Volcanological and petrological evolution
of Nyiragongo volcano,Virunga volcanic field,Zaire |
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Authors: | Alain Demant Patrick Lestrade Ruananza T Lubala Ali B Kampunzu Jacques Durieux |
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Institution: | Laboratoire de Pétrologie Magmatique URA 1277 CNRS, Université Aix-Marseille III, 13397 Marseille Cedex 13, France and Centre de Recherches Volcanologiques,
OPGC 5 rue Kessler, 63038 Clermont-Ferrand, France, FR Laboratoire de Pétrologie, Université de Lubumbashi, BP 1825, Lubumbashi, Za?re, ZR 30 Rue Henri Barbusse, 69008 Lyon, France, FR
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Abstract: | Three major phases are distinguished during the growth of Nyiragongo, an active volcano at the western limit of the Virunga
Range, Zaire. Lavas erupted during phase 1 are strongly undersaturated melilitites characterized by the presence of kalsilite
phenocrysts, perovskite, and the abundance of calcite in the matrix. Such lavas crop out mainly on the inner crater wall and
progressively evolve toward more aphyric melilite nephelinites well represented on the flanks of the volcano. Adventive vents
lying at the base of the cone developed along radial fracture systems and erupted olivine and/or clinopyroxene – rich melilitites
or nephelinites. Stage 2 lavas are melilite-free nephelinites. Clinopyroxene is the main phenocryst and feldspathoids are
abundant in the lavas exposed on the crater wall. These flows result from periodic overflowing of a magma column from an open
crater. Extensive fissure flows which erupted from the base of the cone at the end of this stage are related to widespread
draining out of magma which in turn induces the formation of the summit pit crater. Magmas erupted during stage 3 are relatively
aphyric melilite nephelinites and the main volcanological characteristic is the permanent lava lake observed into the pit
crater until the 1977 eruption. Fluctuations of the level of the lava lake was responsible for the development of the inner
terraces. Periodic overflowing of the lava lake from the central pit formed the nepheline aggregate lava flows. Petrography
and major element geochemistry allow the determination of the principal petrogenetic processes. Melilitites and nephelinites
erupted from the summit crater are lavas derived, via clinopyroxene fractionation, from a more primitive melt. The abundance
of feldspathoids in these lavas is in keeping with nepheline flotation. Aphyric melilite nephelinites covering the flanks
and the extensive fissure flows have a homogeneous chemical composition; rocks from the historical lava lake are slightly
more evolved. All these lavas differentiated in a shallow reservoir. Lavas erupted from the parasitic vents are mainly olivine
and/or clinopyroxene-phyric rocks. Rushayite and picrites from Muja cone are peculiar high-magnesium lavas resulting from
the addition of olivine xenocrysts to melilitic or nephelinitic melts. Fluid and melt inclusions in olivine and clinopyroxene
phenocrysts indicate a crystallization depth of 10–14 km. A model involving two reservoirs located at different depths and
periodically connected is proposed to explain the petrography of the lavas; this hypothesis is in accordance with geophysical
data.
Received: July 8, 1993/Accepted: September 10, 1993 |
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Keywords: | : Nyiragongo – volcanology – petrology – melilitite – nephelinite – feldspathoid flotation |
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