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Tin mineralization of significant economic importance occurs across the continental portion of the Cameroon Line (CL). Tin deposits therein occur as both primary and secondary (residual and alluvial) ore. Though the temporal and, by inference, the genetic link between Sn mineralization and the host granite had long been modeled and widely accepted worldwide, in the CL, however, the age of the granite hosting cassiterite is poorly constrained, preventing a robust assessment of the temporal and genetic relationship between the Sn mineralization and its host rock. Here, we present in-situ zircon and cassiterite laser ablation inductively coupled mass spectrometry (LA-ICP-MS) U-Pb data in order not only to constrain the age of the granitic rock hosting the primary Sn ore but also to bracket the time frame of Sn mineralization, with respect to the magmatic-hydrothermal evolution of the parental magma of the host granite. Zircon from two greisen-altered, cassiterite-bearing granite samples yield overlapping and concordant ages of 64.21 ± 0.59 Ma and 65.46 ± 0.95 Ma, respectively, which are also overlapping with regional granite magmatism in the CL (ca. 65–30 Ma). On the other hand, cassiterite, which is spatially associated with the Paleocene zircon, yields Lower Eocene ages of 54.99 ± 0.35 Ma and 56.08 ± 0.46 Ma. The ca. 10 Myr time gap between zircon and cassiterite suggests that the granite is a passive host not genetically related to the Sn mineralization, which may be linked to a younger, concealed intrusion of ca. 55 Ma. This finding contrasts with the most widely accepted petrogenetic model of tin granite, according to which Sn mineralization and the host granite are cogenetic. [ABSTRACT FROM AUTHOR]

In this study, field descriptions of the dolerite dykes cross‐cutting the Pan‐African basement (DPB) and the dolerite and gabbroic dykes cross‐cutting the Cretaceous sedimentary basin (DGCSB) are used together with petrographic, bulk‐rock geochemistry, as well as Sr–Nd–Pb isotopic data to constrain the geodynamic setting and origin of these mafic (basanite, basalt and basaltic andesite in composition) rocks that outcrop in the Figuil–Léré area. The DPB dykes are alkaline with SiO2 of 46.63–49.03 wt% and MgO of 3.93–7.11 wt%. In contrast, the DGCSB dykes are subalkaline with SiO2 ranging between 50.24 and 53.16 wt% and MgO fluctuating between 5.22 and 7.90 wt%, comparable to dykes described in other regions of the Cameroon Line (CL). The rocks studied belong to the tholeiitic series and were emplaced in within‐plate basalts (WPB) and mid‐ocean ridge basalts (MORBs) setting. The variations in major and trace elements from the basanite to basaltic andesite compositions are dependable with fractional crystallization (FC) of different mineral phases such as opaque minerals, olivine, clinopyroxene, amphibole and alkali feldspar. Most samples exhibit negative Nb–Ta and positive Sr anomalies, which together with the negative correlation between MgO and 87Sr/86Sr, and isotopic ratio of 206Pb/204Pb <19.5 (18.27–19.49) suggests interactions of crustal components. Nevertheless, this contamination seems limited because there is a positive correlation between SiO2 and 143Nd/144Nd plot and low Ba contents (72–210 ppm) of the samples compared to the mid‐continental crust (259–628 ppm). Trace element modelling proposes a derivation of the Figuil–Léré dykes from parental melts generated by 4%–15% partial melting of a source containing garnet peridotite in the mixed lithospheric–asthenospheric mantle. The isotopic data of the dykes studied with hybrid magma composition are characterized by more radiogenic 87Sr/86Sr, 143Nd/144Nd and 206Pb/204Pb isotopic ratios with a dominant mixing of depleted MORB mantle (DMM)–enriched mantle 1 (EM1)–enriched mantle 2 (EM2), like those of other mafic dykes of the CL. [ABSTRACT FROM AUTHOR]

The Mayo-Darley massif is an anorogenic complex of the Cameroon Line that has a mantle origin. Tin-bearing rocks were analyzed by ICP-AES and INAA analytical methods. The purpose of this work was to provide new geochemical data for the Mayo-Darley tin formation and to understand its petrogenesis and the origin of tin mineralization. The Mayo-Darley tin deposit is made up of tin-granite, and tin-greisen, greisenification was developed on the borders of quartz dykes. These rocks belong to the alkaline series and were classified into acid (SiO2 = 61.6%–73.8%; 65.4%–98% respectively) and basic (42.9%–47% SiO2) rocks. They showed enrichment in HSFE, LILE, Ga/Al and chondrite normalized REE patterns indicating LREE enrichment relative to HREE with a negative Eu anomaly, only sample SB8 of tin-granite showed Eu/Eu* = 1.11. Rocks display metaluminous, peralkaline, peraluminous, ferroan, high-K calc-alkaline to shoshonitic, alkalic to calcic affinity, and crystallized at 800 °C. The chemistry of this deposit reflects the primary composition of granite, quartz monzonite, gabbro, and foid gabbro. This complex experienced multi-stage sub-solidus hydrothermal fluid reactions and shows variable alteration of feldspars alkali mobility. The rocks are classified as A1-type granite, overlap with the OIB field, and were derived from a within-plate setting, similar to mantle non plume-derived magmas. The origin of tin mineralization in Mayo-Darley has a complex evolution, tin mineralization was derived from hydrothermal and hydrogenous metal-rich deposits and shows Sn–Hf–Zr and Sn–Tl–Nb association. These new data confirm the complexity of the Mayo-Darley tin complex and elucidate the origin of tin mineralization. [ABSTRACT FROM AUTHOR]

The current work presents the composition and evolutionary trends of amphiboles from alkaline volcanic and subvolcanic complexes along the Cameroon Line (CL). The chemical compositions of amphiboles from the CL cover the domains of calcic, sodi-calcic, sodic, and incidentally ferromagnesian amphiboles. Calcic amphiboles are ubiquitous and cover the full range of composition both in plutonic and volcanic rocks. Sodi-calcic (richterite—Fe-richterite—taramite—Fe-winchite) to sodic amphiboles (arfvedsonite—riebeckite—Fe-eckermannite) are found preferentially in intermediate and felsic rocks, whereas Fe–Mg amphiboles straddle the cummingtonite-grunerite series with Si ranging from 7 to 8 apfu. The crystallization process of amphiboles along the Cameroon Volcanic Line (CVL) is either magmatic or hydrothermal. Two crystallization sequences can be distinguished in the alkaline rocks from the CL: (i) an early sequence related to early magmatic stages during which calcic amphiboles crystallize directly either from magma or from pyroxenes; and (ii) a second sequence characterizes the late stages which corresponds to the crystallization of sodi-calcic amphiboles. These amphiboles are usually associated with clinopyroxene, and the anteriority of one of the above minerals relative to the other might depend on their chemical composition: calcic and sodic-calcic amphiboles crystallize later relative to calcic and sodic-calcic pyroxenes, respectively, while the crystallization of sodic amphiboles is prior to that of sodic pyroxenes. Their main transformations are pseudomorphosis into opaque minerals or pyroxene, chloritisation, and destabilization into calcite. [ABSTRACT FROM AUTHOR]

Brittle deformation and geomorphological studies were carried out to better understand the morphostructure and spatial distribution of rocks of the Nkogam anorogenic complex, located between 5° 35′ 52″ and 5° 47′ 20″ N and 10° 37′ 16″ and 10° 48′ 37″ E. The objective of this study is to correlate the distribution of the main petrographic units with the orientations of major regional fractures and understand the shape of the Nkogam massif. The methodology used to enhance the quality of the images and the extraction of lineaments are pre-processing and processing, performed with Erdas Images 2014. The data used are SRTM and Landsat 8 OLI images acquired by satellites on December 29, 2018. The results highlight a U-shaped form and a central depression that opens to the SW. Major lineaments directions are NE-SW and ENE-WSW which are respectively similar to that of the Cameroon Line and Central Cameroon Shear Zone. However, a fault corridor oriented N170°E cuts across massive basalts, ignimbrites, and granites. The emplacement of all the rocks in the field indicates the following relative chronology: older Pan-African gneissic basement, plateau basalts, granites, syenogranites, ignimbrites, and massive basalts. Granites and syenogranites, then ignimbrites and basalts, cut through the Pan-African basement in three mountain ranges among which two are roughly parallel, one oriented NNE-SSW and the other oriented NE-SW. All these directions locally represent the N070°E-trending Adamawa shear zone on a regional scale. Volcanites overlap plutonites at the northeastern end of the Nkogam anorogenic complex giving the U-shaped form to the massif. This study helps to understand the subcircular shape of the Nkogam massif bearing a central depression and the major directions of the lineaments in order to compare them to similar massifs along the Cameroon Line and elsewhere. [ABSTRACT FROM AUTHOR]

The Late Cretaceous-Quaternary Cameroon Volcanic Line (CVL) is a 1600km long chain of volcanoes that straddles the continent-ocean boundary and extends from the Gulf of Guinea to the interior of the African continent. The magmatic activity started at 70Ma and has continued until the present. The products of this magmatic activity are distinctive in terms of petrology and isotope geochemistry, the variety of volcanic rocks ranging from ultrabasic, alkaline to sub-alkaline lavas to highly evolved alkaline lavas with isotopic compositions indicating complex combinations of both sub-lithospheric (HIMU, EM, DMM) and lithospheric components (sub-continental lithospheric mantle and crust). We conducted a petrological and geochemical study of a set of volcanic rocks, sampled from the rim and interior of the Miocene Mt Bambouto caldera, one of the 12 main volcanic centres of the CVL. The rocks were analysed for their whole-rock major and trace element contents, 40Ar/39Ar ages and whole-rock Sr-Nd-Pb-Os isotopic compositions. Phonolites and quartz-trachytes of the Mt Bambouto caldera are derived by fractional crystallization of highly alkaline and moderately alkaline parental basic magmas, respectively. Assimilation of the shallow crust has affected both alkaline and subalkaline magmas, suggesting that the petrogenesis of the differentiated rocks cannot be explained by crustal contamination alone. Only minor amounts (usually less than 5%) of assimilation of upper crustal silicic rocks from the local Pan-African basement are required to produce the most differentiated compositions. The rocks with the highest crustal contribution are Q-normative trachytes from peripheral cones, as well as one Ne-trachyte. Mt Bambouto basic-ultrabasic rocks, including basanites and alkali-basalts with high 187Os/188Osi, might have experienced some crustal contamination, but it must have been a limited process. Some Mt Bambouto ultrabasic to basic rocks show large ion lithophile element enrichment, notably of Sr, Ba and P compared with Zr. These samples also have relatively radiogenic Sr and unradiogenic Pb isotopic compositions. Such compositions are similar to those of the high-Sr group identified by previous studies. Most of the basic rocks do not show such characteristics and are identified as a low-Sr group. We interpret the geochemical characteristics of the high-Sr group as resulting from the partial melting of a depleted mantle (DMM-like) peridotite source containing pyroxenite veins that had interacted with carbonatitic fluids. To test this hypothesis, we used a new modellingapproach based on Monte Carlo simulation; this method has the advantage of deciphering how different mantle components interacted through time. Our modelling confirms the plausibility of a three-component source. In addition, it suggests that the carbonatitic fluid first mixed with the pyroxenititic component and the resulting melt interacted with a DMM-like mantle. Both high-Sr and low-Sr groups can be produced by such a mixing scenario but with a stronger contribution of the carbonatitic fluid for the high-Sr group. At the time of melting, these source components could have been located in a metasomatized region of the sublithospheric mantle (uppermost section of the asthenosphere) or in the sub-continental lithospheric mantle. [ABSTRACT FROM AUTHOR]

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]

Manganese-bearing rocks consisting of anhedral Mn- and Fe-oxyhydroxides (cryptomelane, goethite), kaolinite, tiny cerianite and zircon grains (< 10 µm), and euhedral to subeuhedral quartz crystals, are hosted within rhyolitic rocks of Hadjer Bigli volcano (SE Lake Chad). Their Ce (1328 ppm) and Zn (1563 ppm) contents are increased, with high SREE values (1573 ppm).The Mn-rich rocks have been shown to be one of the most important environments for adsorption of some trace elements. The high Zn contents are considered to be the result of water-rock chemical reactions at intermediate to high temperatures. The sedimentary processes led likely to the concentration of manganese in the Lake Chad water and beneath, well before the volcanic activity. According to mineral chemistry and the whole-rock data, rhyolitic magma interacted with the near-surface manganese formations already concentrated by sedimentary processes. [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]

The monotypical orphnine genus Stenosternus Karsch is known from a single specimen of S. costatus collected on the São Tomé island (Gulf of Guinea). The holotype of S. costatus Karsch is re-examined and its characters are discussed and illustrated. Although the genus was implicitly placed by Paulian (1984) in the Old World tribe Orphnini Erichson, re-examination of the holotype of S. costatus shows that it has characters similar to those of the members of the New World tribe Aegidiini Paulian. Placement of Stenosternus in the Aegidiini is supported by the metepisternum widened posteriorly (forming posterior metepisternal lock for closed elytra) and a keel separating basal and anterolateral parts of propleurae. Relationships of Stenosternus with other orphnine taxa and possible ways of origin of São Toméan orphnine fauna are discussed. [ABSTRACT FROM AUTHOR]

The Mount Cameroon coastline is believed to be affected by two main morphological, dynamic, environmental problems: the erosion of the Atlantic slopes of the mountains and hills, and the accumulation of sediments on the continental platform. Erosion affects the coastal slopes, as evidenced by the existence of erosion domes observed both on the Atlantic slopes and the coastlines. Three factors explain the deterioration of the Cameroon coastline: (1) its location with a vulnerable region known as the Cameroon Line, (2) the persistent impact of climate and océanographie conditions, and (3) the topographic nature of the region. A high percentage of this coastal area's water infiltrates the soil, weathering the pyroclastic material. Concentrated human population in the area accelerates some of the erosional processes. In fact, human activity could be the primary cause of the degradation; nevertheless, natural-erosion processes remain prominent in the ongoing degradation of this fragile environment. The dynamics of waves and tides alter Mount Cameroon coastal features. The intensity of those waves and tides vary from place to place. The Mabeta mangrove and the surrounding coastal creeks experience a high rate of sedimentation. The area between Man O' War Bay and Mabeta experiences intense weathering because the rocks are old. The area between Limbe and Batoke experiences a high rate of erosion caused primarily by wave action because of the poor erosive action of the few rivers. The high erosion could provide sediments to stabilise the coastline; however, the sediments do not finish their progression to the sea. Therefore, coastal development depends mainly on the coastal current, fluvial discharge, and high rainfall in Debundscha. Tectonic variability associated with landslides also represents a significant natural hazard in this complex environment. [ABSTRACT FROM AUTHOR]

Late Cretaceous peralkaline rhyolites (69.4 ± 0.4 Ma -- 2σ analytical, 1.9 Ma full external uncertainty, 40mr/39Ar sanidine single crystal laser dating) from Lake Chad are the oldest lavas of the "Cameroon Hot Line". These lavas (pantellerite and comendite), consisting of quartz ± alkali feldspar ± fluoro-arfvedsonite ± augite-hedenbergite ± aegirine ± fayalite (Fa97-99) and ilmenite, are characterized by a progressive increase in the total REE contents and the magnitude of the negative Eu anomaly owing to the alkali feldspar-dominated fractionation. Two groups of peralkaline rhyolites have been distinguished according to mineralogical and geochemical data. Both, group- 1 (LaN/YbN: 8.4-9.0; Zr/Nb: ~ 8) and group-2 (LaN/YbN: 10.7-12.6; Zr/Nb: ~ 6) are genetically related. The peralkaline rhyolites from Lake Chad could not be solely the product of fractional crystallization from a basaltic parental magma. The influx of F-rich fluids could have greatly modified the composition of rhyolitic magmas as attested by the presence of fluoro-arfvedsonite. [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]

FeO*-Al2O3-TiO2-rich rocks are found associated with transitional tholeiitic lava flows in the Tertiary Bana plutono-volcanic complex in the continental sector of the Cameroon Line. These peculiar rocks consist principally of iron-titanium oxides, aluminosilicates and phosphates, and occur as layers 1–3 m thick occupying the upper part of lava flows on the southwest (site 1) and northwest (site 2) sites of the complex. Mineral constituents of the rocks include magnetite, ilmenite, hematite, rutile, corundum, andalusite, sillimanite, cordierite, quartz, plagioclase, alkali feldspar, apatite, Fe-Mn phosphate, Al phosphate, micas and fine mixtures of sericite and silica. Texturally and compositionally, the rocks can be subdivided into globular type, banded type, and Al-rich fine-gained massive type. The first two types consist of dark globule or band enriched in Fe-Ti oxides and apatite and lighter colored groundmass or bands enriched in aluminosilicates and quartz, respectively. The occurrence of andalusite and sillimanite and the compositional relations of magnetite and ilmenite in the FeO*-Al2O3-TiO2-rich rocks suggest temperatures of crystallization in a range of 690–830°C at low pressures. The Bana FeO*-Al2O3-TiO2-rich rocks are characterized by low concentrations of SiO2 (25–54.2 wt%), Na2O + K2O (0–1%), CaO (0–2%) and MgO (0–0.5%), and high concentrations of FeO* (total iron as FeO, 20–42%), Al2O3 (20–42%), TiO2 (3–9.2%), and P2O5 (0.26–1.30%). TiO2 is positively correlated with Al2O3 and inversely correlated with FeO*. The bulk rock compositions cannot be derived from the associated basaltic magma by crystal fractionation or by partial melting of the mantle or lower crustal materials. In ternary diagrams of (Al2O3)−(CaO + Na2O + K2O)−(FeO*+ MnO + MgO) and (SiO2)−(FeO*)−(Al2O3), the compositional field of the rocks is close to that of laterite and is distinct from the common volcanic rocks, suggesting that the rocks are derived from lateritic materials by recrystallization when the materials are heated by the basaltic magmas. A hydrothermal origin is discounted because the rocks contain high-temperature mineral assemblages and lack sulfide minerals. It is proposed that the FeO*-Al2O3-TiO2-rich rocks of the Bana complex were formed by pyrometamorphism of laterite by the heat of basaltic magmas. [ABSTRACT FROM AUTHOR]

Abstract: The felsic lavas of the Bamenda Mountains, in the main part of the Cameroon Volcanic Line, are mainly represented by trachytes, with subordinated benmoreites and alkaline to peralkaline rhyolites. New K–Ar geochronological data define two main volcanic episodes, the first one between 18 and 22Ma, and the second one from 12.5 to 13.5Ma. Geochemical data indicate that these felsic rocks mainly originated through a fractional crystallization process from mafic magmas. Crustal contamination also occurred during the magma evolution. These new data confirm that there is no time evolution of the volcanism along the Cameroon Volcanic Line and show a similar magma genesis process through space and time. [Copyright &y& Elsevier]

Abstract: Basaltic lavas of the Kumba Plain contain mantle-spinel peridotite xenoliths. Mineralogical characteristics, together with major and trace element compositions, both in bulk rocks and clinopyroxenes, point out that the upper mantle beneath the Kumba volcanic plain has an essentially lherzolitic composition. This mantle is fertile or only affected by a very low degree of partial melting and cryptic metasomatism. The Kumba upper mantle appears to be homogeneous and very different from the other mantle peridotite occurrences from the Cameroon Line, especially those from the upper mantle beneath the Nyos volcanic plain. [Copyright &y& Elsevier]

Aim Amphibians are a model group for studies of the biogeographical origins of salt-intolerant taxa on oceanic islands. We used the Gulf of Guinea islands to explore the biogeographical origins of island endemism of one species of frog, and used this to gain insights into potential colonization mechanisms. Location São Tomé and Príncipe, two of the four major islands in the Gulf of Guinea, West Africa, are truly oceanic and have an exceptionally high biodiversity. Methods Mitochondrial DNA is used to test the endemic status of a frog from São Tomé and compare it with congeneric taxa from tropical Africa. Existing data on surface currents, surface salinity, atmospheric circulation and bird migration in the Gulf of Guinea are summarized to address hypotheses concerning colonization mechanisms. Results The endemic status of Ptychadena newtoni (Bocage) is supported here by mitochondrial DNA sequences, and analysis of this and other molecular data indicates that an East African species close to Ptychadena mascareniensis (Duméril and Bibron) is its nearest relative. We refute the possibility that this population was anthropogenically introduced, in favour of a natural dispersal mechanism. Main conclusions With six endemic frogs and one caecilian, the Gulf of Guinea islands harbour a diverse amphibian fauna. Five of these species appear to have their closest relatives in East Africa. Insufficient evidence exists for transportation by storms, birds or rafts alone. However, we propose a synergy of rafting, favourable surface currents and a reduction in salinity of surface waters. Catastrophic events, or wet periods in climatic history, could allow freshwater paths to open far enough to enable continental flora and fauna to reach these and other isolated oceanic islands. [ABSTRACT FROM AUTHOR]

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]

Abstract: Camptonites have been discovered at Mount Cameroon. They contain phenocrysts of olivine + clinopyroxene + plagioclase ± kaersutite ± bi and ocelli of nepheline or analcite and carbonates. 87Sr/86Sri values are similar to those of the basalts of Mount Cameroon . The camptonites are interpreted as resulting from fractional crystallization of a basaltic magma generated by a low degree of melting of an infra-lithospheric metasomatized mantle tapped by huge lithospheric cracks. To cite this article: I. Ngounouno et al., C. R. Geoscience 338 (2006). [Copyright &y& Elsevier]

Abstract: Mount Bangou, an Eocene volcano (40K–40Ar ages between 44.7 and 43.1±1 Ma) is the oldest dated volcano of the Cameroon Line. In this region, two magmatic series, evolving by fractional crystallization, show transitional affinities that are exceptionally known in this sector. Mineral compositions of basaltic rocks (scarce modal olivine and occurrence of normative hypersthene) as well as geochemical characteristics (low Ba, La, Ta contents and high Y/Nb ratios) are in agreement with this trend. The succession of magmas evolving in time from transitional to more typical alkaline compositions is evidenced in a continental setting. To cite this article: J. Fosso et al., C. R. Geoscience 337 (2005). [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]

Dykes of monchiquites of Cainozoic age (37.5±2.3 Ma) are intrusive in Cretaceous sandstones in the Tchircotche´ area, Upper Benue valley (northern Cameroon). The differentiation of the monchiquites is explained by crystal fractionation of olivine, clinopyroxene, magnetite, ilmenite and apatite. A studied rock has low Sr-isotope ratio (0.703 69±10), which is similar to those of the alkali basalts of the Cameroon Line. Therefore the continental crust appears to have no significant role in the genesis of the monchiquites. [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]

Major element, trace element and Sr−Nd−Pb isotopic compositions of ultramafic xenoliths and megacrysts from the continental Cameroon line provide evidence for metasomatism of the upper most lithospheric mantle by enriched melts during the Mesozoic The megacrysts probably crystallized within the lower continental crust from melts similar to the host magmas. All the xenoliths originated as depleted residues after the extraction of basaltic melts, but some indicate evidence of interaction with enriched partial melts before entrainment. The U−Pb isotopic data on garnet are consistent with cooling through >900°C at >300 Ma. The Sm−Nd isotope systematics in constituent phases appear to have been in equilibrium on a xenolith scale at the time of entrainment, indicating derivation from mantle that remained at temperatures >600°C until eruption. Spinel therzolies that show simple light rare earth element (LREE) depletions are characterized by isotopic compositions that are comparable with, but slightly more depleted than Atlantic N-MORB, suggesting that the unmetasomatized sub-continental lithosphere of the Cameroon line may be isotopically similar to that of sub-oceanic lithosphere. The Nd-depleted mantle model ages of these xenoliths are consistent with late Proterozoic depletion, similar in age to much of the overlying continental crust. In contrast, samples that have LREE-enriched clinopyr-oxenes (La/Yb =4.7–9.4) contain trace amounts of amphibole, are enriched in U and have more radiogenic Pb and Sr. These xenoliths yield U−Pb and Sm−Nd model ages consistent with Mesozoic enrichment, in agreement with the age of enrichment of the source regions of the basalts, as deduced from Pb isotopic compositions. Clinopyroxenes record three orders of magnitude enrichment in U and LREE accompanied by progressive K depletion associated with the growth of trace amphibole, with K/U ratios that range from 12000 to 1. The ratios of the trace elements thought to have similar bulk D in mantle melting, Ce/Pb, Ba/Rb and Nd/Sr ratios, display regional variations related to the time integrated history of enrichments indicated by Nd isotopic compositions. Mass balance calculations suggest that the melts responsible for the most recent enrichment of the lithosphere had higher La/Yb and U/Pb than Cameroon line host magmas, and were probably the product of small degrees of partial melting associated with the earliest stages of the breakup of Pangea. [ABSTRACT FROM AUTHOR]

• 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]