Carbonatite Emplacement Research Articles

Mantle eclogite and carbonate as sources of sodic carbonatites and alkalic magmas

At mantle pressures, partial melts of peridotite + CO2 are carbonate-rich, but contain insufficient sodium to form most carbonatites and associated silicate magmas. A possible source rock is eclogite, and extrapolations from experiments suggest that eclogite + CO2 will coexist with melt along geotherms of low heat flow. Thermochemical calculations yield high carbonate activities, and imply that the melt is carbonatitic. The melt can be modelled in the system jadeite–calcite–CO2 and will, on emplacement, form mineral assemblages characteristic of a wide range of alkalic rocks. The tectonic settings of carbonatites are consistent with an origin as partial melts of eclogite + carbonate + CO2 under areas of low surface heat flow, and suggest that carbonatite emplacement is not related to mantle hotspots.

Anorogenic magmatism, plate motion and Atlantic evolution

Emplacement of carbonatites at 0–15, 25, 40, 60–70, 130 Ma (and possibly 80 and 100 Ma) may coincide with major changes in plate motion. This suggests a common triggering mechanism which previously may have operated at intervals of about 230 Ma.

Apparent Periodicity of Carbonatite Emplacement in Canada

Carbonatites in Canada seem to have been intruded at intervals of 233 m.y. for at least the last 1,700 m.y. and these may also have been particularly favourable times for the emplacement of carbonatites elsewhere. If this can be confirmed a cosmological explanation may be necessary.

THE AGES OF CARBONATITE COMPLEXES IN EASTERN CANADA

There are four chronological groups of carbonatite complexes in Ontario and Quebec. These are about 125 m.y., 565 m.y., 1075 m.y., and 1700 m.y., with a possible fifth at 1560 m.y. The recently proposed St. Lawrence Rift System is not considered to be a very likely control of carbonatite emplacement unless it includes a widespread system of branch faults. The coincidence of carbonatite complexes with the "Kapuskasing High" is striking and probably puts a minimum age of 1750 m.y, on the feature. Niobium is found in carbonatites of each age group.

Potash-trachytes and ultra-potassic rocks associated with the carbonatite complex of the Toror Hills, Uganda

SummaryRocks containing a high percentage of potash feldspar are described for the first time from the Toror Hills in the Karamoja district of eastern Uganda. They include feldspathic fenites, intrusive feldspathic fenite-breccias, potashtrachytes, and orthoclasites. Petrographic data and chemical analyses show the close similarity in composition of these rocks despite their widely differing textures and modes of occurrence. The potash metasomatism associated with the emplacement of carbonatite, and the subsequent mobilization of these feldspathic fenites are compared with similar phenomena observed at other carbonatite centres. The problem of the mechanism of mobilization is discussed.