REE-SR-BA MINERALS FROM THE KHIBINA CARBONATITES, KOLA-PENINSULA, RUSSIA - THEIR MINERALOGY, PARAGENESIS AND EVOLUTION

Carbonatites from the Khibina Alkaline Massif (360-380 Ma), Kola Penin sula, Russia, contain one of the most diverse assemblages of REE miner als described thus far from carbonatites and provide an excellent oppo rtunity to track the evolution of late-stage carbonatites and their su b-solidus (secondary) changes. Twelve rare earth minerals have been an alysed in detail and compared with literature analyses. These minerals include some common to carbonatites (e.g. Ca-rare-earth fluocarbonate s and ancylite-(Ce)) plus burbankite and carbocernaite and some very r are Ba,REE fluocarbonates. Overall the REE patterns change from light rare earth-enriched in the earliest carbonatites to heavy rare earth-e nriched in the late carbonate-zeolite veins, an evolution which is tho ught to reflect the increasing 'carbohydrothermal' nature of the rock- forming fluid. Many of the carbonatites have been subject to sub-solid us metasomatic processes whose products include hexagonal prismatic ps eudomorphs of ancylite-(Ce) or synchysite-(Ce), strontianite and baryt e after burbankite and carbocernaite. The metasomatic processes cause little change in the rare earth patterns and it is thought that they t ook place soon after emplacement.