Using detrital zircon to reconstruct Neoproterozoic crustal thickness variation in the northwestern margin of the...

Trace element geochemistry and chronology of zircon are reliable tools for reconstructing sediment provenance and crustal evolution, particularly in contexts where early crustal and rock records are sparse. We hereby employ in-situ U-Pb dating of Neoproterozoic detrital zircons to refine our understanding of sediment sources, tectonic settings, and crustal evolution along the northwestern margin of the Yangtze Block. Detrital zircons from the Doushantuo and Dengying formations exhibit similar Neoproterozoic age distributions (700-950 Ma). The lithology of the zircon source rocks correlates with the bimodal volcanic rocks extensively developed in the Micangshan-Hannan region along the Yangtze Block's northwestern margin. Using Eu/Eu* ratios derived from detrital zircons, we reconstructed crustal thickness variations in the northwestern Yangtze Block during the Neoproterozoic. The crustal thickening from 1000 to 850 Ma, thinning between 850 and 730 Ma, and thickening between 730 and 539 Ma. The U/Yb-Nb/Yb, Nb/Hf-Th/U, U/Yb-Hf, and U/Nd ratios of the Neoproterozoic detrital zircons mainly suggest island arc or orogenic features. The zircons younger than 850 Ma suggest progressively depleted mantle-type characteristics and extensional intra-plate. The fluctuated Th/U ratios exhibit a general trend of increase during 1000-730 Ma, which is followed by a decrease. The U/Yb ratios (mostly > 0.1) show a decrease from 1000 to 820 Ma and an insignificant change from 820 to 730 Ma followed by an increase. The estimated crystallization temperatures of the dated detrital zircons, calculated using the Ti-in-zircon geothermometer equation, reveal a general temperature increase during 1000-730 Ma, followed by a gradual decrease. The Ce/Nd ratios (oxygen fugacity) show a fluctuation but a general increase between 820 and 730 Ma possibly due to heat influx from the subduction slab rollback and sediment melting. All the trace element analyses of detrital zircons indicate that subduction along the northwestern margin of the Yangtze Block persisted until 730 Ma and the slab rollback around 850-730 Ma. It also supports the hypothesis that the South China Plate was situated at the periphery, rather than the interior, of the Rodinia supercontinent. After 730 Ma, the Yangtze Block experienced internal extension and rifting, forming rift basins. However, the northwestern margin of the Yangtze Block continued to collide with multiple microcontinents, resulting in crustal thickening. After ca.625 Ma, crust extension led to a decrease in crustal thickness, which is also consistent with the subduction environment indicated by the cumulative distribution function (CDF) plot of detrital zircon ages.