transitional tholeiitic basalts in the tertiary bana … · 2006. 10. 25. · oceanic and...

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Journal of African Earth Sciences 45 (2006) 318–332

Transitional tholeiitic basalts in the Tertiary Banavolcano–plutonic complex, Cameroon Line

Gilbert Kuepouo a,b,*, Jean Pierre Tchouankoue b, Takashi Nagao c, Hiroaki Sato a

a Graduate School of Science and Technology, Department of Earth and Planetary Sciences, Faculty of Science, Kobe University,

Nada, Kobe 657-8501, Japanb Department of Earth Sciences, Internal Geodynamics Laboratory, University of Yaounde-I, PO Box 812, Yaounde, Cameroon

c Center for Instrumental Analysis, Yamaguchi University, 1677-1 Yoshida, Yamaguchi 753-8512, Japan

Received 10 May 2005; received in revised form 7 March 2006; accepted 9 March 2006Available online 18 May 2006

Abstract

The Bana transitional tholeiitic basalts occurring in a Tertiary volcano–plutonic complex of the Cameroon Line, Central Africa areplagioclase-bearing and olivine-free. K/Ar dating on separated plagioclases of the transitional tholeiitic basalts yields an Oligocene age of30.1 ± 1.2 Ma. Their clinopyroxene compositions are marked by iron enrichment and calcium depletion in the Wo–En–Fs system. Thewhole-rock major element compositions are characterized by Mg# � 36–48, normative quartz and hypersthene. The youngest alkali bas-alts from the same igneous complex have higher Mg# � 56–66. These two groups of basalt have trace element characteristics of within-plate basalt with Zr/Nb ratios of 3.7–4.5 and 7.5–9.2 respectively, and different LILE/HFSE and LREE/HREE ratios. The overall traceelement characteristics suggest that the transitional tholeiitic basalts of the Bana complex were derived by high degrees of partial meltingin the upper mantle at shallow depths whereas younger alkali basalts in the complex were probably produced by a small degree of meltingof the same source at slightly greater depths. The transitional tholeiitic character of these basalts suggests a significant lithospheric exten-sion and mantle upwelling below the Cameroon Line in the Oligocene.� 2006 Elsevier Ltd. All rights reserved.

Keywords: Transitional tholeiitic basalts; WPB; Tertiary Bana volcano–plutonic complex; Cameroon Line

1. Introduction

Oceanic and continental lavas constitute the extrusivesection of the Cameroon Line (Fig. 1), a line of Eocene–Oligocene anorogenic volcano–plutonic complexes, andOligocene to Present volcanic centers.

The Cameroon Line comprises about 60 anorogenic vol-cano–plutonic complexes, and a large number of polyge-netic and monogenetic volcanoes extending from PagaluIsland in the Atlantic Ocean (SW) to lake Chad (NE) onthe Africa continent. A number of studies of continentaland oceanic basalts of the Cameroon Line have contrib-

1464-343X/$ - see front matter � 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.jafrearsci.2006.03.005

* Corresponding author. Address: Department of Earth Sciences,Internal Geodynamics Laboratory, University of Yaounde-I, PO Box812, Yaounde, Cameroon. Tel.: +237 720 22 71.

E-mail address: [email protected] (G. Kuepouo).

uted to the understanding of geochemical and isotopic fea-tures of alkali basalts (Fitton and Dunlop, 1985; Fitton,1987; Halliday et al., 1988, 1990; Lee et al., 1994; Marzoliet al., 2000; Rankenburg et al., 2005). These studies high-lighted the chemical and isotopic similarities between bas-alts from oceanic and continental sectors, their alkalinenature and the dominance of the HIMU mantle sourceduring their genesis (Halliday et al., 1990).

Deruelle et al. (1991) showed that extensive petrologicalstudies of the Cameroon Line volcanic and plutonic rocksdo not allow the conclusion that magmatism develops atransitional trend in that region as is the case in the East-African Rift. They discarded a transitional character ofsome basalts from Mt Oku, Manengouba and Principeisland (Fitton, 1987), and gabbros from the Mboutouigneous complex (Parsons et al., 1986) on the basis of theirhigh niobium contents compared with typical transitional

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Fig. 1. Geologic map of the Bana plutono–volcanic complex (Kuepouo et al., 2004). Inset shows the position of the Bana volcano–plutonic complex in theCameroon line.

G. Kuepouo et al. / Journal of African Earth Sciences 45 (2006) 318–332 319

basalts of the East-African Rift (Kampunzu and Mohr,1991) and OIB. Further record of transitional basaltswithin the Cameroon Line by some workers (Kampunzuand Lubala, 1991; Moundi et al., 1996; Moundi, 2004;Fosso et al., 2005) is hotly debated.

For clarification, transitional basalts are basalts havingcompositions intermediate between tholeiitic and alkaline

basalts, and typified by their mildly hypersthene-normativeto mildly nepheline-normative character.

The objectives of this paper are: (1) to prove the occur-rence of transitional tholeiitic basalts discovered within theCameroon Line; (2) to discuss their clinopyroxene andwhole-rock chemistries in comparison with those of typicalalkali basalts of the Cameroon Line occurring in the same

320 G. Kuepouo et al. / Journal of African Earth Sciences 45 (2006) 318–332

complex; and (3) to define and understand the genetic rela-tionship between these two groups of basalts in this setting.

2. Geological background

The Tertiary Bana volcano–plutonic complex (5�8 0S,5�11 0N; area ca. 55 km2) is located southeast of the WestCameroon Highlands in the central part of the CameroonLine (Fig. 1). A previous petrological study on the Banacomplex (Nana, 1988) distinguished: (1) the plutonicunit including a biotite ± amphibole granite, an arfvedso-nite ± aegirine granite, and a small lens of leucogabbro;(2) the volcanic unit made up of small lava flows, basalticlapilli tuffs and rhyolitic cinder tuffs forming a ring aroundand on top of the arfvedsonite ± aegirine granite at theNorth.

Attempts to date the Bana complex by the K/Ar methodresulted in values that ranged from 38 ± 1 and 42 ± 8 Maon benmoreite lavas (Cantagrel et al., 1978; Nana, 1988)and 30 Ma (no analytical precision reported) on ‘‘evolvedlava’’ (Lasserre, 1978). The arfvedsonite ± aegirine graniteyielded an Rb/Sr age of 51 ± 1 Ma and an initial 87Sr/86Srratio of 0.7035 ± 0.0001 (Caen-Vachette et al., 1991).

A detailed geologic map (Fig. 1) of the Bana volcano–plutonic complex is available from Kuepouo (2004). TheBana complex forms a prominent mountainous scarp cul-minating at 2097 m above sea level, rising 600 m abovethe general level of the countryside to the south and350 m to the northwest. This complex is bounded to thesouth by an elevated (up to �1500 m) crystalline basementencompassing Neoproterozoic granite and gneisses, crosscut by mafic and felsic dykes. Aerial photographs and fieldobservations reveal a sharp discordant contact between thecomplex and the basement to the south.

The complex includes two crescent-like plutonic unitsforming the ‘‘Southern Intrusions’’ and the ‘‘NorthernIntrusions’’ (Fig. 1). From west to east, the Southern Intru-sions consist of biotite-amphibole granite and biotitemiarolitic granite, whereas the Northern Intrusions aremade of arfvedsonite-eckermannite ± aegirine granite andsyenodiorite and quartz-syenodiorite. Syenodiorites of theouter margin include several alkali feldspar and quartzcrystal basement xenoliths.

The volcanic rocks include lava flows and pyroclasticrocks exposed in the center and north of the complex.The lava flows are porphyritic and subaphyric plagioclase-basalts, olivine-basalts, olivine-clinopyroxene-basalts,andesitic basalts, hawaiite, benmoreite, rhyolite, arfvedso-nite ± aegirine rhyolite, and unusual high Fe2OT

3 –Al2O3–

TiO2 rocks resembling lava flows associated withplagioclase-basalts. The plagioclase-basalt is the volumetri-cally most abundant lava type followed by benmoreite,andesitic basalt and felsic lavas. Small preserved exposuresof olivine-basalts and olivine-clinopyroxene-basalts are theyoungest lava flows of the complex (Fig. 1). Pyroclasticdeposits are chiefly tuff breccias of intermediate to rhyoliticcomposition, ignimbrite and basaltic lapilli.

Previous petrological studies of basalts along the Cam-eroon Line showed that they were derived from a depletedasthenospheric source beneath the subcontinental litho-sphere (Halliday et al., 1988; Sato et al., 1990), or fromamphibole-bearing lithospheric mantle for the continentalbasalts (Marzoli et al., 2000). Geophysical studies revealedthat the Cameroon Line is underlain by a thin crust ofca. 30–34 km thickness (Fairhead and Okereke, 1987;Plomerova et al., 1993; Poudjom Djomani et al., 1995).Previous hypotheses on the origin of the Cameroon Lineare documented elsewhere (Moreau et al., 1987; Deruelleet al., 1991; Burke, 2001). Recent studies by PoudjomDjomani et al. (1997) suggest that the Cameroon Line fol-lows a major structural zone in the lithosphere.

3. Petrography

3.1. Plagioclase-basalts

The plagioclase-basalts from Bana are subaphyric toporphyritic with a primary mineral assemblage dominatedby plagioclase, Fe–Ti oxides and clinopyroxene. In plagio-clase-phyric basalts, plagioclase phenocrysts >3 mm acrossrepresent ca. 30–50 vol.%. Large plagioclase phenocrystsrange from 5 to 15 mm in length. Individual phenocrystsare euhedral to subhedral, although strongly corrodedcrystals were observed. Glomeroporphyritic associationsshow cruciform intergrowth of plagioclase. The intersertalto intergranular groundmass is dominated by plagioclasemicrolites. Phenocryst and microlite compositions varyfrom bytownite to andesine. Interstitial minerals areFe–Ti oxides, apatite needles and occasionally albite andaccessory titanite. Plagioclase phenocrysts are partly saus-suritized in altered samples and the groundmass partlyreplaced by micas, epidote and carbonate. Chlorite replacesglass.

Clinopyroxene constitutes 7.6 vol.% mostly as microlites(<0.3 mm) and occasional microphenocrysts (up to 1 mmacross). Microphenocrysts form isolated grains in thegroundmass or inclusions in plagioclase phenocrysts, andare occasionally chloritized. In the groundmass of porphy-ritic basalts, small grains of clinopyroxene fill the intersti-tial space between plagioclase laths.

Fe–Ti oxide microphenocrysts (titanomagnetite andilmenite) represent 0.4 ± 0.2 vol.%. Extremely fine granulesand fretted opaque crystals occur in the groundmass.Alkali feldspar-quartz-epidote-calcite-amphibole can beintergrown in clots of secondary origin mostly occurringin rare amygdaloidal plagioclase-basalts.

3.2. Olivine-basalts and olivine-clinopyroxene-basalts

Olivine-basalts and olivine-clinopyroxene-basalts areporphyritic and consist of phenocryts of olivine in basaniteand olivine-basalt, olivine and clinopyroxene in hawaiitefrom the northern part, and clinopyroxene alone in hawai-ite from the southern part of the complex. The groundmass


is intersertal to intergranular and locally hyalopilitic. Inter-granular groundmass consists of olivine, clinopyroxene,plagioclase, Fe–Ti oxides, and occasionally apatite needles,secondary carbonate, titanite and zeolites.

Olivine occurs as euhedral, subhedral or skeletal crys-tals. Olivine phenocrysts (0.4–2 mm) and microphenocrystsrepresent about 15–6.5 vol.% in basanite and �3 vol.% inolivine-basalt. Olivine is <0.1 mm across in the ground-mass and converted into iddingsite, serpentine and chloriteand iron oxides.

Clinopyroxene consists of diopside and rare augite. Itforms about 9 vol.% of the rock among which 6 vol.%are phenocrysts. Some of them have small inclusions ofplagioclase and olivine. Fe–Al-spinel inclusions occur inclinopyroxene of hawaiite from the south. The largest crys-tals show extensive internal melting outlined by a sieve tex-ture. Glomerophyric association of augite-plagioclase issparse in all samples. Concentric and sector zoning andtwinning are common in clinopyroxene. Intergrowth ofacicular clinopyroxene and amphibole replace primaryclinopyroxene in olivine-basalt. Less commonly, smallrandomly oriented clinopyroxene prisms occur in plagio-clase in basanite. In the groundmass, clinopyroxene formsrounded or rectangular grains. Plagioclase microlite com-positions vary between bytownite and labradorite. Biotitewith yellow to brownish pleochroism occurs in the ground-mass of hawaiite from the south.

Titanomagnetite (occasionally converted to titanite)coexists with ilmenite. Quartz xenocrysts with wavyextinction jacketed by clinopyroxene corona, and alkalifeldspar with disequilibrium texture occasionally occur inbasanite.

4. Radiometric age

Plagioclase was manually and magnetically separatedfrom fresh plagioclase-phyric basalt. Selected fractionswere briefly treated in HF (5%) to remove any glass inclu-sions and rinsed in distilled water for accurate and preciseK/Ar determinations at the Research Institute of NaturalSciences, Okayama University of Science in Japan. Gener-ally, fresh plagioclases of these basalts have high concen-tration of K2O � 0.3–1 wt% (microprobe analyses) whichis sufficient to perform K/Ar radiometric dating.

Plagioclase is assumed to be a closed-system with respectto radiogenic 40Ar and 40K. The results show an averageradiometric age of 30.1 ± 1.2 Ma (Table 1).

Table 1Analytical results of the K–Ar dating of transitional tholeiitic basalt of theBana complex

Sample No. Potassium(wt%)

Rad. 40Ar(10-8 cc STP/g)

K–Ar age(Ma)

Non-rad.40Ar(%)

K48-B 0.198 ± 0.10 24.35 ± 0.55 31.4 ± 1.7 34.622.34 ± 0.51 28.8 ± 1.6 36.2

30.1 ± 1.2 35.4

5. Clinopyroxene chemistry

Clinopyroxenes were analyzed by electron probe micro-analyzis at the Venture Business Laboratory of Kobe Uni-versity using a JOEL X-8900 electron microprobe equippedwith a wavelength dispersive analytical system. Operatingconditions were 15 kV and 12 nA using a focused beam.Standards were Si: SiO2, Al: Al2O3, Na: NaCl, Mg:MgO, Ti: TiO2, Fe: Fe2O3, Mn: MnO, Ca: CaSiO3. Correc-tions were made using atomic number, absorption andfluorescence incorporated routine methods.

The data confirm the microscopic analysis that clinopy-roxenes are chiefly diopside and augite (Morimoto et al.,1988) in the alkali basalts, and augite in the plagioclase-basalts. Diopside and augite are highly variable interms of Ca, Ti, Al and to some extent Cr contents (Table2).

For example, the Ti/Al ratio varies widely even in clin-opyroxenes from the same lava: 1.8–0.2 in olivine-basalts,�1–0.2 in olivine-clinopyroxene-basalts. In contrast, thecomposition appears fairly constant in porphyritic andaphyric plagioclase-basalts.

Clinopyroxene compositions are in the rangeWo51�43Ens36.7�42.4Fs12.8�15.5 in olivine-basalts and olivine-clinopyroxene-basalts, and Wo47�36Ens35�39.5Fs15.5�24.5 inplagioclase-basalts (Fig. 2).

Ti and Al are lower in clinopyroxene from plagioclase-basalts than those from olivine-basalts and olivine-clinopy-roxene-basalts (Table 2).

This difference is of petrological significance since Tiand Al contents of clinopyroxenes are related to the crys-tallization condition and the initial magma composition(Lundstrom et al., 1998; Hill et al., 2000; Wood et al.,2001). Clinopyroxenes of basalts from the Bana complexcan be distinguished and classified as titaniferous calcic-clinopyroxene and weakly non-titanoferous calcic-clinopy-roxene based on the amount of Ti and Ca entering thecrystal. Titaniferous augites occur in olivine-basalts andolivine-clinopyroxene-basalts whereas non-titaniferousaugites typify plagioclase-basalts.

6. Whole-rock geochemistry

Major and trace elements were determined using X-rayfluorescence spectrometry (Rigaku RIX 3000) at the Centerfor Instrumental Analysis at Yamaguchi University inJapan. Beads were prepared using 0.9 g powder sample,4.5 g lithium tetraborate (Li2B4O7) flux and 0.54 g LithiumIodide. Six GSJ (Geological Survey of Japan) geochemicalstandards were analyzed to verify the accuracy of the anal-yses. The results are within the range of recommended val-ues of the standards. The estimated accuracy X2 � 0.0022 iswithin accepted limit (<0.10–0.15) for major elements.

A subset of olivine-basalt (G6, K14), olivine-clinopyrox-ene-basalt (K68), and three plagioclase-basalts (K56, K94,KG3) were selected for ICP-MS analyses of REE at ACT-Lab in Canada. The results are presented in Tables 3 and 4.

Table 2Selected electronmicroprobe analyses of clinopyroxenes

wt% K62-Bsph

K62-Bsmicro

K62-Bsmicro

G5-HwPhcore

G5-HwPhrim

G5-HwPhcore

G5-Hwgm

G5-Hwgm

G6-Bnphcore

G6-Bnphcore

G6-Bnphrim

G6-Bnphrim

G6-Bngm

G6-Bnphcore

K14-Bnphcore

K14-Bnphrim

K14-Bnphrcore

K14-Bnphrim

K14-Bngm

SiO2 51.40 49.30 49.74 50.24 51.38 50.51 49.27 51.22 47.78 47.37 46.71 46.90 50.91 49.32 45.93 47.40 50.10 48.82 48.33TiO2 1.69 2.73 2.30 1.71 0.83 1.01 2.21 1.65 2.21 2.42 2.50 2.45 1.14 2.80 3.59 3.24 2.23 2.46 2.91A12O3 2.31 3.70 3.40 4.52 5.69 7.20 4.98 2.98 7.50 8.29 8.46 8.47 4.94 5.15 7.05 6.48 4.14 5.23 5.65Cr2O3 0.00 0.00 0.00 0.16 0.03 0.12 0.07 0.03 0.02 0.00 0.03 0.00 0.50 0.04 0.05 0.13 0.19 0.25 0.02FeO 8.73 8.75 9.56 8.50 9.39 9.25 8.32 7.62 7.86 8.34 7.30 8.10 5.21 7.64 7.92 5.11 5.69 6.05 6.49MnO 0.28 0.22 0.20 0.16 0.10 0.17 0.18 0.20 0.19 0.16 0.16 0.18 0.09 0.16 0.11 0.08 0.12 0.08 0.07MgO 14.47 12.77 13.74 15.54 12.40 12.33 13.88 14.69 12.63 12.26 12.05 12.04 15.05 13.47 12.52 12.88 14.58 13.89 13.40CaO 21.48 21.94 20.73 19.44 19.46 19.24 19.96 21.22 21.26 21.15 21.42 21.30 21.58 22.24 22.30 22.65 22.69 22.66 22.28Na2O 0.46 0.80 0.60 0.45 1.24 1.16 0.48 0.40 0.77 0.92 0.85 0.82 0.59 0.45 0.57 0.43 0.34 0.42 0.41

Total 100.83 100.20 100.27 100.73 100.51 101.01 99.36 100.02 100.23 100.93 99.48 100.25 100.01 101.26 100.04 98.41 100.08 99.86 99.60

6 O per formula

Si 1.90 1.85 1.86 1.85 1.90 1.85 1.84 1.90 1.78 1.75 1.75 1.75 1.87 1.82 1.73 1.78 1.85 1.81 1.80Al 0.10 0.16 0.15 0.20 0.25 0.31 0.22 0.13 0.33 0.36 0.37 0.37 0.21 0.22 0.31 0.29 0.18 0.23 0.25Ti 0.05 0.08 0.06 0.05 0.02 0.03 0.06 0.05 0.06 0.07 0.07 0.07 0.03 0.08 0.10 0.09 0.06 0.07 0.08Cr 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.00 0.00 0.00 0.00 0.00 0.00Fet 0.27 0.27 0.30 0.26 0.29 0.28 0.26 0.24 0.24 0.26 0.23 0.25 0.16 0.24 0.25 0.16 0.18 0.19 0.20Mn 0.01 0.01 0.01 0.00 0.00 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.00Mg 0.80 0.71 0.77 0.85 0.68 0.67 0.77 0.81 0.70 0.68 0.67 0.67 0.82 0.74 0.70 0.72 0.80 0.77 0.75Ca 0.85 0.88 0.83 0.77 0.77 0.76 0.80 0.84 0.85 0.84 0.86 0.85 0.85 0.88 0.90 0.91 0.90 0.90 0.89Na 0.03 0.06 0.04 0.03 0.09 0.08 0.03 0.03 0.06 0.07 0.06 0.06 0.04 0.03 0.04 0.03 0.02 0.03 0.03

Total 4.02 4.02 4.02 4.02 4.00 4.00 4.00 4.00 4.02 4.03 4.02 4.03 4.00 4.01 4.03 3.99 4.00 4.01 4.00

Wo 44.15 46.97 43.67 40.65 44.10 43.98 43.47 44.41 47.13 47.16 48.66 47.84 46.25 47.26 48.49 50.76 47.75 48.45 48.38En 41.39 38.03 40.29 45.21 39.11 39.20 42.06 42.80 38.94 38.04 38.10 37.64 44.89 39.82 37.89 40.16 42.70 41.31 40.49Fs 14.46 15.00 16.05 14.13 16.79 16.82 14.46 12.79 13.93 14.80 13.24 14.52 8.86 12.93 13.62 9.08 9.56 10.24 11.12

K91*-l K91*-2 K91*-3 K91*-4 KG3-1 KG3-2 K61-1 K61-2 K61-3 K61-4 K61-5 K61-6 K61-7 K66-1 K66-2 K95-1 K95-2 K95-3 K74-1 K74-2

SiO2 49.94 46.43 49.44 47.98 50.55 51.46 49.54 49.56 50.33 48.98 49.08 49.71 51.31 51.15 52.47 52.88 51.77 50.60 53.17 51.29TiO2 1.31 3.06 1.31 2.29 1.11 0.66 1.53 1.57 1.22 1.64 1.87 1.67 1.28 0.46 0.19 0.20 1.09 1.44 0.22 0.96A12O3 2.55 5.09 2.93 4.26 1.95 1.40 3.11 3.13 2.27 3.68 3.50 2.98 1.78 1.98 0.84 0.67 2.06 2.11 4.36 1.55Cr2O3 0.00 0.01 0.00 0.00 0.00 0.00 0.05 0.02 0.04 0.00 0.00 0.01 0.01 0.01 0.01 0.00 0.00 0.00FeO 12.69 11.70 13.73 12.56 13.09 12.52 9.97 10.12 9.84 10.65 10.25 10.89 12.61 10.60 8.60 7.39 9.58 13.90 14.12 11.92MnO 0.60 0.38 0.50 0.45 0.35 0.32 0.23 0.31 0.34 0.33 0.34 0.36 0.44 0.57 0.65 0.77 0.30 0.43 0.27 0.28MgO 12.05 11.18 11.75 11.37 14.40 11.67 13.92 14.61 14.80 13.67 14.03 14.20 13.85 13.71 13.74 14.72 14.83 12.19 9.76 14.82CaO 19.83 20.49 19.59 20.38 17.42 21.38 20.42 19.73 20.19 20.23 19.57 19.69 17.95 20.03 23.19 23.14 20.09 19.06 15.84 18.27Na2O 0.45 0.70 0.40 0.43 0.32 0.30 0.46 0.68 0.42 0.41 0.45 0.55 0.03 0.35 0.21 0.06 0.32 0.19 1.67 0.00

Total 99.45 99.06 99.66 99.73 99.21 99.71 99.23 99.72 99.47 99.59 99.09 100.09 99.26 99.01 99.91 99.84 100.05 99.93 99.41 99.08

322G

.K

uep

ou

oet

al.

/J

ou

rna

lo

fA

frican

Ea

rthS

ciences

45

(2

00

6)

31

8–

33

2

6O

per

form

ula

Si

1.91

1.79

1.89

1.83

1.92

1.96

1.87

1.87

1.90

1.85

1.86

1.87

1.94

1.94

1.97

1.97

1.93

1.92

2.00

1.94

Al

0.11

0.23

0.13

0.19

0.09

0.06

0.14

0.14

0.10

0.16

0.16

0.13

0.08

0.09

0.04

0.03

0.09

0.09

0.19

0.07

Ti

0.04

0.09

0.04

0.07

0.03

0.02

0.04

0.04

0.03

0.05

0.05

0.05

0.04

0.01

0.01

0.01

0.03

0.04

0.01

0.03

Cr

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

Fet

0.41

0.38

0.44

0.40

0.42

0.40

0.32

0.32

0.31

0.34

0.32

0.34

0.40

0.34

0.27

0.23

0.30

0.44

0.44

0.38

Mn

0.02

0.01

0.02

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.01

0.02

0.02

0.02

0.01

0.01

0.01

0.01

Mg

0.69

0.64

0.67

0.65

0.82

0.66

0.79

0.82

0.83

0.77

0.79

0.80

0.78

0.78

0.77

0.82

0.82

0.69

0.55

0.84

Ca

0.81

0.85

0.80

0.83

0.71

0.87

0.83

0.80

0.82

0.82

0.79

0.79

0.73

0.81

0.93

0.92

0.80

0.78

0.64

0.74

Na

0.03

0.05

0.03

0.03

0.02

0.02

0.03

0.05

0.03

0.03

0.03

0.04

0.00

0.03

0.01

0.00

0.02

0.01

0.12

0.00

To

tal

4.01

4.03

4.02

4.02

4.02

4.00

4.03

4.04

4.03

4.03

4.03

4.04

3.98

4.01

4.02

4.01

4.01

4.00

3.96

4.00

Wo

42.2

145

.06

41.6

343

.97

36.3

344

.86

42.7

640

.92

41.4

342

.29

41.3

140

.82

37.8

741

.87

46.8

146

.28

41.4

840

.36

38.9

637

.73

En

35.7

034

.19

34.7

534

.12

41.7

934

.09

40.5

642

.18

42.2

639

.78

41.2

340

.97

40.6

439

.89

38.6

040

.97

42.6

135

.94

33.4

142

.59

Fs

22.0

920

.74

23.6

221

.91

21.8

921

.04

16.6

716

.89

16.3

117

.93

17.4

618

.22

21.4

918

.23

14.5

812

.76

15.9

123

.70

27.6

219

.67

Fig. 2. Clinopyroxene compositions of alkali basalts (open circle) andtransitional tholeiitic basalts from Bana (shaded field) complex obtainedfrom plots in comparison with clinopyroxene from Ethiopian transitionaland tholeiitic basalts. Symbols: diamond, clinopyroxene in alkali basalts;filled circles, clinopyroxene in transitional tholeiitic basalts. Abbreviations:

AB, alkali basalt; TAB, transitional alkali basalt; TTB, transitionaltholeiitic basalt; TB, tholeiitic basalt; S line, Skaeggard pyroxene trend.


6.1. Chemical alteration

The effects of alteration were minimized in the firstinstance by careful selection of freshest samples. The mostsensitive elements to weathering are alkali elements (Na, K,Rb) as their concentration is affected even in young andfresh basalts. This mobility is influenced by breakdown ofinterstitial glass and alkali bearing phases, and associateddevelopment of secondary minerals.

The mobility of Na and K is manifested by the scatter inSiO2 vs. K2O + Na2O plot (Fig. 3). Furthermore, LIO upto �4 wt% in a few samples indicate alteration.

However, the overall coherent variation of most majorand trace elements suggests that alteration was limited tohighly mobile alkali elements. Therefore, the use of mobilein conjunction with immobile major (e.g. Ti) and trace

Table 3XRF whole-rocks analyses of Bana alkali basalts

wt% Bn K14 Bn G6 B K62 Hw G5 Hw K68

SiO2 44.42 44.36 46.36 48.46 48.73TiO2 3.16 3.31 3.25 2.79 2.55Al2O3 14.59 14.08 15.44 14.56 15.34Fe2O3 11.86 13.14 12.99 11.95 11.69MnO 0.16 0.20 0.17 0.17 0.17MgO 9.34 8.74 6.84 7.44 6.59CaO 9.59 10.49 8.48 7.94 7.35Na2O 3.17 3.01 3.25 3.69 3.70K2O 1.48 0.94 1.17 1.57 1.53P2O5 0.72 0.60 0.53 0.72 0.74

Total 98.49 98.88 98.47 99.27 98.37

Mg# 66.11 62.22 56.60 63.80 61.49A.I 0.47 0.43 0.50 0.42 0.53

Norm qz

ab 16.57 17.95 27.91 31.43 31.78an 21.54 22.36 24.48 18.67 21.11or 8.88 5.63 7.01 9.34 9.16di 17.68 21.26 11.95 13.07 8.90hy 2.02 2.63 9.05mt 3.49 3.85 3.82 5.23 5.17ilm 6.10 6.36 6.27 5.35 4.93ol 17.32 15.88 14.23 11.78 7.34ne 5.76 4.24ap 1.73 1.43 1.28 1.71 1.77

ppm

Ba 605 422 499 465 592Rb 37 23 31 34 29Th 2 3 9 8 3Sr 954 759 843 659 861Nb 74 52 67 68 71Ta 4 5 7Zr 245 190 234 264 278Y 27 27 23 32 31Co 58 54 68 60 52Cr 273 117 152 334 166Cu 32 35 30 56 40Ga 18 20 17 19 19Ni 162 80 106 134 119Pb 1 0 0 3 5V 225 207 183 282 183Zn 111 105 111 98 111

REE

La 43.1 44.9 48.4Ce 81.1 87.2 90.0Pr 9.80 10.6 10.9Nd 40.8 44.7 43.9Sm 9.05 9.89 8.75Eu 2.85 2.93 2.70Gd 7.38 8.38 6.90Tb 0.97 1.17 0.86Dy 5.00 6.51 4.56Ho 0.87 1.19 0.76Er 2.15 3.11 1.92Tm 0.281 0.440 0.252Yb 1.72 2.66 1.54Lu 0.225 0.358 0.203


elements (Zr, Nb, Y and HREE) of these rocks for petro-genetic interpretations is a valid approach (e.g. Pearceand Cann, 1973; Winchester and Floyd, 1976).

6.2. Major element compositions

Major element compositions (Fig. 4) show a great vari-ation of MgO and similar TiO2 contents (2.1–3.6 wt%) atsmall differences in SiO2 content between olivine-basaltsand olivine-clinopyroxene-basalts; and plagioclase-basalts.

Olivine-basalts are characterized by 8.5–9.5 wt% MgOand 44–44.5 wt% SiO2 and a total range of Mg# � 62–66.1, olivine-clinopyroxene-basalts have 6.6–7.5 wt% MgOand 46.3–48.7 wt% SiO2 and a total range of Mg# �56.6–63.8; and plagioclase-basalts have 2.2–5.2 wt% MgOand 47.3–51.1 wt% SiO2 and a total range of Mg# � 36–47.7 (Tables 3 and 4). Among the plagioclase-basalts, thesubaphyric and weakly porphyritic samples (K91, K74,K64 and K56) show the highest MgO contents �5.2–4.5 wt% relative to the highly porphyritic samples at similarsilica contents. Following the classification grid of Le Baset al. (1986) (Fig. 3), olivine-basalts are basanites, olivine-clinopyroxene-basalts are basalts and hawaiites, and pla-gioclase-basalts are basalts and rare hawaiites. Furtherdivision (Irvine and Baragar, 1971) allows distinction ofalkaline and subalkaline magma series in the Bana com-plex. The olivine-basalts and olivine-clinopyroxene-basaltsare alkaline whereas plagioclase-basalts are subalkaline(Fig. 3).

The distinction between the Bana alkali and subalkalicbasalts also appears on the Ne–Ol–Di–Hy–Q normativesystem (Fig. 5). Normative minerals were calculated usingthe adjusted Fe2O3/FeO = 0.2 (Middlemost, 1989) for thetwo groups. The compositions range from quartz-tholeiitebasalts, through olivine- to nepheline-bearing alkali bas-alts. The absence of calcium-poor pyroxene in subalkalicbasalts, an essential and critical feature of tholeiites, pro-hibits their classification as true oversaturated tholeiitesand suggests instead that they are transitional tholeiiticbasalts, in opposition to transitional alkalic basalts follow-ing the terminology by Tilley (1950) and Coombs (1963).Basalts having similar characteristics occur in the Turkanalocality in northern Kenya (Bellieni et al., 1981) and inAiba in Ethiopia (Zanettin et al., 1980). The field (Fig. 4)for the Bana transitional tholeiitic basalts is mainly withinthe 1 atm cotectic, indicating equilibration in a very shal-low magma reservoir. The common features between alkaliand transitional tholeiitic basalts in the Bana complex arethe high titanium (�2.2–3.5 wt% TiO2) character, similarCaO/Na2O ratios (�2–3.5) and the high potassium con-tents (0.9–1.6 wt% in alkali basalts; 0.6–1.8 wt% in transi-tional tholeiitic basalts). Five samples of transitionaltholeiitic basalts (G1, G9a, K93, K95 and K105) have highAl2O3 contents (18.36–20.35 wt%).

6.3. Trace element compositions

Relative to transitional tholeiitic basalts, the alkali bas-alts have twice or more Nb contents, and higher Ni, and toa certain degree Sr contents (Fig. 4). Other trace elementsnotably the most incompatible ones such as Ba and Rb

Table 4XRF whole-rocks analyses of Bana transitional tholeiitic basalts (TrTB)

wt% TrTB

K91 K64 G9a G1 K95 K104 K54 K93 K67 K74 K58 K77 K39’ K48-b K56 KG3 K107

SiO2 47.36 45.56 49.68 49.36 49.29 47.12 49.06 49.56 49.43 48.99 49.13 48.54 51.16 48.50 48.94 49.66 48.67TiO2 3.11 3.29 2.71 2.15 2.70 2.66 3.78 2.82 3.10 3.37 3.87 3.43 3.10 3.52 3.38 3.52 3.40Al2O3 13.31 14.50 19.14 20.35 19.39 19.10 14.60 18.36 15.28 14.85 14.52 15.60 15.23 15.97 15.20 15.14 14.35Fe2O3 14.18 14.23 10.63 9.86 10.53 10.11 14.75 11.33 13.24 14.11 13.70 13.18 12.34 13.72 14.19 13.72 13.04MnO 0.26 0.18 0.13 0.11 0.14 0.12 0.20 0.14 0.21 0.18 0.19 0.18 0.19 0.17 0.16 0.17 0.18MgO 4.32 5.25 2.76 2.24 2.85 2.68 3.84 2.89 3.74 4.70 3.70 3.78 3.27 3.96 4.49 4.19 3.03CaO 8.29 9.61 9.82 10.44 10.15 9.92 8.18 9.80 7.21 7.34 7.55 9.07 8.42 9.18 8.72 7.48 8.46Na2O 2.70 2.20 3.35 3.34 3.12 3.27 2.76 3.24 3.49 3.07 3.27 3.12 3.12 3.33 3.11 3.45 2.96K2O 1.40 0.89 0.78 0.55 0.80 0.62 1.05 0.65 1.80 1.00 1.09 1.18 1.67 0.92 1.08 1.27 1.08P2O5 1.97 0.39 0.32 0.31 0.35 0.28 0.56 0.36 0.94 0.52 0.55 0.45 0.60 0.44 0.46 0.56 0.57

Total 96.90 96.09 99.30 98.70 99.32 95.89 98.78 99.13 98.43 98.13 97.57 98.54 99.09 99.71 99.72 99.14 95.73

Ms# 43.00 47.75 39.15 36.02 40.08 39.66 39.19 38.69 41.15 45.22 40.10 41.55 39.63 41.69 43.92 43.05 36.55A.I 0.45 0.32 0.33 0.30 0.31 0.32 0.39 0.33 0.50 0.41 0.45 0.41 0.46 0.41 0.41 0.46 0.42

Norm

qz 4.21 0.81 1.88 1.73 1.95 0.39 5.04 2.73 1.01 3.05 4.07 1.26 4.44 0.17 1.07 1.83 5.43ab 23.57 19.37 28.49 28.59 26.57 28.88 23.62 27.66 30.01 26.43 28.32 26.75 26.60 28.27 26.35 29.40 26.12an 20.71 28.14 35.16 39.45 36.78 37.12 24.67 33.92 21.04 24.24 22.28 25.46 22.85 25.97 24.42 22.30 23.71or 8.56 5.48 4.61 3.27 4.78 3.83 6.26 3.88 10.80 6.04 6.60 7.09 9.94 5.47 6.37 7.54 6.67di 7.01 15.49 9.83 9.18 9.67 10.19 10.55 10.62 7.53 7.80 10.32 14.30 12.75 13.95 13.18 9.48 13.58hy 19.72 17.78 10.15 9.22 10.36 9.74 15.74 10.73 16.44 19.38 14.38 12.52 11.47 13.35 15.84 16.29 11.34mt 4.24 4.29 3.10 2.90 3.08 3.06 4.33 3.31 3.90 4.17 4.07 3.88 3.61 3.99 4.13 4.01 3.95ilm 6.10 6.50 5.18 4.13 5.16 5.27 7.27 5.40 5.99 6.52 7.53 6.61 5.93 6.71 6.43 6.74 6.74olneap 4.81 0.96 0.76 0.74 0.83 0.69 1.35 0.85 2.25 1.24. 1.33 1.07 1.44 1.04 1.09 1.33 1.40

ppm

Ba 798 258 282 256 261 222 467 265 517 507 409 320 509 361 327 540 451Rb 58 22 13 12 18 13 19 10 38 24 20 32 42 20 17 36 25Th 15 5 6 4 6 6 9 5 11 4 6 6 5 4 6 3 16Sr 551 551 584 632 620 574 451 564 518 512 439 507 514 518 485 480 522Nb 36 23 24 23 23 21 32 24 44 33 39 33 37 31 32 34 33Ta 3 3 2Zr 254 168 174 168 168 154 265 164 340 266 282 230 278 223 229 287 244Y 47 27 23 24 26 23 36 26 51 41 44 36 40 32 35 42 37Co 31 55 64 43 44 33 58 48 43 52 43 43 46 46 51 46 41Cr 6 67 8 24 7 13 15 8 6 29 6 16 14 11 22 19 9Cu nd nd 26 48 12 15 21 28 nd 7 18 88 nd 69 27 17 13Ga 19 19 23 22 24 23 24 24 28 24 24 22 24 22 23 23 23Ni nd 53 16 23 6 13 17 8 nd 33 7 16 10 11 20 24 8Pb 2 0 0 nd nd nd 4 0 2 2 0 nd 3 1 nd 1 1V 129 324 218 178 210 231 343 221 158 222 286 262 203 309 285 226 258Zn 161 116 96 91 99 87 137 97 175 146 147 115 137 123 130 139 131

REE

La 18.97 24.23 32.23Ce 39.40 50.80 66.70Pr 5.30 6.70 9.10Nd 24.00 30.70 40.40Sm 6.04 7.94 9.76Eu 2.05 2.62 3.12Gd 5.85 7.60 9.58Tb 0.85 1.14 1.36Dy 4.77 6.41 7.74Ho 0.87 1.19 1.40Er 2.25 3.06 3.70Tm 0.32 0.44 0.53Yb 1.86 2.51 3.04Lu 0.25 0.34 0.41


have similar abundances in transitional tholeiitic basaltsthan and alkali basalts (Fig. 4). The two groups of basaltic

rocks are characterized by a narrow range of Rb/Sr ratio0.03–0.04, except for one sample (K91) of transitional

0

2

4

6

8

10

12

40 45 50 55 60 65 70 75 80

wt% SiO2

wt%

(Na 2

O+K

2O)

Mu

Be

Tr

Rhyolite

Subalkaline

Alkaline

Ba

BnHw

Fig. 3. Total Alkali – Silica classification (Le Bas et al., 1986). Straightline in the basaltic field delineates the boundary between alkaline andsubalkaline basaltic series after Irvine and Baragar (1971). Symbols: alkalibasalt group (basanite and basalts (filled diamonds); hawaiites (filledsquare)); transitional tholeiitic basalts (filled circle). The circled areadelineates the composition of transitional basalts of the Bamoun Plateau(Moundi, 2003).

0

2

4

6

8

10

MgO

wt%

1

2

3

4

5

TiO

2w

t%

9

10

11

12

13

14

15

16

Fe 2

O3

wt%

810121416182022

44 46 48 50 52SiO2 wt%

Al 2

O3

wt%

Fig. 4. Variation diagrams of majors and trace elements vs SiO2 for transitionfrom Fig. 3.


tholeiitic basalts with a ratio of 0.08. This narrow ratiorange is within the limit of mantle peridotites (Rb/Sr < 0.07).

The Y/Nb and Zr/Nb ratios (Fig. 6a) are consistent dis-criminants between the two groups of basalts: alkali basaltshave low ratios of Y/Nb < 1 and Zr/Nb < 4 while the tran-sitional tholeiitic basalts are characterized by their highratios of Y/Nb > 1 and Zr/Nb > 6.

The Zr/Nb–Zr/Y and Zr–Nb diagrams (Weaver et al.,1972; Menzies and Kyle, 1990) discriminate between alkalibasalts and transitional tholeiitic basalts of the Bana com-plex (Fig. 6a–c).

The Primary Mantle-normalized (Sun and McDonough,1989) trace element patterns (Fig. 7a and b) have negativeTh and K anomalies relative to similar incompatible ele-ments (Ba and Ta) in alkali basalts and transitional tholei-itic basalts.

In spite of these differences, all of these lavas carry thetypical OIB signature of Nb–Ta enrichment (Fig. 7a and b).

9

59

109

159

209

Nip

pm

10

30

50

70

Srpp

m

0

200

400

600

800

1000

Ba

ppm

10

20

30

40

44 46 48 50 52SiO2 wt%

Rb

ppm

al tholeiitic basalts and alkali basalts from the Bana complex. Symbols are

Ol Hy

DiQzNe

Parana low-Ti lavas

Afar Riftttransitional basalts

1 atm

Fig. 5. Ne–Ol–Di–Hy–Q normative system showing the plot of theplagioclase-basalts from Bana in the field of quartz-tholeiite. Oval fielddelineates the fields for transitional tholeiitic basalt from Afar rift zone (inNorthern Turkana, Kenya by Bellieni et al. (1981); and in Aiba, Ethiopiaby Zanettin et al. (1980)) and Dash oval line delineates the fields forParana-Etendeka low-Ti lavas (Bellieni et al., 1984). Cotectic at 1 atm forequilibrium Ol–Pl–Cpx-basaltic liquid is from Thompson (1982); arrowsindicate the direction of falling temperature. Abbreviations: Ne, Nephe-line; Ol, Olivine; Di, Diopside; Hy, Hypersthene; Q, Quartz. Symbols arefrom Fig. 3.

4

6

8

10

12

0 2 4 6 8 10Zr/Nb

Zr/

Y

Low degree PM

High degree PM

(a)

0

10

20

30

40

50

60

70

80

0 100 200 300 400Zr ppm

Nb

ppm

(b)

(c)

100

150

200

250

300

0.20 0.40 0.60 0.80 1.00 1.20 1.40

Y/Nb

Zr

ppm

Alkali basaltsTransitionalbasalts

Fig. 6. (a) Zr/Y against Zr/Nb plot showing qualitative trend ofcompositions resulting from low vs high melting degrees of the Banabasaltic rocks. Also note the depleted character of transitional tholeiiticbasalts compared with alkali basalts. Abbreviation: PM, partial melting.(b) Nb against Zr plot showing that olivine-, olivine-clinopyroxene-basalts(alkali basalts) and plagioclase-basalts (transitional tholeiitic basalts) fromthe Bana complex represent two different magma types as indicates thepresence of two covariant trends. (c) Plot of Zr against Y/Nb ratiosdistinguishing the basaltic rocks from the Bana complex into alkali basaltsand transitional basalts. The boundary (discontinued line) is from Pearceand Cann (1973). Symbols are from Fig. 3.


The rare earth element (REE) composition (Fig. 8a and b)provides a good distinction and reveals the intrinsictransitional tholeiitic character of these basalts.

There is an increase of light vs heavy REE enrichmentfrom transitional tholeiitic basalt to alkali basalts: the[La/Yb]N ratios = 5.37–5.89 in transitional tholeiitic bas-alts and 9.37–17.48 in alkali basalts; a relatively flat heavyrare earth element (HREE) curve ([Tb/Yb]N = 1.81–2.32)in both basaltic types portraying the general array ofchondrite-normalized (Anders and Grevesse, 1989) pat-terns of Afar Rift transitional basalts (Barberi et al.,1975) (Fig. 8b). The [Tb/Yb]N ratios are comparable withthose of alkali basalts from Hawaii [Tb/Yb]N = 1.89–2.45which are commonly considered to have been generatedin a garnet-bearing lherzolitic mantle (McKenzie andO’Nions, 1991, 1995; Frey et al., 1991).

7. Discussion

From the data summarized above, olivine-basalts andolivine-clinopyroxene-basalts (i.e. alkali basalts) on theone hand, and plagioclase-basalts on the other hand (i.e.transitional tholeiitic basalts) represent basic magmas clo-sely associated in space. The plagioclase-basalts are moreevolved magmatic products (Mg# � 36–47.7) than the for-mer (Mg# � 46.3–66.1). In contrast, primitive lava fromthe upper mantle has a Mg# of 68–72 (Green, 1971). Theoccurrences of titaniferous augites in olivine-basalts andolivine-clinopyroxene-basalts, and the predominance ofnon-titaniferous augites in plagioclase-basalts reflect thesilica-undersaturated character of the former. The compo-sition of clinopyroxene of undersatured basaltic magmas isconsidered to be highly sensitive to the order of appearanceof clinopyroxene-calcic plagioclase if pressure effects are

neglected (Barberi et al., 1971). In the Bana plagioclase-basalts, the early crystallization and withdrawal of clinopy-roxene phenocrysts out of the system followed by theabundant crystallization of calcic plagioclase phenocrysts,may have led to the crystallization of groundmass clinopy-roxene with low Ti content. Such a sequence of crystalliza-tion has been reported in the Erta’Ale plagioclaseporphyritic rocks of mildly alkaline series (Barberi et al.,1971). Progressive Ca depletion and Fe enrichment of clin-opyroxenes in passing from alkali to transitional tholeiiticbasalts in Bana may suggest a genetic relationship at firstglance.

1

10

100

1000

Rb Ba Th K Ta Nb La Ce Sr Nd Hf Zr Sm Ti Tb Y Tm Yb

Roc

ks/P

rim

itiv

eM

antl

e Alkali basalts

(a)

1

10

100

1000

Roc

ks/P

rim

itiv

eM

antl

e

Transitional tholeiitic basalts

(b)

P

Rb Ba Th K Ta Nb La Ce Sr Nd Hf Zr Sm Ti Tb Y Tm YbP

Fig. 7. Spider diagram using the Primary Mantle normalization valuesrecommended by Sun and McDonough (1989) for: (a) Olivine-, olivine-clinopyroxene-basalts (alkali basalts), and (b) Plagioclase-basalts (transi-tional tholeiitic basalts). Symbols are from Fig. 3.

1

10

100

1000

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Roc

k/C

hond

rite

Alkali basalts

1

10

100

1000

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Roc

k/C

hond

rite

Transitionalt holeiitic basalts

(b)

(a)

Fig. 8. Rare earth element distributions diagrams normalized to Chon-drite values recommended by Anders and Grevesse (1989) for: (a) Olivine-,olivine-clinopyroxene-basalts (alkali basalts), and (b) Plagioclase-basalts(transitional tholeiitic basalts). Symbols are from Fig. 3.


The transitional tholeiitic basalts were not derived fromthe exposed alkali basaltic melt by fractionation of olivineand/or clinopyroxene, because of their low concentrationsin LILE, Nb, Zr and Y compared with those of alkalibasalts.

Other lines of evidence such as the high silica and lowmagnesia contents, La/Yb vs. Dy/Yb and La/Sm vs. Sm/Yb diagrams (Fig. 9a and b) suggest that the parental meltof transitional tholeiitic basalts were probably formed byhigh degree partial melting of the mantle at shallow depth,compared to alkali basalts (Thirlwall et al., 1994; Bogaardand Worner, 2003). Geophysical data (Fairhead andOkereke, 1987; Plomerova et al., 1993; Poudjom Djomaniet al., 1995) give supporting evidence of crustal thinningbeneath the Cameroon Line. Thus occurrence of basaltsderiving from partial melting of the mantle at shallowdepth in these settings seems probable.

The distinctive high alumina contents (Al2O3: 18.36–20.35 wt%) in the highly porphyritic transitional tholeiitic

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

0 10 20 30 40La/Yb

Dy/

Yb

Garnet peridotite

Spinel peridotite

8%10%

12%

14%

16%

1%10%15%

G6K14

K68

TrTB

(a)

0

1

2

3

4

5

6

7

0 10Sm/Yb

La/

Sm

1%

5%10%

15%

1%5%10%15%

G6

K14K68

TrTB

Melt model

MORB-source

(b)2 4 6 8

Fig. 9. (a) La/Yb vs Dy/Yb for olivine basalts (G6, basanite; K14,basanite; olivine-clinopyroxene-basalt (K68, hawaiite); and plagioclase-basalts (transitional tholeiitic basalts, TrTB) from the Bana complex.Curves are degrees of melting for garnet peridotite, and spinel peridotite(Thirlwall et al., 1994; Bogaard et al., 2003). The good fit with garnetperidotite as possible source region of basaltic rocks from Bana should benoted as well as higher degree of melting to generate the TrTB. (b) Sm/Ybvs La/Sm plot gives the melt curve for inverse batch melting model and thedepleted MORB source composition. TrTB apart from higher degree ofmelting generated from enriched source composition relative to N-MORBfrom Sun and McDonough (1989). Symbols are from Fig. 3.


basalts (G1, G9a, K93, K95 and K104) contrasting withabsence of positive Eu anomalies (Eu/Eu* = 0.96–1.0;Fig. 8a) might reflects the abundance of plagioclase ratherthan its accumulation by gravitational sinking.

Tatsumi et al. (1986) showed that Zr and Y are both rel-atively immobile in aqueous fluids and can provide infor-mation on the mantle source region of lavas withoutcontamination by crustal aqueous fluids. Fig. 6a–c clearlysuggest again that alkali basalts and transitional tholeiiticbasalts from the Bana complex are two different types ofbasalts rather than one being derived from the other bycrystal fractionation. Another process is therefore requiredto explain the origin of these two groups of basalt. Smalland high degrees of partial melting of the mantle at highpressure (>15 kbar) could yield high variation of Zr/Yratios in the resulting melt due to retention, or release ofY in garnet and clinopyroxene (Tatsumi and Eggins,1995). This may explain the occurrences of basanite andhawaiite (Fig. 6a).

The highly incompatible element ratios such as Zr/Nb,K/Nb, Ba/Th, Th/La, Ba/La and Ba/Nb are shown to beleast susceptible to fractionation during partial melting(Table 5), and are not significantly fractionated during lim-ited degrees of low-pressure crystallization of OIB magmas(Weaver, 1991), hence they could be useful indicators forbasaltic end-member characterization in Bana. These ratiossuggest that: (1) the alkali basalts source is influenced bythe HIMU end-member in terms of Zr/Nb and Th/La;by EM2 end-member in terms of Ba/Th; Ba/La suggeststhe contribution of EM1; whereas (2) the transitionaltholeiitic basalts source is largely influenced by the EM1end-member except for the Zr/Nb ratio that suggestscontribution of EM2 end-member.

The overall OIB character of the two groups of basaltsin accordance with the models for the genesis of oceanic

Table 5Trace element ratios of selected olivine-basalts (K14, G6), olivine-clinopyroxeBana compared with different OIB end-menbers, N-MORB, PM, and average

Sm/Nd Zr/Nb Ba/Th Th/La

K14 0.22 3.76 122.49 0.12G6 0.22 4.43 91.51 0.12K68 0.20 3.98 105.47 0.10K56 0.26 7.50 112.55 0.11K95 0.25 7.67 125.89 0.10KG3 0.24 9.20 187.37 0.09

Reference values from Weaver (1991)Chondrite 0.33Upper Cru 0.17HIMU-mini 2.7 39.0 0.1HIMU-max 5.5 85.0 0.2EM1-mini 3.5 80.0 0.1EM1-max 13.1 204.0 0.2EM2-mini 4.4 57.0 0.1EM2-max 7.8 105.0 0.2N-MORB 30.0 60.0 4.0P.M 14.8 77.0 0.1Cont. C. av. 16.2 124.0 0.2

island basalts (OIB) can generally be either (1) direct par-tial melting of plumes that have risen from below a thermalboundary layer separating the lower mantle from the con-vecting upper mantle or (2) preferential melting of dis-persed HIMU-OIB streaks or blobs from a digestedoceanic crust within the convecting upper mantle (e.g.Zindler et al., 1984). Model number 2 was discarded byHalliday et al. (1990) for the genesis of Cameroon Linebasalts. Hence model number 1 could be considered toexplain the above variations between the two groups ofbasalts in Bana whereby the parent magmas of alkali bas-alts originated from a secondary enriched plume comingfrom the top of the dome near the depth of the transitionzone at the location of the African superswell (Courtillotet al., 2003) whereas that of transitional tholeiitic basaltsoriginated in a depleted asthenosphere as evidenced bythe high Zr/Nb, low Nb and low [Ce/Yb]N ratios. Condie(2003, 2005) suggests that in modern and Archean basalts,it is possible to characterize plume and non-plume mantlesources with four incompatible element ratios: Nb/Th,Zr/Nb, Zr/Y, and Nb/Y. Using these four ratios, alkaliand transitional tholeiitic basalts from the Bana complexplot above the DNb line in the mantle plume field asdefined by Fitton et al. (1997).

It is suggested that these basalts have chemical featuresconsistent with plume activity outlined by their OIB char-acters (Fig. 10a and b).

The HREE ratios and simple interpretation of Fig. 8suggest moderate fractionation and a garnet peridotite res-idue expressed in [Tb/Yb]N = 1.81–2.32 for alkali basaltsand [Tb/Yb]N = 1.84–1.88 for transitional tholeiitic bas-alts. This common source region should be located in thespinel-lherzolite and garnet-lherzolite transition zone cor-responding to depths of about 70–80 km (Frey et al.,1991; McKenzie and O’Nions, 1991) in the upper mantle.

ne-basalt (K68); and transitional tholeiitic basalts (K56, K95, KG3) fromcontinental crust values from Weaver et al. (1991)

Ba/La Ba/Nb K/Nb Rb/Sr

14.15 8.30 167.15 0.0410.57 6.97 115.12 0.0610.76 7.51 182.53 0.0411.84 8.75 272.31 0.0413.07 10.12 272.41 0.0316.57 15.30 301.12 0.08

6.29.3

11.319.17.3

13.54.39.0

54.0

0.01

0.10

1.00

10.00

1 10

Zr /Y

Nb/

Y

Plume source

Non-Plume source

Nb line

REC

PM

DEP

OIB

OPB

EM2H HIMU-EM1

(b)

1

10

100

0 10 20Nb/Th

Zr/

Nb OPB

OIBHIMUREC

EM1

EM2

DEP

PM

EN

DM NMORBARC

(a)

Fig. 10. Plot of basalts from Bana relative to the mantle compositionalcomponents (filled star) and fields for basalts from various tectonicsettings as defined by Weaver (1991) and Condie (2003). (a) Nb/Th–Zr/Nband. (b) Zr/Y–Nb/Y. Abbreviations: PM, primitive mantle; DM,shallow depleted mantle; HIMU, high mu (U/Pb) source; EM1 and EM2,enriched mantle sources; ARC, arcrelated basalts; N-MORB, normalocean ridge basalt; OIB, oceanic island basalt; DEP, deep depleted mantle;EN, enriched component; REC, recycled component; OPB, oceanicplateau basalt. Symbols are from Fig. 3.


Differences in LREE/HREE such as [La/Yb]N = 9.37–17.48 in alkali basalts and [La/Yb]N = 5.37–5.89 in transi-tional tholeiitic basalts also agree with the variable degreesand depths of partial melting of the mantle source.

The K/Ar radiometric age of 30.1 ± 1.2 Ma of the Banatransitional tholeiitic basalts is younger than those of tran-sitional alkali basalts of Mount Bangou �42–44 Ma (Fossoet al., 2005) and plateau Bamoun � 51 Ma (Moundi, 2004)in the Western Cameroon Highlands. Similar age ranges(44–51 Ma) of transitional basalts of the East African Riftzone imply that the lithospheric extension was continental-wide in Africa, at least in the Cenozoic.

8. Conclusion

(1) Plagioclase-basalts of the Bana complex are transi-tional tholeiitic basalts in their mineralogy (norma-tive) and geochemistry,

(2) these transitional tholeiitic basalts are of Oligoceneage � 30.1 ± 1.2 Ma, therefore belong to the oldestlava sequences of the Cameroon Line,

(3) the transitional tholeiitic and alkali basalts of theBana complex represent two spatially close, but tem-porally different magmatic series,

(4) temporal transition from transitional tholeiitic bas-alts to alkali basalts in the Bana system may beascribed to decreasing degrees of partial melting ofthe mantle coupled with the increase of pressure ordepth of melting through time,

(5) low La/Nb, Ba/Nb, and Rb/Sr ratios suggest insignif-icant contamination of these basalts by the crust,however isotopic data are required to confirm thisaspect of the work,

(6) the Cenozoic basalts from the Bana complex aredominantly transitional tholeiitic with subsidiaryalkali basalts in contrast to the overall CameroonLine basaltic composition,

(7) given the genetic relationship between basaltic rockseries and tectonic setting, the finding of transitionaltholeiitic basalts in Bana may imply that this arearepresents a zone of maximum extension of the litho-sphere below the Cameroon Line in Cenozoic times.

Acknowledgements

The authors wish to thank the administration of theVenture Business Laboratory at the Kobe University forallowing the electron probe microanalyses. We are gratefulto C.I. Chalokwu, J.M. Bardinzeff and late A.B. Kampu-nzu for their constructive comments and corrections onmanuscript. F. Schwandner (University of Arizona) isacknowledged for improving the edition of the early man-uscript. The manuscript has been greatly improved thanksto the thorough and constructive comments of reviewers S.Muhongo and an anonymous colleague. The senior authorwas supported by a fellowship from the Japanese Ministryof Education, Science, Sport and Culture (Monbushu).F.M. Tchoua, E. Njonfang and P. Kamgang of Universityof Yaounde I (Cameroon) are acknowledged for fruitfuldiscussions during the field trip.

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