PATTERNS OF ZONATION IN RARE-EARTH-BEARING MINERALS IN NEPHELINE SYENITES OF THE NORTH QOROQ CENTER, SOUTH GREENLAND

The North Qoroq nepheline syenites form part of the rift-related Garda r Province of south Greenland. In situ fractionation of the syenitic m agmas resulted in a peralkaline residual magma of lujavritic compositi on, with concentration of rare-earth elements (REE), Y, Zr and Nb. Suc h a residual magmatic liquid crystallized eudialyte, the major reposit ory of these elements. Both syenites and country rocks were affected b y metasomatism, associated with the intrusion and evolution of younger syenite units. From each new syenite, contrasting fluids evolved, com monly producing two compositionally distinct metasomatic events. Metas omatism associated with one of these fluids resulted in extensive redi stribution of rare-earth and related elements, with apatite, titanite and fluorcarbonate minerals as the major repositories. These phases an d the margins of primary eudialyte crystals show complex, cross-cuttin g patterns of zonation under back-scattered electron imagery and, in t he case of apatite, cathodoluminescence. These patterns can be qualita tively related to successive pulses of metasomatic fluid - containing variable concentrations of REE. In apatite, zoning involved the couple d exchange Ca2+ + P5+ reversible arrow REE(3+) + Si4+, whereas in eudi alyte and titanite, variation was less systematic, but involved Ca, Na , Si, REE, Y, Zr and Nb. The fluid responsible for the metasomatism mu st have been capable of transporting these elements and is considered to be rich in F-, CO32- and PO43- and of probable carbonatitic affinit ies. It evolved from fractionating syenitic magmas at a late stage, pr obably as a result of liquid immiscibility.