POSTMAGMATIC ALTERATION IN EUDIALYTE FROM THE NORTH QOROQ CENTER, SOUTH GREENLAND

The North Qoroq centre comprises a series of nested nepheline syenite intrusions and forms part of the mid-late Proterozoic Gardar province of South Greenland. Within the centre fractionation has produced varie d rock types ranging from augite-syenite to lujavrite, a eudialyte mic rosyenite. Samples of eudialyte from the lujavrites of unit SN1B of th e centre show evidence for two-stage alteration. This alteration range s from slight modification along crystal margins to complete breakdown and replacement by new pseudomorphing phases. Modification to crystal margins is accompanied by increasing Nb and Zr contents and is relate d to metasomatism produced by the intrusion of younger syenite units o f the North Qoroq centre. More extensive alteration is as a result of metasomatism followed by lower-temperature supergene alteration. Simpl ified reactions for this breakdown include eudialyte + metasomatic flu id = allanite + nepheline; eudialyte + metasomatic fluid = titanite aegirine + mosandrite + wohlerite; eudialyte + fluid = zirfesite + flu id. Mass balance calculations for altered compared with unaltered samp les of lujavrite show that alteration took place at approximately cons tant volume with an overall increase in Fe (+2.41 g/100g), Si and K (0.65 and +0.61 g/100g), whilst Na (-2.67 g/100g) and all trace element s, particularly La, Y, Nb and Zr (-5.6 to -166 g/1000g)are lost from t he system.