Generalization of available literature and new original data showed that the Early Cretaceous high- Mg alkaline magmatism is confined to the ancient Lambert rift zone. Alkaline ultramafic rocks developed in the areas of this zone (Jetty Oasis, western flank of the Beaver and Radok lakes, Fisher and Meredith massifs) were derived through melting of metasomatized continental mantle at ~1270°С and at depths of 130–140 km. Variations of major and trace-element composition and the wide range in olivine composition (Fo₉₁–Fo₈₀) are consistent with its change through intrachamber crystallization. The average values of initial isotope composition of ultra-alkaline high-Mg basalts are as follows:¹⁴³Nd/¹⁴⁴Nd—0.512485,⁸⁷Sr/⁸⁶Sr—0.70637,²⁰⁷Pb/²⁰⁴Pb—15.671,²⁰⁶Pb/²⁰⁴Pb—18.391,²⁰⁸Pb/²⁰⁴Pb—38.409. They are close to the model ЕМII source and can be arbitrarily taken as the preliminary assessment of isotope composition of a source of Mesozoic melts. Based on lithophile element and isotope compositions, the alkaline high-Mg basaltic magmatism is thought to be related to the thermal impact of the Kerguelen plume on the lithospheric mantle of East Gondwana 120–110 Ma. Similar ancient deep-seated metasomatized eastern Gondwanan mantle, which contains carbonates and biotite and has an age of 2.4 Ga, was found in southern East Antarctica as well as in the north within eastern India.