Abstract— <p>New ⁴⁰Ar/³⁹Ar age data have been obtained for phlogopite and amphibole from alkaline-ultramafic lamprophyres of the Zhlobin and North Uvarovichi fields, located within the Pripyat segment of the Pripyat–Dnieper–Donets rift system. The results indicate that the emplacement of the lamprophyre pipes occurred between 374–383 Ma, corresponding to the Givetian–Frasnian boundary. The deepest rocks—kimberlites and related alkaline ultramafic rocks–were formed during the early rifting/initial magmatic phase, which occurred sub-synchronously in different parts of the developing rift system. This suggests the existence of a weakened, metasomatized mantle lithosphere beneath the future rift zone.</p>