Archean Accretion and Crustal Evolution of the Kalahari Craton—the Zircon Age and Hf Isotope Record of Granitic Rocks from Barberton/Swaziland to the Francistown Arc
Аннотация:U–Pb and Lu–Hf isotope analyses, obtained by laser ablation-sector field-inductively coupled plasma-mass spectrometry on zircon grains from 37 granitoid samples indicate that the Kalahari Craton consists of at least five distinct terranes—Barberton South (BS), Barberton North (BN), Murchison–Northern Kaapvaal (MNK), Limpopo Central Zone (LCZ), and Francistown—which underwent different crustal evolutions, and were successively accreted at c. 3·23 Ga, 2·9 Ga and 2·65–2·7 Ga. The investigated granitoids were emplaced over a period of c. 1.5 billion years, and are exposed along a c. 1000 km long traverse from the Barberton Mountain Land/Swaziland to the Francistown arc complex, Botswana. The presented datasets reveal that most granitoids of the BS (3·45–3·10 Ga), MNK (2·93–2·67 Ga), Francistown (2·70–2·65 Ga) and LCZ terranes (3·2–2·03 Ga) show near-chondritic to subchondritic ϵHft (BS = –1·7 to + 0·5; MNK = –3·4 to + 0·7; Francistown = –0·5 to + 1·1; LCZ = –12·4 to –1·8), indicating that crustal recycling—perhaps by mixing of an older crust with a depleted mantle reservoir—played an important role during their formation and growth. Higher, superchondritic ϵHft values, as indicative for an important depleted mantle influence, were obtained only from some granitoids of the BN terrane (ϵHf3·23Ga = +2·5 ± 0·8), the Gaborone Granite Suite (ϵHf2·80Ga = +2·0 ± 1·6), and from a few detrital zircons from the Mahalapye complex of the Limpopo Belt. In addition, the datasets show that the internal Hf isotope variation of magmatic zircon domains from most granitoids is commonly less than ±1·5 ϵ-units, and only in rare cases up to ±3·1 ϵ-units. The rare significant ϵHft variations may be ascribed to incomplete mixing of different sources during magma crystallization. It is also shown that the combined approach of cathodoluminescence imaging with U–Pb and Lu–Hf isotope analysis provides a powerful tool to distinguish zircon domains formed and/or altered at different times.
