THE OCCURRENCE OF NIOBIAN ZIRCONOLITE, PYROCHLORE AND BADDELEYITE IN THE KOVDOR CARBONATITE COMPLEX, KOLA-PENINSULA, RUSSIA

The compositions and textural relationships of the oxide minerals zirc onolite, pyrochlore and baddeleyite are described. These occur as acce ssory minerals, often intergrown with each other, from a phoscorite ro ck associated with the Kovdor carbonatite complex. Both the zirconolit e and baddeleyite have relatively high concentrations of Nb and Ta; th e pyrochlore is rich in U and Ta. Backscattered electron images, coupl ed with detailed microprobe analyses, reveal complex compositional zon ing in zirconolite and pyrochlore which reflect changes in the fluid c omposition during growth of these minerals. A comparison is made of in compatible element ratios Zr/Hf, Nb/Ta and Th/U between the three acce ssory minerals.