Does Neoproterozoic Nam Co formation in Northwest Vietnam belong to South China or Indochina?

Abstract

The Nam Co belt is located to the northeast of Song Ma fault in Northwestern Vietnam. The nature of the Precambrian basement rocks in this area and its tectonic attribute have still been controversial. In this study, zircon U-Pb-Hf isotopic compositions and whole rock geochemical compositions of meta-sedimentary rocks from the Nam Co Formation in this belt have been integrated to reveal the components and tectonic setting of the formation of the basement rocks in the Nam Co belt. Zircon U-Pb dating results show that the Nam Co Formation is of middle-late Neoproterozoic age. The lower Nam Co Formation probably formed between 821 and 726 Ma, and the upper Formation was deposited later than 726–721 Ma. The sedimentary rocks from the lower and upper Nam Co formations are characterized by numerous Neoproterozoic (900–800 Ma) detrital materials, but the upper part contains more Meso- to Paleoproterozoic sediments than the lower part. Detrital zircon age distribution and Hf-isotope compositions of the Nam Co Formation are similar to those of Neoproterozoic sedimentary rocks in the southern Yangtze Block. Both zircon U-Pb-Hf isotopic and whole rock geochemical data suggest that the source materials of the Neoproterozoic Nam Co Formation were dominated by felsic rocks with minor mafic materials, and the detrital materials were mainly derived from the Jiangnan Orogenic Belt in the southern Yangtze Block, with minor contribution from the Panxi belt in the western Yangtze Block, Ailaoshan-Song Hong and Phan Si Pan belts. The lower Nam Co Formation contains more sediments derived from the Panxi belt than the upper part. The middle-late Neoproterozoic Nam Co Formation probably was deposited in a continental extensional basin, and the sedimentation environment in later period (<726–721 Ma) was more stable. Integration of our data with other lines of evidence suggests that the Nam Co Formation belongs to the basement of the Yangtze Block, and the Nam Co belt was originally located in the southwest corner of the Yangtze Block. This further suggests that the Song Ma fault probably is the boundary between the South China and Indochina blocks.