Pierre Kamgang

The geochemical characteristics of some plutonic-volcanic complexes along the southern part of the Cameroon line

Abstract About 60 volcanic-plutonic alkaline complexes of Tertiary age are located along the continental part of the Cameroon Line. This paper reports the chemical nature and evolution of six of them (Gwenfalabo, Ntumbaw, Nkogam, Nda Ali, Bana and Koupe), all located in the southern part of this line. The geochemical data (major, trace and REE) of these six complexes reveal the following general behaviour: • -differentiation from a primary magma of intermediate composition situated in the well-known Daly gap. The absence of any discrimination between the different complexes in geochemical diagrams suggests that they have a unique source. This model is defined by three co-magmatic rock-types: cumulative basic rocks composed of anorthositic gabbros, residual acidic rocks with abundant syenites and granites and primitive intermediate rocks mainly represented by monzodiorites and monozonites of the Ntumbaw complex; • -emplacement of the acidic rocks by successive pulses, as illustrated in the Rb/Sr versus Sr diagram by three subparallel trends, corresponding (with increasing Sr contents) to fayalite syenites, other syenites, other syenites and granites respectively. From Rb/Sr isotopic data, the Nkogam granite is 67 Ma-old, while the fayalite syenite of the Mount Koupe is 46 – 50 Ma-old. Rb/Sr isotopic data of acidic formations of the other complexes and stable isotopes are needed.

Mapping and Assessment of Volcanic Hazards Related to the Ignimbritic Eruption by AMS in Bambouto Volcano (Cameroon Volcanic Line)

Pyroclastic deposits constitute major components of explosive volcanic activity. To help improve the safety of the population faced with natural disasters, a study is carried out at Bambouto volcano with a view to map potential hazards related to pyroclastic flows. The Bambouto volcano is indeed considered to be still active since the recent discovery of Quaternary basalts (0.5 Ma) at Totap, a locality situated near the Bambouto Caldera. This discovery has led to reclassify Mount Bambouto among active volcanoes of Cameroon and, therefore, considered as potentially dangerous. The dangerousness of this volcano is accentuated by the presence of ignimbrites that are witnesses of ancient pyroclastic flows. Because a map of volcanic hazards is non-existent on the volcano, anisotropy of magnetic susceptibility (AMS) is the method used in this paper to characterize magnetic fabrics and provides an estimate of flow direction of each ignimbrite sheet (represented by massive lapilli tuff and massive lithic breccia facies). 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. These results have enabled us to produce a new hazard map related to potential future pyroclastic flows.