De Kraalen and Witrivier Greenstone Belts, Kaapvaal Craton, South Africa: Characterisation of the Palaeo-Mesoarchaean evolution by rutile and zircon U-Pb geochronology combined with Hf isotopes

The formation and evolution of Palaeoarchaean De Kraalen and Witrivier Greenstone Belts (DKGB and WGB) of the Kaapvaal Craton are poorly known. Mere we report zircon and rutile in situ U-Pb ages and zircon Hf isotopic data from a variety of supracrustal rocks. The zircon cores from a metamafic amphibole-bearing gneiss from the DKGB give a protolith age of 3441 +/- 5 Ma, whereas the zircon mantle domains give a metamorphic age of 3211 +/- 16 Ma. The Hf-176/Hf-177(t) values for all zircon domains give a tight cluster around 0.280596 +/- 0.00006 (2 SD). U-Pb analyses of zircon for an amphibolite intercalated with thin talc-silicate layers from the WGB give a single crystallisation age of 3 230 +/- 3 Ma, but the Hf isotope ratios of these zircon grains define two different populations. The first population yields Hf-176/Hf-17(7)similar to 3.23(Ga) = 0.28064 +/- 0.00004, corresponding to epsilon Hf-similar to 3.(23)Ga = 2.4 +/- 1.9 (2SD) and Hf model ages between ca. 3.51 to 3.30 Ga. These are Hf isotope characteristics for zircons from a relatively juvenile source extracted from a depleted mantle source ca. 0.28 to 0.07 Ga prior zircon crystallisation. The second population yields Hf-176/(HF similar to 3.23Ga)-H-177 = 0.28093 +/- 0.00004 with epsilon Hf-similar to(3.23)Ga = 8.1 +/- 1.3 (2SD). These Hf data combined with the Pb-206/ Pb-207 ages lead to isotope ratios that lie above those of Depleted Mantle. The unusually high Hf isotope signature for the cores of the zircons from the WGB amphibolite most likely represent a contribution from an early highly depleted mantle source. A rutile in situ U-Pb age of 3.085 Ga from a recrystallised quartzite indicate that the rocks from the DKGB experienced slow cooling following the 3.21 Ga metamorphic event or (partial) resetting due to elevated geothermal gradient caused by the ca. 3.1 Ga intrusions of the Vrede Granitiod Suite. The latter interpretation is preferred because similar to 145 Ma of slow cooling from the amphibolite facies conditions of peak metamorphism to the blocking temperature for mass diffusion of Pb in rutile is unlikely. While the Zr-in-rutile temperature of ca. 710 degrees C at 7 kbar for DKGB most likely records the peak temperature of the similar to 3.23 to 3.21 Ga event. The trace element concentrations of the metamorphic rutile grains within the quartzite of the DKGB indicate that the source rock was enriched in Cr. Either due to silification during hydrothermal alteration of the (ultra)mafic country rock or during deposition in an atmosphere that allowed for chromite grains to be part of the sediment.

Automated summary

The formation and evolution of Palaeoarchaean De Kraalen and Witrivier Greenstone Belts (DKGB and WGB) of the Kaapvaal Craton are poorly understood. This study reports U-Pb ages and Hf isotopic data of zircon and rutile from supracrustal rocks in these areas. The results indicate different protolith and metamorphic ages for the rocks in DKGB and WGB, as well as variations in Hf isotopic compositions.