Abstract:

Through the 1∶250,000 regional geological survey of the Yulin sheets of the southeastern Guangxi，this paper has distinguished an archaic unconformity event from metamorphic series that had been named Yunkai Group formerly，showing that the middle-shallow metamorphic series unconformity cover on the middle-deep metamorphic series. They have great distinction in the composition of rocks，sedimentary circumstance of originality rocks and deformation and metamorphism. The study of isotope chronology and P-T-t path of metamorphism indicates that the middle-shallow metamorphic series were formed posterior to the middle-deep metamorphic series. The ages of garnetpyroxenite of the middle-deep metamorphic series were (1,8944±17.36)Ma，(1,846.8±58.58)Ma and three zircon dating gave a concordia ²⁰⁷Pb/²⁰⁶Pb  age of (1,817±36)Ma using the SHRIMP zircon U-Pb method，indicating that it is the product of Paleoproterozoic. Some microfossils of the middle-shallow metamorphic series were well conserved，the ages of microfossils assemble are Changcheng period, Jixian period or Qingbaikou period.The age of the metabasic volcanic rocks of the middle-shallow metamorphic series is (1,462±28)Ma using the SHRIMP zircon U-Pb method，indicating that it is the product of Meso-Neoproterozoic. It can be concluded that an obvious tremendous disaster event has occurred between the middle-shallow metamorphic series and the middle-deep metamorphic series，indicating that they are the products of Lüliang period of last Paleoproterozoic collision orogenics. The middle-deep metamorphic series were separated from “Yunkai Group”and named “Tiantangshan Group Complex”.

Key words: Proterozoic, Tiantangshan Group Complex, Yunkai Group, unconformity event, Yunkai area