Contrasting zircon Hf and O isotopes in the two episodes of Neoproterozoic granitoids in South China: Implications for growth and reworking of continental crust

The genetic links among rift magmatism, crustal growth and water-rock interaction are an important issue about mass and heat transfer between mantle and crust during supercontinent breakup. A combined study of Hf and O isotopes in zircons from Neoproterozoic granitoids in South China provides evidence for growth and reworking of juvenile and ancient crusts with different styles of water-rock interactions along rift tectonic zones. Two generations of the granitoids show contrasting features in both zircon Hf and O isotope compositions, indicating their distinct petrogenesis. The similar to 825 Ma granitoids exhibit negative epsilon(Hf)(t) values of -3.4 +/- 0.8 to -1.6 +/- 0.8 with old model Hf ages of 1.81 +/- 0.07 to 1.92 +/- 0.10 Ga, and high delta O-18 values of 8.7 to 10.4%. These indicate that the source material of granitoid magmas was derived from melting of Paleoproterozoic basement that has the Hf isotope signature similar to the enriched mantle but experienced chemical weathering process before anatexis. Reworking of ancient crust is demonstrated to occur at similar to 825 Main the orogenic collapse zone, with overprinting of subsolidus hydrothermal alteration during magma emplacement. In contrast, the 760750 Ma bimodal intrusives are characterized by positive epsilon(Hf)(t) values of 3.5 +/- 0.8 to 9.9 +/- 0.8 with young model Hf ages of 0.94 +/- 0.06 to 1.18 +/- 0.06 Ga, and both low and high delta O-18 values of 4.2 to 6.2% relative to 5.3 +/- 0.3% for the normal mantle zircon. Prompt reworking of juvenile crust is suggested to occur at similar to 750 Main the rifted tectonic zone, with occurrence of supersolidus hydrothermal alteration and local low-O-18 magmatism during supercontinent breakup. Contributions of the depleted mantle to their magma sources are contrasting in the two episodes of magmatism in association with breakup of the supercontinent Rodinia. While the change in melt source from the crust to the mantle keeps pace with the advance from continental rifting to supercontinent breakup, significant transport of both heat and material from the depleted mantle to the continental crust only occurred along the active rifting zone. In either case, the growth and reworking of continental crust are episodically associated with rift magmatism. (C) 2006 Elsevier B.V All rights reserved.