The role of CO2 in alkali rock genesis

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Abstract
Summary: The extreme rarity of alkaline rock suites bearing both calcic plagioclase and magmatic carbonatites is believed to reflect a fundamental bifold division between Gabbroic and Carbonatitic types, plagioclase being present only in the former and carbonatite only in the latter. Alkali basalt magma may be parental to both lineages, the gabbroic lineage deriving from normal differentiation under low CO2 pressure, and the carbonatitic by suppression of plagioclase crystallization under high pressures of CO2, leading through pyroxene fractionation to a ‘secondary parental’ olivine-poor nephelinite magma. Support for this hypothesis is found in evidence for the suppression of plagioclase in CO2-rich alkali basaltic magmas and for the secondary origin of olivine-poor nephelinites, in the nature of xenoliths and cumulates at carbonatite complexes, in Sr isotopic data, and in major and trace element compositions of the magmas. The possible origin of melilitic rocks at carbonatite complexes is also briefly discussed.