Episodic crustal growth of South China as revealed by U–Pb–Lu–Hf–O isotopic studies of detrital zircons from Yangtze river
Abstract:Detrital zircons of eight sand samples from 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 were analyzed for U–Pb–Lu–Hf–O isotope compositions. Concordant magmatic zircons from the upper Yangtze river show age peaks of 213 Ma, 423 Ma, 700–1000 Ma, 1.8–1.9 Ga and 2.4–2.7 Ga with a striking peak at 38 Ma diagnostic of the Tibetan plateau, whereas those from the middle and lower Yangtze river and its tributaries exhibit age peaks of 100–300 Ma, 410–520 Ma, 720–910 Ma, 1.7–2.0 Ga and 2.4–2.6 Ga, and lack Cenozoic ages. Proportion of the Cenozoic zircons from the upper Yangtze river decreases remarkably down the stream from elevation 4540 m to 936 m. The middle and lower Yangtze river is below 50 m in elevation. We suggest that the single sand sample collected at the mouth of the Yangtze river is not representative of the entire drainage area, as generally assumed for studies of detrital zircons from modern rivers. This may be a common case for rivers with large variations in elevation.
Depleted mantle and arc mantle have been used to calculate crust formation model ages based on zircon Hf isotope composition. We derived theoretical equations for relationship and differences between the two model ages. Our data show excellent agreement with the theoretical derivations. They indicate linear relationship between the two model ages and their differences ranging from ~160 Ma to ~300 Ma for our data set, depending on zircon crystallization age, initial Hf isotopic ratio and 176Lu/177Hf ratio of the crustal source.
Our zircons with mantle-like O isotope composition (δ18O =4.7–6.0 ‰) reveal four periods of crust formation at 1.2 Ga, ~1.4 Ga, 1.9 Ga and 2.8–3.2 Ga, if depleted mantle is applied. If arc mantle is adopted, they correspond to 800–920 Ga, 1.2 Ga, 1.8 Ga and 2.6–3.0 Ga, which better match the known magmatic events in the drainage area. In addition, three 290–355 Ma zircons have coeval depleted or arc mantle Hf isotope compositions, indicating Paleozoic crust growth.