Volcanologie, pétrologie, géochimie et cindynique des laves du secteur Koutaba – Nkoumelap (Ouest-Cameroun)

Abstract

The present doctoral research work on the Koutaba - Nkoumelap sector is based on (1) geological data collection, (2) sampling of different magmatic formations, and (3) volcanological, petrographic, mineralogical, geochemical and risk studies of lava samples selected and classified into four groups: ankaramites, olivine basalts, olivine-free basalts and rhyodacitic lavas. The mineralogical and geochemical analyses helped to obtain an important new database and the results make it possible to better characterize and apprehend the sources, the nature and the evolution of the studied lavas. Three types of eruptive dynamisms were evident in the sector: (1) a fissural dynamism that produced plateau basalts at Nkoumelap and hexagonal prisms at Koutaba and Mamevouo; (2) an extrusive dynamics which have built domes at Nka’nyam and felsic flows at Nkoumelap; (3) a moderately explosive dynamism at the origin of the varied volcanic projections observed in the localities of Tailandier, Nkoumelap and Nka'nyam. So, petrographic, mineralogical and geochemical studies show a transitional affinity for olivine-free basalts, moderately alkaline to olivine basalts and alkaline to ankaramites (basanites). The transitional basalts originate from the melting at a relatively high level of a shallow mantle source, as evidenced by their low ratios of (La/Yb)N and (Ce/Yb)N of 7.46 – 8.83 and 6.52 - 7.46 respectively. Their evolution is controlled by the fractional crystallization of augite, plagioclase and ferrotitanium minerals. The alkali basalts and basanites are possibly derived from the deep and deeper sources respectively, as reflected by the high ratios of (La/Yb)N 10.79 - 11.76 and 28.07 - 28.60, (Ce/Yb)N of 9.19 - 10.17 and 19.37 - 19.77. Their evolution is controlled by fractional crystallization of olivine, clinopyroxene and plagioclase during differentiation. The different discriminating patterns, the appearance of rare earth spectra and the relationships of incompatible elements do not suggest any kind of relationship between the different groups of mafic rocks. Transitional lava magma were possibly formed by high melting of a sublithospheric mantle of the EM1 type, while alkaline lavas might have resulted from the melting, of an uncontaminated mantle, highly enriched in incompatible elements, of the HIMU type. The melting rate is high in alkali basalts and low in basanites. The rhyodacitic lavas originate from the differenciation of the magma at the origin of the transitional basalts. The study of the hazards and risks related to the volcanic activity of the sector makes it possible to identify the different volcanic hazards, to identify the zones that may be affected by these volcanic hazards and to give an estimate of the populations and structures directly exposed to the effects of these volcanic risks. On the other hand, the geomorphosites of the sector could also constitute real tourist destinations. Moreover, these geomorphosites contain hydrological and geotechnical resources, while products such as basalts and volcanic ash present agricultural, decorative and therapeutic interests, thus making the geological formations of the Koutaba - Nkoumelap sector economically profitable.