Zircon formation versus zircon alteration — New insights from combined U-Pb and Lu-Hf in-situ LA-ICP-Ms analyses, and consequences for the interpretation of Archean zircon from the Central Zone of the Limpopo Belt

摘要

In this study we demonstrate that the combination of U-Pb and Lu-Hf isotope data obtained by laser ablation-inductive coupled plasma-mass spectrometry provides a powerful tool for distinguishing zircon domains grown during several metamorphic or magmatic events from altered domains affected by complete single or multiple Pb-loss. This discrimination is possible because the U-Pb and Lu-Hf isotope systems are decoupled during zircon alteration. Initial ~(176)Hf/~(177)Hf once "incorporated" into the zircon lattice remains nearly unaffected during later alteration processes which cause Pb-loss. Zircon overgrowths always have higher initial ~(176)Hf/~(177)Hf than previously grown domains, due to the incorporation of additional radiogenic ~(176)Hf formed by ~(176)Lu decay in the rock's matrix between successive zircon growth events. Incorporation of such ~(176)Hf into zircon domains affected by post-growth alteration is negligible. By applying the U-Pb and Lu-Hf systems, in combination with cathodoluminescence imaging, we show that zircon in the sand River orthogneiss of the Limpopo Belt grew during magma crystallization at 3.28 Ga and anatectic events at 2.64 Ga and 2,02 Ga. Our data demonstrate that abundant zircon domains underwent multiple Pb-loss, and that many of the 2.02 Ga domains result from complete Pb-loss from zircon overgrowth domains already formed at 2.64 Ga. In contrast, all domains of complex zoned zircon grains from the Zanzibargranodiorite gneiss were formed during a single magmatic event at 2.61 Ga, as indicated by their uniform Hf isotope composition, but most subsequently suffered multiple Pb-loss.