Genesis of carbonatites: inferences from a world map and database
Department of Mineralogy,
A recently published world map and database of carbonatites (Woolley and Kjarsgaard, 2008) includes data that provide insights, some new, into the spatial and temporal distribution and petrogenesis of carbonatites. 527 carbonatite occurrences are briefly described and their plotting on a world geology base map demonstrates (a) that in many provinces there has been repeated emplacement of carbonatites over geological time, (b) that there are few carbonatites in oceanic areas, most of which lie close to continental margins, (c) that there is an overwhelming concentration in Precambrian cratons (perhaps as much as 86%), including occurrences of Phanerozoic age, and (d) that many of these cratonic areas are ‘domed’ i.e. are topographically elevated. The repetition of carbonatite emplacement in restricted areas of continental lithosphere/plate, which have generally drifted considerable distances between episodes of carbonatitic volcanism, must preclude any genesis involving mantle plumes, and underlines the crucial role that the lithosphere undoubtedly plays in carbonatite generation. The doming of the carbonatite-bearing cratonic areas is interpreted as reflecting metasomatism of the lithosphere, the metasomatism being concentrated, initially at least, by structural features leading to ‘lithosphere focussing’.
The database comprises short descriptions of all 527 carbonatites together with tabulated information on economic aspects, including working deposits, and a range of subsidiary maps. Some 50 occurrences include extrusive carbonatite and these are particularly significant because 20 of them contain mantle xenoliths and/or xenocrysts, with none found in intrusive carbonatites, which gives direct evidence of a mantle origin and metasomatism, and pressure (depth) and temperature data. The igneous silicate rocks found in association with carbonatites are tabulated and form an extensive range of rock types which fall into six series i.e. kimberlite, melilitite (melilitolite), nephelinite (ijolite), phonolite (foid syenite), basanite (alkali gabbro) and trachyte (syenite). About 20% of carbonatites have no associated silicate rocks. The extrusive carbonatites demonstrate one major difference from the intrusive ones in that nearly half of them are associated with melilite-bearing rocks, whereas the equivalent association in the intrusives is found in <10% of occurrences. The relative abundance of melilite-bearing rocks amongst the extrusive carbonatites is believed to reflect one of the major, if not the main, paths of carbonatite evolution, while the relative paucity of these rocks amongst intrusive carbonatites is considered to be owing to loss during multiple high-level emplacement and volcano growth as well as reflecting the instability of melilite in later melt fractions. Finally, essentially utilising the evidence of the map and database, a model is presented in the form of a lithosphere section, metasomatised in the lower part, in which carbonatites are either generated at depth directly in the mantle or at higher levels by fractionation from at least five of the six silicate series. The silicate series vary in depth of origin from kimberlite at the base of the lithosphere, or within the uppermost asthenosphere,š through melilitites and nephelinites to relatively shallow originating phonolites and basanites. Some cognisance is also taken of known alkali silicate differentiation series. All the silicate series rocks also occur in carbonatite-free associations so that the impression is gained that carbonatites are only generated when a particular type of metasomatism is superimposed on lithosphere mantle which, in its absence, has the potential to produce the silicate rocks anyway. However, the silicate rocks, being essentially alkaline, themselves probably have antecedants involving mantle metasomatism, albeit chemically distinct from that leading to carbonatite generation.
A.R. and Kjarsgaard, B.A. 2008. Carbonatite
occurrences of the world: map and database. Geological Survey