Granitoids and Greenstone Belts of the Pietersburg Block—Witnesses of an Archaean Accretionary Orogen Along the Northern Edge of the Kaapvaal Craton

2019 ◽  
pp. 83-107 ◽  
Author(s):  
Oscar Laurent ◽  
Armin Zeh ◽  
Günther Brandl ◽  
Adrien Vezinet ◽  
Allan Wilson
Keyword(s):  
Kaapvaal Craton ◽  
Northern Edge ◽  
Greenstone Belts

scholarly journals Seismic reflections from a lithospheric suture zone below the Archaean Yilgarn Craton

2021 ◽  
Vol 12 (1) ◽  
Author(s):  
Andrew J. Calvert ◽  
Michael P. Doublier ◽  
Samantha E. Sellars
Keyword(s):  
Seismic Reflection ◽  
Suture Zone ◽  
Plate Tectonic ◽  
Yilgarn Craton ◽  
Uppermost Mantle ◽  
Northern Edge ◽  
Greenstone Belts ◽  
Seismic Reflection Profiles ◽  
Seismic Reflections ◽  
Seismic Reflectors

AbstractSeismic reflectors in the uppermost mantle, which can indicate past plate tectonic subduction, are exceedingly rare below Archaean cratons, and restricted to the Neoarchaean. Here we present reprocessed seismic reflection profiles from the northwest Archaean Yilgarn Craton and the Palaeoproterozoic Capricorn Orogen of western Australia that reveal the existence of a ~4 km thick south-dipping band of seismic reflectors that extends from the base of the Archaean crust to at least 60 km depth. We interpret these reflectors, which lie south of a ~50 km deep crustal root, as a relict suture zone within the lithosphere. We suggest that the mantle reflectors were created either by subduction of an oceanic plate along the northern edge of the Yilgarn Craton, which started in the Mesoarchaean and produced the rocks in northern Yilgarn greenstone belts that formed in a supra-subduction zone setting, or, alternatively, by underthrusting of continental crust deep into the lithosphere during the Palaeoproterozoic.

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

2021 ◽  
Author(s):  
V. van Schijndel ◽  
G. Stevens ◽  
C. Lana ◽  
T. Zack ◽  
D. Frei
Keyword(s):  
Mantle Source ◽  
Country Rock ◽  
Isotope Ratios ◽  
Hf Isotope ◽  
Kaapvaal Craton ◽  
Blocking Temperature ◽  
Slow Cooling ◽  
Depleted Mantle ◽  
Greenstone Belts

Abstract The formation and evolution of Palaeoarchaean De Kraalen and Witrivier Greenstone Belts (DKGB and WGB) of the Kaapvaal Craton are poorly known. Here 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 3 441 ± 5 Ma, whereas the zircon mantle domains give a metamorphic age of 3 211 ± 16 Ma. The 176Hf/177Hft 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 calc-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 176Hf/177Hf~3.23 Ga = 0.28064 ± 0.00004, corresponding to εHf~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 176Hf/177Hf~3.23 Ga = 0.28093 ± 0.00004 with εHf~3.23 Ga = 8.1 ± 1.3 (2SD). These Hf data combined with the 206Pb/207Pb 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 ~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°C at 7 kbar for DKGB most likely records the peak temperature of the ~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.

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