TITLE:
Upper Mantle Dynamic and Evolution beneath Lake Nyos Volcano (Cameroon Volcanic Line), Documented by Peridotite Xenoliths Enclosed in Alkali Basalts
AUTHORS:
Merlin Isidore Teitchou, Joseph Legrand Tchop, Michel Grégoire, Eddy Ferdinand Mbossi, Pauline Wokwenmendam Nguet, Sahabo Aboubakar Abdoulaye, Ateba Bekoa
KEYWORDS:
Nyos Xenoliths, Metasomatism, P-T Estimated, Mantle Diapirs, Rift-Zones
JOURNAL NAME:
International Journal of Geosciences,
Vol.13 No.6,
June
23,
2022
ABSTRACT: 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, CO2, H2O) 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 FO (%) 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.