The petrologic and isotopic-geochronologic study of basement rocks that were penetrated by a deep borehole in the marginal part of the East European Platform revealed that its section overlain by the Vendian-Paleozoic sedimentary cover is Early Proterozoic in age and largely consists of aluminous migmatized biotite, biotite-cordierite, and biotite-cordierite-sillimanite gneisses, which are intruded by granites, plagiogranites, and metatonalites. The lower part of the section is dominated by amphibole schists and amphibolites with subordinate nonmetamorphosed dolerite dikes and pegmatite veins. By metamorphism parameters (T = 630-680°C, P = 2-4 kbar), the metamorphic complex may be considered as the shallowest one, compared with other Early Proterozoic complexes, developed at least in the southwestern part of Fennoscandia. The progressive decrease in the mineral-formation pressure observed in the Lower Proterozoic metamorphic rocks southward, away from the Karelian Craton is likely explained by the tectonic transport (thrusting) of the Svecofennides over the margin of the Karelian Craton and their subsequent deeper erosion near the craton. The magmatic crystallization of metamorphic palgiogranites, penetrated at depths of 925-928 and 1004 m, is estimated by the U-Pb ID-TIMS method on zircons to occur 1860 ± 9 Ma ago. It is shown that by their age, the REE composition, and isotopic-geochemical characteristics, these rocks are close to the plagiogranites formed in the southeastern extremity of the Svecofennnian belt in the present-day northern Ladoga region and the Karelian Isthmus. No rocks, which could be correlated by their lithology with the Archean rocks of the Karelian Craton, are found.