The palaeoproterozoic Northwestern region of the Ukrainian shield hosts two compositional types of mafic dykes and associated magmatism that intruded at c. 1800–1760 Ma: (1) high-Ni dolerite dykes and layered intrusions of tholeiitic affinity and (2) high-Ti dolerite dykes of jotunitic affinity associated with anorthosite–mangerite–charnockite–granite (AMCG) suites. The jotunitic dykes represent initial melts for basic rocks of the Korosten AMCG plutonic complex, whereas tholeiitic dykes may reflect emplacement of a mantle plume and formation of a large igneous province (LIP). New U–Pb baddeleyite ages indicate that both compositional types can be coeval: the jotunitic Rudnya Bazarska dyke was emplaced at 1793 ・} 3 Ma, and the Zamyslovychi tholeiitic dolerite dyke at 1789 +/-9 Ma. In our model, the mantle plume-derived tholeiitic melts (underplate) supplied heat required for melting of the mafic lower crust and the production of jotunitic melts. As formation of the jotunite melts requires pressures in the range 10–13 kbar, either a thickened crust is needed or the lower crust must be subducted, or downthrusted, into the mantle. Alternatively, emplacement and ponding of large volume of tholeiitic melts might cause delamination of the lower crust, its sinking into the mantle, and further fusion to produce jotunitic melts.
2016. Vol. 138, 79-85 p.