Abstract
The mineralogy of the Dismal Nepheline Syenite indicates crystallisation from a pulse of chemically evolved, anhydrous, alkaline magma concordantly intruded into Koettlitz Group metasediments. In the main intrusion, orthomagmatic phases exhibit only minor mineralogical variation with typical compositions being hedenbergite (mg#0.050-0.080), nepheline (Ne (sub 76) Ks (sub 19) Qtz (sub 5) ), alkali-feldspar (Or (sub 79-86) for orthoclase host and Ab (sub 85-98) for albite lamallae) and carbonate (>95 mol.% CaCO (sub 3) , <1.6 mol.% SrCO (sub 3) ). The Fe (super 2+) -rich nature of the hedenbergite indicates that the orthomagmatic assemblage formed under low f (sub O (sub 2) ) conditions as a result of internal buffering of the oxygen and CO (sub 2) fugacities, during crystallisation, by the graphite-CO (sub 2) equilibrium. Subsolidus coronas composed of hastingsite (mg# 0.015-0.070)+andradite-rich garnet (And (sub 25-72) Gross (sub 22-61) Ti-And (sub 2-15) Alm (sub 2-9) Spess (sub < or =2) )+a lbite (Ab (sub > or =98) ) have formed at the expense of hedenbergite and nepheline. This corona assemblage indicates an increase in the f (sub H2O) and relative f (sub O2) of the system, as a result of gradual equilibration with hydrous fluids from the surrounding Koettlitz Group metasediments. Cross-cutting fine-grained and pegmatitic dykes containing diopside (mg#0.6-0.7), ferroan pargasite (mg# approximately 0.4) and oligoclase (An (sub 22-23) ) are thought to have been fed by a less-evolved parental source magma than that from which the Dismal Nepheline Syenite was derived.