The Chronological Characteristics And Significance Of "WU LING SHANGROUP" InChangNing,Western Yunnan

The wu-liang-shan Group consists of thick epimetamorphic terrigenous clasolite，which occurs widely around the lancang River. The deposition age of the typicalwu-liang-shan Group is late Paleozoic period. The deposition age of “wu-liang-shan”Group grouthed in the region of interest is controversial about the late Paleozoicperiod and the Mesozoic period.The “wu liang shan” Group of the could be made up by4segments according tothe rock assemblages which have something different from the typical Group. Therock assemblages are grayish green-white deterioration in thin-bedded fine–siltstone with small amounts of calcium nodules and limestone and thick-beddedsandstone gray in a small amount of thick-bedded medium-grained sandstone andthin-bedded gray-green slate and phyllite and purple-thin-bedded gray-green slateand phyllite.U-Pb dating of detrital zircons from the “wu-liang-shan” Group yieldstheyoungest detrital zircons age of127±1Ma～194±1Ma， as the maximumdeposition age of the wu-liang-shan Group. Therefore，the deposition age of thewu-liang-shan Group can be constrained to be mesozoic era.Detrital zircons from the sandstone on the top of the sandwich samples of phyllitehave the age composition of the latest age of194±1Ma; the age spectrum of detritalzircon of the gray sandstone contains the newest age of135±1Ma. They make theconclusion that the depositional age of the “wu liang shan”Group in the research areais after the Mesozoic.Detrital zircon dating results of the research area is limited thedepositional age of the “wu liang shan”Group to the Mesozoic period. Then making acomparison with the adjacent strata,we get that: The first segment is the equivalent ofthe formation of huakaizuo(J2h) in Middle Jurassic; The second segment is theequivalent of the formation of jingxing（K1j）in Lower Cretaceous; The third segmentis the equivalent of the formation ofjingxing（K1j） in Lower Cretaceous; The fourthsegment is the equivalent of the formation of huakaizuoformation (J2h) in MiddleJurassic.Differences in the composition of the different samples of detrital zircon agesreveal that provenance differences exist between the second segment and thefourthsegment. The presence of magmatic zircons Permian–Carboniferous revealsthat the diving of the lancang ocean has began in the Carboniferous-Permianperiod.The presence of magmatic zircons which has age of135±1Ma shows that therewas volcanic activity in the lancang orogen at Jurassic.