Volcanisme du secteur de baossi (Ngaoundere ne, Adamaoua) : pétrographie, mineralogie, géothermométrie et géochimie

Abstract

The Baossi area is located in Ngaoundéré NE, Adamawa Plateau, between N7°30’–7°35’ and E13°52’–13°58’, and belongs to the Cameroon Volcanic Line (CVL). In this area, autcropped 03 (three) monogenetic cones (Warack, Baossi 1 and Baossi 2) and basaltic flows along the road cut. The lava flows beeing examined in the thesis in order to better understand the magmatic processes that make difficult the study of such volcanoes. The low volumes of basaltic lavas (< 1 km3) emitted at these volcanoes have been mapped and the samples collected for thin sections preparation for petrography, mineral chemistry, geothermometry and geochemistry studies. Whole rock analysis of about thirty of these lavas have been done using XRF and ICP methods. Microprobe analysis of matrix glass and olivine hosted melt inclusions in some lava samples has also been done. The results show that the mineralogical assemblage of the basaltic lavas in the study area is mainly composed of olivine (8–11%) and clinopyroxene (~11%) phenocrysts, plagioclase microphenocrysts (10–13%), set in a vitreous groundmass. Accessory phases are composed of Fe-Ti oxides (2–5%), nepheline, apatite, quartz and amphibole xenocrysts. Some mineral phases exhibit magma mixing characteristics such as resorpted margins, thin and well preserved rim subrounding olivine and clinopyroxene crystals and oscillatory zoning of plagioclase. Chemical compositions of mineral phases are chrysolite (Fo86-74) for olivine, diopside (Wo50-47En40-34Fs15-10) and augite (Wo43, 88En44, 42Fs11, 71) for clinopyroxene and andesine (An43-40Ab53-48Or8-6) for plagioclase. Rare alkali feldspar crystals analyzed are anorthoclase (An3-1Ab92-78Or18-5) and sanidine (An2Ab32Or65). Fe-Ti oxides are titano-magnetites (Usp69-87) and titano-hematites (Usp70). Amphiboles lamellae found in some lavas are edenite, ferroedenite and ferro-kaersutite, which are new mineral species in the Adamawa Plateau. Two generations of olivine and clinopyroxene are distinguishable in the lavas examined and some olivine crystals are hosting melt inclusions. The lavas studied are alkaline lava flows with normative nepheline, olivine, and diopside. These lavas are silica-undersaturated and sodic alkaline. The matrix glass analyzed is an andesitic basalt, the melt inclusions are picro-basalts and basalts, while the lava flows are foidites, basanites, basalts and hawaiites (SiO2: 40.76–51.39 %; MgO: 4.98–9.99 %). The Baossi lavas shows two liquid lines of descent probably linked to two batches of magma in the binary plot K2O versus SiO2. These lavas are characterized by OIB-like trace elements patterns, enriched in LILE including Pb, and display negative anomalies in Th, U, K, Sm and Ti. The enrichment in Pb and depletion in Th and U in some lavas are probably linked to crustal contamination. REE patterns suggest cogenetic magmas below Baossi 1 and Baossi 2 volcanoes, and distinct sources below Warack volcano and nearby lavas. The lavas studied also show affinity to High-μ (HIMU), Enriched type I (EM1) and Oceanic Island Basalt (OIB)-like mantle signatures and thus indicate a heterogeneous mantle source underneath the vents as observed at other monogenetic and polygenetic volcanoes along the CVL. Primary melts derived from low degrees of partial melting (0.5–2 %) with different compositions of garnet lherzolite (4–8%) in the parent material source and encountered low percentage of fractionation, and crustal contamination coupled with magma mixing. The crystallization and equilibrium temperatures of the minerals also testified weak fractional crystallizations, during the evolution of the magmas through fractures of the basement en route to the surface.