| Popis: |
Mount Cameroon volcano has erupted several times in the 20th Century with documented eruptions in 1909, 1922, 1954, 1959, 1982, 1999 and 2000. Evidence of historic volcanism is represented by several older lava flows and lahar deposits around the flanks of the volcano. This study aims to assess the evolution of Mount Cameroon volcanism through its eruptive history via interpretation of mineralogical, whole rock geochemical and Pb, Sr, Nd isotope data generated from historic and recent lava flows. In this study, samples were collected from the 1959, 1982, 1999 and 2000 eruptions and from several historic eruption sites with unknown eruption dates.Evaluation of major and trace element data demonstrates that Mount Cameroon is geotectonically associated with within-plate Ocean Island Basalt Settings. More than 90% of the studied historic lavas (n = 29) classify as tephrites and basanites whereas the modern lavas (n = 38) are predominantly trachybasalts, demonstrating evolution from primitive to evolved lavas over time typically in response to fractional crystallization. Petrographically, the lavas are porphyritic with main mineral phases being olivine, clinopyroxene, plagioclase feldspars and Fe–Ti–Cr oxides. The 1982 lavas are predominantly aphyric and dominated by lath-shaped flow-aligned plagioclase in the groundmass. Olivine chemistry shows variable forsterite compositions from Fo60–89. Clinopyroxenes vary from diopside through augite to titanaugite with chemical composition ranges from Wo45En32Fs7 to Wo51En47Fs17. Plagioclase feldspars vary from labradorite (An56–70) to bytownite (An80–87). For the Fe–Ti–Cr oxides, calculated ulvöspinel component shows a wide variation from ulv38–87. CIPW-normative classification on the Di-Ol-Hy-Qz-Ne system shows that all Mount Cameroon lavas are nepheline-normative (Ne ranges from 4.20 wt.% to 11.45 wt.%).Radiogenic isotope data demonstrate that Mount Cameroon lavas are HIMU (or high μ = 238U/204Pb), characterized by 206Pb/204Pb = 20.19–20.46, 207Pb/204Pb = 15.63–15.69, 208Pb/204Pb = 40.01–40.30, 87Sr/86Sr = 0.70322–0.70339 (εSr = −21.37 to −18.96) and 143Nd/144Nd = 0.51276–0.51285 (εNd = +2.29 to +4.05). The historic lavas show stronger HIMU signature relative to the modern lavas, suggesting evolution towards less HIMU signatures over time. This study has revealed that Mount Cameroon volcanism has evolved from primitive magmas characterized by stronger HIMU signatures with high 206/204Pb and 208/204Pb isotopes, low SiO2 and high Mg, Ni, Cr content towards lower HIMU signatures with relatively higher SiO2, lower Mg, Cr and Ni compositions. The geochemical and isotopic changes, which account for the evolution of magmatism on Mount Cameroon occur over long periods of time because all the modern lavas erupted within the last 100 years are isotopically homogeneous, with very limited variation in SiO2 compositions. |
