Zn isotope heterogeneity in the continental lithosphere: New evidence from Archean granitoids of the northern Kaapvaal craton, South Africa

Abstract

The Zn isotope data (expressed as δ66Zn) of 25 Archean crustal rocks (3.4–2.7 Ga) from the Pietersburg block in the northern part of the Kaapvaal craton (South Africa) exhibit a range from + 0.26 ± 0.04‰ to + 0.46 ± 0.04‰. This indicates the existence of resolvable Zn isotope heterogeneity in the continental lithosphere. Because the samples are representative of the processes of continental crust formation and evolution in the Archean, we propose that such Zn isotope heterogeneity is linked to early continental lithosphere formation and stabilisation. Among the crustal rock samples, two samples from the 2.97 Ga-old Rooiwater layered intrusion show indistinguishable δ66Zn of ca. + 0.28‰, which suggests that the Archean mafic mantle-derived rocks had the same isotopic composition as modern basalts (ca. + 0.22 to + 0.36‰) and the Earth's mantle (ca. + 0.30‰). Tonalite-trondhjemite-granodiorite (TTG) samples with ages of 3.43, 2.95 and 2.78 Ga, representing the first felsic continental crust formed in the studied area, have similar δ66Zn composition as the Earth's mantle (ca. + 0.30‰), irrespective of their ages, tectonic setting and petrogenesis. This indicates that formation of early, juvenile felsic crust either by melting or crystallization of basalts did not significantly fractionate Zn isotopes, or at least was associated with an equilibrium Zn fractionation process. The biotite-granites (2.85–2.75 Ga) have homogenous Zn isotope compositions with an average of δ66Zn = + 0.44 ± 0.04‰. The biotite-granites in Archean terranes are interpreted as partial melts from pre-existing TTGs. This suggests that reworking of the early felsic crust through partial melting does fractionate Zn isotopes up to + 0.15‰. This presumably results from disequilibrium kinetic fractionation during partial melting process, rather than an influence from the source component-related signature. Enriched mantle-derived sanukitoids have δ66Zn identical to that of the mantle (ca. + 0.30‰), indicating that Zn isotope fractionation would be relatively insensitive to mantle metasomatism and indicate the ultimate mantle origin of mafic sanukitoids rocks. Zinc isotopic compositions are useful to clarify the complex petrogenesis of intermediate, felsic sanukitoids and high-K cac-alkaline granites, especially to discriminate differentiation processes from the influence of interactions with the local felsic crust and its melting products at the time of magma emplacement.