from variable amounts of mixing between melts derived from an The volcanic activity of Mts Bambouto and Oku (Western Highlands) and of the Ngaoundere Plateau, in the continental sector of anhydrous lherzolite source (asthenospheric component) and melts the Cameroon Volcanic Line, Equatorial West Africa, ranges in from an amphibole-bearing peridotite source (lithospheric HSr age from Oligocene to Recent. It is characterized by basanitic, alkali component). New Ar/Ar ages for Mts Oku and Bambouto basaltic and transitional basaltic series. Mineral chemistry, major basalts, combined with previous Ar/Ar and K/Ar ages of and trace element bulk-rock compositions, and geochemical modelling basaltic and silicic volcanics, and with volcanic stratigraphy, suggest suggest that the magmatic series evolved mainly at low pressure a NE–SW younging of the peak magmatic activity in the Western (2–4 kbar) through fractional crystallization of clinopyroxene and Highlands. This SW younging trend, extending from the Oligocene olivine±magnetite, at moderately hydrated (H2O= 0·5–1 wt %) volcanism in northern Cameroon (e.g. Mt Oku) to the still active and QFM (quartz–fayalite–magnetite) to QFM + 1 fO2 conMt Cameroon, suggests that the African plate is moving above a ditions. Basalts from Ngaoundere (Miocene to Quaternary) and deep-seated mantle thermal anomaly. However, the age and location from the early activity (31–14 Ma) of the Western Highlands have of the Ngaoundere volcanism does not conform to the NE–SW incompatible trace element and Sr–Nd isotopic compositions similar younging trend, implying that the continental sector of the Cameroon to those of oceanic Cameroon Line basalts, pointing to a similar Volcanic Line cannot be easily interpreted as the surface expression asthenospheric mantle source. By contrast, the late (15–4 Ma) of a single hotspot system. Western Highlands basanites and alkali basalts have anomalously high concentrations of Sr, Ba and P, and low concentrations of Zr, which are exclusive features of continental Cameroon basalts. The genesis of these latter magmas is consistent with derivation from an incompatible element enriched, amphibole-bearing lithospheric

/pdf/the-cameroon-volcanic-line-revisited-petrogenesis-of-3fw106gq0f.pdf

The Cameroon Volcanic Line Revisited: Petrogenesis of Continental Basaltic Magmas from Lithospheric and Asthenospheric Mantle Sources

The Cameroon Hot Line (CHL): A unique example of active alkaline intraplate structure in both oceanic and continental lithospheres

A petrogenetic study of anorogenic felsic magmatism in the Cretaceous Paresis ring complex, Namibia: evidence for mixing of crust and mantle-derived components

The origin of the Cameroon Volcanic Line (CVL), a 1600 km long linear volcanic chain without age progression that crosses the ocean-continent boundary in west-central Africa, is investigated using body wave tomography. Relative arrival times from teleseismic P and S waves recorded on 32 temporary seismic stations over a 2-year period were obtained using a multichannel cross-correlation technique and then inverted for mantle velocity perturbations. The P and S wave models show a tabular low-velocity anomaly directly beneath the CVL extending to at least 300 km depth, with perturbations of −1.0 to −2.0% for P and −2.0 to −3.0% for S. The S wave velocity variation can be attributed to a 280 K or possibly higher thermal perturbation, if composition and other effects on seismic velocity are negligible. The near vertical sides of the anomaly and its depth extent are not easily explained by models for the origin of the CVL that invoke plumes or decompression melting under reactivated shear zones, but are possibly consistent with a model invoking edge-flow convection along the northern boundary of the Congo Craton lithosphere. If edge-flow convection in the sublithospheric upper mantle is combined with lateral flow channeled along a fracture zone beneath the oceanic sector of the CVL, then the oceanic sector can also be explained by flow in the upper mantle deriving from variations in lithospheric thickness.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2010GC003200

Upper mantle structure beneath Cameroon from body wave tomography and the origin of the Cameroon Volcanic Line

Petrology of the bimodal Cenozoic volcanism of the Kapsiki plateau (northernmost Cameroon, Central Africa)

The intraplate Cameroon Volcanic Line (CVL) straddles the African-South Atlantic continent-ocean boundary and is composed mainly of alkaline basic volcanic rocks. Voluminous silicic volcanics characterize the continental sector of the CVL. We present here new geochemical, isotopic (Sr-Nd-O) and 40Ar/39Ar geochronological data on the main silicic volcanic centres of the Western (Mt. Oku, Sabga and Mt. Bambouto) and Eastern (Ngaoundere plateau) Cameroon Highlands. The silicic volcanism of Mt. Oku, Sabga and Mt. Bambouto occurred between 25 and 15 Ma and is represented by voluminous quartz-normative trachytes and minor rhyolitic ignimbrites. At Mt. Bambouto central volcano about 700 m of silicic volcanics erupted in less than 2.7 million years. These silicic volcanics are associated with slightly to moderately alkaline basalts and minor basanites. In general, onset of the silicic volcanism migrated from NE (Oku: 25 Ma) to SW (Sabga: 23 Ma; Bambouto: 18 Ma; and Mt. Manengouba: 12 Ma). The silicic volcanism of the Ngaoundere plateau (eastern branch of the CVL) is instead dominated by nepheline-normative trachytes which are associated with strongly alkaline basalts and basanitic rocks. These Ne-trachytes are younger (11-9 Ma) than the Q-trachytes of the Western Highlands. The least differentiated silicic volcanics are isotopically similar (87Sr/86Sr  0.51278) to the associated alkaline basalts suggesting differentiation processes without appreciable interaction with crustal materials. Such interactions may, however, have played some role in the genesis of the most evolved silicic volcanics which have 87Sr/86Sr as high as 0.705–0.714. Fractional crystallization is the preferred mechanism for genesis of the silicic melts of both Western and Eastern Highlands, as shown by modeling major and trace element variations. The genesis of the least evolved Q-trachytes from the Western Highlands, starting from slightly to moderately alkaline basalts, is compatible with fractionation of dominantly plagioclase, clinopyroxene and magnetite. Crystal fractionation may have occurred at low pressure and at QFM buffer f
O2conditions. Parental magmas of the Ngaoundere Ne-trachytes are likely instead to have been strongly alkaline basalts which evolved through crystal fractionation at higher P (6-2 kbar) and f
O2 (QFM + 2). The migration (25 to 12 Ma) of the silicic volcanism from NE to SW in the continental sector of the CVL is reminiscent of that (31-5 Ma) of the onset of the basic volcanism in the oceanic sector (Principe to Pagalu islands) of the CVL. These ages, and that (11-9 Ma) of the silicic volcanism of the Ngaoundere plateau, indicate that the Cameroon Volcanic Line as a whole may not be easily interpreted as the surface expression of hot-spot magmatism.

Silicic magmas from the continental Cameroon Volcanic Line (Oku, Bambouto and Ngaoundere): 40 Ar- 39 Ar dates, petrology, Sr-Nd-O isotopes and their petrogenetic significance

Biogenic limestones from three sections (north, central, and south) across peninsular Italy have been analysed for major and trace elements and Nd, Pb, and Sr isotopic ratios. These data are used to monitor the evolution of the Tethys Ocean from the Triassic through to the Miocene. Limestones’ major, trace, and REE elements contents are consistent with their formation in seawater with little sign of crustal, volcanic, or hydrothermal input. V/Cr and Ce/Ce* ratios indicate their deposition in oxygenated waters. Rb-Sr-Ba discrimination diagram, consistent with the immobile trace element distribution, indicates that limestone deposition took place in either marginal or open ocean environments. Ages based on stratigraphy are in good agreement with the chronostratigraphic Sr curves implying that the Tethys ocean, throughout its history, was in contact with the open, global, ocean system. Although the isotopic values of Sr and Nd are relatively restricted, Pb is extremely variable and highly radiogenic. High Pb isotope ratios characterise limestones deposited during the rifting of the southern Tethyan ocean in the Lower Jurassic and in the Lower Cretaceous, suggesting stronger crustal inputs in small basins. The weighted average, present-day, isotope values (AIL = average Italian limestone) for the Italian limestones, excluding anomalous samples, are 87Sr/86Sr=0.70785, 143Nd/144Nd=0.51227, and 206Pb/204Pb=18.94, 207Pb/204Pb=15.69, 208Pb/204Pb=38.66. These values are useful in monitoring the fate of limestones during orogenesis and the role that they may have played in magma genesis. 

Elemental abundances and isotopic composition of Italian limestones: glimpses into the evolution of the Tethys

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Silicic magmas from the continental Cameroon Volcanic Line (Oku, Bambouto and Ngaoundere): 40 Ar- 39 Ar dates, petrology, Sr-Nd-O isotopes and their petrogenetic significance

Elemental abundances and isotopic composition of Italian limestones: glimpses into the evolution of the Tethys