Evolution of the Nyiragongo magma

Abstract

This paper deals with the rocks, mainly foiditic in composition, of the Nyiragongo volcano and compares them with the mainly tephritic to phonolitic rocks of the neighbouring Nyamuragira volcano and with the South Kivu basaltic rocks. AH these rocks occur in the Lake Kivu area of the Western Rift Valley of Central Africa. Two hundred and sixty-four chemical analyses considered representative were selected of these rock groups and were subjected to statistical evaluation. The Nyiragongo magma is considered to be of mantle origin without significant crustal admixture. The Nyamuragira magma and the Nyiragongo magma are both potassic in character and, despite being geographically closely related, display pronounced differences in chemistry. The fairly extreme composition of the Nyiragongo magma is interpreted as a result of a locally strong transfer and enrichment of mainly alkalis with a passive depletion of silica