Volcanic Line Research Articles

Origin and evolution of primitive melts from the Debunscha Maar, Cameroon: Consequences for mantle source heterogeneity within the Cameroon Volcanic Line

Debunscha Maar is a monogenetic volcano forming part of the Mt. Cameroon volcanic field, located within the Cameroon Volcanic Line (CVL). Partly glassy cauliflower bombs have primitive basanite-picrobasalt compositions and contain abundant normally and reversely zoned olivine (Fo 77–87) and clinopyroxene phenocrysts. Naturally quenched melt inclusions in the most primitive olivine phenocrysts show compositions which, when corrected for post-entrapment modification, cover a wide range from basanite to alkali basalt (MgO 6.9–11.7wt%), and are generally more primitive than the matrix glasses (MgO 5.0–5.5wt%) and only partly fall on a common liquid line of descent with the bulk rock samples and matrix glasses. Melt inclusion trace element compositions lie on two distinct geochemical trends: one (towards high Ba/Nb) is thought to represent the effect of various proportions of anhydrous lherzolite and amphibole-bearing peridotite in the source, while the other (for example, high La/Y) reflects variable degrees of partial melting. Comparatively low fractionation-corrected CaO in the melt inclusions with the highest La/Y suggests minor involvement of a pyroxenite source component that is only visible at low degrees of melting. Most of the samples show elevated Gd/Yb, indicating up to 8% garnet in the source. The range of major and trace elements represented by the melt inclusions covers the complete geochemical range given by basalts from different volcanoes of the Cameroon volcanic line, indicating that geochemical signatures that were previously thought to be volcano-specific in fact are probably present under all volcanoes. Clinopyroxene-melt barometry strongly indicates repeated mixing of compositionally diverse melts within the upper mantle at 830±170MPa prior to eruption. Mantle potential temperatures estimated for the primitive melt inclusions suggest that the thermal influence of a mantle plume is not required to explain the magma petrogenesis.

New thermoluminescence age estimates for the Nyos maar eruption (Cameroon Volcanic Line).

Nyos maar is located in the Cameroon Volcanic Line and generates a multitude of primary and secondary hazards to the local population. For risk assessment and hazard mitigation, the age of the Nyos maar eruption provides some vital information. Since previous dating efforts using a range of techniques resulted in vastly varying eruption ages, we applied thermoluminescence (TL) methods to obtain independent and direct chronological constraints for the time of maar formation. Target minerals were granitic quartz clasts contained in pyroclastic surge deposits. Thermoluminescence plateau results prove that heat and/or pressure during the phreatomagmatic eruption was sufficient to reset the inherited luminescence signal of granitic bedrock quartz. Parallel application of three TL measurement protocols to one of the two samples gave consistent equivalent doses for the quartz ultra-violet emission. Despite the robustness of our dose estimates, the assessment of the dose rate was accompanied by methodological challenges, such as estimation of the original size distribution of quartz grains in the pyroclastic deposits. Considering results from additional laboratory analyses to constrain these uncertainties, we calculate an average maximum TL age of 12.3 ± 1.5 ka for the Nyos maar eruption. Based on these new data, a more solid risk assessment can be envisaged.

Strike-slip tectonics within the northernmost Philippine Sea plate in an arc-continent collisional setting

The geological processes in the northernmost Philippine Sea plate, which is bounded by the Suruga and Sagami troughs, are a typical example of an active collision zone. We attempt to illustrate the stress field through seismic estimations and geodetic analysis and propose the kinematic mode of the northernmost tip of the Philippine Sea plate. Seven events (M≥4.0) are chosen for waveform inversion by the ISOLA software to distinguish the stress field. In particular, six of the chosen events, which exhibit strike-slip motion, are distributed in the eastern area, where few focal mechanisms have been reported by previous studies. According to the available focal mechanisms, strike-slip faults with similar P and T axes are widely distributed in the study area. The stress inversion suggests that the northern area is characterized by a NW-SE compression and a NE-SW extension stress regime, although some spatial differences exist. As indicated by an analysis of the geodesy, epicenters, focal mechanisms, gravity anomalies and velocity structure, the deformation in the northernmost tip is mainly accommodated by several conjugate strike-slip fault systems with steep dips that center on the Izu volcanic line. Generally, the maximum principal stress of the kinematics is derived from the collision between the Philippine Sea plate and Central Japan. Because of the different subduction angles, rates and directions of the down-going plate, diverging slab-pull forces along the Suruga and Sagami troughs may be causing the NE-NNE extension in most of the areas that are bounded by the two troughs. The extension propagates southwards along the Izu volcanic line and reaches the area adjacent to Miyake-jima.

Pleistocene refugia and genetic diversity patterns in West Africa: Insights from the liana Chasmanthera dependens (Menispermaceae).

Processes shaping the African Guineo-Congolian rain forest, especially in the West African part, are not well understood. Recent molecular studies, based mainly on forest tree species, confirmed the previously proposed division of the western African Guineo-Congolian rain forest into Upper Guinea (UG) and Lower Guinea (LG) separated by the Dahomey Gap (DG). Here we studied nine populations in the area of the DG and the borders of LG and UG of the widespread liana species, Chasmanthera dependens (Menispermaceae) by amplified fragment length polymorphism (AFLP), a chloroplast DNA sequence marker, and modelled the distribution based on current as well as paleoclimatic data (Holocene Climate Optimum, ca. 6 kyr BP and Last Glacial Maximum, ca. 22 kyr BP). Current population genetic structure and geographical pattern of cpDNA was related to present as well as historical modelled distributions. Results from this study show that past historical factors played an important role in shaping the distribution of C. dependens across West Africa. The Cameroon Volcanic Line seems to represent a barrier for gene flow in the present as well as in the past. Distribution modelling proposed refugia in the Dahomey Gap, supported also by higher genetic diversity. This is in contrast with the phylogeographic patterns observed in several rainforest tree species and could be explained by either diverging or more relaxed ecological requirements of this liana species.

Restitic or not? Insights from trace element content and crystal — Structure of spinels in African mantle xenoliths

The lithospheric architecture of Africa consists of several Archean cratons and smaller cratonic fragments, stitched together and flanked by polycyclic fold belts. Here we investigate the structure and chemistry of spinels from lithospheric mantle xenoliths from distinct tectonic settings, i.e. from the Saharan metacraton in Libya (Waw-En-Namus) which could show archaic chemical features, Cameroon (Barombi Koto and Nyos Lakes) where the Sub Continental Lithospheric Mantle was modified during the Pan-African event and fluxed by asthenospheric melts of the Tertiary Cameroon Volcanic Line and Morocco (Tafraoute, Bou-Ibalrhatene maars) in the Middle Atlas where different metasomatic events have been recorded. From a structural point of view it is to notice that the Libyan spinels can be divided into two groups having different oxygen positional parameter (u>0.2632 and u<0.2627, respectively), while those from Cameroon are in between those values as the Moroccan ones already studied by other authors. The intracrystalline closure temperature (Tc) of the here studied spinels is different among the different samples with one Libyan group (LB I) showing Tc in the range 490–640°C and the other 680–950°C (LB II). Cameroon and Morocco spinels show a Tc in the range 630–760°C. About 150 different spinels have been studied for their trace element content and it can be seen that many of them are related to Cr content, while Zn and Co are not and clearly distinguish the occurrences. Differences in the trace element chemistry, in the structural parameters and in the intracrystalline closure temperatures suggest that a different history should be considered for Cameroon, Morocco and LB I and LB II spinels. Even if it was not considered for this purpose, we tentatively used the Fe2+/Fe3+ vs. TiO2 diagram that discriminate between peridotitic and the so-called “magmatic” spinels, i.e. spinel crystallized from melts. LB I and LB II spinels plot in the peridotitic field while Cameroon and Morocco spinels fall in the magmatic one. Consequently, the xenoliths sampled from a probably juvenile SCLM at the edge of the most important lithospheric roots (i.e. Cameroon and Morocco) apparently have spinels possibly fractionated in situ from percolating melts and do not represent a real spinel-peridotite facies. On the contrary mantle xenoliths from Libya exhibit spinels with peridotitic features compatible with a slow ascent of a mantle diapir (plume).

Chapter 23 Gem deposits of Myanmar

The aim of this chapter is to review the locations, mineralogy and briefly the origins of the major gem deposits in Myanmar. Myanmar is fortunate in containing world-class gem deposits, such as the finest pigeon's-blood-coloured ruby from the Mogok Stone Tract in Mandalay Division, the Mong Hsu ruby deposit in Shan State and imperial coloured jadeite jade from Kachin State (Fig. 23.1). For the purposes of description the gemstone deposits in Myanmar are classified into: (1) primary hard-rock deposits in which the gemstones are hosted in the rocks in which they were crystallized or in their weathered products; and (2) secondary deposits in which the gemstones have been weathered out of their original (primary) host rock, transported and deposited as placer deposits in alluvial sediments. Fig. 23.1. Locations of the Jade Mine, Mogok Stone Tract and Mong Hsu areas and the tectonic zones in Myanmar: 1, the Arakan (Rakhine) coastal strip; 2, Indo-Myanmar Ranges; 3, Western Inner-Myanmar Tertiary Basin; 4, Central Volcanic Belt (Central Volcanic Line); 5, Eastern Inner-Myanmar Tertiary Basin; 6, Shan-Tenasserim Massif (map modified after Bender 1983; Khin Zaw 1989, 1990; Khin Zaw et al. 2015). Small white circles represent extinct or dormant Tertiary volcanoes. Examples of the primary-type gems are ruby in marble (Mogok, Mong Hsu and Sagyin), sapphire in syenite pegmatite, syenite and nepheline syenite (Mogok) and jadeite in serpentinized peridotite (Tawmaw). Other precious gemstones from primary deposits include spinel in marble (Mogok), peridot in dunite/peridotite (Mogok), topaz, aquamarine, tourmaline, quartz, etc. in pegmatite (Mogok, Mong Hsu and Momeik) and kyanite in quartzite (Moehnyin). Rare gemstones are often found in primary gem deposits in the Mogok Stone Tract and include johachidolite and hackmanite from pegmatites, painite from leucogranite and skarn, pollucite, petalite, phenakite, pezzottaite, monazite, danburite, jeremejevite and hambergite from pegmatites in the Momeik area. …

Chapter 8 The Eastern (Back-arc) Basin of Central Myanmar: Basement rocks, lithostratigraphic units, palaeocurrents, provenance and developmental history

The Eastern Basin (Back-arc Trough) was initiated on the West Myanmar Block (WMB) and occupies the eastern half of the Central Myanmar Basin (CMB), between the Central Volcanic Line (CVL) and the Sagaing Fault. Sub-basins of the CMB share a similar origin, related to the collision between India and Asia. Field evidence shows that Upper Palaeozoic carbonates and Mesozoic ophiolitic sediments and their metamorphosed derivatives form the basement rocks in the eastern and northern parts of Eastern Basin (EB), while igneous rocks of the Wuntho-Salingyi Mesozoic Arc form the basement in the western portion of the basin. The basal Tertiary unit of the EB is the Lower Eocene Tonkyauk Chaung Conglomerate, overlying the eroded surface of the pre-Tertiary rocks in the Shwebo Basin (SBB). With the exception of the SBB, Oligocene strata have not been recorded in the Eastern Basin province. The ages of the Tertiary units in the Bago Yoma and Shwebo basins, assigned by Myint Thein (1966) and Myint Thein et al. (1982), are now supported by detrital zircon ages and palaeobotanical findings. More than 3000 measurements of cross-bedding azimuths reveal that the siliciclastic sediments were brought into the basin by southwards-directed longitudinal currents, causing the retreat of the sea. Recent research on U–Pb and Hf isotopic data of detrital zircons from the CMB has demonstrated that the Gangdese Batholith of the Lhasa Terrane in South Tibet was the dominant source during the Palaeogene. By the Early Miocene, terrains of the eastern Transhimalayan and intra-Myanmar Dianxi-Burma batholiths became important areas of provenance while the western section of the Sibumasu Block, with its S-type batholiths, played a secondary role and the Wuntho-Salingyi Magmatic Arc provided a relatively minor provenance. The Tertiary sequence of the CMB contains a record of filling of the foreland (orogen-parallel) basin by river-borne sediments; …

The Twin Bambili Maars (Bamenda Mountains, Cameroon Volcanic Line): Simulative Model of Formation

The Bambili maars are twin contemporaneous maars embedded in trachytic rocks. The two maars are separated by a low lying inter-Maar dome whose inner walls dip towards the Southern Maar. The Bambili Northern Maar (BNM) contains water and its floor is found at an altitude lower than the Bambili Southern Maar (BSM) (50 m) which is swampy and drains into the BNM. Both maars have very steep vertical walls. Three different units are identified in the area. Unit 1 is 1 m thick sequence, characterize by the presence of matrix supported fine-grained fallouts (ash and lapilli size) with stratified diffused bedding planes. This unit is further subdivided into three different layers: L1, L2 and L3 from bottom to top with increasing sizes of particles respectively. This layer shows reverser grading and its grain sizes rarely exceed 2 cm and this unit shows no depositional structures. Unit 2 is an unstratified unit predominantly made up of tephra deposits. Within these deposits are found blocks of sizes ranging between 2 and 10 cm of different petrographic types. They include scoria, basalts, trachyte, and granite cognate. The last unit, Unit 3 is distinctly stratified characterized by well sorted, heterolithologic, graded bedding layers which are matrix supported. Base on the composition and grain sizes, this unit was subdivided into 7 horizons: h1 - h7. Within this unit, two sequences of deposits are distinguished: U1 and U2. Both sequences are characterized by alternation of lapilli and tuff beds. The clast comprises of juvenile materials, mudstone which are highly vesicular and altered rock fragments. Other products identified within the area are ferolithic volcanic bombs (5 - 20 cm), xenocrystals (2 - 4 cm) within finer deposits. They are elongated and show visible twinned planes. Explosion breccias are equally identified occurring as bolders. Along the inner walls of the maars are also found large basaltic and trachytic blocks which are remnants of pre-maar unit within maar deposits. The model proposed indicates the maars were emplaced simultaneously from adjacent vents followed by a succession of lava flows and pyroclastic ejections.

Chapter 6 The mafic–ultramafic (ophiolitic) rocks of Myanmar

Mafic and ultramafic rocks are widely distributed in Myanmar and form major tectonic belts and suture zones representing remnants of the Mesotethyan Ocean. They occur as deformed and disrupted ophiolitic sequences and in forearc slivers as part of an accretionary wedge, intimately associated with siliciclastic and limestone units. They commonly host chromite–nickel–manganese, copper (gold) and jadeite mineralization. In this chapter, the nature, distribution, geological setting, petrological, mineralogical and ore characteristics of the mafic and ultramafic rocks in Myanmar are reviewed and described, and their tectonic setting and mineralization potential are discussed. Myanmar is characterized by north–south-trending major tectonic domains. From west to east, these include: the Rakhine coastal strip, an ensimatic foredeep; the Indo-Myanmar Ranges, an outer arc or forearc; the Western Inner-Myanmar Tertiary Basin, an inter-arc basin; the Central Volcanic Belt (Central Volcanic Line), an inner magmatic-volcanic arc; the Eastern Inner-Myanmar Tertiary Basin, a back-arc basin; and the Shan-Tenasserim massif, the ensialic Sino-Myanmar Ranges (Bender 1983; Khin Zaw 1989, 1990, 2017; Khin Zaw et al. 2015). Among these tectonic domains, ophiolitic rock associations occur in three belts trending nearly north–south parallel with each other (from west to east): the Western Ophiolitic Belt (WOB); the Central Ophiolitic Belt (COB); and the Eastern Ophiolitic Belt (EOB) (Fig. 6.1). The narrow WOB forms the longest belt, extending from the north to the southern part of Myanmar covering the eastern hills of the Naga, Chin and Rakhine ranges. According to Hutchison (1975), there are two ophiolite belts in Myanmar: the Naga Hill Line and the Mandalay Line. In this chapter the Naga Hill Line is termed the WOB. The Mandalay Line is the combination of the COB and the EOB. The southern continuation of the Mandalay Line is uncertain, due to a lack of comprehensive geological information. These …

Structural Relationship between Brittle Deformation and Paleozoic to Mesozoic Basalt Dykes in the Precambrian Basement of the Southern Continental Part of the Cameroon Volcanic Line

The work is a vivid description of the structural relationship between brittle deformation of the Precambrian basement in the southern continental part of the Cameroon Line and intrusive Paleozoic and Mesozoic basalt dykes swarms. A multidisciplinary approach that involves a combination of remote sensing techniques and field studies show that the major trend of brittle structures correspond to well-known regional structures: N70°E (Adamawa Shear Zone), N135°E (upper Benue trend) and N30°E (Cameroon Volcanic Line) corresponding to E-W and N-S directions respectively. Basalt dykes are associated to NE-SW, E-W and NW-SE oriented fractures. An integration of the available information on brittle structures and basalt dykes directions suggest an emplacement of the Mesozoic and Paleozoic basalt dykes structurally controlled by Precambrian structures that were originated through Riedel’s fracture kinematic model with dextral strike-slip Adamawa Shear Zone as the main shear zone during late stage of the Pan-African collision. Spatially, the restriction of the basalt dykes to the corridor of the Adamawa Shear Zone indicate that a rejuvenation of Precambrian faults may very well be the origin of the dykes with possibility that they may have been reworked several times during the Phanerozoic eon.

Geochemical investigation of stream sediments from the nlonako area; littoral, Cameroon: implications for Au, Ag, Cu, Pb and Zn mineralization potentials

Nlonako area belongs to Cameroon Volcanic Line. To determine the mineralization potentials of the area, stream sediment survey and geochemistry were carried out. The results show weak anomalies in Gold and Silver with threshold values of 0.001 and 0.073 respectively. Copper, Lead and Zinc have some relatively high values which are 690ppm, 182ppm and 378ppm with mean values statistically calculated to be 73.90, 46.51 and 198.58 respectively. The data analyzed using multivariate statistical methods yielded 5 factors from Principal Component Analysis (PCA). These five factors are; Factor 1 (Ce, La, Nb, Y), Factor 2 (W, Mo, Sr, As, Cd, Ni), Factor 3 (Pb, Sn, Zn, Sb), Factor 4 (Au, Ag, Cu) and Factor 5 (Cr, Co, Bi). These factors point mostly to a possible sulphide mineralization. Gold shows high correlation with Ag and Cu while Y correlates more highly and positively with Ce and La. There is a relatively high input of the light rare-earth elements which form the highest factor (34.363%) of the total PCA variance and possibly points to a granitic source rock. The existence of mafic–ultramafic igneous rocks in the study area underlies high correlations between Cr and Co and other factors gotten from PCA.

New K–AR ages of tchabal mbabo alkaline volcano massif, Cameroon volcanic line and adamawa plateau (central Africa)

Tchabal Mbabo in Central Africa is a voluminous volcano massif composed of alkali lava series. K-Ar geochronology data obtained from three samples of basanite - trachyandesite composition defined at least two volcanic activities. The first at end Eocene (38.22 ± 0.80 Ma) and the second during Oligocene (28.88 ± 0.61 and 28.60 ± 0.60Ma). The distribution of different ages along the Cameroon Volcanic Line is difficult to council with any migration of magmatic activity, as previously suggested. The most realistic scenario for the formation of Cameroon Line is that the ascent of lavas has been favored by the crustal discontinuities inherited from the Pan-African orogeny and reactivated during Mesozoic and Cenozoic. ICP MS and ICP AES analyses show that basanite lavas are the result of 2 % melting of sub-lithospheric mantle source containing garnet and phlogopite phases; the trachyandesites are formed by fractional crystallization of K-feldspar, amphibole, clinopyroxene and Fe-Ti oxides.

Age and Sr-Nd-Hf isotopes of the sub-continental lithospheric mantle beneath the Cameroon Volcanic Line: Constraints from the Nyos mantle xenoliths

To better constrain the age and composition of the sub-continental lithospheric mantle (SCLM) beneath the Cameroon Volcanic Line (CVL), we present Sr-Nd-Hf-Os isotopes of the Nyos mantle xenoliths (nine lherzolites and four harzburgites). The Nyos lherzolites have fertile compositions with 37.69–41.12wt.% MgO and 2.49–4.38wt.% Al2O3, whereas the harzburgites are more refractory containing 43.2–45.42wt.% MgO and 1.43–2.1wt.% Al2O3. The MgO contents of the Nyos xenoliths overall show negative correlations with CaO, Al2O3, Na2O and TiO2 contents, indicating that they represent mantle residues after melt depletion. Modal minerals like pargasitic amphibole have only been discovered in one Nyos lherzolite, suggesting modal metasomatism locally occurred. Clinopyroxenes in most Nyos xenoliths show enriched trace element compositions, indicating pervasive cryptic metasomatism within the SCLM. Clinopyroxenes in the Nyos lherzolites have less radiogenic Sr and more radiogenic Nd isotopes than the Nyos harzburgites. Clinopyroxene Hf isotopes of the harzburgites are comparable to those of the lherzolites. Although the CVL basalts have similar Sr-Nd isotopes with the Nyos harzburgites, they have different Hf isotopes. Therefore, the CVL basalts could not be sourced solely from the lithospheric mantle. The Nyos lherzolites show consistent HSE patterns, with slight depletion in both Re and Pt over Pd. The harzburgites are remarkably depleted in Pt, Pd and Re relative to Os, Ir and Ru. The Nyos lherzolites have variable 187Os/188Os ratios ranging from 0.11700 to 0.13129, yielding Os model ages (TMA) relative to the primitive upper mantle (PUM) of 0.27–2.18Ga and Re depletion ages (TRD) of 0.12–1.76Ga. In comparison, four harzburgites have more unradiogenic 187Os/188Os ratios of 0.11485–0.11705. Three of the four harzburgites with low Re/Os ratios yield similar TMA and TRD ages, i.e., 1.91–2.43Ga vs. 1.87–2.05Ga, respectively. The 187Os/188Os ratios of the Nyos xenoliths show a good correlation with bulk Al2O3 contents. An initial 187Os/188Os ratio of 0.11064 can be inferred from the 187Os/188Os-Al2O3 correlation, giving an Archean TRD age of ~2.6Ga. This suggests that the Nyos mantle xenoliths have been subjected to melt depletion events during the Paleoproterozoic convergence of the Congo and West Africa cratons, which formed the lithospheric mantle intruded by the CVL. The SCLM beneath the CVL is coeval with or slightly younger than the charnockite and tonalite-trondhjemite-granodiorite in the Ntem Complex of NW Congo Craton. We proposed that the old SCLM was a part of the Congo cratonic mantle. Therefore, our results suggest that old cratonic mantle beneath the CVL has not been completely removed during the Pan-African Orogeny, although the overlying crust might have been strongly deformed and reworked.

Late Cretaceous intraplate silicic volcanic rocks from the Lake Chad region: An extension of the Cameroon volcanic line?

AbstractSilicic volcanic rocks at Hadjer el Khamis, near Lake Chad, are considered to be an extension of the Cameroon volcanic line (CVL) but their petrogenetic association is uncertain. The silicic rocks are divided into peraluminous and peralkaline groups with both rock types chemically similar to within‐plate granitoids. In situ U/Pb zircon dating yielded a mean 206Pb/238U age of 74.4 ± 1.3 Ma indicating the magmas erupted ∼10 million years before the next oldest CVL rocks (i.e., ∼66 Ma). The Sr isotopes (i.e., ISr = 0.7021–0.7037) show a relatively wide range but the Nd isotopes (i.e., 143Nd/144Ndi = 0.51268–0.51271) are uniform and indicate that the rocks were derived from a moderately depleted mantle source. Thermodynamic modeling shows that the silicic rocks likely formed by fractional crystallization of a mafic parental magma but that the peraluminous rocks were affected by low temperature alteration processes. The silicic rocks are more isotopically similar to Late Cretaceous basalts identified within the Late Cretaceous basins (i.e., 143Nd/144Ndi = 0.51245–0.51285) of Chad than the uncontaminated CVL rocks (i.e., 143Nd/144Ndi = 0.51270–0.51300). The age and isotopic compositions suggest the silicic volcanic rocks of the Lake Chad region are related to Late Cretaceous extensional volcanism in the Termit basin. It is unlikely that the silicic volcanic rocks are petrogenetically related to the CVL but it is possible that magmatism was structurally controlled by suture zones that formed during the opening of the Central Atlantic Ocean and/or the Pan‐African Orogeny.

Mantle sources beneath the Cameroon Volcanic Line: geochemistry and geochronology of the Bamoun plateau mafic rocks

The age and origin of the volcanism along the Cameroon Volcanic Line (CVL) are still a matter of debate. We present major and trace element compositions as well as Sr–Nd–Pb–Hf isotopic results for mafic rocks from the Bamoun area, in the central part of the Cameroon Volcanic Line, as well as for two samples from recent eruptions of Mt. Cameroon. Lava flows are mostly basalts and hawaiites with transitional affinity. Some samples are among the oldest rocks of the CVL, with ages older than 51 My while some rocks are very young, around 0.05 My. All the samples are enriched in incompatible elements, indicating that melts were formed in a garnet-bearing mantle source. Different mantle sources participated to the formation of the Bamoun lavas. One source is isotopically similar to the main volcanic rocks of the CVL and probably represents an important part of the subcontinental lithospheric mantle. The second source is enriched in incompatible elements and shows a marked positive Eu anomaly, probably related to the participation of pyroxenites in the partial melting processes. The third mantle source is similar to the source of the Mt. Cameroon. This mantle source was known previously only in the Mt. Cameroon lavas, and we report its occurrence for the first time in old lavas and in other location along the CVL.

Chemistry and origin of the Mayo Kila sapphires, NW region Cameroon (Central Africa): Their possible relationship with the Cameroon volcanic line

Mineralogical, chemical and geochronological studies constrained the origin of sapphires from Mayo Kila, Northwest Cameroon. The sapphires are mostly blue, with sizes ranging from 2 to 5 mm. The pale blue grains are transparent, whereas, other corundums are transparent to translucent and/or opaque. The sapphires are dominantly euhedral to sub-hedral with few polished lustrous grains, acquired features during moderate to short distance transport from a proximal source rock. Solid inclusions are limited to rutile and zircon.Trace element analysis of sapphires shows significant concentration (in ppm) in some elements: Fe (2208–14,473), Ti (82–1783), Ga (77–512), Mg (0.9–264.9), Cr (b.d.l −168) and V (1.3–82). The other elements (e.g. Sn, Nb, Ta, Th, Zr, Ni, Ce) are generally below 10 ppm. The calculated ratios for some of the selected elements show an extreme variation: Fe/Mg (43–3043), Fe/Ti (2–76), Ti/Mg (1–328), and Ga/Mg (0.4–363). They are dominantly corundum crystallized in alkaline magma (s) with few from metamorphic source (s).Trace elemental features with Hf (13,354–26,238 ppm), Th (4018–45,584 ppm) and U (7825–17,175 ppm), and Th/U (0.39–2.65) found in zircon inclusions are compatible with quantified values in magmatic crystallized zircons. The Cenozoic age (mean of 30.78 ± 0.28 Ma) obtained for zircon inclusions is close to the age of some igneous rocks found within the Cameroon Volcanic Line (e.g. rocks of the Mount Oku: 31–22 Ma), showing the same period of formation. The most probable source of the zircon host sapphires is the Oku Mountain located SW of Mayo Kila.

Contrasting Effects of Historical Sea Level Rise and Contemporary Ocean Currents on Regional Gene Flow of Rhizophora racemosa in Eastern Atlantic Mangroves

Mangroves are seafaring taxa through their hydrochorous propagules that have the potential to disperse over long distances. Therefore, investigating their patterns of gene flow provides insights on the processes involved in the spatial genetic structuring of populations. The coastline of Cameroon has a particular geomorphological history and coastal hydrology with complex contemporary patterns of ocean currents, which we hypothesize to have effects on the spatial configuration and composition of present-day mangroves within its spans. A total of 982 trees were sampled from 33 transects (11 sites) in 4 estuaries. Using 11 polymorphic SSR markers, we investigated genetic diversity and structure of Rhizophora racemosa, a widespread species in the region. Genetic diversity was low to moderate and genetic differentiation between nearly all population pairs was significant. Bayesian clustering analysis, PCoA, estimates of contemporary migration rates and identification of barriers to gene flow were used and complemented with estimated dispersal trajectories of hourly released virtual propagules, using high-resolution surface current from a mesoscale and tide-resolving ocean simulation. These indicate that the Cameroon Volcanic Line (CVL) is not a present-day barrier to gene flow. Rather, the Inter-Bioko-Cameroon (IBC) corridor, formed due to sea level rise, allows for connectivity between two mangrove areas that were isolated during glacial times by the CVL. Genetic data and numerical ocean simulations indicated that an oceanic convergence zone near the Cameroon Estuary complex (CEC) presents a strong barrier to gene flow, resulting in genetic discontinuities between the mangrove areas on either side. This convergence did not result in higher genetic diversity at the CEC as we had hypothesized. In conclusion, the genetic structure of Rhizophora racemosa is maintained by the contrasting effects of the contemporary oceanic convergence and historical climate change-induced sea level rise.

Deformation of oceanic crust in the eastern Gulf of Guinea: role in the evolution of the Cameroon Volcanic Line and influence on the petroleum endowment of the Douala-Rio Muni Basin

Abstract Evidence is presented from the interpretation of seismic data for the compressional deformation of intra-plate oceanic crust in the eastern Gulf of Guinea, a region occupied by the Douala-Rio Muni Basin and the Cameroon Volcanic Line. The deformation has taken the form of the reactivation of syn-kinematic structures in the fabric of the oceanic crust. The origin of the Cameroon Volcanic Line is usually attributed to a mantle hotspot, but interpretation of the seismic data indicates that strike-slip and/or compressional tectonics and reactivation of the fabric of the oceanic crust have all played an important role in its evolution. A regional tectonic model is presented that associates deformation in the eastern Gulf of Guinea with regional alpine-related far-field compressive stress. In this way, a causal link is invoked between deformation of the oceanic crust and strike-slip movement on the Central African Shear Zone, establishing a continent–ocean tectonic link. The evolution of the Cameroon Volcanic Line and the continent–ocean tectonic link has imparted a unique tectonostratigraphic history to the Douala-Rio Muni Basin and has influenced its petroleum endowment. The structural and combination trapping potential has been greatly enhanced in this environment and the structural architecture of the basin has influenced the depositional patterns of the deep water reservoir sands.

Ambient noise tomography of the Cameroon Volcanic Line and Northern Congo craton: new constraints on the structure of the lithosphere

Ambient noise tomography of the Cameroon Volcanic Line and Northern Congo craton: new constraints on the structure of the lithosphere

Shallow Marine Cretaceous Sequences and Petroleum Geology of the Onshore Portion Rio del Rey Basin, Cameroon, Gulf of Guinea

Extensive thickly folded sedimentary sequences are exposed onshore of the Rio del Rey Basin contrary to previous reports of their rarity. Shales predominate, frequently intercalated by marlstones, sandstones, mudstones, limestones and marls and have been intruded in places by volcanic rocks of the Cameroon Volcanic Line (CVL). An integration of lithostratigraphic, paleontologic, granulometric and petrographic data generated from field and laboratory studies, permitted the identification of eleven (11) lithofacies which were subsequently grouped into three facies associations to facilitate the reconstruction of the paleo-depositional environment. Graphic logs constructed from the different outcrops studied revealed general coarsening upward sequences. Mega body fossils recovered from these sediments include severely altered, ferruginized fish and turtle-like heads, bivalve shells and moulds. Two bivalve taxa were identified: Exogyra ponderosa (Roemer) and members of theVenericardiinae sub-family. Textural and mineralogical details were obtained from granulometric and thin sections (microscopy). Interpretation of these data inferred an open relatively deeper marine, probably middle neritic to near shore transitional intertidal environments into which these sediments previously dated as Lower to Upper Cretaceous were deposited. Elements of a possible petroleum system: source and reservoir rocks, migratory pathways and trapping conditions, have been pointed out and thought to exist in space and time.