Abstract

The melilite of the Oka complex is chemically equivalent to a mixture of nepheline, wollastonite, and diopsidic pyroxene, a mixture which locally coexists with melilite. The melilite is proposed to result from reaction of this mixture with calcite. Thermodynamic analyses of the possible reactions suggest melilite formed between 800 and 890 °C (1073–1163 °A), and at CO2 pressures less than 2 kbars. Monticellite, a characteristic accompanier of melilite would form from dolomitic carbonate under the same conditions. Experimental data appear to show that all wollastonite-bearing alkaline rocks are emplaced at total pressures less than 3.5 kbars, and all plutonic nephelinitic rocks equilibrated at temperatures of less than 1000 °C (1273 °A).

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