Archean magmatism and deformation were not products of plate tectonics

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 terranescBarberton South (BS), Barberton North (BN), Murchison^Northern Kaapvaal (MNK), Limpopo Central Zone (LCZ), and Francistowncwhich 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 eHft (BS ¼ ^1� 7t oþ 0� 5; MNK ¼ ^3� 4t oþ 0� 7; Francistown ¼ ^0� 5t oþ 1� 1; LCZ ¼ ^12� 4t o ^1 � 8), indicating that crustal recyclingcperhaps by mixing of an older crust with a depleted mantle reservoircplayed an important role during their formation and growth. Higher, superchondritic eHft values, as indicative for an important depleted mantle influence, were obtained only from some granitoids of the BN terrane (eHf3� 23Ga ¼þ 2� 5 � 0� 8), the Gaborone Granite Suite (eHf2� 80Ga ¼þ 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 e-units, and only in rare cases up to � 3� 1 e-units. The rare significant eHft 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.

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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

A combined set of U^Pb and Lu^Hf in situ laser ablation ICP-(MC)-MS zircon analyses were obtained from orthogneisses and granitoids in the Central Zone of the Limpopo Belt, which comprises the Beit Bridge and Mahalapye complexes. The results indicate that by combining the two isotope systems primary magmatic zircon domains can be distinguished from those formed during later metamorphic events, even if the distinct zircon domains underwent multiple Pb loss and the texture^age relationships, as obtained by cathodoluminescence images and U^Pb analyses, are ambiguous. Furthermore, the applied technique allows distinction of zircon grains formed in juvenile magmas from those generated by melting of older continental crust or affected by substantial crustal contamination. The combined U^Pb and Lu^Hf data reveal that the Sand River gneiss suite of the Beit Bridge Complex was emplaced at 3283 8Ma and formed from melting of an older Archaean crust, which was derived from a depleted mantle source at around 3 65 Ga.The hafnium model age (TDM) is significantly older than those obtained from zircons from numerous Neoarchaean granitoids of the Beit Bridge Complex, comprising the Singelele gneiss (2647 12Ma), the Bulai granite (2612 7Ma), the Regina gneiss (2649 9Ma) and two samples of the Zanzibar gneiss (2613 6Ma). These granitoids show initial eHf(t) values between þ 0 5 and 7 1, which correspond to initialTDM between 3 46 and 3 01Ga. These variable TDMinitial and eHf(t)initial values are interpreted to be the result of different mixtures of reworked 3 65 Ga Palaeoarchaean crust with juvenile magmas extracted from the depleted mantle during the Neoarchaean at 2 65 Ga.This conclusion is supported by results obtained from the Mahalapye Complex, which was affected by migmatization and granite intrusions during the Palaeoproterozoic at 2 02^2 06 Ga. The Mokgware granite (2019 9Ma) contains zircon xenocrysts with Pb^Pb ages of 2 52^2 65 Ga and 2 93 Ga and hafnium model ages of 3 0^3 4 Ga, indicating that this granite is derived from remelting of Archaean crust. In contrast, uniformTDM Hf initial ages of 2 61^2 67 Ga obtained from a diorite gneiss (2061 6Ma) of the Mahalapye Complex indicate that its protolith may have been formed from remelting of a Neoarchaean juvenile crust. Variable eHf(t)initial values from 3 7 to þ6 3 of zircon cores (2711 11Ma) in an adjacent leucosome also support a model of mixing of juvenile mantle derived matter with older crust in the Neoarchaean.

Archaean to Proterozoic Crustal Evolution in the Central Zone of the Limpopo Belt (South Africa^Botswana): Constraints from Combined U^Pb and Lu^Hf Isotope Analyses of Zircon

The evolving nature of terrestrial crust from the Hadean, through the Archaean, into the Proterozoic

Single zircon ages for granitoid gneisses in the Central Zone of the Limpopo Belt, Southern Africa and geodynamic significance

The Umlali Thrust Zone has been mapped between the granite–greenstone rocks of the Zimbabwe craton and the granulite-facies rocks of the Limpopo Belt. A tectonic break had not been previously recognized in the southwest because of the intrusion of syn- to post-kinematic porphyritic granites. These intrusions locally obliterate evidence for the tectonic break, although in some localities the associated mylonites are preserved as xenoliths within the intrusive porphyritic granite bodies. The porphyritic granite bodies were emplaced during and after the main regional fabric forming event. A U–Pb zircon age of 2627 ± 7 Ma obtained from a late-kinematic microgranite is interpreted to be the minimum time of thrusting. A whole-rock Rb–Sr age of 2583 ± 52 Ma obtained from plutonic rocks and retrogressed zones within the North Marginal Zone falls within error of the zircon age, suggesting widespread magmatic and hydrothermal activity coeval with the thrusting.

S. Mkweli

Papers

Westward continuation of the craton-Limpopo Belt tectonic break in Zimbabwe and new age constraints on the timing of the thrusting