—The Yllymakh massif is one of the Mesozoic ring intrusions widespread in Central Aldan. The alkaline rocks composing it are greatly diverse in composition. Based on the obtained petrological, geochemical, and geochronological data, we have recognized three groups of rocks, marking the different phases of the massif formation. The rocks of two groups resulted from the fractionation of rock-forming minerals (pyroxene and plagioclase) and accessory apatite, which is reflected in the composition trends in the variation diagrams and in the REE patterns. Assimilation of the crustal material also significantly contributed to the diversity of rock-forming melts, which is evidenced from the increase in 87Sr/86Sr from group to group. At the same time, the εNd value is steadily extremely low throughout the study area (on average, –13.5). The obtained Ar–Ar geochronological data indicate three independent stages of the massif formation: 140.0 ± 1.9, 130.0 ±1.9–131.0 ± 2.4, and 125.0 ± 1.9 Ma, which are close in time to the evolution stages of other Mesozoic massifs in Aldan.