Petrology of combeite- and götzenite-bearing nephelinite at Nyiragongo, Virunga Volcanic Province in the East African Rift

Abstract

The lavas and pyroclastic rocks of Nyiragongo volcano (East African Rift) range in composition from olivine melilitite to nephelinite and minor alkali olivine basalt, and include rare examples of strongly peralkaline combeite nephelinite. In peralkaline nephelinites at Nyiragongo, titanium is hosted in mineral assemblages with Ti-bearing magnetite±perovskite±Ti-rich clinopyroxene±götzenite. Combeite and götzenite occur as groundmass minerals in holocrystalline melilite nephelinite, which also carries kirschsteinite (replacing melilite phenocrysts), recrystallized nepheline+kalsilite phenocryst aggregates and a range of late accessory minerals including delhayelite. The compositions of coexisting nepheline and kalsilite in phenocryst aggregates and groundmass suggest a crystallization temperature of ca. 600°C for the götzenite- and combeite bearing mineral assemblages. The textural features of the rock agree with an origin of holocrystalline nephelinite (with or without götzenite and combeite) by recrystallization of glass-bearing, nepheline-kalsilite and melilite porphyritic peralkaline nephelinite due to thermal metamorphism and metasomatism within the volcanic edifice. A chemographic analysis of the Ti-bearing mineral assemblages of götzenite-bearing and götzenite-free peralkaline nephelinite suggests that götzenite is stabilized by elevated fluorine activity combined with moderately high (for nephelinite) silica activity. At increasing peralkalinity, götzenite is likely to break down to perovskite-bearing mineral assemblages coexisting with combeite.