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Peraluminous (P.I. < 1) and peralkaline (P.I. > 1) rhyolites from SE Lake Chad consisted of quartz, alkali feldspar and oxides–hydroxides phenocrysts, display negative Eu anomaly. F-arfvedsonite, augite–hedenbergite, aegirine and aegirine-augite are present in peralkaline rhyolites. The peralkaline and peraluminous rhyolitic magmas of the SE Lake Chad derive likely from the same source, according to their coexistence on the same volcano (Hadjer Bigli); their similar Zr/Nb, Zr/Hf ratios and their linear trends established in Zr vs Nb, Zr vs Ta and Zr vs Y diagrams, which linked to removal of successive batches of magma from a mantle source or to different degrees of partial melting. These magmas generated from the partial melting model of the underplated mantle, which would have induced the intrusion of crustal materials, triggering the hydrothermal reactions. Although, the high Zr amounts are not related to bulk assimilation of partially melted crustal materials. [ABSTRACT FROM AUTHOR]

At Mt. Bambouto, a continental stratovolcano of the Cameroon Line, magmatic activity lasted for over 20 Ma and was characterized by at least two caldera formation events. Here we present detailed mineral and whole-rock compositions of Mt. Bambouto basanites, hawaiites, trachytes and phonolites, with emphasis on caldera related volcanic rocks. These data show that differentiation took place within a complex magma plumbing system, with magma chambers occurring at different depths within the crust. Though differentiation was chiefly dominated by fractional crystallization, chemical mineral zoning of olivines, clinopyroxenes, and feldspars is also indicative of open-system processes such as magma mixing and magma chamber recharge. Chemical zoning is evident mainly in the outer 100 microns of the analyzed crystals, suggesting that magma mixing occurred shortly before eruption. The last caldera collapse at about 15 Ma also marked a clear change in the magma plumbing system. Before caldera collapse, Mt. Bambouto was characterized by a dominant production of peralkaline quartz trachytic magmas in shallow magma chambers. During this phase, evolved basic magmas (hawaiites) and strongly evolved alkaline magmas were formed in middle and upper crustal magma chambers, respectively. After emptying of the shallow quartz trachytic magma chamber and caldera collapse, magmas from the deep magmatic plumbing system were mobilized and partially mixed. This triggered eruptions of magmas on the caldera rims. [ABSTRACT FROM AUTHOR]

The near 1700 km long Cameroon Line (CL) is an African intraplate ‘fan-shaped’ alkaline volcano-plutonic rift zone of variable width (< 200 km), with a ca. 66 Ma history of magmatic activity without any systematic internal pattern of age variation. Although the CL features prominently in discussions about active rift tectonics and magmatism during the growth of the African Plate and the opening of the Central Atlantic Ocean, there is a significant diversity of opinions about its local geometry, origin and evolution. Here, we review various geodynamic models for the CL based on large data-sets of field geology, petrology, geochemistry and geochronology. Results indicate that the Line traverses both oceanic and continental domains, extending NE–SW for about 700 km on oceanic lithosphere of the Gulf of Guinea (ca. 80–120 Ma), cutting obliquely across oceanic transform faults and across the ocean–continent boundary (OCB), and then continuous for ~ 1000 km (to Kapsiki), with a significant bifurcation at about 320 km, to the Adamawa and the Biu Plateaus along complex Neoproterozoic continental lithosphere of Central and West Africa (500–800 Ma, with embedded fragments of Paleoproterozoic–Archean crust). The continental lithosphere is transected by a dense network of major late Neoproterozoic shear zones (500–650 Ma) that were linked to conterminous extensions in South America before the opening of the Atlantic Ocean. The shear zones define a “Cameroon Block” to which the continental sector of the Cameroon Line is confined, but neither this continental tectonic anisotropy, nor its oceanic transform faults, appear to have influenced the geometry of the CL. Geochemical data confirm that the mostly alkaline CL magmas originate from deep mantle upwelling with variable input from at least four different mantle end-members (DMM, HIMU, FOZO and EM-I), and with different degrees of partial melts derived from above the 410-discontinuity. These melts then interacted diversely with lithospheric mantle, but without extensive crustal contamination. The oldest igneous activity is documented on the continent at the far end of the CL by the Late Cretaceous–Early Paleocene (on the Kapsiki Plateau at Golda Zuelda), and by the Miocene, contemporaneous volcanic activity spread along the oceanic and continental sectors. The intraplate magmatism along the CL is coincident with continent-wide extensional tectonics across central and northern Africa during the Cretaceous and throughout the Cenozoic, a time during which the African Plate experienced rotation of ~ 7° and slow but variable rates of convergence with Europe, involving several episodic changes in the pole of rotation between the African and South American plates that affected the opening of the South-Central Atlantic. The CL reflects a unique long-lived geodynamic setting and there appears to be no simple explanation for its origin. [ABSTRACT FROM AUTHOR]

Highlights: [•] 40Ar/39Ar dating of basalt dykes from the Cameroon Line gave Devonian to Jurassic ages. [•] These ages not observed previously fill the existing magmatic gap in the region. [•] New clues are given for the better understanding of regional geodynamics. [Copyright &y& Elsevier]

We report whole-rock geochemistry and Sr–Nd–Pb isotopic compositions of mafic dykes intruded in the Precambrian granito-gneissic basement complex, exposed at Nyos, Batibo, Dschang and Foumban on the Cameroon Line. The dykes are alkaline (Batibo), transitional (Foumban), and subalkaline (Nyos, Batibo and Dschang) with SiO 2 of 45–54 wt% and MgO of 2–9 wt%, similar to dykes reported in other areas of the Cameroon Line (CL) and the Central Atlantic Magmatic Province (CAMP). The abundances of rare earth elements (REE) and the Primitive Mantle normalised patterns for the Nyos, Batibo and Dschang dykes are similar to those of MORB, indicating that the dykes formed at shallower depths by a higher degree of partial melting relative to the Foumban dykes and the alkaline lavas of the CL. The transitional basaltic dykes with steeper REE patterns have their sources at deeper levels in the lithospheric mantle, possibly the garnet-spinel transition zone and were generated by a lower degree partial melting of the lithospheric and plume components. The Nyos and Batibo subalkaline dykes show similar isotopic compositions with a spectrum extending from depleted (DMM-like) to enriched (EM1-like) mantle, indicating the similarity in their source components. The Dschang dykes show distinct isotopic characteristics with relatively unradiogenic Nd-Pb isotope compositions compared to the Batibo and Nyos dykes. The Foumban transitional dykes with characteristic wide ranges in Sr-Nd-Pb isotopic compositions reveal varying contributions from enriched mantle components (EM1 and EM2) in addition to its plume signature similar to those of CL lavas. The Nyos and Batibo dykes alongside other dykes on the CL have low TiO 2 abundances (<2 wt%), negative PM-normalised Nb-anomalies, and moderately to strongly enriched REE patterns, and isotopic composition that overlaps with those of CAMP, suggesting a similar lithospheric origin. • Dykes are alkaline, transitional and subalkaline. • Petrogenesis of dykes is different from Cameroon Volcanic Line activity. • Origin from the lithospheric mantle • Mixing of at least three mantle components [ABSTRACT FROM AUTHOR]

Abstract: At the north-eastern flank of Mount Bambouto, a lateral cone, the Totap volcano, is dated at 0.480±0.014Ma, which corresponds to the most recent activity of this area. The lava is a basanite similar to the older basanites of Mount Bambouto. Two new datations of the lavas of the substratum are 11.75±0.25Ma, and 21.12±0.45Ma. A synthetic revision of the volcanic story of Mount Bambouto is proposed as follows. The first stage, ca. 21Ma, corresponds to the building of the initial basaltic shield volcano. The second stage, from 18.5 to 15.3Ma, is marked by the collapse of the caldera linked to the pouring out of ignimbritic rhyolites and trachytes. The third stage, from 15 to 4.5Ma, renews with basaltic effusive activity, together with post-caldera extrusions of trachytes and phonolites. The 0.5Ma Totap activity could be a fourth stage. In the recent Quaternary, a number of basaltic activities, similar to that of the Totap volcano, are encountered elsewhere in the Cameroon Line, from Mount Oku to Mount Cameroon. The very long-live activity at Mount Bambouto and the volcanic time-space distribution in the southern Cameroon Line are linked to the working of a hotline. [Copyright &y& Elsevier]

Abstract: The Bana transitional tholeiitic basalts occurring in a Tertiary volcano–plutonic complex of the Cameroon Line, Central Africa are plagioclase-bearing and olivine-free. K/Ar dating on separated plagioclases of the transitional tholeiitic basalts yields an Oligocene age of 30.1±1.2Ma. Their clinopyroxene compositions are marked by iron enrichment and calcium depletion in the Wo–En–Fs system. The whole-rock major element compositions are characterized by Mg#∼36–48, normative quartz and hypersthene. The youngest alkali basalts from the same igneous complex have higher Mg#∼56–66. These two groups of basalt have trace element characteristics of within-plate basalt with Zr/Nb ratios of 3.7–4.5 and 7.5–9.2 respectively, and different LILE/HFSE and LREE/HREE ratios. The overall trace element characteristics suggest that the transitional tholeiitic basalts of the Bana complex were derived by high degrees of partial melting in the upper mantle at shallow depths whereas younger alkali basalts in the complex were probably produced by a small degree of melting of the same source at slightly greater depths. The transitional tholeiitic character of these basalts suggests a significant lithospheric extension and mantle upwelling below the Cameroon Line in the Oligocene. [Copyright &y& Elsevier]

The Bambouto Mountains, resting on a large Pan-African metamorphic horst of the Cameroon Line, lie between 9°57′ and 10°15′E and 5°27′ and 5°48′N in Western Cameroon. Two imbricated calderas (13×8km) are located at its summit area. This volcanic region is made up of a complex sequence of basalt, trachyte and phonolite lava flows and ignimbrites. New field investigations show that the ignimbrites, formerly considered as the first products of volcanic activity, have incorporated xenoliths of basaltic scoriae in the southern flank of the massif and of trachyte within the calderas. These observations are consistent with the occurrence of early volcanic phases: strombolian and pre-ignimbritic in the south, trachytic and penecontemporaneous with the ignimbrites within the calderas. [Copyright &y& Elsevier]

Hossere Nigo is one of several anorogenic plutonic complexes in the north-eastern part of the Cameroon Line (CL), West Africa. It is a dominantly gabbroic ring structure with a core of monzonitic to syenitic intrusive bodies. Previous geochronological work in the CL indicates a time span from 67 to 31 Ma for plutonism and from 42 Ma to Recent for volcanism. No age data for Hossere Nigo were available so far. Therefore, six whole rock samples were dated by the conventional K/Ar method. The model ages obtained range from 82 to 69 Ma but are, however, in conflict with the relative succession of emplacement. We therefore performed 40Ar/39Ar laser fusion analyses on single crystals of phlogopite separated from one of the conventionally dated rocks. Their 40Ar/39Ar isochron age of 65.0±0.8 Ma (1σ) is about 7 Ma younger than the related whole rock K/Ar date. Elevated whole rock ages are ascribed to hydrothermal overprinting and associated uptake of excess argon. By contrast, the reported Ar/Ar age provides a precise and accurate fix point for an early stage of CL intrusive magmatism. [Copyright &y& Elsevier]

• Cameroon line has an equivalent magmatic province on the NE Brazil conjugate margin. • Alkaline magmatism postdates continental separation by 40-50 m.y. and still active. • Mafic rocks in two provinces chemically indistinguishable and lack age progression. • Two hotspots can't be caused by mantle plumes but must originate in shallow mantle. • Sublithospheric drainage of enriched lithospheric mantle beneath Pangea is proposed. Intraplate magmatism is enigmatic in origin despite its importance in our understanding of crustal cycling through the deep mantle. A mantle plume origin is justified for some intraplate magmatism, but not in the case of a large number of occurrences. Here we present a novel explanation for intraplate magmatism in situations where evidence for a plume origin is either lacking or equivocal. Specifically, we highlight two voluminous, long-lived intraplate magmatic provinces located on the precisely conjugate continental margins of Cameroon and NE Brazil and which lasted, respectively, from 65 and 52 Ma to the present. New Ar dating and geochemical data show that magmatism in the two provinces was contemporaneous, identical in incompatible-element composition, and started >40 Myr after continental separation, when the two margins were ∼2000 km apart. Lack of age progression in magmatic activity rules out a mantle plume origin. We propose an origin in sub-continental lithospheric mantle that was thickened during Gondwana supercontinent assembly. Thermal re-equilibration of the thickened lithosphere accompanied by percolation of carbonate-rich melt led to the formation of a thick zone of newly created, enriched asthenosphere that is held in place by buoyancy, prevented from dispersing by adjacent lithospheric blocks, and heated by radioactive decay. Following continental breakup at 105 Ma, slow outward drainage of this enriched and heated asthenosphere was channelled into thinner parts of the continental and oceanic lithosphere. Sublithospheric drainage and decompression melting of this enriched mantle provides a viable explanation for these and many other intraplate magmatic occurrences. [ABSTRACT FROM AUTHOR]