Contrasting zircon Hf and O isotopes in the two episodes of Neoproterozoic granitoids in South China: Implications for growth and reworking of continental crust
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The genetic links among rift magmatism, crustal growth and water–rock interaction are an important issue about mass and heat transfer betweenmantle and crust during supercontinent breakup.Acombined study ofHf andOisotopes in zircons fromNeoproterozoic 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 ~825 Ma granitoids exhibit negative eHf (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 d18O values of 8.7 to 10.4‰. These indicate that the source material of granitoidmagmaswas derived frommelting of Paleoproterozoic basement that has theHf isotope signature similar to the enrichedmantle but experienced chemical weathering process before anatexis. Reworking of ancient crust is demonstrated to occur at ~825 Ma in the orogenic collapse zone, with overprinting of subsolidus hydrothermal alteration during magma emplacement. In contrast, the 760– 750Ma bimodal intrusives are characterized by positive eHf(t) values of 3.5±0.8 to 9.9±0.8 with youngmodel Hf ages of 0.94±0.06 to 1.18±0.06 Ga, and both low and high d18O 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~750Ma in the rifted tectonic zone,with occurrence of supersolidus hydrothermal alteration and local low-18Omagmatism during supercontinent breakup. Contributions of the depletedmantle to their magma sources are contrasting in the two episodes of magmatismin association with breakup of the supercontinent Rodinia.While the change in melt source fromthe 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.
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