THE ORIGIN OF DOLOMITIC CARBONATITES - FIELD AND EXPERIMENTAL CONSTRAINTS

Carbonatites are most commonly either calcitic or dolomitic/ankeritic with very few types in between - what is generally referred to as the bimodal distribution. There is a widely held view that dolomitic carbo natites are less abundant than calcitic carbonatites and that these ar ise as sub-solidus alteration products of primary calcitic carbonatite s. It is demonstrated here that dolomitic types are far more common th an is sometimes appreciated and that they are particularly abundant in old Precambrian cratonic regions such as the Zimbabwean and Kaapvaal Cratons and the Archaean parts of the Canadian Shield. The field, petr ographic and chemical features favour these dolomitic carbonatites bei ng magmatic rather than arising from sub-solidus replacement of calcit ic carbonatites. Experimental studies show that partial melting of car bonated mantle peridotite produces a carbonate liquid with high Mg# an d MgO content and an alkali content of up to 6%. On ascent through the mantle from its original generation site this primitive carbonatite w ill be destroyed by reaction with Iherzolite and harzburgite if it rem ains in equilibrium with the surrounding mantle. If, however, the melt is shielded from the surrounding mantle by a lining of metasomatic we hrlite on the conduits or if it rises too rapidly for equilibrium to b e maintained, it is able to escape the mantle and rise into the crust. Reaction between primitive magnesian carbonate melt and wall-rock weh rlite shifts the composition of the melt to more Ca-rich (''calcitic'' ) compositions. IP is argued that such liquids are capable of generati ng the complete range of carbonatite compositions recognised at the su rface. Dolomite melts incongruently at low pressures and so will only crystallise from a magnesian carbonate magma at temperatures below the dolomite dissociation reaction. These conditions are dictated by the P-T trajectory of the ascending melt as well as the nature and concent ration of minor ''fluxing'' constituents in the melt such as fluorine and alkalis. As a result calcite is the liquidus phase over a wide ran ge of P-T-X conditions. Several authors have suggested that many calci te-rich carbonatites formed as cumulate-enriched crystal mushes. Such calcite mushes could be readily generated from magnesian, essentially ''dolomitic'', parental magmas. It is argued that no reasonable petrog enetic mechanism exists whereby magnesian carbonatite magmas could be generated from calcitic parental melts: it is argued that the reverse is true. (C) 1997 Elsevier Science Limited.