RARE-EARTH.RICH EUDIALYTE AND DALYITE FROM A PERALKALINE GRANITE DYKE AT STRAUMSVOLA, DRONNING MAUD LAND, ANTARCTICA

A microgranite dyke that cuts the Straumsvola nepheline syenite complex, Antarctica, contains RE'E-rich eudialyte (REE2O3 + Y2O3 ca, 8 wt,9o) and the rare mineral dalyite. The dyke rock is not particularly potassic (wt.Yo K,zNa 0.83) compared to rocks in which dalyite has recently been discovered (amproites), but it is similar in composition to the granite xenoliths of Ascension Island, in which dalyite was firs[ recognized. Eudialyte in the dyke is richer in R,EE than that from undersaturated peralkaline rocks, but this is not true of rhe eudialyte from wgs in the surrounding nepheline syenite. The lack of Ca in oversaturated peralkaline magmas in general enables euclialyte to act as an effectivei?EE-sink in such rocks. Although rare, eudi alyte in peralkaline granites should always be REE-rich, whereas tlut in nepheline syenites need not be, as tlese rocks invariably are more calcic.