Age Determination And Tectonic Environment Of Lancang Group In Changning-Menglian Belt

The Changning-Menglian tectonic belt in Sanjiang,western Yunnan,located at the junction of Laurentia and Gondwana,is an important part of the Proto-Tethys and Paleo-Tethys tectonic domains.It has become a hot spot of research of Tethys because it preserves relatively complete geological records of Tethys tectonic that can represent various evolutionary stages.Many representative achievements have been published in mainstream academic journals at home and abroad,which has attracted wide attention of domestic and international geologists,especially the controversy over the age,provenance characteristics and metamorphic age of the Lancang Group on the east side of the Changning-Menglian tectonic belt.In this paper,the Lancang Group in LincangShuangjiang area is selected as the research object,and comprehensive studies on tectonic geology,petrology and isotope geochronology are carried out.Finally,some achievements and new understandings are obtained.1)Based on the contact relationship between geological units on both sides of the tectonic belt and the field geological survey of structural deformation characteristics,the difference of deformation at different tectonic levels is compared.we hold the opinion that there is a fault contact between the strata of Lancang Group,Nanduan Formation and Laba Formation on the eastern side of the tectonic belt and an angular unconformity contact with the clastic rock strata of Sanchahe Formation of Late Triassic in the overlying strata.From bottom to top,the metamorphic-deformation degree of Lancang Group,Nanduan Formation and Laba Formation changes from deep to shallow.The structural types of gneiss,mylonite,schist and phyllite change.The metamorphic degree changes from greenschist facies to eclogite facies,and a large number of rootless intrafolial folds,boudinage and quartz vein crumpled texture develop.Kinematic characteristics indicate that Changning-Menglian Belt underwent the Indosinian Paleo-Tethys collision from east to west.Orogenic thrust faulting.2)The LA-ICP-MS U-Pb chronology and cathodoluminescence(CL)microscopic characteristics of detrital zircons from two samples of quartz mica schist and quartz sandy schist of Lancang Group were studied.The zircon cathodoluminescence images(CL)showed cylindrical and quasi-circular structures.We take it that detrital zircons formed by transportation and abrasion to a certain extent were obtained.Zircon U-Pb isotope chronology obtained that the Th/U ratio of all zircons was between 0.68 and 2.17,indicating that all zircons are magmatic zircons.Seven 206Pb/238 U and 207Pb/206 Pb ages are obtained: 478-650Ma(peak-558Ma),700-843Ma(peak-753Ma),916-1107Ma(peak-997Ma),1142-1311 Ma(peak-1182Ma),1402-1455Ma(peak-1450Ma),1627-1862 Ma(peak-1759Ma)and 2421-2548 Ma(peak-2498Ma).The youngest clastic zircon age range is 478-559 Ma(peak-507 Ma),indicating that the clastic rocks of Lancang Group were formed not earlier than 507 Ma,that is,later than Cambrian,which is consistent with the previous understanding that the sedimentary age of Lancang Group was Ordovician(462-545 Ma).In the comparative study of the clastic zircon age spectrum of the Lancang Group,we hold opinion that the provenance characteristics of the Lancang Group are similar to those of the South Qiangtang and Tethyan Himalaya,showing the characteristics of the affiliated Gondwana.3)Microscopic characteristics,zircon SHRIMP U-Pb geochronology and Lu-Hf isotopes of retrograde eclogites and wall rocks from Lancang Group in Shuangjiang Genhendazhai area of Changning-Menglian structural belt are comprehensively studied.The weighted average ages of 206Pb/238 U obtained from retrograde eclogites are 226.4+2.1Ma(n=13,MSWD=1.11)and 243.2+4.5Ma(n=5,MSWD=0.14),respectively.The ?Hf(t)values of the two stages were-2.99~-1.63 and-19.43~-8.83,respectively,which implies that retrograde eclogites record the process of deep subduction mantle metamorphism and retrograde crustal metamorphism.The weighted average age of 206Pb/238 U obtained from amphibolite-mica schist in Eclogite wall rocks is 244+4 Ma(n=13,MSWD=1.5),and the 206Pb/238 U frequency spectrum obtained from 9 quartz-feldspathic accessory gneiss zircons in three groups were 235.9~242.1 Ma,553.2 Ma and 1877-2562 ma,respectively.The weighted average age of the youngest 3 zircons was 239.9 4.4Ma(n=3,MSWD=0.8).These data further confirm the Lancang Group in Shuangjiang Genhendazhai area underwent deep subduction of the Indosinian continental crust.This data is quite different from previous understanding of the age of wanhe ophiolite(or lancang group)at 470.8 5.3Ma.4)Based on the above research,this paper concluded that the Cambrian-ordovician Lancang group on the east side of the Changning-Menglian structure experienced the indosinian apaleo-tethys ocean closed and continental collision orogeny,that this orogeny formed eclogite facies-amphibolite facies metamorphic rocks,such as eclogite,amphibolic mica schist and quartz-feldspathic accessory gneiss,Micaceous quartz schist,and that the process of the prograde metamorphism which from Deep subduction of retrograde eclogites dives into the mantle and the retrograde metamorphism which from the retrograde eclogites fold back into the crust.