Age and Sr-Nd-Hf isotopes of the sub-continental lithospheric mantle beneath the Cameroon Volcanic Line: Constraints from the Nyos mantle xenoliths

To better constrain the age and composition of the sub-continental lithospheric mantle (SCLM) beneath the Cameroon Volcanic Line (CVL), we present Sr-Nd-Hf-Os isotopes of the Nyos mantle xenoliths (nine lherzolites and four harzburgites). The Nyos lherzolites have fertile compositions with 37.69-41.12 wt.% MgO and 2.49-4.38 wt.% Al2O3, whereas the harzburgites are more refractory containing 43.2-45.42 wt.% MgO and 1.43-2.1 wt.% Al2O3. The MgO contents of the Nyos xenoliths overall show negative correlations with CaO, Al2O3, Na2O and TiO2 contents, indicating that they represent mantle residues after melt depletion. Modal minerals like pargasitic amphibole have only been discovered in one Nyos Iherzolite, suggesting modal metasomatism locally occurred. Clinopyroxenes in most Nyos xenoliths show enriched trace element compositions, indicating pervasive cryptic metasomatism within the SCLM. Clinopyroxenes in the Nyos lherzolites have less radiogenic Sr and more radiogenic Nd isotopes than the Nyos harzburgites. Clinopyroxene Hf isotopes of the harzburgites are comparable to those of the lherzolites. Although the CVL basalts have similar Sr-Nd isotopes with the Nyos harzburgites, they have different Hf isotopes. Therefore, the CVL basalts could not be sourced solely from the lithospheric mantle. The Nyos lherzolites show consistent HSE patterns, with slight depletion in both Re and Pt over Pd. The harzburgites are remarkably depleted in Pt, Pd and Re relative to Os, Ir and Ru. The Nyos lherzolites have variable Os-187/Os-188 ratios ranging from 0.11700 to 0.13129, yielding Os model ages (T-MA) relative to the primitive upper mantle (PUM) of 0.27-2.18 Ga and Re depletion ages (T-RD) of 0.12-1.76 Ga. In comparison, four harzburgites have more unradiogenic Os-187/Os-188 ratios of 0.11485-0.11705. Three of the four harzburgites with low Re/Os ratios yield similar T-MA and T-RD ages, i.e., 1.91-2.43 Ga vs. 1.87-2.05 Ga, respectively. The Os-187/Os-188 ratios of the Nyos xenoliths show a good correlation with bulk Al2O3 contents. An initial Os-187/Os-188 ratio of 0.11064 can be inferred from the Os-187/Os-188-Al2O3 correlation, giving an Archean T-RD age of similar to 2.6 Ga. This suggests that the Nyos mantle xenoliths have been subjected to melt depletion events during the Paleoproterozoic convergence of the Congo and West Africa cratons, which formed the lithospheric mantle intruded by the CVL. The SCLM beneath the CVL is coeval with or slightly younger than the charnockite and tonalite-trondhjemite-granodiorite in the Ntem Complex of NW Congo Craton. We proposed that the old SCLM was a part of the Congo cratonic mantle. Therefore, our results suggest that old cratonic mantle beneath the CVL has not been completely removed during the Pan-African Orogeny, although the overlying crust might have been strongly deformed and reworked. (C) 2016 Elsevier B.V. All rights reserved.