Magnetic fabrics of the Miocene ignimbrites from West-Cameroon: Implications for pyroclastic flow source and sedimentation

The Miocene ignimbrites of Mounts Bambouto and Bamenda located in the central part of Cameroon Volcanic Line are generally made of welded and non-welded massive lapilli tuff and lithic breccias. These discontinuous deposits cover a total area of 180 km2 with thickness ranging from 25 to 200 m. The different facies contain several lithic fragments of mainly trachytic nature. The devitrified matrix of the welded ignimbrites is constituted by sanidine, anorthoclase, quartz, plagioclase, clinopyroxene, biotite, Fe–Ti oxides and devitrified fiammes. Anisotropy of magnetic susceptibility (AMS) is used to characterize magnetic fabrics and to provide an estimate of flow direction of each ignimbrite sheet. Magnetic mineralogy results from different flow units show that titanomagnetite, titanohematite, maghemite and goethite with grain size ranging from coarse MD to very fine SP are the main magnetic carriers of these ignimbrites. Inferred transport directions based on the AMS data and field indicators show that Bambouto caldera is the source of main pyroclastic deposits of Mount Bambouto. In southwestern Mount Bamenda, Santa-Mbu caldera or Bambouto caldera constituted the probable emission center of Mbengwi, Bamenda and Mbu ignimbrite sheets, whereas magnetic fabrics of Bambili, Sabga and Big Babanki ignimbrites demonstrate that these deposits were emitted from a northeastern source, namely Oku vent in Mount Oku. A small number of subvertical AMS fabrics correspond to rocks possibly modified by an elutriation process.