Dismal geology : a study of magmatic and subsolidus processes in a carbonated alkaline intrusion, Southern Victoria Land, Antarctica

Worley, Brenton A. (Brenton Alan), 1968-

The Dismal Nepheline Syenite represents a pulse of extremely evolved, alkaline magmatism with carbonatitic affinities. An anhydrous assemblage dominated by hedenbergite, nepheline and alkali-feldspar crystallized at temperatures -600°C, and pressures of -5 kb. Oxygen and C02 fugacities were controlled by the graphite-C02 and FMQ equilibria with logfo2=-19.4 and logfco2=4.4, equivalent to Xco2'"'0.15. Variations in the whole rock chemistry can be explained by fractional crystallization of hedenbergite, titanite, nepheline and to a lesser extent apatite.
Cross-cutting micro melteigite and ijolitic pegmatite lithologies indicate a probable parental magma composition. The Dismal Nepheline Syenite may have been derived from this composition by fractionation of a systematically evolving pyroxene phase. Following crystallization, increasing fn2o and f o2 along with decreasing temperature resulted in the reequilibration of the magmatic assemblage, producing corona assemblages of amphibole, grandite garnet and albite at contacts between hedenbergite and nepheline.
Carbon and oxygen isotope ratios for carbonates range from 813Cp03:=:-2 to 1 %o and 8180sMow=14 to 16%o. These heavy values are attributed to the evolved state of the Dismal Nepheline Syenite magma and isotopic fractionation between calcite and graphite.
There is unequivocal evidence of interaction between the intrusion and surrounding Koettlitz Group marbles. The effect of marble assimilation, however, appears to be confined to the contacts and is not considered to be the driving force responsible for the undersaturated nature of the intrusion, the formation of the corona textures, and the heavy carbon and oxygen isotopic signatures .

160 p. (some folded) : ill. (some col.), maps ; 30 cm.

1992Worley

Abstract Only

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