High-Ba (~ 11 wt.% BaO) phlogopite was found for the first time in olivinites of the Guli intrusion in the Maimecha–Kotui province of ultrabasic alkaline rocks and carbonatites. The high-Ba mica occurs in assemblage with a paragenesis of olivinite minerals—clinopyroxene, titanomagnetite, apatite, and ilmenite. High-Ba mica is an early phlogopite generation. Its magmatic crystallization led to a decrease in Ba content. Low-Ba mica is a late phlogopite generation. The high Ba/K ratios at the early stages of evolution of a mantle magmatic system are necessary for the formation of high-Ba minerals and point to magma formation at great depths and the contribution of mantle metasomatism to the geochemical characteristics of parental magmas.