The accidental fragments called rock xenoliths, piked up by the turbulent host magma on its ascent, allowed to characterize petrologically the Earth’s upper mantle. They are an important source of information about lithospheric composition and thermal evolution in mantle regions associated with alkaline volcanism  . In the Adamawa Plateau (Figure 1), previously ultramafic xenoliths data are from Dibi area (   ), Youkou maar  , Ngao Voglar  and Hosséré Garba  . These ultramafic xenoliths studied are chiefly of dunite and lherzolite composition. Thus, the discovery of the wehrlite samples from the SW of Hosséré Dammougalré could improve the knowledge of the upper mantle beneath the Adamawa Plateau. Indeed, wehrlite samples hosted in basaltic lavas from the SW of Hosséré Dammougalré are located in the western Adamawa Plateau (Figure 1). Adamawa Plateau is a tectonomagmatic domain bordered by faults oriented N70˚E  . This plateau presented as the horst, is situated to the north of the Congo Craton, as the result of the fold belt shearing, oriented WSW-ENE  .
Figure 1. Location of the studied zone in the Adamawa Plateau domain.
The aims of this paper are to present the first petrographical, mineral chemistry and thermobarometric data of the wehrlite xenoliths from the SW of Hosséré Dammougalre, chiefly to enhance the knowledge of upper mantle beneath the Adamawa Plateau.
2. Geological Description of the Studied Zone
The geological sketch established for the studied zone is presented (Figure 2). The trachytic outcrops of Hosséré Dammougalré and Hosséré Doro are surrounded by several basaltic units (lower, middle and upper). The lower basaltic units were almost completely weathered and converted into black reddish ferralitic soils or the clusters of residual ferruginous vacuolar cuirasses. The intermediate or middle basaltic units were fragmented into angular or rounded blocks of various sizes (5 - 50 cm), slightly weathered and dispersed. The upper basaltic units were represented by the blocks accumulated as small isolated hills (diameter: ~500 m; height <30 m). These hills consist of small prismatic blocks (0.2 - 1.7 m) and vertical columnar jointing (0.5 to 2 m) of basalt. The studied xenoliths (size: ~4 cm) were sampled within these upper basaltic units at the SW of Hosséré Dammougalré. The Hosséré Dammougalré (altitude: ~1600 m a.s.l) is a neck of needle-shaped lava which have circular basis (diameter: ≈200 m) and more or less steep slopes (45˚ - 90˚). This formation is strongly prismatic and dismantled into blocks, disseminated in chaos on the soil. However, the Hosséré Doro is a dome bounded by small steep cliffs (20 - 40 m high) and the lava blocks are accumulated to the foothill.
3. Analytical Methods
The mineral phases were analyzed in polished thin sections with the electron microprobe analyzer using a Cameca microprobe SX100 at the “Université Pierre et Marie Curie”, Paris VI (France). The standards data used for analysis are from natural (Si, Al and K on orthoclase, Ca on anorthite, Na on albite, P on apatite, Zr on zircon) and synthetic (Fe on Fe2O3, Ba on BaO4, Sr on SrSiO3)
Figure 2. Sketch of the geological map of the studied zone.
The measurements were carried out with a beam size of 10 - 100 µm, under the following conditions expressed in kV (accelerating voltage), nA (beam current) and s (counting times at the peak): Olivine (15 kV, 40 nA, 20 s for all elements, except Si (10 s)), clinopyroxene (15 kV, 40 nA, 20 s for Si, Al, Fe, Mg, Ca, Na, Mn and 30 s for Ti and Zr), feldspar (15 kV, 10 nA, 5 s for all elements), Fe- Ti oxides and spinel (15 kV, 40 nA, 40 s for Ti, Fe, Mn, Mg, 10 s for Si, 15 s for Cr and 30 s for Al). Measurements correction was carried out using the “PAP” program  .
Host Basalt is characterized by a porphyritic texture. The phenocrysts of olivine (~2 mm), clinopyroxene (1.2 - 2.1 mm) and Fe-Ti oxides (0.5 - 0.6 mm) are scattered in a groundmass consisting of clinopyroxene, Fe-Ti oxides and plagioclase microlites. Olivine phenocrysts are subhedral, cracked and their edges are sometimes destabilized into iddingsite. Clinopyroxene phenocrysts are euhedral and twinned.
Wehrlite xenoliths (~4 cm size) are characterized by an allotriomorphic texture, with inequigranular olivine (~40 vol.%), clinopyroxene (~55 vol.%) and spinel (~5 vol.%) crystals. Spinel crystals are red brown and interstitial between the crystals of olivine and clinopyroxene. The contact between the host basalt and wehrlite xenoliths is materialized by the accumulation of small Fe-Ti oxides and plagioclase crystals or the tiny host basalt veins.
4.2. Mineral Chemistry
In the host basalt, forsterite (Fo) values calculated for the analyzed olivine phenocrysts (Table 1) reach 73 - 74 and 68 - 72, respectively for the core and the rim. The CaO contents are high (rim, up to 0.43 wt% and core, up to 0.41 wt%). Using bulk-rock composition as a liquid at the total pressure of 1 atm, the crystallization temperatures were estimated after  for the core and the rim respectively to 1224˚C ± 60˚C and 1166˚C ± 48˚C.
For wehrlite xenoliths, the forsterite component (Table 1) range from 76 to 88. These values are similar to those calculated for the wehrlites from the Mount Cameroun (Fo82-86;  ). NiO contents reaching 0.19 wt% and CaO up to 0.39 wt%.
The clinopyroxene phenocrysts (Table 2) of the host basalt are both diopside and augite (after the nomenclature of  ; Figure 3). Their Cr2O3 contents range from 0 wt% to 0.38 wt%), while TiO2 and Al2O3 contents are high (up to 3.9 wt% and 8.7 wt% respectively). The high contents of Ti and Aliv (apfu: atom per formula unit) in diopside and augite are linked to a low-pressure of crystallization  .
Wehrlites contain also diopside and augite, but poor in Al2O3 (1.8 - 5.1 wt%), TiO2 (0.8 - 1.7 wt%) and rich in Cr2O3 (up to 0.59 wt%) with Cr# (= 100 × atomic Cr/(Cr+Al)) up to 15.6.
Figure 3. Wo-En-Fs ternary diagram (after Morimoto, 1989) for the classification of clinopyroxene from the studied rocks.
Table 1. Representative compositions of olivine of the host basalt and the studied wehrlite.
ph.r: phenocryst. rim; ph.c: phenocryst core.
Table 2. Representative compositions of clinopyroxene of the host basalt and the studied wehrlite.
According to the Alvi vs Aliv diagram (Figure 4), these clinopyroxene crystals crystalized at high pressure near of the RMC (Refractory Mantle Clinopyroxene,  ) field.
Figure 4. Plot of Cr# vs. Fe2+/(Fe2+ + Mg) values of the spinel for xenoliths from the SW of the Hosséré Dammougalré and those of the others sector of the Adamawa Plateau and the Cameroon Volcanic Line.
4.2.3. Fe-Ti Oxides
The titano-magnetite is present in the host basalt (Table 3), with TiO2 and FeO contents reaching respectively 25.0 wt% and 69.5 wt%.
Spinel of wherlites (Al > Cr, Mg > Fe2+) is chromiferous with the values of Cr# (Cr × 100/(Cr + Al) and Fe2+/(Fe2+ + Mg) positively correlated (Table 4; Figure 4). The correlation trend displays on the Figure 4, present two distinct evolutions, one for the Adamawa Plateau (Nganha;  , Dibi;  ) and another for the Cameroon Volcanic Line (São Tomé;  , Mbarombi Mbo;  ), Mount Cameroon;   and Kapsiki Plateau;  ). The distribution of the spinel compositions in Cr-Al-Fe3+ ternary diagram (Figure 5) both for Adamawa Plateau and the Cameroon Volcanic Line reflects the heterogeneous nature of the magmatic source. The Ti (0.44 - 0.52 a.p.f.u) and Fe2+ (3.15 - 3.59 a.p.f.u) contents are high and could be link to the re-equilibration during the crystallization of the clinopyroxene from the host basalt.
The compositions of plagioclase (Table 5) from the host basalt range from bytownite (An71Ab28) to labradorite (An61-69Ab37-30).
5.1. Origin of Wehrlites
Ultramafic xenoliths are considered as fragments of lithospheric mantle  or the residues of the partial melting of mantle  .
Clinopyroxenes in the studied wehrlites are low in TiO2 (0.8 - 1.7 wt%) and Al2O3 (1.8 - 5.1 wt% 3 - 4 wt%) as typical for clinopyroxenes of residual mantle origin   . The refractory elements as Cr are enriched in the residue, as
Figure 5. Fe3+-Cr-Al ternary diagram of the spinel for xenoliths from the SW of the Hosséré Dammougalré and those of the others sector of the Adamawa Plateau and the Cameroon Volcanic Line.
Table 3. Representative compositions of Fe-Ti oxides of host basalt.
Table 4. Representative compositions of the spinel of the studied wehrlite.
Table 5. Representative compositions of the plagioclase of the host basalt.
demonstrated for the Cr-rich clinopyroxene from the studied xenoliths. Thus, the studied spinel-bearing wehrlite xenoliths, characterized by the presence of Cr-diopside, represent probably the residual portions of the upper mantle  . The range of Ni contents (up to 1900 ppm) of olivine of wehrlites from the SW of Hosséré Dammougalré confirms that they are residues of melting. However, the compositions of olivine crystals which are characterized by the forsterite contents lower than 90%, may, have been modified by Fe-Mg equilibration between wehrlite olivine and the host basalt. Re-equilibration between Fe-Mg and host basalt at low temperatures is corroborated by slight similarity between wehrlite olivine Mg# (Fo76 - 88), NiO (0.04 - 0.19 wt.) and CaO (0.15 - 0.39) contents and olivine in host basalt Mg# (Fo68 - 74), NiO (0.04 - 0.16 wt%) and CaO (0.30 - 0.43). Thus, the occurrence of small plagioclase and oxides crystal surrounding the wehrlites indicates that olivine re-equilibration took place at low pressures (<0.2 GPa;  ).
5.2. Geothermometry and Geobarometry
Equilibrium temperatures calculated from Fe/Mg exchange reaction for olivine/ spinel using geothermometers  vary between 944˚C and 1102˚C. These temperatures are near of that estimated at liquidus above 1150˚ C after  for the crystallization of the Cr-spinel.
The CaO contents (0.15 - 0.39 wt%) of the studied wehrlite olivine are obviously higher than that of olivine of mantle origin (0.05% - 0.1%:  ), which involve low pressure (<0.2 GPa;  ) equilibrium environment, probably near the surface. However, as evidenced by the Alvi vs Aliv diagram (see Figure 6) all the analyzed wehrlite clinopyroxenes have crystalized at high pressures  near of the RMC (Refractory Mantle Clinopyroxene;  ) field. These contrasted results, would indicate that the wehrlite clinopyroxene would have been re-equilibrated in the host basalt at high pressures (shallow depths) during their as
Figure 6. AlVI vs. AlIV (apfu) plot of clinopyroxene from ultramaﬁc xenoliths and host basalt. Field for refractory mantle clinopyroxenes (RMC) after Jagoutz et al. (1979).
cending towards the surface. As estimated for Ngao Voglar xenoliths  , which are situated in the same geological domain (Adamawa Plateau), the wehrlite xenoliths from the SW of Hosséré Dammougalré have likely been entrained by host basalt and carried up to the earth’s surface from the depth limit of 57 - 58 km.
Petrographical, mineral chemistry and thermobarometric data indicated that the wehrlite xenoliths from the SW of Hosséré Dammougalré are likely residual portions of the mantle partial melting, strongly re-equilibrated by the host melt which were extracted from shallow depths (~60 km) at temperatures estimated around 1100˚C.
The French “Ministère de la Coopération” is acknowledged for providing a grants to I.B.G.M. for nine-month stay in France at the “Laboratoire de magmatologie et de géochimie inorganique et expérimentale, Université Pierre-et-Marie-Curie”, Paris. Pr Bernard Déruelle is acknowledged for his assistance and his supervision during the laboratory-work at this university.