—The origin of the Late Jurassic–Early Cretaceous volcanism within the northern part of the Argun terrane (eastern Transbaikalia) is considered. New data on the geology, age, and composition of late Mesozoic volcanic complexes of the Ust’-Kara basin are presented. Three stages of volcanism have been identified: Tithonian–Berriasian (~150–143 Ma), Valanginian (~140–136 Ma), and Hauterivian (~134–131 Ma), during which volcanic rocks and sediments of three formations (Udyugan, Ust’-Kara, and Shilka, respectively) were deposited. The petrochemical and geochemical characteristics of the rocks of these formations are considered. The compositions of chemically similar rocks evolved toward an increase in the contents of incompatible elements. The rocks of the Ust’-Kara basin are compared with the coeval igneous rocks of the Great Xing’an and East Mongolian belts, which formed in the settings of an active margin and intracontinental rifting, respectively. It is shown that the rocks of the basin are similar in composition to the volcanics of the Great Xing’an belt. A conclusion has been drawn that the late Mesozoic magmatism in the northern part of the Argun terrane was controlled by subduction processes, which led to the formation of the late Mesozoic active continental margin of the Asian continent.