Abstract
In the western Cameroon, crop out several dyke swarms of Paleozoic–Mesozoic age. These dykes intrude the Precambrian basement in the southern continental part of the Cretaceous Cameroon Volcanic Line. In the Njimom area, two groups of mafic dykes that crosscut the Neoproterozoic basement rocks have been observed. A first group intrudes the mylonites whereas the second group intrudes the granites. The dykes are alkaline basalts and hawaiites. The mineralogical assemblage of both groups of dykes consists of plagioclase, clinopyroxene, altered olivine, and opaque oxides. The dykes that cross-cut the Precambrian mylonitic gneisses show moderate TiO2 (1.7–2.0 wt.%), low MgO (4.4–7.1 wt.%), and compatible trace element concentrations (e.g., Cr = 70–180 ppm; Ni = 30–110 ppm). The dykes that intrude the granites have TiO2 contents between 2.3 and 2.5 wt.% and moderate compatible trace element concentrations (e.g., Cr = 260–280 ppm; Ni = 170–230 ppm). MgO varies from 5.9 to 9.2 wt.%. All mafic dykes are enriched in light lanthanide element and show moderate Zr/Nb and high Zr/Y, Nb/Yb, and Ti/V ratios similar to those of average ocean island basalt (OIB)-type magmas. Some dykes that intrude the mylonites show evidence of contamination by continental crust. The composition of the clinopyroxenes of the dykes that intrude the mylonites clearly indicate different and unrelated parental magmas from dykes that intrude the granites. Contents and fractionation of the least and the most incompatible elements suggest low degrees of partial melting (3–5%) of heterogeneous source slightly enriched in incompatible elements in the spinel stability field. The geochemical features of Njimom dykes (in particular the dykes that intrude the granites) are similar to those of Paleozoic and Mesozoic dykes recorded in the southern continental part of the Cameroon Volcanic Line, suggesting multiple reactivations of pre-existing fractures that resulted in the fragmentation of western Gondwana and the opening of the South Atlantic Ocean.
Highlights
Volcanic activity in Cameroon is concentrated along the well-known Cameroon Volcanic Line (CVL), an alignment of oceanic and continental magmatic centers that straddle the boundary continental/oceanic crust in Central Africa (e.g., [1])
Orientations of basaltic dykes are aggregate to the range NNE-ENE with a predominance of the ENE orientation which is locally known as the Adamawa Shear Zone (ASZ) which runs from the Gulf of Guinea to Sudan and in a pre-drift reconstitution, is thought to prolong to Brazil as the Pernambuco Shear Zone
Our field works along the ASZ in the Njimom area indicate the existence of NW-SE oriented basaltic dyke swarms
Summary
Volcanic activity in Cameroon is concentrated along the well-known Cameroon Volcanic Line (CVL), an alignment of oceanic and continental magmatic centers that straddle the boundary continental/oceanic crust in Central Africa (e.g., [1]). In the southern continental part of the CVL, the Paleozoic–Mesozoic dyke swarms show mineralogical and geochemical characteristics different from the common CVL rocks (e.g., [5,6]). Orientations of basaltic dykes are aggregate to the range NNE-ENE with a predominance of the ENE orientation which is locally known as the Adamawa Shear Zone (ASZ) which runs from the Gulf of Guinea to Sudan and in a pre-drift reconstitution, is thought to prolong to Brazil as the Pernambuco Shear Zone. Our field works along the ASZ in the Njimom area indicate the existence of NW-SE oriented basaltic dyke swarms. Petrographic and whole-rock geochemical (major and trace element) data, we decipher the petrogenesis of this dyke suite and the signification of the NW-SE dykes group in the framework of the opening of the Southern Atlantic Ocean
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