The geochemistry of recent Nyamulagira and Nyiragongo potassic lavas, Virunga Volcanic Province, and implications on the enrichment processes in the mantle lithosphere of the Tanzania-Congo craton

We here investigate the geochemical and isotopic variability of primitive and evolved magmas of the Nyamulagira and Nyiragongo volcanic complexes (Virunga Volcanic Province, western branch of the east African Rift), including the very last products of the Nyiragongo's May 22, 2021 eruptive event and the Nyamulagira lava lake in February 2020. The different degree of silica undersaturation (i.e., potassic basanites/tephrites at Nyamulagira vs. potassic olivine melilitites/melilite nephelinites at Nyiragongo) and distinct incompatible element enrichment between the two volcanoes (e.g., Zr/Nb = 3.3–4 at Nyamulagira vs. Zr/Nb = 1.2–2.1 at Nyiragongo) are remarkable. Concentration of volatile elements (especially F and S) increases with the degree of magmatic evolution, and is also markedly different at the same level of magma evolution for the products of the two volcanic complexes, suggesting distinct volatile concentration of the primary magmas. The Sr-Nd-Pb isotopic range (e.g., 87Sr/86Sr = 0.7052–0.7059 at Nyamulagira vs. 87Sr/86Sr = 0.7045–0.7047 at Nyiragongo; 206Pb/204Pb = 19.19–19.31 at Nyamulagira vs. 206Pb/204Pb = 19.41–19.75 at Nyiragongo) overlaps with previous analyses obtained in the Virunga Volcanic Province (VVP), and is discussed with respect to other potassic/ultrapotassic rocks from different tectonic settings. The Ba/Nb and La/Nb ratios and Cs concentration of Nyamulagira and Nyiragongo indicate a negligible role for subducted sediments as a mantle-added geochemical component, as instead took place in the source of other primitive potassic/ ultrapotassic rocks such as those of the Roman Volcanic Province. The genesis of the primitive lavas of Nyamulagira and Nyiragongo is related to partial melting of a heterogeneous lithospheric peridotite hosting phlogopite and variable amounts of carbonates, which was moderately to highly enriched in incompatible elements, particularly Nb, Ta, LREE, (K), Ba and Sr. No high-temperature (plume), asthenospheric components or pyroxenites are evident or unambiguously detectable.