Through the successive 1∶50000 regional geological survey in Huili—Dongchuan—Yimen area, a large set of rhyolitic tuff interlayers with a width of about 20~45 m at the base of the Heishantou Formation of the "Kunyang Group" in the Liujie area of Yimen, central Yunnan.Results: Zircon laser ablation inductively coupled plasma mass spectrometry (LA- ICP- MS) was employed to date the rhyolitic tuff , with a weighted average zircon U- Pb age of 1412±20 Ma (MSWD=0.72, n=15), representing the formation age of the tuff, This result shows that the sedimentary age of the Heishantou Formation is in the middle Proterozoic, which is the first 1.4Ga magmatic record found in central Yunnan, and defines the sedimentary age of the Kunyang Group as the middle Proterozoic to the early Neoproterozoic (1.5~0.9Ga). Conclusions: The study of rock geochemistry and Hf isotopes shows that the rhyolitic tuff is rich in large ion lithophile elements such as K, Rb, and Th, and deficient in high field strength elements such as Nb, Ta, and Ti, displaying characteristics similar to island arc calc- alkaline volcanic rocks, zircon εHf(t) value has both positive and negative values, indicating that the magma source area was mainly formed by the melting of subducted oceanic crust sheets in the Paleoproterozoic (2438~2030 Ma), accompanied by the mixing of crustal materials. This age data is similar to that the age of the important end- member gabbro (~1.4 Ga) in the Mesoproterozoic Caiziyuan ophiolite melange belt, and may be the product of the beginning of the reduction of Caiziyuan—Yimen Ocean, which is an important period of the splitting and subduction transformation of Caiziyuan—Yimen Ocean. Combined with the high pressure metamorphic rocks and ophiolite melanges successively discovered in Hekou, Xinping and Cuoke, the tectonic melange phenomenon of the L zhijiang fault is found in the L zhizhen — Yuanmou area, which infers that Caiziyuan—Yimen Ocean is likely to be distributed along the L zhijiang fault zone from the west to the south along the Jinsha River in the Hekou from Tong’an—Hekou— Yuanmou—L zhi—Xinping.