Minissale, Silvia (2018) Petrogenesis of alkaline and strongly alkaline volcanism: examples from African Rift. [Tesi di dottorato]

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The Virunga Volcanic Province (VVP) lies in the western segment, the Albert Rift, of the East African Rift System (EARS). The volcanism there started about 11 Ma yrs ago and continues to the present. The two active volcanoes of VVP, Nyamuragira and Nyiragongo lie along the seismically active sector of the western rift. Nyiragongo is a stratovolcano characterized by rock types such as melilitite, melilite nephelinite, pyroxene nephelinite, leucite nephelinite, leucitite and leucite tephrite. Nyamuragira located 15 km to the north of Nyiragongo, together with it, is one of the most effusive global volcanoes erupting several times in the past few decades, with rocks ranging from basanite to tephrite and basalt. Concerning to this thesis work the samples include parasitic cones and lava fields of the volcanic complexes from 1938 eruption until the last one in 2016, which products were sampled from the proximal vent area to the distal outcrops. The mafic rocks of Nyamuragira are porphyritic with phenocrysts of olivine and clinopyroxene. Some of the basanites studied 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) contents than basanites. Such enrichment in these elements is probably 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 mantle-normalized incompatible element patterns of Nyamuragira show positive peaks at Ba and Nb and smoothly decreasing normalized-abundances from Nbn to Lun and a high Lan/Ybn ratio (18). The Nyiragongo melilite nephelinites and olivine melilitites have a porphyritic fabric as well as Nyamuragira products. Olivine ranges from forsterite-fayalite to kirschsteinite, clinopyroxene is diopside and melilite has akermanite composition. 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 Light Rare Earth Elements/Heavy Rare Earth Elements LREE/HREE (Lan/Ybn = 42); in general they present a low heavy REE contents. The low Zr/Nb (2.1) value of the olivine melilitites indicates that the Nyiragongo olivine melilitites are melt products from an incompatible element-enriched source. Based on the major and trace elemental compositions of these volcanic rocks, a bimodal characther emerges between the nephelinitic Nyiragongo lavas and the Nyamuragira basanites which suggests that the nephelinitic lavas have different origin. The difference between these types of rocks depends on the degree of partial melting of a garnet-carbonate lherzolite source and it depends on the pressure and depth formation of these lithotypes: deeper source for the melilitites and shallower source for the basanites.

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