Mineral Chemistry of Wehrlite Xenoliths Hosted in Basalts from the SW of Hosséré Dammougalré (Adamawa Plateau, Cameroon): Thermobarometric Implications

Wehrlite samples (size: ~4 cm) hosted in basaltic lavas from the SW of Hosséré Dammougalré are located in the western Adamawa Plateau. Porphyritic and allotriomorphic texture characterize respectively host Basalt and wehrlite xenoliths. The phenocrysts of olivine (Fo68−74), and Ti-magnetite are scattered in host basalt. Wehrlite xenoliths (~4 cm size) contain Cr-rich clinopyroxene (diopise-augite), olivine (Fo76−88) and chromiferous spinel. Equilibrium temperatures calculated from Fe/Mg exchange reaction for olivine/spinel vary between 944 ̊C and 1102 ̊C. The wehrlite olivine crystals with low Fo (<90) indicate a re-equilibration of Fe-Mg in the host basalt at low temperatures. All the analyzed wehrlite clinopyroxenes have crystallized at high pressures as evidenced by the Alvi and Aliv contents. The studied spinelbearing wehrlite xenoliths represent probably the residual portions of the upper mantle, which are an important source of information about lithospheric composition and thermal evolution beneath the Adamawa Plateau.


Introduction
The accidental fragments called rock xenoliths, piked up by the turbulent host 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.

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.

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 Fe 2 O 3 , Ba on BaO 4 , Sr on SrSiO 3 ) Figure 2. Sketch of the geological map of the studied zone.

phases.
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 [9].

Olivine
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 [10] for the core and the rim respectively to 1224˚C ± 60˚C and 1166˚C ± 48˚C.

Fe-Ti Oxides
The titano-magnetite is present in the host basalt (Table 3), with TiO 2 and FeO contents reaching respectively 25.0 wt% and 69.5 wt%.

Origin of Wehrlites
Ultramafic xenoliths are considered as fragments of lithospheric mantle [1] or the residues of the partial melting of mantle [20]. Clinopyroxenes in the studied wehrlites are low in TiO 2 (0.8 -1.7 wt%) and Al 2 O 3 (1.8 -5.1 wt% 3 -4 wt%) as typical for clinopyroxenes of residual mantle origin [14] [13]. The refractory elements as Cr are enriched in the residue, as

Geothermometry and Geobarometry
Equilibrium temperatures calculated from Fe/Mg exchange reaction for olivine/ spinel using geothermometers [23] vary between 944˚C and 1102˚C. These temperatures are near of that estimated at liquidus above 1150˚ C after [24] 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%: [17]), which involve low pressure (<0.2 GPa; [22]) equilibrium environment, probably near the surface. However, as evidenced by the Al vi vs Al iv diagram (see Figure 6) all the analyzed wehrlite clinopyroxenes have crystalized at high pressures [13] near of the RMC (Refractory Mantle Clinopyroxene; [14]) 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 cending towards the surface. As estimated for Ngao Voglar xenoliths [5], 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.

Conclusion
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.