Abstract

The North Qoroq nepheline syenites form part of the rift-related Gardar Province of south Greenland. In situ fractionation of the syenitic magmas resulted in a peralkaline residual magma of lujavritic composition, with concentration of rare-earth elements (REE), Y, Zr and Nb. Such a residual magmatic liquid crystallized eudialyte, the major repository of these elements. Both syenites and country rocks were affected by metasomatism, associated with the intrusion and evolution of younger syenite units. From each new syenite, contrasting fluids evolved, commonly producing two compositionally distinct metasomatic events. Metasomatism associated with one of these fluids resulted in extensive redistribution of rare-earth and related elements, with apatite, titanite and fluorcarbonate minerals as the major repositories. These phases and the margins of primary eudialyte crystals show complex, cross-cutting patterns of zonation under back-scattered electron imagery and, in the case of apatite, cathodoluminescence. These patterns can be qualitatively related to successive pulses of metasomatic fluid containing variable concentrations of REE. In apatite, zoning involved the coupled exchange Ca (super 2+) +P (super 5+) <-->REE (super 3+) +Si (super 4+) , whereas in eudialyte and titanite, variation was less systematic, but involved Ca, Na, Si, REE, Y, Zr and Nb. The fluid responsible for the metasomatism must have been capable of transporting these elements and is considered to be rich in F (super -) , CO (super 2-) 3 and PO (super 3-) 4 and of probable carbonatitic affinities. It evolved from fractionating syenitic magmas at a late stage, probably as a result of liquid immiscibility.

First Page Preview

First page PDF preview
You do not currently have access to this article.