Sur l'éruption du Nyamulagira de décembre 1981 à janvier 1982: cône et coulée du Rugarambiro (Kivu, Zaïre)

The December 1981 - January 1982 eruption which started in the Christmas night on the SE side of Nyamulagira, gave the longest historical flow (26 km) representing the highest production rate of this volcano in this century (280×106m3 of erupted magmas in 19 days). This eruption built Rugarambiro, a composite spatter cinder-cone. The ejected lava is a K-hawaiite (kivite) whose basicity decreased during the eruption (first emission: D.I. = 40; last products: D.I. = 35). This chemical evolution is reflected by:--the modal composition of lavas. The first emissions are poor in ferromagnesian phenocrysts (olivine + clinopyroxene: 3%) and rich in plagioclase (12%); the contrary is observed in the last ejected lavas (livine + Cpx: 16%; plagioclase: 1%); --the nature of the crystallizing minerals in the groundmass. In fact, only the first ejections include alkaline feldspars, nepheline and Tiphlogopite; --the glassy phase composition which is more differentiated in the first lavas (D.I. 68-84) than in the last ones (D.I. 42-61). A stratification of the Nyamulagira magmatic chamber is proposed where magmatic differentiation has probably occurred for fractional crystallization. Mineralogical thermobarometers lead to locate this magmatic reservoir at the depth of 6-7 km that we had already hypothesized. The presence of phenocrysts of bytownite, basic chrysolite, diopside and salite indicates a basaltic paragenesis which marks a hawaiitic magma chamber feeding. © 1984 Stabilimento Tipografico Francesco Giannini & Figli.