Mineralogy, geochemistry and petrogenesis of Nyiragongo and Nyamuragira volcanic rocks (Virunga Province D.R. Congo)

The Virunga Volcanic Province (VVP) lies close to the northern end of the western branch of the East African Rift System (EARS). Volcanism started about 11 Ma ago and continuing to the present. The two active volcanoes of VVP, Nyamuragira and Nyiragongo are located along the seismically active sector of the western rift. Nyamuragira (3058 a.s.l.) is a large volcanic shield characterized by alkaline rocks ranging from basanite to tephrite and rare transitional basalt. Nyiragongo is a stratovolcano (3469 a.s.l.) characterized by rock types such as melilitite, melilite nephelinite, pyroxene nephelinite, leucite nephelinite, leucitite and leucite tephrite. Samples include parasitic cones and lava fields of the volcanic complexes from 1938 products until now, and products of 2002 eruption sampled from the proximal vent area to the distal outcrops. Nyamuragira basanites and tephrites are porphyritic with phenocrysts of olivine and clinopyroxene. Basanites have MgO (12.05-13.60 wt.%), Cr (790-926 ppm) and Ni (245-309 ppm) contents within the ranges expected for mantle-derived liquids. The transitional basalts have higher MgO (> 15 wt.%), Cr (> 969 ppm) and Ni (> 750 ppm) than basanites. Such enrichment in these elements is due to excess of olivine phenocrysts. Nyamuragira basanites have Zr/Nb (3.9-4.0), Ba/Nb (11-12) and La/Nb (0.86-0.9) ratios typical of mantle or OIB values. The primitive mantlenormalized incompatible element patterns of Nyamuragira show peaks at Ba and Nb and smoothly decreasing normalizedabundances from Nb to Lu. The high Lan/Ybn ratio (18) indicates that the Nyamuragira basanites are low degree partial melts of a slightly incompatible element-enriched mantle source in the garnet stability field. The least differentiated mafic rocks of Nyiragongo are melilite nephelinites and olivine melilitites. Melilite has akermanite composition, olivine ranges from forsterite-fayalite to kirschsteinite and clinopyroxene is diopside. All samples are feldspar-free. The composition of the glass is often rich in Ba content (up to 5 wt.% BaO). These rocks have higher CaO (~16.3 wt.%) and lower SiO2 (~ 40 wt.%), MgO (8.7-9.1 wt.%) and compatible elements concentrations (Cr = 380-395 ppm; Ni = 155-169 ppm) than Nyamuragira basanites. Their incompatible element patterns are also more enriched than those of Nyamuragira basanites with high LREE/HREE (Lan/Ybn = 42). The low Zr/Nb (2.1) of the olivine melilitites indicate that the Nyiragongo olivine melilitites are melt products of an incompatible element-enriched source. In addition, their low heavy REE contents suggest that they were generated within the garnet-peridotite stability field within the lithospheric mantle. The compositional variation within the Nyiragongo volcanic rocks was largely controlled by low pressure fractional crystallization of the observed phases.