Niz'yavrski alkaline massif: age, isotopic characteristics and rare-metal mineralization

The Nizʼyavrski alkaline massif includes the rocks of two intrusive phases: (1) alkaline syenites and (2) alkaline quartz syenites. The crystallization age of the latter determined by the U-Pb method for zircon is 2656 ± 3 Ma. According to isotopic (Rb-Sr and Sm-Nd) data, the rocks of the massif have a mantle origin. It is suggested that the source of their melts, as well as the source of melts for other Neoarchean alkaline intrusions of the Kola province belonging to the association of alkaline gabbro, nepheline syenites, alkaline syenites, and granosyenite, was the BSE mantle reservoir. Rocks of the massif contain Ta-Nb mineralization represented by pyrochlore, which distinguishes the studied intrusion from the other Neoarchean alkaline magmatic bodies of the province.