U-Th-Pb and Lu-Hf systematics of zircon from TTG's, leucosomes, meta-anorthosites and quartzites of the Limpopo Belt (South Africa): Constraints for the formation, recycling and metamorphism of Palaeoarchaean crust

摘要

Results of more than 400 in situ U-Th-Pb and Lu-Hf zircon isotope analyses combined with cathodoluminescence images and field relationships reveal that the oldest magmatic rocks of the Limpopo Belt (Musina area) formed during the Palaeoarchaean between 3.40 and 3.27 Ga, experienced a first anatexis at 3.24-3.07 Ga (M1) and were overlain by sediments mainly between 3.24 and 3.0 Ga. Subsequently, the infra- and supracrustal rocks together underwent two high-grade metamorphic overprints, a first during the Neoarchaean at 2.65 Ga (M2), and a second at 2.025 Ga (M3). Hafnium isotope data additionally reveal that the Palaeoarchaean magmatic rocks result from substantial recycling of much older crust. This is well reflected by nearly chondritic εHf_(int) obtained from primary magmatic zircon domains of four Sand River TTG-gneiss samples (εHf_(3.27-3.28 Ga) = -1.3 to -1.8), from leucosomes within the Sand River Gneisses (εHf_(3.40 Ga) = -2.0; εHf_(3.28 Ga) = -1.5), and from two meta-anorthosite samples of the Messina layered intrusion (εHf_(3.35 Ga) = 0.1-1.4). Crustal recycling is also supported by hafnium isotope data of detrital zircon grains from two quartzite samples. These data reflect a general increase of the εHf_(int) between 3.65 and 3.2 Ga from ca. -4.0 to 0.0, indicating that an Eo- to Palaeoarchaean crust was mixed with juvenile mantel material in the hinterland of the Limpopo Belt. The present study also demonstrates that Hf isotope data provide a powerful tool, which makes it possible to distinguish zircon domains formed during distinct magmatic or metamorphic events, from such affected by a pseudomorphic alteration, even if the investigated zircon grain underwent multiple alteration processes.