title: Evolution Of The Subcontinental Lithospheric Mantle Below A Major Tectono-Volcanic Structure: Constraints From Mantle Xenoliths Along The Cameroon Volcanic Line
Creators: name: Nformidah-Ndah, Siggy Signe
Publisher: name: Universität Bern
Year: 2023
Subjects: subject: 550 Earth sciences &amp; geology

description: The Cameroon Volcanic Line represents an important but enigmatic geologic structure on the African continent. Itis an oceanic and a continental alignment of volcanoes and plutons with no spatial age progression along a distance of ~1600 km, eventhough magmatism has been ongoing for the last 70 Ma. A single process whereby magma is generated from one region to another is therefore unlikely in this case and different mechanisms could be involved especially if melting occurred at different times under different conditions. Tracing the evolution of the sub-continental lithospheric mantle below the Cameroon Volcanic Line is therefore essential to understand surface manifestations through time, since it is possible that the SCLM was significantly modified during the Pan-African orogeny and the geodynamic processes that occurred during this time are poorly understood.
The CVL is built across two distinct lithostructural domains in Cameroon; the North-West Cameroon and the Adamawa Yade domains delimited by NNE-SSW faults. Xenoliths were collected from these domains at the Oku Volcanic Field and the Kapsiki Plateau for the North-West Cameroon Domain and the Adamawa Volcanic Massif for the Adamawa Yade Domain. These domains correspond to a juvenile Proterozoic area and a Paleoproterozoic area with Archean relics respectively. Major and trace element systematics including the evaluation of water contents in 33 mantle xenolith samples associated with alkaline basalts from the CVL were investigated to constrain mantle processes and equilibrium conditions.
The xenoliths sampled are mainly clinopyroxene bearing spinel harzburgites for the Kapsiki plateau and spinel lherzolites for the Oku Volcanic Field and Adamawa Volcanic Plateau. Subordinate olivine websterites are also found in the Oku Volcanic Field. Two-pyroxene thermometry based on major element concentrations, Ca in orthopyroxene and Alin olivine thermometers shows two temperature groups, ranging from 810 to 1030°C and 1020 to 1160°C. The high and low-temperature groups are restricted to xenoliths from the Oku Volcanic Field and Adamawa Volcanic Plateau, whereas the Kapsiki xenoliths fall in the low-temperature group.
A simple non-modal fractional partial melting model applied to trace element concentration of orthopyroxenes show that the CVL xenoliths have been affected by variable degrees of partial melting ranging from 0-30%. The xenoliths from the Kapsiki are the most refractory with partial melting always above 20%. For the Oku Volcanic Field and Adamawa Volcanic Plateau, partial melting is below 15%.
Xenoliths from the CVL show evidence of modal metasomatism with the presence of secondary clinopyroxenes and amphibole. Cryptic metasomatism is shown by variable enrichments in LREEs and MREEs and incompatible trace elements. Discrimination diagrams of Ti/Eu vs. (La/Yb)N in clinopyroxene show that xenoliths from the Kapsiki plateau were metasomatized by carbonatite-silicate melts/fluids; those from the Oku Volcanic Field show both metasomatism by silicate and carbonatite melts/fluids while the Adamawa Volcanic Plateau xenoliths are dominated by silicate metasomatism with minor indications of carbonatite metasomatism. A group of samples from the Oku Volcanic Field do not show any evidence for metasomatism but are rather similar to a MORB source mantle. Amphiboles from the Oku Volcanic Field display trace element patterns that are consistent with metasomatic activity that was linked to subduction, likely related to the Pan African orogeny. Other trace element ratios such as Ti/Sr and Zr/Hfin orthopyroxene efficiently distinguish between the different metasomatic agents, which indicates that orthopyroxene could be as useful as clinopyroxenes in tracing mantle metasomatism especially when clinopyroxene is absent. On the other hand, trace elements in olivine are more challenging due to very low concentration and a dominant control of temperature on trace element incorporation. 
The water contents of olivine and pyroxenes were obtained by FTIR spectroscopy using unpolarised light on unoriented crystals. Olivine water contents are very low <2ppm and have been attributed to its depleted chemistry e.g., low Ti (0.6-34 ug/g) and low temperature of equilibration with Cr (31-108 ug/g), which hindered hydrogen incorporation. Orthopyroxene water contents fall within a narrow range and are similar in all locations along the CVL. Clinopyroxenes show two sets of water contents. The water contents for the Kapsiki plateau and the Oku Volcanic Field are low (15-320ppm) whereas, for the Adamawa Volcanic Plateau, they are extremely high (560-910ppm). These differences in water contents were interpreted to reflect the low and high-water activity of metasomatizing fluids for the North-West Cameroon Domain and Adamawa Yade Domain respectively. Calculated bulk rock water contents are very low (16±10 ppm), which indicates that metasomatism did not result in the weakening of the mantle below the CVL by hydrogen related defects.
While there are no traces of carbonatite magmatism along the CVL, there is widespread evidence for carbonatite-silicate melts that reacted with the mantle and that could be the source for mantle derived CO2outgassing in the Oku Volcanic Field. The calculated fO2 of the mantle is overall reducing (0.09-2.28), suggesting that carbon mantle outgassing occurs in the form of CH4.
Our new data provides evidence for local and regional heterogeneity of the SCLM below the CVL and the geochemical features observed are in favour of small-scale local convection proposed for by the edge convection model for the origin of the CVL magmatism.