Contrasting trends of agpaitic crystallization in nepheline syenite in the Pilanesberg Alkaline Complex, South Africa

Different types of peralkaline nepheline syenite in the Pilanesberg Alkaline Complex show contrasting Zr- and Ti-bearing mineral assemblages: In some members of the green foyaite suite, eudialyte is the dominant Zr mineral, as is common in many other agpaitic nepheline syenites. Eudialyte has formed together with aegirine ± arfvedsonite, alkali feldspar(s), sodalite and nepheline. Aenigmatite is the main Ti-bearing mineral, but some samples have normandite (a Ti-Zr disilicate mineral) as a late magmatic mineral. Other varieties of green foyaite do not have eudialyte, but carry lamprophyllite (a Ti-Sr disilicate mineral) as a prominent, early crystallizing mineral. In contrast, pockets of trapped melt in white foyaite crystallized a series of titanium minerals starting with clearly miaskitic assemblages (with titanite and ilmenite), through increasingly agpaitic assemblages with astrophyllite, aenigmatite and lorenzenite, with hilairite forming at the end, as the only mineral with essential Zr. In both green and white foyaite, aegirine has percent- to sub-percent amounts of ZrO₂. Chemographic analysis of melt-mineral equilibria suggests that the main driving force behind the evolution of mineral assemblages in both white and green foyaite is increasing peralkalinity of the melt. In the white foyaite, this took place at high a_H2O throughout, whereas the green foyaite started its evolution at much lower a_H2O. The different Zr and Ti mineral assemblages observed in the latter rock type can be explained by differences in water activity during crystallization of the magma, reflecting different pre-emplacement fractionation and degassing histories of batches of genetically related nepheline syenite magma.