Sedimentology And Detrital Zircon Chronology Of Late Paleozoic Siliciclastic Successions In The Eastern Part Of The Changning-menglian Belt

The geological evolution of Tethys is one of the most complicated issues in geological research.Western Yunnan is a key area to explore the evolution of Tethys for the significant geological documents kept in this region.The Changning-Menglian Belt,a N–S striking structural zone,located between the Baoshan Block and Simao Block in western Yunnan has been considered as the remains of Paleo-Tethys and the boundery between the Gondwana-affinity blocks and Cathaysian-affinity blocks.The siliciclastic Nanduan Formation and Laba Formation widely distributed in the eastern part of the Changning-Menglian Belt are the objects of this study.Methods of biostratigraphy,petrography,geochemistry,and detrital zircon geochronology are employed to explore the sedimentrary characteristics and provenance information of the Nanduan Formation and the Laba Formation.Furthermore,the result could make us better understand the geological evolution of the Paleo-Tethys with the clues obtained.We found in two horizons rich small ammonoids in the upper part of the Nanduan Formation.From the specimens obtained we identified eight ammonoid genera,including Epicanites sp.,Megapronorites sp.,Syngastrioceras sp.,Somoholites sp.,Prolecanites? sp.,Kardailites? sp.,Homoeras? sp.and Fayettevillea.These ammoniod fossils enable us to assign a Serpukhovian age to the beds containing the fossils.No identifiable fossils have been found in the clastics of the the Laba Formation.However,previous works reported Permian fusulinids in the limestone lenses intercalated in the middle part of the Laba Formation.Our field observations show that the so-called limestone lenses very probably resulted from later tectonic movement instead of sedimentary process.We speculate at the moment that the Laba Formation may have a Carboniferous(to Permian)age according to the fossil evidence reported in previous work.Grain-size analysis exhibits three populations in probability cumulative curves in most of the Nanduan Formation sandstone samples,including traction population,saltation population and suspension population.The probability cumulative curves show similar characteristics with those of shallow marine environments which are quite different from those of the turbidite.Sedimentary environment discriminant functions and empirical graphs also show that the sandstone of the Nanduan Formation demonstrates similar features of shallow marine environments.Moreover,environmental discriminant diagrams show that samples of the Nanduan Formation distribute close to those of the shallow marine environment.Thus the Nanduan Formation is likely to be shallow marine deposits rather than deep basin turbidity deposits.Compared with the Nanduan Formation,the overlying Laba Formation contains more mudstone and bioturbation.Also considering other sedimentary evidence reported in literature,the Laba Formation is interpreted to be deposited in deep shelf or outer shelf slope,which represent deeper environment than where the Nanduan Formation were deposited.Petrologic characteristic shows highly mature composition of the Nanduan Formation.Monocrystalline quartz in some samples is up to 90% with a rounded shape.And lithic fragments in the sandstone are mainly sedimentary rocks which indicate a sedimentary provenance.Heavy mineral assemblages show a dominant content of the ZTR,and few unstable heavy mineral grains.Sm+Gd+Dy vs La+Ce+Nd-Yb+Y diagram shows that the mudstone of the siliciclastic succession was derived from the terrigenous region instead of the plegic environment.Petrologic characteristic of the Laba Formation is similar to the Nanduan Formation except that the maturity of the Laba Formation is somewhat poorer than the Nanduan Formation.Geochemistry analysis shows that both of the Nanduan Formation and the Laba Formation were deposited in passive continental margin.Dominant age populations of ca.950 Ma and ca.550 Ma of the Nanduan Foramtion and Laba Formation can be correlated with the magmatic events that occurred in Eastern Ghats-northern Prince Charles(India–Antarctica)(ca.950 Ma)and Pan-African orogens(Australia–India)(ca.550 Ma),which are considered,therefore,to be the main provenance of the Late Paleozoic siliciclastic succession in the eastern part of the Changning-Menglian Belt.Provenance of the Nanduan Formation and Laba Formation is similar with that of the Paleozoic metaclastic rocks,Lancang Group located in the eastern part of the Changning-Menglian Belt,Ximeng Group and Mentong Group located in the western part of the Changning-Menglain Belt.We interpreted these clastic sequences are derived from the north margin of the Gondwanaland.In contrast,provenance of the Paleozoic sequence in the western part of the Changning-Menglian Belt named Nanpihe Group,shows similarity with that of the Paleozoic clastic rocks in Simao block according to our study.Therefore,the Nanduan Formation,Laba Formation and Paleozoic metaclastic rocks in the Changning-Menglian Belt are autochthonous sedimentary successions of the north margin of the Gondwanaland.The Nanpihe Group,Wenquan Formation(Lalei Formation)which shows similar sedimentary characterics with the Nanpihe Group,and the limestones of seamount resemblance,the oceanic bedded cherts and basic/ultra-basic igneous rocks which were considered as the remains of the Paleo-Ththys in the central part of the Changning-Menglian Belt,are very probably overthrusted slices(allochthonous)when the Paleo-Tethys vanished.Thus the Chaning-Menglian Belt is unlikely the suture of the Paleo-Tethys as previously believed.We tend to believe that the Paleo-Tethys suture is probably located in the Lancangjiang zone.