Retreating subduction-related intracratonic rifting in the Ediacaran Sichuan Basin (SW China)

Abstract

Ediacaran continental rift basins in the Sichuan Basin (SW China) overlap in time with the breakup of Rodinia and the assembly of Gondwana, but whether the rifts occurred in response to a mantle plume or to a subduction-related mechanism at the outer margin of the supercontinent remains uncertain. In the absence of Ediacaran igneous rocks, we provide zircon LA-ICP-MS U-Pb age data and whole-rock geochemistry from exposed Ediacaran-Lower Cambrian siliciclastic rocks in the Sichuan Basin in an effort to decipher the origin of the Ediacaran rifting. Together with new seismic and drilling data, the Ediacaran period is characterized by a magma-poor rift succession deposited in a faulted depression within the basin. U-Pb ages of the Ediacaran detrital zircons are concentrated in the range of 850–630 Ma, which suggests a weak magmatic event during the Ediacaran rifting. Geochemically, the siliciclastic rocks are characterized by relatively flat rare earth elements (REE) with negative Eu* anomalies, enrichment in most high field strength elements (HFSE), but depletion in Nb, Ta, Sr and Ti. The geochemical data suggest that the Ediacaran sedimentary rocks formed in a continental island arc setting. Together with compiled zircon εHf(t) values, Ediacaran rifting in the Sichuan Basin is consistent with retreating subduction. We propose a weak, flat, retreating subduction-related intracontinental passive rift model for the Ediacaran South China Block. This case study suggests that the retreating subduction could not only trigger back-arc basin development, but could also lead to the magma-poor passive rift basin.