GEODYNAMIC CONTROLS OF KIMBERLITES - WHAT ARE THE ROLES OF HOTSPOT AND PLATE-TECTONICS

Geodynamic controls of kimberlite magmatism are divided into those aff ecting 1) the crustal tectonic environment during emplacement, 2) trig gering of partial melting, and 3) enrichment of the kimberlite source rocks. Although crustal structures exert a strong control on surface a nd near-surface emplacement of kimberlites, a general correlation betw een crustal tectonics and kimberlite source dynamics in the upper mant le cannot be recognized. The relative roles of hotspot and plate tecto nics as triggering and enrichment processes are examined in light of t he geodynamic setting of Mesozoic southern African kimberlites. Plate tectonic reconstructions of known hotspot tracks are incompatible with a significant role of hotspots or plumes in enriching upper mantle so urce rocks. The same reconstructions are consistent, however, with the presence of a megalith of remnants of subducted oceanic lithosphere u nder much of southern Gondwana that may have been responsible for the enriched geochemical signatures of Group II kimberlites and DUPAL-type hotspots. The apparent hotspot track of Group II kimberlites suggests that melting was triggered by slab-derived volatiles or diapirs as so uthern Africa slid off the megalith after opening of the South Atlanti c.