Detrital zircon geochronology of quartzites from the Central Zone of the Limpopo Complex in southern Africa: Implications for different models and the timing of Zimbabwe Craton–Kaapvaal Craton amalgamation

Abstract

U–Pb geochronological analyses have been conducted on 765 detrital zircon grains from 11 quartzite samples in the Central Zone of the Limpopo Complex, southern Africa, for constraining the depositional age and provenance of the protolith quartz-sandstone and evaluating the effect of high-grade metamorphism on zircon age data in polymetamorphic terranes. The age data have also been used to evaluate two contrasting tectonic models for the evolution of the complex; collision of the Zimbabwe and Kaapvaal Cratons and diapir-driven intracratonic orogeny, both during the Neoarchean. Magmatic cores of the zircons from the Beit Bridge, Phikwe, and Motloutse Complexes in the Central Zone show a wide age range from 3828 Ma to 1995 Ma. Ages from the Beit Bridge Complex in Zimbabwe (ca. 3.6–3.1 Ga) and South Africa (ca. 3.5–2.4 Ga) are dominantly Paleoarchean to Neoarchean with minor Paleoproterozoic ages. Similar Eoarchean to Neoarchean ages were also obtained from the Phikwe Complex (ca. 3.8–2.6 Ga), whereas ages from the Motloutse Complex are slightly younger (early Mesoarchean to Paleoproterozoic; ca. 3.0–2.4 Ga). Although the age patterns from the three complexes are different, the distribution patterns of discordant and concordant data suggest Eoarchean to Mesoarchean (ca. 3.8–3.1 Ga) ages for magmatic cores of detrital zircon grains. Homogeneous zircon rims around magmatic cores gave metamorphic ages of ca. 2.61 Ga and ca. 2.27–2.02 Ga, probably formed by Neoarchean and Paleoproterozoic high-grade metamorphisms. Similar Neoarchean (ca. 2.7–2.6 Ga) and Paleoproterozoic concordant ages obtained from magmatic zircon cores are interpreted as reset ages due to Pb loss during the metamorphic events. The smaller peak of the Paleoproterozoic age population compared to that of the Neoarchean age data suggests the dominant high-grade metamorphism of the Central Zone took place at ca. 2.7–2.6 Ga, either by collision of the Zimbabwe and Kaapvaal Cratons or by diapir-driven intracratonic orogeny. Therefore, cratonization of the Azanian (Kalahari) Craton probably occurred before ca. 2.6 Ga. Based on the age patterns, the depositional age of the protolith quartz-rich sandstone is constrained as 3.1–2.7 Ga. The provenance of the Paleoarchean to Mesoarchean (3.6–3.1 Ga) detrital zircons has been inferred as orthogneisses and granitoids of the southern Zimbabwe Craton (e.g., Tokwe Segment). The oldest Eoarchean detrital zircon (3828 Ma) obtained from the Phikwe Complex suggests the presence of unknown older crust in this provenance.