Carbonatites and carbonatites and carbonatites

Carbonatites are redefined using a mineralogical-genetic classification, and divided into two groups: primary carbonatites and carbothermal residua. Attention is drawn to the fact that carbonatite is both a petrographic term applicable to a particular rock-type and a group name applied to a complex of related carbonate and silicate rocks in a magmatic or extrusive complex. Primary carbonatites (in terms of mineralogical-genetic classifications rather than simple modal classifications) can be divided into a group of bona fide magmatic carbonatites formed from diverse mantle-derived magmas, i.e. carbonatites associated with the melilitite, nephelinite, aillikite and kimberlite clans, with the latter best being termed calcite kimberlites. Each magma type and associated carbonatites are considered to be genetically distinct, and formed at different depth in the upper mantle by different degrees of partial melting. Carbonatites associated with the melilitite and nephelinite clans can have a multiplicity of origins, and may be formed by fractional melting, fractional crystallization or liquid immiscibility. Calcite kimberlites are small-volume, late-forming differentiates not related to other carbonatites or their parent magmas. The origin and genetic relationships of the Oldoinyo Lengai natrocarbonate cannot be unambiguously detemined. Carbonate-rich rocks associated with various potassic or sodic peralkaline saturated to undersaturated magmas derived mainly from metasomatized lithospheric mantle, together with REE-carbonate-rich rocks of undetermined genesis, are best termed carbothermal residua rather than carbonatite; there can be mineralogical (or modal) convergence between these rocks and low-pressure REE-rich derivatives of bona fide primary carbonatites. Carbonate-rich rocks formed by pneumatolytic reactions or anatectic melting of crustal rocks should not be considered to be carbonatites.