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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

V. van Schijndel, G. Stevens, C. Lana, T. Zack and D. Frei
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
South African Journal of Geology (February 2021) 124 (1): 17-36

Abstract

The formation and evolution of Paleoarchaean 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 + or - 5 Ma, whereas the zircon mantle domains give a metamorphic age of 3 211 + or - 16 Ma. The (super 176) Hf/ (super 177) Hft values for all zircon domains give a tight cluster around 0.280596 + or - 0.00006 (2 SD). U-Pb analyses of zircon for an amphibolite intercalated with thin calc-silicate layers from the WGB give a single crystallization age of 3,230 + or - 3 Ma, but the Hf isotope ratios of these zircon grains define two different populations. The first population yields (super 176) Hf/ (super 177) Hf approximately 3.23 Ga = 0.28064 + or - 0.00004, corresponding to epsilon Hf approximately 3.23 Ga = 2.4 + or - 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 crystallization. The second population yields (super 176) Hf/ (super 177) Hf approximately 3.23 Ga = 0.28093 + or - 0.00004 with epsilon Hf approximately 3.23 Ga = 8.1 + or - 1.3 (2SD). These Hf data combined with the (super 206) Pb/ (super 207) Pb 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 recrystallized 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 approximately 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 approximately 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.


ISSN: 1012-0750
EISSN: 1996-8590
Serial Title: South African Journal of Geology
Serial Volume: 124
Serial Issue: 1
Title: 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
Affiliation: Stellenbosch University, Department of Earth Sciences, Centre for Crustal Petrology, Potsdam, Germany
Pages: 17-36
Published: 20210205
Text Language: English
Publisher: Bureau for Scientific Publications, Pretoria, South Africa
References: 120
Accession Number: 2021-022994
Categories: GeochronologyIgneous and metamorphic petrology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. geol. sketch maps
S27°00'00" - S18°00'00", E18°00'00" - E30°00'00"
Secondary Affiliation: Universidade Federal de Ouro Preto, BRA, BrazilUniversity of Gothenburg, SWE, Sweden
Country of Publication: South Africa
Secondary Affiliation: GeoRef, Copyright 2021, American Geosciences Institute. Abstract, Copyright, Geological Society of South Africa. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 202116
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