Petrogenesis of basaltic dikes from the Manjo area (Western Cameroon): insights into the Paleozoic magmatism at the northern margin of the Congo craton in Cameroon

A suite of basaltic dikes with a N60–80° E strike cross-cut the Paleo- to Neoproterozoic basement rocks in the Manjo area (southern continental part of the Cameroon Volcanic Line) along the northern border of the Congo craton in Cameroon. The dikes show a tholeiitic/transitional affinity as revealed by whole-rock and mineral chemistry data. Plagioclase and clinopyroxene are the dominant phases, with subordinate olivine and opaque oxides. The Manjo dikes have low MgO (5.2–7.4 wt%), Cr (119–274 ppm), and Ni (27–114 ppm) indicating that the samples are evolved. All basaltic dikes are mildly light lanthanide element enriched with Lan/Ybn values of 4.1–5.9. The Zr/Nb (4–10), Ba/Nb (6–27), and La/Nb (0.7–1.0) ratios are similar to those of basaltic rocks found in intraplate tectonic setting. Initial 87Sr/86Sr and 143Nd/144Nd isotope ratios range from 0.70472 to 0.70539 and from 0.512178 to 0.512377 (eNdi = + 1.0 to + 4.9), respectively. The Pb isotope compositions are in the ranges 206Pb/204Pb = 19.121–19.861 and 207Pb/204Pb = 15.624–15.703. The observed variations in major and trace elements cannot be fully attributed to plagioclase and clinopyroxene (± olivine) fractionation but indicate different parental magmas. The geochemical and isotopic data indicate no or minor crustal contamination processes in the petrogenesis of Manjo dikes. Trace element modeling suggests that the dikes were likely derived by partial melting of a spinel peridotite mantle source. The mildly enriched incompatible element contents of the Manjo dikes are similar to those of Paleozoic and Mesozoic mafic rocks recorded in Western Cameroon. The enrichment processes of the mantle sources of Manjo dikes may have occurred during Proterozoic subduction at the northern border of the Congo craton in Cameroon. The magmatism in the Manjo area resulted from the partial melting of this old, enriched continental lithospheric mantle induced by extensional processes during the Paleozoic.