A comparative review of petrogenetic processes beneath the Cameroon Volcanic Line: Geochemical constraints

Geoscience Frontiers xxx (2014) 1e17

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Research paper

A comparative review of petrogenetic processes beneaththe Cameroon Volcanic Line: Geochemical constraints

Asobo N.E. Asaah a,b,*, Tetsuya Yokoyama a, Festus T. Aka c, Tomohiro Usui a,Mengnjo J. Wirmvemd, Boris Chako Tchamabe d, Takeshi Ohba d, Gregory Tanyileke c,J.V. Hell c

aDepartment of Earth & Planetary Sciences, Tokyo Institute of Technology, 2-12-1, Ookayama, Meguro-ku, Tokyo 152-8551, JapanbDepartment of Mines, Ministry of Mines, Industry and Technological Development, Yaoundé, Cameroonc Institute of Mining & Geological Research, P.O. Box 4110, Yaoundé, CameroondDepartment of Chemistry, School of Science, Tokai University, Hiratsuka 259-1211, Japan

a r t i c l e i n f o

Article history:Received 24 September 2013Received in revised form8 April 2014Accepted 17 April 2014Available online xxx

Keywords:PetrogenesisCameroon volcanic lineDepleted MORB mantleEnriched mantleMantle heterogeneity

* Corresponding author. Department of Earth &Institute of Technology, 2-12-1, Ookayama, MeguroTel.: þ81 3 5734 3539; fax: þ81 3 5734 3538.E-mail address: [email protected] (A.N.E. Asaah)

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The origin and petrogenesis of the Cameroon Volcanic Line (CVL), composed of volcanoes that form onboth the ocean floor and the continental crust, are difficult to understand because of the diversity, het-erogeneity, and nature of available data. Major and trace elements, and Sr-Nd-Pb isotope data of volcanicrocks of the CVL spanning four decades have been compiled to reinterpret their origin and petrogenesis.Volcanic rocks range from nephelinite, basanite and alkali basalts to phonolite, trachyte and rhyolite withthe presence of a compositional gap between SiO2 58e64 wt.%. Similarities in geochemical characteristics,modeled results for two component mixing, and the existence of mantle xenoliths in most mafic rocksargue against significant crustal contamination. Major and trace element evidences indicate that themelting of mantle rocks to generate the CVL magma occurred dominantly in the garnet lherzolite stabilityfield. Melting models suggest small degree (<3%) partial melting of mantle bearing (6e10%) garnet for Mt.Etinde, the Ngaoundere Plateau and the Biu Plateau, and <5% of garnet for the oceanic sector of the CVL,Mt. Cameroon, Mt. Bambouto, Mt. Manengouba and the Oku Volcanic Group. The Sr-Nd-Pb isotope sys-tematics suggest that mixing in various proportions of Depleted MORB Mantle (DMM) with enrichedmantle 1 and 2 (EM1 and EM2) could account for the complex isotopic characteristics of the CVL lavas.Low Mg number (Mg# ¼ 100 � MgO/(MgO þ FeO)) and Ni, Cr and Co contents of the CVL mafic lavasreveal their crystallization from fractionatedmelts. The absence of systematic variation in Nb/Ta and Zr/Hfratios, and Sr-Nd isotope compositions between the mafic and felsic lavas indicates progressive evolutionof magmas by fractional crystallization. Trace element ratios and their plots corroborate mantle het-erogeneity and reveal distinct geochemical signatures for individual the CVL volcanoes.

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1. Introduction

Volcanic rocks of continental rifts have received considerableattention in recent years. However, the diversity in their compo-sition means they are not explainable by a single process (Jung andMasberg, 1998). Although continental and oceanic intraplatemagmas show considerable variation in trace element abundancesand ratios, they are typically more enriched in incompatible traceelements than basalts erupted at subduction zones and mid-oceanridges (MORs). As a result, trace element modeling for petrogenesisof intraplate magmas invariably requires their derivation from amantle source that is chemically distinct from the upper-mantle

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A.N.E. Asaah et al. / Geoscience Frontiers xxx (2014) 1e172

magma source that produces the MORBs (Weaver, 1991). Conti-nental intraplate rift magmatism is geochemically similar to oceanisland basalts (OIB) (Weaver, 1991) and is thought to be generatedfrom a convectively upwelling asthenosphere during continentalextension associated with the formation of rift systems (Zou et al.,2000). The upper mantle beneath intraplate rift-related Cenozoicvolcanoes is thought to have experienced metasomatism in thepast (Wilson and Downes, 1991). These processes result in theproduction of hydrous minerals (amphibole and phlogopite), arange of accessory mineral phases, and enrichment in incompatibleelements (Wilson and Downes, 1991; Wedepohl et al., 1994).

Unique to other intraplate rift related volcanoes, the CameroonVolcanic Line (CVL) in West Africa, consists of a chain of Cenozoicvolcanoes developed on both the Atlantic oceanic floor and thecontinental crust of the African plate (Fig.1). There is no evidence ofsystematic age migration from one volcano to another (Lee et al.,1994; Marzoli et al., 2000; Aka et al., 2004) as observed in atypical hot spot settings such as the Hawaii-emperor chain. Thesecomplex characteristics (clear volcano alignment but lack of aconsistent time-space migration) of the CVL have generated somedebate concerning its origin. Geological (Grunau et al., 1975; Gorini

Figure 1. Location of the Cameroon Volcanic Line. COB ¼ Continent Ocean Boundary, CAR ¼System (Modified from Marzoli et al., 2000; Rankenburg et al., 2005; Ngako et al., 2006; N

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and Bryan, 1976; Freeth, 1978; Sibuet and Mascle, 1978; Fitton,1980; Morgan, 1983; Burke, 2001), geochemical (Fitton andDunlop, 1985; Deruelle et al., 1991; Lee et al., 1994; Njonfanget al., 2011), structural (Moreau et al., 1987; King and Anderson,1995), geophysical (Fairhead, 1988; Fairhead and Binks, 1991;Meyers et al., 1998; King and Ritsema, 2000; Reusch et al., 2010;Milelli et al., 2012) and geochronological (Dunlop and Fitton,1979; Aka et al., 2004) data have been used to develop comple-mentary models to explain the origin of the CVL. Overall, maficrocks of the CVL show chemical features consistent with plumeactivity outlined by their OIB major and trace elements andradiogenic isotope characteristics (Mbassa et al., 2012).

Geochemical data and its interpretation are challenging for theCVL system, because of large variation. Lavas from the CVL rangefrom mafic end-members e.g., nephelinite, basanite and alkalibasalt to felsic end-members e.g., phonolites, trachytes and rhyo-lites. Available data show that some volcanoes (Mt. Etindé, Mt.Cameroon and Bui Plateau, Fig. 1) are composed only of mafic lavaswhile others (Mounts Manengouba, Bambouto, the Oku VolcanicGroup or OVG and the Ngaounderé Plateau) are bimodal, with thepresence of a Daly gap. The degree of differentiation of lavas varies

Central African Republic, CASZ ¼ Central African Shear Zone, EARS ¼ East African Riftkouathio et al., 2008).


A.N.E. Asaah et al. / Geoscience Frontiers xxx (2014) 1e17 3

from one volcano to another with no well-defined trend. There isheterogeneity even at a local scale within each central volcano(Nkouandou and Temdjim, 2011) which is thought to be due to adifferent degree and depth of partial melting (Marzoli et al., 2000;Kamgang et al., 2013). The CVL magmatism is characterized bymelting in the garnet lherzolite stability fields (Marzoli et al., 2000;Yokoyama et al., 2007; Kamgang et al., 2013), although melting inthe spinel lherzolite stability fields have been reported in theNgaoundere Plateau region (e.g., Lee et al., 1996; Nkouandou andTemdjim, 2011) where the lithosphere is significantly thin(Poudjom Djomani et al., 1992, 1997). In addition, a mixing of bothgarnet and spinel melting fields has been reported inMt. Cameroon(Tsafack et al., 2009). Both mafic and felsic rocks show chemicalfeatures consistent with plume activity outlined by their OIBcharacters, and their isotopic ratios (Mbassa et al., 2012).

Information based on trace element abundances and isotopesystematics of the CVL lavas shows that far from being a simplealignment of individual volcanoes, the CVL is divided into threedistinct sectors e the oceanic, ocean-continent boundary and thecontinental sectors (Halliday et al., 1990; Lee et al., 1994, 1996).These data can be complemented by in compatible trace elementswith similar mineral/melt partition coefficients, as these are lesssensitive to variations in partial melting of the source mantle andmineral crystallization in the magma plumbing system. Thegeochemical complexity and tectonic setting of the CVL make it anideal environment to investigate characteristics of source magmasin an intraplate setting, and their petrogenetic evolution. In addi-tion, the parallelism (Fig. 1) between the CVL alignments and thecentral African shear zone strike (Ngako et al., 2006) could provideinsights into the potentials of mineral deposits associated withthese structures (Njonfang et al., 2011).

More recent data from 2007e2013 (e.g., Njilah et al., 2007;Yokoyama et al., 2007; Aka et al., 2008; Nkouathio et al., 2008;Suh et al., 2008; Tsafack et al., 2009; Kamgang et al., 2010;Nkouandou and Temdjim, 2011; Fagny et al., 2012; Mbassa et al.,2012; Tchouankoue et al., 2012; Kamgang et al., 2013) have beenadded to data sets for the CVL used in a review by Deruelle et al.(2007). With the increase in published geochemical data availableafter this review from 2007, other models and interpretations forthe CVLmagmatism have been proposed. As a result, the debates onits origin and petrogenesis are attracting more scientific interest. Toenhance the debate, a framework based on updated data is needed.Therefore, the aim of this study is to (1) summarise existing modelsfor the origin of the CVL, and (2) investigate petrogenetic processesof the CVL lavas including crustal contamination, nature of sourceand degree of melting, mantle reservoirs and their interactions, aswell as the evolution from mafic to felsic lavas. Simultaneouscomparison of lavas from all the CVL central volcanoes can givevaluable information on the general nature and evolution of indi-vidual volcanoes with respect to others along the chain.

2. Geologic and tectonic setting

The Cameroon Volcanic Line (CVL) is a 1600 km Y-shape chain ofTertiary to recent volcanoes (Fitton and Dunlop, 1985) that trendsfrom the Annobon Island in the Gulf of Guinea into the African plate(Fig. 1). The tectonic setting of the CVL is described in detail inprevious studies (Grunau et al., 1975; Gorini and Bryan, 1976; Fittonet al., 1983; Moreau et al., 1987; Ngako et al., 2006). The intraplatetectono-magmatic alignment is remarkable in that it developedsimultaneously on the oceanic and continental sectors (Fitton,1987). The CVL is oriented N30�E and segmented by severalN70�E shear zones of Pan-African age (Deruelle et al., 2007),particularly evident on the continental sector. The oceanic sectorconsists of two seamounts in the Gulf of Guinea and four islands


(Annobon, São Tomé, Principé and Bioko) (Lee et al., 1994). On theother hand, the continental sector consists of anorogenic plutoniccomplexes and volcanic constructs of varying sizes and shapes(Deruelle et al., 2007). Magmas of the continental CVL intruded intothe overlying Proterozoic meta-sedimentary, meta-granitic, andgranitic rocks. These basement rocks are either of Precambrian age(Geze, 1943; Gouhier et al., 1974; Le Marechal, 1976; Lasserre, 1978;Dunlop, 1983) or bear imprints of a penetrative Neo-ProterozoicPan African deformation (Annor and Freeth, 1985; Toteu et al.,2004).

The central volcanoes in the continental sector of the CVLinclude Mounts Etindé, Cameroon, Manengouba, Bambouto, andOku, as well as two main plateaus; the Ngaounderé and Biu (Fitton,1987; Halliday et al., 1988; Marzoli et al., 1999; Marzoli et al., 2000;Ngounouno et al., 2000; Nono et al., 2004; Rankenburg et al., 2005).Mount Oku forms part of the Oku range referred to in this studyas the Oku Volcanic Group (OVG). Some central volcanoes(Mt. Bambouto, the OVG, and the Ngaounderé Plateau) producemore chemically evolved lavas than others. They are dominantlypolygenetic, and alternate with less evolved monogenetic grabens(Sato et al., 1990; Nkouathio et al., 2008). North of the Oku massif(Fig. 1), the volcanic line bifurcates into a Y-shape; Mt. Mandara-BiuPlateau branch in the north-west and Ngaounderé Plateau branchin the north-east (Fitton et al., 1983). Of these volcanoes, Bioko, Mt.Etindé, and Mt. Cameroon volcanic centers are located close to theseismically-defined “ocean-continent boundary” (OCB) zone(Emery and Uchupi, 1984).

There are four volcanoes in the oceanic sector of the CVL:Annobon (813 m a.s.l.), São Tomé (2024 m a.s.l.), Principé (984 ma.s.l.) and Bioko (2872 m a.s.l.). They are strato volcanoes (Annobonand São Tomé) and lava flows (Principé and Bioko). These volcanicislands are made up of volcanic rocks ranging from nephelinite,basanite and basalt to trachyte and phonolite (Piper andRichardson, 1972; Dunlop and Fitton, 1979; Cornen and Maury,1980; Fitton and Dunlop, 1985; Fitton, 1987; Halliday et al., 1988,1990; Deruelle et al., 1991; Lee et al., 1994).

Mt. Etindé is a small densely forested and highly dissectedvolcano located on the SW flank of Mt. Cameroon. It rises to aheight of 1713 m a.s.l and is made up almost entirely of nepheliniticlavas rich in euhedral (3e7 mm) highly zoned clinopyroxene phe-nocrysts (Nkoumbou et al., 1995).

Mt. Cameroon is the highest (4095m a.s.l) volcano inWest Africaoccupying a surface area of about 1300 km2. This volcano is the onlypresently active member of the CVL with seven eruptions recordedin the last 100 years, i.e., 1909, 1922, 1954, 1959, 1982, 1999, and2000 (Geze, 1943; Fitton et al., 1983; Suh et al., 2003). It is a com-posite volcano made of alkaline basanitic and basaltic flows inter-bedded with small amounts of pyroclastic materials and numerouscinder cones (Suh et al., 2003, 2008; Yokoyama et al., 2007).

Mounts Bambouto (2700 m a.s.l.) and Oku (3011 m a.s.l) areOligocene to Quaternary strato volcanoes with lava successionscomprising a strongly bimodal basalt-trachyte-rhyolite suite(Marzoli et al., 2000; Njilah et al., 2004, 2007; Kamgang et al., 2010,2013; Tchouankoue et al., 2012). Mt. Manengouba (2411 m a.s.l) is awell preserved central volcano whose summit hosts two concentriccalderas (Elengoum and Eboga). Lavas range from basalts to tra-chytes, quartz trachytes, and rare rhyolites (Fitton, 1987; Kagouet al., 2010).

The Ngaounderé Plateau, in the northeastern part of the CVL,consists of alkaline basalts and basanites capped by trachytes andphonolitic flows. The most recent volcanism in this area consists ofcinder cones aligned in a WNWeESE direction, sometimes pro-ducing small lava flows (Fitton, 1987; Nono et al., 2004). The BiuPlateau, which is located in the northern part of the NgaounderéPlateau, consists of basaltic flows with a maximum thickness of


Figure 2. (a) Total alkali vs. silica classification diagram (after Le Bas et al., 1986), (b)K2O vs. SiO2 (Le Maitre, 2002), (c) Mg# vs. SiO2 for the CVL lavas. Hypothetical frac-tionation trends are also illustrated in Fig. 2c (subscripts 1, 2, 3 and 4 are labels todifferentiate dominant trends or directions identified on the diagram). Volcanic suiteranging from primitive basanite and alkali basalts to more evolved trachyte, phonoliteand rhyolite with paucity in the trachy-andesite field. Nephelinite of Mt. Etinde con-stitutes a separate group. (F ¼ foidite, BS ¼ basanite, PB ¼ pico-basalt, B ¼ basalt,TB ¼ trachy-basalt, BA ¼ basaltic andesite, BTA ¼ basaltic trachy-andesite,PhT ¼ phono-tephrite, A ¼ andesite, TA ¼ trachy-andesite, TPh ¼ tephri-phonolite,Ph ¼ phonolite, T ¼ trachyte, D ¼ dacite, R ¼ rhyolite; Data source: The CVL literaturedata up to 2007 and recent data including: Njilah et al., 2007; Yokoyama et al., 2007;Aka et al., 2008; Nkouathio et al., 2008; Suh et al., 2008; Tsafack et al., 2009; Kamganget al., 2010; Nkouandou and Temdjim, 2011; Fagny et al., 2012; Mbassa et al., 2012;Tchouankoue et al., 2012; Kamgang et al., 2013).


250 m. This plateau is composed of basanite to transitional basalts(Rankenburg et al., 2005).

The history of tectono-magmatic activity in the CVL and adja-cent Jos Plateau and the Benoue Trough reveals four key points witha general age progression from the northwest to southeast. Theseinclude: (1) The initial phase of magmatic activity in the BenueTrough (147 Ma; Coulon et al., 1996) occurred at about the sametime as the emplacement of anorogenic granite ring complexeswhen the formation of the Jos plateau ended; (2) The transitionfrom the first to second stages of magmatic activity in the BenueTrough (106e95 Ma) overlaps with the period (100 Ma) when theEquatorial Atlantic opened, leading to the separation between theSouth American and African plates (Wilson, 1992; Wilson andGuiraud, 1992); (3) The last stage of magmatic activity in the BenueTrough (68e49 Ma) was contemporaneous with the initial phase ofintrusive activity of the continental sector of the CVL (66e30 Ma)(Lasserre, 1966); (4) Volcanic activity in the CVL (45 Maepresent,Fitton and Dunlop, 1985; Marzoli et al., 2000; Aka et al., 2004)(Fig. 1).

3. Data selection and treatment

The geochemical data used in this study consist of major andtrace elements, and radiogenic isotopes (Sr-Nd-Pb) from pub-lished literature on the CVL spanning four decades. More recentdata from 2007e2013 have been added to data sets for the CVLused in a previous review by Deruelle et al. (2007). A large dataset of 580 samples covering the entire CVL is used here to addresspetrogenetic processes and source characteristics of the CVLmagmas. Major elements were recalculated on a 100% volatile-free basis. Only representative internally consistent data setswere selected, with priority given to samples in which both majorand trace elements and isotopes were analysed. Data for Annobon,São Tomé, Principé and Bioko that constitute the oceanic sector ofthe CVL are grouped and referred to as the oceanic CVL in thefigures. On the other hand, data for each volcanic centre on thecontinental sector of the CVL (Mounts Cameroon, Etindé, Man-engouba, Bambouto, the OVG, and the Ngaounderé & the BiuPlateaux), are presented separately. PETROGRAPH (Petrelli et al.,2005), PETROMODELER (Ersoy, 2013) and AFC-Modeler (Keskin,2013) software were used to generate diagrams and performmodel calculations.

4. Geochemistry

4.1. Major elements

Representative samples (400) selected for major element sys-tematics have SiO2 ranging from 38 wt.% in a sample from theoceanic CVL to 79 wt.% in a sample from the OVG (Oku VolcanicGroup). Based on the total alkali versus silica (TAS) classification(Fig. 2a; Le Bas et al., 1986), the samples representative of both theoceanic and continental sectors of the CVL are dominantly alkalibasalts and basanites which are parental magmas of two majordifferentiation series: (1) alkali basalt e trachyandesite e trachyteerhyolite, and (2) basanite e tephrite - phonolite (Wilson, andDownes, 1991). Nephelinite occurs in Mt. Etindé but has markedlydifferent geochemical characteristics from the basanites and alkalibasalts. The nephelinites constitutes a separate group, not associ-ated with differentiated lavas. Using the classification criteria pro-posed by Le Maitre (2002), the lavas dominantly range from highlypotassic to ultrapotassic (Fig. 2b). The Mg# (¼100 � MgO/(MgO þ FeO)) of mafic samples ranges from 35e69. This indicatesthat the lavas underwent some fractionation. The under-saturatednephelinite from Mt. Etindé has a lower Mg# (37e53) when

Please cite this article in press as: Asaah, A.N.E., et al., A comparative review of petrogenetic processes beneath the Cameroon Volcanic Line:Geochemical constraints, Geoscience Frontiers (2014), http://dx.doi.org/10.1016/j.gsf.2014.04.012

Table 1Major element (wt.%) compositions (ranges) of representative CVL volcanic rocks.

Volcanic centres SiO2 Al2O3 Fe2Ot3 MgO CaO Na2O K2O TiO2 P2O5 MnO Mg#

Oceanic CVL 40e67 10e22 2e14 0e13 0e13 22e9 0e6 0e4 0e1 0e0.02 0e66Mt. Etinde 39e43 13e17 11e15 3e9 12e16 2e6 0e4 4e5 1e2 0e0.4 37e53Mt. Cameroon 43e50 10e17 11e14 5e12 9e14 2e5 1e2 3e4 0e1 0e0.3 46e65Mt. Manengouba 44e54 12e18 2e14 2e12 5e12 3e6 1e3 2e3 0e1 0e0.2 35e65Mt. Bambouto 41e70 12e20 4e14 0e11 0e11 3e8 1e6 0e4 0e1 0e0.4 1e63OVG 42e79 10e17 1e15 0e13 0e11 2e7 0e6 0e5 0e2 0e0.3 1e65Ngaoundere

Plateau41e68 12e22 2e15 0e11 0e11 2e11 0e6 0e4 0e1 0e0.4 7e68

Biu Plateau 44e53 13e16 8e13 6e13 7e11 2e4 11e3 2e3 0e1 0e0.2 58e69


compared to those of basanites and alkali basalts (Fig. 2c). Thissuggests an earlier fractionation of magnesian olivine and pyrox-enes (possibly augite). Hypothetical fractionation trends for the CVLlavas are illustrated in Fig. 2c (subscripts 1, 2, 3 and 4 are labels todifferentiate supposed trends or directions identified on thediagram).

The ranges for major element oxides are presented in Table 1.Lavas of the oceanic CVL, Mounts Bambouto, the OVG andNgaounderé Plateau have a wider range of major element oxidesespecially for SiO2 when compared to those of Mt. Etindé, Mt.Cameroon, Mt. Manengouba and Biu Plateau. This suggests that theformer have experienced a higher degree of differentiation byfractional crystallization than the latter.

Thewell-defined trends of compositional similarity among lavasof the oceanic and continental sectors of the CVL indicates rapidascent of magma in the continental CVL, avoiding significantcontamination from the continental crust. This is supported by thepresence of mantle xenoliths in most basalts and basanite. Theobserved insignificant crustal contamination of mafic rocks of thecontinental sector of the CVL is reported in previous studies (Fittonand Dunlop, 1985; Marzoli et al., 2000; Rankenburg et al., 2005;Deruelle et al., 2007), though there are some who have reportedcrustal contamination in some lavas (see Section 5.2).

Calcium oxide, MgO, Fe2Ot3 , P2O5, and TiO2 decreasewhile Al2O3,

Na2O, and K2O increase with increasing SiO2 content (Fig. 3aeh).Negative correlation of Na2O, K2O, and Al2O3 with SiO2 is observedfor felsic samples (SiO2 >60 wt.%) (Fig. 3a, b, and f). The observedvariation in major element oxides is consistent with the fraction-ation of (1) olivine, clinopyroxenes, titanomagnetite and apatite(decreasing CaO, MgO, Fe2O

t3 , and TiO2) at an early crystallization

stage, and (2) dominantly feldspar in later stages. The K2O contentcorrelates positively with SiO2. This trend could indicate varyingdegrees of melting of a common source for all the CVL lavas orcrustal contamination. The trend for each major oxide also variesfrom one volcano to the other.

4.2. Trace elements

The ranges for Ni, Cr, and Co (Ni: 90e400 ppm, Cr: 96e860 ppm,Co: 36e70 ppm) in the mafic rocks overlap with those assumed forprimary magmas (Ni: 300e400 ppm, Cr: 300e500 ppm, Co:50e70 ppm) (Frey et al., 1978). Nickel and Cr contents decreasewith an increase in SiO2 content (Fig. 4a and b), suggesting frac-tionation of these elements along with crystallization of olivine andclinopyroxene. Lavas from the oceanic sector of the CVL, Mt.Cameroon, Biu Plateau, and Ngaounderé Plateau are relativelyricher in these elements than Mt. Bambouto and the OVG. This mayindicate that magmas from Mt. Bambouto and the OVG underwentsignificant differentiation during their ascent, as well as storage in ashallow magma chamber (Suh et al., 2008). Hypothetical fraction-ation trends for the CVL magmas are illustrated in Fig. 4b and aresimilar to the trends in Fig. 2c. On the other hand, the compositions


of Nd, La, and Rb show scattered trends with increasing SiO2 con-tent (Fig. 4c, e and f). The curved negative correlation of Sr with SiO2would suggest fractionation of feldspars.

Primitive-mantle normalized trace element patterns (Palme andO’Neill, 2003) for mafic rocks from the oceanic and continentalsectors of the CVL are similar to each other (Fig. 5). The patterns areakin to OIB, suggesting the origin from a similar source. All thesamples show a marked (LREEs) enrichment and a strong frac-tionation of Heavy Rare Earth Elements (REEs) relative to LREEs.The absence of negative Eu anomaly defined by Eu* ¼ Eu(CI)/[Sm(CI)þ Gd(CI)] (CI refers to chondrite normalized concentration)in the mafic rocks suggests an insignificant plagioclase fraction-ation. These samples also show elevation of Ta and Nb compared tothe neighboring elements (K, La) in this diagram (Fitton andDunlop, 1985; Lee et al., 1994), and is similar to other oceanislands and mid ocean ridge basalts (Hofmann, 1988). Conversely,island arcs and continental crustal rocks show negative Ta and Nbpatterns. The most striking features of Fig. 5 are: (1) higher traceelemental abundances in nephelinite of Mt. Etindé, but similarabundances for highly incompatible elements (from Cs to K)compared to the other CVL alkaline basalts; (2) relatively highpositive anomalies of Nb and Ta, high U and Th, alongside Ta, La, Ce,Pb, and Eu for Mt. Cameroon and Biu Plateau which are notobserved for the OVG, the oceanic sector, and Mt. Manengoubalavas; and (3) the occurrence of a K-trough.

The Ba/Nb ratio varies from 4.65 in a sample from Mt.Cameroon to 15.91 in a sample from the OVG. The average Ba/Nbvalue is 10.03 for primitive mantle (McDonough and Sun, 1995)and 57.00 for continental crust (Rudnick and Gao, 2003). The CVLsamples with Ba/Nb ratios that fall out the range of the primitivemantle (10 � 3) would have been affected by contamination and/orpost magmatic alteration processes. This may be element enrich-ment or depletion at shallower depths within the crust. Somesamples fromMt. Cameroon and the Ngaounderé Plateau (and alsoBiu) have Ba/Nb ratios less than 7.00 while those from the OVG, Mt.Bambouto and the oceanic CVL occur within or slightly above thisrange. In addition, Ba/Nb and La/Nb ratios show no clear rela-tionship suggesting that there is selective enrichment or removalof Ba or La.

Barium, which is also a fluid mobile element like K, does notshow depletion. As a result, K-depletion cannot be attributed tosurface processes like weathering. However, the relatively constantbut low concentrations of K, coupled with high concentrations ofNb and La in alkali basalts and nephelinite suggests that K in themelts could have been buffered by residual amphibole, phlogopiteand/or clinopyroxene during source enrichment processes and/ormagma generation (e.g. Sun and Hanson, 1975; Clague and Frey,1982). Such residual K-bearing minerals are accompanied by de-pletions of Rb and Cs in the CVL samples. In addition, nearly con-stant elemental ratios of incompatible elements suggest thatmagma processes such as zone refining melting, magma mixing,and extensive fractionation and replenishment, all of which can


Figure 3. Major element (wt.% oxide) variation plots against SiO2 (wt.%) for the CVL lavas. Symbol, color and data source are same as in Fig. 2.


Please cite this article in press as: Asaah, A.N.E., et al., A comparative review of petrogenetic processes beneath the Cameroon Volcanic Line:Geochemical constraints, Geoscience Frontiers (2014), http://dx.doi.org/10.1016/j.gsf.2014.04.012

Figure 4. Trace element (ppm) variation plots against SiO2 (wt.%) for the CVL lavas. Symbols, color and data source are same as in Fig. 2. Hypothetical fractionation trends are alsoillustrated in Fig. 4b (subscripts 1, 2, 3 and 4 are labels to differentiate dominant trends or directions identified on the diagram).


efficiently fractionate incompatible elements, were not dominantprocesses during the generation of the CVL mafic lavas.

4.3. Sr, Nd and Pb isotopes

Sr-Nd-Pb isotopic compositions of the CVL basalts overlap thoseof OIB. In the 207Pb/204Pb vs.

206Pb/204Pb diagram (Fig. 6), the CVL

data plot along the Northern Hemisphere Reference Line (NHRL) ofHart (1984), and to the right of the 4.53 Ga geochron. However,some data from the OVG with low 206Pb/204Pb and 207Pb/204Pbratios overlap with MORB end members (Atlantic, Indian, and Pa-cificMORBs). Themafic rocks showa limited range in their 87Sr/86Srand 143Nd/144Nd ratios. The 87Sr/86Sr ratios range from 0.70515 in asample from Mt. Bambouto to 0.70286 in a sample from the BiuPlateau and 143Nd/144Nd ratios vary from 0.52771 in a sample fromMt. Cameroon to 0.51302 in a sample from the Biu Plateau. Some


lavas from the Biu Plateau and the oceanic CVL show relatively low87Sr/86Sr and high 143Nd/144Nd ratios, implying that they are moreprimitive than other continental volcanic rocks. (Mt. Bambouto, Mt.Manengouba, and the OVG). Isotope data for the OVG show a widespread than those of other CVL volcanoes. This difference is con-spicuous in Pb isotopes. A negative correlation is displayed betweenthe 143Nd/144Nd and 87Sr/86Sr (Fig. 7) inwhich the correlation slopematches the mantle array of MORB-OIB samples.

On the contrary, plots of 87Sr/86Sr vs. 206Pb/204Pb (Fig. 8a and b)and 143Nd/144Nd vs. 206Pb/204Pb (Fig. 9a and b) show double cor-relation (positive and negative) with different slopes, suggestingthe involvement of enriched mantle components. Therefore, interms of identifying enriched mantle sources (EM1 and EM2), the87Sr/86Sr vs. 206Pb/204Pb and 143Nd/144Nd vs. 206Pb/204Pb diagramsare more powerful and easier than the 143Nd/144Nd vs. 87Sr/86Srdiagram.


Figure 5. Trace element patterns normalized to primitive mantle (Palme and O’Neill, 2003) for representative mafic samples. All samples show positive Ta and Nb anomalies. Datasource are same as in Fig. 2.


5. Discussion

5.1. Origin of the CVL

The origin of the CVL is still controversial. Numerous modelshave been proposed, but there is no unifying model that cancollectively explain the complex traits of the CVL. In the following,the various models that have been presented in the literature arediscussed; they are based on the geology, and geochemical, struc-tural, geophysical, and/or geochronological data.

5.1.1. Structural interpretationsThe magmatic evolution of the CVL has been interpreted to be

related to the development of tectonic structures, including thereactivation of structures in the Cenozoic (in this case, reactivationrefers to the displacement of pre-existing tectonic structures andassociated crustal melting) (Gorini and Bryan, 1976; Moreau et al.,1987; Fairhead, 1988; Deruelle et al., 1991). However, seismic pro-file transects close to Annobon and São Tomé, the southwesternmost part of the CVL alignment of the oceanic sector, do not displayfaults associated with the northeast-trending volcano alignmentthat would otherwise indicate magmatic-tectonic activity (Grunauet al., 1975). In addition, a reconstruction of the south American andAfrican continents in the Santonian (87e83 Ma) by Sibuet andMascle (1978) shows that the CVL is oblique to the ascensionfracture system, which is located further to the southwest of thealignment (Moreau et al., 1987; Fairhead, 1988; Fairhead and Binks,1991). Thus, the fault-reactivation hypothesis to explain the gen-eration of the CVL magmas in the southwestern-most part of theoceanic sector is difficult to confirm. In addition, if the reactivationmodel was possible, then crustal contamination of mafic lavaswould be more evident. Also, the Central African Shear Zone (CASZ)with extension to the Central African Republic should have acomplete alignment of volcanoes (i.e., there is a gap between Mt.Bambouto and the Ngaounderé Plateau (Fig. 1). Therefore, it seemsthat basement tectonic structures rather played a role in chan-neling magmas to the surface than in the generation of themagmas. The alternating horst (islands and continental volcanoes)


and graben structure of the CVL (Deruelle et al., 1987, 1991) isrelated tomembrane stresses developed as the African platemovedaway from the Equator (Freeth, 1978). However, according to thismodel, the intrusive complexes were emplaced when the areashould have been under compression.

Based on the similarity in the structure of the Benoue Trough,the CVL and the East African Rift Systemwith its extension in SouthAfrica, Njonfang et al. (2011) suggested a mechanism of episodicemplacement of alkaline magmas through the entire Africancontinent. The alignment of magmatic complexes resulted from acomplex interaction between hot spots (mantle plumes acting insuccession) and lithospheric fractures (including Precambrianfaults) during African plate motion.

5.1.2. Displacement of the African plateThis model is based on the size and shape similarities between

the CVL and the adjacent Benoue Trough (Fitton, 1980). Bothgeologic features are Y-shaped, and can be superimposed byrotating one with respect to the other. According to this model,there was a motion of the African Plate with respect to theasthenosphere (80e65 Ma) such that the hot asthenosphericwedge, which should have underlain the Benue Trough, was ori-ented beneath Cameroon and the Gulf of Guinea. Therefore, itsuggests that there was migration of magmatism from the Josplateau to the Benue Trough and finally to the currently active CVL.This is in agreement with the decrease in age of magmatic activityfrom Jos Plateau to the CVL (Fig. 1).

5.1.3. Hotspots and hotlinesThe fact that the CVL volcanoes are neatly aligned led to the

“hotspot’’ hypothesis. According to this hypothesis, the CVL origi-nated from the movement of the African plate over a stationarysub-lithospheric hotspot that was periodically fed and melted by adeep mantle plume head (Morgan, 1983; Lee et al., 1994). Thehotspot model is not applicable to the CVL because: (1) Geochro-nologic data do not show any systematic age migration of volca-nism (Fitton and Dunlop, 1985; Lee et al., 1994; Aka et al., 2004),and (2) the only presently active volcano (Mt. Cameroon) is located


Figure 6. 207Pb/204Pb vs. 206Pb/204Pb diagram for the CVL lavas studied. NHRL ¼ Northern Hemisphere Reference Line (Hart, 1984). Data plot on the right of the 4.53 Ga geochron.Samples are characterized by high 207Pb/204Pb and 206Pb/204Pb isotopic ratios. The OVG is distinct with a wide range in Pb isotopic ratios. Data source is same as in Fig. 2.


at the center of the line (Fig.1). Because of this, an alternativemodelto explain volcanic alignment along the CVL, referred to as the‘hotline’ has been proposed (Meyers et al., 1998). According to thismodel, supported by seismic and gravimetric data in the oceanicsector, the mantle plume independently supplies each volcano ofthe CVL (Deruelle et al., 1991; Meyers et al., 1998).

Figure 7. 143Nd/144Nd vs. 87Sr/86Sr diagram for the CVL lavas studied. Positive correlati


5.1.4. Plate-wide swellsBurke (2001) suggested a plate-wide shallow-mantle convec-

tion system localized under the zone of extension. According to thismodel, the CVL originates from a series of three distinct phenom-ena (Burke, 2001). First, its position over the so-called 711 plume(‘711’ indicates latitude 70�N and longitude 11�500E) for the past

on with different slopes for each volcanic centre. Data source is same as in Fig. 2.


Figure 8. 87Sr/86Sr vs. 206Pb/204Pb diagram for the CVL lavas studied. (a) The CVL lavas plot dominantly in the FOZO field. High 87Sr/86Sr ratio for the OVG lavas. (b) Double cor-relation (negative and positive) trends for the CVL lavas. Data source is same as in Fig. 2.


140 million years. Subsequently, the location shifted at a right-angle bend with respect to the continental margin, remaining atthis position for nearly 125 million years. Finally, a new plate-widepattern of shallow-mantle convection was established at 30 Mawhen the African plate came to rest.

5.1.5. Edge convection and lithospheric instabilityRecently, a new geophysical model has been developed to

explain the origin of the CVL. Based on evidence from seismic wavebehavior in the upper mantle, Reusch et al. (2010) proposed an‘edge convection’ model that is driven by lateral variations of lith-ospheric thickness. According to this model, lower temperatures inthick continental roots compared to those of the adjacent thinnerlithosphere generate an edge ‘downwelling’ which feeds an ‘up-welling’ beneath the thinner region (King and Anderson,1995; Kingand Ritsema, 2000). The structure of the CVL, which includes the


alternating horsts (main volcanoes) and grabens (plains), maysupport this model. In an attempt to improve upon the work ofReusch et al. (2010), Milelli et al. (2012) designed a new laboratory-based model, known as ‘lithospheric instability’. Their work in-dicates that there may be instability within the sub-continentallithospheric mantle at the edge of a continent.

5.1.6. Summary of current CVL modelsThe models presented above are complimentary, each covering

the various aspects of the CVL origin and its evolution, includingtectono-magmatic activity through time, and the manifestation ofplumes including convection. The ‘hotline’ model collectively ad-dresses the various aspects of the CVL. This model invokes multipleplumes originating from the same source (the upper mantle), eachof which manifests independently to form volcanoes at the surfaceof the Earth. It should be noted that this model is not only


Figure 9. (a) 143Nd/144Nd vs. 206Pb/204Pb diagram for the CVL lavas studied. (b) Double correlation (positive and negative) trends for the CVL lavas with a point of intersection. Datasource is same as in Fig. 2.


consistent with the geophysical, structural and geochemical infor-mation, it also may explain the observed absence in the migrationof volcanic activity through time from one volcano to another(Fitton, 1987). This model involves the interaction of deep mantleplumes with the subcontinental lithospheric mantle.

5.2. Assimilation and fractionation crystallization (AFC)

Apart from using isotopes to determine genetic relationships involcanic suites, the systematic behavior of some highly incompat-ible element ratios including Nb/Ta, Sr/Pr, Zr/Hf, Ce/Pb, Nb/U arepowerful indicators. These ratios do not change significantly inlavas from the same source. Nb/Ta ratio for more than 90% of bothmafic and felsic CVL lavas range from 13e18 with a mean of 16.There is no systematic variation in this ratio from mafic to felsic


lavas thus the lavas plot roughly horizontally on the Nb/Ta vs. SiO2diagram (Fig. 10). This indicates a genetic relationship amongst themafic and felsic lavas of the CVL.

The degree of differentiation, expressed by the differentiationindex (DI) for the CVL lavas varies from one volcano to another.However, we group them into: low (DI ¼ 19e63) e.g., Mt.Cameroon, Mt. Etinde, the Biu Plateau, and Mt. Manengouba; andhigh (22e97) e.g., Mt. Bambouto, the oceanic CVL, the NgaounderePlateau, and the OVG. Fig. 2c shows hypothetical multiple fractionalcrystallization trends (FC1e4) for all the CVL lavas. FC1 and FC2 arethe most common fractionation trends for the CVL lavas. It isexhibited by the oceanic CVL, Mt Cameroon, Mt. Manengouba, Mt.Bambouto, the OVG and the Ngaoundere Plateau. FC3 and FC4trends are typical of Mt. Etinde and the Biu Plateau respectively.Nevertheless, trends of FC1e3 show similar source characteristics


Figure 10. Nb/Ta vs. SiO2 (wt.%) diagram for the CVL lavas. Limited variation in Nb/Taratios for most samples. Symbols, color and data source are same as in Fig. 2.


and indicates that they might have resulted from a similar up-welling mantle plume, with subsequent divergence in evolutiondue to different conditions.

Magmas traveling to the surface from the mantle may becomecontaminated by assimilation of crustal material. However, theimportance of crustal contamination and processes such as FC andAFC in producing the variety of magma composition remainscontroversial (e.g McBirney and Noyes, 1979). The striking simi-larity in Sr-Nd isotopic compositions between highly mafic rocks ofthe continental and oceanic sectors of the CVL strongly arguesagainst significant crustal contamination. In addition, the existenceof mantle xenoliths in most of the CVL mafic lavas (Aka et al., 2004,2008; Temdjim et al., 2004) suggests that magmas for these rocksascended rapidly, thus avoiding significant contamination andextensive fractional crystallization in crustal magma chambers.However, in some samples (more evolved lavas) the possibility ofcrustal contamination is likely, especially in samples from Mt.Bambouto and the OVG (Marzoli et al., 2000; Deruelle et al., 2007;Kamgang et al., 2008). This corroborates with the relatively high Srisotopic compositions in these lavas (e.g. 0.70472 for the OVG and0.70512 for Mt. Bambouto). This range is higher than the Sr isotopicratios for the oceanic CVL, and some of the continental CVL lavas(Lee et al., 1994; Rankenburg et al., 2005; Yokoyama et al., 2007).

The assimilation fractional crystallization (AFC) model using theequations of DePaolo (1981) was applied to CVL mafic and felsiclavas. The least fractionated sample P18 (MgO ¼ 12%, SiO2 ¼ 38%)was selected to represent the initial magma. In the model, traceelement composition of a magma affected by AFC process (CAFC

lc ) isexpressed by:

CAFClc ¼ Cf

0

"F�z þ

� rr � 1

� Ca

zCf0

�1� F�z

�#(1)

where Cf0 is the initial trace element composition of an element in

magma, F is the mass fraction of the residual magma, and Ca is theconcentration of the element in the assimilated material (wallrock). The r value describes the relative ratio of assimilatedmaterialto crystallized material and is expressed by:

r ¼ ma

mc(2)

where ma is the amount of assimilated material and mc is theamount of crystallized material. The z value in the AFC equation isexpressed as:

z ¼ r þ D0 � 1r � 1

(3)

where, D0 is the bulk partition coefficient for the mineralassemblage.

Modeled results using Rb and Zr (highly incompatible elementswith large contrast between initial magma and crustal material) areshown in Fig. 11. Most mafic lavas plot around the initial magmacomposition indicating low degree fractional crystallization andinsignificant assimilation. However, evolved samples (increasing Zrand Rb) indicate crustal assimilation and plot within the AFC curves(r ¼ 0 to r ¼ 0.85). Majority of samples in this range plot betweenr ¼ 0 (no contamination) and r ¼ 0.25 (less than 25% contamina-tion). Some samples indicate contamination of up to 70% and above.Lavas most affected by AFC include the oceanic CVL, the Ngaoun-dere Plateau, Mt. Etinde, the OVG and Mt. Bambouto. The source ofcrustal input to the oceanic CVL lavas is not clear at moment.However, different hypothesis may be included: (1) convection inthe upper mantle, (2) infiltrating fluids and (3) contamination fromsediments or during sampling. On the other hand, contamination in


lavas from continental volcanoes arise from assimilation of wallrock during magma ascend through the continental crust or resi-dence at shallower magma chambers.

5.3. Nature of the source and melting

Previous studies have concluded that the CVL basic lavas werederived from a similar metasomatized asthenospheric mantle.These findings were based on Sr-Nd-Pb isotopes (Halliday et al.,1990; Lee et al., 1994), Hf isotopes (Ballentine et al., 1997), traceelement variations (Marzoli et al., 2000) and U-Th-Pb and Osisotope systematic (Rankenburg et al., 2005; Yokoyama et al., 2007).Major and trace elements in most of the CVL mafic lavas are char-acterized by elevated CaO/Al2O3, (La/Yb)N, Zr/Y ratios with nearlyconstant Y and Yb concentrations suggesting that the parentalmagmas originated from a dominantly garnet-bearing mantlesource. However, some exceptional cases of melting in the spinellherzolite have been reported in the continental sector of the CVL(e.g Lee et al., 1996; Nkouandou and Temdjim, 2011), suggestingsome degree of heterogeneity in the upper mantle of the conti-nental sector of the CVLwhich is controlled by the depth of melting.

During partial melting of mantle peridotite, incompatible ele-ments are concentrated in the liquid phase. When two incompat-ible elements have identical mineral/melt partition coefficients (Kd)during partial melting, their abundance ratio in the magma reflectsthat of their source, regardless of the degree of partial melting; i.e.,their abundance ratios do not vary with increasing concentrationsin themagma (Sun andMcDonough,1989). In contrast, the ratios ofincompatible elements having different Kd values would vary alongwith the degree of partial melting. The more incompatible anelement is, the more it will be preferentially enriched in the melt.

Based on this assumption, the mantle melting process wasmodeled for the CVL lavas using Zr/Nb (high field strength elementsthat are highly incompatible and are not fractionated duringmagmatic processes) and La/Yb (La is incompatible while Yb iscompatible and is fractionated by garnet). These parameters aregood in distinguishing melting of spinel and garnet lherzolitemantle. The model calculations indicate that melting occurs in thegarnet stability field (Fig. 12). However, the percentage of garnet inthe mantle varies alongside the degree of melting. With theexception of Mt. Etinde, the Ngaoundere Plateau and the BiuPlateau, where the melts were generated in the presence of 6e10%garnet, melts in the other CVL volcanoes were generated below 5%



garnet by a small (less than 3%) degree partial melting of themantlesource (Fig. 12).

The high concentration of REEs in the nephelinites of Mt. Etindéis consistent with a low degree of partial melting in the presence ofmore garnet at relatively deeper levels than the basanites and alkalibasalts.

5.4. Mantle source characteristics

Discrete mantle components have been identified from isotopicstudies of MORB and OIB (Zindler and Hart, 1986). The followingend members as defined in Stracke et al. (2005) will be used in thispaper; these are depleted MORB mantle (DMM), high-m (HIMU),Enriched mantle 1 and 2 (EM1 and EM2), and Focal Zone (FOZO).

Combined Sr, Nd and Pb isotope studies of the CVL basaltsdemonstrate that the observed isotopic variations in these rockscannot be generated by a single mantle process or mixing of twokinds of mantle components with well defined isotopic character-istics. Based on the enrichedmantle (EM1 and EM2) characteristics,Zou et al. (2000) suggested that mixing between DMM and EM2produces a positive correlation in the 87Sr/86Sr vs. 206Pb/204Pb di-agram, and a negative correlation in the 143Nd/144Nd vs. 206Pb/204Pbdiagram and the reverse is true for the mixing between DMM andEM1. Unlike basalts from SE China where this assumption issimplistic (two-component mixing) (Zou et al., 2000), basalts of theCVL show a more complex pattern with the contribution of morethan two components. Mixing between two end members (e.g.,DMM with EM1 or EM2) will produce a linear array. However, theCVL lavas do not plot on any of these arrays and cannot beexplained by only two component mixing. Nevertheless, in a threecomponent mixing, the proportion of each end member (EM1 andEM2) may significantly change the mixing slopes.

In the 143Nd/144Nd vs. 87Sr/86Sr diagram (Fig. 7) all plots showa negative slope. Conversely, in the 87Sr/86Sr vs. 206Pb/204Pb

Figure 11. Modeled AFC (Rb/Zr vs. Zr) for the CVL mafic and felsi


(Fig. 8) and 143Nd/144Nd vs. 206Pb/204Pb (Fig. 9) diagrams, doublecontrasting slopes; negative followed by a positive slope in Fig. 8aand b, and positive followed by a negative slope in Fig. 9a and bare observed. These contrasting slopes suggest different degreesof contributions from EM1 and EM2 to DMM in the generation ofthe CVL magmas. In the first case (negative slope in 87Sr/86Sr vs.206Pb/204Pb and positive slope in 143Nd/144Nd vs. 206Pb/204Pb),there is a greater contribution of EM1 to DMM when compared tothe contribution from EM2. This leads to progressive depletion in87Sr/86Sr, but increasing 143Nd/144Nd with an increase in206Pb/204Pb. At the intersection point, equilibrium in contribu-tions from both EM1 and EM2 to DMM is attained (Figs. 8b and9b). In the second case (positive slope in the 87Sr/86Sr vs.206Pb/204Pb and negative slope in 143Nd/144Nd vs. 206Pb/204Pb),the role of EM2 becomes dominant over EM1 and thus 87Sr/86Srstarts increasing while 143Nd/144Nd decreases with an increase in206Pb/204Pb.

Even though basalts that plot on the recently redefined FOZOzone (Stracke et al., 2005) were initially interpreted as originatingfrom a HIMU source probably due tometasomatism from St. Helena(Halliday et al., 1990; Lee et al., 1994), the initial concept lackssufficient evidence to support their high Pb isotope signature.Although FOZO and HIMU may have similar sources and closecharacteristics, FOZO differs from HIMU in having lower206,207,208Pb/204Pb, but higher 207Pb/206Pb and 208Pb/206Pb ratios.87Sr/86Sr ratios in FOZO basalts are higher than in HIMU basalts andpositively correlate with 206,207,208Pb/204Pb ratios. These charac-teristics are observed in the CVL lavas formed by dominantcontribution of EM2 to DMM (dominantly Mt. Cameroon, Mt.Etinde, the Oceanic CVL and Biu plateau). In contrast, HIMU-typeOIBs seems to be a distinct group of OIBs, which only occurs attwo intra-oceanic localities, St. Helena in the Atlantic Ocean and theCook-Austral island chain in the South Pacific Ocean (Stracke et al.,2005).

c lavas. Symbols, color and data source are same as in Fig. 2.


Figure 12. Modeled melting (Zr/Nb vs. La/Yb) results for the CVL mafic rocks with MgO>4. Melts that produced most basanites and alkali basalts were generated by <3% partialmelting of a dominantly garnet (<6%) bearing mantle lherzolite. Symbols, colors and data source are same as in Fig. 2.


Contributions of EM-type end members to DMM accounts forthe complex Sr-Nd-Pb characteristics of the CVL lavas. Mt.Cameroon and Mt. Etinde lavas show greater contribution fromEM2. An intriguing aspect is the simultaneous involvement in bothcases described above in individual volcanic centers of the CVL,especially in the Biu Plateau and the oceanic CVL. This, coupledwithtrace element variations, suggests mantle heterogeneity at a local(single volcano) scale. The intersection between the two slopes inboth Figs. 8 and 9 are characterized by a zone with the lowest87Sr/86Sr, highest 143Nd/144Nd, and intermediate 206Pb/204Pb. Thiszone might represent the isotopic composition for the commonmantle end-member for the CVL basalts (parent magma). It is thusreasonable to infer that this common mantle component is theasthenospheric mantle source. This zone constitutes lavas domi-nantly from the Biu Plateau, the oceanic CVL and Mt. Manengoubawhich are considered to be less evolved. The OVG is very peculiarwith its wide range in isotopic data. It presents an array whichextends towards EM1 and thus indicates the involvement ofdominantly EM1-like material during recycling.

UnlikeMORBswith a homogeneous chemical composition of theupper mantle, the CVL magmas are plumes derived from deeperlevels. The presence of mantle xenoliths in most alkaline basalts ofthe CVL suggests that the plumes from deeper levels, entrainedperidotite xenoliths at shallower levels (upper mantle). The inter-action of the plumes with the upper mantle at varying degrees


imparts additional signatures to the magma and thus accounts forheterogeneity within the volcanoes. This, therefore, implies thatmultiple plume strandswith a homogenous composition interactedwith a heterogeneous subcontinental lithospheric (SCLM)mantle atvarying degrees. The isotopic composition of the SCLM is locallydifferent because of the complex history of mantle metasomatism.The insignificant crustal contamination in mafic rocks suggests thatboth EM1 and EM2 components owe their origin to metasomatismand of the SCLM. However, the existence of multiple chemical res-ervoirs is indicative that themantle is not mixed up as would be thecase in mid-ocean ridges, where the whole mantle is convecting.

6. Conclusions

This study includes the compilation, assessment, and interpre-tation of existing geochemical data of the ocean and continentalsectors of the CVL to investigate petrogenetic processes and mantlesources in the volcanic system. Main results are:

(1) The origin of the CVL stems from a series of related events,including mantle plumes and lithospheric structures.Geochemical perspective is the most optimal vantage on theorigin and evolution of the CVL.

(2) The CVL lavas were not significantly contaminated by thecontinental crust. The CVL magmas were generated by small



(<3%), but different degrees of partial melting of a dominantlygarnet lherzolite (<10%) mantle. This accounts for the minorvariations in geochemical characteristics at both local (scale ofindividual volcano) and regional (entire CVL) scales

(3) Contribution in various proportions of EM1 and EM2 compo-nents to DMMaccounts for the complex isotopic characteristicsof the CVL lavas. The FOZO characteristics of the CVL can beexplained by mixing between DMM and EM2. The isotopiccomposition for the common end-member for the CVL basaltsis defined by the zone of intersection between the mixingtrends of DMM with EM1 and DMM with EM2. This zoneconsisting of less evolvedmagmas might represent the isotopiccomposition for the common mantle end-member for the CVLbasalts.

(4) The evolution of the CVL magmas was controlled by a smalldegree fractional crystallization (for mafic lavas) and AFC frommafic to felsic lavas. Four major fractional crystallization trendsFC1e4 characterize the differentiation pattern for the CVL lavas.

6.1. Recommendation

Although the entire CVL has been fairly studied, intriguingquestions still remain unanswered. The OVG which is located closeto the junction where the CVL bifurcates and also hosts the infa-mous lake Nyos, presents the most variable geochemical charac-teristics. Existing Sr-Nd-Pb data for the OVG show wide variationwith arrays that are uncommon in other CVL volcanoes. However, itis one of the least studied along the CVL in terms of isotope sys-tematics. Future updates of isotopic data, especially on the entirelycontinental volcanoes, would significantly complement existingstudies to better constrain the origin of the CVL and its petrogen-esis. Application of statistical methods like independent compo-nent analysis and principal component analysis might be aworthwhile additional analytical tool in unraveling valuable infor-mation. This work is on-going.

Acknowledgments

The numerous sources of published data on the Cameroon Vol-canic Line used in this paper are heartily acknowledged. Nick M.W.Roberts and twoanonymous reviewers are thanked forhandling andreviewing the manuscript. We are grateful to James DOHM forhelpful discussions and corrections made on the earlier version ofthe manuscript. This work is part of a running Ph.D by ANEA sup-ported by Science and Technology Research Partnership for Sus-tainable Development (SATREPS) project titled: Magmatic FluidSupply into Lakes Nyos and Monoun, and Mitigation of Natural Di-sasters through capacity building in Cameroon. Material andfinancial support is being provided by the Japan Science and Tech-nology Agency (JST), Japan International Cooperation Agency (JICA)and the Institute of Geological and Mining Research (IRGM) of theCameroonMinistry of Scientific Research and Innovation (MINRESI).

Appendix A. Supplementary data

Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.gsf.2014.04.012.

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A comparative review of petrogenetic processes beneath the Cameroon Volcanic Line: Geochemical constraints