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