The Geochemistry of the melilite-bearing lavas of the Nyiragongo volcanic complex (D.R. Congo)

The extremely active Mount Nyiragongo, located in the Virunga Volcanic Province on the edge of the Tanzanian craton, is formed by highly silica undersaturated, feldspar-free, melilite-bearing rocks (olivine melilitites and melilite-leucite nephelinites); nepheline is more abundant than leucite. The Nyiragongo primitive rocks are characterized by high contents of MgO (>8 wt.%), CaO (>12 wt.%) and Sr (>2000 ppm) and low Zr/Nb (2.1). The REE patterns show high La/Ybn (42), no negative anomalies in Eu, and the mantle normalized diagrams display troughs at K and Pb. The 87 Sr/86Sr (0.70447 - 0.70469), 143Nd/144Nd (0.51272), 206Pb/204Pb (19.413 - 19.751), 207Pb204Pb (15.663 - 15.749) 208Pb/204Pb (39.629 - 39.814) isotope compositions has also been determined. A volatile (H2O, CO2, S, Cl and F) and incompatible element-rich lithosperic source is proposed also from the evaluation of the isotopic compositions of the lavas, far removed from that of astenosferic or plume mantle, and expected in metasomatized cratonic/pericratonic areas throughout the African plate. Melting was favoured by the rifted tectonic regime and by the decreasing of melting temperature caused by fluid-rich sources. Melilitites and nephelinites are silicate melts generated by CO2-rich sources, and occur preferentially at the propagation tip and on the flanks of the rifts which impinge on a craton that appears to be at an ntermediate stage of destabilization (e.g., Chakrabarti et al., 2009; Foley et al., 2012; Foley and Fischer, 2017).