Baossi-Warack monogenetic volcanoes,Adamawa Plateau,Cameroon:petrography,mineralogy and geochemistry

Anicet Feudjio Tiabou·Robert Temdjim·Pierre Wandji·Jacques-Marie Bardintzeff·Vivian Bih Che·Edith Ekatah Bate Tibang·Caroline Neh Ngwa ·Franc?ois Xavier Onana Mebara

Abstract Three monogenetic cones in the Baossi-Warack area,Ngaoundéré,Adamawa Plateau forming part of the Cameroon Volcanic Line(CVL)are documented in this study.Basaltic lavas(＜1 km3)scattered around these vents and restricted volcaniclastic deposits were emplaced by Hawaiianandmildstrombolianstyleeruptions.Thelavasare porphyritic,mainly composed of olivine(chrysolite)and clinopyroxene(diopside and augite)phenocrysts and plagioclase(andesine)microphenocrysts.Accessory minerals include titano-magnetite and titano-hematite,nepheline,apatite and amphibole xenocrysts.Sanidine occurs in some samplesandsodi-potassicalbiteinothers.Someolivinesand clinopyroxenes exhibit resorbed margins and thin reaction rims while plagioclase displays oscillatory zoning,and sieved textures as a result of magma mixing.Whole-rock geochemistry data indicates that the lavas are silica-undersaturated,composed of basanites and basalts,showing little compositional variations(SiO2:39.20 wt.%-48.01 wt.%,MgO:5.29 wt.%-9.70 wt.%).Trace elements patterns of these lavas suggest they are enriched in LILE including Pb,probablyduetocrustalcontamination.REEpatternssuggest cogenetic magmas below Baossi 1 and Baossi 2 volcanoes,anddistinctsourcesbelowWarackvolcanoandnearbylavas.The lavas studied show affinity to high-μ(HIMU),enriched type I(EM1)and Oceanic Island Basalt(OIB)-like mantle signatures and thus indicate a heterogeneous mantle source underneath the vents as noted at other monogenetic and polygenetic volcanoes along the CVL.Primary melts derived from low degrees of partial melting(0.5%-2%)and encountered low rates of fractionation,and crustal contamination coupled with magma mixing.These melts evolved independently through structuralweaknesses in the basement.

Keywords Cameroon volcanic line·Adamawa Plateau·Volcanic field·Monogenetic volcano·Magma mixing·Petrogenesis

1 Introduction

Small-scale volcanic systems are the most widespread type of volcanism on Earth and occur in diverse tectonic settings(Ca?ón-Tapia 2016;Smith and Németh 2017).They are the smallest in terms of the volume of magma that erupts(McGee and Smith 2016).In such systems,the volcanic edifices are usually basaltic in nature with restricted volumes and their eruptions are inferred to be short-lived events(Valentine and Gregg 2008;Németh 2010;Bardintzeff 2016).Monogenetic volcanoes have long been regarded as being geochemically and volcanologically simple compared to long-lived polygenetic volcanoes where greater degrees of magma evolution are expected(Brenna et al.2010).However,field data at a number of monogenetic volcanoes have identified eruption products such as lava flows,pyroclastics of strombolian style eruptions as well as phreatomagmatic deposits pointing to a range of eruption styles and chemical compositions of erupting magma at these edifices(Németh et al.2003;Martin and Németh 2006;Valentine et al.2007;Genareau et al.2010;Kereszturi et al.2011;Bardintzeff et al.2012;McGee et al.2012,2013).For example,compositional discontinuities resulting from successive partial melting of distinct,but contiguous source components with differing melting characteristics in a heterogeneous source were reported in the Auckland Volcanic Field(McGee et al.2012,2013).The sequential eruption of two magma batches has also been reported at Par?`cutin in the Michoacán-Guanajuato volcanic field(Erlund et al.2010)and at Mt.Gambier in Australia(Van Otterloo et al.2014).Thereby,it is noticed that monogenetic volcanoes can exhibit great complexities in their compositional features and petrogenesis(e.g.Van Otterloo et al.2015)but the wide spectrum of compositional changes accompanying the magmatic activities at these volcanoes are not fully understood.This can be investigated through detailed stratigraphic analysis alongside systematic mineralogicaland geochemical sampling.

Prominent among numerous intraplate volcanic fields worldwide(e.g.Cook et al.2005;Brenna et al.2010,2012;McGee et al.2012,2013;Kereszturi et al.2013;Jordan et al.2015)is the Cameroon Volcanic Line(CVL),an active large intraplate volcanic province that comprises several eruptive centers separated by uplifted and eroded plutons(Déruelle et al.2007).The CVL is a ～ 1700 km long chain of numerous volcanic edifices that trends N030°E from Pagalu Island in the Atlantic Ocean to Lake Chad on the African plate,and spans 70 million years of activity,with Mt.Cameroon still active today(see summary in Njome and de Wit 2014;Fig.1).Numerous studies along this chain are concentrated on the large volcanic edifices.Small volume volcanoes have so far received little attention with previous works focused on maars and cinder cones scattered in the continental southern end of the CVL(e.g.Sato et al.1990;Nkouathio et al.2002;Tamen et al.2007;Ngwa etal.2010,2017;Tchamabéetal.2014,2015,2016).These types of volcanoes also exist around Ngaoundéréon the Adamawa Plateau and form the central part of the CVL,but they have not yet been studied.This work focuses on volcanic cones within the Baossi-Warack area in Ngaoundérénortheast.We present new petrographic,mineralogical and whole-rock geochemical data for a suite of samples dispersed over the entire Baossi-Warack complex in a bid to contribute to our understanding of monogenetic style eruptions and their compositional variation.if eld located northeast of Mt.Cameroon reported the presence of ten cinder cones and two maars(see Nkouathio et al.2002;Tamen et al.2007;Tchamabéet al.2013).In this volcanic field,the reconstruction of the stratigraphic sequence at Barombi Mbo maar highlighted the polycyclic nature of this maar,demonstrating the complexities of such volcanoes(Tchamabéet al.2015,2016).Ngwa et al.(2017)also investigated the origin and evolution of primitive melts from the Debunscha maar,a monogenetic

2 Geological setting

The CVL has been extensively discussed in the literature(see Fitton and Dunlop 1985;Sato et al.1990,1991;Wandji et al.2000;Aka et al.2004;Temdjim et al.2004;Déruelle et al.2007;Suh et al.2008;Nkouathio et al.2008;Kamgang et al.2013;Marzoli et al.2015;Bate Tibang et al.2017;Ngwa et al.2017;Njombie et al.2018;Ziem àBidias et al.2018).Along this megastructure,the volcanic edifices are simultaneously developed into both oceanic and continental domains.The oceanic sector is composed of Pagalu,S?o Tomé,Principe,and Bioko islands and the continental part of Mts Etinde,Cameroon,Manenguba,Bambouto,Oku,Mandaras and NgaoundéréPlateau,which are built upon horsts that alternate with grabens towards the continental southern end of the chain,where monogenetic cones predominate(Déruelle et al.2007).The earlier work on monogenetic volcanoes along the CVL includes a comparative study of basalts between polygenetic and monogenetic volcanoes(see Sato et al.1990).This study suggests that the major sources of magma feeding polygenetic volcanoesalong the CVL is theupwelling asthenosphere underneath the African plate,while those feeding monogenetic volcanoes result from partial melting in the lowest portion of the subcontinental lithosphere.Other researchers working on the Barombi Koto volcanic

volcano forming part of the Mt.Cameroon volcanic field,composed of over a hundred unstudied pyroclastic cones and craters scattered around it(Suh et al.2003).In the Debunscha area,there is evidence for repeatedly mixing of compositionally diverse melts in a magma chamber at upper mantle depths prior to eruption(Ngwa et al.2017).

Fig.1 Structural disposition of the Cameroon Volcanic Line(CVL,in black)with ages of the principal volcanic centers(after Marzoli et al.1999)indicated.The study area with the Baossi-Warack cones is part of the Adamawa complex around Ngaoundéré

Situated between latitudes 7°30′-7°35′N and longitudes 13°53′-13°58′E,Baossi-Warack belongs to the volcanic district of Ngaoundéré,Adamawa Plateau,in the eastern branch of the CVL(Fig.1).Most volcanic manifestations around Ngaoundéréare believed to be of Oligocene(Itiga et al.2013),to Mio-Pliocene(Letterman 1984)and Pleistocene in age(Temdjim et al.2004).More than sixty eruptive centers including plugs and maars associated with hydro-magmatic explosive activity have been reported in theNgaoundéré area(Temdjim etal.2003,2006;Nkouandou et al.2008).The diversity of these volcanic centers and their products implies different eruptive styles at play in Ngaoundéré.The current study intends to contribute on the monogenetic style eruptions within the Baossi-Warack area.

3 Sampling and analytical techniques

The Baossi-Warack area was carefully mapped and a new geologic map of the study area was produced(Fig.2).Juvenile lava samples were collected at various stratigraphic levels exposed at the vents.Thin sections were prepared for selected samples and observed under a polarizing microscope.Two lava samples,BA21 and WA10 from Baossi 2 and Warack vents,respectively,were examined using a Jeol IT300 Scanning Electron Microscope at the University of Bristol.Three other lava samples BA11,BA16 and WA8 from different vents were examined using a CAMECA SX 100(15 kV,10 nA)electron microprobe at the UniversitéPierre et Marie Curie,Paris VI,France.Kα lines were used.Standards were diopside for Si,Ca and Mg,Fe2O3for Fe,MnTiO3for Ti and Mn,Cr2O3for Cr,albite for Na,orthoclase for K and Al.Counting times were 10 s for both peaks and background,with a 5 μm defocused beam.The operation was carried out automatically for BA11 and WA8(with 40 points analyzed,each spaced with 433 μm)and BA16(with 60 points analyzed,each spaced with 260 μm).

Major element concentrations of 27 lava samples were determined by X-ray fluorescence(XRF)at GEOMAR,University of Kiel.After ignition at 1000°C over a duration of 4 h,fused discs were prepared using a Li-metaborate-tetraborate flux at 1050-1100°C.Major element results had analytical errors better than± 1%at 1σ standard deviation.Ferric and ferrous irons are reported together as total Fe2O3.Trace element and rare earth elements(REE)analyses of 20 lava samples were carried out by inductively coupled plasma mass spectrometry(ICP-MS)at Activation Laboratories Ltd.(Actlabs).Sample solutions were produced from～30 mg of sample powder.Here,the powders were mixed with HF-HNO3-HCl in Teflon vials and placed on a hot plate at 150 °C for～ 48 h.Subsequently the samples were dried down and taken back into solution using 3%HNO3for final analysis.ICP-MS count rates were externally standardized by means of calibration curves based on the international standards(Actlabs).Reproducibility on replicate analyses and accuracy is of the order of 5%for all elements.

4 Results

4.1 Volcanology and distribution of vents

Three volcanoes,Warack,Baossi 1,and Baossi 2,and basaltic lavas exposed at a road cut in the study area define four eruptive vents sampled for this work(Fig.2).These vents covered～5 km distance within the area.They are small volume volcanic cones,with Warack the largest,～105 m high,and 1500 m in diameter at the base(Fig.3a).The Baossi 1 and Baossi 2 volcanoes(Fig.3b,c)are～45 m high and have diameters of 1400 and 800 m at the base,respectively.The volcanoes almost have the samefield characteristics,with gentle flanks entirely covered with blocky basalts,overlying a granitic basement.These basaltic lavas resulted from fissural eruptions during a quiet episode of Hawaiian style activity.

Within the study area,basaltic lavas are well exposed in a～1 km long road cut,displaying a stratigraphic sequence～0.12 km thick(Fig.4a).In this sequence,two morphological types of lavas intercalated with volcaniclastic deposits with variable thicknesses are distinguishable.From the bottom to the top,a layer of altered lavas with boulders(～4 m)showing onion-skin weathering is overlain by layered volcaniclastic deposits(～2 m).This layer of volcaniclastic deposits(＜2 mm in grain size)comprises,from the bottom to the top,thinly bedded and indurated layers of black ash(0.8 m),laminated gray ash(0.4 m)and lapilli-tuff(0.8 m),all with embedded fragments of the basement rocks,lavas,and charred wood.The thickness of these vocaniclastic deposits varies along the road cut and disappears under the lavas at the eastern end of the road cut.On top of the vocaniclastic deposits lies a layer of altered slab-shaped lava flows(～2.5 m).These are overlain by a～6 m thick layer of pseudo prisms and blocks of coherent lavas,and the whole stratigraphic sequence is covered by a thin layer of soil(Fig.4b).Further north along the road cut,the lava outcrops as columnar blocks immediately covering the basement rocks,the latter crosscut by dolerites.The nature of the products observed at this site suggests that the altered lavas at the base of the sequence belong to an older eruptive phase,the vent being hidden in the area.These products points to two effusive episodes,or at least two different types of lavas,both preceded by the explosive phase that resulted in the emplacement of the volcaniclastic deposits(Fig.5a)with embedded fragments of charred wood(Fig.5b,c),and showing rounded to subrounded particles(Fig.5d).

Fig.2 Geological map of the Baossi-Warack volcanic field,Adamawa Plateau

4.2 Petrography and mineral chemistry

Fig.3 Field photographs of Baossi-Warack monogenetic volcanoes.a Warack volcano,b Baossi 1 volcano,and c Baossi 2 volcano

Texture,zoning and chemical composition of rock-forming minerals in basalts can provide insight into the details on magma evolution and processes operating in the magmatic systems(Roeder et al.2003;Longpréet al.2014).Based on petrography,samples from the study area are homogeneous with no clear distinction in hand specimens.These lavas are porphyritic,with euhedralto subhedralolivine(～ 11%)and clinopyroxene(～ 10%)phenocrysts as dominant minerals in the matrix,followed by plagioclase microlites and phenocrysts(～15%),and quartz xenocrysts in some samples.Olivine and clinopyroxene phenocrysts are surrounded by oriented plagioclase microlites following the flow direction(Fig.6a),while some crystals are skeletal and corroded,surrounded by a very thin reversely zoned rim(Fig.6b,c).Some plagioclase phenocrysts display oscillatory zoning(Fig.6d),while others show sieved textures.Accessory phases include Fe-Ti oxides(2%-5%),rare nepheline,apatite,matrix glass(～1%),and amphibole xenocrysts.The volcaniclastic deposits are made of porphyritic scoria with higher proportion of xenocrysts(quartz,microcline,and orthoclase)than in the lavas.

Chemical compositions of major mineral phases such as olivine(Table 1,Supplementary material)and clinopyroxene(Table 2,Supplementary material)show chrysolite(Fo84-76;Fig.7a)and diopside(Wo50-47En40-34Fs15-10)and augite(Wo43.88En44.42Fs11.71;Fig.7b),respectively.The compositions of olivine are slightly less magnesian than those of Tombel graben(Fo86-70;Nkouathio et al.2002),in the southern part of the CVL.In this part of the CVL,more precisely in the Noun plain,the compositions of clinopyroxene are more titaniferous and more aluminous(Wandji et al.2000)than those of our study area.Feldspars were also analyzed and the compositions(Table 3,Supplementary material)show potassic andesine(An43-40-Ab53-48Or8-6)and rare sanidine crystals(An2Ab32Or65)in Warack lavas and sodi-potassic albite(An3-1Ab92-78-Or18-5)in Baossi 2 lavas(Fig.7c).

Fig.4 a Stratigraphic column exposed at the Baossi-Warack road cut.b Detailed view of(a),showing the different layers of lavas and volcaniclastic deposits with embedded charred wood at this road cut

Chemical compositions of accessory mineral phases such as Fe-Ti oxides are presented in Table 4(Supplementary material).These compositions show titano-magnetite and titano-hematite (Fig.7d). Ulv?spinel(Usp87.63-69.89)contents are slightly higher than those of alkali basalts from Ngaoundéré(Usp73-47;Nkouandou et al.2008)in the same volcanic region.In this region,amphiboles are not frequent in basaltic lavas and are considered xenocrysts.The lamellae of these minerals were analyzed in sample WA8.The compositions obtained(Table 5,Supplementary material)show that they are calcic(CaO=12.06%-15.65%;Ca＞1.5 and Na+K＞0.5),aluminous(Al2O3=9.07%-17.75%)and rich in FeOt(4.89%-20.31%).These amphiboles are edenite and ferro-edenite for Ti＜0.5 and ferro-kaersutite for Ti＞0.5(Fig.7e).They are therefore different from the kaersutite previously described in Ngaoundérélavas(Nono et al.1994;Nkouandou et al.2008).Trace of carbonate minerals were also detected in the lavas studied.In this study area,carbonate minerals were earlier reported in dolerites(Nkouandou et al.2016).

4.3 Whole-rock geochemistry

Whole-rock geochemical data for Baossi-Warack lavas are presented in Table 1.In the total alkali versus silica diagram(Le Maitre 2002),these lavas plot as basanites(normative olivine＞10%)and basalts(Fig.8).They are silica-undersaturated(normative nepheline=0.11 wt.%-20.52 wt.%; Table 1), and alkaline sodic (Na2O/K2O=1.56-2.56).The undersaturated signature of these lavas is one of the characteristics of small magma batch volumes(McGee and Smith 2016),which is observed at other monogenetic volcanoes along the CVL(e.g.Nkouathio et al.2002;Tamen et al.2007),and at large edifices such as Mts Cameroon(see Tsafack et al.2009)and Manenguba(see Kagou Dongmo et al.2001).The Mg numbers(Mg#=42-61)of the lavas studied are lower than the values assumed for primary magmas in some case studies(Mg#=68-72;Tatsumi et al.1983;Jung and Masberg 1998,Kamgang et al.2013),suggesting that these lavas are slightly evolved.These two rock types have also been described at other monogenetic volcanoes along the CVL,including the most primitive picro-basalt(Ngwa et al.2017),and the most evolved hawaiite(Nkouathio et al.2002;Tamen et al.2007).

Fig.5 a Volcaniclastic deposits composed of black ash,gray ash and lapilli-tuff from bottom to top.b Black ash layer with embedded charred wood(c).d Close up view of gray ash showing rounded to subrounded particles

In the major oxides versus SiO2diagram(Fig.9),slight positive trends in Al2O3,Na2O and K2O and negative trends in Fe2O3t,MgO,TiO2and P2O5can be deciphered.The positive trend in Al2O3versus SiO2suggest non fractionation of plagioclase while the negative trends in MgO versus SiO2,TiO2versus SiO2and P2O5versus SiO2suggest minor fractionations of olivine,Fe-Ti oxides and apatite,respectively.In the K2O versus SiO2diagram,two liquid lines of descent are indicated,possibly as the result of different magma batches.

Thorium(1-19 ppm)has a mineral-liquid distribution coefficient less than 0.01 and enriches the most during the evolution process(Laridhi-Ouazaa 1989).This element can serve as differentiation index for variation diagrams(Fig.10).The concentrations of compatible trace elements such as Sc (3-31 ppm), V (169-229 ppm), Cr(87-417 ppm),Co(25-51 ppm),Ni(89-422 ppm),Cu(26-62 ppm)and Zn(88-149 ppm)are variable(Table 1)and consistent with fractional crystallization.Ni and Co for example strongly support fractionation of olivine across the vents,as the values of these elements are lower than those assumed for primitive melts in some case studies(Ni=300-400 ppm;Co=50-70 ppm;Tatsumi et al.1983;Jung and Masberg 1998;Kamgang et al.2013).The concentrations of incompatible trace elements such as Sr(701-1471 ppm),Ba(622-1111 ppm),Zr(255-447 ppm),Nb(54-151 ppm),Hf(5-8.2 ppm),Ta(3-8.5 ppm)and U(0-5 ppm)are also variable.

Fig.6 Photomicrographs of Baossi-Warack basaltic lavas.a Olivine phenocryst surrounded by plagioclase microlites(dark-grey),following theflow direction(sample BA21).b Clinopyroxene phenocryst showing a thin rim surrounding the crystal(sample BA21).c Olivine and clinopyroxene in fine grained groundmass,with a thin rim surrounding the clinopyroxene phenocrystal(WA10).d Sample rich in plagioclase,showing a microphenocryst displaying oscillatory zoning at the center of the image(sample WA8).NB:a-c are backscatter images(SEM)and d is observed under a polarizing microscope

In the primitive mantle-normalized diagram(McDonough 2003),Baossi-Warack lavas show profiles with values about 3-300 times higher than those of the primitive mantle(Fig.11a).The profiles are quite similar to those of Oceanic Island Basalts(OIB)and clearly different from those of Enriched Mid-Oceanic Ridge Basalts(E-MORB)and Normal Mid-Oceanic Ridge Basalts(N-MORB)of Sun and McDonough(1989).A similar trend was reported for other monogenetic volcanoes in Tombel graben(see Nkouathio et al.1998,2002)and large edifices such as Mt.Manenguba(Kagou Dongmo et al.2001)and Mt.Cameroon(Yokoyama et al.2007).These profiles also illustrate an enrichment of large ion lithophile elements(LILE)including Pb and a slight depletion of high field strength elements(HFSE)such as Th,U and Y in some lavas.The strong positive anomaly in Pb is remarkable for the lavas from the volcanoes studied.This anomaly in Pb could be due to crustal contamination as suggested by Zhang et al.(2013).Crustal contamination might also be responsible for the depletion in Th and U,as Th-U bearing minerals are expected in the crust with the example of the Kitongo uranium mineralization found in the northern Cameroon,which lies within the Central African Fold Belt as well as the Adamawa Plateau(Kouske et al.2012).The lavas examined are relatively richer in LILE compared to those of Ngaoundéré(see Nkouandou et al.2008).In addition,the LILE profiles for Baossi 1 and Baossi 2 volcanoes are substantially situated above those of Mt.Cameroon while those of Warack and the road cut are situated under,with similar negative anomalies in K,Sm and Ti on these profi les(Yokoyama et al.2007).This suggests perhaps similar characteristics of the mantle plume across these vents,discussed further.

Fig.7 Classification of mineral phases of Baossi-Warack basaltic lavas.a Mg/(Mg+Fe2+)versus Fe2+/(Mg+Fe2+)diagram of olivine after Dick(1989).b Wo-En-Fs diagram of clinopyroxene after Morimoto et al.(1988).Data for samples BA21 and WA10 are compiled in Tiabou et al.2015.c An-Ab-Or diagram of feldspars in the Baossi-Warack lavas after Smith and Brown(1988).The blue circles represent feldspars of basalt BA16;green crosses are those of basalt WA8.d TiO2-Fe2O3-FeO diagram of Fe-Ti oxides in Baossi-Warack basaltic lavas after Buddington and Lindsley(1964).The red triangles are the Fe-Ti oxides of basalt WA8;the blue crosses are those of basanite BA11.e Mg/Mg+Fe2+versus Si diagram of amphibole in the basalt WA8 after Leake et al.(1997)

Table 1 Whole-rock geochemistry data for Baossi-Warack lava flows

Table 1 continued

Table 1 continued

Table 1 continued

Table 1 continued

Fig.8 Total Alkali versus Silica diagram(Le Maitre 2002)showing bulk-rock compositions for Baossi-Warack lavas.The curved solid line is after Irvine and Baragar(1971).Major oxide contents are based on analyses normalized to 100 wt.%volatile-free.Data for monogenetic volcanoeas along the CVL are from Sato et al.(1990),Nkouathio et al.(2002),Tamen et al.(2007)and Ngwa et al.(2017)

In the chondrite-normalized diagram(Anders and Grevesse 1989),the profiles for the Baossi-Warack lavas show a negative slope with a significant enrichment in Light Rare Earth Element(LREE)to Middle Rare Earth Element(MREE)and moderate depletion in Heavy Rare Earth Element(HREE;Fig.11b).These profiles are relatively parallel for HREE(Tm-Lu)compared to the E-MORB and N-MORB profiles and reflect low or no fractionation for these elements.The enrichment of LILE and most incompatible LREE can be caused by the melting of discrete components such as pyroxenite and/or carbonated,hydrated or recycled components which are subsequently incorporated into melts as indicated in other intraplatevolcanic fields(McGee et al.2012;Van Otterloo et al.2014;Castillo 2015).This is supported by traces of carbonate minerals in the lavas studied and the presence of garnet-bearing pyroxenites reported at Youkou maar in the same volcanic region(France et al.2015;Njombie et al.2018).The REE profiles of Baossi 1 and Baossi 2 are superimposed/parallel while those of the road cut and Warack are largely similar.The parallelism of these profi les points to cogenetic magmas below Baossi 1 and Baossi 2 volcanoes as seen in other volcanic regions on the African plate such as in Tunisia(Laridhi-Ouazaa 1989)and in Algeria(Bendoukha et al.2009).The magma below Warack and the basaltic flows at the road cut could be generated from a distinct parental melt.

Table 1 continued

5 Discussion

5.1 Emplacement mechanism and eruptive style

According to Le Corvec et al.(2013)tectonic setting as well as local or regional stress field are factors that influence the genesis of some volcanic fields.The Baossi-Warack area in Ngaoundéré,Adamawa Plateau,belongs to the Central Pan-African orogenic belt in Cameroon,where the basement has suffered intense deformation during the Pan-African orogeny(Moreau et al.1987;Tchameni et al.2006).This area is also part of an intraplate volcanic chain.In such a tectonic setting,mantle plume increases temperature and induces partial melting of the upper mantle.The melts obtained then erupts on the Earth's surface through fissures and form aligned edifices(Courtillot et al.2006;Keating et al.2008).On the Adamawa Plateau and in the study area in particular,the volcanoes erupted low volumes of basaltic lavas(＜1 km3)through single vents.These vents are slightly aligned and probably follow NWSE trending fractures induced by a Pan-African shortening episode that affected the northern edge of the Adamawa Plateau(Tchameni et al.2006;Tchakountéet al.2017).The contact between the lavas and the basement is observable;the latter is frequently intersected or cross cut by doleritic dykes(Nkouandou et al.2016).These suggest that the volcanism of the Baossi-Warack area typically occurred along fissures and eruptions were basically of haiwaiian style with a component of strombolian style activity.The eruptive style variation is due to the fact that Hawaiian style fissure eruptions generally focuses locus of eruption and start forming cones along a fissure that generate more explosive activity.

The section at the road cut showing different types of lavas and volcaniclastic deposits is complex as the source vent has not yet been identified.The pseudo columns at the top ofthe sequence and slab-shaped basalticflow underneath,suggest that the latter was hotter and stillflowing,when the top of the flow exposed to the atmosphere has cooled and solidified.These lavas were probably emplaced during the same episode while the layer at the bottom of the sequence showing onion-skin weathering would belong to a different and older eruption.The volcaniclastic deposits(～2 m thick)with rounded to subrounded particles,fragments of the basement rocks and charred wood suggest that these materials may have experienced a short transportation.Therefore,it is possible that these materials were ejected during the explosive phase of the volcanic cones nearby and latter transported on a short distance by a river.

5.2 Petrogenesis

5.2.1 Fractional crystallization and crustal contamination

Primary magmas are only rarely erupted at the Earth's surface(Smith and Németh 2017).Monogenetic volcanoes typically erupt relatively primitive magmas,that have undergone a minor amount of fractional crystallisation,and sometimes with a component of crustal contamination(e.g.Smith et al.2008;Németh et al.2003).These processes are observed at the Baossi-Warack area where major oxides variation with silica and the crystal-poor nature of the rocks indicated minor fractionations of olivine,clinopyroxene,Fe-Ti oxides,apatite and plagioclase(Fig.9).Two fractional crystallization trends for these lavas can also be deciphered in a Sr versus Rb diagram(Fig.12),showing constant values of Sr with increasing Rb(see Xu et al.2007).This fractionation process is also constrained by the lower Mg numbers(Mg#=42-61)compared to those of primary melts in some case studies(Mg#=68-72;Tatsumi et al.1983;Jung and Masberg 1998;Kamgang et al.2013).The main mineral phases that fractionated are olivine,clinopyroxene,and plagioclase,followed by Fe-Ti oxides and accessory apatite.The zoning in some plagioclase crystals also suggestfractionalcrystallisation,although these crystals were mostly accumulated with Eu/Eu*＞1(Table 1).

Although magma batches rise quickly to the surface through simple conduit systems,at small volume volcanoes,interaction with crustal rocks encountered en route to the surface cannot be excluded(Smith and Németh 2017).Along the CVL,crustal contamination has been established for mafic and felsic lavas associated to large edifices(e.g.Halliday et al.1988;Marzoli et al.1999;Kamgang et al.2013;Wotchoko et al.2017;Bate Tibang et al.2017),while at monogenetic volcanoes,previous works indicated no significant or no strong crustal contamination in the continental southern end of the chain(Nkouathio et al.2002;Tamen et al.2007).Xenoliths of crustal rocks were reported in pyroclasts at Barombi Mbo maar(Tchamabéet al.2014),although no information on their interaction with the magma is available.In the Baossi-Warack area,quartz and amphibole xenocrysts are observed in hand specimen,combined with microscopic sodi-potassic albite and sanidine in the lava flows,suggesting that crustal contamination did occur during magma ascent.As previously noted,the strong positive anomaly in Pb and depletion in Th and U can also be linked to crustal contamination.In addition,according to Kamgang et al.(2013),the most primitive lavas(MgO＞6 wt.%)have La/Nb ratios lower than 0.8.These authors stipulated that when MgO decreases by crystal fractionation process,La/Nb and87Sr/86Sr ratios increase,suggesting a contamination process by a high La/Nb-87Sr/86Sr component,characteristic of the upper continental crust.Although isotopic data are not available for this study,the Baossi-Warack lavas with MgO greater than 6 wt.%show La/Nb ratios ranging from 0.61 to 1.36(Table 1).The samples with ratios slightly higher than 0.8 suggests that crustal contamination once affected primary melts across the vents studied.

5.2.2 Magma mixing

Magma mixing has been pointed out at some monogenetic volcanoes(e.g.Koyaguchi 1986;Galipp et al.2006;Johnson et al.2008;Stroncik et al.2009;Le Corvec et al.2013;Ngwa et al.2017).In this study,two generations of olivine and clinopyroxene were identified at the Baossi-Warack lavas,but the main phases are chrysolite and diopside respectively.These minerals show textural and geochemicalevidenceofmagmadiversity including skeletal and corroded crystal margins.The oscillatory zoning and sieved textures of plagioclase and clinopyroxene moreover may suggest mixing of different magma batches prior to eruption(Koyaguchi 1986;Suh et al.2003).A similar textural approach was reported at Mt.Cameroon(Suh et al.2003).In the study area,fresh pulses of magma were supplied into a magma reservoir containing a more evolved,partly fractionated,and crystallized residual melt and crystal mush.Preliminary results of microanalyses on olivine-hosted melt inclusions(unpublished data)in the lavas examined suggest two pools of magma;one of more silicic/residual melt and the other more mafic/fresh batch.It is possible that small pockets of basaltic melt probably stored in the crust interacted with each other before the eruption,resulting in a broad preeruption liquidus temperature of 1040-1156± 6°C(Tiabou et al.2015).This temperature was calculated using mineral chemistry of equilibrium olivine-clinopyroxene pairs following the geothermometric method of Loucks(1996).The peaks of temperature(～ 100 °C)observed strongly support the injection of new magma batch into a magma reservoir.In other eruptive centers such as the Canary Islands(Galipp et al.2006;Stroncik et al.2009)and Mt.Cameroon(Ngwa et al.2017),clinopyroxene-melt barometry have shown that magma batches partially crystallize and mix with pre-existing magma in a zone of temporary magma storage within the upper mantle at 830±170 MPa prior to eruption.

5.2.3 Partial melting and plume indications

Basaltic magmas are fundamentally the products of partial melting of a peridotite mantle source in the Earth's interior,that can be constrained with trace elements concentrations(Allègre and Minster 1978;Villemant and Treuil 1983;Albarède 2009;and recent review in Smith and Németh 2017).As pointed out in paragraph 4.3,the rather low contents of transition elements(e.g.Ni,Cr and Co)in some Baossi-Warack basalts,compared with contents in true primitive magmas,witness of a slight differentiation.This is supported by a Gd/Yb versus La/Yb plot(Fig.13)consistent with low degrees of partial melting underneath the volcanoes studied(0.5%-2%);the elevated Gd/Yb ratios indicate＞8%garnet in the source(see Bogaard and W?rner 2003;Yokoyama et al.2007).These degrees of partial melting are slightly lower than the ones inferred at Mt.Cameroon(Yokoyama et al.2007),while a similar content of garnet was recently reported in the source of Debunscha maar(Ngwa et al.2017).In the Baossi-Warack area,the presence of greater amount of garnet in the source component suggests that,melting probably took place at a garnet-stable depth(≥.80 km;Niu 2005).Nkouandou et al.(2008)reported low degrees of partial melting(1%-2%)of a FOZO(Focal Zone)-type mantle that produced basaltic flows in the Ngaoundéréarea.But in this study area,where the degree of partial melting is similar to that reported by Nkouandou et al.(2008),a plume source is suggested by the ΔNb values greater than zero(Table 1;Fitton et al.1997)and the Oceanic Island Basalt(OIB)signature with affinity to High-μ(HIMU),Enriched type I(EM1)and recycled component(Fig.14).Such mantle characteristics were also noted in Barombi Koto(Tamen et al.2007)and in Bafang areas(Tchuimegnie Ngongang et al.2015)in the continental southern end of the CVL.In this part of the CVL,a dominant FOZO-type mantle component and other components,like depleted MORB

sediments(Woodhead 1996;Condie 2005),but this need to be further constrained by isotope analyses.

Fig.9 Major oxides variation for Baossi-Warack lavas plotted against their SiO2respective contents.Major oxide contents are based on analyses normalized to 100 wt.%volatile-free.Symbols as in Fig.8

Fig.10 Trace element variations for Baossi-Warack lavas plotted against their respective Th contents.Symbols as in Fig.8

Fig.11 a Primitive mantle-normalized multi-element patterns and b chondrite-normalized REE patterns for the Baossi-Warack basaltic lavas.Chondrite normalization values are from Anders and Grevesse(1989),primitive mantle normalization values are from McDonough(2003).Data for OIB,N-MORB and E-MORB are from Sun and McDonough(1989).The data for Mt.Cameroon(Yokoyama et al.,2007)are shown for comparison.OIB Oceanic Island Basalts,E-MORB Enriched Mid Oceanic Ridge Basalt,N-MORB Normal Mid Oceanic Ridge Basalt

Mantle source heterogeneity with HIMU-like signature has been regularly proposed for both large volcanoes mantle and variously enriched mantle(EMI and perhaps EMII)were also reported in the Tombel graben as well as at Mt.Bambouto(Nkouathio et al.2008).These components suggest a plume-lithosphere interaction as invoked in some volcanoes along the CVL(e.g.Yokoyama et al.2007)or a mixing between subducted oceanic crust and sediment sources(Woodhead 1996).Thus,it appears that the mantle source region was probably modified prior to melting by recycled oceanic crust or subducted continental(Barfod et al.1999;Aka et al.2004;Yokoyama et al.2007;Wotchoko et al.2017;Ziem àBidias et al.2018)and monogenetic volcanoes(Tamen et al.2007;Ngwa et al.2017)along theCVL.In thisstudy,Nb/U ratios(21.71-43.19;Table 1)support an OIB signature(Nb/U=47±10;Hofmann et al.1986)except for the samples from the road cut which deviates from the standard values,probably due to crustal contamination.Although these samples deviate from the standard values,they preserve their OIB signature as previously noted(Fig.11a).In all three areas,the Zr/Nb ratios(2.47-4.85)are similar to those reported as standard values for HIMU-OIB(3.2-5.0;Weaver 1991,Sun and McDonough 1989).The transition elements such as Ni,Cr and Co are higher in basalts from Warack(Ni=168-422 ppm,Cr=159-427 ppm,Co=40-51 ppm)than in basanites from Baossi 1 and Baossi 2(Ni=89-175 ppm, Cr=104-188 ppm, Co=39-46 ppm).The variation of these concentrations suggest that primary melts below Baossi 1 and Baossi 2 are generated from the same source as previously noted,while at Warack,these melts are likely generated from a distinct mantle source.These melts evolved independently through deep seated faults or fissures in the basement and encountered different rates of fractional crystallization en route to the surface.

Fig.13 Partial melting for Baossi-Warack basaltic lavas illustrated in Gd/Yb versus La/Yb diagram after Yokoyama et al.(2007).The curves marked Grt 4%and 8%represent garnet content of the source after Halliday et al.(1995).PM is the primitive mantle.Symbols as in Fig.8

Fig.14 Zr/Y versus Nb/Y for rocks from Baossi-Warack along with alkaline rocks from Barombi Koto volcanic field(Tamen et al.2007)and those from Bafang in the southern part of the CVL(Tchuimegnie Ngongang et al.2015).DEP deep depleted mantle,DM depleted mantle,PM primitive mantle,UC upper crust,HIMU high U/Pb mantle source,EM1 and EM2 enriched mantle sources,EN enriched component,REC recycled component(Condie 2005).Symbols as in Fig.8

5.3 Compositional variations

Numerous studies have considerably contributed on compositional variability in mafic magmas over short spatial and temporal scales worldwide(e.g.Reiners 2002;Strong and Wolff 2003;Brenna et al.2010;McGee et al.2012;Smith et al.2008;McGee and Smith 2016;Rawson et al.2016).Such studies highlighted the fact that basalts rarely eruptfrom truly primary magmas,implying that modifications almost always occur before the arrival on the surface of such magmas.The basaltic lavas studied here show a narrow range from basanites to basalts.The most evolved basalts are observed at Warack.In this study,the lava samples show variable concentrations of both major and trace elements,with two fractional crystallisation trends.The narrow chemical variations in these samples are discontinuous,as primary melts derived from low but slightly differentdegrees of partialmelting of a

heterogeneous mantle source.Such different degrees of partial melting were also reported in other volcanic fields(e.g.Farmer et al.1995;Strong and Wolff 2003)and interpreted as a dominant factor controlling magma compositional variation.The variation of transition element concentrations is clearly defined within the volcanoes studied and support limited fractionation while between these vents,incompatible element ratios such as Zr/Nb,La/Nb,Ba/Nb,Rb/Nb,La/Ta,Th/Ta,Ba/Th,Rb/Sr and La/Y show no significant difference between Baossi 1 and Baossi 2 volcanoes which are assumed to be cogenetic(Table 2).Values for these ratios are generally lower than those of Warack and the road cut outcrop as their primary magmas are generated from distinct zone of heterogeneous mantle source.These values also show a narrow range compared to those of some monogenetic(e.g.Tombel,Barombi Koto;Nkouathio 1997,Nkouathio et al.2002;Tamen et al.2007)and polygeneticvolcanoes(e.g.Mts Cameroon and Manenguba;Déruelle et al.2000;Kagou Dongmo et al.2001)along the CVL and preserve their OIB signature compared to primitive mantle and continental crust(Sun and McDonough 1989;Table 2).These ratios also show quite similar values when compared with other intraplate setting such as Udo volcano on Jeju Island,South Korea(Brenna et al.2010),except for Zr/Nb ratio showing high values.On this Island,distinct mantle sources of garnet peridotite are also inferred as seen in this study(Brenna et al.2010).

Table 2 Incompatibleelement ratiosofofthe Baossi-Warack lavas comparedtothose ofprimitive mantle(PM),oceanicIslandbasalts(OIB),continentalcrust(CC),and othersvolcanic edif i ces along the CVL.Sourceofdata:PM,OIB and CC(Sunand McDonough 1989);Tombel(Nkouathio1997;Nkouathioetal.2002);BarombiKoto(Tamenetal.,2007);Mt.Cameroon(Déruelle etal.2000);Mt.Manenguba(Kagou Dongmoetal.2001);Udo,Jeju Island(Brenna etal.2010).NB:Datafor Udo are the average valuesoftwo lavashields(LS)analyzed bythe authors

6 Conclusion

The Baossi-Warack area is a poorly known volcanic field in Ngaoundéré,Adamawa Plateau on the eastern branch of the CVL.The small volume volcanoes(＜1 km3)found in this area were emplaced by fissural-type volcanism,along fractures in the basement formed during the Pan-African orogeny.The products of this volcanism are basaltic lavaflows and volcaniclastic deposits which erupted from Hawaiian and mild strombolian style eruptions.These lavaflows composed of basanites and basalts are silica undersaturated showing little variation in their chemistry,which is a common signature observed at monogenetic volcanoes along the CVL and at other intraplate settings.Baossi 1 and Baossi 2 volcanoes are clearly different than Warack volcano and show a heterogeneity at such a small spatial scale.Primary magmas underneath these vents were derived from low degrees of partial melting of a heterogeneous mantle source.These melts evolved independently through fissures and were modified by limited amounts of fractional crystallization,crustal contamination and magma mixing.These processes and the mantle source characteristics should be investigated further through olivine-hosted melt inclusions in the lavas and isotopic data.Dating these lavas using radiometric techniques will also enable to further constrain the time scales and the magma recharge in this area.

AcknowledgementsThis paper is part of ongoing Ph.D.thesis by Anicet Feudjio Tiabou.It is dedicated to late Prof.Pierre Wandji,with whom we started this study.Field work and part of whole-rock geochemistry data have been financially supported by the Ministry of Higher Education,Cameroon,through the Special Allocation for the Modernization of Research(SAMR)granted to the first author.Microprobe analyses have been made by JMB at Camparis,UniversitéPierre et Marie Curie,Paris.We highly appreciate the support of Prof.Cheo Emmanuel Suh with part of whole-rock geochemistry data and backscatter images.The authors are grateful to Prof.Károly Németh(Massey University)and Dr.Karen Fontijn(University of Oxford)for their insightful criticisms that significantly improved the clarity of this contribution.

Compliance with ethical standards

Conflict of interestThe authors declare that they have no conflict of interest.