Neoproterozoic Jiangnan Orogeny in southeast Guizhou, South China: evidence from U-Pb ages for detrital zircons from the Sibao Group and Xiajiang Group

摘要

The Jiangnan Orogeny generated regional angular unconformities between the Xiajiang Group and the underlying Sibao Group in the western Jiangnan Orogen along the southeastern margin of the Yangtze Block in southeast Guizhou, South China. Laser ablation - inductively coupled plasma - mass spectrometry (LA-ICP-MS) U-Pb zircon dating of two samples of the Motianling granitic pluton yielded U-Pb zircon ages of 826.2 +/- 3.4 and 825.5 +/- 6.1 Ma, with an average age of 825.6 +/- 3.0 Ma, which is considered the minimum depositional age of the Sibao Group. The U-Pb ages of the youngest detrital zircon grains from the Sibao Group and the Xiajiang Group yielded average ages of 834.9 +/- 3.8 and 794.6 +/- 4.2 Ma, respectively. The depositional age of the Sibao Group can be constrained at 825-835 Ma, and deposition of the Xiajiang Group did not begin before ca. 800 Ma. These results suggest that the Jiangnan Orogeny, which led to the assembly of the Yangtze and Cathaysia blocks, ended at 795-835 Ma on the western segment of the Jiangnan Orogen. The detrital zircon distribution spectrums of the Sibao and Xiajiang groups suggest a provenance from Neoproterozoic basement sedimentary sequences along with a mixture of local Neoproterozoic subduction-related felsic granitoids, distant plutons from the western Yangtze Block and eastern Jiangnan Orogen, and recycled materials from the interior of the Yangtze Block. By comparing the basin evolution histories and magmatic and metamorphic events along the continental margins of the Rodinia supercontinent, it is proposed that the South China Block might have been located at the periphery, adjacent to North India and East Antarctica, rather than in the interior of Rodinia in Neoproterozoic time.