Abstract
The provenance of the Neoarchaean Ventersdorp Supergroup and several age-related
supracrustal successions was analysed to gain insight into the geotectonic evolution of
the Kaapvaal Craton during the transition from the Archaean to Proterozoic Eras. The
studied successions include, besides the siliciclastic formations of the Ventersdorp
Supergroup, the upper Wolkberg and Buffelsfontein Groups, the Godwan Formation
and the Schmidtsdrift Subgroup of the basal Transvaal Supergroup in Griqualand
West. Petrographic, whole rock geochemical and Sm-Nd isotopic analyses were
combined with SHRIMP U-Pb age dating of detrital zircons. Furthermore, Rb-Sr
isotopic studies were carried out on carefully selected suites of samples from surface
exposure or, wherever possible, on deep diamond drill core.
The Ventersdorp Supergroup is an up to 5 km thick undeformed, only slightly
metamorphosed volcano-sedimentary succession deposited on the Kaapvaal Craton
between 2714 Ma and 2665 Ma. A lack of major time hiati to the underlying
Mesoarchaean Witwatersrand Supergroup and covering Neoarchaean to
Palaeoproterozoic Transvaal Supergroup render the Ventersdorp Supergroup very
well suited for the investigation of the geotectonic evolution of the Kaapvaal Craton
near the Archaean-Proterozoic boundary. This is supported by its excellent
preservation, which also allowed detailed studies of sedimentological structures, such
as seismites indicating Neoarchaean earthquakes.
The provenance analyses carried out on the clastic formations of the Ventersdorp
Supergroup point to a gradual change in tectonic evolution from typically Archaean to
post-Archaean processes rather than a drastic, unique transition in the case of the
Kaapvaal Craton. Texturally immature wackes of the Kameeldoorns Formation,
representing the oldest clastic units of the Ventersdorp Supergroup, are derived
mainly from Mesoarchaean source rocks, whereas the stratigraphically younger
Bothaville Formation displays geochemical signatures comparable with Archaean
trondhjemite-tonalite granodiorite-suites (TTGs), thus suggesting crustal addition in
the so-called ‘Archaean-style’.
The extension of provenance analyses to supracrustal successions that are tentatively
correlated with the Bothaville Formation, revealed contributions from granitoid
V
sources that formed under post-Archaean and Archaean conditions. Furthermore, the
geochemical data for all analysed formations support a passive margin setting. Arc
settings, as indicated in some samples, are due to the input of less fractionated
volcanic material that provides evidence of distal volcanism. Analyses of Nd-isotopic
systematics and U-Pb ages of detrital zircons revealed a Mesoarchaean age for the
source rocks of the formations. U-Pb age dating of detrital zircons of the Godwan
Formation suggests that this formation is of Mesoarchaean age, and therefore not a
correlative of the other Neoarchaean successions.
Hence, the results suggest that the continental crust of the Kaapvaal Craton was thick
enough since the Mesoarchaean (2.8 - 3.1 Ga) to allow long-term crustal recycling,
and therefore modern plate tectonic processes could have operated earlier than on
other well-studied cratonic blocks. During the Neoarchaean, however, crustal
thickening of the Kaapvaal Craton took place by accretion of Archaean-style TTGs
along the margins of the craton. Thus, Archaean and post-Archaean tectono-magmatic
processes co-existed. Furthermore, the Neoarchaean supracrustal successions
represent the first sedimentation events on an entirely stabilised and tectonically
quiescent Kaapvaal Craton. Input from distal volcanic sources marks the last sign of
volcanic activity prior to the craton-wide deposition of carbonate rocks of the
Transvaal Supergroup. Geochronological data also imply a connection of the
Neoarchaean Kaapvaal Craton to further cratonic blocks that may hold source rocks
for the studied formations, as for some small age populations of older detrital zircons
(ca. 3.1 - 3.4 Ga), no suitable source area could be identified on the Kaapvaal Craton
itself.
However, it seems unlikely that the Zimbabwe Craton was one of these cratonic
blocks, because the Rb-Sr whole rock ages of all studied formations yield a model age
of 2092 ± 55 Ma, which is thought to correspond to a craton-wide influence of the
2.05 Ga old Bushveld Igneous Complex on the Rb-Sr isotope systematics of all
analysed clastic successions. This influence is apparently missing in the Southern and
Central Marginal Zones of the Limpopo Belt, suggesting that the collision between
the Kaapvaal and Zimbabwe Cratons only took place after the emplacement of the
Bushveld Igneous Complex, i.e. after 2.05 Ga.
Dr. U. Zimmermann
Prof. J. Gutzmer