al., 2002; Hirdes and Davis, 2002).

In SE Senegal, the relationships between linear to arcu-ate greenstone belts, plutonic belts and surrounding gneis-sic, amphibolitic terranes, that built a part of theKedougou Kenieba Inlier (KKI) is poorly understood. Anumber of workers have focussed on the geochemical andgeochronological evolution of the volcanic and plutonicbelts (Dia et al., 1997; Gueye et al., in press; Pawliget al., 2006). However, the overall relationship betweenthe different plutonic rocks, and their significance in terms

tion of the Kedougou-Kenieba Inlier (KKI). Modified after Gasquet et al.c and Paleozoic. (4) Pan-African and Hercynian belts. (5) Archean andsition of Archean domains.

90 M. Gueye et al. / Journal of African Earth Sciences 50 (2008) 88–110

of the wider tectonomagmatic evolution of the KKI, arepoorly understood. These terranes are considered to havebeen formed by lateral crustal growth through processesof arc and (or) terrane accretion during the Eburnean oro-genic event, where granitic magmatism also played animportant role (Abouchami, 1990; Dia, 1988; Dia et al.,1997).

In SE Senegal the Kakadian batholith provides a uniqueopportunity to study the emplacement of granitic magmasat different crustal levels. Therefore, this paper aims to givethe first complete characterization and synthesis of theigneous components of the Mako belt and to place theminto a coherent geological and chronological framework.This not only provides insight into the early geologicaldevelopment of southeastern Senegal, but also is essentialfor a full appreciation of the evolution of the Eburneanorogeny.

In a broader sense, this paper presents a case study ofthe interaction between deformation, metamorphism andplutonism in the Paleoproterozoic (Eburnean) granite-greenstone terrains of SE Senegal. Our study is focussedon the Mako supergroup and the surrounding gneissicamphibolitic basement.

2. Geological setting

The Kedougou-Kenieba-Inlier (KKI) in SE Senegal is aportion of the West African Craton. Defined by Bassot(1966), the KKI is bordered on its western side by a Panaf-rican belt and in the east it is covered by Phanerozoic sed-iments of the Taoudeni basin. The inlier, attributed to be aBirimian terrane, is composed of granitic gneiss terranesseparated by linear belts (greenstone belts) of metasedi-mentary and metavolcanic material accreted during theEburnean orogeny (at ca 2.0 Ga). The Birimian terraneformed during a time interval of 2.25–2.0 Ga (Tayloret al., 1988, 1992; Abouchami et al., 1990; Liegeois et al.,1991; Hirdes et al., 1992) and represents a major Paleopro-terozoic juvenile crust-forming event. This granite-green-stone assemblage of the KKI, dated between 2213 and2198 Ma, was intruded by granitoids yielding ages between2160 and 2070 Ma (Dia et al., 1997; Gueye et al., in press;Pawlig et al., 2006). Some of the granitoids exhibit a rangeof deformation states from undeformed to highly sheared,and they are locally partially melted (Gueye et al., in press).The geological evolution of the KKI was establishedthrough structural and geochronological studies (Bassot,1966; Ledru et al., 1989a; Milesi et al., 1991; Hirdes andDavis, 2002).

The structural evolution of the Paleoproterozoic in WestAfrica is interpreted by different authors as monocyclic,based on a geosynclinal model or a Phanerozoic plate-tec-tonic model (Bassot, 1966; Leube et al., 1990; Abouchamiet al., 1990). For others, the evolution is polycyclic (Ledruet al., 1989a, 1991; Milesi et al., 1992) and is characterizedby a collision phase (D1) followed by transcurrent phases,D2 and D3.

Geochronological data show ages ranging from 2.3 to1.9 Ga for these units (Bassot and Caen-Vachette, 1984;Dia et al., 1997; Hirdes and Davis, 2002), most of whichare intruded by Paleoproterozoic granites that have beendated between 2.2 and 1.9 Ga (Gueye et al., in press; Paw-lig et al., 2006). All rocks have been affected by medium tolow-grade metamorphism. Indeed higher-grade metamor-phic rocks can be found locally, such as the Sonfara-San-dikounda amphibolitic-gneissic complex and aroundsome plutons. Late tectonic cooling is evidenced by a num-ber of K–Ar and Ar–Ar ages, ranging from ca. 2051 to2030 Ma (Gueye et al., in press). From recent geochemicalstudies Abouchami (1990), Abouchami et al. (1990), Boher(1991) Boher et al. (1992) and Pawlig et al. (2006) considerthat there has been no contamination of the Birimian rocksof the KKI by an Archean basement.

2.1. KKI volcanic belt and volcano-sedimentary basin

Paleoproterozoic rocks of the KKI are divided into theMako supergroup in the West and the Diale–Dalemasupergroups in the East (Bassot, 1966; Bassot, 1987)(Fig. 2). The lowermost unit is the Mako supergroup, com-prising mafic–ultramafic and felsic volcanic rocks intrudedby granitoids that form the Kakadian batholith. The volca-nic packages and the granitoids are interpreted to haveisland arc affinities on the basis of geochemical and petro-logical constraints (Dia, 1988; Dia et al., 1997; Diallo,1994; Diallo et al., 1993; Pawlig et al., 2006).

A regional crustal scale shear zone, the Main Transcur-rent Shear Zone (MTZ) with NE–SW trend and rotatedinto a north direction as it crosses the Faleme river intoMali (Milesi et al., 1989a,b) usually follows the lithologicalcontact between the Mako and the Diale–Dalema super-groups. The lithostratigraphy of the KKI is discussedbelow, based on the work of Bassot (1966), Bertrandet al. (1989), Milesi et al. (1992) and Ledru et al. (1989b).According to the classical sequence of the greenstone beltrecently indicate by geochemical data, Bassot (1987), Dia(1988), Ngom (1989) and Abouchami et al. (1990) put thevolcanic series in a lower position in reference to theArchean greenstone belts assemblages, which are appar-ently very similar.

This classical lithological arrangement, containing a vol-canic sequence (Mako supergroup) at the bottom and vol-canosedimentary to sedimentary rocks (Diale–Dalemasupergroups) at the top, with associated granitic-gneissterranes, is a typical Archean greenstone belt assemblage.However, Milesi et al. (1989a,b) and Ledru et al. (1989b)while agreeing on the polycyclic character, invert the suc-cession; in their view the sedimentary series underlie thevolcanic series, which is younger.

2.1.1. The Mako volcanic beltThe NW–SE-trending Mako volcanic belt is located in the

northwestern part of the KKI, west of the MTZ, andcontains a series of volcanic, volcaniclastic and igneous

Fig. 2. The simplified geological map of the Kedougou-Keniba Inlier (KKI) (modified after Ledru et al., 1991). (A) Mako plutonic belt: 1 –, 2 –, 3 –, 4 –and 5 – granitoids. (B) Mako greenstone belt: 6 – basalts, 7 – andesites, 8 – and 9 – volcanosediments and sediments, 10 – Diale–Dalema-Dalemasupergroup, 11 – undifferentiated Paleozoic and Neoproterozoic, 11 – and 12 – fault; dashed line Senegal – Mali border.


assemblages assigned to the Mako supergroup (Bassot,1966). Massive basalt flows outcrop in the southern partsof Mako, over pillowed basalts. Their thickness is less thanthe submarine flows. The rock displays an amygdaloidalstructure filled by quartz with chlorite and epidote. The lavas

are associated with abundant volcanic agglomerates, brec-cias and banded tuffs at the top of the sequence. The maficvolcanism of the Mako supergroup is tholeiitic; the pillowedbasalts have NMORB affinities whereas the massive basaltshave an enriched MORB composition (Ngom, 1995).


The metavolcanic rocks are basalts to rhyolites of volca-nic arc affinities, intruded by deformed, dioritic to granodio-ritic plutons ranging in age from 2198 to 2130 Ma (Gueyeet al., in press; Dia et al., 1997). Late felsic stocks, like theTinkoto and the Mamakono plutons are present and yieldeda Pb–Pb age of around 2080–2090 Ma (Hirdes and Davis,2002; Gueye et al., in press). This belt is dominantly charac-terized by folded low-grade metamorphic rocks intruded byvarious granitic and mafic rocks. It comprises NE-trendinggreenstone belts whose shape is at least in part controlledby a network of anastomosing N- to NE-trending linea-ments, shear zones and plutonic belts forming the largeKakadian Batholith. These form the boundaries of the rockunits and divide them into corridors or domains. The MakoBelt is composed close to its western side by metamorphosedthick-pillowed tholeiitic basalts with a well-preserved struc-ture, and mafic and ultramafic rocks.

The Kakadian batholith (KB) is located in the westernpart of the Mako greenstone belt. It extends almost120 km from Badon in the south to Kakadian in the northand is described by Bassot (1966), Witschard (1965) asbeing composed of numerous plutons, and provides a unu-sual opportunity to study the emplacement of granitic mag-mas at different crustal levels. Due to its elongate shape, themain body of the Kakadian batholith has been suggestedto be syntectonically emplaced during strike-slip tectonismby Bassot (1966).

2.1.2. The volcanosedimentary basin

The Diale–Dalema sedimentary basin is intruded in itscentre by the plutonic complex of Saraya and the plutonsof Balangouma and Boboti (Ndiaye et al., 1997). TheDalema and the Diale series comprise (Bassot, 1966,1987) a detrital sedimentary pile (quartz-bearing wackes,turbidites, argillite and carbonate) interbeded with calcal-kaline volcanics. All the sedimentary rocks are isoclinallyfolded. Folds are upright or slightly overturned to theSE. The Saraya batholith, interpreted to be syntectonic,is generally foliated and medium-grained and consistsof several coalescent biotite-bearing plutons (Ponset al., 1992).

2.1.3. Intrusive rocks of the northern part of the KKI

The Mako supergroup is intruded by the 120 km longKakadian batholith (KB) and small plutons. The KB, orig-inally mapped by Bassot (1966), is a composite body con-sisting of granitic-dioritic rocks, layered complexes, andgranodiorite with MFK, gabbros, granodiorite to monzog-ranite enclosing a variety of mafic enclaves and dykes.Recent field mapping and associated petrological studies(Dia et al., 1997; Dioh, 1991; Gueye, 1995) have led tothe recognition of two main comagmatic plutonic suitesin the northern area (Fig. 3):

– an early phase of differentiated trondhjemitic calc-alka-line plutonism represented by the Sandikounda LayeredPlutonic Complex (SLPC);

– the Laminia Kaourou Plutonic Complex (LKPC) thatconsists of granodioritic, tonalitic and adamelitic phases.Gueye (2001) indicates that the margins of the LKPC aredeformed by the Leoba Moussala Fault (LMF), whichextends several hundred meters into the pluton. Recentresearch by Gueye et al. (in press) shows that the KBresults from successive magmatic pulses where the olderone encompasses the Badon granodiorite. The geochem-istry of the intrusive rocks of the Mako Belt indicates thatthey are derived from an amphibolite source, possibly bypartial melting of basalts (Pawlig et al., 2006).

Using petrographical and geochemical constraints, Diohet al. (2006) distinguished:

– a layered Plutonic Complex consisting of sodic calcalkalinegranitoids, defining a trend from Opx-Cpx- and amphi-bole-bearing layered gabbros to amphibole-bearing tona-lite; the latter shows similarities with adakite-like rocks;

– amphibole bearing granitoids with TTG features, occur-ring as composite batholiths and as isolated oval-shapedplutons cross-cutting the entire Birimian formation;

– potassic granitoids defining a trend from Opx-, Cpx-bear-ing gabbros to amphibole bearing monzodiorite, and pref-erentially emplaced within calcalkaline metavolcanites;

– peraluminous biotite-bearing granitoids emplacedwithin the metasediments of the upper part of the Biri-mian lithostratigraphic pile.

2.1.4. Sonfara-sandikounda intrusive rocks

The middle part of the Kakadian batholith was mapped indetail and renamed by Dia (1988), Dia et al. (1997), Dioh(1995) and Dioh et al. (2006), as being composed of volcanicxenoliths, metamorphic and plutonic complexes. Systematicgeological mapping supported by geochronological, isoto-pic, and geochemical data (Dia et al., 1997; Dia, 1988) hasled to the characterization of the Sandikounda Amphib-ologneisse Complex SAG and the Sandikounda LayeredPlutonic Complex (SLPC) (Fig. 3). The latter was describedby Dia et al. (1997) as an early phase of trondhjemitic pluto-nism, that contains mafic to ultramafic rocks (wherlite topyroxenite, layered hornblende gabbros, diorite and hornb-lendite), felsic rocks (quartz diorite and tonalite), and minormicrodioritic and microgabbroic dykes.

The older rocks from this area consist mainly of inter-layered mafic dioritic and tonalitic gneisses, migmatite,amphibolite associated with mafic to ultramafic bodies.These bodies include peridotites and pyroxenites associatedwith sheet-like gabbros. Amphibolites, gneisses and perido-tites (wherlite) occur within the layered sequence as bothdiscrete bands, presumably representing deformed xeno-liths, and as more equant xenoliths. Diorite and trondhje-mite veins intrude all rock types in this layered sequence.

2.1.5. The Sandikounda Layered Complex

We distinguished three two main units in this complex:

Fig. 3. Geological sketch map of the Sandikounda area showing the main rock units: 1 – and 2 – Sandikounda Amphibologneissic Complex (SAG); 3 –and 4 – Sandikounda Layered Plutonic Complex (SLPC); 5 – metavolcanic rocks; 6 – and 7 – Laminia and Kaourou plutons (LKPC); 8 – undifferentiatedPaleozoic and Neoproterozoic; 9 – strike slip fault; 10 – thrust fault; 11 – lineation.


– The Tonkouto unit

Located West of Sandikounda, and consists mainlyof interlayered diorite and gabbros associated withnumerous large mafic to ultramafic bodies. These

mafic bodies include wherlite, pyroxenite and gab-bros, with preserved igneous textures. Wherlite xeno-liths are sometimes observed within the gabbro sillsand show evidence of competent behaviour.


– The Guandamaka unit

Rocks from this unit are composed of quartz diorite,tonalite, and gabbros that show evidence for a submag-matic deformation. This shows that deformation contin-ued in the SLPC after crystallization of the sheets toproduce the sub-solidus, mylonitic overprint, whichresulted in the recrystallized asymmetric tails of theK-feldspar phenocrysts.

2.1.6. The Sandikounda amphibologneissic complex

Country rocks in the Sandikounda area include narrowbands of metamorphic and ultramafic lithologies. The maincomponent is banded grey gneisses dated between 2200 and2194 Ma (Pb/Pb ages; Dia et al., 1997; Gueye et al., in press).

– The banded gneisses

The oldest recognised components of the Sandikoundaamphibologneissic complex comprise banded grey, dio-ritic and tonalitic gneisses, with a Pb–Pb crystallizationage of around 2.2 Ga providing a minimum age for thegneissic fabric (Dia et al., 1997; Gueye et al., in press).Diorite and gabbro bodies from the SLPC intrude thebanded gneisses. Concordant with respect to the gneissicbanding, millimetre scale leucogranitoid sheets, ofunknown age, have become an integral component ofthe gneisses. In places, both the banded gneisses andthe leucogranitoids are migmatised.

– Sonfara unitDetailed mapping in the western SPLC (Dia, 1988; Dioh,1986; Gueye et al., this work) revealed trains of 500 m-scale amphibolite xenoliths. The typical amphibolite islight grey, fine- to medium-grained, and has a metamor-phic compositional banding exemplified by the alterna-tion of millimetre- to meter-wide melanocratic bands,and millimetre- to centimetre-wide leucocratic bands.White Plagioclase+Quartz-rich leucosomes with the dar-ker melanosome layers give the rock its migmatiticappearance. The melanocratic bands have a tonalitic com-position, and their major minerals are hornblende with,sometimes, diopside, saussuritized oligoclase and quartz,with subordinate biotite. Hornblende is commonly trans-formed into biotite and titanite. Zircon, apatite, epidote,titanite, and locally microcline are accessory, whereaschlorite, sericite, and clinozoisite are secondary. The leuc-ocratic bands are made up of albite and quartz, with minorbiotite and zircon, and locally microcline. They are com-monly overprinted by a faint cross-cutting foliation givenby the orientation of biotite. Discordant leucosomes alsoappear and have the appearance of dykes and veins thathave been injected into the amphibolite. The proportionsof plagioclase/quartz in these leucosomes range from 50–65:50–35, characteristic of trondhjemite. Dia et al. (1997)consider these trondhjemitic leucocratic bands to reflectdifferentiated liquid from the SLPC.

2.1.7. Badon plutonic rocks

The southern margin of the largest exposed mass of theKakadian batholith is situated around Badon, where a dis-

tinctive range of igneous rocks, from pyroxenite and gab-bro to diorite, granodiorite, outcropping for �6 km, ishere named the Badon Igneous Complex. The easternand southern contacts of this assemblage are exposedagainst metavolcanic-pillowed rocks. Biotite-rich grey rockis predominant and is composed primarily of granodiorite.The dominant phase in the Badon granodiorite is a sphene-bearing, felsic porphyry. Subordinate phases of medium-grained pink-grey granodiorite occupy approximatelyequal areas and are located toward the core of the complexand its northeastern margin. Some schlieren are also pres-ent and consist of high concentrations of biotite. Diorite,minor amounts of gabbros, and other mafic rocks typesform the western margin of the complex.

Some magmatic breccias are locally developed at themargin of the diorite. The contact between the biotitegranites and the diorite was not observed in the field.However, the less-evolved hornblende-bearing granodio-rite of Soukourtou is restricted to the border of the plu-ton and is considered to be the younger phase within theBadon pluton. The Badon granodiorite is geochemicallyand texturally distinct from other intrusive rocks of theKKI. A light grey rock type, at �76% SiO2 and�1.24% CaO is significantly more acid than the otheranalysed rocks of the KKI. Radiometric dating of thegranodiorite at 2198 ± 2 Ma, Pb–Pb zircon (Gueyeet al., in press), permits interpretation of the Badongranodiorite as an early Birimian intrusive phase.

2.1.8. The Laminia Kaourou plutonic complex (LKPC)The LKPC east of Sandikounda, one of the largest

Eburnean calc-alkaline granite plutons of the Mako belt,has been previously mapped by Dia (1988) and Gueye(1995). This complex, roughly elongated in shape anddated at 2140 Ma by the Pb/Pb method (Dia et al.,1997) was emplaced in a sinistral transpressional N-trending shear zone, as indicated by the structural studiesof Gueye (2001). This complex is divided into two mainplutons:

– The Kaourou pluton is located in the NW part of theKakadian (Fig. 3). The main mass of the pluton occu-pies an area of about 200 km2. Xenoliths of wall rockmaterial are widely preserved throughout the plutonand particularly abundant close to the western contact.It is a calc-alkaline granite which is heterogeneouslydeformed. Some of these granites are gneissose withlocally well-developed mylonitic zones. In the Kaourouarea the LKPC granites are spatially associated withthe Leoba Moussala Shear Zone (LMFZ) (Fig. 3).

– The Laminia pluton is composed primarily of tonaliteand granodiorite with minor amounts of diorite, gabbro,hornblendite, aplite and pegmatite. It is a composite plu-ton in that mafic compositions, predominantly tonaliteand minor amounts of gabbros, diorite and other maficrocks types, form the margins of the pluton with grano-diorite occupying the core and northwestern margin. A


zone of mingling is often present between the granodio-rite and tonalite. Mafic dykes are present in the aureoleof the Laminia pluton just east of the LMFZ fault.

2.1.9. Bouroumbourou pluton

This body is a concordant, ovoid/elongated north-southintrusion (Fig. 4) and is composed of three major facies: amedium- to coarse-grained granite that occurs on the wes-tern and northeastern margins of the body; medium- tocoarse-grained biotite granite with a fine-grained porphy-ritic marginal phase that constitutes most of the pluton;fine-grained mafic rock that occurs as a narrow dyke-likeintrusion in its western margin, parallel to its E–W axis.The major phase of the Bouroumbourou pluton is med-ium-grained granodiorite to monzogranite composedessentially of plagioclase, quartz, K-feldspar, hornblende,biotite, titanite and magnetite with accessory apatite andzircon. Quartz is strained and partially recrystallized, withcrystals and crystal aggregates commonly elongated paral-lel to the dominant foliation. Poikilitic K-feldspar crystals(up to 12 mm across, square to rectangular), enclosingeuhedral quartz, plagioclase laths, biotite and hornblendeare also typical. Dykes of aplite and pegmatite, 4 cm to3 m thick, cross-cut the pluton.

2.1.10. Diombalou pluton

The pluton occupies an area of about 17 km2 and has aregionally NW–SE trending ellipsoidal dome shape(Fig. 5). The Diombalou pluton is composed dominantlyof granodiorite that ranges in texture from equigranularto porphyritic, the latter being characterized by K-feldsparmantled by oligoclase; rare clinopyroxene occurs as iso-lated grains and as cores in amphibole crystals, whilesphene, magnetite, aplite, zircon and allanite are the acces-sory phases. The intrusion is treated here as a single plutonalthough there are different units that probably representdistinct intrusive events. Pegmatitic dykes are abundantalong the western pluton margin but are generally rare inthe pluton interior.

2.1.11. Birmassou-Tomoromadji pluton

This body is a concordant elongated north-south intru-sion (Fig. 5). The rock is foliated and inequigranular, withgneissic borders containing schlieren and pegmatites and aporphyritic core, suggesting a zoned intrusion. The Birmas-sou-Tomoromadji pluton is the largest granitoid bodyinside the greenstone belt (60 km � 10 km). The plutonconsists mostly of granodiorite with subordinate diorite,tonalite and marginal migmatite.

2.1.12. Sanssakhoto pluton

The pluton is roughly concentric; gabbros dominate inthe western part, whereas acid rocks such as monzograniteprevail in the outer areas. Petrographic studies from Dioh(1995) and fabric studies (this paper) have evidenced a pro-gressive reverse zonation of the pluton, from Hb-Biomonzogranite to gabbros. The mafic bodies within the

Sanssakhoto pluton consist of fine-grained homogeneousleucogabbro. Mafic phases include biotite (10–25%), horn-blende (up to 20%) and clinopyroxene (up to 5%). Disequi-librium textures include ilmenite mantled by titanite,clinopyroxene mantled by biotite and hornblende, andstrongly myrmekitic alkali feldspar at the margins of pla-gioclase phenocrysts.

2.1.13. Kenieba pluton

This small pluton displays a very high shape ratio(Fig. 5). The pluton was emplaced late and the high aspectratio was essentially developed during emplacement. TheKenieba pluton is 1 km long and 500 m wide. It is a homo-geneous biotite-amphibole granite and also has a pro-nounced NE–SW elongation. It is surrounded by volcanicand volcaniclastic rocks that form a narrow corridor sepa-rating it from the Diombalou pluton. The Kenieba plutonalso contains two major facies: a coarse-grained, equigran-ular to porphyritic granite that occurs on the northeasternborder; fine- to medium-grained, equigranular tonalite inthe southwestern portions of the pluton.

2.1.14. Late kinematic plutons

Toward the end of the Eburnean event (2090–2070; afterGueye et al., in press), numerous basic, intermediate to acidcalcalkaline plutons intruded the Mako belt forming circularto elliptic bodies, each covering some 50–150 km2. Theseoften-unfoliated crosscutting plutons, nearly circular inshape, such as Dioudioukonko, Mamakono, Tinkoto, Sou-kouta, Diakhali and Koulountou, have traditionally beenconsidered post-tectonic (Bassot, 1966; Witschard, 1965).Pb–Pb in zircon age dating of granitoids from the associationrange from 2074 ± 5 Ma (Hirdes and Davis, 2002) to 2080(Gueye et al., in press). These ages are in broad agreementwith field observations and textures. These plutons associ-ated with the supracrustal sequences of the Mako Belt, ingeneral are small, rounded to elliptical granitic stocks. Someof them are not represented in Fig. 2 due to scale limitations.The largest of these bodies is the Dioudioukonko Granite,which crops out in the southeastern segment of the MakoBelt, near the village of Koulountou.

– Mamakono granodiorite

It is a small stock (1 km2) of medium-grained, homoge-neous, biotite-amphibole granodiorite intruded into vol-canic and volcanosedimentary rocks in the vicinity ofMamakono village. The Mamakono granodiorite showsa prevailing porphyritic character. Plagioclase, ortho-clase, quartz, biotite and hornblende form phenocrystsin a microgranular mass of quartz, K-feldspar and bio-tite, with grain sizes of 0.05 ± 0.25 mm. Plagioclase phe-nocrysts occur, with a well-developed zonal structure.Biotite shows syngenitic rutile needles, zircon inclusions,and is sometimes altered partly to chlorite. Incipientsubstitution by calcite may be observed locally. Ortho-clase occurs as small crystals, frequently with a perthitictexture.

Fig. 4. Geological sketch map of the Bouroumbourou area showing the main rock units 1 – metavolcanic rocks, 2 – Badon intrusive rocks 3 – andesites,4 –, 5 – and 6 – volcanosediments and sediments 7 – and 8 – granitoids, 9 – thrust fault, 10 – strike slip fault.


– Tinkoto plutonThe Tinkoto pluton is made up of massive monzodiorit-ic to monzogranitic rocks in the SE and granodioriticrocks in the NE. Xenoliths of country rocks are presentand the country rock has been invaded by both concor-

dant and discordant aplitic pegmatite and monzodioriticdykes. This pluton is texturally heterogeneous, variesfrom coarse to medium-grained, and has an uneven dis-tribution of poikilitic K-feldspar phenocrysts and vari-ability in biotite content. The granite is either equant

Fig. 5. Geological sketch map of the Diombalou area showing the main rock units: 1 – Badon intrusive rocks, 2 – andesites, 3 – volcanosediments andsediments 4 –, 5 – and 6 – granitoids, 8 – thrust fault, 9 – strike slip fault, 10 – lineation, 11 – pegmatite.


to slightly oriented (discrete magmatic fabric), or showsa commonly visible foliation, especially near its margins.The main components are K-feldspar, quartz and pla-gioclase, with biotite as a minor component, and zirconand apatite as accessories.

– Dioudioukonko pluton

The oval-shaped, N–S-elongated Dioudioukonko plu-ton covers an area of approximately 8 km2. The ovoidshape of the pluton probably results from emplacementin a zone where the influence of the regional stress was


lowered. The composition of the pluton is unknown,because of the limited outcrops in such a tropical zone.

3. Structural and metamorphic data

3.1. Deformation within the country rocks

3.1.1. Deformation in the country rock adjacent to the

Bouroumbourou granite

East of Bouroumbourou only a relatively thin amphib-olite facies aureole grades southwards into an actinoliticchlorite schist. In this zone, mica schists with andalusiteporphyroblasts are consistent with contact metamorphismrelated to the widespread intrusion of plutonic rocks inthe Bouroumbourou. These mica schists preserve evidenceof a continuous sequence of porphyroblast growth and foli-ation development (Fig. 6A), consistent with relatively lowsyntectonic emplacement pressure. Outside the aureole ofthe pluton, metamorphism reached biotite or chloritegrade.

3.1.2. Deformation in the country rock adjacent to the

Diombalou granite

The country rocks include migmatite, mica schist, para-and orthogneisses, usually displaying a gently dippingmetamorphic foliation. Contact metamorphism within thewestern margin is defined by the assemblage staurolite-kya-nite and kyanite-sillimanite in the mica schist. Sillimaniteoccurs as fibrolitic needles developed close to staurolite(Fig. 6B and C). However in the eastern margin, we haveobserved the appearance of garnet and biotite. The meta-morphic porphyroblasts overgrow a penetrative schistosityand are themselves rotated and sometimes deformed by anoverprint fabric, which developed synchronously with por-phyroblast growth. Garnet shows variable kinematic rela-tionships to the fabric (i.e. syn- to post-tectonic). Thepresence of kyanite in these mica schists suggests that therocks were metamorphosed under much higher-pressureconditions than the rocks from the Bouroumbourou aure-ole. Outside the aureole of the pluton, metamorphismreached biotite or chlorite grade. Granodiorite borderphases of the pluton contain igneous epidote consideredby various authors (Zen and Hammarstrom, 1984; Sialet al., 1999) to be formed at high pressure.

3.2. Deformation within the granitoids

3.2.1. Sonfara-Sandikounda intrusive rocks

– In the Tonkouto area the main magmatic foliation in thegabbroic rocks wraps around the rigid mafic blocks.Mafic xenoliths are also asymmetrically boudinaged inthe plane of the foliation (Fig. 7A). At another locality,rocks are composed of gabbro sheets and have a strongshape-preferred alignment of plagioclase, suggestingmagmatic flow, while the asymmetric recrystallization

of the grain boundaries indicates that non-coaxial defor-mation continued acting upon the sheets under sub-sol-idus conditions.These zones of heterogeneouscentimetre- to metre-scale sheeted magma are inter-preted to represent high-level, syn-magmatic shearzones. Evidence for the syn-magmatic nature of theshear zones includes imbricated and asymmetricallyrotated centimetre-scale ultramafic xenoliths that areenveloped by leucogranite sheets that show no signifi-cant internal strain (Fig. 7B). The trains of imbricatedgneissic xenoliths within the leucogranite sheets areinterpreted to have undergone rotation during non-coaxial flow, to become aligned perpendicular to thelocal NNW-directed shortening direction (e.g. Hanmerand Passchier, 1991; Simpson, 1998).It is proposed thatthe gabbro sheets were formed by the incremental injec-tion of magmas into active sinistral shear zones. Magmawas sheared during laminar flow to produce the sheets,that are aligned sub-parallel to the ENE-trending shearzone boundary. Close to the contact with the volcanicrocks, the fault is intruded by 15–20 cm wide ultramaficdykes (Fig. 7C). Many of the dykes are strongly asym-metrically boudinaged, suggesting that the basic magmaintruded during sinistral strike-slip faulting.

– The diorite from Sandikounda, located east of the Ton-kouto area, shows only a locally well-developed mag-matic foliation. Indeed, some folded microdioriticdykes and trondhjemitic veins are found in the contactzone. Several observations when taken together suggestthat the Sandikounda Layered Plutonic Complex(SLPC) was emplaced syntectonically with respect todeformation in its wall rocks. Internal shears areintruded by ultramafic rocks, gabbro and diorite, whichare themselves sheared, suggesting a syntectonic originof the gabbro-diorite complex. An important structuralfeature in the wall rocks is decimetre- to metre-scaleextensional shear bands, which are widespread through-out the complex. They are NS-trending, steeply dippingon outcrop scale, cut across the layers and show a dex-tral sense of movement (Fig. 7D). The shear bands areloci for melt segregation of trondhjemitic composition.A systematic investigation of microstructure in a samplefrom the tonalite of the Sandikounda area shows thatthe rocks preserve evidence of a continuum of deforma-tion from magmatic to high temperature solid-state.Solid-state deformation and gneissification occurredunder high temperature conditions as demonstrated bythe co-stability of hornblende and biotite within the foli-ation and C-planes.

– The tonalitic gneisses generally display a well developedtectonic foliation that forms parallel and locally obli-quely to the early igneous layering of the gneiss, andwhich dips moderately to the SW (Fig. 7E and F). Thelayer/foliation obliquity represents a form of hypersoli-dus S–C fabric such that the layer represents the shearplane whereas the foliation represents flattening planes(e.g. Pawley and Collins, 2002). The foliation plane is


commonly defined by elongate quartz grains and feld-spar aggregates, but also by inequidimensional high-grade metamorphic minerals (Opx-Cpx-Hbl-Bio)(Fig. 8A). The presence of metamorphic Opx in the ton-alitic gneiss indicates that the regional foliation devel-oped during granulite facies metamorphism (Fig. 8B).Locally plastic deformation of plagioclase occurs (bend-ing of the twin planes and undulatory extinction) butthere is no evidence for dynamic recrystallization or neo-crystallization, i.e. nucleation and growth of new grainsof different composition.The leucocratic component ofthe gneiss shows textures that are indicative of partialmelting. These include tiny cuspate intergrowths at theedge of some plagioclase crystals (Fig. 8C), developmentof rounded quartz grains in plagioclase, indenting rela-tionships between quartz and feldspar grains, lobategrain boundaries between quartz and plagioclase(Fig. 8D–F). Cuspate boundaries between quartz andfeldspars have previously been interpreted as the resultof diffusion creep (Grower and Simpson, 1992) inamphibolite facies granodioritic gneisses, and, by Mart-elet et al. (1999) in granulite-facies rocks.These olderintrusive sequences had to have been deformed priorto emplacement of the SLPC. However outcrop scaletrondhjemitic veins, some of which may be related tothese younger plutons, are generally sub-parallel to foli-ation layering.

Fig. 6. (A) Sigmoidal andalousite porphyroclast (and) indicating sinistral shear(C) metamorphic foliation defined by Kyanite (Ky), Biotite (Bi), Staurotide (S

– In the Sonfara area deformation in the migmatiteamphibolite zone, including foliation development andfolding of that foliation, occurred during peak metamor-phism and partial melting. Close to Sonfara the structuralpattern is characterized by a constant west-orientedsteeply dipping and ENE-trending foliation, bearing asubhorizontal to locally oblique lineation. An importantstructural feature in the amphibolite is decimetre- tometre-scale shear bands, which are widespread through-out the amphibolite complex. They occur as a NNE–SSW-trending band on outcrop-scale and cut acrossthe foliation (Fig. 9A and B). Close to the west of theSonfara unit, where the foliation is weakly dipping,shear bands dip to the WNW, with displacementsindicating layer-parallel compression (Fig. 9C). Earlyleucocratic bands are folded and boudinaged (Fig. 9C)whereas trondhjemitic sheets show asymmetrical folds(Fig. 9A and B) that occurred syntectonically.

The presence of trondhjemitic leucosome along axialplanes and hinge zones of folds (Fig. 9D), within shearbands in boudin necks and in numerous sills parallel tothe country rock foliation, is also consistent with the pres-ence and migration of magma during deformation (Collinsand Sawyer, 1996; Davidson et al., 1996; Brown and Rush-mer, 1997; Brown and Solar, 1998). Isoclinaly folded andboudinaged trondhjemitic sheets (Fig. 9B) suggest that

ing with new biotite and muscovite growth in the pressure shadow; (B) andt).


deformation continued after the SLPC crystallized (e.g.Rothstein et al., 1994). Macroscopic structures show evi-dence for progressive truncation of the earlier formedsheet. In Fig. 9B sheets show no evidence for foliationdevelopment and therefore were partly crystalline duringrotation. The folds also indicate a rheological contrastbetween a rigid block and a more ductile host (e.g. VanDen Driessche and Burg, 1987), suggesting the granitesheets were partly crystalline during shearing. The minorsinistral shear zone in Fig. 9B is interpreted as a high strainband that developed during fold tightening (e.g. Van DenDriessche and Burg, 1987; Pawley and Collins, 2002). Thisdemonstrates that sheeting occurred by repeated injectionof small magma increments into active dextral (Sonfara)and sinistral (Tonkouto) conjugate shear zones.

Fig. 7. (A) Mafic xenoliths that have been ‘‘wrapped” by granitic sheets durigneiss xenoliths rotated into sinistral sense of shear (SLPC). (C) Asymmetric bto have migrated along the tectonic foliation and into a dextral shear zone (SLdecimetre scale compositional banding made of interlayering of dioritic and tonand foliation in the gneiss (La: layering).

Within the trondhjemitic sheets, a range of microstruc-tures provides evidence for deformation during cooling ofthe SLPC. They contain randomly oriented biotite blades,‘‘checkerboard” subgrain boundaries, undulose extinction,and kink-bands in plagioclase (Fig. 10A and B). Indeed,microscopic structures from the early leucocratic bandsshow evidence for partial melting (Fig. 10C and D).

3.2.2. Badon intrusive rocks

Close to the west of Badon village, abrupt and grada-tional contacts between the diorite and fine-grained mar-ginal phase, and metabasalts have been observed,associated with mingling and mixing of the two intrusivephases. Rounded dioritic enclaves, fluidal textures and asso-ciated intragranitic shear planes occur in the fine-grained

ng apparent anticlockwise rotation (SLPK). (B) Tilted angular ultramaficoudins structure formed by ultramafic dykes (SLPC). (D) Melt interpretedPC). (E) Banded gneiss (Tonkouto area): the sequence shows a centimetre-alitic bands. (F) Stereographic projection, lower hemisphere of the banding

Fig. 8. (A) Photomicrograph illustrating the tectonic foliation in the tonalitic gneiss of Sandikounda defined by the alignment of orthopyroxene (Opx),plagioclase and annealed quartz. The presence of metamorphic Opx defining the foliation indicates that it formed at granulite facies conditions. (B)Alignment of Opx-Pl-Qz-Cpx. The presence of Opx and Cpx defining the foliation indicate that it formed at granulite facies conditions. (C) and (D) Smallcuspate extension of feldspar grain developed along quartz–quartz grain boundaries (arrow in D) with low dihedral angle. (E) and (F) Quartz-plagioclase-Hornblende and clinopyroxene border-mobility indicative of partial melting (apparent indenting).


marginal phase and suggest liquid-liquid interaction of twodifferent magmas during commingling and shearing. Theobliquity of the intramagmatic parallel shear plane indi-cates a dextral sense of movement (Fig. 11A).

In contrast, the presence of in situ melted anatectic gran-ite at both the pluton contact where anatectic melt is inter-foliated, folded and boudinaged with the basaltic rocks(Fig. 11B) indicates that intrusion of the Soukourtougranodiorite occurred within the ductile domain of the con-tinental crust.

3.2.3. The Laminia Kaourou plutonic complex (LKPC)

The main structural element in the LKPC is a NNW–SSE trending foliation. It is well developed in the Kaouroumonzogranite but absent in the Laminia granodiorite and

tonalite. Deformation of the LKPC is heterogeneous, withlow strain domains (well preserved magmatic textures) andorthogneissic domains displaying a N–S trending mainlysubvertical foliation, associated with subhorizontal linea-tion and shear zones.

The pluton has a well developed magmatic–submagmat-ic fabric (Fig. 12A), which is subparallel to variably ori-ented Paleoproterozoic country rock foliations and shearzones, indicating that pluton emplacement was stronglyinfluenced by crustal anisotropy. The pluton-core is freeof solid-state deformation but displays evidence of pre-fulldeformation (Gueye, 2001). Close to its boundary there isevidence of a faint solid-state deformation and shear zonedevelopment. Orthogneissification appears along theeastern margin of the Kaourou pluton. In these zones,

Fig. 9. (A) and (B) Development of patterns of asymmetric fold-like structures: (A) deformed agmatite of amphibolite in gneiss nearly horizontal sectionparallel to XZ. Leucocratic vein deflected asymmetrically around a mafic xenolith. The asymmetry gives a dextral sense of movement. The shearing of theamphibolite and exploitation by leucogranite dykes, indicating strong interaction between intrusion and deformation; (B) folding and shearing oftrondhjemitic veins; (C) amphibolite-layered gneiss: the layers are crossed by shear zone Sheared and boudinaged veins. NW-dipping migmatitic shearbands cutting across amphibolite of the Sonfara area; and (D) leucosome in the axial surfaces of asymmetrical folds.


solid-state deformation and gneissification occurred underHT conditions as demonstrated by the co-stability of horn-blende and biotite within the foliation and C-plane(Fig. 12B).

Mafic dykes are present in the aureole of the Laminiapluton, just at the contact with the Kaourou pluton andoften intrude parallel to the foliation. Some leucogranitedykes and sills are parallel or oblique to the foliation,and are sometimes folded along with the country rock foli-ation; other granite sills are boudinaged. Two feldspar-quartz leucocratic segregations occur as thin planar segre-gations that are subparallel to the axial surface folds, oras infilling between boudins. Microscopic observationsconfirm that folding of the country rocks and pluton con-tact occurred during emplacement. Segregations are associ-ated with folding and are aligned largely within the XY

plane of the strain ellipsoid (Fig. 12C).The relative rarity of solid-state fabrics in the Laminia

pluton appears to indicate that deformation was weak inthose domains, and that strain was partitioned into faultzones and country rocks fabrics and/or that the pluton hadnot crystallized to a point below the critical melt fraction(Arzi, 1978) where the crystal framework would recordsolid-state deformation. Microstructures within the aureolesuggest that the pluton intruded during fault movement(Fig. 12C).

3.2.4. Bouroumbourou

The Bouroumbourou pluton intrudes the LKPC to thewest (Fig. 4). The textures of the intrusive rocks in the Bou-roumbourou pluton are distinguished from the LKPC,which shows a well-developed solid-state deformation closeto the contact.

Some aplitic dykes that cross-cut the granite and folia-tion show some microstructural evidence of ‘‘solid-state”

deformation. The solid-state signatures within the apliticphases become a submagmatic fabric in the granite. Sub-magmatic foliation and lineation are defined by alignedplagioclase, hornblende, and biotite. The Bouroumbouroupluton is bounded by strike-slip shear zones involving thedevelopment of submagmatic shear zones, but preservesgently to moderately dipping magmatic foliation awayfrom the shear zone. The presence of magmatic and sub-magmatic fabrics in the Bouroumbourou pluton supportsemplacement during deformation (Fig. 12D).

3.2.5. Diombalou

An intense foliation is developed in all phases except in ahomogeneous granitic variety, and is orientated subparallelto the pluton margin (Fig. 5). A dyke complex consisting ofvariably mylonitized microdioritic to undeformed apliticdykes was emplaced parallel to or slightly oblique to thefoliation.

Fig. 10. (A) Layer quartz grain displaying a chessboard extinction pattern. (B) Interstitial xenomorphic texture of quartz around euhedral plagioclase. (C)K-feldspar melt pools at quartz grain junctions. (D) Photomicrograph showing quartz grains with interstitial feldspar at quartz triple grain junctions.

Fig. 11. Microfabics of the Badon Granodiorite, crossed polarizers, Plagioclase (Pl), quartz (Qz), feldspar (Kf), biotite (Bi). (A) Melt filled shear zone inthe Badon Intrusive Complex. Intragranitic shears are oblique to the shear zone. (B) Isoclinaly folded early granitic dyke in the Badon Intrusive Complex.


The late homogeneous granite that cross-cuts the folia-tion indicates that solid state deformation predates totalrecrystallization of the pluton. Magmatic shear zones arepresent at the margin but did not evolve to a mylonitic belt.The magmatic nature of the microfabric indicates that thedeformation of the magma took place while it was notentirely crystallized, and that there was no significantpost-emplacement deformation (Fig. 13A and B).

The pluton is intruded by dyke-like to elliptical bodies ofleucogranite. The presence of muscovite-biotite as well asgarnet for the granitic suite suggests deviation from a crus-tal and/or sedimentary source, supporting late-tectonic

emplacement. The geochemical signature of the leucogra-nites attests to crustal partial melting.

Our combined structural and petrographic investiga-tions suggest that the Diombalou pluton intruded syntec-tonically into the northern branch of the LMFZ, withsinistral strike-slip kinematics. Magmatic lineations nearthe border of the pluton are parallel to stretching lineationsof the mylonitic border zone, which supports the interfer-ence of emplacement during strike-slip shearing of theLMFZ.

The deep emplacement of the Diombalou pluton isattested to by the lack of grain size reduction in the granite

Fig. 12. (A) Well-defined foliation is underlined by preferred orientation of K-feldspar megacrysts, enclaves and aplitic dykes. (B) Photomicrograph ofmylonite with hornblende porphyroclast. Note plagioclase grains with bookshelf structure indicative of dextral shearing. Dominant shear bands withstable hornblende are dextral. (C) Deformed tonalitic veins during dextral shearing: tonalitic melt infill discrete extensional structures. (D) High-temperature submagmatic shearing at the border of the Bouroumbourou pluton. Noted the presence of ductilely deformed hornblende.


near its margins, and is in agreement with the absence oflow T deformation features in the neighbouring countryrocks. Instead, the presence of in situ melted anatecticgranite (leucogranite) at the pluton contact indicates thatintrusion occurred within the ductile domain of the conti-nental crust.

3.2.6. Birmassou-Tomoromadji pluton

The main structural element in this pluton is a N–S sub-vertical foliation. Folded and sheared xenoliths are com-mon in the granodiorite near Tomoromadji village. Someof them contain an intense foliation that is continuous withthat in the surrounding granodiorite (Fig. 13C). Well-developed pervasive foliation is defined by elongate, recrys-tallized, centimetre-scale quartz aggregates. Within themonzogranite, a range of microstructures provides evi-dence for high temperature deformation. Quartz preserves‘‘checkerboard” subgrain boundaries indicating basal (hci)and prismatic slip (Kruhl, 1996).

Tonalitic rocks in the core of the pluton show a well-pre-served N–S-trending magmatic foliation. Evidence of mag-matic flow includes imbrications (tiling) of euhedral biotitecrystals that are not internally deformed (Fig. 13D).

In the margin of the pluton the typical phase assem-blages are Act + Chl + Pl ± Bt ± Ep ± Mt ± Qtz inmetabasites, and Qtz + Ms + Bt in metapelites, indicativeof low-grade metamorphic conditions. This indicates that

both the pluton and its country rock were overprinted bya post-emplacement deformation event.

3.2.7. Sanssakhoto pluton

The foliation is defined in the gabbroic rocks by plagio-clase-rich layers. A gradational contact between the gab-bros and the granodiorite of the LKPC exists and islocated at the northern margin. In this zone, hybrid rocksresulting from commingling of basic and acid magma arewidespread. The mingling features between granodioriteand gabbros are observed in a shear zone linking thesemafic rocks.

The preferred orientation of the plagioclase in the gab-bros was not developed during a solid-state deformation,but resulted from rigid-body rotation in response to flowof the magma. On the basis of experimental work and fieldobservations, such orientation can be used as a shear senseindicator (Fernandez et al., 1983; Blummenfeld and Bou-chez, 1988). In our case the observed asymmetry indicatesa sinistral sense of transport, in agreement with the postu-lated large scale shear sense direction.

3.2.8. Kenieba

A pervasive solid-state deformation resulted in thedevelopment of S/C mylonites. A magmatic fabric is pre-served locally. Alkali-feldspar phenocrysts are abundant,whereas the groundmass includes quartz, alkali-feldspar,

Fig. 13. (A) Intragranular fracturing of hornblende (Hb) preceded the crystallization of quartz (Q) and plagioclase (Pl). (B) Interstitial nature ofhornblende grains in hornblende rich aggregate. (C) Desegregations of xenolith materials. Xenoliths contain folded and sheared granitic veins that areclearly connected to the host granodiorite, indicating that folding and dextral shearing are synmagmatic. (D) Tiling of biotite crystals.


biotite, and hematite, with minor plagioclase, amphibole,magnetite, epidote, chlorite, titanite, and zircon. Quartzoccurs interstitially, is sometimes recrystallized, and formsrare myrmekitic intergrowths with alkali-feldspar.

3.2.9. Late kinematic plutons

– Mamakono

Quartz crystals are usually xenomorphic with develop-ment of a strong chessboard texture. Amphibole is repre-sented by green hornblende crystals, replaced in parts bychlorite. Biotite is sometimes kinked and shows fishforms. Other microstructures indicate that the foliatedgranitoid rocks experienced heterogeneous deformation.Quartz commonly shows development of blocky sub-grains with evidence of both c-slip and a-slip. The generallack of elongation of quartz constitutes evidence for onlyminor solid-state strain as a result of this slip. In places,plagioclase grains show undulose extinction, deforma-tion bands, tapered deformation twins, and bent crystals,all characteristic of ductile deformation, along with somebrittle microfracturing. The microcline grains have cross-hatch twinning, and some development of flame perthite.The micas define the plane of foliation, and show someevidence of ductile deformation in the form of kinksand bent crystals, and the development of mica fish.

– Tinkoto

Quartz shows sometimes a well-developed chessboardpattern, indicating strain under high-grade conditions.

Other quartz grains are flattened to produce quartz rib-bons with irregular, serrated edges. Grain boundaries inquartz are strongly lobate due to a high amount ofgrain-boundary migration recrystallization. Quartzforms rare myrmekitic intergrowths with alkali-feldspar.Potassic feldspars appear as megacrystic and shownumerous inclusions of biotite and plagioclase, andperthitic inclusion. Plagioclase defines, with biotite, aweak shape preferred orientation (SPO). A few biotitegrains have been retrogressed to chlorite, but other fea-tures of low-grade retrogression are absent. Non-coaxialmagmatic deformation is attested to by imbrications ofbiotite and plagioclase (Paterson et al., 1989; Fernandezet al., 1983; Blummenfeld and Bouchez, 1988). Quartzand K-feldspar occur as anhedral interstitials in theframework of plagioclase laths and biotite.

4. Plutonism and deformation

4.1. Structural and magmatic interpretation: synchroneity of

plutonism and deformation

The Sandikounda Gneiss Complex in the Eburnean oro-genic terrane of Western Africa provides an example of theclose links between metamorphism, regional-scale defor-mation, and granite generation and emplacement.

The layered intrusion in the Tonkouto area is interpretedto have formed by the injection of discrete magma pulses


into active high temperature shear zones. These magmapulses forming the SLPC were entrained by laminar flowto form sheets that are aligned sub-parallel to the shear zoneboundary. The non-coaxial flow is indicated by the com-mon asymmetric structures, such as the imbricated androtated ultramafic xenoliths. However, melt-filled exten-sional shear zones in the SLPC may also mark a period oflate stage extension, which could have facilitated the accu-mulation of magma in the middle crust during contraction.

Several observations, when taken together, suggest thatthe SLPC was emplaced syntectonically with respect todeformation in its wall rocks. Emplacement of the SLPCis interpreted as being coeval with an episode of sinistraldisplacement parallel to the current strike of the high-grademetamorphic foliation (i.e. ENE–WSW-trending), proba-bly predating the development of the shear zone network.

The SLPC, emplaced at 2160 Ma (Dia et al., 1997), canbe used as a time marker to constrain the evolution of theMako Belt. Ar–Ar and K–Ar hornblende age data fromthe tonalitic gneisses (Gueye et al., in press) show thatthe SLPC provided sufficient heat for the observed migma-tization. The deformation within the main shear zone ofthe Sandikounda area was coeval with partial melting ofgneisses and metavolcanic lithologies of the country rocks.

A number of features imply that shearing was broadlysynchronous with the emplacement of the late trondhjemit-ic phase of the SLPC.

The internal structures of syntectonic intrusions are bestexhibited by the LKPC, which shows an outward progres-sion from magmatic to solid-state mylonitic fabrics. Mag-matic foliation and lineation in the core of such plutonsare well defined by an alignment of tabular euhedral pla-gioclase and alkali-feldspar crystals, and individual lathsof aggregates of biotite and hornblende. Quartz-filledmicrofractures and serrate boundaries of plagioclase grainsare likely to reflect fracturing and abrasion between grains,respectively, at the submagmatic stage (Bouchez et al.,1992; Gueye, 2001). The plutonic complex was emplacedalong older Paleoproterozoic structures, which show reac-tivation. Emplacement of the Laminia granite occurredwithin the transitional crust, which marks the boundarybetween the older basaltic crust and the young ‘‘supracru-stal crust”. Based on both magmatic and solid-state struc-tures and kinematic features from the aureole, we infer thatemplacement of these plutons occured within the NNW-SSE contractional setting.

The LKPC is cut in its northern and eastern parts by theDiombalou, Sanssankhoto and Bouroumbourou plutons,which show a well-developed magmatic, submagmatic foli-ation, well developed metamorphic contact aureole, andhigh temperature shear zones. The plutons of Diombalouand Bouroumbourou show a dome form associated withmarginal syncline. In the Diombalou and Dioubeba area,a last magmatic event is characterized by the intrusion ofmuscovite garnet pegmatite and leucogranite.

In contrast, granites within the structurally higher levelsappear to have been emplaced tarditectonically with

respect to deformation in the greenschist country rocks.The granites are mainly undeformed although weakpre-full-crystallization fabrics are present locally. Theseplutons of the late syn-compressional association are wide-spread across the Mako Belt and encompass the Tinkoto,Mamakono and Koulountou magma types. These late plu-tons separate and cross-cut the tectonic fabrics. They lackpervasive tectonic fabrics and have unmodified igneous tex-tures; records of minor deformational features may besuperimposed by features from a late deformation eventonly (mainly intense jointing and localised high-strainzones).

With the new data we can differentiate between:

– A pre-tectonic GI group of granites (Badon IntrusiveComplex, Banded gneisses, probably pre-Eburnean)(e.g. Gueye et al., in press); the oldest type (GI) is madeup of TTGs that form oval shaped structures or bandedxenoliths, elongated NE-SW to ENE–WSW. Thesegranites have low- to high-K calc-alkaline compositions.

– A set of syn-tectonic Paleoproterozoic groups (GII:SLPC, LKPC, Tomoromadji-Birmassou) and (GIII:Diombalou, Bouroumbourou and Sanssankhoto) asso-ciated with a different period of Eburnean igneous activ-ity. Pervasive high temperature shear bands developedin GII and GIII granites suggest synmagmatic deforma-tion and they intrude into volcanic, volcanodetritic andplutonic rocks (Fig. 3). Within the GIII type, magmaticstructures are still preserved and their shapes are slightlyelongated when compared with G2. They form dome/oval shaped structures.

– A late group, GIV, corresponding to the small intru-sions. The plutons are fresh, undeformed (only igneousfoliations), discordant to regional fabrics and invadehighly foliated greenstone belt lithologies, in some casesdisplaying narrow contact metamorphic aureoles. Theiremplacement does not seem to be separated by a largetime gap from the syn-tectonic group.

The first stage produced the GII and GIII granites,related to the greenstone belt volcanism, but a short timelater, a second phase of intrusive activity created morepotassic granitic rocks and other intrusions ranging fromtonalite, granodiorite to granite (GIV: Mamakono, Dio-udioukonko), which do not show any or only a few defor-mational characteristics of their host rocks.

The syn-tectonic plutonic group involves two types ofgranites (GII and GIII), with ages varying from around2160 Ga to 2105 Ga (Dia et al., 1997; Gueye et al., inpress), which are thought to have intruded during regionaldeformation. The geometry and kinematics of synmagmat-ic deformation show that the SLPC was emplaced whilstthe crust was undergoing WNW-ESE directed extensionat 2160 Ma (Gueye et al., in preparation). It can be envis-aged that the SLPC was emplaced as a series of sheets thatinterfingered into its wall rocks whilst they were undergo-ing apparently WNW–ESE extension. The NNE–SSW


compression event, which we will name D2 in accordancewith the classical terminology, is undoubtedly responsiblefor the map-scale structures. These ages, which fit withthe period of crustal thickening of the Eburnean belt pro-vide a base for the dating of D2 in the KKI. During theD2 event large amounts of magma were transferredthrough the crust, related to strain heterogeneities duringtranspression.

Differentiation of continental crust in the KKI is furtherachieved by emplacement of monzogranitic to granodio-ritic calcalkaline plutons (LKPC) intruding the volcanicsequence of the belt, respectively at 2130 and 2105 Ma.The late evolution of the plutonic complex is marked bythe emplacement at 2080 Ma of highly potassic granitoids,with a geochemical signature indicating an enrichment ofthe magmatic source, and suggesting thickening of thecrust.

5. Interpretation and discussion

Paleoproterozoic plutons are important as evolutionarymarkers, providing means of constraining the time ofregional tectonism in KKI. In West Africa, the majorgranitoids are divided into two groups: the ‘‘belt-type”,now interpreted as pre/syn-orogenic bodies (2108–2190 Ma) and the ‘‘basin-type”, which is younger, withages ranging from 2080–2110 Ma (Hirdes et al., 1992;Davis et al., 1994; Hirdes and Davis, 2002). This orogenicclassification is related to the Eburnean Orogeny. Theseauthors limit the intrusive event in this region within therange 2120–2094 Ma. This time gap seems similar to thatin the KKI (2127–2074 Ma; Dia et al., 1997; Gueye et al.,in press).

The Paleoproterozoic domains of West Africa have beeninterpreted to be a consequence of interference folding, plu-ton ballooning or a combination of both processes.According to Cheilletz et al. (1994), in southern Niger, gra-nitic plutons emplaced in the Birimian greenstones are inthe age range 2158–2115 Ma (U–Pb and Rb–Sr ages) andare syn-D2. In the Ghana Belt, the granitoids all showU–Pb ages in the range 2170–2180 Ma (Hirdes et al.,1992). In northern Ivory Coast, Doumbia et al. (1998)define two generations of granite:

– the first generation of granitoid (2123–2108 Ma)intruded and metamorphosed the greenstone belts;

– the second generation (2108–2097 Ma) intruded andmetamorphosed the sedimentary pile and subordinatevolcanic rocks.

In the Dabakala area, Gasquet et al. (2003) describedfour syntectonic types of granitoids (2160 Ma age ofemplacement of the Dabakala tonalite). These granitoidsdisplay deformation patterns typical of an interferencebetween transpressional tectonics and pluton emplacement,as reported in this study, and by others, in Niger (Ponset al., 1995), or Burkina Faso (Lompo et al., 1995). This

corresponds to the characteristics of the D2 tectonic phasereported in the literature.

The period between 2170 and 2160 Ma seems to havebeen an important period of granite formation and trans-pressional tectonics.

Our structural data and deformation sequence indicatethat the Mako Greenstone Belt and the adjacent granitoidcomplexes have undergone a multi-phase evolutionary his-tory that is spread over a prolonged period.

The first manifestation of more silicic magmatism wasintrusion of granite (Badon), mafic-layered diorite-tonalite(Sandikounda)(GI). A recent geochemical study of Pawliget al. (2006) shows TDM ages that provide further evidencefor an early Birimian magmatic phase at around 2.3 Ga.The Badon and Sandikounda intrusive bodies pre-datethe major D2 deformational episode and the developmentof the SLPC and LKPC (GII), and may, therefore, haveformed very early in the convergent Birimian orogeny.Coeval mafic magmatism is indicated by the swarmed igne-ous-textured mafic enclaves in SLPC types and possibly bythe presence of mafic to ultramafic rocks at Tonkouto. Arecent study of Naba et al. (2004) in Burkina Faso suggeststhat the 2.2 Ga TTGs, which form most of the Birimianterrains in West Africa, were rapidly cooled and reacheda brittle state before being passively intruded by a new gen-eration of granites.

The second major peak of magmatic activity occurredafter D2 with the development of the oval-shaped Diomba-lou and Bouroumbourou plutons (GIII). The orientationof these plutons parallel to the regional strike of the schis-tosity indicates structural control on granitic emplacement.Eburnean magmatism ceased in the Mako Belt followingcompressional Eburnean deformation with the emplace-ment of the Tinkoto and Mamakono plutons (GIV) inthe east of the complex. These include contemporaneousgabbros, which extend under the Diale Basin. Such mag-mas are thought to have intruded in a relaxational orextensional phase following crustal thickening.

The larger number of plutons resolved in the Mako Beltdefines many petrographically and geochemically distinctmagma types. We propose the term Mako Plutonic Beltinstead of Kakadian Batholith.

The tectonic framework in the Sandikounda area duringthe intrusion of the SLPC and the LKPC was dominatedby crustal thickening. Thickening of the crust was pro-posed to have caused heating and the diapiric rise of grani-toids (Diombalou and Bouroumbourou). Active WNW–ESE extension caused doming and steepening of the shearzone during ongoing deformation.

The presence of andalusite and pseudomorphs of anda-lusite in pelites suggests that the rocks were metamor-phosed under much lower pressure conditions than werethe rocks of Diombalou. Outside the aureole, metamor-phism reached biotite or chlorite grade. The domal geom-etry of these two plutons was obtained during activeWNW–ESE extension, which appears to have beenin response to NNW–SSE compression. Post-doming


deformation involved WNW–ESE compression followedby sinistral transcurrent deformation (Gueye et al., in prep-aration). Comparison of this deformation sequence to oth-ers within the Borom-Goren province in Burkina Faso(Hein et al., 2004) suggests a common tectonic history.

The above model, in which partial melting was causedby crustal thickening and consequent heating of the crust,is consistent with the geochemical data (e.g. Pawlig et al.,2006). We suggest that the kyanite (Diombalou) formedduring this phase of crustal shortening, and that the latergeneration of andalusite (Bouroumbourou), preferentiallyin structures related to WNW–ESE extension, reflectsexhumation during the extension, related to doming of,and magmatism within the Mako Belt.

Emplacement of granodiorite in a dominantly basalticcrust induced stronger density contrasts, and thus madediapiric ascent easier than in older recycled granitic conti-nental crust. Heat provided by the granodioritic plutonsallowed softening of the surrounding crust and, ‘therefore,its local ductile behaviour. The NNW–SSE compressionresulted in crustal thickening and partial melting, probablyof the 2.2 Ga amphibologneissic basement, causing crustalweakening, collapse and extension of the granitoids. Regio-nal tectonics, involving both extension and compressionare the dominant causes for deformation and granite intru-sion in the Mako Belt.

6. Conclusion

The Mako Belt in the KKI of Eastern Senegal provides anexample of the close links between metamorphism, regional-scale deformation, and granite generation and emplacement.Combined structural and petrological data allow us to con-strain the Paleoproterozoic evolution of a part of the KKI.The Mako Greenstone Belt and adjacent Granitoid Com-plexes have had a prolonged history with at least four maindeformation phases between ca. 2.2 and 2.0 Ga.

Detailed studies (Pawlig et al., 2006; Dia et al., 1997) ofthe geochemistry of the KKI intrusive rocks have demon-strated that the plutons are calc-alkaline and metalumi-nous. Pb–Pb and U/Pb zircon age determinations (Diaet al., 1997; Gueye et al., in press) indicate the presenceof older granitic material. However, there are slight isoto-pic variations between plutons across the Mako Belt.

An important doming phase occurred during activeWNW-ESE extension and granitoid emplacement (Diom-balou and Bouroumbourou) in the Granitoid Complex(LKPC) at around 2.13 Ga. Extension may have been theresult of an earlier phase of NNW–SSE compression atca. 2.16 Ga. This may have caused crustal thickening andresulted in partial melting, possibly of the old basement,causing crustal weakening, collapse and the emplacementof the 2.1 Ga granitoids. The basement in this area is com-posed of early Proterozoic rocks. The mafic, ultramafic andgranitic intrusions dated at 2160 Ma are interpreted as thejuvenile products of an arc magmatism (e.g. Dia et al.,1997; Pawlig et al., 2006).

Acknowledgements

We are grateful to the Volkswagen-Stiftung (Az: I/77340) for financial support. The paper benefited from thethoughtful reviews by M. Lompo and R. Muzio.

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intrusive rocks and tectono-metamorphic evolution of the mako paleoproterozoic belt (eastern...

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