Petrology and geochemistry of the Tchabal Mbabo volcano in Cameroon volcanic line (Cameroon, Central Africa): An intra-continental alkaline volcanism

Abstract

Tchabal Mbabo massif within the Cameroon Volcanic Line (CVL) at the junction with Adamawa line in Central Africa is a huge (55 × 30km × 2340 m) Eocene-Oligocene volcano composed of frankly alkali lavas of basanites and basalts, trachybasalts, basaltic trachyandesites, trachyandesites, peralkaline trachytes and rhyolites. Lavas show flows and domes shape features and crop up along N160E, N135E and N70E Panafrican cracks. They have microlitic porphyritic textures and are composed of euhedral and subhedral phenocrysts and microcrysts of olivine, clinopyroxene, Ti-magnetite, ilmenite, amphibole (in basanite, basalt and trachybasalt as well as in peralkaline trachyte), plagioclase (in trachybasalt and basaltic trachyandesite), biotite and orthoclase (in trachyte and rhyolite). Same minerals occur in the matrices of the lavas (and moreover plagioclase in basalt). Spinel, olivine and diopside xenocrysts of mantle origin are present in some lavas. ICP-MS and ICP-AES geochemical analyses show that the basanites mostly contain one pyroxene (diopside), with abundant nepheline and olivine in their norms, linked to strong silica undersaturation. Trace element concentrations and ratios (Zr, Nb, Y) witness of alkali affinities, in intra-continental/within-plate anorogenic setting. Increased (Na2O, K2O, Ta, a.s.o.) or decreased (MgO, Fe2O3, TiO2, CaO, a.s.o.) compositional variations of majors and trace elements from basaltic lavas (dated at 28.6–28.9 Ma) to felsic lavas deal to a differentiated through fractional crystallization process. Trachyandesite lavas dated at 38.2 Ma do not belong to this series. Tchabal Mbabo basalts are the result of low partial melting rate of a lherzolite mantle source containing residual phlogopite + garnet, similar to those of the continental and oceanic sectors of the CVL. Melting of mantle source took place just at the base of the lithospheric mantle. Sr, Nd and Pb isotopic ratios of Tchabal Mbabo basaltic lavas are closed to the FOZO reservoir with (206Pb/204 Pb)i: 17.869407 to 19.747896. Tchabal Mbabo mafic and felsic lavas display variable but moderate (87Sr/86Sr)i ratios (0.703481–0.705053) and similar range of Ndi and Pbi isotopic ratios: 0.512526 < (143Nd/144Nd)i < 0.512771, 15.568709 < (207Pb/204 Pb)i < 15.735532 and 37.905174 < (208Pb/204 Pb)i < 39.579573. Values for trachyandesite and peralkaline trachyte are close to OIB rather than FOZO reservoir. Low (87Sr/86Sr)i ratios (0.703481) and positive εNd (+2.59) of basalt preclude any important crustal contamination. Meanwhile, magma mixing, fluid-rocks interaction and selective crustal contamination of felsic lava by underline Pan-African crust might have occurred later, during the differentiation of Tchabal Mbabo lavas series.