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Sub-continental lithospheric mantle structure beneath the Adamawa plateau inferred from the petrology of ultramafic xenoliths from Ngaoundéré (Adamawa plateau, Cameroon, Central Africa)
Accepted Manuscript Sub-continental lithospheric mantle structure beneath the Adamawa plateau inferred from the petrology of ultramafic xenoliths from Ngaoundéré (Adamawa plateau, Cameroon, Central Africa) Oumarou F. Nkouandou, Jacques-Marie Bardintzeff, Aminatou M. Fagny PII:
S1464-343X(15)30010-8
DOI:
10.1016/j.jafrearsci.2015.07.004
Reference:
AES 2315
To appear in:
Journal of African Earth Sciences
Received Date: 6 January 2015 Revised Date:
12 June 2015
Accepted Date: 3 July 2015
Please cite this article as: Nkouandou, O.F., Bardintzeff, J.-M., Fagny, A.M., Sub-continental lithospheric mantle structure beneath the Adamawa plateau inferred from the petrology of ultramafic xenoliths from Ngaoundéré (Adamawa plateau, Cameroon, Central Africa), Journal of African Earth Sciences (2015), doi: 10.1016/j.jafrearsci.2015.07.004. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Sub-continental lithospheric mantle structure beneath the Adamawa plateau inferred from the
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petrology of ultramafic xenoliths from Ngaoundéré (Adamawa plateau, Cameroon, Central Africa)
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Oumarou F. Nkouandou1, Jacques-Marie Bardintzeff2, 3, 4, *, Aminatou M. Fagny1
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Ngaoundéré, Cameroon.
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Univ Cergy-Pontoise, ESPE-IE, F-95000 Cergy-Pontoise, France.
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Univ Paris-Sud, Géosciences, Volcanologie, Planétologie, Bât. 504, F-91405 Orsay, France.
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CNRS, UMR GEOPS 8148, F-91405 Orsay, France.
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Department of Earth Sciences, Faculty of Sciences, University of Ngaoundéré, P.O. Box 454
*Corresponding author: [email protected]
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Abstract
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Ultramafic xenoliths (lherzolite, harzburgite and olivine websterite) have been discovered in
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basanites close to Ngaoundéré in Adamawa plateau. Xenoliths exhibit protogranular texture
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(lherzolite and olivine websterite) or porphyroclastic texture (harzburgite). They are composed of
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olivine Fo89-90, orthopyroxene, clinopyroxene and spinel. According to geothermometers,
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lherzolites have been equilibrated at 880 to 1060 °C; equilibrium temperatures of harzburgite are
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rather higher (880―1160 °C), while those of olivine websterite are bracketed between 820 and
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1010 °C. The corresponding pressures are 1.8―1.9 GPa, 0.8―1.0 GPa and 1.9―2.5 GPa,
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respectively, which suggests that xenoliths have been sampled respectively at depths of 59―63 km,
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26―33 km and 63―83 km. Texture and chemical compositional variations of xenoliths with
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temperature, pressure and depth on regional scale may be ascribed to the complex history
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undergone by the sub-continental mantle beneath the Adamawa plateau during its evolution. This
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may involve a limited asthenosphere uprise, concomitantly with plastic deformation and partial
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melting due to adiabatic decompression processes. Chemical compositional heterogeneities are also
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proposed in the sub-continental lithospheric mantle under the Adamawa plateau, as previously
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suggested for the whole Cameroon Volcanic Line.
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Keywords: lherzolite, harzburgite, olivine websterite, lithospheric mantle, Adamawa plateau,
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Cameroon
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1. Introduction: the Cameroon Volcanic Line
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The Cameroon Volcanic Line (CVL) in Central Africa (Figure 1) is an alignment of volcanoes and
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plutonic complexes, more than 1500 km long, trending mostly N30°E. It straddles the continent-
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ocean boundary, from Pagalú Island in the Atlantic Ocean, up to Lake Chad and Adamawa plateau
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(see the recent paper of Njome and de Wit, 2014 and references therein). This large province, in
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which graben and horst structures alternate (Nkouathio et al., 2002, 2008), has been active since
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Paleogene to Quaternary as illustrated by the recent activity of the Mt. Cameroon volcano (1999
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and 2000 eruptions, Déruelle et al., 2000). No spatial age progression is detected along the Line.
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The lithospheric mantle beneath the whole Cameroon Volcanic Line has been particularly studied
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through mantle xenoliths sampled by alkali basaltic lava. Significant works on the mantle xenoliths
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sampled along the CVL were performed from islands of Gulf of Guinea (São Tomé, Caldeira and
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Munhá, 2002; Bioko and Palagú, Matsukage and Oya, 2010), in CVL s.s. (Mount Cameroon,
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Ngounouno et al., 2001; Ngounouno and Déruelle, 2007; Wandji et al., 2009; Lake Mbarombi, Lee
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et al., 1996, and Kumba plain, Teitchou et al., 2007, close to Mt Cameroon; Lake Enep, Lee et al.,
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1996; Lake Nyos, Temdjim et al., 2004; Temdjim, 2012, and Lake Nji, Princivalle et al., 2000,
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areas, close to Mt Oku), until Biu plateau (Lee et al., 1996), Kapsiki plateau (Tamen et al., 2015,
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and neighbouring Liri region (Nguihdama, 2007). Most publications have focused on mineral
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chemistry of mantle xenoliths and their implication on the origin of the CVL.
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The aim of this contribution is to describe the petrography, mineralogy and mineral chemistry of a
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series of twelve representative ultramafic xenoliths newly discovered in Miocene basanitic lava
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flows of Ngaoundéré region (Figure 1). Compositional variations of xenoliths from one centre to
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another could reflect the structure and compositional variations within upper mantle. Then, possible
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vertical or horizontal structural variation and heterogeneities under the Adamawa SCLM will be
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assessed.
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2. Geological setting of Adamawa plateau
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2.1. Geology of the area: basement and volcanic formations
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The Adamawa plateau is a volcanic horst (of about 200 km wide) bounded north and south by Pan
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African N70°E-trending faults. The Adamawa plateau formed during the Tertiary and then was
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uplifted up to 1 km relatively to the surrounding areas (Okereke, 1988; Nnange et al., 2001).
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The Adamawa basement belongs to the Central Africa Pan African Fold Belt Chain, crosscut by
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N70°E-trending strike-slip fault system (Moreau et al., 1987). It is constituted mainly by
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metamorphic rocks crosscut by neoproterozoic granitoids with U–Pb ages of 615 ± 27 Ma and circa
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575 Ma (Tchameni et al. 2006; Ganwa et al., 2008). Whether the volcanism of the Adamawa
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plateau belongs to the Cameroon Volcanic Line or not is still debated (Gouhier et al., 1974, Fitton,
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1980; Aka et al., 2009) and remains the subject of ongoing discussion. Indeed, the CVL presents a
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distribution of volcanoes along a “Y” shape, where Adamawa represents the NE hand of “Y”. For
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some authors, Adamawa is a part of CVL but not for others as the Adamawa volcanoes may be
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related to the Pan African faults contrary to the CVL. Numerous types of volcanic rocks outcrop on
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Adamawa plateau: basanite, basalt, hawaiite, mugearite, benmoreite, trachyte, rhyolite (North of
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Ngaoundéré and Tchabal Mbabo), phonolite. Differentiated lavas yield peralkaline affinities
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(Nkouandou et al., 2008; Fagny et al., 2012).
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2.2. Geophysical data
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Various informations on crust and lithospheric mantle beneath the Adamawa plateau have been
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provided through (1) geophysical studies and (2) petrological studies. Geophysical results are
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obtained beneath the CVL, and particularly beneath the Adamawa, by gravimetry (Browne and
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Fairhead 1983; Fairhead and Okereke, 1988; Poudjom Djomani et al., 1992, 1997; Nnange et al.,
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2000, 2001) and seismology (Stuart et al., 1985; Tokam et al., 2010; Koch et al., 2012; De Plaen et
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al., 2014).
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Gravimetric data suggest a Moho between 18 and 30 km. Stuart et al. (1985) estimated the depth of
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Moho at 33 km using seismic refraction. According to Tokam et al. (2010), Moho discontinuity is
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between 33 and 36 km deep beneath Adamawa plateau, more precisely 33 km beneath Ngaoudéré
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area. De Plaen et al. (2014) evidenced the isotropic character of the upper mantle beneath the CVL.
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The lithosphere-asthenosphere boundary (LAB) is only 100 km deep beneath the CVL, against 250
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km beneath the Congo Craton. LVZ is 200 km deep beneath the CVL. Milelli et al. (2012) made
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laboratory experiments and invoked a lithospheric instability that may develop within the
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subcontinental lithospheric mantle at the edge of a continent.
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3. Analytical methods
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Major element analyses of host lavas were done using ICP-AES and trace elements using ICP-MS
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at CRPG, Nancy, France, following analytic procedures of Carignan et al. (2001).
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Modal proportions of the three major mineral phases (olivine, orthopyroxene, clinopyroxene) and
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one minor phase (spinel) in ultramafic xenoliths selected for this study were determined with a
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Scanning Electron Microscope (SEM), in the Laboratory of Physics, University of Alexandru Ioan
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Cuza, Iasi, România. Mineral analyses of host lavas and xenoliths were performed on a Camebax
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SX100 Microprobe at the service Camparis of the University of Pierre et Marie Curie, Paris 6,
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France. The operating conditions were an accelerating voltage and a beam current as follow: olivine
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and clinopyroxene: 15 kV and 40 nA, 20 s except Si for olivine (10 s) and Ti for clinopyroxene (30
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s); plagioclase, K-feldspath, nepheline and noseane: 15 kV, 10 nA, 10 s; titanomagnetite: 20 kV and
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40 nA, Si, Ca, Ni:10 s; Mn: 25 s; Cr; 15 s; Al: 30 s; Ti, Fe, Mg: 40 s. Standard used were a
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combination of natural and synthetic minerals. Data corrections were made using the PAP
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correction of Pouchou and Pichoir (1991).
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4. Petrology of host basanite
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4.1. Petrography
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Ultramafic xenoliths have been found in basanitic lavas. These host-lavas are OIB intraplate lava
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flows, and Miocene in age (Nkouandou et al., 2008; Fagny et al., 2012). They cover more than 30
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% of the surface (Lasserre, 1961) and three sequential eruptive units are distinguished: Lower Flow
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Unit (LFU), Middle Flow Unit (MFU) and Upper Flow Unit (UFL) (Figure 1). All sequences
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contain numerous peridotite xenoliths. Host-lavas exhibit fluidal porphyritic to glomeroporphyric
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textures. Phenocryst phases consist of euhedral to subhedral large olivine crystals (1.5 to 3 mm, 15
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to 20 volume %), euhedral clinopyroxene (1 to 2 mm, 15 %), Fe-Ti oxides (0.5 to 1 mm, 5 %), set
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in a groundmass (60 %) containing the same minerals and needle plagioclase microlites. Host-lavas
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are CIPW nepheline normative (4.5 < nenorm < 10.5).
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4.2. Mineralogy
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Chemical compositions of representative phases of host-basaltic lavas are listed in Table 1. Olivine
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phenocryst compositions vary from Fo 84 in the core to Fo 81 in the rim. Fo content of microcrysts
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and microphenocrysts (78 %) are not quite different. CaO contents (up to 0.23 wt %) are rather
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high, while NiO contents are less than 0.24 wt %. Olivine xenocrysts (especially in NG125 and
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NG137) are characterized by high Fo (89-90 wt %), high NiO (0.33 wt %) and low CaO (0.08 wt
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%) contents, typical of mantle peridotite (Takahashi, 1980). Clinopyroxene phenocrysts are
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diopside, according to Morimoto et al. (1988) classification, with Wo51.4―45.7En43.7―46.8Fs7.5―4.9.
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Some phenocrysts (Table 1) exhibit a fassaitic trend with high Al2O3 (up to about 9 wt %), TiO2 (4
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wt %) and CaO (22.5 wt %), like those described in the Plio-Quaternary undersaturated alkaline
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volcanic episode in the Noun plain (Wandji et al., 2000). No significant variations exist between
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cores and rims or between phenocrysts and microlites. TiO2 (2.4 wt %) and Al2O3 (5.2―6.4 wt %)
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contents are rather high, the highest value content being found in microlites (up to 4 wt % and 9 wt
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% respectively in basanite NG125 microlites). Plagioclase microlites described in NG115 and
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NG137 basalts are andesine in composition (Ab32.8―34.8 An66.3―63.9), with FeO* contents up to 0.7
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wt %. Fe-Ti oxides are ulvöspinel (65-68 wt % FeO*, 22 wt % TiO2, Usp mole % ranging between
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61 and 65). Numerous basanitic lavas with similar mineral compositions have been described all
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along the Cameroon Volcanic Line and the Adamawa plateau, including Ngaoundéré region.
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4.3. Geochemistry
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Whole-rock chemical analyses of xenoliths host lavas are listed in Table 2. They are sodic alkaline
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basanite (40-42.5 wt % SiO2, 3.9-4.5 wt % Na2O + K2O, Na2O/K2O > 2.0), with Thornton and
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Tuttle (1960) D.I. ranging from 21 to 30. Compositions are silica-undersaturated with CIPW-
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normative nepheline close to 10 wt %, typical of intraplate alkaline series, resulting from a low
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degree (< 5 %) of mantle partial melting. NG137 is altered/weathered as shown by high LOI of 4.2
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wt %.
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Low contents of transitional elements (Table 2, V: 309-372 ppm, Co: 37-56 ppm, Cr: 25-303 ppm,
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Cu: 23-31 ppm and Ni: 23-124 ppm) evidence that these xenoliths host-lavas do not present
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primitive characters, except NG115, Mg-, Cr- and Ni-rich though Co is too low. Indeed, the
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basanitic magma was not directly produced from upper mantle melting but was slightly
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differenciated as shown by D.I., increasing up to 30. It differs from rocks with high Ni (300-500
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ppm), Co (> 300 ppm) and Cr (500-700 ppm) contents, that typically crystallized from a primitive
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magma (Green et al., 1974). Sr, Ba and Rb contents are high, a typical feature of alkali basaltic lava
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series. Zr, Hf, Ta and Th contents are in the range of intraplate basaltic magmas, without any sign
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of crustal contamination; particularly the Zr contents (332-455 ppm) remain rather high, compared
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to low Zr contents in materials of crustal origin (173 ppm, Rudnick and Fountain, 1995).
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REE contents are in the same range as previous REE data from North and East of Ngaoundéré area
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(Nono et al., 1994; Nkouandou et al., 2008). Spidergrams of the host-basanitic lavas present very
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high incompatible element values (Figure 2), up to 100-200 times the mantle values. Positive
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anomalies are noted in Nb, Ta, Sr and Zr, veryfying the no-contribution of crustal materials.
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Negative anomalies, observed for K and Ti, suggest the presence of Ti-bearing phlogopite in the
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mantle source, as already mentioned by Nkouandou et al. (2010). The same conclusion has been
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reached for basaltic lavas from the Cameroon Volcanic Line (Ngounouno et al., 2000, 2003). The
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mantle source of host basanite lavas was probably melted at a low degree of a residual garnet
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mantle, as attested by high (Ce/Yb)n ratios (12-14), at more than 80 km depth. The occurrence of
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numerous mantle xenoliths and the absence of crustal contamination indicate rapid ascent of the
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host magmas from the upper mantle, probably through the Pan-African fault network that crosscut
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the entire Adamawa plateau lithosphere (Moreau et al., 1987).
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5. Petrology of mantle xenoliths
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5.1. Field data: mantle xenolith occurences
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Basanitic lavas have entrained numerous mantle peridotite xenoliths during their ascent to the
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Earth’s surface.
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The peridotite xenoliths, which are studied here, have been sampled in Mio-Pliocene eruptive
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products (Nkouandou et al., 2010), North (Bambi, 40 km north from Ngaoundéré; 7°33'30'' N,
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13°34' E, 1365 m), Center (Ngao Sey, 12 km east from Ngaoundéré; 7°18' N, 13°40' E, 1240 m)
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and East (Foulféké, 38 km east from Ngaoundéré; 7°18'10'' N, 13°45' E, 1160 m) of the
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Ngaoundéré region in Adamawa plateau (Figure 1). Ultramafic xenoliths sampled in these areas are
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respectively harzburgite at Bambi Upper Flow Unit, lherzolite at Ngao Sey Middle Flow Unit and
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pyroxenite (olivine websterite) at Foulféké Lower Flow Unit. They have been named following
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IUGS recommendation (Streckeisen, 1976) (Figure 3). In this study, the abbreviation “NG”
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corresponds to host xenoliths lavas while “NK” correspond to xenoliths (i.e., host lava NG137
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contains xenolith NK137).
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Published works on petrology, focused on the nature and composition of the lithospheric mantle
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beneath the Adamawa plateau are rare. Studies of spinel- and plagioclase-bearing lherzolites from
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Dibi volcano, 25 km south of Ngaoundéré, further south of our investigated area (Girod et al., 1984;
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Dautria and Girod, 1986) localized the Moho at 20 km below the Adamawa plateau. Lee et al.
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(1996) described rare occurrence of spinel-garnet-pargasite websterite in the Ngaoundéré plateau.
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Temdjim (2005) studied keliphytic garnet-bearing lherzolite from Youkou volcano (situated 15-20
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km SW of Ngaoundéré, that is outside of Figure 1 inset), evidencing retromorphic metasomatism by
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fluids. The most recent contribution concerns the equilibrium conditions and mantle characteristics
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inferred from the petrology of spinel lherzolite xenoliths and host basaltic lava from Ngao Voglar
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volcano (close to Bambi, Figure 1) (Nkouandou and Temdjim, 2011).
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5.2. Petrography
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Twelve samples have been selected. They belong to the group I of “ultramafic inclusions” defined
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by Frey and Prinz (1978) at San Carlos, Arizona. They are subdivided into three types, based on
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texture and mineral characteristics. Each type is, respectively, represented by samples NK115
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(lherzolite), NK125 (harzburgite) and NK137 (olivine websterite) (Table 3). Samples are chiefly
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composed of four phases, typical of mantle composition: olivine, clinopyroxene, orthopyroxene and
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spinel. They are rounded (NK125) to prismatic or angular in shape (NK115 and NK137), ranging
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from 10 to 15 cm in size, and are distinguished according to modal mineral proportions (Figure 3).
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Neither hydrous minerals (e.g., biotite, amphibole, or apatite) nor aluminous phases (plagioclase or
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garnet, but spinel is present) were observed, contrary to some previous findings in Adamawa
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(Dautria and Girod, 1986; Lee et al., 1996). All xenoliths display a sharp contact with host lavas.
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According to Mercier and Nicolas nomenclature (1975), transition from protogranular (lherzolite
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NK115 and olivine websterite NK137) to porphyroclastic texture (harzburgite NK125) was
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acquired by slightly increasing shear deformation. Such mechanism could have induced differences
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in mineral composition.
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NK115 is spinel lherzolite (Figure 3). It exhibits a protogranular texture and is composed of
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abundant olivine crystals (64 volume %), clinopyroxene (25 %), orthopyroxene (10 %) and spinel
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(1 %). Interstitial glass is absent. Olivine crystals are large (2 to 6 mm in size) and sometimes
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display clear deformation bands and parallel twin planes, a typical feature of mantle peridotite
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(Mercier and Nicolas, 1975; Conticelli and Peccerillo, 1990). They are iddingsitized along fine
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irregular cracks and show a sharp contact with host lava. Triple points are common between
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crystals, particularly olivine (Figure 4, photo 1). Elongated orthopyroxene crystals (1.5 to 2 mm in
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length) are often associated with olivine crystals at their edges and are anhedral with grayish color.
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Some crystals show parallel twin planes, while others present rounded or polygonal shapes without
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exsolution lamellae. Clinopyroxene crystals (1.5 to 2.5 mm) are anhedral and show regular molten
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boundaries (1 mm large) when they are in contact with groundmass of the host rock, probably
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acquired during their transport by the host lava. Some crystals present undulose extinction, while
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others are colorless. Many clinopyroxene crystals are interstitial between orthopyroxene and olivine
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and often associated with spinel crystals. Spinel crystals (0.7 to 1 mm) are brown in color and are
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interstitial between olivine and orthopyroxene crystal, or occur as intergranular crystals with
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clinopyroxene. Lherzolite samples strongly resemble type I lherzolites (63 modal % olivine, 18 %
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orthopyroxene, 16 % orthopyroxene, 2.5 % spinel), described by Dautria and Girod (1986) in the
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neighbouring Dibi volcano, except that they do not bear a small amount (< 1 %) of plagioclase.
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NK125 represents the harzburgite group (cpx < 5 modal %, after IUGS nomenclature, Figure 3). It
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presents a porphyroclastic texture with plastically deformed minerals (this texture may be
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considered as transitional between protogranular type and typical porphyroclastic texture, Mercier
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and Nicolas, 1975) and is composed of about 75-85 volume % olivine crystal, 10-20 % opx, less
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than 5 % cpx and 1-2 % spinel (Table 3). Olivine crystals (1.5 to 3 mm) are the most abundant
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phase. They are iddingsitized along cracks and show sharp contact with host lavas. Some crystals
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are intimately associated with orthopyroxene and/or present a thin molten edge. Occurrence of
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triple junctions is frequent between olivine-olivine and olivine-orthopyroxene crystals (Figure 4,
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photo 2). As noted in olivine description, some orthopyroxene grains intergrown with olivine
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present a vermicular shape or are associated to olivine by their edge. Clinopyroxene crystals
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resemble those of NK115 and show a thin molten edge, described as spongy clinopyroxene (Klügel,
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1998; Carpenter et al., 2002). They are 1.5 to 2.5 mm in size, light brown in color, and weakly
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pleochroic. No exsolution lamellae were observed in cpx and in opx crystals, but both crystals form
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frequent aggregates with spinel. Anhedral spinel crystals are brown in color and located within
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interstices between clinopyroxene and orthopyroxene minerals. They are elongated, or flattened,
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and sometimes display a holly-leaf shape. Their mode is < 2 volume % and no crystal is larger than
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2 mm in size. This texture may provide evidence of plastic deformation in upper mantle conditions
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as suggested by Basu (1977).
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NK137, a pyroxenite, has a composition of olivine websterite (olivine < 40 % modal, Figure 3) and
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displays a protogranular texture (Figure 4, photo 3). Composed of orthopyroxene, clinopyroxene,
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olivine, and spinel, it presents a sharp contact with host lava, sometimes separated by curvilinear
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boundaries. There is no basaltic liquid infiltration between crystals. Olivine crystals (2 mm to 5 mm
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in size) are fairly abundant (21 to 31 volume %). They are frequently iddingsitized in cracks and
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define triple point junctions. Some olivine grains present interpenetration edges with orthopyroxene
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and spinel crystals. Anhedral orthopyroxene crystals (1.5 to 3 mm in size and 26 to 38 %) are
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grayish to weakly greenish in color, and, frequently associated with olivine. Some crystals are
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deformed and kinked. Clinopyroxene crystals (2 to 3.5 mm in size and 38 to 40 %) are anhedral,
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without visible exsolution lamellae. They are brownish or yellowish in color and have porous or
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spongy textured rims consisting of glass and small clinopyroxene grains, which may result from
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partial melting effect within the host magma during transport to the surface (Glaser et al., 1999).
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Red-brown spinel crystals (0.5 to 1.5 mm in size and 2 to 3 volume %) are anhedral, and,
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crystallized in interstices. By comparison, Tamen et al. (2015) have observed a pyroxenite of
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websterite type (83 volume % clinopyroxene, 8 % orthopyroxene, 4 % spinel, only 4 % of olivine
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and less than 1 % of plagioclase) in nearby Kapsiki plateau. Note that a typical clinopyroxenite is
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described in Mount Cameroon, together with dunite (Wandji et al., 2009, Figure 3).
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5.3. Mineralogy
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Mineral compositions of xenoliths with porphyroclastic texture (harzburgite NK125) differ
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markedly from xenoliths with protogranular texture (lherzolite NK115, olivine websterite NK137).
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Mg-rich olivine crystals (Fo 89-90, Table 4) have a typical mantle origin. Olivine crystals of
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NK115 and NK137 are characterized by low CaO contents (< 0.09 wt %), suggesting a high
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pressure equilibration (Köhler and Brey, 1990). Higher CaO contents (0.12 wt %) are found in
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NK125 xenolith. On the contrary, NiO contents are higher (up to 0.7 wt %) in NK115 and NK137
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(up to 0.4 wt %) than in NK125 (0.3 wt %). High NiO contents (average of 0.402 wt % for 90
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samples) characterize xenoliths of mantle origin according to Sato (1977). Note that olivine
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xenocrysts found in host basalts NG125 and NG137 display the same characteristics. Similar
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olivine compositions have been described in mantle peridotites worldwide, including the Cameroon
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Volcanic Line (Lee et al., 1996; Princivalle et al., 2000; Caldeira and Munha, 2002; Temdjim et al.,
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2004; Teitchou et al., 2007).
276
Orthopyroxene crystals (Table 5) are enstatite (En91―89 Fs7.6―9.6). Fairly high Wo contents (up to
277
2.7 %) found in NK125 (Figure 5), reflect high temperature equilibration. CaO contents increase
278
from NK137 (0.5 wt %) to NK125 (1.3 wt %), NK115 showing intermediate value (0.75). All
279
crystals are TiO2-poor (0.11-0.21 wt %). Orthopyroxene in lherzolite NK115 and olivine websterite
280
NK137 contains 0.16-0.29 wt % Cr2O3 and 3.75-4.49 wt % Al2O3. In harzburgite NK125, they are
281
Cr- (up to 0.6 wt % Cr2O3) and Al- (up to 6.2 wt % Al2O3) richer. Cr# (= Cr/Cr+Al3+) range
282
between 0.03 and 0.06. All crystals are highly magnesian (Mg# = 100*Mg/Mg+ΣFe2+ = 90-92),
283
which is typical of type I ultramafic xenoliths defined by Frey and Prinz (1978). Orthopyroxene
284
show higher Mg# than coexisting olivine (Mg#Opx > Mg#Ol), and a positive correlation between
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(Al2O3)opx and (Cr2O3)opx (Figure 6a). On the contrary, (Al2O3)spinel vs. (Al2O3)opx are inversely
286
correlated from lherzolite to harzburgite type xenoliths (Figure 6b). Particular attention should be
287
paid to orthopyroxene of harzburgite NK125, as it is characterized by high Al2O3, CaO and Na2O
288
contents, which usually characterize more enriched xenoliths, while high Cr2O3 contents rather
289
suggest depleted features.
290
Clinopyroxene (Table 6) is diopside (Wo46.8-49.9 En49.0-51.1 Fs3.2-0.5) in NK115 and NK137.
291
Clinopyroxenes in NK125, is augite (Wo36.9-43.3 En56.6-60.3 Fs0.1-3.4, Table 6 and Figure 5) according
292
to the IMA classification (Morimoto et al., 1988). It contains 0.58 to 0.94 wt % Cr2O3, i.e. Cr
293
(a.p.f.u.) > 0.01 with a mean Cr# of 0.07. They are typical of Group I xenoliths as previously
294
distinguished by Frey and Prinz (1978). Mg-numbers (0.99-0.94) of clinopyroxene are higher than
295
of coexisting olivine and orthopyroxene (Mg# cpx> Mg# opx > Mg# ol). Clinopyroxene of
296
harzburgite is distinguished by high Al2O3 (up to 7.6 wt %) and FeO* (> 4.0 wt %) contents. Higher
297
Na2O (1.80 wt %) but lower TiO2 (< 0.45 wt %) and FeO* (3.5 wt %) contents are found in NK115
298
lherzolite vs. NK125 harzburgite. This is a typical feature of fertile lherzolites (Jacques and Green,
299
1980). A good correlation (figure not shown) can be seen between Al2O3 of cpx and Cr# of
300
coexisting spinel. Last, clinopyroxene resembles diopside described at Dibi volcano (Dautria and
301
Girod, 1986), particularly concerning Na (0.10-0.15) and Cr (0.01-0.03) amounts.
302
Spinel (Table 7) is Al-Cr-rich with 54-57 wt % Al2O3, and 9-11 wt % Cr2O3. TiO2 contents are
303
always low and less than 0.36 wt %. Cr# (= 100*Cr/Cr+Al) is relatively low and varies slightly (9
304
% in NK115, 12 % in NK125 and 11 % in NK137). Mg# remains high and homogeneous, with a
305
mean value of 0.8. Fe3+# is low (Fe3+# = 100*Fe3+/(Al+Cr+Fe3+) < 0.05), witness of low fO2
306
compatible with mantle origin (see discussions in Preß et al., 1986; Webb and Wood, 1986; Witt-
307
Eickschen and Seck, 1991). Note that the spinel compositions are rather homogeneous, contrary to
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Kapsiki compositions (Tamen et al., 2015) that reveal high heterogeneity (17-55 wt % Al2O3, 11-29
309
wt % Cr2O3, 0.12-0.44 Cr#).
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The plot of Cr# versus Mg# of spinel (figure not shown) typically fall within the field of ultramafic
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xenoliths (according to Conticelli and Peccerillo, 1990) and exhibit a negative correlation from the
312
lherzolite to the harzburgite type xenoliths. This is confirmed as in spinel Cr# vs. olivine Fo plot
313
(Figure 7), all samples occupy the OSMA field (Arai, 1994). For comparison, dunite and wehrlite
314
samples from Mt. Cameroon (Wandji et al., 2009) as well as some lherzolite and harzburgite
315
samples from São Tomé (Caldeira and Munha, 2002), are out of OSMA field and reflect fractional
316
crystallization and accumulation processes. Cr partitioning between spinel and pyroxene yields the
317
relation Cr#sp > Cr#cpx > Cr#opx. Spinel plots in Fe# (= Fe2+/Fe2++Mg2+) versus Cr# (= Cr/Cr+Al)
318
(Figure 8a) and Cr# versus TiO2 (Figure 8b) show a regular increase from lherzolite to olivine
319
websterite to harzburgite. Similar variations have been observed in the ultramafic xenoliths of Lake
320
Nyos (Temdjim et al., 2004).
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5.4. Thermodynamic parameters
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Equilibrium conditions
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All studied ultramafic xenoliths exhibit the characteristics of differing, yet homogeneous mantle
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pockets as indicated by their petrographic features and mineral compositions. As previously noted,
326
there is a correlation between the texture and mineral composition in each xenoliths group,
327
particularly their Mg#, Cr# and Al2O3 content of orthopyroxene crystals. Each xenoliths group with
328
the same texture provide evidence of homogeneous equilibrium environment as illustrated in the
329
different variations diagrams where each group shows grouping points. This would suggest that
330
distinct parts or layers of the upper mantle under the Adamawa plateau, were sampled by Mio-
331
Pliocene Ngaoundéré basanite flow units. Lack of exsolution lamellae in pyroxene and the absence
332
of hydrous phases (amphibole, biotite and apatite) point out no late compositional modification
333
(particularly metasomatic events, Brearley et al., 1984). Low spinel Cr# vary according to xenolith
334
textures. This could attest for the different partial melting degrees (Frey and Prinz, 1978) or
335
continuous depletion of the upper mantle (Dick and Bullen, 1984; Cabanes and Mercier, 1988).
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Equilibrium temperature
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Equilibrium temperature and pressure of representative ultramafic xenoliths have been estimated,
339
considering that those samples are homogeneous. In order to avoid influences due to exchange
340
between mineral phases and host lava, only compositions of cores of minerals have been
341
considered.
342
Five geothermometers were used in the estimation of upper mantle temperature beneath the
343
Adamawa plateau, to get a good estimate of the uncertainties on the obtained numbers. They are:
344
(1) Witt-Eickschen and Seck (1991) geothermometer, based on solubility of Ca and Al in
345
orthopyroxene in equilibrium with olivine, clinopyroxene and spinel, in spinel peridotite (2)
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Mercier (1980) single pyroxene thermobarometry, (3) Brey and Köhler (1990) two-pyroxenes
347
thermometer, (4) Sachtleben and Seck (1981) Al-solubility in orthopyroxene, and (5) Witt-
348
Eickschen and O’Neill (2005) Ca―Mg exchange between pyroxenes.
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The results are listed in Table 8. We have carefully selected 2 fresh samples in each group for a best
350
representation (lherzolites NK115 and 117, harzburgites NK125 and 128, and olivine websterites
351
NK137 and 139): textures are the same in each groupe, only percentages of mineral phases show
352
small differences. Calculations are made for a total mean pressure of 15 kbar (1.5 GPa) that
353
corresponds to the most probable mean value (O’Neill, 1981; Gasparik, 1987).
354
The temperatures range between 883 and 1055 °C for lherzolite, and between 821 and 1005 °C for
355
olivine websterite. Whatever the method used is, the highest temperatures (see Table 8) are always
356
found in harzburgite (between 880 and 1162 °C). Uncertainties are in the range of 20-25 °C for
357
geothermometers (1), (3) and (4). Harzburgites which exhibit porphyroclastic textures present
358
rather higher temperatures than lherzolites and websterites, which exhibit the same protogranular
359
textures: this is in agreement with mean values compiled by Mercier and Nicolas (1975), i.e. 1000-
360
1050 °C for protogranular peridotites against 1000-1260 °C for porphyroclastic peridotites.
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From these rather high temperatures, it is deduced that low temperature metasomatism did not
362
affect the xenoliths, confirming observations under the microscope (see discussions in Kourim et
363
al., 2014) who calculated low-temperature of 750-900 °C for lherzolite xenoliths in Hoggar,
364
Algeria.
365
Interestingly, all calculated equilibrium temperatures are within the temperature range (800 to
366
1200°C) estimated from ultramafic xenoliths along the whole Cameroon Volcanic Line, including
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Mt. Cameroun, Lake Nyos and Adamawa plateau.
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Pressure
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Determination of real pressure of mantle spinel peridotite is unanimously (Medaris et al., 1999;
371
Christensen et al., 2001) recognized as unrealistic. All methods lead to dissatisfying results, because
372
application of experiments to natural systems remains more or less approximate. However, lack of
373
garnet and plagioclase in studied spinel lherzolite, puts limits of equilibrium pressure only within
374
the spinel lherzolite stability field, i.e. between 8 and 20 kbar (0.8-2 GPa) as previously suggested
375
(O’Neill, 1981; Gasparik, 1987). The single pyroxene thermobarometry of Mercier (1980) using
376
clinopyroxene composition (in the spinel facies) may be retained for pressure estimation, as
377
calculated results lay among those of spinel lherzolite of natural system (O’Neill, 1981; Gasparik,
378
1987). This barometer yields pressures of 1.8-1.9, 0.8-1.0, and 1.9-2.5 GPa, respectively for
379
lherzolite, harzburgite and olivine websterite, with corresponding temperatures (Table 8). Pressures
380
correspond to depths of 59-63, 26-33 and 63-83 km, respectively, when using the conversion factor
381
of 33 km × P (GPa). Results compare favorably with earlier estimations for the Cameroon Volcanic
382
Line (Teitchou et al., 2007; Wandji et al., 2009; Temdjim, 2012). They have been already being
383
used to constrain geophysical data on the Adamawa plateau (Nnange et al., 2000).
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6. Discussion
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6.1. Three types of enclaves, components of the lithosphere?
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Miocene basanitic lavas of the Ngaoundéré area, sampled three types of enclaves, each of them in a
388
precise location, several ten kilometers far from each other: (i) protogranular lherzolite, (ii)
389
porphyroclastic harzburgite, (iii) protogranular olivine websterite.
390
Both protogranular and porphyroclastic type textures were observed in the spinel lherzolite from
391
other localities in the Adamawa plateau (Lee et al., 1996). Mercier and Nicolas (1975) classification
392
described a continuous suite from protogranular texture to equigranular texture and porphyroclastic
393
texture with increasing deformation. Note that our samples present contrasting textures, that do not
394
argue (or not) for continuous variations for Adamawa upper mantle, as suggested by Brown et al.
395
(1980) for French Massif Central. According to Ambeh et al. (1989), deformation might be related
396
to mantle uplift beneath Adamawa, as a consequence of upwards migration of lithosphere-
397
asthenosphere boundary during the Tertiary (Okereke, 1988). But it is unlikely that asthenosphere
398
can produce such a deformation. It is much more likely that deformation in lithosphere induced
399
asthenosphere uprise.
400
Garnet was not observed but suggested by clinopyroxene - spinel aggregates (Nicolas et al., 1987),
401
which might be interpreted as initial equilibration in garnet stability field, followed by
402
reequilibration in spinel stability field, during adiabatic upwelling. As a matter of fact, pink garnet
403
(mg number closed to 79) is described around grey/green spinel in two samples of spinel - garnet
404
websterite from Ngaoundéré plateau by Lee et al. (1996).
405
Rocco et al. (2013) described Miocene basanites bearing the three types of co-existing peridotitic
406
enclaves - lherzolite, harzburgite and wehrlite - in the small (less than 4 km in diameter) Nosy
407
Sakatia Island, Madagascar. These authors pointed out the decrease of amount of modal
408
clinopyroxene
409
clinopyroxene/orthopyroxene ratio. This would evidence that xenoliths represent different sections
410
of the lithospheric mantle (vertical variation as these xenoliths are found very close each other, in
411
the same volcanic center) affected by variable degrees of melt extraction. In the case of Ngaoundéré
412
area, we note variations with depths, but not on the same vertical line, because xenoliths are found
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from
lherzolite
to
harzburgite
as
well
as
the
decrease
of
the
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in different locations (both “horizontal” and “vertical” sampling). Like Rocco et al. (2013), we
414
noted increase of Cr# (0.09 to 0.12) in Al-Cr-spinel, a slight increase of Mg# in pyroxenes (0.90 to
415
0.92) from lherzolite to harzburgite. However we did not note clear variations of Fo content in
416
olivine (0.89 to 0.90).
417
In the French Massif Central, Coisy and Nicolas (1978) show that distribution of different textures
418
of xenoliths is not random but might suggest (1) the presence of distinct mantle bodies, or (2) the
419
presence of a single mantle materiel progressively deformed during upwelling, or even (3) a
420
compositional stratification of mantle.
421
Ultramafic (lherzolite, harzburgite, olivine websterite) xenoliths entrained in basanite during ascent
422
to the surface in Ngaoundéré region display features of sub-continental lithosphere fragments, as
423
suggested by Basu (1975) and Menzies et al. (1987).
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6.2 Mantle heterogeneity, melting and metasomatism
426
“Fertile” lherzolite within continental lithosphere does not reflect primitive mantle. More likely, it
427
corresponds to re-fertilized previously depleted mantle, which means at least one metasomatic
428
event. Harzburgite is more likely original depleted lithosphere mantle, while olivine websterite
429
corresponds almost certainly to mantle more metasomatized than lherzolite. Thus, we observe a
430
sequence, which substantiates increasing metasomatism, from harzburgite to lherzolite and olivine
431
websterite (with increasing clinopyroxene amount), either late Pan-African, or Tertiary in age.
432
From petrographical observations, spongy rims of clinopyroxene phase of harzburgite NK125 exist
433
in low modal contents (2-5 %). Numerous hypotheses explain spongy rims: (1) melt infiltration,
434
causing incongruent dissolution, probably during upwelling and incipient melting of low melting
435
point minerals (Rocco et al., 2013), (2) melting event after incorporation in host lava inducing
436
higher temperature and decompression (Lustrino et al., 1999; Carpenter et al., 2002; Shaw et al.,
437
2006). Otherwise, spinel, as all phases of this sample NK125, exhibit flattening and elongation,
438
which are signs of plastic deformation. NK125 is also characterized by high CaO content in olivine,
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low Mg# and Al2O3 in spinel and high Al2O3 and Cr2O3 in orthopyroxene. Moreover, Fe# and TiO2
440
content of spinel phases of all representative samples show an increasing evolution with Cr# from
441
lherzolite NK115 to harzburgite NK125 through olivine websterite NK137 (Figure 8). Hypothesis
442
(2) may be retained to explain the structure of the sub-continental mantle beneath the Adamawa
443
plateau, as it is strengthened by the progressive deformation textures coupled with the continuous
444
compositional chemical variations of their minerals. Furthermore, estimated depths show that
445
harzburgite type xenoliths have been sampled at rather shallow depth (26-33 km, that is to say close
446
below Moho), olivine websterite group xenoliths have been sampled between 60 and 80 km,
447
whereas lherzolite type xenoliths originated from nearly 60 km depth, that is to say in the middle-
448
lower part of the lithospheric mantle in the thermal boundary layer. This is confirmed by higher
449
CaO contents (0.12 wt %) in olivine of harzburgite NK125, witness of lower pressure. The
450
estimated depths fall within those of four calculated major density discontinuities (7-13 km and 19-
451
25 km corresponding to crustal contrasts, 30-37 km corresponding to Moho, 75-149 km
452
corresponding to mantle anomaly) which have been deduced from the spectral analysis of the
453
gravity data (Nnange et al., 2000) beneath the Adamawa plateau. Accordingly, harzburgite
454
xenoliths may have been sampled at the top of the mantle, e.g. between 26 and 33 km, value
455
obtained from thermodynamic calculations. Such depth is close below the Moho discontinuity,
456
assuming a crustal thickness below the Adamawa plateau of 33 km (Tokam et al., 2010).
457
The presence of a hot mantle dome beneath Adamawa at a depth of 60 km that has been proposed
458
(Browne and Fairhead, 1983; Fairehead, 1988) is a matter of debate (for discussion, see De Plaen et
459
al., 2014). The major evidence is the low P-wave velocity of 7.8 km/s (Stuart et al., 1985; Dorbath
460
et al., 1984, 1986). Ambeh et al. (1989) consider that crustal uplift results from LAB upwards
461
migration. Likewise, the same two contrasting models are advocated to explain the Hoggar swell, in
462
Algerian Sahara: (i) mantle plume-induced LAB upwards migration (Dautria et al., 1987), and (ii)
463
lithosphere delamination (Liégeois et al., 2005; Beccaluva et al., 2007; Bouzid et al., in press). The
464
latter model postulates linear/planar delamination along lithosphere-scale shear zones, inducing
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limited asthenosphere uprise, in agreement with normal heat flow and magnetotelluric data.
466
Topographic highs in Tibesti, in Chad, and Darfour, in Sudan, are currently the subject of similar
467
debates.
468
7. Conclusion
470
Two models could explain the volcanic activity in Adamawa plateau: (i) mantle plume-induced
471
LAB upwards migration and (ii) linear/planar delamination along lithosphere-scale shear zones,
472
inducing limited asthenosphere uprise. The second hypothesis is favoured according to a rather
473
normal heat flow. Moreover, volcanic activity in Adamawa plateau might be controlled by the
474
reworking of the Pan-African fault swarm, crosscutting both continental crust and upper mantle
475
down to a depth of 190 km.
476
Occurences of three types of xenoliths evidence geochemical heterogeneities of subcontinental
477
lithospheric mantle structure beneath the Adamawa plateau, as it was already invokated elsewhere
478
beneath the CVL.
479
On its way to the surface, basanitic liquid produced from a weak differentiation of a primary
480
magma would have sampled lherzolite upper mantle between 59 and 63 km (Figure 9). In the sub-
481
continental structure, olivine websterite exhibits features close to lherzolite (temperature and
482
pressure, depth of formation close to 60 km, occurrence of olivine, clinopyroxene, orthopyroxene
483
and spinel, and protogranular texture). We suggest that it may have been emplaced as dykes or sills,
484
crosscutting lherzolite mantle. Harzburgitic xenoliths evidence that sheared subcontinental
485
lithospheric mantle occurs at shallow depths (Figure 9).
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Acknowledgments. Authors warmly thank the “Agence Universitaire de la Francophonie (AUF)”
488
through the BAGL (Bureau Afrique Centrale et des Grands Lacs), for financial support of “Le
489
Projet de soutien aux équipes de recherche 2012/2013_No 51110SU201“, for field works to
490
laboratory analyses. OFN and AMF spent three months in 2013 at the “Department of Earth
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491
Sciences, UMR 8148 GEOPS-CNRS” of the University of Paris-Sud XI, Orsay, France. L. Daumas
492
is thanked for drawing figures. B. Bonin and an other, anonymous, reviewer are thanked for useful
493
remarks.
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upper mantle peridotite. Chemical Geology, 221, 65–101.
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peridotite: an improved version of an empirical geothermometer. Contrib. Mineral. Petrol.
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106, 4, 431–439.
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Figure captions
769
Figure 1. Sampled location and geological map of Ngaoundéré region after Nkouandou et al.
770
(2010), modified. The cross section presented in Figure 9 is localized (bold straight line). Left
771
inset shows the main volcanic zones: Cameroon Volcanic Line and Adamawa plateau. Upper
772
inset: relationships between Cameroon Volcanic Line and African cratons (Kampunzu and
773
Popoff, 1991).
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Sketch showing the possible structure of sub-lithospheric mantle beneath Ngaoundéré area in
775
Adamawa plateau (relief after topographic map NB 33 XX, scale 1/200000). This NW-SE
776
section is shown in Figure 1.
778
Figure 2. Host basanite lava, Primitive Mantle-normalized (McDonough and Sun, 1995) multi-
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element diagrams.
779
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Figure 3. Modal classification of Ngaoundéré ultramafic xenoliths, according to Streckeisen (1976). Data from Cameroon Volcanic Line and Madagascar are plotted for comparison
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Figure 4. Thin sections of the three types of peridotites observed with optical microscope under
784
crossed nicols. Photo 1 (NK115) Triple point junctions between curvilinear or slightly curved
785
boundaries of olivine and pyroxene in protogranular texture of lherzolite. Photo 2 (NK125)
786
Strained grains of clinopyroxene with thin molten edge, orthopyroxene and olivine crystals in
787
porphyroclastic texture of harzburgite. Photo 3 (NK137) Olivine and pyroxene large crystals
788
in protogranular texture of olivine websterite (size of photos 1 and 2 = 8,8 x 6,5 mm, size of
789
photo 3 = 14,8 x 11,2 mm; cpx = clinopyroxene, ol = olivine, opx = orthopyroxene and sp =
790
spinel, abbreviation after Kretz, 1983).
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Figure 5. Chemical composition of ultramafic xenoliths pyroxene, projected in the En-Wo-Fs
793
triangle (after Morimoto et al., 1988). Xenoliths from other localities of Cameroon Volcanic
32
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Line (São Tomé, Caldeira and Munha, 2002; Lake Nyos, Temdjim, 2012) are added for
795
comparison.
796
Figure 6. Variation diagrams illustrating increasing Cr2O3 versus Al2O3 of orthopyroxene (a) and
798
decreasing (Al2O3)Sp versus (Al2O3)Opx content (b), from olivine websterite and lherzolite to
799
harzburgite, explained by partial melting processes.
800
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Figure 7. Plot of Cr# (Cr/(Cr+Al)) of spinel versus Fo of olivine in the Olivine Spinel Mantle Array
SC
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(OSMA) diagram of Arai (1994).
805
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Figure 8. Fe# (= 100*Fe2+/Fe2++Mg) versus Cr# (= 100*Cr/Cr+Al) (a) and Cr# versus TiO2 (b) diagrams of spinel, showing an increase from lherzolite to harzburgite.
806
Figure 9. Sketch showing the possible structure of sub-lithospheric mantle beneath Ngaoundéré
808
area in Adamawa plateau (relief after topographic map NB 33 XX, scale 1/200000). This
809
NW-SE section is shown in Figure 1. Depth of Moho according to various geophysical data
810
(see text), depth of lithosphere-asthenosphere boundary (LAB) according to De Plaen et al.
811
(2014).
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Table captions
814
Table 1. Electron microprobe chemical analyses of representative minerals of host basanite. ph =
815
phenocryst, c = core, r = rim, ml = microlite, mic = microcryst, cpx = clinopyroxene, mt =
816
magnetite, pl = plagioclase, ol = olivine (abbreviation after Kretz, 1983).
817 818 819
Table 2. Whole-rock chemical analyses of host basanite of ultramafic xenoliths.
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820 821
Table 3. Modal proportions of mineral phases in ultramafic xenoliths, determined from SEM images of entire areas of thin sections.
822
824
Table 4. Electron microprobe chemical analyses of olivine in xenoliths. Structural formulae calculated on the basis of 4 anions oxygen.
825
827
Table 5. Electron microprobe chemical analyses of orthopyroxene in xenoliths. Structural formulae calculated on the basis of 6 anions oxygen.
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828
830
Table 6. Electron microprobe chemical analyses of clinopyroxene in xenoliths. Structural formulae
M AN U
829
calculated on the basis of 6 anions oxygen.
831
833
Table 7. Electron microprobe chemical analyses of spinel in xenoliths. Structural formulae calculated on the basis of 32 anions oxygen.
TE D
832
RI PT
823
834
Table 8. Estimated temperature (°C), pressure (GPa) and corresponding depth (km) of ultramafic
836
xenoliths. References: BK (Brey and Köhler, 1990), WS (Witt-Eickschen and Seck, 1991),
837
SS (Sachtleben and Seck, 1981), M (Mercier, 1980), WO (Witt-Eickschen and O’Neill,
838
2005).
AC C
839
EP
835
ACCEPTED MANUSCRIPT Table 1. cpx NG137 NG115 NG115 NG125 mic ph.c ph.r ml ml 0.94 47.39 47.61 47.50 43.07 21.93 2.31 2.42 2.44 4.02 2.68 5.59 5.20 5.11 8.87 0.24 0.37 0.15 0.14 0.27 65.34 6.56 7.33 7.21 8.31 0.94 0.16 0.06 0.17 0.14 4.01 14.04 13.38 13.57 11.31 0.24 21.50 22.57 22.03 22.19 0.50 0.43 0.46 0.52
EP
TE D
M AN U
SC
RI PT
pl mt xenocryst xenocryst xenocryst NG115 NG125 NG137 NG115 NG137 NG115 NG125 ph.c ph.c ph.c ml ml mic mic 38.67 39.23 39.74 40.08 50.15 50.89 0.08 0.69 21.89 21.79 30.91 30.60 1.75 1.58 0.12 0.17 16.48 15.01 9.99 10.19 0.64 0.49 67.79 65.08 0.19 0.27 0.19 0.12 0.90 1.03 41.33 44.52 47.99 48.13 3.54 3.41 0.16 0.09 0.08 0.09 13.56 13.13 0.07 0.70 3.69 3.94 0.15 0.22 0.22 0.23 0.33 0.32 97.05 99.35 98.32 98.94 99.22 99.22 96.150 94.384 1.009 0.993 0.994 0.995 2.306 2.334 0.024 0.206 4.813 4.872 1.676 1.654 0.603 0.552 0.029 0.040 5.527 5.081 0.360 0.318 0.209 0.212 0.022 0.017 11.047 11.082 0.004 0.006 0.000 0.000 0.224 0.259 1.608 1.680 1.789 1.781 1.545 1.513 0.004 0.003 0.002 0.002 0.668 0.645 0.022 0.224 0.33 0.351 0.009 0.013 0.005 0.005 0.007 0.006 0.9 1.3 32.8 34.8 66.3 63.9 81.5 83.9 89.5 89.4 61.5 64.5
96.331 0.272 4.768 0.911 0.052 4.793 11.009 0.231 1.727 0.073
98.14 1.783 0.065 0.248 0.011 0.081 0.116 0.005 0.788 0.867 0.036
AC C
phase ol type sample NG115 NG125 description ph.r mic ph.c SiO2 wt % 38.53 38.10 39.05 TiO2 Al2O3 Cr2O3 FeO 17.58 20.08 15.54 MnO 0.30 0.38 0.22 MgO 42.10 40.34 44.32 CaO 0.12 0.23 0.10 Na2O K2O NiO 0.20 0.22 0.24 Sum 98.84 99.35 99.48 Si (apfu) 0.993 0.989 0.990 Ti Al Cr Fe3+ Fe2+ 0.379 0.436 0.329 Mn 0.000 0.000 0.000 Mg 1.618 1.562 1.675 Ca 0.003 0.006 0.003 Na K Ni 0.004 0.005 0.005 Or (%) Ab An Fo (%) 81.0 78.2 83.6 Mol % Usp Wo (%) En Fs
0.04 99.20 1.786 0.068 0.230 0.004 0.087 0.133 0.002 0.749 0.907 0.031
0.02 98.65 1.790 0.069 0.227 0.004 0.084 0.134 0.006 0.763 0.890 0.033
0.03 98.72 1.639 0.115 0.398 0.008 0.124 0.127 0.004 0.642 0.904 0.038
0.001
0.001
0.001
46.8 45.9 7.3
45.7 46.8 7.5
46.8 45.7 7.5
NG137 ml ph.c 46.96 46.75 2.50 2.41 5.43 6.44 0.05 0.50 7.83 6.48 0.15 0.09 13.25 13.60 21.93 22.27 0.49 0.52
98.58 1.775 0.071 0.242 0.002 0.101 0.135 0.005 0.746 0.888 0.036
0.06 98.76 1.749 0.068 0.284 0.015 0.106 0.085 0.003 0.758 0.892 0.038 0.002
64.5 49.0 44.5 6.5
45.7 46.8 7.5
51.4 43.7 4.9
45 835 539 372 303.0 56.0 124.0 23 29.5 332 7.0 5.20 5.10 1.50
AC C
EP
(ppm) Rb Sr Ba V Cr Co Ni Cu Y Zr Hf Ta Th U La Ce Nd Sm Eu Gd Tb Dy Er Yb Lu
8.8 14.0 29.2
9.4 15.0 30.3
45 967 564 319 25.5 43.1 23.6 31 32.1 435 9.3 5.86 6.12 1.77
61 996 663 309 195.0 37.0 93.0 24 35.2 455 8.9 7.10 6.80 2.00
52.2 98 48 10.3 2.98 9.53 0.970 5.80 2.54 1.95 0.28
53.1 115 56 11.4 3.56 9.67 1.314 6.88 2.91 2.29 0.34
68.8 131 63 12.8 3.67 9.95 1.200 6.89 2.91 2.35 0.35
SC
11.4 19.6 21.2
NG137 42.53 4.16 13.82 13.28 0.24 6.26 10.36 3.40 0.91 1.07 4.16 100.19 3.94
TE D
Ne norm Ol norm D.I.
NG125 41.66 4.50 14.59 14.23 0.20 6.57 10.20 3.55 0.92 0.87 1.78 99.07 4.55
M AN U
lava basanite sample NG115 SiO2 (wt %) 39.98 TiO2 3.89 Al2O3 12.62 Fe2O3 15.18 MnO 0.20 MgO 10.05 CaO 11.11 Na2O 2.76 K2O 1.13 P2O5 0.68 LOI 1.28 Total 98.88 Na2O/K2O 2.44
RI PT
ACCEPTED MANUSCRIPT
Table 2.
ACCEPTED MANUSCRIPT
Table 3. sample
olivine
clinopyroxene orthopyroxene spinel
lherzolite
NK115
64
25
10
1
NK113
63
23
12
2
NK117
67
19
13
1
NK119
67
19
12
2
NK125
85
3
10
NK120
76
2
20
NK122
82
5
12
NK123
75
4
ol. websterite NK137
31
40
NK139
27
NK136
21
NK134
25
EP AC C
SC
2
2 1 3
26
3
39
31
3
39
38
2
38
35
2
M AN U
18
TE D
harzburgite
RI PT
xenoliths
ACCEPTED MANUSCRIPT
Table 4.
40.01 10.20 0.18 48.19 0.09 0.40 99.08
39.76 10.29 0.20 47.84 0.07 0.45 98.61
39.98 39.64 39.94 40.04 10.32 9.93 10.03 10.28 0.17 0.14 0.12 0.18 48.09 48.27 48.52 48.27 0.12 0.12 0.12 0.09 0.32 0.30 0.33 0.33 99.00 98.40 99.06 99.19
40.22 10.28 0.10 48.46 0.06 0.37 99.50
39.74 10.50 0.13 48.47 0.04 0.39 99.27
(a.p.f.u) Si Fe2+ Mn Mg Ca Ni
0.994 0.992 0.993 0.212 0.217 0.215 0.003 0.004 1.784 1.785 1.781 0.003 0.001 0.002 0.008 0.007 0.009
0.986 0.990 0.991 0.993 0.216 0.207 0.210 0.213
0.994 0.213
0.987 0.218
1.794 1.797 1.791 1.784 0.003 0.003 0.003 0.002 0.007 0.006 0.007 0.007
1.786 0.002 0.007
1.795 0.001 0.008
0.89
0.89
0.89
0.89
0.89
EP AC C
olivine websterite NK137
SC
M AN U
39.93 10.38 0.14 47.93 0.05 0.67 99.09
harzburgite NK125
0.89
TE D
Fo
lherzolite NK115
RI PT
rock sample (wt %) SiO2 FeO* MnO MgO CaO NiO Total
0.90
0.90
0.89
ACCEPTED MANUSCRIPT Table 5. lherzolite harzburgite NK115 NK125 54.69 54.70 54.31 52.61 52.55 0.16 0.12 0.14 0.19 0.18 4.46 4.34 4.49 5.83 6.20 0.24 0.22 0.29 0.58 0.49 6.76 6.70 6.70 6.81 6.82 0.13 0.16 0.12 0.20 0.17 32.36 32.14 32.25 31.04 30.98 0.73 0.74 0.75 1.28 1.24 0.10 0.10 0.10 0.18 0.18 99.63 99.23 99.15 98.72 98.81
52.63 0.21 6.07 0.48 6.98 0.14 31.17 1.32 0.20 99.21
olivine websterite NK137 54.81 54.56 54.93 0.11 0.11 0.14 3.75 4.07 3.84 0.23 0.24 0.16 6.61 6.70 6.54 0.19 0.08 0.19 32.89 32.80 32.67 0.51 0.59 0.54 0.10 0.08 0.07 99.19 99.21 99.09
(a.p.f.u.) Si Ti Al Cr Fe3+ Fe2+ Mn Mg Ca Ni Na
1.898 0.004 0.182 0.007 0.014 0.180 0.004 1.674 0.027 0.003 0.007
1.899 0.003 0.179 0.006 0.017 0.177 0.005 1.676 0.028 0.003 0.007
1.4 89.0 9.6 0.90
1.5 89.1 9.4 0.90
SC
1.848 0.005 0.241 0.016 0.049 0.145 0.006 1.625 0.048 0.004 0.012
1.844 0.005 0.256 0.013 0.046 0.149 0.005 1.620 0.047 0.003 0.012
1.840 0.006 0.250 0.013 0.059 0.138 0.004 1.624 0.049 0.003 0.013
1.901 0.003 0.155 0.006 0.039 0.150 0.006 1.713 0.019 0.003 0.006
1.899 0.003 0.167 0.007 0.029 0.163 0.002 1.702 0.022 0.002 0.005
1.918 0.004 0.150 0.005 0.008 0.182 0.006 1.701 0.020 0.003 0.005
1.5 89.4 9.1 0.91
2.6 89.4 8.0 0.92
2.6 89.2 8.2 0.92
2.7 89.7 7.6 0.92
1.0 91.0 8.0 0.92
1.2 90.2 8.6 0.91
1.0 89.4 9.6 0.90
TE D
M AN U
1.892 0.004 0.185 0.008 0.023 0.170 0.003 1.678 0.028 0.003 0.007
EP
AC C
Wo En Fs Mg#
RI PT
rock sample SiO2 (wt%) TiO2 Al2O3 Cr2O3 FeO* MnO MgO CaO Na2O Total
ACCEPTED MANUSCRIPT
Table 6.
51.48 0.59 4.38 0.94 4.02 0.11 16.64 20.90 0.57 99.62
(a.p.f.u.) Si Ti Al Cr Fe3+ Fe2+ Mn Mg Ca Na
1.845 0.017 0.192 0.028 0.098 0.016 0.003 0.925 0.835 0.041
1.863 0.018 0.279 0.021 0.066 0.020 0.001 0.800 0.803 0.129
1.872 0.017 0.261 0.021 0.067 0.016 0.005 0.808 0.804 0.128
1.868 0.018 0.266 0.021 0.068 0.018 0.003 0.795 0.811 0.128
1.866 0.015 0.258 0.020 0.081 0.008 0.003 0.822 0.802 0.122
43.3 56.6 0.1 0.98
49.5 49.3 1.2 0.98
49.4 49.6 1.0 0.98
49.9 49.0 1.1 0.98
49.2 50.3 0.5 0.99
1.873 0.014 0.279 0.022 0.044 0.057 0.004 0.815 0.773 0.119
1.830 0.014 0.327 0.023 0.073 0.050 0.003 0.874 0.695 0.110
1.823 0.014 0.330 0.025 0.083 0.042 0.002 0.878 0.688 0.114
46.9 50.4 2.7 0.95
46.9 51.1 2.0 0.96
46.8 50.0 3.2 0.94
47.0 49.6 3.4 0.94
37.2 59.4 3.4 0.95
36.9 60.3 2.9 0.95
TE D
EP
SC
1.878 0.012 0.270 0.017 0.052 0.053 0.002 0.825 0.772 0.120
M AN U
1.867 0.012 0.276 0.020 0.071 0.032 0.002 0.828 0.761 0.127
AC C
Wo En Fs Mg#
1.863 0.013 0.291 0.019 0.058 0.045 0.002 0.823 0.766 0.120
olivine websterite NK137 51.94 51.81 51.87 0.65 0.61 0.64 6.43 6.05 6.16 0.71 0.72 0.74 3.01 2.94 3.04 0.04 0.16 0.09 14.55 14.80 14.53 20.32 20.49 20.61 1.80 1.80 1.80 99.45 99.39 99.47
RI PT
rock lherzolite harzburgite sample NK115 NK125 SiO2(wt%) 51.83 51.61 51.87 51.85 51.71 50.77 TiO2 0.47 0.43 0.44 0.52 0.51 0.52 Al2O3 6.75 6.41 6.33 6.49 7.53 7.64 Cr2O3 0.67 0.70 0.58 0.75 0.78 0.86 FeO* 3.56 3.64 3.64 3.46 4.24 4.40 MnO 0.06 0.07 0.07 0.12 0.09 0.06 MgO 15.07 15.17 15.27 14.96 15.92 16.07 CaO 19.50 19.51 19.89 19.75 17.62 17.54 Na2O 1.69 1.79 1.70 1.68 1.54 1.60 Total 99.59 99.32 99.80 99.57 99.94 99.46
51.82 0.54 5.93 0.68 3.19 0.10 14.95 20.31 1.70 99.24
ACCEPTED MANUSCRIPT
Table 7.
0.81 0.09
AC C
Mg# Cr#
EP
TE D
M AN U
SC
RI PT
rock lherzolite harzburgite olivine websterite sample NK115 NK125 NK137 SiO2 (wt%) 0.01 0.04 0.04 0.03 0.08 0.11 0.02 0.04 TiO2 0.14 0.14 0.14 0.36 0.33 0.29 0.06 0.18 Al2O3 57.00 57.14 57.18 53.94 54.15 53.98 56.76 55.36 Cr2O3 8.81 8.69 9.15 11.13 11.13 11.17 10.23 10.65 FeO 12.40 12.42 12.35 13.94 13.40 13.44 12.05 12.23 MnO 0.11 0.13 0.08 0.11 0.01 0.10 0.12 0.15 MgO 20.78 20.74 20.81 20.51 20.31 20.62 20.30 20.72 CaO 0.02 0.04 0.01 0.01 Na2O 0.02 0.01 NiO 0.38 0.36 0.38 0.41 0.43 0.43 0.36 0.35 total 99.65 99.69 100.18 100.44 99.87 100.15 99.90 99.69 (a.p.f.u.) Si 0.003 0.009 0.008 0.006 0.016 0.022 0.003 0.009 Ti 0.022 0.022 0.022 0.057 0.052 0.046 0.010 0.028 Al 13.934 13.958 13.912 13.274 13.378 13.299 13.893 13.606 Cr 1.445 1.424 1.494 1.838 1.844 1.846 1.679 1.755 Fe3+ 0.572 0.556 0.535 0.762 0.647 0.719 0.401 0.570 Fe2+ 1.515 1.535 1.538 1.587 1.630 1.551 1.647 1.499 Mn 0.019 0.022 0.014 0.020 0.002 0.018 0.020 0.027 Mg 6.425 6.410 6.404 6.384 6.348 6.426 6.286 6.442 Ca 0.000 0.004 0.009 0.003 0.003 0.000 0.000 0.000 Na 0.001 0.000 0.000 0.000 0.007 0.000 0.000 0.005 Ni 0.064 0.061 0.064 0.069 0.073 0.073 0.060 0.059 0.81 0.09
0.81 0.09
0.80 0.12
0.80 0.12
0.81 0.12
0.79 0.11
0.81 0.11
ACCEPTED MANUSCRIPT
Table 8. Xenolith type
lherzolite
Thermometers in °C
NK115
TBK (Ca-opx)
harzburgite
NK117
NK125
979
984
1125
929
883
1107
912
919
891
TM (opx)
1054
1055
1156
TM (cpx)
998
994
1151
TWO (opx)
979
PM (opx)
1.9
olivine websterite
NK128 1117
NK137
NK139
902
931
TWS (Al-Ca-opx) ± 25 ºC TSS (Al-opx)
M AN U
62.7
EP AC C
902
913
880
940
928
1162
1001
1005
992
821
843
986
1125
1117
902
916
1.8
0.8
1.0
2.5
1.9
59.4
26.4
33.0
82.5
62.7
TE D
depth (km)
1081
SC
± 25 ºC
RI PT
± 19 °C
ACCEPTED MANUSCRIPT
Chad 16° Lake
12°
8°
N
ue
Be
Mboutou Garoua
nu
Poli
ug Tro
e
Ngaoundéré Mayo Dark
Mt Manengouba Mt Etinde
4°
Mt Cameroon
Bioko
ey
frica ral A Cent r Zone Shea
Béka
awa
Adam
Mt Bambouto
Borongo Makan
Va ll
Tchegui
Mt Oku
10°
6°
14°
aga San ne r Zo a e Sh
Lac Bini
Yaoundé
Upper Flow Unit Middle Flow Unit
TE D
Mio-Pliocene basaltic lava flow
Lower Flow Unit Cretaceous volcanism Basement rock Road Locality 13°43'
Dang
Principe 7°23' Säo Tomé
Pagalu
EP 0
Madep
Ngao Sey
Ngaoundéré
Mbalang Lake
Foulféké
7°20' Towards Nganha
N
10°
AC C
6°
Pyroclastic deposit
Lake Dang
Gulf of Guinea
0°
RI PT
Upper
h
Ben
Wasa Golda Zuelva
Swampy area with Quaternary alluvium
SC
8°
Bambi
Kapsiki Plateau
Xenolith sample location
7°35'
M AN U
200 km
Biu Plateau
Towards Nyassar
ola T rough
Congo Craton
Gong
0°
12°
13°41'
Towards Tignère
West African Craton
Towards Garoua
13°30' 7°35'
Mbalang-Djalingo
20 km
7°17' 13°38'
Towards Bélel
13°51
ACCEPTED MANUSCRIPT xenoliths host lava NG 115 (lherz.) NG 125 (harz.) NG 137 (ol webs.)
100
Cs
Th Ba
Nb U
K Ta
Ce La
P Pr
Sr Nd
SC
Rb
Hf
Sm
Ti
Zr
M AN U
1
a
Gd
Eu
Dy Tb
Yb Y
Lu
TE D
Sample / Primitive Mantle
10
RI PT
100
10
AC C
EP
b
1
La
Ce
Pr Nd Sm Eu Gd Tb Dy Ho
Er Tm Yb Lu
ACCEPTED MANUSCRIPT
This study Mt. Cameroon (Wandji et al., 2009)
Ol
Lake Nyos, Cameroon (Temdjim, 2012) Lake Nji, Cameroon (Princivalle et al., 2000)
RI PT
Kumba, Cameroon (Teitchou et al., 2007)
Dunite
Kapsiki plateau, Cameroon (Tamen et al., 2015)
90
Nosy Be Archipelago, Madagascar (Rocco et al., 2013)
123
M AN U
ur rzb
115
TE D
113
ite
Ha
119
hrl We
git e
117
Peridotite
120
SC
125
122
Lherzolite
EP
rox py ho ort ne ivi Ol
AC C
Clinopyroxenite
Orthopyroxenite
Websterite
Pyroxenite
ite
Olivine websterite
en rox
134
py no
Opx
139
cli
5
136
ine
10
137
iv Ol
en it
e
40
10 5 Cpx
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
ACCEPTED MANUSCRIPT
50
Wo
M AN U
this study lherzolite harzburgite olivine websterite host lava
TE D
augite
AC C
EP
20
En
hedenbergite
SC
diopside
45
5
RI PT
Wo
enstatite
Cameroon Volcanic Line Sao Tomé (Caldeira and Munha, 2002) Lake Nyos (Temdjim, 2012)
pigeonite ferrosilite
Fs
Cr2O3 Opx (wt %)
ACCEPTED MANUSCRIPT
0.7
a
0.6 0.5 0.4
0.2 0.1 0.0 3.0
3.5
4.0
4.5
5.0
5.5
RI PT
0.3
6.0
6.5
SC
57.5
M AN U
57.0 56.5 56.0 55.5 55.0 54.0 53.5 3.0
3.5
4.0
4.5
EP
53.0
TE D
54.5
AC C
Al2O3 Sp (wt %)
Al2O3 Opx (wt %)
5.0
b
lherzolite harzburgite olivine websterite
5.5 6.0 6.5 Al2O3 Opx (wt %)
ACCEPTED MANUSCRIPT lherzolite this study: 1.0
dunite (Wandji et al., 2009)
lherzolite (Arai, 1994)
wehrlite (Wandji et al., 2009) lherzolite (Rocco et al., 2013) harzburgite (Rocco et al., 2013)
harzburgite (Temdjim, 2012)
lherzolite (Caldeira and Munha, 2002)
websterite (Temdjim, 2012)
90
Fo olivine
AC C
EP
TE D
95
M AN U
SC
this study
harzburgite (Caldeira and Munha, 2002)
RI PT
0.5
0.0
olivine websterite
harzburgite (Arai, 1994)
lherzolite (Temdjim, 2012)
A M OS
Cr = Cr/(Cr+Al) spinel
harzburgite
85
80
100 * Fe2+ / Fe2+ + Mg
ACCEPTED MANUSCRIPT
21.5
a
21.0 20.5 20.0
RI PT
19.5 19.0 18.5 11
12 13 100 * Cr / Cr + Al
SC
10
13.0
M AN U
12.5 12.0 11.5 11.0
10.0 9.5 9.0
0.1
EP
0.0
TE D
10.5
AC C
100 * Cr / Cr + Al
9
0.2
b
lherzolite harzburgite olivine websterite
0.3 TiO2 (wt %)
0.4
ACCEPTED MANUSCRIPT NW
Bambi (harzburgite)
Depth (km)
Ngao Sey (lherzolite)
Ngaoundéré
F
F
Foulféké (olivine SE websterite)
F
F
F
00
F
P (kb)
RI PT
00
continental crust
10 Moho discontinuity
30
sub-lithospheric mantle
T>1000°C
20
120 mantle uplift
M AN U
zone of xenolith extraction 800
90
SC
60
30 asthenosphere lithosphere mantle boundary 40
asthenosphere
150
TE D
zone of mantle melting
network fault cross cutting the granitoid basement down to the mantle basaltic liquid
AC C
EP
sheared mantle lithosphere of harzburgitic composition
continental crust alkali volcanism dyke of websterite composition lherzolitic lithosphere asthenosphere
50
1
ACCEPTED MANUSCRIPT
Highlights are:
Petrology of newly discovered ultramafic xenoliths vs. host lava, in Adamawa Plateau Study of mineralogical phases and equilibrium conditions
AC C
EP
TE D
M AN U
SC
RI PT
Discussion on mantle/lithosphere composition and melting
S1464-343X(15)30010-8
DOI:
10.1016/j.jafrearsci.2015.07.004
Reference:
AES 2315
To appear in:
Journal of African Earth Sciences
Received Date: 6 January 2015 Revised Date:
12 June 2015
Accepted Date: 3 July 2015
Please cite this article as: Nkouandou, O.F., Bardintzeff, J.-M., Fagny, A.M., Sub-continental lithospheric mantle structure beneath the Adamawa plateau inferred from the petrology of ultramafic xenoliths from Ngaoundéré (Adamawa plateau, Cameroon, Central Africa), Journal of African Earth Sciences (2015), doi: 10.1016/j.jafrearsci.2015.07.004. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
1
ACCEPTED MANUSCRIPT
1
Sub-continental lithospheric mantle structure beneath the Adamawa plateau inferred from the
2
petrology of ultramafic xenoliths from Ngaoundéré (Adamawa plateau, Cameroon, Central Africa)
3 4
Oumarou F. Nkouandou1, Jacques-Marie Bardintzeff2, 3, 4, *, Aminatou M. Fagny1
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Ngaoundéré, Cameroon.
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Univ Cergy-Pontoise, ESPE-IE, F-95000 Cergy-Pontoise, France.
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Univ Paris-Sud, Géosciences, Volcanologie, Planétologie, Bât. 504, F-91405 Orsay, France.
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CNRS, UMR GEOPS 8148, F-91405 Orsay, France.
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Department of Earth Sciences, Faculty of Sciences, University of Ngaoundéré, P.O. Box 454
*Corresponding author: [email protected]
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Abstract
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Ultramafic xenoliths (lherzolite, harzburgite and olivine websterite) have been discovered in
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basanites close to Ngaoundéré in Adamawa plateau. Xenoliths exhibit protogranular texture
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(lherzolite and olivine websterite) or porphyroclastic texture (harzburgite). They are composed of
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olivine Fo89-90, orthopyroxene, clinopyroxene and spinel. According to geothermometers,
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lherzolites have been equilibrated at 880 to 1060 °C; equilibrium temperatures of harzburgite are
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rather higher (880―1160 °C), while those of olivine websterite are bracketed between 820 and
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1010 °C. The corresponding pressures are 1.8―1.9 GPa, 0.8―1.0 GPa and 1.9―2.5 GPa,
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respectively, which suggests that xenoliths have been sampled respectively at depths of 59―63 km,
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26―33 km and 63―83 km. Texture and chemical compositional variations of xenoliths with
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temperature, pressure and depth on regional scale may be ascribed to the complex history
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undergone by the sub-continental mantle beneath the Adamawa plateau during its evolution. This
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may involve a limited asthenosphere uprise, concomitantly with plastic deformation and partial
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melting due to adiabatic decompression processes. Chemical compositional heterogeneities are also
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proposed in the sub-continental lithospheric mantle under the Adamawa plateau, as previously
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suggested for the whole Cameroon Volcanic Line.
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Keywords: lherzolite, harzburgite, olivine websterite, lithospheric mantle, Adamawa plateau,
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Cameroon
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1. Introduction: the Cameroon Volcanic Line
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The Cameroon Volcanic Line (CVL) in Central Africa (Figure 1) is an alignment of volcanoes and
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plutonic complexes, more than 1500 km long, trending mostly N30°E. It straddles the continent-
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ocean boundary, from Pagalú Island in the Atlantic Ocean, up to Lake Chad and Adamawa plateau
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(see the recent paper of Njome and de Wit, 2014 and references therein). This large province, in
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which graben and horst structures alternate (Nkouathio et al., 2002, 2008), has been active since
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Paleogene to Quaternary as illustrated by the recent activity of the Mt. Cameroon volcano (1999
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and 2000 eruptions, Déruelle et al., 2000). No spatial age progression is detected along the Line.
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The lithospheric mantle beneath the whole Cameroon Volcanic Line has been particularly studied
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through mantle xenoliths sampled by alkali basaltic lava. Significant works on the mantle xenoliths
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sampled along the CVL were performed from islands of Gulf of Guinea (São Tomé, Caldeira and
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Munhá, 2002; Bioko and Palagú, Matsukage and Oya, 2010), in CVL s.s. (Mount Cameroon,
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Ngounouno et al., 2001; Ngounouno and Déruelle, 2007; Wandji et al., 2009; Lake Mbarombi, Lee
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et al., 1996, and Kumba plain, Teitchou et al., 2007, close to Mt Cameroon; Lake Enep, Lee et al.,
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1996; Lake Nyos, Temdjim et al., 2004; Temdjim, 2012, and Lake Nji, Princivalle et al., 2000,
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areas, close to Mt Oku), until Biu plateau (Lee et al., 1996), Kapsiki plateau (Tamen et al., 2015,
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and neighbouring Liri region (Nguihdama, 2007). Most publications have focused on mineral
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chemistry of mantle xenoliths and their implication on the origin of the CVL.
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The aim of this contribution is to describe the petrography, mineralogy and mineral chemistry of a
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series of twelve representative ultramafic xenoliths newly discovered in Miocene basanitic lava
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flows of Ngaoundéré region (Figure 1). Compositional variations of xenoliths from one centre to
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another could reflect the structure and compositional variations within upper mantle. Then, possible
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vertical or horizontal structural variation and heterogeneities under the Adamawa SCLM will be
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assessed.
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2. Geological setting of Adamawa plateau
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2.1. Geology of the area: basement and volcanic formations
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The Adamawa plateau is a volcanic horst (of about 200 km wide) bounded north and south by Pan
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African N70°E-trending faults. The Adamawa plateau formed during the Tertiary and then was
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uplifted up to 1 km relatively to the surrounding areas (Okereke, 1988; Nnange et al., 2001).
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The Adamawa basement belongs to the Central Africa Pan African Fold Belt Chain, crosscut by
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N70°E-trending strike-slip fault system (Moreau et al., 1987). It is constituted mainly by
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metamorphic rocks crosscut by neoproterozoic granitoids with U–Pb ages of 615 ± 27 Ma and circa
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575 Ma (Tchameni et al. 2006; Ganwa et al., 2008). Whether the volcanism of the Adamawa
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plateau belongs to the Cameroon Volcanic Line or not is still debated (Gouhier et al., 1974, Fitton,
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1980; Aka et al., 2009) and remains the subject of ongoing discussion. Indeed, the CVL presents a
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distribution of volcanoes along a “Y” shape, where Adamawa represents the NE hand of “Y”. For
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some authors, Adamawa is a part of CVL but not for others as the Adamawa volcanoes may be
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related to the Pan African faults contrary to the CVL. Numerous types of volcanic rocks outcrop on
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Adamawa plateau: basanite, basalt, hawaiite, mugearite, benmoreite, trachyte, rhyolite (North of
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Ngaoundéré and Tchabal Mbabo), phonolite. Differentiated lavas yield peralkaline affinities
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(Nkouandou et al., 2008; Fagny et al., 2012).
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2.2. Geophysical data
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Various informations on crust and lithospheric mantle beneath the Adamawa plateau have been
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provided through (1) geophysical studies and (2) petrological studies. Geophysical results are
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obtained beneath the CVL, and particularly beneath the Adamawa, by gravimetry (Browne and
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Fairhead 1983; Fairhead and Okereke, 1988; Poudjom Djomani et al., 1992, 1997; Nnange et al.,
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2000, 2001) and seismology (Stuart et al., 1985; Tokam et al., 2010; Koch et al., 2012; De Plaen et
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al., 2014).
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Gravimetric data suggest a Moho between 18 and 30 km. Stuart et al. (1985) estimated the depth of
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Moho at 33 km using seismic refraction. According to Tokam et al. (2010), Moho discontinuity is
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between 33 and 36 km deep beneath Adamawa plateau, more precisely 33 km beneath Ngaoudéré
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area. De Plaen et al. (2014) evidenced the isotropic character of the upper mantle beneath the CVL.
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The lithosphere-asthenosphere boundary (LAB) is only 100 km deep beneath the CVL, against 250
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km beneath the Congo Craton. LVZ is 200 km deep beneath the CVL. Milelli et al. (2012) made
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laboratory experiments and invoked a lithospheric instability that may develop within the
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subcontinental lithospheric mantle at the edge of a continent.
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3. Analytical methods
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Major element analyses of host lavas were done using ICP-AES and trace elements using ICP-MS
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at CRPG, Nancy, France, following analytic procedures of Carignan et al. (2001).
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Modal proportions of the three major mineral phases (olivine, orthopyroxene, clinopyroxene) and
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one minor phase (spinel) in ultramafic xenoliths selected for this study were determined with a
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Scanning Electron Microscope (SEM), in the Laboratory of Physics, University of Alexandru Ioan
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Cuza, Iasi, România. Mineral analyses of host lavas and xenoliths were performed on a Camebax
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SX100 Microprobe at the service Camparis of the University of Pierre et Marie Curie, Paris 6,
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France. The operating conditions were an accelerating voltage and a beam current as follow: olivine
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and clinopyroxene: 15 kV and 40 nA, 20 s except Si for olivine (10 s) and Ti for clinopyroxene (30
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s); plagioclase, K-feldspath, nepheline and noseane: 15 kV, 10 nA, 10 s; titanomagnetite: 20 kV and
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40 nA, Si, Ca, Ni:10 s; Mn: 25 s; Cr; 15 s; Al: 30 s; Ti, Fe, Mg: 40 s. Standard used were a
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combination of natural and synthetic minerals. Data corrections were made using the PAP
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correction of Pouchou and Pichoir (1991).
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4. Petrology of host basanite
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4.1. Petrography
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Ultramafic xenoliths have been found in basanitic lavas. These host-lavas are OIB intraplate lava
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flows, and Miocene in age (Nkouandou et al., 2008; Fagny et al., 2012). They cover more than 30
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% of the surface (Lasserre, 1961) and three sequential eruptive units are distinguished: Lower Flow
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Unit (LFU), Middle Flow Unit (MFU) and Upper Flow Unit (UFL) (Figure 1). All sequences
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contain numerous peridotite xenoliths. Host-lavas exhibit fluidal porphyritic to glomeroporphyric
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textures. Phenocryst phases consist of euhedral to subhedral large olivine crystals (1.5 to 3 mm, 15
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to 20 volume %), euhedral clinopyroxene (1 to 2 mm, 15 %), Fe-Ti oxides (0.5 to 1 mm, 5 %), set
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in a groundmass (60 %) containing the same minerals and needle plagioclase microlites. Host-lavas
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are CIPW nepheline normative (4.5 < nenorm < 10.5).
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4.2. Mineralogy
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Chemical compositions of representative phases of host-basaltic lavas are listed in Table 1. Olivine
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phenocryst compositions vary from Fo 84 in the core to Fo 81 in the rim. Fo content of microcrysts
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and microphenocrysts (78 %) are not quite different. CaO contents (up to 0.23 wt %) are rather
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high, while NiO contents are less than 0.24 wt %. Olivine xenocrysts (especially in NG125 and
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NG137) are characterized by high Fo (89-90 wt %), high NiO (0.33 wt %) and low CaO (0.08 wt
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%) contents, typical of mantle peridotite (Takahashi, 1980). Clinopyroxene phenocrysts are
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diopside, according to Morimoto et al. (1988) classification, with Wo51.4―45.7En43.7―46.8Fs7.5―4.9.
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Some phenocrysts (Table 1) exhibit a fassaitic trend with high Al2O3 (up to about 9 wt %), TiO2 (4
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wt %) and CaO (22.5 wt %), like those described in the Plio-Quaternary undersaturated alkaline
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volcanic episode in the Noun plain (Wandji et al., 2000). No significant variations exist between
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cores and rims or between phenocrysts and microlites. TiO2 (2.4 wt %) and Al2O3 (5.2―6.4 wt %)
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contents are rather high, the highest value content being found in microlites (up to 4 wt % and 9 wt
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% respectively in basanite NG125 microlites). Plagioclase microlites described in NG115 and
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NG137 basalts are andesine in composition (Ab32.8―34.8 An66.3―63.9), with FeO* contents up to 0.7
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wt %. Fe-Ti oxides are ulvöspinel (65-68 wt % FeO*, 22 wt % TiO2, Usp mole % ranging between
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61 and 65). Numerous basanitic lavas with similar mineral compositions have been described all
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along the Cameroon Volcanic Line and the Adamawa plateau, including Ngaoundéré region.
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4.3. Geochemistry
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Whole-rock chemical analyses of xenoliths host lavas are listed in Table 2. They are sodic alkaline
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basanite (40-42.5 wt % SiO2, 3.9-4.5 wt % Na2O + K2O, Na2O/K2O > 2.0), with Thornton and
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Tuttle (1960) D.I. ranging from 21 to 30. Compositions are silica-undersaturated with CIPW-
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normative nepheline close to 10 wt %, typical of intraplate alkaline series, resulting from a low
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degree (< 5 %) of mantle partial melting. NG137 is altered/weathered as shown by high LOI of 4.2
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wt %.
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Low contents of transitional elements (Table 2, V: 309-372 ppm, Co: 37-56 ppm, Cr: 25-303 ppm,
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Cu: 23-31 ppm and Ni: 23-124 ppm) evidence that these xenoliths host-lavas do not present
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primitive characters, except NG115, Mg-, Cr- and Ni-rich though Co is too low. Indeed, the
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basanitic magma was not directly produced from upper mantle melting but was slightly
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differenciated as shown by D.I., increasing up to 30. It differs from rocks with high Ni (300-500
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ppm), Co (> 300 ppm) and Cr (500-700 ppm) contents, that typically crystallized from a primitive
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magma (Green et al., 1974). Sr, Ba and Rb contents are high, a typical feature of alkali basaltic lava
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series. Zr, Hf, Ta and Th contents are in the range of intraplate basaltic magmas, without any sign
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of crustal contamination; particularly the Zr contents (332-455 ppm) remain rather high, compared
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to low Zr contents in materials of crustal origin (173 ppm, Rudnick and Fountain, 1995).
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REE contents are in the same range as previous REE data from North and East of Ngaoundéré area
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(Nono et al., 1994; Nkouandou et al., 2008). Spidergrams of the host-basanitic lavas present very
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high incompatible element values (Figure 2), up to 100-200 times the mantle values. Positive
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anomalies are noted in Nb, Ta, Sr and Zr, veryfying the no-contribution of crustal materials.
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Negative anomalies, observed for K and Ti, suggest the presence of Ti-bearing phlogopite in the
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mantle source, as already mentioned by Nkouandou et al. (2010). The same conclusion has been
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reached for basaltic lavas from the Cameroon Volcanic Line (Ngounouno et al., 2000, 2003). The
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mantle source of host basanite lavas was probably melted at a low degree of a residual garnet
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mantle, as attested by high (Ce/Yb)n ratios (12-14), at more than 80 km depth. The occurrence of
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numerous mantle xenoliths and the absence of crustal contamination indicate rapid ascent of the
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host magmas from the upper mantle, probably through the Pan-African fault network that crosscut
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the entire Adamawa plateau lithosphere (Moreau et al., 1987).
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5. Petrology of mantle xenoliths
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5.1. Field data: mantle xenolith occurences
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Basanitic lavas have entrained numerous mantle peridotite xenoliths during their ascent to the
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Earth’s surface.
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The peridotite xenoliths, which are studied here, have been sampled in Mio-Pliocene eruptive
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products (Nkouandou et al., 2010), North (Bambi, 40 km north from Ngaoundéré; 7°33'30'' N,
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13°34' E, 1365 m), Center (Ngao Sey, 12 km east from Ngaoundéré; 7°18' N, 13°40' E, 1240 m)
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and East (Foulféké, 38 km east from Ngaoundéré; 7°18'10'' N, 13°45' E, 1160 m) of the
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Ngaoundéré region in Adamawa plateau (Figure 1). Ultramafic xenoliths sampled in these areas are
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respectively harzburgite at Bambi Upper Flow Unit, lherzolite at Ngao Sey Middle Flow Unit and
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pyroxenite (olivine websterite) at Foulféké Lower Flow Unit. They have been named following
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IUGS recommendation (Streckeisen, 1976) (Figure 3). In this study, the abbreviation “NG”
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corresponds to host xenoliths lavas while “NK” correspond to xenoliths (i.e., host lava NG137
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contains xenolith NK137).
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Published works on petrology, focused on the nature and composition of the lithospheric mantle
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beneath the Adamawa plateau are rare. Studies of spinel- and plagioclase-bearing lherzolites from
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Dibi volcano, 25 km south of Ngaoundéré, further south of our investigated area (Girod et al., 1984;
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Dautria and Girod, 1986) localized the Moho at 20 km below the Adamawa plateau. Lee et al.
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(1996) described rare occurrence of spinel-garnet-pargasite websterite in the Ngaoundéré plateau.
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Temdjim (2005) studied keliphytic garnet-bearing lherzolite from Youkou volcano (situated 15-20
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km SW of Ngaoundéré, that is outside of Figure 1 inset), evidencing retromorphic metasomatism by
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fluids. The most recent contribution concerns the equilibrium conditions and mantle characteristics
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inferred from the petrology of spinel lherzolite xenoliths and host basaltic lava from Ngao Voglar
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volcano (close to Bambi, Figure 1) (Nkouandou and Temdjim, 2011).
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5.2. Petrography
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Twelve samples have been selected. They belong to the group I of “ultramafic inclusions” defined
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by Frey and Prinz (1978) at San Carlos, Arizona. They are subdivided into three types, based on
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texture and mineral characteristics. Each type is, respectively, represented by samples NK115
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(lherzolite), NK125 (harzburgite) and NK137 (olivine websterite) (Table 3). Samples are chiefly
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composed of four phases, typical of mantle composition: olivine, clinopyroxene, orthopyroxene and
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spinel. They are rounded (NK125) to prismatic or angular in shape (NK115 and NK137), ranging
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from 10 to 15 cm in size, and are distinguished according to modal mineral proportions (Figure 3).
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Neither hydrous minerals (e.g., biotite, amphibole, or apatite) nor aluminous phases (plagioclase or
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garnet, but spinel is present) were observed, contrary to some previous findings in Adamawa
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(Dautria and Girod, 1986; Lee et al., 1996). All xenoliths display a sharp contact with host lavas.
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According to Mercier and Nicolas nomenclature (1975), transition from protogranular (lherzolite
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NK115 and olivine websterite NK137) to porphyroclastic texture (harzburgite NK125) was
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acquired by slightly increasing shear deformation. Such mechanism could have induced differences
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in mineral composition.
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NK115 is spinel lherzolite (Figure 3). It exhibits a protogranular texture and is composed of
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abundant olivine crystals (64 volume %), clinopyroxene (25 %), orthopyroxene (10 %) and spinel
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(1 %). Interstitial glass is absent. Olivine crystals are large (2 to 6 mm in size) and sometimes
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display clear deformation bands and parallel twin planes, a typical feature of mantle peridotite
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(Mercier and Nicolas, 1975; Conticelli and Peccerillo, 1990). They are iddingsitized along fine
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irregular cracks and show a sharp contact with host lava. Triple points are common between
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crystals, particularly olivine (Figure 4, photo 1). Elongated orthopyroxene crystals (1.5 to 2 mm in
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length) are often associated with olivine crystals at their edges and are anhedral with grayish color.
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Some crystals show parallel twin planes, while others present rounded or polygonal shapes without
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exsolution lamellae. Clinopyroxene crystals (1.5 to 2.5 mm) are anhedral and show regular molten
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boundaries (1 mm large) when they are in contact with groundmass of the host rock, probably
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acquired during their transport by the host lava. Some crystals present undulose extinction, while
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others are colorless. Many clinopyroxene crystals are interstitial between orthopyroxene and olivine
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and often associated with spinel crystals. Spinel crystals (0.7 to 1 mm) are brown in color and are
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interstitial between olivine and orthopyroxene crystal, or occur as intergranular crystals with
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clinopyroxene. Lherzolite samples strongly resemble type I lherzolites (63 modal % olivine, 18 %
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orthopyroxene, 16 % orthopyroxene, 2.5 % spinel), described by Dautria and Girod (1986) in the
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neighbouring Dibi volcano, except that they do not bear a small amount (< 1 %) of plagioclase.
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NK125 represents the harzburgite group (cpx < 5 modal %, after IUGS nomenclature, Figure 3). It
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presents a porphyroclastic texture with plastically deformed minerals (this texture may be
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considered as transitional between protogranular type and typical porphyroclastic texture, Mercier
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and Nicolas, 1975) and is composed of about 75-85 volume % olivine crystal, 10-20 % opx, less
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than 5 % cpx and 1-2 % spinel (Table 3). Olivine crystals (1.5 to 3 mm) are the most abundant
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phase. They are iddingsitized along cracks and show sharp contact with host lavas. Some crystals
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are intimately associated with orthopyroxene and/or present a thin molten edge. Occurrence of
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triple junctions is frequent between olivine-olivine and olivine-orthopyroxene crystals (Figure 4,
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photo 2). As noted in olivine description, some orthopyroxene grains intergrown with olivine
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present a vermicular shape or are associated to olivine by their edge. Clinopyroxene crystals
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resemble those of NK115 and show a thin molten edge, described as spongy clinopyroxene (Klügel,
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1998; Carpenter et al., 2002). They are 1.5 to 2.5 mm in size, light brown in color, and weakly
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pleochroic. No exsolution lamellae were observed in cpx and in opx crystals, but both crystals form
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frequent aggregates with spinel. Anhedral spinel crystals are brown in color and located within
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interstices between clinopyroxene and orthopyroxene minerals. They are elongated, or flattened,
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and sometimes display a holly-leaf shape. Their mode is < 2 volume % and no crystal is larger than
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2 mm in size. This texture may provide evidence of plastic deformation in upper mantle conditions
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as suggested by Basu (1977).
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NK137, a pyroxenite, has a composition of olivine websterite (olivine < 40 % modal, Figure 3) and
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displays a protogranular texture (Figure 4, photo 3). Composed of orthopyroxene, clinopyroxene,
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olivine, and spinel, it presents a sharp contact with host lava, sometimes separated by curvilinear
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boundaries. There is no basaltic liquid infiltration between crystals. Olivine crystals (2 mm to 5 mm
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in size) are fairly abundant (21 to 31 volume %). They are frequently iddingsitized in cracks and
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define triple point junctions. Some olivine grains present interpenetration edges with orthopyroxene
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and spinel crystals. Anhedral orthopyroxene crystals (1.5 to 3 mm in size and 26 to 38 %) are
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grayish to weakly greenish in color, and, frequently associated with olivine. Some crystals are
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deformed and kinked. Clinopyroxene crystals (2 to 3.5 mm in size and 38 to 40 %) are anhedral,
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without visible exsolution lamellae. They are brownish or yellowish in color and have porous or
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spongy textured rims consisting of glass and small clinopyroxene grains, which may result from
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partial melting effect within the host magma during transport to the surface (Glaser et al., 1999).
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Red-brown spinel crystals (0.5 to 1.5 mm in size and 2 to 3 volume %) are anhedral, and,
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crystallized in interstices. By comparison, Tamen et al. (2015) have observed a pyroxenite of
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websterite type (83 volume % clinopyroxene, 8 % orthopyroxene, 4 % spinel, only 4 % of olivine
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and less than 1 % of plagioclase) in nearby Kapsiki plateau. Note that a typical clinopyroxenite is
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described in Mount Cameroon, together with dunite (Wandji et al., 2009, Figure 3).
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5.3. Mineralogy
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Mineral compositions of xenoliths with porphyroclastic texture (harzburgite NK125) differ
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markedly from xenoliths with protogranular texture (lherzolite NK115, olivine websterite NK137).
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Mg-rich olivine crystals (Fo 89-90, Table 4) have a typical mantle origin. Olivine crystals of
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NK115 and NK137 are characterized by low CaO contents (< 0.09 wt %), suggesting a high
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pressure equilibration (Köhler and Brey, 1990). Higher CaO contents (0.12 wt %) are found in
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NK125 xenolith. On the contrary, NiO contents are higher (up to 0.7 wt %) in NK115 and NK137
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(up to 0.4 wt %) than in NK125 (0.3 wt %). High NiO contents (average of 0.402 wt % for 90
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samples) characterize xenoliths of mantle origin according to Sato (1977). Note that olivine
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xenocrysts found in host basalts NG125 and NG137 display the same characteristics. Similar
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olivine compositions have been described in mantle peridotites worldwide, including the Cameroon
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Volcanic Line (Lee et al., 1996; Princivalle et al., 2000; Caldeira and Munha, 2002; Temdjim et al.,
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2004; Teitchou et al., 2007).
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Orthopyroxene crystals (Table 5) are enstatite (En91―89 Fs7.6―9.6). Fairly high Wo contents (up to
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2.7 %) found in NK125 (Figure 5), reflect high temperature equilibration. CaO contents increase
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from NK137 (0.5 wt %) to NK125 (1.3 wt %), NK115 showing intermediate value (0.75). All
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crystals are TiO2-poor (0.11-0.21 wt %). Orthopyroxene in lherzolite NK115 and olivine websterite
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NK137 contains 0.16-0.29 wt % Cr2O3 and 3.75-4.49 wt % Al2O3. In harzburgite NK125, they are
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Cr- (up to 0.6 wt % Cr2O3) and Al- (up to 6.2 wt % Al2O3) richer. Cr# (= Cr/Cr+Al3+) range
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between 0.03 and 0.06. All crystals are highly magnesian (Mg# = 100*Mg/Mg+ΣFe2+ = 90-92),
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which is typical of type I ultramafic xenoliths defined by Frey and Prinz (1978). Orthopyroxene
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show higher Mg# than coexisting olivine (Mg#Opx > Mg#Ol), and a positive correlation between
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(Al2O3)opx and (Cr2O3)opx (Figure 6a). On the contrary, (Al2O3)spinel vs. (Al2O3)opx are inversely
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correlated from lherzolite to harzburgite type xenoliths (Figure 6b). Particular attention should be
287
paid to orthopyroxene of harzburgite NK125, as it is characterized by high Al2O3, CaO and Na2O
288
contents, which usually characterize more enriched xenoliths, while high Cr2O3 contents rather
289
suggest depleted features.
290
Clinopyroxene (Table 6) is diopside (Wo46.8-49.9 En49.0-51.1 Fs3.2-0.5) in NK115 and NK137.
291
Clinopyroxenes in NK125, is augite (Wo36.9-43.3 En56.6-60.3 Fs0.1-3.4, Table 6 and Figure 5) according
292
to the IMA classification (Morimoto et al., 1988). It contains 0.58 to 0.94 wt % Cr2O3, i.e. Cr
293
(a.p.f.u.) > 0.01 with a mean Cr# of 0.07. They are typical of Group I xenoliths as previously
294
distinguished by Frey and Prinz (1978). Mg-numbers (0.99-0.94) of clinopyroxene are higher than
295
of coexisting olivine and orthopyroxene (Mg# cpx> Mg# opx > Mg# ol). Clinopyroxene of
296
harzburgite is distinguished by high Al2O3 (up to 7.6 wt %) and FeO* (> 4.0 wt %) contents. Higher
297
Na2O (1.80 wt %) but lower TiO2 (< 0.45 wt %) and FeO* (3.5 wt %) contents are found in NK115
298
lherzolite vs. NK125 harzburgite. This is a typical feature of fertile lherzolites (Jacques and Green,
299
1980). A good correlation (figure not shown) can be seen between Al2O3 of cpx and Cr# of
300
coexisting spinel. Last, clinopyroxene resembles diopside described at Dibi volcano (Dautria and
301
Girod, 1986), particularly concerning Na (0.10-0.15) and Cr (0.01-0.03) amounts.
302
Spinel (Table 7) is Al-Cr-rich with 54-57 wt % Al2O3, and 9-11 wt % Cr2O3. TiO2 contents are
303
always low and less than 0.36 wt %. Cr# (= 100*Cr/Cr+Al) is relatively low and varies slightly (9
304
% in NK115, 12 % in NK125 and 11 % in NK137). Mg# remains high and homogeneous, with a
305
mean value of 0.8. Fe3+# is low (Fe3+# = 100*Fe3+/(Al+Cr+Fe3+) < 0.05), witness of low fO2
306
compatible with mantle origin (see discussions in Preß et al., 1986; Webb and Wood, 1986; Witt-
307
Eickschen and Seck, 1991). Note that the spinel compositions are rather homogeneous, contrary to
308
Kapsiki compositions (Tamen et al., 2015) that reveal high heterogeneity (17-55 wt % Al2O3, 11-29
309
wt % Cr2O3, 0.12-0.44 Cr#).
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The plot of Cr# versus Mg# of spinel (figure not shown) typically fall within the field of ultramafic
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xenoliths (according to Conticelli and Peccerillo, 1990) and exhibit a negative correlation from the
312
lherzolite to the harzburgite type xenoliths. This is confirmed as in spinel Cr# vs. olivine Fo plot
313
(Figure 7), all samples occupy the OSMA field (Arai, 1994). For comparison, dunite and wehrlite
314
samples from Mt. Cameroon (Wandji et al., 2009) as well as some lherzolite and harzburgite
315
samples from São Tomé (Caldeira and Munha, 2002), are out of OSMA field and reflect fractional
316
crystallization and accumulation processes. Cr partitioning between spinel and pyroxene yields the
317
relation Cr#sp > Cr#cpx > Cr#opx. Spinel plots in Fe# (= Fe2+/Fe2++Mg2+) versus Cr# (= Cr/Cr+Al)
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(Figure 8a) and Cr# versus TiO2 (Figure 8b) show a regular increase from lherzolite to olivine
319
websterite to harzburgite. Similar variations have been observed in the ultramafic xenoliths of Lake
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Nyos (Temdjim et al., 2004).
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5.4. Thermodynamic parameters
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Equilibrium conditions
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All studied ultramafic xenoliths exhibit the characteristics of differing, yet homogeneous mantle
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pockets as indicated by their petrographic features and mineral compositions. As previously noted,
326
there is a correlation between the texture and mineral composition in each xenoliths group,
327
particularly their Mg#, Cr# and Al2O3 content of orthopyroxene crystals. Each xenoliths group with
328
the same texture provide evidence of homogeneous equilibrium environment as illustrated in the
329
different variations diagrams where each group shows grouping points. This would suggest that
330
distinct parts or layers of the upper mantle under the Adamawa plateau, were sampled by Mio-
331
Pliocene Ngaoundéré basanite flow units. Lack of exsolution lamellae in pyroxene and the absence
332
of hydrous phases (amphibole, biotite and apatite) point out no late compositional modification
333
(particularly metasomatic events, Brearley et al., 1984). Low spinel Cr# vary according to xenolith
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textures. This could attest for the different partial melting degrees (Frey and Prinz, 1978) or
335
continuous depletion of the upper mantle (Dick and Bullen, 1984; Cabanes and Mercier, 1988).
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Equilibrium temperature
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Equilibrium temperature and pressure of representative ultramafic xenoliths have been estimated,
339
considering that those samples are homogeneous. In order to avoid influences due to exchange
340
between mineral phases and host lava, only compositions of cores of minerals have been
341
considered.
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Five geothermometers were used in the estimation of upper mantle temperature beneath the
343
Adamawa plateau, to get a good estimate of the uncertainties on the obtained numbers. They are:
344
(1) Witt-Eickschen and Seck (1991) geothermometer, based on solubility of Ca and Al in
345
orthopyroxene in equilibrium with olivine, clinopyroxene and spinel, in spinel peridotite (2)
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Mercier (1980) single pyroxene thermobarometry, (3) Brey and Köhler (1990) two-pyroxenes
347
thermometer, (4) Sachtleben and Seck (1981) Al-solubility in orthopyroxene, and (5) Witt-
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Eickschen and O’Neill (2005) Ca―Mg exchange between pyroxenes.
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The results are listed in Table 8. We have carefully selected 2 fresh samples in each group for a best
350
representation (lherzolites NK115 and 117, harzburgites NK125 and 128, and olivine websterites
351
NK137 and 139): textures are the same in each groupe, only percentages of mineral phases show
352
small differences. Calculations are made for a total mean pressure of 15 kbar (1.5 GPa) that
353
corresponds to the most probable mean value (O’Neill, 1981; Gasparik, 1987).
354
The temperatures range between 883 and 1055 °C for lherzolite, and between 821 and 1005 °C for
355
olivine websterite. Whatever the method used is, the highest temperatures (see Table 8) are always
356
found in harzburgite (between 880 and 1162 °C). Uncertainties are in the range of 20-25 °C for
357
geothermometers (1), (3) and (4). Harzburgites which exhibit porphyroclastic textures present
358
rather higher temperatures than lherzolites and websterites, which exhibit the same protogranular
359
textures: this is in agreement with mean values compiled by Mercier and Nicolas (1975), i.e. 1000-
360
1050 °C for protogranular peridotites against 1000-1260 °C for porphyroclastic peridotites.
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From these rather high temperatures, it is deduced that low temperature metasomatism did not
362
affect the xenoliths, confirming observations under the microscope (see discussions in Kourim et
363
al., 2014) who calculated low-temperature of 750-900 °C for lherzolite xenoliths in Hoggar,
364
Algeria.
365
Interestingly, all calculated equilibrium temperatures are within the temperature range (800 to
366
1200°C) estimated from ultramafic xenoliths along the whole Cameroon Volcanic Line, including
367
Mt. Cameroun, Lake Nyos and Adamawa plateau.
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Pressure
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Determination of real pressure of mantle spinel peridotite is unanimously (Medaris et al., 1999;
371
Christensen et al., 2001) recognized as unrealistic. All methods lead to dissatisfying results, because
372
application of experiments to natural systems remains more or less approximate. However, lack of
373
garnet and plagioclase in studied spinel lherzolite, puts limits of equilibrium pressure only within
374
the spinel lherzolite stability field, i.e. between 8 and 20 kbar (0.8-2 GPa) as previously suggested
375
(O’Neill, 1981; Gasparik, 1987). The single pyroxene thermobarometry of Mercier (1980) using
376
clinopyroxene composition (in the spinel facies) may be retained for pressure estimation, as
377
calculated results lay among those of spinel lherzolite of natural system (O’Neill, 1981; Gasparik,
378
1987). This barometer yields pressures of 1.8-1.9, 0.8-1.0, and 1.9-2.5 GPa, respectively for
379
lherzolite, harzburgite and olivine websterite, with corresponding temperatures (Table 8). Pressures
380
correspond to depths of 59-63, 26-33 and 63-83 km, respectively, when using the conversion factor
381
of 33 km × P (GPa). Results compare favorably with earlier estimations for the Cameroon Volcanic
382
Line (Teitchou et al., 2007; Wandji et al., 2009; Temdjim, 2012). They have been already being
383
used to constrain geophysical data on the Adamawa plateau (Nnange et al., 2000).
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6. Discussion
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6.1. Three types of enclaves, components of the lithosphere?
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Miocene basanitic lavas of the Ngaoundéré area, sampled three types of enclaves, each of them in a
388
precise location, several ten kilometers far from each other: (i) protogranular lherzolite, (ii)
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porphyroclastic harzburgite, (iii) protogranular olivine websterite.
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Both protogranular and porphyroclastic type textures were observed in the spinel lherzolite from
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other localities in the Adamawa plateau (Lee et al., 1996). Mercier and Nicolas (1975) classification
392
described a continuous suite from protogranular texture to equigranular texture and porphyroclastic
393
texture with increasing deformation. Note that our samples present contrasting textures, that do not
394
argue (or not) for continuous variations for Adamawa upper mantle, as suggested by Brown et al.
395
(1980) for French Massif Central. According to Ambeh et al. (1989), deformation might be related
396
to mantle uplift beneath Adamawa, as a consequence of upwards migration of lithosphere-
397
asthenosphere boundary during the Tertiary (Okereke, 1988). But it is unlikely that asthenosphere
398
can produce such a deformation. It is much more likely that deformation in lithosphere induced
399
asthenosphere uprise.
400
Garnet was not observed but suggested by clinopyroxene - spinel aggregates (Nicolas et al., 1987),
401
which might be interpreted as initial equilibration in garnet stability field, followed by
402
reequilibration in spinel stability field, during adiabatic upwelling. As a matter of fact, pink garnet
403
(mg number closed to 79) is described around grey/green spinel in two samples of spinel - garnet
404
websterite from Ngaoundéré plateau by Lee et al. (1996).
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Rocco et al. (2013) described Miocene basanites bearing the three types of co-existing peridotitic
406
enclaves - lherzolite, harzburgite and wehrlite - in the small (less than 4 km in diameter) Nosy
407
Sakatia Island, Madagascar. These authors pointed out the decrease of amount of modal
408
clinopyroxene
409
clinopyroxene/orthopyroxene ratio. This would evidence that xenoliths represent different sections
410
of the lithospheric mantle (vertical variation as these xenoliths are found very close each other, in
411
the same volcanic center) affected by variable degrees of melt extraction. In the case of Ngaoundéré
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area, we note variations with depths, but not on the same vertical line, because xenoliths are found
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from
lherzolite
to
harzburgite
as
well
as
the
decrease
of
the
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in different locations (both “horizontal” and “vertical” sampling). Like Rocco et al. (2013), we
414
noted increase of Cr# (0.09 to 0.12) in Al-Cr-spinel, a slight increase of Mg# in pyroxenes (0.90 to
415
0.92) from lherzolite to harzburgite. However we did not note clear variations of Fo content in
416
olivine (0.89 to 0.90).
417
In the French Massif Central, Coisy and Nicolas (1978) show that distribution of different textures
418
of xenoliths is not random but might suggest (1) the presence of distinct mantle bodies, or (2) the
419
presence of a single mantle materiel progressively deformed during upwelling, or even (3) a
420
compositional stratification of mantle.
421
Ultramafic (lherzolite, harzburgite, olivine websterite) xenoliths entrained in basanite during ascent
422
to the surface in Ngaoundéré region display features of sub-continental lithosphere fragments, as
423
suggested by Basu (1975) and Menzies et al. (1987).
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6.2 Mantle heterogeneity, melting and metasomatism
426
“Fertile” lherzolite within continental lithosphere does not reflect primitive mantle. More likely, it
427
corresponds to re-fertilized previously depleted mantle, which means at least one metasomatic
428
event. Harzburgite is more likely original depleted lithosphere mantle, while olivine websterite
429
corresponds almost certainly to mantle more metasomatized than lherzolite. Thus, we observe a
430
sequence, which substantiates increasing metasomatism, from harzburgite to lherzolite and olivine
431
websterite (with increasing clinopyroxene amount), either late Pan-African, or Tertiary in age.
432
From petrographical observations, spongy rims of clinopyroxene phase of harzburgite NK125 exist
433
in low modal contents (2-5 %). Numerous hypotheses explain spongy rims: (1) melt infiltration,
434
causing incongruent dissolution, probably during upwelling and incipient melting of low melting
435
point minerals (Rocco et al., 2013), (2) melting event after incorporation in host lava inducing
436
higher temperature and decompression (Lustrino et al., 1999; Carpenter et al., 2002; Shaw et al.,
437
2006). Otherwise, spinel, as all phases of this sample NK125, exhibit flattening and elongation,
438
which are signs of plastic deformation. NK125 is also characterized by high CaO content in olivine,
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low Mg# and Al2O3 in spinel and high Al2O3 and Cr2O3 in orthopyroxene. Moreover, Fe# and TiO2
440
content of spinel phases of all representative samples show an increasing evolution with Cr# from
441
lherzolite NK115 to harzburgite NK125 through olivine websterite NK137 (Figure 8). Hypothesis
442
(2) may be retained to explain the structure of the sub-continental mantle beneath the Adamawa
443
plateau, as it is strengthened by the progressive deformation textures coupled with the continuous
444
compositional chemical variations of their minerals. Furthermore, estimated depths show that
445
harzburgite type xenoliths have been sampled at rather shallow depth (26-33 km, that is to say close
446
below Moho), olivine websterite group xenoliths have been sampled between 60 and 80 km,
447
whereas lherzolite type xenoliths originated from nearly 60 km depth, that is to say in the middle-
448
lower part of the lithospheric mantle in the thermal boundary layer. This is confirmed by higher
449
CaO contents (0.12 wt %) in olivine of harzburgite NK125, witness of lower pressure. The
450
estimated depths fall within those of four calculated major density discontinuities (7-13 km and 19-
451
25 km corresponding to crustal contrasts, 30-37 km corresponding to Moho, 75-149 km
452
corresponding to mantle anomaly) which have been deduced from the spectral analysis of the
453
gravity data (Nnange et al., 2000) beneath the Adamawa plateau. Accordingly, harzburgite
454
xenoliths may have been sampled at the top of the mantle, e.g. between 26 and 33 km, value
455
obtained from thermodynamic calculations. Such depth is close below the Moho discontinuity,
456
assuming a crustal thickness below the Adamawa plateau of 33 km (Tokam et al., 2010).
457
The presence of a hot mantle dome beneath Adamawa at a depth of 60 km that has been proposed
458
(Browne and Fairhead, 1983; Fairehead, 1988) is a matter of debate (for discussion, see De Plaen et
459
al., 2014). The major evidence is the low P-wave velocity of 7.8 km/s (Stuart et al., 1985; Dorbath
460
et al., 1984, 1986). Ambeh et al. (1989) consider that crustal uplift results from LAB upwards
461
migration. Likewise, the same two contrasting models are advocated to explain the Hoggar swell, in
462
Algerian Sahara: (i) mantle plume-induced LAB upwards migration (Dautria et al., 1987), and (ii)
463
lithosphere delamination (Liégeois et al., 2005; Beccaluva et al., 2007; Bouzid et al., in press). The
464
latter model postulates linear/planar delamination along lithosphere-scale shear zones, inducing
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limited asthenosphere uprise, in agreement with normal heat flow and magnetotelluric data.
466
Topographic highs in Tibesti, in Chad, and Darfour, in Sudan, are currently the subject of similar
467
debates.
468
7. Conclusion
470
Two models could explain the volcanic activity in Adamawa plateau: (i) mantle plume-induced
471
LAB upwards migration and (ii) linear/planar delamination along lithosphere-scale shear zones,
472
inducing limited asthenosphere uprise. The second hypothesis is favoured according to a rather
473
normal heat flow. Moreover, volcanic activity in Adamawa plateau might be controlled by the
474
reworking of the Pan-African fault swarm, crosscutting both continental crust and upper mantle
475
down to a depth of 190 km.
476
Occurences of three types of xenoliths evidence geochemical heterogeneities of subcontinental
477
lithospheric mantle structure beneath the Adamawa plateau, as it was already invokated elsewhere
478
beneath the CVL.
479
On its way to the surface, basanitic liquid produced from a weak differentiation of a primary
480
magma would have sampled lherzolite upper mantle between 59 and 63 km (Figure 9). In the sub-
481
continental structure, olivine websterite exhibits features close to lherzolite (temperature and
482
pressure, depth of formation close to 60 km, occurrence of olivine, clinopyroxene, orthopyroxene
483
and spinel, and protogranular texture). We suggest that it may have been emplaced as dykes or sills,
484
crosscutting lherzolite mantle. Harzburgitic xenoliths evidence that sheared subcontinental
485
lithospheric mantle occurs at shallow depths (Figure 9).
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Acknowledgments. Authors warmly thank the “Agence Universitaire de la Francophonie (AUF)”
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through the BAGL (Bureau Afrique Centrale et des Grands Lacs), for financial support of “Le
489
Projet de soutien aux équipes de recherche 2012/2013_No 51110SU201“, for field works to
490
laboratory analyses. OFN and AMF spent three months in 2013 at the “Department of Earth
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Sciences, UMR 8148 GEOPS-CNRS” of the University of Paris-Sud XI, Orsay, France. L. Daumas
492
is thanked for drawing figures. B. Bonin and an other, anonymous, reviewer are thanked for useful
493
remarks.
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wave group velocities and receiver functions. Geophys. J. Int., 183, 1061-1076.
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Webb, S.A.C., Wood, B.J., 1986. Spinel–pyroxene–garnet relationships and their dependence on Cr/Al ratio. Contrib. Mineral. Petrol., 92, 4, 471–480.
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upper mantle peridotite. Chemical Geology, 221, 65–101.
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peridotite: an improved version of an empirical geothermometer. Contrib. Mineral. Petrol.
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106, 4, 431–439.
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Figure captions
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Figure 1. Sampled location and geological map of Ngaoundéré region after Nkouandou et al.
770
(2010), modified. The cross section presented in Figure 9 is localized (bold straight line). Left
771
inset shows the main volcanic zones: Cameroon Volcanic Line and Adamawa plateau. Upper
772
inset: relationships between Cameroon Volcanic Line and African cratons (Kampunzu and
773
Popoff, 1991).
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Sketch showing the possible structure of sub-lithospheric mantle beneath Ngaoundéré area in
775
Adamawa plateau (relief after topographic map NB 33 XX, scale 1/200000). This NW-SE
776
section is shown in Figure 1.
778
Figure 2. Host basanite lava, Primitive Mantle-normalized (McDonough and Sun, 1995) multi-
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element diagrams.
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Figure 3. Modal classification of Ngaoundéré ultramafic xenoliths, according to Streckeisen (1976). Data from Cameroon Volcanic Line and Madagascar are plotted for comparison
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Figure 4. Thin sections of the three types of peridotites observed with optical microscope under
784
crossed nicols. Photo 1 (NK115) Triple point junctions between curvilinear or slightly curved
785
boundaries of olivine and pyroxene in protogranular texture of lherzolite. Photo 2 (NK125)
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Strained grains of clinopyroxene with thin molten edge, orthopyroxene and olivine crystals in
787
porphyroclastic texture of harzburgite. Photo 3 (NK137) Olivine and pyroxene large crystals
788
in protogranular texture of olivine websterite (size of photos 1 and 2 = 8,8 x 6,5 mm, size of
789
photo 3 = 14,8 x 11,2 mm; cpx = clinopyroxene, ol = olivine, opx = orthopyroxene and sp =
790
spinel, abbreviation after Kretz, 1983).
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Figure 5. Chemical composition of ultramafic xenoliths pyroxene, projected in the En-Wo-Fs
793
triangle (after Morimoto et al., 1988). Xenoliths from other localities of Cameroon Volcanic
32
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Line (São Tomé, Caldeira and Munha, 2002; Lake Nyos, Temdjim, 2012) are added for
795
comparison.
796
Figure 6. Variation diagrams illustrating increasing Cr2O3 versus Al2O3 of orthopyroxene (a) and
798
decreasing (Al2O3)Sp versus (Al2O3)Opx content (b), from olivine websterite and lherzolite to
799
harzburgite, explained by partial melting processes.
800
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Figure 7. Plot of Cr# (Cr/(Cr+Al)) of spinel versus Fo of olivine in the Olivine Spinel Mantle Array
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(OSMA) diagram of Arai (1994).
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Figure 8. Fe# (= 100*Fe2+/Fe2++Mg) versus Cr# (= 100*Cr/Cr+Al) (a) and Cr# versus TiO2 (b) diagrams of spinel, showing an increase from lherzolite to harzburgite.
806
Figure 9. Sketch showing the possible structure of sub-lithospheric mantle beneath Ngaoundéré
808
area in Adamawa plateau (relief after topographic map NB 33 XX, scale 1/200000). This
809
NW-SE section is shown in Figure 1. Depth of Moho according to various geophysical data
810
(see text), depth of lithosphere-asthenosphere boundary (LAB) according to De Plaen et al.
811
(2014).
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Table captions
814
Table 1. Electron microprobe chemical analyses of representative minerals of host basanite. ph =
815
phenocryst, c = core, r = rim, ml = microlite, mic = microcryst, cpx = clinopyroxene, mt =
816
magnetite, pl = plagioclase, ol = olivine (abbreviation after Kretz, 1983).
817 818 819
Table 2. Whole-rock chemical analyses of host basanite of ultramafic xenoliths.
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820 821
Table 3. Modal proportions of mineral phases in ultramafic xenoliths, determined from SEM images of entire areas of thin sections.
822
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Table 4. Electron microprobe chemical analyses of olivine in xenoliths. Structural formulae calculated on the basis of 4 anions oxygen.
825
827
Table 5. Electron microprobe chemical analyses of orthopyroxene in xenoliths. Structural formulae calculated on the basis of 6 anions oxygen.
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Table 6. Electron microprobe chemical analyses of clinopyroxene in xenoliths. Structural formulae
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calculated on the basis of 6 anions oxygen.
831
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Table 7. Electron microprobe chemical analyses of spinel in xenoliths. Structural formulae calculated on the basis of 32 anions oxygen.
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Table 8. Estimated temperature (°C), pressure (GPa) and corresponding depth (km) of ultramafic
836
xenoliths. References: BK (Brey and Köhler, 1990), WS (Witt-Eickschen and Seck, 1991),
837
SS (Sachtleben and Seck, 1981), M (Mercier, 1980), WO (Witt-Eickschen and O’Neill,
838
2005).
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ACCEPTED MANUSCRIPT Table 1. cpx NG137 NG115 NG115 NG125 mic ph.c ph.r ml ml 0.94 47.39 47.61 47.50 43.07 21.93 2.31 2.42 2.44 4.02 2.68 5.59 5.20 5.11 8.87 0.24 0.37 0.15 0.14 0.27 65.34 6.56 7.33 7.21 8.31 0.94 0.16 0.06 0.17 0.14 4.01 14.04 13.38 13.57 11.31 0.24 21.50 22.57 22.03 22.19 0.50 0.43 0.46 0.52
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SC
RI PT
pl mt xenocryst xenocryst xenocryst NG115 NG125 NG137 NG115 NG137 NG115 NG125 ph.c ph.c ph.c ml ml mic mic 38.67 39.23 39.74 40.08 50.15 50.89 0.08 0.69 21.89 21.79 30.91 30.60 1.75 1.58 0.12 0.17 16.48 15.01 9.99 10.19 0.64 0.49 67.79 65.08 0.19 0.27 0.19 0.12 0.90 1.03 41.33 44.52 47.99 48.13 3.54 3.41 0.16 0.09 0.08 0.09 13.56 13.13 0.07 0.70 3.69 3.94 0.15 0.22 0.22 0.23 0.33 0.32 97.05 99.35 98.32 98.94 99.22 99.22 96.150 94.384 1.009 0.993 0.994 0.995 2.306 2.334 0.024 0.206 4.813 4.872 1.676 1.654 0.603 0.552 0.029 0.040 5.527 5.081 0.360 0.318 0.209 0.212 0.022 0.017 11.047 11.082 0.004 0.006 0.000 0.000 0.224 0.259 1.608 1.680 1.789 1.781 1.545 1.513 0.004 0.003 0.002 0.002 0.668 0.645 0.022 0.224 0.33 0.351 0.009 0.013 0.005 0.005 0.007 0.006 0.9 1.3 32.8 34.8 66.3 63.9 81.5 83.9 89.5 89.4 61.5 64.5
96.331 0.272 4.768 0.911 0.052 4.793 11.009 0.231 1.727 0.073
98.14 1.783 0.065 0.248 0.011 0.081 0.116 0.005 0.788 0.867 0.036
AC C
phase ol type sample NG115 NG125 description ph.r mic ph.c SiO2 wt % 38.53 38.10 39.05 TiO2 Al2O3 Cr2O3 FeO 17.58 20.08 15.54 MnO 0.30 0.38 0.22 MgO 42.10 40.34 44.32 CaO 0.12 0.23 0.10 Na2O K2O NiO 0.20 0.22 0.24 Sum 98.84 99.35 99.48 Si (apfu) 0.993 0.989 0.990 Ti Al Cr Fe3+ Fe2+ 0.379 0.436 0.329 Mn 0.000 0.000 0.000 Mg 1.618 1.562 1.675 Ca 0.003 0.006 0.003 Na K Ni 0.004 0.005 0.005 Or (%) Ab An Fo (%) 81.0 78.2 83.6 Mol % Usp Wo (%) En Fs
0.04 99.20 1.786 0.068 0.230 0.004 0.087 0.133 0.002 0.749 0.907 0.031
0.02 98.65 1.790 0.069 0.227 0.004 0.084 0.134 0.006 0.763 0.890 0.033
0.03 98.72 1.639 0.115 0.398 0.008 0.124 0.127 0.004 0.642 0.904 0.038
0.001
0.001
0.001
46.8 45.9 7.3
45.7 46.8 7.5
46.8 45.7 7.5
NG137 ml ph.c 46.96 46.75 2.50 2.41 5.43 6.44 0.05 0.50 7.83 6.48 0.15 0.09 13.25 13.60 21.93 22.27 0.49 0.52
98.58 1.775 0.071 0.242 0.002 0.101 0.135 0.005 0.746 0.888 0.036
0.06 98.76 1.749 0.068 0.284 0.015 0.106 0.085 0.003 0.758 0.892 0.038 0.002
64.5 49.0 44.5 6.5
45.7 46.8 7.5
51.4 43.7 4.9
45 835 539 372 303.0 56.0 124.0 23 29.5 332 7.0 5.20 5.10 1.50
AC C
EP
(ppm) Rb Sr Ba V Cr Co Ni Cu Y Zr Hf Ta Th U La Ce Nd Sm Eu Gd Tb Dy Er Yb Lu
8.8 14.0 29.2
9.4 15.0 30.3
45 967 564 319 25.5 43.1 23.6 31 32.1 435 9.3 5.86 6.12 1.77
61 996 663 309 195.0 37.0 93.0 24 35.2 455 8.9 7.10 6.80 2.00
52.2 98 48 10.3 2.98 9.53 0.970 5.80 2.54 1.95 0.28
53.1 115 56 11.4 3.56 9.67 1.314 6.88 2.91 2.29 0.34
68.8 131 63 12.8 3.67 9.95 1.200 6.89 2.91 2.35 0.35
SC
11.4 19.6 21.2
NG137 42.53 4.16 13.82 13.28 0.24 6.26 10.36 3.40 0.91 1.07 4.16 100.19 3.94
TE D
Ne norm Ol norm D.I.
NG125 41.66 4.50 14.59 14.23 0.20 6.57 10.20 3.55 0.92 0.87 1.78 99.07 4.55
M AN U
lava basanite sample NG115 SiO2 (wt %) 39.98 TiO2 3.89 Al2O3 12.62 Fe2O3 15.18 MnO 0.20 MgO 10.05 CaO 11.11 Na2O 2.76 K2O 1.13 P2O5 0.68 LOI 1.28 Total 98.88 Na2O/K2O 2.44
RI PT
ACCEPTED MANUSCRIPT
Table 2.
ACCEPTED MANUSCRIPT
Table 3. sample
olivine
clinopyroxene orthopyroxene spinel
lherzolite
NK115
64
25
10
1
NK113
63
23
12
2
NK117
67
19
13
1
NK119
67
19
12
2
NK125
85
3
10
NK120
76
2
20
NK122
82
5
12
NK123
75
4
ol. websterite NK137
31
40
NK139
27
NK136
21
NK134
25
EP AC C
SC
2
2 1 3
26
3
39
31
3
39
38
2
38
35
2
M AN U
18
TE D
harzburgite
RI PT
xenoliths
ACCEPTED MANUSCRIPT
Table 4.
40.01 10.20 0.18 48.19 0.09 0.40 99.08
39.76 10.29 0.20 47.84 0.07 0.45 98.61
39.98 39.64 39.94 40.04 10.32 9.93 10.03 10.28 0.17 0.14 0.12 0.18 48.09 48.27 48.52 48.27 0.12 0.12 0.12 0.09 0.32 0.30 0.33 0.33 99.00 98.40 99.06 99.19
40.22 10.28 0.10 48.46 0.06 0.37 99.50
39.74 10.50 0.13 48.47 0.04 0.39 99.27
(a.p.f.u) Si Fe2+ Mn Mg Ca Ni
0.994 0.992 0.993 0.212 0.217 0.215 0.003 0.004 1.784 1.785 1.781 0.003 0.001 0.002 0.008 0.007 0.009
0.986 0.990 0.991 0.993 0.216 0.207 0.210 0.213
0.994 0.213
0.987 0.218
1.794 1.797 1.791 1.784 0.003 0.003 0.003 0.002 0.007 0.006 0.007 0.007
1.786 0.002 0.007
1.795 0.001 0.008
0.89
0.89
0.89
0.89
0.89
EP AC C
olivine websterite NK137
SC
M AN U
39.93 10.38 0.14 47.93 0.05 0.67 99.09
harzburgite NK125
0.89
TE D
Fo
lherzolite NK115
RI PT
rock sample (wt %) SiO2 FeO* MnO MgO CaO NiO Total
0.90
0.90
0.89
ACCEPTED MANUSCRIPT Table 5. lherzolite harzburgite NK115 NK125 54.69 54.70 54.31 52.61 52.55 0.16 0.12 0.14 0.19 0.18 4.46 4.34 4.49 5.83 6.20 0.24 0.22 0.29 0.58 0.49 6.76 6.70 6.70 6.81 6.82 0.13 0.16 0.12 0.20 0.17 32.36 32.14 32.25 31.04 30.98 0.73 0.74 0.75 1.28 1.24 0.10 0.10 0.10 0.18 0.18 99.63 99.23 99.15 98.72 98.81
52.63 0.21 6.07 0.48 6.98 0.14 31.17 1.32 0.20 99.21
olivine websterite NK137 54.81 54.56 54.93 0.11 0.11 0.14 3.75 4.07 3.84 0.23 0.24 0.16 6.61 6.70 6.54 0.19 0.08 0.19 32.89 32.80 32.67 0.51 0.59 0.54 0.10 0.08 0.07 99.19 99.21 99.09
(a.p.f.u.) Si Ti Al Cr Fe3+ Fe2+ Mn Mg Ca Ni Na
1.898 0.004 0.182 0.007 0.014 0.180 0.004 1.674 0.027 0.003 0.007
1.899 0.003 0.179 0.006 0.017 0.177 0.005 1.676 0.028 0.003 0.007
1.4 89.0 9.6 0.90
1.5 89.1 9.4 0.90
SC
1.848 0.005 0.241 0.016 0.049 0.145 0.006 1.625 0.048 0.004 0.012
1.844 0.005 0.256 0.013 0.046 0.149 0.005 1.620 0.047 0.003 0.012
1.840 0.006 0.250 0.013 0.059 0.138 0.004 1.624 0.049 0.003 0.013
1.901 0.003 0.155 0.006 0.039 0.150 0.006 1.713 0.019 0.003 0.006
1.899 0.003 0.167 0.007 0.029 0.163 0.002 1.702 0.022 0.002 0.005
1.918 0.004 0.150 0.005 0.008 0.182 0.006 1.701 0.020 0.003 0.005
1.5 89.4 9.1 0.91
2.6 89.4 8.0 0.92
2.6 89.2 8.2 0.92
2.7 89.7 7.6 0.92
1.0 91.0 8.0 0.92
1.2 90.2 8.6 0.91
1.0 89.4 9.6 0.90
TE D
M AN U
1.892 0.004 0.185 0.008 0.023 0.170 0.003 1.678 0.028 0.003 0.007
EP
AC C
Wo En Fs Mg#
RI PT
rock sample SiO2 (wt%) TiO2 Al2O3 Cr2O3 FeO* MnO MgO CaO Na2O Total
ACCEPTED MANUSCRIPT
Table 6.
51.48 0.59 4.38 0.94 4.02 0.11 16.64 20.90 0.57 99.62
(a.p.f.u.) Si Ti Al Cr Fe3+ Fe2+ Mn Mg Ca Na
1.845 0.017 0.192 0.028 0.098 0.016 0.003 0.925 0.835 0.041
1.863 0.018 0.279 0.021 0.066 0.020 0.001 0.800 0.803 0.129
1.872 0.017 0.261 0.021 0.067 0.016 0.005 0.808 0.804 0.128
1.868 0.018 0.266 0.021 0.068 0.018 0.003 0.795 0.811 0.128
1.866 0.015 0.258 0.020 0.081 0.008 0.003 0.822 0.802 0.122
43.3 56.6 0.1 0.98
49.5 49.3 1.2 0.98
49.4 49.6 1.0 0.98
49.9 49.0 1.1 0.98
49.2 50.3 0.5 0.99
1.873 0.014 0.279 0.022 0.044 0.057 0.004 0.815 0.773 0.119
1.830 0.014 0.327 0.023 0.073 0.050 0.003 0.874 0.695 0.110
1.823 0.014 0.330 0.025 0.083 0.042 0.002 0.878 0.688 0.114
46.9 50.4 2.7 0.95
46.9 51.1 2.0 0.96
46.8 50.0 3.2 0.94
47.0 49.6 3.4 0.94
37.2 59.4 3.4 0.95
36.9 60.3 2.9 0.95
TE D
EP
SC
1.878 0.012 0.270 0.017 0.052 0.053 0.002 0.825 0.772 0.120
M AN U
1.867 0.012 0.276 0.020 0.071 0.032 0.002 0.828 0.761 0.127
AC C
Wo En Fs Mg#
1.863 0.013 0.291 0.019 0.058 0.045 0.002 0.823 0.766 0.120
olivine websterite NK137 51.94 51.81 51.87 0.65 0.61 0.64 6.43 6.05 6.16 0.71 0.72 0.74 3.01 2.94 3.04 0.04 0.16 0.09 14.55 14.80 14.53 20.32 20.49 20.61 1.80 1.80 1.80 99.45 99.39 99.47
RI PT
rock lherzolite harzburgite sample NK115 NK125 SiO2(wt%) 51.83 51.61 51.87 51.85 51.71 50.77 TiO2 0.47 0.43 0.44 0.52 0.51 0.52 Al2O3 6.75 6.41 6.33 6.49 7.53 7.64 Cr2O3 0.67 0.70 0.58 0.75 0.78 0.86 FeO* 3.56 3.64 3.64 3.46 4.24 4.40 MnO 0.06 0.07 0.07 0.12 0.09 0.06 MgO 15.07 15.17 15.27 14.96 15.92 16.07 CaO 19.50 19.51 19.89 19.75 17.62 17.54 Na2O 1.69 1.79 1.70 1.68 1.54 1.60 Total 99.59 99.32 99.80 99.57 99.94 99.46
51.82 0.54 5.93 0.68 3.19 0.10 14.95 20.31 1.70 99.24
ACCEPTED MANUSCRIPT
Table 7.
0.81 0.09
AC C
Mg# Cr#
EP
TE D
M AN U
SC
RI PT
rock lherzolite harzburgite olivine websterite sample NK115 NK125 NK137 SiO2 (wt%) 0.01 0.04 0.04 0.03 0.08 0.11 0.02 0.04 TiO2 0.14 0.14 0.14 0.36 0.33 0.29 0.06 0.18 Al2O3 57.00 57.14 57.18 53.94 54.15 53.98 56.76 55.36 Cr2O3 8.81 8.69 9.15 11.13 11.13 11.17 10.23 10.65 FeO 12.40 12.42 12.35 13.94 13.40 13.44 12.05 12.23 MnO 0.11 0.13 0.08 0.11 0.01 0.10 0.12 0.15 MgO 20.78 20.74 20.81 20.51 20.31 20.62 20.30 20.72 CaO 0.02 0.04 0.01 0.01 Na2O 0.02 0.01 NiO 0.38 0.36 0.38 0.41 0.43 0.43 0.36 0.35 total 99.65 99.69 100.18 100.44 99.87 100.15 99.90 99.69 (a.p.f.u.) Si 0.003 0.009 0.008 0.006 0.016 0.022 0.003 0.009 Ti 0.022 0.022 0.022 0.057 0.052 0.046 0.010 0.028 Al 13.934 13.958 13.912 13.274 13.378 13.299 13.893 13.606 Cr 1.445 1.424 1.494 1.838 1.844 1.846 1.679 1.755 Fe3+ 0.572 0.556 0.535 0.762 0.647 0.719 0.401 0.570 Fe2+ 1.515 1.535 1.538 1.587 1.630 1.551 1.647 1.499 Mn 0.019 0.022 0.014 0.020 0.002 0.018 0.020 0.027 Mg 6.425 6.410 6.404 6.384 6.348 6.426 6.286 6.442 Ca 0.000 0.004 0.009 0.003 0.003 0.000 0.000 0.000 Na 0.001 0.000 0.000 0.000 0.007 0.000 0.000 0.005 Ni 0.064 0.061 0.064 0.069 0.073 0.073 0.060 0.059 0.81 0.09
0.81 0.09
0.80 0.12
0.80 0.12
0.81 0.12
0.79 0.11
0.81 0.11
ACCEPTED MANUSCRIPT
Table 8. Xenolith type
lherzolite
Thermometers in °C
NK115
TBK (Ca-opx)
harzburgite
NK117
NK125
979
984
1125
929
883
1107
912
919
891
TM (opx)
1054
1055
1156
TM (cpx)
998
994
1151
TWO (opx)
979
PM (opx)
1.9
olivine websterite
NK128 1117
NK137
NK139
902
931
TWS (Al-Ca-opx) ± 25 ºC TSS (Al-opx)
M AN U
62.7
EP AC C
902
913
880
940
928
1162
1001
1005
992
821
843
986
1125
1117
902
916
1.8
0.8
1.0
2.5
1.9
59.4
26.4
33.0
82.5
62.7
TE D
depth (km)
1081
SC
± 25 ºC
RI PT
± 19 °C
ACCEPTED MANUSCRIPT
Chad 16° Lake
12°
8°
N
ue
Be
Mboutou Garoua
nu
Poli
ug Tro
e
Ngaoundéré Mayo Dark
Mt Manengouba Mt Etinde
4°
Mt Cameroon
Bioko
ey
frica ral A Cent r Zone Shea
Béka
awa
Adam
Mt Bambouto
Borongo Makan
Va ll
Tchegui
Mt Oku
10°
6°
14°
aga San ne r Zo a e Sh
Lac Bini
Yaoundé
Upper Flow Unit Middle Flow Unit
TE D
Mio-Pliocene basaltic lava flow
Lower Flow Unit Cretaceous volcanism Basement rock Road Locality 13°43'
Dang
Principe 7°23' Säo Tomé
Pagalu
EP 0
Madep
Ngao Sey
Ngaoundéré
Mbalang Lake
Foulféké
7°20' Towards Nganha
N
10°
AC C
6°
Pyroclastic deposit
Lake Dang
Gulf of Guinea
0°
RI PT
Upper
h
Ben
Wasa Golda Zuelva
Swampy area with Quaternary alluvium
SC
8°
Bambi
Kapsiki Plateau
Xenolith sample location
7°35'
M AN U
200 km
Biu Plateau
Towards Nyassar
ola T rough
Congo Craton
Gong
0°
12°
13°41'
Towards Tignère
West African Craton
Towards Garoua
13°30' 7°35'
Mbalang-Djalingo
20 km
7°17' 13°38'
Towards Bélel
13°51
ACCEPTED MANUSCRIPT xenoliths host lava NG 115 (lherz.) NG 125 (harz.) NG 137 (ol webs.)
100
Cs
Th Ba
Nb U
K Ta
Ce La
P Pr
Sr Nd
SC
Rb
Hf
Sm
Ti
Zr
M AN U
1
a
Gd
Eu
Dy Tb
Yb Y
Lu
TE D
Sample / Primitive Mantle
10
RI PT
100
10
AC C
EP
b
1
La
Ce
Pr Nd Sm Eu Gd Tb Dy Ho
Er Tm Yb Lu
ACCEPTED MANUSCRIPT
This study Mt. Cameroon (Wandji et al., 2009)
Ol
Lake Nyos, Cameroon (Temdjim, 2012) Lake Nji, Cameroon (Princivalle et al., 2000)
RI PT
Kumba, Cameroon (Teitchou et al., 2007)
Dunite
Kapsiki plateau, Cameroon (Tamen et al., 2015)
90
Nosy Be Archipelago, Madagascar (Rocco et al., 2013)
123
M AN U
ur rzb
115
TE D
113
ite
Ha
119
hrl We
git e
117
Peridotite
120
SC
125
122
Lherzolite
EP
rox py ho ort ne ivi Ol
AC C
Clinopyroxenite
Orthopyroxenite
Websterite
Pyroxenite
ite
Olivine websterite
en rox
134
py no
Opx
139
cli
5
136
ine
10
137
iv Ol
en it
e
40
10 5 Cpx
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
AC C
EP
TE D
M AN U
SC
RI PT
ACCEPTED MANUSCRIPT
ACCEPTED MANUSCRIPT
50
Wo
M AN U
this study lherzolite harzburgite olivine websterite host lava
TE D
augite
AC C
EP
20
En
hedenbergite
SC
diopside
45
5
RI PT
Wo
enstatite
Cameroon Volcanic Line Sao Tomé (Caldeira and Munha, 2002) Lake Nyos (Temdjim, 2012)
pigeonite ferrosilite
Fs
Cr2O3 Opx (wt %)
ACCEPTED MANUSCRIPT
0.7
a
0.6 0.5 0.4
0.2 0.1 0.0 3.0
3.5
4.0
4.5
5.0
5.5
RI PT
0.3
6.0
6.5
SC
57.5
M AN U
57.0 56.5 56.0 55.5 55.0 54.0 53.5 3.0
3.5
4.0
4.5
EP
53.0
TE D
54.5
AC C
Al2O3 Sp (wt %)
Al2O3 Opx (wt %)
5.0
b
lherzolite harzburgite olivine websterite
5.5 6.0 6.5 Al2O3 Opx (wt %)
ACCEPTED MANUSCRIPT lherzolite this study: 1.0
dunite (Wandji et al., 2009)
lherzolite (Arai, 1994)
wehrlite (Wandji et al., 2009) lherzolite (Rocco et al., 2013) harzburgite (Rocco et al., 2013)
harzburgite (Temdjim, 2012)
lherzolite (Caldeira and Munha, 2002)
websterite (Temdjim, 2012)
90
Fo olivine
AC C
EP
TE D
95
M AN U
SC
this study
harzburgite (Caldeira and Munha, 2002)
RI PT
0.5
0.0
olivine websterite
harzburgite (Arai, 1994)
lherzolite (Temdjim, 2012)
A M OS
Cr = Cr/(Cr+Al) spinel
harzburgite
85
80
100 * Fe2+ / Fe2+ + Mg
ACCEPTED MANUSCRIPT
21.5
a
21.0 20.5 20.0
RI PT
19.5 19.0 18.5 11
12 13 100 * Cr / Cr + Al
SC
10
13.0
M AN U
12.5 12.0 11.5 11.0
10.0 9.5 9.0
0.1
EP
0.0
TE D
10.5
AC C
100 * Cr / Cr + Al
9
0.2
b
lherzolite harzburgite olivine websterite
0.3 TiO2 (wt %)
0.4
ACCEPTED MANUSCRIPT NW
Bambi (harzburgite)
Depth (km)
Ngao Sey (lherzolite)
Ngaoundéré
F
F
Foulféké (olivine SE websterite)
F
F
F
00
F
P (kb)
RI PT
00
continental crust
10 Moho discontinuity
30
sub-lithospheric mantle
T>1000°C
20
120 mantle uplift
M AN U
zone of xenolith extraction 800
90
SC
60
30 asthenosphere lithosphere mantle boundary 40
asthenosphere
150
TE D
zone of mantle melting
network fault cross cutting the granitoid basement down to the mantle basaltic liquid
AC C
EP
sheared mantle lithosphere of harzburgitic composition
continental crust alkali volcanism dyke of websterite composition lherzolitic lithosphere asthenosphere
50
1
ACCEPTED MANUSCRIPT
Highlights are:
Petrology of newly discovered ultramafic xenoliths vs. host lava, in Adamawa Plateau Study of mineralogical phases and equilibrium conditions
AC C
EP
TE D
M AN U
SC
RI PT
Discussion on mantle/lithosphere composition and melting