Abstract: The basement component of the Pingbian area, SE Yunnan, has not been well studied before. The tectonic attribution
of this area is important for constraining the western boundary between the Yangtze and Cathaysia blocks. U-Pb-Hf isotopic
compositions of detrital zircons from the Sinian-Cambrian sedimentary rocks in the Pingbian area are used to provide new
evidence for determining this boundary. Zircon U-Pb dating results indicate that two sedimentary rocks from the Sinian and
Cambrian strata share the similar detritus components. Most of them were formed in Neoproterozoic (700-937 Ma) with an
age peak at ~815 Ma. Both sedimentary rocks also contain a small amount of Paleo- to Mesoproterozoic detritus. Six youngest
concordant zircons have ages of 696-761 Ma, suggesting that the formation age of the Pingbian Group sedimentary rocks probably
formed in the Late Neoproterozoic, equivalent to Sinian time. The Pingbian Group sample shows the unimodal age spectrum
with dominant Neoproterozoic detrital zircons, similar to those sedimentary rocks in the southern and western Yangtze Block,
but different from the sedimentary rocks in the Cathaysia and Indochina blocks. Hf isotopic characteristics of detrital zircons
also suggest the affinity of the Pingbian Group with the Yangtze Block (especially the western margin). According to these data
and other evidence, we believe that the Pingbian area in SE Yunnan Province belongs to the Yangtze Block. Thus, the western
boundary between the Yangtze and Cathaysia blocks should be located at south or southeast of the study area, rather than the
Shizong-Mile-Luodian Fault. The detrital zircon age distribution patterns also indicate that the Sinian Pingbian Group was
deposited in the back-arc basin, implying that Neoproterozoic subduction along the southwestern margin of the Yangtze Block
lasted until ~752 Ma.

Key words: Yangtze/Cathaysia boundary, Sinian sedimentary rock, detrital zircon U-Pb-Hf isotope, Southeastern Yunnan