Early crustal evolution in the western Yangtze Block: Evidence from U–Pb and Lu–Hf isotopes on detrital zircons from sedimentary rocks
Abstract In situ U–Pb and Lu–Hf data on detrital zircons from Paleo- to Neoproterozoic sediments have been used to gain a clearer picture of the regional tectonic background and crustal evolution in the western part of the Yangtze Block. The youngest concordant zircon ages for sedimentary units suggest maximum depositional ages of 1014 Ma for the Kunyang Group, 750–649 Ma for the Sinian sequences, and 525 Ma for the Cambrian sediments. The Yinmin Formation, previously assigned to the middle part of the Kunyang Group, was actually deposited after 1667 Ma and contains zircons whose ages are dominantly from late Archean to Paleoproterozoic (2.8–2.7 Ga, 2.5–2.3 Ga and ∼1.85 Ga). The Heishantou Formation in the lower part of the Kunyang Group has two major age populations of ∼1.0 Ga and 1.8–1.6 Ga. The Sinian and Cambrian sedimentary rocks are dominated by Neoproterozoic zircons with age peaks at ∼760 Ma and ∼825 Ma, consistent with the ages of widespread igneous rocks around the Yangtze Block. The Lu–Hf isotope data suggest that a significant juvenile input took place during Archean and Neoproterozoic times, respectively, while crustal reworking was dominant during the Paleoproterozoic time. The Archean–Paleoproterozoic detritus is isotopically distinct from the Archean–Paleoproterozoic basement exposed in the northern part of the Yangtze Block, suggesting that a subarea of old crust lies beneath the young sediments covering the craton. The presence of abundant ∼1.85 Ga zircons suggests that the Yangtze Block was probably part of the Columbia supercontinent during Paleoproterozoic time. Comparisons to the other parts of Columbia suggest that the Yangtze Block could have been adjacent to the North China Craton and/or Australia.