Transition from alkaline to calc-alkaline volcanism during evolution of the Paleoproterozoic Francevillian basin of eastern Gabon (Western Central Africa)

Journal of African Earth Sciences xxx (2014) xxx–xxx

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Journal of African Earth Sciences

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Transition from alkaline to calc-alkaline volcanism during evolutionof the Paleoproterozoic Francevillian basin of eastern Gabon (WesternCentral Africa)

1464-343X/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.jafrearsci.2013.12.007

⇑ Corresponding author. Tel.: +33 2 38 64 35 08; fax: +33 2 38 64 36 85.E-mail address: [email protected] (D. Thiéblemont).

Please cite this article in press as: Thiéblemont, D., et al. Transition from alkaline to calc-alkaline volcanism during evolution of the Paleoproterozoicevillian basin of eastern Gabon (Western Central Africa). J. Afr. Earth Sci. (2014), http://dx.doi.org/10.1016/j.jafrearsci.2013.12.007

Denis Thiéblemont a,⇑, Pascal Bouton b, Alain Préat c, Jean-Christian Goujou d, Monique Tegyey a,Francis Weber e, Michel Ebang Obiang f, Jean Louis Joron g, Michel Treuil g

a Bureau de Recherches Géologiques et Minières, 3 av. Claude Guillemin, BP 360009, F-45060 Orléans Cedex 2, Franceb Oolite, 102 La Bournaire, 44690 Monnières, Francec Department of Earth Science and Environmental Sciences, University of Brussels, 50 av. FD Roosevelt, B-1050 Brussels, Belgiumd JCG-consult, Les Grands Goulets, 26420 La Chapelle-en-Vercors, Francee LHYGES, Université de Strasbourg, CNRS, 1, rue Blessig, 67084 Strasbourg Cedex, Francef DGMG, Libreville, Gabong Laboratoire P. Sue, CEN Saclay, 91140 Gif-sur-Yvette, France

a r t i c l e i n f o

Article history:Available online xxxx

Keywords:Trace elementVolcanismPaleoproterozoicFrancevillian GroupGabonCentral Africa

a b s t r a c t

We report new geochemical data for the volcanic and subvolcanic rocks associated with the evolution ofthe Francevillian basin of eastern Gabon during Paleoproterozoic times (c. 2.1–2 Ga). Filling of this basinhas proceeded through four main sedimentary or volcano-sedimentary episodes, namely FA, FB, FC andFD. Volcanism started during the FB episode being present only in the northern part of the basin (Okondjasub-basin). This volcanism is ultramafic to trachytic in composition and displays a rather constant alka-line geochemical signature. This signature is typical of a within-plate environment, consistent with therift-setting generally postulated for the Francevillian basin during the FB period. Following FB, the FC unitis 10–20 m-thick silicic horizon (jasper) attesting for a massive input of silica in the basin. Following FC,the FD unit is a c. 200–400 m-thick volcano-sedimentary sequence including felsic tuffs and epiclasticrocks. The geochemical signatures of these rocks are totally distinct from those of the FB alkaline lavas.High Th/Ta and La/Ta ratios attest for a calc-alkaline signature and slight fractionation between heavyrare-earth suggests melting at a rather low pressure. Such characteristics are comparable to those of felsiclavas associated with the Taupo zone of New Zealand, a modern ensialic back-arc basin. Following FD, theFE detrital unit is defined only in the Okondja region, probably associated with a late-stage collapse of thenorthern part of the basin.

It is suggested that the alkaline to calc-alkaline volcanic transition reflects the evolution of the Franc-evillian basin from a diverging to a converging setting, in response to the onset of converging movementsin the Eburnean Belt of Central Africa.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

The Francevillian basin system (‘Francevillian basin’) of easternGabon (stratigraphically, the ‘Francevillian Group’, Gérard, 1958;Weber, 1968; Donnot and Weber, 1968–1969) is an exceptionallywell preserved intracratonic domain of Paleoproterozoic agelocated to the eastern ‘piedmont’ of a folded and thrusted orogenicbelt known as the Ogooué orogenic belt (Figs. 1 and 2). Recently, ithas focused much attention because of the report by El Albani et al.(2010) of probable fossil forms suggesting the existence of multi-cellular organisms as early as c. 2.1 Ga.

The basin series is mainly composed of coarse- (sandstones)and fine-grained (shales and black shales) sedimentary rocks,but also includes magmatic (both plutonic and volcanic) unitsat different stratigraphic levels. The most largely developed isthe Ngoutou subvolcanic Complex, located in the northern partof the domain (Fig. 2). This formation is composed of saturated(granite) as well as undersaturated (syenite) magmatic rocks,both showing a constant alkaline affinity (Moussavou andEdou-Minko, 2006) suggestive of an emplacement in a within-plate tectonic setting. Higher in the series, and especially in theFD unit, other magmatic rocks exist (Weber, 1968) which havenot been subject to a detailed petrological study. Thus, the petro-logical knowledge of the Francevillian magmatism is at presentincomplete.

Franc-

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Fig. 1. Simplified regional geological map of Central Africa (from Milesi et al., 2004) showing the major lithostratigraphic domains with discrimination between thesedimentary, plutonic and metamorphic domains within the Eburnean orogeny.

Fig. 2. Simplified geological map of the Gabonese Republic (from Thiéblemont et al., 2009) showing the major lithostructural domains including the Paleoproterozoic OgoouéBelt and the Francevillian basin.

2 D. Thiéblemont et al. / Journal of African Earth Sciences xxx (2014) xxx–xxx

Considering the spatial relationship between the Francevillianbasin and Ogooué orogenic belt (Fig. 2), the basin system shouldbe better defined as a foreland basin (Feybesse et al., 1998) than

Please cite this article in press as: Thiéblemont, D., et al. Transition from alkalineevillian basin of eastern Gabon (Western Central Africa). J. Afr. Earth Sci. (201

as a strictly anorogenic domain. Indeed, the Francevillian seriesof the western part of the basin system, i.e. the Booué basin(Fig. 2), show significant deformation (Prian et al., 1990; Feybesse

to calc-alkaline volcanism during evolution of the Paleoproterozoic Franc-4), http://dx.doi.org/10.1016/j.jafrearsci.2013.12.007


D. Thiéblemont et al. / Journal of African Earth Sciences xxx (2014) xxx–xxx 3

et al., 1998) thus constituting some kind of transition between theorogenic belt and ‘peri-orogenic’ domain.

Following a mapping project of the Gabonese Republic spon-sored by the E.U. (Sysmin funds) and Gabonese Ministry of Mines(Bouton et al., 2009a, 2009b, 2009c, 2009d; Ebang Obiang et al.,2009a, 2009b; Thiéblemont, 2009; Thiéblemont et al., 2009), thispaper presents new data on the Francevillian magmatic rocks,and especially the volcanic and epiclastic rocks located towardthe upper part of the lithostratigraphic succession (FD unit), withthe aim of drawing a complete picture of the Francevillian magma-tism and placing new constraints on the Paleoproterozoic evolu-tion of Gabon.

2. Geological setting

The Francevillian Group refers to a sedimentary and volcanicseries outcropping in the central and eastern parts of Gabon (Figs. 1and 2). These rocks emplaced on an Archean basement referred toas the East-Gabonian block (Thiéblemont et al., 2009), mainly com-posed of granitoids of Mesoarchean age (c. 3–2,8 Ga) and variedgreenstone belts (Bassot, 1988; Mayaga-Mikolo, 1996; Thomaset al., 2001; Chevallier et al., 2002; Thiéblemont et al., 2009). Geo-chronological data show that the Francevillian Group emplaced be-tween c. 2100 and 2000 Ma (Ruffenach et al., 1976; Gancarz, 1978;Bros et al., 1992; Horie et al., 2005) in the external zone of a broadorogenic belt (Eburnean Belt) extending from Cameroon to Angola(Fig. 1), covering the central and eastern parts of Gabon (Fig. 2).

In central Gabon, this orogenic belt is represented by the pre-Eburnean (c. 2500 Ma) to Eburnean (c. 2200–2000 Ma) Ogoouéthrust-belt (Ogooué orogenic belt) lying to the west of the Franc-evillian Group (Figs. 1 and 2). To the West, the Ogooué belt is sep-arated from the crystalline terranes of the West-Gabonian Block bya major strike-slip fault: the Ikoy-Ikobé shear-zone (Fig. 2). Thewestern block is mainly composed of high-grade pre-Eburneancrystalline terranes intruded by c. 2080–2040 Ga-old high-K grani-toids (Thiéblemont et al., 2009). These plutonic rocks witness anintensive event of Eburnean granitization.

The Francevillian Group overlaps a group of sub-basins (collec-tively referred to as the Francevillian basin) which extend east-ward from the thrust belt, and end close to the border withCongo, where the Precambrian basement is covered by Cenozoiccontinental deposits related to the Congo Basin.

Eastwards from the orogenic front, the tectonic imprint in theFrancevillian basin tends to attenuate so that the easternmostsub-basins are free of any metamorphism and only slightly de-formed. These sub-basins are known as the Okondja and France-ville sub-basins, respectively in the North and South (Fig. 2). Dueto the limited deformation, their lithostratigraphic succession hasbeen precisely established more than 40 years ago (Donnot andWeber, 1968–1969; Weber, 1968) and has been only slightly mod-ified during the recent mapping project (Bouton et al., 2009b,2009d) (Fig. 3). From the South to the North the whole sequenceappears to be organized into two narrow basins including five suc-cessive formations known as the Francevillian A–E units (FA–FE).

The basal FA unit rests unconformably on the Archean base-ment. It is mainly composed of coarse-grained detrital rocks(quartz-rich sandstones, microconglomerates) passing upward toalternating sandstones and pelites. The environment of depositionis generally considered as fluvial in the lower part and deltaic tomarine in the upper part, which implies a progressive marine inva-sion as subsidence proceeded. The total thickness of the FA unitmay locally reach 1000 m, but more generally it is about 500 m,with strong lateral variations. An important aspect of the FA unitis the occurrence of uranium deposits (Gauthier-Lafaye, 1986;Gauthier-Lafaye and Weber, 1989); they are located in the unit


itself or at the top of the unit, in frequent contact with the overly-ing FB shale.

This FB unit differs from the previous one by a predominance offine-grained rocks with abundant shales and black shales. Coarse-grained rocks including polygenic breccias and quartz-rich sand-stones occur as local horizons of cartographic extension (Fig. 4). Fi-nely alternating medium- and fine-grained mudrocks have beeninterpreted as turbiditic sequences (Azzibrouck-Azziley, 1986; Par-ize et al., 2013). The FB unit is characterized by important thick-ness and facies lateral variations, and widespread evidence ofsynsedimentary tectonic activity (slump bedding, synsedimentaryfaulting, etc.). The thickness locally reaches several hundreds ofmeters in the axis of the basins but decreases toward the marginswhere siliceous rocks (jaspers) locally occur (Fig. 2). The suddenthickness increase suggests that the axial parts of the basins havesuffered intense subsidence in response to a rifting process. Thepolygenic breccias are considered to have formed by synsedimen-tary reworking along the basin margins. Likewise, turbidites arethought to have formed along the slopes of the basin.

It may be noted that the FB unit does not exist to the West (Boo-ué sub-basin; Fig. 2), (Donnot and Weber, 1968–1969; Prian et al.,1990) where the Francevillian Group as a whole appears as a fine‘blanket’ of variably folded sedimentary rocks covering the Arche-an basement (Prian et al., 1990; Ebang Obiang et al., 2009a, 2009b).Thus, the development of the FB unit in the Franceville and Okon-dja sub-basins appears intimately associated with the riftingaffecting the eastern region.

In the northern part of this region, along the margin of theOkondja basin (Fig. 4), the Francevillian rifting was accompaniedby the emplacement of the subvolcanic Ngoutou Complex. Thiscomplex shows a typical alkaline within-plate signature with coex-istence of two magmatic trends: an undersaturated trend charac-terized by peralkaline syenite and an oversaturated trendcharacterized by amphibole granite (Moussavou and Edou-Minko,2006). Evidence for the Francevillian age of this complex is pro-vided by an imprecise U/Pb zircon age at 2027 ± 55 Ma (Moussa-vou and Edou-Minko, 2006) and by the occurrence of widespreadalkaline lavas (alkaline basalts, trachytes, etc.) within the adjacentFB unit along the NE border of the Okondja basin (Fig. 4).

Finally, one important aspect of the FB unit in the eastern basinsis the occurrence of world-class manganese deposits. Supergeneenrichment was fundamental in the production of an economicore, but this process appears to have affected anomalously Mn-richrocks produced during the course of the FB sedimentary period(Martini and Makanga, 2001; Pambo, 2004; Pambo et al., 2006).The FC is a decametric-thick bed of massive siliceous rocks (mainlyjaspers with widespread sulfides), sometimes composite (two suc-cessive beds separated by a black shale horizon), which appears tocover indifferently all the previous formations, including the Arche-an basement (Figs. 3 and 4). For the most part, this siliceous horizonappears to have developed from the secondary silicification of bio-genic sediments (i.e. mainly stromatolites) and black shales (Boutonet al., 2009c). Likewise, the FC unit includes shallow marine dolom-ites (Préat et al., 2011) which are mainly represented to the west ofthe Franceville and Okondja sub-basins (Fig. 4), in a zone consid-ered as a shoal (Lastoursville shoal) separating these eastern sub-basins from the western Booué sub-basin (Fig. 3). Due to the strat-iform aspect of the siliceous horizon, the FC unit is considered as amajor lithostratigraphic marker in the Francevillian succession.Nevertheless, it may be noted that this horizon is not purely strati-graphic as it has developed secondary from various rocks, includingvitroclastic tuffs (Donnot and Weber, 1968–1969). It is envisagedthat silicification was proceeded synsedimentary in response to asudden input of silica in the basin (Bouton et al., 2012).

The FD unit does not differ very much from the FB unit, so thatthe occurrence of the siliceous FC horizon is the major argument

to calc-alkaline volcanism during evolution of the Paleoproterozoic Franc-), http://dx.doi.org/10.1016/j.jafrearsci.2013.12.007


Fig. 3. Synthetized lithostratigraphic sketch of the Francevillian basin (from Bouton et al., 2009b; modified after Donnot and Weber, 1968–1969).


for defining the FB and FD units as two different lithostratigraphicentities. The FD unit shows a rather constant thickness of somehundreds of meters (�200 to 400 m) and constitutes the main partof the Francevillian Group in the Booué sub-basin (Prian et al.,1990). It is mainly composed of fine- to medium-grained detritalrocks including shales, black shales and subordinate sandstones.Evidence for a synsedimentary volcanic activity is classically re-corded in the Franceville region, where the volcanic products aremainly pyroclastic rocks of felsic composition (Donnot and Weber,1968–1969; Chevallier et al., 2002). A U/Pb age of 2083 ± 6 Ma hasbeen obtained on a FD tuff by Horie et al. (2005), consistent with anage of 2072 ± 29 Ma obtained by Thiéblemont et al. (2009) on zir-cons extracted from an epiclastic sandstone of the same unit. Newmapping (Bouton et al., 2009a, 2009b, 2009c, 2009d) (Fig. 3) hasrevealed the existence of a basal horizon including more or lesssilicified black shales and discontinuous pyroclastic beds. Volcanic(tuffs) and volcanogenic (tuffaceous) rocks have also been ob-served at all levels in the unit.

The FE unit was formerly described as a medium-grainedsedimentary unit including sandstones and feldspathic sandstonesdistinct from the underlying FD rocks by its coarser character(Donnot and Weber, 1968–1969). Such a distinction was impossi-ble to maintain in most of the eastern basins where the only unitallowing the definition of a lithostratigraphic entity distinct fromthe FD unit is a �400 m-thick series of feldspathic sandstonesforming a lenticular outcrop in the northwestern part of the Okon-dja sub-basin (Mont N’Gouadi sandstones of Donnot and Weber,1968–1969) (Fig. 4). These sediments are assumed to have filleda local depression formed at the vanishing stage of the Francevil-lian sedimentation.


3. Nature and emplacement of the subvolcanic and volcanicrocks

3.1. Ngoutou Complex

The Ngoutou Complex is a subvolcanic ring-complex consistingof three successive intrusive units (Moussavou and Edou-Minko,2006): (i) an early external unit (N1) including syenites and micro-syenites (marginal facies) to the North, and alkaline granites to theSouth; (ii) a former internal unit (N2) made of microgranites; and(iii) a later internal unit including porphyritic to pegmatitic sye-nites (N3).

The microsyenites of the N1 unit are mainly composed of perth-itic microcline and peralkaline ferromagnesian minerals (riebeck-ite, aegyrine). In these unit and the N3 unit as well, the syenitesalso contain large biotite crystals with inclusions of feldspathoids(nepheline, cancrinite, sodalite).

The granites of the N1 and N3 units are composed of quartz,alkaline feldspar, amphibole and varied accessory minerals.

This petrographical diversity reflects a large compositionalrange; from peralkaline undersaturated facies (syenites) to metalu-minous saturated rocks (granites) (Moussavou and Edou-Minko,2006).

3.2. FB volcanic rocks

The FB volcanic rocks show a large range of mineralogical andtextural features, suggesting differences in the composition andmode of emplacement of the lavas. Their degree of alteration is also



Fig. 4. Simplified geological map of the eastern part of the Francevillian basin(modified from Bouton et al., 2009a, 2009c).

Fig. 5. General view of a slightly porphyritic alkaline basalt (FB unit) showingzoned euhedral clinopyroxene (Cpx), plagioclase (Pl) and clinopyroxene (Cpx)microlites. Vesicles (Ves) filled with calcite and chlorite are observed in differentplaces. XPL.


strongly variable and generally more pronounced in the differenti-ated pyroclastic rocks than in the mafic massive lavas and subvol-canic dikes. The volcanic series generally appears as a succession ofpoorly developed extrusive units with proximal facies (lava flowsand hyalocastitc breccias) thus suggesting an emplacement closeto the alimentation zones.

The most mafic facies occurs as a c. 10 m-thick intrusive bodyintercalated in the volcanic succession. The rock is remarkablyfresh, showing an intersertal texture with abundant pink clinopy-roxene (phenocrysts and microlites), red biotites, pseudomor-phosed olivines, large apatite crystals and secondary sphene and


calcite. The absence of feldspar indicates an ultramafic composi-tion. The titaniferous nature of the ferromagnesian minerals (clino-pyroxene, biotite) suggests and alkaline affinity. Locally, gabbroshave been observed with ultramafic rocks.

The basaltic rocks occur as lava flows and pyroclastites, withlocal pillowed structures indicating a subaqueous emplacement.Pink clinopyroxene and olivine (generally altered) occur as pheno-crysts set in a microlitic matrix made of pyroxene grains and feld-spar microlites (Fig. 5). Chlorite and calcite are the most frequentsecondary minerals, being present in the matrix and in amygdules.Ti-rich amphibole and biotite are secondary minerals. Again, theobserved paragenesis suggests an alkaline affinity.

Different pyroclastic facies are observed amongst the maficrocks including volcanic breccias (Fig. 6) and hyloclastites. Someof these rocks show important recrystallizations with a completereplacement of the ferromagnesian minerals by secondary phasessuch as actinolite, epidote (Fig. 6) and chlorite.

The differentiated rocks in the FB volcanic succession are de-fined as trachytes or trachyandesites. They include both feldspar-rich porphyritic lavas and pyroclastites (tuffs). Typical examplesare shown in Figs. 7 and 8. Despite well-preserved textures, mostof the rocks show important recrystallizations (Fig. 7) with areplacement of primary minerals by low temperature phases suchas calcite, chlorite or sericite.

3.3. FD pyroclastites and epiclastites

Volcanogenic rocks are present at all levels in the FD unit. Theyare mapped as a separate formation in the central part of the re-gion (Fig. 4), but frequently they appear as isolated outcrops withinthe dominantly sedimentary terrains. This volcanic activity isstrictly of explosive type and never includes lava flows. It is con-centrated in the Franceville sub-basin (Fig. 4) but also occurs inthe Booué (Fig. 2) sub-basin (Prian et al., 1990). The pyroclasticrocks are mainly vitroclastic tuffs of felsic composition (Figs. 9and 10). Their initial textures are well preserved but the originalmagmatic phases are generally replaced by low temperature min-erals such as sericite, iron hydroxide and clays. The epiclastic rocksrange from coarse-grained graywackes (Fig. 11) to fine-grained tuf-fites (Fig. 12). They include variable proportions of volcanogenicelements (lava fragments, feldspar phenocrysts, volcanic clasts)mixed with sedimentary ones (quartz grains, detrital micas)(Fig. 11) and a more or less abundant cryptocrystalline matrix.



Fig. 6. General view of a brecciated basaltic lava flow showing microlitic lavafragments (Lpl) in a recrystallized matrix composed of a secondary actinote–epidote (A–E) assemblage with some feldspar (K-feldpar ?) grains (F). XPL.

Fig. 7. Detailed view of an altered porphyritic trachyte (FB unit) showingrecrystallized K-feldspar phenocrysts (Ksp) set in an oriented matrix with abundantfeldspar microlites (F). Alteration products (dark zones) include ferriferouscarbonate and oxide grains. PPL.

Fig. 8. Detailed view of chloritized vesicular glass shard (Gl) in a polygenicpyroclastite (FB unit). The pumiceous shard is about 2 cm-large. PPL.

Fig. 9. Detailed view of a crystal-rich vitroclastic tuff (FD unit) showing a layerenriched in more or less clastic quartz (Q) and feldspars (F). PPL.

Fig. 10. Detailed view of a felsic vitroclastic tuff from the FD unit showing orientedlenses interpreted as fiammes (F). XPL.

Fig. 11. View of an epiclastic feldspathic sandstone from the FD unit with more orless clastic quartz grains (Q), subhedral feldspars (F) and rare flake mica (M). XPL.


Only epiclastic rocks (‘cinerites’) are known in the Booué sub-basin(Prian et al., 1990).


The occurrence of welded facies (vitroclastic tuffs, ignimbrites)(Weber, 1968) in the Franceville sub-basin implies that the mag-mas were erupted from a volcanic center(s) located at a moderatedistance.



Fig. 12. View of a fine-grained epiclastic tuffite from the FD unit with abundantclastic quartz (Q). PPL.


4. Geochemical study

4.1. Sampling and analyses

Sixty-six new geochemical analyses have been performed in thedifferent volcanic and volcanogenic formations: (i) the NgoutouComplex (6 analyses); (ii) the FB lavas and tuffs (27 analyses)and associated ultramafic (peridotite) to mafic (gabbro) intrusives;(iii) the FD tuffs (11 analyses); and (iv) the epiclastic rocks (mostlyin the FD unit, 17 analyses). Location of the samples is indicated onFig. 4, and the GPS-coordinates for all of them are given in the ta-bles provided as Supplementary material (Appendix A). Most of theFD volcanic rocks were collected in the southern part of the region,south of the town of Franceville where favorable outcrops wererecovered.

In addition to the volcanic and volcanosedimentary rocks, a lotof analyses have been performed on the purely sedimentary rocks,namely the quartz-sandstones (FA and FB) and shales–black shakes(mainly from the FB unit), and also on the FC jaspers (Thiéblemontet al., 2009).

The major elements were analyzed by ICP – AES at the OMAClaboratory and the trace elements by Instrumental Neutron Activa-tion (INA) at the P. Sue Laboratory (CEN Saclay, France). Precisionon the INA techniques and accuracies are given in Chayla et al.(1973).

Representative analyses are reported in Table 1;the whole set ofdata (from Thiéblemont et al., 2009) is provided as Supplementarymaterial (Appendix A).

4.2. Alteration effects

The petrographical study reveals a highly variable rate of alter-ation within the whole volcanic and subvolcanic units. From thedata reported by Moussavou and Edou-Minko (2006) on the Ngou-tou subvolcanic rocks, and especially the mention of fresh primaryminerals (feldspars, feldspathoids, ferromagnesian phases andaccessory minerals) in all the types of rocks, one may conclude thatthe complex has avoided intense weathering.

In the SiO2 vs. Na2O + K2O plot (‘TAS’ classification of Le Bas et al.,1986) (Fig. 13), the dispersion of the Ngoutou samples is indeedconsistent with the variations deduced from the petrographicalstudy, with the undersaturated rocks (nepheline-syenites) overlap-ping the fields of tephritic phonolite and phonolite, the saturatedrocks (syenites and quartz-syenites) being concentrated in the tra-chyte field and the oversaturated rocks (granites, microgranites) in


the rhyolite field. As far as trace elements are concerned regularchondrite-normalized REE patterns and linear La/Yb vs. Gd/Ybcovariations (Moussavou and Edou-Minko, 2006) provide strongarguments that post-emplacement processes have induced littlechemical remobilization.

Petrographical observations show that post-magmatic recrys-tallizations were generally more pronounced in the lava series thanin the Ngoutou Complex, with the possibility that such recrystalli-zations would have deeply modify the original chemical composi-tions. Indeed, most of the volcanic and epiclastic rocks display highvolatile contents (i.e. LOI generally P5%, Table 1) consistent withthe abundance of secondary hydrated phases.

In the SiO2 vs. Na2O + K2O diagram (Fig. 13) five out of the sixanalyzed basaltic samples actually plot in the field of alkaline ba-salt and the two ultrabasites are located in the field of picrobasalt.On the other hand, the location of the FB trachy-andesites andtrachytes is highly variable, and some of them plot in the field ofsubalkaline lavas (andesite, dacite). The FD tuffs show the most fel-sic compositions (dacite, rhyolite), but their total alkali contents(Na2O + K2O) are abnormally low. The Na2O and K2O contents inall the differentiated rocks, as well as in the epiclastites (Fig. 14),are strongly variable and many of them are almost devoid of Naand/or K. Most probably, this erratic behavior of alkali was causedby weathering thus precluding the use of these elements for petro-logical considerations. This let us to restrict the geochemical inter-pretations to the immobile trace elements.

4.3. Trace elements variability

Owing to the highly incompatible character of Th during mag-matic processes (Wood et al., 1979), binary plots using Th as a dif-ferentiation index enable petrological models to be easily tested.Binary plots using Ta, La, Hf and Ni (Fig. 15A–D) against Th estab-lished for Francevillian magmatic rocks show regular variationsindicative of the relative immobility of these elements duringweathering.

In the Th vs. Ta plot (Fig. 15A) a distribution of the samplesalong two linear correlation lines passing through the origin is ob-served: one for the Ngoutou intrusives and FB lavas (Th/Ta � 1.5),the other for the FD tuffs and epiclastic rocks (Th/Ta � 10–20).Within the first group, a broad increase is observed from the mafic(peridotites, basalts) to the differentiated (trachyandesites, trachy-tes) rocks. The range of the Th and Ta contents (�5 to 40 ppm forTh and �5 to 28 ppm for Ta) in the latter partially overlaps therange in the mafic rocks (�2.5 to 6 ppm for Th and 2–5 ppm forTa). In the second group, the Th contents range from �13 to28 ppm in the FD tuffs, and �4 to 25 ppm in the epiclastites, andTa is generally <1 ppm.

The Th vs. La plot (Fig. 15B) exhibits less well-defined correla-tion lines but a clear discrimination still appears between thetwo preceding groups. The first group (Ngoutou intrusives and FBlavas) shows a good La–Th correlation at low Th contents (maficrocks), with a mean La/Th ratio of 10, but this correlation is alteredfor higher values of La. The La content in many of the differentiatedrocks reaches more than 100 ppm, a typical feature of within-platealkaline rocks (Moussavou and Edou-Minko, 2006). La contents aregenerally lower in the second than in the former group, and the La/Th ratio is also more variable in the second group (�6 to 0.5).

The Th vs. Hf plot (Fig. 15C) shows a tight discriminationbetween the two groups, with lower Hf contents in the latter. Inthe former (Ngoutou intrusives and FB lavas) the Hf/Th tends todecrease from the mafic to the differentiated lavas, thus suggestingthat Hf could have been less incompatible than Th duringdifferentiation. The second group delineates a broad correlationindicating comparable values of the Hf/Th ratio in the FD tuffsand epiclastites.



Table 1Representative geochemical analyses of Francevillian volcanic and epiclastic rocks.

NgoutouComplex

Ultrabasite Alkaline basalts Alkalinetrachytes

Pyroclastites Epiclasticrocks

Sample BAT1017 OKO0823

OKO0538

OKO1231

BAT1038A

OKO0514

OKO1732

OKO0567

FRA0008

FRA0285

FRA0056

FRA0063

OKO0571

FRA0283

OKO0367C

FRA0211

Unit Ngoutou FB FB FB FB FB FB FD FD FD FD/FC FD FD FD FD FD/FC

SiO2 (%) 51.33 41.57 46.79 50.21 45.65 57.61 60.97 52.70 72.05 64.67 79.89 74.49 76.24 76.32 84.32 76.79TiO2 1.14 1.93 2.69 2.25 3.25 0.57 0.45 0.24 0.36 0.46 0.25 0.43 0.40 0.34 0.45 0.28Al2O3 20.30 8.01 12.41 15.57 13.81 14.74 15.06 14.97 16.15 16.18 8.63 10.76 10.52 11.50 9.87 12.34Fe2O3t 7.16 12.39 13.27 15.68 14.71 10.92 8.43 13.29 1.06 4.55 1.58 4.21 3.53 2.23 0.51 1.68MnO 0.142 0.156 0.181 0.400 0.103 0.438 0.305 0.683 0.004 0.117 0.005 0.065 0.059 0.041 0.013 0.042MgO 0.73 18.90 6.64 6.75 4.71 0.50 0.92 2.23 1.10 1.70 0.23 1.04 1.01 0.32 0.43 0.38CaO 1.47 9.82 10.05 0.65 6.41 0.31 0.53 0.26 0.04 0.24 0.03 1.57 0.27 0.16 0.04 0.05Na2O 5.82 0.25 1.93 2.07 3.65 0.06 3.06 1.52 0.02 1.14 1.96 3.06 2.21 5.09 0.04 3.69K2O 9.92 1.31 1.86 0.57 0.20 4.69 3.96 5.03 3.16 4.56 1.12 1.28 2.69 0.81 1.19 0.73P2O5 0.07 0.42 0.47 0.26 0.44 0.04 0.10 0.05 0.02 0.08 0.05 0.06 0.10 0.07 0.41 0.03LOI 2.19 5.56 4.97 6.63 8.24 8.44 5.83 8.18 4.89 3.98 5.00 2.41 2.23 2.61 3.86 4.50TOT 100.27 100.31 101.27 101.03 101.16 98.31 99.61 99.14 98.85 97.68 98.74 99.37 99.26 99.48 101.14 100.51Cs (ppm) 4.85 5.56 2.45 1.71 1.14 2.85 1.71 6.21 4.52 6.32 1.49 1.44 2.24 1.06 3.43 1.31Rb 582 505 61 30 2 177 211 155 139 176 49 58 93 29 57 33Ba 1395 1111 820 236 91 168 285 829 2546 723 489 329 679 511 2517 265Sr 985 556 526 62 440 8 61 56 21 51 33 150 68 141 440 76U 7.92 0.92 0.91 1.32 1.17 3.89 3.39 4.97 5.11 4.87 9.19 3.16 2.95 3.87 2.75 6.81Th 93.70 5.14 4.78 7.47 5.56 19.81 19.06 19.56 24.64 18.34 28.80 19.47 13.18 15.45 8.37 25.36Ta 28.58 3.82 3.89 3.84 4.16 13.82 11.56 1.00 1.34 1.41 2.76 0.89 0.70 0.91 0.60 1.57Hf 15.38 4.53 7.01 5.09 8.13 16.25 14.50 3.79 8.17 8.38 7.95 9.91 5.51 5.63 1.93 5.98Zr 973 183 526 210 320 702 615 131 289 316 216 379 204 209 78 195Sc 2.7 26.1 25.3 26.8 25.7 3.4 4.4 10.7 7.9 8.9 5.8 7.4 7.7 0.6 12.4 5.9Co 8.3 85.5 56.6 61.4 61.3 3.3 1.8 26.8 0.3 10.3 0.5 15.2 10.9 8.0 0.7 3.3Cr 36.0 1161.0 427.0 381.0 225.0 6.0 8.0 77.0 14.0 11.0 42.0 64.0 66.0 30.0 68.0 33.0Ni 5.0 768.0 204.0 279.0 155.0 3.8 0.2 32.0 12.0 14.3 10.6 31.0 31.0 8.3 15.4 21.7La 377.0 50.8 53.0 52.7 56.4 154.6 131.7 52.8 17.0 83.4 17.1 48.2 32.6 45.5 51.8 80.6Ce 1280 101 111 111 124 313 254 115 39 172 40 95 63 99 121 144Nd 540 44 48 47 52 106 74 33 66 15 30 22 34 39 57Sm 95.60 6.80 9.22 9.25 11.41 19.70 15.80 6.40 4.58 11.62 4.13 6.19 4.50 6.40 9.59 10.38Eu 30.60 2.09 2.68 2.42 3.14 1.88 2.78 1.54 1.14 2.15 0.46 1.29 1.15 1.42 2.86 1.10Tb 10.48 0.62 0.97 0.99 1.02 2.01 1.36 0.76 0.75 1.32 1.00 0.72 0.58 0.65 1.26 1.09Yb 23.30 1.16 1.95 2.33 2.03 6.36 4.31 3.08 3.16 4.26 5.79 2.53 1.88 2.60 3.28 4.26As nd 0.15 0.38 1.58 0.14 1.21 0.35 27.04 5.15 4.61 13.50 0.89 11.30 6.81 1.46 3.48Br 14.70 nd 0.69 0.58 nd 0.30 nd 0.43 0.48 0.12 4.58 0.45 0.66 0.27 0.42 1.44Mo 1.73 1.70 2.93 1.43 0.99 3.16 0.81 5.80 2.13 2.00 5.08 0.37 1.14 1.70 0.47 1.06Sb 0.06 0.03 0.02 0.27 0.06 0.32 0.14 5.06 0.66 0.66 0.62 0.27 0.70 0.43 0.93 0.26W nd 0.38 1.57 1.37 0.62 9.48 3.90 2.09 0.93 1.96 2.49 1.39 1.97 0.60 2.62 1.51Zn 84 94 127 163 135 181 112 33 11 54 26 123 42 20 60 26Ag (ppb) nd nd 39 114 90 nd 49 347 80 74 40 60 65 74 56 124Au 4.1 0.8 1.6 2.6 1.1 1.2 nd 10.6 1.8 1.5 10.2 0.6 2.5 1.7 1.0 29.8

LOI – Loss on ignition, nd – not detected.


The Th vs. Ni plot (Fig. 15D) strongly contrasts with the preced-ing owing to the highly compatible character of Ni. A broad de-crease of the Ni contents at increasing Th contents is consistentwith the well-known fractionation of Ni in early-crystallizing maficphases (especially olivine). Very low Ni contents in the FD tuffs areconsistent with the felsic composition of these rocks. Ni reaches750 ppm in the ultramafic intrusives and is close to 100 ppm inan associated gabbro. The high Th contents in these rocks(�5 ppm) preclude that their ultramafic composition could be aconsequence of olivine accumulation. This is consistent with thepetrographic observations indicating the subvolcanic nature ofthese rocks.

From the above observations we may conclude that many of thetrace element features of the Francevillian magmatic rocks havebeen preserved despite the recrystallizations affecting most ofthem, allowing the nature of the original magmas to be preciselydefined.

In the (Th/Ta)N vs. (Tb/Ta)N diagram (Thiéblemont et al., 1994)(Fig. 16) the two petrological groups revealed by the binary plots(Fig. 15) are actually located in well-separated fields, thus indicat-ing clearly distinct geochemical signatures. The Ngoutou intru-sives, FB lavas and associated ultramafic intrusives plot as an


elongated cloud almost entirely included within the field of with-in-plate alkaline lavas. This is consistent with the well-establishedalkaline nature of the Ngoutou Complex (Pascal, 1963; Moussavouand Edou-Minko, 2006). The grouping of the FB basalts in the dia-gram suggests a rather homogeneous mantle source. The ultra-mafic intrusives plot below the basalts, indicating a lower Tb/Taratio. This lower ratio is consistent with the higher Th and Ta con-tents (Fig. 15A) and could be a consequence of a lower degree ofmelting (Thiéblemont et al., 1994). Like the ultramafic intrusives,some of the differentiated lavas plot below the basalts, which couldindicate fractionation of Tb during differentiation. A precise petro-logical modeling is beyond the scope of this paper, nevertheless itmay be noted that despite its very large compositional range, theFrancevillian alkaline magmatism shows a rather constant geo-chemical signature (incompatible elements) suggesting that a largepart of its evolution was driven by closed-system differentiationprocesses.

In strong contrast with the alkaline rocks, all the FD tuffs plot inthe field of calc-alkaline lavas (Fig. 16). This points to the ‘orogenic’signature of these rocks, a question which is discussed below. Thereport of the epiclastic rocks in this diagram reveals their similaritywith the FD tuffs. On the other hand, it is worth noting that the



Fig. 13. SiO2 vs. Na2O + K2O plot (Le Bas et al., 1986) for the different magmaticunits: (i) Ngoutou Complex (including analyses from Moussavou and Edou-Minko,2006), (ii) FD tuffs; and (iii) FB lava series (including ultramafic intrusives).

Fig. 14. Na2O vs. K2O plot for the differentiated volcanic and epiclastic rocks of theFB and FD units.


epiclastites are totally different from the alkaline rocks, a conclu-sion also evident in the binary plots (Fig. 15A–C).

The FD tuffs have been plotted in the Yb vs. Ta diagram (Fig. 17)(Pearce et al., 1984), a plot suited to the geotectonic discriminationof felsic rocks. All the analyses but two are located in the field ofvolcanic arc magmatism, therefore confirming the ‘orogenic’signature of the FD tuffs. Likewise the epiclastic rocks are almostentirely included in the volcanic arc field, but this location hasno geotectonic significance owing to the volcanosedimentarynature of these rocks.

4.4. Sedimentary rocks

Two triangular plots (La–Th–Sc, Th–Sc–Zr/10) suited to the dis-crimination of graywackes (Fig. 18A and B) (Bhatia and Crook,1986) have been used with the aim of testing the compatibilityof the tectonic discrimination obtained from the FD tuffs with thatderived from the epiclastic rocks. Analyses of the Francevillianquartz-sandstones (mainly from the FA unit) and shales – blackshales (mainly from the FB unit) have also been plotted in orderto identify possible variations within the Francevillian sedimentarysequence. In the two diagrams (Fig. 18A and B) a significant part ofthe analyses plot outside the discriminant fields. Such shifts arementioned by Bhatia and Crook (1986) for Archean rocks but arenot considered by these authors as a cause for rejecting the useof the diagram.


In the La–Th–Sc diagram (Fig. 18A), the epiclastic rocks andshales–black shales plot very close to each other, overlapping theB (continental island arc) and C and D (undiscriminated active con-tinental margin and passive margin) fields. The quartz-sandstonesplot mainly in the C and D field or close to it. No sample plot in theA field (oceanic island arc). A partial discrimination is observed be-tween the quartz-sandstones and epiclastic rocks (+shales–blackshales).

In the La–Th–Zr/10 diagram (Fig. 18B), the epiclastic rocks arerather dispersed though being never located in the A field. Mostsamples plot in the B field or close to it and two samples are situ-ated in the D field. This repartition is consistent with that observedin the La–Th–Sc diagram (Fig. 18A), pointing to significant analo-gies between the Francevillian epiclastic rocks and recent graywac-kes associated with continental arcs. All the quartz-sandstones plotin the D field (Passive margin) or close to it. Their location close tothe Zr/10 summit reflects a selective partitioning of Zr in the origi-nal sediments relative to La and Sc. The behavior of Zr during sed-imentary processes is mainly controlled by zircon, a mineral likelyto remain in the medium- to coarse-grained fraction of maturesediments such as the passive margin-related graywackes.

5. Discussion

Most researches on the Francevillian Group have been focusedon the sedimentary succession with little attention brought tothe associated magmatism. For instance, magmatism is almost to-tally omitted in recent articles dealing with the Francevillian litho-stratigraphic succession (e.g. Parize et al., 2013). It is unfortunatethat this aspect of the geological evolution has received so littleattention because volcanism may have fundamental connectionswith the thermic structure of the basin (and therefore its mineral-izations) and constitute a significant source of sediments supply.Moreover, magmatism is a precise indicator of geotectonic con-texts, and variations in the nature of magmatism may reveal fun-damental transitions of the geodynamic regime.

At first sight, the occurrence of an alkaline magmatic event(Ngoutou Complex and FB lavas and intrusives) may be consideredas consistent with the location of the basin within and old base-ment region, in a typical within-plate setting. Following theemplacement of the FA detrital series in a wide continental depres-sion under a fluvial then marine (‘tide-related’) environment (Gau-thier-Lafaye, 1986; Pambo et al., 2006), the alkaline volcanism andconcomitant deposit of the FB sequence appear to have accompa-nied a NNE-directed rifting phase restricted to the Francevilleand Okondja sub-basins (Figs. 2–4) (Pambo et al., 2006; Thiéble-mont et al., 2009).

A within-plate environment and strictly diverging geodynamicregime of the Francevillian basin during its early stages of evolu-tion is also consistent with the passive margin ‘signature’ of theFrancevillian FA–FB quartz-sandstones (Fig. 18B). Such a signaturereflects the mature character of this sedimentation precluding thatit could have been fed by juvenile materials. On the other hand, theassociated shales and black shales display a clear orogenic signa-ture (Fig. 18A and B). Because of their pelitic composition theserocks are not suited for plots based on coarse-grained rocks (‘gray-wackes’, Bhatia and Crook, 1986). Indeed, pelitic rocks inherit mostof their trace element characteristics from their source (Taylor andMcLennan, 1985), so that the orogenic signature of the Francevil-lian shales and black shales most probably indicates that they werederived from a basement mainly composed of calc-alkaline grani-toids of TTG type (Chevallier et al., 2002; Thiéblemont et al., 2009).

It is worth noting that none of the Francevillian sedimentaryrocks have inherited the trace element characteristics of the FBalkaline lavas. A likely explanation for that is that these lavas were



Fig. 15. Binary plots of incompatible (Ta, La, Hf) and incompatible (Ni) elements against Th for the different magmatic and epiclastic rocks of the Francevillian basin.Comparison with representative rhyolites from the Taupo volcanic zone (analyses from Sutton et al., 1995). A – Th vs. Ta plot; B – Th vs. La plot; C – Th vs. Hf plot; and D – Thvs. Ni plot. In this plot, the line on the left connects the ultramafic to mafic (gabbro) intrusives.

Fig. 16. (Th/Ta)N vs. (Tb/Ta)N plot (Thiéblemont et al., 1994) for the Francevillianmagmatic and epiclastic rocks. Th, Ta and Tb contents are normalized to theprimordial mantle (after Hofmann, 1988). Discriminant fields are as follows:N-MORB – field of depleted mid-ocean ridge basalt; E-MORB – field of enrichedmid-ocean ridge basalt; WPB – field of within-plate alkaline basalt; BAB – field ofback-arc basin basalt; CFB – field of continental tholeiite; IAT – field of islandtholeiite; CAB – field of calc-alkaline basalt.

Fig. 17. Yb vs. Ta plot (Pearce et al., 1984) for the FD tuffs and epiclastic rocks.Comparison with representative rhyolites from the Taupo volcanic zone (analysesfrom Sutton et al., 1995). Same symbols as on Fig. 14. Discriminant fields are asfollows: Syn-COLG – syn-collision granite; VAG – volcanic arc granite; ORG – oceanridge granite; WPG – within-plate granite.


emplaced in a subaqueous environment and not reworked by ero-sion after their emplacement.

The volcano-sedimentary FB period leads to the filling of theFranceville and Okondja sub-basins. Thus this filling could be con-sidered as the ultimate phase of the within-plate evolution and thefurther FC silicic event as the onset of a new phase. A possible


explanation for this intensive event of silicification could be a mas-sive input of silica caused by phreatomagmatic explosions in theinitial stage of the FD volcanism.

The geochemical signature of the FD volcanic rocks is clearly of‘orogenic’ type, and a wide gap separates these rocks from the FBalkaline products. The geochemical data preclude any kind of



Fig. 18. La–Th–Sc (A) and Th–Sc–Zr/10 (B) discriminatory plots (Bhatia and Crook,1986) of the Francevillian sedimentary and volcano-sedimentary rocks (FA–FBquartz-sandstones, FA–FD shales – black shales, FB–FD epiclastic rocks). Discrim-inant fields are as follows: A – greywackes from oceanic island arc; B – greywackesfrom continental island arc; C – greywackes from active continental margin; D –greywackes from passive margin.

Fig. 19. Chondrite-normalized (chondritic abundances from Anders and Grevesse,1989) rare-earth element patterns for the FD tuffs (continuous line) and Tauporhyolites (dashed line) (analyses from Sutton et al., 1995).


transition or mixing between the two groups thus suggesting thattheir geneses were driven by totally distinct petrological (and geo-tectonic) processes. On the other hand the shales and black shaleswhich form a major part of the FD units (Fig. 3) show no systematicdifference with those of the FB units. This fine-grained sedimenta-tion started in the FA unit and continued until the end of the FDperiod.

The strictly acidic composition of the FD tuffs is reminiscent ofthe mainly felsic magmatism that occurs in large calc-alkaline toalkaline provinces in post-orogenic geotectonic settings (Liégeoiset al., 1987; Richard et al., 1989). However such a setting is precludedfor the Francevillian basin system: (i) deformation of the FD rocks inthe Booué sub-basin clearly indicates that the deposit of this unittook place in the course of the Eburnean orogenesis, and (ii) the U/Pb ages of 2083 and 2072 Ma reported for FD rocks (Horie et al.,2005; Thiéblemont et al., 2009) are actually older than the age of c.2050–2000 Ma estimated for the major Eburnean compressivephase in the Ogooué orogenic belt (Thiéblemont et al., 2009).

Episodes of calc-alkaline volcanism of mainly felsic compositionare also reported in ensialic back-arc basins. A well-known exam-ple is the Taupo volcanic depression, in the North Island of NewZealand. Representative analyses of the Taupo rhyolites (from Sut-ton et al., 1995) are reported on Figs. 15 and 17. Despite differencesin absolute concentrations the Taupo rhyolites and FD tuffs display


comparable ranges of incompatible element ratios and both volca-nic series are located in the Volcanic arc field of the Yb vs. Ta plot(Pearce et al., 1984).

The chondrite-normalized rare earth element (REE) patterns(Fig. 19) attest for a larger range of light REE-enrichment in theFD tuffs than in the Taupo rhyolites, but both groups are character-ized by a slight fractionation between heavy REE. Such patternsindicate an origin of the felsic magmas either by fractional crystal-lization of a tholeiitic mafic magma or partial melting of a crustalprotolith at rather low pressure (Arth, 1979).

The transition from the alkaline FB magmatism to the calc-alka-line FD volcanism suggests a major change in the geodynamic evo-lution of the Francevillian basin during the FC episode. To the eastof the Francevillian basin, the occurrence of 2.118 Ga-old zirconsreworked in metasedimentary rocks of the Ogooué belt (Thiéble-mont et al., 2009) indicates that the sedimentation in the Ogoouébasin started after 2.118 Ga. The onset of converging movementsin the Gabonese Paleoproterozoic domain is indicated by c. 2.08–2.04 Ga-old K-rich granites in the West-Gabonian block (Fig. 1)(Thiéblemont et al., 2009), which is consistent with the age of2.083 Ga obtained on FD tuffs (Horie et al., 2005). Comparableresults are obtained in South-Western Cameroon, in the Northernpart of the Eburnean Belt (Fig. 1). In this region, ages of c. 2.07–2.05 Ga are obtained from high-grade metamorphic rocks (char-nockites) and associated syntectonic plutons belonging to theNyong Group (Lerouge et al., 2006). Thus it is probable that thealkaline to calc-alkaline volcanic transition in the Francevillian ba-sin occurred in response to a change of geodynamic regime that af-fected the whole Eburnean Belt. Considering the apparentorganization of this orogen, it may be envisaged that the calc-alka-line volcanism could have occurred at a time where the Francevil-lian basin was located in the back-arc side of a magmatic arcassociated with a NE- to E-directed subduction zone (Fig. 1). Crus-tal overthickening in the arc domain could have induced a markedpost-tectonic uplift leading to the exhumation of high-grade meta-morphic rocks (Lerouge et al., 2006) and subduction-related plu-tons (Thiéblemont et al., 2009). At present, such a model remainshighly speculative and further works will be necessary to deter-mine the source of the FD magmas and how they were produced.

6. Conclusion

This study of the Francevillian magmatism in the entire litho-stratigraphic interval where this volcanism occurs indicates the




strongly different nature of magmas respectively in the FB and FDunits. The alkaline signature of the former is consistent withemplacement in within-plate tectonic setting, whereas the calc-alkaline nature of the FD volcanism indicates an emplacement inconverging regime. Quartz-sandstones associated with the FAand FB units show the common signatures of mature sedimentaryrocks emplaced in anorogenic settings. Epiclastic rocks associatedwith the FD tuffs display ‘orogenic’ signatures consistent with aconverging setting. Between the FB and FD units, the FC silicichorizon could correspond to a phreatomagmatic event precursorto the FD pyroclastic episode. It is suggested that the onset ofcalc-alkaline volcanism in the Francevillian basin resulted from atransition from a diverging to a converging regime at a regionalscale. Nevertheless, further works will be necessary to defineprecisely the setting of the FD volcanism and it possiblerelationship with an ‘Eburnean’’ subduction zone.

Acknowledgments

Geological mapping in Gabon was sponsored by E.U. (Sysminfunds) and the Gabonese Ministry of Mines. The BRGM ScientificDirection provided financial support for manuscript redaction.The team of the ‘‘Sysmin’’ Project is greatly acknowledged for theinvaluable years we spent together in the Gabonese forest. Con-structive reviews by P. Barbey, J.P. Liégeois and J.E. Scandolara weregreatly appreciated.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.jafrearsci.2013.12.007.

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Transition from alkaline to calc-alkaline volcanism during evolution of the Paleoproterozoic Francevillian basin of eastern Gabon (Western Central Africa)