Detrital zircon evidence for Hf isotopic evolution of granitoid crust and continental growth

We have determined U–Pb ages, trace element abundances and Hf isotopic compositions of approximately 1000 detrital zircon grains from the Mississippi, Congo, Yangtze and Amazon Rivers. The U–Pb isotopic data reveal the lack of >3.3 Ga zircons in the river sands, and distinct peaks at 2.7–2.5, 2.2–1.9, 1.7–1.6, 1.2–1.0, 0.9–0.4, and The striking attribute of our new data set is the non-uniformitarian secular change in Hf isotopes of granitoid crusts; Hf isotopic compositions of granitoid crusts deviate from the mantle evolution line from about 3.3 to 2.0 Ga, the deviation declines between 2.0 and 1.3 Ga and again increases afterwards. Consideration of mantle-crust mixing models for granitoid genesis suggests that the noted isotopic trends are best explained if the rate of crust generation globally increased in two stages at around (or before) 3.3 and 1.3 Ga, whereas crustal differentiation was important in the evolution of the continental crust at 2.3–2.2 Ga and after 0.6 Ga. Reconciling the isotopic secular change in granitoid crust with that in sedimentary rocks suggests that sedimentary recycling has essentially taken place in continental settings rather than active margin settings and that the sedimentary mass significantly grew through addition of first-cycle sediments from young igneous basements, until after ∼1.3 Ga when sedimentary recycling became the dominant feature of sedimentary evolution. These findings, coupled with the lack of zircons older than 3.3 Ga in river sands, imply the emergence of large-scale continents at about 3.3 Ga with further rapid growth at around 1.3 Ga. This resulted in the major growth of the sedimentary mass between 3.3 and 1.3 Ga and the predominance of its cannibalistic recycling later.