Petrogeochemical Assessment of the Erelu-Oyo Dolerite Dyke and Associated Older Granites, Southwestern Nigeria

The petrochemical studies of the medium- to fine-grained discordant dolerite dyke of Erelu, located within latitude 07°52′N to 07°54′N and longitude 003°52′E to 003°57′E, were carried out to characterize the rock, infer the petrogenesis in relation to the host rocks in the area, and advance the present knowledge on the dolerite dykes of the basement complex of southwestern Nigeria. The study involved fieldwork and random sample collection, where four dolerite, four granite, and six banded gneiss samples were collected and labeled. Thin sections of representative samples of the rocks were prepared following standard procedures, while the remaining rock samples were pulverized for major oxides and trace element determination using the Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) method at Activation Laboratory, Ontario, Canada. The modal analysis results show that the dolerite is composed of olivine (40%), pyroxene (30%), plagioclase (10%), biotite (5%), and quartz (15%). Geochemical results revealed the average major elemental composition (%) as SiO₂ (48.62), Al₂O₃ (15.27), Fe₂O₃ (11.48), MgO (8.00), CaO (8.28), Na₂O (3.11), K₂O (0.82), TiO₂ (1.51), P₂O₅ (0.24), MnO (0.16), Cr₂O₃ (0.03), and LOI (1.85). The average trace element concentrations (ppm) are Ba (285.75), Ce (10), Co (51), Cu (54.5), Nb (7.75), Ni (162.25), Sc (19.75), Sr (436.75), Y (15.25), Zn (98), and Zr (111.25). The occurrence of pyroxene and olivine classifies the rock as olivine-dolerite and pyroxene-dolerite, while geochemical evidence suggests a tholeiitic origin for the dolerite. The petrochemistry of the rocks shows that the dolerite dyke is peraluminous with moderately high Al, Fe, Mg, and Ca contents compared to other rocks in the same area.