Episodic Precambrian crust growth: Evidence from U–Pb ages and Hf–O isotopes of zircon in the Nanhua Basin, central South China

Abstract

We report here the first integrated U–Pb dating and hafnium–oxygen isotope analyses of detrital zircon grains from sedimentary and volcaniclastic rocks in the Neoproterozoic Nanhua Basin, central South China. These data provide vital information on the depositional cycles of the Cryogenian strata and their bearings on the breakup of the Rodinia supercontinent, and the characteristics of Precambrian crustal growth in the South China Block and neighboring continents. Zircon SIMS U–Pb ages indicate that the Qingshuijiang Formation was deposited at 772±5Ma and the Fanzhao Formation was deposited between 800 and 770Ma. The onset time of the Sturtian (Jiangkou) glaciation is constrained to be no older than 730Ma. Together with compilations of all available high precision zircon U–Pb data on tuff beds in the Nanhua Basin, our work demonstrates that the Cryogenian deposition in South China began at about 850Ma and had three depositional cycles at ca. 850–820Ma, 820–720Ma and 720–635Ma. Hf isotope analyses and U–Pb dating, combined with existing O isotope analyses, of detrital zircon have revealed three striking features for grains with 1.0–0.7Ga ages: (1) about 22% of total analysed grains have Hf isotope compositions that plot between the new continental crust and depleted mantle growth curves; (2) about 80% of zircon grains have δ18O values below 6.5‰; and (3) 950–700Ma is the most dominant age group in the total of 1616 concordant detrital zircon ages from the Cryogenian sedimentary rocks. The Early Neoproterozoic (1.0–0.7Ga) thus appears to have witnessed significant juvenile crustal growth in South China. In addition, zircon analyses of the Cryogenian sedimentary rocks revealed 3.2–2.9Ga and possibly 2.7–2.4Ga as two additional episodes of juvenile crust growth in South China and neighboring continents.