| The western Yunnan region has experienced a relatively complete tethys evolution process,and has retained rich geological records related to it,which is the key position for studying the global Tethys evolution.Changning-Menglian suture zone is considered to be the ancient tethys ocean basin closed the main suture zone,in recent years for the Proto-Paleo Tethys ocean evolution relationship,the Paleo Tethys ocean closed time limit,the original Precambrian crystalline basement,high-pressure metamorphic belt and other hot spots of research more in-depth,the understanding of Changning-Menglian suture zone is more clear,but on the Proto-Paleo Tethys ocean whether continuous evolution there are larger issues,Besides,the evidence of the closure of the Paleo Tethys ocean except ophiolite is not sufficiently studied.So this article selected Changning-Menglian suture zone in Tongchangjie-Nantinghe basic rock,Wanhe basic rock,Genheng eclogite,Suyi blueschist petrography,LA-ICP-MS zircon U-Pb chronology,whole rock geochemical research,get relevant formation age,rock formation,tectonic setting,further in-depth study Changning-Menglian Tethys evolution.Two ophiolite melange belts related to the closure of the Proto-Paleo Tethyan ocean have been discovered in the suture zone.Based on this,we have identified a series of greenschist phase transition mafic rocks related to the Tethys evolution,whose formation age is later than the Late Triassic.The fractionation of light and heavy rare earth elements is obvious,and Pr and Nd elements are enriched.The young captured zircons range in age from 222 to 201Ma.It can be specifically divided into two types:Tongchangjie-Nantinghe metabasic rocks,with Si O2 content ranging from 49.1 to 50.3%,belonging to the low-potassium tholeck series.The Ree distribution curve and trace element cobweb diagram are similar to N-MORB type on the whole.The source area may be garnet peridotite loss mantle,which was subjected to crystallization differentiation and assimilation and contamination to a certain extent during the emplacement of magma.Both of them have the chemical elements characteristics of island arc and mid-ocean ridge,corresponding to the island arc front arc tectonic environment in the continental margin.The Si O2 content of wanhe mafic rocks ranges from 43.1 to 45.8%,belonging to medium-k calc-alkali series.The Ree distribution curve and trace element cobweb diagram are similar to E-MORB type on the whole,and the source area may be garnet peridotite enriched mantle.The magma evolution process is simple,with a small amount of separation crystallization and assimilation mixing,which is related to the upwelling of mantle magma in the continental asthenosphere.Corresponding to the tectonic environment of the continental plate,mantle plumes may be involved.In addition to the above mentioned green schist phase transition basalt,eclogite in Genheng area and blue schist in Suyi area were also collected.Compared with the high pressure metamorphic rocks discovered in Changning-Menglian belt,the protolith ages of genhian eclogite and Suyi blue schist are younger,243.8Ma and 245.3Ma respectively.Their protoliths are younger,such as the Genhen eclogite protoliths 243.8Ma and the Suyi blue schist protoliths 245.3Ma,both of which were derived from the new oceanic crust and subducted under the Lanping-Simao plate under high pressure metamorphism due to the strong plate dragging force of the old oceanic crust.Combined with regional data,the study of mafic rocks in the Changning-Menlian Paleo Tethys zone provides new evidence for subduction-collision and post-collision extension events in the last phase of the Changning-Menlian Paleo Tethys Ocean Wilson cycle.The Changning-Menlian Paleo Tethys Ocean was in the stage of subduction-collision during Early-Middle Triassic,and the fresh oceanic crust underwent high-pressure metamorphism.The basin-mountain transition occurred in the Late Triassic,which resulted in the formation of mafic rocks with the prearc and intraplate properties of continental margin island arc,and then the collision orogenic stage,which ended the evolution of the Changning-Menlian Paleo Tethys ocean. |
