Component variation in the late Neoproterozoic to Cambrian sedimentary rocks of SW China – NE Vietnam, and its tectonic significance

The Nanwenhe-Song Chay (NSC) area straddles the border of SW China and NE Vietnam, at the tectonic junction of the Yangtze, Cathaysia, and Indochina blocks. However, the lack of a systematic understanding of the basement rocks in this area has led to a debate about its tectonic affinity. This study presents integrated bulk geochemical and zircon U-Pb-Hf isotopic data for the basement rocks in the NSC area. Zircon U-Pb dating results suggest that the Mengdong Group in SW China and the Thac Ba Formation in NE Vietnam were deposited in the late Neoproterozoic (<619–592 Ma), while the Tianpeng Formation in SW China and the Ha Giang Formation in NE Vietnam were deposited in the Cambrian (<527–507 Ma). Bulk geochemical data and zircon Hf-isotope compositions indicate that the sources of these sedimentary rocks were dominated by felsic rocks. The sources of the late Neoproterozoic sedimentary rocks probably underwent weaker weathering under drier and colder conditions than those of the Cambrian ones. The late Neoproterozoic Mengdong Group and the Thac Ba Formation contain abundant 802–747 Ma zircons. Their age distributions and the Hf-isotope compositions are similar to those of Neoproterozoic sedimentary rocks in the southern Yangtze Block (Fanjingshan-Sibao area). In contrast, the Cambrian Tianpeng and Ha Giang formations are characterized by Grenvillian detrital zircons with an age peak at ∼980 Ma. The age spectra and Hf-isotope compositions of the detrital zircons from these rocks are similar to those in the Cathaysia Block (Nanling-Yunkai area) and/or Indochina Block. Integration of our data with published geological data suggests that the Precambrian basement under the NSC area is the part of the Yangtze Block. Thus, the western boundary between Yangtze and Cathaysia blocks must be located to the south or southeast and the boundary between the South China and Indochina blocks should be to the southwest of the NSC area. Numerous Pan-African (600–500 Ma) detrital zircons and the diagnostic age spectra of detrital zircons in the early Paleozoic sedimentary rocks indicate that the SCB was located at the northern margin of East Gondwana during the early Paleozoic. The dramatic change in the clastic components from the late Neoproterozoic to Cambrian sedimentary rocks suggests that the NSC area was probably affected by the Pan-African orogeny.