a b s t r a c tUnderstanding the mechanisms involved in the formation of maars and their diatreme growth processes has been a subject of contention. While there is no direct evidence of the presence of diatremes beneath most of the young maars, their existence is inferred based on the amount and type of country rocks excavated at different depths and deposited as pyroclastic ejecta around their craters. Properly tracing fragmented country rocks in ejecta to interpret their depths of origin and thus the depths of phreatomagmatic explosions require good and detailed information on the substrate geology that is generally lacking at many maars. As an alternative, this paper explores the role of juvenile components in deposits of a maar for understanding the cratering and growth of diatremes during maar-forming eruptions. Based on field investigations, pyroclast distribution, componentry and grain morphology examinations this study reports on the eruptive mechanisms that led to the formation of the Barombi Mbo Maar (BMM), a polygenetic maar volcano in Cameroon. The BMM consists of three diatremes that formed during distinct eruptive events and coalesced to produce an "amalgamated maar-diatreme". Two end-member types of eruption styles from the "dry" magmatic to the "wet" phreatomagmatic explosions governed the formation of the maar. In total, a minimum of~0.0658 km 3 of magma (Dense Rock Equivalent corrected) was ejected based on calculation by applying interpolation techniques on digital elevation models obtained from SRTM30m data corrected by rock textural data collected from the field. The distribution of juvenile clasts throughout the stratigraphic sequence suggests a complex subsurface eruptive process, which originated probably within the uppermost part of the diatreme. From the distribution and morphology of juvenile clasts in the deposits, it is inferred that cratering and country rock excavation during the growth of each of the small diatremes developed mainly from shallow level explosions, sometimes with lateral and vertical variations in the position of the explosion loci. A prospective juvenile-based conceptual model is proposed for the formation of the BMM. The model suggests that, during maar-forming eruptions, explosions taking place at a deeper position might entrain extensive amount of lithics from the mostly lithic-dominated upper crater infill to deposit juvenile-poor (b10 vol.%) tephra beds. Layers with a juvenile content of 10-60 vol.%, for example, might result from deep to shallow-seated explosions, with a common entrainment of lithics from the crater infill region, and with much of the remobilized tephra being transported to the ejecta ring sequence. In contrast, explosions occurring at shallower positions will produce mainly juvenile-rich beds (juvenile N 90 vol.%).

his study presents the first and detail field investigations of exposed deposits at proximal sections of the Barombi Mbo Maar (BMM), NE Mt Cameroon, with the aim of documenting its past activity, providing insight on the stratigraphic distribution, depositional process, and evolution of the eruptive sequences during its formation. Field evidence reveals that the BMM deposit is about 126m thick, of which about 20m is buried lowermost under the lake level and covered by vegetation. Based on variation in pyroclastic facies within the deposit, it can be divided into three main stratigraphic units: U 1 , U 2 and U 3 . Interpretation of these features indicates that U 1 consists of alternating lapilli-ash-lapilli beds series, in which fallout derived individual lapilli-rich beds are demarcated by surges deposits made up of thin, fine-grained and consolidated ash-beds that are well-defined, well-sorted and laterally continuous in outcrop scale. U 2 , a pyroclastic fall-derived unit, shows crudely lenticular stratified scoriaceous layers, in which many fluidal and spindle bombs-rich lapilli-beds are separated by very thin, coarse-vesiculatedash-beds, overlain by a mantle xenolith-and accidental lithic-rich explosive breccia, and massive lapilli tuff and lapillistone. U 3 displays a series of surges and pyroclastic fall layers. Emplacement processes were largely controlled by fallout deposition and turbulent diluted pyroclastic density currents under "dry" and "wet" conditions. The eruptive activity evolved in a series of initial phreatic eruptions, which gradually became phreatomagmatic, followed by a phreato-Strombolian and a violent phreatomagmatic fragmentation. A relatively long-time break, demonstrated by a paleosol between U 2 and U 3 , would have permitted the feeding of the root zone or the prominent crater by the water that sustained the next eruptive episode, dominated by subsequent phreatomagmatic eruptions. These preliminary results require complementary studies, such as geochemistry, for a better understanding of the changes in the eruptive styles, and to develop more constraints on the maar's polygenetic origin.

The Barombi Mbo Maar (BMM), which is the largest maar in Cameroon, possesses about 126 mthick well-preserved pyroclastic deposits sequence in which two successive paleosoil beds have been identified. The maar was thought to have been active a million years ago. However, layers stratigraphically separated by the identified paleosoils have been dated to shed lights on its age and to reconstruct the chronology of its past activity. The results showed that the BMM formed through three eruptive cycles: the first ~0.51 Ma ago, the second at ~0.2 Ma and the third ~0.08 Ma B.P. The ages indicate that the BMM maar-forming eruptions were younger than a million years. The findings also suggested that the maar is polygenetic. At a regional scale, the eruptive events would have occurred during some volcanic manifestations at Mt Manengouba and Mt Cameroon. Therefore, with the decrease in the recurrence time of eruptions from ~0.3 Ma to 0.1 Ma, and given the possible relation between its eruptive events and those of its neighboring polygenetic volcanoes, the BMM is expected to erupt within the next 20 ka.

Three meta-igneous bodies from the Yaounde Group have been analyzed for their petrography, geochemistry, and 207 Pb/ 206 Pb zircon ages. According to their geochemical patterns, they represent meta-diorites. The meta-plutonites yielded identical zircon ages with a mean of 624 ± 2 Ma interpreted as their intrusion age. This age is in agreement with previously published zircon ages of meta-diorites from the Yaounde Group. The meta-diorites derived mainly from crustal rocks with minor contribution from mantle material. The 87 Rb/ 86 Sr isochron ages of one meta-diorite sample and three meta-sedimentary host rocks are significantly younger than the obtained intrusion age. Therefore, they are not related to igneous processes. 87 Rb/ 86 Sr isochron ages differ from sample to sample (599 ± 3, 572 ± 4, 554 ± 5, 540 ± 5 Ma) yielding the oldest Neoproterozoic age (*600 Ma) for a paragneiss sample at a more northern location. The youngest Rb/Sr isochron age (*540 Ma) was obtained for a mica schist sample at a more southern location closer to the border of the Congo Craton. The 87 Rb/ 86 Sr whole rock-biotite ages are interpreted as cooling ages related to transpressional processes during exhumation. Therefore, several discrete metamorphic events related to the exhumation of the Yaounde Group were dated. It could be shown by Rb/Sr dating for the first time that these late tectonic processes occurred earlier at more distant northern locations of the Yaounde Group and lasted at least until early Cambrian (*540 Ma) more closely to the border of the Congo Craton.

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