Leucosomes of two types: anatectic ones and those formed under the effect of K-rich fluid, were studied in migmatites of the K-rich Priozersksaya zone of the Northern Ladoga region. When aluminiferous gneisses are partially melted at temperatures up to 800–850 С melt of granitic composition, similar to that of early generations of leucosomes, is formed. For the presumed РТ parameters of rock metamorphism: Т = 750–800 С, Р = 4.5–5.5 kbar, the proportion of melt would be at least 10–15 w/w % of the initial protolith mass. A rise in water content from 0.5 w/w % to 2.5 w/w % results in a nearly 4-fold increase in the melt volume under the same РТ parameters of ultrametamorphism. The composition and structural characteristics of later generation leucosomes suggest there was a multiple genesis (anatexis-metasomatosis), where K- rich fluid played a significant role in the evolution of the matter. The fluid could originate from potassic porphyraceous granite plutons, which are widespread in the region.