ALKALIC ULTRABASIC ROCK AND CARBONATITES IN THE U.S.S.R.

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Abstract
This article gives a brief review of recent Soviet publications on the subject of alkalic ultrabasic rocks associated with carbonatites in the Kola Peninsula and Siberia. These rocks are found mainly as complexes infilling diatremes or cycloliths, and, as a rule consist of one, two or more rock series, of which the principal are the following: 1) Normal talc-alkali or alkali series ranging from ultrabasic to intermediate, 2) per-alkalic jacupirangite-urtite series, 3) nepheline syenites and fenites, and 4) carbonatites. Taking into consideration this Soviet work and using the Kola Peninsula and Siberian material it seems possible to produce a genetic scheme embracing all the four series, if it is accepted that, in addition to the crystallization-differentiation, there is a diffusion-differentiation, conditioned by an upward thermodiffusion of alkalies and volatiles, and among these, more particularly, carbon dioxide and water. Such a diffusion-differentiation, would promote the formation of the per-alkalic magma from the normal alkalic magma. The newly formed jacupirangite-urtite magma being highly chemically active, would give rise to two parallel processes: -nephelinitization and carbonatization. These two processes would be conditioned by an exchange reaction between the alkali carbonates present in the fluidal fraction of the magma and the calcium and magnesium silicates present either as minerals or molecules in the magma. Such a reaction can be visualized as The production of a nepheline molecule would lead to an intensive alkali auto- and hetero-metasomatism, transforming the granitic country rock first to fenite and this to a mobilized fenite and eventually to nepheline syenite. At the same time the production of calcium, magnesium and iron carbonates, would lead to an equally intensive carbonate auto- and hetero- metasomatism. The remaining solution would give rise to nephelinoliths and carbonatites. Postulating alkali carbonates as the transported material during the diffusion-differentiation, one must abandon the idea of an existing “carbonatite magma” in favor of “magmatic carbonatesrdquo; forming through an exchange reaction during the hydrothermal stages of the magmatic consolidation. --Author.

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