MINERALOGY OF THE DISMAL NEPHELINE SYENITE, SOUTHERN VICTORIA LAND, ANTARCTICA

The mineralogy of the Dismal Nepheline Syenite indicates crystallisati on from a pulse of chemically evolved, anhydrous, alkaline magma conco rdantly intruded into Koettlitz Group metasediments. In the main intru sion, orthomagmatic phases exhibit only minor mineralogical variation with typical compositions being hedenbergite (mg# 0.050-0.080), nephel ine (Ne(76)Ks(19)Qtz(5)), alkali-feldspar (Or(79-86) for orthoclase ho st and Ab(85-98) for albite lamallae) and carbonate (> 95 mol.% CaCO3, < 1.6 mol.% SrCO3). The Fe2+-rich nature of the hedenbergite indicate s that the orthomagmatic assemblage formed under low f(O2) conditions as a result of internal buffering of the oxygen and CO2 fugacities, du ring crystallisation, by the graphite-CO2 equilibrium. Subsolidus coro nas composed of hastingsite (mg# 0.015-0.070) + andradite-rich garnet d(25-72)Gross(22-61)Ti-And(2-15)Alm(2-9)Spess(less than or equal to 2) ) + albite (Ab(greater than or equal to 98)) have formed at the expens e of hedenbergite and nepheline. This corona assemblage indicates an i ncrease in the f(H2O) and relative f(O2) of the system, as a result of gradual equilibration with hydrous fluids from the surrounding Koettl itz Group metasediments. Cross-cutting fine-grained and pegmatitic dyk es containing diopside (mg# 0.6-0.7), ferroan pargrtsite (mg# similar to 0.4) and oligoclase (An(22-23)) are thought to have been fed by a l ess-evolved parental source magma than that from which the Dismal Neph eline Syenite was derived.