The Mount Cameroon 1959 compound lava flow field: morphology, petrography and geochemistry

The Mount Cameroon 1959 compound lava flow field (volume ~3.4 × 107 m3; mean effusion rate ~13 m3 s–1–18 m3 s–1) is the only twentieth century eruption not following the NE–SW fracture system, the pathway for other recorded eruptions. The eruption occurred at two sites: 2006 m–1961 m and 1500 m, typically an aa flow with small pahoehoe channels in proximal regions. Morphologically, lava tubes and inflation hollows characterize upper flow regions while compression ridges and lava pinnacles characterize distal regions. Evidence of lava flow stagnation with time is provided by distinct layers at channel walls in proximal zones. Such channels are very narrow, deep, with poorly developed levee structures, attesting to high effusion rate and initial rapid advance of lava. The 1959 basanites show geochemical signatures and textural features indicating early fractionation of olivine and diopside, followed by Fe-Ti oxides and plagioclase. More evolved 1961 m vents lavas show higher incompatible and lower compatible element contents than those from the 1500 m sites. We interpret this to mean that residual more evolved magma of the 1954 eruption was initially forced out by degassing of a new less evolved magma batch, which finally drained out at the lower 1500 m site, the two magma batches not mixing. Incompatible elements indicate that the 1959 lava is similar to other historic lavas. However, subtle differences in compatible and incompatible element ratios (e.g. V/Rb) suggest that magma is erupted from the volcano in discrete batches from different magma storage regions rather than from a single large evolving chamber. The low whole-rock Mg contents (5.5 wt% MgO, average Mg# = 33.9) and high differentiation index values (36.7–38.5) for the 1959 lavas show that they are the most fractionated of all twentieth century Mt. Cameroon lavas.