Low-pressure fractionation of the Nyiragongo volcanic rocks, Virunga Province, D.R. Congo

A new petrogenetic model for the Nyiragongo volcanics based on petrological and geochemical studies of a wide range of highly alkaline and silica- undersaturated fractionated rocks (Mg# 23-44) invokes low-pressure fractionation occurring mainly within the Nyiragongo volcanic edifice. The most primitive Nyiragongo rocks seem correspond to a pyroxene-nephelinitic melt which evolved to melilite nephelinite and melilitite by fractionation of leucite, together with varying amounts of nepheline. Leucite nephelinite, leucitite and leucite tephrite represent rock types originating partly by phenocryst accumulation. Mineral and bulk rock geochemistry of pyroxene nephelinites indicate an already strongly fractionated character (e.g. Mg# 27-40), which is explained by them originating from an olivine melilitite parental melt represented by rocks exposed in the Rushayo chain, SW of Nyiragongo. Calculations show that fractionation of olivine and melilite relates the olivine melilitites with pyroxene nephelinite.The presence of a large crater which has been occupied commonly by a lava lake in historical times may be related to roof collapse of the magma chamber in which these very low-pressure fractionation processes occurred.