MANTLE CARBONATITE ERUPTIONS - CRUSTAL CONTEXT AND IMPLICATIONS

High level carbonatites in south-east Zambia have sub-volcanic intrusion sequences similar to those of the classic Chilwa complex, with the significant addition of magnesiocarbonatite pyroclastic eruptions. Alkali silicate and alkali carbonate magmas are absent, indicating that the carbonatites are primary and close to their erupted compositions. Melt lapillae in some of the pyroclastics were erupted directly from a mantle source. In composition these pyroclastics are essentially similar to average magnesiocarbonatite (except for Cr, Ni and V). At least two other types of magnesiocarbonatite exist, one from slower moving magma, the other formed by replacement. Sovites (calciocarbonatites) in the complexes also show multiple origins, including calcitic melts. These relationships dramatically strengthen the recent growing awareness that carbonatite activity cannot be encompassed by any single petrogenetic scheme, but must be treated in terms of multiple processes operating on melts from different sources.