Bulk-rock geochemical compositions of hypabyssal
kimberlites, emplaced
through the Archaean Kaapvaal craton and Proterozoic
Namaqua-Natal belt, are
used to estimate close-to-primary magma compositions of Group I
kimberlites (Mg-number = 0·82-0·87; ∼22-28 wt % MgO; ∼21-30 wt % SiO 2 ; ∼10-17
wt % CaO; ∼0·2-1·7 wt % K 2 O) and Group II kimberlites (Mg-number =
0·86-0·89; ∼23-29 wt % MgO; ∼28-36 wt % SiO 2 ; 8-13 wt % CaO;
∼1·6-4·6 wt % K 2 O). Group I kimberlites are distinguished from
Group II by their lower Ba/Nb (<12), Th/Nb (<1·1) and La/Nb (<1·1) but
higher Ce/Pb (>22) ratios. The distinct rare earth element patterns of the
two types of kimberlites indicate a more highly metasomatized source for Group
II kimberlites, with more residual clinopyroxene and less residual garnet. The
similarity of Sr and Nd isotope ratios and diagnostic trace element ratios
(Ce/Pb, Nb/U, La/Nb, Ba/Nb, Th/Nb) of Group I kimberlites to ocean island
basalts (OIB), but more refractory Mg-numbers and Ni contents, are consistent
with derivation of Group I kimberlites from subcontinental lithospheric mantle
(SCLM) that has been enriched by OIB-like melts or fluids. Source enrichment
ages and plate reconstructions support a direct association of these melts or
fluids with Mesozoic upwelling
beneath southern Africa of a mantle plume(s), at
present located beneath the southern South Atlantic Ocean. In contrast, the
geochemical characteristics of both on- and off-craton Group II kimberlites show
strong similarity to calc-alkaline magmas, particularly in their Nb and Ta
depletion and Pb enrichment. It is suggested that Group II kimberlites are
derived from both Archaean and Proterozoic lithospheric mantle source regions
metasomatized by melts or fluids associated with ancient subduction events,
unrelated to mantle plume upwelling. The upwelling of mantle plumes beneath
southern Africa during the Mesozoic, at the time of Gondwana break-up, may have
acted as a heat source for partial melting of the SCLM and the generation of
both Group I and Group II kimberlite magmas.