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 ∼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 ∼890 Ma igneous event. Gabbros from the Liushudian intrusion have rather depleted zircon ɛ Hf( t ) (mean = 10.4) and normal mantle-like zircon δ 18 O (mean = 5.97‰). 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 ɛ Hf( t ) (mean = 5.7) and anomalously high zircon and apatite δ 18 O (mean = 13.76‰ and 13.80‰, respectively). Such a characteristic δ 18 O 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-δ 18 O supracrustal materials (e.g., limestone, chert) during magma evolution. An intra-plate extensional environment is suggested for the ∼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 ∼1000 and ∼870 Ma, and the ∼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. [ABSTRACT FROM AUTHOR]