EVIDENCE FOR FORMATION AND MODIFICATION OF THE
KAAPVAAL CRATON (SOUTHERN AFRICA) IN ARCHAEAN
SUBDUCTION ZONES
Nina S.C. Simon1* , Richard W. Carlson2, 1D. Graham Pearson3 and Gareth R.
Davies
1
FALW, VU Amsterdam, De Boelelaan 1085, 1081HV Amsterdam, The Netherlands;
* now at: Physics of Geological Processes, University of Oslo, PO Box 1048-Blindern,
0316 Oslo, Norway; nina.simon@geo.uio.no
2
DTM, Carnegie Inst. Washington, 5241 Broad Branch Road N.W., Washington, D.C.
20015, USA; carlson@dtm.ciw.edu
3
Dep. of Geological Sciences, Durham University, South Rd, Durham DH1 3LE, UK;
d.g.pearson@durham.ac.uk
Considerable progress has been made in the understanding of craton formation and
evolution in southern Africa in recent years, thanks to newly available geophysical and
geochemical data. Recent tomographic studies and Re-Os analyses of on- and off-craton
mantle xenoliths have clearly shown that the Kaapvaal craton is underlain by a, cold,
depleted lithosphere at least 200 km thick that was stabilized in the Archaean.
Several important questions regarding the consolidation and modification of the
lithosphere, however, remain unanswered or highly debated. Firstly, the sub-cratonic
lithospheric mantle (SCLM) is distinct from oceanic and most continental mantle in that
it is more depleted in magmaphile elements (Fe, Al, Ca, HREE), but at the same time
strongly enriched in incompatible trace elements like LILE and LREE. The Kaapvaal
SCLM also has a distinctly higher Si/Mg ratio and therefore higher modal orthopyroxene
content (at a given Mg/Fe) than oceanic and most continental upper mantle. Resolution of
this issue is critical to understanding the mechanism of lithosphere formation.
We carried out a detailed petrological and geochemical study on low-temperature
peridotite xenoliths from Kimberley (located at the craton core) and northern Lesotho (at
the south-eastern craton edge) in order to obtain information about the processes that led
to the depletion and re-enrichment of the Kaapvaal SCLM. Samples have been
characterized for Re-Os isotope systematics, major and trace element concentrations in
whole rocks and minerals, and garnet and clinopyroxene Lu-Hf, Sm-Nd and Rb-Sr
isotopes.
The combined results require a multistage history of the Kaapvaal mantle. They are
most consistent with a model that involves introduction of orthopyroxene and trace
element enrichment of the garnets by aqueous fluids, possibly derived from a subducting
oceanic slab. This is consistent with the observed enrichment in Si (orthopyroxene) and
the Re-Os systematics. Infiltration of hydrous fluids would also increase the degree of
mantle melting and could therefore explain the strong major and HREE depletion of the
SCLM. The Nd-Hf isotope characteristics of the garnets require the trace element
enrichment to be ancient. We therefore suggest that melting and metasomatism of the
Kaapvaal SCLM took place in subduction zone settings, probably during amalgamation
of smaller pre-existing terranes in the Late Archaean (~2.9 Ga).
Trace element and Nd and Hf isotope disequilibrium between garnet and
clinopyroxene is preserved in many samples and indicates that garnet and clinopyroxene
are not co-genetic. Calculated equilibrium liquids and Hf-Nd isotopic compositions for
clinopyroxene suggest that most diopside in the xenoliths studied crystallized from an
infiltrating kimberlite-like melt, either during group II kimberlite magmatism at ~200110 Ma ago (Kimberley) or shortly prior to eruption of the host kimberlite around 90 Ma
(N. Lesotho).
Simon, N. S. C., Carlson, R. W., Pearson, D. G. & Davies, G. R. (2005). Evidence for
formation and modification of the Kaapvaal craton (Southern Africa) in Archaean
subduction zones. Ofioliti, 30, 218.