PETROGENESIS OF CARBONATE-BEARING NEPHELINE SYENITES AND CARBONATITESFROM SOUTHERN VICTORIA LAND, ANTARCTICA - ORIGIN OF CARBON AND THE EFFECTS OF CALCITE-GRAPHITE EQUILIBRIUM

Chemically-evolved carbonate-bearing nepheline syenites are intruded i nto basement metasediments of the Koettlitz Group on Dismal and Radian Ridges in the Pipecleaner Glacier region of Southern Victoria Land, A ntarctica. Whole rock XRF data from the Dismal Nepheline Syenite defin es a broad trend which is consistent with the removal of a cumulate fr action of approximate composition 70% hedenbergite, 15% nepheline, 10% titanite and 5% apatite. Stable isotope, major and trace element and mineralogical characteristics of the syenites are very similar to thos e of cross-cutting calcite-rich dykes indicating derivation from close ly-related source magmas. The general association of carbonatites and nepheline syenites with extensional environments, suggests that the Di smal and Radian Ridge nepheline syenites and carbonatite dykes indicat e a period of early Paleozoic (531 Ma) rifting or intrusion into local ised tensional structures in an overall compressional regime. Assimila tion of marble by the syenite magmas is evidenced by abundant rafts an d xenoliths within the Dismal Nepheline Syenite, however, carbon and o xygen isotopic ratios from syenite and carbonatite calcites are distin ctly lighter than values from the marble country rock indicating a mag matic source. Graphite and calcite commonly occur as aggregates in the Dismal Nepheline Syenite suggesting equilibrium between these two car bon-bearing phases. Isotopic fractionation between calcite and graphit e, via the equilibrium: C + O-2 = CO2 has resulted in enrichment of th e C-13 isotope in calcites from the Dismal Nepheline Syenite.