Unraveling the Precambrian crustal evolution by Neoproterozoic conglomerates, jiangnan orogen: U-Pb and Hf isotopes of detrital zircons

Combined LA-ICP-MS U-Pb dating and LA-MC-ICP-MS Lu-Hf isotopic analyses were carried out on detrital zircons from the conglomerates of three representative Neoproterozoic metasedimentary cover sequences, South China Block (SCB): the Luojiamen, Baizhu and Xiuning Formations. U-Pb dating results suggest that the maximum depositional ages for the Luojiamen, Baizhu and Xiuning Formations are 863±10Ma, 826±7Ma and 763±10Ma, respectively. One zircon grain from the Baizhu Formation gives a concordant U-Pb age at 3918±32Ma, with Hf model age of ca. 4290Ma, indicating Hadean crust growth in its source. The integrated U-Pb ages and Hf-isotopes in zircons from each formation show different crustal evolution in their source areas. The Luojiamen detrital zircons in the eastern JO are all of Early Neoproterozoic (ca. 965-855Ma) with positive ε_Hf(t) (+6 to +15), suggesting a provenance featured by juvenile arc-related igneous rocks with a duration of ca. 965-855Ma. Their ε_Hf(t) values intersect the depleted mantle curve at ca. 920-880Ma and decrease steadily with ages, probably indicating a transition from oceanic arc to continental arc magmatism in their source. In contrast, recycling of old continental crusts is significant in the sources of the Baizhu and Xiuning Formations. Hf isotope results of detrital zircons from the Baizhu Formation suggest two major crust reworking periods: one at 1600-800Ma by 1700-1600Ma juvenile crusts, and the other at 2500-1700Ma by 3300-2500Ma juvenile crusts. The ca. 2000Ma age peak of the Xiuning Formation is more significant than those of the underlying folded metasedimentary sequences in the eastern JO, suggesting that the Xiuning Formation received more detritus from the Yangtze Block and thus the back-arc basins or peripheral basins along the JO may have been closed at ca. 765Ma. Based on comparisons of detrital zircon U-Pb age spectrums and Hf isotopic characters with worldwide continents, we propose that the Early Neoproterozoic sediments at the southeastern margin of the Yangtze Block have close affinities with those from the India and East Antarctica, which matches a northwestern margin position for the SCB in the configuration of supercontinent Rodinia. Our results suggest that conglomerates can provide important and unrevealed information for crust growth and evolution especially when their sources were eroded or poor in the field.