ANCYLITE-BEARING CARBONATITES OF THE SEBLYAVR MASSIF, KOLA-PENINSULA,RUSSIA

Dolomite carbonatites are the youngest carbonatites in the Seblyavr ul trabasic-alkaline massif, Kola peninsula, Russia. The main rock-formin g minerals include dolomite Ca-1.01(Mg0.90Fe0.05Mn0.03Sr0.01)(Sigma 0. 99)(CO3)(2)-ferroan dolomite Ca-0.99(Mg0.83Fe0.13Mn0.03Sr0.01)(Sigma 1 .00)(CO3)(2) and chlorite. The typical minor mineral is ancylite-(Ce), 9-0.45Nd0.10-0.16Pr0.05-0.07Gd0-0.02Sm0.01Th0-0.01 (Sigma .26)(Sr0.57 -0.65Ca0.03-0.05Fe0-0.03Ba0-0.01)(Sigma 0.62-0.77)(CO3)(2)(OH) . H2O, which usually occurs in cavities and is accompanied by strontianite an d barite, with late chlorite, pyrite, sphalerite, galena and quartz. T he subparallel orientation of the dolomite crystals normal to the cont act of carbonatite with wall rock indicates that carbonatites form in fractures as a result of two contemporary processes: (1) growing of do lomite crystals from edges of fractures and (2) opening (movement apar t) at the same time of the fracture. Mineral composition of the carbon atite indicates crystallisation of the ancylite-(Ce) and associated mi nerals from a carbohydrothermal solution at a period of changing chemi cal environment from alkaline to acid conditions.