Early Cretaceous Kalakan Magmatic Area (Vitim Region, Northern Transbaikalia): Stages of Formation, Magmatic Sources, and Tectonic Setting

The Kalakan volcanic area in the Vitim region was formed in three stages: 137–121, 119–117, and 110 Ma. The first two stages were responsible for the formation of the mild-alkaline basaltic lavas of the Vitim Group and a complex of minor intrusions of the dolerite–granite porphyry series. The Early Cretaceous sequence is completed by a terrigenous sequence, which is intruded by a differentiated teschenite sill. The differences between the teschenite and the earlier volcanic rocks are best expressed in the distribution of incompatible elements. In the teschenite, their content steadily increases with SiO2 increase (crystallization differentiation trend). The early volcanic rocks show no such trend, evolving chaotically, which can be explained by the contamination of mantle melts with crust and/or anatectic melts. The differences in both groups of the Kalakan magmatic area are caused by a change of magmatic sources prior to the late Early Cretaceous.