Geochemical Constraints on the Origin of Primitive Potassic Lavas in the Eastern Virunga Volcanic Province

Young mafic lavas from the East African Western Rift record melting of subcontinental lithospheric mantle that was metasomatically modified by multiple tectonic events. We report new isotope data monogenetic cinder cones near Bufumbira, Uganda, in Virunga Volcanic Field: 87Sr/86Sr = 0.7059–0.7079, εNd −6.5 to −1.3, εHf −6.3 +0.9, 208Pb/204Pb 40.1–40.7, 207Pb/204Pb 15.68–15.75, and 206Pb/204Pb 19.27–19.45. Olivine phenocrysts Bufumbira have 3He/4He 6.0–7.4 RA. The isotopic data, conjunction with major trace element systematics, indicate primitive magmas are derived two different metasomatized source domains. Melts generated lower degrees greater contributions ∼1 2 Ga isotopically enriched garnet-amphibole-phlogopite pyroxenite veins within lithosphere. As progresses, these vein melts become increasingly diluted originate lithosphere/asthenosphere boundary, shifting compositions toward common (CLM) proposed Furman Graham (1999, https://doi.org/10.1016/s0024-4937(99)00031-6). This ∼450–500 Ma domain appears underlie all volcanic provinces is characterized ∼ 0.705, 0, +1 +3, 19.0–19.2, 39.7, 7 Basal portions dense may gravitationally unstable founder into underlying warmer asthenosphere, exposing surfaces where locally heterogeneous produces small-volume, alkaline melts. Mafic mixing between CLM variable domains, suggesting this style melt generation fundamental development magma-poor rifts.