SUCCESSIONS OF THE MISSISSIPPIAN NANDUAN FORMATION IN THE"MIDDLE-UPPER DEVONIAN"AND THE"PERMIAN LABA FORMATION"OF THE CHANGNING-MENGLIAN BELT,WESTERN YUNNAN,CHINA AND ITS GEOLOGICAL SIGNIFICANCE
The Changning-Menglian Belt in western Yunnan,China,is an area of great significance to decipher the geological history of the Paleo-Tethys.Identifying the components of various stratigraphic units and tracing their distribution pattern are fundamental for deep understanding of the geological evolution of the belt.The Mississippian Nanduan Formation,which has a large thickness and high maturity,distributed in the east zone of the Changning-Menglian Belt,was deposited in a shallow marine environment in a passive continental margin.Recently,we identified clastic rock sequences from the"Middle-Upper Devonian"and"Permian Laba Formation"in the west zone of the Changning-Menglian Belt.Fossil assemblages,lithological features,and zircon age spectra indicate that they are outcrops of the Nanduan Formation.The newly found sequences of the Nanduan Formation in the west zone exist randomly,normally with fault contact and with siliceous,clastic,and limestone slices of varied ages.Previous studies show that the Nanduan Formation represents"autochthonous sediments"in a passive continental margin of Gondwana-derived Baoshan-Shan block.The siliceous,clastic rock slices,limestones,and basalts of different ages in the west zone are allochthons of Paleo-Tethyan origin.During the closing period of the Paleo-Tethys,these allochthonous slices were emplaced on to the"autochthonous sediments"of the Baoshan-Shan block.Meanwhile,folding,faulting and other deformations,and also later erosion resulted in the pattern of mixed distribution of the"autochthonous"Nanduan Formation and the allochthonous slices of different ages,and varied tectono-sedimentary settings in the west zone of the Changning-Menglian Belt.
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