Volcanic carbonatites of the Kaluwe complex, Zambia

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Abstract
The Kaluwe carbonatite forms a thick, gently dipping sheet with a crescentic outcrop nearly 10 km long. It is composed largely of layered fragmental rocks and was originally interpreted as a sill-like intrusion, concordant with enveloping Karoo sandstones and emplaced episodically as fluidized suspensions of carbonatite fragments. The complex is re-interpreted as a sub-aerial volcanic accumulation of tuffs, agglomeratic tuffs, carbonatite lavas, and associated volcaniclastic sediments. Clasts in the tuffs include crystal and sövite fragments, and also fine-grained carbonatite fragments interpreted as blocks and lapilli of lava. The latter commonly have a trachytic texture with tabular calcite crystals and calcite pseudomorphs after nyerereite, indicating an original natrocarbonatite composition. A similar rock forms several non-fragmental layers, considered to be lava flows. The extrusive origin of the complex is shown by the presence of weathering and palaeosol horizons, of rubbly tops to lava flows, and of interbedded volcaniclastic arenites near the base of the succession. The entire upper part of the succession consists of conglomerates and arenites composed of re-deposited carbonatite volcanic material. Plutonic carbonatites, found as clasts in tuffs, are cumulates of calcite with layers in which apatite and magnetite are concentrated. The parent magma is considered to have been alkali carbonatite from which calcite and the minor non-carbonates crystallized to give sövite cumulates, with the residual alkaline melt being erupted as lavas and pyroclastics.