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