The Mount Manengouba, a complex volcano of the Cameroon Line: Volcanic history, petrological and geochemical features

Abstract

The volcanic story of Mount Manengouba is related to four chronological stages: (1) forming of the early Manengouba shield volcano between 1.55 and 0.94Ma, (2) building of the Eboga strato-cone between 0.94 and 0.89Ma, (3) caldera collapse and silicic extrusions of the Elengoum Complex between 0.89 and 0.70Ma, and (4) intra-caldera and flank activity between 0.45 and 0.11Ma. The volume of the volcano is calculated at 320km3±5%. The volcanic rocks are attributed to two magmatic outputs. The first and main magma generation produced the shield volcano, the strato-cone, and the syn- to post-caldera extrusions, displaying a complete series from basanites to trachytes (magmatic Group 1). The second magma generation is limited to the late and flank activity evolving from basanites to trachy-phonolite (magmatic Group 2). Both magmatic groups belong to the under-saturated alkaline sodic series. Petrological calculations locate the magmatic reservoir between 37 and 39km in the upper mantle for the Group 1 lavas, and between 42 and 44km for the Group 2 lavas. Trachytes were generated in a secondary crustal reservoir. Magmatic series evolve with medium to low pressure fractional crystallization of olivine, pyroxene, oxides, feldspar, and apatite. Significant crustal assimilation is evidenced in trachytes. The magma of Group 1 was generated with 3–6% of partial melting of a moderately enriched source containing 3–7% of garnet. Melting took place in the spinel to garnet transition zone located at 70–90km and around 25kb. The magma of Group 2 resulted from a slightly higher partial melting from a less garnet-rich source that indicates uprising of the melting column in the upper part of transition zone. Sr, Nd, and Pb isotope data of the Manengouba rocks and neighboring lavas are analyzed and compared with those of the mafic lavas of the CVL. Three source components are distinguished: a depleted component originated from the asthenospheric swell, a radiogenic component linked to the contaminated lithosphere of the Neoproterozoic mobile belt, and an enriched component or the lithosphere possibly related to pre-rifting magmatic processes.