The occurrence and formation of Ti-Aegirines in peralkaline syenites

Abstract

The formation and stabilities of Fe2+-NAT (NaTi1/2Fe2+ 1/2Si2O6) and Mg-NAT (NaT1/2Mg1/2 Si2O6) clinopyroxene end-members in peralkaline nepheline syenites of a shonkinite-nepheline syenite suite in the Tertiary ultramafic alkaline Gardiner complex, East Greenland is described. Based on the composition of coexisting aegirines and Ti-minerals it is suggested that the formation of Mg-NAT is favoured by high f O2 and high Nds- and TiO2-activity. The instability of ilmenite at a late magmatic stage leads to the breakdown of a peralkaline acmite-ilmenite oxygen-buffer. Lack of buffering results in strong enrichment of aegirines in Fe2+-NAT. Up to 8% TiO2 and 20% Mg-NAT and 20% Fe2+-NAT is recorded from these aegirines. The most evolved compositions show decreasing NAT and increasing Ac. The breakdown of sphene at a late magmatic to subsolidus stage results in increasing Wo in coexisting aegirines. Based on comparison with other occurrences it is concluded that NAT endmembers form at low pressure, but not necessarilly at low T., and that high f O2 favors Mg-NAT formation and low f O2 Fe2+-NAT formation.