ORIGINAL PAPER

Microstructures and temperature variability during the Eburneandeformations in the Daléma area, Eastern Senegal

Tahar Aïfa & Moussa Dabo

Received: 20 June 2013 /Accepted: 17 December 2013# Saudi Society for Geosciences 2014

Abstract In the Daléma area, the eastern part of theKédougou-Kéniéba inlier, the petrographic study of feldsparand quartz microstructures revealed the variability of temper-atures related to partitioned D2 Eburnean deformation andretrograde metamorphism from the superimposed D3

Eburnean deformation. Two major thermal events, Q1 andQ2, related to D2 and D3 Eburnean deformation phases, wereidentified in intracrystalline microstructures in the Dalémaarea. The first identified thermal event called Q1, which wouldhave created the largest heat flow, was recorded especially inthe shear zones. The microstructure features of this thermalevent are syntectonic to the emplacement of granitoids around2,100 Ma, during the D2 Eburnean deformation phase. ThisD2 Eburnean deformation phase is characterized by thepartitioning of sinistral transpressive deformation betweenpure shear to simple shear-dominated domains. The thermalgradient associated with this event is very important in theinner shear zones (≥750 °C) and decreases towards theirborders, where it would be estimated at ∼450 °C. Outsidethe shear zones, the metamorphic temperature peaks are lessimportant with thermal gradients lower than 400 °C. Theywould be very low (≤250 °C) in undeformed rocks with wellpreserved primary beddings. The intensity of the second ther-mal event is weaker than in the first one, with temperatures notexceeding 500 °C. The intracrystalline microstructures whichcharacterize this second thermal event are closely connectedto those of the D3 Eburnean deformation phase, and cross-cutor are superimposed on those of the first thermal event related

to the D2 Eburnean deformation phase. This second thermalevent is then subsequent to the late D3 Eburnean deformationphase. The intracrystalline microstructures related to the D1

Eburnean deformation phase were not identified, and theirabsence is interpreted in various ways.

Keywords Microstructures . Temperature . Schistosity .

Eburnean . Petrography . Kédougou-Kéniéba

Introduction

The relation between microstructure and temperature in thestudy of both experimentally and naturally deformed rocks isconsidered by many authors (Simpson 1985; Tullis and Yund1985; White and Mawer 1986; Hirth and Tullis 1992; Pryer1993; Srivastava andMitra 1996). Physical conditions such astemperature, pressure, and fluids influence the processes bywhich a rock and its constituent minerals deformwhen subjectto stress. At grain-scale, various mechanisms of deformationmay take place under different physical conditions; for eachone of these deformation mechanisms, various microstruc-tures will result. Because the temperature may be a control-ling, predominant factor in the activation the deformationmechanism, the microstructures of certain minerals can beused as a relatively reliable geothermometer. In the Birimianformations of the West African Craton (WAC; Fig. 1a), rela-tions between temperature and deformation were studied bysome authors (Caby et al. 2000; Pitra et al. 2010; Ganne et al.2012). Contemporaneously to the intrusion of granitoids,Eburnianmetamorphism of the Birimian Supergroups reachedamphibolite facies conditions and greenschist facies retrogres-sion as shown by petrologic investigations in the Ashanti,Sefwi and Kibi-Winneba volcanic belts of Ghana (John et al.1999; Klemd et al. 2002; Galipp et al. 2003). Peak metamor-phic conditions were calculated to 500–610 °C and 4–6 kbars.

T. Aïfa (*) :M. DaboGéosciences Rennes, CNRS UMR 6118, Université de Rennes 1,Bat.15, Campus de Beaulieu, 35042 Rennes Cedex, Francee-mail: tahar.aifa@univ-rennes1.fr

M. DaboDépartement de Géologie, Faculté des Sciences et Techniques,Université Cheikh Anta Diop de Dakar, 5005, Dakar-Fann, Senegal

Arab J GeosciDOI 10.1007/s12517-013-1254-1

This peak metamorphism was dated by U-Pb on sphenes at2.1 Ga (Loh and Hirdes 1996; Oberthür et al. 1998). Thus,John et al. (1999) and Klemd et al. (2002) conclude that largeparts of the Birimian Supergroup in Ghana and probablyWestAfrica were metamorphosed under peak amphibolite faciesconditions. Furthermore, Debat et al. (2003) show thatamphibolite facies assemblages are locally overprinted byregional greenschist facies metamorphism.

The Birimian formations of the Kédougou-Kéniéba inlier(KKI) (Fig. 1b) are generally affected by a regional epizonalmetamorphism which can be mesozonal within the granitoidcontacts (Bassot 1966; Dia et al. 1997). This paper presentsthe results of a petrographic study of feldspar and quartzmicrostructures in the Paleoproterozoic formations of theDaléma area, the eastern part of the KKI. The purpose is tobetter understand the evolution of the temperature distributionduring the Eburnean orogeny in this area. Intracrystallinemicrostructures were examined in thin oriented sections fromvarious samples (metapelites, metagraywackes, carbonatedbreccias, granite, albitized diorites, rhyolites, etc.) collectedwithin the core and borders of the shear zones and alongweakly deformed domains outside the shear zones (Fig. 2).This is to constrain the variability of temperatures possiblyrelated to differential deformation from outer to inner shearzones and from the superimposed Eburnean deformation. Thetemperature estimation is based on a compilation ofinterpreted correlations between temperatures and microstruc-tures from naturally (field) and experimentally (laboratory)deformed samples presented in the extant literature. In thispaper, we essentially use the intracrystalline microstructuresof quartz and feldspar grains to characterize temperaturesassociated with Eburnean regional deformation in theDaléma area, eastern KKI (Fig. 1b).

Geological setting

The Birimian terranes of the WAC are marked by an alterna-tion of volcanic belts and sedimentary basins cross-cut byvarious Eburnean granitoids and shear zones (Bassot 1966;Milési et al. 1986; Ledru et al. 1989). The Birimian formationsof the KKI (Fig. 1a, b) represent a part of the westernPaleoproterozoic domain of the Leo shield in the southWAC. They consist of volcanic and sedimentary rocks whichare subdivided into two Supergroups (Bassot 1987): (a) theMako supergroup in the west consist of volcano-plutonictholeiitic-dominated formations, dated between 2,213±68and 2,158±8 Ma (Bassot and Caen-Vachette 1984; Dia et al.1997); (b) the Dialé-Daléma Supergroup to the east is mainlycomposed of detrital turbiditic and carbonated metasediments,dated between 2,096±8 and 2,165±1 Ma (Calvez et al. 1990;Hirdes and Davis 2002), and associated with volcanic tohypovolcanic rocks (andesite, dacite). Both Supergroups are

separated by a major transcurrent zone (MTZ; Ledru et al.1991). Badon-Kakadian and Saraya batholilths represent thegranitoid intrusions within the Mako and the Dialé-DalémaSupergroups, respectively (Bassot 1987). Recently,Thiéveniaut et al. (2010) distinguished only a singleB i r im i an Supe rg roup con t a i n i ng a l l t h e KKIPaleoproterozoic terranes. This Birimian Supergroup issubdivided into two groups: the Mako (2,200–2,170 Ma)and the Dialé-Daléma (2,140–2,100 Ma), separated by theMTZ. Both groups are cross-cut by the suites of plutonicintrusions (granitoids and gabbros). The Mako group containsthe Sandikounda-Soukouta suite (2,170–2,140 Ma). TheDialé-Daléma group is cross-cut by the Saraya (2,100–2,060 Ma) and Boboti (2,080–2,060 Ma) suites. The hiatusbetween the deposit of the Birimian formations in both groups(Mako and Dialé-Daléma) and in the emplacement of thegranitoid intrusions is a controversial subject within theWAC. For the Birimian deposits of the KKI, most geochro-nological data strongly suggest an anteriority of the volcanicdeposits of the Mako group on the sedimentary deposits ofDialé-Daléma (Dia et al. 1997; Hirdes and Davis 2002; Gueyeet al. 2007). Moreover, the hiatus between the emplacement ofthe plutonic intrusions in the Mako group (mainly volcanics)and that of Dialé-Daléma (mainly sedimentary) are notcompletely attested by geochronological data. Thus, theTinkoto pluton in the Mako group, dated by U-Pb on zirconwith 2,074±9 Ma (Gueye et al. 2007) remains apparentlyyounger than that of Boboti in the Dialé-Daléma group whichis dated at 2,080±9 Ma by U-Pb on zircon (Hirdes and Davis2002).

The Birimian of the WAC was deformed during theEburnean tectono-magmatic events, which include three ma-jor phases of deformation: D1, thrust; D2 and D3, transcurrent(Feybesse et al. 1989; Milési et al. 1989). These three phaseswere recognized in several Birimian provinces of the WAC,particularly in Côte d’Ivoire (Ledru et al. 1988; Pitra et al.2010), Burkina Faso (Ouedraogo and Prost 1986; Ouedraogo1987; Baratoux et al. 2011), Ghana (Feybesse et al. 2006), andGuinea and Mali (Feybesse et al. 1989).

In the KKI, two major phases of Eburnean deformations(D1 and D2) are generally identified (Ledru et al. 1989, 1991):D1 of thrust tectonics assigning only the formations of the B1

basic unit, and D2 transcurrent tectonics with NS to NE-SWsinistral wrench faults. This second D2 phase can be compres-sive (D2a), sinistral transpressive (D2b), and locally thrusting(D2c) (Dabo and Aïfa 2010). It is associated with two large NSto NE oriented shear zones: Senegalo-Malian Fault (SMF) inthe Dialé-Daléma Supergroup (Bassot and Dommanget 1986)and Main Transcurrent Zone (MTZ) which marks the limitbetween Mako and Dialé-Daléma Supergroups (Fig. 1b). Themain architecture of the D2 Eburnean deformation comprisesalternating large shear zones relatively well-separated byweakly deformed surrounding rock domains due to the

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partitioning of deformation (Dabo and Aïfa 2013). The D3

tectonic phase, defined in several Birimian segments of theWAC (Milési et al 1989), was identified and characterized inthe Dialé-Daléma Supergroup (Dabo and Aïfa 2011). Themetamorphism is generally epizonal facies in the KKIBirimian formations. However, hornfels mesozonal faciesappear around granitoid intrusions. So far, no clear evidencefor large-scale amphibolite facies metamorphic conditions hasbeen reported for the KKI (Dia et al. 1997; Debat et al. 2003).

However, local occurrences of staurolite-sillimanite-garnetassemblages found in metasedimentary rocks in the vicinityof Eburnian granitoid plutons seem to imply that amphibolite-facies assemblages are restricted to contact aureoles of thegranitoids (Ndiaye et al. 1989 ; Dabo 1999). In addition,amphibolo-gneissic metamorphosed rocks dated by Pb/Pbbetween 2,194±4 and 2,202±6 Ma, were identified at thenorthern part of the Mako Supergroup (Dia et al. 1997).These amphibolo-gneissic metamorphosed rocks dated by

Fig. 1 aSchematic map of the major Precambrian shields of theWAC (simplified from Gueye et al. 2008). bLocation map of Kédougou-Kéniéba inlier(KKI) in the West African Craton (WAC)

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Pb/Pb between 2,194±4 and 2,202±6 Ma, were identified asthe basement of the Mako Supergroup (Dia et al. 1997).Migmatitization appears locally in the host rock adjacent tothe Diombalou and Sonfara granites respectively dated to2,102 and 2,171 Ma (U-Pb on Zircon, Théveniaut et al.2010) in the margins of Birmassou-Tomoromadji pluton(Gueye et al. 2008) in the Mako Supergroup. Temperaturevariability during the Eburnean deformation in the KKI ispoorly investigated within Birimian rocks and along subse-quent shear zones in the existing literature (Bassot 1966; Diaet al. 1997).

Geological architecture of the Daléma area

The Daléma area consists of sedimentary, volcanic and gran-itoid rocks which are affected by a general greenschist faciesmetamorphism (Fig. 2). In the vicinity of the Boboti granit-oids, this epizonal metamorphism becomes a mesozonal con-tact metamorphism.

The sedimentary formations comprise an alternation ofpelites, quartzites, sandstones, graywakes which are oftentourmalinized, and ofmarbles. The volcano–plutonic complexforms a NS lengthened band cross-cutting its sedimentary hoston an approximate length of 80 km and a width of 10 to20 km, along the western bank of the Falémé river. Thevolcanic rocks are made up of pyroclasts (tuffs, agglomerates,breccias) and lava flows (andesites, dacites). Thehypovolcanic rocks, emplaced as dykes, consist ofmicrodiorites, microgranodiorites and albitites. The plutonicterms are related to the Boboti granitoids which intruded intovolcano-sedimentary host rocks. The petrographic descriptionof these rocks is recorded in Table 1.

From a tectonic point of view, three Eburnean deformationphases (D1, D2 and D3) have been characterized in this area(Ledru et al. 1989, 1991; Dabo and Aïfa 2010, 2011). A D1

deformation phase marked by a N45–70° oriented S1 schis-tosity and by frequently obliterated P1 folds of variable geom-etry (Dabo and Aïfa 2010). The style of deformation associ-ated with this D1 phase still remains controversial throughoutthe WAC, probably related to tangential or peri-batholitictectonics (Ledru et al. 1989; Vidal et al. 2009; Lompo2010). The D2 deformation phase is progressive with threestages (Dabo and Aïfa 2010): (a) compression D2a, (b) sinis-tral transpression D2b, and (c) D2c overlaps. The main short-ening direction associated with the D2 deformation phase is∼N110–130°. Analysis of the various structures in relation to

this major D2 Eburnean deformation phase indicatespartitioning of sinistral transpressive deformation betweenpure shear to simple shear-dominated domains (Dabo andAïfa 2013). The D3 deformation phase is characterized bydextral transcurrent shear zones, associated with N45° andN160° oriented S3 schistosity and Z-shaped P3 dissymmetricalfolds (Dabo and Aïfa 2011). The main shortening directionassociated with the D3 Eburnean deformation in the Dalémaarea is EW to NE-SW (Dabo and Aïfa 2011).

The temperatures throughout metamorphic conditions, as-sociated with the Eburnean deformations in the Daléma area,are estimated below using the intracrystalline microstructuresof quartz and feldspar grains within highly deformed domainsinside shear zones and within weakly deformed domainsoutside shear zones.

Microstructures and deformation temperatures

For the best estimation of the variability of deformation tem-peratures based on intracrystalline microstructures analysis,samples were taken from the core and borders of shear zonesas well as from the weakly folded domains outside the shearzones (Fig. 2). Deformation temperatures were estimated bycomparison with the results of Eggleton and Buseck (1980),Tullis and Yund (1987), Blumenfeld and Bouchez (1988),Davidson et al. (1994), Pryer and Robin (1995), Dell’Angelo and Tullis (1996) and other authors mentioned belowin the text.

Microstructures of the Eburnean major thermal event (Q1)

Microstructures within the borders of shear zones

Samples taken from the borders of shear zones correspond tomica graywacke (K5), mylonitic granite (D28) and quartzite(K16, D19; Fig. 2). Quartz and feldspar porphyroclasts ofthese rocks mainly stretch N10-25° along the S2 major schis-tosity direction of the D2 Eburnean deformation phase (Daboand Aïfa 2010). Quartz grains show undulatory extinction,joint migration and boundary subgrain recrystallizations lead-ing to jagged boundaries (Fig. 3a). Sometimes recrystallizedsubgrains can be stretched obliquely to the S2 major schistos-ity direction (Fig. 3a). They are generally fine-grained wrap-ping porphyroclast relics (Fig. 3b) in a shape of core andmantle texture (White 1976).

Feldspars show undulatory extinction, twinning withsometimes ductile deformation of twins in kink shape(Fig. 3c). Some feldspar porphyroclasts contain albite flamelamellae (Debat et al. 1978; Pryer 1993) slightly orthogonal tothe S2 schistosity direction (Fig. 3d). They also show jointmigration in their jagged boundaries. Ductile deformation offeldspar grains generally occurs at temperatures ≥450 °C (Voll

�Fig. 2 Geological map of the study area with the location of the differentsamples which have been studied (inside circles). MSZ Mandankhotoshear zone, SMF Senegalo-Malian fault, KSZ Kolia shear zone. Blackstar indicates sample position whose number is indicated in thecorresponding circle

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1976; Debat et al. 1978; Hanmer 1982; Tullis 1983; Bell andJohnson 1989). These different microstructures and recrystalli-zations of quartz and feldspar grains occur during the major D2

Eburnean deformation as evidenced by porphyroblast andsubgrain elongation along S2 major schistosity direction. Theysuggest that deformation temperature is ∼450 °C in the bordersof shear zones during the D2 Eburnean deformation phase.

Microstructures within the cores of shear zones

Within the cores of shear zones, investigations of samples weremade on mylonitic granite (K7), pelitic micaschist (D21),albitites (K18, K45), microdiorite (K14) and carbonated brec-cias (D10, D12; Fig. 2). In the granite (K7), pelitic micaschist(D21) and carbonated breccias (D10, D12), quartz are almostentirely recrystallized. Subgrains are sometimes equant andarranged in triple junction with straight or slightly jaggedboundaries (Fig. 4a). Locally, in pelitic micaschist, large rod-shaped quartz porphyroclasts with straight boundaries formribbons perpendicular to the microcrystalline subgrain ribbonshighlighting S2 schistosity (Fig. 4a). Quartz is poorly repre-sented in albitites (K18, K45) with equant neoblasts arrangedin triple junction with straight or jagged boundaries.

In these shear zones, rocks (micaschists, carbonated brec-cias, granites) generally present feldspar porphyroclasts show-ing undulatory extinction and recrystallized subgrains bound-aries. The recrystallized subgrains are elongated parallel to S2schistosity. Feldspar porphyroclasts also show deformationtwins (kinks) and conjugated jogs (Fig. 4b). In some feldsparporphyroclasts, the geometry of conjugated jogs (Fig. 4b) sug-gests a WNW-ESE oriented main shortening stress (σ1) similarto that of the D2 Eburnean deformation in the area, estimated at∼N110° (Dabo and Aïfa 2011). In natural feldspar, fracturesmay commonly occur even during high temperature deforma-tion (e.g. 600-800 °C) where dislocation creep is dominant

(Goode 1978; Sodre Borges and White 1980; Brown andMacaudiere 1984; Kruse and Stünitz 1999; Kruse et al.2001). Microcline texture and biotite recrystallization withinS2 schistosity planes are locally recorded in the rocks(micaschists, carbonated breccias and granites) within the shearzones (Fig. 4c). These different intracrystalline microstructureswithin shear zones support a deformation at solidus stage undertemperatures between ∼500 °C and 600 °C. In addition, granite(K7), albitite (K18), pelitic micaschist (D21), carbonated brec-cia (D12), andmicrodiorite (K14) reveal local high temperature(≥700 °C) intracrystalline microstructures marked by solid-state diffusion and sub-solidus deformation. In the myloniticgranite, some large quartz porphyroclasts show subgrains withcheckerboard texture (Fig. 4d) evidencing high temperaturedeformation (∼700 °C; Kruhl 1996; Auréjac et al. 2004).Monocrystalline quartz roddings or ribbons bereft ofintracrystalline deformation, elongated along the S2 schistositydirection, appear in micaschists (D21) and carbonated breccias(D10). They are bounded by fine recrystallized subgrains(Fig. 4e). These monocrystalline quartz ribbons appear duringthe high deformation (Boullier and Bouchez 1978) followed bydestabilization, remobilization and recrystallization ofsubgrains around original grains. Furthermore, free growth ofquartz and feldspar grains (K15) characterized by their shin-gling, diffuse and lobate boundaries (Fig. 4d), evidence a liquidmixture. This mixture results from a diffusion of grain bound-aries under high temperature deformation conditions of typicalgneiss facies (Behrmann and Mainprice 1987; Tullis et al.1990). These gneissic rocks locally occur in the core of shearzones, where they show augen or banded texture with alternat-ing quartz, feldspars and micas bands (Fig. 4f). Augen texturealso appears within feldspars of albitite and is characterized bysinistral rotation of feldspar porphyroclasts (Fig. 5a) coatedwith fine recrystallized subgrains with undulatory extinction.These recrystallized subgrains are stretched along S2 schistosity

Table 1 Summary of the main petrographic facies and mineralogy compositions of the Paleoproterozoic rocks of the Daléma area (observations of thinsections under an optical microscope)

Fms. Petrography Description Mineralogy

Metasediments Pelites Stratified and locally tourmalinized fine sediments qz-se-ca-tm±op±bi

Graywakes Quartz and feldspars grains in clay–sericite matrix qz-se-ab-ca-tm±bi±op

Tourmalinized sandstones Quartz grains in tourmalinized matrix qz-tm-se±op

Quartzites Quartz grains with poor silicate-carbonated matrix qz-tm-ab-ca±op

Carbonates Some calcite and quartz grains in a carbonated matrix ca-qz-se±op

Volcanic rocks Pyroclastites Feldspars, quartz, calcite, amphibole, biotite, opaque grains in volcanic matrix qz-fd-op-ca-bi-se-am

Rhyodacites Feldspars, amphibole, biotite, opaque minerals in quartzo-feldspathicmesostasis

qz-fd-se-am-op±ca±ch

Andesites Feldspars and ferromagnesian minerals in poor mesostasis qz-fd-am-ca±op±ep

Plutonic rocks Albitized microdiorites(albites)

Altered plagioclase microcrystal, amphibole, and epidote qz-se-ca-bi-am-ab±op

Granodiorites Quartz, feldspars and some biotite, muscovite, and amphibole qz-fd-se-ca-ab±op

ab albite, am amphibole, bi biotite, ca calcite, fd feldspar, op opaque, tm tourmaline, qz quartz, se sericite, ch chalcopyrite, ep epidote

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direction. According to Dell’ Angelo and Tullis (1996), augentexture is characteristic of high temperature conditions of mod-erate deformation.

In addition, intragranular fractures of feldspar porphyroclastsare followed by recrystallization of magmatic material (Fig. 5a),which suggest a brittle deformation at the sub-solidus stage. Inthis case, remobilized liquidus would come from size reductionand intracrystalline deformation of the matrix grains. Antitheticshape and arrangement of joints compared to S2 schistositytrend (Fig. 5b) suggest a sinistral shear (Bouchez et al. 1992),compared to the sinistral transpression of D2 Eburnean defor-mation phase (Dabo and Aïfa 2010). The trend of the mainshortening stress (σ1) associated with this joint arrangement issimilar to those of D2 Eburnean deformation (∼N110°) in thesector (Dabo and Aïfa 2010). Twin ductile deformations, man-tle and core texture and recrystallized ribbons with undulatoryextinction (White 1975; Simpson 1985; Simpson and Wintsch1989; Anderson 1996; Dell’ Angelo and Tullis 1996) are alsorecorded in feldspars (Fig. 5b, c, d). These different microstruc-tures suggest high temperature (≥700 °C) deformation at sub-solidus stage, probably during the emplacement of magmaticintrusions (Nédélec and Bouchez 2011) of the D2 Eburneandeformation phase (Ledru et al. 1991; Pons et al. 1992).

Microstructures outside the shear zones

In the study area, temperatures of metamorphism are lowergrade and variable outside the shear zones. Moreover, somerocks are not deformed and show a well preserved primary

bedding (Fig. 6a). In albitites (K8, K9) some recrystallizedquartz grains with undulatory extinction are isolated betweenthe feldspar grains. Their limits with feldspar grains are oftendiffuse (Fig. 6b). Intracrystalline deformation is weaklyexpressed in quartzites (K20, D18); original quartz grainsand surrounding recrystallized subgrains are flattened andelongated along S2 schistosity direction (Fig. 6c). In somesamples (e.g. D20) the flattening of original coarse grains isaccommodated by undulatory extinction, lamellae deforma-tion and fine subgrain recrystallizations at their boundaries(Fig. 6c). Feldspars of albitites (K8, K9) show a weak undu-latory extinction and straight or jagged boundaries (Fig. 6b).Particularly in rhyodacite (D39) fractures, feldspars exhibitweakly recrystallized quartz grains (Fig. 6d).

Microstructures recorded in rocks outside the shear zonessuggest lower metamorphic temperature conditions(≤400 °C). These temperatures are weaker than those recordedwithin the shear zones (≥700 °C) and at their borders (500–600 °C). In the well preserved primary bedding rocks, tem-perature intensities are much lower (≤250 °C).

Microstructures of the Eburnean late thermal event (Q2)

In some cases the superimposition of microstructures producedunder different metamorphic conditions occurred, but we candistinguish lower grade deformation since it is always restrictedto a much narrower zone than that of higher grade (Pryer 1993).In the borders and in the cores of shear zones lateintracrystalline microstructures appear, superimposing or

Fig. 3 aQuartz crystals withundulatory extinction and jaggedboundaries showing migrationjoints and subgrainrecrystallizations which areelongated parallel or obliquely(Ob) to the S2 schistosity direction.bCore and mantle texture formedby quartz porphyroclast andsurrounded subgrains (Sg). Notethe biotite lamellae (Bi) which iselongated along the direction ofschistosity S2. c Feldsparporphyroclast with twin kinks (Mk)and recrystallized subgrains (Sg)on the boundaries. d Feldsparporphyroclast with perthiticlamellae (Lp) elongatedperpendicular to the major S2schistosity; recrystallizedsubgrains and migration joints(Jm) are located on the boundaries.Fd feldspar, Qz quartz

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cross-cutting those of the D2 Eburnean deformation phase.Their structural behaviour reminds the D3 Eburnean phase ofdeformation (Dabo and Aïfa 2011). Quartz and feldspar grainspreviously elongated along the S2 major schistosity of the D2

Eburnean deformation phase are remobilized and deformed bylate superimposed microstructures. In quartzites (K16, D19,K20, andD18) and carbonated breccias (D10, D12), undulatoryextinction quartz grains show antithetic fractures, rotated grainsand lamellae deformation indicating a dextral movement(Fig. 7a). In some microshear zones, quartz grains exhibit σ-type porphyroclast shape or intragranular fractures related to adextral rotation (Fig. 7b). Moreover, rhyodacite (D39) andmicaschists (D21, K5) contain biotite porphyroclasts affectedby dextral slip joints creating cleavage shears (Fig. 7c). In themicaschists, fine-grained biotite recrystallizations along S2schistosity are crenulated by N70° oriented S3 schistosity, re-lated to D3 Eburnean deformation (Fig. 5d). Feldspars presentsome subgrain recrystallizations, undulating extinction and

predominantly SW-NE oriented jogs with frequent dextralshears (Fig. 7d). In albitites (K18, D45), feldspar porphyroclastsshow dextral jogs associated locally with sigmoid subgrains(Fig. 7e). Recrystallized subgrains cross-cut the originalporphyroblasts oriented along the S2 schistosity (Fig. 7e).Feldspars show also twin (kink) feature deformation (Fig. 7e).Predominance of antithetic fractures associated with brittlemicrostructures indicate a temperature possibly between300 °C and 400 °C (Pryer, 1993). However, feldspar ductiledeformation generally occurs at temperatures ≥450 °C (Debatet al. 1978; Hanmer 1982; Tullis 1983; Bell and Johnson 1989).

Outside the shear zones, microstructures related to the D3

Eburnean deformation phase are poorly recorded.Nevertheless, locally in quartzites some quartz porphyroclastspreviously elongated along S2 schistosity were deformed dur-ing this late Eburnean deformation phase (Fig. 7f).

All these intracrystalline microstructures post-D2 Eburneandeformation characterize temperatures (Q2) ranging between

Fig. 4 a Straight boundaries oflarge quartz crystals which areelongated perpendicularly to thedirection of surrounding subgrains.Some recrystallized subgrains arearranged in triple junction (Pt) orstretched parallel (Qp) or obliquely(Qo) to the S2 schistosity direction.b Feldspar porphyroclasts withtwinning (Mc) and jogs (Mj),subgrain recrystallizations andjoint migrations (Jm) on itsboundaries. The conjugatedmicrojogs are associated with∼N110° oriented principalshortening stress σ1. c Feldsparwith undulatory extinctionshowingmicroclinemicrostructure(Mi) and biotite recrystallization(Bi) parallel to S2 schistosity. dQuartz showing completerecrystallization and chessboardtexture (Eq) related to subgrainsarchitecture (Sg). eMonocrystalline rodding orribbons of quartz (Bq), withoutintracrystalline deformation andelongated along S2 schistosity.Small subgrains are recrystallizedon the boundaries. fMicaschistshowing a schistosity highlightedby alternation of micaceous (Bm)and quartzo-feldspathic bands (Bs).Feldspars are transformed intocalcite (Ca) and sericite (Se) and S2schistosity is affected by S3crenulation cleavage

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400 °C and 500 °C. These temperatures are lower than thoserecorded in the core of shear zones during the Q1 Eburneanthermal event. Dextral shears within the microstructureswould be associated to dextral transcurrent deformation relat-ed to D3 Eburnean phase (Dabo and Aïfa 2011).

Discussion and conclusions

Two phases of metamorphic temperatures (Q1 and Q2) areidentified in the study area, based essentially on analysis ofquartz and feldspar intracrystalline microstructures from

Fig. 5 a Feldspar porphyroclasts(Fd) affected by sinistral rotationand wrapped in recrystallizedsubgrains (Sg) parallel to S2schistosity. b Feldsparporphyroclasts showing antitheticfractures (Ri) associated withquartzo-feldspathic recrystallizedsolution. Fracture orientation anddisplacement assume a sinistralmovement related to ∼N110°oriented shortening main stress.Note the twinning (Md)deformation and quartz crystalrecrystallization along the S2schistosity direction. cCore andmantle structure resulted fromneoblasts (Sg) recrystallizationaround feldspar porphyroclastwhich shows dissolution features(Fi) on its boundaries. dRoddingfeldspar neoblasts, recrystallizedobliquely to schistosity (S2) andshowing lamellae and banddeformations (Bl). Bi biotite, Cacalcite, Qz quartz

Fig. 6 aQuartz porphyroclastsslightly flattened along the S2schistosity direction and showinga low undulatory extinction. bFeldspar (Fd) lamellaes elongatedalong S2 schistosity direction andassociated with small quartz grainrecrystallization which showdiffuse boundaries in the albitites.cUndulatory extinction in quartzporphyroclasts with partialsubgrain recrystallization alongS2 schistosity. d Fracturedfeldspars with lowrecrystallization of quartz in thefracture (Fr). Mamatrix

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samples taken in the core and borders of the shear zones aswell as in weakly folded domains outside the shear zones. Thefirst temperature phase (Q1) is the most important and can be≥700 °C. It occurred during the D2 Eburnean deformationphase, associated with the emplacement of Eburnean granit-oids between 2,105±8 and 2,080±0.9 Ma (Liégeois et al.1991; Dia et al. 1997; Hirdes and Davis 2002). In fact,intracrystalline microstructures related to the first thermalevent show similar tectonic features with D2 Eburnean defor-mation structures in the area (Dabo and Aïfa 2010).

The planar fabric of quartz and feldspar grains in granitoidsis parallel to S2 schistosity and some microstructures showsyntectonic features which are related to the D2 Eburneandeformation phase. The thermal gradient associated with Q1

event decreases from the core of shear zones towards weaklyfolded domains outside the shear zones. It is high within thecore of shear zones (≥750 °C) and decreases towards theirborders where it reaches ∼450 °C. Outside the shear zones, Q1

metamorphic temperatures are lower with weaker (≤400 °C)or very low (≤250 °C) thermal gradients in undeformed andbedded rocks. This temperature distribution between the dif-ferent study domains (core and borders of shear zones, weaklyfolded domains) during the D2 Eburnean deformation phasewould be related to partitioning of the D2 deformation (Daboand Aïfa 2013). The second thermal peak (Q2) corresponds toretrograde metamorphism with temperatures not exceeding500 °C. The geometry and kinematics of the microstructuresassociated with this thermal event are characteristic of the D3

Eburnean deformation phase (Dabo and Aïfa 2011). Themicrostructures of Q2 thermal event cross-cut or aresuperimposed on the microstructures related to D2 majordeformation phase. The D3 Eburnean deformation isresponsible for the Q2 thermal event at around 2064±4 Maand 1989±28 Ma (Bassot and Caen-Vachette 1984; Hirdesand Davis 2002). The lack of intracrystalline microstructuresrelated to the D1 Eburnean deformation phase can be

Fig. 7 aQuartz porphyroclastwith undulatory extinctionshowing microfractures andlamellae deformation (Ld) relatedto a dextral rotation associatedwith ∼N70° shortening mainstress. bUndulatory extinction inquartz porphyroclast affected bysigmoid fracture along a dextral∼N25° oriented microshear band.The main shortening stress is inagreement with the orientation ofthe D3 Eburnean deformationphase in the area (∼N70°). cBiotite kinked by a dextral N140°oriented microshear band,creating its cleavage deformation(Cv). d Feldspar porphyroclastsstraightened N45° along S3schistosity direction and showingperthitic lamellae (Lp) affected bydextral microjog (Mj). eRoddingof feldspars (Fd) elongated alongthe S2 schistosity and affected by∼N110° oriented dextralmicroshear bands in albitites.Subgrains (Sg) are recrystallizedinner and at the boundaries of theporphyroclasts. fQuartzporphyroclasts with undulatoryextinction and lamellaedeformation (Ld), straightenedand distorted following S3schistosity direction (∼N45°). Bibiotite, Ca calcite, Mc twinning,Qz quartz, Se sericite

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supported by two assumptions: (a) D1 estimated at ∼2,137 Ma(Hirdes et al. 1996) is prior to the sediment deposition(>2,124 Ma; Bossière et al. 1996) and granitoid emplacement(∼2,120 Ma; Hirdes and Nunoo 1994) in the WAC (BurkinaFaso, Côte d’Ivoire); (b) D1 a peri-batholithic deformation(Pouclet et al. 1996) was recorded just around plutoniccontacts.

In other parts of the WAC, similar metamorphic tempera-ture conditions are also recognized. In Paleoproterozoic rocksof Côte d’Ivoire, Caby et al. (2000) distinguished two meta-morphic facies: (a) an amphibolite facies (∼550 °C) locallyassociated with a high temperature contact metamorphism; (b)a granulite facies with kinzigites (700–800 °C) in themetapelite located within the Sassandra sinistral shear fault.The granulite facies conditions with two metamorphismpeaks, M1 and M2 at 13 kbar, 850 °C and <7 kbar, 700–800 °C, respectively, are also distinguished in thePaleoproterozoic rocks of Côte d’Ivoire (Pitra et al. 2010).These latter authors suggest a metamorphic evolution domi-nated by decompression accompanied by moderate cooling.Metamorphic cooling conditions (400–450 °C) are also de-scribed in Paleoproterozoic rocks of Burkina Faso, probablyrelated to subduction tectonics (Ganne et al. 2012).

Acknowledgments We gratefully acknowledge D.MBaye and the staffof the Randgold Resources (Senegal) for their assistance and on-fieldlogistics. One of the authors (M.D.) is indebted to the AUF for thefinancial support received during his stay in Géosciences-Rennes toachieve his PhD thesis.

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