Upper Mantle Dynamic and Evolution beneath Lake Nyos Volcano (Cameroon Volcanic Line), Documented by Peridotite Xenoliths Enclosed in Alkali Basalts

Volcanic deposits from the lake Nyos contain ultramafic xenoliths: lherzolites, harzburgites and wehrlites, sometimes containing amphiboles phlogopites. The lithospheric mantle beneath Nyos, as inferred chemical diagrams, has experienced partial melting variably cryptic modal metasomatism of two groups samples that have been distinguished: Group 1 are characterized by spoon-shaped REE patterns, 2 show light (L) REE-enriched patterns. Metasomatic events were associated with pervasive infiltration volatile (Ti, CO2, H2O) or alkali-rich small melts fractions fluids. Later on, hydrous phases, Ti-rich Cpx, CaO-rich Ol, Cr poor low values NiO FO (%) in wehrlite compared to other xenoliths, precipitated alkali enrichments due percolation basaltic magmas. metasomatic liquid which percolates column was a dense alkaline silicate rich volatile, displaying HFSE abundances LILE. It is suggested 1) affected samples, 2) spinel-free sample corresponding cumulate similar melt 3) amphibole may be potassium-bearing mineral addition phlogopite at shallower levels upper mantle. P-T estimated indicates xenoliths initially equilibrated garnet stability field, depth 85 Km, then they re-equilibrated spinel field owing isobaric heating up 1000℃. Adiabatic decompressions occur 50 Km materialized NK14 showing transitional porphyroclastic equigranular texture pyroxene-Cr symplectites, 30 8 - 18 Kbar, pressures most incorporated host lavas. Therefore, presence rising plumes (sample NK14) (all studied samples) probably related evolution heterogeneous translithospheric diapirs this section continental Cameroon Line. Local here interpreted rift-zone initiators.