Ca. 890 Ma magmatism in the northwest Yangtze block, South China: SIMS U-Pb dating, in-situ Hf-O isotopes, and tectonic implications

Early Neoproterozoic tectonics of the Yangtze block remains poorly understood because very limited igneous records are available from the time interval of similar to 1000-870 Ma. In this paper, our new SIMS U Pb dating results demonstrate that the Liushudian mafic intrusion and Pinghe alkaline complex in the northwest Yangtze block were emplaced at 888 +/- 6 Ma and 891 +/- 7 Ma, respectively, representing the products of a similar to 890 Ma igneous event. Gabbros from the Liushudian intrusion have rather depleted zircon epsilon Hf(t) (mean = 10.4) and normal mantle-like zircon delta O-18 (mean = 5.97 parts per thousand). Their parental magma was thus probably derived from asthenospheric mantle. Geochemically, these mafic rocks have an affinity to continental flood tholeiitic basalts rather than ocean island basalts, as previously thought. In contrast, an ijolite sample from the Pinghe complex has less depleted zircon epsilon Hf(t) (mean = 5.7) and anomalously high zircon and apatite delta O-18 (mean = 13.76 parts per thousand and 13.80 parts per thousand, respectively). Such a characteristic delta O-18 signal, among the highest yet known for igneous zircons, could be either inherited from a magma source in metasomatized lithospheric mantle or acquired by assimilation of high-delta O-18 supracrustal materials (e.g., limestone, chert) during magma evolution. An intra-plate extensional environment is suggested for the similar to 890 Ma igneous event in the northwest Yangtze block, although it is as yet unclear whether this igneous event is related to a mantle plume or not. It could be concluded that magmatism on the western periphery of the Yangtze block was not shut down between similar to 1000 and similar to 870 Ma, and the similar to 890 Ma intra-plate igneous event may mark either the onset of Neoproterozoic continental rifting or the ending of Late Mesoproterozoic to Early Neoproterozoic lithospheric extension.