New chemical and petrographic data of some undersature lavas from Nyiragongo and Mikeno (Virunga Western African-Rift-Zaire)

The lavas investigated from Nyiragongo and Mikeno volcanoes (Virunga volcanic field) are leucite - melilite - nephelinites, melilite - nephelinites (Nyiragongo) and leucitites (Mikeno): melilite characterizes the Nyiragongo lavas whereas clino-pyroxene is the prevalent mafic phase at Mikeno. The mineralogy is consistent with the highly alkaline (Ealk = 7.6-15.3 wt.%), potash-rich (K.20/Na20 > 1) and strongly silica undersaturated nature of the lavas. The samples analyzed are more or less fractionated. At Nyiragongo, addition of nepheline and leucite (in a very constant ratio) possibly to a bergalite-type magma may explain the “dilution” of both “incompatible” and “compatible” elements with increasing fractionation. On the contrary, at Mikeno, crystal fractionation processes seem to be dominated by the separation of mafic phases. Geochemical features indicate that the analyzed lavas from Baruta, and those from Nyiragongo and Mikeno were generated from different sources.