Step-like growth of the continental crust in South China: evidence from detrital zircons in Yangtze River sediments

Abstract

Detrital zircons from nine river sand samples from the upper, middle and lower streams of the Yangtze River (the world's third largest river) and its two largest tributaries, the Han and Jialing rivers have been analyzed for U–Pb–Lu–Hf–O isotope compositions. Zircons from the upper Yangtze River cluster in age groups at 0–100, 200–300, 400–500, 700–1000, 1800–1900 and 2300–2500 Ma, with a marked peak at 41 Ma diagnostic of magmatism on the Tibetan Plateau. Zircons from the middle and lower Yangtze River and its tributaries exhibit broadly similar age groups at 100–300, 400–500, 700–900, 1800–2000 and 2300–2700 Ma, except for the lack of Cenozoic ages.The Yangtze River catchment is covered by thick Phanerozoic sedimentary rocks and so Archean-aged zircons are likely to be under-represented in modern river sands. We therefore applied the approach of Dhuime et al. (2012) to calculate a crustal growth curve for South China, and the resultant curve has two inflections. The older inflection at ~2.8 Ga is slightly younger than that of Dhuime et al. (2012), suggesting the onset of plate tectonics in the Yangtze Craton. The younger inflection at ~1.8 Ga appears to mark the onset of another period of relatively high crustal growth rate in South China. This period of increased crustal growth rate in the Mesoproterozoic is coeval with the breakup of the supercontinent Columbia, suggesting that the Mesoproterozoic crustal growth in South China may be related to supercontinent dispersal.Comparison with global crustal growth curves based on detrital zircons highlights the distinct step-like pattern of crustal growth in South China. This emphasizes the potential of comparing regional with global crustal growth curves, and the changes in the rates of crustal growth in South China appear to have been controlled by changes in regional geodynamics. The maximum δ18O values in the zircons analyzed increase markedly at the end of the Archaean, and these increases therefore accompany the estimated increases in atmospheric oxygen at that time. It is therefore suggested the atmospheric O2 levels were associated with increased crustal weathering, the development of more elevated δ18O sediments and their increasing incorporation in the generation of crustally derived magmas from ca. 2.5 Ga.