Pyroxenites of the Krestovskaya alkaline-ultramafic intrusion: Composition of parental magmas and their sources

Abstract

The main rock-forming minerals of pyroxenites in the Krestovskaya intrusion in the Maimecha-Kotui alkaline-ultramafic province are Al- and Ti-fassaite and low-Al high-Mg diopside. Both clinopyroxene varieties bear primary inclusions of alkaline-ultramafic melts enriched in incompatible elements, F (up to 0.3–0.4 wt %), and probably also CO2. The homogenization temperatures of the inclusions are approximately equal and lie within the range of 1200–1300° C. However, the melts preserved in the diopside are undersaturated in Si and Al and richer in Fe, Ba, Sr, Na, and incompatible elements than melt inclusions in the fassaite; they are free in H2O (no more than 0.003 wt %); and are close in composition to katungite-mafurite. Melt inclusions in the fassaite are richer in Si, Mg, and Al; contain up to 0.435 wt % H2O; and compositionally approach alkaline picritoids. Melts of such composition cannot be produced by the differentiation of a single parental magma and were most probably derived from different mantle sources. Judging from the high concentrations of incompatible elements and their distribution in the melt inclusions, these sources were localized in the undepleted mantle at various depths (the picritoid melts were derived from a deeper source) and underwent different degrees of partial melting, with garnet and plagioclase remaining in the residue. The coexistence of diopside and fassaite in a single rock can be explained by the concurrent development of magmatic chambers at different depths during rifting, when this process was repeatedly reactivated and it facilitated the arrival of primitive melts derived from different mantle sources into the same magmatic chambers, in which these melts mixed and evolved. These processes probably predetermined the origin of the alkaline-ultramafic carbonatite intrusions and perhaps also the potassic series in the East African Rift.