Phlogopitization of pyroxenite; its bearing on the composition of carbonatite magmas

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Abstract
Summary: Phlogopitization of pyroxenite is common in contact zones between clinopyroxenites and carbonatite dikes of the Cargill ultramafic rock—carbonatite complex near Kapuskasing, Ontario. The most typical development is a mica zone 1–10 cm wide but phlogopite is also developed in a more pervasive manner throughout the groundmass of several types of ultramafic rock. Fenitization is most commonly thought of as a process whereby aegirine and riebeckitic amphiboles are formed in the host rock while feldspar is recrystallized and silica progressively removed. Phlogopitization of pyroxenite can properly be referred to, however, as a type of fenitization. It is clearly related to the intrusion of carbonatite into pyroxenite and is further testimony to the fact that many carbonatite magmas are initially alkalic but lose alkalies to the surrounding rocks and crystallize as calcitic and dolomitic carbonatite with alkali contents restricted to the amounts that could be fixed as micas, pyroxenes or amphiboles. This in turn is controlled by the silica and alumina activity of the carbonatite magma. Abundant evidence for considerable amounts of fluorine in carbonatite magmas suggests that alkalies may be transported into the country rocks as fluorides. It is further suggested that late-stage feldspathization in carbonatite complexes is explained by the abstraction of potassic halide solutions from the crystallizing carbonatite magma. The conclusion seems inescapable that alkali carbonatite magmas, far from being the curiosity thought by many petrologists, are in fact very common during the evolutionary history of carbonatites. The common calcitic and dolomitic carbonatites have not generally crystallized from a magma of the same composition but are the residue remaining after the abstraction of an alkali-rich aqueous fluid. Consequently, there is a need to redesign the experimental phase equilibrium approach to problems of carbonatite genesis in order to take account of the presence of alkalies in most carbonatite magmas.