Mineralogy of the Dismal Nepheline Syenite, Southern Victoria Land, Antarctica

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 76Ks 19Qtz 5), alkali-feldspar (Or 79 − 86 for orthoclase host and Ab 85 −98 for albite lamallae) and carbonate ( > 95 mol.% CaCO 3, < 1.6 mol.% SrCO 3). The Fe 2+-rich nature of the hedenbergite indicates that the orthomagmatic assemblage formed under low ƒ O 2 conditions as a result of internal buffering of the oxygen and CO 2 fugacities, during crystallisation, by the graphite-CO 2 equilibrium. Subsolidus coronas composed of hastingsite (mg # 0.015-0.070) + andradite-rich garnet (And 25 − 72Gross 22 − 61Ti-And 2 −15Alm 2 −9Spess ⩽ 2) + albite (Ab ⩾ 98) have formed at the expense of hedenbergite and nepheline. This corona assemblage indicates an increase in the ƒ H 2O and relative ƒ O 2 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 # ∼ 0.4) and oligoclase (An 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.