The paper considers the age, composition, and genesis of granitoids of the Soktui massif, a petrotype of the Kukul’bei Complex of Mesozoic rare-metal granites in Transbaikalia. The Soktui massif is heterogeneous; it comprises several petrographic varieties: monzogranite–monzoleucogranites and microleucogranites of the major intrusive phase and alaskites, alkali–feldspathic granite porphyry, granodiorites, and quartz syenites of the phase of additional intrusions. According to the obtained U–Pb geochronological data, the ages of all varieties are the same within the analytical error and correspond to the Early Cretaceous. The granitoids are diverse in geochemical characteristics: The monzogranite–monzoleucogranites of the major intrusive phase belong to the rare-metal plumasite type, and the rocks of the phase of additional intrusions show signs of A-type granitoids. The geochemical and isotope characteristics of the rocks confirm the contribution of both continental crustal substrates and deep-seated mantle-derived magmas to their formation. The contents of volatiles in micas and the composition of fluid and melt inclusions indicate the involvement of two types of fluids in the magma generation: reduced chloride (probably mantle) and more oxidized fluoride (presumably of lower-crust origin). All melts were generated at depths of no more than 30 km, and the depth of the massif formation was shallower than 8 km. Based on the obtained data, we propose a model for the formation of the massif rocks.

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